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T A Gasiewicz

Publications and source records attributed to T A Gasiewicz.

At least 37 records · Page 2Linked to original sources
PubMed1999

A chimeric aryl hydrocarbon receptor knockout mouse model indicates that aryl hydrocarbon receptor activation in hematopoietic cells contributes to the hepatic lesions induced by 2,3,7, 8-tetrachlorodibenzo-p-dioxin.

Pathologic changes associated with 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) exposure have been reported in the livers of a wide range of species. While these changes have been extensively described, the mechanisms of toxic interaction(s) that produce these lesions remain unclear. Using an aryl hydrocarbon receptor (Ahr) knockout male mouse chimeric model, we investigated whether the presence of this receptor in hematopoietic and/or parenchymal cells affects TCDD-induced hepatotoxicity. Bone marrow chimeras were produced by hematopoietic reconstitution of irradiated mice. Specifically, chimeras were generated with aryl hydrocarbon receptor (AHR) positive hematopoietic and parenchymal cells (Ahr+/+ animal bone marrow cells into irradiated Ahr+/+ animals), AHR positive hematopoietic and negative parenchymal cells (Ahr+/+ into Ahr-/-), AHR negative hematopoietic and positive parenchymal cells (Ahr-/- into Ahr+/+), and AHR negative hematopoietic and parenchymal cells (Ahr-/- into Ahr-/-). Male wild-type (Ahr+/+) and knockout (Ahr-/-) animals were used as nonchimeric controls. Following TCDD treatment (30 microg/kg body wt), liver sections from mice in each control and chimeric group were histologically evaluated for necrotic and inflammatory changes. TCDD treatment produced moderate inflammation in Ahr+/+ controls and Ahr+/+ into Ahr+/+ chimeras. This response was mild in TCDD-treated Ahr-/-, Ahr-/- into Ahr-/-, Ahr+/+ into Ahr-/-, and Ahr-/- into Ahr+/+ animals and was not different from the corresponding vehicle-treated groups. Moderate necrosis was observed in all TCDD-treated controls or chimeras with AHR-positive parenchyma. No or mild necrosis was observed in TCDD- and vehicle-treated animals containing AHR-negative parenchyma. These data indicate that the presence of AHR in hepatic parenchyma alone is sufficient for TCDD induction of hepatic necrosis, and its presence in hematopoietic cells is necessary for the inflammatory response to TCDD-induced hepatic lesions.

Animals↗
PubMed1999

Flavone antagonists bind competitively with 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) to the aryl hydrocarbon receptor but inhibit nuclear uptake and transformation.

Previous analyses suggested that potent aryl hydrocarbon receptor (AhR) antagonists were planar, with a lateral electron-rich center. To further define structural requirements and mechanism for antagonism, ten additional flavone derivatives were synthesized. Based on their ability to 1) compete with 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) for binding to the AhR; 2) inhibit TCDD-elicited binding of AhR to dioxin-responsive elements (DRE) in vitro; and 3) inhibit TCDD-induced transcription of DRE-dependent luciferase in stably transfected hepatoma cells, the most potent flavones contained a 3'-methoxy group and a 4'-substituent having one or more terminal atoms of high electron density (-N3, -NO2, or -NCS). Furthermore, these had low agonist activity as assessed by their inability to elicit AhR. DRE binding or to induce luciferase. Compounds containing bulkier 3' or 4'-substituents, or a 3'-OH group were less potent antagonists, and some were partial agonists. In rat liver cytosol, 3'-methoxy-4'-azido- and 3'-methoxy-4'-nitroflavones bound competitively (with TCDD) to the AhR, indicating that they bind to the TCDD-binding site. When hepatoma cells were exposed to these flavones, AhR complexes were primarily immunoprecipitable from the cytosol and contained 90 kDa heat shock protein. In contrast, AhR in TCDD-treated cells was primarily immunoprecipitated from nuclear extracts and was associated with Arnt but not 90 kDa heat shock protein. Immunocytofluorescence analysis in intact cells further indicated that the potent antagonist inhibited nuclear uptake of AhR and blocked TCDD-dependent down-regulation of AhR. Together, these data indicate that the most potent antagonists bind the AhR with high affinity but cannot initiate receptor transformation and nuclear localization.

