BubR1 deficiency results in enhanced activation of MEK and ERKs upon microtubule stresses

Disruption of microtubules activates the spindle checkpoint, of which BubR1 is a major component. Our early studies show that BubR1 haplo-insufficiency results in enhanced mitotic slippage in vitro and tumorigenesis in vivo. Objective: Given that both MAPKs/ERKs and MEK play an important role during mitosis, we investigated whether there existed regulatory relationship between the MAPK signalling pathway and BubR1. Method and Results: Here, we have demonstrated that BubR1 deficiency is correlated with enhanced activation of MEK and ERKs after disruption of microtubule dynamics. Specifically, treatment with nocodazole and paclitaxel resulted in hyper-activation of ERKs and MEK in BubR1(+/-) murine embryonic fibroblasts (MEF) compared to that of wild-type MEFs. This enhanced activation of ERKs and MEK was at least partly responsible for more successful proliferation completion when cells were treated with nocodazole. BubR1 knockdown via RNAi resulted in enhanced activation of ERKs and MEK in HeLa cells, correlating with inhibition of PP1, a negative regulator of MEK. Moreover, when BubR1 was partially inactivated due to premature missegregation of chromosomes after Sgo1 depletion, phosphorylation of ERKs and MEK was enhanced in mitotic cells; in contrast, little, if any activated ERKs and MEK were detected in mitotic cells induced by nocodazole. Furthermore, BubR1, activated ERKs and activated MEK all localized to spindle poles during mitosis, and also, the proteins physically interacted with each other. Conclusion: Our studies suggest that there exists a cross-talk between spindle checkpoint components and ERKs and MEK and that BubR1 may play an important role in mediating the cross-talk.