Protein-tyrosine phosphatase 1B (PTP-1B) is a major protein-tyrosine phosphatase that has been implicated in the regulation of insulin action, as well as in other signal transduction pathways. To investigate the role of PTP-1B in vivo, we generated homozygotic PTP-1B-null mice by targeted gene disruption. PTP-1B-deficient mice have remarkably low adiposity and are protected from diet-induced obesity. Decreased adiposity is due to a marked reduction in fat cell mass without a decrease in adipocyte number. Leanness in PTP-1B-deficient mice is accompanied by increased basal metabolic rate and total energy expenditure, without marked alteration of uncoupling protein mRNA expression. In addition, insulin-stimulated whole-body glucose disposal is enhanced significantly in PTP-1B-deficient animals, as shown by hyperinsulinemic-euglycemic clamp studies. Remarkably, increased insulin sensitivity in PTP-1B-deficient mice is tissue specific, as insulin-stimulated glucose uptake is elevated in skeletal muscle, whereas adipose tissue is unaffected. Our results identify PTP-1B as a major regulator of energy balance, insulin sensitivity, and body fat stores in vivo.

Increased Energy Expenditure, Decreased Adiposity, and Tissue-Specific Insulin Sensitivity in Protein-Tyrosine Phosphatase 1B-Deficient Mice

The earliest defect in developing type 2 diabetes is insulin resistance, characterized by decreased glucose transport and metabolism in muscle and adipocytes. The glucose transporter GLUT4 mediates insulin-stimulated glucose uptake in adipocytes and muscle by rapidly moving from intracellular storage sites to the plasma membrane. In insulin-resistant states such as obesity and type 2 diabetes, GLUT4 expression is decreased in adipose tissue but preserved in muscle. Because skeletal muscle is the main site of insulin-stimulated glucose uptake, the role of adipose tissue GLUT4 downregulation in the pathogenesis of insulin resistance and diabetes is unclear. To determine the role of adipose GLUT4 in glucose homeostasis, we used Cre/loxP DNA recombination to generate mice with adipose-selective reduction of GLUT4 (G4A-/-). Here we show that these mice have normal growth and adipose mass despite markedly impaired insulin-stimulated glucose uptake in adipocytes. Although GLUT4 expression is preserved in muscle, these mice develop insulin resistance in muscle and liver, manifested by decreased biological responses and impaired activation of phosphoinositide-3-OH kinase. G4A-/- mice develop glucose intolerance and hyperinsulinaemia. Thus, downregulation of GLUT4 and glucose transport selectively in adipose tissue can cause insulin resistance and thereby increase the risk of developing diabetes.

Adipose-selective targeting of the GLUT4 gene impairs insulin action in muscle and liver

Adipose tissue selective insulin receptor knockout protects against obesity and obesity-related glucose intolerance.

Potential mechanism of insulin action on glucose transport in the isolated rat diaphragm. Apparent translocation of intracellular transport units to the plasma membrane.

This study was initiated to develop an animal model of type 2 diabetes in a non-obese, outbred rat strain that replicates the natural history and metabolic characteristics of the human syndrome and is suitable for pharmaceutical research. Male Sprague-Dawley rats (n = 31), 7 weeks old, were fed normal chow (12% of calories as fat), or high-fat diet (40% of calories as fat) for 2 weeks and then injected with streptozotocin (STZ, 50 mg/kg intravenously). Before STZ injection, fat-fed rats had similar glucose concentrations to chow-fed rats, but significantly higher insulin, free fatty acid (FFA), and triglyceride (TG) concentrations (P < .01 to .0001). Plasma insulin concentrations in response to oral glucose (2 g/kg) were increased 2-fold by fat feeding (P < .01), and adipocyte glucose clearance under maximal insulin stimulation was significantly reduced (P < .001), suggesting that fat feeding induced insulin resistance. STZ injection increased glucose (P < .05), insulin (P < .05), FFA (P < .05), and TG (P < .0001) concentrations in fat-fed rats (Fat-fed/STZ rats) compared with chow-fed, STZ-injected rats (Chow-fed/STZ rats). Fat-fed/STZ rats were not insulin deficient compared with normal chow-fed rats, but had hyperglycemia and a somewhat higher insulin response to an oral glucose challenge (both P < .05). In addition, insulin-stimulated adipocyte glucose clearance was reduced in Fat-fed/STZ rats compared with both chow-fed and Chow-fed/STZ rats (P < .001). Finally, Fat-fed/STZ rats were sensitive to the glucose lowering effects of metformin and troglitazone. In conclusion, Fat-fed/STZ rats provide a novel animal model for type 2 diabetes, simulates the human syndrome, and is suitable for the testing of antidiabetic compounds.

Samuel W. Cushman

Papers

Determinations of adipose cell size and number in suspensions of isolated rat and human adipose cells.

Cell-associated nonesterified fatty acid levels and their alteration during lipolysis in the isolated mouse adipose cell.

Absence of an early effect of glucocorticoids on nonesterified fatty acid accumulation in isolated rat thymus cells