Abstract 1702: Androgen receptor antagonist bicalutamide sensitizes androgen-independent prostate cancer cells to mTOR inhibitor rapamycin by regulating phosphorylation of translation initiation suppressor eIF4E-binding protein 1 (4E-BP1)

Abstract

Abstract We previously showed that resistance of prostate cancer cells to the mTOR inhibitor rapamycin results from its ability to induce androgen receptor (AR) transcriptional activity, whereas co-treatment with the anti-androgen bicalutamide prevented this effect. Based on our data, collaborators at UC Davis Cancer Center are conducting a clinical trial to test the efficacy of this combination in patients with advanced prostate cancer. However, the mechanism by which the combination of rapamycin and bicalutamide work remains unknown. Investigation of the cause for resistance of prostate cancer cells to rapamycin showed that rapamycin-induced growth inhibition as well as AR transcriptional activity is mediated primarily by p70S6 kinase (S6K1), a downstream target of mTOR, but not by the 4E-BP1/eIF4E/eIF4G pathway, another downstream target of mTOR. Additionally, 4E-BP1 phosphorylation at Ser 65 was dose dependently inhibited by rapamycin in androgen-dependent LNCaP, but not in androgen-independent C4-2 cells, although rapamycin equally affected S6K1phosphorylation in both cells. Comparison of AR expression and 4E-BP1 phosphorylation in various prostate cancer cell lines demonstrated that those expressing high levels of the full-length AR also had increased 4E-BP1 phosphorylation, whereas no such relationship existed with S6K1. Hence, we investigated whether increased AR activity contributed to resistance of C4-2 cells to rapamycin-induced 4E-BP1 dephosphorylation. Bicalutamide treatment as well as culture in medium containing decreased levels of androgens repressed 4E-BP1 phosphorylation, showing that the AR is a positive regulator of 4E-BP1 phosphorylation. The combination of bicalutamide and rapamycin, but not rapamycin alone, inhibited 4E-BP1 phosphorylation in C4-2 cells. Additionally, bicalutamide sensitized PC-3 cells stably transfected with the AR, but not wild-type PC-3 cells that are AR null, to rapamyin-induced 4E-BP1 dephosphorylation. We also compared the effect of bicalutamide in two cell lines derived from CWR22 relapsed tumors: in CWR22R1 cell, which expressed significant levels of the full-length AR, 4E-BP1 phosphorylation was inhibited by bicalutamide, whereas in CWR22Rv1 cells, where the AR lack the ligand-binding domain (LBD), bicalutamide was unable to affect 4E-BP1 phosphorylation. Thus, in cells where bicalutamide is able to bind the AR, this drug sensitizes the cells to the effects of rapamycin. These results were verified in a CWR22 mouse xenograft model of prostate cancer progression. Taken together, our data show that rapamycin treatment increased AR activity by inhibiting S6K1, and that this increase in AR activity in turn induced resistance to rapamycin's ability to repress 4E-BP1 phosphorylation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1702.