The SIRT1 deacetylase inhibits fat synthesis and stimulates fat oxidation in response to fasting, but the underlying mechanisms remain unclear. Here we report that SREBP-1c, a key lipogenic activator, is an in vivo target of SIRT1. SIRT1 interaction with SREBP-1c was increased during fasting and decreased upon feeding, and consistently, SREBP-1c acetylation levels were decreased during fasting in mouse liver. Acetylated SREBP-1c levels were also increased in HepG2 cells treated with insulin and glucose to mimic feeding conditions, and down-regulation of p300 by siRNA decreased the acetylation. Depletion of hepatic SIRT1 by adenoviral siRNA increased acetylation of SREBP-1c with increased lipogenic gene expression. Tandem mass spectrometry and mutagenesis studies revealed that SREBP-1c is acetylated by p300 at Lys-289 and Lys-309. Mechanistic studies using acetylation-defective mutants showed that SIRT1 deacetylates and inhibits SREBP-1c transactivation by decreasing its stability and its occupancy at the lipogenic genes. Remarkably, SREBP-1c acetylation levels were elevated in dietinduced obese mice, and hepatic overexpression of SIRT1 or treatment with resveratrol, a SIRT1 activator, daily for 1 week decreased acetylated SREBP-1c levels with beneficial functional outcomes. These results demonstrate an intriguing connection between elevated SREBP-1c acetylation and increased lipogenic gene expression, suggesting that abnormally elevated SREBP-1c acetylation increases SREBP-1c lipogenic activity in obese mice. Reducing acetylation of SREBP-1c by targeting SIRT1 may be useful for treating metabolic disorders, including fatty liver, obesity, and type II diabetes.The NAD ϩ -dependent SIRT1 (sirtuin 1) deacetylase plays a critical role in cellular metabolism, stress responses, and possibly aging by modulating the activity of transcription factors and cofactors by protein deacetylation (1-4). In response to low nutritional availability, SIRT1 functions as a master switch to maintain lipid and glucose homeostasis and energy balance by regulating important metabolic regulators, such as PGC-1␣ (PPAR␥ coactivator ␣), Foxo-1, and liver X receptor (1, 5-7). We recently identified the nuclear bile acid receptor, farnesoid X receptor (FXR), 3 as an important in vivo target of SIRT1 in the regulation of hepatic lipid metabolism (8). Of these reported regulators, the function of SIRT1 in deacetylating and enhancing the activity of PGC-1␣ has been well established (1,5,9,10).

Summary The nuclear bile acid receptor FXR is critical for regulation of lipid and glucose metabolism. Here we report that FXR is a target of SIRT1, a deacetylase that mediates nutritional and hormonal modulation of hepatic metabolism. Lysine 217 of FXR is the major acetylation site targeted by p300 and SIRT1. Acetylation of FXR increases its stability but inhibits heterodimerization with RXRα, DNA binding, and transactivation activity. Down-regulation of hepatic SIRT1 increased FXR acetylation with deleterious metabolic outcomes. Surprisingly, in mouse models of metabolic disease, FXR interaction with SIRT1 and p300 was dramatically altered, FXR acetylation levels were elevated, and overexpression of SIRT1 or resveratrol treatment reduced acetylated FXR levels. Our data demonstrate that FXR acetylation is normally dynamically regulated by p300 and SIRT1 but is constitutively elevated in metabolic disease states. Small molecules that inhibit FXR acetylation by targeting SIRT1 or p300 may be promising therapeutic agents for metabolic disorders.

FOXO1 orchestrates wound healing through the up-regulation of TGF-β1 and protection against oxidative stress, which together act to promote keratinocyte migration and decrease apoptosis.

Diabetes is a chronic metabolic disorder, characterized by hyperglycemia resulting from insulin deficiency and/or insulin resistance. Recent evidence suggests that high levels of reactive oxygen species (ROS) and subsequent oxidative stress are key contributors in the development of diabetic complications. The FOXO family of forkhead transcription factors including FOXO1, FOXO3, FOXO4, and FOXO6 play important roles in the regulation of many cellular and biological processes and are critical regulators of cellular oxidative stress response pathways. FOXO1 transcription factors can affect a number of different tissues including liver, retina, bone, and cell types ranging from hepatocytes to microvascular endothelial cells and pericytes to osteoblasts. They are induced by oxidative stress and contribute to ROS-induced cell damage and apoptosis. In this paper, we discuss the role of FOXO transcription factors in mediating oxidative stress-induced cellular response.

SHP has been implicated as a pleiotropic regulator of diverse biological functions by its ability to inhibit numerous nuclear receptors. Recently, we reported that SHP inhibits transcription of CYP7A1, a key gene in bile acid biosynthesis, by recruiting histone deacetylases (HDACs) and a Swi/Snf-Brm complex. To further delineate the mechanism of this inhibition, we have examined whether methylation of histones is also involved and whether a functional interplay between chromatin-modifying enzymes occurs. The histone methyltransferase G9a, but not SUV39, was colocalized with SHP in the nucleus and directly interacted with SHP in vitro. G9a, which was coimmunoprecipitated with hepatic SHP, methylated Lys-9 of histone 3 (H3K9) in vitro. Expression of G9a enhanced inhibition of CYP7A1 transcription by SHP, while a catalytically inactive G9a dominant negative (DN) mutant reversed the SHP inhibition. G9a was recruited to and H3K9 was methylated at the CYP7A1 promoter in a SHP-dependent manner in bile acid-treated HepG2 cells. Expression of the G9a-DN mutant inhibited H3K9 methylation, blocked the recruitment of the Brm complex, and partially reversed CYP7A1 inhibition by bile acids. Inhibition of HDAC activity with trichostatin A blocked deacetylation and methylation of H3K9 at the promoter, and, conversely, inhibition of H3K9 methylation by G9a-DN partially blocked deacetylation. Hepatic expression of G9a-DN in mice fed cholic acid disrupted bile acid homeostasis, resulting in increased bile acid pools and partial de-repression of Cyp7a1 and Cyp8b1. Our studies establish a critical role for G9a methyltransferase, histone deacetylases, and the Swi/Snf-Brm complex in the SHP-mediated inhibition of hepatic bile acid synthesis via coordinated chromatin modification at target genes.The small heterodimer partner (SHP, or NR0B2), an unusual orphan nuclear receptor that lacks a DNA binding domain but contains a putative ligand binding domain, interacts with a number of nuclear receptors, including liver receptor homologue 1, hepatic nuclear factor 4 (HNF-4), estrogen-related receptor, constitutive androstane receptor, liver X receptor, glucocorticoid receptor, and estrogen receptor, and inhibits their transcriptional activities (1,4,7,30,39,40). Thus, SHP acts as a pleiotropic transcriptional repressor affecting diverse biological functions, including cholesterol and glucose metabolic pathways, energy homeostasis, and reproductive biology (2). SHP has been shown to be a key regulator in the negative feedback regulation of bile acid biosynthesis from cholesterol in the liver (15,25,31,48). Bile acid-activated farnesoid X receptor (FXR), upon heterodimerization with retinoid X receptor, binds to the promoter of the SHP gene and increases transcription (15,31). The bile acid-induced SHP then suppresses transcription of CYP7A1 and CYP8B1, key hepatic genes involved in the neutral pathway of bile acid biosynthesis, by interacting with liver receptor homologue 1 and/or HNF-4, which are bound to the promoters of the genes. In addit...