MED12 Mutations Promote Castration Resistant Prostate Cancer Through Hyper‐activated SHH Signaling

Abstract

Prostate cancer is the most common malignancy and the second leading cause of cancer associated deaths among US men. As an androgen-driven disease, prostate cancer is critically dependent upon androgen receptor (AR) signaling. Accordingly, most prostate cancer patients respond very well to androgen deprivation therapy. The current problem lies with the fact that nearly all patients progress to a lethal form of the disease after androgen deprivation therapy known as castration resistant prostate cancer (CRPC). Once patients reach the stage of CRPC, treatment options are frustratingly limited and average survival times range from two to three years only. Thus, there is a compelling need to develop effective treatments for patients suffering from CRPC. Sonic hedgehog (SHH) signaling is a stromal-epithelial interacting pathway that is critical for prostate development and cell growth. SHH signaling is induced by androgen deprivation thereby leading to re-activation of AR target genes to promote prostate cancer cell growth in the absence of androgens. Furthermore, AR interacts with GLI proteins, downstream effectors in the SHH pathway. Therefore, GLI3-dependent SHH signaling in androgen deprived cells could promote re-activation of AR target genes and consequently lead to CRPC. It is currently unknown how GLI3-dependent SHH signaling is induced in CRPC. Several studies have found that MED12, a subunit of Mediator, is commonly mutated in prostate cancer and, interestingly, MED12 has been shown to play a critical role in restricting GLI3-dependent SHH signaling. We therefore hypothesize that MED12 mutations promote GLI3-dependent SHH signaling in prostate cancer to drive progression towards CRPC. We generated stable MED12 knockdown cells, which likely recapitulates a MED12 mutant setting, followed by proliferation and qPCR assays in androgen replete and androgen deprived conditions to test our hypothesis. Our results show that MED12 knockdown promotes prostate cancer growth specifically in an androgen deprived setting and that this hyper-active growth is dependent upon GLI3. Furthermore, we also show that GLI3 target genes are sharply upregulated when MED12 expression is low and androgens are absent. The collective results of our study therefore indicate that prostate cancer patients with MED12 mutations could relapse to CRPC after androgen deprivation therapy by hyperactivating the GLI3 dependent SHH signaling pathway. Importantly, our results thus suggest that therapeutic agents that target the SHH signaling pathway could prove to be beneficial for MED12 mutant prostate cancer patients.