ALDHHigh Breast Cancer Stem Cells Exhibit a Mesenchymal–Senescent Hybrid Phenotype, with Elevated Metabolic and Migratory Activities

Cancer stem cells (CSCs) account for 0.01 to 2% of the total tumor mass; however, they play a key role in tumor progression, metastasis and resistance to current cancer therapies. The generation and maintenance of CSCs are usually linked to the epithelial–mesenchymal transition (EMT), a dynamic process involved in reprogramming cancer cells towards a more aggressive and motile phenotype with increased stemness potential. Cells that undergo an EMT process have shown to be more resistant to conventional chemo/radiotherapies. In this context, aldehyde dehydrogenase (ALDH) enzymes, known for their role in the cellular detoxification of aldehydes and enhancement of cell survival, are often upregulated in cancer cells, promoting their resistance to conventional cancer treatments. Indeed, high ALDH levels have become a hallmark biomarker of CSCs and are often used to isolate this sub-population from the more abundant cancer cell populations. Herein, we isolated human breast cancer epithelial cells with higher ALDH abundance (ALDHHigh) and compared them to those with low ALDH abundance (ALDHLow). ALDHHigh sub-populations exhibited more characteristic EMT biomarkers by adopting a more mesenchymal phenotype with increased stemness and enhanced migratory potential. Furthermore, ALDHHigh sub-populations displayed elevated senescent markers. Moreover, these cells also demonstrated higher levels of mitochondria DNA/mass, as well as greater mitochondrial and glycolytic metabolic function. Conversely, ALDHLow sub-populations showed a higher efficiency of mammosphere/colony formation and an increased proliferative capacity. Therefore, we demonstrated that these ALDH sub-populations have distinct characteristics, underscoring their role in EMT, the formation of tumors and the mechanisms of metastasis.