Iron behaving badly: inappropriate iron chelation as a major contributor to the aetiology of vascular and other progressive inflammatory and degenerative diseases

Abstract

Background

The production of peroxide and superoxide is an inevitable consequence of aerobic metabolism, and while these particular 'reactive oxygen species' (ROSs) can exhibit a number of biological effects, they are not of themselves excessively reactive and thus they are not especially damaging at physiological concentrations. However, their reactions with poorly liganded iron species can lead to the catalytic production of the very reactive and dangerous hydroxyl radical, which is exceptionally damaging, and a major cause of chronic inflammation.

Review

We review the considerable and wide-ranging evidence for the involvement of this combination of (su)peroxide and poorly liganded iron in a large number of physiological and indeed pathological processes and inflammatory disorders, especially those involving the progressive degradation of cellular and organismal performance. These diseases share a great many similarities and thus might be considered to have a common cause (i.e. iron-catalysed free radical and especially hydroxyl radical generation).

The studies reviewed include those focused on a series of cardiovascular, metabolic and neurological diseases, where iron can be found at the sites of plaques and lesions, as well as studies showing the significance of iron to aging and longevity. The effective chelation of iron by natural or synthetic ligands is thus of major physiological (and potentially therapeutic) importance. As systems properties, we need to recognise that physiological observables have multiple molecular causes, and studying them in isolation leads to inconsistent patterns of apparent causality when it is the simultaneous combination of multiple factors that is responsible.

This explains, for instance, the decidedly mixed effects of antioxidants that have been observed, since in some circumstances (especially the presence of poorly liganded iron) molecules that are nominally antioxidants can actually act as pro-oxidants. The reduction of redox stress thus requires suitable levels of both antioxidants and effective iron chelators. Some polyphenolic antioxidants may serve both roles.

Understanding the exact speciation and liganding of iron in all its states is thus crucial to separating its various pro- and anti-inflammatory activities. Redox stress, innate immunity and pro- (and some anti-)inflammatory cytokines are linked in particular via signalling pathways involving NF-kappaB and p38, with the oxidative roles of iron here seemingly involved upstream of the IkappaB kinase (IKK) reaction. In a number of cases it is possible to identify mechanisms by which ROSs and poorly liganded iron act synergistically and autocatalytically, leading to 'runaway' reactions that are hard to control unless one tackles multiple sites of action simultaneously. Some molecules such as statins and erythropoietin, not traditionally associated with anti-inflammatory activity, do indeed have 'pleiotropic' anti-inflammatory effects that may be of benefit here.

Conclusion

Overall we argue, by synthesising a widely dispersed literature, that the role of poorly liganded iron has been rather underappreciated in the past, and that in combination with peroxide and superoxide its activity underpins the behaviour of a great many physiological processes that degrade over time. Understanding these requires an integrative, systems-level approach that may lead to novel therapeutic targets.

Background and preamble

The 'balkanisation' of the literature is in part due to the amount of it (some 25,000 journals with presently 2.5 million peer-reviewed papers per year, i.e. ~5 per minute [1]), with a number http://www.nlm.nih.gov/bsd/medline_cit_counts_yr_pub.html increasing by something approaching 2 per minute at PubMed/Medline alone. In addition, the disconnect between the papers in the literature (usually as pdf files) and the metadata describing them (author, journal, year, pages, etc) is acute and badly needs filling [2]. Without solving this problem, and without automation of the processes of reading, interpreting and exploiting this literature and its metadata in a digital format, we cannot make use of the existing tools for text mining and natural language processing (e.g. [3–5]), for joining disparate concepts [6], for literature-based discovery (e.g. [7–11], and for studies of bibliometrics [12, 13], literature dynamics [14], knowledge domains [15], detecting republication [16] and so on. Until we recognise these possibilities we are unlikely to seek to realise them.

The present article (and see [17] for a preprint) serves to show some of the benefits than can accrue from a more overarching view of the otherwise highly disparate literature in a particular domain (see also [18]), but was done 'the hard way', i.e. with a few bibliographic and bibliometric tools but without the kind of automation implied above. For the record, the main tools used (see a review in [2]) were Web of Knowledge and Scopus for literature and citation searching, supplemented by Google Scholar. Some use was also made of ARROWSMITH [6, 19, 20] and GOPubMed [21], as well as various workflows in the Taverna environment [22–26], including the BioAID_DiseaseDiscovery workflow http://www.myexperiment.org/workflows/72 written by Marco Roos. Citations and attendant metadata were stored in Endnote (latterly version X).

Introduction

Even under 'normal' conditions, as well as during ischaemia when tissue oxygenation levels are low, the redox poise of the mitochondrial respiratory chain is such that the normally complete four-electron reduction of dioxygen to water is also accompanied by the production, at considerable rates (ca 1–4% of O₂ reduced), of partially reduced forms of dioxygen such as hydrogen peroxide and superoxide (e.g. [27–45]). These 1- and 2-electron reductions of O₂ are necessarily exacerbated when the redox poise of the b-type cytochromes is low, for instance when substrate supplies are in excess or when the terminal electron acceptor O₂ is abnormally low due to hypoxia or ischaemia. Various other oxygenases, oxidases and peroxidases can also lead directly to the production of such 'reduced' forms of dioxygen in vivo (e.g. [46–48]), with H₂O₂ from xanthine oxidase being especially implicated in ischaemia/reperfusion injury (e.g. [47, 49–54]). These molecules (peroxide and superoxide) can cause or contribute to various kinds of oxidative stress. However, this is mainly not in fact because they can react directly with tissue components themselves, since they are comparatively non-toxic, cells have well-known means of dealing with them [55], and they are even used in cellular signalling (e.g. [56–60]). Much more importantly, it is because they can react with other particular species to create far more reactive and damaging products such as hydroxyl radicals, with all these agents nevertheless being known collectively (and indiscriminately) as reactive oxygen species (ROSs). Possibly the commonest means by which such much more damaging species, in particular the hydroxyl radical, are created is by reaction with unliganded or incompletely liganded iron ions [61–63]. The themes of this review are thus (i) that it is this combination of poorly liganded iron species, coupled to the natural production of ROSs, that is especially damaging, (ii) that the role of iron has received far less attention than has the general concept of ROSs, albeit the large literature that we review, and (iii) that this basic combination underpins a great many (and often similar) physiological changes leading to a variety of disease manifestations, and in particular those where the development of the disease is manifestly progressive and degenerative.

An overview of the structure of the review is given in Fig 1, in the form of a 'mind map' [64]. The main literature review for this meta-analysis was completed on June 30^th, 2008, with some updates being added following the refereeing process.

Figure 1

An overview of this article, set out in the form of a 'mind map' [64].

Some relevant chemistry of iron and reduced forms of oxygen

While superoxide and peroxide are the proximate forms of incomplete O₂ reduction in biology, a reaction catalysed by the enzyme superoxide dismutase [65] serves to equilibrate superoxide and peroxide:

(1)

Arguably the most important reaction of hydrogen peroxide with (free or poorly liganded) Fe(II) is the Fenton reaction [66], leading to the very reactive and damaging hydroxyl radical (OH^•)

(2)

Superoxide can also react with ferric iron in the Haber-Weiss reaction [67] to produce Fe(II) again, thereby effecting redox cycling: