The pathology of hepatic iron overload: a free radical--mediated process?

Comparison of cytosolic products formed in rat liver in response to parenteral and dietary iron loading

Two different methods were used to create a situation of iron (Fe) overload in rats. One group of rats received Fe dextran, and another group of rats received a carbonyl Fe-enriched diet. The ferritins present in the liver cytosol of these rats were isolated and compared. From each group, two cytosolic products were isolated with the use of ultracentrifugation: a cytosolic ferritin fraction (CF) and a (slower sedimenting) light ferritin fraction (CLF). There were no differences with respect to the protein coat (subunit composition and amino acid analysis). Analysis of the Fe core revealed that the two CF fractions were similar, whereas the two CLF fractions differed with respect to their Fe content and to the packing of their cores. The carbonyl CLF product contained less Fe atoms/molecule, which, moreover, seemed to be packed in a less compact way.

Brown fat thermogenesis and exercise: two examples of physiological oxidative stress?

Both brown fat tissue (BAT) and skeletal muscle experience large increases of oxygen consumption and oxygen radical generation during activation. This, together with the relatively low activities of antioxidant enzymes in these two tissues and the high lipid content and free fatty acid liberation of BAT, can produce a physiological oxidative stress. Increases of in vivo or in vitro (BAT) lipid peroxidation have been described in these tissues after activation. They react to this oxidative stress in an adaptive way after chronic stimulation. Cold acclimation increases antioxidant enzymes, ascorbate, and especially reduced glutathione (GSH) in BAT. There is controversy about the variations of antioxidants in skeletal muscle after acute exercise. Nevertheless, exercise training seems to increase muscle antioxidant enzymes and GSH. Many reports show that vitamin E levels decrease in the muscle and increase in plasma during exercise. Studies of vitamin E deficiency and supplementation strongly suggest that this vitamin is of protective value during exercise.

Vitamin E dietary supplementation protects against carbon tetrachloride-induced chronic liver damage and cirrhosis

Previous studies have shown that alpha-tocopherol (vitamin E) pretreatment of experimental animals can protect against acute liver necrosis induced by carbon tetrachloride. In this study we investigated whether the increase of vitamin E liver content by dietary supplementation influences chronic liver damage and cirrhosis induced by carbon tetrachloride in the rat. Our data indicate that vitamin E supplementation did not interfere with the growth rate of the animals and increased about threefold the liver's content of the vitamin. Vitamin E supplementation significantly reduced oxidative liver damage, but it was not effective in protecting against development of fatty liver and did not interfere with metabolic activation of carbon tetrachloride. Moreover, vitamin E-fed animals showed incomplete but significant prevention of liver necrosis and cirrhosis induced by carbon tetrachloride. This has been shown by means of histological examination, analysis of serum parameters and biochemical evaluation of collagen content. These results show that an increased liver content of vitamin E can afford a significant degree of protection against carbon tetrachloride-induced chronic liver damage and cirrhosis.

Iron-mediated DNA damage: sensitive detection of DNA strand breakage catalyzed by iron

Oxidative DNA damage is involved in mutagenesis, carcinogenesis, aging, radiation effects, and the action of several anticancer drugs. Accumulated evidence indicates that iron may play an important role in those processes. We studied the in vitro effect of low concentrations of Fe(II) alone or Fe(III) in the presence of reducing agents on supercoiled plasmid DNA. The assay, based on the relaxation and linearization of supercoiled DNA, is simple yet sensitive and quantitative. Iron mediated the production of single and double strand breaks in supercoiled DNA. Iron chelators, free radical scavengers, and enzymes of the oxygen reduction pathways modulated the DNA damage. Fe(III)-nitrilotriacetate (NTA) plus either H2O2, L-ascorbate, or L-cysteine produced single and double strand breaks as a function of reductant concentration. A combination of 0.1 microM Fe(III)-NTA and 100 microM L-ascorbate induced detectable DNA strand breaks after 30 min at 24 degrees C. Whereas superoxide dismutase was inhibitory only in systems containing H2O2 as reductant, catalase inhibited DNA breakage in all the iron-mediated systems studied. The effect of scavengers and enzymes indicates that H2O2 and .OH are involved in the DNA damaging process. These reactions may account for the toxicity and carcinogenicity associated with iron overload.

