Microtubules in apoptosis induction: are they necessary?

Microtubule-Targeting Agents (MTAs) constitute a class of drugs largely used in cancer treatment. Among them, both taxanes and Vinca-alkaloids are known to inhibit cancer cell proliferation by inducing cell cycle arrest and subsequent apoptosis. These agents modify the cytoskeleton by affecting the tubulin/microtubule system. In cancer cells, both classes suppress microtubule dynamics through inhibition of microtubule dynamic instability and treadmilling, and commonly induce diverse signals responsible for cell death initiation and execution via the mitochondrial intrinsic pathway. However, links between microtubule network disturbance and the involvement of mitochondria in apoptosis are not obvious, and one may think that they could be independent. Nevertheless, several intracellular proteins could connect microtubules and the apoptotic machinery. The aim of the present review is to provide elements that could answer to the question: is microtubule disruption dispensable for MTA-induced apoptosis? The first section is focused on the mechanisms responsible for the MTA-mediated apoptosis. Then, links between cell cycle regulators and apoptosis are underlined since MTA induce cell cycle arrest by inhibiting microtubules. In the third part, the potential involvement of microtubule-sequestered and/or -transported proteins in apoptotic signalisation is discussed. Lastly, the possible role of the tubulin/microtubule system in direct effects of MTAs on mitochondria is summarized. Thus, it becomes clear that microtubule network and apoptosis are deeply linked in MTA effectiveness, through a cascade of cellular events. It could lead to identification of new biomarkers of MTA effectiveness, that could improve combinatorial therapy with MTAs and provide crucial arms to circumvent resistance of cancer cells.