Hemochromatosis (HFE) genotype and atherosclerosis: Increased susceptibility to iron-induced vascular damage in C282Y carriers?

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.

New insights into the role of iron in inflammation and atherosclerosis

Iron is fundamental for life-essential processes. However, it can also cause oxidative damage, which is thought to trigger numerous pathologies, including cardiovascular diseases. The role of iron in the pathogenesis of atherosclerosis is still not completely understood. Macrophages are both key players in the handling of iron throughout the body and in the onset, progression and destabilization of atherosclerotic plaques. Iron itself might impact atherosclerosis through its effects on macrophages. However, while targeting iron metabolism within macrophages may have some beneficial effects on preventing atherosclerotic plaque progression there may also be negative consequences. Thus, the prevailing view of iron being capable of accelerating the progression of coronary disease through lipid peroxidation may not fully take into account the multi-faceted role of iron in pathogenesis of atherosclerosis. In this review, we will summarize the current understanding of iron metabolism in the context of the complex interplay between iron, inflammation, and atherosclerosis.

Balance of cardiac and systemic hepcidin and its role in heart physiology and pathology

Hepcidin is the main regulator of iron metabolism in tissues. Its serum levels are mostly correlated with the levels of hepcidin expression from the liver, but local hepcidin can be important for the physiology of other organs as well. There is an increasing evidence that this is the case with cardiac hepcidin. This has been confirmed by studies with models of ischemic heart disease and other heart pathologies. In this review the discussion dissects the role of cardiac hepcidin in cellular homeostasis. This review is complemented with examination of the role of systemic hepcidin in heart disease and its use as a biochemical marker. The relationship between systemic vs local hepcidin in the heart is important because it can help us understand how the fine balance between the actions of two hepcidins affects heart function. Manipulating the axis systemic/cardiac hepcidin could serve as a new therapeutic strategy in heart diseases.

The role of iron in the pathogenesis of atherosclerosis

Ferritin and increased iron stores first appeared on the list of cardiovascular risk factors more than 30 years ago and their causal role in the pathogenesis of atherosclerosis has been heavily discussed since the early 1990s. It seems that besides traditional factors such as hyperlipoproteinemia, hypertension, diabetes mellitus, obesity, physical inactivity, smoking and family history, high iron stores represent an additional parameter that could modify individual cardiovascular risk. The role of iron in the pathogenesis of atherosclerosis was originally primarily associated with its ability to catalyze the formation of highly reactive free oxygen radicals and the oxidation of atherogenic lipoproteins. Later, it became clear that the mechanism is more complex. Atherosclerosis is a chronic fibroproliferative inflammatory process and iron, through increased oxidation stress as well as directly, can control both native and adaptive immune responses. Within the arterial wall, iron affects all of the cell types that participate in the atherosclerotic process (monocytes/macrophages, endothelial cells, vascular smooth muscle cells and platelets). Most intracellular iron is bound in ferritin, whereas redox-active iron forms labile iron pool. Pro-inflammatory and anti-inflammatory macrophages within arterial plaque differ with regard to the amount of intracellular iron and most probably with regard to their labile iron pool. Yet, the relation between plasma ferritin and intracellular labile iron pool has not been fully clarified. Data from population studies document that the consumption of meat and lack of physical activity contribute to increased iron stores. Patients with hereditary hemochromatosis, despite extreme iron storage, do not show increased manifestation of atherosclerosis probably due to the low expression of hepcidin in macrophages.

Vascular Damage in Patients with Nonalcoholic Fatty Liver Disease: Possible Role of Iron and Ferritin

Non Alcoholic Fatty Liver Disease (NAFLD) is the most common chronic liver disease in Western countries. Recent data indicated that NAFLD is a risk factor by itself contributing to the development of cardiovascular disease independently of classical known risk factors. Hyperferritinemia and mild increased iron stores are frequently observed in patients with NAFLD and several mechanisms have been proposed to explain the role of iron, through oxidative stress and interaction with insulin metabolism, in the development of vascular damage. Moreover, iron depletion has been shown to decrease atherogenesis in experimental models and in humans. This review presents the recent evidence on epidemiology, pathogenesis, and the possible explanation of the role of iron and ferritin in the development of cardiovascular damage in patients with NAFLD, and discusses the possible interplay between metabolic disorders associated with NAFLD and iron in the development of cardiovascular disease.

