Estrogen-related receptor (ERR) alpha-1 shares a high amino acid sequence homology with
estrogen receptor alpha. Although
estrogens are not
ligands of ERR alpha-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 ERR alpha-1-mediated expression of the reporter
enzyme beta-galactosidase in a yeast-based assay. The screen was developed by expressing the hERR alpha-1-yeast Gal 4 activation domain fusion
protein in yeast cells carrying the
beta-galactosidase reporter plasmid, which contains an ERR alpha-1-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 ERR alpha-1-mediated expression of the reporter
chloramphenicol acetyltransferase. In contrast to the findings with ERR alpha-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 ERR alpha-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 ERR alpha-1 activity in SK-BR-3 cells. In addition, a stable ERR alpha-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 ERR alpha-1-modulating promoter. A 24-h incubation of an ERR alpha-1-transfected HepG2 cell line with 10 microM
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 ERR alpha-1 transfected HepG2 cells following
toxaphene treatment resulted from a suppression of the
aromatase expression by
toxaphene acting as the antagonist of ERR alpha-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 ERR alpha-1 should not be overlooked.