Akt-activated endothelium constitute the niche for residual disease and resistance to bevacizumab in ovarian cancer
Authors: Guerrouahen Bella Samia 
Published: November 2013

Background. Ovarian cancer is the second leading cause of cancer-related death in women worldwide. Despite optimal cytoreduction and adequate adjuvant therapy, initial tumor response is often followed by relapse. Targeted therapies have been evaluated in ovarian cancer to overcome resistant disease. Among them anti-angiogenic therapies inhibit new blood vessel growth, induce endothelial cell apoptosis, and block the incorporation of haematopoietic and endothelial progenitor cells into new blood vessels. Despite in-vitro and in vivo successes anti-vascular therapy with bevacizumab targeting VEGF has limited efficacy in ovarian cancer. Anti-angiogenic treatment increases hypoxia, and might lead to tumor rebound and drug resistance. The precise molecular mechanisms underlying clinical resistance to anti-VEGF therapies are not well understood. But it became clear that the multiple changes in the stroma may determine the treatments outcome, Hypothesis. We hypothesized that abnormalities in the tumor endothelium may contribute to treatment resistance and produce and promote a residual microscopic disease and resistance to bevacizumab. Methods. We showed that Akt pathway is activated in vitro and in vivo in ovarian cancer endothelium. We used Akt-activated endothelial cells (E4+EC) that replicate tumor endothelium biology, and their control, HUVEC to investigate the anti-angiogenic activity of bevacizumab by angiogenesis and migration assays. We conducted XTT assay to examine the effect of bevacizumab on proliferation of VEGF producing human ovarian cancer cell lines. Expression of FGF-2, phospho-AKT was assessed by western blotting and flow cytometry. Finally, using a feeder-free matrigel and spheroid models of ovarian cancer we examined the effect of bevacizumab on residual disease. Results. Our study describes a comprehensive observational and functional investigation on the pivotal role played by the endothelium in the resistance to bevacizumab. We showed that the cross-talk between ovarian cancer cells and the endothelium activate PI3k/Akt. According to our findings, activated ECs expressing higher amount of VEGF-A tend to be less susceptible to the inhibitory effect of bevacizumab. Bevacizumab had no effect on the proliferation of Akt-activated EC, but significantly inhibited angiogenesis and delayed wound healing in HUVEC. We were able to show most primary ovarian cancer cells and ovarian cancer cells cultures secrete a large quantity of FGF-2 and showed that FGF-2 is able to revert the effect of bevacizumab on HUVEC. Our data suggest that an FGF-2/FGFR mediates a cross-talk between cancer and endothelium and is inviolved in an angicrine switch. We demonstrate the role of Akt-activated EC in supporting expansion and self-renewal of OCC in a residual disease context. Conclusion. We used the E4+ECs as a surrogate for tumor associated endothelium. We showed that an FGF-2/PI3K-AKT autocrine loop is required in ECs to perturb bevacizumab treatment. In summary, our study point out the role of an activated endothelium in the constitution of the residual disease and resistance to bevacizumab.