Decreased cardiovascular and extrahepatic cancer-related mortality in treated patients with mild HFE hemochromatosis

BACKGROUND & AIMS

Mortality studies in patients with hemochromatosis give conflicting results especially with respect to extrahepatic causes of death. Our objective was to assess mortality and causes of death in a cohort of patients homozygous for the C282Y mutation in the HFE gene, diagnosed since the availability of HFE testing.

METHODS

We studied 1085 C282Y homozygotes, consecutively diagnosed from 1996 to 2009, and treated according to current recommendations. Mortality and causes of death were obtained from death certificates and compared to those of the general population. Standardized mortality ratios (SMRs) were used to assess specific causes of death and the Cox model was used to identify prognostic factors for death.

RESULTS

Patients were followed for 8.3±3.9 years. Overall the SMR was the same as in the general population (0.94 CI: 0.71-1.22). Patients with serum ferritin⩾2000 μg/L had increased liver-related deaths (SMR: 23.9 CI: 13.9-38.2), especially due to hepatic cancer (SMR: 49.1 CI: 24.5-87.9). Patients with serum ferritin between normal and 1000 μg/L had a lower mortality than the general population (SMR: 0.27 CI: 0.1-0.5), due to a decreased mortality, related to reduced cardiovascular events and extrahepatic cancers in the absence of increased liver-related mortality. Age, diabetes, alcohol consumption, and hepatic fibrosis were independent prognostic factors of death.

CONCLUSIONS

In treated HFE hemochromatosis, only patients with serum ferritin higher than 2000 μg/L have an increased mortality, mainly related to liver diseases. Those with mild iron burden have a decreased overall mortality in relation to reduced cardiovascular and extrahepatic cancer-related events. These results support a beneficial effect of early and sustained management of patients with iron excess, even when mild.

Epidemiological associations between iron and cardiovascular disease and diabetes

Disruptions in iron homeostasis are linked to a broad spectrum of chronic conditions including cardiovascular, malignant, metabolic, and neurodegenerative disease. Evidence supporting this contention derives from a variety of analytical approaches, ranging from molecular to population-based studies. This review focuses on key epidemiological studies that assess the relationship between body iron status and chronic diseases, with particular emphasis on atherosclerosis ,metabolic syndrome and diabetes. Multiple surrogates have been used to measure body iron status, including serum ferritin, transferrin saturation, serum iron, and dietary iron intake. The lack of a uniform and standardized means of assessing body iron status has limited the precision of epidemiological associations. Intervention studies using depletion of iron to alter risk have been conducted. Genetic and molecular techniques have helped to explicate the biochemistry of iron metabolism at the molecular level. Plausible explanations for how iron contributes to the pathogenesis of these chronic diseases are beginning to be elucidated. Most evidence supports the hypothesis that excess iron contributes to chronic disease by fostering excess production of free radicals. Overall, epidemiological studies, reinforced by basic science experiments, provide a strong line of evidence supporting the association between iron and elevated risk of cardiovascular disease and diabetes. In this narrative review we attempt to condense the information from existing literature on this topic.

Serum Hepcidin and Macrophage Iron Correlate With MCP-1 Release and Vascular Damage in Patients With Metabolic Syndrome Alterations

Objective—

Increased body iron stores and hepcidin have been hypothesized to promote atherosclerosis by inducing macrophage iron accumulation and release of cytokines, but direct demonstration in human cells is lacking. The aim of this study was to evaluate the effect of iron on cytokine release in monocytes ex vivo and the correlation with vascular damage and to evaluate the relationship among serum levels of hepcidin, cytokines, and vascular damage in patients with metabolic syndrome alterations.

Methods and Results—

Manipulation of iron status with ferric ammonium citrate and hepcidin-25 induced monocyte chemoattractant protein (MCP)-1 and interleukin-6 in human differentiating monocytes of patients with hyperferritinemia associated with the metabolic syndrome (n=11), but not in subjects with hemochromatosis or HFE mutations impairing iron accumulation (n=15), and the degree of induction correlated with the presence of carotid plaques, detected by echocolor–Doppler. In monocytes of healthy subjects (n=7), iron and hepcidin increased the mRNA levels and release of MCP-1, but not of interleukin-6. In 130 patients with metabolic alterations, MCP-1 levels, as detected by ELISA, were correlated with hepcidin-25 measured by time-of-flight mass spectrometry ( P =0.005) and were an independent predictor of the presence of carotid plaques ( P =0.05).

Conclusion—

Hepcidin and macrophage iron correlate with MCP-1 release and vascular damage in high-risk individuals with metabolic alterations.