Biochemical and cytogenetic effects in rats caused by short-term ingestion of Aroclor 1254 or Firemaster BP6

Effects of Simultaneous Dietary Exposure to 2,2′,4,4′,5,5′-Hexabromobiphenyl and 3,3′,4,4′,5,5′-Hexachlorobiphenyl on Hepatic Tumor Promotion in Rats

Female Sprague-Dawley rats were partially hepatectomized, initiated with diethylnitrosamine (DEN), and fed diets containing 2,2′,4,4′,5,5′-hexabromobiphenyl (245-HBB), 3,3′,4,4′,5,5′-hexachlorobiphenyl (345-HCB), or combinations of 245-HBB and 345-HCB to determine the tumor-promoting ability of these compounds in a two-stage (initiation/promotion) hepatocar-cinogenesis system. Tumor-promoting ability was assessed by measuring hepatic foci positive for gamma glutamyl transpeptidase (GGT) activity. Concentrations of 10 or 100 mg 245-HBB/kg of diet caused significantly increased numbers of GGT-positive hepatic foci. When 245-HBB and 345-HCB were fed simultaneously, an additive effect on tumor-promoting ability was observed at dietary concentrations of 10 mg/kg 245-HBB and 0.1 mg/kg 345-HCB. However, an inhibitory effect on tumor promotion occurred when dietary concentrations of 100 mg/kg 245-HBB and 1.0 mg/kg 345-HCB were fed simultaneously. These results suggest that the tumor-promoting ability of simultaneous exposure to 245-HBB and 345-HCB can be additive or inhibitory depending upon the concentration of each congener in the diet.

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.

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.

Metabolite of 2,2',4',5-tetrabromobiphenyl, 3-methylsulphonyl-2,2',4',5-tetrabromobiphenyl, a potent inducer of CYP2B1/2 in rat

1. 3-Methylsulphonyl- and 4-methylsulphonyl-2,2',4',5-tetrabromobiphenyls (3-MeSO(2)- and 4-MeSO(2)-TetraBrBs) were detected in the liver, lung, kidney, adipose tissue and faeces of the 2,2',4',5-tetrabromobiphenyl (TetraBrB)-dosed rat. 2. The administration of 0.05-2.0 micromol kg(-1) doses of 3-MeSO(2)-TetraBrB produced corrresponding increases in the hepatic concentration of the methyl sulphone metabolite, corresponding increases in the content of total cytochrome P450, and corresponding increases in the activities of 7-benzyloxy-, 7-ethoxy- and 7-pentoxyresorufin O-dealkylases. The inducing effects of the 3-MeSO(2)-TetraBrB (0.2 micromol kg(-1)), both on the content of total P450 and on the activities of the three alkoxyresorufin O-dealkylases, were higher than that of the parent TetraBrB (342 micromol kg(-1)). 3. The major phenobarbital (PB)-inducible forms of P450, CYP2B1, CYP2B2, CYP3A2 and CYP2C6, were substantially induced by 3-MeSO(2)-TetraBrB, but CYP1A1 and CYP1A2 were not. On the other hand, the activities of drug-metabolizing enzymes and the four PB-inducible forms of P450 were unchanged by 4-MeSO(2)-TetraBrB treatment. 4. The induction profiles of these enzymes and P450 forms in rat treated with 3-MeSO(2)-TetraBrB were similar to those treated with PB. 5. The inducing ability of 3-MeSO(2)-TetraBrB (0.5 micromol kg(-1)) both on the activities of the three alkoxyresorufin O-dealkylases and on the contents of four PB-inducible forms of P450 was roughly equal to that of PB (431 micromol kg(-1) twice at a 24-h interval) or 3-MeSO(2)-2,2',4',5-tetrachlorobiphenyl (1 micromol kg(-1)). It is noteworthy that the effects of 3-MeSO(2)-TetraBrB on the drug-metabolizing enzymes CYP2B1 and CYP2B2 were several thousand-fold higher than those of parent TetraBrB, while the effect of its isomeric 4-MeSO(2)-TetraBrB were not. 6. The extent of hepatic accumulation of the 3-MeSO(2) metabolite after the administration of TetraBrB (342 micromol kg(-1)) was almost the same as that after the administration of 3-MeSO(2)-TetraBrB (0.1-0.2 micromol kg(-1)). The relationship between the hepatic concentration of the 3-MeSO(2) metabolite and the extent of enzyme induction after the administration of TetraBrB or 3-MeSO(2)-TetraBrB suggests that 3-MeSO(2)-TetraBrB plays an important role in the induction of microsomal drug-metabolizing enzymes by TetraBrB.

