Differential effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on adipose tissue lipoprotein lipase activity in the guinea pig, rat, hamster, rabbit, and mink

Tyroxine Hydroxylase-Positive Neuronal Cell Population is Increased by Temporal Dioxin Exposure at Early Stage of Differentiation from Human Embryonic Stem Cells

Background: The neurological effects of short-term dioxin exposure during the fetal period is an important health risk in humans. Here, we investigated the effects of dioxin on neural differentiation using human embryonic stem cells (hESCs) to evaluate human susceptibility to dioxin. Methods: Using an enzymatic bulk passage, neural differentiation from human ESCs was carried out. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) was added to various stages of culture. The expression levels of the neuronal markers microtubule-associated protein 2 (MAP2) and thyroxine hydroxylase (TH) were measured by RT-qPCR and image analysis of immunostaining. Results: Although early-stage neuronal cells are quite resistant to TCDD, the numbers of neural rosettes and increases in mRNA expression levels and the number of cells positive for MAP2 and TH were significant by temporal exposure at embryoid body stage (Day9-exposure group). In contrast, the TCDD exposures against ESCs (Day0-exposure group) and differentiated neural cells (Day35-exposure group) were not affected at all. The increment was similarly observed by continuous exposure of TCDD from Day9 through Day60. Conclusions: These results indicated that dioxin exposure during the early stage of differentiation from hESCs increases the contents of neuronal cells, especially TH-positive neuronal cells. Regulations of aryl hydrocarbon receptor (AHR) signaling in an early stage of embryogenesis should be investigated extensively to understand the mechanism underlying the increase in neuronal cell populations and to apply the knowledge to regenerative medicine.

Comparative analysis of temporal and dose-dependent TCDD-elicited gene expression in human, mouse, and rat primary hepatocytes

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-elicited time- and dose-dependent differential gene expression was compared in human, mouse, and rat primary hepatocytes. Comprehensive time course (10 nM TCDD or dimethyl sulfoxide vehicle control for 1, 2, 4, 8, 12, 24, and 48h) studies identified 495, 2305, and 711 differentially expressed orthologous genes in human, mouse, and rat hepatocytes, respectively. However, only 16 orthologs were differentially expressed across all three species, with the majority of orthologs exhibiting species-specific expression (399 human, 2097 mouse, and 533 rat), consistent with species-specific expression reported in other in vitro and in vivo comparative studies. TCDD also elicited the dose-dependent induction of 397 human, 100 mouse, and 443 rat genes at 12h and 615 human, 426 mouse, and 314 rat genes at 24h. Comparable EC50 values were obtained for AhR battery genes including Cyp1a1 (0.1 nM human, 0.05 nM mouse, 0.08 nM rat at 24h) and Tiparp (0.97 nM human, 0.63 nM mouse, 0.14 nM rat at 12h). Overrepresented functions and pathways included amino acid metabolism in humans, immune response in mice, and energy homeostasis in rats. Differentially expressed genes functionally associated with lipid transport, processing, and metabolism were overrepresented in all three species but exhibited species-specific expression consistent with the induction of hepatic steatosis in mice but not in rats following a single oral gavage of TCDD. Furthermore, human primary hepatocytes showed lipid accumulation following 48h of treatment with TCDD, suggesting that AhR-mediated steatosis in mice more closely resembles human hepatic fat accumulation compared with that in rats. Collectively, these results suggest that species-specific gene expression profiles mediate the species-specific effects of TCDD despite the conservation of the AhR and its signaling mechanism.

Growth factors, cytokines and their receptors as downstream targets of arylhydrocarbon receptor (AhR) signaling pathways

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a widespread environmental pollutant, which causes a variety of severe health effects, e.g. immunosuppression, hepatotoxicity, and carcinogenesis. The main mediator of TCDD toxicity is the arylhydrocarbon receptor (AhR), which, upon activation, translocates into the nucleus and enforces gene expression. Since most of the pleiotropic effects caused by TCDD are associated with alterations in cell growth and differentiation, the analysis of the interference of the AhR with factors controlling these cellular functions seems to be a promising target regarding the prevention and treatment of chemical-provoked diseases. Cell growth and differentiation are regulated by numerous growth factors and cytokines. These multifunctional peptides promote or inhibit cell growth and regulate differentiation and other cellular processes, depending on cell-type and developmental stage. They are involved in the regulation of a broad range of physiological processes, including immune response, hematopoiesis, neurogenesis, and tissue remodeling. The complex network of growth factors and cytokines is accurately regulated and disturbances of this system are associated with adverse health effects. The molecular mechanisms by which the AhR interferes with this signaling network are multifaceted and the physiological consequences of this cross-talk are quite enigmatic. The investigation of this complex interaction is an exciting task, especially with respect to the recently described non-genomic and/or ligand-independent activities of AhR. Therefore, we summarize the current knowledge about the interaction of the AhR with three cytokine-/growth factor-related signal transducers -- the epidermal growth factor (EGF) family, tumor necrosis factor-alpha (TNF-alpha), and transforming growth factor-beta (TGF-beta) -- with regard to pathophysiological findings.

