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

6-Methyl-1,3,8-trichlorodibenzofuran (MCDF) as a 2,3,7,8-tetrachlorodibenzo-p-dioxin antagonist in C57BL/6 mice

6-Methyl-1,3,8-trichlorodibenzofuran (MCDF), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and TCDD plus MCDF were administered to C57BL/6 mice and their effects on several aryl hydrocarbon (Ah) receptor-mediated responses including hepatic microsomal aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) induction, immunotoxicity and teratogenicity were determined. MCDF did not induce hepatic microsomal AHH and EROD at doses up to 500 mumol/kg, however, co-administration of MCDF (50 mumol/kg) with a dose of TCDD which elicited a submaximal induction response (i.e. ED80-100, 15 nmol/kg) resulted in some small but significant inhibition of the induction of hepatic microsomal AHH and EROD (14 and 17%, respectively) compared to that observed with TCDD alone. Co-administration of TCDD and other doses of MCDF (10, 100, 200 or 500 mumol/kg) did not effect the induction response. These results were in contrast to the effectiveness of MCDF as an antagonist of the induction of AHH and EROD by TCDD in the rat (up to 50% inhibition of monooxygenase induction). Administration of MCDF (4, 20 and 40 mumol/kg) to C57BL/6 mice caused some inhibition of the splenic plaque-forming cell response to sheep erythrocytes only at the highest dose (26% decrease); the interaction of MCDF (4, 20 and 40 mumol/kg) and an immunotoxic dose of TCDD (3.7 nmol/kg) resulted in significant protection from the immunotoxic effects of TCDD at the 2 higher dose levels of MCDF. Similarly, MCDF (400 mumol/kg) did not cause cleft palate in mice but at this dose level MCDF afforded some protection from TCDD (20 micrograms/kg)-mediated cleft palate in mice. However, studies utilizing [3H]TCDD suggested that the protective effects may be due to modulation of TCDD reaching the palate in the co-treated animals (MCDF plus TCDD). Although both MCDF and Aroclor 1254 were both weak Ah receptor agonists in C57BL/6 mice, the former compound was much less effective as a TCDD antagonist. The observed species-specific effects for these 2 TCDD antagonists may be related species-dependent differences in receptor structure and receptor-ligand (i.e. agonist or antagonist) interactions.

Toxicity of fenclor 42 in mice: effects on immunocompetent cells

The aim of this study is to evaluate the effects of Fenclor 42 (a mixture of trichlorobiphenyls) on the immune system. A prolonged administration of this compound to CD2F1 mice resulted in a reduction of relative spleen and thymus weight according to the dose. Furthermore, spleen weights, total number of splenocytes and relative spleen weights decreased significantly also following a single treatment with 0.5 g/kg or 1 g/kg of Fenclor 42. An analysis of the functional activity of splenocytes pointed out that proliferative response to mitogens was also inhibited. Splenic parameters returned to normal values within 5 days after a single treatment and between 8 and 15 days after a subchronic administration. The functional activity of splenocytes was restored between day +5 and day +8 according to the different schedules of treatment. On the contrary, natural killer cell (NK) activity was never affected by Fenclor 42. Studies are in progress to elucidate the intimate mechanism of the toxicity of Fenclor 42 on immunocompetent cells.

Immunotoxicity studies of PCB (Aroclor 1254) in the adult rhesus (Macaca mulatta) monkey--preliminary report

The effects of PCB (Aroclor 1254) on the immune system of adult female rhesus monkeys were investigated in a chronic study wherein five groups of monkeys (16/group) were administered (orally) PCB at levels of 0.0, 5.0, 20.0, 40.0 or 80.0 micrograms/kg body wt daily. Tests for immunotoxicity were initiated at 23 months of exposure to PCB, at which time the monkeys had achieved an apparent pharmacokinetic steady state based on the PCB concentration in fat and/or blood. A statistically significant (P less than 0.05) dose response reduction in antibody levels (IgG and IgM) to sheep red blood cells (SRBC) was observed following i.v. administration of three immunizing doses of SRBC at weekly intervals. A statistically significant decrease in the percent TH and an increase in the percent and absolute TS lymphocyte levels was found in the 80 micrograms/kg body wt group compared to the control. The TH/TS ratio was also significantly lower in the 80 micrograms/kg body wt group compared to the control. Other parameters investigated including percent of B-lymphocytes and total T-lymphocytes, total serum immunoglobulin levels (IgG, IgM and IgA), other serum proteins, glucocorticosteroid levels and lymphocyte transformation results following stimulation with the mitogens PHA-P and Con A were not affected significantly by PCB treatment. Additional immunologic parameters are currently being investigated to further elucidate the mechanism by which PCB induces immunotoxicity.

Persistency of highly toxic coplanar PCBs in aquatic ecosystems: uptake and release kinetics of coplanar PCBs in green-lipped mussels (Perna viridis Linnaeus)

The bioaccumulation potential of three highly toxic coplanar PCB isomers [3,3',4,4'-tetrachlorobiphenyl (T(4)CB); 3,3',4,4',5-pentachlorobiphenyl (P(5)CB); and 3,3',4,4',5,5'-hexachlorobiphenyl (H(6)CB)] was investigated using green-lipped mussels (Perna viridis Linnaeus) as a bioindicator, through a transplantation experiment at two locations in Hong Kong waters. By contrast to the relatively rapid uptake and release of many other PCB isomers, the non-ortho chlorine substituted coplanar PCB congeners exhibited slow uptake and clearance. The kinetic parameters of coplanar PCBs based on lipid weight-related data, and the degree of bioaccumulation based on the proportion of coplanar PCBs in total PCBs in mussels, clearly indicate that coplanar PCBs are highly bioaccumulative in lower organisms. On the assumption that mussels are unlikely to be particularly unusual with respect to their bioaccumulation of coplanar PCBs, it appears most likely that these highly toxic and persistent PCB congeners are concentrated by all aquatic organisms, and may reach higher consumers (including humans) in quantities of toxicological concern.

Polychlorinated biphenyls (PCBs): mutagenicity and carcinogenicity

The potential mutagenicity and carcinogenicity of commercial PCBs has been investigated in both in vivo and in vitro systems and several conclusions can be drawn from these studies. (1) PCBs can covalently adduct DNA both in vivo and in vitro (using a source of metabolic activation); the more highly chlorinated biphenyls are poorly metabolized and these compounds tend to exhibit very low binding to DNA. Based on the structure-activity relationships for PCBs (Safe, 1984) it is unlikely that the more toxic compounds such as 3,3',4,4',5-penta- and 3,3',4,4',5,5'-hexachlorobiphenyl, would form covalent adducts with DNA. (2) PCB mixtures and individual compounds exhibit minimal mutagenic activity in most assay systems. (3) The more highly chlorinated PCB mixtures (i.e. greater than 50% Cl by weight) are hepatocarcinogens in rodents whereas data from a limited number of studies suggest that the lower chlorinated mixtures are not carcinogenic. (4) In some model systems, the higher chlorinated PCB mixtures act as promoters of preneoplastic lesions and hepatocellular carcinomas in rodents treated with a variety of initiators. (5) Aroclor 1254 acts as a promoter of skin papilloma formation in HRS/J hairless mice and structure-activity and genetic studies suggest that the Ah receptor is necessary but not sufficient for the activity of halogenated aryl hydrocarbons as promoters in hairless mice. (6) Individual PCB congeners and higher chlorinated commercial mixtures also exhibit anti-carcinogenic activity in the CD-1 mouse skin cancer model. (7) Results from occupational studies suggest that individuals exposed to PCBs may have an excess of cancer at some sites, however, the most comprehensive study (Brown, 1987) suggests that there are no significant increases in the overall cancer rate in workers exposed to PCBs. Follow-up and continuing epidemiological studies on the PCB-exposed workers are required to further clarify the potential carcinogenic effects of PCBs on humans. In several strains of rats and mice, there is a high incidence of hepatic preneoplastic lesions and carcinomas and these lesions can be induced by diverse promoting agents (Schulte-Hermann et al., 1983; Weinstein, 1984). Since PCBs are not mutagenic and do not readily form covalent adducts with cellular DNA, it is likely that the higher chlorinated biphenyls are not genotoxic and act as promoters of carcinogenesis in rodents. A comparable mechanism has been suggested for 2,3,7,8-TCDD (Shu et al., 1987; Weinstein, 1984). For PCBs, the role of the Ah receptor in mediating their activity as promoters has not been delineated.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of rat liver microsomal cytochrome P-450 isozymes and epoxide hydrolase by a series of 4'-substituted-2,3,4,5-tetrachlorobiphenyls

