Cryptic receptors for insulin-like growth factor II in the plasma membrane of rat adipocytes--a possible link to cellular insulin resistance.

To further elucidate the mechanisms for short-term regulation of the receptor for insulin-like growth factor II (IGF-II), we investigated effects of insulin, cAMP and phosphatase inhibitors on cell surface 125I-IGF-II binding in rat adipocytes. Preincubation with the serine/threonine phosphatase inhibitor okadaic acid (OA, 1 microM) or the non-hydrolysable cAMP analogue N6-mbcAMP (4 mM) markedly impaired insulin-stimulated 125I-IGF-II binding. Furthermore, addition of OA enhanced the inhibitory effect exerted by N6-mbcAMP. N6-mbcAMP also induced an insensitivity to insulin which was normalized by concomitant addition of the tyrosine phosphatase inhibitor vanadate (0.5 mM). In contrast, vanadate did not affect the impairment in maximal insulin-stimulated 125I-IGF-II binding produced by either OA or N6-mbcAMP. Phospholipase C (PLC), which cleaves phospholipids at the cell surface, markedly enhanced cell surface 125I-IGF-II binding in a concentration-dependent manner. Scatchard analysis demonstrated that the effect of PLC was due to an increased number of binding sites suggesting that "cryptic' IGF-II receptors are associated with the plasma membrane (PM). PLC (5 U/ml) also reversed the N6-mbcAMP-induced decrease of 125I-IGF-II binding at a low insulin concentration (10 microU/ml). Taken together, these data indicate that cAMP, similar to its effects on the glucose transporter GLUT 4 and the insulin receptor, may increase the proportion of functionally cryptic IGF-II receptors in the PM through mechanisms involving serine phosphorylation, possibly of a docking or coupling protein. Tyrosine phosphorylation appears to exert an opposite effect promoting the full cell surface expression of receptors.