ATR and DNA-PK/CHK1 Elicit Distinct Responses in Cancer Cells

Under high levels of DNA replication stress, the ataxia telangiectasia and Rad3-related (ATR) kinase is thought to activate the downstream effector kinase CHK1 in order to inhibit origin firing and prevent massive replication fork collapse. However, recent work shows that replicating cells are more dependent on CHK1 and that inhibitors targeting ATR (ATRi) or CHK1 (CHK1i) induce differential levels of DNA damage, suggesting that ATR and CHK1 may function in a nonlinear pathway in response to intrinsic DNA replication stress. Buisson and colleagues showed that, in the absence of exogenous replication stress, ATR inhibition led to a rapid increase in single-stranded DNA (ssDNA) in a subset of early S-phase cells due to impaired accumulation of the ribonucleotide reductase subunit RRM2. Decreased RRM2 expression in the presence of ATRi was attributed to destabilization of the transcriptional activator E2F1 and was reversed upon inhibition of cyclin-dependent kinase 2, indicating that ATR limits CDK2 activity to induce RRM2 accumulation. In addition, ATR-mediated inhibition of CDK2 also restricted origin firing in early S-phase cells. Compared with ATRi, inhibition of CHK1 led to accumulation of higher levels of ssDNA and DNA damage, suggesting that CHK1 may protect ATRi-treated cells with lower levels of ssDNA. Indeed, DNA-PK–dependent CHK1 activation inhibited further CDK2-driven origin firing and prevented further ssDNA accumulation by destabilizing cell division cycle 25A (CDC25A) in cells treated with ATRi for longer time points. Consistent with a CHK1-mediated threshold effect, cells exposed to high levels of replicative stress were more dependent on ATR, whereas CHK1i-treated cells were sensitive to more moderate levels of replicative stress. Moreover, ATRi-induced ssDNA levels predicted ATRi sensitivity across a panel of cancer cell lines. Together, these data suggest that the ATR–CHK1 pathway protects cells from high DNA replication stress in unperturbed S phase, and that the DNA-PK–CHK1 axis protects cells from lower levels of replicative stress in the absence of ATR.

Buisson R, Boisvert JL, Benes CH, Zou L. Distinct but concerted roles of ATR, DNA-PK, and Chk1 in countering replication stress during S phase. Mol Cell 2015;59:1011–24.

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