New insights into checkpoint kinase 1 in the DNA damage response signaling network

Inhibition of Wee1 sensitizes AML cells to ATR inhibitor VE-822-induced DNA damage and apoptosis

Resistance to standard induction therapy and relapse remain the primary challenges for improving therapeutic effects in acute myeloid leukemia (AML); thus, novel therapeutic strategies are urgently required. Ataxia telangiectasia and Rad3-related protein (ATR) is a key regulator of different types of DNA damage, which is crucial for the maintenance of genomic integrity. The ATR-selective inhibitor VE-822 has proper solubility, potency, and pharmacokinetic properties. In this study, we investigated the anti-leukemic effects of VE-822 alone or combined with Wee1-selective inhibitor AZD1775 in AML cells. Our results showed that VE-822 inhibited AML cell proliferation and induced apoptosis in a dose-dependent manner. AZD1775 significantly promoted VE-822-induced inhibition of AML cell proliferation and led to a decreased number of cells in the G2/M phase. VE-822 and AZD1775 decreased the protein levels of ribonucleotide reductase M1 (RRM1) and M2 (RRM2) subunits, key enzymes in the synthesis of deoxyribonucleoside triphosphate, which increased DNA replication stress. VE-822 combined with AZD1775 synergistically induced AML cell apoptosis and led to replication stress and DNA damage in AML cell lines. Our study demonstrated that AZD1775 synergistically promotes VE-822-induced anti-leukemic activity in AML cell lines and provides support for clinical research on VE-822 in combination with AZD1775 for the treatment of AML patients.

High expression of TRIM36 is associated with radiosensitivity in gastric cancer

Radiotherapy is one of the main adjuvant treatments for gastric cancer (GC) that can effectively reduce local recurrence and improve survival rates. However, radiotherapy may result in cytotoxicity and not benefit all patients. This highlights the requirement for identifying potential radiosensitivity genes in GC. The current study investigated the association between tripartite motif containing 36 (TRIM36) status and the prognosis of patients with GC receiving radiotherapy. A total of 371 patients with GC were selected from The Cancer Genome Atlas and randomly divided into test and the validation groups. The results revealed that TRIM36 expression was not associated with the overall survival (OS) rate. Patients who received radiotherapy with high TRIM36 expression had an improved OS rate compared with patients who did not receive radiotherapy in the test group, as demonstrated by univariate analysis [hazard ratio (HR), 0.062; 95% confidence interval (CI), 0.008–0.462; P=0.007] and multivariate analysis (HR, 0.095; 95% CI, 0.012–0.748; P=0.025). In the validation group, patients with high TRIM36 expression had decreased mortality risk when they received radiotherapy compared with patients who did not receive radiotherapy, as determined by univariate analysis (HR, 0.190; 95% CI, 0.067–0.540; P=0.002) and multivariate analysis (HR, 0.075; 95% CI, 0.020–0.276; P<0.001). However, for patients with low expression, no significant difference was identified in the overall survival rates between the radiotherapy and non-radiotherapy groups. Chi-squared analysis revealed that the expression status of TRIM36 was an independent factor and was not associated with clinicopathological factors. The results indicated that patients with high TRIM36 expression receiving radiotherapy exhibited an improved OS rate. TRIM36 may therefore be an important factor affecting the clinical prognosis of patients with GC receiving radiotherapy and may be considered as a potential radiosensitivity gene signature.

Iron-oxide nanoparticles target intracellular HSP90 to induce tumor radio-sensitization

BACKGROUND

Nanoparticle-based therapies have emerged as a promising approach to overcome limitations of conventional chemotherapy. Present study investigates the potential of oleic acid-functionalized iron-oxide nanoparticles (MN-OA) to enhance the radiation response of fibrosarcoma tumor and elucidates its underlying mechanism.

METHODS

Various cellular and molecular assays (e.g. MTT, clonogenic, cell cycle analysis, cell death, DNA damage/repair) and tumor growth kinetics were employed to investigate the mechanism of MN-OA induced radio-sensitization.