We have shown previously that 2,3,4,5-tetrachlorobiphenyl is ineffective as an inducer of rat liver microsomal cytochrome P-450. Addition of a single para-chloro substituent in the otherwise unsubstituted phenyl ring, to give 2,3,4,4',5-pentachlorobiphenyl, produces a potent cytochrome P-450 inducer with both phenobarbital- and 3-methylcholanthrene-type characteristics. In the present study, 2,3,4,5-tetrachlorobiphenyl was substituted in the para(4') position with 12 other functional groups. The 4'-X-C12H5Cl4 derivatives were tested as inducers of cytochromes P-450a--P-450e and epoxide hydrolase, by immunochemical analysis of liver microsomes prepared 4 days after a single treatment (500 mumol/kg) of 1-month-old male Long Evans rats. When the para' substituent was a halogen (F, Cl, Br or I), the derivative induced both cytochromes P-450b and P-450e, and cytochromes P-450c and P-450d, which are the major phenobarbital- and 3-methylcholanthrene-inducible isozymes, respectively. A similar type of induction was observed with a second group of derivatives substituted with CN, NO2 or CF3. However, a derivative containing CH3CO--(which is also a meta-directing, ring-activating substituent) failed to induce cytochromes P-450a-P-450e at the dosage and time tested. Members of a third group of derivatives, which contained an ortho/para-directing, ring-activating substituent) were either ineffective inducers (OH, CH3, CH3O--), or were inducers of cytochromes P-450c and P-450d (isopropyl or t-butyl). Hence, 4'-substitution with a bulky lipophilic substituent conferred 3-methylcholanthrene- but not phenobarbital-type characteristics on 2,3,4,5-tetrachlorobiphenyl. Some of the derivatives tested, namely those substituted with Cl, Br, I and CF3, were remarkably effective inducers of cytochrome P-450a, causing a 10-11-fold induction of this isozyme. Data on the induction of cytochrome P-450c were analyzed by multiparameter linear regression in an attempt to correlate the biological activity of the 4'-X-C12H5Cl4 derivatives with the physiochemical properties of the various substituents. From these results, and those reported recently, we propose that binding of the 4'-X-C12H5Cl4 derivatives to the rat cytosolic Ah receptor is favored by increasing the electronegativity, lipophilicity and hydrogen bonding characteristics of the 4' substituent, whereas enzyme induction (both in vivo and in cultured rat hepatoma cells) is also governed by a fourth characteristic, the STERIMOL factor, which gives a measure of the width of the substituent.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetic models for association of 2,3,7,8-tetrachlorodibenzo-p-dioxin with the Ah receptor

Saturation binding studies of the interaction between 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and the Ah receptor obtained from the hepatic cytosol of Wistar rats have been carried out. The conventional Scatchard analysis for determination of the equilibrium constant for ligand-receptor binding has been shown to be inappropriate due to thermal inactivation of the unoccupied receptor. Simulation models of the receptor-ligand binding kinetics which take into account receptor degradation have been developed and the results are consistent with two alternative kinetic models. In Model 1, reversible 2,3,7,8-TCDD-receptor binding occurs in parallel with inactivation of the unbound receptor; analysis of the observed data using this model suggests that the previously determined equilibrium constants (Kass) for association of the ligand with the receptor are orders of magnitude too low and the total initial receptor concentrations are somewhat underestimated. In Model 2, the unbound receptor is converted unimolecularly to an activated state which then undergoes competitive degradation or entrapment by ligand. Experiments have been carried out over the temperature range 4-37 degrees C, enabling activation parameters to be obtained. According to Scheme 1, the activation enthalpies for association of receptor with ligand and for thermal inactivation of the unoccupied receptor are high, and numerically almost identical (delta H++ ca 125 kJ mol-1). These reactions are strongly entropically driven and this is consistent with association being accompanied by a conformational change in the receptor protein, and the previously postulated binding of the ligand to a hydrophobic pocket. According to Scheme 2, there is only one enthalpy of activation because both inactivation and entrapment by 2,3,7,8-TCDD are fast processes which follow the same slow activation step. On the basis of this latter model, a 10(-9) M concentration of 2,3,7,8-TCDD is sufficient to trap roughly two-thirds of the activated receptors.

Delayed spatial alternation deficits resulting from perinatal PCB exposure in monkeys

Monkeys exposed to low, chronic levels of polychlorinated biphenyls (PCBs) in utero and during nursing until 4 months after birth were tested at 4-6 years of age on delayed spatial alternation (DSA), a spatial learning and memory task. Deficits in performance accuracy were detected in two cohorts of monkeys whose mothers had been fed 2.5 ppm of the PCB mixture, Aroclor 1248, in their diet for an 18-month period ending at least 12 months prior to pregnancy. The deficit was most apparent at the shorter delays, suggesting that it was not due to memory impairment, but may have been due to impairments in associational or attentional processes. There may also have been a deficit in a group of monkeys whose mothers were fed 1.0 ppm of the PCB mixture, Aroclor 1016. However, the deficit in this group was less pronounced than in the other groups. The appearance of a PCB-induced cognitive deficit more than 3 years after the end of exposure indicated the existence of very long-term adverse consequences of low-level perinatal PCB exposure.

Toxicity and 7-ethoxyresorufin O-deethylase-inducing potency of coplanar polychlorinated biphenyls (PCBs) in chick embryos

The toxicities of the coplanar polychlorinated biphenyls 3,3',4,4'-tetrachlorobiphenyl (TCB), 3,3',4,4',5-pentachlorobiphenyl (PeCB) and 3,3',4,4',5,5'-hexachlorobiphenyl (HCB) were compared in a 72-h study on chick embryos. The substances were injected into the air sacs of hens' eggs preincubated for 7 days. Mortality was measured 72 h later and corresponding LD50 values were calculated. The rank order of toxicity was PeCB greater than TCB greater than HCB. Using the same injection procedure, the potencies of these chlorobiphenyls with regard to their induction of hepatic 7-ethoxyresorufin O-deethylase activity were compared. The ranking order of the substances as inducers was the same as their order when ranked according to toxicity. The three coplanar chlorobiphenyls were considerably more toxic and potent as inducers than the nonplanar 2,2',4,4',5,5'-hexachlorobiphenyl. In a 2-week toxicity study, PeCB and HCB were injected into the yolks of hens' eggs preincubated for 4 days. PeCB was about 50-fold more potent than HCB in causing embryonic death. Both substances caused abnormalities, including edema, liver lesions, microphthalmia and beak deformities.

