Iron and Copper Toxicity in Diseases of Aging, Particularly Atherosclerosis and Alzheimer’s Disease

 

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George J. Brewer1

Department of Human Genetics, Department of Internal Medicine, University of Michigan Medical School, Ann Arbor, Michigan

¹ 5024 Kresge Bldg. II, Ann Arbor, MI 48109-0534. E-mail: brewergj{at}umich.edu

	Abstract

TOP Abstract Introduction Iron Toxicity Copper Toxicity Summary, Recommendations, and… References

 

In this review, we point out that natural selection does notact to lessen human diseases after the reproductive and caregivingperiod and that normal levels of iron and copper that may behealthy during the reproductive years appear to be contributingto diseases of aging and possibly the aging process itself.It is clear that oxidant damage contributes to many of the diseasesof aging, such as atherosclerosis, Alzheimer’s disease,Parkinson’s diseases, diabetes, diseases of inflammation,diseases of fibrosis, diseases of autoimmunity, and so on. Itis equally clear that both iron and copper can contribute toexcess production of damaging reactive oxygen species throughFenton chemistry. Here, we examine the evidence that “normal”levels of iron and copper contribute to various diseases ofaging.

Keywords: iron, copper, atherosclerosis, Alzheimer’s disease

	Introduction

TOP Abstract Introduction Iron Toxicity Copper Toxicity Summary, Recommendations, and… References

 

The concept of normalcy is intrinsic to medicine. We have normalvalues for everything from blood counts to electrolytes. Wemake minor concessions for variation due to age or gender withsome variables, but for the most part, normal values are normalvalues throughout the human lifespan.

We assume that normal values are healthy values, but in thisregard we fail to consider that natural selection works to optimizehealth and survival only during the reproductive and early care-givingperiod, roughly the first 50 years of human life. Thus, whatmight be healthy during that period may not be optimal in termsof health after age 50, that is, the period when diseases ofaging become prevalent. There is no natural selection operatingto prevent or mitigate diseases of aging as long as the diseasesdo not impinge significantly on the reproductive period.

In this essay, I would like to consider the above with respect to iron and copper stores and levels of “free” iron and copper in the body. In general, I wish to point out that “normal” stores of iron and copper during reproductive years provide reserves for such things as hemorrhage or periods of severe dietary restrictions and starvation, thus protecting the individual during their early years, but may contribute in a major way to diseases of aging. Thus, these stores may be in the best interests of the younger person in terms of survival and relative good health. However, both iron and copper are transition elements that fuel generation of damaging reactive oxygen species (ROS) if present in “normal” amounts, and this oxidant damage takes its toll in the later years of life. Normal values for iron and copper variables are summarized in Table 1. Actual values for serum ferritin in adult Americans shown in Table 1 come from Zacharski et al. (1).

Oxidative metabolism is a key aspect of life and is the underpinningof energy generation and use in most organisms, including thehuman. But the tradeoff is generation of ROS and the oxidativedamage they can cause. Organisms have developed antioxidantprotective mechanisms, but these are not perfect, and some ROSescape and cause damage to various molecules. This can becomeworse in times of stress, such as inflammation, when generationof ROS increases. The evidence is increasing that the agingprocess itself, as well as many of the diseases of aging, arecaused at least in part by oxidant damage.

One of the first authors to point to oxidant radicals as a cause of aging was Harman (2, 3). Butterfield’s group has published extensively on the role of oxidant radicals, generated from Fenton reactions dependent on iron or copper, on protein and lipid peroxidation, brain aging, and neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease, and amyotrophic lateral sclerosis (4–8). Their reviews, for example Poon et al. (4), are quite instructive as to theunderlying mechanisms of oxidant radical damage.

The deleterious effects of oxidant damage from iron and copperare not evident early in life, absent mutations causing veryexcessive accumulations of iron (hemochromatosis) or copper(Wilson’s disease). However, the evidence suggests thatthese harmful effects gradually accumulate, and, as we age,take their toll in terms of many of the diseases of aging. Iwill argue and present evidence that lowering the availabilityof both iron and copper might mitigate these diseases and possiblyslow the aging process.

