Are Antagonists for Estrogen-related Receptor Two Organochlorine Pesticides, Toxaphene and Chlordane,

Estrogen-related receptor (ERR) a-1 shares a high amino acid sequence homology with estrogen receptor a. Although estrogens are not ligands of ERRa-1, our recent results suggest that toxaphene and chlordane, two organochlorine pesticides with estrogen-like activity, behave as antagonists for this orphan nuclear receptor. The two compounds increased ERRa-1-mediated expression of the reporter enzyme b-galactosidase in a yeast-based assay. The screen was developed by expressing the hERRa-1-yeast Gal 4 activation domain fusion protein in yeast cells carrying the b-galactosidase reporter plasmid, which contains an ERRa1-binding element. In transfection experiments using mammalian cell lines, such as the SK-BR-3 breast cancer cell line, the compounds were found to have an antagonist activity against ERRa-1-mediated expression of the reporter chloramphenicol acetyltransferase. In contrast to the findings with ERRa-1, the two compounds were found to slightly induce the estrogen receptor a-mediated expression of chloramphenicol acetyltransferase in SK-BR-3 cells. In a ligand-independent manner, the ERRa-1 activity in SK-BR-3 cells was induced 3-fold by cotransfection with the GRIP1 coactivator expression plasmid. Toxaphene was found to be capable of suppressing the GRIP1 coactivator-induced ERRa-1 activity in SK-BR-3 cells. In addition, a stable ERRa-1 expressing HepG2 hepatoma cell line was generated, and the aromatase activity in the transfected cell line was found to be twice that in the untransfected cell line. The enzyme aromatase converts androgens to estrogens, and aromatase expression in HepG2 cells is regulated in part by an ERRa-1modulating promoter. A 24-h incubation of an ERRa-1-transfected HepG2 cell line with 10 mM toxaphene reduced its aromatase activity to the level in the untransfected cell line. Because toxaphene is not an inhibitor of aromatase, it is thought that the decrease of the aromatase activity in ERRa-1 transfected HepG2 cells following toxaphene treatment resulted from a suppression of the aromatase expression by toxaphene acting as the antagonist of ERRa-1. Toxaphene and chlordane are among the 12 persistent organic pollutants identified by the United Nations Environment Programme as requiring urgent attention. Their antagonistic effects on ERRa-1 should not be overlooked.