Background
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. That is, loss of BRCA1 function
Conclusions
Both 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.
Results
Loss 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. Conclusions: Both 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.