The association of biomarkers of iron status with peripheral arterial disease in US adults

Associations Between Genetically Predicted Iron Status and Cardiovascular Disease Risk: A Mendelian Randomization Study

BACKGROUND

Mendelian randomization (MR) studies suggest a causal effect of iron status on cardiovascular disease (CVD) risk, but it is unknown if these associations are confounded by pleiotropic effects of the instrumental variables on CVD risk factors. We aimed to investigate the effect of iron status on CVD risk controlling for CVD risk factors.

METHODS AND RESULTS

Iron biomarker instrumental variables (total iron-binding capacity [n=208 422], transferrin saturation [n=198 516], serum iron [n=236 612], ferritin [n=257 953]) were selected from a European genome-wide association study meta-analysis. We performed 2-sample univariate MR of each iron trait on CVD outcomes (all-cause ischemic stroke, cardioembolic ischemic stroke, large-artery ischemic stroke, small-vessel ischemic stroke, and coronary heart disease) from MEGASTROKE (n=440 328) and CARDIoGRAMplusC4D (Coronary Artery Disease Genome Wide Replication and Meta-Analysis Plus the Coronary Artery Disease Genetics) (n=183 305). We then implemented multivariate MR conditioning on 7 CVD risk factors from independent European samples to evaluate their potential confounding or mediating effects on the observed iron-CVD associations. With univariate MR analyses, we found higher genetically predicted iron status to be associated with a greater risk of cardioembolic ischemic stroke (transferrin saturation: odds ratio, 1.17 [95% CI, 1.03-1.33]; serum iron: odds ratio, 1.21 [95% CI, 1.02-1.44]; total iron-binding capacity: odds ratio, 0.81 [95% CI, 0.69-0.94]). The detrimental effects of iron status on cardioembolic ischemic stroke risk remained unaffected when adjusting for CVD risk factors (all P<0.05). Additionally, we found diastolic blood pressure to mediate between 7.1 and 8.8% of the total effect of iron status on cardioembolic ischemic stroke incidence. Univariate MR initially suggested a protective effect of iron status on large-artery stroke and coronary heart disease, but controlling for CVD factors using multivariate MR substantially diminished these associations (all P>0.05).

CONCLUSIONS

Higher iron status was associated with a greater risk of cardioembolic ischemic stroke independent of CVD risk factors, and this effect was partly mediated by diastolic blood pressure. These findings support a role of iron status as a modifiable risk factor for cardioembolic ischemic stroke.

Associations between genetically predicted iron status and cardiovascular disease risk: A Mendelian randomization study

Background

Mendelian randomization (MR) studies suggest a causal effect of iron (Fe) status on cardiovascular disease (CVD) risk, but it is unknown if these associations are confounded by pleiotropic effects of the instrumental variables (IV) on CVD risk factors. We aimed to investigate the effect of Fe status on CVD risk controlling for CVD risk factors.

Methods

Fe biomarker IVs (total Fe binding capacity (TIBC, n=208,422), transferrin saturation (TSAT, n=198,516), serum Fe (SI, n=236,612), ferritin (n=257,953)) were selected from a European GWAS meta-analysis. We performed two-sample univariate (UV) MR of each Fe trait on CVD outcomes (all-cause ischemic stroke (IS), cardioembolic IS (CES), large artery IS (LAS), small vessel IS (SVS), and coronary heart disease (CHD)) from MEGASTROKE (n=440,328) and CARDIoGRAMplusC4D (n=183,305). We then implemented multivariate (MV) MR conditioning on six CVD risk factors from independent European samples to evaluate their potential confounding and/or mediating effects on the observed Fe-CVD associations.

Results

With UVMR analyses, we found higher genetically predicted Fe status to be associated with a greater risk of CES (TSAT: OR 1.17 [95%CI 1.03, 1.33], SI: OR 1.21 [ 95%CI 1.02, 1.44]; TIBC: OR 0.81 [95%CI 0.69, 0.94]). The detrimental effects of Fe status on CES risk remained unaffected when adjusting for CVD risk factors (all P<0.05). Additionally, we found diastolic blood pressure (DBP) to mediate between 7.1-8.8% of the total effect of Fe status on CES incidence. While UVMR initially suggested a protective effect of Fe status on LAS and CHD, MVMR analyses factoring CVD risk factors revealed a complete annulment of this perceived protective effect (all P>0.05).

Discussion

Higher Fe status was associated with a greater risk of CES independent of CVD risk factors, and this effect was partly mediated by DBP. These findings support a role of Fe status as a modifiable risk factor for CES.

Iron Metabolism Markers and Lower Extremity Arterial Disease in People with Type 2 Diabetes

OBJECTIVE

To determine the levels of serum iron, ferritin, total iron-binding capacity, and hepcidin in patients with type 2 diabetes mellitus (T2DM), and to elucidate the relationship of these biomarkers with lower extremity arterial disease (LEAD).

METHODS

Three hundred fifteen patients with T2DM were selected for the study and divided into non-LEAD (n = 119) and LEAD groups (n=196) based on the ankle-brachial index (ABI) results. Demographic data and clinical test results were collected from all patients. Serum iron, ferritin, total iron-binding capacity, and hepcidin levels were measured, and the transferrin saturation was calculated.

RESULTS

Hepcidin levels were substantially higher in the LEAD group (19.17 ± 8.66 ng/mL) than the non-LEAD group (15.44±7.55 ng/mL, P < 0.001), and there was a negative correlation between the ABI and serum lecithin level (r = -0.349, P < 0.001). There were no other correlations with the other iron metabolism indicators. The results of dichotomous logistic regression with LEAD as the dependent variable revealed that smoking history (OR = 4.442, P = 0.008), hypertension history (OR = 3.721, P = 0.006), cardiovascular disease history (OR = 11.126, P < 0.001), diabetes duration (OR = 1.305, P < 0.001), age (OR = 1.056, P = 0.021), hs-CRP level (OR = 1.376, P = 0.002), HbA1c concentration (OR = 1.394, P = 0.001), and hepcidin level (OR = 1.097, P = 0.003) were independent risk factors for LEAD in T2DM patients.

CONCLUSION

Serum hepcidin levels were elevated in the LEAD group compared with the non-LEAD group, and elevated hepcidin levels were associated with the development of LEAD in T2DM patients, suggesting that hepcidin may be involved in the occurrence and development of LEAD in T2DM patients.

Non-Transferrin-Bound Iron in the Spotlight: Novel Mechanistic Insights into the Vasculotoxic and Atherosclerotic Effect of Iron

Significance: While atherosclerosis is an almost inevitable consequence of aging, food preferences, lack of exercise, and other aspects of the lifestyle in many countries, the identification of new risk factors is of increasing importance to tackle a disease, which has become a major health burden for billions of people. Iron has long been suspected to promote the development of atherosclerosis, but data have been conflicting, and the contribution of iron is still debated controversially. Recent Advances: Several experimental and clinical studies have been recently published about this longstanding controversial problem, highlighting the critical need to unravel the complexity behind this topic. Critical Issues: The aim of the current review is to provide an overview of the current knowledge about the proatherosclerotic impact of iron, and discuss the emerging role of non-transferrin-bound iron (NTBI) as driver of vasculotoxicity and atherosclerosis. Finally, I will provide detailed mechanistic insights on the cellular processes and molecular pathways underlying iron-exacerbated atherosclerosis. Overall, this review highlights a complex framework where NTBI acts at multiple levels in atherosclerosis by altering the serum and vascular microenvironment in a proatherogenic and proinflammatory manner, affecting the functionality and survival of vascular cells, promoting foam cell formation and inducing angiogenesis, calcification, and plaque destabilization. Future Directions: The use of additional iron markers (e.g., NTBI) may help adequately predict predisposition to cardiovascular disease. Clinical studies are needed in the aging population to address the atherogenic role of iron fluctuations within physiological limits and the therapeutic value of iron restriction approaches. Antioxid. Redox Signal. 35, 387-414.

Ferroptosis: the potential value target in atherosclerosis

In advanced atherosclerosis (AS), defective function-induced cell death leads to the formation of the characteristic necrotic core and vulnerable plaque. The forms and mechanisms of cell death in AS have recently been elucidated. Among them, ferroptosis, an iron-dependent form of necrosis that is characterized by oxidative damage to phospholipids, promotes AS by accelerating endothelial dysfunction in lipid peroxidation. Moreover, disordered intracellular iron causes damage to macrophages, vascular smooth muscle cells (VSMCs), vascular endothelial cells (VECs), and affects many risk factors or pathologic processes of AS such as disturbances in lipid peroxidation, oxidative stress, inflammation, and dyslipidemia. However, the mechanisms through which ferroptosis initiates the development and progression of AS have not been established. This review explains the possible correlations between AS and ferroptosis, and provides a reliable theoretical basis for future studies on its mechanism.

Dietary iron overload mitigates atherosclerosis in high-fat diet-fed apolipoprotein E knockout mice: Role of dysregulated hepatic fatty acid metabolism

The atherosclerosis "iron hypothesis" generates a fair amount of debate since it has been proposed. Here, we revisited the "iron hypothesis" by examining whether dietary iron overload would intensify iron deposition in plaques and thus lead to further exacerbation of atherosclerosis in apolipoprotein E knockout (ApoE KO) mice. ApoE KO mice were fed either a normal chow diet (ND) or a high fat diet (HFD) supplemented with or without 2% carbonyl iron (Fe) for 16 weeks. However, contrary to our assumption, dietary iron overloading did not intensify, but rather diminished the atherosclerotic lesion area by 65.3%, which was accompanied by significantly decreased serum total cholesterol, triglyceride and low-density lipoprotein cholesterol contents, together with hepatic lipid accumulation decline, despite the evident existence of aortic iron accumulation and the typical signs of iron overload in ApoE KO mice. Using isobaric tag for absolute quantification (iTRAQ) proteomics approach, hepatic CD36 and fatty acid binding proteins-mediated fatty acid (FA) uptake and trafficking impairment were identified as the key potential pathomechanisms by which iron overload diminishes atherosclerotic lesions. Furthermore, downstream hepatic FA de novo biosynthesis was enhanced and FA oxidation was inhibited to compensate for the FA deficiency triggered by iron overload-impaired fatty acid uptake and trafficking. Our findings suggested that dietary iron overload is not atherogenic in ApoE KO mice, and more research efforts are warranted to revisit the "iron hypothesis" of atherosclerosis.

Atherosclerosis is aggravated by iron overload and ameliorated by dietary and pharmacological iron restriction

AIMS

Whether and how iron affects the progression of atherosclerosis remains highly debated. Here, we investigate susceptibility to atherosclerosis in a mouse model (ApoE-/- FPNwt/C326S), which develops the disease in the context of elevated non-transferrin bound serum iron (NTBI).

METHODS AND RESULTS

Compared with normo-ferremic ApoE-/- mice, atherosclerosis is profoundly aggravated in iron-loaded ApoE-/- FPNwt/C326S mice, suggesting a pro-atherogenic role for iron. Iron heavily deposits in the arterial media layer, which correlates with plaque formation, vascular oxidative stress and dysfunction. Atherosclerosis is exacerbated by iron-triggered lipid profile alterations, vascular permeabilization, sustained endothelial activation, elevated pro-atherogenic inflammatory mediators, and reduced nitric oxide availability. NTBI causes iron overload, induces reactive oxygen species production and apoptosis in cultured vascular cells, and stimulates massive MCP-1-mediated monocyte recruitment, well-established mechanisms contributing to atherosclerosis. NTBI-mediated toxicity is prevented by transferrin- or chelator-mediated iron scavenging. Consistently, a low-iron diet and iron chelation therapy strongly improved the course of the disease in ApoE-/- FPNwt/C326S mice. Our results are corroborated by analyses of serum samples of haemochromatosis patients, which show an inverse correlation between the degree of iron depletion and hallmarks of endothelial dysfunction and inflammation.

