Role of Cancer Stem-like Cells in the Process of Invasion and Mesenchymal Transformation by a Reconstituted Triple-negative Breast Cancer Cell Population Resistant to p53-induced Apoptosis

We investigated the role of cancer stem cells (CSCs) in a population of triple-negative breast cancer (TNBC) cells that are resistant to apoptosis. A human breast cancer cell population capable of inducing p53 expression with doxycycline (Dox) was created and used as an untreated control (UT). After the addition of Dox to UT for 5 days, the cell population reconstituted with cells showing resistance to apoptosis was named RE. Fluorescence-activated cell sorting (FACS) and immunostaining revealed that after the addition of Dox, the ratio of cells in the S and G2/M phases decreased in UT as apoptosis proceeded, but did not markedly change in apoptosis-resistant RE. CSC-like cells in RE exhibited a cell morphology with a larger ratio of the major/minor axis than UT. FACS showed that RE had a higher proportion of CSC-like cells and contained more CD44⁺CD24^- mesenchymal CSCs than ALDH1A3⁺ epithelial-like CSCs. In a Matrigel invasion assay, UT was more likely to form a three-dimensional cell population, whereas RE exhibited a planar population, higher migration ability, and the up-regulated expression of epithelial-mesenchymal transition-related genes. These results provide insights into the mechanisms by which TNBC cells acquire treatment resistance at the time of recurrence.