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.

The genetic toxicology of putative nongenotoxic carcinogens

This report examines a group of putative nongenotoxic carcinogens that have been cited in the published literature. Using short-term test data from the U.S. Environmental Protection Agency/International Agency for Research on Cancer genetic activity profile (EPA/IARC GAP) database we have classified these agents on the basis of their mutagenicity emphasizing three genetic endpoints: gene mutation, chromosomal aberration and aneuploidy. On the basis of results of short-term tests for these effects, we have defined criteria for evidence of mutagenicity (and nonmutagenicity) and have applied these criteria in classifying the group of putative nongenotoxic carcinogens. The results from this evaluation based on the EPA/IARC GAP database are presented along with a summary of the short-term test data for each chemical and the relevant carcinogenicity results from the NTP, Gene-Tox and IARC databases. The data clearly demonstrate that many of the putative nongenotoxic carcinogens that have been adequately tested in short-term bioassays induce gene or chromosomal mutations or aneuploidy.

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.

Interaction with microsomal lipid as a major factor responsible for S9-mediated inhibition of 1,8-dinitropyrene mutagenicity

1,8-Dinitropyrene (1,8-DNP), present in polluted air, is a rodent carcinogen and a potent, direct-acting mutagen in salmonella typhimurium TA98. This mutagenicity is markedly reduced in the presence of mammalian hepatic S9 or microsomes. We demonstrate that at least a substantial part of this effect is attributable to non-enzymatic processes. The microsomal-dependent inhibition was unaffected by omission of an NADPH-generating system or when the cytochrome P-450 inhibitor, SKF-525A, or the cytochrome P-448 inhibitor, ellipticine, was incorporated in the metabolic activation system, suggesting that mixed function oxidases are not involved. Heat inactivation partially decreased the ability of induced S9 to reduce DNP mutagenicity. Substitution of S9 with a similar concentration of bovine serum albumin did not affect DNP activity. Thus non-specific binding to microsomal protein is not involved. However, when lipids, derived from uninduced microsomes, were added to incubations of DNP and S. typhimurium TA98, mutagenicity was decreased. Furthermore, substitution of microsomal lipids with a suspension of phosphatidylcholine (PC), a major lipid constituent of microsomes, affected DNP mutagenicity similarly. An increase in PC concentration resulted in a greater inhibitory effect. The reduction in DNP mutagenicity observed with microsomal lipids or with PC was less than that detected with uninduced S9, whilst the mutagenicity of 2-nitrofluorene was reduced to an approximately equal extent by lipids and S9. This phenomenon may be responsible for the S9-mediated detoxification of other mutagenic nitroaromatic compounds and may have important implications for mutagenicity testing.

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.

Aroclor 1254 treatment and fasting influences on rat liver mitochondrial carbamoyl phosphate synthesis with ADP and ATP

Previous studies have shown that the polychlorinated biphenyl mixture, Aroclor 1254 (ARO), -induced wasting in male rats is associated with increased permeability of hepatic mitochondria. This was correlated with hyperuremia and stimulated urea synthesis, hypoglycemia and suppressed glucogenesis after an ammonium acetate injection, and decreased retention of assimilated nitrogen and food intake. For ARO-toxic rats (100 mg/kg, ip, for 1, 2, and 4 days) versus Tween 80-treated, ad libitum-fed controls, mitochondrial carbamoyl phosphate (CP) formation (the initial step in urea synthesis from NH4+) was progressively stimulated for the duration of treatment from NH4+ and ATP but not from NH4+ and ADP. ARO maximal stimulation of CP formation also correlated with significant loss in body weight. Mitochondrial ornithine transcarbamoylase synthesis of citrulline from ornithine and carbamoyl phosphate was also stimulated. In comparison to fasted rats (24 hr), mitochondrial CP synthesis from NH4+ was enhanced with ADP but not with ATP. This ARO uncoupling of mitochondrial NH4+ metabolism and stimulation of CP formation with exogenous ATP and citrulline synthesis may have resulted from increased availability of substrates and cofactors in the matrix space, leakage of enzymes from the matrix, or a combination of these effects. These results are consistent with an increased inner membrane permeability and fragility during isolation and assays. In agreement with our previous studies, the data show that ARO exposure poises hepatic mitochondria toward the synthesis of urea intermediates.

