The Mre11 Nuclease Is Critical for the Sensitivity of Cells to Chk1 Inhibition

Department of Pharmacology and Toxicology, The Geisel School of Medicine at Dartmouth, Hanover, New Hampshire, United States of America.
PLoS ONE (Impact Factor: 3.23). 06/2012; 7(8):e44021. DOI: 10.1371/journal.pone.0044021
Source: PubMed


The Chk1 kinase is required for the arrest of cell cycle progression when DNA is damaged, and for stabilizing stalled replication forks. As a consequence, many Chk1 inhibitors have been developed and tested for their potential to enhance DNA damage-induced tumor cell killing. However, inhibition of Chk1 alone, without any additional exogenous agent, can be cytotoxic. Understanding the underlying mechanisms of this sensitivity is critical for defining which patients might respond best to therapy with Chk1 inhibitors. We have investigated the mechanism of sensitivity in U2OS osteosarcoma cells. Upon incubation with the Chk1 inhibitor MK-8776, single-stranded DNA regions (ssDNA) and double-strand breaks (DSB) begin to appear within 6 h. These DSB have been attributed to the structure-specific DNA endonuclease, Mus81. The Mre11/Rad50/Nbs1 complex is known to be responsible for the resection of DSB to ssDNA. However, we show that inhibition of the Mre11 nuclease activity leads, not only to a decrease in the amount of ssDNA following Chk1 inhibition, but also inhibits the formation of DSB, suggesting that DSB are a consequence of ssDNA formation. These findings were corroborated by the discovery that Mre11-deficient ATLD1 cells are highly resistant to MK-8776 and form neither ssDNA nor DSB following treatment. However, once complimented with exogenous Mre11, the cells accumulate both ssDNA and DSB when incubated with MK-8776. Our findings suggest that Mre11 provides the link between aberrant activation of Cdc25A/Cdk2 and Mus81. The results highlight a novel role for Mre11 in the production of DSB and may help define which tumors are more sensitive to MK-8776 alone or in combination with DNA damaging agents.

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    • "Incubation of cells with MK-8776 alone for 24 h induced low level phosphorylation of ser345-Chk1. We have previously reported that this phosphorylation occurs prior to the detection of DNA damage as assessed by γH2AX [14], hence this is likely attributable to the inhibition of Chk1 preventing the normal feedback dephosphorylation by protein phosphatase 2A such that ongoing phosphorylation by ATR enhances phosphorylation of Chk1 [15]. When 1 μmol/L MK-8776 was combined with gemcitabine, even at the lowest concentrations tested, there was an increased phosphorylation of ser345-Chk1 but no phosphorylation of ser296-Chk1, an autophosphorylation site, consistent with inhibition of Chk1. "
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    ABSTRACT: Chk1 inhibitors have emerged as promising anticancer therapeutic agents particularly when combined with antimetabolites such as gemcitabine, cytarabine or hydroxyurea. Here, we address the importance of appropriate drug scheduling when gemcitabine is combined with the Chk1 inhibitor MK-8776, and the mechanisms involved in the schedule dependence. Growth inhibition induced by gemcitabine plus MK-8776 was assessed across multiple cancer cell lines. Experiments used clinically relevant "bolus" administration of both drugs rather than continuous drug exposures. We assessed the effect of different treatment schedules on cell cycle perturbation and tumor cell growth in vitro and in xenograft tumor models. MK-8776 induced an average 7-fold sensitization to gemcitabine in 16 cancer cell lines. The time of MK-8776 administration significantly affected the response of tumor cells to gemcitabine. Although gemcitabine induced rapid cell cycle arrest, the stalled replication forks were not initially dependent on Chk1 for stability. By 18 h, RAD51 was loaded onto DNA indicative of homologous recombination. Inhibition of Chk1 at 18 h rapidly dissociated RAD51 leading to the collapse of replication forks and cell death. Addition of MK-8776 from 18-24 h after a 6-h incubation with gemcitabine induced much greater sensitization than if the two drugs were incubated concurrently for 6 h. The ability of this short incubation with MK-8776 to sensitize cells is critical because of the short half-life of MK-8776 in patients' plasma. Cell cycle perturbation was also assessed in human pancreas tumor xenografts in mice. There was a dramatic accumulation of cells in S/G2 phase 18 h after gemcitabine administration, but cells had started to recover by 42 h. Administration of MK-8776 18 h after gemcitabine caused significantly delayed tumor growth compared to either drug alone, or when the two drugs were administered with only a 30 min interval. There are two reasons why delayed addition of MK-8776 enhances sensitivity to gemcitabine: first, there is an increased number of cells arrested in S phase; and second, the arrested cells have adequate time to initiate recombination and thereby become Chk1 dependent. These results have important implications for the design of clinical trials using this drug combination.
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    ABSTRACT: Replication stress and DNA damage activate the ATR-CHK1 checkpoint signaling pathway that licenses repair and cell survival processes. In this study, we examined the respective roles of the ATR and CHK1 kinases in ovarian cancer cells using genetic and pharmacological inhibitors of in combination with cisplatin, topotecan, gemcitabine and the poly(ADP-ribose)-polymerase (PARP) inhibitor veliparib (ABT-888), four agents with clinical activity in ovarian cancer. RNAi-mediated depletion or inhibition of ATR sensitized ovarian cancer cells to all four agents. In contrast, while cisplatin, topotecan and gemcitabine each activated CHK1, RNAi-mediated depletion or inhibition of this kinase in cells sensitized them only to gemcitabine. Unexpectedly, we found that neither the ATR kinase inhibitor VE-821 or the CHK1 inhibitor MK-8776 blocked ATR-mediated CHK1 phosphorylation or autophosphorylation, two commonly used readouts for inhibition of the ATR-CHK1 pathway. Instead, their ability to sensitize cells correlated with enhanced CDC25A levels. Additionally, we also found that VE-821 could further sensitize BRCA1-depleted cells to cisplatin, topotecan and veliparib beyond the potent sensitization already caused by their deficiency in homologous recombination. Taken together, our results established that ATR and CHK1 inhibitors differentially sensitize ovarian cancer cells to commonly used chemotherapy agents, and that CHK1 phosphorylation status may not offer a reliable marker for inhibition of the ATR-CHK1 pathway. A key implication of our work is the clinical rationale it provides to evaluate ATR inhibitors in combination with PARP inhibitors in BRCA1/2-deficient cells.
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    ABSTRACT: Many anticancer agents damage DNA and activate cell cycle checkpoints that permit time for the cells to repair their DNA and recover. These checkpoints have undergone intense investigation as potential therapeutic targets, and Chk1 inhibitors have emerged as promising novel therapeutic agents. Chk1 was initially recognized as a regulator of the G2/M checkpoint, but has since been demonstrated to have additional roles in replication fork stability, replication origin firing and homologous recombination. Inhibition of these pathways can dramatically sensitize cells to some antimetabolites. Current clinical trials with Chk1 inhibitors are primarily focusing on their combination with gemcitabine. Here, we discuss the mechanisms of, and emerging uses for Chk1 inhibitors as single agents and in combination with antimetabolites. We also discuss the pharmacodynamic issues that need to be addressed in attaining maximum efficacy in vivo. Following administration of gemcitabine to mice and humans, tumor cells accumulate in S phase for at least 24 h before recovering. In addition, stalled replication forks evolve over time to become more Chk1 dependent. We emphasize the need to assess cell cycle perturbation and Chk1 dependence of tumors in patients administered gemcitabine. These assessments will define the optimum dose and schedule for administration of these drug combinations.
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