The PKB/Akt substrate AS160 facilitates opioid-induced neurite outgrowth and cell differentiation

The "Proteinkinase B (PKB)/Akt substrate of 160 kDa" (AS160) is known to regulate exocytosis of glucose transporter4 (GLUT4) by stimulating Rab8 proteins in skeletal muscle cells and adipocytes. Its function in Rab8-expressing, but GLUT4-deficient cells such as neurons, however, has still not been identified. Because Rab8 plays a central role in neurite outgrowth and differentiation, AS160 might be also engaged in the cellular processes. To test this, AS160 was examined in neuroblastoma x glioma hybrid (NG108-15) cells treated with opioids leading to neurite outgrowth and differentiation. By using phospho-Thr642 specific antibody and Western Blot technique, incubation of NG108-15 cells with (D-Pen2,5)Enkephalin (DPDPE) and Etorphine was revealed to bring about time- and dose-dependent phosphorylation of AS160. This effect was blocked by both naloxone and PKB/Akt inhibitor indicating that opioid receptor-dependent stimulation of PKB/Akt signaling pathway is implicated in AS160 phosphorylation. Prolonged exposure of NG108-15 cells to DPDPE and Etorphine induced both neurite outgrowth and polymerization of beta-tubulin, which is indicative for cell differentiation. In contrast, cells transfected with siRNA to knock-down AS160 remained unaffected by the opioid treatment. Together the findings suggest that AS160 participates in the mechanisms of opioid-induced neurite outgrowth and differentiation, representing another function of this well-known regulator of GLUT4 exocytosis.