	Iron Toxicity

TOP Abstract Introduction Iron Toxicity Copper Toxicity Summary, Recommendations, and… References

 

Taking iron first, clearly iron is absolutely vital to life. A good review of iron metabolism is given in the first part of the book by Weinberg (9). A short version is given in the introduction of the paper by Zecca et al. (10). Iron is a centralelement of the heme molecule, which is a critical part of hemoglobinand essential for oxygen transport. Heme is also part of manyessential enzymes, such as the cytochrome series. Iron is alsoa vital component of other enzymes and proteins. Having adequateiron is important, because as iron deficiency sets in, anemiaensues, which, depending on its severity, can lead to fatigability,decreased exercise tolerance, and general inanition. Further,such a person becomes much more susceptible to traumatic episodeswith bleeding—simply, their reserves of blood are depleted.Thus, evolution has acted against this during the reproductiveyears by providing for reserve iron stores in times of plentyto balance periods of relative famine and losses from such thingsas menstruation and hemorrhage. Even so, it must be noted thatpremenopausal women are not completely protected from iron deficiencyanemia. Intake of adequate bioavailable iron, usually in theform of meat, is necessary to prevent anemia, and iron deficiencyanemia is common among premenopausal women due to food choiceand bleeding.

Serum ferritin provides one measure of iron stores, and a low ferritin is a reliable indicator of iron deficiency. However, as pointed out by Hallberg and Hulthen (11), an elevated ferritinis not always a true indicator of iron stores, because ferritinis an acute phase reactant. Nonetheless, under most circumstancesferritin is a good marker of iron stores, and it is usuallyhigher in men then in women, particularly menstruating women(Table 1). There tends to be recovery towards the male valuesin menopausal women. Thus, menstruation reduces some of theiron stores, and, at least in current times, this isn’tcompensated for in men.

As already mentioned, the toxicity of iron is related to its involvement in producing oxidant damage. Through Fenton chemistry and other reactions, iron catalyzes the production of the toxic hydroxyl radical as well as other ROS (12–15). The production of ROS in vivo, the role of metals such as iron and copper, and the antioxidant protective mechanisms are thoroughly reviewed in Poon et al. (4). Increasingly, it is apparent, as documentedin the following sections, that oxidant damage is intimatelyinvolved with diseases of aging, such as atherosclerosis, diseasesof autoimmunity, AD, Parkinson’s disease, diabetes, diseasesof fibrosis, diseases of inflammation, and others. Thus, rightfrom the start, the production of ROS by iron has to be onesuspect in diseases of aging.

Iron and Atherosclerosis.

The evidence that relative iron availability contributes to diseases of aging is strongest with atherosclerotic disease, and is of several types (see Table 2 for a summary). The concept was first proposed by Sullivan (16–18), and his major rationale at the start was that the much lower risk of atherosclerotic cardiovascular disease in menstruating women than in men of the same ages was due to the reduced iron stores in the women (see ferritin levels in Table 1). When women stop menstruating they begin to lose this protective effect. Efforts to show that this protective effect in menstruating women is due to hormonal differences during this period have failed. Data from the Framingham study show that the increased risk of coronary heart disease is equal in women who underwent natural as opposed to surgical menopause, independent of oophorectomy as part of the surgery. This finding suggests a uterine (blood loss) etiology to worsening risk as opposed to an ovarian (estrogen) etiology (19, 20). Additionally, postmenopausal hormone replacement therapy is ineffective in reducing the rate of coronary heart disease events (21, 22).

A second line of evidence is epidemiological, looking at atherosclerotic heart disease risk or some other measure of atherosclerosis, such as carotid artery intimal thickness, and correlating it, usually with serum ferritin but occasionally with some other measure of iron stores, such as transferrin saturation. This area has been recently (2005) reviewed by You and Wang (23). They cite 12 epidemiological studies supporting a relationship between stored iron and cardiovascular disease and 27 studies which are nonsupportive. Citations to the original 39 studies can be found in You and Wang (23). One of the problems with this approach is that it is presumably the “free,” “reactive,” or “readily available” iron, sometimes called the “labile iron pool,” that is toxic, and once ferritin levels have reached some minimum, additional stored iron as measured by ferritin (or transferrin saturation) may not influence levels of free iron very much. I agree with Lee and Jacobs (24) that the contraryepidemiological evidence could very well be due to serum ferritinand transferrin saturation not being a good measure of the labileiron pool. This labile pool of iron, nontransferrin-bound, ismuch more likely to be more intimately involved with the generationof oxidant stress and is probably only weakly correlated withserum ferritin and transferrin saturation. Given this reasonableexplanation for the negative epidemiological studies, possiblya good deal more weight should be given to the positive studies,which found an association in spite of the likely poor correlationbetween measures of stored iron and the level of free iron.Of course, we must keep in mind that association does not provecause and effect.

