NAD Metabolism Regulates Proliferation of Macrophages in Atherosclerosis

In genetic studies with the Hybrid Mouse Diversity Panel, we previously identified a chromosome 9 locus for atherosclerosis. We now identify NNMT (nicotinamide N-methyltransferase), an enzyme that degrades nicotinamide, as the causal gene in the locus and show that the underlying mechanism involves salvage of nicotinamide to nicotinamide adenine dinucleotide (NAD).

Gain/loss of function studies in macrophages were performed to examine the role of NAD levels in macrophage proliferation and apoptosis in atherosclerosis.

Global inhibition of NNMT using an antisense oligonucleotide reduced atherosclerosis lesion area 5- to 10-fold in both male and female mice on a hyperlipidemic background. Selective inhibition of NNMT in liver and adipose, the major tissues expressing high levels of the enzyme, using siRNA (small interfering RNA), had little or no effect on atherosclerosis. Therefore, we hypothesized that levels of NAD in macrophages might contribute. This was confirmed by showing that transplantation with bone marrow from Nnmt knockout mice resulted in reduced lesional macrophage proliferation, increased macrophage apoptosis, and reduced atherosclerosis. Consistent with this conclusion, reduced expression of macrophage CD38, an enzyme that degrades NAD, reduced both macrophage proliferation and atherosclerosis. Moreover, cultured macrophages from heterozygous Nnmt knockout mice exhibited reduced proliferation, increased apoptosis, and an increased NAD/NADH (nicotinamide adenine dinucleotide, reduced) ratio.

These findings reveal a role for nicotinamide salvage and NAD turnover in macrophage proliferation and survival in the context of atherosclerosis.