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

Effect of TCDD exposure in adult female and male mice on the expression of miRNA in the ovaries and testes and associated reproductive functions

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant found widely across the world. While animal and human studies have shown that exposure to TCDD may cause significant alterations in the reproductive tract, the effect of TCDD on the expression of miRNA in the reproductive organs has not been previously tested. In the current study, we exposed adult female or male mice to TCDD or vehicle and bred them to study the impact on reproduction. The data showed that while TCDD treatment of females caused no significant change in litter size, it did alter the survival of the pups. Also, TCDD exposure of either the male or female mice led to an increase in the gestational period. While TCDD did not alter the gross morphology of the ovaries and testes, it induced significant alterations in the miRNA expression. The ovaries showed the differential expression of 426 miRNAs, of which 315 miRNAs were upregulated and 111 miRNA that were downregulated after TCDD exposure when compared to the vehicle controls. In the testes, TCDD caused the differential expression of 433 miRNAs, with 247 miRNAs upregulated and 186 miRNAs downregulated. Pathway analysis showed that several of these dysregulated miRNAs targeted reproductive functions. The current study suggests that the reproductive toxicity of TCDD may result from alterations in the miRNA expression in the reproductive organs. Because miRNAs also represent one of the epigenetic pathways of gene expression, our studies suggest that the transgenerational toxicity of TCDD may also result from dysregulation in the miRNAs.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) role in hematopoiesis and in hematologic diseases: A critical review

Dioxin exposure and its effect on hematopoiesis and cancer have been largely investigated in both human and non-human settings. Here we systematically reviewed literature to address the question of what we know about TCDD biology and exposure. Most effects are due to TCDD interaction with a receptor of xenobiotics called AHR, which is ubiquitously represented and also works on hematopoietic myeloid and lymphoid stem cells, inducing proliferation and stem cell release from bone marrow to peripheral circulation. Epidemiologic studies on TCDD exposure demonstrated an association with onco-hematologic diseases, particularly with non Hodgkin lymphomas and multiple myeloma, and non hematologic cancers, such as sarcomas, although these relationships are affected by multiple confounding factors.

Prenatal exposure to TCDD triggers significant modulation of microRNA expression profile in the thymus that affects consequent gene expression

Background

MicroRNAs (miRs) are a class of small RNAs that regulate gene expression. There are over 700 miRs encoded in the mouse genome and modulate most of the cellular pathways and functions by controlling gene expression. However, there is not much known about the pathophysiological role of miRs. TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), an environmental contaminant is well known to induce severe toxicity (acute and chronic) with long-term effects. Also, in utero exposure of fetus to TCDD has been shown to cause thymic atrophy and alterations in T cell differentiation. It is also relevant to understand “the fetal basis of adult disease” hypothesis, which proposes that prenatal exposure to certain forms of nutritional and environmental stress can cause increased susceptibility to clinical disorders later in life. In the current study, therefore, we investigated the effects of prenatal exposure to TCDD on miR profile in fetal thymocytes and searched for their possible role in causing thymic atrophy and alterations in the expression of apoptotic genes.

Methodology/Principal Findings

miR arrays of fetal thymocytes post exposure to TCDD and vehicle were performed. Of the 608 mouse miRs screened, 78 miRs were altered more than 1.5 fold and 28 miRs were changed more than 2 fold in fetal thymocytes post-TCDD exposure when compared to vehicle controls. We validated the expression of several of the miRs using RT-PCR. Furthermore, several of the miRs that were downregulated contained highly complementary sequence to the 3′-UTR region of AhR, CYP1A1, Fas and FasL. Also, the Ingenuity Pathway Analysis software and database was used to analyze the 78 miRs that exhibited significant expression changes and revealed that as many as 15 pathways may be affected.

Conclusions/Significance

These studies revealed that TCDD-mediated alterations in miR expression may be involved in the regulation of its toxicity including cancer, hepatic injury, apoptosis, and cellular development.

Does furan affect the thymus in growing male rats?

