text:
Caveolin-1 Regulates Endothelial Adhesion of Lung Cancer Cells via Reactive Oxygen Species-Dependent Mechanism

The knowledge regarding the role of caveolin-1 (Cav-1) protein on endothelium adhesion of cancer cells is unclear. The present study revealed that Cav-1 plays a negative regulatory role on cancer-endothelium interaction. Endogenous Cav-1 was shown to down-regulate during cell detachment and the level of such a protein was conversely associated with tumor-endothelial adhesion. Furthermore, the ectopic overexpression of Cav-1 attenuated the ability of the cancer cells to adhere to endothelium while shRNA-mediated Cav-1 knock-down exhibited the opposite effect. We found that cell detachment increased cellular hydrogen peroxide and hydroxyl radical generation and such reactive oxygen species (ROS) were responsible for the increasing interaction between cancer cells and endothelial cells through vascular endothelial cell adhesion molecule-1 (VCAM-1). Importantly, Cav-1 was shown to suppress hydrogen peroxide and hydroxyl radical formation by sustaining the level of activated Akt which was critical for the role of Cav-1 in attenuating the cell adhesion. Together, the present study revealed the novel role of Cav-1 and underlying mechanism on tumor adhesion which explain and highlight an important role of Cav-1 on lung cancer cell metastasis.

Introduction
Recently, roles of caveolin-1 (Cav-1) in regulation of cancer progression and metastasis in various types of cancer have been revealed [1][2][3][4] and such a protein perhaps received the most attention in cancer-related research. Although some studies suggested that Cav-1 may play a role in inhibiting cancer progression in certain cancers [5], in lung cancer, Cav-1 potentiates cancer aggressiveness as well as metastasis [6]. Together with the fact that Cav-1 expression in lung cancer was shown to relate to poor prognosis [2], and most of the cancerrelated death in this cancer was shown to link with metastasis, it is of great interest to investigate the entire regulatory role of this protein on cancer metastasis [7]. Metastasis is a multi-step process of cancer cells spreading from their original locations to the distant secondary sites. Starting with the cancer cell detachment from their primary tumor, the cells invade vascular wall, travel in the circulatory system, and adhere to the endothelium to form the secondary tumors. Although roles of Cav-1 on lung cancer cell behaviors have been intensively explored, the role of such a protein on lung cancer cell adhesion to endothelium surface is largely unknown. We and others have suggested the important role of Cav-1 in rendering cancer cells resistant to anoikis after cell detachment [6,8,9,10], enhancing invasion and migration [11], and facilitating growth in anchorage-independent manner [12]. Endogenous Cav-1 level was shown in the previous studies to be controlled by the reactive oxygen species (ROS). In detached cell condition, hydrogen peroxide was shown to increase the cellular level of Cav-1 by inhibiting its degradation [6]. While in the adherent cells, hydroxyl radical was shown to be a key player in up-regulating Cav-1 expression and increased cell migration [11]. These findings highlighted the regulatory role of ROS on Cav-1 expression and their accompany roles on cancer metastasis.
In biology, negative feedback regulations exist to prevent the excessive stimulations. Likewise, Cav-1 protein was shown to suppress oxidative stress caused by hydrogen peroxide exposures [13]. However, it remains unknown whether Cav-1 regulates ROS level in detached cells and such regulation is critical for cancer adhesive property. Using pharmacological and genetic approaches, the present study revealed that Cav-1 plays a key role in inhibition of cancer-endothelium adhesion by attenuating hydrogen peroxide and hydroxyl radical generations after cell detachment. The present study also found that Cav-1 suppressed such ROS formation through Akt-dependent mechanism. Along with the observation that Cav-1 decreased in a time-dependent fashion after cell detachment, we found that at later-time points, cancer-endothelium adhesion significantly increased the concomitant of that Cav-1 depletion. Thus, our study revealed the existence of a novel mechanism of cancer cell adhesion regarding Cav-1 which might be exploited in metastasis and drug design.

Plasmid and Transfection
The Cav-1 expression plasmid pEX_Cav-1 and its control vector; pDS_XB-YFP were acquired from the American Type Culture Collection (Manassas, VA) and Cav-1 knockdown plasmid shRNA-Cav-1 and its control vector; control shRNA plasmid A were obtained from Santa Cruz Biotechnology (Santa Cruz, CA). Stable transfections of Cav-1 expression plasmid or Cav-1 knockdown plasmid were generated by culturing H460 cells in a 6-well plate until they reached 60% confluence. 15 ml of Lipofectamine reagent and 2 mg of Cav-1, and shRNA-Cav-1 plasmid were used to transfect the cells in the absence of serum. After 12 h the medium was replaced with culture medium containing 5% fetal bovine serum. Approximately 36 h after the beginning of transfection, the cells were digested with 0.03% trypsin, and the cell suspensions were plated onto 75-ml culture flasks and cultured for 24 to 28 days with G418 or puromycin for selection of Hcav-1 and shCav-1, respectively. The stable transfectants were pooled and the expression of Cav-1 protein in the transfectants was confirmed by western blotting. The cells were cultured in antibiotic free RPMI 1640 medium for at least two passages before used in each experiment.

ROS Detection
Intracellular ROS were determined by flow cytometry using DCFH 2 -DA as a fluorescent probe. Briefly, cells were incubated with 10 mM DCFH-DA, HPF for 30 min at 37uC, after which they were washed, trypsinized, resuspended in phosphate-buffered saline, and immediately analyzed for fluorescence intensity by FACScan flow cytometer (Beckton Dickinson, Rutherford, NJ) using a 488-nm excitation beam and a 538-nm band-pass filter. Median fluorescence intensity was quantified by CellQuest software (Becton Dickinson) analysis of the recorded histograms. H 2 O 2 was determined by Amplex Red reagent (Invitrogen). Briefly, the reaction mixture containing 50 mM Amplex Red reagent and 0.1 U/ml HRP in Krebs-Ringer phosphate (KRPG) was added into each microplate well. Subsequently, 20 ml of 1.5610 4 H460 cells suspended in KRPG was added to the reaction mixture. After 30 min incubation, the fluorescence signal obtained from the fluorescence microplate reader equipped for excitation in the range of 530-560 nm and emission detection at 590 nm was monitored and measured over the 3 h postdetachment.

Monolayer Cell Adhesion Assay
HUV-EC-C were stimulated with 10 ng/ml IL-1b for 0-4 h. H460 cells (2.5610 4 cells/ml) were stained with 15 mM of calcein AM (Sigma) for 45 min at 37uC and 5% CO 2 , then added onto a semi-confluent monolayer culture of HUV-EC-C, incubated for 20 min at 37uC with rotation at 120 rpm, and washed extensively to exclude nonspecific cell attachment. The number of attached cells was counted directly under a fluorescence microscope as reported previously [14]. For antibody mediated blocking of cell adhesion, the IL-1b -stimulated HUV-EC-C were incubated with antibody to VCAM-1, ICAM-1 and E-selectin (at a final dilution of 1:400 for every antibodies) for 3 h at 37uC in humidified CO2 incubator, and then H460 cells were added.

Western Blotting
After specific treatments, cells were incubated in lysis buffer containing 20 mM Tris-HCl (pH 7.5), 1% Triton X-100, 150 mM sodium chloride, 10% glycerol, 1 mM sodium orthovanadate, 50 mM sodium fluoride, 100 mM phenylmethylsulfonyl fluoride, and a commercial protease inhibitor cocktail (Roche Molecular Biochemicals) for 30 min on ice. Cell lysates were collected and determined for protein content using the Bradford method (Bio-Rad Laboratories, Hercules, CA). Equal amount of proteins of each sample (40 mg) were denatured by heating at 95uc for 5 min with Laemmli loading buffer, and subsequently loaded on 10% SDS-polyacrylamide gel electrophoresis. After separation, proteins were transferred onto 0.45 mm nitrocellulose membranes (Bio-Rad). The transferred membranes were blocked for 1 hour in 5% nonfat dry milk in TBST (25 mM Tris-HCl (pH 7.5), 125 mM NaCl, 0.05% Tween 20) and incubated with the appropriate primary antibodies at 4uC overnight. Membranes were washed twice with TBST for 10 min and incubated with horseradish peroxidase-coupled isotype-specific secondary antibodies for 1 hour at room temperature. The immune complexes were detected by enhanced with chemiluminescence substrate (Supersignal West Pico; Pierce) and quantified using analyst/PC densitometry software (Bio-Rad).

Statistical Analysis
Mean data from independent experiments were normalized to result from cells in the control. All the experiments were repeated at least three times. A statistical analysis between two groups was verified by Student's t test, in comparison to multiple groups, an analysis of variant (ANOVA) with post hoc test was conducted. The strength of relationships, correlation coefficient (r), between each protein level after detachment was determined with SPSS version 16 (SPSS Inc., Chicago, IL, USA). A P-value of less than 0.05 would be considered as statistically significant.

