Stimulation of IRS-1-associated phosphatidylinositol 3-kinase and Akt/protein kinase B but not glucose transport by beta1-integrin signaling in rat adipocytes

The signal transduction pathway by which insulin stimulates glucose transport is not understood, but a role for complexes of insulin receptor substrate (IRS) proteins and phosphatidylinositol (PI) 3-kinase as well as for Akt/protein kinase B (PKB) has been proposed. Here, we present evidence suggesting that formation of IRS-1/PI 3-kinase complexes and Akt/PKB activation are insufficient to stimulate glucose transport in rat adipocytes. Cross-linking of beta1-integrin on the surface of rat adipocytes by anti-beta1-integrin antibody and fibronectin was found to cause greater IRS-1 tyrosine phosphorylation, IRS-1-associated PI 3-kinase activity, and Akt/PKB activation, detected by anti-serine 473 antibody, than did 1 nM insulin. Clustering of beta1-integrin also significantly potentiated stimulation of insulin receptor and IRS-1 tyrosine phosphorylation, IRS-associated PI 3-kinase activity, and Akt/PKB activation caused by submaximal concentrations of insulin. In contrast, beta1-integrin clustering caused neither a change in deoxyglucose transport nor an effect on the ability of insulin to stimulate deoxyglucose uptake at any concentration along the entire dose-response relationship range. The data suggest that (i) beta1-integrins can engage tyrosine kinase signaling pathways in isolated fat cells, potentially regulating fat cell functions and (ii) either formation of IRS-1/PI 3-kinase complexes and Akt/PKB activation is not necessary for regulation of glucose transport in fat cells or an additional signaling pathway is required.