Iron deficiency prevents liver toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin

TCDD promotes liver fibrosis through disordering systemic and hepatic iron homeostasis

2, 3, 7, 8-Tetrachlorodibenzo-p-dioxin (TCDD) is a toxic environmental pollutant which can cause severe health problems, such as fibrosis. However, the toxic effects and related mechanism of TCDD on the liver remain largely unknown. In this study, we established a liver fibrosis mouse model upon exposure of TCDD, as evidenced by increased collagen I, tumor growth factor β1 (TGFβ1), α-smooth muscle actin (α-SMA), and Masson staining. Meanwhile, there was also a significant increase of inflammatory factors and TUNEL-positive hepatocytes in liver, indicating that liver inflammation and hepatic cell apoptosis occurred. In addition, increased serum and liver iron were concomitant with liver injury induced by TCDD. We further investigated the mechanism underlying TCDD-induced hepatocyte apoptosis through apoptosis polymerase chain reaction array, and found that a crucial apoptosis-related gene, cell death-inducing DFF45-like effector b (Cideb), was significantly increased in primary hepatocytes from TCDD-exposed mice, and accompanied by liver iron deposition in hepcidin knockout mice. Therefore, Cideb depletion could effectively attenuated TCDD or iron induced cell death related genes expression. In conclusion, our results showed that iron-induced Cideb expression played a critical role in promoting TCDD-induced hepatocyte apoptosis and liver fibrosis, which provide a novel mechanism for understanding TCDD-induced liver injury.

Convergence of hepcidin deficiency, systemic iron overloading, heme accumulation, and REV-ERBα/β activation in aryl hydrocarbon receptor-elicited hepatotoxicity

Persistent aryl hydrocarbon receptor (AhR) agonists elicit dose-dependent hepatic lipid accumulation, oxidative stress, inflammation, and fibrosis in mice. Iron (Fe) promotes AhR-mediated oxidative stress by catalyzing reactive oxygen species (ROS) production. To further characterize the role of Fe in AhR-mediated hepatotoxicity, male C57BL/6 mice were orally gavaged with sesame oil vehicle or 0.01-30μg/kg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) every 4days for 28days. Duodenal epithelial and hepatic RNA-Seq data were integrated with hepatic AhR ChIP-Seq, capillary electrophoresis protein measurements, and clinical chemistry analyses. TCDD dose-dependently repressed hepatic expression of hepcidin (Hamp and Hamp2), the master regulator of systemic Fe homeostasis, resulting in a 2.6-fold increase in serum Fe with accumulating Fe spilling into urine. Total hepatic Fe levels were negligibly increased while transferrin saturation remained unchanged. Furthermore, TCDD elicited dose-dependent gene expression changes in heme biosynthesis including the induction of aminolevulinic acid synthase 1 (Alas1) and repression of uroporphyrinogen decarboxylase (Urod), leading to a 50% increase in hepatic hemin and a 13.2-fold increase in total urinary porphyrins. Consistent with this heme accumulation, differential gene expression suggests that heme activated BACH1 and REV-ERBα/β, causing induction of heme oxygenase 1 (Hmox1) and repression of fatty acid biosynthesis, respectively. Collectively, these results suggest that Hamp repression, Fe accumulation, and increased heme levels converge to promote oxidative stress and the progression of TCDD-elicited hepatotoxicity.

Beyond the Aryl Hydrocarbon Receptor: Pathway Interactions in the Hepatotoxicity of 2,3,7,8-Tetrachlorodibenzo-p-dioxin and Related Compounds

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the prototypical ligand for a group of environmental halogenated aromatic hydrocarbon contaminants which elicit hepatotoxicity and other toxic responses through activation of the aryl hydrocarbon receptor (AhR). Despite the conservation of the AhR and its signaling pathway, TCDD-elicited differential gene expression networks are species-specific, consistent with differences in sensitivity and toxic responses between species. This review integrates gene expression studies with complementary phenotypic analyses (e.g., metabolomics, clinical biochemistry, and histopathology) to elucidate the pathways through which TCDD and related compounds cause hepatotoxicity beyond AhR activation. We propose that AhR-mediated toxicity is a collective response to the cumulative burden of metabolic reprogramming across multiple pathways. Consequently, nutrition, health status, and genetic background establish the basis for differences in sensitivity and predisposition to adverse outcomes between species, sub-populations, tissues, and cells.

Towards a unifying, systems biology understanding of large-scale cellular death and destruction caused by poorly liganded iron: Parkinson's, Huntington's, Alzheimer's, prions, bactericides, chemical toxicology and others as examples

Exposure to a variety of toxins and/or infectious agents leads to disease, degeneration and death, often characterised by circumstances in which cells or tissues do not merely die and cease to function but may be more or less entirely obliterated. It is then legitimate to ask the question as to whether, despite the many kinds of agent involved, there may be at least some unifying mechanisms of such cell death and destruction. I summarise the evidence that in a great many cases, one underlying mechanism, providing major stresses of this type, entails continuing and autocatalytic production (based on positive feedback mechanisms) of hydroxyl radicals via Fenton chemistry involving poorly liganded iron, leading to cell death via apoptosis (probably including via pathways induced by changes in the NF-κB system). While every pathway is in some sense connected to every other one, I highlight the literature evidence suggesting that the degenerative effects of many diseases and toxicological insults converge on iron dysregulation. This highlights specifically the role of iron metabolism, and the detailed speciation of iron, in chemical and other toxicology, and has significant implications for the use of iron chelating substances (probably in partnership with appropriate anti-oxidants) as nutritional or therapeutic agents in inhibiting both the progression of these mainly degenerative diseases and the sequelae of both chronic and acute toxin exposure. The complexity of biochemical networks, especially those involving autocatalytic behaviour and positive feedbacks, means that multiple interventions (e.g. of iron chelators plus antioxidants) are likely to prove most effective. A variety of systems biology approaches, that I summarise, can predict both the mechanisms involved in these cell death pathways and the optimal sites of action for nutritional or pharmacological interventions.

Porphyria cutanea tarda--when skin meets liver

Porphyria cutanea tarda (PCT) is the most frequent type of porphyria worldwide and results from a catalytic deficiency of uroporphyrinogen decarboxylase (UROD), the fifth enzyme in heme biosynthesis. At least two different types of PCT are currently distinguished: an acquired variant, also referred to as sporadic or type I PCT, in which the enzymatic deficiency is limited to the liver; and an autosomal dominantly inherited form, also known as familial or type II PCT, in which there is a decrease of enzymatic activity in all tissues. The cutaneous findings include increased photosensitivity, skin fragility, blistering, erosions, crusts, and miliae on the sun-exposed areas of the body. Additionally, hyperpigmentation, hypertrichosis, sclerodermoid plaques, and scarring alopecia might be observed. In patients with type I PCT, there is a significant association with liver disease that can be triggered by genetic and environmental factors, such as alcohol abuse, iron overload, haemochromatosis, polychlorinated hydrocarbons, and hepatitis C virus infection. The diagnosis of PCT can be made based on the skin symptoms, a characteristic urinary porphyrin excretion profile, and the detection of isocoproporphyrin in the feces. In red blood cells of individuals with type II PCT, UROD activity is decreased by approximately 50% due to heterozygous mutations in the UROD gene. Here we provide an update on clinical, diagnostic and therapeutic aspects of PCT, a disorder that affects both skin and liver.

Absolute requirement for iron in the development of chemically induced uroporphyria in mice treated with 3-methylcholanthrene and 5-aminolevulinate

Accumulating evidence, including experiments using cytochrome P450 1a2 (Cyp1a2) gene knock-out mice (Cyp1a2(-/-)), indicates that the development of chemically induced porphyria requires the expression of CYP1A2. It has also been demonstrated that iron enhances and expedites the development of experimental uroporphyria, but that iron alone without CYP1A2 expression, as in Cyp1a2(-/-) mice, does not cause uroporphyria. The role of iron in the development of porphyria has not been elucidated. We examined the in vivo effect of iron deficiency on hepatic URO accumulation in experimental porphyria. Mice were fed diets containing low (iron-deficient diet (IDD), 8.5 mg iron/kg) or normal (normal diet (ND), 213.7 mg iron/kg) levels of iron. They were treated with 3-methylcholanthrene (MC), an archetypal inducer of CYP1A, and 5-aminolevulinate (ALA), precursors of porphyrin and heme. We found that uroporphyrin (URO) levels and uroporphyrinogen oxidation (UROX) activity were markedly increased in ND mice treated with MC and ALA, while the levels were not raised in IDD mice with the same treatments. CYP1A2 levels and methoxyresorufin O-demethylase (MROD) activities, the CYP1A2-mediated reaction, were markedly induced in the livers of both ND and IDD mice treated with MC and ALA. UROX activity, supposedly a CYP1A2-dependent activity, was not enhanced in iron-deficient mice in spite of the fact of induction of CYP1A2. We showed that a sufficient level of iron is essential for the development of porphyria and UROX activity.

