Review Peroxynitrite and Inflammatory Bowel Disease
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436 Gut 2000;46:436–439 Review Gut: first published as 10.1136/gut.46.3.436 on 1 March 2000. Downloaded from Peroxynitrite and inflammatory bowel disease Introduction mainly found in granulocytes and is present in high Idiopathic inflammatory bowel diseases (IBD), including amounts in intestinal tissue of patients with IBD. ulcerative colitis and Crohn’s disease, are chronic inflam- Fe3+ matory disorders of the gastrointestinal tract which lead an Hydroxyl radical: O −v +HO→vOH+OH− +O (A) unpredictable clinical course undergoing a succession of 2 2 2 2 exacerbations and remissions of variable intensity. The MPO highest incidence rates for IBD are found in developed Hypochlorus acid: H O +Cl− +H+→HOCl + H O (B) countries of the northern hemisphere (UK, Scandinavia, 2 2 2 Canada, USA) with the average annual incidence ranging It is well documented that these radicals are generated in from six to 13 new cases per year per 100 000 of the gen- high amounts in clinical biopsy samples10 11 and that eral population.1 For the patient, the consequences of IBD neutrophils from patients with IBD are capable of generat- include rectal bleeding, diarrhoea, weight loss, and a ing increased levels of superoxide relative to controls.12 −v reduced quality of life and may even result in death (mor- SOD and catalase (scavengers of O2 and H2O2 respec- tality rate for UK, USA, Canada, Scandinavia ∼3.9 per 106 tively) and glutathione are generally decreased in ulcerative per year (combined statistics from 1950 to 19942)). colitis and Crohn’s disease13 14 suggesting depletion by high Investigators working on the aetiology of IBD have pro- levels of ROMs. Experimental animal models of IBD have posed many factors which are thought to be involved in the demonstrated similarly high levels of ROMs and a benefit initiation or exacerbation, or both, of the inflammatory of antioxidant therapy.15–17 For example, antioxidant process. Some of these factors include increased epithelial therapy has improved the colonic damage and diarrhoea permeability, inappropriate neutrophil infiltration into the observed in the trinitrobenzene sulphonic acid (TNBS) intestine, activation of mast cells, and increased concentra- model of colitis.18 tions of pro-inflammatory mediators (cytokines, leuko- trienes, reactive oxygen metabolites (ROMs)). In addition, Role of nitric oxide in inflammatory bowel disease over the past 10 years, the overproduction of nitric oxide A pathological role for NOv in IBD is less clear-cut. As (NOv) has received considerable attention as an important stated previously NOv is produced from L-arginine via NOS. 3–5 v player in the pathogenesis of IBD. NO is a free radical NOS exists in three distinct isoforms: constitutively produced from L-arginine via the enzyme nitric oxide syn- expressed, vascular endothelium (ecNOS or NOS-3) and http://gut.bmj.com/ v thase (NOS). Alone, NO is a weak free radical; however, it neuronal NOS (bNOS or NOS-1), and an inducible −v can react with superoxide (O2 ) to produce peroxynitrite − isoform that operates independently of calcium (iNOS or (OONO ) a highly toxic reactive nitrogen intermediary. In 19 6 NOS-2). iNOS expression is induced by cytokines and 1990, Beckman and colleagues showed that peroxynitrite requires protein synthesis and is capable of high output v had hydroxyl radical ( OH)-like activity at physiological pH production of NOv at sites of inflammation. A role for NOv and proposed a pathophysiological role for peroxynitrite. in IBD has been suggested by various clinical studies which This work has since generated substantial interest in the demonstrated the presence of increased levels of nitrite/ role of peroxynitrite in IBD as it may provide an explana- nitrates in plasma and NOv in rectal dialysates, as well as on September 25, 2021 by guest. Protected copyright. tion for pathological roles proposed for both O −v and NOv 2 increased NOS activity in biopsy samples from patients in clinical studies and experimental models of colitis. The with ulcerative colitis and Crohn’s disease.3420 The latter role of peroxynitrite in IBD is the focus of this review; was shown to be calcium independent suggesting increased however, we should first briefly review the role fulfilled by iNOS activity. iNOS expression has since been demon- its individual precursors. strated in numerous cell types in biopsy specimens from patients with ulcerative colitis or Crohn’s disease, including Role of superoxide in inflammatory bowel disease − neutrophils, macrophages, epithelial cells, and endothelial v 5 21–23 A pathological role for the O2 radical in IBD is generally cells. Although it is well documented that NOv has 78 −v accepted. Reactive oxygen metabolites like O2 and