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.

Permeability changes in hepatic mitochondria and altered glucose and urea metabolism in aroclor 1254-treated rats

The influence of Aroclor 1254 (ARO) treatment or pair-feeding (PF) on gluconeogenesis and urea synthesis and on isolated hepatic mitochondria was studied in rats of different ages. ARO (300 mg/kg, po on 4 consecutive days) induced variable weight loss in young (153 +/- 10 g (initial wt), -10.9%), intermediate-age (195 +/- 10 g, -17.0%), and old (232 +/- 23 g, -4.9%) rats. Isolated mitochondria contained equal amounts of cytochromes aa3, b, c1 and c with exception that c1 and c were lower in the young ARO rats than in the PF controls. Mitochondria from ARO rats, which lost more weight than ad libitum-fed (AF) rats, showed suppression of ADP-stimulated H+ and oxygen uptake and succinate plus valinomycin maximal swelling in a potassium acetate and sucrose medium. Mitochondria from young ARO rats absorbed less incident light than mitochondria from PF or AF rats. Maximally swollen mitochondria from intermediate-age ARO rats, contracted more rapidly with antimycin addition than those from PF or AF controls. These findings showed greater permeability of ARO mitochondria to impermeable and accumulated ions. In contrast, mitochondria from ARO rats without significant weight loss showed activation of ADP-stimulated H+ and oxygen uptake and maximal swelling in comparison to mitochondria from AF and PF rats, but contracted like these controls after the antimycin addition. Urea synthesis in ARO rats, which lost 9.9% of initial body wt (173 +/- 6 g) and experienced a nitrogen deficit (Ebner et al., 1986), was significantly increased 12 min postinjection of NH4 acetate, and was greater than the urea level in AF rats at this time point. In comparison, gluconeogenesis was significantly increased in AF rats 12 min postinjection of NH4 acetate and was greater than in ARO rats at this time point. These differences were also observed when the data were expressed as the rate of glucose or urea appearance in peripheral blood per 100 g body wt. In PF rats, blood glucose and urea concentrations were intermediate to and indistinguishable from the ARO and AF groups. These data demonstrate that hepatic mitochondria from ARO rats that experienced a significant loss of body weight were suppressed and more permeable to ions than AF and PF controls. These mitochondrial properties may have predisposed the ARO rats toward urea formation rather than glucose synthesis and nitrogen retention.

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.

Effects of PCBs and related compounds on hepatocarcinogenesis in rats and mice

Commercial mixtures of polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs) can cause hepatocellular carcinoma in rats and mice. Present evidence indicates that these chemicals act as promoters and not initiators of hepatocarcinogenesis. Our results show that Firemaster BP-6 (FM) and its nontoxic major congener, 2,2', 4,4', 5,5'-hexabromobiphenyl (HBB), act as promoters in the two-stage model for hepatocarcinogenesis devised by Pitot and associates. A toxic congener, 3,3', 4,4', 5,5'-HBB, also was assessed for tumor-promoting activity. This congener, though not in FM, is similar to TCDD, in that both cause 3-methylcholanthrene (MC)-type induction of hepatic microsomal enzymes and produce similar toxic responses. FM contains several congeners which are mixed-type inducers in that they induce MC-type and phenobarbital (PB)-type enzymes. The toxicity of FM is most likely associated with its congeners which are mixed-type inducers and not to relatively nontoxic congeners such as 2,2', 4,4', 5,5'-HBB which are strictly PB-type inducers. Congener 3,3', 4,4', 5,5'-HBB acted as a tumor promoter only at a dose that was hepatotoxic. A synergistic effect on tumor promoting ability was produced by combining a nontoxic and nonpromoting dose of 3,3', 4,4', 5,5'-HBB with a promoting dose of 2,2', 4,4', 5,5'-HBB. Our results suggest that synergism between toxic and nontoxic congeners in FM may explain why mixtures such as FM have greater promoting ability than individual congeners. Our results also indicate that with PBB, toxicity and carcinogenicity are not necessarily related.

