Published online Jan 26, 2020. doi: 10.4252/wjsc.v12.i1.87
Peer-review started: June 18, 2019
First decision: July 31, 2019
Revised: August 12, 2019
Accepted: October 14, 2019
Article in press: October 14, 2019
Published online: January 26, 2020
Breast cancer has a high degree of phenotype and functional heterogeneity, and the intratumoral variation is obvious. Traditional chemical and endocrine therapies are not effective in treating all cells in a tumor. Researchers have found a small group of highly tumorigenic cell populations in breast tumors. They have stem cell-like properties that are critical for tumorigenesis, progression, progression, and recurrence, and are closely associated with breast cancer metastasis. In the development of tumors, hypoxia-inducible factor-2α (HIF-2α) plays an important role in enhancing the drug resistance and migration of breast cancer stem cells. At present, whether the expression of cancer stem cells can be regulated by interfering with the expression of HIF-2α has not been reported in the literature.
Due to the rapid growth of triple-negative breast cancer (TNBC), and the lack of self-vascular supply to meet the needs of rapidly growing tumor cells, a hypoxic environment is gradually formed. In the long-term chronic hypoxia of tumors, the major regulatory factor is HIF-2α, which increases the malignant biological behavior of tumors by activating its downstream target genes. As a marker of breast cancer stem cells, CD44 is closely related to the invasion and metastasis of tumor cells. Previous studies have found that HIF-2α can regulate the expression of the cell adhesion molecule CD44, but the relationship between the two and the regulatory mechanism are not clear.
This study analyzed the relationship between the expression of HIF-2α and CD44 in patients with TNBC and non-TNBC. Then, the effects of HIF-2α on the expression of CD44 in human breast cancer cell lines MCF-7 and MDA-MB-231 and its possible mechanism were explored.
We analyzed the expression of HIF-2α and CD44 in patients with TNBC (n = 29) and non-TNBC (n = 20) using immunohistochemistry. The co-expression of HIF-2α and CD44 in MDA-MB-231 cells and MCF-7 cells was characterized by double-labeling immunofluorescence. The impact of siRNA-mediated HIF-2α knockdown on the CSCs and apoptosis of MDA-MB-231 cells was detected by real-time fluorescent quantitative PCR, flow cytometry, TUNEL, and mammosphere formation assays. Data were statistically analyzed using the independent t-test and one-way analysis of variance followed by LSD pairwise comparison tests.
Our data showed that HIF-2α had a high level expression in both TNBC and non-TNBC, and HIF-2α and CD44 were located in the same cell. Functionally, HIF-2α silencing significantly reduced the expression of HIF-2α and CD44 mRNAs, but increased cell apoptosis. Flow cytometry and mammosphere formation assay results indicated downregulation of CD44+/CD24− population (P < 0.05) and mammosphere formation upon HIF-2α suppression in hypoxic MDA-MB-231 cells. Moreover, HIF-2α siRNA transfection could decrease the levels of phosphorylated protein-serine-threonine kinase (p-AKT) and phosphorylated mammalian target of rapamycin (p-mTOR) in MDA-MB-231 cells.
HIF-2α plays an important role in the stemness and apoptosis of human breast cancer MDA-MB-231 cells via the CD44/PI3K/AKT/mTOR signaling pathway, thus emerging as a target for the treatment of TNBC.
The molecular regulation mechanism of CSCs in TNBC tissues is still unclear. Inhibition of CSC activation or elimination of CSCs has become a difficult and critical step in the current treatment of TNBC. In this study, the effect of HIF-2α on CSCs was explored by clinicopathological specimens and in vitro experiments, and the mechanism of action was preliminarily analyzed. The conclusions of this study have important clinical and pathological significance for understanding the mechanism of CSC activation in TNBC tissues, blocking the signaling pathway of HIF-2α, inhibiting the malignant progression of TNBC, and improving the prognosis of patients.