Species and congener specific induction of hepatic cytochrome P4504A by polychlorinated biphenyls

Comparative toxicogenomic examination of the hepatic effects of PCB126 and TCDD in immature, ovariectomized C57BL/6 mice

Polychlorinated biphenyls are persistent environmental pollutants that elicit a wide range of effects in humans and wildlife, mediated by the aryl hydrocarbon receptor. 3,3',4,4',5-pentachlorobiphenyl (PCB126) is the most potent congener with relative effect potencies ranging from 0.0026 to 0.857, and a toxic equivalency factor (TEF) of 0.1 set by an expert panel of the World Health Organization. In this study, the hepatic effects elicited by 300 microg/kg PCB126 were compared with 30 microg/kg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in immature, ovariectomized female C57BL/6 mice. Comprehensive hepatic gene expression analyses with complementary histopathology, high-resolution gas chromatograph/high-resolution mass spectrometer tissue analysis, and clinical chemistry were examined. For temporal analysis, mice were orally gavaged with PCB126 or sesame oil vehicle and sacrificed after 2, 4, 8, 12, 18, 24, 72, 120, or 168 h. In the dose-response study, mice were gavaged with 0.3, 1, 3, 10, 30, 100, 300, 1000 microg/kg PCB126, 30 or 100 microg/kg TCDD and sacrificed after 72 h. 251 and 367 genes were differentially expressed by PCB126 at one or more time points or doses, respectively, significantly less than elicited by TCDD. In addition, there was less vacuolization and necrosis, and no immune cell infiltration, despite comparable or higher TEF-adjusted hepatic PCB126 levels. The functional annotation of differentially expressed genes was consistent with the observed histopathology. Collectively, the data indicate that 300 microg/kg PCB126 elicited a subset of weaker effects compared with 30 microg/kg TCDD in immature, ovariectomized C57BL/6 mice.

Suppression of peroxisomal enzyme activities and cytochrome P450 4A isozyme expression by congeneric polybrominated and polychlorinated biphenyls

The purpose of this study was to determine the effects of PCBs and PBBs on peroxisome proliferator-activated receptor-alpha-(PPARalpha-) associated enzyme activities or protein levels. Male Sprague-Dawley rats were administered a single IP injection (150 mu mol/kg) of either 3,3',4,4'-tetrabromobiphenyl, 3,3',4,4'-tetrachlorobiphenyl, 3,3',5,5'-tetrabromobiphenyl, 2',3,3',4,5-pentachlorobiphenyl, 3,3',4,4',5-pentachlorobiphenyl, 2,2',3,3',5,5'-hexachlorobiphenyl, or 3,3',4,4',5,5'-hexabromobiphenyl in corn oil (10 ml/kg). One week later, the activities of catalase, peroxisomal fatty acyl-CoA oxidase, and peroxisomal beta-oxidation as well as cytochrome P450 4A (CYP4A) protein content were determined in subcellular liver fractions. None of the peroxisomal enzyme activities were significantly increased by any of the halogenated biphenyl congeners tested. Except for minor (approx. 25%) increases in the total CYP4A content following treatment with 2,2',3,3',5,5'-hexachlorobiphenyl and 3,3',5,5'-tetrabromobiphenyl, CYP4A protein contents were not increased by any treatment. The two Ah receptor agonists, 3,3',4,4'-tetrabromobiphenyl and 3,3',4,4',5-pentachlorobiphenyl, significantly diminished the liver content of CYP4A proteins and activities of the peroxisomal enzymes studied. Since a range of congeners with different biologic and toxicologic activities were selected for this study, it may be concluded that the polyhalogenated biphenyls do not induce peroxisome proliferation in the male rat, but rather certain members of this class of compounds down regulate peroxisome-associated enzymes. Since PCBs and PBBs do not increase enzyme activities and expression of proteins associated with PPARalpha, these agents are therefore exerting their carcinogenic and promoting activities by some other mechanism.

