p110 beta is up-regulated during differentiation of 3T3-L1 cells and contributes to the highly insulin-responsive glucose transport activity

Activation of p85/p110 type phosphatidylinositol kinase is essential for as pects of insulin-induced glucose metabolism, including translocation of GLU T4 to the cell surface and glycogen synthesis. The enzyme exists as a heter odimer containing a regulatory subunit (e.g. p85 alpha) and one of two wide ly distributed isoforms of the p110 catalytic subunit: p110 alpha or p110 b eta, In the present study, we compared the two isoforms in the regulation o f insulin action. During differentiation of 3T3-L1 cells into adipocytes, p 110 beta was up-regulated similar to 10-fold, whereas expression of p110 al pha was unaltered. The effects of the increased p110 expression were furthe r assessed by expressing epitope tagged p110 beta and p110 alpha in 3T3-L1 cells using adenovirus transduction systems, respectively. In vitro, the ba sal lipid kinase activity of p110 beta was lower than that of p110 alpha. W hen p110 alpha and p110 beta were overexpressed in 3T3-L1 adipocytes, expos ing cells to insulin induced each of the subunits to form complexes with p8 5 alpha and tyrosine-phosphorylated IRS-1 with similar efficiency. However, whereas the kinase activity of p110 beta, either endogenous or exogeneous, was markedly enhanced by insulin stimulation, only very small increases of the activity of p110 alpha were observed. Interestingly, overexpression of p110 beta increased insulin-induced glucose uptake by 3T3-L1 cells without significantly affecting basal glucose transport, whereas overexpression of p110 alpha increased both basal and insulin-stimulated glucose uptake. Fin ally, microinjection of anti-p110 beta neutralizing antibody into 3T3-L1 ad ipocytes abolished insulin-induced translocation of GLUT4 to the cell surfa ce almost completely, whereas anti-p110 alpha neutralizing antibody did onl y slightly. Together, these findings suggest that p110 beta plays a crucial role in cellular activities evoked acutely by insulin.