Iron versus cholesterol--perspectives on the iron and heart disease debate

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.

IHD from copper deficiency: a unified theory

The theory, in brief outline here, implicating deficiency of Cu in the aetiology and pathophysiology of IHD explains more attributes of the disease than any other theory. This theory satisfies several of Hill's criteria of a half-century ago for deducing association between an environmental feature and presence of an illness. Most important is the temporal association between the rise of IHD and the decrease in dietary Cu since the 1930s along with a parallel increase in the supplementation of pregnant women with Fe, a Cu antagonist. There are more than eighty anatomical, chemical and physiological similarities between animals deficient in Cu and individuals with IHD. Few of these similarities have been produced by other dietary manipulations because feeding cholesterol induces Cu deficiency in animals. The most recent of these to be identified is decreased serum dehydroepiandrosterone. Some concomitant aspects of Cu metabolism and utilisation have been identified in other theories about heart disease: fetal programming, homocysteine, and Fe overload.

Deferoxamine stimulates LDLR expression and LDL uptake in HepG2 cells

SCOPE

Iron overload contributes to the pathogenesis of atherosclerosis and iron chelators are beneficial through their antioxidant properties. Hepatic iron loading increases cholesterol synthesis. Whether iron depletion could affect hepatic cholesterol metabolism is unknown.

METHODS AND RESULTS

We examined the effect of the iron chelator deferoxamine (DFO) on mRNA expression of genes involved in cholesterol metabolism and/or cholesterol uptake. Our results revealed that DFO increases LDL receptor (LDLR) mRNA levels in human hepatocyte-derived cell lines HepG2 and Huh7 cells, and in K562 cells. In HepG2 cells, we observed that DFO increases (i) LDLR-mRNA levels in a time- and dose-dependent manner, (ii) LDLR-protein levels; (iii) cell surface LDLR; and (iv) LDL uptake. In contrast, the mRNA levels of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, sterol regulatory element-binding proteins, and the mRNA/protein levels of proprotein convertase subtilisin-kexin 9 were not modulated by DFO, suggesting that the LDLR regulation by DFO is not at the transcriptional or posttranslational levels. Since LDLR-mRNA was stabilized by DFO, a posttranscriptional mechanism is suggested for the DFO-mediated upregulation of LDLR.

CONCLUSION

DFO induced an increase in LDLR expression by a posttranscriptional mechanism resulting in an enhancement of LDL uptake in HepG2 cells, suggesting increased LDLR activity as one of the underlying causes of the hypocholesterolemic effect of iron reduction.

The IDOL-UBE2D complex mediates sterol-dependent degradation of the LDL receptor

We previously identified the E3 ubiquitin ligase IDOL as a sterol-dependent regulator of the LDL receptor (LDLR). The molecular pathway underlying IDOL action, however, remains to be determined. Here we report the identification and biochemical and structural characterization of an E2-E3 ubiquitin ligase complex for LDLR degradation. We identified the UBE2D family (UBE2D1-4) as E2 partners for IDOL that support both autoubiquitination and IDOL-dependent ubiquitination of the LDLR in a cell-free system. NMR chemical shift mapping and a 2.1 Å crystal structure of the IDOL RING domain-UBE2D1 complex revealed key interactions between the dimeric IDOL protein and the E2 enzyme. Analysis of the IDOL-UBE2D1 interface also defined the stereochemical basis for the selectivity of IDOL for UBE2Ds over other E2 ligases. Structure-based mutations that inhibit IDOL dimerization or IDOL-UBE2D interaction block IDOL-dependent LDLR ubiquitination and degradation. Furthermore, expression of a dominant-negative UBE2D enzyme inhibits the ability of IDOL to degrade the LDLR in cells. These results identify the IDOL-UBE2D complex as an important determinant of LDLR activity, and provide insight into molecular mechanisms underlying the regulation of cholesterol uptake.

Marginal hazards model for case-cohort studies with multiple disease outcomes

Case-cohort study designs are widely used to reduce the cost of large cohort studies while achieving the same goals, especially when the disease rate is low. A key advantage of the case-cohort study design is its capacity to use the same subcohort for several diseases or for several subtypes of disease. In order to compare the effect of a risk factor on different types of diseases, times to different events need to be modelled simultaneously. Valid statistical methods that take the correlations among the outcomes from the same subject into account need to be developed. To this end, we consider marginal proportional hazards regression models for case-cohort studies with multiple disease outcomes. We also consider generalized case-cohort designs that do not require sampling all the cases, which is more realistic for multiple disease outcomes. We propose an estimating equation approach for parameter estimation with two different types of weights. Consistency and asymptotic normality of the proposed estimators are established. Large sample approximation works well in small samples in simulation studies. The proposed methods are applied to the Busselton Health Study.

