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Cell_Biology_Alberts. the Composition of the Cell Wall Depends on the Cell type


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Cell_Biology_Alberts. All cell walls in plants have their origin in dividing cells, as the cell plate forms during cytokinesis to create a new partition wall between the daughter cells (discussed in Chapter 17). The new cells are usually produced in special regions called meristems, and they are generally small in comparison with their final size. To accommodate subsequent cell growth, the walls of the newborn cells, called primary cell walls, are thin and extensible, although tough. Once cell growth stops, the wall no longer needs to be extensible: sometimes the primary wall is retained without major modification, but, more commonly, a rigid secondary cell wall is produced by depositing new layers of matrix inside the old ones. These new layers generally have a composition that is significantly different from that of the primary wall. The most common additional polymer in secondary walls is lignin, a complex network of covalently linked phenolic compounds found in the walls of the xylem vessels and fiber


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Cell_Biology_Alberts. that of the primary wall. The most common additional polymer in secondary walls is lignin, a complex network of covalently linked phenolic compounds found in the walls of the xylem vessels and fiber cells of woody tissues.


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Cell_Biology_Alberts. Although the cell walls of higher plants vary in both composition and organization, they are all constructed, like animal extracellular matrices, using a structural principle common to all fiber-composites, including fiberglass and reinforced concrete. One component provides tensile strength, while another, in which the first is embedded, provides resistance to compression. While the principle is the same in plants and animals, the chemistry is different. Unlike the


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Cell_Biology_Alberts. Figure 19–61 Plant cell walls. (a) Electron micrograph of the root tip of a rush, showing the organized pattern of cells that results from an ordered sequence of cell divisions in cells with relatively rigid cell walls. In this growing tissue, the cell walls are still relatively thin, appearing as fine black lines between the cells in the micrograph. (B) Section of a typical cell wall separating two adjacent plant cells. the two dark transverse bands correspond to plasmodesmata that span the wall (see Figure 19–27). (a, courtesy of C. Busby and B. Gunning, Eur. J. Cell Biol. 21:214– 223, 1980. With permission from Elsevier; B, courtesy of Jeremy Burgess.) animal extracellular matrix, which is rich in protein and other nitrogen-containing polymers, the plant cell wall is made almost entirely of polymers that contain no nitrogen, including cellulose and lignin. For a sedentary organism that depends on CO2, H2O, and sunlight, these two abundant biopolymers represent “cheap,” carbon-based


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Cell_Biology_Alberts. of polymers that contain no nitrogen, including cellulose and lignin. For a sedentary organism that depends on CO2, H2O, and sunlight, these two abundant biopolymers represent “cheap,” carbon-based structural materials, helping to conserve the scarce fixed nitrogen available in the soil that generally limits plant growth. Thus trees, for example, make a huge investment in the cellulose and lignin that comprise the bulk of their biomass.


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Cell_Biology_Alberts. In the cell walls of higher plants, the tensile fibers are made from the polysaccharide cellulose, the most abundant organic macromolecule on Earth, tightly linked into a network by cross-linking glycans. In primary cell walls, the matrix in which the cross-linked cellulose network is embedded is composed of pectin, a highly hydrated network of polysaccharides rich in galacturonic acid. Secondary cell walls contain additional molecules to make them rigid and permanent; lignin, in particular, forms a hard, waterproof filler in the interstices between the other components. All of these molecules are held together by a combination of covalent and noncovalent bonds to form a highly complex structure, whose composition, thickness, and architecture depend on the cell type.


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Cell_Biology_Alberts. The plant cell wall thus has a “skeletal” role in supporting the structure of the plant as a whole, a protective role as an enclosure for each cell individually, and a transport role, helping to form channels for the movement of fluid in the plant. When plant cells become specialized, they generally adopt a specific shape and produce specially adapted types of walls, according to which the different types of cells in a plant can be recognized and classified. We focus here, however, on the primary cell wall and the molecular architecture that underlies its remarkable combination of strength, resilience, and plasticity, as seen in the growing parts of a plant. the tensile Strength of the Cell Wall allows plant Cells to Develop turgor pressure


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Cell_Biology_Alberts. The aqueous extracellular environment of a plant cell consists of the fluid contained in the walls that surround the cell. Although the fluid in the plant cell wall contains more solutes than does the water in the plant’s external milieu (for example, soil), it is still hypotonic in comparison with the cell interior. This osmotic imbalance causes the cell to develop a large internal hydrostatic pressure, or turgor pressure, which pushes outward on the cell wall, just as an inner tube pushes outward on a tire. The turgor pressure increases just to the point where the cell is in osmotic equilibrium, with no net influx of water despite the salt imbalance. The turgor pressure generated in this way may reach 10 or more atmospheres, about five times that in the average car tire. This pressure is vital to plants because it is the main driving force for cell expansion during growth, and it provides much of the mechanical rigidity of living plant tissues. Compare the wilted leaf of a


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Cell_Biology_Alberts. is vital to plants because it is the main driving force for cell expansion during growth, and it provides much of the mechanical rigidity of living plant tissues. Compare the wilted leaf of a dehydrated plant, for example, with the turgid leaf of a well-watered one. It is the mechanical strength of the cell wall that allows plant cells to sustain this internal pressure.


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Cell_Biology_Alberts. the primary Cell Wall Is Built from Cellulose Microfibrils Interwoven with a Network of pectic polysaccharides Cellulose gives the primary cell wall tensile strength. Each cellulose molecule consists of a linear chain of at least 500 glucose residues that are covalently linked to one another to form a ribbonlike structure, which is stabilized by hydrogen bonds within the chain (Figure 19–62). In addition, hydrogen bonds between adjacent cellulose molecules cause them to stick together in overlapping parallel arrays, forming bundles of about 40 cellulose chains, all of which have the same polarity. These highly ordered crystalline aggregates, many micrometers long, are called cellulose microfibrils, and they have a tensile strength comparable to that of steel. Sets of microfibrils are arranged in layers, or lamellae, with each microfibril about 20–40 nm from its neighbors and connected to them by long cross-linking


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Cell_Biology_Alberts. Figure 19–62 Cellulose. Cellulose molecules are long, unbranched chains of β1,4-linked glucose units. Each glucose residue is inverted with respect to its neighbors, and the resulting disaccharide repeat occurs hundreds of times in a single cellulose molecule. about 16 individual cellulose molecules assemble to form a strong, hydrogen-bonded cellulose microfibril. glycan molecules, which are attached by hydrogen bonds to the surface of the microfibrils. The primary cell wall consists of several such lamellae arranged in a plywoodlike network (Figure 19–63).


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Cell_Biology_Alberts. glycan molecules, which are attached by hydrogen bonds to the surface of the microfibrils. The primary cell wall consists of several such lamellae arranged in a plywoodlike network (Figure 19–63). The cross-linking glycans are a heterogeneous group of branched polysaccharides that bind tightly to the surface of each cellulose microfibril and thereby help to cross-link the microfibrils into a complex network. There are many classes of cross-linking glycans, but they all have a long linear backbone composed of one type of sugar (glucose, xylose, or mannose) from which short side chains of other sugars protrude. It is the backbone sugar molecules that form hydrogen bonds with the surface of cellulose microfibrils, cross-linking them in the process. Both the backbone and the side-chain sugars vary according to the plant species and its stage of development.


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Cell_Biology_Alberts. Coextensive with this network of cellulose microfibrils and cross-linking glycans is another cross-linked polysaccharide network based on pectins (see Figure 19–63). Pectins are a heterogeneous group of branched polysaccharides that contain many negatively charged galacturonic acid units. Because of their negative charge, pectins are highly hydrated and associated with a cloud of cations, resembling the glycosaminoglycans of animal cells in the large amount of space they occupy (see Figure 19–33). When Ca2+ is added to a solution of pectin molecules, it cross-links them to produce a semirigid gel (it is pectin that is added to fruit juice to make jam set). Certain pectins are particularly abundant in the middle lamella, the specialized region that cements together the walls of adjacent cells (see Figure 19–63); here, Ca2+ cross-links are thought to help hold cell wall components together. Although covalent bonds also play a part in linking the components, very little is known about


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Cell_Biology_Alberts. cells (see Figure 19–63); here, Ca2+ cross-links are thought to help hold cell wall components together. Although covalent bonds also play a part in linking the components, very little is known about their nature. Regulated separation of cells at the middle lamella underlies such processes as the ripening of tomatoes and the abscission (detachment) of leaves in the fall.


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Cell_Biology_Alberts. In addition to the two polysaccharide-based networks that form the bulk of all plant primary cell walls, proteins are present, contributing up to about 5% of the wall’s dry mass. Many of these proteins are enzymes, responsible for wall turnover and remodeling, particularly during growth. Another class of wall proteins, like collagen, contains high levels of hydroxyproline. These proteins are thought to strengthen the wall, and they are produced in greatly increased amounts as a local response to attack by pathogens. From the genome sequence of Arabidopsis, it has been estimated that more than 700 genes are required to synthesize, assemble, and remodel the plant cell wall.


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Cell_Biology_Alberts. Figure 19–63 Scale model of a portion of a primary plant cell wall showing the two major polysaccharide networks. the orthogonally arranged layers of cellulose microfibrils (green) are tied into a network by the cross-linking glycans (red) that form hydrogen bonds with the microfibrils. this network is coextensive with a network of pectin polysaccharides (blue). the network of cellulose and cross-linking glycans provides tensile strength, while the pectin network resists compression. Cellulose, cross-linking glycans, and pectin are typically present in roughly equal amounts in a primary cell wall. the middle lamella is especially rich in pectin, and it cements adjacent cells together.


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Cell_Biology_Alberts. Once a plant cell has left the meristem where it is generated, it can grow dramatically, commonly by more than a thousand times in volume. The manner of this expansion determines the final shape of each cell, and hence the final form of the plant as a whole. Turgor pressure inside the cell drives the expansion, but it is the behavior of the cell wall that governs its direction and extent. Complex wall-remodeling activities are required, as well as the deposition of new wall materials. Because of their crystalline structure, the individual cellulose microfibrils in the wall are unable to stretch, and this gives them a crucial role in the process. For the cell wall to stretch or deform, the microfibrils must either slide past one another or become more widely separated, or both. The orientation of the microfibrils in the innermost layers of the wall governs the direction in which the cell expands. Cells in plants therefore anticipate their future morphology by controlling the


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Cell_Biology_Alberts. orientation of the microfibrils in the innermost layers of the wall governs the direction in which the cell expands. Cells in plants therefore anticipate their future morphology by controlling the orientation of the cellulose microfibrils that they deposit in the wall (Figure 19–64).


