Effects of polychlorinated biphenyls at low dose levels in rats

Evaluation of a marine mammal status and trends contaminants indicator for European waters

Marine mammals are vulnerable to the bioaccumulation, biomagnification and lactational transfer of specific types of pollutants, such as industrial polychlorinated biphenyls (PCBs), due to their long-life spans, feeding at a high trophic level and unique fat stores that can serve as depots for these lipophilic contaminants. Currently, European countries are developing indicators for monitoring pollutants in the marine environment and assessing the state of biodiversity, requirements under both Regional Seas Conventions and European legislation. As sentinel species for marine ecosystem and human health, marine mammals can be employed to assess bioaccumulated contaminants otherwise below current analytical detection limits in water and lower trophic level marine biota. To aid the development of Regional Seas marine mammal contaminants indicators, as well as Member States obligations under descriptor 8 of the EU Marine Strategy Framework Directive, the current study aims to further develop appropriate methodological standards using data collected by the established UK marine mammal pollutant monitoring programme (1990 to 2017) to assess the trends and status of PCBs in harbour porpoises. Within this case study, temporal trends of PCB blubber concentration in juvenile harbour porpoises were analysed using multiple linear regression models and toxicity thresholds for the onset of physiological (reproductive and immunological) endpoints were applied to all sex-maturity groups. Mean PCB blubber concentrations were observed to decline in all harbour porpoise Assessment Units and OSPAR Assessment Areas in UK waters. However, a high proportion of animals were exposed to concentrations deemed to be a toxicological threat, though the relative proportion declined in most Assessment Units/Areas over the last 10 years of the assessment. Recommendations were made for improving the quality of the assessment going forward, including detailing monitoring requirements for the successful implementation of such an indicator.

Juvenile harbor porpoises in the UK are exposed to a more neurotoxic mixture of polychlorinated biphenyls than adults

Polychlorinated biphenyls (PCBs) are a group of 209 persistent and bio-accumulative toxic pollutants present as complex mixtures in human and animal tissues. Harbor porpoises accumulate some of the highest levels of PCBs because they are long-lived mammals that feed at a high trophic level. Studies typically use the sum of a suite of individual chlorobiphenyl congeners (CBs) to investigate PCBs in wildlife. However, toxic effects and thresholds of CB congeners differ, therefore population health risks of exposure may be under or over-estimated dependent on the congener profiles present. In this study, we found congener profiles varied with age, sex and location, particularly between adult females and juveniles. We found that adult females had the highest proportions of octa-chlorinated congeners whilst juveniles had the highest proportions of tri- and tetra-chlorinated congeners. This is likely to be a consequence of pollutant offloading between mothers and calves during lactation. Analysis of the individual congener toxicities found that juveniles were exposed to a more neurotoxic CB mixture at a time when they were most vulnerable to its effects. These findings are an important contribution towards our understanding of variation in congener profiles and the potential effects and threats of PCB exposure in cetaceans.

Subchronic toxicity of 2,4,4'-trichlorobiphenyl in the rat liver: an ultrastructural and biochemical study

2,4,4'-Trichlorobiphenyl or PCB congener 28 was given to Sprague-Dawley weanling rats and the experimental diets were prepared by mixing the congener in 4% corn oil. The congener was administered to animals placed in four groups, each comprising 10 males or females. The diets contained 0.05, 0.5, 5, or 50 ppm congener. The fifth or control group comprised animals that received diets mixed with corn oil. Thirteen weeks after commencement of dosing, animals were euthanized and liver specimens were harvested from the animals and prepared for electron microscopy and biochemical analyses. The hepatocyte architectural modifications included an augmentation of SER profiles and an elevation of peroxisome numbers in animals regardless of gender, and mitochondrial abnormalities in the females only. Mitochondrial aberrations consisted of abnormal shapes and cristae in atypical orientation. The alterations were revealed in animals of the 5-and 50-ppm groups and were more extensive in the females. Ethoxyresorufin-O-de-ethylase activity was significantly high in the animals of the 50-ppm group. The results suggest that the female rats were more sensitive than the males to congener 28, and the no observable adverse effect level (NOAEL) was believed to be 0.5 ppm for the congener.

