PARP-1 Inhibition Increases Mitochondrial Metabolism through SIRT1 Activation

SIRT1 regulates energy homeostasis by controlling the acetylation status and activity of a number of enzymes and transcriptional regulators. The fact that NAD(+) levels control SIRT1 activity confers a hypothetical basis for the design of new strategies to activate SIRT1 by increasing NAD(+) availability. Here we show that the deletion of the poly(ADP-ribose) polymerase-1 (PARP-1) gene, encoding a major NAD(+)-consuming enzyme, increases NAD(+) content and SIRT1 activity in brown adipose tissue and muscle. PARP-1(-)(/-) mice phenocopied many aspects of SIRT1 activation, such as a higher mitochondrial content, increased energy expenditure, and protection against metabolic disease. Also, the pharmacologic inhibition of PARP in vitro and in vivo increased NAD(+) content and SIRT1 activity and enhanced oxidative metabolism. These data show how PARP-1 inhibition has strong metabolic implications through the modulation of SIRT1 activity, a property that could be useful in the management not only of metabolic diseases, but also of cancer.

The activation of hepatic and muscle polyamine catabolism improves glucose homeostasis

Eija Riitta Pirinen

The mitochondrial biogenesis and energy expenditure regulator, PGC-1 alpha, has been previously reported to be induced in the white adipose tissue (WAT) and liver of mice overexpressing spermidine/spermine N (1)-acetyltransferase (SSAT). The activation of PGC-1 alpha in these mouse lines leads to increased number of mitochondria, improved glucose homeostasis, reduced WAT mass and elevated basal metabolic rate. The constant activation of polyamine catabolism produces a futile cycle that greatly reduces the ATP pools and induces 5'-AMP-activated protein kinase (AMPK), which in turn activates PGC-1 alpha in WAT. In this study, we have investigated the effects of activated polyamine catabolism on the glucose and energy metabolisms when targeted to specific tissues. For that we used a mouse line overexpressing SSAT under the endogenous SSAT promoter, an inducible SSAT overexpressing mouse model using the metallothionein I promoter (MT-SSAT), and a mouse model with WAT-specific SSAT overexpression (aP2-SSAT). The results demonstrated that WAT-specific SSAT overexpression was sufficient to increase the number of mitochondria, reduce WAT mass and protect the mice from high-fat diet-induced obesity. However, the improvement in the glucose homeostasis is achieved only when polyamine catabolism is enhanced at the same time in the liver and skeletal muscle. Our results suggest that the tissue-specific targeting of activated polyamine catabolism may reveal new possibilities for the development of drugs boosting mitochondrial metabolism and eventually for treatment of obesity and type 2 diabetes.

2012

Effective treatment of mitochondrial myopathy by nicotinamide riboside, a vitamin B3

Johan Auwerx, Eija Riitta Pirinen

Nutrient availability is the major regulator of life and reproduction, and a complex cellular signaling network has evolved to adapt organisms to fasting. These sensor pathways monitor cellular energy metabolism, especially mitochondrial ATP production and NAD+/NADH ratio, as major signals for nutritional state. We hypothesized that these signals would be modified by mitochondrial respiratory chain disease, because of inefficient NADH utilization and ATP production. Oral administration of nicotinamide riboside (NR), a vitamin B3 and NAD+ precursor, was previously shown to boost NAD+ levels in mice and to induce mitochondrial biogenesis. Here, we treated mitochondrial myopathy mice with NR. This vitamin effectively delayed early- and late-stage disease progression, by robustly inducing mitochondrial biogenesis in skeletal muscle and brown adipose tissue, preventing mitochondrial ultrastructure abnormalities and mtDNA deletion formation. NR further stimulated mitochondrial unfolded protein response, suggesting its protective role in mitochondrial disease. These results indicate that NR and strategies boosting NAD+ levels are a promising treatment strategy for mitochondrial myopathy.

Wiley Open Access2014

Gene regulatory dynamics underlying brown fat cell differentiation

Rachana Narendra Pradhan

Adipose tissue is increasingly recognized as a dynamic, endocrine organ responsible for regulating metabolic homeostasis via the collective action of its two principal subtypes: white adipose tissue (WAT) and brown adipose tissue (BAT). Recently, the discovery of metabolically active brown fat reserves and brown in white fat (also known as "brite" or "beige" fat) in adult humans has prompted fundamental research to understand the development and function of brown adipocytes. The functional property of brown adipocytes is executed by the mitochondrial protein, Un- coupling Protein 1 (UCP1). UCP1 is responsible for uncoupling oxidative phosphorylation from ATP production thereby dissipating heat. The gene expression of Ucp1 is orchestrated by a multitude of transcriptional regulators and co-regulators that are distinct from those governing white adipocyte differentiation and function. However, in contrast to its counter- part, the white adipocyte, the gene regulatory mechanisms and transcriptional drivers of the brown adipocyte differentiation program remain elusive. Thus the focus of this thesis is to elucidate the gene regulatory network underlying brown fat cell differentiation and to decou- ple differentiation from the thermogenesis program using an integrative genomics approach. The main contributions of this work are as follows (1) construction of a comprehensive map integrating signaling, transcriptional and metabolic modules necessary for the activation of a brown adipocyte, (2) systematic, comprehensive analysis of transcriptional and chromatin state dynamics during brown fat cell differentiation leading to the identification of novel transcriptional regulators, (3) genomic characterization of the TF BCL6 during brown fat cell differentiation and (4) phenotypic characterization of a novel conditional BCL6 knockout mouse model.

EPFL2017