▪ Abstract At the interface between the innate and adaptive immune systems lies the high-output isoform of nitric oxide synthase (NOS2 or iNOS). This remarkable molecular machine requires at least 17 binding reactions to assemble a functional dimer. Sustained catalysis results from the ability of NOS2 to attach calmodulin without dependence on elevated Ca2+. Expression of NOS2 in macrophages is controlled by cytokines and microbial products, primarily by transcriptional induction. NOS2 has been documented in macrophages from human, horse, cow, goat, sheep, rat, mouse, and chicken. Human NOS2 is most readily observed in monocytes or macrophages from patients with infectious or inflammatory diseases. Sustained production of NO endows macrophages with cytostatic or cytotoxic activity against viruses, bacteria, fungi, protozoa, helminths, and tumor cells. The antimicrobial and cytotoxic actions of NO are enhanced by other macrophage products such as acid, glutathione, cysteine, hydrogen peroxide, or superoxid...

Nitric oxide and macrophage function

The peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a member of the nuclear receptor superfamily of ligand-dependent transcription factors that is predominantly expressed in adipose tissue, adrenal gland and spleen PPAR-gamma has been demonstrated to regulate adipocyte differentiation and glucose homeostasis in response to several structurally distinct compounds, including thiazolidinediones and fibrates Naturally occurring compounds such as fatty acids and the prostaglandin D2 metabolite 15-deoxy-delta prostaglandin J2 (15d-PGJ2) bind to PPAR-gamma and stimulate transcription of target genes Prostaglandin D2 metabolites have not yet been identified in adipose tissue, but are major products of arachidonic-acid metabolism in macrophages, raising the possibility that they might serve as endogenous PPAR-gamma ligands in this cell type Here we show that PPAR-gamma is markedly upregulated in activated macrophages and inhibits the expression of the inducible nitric oxide synthase, gelatinase B and scavenger receptor A genes in response to 15d-PGJ2 and synthetic PPAR-gamma ligands PPAR-gamma inhibits gene expression in part by antagonizing the activities of the transcription factors AP-1, STAT and NF-kappaB These observations suggest that PPAR-gamma and locally produced prostaglandin D2 metabolites are involved in the regulation of inflammatory responses, and raise the possibility that synthetic PPAR-gamma ligands may be of therapeutic value in human diseases such as atherosclerosis and rheumatoid arthritis in which activated macrophages exert pathogenic effects

The peroxisome proliferator-activated receptor-gamma is a negative regulator of macrophage activation

The peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a member of the nuclear receptor family of transcription factors, a large and diverse group of proteins that mediate ligand-dependent transcriptional activation and repression Expression of PPAR-gamma is an early and pivotal event in the differentiation of adipocytes Several agents that promote differentiation of fibroblast lines into adipocytes have been shown to be PPAR-gamma agonists, including several prostanoids, of which 15-deoxy-delta-prostaglandin J2 is the most potent, as well as members of a new class of oral antidiabetic agents, the thiazolidinediones, and a variety of non-steroidal anti-inflammatory drugs (NSAIDs) Here we show that PPAR-gamma agonists suppress monocyte elaboration of inflammatory cytokines at agonist concentrations similar to those found to be effective for the promotion of adipogenesis Inhibition of cytokine production may help to explain the incremental therapeutic benefit of NSAIDs observed in the treatment of rheumatoid arthritis at plasma drug concentrations substantially higher than are required to inhibit prostaglandin G/H synthase (cyclooxygenase)

PPAR-γ agonists inhibit production of monocyte inflammatory cytokines

S-nitrosylation, the covalent attachment of a nitrogen monoxide group to the thiol side chain of cysteine, has emerged as an important mechanism for dynamic, post-translational regulation of most or all main classes of protein. S-nitrosylation thereby conveys a large part of the ubiquitous influence of nitric oxide (NO) on cellular signal transduction, and provides a mechanism for redox-based physiological regulation.