Animals↗
PubMed1998

Thymic alterations induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin are strictly dependent on aryl hydrocarbon receptor activation in hemopoietic cells.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related congeners affect the immune system, causing immunosuppression and thymic atrophy in a variety of animal species. TCDD is believed to exert its effects primarily through the ligand-activated transcription factor, the aryl hydrocarbon receptor (AhR). Although the AhR is found at high levels in both thymocytes and thymic stroma, it is uncertain in which cells TCDD is activating the AhR to cause alterations in the thymus. Some investigators have suggested that stromal elements, primarily epithelial cells, within the thymus are the primary targets for TCDD. Others have suggested that atrophy is due to a direct effect on thymocytes, either by apoptosis or by altering the development of progenitor cells. By producing chimeric mice with TCDD-responsive (AhR[+/+]) stromal components and TCDD-unresponsive (AhR[-/-]) hemopoietic components, or the reverse, we have clarified the role of stromal vs hemopoietic elements in TCDD-induced thymic alterations. Our results show that the targets for TCDD-induced thymic atrophy and phenotypic alterations are strictly in the hemopoietic compartment and that TCDD activation of epithelial cells in the stroma is not required for thymic alterations. Furthermore, changes observed in the putative stem cell populations of these chimeric mice are also dependent on TCDD activation of the AhR in hemopoietic elements.

Animals↗
PubMed1998

Expression of functional aromatic hydrocarbon receptor and aromatic hydrocarbon nuclear translocator proteins in murine bone marrow stromal cells.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) acting through the aromatic hydrocarbon receptor (AhR) and its dimerization partner, the AhR nuclear translocator protein (arnt), elicits numerous toxicological effects including immunosuppression and thymic atrophy. Previous work has shown that TCDD alters bone marrow prothymocyte populations. These effects could be mediated at the lymphocyte level directly and/or through effects on bone marrow stromal cells, a population important in the support of lymphopoiesis. The purpose of this study was to characterize AhR and arnt expression in three murine bone marrow stromal cell lines (S17, M2-10B4, and BMS2) and in primary stromal cell cultures. Immunoblot analysis detected AhR protein in M2-10B4 and BMS2 cells. AhR protein was also detected in the primary cultures. Arnt protein could be detected in all cell cultures. Electrophoretic mobility shift assays detected TCDD-dependent dioxin-responsive element (DRE) binding in all three cell lines. DNA binding was sequence-specific and dependent on AhR, as demonstrated by the addition of unlabeled DRE DNA or of anti-AhR antibody. Results obtained with the primary cultures paralleled those seen with the stromal cell lines. The ED50 for induction of TCDD-dependent DRE binding in M2-10B4 cells was 0.21 nM. TCDD treatment did not induce stromal P4501A1 mRNA expression but did increase P4501B1 mRNA levels in all three cell lines and in the primary cultures. These results indicate that murine bone marrow stromal cells express AhR and arnt proteins. Furthermore, these proteins are functional in terms of their DRE-binding ability and potential to regulate mRNA levels in a gene-specific fashion.

