當基因遭受損傷時，細胞會啟動檢查控制機制將細胞週期停滯在G1或G2/M期來防止受損傷的基因進入S期或進行有絲分裂。此檢查控制機制是由基因損傷感應子，訊息傳遞子及效應子所組成，但是其作用機制會因細胞處在不同的細胞週期、遭受不同的基因損傷或細胞種類的不同而有差異。而中國倉鼠卵巢細胞 (CHO-K1) 因缺乏p21蛋白質的表現，當其受到紫外線傷害時會無法停滯於G1期中。所以，此細胞可用於研究當細胞受紫外線照射後誘發細胞週期停滯現象(G2/M arrest) 的分子機制。在本研究中發現，在高劑量的紫外線照射下會使中國倉鼠卵巢細胞走向細胞凋亡，而給予低劑量的紫外線照射則會讓細胞停滯於G2/M期中。當在低劑量的紫外線照射後同時處理咖啡因，會讓細胞週期無法停滯於G2/M期中，反而造成大量的細胞走向細胞凋亡。而因紫外線造成G2/M期的停滯現象是透過Chk1-Cdc25C訊息傳遞路徑而非經由p53依賴性的訊息傳遞方式。此外，停滯在G2/M期的細胞會進行基因損傷的修復，當紫外線引起之基因損傷完全修復後會恢復其細胞週期的進行。於本研究中明確顯示細胞週期的停滯是讓中國倉鼠卵巢細胞有足夠的時間進行基因損傷的修復，藉此保護細胞不因紫外線的傷害而走向細胞凋亡。

In responding to DNA damage, cells may trigger checkpoint controls to arrest cell cycle progression at G1 or G2/M phase to prevent the injured DNA to enter S phase or mitosis. Although the checkpoint control in general consists of DNA damage sensors, signal transducers and effectors, the detailed mechanisms of checkpoint controls may be varied due to the differences of cell stages, DNA damages or cell types. Chinese hamster ovary cells (CHO-K1) which fail to arrest at G1 phase following UV irradiation through the absence of p21 provide a simplified system to investigate the molecular mechanism of UV-induced cell cycle arrest. In our study, we found that CHO-K1 underwent apoptosis under high dose of UV irradiation, while arrested at G2/M phase under low dose of irradiation. The G2/M arrest was abolished by caffeine and the cells underwent massive apoptosis. Moreover, the G2/M arrest is regulated by p53-independent fashion through Chk1-Cdc25C signaling pathway. Furthermore, the cell cycle progression was restored and UV-induced DNA damages were completely repaired in cells with G2/M arrest. The present study clearly show the role of G2/M arrest in giving CHO-K1 cells enough time to repair DNA adducts, and protecting the cells from UV-induced apoptosis.

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