Antiestrogenic effects of motorcycle exhaust particulate in MCF-7 human breast cancer cells and immature female rats.

The emissions from 2- and 4-stroke motorcycles pollute the air of urban areas where motorcycle is a popular means of transportation. This study aimed to determine the endocrine-disrupting activity of motorcycle exhaust particulate (MEP) using MCF-7 human breast cancer cells and immature female Wistar rats treated with organic extracts of MEP. Treatments with 1, 10, and 50 μg/ml MEP extract for 2 and 4 days produced dose-dependent inhibition of thymidine incorporation and cell growth, respectively, in untreated and 1 nM 17β-estradiol (E2)-treated cells. Treatments of MCF-7 cells with MEP extract replaced [3H]E2 from the estrogen receptor in a time- and concentration-dependent manner. These antiestrogenic and receptor binding properties of MEP extract were blocked by cotreatment of the cells with 2 μM α-naphthoflavone, a cytochrome P450 inhibitor and aryl hydrocarbon receptor antagonist. E2 metabolism and HPLC analysis showed that treatment of MCF-7 cells with 50 μg/ml MEP extract for 24 h increased E2 2- and 4-hydroxylation in microsomes. The MEP-mediated increase in E2 2-hydroxylation was inhibited by the addition of 1 μM α-naphthoflavone to MCF-7 microsomes. Cotreatment of immature female rats with 10 μg/kg E2 and 10 mg/kg MEP extract intraperitoneally for 3 days decreased the E2-induced uterine weights. MEP extract alone showed no effect on rat uterine weight. The endocrine-disrupting activity of MEP extract was further confirmed in parallel experiments using MCF-7 cells and immature female rats treated with benzo(a)pyrene, an MEP constituent compound. The present findings demonstrate that MEP extract is antiestrogenic in vitro and in vivo and cytochrome P450 induction is an underlying mechanism.