Polychlorinated biphenyls (PCBs) and human health: an update

Polychlorinated biphenyls (PCBs) are a mixture of 209 different chlorinated biphenyl congeners (forms) of which 36 are environmentally relevant. PCBs are lipid (fat)-soluble, stable compounds. PCBs may be contaminated with more highly toxic polychlorinated dibenzofurans (PCDFs). Some PCDFs were primarily responsible for the two poisoning outbreaks--Yusho and Yu-Cheng. Based on the reports on workers and the general population, no clear and convincing evidence that PCB exposures were casually associated with adverse health effects was advanced; this included cancer for a wide range of body burdens and exposures for serum PCB concentrations > 1000 ppb (micrograms/l) and adipose PCB levels > 400 ppm (mg/kg). No meaningful reproductive problems have been identified in female capacitor workers. In the opinion of the review author, the available evidence for cancer and for reproductive effects is inconclusive. Adverse neurobehavioral effects in infants and young children have been reported in a study of women in the general population and a study of fish eaters and their offspring. The adverse effects observed in the two studies were not the same; the exposure assessments in both studies are not well defined and have many uncertainties. Subhuman primates appear to be more sensitive to reproductive and other adverse effects of PCBs than humans. Obvious external clinical signs are observed in the offspring of subhuman primates at dosage levels below those experienced by female capacitor workers and members of the general population prior to the control of PCBs.

Long-term effects of Aroclor 1254 (PCBs) on plasma lipid and carnitine concentrations in rhesus monkey

Sixty-seven female rhesus monkeys (Macaca mulatta) were orally dosed daily for 152 weeks with 0, 5, 20, 40, and 80 micrograms Aroclor 1254 (PCB)/kg body wt. Blood polychlorinated biphenyl (PCB) concentrations were highly positively correlated (r = 0.92, P < 0.001) with doses of PCB administered. A comprehensive analysis of plasma lipids/lipoproteins revealed a PCB-associated increase in plasma triglycerides and decreases in plasma total cholesterol, high-density lipoprotein cholesterol (HDL-chol), very-low plus low-density lipoprotein cholesterol (VLDL+LDL-chol), and total carnitine (which is involved in fatty acid metabolism). All of the lipid/lipoprotein changes were significantly (P < or = 0.05) correlated with blood PCB concentration. These data, obtained after 152 weeks of continuous daily exposure of a primate model to PCB support a causal relationship between plasma lipid changes and PCB intake. Previously, causality has been refuted on the premise that the commonly observed elevation of triglycerides with increasing concentration of blood PCB is a reflection, not of PCB dose, but of the partitioning of PCB between tissues (adipose) and blood in proportion to the blood lipid present. The mechanism of the plasma lipid changes was not investigated in this study but the altered lipid/lipoprotein pattern is discussed with respect to known cardiovascular risk profiles.

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.

Specific binding of polychlorinated biphenyls to rat liver cytosol protein

Specific binding of polychlorinated biphenyls (PCBs) to rat liver cytosol protein has been detected using the 3H-labeled PCB probe 2,2',4,4',5,5'-hexachlorobiphenyl (6-CB). Binding of 6-CB to cytosol protein is displaced by its non-radioactive congener, is of high affinity (Kd approximately 3 nM) and is saturable (maximal binding capacity Bmax approximately 600 pmol/mg protein). 6-CB binding is not found in liver cytosol of animals fed a PCB-supplemented diet (500 ppm PCB for 5 days). Binding is also in vitro inhibited by high concentrations of triglyceride. PCB congeners such as 3,3',4,4',5-pentachlorobiphenyl as well as the thyroid hormones 3,5,3',5'-tetraiodothyronine and 3,5,3'-triiodothyronine (the latter hormone with an order of magnitude lower affinity) compete for the PCB binding site. On the other hand, a number of biochemically important compounds including the PCB core compound biphenyl and the hormone ligands dexamethasone and estradiol, as well as 2,3,7,8-tetrachlorodibenzo-p-dioxin, do not compete for the 6-CB binding site. The data provide the first evidence of specific binding of unmetabolized PCB congeners to distinct binding sites in rat liver cytosol.

