Androgen Receptor-Induced Integrin α6β1 and Adhesion to Laminin Promotes Survival and Drug Resistance in Castration-Resistant Prostate Cancer through BNIP3

Although castration-resistant prostate cancers no longer respond to anti-androgen therapies, the androgen receptor (AR) is still required to promote tumor survival. However, the signaling pathways downstream of AR that promote this survival are not well known. We recently identified an AR-dependent survival pathway whereby AR induction of integrin α6β1 and adhesion to laminin activates NF-κB/RelA signaling and Bcl-xL. This pathway acts in parallel with the PI3K/Akt pathway in Pten-null tumor cells such that combined inhibition of both PI3K and integrin α6β1 is required to kill tumor cells adherent to laminin. However, PTEN-null castration-resistant tumors were not effectively inhibited by this combination. We discovered that BNIP3, a hypoxia-induced BH3-only, pro-mitophagic Bcl2 family member, is induced by androgen in castration-resistant cells through integrin α6β1 signaling to HIF1α. Furthermore, castration-resistant cells adherent to laminin were much more efficient at inducing autophagy in response to androgen. Androgen blocked the ability of the PI3K inhibitor PX-866 to kill castration-resistant tumors, but this was reversed by loss of BNIP3. Although BNIP3 was dispensable for androgen-induced autophagy, its mitophagy function was required for BNIP3 to promote resistance to PI3K inhibition. Thus, adhesion to laminin triggers signaling through AR/α6β1/HIF1α in castration-resistant cells to drive the expression of BNIP3 and cooperates with AR/α6β1-mediated autophagy, both of which contribute to PI3K resistance through induction of mitophagy.