Tarek Moustafa

ATGL-mediated fat catabolism regulates cardiac mitochondrial function via PPAR-[alpha] and PGC-1

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that regulate genes involved in energy metabolism and inflammation. For biological activity, PPARs require cognate lipid ligands, heterodimerization with retinoic X receptors, and coactivation by PPAR-γ coactivator-1α or PPAR-γ coactivator-1β (PGC-1α or PGC-1β, encoded by Ppargc1a and Ppargc1b, respectively). Here we show that lipolysis of cellular triglycerides by adipose triglyceride lipase (patatin-like phospholipase domain containing protein 2, encoded by Pnpla2; hereafter referred to as Atgl) generates essential mediator(s) involved in the generation of lipid ligands for PPAR activation. Atgl deficiency in mice decreases mRNA levels of PPAR-α and PPAR-δ target genes. In the heart, this leads to decreased PGC-1α and PGC-1β expression and severely disrupted mitochondrial substrate oxidation and respiration; this is followed by excessive lipid accumulation, cardiac insufficiency and lethal cardiomyopathy. Reconstituting normal PPAR target gene expression by pharmacological treatment of Atgl-deficient mice with PPAR-α agonists completely reverses the mitochondrial defects, restores normal heart function and prevents premature death. These findings reveal a potential treatment for the excessive cardiac lipid accumulation and often-lethal cardiomyopathy in people with neutral lipid storage disease, a disease marked by reduced or absent ATGL activity.

Bile Acids as Regulators of Hepatic Lipid and Glucose Metabolism

Besides their well-established roles in dietary lipid absorption and cholesterol homeostasis, bile acids (BA) also act as metabolically active signaling molecules. The flux of reabsorbed BA undergoing enterohepatic circulation, arriving in the liver with the co-absorbed nutrients (e.g. glucose, lipids), provides a signal that coordinates hepatic triglyceride (TG), glucose and energy homeostasis. As signaling molecules with systemic endocrine functions, BA can activate protein kinases A and C as well as mitogen-activated protein kinase pathways. Additionally, they are ligands for a G-protein-coupled BA receptor (TGR5/Gpbar-1) and activate nuclear receptors such as farnesoid X receptor (FXR; NR1H4). FXR and its downstream targets play a key role in the control of hepatic de novo lipogenesis, very-low-density lipoprotein-TG export and plasma TG turnover. BA-activated FXR and signal transduction pathways are also involved in the regulation of hepatic gluconeogenesis, glycogen synthesis and insulin sensitivity. Via TGR5, BA are able to stimulate glucagon-like peptide-1 secretion in the small intestine and energy expenditure in brown adipose tissue and skeletal muscle. Dysregulation of BA transport and impaired BA receptor signaling may contribute to the pathogenesis of non-alcoholic fatty liver disease (NAFLD). Thus, BA transport and BA-controlled nuclear receptors and signaling pathways are promising drug targets for treatment of NAFLD. As such, FXR and/or TGR5 ligands have shown promising results in animal models of NAFLD and clinical pilot studies. Despite being a poor FXR and TGR5 ligand, ursodeoxycholic acid (UDCA) improves hepatic ER stress and insulin sensitivity. Notably, norUDCA, a side chain-shortened homologue of UDCA, improves fatty liver and atherosclerosis in Western diet-fed ApoE–/– mice. Collectively, these findings suggest that BA and targeting their receptor/signaling pathways may represent a promising approach to treat NAFLD and closely linked disorders such as obesity, diabetes, dyslipidemia and arteriosclerosis.