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Cell_Biology_Alberts. Unlike most other matrix macromolecules, which are made in the endoplasmic reticulum and Golgi apparatus and are secreted, cellulose is spun out from the surface of the cell by a plasma-membrane-bound enzyme complex (cellulose synthase), which uses as its substrate the sugar nucleotide UDP-glucose supplied from the cytosol. Each enzyme complex, or rosette, has a sixfold symmetry (see Figure 19–65) and contains the protein products of three separate cellulose synthase (CESA) genes. Each CESA protein is essential for the production of a cellulose microfibril. Three CESA genes are required for primary cell wall synthesis and a different three for secondary cell wall synthesis.


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Cell_Biology_Alberts. As they are being synthesized, the nascent cellulose chains assemble into microfibrils. These are spun out on the extracellular surface of the plasma membrane, forming a layer, or lamella, in which all the microfibrils have more or less the same alignment (see Figure 19–63). Each new lamella is deposited internally to the previous one, so that the wall consists of concentrically arranged lamellae, with the oldest on the outside. The most recently deposited microfibrils in elongating cells commonly lie perpendicular to the axis of cell elongation, although the orientation of the microfibrils in the outer lamellae that were laid down earlier may be different (see Figure 19–64B and C). Figure 19–64 Cellulose microfibrils influence the direction of cell elongation.


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Cell_Biology_Alberts. the orientation of cellulose microfibrils in the primary cell wall of an elongating carrot cell is shown in this electron micrograph of a shadowed replica from a rapidly frozen and deep-etched cell wall. the cellulose microfibrils are aligned parallel to one another and perpendicular to the axis of cell elongation. the microfibrils are cross-linked by, and interwoven with, a complex web of matrix molecules (compare with Figure 19–63). (B, C) the cells in (B) and start off with identical shapes (shown here as cubes) but with different net orientations of cellulose microfibrils in their walls. although turgor pressure is uniform in all directions, cell wall loosening allows each cell to elongate only in a direction perpendicular to the orientation of the innermost layer of microfibrils, which have great tensile strength. Cell expansion occurs in concert with the insertion of new wall material. the final shape of an organ, such as a shoot, is determined in part by the direction in which


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Cell_Biology_Alberts. have great tensile strength. Cell expansion occurs in concert with the insertion of new wall material. the final shape of an organ, such as a shoot, is determined in part by the direction in which its component cells can expand. (a, courtesy of Brian Wells and Keith roberts.)


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Cell_Biology_Alberts. An important clue to the mechanism that dictates microfibril orientation came from observations of the microtubules in plant cells. These are frequently arranged in the cortical cytoplasm with the same orientation as the cellulose microfibrils that are currently being deposited in the cell wall in that region. These cortical microtubules form a cortical array close to the cytosolic face of the plasma membrane, held there by poorly characterized proteins. The congruent orientation of the cortical array of microtubules (lying just inside the plasma membrane) and cellulose microfibrils (lying just outside) is seen in many types and shapes of plant cells and is present during both primary and secondary cell wall deposition, suggesting a causal relationship.


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Cell_Biology_Alberts. This suggestion can be tested by treating a plant tissue with a microtubule-depolymerizing drug so as to disassemble the entire system of cortical microtubules. The consequences for subsequent cellulose deposition, however, are not as straightforward as might be expected. The drug treatment does not disrupt the production of new cellulose microfibrils, and in some cases cells can continue to deposit new microfibrils in the preexisting orientation. Any developmental switch in the orientation of the microfibril pattern that would normally occur between successive lamellae, however, is invariably blocked. It seems that a preexisting orientation of microfibrils can be propagated even in the absence of microtubules, but any change in the deposition of cellulose microfibrils requires that intact microtubules be present to determine the new orientation.


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Cell_Biology_Alberts. These observations are consistent with the following model. The cellulose-synthesizing rosettes embedded in the plasma membrane spin out long cellulose molecules. As the synthesis of cellulose molecules and their self-assembly into microfibrils proceeds, the distal end of each microfibril presumably forms indirect cross-links to the previous layer of wall material, orienting the new microfibril in parallel with the old ones as it becomes integrated into the texture of the wall. Since the microfibril is stiff, the rosette at its growing, proximal end has to move as it deposits the new material. Traveling in the plane of the membrane, the rosette moves in the direction defined by the way in which the far end of the microfibril is anchored in the existing wall. In this way, each layer of microfibrils would tend to be spun out from the membrane in the same orientation as the layer laid down previously, with the rosettes following the direction of the preexisting oriented microfibrils


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Cell_Biology_Alberts. microfibrils would tend to be spun out from the membrane in the same orientation as the layer laid down previously, with the rosettes following the direction of the preexisting oriented microfibrils outside the cell. Oriented microtubules inside the cell, however, can force a change in the direction in which the rosettes move: they can create boundaries in the plasma membrane that act like the banks of a canal to constrain rosette movement (Figure 19–65). In this view, cellulose synthesis can occur independently of microtubules; but it is constrained spatially when cortical microtubules are present to define membrane microdomains within which the enzyme complex can move.


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Cell_Biology_Alberts. Figure 19–65 One model of how the orientation of newly deposited cellulose microfibrils might be determined by the orientation of cortical microtubules.


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Cell_Biology_Alberts. (a) the large cellulose synthase complexes, or rosettes, are integral membrane proteins that continuously synthesize cellulose microfibrils on the outer face of the plasma membrane. the distal ends of the stiff microfibrils become integrated into the texture of the wall, and their elongation at the proximal end pushes the synthase complex along in the plane of the membrane. Because the cortical array of microtubules is attached to the plasma membrane in a way that confines this complex to defined membrane channels, the orientation of these microtubules— when they are present—determines the axis along which the new microfibrils are laid down. (B, C) two electron micrographs show the tight association of the cortical microtubules with the plasma membrane. One shows the microtubules in cross section while the other shows a microtubule in longitudinal section. Both emphasize the constant gap of about 20 nm between membrane and microtubule; the connecting molecules responsible remain


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Cell_Biology_Alberts. cross section while the other shows a microtubule in longitudinal section. Both emphasize the constant gap of about 20 nm between membrane and microtubule; the connecting molecules responsible remain obscure. (B and C, courtesy of andrew Staehelin.) 0.1 µm cellulose microfbril being added to preexisting wall plasma membrane microtubule attached to plasma membrane cellulose synthase complex CYTOSOL


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Cell_Biology_Alberts. In this way, plant cells can change their direction of expansion by a sudden What WE DON’t KNOW change in the orientation of their cortical array of microtubules. Because plant cells cannot move (being constrained by their walls), the entire morphology of • What are the regulatory mechanisms a multicellular plant presumably depends on a coordinated, highly patterned that control the rearrangement of deployment of cortical microtubule orientations during plant development. It is cell–cell junctions in epithelia during early development? What roles do not known how these orientations are controlled, although it has been shown that the microtubules can reorient rapidly in response to extracellular stimuli, includ these rearrangements? ing plant growth regulators such as ethylene and auxins (discussed in Chapter 15).


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Cell_Biology_Alberts. ing plant growth regulators such as ethylene and auxins (discussed in Chapter 15). Microtubules are not, however, the only cytoskeletal elements that influence • how do extracellular matrix proteins wall deposition. Local foci of cortical actin filaments can also direct the deposition and carbohydrates influence the of new wall material at specific sites on the cell surface, contributing to the elabo-localization and actions of extracellular rate final shaping of many differentiated plant cells. receptors?


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Cell_Biology_Alberts. coordinate the activation of integrin Plant cells are surrounded by a tough extracellular matrix, or cell wall, which is proteins and their interactions with responsible for many of the unique features of a plant’s lifestyle. The wall is com-cytoskeletal components and their posed of a network of cellulose microfibrils and cross-linking glycans, embedded in response to changes in mechanical a highly cross-linked matrix of pectin polysaccharides. In secondary cell walls, lig-force acting on cell–matrix junctions? nin may be deposited to make them waterproof, hard, and woody. A cortical array of microtubules can control the orientation of newly deposited cellulose microfi molecules have the ability to present brils, which in turn determine the direction of cell expansion and therefore the final ordered arrays of signals to cells, shape of the cell and, ultimately, of the plant as a whole. might the exact spatial relationships between such signals carry a message beyond that of the


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Cell_Biology_Alberts. the final ordered arrays of signals to cells, shape of the cell and, ultimately, of the plant as a whole. might the exact spatial relationships between such signals carry a message beyond that of the individual signals themselves?


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Cell_Biology_Alberts. Which statements are true? explain why or why not. sites of sites for 19–1 Given the numerous processes inside cells that cleavage are regulated by changes in Ca2+ concentration, it seems likely that Ca2+-dependent cell–cell adhesions are also regulated by changes in Ca2+ concentration. 19–2 Tight junctions perform two distinct functions: they seal the space between cells to restrict paracellular flow and they fence off plasma membrane domains to prevent the mixing of apical and basolateral membrane Figure Q19–1 Production of Fab fragments from IgG antibodies by digestion with papain (problem 19–6). proteins.


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Cell_Biology_Alberts. proteins. 19–3 The elasticity of elastin derives from its high con-Fab fragments were generated by digesting the IgG anti-tent of α helices, which act as molecular springs. bodies with papain, a protease, to separate the two binding sites (Figure Q19–1). Why do you suppose it was nec19–4 Integrins can convert mechanical signals into essary to use Fab fragments to block cell aggregation? intracellular molecular signals. 19–7 The food-poisoning bacterium Clostridium per-Discuss the following problems. fringens makes a toxin that binds to members of the claudin family of proteins, which are the main constituents of 19–5 Comment on the following (1922) quote from tight junctions. When the C-terminus of the toxin is bound Warren Lewis, who was one of the pioneers of cell biology.