RESULTS

Mouse (WEHI-164) and human (HT-1080) fibrosarcoma cells treated with MN-OA and gamma-radiation (2 Gy) showed a significant decrease in the cell proliferation. Combination treatment showed significant decrease in clonogenic survival of WEHI-164 cells and was found to induce cell cycle arrest, apoptosis and mitotic catastrophe. The mechanism of radio-sensitization was found to involve binding of MN-OA with HSP90, resulting in down-regulation of its client proteins, involved in cell cycle progression (Cyclin B1 and CDC2) and DNA-double strand break repair (e.g. RAD51 and BRCA1). Consistently, longer persistence of DNA damage in cells treated with MN-OA and radiation was observed in the form of γ-H2AX foci. The efficacy and mechanism of MN-OA-induced radio-sensitization was also validated in an immuno-competent murine fibrosarcoma model.

CONCLUSION

This study reveals the key role of HSP90 in the mechanism of tumor radio-sensitization by MN-OA.

GENERAL SIGNIFICANCE

Present work provides a deeper understanding about the mechanism of MN-OA-induced tumor radiosensitization, highlighting the role of HSP90 protein. In addition to diagnostic and magnetic hyperthermia abilities, present remarkable radiosensitizing activity of MN-OA would further excite the clinicians to test its anti-cancer potential.

Chk1-mediated Cdc25A degradation as a critical mechanism for normal cell cycle progression

Chk1 (encoded by CHEK1 in mammals) is an evolutionarily conserved protein kinase that transduces checkpoint signals from ATR to Cdc25A during the DNA damage response (DDR). In mammals, Chk1 also controls cellular proliferation even in the absence of exogenous DNA damage. However, little is known about how Chk1 regulates unperturbed cell cycle progression, and how this effect under physiological conditions differs from its regulatory role in DDR. Here, we have established near-diploid HCT116 cell lines containing endogenous Chk1 protein tagged with a minimum auxin-inducible degron (mAID) through CRISPR/Cas9-based gene editing. Establishment of these cells enabled us to induce specific and rapid depletion of the endogenous Chk1 protein, which resulted in aberrant accumulation of DNA damage factors that induced cell cycle arrest at S or G2 phase. Cdc25A was stabilized upon Chk1 depletion before the accumulation of DNA damage factors. Simultaneous depletion of Chk1 and Cdc25A partially suppressed the defects caused by Chk1 single depletion. These results indicate that, similar to its function in DDR, Chk1 controls normal cell cycle progression mainly by inducing Cdc25A degradation.

Crosstalk between Phosphoinositide 3-kinase/Akt signaling pathway with DNA damage response and oxidative stress in cancer

The phosphatidylinositol 3-kinases (PI3K)/Akt signaling pathway is one of the well-characterized and most important signaling pathways activated in response to DNA damage. This review discusses the most recent discoveries on the involvement of PI3K/Akt signaling pathway in cancer development, as well as stimulation of some important signaling networks involved in the maintenance of cellular homeostasis upon DNA damage, with an exploration of how PI3K/Akt signaling pathway contributes to the regulation of modulators and effectors underlying DNA damage response, the intricate, protein-based signal transduction network, which decides between cell cycle arrest, DNA repair, and apoptosis, the elimination of irreparably damaged cells to maintain homeostasis. The review continues by looking at the interplay between cell cycle checkpoints, checking the repair of damage inflicted to the DNA before entering DNA replication to facilitate DNA synthesis, and PI3K/Akt signaling pathway. We then investigate the challenges the cells overcome to ameliorate damages induced by oxidative activities, for example, the recruitment of many pathways and factors to maintain integrity and hemostasis. Finally, the review provides a discussion of how cells use the PI3K/Akt signaling pathway to regulate the balance between these networks.

Checkpoint Kinase 1 Expression Predicts Poor Prognosis in Nigerian Breast Cancer Patients

BACKGROUND

Checkpoint kinase 1 (CHEK1), a DNA damage sensor and cell death pathway stimulator, is regarded as an oncogene in tumours, where its activities are considered essential for tumourigenesis and the survival of cancer cells treated with chemotherapy and radiotherapy. In breast cancer, CHEK1 expression has been associated with an aggressive tumour phenotype, the triple-negative breast cancer subtype, an aberrant response to tamoxifen, and poor prognosis. However, the relevance of CHEK1 expression has, hitherto, not been investigated in an indigenous African population. We therefore aimed to investigate the clinicopathological, biological, and prognostic significance of CHEK1 expression in a cohort of Nigerian breast cancer cases.

MATERIAL AND METHODS

Tissue microarrays of 207 Nigerian breast cancer cases were tested for CHEK1 expression using immunohistochemistry. The clinicopathological, molecular, and prognostic characteristics of CHEK1-positive tumours were determined using the Chi-squared test and Kaplan-Meier and Cox regression analyses in SPSS Version 16.

