Downregulation of <scp>NAD</scp> Kinase Expression in β‐Cells Contributes to the Aging‐Associated Decline in Glucose‐Stimulated Insulin Secretion

Nicotinamide adenine dinucleotide kinase (NADK) is essential to the generation of nicotinamide adenine dinucleotide phosphate (NADP(H)), an important metabolic coupling factor involved in glucose-stimulated insulin secretion. In the present study, we showed that the expression of Nadk and Nadk2 transcripts and NADP(H) content were lower in islets of 80-week-old (aged) mice than those of 8-week-old (young) mice. This was associated with diminished oral glucose tolerance of old mice and the glucose-stimulated insulin secretion (GSIS) response of islets. Knockdown (KD) of Nadk or Nadk2 gene expression in NIT-1 cells impaired glucose-stimulated insulin secretion. Metabolomic analysis revealed that Nadk KD specifically affected purine metabolism in glucose-stimulated cells. The levels of 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) were higher in KD cells than in the non-targeting control (NTC) cells. Phosphorylation of AMP-activated protein kinase (AMPK) was elevated in glucose-treated KD cells compared to that of NTC cells. Increased AICAR level and AMPKα phosphorylation were observed in the glucose-stimulated islets of the aged mice. Genetic and pharmacological inhibition of AMPK promoted glucose-stimulated insulin release by KD cells and the aged mouse islets. It is likely that NADK is modulatory to AMPK activation in pancreatic β-cells and to their GSIS response. Enhanced AICAR formation in KD cells was accompanied by significantly increased conversion from inosine monophosphate (IMP) in a tetrahydrofolate (THF)-dependent manner. Folate supplementation augmented the GSIS response of KD cells and aged mouse islets. Taken together, these findings suggest that the aging-associated decline in NADK expression may underlie the reduced insulin secretory capacity of pancreatic β-cells.