Another type of epidemiological evidence is correlation of oxidative damage, in this case oxidative DNA damage measured by circulating 8-hydroxydeoxyguanosine levels, and serum ferritin in a large sample of Japanese men and women. There was a very significant correlation in both sexes (25). Again, this does not prove thathigher levels of iron are causing greater oxidation, but itis consistent with that hypothesis.

A fourth line of evidence is the effect of blood donation, expected to reduce available iron, on atherosclerotic heart disease. Three studies of this type cited by You and Wang (23) have been positive but can be criticized on the basis of the “healthy donor effect,” namely that volunteer blood donors enjoy better health than nondonors. One study of this type not cited by You and Wang (23) was negative (26).

A fifth line of evidence is various types of animal studies which have generally been supportive of a role of free iron in atherosclerosis and related processes. Meyers (27) cites12 animal studies which have been supportive and only one nonsupportive.The positive animal model studies range from animals fed iron-deficientdiets or given iron chelators and showing less atherosclerosisto animals supplemented with iron showing more atherosclerosis.

A sixth line of evidence is molecular studies which have been supportive of the role of iron in atherosclerosis and provide additional insight into mechanisms. These include findings of high iron deposition in human atherosclerotic lesions (28–32), demonstration that H- and L-ferritin mRNAs are higher in human and rabbit atherosclerotic vessels than in normal ones (33), the colocalization of iron with ceroid in human atherosclerotic tissue (34), and inhibition of low-density lipoprotein (LDL) oxidation by an iron chelator (35, 36). In other studies, plasma levels of cholesterol oxidation products, thought to be intimately involved in atherosclerosis, were correlated with ferritin levels in Finnish men (37). Another study suggests that homocysteine, another cardiovascular disease risk factor, promotes iron-catalyzed oxidation of LDL (38). As pointed out by Balla et al. (39),heme oxygenase and ferritin genes are upregulated in endotheliumin the early phase of progression of atherosclerotic lesions,perhaps a response to iron toxicity.

A criticism of some of the molecular studies cited above inwhich iron and iron-related abnormalities occur in the vesselwall relates to the “chicken and egg” question. That is, otherprocesses, such as inflammation, might initiate the vessel wallabnormality, and accumulation of iron and iron related abnormalitiesmight be secondary phenomena, not important in pathogenesis.

Finally, a seventh line of evidence (Table 2) produces mixed results. Heterozygosity for a hemochromatosis (an iron-loading disorder) gene has produced increased risk of atherosclerotic disease in two separate studies (40, 41). There is a mild increase in iron loading in the heterozygous state. However, there is strong iron overloading in the homozygous state, and there the results are mixed and controversial. Failla et al. (42) did find structural abnormalities in the radial arteries of hemochromatosis patients, which largely reverted with iron depletion. However, Niederau (43) has summarized the overall data and concludesbeyond any doubt that atherosclerotic coronary heart disease,stroke, and peripheral artery disease are rare clinical featuresand causes of death in homozygous hemochromatosis. The lackof excess atherosclerosis in hemochromatosis, if this is infact the case, is negative evidence against the hypothesis thatexcess available iron is causative in atherosclerosis. Certainlythe availability of excess iron is high in this disease, aswitness the damage to heart, joints, pancreas, etc. To believein the excess iron/atherosclerosis hypothesis, one has to postulatethat the lack of effect in homozygous hemochromatosis is dueto some type of differences or secondary effects which mitigatethe atherosclerotic effect.