Mixed-function oxidase system induction and propylene hepatotoxicity

Propylene is hepatotoxic to male Charles River COBS Sprague-Dawley rats pretreated with polychlorinated biphenyls (PCB: Aroclor 1254). Four-hour inhalation exposure to 50,000 ppm propylene increased liver weight/body weight ratios and elevated serum enzyme activities in PCB-pretreated animals. Hepatic microsomal cytochrome P-450 content of PCB-pretreated rats dropped profoundly during propylene exposure and remained depressed for at least 24 h. In addition, PCB-pretreated, propylene-exposed rats exhibited a decrease in the specific activity of hepatic microsomal aniline hydroxylase. However, there was no change in activities of either hepatic microsomal aminopyrine demethylase or glucose-6-phosphatase. Propylene exposure of rats pretreated with beta-naphthoflavone (BNF), phenobarbital (PB), or a mixture of BNF and PB was not hepatotoxic. However, there was, in these animals, a substantial decline in hepatic microsomal cytochrome P-450 levels 24 h after the start of propylene exposure. Hence, the propylene-dependent process resulting in hepatic cytochrome P-450 destruction is qualitatively or quantitatively different from the process that causes acute hepatotoxicity. Preexposure fasting had no effect on the hepatotoxicity resulting from a 4-h exposure of PCB-pretreated rats to 50,000 ppm propylene. Administration of SKF-525A to PCB-pretreated rats immediately prior to propylene exposure completely prevented elevations in serum enzyme activities and liver weight/body weight ratios. In vitro incubation of hepatic microsomes prepared from either BNF-, PB-, or PCB-pretreated rats with an atmosphere of 20% propylene/80% air produced in NADPH-dependent decrease in cytochrome P-450 content. These results suggest that PCB pretreatment is a prerequisite for propylene hepatotoxicity in the rat. Cytochrome P-450-dependent bioactivation of propylene is associated with this hepatotoxicity, but further studies are needed to characterize the mechanism of the PCB-propylene interaction.

The PBB episode in Michigan: an overall appraisal

Polybrominated biphenyls (PBB) were used as a fire retardant. In common with other halogenated hydrocarbons, PBBs are lipophilic and resistant to chemical and metabolic degradation. Cattle on about 25 Michigan farms were exposed to as much as 250 g per head of PBB when it was accidentally mixed in cattle feed in 1973 to 1974. Livestock exposures several orders of magnitude lower occurred on several hundred other farms because of carryover and equipment contamination in feed mills. Approximately 85% of the Michigan population received some exposure to PBB because dairy product marketing involves mixing milk from many farms. A few cases of high human exposure, which may have been as great as 10 g, occurred when residents of the more highly exposed farms consumed their own products. Although numerous clinical signs and pathological changes were reported in exposed cattle, only anorexia, lacrimation, emaciation, hyperkeratosis, and kidney damage were confirmed in controlled studies. The acute toxicity of PBB in laboratory animals is low, but a variety of subacute effects have been reported. Induction of microsomal enzymes, enlargement and histopathological changes of the liver, fetotoxicity, and immunosuppression are among the more significant. Epidemiological studies of exposed humans have revealed no pattern of clinical signs or symptoms that were related to PBB exposure. A complete evaluation of the human consequences of exposure to PBB await the conclusion of long-term epidemiological studies.

Aflatoxin B1 metabolism in the rat: polyhalogenated biphenyl enhanced conversion to aflatoxin M1

The effects of polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs) on the formation in vitro of aflatoxin Q1 and aflatoxin M1 from aflatoxin B1 by rat-liver microsomes were investigated. AFB1 metabolism by hepatic microsomes from PBB- and PCB-treated rats resulted in 16- and 30-fold increases, respectively, in levels of aflatoxin M1. The enhanced formation of aflatoxin M1 did not correlate with PBB and PCB stimulation of benzo[a]pyrene hydroxylase (AHH) activity. Studies in vivo clearly demonstrated enhanced secretion of aflatoxin M1 by female lactating rats with prior exposure to PCBs. PCB pretreatment enhanced the activity of mammary as well as hepatic tissue microsomal preparations in converting aflatoxin B1 to aflatoxin M1. Our findings indicate that PCB exposure increases the production of aflatoxin M1 in vitro and also increases the levels of aflatoxin M1 released into the milk.