Dioxin-induced chloracne--reconstructing the cellular and molecular mechanisms of a classic environmental disease

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is among the most toxic pollutants known to date that serves as a prototype for a group of halogenated hydrocarbon compounds characterized by extraordinary environmental persistence and unique ability to concentrate in animal and human tissues. TCDD can elicit a complex array of pleiotropic adverse effects in humans, although chloracne, a specific type of acne-like skin disease, is the only consistent manifestation of dioxin intoxication, thus representing a 'hallmark' of TCDD exposure. Chloracne is considered to be one of the most specific and sensitive biomarkers of TCDD intoxication that allows clinical and epidemiological evaluation of exposure level at threshold doses. The specific cellular and molecular mechanisms involved in pathogenesis of chloracne are still unknown. In this review, we summarize the available clinical data on chloracne and recent progress in understanding the role of the dioxin-dependent pathway in the control of gene transcription and discuss molecular and cellular events potentially involved in chloracne pathogenesis. We propose that the dioxin-induced activation of skin stem cells and a shift in differentiation commitment of their progeny may represent a major mechanism of chloracne development.

The dioxin-like pollutant PCB 126 (3,3',4,4',5-pentachlorobiphenyl) affects risk factors for cardiovascular disease in female rats

Epidemiological studies suggest that exposure to persistent organic pollutants such as organochlorines might induce cardiovascular disorders and diabetes. Some of these organochlorines, such as dioxins and some dioxin-like PCBs, have been characterised as anti-estrogenic due to their inhibition of estrogenic-induced responses. In the present pilot study, 40 female rats were subjected to either exposure to the dioxin-like 3,3',4,4',5-pentachlorobiphenyl (PCB 126) or vehicle, as well as ovariectomy (OVX) or sham operation in a 2 x 2 factorial design over 12 weeks to explore potential interactions between estrogen status and PCB 126 exposure on cardiovascular risk factors. PCB 126 increased heart weight and serum cholesterol levels in both groups. PCB 126 increased blood pressure in the sham-operated animals only. In conclusion, PCB 126 exposure in female rats resulted in effects on cardiovascular risk factors, such as serum cholesterol, blood pressure, and heart weight. Of these effects of PCB 126, the increase in blood pressure was dependent on estrogen status.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on fatty acid availability and neural tube formation in cynomolgus macaque, Macaca fascicularis

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is known to alter carbohydrate utilization and specific steps in lipid metabolism. TCDD interacts with estradiol in mobilizing specific fatty acids in chickens that may be a cause of cranial/beak malformations in this species. This study was designed to test the hypothesis that TCDD simultaneously alters critical fatty acid mobilization during early pregnancy and determine if those changes correlate to morphological defects of the developing neural tube in the nonhuman primate. Cynomolgus macaques were treated with a single dose of 4 microg/kg body weight (BW) TCDD on gestational day 15 or 20. Pregnancies were terminated by hysterectomy on gestational day 24-26 and embryos were examined to determine morphology of the developing neural tube. Maternal blood samples were used for fatty acid quantification. Embryos exhibited cellular changes, mainly increased cell death, and intercellular spaces in the neural tube, suggestive of an adverse effect on the developing nervous system. Significant decreases on fatty acid composition were found on some of the eight classes of lipids analyzed. Particularly, a decrease was observed in the n-3 (40-60%) and n-6 (47-75%) essential fatty acids in treated pregnancies compared to untreated controls. These data demonstrate the effect of TCDD in decreasing maternal levels of n-3 and n-6 fatty acids that are considered necessary for normal development in mammals. Since neural tube development is dependent, in part, on n-3 and n-6 fatty acids, it is possible that the limitation of these essential fatty acids in plasma resulted in the observed detrimental effects on early brain development.

