Involvement of SREBPs in 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced disruption of lipid metabolism in male guinea pig.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has multiple toxic effects causing a wasting syndrome characterized by a loss of body weight accompanied by a decrease in adipose tissue weight. To elucidate the mechanism behind this syndrome, we investigated the changes in lipid metabolism 7 and 21 days after a single intraperitoneal injection of TCDD at 1 microg/kg body weight to male guinea pigs. TCDD caused the symptoms of the syndrome, body weight loss with a decrease in adipose tissue weight, while it increased the levels of triacylglycerols, total cholesterols, and free fatty acids in plasma. On day 7, TCDD decreased the levels of CCAAT/enhancer binding protein (C/EBP) alpha, peroxisome proliferator activated receptor gamma, and glucose transporter 4, adipogenesis-related factors, in adipose tissue, whereas the levels of retinoid X receptor alpha, C/EBPbeta, C/EBPdelta, and c-Myc remained unchanged. TCDD also reduced the levels of both p125 precursor and p68 active forms of sterol regulatory element binding protein (SREBP)-1 and -2, the lipogenesis-related factors, and downregulated their DNA binding activity in adipose tissue, while it raised the levels of their p68 active forms and increased their DNA binding activity in the liver. TCDD decreased mRNA and protein levels of acetyl-CoA carboxylase and HMG-CoA synthase in the liver and adipose tissue. Similar results were obtained on day 21. These results suggest that TCDD disrupts lipid metabolism through changes in the expression levels of the adipogenesis-related and lipogenesis-related proteins in the liver and adipose tissue, and SREBPs would be involved in the development of the wasting syndrome.

Molokhia (Corchorus olitorius L.) extract suppresses transformation of the aryl hydrocarbon receptor induced by dioxins.

Dioxins enter the body mainly through diet and cause the various toxicological effects by binding to the cytosolic aryl hydrocarbon receptor (AhR) followed by its transformation. In recent reports, it has been shown that certain natural compounds suppress AhR transformation in vitro. In this study, we demonstrated that ethanolic extract from molokhia, known as Egyptian spinach, showed the strongest suppressive effect on AhR transformation induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in cell-free system using rat hepatic cytosol among 41 kinds of extracts from vegetables and fruits. The molokhia extract also suppressed TCDD-induced AhR transformation in mouse hepatoma Hepa-1c1c7 cells and in intestinal permeability system constructed with human colon adenocarcinoma Caco-2 cells and human hepatoma HepG2 cells. Moreover, oral administration of the molokhia extract (100mg/kg body weight) decreased 3-methylcholanthrene-induced AhR transformation to the control level by inhibiting translocation of the AhR from cytosol into the nucleus in the liver of rats. The molokhia extract-administered rat liver showed a tolerance to TCDD-induced AhR transformation by ex vivo experiment. These results indicate that molokhia is an attractive food for isolation and identification of a natural antagonist for the AhR.

Black tea theaflavins suppress dioxin-induced transformation of the aryl hydrocarbon receptor.

Dioxins cause various adverse effects through transformation of aryl hydrocarbon receptor (AhR). In this study, we investigated whether black tea extract and its components, theaflavins, suppress AhR transformation in vitro. First, we confirmed that black tea extract strongly suppressed AhR transformation compared to green and oolong tea, although the catechin contents did not change significantly among the extracts. Then we isolated four theaflavins as active compounds from black tea leaves. They suppressed 1 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced AhR transformation in a dose-dependent manner. The IC(50) values of theaflavin, theaflavin-3-gallate, theaflavin-3'-gallate, and theaflavin-3,3'-digallate (Tfdg) were 4.5, 2.3, 2.2, and 0.7 muM, respectively. The suppressive effect of Tfdg was observed not only by pre-treatment but also by post-treatment. This suggests that theaflavins inhibit the binding of TCDD to the AhR and also the binding of the transformed AhR to the specific DNA-binding site as putative mechanisms.

Pigments in green tea leaves (Camellia sinensis) suppress transformation of the aryl hydrocarbon receptor induced by dioxin.

Environmental contaminants such as dioxins enter the body mainly through diet and cause various toxicities through transformation of the aryl hydrocarbon receptor (AhR). We previously reported that certain natural flavonoids at the dietary level suppress the AhR transformation induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In this study, we identified lutein and chlorophyll a and b from green tea leaves as the novel antagonists for AhR. These active compounds suppressed AhR transformation dose-dependently with the 50% inhibitory concentration (IC(50)) values against 0.1 nM TCDD-induced AhR transformation at 3.2, 5.0, and 5.9 microM, respectively. (-)-Epigallocatechin gallate, which is the most abundant flavonoid in green tea leaves, also showed stronger suppressive effects than did other major tea components, with the IC(50) value of 1.7 microM. Thus, these pigments of green tea leaves have the potential to protect from dioxin toxicity through the suppression of AhR transformation.

A new southwestern chemistry-based ELISA for detection of aryl hydrocarbon receptor transformation: application to the screening of its receptor agonists and antagonists.

Halogenated aromatic hydrocarbons (HAHs), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), produce a wide variety of biological and toxic effects mainly through the aryl hydrocarbon receptor (AhR)-dependent mechanism. After the binding of HAHs, the AhR subsequently transforms its form in order to interact with a specific DNA sequence, the dioxin responsive element (DRE). Thus, detection of the transformed AhR is a target for estimation of the biological and toxic potency of ligands. In this study, we have developed a simple method for quantitative assessment of the transformation state of AhR based on an enzyme-linked immunosorbent assay (ELISA) combined with southwestern chemistry technique (SW-ELISA) that detects the complex of transformed AhR:fluorescein isothiocyanate (FITC)-labeled DRE probe. SW-ELISA has shown the response to HAHs including TCDD and other known agonists in a dose-dependent manner. In the case of TCDD, SW-ELISA has revealed a minimum detection limit (MDL) of 2 pM (0.026 pg/assay), a median effective concentration (EC(50)) value of 0.125 nM (1.6 pg/assay), and a maximum response at 10 nM (129 pg/assay). Furthermore, SW-ELISA provides the confirmation that flavonoids, the potent antagonists for AhR as reported previously, show the inhibitory effects on TCDD-induced AhR transformation. These results indicate that SW-ELISA is a new and straightforward method for the detection of AhR transformation and will be useful in screening of agonists or antagonists for AhR.

Black tea extract suppresses transformation of aryl hydrocarbon receptor induced by dioxin.

Dioxins cause various adverse effects through binding to an aryl hydrocarbon receptor (AhR) and transformation of the receptor. In this study, we investigated whether black tea extract suppresses AhR transformation. Dried black tea leaves were extracted with 75% ethanol, and the extract was pretreated to the rat liver cytosol fraction 10 min prior to addition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Transformed AhR was detected by electrophoretic gel mobility shift assay. Black tea extract suppressed AhR transformation in a dose-dependent manner, and the IC50 value against 1 nM TCDD-induced AhR transformation was 8.9 microg/ml. The result suggests that intake of black tea has a potential to suppress the AhR transformation, leading protection from dioxin toxicity.