RESULTS

Nuclear expression of CHEK1 was present in 61% of breast tumours and was associated with tumour size, triple-negative cancer, basal-like phenotype, the epithelial-mesenchymal transition, p53 over-expression, DNA homologous repair pathway dysfunction, and poor prognosis.

CONCLUSIONS

The rate expression of CHEK1 is high in Nigerian breast cancer cases and is associated with an aggressive phenotype and poor prognosis.

HMGA2 gene silencing reduces epithelial-mesenchymal transition and lymph node metastasis in cervical cancer through inhibiting the ATR/Chk1 signaling pathway

Many cervical cancer (CC) patients suffer from cancer invasion and lymph node metastasis, resulting in poor therapeutic outcome. Evidence has indicated the involvement of misexpressed high-mobility group AT-hook 2 (HMGA2) in poor survival of cancer patients. This study hereby aims to investigate the role of HMGA2 in CC cell biological functions via the ATR/Chk1 signaling pathway. The cell line with the highest HMGA2 expression was selected to establish cell lines with wild-type and stable HMGA2 silencing. The underlying regulatory mechanisms of HMGA2 in CC cells were analyzed with the treatment of the ATR/Chk1 signaling pathway activator, inhibitor, shRNA against HMGA2 or pcDNA-HMGA2 plasmids, followed by quantification of expression levels of ATR, Chk1, Bcl-2, Bax, MMP-2, MMP-9, E-cadherin and N-cadherin. CC cell apoptosis, proliferation, migration, invasion and lymph node metastasis in nude mice were evaluated. The HeLa cell line with the highest HMGA2 expression was selected. HMGA2 inhibited the activation of the ATR/Chk1 signaling pathway. Notably, HMGA2 silencing or inhibition of the ATR/Chk1 signaling pathway inhibited epithelial mesenchymal transition (EMT), CC cell proliferation, invasion, migration, tumorigenicity and lymph node metastasis while promoting apoptosis, indicated by reduced expression of Bcl-2, MMP-2, MMP-9 and N-cadherin, with increased expression of E-cadherin and Bax. Collectively, our study provides evidence that HMGA2 gene silencing inhibits the activation of the ATR/Chk1 signaling pathway, whereby repressing EMT, proliferation, migration and invasion of CC cells and lymph node metastasis, and promoting CC cell apoptosis.

Altering the response to radiation: radiosensitizers and targeted therapies in pancreatic ductal adenocarcinoma: preclinical and emerging clinical evidence

Radiation therapy continues to have an evolving role in pancreatic ductal adenocarcinoma. While metastatic failure likely contributes to the majority of patient mortality, achieving local control through surgery and/or radiation appears to be important as certain studies suggest that mortality is contributed by local failure. Many studies support that pancreatic cancer is a relatively radiation resistant tumor type. In addition, the ability to further improve radiation through dose escalation strategies in the non-metastatic setting is hampered by closeness of normal organs, including small bowel and stomach, to the tumor. Thus subverting molecular pathways that promote radiation resistance will be critical to further success of radiation in this disease. There is a wealth of preclinical data supporting the targeting of various molecular pathways in combination with radiation therapy, including DNA repair, cell cycle checkpoint proteins, receptor tyrosine kinases, oncoproteins, stem cells, and immunomodulation. A number of clinical trials have been completed or are on-going with novel molecular inhibitors. In this review, we summarize existing preclinical and clinical molecular strategies for improving the efficacy of radiation in pancreatic cancer, and highlight recent and ongoing clinical trials combining radiation and various targeted therapies.

Multi-Scale Network Model Supported by Proteomics for Analysis of Combined Gemcitabine and Birinapant Effects in Pancreatic Cancer Cells

Gemcitabine combined with birinapant, an inhibitor of apoptosis protein antagonist, acts synergistically to reduce pancreatic cancer cell proliferation. A large‐scale proteomics dataset provided rich time‐series data on proteome‐level changes that reflect the underlying biological system and mechanisms of action of these drugs. A multiscale network model was developed to link the signaling pathways of cell cycle regulation, DNA damage response, DNA repair, apoptosis, nuclear factor‐kappa β (NF‐κβ), and mitogen‐activated protein kinase (MAPK)‐p38 to cell cycle progression, proliferation, and death. After validating the network model under different conditions, the Sobol Sensitivity Analysis was applied to identify promising targets to enhance gemcitabine efficacy. The effects of p53 silencing and combining curcumin with gemcitabine were also tested with the developed model. Merging proteomics analysis with systems modeling facilitates the characterization of quantitative relations among relevant signaling pathways in drug action and resistance, and such multiscale network models could be applied for prediction of combination efficacy and target selection.