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Cell_Biology_Alberts. to a claudin, the N-terminus can insert into the adjacent “Were the various types of cells to lose their stickiness for cell membrane, forming holes that kill the cell. The porone another and for the supporting extracellular matrix, tion of the toxin that binds to the claudins has proven to be our bodies would at once disintegrate and flow off into the a valuable reagent for investigating the properties of tight ground in a mixed stream of cells.” junctions. MDCK cells are a common choice for studies 19–6 Cell adhesion molecules were originally identi-of tight junctions because they can form an intact epithefied using antibodies raised against cell-surface compo-lial sheet with high transepithelial resistance. MDCK cells nents to block cell aggregation. In the adhesion-blocking express two claudins: claudin-1, which is not bound by assays, the researchers found it necessary to use antibody the toxin, and claudin-4, which is. fragments, each with a single binding site (so-called Fab


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Cell_Biology_Alberts. two claudins: claudin-1, which is not bound by assays, the researchers found it necessary to use antibody the toxin, and claudin-4, which is. fragments, each with a single binding site (so-called Fab When an intact MDCK epithelial sheet is incufragments), rather than intact IgG antibodies, which are bated with the C-terminal toxin fragment, claudin-4 Y-shaped molecules with two identical binding sites. The disappears, becoming undetectable within 24 hours. In (A) BASOLATERAL (B) APICAL 10,000


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Cell_Biology_Alberts. Figure Q19–2 Effects of Clostridium toxin on the barrier function of MDCK cells (problem 19–7). (A) Addition of toxin from the basolateral side of the epithelial sheet. (B) Addition of toxin from the apical side of the epithelial sheet. For a given voltage, a higher resistance (ohms cm2) gives less paracellular current. the absence of claudin-4, the cells remain healthy and the epithelial sheet appears intact. The mean number of strands in the tight junctions that link the cells also decreases over 24 hours from about four to about two, and they are less highly branched. A functional assay for the integrity of the tight junctions shows that transepithelial resistance decreases dramatically in the presence of the toxin, but the resistance can be restored by washing out the toxin (Figure Q19–2A). Curiously, the toxin produces these effects only when it is added to the basolateral side of the sheet; it has no effect when added to the apical surface (Figure Q19–2B).


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Cell_Biology_Alberts. A. How can it be that two tight-junction strands remain, even though all of the claudin-4 has disappeared? b. Why do you suppose the toxin works when it is added to the basolateral side of the epithelial sheet, but not when added to the apical side?


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Cell_Biology_Alberts. 19–8 It is not an easy matter to assign particular functions to specific components of the basal lamina, since the overall structure is a complicated composite material with both mechanical and signaling properties. Nidogen, for example, cross-links two central components of the basal lamina by binding to the laminin γ-1 chain and to type IV collagen. Given such a key role, it was surprising that mice with a homozygous knockout of the gene for nidogen-1 were entirely healthy, with no abnormal phenotype. Similarly, mice homozygous for a knockout of the gene for nidogen-2 also appeared completely normal. By contrast, mice that were homozygous for a defined mutation in the gene for laminin γ-1, which eliminated just the binding site for nidogen, died at birth with severe defects in lung and kidney formation. The mutant portion of the laminin γ-1 chain is thought to have no other function than to bind nidogen, and does not affect laminin structure or its ability to assemble into the basal


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Cell_Biology_Alberts. formation. The mutant portion of the laminin γ-1 chain is thought to have no other function than to bind nidogen, and does not affect laminin structure or its ability to assemble into the basal lamina. How would you explain these genetic observations, which are summarized in Table Q19–1? What would you predict would be the phenotype of a mouse that was homozygous for knockouts of both nidogen genes?


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Cell_Biology_Alberts. 19–9 Discuss the following statement: “The basal lamina of muscle fibers serves as a molecular bulletin board, in which adjoining cells can post messages that direct the differentiation and function of the underlying cells.” 19–10 The affinity of integrins for matrix components can be modulated by changes to their cytoplasmic domains: a process known as inside-out signaling. You have identified a key region in the cytoplasmic domains of αIIbβ3 integrin that seems to be required for inside-out signaling (Figure Q19–3). Substitution of alanine for either D723 in the β chain or R995 in the α chain leads to a high level of spontaneous activation, under conditions where the wild-type chains are inactive. Your advisor suggests that you convert the aspartate in the β chain to an arginine (D723R) and the arginine in the α chain to an aspartate (R995D). You compare all three α chains (R995, R995A, and R995D) against all three β chains (D723, D723A, and D723R). You find that all pairs have a


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Cell_Biology_Alberts. and the arginine in the α chain to an aspartate (R995D). You compare all three α chains (R995, R995A, and R995D) against all three β chains (D723, D723A, and D723R). You find that all pairs have a high level of spontaneous activation, except D723 vs R995 (the wild type) and D723R vs R995D, which have low levels. Based on these results, how do you think the αIIbβ3 integrin is held in its inactive state?


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Cell_Biology_Alberts. Figure Q19–3 Schematic representation of αIIbβ3 integrin (problem 19–10). The D723 and R995 residues are indicated. (From P.E. Hughes et al., J. Biol. Chem. 271:6571–6574, 1996. With permission from American Society for Biochemistry and Molecular Biology.) 19–11 The glycosaminoglycan polysaccharide chains that are linked to specific core proteins to form the proteoglycan components of the extracellular space are highly negatively charged. How do you suppose these negatively charged polysaccharide chains help to establish a hydrated gel-like environment around the cell? How would the properties of these molecules differ if the polysaccharide chains were uncharged?


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Cell_Biology_Alberts. 19–12 At body temperature, L-aspartate in proteins racemizes to D-aspartate at an appreciable rate. Most proteins in the body have a very low level of D-aspartate, if it can be detected at all. Elastin, however, has a fairly high level of D-aspartate. Moreover, the amount of D-aspartate increases in direct proportion to the age of the person from whom the sample was taken. Why do you suppose that most proteins have little if any D-aspartate, while elastin has levels of D-aspartate that increase steadily with age? 19–13 Your boss is coming to dinner! All you have for a salad is some wilted, day-old lettuce. You vaguely recall that there is a trick to rejuvenating wilted lettuce, but you cannot remember what it is. Should you soak the lettuce in salt water, soak it in tap water, or soak it in sugar water, or maybe just shine a bright light on it and hope that photosynthesis will perk it up?


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Cell_Biology_Alberts. 19–14 A plant must be able to respond to changes in the water status of its surroundings. It does so by the flow of water molecules through water channels called aquaporins. The hydraulic conductivity of a single aquaporin is 4.4 × 10–22 m3 per second per MPa (megapascal) of pressure. What does this correspond to in terms of water molecules per second at atmospheric pressure? [Atmospheric pressure is 0.1 MPa (1 bar) and the concentration of water is 55.5 M.] Beckerle M ed. (2002) Cell adhesion. Oxford: Oxford University press. hynes rO & Yamada KM (eds) (2011) Extracellular Matrix Biology (Cold Spring harbor perspectives in Biology). Cold Spring harbor: Cold Spring harbor Laboratory press. Brasch J, harrison OJ, honig B & Shapiro L (2012) thinking outside the cell: how cadherins drive adhesion. Trends Cell Biol. 22, 299–310. Gomez Ga, McLachlan rW & Yap aS (2011) productive tension: force-sensing and homeostasis of cell-cell junctions. Trends Cell Biol. 21, 499–505.


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Cell_Biology_Alberts. Gomez Ga, McLachlan rW & Yap aS (2011) productive tension: force-sensing and homeostasis of cell-cell junctions. Trends Cell Biol. 21, 499–505. Goodenough Da & paul DL (2003) Beyond the gap: functions of unpaired connexon channels. Nat. Rev. Mol. Cell Biol. 4, 285–294. Gumbiner BM (2005) regulation of cadherin-mediated adhesion in morphogenesis. Nat. Rev. Mol. Cell Biol. 6, 622–634. harris tJ & tepass U (2010) adherens junctions: from molecules to morphogenesis. Nat. Rev. Mol. Cell Biol. 11, 502–514. King N, hittinger Ct & Carroll SB (2003) Evolution of key cell signaling and adhesion protein families predates animal origins. Science 301, 361–363. Leckband DE, le Duc Q, Wang N & de rooij J (2011) Mechanotransduction at cadherin-mediated adhesions. Curr. Opin. Cell Biol. 23, 523–530. Lecuit t, Lenne pF & Munro E (2011) Force generation, transmission, and integration during cell and tissue morphogenesis. Annu. Rev. Cell Dev. Biol. 27, 157–184.


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Cell_Biology_Alberts. Lecuit t, Lenne pF & Munro E (2011) Force generation, transmission, and integration during cell and tissue morphogenesis. Annu. Rev. Cell Dev. Biol. 27, 157–184. Litjens Sh, de pereda JM & Sonnenberg a (2006) Current insights into the formation and breakdown of hemidesmosomes. Trends Cell Biol. 16, 376–383. Maule aJ, Benitez-alfonso Y & Faulkner C (2011) plasmodesmata— membrane tunnels with attitude. Curr. Opin. Plant Biol. 14, 683–690. McEver rp & Zhu C (2010) rolling cell adhesion. Annu. Rev. Cell Dev. Biol. 26, 363–396. Nakagawa S, Maeda S & tsukihara t (2010) Structural and functional studies of gap junction channels. Curr. Opin. Struct. Biol. 20, 423–430. Shin K, Fogg VC & Margolis B (2006) tight junctions and cell polarity. Annu. Rev. Cell Dev. Biol. 22, 207–236. takeichi M (2007) the cadherin superfamily in neuronal connections and interactions. Nat. Rev. Neurosci. 8, 11–20.


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Cell_Biology_Alberts. takeichi M (2007) the cadherin superfamily in neuronal connections and interactions. Nat. Rev. Neurosci. 8, 11–20. thomason ha, Scothern a, Mcharg S & Garrod Dr (2010) Desmosomes: adhesive strength and signalling in health and disease. Biochem. J. 429, 419–433. The extracellular Matrix of Animals aszodi a, Legate Kr, Nakchbandi I & Fassler r (2006) What mouse mutants teach us about extracellular matrix function. Annu. Rev. Cell Dev. Biol. 22, 591–621. Bulow hE & hobert O (2006) the molecular diversity of glycosaminoglycans shapes animal development. Annu. Rev. Cell Dev. Biol. 22, 375–407. Couchman Jr (2010) transmembrane signaling proteoglycans. Annu. Rev. Cell Dev. Biol. 26, 89–114. Domogatskaya a, rodin S & tryggvason K (2012) Functional diversity of laminins. Annu. Rev. Cell Dev. Biol. 28, 523–553. hynes rO (2009) the extracellular matrix: not just pretty fibrils. Science 326, 1216–1219.


