Time course of hexachlorobenzene-induced alterations of lipid metabolism and their relation to porphyria

Hepatoprotective effects of vitamin E against hexachlorobenzene-induced hepatotoxicity and oxidative stress in rats: histological, biochimical and antioxidant status changes

The protective effects of α-Tocopherol (vitamin E) on liver injury induced by hexachlorobenzene (HCB) were investigated in adult male rats of Wistar strain. Animals were randomly divided into six groups of eight rats each. Group 1 and 2 have received HCB, dissolved in olive oil, at a dose of 4 mg or 16 mg/kg b.w., respectively. Group 3 and 4 were treated by the same doses of HCB (4 mg and 16 mg/kg b.w.) after 1 h of pretreatment with α-tocopherol at a dose of 100 mg kg^-1 b.w. The other two groups served as controls; which received either olive oil only, a solvent of HCB, or α-tocopherol. A significant increase in hepatic lipid peroxidation (LPO) and GSH activity were observed following HCB administration. The activities of antioxidant enzymes like superoxide dismutase and catalase were significantly decreased while glutathione peroxidase was significantly increased following HCB administration. Similarly, a significant increase in plasma levels of various marker enzymes [aminotransferase (aspartate aminotransférase (AST) and alanine aminotransferase (ALT)), alkaline phosphatase (ALP) and lactate dehydrogenase (LDH)] and a decrease of total protein level were observed. Pretreatment with vitamin E of HCB treated rats ameliorated all biochemical parameters to near normal values. Liver histological study confirmed biochemical parameters and the beneficial role of vitamin E.

Vitamin D3 supplementation attenuates the early stage of mouse hepatocarcinogenesis promoted by hexachlorobenzene fungicide

Hexachlorobezene (HCB), a fungicide widely distributed in the environment, promotes the development of hepatocellular preneoplastic lesions (PNL) and tumors in rodents. In contrast, vitamin D₃ (VD₃) supplementation presents a potential role for the prevention/treatment of chronic liver diseases. Thus, we investigated whether VD₃ supplementation attenuates the early stage of HCB-promoted hepatocarcinogenesis. Female Balb/C mice were injected a single dose of diethylnitrosamine (DEN, 50 mg/kg) at postnatal day 15. From day 40 onwards, mice were fed with a standard diet containing 0.02% HCB alone or supplemented with VD₃ (10,000 or 20,000 IU/Kg diet) for 20 weeks. Untreated mice were fed just standard diet. After this period, mice were euthanized and liver and serum samples were collected. Compared to the untreated group, DEN/HCB treatment decreased total hepatic glutathione levels and glutathione peroxidase (GSH-Px) activity while increased lipid peroxidation, p65 protein expression, cell proliferation/apoptosis and the PNL development. In contrast, dietary VD₃ supplementation enhanced vitamin D receptor (VDR) protein expression, total glutathione levels and GSH-Px activity while diminished lipid hydroperoxide levels. Also, VD₃ supplementation decreased p65 protein expression, hepatocyte proliferation, the size and the liver area occupied by PNL. Therefore, our findings indicate that VD₃ supplementation attenuates the early stage of HCB-promoted hepatocarcinogenesis.

Endocrine-disrupting chemicals: associated disorders and mechanisms of action

The incidence and/or prevalence of health problems associated with endocrine-disruption have increased. Many chemicals have endocrine-disrupting properties, including bisphenol A, some organochlorines, polybrominated flame retardants, perfluorinated substances, alkylphenols, phthalates, pesticides, polycyclic aromatic hydrocarbons, alkylphenols, solvents, and some household products including some cleaning products, air fresheners, hair dyes, cosmetics, and sunscreens. Even some metals were shown to have endocrine-disrupting properties. Many observations suggesting that endocrine disruptors do contribute to cancer, diabetes, obesity, the metabolic syndrome, and infertility are listed in this paper. An overview is presented of mechanisms contributing to endocrine disruption. Endocrine disruptors can act through classical nuclear receptors, but also through estrogen-related receptors, membrane-bound estrogen-receptors, and interaction with targets in the cytosol resulting in activation of the Src/Ras/Erk pathway or modulation of nitric oxide. In addition, changes in metabolism of endogenous hormones, cross-talk between genomic and nongenomic pathways, cross talk with estrogen receptors after binding on other receptors, interference with feedback regulation and neuroendocrine cells, changes in DNA methylation or histone modifications, and genomic instability by interference with the spindle figure can play a role. Also it was found that effects of receptor activation can differ in function of the ligand.