ATR is required to complete meiotic recombination in mice

Precise execution of recombination during meiosis is essential for forming chromosomally-balanced gametes. Meiotic recombination initiates with the formation and resection of DNA double-strand breaks (DSBs). Cellular responses to meiotic DSBs are critical for efficient repair and quality control, but molecular features of these remain poorly understood, particularly in mammals. Here we report that the DNA damage response protein kinase ATR is crucial for meiotic recombination and completion of meiotic prophase in mice. Using a hypomorphic Atr mutation and pharmacological inhibition of ATR in vivo and in cultured spermatocytes, we show that ATR, through its effector kinase CHK1, promotes efficient RAD51 and DMC1 assembly at RPA-coated resected DSB sites and establishment of interhomolog connections during meiosis. Furthermore, our findings suggest that ATR promotes local accumulation of recombination markers on unsynapsed axes during meiotic prophase to favor homologous chromosome synapsis. These data reveal that ATR plays multiple roles in mammalian meiotic recombination.

Identification of candidate biomarkers and pathways associated with SCLC by bioinformatics analysis

Small cell lung cancer (SCLC) is one of the highly malignant tumors and a serious threat to human health. The aim of the present study was to explore the underlying molecular mechanisms of SCLC. mRNA microarray datasets GSE6044 and GSE11969 were downloaded from Gene Expression Omnibus database, and the differentially expressed genes (DEGs) between normal lung and SCLC samples were screened using GEO2R tool. Functional and pathway enrichment analyses were performed for common DEGs using the DAVID database, and the protein-protein interaction (PPI) network of common DEGs was constructed by the STRING database and visualized with Cytoscape software. In addition, the hub genes in the network and module analysis of the PPI network were performed using CentiScaPe and plugin Molecular Complex Detection. Finally, the mRNA expression levels of hub genes were validated in the Oncomine database. A total of 150 common DEGs with absolute fold-change >0.5, including 66 significantly downregulated DEGs and 84 upregulated DEGs were obtained. The Gene Ontology term enrichment analysis suggested that common upregulated DEGs were primarily enriched in biological processes (BPs), including ‘cell cycle’, ‘cell cycle phase’, ‘M phase’, ‘cell cycle process’ and ‘DNA metabolic process’. The common downregulated genes were significantly enriched in BPs, including ‘response to wounding’, ‘positive regulation of immune system process’, ‘immune response’, ‘acute inflammatory response’ and ‘inflammatory response’. Kyoto Encyclopedia of Genes and Genomes pathway analysis identified that the common downregulated DEGs were primarily enriched in the ‘complement and coagulation cascades’ signaling pathway; the common upregulated DEGs were mainly enriched in ‘cell cycle’, ‘DNA replication’, ‘oocyte meiosis’ and the ‘mismatch repair’ signaling pathways. From the PPI network, the top 10 hub genes in SCLC were selected, including topoisomerase IIα, proliferating cell nuclear antigen, replication factor C subunit 4, checkpoint kinase 1, thymidylate synthase, minichromosome maintenance protein (MCM) 2, cell division cycle (CDC) 20, cyclin dependent kinase inhibitor 3, MCM3 and CDC6, the mRNA levels of which are upregulated in Oncomine SCLC datasets with the exception of MCM2. Furthermore, the genes in the significant module were enriched in ‘cell cycle’, ‘DNA replication’ and ‘oocyte meiosis’ signaling pathways. Therefore, the present study can shed new light on the understanding of molecular mechanisms of SCLC and may provide molecular targets and diagnostic biomarkers for the treatment and early diagnosis of SCLC.

Gene expression microarray analysis reveals prognostic markers of survival in high grade astrocytomas

OBJECTIVE

High grade astrocytoma (HGA) as an aggressive brain tumor, is always correlated with poor prognosis. In this paper, we aimed to explore the genetic prognostic biomarkers for HGA.

METHODS

The genome-wide expression profile of 26 brain tumor samples obtained from 26 patients with HGA was downloaded from Gene Expression Omnibus. The risk genes for prognosis of HGA were identified and verified by the data in TCGA database. Protein-protein interaction (PPI) network of risk factor genes was constructed and significant module was screened. Function and pathway annotations were performed for risk genes and drug target genes were further analyzed.