Mitochondrial damage and its role in causing hepatocyte injury during stimulation of lipid peroxidation by iron nitriloacetate

Incubation of isolated rat hepatocytes with 0.1 mM iron nitrilotriacetic acid (FeNTA) caused a rapid rise in lipid peroxidation followed by a substantial increase in trypan blue staining and lactate dehydrogenase release, but did not affect the protein and non-protein thiol content of the cells. Hepatocyte death was preceded by the decline of mitochondrial membrane potential, as assayed by rhodamine 123 uptake, and by the depletion of cellular ATP. Chelation of extracellular Ca2+ by ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid or inhibition of Ca2+ cycling within the mitochondria by LaCl3 or cyclosporin A did not prevent the decline of rhodamine 123 uptake. On the other hand, a dramatic increase in the conjugated diene content was observed in mitochondria isolated from FeNTA-treated hepatocytes. Oxidative damage of mitochondria was accompanied by the leakage of matrix enzymes glutamic oxalacetic aminotransferase (GOT) and glutamate dehydrogenase (GLDH). The addition of the antioxidant N,N'-diphenylphenylene diamine (DPPD) completely prevented GOT and GLDH leakage, inhibition of rhodamine 123 uptake, and ATP depletion induced by FeNTA, indicating that Ca(2+)-independent alterations of mitochondrial membrane permeability consequent to lipid peroxidation were responsible for the loss of mitochondrial membrane potential. DPPD addition also protected against hepatocyte death. Similarly hepatocytes prepared from fed rats were found to be more resistant than those obtained from starved rats toward ATP depletion and cell death caused by FeNTA, in spite of undergoing a comparable mitochondrial injury. A similar protection was also observed following fructose supplementation of hepatocytes isolated from starved rats, indicating that the decline of ATP was critical for the development of FeNTA toxicity. From these results it was concluded that FeNTA-induced peroxidation of mitochondrial membranes impaired the electrochemical potential of these organelles and led to ATP depletion which was critical for the development of irreversible cell injury.

Liver pathology in genetic hemochromatosis: a review of 135 homozygous cases and their bioclinical correlations

Liver pathology was assessed in 135 patients with well-defined genetic hemochromatosis ranging from mild disease to severe overload. Three lesions were clearly linked to iron-overload intensity--scarce sidero-necrosis, mild inflammation, and progressive fibrosis. Iron-free foci made of typical or dysplastic hepatocytes were found in 7.4% of the cases. An original grading allowed a reliable quantification of iron and the study of cellular and lobular distribution of iron, which permitted (a) the accurate identification of a decreasing iron gradient in hepatocytes from zone 1 to zone 3 in all cases, (b) the definition of a threshold hepatocytic/mesenchymal iron ratio related to the appearance of sidero-necrosis and to the development of fibrosis, and (c) demonstration that non-iron-related factors (mainly alcoholism) could shift iron from hepatocytes to sinusoidal cells without an increase in the total liver iron amount. This study provides a dynamic view of the iron overload process and suggests that sidero-necrosis and progressive sinusoidal iron overload play a role in the development of fibrosis in human genetic hemochromatosis.

Survival and prognostic factors in 212 Italian patients with genetic hemochromatosis

Two hundred twelve Italian patients with genetic hemochromatosis (181 men, mean age 50 +/- 11 yr; and 31 women, mean age 49 +/- 10 yr) were followed for a median period of 44 mo (range = 3 to 218 mo). Alcohol abuse was present in 31 subjects (15%), and chronic HBV and HCV infection were seen in 19 (9%) and 35 (24%) of 145 cases tested, respectively. Twenty-four patients (11%) had concomitant beta-thalassemia trait. Liver biopsy revealed cirrhosis in 146 and a noncirrhotic pattern in the other 66. Perls' stain was degree III in 37 patients and IV in 171 patients. One hundred eighty-five patients underwent weekly venesection, and iron depletion was achieved in 122 cases after total iron removal of 3 to 41 gm. Death occurred in 44 patients after 3 to 198 mo and was due to hepatocellular carcinoma in 20 cases, liver failure in 10, extrahepatic cancer in six, heart failure in three and hemochromatosis unrelated causes in five. Cancer has developed in seven other patients still alive (hepatocellular in five and extrahepatic in two). No deaths were observed among noncirrhotic patients; cumulative survival rates in cirrhotic patients were 85%, 75%, 60% and 47% at 3, 5, 8 and 10 yr, respectively. Univariate analysis in the 146 cirrhotic patients showed that age greater than 60 yr, alcohol abuse, cardiomyopathy, skin pigmentation, portal hypertension, hypoalbuminemia, hypergammaglobulinemia and Child class B or C had significant negative prognostic value. At multivariate analysis, only alcohol abuse, gamma-globulins greater than 2.0 gm/dl and Child class B or C maintained their negative prognostic values (p less than 0.01, hazard ratio 2.7; p less than 0.001, hazard ratio 2.8; and p less than 0.001, hazard ratio 4.3, respectively).