Caveolin-1 Depletion after Cell Detachment Enhances Tumor-endothelial Cell Adhesion
Cav-1 was shown to associate with metastatic potentials of lung cancer cells [6,12], while its role on cancer cell adhesion is still unclear. To study the role of Cav-1 protein on cancer cell adhesion, we first detached the cancer cells to mimic the early step of metastasis, and evaluated the expression profile of Cav-1 after cell detachment over times. Lung cancer H460 cells cultured in ultralow attachment plates were analyzed for Cav-1 protein expression by western blotting at indicated time points. Figure 1A shows that after cell detachment, Cav-1 gradually decreased in a time-dependent fashion and the significant decrease was detected as early as 6 h after cell detachment. Next, the suspended cells at concurrent time points were subjected to the monolayer cell adhesion assay as described in Materials and Methods. Figures 1A  and B show that after cell detachment, Cav-1 expression gradually decreased over time concomitant with the increase of cancer cell adhesion on endothelial surfaces, suggesting the potential role of Cav-1 in cancer adhesive regulation. We further supported such a correlation, by generating a correlation plot between Cav-1 expression and cancer cell adhesion ( Fig. 1 C). Not only did the reduction of these proteins correlate well with the enhanced ability of cancer cells to adhere on endothelium surfaces, but the plot also revealed a highly correlated profile with the correlation coefficient of 0.922.
To confirm that Cav-1 attenuates cancer cell adhesion to endothelial cells, we explored the effect of different ectopic Cav-1 expression levels on H460 cells adhesion to endothelium. We established Cav-1 overexpressing (Hcav-1) cells, short-hairpin (sh)mediated Cav-1 knockdown (shCav-1) cells by stable transfection. The transfectant clones were analyzed for Cav-1 expression compared to their parental H460 cells by western blotting (Fig. 1D). It is noteworthy that the control transfected cells generated from control plasmids, pDS_XB-YFP and control shRNA plasmid A, were also evaluated for Cav-1 expression, however; the level of the protein was comparable to that of H460 cells (data not shown). Cav-1 overexpressed, Cav-1 knockdown, and H460 cells were detached and subjected to endothelial adhesion assay and the cancer cells adhering on the HUV-EC-C endothelial cells were observed. As expected, analysis of cell adhesion showed that Cav-1 overexpressed cells exhibited the lowest ability to adhere to the HUV-EC-C cells, while the shCav-1 cells had the highest ability. Also, the vector transfected cells were similarly evaluated for adhesive capability and we found no significant alteration in comparison to that of parental cells (data not shown). This indicates that Cav-1 negatively regulates cancer cell adhesion to vascular endothelium.

Caveolin-1 Suppresses Hydrogen Peroxide and Hydroxyl Radical Generation after Cell Detachment
Our previous study has shown that in the adhered condition, Cav-1 function in attenuating oxidative stress caused by hydrogen peroxide treatment [13]. Therefore, it is possible that Cav-1 protein may function to regulate redox status of the cancer cells during metastasis. To study the effect of Cav-1 protein on detachment-induced ROS generation, Cav-1 overexpressed (Hcav-1), Cav-1 knock-down (shCav-1), and control cells (H460) were detached and incubated in adhesion-resistant plates until specific time points. The suspended cells were then analyzed for intracellular ROS levels by flow cytometry using DCFH 2 -DA as a fluorescent probe. Figure 2A shows that cell detachment induced an increase in cellular ROS in a time-dependent manner which aligned with our previous report [6]. The present study revealed for the first time that Cav-1 functioned in attenuating the detachment induced ROS in these cells. ShCav-1 cells possessing the lowest Cav-1 expression was shown to exhibit the highest ROS induction and such an induction could be detected as early as 1 h after detachment ( Fig. 2A), whereas cellular ROS in Cav-1 overexpressed (Hcav-1) cells was not altered in response to cell detachment.
Further, the specific ROS inhibitors were used to evaluate specific ROS which were up-regulated in these cells. Cav-1 overexpressed, Cav-1 knock down, and H460 cells were pretreated with specific ROS inhibitors which were MnTBAP (a superoxide anion inhibitor), catalase (a hydrogen peroxide inhibitor), deferoxamine (a hydroxyl radical inhibitor), and sodium formate (a hydroxyl radical scavenger), and the ROS levels after detachment for 0-3 h were determined. Figure 2B indicates that treatment with catalase, sodium formate, and deferoxamine could block the induction of ROS in these cells, suggesting that hydrogen peroxide and hydroxyl radical were primary specific ROS up-regulated after cell detachment.
The effect of Cav-1 on hydrogen peroxide and hydroxyl radical produced by these cells was next evaluated. Amplex red, a specific detection probe for hydrogen peroxide, and HPF, a specific detection probe for hydroxyl radical, were used to determine such specific ROS in the detached cells. Figures 2C and D indicate that cell detachment increased the signals of hydrogen peroxide and hydroxyl radical specific probes in a time-dependent manner. Importantly, the signal of either hydrogen peroxide or hydroxyl radical was barely detectable in Cav-1 overexpressed cells, whereas such ROS signals were potentiated in the cells expressing a low level of Cav-1. These results indicated that Cav-1 attenuated hydrogen peroxide and hydroxyl radical generations in suspended lung cancer cells.
Hydrogen Peroxide and Hydroxyl Radical Regulate Cancer Cell Adhesion to Endothelial Cells via VCAM-1dependent Mechanism ROS were shown in many studies to play a key role in the regulation of cancer cell behaviors [11]. Together with the above finding indicating that Cav-1 attenuated hydrogen peroxide and hydroxyl radical formations during cell detachment, we investigated whether hydrogen peroxide and hydroxyl radical play a role in regulating cancer cell adhesion. Lung cancer cells were incubated with various known specific ROS modulating agents as described and the cells were subjected to the adhesion assays. Figure 3A indicates that treatment of the cells with a superoxide inhibitor MnTBAP had only an minimal and insignificant effect on cancer cell adhesive activity, whereas the addition of catalase, deferoxamine, and sodium formate significantly inhibited the adhesion of H460 cells to endothelium surface. These results suggested that hydrogen peroxide and hydroxyl radical but not superoxide anion are the ROS that potentiate adhesive capability of these cancer cells to vascular endothelial cells.
Evidences have suggested that endothelial cell adhesion molecules (ECAMs) play an important role on cancer-endothelial cell interaction [15]. Activation of endothelial cells by IL-1b induced the expression of ECAMs namely VCAM-1, ICAM-1 and E-selectin on endothelial cell surfaces, and these proteins facilitated the binding of cancer cells [15]. To confirm the role of hydrogen peroxide and hydroxyl radical on cancer-endothelial cell interaction and to define the affected adhesion molecules, we blocked the endothelial surfaces with specific antibodies against VCAM-1, ICAM-1 and E-selectin, prior to cancer cell adhesion. Also, the cancer cells were treated with exogenous hydrogen peroxide in the absence or presence of ferrous sulfate to confirm the role of hydrogen peroxide and hydroxyl radical, respectively. Figure 3B indicates that hydrogen peroxide and hydroxyl radical significantly enhanced cancer cell adhesion to endothelial cells, confirming the role of mentioned ROS on cancer cell adhesion. Interestingly, blocking endothelium with the VCAM-1 antibodies could abolish the effect of hydrogen peroxide and ferrous sulfate on the adhesive property of these cancer cells while antibodies against ICAM-1 and E-selectin had no significant effects. These results indicated that hydrogen peroxide and hydroxyl radical may, at least in part, regulate the interaction of the cancer cells to endothelial surface through VCAM-1-dependent mechanism. Also, Cav-1 may decrease the adhesive property of the cancer cells by attenuating hydrogen peroxide and hydroxyl radical.

