Okadaic acid stimulates IGF-II receptor translocation and inhibits insulin action in adipocytes

Okadaic acid, an inhibitor of protein phosphatases 2A and 1, stimulates glucose transport in muscle and fat cells, suggesting that serine/threonine phosphorylation steps are involved in the translocation of glucose transporters. Here we have investigated whether such phosphorylation events could also participate in another membrane-associated insulin-stimulated process: insulin-like growth factor II (IGF-II) receptor translocation in adipocytes. Maximally effective concentrations of insulin and okadaic acid stimulated deoxyglucose uptake by 5.5- and 2.5-fold, respectively, whereas IGF-II binding was increased 3.5-fold and 1.5-fold. Subcellular fractionation indicated that the okadaic acid-induced stimulation of IGF-II binding resulted from an increase in the number of IGF-II receptors in the plasma membrane with a concomitant disappearance from the low-density microsomal fraction. These changes occurred in parallel to those observed for the glucose transporter GLUT-4. Both insulin-stimulated glucose transport and IGF-II binding were prevented when cells were pretreated with okadaic acid. To understand the mechanism of this inhibitory effect, insulin receptor autophosphorylation and the tyrosine phosphorylation of endogenous proteins were studied. Insulin induced the tyrosine phosphorylation of its receptor beta-subunit and of proteins at 120 and 185 kDa, whereas okadaic acid alone had no effect. When okadaic acid and insulin were added together, the beta-subunit autophosphorylation was similar to that observed with insulin alone, but the tyrosine phosphorylation of substrates was prevented. Taken together, our data suggest that, in adipocytes, serine/threonine phosphorylation events mimicked by okadaic acid are required for the translocation of IGF-II receptors and glucose transporters.(ABSTRACT TRUNCATED AT 250 WORDS)