Transfer of 2,4,5,2',4',5'-hexachlorobiphenyl across the in situ perfused guinea pig placenta

The transplacental crossover of 14C-2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) from the maternal circulation to the fetal side of the placenta was examined in intact fetuses and following the in situ perfusion of the guinea pig placenta. Fetal, late pregnant, and nonpregnant female guinea pig lipoprotein profiles and the association of 6-CB with these plasma constituents were also determined in vivo. Very low density lipoprotein (VLDL) concentrations were 10-fold higher in fetal than in maternal plasma, and the great majority of 6-CB which was transferred to intact fetuses became associated with this plasma fraction. 6-CB was found primarily in association with low density lipoproteins (LDL) in nonpregnant animals. In the late pregnant guinea pig, 6-CB became primarily associated with plasma protein in spite of circulating protein concentrations lower than those seen in the nonpregnant state. No differences in the levels of the three plasma lipoprotein classes were observed between pregnant and nonpregnant animals. It was found that an amount of 6-CB similar to that found in intact litter mates crossed the perfused placenta over the same time period. Despite the much higher VLDL concentrations on the fetal side of the placenta and the association of 6-CB with VLDL in intact fetuses, addition of 1,000 mg/dl VLDL to the 5.4% bovine serum albumin perfusion medium failed to influence the magnitude of 6-CB crossover. 6-CB crossover was influenced by protein concentration in the perfusion media in a concentration-dependent fashion. It is hypothesized that 6-CB and free fatty acids traverse the placenta and are retained by the fetus via similar mechanisms.

Toxicity of Fenclor 42 on Immunocompetent Cells

Polychlorinated biphenyls (PCBs) are environmental contaminants, whose toxicity is related to the degree and the position of chlorine substitutes. Fenclor 42 is a mixture of trichlorobiphenyls employed for industrial use in Italy. In order to evaluate its effects on the immune system, CD2F1 mice were treated with Fenclor 42 at different doses and schedules. Spleen weights, total number of splenocytes and relative spleen weight decreased significatively following a single or a prolonged exposure to different doses of Fenclor 42.

An analysis of the functional activity of splenocytes showed that the proliferative response to Con A and PWM mitogens was also inhibited by Fenclor 42. Splenic parameters returned to normal values within 5 days after a single injection and between 8–15 days after a subchronic administration of the compound. The capability of splenocytes to proliferate, following mitogen stimulation, was restored between day 5 and day 8 according to the different schedules of treatment. By comparison, Fenclor 42 never affected NK activity.

These data clearly indicate damage at the splenic level by Fenclor 42. Studies are in progress to elucidate the mechanisms involved in the immunotoxic response.

Immunosuppressive activities of polychlorinated dibenzofuran congeners: quantitative structure-activity relationships and interactive effects

The dose-response immunosuppressive effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8- and 1,2,3,7,9-pentachlorodibenzofuran (PeCDF), 2,3,7,8- and 1,3,6,8-tetrachlorodibenzofuran (TCDF) on the splenic plaque-forming cell (PFC) response to sheep red blood cells were determined in C57BL/6 mice. The ED50 values for immunosuppression were 2.4, 3.0, 14.0, 710, and 35,700 nmol/kg for 2,3,7,8-TCDD, 2,3,4,7,8-PeCDF, 2,3,7,8-TCDF, 1,2,3,7,9-PeCDF, and 1,3,6,8-TCDF, respectively, and the results confirmed that lateral chlorine substitutions were important structural determinants for the toxicity of the polychlorinated dibenzofuran congeners. Interaction of both 2,3,7,8-TCDD and 2,3,4,7,8-PeCDF with subimmunotoxic doses of 1,3,6,8-TCDF resulted in significant antagonism of the immunotoxic effects of both 2,3,7,8-TCDD and 2,3,4,7,8-PeCDF. Previous studies have also demonstrated that 1,3,6,8-TCDF also antagonizes the induction of aryl hydrocarbon hydroxylase by 2,3,7,8-TCDD and analysis of competitive receptor binding studies suggests that 1,3,6,8-TCDF acts as a competitive partial antagonist of the action of 2,3,7,8-TCDD. The antagonism of 2,3,7,8-TCDD immunosuppression was found to be dependent on the timing of administration of 1,3,6,8-TCDF. Using a protocol in which 2,3,7,8-TCDD is administered 5 days prior to the antigen and 9 days prior to assessing the splenic PFC response, it was possible to partially antagonize the immunosuppressive effects of 2,3,7,8-TCDD by administering the antagonist up to 5 days after the initial dose of the toxin. Administration of 1,3,6,8-TCDF after the antigen does not afford any significant protection from the effects of 2,3,7,8-TCDD and these results are consistent with the hypothesis that 2,3,7,8-TCDD modulates some early event in B-cell differentiation. However, these results do not exclude a role for 2,3,7,8-TCDD in modulating other cellular processes associated with the PFC response.

Effects of 3,3',4,4'-tetrachlorobiphenyl in the brain of the neonatal mouse

To study the retention of TCB in the brain of the immature mouse and its effects on the muscarinic cholinergic receptors in the cerebral cortex and hippocampus, [14C]TCB and TCB were administered to 10-day-old mice as a single peroral dose of 1.5 MBq/kg body wt and 0.41 mg or 41 mg/kg body wt, respectively. The mice were killed 24 h or 7 days after treatment. The amount of radioactivity in the whole brain decreased by 78% between 24 h and 7 days after administration. The density of muscarinic receptors was measured using the muscarinic antagonist [3H]QNB. A significant decrease (16%) in specific [3H]QNB binding was observed in the hippocampus 7 days after treatment. Despite the relatively rapid elimination of TCB from the immature mouse brain, a single low dose of TCB given during the period of rapid development of the cholinergic system appeared to affect the muscarinic cholinergic receptors in the immature mice.

Chronic toxicity of 3,4,3',4'-tetrachlorobiphenyl in the marmoset monkey (Callithrix jacchus)

Cotton top marmoset monkeys (Callithrix jacchus) were orally dosed with 3, 1, 0.1 or 0 mg 3,4,3',4'-tetrachlorobiphenyl (TCB)/kg body weight twice per week for 18-23 weeks. Severe toxicity occurred in the highest dose group. Clinical signs of toxicity were a rapid decrease in body weight, alopecia, abnormal nail growth, nodular enlargement of the nipple area and scaly skin. Haematological analysis of peripheral blood revealed mild leukocytosis and anemia. Biochemical alterations observed were elevated triglyceride levels and cholesterol levels. Histopathology revealed dose dependent changes in a variety of tissues. Squamous metaplasia was found in skin and adnexa as well as in salivary glands. In the stomach, parietal cells were decreased and mucus producing cells were increased. The duodenal mucosa was hyperplastic. Ovaries showed an absence of corpora lutea. In the thyroid follicular cell hyperplasia and hypertrophy were noted. Toxicity was less severe in marmoset monkeys dosed with 1 mg TCB/kg, while minor toxic effects were observed in the animals dosed with 0.1 mg TCB/kg. The marmoset monkey appears to be less sensitive to the toxic action of TCB than the rhesus monkey. The pattern of histological and biochemical changes induced by TCB in marmoset monkeys is comparable to that described in humans and in other primate species exposed to PCBs. The marmoset monkey model may be valuable for investigations on human-related toxicity of PCBs.

Differences between chick and turkey embryos in sensitivity to 3,3',4,4'-tetrachloro-biphenyl and in concentration/affinity of the hepatic receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin

1. 3,3',4,4'-Tetrachlorobiphenyl (TCB) was 20-100 times more toxic in chick embryos than in turkey embryos when injected into eggs. 2. The ED50-value for induction of AHH activity by TCB in the liver of early chick and turkey embryos was estimated to be 0.6 and 6 micrograms/kg egg, respectively. 3. In both species alpha-naphthoflavone was more effective than metyrapone at inhibiting basal and TCB-induced AHH activities. 4. The TCDD receptor was detected in the liver of 7-day-old chick embryos, while it was not found in 9-day-old turkey embryo liver.