RESULTS

A total of 598 genes were identified as significant risk genes for prognosis, such as checkpoint kinase 1, potassium inwardly-rectifying channel, subfamily J, member 6, leukocyte receptor tyrosine kinase and uncharacterized LOC283887. All risk genes for prognosis of HGA were significantly enriched in cell cycle, mitotic as well as mitotic anaphase. While the genes in the network module mainly participated in functions such as cell cycle, mitotic cell cycle and cell cycle process. Moreover, the genes in the network module mainly participated in the pathways such as cell cycle and cell cycle, mitotic. Drug target analysis showed that seven genes were recorded in Drugbank database, and there were as many as 32 drug records of CHEK1.

CONCLUSION

The prognostic effect of CHEK1 was validated based on the expression profile data of 615 low-grade glioma and glioblastoma samples. We proposed CHEK1 as prognostic biomarker for HGA. Our work might provide the candidate target for HGA therapy.

Chk1 inhibition significantly potentiates activity of nucleoside analogs in TP53-mutated B-lymphoid cells

Treatment options for TP53-mutated lymphoid tumors are very limited. In experimental models, TP53-mutated lymphomas were sensitive to direct inhibition of checkpoint kinase 1 (Chk1), a pivotal regulator of replication. We initially tested the potential of the highly specific Chk1 inhibitor SCH900776 to synergize with nucleoside analogs (NAs) fludarabine, cytarabine and gemcitabine in cell lines derived from B-cell malignancies. In p53-proficient NALM-6 cells, SCH900776 added to NAs enhanced signaling towards Chk1 (pSer317/pSer345), effectively blocked Chk1 activation (Ser296 autophosphorylation), increased replication stress (p53 and γ-H2AX accumulation) and temporarily potentiated apoptosis. In p53-defective MEC-1 cell line representing adverse chronic lymphocytic leukemia (CLL), Chk1 inhibition together with NAs led to enhanced and sustained replication stress and significantly potentiated apoptosis. Altogether, among 17 tested cell lines SCH900776 sensitized four of them to all three NAs. Focusing further on MEC-1 and co-treatment of SCH900776 with fludarabine, we disclosed chromosome pulverization in cells undergoing aberrant mitoses. SCH900776 also increased the effect of fludarabine in a proportion of primary CLL samples treated with pro-proliferative stimuli, including those with TP53 disruption. Finally, we observed a fludarabine potentiation by SCH900776 in a T-cell leukemia 1 (TCL1)-driven mouse model of CLL. Collectively, we have substantiated the significant potential of Chk1 inhibition in B-lymphoid cells.

The HNF-1β-USP28-Claspin pathway upregulates DNA damage-induced Chk1 activation in ovarian clear cell carcinoma

Transcription factor hepatocyte nuclear factor 1-beta (HNF-1β) enhances checkpoint kinase 1 (Chk1) activation and promotes G2/M cell cycle progression in ovarian clear cell carcinoma (CCC) following exposure to diverse genotoxic agents including bleomycin. However, the underlying mechanism leading to checkpoint activation of HNF-1β still remains largely unknown. To clarify the effects of HNF-1β on cell cycle checkpoints, human CCC cell lines were transfected with siRNAs targeting HNF-1β, Claspin, USP28, or a control vector. Ubiquitination and stabilization of Claspin protein by HNF-1β was assessed by immunoprecipitation. Loss-of-function studies using RNAi-mediated gene silencing indicated that HNF-1β facilitated the Claspin expression after treatment with a genotoxic agent bleomycin, resulting in accumulation of phosphorylated Chk1 (p-Chk1) and promotion of survival in CCC cell lines. This study showed for the first time that USP28, a de-ubiquitinase crucial for Claspin expression, is one target gene of HNF-1β. Knockdown of endogenous USP28 suppressed the Claspin expression and p-Chk1 activation and cell viability. Our findings identify a novel pathway of the HNF-1β-USP28-Claspin-Chk1 axis in checkpoint signal amplification in response to DNA damage. Targeting this pathway may represent a putative, novel, anticancer strategy in ovarian CCC.

