Protooncogene expression in rat liver by polychlorinated biphenyls (PCB)

Protein expression profiling in the African clawed frog Xenopus laevis tadpoles exposed to the polychlorinated biphenyl mixture aroclor 1254

Exposure to environmental pollutants such as polychlorinated biphenyls (PCBs) is now taken into account to partly explain the worldwide decline of amphibians. PCBs induce deleterious effects on developing amphibians including deformities and delays in metamorphosis. However, the molecular mechanisms by which they express their toxicity during the development of tadpoles are still largely unknown. A proteomics analysis was performed on developing Xenopus laevis tadpoles exposed from 2 to 5 days postfertilization to either 0.1 or 1 ppm Aroclor 1254, a PCB mixture. Two-dimensional DIGE with a minimal labeling method coupled to nanoflow liquid chromatography-tandem mass spectrometry was used to detect and identify proteins differentially expressed under PCBs conditions. Results showed that 59 spots from the 0.1 ppm Aroclor 1254 condition and 57 spots from the 1 ppm Aroclor 1254 condition displayed a significant increase or decrease of abundance compared with the control. In total, 28 proteins were identified. The results suggest that PCBs induce mechanisms against oxidative stress (peroxiredoxins 1 and 2), adaptative changes in the energetic metabolism (enolase 1, glycerol-3-phosphate dehydrogenase, and creatine kinase muscle and brain types), and the implication of the unfolded protein response system (glucose-regulated protein, 58 kDa). They also affect, at least at the highest concentration tested, the synthesis of proteins involved in normal cytogenesis (alpha-tropomyosin, myosin heavy chain, and alpha-actin). For the first time, proteins such as aldehyde dehydrogenase 7A1, CArG binding factor-A, prolyl 4-hydroxylase beta, and nuclear matrix protein 200 were also shown to be up-regulated by PCBs in developing amphibians. These data argue that protein expression reorganization should be taken into account while estimating the toxicological hazard of wild amphibian populations exposed to PCBs.

Polychlorinated biphenyls promote 1-nitropyrene-induced lung tumorigenesis without the induction of K-ras gene mutation in A/J mice

Although the effects of polychlorinated biphenyls (PCBs) on human lung carcinogenesis are suggested from the massive PCBs poisoning that occurred in Japan designated "Yusho," the detailed molecular mechanism are unknown. 1 nitropyrene (1-NP), an ubiquitous and abundant environmental pollutant, is known to be detected in lung tissues derived from patients with lung cancer in Japan, and its relation to lung carcinogenesis is also suggested. We investigated the effects of PCBs (Kanechlor-400) on 1-NP-induced lung tumorigenesis in A/J mice. PCBs were administered intraperitoneally followed by ip injection of 1-NP. The lung lesions were examined 18 weeks after the final treatment. In the control group, no neoplastic lesions were induced in the lung. In the PCB group, preneoplastic lesions such as hyperplasia and adenoma were induced in 2/10 (20%) mice. In 1-NP group and in PCB + 1-NP group, lung lesions including adenocarcinoma were induced in 16/20 (80%) and 13/13 (100%) mice, respectively. Both the number and the size of tumors in PCB + 1-NP group were significantly greater than those in 1-NP group. K-ras gene mutation, CAA to CGA in codon 61 or GGT to GAT in codon 12, was found in either 1-NP group or PCB + 1-NP group but not in the PCB group. There was no difference in the pattern of K-ras mutation associated with the pretreatment with PCBs. These results suggest that PCBs promote 1-NP-induced lung tumorigenesis and may support, at least in part, the mechanism of the high incidence of lung cancer in patients with Yusho.

Effects of polychlorinated biphenyl mixtures and three specific congeners on growth and circulating growth-related hormones