Furan has been identified in foods such as heat-treated foods, including coffee, canned meat, hazelnuts, and infant foods and formulas. Children may be exposed to furan via either consumption of these foods or their derivatives. We evaluated the effects of furan on the thymus of weaning male rats in the present study. Five separate groups containing male rats were used: control, oil control, and three furan-treated groups. Furan was given orally to rats in the treatment groups at doses of 2, 4, and 8 mg/kg/day for 90 days. At the end of the experiment, thymus of the rats were examined morphologically, histopathologically, and immunohistochemically. We observed that absolute and relative weights of thymus were decreased significantly in rats treated with 4- and 8-mg/kg/day doses of furan. In histopathological examination, enlargement of interstitial connective tissue between the thymic lobules, lymphocyte depletion, and hemorrhage were observed. We detected an increase in apoptotic cell counts in thymus of the treatment groups. In addition, we found significant differences in the distribution of fibronectin and transforming growth factor-beta in the thymus of the treatment groups. In conclusion, we suggest that furan has affected the thymus in growing male rats.

Some clinico and histopathological changes in female goats experimentally exposed to dioxin

Female Baladi goats were used for investigating the toxicological effects of dioxin. Each animal in the treated group was given an oral dose of 4 mL of stock standard solution of dioxin (labelled and native congeners) diluted in 5 mL distilled water (1/3 of LD50) for three times with 2 days interval and slaughtered 16 days post treatment. Blood and tissue samples were taken and subjected for haemogram, biochemical and pathological studies as well as for determination of dioxin residues. Results revealed that exposure of female goats to dioxin induced anemia, leucocytopenia, neutropenia and eosinophilia with non significant increases in activities of serum ALT and AST as compared with untreated group. Meanwhile, activity of ALP and BUN concentration were significantly increased. Histopathological examination showed degenerative and necrotic changes associated with inflammatory reaction in liver and kidney, in addition to cystic glandular hyperplasia and adenomyosis in uterus. In ovarian tissue, marked decrease ofpreantral follicles together with cystic atretic follicle were noticed. The average percentage residues ofpg WHO-TEQ values for dioxins (PCDDs and PCDFs) in liver, kidney, mammary gland, uterus and milk after oral dose were 0.013, 0.0012, 0.0012, 0.009 and 0.0012%, respectively. It was concluded that oral exposure to dioxin in female goats induced adverse effects on liver and kidney. Dioxins had estrogenic like effect as indicated by uterine and ovarian histopathological changes.

Genetic mapping of loci controlling diethylstilbestrol-induced thymic atrophy in the Brown Norway rat

Chronic estrogen administration can lead to thymic atrophy in rodents. In this article we report that the Brown Norway (BN) rat is sensitive to thymic atrophy induced by the estrogen diethylstilbestrol (DES). By contrast, DES does not induce significant thymic atrophy in the August x Copenhagen-Irish (ACI) strain. The sensitivity of the BN rat to DES-induced thymic atrophy appears to segregate as an incompletely dominant trait in crosses between the BN and ACI strains. In a (BN x ACI)F(2) population, we find strong evidence for three major genetic determinants of sensitivity to DES-induced thymic atrophy on rat Chromosome (RNO) 10 and RNO2. Genotypes at these loci, termed Esta1, 2, and 3, do not have a significant impact on the ability of DES to induce pituitary tumorigenesis or inhibit growth of these F(2) rats. These data indicate that the genetic factors that control DES-induced thymic atrophy are distinct from those that control the effects of DES on pituitary mass and body mass. The Esta intervals on RNO10 and RNO2 overlap with loci that control sensitivity to radiation-induced thymocyte apoptosis, as well as susceptibility to a variety of allergic and autoimmune pathologies, including allergic encephalitis, arthritis, and glomerulonephritis in rodents. These observations suggest that common genetic determinants may control sensitivity to estrogen-induced thymic atrophy, maintenance of thymocyte homeostasis, and immune function.