CM363, a novel naphthoquinone derivative which acts as multikinase modulator and overcomes imatinib resistance in chronic myelogenous leukemia

Human Chronic Myelogenous Leukemia (CML) is a hematological stem cell disorder which is associated with activation of Bcr-Abl-Stat5 oncogenic pathway. Direct Bcr-Abl inhibitors are initially successful for the treatment of CML but over time many patients develop drug resistance. In the present study, the effects of CM363, a novel naphthoquinone (NPQ) derivative, were evaluated on human CML-derived K562 cells. CM363 revealed an effective cell growth inhibition (IC50 = 0.7 ± 0.5 μM) by inducing cancer cells to undergo cell cycle arrest and apoptosis. CM363 caused a dose- and time-dependent reduction of cells in G0/G1 and G2/M phases. This cell cycle arrest was associated with increased levels of cyclin E, pChk1 and pChk2 whereas CM363 downregulated cyclin B, cyclin D3, p27, pRB, Wee1, and BUBR1. CM363 increased the double-strand DNA break marker γH2AX. CM363 caused a time-dependent increase of annexin V-positive cells, DNA fragmentation and increased number of apoptotic nuclei. CM363 triggered the mitochondrial apoptotic pathway as reflected by a release of cytochrome C from mitochondria and induction of the cleavage of caspase-3 and -9, and PARP. CM363 showed multikinase modulatory effects through an early increased JNK phosphorylation followed by inhibition of pY-Bcrl-Abl and pY-Stat5. CM363 worked synergistically with imatinib to inhibit cell viability and maintained its activity in imatinib-resistant cells. Finally, CM363 (10 mg/Kg) suppressed the growth of K562 xenograft tumors in athymic mice. In summary, CM363 is a novel multikinase modulator that offers advantages to circumvent imanitib resistance and might be therapeutically effective in Bcrl-Abl-Stat5 related malignancies.

Finite Element Analysis of the Mouse Distal Femur with Tumor Burden in Response to Knee Loading

Breast cancer-associated bone metastasis induces bone loss, followed by an increased risk of bone fracture. To develop a strategy for preventing tumor growth and protecting bone, an understanding of the mechanical properties of bone under tumor burden is indispensable. Using a mouse model of mammary tumor, we conducted finite element analysis (FEA) of two bone samples from the distal femur. One sample was from a placebo-treated mouse, and the other was from a mouse treated with the investigational drug candidate, PD407824, an inhibitor of checkpoint kinases. Mechanical testing and microCT images revealed that bone strength is improved by administration of PD407824. In response to loading to the knee, FEA predicted that the peaks of von Mises stress, an indicator of fracture yielding, as well as the third principal compressive stress, were higher in the placebo-treated femur than the drug-treated femur. Higher peak stresses in trabecular segments were observed in the lateral condyle, a critical region for integrity of the knee joint. Collectively, this FE study supports the notion that mechanical weakening of the femur was observed in the tumor-invaded trabecular bone, and chemical agents such as PD407824 may potentially assist in preventing bone loss and bone fracture.

Radiation-induced G2/M arrest rarely occurred in glioblastoma stem-like cells

PURPOSE

The purpose of this study is to systematically study the cell-cycle alterations of glioblastoma stem-like cells (GSLCs) after irradiation, possibly enriching the mechanisms of radioresistance of GSLCs.

MATERIALS AND METHODS

GSLCs were enriched and identified, and then the radioresistance of GSLCs was validated by analyzing cell survival, cell proliferation, and radiation-induced apoptosis. The discrepancy of the cell-cycle distribution and expression of cell-cycle-related proteins between GSLCs and glioblastoma differentiated cells (GDCs) after irradiation was completely analyzed.

RESULTS

The survival fractions and the cell viabilities of GSLCs were significantly higher than those of GDCs after irradiation. Radiation-induced apoptosis was less prominent in GSLCs than in GDCs. After irradiation with high-dose X-rays, the percentages of GDCs in G2/M phase was evidently increased. However, radiation-induced G2/M arrest occurred less frequently in GSLCs, but S-phase arrest occurred in GSLCs after irradiation with 8 Gy. Further mechanistic studies showed that the expressions levels of Cdc25c, Cdc2, and CyclinB1 in GSLCs were not apparently changed after irradiation, while those of p-ATM and p-Chk1 were sharply increased after irradiation in GSLCs. The basal level of Cdc25c expression in GSLCs was much higher than that in GDCs.

CONCLUSIONS

We explored the cell-cycle alterations and cell-cycle-related proteins expression levels in GSLCs after irradiation, providing a novel mechanism of radioresistance of GSLCs.

