The role of the Mediator complex subunits MED12 and MED15 in the development of androgen-dependent prostate cancer into androgen-independent castration resistant prostate cancer

Hormone sensitive prostate cancer (PCa) is commonly treated with androgen deprivation therapy (ADT), but most tumors relapse resulting in a castration-resistant prostate cancer (CRPC) with poor prognosis. These tumors regain the ability to progress under androgen deprived conditions through ADT-driven molecular alterations. Activation of androgen receptor (AR) signaling and hyper-activation of alternative signaling pathways bypassing the AR are potential molecular changes in metastatic PCa cells. Gene expression profiling of cancer tissues and cell lines untraveled changes in gene expression pattern during androgen ablation, in which resistant tumor cells exhibit a reactivation of the androgen regulated program. AR activity and other signaling pathways are critically dependent upon interactions with co-regulatory proteins and complexes. The Mediator complex is an important co-activator for a broad range of regulatory transcriptional factors including AR. In previous studies, we reported that the involvement of the Mediator complex subunits MED12 and MED15 in PCa progression to CRPC and their direct implication in TGFß signaling. Based on our data so far, we hypothesize that MED12 and MED15 are directly implicated in the development of androgen-dependent PCa into androgen-independent CRPC. We propose proving this hypothesis by pursuing three specific aims. Firstly, we will investigate whether MED12 and MED15 effect the gene expression changes of androgen dependent prostate cancer cells in response to androgen ablation performing Illumina gene chip analysis. Secondly, we will investigate if MED12 and MED15 influence the ability of androgen dependent PCa cells to survive and proliferate under androgen deprived conditions rendering them resistant to androgen deprivation. Thirdly, we will determine the signaling pathway(s) through which MED12 and MED15 may affect the development of drug resistance in androgen dependent PCa cells. Upon the completion of the above aims, we will have evidence whether MED12 or MED15 may serve as predictive markers for development of drug resistance in PCa. Therefore, our results may lead to recommend the targeting of MED12 or MED15 as this may disrupt key signaling pathways which enable cancer cells to survive during androgen deprivation therapy. Thus, this may call for MED12 or MED15 to be established as therapeutic targets in PCa patients harboring high expression levels of MED12 or MED15.

In the first part of this project, we have achieved main aims of this grant focusing on the role of MED15 in PCa. Our results highlight that MED15 is involved in the development of CRPC and connected to cancer-driven pathways including PI3K activation. Inhibiting MED15 under androgen deprived conditions reflecting circumstances under androgen deprivation therapy in patients induced apoptosis of cells suggesting to further investigating MED15 inhibition as therapeutic target for CRPC. Subsequently, we explored the involvement of the Mediator subunit CDK19 in PCa instead of MED12 as initially intended. Hereby, we found that CDK19 is up-regulated in advanced, metastatic PCa and predicts disease recurrence suggesting CDK19 as prognostic biomarker for risk stratification. Results of following in vitro analyses mainly contribute to a better understanding of its molecular functions in PCa. We observed that CDK19 has major impact on both gene transcription as well as post-translational protein modification through phosphorylation of multiple cancer-related substrates. Furthermore, CDK19 inhibition by diverse recently developed small molecule inhibitors resulted in significant anti-tumor effects such as impaired migration and invasion. First results of follow-up projects show that CDK19 inhibition might sensitize CRPC cells to androgen-blockade suggesting that the combined treatment with established androgen deprivation and simultaneous CDK19 inhibition might be a promising therapeutic option for patients. Collectively, these projects contribute to better understand the molecular involvement of the Mediator complex in human cancers. Several Mediator subunits have been linked to cancer-related pathways and specific cancer entities highlighting its crucial involvement in cancer initiation, progression and the development of therapy resistance. CDK8, the gene paralog of CDK19, is a known colon cancer oncogene and contributes to diverse pro-tumorigenic cellular processes. Therefore, multiple small molecule inhibitors targeting CDK8 and CDK19 have been recently developed. Results of our tissue and function in vitro experiments strongly suggest testing these inhibitors for the treatment of patients suffering from metastatic and/or castration-resistant PCa. Results of this study provided the basis for subsequent follow-up projects which are currently ongoing.