Cell Death and Differentiation (2003) 10, 418–430 & 2003 Nature Publishing Group All rights reserved 1350-9047/03 $25.00 www.nature.com/cdd Review Nitric oxide: a key regulator of myeloid inflammatory cell apoptosis Introduction EL Taylor*,1, IL Megson2, C Haslett1 and AG Rossi1 The free radical nitric oxide (NO) was first discovered as an 1 Centre for Inflammation Research, Rayne Laboratory, University of Edinburgh, endogenous vasodilator released from the endothelium to Medical School, Teviot Place, Edinburgh EH8 9AG, UK regulate vascular tone.1 However, it is now known that NO is a 2 Centre for Cardiovascular Science, University of Edinburgh, Hugh Robson key mediator in a great number of physiological and Building, George Square, Edinburgh EH8 9XD, UK pathophysiological processes (see Quinn).2 This ubiquitous * Corresponding author: EL Taylor, Centre for Inflammation Research, Rayne Laboratory, University of Edinburgh, Medical School, Teviot Place, Edinburgh signalling molecule can regulate the rate of apoptosis, or EH8 9AG, UK. Tel.: +44 131 651 1323; Fax: +44 131 650 4384; programmed cell death, in many cell types, including human E-mail: [email protected] inflammatory cells. Whether or not cells undergo apoptosis depends on the net balance of a large number of pro- versus Received 30.4.02; revised 29.8.02; accepted 2.9.02 antiapoptotic factors. Studies have revealed that NO Edited by G. Melino has both pro and antiapoptotic properties, depending largely on the concentration and flux of NO, and the cell type under scrutiny (for reviews, see Nicotera et al.3 and Kim et Abstract al.4). It has been proposed that low concentrations of NO, Apoptosis of inflammatory cells is a critical event in the derived from constitutively active endothelial and neuronal resolution of inflammation, as failure to undergo this form of isoforms of NO synthase (eNOS and nNOS), usually have a cell death leads to increased tissue damage and exacerbation protective effect on cells whereas higher concentrations of the inflammatory response. Many factors are able to derived from the inducible isoform (iNOS) are more likely to 3 influence the rate of apoptosis in neutrophils, eosinophils, drive cell death. monocytes and macrophages. Among these is the signalling In stark contrast to cells undergoing necrosis, apoptotic inflammatory cells fail to release their proinflammatory and molecule nitric oxide (NO), which possesses both anti- and histotoxic contents.5 Furthermore, their clearance by profes- proapoptotic properties, depending on the concentration and sional phagocytes such as macrophages occurs via a flux of NO, and also the source from which NO is derived. This nonphlogistic mechanism, which additionally aids the resolu- review summarises the differential effects of NO on tion of the inflammatory response (see Figure 1).6 Thus, inflammatory cell apoptosis and outlines potential mechan- apoptosis is generally regarded to be noninflammatory and is isms that have been proposed to explain such actions. crucial for the successful resolution of inflammation. Chronic Cell Death and Differentiation (2003) 10, 418–430. doi:10.1038/ inflammatory conditions are frequently characterised by an sj.cdd.4401152 apparent failure of myelocytic inflammatory cells to respond to apoptotic stimuli, or of phagocytes to remove apoptotic cells. Persistence of these cells leads to damage of the surrounding tissue and exacerbation of inflammation, as cells ultimately Keywords: nitric oxide; apoptosis; neutrophil; eosinophil; mono- undergo either primary or secondary (following failed clear- cyte; macrophage ance of apoptotic cells) necrosis, an extremely proinflamma- tory form of cell death (see Figure 1).5,7 Manipulation of the rate of apoptosis in critical inflammatory effector cells, such as neutrophils, eosinophils, monocytes and macrophages, could Abbreviations: NO, nitric oxide; NOS, nitric oxide synthase; therefore be of therapeutic benefit.7 NO is capable of inducing SNP, sodium nitroprusside; SPER/NO, spermine diazeniumdio- inflammatory cell apoptosis and also possesses several other late; DEA/NO, diethylamine diazeniumdiolate; DETA/NO, anti-inflammatory properties, including direct downregulation diethylenetriamine diazeniumdiolate; SIN-1, 3-morpholino- of leukocyte functions, such as neutrophil and monocyte À À sydnonimine; O2 , superoxide anion; ONOO , peroxynitrite; adhesion, and neutrophil chemotaxis, degranulation and À 8–10 GEA 3162, 1,2,3,4-oxatriazolium, 5-amino-3-(3,4-dichorophe- superoxide anion (O2 ) generation. It also acts to maintain nyl)-chloride; LPS, lipopolysaccharide; IFN, interferon; GSNO, the impermeable nature of the vascular endothelium to S-nitrosoglutathione; SNAP, S-nitroso-N-acetylpenicillamine; leukocytes.9 Thus, manipulation of NO