Ligand-independent CXCR7 activation of MAPK signaling leads to prostate cancer enzalutamide resistance

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, there is an urgent need for understanding the molecular mechanisms of Enz-resistant (Enz R ) CRPC in order to identify novel targeted therapies for deadly PC. To approach Enz-resistant CRPC we have created Enz R PC 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 CXCR7 (atypical chemokine receptor ACKR3) as one of the top upregulated genes in Enz R cells. Utilizing ChIP-Seq technique we found that AR directly binds to promoter and, at larger scale, enhancer of CXCR7 gene. We identified AR-binding negatively regulates CXCR7 expression; thus, anti-AR therapy unleashes CXCR7 expression in Enz R PC cells. We determined that Enz R PC cells rely on CXCR7-driven signaling in order to survive anti-AR therapy in vitro and in vivo when injected in immunodeficient mice. Moreover, we found that patients with metastatic CRPC increase CXCR7 expression comparing to localized PC tumors. Further, patients who relapsed on second-line ADT demonstrate even higher level of CXCR7 expression. CXCR7 has a structure of 7-transmembrane (7TM) domains receptor. Activated CXCR7 recruits ARRB2, a cytoplasmic scaffold protein, and internalizes, facilitating activation of MAPK cascade: Raf, MEK, and ERK kinases. CXCR7 can be activated by CXCL12 (SDF1), CXCL11 (ITAC), and MIF. Interestingly, we found that in Enz R PC cells CXCR7 is activated independently from ligand stimulation, causing sustainable ERK phosphorylation (pERK). Thus, we identified that Enz R PC exhibit constitutive accumulation of pro-survival pERK. Accordingly, when we targeted pERK with small molecule inhibitors, we observed decreased Enz R PC survival and growth in vitro and in vivo. Taken together these data suggest that CXCR7-ERK signaling axis plays a pivotal role in progression to Enz-resistant CRPC, facilitating survival and growth of tumor cells during and after second-line of ADT. Currently, we are focusing on further understanding the molecular mechanisms of CXCR7-ERK tumor-promoting activity. In parallel, we are evaluating the efficacy of CXCR7-ERK targeting for Enz-resistant CRPC and its translatability to the clinics as a novel targeted therapy for patients who relapsed on advanced ADT with deadly CRPC. Citation Format: Shangze Li, Ka-Wing Fong, Galina Gritsina, Ali Zhang, Jonathan C. Zhao, Jung Kim, Jindan Yu. Ligand-independent CXCR7 activation of MAPK signaling leads to prostate cancer enzalutamide resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4886.