Figure 1 portrays a scheme in which increased iron could contributeto the pathogenesis of atherosclerosis along with other riskfactors. Thus, it is important to keep in mind that the hypothesisis not that iron acts alone, but that it acts in concert withother risk factors. Thus, there is increasing interest in thehypothesis that atherosclerosis is an inflammatory disease.The excess-iron hypothesis fits well with the inflammatory hypothesisin that oxidant damage is a central feature of each, and theywould be expected to add to one another. Likewise, increasedLDL is a strong risk factor, and it fits with the excess ironhypothesis because iron oxidizes LDL, a central feature of theatherosclerotic process. Similarly, increased serum homocysteineis a risk factor, and there is evidence that homocysteine promotesiron-catalyzed oxidation of LDL.

Admitting that the homozygous hemochromatosis situation is a “fly in the ointment,” I nonetheless believe the weight of the rest of the data is sufficient to implicate iron in the atherosclerotic process. Thus, I conclude at this point that Sullivan (16) islikely correct, and that stored iron, or perhaps, more preciselyput, ready availability of labile iron, contributes to the diseaseof aging, atherosclerosis. At the least, there is enough evidencefor the involvement of excess iron as a causative factor inatherosclerosis that the hypothesis needs thorough testing,including clinical trials.

Iron and AD.

Beyond atherosclerosis, iron has also been implicated in neurodegenerative diseases of aging, including AD. Numerous papers have appeared on this topic (15, 44–48) including a review by Ong and Halliwell (49), who discuss mechanisms, particularly the interaction of iron and cholesterol in promoting oxidative damage in both atherosclerosis and neurodegeneration. Additional evidence of the involvement of iron in AD is the association of what Zecca et al. (10) call “iron management genes” with AD. Thus, mutations in the hemochromatosis gene, HFE, are more common in AD patients than in the general population (50). Patients with the transferrin subtype C2 are also more common in AD than in the general population (51–53). The presence of the C2 variant plus an HFE mutation increased the risk of AD 5-fold (54). A clinical trial of the iron chelator desferrioxamine given for two years to AD patients was very positive in terms of slowing the clinical progression of dementia (55). Dementia was measured by a video recorderhome-behavioral assessment, which had three parts and measured44 tasks, mostly samples of daily living. These videos werescored by two blinded trained raters, who had established a90% agreement with an “expert rater.” The statistically significantbetter performance of the treated group versus the untreatedgroup means that tasks of daily living were significantly betterpreserved in the treated group. Desferrioxamine will chelatealuminum as well as iron, so theoretically the benefit couldresult from reduction of aluminum levels, but aluminum toxicityis not currently believed to play a role in AD.

Other Toxicities of Iron.

It is thought that excess iron accelerates the aging process in general (13). A considerable body of literature has built up, with several recent reviews (15, 56, 57), on the concept of iron, free radicals, and mitochondrial injury as keys to the aging process. It is of considerable interest that lowering total body iron has been shown to increase the life span of some organisms. Good examples are the fruit fly (58) and the housefly (59).

There are possibly interesting data relating elevated transferrin saturation to overall mortality from the NHANES 1 study. Individuals with a transferrin saturation greater than 55% (between 1 and 2% of the population) had increased mortality compared with those with lower saturations (60). It was also found that those with elevated transferrin saturation had higher mortality if they had a high iron or red meat intake (61). How many of thepeople with elevated transferrin saturation might have beenheterozygous or even homozygous for hemochromatosis is unclear.

	Copper Toxicity

TOP Abstract Introduction Iron Toxicity Copper Toxicity Summary, Recommendations, and… References

 

Turning to copper, it is also vital to life. Copper is an essential component of countless enzymes and other proteins and is critical for numerous reactions necessary for life (62). Deficiency leads to anemia and bone marrow suppression, followed by a neurologic syndrome called a myelopathy (63). As with iron, the most bioavailable source of copper is in meat. Although almost all foods have some copper, vegetarian diets are much more borderline in providing adequate copper (64, 65). Perhaps to counter gaps in dietary, particularly meat, provision of copper, evolution has provided for copper storage in the liver, primarily bound to metallothionein, a protein which can bind multiple molecules of copper (as well as other divalent cations, such as zinc). A blood protein, ceruloplasmin (Cp), synthesized by the liver and also containing several molecules of copper, is, like ferritin for iron, one marker of body copper status (66). Plasma Cp levels begin to decrease after the readilymobilizable copper stores of the liver are depleted.

