Ormeloxifene, a non-steroidal selective estrogen receptor modulator induces senescence in breast cancer cells

Ormeloxifene (ORM), a selective estrogen receptor modulator with a non-steroidal structure, exhibits mild estrogenic activity in bone and anti-estrogenic activity in uterine and breast tissues. ORM induces cell-cycle arrest and exhibits antineoplastic activity in breast cancer. However, its direct effect on the cellular senescence phenotype has not been documented.

Two-dimensional gel electrophoresis (2DE)-based proteomic analysis of ORM-treated MDA-MB-231 cells identified differentially regulated proteins by MALDI-TOF/TOF mass spectrometry. Functional categorization and pathway-level integration of these proteins suggested a coordinated cellular response converging on stress-induced cellular senescence. MTT, colony formation, cell-cycle, β-galactosidase and Sudan Black B staining, and immunoblotting were performed to investigate ORM-induced senescence. ORM inhibited viability of MCF7 and MDA-MB-231 cells, with IC₅₀ values ranging from 7 to 9 µM, and reduced their colony-forming ability, with a greater effect in MCF7 cells. ORM induced G0/G1 growth arrest in MCF7 cells by decreasing CDK4/CDK6 and increasing CDK inhibitors p21Cip1/p27Kip1. Furthermore, ORM induced irreversible senescence in a time-dependent manner, as evidenced by β-galactosidase and Sudan Black B staining and increased expression of established senescence markers. Interestingly, ORM enhanced the sensitivity of MCF7 cells to tamoxifen.

Collectively, these findings suggest that ORM restrains tumor progression by promoting irreversible senescence. The combination of ORM and tamoxifen may represent a potential therapeutic strategy for tamoxifen-resistant breast tumors.