Abstract
The androgen receptor (AR) is a transcription factor essential for the proper development of the prostate and crucial for the pathogenesis of prostate cancer (PCa). Indeed, blocking the activity of the androgen receptor by androgen ablation therapies like chemical or surgical castration and/or the application of antiandrogens is a common therapy for the treatment of non-organ-confined PCa. Although the canonical AR signaling pathway requires the binding to its ligand dihydrotestosterone several alternative AR activation mechanisms have been identified which cause an androgen-independent activation of the AR leading to the recurrence of so-called castration-resistant prostate cancers (CRPCa). Epigenetic modifier like CBP/p300 or HDACs are essential for the induction of AR target genes. Thus we hypothesized that mechanism involved in the androgen independent activation of the AR might also influence its interplay with cofactors and might thus affect the expression of AR target genes. Since the IkappaB kinases (IKK) are known to modify the interaction between transcription factors and co-actovators/co-repressors and the AR is known to be activated by specific phosphorylations in an androgen-independent manner, we analyzed the role of the IKKs for the function of the AR in prostate cancer cell lines.
To determine the effect of the IKKs on AR we used the pharmacological IKK inhibitor BMS345541 as well as IKK specific siRNAs. Inhibition of the IKKs was paralleled by a decreased phosphorylation pattern of the AR while IKK1 and IKK2 were capable of phosphorylating the AR in vitro. Moreover, by mass spectrometry analysis we identified several IKK-phosphorylated AR-peptides including one harboring Ser308.
Blocking of IKK1 function by small molecule inhibitors or siRNA-mediated IKK1-knockdown attenuated AR-dependent target gene expression. Inhibition of IKK1 was linked to reduced nuclear localization, diminished DNA binding and reduced transcriptional activity of the AR. Furthermore, applying the CAM in vivo tumor model we observed an attenuated tumor size and proliferation as measured by Ki-67 staining upon treatment with the pharmalogical IKK inhibitor BMS345541. In addition, reduced Ki-67 staining was also observed after siRNA mediated knock-down of IKK1.
Currently, we investigate the potential function of the IKK-mediated AR phosphorylation for its interaction with epigenetic modifiers and its impact on the epigenetic regulation of AR target genes. Interfering with this IKK mediated AR regulation mechanism might be a novel approach for the targeted therapy of castration-resistant prostate cancer.
Citation Format: Lasse Marck, Garima Jain, Marcus V. Cronauer, Felicitas Genze, Ralf Marienfeld. IKK-mediated phosphorylation of the androgen receptor: A mechanism to modulate AR-cofactor interaction. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Jun 19-22, 2013; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2013;73(13 Suppl):Abstract nr B59.
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