Effects of inducers of drug metabolism enzymes on triglyceride and phospholipid biosynthesis

The toxicological effects of halogenated naphthalenes: a review of aryl hydrocarbon receptor-mediated (dioxin-like) relative potency factors

There is no doubt that chloronaphthalenes (PCNs) and their brominated counterparts (PBNs) are dioxin-like compounds, but there is less evidence for mixed bromo/chloronaphthalenes (PXNs). In this article we review information relating to the dioxin-like potency of PCNs and PBNs obtained in vivo, in vitro, and in silico. The aim was to help and improve the quality of data when assessing the contribution of these compounds in the risk analysis of dioxin-like contaminants in foods and other sample types. In vivo and in vitro studies have demonstrated that PCN/PBN congeners are inducers of aryl hydrocarbon hydroxylase, ethoxyresorufin O-deethylase, and luciferase enzymes that are features specifically indicative of planar diaromatic halogenated hydrocarbons such as dioxin and dioxin-like compounds. PCNs in the environment are of multisource origin. The limited data on PBNs in the environment suggest that these also appear to originate from different sources. The toxicological data on these compounds is even scarcer, most of it directed toward explaining the exposure risk from accidental contamination of feed with the commercial PBN containing product, Firemaster BP-6. The occurrence of PBNs and PXNs is possible as ultra-trace environmental and food-chain contaminants produced at least from combustion processes at unknown concentrations. Available toxicological and environmental data enable a focus on PCNs as dioxin analogues to an extent that specific local or regional environmental influences could result in a risk to human health. There is the possibility that they may act synergistically with the better-known classic dioxin and other dioxin-like compounds. PBNs and PXNs are much less studied than the dioxins, but are known to be products of anthropogenic processes that contaminate the environment. A continuously increasing use of bromine for manufacture of brominated flame retardants over the past three decades is anticipated as a stream of "brominated" wastes, that when degraded (combusted), will release PBNs and PXNs. This calls for advanced analytical methods and greater interest toxicologically to understand and control pollution and exposure by PBNs and PXNs. Particular congeners of bromonaphthalene in single studies were found to be much more toxic than their chlorinated counterparts. In addition, brominated/chlorinated naphthalenes also seem to be more potent toxicants than PCNs. About 20% of PCN congeners exhibit a dioxin-like toxicity with relative potencies varying between around 0.003 and 0.000001, but additional and more rigorous data are needed to confirm these figures. Recent food surveys have estimated a small but relevant human exposure to these compounds in foods, giving an additional source of dioxin-like toxicity to those compounds already covered by the World Health Organization-Toxic Equivalency Factors (TEFs) scheme. Given the additivity of response postulated for other dioxin-like compounds, it would seem unwise to ignore this additional contribution. Few data available showed that PBN congeners also exhibit a dioxin-like toxicity and are even more potent than PCN congeners, but the relative potency values were not derived for them until now. There are no toxicological data available for PXNs.

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.

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.

Carcinogenicity of polyhalogenated biphenyls: PCBs and PBBs

Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs) are compounds whose physical/chemical properties led to their widespread commercial use. Although their production has been banned or severely limited in most countries since the 1970s, the persistence and stability of these compounds have resulted in a worldwide distribution, especially of PCBs. PBB contamination is limited principally to the state of Michigan, where a series of tragic errors eventually resulted in the accumulation of residues in livestock and the general human population. Long-term exposure to PCBs and PBBs in animals has been associated with the induction of neoplastic nodules in the liver and in some cases hepatocellular carcinoma. This review discusses the role of PCBs and PBBs in the process of carcinogenesis. The mutagenicity/genotoxicity of these compounds, as well as their initiation/promotion potential is discussed. The epidemiology of PCB and PBB exposure is reported along with an estimation of the risk of cancer to humans. Finally, possible molecular mechanisms of action are suggested for polyhalogenated biphenyls in cancer development.

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.

Studies of transformer repair workers exposed to PCBs: II. Results of clinical laboratory investigations

Thirty-eight transformer repairmen currently exposed to polychlorinated biphenyls (PCBs), 17 former transformer repairmen, and 56 comparison workers not known to be exposed to PCBs were studied. Measurements were made of serum liver function tests, gamma-glutamyl transpeptidase (GGT), lipid profile, thyroid function tests, and other serum biochemistry; hemoglobin; white cell count; 24-hour excretion of delta-aminolevulinic acid, porphyrins, 17-hydroxycorticosteriods and 17-ketosteroids; sperm count; spirometry; and antipyrine half-life to evaluate microsomal mixed function oxidase induction. The total exposed group differed significantly from the comparison group in albumin, LDH, T4, T4-RT3 index, and actual/predicted FEV1. Significant differences among all three exposure groups were seen for albumin, T4, T4-RT3 index, and 17-hydroxycorticosteroid excretion. Differences in FEV1 were attributable to smoking. Significant correlations between serum PCBs and serum lipids were removed by adjustment for confounding variables. After adjustment for confounding variables, there was a statistically significant positive correlation between serum PCBs and GGT and a negative correlation between adipose PCBs and 17-hydroxycorticosteroid excretion. These may reflect subtle metabolic effects of PCBs.