Melatonin formation in pineal gland from rats with hexachlorobenzene experimental porphyria

Hexachlorobenzene produces an experimental hepatic porphyria in rats, which is similar to human porphyria cutanea tarda, with hyperpigmentation as one of its characteristic features. Alterations in tryptophan metabolism have been previously observed in this chronic porphyria. Melatonin formation from tryptophan via serotonin shows diurnal rhythmicity in the pineal gland, and higher values are observed during the dark phase of an imposed light-dark cycle. The purpose of this study was to determine the contents of tryptophan and its metabolites in pineal gland of normal and hexachlorobenzene-treated rats in order to find alterations potentially related to porphyria cutanea tarda. Results show that in animals with this experimental porphyria some tryptophan metabolite levels (serotonin and 5-hydroxyindoleacetic acid) increase only during the light period, whereas tryptophan content remained equal to the controls. Hydroxyindole-O-methyltransferase activity also increases by light in pineal gland from hexachlorobenzene-treated rats. On the other hand, tryptophan is converted to melatonin in the dark period, but this route is not exacerbated in hexachlorobenzene porphyria. The relevance of these alterations is discussed in relation to hyperpigmentation, neoplastic and oxidative stress processes associated with this porphyria.

Hexachlorobenzene as hormonal disruptor--studies about glucocorticoids: their hepatic receptors, adrenal synthesis and plasma levels in relation to impaired gluconeogenesis

In Wistar rats, hexachlorobenzene (HCB) depresses the gluconeogenic enzyme phosphoenolpyruvate-carboxykinase (PEPCK). In the liver, glucocorticoids (GC) normally regulate the glucose synthesis by acting on PEPCK. Thus, the aim of this work was to investigate, in a time-course study, the effects of HCB on plasma GC, its adrenal synthesis and stimulation, and the kinetic parameters of its hepatic receptors (GR) in relation to the gluconeogenic blockage produced by HCB. Plasma corticosterone (CORT) concentration, urinary porphyrins and hepatic PEPCK were determined after 2, 4, 6 and 8 weeks of HCB-treatment. The effect of HCB on kinetic parameters of GR was studied in adrenalectomized porphyric rats after 2, 4 and 8 weeks of treatment. Additionally, adrenal CORT synthesis in the same weeks was measured with or without ACTH. Results show that plasma CORT in intoxicated animals dropped significantly after 2 and 4 weeks of treatment (23% and 58%, respectively), and then remained constant until the 8th week. HCB also promoted a reduction in the number of hepatic GR (50-55%) without modifying affinity. After 8 weeks, when porphyria was well established (40-50-fold increase in urinary porphyrins), a reduction (52%) in hepatic GR number, as well as a decrease in PEPCK activity (56%) were observed. Moreover, CORT biosynthesis in adrenals from intoxicated animals significantly decreased (60%) without changes in ACTH effect. Briefly, this paper shows that HCB causes a disruption in GC and GR. This disturbance could contribute to the negative effect on glucose synthesis through PEPCK regulation, thus modulating porphyria. These results enhance the knowledge about the hormonal disruption produced by chlorinated xenobiotics.

Effects of hexachlorobenzene on antioxidant status of liver and brain of common carp (Cyprinus carpio)

Hexachlorobenzene (HCB)-induced oxidative damages have been published in rats while the effects have not yet been reported in fishes. Juvenile common carps (Cyprinus carpio) were exposed to waterborne HCB from 2 to 200 microg l-1 for 5, 10 or 20 days. Liver and brain were analyzed for various parameters of oxidative stress. There were no significant changes of glutathione (GSH) content and superoxide dismutase (SOD) activity in liver after 5 or 10 days exposure, whereas obvious drops were observed at higher concentrations after 20 days exposure. Significant decreases of GSH content and SOD activity in brain were found during all the exposure days. In brain, HCB also significantly elevated the contents of reactive oxygen species (ROS), thiobarbituric acid- reactive substances (TBARS, as an indicator of lipid peroxidation products), glutathione disulfide (GSSG), and activities of nitric oxide synthase (NOS), glutathione peroxidase (GPx), and glutathione reductase (GR), and inhibited activities of acetylcholinesterase (AchE) and glutathione S-transferase (GST). The results clearly demonstrated that environmentally possible level of HCB could result in oxidative stress in fish and brain was a sensitive target organ of HCB toxicity.

