p110β 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 aspects of insulin-induced glucose metabolism, including translocation of GLUT4 to the cell surface and glycogen synthesis. The enzyme exists as a heterodimer containing a regulatory subunit (e.g. p85 alpha) and one of two widely distributed isoforms of the p110 catalytic subunit: p110 alpha or p110 beta, In the present study, we compared the two isoforms in the regulation of insulin action. During differentiation of 3T3-L1 cells into adipocytes, p110 beta was up-regulated similar to 10-fold, whereas expression of p110 alpha was unaltered. The effects of the increased p110 expression were further assessed by expressing epitope tagged p110 beta and p110 alpha in 3T3-L1 cells using adenovirus transduction systems, respectively. In vitro, the basal lipid kinase activity of p110 beta was lower than that of p110 alpha. When p110 alpha and p110 beta were overexpressed in 3T3-L1 adipocytes, exposing cells to insulin induced each of the subunits to form complexes with p85 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. Finally, microinjection of anti-p110 beta neutralizing antibody into 3T3-L1 adipocytes abolished insulin-induced translocation of GLUT4 to the cell surface almost completely, whereas anti-p110 alpha neutralizing antibody did only slightly. Together, these findings suggest that p110 beta plays a crucial role in cellular activities evoked acutely by insulin.