Comparison of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) disposition following pulmonary, oral, dermal, and parenteral exposures to rats

In evaluating human health risks posed by dioxins, it is necessary to accurately predict systematic dosimetry or the fate of these chemicals. Pharmacokinetic parameters pertaining to inhalation, ingestion, and dermal absorption may be estimated using animal models. The present study was designed to assess absorption, tissue distribution, and elimination of TCDD following intratracheal instillation (itr.), oral gavage (p.o.), or intravenous administration (i.v.) of 1 nmol [3H]TCDD/kg to male rats; experimental conditions were chosen to permit comparison to a previous dermal disposition study (Banks and Birnbaum, Toxicol. Appl. Pharmacol. 107, 302-310, 1991). After treatment, rats were housed in individual metabolism cages for 3 days with daily excreta collection. Following termination, radioactivity was quantified in tissues and excreta. By 3 days postexposure, fecal excretion accounted for 22 (i.v.), 26 (itr.), and 32% (p.o.) of dose, while urinary excretion was only 2.2 (i.v.), 1.3 (itr), and 1.4% (p.o.). Pulmonary absorption was calculated as 95% of administered dose, while oral absorption was 88%. Dermal absorption of an equivalent administered dose was 40% (Banks and Birnbaum, 1991). For all exposure routes by 3 days, major tissue depots for absorbed dose were liver and fat. Distribution of absorbed dose was 37% (i.v.) and 35% (itr.) to liver and 21% (i.v.) and 16% (itr.) to fat. Oral gavage-treated rats had similar dosimetry (28-30% absorbed dose) in both liver and fat. In contrast following dermal exposure, distribution to liver and fat was 52 and 22%, respectively (Banks and Birnbaum, 1991). Results suggest that inhalation can be an important route for systemic absorption of dioxins. Moreover, all environmentally relevant routes of exposure (oral, dermal, and respiration) must be uniquely considered as important routes of systemic exposure for TCDD and related compounds.

Relationship between CYP1A enzyme activities and protein levels in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Induction of CYP1A1 is one of the best characterized responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). EROD activity has been used as an enzymatic marker for CYP1A1 following TCDD treatment. Enzymatic markers for the induction of CYP1A2 by TCDD are not as well characterized. The present study examines the relationship between CYP1A1 and CYP1A2 protein and the corresponding enzymatic markers. Induction of hepatic ethoxyresorufin O-deethylase (EROD) activity and methoxyresorufin O-demethylase (MEROD) and acetanilide 4-hydroxylase (ACOH) activity (both markers for CYP1A2) were analyzed in 8-wk-old male and female Fischer 344 rats treated orally with either 0, 0.1, 0.3, 1.0, or 3.0 micrograms TCDD/kg. There were no sex differences in basal EROD or ACOH activity. MEROD activity was significantly greater in control males than in control females. Significant induction of EROD activity in females occurred at slightly lower doses of TCDD compared to males (0.1 vs. 0.3 micrograms/kg, respectively); however, a greater absolute and a larger fold induction of EROD activity was seen in males compared to females at all doses tested except 0.1 micrograms/kg. EROD activity did not attain a maximum in either sex. Similarly, MEROD activity was induced at lower doses of TCDD in females than in males (0.1 vs. 0.3 micrograms/kg, respectively). MEROD activity was maximally induced at 0.3 micrograms/kg in males. In females, MEROD did not attain maximum induction at the doses tested. ACOH activity was induced at doses as low as 0.3 micrograms/kg in both sexes, and the dose-dependent increases in activity were equivalent in males and females. Both ACOH and MEROD activity correlated well with CYP1A2 levels as determined by Western blot analysis, although there was a greater fold induction of protein than either MEROD or ACOH. Although MEROD and ACOH are both markers for the same response, MEROD activity may be a more useful marker because it is the quicker and more sensitive of the two assays.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on influenza virus host resistance in mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes numerous immunotoxic effects including thymic involution and an immunosuppression of nonspecific as well as specific cell- and humoral-mediated immunity. TCDD administration to laboratory animals also results in a decreased resistance to numerous bacteria, viruses, and parasites. Effects on virus host resistance appear to be among the most sensitive effects of TCDD immunotoxicity. However, previous studies have not achieved a no effect level. The present studies utilized an influenza virus host resistance model in mice to quantify the sensitivity of this model to TCDD and to determine the NOAEL (no observed adverse effect level) of TCDD for influenza virus. Results indicated that a single dose of TCDD at 0.10, 0.05, or 0.01 microgram/kg resulted in an increased mortality to Hong Kong influenza virus when mice were challenged 7 days after TCDD administration. Increased mortality was not correlated with increased virus titers in the lungs. TCDD at 0.005 or 0.001 micrograms/kg had no effect on influenza-induced mortality. TCDD alone did not affect thymus weight at any dose administered in this study. TCDD also did not alter the virus-enhanced increase in lung weight:body weight ratio nor the virus-induced decrease in thymus weight. Thus, low levels of TCDD exposure lead to enhanced mortality to influenza virus; however, the mechanism of this effect remains to be elucidated. Nonetheless, enhanced mortality to influenza virus in mice following a single dose of 10 ng TCDD/kg represents the most sensitive adverse effect yet reported for TCDD.

