MITOGEN-ACTIVATED PROTEIN-KINASE KINASE INHIBITION DOES NOT BLOCK THESTIMULATION OF GLUCOSE-UTILIZATION BY INSULIN

Insulin stimulates the activity of mitogen-activated protein kinase (M APK) via its upstream activator, MAPK kinase (MEK), a dual specificity kinase that phosphorylates MAPK on threonine and tyrosine. The potent ial role of MAPK activation in insulin action was investigated with th e specific MEK inhibitor PD98059. Insulin stimulation of MAPK activity in 3T3-L1 adipocytes (2.7-fold) and L6 myotubes (1.4-fold) was comple tely abolished by pretreatment of cells with the MER inhibitor, as was the phosphorylation of MAPK and pp90(Rsk), and the transcriptional ac tivation of c-fos. Insulin receptor autophosphorylation on tyrosine re sidues and activation of phosphatidylinositol 3'-kinase were unaffecte d. Pretreatment of cells with PD98059 had no effect on basal and insul in stimulated glucose uptake, lipogenesis, and glycogen synthesis. Gly cogen synthase activity in extracts from 3T3-L1 adipocytes and L6 myot ubes was increased 3-fold and 1.7-fold, respectively, by insulin. Pret reatment with 10 mu M PD98059 was without effect. Similarly, the 2-fol d activation of protein phosphatase 1 by insulin was insensitive to PD 98059. These results indicate that stimulation of the MAPK pathway by insulin is not required for many of the metabolic activities of the ho rmone in cultured fat and muscle cells.