As with iron, the reserve stores of copper have no doubt beenselected for by natural selection to maintain adequate copperduring periods of famine, to maximize reproductive potential.However, just as with iron, copper participates in generationof ROS through Fenton chemistry and can produce oxidative damagein much the same manner.

The copper that participates in producing ROS is called “free”copper, probably a somewhat inappropriate term. This term isused to include the noncovalently bound copper, and thus refersto copper more loosely bound to proteins and other molecules,so for the most part it is not really free. As with iron, abetter term might be “labile copper pool,” although we willuse the term “free” copper to refer to this pool in this review.About 90% of blood copper in humans is covalently bound to Cp,while the remaining 10% is free, loosely bound to albumin andother molecules. In cells of all types, there is a considerablepool of free copper, some of it stored in metallothionein. Thisamount of free copper produces ROS throughout life, with atleast partial protection by antioxidant defense mechanisms.It is our hypothesis that damage gradually accrues from thissource and becomes evident in the many diseases of aging.

Copper and Atherosclerosis.

One type of evidence that copper contributes to diseases of aging is, again, epidemiological evidence relating copper levels, or in some cases Cp levels, to atherosclerotic disease. (For a summary of the evidence that copper and/or Cp contributes to development of atherosclerosis, see Table 3.) At this point we must clarify the relation of serum copper to serum Cp levels. Cp normally accounts for about 90% of serum copper. Further, Cp is an acute phase reactant, and plasma levels will go up, for example, in the presence of inflammation. Thus, when serum copper is measured and an increase noted, unless both Cp and copper are measured it is not possible to tell whether the increased copper is due to an increased Cp or an increase in the non-Cp serum copper, or both. Since Cp is increased in inflammation, an increase in Cp levels in atherosclerosis could be a marker of the inflammation of the atherosclerotic process, or perhaps is causally involved in the process unrelated to inflammation. The hypothesis that Cp is causally involved is supported by work that shows that Cp can oxidize LDL through an interaction of one of the copper molecules carried by Cp (Table 3; Refs. 67–69).

In the literature there are papers citing an epidemiological relationship between serum copper and atherosclerotic disease (70–76) and papers citing an epidemiologic relationship between Cp levels and atherosclerotic disease (Table 3; Refs. 77–83). One group controlled for the inflammatory component by adjusting for such things as protein C levels, and found that a substantial risk from elevated Cp remained (84). We found one paper failing to find a relationship between serum copper and coronary heart disease, at least in patients with moderate coronary heart disease (85), and another paper that observed no relationship between Cp and coronary heart disease when patients with inflammation were excluded (86).

Elevated levels of Cp and copper have been found in type I and type II diabetes mellitus (Table 3), which are risk factors for cardiovascular disease (87). Diabetic patients with vascular complications have higher plasma copper levels than diabetic patients without complications or normal controls (87). Patients with the “metabolic syndrome” (patients having in common risk factors such as obesity, hypertension, glucose intolerance, and dyslipidemia) also have elevated Cp levels (88).

Some animal model work has been done on copper and atherosclerosis. First, it is clear that severe copper deficiency damages blood vessels, perhaps as a result of deficiencies of copper-dependent enzymes, such as lysyl oxidase, important in collagen cross-linking, and copper/zinc superoxide dismutase (SOD), important in oxidant protection. For example, Dalle Lucca et al. (89) find increased neointima thickening in the copper-deficient rat carotid artery and attribute it to the lower SOD levels they also find. Similarly, Saari et al. (90) review all the harmful effects on the cardiovascularsystem of severe copper deficiency and also conclude that theseare primarily due to deficiencies of enzymes that depend oncopper for activity.

Lamb et al. (91) have used the cholesterol-fed rabbit model of atherosclerosis and compared a copper-deficient and copper-adequate diet. They found evidence of more damage in the aortas of the copper-deficient animals. This group also studied this model, comparing a copper-adequate to a copper-supplemented sample of rabbits, and found the copper supplemented-group had significantly smaller intimal lesions (92). Thus, this is evidence against the hypothesis that elevated free copper levels are atherogenic, at least in this model. However, Lamb et al. (93) have alsodone a copper dose response in the rabbit model, and find abiphasic atherogenic response. Thus, at both high and low levelsof copper there is greater atherosclerosis than at intermediatelevels. Although it is difficult to put these dietary copperlevels in this rabbit model into context with human levels offree copper, this latter work is at least mildly supportiveof the concept that lowering free copper levels could reduceatherosclerosis. It is clear, of course, that copper levelsmust not be lowered into the range where the activities of copper-dependentenzymes are affected, because that adversely affects the vasculature.

