In the first step in insulin action, insulin binds to its receptor on the surface of the target cell, thereby activating the insulin receptor tyrosine kinase. This leads to receptor autophosphorylation, and to phosphorylation of tyrosine residues in multiple intracellular proteins. We have investigated this pathway whereby insulin stimulates the translocation of GLUT4 glucose transporters to the plasma membrane in isolated rat adipocytes, a physiologically important target cell for insulin action. Using an experimental approach involving transient expression in primary cultures of rat adipocytes, we have previously demonstrated that insulin receptor tyrosine kinase activity is required for this action of insulin, and that IRS-1 appears to be substrates which participate in mediating the effect of insulin upon recruitment of GLUT4. More recently, we have demonstrated that activation of phosphatidyl inositol 3-kinase (PI 3- kinase) activity also appears to play a necessary role in mediating this action on insulin. This conclusion is based upon two types of evidence: first, that the effect of insulin is inhibited by wortmannin (an inhibitor of the catalytic activity of PI 3-kinase) and, second, that expression of a dominant negative mutant of the p85 regulatory subunit of PI 3-kinase inhibits insulin action in rat adipocytes. It is not definitively established whether activation of PI 3-kinase is sufficient to trigger the translocation of GLUT4. Work is currently underway to identify the molecules that function downstream from PI 3- kinase, and also to eludicate the role of other phosphorylation substrates (e.g., insulin receptor substrate-2) in mediating the metabolic actions of insulin in adipose tissue.