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.

Transport and cellular uptake of polychlorinated biphenyls (PCBs)--II. Changes in vivo in plasma lipoproteins and proteins of pigeons in response to PCBs, and a proposed model for the transport and cellular uptake of PCBs

The complex distribution of polychlorinated biphenyl (PCB) isomers and congeners amongst plasma fractions of the pigeon suggests that the lipid and apolipoprotein components of lipoproteins, as well as plasma proteins, may be important in transporting PCBs to tissues (Borlakoglu et al., Biochem. Pharmac. 40, 265 (1990]. Pigeons were injected with the commercial PCB mixture Aroclor 1254 (1.5 mmol/kg body weight). After 120 hr triacylglycerol-like droplets accumulated in hepatocytes ('fatty liver syndrome'), there was proliferation of the hepatic smooth endoplasmic reticulum, and plasma concentrations of triacylglycerol and total cholesterol increased. This was accompanied by significant decreases in plasma concentrations of total protein, total apolipoproteins of the low density lipoprotein (LDL) and the high density lipoprotein (HDL) fractions, and albumin and by a significant increase in that of urea, indicating increased protein breakdown. These results suggest that Aroclor 1254 increased hepatic lipid synthesis, but decreased hepatic production of albumin and apolipoproteins. This would explain the accumulation of triacylglycerol in the liver and the increase in the proportion of triacylglycerol to apolipoprotein in the total lipoproteins. From the evidence presented, a model is proposed based on the association of PCBs with hydrophobic domains of lipids and proteins for the transport of PCBs by plasma fractions, their uptake into cells and intracellular metabolism, and their accumulation in adipose tissue.

Long-term effects of commercial and congeneric polychlorinated biphenyls on ethane production and malondialdehyde levels, indicators of in vivo lipid peroxidation

Ethane exhalation was increased in male Sprague-Dawley rats following a single intraperitoneal (IP) injection of Aroclor 1254 (500 mg/kg). In the first 2 weeks following Aroclor 1254 treatment, the increase in ethane exhalation was due to an inhibition of metabolism of endogenous ethane rather than to an increase in ethane production. In weeks 3 and 4 following Aroclor 1254 administration, metabolic clearance of ethane returned to and exceeded control levels, while ethane production increased to approximately twice the control rates (day 30). The HPLC determination of in situ hepatic malondialdehyde levels revealed a 2-fold increase in malondialdehyde content on day 30 following the Aroclor 1254 injection. Further, parallel increases in in situ malondialdehyde levels and ethane production rates were also found 30 days following a single IP injection of 3,3',4,4'-tetrachlorobiphenyl, 2,3,4,4',5-pentachlorobiphenyl and 2,2',4,4',5,5'-hexachlorobiphenyl (300 mumol/kg). These effects were not reflected in increased diene conjugation. Redox state of the liver was largely unaffected, as evidenced by the relative concentrations of reduced and oxidized NADPH. However, minor changes in reduced and oxidized glutathione were noted.

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

Feeding a 0.05% polychlorinated biphenyl-supplemented diet to rats resulted in an increase of liver 3-hydroxy-3-methylglutaryl coenzyme A reductase activity within 9 days, followed by a decrease towards normal levels. Polychlorinated biphenyls were incorporated into the microsomal membrane. There was a concomitant decrease in the cholesterol/phospholipid ratio in the microsomal fraction. Immunotitration studies strongly suggested that polychlorinated biphenyls modulate preexisting 3-hydroxy-3-methylglutaryl coenzyme A reductase activity by changing the lipid environment of the enzyme. 32P-labeled cDNA probes were used to study the levels of mRNA coding for 3-hydroxy-3-methylglutaryl coenzyme A reductase during polychlorinated biphenyl feeding. Northern dot hybridization experiments showed an increase in the amount of this mRNA. This stimulation correlated with the increase in the activity of the enzyme but was more pronounced. The data suggest that polychlorinated biphenyls act on the regulation of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase activity by enzyme-lipid interaction and at the transcriptional level.