Effect of BCL‐2 Inhibition on the Aryl Hydrocarbon Receptor‐Mediated Human Breast Cancer Stem Cells Development using MCF‐7 cell line as a model

One of the hallmarks of cancer is cell immortality; this feature can be explained by the presence of cancer stem cells (CSCs), which have unlimited proliferative capacity, within the tumor. CSCs are a small subgroup of cells in the tumor mass that have the ability to initiate and repair damages induced by either surgery or chemotherapy. Therefore, CSCs have been a major focus in the cancer research field, and many studies have been performed with the goal of developing efficient CSC‐targeted therapeutic strategies that are able to eradicate tumors and ultimately improve patient clinical outcomes. We have previously reported that activation and development of breast CSCs is regulated by the aryl hydrocarbon receptor (AhR). In which, activation of AhR and its regulated gene, CYP1A1, significantly increased CSCs features, such as ALDH+ and side population. Interestingly, the anti‐apoptotic protein, BCL‐2 was overexpressed in approximately 75% of breast cancer, suggesting its role in chemoresistance and cancer recurrence. However, the role of BCL‐2 in CSCs development and its cross‐talk with AhR/CYP1A1 has not been investigated yet. To test this hypothesis, human breast cancer MCF‐7 cells with estrogen/progesterone expression were utilized as study model. MCF‐7 cells were treated with BCL‐2 inhibitor, venetoclax (VCX, 10 µM), thereafter, the mRNA and protein expression levels of AhR, CYP1A1, and BCL‐2 were determined by RT‐PCR and Western blot analyses. The results were further confirmed by flow cytometry and immunofluorescence assays. Our results showed that blocking of the BCL‐2 by VCX inhibited the activation of breast CSC features; mammospheres and NANOG expression levels by both TCDD and DMBA, well‐known AhR/CYP1A1 inducers. In addition, VCX treatment significantly inhibited CSC expansion though decreasing the percentage of SP cells in a concentration‐dependent manner. Importantly, flow cytometric analysis showed that pretreatment of MCF‐7 cells with TCDD, AhR/CYP1A1 inducer, sensitizes CSC marker, CD44 High CD24 Low , to apoptotic effects of VCX. In conclusion, the present study provides the first evidence that breast CSC expansion and chemoresistance in MCF‐7 cells could be attenuated by blocking BCL‐2 protein through an AhR/CYP1A1 dependent mechanism. The results of this study strongly support the rational use of BCL‐2 inhibitors in the treatment of breast cancer.