Effect of antioxidant phytochemicals on the hepatic tumor promoting activity of 3,3',4,4'-tetrachlorobiphenyl (PCB-77)

Polychlorinated biphenyls (PCBs) have promoting activity in the liver, which may be brought about in part by the induction of oxidative stress. In this study we examined the effects of several antioxidant phytochemicals on the tumor promoting activity of 3,3',4'4-tetrachlorobiphenyl (PCB-77). Female Sprague Dawley rats were first injected with diethylnitrosamine (DEN, 150 mg/kg) and one week later the rats were fed an AIN-93 based purified diet or the same diet containing ellagic acid (0.4%), beta-carotene (0.5%), curcumin (0.5%), N-acetyl cysteine (NAC, 1.0%), coenzyme CoQ10 (CoQ10, 0.4%), resveratrol (0.005%), lycopene (10% as Lycovit, which contains 10% lycopene), or a tea extract (1%, containing 16.5% epigallocatechin-3-gallate [EGCG] and 33.4% total catechins). Rats were fed the diets for the remainder of the study. After three weeks, 2/3 of the control rats and all of the antioxidant diet-fed rats were injected i.p. with PCB-77 (300 micromol/kg) every other week for four injections. All rats were euthanized ten days after the last PCB injection. The rats that received PCB-77 alone showed an increase in the number and size of placental glutathione S-transferase (PGST)-positive foci in the liver. Lycopene significantly decreased the number of foci, while curcumin and CoQ10 decreased the size of the foci. In contrast, ellagic acid increased the number but decreased the size of the foci. All of the other phytochemicals showed only slight or no effects. Compared with the PCB-77 group, CoQ10 increased cell proliferation in normal hepatocytes, whereas the other antioxidants had no effect in either normal or PGST-positive hepatocytes. These findings show that none of the antioxidant phytochemicals produced a clear decrease in the promoting activity of PCB-77.

Substrate-dependent irreversible inactivation of cytochrome P-450: conversion of its haem moiety into modified porphyrins

2-Allyl-2-isopropylacetamide and other drugs containing either an allyl, a vinyl or an ethynyl unsaturated side chain are metabolized by liver cytochrome P-450 to reactive derivatives that irreversibly inhibit the haemoprotein by a suicidal type of inactivation. The main target is the haem moiety of cytochrome P-450 which is converted into abnormal porphyrins. These have been isolated from the liver of treated rats, extensively purified and compared with model porphyrins. The abnormal porphyrins incorporate metal ions in vitro much more readily than does their parent porphyrin, protoporphyrin. They are also much more basic than protoporphyrin, and on titration with a strong acid they readily give rise to a porphyrin monocation which then requires relatively large amounts of acid for conversion to the porphyrin dication. In all these respects and also in the intensity of their bathochromic shifts these abnormal porphyrins closely resemble N-alkylated porphyrins and they markedly differ from porphyrins that are substituted at one of their meso-carbon positions or which bear electron-withdrawing substituents at the beta-positions of the pyrrole rings. This suggests strongly that reactive derivatives of the unsaturated drugs act as electrophilic reagents and alkylate one of the pyrrole nitrogen atoms of cytochrome P-450. A model centred on the apoprotein of cytochrome P-450 is considered for the degradation of liver haem caused by unsaturated drugs. The apocytochrome may accept exchangeable pools of liver haem for degradation, leading to a state of haem depletion and to activation of delta-aminolaevulinate synthase.

Effect of an oral iron chelator or iron-deficient diets on uroporphyria in a murine model of porphyria cutanea tarda

Porphyria cutanea tarda is a liver disease characterized by elevated hepatic iron and excessive production of uroporphyrin (URO). Phlebotomy is an effective treatment that probably acts by reducing hepatic iron. Here we used Hfe(-/-) mice to compare the effects on hepatic URO accumulation of two different methods of hepatic iron depletion: iron chelation using deferiprone (L1) versus iron-deficient diets. Hfe(-/-) mice in a 129S6/SvEvTac background were fed 5-aminolevulinic acid (ALA), which results in hepatic URO accumulation, and increasing doses of L1 in the drinking water. Hepatic URO accumulation was completely prevented at low L1 doses, which partially depleted hepatic nonheme iron. By histological assessment, the decrease in hepatic URO accumulation was associated with greater depletion of iron from hepatocytes than from Kupffer cells. The L1 treatment had no effect on levels of hepatic cytochrome P4501A2 (CYP1A2). L1 also effectively decreased hepatic URO accumulation in C57BL/6 Hfe(-/-) mice treated with ALA and a CYP1A2 inducer. ALA-treated mice maintained on defined iron-deficient diets, rather than chow diets, did not develop uroporphyria, even when the animals were iron-supplemented either directly in the diet or by iron dextran injection.

CONCLUSION

The results suggest that dietary factors other than iron are involved in the development of uroporphyria and that a modest depletion of hepatocyte iron by L1 is sufficient to prevent URO accumulation.

Hexachlorobenzene stimulates uroporphyria in low affinity AHR mice without increasing CYP1A2

Hexachlorobenzene (HCB), a weak ligand of the aryl hydrocarbon receptor (AHR), causes hepatic uroporphyrin (URO) accumulation (uroporphyria) in humans and animals. CYP1A2 has been shown to be necessary in the development of uroporphyria in mice. Using mice expressing the low affinity form of the AH receptor (AHRd), we investigated whether the enhancement of uroporphyria by HCB involves an obligatory increase in CYP1A2 as measured by specific enzyme assays and immunoblotting. We compared the ability of HCB, in combination with iron dextran and the porphyrin precursor, 5-aminolevulinate (ALA), to cause uroporphyria in a strain of mice (C57BL/6) which expresses the high affinity form of the receptor (AHRb(1)), with three strains of mice (SWR and two 129 sublines) expressing the low affinity AHRd. In C57BL/6 mice, HCB-enhanced uroporphyria was associated with a doubling of CYP1A2. HCB treatment produced uroporphyria in iron-loaded mice expressing AHRd, even though there was little or no increase in CYP1A2. Cyp1a2(-/-) mice in a 129 background were completely resistant to HCB-induced uroporphyria, and female Hfe(-/-) 129 mice, in which the levels of hepatic CYP1A2 were half of those of the male levels, responded poorly. The effect of exogenous iron, administered in the form of iron dextran, on HCB enhancement of uroporphryia could be replicated utilizing the endogenous hepatic iron accumulated in 129 Hfe(-/-) mice. In conclusion, some minimal basal expression of CYP1A2 is essential for HCB-mediated enhancement of uroporphyria, but increases in CYP1A2 above that level are not essential.

A porphomethene inhibitor of uroporphyrinogen decarboxylase causes porphyria cutanea tarda

Porphyria cutanea tarda (PCT), the most common form of porphyria in humans, is due to reduced activity of uroporphyrinogen decarboxylase (URO-D) in the liver. Previous studies have demonstrated that protein levels of URO-D do not change when catalytic activity is reduced, suggesting that an inhibitor of URO-D is generated in hepatocytes. Here, we describe the identification and characterization of an inhibitor of URO-D in liver cytosolic extracts from two murine models of PCT: wild-type mice treated with iron, delta-aminolevulinic acid, and polychlorinated biphenyls; and mice with one null allele of Uro-d and two null alleles of the hemochromatosis gene (Uro-d(+/-), Hfe(-/-)) that develop PCT with no treatments. In both models, we identified an inhibitor of recombinant human URO-D (rhURO-D). The inhibitor was characterized by solid-phase extraction, chromatography, UV-visible spectroscopy, and mass spectroscopy and proved to be uroporphomethene, a compound in which one bridge carbon in the uroporphyrinogen macrocycle is oxidized. We synthesized uroporphomethene by photooxidation of enzymatically generated uroporphyrinogen I or III. Both uroporphomethenes inhibited rhURO-D, but the III isomer porphomethene was a more potent inhibitor. Finally, we detected an inhibitor of rhURO-D in cytosolic extracts of liver biopsy samples of patients with PCT. These studies define the mechanism underlying clinical expression of the PCT phenotype, namely oxidation of uroporphyrinogen to uroporphomethene, a competitive inhibitor of URO-D. The oxidation reaction is iron-dependent.

