RESUMEN
There have been many concerns expressed regarding the possible adverse effects of thyroid hormone-disrupting chemicals including polychlorinated biphenyls and polybrominated diphenyl ethers (PBDEs), since thyroid hormones play crucial roles in normal vertebrate development. A vast amount of PBDEs have been used as flame retardants for the last two decades and our environment has been contaminated with them. Some PBDEs, especially hydroxylated PBDEs, reportedly show an affinity to the thyroid hormone receptor (TR) and act as thyroid hormone agonists, but in other studies they were reported to inhibit the actions of thyroid hormones. Therefore, in the present study, we investigated the binding affinities of PBDEs and their metabolites to TR and their ability to induce thyroid hormone-responsive transcription using luciferase reporter gene assays in two different cell lines, a pituitary cell line, MtT/E-2, and Chinese hamster ovary (CHO) cells. The binding assay showed that many of the examined PBDEs have significant affinity to TR. Interestingly, some of these PBDEs, such as 4'-OH-BDE-17 and 2'-OH-BDE-28, acted as agonists in the reporter gene assay in MtT/E-2 cells, while they acted as antagonists in CHO cells. Our results demonstrated that whether PBDEs and their metabolites are TR agonists or antagonists depends on the cell type used in the assay, which may suggest that the thyroid hormone-disrupting actions of PBDEs differ among target tissues or species.
Asunto(s)
Éteres Difenilos Halogenados/toxicidad , Hipófisis/citología , Receptores de Hormona Tiroidea/agonistas , Receptores de Hormona Tiroidea/antagonistas & inhibidores , Animales , Células CHO , Línea Celular , Cricetinae , Cricetulus , Disruptores Endocrinos/toxicidad , Retardadores de Llama/toxicidad , Genes Reporteros , Hipófisis/efectos de los fármacos , ARN Mensajero/genética , ARN Mensajero/metabolismo , Ratas , Reacción en Cadena en Tiempo Real de la Polimerasa , Receptores de Hormona Tiroidea/metabolismo , Hormonas Tiroideas/agonistas , Hormonas Tiroideas/metabolismoRESUMEN
A liver microsome-mediated activation of the proestrogens trans-stilbene and trans-stilbene oxide was found in this study. trans-Stilbene and trans-stilbene oxide were negative in estrogen reporter assay using estrogen-responsive human breast cancer cell line MCF-7 and growth assay in rat pituitary tumor cell line MtT/E-2. However, these compounds exhibited estrogenic activity after incubation with liver microsomes of 3-methylcholanthrene-treated rats in the presence of NADPH. In contrast, cis-stilbene and cis-stilbene oxide did not show estrogenic activity after such incubation. When trans-stilbene was incubated with the liver microsomes of 3-methylcholanthrene-treated rats in the presence of NADPH, two metabolites were detected by HPLC. They were identified unequivocally as trans-4-hydroxystilbene and trans-4, 4'-dihydroxystilbene by mass and UV spectral comparison with authentic samples. The oxidase activity of the liver microsomes toward trans-stilbene was inhibited by SKF 525-A and alpha -naphthoflavone. Minor activity was observed when liver microsomes of untreated or phenobarbital-treated rats were used instead of those from 3-methylcholanthrene-treated rats. trans-4-Hydroxystilbene and trans-4,4'-dihydroxystilbene exhibited significant estrogenic activities. These results suggest that the estrogenic activities of trans-stilbene and trans-stilbene oxide were due to formation of hydroxylated metabolites.