Toxicity of hexachlorobenzene and its transference from microalgae (Chlorella kessleri) to crabs (Chasmagnathus granulatus)

The aim of this work was to study the transference of hexachlorobenzene from a green alga (Chlorella kessleri) to an estuary crab (Chasmagnathus granulatus), and to analyze the toxic effects that the xenobiotic has on the latter. The effect of hexachlorobenzene uptake was evaluated measuring oxidative stress, Uroporphyrinogen decarboxylase activity and morphometric parameter alteration, and also performing a histological analysis of crab hepatopancreas. Results demonstrated that hexachlorobenzene enters the alga, is accumulated in it, and then transferred into the crab, causing a decrease in Uroporphyrinogen decarboxylase activity in both organisms. The high malondialdehyde levels detected in crab hepatopancreas after the toxic treatment suggested the existence of hexachlorobenzene-induced lipid peroxidation. Antioxidant defenses such as superoxide dismutase activity and reduced glutathione content fell below normal values on the fourth week of treatment. At the same time, the hepatosomatic index, used as a morphometric parameter, reduced 20% with respect to the control. The histological analysis revealed epithelium disorganization in hepatopancreas tubules, confirming the existence of structural damage caused by hexachlorobenzene.

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.

Hepatic arachidonic acid metabolism is disrupted after hexachlorobenzene treatment

Hexaclorobenzene (HCB), one of the most persistent environmental pollutants, can cause a wide range of toxic effects including cancer in animals, and hepatotoxicity and porphyria both in humans and animals. In the present study, liver microsomal cytochrome P450 (CYP)-dependent arachidonic acid (AA) metabolism, hepatic PGE production, and cytosolic phospholipase A2 (cPLA2) activity were investigated in an experimental model of porphyria cutanea tarda induced by HCB. Female Wistar rats were treated with a single daily dose of HCB (100 mg kg(-1) body weight) for 5 days and were sacrificed 3, 10, 17, and 52 days after the last dose. HCB treatment induced the accumulation of hepatic porphyrins from day 17 and increased the activities of liver ethoxyresorufin O-deethylase (EROD), methoxyresorufin O-demethylase (MROD), and aminopyrine N-demethylase (APND) from day 3 after the last dose. Liver microsomes from control and HCB-treated rats generated, in the presence of NADPH, hydroxyeicosatetraenoic acids (HETEs), epoxyeicosatrienoic acids (EETs), 11,12-Di HETE, and omega-OH/omega-1-OH AA. HCB treatment caused an increase in total NADPH CYP-dependent AA metabolism, with a higher response at 3 days after the last HCB dose than at the other time points studied. In addition, HCB treatment markedly enhanced PGE production and release in liver slices. This HCB effect was time dependent and reached its highest level after 10 days. At this time cPLA2 activity was shown to be increased. Unexpectedly, HCB produced a significant decrease in cPLA2 activity on the 17th and 52nd day. Our results demonstrated for the first time that HCB induces both the cyclooxygenase and CYP-dependent AA metabolism. The effects of HCB on AA metabolism were previous to the onset of a marked porphyria and might contribute to different aspects of HCB-induced liver toxicity such as alterations of membrane fluidity and membrane-bound protein function. Observations also suggested that a possible role of cPLA2 in the early increase of AA metabolism cannot be excluded. However, the existence of other pathway(s) for metabolizable AA generation different from cPLA2 activation is also proposed.

Abnormal kinetic behavior of uroporphyrinogen decarboxylase obtained from rats with hexachlorobenzene-induced porphyria

