Polychlorinated biphenyls interfere with the regulation of hydroxymethylglutaryl-coenzyme A reductase activity in rat liver via enzyme-lipid interaction and at the transcriptional level

Exposure to a northern contaminant mixture (NCM) alters hepatic energy and lipid metabolism exacerbating hepatic steatosis in obese JCR rats

Non-alcoholic fatty liver disease (NAFLD), defined by the American Liver Society as the buildup of extra fat in liver cells that is not caused by alcohol, is the most common liver disease in North America. Obesity and type 2 diabetes are viewed as the major causes of NAFLD. Environmental contaminants have also been implicated in the development of NAFLD. Northern populations are exposed to a myriad of persistent organic pollutants including polychlorinated biphenyls, organochlorine pesticides, flame retardants, and toxic metals, while also affected by higher rates of obesity and alcohol abuse compared to the rest of Canada. In this study, we examined the impact of a mixture of 22 contaminants detected in Inuit blood on the development and progression of NAFLD in obese JCR rats with or without co-exposure to10% ethanol. Hepatosteatosis was found in obese rat liver, which was worsened by exposure to 10% ethanol. NCM treatment increased the number of macrovesicular lipid droplets, total lipid contents, portion of mono- and polyunsaturated fatty acids in the liver. This was complemented by an increase in hepatic total cholesterol and cholesterol ester levels which was associated with changes in the expression of genes and proteins involved in lipid metabolism and transport. In addition, NCM treatment increased cytochrome P450 2E1 protein expression and decreased ubiquinone pool, and mitochondrial ATP synthase subunit ATP5A and Complex IV activity. Despite the changes in mitochondrial physiology, hepatic ATP levels were maintained high in NCM-treated versus control rats. This was due to a decrease in ATP utilization and an increase in creatine kinase activity. Collectively, our results suggest that NCM treatment decreases hepatic cholesterol export, possibly also increases cholesterol uptake from circulation, and promotes lipid accumulation and alters ATP homeostasis which exacerbates the existing hepatic steatosis in genetically obese JCR rats with or without co-exposure to ethanol.

Transcriptomics identifies differences between ultrapure non-dioxin-like polychlorinated biphenyls (PCBs) and dioxin-like PCB126 in cultured peripheral blood mononuclear cells

Polychlorinated biphenyls (PCBs) remain ubiquitously present in human lipids despite the ban on their production and use. Their presence can be chemically monitored in peripheral blood samples of the general population. We tested whether in vitro exposure to different PCB congeners induced different gene expression profiles in peripheral blood cells. We have isolated peripheral blood mononuclear cells (PBMC) from whole blood of 8 healthy individuals and exposed these cells in vitro to individual non-dioxin-like (NDL)-PCB congeners (PCB52, 138 or 180; 10μM) or dioxin-like (DL)-PCB congener PCB126 (1μM) during 18h. Differential gene expression response was measured using Agilent whole-human genome microarrays. Two-way ANOVA analysis of the data showed that both gender and PCB exposure are important factors influencing gene expression responses in blood cells. Hierarchical cluster analysis of genes influenced by PCB exposure, revealed that DL-PCB126 induced a different gene expression response compared to the NDL-PCBs. Biological interpretation of the results revealed that exposure to PCB126 induced the AhR signaling pathway, whereas the induction of nuclear receptor pathways by the NDL-PCBs was limited in blood cells. Nevertheless, molecular responses of blood cells to individual PCB congeners revealed significantly expressed genes that play a role in biological functions and processes known to be affected by PCB exposure in vivo. Observed gene expression changes in this in vitro model were found to be related to hepatotoxicity, immune and inflammatory response and disturbance of lipid and cholesterol homeostasis.

