Bridge-1 is expressed in human breast carcinomas: Silencing of Bridge-1 decreases Smad2, Smad3 and Smad4 expression in MCF-7 cells, a human breast cancer cell line

Purpose: The aim of the study was to investigate the expression of Bridge-1 in human breast carcinomas, and to determine the in vitro regulation of Bridge-1 by activin A and the influence of Bridge-1 on activin A signaling in the human breast cancer cell line MCF-7. Method: Bridge-1 expression in human breast carcinomas was shown after staining paraffin slides with a specific antibody against Bridge-1. To gain insight into Bridge-1 function, immortalized, human breast cancer cells (MCF-7 cell line) were stimulated with activin A and the expression of Bridge-1 was analyzed by real-time PCR and Western blot. Next, Bridge-1 was downregulated via siRNA treatment in MCF-7 cells and the expression of Bridge-1, Smad2, 3 and 4 was investigated by real-time PCR and Western blot. Results: Human breast carcinoma cells showed nuclear and cytoplasmic localization of Bridge-1. Activin A stimulation of the immortalized human breast adenocarcinoma cell line MCF-7 showed an increase in Bridge-1 expression by real-time PCR and Western blot. Downregulation of Bridge-1 by Bridge-1-siRNA resulted in a decreased expression of Smad2, 3 and 4 of up to 50% compared to the treatment with non-targeting siRNA. Conclusions: This study is the first to demonstrate the expression of Bridge-1 in human breast carcinomas. Bridge-1 expression is increased by activin A stimulation and itself seems to influence activin A signaling by affecting the expression of Smad2, 3 and 4.