Hypoxia mediated sorafenib resistance of HCC cells
First, we confirmed the antitumor effect of the sorafenib in human HCC cell lines, SMMC-7721 and Bel-7402 in normoxia and hypoxia. Cells were treated with serial concentrations of sorafenib for 72 h, and then SRB staining assay was used to detect the survival fractions. The survival curve of sorafenib is shown in Fig. 1A. When cells were exposed to serial concentrations of sorafenib in normoxia or hypoxia, the survival rate of sorafenib in normoxia or hypoxia were determined respectively, and the results suggested that hypoxia significantly reduced the anti-cancer effect of sorafenib. To specifically evaluate the differential antitumor effect in normoxia and hypoxia, we calculated the half maximal inhibitory concentration (IC50) [16, 17]. The IC50 values of sorafenib in normoxia and hypoxia were 10.01 μM and 47.99 μM respectively (Fig. 1B). Similar results were obtained on Bel-7402 (Supplemental Fig. 1). These data demonstrate that hypoxic microenvironment conferred the resistance of HCC cells towards sorafenib.
Cytotoxicity of the combination of sorafenib and CT-707 in human HCC cell lines
Our previous study has uncovered the enhanced anti-cancer capacity of CT-707 (Fig. 2A) against the hypoxic HCC cells [13]; therefore, we were encouraged to ask whether the combination of sorafenib and CT-707 would exert enhanced effects under hypoxia. Normoxic or hypoxic SMMC-7721 cells were pre-treated with serial concentrations of CT-707 for the first 24 h, and then subjected to serial concentrations of sorafenib for the next 48 h. SRB assay was used to monitor the survival fractions of each group. As shown in Fig. 2B, the combination of sorafenib and CT-707 achieved synergistic anti-cancer effects under hypoxia, which was greater than that under normoxia. Specifically, when cells were exposed to sorafenib (20 μM), CT-707 (3 μM), or their combination in normoxia, the inhibition rates were 83.94%, 32.07%, and 92.94%, respectively. In the contrast, those under hypoxia were 29.76%, 40.32%, and 91.29%, respectively (Fig. 2C). These data revealed that the hypoxia-mediated resistance was remarkably attenuated by CT-707, and the combination of these two agents elicited robustly enhanced anti-cancer activities against hypoxic HCC cells than that under normoxia. Similar results were also achieved when cells were co-exposed to CT-707 at 4 μM (Fig. 2C).
To further demonstrate that CT-707 could overcome the hypoxia-mediated resistance of sorafenib, we treated SMMC-7721 cells with sorafenib (10 μM), CT-707 (3 μM) or both under normoxia and hypoxia, respectively; and observed the cell morphology using optical microscope. Representative pictures of SMMC-7721 and Bel-7402 after 72 h in cell culture dishes were displayed in Fig. 2D and Supplemental Fig. 2. Combination treatment resulted in significant inhibition to the proliferation of SMMC-7721 and Bel-7402 under hypoxia, while the mono-treatment induced moderate inhibition. Taken together, these data suggested that the hypoxic resistance of sorafenib in HCC cells could be greatly abolished by CT-707.
CT-707 treatment strengthens the apoptosis-induction by sorafenib in hypoxic HCC cells
To further confirm that CT-707 could overcome sorafenib resistance under hypoxia, we assessed the apoptosis of SMMC-7721 and Bel-7402 cells after 72 h treatment by sorafenib (15 μM), CT-707(4 μM) or both. The results detected by PI staining following FACS analysis were shown in Fig. 3A and Supplemental Fig. 3, the apoptosis ratio (early + late apoptosis) of control, sorafenib, CT-707 and combination groups in SMMC-7721 were 7.49%, 15.90%, 21.60% and 71.03% respectively; and those in Bel-7402 were 0.08%, 34.40%, 27.71% and 61.76% respectively. The results indicated that the combination of these two agents enhanced the apoptosis in HCC compared with mono-treatment.
Because most apoptotic cell death undergoes the caspase-dependent pathway [18], we further examine the activation of the caspase cascade of SMMC-7721 cell line after 72-h treatment by sorafenib (20 μM), CT-707(3μM) or the combination using Western blotting. As demonstrated by Fig. 3A, the combined treatment of CT-707 an sorafenib significantly triggered caspase activation as indicated by the robust cleavage of PARP, the substrate of caspases cascade, which denoting more apoptosis in the combination groups. These findings collectively verified that the combination of CT-707 with sorafenib could significantly enhance the hypoxic anti-cancer activities in HCC.
CT-707 inhibits YAP nuclear translocation in HCC cell lines
Above-mentioned data illustrated the capability of CT-707 that increased the hypoxic HCC cell susceptibility towards sorafenib. Mounting evidence has implicated the critical roles of YAP signaling in hypoxia-mediated drug resistance [19, 20]. Particularly, our previous studies demonstrated that hypoxia-activated YAP pathway contributed to the decreased drug response of HCC cells towards sorafenib or SN-38 [9, 12]. Based on these findings, we performed a functional screening and identified CT-707 as a novel YAP inhibitor, and this agent possessed superior activity under hypoxia by suppressing hypoxia-induced YAP translocation. Therefore, we next investigated whether CT-707 overcome the resistance to sorafenib through its YAP-inhibitory effect.
The core components of the Hippo pathway include the mammalian sterile 20-like kinases (MSTs) and large tumor suppressor kinases (LATSs), impose negative regulation on YAP by phosphorylation on residue Ser127, leading to the cytoplasmic retention of YAP protein. On the contrary, the unphosphorylated YAP would translocate into the nucleus and exert its transactivation function. We assessed the phosphorylated and total levels of YAP in SMMC-7721 cells after 24-h exposure to sorafenib (10 μM), CT-707 (3 μM) or both using Western blot analysis. The results showed that hypoxia caused decreased levels of p-YAP (Ser127), denoting the nuclear translocation of YAP under hypoxic microenvironment (Fig. 4A). While CT-707 exposure under hypoxia significantly induced the phosphorylated YAP, which indicated that in these cells, YAP protein was detained in the cytoplasm (Fig. 4A). These results implied that CT-707 overcome the resistance of sorafenib in hypoxia by preventing YAP nuclear translocation.