Molecular studies are generally supportive of a relationship of copper to the atherogenic process and provide possible insights into mechanisms (Table 3). Elevated levels of copper have been found in human atherosclerotic plaques (32). Copper is capable of oxidizing LDL, and oxidized LDL is part of the atherogenic process. One study showed that apolipoprotein E may owe its antioxidant effects to inhibiting copper oxidation of LDL (94). A number of papers have shown that homocysteine, a risk factor for cardiovascular disease, interacts with copper to produce oxidant stress (95–99). One molecular study with a contrary conclusion is that of Leeuwenburgh et al. (100), who on thebasis of mass spectrophotometric quantification of markers forprotein oxidation in atherosclerotic plaques conclude that freemetal ions are not involved in LDL oxidation in the arterialwall.

The role of copper in atherosclerosis has been reviewed by Ferns et al. (101). In general, they conclude that the relationshipis probably biphasic, with both severe copper deficiency andexcess copper causing enhanced atherogenesis. With respect tothe latter, they emphasize the role of copper in oxidizing LDL,which they point out is important in the early phases of atherogenesis.

I would like to make it clear that severe copper deficiency,that is, copper deficiency severe enough to cause lessened activityof copper-dependent enzymes, is not relevant to the questionI am discussing, which is whether free copper levels are highenough in “normal” people during aging to contribute to diseasesof aging, such as atherosclerosis. If so, the proposal wouldbe to bring these levels down mildly, not so much as to affectcopper-dependent enzymes.

Turning to a summary of the evidence for copper levels beinghigh enough as people age to contribute to atherosclerosis,the epidemiologic evidence of points 2 and 3 of Table 3 arenot as persuasive for copper as were the positive epidemiologicstudies for ferritin, in my opinion. The reason has alreadybeen discussed, that Cp is an acute phase reactant, and whenCp is elevated, so is serum copper. Thus, the inflammatory componentof the atherosclerotic process could be simply elevating Cplevels secondarily. On the other hand, the positive data involvingcopper and homocysteine are more compelling. This interactioncauses production of ROS, known to be involved in atherogenesis,and would be an explanation for homocysteine levels as a cardiovascularrisk factor. The evidence suggesting that iron is contributingto atherogenesis should be kept in mind when considering copper,because the two metals are toxic through identical mechanisms,generation of ROS. Thus, if a clinical trial of iron depletionis positive, it would provide weight to the consideration ofa clinical trial of copper depletion in atherosclerotic disease.

Copper and AD.

Copper may be involved at many steps with the pathogenesis of AD, another disease of aging (see Table 4 for a summary of evidence). It is believed that β-amyloid (Aβ), present in amyloid plaques in AD, is intimately involved with pathogenesis. β-Amyloid is generated from amyloid precursor protein from cleavage by β-secretase. Amyloid precursor protein has a copper-binding domain which reduces copper (II) to copper (I) and then produces oxidative damage (102, 103). β-Secretase itself also binds copper for activity (104). β-Amyloid binds copper and cholesterol, facilitating copper oxidation of cholesterol to 7–0H cholesterol, extremely toxic to neurons (105, 106). One study has shown that amyloid plagues and neurofibrillary tangles, also common in AD brains, are major sites of catalytic redox activity (107). Deferoxamine, an iron chelator, or EDTA, a general metal chelator, abolishes this redox activity, while replenishment with copper or iron restores the activity (107). Tau protein is a major component of neurofibrillary tangles and also binds copper, which appears to be important for its aggregation (108).