Animals↗
PubMed1998

2,3,7,8-Tetrachlorodibenzo-p-dioxin and diethylstilbestrol affect thymocytes at different stages of development in fetal thymus organ culture.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and estrogen induce thymic atrophy and alter thymocyte development. In the present study we investigate whether TCDD and the synthetic estrogen diethylstilbestrol (DES) alter intrathymic development by the same or different mechanisms. We compared the effects of TCDD and DES on thymocyte development in fetal thymus organ culture (FTOC) and found that both compounds caused a reduction in cell yield. TCDD- and DES-treated FTOCs yielded fewer CD4 + CD8+ double-positive cells. However TCDD treatment also led to a greater percentage of cells in the CD8+ single-positive compartment. At lower dioxin concentrations, our results demonstrated an actual increase in CD8+ cells, whereas DES-treated fetal thymocytes were mainly enriched in CD4-CD8- double-negative cells. More alpha beta-TCR+ positive cells were seen in TCDD- but not in DES-exposed cultures. Furthermore, in this study we found that TCDD and DES also alter intrathymic development at different stages in the CD4-CD8- double-negative compartment. TCDD induced a relative increase in c-kit + CD44 + CD25-HSA-thymocytes, while DES induced an relative increase in c-kit-CD44-CD25 + HSA+ cells. RT-PCR revealed that TCDD reduced RAG-1, RAG-2, and TdT gene expression in the CD4-CD8- double-negative thymocytes. Co-treatment by TCDD and DES in FTOC yielded a mixture of effects induced by each agent. Taken together, our results demonstrate that TCDD and DES affect thymocytes at different stages of development, suggesting distinct mechanisms for induction of thymic atrophy.

Adjuvants, Immunologic↗
PubMed1998

Aryl hydrocarbon receptor activation in genital tubercle, palate, and other embryonic tissues in 2,3,7, 8-tetrachlorodibenzo-p-dioxin-responsive lacZ mice.

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the toxicity of 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons. Although the normal function and endogenous ligand for this receptor are not known, it is thought to have a role in growth regulation processes. The AhR has been found in both adult and certain developing tissues, and AhR agonists like the environmental contaminant TCDD cause a number of developmental anomalies. We sought to determine whether the AhR is directly activated to a transcriptionally functional form in tissues known to be adversely affected by AhR agonist exposure. To this end, a transgenic mouse model was developed that could be used to indicate the temporal and spatial context of transcriptionally active AhR following agonist exposure in vivo. A synthetic promoter containing two dioxin-responsive elements (DREs) and a minimal TATA box was strongly induced by TCDD in transfected cells when linked to the lacZ or luciferase reporter gene. Transgenic mice harboring the lacZ construct had TCDD-inducible beta-galactosidase activity in tissues following adult and in utero exposure. Embryonic lacZ expression was induced in hard and soft palates, genital tubercle, certain facial regions, shoulder, as well as other tissues by in utero exposure to 30 microg TCDD/kg at Gestational Day 13. The most intense reporter response was observed in the genital tubercle. Histopathology of the palate and tubercle demonstrated the reporter gene activity to be both cell- and region-specific. This is the first publication to correlate reported TCDD-elicited toxicity (e.g., cleft palate in mice) with TCDD-dependent AhR activation. These data indicate the ability of TCDD to initiate a signal transduction process leading to a transcriptionally active AhR in these tissues, thereby identifying potential targets of dioxin-induced toxicity during development. Weak activation of the reporter gene was consistently observed only in the genital tubercle in the absence of exogenous inducer. This indicates minimal or no endogenous AhR activators at the developmental stage examined. This mouse model will prove useful for both the examination of the endogenous role of the AhR in proliferation or differentiation and of the developmental targets of dioxin-like compounds.

Animals↗
PubMed1998

Overexpression of the anti-apoptotic oncogene, bcl-2, in the thymus does not prevent thymic atrophy induced by estradiol or 2,3,7, 8-tetrachlorodibenzo-p-dioxin.