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Cell_Biology_Alberts. hynes rO (2009) the extracellular matrix: not just pretty fibrils. Science 326, 1216–1219. hynes rO & Naba a (2012) Overview of the matrisome—an inventory of extracellular matrix constituents and functions. Cold Spring Harb. Perspect. Biol. 4, a004903. Kielty CM, Sherratt MJ & Shuttleworth Ca (2002) Elastic fibres. J. Cell Sci. 115, 2817–2828. Larsen M, artym VV, Green Ja & Yamada KM (2006) the matrix reorganized: extracellular matrix remodeling and integrin signaling. Curr. Opin. Cell Biol. 18, 463–471. Lu p, takai K, Weaver VM & Werb Z (2011) Extracellular matrix degradation and remodeling in development and disease. Cold Spring Harb. Perspect. Biol. 3, a005058. ricard-Blum S (2011) the collagen family. Cold Spring Harb. Perspect. Biol. 3, a004978. Sasaki t, Fässler r & hohenester E (2004) Laminin: the crux of basement membrane assembly. J. Cell Biol. 164, 959–963. toole Bp (2001) hyaluronan in morphogenesis. Semin. Cell Dev. Biol. 12, 79–87.


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Cell_Biology_Alberts. Sasaki t, Fässler r & hohenester E (2004) Laminin: the crux of basement membrane assembly. J. Cell Biol. 164, 959–963. toole Bp (2001) hyaluronan in morphogenesis. Semin. Cell Dev. Biol. 12, 79–87. Yurchenco pD (2011) Basement membranes: cell scaffoldings and signaling platforms. Cold Spring Harb. Perspect. Biol. 3, a004911. Calderwood Da, Campbell ID & Critchley Dr (2013) talins and kindlins: partners in integrin-mediated adhesion. Nat. Rev. Mol. Cell Biol. 14, 503–517. Campbell ID & humphries MJ (2011) Integrin structure, activation, and interactions. Cold Spring Harb. Perspect. Biol. 3, a004994. hoffman BD, Grashoff C & Schwartz Ma (2011) Dynamic molecular processes mediate cellular mechanotransduction. Nature 475, 316–323. hogg N, patzak I & Willenbrock F (2011) the insider’s guide to leukocyte integrin signalling and function. Nat. Rev. Immunol. 11, 416–426.


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Cell_Biology_Alberts. hogg N, patzak I & Willenbrock F (2011) the insider’s guide to leukocyte integrin signalling and function. Nat. Rev. Immunol. 11, 416–426. Kanchanawong p, Shtengel G, pasapera aM et al. (2010) Nanoscale architecture of integrin-based cell adhesions. Nature 468, 580–584. Luo Bh & Springer ta (2006) Integrin structures and conformational signaling. Curr. Opin. Cell Biol. 18, 579–586. Moser M, Legate Kr, Zent r & Fässler r (2009) the tail of integrins, talin, and kindlins. Science 324, 895–899. ross tD, Coon BG, Yun S et al. (2013) Integrins in mechanotransduction. Curr. Opin. Cell Biol. 25, 613–618. Shattil SJ, Kim C & Ginsberg Mh (2010) the final steps of integrin activation: the end game. Nat. Rev. Mol. Cell Biol. 11, 288–300. The Plant Cell Wall albersheim p, Darvill a, roberts K et al. (2011) plant Cell Walls: From Chemistry to Biology. New York: Garland Science.


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Cell_Biology_Alberts. The Plant Cell Wall albersheim p, Darvill a, roberts K et al. (2011) plant Cell Walls: From Chemistry to Biology. New York: Garland Science. Braidwood L, Breuer C & Sugimoto K (2013) My body is a cage: mechanisms and modulation of plant cell growth. New Phyto. 210, 388–402. Keegstra K (2010) plant cell walls. Plant Physiol. 154, 483–486. Li S, Lei L, Somerville C et al. (2011) Cellulose synthase interactive protein 1 (CSI1) links microtubules and cellulose synthase complexes. Proc. Natl. Acad. Sci. USA 109, 189–190. Lloyd C (2011) Dynamic microtubules and the texture of plant cell walls. Int. Rev. Cell Mol. Biol. 287, 287–329. McFarlane hE, Döring a & perrson S (2014) the cell biology of cellulose synthesis. Annu. Rev. Plant Biol. 65, 69–94. Somerville C (2006) Cellulose synthesis in higher plants. Annu. Rev. Cell Dev. Biol. 22, 53–78.


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Cell_Biology_Alberts. Somerville C (2006) Cellulose synthesis in higher plants. Annu. Rev. Cell Dev. Biol. 22, 53–78. Szymanski DB & Cosgrove DJ (2009) Dynamic Coordination of cytoskeletal and cell wall systems during cell wall biogenesis. Curr. Biol. 19, r800–r811. Wightman r & turner Sr (2008) the roles of the cytoskeleton during cellulose deposition at the secondary cell wall. Plant J. 54, 794–805. Wolf S, hématy K & höfte h (2012) Growth control and cell wall signaling in plants. Annu. Rev. Plant Biol. 63, 381–407.


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Cell_Biology_Alberts. Wolf S, hématy K & höfte h (2012) Growth control and cell wall signaling in plants. Annu. Rev. Plant Biol. 63, 381–407. About one in five of us will die of cancer, but that is not why we devote a chapter to this disease. Cancer cells break the most basic rules of cell behavior by which multicellular organisms are built and maintained, and they exploit every kind of opportunity to do so. These transgressions help to reveal what the normal rules are and how they are enforced. As a result, cancer research helps to illuminate the fundamentals of cell biology—especially cell signaling (Chapter 15), the cell cycle and cell growth (Chapter 17), programmed cell death (apoptosis, Chapter 18), and the control of tissue architecture (Chapters 19 and 22). Of course, with a deeper understanding of these normal processes, we also gain a deeper understanding of the disease and better tools to treat it.


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Cell_Biology_Alberts. In this chapter, we first consider what cancer is and describe the natural history of the disease from a cellular standpoint. We then discuss the molecular changes that make a cell cancerous. And we end the chapter by considering how our enhanced understanding of the molecular basis of cancer is leading to improved methods for its prevention and treatment.


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Cell_Biology_Alberts. The body of an animal operates as a society or ecosystem, whose individual members are cells that reproduce by cell division and organize themselves into collaborative assemblies called tissues. This ecosystem is very peculiar, however, because self-sacrifice—as opposed to survival of the fittest—is the rule. Ultimately, all of the somatic cell lineages in animals are committed to die: they leave no progeny and instead dedicate their existence to the support of the germ cells, which alone have a chance of continued survival (discussed in Chapter 21). There is no mystery in this, for the body is a clone derived from a fertilized egg, and the genome of the somatic cells is the same as that of the germ-cell lineage that gives rise to sperm or eggs. By their self-sacrifice for the sake of the germ cells, the somatic cells help to propagate copies of their own genes.


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Cell_Biology_Alberts. Thus, unlike free-living cells such as bacteria, which compete to survive, the cells of a multicellular organism are committed to collaboration. To coordinate their behavior, the cells send, receive, and interpret an elaborate set of extracellular signals that serve as social controls, directing cells how to act (discussed in Chapter 15). As a result, each cell behaves in a socially responsible manner—resting, growing, dividing, differentiating, or dying—as needed for the good of the organism.


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Cell_Biology_Alberts. Molecular disturbances that upset this harmony mean trouble for a multicellular society. In a human body with more than 1014 cells, billions of cells experience mutations every day, potentially disrupting the social controls. Most dangerously, a mutation may give one cell a selective advantage, allowing it to grow and divide slightly more vigorously and survive more readily than its neighbors and in this way to become a founder of a growing mutant clone. A mutation that promotes such selfish behavior by individual members of the cooperative can jeopardize the future of the whole enterprise. Over time, repeated rounds of mutation, competition, and natural selection operating within the population of somatic cells can cause matters to go from bad to worse. These are the basic ingredients of cancer: it is a disease in which an individual mutant clone of cells begins by CANCER-CRITICAL GENES: hOW ThEY ARE FOUND AND WhAT ThEY DO


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Cell_Biology_Alberts. CANCER-CRITICAL GENES: hOW ThEY ARE FOUND AND WhAT ThEY DO CANCER PREVENTION AND TREATMENT: PRESENT AND FUTURE prospering at the expense of its neighbors. In the end—as the clone grows, evolves, and spreads—it can destroy the entire cellular society (Movie 20.1). In this section, we discuss the development of cancer as a microevolutionary process that takes place within the course of a human life-span in a subpopulation of cells in the body. But the process depends on the same principles of mutation and natural selection that have driven the evolution of living organisms on Earth for billions of years.


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Cell_Biology_Alberts. Cancer cells are defined by two heritable properties: (1) they reproduce in defiance of the normal restraints on cell growth and division, and (2) they invade and colonize territories normally reserved for other cells. It is the combination of these properties that makes cancers particularly dangerous. An abnormal cell that grows (increases in mass) and proliferates (divides) out of control will give rise to a tumor, or neoplasm—literally, a new growth. As long as the neoplastic cells have not yet become invasive, however, the tumor is said to be benign. For most types of such neoplasms, removing or destroying the mass locally usually achieves a complete cure. A tumor is considered a true cancer if it is malignant; that is, when its cells have acquired the ability to invade surrounding tissue. Invasiveness is an essential characteristic of cancer cells. It allows them to break loose, enter blood or lymphatic vessels, and form secondary tumors called metastases at other sites in the


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Cell_Biology_Alberts. tissue. Invasiveness is an essential characteristic of cancer cells. It allows them to break loose, enter blood or lymphatic vessels, and form secondary tumors called metastases at other sites in the body (Figure 20–1). In general, the more widely a cancer spreads, the harder it becomes to eradicate. It is generally metastases that kill the cancer patient.


