Innovative Techniques: The liver coordinates metabolism of the glucose and TG through the convergence of multiple metabolic signals, including hormonal signals such as insulin and glucagon, and substrate concentrations of glucose and fatty acids. The corollary is that relatively subtle failure this convergent signaling could lead to abnormalities in both glucose and lipid metabolism –such as seen in obesity and diabetes. Traditional methods to study liver metabolism in vivo are confounded by counter-regulatory changes in glucose and insulin action. In our lab, our approach has been to use chronically-catheterized mice and rats. We then incorporate metabolic clamp techniques to control serum insulin, glucose, and glucagon levels, and thus avoid compensatory metabolic changes. This approach is the gold standard to define insulin sensitivity in vivo, but has not been widely applied to studying TG metabolism in rodents. On top of physiologic definition of insulin sensitivity and TG production, we use metabolic tracers to define the metabolic fate glucose and synthesis of TG. We overlay cutting-edge proteomics, metabolomics and transcriptomics techniques to relate lipid metabolism to insulin sensitivity.
Specific research projects include:
1) Sex-Differences in Cardiovascular risk: Compared to men, women have a delay in the onset of cardiovascular disease. In some studies, this is as much as 10 to 20 years. Some of this protection may be due to protection from the metabolic complications of obesity, including diabetes and a dyslipidemia characterized by increased VLDL, and low HDL. Our lab is interested in defining the molecular pathways that contribute to sex-differences in cardiovascular risk. We use genetic models with tissue-specific knock-out of estrogen receptor alpha. We also use a surgical model of ovariectomy, which mimics many aspects of menopause. Our lab has identified important roles of ovarian hormones in protecting from abnormalities in liver metabolism with obesity. We have found that ovarian hormones have a protective role against HDL changes associated with high-fat feeding.
2) HDL composition and function: High density lipoprotein (HDL) protects from coronary heart disease (CHD). HDL prevents inflammation, and accepts cholesterol from tissues by reverse cholesterol transport (RCT). In obesity, HDL may lose its ability to limit inflammation and participate in RCT. In humans, impaired HDL function correlated with the development of obesity, insulin resistance, and fatty liver. Our lab studies how changes in glucose and triglyceride metabolism with obesity contribute to alterations in HDL composition and function. We use both targeted and shotgun proteomics to define HDL composition changes in our obesity models, and relate this to HDL function assays.
3) Coordination of liver glucose and triglyceride metabolism: In the liver metabolism of glucose and triglyceride are exquisitely coordinated to meet changing metabolic needs, including those day-to-day events such as fasting and feeding. This coordination occurs through a convergence of physiologic, signaling, and transcriptional steps in the liver. The implication of this intricate regulation is that subtle defects in the metabolism of one macronutrient influence the other. For instance, liver fat accumulation leads to defects in glucose oxidation. Reciprocally high-carbohydrate diets increase lipogenesis in insulin resistant individuals. Our lab uses both genetic models and metabolic clamp techniques to define how abnormalities in glucose metabolism give rise to defects in triglyceride metabolism. Reciprocally we are interested in defining how defects in hepatic lipid metabolism give rise to defects in liver glucose handling.