Intermittent hypoxia induces proteasome-dependent down-regulation of estrogen receptor alpha in human breast carcinoma.
OBJECTIVE
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.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Cooper
Charlton
C
Manitoba Institute of Cell Biology and Department of Pathology, University of Manitoba, Faculty of Medicine, Winnipeg, Manitoba, Canada.
Liu
Guang-Yu
GY
Niu
Yu-Lian
YL
Santos
Sylvia
S
Murphy
Leigh C
LC
Watson
Peter H
PH
D000951:Antigens, Neoplasm / Q000378:metabolism; D015153:Blotting, Western; D001897:Boronic Acids / Q000494:pharmacology; D000069286:Bortezomib; D001943:Breast Neoplasms / Q000188:drug therapy / Q000378:metabolism* / Q000473:pathology; D000071231:Carbonic Anhydrase IX; D002256:Carbonic Anhydrases / Q000378:metabolism; D002285:Carcinoma, Intraductal, Noninfiltrating / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D015687:Cell Hypoxia; D002470:Cell Survival / Q000187:drug effects; D015536:Down-Regulation; D047628:Estrogen Receptor alpha / Q000235:genetics / Q000378:metabolism*; D005260:Female; D051272:Glucose Transporter Type 1; D006801:Humans; D007124:Immunoenzyme Techniques; D009004:Monosaccharide Transport Proteins / Q000378:metabolism; D009361:Neoplasm Invasiveness / Q000473:pathology; D010100:Oxygen / Q000378:metabolism; D011480:Protease Inhibitors / Q000494:pharmacology; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D061988:Proteasome Inhibitors; D011719:Pyrazines / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D000951:Antigens, Neoplasm; D001897:Boronic Acids; D047628:Estrogen Receptor alpha; D051272:Glucose Transporter Type 1; D009004:Monosaccharide Transport Proteins; D011480:Protease Inhibitors; D061988:Proteasome Inhibitors; D011719:Pyrazines; C495677:SLC2A1 protein, human; D000069286:Bortezomib; D046988:Proteasome Endopeptidase Complex; C089226:CA9 protein, human; D000071231:Carbonic Anhydrase IX; D002256:Carbonic Anhydrases; D010100:Oxygen