Use of toxic equivalency factors for risk assessment for dioxins and related compounds

TCDD is the most toxic member of a class of polyhalogenated aromatic hydrocarbons that are structurally related, have a similar mechanism of action, and cause the same spectrum of responses. Because of the need to assess the risk from complex mixtures of these chemicals, the international community has adopted an interim approach that assigns relative potency factors to this family of chemicals, based on a comparison with the potency of TCDD. Each chemical that fits the criteria for this class is assigned a toxic equivalency factor, TEF, which is some fraction of that of TCDD. The total toxic equivalency of a mixture, TEQ, is the sum of the weighted potency of each compound in the mixture. Although there may be some variability between different responses in the determination of a TEF value for a compound, endpoint-specific TEFs are usually very similar. There may also be some species differences in TEFs. Again, if pharmacokinetic factors are taken into account, they are usually relatively minor. TEFs based on intake values may also exhibit some differences when compared to those based on target tissue concentrations. Using scientific judgment and a broad data base, interim TEF values have been recommended for PCDDs, PCDFs, and dioxin-like PCBs. Using such values, the TEF approach has been successful at predicting the toxicity of real world mixtures. Ongoing studies from our laboratory have validated the approach for synthetic mixtures that approximate congener ratios found in food samples. Whether non-additive interactions occur with nondioxin-like compounds found in environmentally relevant concentrations remain to be determined.

Water chlorination: essential process or cancer hazard?

Chlorine has been successfully used for the control of waterborne infectious disease for nearly a century. In the 1970s it was found that chlorine reacted with natural organic matter present in surface waters to produce disinfection by-products (DBP). Concern focused initially on the trihalomethanes (THM), but a wide variety of DBPs are now known to result from chlorination. Chlorination of drinking water has been one of the most effective public health measures ever undertaken. There are a number of alternatives to chlorination that are in active use in many parts of the world, but the risks associated with their by-products are even less well established than for chlorination. Moreover, the use of these alternatives vary in their effectiveness and some require greater sophistication in their application. This can mean less protection to public health as a result of inappropriate application and control. Therefore, hazards associated with the use of such a clearly beneficial process as chlorination must be carefully considered not only in an absolute sense, but also in the context of alternative approaches for producing a safe drinking water. The key question is whether the hazards associated with by-products have been sufficiently well established to warrant regulations that will undoubtedly have both positive and negative impacts on the public health. This symposium examined the toxicological and epidemiological data on chemical hazards associated with chlorination and attempted to measure this hazard against competing microbial risks. The first presentation discussed the available analytical epidemiological studies. A second presentation dealt with the importance of chlorination to the prevention of waterborne infectious disease. Pharmacokinetic, mechanistic, and modeling information on the prototypical DBP, chloroform, were discussed and contrasted with data on brominated THMs to determine if it was scientifically appropriate to regulate THMs as a single toxicological class. The fifth presentation dealt with the carcinogenic properties of a potent mutagen that is produced by chlorination. The final presentation discussed the haloacetates, carcinogenic DBPs whose concentrations approach and occasionally exceed those of the THMs. Clearly, there is a need to carefully weigh these different types and sometimes competing risks when considering the delivery of drinking water to ever-increasing populations for which there are finite sources of fresh water.

Developmental effects of dioxins and related endocrine disrupting chemicals

Alteration of hormones has long been known to affect development. TCDD and related PHAHs modulate the levels of many hormonal systems. Dioxins cause a spectrum of morphological and functional developmental deficits. Fetotoxicity, thymic atrophy, and structural malformations are often noted. Delayed genitourinary tract effects have been observed, and recent studies reported behavioral effects. Highly exposed human offspring have exhibited developmental problems as well. Recently, hormonal and neurological abnormalities have been reported in infants from the general population. The complex alteration of multiple endocrine systems is likely associated with the spectrum of adverse developmental effects caused by dioxin and related compounds.