Polychlorinated biphenyls (PCB) are ubiquitous environmental contaminants that bioaccumulate in avian species. Exposure to PCBs can result in decreased growth. Thyroid hormones and growth hormone (GH) are important for normal growth. The present studies employed the chicken embryo to investigate effects of Aroclor 1242, Aroclor 1254, 2,2',6,6'-tetrachlorobiphenyl (TCB), 3,3',4,4'-TCB, and 3,3',5,5'-TCB on growth and growth-related hormones. The following indices were measured: embryo mortality, body weights, bone length, pituitary GH content, and plasma concentrations of triiodothyronine (T3), thyroxine (T4), GH, and insulin-like growth factor-1 (IGF-1). Fertile eggs were injected with PCBs on Day 0 and indices determined on Day 17 of incubation. Unexpectedly, 3,3',5,5'-TCB or low-dose Aroclor 1242 treatment increased body weight and bone length (P < 0.05), whereas Aroclor 1242 (high dose), 3,3,4,4'-TCB, or Aroclor 1254 treatment reduced body weights and/or bone length (P < 0.05). Aroclor 1242 or 3,3',4,4'-TCB (low-dose treatment) elevated plasma T4 concentrations (P < 0.05). Both growth and pituitary GH content were increased (P < 0.05) by 3,3',5,5'-TCB (low dose) or Aroclor 1242 treatment. Despite marked differences in growth rates, plasma T3, GH, and IGF-I concentrations were unaffected by PCB treatment. Growth-related hormones may not be responsible for the growth depression observed after PCB treatment. Possibly the decrease in growth occurred because of general toxicity. The importance of chlorine position in causing thyroid hormone axis alterations was not clearly established.

Intranuclear distribution of 2,2',4,4',5,5'-hexachlorobiphenyl and 2,3,7,8,-tetrachlorodibenzo-p-dioxin

The in vitro intranuclear distribution of the chlorinated aromatic hydrocarbon compounds 2,2',4,4',5,5'-hexachlorobiphenyl and 2,3,7,8-tetrachlorodibenzo-p-dioxin was determined in isolated rat liver cell nuclei. Suspended nuclei were incubated with the 3H-labeled congeners, and the incubation terminated by brief UV irradiation. High-intensity UV irradiation at 254 nm changes the reversible association between macromolecules and ligands into covalent linkages and thus stabilizes the equilibrium distribution. The nuclei were then fractionated with the radioactive congeners covalently linked to the purified fractions. The intranuclear distribution of the model compounds was not uniform. The majority of either chemical was attached to the nuclear envelope and to the chromatin fraction. Much lower amounts were bound to nucleoli. The nuclear matrix was almost devoid of the chemicals. Minute amounts of either compound were detected in association with DNA, none with nuclear RNA. The substantial association of these chlorinated hydrocarbon model compounds with chromatin may bear more general biological relevance and point to detrimental effects on the genetic apparatus. The presented method yields an unequivocal profile of the genuine nuclear distribution of photoactivatable chemicals.

Transfer of PCBs via lactation simultaneously induces the expression of P450 isoenzymes and the protooncogenes c-Ha-ras and c-raf in neonates

At the first day of lactation, maternal rats were injected with a single i.p. dose of 100 or 250 mg/kg body weight of a mixture of polychlorinated biphenyls (Aroclor 1254). This treatment caused significant increases in both material and neonatal hepatic cytochrome P-450, cytochrome b5, and cytochrome-c-(P-450) reductase. Transfer of PCBs via lactation resulted in significant increases in hepatic enzyme activities catalysed by neonatal CYP1A1, CYP1A2, CYP2B1, CYP3A1, and CYP2E1 using a variety of substrates. In contrast, the metabolism of dimethylnitrosamine and aminopyrine was only marginally (up to 2-fold) increased in maternal animals four days post treatment. Further measurements showed significant increases in maternal and neonatal epoxide hydrolase, glutathione-S-transferase, and UDP-glucuronyl transferase activities, thus suggesting a coordinated response for an induction of CYP1A1, CYP1A2, CYP2A1, CYP2B1, CYP2E1, CYP3A1, and CYP4A1 in both maternal and neonatal CYP2C6, and at the higher dose the expression of neonatal CYP2E1 was significantly reduced. Northern blot analysis provided further evidence for significant increases in maternal and neonatal hepatic CYP1A1, CYP1A2, CYP2B1, and CYP2E1 mRNA, but reduced amounts of CYP2C7 and CYP4A1 mRNA. Additional Northern blot hybridization experiments may suggest an increased expression of the protooncogenes c-Ha-ras and c-raf in the mother and the neonate upon treatment of maternal rats with Aroclor 1254. Lactation itself may result in an increased expression of the latter protooncogenes, but the mRNA of the protooncogenes c-erb A and c-erb B was not detected in any of the tissues examined.

