Jerome L. Sullivan

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 AND THE GENETICS OF CARDIOVASCULAR DISEASE

The hypothesis that iron depletion protects against ischemic heart disease was proposed in 1981 as an explanation for the sex difference in heart disease rates.1 2 3 4 5 The “iron hypothesis” has generated significant debate, especially in the 1990s after publication of supportive findings from Finland.6 Confusion has been introduced into the debate by inappropriate study designs in tests of the hypothesis. A recurring weakness is that stored iron is measured by inappropriate methods. In addition, evidence against a particular mechanism may be erroneously taken as evidence against the core hypothesis. For example, in a particular study design, a finding of no association of stored iron with coronary atherosclerosis would not invalidate an association of stored iron with cardiovascular mortality. Recent studies relevant to the iron hypothesis have been reviewed.5 7 8 9 10 11 12 In 1994, Ascherio and Willett13 recognized that the iron hypothesis cannot be rejected on the basis of available data, stating that “[s]tronger evidence is needed before the hypothesis is rejected that greater iron stores increase the incidence of coronary heart disease or death from myocardial infarction.” Indeed, nothing in data made available since 1994 excludes the possibility that iron depletion has a large protective effect, large enough to explain the low incidence of myocardial infarction in menstruating women. A 1997 editorial14 on the iron hypothesis suggests that this once-controversial idea has become more acceptable to many scientists. Gillum14 noted that “this important hypothesis and related research fronts have led to many valuable studies like that of Kiechl et al [see Reference 99 ], which will greatly enhance our understanding of atherosclerotic vascular disease and may lead to innovations useful in clinical medicine and public health, perhaps in heretofore unanticipated ways.” Mechanisms of protection have not been …

Stored Iron and Vascular Reactivity

Zheng et al1 report that volunteer blood donors with a high donation frequency have significantly greater flow-mediated dilation than low frequency donors. Iron status among the frequent donors approximated a state of iron depletion by assessment of conventional iron markers. The study provides important support for the “iron hypothesis,” which suggests a protective effect of iron depletion, ie, the absence of storage iron without anemia, against ischemic heart disease.2–4 The blood donor findings suggest a new direction for the study of endothelial iron status in vivo to complement a growing body of work on iron in cultured endothelial cells. In addition, the finding of lower serum nitrotyrosine in frequent donors is consistent with the hypothesis that myeloperoxidase, a powerful emerging cardiovascular risk factor,5–8 is modifiable by manipulation of iron status. See page 1577 In cultured endothelial cells, iron status can be readily manipulated through the use of iron chelators or supplemental iron,9 or by altering the essential fatty acid composition of the culture medium.10 Use of these methods has identified several important effects of iron in endothelial activation and oxidative injury.9,11–20 But the in vitro approach does not define endothelial iron status in vivo, in particular what might represent a physiologically optimal range of endothelial iron concentrations. Iron status parameters such as serum ferritin are imperfect measures of total body iron stores and are likely to be even less adequate in assessing endothelial iron status. There is the additional uncertainty of differences in storage iron concentration in various cell types, even in situations in which total body iron status is well defined. An important premise of the study by Zheng et al1 is that body iron status within the conventionally defined normal range can influence endothelial iron status and endothelial function in vivo. …

Overexpression of transferrin receptor and ferritin related to clinical symptoms and destabilization of human carotid plaques.

Accumulation of tissue iron has been implicated in development of atherosclerotic lesions mainly because of increased iron-catalyzed oxidative injury. However, it remains unknown whether cellular iron import and storage in human atheroma are related to human atheroma development. We found that transferrin receptor 1 (TfR1), a major iron importer, is highly expressed in foamy macrophages and some smooth muscle cells in intimal lesions of human carotid atheroma, mainly in cytoplasmic accumulation patterns. In 52 human carotid atherosclerotic lesions, TfR1 expression was positively correlated with macrophage infiltration, ectopic lysosomal cathepsin L, and ferritin expression. Highly expressed TfR1 and ferritin in CD68-positive macrophages were significantly associated with development and severity of human carotid plaques, smoking, and patient’s symptoms. The findings suggest that pathologic macrophage iron metabolism may contribute to vulnerability of human atheroma, established risk factors, and their clinical symptoms. The cytoplasmic overexpression of TfR1 may be the result of lysosomal dysfunction and ectopic accumulation of lysosomal cathepsin L caused by atheroma-relevant lipids in atherogenesis.