Embryonic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin in chickens: effects of dose and embryonic stage on hatchability and growth

Chicken embryos (Gallus domesticus) were injected with 0, 8, 20 or 50 ng tetrachlorodibenzo-p-dioxin (TCDD) per egg at embryonic day (ED) 4, 8 or 12 to investigate the effects of differential periods of sensitivity to TCDD exposure. At hatch, all chicks were weighed, sexed and examined macroscopically to identify possible malformations. Liver, bursa, heart and spleen masses were recorded from a number of chicks. The remaining chicks were raised until 6 weeks of age and body and organ masses, plasma concentrations of thyroid hormones, triglycerides and glucose were measured. Dose and stage during embryonic development at which injection was performed affected hatchability. Fifty nanogram of TCDD was highly toxic for 4-day-old chicken embryos. TCDD was less toxic for chicken embryos of 8- and especially 12-days old. One-day-old chick and organ weights were not different between TCDD doses at all injection days. However, injection performed at ED4 or ED8 with 20 and 50 ng, respectively, significantly depressed post-hatch body mass gain. Moreover, body mass gain in males was more depressed than in females. The delayed growth in TCDD treated chickens was accompanied by changes in T(3)/T(4) ratio that at some ages were significantly higher compared to control animals. No pronounced changes in plasma triglycerides or glucose concentrations during postnatal life were observed. Absolute and relative organ masses of 6-week-old chickens showed no remarkable changes.

Serum dioxin level in relation to diabetes mellitus among Air Force veterans with background levels of exposure

Data from several epidemiologic studies suggest that exposure to unusually high amounts of dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) increases the risk of diabetes mellitus, and experimental data suggest that the mechanism for this is decreased cellular glucose uptake. To investigate the dose-response relation more closely, we examined the association of serum dioxin level with prevalence of diabetes mellitus and with levels of serum insulin and glucose among 1,197 veterans in the Air Force Health Study who never had contact with dioxin-contaminated herbicides and whose serum dioxin level was within the range of background exposure typically seen in the United States (< or =10 ng/kg lipid). Compared with those whose serum dioxin level was in the first quartile (<2.8 ng/kg lipid), the multivariate-adjusted odds of diabetes among those in the highest quartile (> or =5.2 ng/kg lipid) was 1.71 (95% confidence interval = 1.00-2.91). The association was slightly attenuated after adjustment for serum triglycerides. Whether adjustment for serum triglycerides was appropriate, however, cannot be determined with available data. The association of background-level dioxin exposure with the prevalence of diabetes in these data may well be due to reasons other than causality, although a causal contribution cannot be wholly dismissed.

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on tumor necrosis factor (TNF) production by peritoneal cells

Recent studies in mice have demonstrated that TNF plays a critical role in mediating the TCDD-induced enhanced inflammatory response to intraperitoneal (i.p.) sheep red blood cells. The current studies were designed to evaluate the effects of TCDD on TNF production by ex-vivo peritoneal cells and a peritoneal macrophage cell line (IC-21) stimulated with LPS. In support of the hypothesis that TCDD can act directly on the peritoneal macrophage to increase TNF production, following pretreatment with TCDD, both ex-vivo peritoneal cells and IC-21 cells produced increased levels of bioactive TNF when stimulated with LPS. Flow cytometric analyses of IC-21 cells indicate that TCDD exposure increases intracellular production and secretion of TNF but does not alter levels of membrane associated TNF.

Inhibition of acute TCDD toxicity by treatment with anti-tumor necrosis factor antibody or dexamethasone

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) acute toxicity is characterized in part by a wasting syndrome with depletion of adipose tissue. Tumor necrosis factor (TNF) induces a similar response during chronic infection. The similarities of these toxic effects led to a hypothesis that TNF plays a role in TCDD acute toxicity. To test this hypothesis pharmacologic doses of an antibody specific for murine TNF and the potent anti-inflammatory agent Dexamethasone (DEX) were used to inhibit TCDD toxicity in mice. TNF antibody treatment resulted in a 54% reduction in TCDD-mediated mortality while DEX treatment, a glucocorticoid agonist that inhibits transcription of TNF, reduced mortality by 92%. Cyp 1A1 induction, the most commonly measured TCDD-mediated response, was not blocked by DEX, demonstrating separation of this biochemical effect from acute toxic responses to TCDD. These data suggest that TCDD-mediated changes in the TNF pathway may be an important mechanism for acute TCDD toxicity.