Direct evidence for in vivo hydroxyl-radical generation in experimental iron overload: an ESR spin-trapping investigation

Although the hydroxyl radical is often implicated as the species responsible for the initiation of oxidative damage in iron-overload conditions, no ESR evidence for the formation of the radical in vivo has been reported. We have employed a secondary radical-trapping technique in which the hydroxyl radical reacts with dimethyl sulfoxide to form the methyl radical, which is then detected as its adduct of the spin trap N-t-butyl-alpha-phenylnitrone in the bile of animals given an intragastric dose of ferrous sulfate. The identity of this adduct was verified by isotope-substitution techniques. We show that unless measures are taken to inactivate the iron excreted in the bile of treated animals, reactions between iron, oxygen, dimethyl sulfoxide, N-t-butyl-alpha-phenylnitrone, and bile components lead to the formation of artifacts during sample collection.

Transplantation of a donor liver with haemochromatosis: evidence against an inherited intrahepatic defect

An iron loaded liver from a 40 year old man with occult haemochromatosis was transplanted into a 19 year old woman with acute liver failure secondary to a paracetamol overdose. Increased parenchymal hepatic iron was found in a liver specimen at biopsy undertaken because of mild rejection 30 days after transplantation. After transplantation the patient had two episodes of liver rejection confirmed by biopsy. The hepatic iron concentration fell from 161 mumol/g on day 30 after transplant to 26.5 mumol/g (normal less than 40) on day 210. Iron absorption, measured 45 days after transplant, was in the normal range at 12.4%. The rapid fall in hepatic iron and the normal iron absorption study result suggest that the genetic defect of haemochromatosis is not exclusively an intrahepatic defect.

Iron and the liver

Iron is essential for life, but iron overload is toxic and potentially fatal. The liver is a major site of iron storage and is particularly susceptible to injury from iron overload, especially when (as in primary hemochromatosis) the iron accumulates in hepatocytes. Iron can be taken up by the liver in several forms and by several pathways including: (1) receptor-mediated endocytosis of diferric or monoferric transferrin or ferritin, (2) reduction and carrier-facilitated internalization of iron from transferrin without internalization of the protein moiety of transferrin, (3) electrogenic uptake of low molecular weight, non-protein bound forms of iron, and (4) uptake of heme from heme-albumin, heme-hemopexin, or hemoglobin-haptoglobin complexes. Normally, pathway 2 is probably the major one for uptake of iron by hepatocytes. Iron is stored in the liver in the cores of ferritin shells and as hemosiderin, an insoluble product derived from iron-rich ferritin. Iron in hepatocytes stimulates translation of ferritin mRNA and represses transcription of DNA for transferrin and transferrin receptors. The major pathologic effects of chronic hepatic iron overload are: (1) fibrosis and cirrhosis, (2) porphyria cutanea tarda, and (3) hepatocellular carcinoma. Although precise pathogenetic mechanisms remain unknown, iron probably produces these and other toxic effects by increasing oxidative stress and lysosomal lability. Vigorous efforts at diagnosis and treatment of iron overload are essential since the pathologic effects of iron are totally preventable by early vigorous iron removal and prevention of iron re-accumulation.

Vitamin E and oxidative stress

Oxidative stress can result from or be enhanced by a large variety of conditions, including nutritional imbalance, exposure to chemical and physical agents in the environment, strenuous physical activities, injury, and hereditary disorders. While many enzymes and compounds are involved in protecting cells from the adverse effects of oxidative stress, vitamin E occupies an important and unique position in the overall antioxidant defense. The antioxidant function of vitamin E is closely related to the status of many dietary components. Vitamin E-depleted animals are generally more susceptible to the adverse effects of environmental agents than supplemented animals. Also, vitamin E supplementation is beneficial to certain groups of the population. However, supplementing vitamin E in experimental subjects maintained on a nutritionally adequate diet does not always provide additional protection. Differential metabolic responses in various organs and differences in experimental conditions often contribute in the discrepancies in the literature. The lack of clear evidence for the occurrence of lipid peroxidation or antioxidant function of vitamin E in vivo can be attributed partly to the presence of active pathways for metabolizing hydroperoxides, aldehydes, and other oxidation products. Specific and sensitive techniques for measuring lipid peroxidation products in biological systems are essential for understanding the role of free radical-induced lipid peroxidation in tissue damage and antioxidant function of vitamin E in vivo.

Oxidative stress status and cancer: methodology applicable for human studies

There is convincing evidence to suggest that oxidative stress status might be involved in the etiology of cancer, but relatively little direct human data are available. The National Cancer Institute recently sponsored a workshop evaluating methodology for measuring oxidative stress status with potential application to human studies.

