Prolonged hypoxia promotes enrichment of cytosolic EZH2 in breast cancer stem cells via a HIF-1α-independent mechanism

Abstract

Cancer stem cells (CSCs) make up a small subpopulation of the entire tumour. They have the ability to self-renew as well as differentiate into the cells that make up the bulk tumour. Previously, we have demonstrated that EZH2 is highly enriched in the cytosol of breast cancer stem cells (BCSCs), resulting in EZH2-dependent methylation of Talin1 and subsequent hyperactivation of STAT3. We also identified a subpopulation within BCSCs that highly expressed cytosolic EZH2 (60-80%). Since the tumour microenvironment has been revealed to be a suitable niche for CSCs, we sought to evaluate whether the hypoxic tumour microenvironment could result in further enrichment of cytosolic EZH2 in BCSCs. We discovered that our in vitro CSC-like model exhibited significantly higher cytosolic EZH2 under prolonged hypoxia in a HIF-1α-independent manner. Paradoxically, HIF-1α, EZH2 and STAT3 activation levels were decreased upon acute and prolonged hypoxia, indicating that HIF-2α may be responsible for the observed changes. The activation of PI3K-mTORC2 and biphasic activation pattern of ERK1/2 further supported the preference of HIF-2α over HIF-1α under prolonged hypoxia. In conclusion, this study highlighted a potential HIF-1α-independent hypoxic adaptive response which may have played a role in the further enrichment of cytosolic EZH2 in BCSCs.