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 tryptophan catabolism via the kynurenine pathway. Indoleamine 2,3-dioxygenase, the rate-limiting enzyme for this pathway, is induced by the proinflammatory cytokine interferon-γ. The aim of this study was to investigate the effect of interferon-γ treatment on intracellular NAD concentration in the murine macrophage cell line, RAW 264.7. A significant increase in intracellular NAD concentration was observed following 24 h exposure to interferon-γ. This cytokine-mediated increase in NAD concentration was markedly enhanced by the inhibition of poly(ADP-ribose) polymerase or nitric oxide synthase or following treatment with the synthetic glucocorticoid dexamethasone. NAD production was dependent on both the presence of tryptophan in the culture medium and on functional indoleamine 2,3-dioxygenase activity. In agreement with previous studies a marked increase in nitric oxide production was observed in these cells following activation with interferon-γ. 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-γ. This increase in NAD biosynthesis may provide an improved supply of substrate to the nuclear repair enzyme poly(ADP-ribose) polymerase assisting in DNA repair and hence cell viability.