The role of specific checkpoint-induced S-phase transcripts in resistance to replicative stress

An essential role for dNTP homeostasis following CDK-induced replication stress

Replication stress is a common feature of cancer cells, and thus a potentially important therapeutic target. Here, we show that cyclin-dependent kinase (CDK)-induced replication stress, resulting from Wee1 inactivation, is synthetic lethal with mutations disrupting dNTP homeostasis in fission yeast. Wee1 inactivation leads to increased dNTP demand and replication stress through CDK-induced firing of dormant replication origins. Subsequent dNTP depletion leads to inefficient DNA replication, DNA damage and to genome instability. Cells respond to this replication stress by increasing dNTP supply through histone methyltransferase Set2-dependent MBF-induced expression of Cdc22, the catalytic subunit of ribonucleotide reductase (RNR). Disrupting dNTP synthesis following Wee1 inactivation, through abrogating Set2-dependent H3K36 tri-methylation or DNA integrity checkpoint inactivation results in critically low dNTP levels, replication collapse and cell death, which can be rescued by increasing dNTP levels. These findings support a 'dNTP supply and demand' model in which maintaining dNTP homeostasis is essential to prevent replication catastrophe in response to CDK-induced replication stress.

The Intra-S Checkpoint Responses to DNA Damage

Faithful duplication of the genome is a challenge because DNA is susceptible to damage by a number of intrinsic and extrinsic genotoxins, such as free radicals and UV light. Cells activate the intra-S checkpoint in response to damage during S phase to protect genomic integrity and ensure replication fidelity. The checkpoint prevents genomic instability mainly by regulating origin firing, fork progression, and transcription of G1/S genes in response to DNA damage. Several studies hint that regulation of forks is perhaps the most critical function of the intra-S checkpoint. However, the exact role of the checkpoint at replication forks has remained elusive and controversial. Is the checkpoint required for fork stability, or fork restart, or to prevent fork reversal or fork collapse, or activate repair at replication forks? What are the factors that the checkpoint targets at stalled replication forks? In this review, we will discuss the various pathways activated by the intra-S checkpoint in response to damage to prevent genomic instability.

ATM/CHK/p53 Pathway Dependent Chemopreventive and Therapeutic Activity on Lung Cancer by Pterostilbene

Among the many stilbenoids found in a variety of berries, resveratrol and pterostilbene are of particular interest given their potential for use in cancer therapeutics and prevention. We purified four stilbenoids from R. undulatum and found that pterostilbene inhibits cancer cell proliferation more efficiently than rhapontigenin, piceatannol and resveratrol. To investigate the underlying mechanism of this superior action of pterostilbene on cancer cells, we utilized a reverse-phase protein array followed by bioinformatic analysis and found that the ATM/CHK pathway is modified by pterostilbene in a lung cancer cell line. Given that ATM/CHK signaling requires p53 for its biological effects, we hypothesized that p53 is required for the anticancer effect of pterostilbene. To test this hypothesis, we used two molecularly defined precancerous human bronchial epithelial cell lines, HBECR and HBECR/p53i, with normal p53 and suppressed p53 expression, respectively, to represent premalignant states of squamous lung carcinogenesis. Pterostilbene inhibited the cell cycle more efficiently in HBECR cells compared to HBECR/p53i cells, suggesting that the presence of p53 is required for the action of pterostilbene. Pterostilbene also activated ATM and CHK1/2, which are upstream of p53, in both cell lines, though pterostilbene-induced senescence was dependent on the presence of p53. Finally, pterostilbene more effectively inhibited p53-dependent cell proliferation compared to the other three stilbenoids. These results strongly support the potential chemopreventive effect of pterostilbene on p53-positive cells during early carcinogenesis.