Carcinogenicity of "non-dioxinlike" polychlorinated biphenyls

Complex technical mixtures of polychlorinated biphenyls (PCBs) cause liver and thyroid neoplasms in rodents, whereas very few data are available on the carcinogenic potency of single non-dioxinlike (NDL) PCB congeners. In most genotoxicity assays technical PCB mixtures and individual congeners were inactive, suggesting that PCBs act as indirect, nongenotoxic carcinogens. Various mechanisms, including suppression of apoptosis in preneoplastic cells or inhibition of intercellular communication, have been suggested to be active in liver tumor promotion by PCBs. A decrease in thyroid hormone levels after PCB treatment has been suggested to play a role in the development of thyroid neoplasms in rats; however, other mechanisms may also be involved. Results from a chronic carcinogenicity study in rats indicate that not the dose of total PCBs but the total TCDD or toxic equivalents (TEQs) associated with "dioxinlike" (DL) constituents within a technical mixture are mainly if not exclusively responsible for the development of liver neoplasms in female rats. Quantitative comparison reveals almost identical dose-response curves for the total TEQs in various technical PCB mixtures and for TCDD as inducers of hepatic neoplasms in female rats. Tumor promotion experiments have shown, however, that, after initiation with a genotoxic carcinogen, technical PCB mixtures and individual DL-and NDL-PCBs act as liver tumor promoters in rodents. Based on these data, a weak carcinogenic potency of individual NDL-PCB congeners cannot be excluded. In epidemiological studies, increased mortality from cancers of the liver, gallbladder, biliary tract, gastrointestinal tract, and from brain cancer and malignant melanoma were observed in workers exposed to a series of technical PCB mixtures. A significant association between PCB concentrations in adipose tissue and non-Hodgkins lymphoma was found in another study. While in all human studies mixed exposure to DL-and NDL-PCBs occurred, no comprehensive data are available on the relative contribution of NDL-PCBs to the overall external and/or internal PCB exposure in those cohorts.

Understanding population and individual risk assessment: the case of polychlorinated biphenyls

Decisions about how to improve or protect the public health can be, and sometimes necessarily are, made on imprecise science. The regulation of potential human carcinogens in the environment entails a population-risk assessment process intended to reduce risks to less than one additional cancer in 100,000 or 1,000,000 persons. These risk assessment processes, however, may be miscommunicated or misinterpreted in the context of individual cancer risks by scientists, regulators, the lay media, and the public. This commentary will review methods for establishing a causal relationship between carcinogen exposures and cancer risk. It will use the case of polychlorinated biphenyls (PCB) as an example of how to place scientific data into the context of human exposure and cancer risk. PCBs are widespread environmental contaminants and most people have detectable levels of PCBs in their bodies. The primary source for exposure in the general population is through the diet. PCBs are carcinogens in experimental animal models, but how this information can be extrapolated to human risk remains uncertain. PCB experimental studies provide data that are used to regulate and control human exposure, although the epidemiologic evidence fails to establish PCBs as human carcinogens. Thus, what is used for population-risk assessment may not be appropriate for individual-risk assessment or concluding that a causal relationship exists between PCB exposure and cancer risk. The hazards from a carcinogen designated by regulatory and review agencies as a "probable" human carcinogen is often misunderstood out of context about the magnitude of the risk and in what settings. How scientists communicate their results in scientific articles can strongly influence how others interpret their data. Misunderstandings from both the use of regulatory and review-agency opinions and the conclusions espoused by scientists occur in the media, among private physicians counseling their patients about cancer risk, and in the legal settings where plaintiffs seek compensation for exposure and alleged harm (or future harm). This can lead to false conclusions about what caused a cancer in a specific patient, undue anxiety about future cancer risk, inappropriate cancer screening, and attendant increased morbidity due to increased uses of the medical system and complication rates from medical procedures. The communication of research findings by scientists must be presented with caution, resisting the temptation to extrapolate, inappropriately, research data to the general population.

