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

SIMPLE SUMMARY: Prostate cancer is one of the most frequently diagnosed cancers in men and is the second leading cause of cancer death in developed countries. Current therapeutics that target the androgen receptor (AR) are only transiently effective. Anti-AR therapy-resistant tumors often emerge with vast cellular and molecular alterations and present themselves at the clinic in more deadly forms including metastatic castration-resistant prostate cancer (mCRPC) or neuroendocrine prostate cancer (NEPC). One emerging strategy in effective treatment of the advanced forms of prostate cancer is to target drivers other than AR. The present study shows that the nuclear receptor RORγ and the serine/threonine kinase PBK form a regulatory loop in hyperactive AR signaling. It also demonstrates that orally administered, small-molecule antagonists/inverse agonists of RORγ are effective in blocking the growth of the mCRPC tumors. Our findings provide a rationale for therapeutic targeting of RORγ alone or in combination with PBK inhibitors for the advanced forms of prostate cancer. ABSTRACT: 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.