Metabolic Regulation of Transcription Through Compartmentalized NAD+ Biosynthesis
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Extracellular signaling and nutrient availability are major factors for the cell fate decision. Responds to extracellular information requires metabolic alterations and differential gene expression. Recently emerging concept of metabolic regulation of transcription reveals that fluctuation of metabolite levels could modulate the activities of enzymes involved in gene regulation, which require substrates or cofactors that are intermediates of cell metabolism. However, how cells integrate extracellular signals (e.g. hormones) and cellular metabolic status to coordinate transcriptional outcome is poorly understood. Nicotinamide adenine dinucleotide (NAD+) is an essential small molecule co-factor in metabolic redox reactions as well as a substrate for many NAD+-dependent enzymes, such as poly(ADP-ribose) polymerases (PARPs; e.g., PARP-1) or sirtuins (SIRTs; e.g., SIRT-1), which many of them are known to play an important role in gene regulation. In mammalian cells, NAD+ is synthesized from nicotinamide mononucleotide (NMN) and ATP, by the family of enzymes known as nicotinamide mononucleotide adenylyl transferases (NMNATs). NMNATs exhibit unique subcellular localizations: NMNAT-1 in the nucleus NMNAT-2 in the cytosol and Golgi, and NMNAT-3 in the mitochondria, suggesting the compartmentalized regulation of NAD+ biosynthesis within the cell. However, the biological role of this compartmentalized NAD+ synthesis is largely unknown. Interestingly, NAD+ synthesizing enzymes localize at the subcellular compartment where the transcription (nucleus) or the cellular metabolism (cytoplasm and mitochondria) occurs. Using adipogenesis as a model, we found that compartmentalized NAD+ synthesis acts to integrate cellular glucose metabolism and the adipogenic transcription program during adipocyte differentiation. Nuclear NAD+ is depleted by the induction of cytoplasmic NMNAT-2, whose levels rapidly increase concomitantly with glucose metabolism during differentiation. Competition between nuclear NMNAT-1 and cytoplasmic NMNAT-2 for the common substrate, nicotinamide mononucleotide (NMN), leads to a precipitous reduction in nuclear NAD+ synthesis by NMNAT-1. This inhibits the catalytic activity of poly(ADP-ribose) polymerase 1 (PARP-1), an NAD+-dependent nuclear enzyme that ADP-ribosylates and inhibits the adipogenic transcription factor, C/EBPβ. Subsequent reversal of PARP-1-mediated repression and enhanced binding of C/EBPβ to adipogenic target genes drives differentiation. Thus, compartmentalized NAD+ synthesis functions as an integrator of cellular metabolism and signal-dependent transcriptional programs.