Clarification of signaling pathways mediated by insulin and insulin-like growth factor I receptors in fibroblasts from patients with specific defect in insulin receptor

Receptor binding and biological action of insulin and insulin-like growth factor I (IGF-I) were studied in fibroblasts from a patient with leprechaunism and a patient with type A syndrome of insulin resistance. Insulin binding was reduced to 18.8 and 27.7% of control value, respectively. In contrast, IGF-I binding was normal in both patients. Competitive binding studies, IGF-I had 0.2% of the ability of insulin to compete with ¹²⁵I-labeled insulin binding, and insulin had 0.1% of the ability of IGF-I to complete with ¹²⁵I-labeled IGF-I binding in control subjects and patient fibroblasts. The dose-response curves of insulin stimulation assessed by glucose incorporation and α-aminoisobutyric acid uptake showed normal responsiveness, and ED₅₀ was significantly shifted to the right in fibroblasts from both patients. However, normal responsiveness and sensitivity were observed in thymidine incorporation studies. For IGF-I, dose-response curves of glucose incorporation, α-aminoisobutyric acid uptake, and thymidine incorporation were all normal in both patients. These results indicate that 1) the defect is specific to the insulin-receptor binding in these patients, 2) insulin and IGF-I activate glucose incorporation and α-aminoisobutyric acid uptake mainly through their own specific receptors, but 3) the IGF-I receptor appears to have a more important role in stimulating thymidine incorporation than the insulin receptor in physiological condition or, alternatively, an unknown postreceptor process with cascade signal transmission may overcome the decreased insulin-receptor binding to produce a normal dose-response curve.