RET signaling in prostate cancer

Purpose: Large diameter perineural prostate cancer is associated with poor outcomes. GDNF, with its coreceptor GFRa1, binds RET and activates downstream pro-oncogenic signaling. Because both GDNF and GFRa1 are secreted by nerves, we examined the role of RET signaling in prostate cancer. Experimental Design: Expression of RET, GDNF, and/or GFRa1 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. GFRa1 is only expressed in 22Rv1 cells, which is the only line that responds to exogenous GDNF. In contrast, all cell lines respond to GDNF plus GFRa1. Conditioned medium from dorsal root ganglia contains secreted GFRa1 and promotes transformation-related phenotypes, which can be blocked by anti-GFRa1 antibody. Perineural invasion in the dorsal root ganglion assay is inhibited by anti-GFRa 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.