Pathological mechanisms of hepatic tumour formation in rats exposed chronically to dietary hexachlorobenzene

AhR Activation in Pharmaceutical Development: Applying Liver Gene Expression Biomarker Thresholds to Identify Doses Associated with Tumorigenic Risks in Rats

Aryl hydrocarbon receptor (AhR) activation is associated with carcinogenicity of non-genotoxic AhR-activating carcinogens such as 2, 3, 7, 8-tetrachlorodibenzodioxin (TCDD), and is often observed with drug candidate molecules in development and raises safety concerns. As downstream effectors of AhR signaling, the expression and activity of Cyp1a1 and Cyp1a2 genes are commonly monitored as evidence of AhR activation to inform carcinogenic risk of compounds in question. However, many marketed drugs and phytochemicals are reported to induce these Cyps modestly and are not associated with dioxin-like toxicity or carcinogenicity. We hypothesized that a threshold of AhR activation needs to be surpassed in a sustained manner in order for the dioxin-like toxicity to manifest, and a simple liver gene expression signature based on Cyp1a1 and Cyp1a2 from a short-term rat study could be used to assess AhR activation strength and differentiate tumorigenic dose levels from non-tumorigenic ones. To test this hypothesis, short term studies were conducted in Wistar Han rats with two AhR-activating carcinogens (TCDD and PCB126) at minimally carcinogenic and non-carcinogenic dose levels, and three AhR-activating non-carcinogens (omeprazole, mexiletine, and canagliflozin) at the top doses used in their reported 2-year rat carcinogenicity studies. A threshold of AhR activation was identified in rat liver that separated a meaningful "tumorigenic-strength AhR signal" from a statistically-significant AhR activation signal that was not associated with dioxin-like carcinogenicity. These studies also confirmed the importance of the sustainability of AhR activation for carcinogenic potential. A sustained activation of AhR above the threshold could thus be used in early pharmaceutical development to identify dose levels of drug candidates expected to exhibit dioxin-like carcinogenic potential.

The association between chemical-induced porphyria and hepatic cancer

The haem biosynthetic pathway is of fundamental importance for cellular metabolism both for the erythroid and nonerythroid tissues. There are several genetic variants of the pathway in the human population that cause dysfunction of one or other of the enzymes resulting in porphyrias of varying severity. Serious chronic hepatic and systemic diseases may result. Some of these can be precipitated by exposure to drugs including hormones, barbiturates and antibiotics, as well as alcohol and particular chlorinated aromatic chemicals. In experimental animals some of the steps of this pathway can also be severely disrupted by a variety of environmental chemicals, potential drugs and pesticides, especially in the liver, leading to the accumulation of uroporphyrins derived from the intermediate uroporphyrinogens or protoporphyrin IX, the immediate precursor of haem. With some of these chemicals this also leads to cholestasis and liver cell injury and eventually hepatic tumours. The review evaluates the available evidence linking hepatic porphyria with carcinogenesis in naturally occurring human genetic conditions and in chemically-induced porphyrias in laboratory animals. The existing data showing gender, strain, and species differences in sensitivity to the chemical-induced porphyrias, liver injury and liver tumours are discussed and the role that transgenically altered mouse models have played in defining the varying mechanisms. Finally, the review proposes a novel, unifying hypothesis linking the hepatotoxicity induced by the accumulation of various porphyrins, with the increased risk of developing hepatic cancer as a long term consequence.

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.

Diuron Lacks Promoting Potential in a Rat Liver Bioassay

The promoting activity of the herbicide Diuron was evaluated in a medium-term rat liver carcinogenesis bioassay that uses as endpoint immunohistochemically identified glutathione S-transferase positive (GST-P+) foci. Male Wistar rats were allocated to the following groups: G1 to G6 were initiated for liver carcinogenesis by a single dose of diethylnitrosamine (DEN, 200 mg/kg) while groups G7 and G8 received only 0.9% NaCl (DEN vehicle). From the 2nd week animals were fed a basal diet (G1 and G7) or a diet added with Diuron at 125, 500, 1250, 2500 and 2500 ppm (G2 to G5 and G8, respectively) or 200 ppm Hexaclorobenzene (HCB; G6). The animals were submitted to 70% partial hepatectomy at the 3rd week and sacrificed at the 8th week. The herbicide did not alter ALT or creatinine serum levels. No conspicuous GST-P+ foci development was registered in non-initiated rats fed Diuron at 2500 ppm. While DEN-initiated animals fed Diuron at 1250 or 2500 ppm developed mild centrilobular hypertrophy, DEN-initiated HCB-fed animals showed severe liver centrilobular hypertrophy and significant GST-P+ foci development. These findings indicate that the medium-term assay adopted in this study does not reveal any liver carcinogenesis initiating or promoting potential of Diuron in the rat.