Effects of oestrogen and exercise on caspase-3 activity in primary and secondary lymphoid compartments in ovariectomized mice

This study investigated the effect of oestrogen exposure and exercise on caspase-3 activity, a measure of apoptosis, in lymphocytes from the thymus, spleen, and lymph nodes in ovariectomized mice. Fifty-nine female B6D2F1 mice were randomized to hormone and exercise conditions. Hormone treatment consisted of implantation with oestradiol pellets (0.72 mg oestradiol) or placebo pellets (0 mg) for 21 days following bilateral ovariectomy (OVX). Exercise consisted of a single treadmill exercise bout (26 m min(-1), 6 degrees slope, 90-min) or sedentary condition. Mice were killed and the thymus, spleen and lymph nodes were removed for the determination of caspase-3 expression by enzyme-linked immunosorbent assay (ELISA), serum oestrogen levels by RIA, and tissue weights. Body weights were monitored throughout the study. In the thymus, oestrogen exposure, exercise and both treatments together were associated with higher caspase-3 activity (P < 0.05) and lower thymus weights (P < 0.05). In contrast, oestrogen exposure and exercise treatment were not associated with greater caspase-3 activity or change in tissue weight in secondary lymphoid tissues (spleen, lymph nodes). Oestrogen-replaced OVX mice had a higher concentration of plasma oestradiol than placebo OVX mice (P < 0.05).

CONCLUSION

The results suggest that oestrogen and treadmill exercise are associated with greater apoptosis, as measured by caspase-3 activity, in the thymus but not in the spleen or lymph nodes. Clinical studies will be necessary to determine if women who take oestrogen have higher rates of apoptosis in primary lymphoid tissues and the significance of thymocyte apoptosis for maintenance of cellular immune function during the post-menopausal years.

2,3,7,8-tetrachlorodibenzo-p-dioxin does not directly alter the phenotype of maturing B cells in a murine coculture system

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), acting through the aromatic hydrocarbon receptor (AhR), elicits numerous toxicological effects, including immunosuppression. Previous work from our laboratory has suggested that TCDD exposure in mice is associated with altered lymphopoietic development, in particular altered B-cell phenotype in the bone marrow. It remains to be determined which specific hematopoietic populations or subpopulations within the marrow cavity are directly targeted by TCDD. To examine the effects of TCDD on developing B cells in vitro, a stromal coculture model was used. Primary bone marrow cells from male, 6- to 7-week-old C57Bl/6 mice were cocultured separately on two AhR-containing stromal cell lines (M2-10B4 and S17) that support B-cell development in the presence of IL-7. The cocultures were treated with 0 to 10 nM TCDD. Shifts in phenotype were quantified by cell surface marker staining and flow cytometry. Four populations in the maturing B cell (very early pre-pro-B, pre-pro-B, pro-B, and pre-B) were defined for quantification. The results show that the only statistically significant effect of TCDD was within the pre-pro-B population in cultures with the S17 stromal cell line. The increase in number of cells with this phenotype was seen in cultures with both wild-type and AhR-/- primary bone marrow cells. These results suggest that the maturing B220+ B cell is not the direct target for TCDD-induced bone marrow B-cell alterations.

Effect of 17beta-estradiol and voluntary exercise on lymphocyte apoptosis in mice

During the perimenopause, women may begin estrogen replacement therapy (ERT) and physical activity programs to reduce the symptoms of the climacteric. High-intensity exercise increases lymphocyte apoptosis, and estrogen is also known to have immune modulatory effects. The present study determined whether (1) estrogen exposure in vivo, and (2) low-intensity, voluntary exercise affect thymic and splenic lymphocyte apoptosis in adult female mice. 'Middle-aged' (>1 year), ovarian-intact female B6D2F(1) mice were implanted with 17beta-estradiol (E) pellets (L: 3 microg/day or H: 12 microg/day) or placebo (P: 0 microg/day). Mice were given 1 week to recover from implantation surgery after which they were randomized to wheel-running or no-wheel-running conditions. Twenty one days later, mice were sacrificed and thymus and spleen removed for determination of percent apoptosis and percent necrosis by flow cytometry, serum E levels by RIA, and tissue and body weights. Estrogen-treated, ovarian-intact mice accumulated less cumulative wheel-running activity than mice implanted with placebo (P<.001). E exposure was associated with lighter thymuses (P<.05), higher thymocyte apoptosis (P<.001), and higher serum E levels (P<.001), effects which were not modified by voluntary exercise. In contrast, splenocyte apoptosis and spleen weights did not differ by estrogen treatment or exercise. The results suggest that in vivo exposure to supplemental estrogen is associated with greater spontaneous apoptosis of thymocytes and reduced thymus weights in older ovarian-intact mice. The clinical significance for thymic (cellular) immunity in perimenopausal women given HRT remains to be determined.