CONCLUSION

Our data demonstrate that NTBI-triggered iron overload aggravates atherosclerosis and unravel a causal link between NTBI and the progression of atherosclerotic lesions. Our findings support clinical applications of iron restriction in iron-loaded individuals to counteract iron-aggravated vascular dysfunction and atherosclerosis.

The role of hepcidin and iron homeostasis in atherosclerosis

Atherosclerotic cardiovascular disease is a major burden on global health and a leading cause of death worldwide. The pathophysiology of this chronic disease is complex, involving inflammation, lipoprotein oxidation and accumulation, plaque formation, and calcification. In 1981, Dr. Jerome Sullivan formulated the 'Iron Hypothesis', suggesting that higher levels of stored iron promote cardiovascular diseases, whereas iron deficiency may have an atheroprotective effect. This hypothesis has stimulated research focused on clarifying the role of iron in the development of atherosclerosis. However, preclinical and clinical studies have produced contradictory results and the observation that patients with hemochromatosis do not appear to have an increased risk of atherosclerosis seemed incongruous with Sullivan's initial hypothesis. The 'paradox' of systemic iron overload not being accompanied by an increased risk for atherosclerosis led to a refinement of the iron hypothesis focusing on intracellular macrophage iron. More recent in vitro and animal studies have elucidated the complex signaling pathways regulating iron, with a particular focus on hepcidin, the master regulator of body iron homeostasis. Bone morphogenetic protein (BMP) signaling is the major pathway that is required for induction of hepcidin expression in response to increasing levels of iron. Strong links between iron homeostasis, BMP signaling, inflammation and atherosclerosis have been established in both mechanistic and human studies. This review summarizes the current understanding of the role of iron homeostasis and hepcidin in the development of atherosclerosis and discusses the BMP-hepcidin-ferroportin axis as a novel therapeutic target for the treatment of cardiovascular disease.

Hepcidin Deficiency Protects Against Atherosclerosis

Objective- Inflammatory stimuli enhance the progression of atherosclerotic disease. Inflammation also increases the expression of hepcidin, a hormonal regulator of iron homeostasis, which decreases intestinal iron absorption, reduces serum iron levels and traps iron within macrophages. The role of macrophage iron in the development of atherosclerosis remains incompletely understood. The objective of this study was to investigate the effects of hepcidin deficiency and decreased macrophage iron on the development of atherosclerosis. Approach and Results- Hepcidin- and LDL (low-density lipoprotein) receptor-deficient ( Hamp^-/-/ Ldlr^-/-) mice and Hamp^+/+/ Ldlr^-/- control mice were fed a high-fat diet for 21 weeks. Compared with control mice, Hamp^-/-/ Ldlr^-/- mice had decreased aortic macrophage activity and atherosclerosis. Because hepcidin deficiency is associated with both increased serum iron and decreased macrophage iron, the possibility that increased serum iron was responsible for decreased atherosclerosis in Hamp^-/-/ Ldlr^-/- mice was considered. Hamp^+/+/ Ldlr^-/- mice were treated with iron dextran so as to produce a 2-fold increase in serum iron. Increased serum iron did not decrease atherosclerosis in Hamp^+/+/ Ldlr^-/- mice. Aortic macrophages from Hamp^-/-/ Ldlr^-/- mice had less labile free iron and exhibited a reduced proinflammatory (M1) phenotype compared with macrophages from Hamp^+/+/ Ldlr^-/- mice. THP1 human macrophages treated with an iron chelator were used to model hepcidin deficiency in vitro. Treatment with an iron chelator reduced LPS (lipopolysaccharide)-induced M1 phenotypic expression and decreased uptake of oxidized LDL. Conclusions- In summary, in a hyperlipidemic mouse model, hepcidin deficiency was associated with decreased macrophage iron, a reduced aortic macrophage inflammatory phenotype and protection from atherosclerosis. The results indicate that decreasing hepcidin activity, with the resulting decrease in macrophage iron, may prove to be a novel strategy for the treatment of atherosclerosis.

Haploinsufficiency of Transferrin Receptor 1 Impairs Angiogenesis with Reduced Mitochondrial Complex I in Mice with Limb Ischemia

Limb ischemia (LI) is a major consequence of peripheral artery disease (PAD) with a high mortality rate. Iron is an essential mineral to maintain physiological function of multiple organs. Intracellular iron transport is regulated by transferrin receptor 1 (TfR1). Although increase in serum ferritin levels has been reported in PAD patients, the mechanism of iron metabolism in LI is still unclear. The aim of this study is to investigate whether TfR1 deletion attenuates LI formation. To generate LI, the left femoral artery of 8-10 week-old C57BL6/J male mice was ligated. Adductor muscles were harvested at 28 days after surgery to investigate iron metabolism. The level of ferritin, intracellular iron storage protein, was higher in ischemic adductor muscles compared to non-ischemic adductor muscles. Level of intracellular iron transport protein, TfR1, was decreased in ischemic adductor muscles. LI was then generated in TfR1 heterozygous deleted mice (TfR1^+/-) to examine whether TfR1 contributes to the pathophysiology of LI. Laser Doppler blood flowmetry revealed that blood flow recovery was attenuated in TfR1^+/- mice compared to wild type (WT) littermates, along with decreased expression of ferritin and CD31 in ischemic adductor muscles. Since iron is used for energy production in mitochondria, we then assessed mitochondrial complexes in the ischemic adductor muscle. Of interest, expression of mitochondrial complex I, but not complexes II, III, and V in ischemic adductor muscles was significantly reduced in TfR1^+/- mice compared to WT mice. Haploinsufficiency of TfR1 attenuated angiogenesis via reduction of mitochondrial complex I in LI in mice.