Uroporphyrinogen decarboxylase is an essential enzyme in all organisms and functions in the heme biosynthetic pathway, catalyzing the decarboxylation of the four acetate groups of uroporphyrinogen to form coproporphyrinogen. This work examines whether the four sequential decarboxylations occur at the same active site, and explores whether hexachlorobenzene-induced porphyria affects the behavior of the enzyme. For this purpose, kinetic competition studies were done with mixtures of uroporphyrinogen III and pentacarboxyporphyrinogen III. With the enzyme from normal rats, a constant velocity was obtained with all the mixtures, indicating that uroporphyrinogen and pentacarboxy-porphyrinogen react at the same active site, i.e. the first and fourth decarboxylations occur at the same site. In contrast, in experiments with enzyme from rats with hexachlorobenzene-induced porphyria, the total rate for mixtures was always lower than the reference rate; and a curve with a deep minimum was obtained, indicating that the two reactions occur at functionally different sites, but with cross-inhibition. This suggests that the modifications induced in the enzyme by hexachlorobenzene cause the two active sites to become nonequivalent and functionally different. The question is discussed how the hexachlorobenzene treatment may produce this abnormal kinetic behavior, and alternative hypotheses are considered.

Toxicogenomics of subchronic hexachlorobenzene exposure in Brown Norway rats

Hexachlorobenzene (HCB) is a persistent environmental pollutant with toxic effects in man and rat. Reported adverse effects are hepatic porphyria, neurotoxicity, and adverse effects on the reproductive and immune system. To obtain more insight into HCB-induced mechanisms of toxicity, we studied gene expression levels using DNA microarrays. For 4 weeks, Brown Norway rats were fed a diet supplemented with 0, 150, or 450 mg HCB/kg. Spleen, mesenteric lymph nodes (MLN), thymus, blood, liver, and kidney were collected and analyzed using the Affymetrix rat RGU-34A GeneChip microarray. Most significant (p < 0.001) changes, compared to the control group, occurred in spleen, followed by liver, kidney, blood, and MLN, but only a few genes were affected in thymus. This was to be expected, as the thymus is not a target organ of HCB. Transcriptome profiles confirmed known effects of HCB such as stimulatory effects on the immune system and induction of enzymes involved in drug metabolism, porphyria, and the reproductive system. In line with previous histopathological findings were increased transcript levels of markers for granulocytes and macrophages. New findings include the upregulation of genes encoding proinflammatory cytokines, antioxidants, acute phase proteins, mast cell markers, complements, chemokines, and cell adhesion molecules. Generally, gene expression data provide evidence that HCB induces a systemic inflammatory response, accompanied by oxidative stress and an acute phase response. In conclusion, this study confirms previously observed (immuno)toxicological effects of HCB but also reveals several new and mechanistically relevant gene products. Thus, transcriptome profiles can be used as markers for several of the processes that occur after HCB exposure.

Glycogen metabolism and glucose transport in experimental porphyria

Hexachlorobenzene (HCB) is a fungicide of well-known porphyrinogenic ability, which induces an experimental porphyria that resembles human porphyria cutanea tarda (PCT) in several animal species. It has been demonstrated that high glucose ingestion prevents porphyria development, and high-fat/high-protein diets enhance HCB porphyrinogenic ability. On the contrary, a diet rich in carbohydrates reduces HCB effects. The aim of this work was to study HCB effects on glycogen synthesis and degradation, as well as on glucose synthesis and transport, in order to elucidate whether would justify the beneficial use of carbohydrates in this porphyria. Rats were treated with HCB dissolved in corn oil (five daily doses 100mg/kg body weight). Results showed that: (1) HCB caused an increase in glycogen content; (2) glycogen synthase activity increased three times, and phosphorylase activity decreased about 40% due to fungicide intoxication. The effect of HCB on these two activities accounted for the higher glycogen content observed in treated animals; (3) three gluconeogenic enzymes were reduced 30-50%; (4) glucose uptake in the liver decreased in all weeks studied. The alterations found in glucose synthesis, its uptake in liver and other tissues, and its release from glycogen might contribute to the biochemical porphyria picture and would account for the effect of glucose above mentioned.

Tryptophan metabolism via serotonin in rats with hexachlorobenzene experimental porphyria

