Genome-independent hypoxic repression of estrogen receptor alpha in breast cancer cells.
About 75-80% of breast tumors express the estrogen receptor alpha (ER-α) and are treated with endocrine-target therapeutics, making this the premier therapeutic modality in the breast cancer clinic. However, acquired resistance is common and about 20% of resistant tumors loose ER-α expression via unknown mechanisms. Inhibition of ER-α loss could improve endocrine therapeutic efficacy, benefiting a significant number of patients. Here we test whether tumor hypoxia might commonly produce ER-α loss.



Using standard molecular and cellular biological assays and a work station/incubator with controllable oxygen levels, we analyze the effects of hypoxia on ER-α protein, mRNA, and transcriptional activity in a panel of independently-derived ER-α positive cell lines. These lines were chosen to represent the diverse genetic backgrounds and mutations commonly present in ER-α positive tumors. Using shRNA-mediated knockdown and overexpression studies we also elucidate the role of hypoxia-inducible factor 1-alpha (HIF-1α) in the hypoxia-induced decrease in ER-α abundance.



We present the first comprehensive overview of the effects of bona fide low environmental oxygen (hypoxia) and HIF-1α activity on ER-α abundance and transcriptional activity. We find that stabilized HIF-1α induces rapid loss of ER-α protein in all members of our diverse panel of breast cancer cell lines, which involves proteolysis rather than transcriptional repression. Reduced ER-α severely attenuates ER-α directed transcription, and inhibits cell proliferation without overt signs of cell death in the cell lines tested, despite their varying genomic backgrounds.



These studies reveal a common hypoxia response that produces reduced ER-α expression and cell cycle stalling, and demonstrate a common role for HIF-1α in ER-α loss. We hypothesize that inhibitors of HIF-1α or the proteasome might stabilize ER-α expression in breast tumors in vivo, and work in combination with endocrine therapies to reduce resistance. Our data also suggests that disease re-occurrence in patients with ER-α positive tumors may arise from tumor cells chronically resident in hypoxic environments. We hypothesize that these non-proliferating cells may survive undetected until conditions change to oxygenate the environment, or cells eventually switch to proliferation via other signaling pathways.
Department of Radiation Oncology, University of California, San Francisco, CA, 94115, USA.|Helen Diller Family Comprehensive Cancer Center, University of California, UCSF Mail stop 0875, 2340 Sutter Street, Room N361, San Francisco, CA, 94115, USA.
Mercè
M
Padró
Department of Radiation Oncology, University of California, San Francisco, CA, 94115, USA.|Helen Diller Family Comprehensive Cancer Center, University of California, UCSF Mail stop 0875, 2340 Sutter Street, Room N361, San Francisco, CA, 94115, USA.
Raymond J
RJ
Louie
Department of Radiation Oncology, University of California, San Francisco, CA, 94115, USA.|Helen Diller Family Comprehensive Cancer Center, University of California, UCSF Mail stop 0875, 2340 Sutter Street, Room N361, San Francisco, CA, 94115, USA.
Brian V
BV
Lananna
Department of Obstetrics and Gynecology, Kansas University Medical Center, Kansas City, KS, 66160, USA.
Adam J
AJ
Krieg
Helen Diller Family Comprehensive Cancer Center, University of California, UCSF Mail stop 0875, 2340 Sutter Street, Room N361, San Francisco, CA, 94115, USA. Luika.Timmerman@UCSF.edu.
Luika A
LA
Timmerman
Department of Radiation Oncology, University of California, San Francisco, CA, 94115, USA.|Helen Diller Family Comprehensive Cancer Center, University of California, UCSF Mail stop 0875, 2340 Sutter Street, Room N361, San Francisco, CA, 94115, USA.
Denise A
DA
Chan
D014408:Biomarkers, Tumor / Q000378:metabolism; D001943:Breast Neoplasms / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D015687:Cell Hypoxia; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D047628:Estrogen Receptor alpha / Q000235:genetics / Q000378:metabolism*; D005260:Female; D015870:Gene Expression; D015972:Gene Expression Regulation, Neoplastic; D020868:Gene Silencing; D015894:Genome, Human; D006801:Humans; D009364:Neoplasm Recurrence, Local / Q000378:metabolism
D014408:Biomarkers, Tumor; C506487:ESR1 protein, human; D047628:Estrogen Receptor alpha
Aromatase inhibitor; Breast cancer; Drug resistance; Endocrine therapy; Endocrine therapy resistance; Estrogen receptor; HIF; HIF-1 alpha; HIF-1α; Hypoxia; Tamoxifen