Long-term donors versus non-donor men: Iron metabolism and the atherosclerotic process

BACKGROUND AND AIMS

The increased iron level and the labile iron pool (LIP) in circulating monocytes are connected to a higher frequency of cardiovascular events.

METHODS

The study investigates the relationship between LIP in circulating monocytes and markers of iron metabolism and atherosclerosis (inflammation, oxidative stress, endothelial dysfunction and arterial elasticity) in long-term blood donors and non-donor volunteers.

RESULTS

We found that donors had significantly higher LIP values than the control group (1.89 ± 0.47 μM vs. 1.50 ± 0.41 μM, p = 0.007). Despite the observed tendency for the donor group to have higher blood pressure, cholesterol, glucose and HOMAR-IR (homeostasis model assessment of insulin resistance), the groups did not differ in inflammatory markers, markers of endothelial dysfunction and markers of impaired arterial elasticity. The donor group had significant changes in iron metabolism (higher serum Fe, ceruloplasmin, and TfR/Ft ratio (transferrin receptor/ferritin ratio) and lower hepcidin, ferritin, and CD163), indicating depletion of body iron stores and activation of iron turnover.

CONCLUSIONS

LIP seems to be a good marker of iron turnover activity in these individuals despite the lack of a decrease in the hemoglobin concentration. We did not find a significant correlation between LIP levels and atherosclerosis progression in the two groups. However, further studies are needed to assess long-term donorship as a protective factor against atherosclerosis.

Management of iron overload in hemoglobinopathies: what is the appropriate target iron level?

Patients with thalassemia become iron overloaded from increased absorption of iron, ineffective erythropoiesis, and chronic transfusion. Before effective iron chelation became available, thalassemia major patients died of iron-related cardiac failure in the second decade of life. Initial treatment goals for chelation therapy were aimed at levels of ferritin and liver iron concentrations associated with prevention of adverse cardiac outcomes and avoidance of chelator toxicity. Cardiac deaths were greatly reduced and survival was much longer. Epidemiological data from the general population draw clear associations between increased transferrin saturation (and, by inference, labile iron) and early death, diabetes, and malignant transformation. The rate of cancers now seems to be significantly higher in thalassemia than in the general population. Reduction in iron can reverse many of these complications and reduce the risk of malignancy. As toxicity can result from prolonged exposure to even low levels of excess iron, and survival in thalassemia patients is now many decades, it would seem prudent to refocus attention on prevention of long-term complications of iron overload and to maintain labile iron and total body iron levels within a normal range, if expertise and resources are available to avoid complications of overtreatment.

Association of serum ferritin levels with metabolic syndrome and insulin resistance in a Chinese population

AIMS

Increased iron is associated with type 2 diabetes, dyslipidemia, and high blood pressure. Therefore, serum ferritin may be a suitable biomarker to detect metabolic syndrome (MetS). We investigated the relationship between serum ferritin, and the prevalence of MetS and insulin resistance (IR).

METHODS

This cross-sectional study assessed 2,786 Chinese participants, aged 25-75 years. MetS was defined using the 2006 International Diabetes Federation guidelines. IR was assessed with homeostasis model assessment estimated IR (HOMA-IR). Regression analysis was used to estimate the association between serum ferritin and the prevalence of MetS and IR.

RESULTS

MetS prevalence within each serum ferritin quartile (Q1-4) was 31.7%, 37.1%, 43.6%, and 55.4%, respectively in men (P<0.001), and 30.1%, 34.8%, 48.2%, and 66.9%, respectively in women (P<0.001). Increased serum ferritin correlated with the number of MetS components (P<0.001). The odds ratio for MetS in the ferritin Q4 group was 1.95 (1.39-2.73) for men and 1.66(1.12-2.47) for women, compared with Q1. Serum ferritin correlated positively with HOMA-IR in men (regression coefficient: 0.058, P=0.009) and women (regression coefficient: 0.082, P=0.001).

CONCLUSION

MetS prevalence increased with elevated serum ferritin levels, and serum ferritin levels were independently associated with MetS and IR.

The labile iron pool in monocytes reflects the activity of the atherosclerotic process in men with chronic cardiovascular disease

The study investigates the relationship between the labile iron pool (LIP) in circulating monocytes and markers of iron metabolism, inflammation, oxidative stress, endothelial dysfunction and arterial elasticity in patients with chronic cardiovascular disease and in healthy volunteers. The patients with a history of CVEs had significantly higher LIP values than did the control group (1.94+/-0.46 microM vs. 1.62+/-0.49 microM, p=0.02). Except for the leukocyte number (WBCs), the groups did not differ in other inflammatory markers (CRPus, CD 163, MPO, MMP-1). Similarly, there were no differences in the markers of endothelial dysfunction (ICAM, VCAM, E-selectin, vWF). The CVE group had higher pulse pressures, levels of markers of impaired arterial elasticity (AI, Young´s modulus, pulsatility, stiffness index), IMT values and ABI values. The LIP concentration was significantly correlated with the transferrin receptor/ferritin ratio, hepcidin levels, VFT content and the ABI and ET values. Patients with a history of CVE have significantly higher concentrations of iron in their intracellular LIP in circulating monocytes than do healthy controls. The independent and significant correlation of LIP with markers of the progression of atherosclerosis and arterial stiffness suggests LIP as a possible novel marker of atherosclerotic activity.

