根據先前的研究指出，中國倉鼠細胞株 (CHO.K1) 無法停滯在G1細胞期，特別是接受X射線照射刺激之後。除了此現象之外，我們實驗室還發現如果加入秋水仙素到中國倉鼠細胞的培養皿中，隨即接受紫外線的照射，則會出現細胞大量死亡的現象，且此現象是一種細胞自裁反應 (apoptosis)並且可能和細胞的p21基因不表現有關。因此，為了進一步瞭解上述觀察到的現象是否與p21基因的表現缺失有關。我們建構四環黴素調控的基因表現系統，藉以控制送進細胞的人類p21基因的表現時機。以不同的DNA傷害刺激後，利用細胞流式分析儀來觀察細胞週期的分佈情形。在誘發人類p21蛋白的表現前提之下，無論以X射線(8 Gy)或者紫外線(25 J/m2)的刺激，我們都可以觀察到細胞有G1週期停滯現象。此外，當我們以秋水仙素配合紫外線來刺激有表現人類p21蛋白的細胞，不但沒有出現先前所看到的細胞大量死亡現象，反而卻觀察到G1細胞週期停滯的情形。在細胞存活分析的實驗中，我們還發現有人類p21蛋白表現的細胞在接受紫外線的刺激後，比p21蛋白表現缺失的細胞有較明顯的存活能力。總之，根據實驗結果發現，p21蛋白的表現缺失是造成中國倉鼠細胞珠G1檢查點缺失的重要因素。此外，我們也認為p21蛋白的表現，有助於細胞抵抗與紫外線有關所造成的細胞死亡現象，以增進細胞的存活能力。

The failure of G1 arrest in CHO.K1 cells is widely observed following X-ray or other DNA-damaging treatments. Recent research in our laboratory has found that the expression of p21 (Waf1/Cip1) gene is deficient in CHO.K1 cells. In addition, cells were sensitive to UV irradiation (25 J/m2) and underwent apoptosis in colcemid-containing medium. In this study, the role of p21 (Waf1/Cip1) protein in CHO.K1 cells following environmental assaults was investigated. After constructing a tetracycline-regulated human p21 expression system, we ectopically expressed human p21 (Waf1/Cip1) prior to treatment with DNA damaging agents and diagnosed the cell cycle distribution by flow cytometric analysis. Intriguingly, we observed recovery of G1 checkpoint in CHO.K1 cells with overexpression of human p21 following UV or X-ray irradiation. Furthermore, the survival analysis of CHO.K1 cells indicated that cells with overexpression of human p21 (Waf1/Cip1) protein were significantly resistant to UV than those without p21 expression, which seemed to block the massive apoptotic death exacerbated by UV irradiation in the presence of colcemid. These results demonstrate that overexpression of human p21 (Waf1/Cip1) in CHO.K1 cells can complement the G1 checkpoint defect and promote cell survival ability to against from DNA damaging agents.

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