Sensitivity of embryos from duck, goose, herring gull, and various chicken breeds to 3,3',4,4'-tetrachlorobiphenyl

Yolks in embryonated eggs from duck, goose, herring gull, and various breeds of chicken were injected with 3,3',4,4'-tetrachlorobiphenyl (TCB). Hens' eggs were injected after 4 days of incubation and eggs from the other species were injected after 5 days of incubation. All breeds of chicken tested were very sensitive to TCB. At a dose of 20 micrograms/kg egg the death rate in chick embryos ranged from 70 to 100% at the end of the experiment by Day 18 of incubation. Liver lesions, hydropericardium, subcutaneous edema, shortened beak, and microphthalmia were found in both dead and living TCB-treated chick embryos. Embryos of the other species tested were considerably less sensitive than chick embryos to TCB. The highest dose administered to these species was 5,000 micrograms/kg egg for ducks and 1,000 micrograms/kg egg for geese and herring gulls. These doses did not affect the viability of the embryos and caused no gross abnormalities.

Potentially hazardous residues of non-ortho chlorine substituted coplanar PCBs in human adipose tissue

The non-ortho chlorine substituted coplanar polychlorinated biphenyls (PCBs), namely, 3,3',4,4'-tetra-(T4CB), 3,3'4,4',5-penta-(P5CB), and 3,3',4,4',5,5', hexachlorobiphenyl (H6CB) are found to be the most toxic congeners of PCBs eliciting toxic and biological responses typical of 2,3,7,8-tetrachloro dibenzo-p-dioxin (T4CDD). Analyses of human adipose tissues for these congeners revealed 94-860 pg/g of T4CB, 120-730 pg/g of P5CB, and 36-200 pg/g of H6CB on wet weight basis. These were significantly higher than 2,3,7,8-T4CDD detected in the same samples (less than 1-18 pg/g). On the basis of in vitro and in vivo induction of hepatic microsomal benzo(a)pyrene hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD), "T4CDD toxic equivalents" were calculated for coplanar PCBs in humans. "T4CDD toxic equivalent" for 3,3',4,4',5-P5CB was one order of magnitude higher than 2,3,7,8-T4CDD. Considering the extreme toxic potential and persistence of coplanar PCBs, their presence in the human body may pose a greater toxic threat than dioxins and dibenzofurans.

Chemiluminescence in human whole blood: modulation by the cocarcinogens phorbol diester and polychlorobiphenyls

A human whole blood chemiluminescence (CL) assay was established using zymosan as cell activator. Aroclor 1254 was found to inhibit this CL response in a direct linear relation to its concentration, (50% inhibitory dose, (ID50) equal to 5 x 10(-4)M) in diluted blood samples of 10 normal human subjects. In comparison the ID50 of other inhibitors was 1.3 x 10(-3)M for ethylenediamine tetraacetic acid, 3.3 x 10(-3)M for ascorbic acid, 4 x 10(-3)M for reduced glutathione, 1.2 x 10(-1)M for ethanol, 2.5 x 10(-1)M for methanol and 3.7 x 10(-1)M for dimethyl sulfoxide. Using 12-o-tetradecanoyl-phorbol-13-acetate (TPA) as cell activator the CL response was likewise inhibited by Aroclor 1254 with an ID50 of 4.5 x 10(-4)M. However, it was found that Aroclor 1254 alone has a stimulatory CL effect on otherwise unactivated cells. To compare the mechanisms involved in the CL elicited by the three stimulants zymosan, TPA and Aroclor 1254, the CL signal was measured in the presence of cytochalasin B. Cytochalasin B inhibited zymosan-induced CL, had a smaller inhibitory effect on TPA-induced CL but it could augment the CL response initiated by Aroclor 1254. This pattern of responses implicates Aroclor 1254 in the activation of eicosanoid metabolism as it matches the differential responses reported for arachidonic acid.

PCB problems in the future: foresight from current knowledge

The present paper overviews the forthcoming PCB problems from current knowledge of their use, environmental contamination and toxicology. From a global point of view, PCB levels in the environmental media and biota are unlikely to decline in the near future due to the greater quantities of PCBs still in use than the quantity that has already escaped into the open environment. Considering all the information on the occurrence, distribution and behaviour of PCBs in the ecosystems, the marine mammals are probably the most vulnerable and possible target organisms in forthcoming long-term PCB toxicity. The recent isomer-specific analyses suggest that the intrinsic toxicity of PCBs principally resulted from the coplanar PCB congeners which may impose a greater toxic threat than chlorinated dioxins and furans to humans and wildlife. The measures necessary to reduce further discharge of PCBs into the environment should be set in motion immediately, otherwise there may be a subsequent deleterious biological impact.

Mutagenicity of monochlorodibenzofurans detected in the environment

Recently, monochlorodibenzofuran, which is formed by the reaction of dibenzofuran with residual chlorine, has been detected in tap water in one region of Japan. The mutagenicities of 1-, 2-, 3-, and 4-chlorodibenzofuran were tested using Salmonella typhimurium TA98 and TA100. 1-Chlorodibenzofuran and 4-chlorodibenzofuran proved to be practically non-mutagenic, while 2-chlorodibenzofuran was weakly mutagenic. Unlike these three isomers, 3-chlorodibenzofuran was markedly mutagenic, and the intensity of its mutagenicity in TA98 was about one-fifth and in TA100 about one-twentieth of that of benzo[a]pyrene.

Synthesis and aryl hydrocarbon receptor binding properties of radiolabeled polychlorinated dibenzofuran congeners

Microchlorination of 1,4,9[3H]dibenzofuran gave several polychlorinated dibenzofuran (PCDF) products and 2,3,7,8-[3H]tetrachlorodibenzofuran (TCDF), 1,2,3,7,8-[3H]pentachlorodibenzofuran (PeCDF), and 1,2,3,6,7,8-/1,2,3,4,7,8-hexachlorodibenzofuran (HCDF) of high specific activity (57, 34, and 32.5 Ci/mmol, respectively) were purified by preparative high-pressure liquid chromatography. These compounds were investigated as radioligands for the rat liver cytosolic aryl hydrocarbon (Ah) receptor protein. Like 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD), the radiolabeled PCDF congeners exhibited saturable binding with the receptor protein and sucrose density gradient analysis of the radiolabeled ligand-receptor complexes gave specific binding peaks with comparable sedimentation profiles. The rank order of radioligand binding affinities (Kd values) was 2,3,7,8-TCDD greater than 2,3,7,8-TCDF greater than 1,2,3,6,7,8-HCDF greater than 1,2,3,7,8-PeCDF and the maximum difference in Kd values for the four radioligands was less than 13-fold (0.44-5.9 nM). The interactions of the PCDF radioligands with the cytosolic receptor all exhibited saturable binding curves and linear Scatchard plots and the slopes of their Hill plots were in the range 1.0-1.1, thus indicating that cooperativity was not a factor in these binding interactions. The relative stabilities and dissociation kinetics of the radioligand-receptor complexes were highly dependent on the structure of the radioligand. The dissociation curves of the 2,3,7,8-[3H]TCDD and PCDF receptor complexes were biphasic and this suggests that there may be a temporal shift in ligand binding affinities. However, the rates of dissociation did not correlate with the rank order of ligand binding affinities. The stabilities of the radioligand-receptor complexes were also dependent on the structures of the radioligands; for example, the 2,3,7,8-[3H]TCDD-receptor complex degraded more rapidly than the PCDF-receptor complex and these relative stabilities were clearly not related to the Kd values or the relative in vivo or in vitro biologic potencies of these halogenated aryl hydrocarbons.

