DIFFERENTIAL-EFFECTS OF OKADAIC ACID ON INSULIN-STIMULATED GLUCOSE AND AMINO-ACID-UPTAKE AND PHOSPHATIDYLINOSITOL 3-KINASE ACTIVITY

The effect of okadaic acid, a serine/threonine phosphatase inhibitor, was analyzed in two insulin-responsive systems, the isolated mouse sol eus muscle and 3T3-L1 adipocytes. While okadaic acid alone was a poten t stimulator of glucose transport in both systems, it prevented transp ort stimulation by insulin. To gain insight into this inhibitory actio n, the activation of phosphatidylinositol 3-kinase (PI3-kinase), one o f the earliest postreceptor steps identified so far, was studied. In 3 T3-L1 adipocytes and muscle, insulin increased PI3-kinase activity in immunoprecipitates obtained with antibodies to phosphotyrosine. Okadai c acid alone had no effect but strongly inhibited this hormonal action . Okadaic acid treatment did not interfere with insulin-induced recept or autophosphorylation or with its tyrosine kinase activity toward art ificial substrates. In contrast, in the presence of the phosphatase in hibitor, we did not observe tyrosine phosphorylation of the insulin re ceptor cellular substrate p185 (IRS-1) or immunoprecipitation of PI3-k inase by antibodies to phosphotyrosine. These results suggest that oka daic acid interferes with insulin's stimulation of glucose transport b y inhibiting IRS-1 phosphorylation and its association with PI3-kinase and/or other signaling molecules. However, okadaic acid did not block the insulin stimulation of aminoisobutyric acid uptake in muscle. Thi s would indicate that IRS-1 phosphorylation and PI3-kinase activation are not required for all the effects of insulin and that the serine/th reonine phosphorylation events implicated in the translocation of gluc ose transporters are not controlling amino acid transport in muscle.