Leptin-replacement therapy improved glycemic control and decreased triglyceride levels in patients with lipodystrophy and leptin deficiency. Leptin deficiency contributes to the insulin resistance and other metabolic abnormalities associated with severe lipodystrophy.

To investigate the role of insulin receptor substrate (IRS)-2 in vivo, we generated IRS-2-deficient mice by gene targeting. Although homozygous IRS-2-deficient mice (IRS-2 -/-mice) had a body weight similar to wildtype mice, they progressively developed type 2 diabetes at 10 weeks. IRS-2 -/-mice showed insulin resistance and a defect in the insulin-stimulated signaling pathway in liver but not in skeletal muscle. Despite insulin resistance, the amount of ␤-cells was reduced to 83% of that in wild-type mice, which was in marked contrast to the 85% increase in the amount of ␤-cells in IRS-1-deficient mice (IRS-1 -/-mice) to compensate for insulin resistance. Thus, IRS-2 plays a crucial role in the regulation of ␤-cell mass. On the other hand, insulin secretion by the same number of cells in response to glucose measured ex vivo was significantly increased in IRS-2 -/-mice compared with wild-type mice but was decreased in IRS-1 -/-mice. These results suggest that IRS-1 and IRS-2 may play different roles in the regulation of ␤-cell mass and the function of individual ␤-cells.

NIDDM is a polygenic disease characterized by insulin resistance in muscle, fat, and liver, followed by a failure of pancreatic beta cells to adequately compensate for this resistance despite increased insulin secretion. Mice double heterozygous for null alleles in the insulin receptor and insulin receptor substrate-1 genes exhibit the expected approximately 50% reduction in expression of these two proteins, but a synergism at a level of insulin resistance with 5- to 50-fold elevated plasma insulin levels and comparable levels of beta cell hyperplasia. At 4-6 months of age, 40% of these double heterozygotes become overtly diabetic. This NIDDM mouse model in which diabetes arises in an age-dependent manner from the interaction between two genetically determined, subclinical defects in the insulin signaling cascade demonstrates the role of epistatic interactions in the pathogenesis of common diseases with non-Mendelian genetics.

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