In the past year, our efforts focus on the role of SIRT1 in hepatic and intestinal nutrient metabolism and tissue homeostasis, as well as mechanisms and physiological importance of SIRT1 Thr522 phosphorylation. As a highly conserved NAD+-dependent protein deacetylase, SIRT1 has been shown as a key metabolic sensor that directly links nutrient signals to animal metabolic homeostasis. Although SIRT1 has been implicated in a number of hepatic metabolic processes, the mechanisms by which hepatic SIRT1 modulates bile acid metabolism are still not well understood. In a recent study, we show that deletion of hepatic SIRT1 reduces the expression of farnesoid X receptor (FXR), a nuclear receptor that regulates bile acid homeostasis. We provide evidence that SIRT1 regulates the expression of FXR through hepatocyte nuclear factor 1α(HNF1α). SIRT1 deficiency in hepatocytes leads to decreased binding of HNF1αto the FXR promoter and decreased expression of a number of FXR target genes involved in bile acid transport and metabolism. Furthermore, we show that hepatocyte-specific deletion of SIRT1 leads to decreased biosynthesis of bile acids and reduced biliary transport of bile salts, predisposing the mice to development of cholesterol gallstones on a lithogenic diet. Taken together, our findings indicate that SIRT1 plays a vital role in the regulation of hepatic bile acid homeostasis through the HNF1α/FXR signaling pathway. A paper describing this study was published in Molecular and Cellular Biology in April of 2012. Although the functions of SIRT1 in many organs have been extensively studied in tissue-specific knockout mouse models, the systemic role of SIRT1 is still largely unknown due to the severe developmental defects of the whole-body knockout mice. During the past four years, we investigated the systemic functions of SIRT1 in metabolic homeostasis utilizing a whole body SIRT1 heterozygous mouse model (SIRT1 Het mice). These mice are phenotypically normal under standard feeding condition, therefore allowing us to investigate the systemic role of SIRT1 without the interference of developmental defects. Our data indicate that when chronically challenged with a 40%-fat diet, SIRT1 Het mice become obese and insulin-resistant, display increased serum cytokine levels, and develop hepatomegaly. Hepatic metabolomic analyses revealed that SIRT1 Het mice have elevated gluconeogenesis and oxidative stress. However surprisingly, they are depleted of glycerolipid metabolites and free fatty acids, yet accumulate highly proinflammatory lysolipids, such as lysophosphocholines. Microarray analysis of liver mRNA indicates that these mice have altered expression of genes involved in gluconeogenesis and glycerolipid metabolism, and unexpectedly, steroid metabolism, particularly genes involved in testosterone inactivation. These observations suggest that chronic high-fat feeding may impair hepatic steroid hormone inactivation/clearance in SIRT1 Het mice. Consistent with this possibility, high-fat diet feeding induces elevation of serum testosterone levels and enlargement of seminal vesicles in SIRT1 heterozygous males without alterations in expression of testicular and adrenal steroid hormone metabolism genes. Taken together, our findings indicate that SIRT1 plays a vital role in the regulation of systemic energy and steroid hormone homeostasis. A paper describing this work was published in FASEB Journal in February of 2012. While much attention has been focused on the identification of cellular targets to explore the molecular mechanisms and functional networks controlled by SIRT1, how SIRT1 activity is regulated is still unclear. Therefore, another focus of our group is to understand the molecular mechanism that controls SIRT1 expression and activity. In one of our previous studies, we reported that SIRT1 is activated by phosphorylation at a conserved Thr522 residue in reesponse to environmental stress. In a recent study we demonstrate that phosphorylation of Thr522 activates SIRT1 through modulation of its oligomeric status. We provide evidence that nonphosphorylated SIRT1 protein is aggregation-prone in vitro and in cultured cells. Conversely, phosphorylated SIRT1 protein is largely in the monomeric state. Our findings reveal a novel mechanism for environmental regulation of SIRT1 activity, which may have important implications in understanding the molecular mechanism of stress response, cell survival, and aging. A manuscript describing this study is accepted for publication in Scientific Reports. In addition to hepatic lipid metabolism, we also focus on the role of SIRT1 in intestine, an important metabolic organ that is primarily involved in nutrient absorption. To explore the function of SIRT1 in nutrient metabolism in intestine, we recently generated an intestinal specific SIRT1 knockout mouse strain (SIRT1 IKO) by breeding the flox SIRT1 allele with the Villin-Cre line. The resulting mice have SIRT1 specifically deleted in the epithelial cells of the small and large intestines, including both villi and crypt cells. SIRT1 IKO mice were morphologically normal under standard feeding conditions, however, they displayed significantly lower serum bile acid levels than control animals, suggesting a defect in bile acid metabolism. Interestingly, six-week feeding of lithogenic diet, which contains high-fat high-cholesterol along with 0.5% cholate, results in formation of cholesterol gallstones in control animals, whereas SIRT1 IKO mice were protected from this disease. Further analyses of gene expression profile in intestine and liver indicate that intestine-specific SIRT1 IKO mice had significantly decreased levels of nuclear receptor FXR, and its downstream target genes - organic anion transporters, OSTalpha/beta and ileal bile acid transporter, ASBT, in intestine than control mice. As a result, intestinal absorption of bile acids decreased. The reduced intestinal bile acid signals further stimulated hepatic bile acid synthesis and secretion, leading to decreased incidence in formation of cholesterol gallstones in gall bladder upon lithogenic diet feeding. Taken together, our data identify a critical role for SIRT1 in the regulation of intestinal bile acid absorption and systemic bile acid homeostasis, and suggest that pharmacological modulation of SIRT1 activity may be important for bile acid metabolism-related diseases. Currently, a manuscript describing this study is in preparation.
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