Peroxisome proliferator-activated receptor gamma coactivator-1-dependent uncoupling protein-2 expression in pancreatic islets of rats: a novel pathway for neural control of insulin secretion.

AIMS/HYPOTHESIS: Sympathetic inputs inhibit insulin secretion through alpha2-adrenergic receptors coupled with Gi protein. High adrenergic tonus generated by exposure of homeothermic animals to cold reduces insulin secretion. In this study we evaluate the participation of UCP-2 in cold-induced regulation of insulin secretion. METHODS: Static insulin secretion studies, western blotting and immunohistochemistry were used in this investigation. RESULTS: Exposure of rats to cold during 8 days promoted 60% ( n=15, p<0.05) reduction of basal serum insulin levels concentration accompanied by reduction of the area under insulin curve during i.p. GTT (50%, n=15, p<0.05). Isolated islets from cold-exposed rats secreted 57% ( n=6, p<0.05) less insulin following a glucose challenge. Previous sympathectomy, partially prevented the effect of cold exposure upon insulin secretion. Islets isolated from cold-exposed rats expressed 51% ( n=6, p<0.5) more UCP-2 than islets from control rats, while the inhibition of UCP-2 expression by antisense oligonucleotide treatment partially restored insulin secretion of islets obtained from cold-exposed rats. Cold exposure also induced an increase of 69% ( n=6, p<0.05) in PGC-1 protein content in pancreatic islets. Inhibition of islet PGC-1 expression by antisense oligonucleotide abrogated cold-induced UCP-2 expression and partially restored insulin secretion in islets exposed to cold. CONCLUSION/INTERPRETATION: Our data indicate that sympathetic tonus generated by exposure of rats to cold induces the expression of PGC-1, which participates in the control of UCP-2 expression in pancreatic islets. Increased UCP-2 expression under these conditions could reduce the beta-cell ATP/ADP ratio and negatively regulate insulin secretion.

Early steps of insulin action in the skin of intact rats.

Insulin is an important regulator of growth and initiates its action by binding to its receptor, which undergoes tyrosyl autophosphorylation and further enhances its tyrosine kinase activity towards other intermediate molecules, including insulin receptor substrate 1, insulin receptor substrate 2, and Shc. Insulin receptor substrate proteins can dock various src-homology-2-domain-containing signaling proteins, such as the 85 kDa subunit of phosphatidylinositol 3 kinase and growth-factor-receptor-bound protein 2. The serine-threonine kinase is activated downstream to phosphatidylinositol 3 kinase. Shc protein has been shown to directly induce the association with growth-factor-receptor-bound protein 2 and downstream the activation of the mitogen-activated protein kinase. In this study we investigated insulin signal transduction pathways in skin of intact rats by immunoprecipitation and immunoblotting with specific antibodies, and also by immunohistochemistry with anti-insulin-receptor antibody. Our results showed that skin fragments clearly demonstrated the presence of insulin receptor in cell bodies of the epidermis and hair follicles and some faint staining was also detected in fibroblasts of the dermis. It was also observed that acute stimulation with insulin can induce tyrosyl phosphorylation of insulin receptor, that the insulin receptor substrates and Shc proteins serve as signaling molecules for insulin in skin of rats, and that insulin is able to induce association of insulin receptor substrate 1/phosphatidylinositol 3 kinase and Shc/growth-factor-receptor-bound protein 2 in this tissue, as well as phosphorylation of mitogen-activated protein kinase and serine-threonine kinase, demonstrating that proteins involved in early steps of insulin action are expressed in skin of intact rats and are quickly activated after insulin stimulation.