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The Radioresistance to Killing of A1–5 Cells Derives from Activation of the Chk1 Pathway*

Baocheng Hu,Xiang-yang Zhou,Xiang Wang,Z. Zeng,G. Iliakis,Ya Wang

Published 2001 in Journal of Biological Chemistry

ABSTRACT

Checkpoints respond to DNA damage by arresting the cell cycle to provide time for facilitating repair. In mammalian cells, the G2 checkpoint prevents the Cdc25C phosphatase from removing inhibitory phosphate groups from the mitosis-promoting kinase Cdc2. Both Chk1 and Chk2, the checkpoint kinases, can phosphorylate Cdc25C and inactivate its in vitro phosphatase activity. Therefore, both Chk1 and Chk2 are thought to regulate the activation of the G2 checkpoint. Here we report that A1–5, a transformed rat embryo fibroblast cell line, shows much more radioresistance associated with a much stronger G2 arrest response when compared with its counterpart, B4, although A1–5 and B4 cells have a similar capacity for nonhomologous end-joining DNA repair. These phenotypes of A1–5 cells are accompanied by a higher Chk1 expression and a higher phosphorylation of Cdc2. On the other hand, Chk2 expression increases slightly following radiation; however, it has no difference between A1–5 and B4 cells. Caffeine or UCN-01 abolishes the extreme radioresistance with the strong G2 arrest and at the same time reduces the phosphorylation of Cdc2 in A1–5 cells. In addition, Chk1 but not Chk2 antisense oligonucleotide sensitizes A1–5 cells to radiation-induced killing and reduces the G2 arrest of the cells. Taken together these results suggest that the Chk1/Cdc25C/Cdc2 pathway is the major player for the radioresistance with G2 arrest in A1–5 cells.

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