Apolipoprotein E4 is a risk factor for AD, and it has an arginine at position 112 rather than a cysteine, while the other apolipoprotein alleles have cysteine at this position (109). This cysteine may be involved in copper binding and may be related to the diminished antioxidant effect of the E-4 allele (110). Plasma homocysteine levels are a risk factor for AD (111), and copper mediates LDL oxidation by homocysteine (95). β-Amyloid causes copper-dependent inhibition of cytochrome c-oxidase, an important enzyme of oxidative metabolism (112). Squitti et al. (113) have found a high free copper in the blood of AD patients, and report a high level of serum peroxides, which correlate positively with serum copper (114). Penicillamine (a copper chelator) therapy reduced the level of serum peroxides (114). However, an earlier study (115) did not find a difference betweencopper levels in AD patients and controls.

In animal studies, Sparks (116) has found that trace amounts of copper added to the drinking water in a rabbit model of AD greatly enhances the accumulation of Aβ in the brains of the rabbits and increased learning deficits. In another study, treatment of the rodent model of AD with clioquinol, a copper/zinc chelator, markedly inhibited Aβ deposition in the brain (117).

However, there is significant controversy over whether an excess of copper is involved in the pathogenesis of AD. Data suggesting otherwise include animal studies in which an increase in brain copper due to amplification of a copper transporter resulted in reduction of Aβ in the brain (118) and supplementation with copper in an AD mouse model lowered Aβ production and increased longevity (119), and human AD studies in which cognitive decline correlated positively with low plasma levels of copper (120).

Thus, at this time the evidence is conflicting as to whethertoo much copper is involved in the pathogenesis of AD. Thiswill have to be resolved by further experimentation. Similarly,the involvement of copper in other disease of neurodegeneration,suggested by various findings, remains not definitively established.

Other Potential Toxicities of Copper.

Intervention data, lowering copper levels with drugs, in diabetes,cancer, diseases of fibrosis, diseases of inflammation, andautoimmune diseases, involving primarily animal studies, aresummarized in Table 5 and briefly discussed below.

Cooper and his group (121) have shown that copper metabolism becomes abnormal after induction of diabetes in rats and that the copper chelator trientine, given to these animals, alleviated their heart failure, improved cardiomyocyte structure, and reversed elevations in left ventricular collagen and β₁ integrin without lowering blood glucose. They followed this up with studies in diabetic patients and showed that trientine therapy decreased left ventricular hypertrophy (121). Others have shown beneficial effects of trientine in animal studies of diabetic neuropathy (122).

In a series of mouse studies with the copper-lowering agent tetrathiomolybdate (TM), being developed for Wilson’s disease (123), our group has shown (summarized in Table 5): i) an antiangiogenic effect, useful in inhibiting cancer growth (124); ii) an antifibrotic effect in lung and liver (125, 126); iii) an anti-inflammatory effect (127, 128); and iv) an inhibitory effect on autoimmune diseases (126, 129). In another study with TM, it was shown that neointimal vascular thickening after balloon injury in the rat was inhibited by this drug (130). Another group has shown that TM inhibits adjuvant-induced arthritis in the rat (131). Clinical studies in cancer have been promising (132), and clinical trials in diseases of fibrosis are justbeginning. TM is capable of producing a greater degree of copperdepletion than other drugs and of doing so safely, as long asCp levels are monitored and kept in an intermediate range. Atthis level of copper depletion, copper-dependent enzymes arenot affected. The various disease processes affected by theseintervention studies—cancer, fibrosis, inflammation, autoimmunity,diabetes—are all diseases associated with aging.

	Summary, Recommendations, and Conclusion

TOP Abstract Introduction Iron Toxicity Copper Toxicity Summary, Recommendations, and… References

 

Summarizing, at this point I think the data with iron are compellingenough to warrant a well-designed clinical trial of iron depletionon atherosclerosis. This could be done most safely by phlebotomy.Since the blood donor data have been mostly positive but havebeen criticized on the basis of nonrandomness of the donors,it seems reasonable to design a randomized study in which aunit of blood is taken at periodic intervals from half of thesubjects. These should be subjects who are at risk for atherogenesis,and the endpoints could be carotid artery intimal thickeningas well as cardiovascular events. Alternatively this could bedone with oral iron chelation therapy, using the recently approveddeferasirox (Exjade, Novartis). If this drug proves to be safeenough, it might be preferred by patients to periodic phlebotomy.