Cav-1 Suppresses ROS Generation after Cell Detachment through PI3K/Akt Pathway
Having shown that Cav-1 attenuated cancer ability to adhere to an endothelium by decreasing cellular hydrogen peroxide and hydroxyl radical up-regulations, and evidence indicated that Cav-1 regulates several cell behaviors during cell detachment through PI3K/Akt pathway [11]. To further confirm the role of Cav-1 on Akt signaling, we monitored the Akt activation in terms of phosphorylated (serine 473) Akt. In agreement with previous finding [16], we found the increased level of Akt activation in Hcav-1 cells in comparison to that of H460 and shCav-1 cells (Fig. 4A). Also, we found that cell detachment significantly decreased the level of activated Akt in almost all cells. While phosphorylated (serine 473) Akt in shCav-1 cells found to be most affected by such a cell detachment, the level of activated Akt in Hcav-1 was barely altered (Fig. 4A).
Since phosphatase and tensin homolog deleted on chromosome ten (PTEN) is a major negative regulator of the PI3K/Akt signaling pathway, we also performed the experiment to elucidate the role of Cav-1 on PTEN. Our results indicated that PTEN was up-regulated in the cytoplasmic portion (C) but down-regulated in the caveolae microsomal fraction (M) in Cav-1 overexpressed (Hcav-1) cells in comparison with those of H460 cells (Fig. 4B). In contrast, Cav-1-knock down cells exhibited the opposite profiles, suggesting that Cav-1 regulated the compartmentalization of PTEN. In order to assess the activity of PTEN, the western blot analysis of the C-terminal phosphorylated PTEN at Ser380, Thr382, and Thr383 that promote PTEN stability but decrease PTEN activities [17], was performed. Figure 4B shows that activated PTEN was significantly reduced in the Hcav-1 cells with the observations that the phosphorylated PTEN (at Ser380, Thr382, and Thr383) were significantly increased. Together with the finding indicating that the phosphorylation of such a protein in ShCav-1 cells was decreased, Cav-1 was demonstrated in the presence study that negatively regulated activated status of PTEN and such regulation may, at least in part, sustained the level of phosphorylated Akt.
To test whether the alteration in phosphorylated Akt affects the cellular oxidative stress, a PI3K inhibitor LY294002 was used to suppress Akt activation. H460 cells were incubated with PI3K inhibitor LY294002 at the concentrations of 10 and 50 mM, and Akt and phosphorylated-Akt levels were determined by western blotting (Fig. 4C). LY294002 at 10 and 50 mM were able to reduce p473-Akt (Fig. 4C). The effect of LY294002 on ROS induced by cell detachment was monitored by DCFH 2 -DA, HPF and Amplex red. Apparently, PI3K inhibitor was able to increase cellular oxidative stress as indicated by the significant increase of DCF fluorescence signal. Moreover, the specific ROS, namely hydroxyl radical and hydrogen peroxide, were found to be significantly increased in response to the treatment of PI3K inhibitor in H460 cells (Fig. 4D).
Interestingly, the treatment of LY294002 at 10 and 50 mM could reverse to effect of Cav-1 in suppressing ROS reduction in Hcav-1 cells. Figure 4E shows that ROS signal in Hcav-1 cells treated with LY294002 was remarkably increased, compared to that of non-treated Hcav-1 cells. In contrast, ROS level in shCav-1 cells were barely affected by the treatment of LY294002. Taken together, these findings suggested that Cav-1 regulate ROS formation after cell detachment by sustaining the activation of PI3K/Akt pathway.

Discussion
So far, a number of cancer-related proteins have been identified and used for specific applications, including biomarkers for the early detection, prognosis, and molecular targets for anti-cancer design [18]. According to the widely accepted hypothesis that not all primary tumors are able to adhere on the endothelial surfaces, but certain cancer cells possessing an innate or adaptive ability are able to adhere on the surface of vascular endothelial cells and to form metastases [19]. Many regulatory proteins were identified as tumor suppressor or tumor promoter proteins; however, in the case of Cav-1, both functions were reported. Cav-1 was first described as a tumor suppressor protein since the down-regulation of Cav-1 was found during cell transformation [7]. In contrast, Cav-1 was shown to potentiate cancer progression and increasing evidence has indicated its role as a cancer promoter [20]. Cav-1 was shown to function as an important mediator for multiple prosurvival signaling pathways [21][22][23]. In addition, Cav-1 expression was shown to increase in several types of human cancers, including lung, breast, prostate, and pancreas cancers, and this up-regulation was associated with a high degree of metastasis [20,[24][25][26][27][28]. Even though Cav-1 was shown to potentiate lung cancer metastasis by enhancing anoikis resistance [9] and invasion and migration [11], the present study provided the inhibitory effect of Cav-1 on cancer-endothelium adhesion which has not been demonstrated.
Hydrogen peroxide and hydroxyl radical previously shown to be generated during cell detachment [6] were demonstrated herein to be critical for adhesive action of cancer cells to endothelial surfaces (Fig. 3B). Such a finding has strengthened the role of ROS, especially hydroxyl radical in the regulation of cancer-endothelial interaction since hydroxyl radical was reported to enhance cancer cell adhesion by affecting the surface of endothelial cells [29]. Our finding was added to the growing list of HUV-EC-C adhesion (n = 5). D: Cav-1 overexpressed Hcav-1 or short hairpin Cav-1-transfected shCav-1 cells were constructed and analyzed for Cav-1 expression by Western blotting. Blots were re-probed with b-actin antibody to confirm equal loading of samples. The immunoblot signals were quantified by densitometry, and mean data from independent experiments were normalized to the results. Columns are means 6SD (n = 3), *P,0.05 vs. H460 cells. E: Hcav-1, H460, and shCav-1 cells were detached for 3 h and added onto a culture of HUV-EC-C. Columns are means 6SD (n = 3), *P,0.05 vs. H460 cells. doi:10.1371/journal.pone.0057466.g001 Caveolin-1 Regulates Endothelial Adhesion PLOS ONE | www.plosone.org the functions of specific ROS in regulating cellular pathologic events.
Cell adhesion molecules (CAMs), such as vascular endothelial cell adhesion molecule-1 (VCAM-1), intercellular cell adhesion molecule-1 (ICAM-1), and E-selectin, have been shown to be essential for endothelial cells in interaction with the cancer cells [15]. Such CAMs were shown to be up-regulated when endothelial cells were exposed to inflammatory cytokines, namely interleukin-1 beta (IL-1b) and tumor necrosis factor-alpha (TNF-a) [15]. Among them, VCAM-1 has garnered increasing attention in the anti-cancer field and recognized as a potential therapeutic target in metastasis [30]. In the present study, the regulatory role  Caveolin-1 Regulates Endothelial Adhesion PLOS ONE | www.plosone.org of hydrogen peroxide and hydroxyl radical was found to be associated with the expression of VCAM-1 on the endothelial cells (Fig. 3B). Blocking VCAM-1 on the endothelium surface but not ICAM-1 and E-selectin was shown to abolish the effect of such ROS on cancer cell binding, implying that ROS mediates cancer cell adhesion via VCAM-1-dependent mechanism. Since VCAM-1 is an endothelial ligand for interacting with b1 sub family of integrins [31], we hypothesized that hydrogen peroxide and hydroxyl radical probably affected the expression level and/or function of integrins on cancer cell surfaces.
The down-regulation of Cav-1 in lung cancer cells was shown to be dependent on the cellular redox status [6]. However, the feedback mechanism of Cav-1 on ROS regulation has not been reported elsewhere. We found that Cav-1 suppressed the formation of ROS in response to cell detachment and the inhibition of hydrogen peroxide and hydroxyl radical was essential for anti-adhesive role of Cav-1. The ectopic overexpression of Cav-1 significantly attenuated ROS, including hydrogen peroxide and hydroxyl radical generations induced by cell detachment, while the transfection with shRNA mediating Cav-1 downregulation showed the opposite effect ( Fig. 2A-D). Further, we showed that Cav-1 plays a role in regulating ROS after cell detachment through PI3K/Akt pathway. Considering the fact that Cav-1 was shown to reduce overtimes after cell detachment and this reduction was associated with the increase of cancer cell adhesion (Fig. 1B), at later time the metastatic cells may adhere and form secondary tumors. Also, Cav-1 was shown to reduce anoikis response that allows cancer cells expressing high Cav-1 to survive for long time and adhered to the distant sites (Fig. 5).
Although activated status of Akt was shown to be regulated by several mechanisms, several cancer-related studies suggested that PTEN play a dominant role in attenuating Akt functions [32][33]. Indeed, phosphatase activity of PTEN turns phosphatidyl-inositol 3,4,5 triphosphate (PIP 3 ), an activator of Akt, into PIP 2 , resulting in Akt suppression [32]. In the present study, PTEN expression was found in the caveolin-1 rich domains and was down-regulated in such a cell compartment in the Cav-1-overexpressed cells (Fig. 4B). In contrast, the level of PTEN was found to be increased in cytoplasmic portion of the cells. These results suggested that both Cav-1 and PTEN shared the same localization in these cells. In terms of PTEN function, we found that phosphorylated PTEN was significantly up-regulated in Cav-1-overexpressed cells, while the Cav-1-knock down cells exhibited the opposite results (Fig. 4B) Although further investigations on the underlying mechanisms that Cav-1 regulates PTEN are needed, our findings indicate that Cav-1 sustained the activity of Akt by suppressing PTEN activation.
In summary, the present study has revealed a novel finding on the cancer-endothelium regulatory effect of Cav-1 in human lung carcinoma H460 cells. This effect of Cav-1 involves its ability to Caveolin-1 Regulates Endothelial Adhesion PLOS ONE | www.plosone.org suppress hydrogen peroxide and hydroxyl radical generation by sustaining phosphorylated Akt levels. Because cancer adhesion to endothelial cells is crucial for cancer metastasis, the findings of this study could be beneficial to the understating of cancer metastasis and development of cancer therapy.


extfieldsofstudy: Biology Medicine

s2fieldsofstudy: Biology Medicine Chemistry

text:
Influenza A Virus Entry Inhibitors Targeting the Hemagglutinin

Influenza A virus (IAV) has caused seasonal influenza epidemics and influenza pandemics, which resulted in serious threat to public health and socioeconomic impacts. Until now, only 5 drugs belong to two categories are used for prophylaxis and treatment of IAV infection. Hemagglutinin (HA), the envelope glycoprotein of IAV, plays a critical role in viral binding, fusion and entry. Therefore, HA is an attractive target for developing anti‑IAV drugs to block the entry step of IAV infection. Here we reviewed the recent progress in the study of conformational changes of HA during viral fusion process and the development of HA-based IAV entry inhibitors, which may provide a new choice for controlling future influenza pandemics.

