C-terminal binding proteins play a critical nuclear function in the mitotic fidelity of breast cancer cells

We are interested in finding novel therapeutic targets for breast cancer, particularly focusing on C-terminal binding proteins (CtBPs) as when they are inhibited, cells show an increased sensitivity to apoptotic stimuli. Using siRNA to inhibit CtBP expression, we have found that CtBPs are essential for the survival of breast cancer cells, and in particular those with a more aggressive p53-mutant phenotype. To direct our future studies into therapeutic strategies targeting CtBPs in breast cancer, we need to know more precisely how their loss results in death, and which of their functions are required for this prosurvival role.

Here we show that loss of CtBP function through siRNA treatment suppresses proliferation through a combination of p53-independent apoptosis, reduction in cell-cycle progression into mitosis, and aberrations in transit through mitosis itself. This third phenotype includes errors in mitotic chromosome segregation, activation of, but failure to sustain, the spindle assembly checkpoint, decreased expression of Aurora B, and a high rate of failure to complete cytokinesis. We showed that loss of CtBP in breast cancer cells with a functional p53 response pathway resulted in a marked upregulation of the p53 protein. Here p53 appears to be providing a protective role by arresting aberrant cells in G₁, thus preventing them from entering S-phase with incorrectly segregated DNA.

CtBPs are known to act in the nucleus as transcriptional co-repressors and in the cytoplasm as regulators of Golgi fission. Using a series of dominant negative CtBP mutants microinjected into either the cytoplasm or nucleus, we show that localisation of CtBPs to the nucleus is critical for its function in ensuring the correct division of breast cancer cells. This suggests that CtBPs function in maintaining mitotic fidelity, and thus in the continued proliferation and survival of breast cancer cells through their actions as a transcriptional co-repressor within the nucleus.