Blocking GRP/GRP-R Signaling Decreases Expression of Androgen Receptor Splice Variants and Inhibits Tumor Growth in Castration-resistant Prostate Cancer

Abstract Background: Clinical management of castration-resistant prostate cancer (CRPC) resulting from androgen deprivation therapy (ADT) remains challenging. Previously, we have reported that long-term ADT increases the neuroendocrine (NE) hormone – Gastrin Releasing Peptide (GRP) and its receptor (GRP-R) expression in prostate cancer (PC) cells. Further, we demonstrated that activation of GRP/GRP-R signaling increases androgen receptor (AR) splice variants (ARVs) expression through activating NF-κB signaling thereby contributing cancer progression to CRPC. Most importantly, as a cell surface protein, GRP-R is easily targeted by drugs to block GRP/GRP-R signaling. Here, we aim to investigate if blocking GRP/GRP-R signaling by targeting GRP-R using GRP-R antagonist is sufficient to control CRPC progression, including in therapy-induced (t) neuroendocrine prostate cancer (tNEPC). Methods: Bone-growing NEPC cells were generated by treating androgen dependent LNCaP PC cells with anti-androgen (MDV3100) for more than 3 months. RC-3095, a selective GRP-R antagonist, was used for blocking GRP/GRP-R signaling. The NGL vector [a NF-kB responsive reporter vector which has Luciferase and Green Fluorescent Protein (GFP) reporter genes] was used to measure NF-kB activity and the ARR2PB-Luc vector (an AR responsive reporter vector) was used to measure AR activity in the PC cells. For in vivo experiments, the effect of RC-3095 on CRPC was observed in subcutaneous CRPC and bone-growing tNEPC tumor models.Results: Our studies show that blocking GRP/GRP-R signal by targeting GRP-R using RC-3095 efficiently inhibits NF-κB activity and ARVs (AR-V7) expression in CRPC and tNEPC cells. In addition, blocking of GRP/GRP-R signaling by targeting GRP-R can sensitize CRPC cells to anti-androgen treatment. Further, preclinical animal studies indicate combination of GRP-R antagonist (targeting ARVs) with anti-androgen [targeting full-length AR (AR-FL)] is sufficient to inhibit CRPC and tNEPC tumor growth.Conclusion: Our findings strongly indicate that blocking of GRP/GRP-R signaling in combination with ADT is a potential new approach to control CRPC tumor growth, including ADT induced tNEPC.