Neutral endopeptidase inhibits neuropeptide-mediated transactivation of the insulin-like growth factor receptor-Akt cell survival pathway

G-protein coupled receptor (GPCR) agonists such as neuropeptides activate t he insulin-like growth factor-1 receptor (IGF-IR) or the serine-threonine p rotein kinase Akt, suggesting that neuropeptides-GPCR signaling can cross-c ommunicate with IGF-IR-Akt signaling pathways. Neutral endopeptidase 24.11 (NEP) is a ceh-surface peptidase that cleaves and inactivates the neuropept ides endothelin-l (ET-I) and bombesin, which are implicated in progression to androgen-independent prostate cancer (PC). We investigated the mechanism s of NEP regulation of neuropeptide-mediated cell survival in PC cells, inc luding whether neuropeptide substrates of NEP induce phosphorylations of IG F-IR and Akt in PC cells. Western analyses revealed ET-1 and bombesin treat ment induced phosphorylation of IGF-IR beta and Akt independent of IGF-T in TSU-Prl, DU145, and PC-3 PC cells, which lack NEP expression, but not in N EP-expressing LNCaP cells. Recombinant NEP and induced NEP expression in TS U-Prl cells using a tetracycline-repressive expression system inhibited ET- 1-mediated phosphorylation af IGF-IR beta and Akt, and blocked the protecti ve effects of ET-I against apoptosis induced by serum starvation. Incubatio n of TSU-Prl cells with specific kinase inhibitors together with ET-1 or bo mbesin showed that IGF-IR activation is required for neuropeptide-induced A kt phosphorylation, and that neuropeptide-induced Akt activation is predomi nantly mediated by Src and phosphatidylinositol 3-kinase but not by mitogen -activated protein kinase or protein kinase C. These data show that the neu ropeptides ET-1 and bombesin stimulate ligand-independent activation of the IGF-DR, which results in Akt activation, and that this cross-communication between GPCR and IGF-IR signaling is inhibited by NEP.