CYP1A in TCDD toxicity and in physiology-with particular reference to CYP dependent arachidonic acid metabolism and other endogenous substrates

Toxicologic and physiologic roles of CYP1A enzyme induction, the major biochemical effect of aryl hydrocarbon receptor activation by TCDD and other receptor ligands, are unknown. Evidence is presented that CYP1A exerts biologic effects via metabolism of endogenous substrates (i.e., arachidonic acid, other eicosanoids, estrogens, bilirubin, and melatonin), production of reactive oxygen, and effects on K(+) and Ca(2+) channels. These interrelated pathways may connect CYP1A induction to TCDD toxicities, including cardiotoxicity, vascular dysfunction, and wasting. They may also underlie homeostatic roles for CYP1A, especially when transiently induced by common chemical exposures and environmental conditions (i.e., tryptophan photoproducts, dietary indoles, and changes in oxygen tension).

Highly toxic coplanar PCB126 reduces liver peroxisomal enzyme activities in rats

The effect of the highly toxic coplanar PCB congener, 3,4,5,3',4'-pentachlorobiphenyl (PCB126) on hepatic peroxisomes was studied in rats. The aim of this study was to investigate whether a toxic dose of the dioxin-like coplanar PCB modifies enzyme activities in peroxisomes where plays an important role in lipid metabolism. Treatment with PCB126, at a single i.p. administration of 25 mg/kg which evokes clear suppression of body weight gain, resulted in marked reduction (to about 40-50%) of catalase activity and peroxisomal fatty acyl-CoA β-oxidizing system. Immunoblotting showed that expression of catalase was greatly reduced by the treatment in parallel with the activity. Light microscopy revealed a drastic reduction in granules possessing peroxidase activity, while electron microscopy demonstrated that no apparent morphological changes had taken place. Thus the reduction in catalase activity caused by PCB126 could be attributable to suppression of protein expression. The marked reduction of these peroxisomal enzyme activities might be related to hyperlipidemia caused by dioxin-related compounds in rats and humans.

Significant induction of a 54-kDa protein in rat liver with homologous alignment to mouse selenium binding protein by a coplanar polychlorinated biphenyl, 3,4,5,3',4'-pentachlorobiphenyl and 3-methylcholanthrene

A 54-kDa protein in rat liver cytosol was significantly induced by treatment with 3,4,5,3',4'-pentachlorobiphenyl (25 mg/kg, single i.p.) and 3-methylcholanthrene (20 mg/kg, once a day for 3 days, i.p.). The protein exhibited pI of 6.8 on two-dimensional gel electrophoresis. The amino acid sequences of peptide fragments from the protein digested in situ were highly similar to a selenium binding protein in mice and to the isoform acetaminophen binding protein in mice. The present result clearly demonstrates that a coplanar polychlorinated biphenyl and 3-methylcholanthrene are responsible for induction of selenium binding protein homologues. The physiological role of the mouse proteins, however, is not yet elucidated.

Lactational transfer of 2,4,5,2',4',5'-hexachlorobiphenyl but not 3,4,3',4'-tetrachlorobiphenyl, induces neonatal CYP4A1

In order to study the lactational transfer of polychlorinated biphenyls, lactating rats were treated with a low dose of either 3,4,3',4'-tetrachlorobiphenyl (TCB), 2,4,5,2',4',5'-hexachlorobiphenyl (HCB) or a combination of TCB and HCB. For comparison, animals were also treated with Aroclor 1254, Lactational transfer of these chemicals resulted in the induction of neonatal hepatic CYP4A haemoproteins, the isozymes induced being dependent on the compound used. CYP4A1 was not detected in control, TCB or HCB/TCB-treated animals, but was induced in neonates when mothers were treated with HCB or Aroclor 1254. In the case of Aroclor 1254, the magnitude of the effect appeared to be dependent on the dose used. CYP4A2 and CYP4A3 were induced in the neonates when mothers were treated with Aroclor 1254 but not with the other agents used. It appears that TCB induces novel members of the CYP4A gene family. The present study provides immunochemical evidence for the ability of congeneric polychlorinated biphenyls to modulate differentially the expression of CYP4A isozymes in lactating mothers and their suckling offspring. These findings further support the potential hazards induced by lactational transfer of inert lipophilic chemicals and exemplify the complexity of the regulation of genes within this gene family.