Iron-induced skeletal muscle atrophy involves an Akt-forkhead box O3-E3 ubiquitin ligase-dependent pathway

Skeletal muscle wasting or sarcopenia is a critical health problem. Skeletal muscle atrophy is induced by an excess of iron, which is an essential trace metal for all living organisms. Excessive amounts of iron catalyze the formation of highly toxic hydroxyl radicals via the Fenton reaction. However, the molecular mechanism of iron-induced skeletal muscle atrophy has remained unclear. In this study, 8-weeks-old C57BL6/J mice were divided into 2 groups: vehicle-treated group and the iron-injected group (10 mg iron day(-1)mouse(-1)) during 2 weeks. Mice in the iron-injected group showed an increase in the iron content of the skeletal muscle and serum and ferritin levels in the muscle, along with reduced skeletal muscle mass. The skeletal muscle showed elevated mRNA expression of the muscle atrophy-related E3 ubiquitin ligases, atrogin-1 and muscle ring finger-1(MuRF1), on days 7 and 14 of iron treatment. Moreover, iron-treated mice showed reduced phosphorylation of Akt and forkhead box O3 (FOXO3a) in skeletal muscles. Inhibition of FOXO3a using siRNA in vitro in C2C12 myotube cells inhibited iron-induced upregulation of atrogin-1 and MuRF1 and reversed the reduction in myotube diameters. Iron-load caused oxidative stress, and an oxidative stress inhibitor abrogated iron-induced muscle atrophy by reactivating the Akt-FOXO3a pathway. Iron-induced skeletal muscle atrophy is suggested to involve the E3 ubiquitin ligase mediated by the reduction of Akt-FOXO3a signaling by oxidative stress.

Iron Homeostasis in Health and Disease

Iron is required for the survival of most organisms, including bacteria, plants, and humans. Its homeostasis in mammals must be fine-tuned to avoid iron deficiency with a reduced oxygen transport and diminished activity of Fe-dependent enzymes, and also iron excess that may catalyze the formation of highly reactive hydroxyl radicals, oxidative stress, and programmed cell death. The advance in understanding the main players and mechanisms involved in iron regulation significantly improved since the discovery of genes responsible for hemochromatosis, the IRE/IRPs machinery, and the hepcidin-ferroportin axis. This review provides an update on the molecular mechanisms regulating cellular and systemic Fe homeostasis and their roles in pathophysiologic conditions that involve alterations of iron metabolism, and provides novel therapeutic strategies to prevent the deleterious effect of its deficiency/overload.

Serum ferritin is associated with arterial stiffness in hemodialysis patients: results of a 3-year follow-up study

BACKGROUND

Iron may contribute to vascular injury through reactive oxygen species. Hemodialysis patients frequently receive iron supply for correction of anemia and are at a high risk of cardiovascular disease. We tested the relationship between iron status and change in arterial stiffness in hemodialysis patients.

PATIENTS AND METHODS

We measured iron status in 53 hemodialysis patients and studied the association with clinical, biochemical, and arterial stiffness measured by brachial-ankle pulse wave velocity (baPWV) over 3 years. The blood pressure was controlled to below 140/90 mmHg by anti-hypertensive drugs.

RESULTS

Median and interquartile range of baseline baPWV, baPWV at 3 years, and ΔbaPWV (difference between 3-year baPWV and baseline baPWV) were following: 17.6 (14.8-18.9), 16.9 (15.3-19.9), and 0.2 (-1.2 to 2.7) m/s. At baseline, baPWV was positively correlated with age, serum ferritin, and systolic blood pressure in univariate analysis. However, in multivariate analysis, only age and serum ferritin remained the significant determinants of baseline baPWV. After 3 years, ΔbaPWV was negatively correlated with age and positively with 3-year averaged serum ferritin in univariate analysis. Then, in multivariate analysis, only 3-year averaged serum ferritin was the important determinant of ΔbaPWV. ΔbaPWV was significantly increased in patients with 3-year averaged serum ferritin >500 ng/mL compared to patients with 3-year averaged serum ferritin ≤500 ng/mL.

CONCLUSIONS

In hemodialysis patients, serum ferritin associates with the progressive arterial stiffness, especially when serum ferritin >500 ng/mL.

Iron toxicity and its possible association with treatment of Cancer: lessons from hemoglobinopathies and rare, transfusion-dependent anemias

Exposure to elevated levels of iron causes tissue damage and organ failure, and increases the risk of cancer. The toxicity of iron is mediated through generation of oxidants. There is also solid evidence indicating that oxidant stress plays a significant role in a variety of human disease states, including malignant transformation. Iron toxicity is the main focus when managing thalassemia. However, the short- and long-term toxicities of iron have not been extensively considered in children and adults treated for malignancy, and only recently have begun to draw oncologists' attention. The treatment of malignancy can markedly increase exposure of patients to elevated toxic iron species without the need for excess iron input from transfusion. This under-recognized exposure likely enhances organ toxicity and may contribute to long-term development of secondary malignancy and organ failure. This review discusses the current understanding of iron metabolism, the mechanisms of production of toxic free iron species in humans, and the relation of the clinical marker, transferrin saturation (TS), to the presence of toxic free iron. We will present epidemiological data showing that high TS is associated with poor outcomes and development of cancer, and that lowering free iron may improve outcomes. Finally, we will discuss the possible relation between some late complications seen in survivors of cancer and those due to iron toxicity.

Ferritin levels and risk of metabolic syndrome: meta-analysis of observational studies

Background

Elevated ferritin levels have been associated with single cardiovascular risk factors but the relationship to the presence of metabolic syndrome is inconclusive.

The aim of this systematic review and meta-analysis of published observational studies was to estimate the association between serum ferritin levels and metabolic syndrome in adults.

