RET Signaling in Prostate Cancer

Purpose: Large diameter perineural prostate cancer is associated with poor outcomes. GDNF, with its coreceptor GFR alpha 1, binds RET and activates downstream pro-oncogenic signaling. Because both GDNF and GFR alpha 1 are secreted by nerves, we examined the role of RET signaling in prostate cancer. Experimental Design: Expression of RET, GDNF, and/or GFR alpha 1 was assessed. The impact of RET signaling on proliferation, invasion and soft agar colony formation, perineural invasion, and growth in vivo was determined. Cellular signaling downstream of RET was examined by Western blotting. Results: RET is expressed in all prostate cancer cell lines. GFR alpha 1 is only expressed in 22Rv1 cells, which is the only line that responds to exogenous GDNF. In contrast, all cell lines respond to GDNFplus GFR alpha 1. Conditioned medium from dorsal root ganglia contains secreted GFR alpha 1 and promotes transforma-tion-related phenotypes, which can be blocked by anti-GFR alpha 1 antibody. Perineural invasion in the dorsal root ganglion assay is inhibited by anti-GFR alpha antibody and RET knockdown. In vivo, knockdown of RET inhibits tumor growth. RET signaling activates ERK or AKT signaling depending on context, but phosphorylation of p70S6 kinase is markedly increased in all cases. Knockdown of p70S6 kinase markedly decreases RET induced transformed phenotypes. Finally, RET is expressed in 18% of adenocarcinomas and all three small-cell carcinomas examined. Conclusions: RET promotes transformation associated phenotypes, including perineural invasion in prostate cancer via activation of p70S6 kinase. GFR alpha 1, which is secreted by nerves, is a limiting factor for RET signaling, creating a perineural niche where RET signaling can occur. (C) 2017 AACR.