Analysis the role of Cdk1/Cyclin B and Bax in Mitotic Checkpoint-Induced Apoptosis

There has been extensive clinical usage of antimitotic drugs (spindle poisons), such as Taxol, which is a potent antineoplastic agent, but the manner in which they promote cell death is still unclear. Some antimitotic drugs target the microtubules directly while some agents such as Monastrol target microtubules-associated motor proteins. HeLa cells treated with antimitotic agents activate the spindle assembly checkpoint (SAC also known as mitotic checkpoint) that results in prolonged mitotic arrest and activation of the mitochondrial apoptotic pathway. The signal that links activation of the SAC to activation of the mitochondrial apoptotic pathway has remained unknown. A study conducted in our laboratory has indicated that the signal that links SAC activation to apoptosis is the complex of activated Cdk1and the pro-apoptotic protein Bax. Bax binds to and facilitates the translocation of active Cdk1 to the outer mitochondrial membrane (OMM) to phosphorylate and inactivate B-cell lymphoma-2 (Bcl-2) and B-cell lymphoma-xl (Bcl-xL), both of which are anti-apoptotic proteins located in the OMM. Inactivation of Bcl-2 and Bcl-xL proteins allows Bax/ Bak to form pores in the OMM leading to the release of cytochrome c and ultimately cell death.

This study has found that the Cdk1 and Bax complex is essential for Taxol-induced apoptosis, and Bax/Bak-deficient cells display a reduction in the level of phosphorylation of Bcl 2, Bcl-xL and reactivation of cell death signalling.

Furthermore, it has been shown that the re-introduction of GFP-Bax in to Bax/Bak-deficient cells restored the phosphorylation of Bcl-2 and Bcl-xL and cell death. We have also demonstrated that the Cdk1/Bax complex is formed and translocate from the cytosol to mitochondria when the SAC is activated with both microtubules-targeting and motor protein-targeting drugs. Although, we have found that Cdk 1 interacts with Bax directly in vitro, the exact molecular region of Cdk1 that binds to Bax is unknown. The results have shown that it is the C-terminus of Cdk1 binds to Bax in vitro.