Combined and individual administration of diethyl phthalate and polychlorinated biphenyls and its toxicity in female Wistar rats

Polychlorinated biphenyls (PCBs) are persistent environmental pollutants and known to act as xenoestrogens. PCBs and diethyl phthalate (DEP) are ubiquitous environmental pollutants because both are used as plasticizers and in various other industrial applications. Therefore, a study was undertaken to evaluate the interactive toxicity of DEP and PCBs in young female Wistar rats. Healthy young female albino rats of Wistar strain weighing 100g (7-8 weeks old) were randomly assigned to five groups of six each. Group I female rats were fed on normal diet and water ad libitum. Group II female rats were maintained on normal diet mixed with corn oil at 16.5mg/kg diet/day and 0.94mg/kg body weight/day as oil control. Groups III and IV female rats were given Clophen A60 and DEP dissolved in corn oil mixed with the diet at 50mg/(kgdietday), which is approximately equal to 2.85mg/(kgbodyweightday), individually to each group. Group V female rats received a mixture of DEP and Clophen A60, each dissolved in corn oil mixed with the diet at 50mg/(kgdietday), which is approximately equal to 2.85mg/(kgbodyweightday). Treatment was carried out for 150 days and after the completion of treatment, serum and liver enzymes and other biochemical parameters in the serum and liver were assessed. Liver weight to body weight ratio showed significant increase in Clophen A60 and Clophen A60+DEP treated rats. In the three treated groups, there was significant decrease in liver glutathione (GSH) and glutathione reductase (GR). Alanine amino transferase (ALT) was significantly increased in the liver of the three treated groups and in the serum of Clophen A60 and DEP alone treated groups and significant decrease only in the serum of Clophen A60+DEP treated rats. Significant increase in liver and serum lactate dehydrogenase (LDH) and acid phosphatase (ACP) activity was observed in the three treated groups. Alkaline phosphatase (ALP) activity was significantly increased only in the serum of the Clophen A60 and Clophen A60+DEP treated rats, whereas significant decrease in the serum and liver of DEP alone treated rats was observed. Aspartate aminotransferase (AST) activity and cholesterol levels were highly significant in the liver and serum of DEP treated rats. In addition, cholesterol level was significantly increased in the liver and serum of Clophen A60 treated rats and only in the liver of Clophen A60+DEP treated rats. Succinate dehydrogenase (SDH) activity was significantly increased in the liver of Clophen A60 and Clophen A60+DEP treated rats and highly significant increase in the serum of Clophen A60+DEP treated rats. There was significant increase in triglyceride levels in the liver and serum of Clophen A60 and Clophen A60+DEP treated rats, whereas significant increase in triglyceride levels in the serum of DEP alone treated rats was observed. Glycogen levels were significantly increased in the liver of Clophen A60+DEP treated rats, whereas serum glucose levels showed significant decrease, but in Clophen A60 alone treated rats showed significant increase in liver glycogen and serum glucose, whereas DEP alone treated rats showed significant increase in only serum glucose levels. Lipid peroxidation was increased in the liver of DEP treated rats, which was highly significant, compared to significant increase in Clophen A60 and Clophen A60+DEP treated rats. Histology of liver showed severe vacuolation, loss of hepatic architecture and granular deposits in the hepatocytes of DEP and Clophen A60+DEP treated rats, whereas in Clophen A60 alone treated rats, hepatocytes showed hyper pigmentation mild loss of hepatic architecture in centrilobular and periportal area.

Effects of two prototypic polychlorinated biphenyls (PCBs) on lipid composition of rat liver and serum

Mature male Sprague-Dawley rats received a single IP injection of either 2,2',4,4',5,5'-hexachlorobiphenyl (HCB), 3,3',4,4'-tetrachlorobiphenyl (TCB) (300 microm/kg) in corn oil (10 ml/kg) or the corn oil vehicle alone, and were killed four days later after having been fasted overnight. The vehicle control group consisted of rats which were allowed free access to feed as well as pair-fed animals. Lipid analyses were conducted on liver, hepatic microsomes and serum. TCB- (but no HCB-) treatment resulted in a statistically significant increase in total liver lipids and triglycerides. Liver phospholipids remained unchanged. Both PCBs increased the cholesterol and phospholipids content of the liver microsomal fraction. Serum lipids measured were not statistically different from control values. While HCB had little effect on the fatty acid composition of liver lipids, TCB caused an increase in C 18:1 (n-9) and a decrease in C 20:4 (n-6). Both PCBs increased C 18:0 in the hepatic microsomal fraction, but TCB also decreased C 16:0. Neither PCB altered the fatty acid composition of serum total lipids. These data are consistent with the concept that specific alterations in lipid metabolism are dependent on the structure of the PCB.