The aryl hydrocarbon receptor has a role in the in vivo maturation of murine bone marrow B lymphocytes and their response to 2,3,7,8-tetrachlorodibenzo-p-dioxin

The ligand-activated aryl hydrocarbon receptor (AHR) is a cytosolic DNA binding protein. Although no biologic role for AHR has been elucidated, it mediates the immunotoxicity of xenobiotics such as 2, 3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and its targeted inactivation produces abnormal immune system development. While investigators have demonstrated AHR's involvement in TCDD-induced B lymphocyte functional alterations, little is known about the receptor's possible role in early B cell maturation and whether exogenous ligands change this process. The purpose of this study was to determine, (1) whether bone marrow B lymphocyte maturation is affected by AHR presence, (2) if so, its relative importance in hematopoietic and/or nonhematopoietic elements and, (3) whether TCDD alters this process. Radiation chimeras were produced that were AHR positive (Ahr+/+) or negative (Ahr-/-) in either their nonhematopoietic or hematopoietic elements, or both. Marrow cells were analyzed for alterations in B lymphocyte maturation stage cell numbers in both vehicle- and TCDD-treated animals. Our results showed that (1) Ahr-/- animals had significantly higher numbers of pro/pre-B cells than Ahr+/+ animals, (2) TCDD treatment of Ahr+/+ animals produced a decrease in pro/pre-B cell numbers, whereas no effect was observed on Ahr-/- animals, and (3) AHR is required in both hematopoietic and stromal elements for maintenance of B cell subset maturation profiles.

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.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.

Thymic regression and apoptosis in the rat after treatment with the Leydig cell cytotoxin ethylene dimethanesulphonate (EDS)

Ethylene dimethanesulphonate (EDS) is an alkylating agent which is widely assumed to specifically kill Leydig cells leaving other biological systems intact. However, after EDS treatment of the male rat the thymus reversibly involutes and the gonadal regional lymph nodes are activated. In the present experiments we have demonstrated that EDS has a direct action upon the thymus both in vivo and in vitro. EDS treatment of the intact and castrated male rat and the intact female rat caused regression of the thymus by up to 50% 3 days later. Total cellularity decreased while the proliferative index increased suggesting a compensatory mechanism. Thymocytes were exposed to EDS in vitro and the response compared to the glucocorticoid methylprednisolone (P), a well characterised thymic apoptotic stimulant. EDS and P increased apoptosis in the thymocyte as characterised by the appearance of cells containing nuclei with apoptotic morphology and with DNA fragmentation visualised by a characteristic ladder after agarose gel electrophoresis. The effects of both EDS and P were time and dose dependent but, in contrast to the effects in Leydig cells, P was the most effective apoptotic stimulus (for instance 100%-P compared to 30%-EDS or 7% control/DMSO after 24 h incubation). The immunological responses of the gonadal lymph nodes were not associated with testicular regression as it was seen in the castrated rat but may be related to a direct action upon the epididymis. In conclusion, tissue specificity of the Leydig cell cytotoxin needs to be extended to the thymus and epididymis. The mode of cell death in Leydig cells and thymocytes after both glucocorticoids and EDS is apoptosis which suggests that they possess some common mechanism(s) which is responsible for the toxicity of these diverse compounds.