Methods

The Pubmed, SCOPUS and the Cochrane Library databases were searched for epidemiological studies that assessed the association between ferritin levels and metabolic syndrome and were published before September 2013. There were no language restrictions. Two investigators independently selected eligible studies. Measures of association were pooled by using an inverse-variance weighted random-effects model. The heterogeneity among studies was examined using the I² index. Publication bias was evaluated using the funnel plot.

Results

Twelve cross-sectional, one case–control and two prospective studies met our inclusion criteria including data from a total of 56,053 participants. The pooled odds ratio (OR) for the metabolic syndrome comparing the highest and lowest category of ferritin levels was 1.73 (95% CI: 1.54, 1.95; I² = 75,4%). Subgroup analyses indicate that pooled OR was 1.92 (95% CI: 1.61, 2.30; I² = 78%) for studies adjusting for C-reactive protein (CRP), and 1.52 (95% CI:1. 36, 1.69; I² = 41%) for studies that did not adjust for CRP (P = 0.044). This finding was remarkably robust in the sensitivity analysis. We did not find publication bias.

Conclusions

The meta-analysis suggests that increased ferritin levels are independently and positively associated with the presence of the metabolic syndrome with an odds ratio higher than 1.73.

Atherogenesis and iron: from epidemiology to cellular level

Iron accumulates in human atherosclerotic lesions but whether it is a cause or simply a downstream consequence of the atheroma formation has been an open question for decades. According to the so called "iron hypothesis," iron is believed to be detrimental for the cardiovascular system, thus promoting atherosclerosis development and progression. Iron, in its catalytically active form, can participate in the generation of reactive oxygen species and induce lipid-peroxidation, triggering endothelial activation, smooth muscle cell proliferation and macrophage activation; all of these processes are considered to be proatherogenic. On the other hand, the observation that hemochromatotic patients, affected by life-long iron overload, do not show any increased incidence of atherosclerosis is perceived as the most convincing evidence against the "iron hypothesis." Epidemiological studies and data from animal models provided conflicting evidences about the role of iron in atherogenesis. Therefore, more careful studies are needed in which issues like the source and the compartmentalization of iron will be addressed. This review article summarizes what we have learnt about iron and atherosclerosis from epidemiological studies, animal models and cellular systems and highlights the rather contributory than innocent role of iron in atherogenesis.

Iron-induced fibrin in cardiovascular disease

Accumulating evidence within the last two decades indicates the association between cardiovascular disease (CVD) and chronic inflammatory state. Under normal conditions fibrin clots are gradually degraded by the fibrinolytic enzyme system, so no permanent insoluble deposits remain in the circulation. However, fibrinolytic therapy in coronary and cerebral thrombosis is ineffective unless it is installed within 3-5 hours of the onset. We have shown that trivalent iron (FeIII) initiates a hydroxyl radical-catalyzed conversion of fibrinogen into a fibrin-like polymer (parafibrin) that is remarkably resistant to the proteolytic dissolution and thus promotes its intravascular deposition. Here we suggest that the persistent presence of proteolysis-resistant fibrin clots causes chronic inflammation. We study the effects of certain amphiphilic substances on the iron- and thrombin-induced fibrinogen polymerization visualized using scanning electron microscopy. We argue that the culprit is an excessive accumulation of free iron in blood, known to be associated with CVD. The only way to prevent iron overload is by supplementation with iron chelating agents. However, administration of free radical scavengers as effective protection against persistent presence of fibrin-like deposits should also be investigated to contribute to the prevention of cardiovascular and other degenerative diseases.

Iron: protector or risk factor for cardiovascular disease? Still controversial

Iron is the second most abundant metal in the Earth’s crust. Despite being present in trace amounts, it is an essential trace element for the human body, although it can also be toxic due to oxidative stress generation by the Fenton reaction, causing organic biomolecule oxidation. This process is the basis of numerous pathologies, including cardiovascular diseases (CVD). The relationship between iron and cardiovascular disease was proposed in 1981 by Jerome Sullivan. Since then, numerous epidemiological studies have been conducted to test this hypothesis. The aim of this review is to present the main findings of the chief epidemiological studies published during the last 32 years, since Sullivan formulated his iron hypothesis, suggesting that this element might act as a risk factor for cardiovascular disease. We have analyzed 55 studies, of which 27 supported the iron hypothesis, 20 found no evidence to support it and eight were contrary to the iron hypothesis. Our results suggest that there is not a high level of evidence which supports the hypothesis that the iron may be associated with CVD. Despite the large number of studies published to date, the role of iron in cardiovascular disease still generates a fair amount of debate, due to a marked disparity in results.

Ferritin Is Independently Associated With the Presence of Coronary Artery Calcium in 12 033 Men

Objective—

Ferritin concentrations are often increased in patients with metabolic syndrome and type 2 diabetes mellitus, but few reports have examined the associations between ferritin and atherosclerosis. We investigated whether any relationship between ferritin and coronary artery calcium score (CACS) >0 (as a marker of atherosclerosis) was independent of potential confounders, such as iron-binding capacity (transferrin), low-grade inflammation, and cardiovascular risk factors.

Methods and Results—

Data were analyzed from a South Korean occupational cohort of 12 033 men who underwent a cardiac computed tomography estimation of CACS and measurements of multiple cardiovascular risk factors. One-thousand three- hundred-fifteen of 12 033 (11.2%) subjects had a CACS >0. For people with a CACS >0, median (interquartile range) ferritin concentration was 196.8 (136.3–291.9) compared with 182.2 (128.1–253.6) in people with a CACS=0; P <0.001. In the highest ferritin quartile, 14.7% (442/3008) of subjects had a CACS >0 compared with 9.7% (292/3010) in the lowest quartile ( P <0.0001). With increasing ferritin quartiles, there were also higher proportions of people with diabetes mellitus ( P <0.0001), hypertension ( P <0.0001), coronary heart disease ( P =0.003), and a Framingham Risk Score >10% ( P <0.0001). In logistic regression modeling with CACS >0 as the outcome, ferritin but not transferrin was independently associated with CACS >0 (odds ratio for highest quartile versus lowest quartile, 1.66 [95% CI, 1.3–1.98]; P =0.0001).

