Abstract
BackgroundResistance to anoikis, which is defined as apoptosis induced by loss of integrin-mediated cell attachment to the extracellular matrix, is a determinant of tumor progression and metastasis. We have previously identified the mitochondrial Bit1 (Bcl-2 inhibitor of transcription) protein as a novel anoikis effector whose apoptotic function is independent from caspases and is uniquely controlled by integrins. In this report, we examined the possibility that Bit1 is suppressed during tumor progression and that Bit1 downregulation may play a role in tumor metastasis.Methodology/Principal FindingsUsing a human breast tumor tissue array, we found that Bit1 expression is suppressed in a significant fraction of advanced stages of breast cancer. Targeted disruption of Bit1 via shRNA technology in lowly aggressive MCF7 cells conferred enhanced anoikis resistance, adhesive and migratory potential, which correlated with an increase in active Extracellular kinase regulated (Erk) levels and a decrease in Erk-directed phosphatase activity. These pro-metastasis phenotypes were also observed following downregulation of endogenous Bit1 in Hela and B16F1 cancer cell lines. The enhanced migratory and adhesive potential of Bit1 knockdown cells is in part dependent on their high level of Erk activation since down-regulating Erk in these cells attenuated their enhanced motility and adhesive properties. The Bit1 knockdown pools also showed a statistically highly significant increase in experimental lung metastasis, with no differences in tumor growth relative to control clones in vivo using a BALB/c nude mouse model system. Importantly, the pulmonary metastases of Bit1 knockdown cells exhibited increased phospho-Erk staining.Conclusions/SignificanceThese findings indicate that downregulation of Bit1 conferred cancer cells with enhanced anoikis resistance, adhesive and migratory properties in vitro and specifically potentiated tumor metastasis in vivo. These results underscore the therapeutic importance of restoring Bit1 expression in cancer cells to circumvent metastasis at least in part through inhibition of the Erk pathway.
Highlights
The loss of the anchorage dependence of normal epithelial cells is one of the hallmarks of malignant cells [1,2] and is acquired through resistance to detachment-induced apoptosis [3]
Expression of Bit1 is reduced in invasive breast tumors Since anoikis resistance is a determinant of tumor progression and metastasis in tumor cells, we tested the possibility that the Bit1 anoikis pathway is suppressed in human mammary cancers
These findings indicate that Bit1 expression is selectively lost in invasive breast carcinomas, suggesting that loss of Bit1 may accompany the transition from Ductal Carcinoma In Situ (DCIS) to invasive carcinoma during the progression of breast cancer
Summary
The loss of the anchorage dependence of normal epithelial cells is one of the hallmarks of malignant cells [1,2] and is acquired through resistance to detachment-induced apoptosis (anoikis) [3]. This work analyzes the potential role of the novel anoikis effector Bit in tumor progression and metastasis. Bit (Bcl2-inhibitor of transcription) is a mitochondrial protein that is released to the cytosol following loss of cell attachment and interacts with the Groucho/TLE family transcription factor AES to induce caspase-independent apoptosis. Resistance to anoikis, which is defined as apoptosis induced by loss of integrin-mediated cell attachment to the extracellular matrix, is a determinant of tumor progression and metastasis. We have previously identified the mitochondrial Bit (Bcl-2 inhibitor of transcription) protein as a novel anoikis effector whose apoptotic function is independent from caspases and is uniquely controlled by integrins. We examined the possibility that Bit is suppressed during tumor progression and that Bit downregulation may play a role in tumor metastasis
Sign-in/Register to view highlights & summary 
Published Version (
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