Developmental effects of dioxins

The potent developmental toxicity of dioxin in multiple species has been known for a number of years. However, recent studies have indicated that dioxin also induces functional developmental defects, many of which are delayed. Subtle structural deficits, not detectable at birth, have also been described in multiple species and in both sexes. Certain defects have been reported not only in animals but also in children prenatally exposed to complex mixtures containing dioxinlike compounds. None of the effects can be attributed to modulation of any one endocrine system. For example, dioxin does not bind to the estrogen receptor, but it can cause effects that are both estrogenic and antiestrogenic. However, viewing dioxin and related compounds as endocrine disruptors that may alter multiple pathways sheds some light on the complexities of this potent class of growth dysregulators.

Developmental expression of two members of a new class of transcription factors: I. Expression of aryl hydrocarbon receptor in the C57BL/6N mouse embryo

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor with a basic region/helix-loop-helix (bHLH) motif. AhR has been sequenced and the functional domains defined and there is information on the formation of complexes with other peptides and interactions with DNA, although these areas continue to be investigated. AhR mediates many biological effects such as developmental toxicity, including induction of cleft palate and hydronephrosis. This regulatory protein is expressed in embryonic liver and has been immunohistochemically localized in cells of human and mouse secondary palate. The expression of AhR in embryonic tissues and its ability to disrupt development suggests a significant role for this protein in development. The present study examines the pattern of AhR expression in the C57BL/6N mouse embryo from gestation days (GD) 10-16, using in situ hybridization and immunohistochemical analysis. AhR mRNA was localized with 35S-RNA antisense riboprobe (cAh1 probe, 1.8 Kb amino terminal DNA). AhR protein was localized with purified monoclonal antibody (RPT-9) raised against the N-terminal peptide sequence. AhR mRNA and protein were expressed in GD 10-13 neuroepithelium, and as development progressed the levels in brain decreased. GD 10-12 embryos also showed AhR in branchial arches, heart, somites, and liver. AhR protein and mRNA in heart were highest at GD 10-11 and decreased with age. In liver, AhR mRNA and protein levels increased and nuclear localization became more pronounced with gestational age. In GD 14-16 embryos levels in liver and adrenal were highest, but AhR was present in ectoderm, bone, and muscle. AhR expression was specific for both cell type, organ/tissue, and developmental stage, suggesting that this novel ligand-activated transcriptional regulator may be important in normal embryonic development.

Dioxins: model chemicals for assessing receptor-mediated toxicity

Dioxins and related compounds are chlorinated aromatic hydrocarbons that are persistent in both environmental and biological samples. Many members of this class of compounds produce a similar spectrum of toxicity which is mediated by interaction with the Ah receptor. The toxic effects of these chemicals can best be described by their actions as growth dysregulators. Dioxins disrupt normal homeostatic processes that tightly regulate cellular growth and differentiation. Disruption in these processes produce a variety of toxicities and pathologies. The available data indicate that humans are sensitive to the toxic effects of these chemicals. Clearer definition of human responses and the body burdens associated with such effects requires more research. Comprehensive risk assessments of dioxins should include all Ah receptor ligands such as the halogenated dibenzofurans and biphenyls.

Comparisons of estimated human body burdens of dioxinlike chemicals and TCDD body burdens in experimentally exposed animals

Humans are exposed to mixtures of polyhalogenated aromatic hydrocarbons, and the potential health effects of these exposures are uncertain. A subset of this class of compounds produce similar spectra of toxicity in experimental animals as does 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and these chemicals have been classified as "dioxins." In this study, we compared the body burdens of dioxins that produce effects in experimental animals to body burdens associated with these effects in humans. Human body burdens were estimated from lipid-adjusted serum concentrations of dioxins, assuming dioxins are equally distributed in body fat and an adult has 22% body fat. The toxic equivalency factor (TEF) method was used to calculate body burdens of dioxins in humans. These calculations included dibenzo-p-dioxins, dibenzofurans, and polychlorinated biphenyls. In the general population, average background concentrations were estimated at 58 ng TCDD equivalents (TEQ)/kg serum lipid, corresponding to a body burden of 13 ng TEQ/kg body weight. Populations with known exposure to dioxins have body burdens of 96-7,000 ng TEQ/kg body weight. For effects that have been clearly associated with dioxins, such as chloracne and induction of CYP1A1, humans and animals respond at similar body burdens. Induction of cancer in animals occurs at body burdens of 944-137,000 ng TCDD/kg body weight, while noncancer effects in animals occur at body burdens of 10-12,500 ng/kg. Available human data suggest that some individuals may respond to dioxin exposures with cancer and noncancer effects at body burdens within one to two orders of magnitude of those in the general population.

