Evidence for increased de novo synthesis of NAD in immune-activated RAW264.7 macrophages: A self-protective mechanism?

The parent pyridine nucleotide NAD is the end product of oxidative tryptoph an catabolism via the kynurenine pathway. Indoleamine 2,3-dioxygenase, the rate-limiting enzyme for this pathway, is induced by the proinflammatory cy tokine interferon-gamma, The aim of this study was to investigate the effec t of interferon-gamma treatment on intracellular NAD concentration in the m urine macrophage cell line, RAW 264.7. A significant increase in intracellu lar NAD concentration was observed following 24 h exposure to interferon-ga mma, This cytokine-mediated increase in NAD concentration was markedly enha nced by the inhibition of poly(ADP-ribose) polymerase or nitric oxide synth ase or following treatment with the synthetic glucocorticoid dexamethasone, NAD production was dependent on both the presence of tryptophan in the cul ture medium and on functional indoleamine 2,3-dioxygenase activity. In agre ement with previous studies a marked increase in nitric oxide production wa s observed in these cells following activation with interferon-gamma, These results provide evidence for the first time that de novo synthesis of NAD from tryptophan is increased concomitantly with free radical production in RAW 264.7 macrophages stimulated with interferon-gamma, This increase in NA D biosynthesis may provide an improved supply of substrate to the nuclear r epair enzyme poly(ADP-ribose) polymerase assisting in DNA repair and hence cell viability. (C) 1999 Academic Press.