Selective Activation of Nuclear Bile Acid Receptor FXR in the Intestine Protects Mice Against Cholestasis

BACKGROUND & AIMS Cholestasis is a liver disorder characterized by impaired bile flow, reduction of bile acids (BAs) in the intestine, and retention of BAs in the liver. The farnesoid X receptor (FXR) is the transcriptional regulator of BA homeostasis. Activation of FXR by BAs reduces circulating BA levels in a feedback mechanism, repressing hepatic cholesterol 7α-hydroxylase (Cyp7a1), the rate-limiting enzyme for the conversion of cholesterol to BAs. This mechanism involves the hepatic nuclear receptor small heterodimer partner and the intestinal fibroblast growth factor (FGF) 19 and 15. We investigated the role of activation of intestine-specific FXR in reducing hepatic levels of BAs and protecting the liver from cholestasis in mice. METHODS We generated transgenic mice that express a constitutively active FXR in the intestine. Using FXR gain- and loss-of-function models, we studied the roles of intestinal FXR in mice with intrahepatic and extrahepatic cholestasis. RESULTS Selective activation of intestinal FXR induced FGF15 and repressed hepatic Cyp7a1, reducing the pool size of BAs and changing the BA pool composition. Activation of intestinal FXR protected mice from obstructive extrahepatic cholestasis after bile duct ligation or administration of α-naphthylisothiocyanate. In Mdr2(-/-) mice, transgenic expression of activated FXR in the intestine protected against liver damage, whereas absence of FXR promoted progression of liver disease. CONCLUSIONS Activation of FXR transcription in the intestine protects the liver from cholestasis in mice by inducing FGF15 expression and reducing the hepatic pool of BA; this approach might be developed to reverse cholestasis in patients.

Nucleocytosolic Depletion of the Energy Metabolite Acetyl-Coenzyme A Stimulates Autophagy and Prolongs Lifespan

Summary Healthy aging depends on removal of damaged cellular material that is in part mediated by autophagy. The nutritional status of cells affects both aging and autophagy through as-yet-elusive metabolic circuitries. Here, we show that nucleocytosolic acetyl-coenzyme A (AcCoA) production is a metabolic repressor of autophagy during aging in yeast. Blocking the mitochondrial route to AcCoA by deletion of the CoA-transferase ACH1 caused cytosolic accumulation of the AcCoA precursor acetate. This led to hyperactivation of nucleocytosolic AcCoA-synthetase Acs2p, triggering histone acetylation, repression of autophagy genes, and an age-dependent defect in autophagic flux, culminating in a reduced lifespan. Inhibition of nutrient signaling failed to restore, while simultaneous knockdown of ACS2 reinstated, autophagy and survival of ach1 mutant. Brain-specific knockdown of Drosophila AcCoA synthetase was sufficient to enhance autophagic protein clearance and prolong lifespan. Since AcCoA integrates various nutrition pathways, our findings may explain diet-dependent lifespan and autophagy regulation.

Acetylation dynamics and stoichiometry in Saccharomyces cerevisiae.

Lysine acetylation is a frequently occurring posttranslational modification; however, little is known about the origin and regulation of most sites. Here we used quantitative mass spectrometry to analyze acetylation dynamics and stoichiometry in Saccharomyces cerevisiae. We found that acetylation accumulated in growth‐arrested cells in a manner that depended on acetyl‐CoA generation in distinct subcellular compartments. Mitochondrial acetylation levels correlated with acetyl‐CoA concentration in vivo and acetyl‐CoA acetylated lysine residues nonenzymatically in vitro. We developed a method to estimate acetylation stoichiometry and found that the vast majority of mitochondrial and cytoplasmic acetylation had a very low stoichiometry. However, mitochondrial acetylation occurred at a significantly higher basal level than cytoplasmic acetylation, consistent with the distinct acetylation dynamics and higher acetyl‐CoA concentration in mitochondria. High stoichiometry acetylation occurred mostly on histones, proteins present in histone acetyltransferase and deacetylase complexes, and on transcription factors. These data show that a majority of acetylation occurs at very low levels in exponentially growing yeast and is uniformly affected by exposure to acetyl‐CoA.

