It has been proposed that elevated levels of tissue iron increase the risk for atherosclerosis, perhaps by favoring the formation of pro-atherogenic oxidized LDL. Working with apoE-deficient (apoE–/–) mice, which do not require a high-fat diet to develop atherosclerosis, we compared the effects of standard diet (0.02% iron) or a 2% carbonyl iron diet. After 24 weeks, mice fed the 2% carbonyl iron diet had twice as much iron in their plasma, a ninefold increase in bleomycin-detectable free iron in their plasma, and ten times as much iron in their livers as control mice. Dietary iron overload caused a modest (30%) rise in plasma triglyceride and cholesterol. Nevertheless, this regimen did not exacerbate, but rather reduced the severity of atherosclerosis by 50%, and it failed to elevate hepatic levels of heme oxygenase mRNA, which is induced by many different oxidative insults in vitro. Moreover, hepatic levels of protein-bound dityrosine and ortho-tyrosine, two markers of metal-catalyzed oxidative damage in vitro, failed to rise in iron-overloaded animals. Our observations suggest that elevated serum and tissue levels of iron are not atherogenic in apoE–/– mice. Moreover, they call into question the hypothesis that elevated levels of tissue iron promote LDL oxidation and oxidative stress in vivo.
Elizabeth A. Kirk, Jay W. Heinecke, Renée C. LeBoeuf
Submitter: Jerome L. Sullivan, MD, PhD | email@example.com
University of Florida College of Medicine
Published August 6, 2001
Kirk et al (1) report that massive iron overload reduces the severity of aortic atherosclerosis in apoE-deficient mice. The finding is offered as evidence against the "iron hypothesis," the suggestion that iron depletion protects against ischemic heart disease (2-6).
Key questions at issue include the following: What are the optimal levels of dietary iron and stored iron? Is the dietary concentration required to minimize atherosclerosis lower than the concentrations for maximizing growth or hemoglobin level? The relevance of their data to these questions is uncertain because important control groups were omitted. Two levels of dietary iron were used: 200 mg/kg and 20,000 mg/kg, referred to as "low iron-fed" and "high iron-fed," respectively. However, mice fed a diet containing 200 mg Fe/kg for 24 weeks may be significantly iron overloaded. At a minimum, an additional iron-depleted control group would be necessary to assess the significance of their iron status.
It has been shown that dietary iron restriction significantly decreases the severity of aortic atherosclerosis in the apoE-deficient mouse (7). Kirk et al (1) cite this study, but do not address the discrepancies with their findings. Lee et al (7) used very different dietary iron levels to achieve low and normal iron loads: 11.5 mg/kg and 50 mg/kg for "iron restricted" and "regular" iron diets, respectively. Both iron concentrations used by Lee et al (7) were associated with normal hemoglobin level. The "low iron-fed" control group of Kirk et al (1) received a dietary iron level more than 17 times greater than the iron restricted level used by Lee et al (7) . An iron level 17 times in excess of an amount that produces a normal hemoglobin level may represent significant iron overload, rather than a "low iron-fed" state.
The two sets of data in apoE-deficient mice fed 11.5, 50, 200 or 20,000 mg Fe/kg cannot be directly compared. However, the 11.5 mg/kg group had a significantly lower lesion areas than the 50 mg/kg group in aortic root sections (7), and both of these groups seem to have lower lesion areas than the groups studied by Kirk et al (1). Taken together, the two studies appear to be compatible with the validity of the iron hypothesis. Without appropriate controls, the findings of Kirk et al (1) give a misleading picture of the role of iron in atherogenesis.
1. Kirk EA, Heinecke JW, LeBoeuf RC. Iron overload diminishes atherosclerosis in apoE-deficient mice. J Clin.Invest 2001;107:1545-53.
2. Sullivan JL. Iron and the sex difference in heart disease risk. Lancet 1981;1:1293-94.
3. Sullivan JL. Iron and the genetics of cardiovascular disease. Circulation 1999;100:1260-63.
4. Salonen JT, Nyyssonen K, Korpela H, Tuomilehto J, Seppanen R, Salonen R. High stored iron levels are associated with excess risk of myocardial infarction in eastern Finnish men. Circulation 1992;86:803-11.
5. de Valk B, Marx JJ. Iron, atherosclerosis, and ischemic heart disease. Arch.Intern.Med. 1999;159:1542-48.
6. Zacharski LR, Chow B, Lavori PW, Howes PS, Bell MR, DiTommaso MA et al. The iron (Fe) and atherosclerosis study (FeAST): a pilot study of reduction of body iron stores in atherosclerotic peripheral vascular disease. Am.Heart J. 2000;139:337-45.
7. Lee TS, Shiao MS, Pan CC, Chau LY. Iron-deficient diet reduces atherosclerotic lesions in apoE-deficient mice. Circulation 1999;99:1222- 29.