Induction of transforming growth factor beta by the antiestrogens droloxifene, tamoxifen, and toremifene in MCF-7 cells.

We have previously shown that transforming growth factor beta (TGF beta) is a hormonally regulated negative growth factor in estrogen responsive MCF-7 human breast cancer cells. We have now compared the antiestrogens tamoxifen, droloxifene (3-hydroxytamoxifen), and toremifene in their ability to induce the secretion of autoinhibitory TGF beta by MCF-7 cells. The main results are as follows: induction of TGF beta secretion by droloxifene is about two to three times higher than by identical concentrations of tamoxifen or toremifene. A 5-10 times higher concentration of tamoxifen or toremifene than droloxifene is necessary to reach a similar induction of TGF beta secretion. In contrast to tamoxifen, intermittent application of droloxifene is as effective as continuous treatment in inducing TGF beta secretion. We conclude from these data that TGF beta proteins represent markers of antiestrogen action and might also play a pivotal role in their mechanism of action. Droloxifene is a more effective inducer of TGF beta and a more potent growth inhibitor for estrogen responsive human breast cancer cells than tamoxifen and toremifene in vitro. Therefore, droloxifene might also possess a higher antiestrogenic potential in treatment of human breast cancer.