Abstract
BackgroundBRCA1 has recently been identified as a potential regulator of mammary stem/progenitor cell differentiation, and this function may explain the high prevalence of breast cancer in BRCA1 mutation carriers, as well as the downregulation of BRCA1 in a large proportion of sporadic breast cancers. That is, loss of BRCA1 function results in blocked differentiation with expansion of the mammary stem/progenitor cells. Because BRCA1 also maintains genomic integrity, its loss could produce a pool of genetically unstable stem/progenitor cells that are prime targets for further transforming events. Thus, elucidating the regulatory mechanisms of BRCA1 expression is important to our understanding of normal and malignant breast differentiation.ResultsLoss of BRCA1 expression in the ErbB2-amplified SK-BR-3 cell line was found to be the result of loss of activity of the ets transcription factor GABP, a previously characterized regulator of BRCA1 transcription. The expression of the non-DNA binding GABPβ subunit was shown to be deficient, while the DNA binding subunit, GABPα was rendered unstable by the absence of GABPβ. Deletion analysis of the GABPβ proximal promoter identified a potential NRF-1 binding site as being critical for expression. Supershift analysis, the binding of recombinant protein and chromatin immunoprecipitation confirmed the role of NRF-1 in regulating the expression of GABPβ. The siRNA knockdown of NRF-1 resulted in decreased GABPβ and BRCA1 expression in MCF-7 cells indicating that they form a transcriptional network. NRF-1 levels and activity did not differ between SK-BR-3 and MCF-7 cells, however the NRF-1 containing complex on the GABPβ promoter differed between the two lines and appears to be the result of altered coactivator binding.ConclusionsBoth NRF-1 and GABP have been linked to the regulation of nuclear-encoded mitochondrial proteins, and the results of this study suggest their expression is coordinated by NRF-1's activation of the GABPβ promoter. Their linkage to BRCA1, a potential breast stem cell regulator, implies a connection between the induction of mitochondrial metabolism and breast differentiation.
Highlights
BRCA1 has recently been identified as a potential regulator of mammary stem/progenitor cell differentiation, and this function may explain the high prevalence of breast cancer in BRCA1 mutation carriers, as well as the downregulation of BRCA1 in a large proportion of sporadic breast cancers
Differential expression of BRCA1 in MCF-7 verses SK-BR-3 cell lines Cell lines and tumours overexpressing ErbB2 have been reported to have low BRCA1 levels [29,30]. This phenomenon was confirmed by comparison of the breast cancer cell lines MCF-7, which has low ErbB2 levels, and SK-BR-3, which highly overexpresses the receptor
Consistent with this observation, chromatin immunoprecipitation (ChIP) revealed that the BRCA1 promoter is occupied by RNA polymerase II (RNA pol II) in MCF-7 but not SK-BR-3 cells, suggesting that the promoter is transcribed at a low level in the SK-BR-3 cell line (Figure 1d)
Summary
BRCA1 has recently been identified as a potential regulator of mammary stem/progenitor cell differentiation, and this function may explain the high prevalence of breast cancer in BRCA1 mutation carriers, as well as the downregulation of BRCA1 in a large proportion of sporadic breast cancers. Loss of BRCA1 function results in blocked differentiation with expansion of the mammary stem/progenitor cells. Characterization of epithelial subpopulations in preneoplastic tissue from BRCA1 mutation carriers identified an aberrantly expanded luminal progenitor cell population as the likely target of transformation [7]. This model is consistent with clinical data, i.e. the vast majority of breast tumours in women with germ-line mutations in BRCA1 display a basal-like (stem cell-like) phenotype characterized by a lack of expression of ER, PR and ErbB2 and robust expression of markers of myoepithelial differentiation [8]. There is strong evidence to suggest that loss of BRCA1 generates a cancer stem cell capable of initiating and driving breast tumour formation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