Misconceptions in the debate on the iron hypothesis

“The epidemiological findings and the accumulating data on the protective effects of deferoxamine in myocardial injury are consistent with the iron depletion hypothesis. The hypothesis implies that ferritin is a risk factor for IHD but does not precisely define the epidemiological behavior of ferritin as a risk factor. The hypothesis focuses on protective effects of iron depletion, a state in which there is essentially no ferritin present. The hypothesis does not, and with available data, cannot adequately address the quantitative aspects of the relationship between ferritin and incidence of IHD. The strength of serum ferritin as a risk factor for IHD may

Putative hepatoprotective effects of coffee

Putative hepatoprotective effects of coffee SIRS, Cadden et al. suggest that coffee consumption may protect against abnormal liver biochemistry, cirrhosis and hepatocellular carcinoma. However, the mechanisms underlying this beneficial association are not completely understood. We suggest that the mechanisms involved in this association might, in part, be related to inhibition of iron absorption by polyphenol compounds present in coffee. The main phenolic compound in coffee, chlorogenic acid, is a potent inhibitor of nonhaeme iron absorption. 3 It has been shown that the polyphenolic content typical in a cup of instant coffee reduces iron absorption from a test meal by 60–90%. Among elderly participants in the Framingham Heart Study, each 236 mL ⁄ week (one cup ⁄ wk) of coffee consumed was associated with 1% lower serum ferritin. On the other hand, mounting evidence suggests that even mildly increased amounts of iron in the liver can be damaging, particularly if combined with other factors, such as alcohol use, assumption of hepatotoxic drugs or chronic viral hepatitis. Iron enhances the pathogenicity of microorganisms, adversely affects macrophages and lymphocytes, and enhances fibrogenic pathways, all of which may increase hepatic injury because of iron itself or because of the combination of iron with other factors. Iron may also be a cocarcinogen or a promoter of hepatocellular carcinoma, even in patients without haemochromatosis or cirrhosis. Furthermore, ironreduction therapy has been shown to improve liver functions in a variety of pathological conditions. Therefore, the increased intake of polyphenol compounds present in coffee may maintain a relatively lower iron status and therefore reduce the risk of liver injury.

Neutral amino acid transport and in vitro aging

Human fibroblasts possess five distinguishable systems for amino acid transport. Initial rates for three of these decline with in vitro age. The decline most noticeably affects the L system, whose rate for L-phenylalanine transport at 10 microM declines by 44% between the 27th and 39th population doubling levels. Accumulation of L-phenylalanine is similarly affected. The insensitivity of older cells to reduction of the amino acid concentration in culture medium casts doubt on possible interpretation of the transport decline as a causal factor in limiting growth.

The Effect of Polyphenols in Olive Oil on Heart Disease Risk Factors

TO THE EDITOR: Covas and colleagues (1) found that consumption of polyphenol-rich olive oil increased high-density lipoprotein (HDL) cholesterol levels and lowered levels of oxidative stress markers and oxidized low-density lipoprotein (LDL) cholesterol. We suggest that some of the involved mechanisms might be related to interactions of endogenous iron with polyphenols absorbed from the olive oil. The polyphenols may augment the antioxidant activity of endogenous iron-binding antioxidants (2). Use of phlebotomy to achieve modest decreases in stored iron level (3) or induction of near iron deficiency (4) is associated with increased HDL cholesterol concentration. Polyphenol-rich olive oil may acutely augment the reactive iron-neutralizing activity of endogenous iron-binding antioxidants. In prolonged use, olive oilderived polyphenols are associated with progressive loss of iron stores (5). Lowering the availability of reactive iron in vivo either by decreasing stored iron level or by acute iron chelation may improve antioxidant activity and increase HDL cholesterol levels by closely related mechanisms (6).

Membrane function in cystic fibrosis. II. Methionine transport in normal and cystic fibrosis fibroblasts.

Putrescine transport was examined in normal and cystic fibrosis fibroblasts. No differences were observed in accumulation pattern, kinetics of uptake, or efflux between CF and normal cells. In both growing and growth-arrested CF and normal fibroblasts, exogenously supplied putrescine remained unchanged for at least 60 min. Some differences were observed in the response of CF and normal cells to environmental (media) changes.