Nampt Is Required For Calorie Restriction Induced Metabolic Improvements

The increasing incidence of metabolic dysfunction is believed to be associated withsedentary lifestyle and high-calorie food intake. Calorie restriction (CR), a dietaryintervention that restricts calorie intake, has been reported to produce beneficial biologicalfunctions in metabolism and delay aging process. CR is one of the most robust andreproducible method for weight loss and slowing aging. It has been proofed to besignificant in anti-aging process, prevention of metabolic diseases and improving quality oflife in a range of animal models. However, the mechanism underlying CR inducedmetabolism improvement is yet to be fully understood.Nicotinamide phosphoribosyltransferase (Nampt), also known as visfatin, is acytokine secreted by adipocyte. In mammals, it is a rate-limiting enzyme in NAD+biosynthesis. Nampt has an important role in regulating energy metabolism andtransformation. Evidence had showed that Nampt participates in basic life activities suchas regulating circadian, anti-apoptosis, body immune and inflammatory reactions as well.Clinical studies also demonstrated that Nampt is closely associated with type2diabetes,obesity, insulin resistance, metabolism disorder and other clinical diseases. Recently,nicotinamide mononucleotide(NMN), the enzymatic product of Nampt, was reported to beable to treat pathophysiology of age-induced diabetes in mice.Since Nampt is the rate-limiting enzyme in NAD+biosynthesis. And SIRT1, which isbelieved to be the major director in CR induced improvement of metabolism, is depend onNAD+biosynthesis. Hence, we doubt if Nampt could also be a crucial regulator in CRinduced beneficial effects on longevity and metabolism. Based upon this hypothesis, weused FK866, a specific Nampt inhibitor to systemically block the activity of Nampt inorder to evaluate its functional role involved in CR induced improvement of metabolism.MethodSD rats were randomly divided into four groups: ad labium (AL), calorie restriction(CR), AL+FK866, CR+FK866. FK866was given through drinking water. The CRtreatment was lasted for8weeks. To evaluate the functional role of Nampt in CR inducedmetabolic improvement, we compared the general metabolic parameters, insulin sensitivityand activation level of insulin signaling pathway among the four groups. We furtherinvestigated SIRT1activity among different groups to illustrate the inner mechanismunderlying CR induced beneficial effects. Results1. CR could significantly upregulate Nampt mRNA and protein levels in skeletalmuscle and white adipose tissue (WAT). The activity of Nampt could also be improved byCR, while use of FK866would block Nampt activity enhancement by CR.2. CR could significantly ameliorate body metabolism. General metabolic parametersindicated that CR could markedly reduce fasting serum insulin and glucose levels, reduceserum leptin level as well as HOMA-IR. FK866treatment would also inhibit this metabolicimprovement to some extent.3. CR could improve whole body insulin sensitivity by increasing phosphorylation ofAkt and AS160, which would be blocked by FK866treatment.4. CR could significantly upregulate SIRT1expression in skeletal muscle and WAT andenhance its activity by deacetylation of FOXO-1and PGC-1α. Use of FK866wouldmarkedly attenuate SIRT1activation induced by CR but not affect its expression.Conclusively, our work verified CR induced upregulation of Nampt expression andactivity. Inhibition of Nampt could markedly reduce the effect of CR-induced improvementof metabolism and insulin sensitivity and provide the first evidence for the crucial role ofNampt in CR-mediated beneficial metabolic effects.