SUMMARY 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 PPARα, a nuclear receptor that mediates the adaptive response to fasting and starvation. Hepatocyte-specific deletion of SIRT1 impairs PPARα signaling and decreases fatty acid β-oxidation, whereas overexpression of SIRT1 induces the expression of PPARα targets. SIRT1 interacts with PPARα and is required to activate PPARα co-activator PGC-1α. 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.

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Peroxisome proliferator-activated receptor ␣ (PPAR␣) plays a central role in the cell-specific pleiotropic responses induced by structurally diverse synthetic chemicals designated as peroxisome proliferators. Transcriptional regulation by liganded nuclear receptors involves the participation of cofactors that form multiprotein complexes to achieve cell-and gene-specific transcription. Here we report the identification of such a transcriptionally active PPAR␣-interacting cofactor (PRIC) complex from rat liver nuclear extracts that interacts with full-length PPAR␣ in the presence of ciprofibrate, a synthetic ligand, and leukotriene B 4, a natural ligand. The liganded PPAR␣-PRIC complex enhanced transcription from a peroxisomal enoyl-CoA hydratase͞L-3-hydroxyacyl-CoA dehydrogenase bifunctional enzyme gene promoter template that contains peroxisome proliferator response elements. Rat liver PRIC complex comprises some 25 polypeptides, and their identities were established by mass spectrometry and limited sequence analysis. Eighteen of these peptides contain one or more LXXLL motifs necessary for interacting with nuclear receptors. PRIC complex includes known coactivators or coactivator-binding proteins (CBP, SRC-1, PBP, PRIP, PIMT, TRAP100, SUR-2, and PGC-1), other proteins that have not previously been described in association with transcription complexes (CHD5, TOG, and MORF), and a few novel polypeptides designated PRIC300, -285, -215, -177, and -145. We describe the cDNA for PRIC285, which contains five LXXLL motifs. It interacts with PPAR␣ and acts as a coactivator by moderately stimulating PPAR␣-mediated transcription in transfected cells. We conclude that liganded PPAR␣ recruits a distinctive multiprotein complex from rat liver nuclear extracts. The composition of this complex may provide insight into the basis of tissue and species sensitivity to peroxisome proliferators.

8-oxoguanine DNA glycosylase (Oggl) repairs 8-oxo-7,8-dihydroxyguanine (8-oxoG), one of the most abundant DNA adducts caused by oxidative stress. In the mitochondria, Oggl is thought to prevent activation of the intrinsic apoptotic pathway in response to oxidative stress by augmenting DNA repair. However, the predominance of the β-Oggl isoform, which lacks 8-oxoG DNA glycosylase activity, suggests that mitochondrial Oggl functions in a role independent of DNA repair. We report here that overexpression of mitochondria-targeted human α-hOggl (mt-hOggl) in human lung adenocarcinoma cells with some alveolar epithelial cell characteristics (A549 cells) prevents oxidant-induced mitochondrial dysfunction and apoptosis by preserving mitochondrial aconitase. Importantly, mitochondrial α-hOggl mutants lacking 8-oxoG DNA repair activity were as effective as wild-type mt-hOggl in preventing oxidant-induced caspase-9 activation, reductions in mitochondrial aconitase and apoptosis suggesting that the protective effects of mt-hOgg 1 occur independent of DNA repair. Notably, wild-type and mutant mt-hOggl co-precipitate with mitochondrial aconitase. Furthermore, overexpression of mitochondrial aconitase abolishes oxidant-induced apoptosis whereas hOggl silencing using shRNA reduces mitochondrial aconitase and augments apoptosis. These findings suggest a novel mechanism that mt-hOggl acts as a mitochondrial aconitase chaperone protein to prevent oxidant-mediated mitochondrial dysfunction and apoptosis that might be important in the molecular events underlying oxidant-induced toxicity.