Identification of thiolic sarcolemmal proteins as a primary target of iron toxicity in cultured heart cells

Iron toxicity and chelation therapy

Iron is an essential mineral for normal cellular physiology, but an excess can result in cell injury. Iron in low-molecular-weight forms may play a catalytic role in the initiation of free radical reactions. The resulting oxyradicals have the potential to damage cellular lipids, nucleic acids, proteins, and carbohydrates; the result is wide-ranging impairment in cellular function and integrity. The rate of free radical production must overwhelm the cytoprotective defenses of cells before injury occurs. There is substantial evidence that iron overload in experimental animals can result in oxidative damage to lipids in vivo, once the concentration of iron exceeds a threshold level. In the liver, this lipid peroxidation is associated with impairment of membrane-dependent functions of mitochondria and lysosomes. Iron overload impairs hepatic mitochondrial respiration primarily through a decrease in cytochrome C oxidase activity, and hepatocellular calcium homeostasis may be compromised through damage to mitochondrial and microsomal calcium sequestration. DNA has also been reported to be a target of iron-induced damage, and this may have consequences in regard to malignant transformation. Mitochondrial respiratory enzymes and plasma membrane enzymes such as sodium-potassium-adenosine triphosphatase (Na(+) + K(+)-ATPase) may be key targets of damage by non-transferrin-bound iron in cardiac myocytes. Levels of some antioxidants are decreased during iron overload, a finding suggestive of ongoing oxidative stress. Reduced cellular levels of ATP, lysosomal fragility, impaired cellular calcium homeostasis, and damage to DNA all may contribute to cellular injury in iron overload. Evidence is accumulating that free-radical production is increased in patients with iron overload. Iron-loaded patients have elevated plasma levels of thiobarbituric acid reactants and increased hepatic levels of aldehyde-protein adducts, indicating lipid peroxidation. Hepatic DNA of iron-loaded patients shows evidence of damage, including mutations of the tumor suppressor gene p53. Although phlebotomy therapy is effective in removing excess iron in hereditary hemochromatosis, chelation therapy is required in the treatment of many patients who have combined secondary and transfusional iron overload due to disorders in erythropoiesis. In patients with beta-thalassemia who undergo regular transfusions, deferoxamine treatment has been shown to be effective in preventing iron-induced tissue injury and in prolonging life expectancy. The use of the oral chelator deferiprone remains controversial, and work is continuing on the development of new orally effective iron chelators.

Iron-mediated cardiotoxicity develops independently of extracellular hydroxyl radicals in isolated rat hearts

BACKGROUND

Myocardial iron toxicity is often attributed to free radical damage. Present studies examine the role of extracellular hydroxyl radical formation in this process.

METHODS

In vitro reactions examined the rate of hydroxyl radical formation using salicylate trapping with high-pressure liquid chromatography separation and electrochemical detection of 2,3- and 2,5- dihydroxybenzoic acid. Isolated rat hearts were perfused by the Langendorff technique under the same buffer conditions to determine changes in myocardial contractility, release of tissue lactate dehydrogenase activity, and formation of lipid peroxidation products when iron was added to the perfusate with or without the formation of extracellular radicals.

RESULTS

In vitro reactions, performed in Krebs buffer alone or with addition of iron (25 microM), produced levels of hydroxyl radicals that were nondetectable with salicylate trapping. Addition of iron/ascorbate (FeSO4 = 25 microM, ascorbate = 1 mM), or iron/ascorbate/histidine (FeSO4 = 25 microM, ascorbate = 1 mM, histidine = 15 mM) produced significant and equivalent accumulation of hydroxyl radicals. Isolated rat hearts were perfused under the same 4 conditions. Control heart contractile function was stable with little release of lactate dehydrogenase activity and low levels of thiobarbituric acid reactive substances (TBARS). There was significant and equal injury to contractile function, release of lactate dehydrogenase activity, and accumulation of TBARS in hearts in the presence (iron/ascorbate) and absence (iron alone) of extracellular hydroxyl radicals. In addition, there was significant reduction in injury with iron/ascorbate/histidine, where the formation of extracellular hydroxyl radicals was equal to those observed with iron/ascorbate alone. Additional control hearts, perfused with histidine alone, showed stable heart function.

CONCLUSIONS

These findings indicate that the extracellular formation of hydroxyl radicals is not responsible for iron-mediated cardiotoxicity.

Preclinical cardiac dysfunction in transfusion-dependent children and young adults detected with low-dose dobutamine stress echocardiography

Transfusion-dependent (TD) patients develop cardiac iron overload that will eventually lead to cardiac pump failure. Low-dose dobutamine stress echocardiography may complement resting echocardiography and identify preclinical myocardial dysfunction caused by early cardiac hemosiderosis. Twenty-six iron-overloaded TD patients had stress echocardiography with 5 microg/kg per minute of dobutamine. Indexed left ventricular (LV) mass, LV dimensions, meridional wall stress, and cardiac index were significantly increased. TD patients had similar LV shortening fraction by M-mode (40.5% +/- 5.6% vs 39.4% +/- 4.5%) but had a lower mean LV ejection fraction (53.3% +/- 3.9% vs 46.8% +/- 6.9%, P < .002) and a subnormal increase in cardiac index during dobutamine stress (35% +/- 20% vs 11% +/- 16%, P < .0001). Impairment in LV relaxation was demonstrated by a prolonged isovolumetric relaxation time (0.060 +/- 0.005 vs 0.088 +/- 0.019 seconds, P < .0001), increased peak mitral E wave, and abnormal E/A ratio. Asymptomatic TD patients demonstrate decreased systolic functional reserve and abnormal left ventricular relaxation that may be caused by cardiac hemosiderosis. Low-dose dobutamine stress echocardiography may be useful for detecting and following cardiac dysfunction in patients at risk for cardiac hemosiderosis.