Conclusion—

Increased ferritin concentrations are associated with the presence of a marker of early coronary artery atherosclerosis, independently of traditional cardiovascular risk factors including Framingham risk score, transferrin, preexisting vascular disease, diabetes mellitus, metabolic syndrome factors, and low-grade inflammation.

Iron overload potentiates diet-induced hypercholesterolemia and reduces liver PPAR-α expression in hamsters

Iron stores and lipids are related to the development of cardiovascular disease. Given that peroxisome proliferator-activated receptor alpha (PPAR-α) regulates important physiological processes that impact lipid and glucose homeostasis, we decided to investigate the effects of iron overload on serum lipids and the liver expression of PPAR-α, 3-hydroxy-3-methylglutaryl coenzyme A reductase, and cholesterol 7α-hydroxylase. Hamsters were divided into four groups. The standard group (S) was fed the AIN-93M diet, the SI group was fed the diet and iron injections, the hypercholesterolemic group (H) was fed a standard diet containing cholesterol, and the HI group was fed a high-cholesterol diet and iron injections. Serum cholesterol in the HI group was higher than in the H group. Gene expression analysis of PPAR-α showed that the HI group had a lower PPAR-α expression than H. These data show that iron, when associated with a high-fat diet, can cause increased serum cholesterol levels, possibly due to a reduction in PPAR-α expression.

High risk of cardiovascular disease in iron overload patients

INTRODUCTION

Iron overload (IO) is defined as an increase in storage iron, regardless of the presence or absence of tissue damage. Whether increased iron stores are involved in the pathogenesis of cardiovascular disease remains controversial.

OBJECTIVES

 To study insulin resistance markers, lipoprotein profile, activities of anti and prooxidant enzymes and cholesteryl ester transfer protein (CETP) in patients with IO.

METHODS

Twenty male patients with IO were compared with 20 sex- and age-matched controls. General biochemical parameters, lipoprotein profile, and activities of paraoxonase 1, employing two substrates, paraoxon (PON) and phenylacetate (ARE), lipoprotein-associated phospholipase A(2) (Lp-PLA(2) ) and CETP were determined.

RESULTS

IO patients showed higher levels of HOMA-IR and triglycerides [median (Q1-Q3)] [128 (93-193) vs. 79(51-91) mg dL(-1) , P < 0·0005] while lower high-density lipoprotein (HDL) cholesterol (mean ± SD) (41 ± 9 vs. 52 ± 10 mg dL(-1) , P < 0·0005) in comparison with controls. Moreover, the triglycerides/HDL-cholesterol [3·2 (2·0-5·1) vs. 1·5 (1·0-1·9), P < 0·0005] ratio and oxidized low-density lipoprotein levels [94 (64-103) vs. 68 (59-70) IU L(-1) , P < 0·05] were increased in the patient group. Although no difference was observed in ARE activity, PON activity was decreased in IO patients [246 (127-410) vs. 428 (263-516) nmol mL(-1) min(-1) , P < 0·05]. In addition, CETP and Lp-PLA(2) activities were also increased in the patients (189 ± 31 vs. 155 ± 36% ml(-1)  h(-1) , P < 0·005; and 10·1 ± 2·9 vs. 8·2 ± 2·4 μmol mL(-1) h(-1) , P < 0·05, respectively). Associations between ferritin concentration and the alterations in lipid metabolism were also found. Multiple regression analyses identified HOMA-IR as independent predictor of CETP activity (B = 65·9, P < 0·0001, r(2) = 0·35), as well as ferritin concentration of Lp-PLA(2) activity (B = 3·7, P < 0·0001, r(2) = 0·40) after adjusting for confounding variables.

CONCLUSIONS

  IO patients presented not only insulin resistance but also metabolic alterations that were related to elevated iron stores and are associated with high risk of cardiovascular disease.

Deferoxamine promotes angiogenesis via the activation of vascular endothelial cell function

BACKGROUND

Deferoxamine (DFO), an iron chelator for disorders of excess iron, upregulates the expression of angiogenic factors, such as vascular endothelial growth factor (VEGF) and cyclooxygenase-2 (COX-2), indicating that it affects angiogenesis. Herein, we clarify the effect and mechanism of action of DFO on angiogenesis.

METHODS AND RESULTS

In an in vitro study, DFO increased endothelial nitric oxide synthesis (eNOS) phosphorylation in human aortic endothelial cells (HAECs), which were inhibited by the phosphatidylinositol 3-kinase inhibitor LY294002. Tube formation, cell proliferation, and cell migration in HAECs were promoted by DFO, which were significantly reduced by LY294002. In an in vivo study, DFO promoted blood flow recovery in response to the hindlimb ischemia in mice with unilateral hindlimb surgery. The density of capillaries and arterioles in ischemic muscle was higher in DFO-treated mice compared to vehicle-treated mice. Endothelial cell proliferation increased and oxidative stress and apoptosis decreased in ischemic muscles of DFO-treated mice. The phosphorylation of Akt and eNOS on the ischemic side was elevated and urinary nitric oxide/nitric dioxide (NOx) excretion was higher in DFO-treated mice compared to vehicle-treated mice. The effect of DFO on angiogenesis was abolished in eNOS-deficient mice with hindlimb ischemia.

