Timosaponin A-III Induced Senescence Regulates Oncogenic Phenotype in Breast Cancer Cells by Down Regulating Polycomb Group Proteins
Open AccessThe polycomb group (PcG) proteins, BMI1 and EZH2, are important regulators of senescence, aging, and cancer; and are often overexpressed in several human malignancies including breast cancer. Aberrant expression of these proteins is associated with metastasis and poor prognosis in cancer patients. At present, very little is known about the therapy reagents that can efficiently inhibit the expression of polycomb group proteins such as BMI1 and EZH2. Here, we report that Timosaponin A-III (TAIII), a steroidal saponin obtained from the rhizomes of an herb, Anemarrhena asphodeloides, regulates the expression of polycomb group proteins in breast cancer cells. Treatment of breast cancer cells with Timosaponin A-III caused inhibition of BMI1 and EZH2 expression that was accompanied by downregulation of their respective PRC activity - histone H2A lysine 119 ubiquitinylation (H2AUb) and histone 3 lysine 27 trimethylation (H3K27me3) respectively. We also show that Timosaponin A-III reduces the oncogenic phenotype as seen by invasion and migration assays, and induces cellular senescence as seen by SA-β-galactosidase assay in MCF7 and MDA-MB-231 cells. Quantitative Real Time PCR (qRT-PCR) analyses of mock and Timosaponin A-III treated breast cancer cells show transcriptional upregulation of microRNA expression – specifically the miR-200c/141 cluster that is known to be upregulated during senescence and inhibit BMI1 expression. Thus, our data suggest that Timosaponin A-III can be successfully used to inhibit the growth of tumors where PcG proteins BMI1 and/or EZH2 are overexpressed.
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