Conserved role of SIRT1 orthologs in fasting-dependent inhibition of the lipid/cholesterol regulator SREBP.

The sterol regulatory element-binding protein (SREBP) transcription factor family is a critical regulator of lipid and sterol homeostasis in eukaryotes. In mammals, SREBPs are highly active in the fed state to promote the expression of lipogenic and cholesterogenic genes and facilitate fat storage. During fasting, SREBP-dependent lipid/cholesterol synthesis is rapidly diminished in the mouse liver; however, the mechanism has remained incompletely understood. Moreover, the evolutionary conservation of fasting regulation of SREBP-dependent programs of gene expression and control of lipid homeostasis has been unclear. We demonstrate here a conserved role for orthologs of the NAD(+)-dependent deacetylase SIRT1 in metazoans in down-regulation of SREBP orthologs during fasting, resulting in inhibition of lipid synthesis and fat storage. Our data reveal that SIRT1 can directly deacetylate SREBP, and modulation of SIRT1 activity results in changes in SREBP ubiquitination, protein stability, and target gene expression. In addition, chemical activators of SIRT1 inhibit SREBP target gene expression in vitro and in vivo, correlating with decreased hepatic lipid and cholesterol levels and attenuated liver steatosis in diet-induced and genetically obese mice. We conclude that SIRT1 orthologs play a critical role in controlling SREBP-dependent gene regulation governing lipid/cholesterol homeostasis in metazoans in response to fasting cues. These findings may have important biomedical implications for the treatment of metabolic disorders associated with aberrant lipid/cholesterol homeostasis, including metabolic syndrome and atherosclerosis.

Intestine-specific deletion of SIRT1 in mice impairs DCoH2-HNF-1α-FXR signaling and alters systemic bile acid homeostasis.

BACKGROUND & AIMS: Sirtuin 1 (SIRT1), the most conserved mammalian oxidized nicotinamide adenine dinucleotide-dependent protein deacetylase, is an important metabolic sensor in many tissues. However, little is known about its role in the small intestine, which absorbs and senses nutrients. We investigated the functions of intestinal SIRT1 in systemic bile acid and cholesterol metabolism in mice. METHODS: SIRT1 was specifically deleted from the intestines of mice using the flox-Villin-Cre system (SIRT1 iKO mice). Intestinal and hepatic tissues were collected, and bile acid absorption was analyzed using the everted gut sac experiment. Systemic bile acid metabolism was studied in SIRT1 iKO and flox control mice placed on standard diets, diets containing 0.5% cholic acid or 1.25% cholesterol, or lithogenic diets. RESULTS: SIRT1 iKO mice had reduced intestinal farnesoid X receptor (FXR) signaling via hepatocyte nuclear factor 1α (HNF-1α) compared with controls, which reduced expression of the bile acid transporter genes Asbt and Mcf2l (encodes Ost) and absorption of ileal bile acids. SIRT1 regulated HNF-1α/FXR signaling partially through dimerization cofactor of HNF-1a (Dcoh2) Dcoh2, which increases dimerization of HNF-1α. SIRT1 was found to deacetylate Dcoh2, promoting its interaction with HNF-1α and inducing DNA binding by HNF-1α. Intestine-specific deletion of SIRT1 increased hepatic bile acid biosynthesis, reduced hepatic accumulation of bile acids, and protected animals from liver damage from a diet high in levels of bile acids. CONCLUSIONS: Intestinal SIRT1, a key nutrient sensor, is required for ileal bile acid absorption and systemic bile acid homeostasis in mice. We delineated the mechanism of metabolic regulation of HNF-1α/FXR signaling. Reagents designed to inhibit intestinal SIRT1 might be developed to treat bile acid-related diseases such as cholestasis.

Systemic SIRT1 insufficiency results in disruption of energy homeostasis and steroid hormone metabolism upon high-fat-diet feeding.

SIRT1 is a highly-conserved NAD(+)-dependent protein deacetylase that plays essential roles in the regulation of energy metabolism, genomic stability, and stress response. 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 as a result of severe developmental defects that result from whole-body knockout in mice. Here, we investigated the systemic functions of SIRT1 in metabolic homeostasis by utilizing a whole-body SIRT1 heterozygous mouse model. These mice are phenotypically normal under standard feeding conditions. However, when chronically challenged with a 40% fat diet, they become obese and insulin resistant, display increased serum cytokine levels, and develop hepatomegaly. Hepatic metabolomic analyses revealed that SIRT1 heterozygous mice have elevated gluconeogenesis and oxidative stress. Surprisingly, they are depleted of glycerolipid metabolites and free fatty acids, yet accumulate lysolipids. Moreover, high-fat feeding induces elevation of serum testosterone levels and enlargement of seminal vesicles in SIRT1 heterozygous males. Microarray analysis of liver mRNA indicates that they have altered expression of genes involved in steroid metabolism and glycerolipid metabolism. Taken together, our findings indicate that SIRT1 plays a vital role in the regulation of systemic energy and steroid hormone homeostasis.

