In a patient with severe insulin resistance and acanthosis nigricans, a decrease in the number of insulin receptors has been found on freshly isolated monocytes and cultured fibroblasts compatible with a primary or genetic decrease in cell-surface insulin receptors. To determine the functional characteristics of the remaining receptors on these cells, insulin-stimulated glucose uptake, insulin internalization, and insulin-induced receptor loss were evaluated in monolayer fibroblasts obtained from this subject. Maximal insulin stimulation of 2-deoxyglucose was markedly blunted, compatible with abnormal insulin responsiveness due to a functional impairment of the remaining receptors. In the presence of chloroquine, the acanthotic subject's fibroblasts internalized more insulin per available receptor compared with the normal cell line, suggesting an accelerated rate of insulin internalization. When the rate of insulin internalization was more directly determined by assessing the rate of appearance of acid-resistant, cell-associated radioactivity at 37°C, a similar increase in insulin internalization rate was evident. When downregulation was assessed, insulin's ability to induce receptor loss in the acanthotic subject's cell line was augmented.
Thus, a primary or genetic decrease in insulin receptors on cultured fibroblasts from a patient with acanthosis nigricans and insulin resistance is associated with functional impairment of the remaining receptors leading to significant alterations in ligand processing and subsequent insulin action.