CSCs are the root cause of recurrence and metastasis [
2]. Increasing evidence has implicated breast cancer stem cells (BCSCs) as essential for breast cancer development, progression, recurrence, and treatment resistance. Numerous studies have found that CSCs exist in a special microenvironment referred to as their “niche.” The niche can maintain CSCs in a dormant state,ensure their asymmetric division to maintain the proportion of stem cells, and prevent their differentiation [
3]. The niche comprises various factors including different types of mesenchymal cells, extracellular matrix, nutrients, temperature, pH, and oxygen concentration. Among these factors, hypoxia is essential to the formation of the CSC niche [
4]. Adult stem cell research showed that hypoxia could maintain human embryonic stem cells in an undifferentiated state [
5] and bone marrow stem cells in a dormant resting state [
6]. Additionally, it previously found a similar effect of hypoxiaon mesenchymal stem cells [
7]. In terms of CSCs, hypoxia can enhance cancer cell invasiveness and tumorigenicity as well as strengthening cancer cell stemness [
8]. Moreover, hypoxia has been found to facilitate the dedifferentiation of differentiated cancer cells, so as to acquire stemness [
9]. The use of anti-angiogenic therapy to treat breast cancer results in ischemic hypoxia in the tumor, which is conducive to the development of BCSCs [
10]. Hypoxia can also induce epithelial–mesenchymal transition (EMT) and thus facilitate cancer metastasis [
11]. What’s more, the development and metastasis of cancer cells are affected by the hypoxic microenvironment in target organs and tissues [
12], and hypoxia can also enhance the resistance of cancer cells to radiotherapy and chemotherapy. These factors contribute to the difficulties in treating cancers [
13]. However, there is little information on the role of hypoxia in modulating the stemness of breast cancer cells.
Triple-negative breast cancer (TNBC), which accounts for about 12–17 % of all breast cancer cases, is difficult to treat [
14]. The MDA-MB-231 cell line is a representative TNBC cell line, and studies on the biological characteristics and behavior of MDA-MB-231 cells may thus be of great significance. In the present study, we cultured TNBC MDA-MB-231 cells in a hypoxic gas mixture to simulate the hypoxic environment in tissues. The expression of the stem cell phenotype CD24
−CD44
+ESA
+ was analyzed to assess the effects of hypoxia on stemness transformation in MDA-MB-231 cells.