Dietary fat interacts with PCBs to induce changes in lipid metabolism in mice deficient in low-density lipoprotein receptor

There is evidence that dietary fat can modify the cytotoxicity of polychlorinated biphenyls (PCBs) and that coplanar PCBs can induce inflammatory processes critical in the pathology of vascular diseases. To test the hypothesis that the interaction of PCBs with dietary fat is dependent on the type of fat, low-density lipoprotein receptor-deficient (LDL-R(-/-)) mice were fed diets enriched with either olive oil or corn oil for 4 weeks. Half of the animals from each group were injected with PCB-77. Vascular cell adhesion molecule-1 (VCAM-1) expression in aortic arches was nondetectable in the olive-oil-fed mice but was highly expressed in the presence of PCB-77. PCB treatment increased liver neutral lipids and decreased serum fatty acid levels only in mice fed the corn-oil-enriched diet. PCB treatment increased mRNA expression of genes involved in inflammation, apoptosis, and oxidative stress in all mice. Upon PCB treatment, mice in both olive- and corn-oil-diet groups showed induction of genes involved in fatty acid degradation but with up-regulation of different key enzymes. Genes involved in fatty acid synthesis were reduced only upon PCB treatment in corn-oil-fed mice, whereas lipid transport/export genes were altered in olive-oil-fed mice. These data suggest that dietary fat can modify changes in lipid metabolism induced by PCBs in serum and tissues. These findings have implications for understanding the interactions of nutrients with environmental contaminants on the pathology of inflammatory diseases such as atherosclerosis.

Apolipoprotein A-I gene expression is upregulated by polychlorinated biphenyls in rat liver

Xenobiotics such as polychlorinated biphenyls (PCB) increase serum cholesterol level (especially high density lipoprotein cholesterol) and apolipoprotein A-I (apo A-I) level in rats. The effect of PCB on serum apo A-I and hepatic apo A-I gene expression and the relationship between apo A-I and drug-metabolizing enzymes in rats were investigated. Serum levels of cholesterol and apo A-I were increased by dietary addition of PCB in a dose-dependent manner (0-500 mg/kg diet). Hepatic apo A-I mRNA level was also elevated by PCB in a similar fashion. Serum level of cholesterol gradually increased during feeding period of PCB (200 mg/kg diet, 105 days) and reached a two-fold higher level in PCB group than in controls. The levels of serum apo A-I and hepatic apo A-I mRNA linearly elevated during feeding period of PCB and were increased 3- or 4-fold, respectively, compared to controls. Although acute administration (16 hr) of PCB, 3-methylcholanthrene, and phenobarbital induced cytochrome P-450 gene expression in the liver, hepatic apo A-I gene expression was not increased by these xenobiotics. These results indicated that the serum levels of cholesterol and apo A-I had positive correlation with hepatic level of apo A-I mRNA in rats fed PCB, and that hepatic apo A-I gene expression was dependent upon intake of PCB but was not directly related to the induction of drug-metabolizing enzymes. This study demonstrated that xenobiotic-induced hyper-alpha-cholesterolemia would be caused by the increased apo A-I gene expression and cholesterol synthesis in the liver, coordinately.

The enzyme inducers 3-methylcholanthrene and phenobarbital affect the activities of glucocorticoid hormone-regulated enzymes in rat liver and kidney

3-Methylcholanthrene, an inducer of P448-type cytochromes (mostly 1A1 and 1A2), and phenobarbital, an inducer of P450-type cytochromes (mostly 2B1 and 2B2), are prototypical for the actions of many xenobiotics. They cause endocrine disruption by affecting, among others, steroid hormone levels. Rats were treated with single bolus doses of 3-methylcholanthrene or phenobarbital, and enzyme activities that are controlled by glucocorticoids were measured in liver and kidney. The activities of the cytosolic enzymes L-alanine aminotransferase, indoleamine 2,3-dioxygenase (L-tryptophan pyrrolase), phosphoenolpyruvate carboxykinase, L-serine dehydratase and L-tyrosine aminotransferase were affected in a similar fashion: an initial activity reduction followed by two overshoots of activity 1 and 2 days after dosing. 3-Hydroxy-3-methylglutaryl coenzyme A reductase, the microsomal key enzyme of sterol synthesis, responded with a temporary reduction of activity only and evidently lost its diurnal rhythm. The time course of these changes is most likely caused by a combination of sub-physiological levels of glucocorticoids plus changes of other regulatory hormones elicited by feed intake, postprandial state, etc. A possible role for a combined action of the arylhydrocarbon (Ah) and glucocorticoid receptors in the effects of 3-methylcholanthrene is also suggested.

