Treatment of
cancer cells with microtubule inhibitors causes mitotic arrest, which subsequently leads to cell death via activation of the intrinsic apoptotic pathway. Mitotically arrested cells typically display increased phosphorylation (i.e., inactivation) of two key
anti-apoptotic proteins, Bcl-2 and Bcl-XL , but the mechanisms that regulate their phosphorylation as well as their role in apoptotic cell death following mitotic arrest are still poorly understood at present, which are the focus of this study. We recently showed that
cyclin B1 and cell division cycle 2 (Cdc2)
proteins are strongly up-regulated in human
breast cancer cells following treatment with
nocodazole (a prototypical microtubule inhibitor), and their up-regulation plays a critical role in the development of mitotic prometaphase arrest. In this study, we present evidence showing that the up-regulated
cyclin B1/Cdc2 complex in
nocodazole-treated human
breast cancer cells is also responsible for the increased phosphorylation of Bcl-2 and Bcl-XL . However, only the increased phosphorylation of Bcl-XL , but not the phosphorylation of Bcl-2, contributes to subsequent activation of the intrinsic cell death pathway. In addition, evidence is presented to show that mitotic arrest deficient 2 (MAD2) is a key upstream mediator of the up-regulation of
cyclin B1/Cdc2 as well as the subsequent increase in phosphorylationof Bcl-2 and Bcl-XL in
nocodazole-treated
cancer cells. Together, these results reveal that the up-regulated
cyclin B1/Cdc2 complex not only mediates prometaphase arrest in
nocodazole-treated cells, but also activates the subsequent intrinsic cell death pathway in these cells via increased phosphorylation of Bcl-XL .