Introduction
Main Influenza A viruses (IAV) cause acute respiratory diseases in humans, birds, and other mammals, representing one of the major threats to public health. Wild birds are the reservoir of influenza A viruses. An avian strain can adapt to the human host and attain human-to-human transmission capability through acquired mutations. An unexpected human adaptation of an influenza subtype or strain rather than currently circulating influenza viruses may cause pandemic flu. The pandemics of 1918 H1N1 (Spanish flu), 1957 H2N2 (Asian flu), 1968 H3N2 (Hong Kong flu) and 2009 H1N1 (swine flu) symbolize the devastating public health and socioeconomic impacts of pandemic flu and keep us alert to any such outbreak. Additionally, seasonal flu is responsible for about 50,000 deaths per year. The H5N1 type IAV, which infected 18 patients in Hong Kong and caused 6 death in 1997, is a potentially serious threat to human health in the near future because of its high mortality (about 60%) and potential human-to-human transmission.
Influenza viruses mutate frequently because of their segmented RNA genome, making it almost impossible to produce a timely and sufficiently effective vaccine to prevent the potential oseltamivirresistant H5N1 influenza A viruses epidemic outbreaks. Therefore, it is the only way to use antiinfluenza agents for treatment and prevention at the beginning of pandemic outbreak of a virulent influenza strain, which gives time for the development and widespread dissemination of an effective vaccine. There are two classes of anti-influenza drugs up to now available in the clinic, which targeting the M2 ion channel and neuraminidase (NA) expressed on the virus envelope, respectively. Adamantanes block the ion channel formed by the M2 protein, which is critical in the release of viral ribonucleoprotein complexes (vRNPs) into the cytoplasm [1]. Although ion channel inhibitors can be effective against influenza virus infection, they have been reported to cause central nervous system (CNS) side effects. Also, currently circulating IAV strains are mostly resistant to adamantanes [2]. Thus adamantanes are not recommended for a general and uncontrolled use.
Two neuraminidase inhibitors, oseltamivir and zanamivir, were both approved in 1999 for treatment and prevention for acute uncomplicated flu caused by influenza A and B. Neuraminidase inhibitors interfere with the enzymatic activity of the NA protein, which is critical for the efficient release of newly synthesized viruses from infected cells. However, resistant virus strains are constantly emerging, especially to oseltamivir [3]. Different from the oral administration oseltamivir, zanamivir can only be inhaled due to its low bioavailability, which makes the limited use of this drug. In 2009, a new NA inhibitor, peramivir, was authorized for the emergent treatment of certain hospitalized patients with known or suspected 2009 H1N1 influenza.
It seems quite pressing to seek for new anti-influenza medications. Up to now, the life cycle of influenza virus has been well understood, allowing for the validation of several therapeutic targets. Among them, hemagglutinin (HA) is one of the most appealing ones. Till now 16 subtypes of HA have been identified and can be further subdivided into 5 clades and 2 groups (Figure 1). This malleable nature of HA imposes a great difficulty to conduct rational drug design. Furthermore, the variety of HA may be even strengthened by antigenic drift and antigenic shift [4,5]. Here, we described the functional and structural studies leading to the discovery of HA as a new anti-influenza target, and also how structural information is facilitating the rational design of new IAV entry inhibitors targeting HA.

The Virus Entry Process and the Function of Hemagglutinin
Hemagglutinin is encoded by the fourth negative-stranded RNA segment of influenza A viral genomes. This RNA segment encodes HA0 with 566 residues. HA0 is post-translationally glycosylated and trimerized with chaperon in endoplasmic reticulum (ER) in infected cell. Subsequently, HA0 undergoes an extra-or intra-cellular cleavage process into HA1 and HA2, which is a critical step for the maturation of influenza virus progenies to acquire their infectivity [6]. Most HA subtypes (H1, H2 and H3) of the circulating influenza viruses in human, have a conserved cleavage site with a sole basic amino acid residue R343 in specified sequence Q/E-X-R, which is only recognized by extracellular tissue-restrict trypsin-like proteases [7]. However, for highly pathogenic avian influenza (HPAI) viruses, the multi-basic HA0 cleavage sites (R-X-R/K-R) of H5-and H7-subtyped HA are recognized by ubiquitously expressed intracellular proteases, facilitating systemic virus spread and greater pathogenicity [8].
After proper proteolytic cleavage and glycosylation of HA0, disulfide bound subunits HA1 (about 327 residues) and HA2 (about 222 residues) form the fusogenic homotrimer, whose structure resembles a "mushroom" planted in the viral envelope ( Figure 2). The globular head of the "mushroom" is mainly constructed with HA1 subdomains, including receptor binding subdomain, vestigial esterase subdomain and antigenic epitope [4]. The receptor binding subdomain of HA1 recognizes α-2,3 or α-2,6 linked terminal sialic acids (SAs) in membrane glycoprotein receptor of host cell, therefore can prime the virus-cell adsorption and endocytosis for virus entry [9]. The stem of the "mushroom" is mainly composed of the inner trimeric HA2 ectodomain subunits, embraced by the N-and C-terminal segments of HA1. The first 23 residues of N-terminal of HA2 is the functional fusion peptide (FP). FP is accommodated in a hydrophobic pocket formed partially by the fusion domain of HA1 [10]. The acidification induced rearrangment of HA1 is a prerequisite for the exposure and release of FP subdomain of HA2 from inner pocket [11]. Subsequently, low pH-triggered HA2 reconformation results in the fusion of the viral envelope with endosome membranes (Figure 2) [12]. The fusion allows the release of viral ribonucleocapsid (vRNP) into cytoplasm of an infected cell, leading to the completion of the entry step. Figure 2. Scheme of pH-induced conformational change of HA2 at the pre-fusion and post-fusion states. The approximate location of HA2 residue 106 is marked with "*". The figure was adapted from the paper published in J. Virol. [13].

Antivirals Targeting HA1
The globular head of HA1, which "crowns" HA2, exposes its outmost surface to the environment, plays an incipient and central role in virus-host interactions, such as binding host receptor, being recognized by host antibodies, evading host immunity system through mutations and glycosylations at specific epitopes. Meanwhile, the electrostatic interactions between positive charged inner surface sites of HA1 and negative charged sites of HA2 stabilize trimeric HA at neutral pH. During fusion, dissociation of HA1 from the trimer spike is necessary for completion of HA2 spring unloading. Being so closely involved in these critical steps in viral entry, HA1 probably contains several critical sites for its interactions with host cell or HA2, which are also potential medicinal targets for developing antiviral drugs.

