BRCA1 is a key regulatory
protein participating in cell cycle checkpoint and DNA damage repair networks. BRCA1 plays important roles in protecting numerous cellular processes in response to cell damaging signals.
Transforming growth factor-beta (
TGF-beta) is a potent regulator of growth, apoptosis and invasiveness of
tumor cells. TFG-beta activates Smad signaling via its two
cell surface receptors, the TbetaRII and ALK5/TbetaRI, leading to Smad-mediated transcriptional regulation.
METHODOLOGY/PRINCIPAL FINDINGS: Here, we report an important role of BRCA1 in modulating
TGF-beta signaling during oxidative stress responses. Wild-type (WT) BRCA1, but not mutated BRCA1 failed to activate
TGF-beta mediated transactivation of the
TGF-beta responsive reporter, p3TP-Lux. Further, WT-BRCA1, but not mutated BRCA1 increased the expression of
Smad3 protein in a dose-dependent manner, while silencing of WT-BRCA1 by
siRNA decreased Smad3 and Smad4 interaction induced by
TGF-beta in MCF-7
breast cancer cells. BRCA1 interacted with Smad3 upon
TGF-beta1 stimulation in MCF-7 cells and this interaction was mediated via the domain of 298-436aa of BRCA1 and Smad3 domain of 207-426aa. In addition, H(2)O(2) increased the colocalization and the interaction of Smad3 with WT-BRCA1. Interestingly,
TGF-beta1 induced Smad3 and Smad4 interaction was increased in the presence of H(2)O(2) in cells expressing WT-BRCA1, while the
TGF-beta1 induced interaction between Smad3 and Smad4 was decreased upon H(2)O(2) treatment in a dose-dependent manner in HCC1937
breast cancer cells, deficient for endogenous BRCA1. This interaction between Smad3 and Smad4 was increased in reconstituted HCC1937 cells expressing WT-BRCA1 (HCC1937/BRCA1). Further, loss of BRCA1 resulted in H(2)O(2) induced nuclear export of phosphor-Smad3
protein to the cytoplasm, resulting decreased of Smad3 and Smad4 interaction induced by
TGF-beta and in significant decrease in Smad3 and Smad4 transcriptional activities.
CONCLUSIONS/SIGNIFICANCE: