Cell type- and estrogen receptor-subtype specific regulation of selective estrogen receptor modulator regulatory elements

Selective estrogen receptor modulators (SERMs), such as tamoxifen and raloxifene can act as estrogen receptor (ER) antagonists or agonists depending on the cell type. The antagonistic action of tamoxifen has been invaluable for treating breast cancer, whereas the agonist activity of SERMs also has important clinical applications as demonstrated by the use of raloxifene for osteoporosis. Whereas the mechanism whereby SERMs function as antagonists has been studied extensively very little is known about how SERMs produce agonist effects in different tissues with the two ER types; ERα and ERβ. We examined the regulation of 32 SERM-responsive regions with ERα and ERβ in transiently transfected MCF-7 breast, Ishikawa endometrial, HeLa cervical and WAR-5 prostate cancer cells. The regions were regulated by tamoxifen and raloxifene in some cell types, but not in all cell lines. Tamoxifen activated similar number of regions with ERα and ERβ in the cell lines, whereas raloxifene activated over twice as many regions with ERβ compared to ERα. In Ishikawa endometrial cancer cells, tamoxifen activated 17 regions with ERα, whereas raloxifene activated only 2 regions, which might explain their different effects on the endometrium. Microarray studies also found that raloxifene regulated fewer genes than tamoxifen in U2OS bone cancer cells expressing ERα, whereas tamoxifen was equally effective at regulating genes with ERα and ERβ. Our studies indicate that tamoxifen is a non-selective agonist, whereas raloxifene is a relative ERβ-selective agonist, and suggest that ERβ-selective SERMs might be safer for treating clinical conditions that are dependent on the agonist property of SERMs.