Contrasting Hypoxic Effects on Breast Cancer Stem Cell Hierarchy Is Dependent on ER-α Status

Hannah Harrison,Robert B Clarke,Robert B Clarke,Robert B Clarke,Lynsey Rogerson,Lynsey Rogerson,Lynsey Rogerson,Göran Landberg,Göran Landberg,Göran Landberg,Keith R Brennan,Keith R Brennan,Keith R Brennan,Hannah J Gregson,Hannah J Gregson,Hannah J Gregson

doi:10.1158/0008-5472.can-12-2505

Abstract

Tumor hypoxia is often linked to decreased survival in patients with breast cancer and current therapeutic strategies aim to target the hypoxic response. One way in which this is done is by blocking hypoxia-induced angiogenesis. Antiangiogenic therapies show some therapeutic potential with increased disease-free survival, but these initial promising results are short lived and followed by tumor progression. We hypothesized that this may be due to altered cancer stem cell (CSC) activity resulting from increased tumor hypoxia. We studied the effects of hypoxia on CSC activity, using in vitro mammosphere and holoclone assays as well as in vivo limiting dilution experiments, in 13 patient-derived samples and four cell lines. There was a HIF-1α-dependent CSC increase in ER-α-positive cancers following hypoxic exposure, which was blocked by inhibition of estrogen and Notch signaling. A contrasting decrease in CSC was seen in ER-α-negative cancers. We next developed a xenograft model of cell lines and patient-derived samples to assess the hypoxic CSC response. Varying sizes of xenografts were collected and analyzed for HIF1-α expression and CSC. The same ER-α-dependent contrasting hypoxic-CSC response was seen validating the initial observation. These data suggest that ER-α-positive and negative breast cancer subtypes respond differently to hypoxia and, as a consequence, antiangiogenic therapies will not be suitable for both subgroups.

Highlights

Hypoxia plays a major role in cancer progression, and areas of hypoxia are common in both preinvasive and invasive breast cancer [1, 2]
The breast cancer cell hierarchy is affected by hypoxia To assess hypoxic effects on breast cancer cells, 13 patientderived primary samples and 4 established cell lines were cultured in 21% or 1% oxygen
We describe differing cancer stem cell (CSC) responses to hypoxia in ERa–positive and negative breast cancer with ER-a–positive cancers gaining increased CSC activity and ER-a–negative cancers showing reduced CSC

Summary

Introduction

Hypoxia plays a major role in cancer progression, and areas of hypoxia are common in both preinvasive and invasive breast cancer [1, 2]. Tumor hypoxia is often linked to decreased survival in patients with breast cancer and has been shown to induce specific molecular changes in cells including changes that confer a more malignant phenotype such as increased proliferation [3], survival [4], invasion [5], and metastasis [6]. Cellular responses to hypoxia are mediated by the hypoxiainducible factors (HIF). HIF-a is rapidly degraded in normoxia, meaning signaling does not occur.

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Results

Conclusion