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Therapeutic Stem Cells for Cancer Papers



Despite the considerable progress that has been made towards developing cancer therapeutics, there is still an urgent need for creating alternate and innovative means of cancer treatment. The most notable pitfalls of current treatments are the short half-life of a number of cancer specific drugs, limited drug delivery to some cancer types, and their adverse effects on vital non-cancerous bodily tissues. For the last decade, increasing number of research groups have focused on developing and testing therapeutic stem cells in different animal models of cancer. The ability of adult mesenchymal stem cells (MSCs) and neural stem cells (NSCs) to preferentially migrate towards local and disseminated malignant disease and interact with different tissue environments present them as attractive candidates for cell based therapies in humans. Recently, induced pluripotent stem cells (iPSCs), which provide a novel and practical tool for human disease modeling and correction, have demonstrated therapeutic potential for cancer treatment.

The unmodified stem cells, particularly MSCs, possess anti-tumor effects both in vitro and in different mouse models of cancer. This is attributed to the factors released by MSCs that have antitumor properties, which can reduce the proliferation of glioma, melanoma, lung cancer, hepatoma, and breast cancer cells. Both MSCs and NSCs have been genetically modified, primarily to introduce and over-express exogenous genes for expression/secretion of a desired therapeutic factor for targeted treatment of both primary and metastatic tumor types. Mesenchymal stem cells (MSCs) are multipotent cells that can be isolated and expanded with relative ease from a number of different sources, including bone marrow, cord blood, adipose tissue, and dental pulp.  Although MSCs play a primary role in tissue regeneration, they also have the proven ability to migrate specifically to the site of multiple tumor types in vivo. Although different MSC types have been engineered with therapeutic agents to specifically target numerous tumor types, the ability to image MSC homing to tumors and engraftment in real-time to confirm tumor targeting is essential to increase understanding of this process and  facilitate translation to the clinical setting. Dwyer et al (STEM CELLS 2011;29:1149-117) explored the non-invasive tracking of MSC migration and sodium iodide symporter (NIS) transgene expression in real time prior to therapy in a mouse model of breast cancer. A major advantage to this approach is the ability to noninvasively image uptake of a tracer such as 99mTechnetium pertechnetate (99mTcO_4) or 123Iodide (123I) before administration of a therapeutic dose of 131I. Dwyer et al engineered MSCs to express sodium NIS and utilized MSC-NIS for imaging and subsequent therapy of breast cancer. Tumor bearing animals received an intravenous or intratumoral injection of NIS expressing MSCs (MSC-NIS), followed by (99m) Technetium pertechnetate imaging. SPECT imaging and subsequent bio-distribution studies revealed significant reduction in radionuclide accumulation in non-target tissue as compared to the tumor tissue 2 weeks post MSC-NIS and (131) I injection. Based on imaging/biodistribution data, animals received a therapeutic dose of (131) I 2 weeks after MSC-NIS injection which resulted in a significant reduction in tumor growth.  Non-invasive tracking of MSC migration and transgene expression in real time prior to therapy is a major advantage of this study's technique. As MSCs have the ability to target multiple tumor types, this approach has potential applications in a range of cancers.

