OKADAIC ACID STIMULATES IGF-II RECEPTOR TRANSLOCATION AND INHIBITS INSULIN ACTION IN ADIPOCYTES

Okadaic acid, an inhibitor of protein phosphatases 2A and 1, stimulate s glucose transport in muscle and fat cells, suggesting that serine/th reonine phosphorylation steps are involved in the translocation of glu cose transporters. Here we have investigated whether such phosphorylat ion events could also participate in another membrane-associated insul in-stimulated process: insulin-like growth factor II (IGF-II) receptor translocation in adipocytes. Maximally effective concentrations of in sulin 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-in duced stimulation of IGF-II binding resulted from an increase in the n umber of IGF-II receptors in the plasma membrane with a concomitant di sappearance from the low-density microsomal fraction. These changes oc curred in parallel to those observed for the glucose transporter GLUT- 4. Both insulin-stimulated glucose transport and IGF-II binding were p revented when cells were pretreated with okadaic acid. To understand t he mechanism of this inhibitory effect, insulin receptor autophosphory lation and the tyrosine phosphorylation of endogenous proteins were st udied. Insulin induced the tyrosine phosphorylation of its receptor be ta-subunit and of proteins at 120 and 185 kDa, whereas okadaic acid al one 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 pre vented. Taken together, our data suggest that, in adipocytes, serine/t hreonine phosphorylation events mimicked by okadaic acid are required for the translocation of IGF-II receptors and glucose transporters. Fi nally, okadaic acid would block insulin effect by preventing the tyros ine phosphorylation of insulin receptor cellular substrates.