One of the three pathways for the metabolisation of dietary tryptophan is the formation of serotonin. Tryptophan hydroxylase catalyses the formation of 5-hydroxytryptophan, the first and regulatory step of this biosynthesis. The aim of the present work is to study alterations in this tryptophan metabolism in rats with experimental Porphyria Cutanea Tarda induced by hexachlorobenzene. With this purpose, the content of tryptophan and its metabolites related to the serotonin pathway are determined by HPLC techniques, in tissues (brain, liver and gut) and in fluids (blood, plasma and urine) of controls and hexachlorobenzene-porphyric rats. In these experimental-porphyric animals, we determine a significant increase in the excretion of 5-hydroxyindole acetic acid in urine and a decrease in the content of serotonin in small gut, respect to controls. Significant increases in contents of serotonin in 24-hr urine and tryptophan in liver are also found. No other significant variations for the different metabolites are detected in any of the tissues and fluids studied. Brain and liver activities of the rate-limiting enzyme tryptophan hydroxylase can only be measured in porphyric rats. Our results agree with an increased turnover of gastrointestinal serotonin derived from dietary tryptophan and its excretion as urinary 5-hydroxyindole acetic acid, which is formed in liver. An increased serotonin pathway in porphyric livers is confirmed by the measured increase in the activity of hepatic tryptophan hydroxylase. The absence of neurological symptoms in patients with Porphyria Cutanea Tarda could be related to the absence of a statistically significant variation in serotonin content shown in brain.

Effect of in vivo administered hexachlorobenzene on epidermal growth factor receptor levels, protein tyrosine kinase activity, and phosphotyrosine content in rat liver

In the present study, the effects of hexachlorobenzene (HCB) on epidermal growth factor receptor (EGFR) content of liver microsomes and plasma membrane, and on EGFR-tyrosine kinase activity in the microsomal fraction were investigated. In addition, we studied the parameters of the tyrosine kinase signalling pathway such as protein tyrosine kinase (PTK) activity and phosphotyrosine content in microsomal and cytosolic protein. To determine whether the observed alterations were correlated with a manifestation of overt toxicity, a single very low dose of HCB (1mg/kg body wt) and two much higher doses (100 and 1000 mg/kg body wt), the highest being toxicologically significant in that it reduced serum thyroxine (T(4)) and inhibited uroporphyrinogen decarboxylase (URO-D) (EC 4.1.1.37) activity, were tested. Our results demonstrated that liver microsomes of rats treated with HCB had higher levels of EGFR than untreated rats; treated rats also had less EGFR present in hepatocyte plasma membrane fractions than did untreated rats. HCB altered the phosphotyrosine content and protein phosphorylation of some microsomal and cytosolic proteins in a biphasic dose-response relationship. At the low dose, phosphorylation and phosphotyrosine content of several microsomal proteins were increased; however, these effects were diminished or reversed at the higher doses. Our results suggest that chronic HCB treatment produces a down-regulation of the EGFR and a dose-dependent increase in EGFR-tyrosine kinase activity in the microsomal fraction. This effect may contribute to the alteration of membrane and cytosolic protein tyrosine phosphorylation. The level of sensitivity encountered in our studies is extraordinary, occurring at 1/10 to 1/1000 the doses of HCB known to cause other toxicological lesions.

Ontogeny of 5-aminolevulinic dehydratase and porphobilinogen deaminase activities in the yolk sac membrane and liver of chick embryos

1. Chick embryos of 7, 9, 11, 12, 13, 14, 15, 17 and 19 d of embryonic development were examined to determine the activities of 5-aminolevulinic dehydratase (ALA-D, EC 4.2.1.24) and porphobilinogen deaminase (PBG-D, EC 4.3.1.8). 2. Liver and yolk sac membrane ALA-D specific activities showed a maximum between 12 and 13 d of embryonic development, yolk sac membrane PBG-ase activity a maximum at 9 d and at 7 d in liver. Total activities of ALA-D and PBG-D were not constant during the course of embryonic development but probably related to the changes of intensity of haem synthesis. 3. ALA-D and PBG-ase activities were higher in yolk sac membrane than in liver, showing the importance of the yolk sac membrane as erythropoietic tissue. PBG-D catalysed the rate-limiting reaction of the cytosolic steps in the biosynthetic pathway in both tissues.

