Severe obesity affects 4.7% of the U.S. population about 25% of which suffer from type 2 diabetes due to reduced insulin sensitivity and/or beta-cell function. Several studies have shown that the accumulation of intraabdominal (IA) adipose tissue in particular increases the risk of impaired insulin action but the mechanisms responsible for this phenomenon are poorly understood. In addition, the role of subcutaneous (SC) adipose tissue regarding insulin action - protective, inert or detrimental - is still under debate. This is because SC adipose tissue releases adipocytokines (e.g. IL-6, leptin, TNF-alpha, resistin, adiponectin) that have been demonstrated to affect insulin action. In individuals who are severely obese (SO), excess IA or SC adipose tissue may secrete abnormal (exaggerated or reduced) amounts of adipocytokines.
Our specific aims are: (1) to determine the relative contribution of abdominal IA fat versus SC fat to systemic levels of IL- 6, leptin, TNF-alpha, resistin, and adiponectin in lean and in SO individuals; (2) to determine the effects of systemic adipocytokine concentrations on insulin action in vivo (insulin sensitivity, Si and insulin secretion, AIRg) and in vitro (glucose uptake in biopsied muscle and fat). Hypothesis: (a) In the context of severe obesity, IA fat produces increased quantities of IL-6, leptin, TNF-alpha and resistin and reduced quantities of adiponectin compared to SC fat; (b) in SO subjects undergoing weight loss (WL), insulin action in vivo and in vitro can be predicted by changes in systemic adipocytokines. Methods: Adipose tissue content of adipocytokines in biopsies obtained from IA and SC adipose tissue stores in lean and SO subjects will be determined by ELISA. In addition, changes in computed tomography-determined volumes of abdominal IA and SC fat will be related to concomitant changes in systemic adipocytokines from baseline to 6 mo following WL therapy. For this time period we will also determine changes in insuln action in vivo (Si and AIRg) and in vitro (via glucose uptake into muscle and fat). Longitudinal relationships between systemic adipocytokines and insulin action will be assessed using uni- and multivariate correlation analysis. The proposed studies will determine whether aberrant secretion of adipocytokines by excess IA or SC adipose tissue impairs insulin action in SO individuals. Our findings may provide novel mechanisms that link regional obesity and insulin action.
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