Cardioprotection and lifespan extension by the natural polyamine spermidine

Aging is associated with an increased risk of cardiovascular disease and death. Here we show that oral supplementation of the natural polyamine spermidine extends the lifespan of mice and exerts cardioprotective effects, reducing cardiac hypertrophy and preserving diastolic function in old mice. Spermidine feeding enhanced cardiac autophagy, mitophagy and mitochondrial respiration, and it also improved the mechano-elastical properties of cardiomyocytes in vivo, coinciding with increased titin phosphorylation and suppressed subclinical inflammation. Spermidine feeding failed to provide cardioprotection in mice that lack the autophagy-related protein Atg5 in cardiomyocytes. In Dahl salt-sensitive rats that were fed a high-salt diet, a model for hypertension-induced congestive heart failure, spermidine feeding reduced systemic blood pressure, increased titin phosphorylation and prevented cardiac hypertrophy and a decline in diastolic function, thus delaying the progression to heart failure. In humans, high levels of dietary spermidine, as assessed from food questionnaires, correlated with reduced blood pressure and a lower incidence of cardiovascular disease. Our results suggest a new and feasible strategy for protection against cardiovascular disease.

Side Chain Structure Determines Unique Physiologic and Therapeutic Properties of norUrsodeoxycholic Acid in Mdr2−/− Mice

24‐norursodeoxycholic acid (norUDCA), a side chain–modified ursodeoxycholic acid derivative, has dramatic therapeutic effects in experimental cholestasis and may be a promising agent for the treatment of cholestatic liver diseases. We aimed to better understand the physiologic and therapeutic properties of norUDCA and to test if they are related to its side chain length and/or relative resistance to amidation. For this purpose, Mdr2−/− mice, a model for sclerosing cholangitis, received either a standard diet or a norUDCA‐, tauro norursodeoxycholic acid (tauro‐ norUDCA)‐, or di norursodeoxycholic acid (di norUDCA)‐enriched diet. Bile composition, serum biochemistry, liver histology, fibrosis, and expression of key detoxification and transport systems were investigated. Direct choleretic effects were addressed in isolated bile duct units. The role of Cftr for norUDCA‐induced choleresis was explored in Cftr−/− mice. norUDCA had pharmacologic features that were not shared by its derivatives, including the increase in hepatic and serum bile acid levels and a strong stimulation of biliary HCO3− ‐output. norUDCA directly stimulated fluid secretion in isolated bile duct units in a HCO3− ‐dependent fashion to a higher extent than the other bile acids. Notably, the norUDCA significantly stimulated HCO 3− ‐output also in Cftr−/− mice. In Mdr2−/− mice, cholangitis and fibrosis strongly improved with norUDCA, remained unchanged with tauro‐ norUDCA, and worsened with di norUDCA. Expression of Mrp4, Cyp2b10, and Sult2a1 was increased by norUDCA and di norUDCA, but was unaffected by tauro‐ norUDCA. Conclusion:The relative resistance of norUDCA to amidation may explain its unique physiologic and pharmacologic properties. These include the ability to undergo cholehepatic shunting and to directly stimulate cholangiocyte secretion, both resulting in a HCO3− ‐rich hypercholeresis that protects the liver from cholestatic injury. (HEPATOLOGY 2009;49:1972–1981.)

Farnesoid X Receptor Critically Determines the Fibrotic Response in Mice but Is Expressed to a Low Extent in Human Hepatic Stellate Cells and Periductal Myofibroblasts

The nuclear bile acid receptor, farnesoid X receptor (FXR), may play a pivotal role in liver fibrosis. We tested the impact of genetic FXR ablation in four different mouse models. Hepatic fibrosis was induced in wild-type and FXR knock-out mice (FXR(-/-)) by CCl(4) intoxication, 3,5-diethoxycarbonyl-1,4-dihydrocollidine feeding, common bile duct ligation, or Schistosoma mansoni (S.m.)-infection. In addition, we determined nuclear receptor expression levels (FXR, pregnane X receptor (PXR), vitamin D receptor, constitutive androstane receptor (CAR), small heterodimer partner (SHP)) in mouse hepatic stellate cells (HSCs), portal myofibroblasts (MFBs), and human HSCs. Cell type-specific FXR protein expression was determined by immunohistochemistry in five mouse models and prototypic human fibrotic liver diseases. Expression of nuclear receptors was much lower in mouse and human HSCs/MFBs compared with total liver expression with the exception of vitamin D receptor. FXR protein was undetectable in mouse and human HSCs and MFBs. FXR loss had no effect in CCl(4)-intoxicated and S.m.-infected mice, but significantly decreased liver fibrosis of the biliary type (common bile duct ligation, 3,5-diethoxycarbonyl-1,4-dihydrocollidine). These data suggest that FXR loss significantly reduces fibrosis of the biliary type, but has no impact on non-cholestatic liver fibrosis. Since there is no FXR expression in HSCs and MFBs in liver fibrosis, our data indicate that these cells may not represent direct therapeutic targets for FXR ligands.

