Overexpression Of Aurora-A Upregulates Cyclin B1 Through Inhibiting Its Ubiquitin-mediated Degradation, By Which Plays A Role In Tumorigenesis

The proper cell cycle progression is essential to maintain the integrity of the genome. The cell cycle of eukaryote is under the accurate control of many cell cycle regulators, whose expression and regulation abnormality has been demonstrated can result in genome instability, the hallmark of many human tumors. Both Aurora-A and Cyclin B1 are the important regulators of the cell cycle. Cyclin B1 is a mitotic Cyclin, which functions mainly at late G2 phase and during mitosis as the regulatory subunit of CDK1. Aurora-A belongs to a multigenic serine/threonine protein kinase family found rescently. Several lines of evidences demonstrate that Aurora-A plays several crucial roles in maturation and separation of centrosome, mitosis spindle assembly and the regulation of cell cycle checkpoints. Aurora-A is amplified and overexpressed in many kinds of human cancers and cancer-derived cell lines, including ESCC (esophageal squamous cell carcinomas), suggesting that when overexpressed, Aurora-A might act as a potential oncogene during tumorigenesis. However, the accurate molecular mechanism by which Aurora-A participates in the development and progression of tumor is unclearly known up to now.In this paper, we find that when overexpressed, Aurora-A induces up-regulation of Cyclin B1 protein. According to the result of flow cytometry, Aurora-A overexpression has no evident effect on the distribution of cell cycle, which excludes the possibility that the upregulation of Cyclin B1 is due to only the G2/M arrest induced by Aurora-A overexpression. Aurora-A overexpression is not involved in the regulation of Cyclin B1 expression at the transcriptional level. In order to know whether the overexpression of Aurora-A affects the degradation of Cyclin B1, we use the protein stability experiments and find that the degradation of Cyclin B1 is evidently delayed but not blocked in Aurora-A overexpressing ESCC cells compared with that in control cells, which maybe contribute to the increased level of Cyclin B1 protein induced by Aurora-A overexpression. To investigate the molecular mechanism through which overexpressed Aurora-A enhances the protein stability of Cyclin B1, we employed immunoprecipatation experiments. In Aurora-A overexpressing cell line, the level of ubiquitinated-Cyclin B1 is lower than that in control cell line. Furthermore, the interaction of Cyclin B1 with the proteins involving in its ubiquitin-mediated degradation is substantially reduced in Aurora-A overexpressing cells, suggesting that Aurora-A overexpression might inhibite Cyclin B1 to be recognized by and connected to APC/C and its activators. In addition, we find that Aurora-A interacts with Cyclin B1 in vitro and in vivo. Interestingly, immunohistochemistry experiments show that both Aurora-A and Cyclin B1 proteins are overexpressed in human ESCC. Moreover, the deregulated expression of Cyclin B1 is correlated with the overexpression of Aurora-A in ESCC, suggesting a clinical correlation between these two proteins.Taken together, these findings suggest that overexpression of Aurora-A might upregulate Cyclin B1 protein through inhibiting its ubiquitin-mediated degradation, providing novel insight into understanding the mechanism of how deregulated Aurora-A contributes to genome instability and carcinogenesis.