Metabolic syndrome is devastating our health care system and compromising the quality of life for millions. Understanding the pathogenesis of this condition is paramount to eliminating it. The metabolic syndrome is an epidemic because people have adopted a diet for which they are poorly adapted and a lifestyle that is largely inactive. Hepatic metabolic dysfunction associated with inadequate substrate oxidation, lipid accumulation, and dyslipidemia is a hallmark of metabolic syndrome as it is evident early in its development and is associated with the severity of other symptoms. It has been speculated that liver metabolic dysfunction is a causative step in the natural progression to metabolic syndrome. Despite the central role of liver metabolism to overall "metabolic health," the mechanism for its effectiveness in healthy physically active states, the factors responsible for dysfunction, and the means to correct dysfunction are poorly understood. The protocols that comprise this proposal are designed to define mechanisms that control (i) intra-hepatic energy balance during acute perturbations and (ii) intra-hepatic and whole body energy balance by modifications in diet and physical activity. The finding from the present grant cycle that shapes the aims of this proposal is based on an observation so fundamental to metabolism that it will influence flux control at the most basic level. What we have shown is that the energy state of the "healthy liver" undergoes dramatic deviations. Cellular energy status is tightly controlled in most tissues of the body, so that cells are in a highly charged state (low AMP:ATP). However, physiological conditions such as exercise and fasting can trigger a five- to tenfold increase in the AMP:ATP in liver. We will test whether (a) the increase in hepatic AMP:ATP during glucagon stimulation and exercise is due to ATP hydrolysis associated with the energetics of gluconeogenesis;(b) the nucleotide monophosphates signal the stimulation of hepatic substrate oxidation through the activation of AMPK11 and AMPK12 subunits;and (c) the hepatic adaptations to high fat feeding and physical activity are AMPK-dependent. The regulation of hepatic metabolism will be studied using surgical and experimental tools that allow well-controlled experiments to be carried out in vivo. Glucagon infusion, treadmill exercise, wheel running and high fat feeding will be used as tools to amplify physiological signals and as a means of perturbing hepatic metabolic control. Mechanisms of action and sites of dysfunction will be delineated using well-defined genetic mouse models. Hepatic substrate metabolism will be quantified using sophisticated isotopic approaches employing 2H and 13C NMR analytical techniques. These studies will define how liver substrate fluxes are regulated in the healthy liver and where sites of dysfunction lie in the pathogenesis of metabolic syndrome and hepatic insulin resistance.

Public Health Relevance

. Metabolic syndrome and Type II diabetes are an enormous burden on our health care system. The physiological adaptations to exercise decrease the risk of developing these conditions, at least in part by improving liver metabolic function.
The aim of this proposal is to elucidate the mechanism by which exercise improves metabolic regulation by the liver.

National Institute of Health (NIH)
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
Method to Extend Research in Time (MERIT) Award (R37)
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Skeletal Muscle and Exercise Physiology Study Section (SMEP)
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Laughlin, Maren R
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Vanderbilt University Medical Center
Schools of Medicine
United States
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