Functional aspects of developmental toxicity of polyhalogenated aromatic hydrocarbons in experimental animals and human infants

A scientific evaluation was made of functional aspects of developmental toxicity of polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in experimental animals and in human infants. Persistent neurobehavioral, reproductive and endocrine alterations were observed in experimental animals, following in utero and lactational exposure to PCBs, PCDDs and PCDFs. The lowest observable adverse effect levels (LOAELs) for developmental neurobehavioral and reproduction endpoints, based on body burden of TCDD-toxic equivalents (TEQs) in animals, are within the range of current background human body burdens. Relatively subtle adverse effects on neurobehavioral development and thyroid hormone alterations have also been observed in infants and children exposed to background levels. Exclusive use of the toxic equivalency factor (TEF) approach may underestimate the risk of neurodevelopmental effects, because both Ah receptor dependent and independent mechanisms may be involved in these effects. The use of marker congeners and/or bioassays based on Ah receptor mediated mechanisms are rapid, low cost pre-screening alternatives for expensive and time consuming gas chromatographic-mass spectrometric analysis.

Functional aspects of developmental toxicity of polyhalogenated aromatic hydrocarbons in experimental animals and human infants

A scientific evaluation was made of functional aspects of developmental toxicity of polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in experimental animals and in human infants. Persistent neurobehavioral, reproductive and endocrine alterations were observed in experimental animals, following in utero and lactational exposure to PCBs, PCDDs and PCDFs. The lowest observable adverse effect levels (LOAELs) for developmental neurobehavioral and reproduction endpoints, based on body burden of TCDD-toxic equivalents (TEQs) in animals, are within the range of current background human body burdens. Relatively subtle adverse effects on neurobehavioral development and thyroid hormone alterations have also been observed in infants and children exposed to background levels. Exclusive use of the toxic equivalency factor (TEF) approach may underestimate the risk of neurodevelopmental effects, because both Ah receptor dependent and independent mechanisms may be involved in these effects. The use of marker congeners and/or bioassays based on Ah receptor mediated mechanisms are rapid, low cost pre-screening alternatives for expensive and time consuming gas chromatographic-mass spectrometric analysis.

Workshop on perinatal exposure to dioxin-like compounds. V. Immunologic effects

The immune system comprises a highly integrated network of multiple tissues and cell types with complicated interactions and effects. It is modulated by the endocrine and nervous systems and there is growing realization of its multifunctionality. The session focusing on immunologic effects of dioxin and related compounds following prenatal exposure involved a review of the immunotoxic effects that have been reported for polyhalogenated aromatic hydrocarbons (PHAHs), a discussion of species differences in responses, and development of the immune system, and data from two ongoing epidemiological studies comparing the immune status of children exposed to higher-than-average concentrations of PHAHs both prenatally and lactationally.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) distribution and cytochrome P4501A induction in young adult and senescent male mice

While the developmental toxicology of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and its congeners has received considerable attention, the impact of advanced age on the biochemical effects and the pharmacokinetics of dioxins remains largely undetermined. In the present investigation, TCDD tissue distribution and cytochrome P4501A (CYP1A) induction were characterized in male C57BL/6N mice aged 10 weeks and 28 months at 7 days after administration of single oral [3H]TCDD doses ranging from 0.015 to 15 microgram/kg body wt. Determinations of hepatic marker enzyme activities for CYP1A1 (ethoxyresorufin O-deethylation, EROD) and 1A2 (acetanilide-4-hydroxylation, ACOH) indicated that the dose response curves for EROD induction by TCDD were nearly identical for the 2 age groups, but the ACOH induction response was greater in old mice. After receiving the 15 micrograms/kg dose, an increase (approximately 35%) in relative liver weight was observed 7 days after dosing in the 10-week mice, but not in the aged mice, and the hepatic concentration of TCDD was approximately 25% greater in young than old mice. No age difference was found in hepatic nuclear concentrations of TCDD. A dose-dependent increase in liver:fat tissue concentration ratios was noted at both ages, and adipose tissue and blood concentrations of TCDD did not vary significantly with age. In old mice however, TCDD concentrations in skin, kidney and muscle were all approximately twice those of young mice at the 15 micrograms/kg dose. These results suggest that advanced age may have differential effects on Ah receptor-mediated enzyme induction, while increased TCDD concentrations in certain tissues may have toxicological implications for older animals.

