7-Hydroxystaurosporine (UCN-01) Inhibition of Akt Thr but not Ser Phosphorylation: A Basis for Decreased Insulin-Stimulated Glucose Transport

7-Hydroxystaurosporine (UCN-01) infused for 72 hours by continuous i.v. infusion induced insulin resistance during phase I clinical trials. To understand the mechanism for this observation, we examined the effect of UCN-01 on insulinstimulated glucose transport activity with 3-O-methylglucose in isolated rat adipose cells. UCN-01 inhibits glucose transport activity in a dose-dependent manner at all insulin concentrations. At the clinically relevant concentration of 0.25 mol/L UCN-01, glucose transport is inhibited 66, 29, and 26% at insulin concentrations of 10, 50, and 100,000 (100K) microunits/mL respectively, thus shifting the doseresponse curve to the right. Increasing concentrations of UCN-01 up to 2.5 mol/L progressively shift the insulin dose-response curve even further. As Akt is known to mediate in part action initiated at the insulin receptor, we also studied the effect of UCN-01 on Akt activation in whole-cell homogenates of these cells. Decreased glucose transport activity directly parallels decreased Akt Thr phosphorylation in both an insulin and UCN-01 dose-dependent manner, whereas Akt Ser phosphorylation is inhibited only at the lowest insulin concentration, and then, only modestly. UCN-01 also inhibits insulin-induced Thr but not Ser phosphorylation of Akt associated with the plasma membranes and low-density microsomes and inhibits translocation of GLUT4 from low-density microsomes to plasma membranes as expected from the glucose transport activity measurements. These data suggest that UCN-01 induces clinical insulin resistance by blocking Akt activation and subsequent GLUT4 translocation in response to insulin, and this effect appears to occur by inhibiting Thr phosphorylation even in the face of almost completely unaffected Ser phosphorylation.