Dexamethasone (Dex), estradiol (E2), and 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) all affect the immune system, causing immunosuppression and thymic atrophy. It is still uncertain how and where these compounds act to induce thymic atrophy. However, it has been suggested that these compounds may have similar actions and targets, i.e., apoptosis of immature thymocytes for Dex and TCDD and preferential targeting of double-positive cells by Dex and E2. The lckpr-bcl-2 transgenic mouse has been shown to be protected against Dex-induced thymic atrophy. We used this murine model to determine if bcl-2 expression would also protect against E2- and TCDD-induced thymic atrophy. Our results indicate that, although the bcl-2 transgenic (TG+) mice were fully protected from atrophy induced by a single dose of Dex, atrophy was still induced in these mice following treatment with E2 or TCDD. Phenotypic analysis of thymocytes from TG- and TG+ mice also showed distinct consequences of atrophy induced by Dex, E2, and TCDD. Finally, since there are alternative pathways for apoptosis that are bcl-2 independent, both TG- and TG+ thymocytes were examined directly for indications of apoptosis using the TUNEL assay. After TCDD and E2 treatment there were no detectable signs of apoptosis in either TG- or TG+ mice even at early time points and at elevated dose levels. These results indicate that there are distinct mechanisms for the actions of Dex, E2, and TCDD in the thymus and that apoptosis is not a key mechanism of E2- and TCDD-induced thymic atrophy.

Animals↗
PubMed1998

Influence of aromatic hydrocarbon receptor-mediated events on the genotoxicity of cigarette smoke condensate.

The role of aromatic hydrocarbon receptor (AhR)-mediated events on the genotoxicity of mainstream cigarette smoke condensate was investigated. In vitro studies with mouse hepatoma cells stably transfected with a DRE-dependent luciferase reporter indicate that cigarette smoke condensate is able to transform AhR to an active form which is capable of initiating gene transcription. Micronucleus formation in two hepatoma cell lines was used as an index of genotoxicity. Cigarette smoke condensate was observed to induce a higher frequency of micronuclei in Hepa1c1c7 cells relative to TAOc1BP(r)c1 cells, which express approximately 10-fold less AhR. Furthermore, the frequency of micronuclei was potentiated when Hepa1c1c7 cells were pretreated with 2,3,7,8-tetrachlorodibenzo-p-dioxin, a high affinity ligand of AhR. These in vitro studies were followed by an in vivo experiment with Ahr+/+ and Ahr-/- mice. Animals were dosed for three consecutive days with cigarette smoke condensate (0.5-10 microg/kg/day, i.p. injection). The frequency of micronuclei in reticulocytes and total erythrocytes was determined in peripheral blood samples collected 24 h after the last administration. While condensate was found to increase the incidence of micronucleated reticulocytes in Ahr+/+ mice, no increase was observed in the null allele animals. Furthermore, the frequency of micronucleated erythrocytes, a measure of basal chromosome-damaging activity, was slightly but significantly higher in Ahr+/+ relative to Ahr-/- mice. Together, these data suggest that cigarette smoke contains chemicals which transform the AhR to an active transcription factor and AhR-regulated enzyme induction plays an important role in mediating the genotoxicity of this complex environmental pollutant.

Animals↗
PubMed1997

DNA binding and transcriptional enhancement by purified TCDD.Ah receptor complex.

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcriptional enhancer which mediates the biochemical and toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and structurally related chemicals and which may have a role in the normal development of some tissues. We have previously reported the purification of the transformed TCDD.receptor complex from rat liver cytosol based on binding to its dioxin-responsive enhancer sequence (DRE) and that it comprises the AhR ligand-binding monomer and its dimerization partner, ARNT. The present studies were designed to compare the DRE-binding characteristics of the purified receptor with the cruder preparations that are commonly used and ultimately to determine whether the purified receptor complex itself (in the absence of additional cytosolic or nuclear factors) is capable of enhancing transcription in an in vitro system. The purified AhR retained in vitro DRE binding activity in the presence of carrier protein and dithiothreitol, and its affinity for the DRE oligonucleotide was equivalent to that of the other receptor preparations (crude and partially purified cytosolic and crude nuclear). When the ligand.receptor complex was bound to a DRE oligonucleotide containing BrdU and then UV-irradiated, two proteins in each of the receptor preparations were found to crosslink to BrdU-DRE, and we concluded that they are the AhR monomer and ARNT protein. All receptor preparations also gave a similar footprint of interaction with G-residues within the DRE consensus sequence, as assessed by methylation interference. Furthermore, purified and partially purified receptors were able to stimulate transcription from a DRE-containing template in a cell-free system in the presence of HeLa cell nuclear extract. Transcriptional enhancement was receptor dose-dependent, TCDD-dependent, and specific for the DRE sequence upstream of the promotor in our template construct. These data document for the first time that a purified TCDD.Ah receptor complex retains both specific DNA binding and transcriptional activities. This observation constitutes an important step toward understanding the mechanism of gene regulation by TCDD since it implies that the transformed receptor.ligand complex itself is competent as a transcriptional enhancer without a requirement for other factors.

