Impact of estrogen receptor alpha on non-alcoholic fatty liver disease and liver energy metabolism

Abstract

One of the consequences of the Western lifestyle and high-fat diet is Non-Alcoholic Fatty Liver Disease (NAFLD), which affects about 30% of adults and up to 10% of children in developed countries. NAFLD begins with the accumulation of triacylglycerides (TAG) in the liver and is defined as the presence of lipid droplets in the cytoplasm of more than 5% of hepatocytes. This disease develops when the rate of hepatic TAG synthesis, as a result of increased uptake of fatty acids and their esterification into TAG as well as lipogenesis again from the metabolism of carbohydrates and proteins, exceeds the rate of catabolism of hepatic TAG through the oxidation of fatty acids or the secretion of TAG in the form of very low density lipoproteins (VLDL). One of the major concerns with NAFLD is that the accumulation of liver lipids has been clearly linked to the development of liver resistance to insulin and DM2.Experimental studies demonstrate the importance of estradiol in metabolic homeostasis. Increased body weight associated with increased body fat is observed in both female rats and ovariectomized mice. In addition, both male and female mice that do not have the aromatase enzyme (enzyme responsible for the final stage of estrogen synthesis) showed an increase in body weight, body fat and adipocyte hypertrophy. In fact, we recently demonstrated that estradiol treatment in female ovariectomized mice fed a high-fat diet reduced body weight and fat percentage, in addition to increased insulin sensitivity. These effects of estradiol on body weight and insulin sensitivity were associated with a reduction in the ectopic content of lipids, both in the liver and in the muscle, mainly the reduction of diacylglycerol (DAG) which, in turn, led to a reduction in activation PKCµ (liver) and PKC¸ (muscle), increasing insulin sensitivity.In addition, it has been suggested that these metabolically beneficial actions of estradiol are mediated by the estrogen alpha receptor (ER±), since mice that lack this receptor also have obesity. It has recently been shown that ER± activation increases mitochondrial activity and systemic metabolism, protecting ovariectomized female mice against obesity induced by a high-fat diet. This effect of ER± on mitochondrial metabolism has also been demonstrated specifically in skeletal muscle. Animals without ER± specifically in skeletal muscle demonstrate reduced mitochondrial activity, ectopic lipid increase and defect in mitochondrial turnover, leading to glucose intolerance and insulin resistance. Interestingly, animals without ER± specifically in the liver have increased hepatic TAG deposits and increased NAFLD-induced susceptibility to a high-fat diet. However, the function of ER± in the liver in relation to energy metabolism, insulin sensitivity and deposition of other species of biologically active lipids (such as DAG and ceramides) has not been fully studied. Thus, the general objective of this project is to study (in vivo) the function of ER± on hepatic energy metabolism using animals with ER± knockout specifically in the liver (using Cre-LoxP technology).The specific objectives will be: Assess in vivo the effects of the absence of ER± expression in the liver on; 1. Accumulation of lipids (TAG, DAG and ceramides); 2. Gene and protein expression of inflammatory markers; 3. Glucose tolerance through glucose tolerance test (GTT); 4. Insulin resistance using an euglycemic-hyperinsulinemic clamp (using radioactive tracers to determine glucose metabolism in vivo); 5. Global energy metabolism through indirect calorimetry; (AU)