NSCs isolated from both embryonic and adult human tissues have emerged as attractive candidates for delivering therapeutic proteins that specifically target tumor cells. NSCs can be expanded and manipulated in vitro, and re-engrafted following transplantation. NSCs have shown the ability to migrate extensively to sites of different pathologies and reintegrate into tissue architecture to give rise to progeny consisting of both stem cells and lineage-restricted terminal cell types. Numerous studies have demonstrated the inherent ability of NSCs to seek out and infiltrate invasive tumors and can be used as cellular vehicles for delivering therapeutic proteins to different tumor types like medulloblastoma, melanoma, brain metastases, and disseminated neuroblastoma. NSCs, like MSCs, have been genetically modified to express exogenous genes for targeted treatment of different cancer types. Among these tumor targeting agents,different prodrug activation schemes which convert non-toxic prodrugs into toxic anti-metabolites are available for selective killing of tumor cells. These include cytosine deaminase (CD), herpes simplex virus (HSV)-1, Thymidine kinase (TK), and carboxyesterase genes, which confer sensitivity to 5-fluorocytosine 5-FC, ganciclovir (GCV), and camptothecin-11 (CPT-11), respectively, and are being evaluated in clinical trials. Activation of prodrugs that are non-toxic to NSCs and have a bystander tumor-killing effect are appropriate for the use of NSCs as “pharmacologic pumps”. Zhao et al (STEM CELLS 2012; 30:314-325) show that NSCs preferentially target tumor metastases in multiple organs, including liver, lung, lymph nodes, and femur, versus the primary intramammary fat pad tumor. They also show that increased NSC tropism to breast tumor cell lines is strongly correlated with the invasiveness of cancer cells, and have identified Interleukin 6 (IL-6) as a major cytokine mediating NSC tropism to invasive breast cancer cells (STEM CELLS 2012; 30:314-325). In an effort to explore NSC based therapies in such pre-clinical models, they genetically modified NSCs to secrete rabbit carboxylesterase (rCE), an enzyme that activates the CPT-11 prodrug to SN-38, a potent topoisomerase I inhibitor, to effect tumor-localized chemotherapy. A significant reduction of metastatic tumor burden in lung and lymph nodes was seen post treatment of tumor-bearing mice with NSC-rCE cells, in combination with CPT-11. This study suggests that NSC mediated enzyme/prodrug therapy may be more effective and less toxic than currently available chemotherapy strategies for breast cancer metastases, and warrants further investigation of NSC mediated delivery of targeted therapeutics for advanced metastatic breast cancers.

To realize the full potential of using NSCs for cancer therapeutics, it is desirable to have a reliable and stable supply of human NSCs. Pluripotent stem cells, such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), are attractive cell sources to derive differentiated cells, including NSCs. Human iPSCs appear to be more attractive for clinical applications since these cells are relatively easy to generate through reprogramming of differentiated somatic cells with transcription factors. This reprogramming procedure circumvents the bioethical controversies associated with the derivation of human ESCs from human embryos. Furthermore, transplantation of the differentiated progeny of iPSCs which are reprogrammed from the patient’s own cells, reduces the likelihood of immune rejection. In a recent study by Yang et al (STEM CELLS 2012;30:1021-1029), iPSCs were derived from primary human fibroblasts and then differentiated into NSCs. These iPS-derived NSC (iPS-NSC) displayed a robust homing to established orthotopic mouse mammary tumors in both immuno-deficient and immuno-competent mice. iPS-NSC engineered to express HSV-TK effectively inhibited the growth of orthotopic 4T1 breast tumor and the metastatic spread of the cancer cells post systemic delivery of prodrug, GCV, resulting in a prolonged survival of the tumor-bearing mice.

Although therapeutically engineered NSCs and MSCs are emerging as a very effective tumor specific therapeutic approach for different cancer types, the assessment of the long-term fate and the eradication of therapeutic SC post-tumor treatment is critical if such promising therapies are to be increasingly translated into clinical practice. Tumor necrosis factor apoptosis inducing ligand (TRAIL) has emerged as a prime candidate for the treatment of several cancers due to its ability to induce apoptosis in a tumor-specific manner. The major interest in TRAIL as an anticancer agent was initiated by early studies in which TRAIL-induced killing was shown in a wide variety of tumor cells in vitro and in vivo, while normal cells were unaffected by TRAIL treatment. A number of studies have shown the therapeutic efficacy of different adult stem cell types, including MSCs, engineered to express TRAIL in either tumor cell lines or mouse models of colorectal carcinoma, gliomas, lung, breast, squamous, and cervical cancer. Martinez-Quintanilla et al (STEM CELLS 2013;31:1706-1714) developed an efficient stem cell based therapeutic strategy that simultaneously allows killing of tumor cells with TRAIL and assessment and eradication of MSCs post-treatment of highly malignant glioblastoma multiforme (GBM) brain tumor, utilizing prodrug converting enzyme, HSV-TK. MSCs engineered to co-express HSV-TK and a potent and secretable variant of TRAIL(S-TRAIL) induced caspase mediated GBM cell death and showed selective MSC sensitization to the prodrug GCV. A significant decrease in tumor growth and a subsequent increase in survival were observed when mice bearing a highly aggressive GBM were treated with MSCs co-expressing S-TRAIL and HSV-TK. Furthermore, the systemic administration of GCV post-tumor treatment selectively eliminated therapeutic MSCs expressing HSV-TK in vitro and in vivo, which was monitored inreal time by positron emission-computed tomography (PET) imaging utilizing 18F-FHBG, a substrate for HSV-TK (STEM CELLS 2013;31:1706-1714). This study demonstrates the development and validation of a novel therapeutic/safety strategy that has implications for translating stem cell based therapies into clinics.

