SUN-003 Targeting Androgen Receptor-CXCR7-MARK Signaling Axis in CRPC

Abstract

Prostate cancer (PC) is the most commonly diagnosed cancer among males in the US. The standard of care for patients with metastatic PC is based on androgen-depletion therapy (ADT), including a first-line chemical castration, and a second-line, treatment with high-affinity antagonists for androgen receptor (AR), e.g. enzalutamide (Enz). Unfortunately, most of the patients eventually relapse with castration-resistant metastatic PC (CRPC). Further, use of Enz is thought to cause a growing concern for prevalence of highly aggressive AR-independent CRPC. Therefore, the molecular mechanisms of Enz-resistant (EnzR) CRPC are need to be understood urgently in order to identify novel targeted therapies for deadly CRPC. To approach Enz-resistant CRPC we have created EnzRPC cell lines by passaging cells in the presence of sub-lethal doses of Enz for over 6 months Analyzing the global profiles of expressed genes, we determined chemokine receptor CXCR7 (also known as atypical chemokine receptor ACKR3) as one of the top upregulated genes in EnzRcells. Utilizing molecular techniques we found that AR directly binds to promotor and enhancer regions of CXCR7 gene and inhibits its expression. EnzR PC cells depends on CXCR7-activated signaling in order to survive anti-AR therapy in vitroand in vivowhen injected in immuno-deficient mice. Moreover, we found that patients with metastatic CRPC increase CXCR7 expression comparing to localized PC tumors, and patients who relapse on second-line ADT further increase CXCR7 expression. CXCR7 is a 7-transmembrane (7TM) receptor, which when activated recruits ARRB2, a cytoplasmic scaffold protein. This leads to receptor internalization and activation of MAPK kinases. Normally, CXCR7 is activated by CXCL12 (SDF1a), CXCL11 (ITAC), and MIF. However, we observed that in EnzR PC cells upregulated CXCR7 signaling is constitutive, causing sustained ERK activation. Thus, when we combined MAPKK small molecule inhibitor with enzalutamide we observed superior therapeutic efficacy for CRPC in pre-clinical studies. Besides MAPK, CXCR7 can cross-activate other pro-survival signaling cascades, such as Akt and EGFR. Moreover, GSEA analysis reveals that EnzRPC cells further reduce AR signaling upon CXCR7 knock down suggesting another level of communication between CXCR7 and AR. Currently, CXCR7 lacks known potent antagonists, which would entirely cancel downstream signal activation, thereby, we are searching for novel small molecule CXCR7 inhibitors. Taken together our data suggest that AR and CXCR7 form a multilevel signaling axis, important for EnzR CRPC progression. Currently, we are focusing on further understanding the molecular mechanisms of CXCR7 tumor-promoting activity. In parallel we are developing novel anti-CXCR7 therapies that will be translatable into the clinics as a targeted therapy for patients with deadly CRPC.