Anti-apoptotic proteins of the Bcl-2 family are known to confer resistance against a variety of therapeutic agents. Among them is myeloid leukemia cell differentiation protein (Mcl-1), a highly regulated protein at both transcriptional and post-translational levels by a number of signaling pathways. Overexpression of Mcl-1 has been reported in biopsies of prostate cancer patients with high Gleason grades. Currently, novel anti-cancer approaches that aim to inhibit Mcl-1 are being developed. In this study we have analyzed the influence of androgens and androgen receptor (AR) signaling on Mcl-1 protein expression. Western Blot experiments with androgen-sensitive prostate cancer cell lines LNCaP and VCaP showed a consistent upregulation of Mcl-1 protein expression levels under androgen-depleted conditions, i.e. in steroid-free medium. Addition of 0.1-10 nM of the synthetic androgen R1881 decreased Mcl-1 expression in a concentration-dependent manner in LNCaP and VCaP cells. This effect could also be observed in the castration-resistant LNCaP-abl cell line, but not in AR-negative PC3 cells. Further analysis showed that regulation of Mcl-1 was dependent on the presence of functional AR since it could not be observed in cells co-treated with R1881 and antiandrogens bicalutamide or hydroxyflutamide or after AR downregulation by siRNA. Quantitative PCR on mRNA levels revealed that Mcl-1 regulation by androgens is at a transcriptional level, while downregulation of the three known Mcl-1 E3 ligases APC/C(CDC20), SCF(FBW7), and HUWE1 (MULE) by siRNA did not influence androgenic regulation of Mcl-1 at the post-translational level. To evaluate a possible role of Mcl-1 as a resistance mechanism in cancer stem cells, its expression in basal cells isolated from patient's biopsies was assessed. The highest expression of Mcl-1 mRNA was observed in tumors isolated from castration therapy resistant patients, followed by therapy naïve samples and was the lowest in prostatic cell lines. Interestingly, after isolation of stem cells (SC), transit amplifying (TA) and committed basal (CB) cells, the highest expression of Mcl-1 mRNA could be detected in SC. Our findings indicate that androgenic signaling leads to decreased Mcl-1 expression levels in prostate cancer cell lines, which could be mediated by E2F1 transcription factor known to inhibit Mcl-1 transcription. Vice versa, absent AR signaling leads to upregulated Mcl-1 expression levels. The finding of high Mcl-1 mRNA expression in prostate cancer stem cells that do not express AR supports the hypothesis that Mcl-1 could be involved in resistance mechanisms against first- and second-line therapies. Therefore, anti-Mcl-1 therapies might be considered to improve the efficacy of androgen ablation and could furthermore target prostate cancer stem cells.

Citation Format: Frédéric R. Santer, Holger H. Erb, Birgit Luef, Florian Handle, Su Jung Oh, Susanne Lobenwein, Christian Ploner, Jayant Rane, Anne T. Collins, Norman J. Maitland, Zoran Culig. Androgenic regulation of the anti-apoptotic Bcl-2 family member Mcl-1 in prostate cancer cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1726. doi:10.1158/1538-7445.AM2013-1726