The toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and their relevance for toxicity

Correlation of 2,3,7,8-tetrachlorodibenzo-p-dioxin induction of cytochrome P4501A in vascular endothelium with toxicity in early life stages of lake trout

Edema and cardiovascular dysfunction occur in vertebrates exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) during early development. This study examined cytochrome P4501A (CYP1A) induction in endothelium and its possible association with mortality due to the edema and vascular effects of TCDD in lake trout early life stages. Lake trout (Salvelinus namaycush) eggs were injected at 24-50 hr postfertilization with 0.2 microl of 50 mM phosphatidylcholine liposomes or liposomes containing TCDD to give seven doses ranging from 11 to 176 pg TCDD/g egg. Doses of TCDD greater than 44 pg/g egg elicited hemorrhages; yolk sac, pericardial, and meningial edema; craniofacial malformations; regional ischemia; growth retardation; and mortality at the sac fry stage of development. Expression of CYP1A was assessed at four developmental stages, by immunohistochemical analysis of serial sections of individual fish with monoclonal antibody 1-12-3 to teleost CYP1A. CYP1A staining occurred in endothelial cells of many organs of TCDD-exposed but not vehicle-exposed embryos at 1 week prehatch and sac fry at 2 weeks posthatch. Earlier developmental stages examined were negative for CYP1A expression at any dose of TCDD. The strongest response occurred in sac fry at TCDD doses greater than 88 pg TCDD/g egg but was detected at doses as low as 22 pg TCDD/g egg. CYP1A staining in endothelium appeared at lower doses and was stronger than that in other cell types, in both prehatch embryos and posthatch sac fry. Thus, the vascular system is a major initial site affected by TCDD in lake trout early life stages, and the vascular endothelium is a cell type uniquely sensitive to induction of CYP1A in these developing animals. Based on an index of immunohistochemical staining of CYP1A, endothelial CYP1A induction in sac fry by TCDD occurred with an ED50 of 64-69 pg TCDD/g egg, similar to the dose-response for mortality occurring during the sac fry stage of development (LD50 = 47 pg TCDD/g egg). The correlations seen here suggest that CYP1A or aryl hydrocarbon receptor (AhR) in the endothelium may be linked to early lesions that result in TCDD-induced vascular derangements leading to yolk sac, pericardial, and meningial edema that is associated with lake trout sac fry mortality, but the precise mechanism remains to be determined.

Identification of NIH-shifted metabolites of 1,3,7,8-tetrachlorodibenzo-p-dioxin in the rat by NMR comparison with synthesized isomers

In the rat, 1,3,7,8-tetrachlorodibenzo-p-dioxin was oxidatively metabolized to the NIH-shifted products 2-hydroxy-1,4,7,8-tetrachlorodibenzo-p-dioxin and 3-hydroxy-1, 2,7,8-tetrachlorodibenzo-p-dioxin. The chlorine substitution patterns were determined by comparison with 1H NMR spectra of six synthesized isomers in CDCl3, CD3OD, and acetone-d6. Glucuronide and glucuronide-sulfate conjugates of the monohydroxy dioxins were identified in the bile by FAB-MS. The dominance of the NIH-shift products in the metabolism of tetrachlorodibenzo-p-dioxins indicates that the same isomers may be produced from differently substituted chlorinated dioxins.

Derivation of 2,3,7,8-TCDD toxic equivalent factors (TEFs) for selected dioxins, furans and PCBs with rainbow trout and rat liver cell lines and the influence of exposure time

The effect of exposure time on the induction of 7-ethoxyresorufin O-deethylase (EROD) by 2,3,7,8,-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), and 3,3',4, 4'-tetrachlorobiphenyl (PCB 77) was examined in two liver cell lines: rainbow trout RTL-W1 and rat H4IIE. In RTL-W1, exposure time had no effect on induction EC50s and derivation of TCDD- toxic equivalency factors (TEFs). In H4IIE, EC50s for TCDD also were unchanged. However, for TCDF and PCB 77 in H4IIE, EC50s increased and TEFs decreased as exposure time increased from 6 to 72 h. These results suggest that metabolism of TCDF and PCB 77 can account for some of the previously observed differences between RTL-W1 and H4IIE TEFs.

