HIGH GLUCOSE CONDITION DESENSITIZES INSULIN ACTION AT THE LEVELS OF RECEPTOR KINASE

The mechanisms for the insulin resistance induced by hyperglycemia wer e investigated by studying the in vitro effects of a high glucose conc entration on insulin signaling with Rat 1 fibroblasts expressing human insulin receptors (HIRc). Incubation of HIRc cells for 4 days in 27 m M D-glucose led to impaired insulin-stimulation of both alpha-aminoiso butyric acid uptake (AIB) and phosphorylation of pp185 and receptor be ta-subunits in vivo. In vitro autophosphorylation and tyrosine kinase activities toward poly Glu(80) Tyr(20) Of insulin receptors from cells exposed to high glucose media (HG) were also impaired (46-48% of cont rol), although the binding of insulin to HG cells was unchanged. One p ossible explanation for these high glucose effects is that they are me diated by the activation of protein kinase C (PKC). However, a 4-day-h igh glucose culture had no effect on cytosolic and membrane PKC activi ties or on phorbol dibutyrate binding to whole cells. This is in accor dance with the orthophosphate labeling study, in which basal autophosp horylation activity in HG cells did not increase, suggesting that phos phorylation of serine and threonine residues in the basal state might not increase in HG cells; These results indicate that in cells exposed to high glucose, desensitization of insulin receptors was induced via several intracellular events, but might not be due to persistent acti vation of PKC in HIRc cells.