Ccaat/Enhancer Binding Protein Beta Facilitates Castrate-Resistant Prostate Cancer Cell Growth And Sensitivity To Mtor Inhibitors

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA Adaptive changes that frequently occur in castrate-resistant prostate cancer (CRPC) cells are genomic mutations which enhance the activity of the PI3K-mTOR signaling pathway. Although there have been several promising clinical trials for the treatment of CRPC utilizing inhibitors of mTOR, these efforts failed to show significant improvement of patient outcomes. Thus, identifying potential mechanisms of resistance to these agents could improve their therapeutic efficacy. CCAAT/enhancer binding protein beta (C/EBPβ) is a broadly expressed transcription factor, with three isoforms that are differentially translated by mTOR activity. T reatment of cells with mTOR inhibitors increases the translation of the larger C/EBPβ LAP isoforms and decreases the truncated LIP isoform. The function of C/EBPβ LAP has been previously associated with survival and cellular senescence, while the LIP isoform facilitates cell cycle progression. Our central hypothesis was that the differential translation of LAP and LIP C/EBPβ isoforms was essential for the survival of CRPC cells treated with mTOR inhibitors. We found that treatment of PC3 cells with mTOR inhibitors unexpectedly increased the protein levels of all three C/EBPβ isoforms, while androgen-sensitive LNCaP cells largely showed decreased C/EBPβ protein levels. However, the PI3K inhibitor LY294002, decreased the LIP isoform of C/EBPβ without affecting LAP expression in LNCaP and PC3 cells. C/EBPβ gene expression decreased in both LNCaP and PC3 cells in response to treatment with mTOR inhibitors. We next evaluated whether C/EBPβ facilitated cell survival during mTOR inhibition. We found that PC3 cells deficient in C/EBPβ, via shRNA knockdown or TALEN gene editing, were less sensitive to the mTORC1/2 inhibitor, INK128, as assessed by a significant increase in IC50 value determined by the WST-1 assay. Further, suppression of C/EBPβ through shRNA KD or TALEN targeting dramatically decreased the rate of PC3 cell growth, clonogenic potential and induced a senescent phenotype. Intriguingly, loss of C/EBPβ in LNCaP cells increased clonogenic potential. Collectively, these data suggest that C/EBPβ has a differential role for cell growth in androgen-sensitive versus castrate-resistant prostate cancer and that C/EBPβ levels inversely correlate with mTOR inhibitor resistance. Citation Format: David J. Barakat, Jing Zhang, Alan D. Friedman, Samuel R. Denmeade, Ido Paz-Priel. CCAAT/enhancer binding protein beta facilitates castrate-resistant prostate cancer cell growth and sensitivity to mTOR inhibitors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 85. doi:10.1158/1538-7445.AM2015-85