N-acetylcysteine (NAC)，是一個具有膜通透性的抗氧化劑，常被用來清除細胞中過多的氧化自由基(reactive oxygen species, ROS)，使細胞免於ROS所造成的傷害。先前的研究顯示，紫外線(UV)照射中國倉鼠卵巢細胞(CHO-K1 cells)會促使細胞走向細胞凋亡(apoptosis)。生化分析顯示，CHO-K1 細胞無法表現p21wafl，如果轉殖人類的p21wafl於此細胞中，則可消除UV所造成的細胞凋亡。在本論文研究中，我們發現NAC可以抑制UV所造成的細胞凋亡。雖然NAC可以增加細胞內glutathione(GSH)的量，但是NAC的抑制作用不是透過GSH所造成，因為直接增加細胞內GSH的量並無抑制效果，而且利用DL-buthionine-[S,R]-sulfoximine(BSO)消耗細胞內的GSH並不會加劇UV所造成的細胞凋亡。儘管BSO可以使細胞內ROS的量顯著提升，但ROS的增加並無法加劇UV所造成的細胞凋亡。因此ROS的量與細胞凋亡之間並無直接關係。另外，我們發現利用一種選擇性的cyclin-dependent kinase(CDK)小分子抑制劑-roscovitine，也會抑制UV造成的細胞凋亡。由於NAC可以使細胞生長停滯於G1期，所以我們推論NAC抑制UV所造成的細胞凋亡可能與抑制CDK活性有關，而不是透過抗氧化的機制所造成。

N-acetylcysteine (NAC), a cell membrane permeable antioxidant, is a precursor of glutathione that can remove the excess free radical and protect cells from damage by oxidative stress. Our previously study has indicated that the CHO-K1 cells undergo apoptosis after UV irradiation. The CHO-K1 cells do not express p21wafl, and the UV-induced apoptosis can be inhibited by ectopic expression of human p21wafl gene in the cells. In this study, we found that NAC could protect the cells from the UV-induced apoptosis. Although NAC might increase the intracellular glutathione (GSH) content, the inhibitory effect of NAC on the UV-induced apoptosis was probably not mediated by GSH. Adding GSH into cells failed to produce the similar effect seen with NAC. Moreover, using DL-buthionine-[S,R]-sulfoximine (BSO) to deplete the intracellular GSH content was unable to enhance the UV-induced apoptosis. Despite the large increase of the intracellular ROS content following BSO treatment, the finding that BSO could not promoter the UV-induced apoptosis suggests that the UV-induced apoptosis did not correlate with the intracellular ROS content. In addition, roscovitine, a selective cyclin-dependent kinase (CDK) inhibitor, also inhibited the UV-induced apoptosis in CHO-K1 cells. As NAC was a strong cell cycle inhibitor at G1. The inhibition of NAC upon the UV-induced apoptosis is probably mediated by the modulation of CDK activity as seen with roscovitine.

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