Toxic effects in C57B1/6 and DBA/2 mice following consumption of halogenated aromatic hydrocarbon-contaminated Great Lakes coho salmon (Oncorhynchus kisutch Walbaum)

Diets containing coho salmon (Oncorhynchus kisutch Walbaum) from the Pacific Ocean or from Lakes Erie, Michigan, and Ontario [containing a gradation from low to high of halogenated aromatic hydrocarbons, (HAHs)] were fed to C57B1/6 and DBA/2 mice. Following a 4-month dietary exposure to Lake Ontario salmon, both strains of mice demonstrated hepatomegaly. The ethoxyresorufin-O-deethylase (ERR) enzyme levels were elevated in livers of C57B1/6 mice fed diets of salmon from all of the Great Lakes studied, with exceptionally high levels detected in C57B1/6 mice fed Lake Ontario salmon. Induction of ERR enzyme levels was detected in DBA/2 mice only following dietary exposure to Lake Ontario salmon. Serum levels of L-thyroxine (T4) and triiodo-L-thryonine (T3) were suppressed in C57B1/6 mice following consumption of Lake Ontario coho salmon, but T3 and T4 levels remained unchanged in DBA/2 mice. In general, pathobiological effects correlated with both dietary HAH exposure level and Ah receptor status.

Aroclor 1254 as a 2,3,7,8-tetrachlorodibenzo-p-dioxin antagonist: effects on enzyme induction and immunotoxicity

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and Aroclor 1254 induced the cytochrome P-450 dependent monooxygenases, aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) in rat hepatoma H-4-II E cells and C57BL/6J mice. It has been proposed that both Aroclor 1254 and 2,3,7,8-TCDD induce these enzymes via a common mechanism which features initial binding to the aryl hydrocarbon (Ah) cytosolic receptor protein. The major difference between these compounds was the relative potency (i.e. 2,3,7,8-TCDD much greater than Aroclor 1254). Cotreatment of rat hepatoma H-4-II E cells or C57BL/6J mice with a dose of 2,3,7,8-TCDD which submaximally induces AHH and EROD and a dose of Aroclor 1254 which exhibited little or no induction activity resulted in significant antagonism of the induction effects of 2,3,7,8-TCDD. For example, cotreatment of C57BL/6J mice with 2,3,7,8-TCDD (15 nmol/kg) and Aroclor 1254 (25, 75 and 150 mumol/kg) resulted in up to 23% antagonism of AHH induction by 2,3,7,8-TCDD. Moreover, cotreatment with a higher dose of the 2,3,7,8-TCDD agonist (30 or 50 nmol/kg) partially reversed some of the antagonism by Aroclor 1254. In vivo antagonism was observed only at Aroclor 1254/2,3,7,8-TCDD molar ratios of 1667:1, 5000:1 and 10,000:1. Administration of 2,3,7,8-TCDD (3.72 nmol/kg) to C57BL/6J mice resulted in a 76% decrease in the splenic plaque forming cell response to sheep red blood cells. This T-cell mediated immunotoxic effect of 2,3,7,8-TCDD segregates with the Ah locus. In contrast, administration of 5, 15, 75 and 150 mumol/kg of Aroclor 1254 resulted in impairment of the immune response only at the highest dose level. However, cotreatment of mice with 2,3,7,8-TCDD (3.72 nmol/kg) and Aroclor 1254 (5, 15 or 75 mumol/kg) resulted in no significant decrease in the plaque forming cell response and complete protection from the immunotoxicity of 2,3,7,8-TCDD. Cotreatment of the mice with Aroclor 1254 (75 mumol/kg) and a higher dose of the 2,3,7,8-TCDD agonist resulted in partial reversal of the protective effects of Aroclor 1254. The in vitro and in vivo data suggest that within specific antagonist/agonist dose ratios, Aroclor 1254 can antagonize at least 2 Ah receptor-mediated effects of 2,3,7,8-TCDD, namely AHH induction and immunotoxicity.

Aroclor 1254 as an antagonist of the teratogenicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin

Administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 20 micrograms/kg) to pregnant C57BL/6J mice (on day 10) resulted in 62% fetuses with cleft palate per litter without any observable maternal toxicity. In contrast, Aroclor 1254 administered at a dose of 750 mumol/kg was not teratogenic. Cotreatment of the pregnant mice with both Aroclor 1254 (244 mg/kg) and 2,3,7,8-TCDD (20 micrograms/kg) resulted in an 8.2% incidence of cleft palate per litter. In contrast, Aroclor 1254 did not afford any protection from the teratogenicity of dexamethasone in C57BL/6J mice. Previous studies have shown that Aroclor 1254 can act as a partial antagonist of the microsomal enzyme induction and immunotoxic effects of 2,3,7,8-TCDD in C57BL/6J mice and this paper demonstrates that the commercial polychlorinated biphenyl mixture also antagonizes 2,3,7,8-TCDD-mediated teratogenicity in this strain of mice.

Monoclonal antibodies for dioxin: antibody characterization and assay development

A set of 5 anti-dioxin monoclonal antibodies (mAbs), named DD-1, DD-3, DD-4, DD-5 and DD-6, have been isolated. In order to evaluate the ability of these mAbs to recognize various kinds of polychlorinated dibenzodioxins and dibenzofurans, a competition enzyme-linked immunosorbtion assay (ELISA) was developed. All 5 antibodies recognize tetrachloro- and pentachloro-dibenzodioxins and -dibenzofurans. They fail to bind either non-chlorinated, mono-, hexa-, or octa-chlorinated dibenzodioxins, nor do they recognize non-chlorinated, octachloro- or 1,2,3,4,8,9-hexachloro-dibenzofurans. Chlorine substitution on both rings appears necessary for antibody recognition. In the course of our experiments, 3 of the mAbs did not recognize any of the polychlorinated biphenyls (PCBs) tested, while 2 mAbs (DD-1 and DD-6) weakly recognized the 3,3',4,4'-tetrachloro congener. DD-4 and DD-5 are the most specific of the antibodies for the dibenzodioxin and dibenzofuran structure. They do not recognize any of a panel of chlorinated phenols, benzenes, or pesticides. Significantly, these antibodies do not react with PCBs, pentachlorophenol, 2,4-dichlorophenoxyacetic acid, trichlorophenol, or 2,4,5-trichlorophenoxyacetic acid (the latter is weakly recognized by DD-6), any or all of which might be present in large quantities in some dioxin-contaminated samples. Finally, the competition ELISA is able to easily detect 0.5 ng of the most toxic dioxin congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin. It should thus prove useful as an environmental screen for contamination.

Influences of diethylnitrosamine on longevity of surrounding hepatocytes and progression of transplanted persistent nodules during phenobarbital promotion of hepatocarcinogenesis

The possibility that phenobarbital (PB) selectively promotes liver nodule development by decreasing survival of surrounding hepatocytes previously exposed to diethylnitrosamine (DENA) was evaluated. Livers of F-344 rats were labelled with [3H-methyl]-thymidine (3H-TdR) during developmental or regenerative growth. Neonatal rats given 3H-TdR between days 3 and 12 were subjected at 12 weeks of age to partial hepatectomy (PH) followed 24 hr later by DENA (10 mg/kg) or saline. Subsequent administration of PB (0.1% in drinking water) for 28 weeks reduced total liver label to 46 +/- 10% (saline group) or 40 +/- 4% (DENA group). Adult male rats initiated with DENA (200 mg/kg) and later labelled with 3H-TdR after PH also lost total liver label during 28 weeks' promotion with PB (0.05% in water) at rates similar to those exhibited by noninitiated rats given PB, and by DENA-treated or control rats not given PB. Large persistent (12 weeks) liver nodules generated by DENA in the Solt-Farber model were transplanted as small fragments into the spleens of syngeneic rats previously given 0, 100 or 200 mg/kg of DENA. Subsequent exposure to PB (0.05% in drinking water for 40 weeks) or Aroclor 1254 (6 X 300 mg/kg per month) promoted nodule and cancer development only in livers of DENA-initiated recipients. Surviving transplanted nodules remained as small microscopic clusters even after 40 weeks of promotion. However, PB increased transplant survival (50% vs. 21% in controls) whereas Aroclor reduced it to 8%. These findings indicate that promotion of liver nodules by PB occurs without enhanced mortality of surrounding hepatocytes previously damaged by DENA. They further suggest that promoters such as PB and PCBs do not directly influence the progression of established persistent nodules.

