Implication of bax in apoptosis depends on microtubule network mobility

Paclitaxel and vincristine sulfate, two anti-microtubule agents are known to induce apoptosis. In this study, we tried to apprehend the relationship between the regulation of apoptotic proteins such as the Bcl-2-family proteins and the cytoskeleton structure during apoptosis induction by these two drugs. Paclitaxel and vincristine sulfate were used for a 24-h incubation and resulted in EC50 of 1 µM and 1 µg/ml, respectively. Under these conditions, paclitaxel treatment induced microtubule network polymerization, condensation of chromatin, characteristic features of early and late apoptosis as confirmed by orange acridine and ethydium bromide double staining. However, the shape of cells was not modified, while mitochondria changed their conformation from filamentous to aggregated corpuscles located around the nucleus. In addition, pro-apoptotic Bax protein remained in the cytoplasm, the β-tubulin polymerization induced phosphorylation and inactivation of anti-apoptotic Bcl-2 and/or BclX/L proteins leading to intense mitochondria swelling and membrane disruption that are responsible for observed cytochrome c release and apoptotic proceeding. On the contrary, after vincristine sulfate treatment we observed morphological modifications such as cell shrinkage and nucleus condensation as the result of β-tubulin depolarization and disruption of microtubules. Bax protein was intensively translocated into mitochondria membrane, decreasing the proportion of Bax/Bcl-2 or Bax/Bcl-xL heterodimers allowing the release of cytochrome c from the mitochondria and apoptotic process. In conclusion, our study demonstrated that the two anti-microtubule agents (paclitaxel and vincristine sulfate) induced apoptosis by two different pathways. However, mitochondrial dysfunction followed by cytochrome c release are the crucial events whatever the apoptotic signal, polymerization or disruption of β-tubulin.