homoeostatic regulatory functions in the intestine and has 24–28 hydrogen peroxide (H2O2) are a byproduct of normal cellu- 9 many anti-inflammatory mechanisms of action, studies lar metabolism produced by many enzyme systems (e.g. using inhibitors of NOS in experimental models of colitis xanthine oxidase, NADPH oxidase) in the body. Antioxi- would suggest that inhibition of NOv production will dant enzymes (superoxide dismutase (SOD), catalase, glu- attenuate the intestinal inflammation.29–31 To explain this tathione peroxidase) and free radical scavengers (á- dichotomy, the prevailing view has been that the small tocopherol, ascorbate, uric acid) are contained in all tissues. amount of NOv produced under normal or acute inflamma- Inappropriate oxidative reactions resulting in inflammation tory conditions via constitutive NOS (nNOS or eNOS) may will develop when these antioxidants and scavengers are be an important endogenous inhibitor of inflammation; v −v depleted/overloaded by high levels of ROMs. Like NO ,O2 however, high levels of NOv associated with chronic inflam- is a relatively weak free radical and its cytotoxic eVect is matory conditions such as IBD (via iNOS) may be generally ascribed to its role as a precursor for hypochlorous acid and vOH radical formation. Although the iron catalysed Haber-Weiss reaction has been suggested previously to Abbreviations used in this review: ASA, aminosalicylic acid; − COX, cyclooxygenase; IBD, inflammatory bowel disease; IL, explain the generation of vOH from O v (see A below) the 2 interleukin; MEG, mercaptoethylguanidine; MPO, myeloperoxidase; formation of peroxynitrite is another possible mechanism. NO, nitric oxide; NOS, nitric oxide synthase; NT, nitrotyrosine; Hypochlorous acid (B) is generated from H2O2 and chloride ROM, reactive oxygen metabolites; SOD, superoxide dismutase; (Cl−) via myeloperoxidase (MPO), an enzyme which is TNBS, trinitrobenzene sulphonic acid. Peroxynitrite and inflammatory bowel disease 437 detrimental to intestinal integrity. Our understanding of the In 1996, Southan and colleagues51 described a novel class role of NOv in ulcerative colitis and Crohn’s disease is of NOS inhibitors which had increased selectivity for iNOS further embroiled by a number of other inhibitor studies over both eNOS and nNOS (EC50 11.5 µM v 110 µM, which have shown little32 33 or no34 eVect, or even an exacer- iNOS v ecNOS respectively). These drugs belonged to a Gut: first published as 10.1136/gut.46.3.436 on 1 March 2000. Downloaded from bation in experimentally induced inflammation.35 One pos- series of aminoalkylisothioureas, compounds which are sible explanation for this is the relative lack of specificity of thought to exert their potent inhibitory eVects through the inhibitors used in these studies which have eVects on transguanidine rearrangement to mercaptoalkylguanidines both the constitutively expressed and the inducible isoform such as mercaptoethylguanidine (MEG).51 MEG was of NOS. Yet, when the iNOS gene is genetically deleted shown to be almost 10-fold more potent than aminoguani- from mice using recombinant DNA technology, experimen- dine (another “selective” inhibitor), at inhibiting the tally induced colitis is exacerbated acutely.36 37 In TNBS inducible isoform of NOS. This group later showed that induced inflammation, the lack of iNOS induction made MEG and related compounds could also act as scavengers little diVerence to the developing chronic inflammation in of peroxynitrite.52 MEG was shown to inhibit peroxynitrite one study37 and improved the mortality rate and some induced DNA single strand breaks, suppress mitochon- inflammatory indexes in the surviving mice in another.38 drial respiration and prevent nitration of Furthermore, a chronic colitis which develops spontane- 4-hydroxyphenylacetic acid. Therefore, MEG could poten- ously in interleukin 10 (IL-10) deficient mice, developed at tially reduce peroxynitrite formation, yet maintain the the same rate and intensity in mice which were doubly defi- physiological eVects of NOv derived from cNOS. In fact, cient in IL-10 and iNOS genes.39 In addition to this, in a using the TNBS induced model of experimentally induced clinical condition (collagenous colitis) which is never asso- colitis in rats, Zingarelli and colleagues50 recently reported ciated with obvious macroscopic ulcerations, even higher that administration of MEG significantly attenuated the levels of NOv than in ulcerative colitis are found.40 These clinical signs (diarrhoea and weight loss) of colitis as well as studies suggest that NOv concentrations alone cannot macroscopic and histological damage scores, granulocyte dictate pathological inflammation in the intestine and infiltration, iNOS immunoreactivity, and 3-NT formation. makes the generation of peroxynitrite in vivo an exciting Previously,