Accelerated development of uroporphyria in mice heterozygous for a deletion at the uroporphyrinogen decarboxylase locus

Three weeks after a single dose of iron-dextran and Aroclor 1254, mice maintained continuously on delta-aminolevulinic acid supplemented drinking water showed significantly elevated levels of hepatic uroporphyrin and depressed (25% of normal) uroporphyrinogen decarboxylase (URO-D) activity. Depressed URO-D activity was paralleled by the ability of heat denatured cytosol to inhibit rhURO-D activity. Mice heterozygous for a targeted disruption at the URO-D locus (URO-D+/-) exhibited half the URO-D activity of homozygous controls prior to treatment. After treatment, these animals showed URO-D activity and rhURO-D inhibitory activity comparable to similarly treated wild type (URO-D +/+) mice but with significantly greater uroporphyrin accumulation. With only 10 days of treatment, URO-D +/- but not URO-D +/+ mice showed changes similar in magnitude to those seen after 21 days. Prior to treatment, URO-D genotype did not influence overall hepatic P450 concentration in either sex and there was no significant difference between sexes. The treatment regimen significantly elevated P450 in animals of either URO-D genotype and in both sexes, although the induction response at the 10-day point was attenuated in URO-D +/- mice. From differences in the CO absorbance maximum, and by P450 activity analysis, this attenuated induction response resulted from an attenuation of the CYP2B not the CYP1A induction.

Phospholipid alterations elicited by hexachlorobenzene in rat brain are strain-dependent and porphyria-independent

Hexachlorobenzene (HCB) alters phospholipid and heme metabolisms in the liver and Harderian gland. The effects of HCB on phospholipid metabolism, in an organ considered to be non-responsive to its porphyrinogenic effects, remain to be studied. Therefore, as the brain is an organ with this feature, this paper analyzes the effects of HCB on brain phospholipid composition in order to investigate if there is any relationship between HCB-induced porphyrin metabolism disruption and phospholipid alterations. For this purpose, a time-course study of HCB effects on brain phospholipids was performed in two strains of rats differing in their susceptibility to acquire hepatic porphyria: Chbb THOM (low); and Wistar (high). This paper shows for the first time that rat brain phospholipids are affected by HCB exposure. Comparative studies show that HCB-induced disturbances in brain phospholipid patterns are time and strain-dependent. Thus, whereas major phospholipids, phosphatidylcholine and phosphatidylethanolamine were more altered in Wistar rats, minor phospholipids, phosphatidylinositol and phosphatidylserine were more affected in Chbb THOM rats. HCB intoxication led to a sphingomyelin/phosphatidylcholine molar ratio lower than the normal, in both strains. As was expected, brain porphyrin content was not altered by HCB intoxication in either strain. It can be concluded that HCB is able to alter brain phospholipid metabolism in a strain-dependent fashion, and in the absence of alterations in brain heme metabolism. In addition, HCB-induced disturbances in brain phospholipids were not related to the degree of hepatic porphyria achieved by the rats.

Heme metabolism after discontinued hexachlorobenzene administration in rats: possible irreversible changes and biomarker for hexachlorobenzene persistence

The aim of the present study was to determine whether short-term administration of hexachlorobenzene (HCB) (1 g/kg body wt., suspended in water, 5 days/week), could cause and maintain marked porphyria in the absence of the exogenous drug, and whether porphyria parameters can be useful as biomarkers of HCB persistence in rats. Hepatic uroporphyrinogen decarboxylase activity, its inhibitor formation, porphyrin content and composition were studied in Wistar rats treated with the fungicide for 1, 2, 3, or 4 weeks and then withdrawn for a 20-week period. The time course of urinary porphyrin excretion was studied for 7 weeks either by continuous treatment for the entire period, or a 1-week HCB administration. The degree of porphyria achieved by rats after 20 weeks of suspended HCB administration was severe, independent of the length of the treatment, and even higher than that observed in animals analysed immediately at the end of each treatment. Rats treated with HCB for 1 week showed a modest decrease in uroporphyrinogen decarboxylase and low inhibitor formation, and exhibited a greater enzyme inhibition, inhibitor formation, hepatic porphyrin accumulation, and an altered pattern of porphyrin composition in the absence of the exogenous drug. Independent of the treatment, urinary porphyrins rose after a delay of 5 weeks. Substantial amounts of HCB were still found in fat of rats treated with HCB for 1 week, after a withdrawal period of 20 weeks. These results suggest that the high persistence of HCB in tissues acts as a continuous source of the xenobiotic, and stimulus for heme biosynthesis derangement. The alterations induced by HCB within 1 week of treatment could be regarded as an initial trigger for irreversible damage on heme metabolism. Thus, abnormalities in heme biosynthesis can be considered effective markers of HCB persistence in rats or of irreversible HCB-induced damage. Taking into account the delayed and enhanced metabolic effects of HCB, it is advisable that porphyria parameters should be evaluated not only immediately after exposure, but also some time afterwards, especially in susceptible and occupationally-exposed populations.

