A Noncompetitive Small Molecule Inhibitor of Estrogen-regulated Gene Expression and Breast Cancer Cell Growth That Enhances Proteasome-dependent Degradation of Estrogen Receptor α

The mechanisms responsible for 17 beta-estradiol (E-2)-stimulated breast cancer growth and development of resistance to tamoxifen and other estrogen receptor alpha (ER alpha) antagonists are not fully understood. We describe a new tool for dissecting ER alpha action in breast cancer, p-fluoro-4-(1,2,3,6,-tetrahydro-1,3-dimethyl-2-oxo-6-thionpurin-8-ylthio) (TPSF), a potent small-molecule inhibitor of estrogen receptor alpha that does not compete with estrogen for binding to ER alpha. TPSF noncompetitively inhibits estrogen-dependent ER alpha-mediated gene expression with little inhibition of transcriptional activity by NF-kappa B or the androgen or glucocorticoid receptor. TPSF inhibits E-2-ER alpha-mediated induction of the proteinase inhibitor 9 gene, which is activated by ER alpha binding to estrogen response element DNA, and the cyclin D1 gene, which is induced by tethering ER alpha to other DNA-bound proteins. TPSF inhibits anchorage-dependent and anchorage-independent E-2-ER alpha-stimulated growth of MCF-7 cells but does not inhibit growth of ER-negative MDA-MB-231 breast cancer cells. TPSF also inhibits ER alpha-dependent growth in three cellular models for tamoxifen resistance; that is, 4-hydroxytamoxifen-stimulated MCF7ER alpha HA cells that overexpress ER alpha, fully tamoxifen-resistant BT474 cells that have amplified HER-2 and AIB1, and partially tamoxifen-resistant ZR-75 cells. TPSF reduces ER alpha protein levels in MCF-7 cells and several other cell lines without altering ER alpha mRNA levels. The proteasome inhibitor MG132 abolished down-regulation of ER alpha by TPSF. Thus, TPSF affects receptor levels at least in part due to its ability to enhance proteasome-dependent degradation of ER alpha. TPSF represents a novel class of ER inhibitor with significant clinical potential.