Abstract B4: Elevated expression of c-FLIP in castrate-resistant prostate cancer antagonizes response to androgen receptor-targeted therapy

Abstract

Abstract Cellular Fas-associated death domain (FADD)-like interleukin 1β-converting enzyme (FLICE) inhibitory protein (c-FLIP) is a key inhibitor of caspase 8 (FLICE)-promoted apoptosis. We have previously shown c-FLIP to be a transcriptional target of the androgen receptor (AR) and NF-κB. Accordingly, we proposed that cells with elevated expression of c-FLIP may remain viable and withstand androgen-deprivation. Expression of c-FLIP was assessed in prostatectomy tissue while in vitro experiments were performed to determine its importance in regulating response to the androgen receptor (AR)-targeted therapeutic bicalutamide. c-FLIP expression was increased in pre-malignant prostatic intra-epithelial neoplasia (PIN) (P<0.001) and each of Gleason 3 and Gleason 4 prostate cancer (CaP) tissue (P<0.001) relative to normal prostate epithelium. Maximal expression of c-FLIP was detected in castrate-resistant prostate cancer (CRPC) (P<0.001 relative to Gleason 3/4 cancer. In vitro, siRNA-mediated silencing of c-FLIP in 22Rv1 and LNCaP cells induced spontaneous apoptosis and increased their response to bicalutamide, determined by flow cytometry, PARP cleavage and caspase activity assays. Two histone deacetylase inhibitors (HDACi), droxinostat and SAHA, were also shown to down-regulate c-FLIP expression, resulting in caspase-8 and caspase-3/7 mediated apoptosis. HDACi also potentiated the sensitivity of 22Rv1 cells to bicalutamide, while maximal apoptosis induction in LNCaP cells was not changed in the presence of bicalutamide. The combination of SAHA and bicalutamide promoted cell-specific inhibition of the AR or NF-κB signaling pathways consistent with decreased transcription of c-FLIP in these cells. Conversely, the elevated expression of c-FLIP and Bcl-2 was consistent with the insensitivity of the castrate-resistant VCaP cell line to SAHA in vitro, and the sub-optimal performance of this class of agents in castrate-resistant patients. Therefore, HDACi may prove more effective in enhancing androgen-deprivation therapy (ADT) in early-stage disease. Moreover, c-FLIP expression may serve as a predictive biomarker for androgen-deprivation strategies including bicalutamide. Citation Format: Clare McCourt, Pamela Maxwell, Roberta Mazzucchelli, Manuel Salto-Tellez, Joe O'Sullivan, Rodolfo Montironi, Marina Scarpelli, Daniel Longley, David Waugh. Elevated expression of c-FLIP in castrate-resistant prostate cancer antagonizes response to androgen receptor-targeted therapy [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr B4.