Deletion of Asn281 in the alpha-subunit of the human insulin receptor causes constitutive activation of the receptor and insulin desensitization.

We studied the structure and function of the insulin receptor (IR) in two sisters with leprechaunism. The patients had inherited alterations in the IR gene and were compound heterozygotes. Their paternal IR allele carried a major deletion, including exons 10-13, which shifted the reading frame and introduced a premature chain termination codon in the IR sequence.

Lipoatrophic diabetes: genetic exclusion of the insulin receptor gene.

Lipoatrophic diabetes (LD) is a syndrome with congenital or delayed onset, characterized by severe insulin resistance and generalized lipoatrophy. Using denaturing gradient gel electrophoresis and sequencing, we have investigated the contribution of defects in the insulin receptor (IR) gene in LD. First, we performed an association study between the IR gene and congenital lipoatrophy in two families with consanguineous parents and one or two affected children (patients D1, D2, and D3).

Increased insulin action in cultured hepatocytes from rats with diabetes induced by neonatal streptozotocin.

Previous studies have shown that Wistar rats injected at birth (n0) with STZ (n0-STZ) develop as adults a noninsulin-dependent diabetic state characterized by a lack of insulin response to glucose in vivo, a mild basal hyperglycemia, and an impaired glucose tolerance. Our former in vivo studies using the insulin-glucose clamp technique revealed an increased insulin action upon hepatic glucose production in these animals. We have now cultured hepatocytes from these mildly diabetic rats in parallel with hepatocytes from control rats, to examine more closely basal and insulin-regulated glucose production and glucose incorporation into glycogen.

Dual effect of metformin in cultured rat hepatocytes: potentiation of insulin action and prevention of insulin-induced resistance.

The ability of the biguanide hypoglycemic agent metformin to improve the acute effects of insulin on glucose and/or lipid metabolism was investigated in both insulin-responsive and insulin-resistant cultured rat hepatocytes: (1) metformin (20 micrograms/mL, 16 hours) increased the insulin-dependent stimulation of glycogen and lipid synthesis through an exclusive enhancement of the responsiveness without modification of the cell sensitivity to the hormone; (2) metformin neither altered basal glycogenesis from [U-14C]glucose and basal lipogenesis from [1-14C]acetate nor insulin binding. These results indicate the ability of this drug to selectively potentiate the acute action of insulin at a postreceptor step in normal liver cells. A prolonged incubation with insulin (16 hours, 5 x 10(-7) mol/L) led the hepatocytes to a state of resistance evidenced by a 50% decrease in their maximal responsiveness and sensitivity to a subsequent acute stimulation by the hormone, as assessed on lipogenesis.

In vitro studies of insulin resistance in patients with lipoatrophic diabetes. Evidence for heterogeneous postbinding defects.

We studied the binding and action of insulin in cultured fibroblasts from six patients with lipoatrophic diabetes and marked in vivo insulin resistance and from seven control subjects. The binding of insulin was not altered, which corresponds well with studies with circulating erythrocytes. Similarly, the action of the hormone on amino acid uptake (estimated by active transport of aminoisobutyric acid) was comparable in patient and control cells.

Degradation of insulin receptors by hepatoma cells: insulin-induced down-regulation results from an increase in the rate of basal receptor degradation.

The degradation of insulin receptors was studied in cultured Zajdela hepatoma cells (ZHC). Receptor distribution within the cell was evaluated by estimating: i) surface receptor level on entire cells, ii) total cell receptors solubilized by Triton from cell membranes and iii) intracellular receptors solubilized from cells whose surface receptors had been inactivated with trypsin. In the absence of insulin, 80-90% of the insulin binding sites were located on the cell surface.