1. Investigate role of SIRT1 in hepatic lipid metabolism? The efforts of our first year at NIEHS focus on the role of SIRT1 in hepatic lipid metabolism. The liver is the central metabolic organ that governs several major aspects of lipid metabolism, including lipogenesis, fatty acid β-oxidation, and lipoprotein uptake and secretion. These pathways are tightly controlled by nutritional and hormonal signals to maintain hepatic and systemic energy homeostasis. For example, during fed state, dietary glucose stimulates insulin secretion from the pancreas. Increased plasma insulin directly go to the liver via the portal vein and increase the hepatic lipogenesis and lipoprotein secretion through transcription factor SREBP-1c. Upon fasting, on the other hand, the hepatic fat oxidation is enhanced as a result of increased availability of fatty acids and transcriptional activation of peroxisomal and mitochondrial fatty acid oxidation genes. The imbalance of lipid uptake and clearance in the liver results in hepatic steatosis characterized by increased accumulation of lipids in hepatocytes, and contributes to the development of chronic hepatic inflammation, insulin resistance, and eventually liver damage. ? Previous studies have demonstrated that SIRT1 is an important regulator of nutrient homeostasis in many tissues involved in metabolism. To study the function of SIRT1 in the hepatic lipid metabolism, we generated hepatocyte-specific SIRT1 KO (SIRT1 HKO) mice in C57BL/6 background by crossing a floxed SIRT1 allele with a cre-recombinase expressing line driven by the rat Albumin promoter. SIRT1 HKO were phenotypically normal under chow, however, when challenged with a high-fat high cholesterol diet, SIRT1 HKO mice show hepatic steatosis and inflammation. Microarray analysis showed that a number of signaling pathways involved in lipogenesis and fatty acid metabolism are significantly altered in these mice, especially after an overnight fasting. Cultured primary hepatocytes isolated from SIRT1 HKO mice also show similar defects as in animals, suggesting SIRT1 cell-autonomously regulate hepatic lipid homeostasis. Currently we are working on the molecular mechanisms underlying these defects. Our data indicate that SIRT1 plays a vital role in the regulation of hepatic lipid homeostasis, and suggest that activation of SIRT1 may be important for the prevention/attenuation of obesity/aging-associated metabolic diseases.? ? 2. Study the role of SIRT1 in atherosclerosis? Our previous studies predict that high levels/activities of SIRT1 may decrease the risk of cholesterol diseases and other metabolic syndromes associated with diet and aging. Inhibition of SIRT1, on the other hand, could enhance the malignancy of these diseases. We will directly test this prediction by analyzing the onset and progression of age-associated metabolic diseases, particularly atherosclerosis, in tissue specific conditional SIRT1 knockout under various conditions.? To analyze the role of SIRT1 in atherosclerosis, we generated liver and macrophage specific SIRT1 knockout mice and fed them with atherogenic high fat high cholesterol diet. We will analyze atherosclerosis lesion in arterial wall of these mice. We will also breed these animals into (e.g. apoE-/- or LDLR-/- mice), and directly analyze the atherosclerosis phenotypes. We also used a SIRT1 whole body knockout mouse model to study the role of SIRT1 in atherosclerosis. We bred the whole body knockout allele into ApoE null animal, collected the arteries, and will analyze atherosclerosis phenotypes in the near future.