Exposure to TCDD during development permanently alters reproductive function in male Long Evans rats and hamsters: reduced ejaculated and epididymal sperm numbers and sex accessory gland weights in offspring with normal androgenic status

Prenatal administration of relatively low doses of TCDD alters reproductive development and fertility of the progeny. Fertility was reduced in the progeny of Wistar rats exposed to 0.5 micrograms TCDD/kg/day from Gestational Day (GD) 6 to GD 15. In a three-generation reproduction study, TCDD reduced fertility of Sprague-Dawley rats in the F1 and F2 but not the F0 (no developmental exposure) generation at 0.01 microgram/kg/day in the diet. Furthermore, administration of TCDD on GD 15 (at 0.064 to 1 microgram/kg) both demasculinized and feminized morphology and behavior of Holtzman male rat offspring. Our objectives were to expand the observations of Mably et al. (1992, Toxicol, Appl. Pharmacol. 114, 97-107, 108-117, 118-126) on the effects of gestational administration of a single dose of TCDD to another strain of rat and another species, the hamster. In the first study, Long Evans (LE) hooded rats were dosed by gavage with 1 microgram TCDD/kg on GD 8 (during the period of major organogenesis) or GD 15 (the gestational day used by Mably et al.). In the second study, pregnant Syrian hamsters, a species relatively insensitive to the lethal effects of TCDD, were dosed on GD 11, equivalent to GD 15 in the rat, with TCDD at 2 micrograms/kg. When LE rats were dosed on GD 15, or when hamsters were dosed on GD 11, puberty (preputial separation) was delayed by about 3 days, ejaculated sperm counts were reduced by at least 58%, and epididymal sperm storage was reduced by 38%. Testicular sperm production was less affected. The sex accessory glands were also reduced in size in LE rat offspring treated on GD 15 despite the fact that serum testosterone (T), T production by the testis in vitro, and androgen receptor (AR) levels were not reduced. Some reproductive measures, such as anogenital distance and male sex behavior, were altered by TCDD treatment in rat but not hamster offspring. Since T and AR levels appeared normal in the sex accessory glands and the epididymis following perinatal TCDD exposure, the alterations in these tissues are not likely to have resulted from an alteration of the androgenic status of the male offspring.

Dose-response relationships of tissue distribution and induction of CYP1A1 and CYP1A2 enzymatic activities following acute exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in mice

Tissue disposition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been shown to be dose-dependent in rats. However, no reported studies in mice have demonstrated dose- and time-dependent distribution of TCDD and the potential sensitivities of target tissues to enzyme induction. The objectives of this study were to determine in mice the effects of dose (0, 0.1, 1, or 10 micrograms [3H]TCDD/kg) and time (7, 14, 21, and 35 days posttreatment) on tissue distribution (18 tissues) and enzyme induction (CYP1A1 in liver, skin, and lung and CYP1A2 in liver). Distribution of TCDD-derived radioactivity in all tissues was dose- and time-dependent with nonlinear distribution. Liver-to-adipose tissue concentration ratios range from 0.6 to 3.1 (low to high dose at Day 7) demonstrating a dose-dependent shift in the disposition of TCDD. In contrast to liver, relative concentrations of percentage dose/g and percentage dose/total tissue decreased with increasing doses in all other tissues. At Day 7 and lowest dose, all tissues contained < 3% dose/g except for thyroid, adrenals, skin, liver, and adipose tissue which had 3, 6, 6, 15, and 24% dose/g, respectively. Induction of EROD activity, a marker for CYP1A1, was dose-dependent in liver, lung, and skin but did not parallel tissue concentrations of TCDD. At the highest dose, fold induction of EROD activity was two times greater in lung than liver, while the concentration in liver was 100 times greater than that in lung. Fold inductions of EROD activity in liver and skin were similar but the concentration was 20 times greater in liver than that in skin. Induction of hepatic acetanilide-4-hydroxylase (ACOH) activity, a CYP1A2 marker, was dose-dependent. Results of the present study demonstrated dose and time dependency in tissue distribution and induction of CYP1A1 and CYP1A2 as well as tissue sensitivities for enzyme induction in the female B6C3F1 mouse. These results provide important considerations for high- to low-dose extrapolations in risk assessments and use of sensitive markers of enzyme induction as surrogates for estimating exposure and in predicting risk.