Histology and histochemistry of the liver of chickens after DENA induced hepatocarcinogenesis and ingestion of low chlorinated biphenyls

Studies are presented which demonstrate the pathological effects of diethylnitrosamine (DENA) and polychlorinated biphenyls (PCB) in the liver of chickens. DENA, which has been shown to cause tumors in rats and chickens, was tested on 80 laying hens to determine whether changes in the liver during hepatocarcinogenesis were similar to those observed in rats. In addition, the hepatocarcinogenic and cocarcinogenic properties of Clophen C (CC), a technical mixture of low chlorinated biphenyls was tested on chickens. The livers of test animals were examined histologically for preneoplastic and neoplastic lesions. Histochemical methods were used to determine lipid and glycogen contents as well as changes in activity of alkaline phosphatase and gamma-glutamyl-transpeptidase. Putative preneoplastic lesions in the liver were first observed according to the substance and dosage applied as follows: DENA 1 X weekly (group II), beginning on day 258; DENA 2 X weekly (group III), beginning on day 183; and CC + DENA 2 X weekly (group V), beginning on day 231. Application of CC alone did not induce lesions. The concept that CC has cocarcinogenic capability was substantiated by the fact that animals from group V (CC + DENA 2 X weekly) developed twice as many tumors as did animals from group III (DENA 2 X weekly). Additionally, tumors developed earlier in the former group than in the latter.

Absence of positive co-operativity in the binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin to its cytosolic receptor protein

The role of positive co-operativity in stabilizing the binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to the rat hepatic cytosolic TCDD receptor protein (Ah receptor) was investigated. The binding mechanism of TCDD was determined by kinetic means through equilibrium and saturation binding studies, and Scatchard and Hill plot analysis. In all studies, the slope of the Hill plot was close to 1.0, indicating the absence of positive co-operativity. Interpretation of the Scatchard plot was however complicated by the fact that both linear and nonlinear plots were experimentally obtained. The nonlinearity was shown to be an experimental artifact and a consequence not of co-operativity, but of high levels of nonspecific binding. The high level of nonspecific binding could be attributed to: (1) lipophilicity of the TCDD ligand, and (2) inefficient competition of receptor-bound [3H]TCDD. When nonspecific binding was minimized, the Scatchard slope was linear and in agreement with the Hill coefficient, thus indicating the lack of positive co-operativity in the binding of TCDD to the Ah receptor.

Purification and characterization of the dog hepatic cytochrome P-450 isozyme responsible for the metabolism of 2,2',4,4',5,5'-hexachlorobiphenyl

The biochemical basis for the marked difference in the rate of the hepatic metabolism of 2,2',4,4',5,5'-hexachlorobiphenyl (245-HCB) by Beagle dogs and Sprague-Dawley rats has been investigated. Control dog liver microsomes metabolize this substrate 15 times faster than control rat liver microsomes. Upon treatment with phenobarbital (PB), at least two cytochrome P-450 isozymes are induced in the dog, and the hepatic microsomal metabolism of 245-HCB is increased on both a per nanomole P-450 basis (twofold) and a per milligram protein basis (fivefold). One of the PB-induced isozymes, PBD-2, has been purified to a specific content of 17-19 nmol/mg protein and to less than 95% homogeneity, as evidenced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In a reconstituted system containing cytochrome b5, this isozyme shows an activity toward 245-HCB which is greater than threefold that seen in intact liver microsomes from PB-induced dogs. A reconstituted system containing the major isozyme induced by PB in the rat (PB-B) metabolizes 245-HCB at 1/10 the rate observed with purified PBD-2. Antibody inhibition studies have shown that PBD-2 accounts for greater than 90% of the hepatic microsomal metabolism of 245-HCB in control and PB-induced dogs, while PB-B only accounts for about half of the metabolism of this compound by microsomes obtained from PB-treated rats. Immunoblot analysis has revealed that the level of PBD-2 in dog liver microsomes increases nearly sixfold with PB treatment, and this increase correlates well with the fivefold increase in the rate of hepatic microsomal metabolism of 245-HCB by dogs. Together these data support a primary role for isozyme PBD-2 in the hepatic metabolism of 245-HCB in control and PB-induced dogs. In addition, these results suggest that, in contrast to rats, dogs can readily metabolize 245-HCB as a result of the presence of a cytochrome P-450 isozyme with efficient 245-HCB metabolizing activity.

Synergistic interactions of 2,3,7,8-TCDD and 2,2',4,4',5,5'-hexachlorobiphenyl in C57BL/6J and DBA/2J mice: role of the Ah receptor

Treatment of C57BL/6J mice with 2,2',4,4',5,5'-hexachlorobiphenyl (HCBP, 500 mumol/kg) elevated hepatic cytosolic Ah receptor levels 82-107% for up to 14 days. Scatchard analysis of the [3H]2,3,7,8-TCDD (TCDD)-Ah receptor saturation binding curves from corn oil and HCBP treated rats gave KD values of 0.80 and 0.90 nM, respectively and confirmed that treatment with HCBP did not significantly alter receptor-radioligand affinities. Administration of HCBP to DBA/2J mice did not result in detectable hepatic cytosolic Ah receptor levels. Cotreatment of C57BL/6J mice with HCBP (500 mumol/kg) at a dose level of TCDD (1 nmol/kg) which elicited less than 10% of the maximum induction response resulted in significant synergistic induction of hepatic EROD and AHH [compared to animals treated only with TCDD (1 nmol/kg)]. In contrast, cotreatment of C57BL/6J mice with HCBP (500 mumol/kg) and maximally inducing dose levels of TCDD (100 or 500 nmol/kg) resulted in either a slight or no difference in the induction of AHH or EROD compared to the induction responses observed in mice treated only with TCDD. In contrast, cotreatment of DBA/2J mice with TCDD and HCBP (500 mumol/kg) resulted in significant synergistic induction of AHH and EROD at both submaximal (10-500 nmol/kg) and maximal (5000 nmol/kg) induction levels of TCDD. The only significant interactive effect of HCBP (500 mumol/kg) on the toxicity of TCDD in C57BL/6J and DBA/2J was protection from body weight loss observed after cotreatment of HCBP and TCDD in DBA/2J mice.

Induction of rat hepatic monooxygenase activity by polychlorinated diphenyl ethers

Polychlorinated diphenyl ethers are recognized environmental contaminants. Twelve of these compounds were tested for their ability to induce liver cytochrome P-450 and monooxygenase activities in Sprague-Dawley rats. All the compounds increased P-450 levels or increased monooxygenase activities in a manner resembling 3-methylcholanthrene, phenobarbital or a combination of both (mixed). The responses obtained resembled those of the polychlorinated biphenyls, some of which are known to be toxic.