Protective and damaging effects of platelets in acute cholestatic liver injury revealed by depletion and inhibition strategies

Alpha-naphthylisothiocyanate (ANIT) causes cholestatic hepatitis characterized by intrahepatic bile duct epithelial cell injury and periportal hepatocellular necrosis. The progression of ANIT-induced hepatocyte injury is reported to involve extrahepatic cells including platelets. We showed recently that the procoagulant protein tissue factor (TF) is essential for ANIT-induced coagulation and contributes to ANIT-induced liver necrosis. Platelets have been shown to express TF and can contribute to coagulation cascade activation. To this end, we tested the hypothesis that platelet-dependent coagulation contributes to ANIT-induced liver injury. In ANIT (60 mg/kg)-treated mice, activation of the coagulation cascade occurred prior to a decrease of platelets in the blood. Immunostaining for glycoprotein IIb (CD41) revealed platelet accumulation along the borders of necrotic foci in livers of ANIT-treated mice. Antibody-mediated platelet depletion did not affect coagulation but markedly affected liver histopathology in ANIT-treated mice. Platelet depletion induced marked pooling of blood within necrotic lesions consistent with parenchymal-type peliosis as early as 24 h after ANIT treatment. In contrast, treatment with the P2Y(12) inhibitor clopidogrel significantly reduced ANIT-induced hepatocyte necrosis and serum alanine aminotransferase activity but did not exaggerate bleeding into necrotic foci. Clopidogrel also reduced hepatic neutrophil accumulation but did not affect induction of Intercellular adhesion molecule-1 or chemokine CxC motif ligand-1 messenger RNA expression in liver. The data indicate that ANIT-induced coagulation is platelet independent and that platelets contribute to ANIT-induced hepatocyte necrosis by promoting neutrophil accumulation. In contrast, severe thrombocytopenia induces parenchymal-type peliosis in the livers of ANIT-treated mice, a rare hepatic lesion associated with pooling of blood in the liver.

Mode of Action in Relevance of Rodent Liver Tumors to Human Cancer Risk

Hazard identification and risk assessment paradigms depend on the presumption of the similarity of rodents to humans, yet species specific responses, and the extrapolation of high-dose effects to low-dose exposures can affect the estimation of human risk from rodent data. As a consequence, a human relevance framework concept was developed by the International Programme on Chemical Safety (IPCS) and International Life Sciences Institute (ILSI) Risk Science Institute (RSI) with the central tenet being the identification of a mode of action (MOA). To perform a MOA analysis, the key biochemical, cellular, and molecular events need to first be established, and the temporal and dose-dependent concordance of each of the key events in the MOA can then be determined. The key events can be used to bridge species and dose for a given MOA. The next step in the MOA analysis is the assessment of biological plausibility for determining the relevance of the specified MOA in an animal model for human cancer risk based on kinetic and dynamic parameters. Using the framework approach, a MOA in animals could not be defined for metal overload. The MOA for phenobarbital (PB)-like P450 inducers was determined to be unlikely in humans after kinetic and dynamic factors were considered. In contrast, after these factors were considered with reference to estrogen, the conclusion was drawn that estrogen-induced tumors were plausible in humans. Finally, it was concluded that the induction of rodent liver tumors by porphyrogenic compounds followed a cytotoxic MOA, and that liver tumors formed as a result of sustained cytotoxicity and regenerative proliferation are considered relevant for evaluating human cancer risk if appropriate metabolism occurs in the animal models and in humans.

Effects of the porphyrinogenic compounds hexachlorobenzene and 3,5-diethoxycarbonyl-1,4-dihydrocollidine on polyamine metabolism

The naturally occurring polyamines--putrescine, spermidine and spermine--are organic cations present in all living cells and essential for cell growth and differentiation. The aim of the present study was to extend the investigations on the effects of porphyrinogenic compounds on polyamine metabolism. This was achieved by studying putrescine, spermidine and spermine levels in a model of acute porphyria, i.e. 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-induced porphyria, and in a model of non-acute porphyria, i.e. hexachlorobenzene (HCB)-induced porphyria. HCB administration to female Wistar rats for 7, 14, 21, 28 and 56 days did not alter polyamine levels in liver, even though rats presented clear signs of HCB-induced porphyria. In contrast to HCB, DDC treatment resulted in a remarkable increase in putrescine levels in the liver of female and male Sprague-Dawley rats. This increase was due, at least in part, to ornithine decarboxylase (ODC) activation. DDC induction of putrescine levels did not show organ specificity, since it could also be seen in adrenal gland. Interestingly, the deregulation of polyamine biosynthesis occurred concomitantly with the deregulation of the heme biosynthetic pathway. In addition to porphyria, it is known that DDC intoxication affects several proteins of the hepatocyte cytoskeleton. It is suggested that DDC-induced increase in ODC activity and putrescine levels may be an early event contributing to alter the cytoskeleton.

