| Summary: | 碩士 === 國立成功大學 === 生物化學暨分子生物學研究所 === 100 === Human Rad9 (hRad9), a structural homologue of S.pombe rad9, is a multifunction protein which participates in DNA repair, cell cycle regulation, apoptosis. The ten phosphorylation sites located in the C-terminal domain of hRAD9 were suggested to be critical for modulating its functions. In this study, we found hRAD9 was aberrantly hypre-phosphorylated in cancerous colon tissues, thus we investigated whether its functions were modulated by protein phosphorylation. In our previous findings, hRad9 could promote p53- independent senescence in breast, lung, and colon cancers. By ectopic expression of phosphorylated or de-phosphorylated mimicking hRAD9 (hRad9-10D and -10A), we found hRad9-10A could induce senescence, even was stronger than hRad9, but overexpression of hRad9-10D made it lose the ability of inducing senescence. Two protein kinases, CDC2 and CK2α, which contributed to phosphorylation of hRAD9, were suggested to be oncogenic in tumorigenesis. By silencing CDC2 or CK2α, it caused the accumulation of de-phosphorylated hRAD9, and caused senescence accompanying with upregulation of p21. Furthermore, the effects caused by loss of CDC2 or CK2α were reversed by hRad9 knockdown. Otherwise, we accessed the levels of CDC2 and CK2α in colon cancer specimens, expressions of CDC2 and CK2α were consistent with the patterns of hyper-phophorylated hRAD9. In conclusion, our findings reveal that protein phosphorylation of hRad9, which is caused by CDC2 and CK2α result in a negative effect on hRAD9-triggered senescence.
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