Regulation of Phosphatidylinositol 3,4,5-Trisphosphate 5′-Phosphatase Activity by Insulin

Polyphosphoinositides are thought to be mediators of cellular signaling pathways as well as regulators of cytoskeletal elements and membrane trafficking events. It has recently been demonstrated that a class of phosphatidylinositol (PI) 3,4,5-P3 5′-phosphatases contains SH2 domains and proline-rich regions, which are present in many signaling proteins. We report here that insulin stimulation of Chinese hamster ovary cells (CHO-T) expressing human insulin receptors causes an 8-10-fold increase in PI 3,4,5-P3 5′-phosphatase activity in anti-phosphotyrosine immunoprecipitates of the cell lysates. This insulin-sensitive polyphosphoinositide 5′-phosphatase did not catalyze dephosphorylation of PI 4,5-P2. No change in 5′-phosphatase activity was detected in insulin receptor or IRS-1 immune complexes in response to insulin. However, insulin treatment of CHO-T cells markedly increased the PI 3,4,5-P3 5′-phosphatase activity associated with Shc and Grb2. The insulin-regulated polyphosphoinositide 5′-phosphatase was not immunoreactive with antibody raised against the recently cloned SHIP 5′-phosphatase reported to associate with Shc and Grb2 in B lymphocytes. These data demonstrate that insulin causes formation of complexes containing a PI 3,4,5-P3 5′-phosphatase, and Shc or Grb2, or both, suggesting an important role of this enzyme in insulin signaling.