NAD kinase regulates the size of the NADPH pool and insulin secretion in pancreatic beta-cells

TitleNAD kinase regulates the size of the NADPH pool and insulin secretion in pancreatic beta-cells
Publication TypeJournal Article
Year of Publication2012
AuthorsGray J.P, Alavian K.N, Jonas E.A, Heart E.A
JournalAm J Physiol Endocrinol MetabAm J Physiol Endocrinol Metab
Volume303
PaginationE191-9
Date PublishedJul 15
ISBN Number1522-1555 (Electronic)<br/>0193-1849 (Linking)
Accession Number22550069
KeywordsAnimals, Glucose/pharmacology, HEK293 Cells, Humans, Insulin-Secreting Cells/enzymology/*secretion, Insulin/*secretion, Male, Mice, Mice, Inbred Strains, NADP/*metabolism, Oxidative Stress/drug effects, Phosphotransferases (Alcohol Group Acceptor)/genetics/*metabolism, Rats, Rats, Sprague-Dawley
Abstract

NADPH is an important component of the antioxidant defense system and a proposed mediator in glucose-stimulated insulin secretion (GSIS) from pancreatic beta-cells. An increase in the NADPH/NADP(+) ratio has been reported to occur within minutes following the rise in glucose concentration in beta-cells. However, 30 min following the increase in glucose, the total NADPH pool also increases through a mechanism not yet characterized. NAD kinase (NADK) catalyzes the de novo formation of NADP(+) by phosphorylation of NAD(+). NAD kinases have been shown to be essential for redox regulation, oxidative stress defense, and survival in bacteria and yeast. However, studies on NADK in eukaryotic cells are scarce, and the function of this enzyme has not been described in beta-cells. We employed INS-1 832/13 cells, an insulin-secreting rat beta-cell line, and isolated rodent islets to investigate the role of NADK in beta-cell metabolic pathways. Adenoviral-mediated overexpression of NADK resulted in a two- to threefold increase in the total NADPH pool and NADPH/NADP(+) ratio, suggesting that NADP(+) formed by the NADK-catalyzed reaction is rapidly reduced to NADPH via cytosolic reductases. This increase in the NADPH pool was accompanied by an increase in GSIS in NADK-overexpressing cells. Furthermore, NADK overexpression protected beta-cells against oxidative damage by the redox cycling agent menadione and reversed menadione-mediated inhibition of GSIS. Knockdown of NADK via shRNA exerted the opposite effect on all these parameters. These data suggest that NADK kinase regulates intracellular redox and affects insulin secretion and oxidative defense in the beta-cell.