Attenuation of polychlorinated biphenyl induced uroporphyria by iron deprivation

A toxic sequel to polyhalogenated aromatic hydrocarbon exposure in humans is the development of porphyria cutanea tarda. In a mouse model (experimental uroporphyria) utilizing an environmentally relevant polychlorinated biphenyl (PCB) mixture, we show that the toxicity can be markedly influenced by nutritional status. In mice made susceptible to uroporphyria through a targeted deletion of one allele of uroporphyrinogen decarboxylase (Uro-D+/-), an iron deficient diet prevented the development of the uroporphyria and the changes in associated parameters normally seen within three weeks following a single exposure to Aroclor 1254. Iron deprivation also completely prevented PCB-induced uroporphyria in mice wild-type at the Uro-D locus (Uro-D+/+), a model that requires δ-aminolevulinic acid administration for the development of uroporphyria. In Uro-D+/- mice consuming δ-aminolevulinic acid, PCB exposure produced a severe uroporphyria that was attenuated, not prevented, by iron deficiency. This attenuation moderated hepatic uroporphyrin and uroporphyrinogen decarboxylase inhibitor levels, but not the depression of cytosolic uroporphyrinogen decarboxylase activity.

Experimental hepatic uroporphyria induced by the diphenyl-ether herbicide fomesafen in male DBA/2 mice

Hepatic uroporphyria can be readily induced by a variety of treatments in mice of the C57BL strains, whereas DBA/2 mice are almost completely resistant. However, feeding of the protoporphyrinogen oxidase-inhibiting herbicide fomesafen (0.25% in the diet for 18 weeks) induced hepatic uroporphyria in male DBA/2N mice (liver porphyrin content up to 150 nmol/g, control animals 1 nmol/g), whereas fomesafen-treated male C57BL/6N mice displayed only a slight elevation of liver porphyrins (approximately 5 nmol/g). The profile of accumulated hepatic porphyrins in fomesafen-treated DBA/2N mice resembled the well-characterised uroporphyria induced by polyhalogenated aromatic hydrocarbons, while histological examination confirmed the presence of uroporphyria-specific cytoplasmic inclusions in the hepatocytes. Uroporphyrinogen decarboxylase activity decreased to about 30% of control values in fomesafen-treated DBA/2N mice; microsomal methoxyresorufin O-dealkylase activity was slightly reduced. The amount of CYP1A1 and CYP1A2 mRNA, as determined by real-time PCR, was not significantly changed; mRNA encoding the housekeeping 5-aminolevulinic acid synthase was elevated 10-fold. Total liver iron was slightly increased. A similar uroporphyria was induced by the herbicide formulation Blazer, containing a structurally related herbicide acifluorfen, when fed to DBA/2N mice at a dose corresponding to 0.25% of acifluorfen in the diet. Since DBA/2 mice are almost completely resistant to all well-characterised porphyrogenic chemicals, the results suggest the possible existence of a yet unknown mechanism of uroporphyria induction, to which the DBA/2 mouse strain is more sensitive than the C57BL strain.

Uroporphyria caused by ethanol in Hfe(-/-) mice as a model for porphyria cutanea tarda

Two major risk factors for the development of porphyria cutanea tarda (PCT) are alcohol consumption and homozygosity for the C282Y mutation in the hereditary hemochromatosis gene (HFE). To develop an animal model, Hfe knockout mice were treated continuously with 10% ethanol in drinking water. By 4 months, uroporphyrin (URO) was detected in the urine. At 6 to 7 months, hepatic URO was increased and hepatic uroporphyrinogen decarboxylase (UROD) activity was decreased. Untreated Hfe(-/-) mice or wild-type mice treated with or without ethanol did not show any of these biochemical changes. Treatment with ethanol increased hepatic nonheme iron and hepatic 5-aminolevulinate synthase activity in Hfe(-/-) but not wild-type mice. The increases in nonheme iron in Hfe(-/-) mice were associated with diffuse increases in iron staining of parenchymal cells but without evidence of significant liver injury. In conclusion, the results of this study suggest that the uroporphyrinogenic effect of ethanol is mediated by its effects on hepatic iron metabolism. Ethanol-treated Hfe(-/-) mice seem to be an excellent model for studies of alcohol-mediated PCT.

Effects of chronic dietary exposure to environmentally relevant concentrations to 2,3,7,8-tetrachlorodibenzo-p-dioxin on survival, growth, reproduction and biochemical responses of female rainbow trout (Oncorhynchus mykiss)

Adult female rainbow trout were exposed to dietary 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at concentrations of 1.8, 18 and 90 ng TCDD/kg (ww) food for up to 300 day. At the end of the exposure fish were spawned and the reproductive outcomes were assessed. TCDD was accumulated into tissues and eggs in a dose-dependent manner with steady state being achieved after 50-100 day of exposure. Biochemical and hematological parameters were monitored at 50, 100, 150, and 200 day after the beginning of exposure. The survival of adult female trout was reduced in a dose-dependent manner by exposure to TCDD in the diet. Fish fed 1.8 ng TCDD/kg, moist weight of diet, showed significantly reduced survival compared with those fed the control diet. TCDD also affected survival of fry from females fed 1.8 ng TCDD/kg. Observed adverse effects in adult fish were as sensitive as early life-stage endpoints. Liver EROD activity was only moderately increased in all exposure groups after 250+ day of exposure. Low rates of edema and deformities were observed in fry from all treatment groups including controls. This study has demonstrated adverse effects of TCDD to both adults and fry at concentrations comparable to current environmental concentrations. This suggests that direct adult toxicity as well as reproductive endpoints need to be incorporated in the current risk assessment paradigm for these compounds.

Uroporphyria in mice: thresholds for hepatic CYP1A2 and iron

In mice treated with 5-aminolevulinic acid (ALA) and polyhalogenated aromatic compounds, the levels of both hepatic cytochrome P450 (CYP)1A2 and iron-which can be quite different among inbred strains-are critical in causing experimental uroporphyria. Here we investigate the development of uroporphyria as a function of CYP1A2 and iron levels in the liver of mice having a common C57BL/6 genetic background. We compared Cyp1a2(-/-) knockout mice, Cyp1a2(+/-) heterozygotes, Cyp1a2(+/+) wild type, and Cyp1a2(+/+) mice pretreated with a low dose of 3,3',4,4',5-pentachlorobiphenyl (PCB126) (4 microg/kg). Cyp1a2(+/-) mice contain about 60% of the hepatic CYP1A2 content of Cyp1a2(+/+) mice, and the PCB126-pretreated Cyp1a2(+/+) mice have about twice the wild-type levels of CYP1A2. ALA- and iron-treated Cyp1a2(+/+) mice are known to accumulate hepatic uroporphyrin; this accumulation was increased 7-fold by pretreatment with the low dose of PCB126. ALA- and iron-treated Cyp1a2(+/-) heterozygote mice accumulated no uroporphyrin in 4 weeks, but by 8 weeks accumulated significant amounts of uroporphyrin. As previously reported, the ALA- and iron-treated Cyp1a2(-/-) knockout mouse has no CYP1A2 and exhibits no detectable uroporphyrin accumulation. Iron dose-response curves in ALA- and PCB126-treated Cyp1a2(+/+) mice showed that hepatic iron levels greater than 850 microg/g liver were required to produce significant uroporphyrin accumulation in the liver. Other measures of hepatic effects of iron (iron-response element-binding protein [IRP]-iron response element [IRE] binding activity and accumulation of protoporphyrin from ALA) decreased when the level of iron was considerably lower than 850 microg/g liver. At low iron doses, accumulation of iron was principally in Kupffer cells, whereas at the higher doses (required to stimulate uroporphyrin accumulation), more iron was found in parenchymal cells. We conclude that small changes in hepatic CYP1A2 levels can dramatically affect uroporphyria in C57BL/6 mice, providing the animals have been sufficiently loaded with iron; these data might be clinically relevant to acquired (sporadic) porphyria cutanea tarda, because humans show greater than 60-fold genetic differences in hepatic basal CYP1A2.