The importance of pharmacokinetics in determining the relative potency of 2,3,7,8-tetrachlorodibenzo-p-dioxin and 2,3,7,8-tetrachlorodibenzofuran

Polychlorinated dibenzo-p-dioxins and dibenzofurans induce exthoxyresorufin-O-deethylase (EROD) activity, a marker for CYP1A1. Differences in potency of these compounds can be attributed to differences in their affinity for the Ah receptor as well as differences in pharmacokinetics. To test the role of pharmacokinetics in the in vivo potency of these chemicals, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-tetrachlorodibenzofuran (TCDF) were administered to female B6C3F1 mice for 4 or 13 weeks of treatment and EROD activity in liver and skin was determined. The doses were designed to be equally potent based on the published Toxic Equivalency Factor (TEF) values for these compounds. Mice received either 150 ng TCDD/kg/day or 1500 ng TCDF/kg/day, 5 days/week for either 4 or 13 weeks. At 4 weeks, hepatic EROD was induced 11- and 7-fold by TCDD and TCDF, respectively. These data indicate that the published TEFs accurately estimated the relative potency of TCDF after 4 weeks of treatment. After 13 weeks, hepatic EROD was induced 41- and 6-fold by TCDD and TCDF, respectively. The TEFs did not accurately estimate the relative inductive potency of these compounds when compared after 13 weeks of treatment. The inability of the TEFs to predict the relative potency of these compounds after 13 weeks of treatment may be due in part to the differences in the pharmacokinetic properties of each congener. The half-life of TCDF and TCDD is approximately 2 and 15 days, respectively. Steady-state levels of TCDD were not attained by 4 weeks, which is reflected in the increase in hepatic EROD between 4 and 13 weeks. In contrast, steady-state levels of TCDF were reached within 4 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes in benzene disposition in male C57BL/6N mice described by a physiologically based pharmacokinetic model

A physiologically based pharmacokinetic (PBPK) model was developed to describe the disposition of benzene in 3- and 18-month C57BL/6N mice and to examine the relevant physiologic and/or biochemical parameters governing previously observed age-related changes in the disposition of benzene. The model developed was based on that of Medinsky et al. (Toxicol. Appl. Pharmacol. 99 (1989) 193-206), with the inclusion of an additional rate constant for urinary elimination of benzene metabolites. Experimentally determined tissue partition coefficients for benzene in 3- and 18-month mice, as well as actual body weights and fat compartment volumes, were included as part of the model. Model simulations were conducted for oral exposure of 3-month mice to 10 and 200 mg benzene/kg and for oral exposure of 18-month mice to 10 and 150 mg benzene/kg. Total amount of benzene metabolized, as well as metabolism of benzene to specific metabolites and their elimination, was simulated. Modeling results for total amount of benzene metabolites eliminated in urine over a 24-h period at 10 mg/kg showed that a greater total amount of benzene metabolites would be excreted by 18-month versus 3-month old mice. At saturating doses of 150 and 200 mg/kg, total amount of benzene metabolites excreted 24 h post-dose was predicted to be equivalent in 18-month mice and 3-month old mice, but the rate of elimination over time was shown to be decreased in 18-month vs. 3-month mice. Decreased urinary elimination of total benzene metabolites was simulated by a smaller renal elimination rate constant in 18-month vs. 3-month mice, which is consistent with decreased renal blood flow noted in aging rodents. These model predictions were consistent with observed in vitro and in vivo experimental data. Model simulations for production of specific metabolites from benzene and elimination in urine agreed well with experimental data in showing no significant age-related changes in formation of benzene metabolites, with the exception of hydroquinone conjugates. Model simulations and experimental data showed decreased total urinary elimination of hydroquinone conjugates in 18-month vs. 3-month mice. The change in hydroquinone conjugate elimination with age was simulated in modeling experiments as an age-related increase in Km for production of hydroquinone conjugates from benzene. The results of this study indicate that age-related changes in physiology are primarily responsible for altered disposition of benzene in aged mice and suggest that concentrations for toxicity of benzene and/or metabolites may differ in target tissues of aged mice.

