TNF regulates cellular NAD+ metabolism in primary macrophages

The inflammatory cytokine TNF is known to affect glucose and lipid metabolism, where its action leads to a cachexic state. Despite a well-established connection of TNF to metabolism, the relationship between TNF and NAD+ metabolism remains unclear. In this report, we evaluated the effects of TNF on NAD+ metabolism in cells that are TNF’s primary autocrine target—macrophages. We designed real-time PCR primers to all NAD+ metabolic enzymes, which we used to examine TNF-induced changes over time. We found that TNF paradoxically up-regulated enzymes that served to increase NAD+ levels, such as IDO and PBEF, as well as enzymes that decrease NAD+ levels, such as CD38 and CD157. The significance of these mRNA changes was evaluated by examining TNF-mediated changes in cellular NAD+ levels. Treatment of macrophages with TNF decreased NAD+ levels over time, suggesting that increases in NAD+-degrading enzymes were dominant. To evaluate whether this was the case, we measured TNF-mediated changes in NAD+ levels in animals where CD38 was genetically deleted. In CD38−/− macrophages, the effects of TNF were reversed, with TNF increasing NAD+ levels over time. The significance of our findings is threefold: (1) we establish that TNF affects NAD+ metabolism by regulating the expression of major NAD+ metabolic enzymes, (2) TNF-induced decreases in cellular NAD+ levels were carried out through the up-regulation of extracellularly situated enzymes, and (3) we provide a mechanism for the observed clinical connection of TNF-dependent diseases to tissue reductions in NAD+ content.