The therapeutic role of NAD+ modulation in fatty liver disease

NAFLD encompasses a disease spectrum ranging from hepatic steatosis to nonalcoholic steatohepatitis (NASH), fibrosis, and finally cirrhosis, which predisposes to the development of hepatocellular carcinoma. With no approved pharmacological treatment, NAFLD is currently the most common etiology of chronic liver disease in western countries and its prevalence continues to growth concurrent to the rising obesity epidemic. SIRT1 may prove to be a critical target for the treatment of NAFLD since it has been demonstrated to induce transcription of genes involved in mitochondrial metabolism and antioxidant protection, while also inhibiting inflammatory and death signaling. In my PhD thesis projects, we aimed to explore the effect of SIRT1 activation through NAD+ modulation in the treatment of NAFLD. Protection against NAFLD by NAD+ repletion. In this study we evaluated the potential of Nicotinamide Riboside (NR) as a therapeutic agent against the development of NAFLD. First, we observed a positive correlation between gene sets for Î²-oxidation and genes involved in NAD+ biosynthesis in two distinct data sets of healthy human liver samples supporting the function of NAD+ as a coenzyme in key metabolic processes. Then we showed that NAD+ repletion using NR, an NAD+ booster, attenuated mitochondrial dysfunction and prevented and reversed fatty liver development in a several mouse models of NAFLD. Inhibiting PARPs protects against NAFLD development. We hypothesized that overactivation of PARP proteins, NAD+ consuming enzymes that are involved in various cell stress responses, promotes NAD+ depletion in NAFLD. To evaluate this hypothesis, we tested the therapeutic effect of a PARP inhibitor, olaparib in two mouse models of NAFLD. We found that PARP inhibition repletes NAD+ levels, protects against NAFLD and reduces ER-stress and fibrosis. The beneficial effect of PARP was dependent on Sirt1. In summary, our studies demonstrated a major role for NAD+ levels and PARP activity in the development of NAFLD. Repletion of NAD+, by NR administration, or PARP inhibition may therefore represent attractive strategies to counteract the development of NAFLD.

Inhibiting poly ADP-ribosylation increases fatty acid oxidation and protects against fatty liver disease

Johan Auwerx, Karim Laurent Gariani, Pooja Jha, Elena Katsyuba, Jun Yong Kim, Vera Monica Lemos Da Silva, Keir Joe Menzies, Alessia Perino, Dongryeol Ryu, Kristina Schoonjans, Matthias Alexander Sokrates Stein, Hongbo Zhang

Background & Aims: To date, no pharmacological therapy has been approved for non-alcoholic fatty liver disease (NAFLD). The aim of the present study was to evaluate the therapeutic potential of poly ADP-ribose polymerase (PARP) inhibitors in mouse models of NAFLD. Methods: As poly ADP-ribosylation (PARylation) of proteins by PARPs consumes nicotinamide adenine dinucleotide (NAD+), we hypothesized that overactivation of PARPs drives NAD+ depletion in NAFLD. Therefore, we assessed the effectiveness of PARP inhibition to replenish NAD+ and activate NAD(+)-dependent sirtuins, hence improving hepatic fatty acid oxidation. To do this, we examined the preventive and therapeutic benefits of the PARP inhibitor (PARPi), olaparib, in different models of NAFLD. Results: The induction of NAFLD in C57BL/6J mice using a high fat high-sucrose (HFHS)-diet increased PARylation of proteins by PARPs. As such, increased PARylation was associated with reduced NAD+ levels and mitochondrial function and content, which was concurrent with elevated hepatic lipid content. HFHS diet supplemented with PARPi reversed NAFLD through repletion of NAD+, increasing mitochondrial biogenesis and beta-oxidation in liver. Furthermore, PARPi reduced reactive oxygen species, endoplasmic reticulum stress and fibrosis. The benefits of PARPi treatment were confirmed in mice fed with a methionine- and choline-deficient diet and in mice with lipopolysaccharide-induced hepatitis; PARP activation was attenuated and the development of hepatic injury was delayed in both models. Using Sirt1(heP-/-) mice, the beneficial effects of a PARPi-supplemented HFHS diet were found to be Sirt1-dependent. Conclusions: Our study provides a novel and practical pharmacological approach for treating NAFLD, fueling optimism for potential clinical studies. Lay summary: Non-alcoholic fatty liver disease (NAFLD) is now considered to be the most common liver disease in the Western world and has no approved pharmacological therapy. PARP inhibitors given as a treatment in two different mouse models of NAFLD confer a protection against its development. PARP inhibitors may therefore represent a novel and practical pharmacological approach for treating NAFLD. (C) 2016 European Association for the Study of the Liver. Published by Elsevier B.V.

