The insulin receptor, though understood in fine molecular detail, remains problematic with regards to its mechanism(s) of cellular signalling. To better understand the normal mechanisms of plasma membrane signalling by insulin, we studied the family of a patient with insulin resistance, hypoglycemia and leprechaunism. Skin fibroblasts were cultured from the proband and his parents. High-affinity insulin binding to the proband’s cells was absent. Both parents’ fibroblasts showed high-affinity insulin binding intermediate between the controls and the proband. Thus impaired binding conformed to an autosomal recessive pattern of inheritance. The transport of neutral amino acids by the proband’s cells was comparable to controls and was insulin-sensitive. By contrast, hexose transport by the proband’s fibroblasts was very high and insulin-insensitive. This increased uptake was due to an increased number of glucose transporters on the plasma membrane and to a posttranslational mechanism. Similar levels of glucose transporter mRNA were observed in control and mutant cells. Thus, this inborn error of the insulin receptor offers unique insight into a regulatory signal by its a subunit which, when altered, results in constitutively increased glucose transport.

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