No abstract

Breast cancer stem cells (BCSCs), which are characterized by a capacity for unlimited self-renewal and for generation of the bulk cancer cell population, play a critical role in cancer relapse and metastasis. Hypoxia is a common feature of the cancer microenvironment that stimulates the specification and maintenance of BCSCs. In this study, we found that hypoxia increased expression of adenosine receptor 2B (A2BR) in human breast cancer cells through the transcriptional activity of hypoxia-inducible factor 1. The binding of adenosine to A2BR promoted BCSC enrichment by activating protein kinase C-δ, which phosphorylated and activated the transcription factor STAT3, leading to increased expression of interleukin 6 and NANOG, two key mediators of the BCSC phenotype. Genetic or pharmacological inhibition of A2BR expression or activity decreased hypoxia- or adenosine-induced BCSC enrichment in vitro, and dramatically impaired tumor initiation and lung metastasis after implantation of MDA-MB-231 human breast cancer cells into the mammary fat pad of immunodeficient mice. These data provide evidence that targeting A2BR might be an effective strategy to eradicate BCSCs.

Triple-negative breast cancer (TNBC) has a poor prognosis due to its aggressive characteristics and lack of targeted therapies. Cytotoxic chemotherapy may reduce tumor bulk, but leaves residual disease due to the persistence of chemotherapy-resistant breast cancer stem cells (BCSC), which are critical for tumor recurrence and metastasis. Here, we demonstrate that hypoxia-inducible factor (HIF)-1-dependent regulation of mitogen-activated protein kinase (MAPK) signaling pathways contributes to chemotherapy-induced BCSC enrichment. Chemotherapy increased DUSP9 expression and decreased DUSP16 expression in a HIF1-dependent manner, leading to inhibition of ERK and activation of p38 signaling pathways, respectively. Inhibition of ERK caused transcriptional induction of the pluripotency factor Nanog through decreased inactivating phosphorylation of FoxO3, while activation of p38 stabilized Nanog and Klf4 mRNA through increased inactivating phosphorylation of RNA-binding protein ZFP36L1, both of which promoted specification of the BCSC phenotype. Inhibition of HIF1 or p38 signaling blocked chemotherapy-induced pluripotency factor expression and BCSC enrichment. These surprising results delineate a mechanism by which a transcription factor switches cells from ERK to p38 signaling in response to chemotherapy and suggest that therapeutic targeting of HIF1 or the p38 pathway in combination with chemotherapy will block BCSC enrichment and improve outcome in TNBC. These findings provide a molecular mechanism that may account for the increased relapse rate of women with TNBC who are treated with cytotoxic chemotherapy and suggest that combining chemotherapy with an inhibitor of HIF1 or p38 activity may increase patient survival. .