Characterization of Uncleaved Insulin Proreceptor Leading to Extreme Insulin Resistance

We investigated the binding characteristics and signal transmission of the uncleaved insulin proreceptor in transformed lymphocytes and cultured fibroblasts from a patient with extreme insulin resistance. Insulin binding was extremely decreased to 20 % and 27 %, respectively, of the control values. Scatchard analysis revealed that the reduced receptor binidng was due to decreased receptor affinity in both types of cells. An affinity cross-linking study with dithiothreitol treatment revealed that the molecular weight of the insulin receptor was 210 KDa in both kinds of cells from the patient. Since IGF-I binding to the fibroblasts from the patient was normal, the defcet was specific to the insulin receptor. Decreased insulin-stimulated autophosphorylation of the proreceptor was proportional to the decreased insulin binding. In the fibroblasts from the patient, the dose-response curve of insulin-stimulated α-aminoisobutyric acid uptake showed a 5-fold shift to the right (ED50 was 20 ng/ml for the patient vs. 3.5 ng/ml for the control subjects), but the maximally stimulated uptake was normal. With 0.025 % trypsin treatment, the binding abnormalities, autophosphorylation and α-aminoisobutyric acid uptake were all normalized. These results suggest that extreme insulin resistance is exclusively due to uncleavage of the insulin proreceptor, and that the proreceptor has a three -dimensional structural alteration affecting insulin binding but not intramolecular signal transmission. The proreceptor's failure to cleave produces a new disease entity for hormone resistance.