The effect of antibodies against the insulin receptor (anti-R) found in a patient with the type B syndrome of insulin resistance and acanthosis nigricans was characterized using 3T3-L1 cultured fat cells. Anti-R acutely mimicked the action of insulin by stimulating deoxyglucose uptake. With more prolonged exposure, this insulinomimetic effect decayed, glucose metabolism returned to basal levels, and the cells became severely resistant to the actions of insulin. As seen with anti-R from a previous patient, desensitization consisted of both a dramatic decrease in the maximal responsiveness of the cells to insulin and a shift in the dose-response curve for insulin-stimulated glucose oxidation.
The acute and chronic effects of anti-R were then compared. The concentration of anti-R required to halfmaximally inhibit insulin binding averaged more than twice that required for half-maximal stimulation of deoxyglucose uptake, consistent with the amount of spare receptors in 3T3-L1 cells. After prolonged exposure, the insulinomimetic activity was completely lost at all concentrations of anti-R, even at those that did not completely induce insulin resistance. Thus, loss of the insulinomimetic activity of anti-R is necessary, but not sufficient, to cause desensitization. Less anti-R was required to desensitize cells to insulin than would have been predicted on the basis of the acute inhibition of binding and the number of spare receptors. In addition, the amount of anti-R required to half-maximally inhibit insulin binding after prolonged exposure was an order of magnitude less than that during acute exposure, implying a progressive loss of insulin receptor binding during antibody-induced desensitization. When cells were treated with subsaturating concentrations of anti-R, insulin binding decreased over a period of several hours.
Finally, an acid wash (capable of dissociating antigen-antibody complexes) could not restore insulin binding after desensitization, but could restore insulin binding if cells were exposed to anti-R at 4°C. These results support the theory that insulin receptors are removed from the cell surface during antibody-induced desensitization.