Inhibitors Targeting Receptor Binding Sites in HA1
The receptor binding site of HA resembles a shallow pocket, whose structure is composed of β-barrel motif and α-helices, located on the surface of globular head of the HA1 (residues 116-261). Terminal SAs of cell membrane glycoprotein, the natural receptor for HA-mediated viral adsorption, sit on top of two aromatic residues of HA1 (Y98 and W153 at the bottom of the binding pocket) [14]. Accordingly, a theoretical investigation by molecular dynamics (MD) simulation revealed that Y91 at the bottom of binding pocket of HA1 forms hydrogen bonds with α-2,3 or α-2,6 linked terminal SAs in various HA subtypes [15]. Besides, the binding sites in the pocket are of conserved amino acids that is edged by 190-helix at the top of the pocket, and the 130-and 220-loops located at the front edge and the left side of the pocket, respectively [9]. Above all, the receptor binding sites of HA mainly consist of residues 91, 98, 153 at bottom, 134 to 138 at 130-loop, 183, 190, 194 at 190-helix and 224-228 at 220-loop. Most of these residues are highly conserved among all 16 subtypes of HA.
In association with above reports, several mutations in correspondent sites in 130-loop, 190-helix or 220-loop result in altered HA1 binding specificity for SAs, and therefore may even create a strain of avian influenza A virus transmissible in mammal, such as ferrets [16]. Recent researches indicate two mutations in the avian influenza virus H1N1-typed HA1 (E190D in 190-helix and G225D in 220-loop) change the receptor binding preference from the avian α-2,3 to the human α-2,6 SAs, which resulted in the 1918 H1N1 and 2009 H1N1 virus capable of respiratory transmission between human, and maintaining its lethality and replication [17,18]. However, these two mutations (E190D and G225D) in HPAI H5N1 viruses do not cause a switch in binding specificity [5], though substitutions at positions 129 and 134 in 130-loop of HA1 could change the receptor-binding preference of H5N1 HA from α-2,3 to both α-2,3 and α-2,6 SAs [19]. Strikingly, in 2012, Imai et al. identified four mutations (N158D, N224K, Q226L and T318I) in the H5N1 HA1, which allow this highly pathogenic virus to be transmitted by respiratory droplets between ferrets [16].
Based on the fact that SA is HA receptor, SA-based inhibitors can be exploited as potential anti-HA agents. Because one influenza virion usually contains about 350-400 HA trimers on its surface [9], monovalent SA analogs ( Figure 3) would be unable to compete with the highly multivalent interactions between viruses and their host cells. Therefore, inhibitors of the bivalent, tetravalent and even polyvalent sialosides afford an enhanced inhibitory activity over monovalent ligands [20]. The polyvalent sialosides show high potency in vitro [20]. It is currently acknowledged that recognition of α-2,6 SAs is indispensable for the influenza viruses to gain their efficient transmissibility in human.
Therefore, multivalent α-2,6-sialyloligosaccharides could be effective for the protection against newly emerging pandemic influenza virus strains. Otherwise, it is possible to design transition-state α-2,6-SA binding analogues to inhibit receptor-binding and viral adsorption. As alternatives to SAs analogs, Teruhiko et al. identified peptides ( Figure 3) that bind to receptorbinding sites of H1 and H3-typed HAs through a multiple serial selection from phage-displayed random peptide libraries [21]. A docking simulation suggested that the peptides mimic the structure and binding ability of SAs. For example, N-steroyl peptides C 17 H 35 CO-ARLPRTMV-NH 2 (C18-s2(1-8)) and C 17 H 35 CO-ARLPR-NH2 (C18-s2(1-5)) could inhibit the infection of influenza A/H1N1/Puerto Rico/8/34 virus with IC 50 values of 3.0 and 1.9 μM, respectively [21]. In 2006, a 20-amino-acid peptide (EB) was found to exhibit broad-spectrum antiviral activity against influenza viruses in vitro and in vivo, even administered post-infection. Further analysis demonstrated that this EB peptide can specifically bind to the viral HA protein as an entry blocker [22]. In 2009, a smaller peptide P1 (CNDFRSKTC) with similar inhibitory mechanism to EB was also derived [23]. These results indicate that the HA-binding peptides are promising candidates for antiviral drugs as well as anti-HA antibodies.

Inhibitors Targeting Glycosylation Sites of HA1
The HA of IAV often undergoes N-linked glycosylation (NLG) at several sites. NLG of the globular head of HA is known to modulate the antigenicity, fusion activity, virulence, receptor-binding specificity, virus susceptibility to neuraminidase inhibitors, and immune evasion of IAV [24,25].
By interfering glycosylation sites, surfactant protein D (SP-D) plays an important role in innate defense against IAV and other pathogens. SP-D (and other collectins including mannose binding lectin (MBL), conglutinin and CL43) inhibits IAV by binding to glycans on the viral HAs. Binding of these collectins to high mannose oligosaccharides on the hemagglutinin (HA) appears to be most important for viral neutralization [26]. Inspired by reported antiviral activity of glycosylation inhibitors, such as nonactin and N-butyldeoxynojirimycin (NB-DNJ) [27,28], it may be possible to achieve an anti-influenza activity by appropriate glycosylation inhibition strategy. No clear examples of antibodies targeting glycans on HA have yet been reported, suggesting this may be a fertile area for future discovery efforts.
By binding antigenic epitopes, antibodies play a central role in the recognition and elimination of invading viruses. Therefore, IAV has evolved low-fidelity polymerases that result in high mutation rates and diverse glycosylations in antigenic sites to escape recognition by neutralizing antibodies [24,31]. Due to viral antigenic shift and drift, most influenza antibodies and vaccines often expire in a short period. Therefore, it is a leading edge to isolate or discover novel broadly neutralizing monoclonal antibodies (bnmAbs) recognizing highly conserved sites. The conserved HA stem domain elicits cross-reactive antibodies, but epitopes in the globular head typically elicit strain-specific responses because of the hyper-variability of this region. However, receptor binding sites are more conserved, since any mutations interfering receptor binding are most likely to impair viral adsorption and infectivity. Based on the fact that some antigenic sites overlap receptor binding sites, these sites are exploited by naturally occurring antibodies in humans. For example, F045-092 neutralizes not only H3N2 but also H1N1, H2N2 and H5N1 viruses. F045-092 shows hemagglutination inhibition activity and do not compete with C179, an antibody thought to bind to the stem region in HA2, but competes with antibodies that recognize sites A and B in HA1, especially residue S136 in 130-loop [32]. Strikingly, another bnmAbs C05 achieve a nanomolar binding with a minimal footprint in the tiny SA binding site in HA1, mainly by inserting just a single heavy-chain complimentarily determining region 3 (CDR3) loop [33]. Similarly, another bnmAbs CH65 also inserts its CDR3 loop into the receptor binding pocket on HA1 [34]. Moreover, receptor-binding bnmAbs F045-092 and CH65 have been isolated from B cells [32,34], suggesting that receptor-binding site antibodies can be found naturally and are potentially inducible by vaccination strategies.

Host-Targeting Drug Candidates Blocking Viral Adsorption
Though this review talks mainly about the antivirals targeting virus HA, the development of DAS181 still catch our nerves. DAS181 is the only host-targeting drug candidate under clinical development in influenza field. DAS181 is a 46 kDa recombinant fusion protein composed of a sialidase catalytic domain derived from Actinomyces viscosus fused with a mucosal cell surface-anchoring domain [35]. Through cleaving SAs from the host mucosal cell surface, DAS181 inactivates the host cell receptors for viral adsorption, thereby consequently blocks viral entry into respiratory epithelial cells. DAS181 cleaves both the α-2,6-SA and the α-2,3-SA, presents a potent inhibitory effect against a panel of laboratory strains and clinical isolates of influenza A and B viruses, including HPAI strains (H5N1), with in vitro EC 50 values range from 0.04 to 0.9 nM [35]. Recent research revealed that DAS181 is also active against oseltamivir-resisitant H1N1 strains [36]. Besides broad-spectrum antiviral activity, DAS181 treatment may also protect the airway epithelium from inflammation and denudation, which underlies mechanism of preventing secondary bacterial infection [37]. These findings highlight the potential broad spectrum activity of DAS181 against novel and drug-resistant influenza virus strains.
In contrast to DAS181 cleaving receptor SAs from host cell membrane, shielding the receptor SAs on cell membrane is another strategy to interfere the binding event of virus to host cell. Alkylation of two peptides c01 and c03 using N-sterol peptides (C18-peptides), derived C18-c01 and C18-c03 respectively. The alkyl groups of the two peptides are able to promote the formation of peptide assembly that ensures multivalent binding with SA-containing receptors. C18-c01 and C18-c03 respectively could inhibit the infection of influenza A/PR/8/34 virus with IC 50 values of 3.2 and 6.5 μM [38].

Antivirals Targeting HA2
HA2 subunit, which is linked with virus capsule membrane and owns a highly conservative sequence, conducts the irreversible fusion between viruses and cell endosome membrane at low pH condition and makes the virus genetic material released into the cells. HA2 is constructed by two different length of trend reverse parallel α-helix. There is a B ring formed by two spirals, which connected α-helix and a fusion peptide located in the N-terminal of HA2 with twenty residues (Figure 2).

Peptide Entry Inhibitors Targeting HA2 Trimer Structure
In the early 1990s, some peptides deriving from the HIV envelope protein gp41 were found to have very strong activity against HIV and to be capable of preventing HIV membrane fusion [39,40]. Among them, a polypeptide, T20, has become the first HIV entry inhibitors targeting gp41 trimer structure with the approval by US FDA [41]. It is a kind of new type of antiviral drugs, suitable for HIV patients with drug resistance. The discovery of HIV peptide enlightened scientists to study type 1 envelope virus entry inhibitors, such as respiratory syncytial virus (RSV), measles virus [42], ebola virus [43], viral encephalitis [44] and severe acute respiratory distress syndrome coronavirus (SARS-CoV) [45].
Because the influenza virus also belongs to type 1 envelope, scientists have tried to find similar polypeptides like T20 capable of fighting against IAV. But so far there has been no successful report, maybe resulting from the obstacles in the process of developing anti-influenza polypeptides. Unlike HIV, the membrane fusion of influenza virus happened in the endosome with an environment of acidic pH. A polypeptide derived from HA2 subunit cannot pass through the cell membrane to get in the endosome, or become unstable in the acidic conditions, so these peptides cannot effectively prevent membrane fusion mediated by hemagglutinin. A chemical method called hydrocarbon stapling is applied to design spiral polypeptides with the cell membrane penetrability in the study of HIV, with which it was found some peptides have activity to fight against HIV [46,47]. This method may be applicable to search for antiviral polypeptide with the haemagglutinin as the target.

