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 r eceptor for insulin-like growth factor II (IGF-II), we investigated ef fects of insulin, cAMP and phosphatase inhibitors on cell surface I-12 5-IGF-II binding in rat adipocytes, Preincubation with the serine/thre onine phosphatase inhibitor okadaic acid (OA, 1 mu M) or the non-hydro lysable cAMP analogue N-6-mbcAMP (4 mM) markedly impaired insulin-stim ulated I-125-IGF-II binding. Furthermore, addition of OA enhanced the inhibitory effect exerted by N-6-mbcAMP. N-6-mbcAMP also induced an in sensitivity to insulin which was normalized by concomitant addition of the tyrosine phosphatase inhibitor vanadate (0.5 mM). In contrast, va nadate did not affect the impairment in maximal insulin-stimulated I-1 25-IGF-II binding produced by either OA or N-6-mbcAMP. Phospholipase C (PLC), which cleaves phospholipids at the cell surface, markedly enha nced cell surface I-125-IGF-II binding in a concentration-dependent ma nner. Scatchard analysis demonstrated that the effect of PLC was due t o an increased number of binding sites suggesting that 'cryptic' IGF-I I receptors are associated with the plasma membrane (PM). PLC (5 U/ml) also reversed the N-6-mbcAMP-induced decrease of I-125-IGF-II binding at a low insulin concentration (10 mu U/ml). Taken together, these da ta indicate that cAMP, similar to its effects on the glucose transport er GLUT 4 and the insulin receptor, may increase the proportion of fun ctionally cryptic IGF-II receptors in the PM through mechanisms involv ing serine phosphorylation, possibly of a docking or coupling protein. Tyrosine phosphorylation appears to exert an opposite effect promotin g the full cell surface expression of receptors.