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ObjectiveNur77 is an orphan nuclear receptor with pleotropic functions. Previous studies have identified Nur77 as a transcriptional regulator of glucose utilization genes in skeletal muscle and gluconeogenesis in liver. However, the net functional impact of these pathways is unknown. To examine the consequence of Nur77 signaling for glucose metabolism in vivo, we challenged Nur77 null mice with high-fat feeding.Research design and methodsWild-type and Nur77 null mice were fed a high-fat diet (60% calories from fat) for 3 months. We determined glucose tolerance, tissue-specific insulin sensitivity, oxygen consumption, muscle and liver lipid content, muscle insulin signaling, and expression of glucose and lipid metabolism genes.ResultsMice with genetic deletion of Nur77 exhibited increased susceptibility to diet-induced obesity and insulin resistance. Hyperinsulinemic-euglycemic clamp studies revealed greater high-fat diet-induced insulin resistance in both skeletal muscle and liver of Nur77 null mice compared with controls. Loss of Nur77 expression in skeletal muscle impaired insulin signaling and markedly reduced GLUT4 protein expression. Muscles lacking Nur77 also exhibited increased triglyceride content and accumulation of multiple even-chained acylcarnitine species. In the liver, Nur77 deletion led to hepatic steatosis and enhanced expression of lipogenic genes, likely reflecting the lipogenic effect of hyperinsulinemia.ConclusionsCollectively, these data demonstrate that loss of Nur77 influences systemic glucose metabolism and highlight the physiological contribution of muscle Nur77 to this regulatory pathway.

Insulin regulates the uptake of glucose into skeletal muscle and adipocytes by redistributing the tissue-specific glucose transporter GLUT4 from intracellular vesicles to the cell surface. To date, GLUT4 is the only protein involved in insulin-regulated vesicular traffic that has this tissue distribution, thus raising the possibility that its expression alone may allow formation of an insulin-responsive vesicular compartment. We show here that treatment of differentiating [C.sub.2][C.sub.12] myoblasts with dexamethasone, acting via the glucocorticoid receptor, causes a [greater than or equal to] 10-fold increase in GLUT4 expression but results in no significant change in insulin-stimulated glucose transport. Signaling from the insulin receptor to its target, Akt2, and expression of the soluble N-ethylmaleimide-sensitive factor-attachment protein receptor, or SNARE, proteins syntaxin 4 and vesicle-associated membrane protein are normal in dexamethasone-treated [C.sub.2][C.sub.12] cells. However, these cells show no insulin-dependent trafficking of the insulin-responsive aminopeptidase or the transferrin receptor, respective markers for intracellular GLUT4-rich compartments and endosomes that are insulin responsive in mature muscle and adipose cells. Therefore, these data support the hypothesis that GLUT4 expression by itself is insufficient to establish an insulin-sensitive vesicular compartment. glucocorticoid; skeletal muscle; insulin; glucose transporter; [C.sub.2][C.sub.12] cells; protein trafficking

The effects of cold exposure on changes in muscle uncoupling protein-3 (UCP-3) and GLUT-4 messenger RNAs and the expression of GLUT-4 co-resident proteins were studied to illustrate the influence of UCP-3 in skeletal muscle as a regulator of adaptive thermogenesis and energy balance. Findings indicated that the two proteins exhibited almost identical response in skeletal muscles. However, cold exposure did not significantly affect the expression of GLUT-4 coproteins such as sortilin and transferrin receptor protein.

A study showed that 20-30% of the insulin-like growth factor II/mannose 6-phosphate and transferrin receptors are colocalized with GLUT-4 in GLUT-4-containing vesicles. The finding was derived through a simple method for separating intracellular pools of glucose transporter isoform GLUT-4-containing vesicles from surface sarcolemma and T tubule membranes of rat skeletal muscle cells. Results also showed that insulin stimulation in vivo was succeeded by the translocation of the receptors from their intracellular compartments to the sell surface together with GLUT-4.

Fluorescence spectroscopy was done to characterize the conformational states of recombinant forms of insulin holoreceptor in which insulin binding and ligand-induced autophosphorylation are normal. The results confirmed the applicability of the fluorescence spectroscopic technique in monitoring the real time changes in insulin receptor conformation. The method was also found useful for elucidating the mechanism of insulin action in terms of receptor autophosphorylation and activation.

Sucrose (Suc) feeding, dexamethasone (Dex) administration and Suc/Dex combination models of insulin resistance were analyzed to determine the level of expression of GLUT-4 glucose transport protein. The degree of expression of GLUT-4 protein depends on the type of model for insulin resistance and the kind of tissue in which the transporter is expressed. Adipocyte insulin resistance is characterized by an increase in basal state GLUT-4 in the plasma membrane. An increase in GLUT-4 expression was exhibited by the skeletal muscle during Dex-induced insulin resistance.

Insulin induces translocation of glucose transporting activity from an intracellular pool to the cell surface in insulin-sensitive cells such as adipocytes and myocytes. This insulin-sensitive glucose transport is regulated by several integral membrane proteins. Analysis of the compartmentalization of these proteins in adipocytes identified several vesicular populations which contain other types of proteins that show insulin-dependent cell-surface translocation.

The insulin receptor is a ligand-activated transmembrane signaling protein, which participates primarily in acute and chronic metabolic regulation. The clinical applications of insulin derivatives in diabetes has enabled a good understanding of the structure and chemistry of insulin receptors. Insulin receptors remain functionally as dimers due to covalent disulfide bonds. Insulin receptor phosphorylates a large protein which then binds the affector molecules.