BRCA1 plays a role in the hypoxic response by regulating HIF-1alpha stability and by modulating vascular endothelial growth factor expression.
A recent study of breast cancer patients with and without BRCA1 gene mutations found significantly lower levels of VEGF in serum from patients with BRCA1 mutations (Tarnowski, B., Chudecka-Glaz, A., Gorski, B., and Rzepka-Gorska, I. (2004) Breast Cancer Res. Treat. 88, 287-288). Here, we describe a possible mechanistic explanation for this correlation. Because hypoxia in tumors stimulates VEGF expression and secretion we hypothesized that altered BRCA1 protein levels in breast tumors could affect hypoxia-stimulated VEGF promoter activity. This possibility was tested in cells transfected with various combinations of expression plasmids for BRCA1, BRCA1 specific inhibitory RNAs (BRCA1-siRNAs), HIF-1alpha, and a VEGF promoter-reporter and then incubated in normoxia (21%, O2) or hypoxia (0.1%, O2). As predicted, increased BRCA1 levels enhanced both hypoxia-stimulated VEGF promoter activity and the amounts of VEGF mRNA, as determined by semiquantitative RT-PCR and quantitative real time PCR. Using the ChIP assay, we discovered that BRCA1 could be recruited to the endogenous human VEGF promoter along with HIF-1alpha following hypoxia. An interaction between BRCA1 and HIF-1alpha was found in human breast cancer cells. We also found that hypoxia-stimulated VEGF promoter activity and secretion was reduced in cells containing reduced amounts of endogenous BRCA1 protein (obtained by transfecting with BRCA1 siRNAs). A mechanistic explanation for these effects is provided by our finding a reduced half-life and reduced accumulation of HIF-1alpha in hypoxic cells transfected with BRCA1-siRNAs and that proteasome inhibitors blocked these effects of BRCA1-siRNAs. Thus, our results suggest that normal amounts of BRCA1 function in hypoxia to regulate HIF-1alpha stability, probably by interacting with HIF-1alpha.
Kang
Hyo Jin
HJ
Department of Oncology, Lombardi Comprehensive Cancer Center, Georgetown University, Washington, D. C. 20057.
Kim
Hee Jeong
HJ
Department of Oncology, Lombardi Comprehensive Cancer Center, Georgetown University, Washington, D. C. 20057.
Rih
Jeong-Keun
JK
Department of Oncology, Lombardi Comprehensive Cancer Center, Georgetown University, Washington, D. C. 20057.
Mattson
Thomas L
TL
Department of Oncology, Lombardi Comprehensive Cancer Center, Georgetown University, Washington, D. C. 20057.
Kim
Kyu Won
KW
Division of Pharmaceutical Bioscience, College of Pharmacy, Research Institute of Pharmaceutical Sciences, Seoul National University, Seoul 135-725, Republic of Korea.
Cho
Chi-Heum
CH
Department of Obstetrics and Gynecology, Keimyung University School of Medicine, Daegu 700-712, Republic of Korea.
Isaacs
Jennifer S
JS
Department of Cell and Molecular Pharmacology, Medical University of South Carolina, Charleston, South Carolina 29425.
Bae
Insoo
I
Department of Oncology, Lombardi Comprehensive Cancer Center, Georgetown University, Washington, D. C. 20057. Electronic address: ib42@georgetown.edu.
D019313:BRCA1 Protein / Q000378:metabolism*; D045744:Cell Line, Tumor; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D014158:Transcription, Genetic; D042461:Vascular Endothelial Growth Factor A / Q000235:genetics / Q000378:metabolism*
D019313:BRCA1 Protein; C497442:HIF1A protein, human; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D042461:Vascular Endothelial Growth Factor A; D010100:Oxygen