Influence of target hemoglobin in dialysis patients on morbidity and mortality

Since the introduction of recombinant human erythropoietin (rHuEPO) in 1989, the level of anemia correction has been debated. Information developed from clinical trials and observational studies have given conflicting results. The normal hematocrit (Hct) trial of Besarab et al showed no benefit, and a possible risk, of correcting the Hct from 30% to 42% in hemodialysis patients with NYHA class I to III cardiac disease. The study of Moreno et al in non-cardiac patients showed improved Sickness Impact Profile and Karnofsky scores in hemodialysis patients when their Hct was increased from 31% to 38.5%. Hospitalizations were significantly reduced. Observational studies from Madore et al, Xia et al, Ma et al, and Collins et al all show that increased morbidity and mortality are significantly associated with Hct <33%. Recent data on incident hemodialysis patients indicate the associated risk of death was not different for patients with Hct 33 to <36% versus Hct 36 to <39%. Hospitalization risks and associated costs were significantly less in the patients with Hct 36 to <39%, suggesting that higher Hct values may be less of a concern than previously considered. The current data suggest that patents with advanced cardiac disease should avoid Hct values in the normal range. In others, Hcts at least up to 39% appear to be safe and effective. Based on this review, a reasonable target Hct range may be 33 to 39%, which balances the risks and potential benefits.

Impact of body iron status on myocardial perfusion, left ventricular function, and angiographic morphologic features in patients with hypercholesterolemia

BACKGROUND

Previous studies have shown that the effects of iron stores on atherogenesis through promotion of free radical formation and low-density lipoprotein (LDL) oxidation largely depend on the state of hypercholesterolemia (HCL) in animal models. A synergistic association of serum ferritin and LDL cholesterol with the risk of myocardial infarction has also been observed in humans.

METHODS

We sought to assess the relationship of serum iron parameters to myocardial perfusion and wall motion abnormalities and to the extent of angiographic coronary artery disease (CAD) in patients with HCL. Sixty-eight male patients (mean age 58 +/- 9 years) with hypercholesterolemia (LDL cholesterol >130 mg/dL) who had never been treated and 52 normocholesterolemic male subjects of similar age underwent coronary angiography and exercise technetium-99m sestamibi gated single-photon emission computed tomography imaging within 10 days.

RESULTS

Serum ferritin had a significant correlation with the perfusion index (r = 0.70, P <.001), the reversibility index (r = 0.68, P <.01), and the wall motion index (r = 0.54, P <.05), whereas a relatively weak correlation was observed between total iron binding capacity and perfusion index (inversely) (r = -0.59, P <.01) in patients with HCL. Iron parameters were not associated with either perfusion or wall motion indices in the normocholesterolemic group. Stepwise multiple regression analysis confirmed these results. Ferritin was a strong determinant of perfusion in patients with HCL only (beta =.55, P =.002). Iron parameters were not related to the angiographic extent of CAD as defined by angiographic vessel or extent score in either group.

CONCLUSIONS

Our data suggest that increased iron stores are closely associated with a greater extent and severity of perfusion and functional abnormalities but not with the angiographic extent of CAD in patients with HCL. Enhanced iron-mediated oxidative stress and LDL peroxidation may contribute to the hypercholesterolemia-related endothelial dysfunction and cause further impairment of myocardial perfusion and wall motion.

Marginal vitamin and mineral intake of Costa Rican adolescents

BACKGROUND

Although a great deal of attention is given to macronutrient and energy intake in the diet, elements essential to dietary metabolic balance include important micronutrients. Very little information exists on vitamin and mineral intake during adolescence, especially in developing countries.

METHODS

The micronutrient intake of urban and rural adolescents aged 12-19 years from the Costa Rican capital city, San José, was evaluated. Prospective 3-day diet records including 2 weekdays and 1 weekend day were used for dietary data. We evaluated the micronutrient intake to determine the nutrient adequacy ratio.

RESULTS

Approximately 30% of adolescents did not meet the 70% level of the daily recommended intake (DRI) for vitamin Bl2. Additionally, 40% of the Costa Rican youngsters presented a vitamin A, E, and iron intake of between 30 and 69% of the DRI or recommended daily dose (RDA) for these nutrients. Likewise, approximately 15% of adolescents did not meet 30% of the DRI or RDA indicators for these nutrients. The micronutrients most at risk for inadequate intake were zinc, calcium, magnesium, and folate: more than 25% of the adolescents did not meet 50% of DRI or RDA indications for these micronutrients. Contrariwise, vitamin C, vitamin B6, thiamin, riboflavin, and niacin were the nutrients less at risk for inadequate intake. Fast foods prepared in school cafeterias provided approximately 15-30% of the DRI for vitamins B1, B2, B6, Bl2, and niacin, approximately 18% of the RDA for iron, and over 40% of the RDA for vitamin E.

