Is homocysteine an iron-dependent cardiovascular risk factor?

A comprehensive study of the genotoxic and anti-genotoxic effects of homocysteine in HUVECs and mouse bone marrow cells

Elevated Homocysteine (Hcy) is associated with increased risk of vascular disease, but whether it induces genotoxicity to vascular endothelial cells remains unknown. Here, we conducted a comprehensive study of the genotoxicity, and unexpected anti-genotoxicity, of Hcy by cytokinesis-blocked micronucleus assay in HUVECs and erythrocyte micronucleus test in mouse bone marrow cells. Our experiments led to several important findings. First, while supraphysiological Hcy (SP-Hcy) exhibited remarkable genotoxicity, physiologically-relevant Hcy (PR-Hcy) reduced the basal genotoxicity. Second, among the metabolites of Hcy, cysteine phenocopied the anti-genotoxicity of PR-Hcy and, methionine, S-adenosylhomocysteine and H₂S phenocopied the genotoxicity of SP-Hcy. Third, the genotoxicity of SP-Hcy was mitigated by vitamin B₆, Fe²⁺ and Cu²⁺, but was exacerbated by N-acetylcysteine. Fourth, under pre-, co- or post-treatment protocol, both SP-Hcy and PR-Hcy attenuated the genotoxicity of cisplatin, mitomycin-C, nocodazole or deoxycholate. Finally, 100 and 250 mg/kg Hcy ameliorated cisplatin-induced genotoxicity in bone marrow cells of CF-1 and Kunming mice. Our results suggest that genotoxicity may be one mechanism through which Hcy confers an increased risk for vascular disease, but more importantly, they challenge the long-standing paradigm that Hcy is always harmful to human health. Our study calls for a more systematic effort in understanding the molecular mechanisms underlying the anti-genotoxicity of Hcy.

Homocysteine-induced decrease in HUVEC cells' resistance to oxidative stress is mediated by Akt-dependent changes in iron metabolism

PURPOSE

Hyperhomocysteinemia is an independent risk factor for cardiovascular diseases and also promotes neuronal death in various neurodegenerative diseases. There is evidence that iron can mediate homocysteine (Hcy) toxicity. Thus, the aim of this study was to investigate the effect of Hcy on iron metabolism in HUVEC and SH-SY5Y cells.

METHODS

HUVEC and SH-SY5Y cells were treated with 3 mM Hcy for a defined time.

RESULTS

We demonstrate that Hcy induced the upregulation of ferritins type L and H in HUVEC cells in a time-dependent manner and had no effect on the ferritins in SH-SY5Y cells. The change in ferritin expression was preceded by a significant decrease in the cellular level of the active form of Akt kinase in HUVEC but not in SH-SY5Y cells. An increase in ferritin L and H protein levels was observed in the Akt1, Akt2, Akt3 siRNA transfected cells, while in the cells transfected with FOXO3a siRNA, a decrease in both ferritins levels was noticed. Moreover, in the HUVEC cells treated with Hcy for 6 days, the active form of kinase Akt returned to the control level and it was accompanied by a drop in ferritin L and H protein levels. Cytotoxicity of hydrogen peroxide significantly increased in HUVEC cells pre-treated with Hcy for 24 h.

CONCLUSIONS

These data indicate that Hcy induces an increase in cellular ferritin level, and the process is mediated by alterations in Akt-FOXO3a signaling pathway.

Impact of Iron and Homocysteine Levels on T Peak-to-End Interval and Tp-e/QT Ratio in Elite Athletes

BACKGROUND

Electrocardiography (ECG) is frequently used in preparticipation evaluation of competitive athletes. Repolarization heterogeneities on ECG is a well-known indicator for malignant ventricular arrhythmias and sudden cardiac death. We aimed to investigate the effect of iron and homocysteine levels on arrhythmogenic indicators, T peak-to-end (Tp-e) interval, and Tp-e/QT ratio in elite athletes.

METHODS

A total of 72 players (48 football and 24 basketball) with a mean age of 25.4 ± 5.0 years were included to the analysis. Blood biochemistry, homocysteine level, and iron parameters (transferrin saturation and serum iron) were obtained by standard methods. Duration of QRS, QT interval, and Tp-e interval were measured manually on the precordial leads and Tp-e/QT ratio was calculated.

