PO-284 Hypoxia mediates cancer stem cell (CSC) enrichment through downregulation of oestrogen receptor alpha (ERα) expression in ER positive breast cancer cells

Abstract

IntroductionCSCs are a subpopulation of cancer cells which are responsible for tumour initiation and progression. It is well known that hypoxia regulates cancer progression by upregulating CSC phenotype. However, molecular mechanism by which hypoxia regulates CSC phenotype is yet to be elucidated. In our study, we proposed a novel mechanism by which hypoxia regulates CSC phenotype in breast cancer cells through regulation of ERα.Material and methodsThe effect of hypoxia on regulation of breast CSC phenotype and epithelial-mesenchymal transition (EMT) marker expression was examined in MCF-7 cells using various in vitro assays. The expression of ERα was analysed by using Immunoblotting and qRT-PCR. CSC and non-CSC populations were sorted by FACS using breast CSC-specific markers. Role of ERα in regulation of CSC phenotype was evaluated by using ERα specific siRNA. Effect of selective oestrogen receptor modulators (SERMs) in regulating CSC marker expression was assessed by immunoblotting.Results and discussionsWe observed that hypoxia enriches CSC phenotype and upregulates EMT marker expression by targeting the expression of ERα. CSC phenotype negatively correlates with ERα status of breast cancer cells. Isolated CSCs express low levels of ERα compared to non-CSCs. Hypoxia inducible factor 1 alpha (HIF1α) downregulates the expression of ERα under hypoxic condition, that leads to the enrichment of CSC population in ER positive breast cancer cells. Interestingly, knockdown of ERα by specific siRNA enriches CSC population in normoxic condition. Treatment of breast cancer cells with SERMs upregulates the expression of EMT specific genes and CSC specific transcription factors in ER positive breast cancer cells.ConclusionThese findings suggest that HIF1α downregulates ERα expression under hypoxic condition. Downregulation of ERα upregulates CSC phenotype and promotes EMT in breast cancer cells. Treating breast cancer cells by targeting ERα using SERMs may contribute to therapeutic resistance by enriching CSC phenotype.