Targeting Feedforward Loops Formed by Nuclear Receptor RORγ and Kinase PBK in mCRPC with Hyperactive AR Signaling

Metastatic castration-resistant prostate cancer (mCRPC) is a highly aggressive disease with few therapeutic options. Hyperactive androgen receptor (AR) signaling plays a key role in CRPC progression. Previously, we identified RAR-related orphan receptor gamma (RORγ) as a novel key driver of AR gene overexpression and increased AR signaling. We report here that several RORγ antagonists/inverse agonists including XY018 and compound 31 were orally effective in potent inhibition of the growth of tumor models including patient-derived xenograft (PDX) tumors. RORγ controls the expression of multiple aggressive-tumor gene programs including those of epithelial-mesenchymal transition (EMT) and invasion. We found that PDZ binding kinase (PBK), a serine/threonine kinase, is a downstream target of RORγ that exerts the cellular effects. Alterations of RORγ expression or function significantly downregulated the mRNA and protein level of PBK. Our further analyses demonstrated that elevated PBK associates with and stabilizes RORγ and AR proteins, thus constituting novel, interlocked feed-forward loops in hyperactive AR and RORγ signaling. Indeed, dual inhibition of RORγ and PBK synergistically inhibited the expression and function of RORγ, AR, and AR-V7, and the growth and survival of CRPC cells. Therefore, our study provided a promising, new strategy for treatment of advanced forms of prostate cancer.