Background: PTEN-loss is a highly prevalent genomic event associated with higher risk of metastatic progression and resistance to hormonal therapy in prostate cancer. Akt inhibitor therapy in combination with abiraterone demonstrates early promise in the clinical control of PTEN-deficient prostate cancers. We have previously implicated BCL-XL in the survival of PTEN-null PC-3 prostate cancer cells in vitro, regulated by integrin alpha 5 (ITGA5) (Ren, MCR 2016). Toward advancing therapeutic strategies in PTEN-deficient prostate cancer we further assessed the link between PTEN and BCL-XL mediated prostate cancer cell survival.

Methods: Isogenic PTEN knockdown stable cell lines (VCAP and DU145) were generated by lentiviral transduction of pLKO-control-shRNA and two pLKO-PTEN-shRNAs, respectively. PTEN was exogenously expressed in PTEN-null prostate cancer cell lines (PC-3, LNCaP and C4-2B) by 1) transfecting cells with pCMV-Flag-vector or pCMV-Flag-PTEN 2) transfecting cells with pcDNA3-GFP-PTEN and then sorting GFP-PTEN-negative and GFP-PTEN-positive cells by flow cytometry. Cells were then treated with a single agent BCL-XL inhibitor (A1331852 or navitoclax (ABT263); AbbVie), PI3K (buparlisib; Selleck Chem) or Akt (ipatasertib, Selleck Chem) inhibitors or combined PI3K and BCL-2 family inhibitors. Cell survival was assessed by cell viability assay. Expression of pAkt, ITGA5 and apoptotic markers was assayed by Western blot analysis and/or flow cytometry.

Results: Notwithstanding the reduction of phospho-bad expression with PI3K inhibition, the potent BCL-XL-targeting BH3 mimetic, A1331852, compared to single agent PI3K or Akt inhibitors in titrated doses was alone sufficient to induce apoptosis and loss of cell viability in all PTEN-null prostate cancer lines tested. PTEN expression reversed the apoptotic response to BCL-XL targeting and blocked the anti-proliferative effect of PI3K or Akt inhibition in PTEN-mutant cells. PTEN knockdown induced membrane expression of ITGA5 whereas PTEN overexpression diminished ITGA5 expression.

Conclusions: PTEN regulates the apoptotic threshold in prostate cancer cells via BCL-XL independent of the PI3K/Akt pathway. The induction of ITGA5 membrane localization by PTEN loss defines a potential upstream signaling mechanism for the BCL-XL regulated apoptotic threshold in these cells. Highly potent BCL-XL inhibition requires further investigation as a therapeutic strategy in PTEN-deficient prostate cancer.

Citation Format: Wenying Ren, Raghav Joshi, Paul Mathew. A potent BH3 mimetic targeting BCL-XL induces apoptosis regulated by PTEN loss and integrin alpha 5 in prostate cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5499.