Troglitazone enhances tamoxifen-induced growth inhibitory activity of MCF-7 cells

Peroxisome proliferator-activated receptor γ (PPARγ) ligands have been identified as a potential source of therapy for human cancers. However, PPARγ ligands have a limitation for breast cancer therapy, since estrogen receptor α (ERα) negatively interferes with PPARγ signaling in breast cancer cells. Here we show that ERα inhihits PPARγ transactivity and ERα-mediated inhibition of PPARγ transactivity is blocked by tamoxifen, an estrogen receptor blocker. The activation of ERα with 17-β-estradiol blocked PPRE transactivity induced by troglitazone, a PPARγ ligand, indicating the resistance of ERα-positive breast cancer cells to troglitazone. Indeed, troglitazone inhibited the growth of ERα-negative MDA-MB-231 cells more than that of ERα-positive MCF-7 cells. Combination of troglitazone with tamoxifen led to a marked increase in growth inhibition of ERα-positive MCF-7 cells compared to either agent alone. Our data indicates that troglitazone enhances the growth inhibitory activity of tamoxifen in ERα-positive MCF-7 cells.