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Cell_Biology_Alberts. Cancers are traditionally classified according to the tissue and cell type from which they arise. Carcinomas are cancers arising from epithelial cells, and they are by far the most common cancers in humans. They account for about 80% of cases, perhaps because most of the cell proliferation in adults occurs in epithelia. In addition, epithelial tissues are the most likely to be exposed to the various forms of physical and chemical damage that favor the development of cancer. Sarcomas arise from connective tissue or muscle cells. Cancers that do not fit in either of these two broad categories include the various leukemias and lymphomas, derived from white blood cells and their precursors (hemopoietic cells), as well as cancers derived from cells of the nervous system. Figure 20–2 shows the types of cancers that are common in the United States, together with their incidence and death rates. Each broad category has many subdivisions according to the specific cell type, the location in the


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Cell_Biology_Alberts. of cancers that are common in the United States, together with their incidence and death rates. Each broad category has many subdivisions according to the specific cell type, the location in the body, and the microscopic appearance of the tumor.


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Cell_Biology_Alberts. In parallel with the set of names for malignant tumors, there is a related set of names for benign tumors: an adenoma, for example, is a benign epithelial tumor with a glandular organization; the corresponding type of malignant tumor is an adenocarcinoma (Figure 20–3). Similarly, a chondroma and a chondrosarcoma are, respectively, benign and malignant tumors of cartilage.


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Cell_Biology_Alberts. Most cancers have characteristics that reflect their origin. Thus, for example, the cells of a basal-cell carcinoma, derived from a keratinocyte stem cell in the skin, generally continue to synthesize cytokeratin intermediate filaments, whereas the cells of a melanoma, derived from a pigment cell in the skin, will often (but not always) continue to make pigment granules. Cancers originating from different cell types are, in general, very different diseases. Basal-cell carcinomas of the skin, for example, are only locally invasive and rarely metastasize, whereas melanomas can become much more malignant and often form metastases. Basal-cell carcinomas are readily cured by surgery or local irradiation, whereas malignant melanomas, once they have metastasized widely, are usually fatal.


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Cell_Biology_Alberts. Later, we shall see that there is also a different way to classify cancers, one that cuts across the traditional classification by site of origin: we can classify them in terms of the mutations that make the tumor cells cancerous. The final section of the chapter will show how this information can be crucial to the design and choice of treatments.


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Cell_Biology_Alberts. Figure 20–1 Metastasis. Malignant tumors typically give rise to metastases, making the cancer hard to eradicate. Shown in this fusion image is a whole-body scan of a patient with metastatic non-hodgkin’s lymphoma (NhL). The background image of the body’s tissues was obtained by CT (computed x-ray tomography) scanning. Overlaid on this image, a PET (positron emission tomography) scan reveals the tumor tissue (yellow), detected by its unusually high uptake of radioactively labeled fluorodeoxyglucose (FDG). high FDG uptake occurs in cells with unusually active glucose uptake and metabolism, which is a characteristic of cancer cells (see Figure 20–12). The yellow spots in the abdominal region reveal multiple metastases. (Courtesy of S. Gambhir.) respiratorycancers of system epithelia: carcinomas breast blood: myelomas, leukemias, and lymphomas bones, connective tissue, new cases muscles, and vasculature KEY: deaths/year endocrine system, thyroid


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Cell_Biology_Alberts. KEY: deaths/year endocrine system, thyroid Even when a cancer has metastasized, we can usually trace its origins to a single primary tumor, arising in a specific organ. The primary tumor is thought to derive by cell division from a single cell that initially experienced some heritable change. Subsequently, additional changes accumulate in some of the descendants of this cell, allowing them to outgrow, out-divide, and often outlive their neighbors. By the time it is first detected, a typical human cancer will have been developing for many years and will already contain a billion cancer cells or more (Figure 20–4). Tumors will usually also contain a variety of other cell types; for example, fibroblasts will be present in the supporting connective tissue associated with a carcinoma, in addition to inflammatory and vascular endothelial cells. How can we be sure that the cancer cells are the clonal descendants of a single abnormal cell?


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Cell_Biology_Alberts. One way of proving clonal origin is through molecular analysis of the chromosomes in tumor cells. In almost all patients with chronic myelogenous leukemia (CML), for example, we can distinguish the leukemic white blood cells from the patient’s normal cells by a specific chromosomal abnormality: the so-called Philadelphia chromosome, created by a translocation between the long arms of chromosomes 9 and 22 (Figure 20–5). When the DNA at the site of translocation is cloned and sequenced, it is found that the site of breakage and rejoining of the translocated fragments is identical in all the leukemic cells in any given patient, but that this site differs slightly (by a few hundred or thousand base pairs) from one patient to another. This is the expected result if, and only if, the cancer in each patient arises from a unique accident occurring in a single cell. We will see later


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Cell_Biology_Alberts. Figure 20–2 Cancer incidence and mortality in the United States. The total number of new cases diagnosed in 2012 in the United States was 1,665,540, and total cancer deaths were 585,720. Note that deaths reflect cases diagnosed at many different times and that somewhat less than half of the people who develop cancer die of it. In the world as a whole, the five most common cancers are those of the lung, stomach, breast, colon/rectum, and uterine cervix (included in the figure under the heading of reproductive tract), and the total number of new cancer cases recorded per year is just over 6 million. Skin cancers other than melanomas are not included in these figures, since almost all are cured easily and many are unrecorded.


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Cell_Biology_Alberts. The data for the United Kingdom are similar. however, incidences are different in some other parts of the world, reflecting widespread exposures to different infectious agents and environmental toxins. (Data from American Cancer Society, Cancer Facts and Figures, 2014.) Figure 20–3 Benign versus malignant tumors. A benign glandular tumor (pink cells; an adenoma) remains inside the basal lamina (yellow) that marks the boundary of the normal structure (a duct, in this example). In contrast, a malignant glandular tumor (red cells; an adenocarcinoma) can develop from a benign tumor cell, and it destroys the integrity of the tissue, as shown. There are many different forms that such tumors may take.


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Cell_Biology_Alberts. Figure 20–4 The growth of a typical human tumor, such as a tumor of the breast. The diameter of the tumor is plotted on a logarithmic scale. Years may elapse before the tumor becomes noticeable. The doubling time of a typical breast tumor, for example, is about 100 days. however, particularly virulent tumors may grow much more rapidly. diameter of tumor (mm) how this particular translocation promotes the development of CML by creating a novel hybrid gene encoding a protein that promotes cell proliferation. Many other lines of evidence, from a variety of cancers, point to the same con clusion: most cancers originate from a single aberrant cell. 0.1


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Cell_Biology_Alberts. 0.1 If a single abnormal cell is to give rise to a tumor, it must pass on its abnormality to its progeny: the aberration has to be heritable. Thus, the development of a clone of cancer cells depends on genetic changes. The tumor cells contain somatic mutations: they have one or more shared detectable abnormalities in their DNA sequence that distinguish them from the normal cells surrounding the tumor, as in the example of CML just described. (The mutations are called somatic because they occur in the soma, or body cells, not in the germ line). Cancers are also driven by epigenetic changes—persistent, heritable changes in gene expression that result from modifications of chromatin structure without alteration of the cell’s DNA sequence. But somatic mutations that alter DNA sequence appear to be a fundamental and universal feature, and cancer is in this sense a genetic disease.


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Cell_Biology_Alberts. Factors that cause genetic changes tend to provoke the development of cancer. Thus, carcinogenesis (the generation of cancer) can be linked to mutagenesis (the production of a change in the DNA sequence). This correlation is particularly clear for two classes of external agents: (1) chemical carcinogens (which typically cause simple local changes in the nucleotide sequence), and (2) radiation such as x-rays (which typically cause chromosome breaks and translocations) or ultraviolet (UV) light (which causes specific DNA base alterations). As would be expected, people who have inherited a genetic defect in one of several DNA repair mechanisms, causing their cells to accumulate mutations at an elevated rate, run a heightened risk of cancer. Those with the disease xeroderma pigmentosum, for example, have defects in the system that repairs DNA damage induced by UV light, and they have a greatly increased incidence of skin cancers.


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Cell_Biology_Alberts. A Single Mutation Is Not Enough to Change a Normal Cell into a Cancer Cell


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Cell_Biology_Alberts. An estimated 1016 cell divisions occur in a normal human body in the course of a typical lifetime; in a mouse, with its smaller number of cells and its shorter lifespan, the number is about 1012. Even in an environment that is free of mutagens, mutations would occur spontaneously at an estimated rate of about 10–6 mutations per gene per cell division—a value set by fundamental limitations on the accuracy of DNA replication and repair (see pp. 237–238). Thus, in a typical lifetime, every single gene is likely to have undergone mutation on about 1010 separate occasions in a human, or on about 106 occasions in a mouse. Among the resulting mutant cells, we might expect a large number that have sustained deleterious mutations in genes that regulate cell growth and division, causing the cells to disobey the normal restrictions on cell proliferation. From this point of view, the problem of cancer seems to be not why it occurs, but why it occurs so infrequently.


Cell_Biology_Alberts

Cell_Biology_Alberts. Clearly, if a mutation in a single gene were enough to convert a typical healthy cell into a cancer cell, we would not be viable organisms. Many lines of evidence indicate that the development of a cancer typically requires that a substantial number of independent, rare genetic and epigenetic accidents occur in the lineage that emanates from a single cell. One such indication comes from epidemiological studies of the incidence of cancer as a function of age (Figure 20–6). If a Figure 20–5 The translocation between chromosomes 9 and 22 responsible for chronic myelogenous leukemia. The normal structures of chromosomes 9 and 22 are shown at the left. When a translocation occurs between them at the indicated site, the result is the abnormal pair at the right. The smaller of the two resulting abnormal chromosomes (22q–) is called the Philadelphia chromosome, after the city where the abnormality was first recorded.


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Cell_Biology_Alberts. Figure 20–6 Cancer incidence as a function of age. The number of newly 180 diagnosed cases of colon cancer in women in England and Wales in 1 year is plotted as a function of age at diagnosis, relative to the total number of individuals in each age group. The incidence of cancer rises steeply as a function of age. If only a single mutation were required to trigger the cancer and this mutation had an equal chance of occurring at any time, the incidence of this cancer would be the same at all ages. Analyses of this type suggest that the development of a solid tumor instead requires five to eight independent accidents (“hits”) that occur randomly over time. This calculation incidence rate per 100,000 assumes that the mutation rate remains constant as a cancer evolves, where in fact it often increases (see p. 1097). (Data from C. Muir et al., Cancer


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Cell_Biology_Alberts. Incidence in Five Continents, Vol. V. Lyon: International Agency for Research on Cancer, 1987.) single mutation were responsible for cancer, occurring with a fixed probability per year, the chance of developing cancer in any given year of life should be indepen dent of age. In fact, for most types of cancer, the incidence rises steeply with age— as would be expected if cancer is caused by a progressive, random accumulation 20 of a set of mutations in a single lineage of cells. As discussed later, these indirect arguments have now been confirmed by sys-0 tematically sequencing the genomes of the tumor cells from individual cancer patients and cataloging the mutations that they contain.