Polybrominated biphenyls are nongenotoxic and produce an epigenetic membrane effect in cultured liver cells

Polybrominated biphenyls (PBB) were studied for their genotoxic and epigenetic effects in cultured liver cells. PBB did not elicit DNA repair synthesis in rat, mouse, or hamster hepatocytes in primary cultures and did not cause mutations at the hypoxanthine-guanine phosphoribosyl transferase locus in a line of rat liver epithelial cells or in human fibroblasts cocultivated with rat hepatocytes as an activating system. In contrast, PBB inhibited intercellular molecular exchange between rat hepatocytes and liver epithelial cells indicating an epigenetic membrane effect. These data are consistent with the interpretation that PBB act as neoplasm promoters in the production of rodent liver neoplasms.

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.

Agents affecting the male reproductive system: effects of structure on activity

This paper places into perspective the available data on agents inducing male reproductive toxicity, structural correlates that can be drawn from these data, and a scenario of action to resolve potential concerns.

Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs): biochemistry, toxicology, and mechanism of action

Polychlorinated and polybrominated biphenyls are industrial chemical mixtures which have been implicated in numerous human poisonings in Taiwan and Japan (PCBs) and Michigan (PBBs). Moreover, these polyhalogenated biphenyls have been widely detected in the environment including the air, water, fish, wildlife, human adipose tissue, and blood and breast milk. A major problem associated with the analysis and toxicology of this group of chemicals is their chemical complexity (e.g., there are 209 possible PCB isomers and congeners) and the remarkable effects of structure on activity. This article will discuss the effects of structure on the biologic and toxic effects of individual PCB and PBB congeners as well as reconstituted mixtures. The results clearly show that like "dioxin" (or 2,3,7,8-TCDD), the PCBs and PBBs elicit their effects through a cytosolic receptor protein which preferentially binds with the toxins which are approximate isostereomers of 2,3,7,8-TCDD. The evidence for this mechanism of action will be discussed in detail.

The comparative biologic and toxic potencies of polychlorinated biphenyls and polybrominated biphenyls

Aroclor 1254 and fireMaster BP-6, two commercial polychlorinated biphenyl (PCB) and polybrominated biphenyl (PBB) preparations, exhibit comparable biologic and toxic effects. In the present study the commercial PBB was more active than Aroclor 1254 in causing thymic atrophy in male Wistar rats. However, a direct comparison of the relative effects of bromine vs chlorine substituents is not possible with the commercial PBB and PCB mixtures due to their complex congeneric composition. This study reports the synthesis and biologic and toxic effects of a series of laterally substituted 3,3',4,4'-tetrahalobiphenyls which contain the following variable molecular Cl/Br ratios; Br4, Br3Cl, Br2Cl2 (two isomers), BrCl3, and Cl4. 3,3',4,4'-Tetrabromobiphenyl and 3,4,4'-tribromo-3'-chlorobiphenyl (150 mumol/kg)-pretreated animals significantly inhibited the growth rate of and caused thymic atrophy in immature male Wistar rats whereas those isostereomers with reduced Br (and increased Cl) content were either less active or inactive. Pretreatment of male Wistar rats with 10 mumol/kg of the 3,3',4,4'-tetrahalobiphenyls and determination of their effects as inducers of the hepatic microsomal drug-metabolizing enzymes also illustrated the effects of the relative Cl/Br ratios on induction potencies. Both 3,3',4,4'-tetrabromo- and 3,4,4'-tribromo-3'-chlorobiphenyl maximally induced the cytochrome P-448-dependent monooxygenases, benzo[a]pyrene and 4-chlorobiphenyl hydroxylase; the order of potency of the other isostereomers was 4,4'-dibromo-3,3'-dichloro- congruent to 3,4-dibromo-3',4'-dichlorobiphenyl greater than 4-bromo-3,3',4'-trichloro- greater than 3,3',4,4'-tetrachlorobiphenyl. With few exceptions this order of potency was observed for the induction of benzo[a]pyrene hydroxylase and ethoxyresorufin O-deethylase in rat hepatoma cells in culture and for their relative binding affinities to the rat cytosolic receptor protein. The data clearly demonstrate that the biologic activities of this group of isosteric halogenated biphenyls are enhanced with increasing bromine substitution and also support the hypothesis that the activities of this class of chemicals are mediated through the receptor.

