Regulation of NAD biosynthesis, Sirt1 activity, and glucose homeostasis by the mammalian nicotinamide phosphoribosyltransferase.

摘要

Nicotinamide adenine dinucleotide (NAD) is a classic coenzyme involved in a variety of biological processes. However, despite its importance, little is known about how NAD biosynthesis is regulated, particularly in mammals. Mammals utilize nicotinamide phoshoribosyltransferase (Nampt) to initiate NAD biosynthesis from nicotinamide. Intriguingly, Nampt has also been suggested to be a cytokine (PBEF) or a hormone (visfatin). I hypothesized that intra- and extracellular Nampt might play a key role in synthesizing nicotinamide mononucleotide (NMN), a precursor of NAD, and thereby regulate the activity of NAD-dependent Sirt1 and other biological processes.; I have characterized the biochemical properties of Nampt and determined that it is the rate-limiting step in the biosynthesis of NAD from nicotinamide. Additionally, increasing the dosage of intracellular Nampt increases intracellular NAD levels and enhances the activity of Sirt1 in mammalian cells. In collaboration with Dr. Cynthia Wolberger, we determined the crystal structure of this protein, and it clearly shows that Nampt is a type II phosphoribosyltransferase enzyme involved in NAD biosynthesis. Furthermore, I found that Nampt is positively secreted through a non-classical secretory pathway by adipocytes and that this extracellular Nampt shows robust NAD biosynthetic activity. To elucidate the physiological significance of intra- and extracellular Nampt, I characterized Nampt heterozygous (Nampt+/-) mice. I found that Nampt +/- female mice show impaired glucose tolerance and reduced glucose-stimulated insulin secretion. Also, primary islets isolated from Nampt+/- mice show reduced insulin secretion in response to glucose. These phenotypes can be corrected by administration of NMN, the product of the Nampt enzymatic reaction. Taken together, these results demonstrate that Nampt is an intra- and extracellular NAD biosynthetic enzyme that promotes the activity of Sirt1 and regulates glucose homeostasis.