Proteomic Analysis of Combined Gemcitabine and Birinapant in Pancreatic Cancer Cells

Pancreatic cancer is characterized by mutated signaling pathways and a high incidence of drug resistance. Comprehensive, large-scale proteomic analysis can provide a system-wide view of signaling networks, assist in understanding drug mechanisms of action and interactions, and serve as a useful tool for pancreatic cancer research. In this study, liquid chromatography-mass spectrometry-based proteomic analysis was applied to characterize the combination of gemcitabine and birinapant in pancreatic cancer cells, which was shown previously to be synergistic. A total of 4069 drug-responsive proteins were identified and quantified in a time-series proteome analysis. This rich dataset provides broad views and accurate quantification of signaling pathways. Pathways relating to DNA damage response regulations, DNA repair, anti-apoptosis, pro-migration/invasion were implicated as underlying mechanisms for gemcitabine resistance and for the beneficial effects of the drug combination. Promising drug targets were identified for future investigation. This study also provides a database for systems mathematical modeling to relate drug effects and interactions in various signaling pathways in pancreatic cancer cells.

MK-8776, a novel chk1 kinase inhibitor, radiosensitizes p53-defective human tumor cells

Radiotherapy is commonly used to treat a variety of solid tumors but improvements in the therapeutic ratio are sorely needed. The aim of this study was to assess the Chk1 kinase inhibitor, MK-8776, for its ability to radiosensitize human tumor cells. Cells derived from NSCLC and HNSCC cancers were tested for radiosensitization by MK-8776. The ability of MK-8776 to abrogate the radiation-induced G2 block was determined using flow cytometry. Effects on repair of radiation-induced DNA double strand breaks (DSBs) were determined on the basis of rad51, γ-H2AX and 53BP1 foci. Clonogenic survival analyses indicated that MK-8776 radiosensitized p53-defective tumor cells but not lines with wild-type p53. Abrogation of the G2 block was evident in both p53-defective cells and p53 wild-type lines indicating no correlation with radiosensitization. However, only p53-defective cells entered mitosis harboring unrepaired DSBs. MK-8776 appeared to inhibit repair of radiation-induced DSBs at early times after irradiation. A comparison of MK-8776 to the wee1 inhibitor, MK-1775, suggested both similarities and differences in their activities. In conclusion, MK-8776 radiosensitizes tumor cells by mechanisms that include abrogation of the G2 block and inhibition of DSB repair. Our findings support the clinical evaluation of MK-8776 in combination with radiation.

Checkpoint kinase 1 expression is an adverse prognostic marker and therapeutic target in MYC-driven medulloblastoma

Checkpoint kinase 1 (CHK1) is an integral component of the cell cycle as well as the DNA Damage Response (DDR) pathway. Previous work has demonstrated the effectiveness of inhibiting CHK1 with small-molecule inhibitors, but the role of CHK1 mediated DDR in medulloblastoma is unknown. CHK1, both at the mRNA and protein level, is highly expressed in medulloblastoma and elevated CHK1 expression in Group3 medulloblastoma is an adverse prognostic marker. CHK1 inhibition with the small-molecule drug AZD7762, results in decreased cell growth, increased DNA damage and cell apoptosis. Furthermore, AZD7762 acts in synergy with cisplatin in reducing cell proliferation in medulloblastoma. Similar phenotypic changes were observed with another CHK1 inhibitor, PF477736, as well as genetic knockdown using siRNA against CHK1. Treatments with small-molecule inhibitors of CHK1 profoundly modulated the expression of both upstream and downstream target proteins within the CHK1 signaling pathways. This suggests the presence of a feedback loop in activating CHK1. Overall, our results demonstrate that small-molecule inhibition of CHK1 in combination with, cisplatin, is more advantageous than either treatment alone, especially for Group 3 medulloblastoma, and therefore this combined therapeutic approach serves as an avenue for further investigation.

Chk1 and 14-3-3 proteins inhibit atypical E2Fs to prevent a permanent cell cycle arrest

The atypical E2Fs, E2F7 and E2F8, act as potent transcriptional repressors of DNA replication genes providing them with the ability to induce a permanent S-phase arrest and suppress tumorigenesis. Surprisingly in human cancer, transcript levels of atypical E2Fs are frequently elevated in proliferating cancer cells, suggesting that the tumor suppressor functions of atypical E2Fs might be inhibited through unknown post-translational mechanisms. Here, we show that atypical E2Fs can be directly phosphorylated by checkpoint kinase 1 (Chk1) to prevent a permanent cell cycle arrest. We found that 14-3-3 protein isoforms interact with both E2Fs in a Chk1-dependent manner. Strikingly, Chk1 phosphorylation and 14-3-3-binding did not relocate or degrade atypical E2Fs, but instead, 14-3-3 is recruited to E2F7/8 target gene promoters to possibly interfere with transcription. We observed that high levels of 14-3-3 strongly correlate with upregulated transcription of atypical E2F target genes in human cancer. Thus, we reveal that Chk1 and 14-3-3 proteins cooperate to inactivate the transcriptional repressor functions of atypical E2Fs. This mechanism might be of particular importance to cancer cells, since they are exposed frequently to DNA-damaging therapeutic reagents.

