| Summary: | 博士 === 國立清華大學 === 生命科學系 === 94 === DNA damage checkpoint control which involves the pathway of ATR/ATM- p53-p21(Waf1/Cip1) provides a mechanism to protect the genomic integrity. Chinese hamster ovary cells (CHO-K1) which cannot express p21 fail to arrest at G1 phase following UVC irradiation. The UV-irradiated CHO-K1 cells exhibit substantial apoptosis at 24 h after irradiation. Activities of cyclin-dependent kinases especially, Cdk2 are found elevated following the irradiation. The elevation of Cdk2 activity correlates with the increase of UV-induced apoptosis. Further, the increase in Cdk2 activity causes hyperphosphorylation of retinoblastoma (Rb) protein, which consequently activates E2F protein. Moreover, the UV-induced apoptosis was suppressed by roscovitine, a small-molecule Cdk2 inhibitor. On the other hand, the reactive oxygen species (ROS) seems not to be involved in UVC-induced apoptosis, which was proved by pretreatment with different kinds of antioxidant except pyrrolidine dithiocarbamate (PDTC). The PDTC reduces cell proliferation, and its inhibition of UV-induced apoptosis is likely to due to the inhibition of Cdk2 activity. Thus, we hypothesize that deregulation of Cdk2 may lead to apoptosis via the Rb/E2F-mediated pathway. Besides, p53 is induced in UV-irradiated CHO-K1 cells, but the expression of pro-apoptotic Bax protein, a downstream effector of p53, is not elevated. Further study indicates decrease of endogenous p53 by p53 specific siRNA cannot suppress UV-induced apoptosis. In fact, p53 plays an important role in excision of UV-induced DNA adducts. Taken together, my study demonstrates that deregulation of Cdk2 via Rb/E2F and p53-independent pathway induces apoptosis in UV-irradiated cells. Furthermore, p53 is involved in early stage of nucleotide excision repair.
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