Metabolic degradation, inducing potency, and metabolites of fluorinated and chlorinated-fluorinated dibenzodioxins and dibenzofurans

The metabolic degradation of fluorinated, chlorinated-fluorinated and chlorinated congeners was measured in liver homogenate of NMRI mice. While in the time period between 0 and 240 min no degradation of the 2,3,7,8-TCDD/TCDF could be detected, for all fluorinated congeners a perceptible degradation was found, even for the 2,3,7,8-TFDD. Stepwise chlorination of the 2,3,7,8-fluorinated congeners leads to a decrease of the degradation rate. In the EROD test, the exchange of chloro- with fluorosubstituents in the 2,3,7,8-TCDF leads to a decrease of induction potency. 3,7-Dichloro-2,8-difluorodibenzofuran was about 1/1000th as potent as 2,3,7,8-TCDF, while 2,3,7,8-TFDF was complete inactive. Comparison of the metabolic rates of different TCDD with those of the analogous TFDD demonstrates that the order of enzymatic degradation of different TCDD and the analogous TFDD is identical. The TFDD are degraded slightly faster than the corresponding TCDD. Surprisingly 1,4,6,9-TXDD showed the second slowest metabolic rate of the fluorinated and chlorinated TXDD after 2,3,7,8-TXDD although none of the 2,3,7,8-positions were substituted. Judging from 2,3,7,8-TFDD and 1,7-dichloro-2,8-difluorodibenzofuran the metabolic pathway of fluorinated and chlorinated-fluorinated congeners seem to be comparable to the chlorinated congeners.

Intake, fecal excretion, and body burden of polychlorinated dibenzo-p-dioxins and dibenzofurans in breast-fed and formula-fed infants

To assess toxicokinetics of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), oral intake and fecal excretion were measured in two breast-fed infants and one formula-fed infant during the 1st y of life. The intake of these compounds was up to 50 times higher in the breast-fed infants. In these children, fecal excretion of the main tetra- to hexachlorinated congeners was less than 9% of the intake at age of 1 and 5 mo, indicating almost complete intestinal absorption during breast-feeding. In contrast, distinctly higher fecal excretion rates were observed for the hepta- and octachlorinated compounds. Despite much lower PCDD/PCDF intake after weaning, concentrations in stool fat did not decrease substantially. We conclude that concentrations in fecal fat more or less reflect those in body fat. Additionally, PCDD/PCDF concentrations were measured in blood fat of all infants (and in a second formula-fed baby) at the age of 11 mo. International toxicity equivalent (I-TEq) concentrations in the formula-fed infants were less than 25% of maternal values and about 10 times lower than in the infants breast-fed for 6-7 mo. In the latter, a distinct accumulation was found for the tetra- to hexachlorinated congeners compared with maternal concentrations. We conclude that accumulation of PCDDs and PCDFs in infants is as high as expected on the basis of intake data and assuming complete absorption and negligible elimination during the 1st y of life.

Intranuclear distribution of 2,2',4,4',5,5'-hexachlorobiphenyl and 2,3,7,8,-tetrachlorodibenzo-p-dioxin

The in vitro intranuclear distribution of the chlorinated aromatic hydrocarbon compounds 2,2',4,4',5,5'-hexachlorobiphenyl and 2,3,7,8-tetrachlorodibenzo-p-dioxin was determined in isolated rat liver cell nuclei. Suspended nuclei were incubated with the 3H-labeled congeners, and the incubation terminated by brief UV irradiation. High-intensity UV irradiation at 254 nm changes the reversible association between macromolecules and ligands into covalent linkages and thus stabilizes the equilibrium distribution. The nuclei were then fractionated with the radioactive congeners covalently linked to the purified fractions. The intranuclear distribution of the model compounds was not uniform. The majority of either chemical was attached to the nuclear envelope and to the chromatin fraction. Much lower amounts were bound to nucleoli. The nuclear matrix was almost devoid of the chemicals. Minute amounts of either compound were detected in association with DNA, none with nuclear RNA. The substantial association of these chlorinated hydrocarbon model compounds with chromatin may bear more general biological relevance and point to detrimental effects on the genetic apparatus. The presented method yields an unequivocal profile of the genuine nuclear distribution of photoactivatable chemicals.

Exposure of laboratory animals to polychlorinated dibenzodioxins and polychlorinated dibenzofurans from commerical rodent chow

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/PCDFs) are ubiquitous low level environmental contaminants. Present finding of PCDDs and PCDFs in the human food supply suggested that laboratory animals maintained under controlled conditions may also receive unwanted exposure to these compounds from food. In this study, a commercial rodent chow, Prolab-RMH 1000, was analyzed for the toxicologically active 2,3,7,8-substituted PCDDs and PCDFs. The results show that the feed had a total PCDD/F dioxin toxic equivalence (I-TEq) of 0.13 pg/kg (ppt) wet weight. OCDD was the most abundant congener present, followed by 1,2,3,4,6,7,8-HpCDD, with these congeners representing the 5th and 6th greatest contributors to the calculated TEq. Daily ingestion of I-TEqs for the rats is calculated to be 3.9-6.5 pg/kg/day, and 13.0-32.5 pg/kg/day for mice. It is therefore important to consider the potential for background exposure to PCDDs and PCDFs from the diet when conducting studies with laboratory animals. This is particularly important when sensitive responses, such as enzyme induction, immunotoxicity, and developmental/reproductive toxicity are being investigated at very low exposures to this class of compounds.

