Abstract 5091: Synergistic apoptosis induced by combined PI3 kinase and bcl-xl inhibition in genotypic subsets of prostate cancer is attenuated by a feedback signaling loop via the alpha-5 integrin

Background: Integrin-mediated cellular adhesion to extracellular matrix components is a crucial regulator of tumor cell survival and chemoresistance. The integrin alpha 5 (ITGA5) plays a vital role in the migration and adhesion of prostate cancer (PCa) cells to fibronectin secreted by bone-derived mesenchymal stromal cells (Joshi, 2015), architects of a metastatic niche (Shiozawa, 2011). Hypothesis: The mechanism(s) whereby ITGA5 regulates the survival and chemoresistance in PCa may define novel therapeutic approaches. Methods and Results: To assess the pro-survival function of ITGA5 in PCa, we found that ITGA5 shRNA reduced bcl-2 and bcl-xl expression, and induced apoptosis in PC-3 cells. In these PTEN-mutant (mt) AR-negative(-) cells, pharmacological inhibition of the PI3K signaling pathway (BKM120) in combination with ITGA5 knockdown synergistically induced apoptosis as assessed by induction of cleaved caspase-3 and cleaved PARP. Similarly, BKM120 in combination with pharmacological inhibition of bcl-2/bcl-xL (ABT263) also induced apoptosis in synergistic fashion in PC-3 cells. Synergistic apoptosis with PI3-kinase inhibitors (BKM120, pictilisib) and ABT-263 was restricted to PTEN-mt PC-3 and LNCaP cells and was not observed in PTEN-wt, AR- DU145 cells. The pan-Akt-inhibitor GDC-0068 in combination with ABT263 induced apoptosis specifically in PTEN-mt AR+ LNCaP and C4-2B cells. By contrast, there was diminished evidence of synergy of PI3-Kinase inhibitors and bcl-2 specific inhibition with ABT199 pointing toward the requirement of bcl-xl inhibition. However, resistance to longer-term therapy with PI3K and bcl-xl/bcl-2 inhibition was observed in PC-3 but not LNCaP cells. Although single-agent PI3K inhibitor therapy downregulates ITGA5 expression in PC-3 cells, by contrast bcl-2/bcl-xl inhibitor therapy, alone or in combination with PI3K inhibitor therapy, strongly upregulates ITGA5 expression together with Akt activation suggesting that ITGA5 represents a pathway of resistance to combinatorial therapy through feedback loop signals generated with bcl-xl inhibition. In support of this hypothesis, ITGA5 knockdown together with BKM120 and ABT263 combinatorial therapy blocked the upregulation of ITGA5 and associated Akt activation, further reduced bcl-xl expression, and enhanced apoptosis in PC-3 cells. In PTEN-wild type AR-positive VCAP cells, evidence for synergistic apoptosis with PI3K and bcl-2/bcl-xl inhibition was also obtained suggesting that the TMPRSS2-ERG fusion may phenocopy the pro-survival pathways of PTEN-deficient cells. Conclusions: These data point toward a personalized pharmacological strategy combining PI3-kinase or Akt inhibition with bcl-2/bcl-xL inhibition in genotypic subsets of prostate cancer and identify a potential resistance pathway mediated by ITGA5-mediated signaling. Citation Format: Wenying Ren, Raghav Joshi, Paul Mathew. Synergistic apoptosis induced by combined PI3 kinase and bcl-xl inhibition in genotypic subsets of prostate cancer is attenuated by a feedback signaling loop via the alpha-5 integrin. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 5091.