β-Nicotinamide Adenine Dinucleotide (β-NAD) Inhibits ATP-Dependent IL-1β Release from Human Monocytic Cells

While interleukin-1 beta (IL-1 beta) is a potent pro-inflammatory cytokine essential for host defense, high systemic levels cause life-threatening inflammatory syndromes. ATP, a stimulus of IL-1 beta maturation, is released from damaged cells along with beta-nicotinamide adenine dinucleotide (beta-NAD). Here, we tested the hypothesis that beta-NAD controls ATP-signaling and, hence, IL-1 beta release. Lipopolysaccharide-primed monocytic U937 cells and primary human mononuclear leukocytes were stimulated with 2'(3')-O-(4-benzoyl-benzoyl) ATP trieethylammonium salt (BzATP), a P2X7 receptor agonist, in the presence or absence of beta-NAD. IL-1 beta was measured in cell culture supernatants. The roles of P2Y receptors, nicotinic acetylcholine receptors (nAChRs), and Ca2+-independent phospholipase A2 (iPLA2 beta, PLA2G6) were investigated using specific inhibitors and gene-silencing. Exogenous beta-NAD signaled via P2Y receptors and dose-dependently (IC50 = 15 mu M) suppressed the BzATP-induced IL-1 beta release. Signaling involved iPLA2 beta, release of a soluble mediator, and nAChR subunit alpha 9. Patch-clamp experiments revealed that beta-NAD inhibited BzATP-induced ion currents. In conclusion, we describe a novel triple membrane-passing signaling cascade triggered by extracellular beta-NAD that suppresses ATP-induced release of IL-1 beta by monocytic cells. This cascade links activation of P2Y receptors to non-canonical metabotropic functions of nAChRs that inhibit P2X7 receptor function. The biomedical relevance of this mechanism might be the control of trauma-associated systemic inflammation.