Binding of polynuclear aromatic hydrocarbons to the rat 4S cytosolic binding protein: structure-activity relationships

The relative competitive binding affinities of benzo[a]pyrene (B[a]P), benzo[e]pyrene, benzo[g, h, i]perylene, picene, 7,12-dimethylbenz [a]anthracene, 1,2,3,4-dibenz[a]anthracene, 1,2,5,6-dibenz[a]anthracene, perylene, 4H-cyclopenta[d,e,f]-phenanthrene, benz[a] anthracene, triphenylethylene and triptycene for the rat hepatic cytosolic 4S binding protein were determined using [3H]benzo[a]pyrene as the radioligand. With the exception of triphenlethylene, triptycene and 4H-cyclopenta[d,e,f]phenanthrene, the EC50 values for the remainder of these compounds were between 1.25 X 10(-7) and 2.5 X 10(-8) M with 1,2,5,6-dibenz[a]anthracene being the most active ligand. A comparison of the relative cytosolic Ah (9S) receptor binding affinities and aryl hydrocarbon hydroxylase (AHH) induction potencies of these hydrocarbons with their 4S protein binding affinities demonstrated the following: five compounds, namely 1,2,5,6-dibenz[a]-anthracene, 1,2,3,4-dibenz[a]anthracene, picene, benzo[a]pyrene and 3-methylcholanthrene exhibited high to moderate binding affinities for the 4S and 9S cytosolic proteins (EC50 values less than 10(-6) M) and induced AHH in rat hepatoma cells; three compounds, namely perylene, benzo[e]pyrene and benzo[g,h,i]perylene exhibited high affinities for the 4S binding protein (1.25 X 10(-7), 4.4 X 10(-8) and 2.9 X 10(-8) M, respectively) and low affinities (EC50 values greater than 10(-5) M) for the Ah receptor protein; moreover these three compounds did not induce AHH in rat hepatoma H-4-II E cells in culture. These data suggest that the 4S binding protein may not play a significant role in AHH induction although the results do not rule out a function for this protein in the transregulation of AHH and its associated cytochromes P-450.

Effects of polychlorinated biphenyls on porphyrin synthesis and cytochrome P-450-dependent monooxygenases in small intestine and liver of Japanese quail

The effects of acute exposure to polychlorinated biphenyls (PCBs) on porphyrin synthesis and cytochrome P-450-dependent monooxygenases in the small intestine and liver were studied in male Japanese quail. The birds were dosed orally with the PCB mixture, Aroclor 1242, or the individual PCB isomers, 2,4,2',4'-tetrachlorobiphenyl (2-TCB) and 3,4,3',4'-tetrachlorobiphenyl (3-TCB), and were killed 48 h later. All the PCB compounds caused a significant increase in porphyrin content and delta-aminolevulinic acid synthetase (ALA-S) activity in the small intestine and liver. Increases in porphyrins were greater in the small intestine than in liver. However, a smaller increase in ALA-S activity occurred in the small intestine than in liver, suggesting that ALA-S induction is not a major mechanism for the increased porphyrin content of small intestine. All the test compounds significantly increased the cytochrome P-450 content of liver. In the small intestine, cytochrome P-450 content was increased by Aroclor 1242 and 2-TCB but not by 3-TCB. The activity of 7-ethoxyresorufin O-deethylase, however, was increased by all test compounds in both liver and small intestine. In contrast, there was a striking difference between small intestine and liver in the induction of 7-ethoxycoumarin O-deethylase (ECOD) activity by Aroclor 1242. In the liver, ECOD activity was unchanged or decreased, but in the small intestine, ECOD activity increased linearly with dose. No tissue difference in ECOD activity was observed after treatment with 2-TCB or 3-TCB. These findings suggest that acute exposure to a given PCB results in marked differences between small intestine and liver in porphyrin metabolism and in the induction of cytochrome P-450 isozymes and associated monooxygenases.

Highly toxic coplanar PCBs: occurrence, source, persistency and toxic implications to wildlife and humans

Isomer-specific determinations of PCB congeners in a wide variety of animal species such as fish, marine mammals (whale, dolphin and porpoise) and terrestrial mammals (dog, cat and human) revealed the environmental occurrence of highly toxic coplanar 3,3',4,4'-tetrachlorobiphenyl (T(4)CB), 3,3',4,4',5-pentachlorobiphenyl (P(5)CB) and 3,3',4,4',5,5'-hexachlorobiphenyl (H(6)CB) within a range of few pg g(-1) to several ten ng g(-1) in fat tissues (except fish) on a wet weight basis. Detection of these toxic residues in wild specimens collected from remote areas such as the North Pacific suggests the already widespread distribution of coplanar PCBs as in the case of general PCB pollution. The clear positive correlations between concentrations of total PCBs and each of the three coplanar PCBs obtained in all mammals analysed suggest that the sources of coplanar PCB contamination to the environment are mainly commercial PCB preparations. Comparison of the composition of three toxic coplanar PCBs in commercial PCB mixtures and in the various animals indicates the relative metabolisability of these congeners as follows: 3,3',4,4'-T4CB>3,3',4,4',5-P5CB>3,3',4,4',5,5'-H6CB. Moreover, marine mammals seem to have lower potency to metabolise the coplanar PCBs in comparison with terrestrial mammals. In human adipose tissues, the concentrations of coplanar PCBs were found to be much higher than 2,3,7,8-tetrachlorodibenzo-p-dioxin (T(4)CDD), 2,3,4,7,8-pentachlorodibenzofuran (P(5)CDF) and other toxic congeners. 'T(4)CDD-equivalent' analysis based on the enzyme induction potencies and the residues of these toxic chemicals indicates that 3,3',4,4',5-P(5)CB may impose a greater toxic threat than dioxins and furans to the humans and probably to wildlife also.

Effects of polychlorinated biphenyls and lipemia on serum analytes

Twelve serum analytes [triglycerides, cholesterol, total and conjugated bilirubin, high-density-lipoprotein cholesterol (HDL-C), alkaline phosphatase (AP), gammaglutamyl transferase (GGT), aspartate aminotransferase (AST), alanine aminotransferase (ALT), beta-glucuronidase (beta-glu), alanine aminopeptidase (AAP), and 5'-nucleotidase (5'nuc)] were measured to investigate their correlation with exposure to polychlorinated biphenyls (PCBs) and 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT). The relationship between serum lipids, lipophilic toxicants, and the analytes was also evaluated. The beta-glu, 5'nuc, triglycerides, cholesterol, and total bilirubin correlated positively and significantly with log concentrations of serum total PCBs and 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE), a metabolite of DDT. The more highly chlorinated PCBs (Aroclor 1260) had significant, positive correlations with several serum analytes, but the less chlorinated PCBs (Aroclor 1242) correlated significantly and negatively only with HDL-cholesterol. Triglyceride- and cholesterol-rich lipoproteins were added to serum to determine the effects of lipids on these assays. Several were spuriously elevated. AP and beta-glu were not affected by lipoprotein addition with the methods used in this study. AAP was increased significantly only at triglyceride concentrations exceeding 400 mg/dl. Lipoproteins may be elevated because of deranged lipid metabolism in response to PCBs, or PCBs may be elevated because elevated lipoproteins are present, as in familial triglyceridemia, a relatively common dyslipoproteinemia. Because this relationship is not well understood with respect to cause and effect, we propose the further use in epidemiological investigations of assay methods that are little affected by blood lipids yet are correlated with PCB concentrations. Congener-specific quantification of PCBs would help elucidate the effects of PCBs on assays used to monitor health effects.