concentrations is a SOD, superoxide dismutase; GSH, glutathione; PS, phosphati- particularly promising candidate to alter leukocyte function dylserine; SNOC, S-nitrosocysteine; IL, interleukin; cGMP, cyclic and rates of apoptosis in inflammatory conditions. guanosine monophosphate; sGC, soluble guanylate cyclase; GC, A number of different NO donor classes have been utilised glucocorticoid; cAMP, cyclic adenosine monophosphate; NF-kB, to investigate the role of NO and NO-related species in nuclear factor kappa B; MPTP, mitochondrial permeability apoptotic processes. These fall into several broad categories transition pore defined by the species derived and the metabolic process Nitric oxide and inflammatory cell apoptosis EL Taylor et al 419 Figure 1 Death pathways and phagocytic clearance of inflammatory cells. Cells may undergo apoptosis, the noninflammatory form of cell death, in which granule contents are retained within the cell, and cells are cleared by macrophages that release anti-inflammatory mediators. Alternatively, they may undergo primary necrosis, during which the cell membrane integrity is compromised and granule contents are lost, exacerbating inflammation and tissue injury. Phagocytosis of these cells leads to release of proinflammatory mediators from the macrophages. In the event of failure of phagocytosis, or if the capacity of the phagocytes to ingest cells is exceeded, then apoptotic cells may enter into secondary necrosis. Again, this leads to release of toxic granule contents and clearance by a proinflammatory mechanism. TGF, transforming growth factor; IL, interleukin; TNF, tumour necrosis factor; Tx, thromboxane underlying NO release.11 Existing drugs such as sodium in studies into the effects of NO. These compounds, such as À nitroprusside (SNP), azide and hydroxylamine require com- SIN-1, generate equal amounts of NO and O2 , which combine plex metabolism to generate intracellular NO. Release of NO rapidly to form peroxynitrite (ONOOÀ); as a result, they are from azide and hydroxylamine is catalase-dependent, while generally considered to be ONOOÀ donors.15 As the membrane-bound proteins are thought to have a role in NO oxatriazole-5-imine derivative GEA 3162 is structurally similar generation from SNP.11,12 Another class of NO donors is the to molsidomine, the precursor of SIN-1, it is possible that GEA S-nitrosothiols, which can release NO spontaneously, prob- 3162 generates ONOOÀ rather than pure NO, although this ably both inside and outside the cell. However, they also have issue remains to be resolved. the ability to transfer NO+ to free reduced thiols and cysteine This review will examine our current understanding of residues in proteins, thus modulating enzyme activity, and this the regulation of inflammatory cell apoptosis by NO and may constitute an important component of their cellular the mechanisms through which the process is mediated. effects.12,13 The diazeniumdiolate, or ‘NONOate’, compounds Primary human inflammatory cells will constitute the main are nucleophiles with two molecules of NO, which are focus of the review since cell lines are usually transformed spontaneously released in aqueous solution in a temperature- and, therefore, their apoptotic machinery is likely to be altered. and pH-dependent reaction. A wide variety of diazeniumdio- The biology and chemistry of NO,2 the various classes of NO lates are available, such as spermine (SPER/NO), diethyla- donor11 and the apoptotic process itself16 have all previously mine (DEA/NO) and diethylenetriamine (DETA/NO)-based been extensively reviewed elsewhere and will not be covered compounds, each with a different rate of release of NO that in detail here. The regulation of apoptosis in distinct depends on the nature of the nucleophile.14 Finally, although inflammatory cells of the myeloid lineage will be considered not strictly NO donors, the sydnonimines are frequently used first, and summarised in Table 1, then potential mechanisms Cell Death and Differentiation Nitric oxide and inflammatory cell apoptosis EL Taylor et al 420 Table 1 Summary of the effects of NO on inflammatory cell apoptosis Cell type Species/cell line Source of NO ConcN Effect on Reference (lM unless otherwise stated) apoptosis Monocytes Human 11 monocytes SIN-1 250–2000 Induction Adrie et al.33 and macrophages SNAP 500–2000 No effect Murine 11 macrophages iNOS ? Induction Sarih et al.17 Murine 11 peritoneal iNOS ? Induction Albina et al.18 macrophages Murine 11 peritoneal iNOS ? Induction Hortelano et al.23 macrophages/RAW GSNO 1000 264.7 Human 11 MDM/RAW GSNO 1000 Induction Von Knethen et al.25 264.7 SPER/NO 1000 Human canine LTMC SIN-1 0.006–0.6 Induction Lee et al.39 J774 75 Murine macrophages in L-arginine 900 Induction Niebauer et al. vivo 74 Murine macrophages