Summary 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, 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 not only could be useful in the management of metabolic diseases but also of cancer.

Poly(ADP-ribose) polymerase (PARP; EC 2.4.2.30) is a zinc-finger DNA-binding protein that detects and signals DNA strand breaks generated directly or indirectly by genotoxic agents. In response to these breaks, the immediate poly(ADP-ribosyl)ation of nuclear proteins involved in chromatin architecture and DNA metabolism converts DNA damage into intracellular signals that can activate DNA repair programs or cell death options. To have greater insight into the physiological function of this enzyme, we have used the two-hybrid system to find genes encoding proteins putatively interacting with PARP. We have identified a physical association between PARP and the base excision repair (BER) protein XRCC1 (X-ray repair cross-complementing 1) in the Saccharomyces cerevisiae system, which was further confirmed to exist in mammalian cells. XRCC1 interacts with PARP by its central region (amino acids 301 to 402), which contains a BRCT (BRCA1 C terminus) module, a widespread motif in DNA repair and DNA damage-responsive cell cycle checkpoint proteins. Overexpression of XRCC1 in Cos-7 or HeLa cells dramatically decreases PARP activity in vivo, reinforcing the potential protective function of PARP at DNA breaks. Given that XRCC1 is also associated with DNA ligase III via a second BRCT module and with DNA polymerase ␤, our results provide strong evidence that PARP is a member of a BER multiprotein complex involved in the detection of DNA interruptions and possibly in the recruitment of XRCC1 and its partners for efficient processing of these breaks in a coordinated manner. The modular organizations of these interactors, associated with small conserved domains, may contribute to increasing the efficiency of the overall pathway.The genomic integrity of cells is controlled by a network of protein factors that assess the status of the genome and either cause progression of proliferation or induce a halt in the cell cycle. In eukaryotes, DNA strand breaks, introduced either directly by ionizing radiation or indirectly following enzymatic incision of a DNA lesion, trigger the synthesis of poly(ADPribose) by the enzyme poly(ADP-ribose) polymerase (PARP) (1,13,39). At the site of breakage, PARP catalyzes the transfer of the ADP-ribose moiety from its substrate, NAD ϩ , to a limited number of protein acceptors involved in chromatin architecture and DNA metabolism, including the enzyme itself. These modified proteins, which carry long chains of negatively charged ADP-ribose polymers, lose their affinity for DNA and are thus inactivated. The short half-life of the polymer is attributed to the high activity of poly(ADP-ribose) glycohydrolase, which cleaves the ribose-ribose bond (28, 30). Therefore, poly(ADP-ribosylation) is an immediate but transient postranslational modification of nuclear proteins, induced by DNA-damaging agents.The physiological role of PARP has been much debated in the last decade, but recent molecular and genetic approaches, including expression of either a dominant-negative mutant (26,36,44) or antisense oligonucleo...