| Mitosis can be divided into prophase、prometaphase、metaphase、anaphase、 and telophase. The spindle assembly checkpoint (SAC), which is between prometaphase and metaphase, prevents mis-segregation of chromosomes by retaining cells in metaphase until every chromosome has successfully attached to the spindles. The SAC can be activated by a single mis-segregated chromosome. Lack of fidelity of chromosome partiotion due to SAC defect can cause aneuploidy, which is a general phenomenon of tumor cells and has a promoting effect on tumorigenesis. So the SAC plays an important role in the maintenance of genome stability.Monopolar spindle 1 (Mpsl) was first found in the budding yeast (Saccharomyces cerevisiase) and to be named for its mutant alleles form a monopolar spindle polar body. Mpsl is a dual-specificity protein kinase, which phosphorylates serines、threonines and tyrosines of the substrates and itself. The transcriptional level of Mpsl is regulated by the E2F family of transcription factors and its subcellular localization is regulated by the cell cycle which is located in the cytoplasm during G1 and enters into the nucleus at the G2/M boundary. Mpsl is a multifacet player, which functions in SAC regulation, chromosomes alignnment、cytokinesis、centrosome duplication in cell and the develoment of organs. Mpsl is highly expressed in a varitey of tumors and correlates with tumor grade in breast cancer. Loss of Mpsl functions by using Mpsl-specific siRNAs or specific inhibitors can effectively kill tumor cells but leave normal cells unaffected, enable Mpsl a potential new target for tumro therapy.Although Mpsl is overexpressed in a variety of tumors, however, the relationship between high levels of Mpsl and tumorigenesis is not clear. In this study we find Mpsl is highly expressed in colon cancer tissues and a variety of tumor cells including HelaS3, MCF-7, HepG2, SMMC7701 and SW480. Further, we constructed a Flag-Mpsl stably overexpressed in a S W480 cell line and find that overexpression of Mpsl slightly decreases the threshold for checkpoint generation but attenuates the maintenance function of the spindle assembly checkpoint. Overexpression of Mpsl also increases genome instability in tumor cells. Collectively, our findings suggest that high levels of Mpsl contribute to tumorigenesis by attenuating the spindle assembly checkpoint.Mpsl has been suggested in regulating cytokinesis, but the conclusive result is unavailable yet. We designed a degradable Mpsl (degMpsl) by fusing Mpsl to CyclinB10-107 fragment, the latter of which is responslbe for CyclinB degradation upon onset of anaphase. The degMpsl show the similar degradation pattern compared to the Cyclin B when stably expressed in SW480 cells. Using this cell model, we first find the degradation of Mps1 after metaphase affects neither cell viability nor cytokinesis. Interestingly, the degradation of Mpsl after metaphase promotes the phosphorylation of PLK1 and AuroraB. Our results suggest Mpsl is involved in but dispensable for cytokinesis.Based on the finding that overexpression of Mpsl attenuates the maintenance function of the spindle assembly checkpoint, we want to know if the spindle assembly checkpoint is necessary to tumor cells survive. Using a specific inhibitor (Reversine) that inhibit the activity of Mpsl after prometaphase, we find the survival ability of tumor cells decreased significantly. As we know the degradation of Mpsl after metaphase doesn’t affect cell viability, this suggests SAC is an indispensable element in tumor cells survival. |
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