Non-ahr gene susceptibility Loci for porphyria and liver injury induced by the interaction of 'dioxin' with iron overload in mice

Among the actions of 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin) in mice is the induction of hepatic porphyria. This is similar to the most common disease of this type in humans, sporadic porphyria cutanea tarda (PCT). Evidence is consistent with the actions of dioxin being mediated through binding to the aryl hydrocarbon receptor (AHR) with different Ahr alleles in mouse strains apparently accounting for differential downstream gene expression and susceptibility. However, studies of dioxin-induced porphyria and liver injury indicate that the mechanisms must involve interactions with other genes, perhaps associated with iron metabolism. We performed a quantitative trait locus (QTL) analysis of an F(2) cross between susceptible C57BL/6J (Ahr(b1) allele) and the highly resistant DBA/2 (Ahr(d) allele) strains after treatment with dioxin and iron. For porphyria we found QTLs on chromosomes 11 and 14 in addition to the Ahr gene (chromosome 12). Studies with C57BL/6.D2 Ahr(d) mice confirmed that the Ahr(d) allele alone did not completely negate the response. SWR mice are syngenic for the Ahr(d) allele with the DBA/2 strain but are susceptible to porphyria after elevation of hepatic iron. Analysis of SWRxD2 F(2) mice treated with iron and dioxin showed a QTL on chromosome 11, as well as finding other loci on chromosomes 1 (and possibly 9), for both porphyria and liver injury. These findings show for the first time the location of genes, other than Ahr, that modulate the mechanism of hepatic porphyria and injury caused by dioxin in mice. Orthologous loci may contribute to the pathogenesis of human sporadic PCT.

Protection of the Cyp1a2(-/-) null mouse against uroporphyria and hepatic injury following exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin

The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the liver of C57BL/6J mice is a model for clinical sporadic porphyria cutanea tarda (PCT). There is massive uroporphyria, inhibition of uroporphyrinogen decarboxylase (UROD) activity, and hepatocellular damage. A variety of evidence implicates the CYP1A2 enzyme as necessary for mouse uroporphyria. Here we report that, 5 weeks after a single oral dose of TCDD (75 microg/kg), Cyp1a2(+/+) wild-type mice showed severe uroporphyria and greater than 90% decreases in UROD activity; in contrast, despite exposure to this potent agent Cyp1a2(-/-) knockout mice displayed absolutely no increases in hepatic porphyrin levels, even after prior iron overload, and no detectable inhibition of UROD activity. Plasma levels of alanine-aminotransferase (ALT) and aspartate aminotransferase (AST)-although elevated in both genotypes after TCDD exposure-were significantly less in Cyp1a2(-/-) than in Cyp1a2(+/+) mice, suggesting that the absence of CYP1A2 also affords partial protection against TCDD-induced liver toxicity. Histological examination confirmed a decrease in hepatocellular damage in TCDD-treated Cyp1a2(-/-) mice; in particular, there was no bile duct damage or proliferation that in the Cyp1a2(+/+) mice might be caused by uroporphyrin. We conclude that CYP1A2 is both necessary and essential for the potent uroporphyrinogenic effects of TCDD in mice, and that CYP1A2 also plays a role in contributing to TCDD-induced hepatocellular injury. This study has implications for both the toxicity assessment of TCDD and the hepatic injury seen in PCT patients.

CYP1A2 is essential in murine uroporphyria caused by hexachlorobenzene and iron

Using Cyp1a2(-/-) mice we previously showed that CYP1A2 is absolutely required for hepatic uroporphyrin accumulation caused by iron and 5-aminolevulinate (ALA) treatment, both in the presence and absence of an inducer of CYP1A2. In this study we have used these mice to investigate whether CYP1A2 has an obligatory role in hepatic uroporphyria caused by hexachlorobenzene (HCBZ), an inducer of CYP2B and CYP3A, as well as CYP1A2. Here we treated mice with HCBZ and iron, with and without the porphyrin precursor, ALA, in the drinking water. In iron-loaded wild-type mice given a single dose of HCBZ and ALA, hepatic uroporphyrin (URO) accumulated to 300 nmol/g liver after 37 days, whereas in Cyp1a2(-/-) mice, there was no hepatic URO, even after an additional dose of HCBZ, and a further 29 days of ALA treatment. A similar requirement for CYP1A2 was found in uroporphyria produced in HCBZ and iron-treated mice in the absence of ALA. As detected by Western immunoblotting, HCBZ induced small increases in CYP2B and CYP3A in the livers of all animals. In the wild-type animals, HCBZ also induced CYP1A2 and associated enzyme activities, including uroporphyrinogen oxidation, by about 2-3-fold. In the Cyp1a2(-/-) mice, HCBZ did not increase hepatic microsomal uroporphyrinogen oxidation. These results indicate that, in mice, CYP1A2 is essential in the process leading to HCBZ-induced uroporphyria. Contributions by other CYP forms induced by HCBZ appear to be minimal.

Uroporphyria produced in mice by iron and 5-aminolaevulinic acid does not occur in Cyp1a2(-/-) null mutant mice

In the present study we have investigated the putative requirement for the cytochrome P-450 isoform CYP1A2 in murine uroporphyria, by comparing Cyp1a2(-/-) knockout mice with Cyp1a2(+/+) wild-type mice. Uroporphyria was produced by injecting animals with iron-dextran and giving the porphyrin precursor 5-aminolaevulinic acid in the drinking water. Some animals also received 3-methylcholanthrene (MC) to induce hepatic CYP1A2. In both protocols, uroporphyria was elicited by these treatments in the Cyp1a2(+/+) wild-type mice, but not in the null mutant mice. Uroporphyrinogen oxidation activity in hepatic microsomes from untreated Cyp1a2(+/+) mice was 2.5-fold higher than in Cyp1a2(-/-) mice. Treatment with MC increased hepatic CYP1A1 in both mouse lines and hepatic CYP1A2 only in the Cyp1a2(+/+) line, as determined by Western immunoblotting. MC increased hepatic ethoxy- and methoxy-resorufin O-dealkylase activities in both mouse lines, but increased uroporphyrinogen oxidation activity in the Cyp1a2(+/+) wild-type mice only. These results indicate the absolute requirement for hepatic CYP1A2 in causing experimental uroporphyria under the conditions used.

Interaction between iron metabolism and 2,3,7,8-tetrachlorodibenzo-p-dioxin in mice with variants of the Ahr gene: a hepatic oxidative mechanism

The binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) with the aryl hydrocarbon (AH) receptor and subsequent changes in gene expression have been studied intensively, but the mechanisms by which these lead to toxicity are unclear. We investigated the influence of iron, previously implicated in TCDD-induced hepatic porphyria, in mice with alleles of Ahr that encode receptors with varied affinity for TCDD. The administration of iron to Ahrb-1 C57BL/6J (AH-responsive) mice before a single dose of TCDD (75 micrograms/kg) markedly potentiated not only the hepatic porphyria but also general hepatocellular damage and elevation of plasma hepatic enzymes. The formation of hydroxylated and peroxylated derivatives of uroporphyrins formed from uroporphyrinogen and the induction of a mu-glutathione transferase (GST) were consistent with the operation of an oxidative mechanism. In a comparison of C57BL/6J mice with Ahrb-2 BALB/c (AH-responsive) and Ahrd SWR and DBA/2 (AH-nonresponsive) mice, iron overcame the weak hepatic porphyria and toxicity responses in BALB/c and SWR strains but not in DBA/2. CYP1A isoforms are strongly implicated in the mechanism of porphyria, but activities were lowered by 20-30% with iron treatment, and a comparison of levels between strains did not fully account for the resistance of DBA/2 mice. Studies with the use of gel shift assays and cytosolic aconitase of the capacity of the iron regulatory protein controlling the translation of some iron metabolism proteins showed a significant difference between C57BL/6J and DBA/2 mice after the administration of TCDD. We conclude that iron potentiates both the hepatic porphyria and toxicity of TCDD in susceptible mice in an oxidative process with disturbance of iron regulatory protein capacity. Iron even overcomes the AH-nonresponsive Ahrd allele in the SWR strain but not in DBA/2 mice, which remain resistant.

