Metformin inhibits androgen-induced IGF-IR upregulation in prostate cancer cells by disrupting membrane-initiated androgen signaling

We have previously demonstrated that, in prostate cancer cells, androgens up-regulate IGF-I re- ceptor (IGF-IR) by inducing cAMP-response element-binding protein (CREB) activation and CREB- dependent IGF-IR gene transcription through androgen receptor (AR)-dependent membrane-ini- tiated effects. This IGF-IR up-regulation is not blocked by classical antiandrogens and sensitizes cells to IGF-I-induced biological effects. Metformin exerts complex antitumoral functions in various models and may inhibit CREB activation in hepatocytes. We, therefore, evaluated whether met- formin may affect androgen-dependent IGF-IR up-regulation. In the AR LNCaP prostate cancer cells, we found that metformin inhibits androgen-induced CRE activity and IGF-IR gene transcrip- tion. CRE activity requires the formation of a CREB-CREB binding protein-CREB regulated tran- scription coactivator 2 (CRTC2) complex, which follows Ser133-CREB phosphorylation. Metformin inhibited Ser133-CREB phosphorylation and induced nuclear exclusion of CREB cofactor CRTC2, thus dissociating the CREB-CREB binding protein-CRTC2 complex and blocking its transcriptional activity. Similarly to metformin action, CRTC2 silencing inhibited IGF-IR promoter activity. More- over, metformin blocked membrane-initiated signals of AR to the mammalian target of rapamycin/p70S6Kinase pathway by inhibiting AR phosphorylation and its association with c-Src. AMPK signals were also involved to some extent. By inhibiting androgen-dependent IGF-IR up-regu- lation, metformin reduced IGF-I-mediated proliferation of LNCaP cells. These results indicate that, in prostate cancer cells, metformin inhibits IGF-I-mediated biological effects by disrupting membrane- initiated AR action responsible for IGF-IR up-regulation and suggest that metformin could represent a useful adjunct to the classical antiandrogen therapy. (Endocrinology 155: 1207–1221, 2014)