Assessing estrogenic activity of pyrethroid insecticides using in vitro combination assays

Pyrethroid insecticides are among the most commonly used classes of insecticides worldwide, but their endocrine disrupting activities remain unclear. Therefore, in the present study, we examined the estrogenic activities of pyrethroid insecticides in E-screen and competition binding assays. In addition, we measured estrogen receptor (ER) protein and pS2 mRNA levels in human breast cancer cells (MCF-7 BUS) to clarify the mechanism of their estrogenicity. Seven pyrethroid insecticides (bioallethrine, cypermethrin, deltamethrin, fenvalerate, permethrin, sumithrin, and tetramethrin) were tested because of their worldwide usage. In addition, 17beta-estradiol was tested as a positive control. As expected, 17beta-estradiol significantly increased MCF-7 BUS cell proliferation at concentrations of 10(-11) M and above. Of the pyrethroid insecticides tested, only sumithrin increased MCF-7 BUS cell proliferation in a dose-dependent manner; the maximum induction of cell proliferation was observed at a dose of 10(-5) M. In the anti-estrogenic activity test, bioallethrin, fenvalerate, and permethrin significantly inhibited 17beta-estradiol-induced MCF-7 BUS cell proliferation at 10(-6) M, a concentration comparable to the effective dose (10(-9) M) of ICI 182,780, a pure ER antagonist. However, none of the pyrethroid insecticides competitively inhibited the binding of [(3)H]estradiol to rat uterus ERs in competition binding assays. Both 17beta-estradiol (10(-10) M) and sumithrin (10(-5) M) decreased the levels of cytosolic ERalpha and ERbeta protein expression significantly as compared with the vehicle control. In addition, 17beta-estradiol (10(-10) M) increased pS2 mRNA expression markedly, and sumithrin significantly increased pS2 mRNA levels in a dose-dependent manner. The other six compounds tested in the present study did not affect ER protein levels or pS2 mRNA levels. These results suggest that certain pyrethroid insecticides may be considered to be estrogen-like chemicals that act through pathways other than direct ER binding, and may function as endocrine modulators in both wildlife and humans.