Conjugated linoleic acid does not reduce body fat but decreases hepatic steatosis in adult Wistar rats.

The dietary fatty acid conjugated linoleic acid (CLA) reduces hepatic lipid accumulation in some rodent models for obesity and hepatic steatosis. However, these effects are variable and complex due to differences in isomer responses and degree and sensitivity to changes in adiposity. Here, we hypothesized that CLA decreases hepatic steatosis in a diet-induced model of obesity in rats which are resistant to the adipose-lowering effects of CLA. To investigate this, we fed male Wistar rats a high-fat (20%) diet for 4 weeks to induce obesity and hepatic steatosis followed by low-fat (6.5%) experimental diets containing either 6.5% soybean oil (CON) or 1.5% CLA triglyceride mix plus 5% soybean oil (CLA). Dietary CLA significantly lowered hepatic triglycerides without changing weight, adiposity or adipokines, and was associated with significantly lower hepatic fatty acid synthase and stearoyl CoA desaturase-1 (SCD-1) mRNA levels and SCD-1 index along with significantly lower sterol regulatory element binding protein-1 mRNA, a transcription factor that regulates lipogenesis. Furthermore, the lower lipogenesis was associated with significantly higher mRNA expression of lipid oxidation gene peroxisome proliferator-activated receptor-alpha and acetyl CoA oxidase in the livers of rats fed dietary CLA. The lipid-lowering effects of CLA in the liver were observed in the absence of changes in adipose tissue and body weight. Thus, we conclude that in the Wistar rat model, where adipose levels remain static after feeding dietary CLA, hepatic lipid accumulation is reduced and these effects are not due to an improvement in overall adiposity.

Hepatic deletion of SIRT1 decreases hepatocyte nuclear factor 1α/farnesoid X receptor signaling and induces formation of cholesterol gallstones in mice.

SIRT1, a highly conserved NAD(+)-dependent protein deacetylase, is 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. Here we report 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 HNF1α to the FXR promoter. Furthermore, we show that hepatocyte-specific deletion of SIRT1 leads to derangements in bile acid metabolism, 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.

Myeloid deletion of SIRT1 induces inflammatory signaling in response to environmental stress.

Macrophage activation and infiltration into resident tissues is known to mediate local inflammation and is a hallmark feature of metabolic syndrome. Members of the sirtuin family of proteins regulate numerous physiological processes, including those involved in nutrient regulation and the promotion of longevity. However, the important role that SIRT1, the leading sirtuin family member, plays in immune response remains unclear. In this study, we demonstrate that SIRT1 modulates the acetylation status of the RelA/p65 subunit of NF-κB and thus plays a pivotal role in regulating the inflammatory, immune, and apoptotic responses in mammals. Using a myeloid cell-specific SIRT1 knockout (Mac-SIRT1 KO) mouse model, we show that ablation of SIRT1 in macrophages renders NF-κB hyperacetylated, resulting in increased transcriptional activation of proinflammatory target genes. Consistent with increased proinflammatory gene expression, Mac-SIRT1 KO mice challenged with a high-fat diet display high levels of activated macrophages in liver and adipose tissue, predisposing the animals to development of systemic insulin resistance and metabolic derangement. In summary, we report that SIRT1, in macrophages, functions to inhibit NF-κB-mediated transcription, implying that myeloid cell-specific modulation of this sirtuin may be beneficial in the treatment of inflammation and its associated diseases.

The citrus fruit flavonoid naringenin suppresses hepatic glucose production from Fao hepatoma cells.

Hepatic gluconeogenesis is the major source of fasting hyperglycemia. Here, we investigated the role of the citrus fruit flavonoid naringenin, in the attenuation of hepatic glucose production from hepatoma (Fao) cells. We show that naringenin, but not its glucoside naringin, suppresses hepatic glucose production. Furthermore, unlike insulin-mediated suppression of hepatic glucose production, incubation of hepatocytes with the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor Ly294002 had no effect on the ability of naringenin to suppress hepatic glucose production. Further, naringenin did not increase phosphorylation of Akt at Ser473 or, Thr308, indicating this down-stream target of PI3-kinase is also not a player in naringenin-mediated suppression of hepatic glucose production. Importantly, like the dimethylbiguanide, metformin, naringenin significantly decreased cellular ATP levels without increasing cell cytotoxicity. Together, these results suggest that the aglycone, naringenin, has a role in the attenuation of hyperglycemia and may exert this effect in a manner similar to the drug, metformin.