Bald eagle mortality and chlorinated hydrocarbon contaminants in livers from British Columbia, Canada, 1989-1994

Between 1989 and 1994, we obtained 278 carcasses of bald eagles (Haliaeetus leucocephalus) found dead or dying in British Columbia, Canada. All specimens were necropsied and the cause of death determined wherever possible. Livers from a subset of 75 birds were analyzed for polychlorinated biphenyl (PCB) and organochlorine (OC) pesticide residues. A further subset of 19 eagles found dead around the Strait of Georgia, an area of known pulp mill pollution, in summer, and therefore presumably resident birds, were analyzed for polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) and non-ortho PCBs. Liver concentrations ranged from less than 1 to 190 mg/kg for DDE, and up to 72 mg/kg for total PCBs. Concentrations of other OCs were generally less than 1 mg/kg, with the exception of chlordane-related compounds which were occasionally over 2 mg/kg. All birds analyzed for PCDDs and PCDFs contained detectable concentrations of the major 2,3,7,8-substituted isomers. Some birds were very contaminated; one eagle found near a kraft pulp mill site in 1990 contained: 400 ng/kg 2,3,7,8-TCDD, 1400 ng/kg 1,2,3,7,8-PnCDD and 4400 ng/kg 1,2,3,6,7,8-HxCDD. Birds with higher PCB and dichlorodiphenyldichloroethane (DDE) concentrations appeared to weigh less, and there was a significant negative relationship between both PCBs and DDE and numeric scoring of body condition, reflecting the well known process of starvation-induced mobilization of body lipids and contaminants. Birds with higher 2,3,7,8-TCDD concentrations tended to have unusually low concentrations of 2,3,7,8-TCDF, interpreted to indicate hepatic cytochrome P4501A-type induction by TCDD and subsequent metabolism of TCDF.

Immunosuppressive potential of several polycyclic aromatic hydrocarbons (PAHs) found at a Superfund site: new model used to evaluate additive interactions between benzo[a]pyrene and TCDD

Exposure to environmental pollution is rarely limited to a single compound or even a single class of compounds. The Superfund site located in Massena, NY, is contaminated by both halogenated aromatic hydrocarbons (HAHs) and polycyclic aromatic hydrocarbons (PAHs). Since representatives of both HAHs and PAHs are capable of binding to the aromatic hydrocarbon receptor (AhR), two well-documented AhR-mediated effects, immunosuppression and induction of hepatic aryl hydrocarbon hydroxylase (AHH) activity, were used to evaluate the individual and interactive toxicity of these compounds. Fifteen PAHs were first screened for their ability to suppress the antibody response in C57BL/6 (Ah+/+) mice immunized 12 h after a single oral dose of 0.1, 1, 10, or 100 mg/kg. Acenaphthene, anthracene, benzo[g,h,i]perylene, fluoranthene, fluorene, naphthalene, phenanthrene, and pyrene had little or no effect. Seven PAHs caused > 50% suppression at 100 mg/kg. Listed in order of decreasing potency they were benzo[k]fluoranthene, benzo[b]fluoranthene, indeno[1,2,3,c,d]pyrene, benzo[a]pyrene, chrysene, dibenzo[a,h]anthracene, and benz[a]anthracene. Chrysene and benzo[a]pyrene (B[a]P), were further evaluated to determine the dependence of these effects on the Ah phenotype by comparing responses of C57BL/6 and congenic B6.D2 (Ah-/-) mouse strains. Chrysene immunosuppression was maximal at 0.1 mg/kg and was Ah phenotype-independent whereas chrysene AHH induction was Ah phenotype-dependent, but a 100-fold less sensitive indicator of exposure. In contrast, B[a]P immunosuppression and AHH induction were coincident in B6 mice and Ah phenotype-dependent. In the final phase, a new approach was used to evaluate toxic interactions. This approach considers the mechanism of action of each compound and accounts for the fact that the extent of increase in toxic response caused by an incremental change of dose is determined by its position on the dose-response curve rather than on the absolute amount of dose administered. Thus, the immunotoxic effects of combined exposure to B[a]P and the AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a representative HAH, were evaluated by combining the ED20 of B[a]P with the difference between the ED20 and ED40 of TCDD, and vice versa, to produce 40% suppression. The results of the combination were consistent with additivity regardless of the composite arrangement or phenotype although some antagonism could not be excluded with certainty.

