Artemisinin selectively decreases functional levels of estrogen receptor-alpha and ablates estrogen-induced proliferation in human breast cancer cells

MCF7 cells are an estrogen-responsive human breast cancer cell line that expresses both estrogen receptor (ER) alpha and ER beta. Treatment of MCF7 cells with artemisinin, an antimalarial phytochemical from the sweet wormwood plant, effectively blocked estrogen-stimulated cell cycle progression induced by either 17 beta-estradiol (E-2), an agonist for both ERs, or by propyl pyrazole triol (PPT), a selective ER alpha agonist. Artemisinin strongly downregulated ER alpha protein and transcripts without altering expression or activity of ER beta. Transfection of MCF7 cells with ER alpha promoter-linked luciferase reporter plasmids revealed that the artemisinin downregulation of ER alpha promoter activity accounted for the loss of ER alpha expression. Artemisinin treatment ablated the estrogenic induction of endogenous progesterone receptor (PR) transcripts by either E-2 or PPT and inhibited the estrogenic stimulation of a luciferase reporter plasmid driven by consensus estrogen response elements (EREs). Chromatin immunoprecipitation assays revealed that artemisinin significantly downregulated the level of endogeneous ER alpha bound to the PR promoter, whereas the level of bound endogeneous ER beta was not altered. Treatment of MCF7 cells with artemisinin and the pure antiestrogen fulvestrant resulted in a cooperative reduction of ER alpha protein levels and enhanced G(1) cell cycle arrest compared with the effects of either compound alone. Our results show that artemisinin switches proliferative human breast cancer cells from expressing a high ER alpha:ER beta ratio to a condition in which ER beta predominates, which parallels the physiological state linked to antiproliferative events in normal mammary epithelium.