HIF-2α regulates CD44 to promote cancer stem cell activation in triple-negative breast cancer via PI3K/AKT/mTOR signaling

BACKGROUND Breast cancer is a common malignant tumor that seriously threatens women's health. Breast cancer stem cell (CSC)-like cell population may be the main factor for breast cancer metastasis. Therefore, targeted therapy for CSCs has great potential significance. Hypoxia-inducible factor is a transcription factor widely expressed in tumors. Studies have shown that down-regulation of the hypoxia signaling pathway inhibits tumor stem cell self-renewal and increases the sensitivity of stem cells to radiotherapy and chemotherapy mediated by hypoxia-inducible factor-2 alpha (HIF-2 alpha). However, the specific mechanism remains unclear and further research is necessary. AIM To investigate the effect of HIF-2 alpha down-regulation on stem cell markers, microsphere formation, and apoptosis in breast cancer cell line MDA-MB-231 under hypoxia and its possible mechanism. METHODS Immunohistochemistry was used to detect the expression of HIF-2 alpha and CD44 in triple-negative breast cancer (TNBC) and non-TNBC tissues. Double-labeling immunofluorescence was applied to detect the co-expression of HIF-2 alpha and CD44 in MDA-MB-231 cells and MCF-7 cells. HIF-2 alpha was silenced by RNA interference, and the expression of CD44 and transfection efficiency were detected by real-time fluorescent quantitative PCR. Further, flow cytometry, TdT-mediated X-dUTP nick end labeling, and mammosphere formation assays were used to evaluate the effect of HIF-2 alpha on CSCs and apoptosis. The possible mechanisms were analyzed by Western blot. RESULTS The results of immunohistochemistry showed that HIF-2 alpha was highly expressed in both TNBC and non-TNBC, while the expression of CD44 in different molecular types of breast cancer cells was different. In in vitro experiments, it was found that HIF-2 alpha and CD44 were expressed almost in the same cell. Compared with hypoxia + negative-sequence control, HIF-2 alpha small interfering ribonucleic acid transfection can lower the expression of HIF-2 alpha and CD44 mRNA(P < 0.05), increase the percentage of apoptotic cells (P < 0.05), and resulted in a reduction of CD44(+)/CD24(-) population (P < 0.05) and mammosphere formation (P < 0.05) in hypoxic MDA-MB-231 cells. Western blot analysis revealed that phosphorylated protein-serine-threonine kinase (p-AKT) and phosphorylated mammalian target of rapamycin (p-mTOR) levels in MDA-MB-231 decreased significantly after HIF-2 alpha silencing (P < 0.05). CONCLUSION Down-regulation of HIF-2 alpha expression can inhibit the stemness of human breast cancer MDA-MB-231 cells and promote apoptosis, and its mechanism may be related to the CD44/phosphoinosmde-3-kinase/AKT/mTOR signaling pathway.