A study of the effect of phlebotomy or deferasirox on AD progression should also be undertaken, in view of the increasingly obvious role of oxidant damage in this disease and the one study showing a beneficial effect of the iron chelator desferrioxamine (55). Particularly, if these studies are positive, the effects of iron depletion on other diseases of aging should be evaluated. As mentioned above, phlebotomy is probably safer for iron depletion than currently available iron chelators, but it has the disadvantage that it doesn’t involve a pharmaceutical product, and thus there is no pharmaceutical company financing. Thus, if phlebotomy is the method of choice, these studies will have to be undertaken by academic investigators, presumably with financing from federal agencies, such as the National Institutes of Health. These studies should include the best measures of the labile iron pool, as well as other measures of iron status. A pilot study on the feasibility of phlebotomy to reduce iron stores (as measured by serum ferritin) to predictable levels has already been done (133).

Regarding copper, it is probably premature to plan a major clinicalintervention study to evaluate copper-lowering effects on atherosclerosis.A good way to proceed would be to carry out animal model studiesto see if copper depletion mitigates atherogenesis and if so,to what degree the copper depletion need be carried out. Trientinehas shown effects in diabetes, as discussed, but is not generallyas capable of producing the degree of copper depletion thatTM can produce. TM is not yet commercially available, althoughit is expected to be approved for Wilson’s disease withinabout a year. Other anticopper drugs on the market are not ideal.Penicillamine is too toxic, and zinc may be too slow-actingand mild to effect the kind of copper depletion readily availablewith TM. If animal studies are positive, and once TM comes onthe market, for example for Wilson’s disease, an interventionstudy should be undertaken using either trientine or TM, lookingat effects on atherogenesis, with similar endpoints to the proposediron study. If both iron and copper studies are positive, astudy of the combined effects of iron and copper depletion shouldthen be undertaken.

Regarding the other diseases in which copper depletion may bebeneficial, research extending the animal work into the clinicshould continue.

It has been hypothesized that even mild copper deficiency might be atherogenic (134). This theory is based in part upon observations that the higher the ratio of zinc to copper, the higher the blood cholesterol (135, 136). It should be noted that zinc levels are unchanged with copper depletion by TM. Further, in our various studies of lowering copper levels with TM, we have not seen an adverse effect on lipid levels or on development of heart disease, as suggested by Klevay (134).

In conclusion, oxidant stress is now viewed as a major culpritin the aging process and in many diseases of aging. Iron andcopper are extremely redox-active and are constantly involvedin generation of ROS, which may be important in the aging processand in the pathogenesis of many diseases of aging. Evolutionhas resulted in extra stores of iron and copper, because theyare so vital to life and health during the reproductive period.However, these stores may be contributing to diseases as weage, and it is time to begin evaluating what levels of thesemetals are optimal during the latter part of life.

 

 

 

View this table: [in this window] [in a new window] Table 1. Normal Ranges for Iron, Copper and Ferritin

 

View this table: [in this window] [in a new window] Table 2. Evidence on the Question of Whether Relative Iron Availability Contributes to Development of Atherosclerosis

 

View this table: [in this window] [in a new window] Table 3. Evidence on the Question of Whether Relative Copper Availability Contributes to Development of Atherosclerosis

 

View this table: [in this window] [in a new window] Table 4. Evidence on the Question of Whether Relative Copper Availability Contributes to Development of Alzheimer’s Disease

 

View this table: [in this window] [in a new window] Table 5. Evidence on the Question of Whether Relative Copper Availability Contributes to a Variety of Diseases of Aging

 

View larger version (11K): [in this window] [in a new window] Figure 1. This figure identifies the major risk factors leading to atherosclerosis, and suggest that production of ROS by excessive free iron and copper, oxidation of LDL by iron and copper, and interaction of iron and copper with homocysteine, fits into this scheme. ROS, reactive oxygen species; LDL, low-density lipoprotein.

	Footnotes

The investigations in our laboratory have been supported inpart by Attenuon LLC, San Diego, California, and Pipex Therapeutics,Inc., Ann Arbor, Michigan, who are developing tetrathiomolybdatefor various uses. Dr. Brewer has equity in, and is a paid consultant,for both companies.

Received for publication April 13, 2006.

Accepted for publication July 13, 2006.

	References

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