Animals↗
PubMed1997

Dioxins and the Ah receptor: probes to uncover processes in neuroendocrine development.

The developing neuroendocrine system is thought to be a sensitive target for a number of environmental chemicals. Many of these chemicals act, not by directly damaging macromolecules but through the inappropriate modulation of normal cellular processes that regulate cell growth and differentiation patterns. As our knowledge of the specific hormones and signaling pathways involving in these functions has advanced, so has our understanding that these chemicals may act by a number of different molecular mechanisms. The dioxin-like compounds are persistent environmental contaminants. The findings that they affect cell proliferation and differentiation, are tumor promoters, and are potent immuno-, developmental, and reproductive toxicants by mechanisms not dependent on cytotoxicity, are consistent with the hypothesis that these compounds act by modulating normal cell and tissue growth processes. Furthermore, findings in exposed experimental animals and humans suggest that the developing neuroendocrine system is a sensitive target for these chemicals. All evidence to data indicates that these compounds produce their biological and toxicology affects by binding to a gene regulatory protein, the Ah receptor, whose normal function has not been clearly delineated and whose normal endogenous ligand has not been identified. Yet data on the biochemistry and molecular biology of this transcription factor indicates that well regulated and conserved pathways exist for it to mediate the transduction of biochemical signals for the control of a number of hormones and growth factors. These and other data provide evidence to suggest that the Ah receptor may be necessary for the normal development of many tissues including those in the neuroendocrine system. Further research is necessary to understand if and how it may function in this system, what the normal endogenous ligand is, and how perturbations in its activity may affect normal development processes.

Animals↗
PubMed1996

Analysis of structural requirements for Ah receptor antagonist activity: ellipticines, flavones, and related compounds.

A number of studies have examined the structure-activity relationships for the agonist activity of Ah receptor (AhR) ligands. Fewer studies have considered the structural basis for potential antagonist properties. Certain ellipticine derivatives have been reported to bind to the AhR and inhibit the ability of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to transform the AhR to a form that recognizes a dioxin-responsive enhancer element (DRE) upstream of the cytochrome P4501A1 gene. In the present study, over 30 ellipticine derivatives and structurally related compounds were examined for their ability to bind to the AhR, activate it to a DRE-binding form, induce the luciferase gene under control of a DRE-containing enhancer, and block activation of the AhR by TCDD. The ability of several ellipticine derivatives to inhibit TCDD-elicited DRE binding and TCDD-induced luciferase activity was inversely related to their ability to alone stimulate these responses. The most potent antagonist activity was related to good AhR binding characteristics in terms of conforming to previously predicted 14 x 12 x 5 A van der Waals dimensions and the presence of an electron-rich ring nitrogen at or near a relatively unsubstituted X-axis terminal position. Based on these data, a number of flavone derivatives were synthesized and tested for their relative agonist/antagonist activity. These additional data were consistent with the hypothesis that an electron-rich center near or along a lateral position of the van der Waals binding cavity is a characteristic that enhances AhR antagonist activity.

Animals↗
PubMed1996

Induction of CYP1A1 and ALDH-3 in lymphoid tissues from Fisher 344 rats exposed to 2,3,7,8-tetrachlorodibenzodioxin (TCDD).