Synergy of iron in the toxicity and carcinogenicity of polychlorinated biphenyls (PCBs) and related chemicals

In Ah-responsive C57BL/10ScSn mice a single dose of iron significantly potentiated the property of the polychlorinated biphenyl (PCB) mixture Aroclor 1254 to induce porphyria by inhibition at the uroporphyrinogen decarboxylase stage of hepatic haem biosynthesis. The induction of liver tumors and other lesions were also enhanced markedly by iron overload suggesting a link between porphyria and cancer. The cellular, molecular and biochemical processes involved have been investigated in attempts to explain these phenomena by an iron-catalysed 'oxidative stress' mechanism.

Ferritin accumulation and uroporphyrin crystal formation in hepatocytes of C57BL/10 mice: a time-course study

To establish the time-sequence relationship between ferritin accumulation and uroporphyrin crystal formation in livers of C57BL/10 mice, a biochemical, morphological and morphometrical study was performed. Uroporphyria was induced by the intraperitoneal administration of hexachlorobenzene plus iron dextran and of iron dextran alone. Uroporphyrin crystal formation started in hepatocytes of mice treated with hexachlorobenzene plus iron dextran at 2 weeks and in mice treated with iron dextran alone a 9 weeks. In the course of time, uroporphyrin crystals gradually increased in size. Uroporphyrin crystals were initially formed in hepatocytes in the periportal areas of the liver, in which also ferric iron staining was first detected. The amount and the distribution of the main storage form of iron in hepatocytes, ferritin, did not differ between the two treatment groups. Ferritin accumulation preceded the formation of uroporphyrin crystals in hepatocytes in both treatment groups. Moreover, uroporphyrin crystals were nearly always found close to ferritin iron. We conclude that uroporphyrin crystals are only formed in hepatocytes in which also iron (ferritin) accumulates. Hexachlorobenzene accelerates the effects of iron in porphyrin metabolism, but does not influence the accumulation of iron into the liver.

The effect of desferrioxamine on iron metabolism and lipid peroxidation in hepatocytes of C57BL/10 mice in experimental uroporphyria

The effects of the iron chelator desferrioxamine (DFx) on liver iron accumulation, malondialdehyde (MDA) production, porphyrin accumulation and uroporphyrinogen decarboxylase (URO-D; EC 4.1.1.37) activity were investigated over a period of 14 weeks in C57BL/10 mice, made porphyric by the administration of hexachlorobenzene (HCB) and iron-dextran (Imferon, IMF) or IMF alone. In addition, we measured the amount of low molecular weight (LMW) iron in liver tissue to determine a possible correlation with MDA production. These experiments showed that combined treatment with HCB + IMF, as well as IMF alone, resulted in porphyrin accumulation, increased MDA production and reduced URO-D activity, whereas HCB alone had no effect. DFx caused a reduction in hepatic porphyrins, this reduction being more distinct in the IMF group than in the HCB + IMF group. The effect of DFx on MDA production and URO-D activity was in agreement with the results on porphyrin accumulation. LMW iron pool measurements at 11 weeks correlated well with data on MDA production in all treated groups in that period (r2 = 0.84), suggesting both variables are interdependent. In conclusion, these results suggest an important role for iron in porphyrin accumulation, probably through its catalytic role in the generation of oxygen-related free radicals, resulting in direct damage to URO-D. The effectiveness of DFx in reducing porphyrin accumulation is probably the result of a reduction in LMW iron, thus diminishing the amount of iron available for a catalytic role in the generation of oxygen-related free radicals.

Genetic variation of iron-induced uroporphyria in mice

Iron overload causes inhibition of hepatic uroporphyrinogen decarboxylase (UROD) and uroporphyria in C57BL/10ScSn but not DBA/2 mice [Smith, Cabral, Carthew, Francis and Manson (1989) Int. J. Cancer 43, 492-496]. We have investigated the induction of uroporphyria in 12 inbred strains of mice 25 weeks after iron treatment (600 mg/kg) to determine if there was any correlation with the Ah locus. Under these conditions, inhibition of UROD occurred to varying degrees in Ahd mice (SWR and AKR) as well as nominally Ahb-1 (C57BL/6J, C57BL/10ScSn and C57BL/10-cc) and Ahb-2 strains (BALB/c and C3H/HeJ). Five other Ahb or Ahd strains (C57BL/Ks, A/J, CBA/J, LP and DBA/2) were unaffected. Thus there appeared to be no correlation with the Ah phenotype and this illustrated that some other variable inherited factors are involved. Comparisons between another susceptible strain, A2G, and the congenic A2G-hr/+strain (carrying the recessive hr gene) showed a modulating influence associated with the hr locus. In contrast with individual mice of inbred strains, which showed consistent responses to iron, those of the outbred MF1 strain showed a spectrum of sensitivities as might be expected for a heterogeneic stock. The rate of porphyria development was accelerated by administration of 5-aminolaevulinic acid (5-ALA) in the drinking water, but this did not overcome strain differences. Among four strains the order of susceptibility was SWR > C57BL/10ScSn > C57B1/6J > DBA/2 (the last strain was completely resistant). With degrees of iron loading greater than 600 mg of Fe/kg (1200-1800 mg of Fe/kg) C57BL/10ScSn mice (after 20 weeks) and SWR mice (after 5 weeks which included 4 weeks of 5-ALA treatment) had less inhibition of UROD and a lower uroporphyric response, showing that there was an optimum level of liver iron concentration. Studies on selected microsomal enzyme activities associated with cytochrome P-450 showed no correlation with the propensities of strains to develop porphyria. These activities included the NADPH-dependent oxidation of uroporphyrinogen I to uroporphyrin I.

The role of iron in experimental porphyria and porphyria cutanea tarda

Porphyria cutanea tarda (PCT) and experimental porphyria are characterized by a decreased activity of the enzyme uroporphyrinogen decarboxylase, and accumulation of uroporphyrins and heptacarboxylporphyrins in the liver. Iron (Fe) plays an important role in PCT and experimental porphyria. Biochemically and electron microscopically, we examined the relationship between Fe and porphyrins in liver tissue of C57BL/10 mice made porphyric by administration of iron dextran as Imferon (IMF), and in liver biopsies of patients with symptomatic PCT. Accumulation of uroporphyrins and heptacarboxylporphyrins, and an increased amount of Fe were observed in livers of mice treated with IMF and in liver biopsies of patients with PCT. In mice treated with IMF, the activity of uroporphyrinogen decarboxylase was decreased. Both in livers of mice treated with IMF and in livers of patients with PCT, needle-like structures, representing uroporphyrin crystals, were observed by electron microscopy. Uroporphyrin crystals and Fe (as ferritin) were observed in the same hepatocyte. Moreover, there was a striking morphological correlation between uroporphyrin crystals and ferritin-Fe, suggesting a role for (ferritin-)Fe in the pathogenesis of porphyria.

Increased copper concentrations in rat tissues after acute intoxication with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Recently, acutely toxic doses of the environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) have been reported to affect the hepatic distribution of essential metals in the rat. However, the reduced food intake by TCDD was not taken into account. Therefore, metal concentrations were determined in different rat tissues at the end of a toxicity study with TCDD in which a pair-fed control group was introduced. Male Sprague-Dawley rats received a single i.p. injection of corn oil/acetone with or without TCDD at 125 micrograms/kg. Controls and TCDD-treated rats were fed ad libitum; additionally, pair-fed controls received the amount of food consumed by their TCDD-treated partners 1 day before. Twenty-one days after dosing rats were killed and samples of liver, kidney and jejunum were taken for the analysis of Ca, Cu, Fe, Mg, Mn, and Zn. After acid digestion of the tissues metals were determined by atomic emission spectrometry (AES). The most outstanding effect of TCDD treatment was an increase of the copper levels in the kidney (4-fold, versus pair-fed controls) and in the liver (greater than 2-fold, versus pair-fed controls). Other metals were mainly affected by the reduced food intake only. Since Cu represents a trace metal the homeostasis of which depends on its biliary excretion and since TCDD is known to impair biliary flow and excretion, an impaired biliary excretion of Cu by TCDD is suggested as the causal mechanism.