Effect of postnatal exposure to polychlorinated biphenyls on adult male reproductive function

Polychlorinated biphenyls (PCBs) have been found widely distributed in the ecosystem, including human milk, yet there is limited information concerning the effect that PCB exposure during the postnatal period might have on subsequent reproductive capacity of adult male offspring. In this study, fertility and condition of reproductive organs were examined in male offspring of rats that received Aroclor 1254 orally (8, 32, or 64 mg/kg) on Days 1-3, 5, 7, and 9 of lactation. It was found that the experimental male offspring as adults were less successful in mating and reproducing when compared with control offspring. Virgin females mated with males exposed through suckling to the two higher doses of PCBs had a significantly lower proportion of ovulated eggs that implanted, a significantly lower number of live fetuses, and a significantly higher rate of resorption. At autopsy, although body weights were not significantly different, ventral prostate weights were significantly less in all treated male offspring; there were fewer alveoli and flattened epithelial cells in these glands as compared to controls. Seminal vesicle weights were significantly less in males exposed to the two higher doses of PCBs, while the testes were significantly larger. PCB-derived material remained in adipose tissue and liver at the time of autopsy. These data support the hypothesis that exposure of male offspring to PCBs during early postnatal development results in an hypoandrogenic condition that is detrimental to normal reproductive functioning in the adult.

Long-term effects of a single oral dose of polybrominated biphenyls on serum and liver lipids in rats

Adult (5 months) male Sherman strain rats received a single dose of either 0 or 500 mg polybrominated biphenyls (PBB) in corn oil/kg body weight by stomach tube. After an 18-month recovery period, serum and liver samples were examined. The primary serum lipid response was an increase in cholesterol (both free and esterified) and in total phospholipids. The percentage of esterified cholesterol was not significantly different from that of the controls, and no significant differences in the cholesterol ester fatty acid composition were observed. Serum triglycerides were also unaffected. In the PBB-dosed animals, the total hepatic fatty acids contained significantly less palmitic acid and more stearic acid, consistent with an increase in palmitic acid chain elongation activity. No significant differences could be detected in the n-3 or n-6 acids except for a slight decline in the content of 22:6 (n-3). Hepatic microsomal phospholipids were slightly higher (per milligram protein) in the PBB-dosed animals, and the cholesterol content was lower. Consequently, the cholesterol-phospholipid ratio was reduced, and microsomes from the latter group appeared to have an altered lipid domain on the basis of steady-state fluorescence anisotrophy measurements. In addition, total hepatic thiobarbituric acid-reactive substances (assayed as malondialdehyde) were significantly increased in the PBB-dosed animals. This observation appeared to reflect an increased susceptibility to peroxidative stress in the latter group, probably resulting from reduced membrane antioxidant concentrations. The PBB-dosed rats had significantly lower serum retinol levels and a reduced content of this vitamin in liver microsomes. Microsomes were also deficient in alpha-tocopherol in the PBB-dosed animals, although serum levels were normal.

Cardiac and hepatic effects of pre- and postnatal exposure to polybrominated biphenyls in rats

Body weight gain and hepatic concentrations of vitamin A were reduced in Sprague-Dawley rats by pre- and postnatal exposure to 100 ppm polybrominated biphenyls (PBBs). The ratio of liver weight to body weight, activity of hepatic delta-aminolevulinic acid (ALA) synthetase, and urinary excretion of uro- and coproporphyrins were increased by PBBs. Treatment with PBBs also increased the left atrial inotropic response to calcium. However, PBBs had no effect on development of the adrenergic neuronal transport system in heart, left atrial baselike peak tension, or inotropic response to ouabain. Thus PBBs retarded body weight gain and produced a variety of alterations in liver, but had little effect on cardiac contractile function.

Nonrespiratory metabolic function and morphology of lung following exposure to polybrominated biphenyls in rats

Exposure to polybrominated biphenyls (PBBs) resulted in increased activity of microsomal arylhydrocarbon hydroxylase and ethoxyresorufin-O-deethylase in rat lung. Clearance of 5-hydroxytryptamine (5-HT) and angiotensin 1 by perfused lungs was decreased by PBBs. However, PBBs had no effect on the activity of epoxide hydrolase, monoamine oxidase, or angiotensin-converting enzyme in lung. The only histopathologic change detected in lungs from PBB-treated rats was an increase in alveolar type II cell lamellar bodies. Selective accumulation of certain PBB congeners by lung was not observed in this investigation.