Effects of individual terphenyls and polychlorinated terphenyls on rat hepatic microsomal cytochrome P-450-dependent monooxygenases: structure-activity relationships

The effects of o-, m- and p-terphenyl, 2,4-dichloro-, 2,4,6-trichloro-, 2,3,5,6-tetrachloro-, 2,3,4,6-tetrachloro-, 2,4,4''',6- tetrachloro- and 2,3,4,5-tetrachloro-p-terphenyl, 2,3,4,5-tetrachloro-m- and o-terphenyl as inducers of hepatic drug-metabolizing enzymes were determined in immature male Wistar rats. o-Terphenyl, 2,4-dichloro-, 2,4,6-trichloro-p-terphenyl and 2,3,4,5-tetrachloro-o-terphenyl induced 4,4'-dimethylamino antipyrine N-demethylase at total dose levels of 300 mumol/kg and the 2,3,4,5-tetrachloro-p-terphenyl induced ethoxyresorufin O-deethylase (EROD). In contrast, none of the other terphenyls or polychlorinated terphenyls (PCTs) induced these enzyme activities. Previous studies have demonstrated that 2,3,4,5-tetrachloro-p-terphenyl did not exhibit a high affinity for the 2,3,7,8-tetrachlorodibenzo-p-trachlorodibenzo-p-dioxin (TCDD) receptor protein (EC50 = 6.6 X 10(-6) M). In contrast, this study showed that 2,3,4,5-tetrachloro-p-terphenyl was more active than either 2,3,4,5-tetrachloro-o- or m-terphenyl as an inducer of EROD. Moreover, the competitive receptor binding EC50 values for the latter two isomers were greater than 10(-5) M and this result was also consistent with their lack of EROD induction activity. Previous studies showed that analysis of the data for a series of 4'-substituted-2,3,4,5-tetrachlorobiphenyls indicated that the p-terphenyl structural moiety (i.e. 4'-substituent = phenyl) did not interact with high affinity with the receptor protein binding site. Since the 2,3,4,5-tetrachloro o- and m-terphenyls are also poor ligands for the receptor protein, this data and results from other studies indicate that PCT congeners (and commercial mixtures) are therefore unlikely to elicit significant 2,3,7,8-TCDD-like biologic or toxic effects in target species.

Polychlorinated dibenzo-p-dioxins: quantitative in vitro and in vivo structure-activity relationships

There were marked effects of structure on the activities of 14 polychlorinated dibenzo-p-dioxins (PCDDs) as competitive ligands for the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) receptor and as inducers of aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) in rat hepatoma H-4-II E cells in culture. 2,3,7,8-TCDD was the most active compound in both assays and several PCDD congeners which were fully substituted in the lateral 2, 3, 7 and 8 positions but also contained additional chlorosubstituents in non-lateral 1, 4, 6 and 9 positions were less active. It was also evident that there was a decrease in in vitro binding and induction activities with decreasing lateral chlorine substitution. Although comparable structure-activity relationships (SARs) for the PCDDs were observed for the induction and receptor binding assays, there was not a linear or rank order correlation between the 2 sets of data. Several in vivo biologic and toxic activities of 2,3,7-trichloro-, 2,3,7,8- and 1,3,7,8-tetrachloro-, 1,2,4,7,8- and 1,2,3,7,8-pentachloro- and 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin were determined in a dose-response fashion in immature male Wistar rats. The ED50 values for hepatic microsomal AHH and EROD induction, body weight loss and thymic atrophy were obtained. There was an excellent linear correlation between the -log EC50 values for AHH or EROD induction in cell culture and the -log ED50 values for enzyme induction, body weight loss and thymic atrophy in the rat. The in vitro enzyme induction data could be used to quantitatively estimate the toxicity of the PCDD congeners in the rat: this latter correlation has previously been observed for a series of polychlorinated dibenzofurans.

Polychlorinated biphenyls, classified as either phenobarbital- or 3-methylcholanthrene-type inducers of cytochrome P-450, are both hepatic tumor promoters in diethylnitrosamine-initiated rats

The cytochrome P-450 isozymes, cytochrome P-450 MC1 and MC2, purified from rats treated with 3-methylcholanthrene (MC), were found by immunohistochemical staining to be strongly induced in the livers of rats treated with 3,3', 4,4'-tetrachlorobiphenyl (TCBP), while the cytochrome P-450 isozymes, PB1 and PB2, purified from the livers of rats treated with phenobarbital (PB), were shown to be induced in the livers of rats treated with 2,2', 4,4', 5,5'-hexachlorobiphenyl (HCBP). The latter compound also strongly induced NADPH-cytochrome P-450-reductase. Following induction, all 5 enzymes were located preferentially in the centrilobular and midzonal region of the liver acinus. The influence of these polychlorinated biphenyls (PCBs) on diethylnitrosamine (DEN)-initiated hepatocarcinogenesis was investigated by analyzing the evolution of adenosine triphosphatase-deficient focal lesions. Whereas DEN alone produced very few islets, the administration of either PCB congener (150 mumol/kg, i.p., once weekly over a period of 8 weeks) subsequent to DEN treatment (50 ppm in the drinking water, 10 days) strongly enhanced the number of islets as well as the relative volume of liver occupied by islet tissue. These effects were evident, both 1 and 9 weeks, after cessation of PCB treatment. Unexpectedly the less persistent PCB congener, TCBP, showed a much more potent enhancing effect after the 9 weeks recovery period than did (HCBP).

Effects of a single dose of polychlorinated biphenyls to infant mice on N-nitrosodimethylamine-initiated lung and liver tumors

Polychlorinated biphenyls (PCBs) are widespread, chemically-stable environmental contaminants; some congeners are commonly found in human adipose tissue and breast milk. We investigated the effects of a single dose of one PCB mixture (Aroclor 1254) on tumors initiated by N-nitrosodimethylamine (NDMA), also a common environmental agent. Infant outbred Swiss male mice were treated with NDMA (5 mg/kg) i.p. on the 4th day of life, to initiate lung and liver tumors. Four days later each received a single intragastric dose of PCBs (50, 250, or 500 mg/kg of Aroclor 1254) or oil. Groups were killed 16 and 28 weeks later. At both endpoints the mice given 500 mg/kg PCBs after NDMA developed twice as many lung tumors (alveologenic adenomas) as those treated with NDMA only, a significant difference. The PCBs alone did not cause lung tumors. This is the first demonstration of tumor promotion by PCBs in an extrahepatic organ, and it occurred after a single exposure. There were also complex, multiple effects on NDMA-caused liver tumors (adenomas and carcinomas) and on focal hepatocellular proliferative lesions: PCB treatment after the NDMA was associated with decreased number but increased size of these tumors and foci. All of these changes were accompanied by retention in the bodies of 0.1-6 ppm PCBs, as indicated by gas chromatography with electron capture detection. Of this, 80% or more consisted of 2,4,5,2',4',5'-and 2,3,4,2',4',5'-hexachlorobiphenyls in about equal amounts for periods up to 28 weeks. These results point to a need for both experimental and epidemiological studies of the effect of PCB body burden on tumor development.

Induction of immunotoxicity in mice by polyhalogenated biphenyls

Acute administration of Aroclor-1254 (500 mg/kg) or 3,4,5,3',4',5'-hexabromobiphenyl (HBB) (2-6 mg/kg) IP, profoundly inhibited the plaque forming response to subsequent challenge with sheep erythrocytes in Ah locus positive (C57Bl/6N or B6C3F1N) mice. These studies showed: the immunotoxicity results paralleled enzyme induction results insofar as HBB was approximately 100 times more potent than Aroclor 1254; neither Aroclor nor HBB treatment caused significant induction in the Ah locus negative DBA/2N mice; when B6C3F1 mice were challenged with sheep red blood cells (SRBC) 6 or 16 weeks post Aroclor 1254 treatment, substantial recovery of a PFC response was observed; when these compounds were administered to older (76-week-old) (B6C3F1 mice, severe depression of a PFC response was observed. In contrast to its profound depression of a PFC response, Aroclor-1254 (up to 1250 mg/kg) caused slight increases in lymphocyte proliferation induced by either T or B cell mitogens. A single 500 mg/kg dose of Aroclor-1254 also suppressed the ability of recipient B6C3F1 animals to reject a challenge with either the syngenic fibrosarcoma (PYB6) or the gram negative pathogen (Listeria monocytogenes).