Histone Deacetylase Inhibitor Entinostat Inhibits Tumor-Initiating Cells in Triple-Negative Breast Cancer Cells

Mortality following breast cancer diagnosis is mainly due to the development of distant metastasis. To escape from the primary site, tumor cells undergo the epithelial to mesenchymal transition (EMT), which helps them acquire a more motile and invasive phenotype. In our previous study, we showed that class I selective HDAC inhibitor entinostat (ENT) reverses the EMT phenotype through reversal of epigenetic repression of E-cadherin. Recent evidence suggests that a subset of cells within a breast tumor may drive the metastatic outgrowth following escape from the primary site. These cells, termed as tumor-initiating cells (TICs), represent a great threat to overall prognosis. They are critical in terms of drug resistance and tumor initiation at metastatic sites. Acquisition of EMT traits has also been shown to impart TIC phenotype to the cells, making EMT a "dual-threat" for prognosis. In the current study, we show that ENT treatment can reduce the percentage of TIC cells from TNBC cells. ENT treatment was able to reduce the CD44high/CD24low cell population, ALDH-1 activity as well as protein and mRNA expression of known TIC markers such as Bmi-1, Nanog and Oct-4. Next, we inoculated MDA-MB-231 cells transfected with firefly luciferase (231/Luc) in mammary fat pad of NSG mice. The mice were then treated with ENT (2.5mg/kg/day) and tumor development and formation of metastasis was assessed by bioluminescence imaging. Treatment with ENT significantly reduced tumor formation at the primary site as well as lung metastasis. As such, ENT may help prevent development of distant metastasis.