RESULTS

Baseline demographic and clinical characteristics of the study population were compared in two groups according to the median value of Tp-e/QT = 0.219. Both iron and transferrin saturations were higher in the above median group (P = 0.001 and P = 0.002, respectively), however, homocysteine levels were not statistically different among two groups (P = 0.405). In correlation analysis, both serum iron and transferrin saturation were significantly correlated with Tp-e interval (r = 0.368; P = 0.001 and r = 0.394; P = 0.00, respectively) and Tp-e/QT ratio (r = 0.357; P = 0.002 and r = 0.372; P = 0.001, respectively). Multiple stepwise regression analysis revealed that transferrin saturation was an independent predictor of Tp-e interval and Tp-e/QT ratio (β = 0.325; P = 0.002 and β = 0.372; P = 0.001, respectively).

CONCLUSION

This study showed an independent relationship between iron status and Tp-e interval and Tp-e/QT ratios of elite sport players which were also not correlated with serum homocysteine levels.

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

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.

Macrophage iron, hepcidin, and atherosclerotic plaque stability

Hepcidin has emerged as the key hormone in the regulation of iron balance and recycling. Elevated levels increase iron in macrophages and inhibit gastrointestinal iron uptake. The physiology of hepcidin suggests an additional mechanism by which iron depletion could protect against atherosclerotic lesion progression. Without hepcidin, macrophages retain less iron. Very low hepcidin levels occur in iron deficiency anemia and also in homozygous hemochromatosis. There is defective retention of iron in macrophages in hemochromatosis and also evidently no increase in atherosclerosis in this disorder. In normal subjects with intact hepcidin responses, atherosclerotic plaque has been reported to have roughly an order of magnitude higher iron concentration than that in healthy arterial wall. Hepcidin may promote plaque destabilization by preventing iron mobilization from macrophages within atherosclerotic lesions; the absence of this mobilization may result in increased cellular iron loads, lipid peroxidation, and progression to foam cells. Marked downregulation of hepcidin (e.g., by induction of iron deficiency anemia) could accelerate iron loss from intralesional macrophages. It is proposed that the minimally proatherogenic level of hepcidin is near the low levels associated with iron deficiency anemia or homozygous hemochromatosis. Induced iron deficiency anemia intensely mobilizes macrophage iron throughout the body to support erythropoiesis. Macrophage iron in the interior of atherosclerotic plaques is not exempt from this process. Decreases in both intralesional iron and lesion size by systemic iron reduction have been shown in animal studies. It remains to be confirmed in humans that a period of systemic iron depletion can decrease lesion size and increase lesion stability as demonstrated in animal studies. The proposed effects of hepcidin and iron in plaque progression offer an explanation of the paradox of no increase in atherosclerosis in patients with hemochromatosis despite a key role of iron in atherogenesis in normal subjects.

Iron-dependent formation of homocysteine from methionine and other thioethers

OBJECTIVE

We tested whether homocysteine is formed from methionine and other thioethers in vitro and in vivo, because methionine can be chemically demethylated to homocysteine.

DESIGN

In in vitro studies, chemical conversions of thioethers (methionine, S-adenosylhomocysteine and cystathionine) into homocysteine were measured under various aerobic conditions. In humans, oral methionine (0.17 mmol/kg body weight) loading tests with and without an oral iron dose (ferrous sulfate, 13 mumol/kg) were performed.

SETTING

A university setting in Birmingham, AL, USA.

SUBJECTS

A total of five healthy adult subjects volunteered.

RESULTS

The in vitro incubation of methionine, S-adenosylhomocysteine or cystathionine with chelated iron resulted in the formation of homocysteine. These conversions were iron- and pH-dependent (pH optima between 5.0 and 6.0) and it was also chelator-dependent. In humans, oral methionine loading tests resulted in a 45% increase in the area-under-the-curve for plasma total homocysteine concentrations, when iron was given together with methionine.

CONCLUSION

Our data suggest that iron-dependent chemical formation of homocysteine can occur in vivo, and contribute to the plasma total homocysteine pool, since this formation can occur ceaselessly. We hypothesize that plasma total homocysteine concentrations reflect, in part, non-protein-bound iron in the body.