SIRT1 performs a balancing act on the tight-rope toward longevity.

Our recent study defined a new role for SIRT1 as a regulator of hepatic lipid metabolism. In the liver a major target of this sirtuin is the PPARalpha/PGC-1alpha signaling axis. Ablation of SIRT1 in the liver results in disrupted fatty acid oxidation, increased cellular stress, and elevations in proinflammatory cytokines. However, contrary to previous studies, we observed no changes in glucose production in the absence of SIRT1, despite impaired PGC-1alpha signaling. These findings point toward the involvement of other players in SIRT1-regulated hepatic metabolism. Here we discuss our findings, and comment on some of the controversy surrounding this protein in the current literature.

Conjugated linoleic acid induces uncoupling protein 1 in white adipose tissue of ob/ob mice.

Conjugated linoleic acid (CLA) reduces body weight and adipose mass in a variety of species. The mechanisms by which CLA depletes adipose mass are unclear, but two independent microarray analyses indicate that in white adipose tissue (WAT), uncoupling protein 1 (UCP1) was among genes most changed by CLA. The objective of this study was to determine whether CLA induces ectopic expression of UCP1 in WAT, which may contribute to increased energy expenditure and weight loss. Six-week old, male ob/ob mice were fed either a control diet (CON) or a diet supplemented with 1.5% mixed isomer CLA (CLA) for 4 weeks. A third group of mice (LEPTIN) was fed the control diet and received daily injections of recombinant leptin as a positive control for adipose depletion in ob/ob mice. CLA did not alter several mRNA markers of lipid oxidation in epididymal white adipose tissue (eWAT) , but significantly increased carnitine palmitoyltransferase-1b (CPT1b) and PPAR gamma coactivator-1alpha (PGC1alpha) expression. Notably, CLA increased both mRNA and protein expression of uncoupling protein-1 (UCP1). beta3-adrenoceptor mRNA and phosphorylated-p38 mitogen activated protein kinase (MAPK) protein levels were not affected by CLA, but were upregulated by LEPTIN. These data suggest the increased CPT1b, PGC1alpha, and UCP1, in WAT of CLA-fed mice may contribute to the depletion of adipose, and CLA does not appear to increase UCP1 through beta3-adrenergic signaling. Future studies will focus on understanding how CLA increases mitochondrial oxidation and energy dissipation in white adipose tissue.

Hepatocyte-specific deletion of SIRT1 alters fatty acid metabolism and results in hepatic steatosis and inflammation.

Hepatic metabolic derangements are key components in the development of fatty liver, insulin resistance, and atherosclerosis. SIRT1, a NAD+-dependent protein deacetylase, is an important regulator of energy homeostasis in response to nutrient availability. Here we demonstrate that hepatic SIRT1 regulates lipid homeostasis by positively regulating peroxisome proliferators-activated receptor alpha (PPARalpha), a nuclear receptor that mediates the adaptive response to fasting and starvation. Hepatocyte-specific deletion of SIRT1 impairs PPARalpha signaling and decreases fatty acid beta-oxidation, whereas overexpression of SIRT1 induces the expression of PPARalpha targets. SIRT1 interacts with PPARalpha and is required to activate PPARalpha coactivator PGC-1alpha. When challenged with a high-fat diet, liver-specific SIRT1 knockout mice develop hepatic steatosis, hepatic inflammation, and endoplasmic reticulum stress. Taken together, our data indicate that SIRT1 plays a vital role in the regulation of hepatic lipid homeostasis and that pharmacological activation of SIRT1 may be important for the prevention of obesity-associated metabolic diseases.

Conjugated linoleic acid fails to worsen insulin resistance but induces hepatic steatosis in the presence of leptin in ob/ob mice.