Tolerance of deregulated G1/S transcription depends on critical G1/S regulon genes to prevent catastrophic genome instability

Expression of a G1/S regulon of genes that are required for DNA replication is a ubiquitous mechanism for controlling cell proliferation; moreover, the pathological deregulated expression of E2F-regulated G1/S genes is found in every type of cancer. Cellular tolerance of deregulated G1/S transcription is surprising because this regulon includes many dosage-sensitive proteins. Here, we used the fission yeast Schizosaccharomyces pombe to investigate this issue. We report that deregulating the MBF G1/S regulon by eliminating the Nrm1 corepressor increases replication errors. Homology-directed repair proteins, including MBF-regulated Ctp1(CtIP), are essential to prevent catastrophic genome instability. Surprisingly, the normally inconsequential MBF-regulated S-phase cyclin Cig2 also becomes essential in the absence of Nrm1. This requirement was traced to cyclin-dependent kinase inhibition of the MBF-regulated Cdc18(Cdc6) replication origin-licensing factor. Collectively, these results establish that, although deregulation of G1/S transcription is well tolerated by cells, nonessential G1/S target genes become crucial for preventing catastrophic genome instability.

DNA replication stress differentially regulates G1/S genes via Rad53-dependent inactivation of Nrm1

MBF and SBF transcription factors regulate a large family of coordinately expressed G1/S genes required for early cell-cycle functions including DNA replication and repair. SBF is inactivated upon S-phase entry by Clb/CDK whereas MBF targets are repressed by the co-repressor, Nrm1. Using genome-wide expression analysis of cells treated with methyl methane sulfonate (MMS), hydroxyurea (HU) or camptothecin (CPT), we show that genotoxic stress during S phase specifically induces MBF-regulated genes. This occurs via direct phosphorylation of Nrm1 by Rad53, the effector checkpoint kinase, which prevents its binding to MBF target promoters. We conclude that MBF-regulated genes are distinguished from SBF-regulated genes by their sensitivity to activation by the S-phase checkpoint, thereby, providing an effective mechanism for enhancing DNA replication and repair and promoting genome stability.

Phosphorylation of the MBF repressor Yox1p by the DNA replication checkpoint keeps the G1/S cell-cycle transcriptional program active

Background

In fission yeast Schizosaccharomyces pombe G1/S cell-cycle regulated transcription depends upon MBF. A negative feedback loop involving Nrm1p and Yox1p bound to MBF leads to transcriptional repression as cells exit G1 phase. However, activation of the DNA replication checkpoint response during S phase results in persistent expression of MBF-dependent genes.

Methodology/Principal Findings

This report shows that Yox1p binding to MBF is Nrm1-dependent and that Yox1p and Nrm1p require each other to bind and repress MBF targets. In response to DNA replication stress both Yox1p and Nrm1p dissociate from MBF at promoters leading to de-repression of MBF targets. Inactivation of Yox1p is an essential part of the checkpoint response. Cds1p (human Chk2p) checkpoint protein kinase-dependent phosphorylation of Yox1p promotes its dissociation from the MBF transcription factor. We establish that phosphorylation of Yox1p at Ser114, Thr115 is required for maximal checkpoint-dependent activation of the G1/S cell-cycle transcriptional program.

Conclusions/Significance

This study shows that checkpoint-dependent phosphorylation of Yox1p at Ser114, Thr115 results in de-repression of the MBF transcriptional program. The remodeling of the cell cycle transcriptional program by the DNA replication checkpoint is likely to comprise an important mechanism for the avoidance of genomic instability.

Yox1 links MBF-dependent transcription to completion of DNA synthesis

When DNA replication is challenged cells activate a DNA synthesis checkpoint, blocking cell cycle progression until they are able to overcome the replication defects. In fission yeast, Cds1 is the effector kinase of this checkpoint, inhibiting M-phase entry, stabilizing stalled replication forks and triggering transcriptional activation of S-phase genes. The molecular basis of this last effect is largely unknown. The Mlu1 binding factor (MBF) complex controls the transcription of S-phase genes. We purified novel interactors of the MBF complex and identified the repressor Yox1. When the DNA synthesis checkpoint is activated, Yox1 is phosphorylated, which abrogates its binding to MBF. MBF-dependent transcription therefore remains active until cells are able to overcome this challenge.