Insulin stimulates the kinase activity of RAC‐PK, a pleckstrin homology domain containing ser/thr kinase

In the present study, insulin is shown to rapidly stimulate by 8‐ to 12‐fold the enzymatic activity of RAC‐PK alpha, a pleckstrin homology domain containing ser/thr kinase. In contrast, activation of protein kinase C by phorbol esters had almost no effect on the enzymatic activity of RAC‐PK alpha. Insulin activation was accompanied by a shift in molecular weight of the RAC‐PK alpha protein, and the activated kinase was deactivated by treatment with a phosphatase, indicating that insulin activated the enzyme by stimulating its phosphorylation. This insulin‐induced shift in RAC‐PK was also observed in primary rat epididymal adipocytes, as well as in a muscle cell line called C2C12 cells. The insulin‐stimulated increase in RAC‐PK alpha activity was inhibited by wortmannin (an inhibitor of phosphatidylinositol 3‐kinase) in a dose‐dependent manner with a half‐maximal inhibition of 10 nM, but not by 20 ng/ml of rapamycin. Activation of RAC‐PK alpha activity was also observed in a variant RAC lacking the pleckstrin homology domain. These results indicate that RAC‐PK alpha activity can be regulated by the insulin receptor. RAC‐PK alpha may therefore play a general role in intracellular signaling mediated by receptor tyrosine kinases.