Hepatic Deletion of SIRT1 Decreases Hepatocyte Nuclear Factor 1 /Farnesoid X Receptor Signaling and Induces Formation of Cholesterol Gallstones in Mice

Article (PDF Available)inMolecular and Cellular Biology 32(7):1226-36 · April 2012with36 Reads
DOI: 10.1128/MCB.05988-11 · Source: PubMed
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.

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Available from: Dong-hyun Kim, Mar 31, 2015
    • "Sirt1-LKO mice have lower expression of FXR in liver and Fgf15 in ileum than WT mice (Fig 5). This is consistent with a previous report of decreased FXR expression with Sirt1 deficiency in liver [32]. The CR-induced Cyp7a1 alterations in these genotypes of mice (Fig 4A) do not correlate with the negative regulators in FXR signaling (SHP in liver and Fgf15 in ileum) or the positive regulators (HNF4α and LRH-1 in liver) (Fig 5). "
    [Show abstract] [Hide abstract] ABSTRACT: Sirtuin 1 (Sirt1) is an NAD+-dependent protein deacetylase that is proposed to mediate many health-promoting effects of calorie restriction (CR). We recently reported that short-term CR increased the bile acid (BA) pool size in mice, likely due to increased BA synthesis in liver. Given the important role of Sirt1 in the regulation of glucose, lipid, as well as BA metabolism, we hypothesized that the CR-induced increase in BAs is Sirt1-dependent. To address this, the present study utilized genetically-modified mice that were Sirt1 loss of function (liver knockout, LKO) or Sirt1 gain of function (whole body-transgenic, TG). Three genotypes of mice (Sirt1-LKO, wild-type, and Sirt1-TG) were each randomly divided into ad libitum or 40% CR feeding for one month. BAs were extracted from various compartments of the enterohepatic circulation, followed by BA profiling by UPLC-MS/MS. CR increased the BA pool size and total BAs in serum, gallbladder, and small intestine. The CR-induced increase in BA pool size correlated with the tendency of increase in the expression of the rate-limiting BA-synthetic enzyme Cyp7a1. However, in contrast to the hypothesis, the CR-induced increase in BA pool size and Cyp7a1 expression was still observed with ablated expression of Sirt1 in liver, and completely suppressed with whole-body overexpression of Sirt1. Furthermore, in terms of BA composition, CR increased the ratio of 12α-hydroxylated BAs regardless of Sirt1 genotypes. In conclusion, the CR-induced alterations in BA pool size, BA profiles, and expression of BA-related genes do not appear to be dependent on Sirt1.
    Full-text · Article · Sep 2015
    • "SIRT1 deletion in mouse pancreatic beta cells disrupts glucose sensing, impairing insulin secretion [4]. Deletion in the liver prompts hepatic steatosis [5, 6] and formation of cholesterol gallstones [7]. Endothelial SIRT1 deficiency induces vascular senescence, producing nephro- sclerosis [8] . "
    [Show abstract] [Hide abstract] ABSTRACT: SIRT1 is central to the lifespan and vascular health, but undergoes degradation that contributes to several medical conditions, including diabetes. How SIRT1 turnover is regulated remains unclear. However, emerging evidence suggests that endothelial nitric oxide synthase (eNOS) positively regulates SIRT1 protein expression. We recently identified NO as an endogenous inhibitor of 26S proteasome functionality with a cellular reporter system. Here we extended this finding to a novel pathway that regulates SIRT1 protein breakdown. In cycloheximide (CHX)-treated endothelial cells, NONOate, an NO donor, and A23187, an eNOS activator, significantly stabilized SIRT1 protein. Similarly, NO enhanced SIRT1 protein, but not mRNA expression, in CHX-free cells. NO also stabilized an autophagy-related protein unc-51 like kinase (ULK1), but did not restore SIRT1 protein levels in ULK1-siRNA-treated cells or in mouse embryonic fibroblasts (MEF) from Ulk1-/- mice. This suggests that ULK1 mediated the NO regulation of SIRT1. Furthermore, adenoviral overexpression of ULK1 increased SIRT1 protein expression, while ULK1 siRNA treatment decreased it. Rapamycin-induced autophagy did not mimic these effects, suggesting that the effects of ULK1 were autophagy-independent. Treatment with MG132, a proteasome inhibitor, or siRNA of β-TrCP1, an E3 ligase, prevented SIRT1 reduction induced by ULK1-siRNA. Mechanistically, ULK1 negatively regulated 26S proteasome functionality, which was at least partly mediated by O-linked-GlcNAc transferase (OGT), probably by increased O-GlcNAc modification of proteasomal subunit Rpt2. The NO-ULK1-SIRT1 axis was likely operative in the whole animal: both ULK1 and SIRT1 protein levels were significantly reduced in tissue homogenates in eNOS-knockout mice (lung) and in db/db mice where eNOS is downregulated (lung and heart). Taken together, the results show that NO stabilizes SIRT1 by regulating 26S proteasome functionality through ULK1 and OGT, but not autophagy, in endothelial cells.
    Full-text · Article · Dec 2014
    • "We have shown that acetylation of FXR is tightly regulated by p300 and SIRT1 under physiological conditions, which is important for dynamic transcriptional regulation, but that in obesity, SIRT1 levels and activity are low, resulting in persistently elevated acetylation of FXR (Kemper et al, 2009 ). Further, FXR acetylation was elevated in liver-specific SIRT1-KO mice (Purushotham et al, 2012), and conversely, acetylation of FXR as well as that of SREBP-1c, PGC-1a, and NF-jB was decreased by overexpression of SIRT1 (Kemper et al, 2009; Ponugoti et al, 2010; Choi et al, 2013). Intriguingly, expression of the corepressor complex subunits, GPS2 and SMRT, was reduced in adipocytes from obese individuals, and their expression was restored by weight loss after gastric bypass surgery (Toubal et al, 2013 ). "
    [Show abstract] [Hide abstract] ABSTRACT: Acetylation of transcriptional regulators is normally dynamically regulated by nutrient status but is often persistently elevated in nutrient-excessive obesity conditions. We investigated the functional consequences of such aberrantly elevated acetylation of the nuclear receptor FXR as a model. Proteomic studies identified K217 as the FXR acetylation site in diet-induced obese mice. In vivo studies utilizing acetylation-mimic and acetylation-defective K217 mutants and gene expression profiling revealed that FXR acetylation increased proinflammatory gene expression, macrophage infiltration, and liver cytokine and triglyceride levels, impaired insulin signaling, and increased glucose intolerance. Mechanistically, acetylation of FXR blocked its interaction with the SUMO ligase PIASy and inhibited SUMO2 modification at K277, resulting in activation of inflammatory genes. SUMOylation of agonist-activated FXR increased its interaction with NF-κB but blocked that with RXRα, so that SUMO2-modified FXR was selectively recruited to and trans-repressed inflammatory genes without affecting FXR/RXRα target genes. A dysregulated acetyl/SUMO switch of FXR in obesity may serve as a general mechanism for diminished anti-inflammatory response of other transcriptional regulators and provide potential therapeutic and diagnostic targets for obesity-related metabolic disorders. © 2014 The Authors.
    Full-text · Article · Nov 2014
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