BRCA1 plays a role in the hypoxic response by regulating HIF-1alpha stability and by modulating vascular endothelial growth factor expression

J Biol Chem. 2006 May 12;281(19):13047-13056. doi: 10.1074/jbc.M513033200. Epub 2006 Mar 16.

Abstract

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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • BRCA1 Protein / metabolism*
  • Cell Line, Tumor
  • Gene Expression Regulation, Neoplastic
  • Humans
  • Hypoxia-Inducible Factor 1, alpha Subunit / metabolism*
  • Oxygen / metabolism*
  • Promoter Regions, Genetic
  • Transcription, Genetic
  • Vascular Endothelial Growth Factor A / genetics
  • Vascular Endothelial Growth Factor A / metabolism*

Substances

  • BRCA1 Protein
  • HIF1A protein, human
  • Hypoxia-Inducible Factor 1, alpha Subunit
  • Vascular Endothelial Growth Factor A
  • Oxygen