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Cell_Biology_Alberts. For those cancers known to have a specific external cause, the disease does not usually become apparent until long after exposure to the causal agent. The incidence of lung cancer, for example, does not begin to rise steeply until after decades of heavy smoking (Figure 20–7). Similarly, the incidence of leukemias in Hiroshima and Nagasaki did not show a marked rise until about 5 years after the explosion of the atomic bombs, and industrial workers exposed for a limited period to chemical carcinogens do not usually develop the cancers characteristic of their occupation until 10, 20, or even more years after the exposure. During this long incubation period, the prospective cancer cells undergo a succession of changes, and the same presumably applies to cancers where the initial genetic lesion has no such obvious external cause.


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Cell_Biology_Alberts. The concept that the development of a cancer requires a gradual accumulation of mutations in a number of different genes helps to explain the well-known phenomenon of tumor progression, whereby an initial mild disorder of cell behavior evolves gradually into a full-blown cancer. Chronic myelogenous leukemia again provides a clear example. It begins as a disorder characterized by a nonlethal overproduction of white blood cells and continues in this form for several years before changing into a much more rapidly progressing illness that usually ends in death within a few months. In the early chronic phase, the leukemic cells are distinguished mainly by the chromosomal translocation (the Philadelphia chromosome) mentioned previously, although there may well be other, less visible


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Cell_Biology_Alberts. Figure 20–7 Smoking and the onset of lung cancer. A major increase in cigarette smoking (red line) has caused a dramatic rise in lung cancer deaths (green line), with a lag time of about 35 years. Because global cigarette smoking peaked in 1990, global lung cancer deaths are expected to decline after a similar lag. (Data from R.N. Proctor, Nat. Rev. Cancer 1:82–86, 2001). genetic or epigenetic changes. In the subsequent acute phase, cells that show not only the translocation but also several other chromosomal abnormalities overrun the hemopoietic (blood-forming) system. It appears that cells from the initial mutant clone have undergone further mutations that make them proliferate even more vigorously, so that they come to outnumber both the normal blood cells and their ancestors with the primary chromosomal translocation.


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Cell_Biology_Alberts. Carcinomas and other solid tumors evolve in a similar way (Figure 20–8). Although many such cancers in humans are not diagnosed until a relatively late stage, in some cases it is possible to observe the earlier steps and, as we shall see later, to relate them to specific genetic changes Tumor Progression Involves Successive Rounds of Random Inherited Change Followed by Natural Selection From all the evidence, therefore, it seems that cancers arise by a process in which an initial population of slightly abnormal cells—descendants of a single abnormal ancestor—evolve from bad to worse through successive cycles of random inherited change followed by natural selection. Correspondingly, tumors grow in fits and starts, as additional advantageous inherited changes arise and the cells bearing them flourish. Tumor progression involves a large element of chance and usually takes many years, which may be why the majority of us will die of causes other than cancer.


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Cell_Biology_Alberts. At each stage of progression, some individual cell acquires an additional mutation or epigenetic change that gives it a selective advantage over its neighbors, making it better able to thrive in its environment—an environment that, inside a tumor, may be harsh, with low levels of oxygen, scarce nutrients, and the natural barriers to growth presented by the surrounding normal tissues. The larger the number of tumor cells, the higher the chance that at least one of them will undergo a change that favors it over its neighbors. Thus, as the tumor grows, progression accelerates. The offspring of the best-adapted cells continue to divide, eventually producing the dominant clones in the developing lesion (Figure 20–9).


Cell_Biology_Alberts

Cell_Biology_Alberts. Just as in the evolution of plants and animals, a kind of speciation often occurs: the original cancer cell lineage can diversify to give many genetically different vigorous subclones of cells. These may coexist in the same mass of tumor tissue; or they may migrate and colonize separate environments suited to their individual quirks, where they settle, thrive, and progress as independently evolving metastases. As new mutations arise within each tumor mass, different subclones may gain an advantage and come to predominate, only to be overtaken by others or outgrown by their own sub-subclones. The increasing genetic diversity as a cancer progresses is one of the chief factors that make cures difficult.


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Cell_Biology_Alberts. Figure 20–8 Stages of progression in the development of cancer of the epithelium of the uterine cervix. Pathologists use standardized terminology to classify the types of disorders they see, so as to guide the choice of treatment. (A) In a stratified squamous epithelium, dividing cells are confined to the basal layer. (B) In this low-grade intraepithelial neoplasia (right half of image), dividing cells can be found throughout the lower third of the epithelium; the superficial cells are still flattened and show signs of differentiation, but this is incomplete. (C) In high-grade intraepithelial neoplasia, cells in all the epithelial layers are proliferating and exhibit defective differentiation. (D) True malignancy begins when the cells move through or destroy the basal lamina that underlies the basal layer of epithelium and invade the underlying connective tissue. (Photographs courtesy of Andrew J. Connolly.)


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Cell_Biology_Alberts. Figure 20–9 Clonal evolution. In this schematic diagram, a tumor develops through repeated rounds of mutation and proliferation, giving rise eventually to a clone of fully malignant cancer cells. At each step, a single cell undergoes a mutation that either enhances cell proliferation or decreases cell death, so that its progeny become the dominant clone in the tumor. Proliferation of each clone hastens the occurrence of the next step of tumor progression by increasing the size of the cell population that is at risk of undergoing an additional mutation. The final step depicted here is invasion through the basement membrane, an initial step in metastasis. In reality, there are more than the three steps shown here, and a combination of genetic and epigenetic changes are involved. Not shown here is the fact that, over time, a variety of competing subclones will often arise in a tumor. As we will discuss later, this heterogeneity complicates cancer therapies (see Figure 20–30).


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Cell_Biology_Alberts. Most human cancer cells accumulate genetic changes at an abnormally rapid rate and are said to be genetically unstable. The extent of this instability and its molecular origins differ from cancer to cancer and from patient to patient, as we shall discuss in a later section. The basic phenomenon was evident even before modern molecular analyses. For example, the cells of many cancers show grossly abnormal sets of chromosomes, with duplications, deletions, and translocations that are visible at mitosis (Figure 20–10). When the cells are maintained in culture, these patterns of chromosomal disruption can often be seen to evolve rapidly and in a seemingly haphazard way. And for many years, pathologists have used an abnormal appearance of the cell nucleus to identify and classify cancer cells in tumor biopsies; in particular, cancer cells can contain an unusually large amount of heterochromatin—a condensed form of interphase chromatin that silences genes (see pp. 194–195). This suggested


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Cell_Biology_Alberts. in tumor biopsies; in particular, cancer cells can contain an unusually large amount of heterochromatin—a condensed form of interphase chromatin that silences genes (see pp. 194–195). This suggested that epigenetic changes of chromatin structure can also contribute to the cancer cell phenotype, as recently confirmed by molecular analysis.


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Cell_Biology_Alberts. The genetic instability observed in cancer cells can arise from defects in the ability to repair DNA damage or to correct replication errors of various kinds. These alterations lead to changes in DNA sequence and produce rearrangements such as DNA translocations and duplications. Also common are defects in chromosome segregation during mitosis, which provide another possible source of chromosome instability and changes in karyotype. From an evolutionary perspective, none of this should be a surprise: anything that increases the probability of random changes in gene function heritable from one cell generation to the next—and that is not too deleterious—is likely to speed the evolution of a clone of cells toward malignancy, thereby causing this property to be selected for during tumor progression. epithelial cells growing on accidental production basal lamina of mutant cell cell with cell with


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Cell_Biology_Alberts. epithelial cells growing on accidental production basal lamina of mutant cell cell with cell with Figure 20–10 Chromosomes from a breast tumor displaying abnormalities in structure and number. Chromosomes were prepared from a breast tumor cell in metaphase, spread on a glass slide, and stained with (A) a general DNA stain or (B) a combination of fluorescently labeled DNA molecules that color each normal human chromosome differently (see Figure 4–10). The staining (displayed in false color) shows multiple translocations, including a doubly translocated chromosome (white arrow) that is made up of two pieces of chromosome 8 (green-brown) and a piece of chromosome 17 (purple). The karyotype also contains 48 chromosomes, instead of the normal 46. (Courtesy of Joanne Davidson and Paul Edwards.) Cancer Cells Display an Altered Control of Growth


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Cell_Biology_Alberts. Mutability and large cell population numbers create the opportunities for mutations to occur, but the driving force for development of a cancer has to come from some sort of selective advantage possessed by the mutant cells. Most obviously, a mutation or epigenetic change can confer such an advantage by increasing the rate at which a clone of cells proliferates or by enabling it to continue proliferating when normal cells would stop. Cancer cells that can be grown in culture, or cultured cells artificially engineered to contain the types of mutations encountered in cancers, typically show a transformed phenotype. They are abnormal in their shape, their motility, their responses to growth factors in the culture medium, and, most characteristically, in the way they react to contact with the substratum and with one another. Normal cells will not divide unless they are attached to the substratum; transformed cells will often divide even if held in suspension. Normal cells become inhibited


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Cell_Biology_Alberts. substratum and with one another. Normal cells will not divide unless they are attached to the substratum; transformed cells will often divide even if held in suspension. Normal cells become inhibited from moving and dividing when the culture reaches confluence (where the cells are touching one another); transformed cells continue moving and dividing even after confluence, and so pile up in layer upon layer in the culture dish (Figure 20–11). In addition, transformed cells no longer require all of the positive signals from their surroundings that normal cells require.


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Cell_Biology_Alberts. Their behavior in culture gives a hint of the ways in which cancer cells may misbehave in their natural environment, embedded in a tissue. But cancer cells in the body show other peculiarities that mark them out from normal cells, beyond those just described.


