Low Cellular NAD+ Compromises Lipopolysaccharide-Induced Inflammatory Responses via Inhibiting TLR4 Signal Transduction in Human Monocytes

Abstract

NAD⁺ is an essential cofactor in reduction-oxidation metabolism with impact on metabolic and inflammatory diseases. However, data elucidating the effects of NAD⁺ on the proinflammatory features of human primary monocytes are scarce. In this study, we explored how NAD⁺ affects TLR4 and NOD-like receptor with a PYD-domain 3 (NLRP3) inflammasome activation, two key innate immune responses. Human primary monocytes were isolated from buffy coats obtained from healthy individuals. Intracellular NAD⁺ was manipulated by nicotinamide riboside and the NAMPT inhibitor FK866. Cells were primed with LPS with or without subsequent NLRP3 activation with ATP or cholesterol crystals to analyze the effects of NAD⁺ levels on TLR4-mediated NF-κB activation and NLRP3 activity, respectively. Cytokine release was quantified, and the downstream signal pathway of TLR4 was investigated with Western blot and proteomic analysis. The impact of sirtuin and PARP inhibition was also explored. Our main findings were: 1) elevated NAD⁺ enhanced IL-1β release in LPS-primed human monocytes exposed to ATP in vitro, 2) both NLRP3-dependent and -independent inflammatory responses in LPS-exposed monocytes were inhibited by NAD⁺ depletion with FK866, 3) the inhibition was not caused by suppression of sirtuins or PARP1, and 4) phosphorylation of several proteins TLR4 signal pathway was inhibited by FK866-mediated NAD⁺ depletion, specifically TAK1, IKKβ, IkBα, MEK 1/2, ERK 1/2, and p38. Hence, we suggest a novel mechanism in which NAD⁺ affects TLR4 signal transduction. Furthermore, our data challenge previous reports of the interaction between NAD⁺ and inflammation and question the use of nicotinamide riboside in the therapy of inflammatory disorders.