The anti-tumor effects of unmodified MSCs have been attributed to the factors released by MSCs that reduce the proliferation of glioma, melanoma, lung cancer, hepatoma, and breast cancer cells. In a recently published study, Nasuno et al (STEM CELLS 2013) studied the therapeutic effect of unmodified MSCs in different models of colon carcinogenesis. The study utilized three different models: an azoxymethane (AOM)/dextran sulfate sodium colitis-associated carcinoma model, an aberrant crypt foci (ACF) model, and a model to assess the acute apoptotic response of a genotoxic carcinogen (AARGC). MSCs were shown to partially cancel the AOM-induced tumor initiation, but not tumor promotion, suggesting that MSCs do not reduce aberrant crypt foci but block ACF formation.  AARGC is accepted as one of the in vivo mechanisms that suppresses tumorigenicity. MSCs inhibited AARGC in colonic epithelial cells because of the removal of O6-methylguanine adducts through O6-methylguanine-DNA methyltransferase activation. The anti-carcinogenetic properties of MSCs in vitro required transforming growth factor (TGF)-β signaling because such properties were completely abrogated by absorption of TGF-β under indirect co-culture conditions. MSCs also inhibited AOM-induced tumor initiation by preventing the initiating cells from sustaining DNA insults and subsequently inducing G1 arrest in the initiated cells that escaped AARGC. This study confirms that exogenous unmodified MSCs possess anti-tumor properties. Although a complete mechanistic insight into the properties of MSCs has yet to be achieved, it is clear from this study that MSCs act on tumors by reducing the number of initiating cells and/or to induce G1 arrest in early initiated cells in colon carcinogenesis models.

To conclude, athorough understanding of stem cell biology and fate in different tumor models which closely recapitulate clinical settings of tumor growth and heterogeneity is critical when developing stem cell based therapies for clinical translation in cancer patients.

Editor's Note provided by Khalid Shah, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts, USA.

Mesenchymal Stem Cell-mediated delivery of the sodium iodide symporter supports radionuclide imaging and treatment of breast cancer.
Dwyer RM, Ryan J, Havelin RJ, Morris JC, Miller BW, Liu Z, Flavin R, O'Flatharta C, Foley MJ, Barrett HH, Murphy JM, Barry FP, O'Brien T, Kerin MJ

Tumor tropism of intravenously injected human-induced pluripotent stem cell-derived neural stem cells and their gene therapy application in a metastatic breast cancer model.
Yang J, Lam DH, Goh SS, Lee EX, Zhao Y, Tay FC, Chen C, Du S, Balasundaram G, Shahbazi M, Tham CK, Ng WH, Toh HC, Wang S

Human neural stem cell tropism to metastatic breast cancer.
Zhao D, Najbauer J, Annala AJ, Garcia E, Metz MZ, Gutova M, Polewski MD, Gilchrist M, Glackin CA, Kim SU, Aboody KS

Mesenchymal stem cells cancel azoxymethane-induced tumor initiation.
Masanao Nasuno, Yoshiaki Arimura, Kanna Nagaishi, Hiroyuki Isshiki, Kei Onodera, Suguru Nakagaki, Shuhei Watanabe, Masashi Idogawa, Kentaro Yamashita, Yasuyoshi Naishiro, Yasushi Adachi, Hiromu Suzuki, Mineko Fujimiya, Kohzoh Imai and Yasuhisa Shinomura

Therapeutic efficacy and fate of bimodal engineered stem cells in malignant brain tumors.
Martinez-Quintanilla J, Bhere D, Heidari P, He D, Mahmood U, Shah K.