C-raf expression in early rat liver tumorigenesis after promotion with polychlorinated biphenyls or phenobarbital

1. The expression of c-raf protooncogene in early stages of chemically induced rat liver tumorigenesis was studied in weanling female and adult male Sprague-Dawley rats. After initiation with diethylnitrosamine, promotion by polychlorinated biphenyls (PCBs) or phenobarbital (PB) was studied in the female. Male rats were promoted with PCBs only. 2. The incidence of enzyme-altered foci was evaluated histochemically by demonstrating a deficiency in adenosine-5'-triphosphatase and the emergence of gamma-glutamyl-transpeptidase. C-raf expression was measured in liver tissue containing preneoplastic foci, and in small (< 3 mm in diameter) and large (> 3 mm in diameter) neoplastic nodules up to 36 weeks. 3. Foci numbers amounted to 60-70 per cm2 liver section with both histochemical markers and both promoters in female rats. In male rats foci numbers were about 20-40 per cm2 liver section with both markers and with PCBs as promoting agents. Foci area developed more rapidly in female rats. 4. Small and large nodules were found in females during the entire observation period with both promoting agents, PCBs being more effective than PB. C-raf expression in nodules was increased up to 10-fold in PCB-treated animals compared with untreated controls. No dependence on the size of the nodules was seen. In male rats nodule incidence was very low and c-raf induction was marginal. 5. In conclusion, c-raf proto-oncogene expression correlated with the incidence of foci and nodules, female rats being more sensitive than males.

Polychlorinated biphenyls modulate protooncogene expression in Chang liver cells

n Chang liver cells we studied the influence of polychlorinated biphenyls (PCB) on the expression of different protooncogenes. In cells incubated with medium supplemented with PCB we observed an early effect after 3 h on c-erbA and c-erbB RNA level. This reduction of RNA was due to a delayed transcriptional activation of the genes. The PCB congener 3,3',4,4',5-pentachlorobiphenyl (5-CB) in a 1,000-fold lower concentration influenced protooncogene expression in the same manner. In contrast c-raf RNA level increased transiently after 24 h. The fact that persistent chemicals like PCB interfere with protooncogene expression is particularly interesting in view of their tumor promoting activity.

Polychlorinated biphenyls (PCBs): environmental impact, biochemical and toxic responses, and implications for risk assessment

Commercial polychlorinated biphenyls (PCBs) and environmental extracts contain complex mixtures of congeners that can be unequivocally identified and quantitated. Some PCB mixtures elicit a spectrum of biochemical and toxic responses in humans and laboratory animals and many of these effects resemble those caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons, which act through the aryl hydrocarbon (Ah)-receptor signal transduction pathway. Structure-activity relationships developed for PCB congeners and metabolites have demonstrated that several structural classes of compounds exhibit diverse biochemical and toxic responses. Structure-toxicity studies suggest that the coplanar PCBs, namely, 3,3',4,4'-tetrachlorobiphenyl (tetraCB), 3,3',4,4',5-pentaCB, 3,3',4,4',5,5'-hexaCB, and their monoortho analogs are Ah-receptor agonists and contribute significantly to the toxicity of the PCB mixtures. Previous studies with TCDD and structurally related compounds have utilized a toxic equivalency factor (TEF) approach for the hazard and risk assessment of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) congeners in which the TCDD or toxic TEQ = sigma([PCDFi x TEFi]n)+sigma([PCDDi x TEFi]n) equivalent (TEQ) of a mixture is related to the TEFs and concentrations of the individual (i) congeners as indicated in the equation (note: n = the number of congeners). Based on the results of quantitative structure-activity studies, the following TEF values have been estimated by making use of the data available for the coplanar and monoortho coplanar PCBs: 3,3',4,4',5-pentaCB, 0.1; 3,3',4,4',5,5'-hexaCB, 0.05; 3,3',4,4'-tetraCB, 0.01; 2,3,3',4,4'-pentaCB, 0.001; 2,3',4,4',5-pentaCB, 0.0001; 2,3,3',4,4',5-hexaCB, 0.0003; 2,3,3',4,4',5'-hexaCB, 0.0003; 2',3,4,4',5-pentaCB, 0.00005; and 2,3,4,4',5-pentaCB, 0.0002. Application of the TEF approach for the risk assessment of PCBs must be used with considerable caution. Analysis of the results of laboratory animal and wildlife studies suggests that the predictive value of TEQs for PCBs may be both species- and response-dependent because both additive and nonadditive (antagonistic) interactions have been observed with PCB mixtures. In the latter case, the TEF approach would significantly overestimate the toxicity of a PCB mixture. Analysis of the rodent carcinogenicity data for Aroclor 1260 using the TEF approach suggests that this response is primarily Ah-receptor-independent. Thus, risk assessment of PCB mixtures that uses cancer as the endpoint cannot solely utilize a TEF approach and requires more quantitative information on the individual congeners contributing to the tumor-promoter activity of PCB mixtures.