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Cell_Biology_Alberts. Given sufficient oxygen, normal adult tissue cells will generally fully oxidize almost all the carbon in the glucose they take up to CO2, which is lost from the body as a waste product. A growing tumor needs nutrients in abundance to provide the building blocks to make new macromolecules. Correspondingly, most tumors have a metabolism more similar to that of a growing embryo than to that of normal adult tissue. Tumor cells consume glucose avidly, importing it from the blood at a rate that can be as much as 100 times higher than neighboring normal cells. Moreover, only a small fraction of this imported glucose is used for production of ATP by oxidative phosphorylation. Instead, a great deal of lactate is produced, and many of the remaining carbon atoms derived from glucose are diverted for use as raw materials for synthesis of the proteins, nucleic acids, and lipids required for tumor growth (Figure 20–12).


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Cell_Biology_Alberts. This tendency of tumor cells to de-emphasize oxidative phosphorylation even when oxygen is plentiful, while at the same time taking up large quantities of glucose, can be shown to promote cancer cell growth and is called the Warburg contact-inhibited monolayer transformed cells foci of uninhibited of normal cells in lose contact inhibition transformed cells tissue-culture dish Figure 20–11 Loss of contact inhibition by cancer cells in cell culture. Most normal cells stop proliferating once they have carpeted the dish with a single layer of cells: proliferation seems to depend on contact with the dish, and to be inhibited by contacts with other cells—a phenomenon known as “contact inhibition.” Cancer cells, in contrast, usually disregard these restraints and continue to grow, so that they pile up on top of one another, as shown (Movie 20.2). (A) Schematic drawing. (B and C) Light micrographs of normal (B) and transformed (C) fibroblasts. (B and C, courtesy of Lan Bo Chen.)


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Cell_Biology_Alberts. NET PRODUCTS: NET PRODUCTS: ENERGY, CO2, H2O ENERGY, BUILDING BLOCKS, NADPH effect—so named because Otto Warburg first noticed the phenomenon in the early twentieth century. It is this abnormally high glucose uptake that allows tumors to be selectively imaged in whole-body scans (see Figure 20–1), thereby providing a way to monitor cancer progression and responses to treatment. Cancer Cells have an Abnormal Ability to Survive Stress and DNA Damage In a large multicellular organism, there are powerful safety mechanisms that guard against the trouble that can be caused by damaged and deranged cells. For example, internal disorder gives rise to danger signals in the faulty cell, activating protective devices that can eventually lead to apoptosis (see Chapter 18). To survive, cancer cells require additional mutations to elude or break through these defenses against cellular misbehavior.


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Cell_Biology_Alberts. Cancer cells are found to contain mutations that drive the cell into an abnormal state, where metabolic processes may be unbalanced and essential cell components may be produced in ill-matched proportions. States of this type, where the cell’s homeostatic mechanisms are inadequate to cope with an imposed disturbance, are loosely referred to as states of cell stress. As one example, chromosome breakage and other forms of DNA damage are commonly observed during the development of cancer, reflecting the genetic instability that cancer cells display. Thus, to survive and divide without limit, a prospective cancer cell must accumulate mutations that disable the normal safety mechanisms that would otherwise induce a cell that is stressed, in this or in other ways, to commit suicide. In fact, one of the most important properties of many types of cancer cells is that they fail to undergo apoptosis when a normal cell would do so (Figure 20–13).


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Cell_Biology_Alberts. While cancer cells tend to avoid apoptosis, this does not mean that they rarely die. On the contrary, in the interior of a large solid tumor, cell death often occurs on a massive scale: living conditions are difficult, with severe competition among the cancer cells for oxygen and nutrients. Many die, but typically much more by necrosis than by apoptosis (Figure 20–14). The tumor grows because the cell birth rate outpaces the cell death rate, but often by only a small margin. For this reason, the time that a tumor takes to double in size can be far longer than the cell-cycle time of the tumor cells. human Cancer Cells Escape a Built-in Limit to Cell Proliferation Many normal human cells have a built-in limit to the number of times they can divide when stimulated to proliferate in culture: they permanently stop dividing Figure 20–12 The Warburg effect in tumor cells reflects a dramatic change in glucose uptake and sugar metabolism.


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Cell_Biology_Alberts. (A) Cells that are not proliferating will normally oxidize nearly all of the glucose that they import from the blood to produce ATP through the oxidative phosphorylation that takes place in their mitochondria. Only when deprived of oxygen will these cells generate most of their ATP from glycolysis, converting the pyruvate produced to lactate in order to regenerate the NAD+ that they need to keep glycolysis going (see Figure 2–47). (B) Tumor cells, by contrast, will generally produce abundant lactate even in the presence of oxygen. This results from a greatly increased rate of glycolysis that is fed by a very large increase in the rate of glucose import. In this way, tumor cells resemble the rapidly proliferating cells in embryos (and during tissue repair), which likewise require for biosynthesis a large supply of the small-molecule building blocks that can be produced from imported glucose (see also Figure 20–26).


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Cell_Biology_Alberts. 1100 Chapter 20: Cancer after a certain number of population doublings (25–50 for human fibroblasts, for example). This cell-division-counting mechanism is termed replicative cell senescence, and it generally depends on the progressive shortening of the telomeres at the ends of chromosomes, a process that eventually changes their structure (discussed in Chapter 17). As discussed in Chapter 5, the replication of telomere DNA during S phase depends on the enzyme telomerase, which maintains a special telomeric DNA sequence that promotes the formation of protein cap structures to protect chromosome ends. Because many proliferating human cells (stem cells being an exception) are deficient in telomerase, their telomeres shorten with every division, and their protective caps deteriorate, creating a DNA damage signal. Eventually, the altered chromosome ends can trigger a permanent cell-cycle arrest, causing a normal cell to die.


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Cell_Biology_Alberts. Human cancer cells avoid replicative cell senescence in one of two ways. They can maintain the activity of telomerase as they proliferate, so that their telomeres do not shorten or become uncapped, or they can evolve an alternate mechanism based on homologous recombination (called ALT) for elongating their chromosome ends. Regardless of the strategy used, the result is that the cancer cells continue to proliferate under conditions when normal cells would stop. The Tumor Microenvironment Influences Cancer Development While the cancer cells in a tumor are the bearers of dangerous mutations and are often grossly abnormal, the other cells in the tumor—especially those of the supporting connective tissue, or stroma—are far from passive bystanders. The


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Cell_Biology_Alberts. Figure 20–13Both increased cell division and decreased apoptosis can contribute to tumorigenesis. In normal tissues, apoptosis balances cell division to maintain homeostasis (see Movie 18.1). During the development of cancer, either an increase in cell division or an inhibition of apoptosis can lead to the increased cell numbers important for tumorigenesis. The cells fated to undergo apoptosis are gray in this diagram. Both an increase in cell division and a decrease in apoptosis normally contribute to tumor growth.


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Cell_Biology_Alberts. Figure 20–14Cross-section of a colon adenocarcinoma that has metastasized to the lung. This tissue slice shows well-differentiated colorectal cancer cells forming cohesive glands in the lung. The metastasis has central pink areas of necrosis where dying cancer cells have outgrown their blood supply. Such anoxic regions are common in the interior of large tumors. (Courtesy of Andrew J. Connolly.) development of a tumor relies on a two-way communication between the tumor cells and the tumor stroma, just as the normal development of epithelial organs relies on communication between epithelial cells and mesenchymal cells (discussed in Chapter 22).


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Cell_Biology_Alberts. The stroma provides a framework for the tumor. It is composed of normal connective tissue containing fibroblasts and inflammatory white blood cells, as well as the endothelial cells that form blood and lymphatic vessels with their attendant pericytes and smooth muscle cells (Figure 20–15). As a carcinoma progresses, the cancer cells induce changes in the stroma by secreting signal proteins that alter the behavior of the stromal cells, as well as proteolytic enzymes that modify the extracellular matrix. The stromal cells in turn act back on the tumor cells, secreting signal proteins that stimulate cancer cell growth and division as well as proteases that further remodel the extracellular matrix. In these ways, the tumor and its stroma evolve together, like weeds and the ecosystem that they invade, and the tumor becomes dependent on its particular stromal cells. Experiments using mice indicate that the growth of some transplanted carcinomas depends on the tumor-associated fibroblasts


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Cell_Biology_Alberts. invade, and the tumor becomes dependent on its particular stromal cells. Experiments using mice indicate that the growth of some transplanted carcinomas depends on the tumor-associated fibroblasts and normal fibroblasts will not do. Such environmental requirements help to protect us from cancer, as we discuss next in considering the critical phenomenon called metastasis.


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Cell_Biology_Alberts. Cancer cells generally need to spread and multiply at new sites in the body in order to kill us, through a process called metastasis. This is the most deadly—and least understood—aspect of cancer, being responsible for 90% of cancer-associated deaths. By spreading through the body, a cancer becomes almost impossible to eradicate by either surgery or local irradiation. Metastasis is itself a multi-step process: the cancer cells first have to invade local tissues and vessels, move through the circulation, leave the vessels, and then establish new cellular colonies at distant sites (Figure 20–16). Each of these events is complex, and most of the molecular mechanisms involved are not yet clear.


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Cell_Biology_Alberts. For a cancer cell to become dangerous, it must break free of constraints that keep normal cells in their proper places and prevent them from invading neighboring tissues. Invasiveness is thus one of the defining properties of malignant tumors, which show a disorganized pattern of growth and ragged borders, with extensions into the surrounding tissue (see, for example, Figure 20–8). Although the underlying molecular changes are not well understood, invasiveness almost certainly requires a disruption of the adhesive mechanisms that normally keep cells tethered to their proper neighbors and to the extracellular matrix. For carcinomas, this change resembles the epithelial–mesenchymal transition (EMT) that occurs in some epithelial tissues during normal development (see p. 1042). The next step in metastasis—the establishment of colonies in distant organs— begins with entry into the circulation: the invasive cancer cells must penetrate the


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Cell_Biology_Alberts. The next step in metastasis—the establishment of colonies in distant organs— begins with entry into the circulation: the invasive cancer cells must penetrate the Figure 20–15 The tumor microenvironment plays a role in tumorigenesis. Tumors consist of many cell types, including cancer cells, endothelial cells, pericytes (vascular smooth muscle cells), fibroblasts, and inflammatory white blood cells. Communication among these and other cell types plays an important part in tumor development. Note, however, that only the cancer cells are thought to be genetically abnormal in a tumor. 1102 Chapter 20: Cancer normal epithelium cells grow as benign tumor in epithelium cells become invasive and enter capillary (fewer than 1 in 1000 cells will survive to form metastases) adhere to blood vessel escape from blood vessel colonize liver, forming wall in liver to form micrometastasis full-blown metastasis


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Cell_Biology_Alberts. Figure 20–16Steps in the process of metastasis. This example illustrates the spread of a tumor from an organ such as the bladder to the liver. Tumor cells may enter the bloodstream directly by crossing the wall of a blood vessel, as diagrammed here, or, more commonly perhaps, by crossing the wall of a lymphatic vessel that ultimately discharges its contents (lymph) into the bloodstream. Tumor cells that have entered a lymphatic vessel often become trapped in lymph nodes along the way, giving rise to lymph-node metastases. Studies in animals show that typically far fewer than one in every thousand malignant tumor cells that enter the bloodstream will colonize a new tissue so as to produce a detectable tumor at a new site.