Toxic equivalency factors do not predict the acute toxicities of dioxins in rats

Risk evaluation of complex environmental mixtures of polychlorinated dibenzo-p-dioxins and related halogenated aromatic hydrocarbons (polychlorinated dibenzofurans, azo- and azoxybenzenes, naphthalenes and some of the biphenyls) is currently carried out by measuring the concentration of each congener in the mixture and then multiplying every figure by its specific constant, toxic equivalency factor (TEF). All congeners are thought to produce highly similar effects albeit at different doses, and the TEFs are believed to represent the potencies of the congeners relative to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), considered the most toxic derivative of this class of environmental contaminants. Here we compared the acute toxicities of TCDD, 1,2,3,7,8-penta-, 1,2,3,4,7,8-hexa- and 1,2,3,4,6,7,8-heptachloro-dibenzo-p-dioxin in the most TCDD-susceptible (Long-Evans Turku AB; L-E) and the most TCDD-resistant (Han/Wistar kuopio; H/W) rat strain. While L-E rats exhibited the expected rank order of sensitivities to the four dioxins, the higher chlorinated dioxins were more toxic than TCDD (in terms of acute lethality) to H/W rats, with the hexachlorodioxin showing the greatest potency. Even if the doses were adjusted according to the LD50 values, both biochemical and morphological effects elicited by the dioxins turned out to depend, often critically, on strain, congener or the interaction of these two determinants. These findings demonstrate that the dioxins have distinct profiles of acute toxicities and underscore the importance of response and test organism in defining the TEFs.

Occupational exposure to polychlorinated dibenzo-p-dioxins and dibenzofurans in a magnesium production plant

OBJECTIVES

The production of magnesium is a well known source of both aliphatic and aromatic chlorinated compounds, among others the polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). The aim of this study was to establish whether increased concentrations of PCDDs and PCDFs could be found in the blood of workers in a magnesium plant.

METHODS

Blood plasma from 10 workers, employed at a magnesium plant for 10 to 36 years, and from a control group consisting of nine people who had no direct contact with the production were studied. Isomer specific analyses of PCDDs and PCDFs by means of high resolution gas chromatography/high resolution mass spectrometry (HRGC/HRMS) techniques were performed.

RESULTS

A significant increase was found in the concentrations of some of the congeners, mainly PCDFs, in the workers compared with the control group. Octachlorodibenzofuran (OCDF) is the congener that most strongly correlates with occupational exposure in the magnesium plant. Low concentrations of 1,2,3,4,6,8,9-heptachlorodibenzofuran were found in seven of the workers. Such isomers--that is, not 2,3,7,8-substituted--are rarely found in human samples.

CONCLUSION

Due to the long biological half lives and lipid solubility of PCDDs and PCDFs, blood analyses may serve as an index of past cumulative occupational exposure and a means of assessing a person's exposure situation.

Short term fasting does not aggravate immunosuppression in harbour seals (Phoca vitulina) with high body burdens of organochlorines

Two groups of 11 harbour seals (Phoca vitulina) with different body burdens of organochlorines were subjected to an experimental 15-day fasting period, during which they lost an average 16.5% of their body weights. Blood levels of the most persistent organochlorines showed an approximate twofold increase, while levels of aryl hydrocarbon receptor-binding organochlorines remained largely unaffected. Few differences in immunological parameters were observed between the two dietary groups. Numbers of circulating lymphocytes dropped to about 65% of the initial values and NK cell activity showed a slight increase in both groups. Mitogen- and antigen-induced lymphoproliferative responses of the Baltic group of seals remained within normal ranges. These results suggest that relatively short-term fasting periods do not present an additional immunotoxicological risk to seals with high body burdens of organochlorines.

Individual PCBs as predictors for concentrations of non and mono-ortho PCBs in human milk

