|dc.description.abstracteng||The developmental program of Epithelial Mesenchymal Transition (EMT) is assumed to be involved in tumor progression, enabling cancer cells to spread from the primary tumor and form metastasis in distant tissues. At least in some cases, disseminated tumor cells display tumor-initiating properties similar to stem cells. In addition, these cells tend to resist chemo-therapy. However, it is unclear how cells gain the competency to escape chemotherapeutic treatment upon EMT.
In order to investigate the role of EMT in chemotherapeutic resistance and the underlying mechanisms, we used a HMLE RAS (Human Mammary Large T-Antigen immortalized Epi-thelial HaRas expressing cell) model system. This EMT system is composed of two different cell lines. On the one hand, the so called 24+ RAS cells display an epithelial morphology and lack efficient migration. On the other hand, the MSP RAS cells (mesenchymal subpopulation) that originate from the 24+ RAS cells have undergone EMT, accompanied by a mesenchymal cell shape and high invasivity. In this study, we applied several chemotherapeutic treatments to both cell lines to assess their response to chemotherapy.
We found that, in contrast to 24+ RAS cells, MSP RAS cells are resistant to the chemothera-peutics Cisplatin, Doxorubicin and Neocarzinostain, but also to the death ligands Trail and TNFα, as seen by reduced PARP and Caspase 3 cleavage upon treatment. Beyond that, we showed that the anti-apoptotic Bcl-2 family member Bcl-xL is overexpressed in MSP RAS cells, in comparison to 24+ RAS, which enables former cells to survive chemotherapy and to continue proliferation after treatment.
We have demonstrated that retrovirus-mediated Bcl-xL overexpression in 24+HMLE renders these cells chemoresistant, as well. Conversely, knocking down Bcl-xL in MSP RAS cells rendered them sensitive to death stimuli. Strikingly, we found Bcl-xL overexpressed in human breast cancer cells at the tumor-stroma-interface, suggesting a general role of Bcl-xL in mi-grating cancer cells. We found that inhibition of Bcl-xL with the BH3 mimic Gossypol results in chemosensitivity of MSP RAS cells. The same observation was obtained upon Bcl-xL de-stabilization, mediated by the inhibition of the chaperone HSP90 in vitro and in vivo, demon-strating that we found an option to overcome chemoresistance of cells that have undergone EMT.
In conclusion, our data reveal Bcl-xL as a key mediator of apoptosis-resistance in cells upon EMT. We propose a causal relation between EMT, Bcl-xL overexpression and chemo-resistance, and we suggest Bcl-xL as a promising target in cancer therapies.||de