Accumulation of a commercial polychlorinated biphenyl mixture (Aroclor 1016) in adult rhesus monkeys and their nursing infants

Female rhesus monkeys (24) were divided into 3 groups and fed diets calculated to contain 1.0, 0.25 or 0 parts per million (ppm) of a commercial mixture of polychlorinated biphenyls (PCBs), Aroclor 1016. The animals consumed a calculated total of 18.1 +/- 3.1, 4.5 +/- 0.6 and 0 mg Aroclor 1016/kg of body weight over the 87 +/- 9 week experimental period. All animals were bred, conceived and experienced uncomplicated pregnancies. The birth weights of the infants born to females receiving the 1.0 ppm PCB diets were significantly less than those of the control infants. There was a positive relationship between the levels of Aroclor 1016 in the tissues and the dose administered in the diets. Aroclor 1016 content of infant tissues was consistent with maternal adipose tissue PCB levels. The content of Aroclor 1016 in the milk fat of the animals more closely approximated that in the individual's adipose tissue than in the serum. At weaning, the mesenteric fat of the experimental infants contained concentrations of Aroclor 1016 which were 4-7 times that found in the mothers. Analyses of the tissues and fluids of the experimental animals demonstrated accumulation of PCB isomerides. The number of isomerides present and the ratio of those detected differed between adult and infants as well as between milk and serum. After weaning, the pattern of Aroclor 1016 isomerides present in the infants' adipose was similar to that observed at weaning although the concentrations had decreased.

Acute response of rat liver microsomal lipids, lipid peroxidation, and membrane anisotropy to a single oral dose of polybrominated biphenyls

Liver microsomal lipids and lipid peroxidation activities were examined in adult male rats at intervals over a 2-mo period after the administration of a single oral dose of 0 or 500 mg/kg of FireMaster BP-6 in corn oil. Microsomal lipids were markedly altered in the polybrominated biphenyl- (PBB-) dosed animals at the earliest time examined (1 wk), and these changes persisted throughout the remainder of the study. An early decrease in the cholesterol-phospholipid ratio was noted, which probably contributed to the significant decrease in the steady-state fluorescence anisotropy demonstrable in both intact microsomes and in liposomes prepared from microsomal lipid extracts. Significant concentrations of PBBs were present in dosed rat microsomes, but the changes in anisotropy appeared to result from membrane lipid alterations rather than from a direct perturbation by PBBs. Iron ascorbate-induced peroxidation was also greatly enhanced in dosed rat microsomes, even when rats were maintained on a low-iron (25 ppm) diet. These early alterations in membrane fluidity and peroxidative capacity of microsomes may ultimately contribute to the hepatotoxicity of PBBs.

Hexabromonaphthalene contaminants of polybrominated biphenyls: chemical composition and disposition in the rat

Hexabromonaphthalene (HBN) has been identified as a toxic contaminant of Firemaster BP-6, a mixture of polybrominated biphenyls (PBBs) that is used commercially as a fire retardant and that was responsible for a major public health emergency in Michigan in 1974. Previously reported to be a single compound, the hexabrominated naphthalene derived from direct bromination of naphthalene was here shown by high-field nuclear magnetic resonance (NMR) to be a mixture of two closely related isomers, 1,2,3,4,6,7-HBN and 2,3,4,5,6,7-HBN, in a ratio of 60:40. The absorption, distribution, metabolism, and excretion of the HBN mixture was studied in the male Fischer-344 rat. [14C] HBN was incompletely absorbed after oral doses of 0.4 and 4.0 mumol/kg body weight. After iv treatment, fecal excretion accounted for 24% of the total dose by the end of d 1, 44% by d 3, and 62% by d 35. Urinary excretion was negligible. The excreted radioactivity was in the form of metabolite(s). Biliary excretion studies confirmed the nature of the excreted dose. The tissue distribution pattern showed accumulation and redistribution during early time points (0.5-3 h). By the end of the first day, 30.7% and 12.3% of the total dose remained in the liver and fat, respectively. All other tissues accounted for less than 5% of the total dose by d 1. The major tissue depots remained liver and adipose tissue, which contained 26% and 4.6% of the total dose, respectively, by d 35. This residual radioactivity was found to be unmetabolized HBN. Thus, over 60% of the dose of HBN is readily metabolized and excreted and was tentatively identified with the toxic isomer 1,2,3,4,6,7-HBN. The remainder, probably 2,3,4,5,6,7-HBN, was relatively nontoxic and extremely persistent, with the liver being the primary site of long-term accumulation.