The mechanism of dioxin toxicity: relationship to risk assessment

Risk characterization involves hazard identification, determination of dose-response relationships, and exposure assessment. Improvement of the risk assessment process requires inclusion of the best available science. Recent findings in the area of dioxin toxicity have led to a major effort to reassess its risk. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), commonly referred to as "dioxin," is the most toxic member of a class of related chemicals including the polyhalogenated dibenzo-p-dioxins, dibenzofurans, biphenyls, naphthalenes, azo- and azoxy-benzenes, whose toxicities can be expressed as fractional equivalencies of TCDD. These chemicals exert their effects through interaction with a specific intracellular protein, the Ah receptor. While binding to the receptor is necessary, it is not sufficient to bring about a chain of events leading to various responses including enzyme induction, immunotoxicity, reproductive and endocrine effects, developmental toxicity, chloracne, tumor promotion, etc. Some of these responses appear to be linear at low doses. Immunotoxicity and effects on the reproductive system appear to be among the most sensitive responses. The Ah receptor functions as a transcriptional enhancer, interacting with a number of other regulatory proteins (heat shock proteins, kinases, translocases, DNA binding species). Interaction with specific base sequences in the DNA appear to be modulated by the presence of other growth factors, hormones and their receptors as well as other regulatory proteins. Thus, dioxin appears to function as a hormone, initiating a cascade of events that is dependent upon the environment of each cell and tissue. While Ah receptor variants exist, all vertebrates examined have demonstrated such a protein with similar numbers of receptors and binding affinity for TCDD. Most species respond similarly to dioxin and related compounds. While a given species may be an outlier for a given response, it will behave like other animals for other responses. For both in vivo and in vitro end points where animal and human data exist, such as enzyme induction, chloracne, immunotoxicity, developmental toxicity, and cancer, the sensitivity of humans appears similar to that of experimental animals. Current levels of environmental exposure to this class of chemicals may be resulting in subtle responses in populations at special risk such as subsistence fisherman and the developing infant, as well as in the general population. Increased understanding of the mechanism of dioxin's effects as well as elucidation of exposure-dose relationships is leading to the development of a biologically based dose-response model in the ongoing process of incorporating the best science into the risk assessment of TCDD and related compounds.

Interactive regulation of Ah and glucocorticoid receptors in the synergistic induction of cleft palate by 2,3,7,8-tetrachlorodibenzo-p-dioxin and hydrocortisone

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a wide-spread environmental contaminant that produces adverse biological effects including carcinogenesis, reproductive toxicity, immune dysfunction, hyperkeratosis, hepatotoxicity, thymic involution and teratogenesis. In the mouse embryo, TCDD induces cleft palate and hydronephrosis. Glucocorticoids are endogenous steroid compounds that have an important role in development, but are teratogenic at pharmacological doses. The synthetic glycocorticoid, hydrocortisone (HC), induces cleft plate and a potent, synergistic interaction has been observed between TCDD and HC. Both TCDD and HC act through receptor-mediated mechanisms and each compound has its own receptor, the Ah receptor (AhR) and the glucocorticoid receptor (GR), respectively. The morphology and etiology of TCDD- and HC-induced clefts are distinctly different, as HC clefting is due to formation of small palatal shelves, while TCDD-treated shelves fail to fuse due to effects on epithelial cell proliferation and differentiation. The present study examines the expression of AhR and GR in the embryonic palate following exposure to TCDD, HC, and HC + TCDD. C57BL/6N pregnant mice were treated with HC (25 or 100 mg/kg/day GD10-13, sc), TCDD (3 micrograms/kg/day GD10-13, or 24 micrograms/kg GD10, orally), or HC + TCDD (25 mg/kg/day sc and 3 micrograms/kg/day orally, GD10-13). Craniofacial tissues were collected from the embryos on GD14 and examined for AhR and GR expression using in situ hybridization. Northern blots, and immunohistochemistry. We found that in the embryonic palate exposed to TCDD, the AhR was downregulated and the GR expression increased. Conversely, following HC exposure, the GR was downregulated and AhR levels were elevated. HC + TCDD produced increased expression of both receptors. Effects on AhR appeared to be regulated at the transcriptional level, as both protein and mRNA were altered in similar directions. The observed cross-regulation of the receptors is believed to be important in the synergistic interaction between TCDD and HC for the induction of cleft palate.

Endocrine effects of prenatal exposure to PCBs, dioxins, and other xenobiotics: implications for policy and future research

Recent reports have suggested that environmental chemicals may be associated with endocrine alterations in people, wildlife, and experimental animals. Pharmacological investigations as well as natural poisoning episodes have led to the association between exogenous chemicals and alterations in multiple hormonal systems. Persistent environmental contaminants such as dioxins and PCBs have been shown to modulate the activities of several different hormones. The unborn child or the neonate may be at special risk from these chemicals because of rapid growth and development, in addition to enhanced exposure. Because most exposure to these persistent chemicals is via food, changes in dietary habits and/or reduced contamination of the food supply may be required.

