Abstract 157: Targeting distinct nodes of the PI3K/AKT/mTOR cascade in prostate cancer cells: impact on cell proliferation, apoptosis and pathway signaling

Abstract

Abstract Inactivation of the PTEN gene is a common event in PCa, leading to activation of the PI3K/AKT/mTOR pathway and promoting PCa progression. Inhibitors targeting this pathway are currently being developed as anticancer agents. While recent clinical trials with mTORC1 inhibitors have shown limited single-agent efficacy in castration-resistant prostate cancer (CRPC), it is still not known whether targeting alternative nodes of the PI3K/AKT/mTOR cascade may improve response. The current study aimed to characterize the response of PCa cell lines to multiple inhibitors targeting the PI3K, AKT and mTOR nodes of the cascade and identify possible mechanisms of resistance. The impact of the different PI3K/AKT/mTOR inhibitors on cell proliferation and apoptosis was measured by WST-1 and ApoTox-Glo assays, respectively, in a broad panel of established PCa cell lines and primary cell cultures generated from patient-derived xenografts. PTEN-negative PCa cell lines showed strong response to PI3Kβ/δ, AKT and mTOR inhibition, whereas PTEN-positive cells were resistant to PI3Kβ/δ and showed variable sensitivity to mTOR inhibitors. MTORC1 inhibitor showed low IC50 but reached plateau at about 70-80% growth inhibition. In contrast, mTORC1/2 inhibitor could reach complete growth inhibition at ~1μM concentration, in sensitive cell lines. Furthermore, treatment with PI3K or AKT inhibitor in combination with androgen depletion induced apoptosis in various PTEN-negative models, which is consistent with previously reported synergistic effect of PI3K/AKT and androgen receptor co-targeting. The effect of the inhibitors on phosphorylation of PI3K downstream targets (AKT, PRAS40, GSK3, S6K1 and 4EBP1) was analyzed by phospho-specific western blotting. Perturbation of different nodes of the PI3K/AKT/mTOR cascade resulted in differential phosphorylation of downstream targets, but there was no evident correlation with in vitro growth inhibition. In conclusion, the current study validates PI3Kβ and AKT as alternative targets in PTEN-negative PCa, whereas PTEN-positive models showed preferential yet variable responses towards mTOR inhibitors. Citation Format: Rute B. Marques, Ashraf Aghai, Wendy Stam, Wytske M. van Weerden. Targeting distinct nodes of the PI3K/AKT/mTOR cascade in prostate cancer cells: impact on cell proliferation, apoptosis and pathway signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 157. doi:10.1158/1538-7445.AM2017-157