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Vol. 17, Issue 1, 402-412, January 2006

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Chk1 and p21 Cooperate to Prevent Apoptosis during DNA Replication Fork Stress

Institute for Cancer Studies, School of Medicine and Biomedical Sciences, University of Sheffield, Sheffield S10 2RX, United Kingdom

Submitted July 5, 2005; Revised September 29, 2005; Accepted October 27, 2005
Monitoring Editor: Gerard Evan

Cells respond to DNA replication stress by triggering cell cycle checkpoints, repair, or death. To understand the role of the DNA damage response pathways in determining whether cells survive replication stress or become committed to death, we examined the effect of loss of these pathways on cellular response to agents that slow or arrest DNA synthesis. We show that replication inhibitors such as excess thymidine, hydroxyurea, and camptothecin are normally poor inducers of apoptosis. However, these agents become potent inducers of death in S-phase cells upon small interfering RNA-mediated depletion of the checkpoint kinase Chk1. This death response is independent of p53 and Chk2. p21-deficient cells, on the other hand, produce a more robust apoptotic response upon Chk1 depletion. p21 is normally induced only late after thymidine treatment. In Chk1-depleted cells p21 induction occurs earlier and does not require p53. Thus, Chk1 plays a primary role in the protection of cells from death induced by replication fork stress, whereas p21 mediates through its role in regulating entry into S phase. These findings are of potential importance to cancer therapy because we demonstrate that the efficacy of clinically relevant agents can be enhanced by manipulation of these signaling pathways.

Abbreviations used: ATM, ataxia-telangiectasia mutated; ATR, ATM- and Rad3-related; CPT, camptothecin; DSB, double-strand break; HRR, homologous recombination repair; HU, hydroxyurea; IR, ionizing radiation; MMR, mismatch repair; RPA, replication protein A.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

Address correspondence to: Mark Meuth (m.meuth{at}sheffield.ac.uk).

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