The role of the immune system in hexachlorobenzene-induced toxicity

Hexachlorobenzene (HCB) is a persistent environmental pollutant. The toxicity of HCB has been extensively studied after an accidental human poisoning in Turkey and more recently it has been shown that HCB has immunotoxic properties in laboratory animals and probably also in man. Oral exposure of rats to HCB showed stimulatory effects on spleen and lymph node weights and histology, increased serum IgM levels, and an enhancement of several parameters of immune function. Moreover, more recent studies indicate that HCB-induced effects in the rat may be related to autoimmunity. In Wistar rats exposed to HCB, IgM antibodies against several autoantigens were elevated; in the Lewis rat, HCB differently modulated two experimental models of autoimmune disease. Oral exposure of rats to HCB induces skin and lung pathology in the rat. Recently several studies have been conducted to investigate whether these skin and lung lesions can be related to HCB-induced immunomodulation, and these studies will be discussed in this review. HCB-induced skin and lung lesions probably have a different etiology; pronounced strain differences and correlation of skin lesions with immune parameters suggest a specific involvement of the immune system in HCB-induced skin lesions. The induction of lung lesions by HCB was thymus independent. Thymus-dependent T cells were not likely to be required for the induction of skin lesions, although T cells enhanced the rate of induction and the progression of the skin lesions. No deposition of autoantibodies was observed in nonlesional or lesional skin of HCB-treated rats. Therefore, we concluded that it is unlikely that the mechanism by which most allergic or autoimmunogenic chemicals work, i.e., by binding to macromolecules of the body and subsequent T- and B-cell activation, is involved in the HCB-induced immunopathology in the rat. Such a thymus-independent immunopathology is remarkable, as HCB strongly modulates T-cell-mediated immune parameters. This points at a very complex mechanism and possible involvement of multiple factors in the immunopathology of HCB.

Hexachlorobenzene, a dioxin-type compound, increases malic enzyme gene transcription through a mechanism involving the thyroid hormone response element

Hexachlorobenzene (HCB) is a dioxin-type chemical that acts mainly through the aryl hydrocarbon receptor. Chronic exposure of rats to HCB increases the activity of malic enzyme (ME). In this report, we show that this increase is correlated with an induction of ME messenger RNA (mRNA) levels, with the maximal HCB effect achieved after 9 days of intoxication. This effect is specific for ME, as other liver enzymes, such as glyceraldehyde-3-phosphate dehydrogenase, phosphoenol pyruvate carboxykinase, and mitochondrial alpha-glycerol-3-phosphate dehydrogenase, are not affected by HCB. The induction of ME mRNA levels is accompanied by an increase in ME promoter activity, as demonstrated by transient transfection experiments performed in rat hepatoma H35 cells. In an attempt to identify the cis-regulatory elements responsible for the HCB effect, different promoter deletions and mutations were used. The results obtained localize the responsive region between positions -315 and -177. This region does not contain either consensus xenobiotic response or activating protein-1 elements, the two main mediators of dioxin compounds described to date. In contrast, a thyroid hormone response element (TRE) is located between -281 to -261. Deletions and mutations of the TRE element do not respond to HCB, demonstrating that this element mediates the response of this dioxin-type compound. As ME gene expression is regulated mainly by thyroid hormones, we next investigated the role of T3 receptor (T3R) in the ME gene transcriptional induction mediated by HCB. Using Scatchard analysis, we show that neither T3R binding features for its ligand nor alpha1 or beta1T3R mRNA levels are changed with the toxic. In gel shift assays, however, we observed that protein/DNA complexes formed on TRE from the ME promoter were induced by HCB. Using an oligonucleotide with a mutation that eliminates the TRE function, we demonstrate a loss of the induced protein/DNA complexes. Together, these data suggest that the dioxin-type compound HCB increases ME gene transcription by modulating the levels of still unidentified nuclear proteins that bind to the TRE element of the ME promoter.

Metabolism of organochlorine pesticides: the role of human cytochrome P450 3A4

Organochlorine compounds are widely used as pesticides and are substantial environmental pollutants and carcinogens due to their extensive environmental release. In the present study biotransformation of these pesticides was observed in the microsomal fractions and whole cells of Saccharomyces cerevisiae expressing human cytochrome P450 3A4. In both in vitro and in vivo studies, hexachlorobenzene and pentachlorobenzene were metabolised into pentachlorophenol which was further transformed into tetrachlorohydroquinone. Metabolites were identified by thin layer chromatography and 13C-NMR spectroscopy. The formation of products was observed only in the presence NADPH in microsomal fractions and no activity was observed in control microsomal fractions, or in whole cells.