Transplacental transfer of polychlorinated biphenyls induces simultaneously the expression of P450 isoenzymes and the protooncogenes c-Ha-ras and c-raf

At day 15 of gestation, rats were injected with a single i.p. dose of 100, 250 and 500 mg/kg body weight of a mixture of polychlorinated biphenyls (PCBs) (Aroclor 1254). Seven days later, significant increases in maternal and foetal cytochrome P450, cytochrome b5 and cytochrome c (P450) reductase were found. Concomitantly, the metabolism of nitroanisole, aniline, ethoxyresorufin and benzo[a]pyrene was significantly increased, but foetal metabolism of dimethylnitrosamine was not detectable and only marginal increases in the metabolism of aminopyrine and aldrin were seen. In contrast, maternal metabolism of dimethylnitrosamine, aminopyrine and aldrin was measurable, but significant increases were determined only with the latter substrate. Transplacental transfer of PCBs resulted in increased metabolism of substrates catalysed by foetal CYP1A1 and CYP2B1, but there was no evidence for CYP2E1-catalysed reactions. Further measurements show significant increases in foetal and maternal epoxide hydrolase, glutathione-S-transferase and UDP-glucuronyl transferase activities, thus suggesting that treatment with Aroclor 1254 resulted in coordinated increases in foetal and maternal oxidative and post-oxidative drug metabolism. Western blot analysis of microsomal proteins shows the induction of foetal and maternal CYP1A1, CYP1A2, CYP2A1, CYP2B1, CYP3A1 and CYP4A1. In addition, increased expression of CYP2C6 was seen with the mother but not the foetus. Unlike the mother, foetal rats did not express CYP2E1 and the expression of the above-listed P450 isoenzymes was greater in the mother than the foetus. Northern blot analysis shows significant increases in maternal and foetal CYP1A1, CYP1A2 and CYP2B1 mRNA. An increased amount of CYP3A1 mRNA was only seen with the mother, but not the foetus. Treatment of mothers with Aroclor 1254 resulted in reduced CYP2A1, CYP2C7, CYP2E1 and CYP4A1 mRNA. Insignificant differences in the expression of foetal CYP2A1 and CYP4A1 mRNA were found, but in utero exposure to PCBs reduced the amounts of CYP2E1 mRNA and there was no foetal CYP2C7 mRNA transcript. Treatment with Aroclor 1254 increased the expression of the protooncogenes c-Ha-ras and c-raf in the mother and the foetus, but at varying intensities. Pregnancy itself was linked to an increased expression of these protooncogenes. erbA and erbB mRNA was not detected.

Partial purification and characterization of a rat liver polychlorinated biphenyl (PCB) binding protein

A protein capable of specifically binding polychlorinated biphenyls (PCB) was partially purified from rat liver cytosol. After labeling with [3H]2,2',4,4',5,5'-hexachlorobiphenyl (6-CB), protein enrichment was guided by monitoring the protein-bound radioactivity through a sequence of purification steps including ion exchange chromatography and preparative gel electrophoresis. In addition, specific binding tests of individual fractions were carried out. An average 100-fold enrichment of the 40 kDa protein was achieved. A variety of ligands were tested in competitive binding studies with 6-CB. Whereas penta- and hexachloro-PCB congeners are high affinity competitors, the 3,3',4,4'-tetrachlorobiphenyl congener does not compete for 6-CB binding. Studies on the species and tissue distribution suggest a prevalence of the binding protein in tissues of the rat. Since the natural physiological ligand of the protein has not yet been identified, the function of the protein can only be speculated on.