Differential effects of constitutively active phosphatidylinositol 3-kinase on glucose transport, glycogen synthase activity, and DNA synthesis in 3T3-L1 adipocytes

Phosphatidylinositol 3-kinase (PI3K) activation is necessary for manly insulin-induced metabolic and mitogenic responses. However, it is unclear whether PI3K. activation is sufficient for any of these effects, To address this question we increased PI3K activity in differentiated 3T3-L1 adipocytes by adenovirus-mediated expression of both the inter-SH2 region of the regulatory p85 subunit of PI3K (iSHZ) and the catalytic p110 alpha subunit (p110), Coexpression resulted in PI3K activity that exceeded insulin-stimulated activity by two- to fivefold in cytosol, total membranes, and the low density microsome (LDM) fraction, the site of greatest insulin stimulation, While insulin increased glucose transport 15-foId, coexpression of iSH2-p110 increased transport 5.2- +/- 0.7-fold with a parallel increase in GLUT-I translocation to the plasma membrane, Constitutive activation of PI3K had no effect on maximally insulin-stimulated glucose transport, Neither basal nor insulin-stimulated activity of glycogen synthase or mitogen-activated protein kinase was altered by iSH2-p110 coexpression, DNA synthesis aas increased twofold by insulin in control 3T3-L1 adipocytes transduced with beta-galactosidase-encoding recombinant adenovirus, while iSH2-p110 expression increased DNA synthesis fivefold, These data indicate that (i) increased PI3K activity is sufficient to activate some but not all metabolic responses to insulin, (ii) activation of PI3K; to levels exceeding the effect of insulin in adipocyte LDM results in only a partial stimulation of glucose transport, and (iii) increased PI3K in activity in the absence of growth factor or oncoprotein stimulation is a potent stimulus of DNA synthesis.