Inducible nitric oxide synthase expression is inhibited by myeloperoxidase

Nitric oxide (NO) plays key roles in vasodilation and host defense, yet the overproduction of NO by inducible nitric oxide synthase (iNOS) at inflammatory sites can also be pathogenic. Here, we investigate the role of MPO in modulating the induction of iNOS by IFN gamma/LPS IL). In monocyte-macrophages (M phi) treated with IL, MPO gene expression was found to be downregulated as iNOS was upregulated. In M phi from MPO-knockout (KO) mice, the induction of iNOS by IL was earlier and higher than in MPO-positive cells, suggesting NIPO is inhibitory. Consistent with that interpretation, the addition of purified MPO enzyme to cultured macrophages inhibited iNOS induction by IL. In addition, an inhibitor of MPO enzyme, 4-aminobenzohydrazide, enhanced iNOS induction in MPO-positive cells, but not in MPO-KO cells. Similarly, taurine, a scavenger of NIPO-generated HOCl, enhanced iNOS induction in MPO-positive cells, but not in MPO-KO cells. MPO affects an early event, suppressing iNOS induction when added within 2 h of IL, but not when added several hours after IL. The suppression by MPO was alleviated by NO donor, sodium nitroprusside, suggesting the suppression results from scavenging of NO by MPO. This interpretation is consistent with earlier reports that MPO consumes NO, and that low levels of NO donor augment induction of iNOS by IFN gamma/LPS. The implication of these findings is that MPO acts as gatekeeper, suppressing the deleterious induction of iNOS at inflammatory sites by illegitimate signals. The combined signaling of IFNy/LPS overrides the gatekeeper function by suppressing MPO gene expression. (c) 2005 Elsevier Inc. All rights reserved.