Abstract A083: Cell adhesion to laminin, by integrin α6β1, mediates drug resistance to PI3K in castration-resistant prostate cancer through AR-dependent induction of BNIP3

Pten loss leading to constitutive PI3K/Akt/mTor activation is a well-characterized PCa survival pathway frequently activated in castration-resistant prostate cancer (CRPC). Unfortunately, targeted mono-therapies to PI3K/Akt/mTor with or without anti-androgen therapy failed to achieve significant response in clinical trials with CRPC patients. The mechanisms that drive PI3K resistance in CRPC are unknown. The androgen receptor (AR) is also a major driver of prostate cancer (PCa) survival, even in CRPC. However, surprisingly very little is known about how AR promotes PCa survival. We previously identified an AR-dependent survival pathway in androgen-dependent tumor cells whereby AR induction of integrin α6β1 and adhesion to laminin in the tumor microenvironment activates NF-κB to induce Bcl-xL. This pathway acts in parallel to the PI3K/Akt pathway in Pten-null tumor cells such that combined inhibition of both PI3K and integrin α6β1 is required to effectively kill tumor cells and suppress tumor growth. While this drug combination was effective in androgen-responsive tumors in vivo, it poorly suppressed CRPC. Thus, we hypothesized there are additional survival mechanisms operating in CRPC. To identify CRPC survival pathways, we mined the gene expression data comparing androgen-sensitive to CRPC tumors in a series of human PDX LuCaP models. We found the hypoxia-inducible protein, BNIP3, to be preferentially elevated in CRPC tumors, and this correlated with increased integrin α6β1 expression and enhanced AR signaling. Furthermore, elevated BNIP3 predicts for poor outcome in PCa patients. We discovered that androgen induces BNIP3 expression specifically in CRPC C4-2 cell lines, but not the androgen-dependent parental LNCaP cells. BNIP3 expression is dependent on AR induction of integrin α6β1, adhesion to laminin, and nuclear translocation of HIF-1α. The CRPC C4-2 cells are highly resistant to PI3K inhibition by PX866 compared to LNCaP when adherent to laminin, and siRNA-mediated loss of integrin α6 or BNIP3 sensitizes these cells to PI3K inhibition. Blocking Bnip3 expression in C4-2 cells injected orthotopically into mice reduced tumor growth and induced apoptosis. BNIP3 can promote survival by activating autophagy and/or mitophagy. Even though autophagic flux was specifically enhanced in laminin-adherent CRPC C4-2 cells, but not in LNCaP cells, this was not dependent on BNIP3. WT BNIP3 can restore PI3K drug resistance in shBNIP3-expressing cells, but the BNIP3 ΔLIR mutant, which is unable to target mitochondria to autophagosomes, cannot restore drug resistance. We have identified an AR-dependent survival pathway in CRPC involving BNIP3-induced mitophagy that is dependent on and mediated by integrin α6β1 adhesion to laminin that promotes resistance to PI3K inhibition. Laminin is abundant in prostate tissue, lymph nodes, and bone—the primary sites for PCa growth. Thus, laminin within the tumor microenvironment of both primary and metastatic PCa drives drug resistance in CRPC. Citation Format: Eric Nollet, Sourik Ganguly, Veronique Schulz, Eva Corey, Cindy K. Miranti. Cell adhesion to laminin, by integrin α6β1, mediates drug resistance to PI3K in castration-resistant prostate cancer through AR-dependent induction of BNIP3 [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr A083.