Chlorinated biphenyls induce cytochrome P450IA2 and uroporphyrin accumulation in cultures of mouse hepatocytes

Previous enzymatic and immunological studies from this laboratory have indicated a critical role for cytochrome P450IA2-catalyzed uroporphyrinogen oxidation in the development of uroporphyria caused by halogenated aromatic hydrocarbons. To extend these studies, we investigated whether primary cultures of mammalian hepatocytes which are inducible for cytochrome P450IA2 are also inducible for chemically mediated uroporphyria. Hepatocytes were isolated from C57BL/6 mice and maintained on Matrigel, an extracellular matrix isolated from a mouse tumor. When these cultures were treated with 3,4,5,3',4',5'-hexachlorobiphenyl (HCB) and 5-aminolevulinic acid (ALA), they accumulated cytochrome P450IA2 as well as uroporphyrin (URO) and heptacarboxyporphyrin for up to 12 days. Cultures treated with ALA alone accumulated no P450IA2 and very little URO. Neither URO accumulation nor the level of P450IA2 was affected by addition of iron as the nitrilotriacetate complex. Other inducers of P450IA2 in vivo (3,4,5,3',4'-pentachlorobiphenyl, 3,4,3',4'-tetrachlorobiphenyl, and 3-methylcholanthrene) also increased P450IA2 in the cultures and caused URO accumulation in the presence of added ALA. The tetrachlorobiphenyl and methylcholanthrene caused these effects only when given repeatedly. Inducers of other forms of P450 failed to cause URO accumulation in the presence of ALA and iron. Cultures of hepatocytes from DBA mice (which are resistant to the uroporphyria in vivo) accumulated much less P450IA2 or URO when treated with HCB and ALA. These primary cultures of mammalian hepatocytes represent a new experimental model to investigate the role of cytochrome P450IA2 in the mechanism of chemically induced uroporphyria.

Free radical mechanism of oxidation of uroporphyrinogen in the presence of ferrous iron

Human porphyria cutanea tarda is an unusual consequence of common hepatic disorders such as alcoholic liver disease. Hepatic iron plays a key role in the expression of the metabolic lesions, i.e., defective hepatic decarboxylation of porphyrinogens, catalyzed by uroporphyrinogen decarboxylase. This prompted the present study to determine the in vitro effects of iron on the uroporphyrinogen substrate in the absence and presence of atmospheric oxygen. We observed that (i) unless oxygen is the limiting reactant, autoxidation of ferrous iron and iron-catalyzed oxidation of uroporphyrinogen occurred soon after initiating the reaction at pH 7.4 and 30 degrees C in buffers which are non- or poor chelators of iron; (ii) the rates of uroporphyrinogen oxidation were proportional to the initial concentration of ferrous ion; (iii) about 70% of the oxidations of uroporphyrinogen were accountable due to a free-radical chain reaction pathway involving superoxide radical and hence inhibitable by superoxide dismutase; (iv) uroporphyrinogen could be further oxidized to completion by the hydroxyl radical since the reaction was partially inhibited by both mannitol and catalase which prevent hydroxyl radical production; (v) the oxidizing effects of ferric ion on uroporphyrinogen were none or negligible as compared to those of ferrous ion. Ferric was reduced to ferrous ion in the presence of dithiothreitol. When the ferrous ion thus formed was reoxidized in the presence of atmospheric oxygen, minor but definite oxidations of both uroporphyrinogen and dithiothreitol were observed. The oxidations of Fe2+ and uroporphyrinogen could be blocked by 1,10-phenanthroline, a ferrous iron chelator. The data suggest that ferrous is the reactive form of iron that may contribute to pathogenic development of the disease by irreversibly oxidizing the porphyrinogen substrates to nonmetabolizable porphyrins, which accumulate in porphyric liver.

Oxidative injury mediated by the hepatic cytochrome P-450 system in conjunction with cellular iron. Effects on the pathway of haem biosynthesis

1. Some polyhalogenated aromatic chemicals such as 2,3,7,8-tetrachloro-p-dioxin, brominated and chlorinated biphenyls, and hexachlorobenzene cause in humans, animals and hepatocyte systems a partial block in haem biosynthesis leading to accumulation and excretion of uroporphyrin, the oxidation product of the unstable biosynthetic intermediate uroporphyrinogen. 2. The involvement of reactive toxic metabolites of the halogenated chemicals has previously been suggested. The evidence presented in this paper supports a different mechanism involving chronic induction of the microsomal cytochrome P-450 system, mobilization of hepatocellular iron and associated oxidative stress. Besides oxidation of uroporphyrinogen to uroporphyrin, an inhibitor of uroporphyrinogen decarboxylase may also be formed. 3. Studies with iron-loaded mice and chicken embryo hepatocytes show that under appropriate conditions iron alone, or chemicals such as beta-naphthoflavone which induce the same cytochromes P-450 isozymes as do the chlorinated aromatics, will cause a similar uroporphyria. These findings provide an experimental model for the human disease porphyria cutanea tarda, sometimes occurring in patients with liver damage. 4. Experiments with rats and iron-loaded mice indicate that there may also be an association between the induction of uroporphyria and the development of liver tumours after administration of polyhalogenated aromatic chemicals.

Oxidative stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is one of the most potent toxins and tumor promoters known to man. It is prototypical of many halogenated polycyclic hydrocarbons that occur as environmental contaminants. Pathologic lesions produced by these compounds are mediated by an intracellular receptor protein called the TCDD (Ah) receptor which functions as a trans-acting effector of gene expression. However, the ultimate posttranslational pathways and mechanisms involved in the expression of the toxic manifestations of TCDD have received little attention and remain unclear, yet constitute an important segment in our understanding of the overall mechanism of action of TCDD. Recent studies have demonstrated that an oxidative stress occurs in various tissues of TCDD-treated animals. Evidence indicating production of an oxidative stress by TCDD in rodents is reviewed and includes:enhanced in vitro and in vivo hepatic and extrahepatic lipid peroxidation; increased hepatic and macrophage DNA damage; increased urinary excretion of malondialdehyde; decreased hepatic membrane fluidity; increased production of superoxide anion by peritoneal macrophage; and decreased glutathione, nonprotein sulfhydryl, and NADPH contents in liver. The potential role of reactive oxygen species in tumor promotion by TCDD is discussed. Possible sources and mechanisms of production of reactive oxygen species in response to TCDD are considered in light of current information. Evidence demonstrating the involvement of iron in TCDD-induced formation of reactive oxygen species and DNA damage is reviewed. Oxidative damage may contribute to many of the toxic responses produced by TCDD and its bioisosteres, and may be common to most of the tissue-damaging effects.

Hepatic mitochondrial oxidative metabolism and lipid peroxidation in experimental hexachlorobenzene-induced porphyria with dietary carbonyl iron overload

Both human porphyria cutanea tarda and experimental hexachlorobenzene-induced porphyria are associated with hepatic injury and are potentiated by excess hepatic iron. The mechanisms whereby cellular injury occurs and the synergistic role of iron overload are unknown. In the present experiments, we studied hepatic mitochondrial function and lipid peroxidation in rats with hexachlorobenzene-induced porphyria in which iron loading was achieved by dietary carbonyl iron supplementation. Female rats were treated for 8 weeks, receiving a chow diet supplemented with hexachlorobenzene (0.2%, w/w), carbonyl iron (1.0%, w/w) or hexachlorobenzene + iron. Hepatic total porphyrins were increased 100-fold in rats receiving hexachlorobenzene (hexachlorobenzene alone and hexachlorobenzene + Fe), and total hepatic iron was increased approximately 10-fold in rats receiving iron supplementation (Fe alone and hexachlorobenzene + Fe). There was a significant increase in mitochondrial lipid peroxidation in rats treated with hexachlorobenzene alone and hexachlorobenzene + Fe. A significant reduction in mitochondrial respiratory control ratios and in oxidative phosphorylation (ADP/O ratios) using glutamate and succinate as substrates was demonstrated when rats were treated with hexachlorobenzene + iron. The reductions in respiratory control ratios were due to a combination of an inhibitory defect in electron transport as evidenced by an irreversible decrease in State 3 respiration and an uncoupling effect as evidenced by an increase in State 4 respiration. These findings suggest that lipid peroxidation and mitochondrial dysfunction may contribute to the hepatotoxicity seen in hexachlorobenzene-induced porphyria.

