The tumor suppressor PTEN negatively regulates insulin signaling in 3T3-L1adipocytes

PTEN is a tumor suppressor with sequence homology to protein-tyrosine phosp hatases and the cytoskeleton protein tensin. PTEN is capable of dephosphory lating phosphatidylinositol 3,4,5-trisphosphate in vitro and down-regulatin g its levels in insulin-stimulated 293 cells. To study the role of PTEN in insulin signaling, we overexpressed PTEN in 3T3-L1 adipocytes similar to 30 -fold above uninfected or control virus (green fluorescent protein)-infecte d cells, using an adenovirus gene transfer system. PTEN overexpression inhi bited insulin-induced 2-deoxy-glucose uptake by 36%, GLUT4 translocation by 35%, and membrane ruffling by 50%, all of which are phosphatidylinositol S -kinase-dependent processes, compared with uninfected cells or cells infect ed with control virus. Microinjection of an anti-PTEN antibody increased ba sal and insulin stimulated GLUT4 translocation, suggesting that inhibition of endogenous PTEN function led to an increase In intracellular phosphatidy linositol. 3,4,S-trisphosphate levels, which stimulates GLUT4 translocation . Further, insulin-induced phosphorylation of downstream targets Akt and p7 0S6 kinase were also inhibited significantly by overexpression of PTEN, whe reas tyrosine phosphorylation of the insulin receptor and IRS-1 or the phos phorylation of mitogen-activated protein kinase were not affected, suggesti ng that the Ras/mitogen-activated protein kinase pathway remains fully func tional. Thus, we conclude that PTEN may regulate phosphatidylinositol 3-kin ase-dependent insulin signaling pathways in 3T3-L1 adipocytes.