CONCLUSIONS

Nutritional interventions and educational strategies are needed to promote the adoption of healthful eating habits among adolescents.

Serum ferritin and death from all causes and cardiovascular disease: the NHANES II Mortality Study. National Health and Nutrition Examination Study

PURPOSE

The purpose of this study was to assess the association between serum ferritin and death from all causes, cardiovascular diseases (CVD), CHD and myocardial infarction (MI). Positive body iron stores have been proposed as a risk factor for coronary heart disease (CHD). While most epidemiologic studies using serum ferritin and other measures of body iron stores have not found an association between iron and heart disease risk, the hypothesis remains controversial. As a result, we examined the relationship of serum ferritin, the principle blood measure of body iron stores, to risk of death in a cohort with a standardized exam and long follow-up.

METHODS

The baseline data for this prospective cohort study were collected in 1976-1980 as part of the second National Health and Nutrition Examination Study (NHANES II) with mortality follow-up using the National Death Index (NDI) through December 31, 1992. The analytic sample (n = 1604) consisted of 128 black men, 658 white men, 100 black women and 718 white women 45-74 years of age at baseline who, based on self-reported data, were free of coronary heart disease at baseline and had no missing data. The main outcome measures were the relative risk of death for persons with serum ferritin levels: <50 microg/L; or 100-199 microg/L; or > or =200 microg/L was compared to persons with serum ferritin levels of 50-99 microg/L adjusted for possible confounding using the Cox proportional hazards model.

RESULTS

Most of the deaths were among white men (n = 254) and women (n = 168). There were relatively few deaths among black men (n = 50) and too few in women (n = 23) to reliably model. The largest number of CVD (n = 119), CHD (n = 82), and MI (n = 49) deaths were in white men while there were 69 CVD, 45 CHD and 13 MI deaths in white women. Black men with a serum ferritin level of <50 microg/L had a significantly higher adjusted risk of death from all causes (RR = 3.1 with 95% confidence limits of 1.5-6.5). There were no other statistically significant associations for all causes mortality for the other three race/sex groups. Additionally, there were no statistically significant associations between serum ferritin and any of the cardiovascular endpoints for any of the groups. There was an apparent but nonsignificant u-shaped association between serum ferritin and all causes mortality in black men and between serum ferritin and CVD death in white women. However, in both cases very wide confidence limits preclude further interpretation.

CONCLUSIONS

Overall, the results do not support the hypothesis that positive body iron stores, as measured by serum ferritin, are associated with an increased risk of CVD, CHD or MI death or between serum ferritin and all causes mortality. Most of the research to date with serum ferritin has been conducted in European men or in European American men. Our results are consistent with the primarily negative results for that race/sex group. More research is needed in women and minority groups, including an explanation of why such an association would exist in these groups but not in white men before an association can be established in them.

Novel risk factors for coronary heart disease: emerging connections

In only 50% of cases of coronary heart disease (CHD) is the cause attributable to the established risk factors of hypertension, cigarette smoking, and elevated total and low-density lipoprotein (LDL) cholesterol levels. This finding has led to research examining other markers for CHD that may have a causal link to the atherothrombotic process. Several of these "emerging" risk factors are reviewed in this article: lipoprotein(a); small dense LDL particle size; hyperhomocysteinemia; and inflammatory, infectious, and hemostatic factors. Evaluation of each of these factors includes a review of the epidemiologic evidence, examination of the pathologic mechanism(s) by which the factor might participate in atherothrombosis, and the clinical utility of screening. Finally, and most relevant for the practicing clinician, the following is addressed: Does evidence exist that selective modification of these risk factors is associated with net clinical benefit?

Iron therapy and cardiovascular disease

Intensive iron therapy is now a generally accepted adjunct for the treatment of renal anemia with recombinant human erythropoietin. However, with the emerging role of iron in cardiovascular disease, carcinogenesis, infectious diseases, and other disorders, it is no longer appropriate to assume that any amount of stored iron is safe until proven otherwise. In this article, the history and current status of the "iron hypothesis" on ischemic heart disease are briefly reviewed, followed by comments on iron management practices for renal patients.