Ah receptor in embryonic mouse palate and effects of TCDD on receptor expression

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the most potent member of a family of halogenated aromatic hydrocarbons which are widespread environmental contaminants. In animals the adverse biological effects of TCDD include carcinogenesis, reproductive toxicity, immune function alteration, hyperkeratosis, hepatotoxicity, thymic involution, and teratogenesis. In the mouse embryo, TCDD induces cleft palate through a mechanism which involves altered differentiation and proliferation of the palatal cells, resulting in the failure of opposing shelves to fuse. Cleft palate induction by TCDD requires the Ah receptor. This study examines the expression of the Ah receptor in secondary palate of control and TCDD-exposed C57BL/6N embryos using in situ hybridization, Northern blots, and immunohistochemistry. Ah receptor protein expression was significantly higher in epithelial versus mesenchymal cells, and regional differences in expression within the epithelium were statistically significant. TCDD exposure was shown to downregulate Ah receptor mRNA and protein throughout the palatal shelf and this occurred at both the teratogenic dose and the dose which was not sufficient to produce cleft palate. This study represents the first demonstration of the tissue and cellular localization of the Ah receptor, raising questions about the extrapolation of results from cultured tumor cells to those observed in vivo.

Assessment of host resistance to Trichinella spiralis in mice following preinfection exposure to 2,3,7,8-TCDD

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been reported to decrease host resistance to a variety of infectious agents when exposure occurs prior to infection. Resistance to viral infection has been observed at doses as low as 0.1 microgram TCDD/kg body wt, well below the thymolytic dose in mice. In the present study, female B6C3F1 mice were exposed to a single intraperitoneal injection of 0, 0.1, 1.0, 10.0, or 30.0 micrograms TCDD/kg 7 days prior to infection to determine the effects of TCDD exposure on resistance to the nematode parasite Trichinella spiralis. Exposure to 10 or 30 micrograms TCDD/kg delayed adult parasite elimination from the small intestine. Significantly more larvae were released by female parasites and greater numbers of encysted larvae were recovered from the muscle of mice exposed to TCDD. Proliferative responses of splenocytes and mesenteric lymph node cells stimulated with T. spiralis antigen were significantly suppressed at exposure levels of TCDD > or = 1.0 microgram/kg 7 days after infection and in splenocytes only at 14 days after infection, demonstrating the greater sensitivity of proliferative responses to TCDD exposure than actual host resistance to Ts infection. Suppressed proliferation was observed at doses which produced TCDD concentrations > or = 0.2 pmol/g of lymphoid tissue on Day 7 of infection. In addition, it was determined that infected mice had higher TCDD levels than noninfected mice given the same dose. These results suggest an interaction between TCDD exposure and infection, i.e., that exposure to TCDD altered the host response to infection, while infection delayed elimination of TCDD from the host.

Chemical characterization and disposition studies with 1,2,7,8-tetrabromodibenzofuran in the rat

Polybrominated dibenzo-p-dioxins and dibenzofurans have been identified as potential environmental contaminants. The present studies were designed to characterize the chemical disposition of a tetrabrominated dibenzofuran. The isomer-specific pattern of 1,2,7,8-tetrabromodibenzofuran (TBDF) was chemically characterized using high-pressure liquid chromatography, gas chromatography/mass spectrometry, infrared absorption, and proton nuclear magnetic resonance techniques. The absorption, distribution, and elimination of 1,2,7,8-[4,6-3H]-TBDF were examined in the rat following a single oral, dermal, or intravenous dose of 1 nmol/kg. The 1,2,7,8-TBDF was rapidly excreted in the bile (approximately 50% of the dose in 8 h). Likewise, over half of the administered dose was found in the feces and intestine contents 24 h after iv administration and in feces 72 h after oral administration. Thus, the half-life of 1,2,7,8-TBDF is approximately 1 d. Major tissue depots included the liver, adipose tissue, and skin. The decline in hepatic concentrations observed in the iv and bile studies occurred in conjunction with metabolic elimination as well as a slight accumulation in adipose tissue. Dermal absorption of 1,2,7,8-TBDF, quantified as the amount contained in tissues (excluding the skin site) and excreta at 72 h, was estimated to be 29% of the administered dose. Thus, the general disposition profile of 1,2,7,8-TBDF in the rat is similar to that of other polyhalogenated aromatic hydrocarbons. Due to its rapid elimination, which is consistent with its predicted susceptibility to metabolic elimination, acute exposure to 1,2,7,8-TBDF would not be expected to result in the degree of toxicity associated with other more persistent congeners.