Characterization and accumulation of ferritin in hepatocyte nuclei of mice with iron overload

After a single subcutaneous dose of iron-dextran (600 mg of iron/kg), iron overload developed in C57BL/10ScSn mice. At 4, 24 and 78 wk liver nonheme iron concentrations were 67-, 42- and 21-fold higher than controls, respectively. Much of the iron was in macrophages, but hepatocytes were also strongly positive for Perls' stainable iron. One feature was the development of iron-positive nuclear inclusions in hepatocytes. After a delay of at least 8 wk when no stainable iron was evident, a maximum of 37% of periportal hepatocytes contained inclusions by 24 wk. Although this proportion remained constant for the remainder of the study, the size of the inclusions (which were not membrane-limited) increased to greater than 3 microns in diameter, occupying greater than 25% of the nuclear volume. The presence of iron in the inclusions was confirmed by energy dispersive x-ray microanalysis. Immunocytochemical studies showed that the iron was present as aggregates of ferritin. Quantitation of nonaggregated ferritin molecules by image analyses after electron microscopy demonstrated that within 4 wk ferritin levels in cytoplasm and nucleoplasm had greatly increased but that there was a concentration gradient of approximately one order of magnitude across the nuclear envelope. These findings are consistent with the hypothesis that in iron-loaded mouse hepatocytes there is a slow passage of ferritin-molecules through the nuclear pores; the gradient is maintained by the continual aggregation of ferritin within the nucleus. Intranuclear ferritin may provide a source of iron for catalyzing hydroxyl radical formation in nuclei during some toxic, carcinogenic and aging processes.

Analysis of iron-containing compounds in different compartments of the rat liver after iron loading

The livers of iron-loaded rats were fractionated and a cytosolic fraction, a lysosomal fraction, a siderosomal fraction and haemosiderin were obtained. All iron-containing compounds from these fractions were isolated and their morphology, Fe/P ratios, iron core diameter and peptide content were compared. The cytosolic fraction contained ferritin (CF) and a slower sedimenting, light ferritin (CLF). The lysosomal fraction also contained ferritin (LF) and a slower sedimenting light ferritin (LLF). The siderosomal fraction contained ferritin (SF), a faster sedimenting non-ferritin iron compound (SIC) and haemosiderin (HS). SIC and HS did not resemble ferritin as much as the other products did, but were found to be water-insoluble aggregates. The Fe/P ratios of CF and CLF were lower than the Fe/P ratios of LF and LLF and these in turn had lower Fe/P ratios than SF, SIC and HS. The iron core diameter of the cytosolic ferritin was increased after lysosomal uptake. The iron core diameters of the siderosomal products were smaller. CLF, CF, LF, LLF and SF contained one kind of subunit of approximately 20.5 kDa. SIC and HS contained other peptides in addition to the 20.5-kDa subunit. The results indicate that storage of ferritin molecules is not limited to the cytosolic compartment, but is also the case in the lysosomes. Extensive degradation of the ferritin molecule seems to be confined to the siderosomes.

Aetiology and pathophysiology of chronic liver disorders

The aetiology of chronic liver disease covers a wide range of congenital or acquired abnormalities of the hepatocellular biochemical network. Although our knowledge has considerably increased in recent years, the aetiology of chronic liver disease often remains obscure. Acquired irreversible disturbances of normal liver function can be mediated by hepatotrophic viruses, chemicals, chronic oxygen depletion, or interference with the immune system. Considerable progress has been made in the detection and characterisation of hepatitis B, C, and D viruses as causative agents of chronic active hepatitis. Alcohol abuse remains the predominant cause of chronic liver disease in the Western world. The targets of autoantibodies used to diagnose autoimmune diseases of the liver and primary biliary cirrhosis continue to be biochemically defined. Their significance for the aetiology of the disease, however, remains to be established. Nonparenchymal cells play an important role in the sequence of events following hepatocellular injury and ultimately leading to liver cirrhosis. They release vasoactive compounds, cytokines, and other important mediators, and participate in the modulation of the extracellular matrix that is characteristic of liver fibrosis and cirrhosis. The biochemical basis of liver cell necrosis remains poorly defined. In spite of recent progress, and the detection of some new pathogenic principles that help in the understanding of the complications of chronic liver disease such as portal hypertension, oesophagogastric variceal bleeding, portosystemic encephalopathy, ascites, and other metabolic disturbances, many questions concerning the aetiology and pathophysiology of chronic liver disease and its complications remain to be answered.