Insulin resistant states such as obesity, Type 2 diabetes (T2DM), and the Metabolic Syndrome (MetS) are associated with low circulating levels of adiponectin, a protein produced exclusively by adipocytes. Adiponectin has insulin sensitizing actions in muscle and liver, and anti-atherogenic properties in the vasculature. Thiazolidinediones (TZDs), an insulin sensitizing class of drugs, increase adiponectin levels independently of their actions on glucose metabolism. The mechanisms for this response and for decreased adiponectin in insulin resistant states are not yet understood. The proposed study will test the hypothesis that the mechanism underlying decreased adiponectin in insulin resistant states involves impaired insulin signaling in adipocytes and that therapeutic interventions that raise plasma adiponectin do so by reversing this impairment. This study will aim to establish a correlation between circulating adiponectin levels and adipocyte insulin signaling in controls, T2DM first-degree relatives, and obese subjects with and without T2DM. We plan to assess adiponectin production using RT-PCR and protein analysis methods. Adiponectin rate of production and secretion will be examined with pulse chase experiments in isolated adipocytes, and plasma adiponectin will be measured by radioimmunoassay. Adipocyte insulin sensitivity (anti-lipolytic effect of insulin) will be evaluated in vivo using isotope-labeled metabolites during hyperinsulinemic euglycemic clamp. The activity of the PI3-kinase signaling pathway, known to be impaired in insulin resistance, will be measured in biopsied adipose tissue. We will then attempt to demonstrate that therapeutic interventions which increase adiponectin levels do so by improving adipocyte insulin signaling though the PI 3-kinase pathway. T2DM subjects will be treated for two months with calorie restriction, metformin, or a thiazolidinedione. After treatment, adiponectin, insulin sensitivity, and insulin signaling measures will be repeated. The prevention of complications in patients with obesity, T2DM, and MetS continues to be a public health priority. The anticipated results of this clinical study may provide new insight into potential therapeutic targets in patients with T2DM, obesity, and MetS.
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