Intermittent Hypoxia Induces Proteasome-Dependent Down-Regulation of Estrogen Receptor α in Human Breast Carcinoma

Abstract

Hypoxia may influence gene expression to promote malignancy, and acute hypoxia has been shown to transiently repress estrogen receptor (ER)-alpha expression in breast cell lines. However, the effect of intermittent hypoxia, which is likely more prevalent in breast cancers, remains to be determined. ER-alpha expression was assessed by Western blot and immunohistochemistry in a selected cohort of 51 ER-alpha-positive breast carcinomas, in relation to markers of hypoxia. The effect of acute and intermittent hypoxia on ER-alpha expression was also determined in MCF7 and ZR-75 breast cell lines, together with the role of proteasome function with the proteasome inhibitor bortezomib. Regional loss of ER-alpha expression occurs in breast tumors and is consistently present in hypoxic regions defined by the proximity of necrosis and induction of hypoxia-induced genes carbonic anhydrase IX (CA-IX) and glucose transporter 1 (Glut-1), in both in situ (n = 29; P < 0.0001) and invasive (n = 20; P = 0.0001) carcinomas. In MCF7 and ZR-75 cells, ER-alpha is transiently down-regulated by acute hypoxia and rapidly restored by reoxygenation. However, intermittent, acute hypoxia can cause a similar down-regulation of ER-alpha that is not attributable to decreased mRNA and persists in MCF7 cells despite reoxygenation for up to 14 days. This effect occurs with no change in cell viability but a corresponding reduction in growth response to estradiol. However, ER-alpha expression can be restored by bortezomib. Intermittent hypoxia can cause persistent changes in proteasome function that may contribute to reduced ER-alpha expression in breast tumors and consequently to diminished response and development of resistance to endocrine therapy.