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Cell_Biology_Alberts. wall of a blood or lymphatic vessel. Lymphatic vessels, being larger and having more flimsy walls than blood vessels, allow cancer cells to enter in small clumps; such clumps may then become trapped in lymph nodes, giving rise to lymph-node metastases. The cancer cells that enter blood vessels, in contrast, seem to do so singly. With modern techniques for sorting cells according to their surface properties, it has become possible in some cases to detect these circulating tumor cells (CTCs) in samples of blood from cancer patients, even though they are only a minute fraction of the total blood-cell population. These cells, in principle at least, provide a useful sample of the tumor-cell population for genetic analysis.


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Cell_Biology_Alberts. Of the cancer cells that enter the lymphatics or bloodstream, only a tiny proportion succeed in making their exit, settling in new sites, and surviving and proliferating there as founders of metastases. Experiments show that fewer than one in thousands, perhaps one in millions, manage this feat. The final step of colonization seems to be the most difficult: like the Vikings who landed on the inhospitable shores of Greenland, the migrant cells may fail to survive in the alien environment; or they may only thrive there for a short while to found a little colony—a micrometastasis—that then dies out (Movie 20.3). Many cancers are discovered before they have managed to found metastatic colonies and can be cured by destruction of the primary tumor. But on occasion, an undetected micrometastasis will remain dormant for many years, only to reveal its presence by erupting into growth to form a large secondary tumor long after the primary tumor has been removed.


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Cell_Biology_Alberts. Many Properties Typically Contribute to Cancerous Growth Clearly, to produce a cancer, a cell must acquire a range of aberrant properties—a collection of subversive new skills—as it evolves. Different cancers require different combinations of these properties. Nevertheless, cancers all share some common features. By definition, they all ignore or misinterpret normal social controls so as to proliferate and spread where normal cells would not. These defining properties are commonly combined with other features that help the miscreants to arise and thrive. A list of the key attributes of cancer cells in general would include the following, all of which we have just discussed: 1. They grow (biosynthesize) when they should not, aided by a metabolism shifted from oxidative phosphorylation toward aerobic glycolysis. 2. They go through the cell-division cycle when they should not. 3.


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Cell_Biology_Alberts. 2. They go through the cell-division cycle when they should not. 3. They escape from their home tissues (that is, they are invasive) and survive and proliferate in foreign sites (that is, they metastasize). 4. They have abnormal stress responses, enabling them to survive and continue dividing in conditions of stress that would arrest or kill normal cells, and they are less prone than normal cells to commit suicide by apoptosis. 5. They are genetically and epigenetically unstable. 6. They escape replicative cell senescence, either by producing telomerase or by acquiring another way of stabilizing their telomeres. In the next section of the chapter, we examine the mutations and molecular mechanisms that underlie these and other properties of cancer cells.


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Cell_Biology_Alberts. In the next section of the chapter, we examine the mutations and molecular mechanisms that underlie these and other properties of cancer cells. Cancer cells, by definition, grow and proliferate in defiance of normal controls (that is, they are neoplastic) and are able to invade surrounding tissues and colonize distant organs (that is, they are malignant). By giving rise to secondary tumors, or metastases, they become difficult to eradicate by surgery or local irradiation. Cancers are thought to originate from a single cell that has experienced an initial mutation, but the progeny of this cell must undergo many further changes, requiring additional mutations and epigenetic events, to become cancerous. Tumor progression usually takes many years and reflects the operation of a Darwinian-like process of evolution, in which somatic cells undergo mutation and epigenetic changes accompanied by natural selection.


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Cell_Biology_Alberts. Cancer cells acquire a variety of special properties as they evolve, multiply, and spread. Their mutant genomes enable them to grow and divide in defiance of the signals that normally keep cell proliferation under tight control. As part of the evolutionary process of tumor progression, cancer cells acquire a collection of additional abnormalities, including defects in the controls that permanently stop cell division or induce apoptosis in response to cell stress or DNA damage, and in the mechanisms that normally keep cells from straying from their proper place. All of these changes increase the ability of cancer cells to survive, grow, and divide in their original tissue and then to metastasize, founding new colonies in foreign environments. The evolution of a tumor also depends on other cells present in the tumor microenvironment, collectively called stromal cells, that the cancer attracts and manipulates.


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Cell_Biology_Alberts. Since many changes are needed to confer this collection of asocial behaviors, it is not surprising that most cancer cells are genetically and/or epigenetically unstable. This instability is thought to be selected for in the clones of aberrant cells that are able to produce tumors, because it greatly accelerates the accumulation of the further genetic and epigenetic changes that are required for tumor progression. CANCER-CRITICAL GENES: hOW ThEY ARE FOUND AND WhAT ThEY DO


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Cell_Biology_Alberts. As we have seen, cancer depends on the accumulation of inherited changes in somatic cells. To understand it at a molecular level we need to identify the mutations and epigenetic changes involved and to discover how they give rise to cancerous cell behavior. Finding the relevant cells is often easy; they are favored by natural selection and call attention to themselves by giving rise to tumors. But how do we identify those genes with the cancer-promoting changes among all the other genes in the cancerous cells? A typical cancer depends on a whole set of mutations and epigenetic changes—usually a somewhat different set in each individual patient. In addition, a given cancer cell will also contain a large number of somatic mutations that are accidental by-products—so-called passengers rather than drivers—of its genetic instability, and it can be difficult to distinguish these meaningless changes from those changes that have a causative role in the disease. Despite these difficulties,


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Cell_Biology_Alberts. than drivers—of its genetic instability, and it can be difficult to distinguish these meaningless changes from those changes that have a causative role in the disease. Despite these difficulties, many of the genes that are repeatedly altered in human cancers have been identified over the past 40 years. We will call such genes, for want of a better term, cancer-critical genes, meaning all genes whose alteration contributes to the causation or evolution of cancer by driving tumorigenesis.


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Cell_Biology_Alberts. In this section, we shall first discuss how cancer-critical genes are identified. We shall then examine their functions and the parts they play in conferring on cancer cells the properties outlined in the first part of the chapter. We shall end the section by discussing colon cancer as an extended example, showing how a succession of changes in cancer-critical genes enables a tumor to evolve from one pattern of bad behavior to another that is worse. The Identification of Gain-of-Function and Loss-of-Function Cancer Mutations has Traditionally Required Different Methods


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Cell_Biology_Alberts. Cancer-critical genes are grouped into two broad classes, according to whether the cancer risk arises from too much activity of the gene product or too little. Genes of the first class, in which a gain-of-function mutation can drive a cell toward cancer, are called proto-oncogenes; their mutant, overactive or overexpressed forms are called oncogenes. Genes of the second class, in which a loss-of-function mutation can contribute to cancer, are called tumor suppressor genes. In either case, the mutation may lead toward cancer directly (by causing cells to proliferate when they should not) or indirectly—for example, by causing genetic or epigenetic instability and so hastening the occurrence of other inherited changes that directly stimulate tumor growth. Those genes whose alteration results in genomic instability represent a subclass of cancer-critical genes that are sometimes called genome maintenance genes.


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Cell_Biology_Alberts. As we shall see, mutations in oncogenes and tumor suppressor genes can have similar effects in promoting the development of cancer; overproduction of a signal for cell proliferation, for example, can result from either kind of mutation. Thus, from the point of view of a cancer cell, oncogenes and tumor suppressor genes—and the mutations that affect them—are flip sides of the same coin. The techniques that led to the discovery of these two categories of genes, however, are quite different.


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Cell_Biology_Alberts. The mutation of a single copy of a proto-oncogene that converts it to an oncogene has a dominant, growth-promoting effect on a cell (Figure 20–17A). Thus, we can identify the oncogene by its effect when it is added—by DNA transfection, for example, or through infection with a viral vector—to the genome of a suitable type of tester cell or experimental animal. In the case of the tumor suppressor gene, on the other hand, the cancer-causing alleles produced by the change are generally recessive: often (but not always) both copies of the normal gene must be removed or inactivated in the diploid somatic cell before an effect is seen (Figure 20–17B). This calls for a different experimental approach, one focusing on discovering what is missing in the cancer cell. functionally eliminate the tumor suppressor gene, promoting cell transformation


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Cell_Biology_Alberts. functionally eliminate the tumor suppressor gene, promoting cell transformation We begin by discussing some examples of each class of cancer-critical genes to illustrate basic principles. These examples are chosen also for their historical importance: the experiments that led to their discovery—at different times and by different methods—marked turning points in the understanding of cancer. Retroviruses Can Act as Vectors for Oncogenes That Alter Cell Behavior The search for the genetic causes of human cancer took a devious route, beginning with clues that came from the study of tumor viruses. Although viruses are involved only in a minority of human cancers, a set of viruses that infect animals provided critical early tools for studying cancer.


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Cell_Biology_Alberts. One of the first animal viruses to be implicated in cancer was discovered over 100 years ago in chickens, when an infectious agent that causes connective-tissue tumors, or sarcomas, was characterized as a virus—the Rous sarcoma virus. Like all the other RNA tumor viruses discovered since, it is a retrovirus. When it infects a cell, its RNA genome is copied into DNA by reverse transcription, and the DNA is inserted into the host genome, where it can persist and be inherited by subsequent generations of cells. Something in the DNA inserted by the Rous sarcoma virus made the host cells cancerous, but what was it? The answer was a surprise. It turned out to be a piece of DNA that was unnecessary for the virus’s own survival or reproduction; instead, it was a passenger, a gene called v-Src, that the virus had picked up on its travels. v-Src was unmistakably similar, but not identical, to a gene—c-Src—that was discovered in the normal vertebrate genome. c-Src had evidently been caught up