Insulin action rapidly modulates the apparent affinity of the insulin-like growth factor II receptor

Incubation of intact rat adipocytes with physiological concentrations of insulin stimulates binding of insulin-like growth factor II (IGF-II) to its receptor by 3- to 10-fold. The effect is temperature- and dose-dependent, with 0.1 nM insulin giving half-maximal stimulation. Scatchard analysis of IGF-II binding to intact adipocytes indicates that this effect is due to an apparent increase in receptor affinity, from Kd = 63 nM in the absence of insulin to Kd = 5.8 nM in the presence of 10 nM insulin, with no apparent change in the number of cell surface binding sites (220,000/cell). Scatchard analysis of 125I-IGF-II binding to isolated membrane fractions demonstrated that all IGF-II receptors in plasma membranes and low density microsomes from control cells are converted during homogenization to the high affinity form (Kd = 2 to 6 nM) seen in insulin-treated intact adipocytes. No significant difference in affinity was observed between plasma membranes from control or insulin-treated adipocytes or between low density microsomes from control or insulin-treated cells. However, in apparent contrast to the results obtained in intact adipocytes, the number of binding sites is increased in the plasma membrane fraction from insulin-treated cells by an average of 60%, while the number of receptors is decreased by 40% in low density microsomes from insulin-treated cells compared to control cells. These results were confirmed by direct visualization of the Mr = 270,000 IGF-II receptor band on dodecyl sulfate gels following affinity labeling with 125I-IGF-II and the cross-linker disuccinimidyl suberate. Scatchard analysis of the total cellular membranes showed no difference in the total number of binding sites between control and insulin-treated cells. These results demonstrate that insulin has two effects on the IGF-II receptor in adipocytes. 1) It rapidly increases the apparent affinity of the receptor in the intact cell without changing the apparent number of receptors on the cell surface; and 2) it induces a redistribution of the high affinity IGF-II receptor between plasma membranes and low density microsomes upon homogenization of cells and preparation of membranes. The latter effect closely parallels the insulin-induced membrane redistribution of the glucose transporter that occurs in the rat adipocyte by an unknown mechanism.