Elsevier Science Bv2017

Reversible acetylation of PGC-1: connecting energy sensors and effectors to guarantee metabolic flexibility

Johan Auwerx, Ellen Helena Jeninga, Kristina Schoonjans

rganisms adapt their metabolism to meet ever changing environmental conditions. This metabolic adaptation involves at a cellular level the fine tuning of mitochondrial function, which is mainly under the control of the transcriptional co-activator proliferator-activated receptor gamma co-activator (PGC)-1alpha. Changes in PGC-1alpha activity coordinate a transcriptional response, which boosts mitochondrial activity in times of energy needs and attenuates it when energy demands are low. Reversible acetylation has emerged as a key way to alter PGC-1alpha activity. Although it is well established that PGC-1alpha is deacetylated and activated by Sirt1 and acetylated and inhibited by GCN5, less is known regarding how these enzymes themselves are regulated. Recently, it became clear that the energy sensor, AMP-activated kinase (AMPK) translates the effects of energy stress into altered Sirt1 activity by regulating the intracellular level of its co-substrate nicotinamide adenine dinucleotide (NAD)(+). Conversely, the enzyme ATP citrate lyase (ACL), relates energy balance to GCN5, through the control of the nuclear production of acetyl-CoA, the substrate for GCN5's acetyltransferase activity. We review here how these metabolic signaling pathways, affecting GCN5 and Sirt1 activity, allow the reversible acetylation-deacetylation of PGC-1alpha and the adaptation of mitochondrial energy homeostasis to energy levels.

2010

Metabolic control of muscle and muscle stem cell function

Hongbo Zhang

Skeletal muscle composed of myofibers and a small amount of muscle stem cells (MuSCs). It plays important roles in energy metabolism. Some of the key metabolites, such as NAD+ and acetyl-CoA were recently found to regulate muscle and MuSCs function. Most of these functions are related to the cellular mitochondria. In my PhD thesis projects, we aimed at exploring the link between NAD+ levels, mitochondrial function, muscle structural homeostasis and MuSCs senescence. We also investigated the impact of acetyl-CoA on MuSCs activation and proliferation, through the control of protein acetylation. These projects mainly focus on three aspects: Rejuvenation of adult stem cells by NAD+ repletion. Aging is accompanied by SCs senescence. However, the initial signaling events mediating SCs senescence are still not well defined. We demonstrate the importance of mitochondrial activity as a pivotal modulator of MuSCs senescence. MuSCs from aged mice have reduced NAD+ levels, mitochondrial content and capacity for oxidative respiration. Importantly, the induction of the mitochondrial unfolded protein response (UPRmt), subsequent to increasing cellular NAD+ with the precursor nicotinamide riboside (NR), prevents MuSCs senescence and improves muscle function and regeneration in aged mice. NR also prevents MuSCs dysfunction in the mdx mouse, a known mouse model of accelerated MuSCs senescence. Extending these observations to other SC pools and on the organism as a whole, we demonstrate that NR delays neural and melanocyte SCs senescence, while also increasing mouse lifespan. Control of MuSCs function by acetyltransferase KAT2A. MuSCs are essential for skeletal muscle homeostasis and repair. The complete mechanism controlling MuSCs maintenance and differentiation, however, remains elusive. We found that the acetyltransferase KAT2A expression is essential for myogenic differentiation of MuSCs. KAT2A loss-of-function in cells blocks MuSCs differentiation, and in mice impairs muscle regeneration after damage. The regulation of KAT2A in MuSCs function might rely on its ability to enzymatically acetylate other proteins, and in particular the transcription factor PAX7. NAD+ repletion improves muscle function in muscular dystrophy. Duchenne muscular dystrophy (DMD) is caused by dystrophin gene mutations that lead to progressive muscle degeneration. Our bioinformatics analysis in mice and DMD patients suggests a role for NAD+ in protecting the muscle from metabolic and structural degeneration. Validating these findings, we reveal that the mdx mouse is characterized by reductions in muscle NAD+ levels, concurrent to increased PARP activity and reduced expression of nicotinamide phosphoribosyltransferase (NAMPT), the key enzymes for NAD+ consumption and biosynthesis. Replenishing the NAD+ pool by dietary NR supplementation benefits muscle function in mdx and mdx/Utr-/- mice, an effect replicated in C. elegans models of DMD. The beneficial effects of NAD+ repletion in muscular dystrophy are pleiotropic and rely on the improvement in mitochondrial function, structural protein expression and on reductions in general PARylation, inflammation and fibrosis. In summary, our work demonstrates the critical role of NAD+ and Acetyl-coA in maintaining muscle and MuSCs function. Repletion of NAD+ levels, by NR administration, and/or boosting the function of acetyltransferase KAT2A might be attractive strategies to maintain muscle homeostasis and MuSCs function in aging and muscular dystrophy.

EPFL2016

Inhibiting poly ADP-ribosylation increases fatty acid oxidation and protects against fatty liver disease

Elsevier Science Bv2017

Metabolic control of muscle and muscle stem cell function

Hongbo Zhang

EPFL2016