Iron stimulation of free radical-mediated porphyrinogen oxidation by hepatic and renal mitochondria

Mitochondria from rat liver and kidney catalyze oxidation of uroporphyrinogen in the presence of NADH or succinate and the respiratory chain inhibitor, NaN3. The rate of porphyrinogen oxidation was substantially accelerated when iron as Fe+3-EDTA was added to reaction mixtures. This effect was partially attenuated by catalase, reduced glutathione (GSH) and other free radical scavengers. These results suggest that iron stimulates free radical-mediated porphyrinogen oxidation by tissue mitochondria under conditions of perturbed mitochondrial respiratory function. These observations suggest a mechanism by which iron could contribute to excess porphyrin excretion in various inherited or chemically-induced porphyrias.

Carcinogenicity of iron in conjunction with a chlorinated environmental chemical, hexachlorobenzene, in C57BL/10ScSn mice

Pre-loading of C57BL/10ScSn mice with iron greatly sensitizes them to the induction of hepatic porphyria caused by hexachlorobenzene (HCB) (Smith and Francis, 1983). HCB will also cause liver tumours in experimental animals. Elevated liver iron stores are implicated in the development of some human liver cancers. To determine whether iron overload will potentiate the hepatocarcinogenicity of HCB, male C57BL/10ScSn mice received a single dose of iron-dextran complex or the dextran carrier and were then fed HCB in the diet (0.01%) for up to 18 months. A very clear potentiation by iron of liver tumour incidence occurred. Of the mice which received HCB, but no iron, for 12 months a low incidence of liver hyperplastic nodules was observed, whereas preadministration of iron increased the incidence. After 18 months, all surviving mice initially given iron followed by HCB developed advanced hepatic nodules and 90% had hepatocellular carcinomas. No tumours were seen in animals that only received HCB. Nodules were not seen in iron-dosed mice receiving control diet, but another novel observation was that these animals developed porphyria by 6 months. Although iron enhanced tumour formation in the presence of HCB, the foci, nodules and carcinomas formed all excluded iron. Preliminary studies with the DBA/2 strain suggested that these mice were considerably less susceptible to potentiation by iron.

Uroporphyria produced in mice by 20-methylcholanthrene and 5-aminolaevulinic acid

Iron-loaded male C57BL/6 mice allowed free access to an aqueous solution of 5-aminolaevulinic acid (ALA) (2 mg/ml) as their only drink, develop severe uroporphyria within 9 days of a single intraperitoneal dose of 20-methylcholanthrene (MC) (125 mg/kg). At 21 days, uroporphyrinogen decarboxylase (EC 4.1.1.37) activities are less than 10% of control activities. The porphyria is not dependent on pretreatment with iron and persists for at least 21 days after withdrawal of ALA. The same intraperitoneal dose of MC does not produce porphyria within 21 days when given without ALA. Continuous administration of ALA markedly accelerates the onset of porphyria in iron-loaded male C57BL/6 mice after a single intraperitoneal dose of hexachlorobenzene (200 mg/kg); mice given phenobarbitone and ALA do not become porphyric. MC with ALA does not produce porphyria in iron-loaded male DBA/2 mice. At least two separate events are needed to produce uroporphyria in mammals: induction of a specific form of cytochrome P-450 and stimulation of the formation of intermediates of haem biosynthesis in the liver. These results show that severe, persistent porphyria can be produced in mammals by compounds other than polyhalogenated aromatic hydrocarbons and suggest that a similar mechanism underlies the porphyrogenic action of halogenated and non-halogenated compounds.

Investigations on the role of free radical processes in hexachlorobenzene-induced porphyria in mice

Male C57Bl/10 mice were chronically fed hexachlorobenzene (HCB) (0.02% of the diet) alone or in combination with a single subcutaneous dose of iron (12.5 mg iron per mouse). After eight weeks the group of mice pretreated with the iron overload was highly sensitized to the porphyrogenic effect of HCB, as shown by liver porphyrin accumulation. A synergistic effect of iron was evident on other parameters too, such as HCB-induced hepatic damage, activation of type O of xanthine oxidase, and decreased activity of copper zinc superoxide dismutase and glutathione peroxidase(s). None of these parameters was affected by iron alone. Iron alone and in association with HCB markedly raised the level of lipid peroxides, the increase in the HCB group being smaller. The combined treatment resulted in a significant reduction of HCB's inductive effects on microsomal heme and cytochromes P-450 and b5 and on the activity of aryl hydrocarbon hydroxylase. The content of nonprotein sulfhydryl groups was reduced to the same extent in mice treated with HCB or HCB plus iron. The results suggest that reactive intermediates such as are formed by lipid peroxidation are not sufficient on their own to create the conditions for uroporphyrinogen decarboxylase impairment, as evident in the group of mice receiving iron overload alone. Conversely, HCB administration induced a specific condition of imbalance in the liver between formation and inactivation of reactive intermediates which was associated with hepatic porphyrin accumulation and was potentiated by concomitant administration of iron.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on the hepatic distribution of iron, copper, zinc, and magnesium in rats

The distribution of iron, copper, zinc, and magnesium in hepatic subcellular fractions of male and female rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was determined. Animals received 40 micrograms TCDD per kilogram per day for three days by mouth (po) or the vehicle and were killed seven or nine days posttreatment. Iron, copper, zinc, and magnesium were determined by atomic absorption spectroscopy. The iron content of liver from female animals was twofold higher than male animals. The administration of TCDD increased the iron content of mitochondria in female and male rats and decreased iron content of microsomes of both sexes. Significant increases occurred in the copper content of whole liver, mitochondria, and cytosol of male rats and in whole liver and cytosol of female rats. Decreases in the copper content of the microsomes of male rats were observed following TCDD treatment; however, TCDD produced no changes in the zinc content of hepatic subcellular fractions of either sex. The magnesium content of female TCDD-treated rats increased in whole liver, mitochondria, and cytosol, while the magnesium content of microsomes was not altered. With respect to the subcellular distribution of iron, copper, zinc, and magnesium, TCDD produces differential effects. The altered distribution of some cations may contribute to the broad range of effects of TCDD.

Dietary iron and 2,3,7,8-tetrachlorodibenzo-p-dioxin induced alterations in hepatic lipid peroxidation, glutathione content and body weight

The effects of various levels of dietary iron on hepatic lipid peroxidation (malondialdehyde [MDA] content), reduced glutathione (GSH) and GSH peroxidase (GSH-PX) activity as well as liver and body weights of female rats following TCDD administration were examined. Rats were fed diets containing deficient (6 ppm), normal (35 ppm) and supplemented (120 ppm) iron for 17, 24 and 31 days. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD, 40 micrograms/kg/day P.O.) in corn oil or the vehicle was given on days 9, 8 and 7 prior to sacrifice. TCDD treatment produced a 3-fold increase in hepatic MDA content in animals on normal iron diet. TCDD administration failed to increased MDA content in iron deficient animals. In the iron supplemented groups, TCDD resulted in 2.5 fold increases in lipid peroxidation. Dietary iron had no effect on hepatic GSH-PX activity. Animals on the iron deficient diet had 12-21% decreases in hepatic GSH content. TCDD administration resulted in 15-22% decreases in GSH content in animals on the control and iron supplemented diets. TCDD treatment resulted in significant decreases in body weights of animals on all 3 diets. TCDD induced lipid peroxidation appears to be iron dependent. However, the loss in body weight due to TCDD toxicity may not be dependent on lipid peroxidation.

Comparison of the effects of carbon tetrachloride and of 2,3,7,8-tetrachlorodibenzo-p-dioxin on the disposition of linoleic acid in rat liver in vitro

Both 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and carbon tetrachloride (CCl4) have conspicuous effects on lipid metabolism in rat liver. Although it is generally accepted that CCl4 administration leads to hepatic lipid peroxidation in vivo, conflicting reports from different laboratories make it unclear whether or not lipid peroxidation is involved in the mechanism of toxicity of TCDD. The present study involved pretreating F344 rats with CCl4 or TCDD, then at predetermined times thereafter, giving [U-14C]linoleic acid. A variety of compound classes were monitored in extracts of liver taken 30 min after the label was given. A previously unreported effect of CCl4 was a conspicuous increase in turnover of 1,2-diglycerides. That CCl4 did cause lipid peroxidation was evident from the presence of allylic hydroxyacids not seen in vehicle-treated controls, greatly increased radioactivity in protein-bound material, and decreased levels of arachidonate without decreased synthesis from linolate. Where effects of TCDD pretreatment could be seen, they were much less than the corresponding effects of CCl4. No allylic hydroxyacids were detected in livers of TCDD-treated rats. The concentration of arachidonate was not reduced, and elongation of linolate was not stimulated, indicating that TCDD did not cause extensive-but-repaired peroxidation. It is concluded that while TCDD may slightly increase hepatic lipid peroxidation in rats in vivo, the extent of such stimulation appears to be too slight to account for the toxicity of TCDD.

