Acromegaloid patients with type A insulin resistance: parallel defects in insulin and insulin-like growth factor-I receptors and biological responses in cultured fibroblasts

A subset of patients with the syndrome of acanthosis nigricans and insulin resistance type A is characterized by acromegaloid features in addition to hyperinsulinemia, hyperandrogenemia, and an inherent defect in insulin receptor function. It has been proposed that the acromegaloid features result from the interaction of insulin at concentrations encountered in vivo, with a functionally intact insulin-like growth factor-I (IGF-I) receptor closely related to the insulin receptor. We investigated this possibility by examining binding and hormone-stimulated 14C]glucose uptake, 3H]thymidine uptake, and receptor autophosphorylation by both insulin and IGF-I in cultured fibroblasts from two affected patients. In comparison to normal fibroblasts, 125I]insulin binding, insulin-stimulated 14C]glucose, and 3H]thymidine uptake, and insulin-stimulated autophosphorylation were each reduced by approximately 50-60% of the absolute values in controls. In contrast to expectation, each of these apparent defects in insulin binding and action were mirrored by a parallel decrease in IGF-I binding and action. Thus, 125I]IGF binding was approximately 50%, IGF-I stimulated 3H]thymidine uptake was approximately 40% and 60% of the control value, and IGF-I-stimulated receptor autophosphorylation was reduced by 40%. Incubation of fibroblasts with insulin at 25 ng/mL reduced subsequent binding of 125I]IGF-I by approximately 20% and did not enhance maximal stimulation of 3H]thymidine incorporation. We conclude that in some patients with acanthosis nigricans and acromegaloid features, IGF-I receptors of cultured fibroblasts may share the inherent defects of insulin receptor function. These in vitro data do not explain the acromegaloid features observed in vivo, suggesting that acromegaloid features are mediated by other mechanisms.