Glucose Regulates Diacylglycerol Intracellular Levels and Protein Kinase C Activity by Modulating Diacylglycerol Kinase Subcellular Localization

Abstract

Although chronic hyperglycemia reduces insulin sensitivity and leads to impaired glucose utilization, short term exposure to high glucose causes cellular responses positively regulating its own metabolism. We show that exposure of L6 myotubes overexpressing human insulin receptors to 25 mm glucose for 5 min decreased the intracellular levels of diacylglycerol (DAG). This was paralleled by transient activation of diacylglycerol kinase (DGK) and of insulin receptor signaling. Following 30-min exposure, however, both DAG levels and DGK activity returned close to basal levels. Moreover, the acute effect of glucose on DAG removal was inhibited by >85% by the DGK inhibitor R59949. DGK inhibition was also accompanied by increased protein kinase C-alpha (PKCalpha) activity, reduced glucose-induced insulin receptor activation, and GLUT4 translocation. Glucose exposure transiently redistributed DGK isoforms alpha and delta, from the prevalent cytosolic localization to the plasma membrane fraction. However, antisense silencing of DGKdelta, but not of DGKalpha expression, was sufficient to prevent the effect of high glucose on PKCalpha activity, insulin receptor signaling, and glucose uptake. Thus, the short term exposure of skeletal muscle cells to glucose causes a rapid induction of DGK, followed by a reduction of PKCalpha activity and transactivation of the insulin receptor signaling. The latter may mediate, at least in part, glucose induction of its own metabolism.