Changes in hamster hepatic cytochrome P-450, ethoxycoumarin O-deethylase, and reduced NAD(P): menadione oxidoreductase following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin. Partial dissociation of temporal and dose-response relationships from elicited toxicity

The temporal and dose-related characteristics of hepatic enzymes induced in the hamster by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were examined. Male Syrian golden hamsters received a single intraperitoneal injection of TCDD at a dose of 0-500 micrograms/kg. At various times up to 35 days, a number of variables were determined and compared: whole body, liver, and thymus weights; hepatic concentrations of cytochrome P-450 (P-450); and activities of 7-ethoxycoumarin O-deethylase (ECOD) and reduced NAD(P): menadione oxidoreductase (NMOR). Increased liver weights and decreased thymus weights were observed to be dose related. At day 7 following treatment, the approximate ED50 values for these responses were 15 and 100 micrograms/kg respectively. The ED50 values for the increase in hepatic P-450 concentrations and activities of ECOD and NMOR ranged from 0.5 to 2.0 micrograms/kg. At 10 and 500 micrograms/kg, NMOR activity remained maximally induced for up to 35 days. This was also the case for P-450 and ECOD activity at a dose of 10 micrograms/kg. At 500 micrograms/kg, both P-450 and ECOD demonstrated an induction up to day 4 followed by a decrease to near control levels by day 14. This decrease appeared to correlate with changes in hepatic morphology. These results demonstrate a dissociation of the induction of these hepatic enzymes from TCDD-induced lethality, in this species.

Effect of desferrioxamine on the development of hexachlorobenzene-induced porphyria

The present work deals with the effect of desferrioxamine (DF) on hexachlorobenzene (HCB)-induced porphyria in female rats with the purpose of further investigation of the role of iron in the development of this porphyria. The results obtained show that the repeated injection of DF (three times a week: 100 mg/kg each i.m.) delayed and diminished remarkably the urinary excretion of precursors and porphyrins as well as the accumulation of the latter in liver promoted by HCB (1 g/kg daily given by stomach tube). This was probably due to attenuation by DF of the alterations produced by the fungicide in the two key enzymes: porphyrinogen carboxy-lyase (PCL) and delta-aminolaevulinate synthase (ALA-S). In fact, DF by reducing liver iron levels produced a smaller decrease of the target enzyme (PCL) and a concomitant smaller induction of ALA-S. DF alone did not modify any of these variables or the liver to body weight ratio. DF added at 10(-2) and 10(-3) M to the incubation media of ALA-S and PCL did not alter either of the enzymatic activities, whether in normal or HCB-porphyric preparations. The results obtained show that DF improved the biochemical picture during HCB porphyria. They suggest that iron plays an indirect role in the decrease of PCL enzyme, possibly at the HCB metabolization step. A common iron-involving mechanism for the production of porphyria by different chlorinated compounds is suggested.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on lipid peroxidation in microsomal systems in vitro

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) when added to suspensions of rat hepatic microsomes in the presence of NADPH has little influence on the peroxidation of microsomal lipids unless the system also contains complexed ferric ion, in which case TCDD stimulates. This stimulation does not appear to require metabolism of the TCDD. Peroxidation was monitored by production of thiobarbiturate-reactive substances (malondialdehyde and dienals), production of conjugated dienes, and disappearance of polyunsaturated fatty acids. Stimulation of lipid peroxidation by TCDD in a mixed lysosome-microsome preparation resulted in significantly decreased 'leakage' of acid phosphatase into the medium, implying an effect on lysosomal membranes. Consideration both of the present results and data in the literature leads to the conclusion that it is premature to attempt to define the relationship between enzyme induction, lipid peroxidation and TCDD lethality.

Effects of iron overload on bile secretion and hepatic porphyrin metabolism in ethinyl estradiol-treated rats

We investigated the effects of ethinyl estradiol (5 mg/kg body wt daily for 5 days, orally) and/or iron sorbitol (50 mg/kg body wt daily for 5 days, i.m.) on bile flow, bile salt independent fraction (BSIF), hepatic delta-aminolevulinate synthase (ALA-S) and uroporphyrinogen decarboxylase (URO-D) in female rats. Ethinyl estradiol administration was associated with a significant decrease of bile flow and BSIF and an increase in URO-D activity in comparison to control values. Iron alone did not modify biliary parameters, but significantly increased the activity of ALA-S. Combined treatment with ethinyl estradiol plus iron partially corrected the reduction of BSIF and restored the activity of ALA-S and URO-D to control levels. Thus iron appears to exert a partially protective effect against ethinyl estradiol-induced cholestasis. No porphyrinogenic effect was observed.

In vitro studies of the mechanism of inhibition of rat liver uroporphyrinogen decarboxylase activity by ferrous iron under anaerobic conditions

Human porphyria cutanea tarda (PCT) is an unusual consequence of common hepatic disorders such as alcoholic liver disease and iron overload, where hepatic iron plays a key role in the expression of the metabolic lesion, i.e., defective hepatic decarboxylation of porphyrinogens. In this investigation, kinetic studies on a partially purified rat liver uroporphyrinogen decarboxylase have been conducted under controlled conditions to determine how iron perturbs porphyrinogen decarboxylation in vitro. The enzyme, assayed strictly under anaerobic conditions in the dark, was inhibited progressively by ferrous iron. Approximately 0.45 mM ferrous ammonium sulfate was required to observe about 50% inhibition of enzyme activity measured with uroporphyrinogen I as substrate. We showed that (a) all the steps of enzymatic decarboxylation (octa-, hepta-, hexa-, and pentacarboxylic porphyrinogen of isomer I series) were inhibited by ferrous iron. The inhibition was competitive with respect to uroporphyrinogen I and III substrates; (b) the cations, e.g., Fe3+ and Mg2+, had no effect, whereas sulfhydryl group specific cations and compounds such as Hg2+, Zn2+, p-mercuribenzoate, and 5,5'-dithiobis(2-nitrobenzoate) all inhibited the enzyme; (c) the enzyme could be protected from inhibition by Fe2+ and p-mercuribenzoate by preincubation with pentacarboxylic porphyrinogen, a natural substrate and competitive inhibitor. These data suggest for the first time a direct interaction of ferrous iron with cysteinyl residue(s) located at the active site(s) of the enzyme.

Porphyria cutanea tarda, or the uroporphyrinogen decarboxylase deficiency diseases

The term "porphyria cutanea tarda" originally described the dermatological manifestations of various chronic porphyrias. Its usage now is usually restricted to disorders associated with a deficiency of uroporphyrinogen decarboxylase (UROD), for which the term "UROD-deficiency" may be more appropriate. Four etiologic agents have been implicated in this condition: alcohol, oral estrogens, halogenated aromatic hydrocarbons, and iron. An inherited deficiency of UROD is also recognized, with increased susceptibility to these agents. Certain halogenated aromatic hydrocarbons can cause UROD-deficiency in animals and synergism with iron is demonstrable in this model. Neither ethanol nor estrogen has been shown to cause UROD-deficiency in animals. Treatment by venesection to reduce total body iron is safe and effective. The 4-aminoquinoline antimalarial drugs also provide effective treatment, possibly by lysis of affected liver cells. Unlike venesection, they may not reverse the biochemical lesion which causes porphyrins to accumulate. The mechanism of acquired UROD-deficiency is not clear but animal studies suggest a role for the hepatic mixed function oxygenases which initiate iron-dependent inactivation of UROD. Diagnosis is simple, often requiring only appropriate clinical data and testing of a random urine sample. Although not common, the disorder is the most frequently diagnosed disturbance of porphyrin metabolism in many countries, and further insight into its unusual pathogenesis may clarify the hepatotoxic effects of the 4 etiologic agents.