The immune system is a primary target for toxic insult by a number of drugs and environmental chemicals, many of which require activation to toxic metabolites by drug-metabolizing enzymes. We compared the induction of drug-metabolizing enzymes, including cytochrome P450 1A1 (CYP1A1) and aldehyde dehydrogenase (ALDH), in lymphoid tissues of F344 rats following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). ALDH was induced in both the spleen and the thymus after TCDD treatment, with maximal expression at 9 and 15 days, respectively. Thymic microsomal preparations from TCDD-treated animals expressed elevated levels of inducible CYP1A1 as compared to microsomes from the spleens of treated animals or tissues from control rats. TCDD treatment also resulted in increased ethoxyresorufin-O-deethylase (EROD) activity in the thymus. There were no detectable mRNA transcripts for CYP1A1 in peripheral blood or splenic lymphocytes from treated animals; however, CYP1A1 transcripts were induced in isolated thymocytes, whole spleen, and whole thymus. In vitro exposure to TCDD did not result in induction of immunoreactive CYP1A1 in thymocytes unless simultaneously activated with the mitogen, phytohemagglutinin (PHA). Immunohistochemical localization of CYP1A1 in immune tissues indicated that cells other than the lymphoid populations are responsible for the increased CYP1A1 expression. The pattern of CYP1A1 induction was related to the expression of the Ah receptor (AhR) in immune tissues. Western blot analyses demonstrated less AhR present in peripheral blood lymphoid cells and spleen, as compared to whole tissues. These studies indicate that while drug-metabolizing enzymes are present in immune tissues, the induction of enzymes is selective in different lymphoid cells.

Aldehyde Dehydrogenase↗
PubMed1995

Ah receptor phosphorylation: localization of phosphorylation sites to the C-terminal half of the protein.

The aryl hydrocarbon receptor (AhR) is a transcriptional enhancer activated by the binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related xenobiotics. Ligand binding initiates a series of poorly understood molecular events which confers recognition of cis-acting elements located in regulatory regions of particular structural genes, such as CYP1A1. Several studies have suggested that AhR phosphorylation may be instrumental in activating the AhR to a DNA-binding state. In agreement with previous investigations, treatment of the AhR with acid phosphatase resulted in the loss of DNA-binding activity. To further evaluate the functional role of AhR phosphorylation we determined whether TCDD binding altered total AhR phosphorylation, and identified phosphorylated regions by the examination of chemical cleavage patterns. The AhR was isolated by immunoprecipitation from [32P]-orthophosphate-labeled Hepa 1 cells grown in the presence or absence of TCDD. Examination of the amount of 32P associated with the AhR indicated that the total level of AhR phosphorylation was not affected by ligand binding. Chemical cleavage with hydroxylamine and cyanogen bromide also revealed a similar pattern for liganded and unliganded AhR. The shortest regions of overlap determined by the chemical cleavage patterns localized phosphorylation sites to two regions in the C-terminal half of the AhR. One region is centrally located between amino acids 368 and 605 and within or adjacent to a DNA binding repressor domain. The other region is located at the glutamine-rich carboxyl terminus between amino acids 636 and 759. These data coupled with previous observations imply that total AhR phosphorylation is not altered by the ligand-elicited transformation to a DNA-binding form, but that phosphorylation nevertheless plays an important role in the ability of an active AhR-Arnt complex to associate with cis-acting regulatory elements.

Acid Phosphatase↗
PubMed1995

Weanling female Sprague-Dawley rats are not sensitive to the antiestrogenic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).

Investigators have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) can inhibit certain estrogenic events in vivo and in vitro. To further investigate this phenomenon, the effects of estradiol (E2) alone or TCDD plus estradiol on several estrogen-dependent parameters were evaluated in weanling female Sprague-Dawley rats. E2 (10 micrograms/kg/day, Postnatal Days (PND) 21 and 22) caused significant increases in relative uterine weight and keratinization of the vaginal epithelium (PND 23). E2 significantly reduced uterine estrogen receptor (ER) protein levels and serum FSH levels, with a trend toward reduction of ER mRNA levels. None of these parameters were affected by pretreatment with 20, 40, or 80 micrograms/kg TCDD (PND 19). Uterine progesterone receptor levels were not affected by E2 or TCDD in the present study. In contrast, TCDD significantly decreased body weight (40 or 80 micrograms/kg) by PND 21, significantly decreased relative thymic weights, and significantly increased relative hepatic weights (20, 40, and 80 micrograms/kg, by PND 23). In addition, TCDD dramatically induced CYPIA1 hepatic mRNA levels, indicating that TCDD was properly delivered and could mediate other well-documented Ah receptor-dependent events. Thus, weanling female Sprague-Dawley rats are not sensitive to the antiestrogenic effects of TCDD at doses which cause overt toxicity. The results provide evidence that the previously reported antiestrogenic effects of TCDD are probably species, strain, and age dependent.