Protein S-nitrosylation: purview and parameters.

Human heat shock protein 60 (hsp60) elicits a potent proinflammatory response in cells of the innate immune system and therefore has been proposed as a danger signal of stressed or damaged cells We report here that macrophages of C3H/HeJ mice, carrying a mutant Toll-like-receptor (Tlr) 4 are nonresponsive to hsp60 Both the induction of TNF-alpha and NO formation were found dependent on a functional Tlr4 whereas stimulation of macrophages by CpG DNA was Tlr4 independent We conclude that Tlr4 mediates hsp60 signaling This is the first report of a putative endogenous ligand of the Tlr4 complex

Cutting Edge: Heat Shock Protein 60 Is a Putative Endogenous Ligand of the Toll-Like Receptor-4 Complex

The promoter region of the mouse gene for macrophage-inducible nitric oxide synthase (mac-NOS; EC 1.14.13.39) has been characterized. A putative TATA box is 30 base pairs upstream of the transcription start site. Computer analysis reveals numerous potential binding sites for transcription factors, many of them associated with stimuli that induce mac-NOS expression. To localize functionally important portions of the regulatory region, we constructed deletion mutants of the mac-NOS 5' flanking region and placed them upstream of a luciferase reporter gene. The macrophage cell line RAW 264.7, when transfected with a minimal promoter construct, expresses little luciferase activity when stimulated by lipopolysaccharide (LPS), interferon gamma (IFN-gamma), or both. Maximal expression depends on two discrete regulatory regions upstream of the putative TATA box. Region I (position -48 to -209) increases luciferase activity approximately 75-fold over the minimal promoter construct. Region I contains LPS-related responsive elements, including a binding site for nuclear factor interleukin 6 (NF-IL6) and the kappa B binding site for NF-kappa B, suggesting that this region regulates LPS-induced expression of the mac-NOS gene. Region II (position -913 to -1029) alone does not increase luciferase expression, but together with region I it causes an additional 10-fold increase in expression. Together the two regions increase expression 750-fold over activity obtained from a minimal promoter construct. Region II contains motifs for binding IFN-related transcription factors and thus probably is responsible for IFN-mediated regulation of LPS-induced mac-NOS. Delineation of these two cooperative regions explains at the level of transcription how IFN-gamma and LPS act in concert to induce maximally the mac-NOS gene and, furthermore, how IFN-gamma augments the inflammatory response to LPS.

https://www.pnas.org/content/pnas/90/20/9730.full.pdf

Macrophage nitric oxide synthase gene: two upstream regions mediate induction by interferon gamma and lipopolysaccharide

The purpose of this study was to determine whether the female hormones estradio1-17f� (E2) and progesterone (P4) influence inducible nitric oxide synthase (iNOS) and the production of nitric oxide (NO) by mterferon-'y(IFN-y)- and lipopolysaccharide ( LPS)-activated mouse mac rophages. Treatment with P4 alone caused a time- and dose-dependent inhibition ofNO production by macrophage cell lines (RAW 264.7, J774) and mouse bone marrow cul- ture-derived macrophages as assessed by nitrite ac- cumulation. RAW 264.7 cells transiently transfected with an iNOS gene promoter/luciferase reporter- gene construct that were stimulated with IFN-y/LPS in the presence of P4 displayed reduced luciferase activity and NO production. Analysis of RAW 264.7 cells by Northern blot hybridization revealed con- current P4-mediated reduction in INOS mRNA. These observations suggest that P4-mediated inhibi- tion of NO may be an important gender-based differ- ence within females and males that relates to macrophage-mediated host defense. J. Leukoc. Biol. 59: 442-450; 1996.

Evan W. Alley

Papers

Macrophage nitric oxide synthase gene: two upstream regions mediate induction by interferon gamma and lipopolysaccharide

Progesterone inhibits inducible nitric oxide synthase gene expression and nitric oxide production in murine macrophages