Alterations in Lipid Metabolism Mediate Inflammation, Fibrosis, and Proliferation in a Mouse Model of Chronic Cholestatic Liver Injury

BACKGROUND & AIMS The liver controls central processes of lipid and bile acid homeostasis. We aimed to investigate whether alterations in lipid metabolism contribute to the pathogenesis of chronic cholestatic liver disease in mice. METHODS We used microarray and metabolic profiling analyses to identify alterations in systemic and hepatic lipid metabolism in mice with disruption of the gene ATP-binding cassette sub-family B member 4 (Abcb4(-/-) mice), a model of inflammation-induced cholestatic liver injury, fibrosis, and cancer. RESULTS Alterations in Abcb4(-/-) mice, compared with wild-type mice, included deregulation of genes that control lipid synthesis, storage, and oxidation; decreased serum levels of cholesterol and phospholipids; and reduced hepatic long-chain fatty acyl-CoAs (LCA-CoA). Feeding Abcb4(-/-) mice the side chain-modified bile acid 24-norursodeoxycholic acid (norUDCA) reversed their liver injury and fibrosis, increased serum levels of lipids, lowered phospholipase and triglyceride hydrolase activities, and restored hepatic LCA-CoA and triglyceride levels. Additional genetic and nutritional studies indicated that lipid metabolism contributed to chronic cholestatic liver injury; crossing peroxisome proliferator-activated receptor (PPAR)-α-deficient mice with Abcb4(-/-) mice (to create double knockouts) or placing Abcb4(-/-) mice on a high-fat diet protected against liver injury, with features similar to those involved in the response to norUDCA. Placing pregnant Abcb4(-/-) mice on high-fat diets prevented liver injury in their offspring. However, fenofibrate, an activator of PPARα, aggravated liver injury in Abcb4(-/-) mice. CONCLUSIONS Alterations in lipid metabolism contribute to the pathogenesis and progression of cholestatic liver disease in mice.

Analysis of acetylation stoichiometry suggests that SIRT3 repairs nonenzymatic acetylation lesions

Acetylation is frequently detected on mitochondrial enzymes, and the sirtuin deacetylase SIRT3 is thought to regulate metabolism by deacetylating mitochondrial proteins. However, the stoichiometry of acetylation has not been studied and is important for understanding whether SIRT3 regulates or suppresses acetylation. Using quantitative mass spectrometry, we measured acetylation stoichiometry in mouse liver tissue and found that SIRT3 suppressed acetylation to a very low stoichiometry at its target sites. By examining acetylation changes in the liver, heart, brain, and brown adipose tissue of fasted mice, we found that SIRT3‐targeted sites were mostly unaffected by fasting, a dietary manipulation that is thought to regulate metabolism through SIRT3‐dependent deacetylation. Globally increased mitochondrial acetylation in fasted liver tissue, higher stoichiometry at mitochondrial acetylation sites, and greater sensitivity of SIRT3‐targeted sites to chemical acetylation in vitro and fasting‐induced acetylation in vivo, suggest a nonenzymatic mechanism of acetylation. Our data indicate that most mitochondrial acetylation occurs as a low‐level nonenzymatic protein lesion and that SIRT3 functions as a protein repair factor that removes acetylation lesions from lysine residues.