Animals↗
PubMed1995

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.

Animals↗
PubMed1994

2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced thymic atrophy and lymphocyte stem cell alterations by mechanisms independent of the estrogen receptor.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has both agonist and antagonist effects on estrogen-mediated activities and estrogen receptor (ER) levels in epithelial tissues following exposure. We previously demonstrated that TCDD alters bone marrow lymphocyte stem cells, including prothymocytes, as measured by functional assays and alterations in the lymphocyte stem cell-specific markers terminal deoxynucleotidyl transferase (TdT) and recombinase activating gene-1 (RAG-1). We have also shown that 17 beta-estradiol valerate (E2V) affects lymphocyte stem cells by reducing TdT and RAG-1 mRNA. It has been suggested that the effect of TCDD on these lymphocyte stem cells may be mediated directly or indirectly through estrogenic action and/or the ER. Studies were designed to evaluate whether endogenous estrogens or the ER mediate TCDD-elicited bone marrow alterations and thymic atrophy. Ovariectomy did not alter the sensitivity of mice to TCDD-induced thymic atrophy or to a reduction in TdT biosynthesis in bone marrow cells compared with either intact or sham-operated mice. The pure estrogen antagonist ICI 164,384 blocked E2V-induced uterine hypertrophy, thymic atrophy and reductions in lymphocyte stem cell markers. However, the antiestrogen failed to protect against TCDD-elicited thymic atrophy or bone marrow alterations in intact animals. The results are consistent with the hypothesis that the effects of TCDD on the thymus and/or bone marrow are mediated by mechanisms independent of estrogens or the ER.

Animals↗
PubMed1994

The thymus does not mediate 2,3,7,8-tetrachlorodibenzo-p-dioxin-elicited alterations in bone marrow lymphocyte stem cells.

Our previous studies have shown that bone marrow lymphocyte stem cells are affected following perinatal or adult exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). These alterations may, in part, be responsible for thymic atrophy that is also observed following TCDD exposure. However, other investigators have suggested that the thymus or thymic-derived lymphocytes can affect bone marrow stem cell development. The purpose of these studies was to determine whether the TCDD-elicited effects that we have observed on lymphocyte stem cells in bone marrow were secondary to the actions of this chemical on the thymus. A single intraperitoneal dose of TCDD (30 micrograms/kg) to sham-operated or neonatally thymectomized female BALB/c mice reduced the levels of mRNA in the bone marrow for the lymphocyte stem cell-specific enzymes terminal deoxynucleotidyl transferase (TdT) and recombinase activating gene (RAG-1). TdT biosynthesis was also reduced by TCDD treatment. Thus, neonatal thymectomy had no effect on the TCDD-elicited reduction of TdT or RAG-1 mRNAs or TdT biosynthesis. Genetically athymic (nu/nu) mice were used to further determine if the actions of TCDD on the thymus or long-lived T-cells altered lymphocyte stem cell development. As observed in BALB/c mice, TCDD treatment decreased the expression of TdT and RAG-1 mRNAs in bone marrow from athymic nu/nu and intact nu/+ littermates. We conclude that TCDD-elicited alterations in bone marrow lymphocyte stem cells are not secondary to any actions, direct or indirect, that TCDD has on the thymus or thymic-derived T-cells.

Animals↗