Chronic exposure of the liver to elevated FAs, as occurs on a high-fat diet (HFD), promotes liver gluconeogenesis, reduces enzymes of FA oxidation and increases hepatic lipogenesis, leading to increased liver triglyceride content (i.e., fatty liver) and insulin resistance. This is especially the case as visceral obesity develops. While there is evidence that dysregulation of FA storage within adipose tissue provides a fundamental link among obesity, insulin resistance and type-2 diabetes (T2D), the contribution of fatty acid trafficking in visceral fat is often underemphasized. Based on our preliminary data, we have hypothesized that excess FA flux to the liver via the hepatic portal vein is a key factor, and that it can occur for different reasons. We believe that this situation can be ameliorated, at least temporarily, by increasing the capacity of visceral fat (i.e., mesenteric white adipose tissue or WAT) to sequester FA from the blood, thereby reducing flux to the liver. In support of this, we have evidence that increasing visceral adipose mass due to local adipocyte proliferation or transplantation of adipose tissue into the mesenteric region improves glucose tolerance;i.e., visceral adipose tissue mass per se is not a reliable predictor of insulin resistance. We propose to test the hypothesis that insulin resistance/glucose intolerance can be reversed by procedures that promote adipocyte expansion and FA retention in the visceral bed.
Specific Aims are: 1.) To test the hypothesis that DIO (HFD- induced obesity) increases hepatic insulin resistance in part due to insufficient removal of FA from blood by proliferating adipocytes. This will be accomplished by investigating small adipocyte growth-induced improvements in glucose tolerance via characterizing specific adipocyte cell stage and anatomic location in terms of the capacity to store and retain FA. We will also demonstrate if adipocyte proliferation induction is prevented, then concurrent improvement of glucose tolerance is prevented as well. Lastly, we will induce insulin resistance via a selective increase of portal FA influx to the liver. 2.) To test the hypothesis that the development of FA dysregulation in mature adipocytes is due to reduced insulin-mediated FA retention. We predict that transplantation-induced glycemic improvements result from adipose depot-specific differences in insulin-mediated actions within mature adipocytes, and that these in turn are related to the relative neural (i.e., sympathetic) activity in specific fat depots. We will compare the individual contribution of adrenergic-induced lipolytic effects and insulin FA uptake/anti-lipolytic effects on glucose tolerance in animals receiving different transplants. Collectively, these experiments will provide important new information on the regulation of liver functioning by FA released from adipocytes and/or diet. Further, the studies proposed have direct clinical relevance in that they may provide novel mechanisms as to how adipocyte dysregulation increases the risk of diabetes.
Project Narrative: Chronic exposure of the liver to elevated FAs, as occurs on a high-fat diet (HFD), promotes liver gluconeogenesis, reduces enzymes of FA oxidation and increases hepatic lipogenesis, leading to increased liver triglyceride content (i.e., fatty liver) and insulin resistance. While there is evidence that dysregulation of FA storage within adipose tissue provides a fundamental link among obesity, insulin resistance and type-2 diabetes (T2D), the contribution of fatty acid trafficking in visceral fat is often underemphasized. The goal of this proposal is to determine the link between adipocyte expansion and FA retention in the visceral bed and insulin resistance/glucose intolerance.
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