Conjugated linoleic acid (CLA) induces insulin resistance preceded by rapid depletion of the adipokines leptin and adiponectin, increased inflammation, and hepatic steatosis in mice. To determine the role of leptin in CLA-mediated insulin resistance and hepatic steatosis, recombinant leptin was coadministered with dietary CLA in ob/ob mice to control leptin levels and to, in effect, negate the leptin depletion effect of CLA. In a 2 x 2 factorial design, 6 week old male ob/ob mice were fed either a control diet or a diet supplemented with CLA and received daily intraperitoneal injections of either leptin or vehicle for 4 weeks. In the absence of leptin, CLA significantly depleted adiponectin and induced insulin resistance, but it did not increase hepatic triglyceride concentrations or adipose inflammation, marked by interleukin-6 and tumor necrosis factor-alpha mRNA expression. Insulin resistance, however, was accompanied by increased macrophage infiltration (F4/80 mRNA) in adipose tissue. In the presence of leptin, CLA depleted adiponectin but did not induce insulin resistance or macrophage infiltration. Despite this, CLA induced hepatic steatosis. In summary, CLA worsened insulin resistance without evidence of inflammation or hepatic steatosis in mice after 4 weeks. In the presence of leptin, CLA failed to worsen insulin resistance but induced hepatic steatosis in ob/ob mice.

Combined effects of rosiglitazone and conjugated linoleic acid on adiposity, insulin sensitivity, and hepatic steatosis in high-fat-fed mice.

Dysfunctional cross talk between adipose tissue and liver tissue results in metabolic and inflammatory disorders. As an insulin sensitizer, rosiglitazone (Rosi) improves insulin resistance yet causes increased adipose mass and weight gain in mice and humans. Conjugated linoleic acid (CLA) reduces adipose mass and body weight gain but induces hepatic steatosis in mice. We examined the combined effects of Rosi and CLA on adiposity, insulin sensitivity, and hepatic steatosis in high-fat-fed male C57Bl/6 mice. CLA alone suppressed weight gain and adipose mass but caused hepatic steatosis. Addition of Rosi attenuated CLA-induced insulin resistance and dysregulation of adipocytokines. In adipose, CLA significantly suppressed lipoprotein lipase and fatty acid translocase (FAT/CD36) mRNA, suggesting inhibition of fatty acid uptake into adipose; addition of Rosi completely rescued this effect. In addition, CLA alone increased markers of macrophage infiltration, F4/80, and CD68 mRNA levels, without inducing TNF-alpha in epididymal adipose tissue. The ratio of Bax to Bcl2, a marker of apoptosis, was significantly increased in adipose of the CLA-alone group and was partially prevented by treatment of Rosi. Immunohistochemistry of F4/80 demonstrates a proinflammatory response induced by CLA in epididymal adipose. In the liver, CLA alone induced microsteatotic liver but surprisingly increased the rate of very-low-density lipoprotein-triglyceride production without inducing inflammatory mediator-TNF-alpha and markers of macrophage infiltration. These changes were accompanied by significantly increased mRNA levels of stearoyl-CoA desaturase, FAT/CD36, and fatty acid synthase. The combined administration of CLA and Rosi reduced hepatic liver triglyceride content as well as lipogenic gene expression compared with CLA alone. In summary, dietary CLA prevented weight gain in Rosi-treated mice without attenuating the beneficial effects of Rosi on insulin sensitivity. Rosi ameliorated CLA-induced lipodystrophic disorders that occurred in parallel with rescued expression of adipocytokine and adipocytes-abundant genes.

Maintenance of adiponectin attenuates insulin resistance induced by dietary conjugated linoleic acid in mice.

Conjugated linoleic acid (CLA) causes insulin resistance and hepatic steatosis in conjunction with depletion of adipokines in some rodent models. Our objective was to determine whether the maintenance of adipokines, mainly leptin and adiponectin, by either removing CLA from diets or using an adiponectin enhancer, rosiglitazone (ROSI), could attenuate CLA-induced insulin resistance. Male C57BL/6 mice were consecutively fed two experimental diets containing 1.5% CLA mixed isomer for 4 weeks followed by a diet without CLA for 4 weeks. CLA significantly depleted adiponectin but not leptin and was accompanied by hepatic steatosis and insulin resistance. These effects were attenuated after switching mice to the diet without CLA along with restoration of adiponectin. To further elucidate the role of adiponectin in CLA-mediated insulin resistance, ROSI was used in a subsequent study in male ob/ob mice fed either control (CON) or CLA diet. ROSI maintained significantly higher adiponectin levels in CON- and CLA-fed mice and prevented the depletion of epididymal adipose tissue and the development of insulin resistance. In conclusion, we show that insulin resistance induced by CLA may be related more to adiponectin depletion than to leptin and that maintaining adiponectin levels alone either by removing CLA or using ROSI can attenuate these effects.