Inhibitors Preventing Low pH-Triggering HA Conformation Change
Another strategy is to find small molecular compounds with the cell membrane penetrability or oral bioavailable compounds which can have interaction with HA to stop the membrane fusion ( Figure 4). However, before screening anti-influenza compounds targeting HA2 by high throughput, it is necessary to confirm whether the three polymers possesses the target point suitable for combination before fusion or during the fusion transition state. The target cavity of HIV gp41 is highly conservative, thus it is not easy to induce resistance for compounds interacted with the cavity, such as T-2544 [48] and ADS-J1 [49], because the resistance mutation of this segment may lead to the loss of HIV virulence. To study of antiviral compounds, it is advisable to devote more efforts to those conservative target points. According to the hemagglutinin, conservative target holes exist in HA2 spiral-screw parts.
In the process of the membrane fusion mediated by HA, after endocytosis into endosome, HA happens irreversible restructuring with the drop of pH. A few small molecular compounds can prevent low pH from triggering HA restructuring, such as benzene anthraquinones and hydroquinone compounds. The most promising benzodiazepine derivative is TBHQ, which can restrain virus infection in the μM concentration [50]. Under the pressure of TBHQ, mutations leading to resistance appear on HA2 subunit which shows that TBHQ is applied to HA2 subunit. The cocrystallization of TBHQ and HA has also been analytic [51]. TBHQ is combined with HA2 subunit, and stabilizes the trimer structure of HA2 under the neutral condition, thus making it disable to restructure in the fusion low pH. Three residues not only owning the competence of ionization but also capable of interacting with TBHQ are the R54 in a HA2 monomer, and E57/E97 in the adjacent HA2 monomer. TBHQ can stop the infection of H14-subtype influenza virus rather than H5N1, which is because in HA2 subunit of H5-subtype hemagglutinin, K58 and E97 form the salt bridge, and E97 on HA2 subunit of H14-subtype haemagglutinin tends to form the salt bridge with R54. Therefore, K58, existing in HA2 subunit of H14-subtype, can combines with TBHQ. Through the molecular docking screening, two compounds was found with better antiviral activity, S19 (IC50 = 0.8 μM) and C22 (IC50 = 8 μM) [52].
Russia approved an anti-influenza drug, Arbidol, in 1993, which acted by increasing influenza virus HA stability and preventing low pH-induced HA transition to its fusogenic state [53]. However, this potent broad-spectrum antiviral drug is only well-known in Russia and China. A recent study exploiting the characteristics of arbidol-resistant mutants of influenza virus has confirmed its inhibitory mechanism [54].
Scientists from Wyeth-Ayerst found three compounds, CL-61917 (N-replaced piperidine), CL-385319 and CL-62554, can inhibit the activity of H1-and H2-subtype avian influenza, and IC50 reaches a micromole level [55]. When testing the resistant virus strains, it was found that in HA protein sequences the mutation happened on an amino acid located in HA trimer stem area near the HA2 fusion peptide part. By computer-aided drug design the scientists revealed that CL-61917 is able to bind this part of a cavity and may prevent HA restructuring triggered by the low pH, which involves two pathways: (1) Enhance the stability of the residues around the cavity and prevent amino acid protonation caused by acid from damaging interaction force between residues; (2) Stop fusion peptide from moving, and make it disable to form the structure necessary for fusion. CL-385319, the 5-fluoro analog of CL-61917, is more potent than CL-61917 in inhibiting H1 and H2 influenza virus infection. We further found that CL-385319 is also potent in inhibiting H5N1 influenza virus by targeting hemagglutinin on a cavity other than the receptor binding, which at least including the M24 in HA1 and F110 in HA2 [56]. Further mutation study showed V48 in HA2 is also critical for the inhibitory activity of CL-385319 [57]. These critical residues clustered in the stem region of the HA trimer. Extensive computational simulations, including MD simulations, molecular mechanics generalized Born surface area (MM_GBSA) calculations have been carried out to uncover the detailed molecular mechanism. It was found that the recognition and binding of CL-385319 to HA proceeds by a process of ''induced fit'', whereby the binding pocket is formed during their interaction [57]. Occupation of this pocket by CL-385319 stabilizes the neutral pH structure of hemagglutinin, and thus inhibiting the conformational rearrangements required for membrane fusion. Recently, we further optimized the structure of CL-385319, and found that the 3-fluoro-5-(trifluoromethyl)benzamide segment is critical to the anti-H5N1 virus activity. Substitution the pyrrolidine ring of CL-385319 with thiophene ring can increase the activity to about one fold, making the compound 1l more attractive candidate for developing as an oral available anti-influenza drug [58].
Researchers from Japan found a few anti-influenza membrane fusion inhibitors, which are stachyflin and its derivatives separated from fermentation broth medium of botryoid ear mould [59]. Stachyflin can effectively prevent the infection of H1 subtype and H2 subtype avian influenza with IC50 value of a micromole level. Stachyflin stops the fusion of virus membrane through the interfering with HA restructuring triggered by low pH. By analyzing the HA gene sequence of stachyflin resistant strain, two point mutations was found, K51R and K121E, which are located in HA2 subunit [60]. It suggests that stachyflin's binding site is in the HA2 subunit. The same team also designed and synthesized stachyflin derivatives, such as acetylstachyflin, which have better bioavailability and antiviral activity [61]. But now there is no further report about the progress of this type of antiviral drugs.
Scientists in Bristol Myers Squibb have found another new kind of fusion inhibitors, BMY-27709, which can inhibit H1-and H2-subtypes influenza virus with IC50 in 3 to 8 μM [62]. Further study found that BMY-27709, combined with HA, prevents low pH triggering HA restructuring. The HA gene sequence analysis of BMY-27709 drug-resistant strain found that the mutation site is located in the N-terminal of HA2 subunit, which showed that compounds prevent HA mediated membrane fusion with an interaction to the N-terminal of HA2 subunit [63].
Eli Lilly researchers found the LY-180299 has a strong inhibition toward A/Kawasaki/86 H1N1 in the early stage of virus replication [64]. After analyzing the HA gene sequence of drug-resistant strain, it was found that the resistant mutations exist on HA1 and HA2 subunit adjacent to interface and the position near the fusion peptide. The pH in which membrane fusion occurs by the drug-resistant strains is 0.3-0.6 units higher than that of the wild type. It suggests that LY-180299 can stabilize the structure of HA neutral state and prevent low pH-triggering HA restructuring.
Tang et al. reported a series of benzenesulfonamide derivatives recently with potent anti-influenza activity by targeting HA. These compounds are modified from a salicylamide-based HA inhibitor cis-2-hydroxy-N-(5-hydroxy-1,3,3-trimethyl cyclohexylmethyl)-benzamide [65]. The lead compound RO5464466 and its 2-chloro analogue RO5487624 can effectively prevent the infection of influenza A/Weiss/43 strain (H1N1) with EC50 values of 210 and 86 nM, respectively. In vivo data suggest that RO5487624 has a protective effect on mice that were lethally challenged with influenza H1N1 virus [66]. Mechanism of action studies indicated that these compounds inhibit the virus membrane fusion with host endosome membrane by binding to HA and stabilizing the prefusion HA structure. These compounds also have significantly improved metabolic stability which could be developed as the first generation of orally bioavailable HA inhibitors.
Chloroquine is an inexpensive and widely available anti-malaria drug. Recently, in vitro assays suggest that chloroquine may have utility in the treatment of several viral infections including influenza. Vigerust et al. tested the in vivo effectiveness in ferret models. The results showed that clinical signs and viral replication in the nose of ferrets were not altered by treatment [66]. Just recently, a finished randomised, double-blind, placebo controlled trial showed that chloroquine does not prevent infection with influenza [67]. More cases in the chloroquine group are found catching laboratory-confirmed influenza infection, although generally well tolerated by a healthy community population. This dampens the enthusiasm for potential utility of the drug for humans with influenza.
Most of the founded small molecules are more potent in inhibiting group 1 IAV than group 2 IAVs. Vanderlinden et al. reported a new series of N-(1-thia-4-azaspiro [4.5]decan-4-yl)carboxamide inhibitors which show marked antiviral activity against group 2 influenza A/H3N2 viruses [68]. The lead compound 4c has an IC50 of 9.6 μM and a selectivity index of 10. This compound has specific activity against the H3 subtype, but not the H1, H5 and H7 subtypes.