Hexachlorobenzene-induced alterations on neutral and acidic sphingomyelinases and serine palmitoyltransferase activities. A time course study in two strains of rats

Hexachlorobenzene (HCB) induces porphyria both in humans and rodents, and hepatocarcinoma in rodents. In a previous work we observed that HCB produces a continuous decrease in hepatic sphingomyelin (SM) content in Wistar rats. A distinguishing characteristic of sphingolipids breakdown products is their participation in anti-proliferative and apoptotic processes and in the suppression of oncogenesis. As a first step to elucidate the role of SM decrease in the hepatotoxicity induced by HCB, the present study evaluates the metabolic causes of the continuous decrease in hepatic SM content observed in Wistar rats with HCB intoxication, and its relation with porphyria development. For this purpose, the time-course (3, 7, 15, 21 and 28 days) of the effects of HCB on hepatic SM levels and on some of the enzymes of SM synthesis (serine palmitoyltransferase, SPT) and catabolism (sphingomyelinases, SMases) was followed, using two strains of rats differing in their susceptibility to acquire porphyria: Chbb THOM (low) and Wistar (high). HCB (1 g kg(-1) b.w. per day) was administered by gastric intubation as an aqueous suspension. After 5 days of HCB treatment, animals were allowed a 2-day recovery period without HCB administration. Two phases in the HCB-induced damages to sphingolipid metabolism were observed. The first stage (7 days of treatment), common to both strains of rats, was characterized by a decrease in hepatic SM levels (17-25%) and in SPT activity (50-43%), while strain differences were found for the later stage. In Chbb THOM rats, hepatic SM content was restored to normal values concomitantly with an increase in SPT activity (44%, at day 28), and without any increase in SM catabolism. In addition, the level of the other phospholipids was not altered. In Wistar rats, hepatic SM levels decreased continuously throughout the experiment, accompanied by increases in SPT, acidic sphingomyelinase (A-SMase) and neutral sphingomyelinase (N-SMase) activities (86, 28.5 and 78% increase, respectively). A role for glutathione (GSH) in the interstrain differences or a direct effect of HCB on SM metabolism was not found. The present study: (a) demonstrates that N-SMase, A-SMase, and SPT are some of the enzymes that play a role in the HCB-induced decrease of hepatic SM content; (b) finds that HCB-induced alterations of SM metabolism do not correlate with HCB-induced accumulation of hepatic porphyrins; and (c) proposes a link between HCB-induced alterations in phospholipid pattern and in SM metabolism. The increased SM hydrolysis produced as a consequence of SMases induction could be regarded as a cellular response to liver injury elicited by HCB, perhaps acting through the activation of SM signal transduction pathway delaying the proliferative processes observed after long-term treatment with HCB in some rodent species. However, such protective mechanism appears to be strain-dependent.

Effects of hexachlorobenzene on phospholipid and porphyrin metabolism in Harderian glands: a time-course study in two strains of rats

Hexachlorobenzene, one of the most persistent environmental pollutants, induces uroporphyria and phospholipid alterations in rat liver. Harderian glands produce a secretion that is rich in lipids and accumulate large amounts of protoporphyrin. The aim of the present study was to determine if hexachlorobenzene administration to rats affects phospholipid and porphyrin metabolisms in Harderian glands and if these effects are strain dependent. For this purpose, a time-course study (2, 3 and 4 weeks of hexachlorobenzene treatment) of phospholipid pattern and porphyrin content was performed comparatively in two strains of rats (Wistar and Chbb THOM) which differ in their susceptibility to develop HCB-induced porphyria. Hexachlorobenzene produced decreases in several phospholipid contents, but no changes in phosphatidylcholine levels. While the sphingomyelin/phosphatidylcholine molar ratio remained essentially constant until the third week in Chbb THOM rats, it showed a constant drop in Wistar rats, suggesting a more pronounced alteration of membrane fluidity in the later strain. In regard to porphyrin metabolism, Wistar rats showed an increase in the porphyrin content of the gland, while Chbb THOM animals showed a decrease. The study revealed that not only are the normal parameters of phospholipid and porphyrin metabolism in rat Harderian glands strain dependent, but the response to hexachlorobenzene is also.