Insulin resistance of the skeletal muscle plays a key role in the development of the metabolic endocrine syndrome and its further progression to type II diabetes. Impaired signaling from the insulin receptor to the glucose transport system and to glycogen synthase is thought to be the cause of skeletal muscle insulin resistance. An incomplete activation of the insulin receptor tyrosine kinase, which is found in type II diabetes, appears to contribute to the pathogenesis of the signaling defect. Available data suggest that the impaired tyrosine kinase function of the insulin receptor is not due to an inherited defect but rather is caused by a modulation of insulin receptor function. We used rat-1 fibroblasto and NIH-3T3 cells stably overexpressing human insulin receptor and 293 cells transiently overexpressing human insulin receptor to characterize conditions modulating the signaling function of the insulin receptor kinase. Using these cell models, we could demonstrate that activation of different protein kinase C (PKC) isoforms by high glucose levels or phorbol esters causes a rapid inhibition of the receptor tyrosine kinase activity. This effect is most likely mediated through serine phosphorylation of the receptor β-subunit. It can be prevented by PKC inhibitors and the new oral antidiabetic agent thiazolidindione. The data suggest that PKC might be an important negative regulator of insulin receptor function. Because we have recently shown that bradykinin activates different isoforms of PKC in these cell types, an inhibitory cross talk between the bradykinin receptor and the insulin receptor through PKC activation seemed possible. However, we were unable to observe an insulin receptor tyrosine kinase inhibition through bradykinin, suggesting that different isoforms of PKC are activated by hyperglycemia and bradykinin. On the other hand, a modulation of bradykinin signals by insulin could be demonstrated in these cells. Bradykinin-induced tyrosine phosphorylation of proteins of ∼130 and 70 kDa was inhibited by insulin treatment of rat-1 flbroblasts. These data suggest that signals from the insulin receptor modify signaling from the bradykinin receptor to tyrosine phosphorylation of different cellular proteins.

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