Anti-HA2 Monoclonal Antibodies
Among all structural proteins of influenza viruses, hemagglutinin plays an important role in viral binding to receptors, virus entry and membrane fusion. Antibodies specific for HA have been shown to inhibit and/or neutralize virus infection in vaccinated hosts and anti-HA monoclonal antibodies (mAbs) are regarded as one of the important passive therapeutics for treatment of influenza. There are some companies, such as Theraclone, intend to screen human donors for broadly neutralizing, cross-reactive human antibodies to hemagglutinin. Current flu vaccines mainly provoke antibodies that target the head region of HA, but that part of this protein changes easily. Therefore, mABs recognizing a more conserved and efficacious epitope may broadly neutralize homologous and heterologous strains with different clades of IAV infection and prevent formation of escape mutants.
Although antibodies induced by HA2 do not have virus-neutralizing activity, monoclonal antibodies (mAbs) specific to the fusion peptide were able to inhibit the HA fusion activity and to reduce virus replication in vitro, and also in vivo [69,70]. Antibody CR6261 was first isolated from a person who had been vaccinated against seasonal flu [71]. Virus neutralization test showed CR6261 exhibits broad neutralizing activity against antigenically diverse H1-, H2-, H5-, H6-, H8-and H9-typed group 1 influenza viruses. More recent tests have shown that CR6261 also combats the novel H1N1 virus that caused the 2009 pandemic. Moreover, CR6261 strongly outperformed oseltamivir for influenza prevention and treatment in a preclinical study. Researches revealed that the Crucell's mAb CR6261 binds to a part of the H surface protein that does not change when the virus mutates-which it does with great ease. The H surface protein is crucial for replication of the influenza virus. Once this site is bound by an antibody, the virus cannot fuse with membranes inside the host's cells and therefore cannot reproduce. This mAb neutralized the virus by stabilizing the prefusion state and strongly inhibiting the conformational change of HA at the fusogenic pH. CR6261 cannot neutralize the H3-subtype likely due to glycosylation at residue position of N38. It was also undesired that the conserved epitope recognized by CR6261 was not particularly exposed in intact virus. The CR6261 epitope identified here should accelerate the design and implementation of improved vaccines that can elicit CR6261-like antibodies, as well as antibody-based therapies for the treatment of influenza [72]. Besides CR6261, new broadly neutralizing mAbs F10 and FI6 are also bind to a common conserved region of HA [73].
Monoclonal antibodies that target highly conserved viral epitopes might offer an alternative protection paradigm. Anti-M2e mAbs TCN-032 is an IgG monoclonal antibody which provide a broadly efficient protection for the treatment of influenza A virus. TCN-032 can binds to a novel conformational epitope on the N-terminus of the M2e protein [74]. The epitope recognized by TCN-032 is conserved in over 98% of known influenza A strains, including avian and swine strains. TCN-032 also has demonstrated potent protection in vivo against H5N1 and 2009 S-OIV H1N1 influenza strains. Clinical trials are undergoing to show its safety and effectiveness. Besides, broadly neutralizing, cross-reactive human antibodies to hemagglutinin are screening from human donors. To date, several candidates have been identified that bind and broadly neutralize Group 1 as well as the H3 subtype of Group 2 influenza strains [75].

Inhibitors Targeting the Cleavage of the HA0 to HA1 and HA2
As the H3-subtype avian flu virus X31 HA crystal structure and H1, H5, H9, H7-subtype avian influenza virus HA crystal structure are unfolded, people get a better understanding of haemagglutinin. The crystal structure show that HA is trimer forms. Each monomer of HA can be hydrolyzed into subunits HA1 and HA2 by host protease. The two hydrolyzed subunits are then connected by disulfide ( Figure 2). The structure of HA precursor (HA0) and the structure of the hydrolysis are very alike. Only after six residues of the C-terminal of HA1 and 12 residues of the N-terminal of HA2 subunit distributing can they fuse. Especially the 12 residues of HA2 cover polar amino acids in HA0 which is exposed to liquid environment. This makes the HA vulnerable to hydrogen ion excitation and its structure changes [76].
The hydrolysis of HA0 into HA1 and HA2 is a necessary step for avian influenza virus to be contagious. Therefore, if any molecule can prevent HA0 from being hydrolyzed into the subunits of HA1 and HA2, it possesses the ability to fight against viral infection. Some serine protease inhibitors, such as anti-protease peptide constituted of 58 amino acids [77], e-aminocaproic acid [7], Nafamostat [78], pulmonary surfactant (a kind of surface active lipoprotein complex) [79] and mucous protease inhibitors [80], can reduce the hydrolysis of HA0 and the infection of influenza viruses in both cell models and animal models ( Figure 5). As a kind of injections to treat the symptoms of bleeding, cow pancreas inhibitor, protease peptide (Trasylol bayer), has been applied to the clinical practice. However, it was withdrawn from the clinical application with the study finding that the drug could increase mortality in 2008 [81].   There are some FDA approval pulmonary surfactants, such as Exosurf, Curosurf, Infasurf and Survanta, all of which can enhance lung compliance, and prevent the baby respiratory distress syndrome. Quasi peptide inhibitor (dec-RVKR-CMK) resisting alkaline amino acid protease can inhibit the replication of H7 avian flu with high pathogenicity [82]. Recently, some researchers reported the MPSL/TMPRSS13 can hydrolyze H5 and H7 subtypes [83]. Therefore, natural inhibitors in the human body against MPSL/TMPRSS13 can serve as a lead compound to develop antiviral drugs which targeting haemagglutinin. It was found that the quasi peptide inhibitor also can restrain MPSL/TMPRSS13 and other insulin proteases such as plasmin, which may be the mechanism of action for this inhibitory peptide against influenza virus infection.

HA-Targeting Natural Products
In addition to above inhibitors, some natural molecules also possess good inhibitory activity against influenza A virus infection ( Figure 6). For example, catechins isolated from green tea, like EGCG, were found to exhibit mild anti-influenza effect [84]. Further modifications of catechins derived a set of better inhibitors. To be better anti-influenza drug candidates, catechin derivatives possess broad spectrum anti-influenza activity. Besides, curcumin, the widely used spice and coloring agent in Indian food, was proved to be a good virus entry inhibitor targeting HA with EC 50 value of 0.47 μM [85]. Modification of curcumin may produce a series of novel HA targeting inhibitors. Another kind of small molecule inhibitor is derived from andrographolide, like AL-1. AL-1 showed significant activity against avian influenza A (H9N2 and H5N1) and human influenza A H1N1 viruses in vitro [86]. AL-1 is capable of direct interfering with viral HA to block viral binding to cellular receptors as was demonstrated by its inhibitory activity on viral adsorption to red blood cells. In 2009, Li's group in China reported the first three small saponins molecule which inhibit HA. These three compounds can potently inhibit the entry of a H5N1 virus (A/Viet Nam/1203/2004) with IC 50 at low μM level. Further modifications revealed that the reduction of R1 to a disaccharide chain would abolish the inhibitory activity of these inhibitors [87]. Therefore, anti-influenza agents from natural products, especially those from Traditional Chinese Medicine (TCM), are promising lead compounds. Some lead compounds from TCM are extensively and intensively reviewed by Xu et al. in 2010 [88].

Conclusions
HA, the major surface protein of IAV, mediates the viral binding, membrane fusion and viral entry. Determination of the crystal structures of HA0 provides important information for understanding the pH-induced conformational changes of HA at the pre-fusion, intermediate and post-fusion states for development of anti-influenza drugs. Although a series of anti-influenza drugs targeting the NA and M2 ion channel are currently available, the emergency of drug-resistance viruses has raised the great concern on their ineffectiveness against the newly emerging IAVs and the HPAI viruses. Thus, it is essential to develop novel anti-IAV drugs with new targets. A number of protein-based or small molecule anti-IAV agents have been shown to interfere with the HA-mediated membrane fusion by targeting the receptor binding, blocking the cleavage of HA0, or by inhibiting the low pH-mediated conformation changes of HA. It seems not easy to find or design small compounds targeting the binding event of influenza virus, but the conformational change of HA2 which mediates membrane fusion, is a promising target for developing anti-influenza drugs. Novel influenza virus entry inhibitors may provide more selections for combination therapy with NA inhibitors and M2 ion channel blockers for treating and preventing influenza virus infection and potential pandemic outbreak.


SPPS Resins Impact the PNA-Syntheses' Improvement

The personalized medicine, also documented as “individualized medicine”, is an effective and therapeutic approach. It is designed to treat the disease of the individual patient whose precise differential gene expression profile is well known. The trend in the biomedical and biophysical research shows important consequences for the pharmaceutical drug and diagnostics research. It requires a high variability in the design and safety of target-specific pharmacologically active molecules and diagnostic components for imaging of metabolic processes. A key technology which may fulfill the highest demands during synthesis of these individual drugs and diagnostics is the solid phase synthesis which is congenial to automated manufacturing. Additionally the choice of tools like resins and reagents is pivotal to synthesize drugs and diagnostics in high quality and yields. Here we demonstrate the solid phase synthesis effects dependent on the choice of resin and of the deprotection agent.