CONCLUSION

These findings indicate that DFO promotes revascularization via the activation of vascular endothelial cell function by an Akt-eNOS-dependent mechanism.

Towards a unifying, systems biology understanding of large-scale cellular death and destruction caused by poorly liganded iron: Parkinson's, Huntington's, Alzheimer's, prions, bactericides, chemical toxicology and others as examples

Exposure to a variety of toxins and/or infectious agents leads to disease, degeneration and death, often characterised by circumstances in which cells or tissues do not merely die and cease to function but may be more or less entirely obliterated. It is then legitimate to ask the question as to whether, despite the many kinds of agent involved, there may be at least some unifying mechanisms of such cell death and destruction. I summarise the evidence that in a great many cases, one underlying mechanism, providing major stresses of this type, entails continuing and autocatalytic production (based on positive feedback mechanisms) of hydroxyl radicals via Fenton chemistry involving poorly liganded iron, leading to cell death via apoptosis (probably including via pathways induced by changes in the NF-κB system). While every pathway is in some sense connected to every other one, I highlight the literature evidence suggesting that the degenerative effects of many diseases and toxicological insults converge on iron dysregulation. This highlights specifically the role of iron metabolism, and the detailed speciation of iron, in chemical and other toxicology, and has significant implications for the use of iron chelating substances (probably in partnership with appropriate anti-oxidants) as nutritional or therapeutic agents in inhibiting both the progression of these mainly degenerative diseases and the sequelae of both chronic and acute toxin exposure. The complexity of biochemical networks, especially those involving autocatalytic behaviour and positive feedbacks, means that multiple interventions (e.g. of iron chelators plus antioxidants) are likely to prove most effective. A variety of systems biology approaches, that I summarise, can predict both the mechanisms involved in these cell death pathways and the optimal sites of action for nutritional or pharmacological interventions.

Hepatic iron loading in mice increases cholesterol biosynthesis

Iron and cholesterol are both essential metabolites in mammalian systems, and too much or too little of either can have serious clinical consequences. In addition, both have been associated with steatosis and its progression, contributing, inter alia, to an increase in hepatic oxidative stress. The interaction between iron and cholesterol is unclear, with no consistent evidence emerging with respect to changes in plasma cholesterol on the basis of iron status. We sought to clarify the role of iron in lipid metabolism by studying the effects of iron status on hepatic cholesterol synthesis in mice with differing iron status. Transcripts of seven enzymes in the cholesterol biosynthesis pathway were significantly up-regulated with increasing hepatic iron (R(2) between 0.602 and 0.164), including those of the rate-limiting enzyme, 3-hydroxy-3-methylglutarate-coenzyme A reductase (Hmgcr; R(2) = 0.362, P < 0.002). Hepatic cholesterol content correlated positively with hepatic iron (R(2) = 0.255, P < 0.007). There was no significant relationship between plasma cholesterol and either hepatic cholesterol or iron (R(2) = 0.101 and 0.014, respectively). Hepatic iron did not correlate with a number of known regulators of cholesterol synthesis, including sterol-regulatory element binding factor 2 (Srebf2; R(2) = 0.015), suggesting that the increases seen in the cholesterol biosynthesis pathway are independent of Srebf2. Transcripts of genes involved in bile acid synthesis, transport, or regulation did not increase with increasing hepatic iron.

CONCLUSION

This study suggests that hepatic iron loading increases liver cholesterol synthesis and provides a new and potentially important additional mechanism by which iron could contribute to the development of fatty liver disease or lipotoxicity.

Do high ferritin levels confer lower cardiovascular risk in men with Type 2 diabetes?

AIMS

High ferritin levels are associated with insulin resistance and liver steatosis, both thought of as emerging cardiovascular risk factors. The association between ferritin and cardiovascular disease is poorly documented in cardiometabolic states with higher cardiovascular risk, such as diabetes and metabolic syndrome. We therefore characterized a cohort of males with Type 2 diabetes mellitus (T2DM) according to ferritin levels and prevalent macroangiopathy.

METHODS

The presence of overall macroangiopathy, peripheral and/or coronary artery disease was documented in 424 consecutive T2DM males, who were divided according to ferritin quartiles (Q) as follows: QI-III, normal ferritin (NF; n=318), mean+/-1 sd ferritin 133+/-72 ng/ml; and QIV patients, high ferritin (HF; n=106), ferritin 480+/-228 ng/ml.

RESULTS

Age, age at diabetes diagnosis, smoking, ethanol intake, body mass index, waist circumference, blood pressure and presence of metabolic syndrome did not differ between groups. However, the prevalence of macroangiopathy was unexpectedly much lower in patients with high ferritin, as follows: 25% vs. 43% for overall macroangiopathy; 7% vs. 16% for peripheral artery disease; and 16% vs. 31% for coronary artery disease (P=0.0009, P=0.0140 and P=0.0035, respectively, vs. NF patients). Insulin resistance index and prevalence of liver steatosis were higher in HF compared with NF patients as follows: 2.17% vs. 1.89% and 78% vs. 64% (P=0.0345 and P=0.0059, respectively). Liver enzymes (aspartate aminotransferase, alanine aminotransferase and gamma-glutamyl transferase) were significantly higher in HF, by 33%, 42% and 72%, respectively (all P<0.0002), suggesting a higher prevalence of steatohepatitis. Glycated haemoglobin, low-density lipoprotein-cholesterol, high-density lipoprotein-cholesterol, triglycerides, urate, high-sensitivity C-reactive protein and albuminuria were not different between groups.

CONCLUSIONS

Our results demonstrate that T2DM males with high ferritin levels exhibit a markedly decreased prevalence of macroangiopathy, despite more severe insulin resistance and higher markers of steatohepatitis. High ferritin levels and/or steatosis may thus paradoxically confer a lowered cardiovascular risk in diabetic males.