32 Dutch human milk samples were analyzed for PCBs with either HRGC-ECD or HRGC-LRMS in the NCI mode. Samples were collected from three different locations in The Netherlands: Amsterdam, Rotterdam and Groningen. Quantitatively, no differences could be observed between the three localities, while in addition the congener specific pattern showed a striking similarity for all individual samples. Only principal component analysis revealed slight individual differences. Based on similarities in the PCB profiles, linear relationships were calculated between 2,3'4,4',5-PnCB (#118) or 2,2'4,4'5,5'HxCB (#153) and the most relevantnon andmonoortho PCBs exhibiting dioxinlike activity. These PCBs included 2,3,3',4,4'-PnCB (#105), 3,3',4,4'5-PnCB (#126) 2,3,3',4,4',5-HxCB (#156), 2,3,3',4,4',5'-HxCB (#157), 2,3',4,4',5,5'-HxCB (#167) and 3,3',4,4',5'5-HxCB (#169).Good linear relationships were observed between individual PCBs. Based on the results of this study, PCB #118 can be used to predict concentrations of the PCBs #105 and #126. PCB #153 can be used as a predictor for the PCBs #156, #157, #167 and #169, but also for the total toxic equivalencies (TEQs) ofnon andmonoortho PCBs present in human milk. This method using certain PCBs as predictors for other toxicological relevant congeners, can be useful and cost effective, e.g. for epidemiological studies. However, before applied a number of conditions should be met. These are: 1) A stable composition of the PCB matrix should be established. 2) A possible time dependent change in composition of the matrix should first be excluded when used over different time periods.

A teratologic suppressor role for p53 in benzo[a]pyrene-treated transgenic p53-deficient mice

DNA damage may mediate birth defects caused by many drugs and environmental chemicals, therefore p53, a tumour suppressor gene that facilitates DNA repair, may be critically embryoprotective. We have studied the effects of the environmental teratogen, benzo[a]pyrene, on pregnant heterozygous p53-deficient mice. Such mice exhibited between 2- to 4-fold higher embryotoxicity and teratogenicity than normal p53-controls. Fetal resorptions reflecting in utero death were genotyped using the polymerase chain reaction and found to be increased 2.6-fold and 3.6-fold respectively with heterozygous and homozygous p53-deficient embryos. These results provide the first direct evidence that p53 may be an important teratological suppressor gene which protects the embryo from DNA-damaging chemicals and developmental oxidative stress.

Peri- and postnatal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin: effects on physiological development, reflexes, locomotor activity and learning behaviour in Wistar rats

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the development of rat offspring were studied after administration of a loading dose of 300 or 1000 ng TCDD/kg body wt on day 19 of pregnancy, followed by weekly maintenance doses of 120 or 400 ng TCDD/kg body wt. The dose regimens led to a fluctuation of average TCDD concentrations in the liver of the offspring of 4.9-14.9 ng/g (TCDD1000/400 group) or 1.4-6.3 ng/g (TCDD300/120 group) during the course of the experiment. In both TCDD-exposed groups the body weight of the offspring was significantly lower on postnatal day 7 (PND 7); in the high dose group from PND 7 to PND 31. Some landmarks of postnatal development were retarded in the exposed groups; in particular, the vaginal opening was delayed for several days in both TCDD-exposed groups. The TCDD-exposed animals revealed a reduced ability to remain on a rotating rod. During reflex testing, the rate of successfully responding animals was higher in the exposed groups. No statistically significant differences in the locomotor activity between controls and TCDD-exposed offspring were detectable under our experimental conditions. In a discrimination learning test no effects on the learning ability were found. However, TCDD-exposed offspring showed an increase in unanswered trials during critical phases of the task. They also exhibited increased locomotor activity in a novel environment; prior to an amphetamine challenge dose of 1 mg/kg body weight. Amphetamine-induced activity was decreased in a dose-dependent manner.

Effects of polychlorinated biphenyls (PCBs) and dioxins on growth and development

Polychlorinated biphenyls (PCBs) and dioxins are potentially toxic compounds which occur widely in the environment. Their effects on the growth and development of infants at the levels currently found in highly industrialised western countries is not well known. This Dutch multicenter study, combining animal and human studies, tries to answer this question. Animal studies showed that PCB 169, given once during pregnancy at a dose of 1.8 g kg-1 bodyweight, has an effect on developmental parameters, dopamine regulation and fertility. Effects on thyroid hormones were also found in animals, probably due to both a competitive binding of PCB metabolites to the thyroxine binding protein and increased glucuronidation. Perhaps to compensate for this, an increased diodase activity in the brain was found. Human studies involved 400 mother-infant pairs, half of them being breast-fed, the other half were fed a formula devoid of PCBs and dioxins. PCB levels were measured in serum and dioxin and PCB levels in breastmilk. Levels were found to be as high as previously found in highly industrialised countries. Growth and development were carefully documented, but no data are as yet available. In pregnant women, a significant negative correlation was found between some dioxin and PCB congeners in milk and plasma thyroid hormones, while newborn infants showed higher thyroid stimulating hormone (TSH) at higher levels of dioxin exposure. In summary, data from this combined multicenter study involving animals and humans increases our insight into the potentially negative effects of PCBs and dioxins on growth and development.