Introduction
The increasingly expanding biomedical and biophysical research areas trace back to the human genome project (HGP) founded and started in 1990 as a manhood's big challenge for providing scientific findings to better understand biochemistry, molecular biology and medical sciences [1]. In the year 2001 the human's genome decryption was first documented by the International Human Genome Sequencing Consortium and by Greg Venter's founded Celera Corporation [2,3] simultaneously. With the release in 2003 of the genome's complete decoding in 2003, the HUGO was considered as finished [4,5]. It was then taken as the basis for the development of new diagnostic tools [6][7][8] and therapeutic approaches [9,10] with a previously unreached precision in sensitivity and sensibility. This led to the Gene Ontology Anno-tation (GOA) project and the proteome [11,12]. Also important is on the combination of both which describes the new expanding research field in the development of theranostic tools [13] enabling a successful pharmacotherapy with a minimum of adverse reactions, realizing a perfect fit to the patient's differential gene expression profile.. Due to the lack of stability of natural DNA and RNA against nucleases their use as a drug is not possible till now, and modifications are inevitable. Derivatives like peptide nucleic acids (PNAs) however, are not a substrate for the cell immanent enzymes and therefore they are resistant, highly sensitive and specific tools for antisense strategies [14][15][16] and can be applied both in cancer diagnostics and in therapy [23][24][25]. To increase their efficiency further they can be conjugated with Ivyspring International Publisher cell penetrating peptides (CPP) and peptide-based sequences for subcellular targeting [17][18][19] and are individually designed by solid phase peptide synthesis (SPPS) methodologies. This is carried out by coupling of building blocks combined with protection group chemistry [20][21][22]. Despite the improvement in the PNA syntheses, high quality PNAs' syntheses remain a challenge in ligation and deprotection chemistry. The success of synthesis strongly depends on different parameters, like activator's quality and deproctection kinetics which also correlate with the length of the PNA SPPS polymer. Modifications of the solid phase PNA synthesis's methods like micro wave [26] and the use of suitable deprotection reagents, like piperidine and pyrrolidine [27] which optimize yield and quality, were established and documented. It became increasingly apparent, that the choice of resin matrices [28] with physical properties especially for a high quality PNA synthesis is pivotal to a considerable extent and additional investigations in the resin development are required [29]. Here we report how we synthesized a PNA (see Figure 5) targeted to the translation initiation region which is part of the complementary coding sequence of the human c-myc Exon II [30]. We compared, analyzed and optimized the synthesis strategy for forthcoming functional PNA studies dealing with the cell cycle behavior, apoptosis, alterations of the cell phenotype and differential gene expression.

Chemical Procedure
The synthesis of the human c-myc specific PNA TACGGGGAGTTGCAA-NH2 shown in Figure 5, was performed on the ABI synthesizer 433A (Applied Biosystems, Foster City, USA). We used 9-fluorenylmethoxycarbonyl (Fmoc)-building blocks with blocked side chains of Adenine A, Cytosine C, and Guanine G by benzhydroxylcarbonyl (Bhoc) groups. The syntheses were performed in a 5 μmol scale on a H-Rink-Amide-ChemMatrix® resin (PCAS BioMatrix, Saint-Jean-sur-Richelieu, Canada) loading 0.52 mmol/g and on a TentaGel® R RAM high swell resin (RAPP Polymers, Tuebingen, Germany) loading 0.62 mmol/g. In the first step the Fmoc-groups were cleaved with 20% piperidine in N,N'-dimethylformamide (DMF) and 20% pyrrolidine in N-methylpyrrolidone for 10 min at 20°C. After that, the resin was washed 5 times with DMF. The coupling reaction was performed with 2-(1H-7-azabenzotriazole-1-yl)-1,1,3,3-tetramethyl uronium hexafluorophosphate (HATU) and N,N'-diisopropylethylamine (DIPEA) for 40 min. After the coupling all residual free amino groups were capped with acetic acid anhydride in DIPEA (1eq:2eq) in DMF and the resin was washed with DMF. At the end of the synthesis the N-terminal protecting group was removed with piperidine or pyrrolidine respectively and the reaction product was finally cleaved from the resin with TFA (95%) and scavanger triisopropylethylsilane/water (2.5%/2.5%) for 2.5 h at room temperature. The PNA product was precipitated in diethylether and subsequently lyophilized. The crude material was characterized with analytical HPLC on a Kromasil 100-5µm C 18 reverse phase column (30 × 250 mm) using an eluent of 0.1% trifluoro acetic acid in water (A) and 80% acetonitrile in water (B). The PNA product was eluted with a successive linear gradient of 10% B to 80% B in 30 min at a flow rate of 1 ml/min. (Figure 1 and Figure 2) and mass spectrometry (ESI-MS, Finni-gan TSQ 7000) ( Figure 3).

Results and Discussion
The solid phase peptide synthesis (SPPS) introduced by Merrifield is considered as the key technology in the automatized technique for the synthesis of functional peptides as well as for DNA derivatives like the peptide nucleic acids based on amino acids and peptide nucleic acid building blocks. Whereas at the beginning of the "SPPS age" time and efforts were immense, the progress in the synthesis' development led to reaction products with high puritiy and yield. The use of different modifications like microwaves [31,32] and ultrasound [33] resulted in a clear diminishment of the time rate in the reaction steps and resulted in simultaneously improvement of the quality. The resin acts as the solid phase platform originally described as "Merrifield-Resin" [34,35]. Its enormous advantage is shown in the following data which highlight different aspects of a PNA SPPS. The diagrams (left & right) document the HPLC analysis (Figure 1 and 2) and the corresponding mass ( Figure  3) of the PNA product. It was exemplarily synthesized by SPPS dependent on the resin chosen [22] either TentaGel® R RAM high swell Rapp Polymers ( Figure  1) or H-Rink Amide-ChemMatrix® Polymer ( Figure  2) and the basicity of the protection reagent.
The HPLC diagrams in Figure 1 and in Figure 2 show the peaks of the PNA synthesis products synthesized on TentaGel® R RAM high swell Rapp Polymer and H-Rink-Amide-ChemMatrix® Resin as a solid phase. The higher number of peaks in the diagrams after using the piperidine-step (left diagrams) indicates a higher number of truncated products.  The ESI-MS diagrams in Figure 3 show the correct mass peaks after deprotection with pyrrolidine but independent from the used resins. More or less conspicuous but mentionable is the detection of the fivefold loaded [M+5H] 5+ substance peak in the TentaGel® R RAM high swell Rapp Polymer representing diagram (right picture). The slight differences of the purities could be explained by the resins' different spatial patterns and surface characteristics which influence the syntheses: the TentaGel® R RAM high swell Rapp Polymer synthesis occurs spacer-mediated, whereas the synthesis runs directly at the ChemMatrix® Resin's backbone.
Furthermore, the graphs (Figure 4 A and B) highlight the discrepancies of the dissociated PNA building blocks as a function of the deprotection potential.
Since the liberated fluorenyl group is a chromophore, a deprotection by Fmoc can be monitored by UV absorbance of the fractions, a strategy which is employed in automated synthesizers. In the chart diagrams the bars represent the extent of the UV absorption at 301 nm of the separated fluorenyl groups. If a middle bar (as shown in B) occurs, a third deprotection step is carried out by the synthesizer's program routinely.
As shown in the left diagrams in the part A and B of Figure 4, we observed a more efficient deprotection behavior by pyrrolidine as a cleavage agent. Whereas one cleavage step was here sufficient, the use of piperidine as agent required three deprotection steps at all purines and also at the thymine. Additionally, the TentaGel® R RAM high swell resin seems to be more suitable for the deprotection steps during the PNA synthesis.   With this experience we could achieve the best reaction product's purity on the TentaGel® R RAM high swell resin with pyrrolidine as cleaving reagent. This result can be explained with the different physical properties of the TentaGel® R RAM high swell resin and the ChemMatrix resin. Namely, in the first case we performed the synthesis on a spacer and in the second case the synthesis occurred on the backbone of the resin and these properties gave rise to different swelling conditions [36].

Conclusion
Modern drugs consist of combined peptide-based functional modules with delivery and targeting properties. They are designed for use as a cargo of genetic material [38,39], highly pharmacologically active substances like temozolomide (TMZ) [40,41] or components for imaging of morphologic structures and metabolic processes in PET, SPECT [42], MRT [43], and CT [44] as well as for optical imaging (OI) [45][46][47][48]. In case of theranostic agents [13,[49][50][51] both groups of cargos (therapeutic and diagnostic) may be combined and a monitoring of the therapeutic success is much easier. The efficacy of the delivery and the specificity of the targeting allow a successful therapy and medical diagnosis by application of drastically reduced doses resulting in minimized adverse reactions which then would lead to discontinuation of the therapy. The criteria for therapeutic and diagnostic use of these new conjugates pose highest requirements on the synthesis as well as on optimized ligation procedures [52][53][54][55]. The SPPS can be considered as an excellent technology and the choice of appropriate resins and deprotection chemicals is indispensable.