Calmodulin antagonists inhibit insulin-stimulated GLUT4 (glucose transporter 4) translocation by preventing the formation of phosphatidylinositol 3,4,5-trisphosphate in 3T3L1 adipocytes

It has been previously reported that calmodulin plays a regulatory role in the insulin stimulation of glucose transport. To examine the basis for this observation, we examined the effect of a panel of calmodulin antagonists t hat demonstrated a specific inhibition of insulin-stimulated glucose transp orter 4 (GLUT4) but not insulin- or platelet-derived growth factor (PDGF)-s timulated GLUT1 translocation in 3T3L1 adipocytes. These treatments had no effect on insulin receptor autophosphorylation or tyrosine phosphorylation of insulin receptor substrate 1 (IRS1). Furthermore, IRS1 or phosphotyrosin e antibody immunoprecipitation of phosphatidylinositol (PI) 3-kinase activi ty was not affected. Despite the marked insulin and PDGF stimulation of PI 3-kinase activity, there was a near complete inhibition of protein kinase B activation. Using a fusion protein of the Grp1 pleckstrin homology (PH) do main with the enhanced green fluorescent protein, we found that the calmodu lin antagonists prevented the insulin stimulation of phosphatidylinositol 3 ,4,5-trisphosphate [PI(3,4,5)P3] formation in vivo. Similarly, although PDG F stimulation increased PI 3-kinase activity in in vitro immunoprecipitatio n assays, there was also no significant formation of PI(3,4,5)P3 in vivo. T hese data demonstrate that calmodulin antagonists prevent insulin-stimulate d GLUT4 translocation by inhibiting the in vivo production of PI(3,4,5)P3 w ithout directly affecting IRS1- or phosphotyrosine-associated PI 3-kinase a ctivity. This phenomenon is similar to that observed for the PDGF stimulati on of 3T3L1 adipocytes.