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.

Protooncogene expression in rat liver by polychlorinated biphenyls (PCB)

1. The expression of 10 protooncogenes was studied in control rat liver and at various times after exposure to polychlorinated biphenyls (PCB), a known tumour promoter. 2. The expression of protooncogenes in liver is more pronounced in those rats treated with PCB beginning at weaning ('weanlings') than in adult rats. 3. The RNA levels of c-Ha-ras, c-raf, c-yes, c-erbA and c-erbB are elevated after PCB feeding. 4. Nuclear run-on transcription analysis revealed that the altered expression of the protooncogenes is transcriptionally regulated. 5. In one group the prompt rise of the protooncogene transcription rate is followed by a decline (c-Ha-ras, c-raf c-yes). In a second group a further increase in transcription at later feeding times (c-erbA, c-erbB) was observed. 6. A correlation between the altered expression of these protooncogenes and the action of PCB as a tumour promotor remains to be determined.

Modes of action and combination effects of polychlorinated biphenyls and gamma-hexachlorocyclohexane on the regulation of rat liver 3-hydroxy-3-methylglutaryl coenzyme A reductase

The effect of polychlorinated biphenyls, gamma-hexachlorocyclohexane and the effect of a combination of these substances on the regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase were investigated. As known from previous investigations polychlorinated biphenyls interfere with the regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in rat liver via enzyme-lipid interaction and at the pretranslational level. In contrast to polychlorinated biphenyls, gamma-hexachlorocyclohexane did not alter the lipid status of the microsomal membrane. Thus the location of the 3-hydroxy-3-methylglutaryl coenzyme A reductase, and consequently the catalytic activity of the enzyme was not changed. As with polychlorinated biphenyls, gamma-hexachlorocyclohexane interacted with enzyme regulation at the pretranslational level. Northern dot hybridization experiments showed a decrease in the level of m-RNA coding for 3-hydroxy-3-methylglutaryl coenzyme A reductase. The effect of combination of gamma-hexachlorocyclohexane and polychlorinated biphenyls was not additive. The gamma-hexachlorocyclohexane effect appeared to play a more important role than that of the polychlorinated biphenyls. The results indicate that the combination effects are as important as the effects of the single compounds when making risk assessments for xenobiotics.

[Concentration of polychlorobiphenyl in the population of Greater São Paulo, Brazil]

Os policloretos de bifenila (PCBs), cuja utilização iniciou-se em 1930 em sistemas fechados (capacitores e transformadores elétricos) foram observados no ambiente em 1966, atingindo a cadeia alimentar e acumulando-se no tecido subcutâneo de seres humanos; ao mesmo tempo, foi possível mostrar que esses produtos químicos são potencialmente nocivos para a saúde humana. Como em 1976 os PCBs foram encontrados no tecido gorduroso de pequena amostra da população brasileira, decidiu-se fazer um estudo mais amplo, analisando-se por cromatografia de fase gasosa a gordura do tecido celular subcutâneo de 189 pessoas, de ambos os sexos, que tinham sofrido morte violenta. Foram encontrados níveis médios de 0,8661 ppm em indivíduos do sexo masculino, que apresentaram aumento linear das concentrações de acordo com a idade, o que não foi observado entre os do sexo feminino, onde os valores médios de acúmulo foram de 1,4530 ppm. Do total de amostras, 12,7% apresentaram níveis superiores a 1,95 ppm. É discutida a importância do achado e são assinaladas as medidas de proteção da população brasileira exposta aos PCBs.