Cdc6 contributes to cisplatin-resistance by activation of ATR-Chk1 pathway in bladder cancer cells

High activation of DNA damage response is implicated in cisplatin (CDDP) resistance which presents as a serious obstacle for bladder cancer treatment. Cdc6 plays an important role in the malignant progression of tumor. Here, we reported that Cdc6 expression is up-regulated in bladder cancer tissues and is positively correlated to high tumor grade. Cdc6 depletion can attenuate the malignant properties of bladder cancer cells, including DNA replication, migration and invasion. Furthermore, higher levels of chromatin-binding Cdc6 and ATR were detected in CDDP-resistant bladder cancer cells than in the parent bladder cancer cells. Intriguingly, down-regulation of Cdc6 can enhance sensitivity to CDDP both in bladder cancer cells and CDDP-resistant bladder cancer cells. Cdc6 depletion abrogates S phase arrest caused by CDDP, leading to aberrant mitosis by inactivating ATR-Chk1-Cdc25C pathway. Our results indicate that Cdc6 may be a promising target for overcoming CDDP resistance in bladder cancer.

RNA-Sequencing of Drosophila melanogaster Head Tissue on High-Sugar and High-Fat Diets

Obesity has been shown to increase risk for cardiovascular disease and type-2 diabetes. In addition, it has been implicated in aggravation of neurological conditions such as Alzheimer’s. In the model organism Drosophila melanogaster, a physiological state mimicking diet-induced obesity can be induced by subjecting fruit flies to a solid medium disproportionately higher in sugar than protein, or that has been supplemented with a rich source of saturated fat. These flies can exhibit increased circulating glucose levels, increased triglyceride content, insulin-like peptide resistance, and behavior indicative of neurological decline. We subjected flies to variants of the high-sugar diet, high-fat diet, or normal (control) diet, followed by a total RNA extraction from fly heads of each diet group for the purpose of Poly-A selected RNA-Sequencing. Our objective was to identify the effects of obesogenic diets on transcriptome patterns, how they differed between obesogenic diets, and identify genes that may relate to pathogenesis accompanying an obesity-like state. Gene ontology analysis indicated an overrepresentation of affected genes associated with immunity, metabolism, and hemocyanin in the high-fat diet group, and CHK, cell cycle activity, and DNA binding and transcription in the high-sugar diet group. Our results also indicate differences in the effects of the high-fat diet and high-sugar diet on expression profiles in head tissue of flies, despite the reportedly similar phenotypic impacts of the diets. The impacted genes, and how they may relate to pathogenesis in the Drosophila obesity-like state, warrant further experimental investigation.

Inhibition of CHK1 enhances cell death induced by the Bcl-2-selective inhibitor ABT-199 in acute myeloid leukemia cells

Resistance to standard chemotherapy agents remains a major obstacle for improving treatment outcomes for acute myeloid leukemia (AML). The Bcl-2-selective inhibitor ABT-199 has demonstrated encouraging preclinical results, drug resistance remains a concern. Mcl-1 has been demonstrated to contribute to ABT-199 resistance, thus combining with therapies that target Mcl-1 could overcome such resistance. In this study, we utilized a CHK1 inhibitor, LY2603618, to decrease Mcl-1 and enhance ABT-199 efficacy. We found that LY2603618 treatment resulted in abolishment of the G2/M cell cycle checkpoint and increased DNA damage, which was partially dependent on CDK activity. LY2603618 treatment resulted in decrease of Mcl-1, which coincided with the initiation of apoptosis. Overexpression of Mcl-1 in AML cells significantly attenuated apoptosis induced by LY2603618, confirming the critical role of Mcl-1 in apoptosis induced by the agent. Simultaneous treatment with LY2603618 and ABT-199 resulted in synergistic induction of apoptosis in both AML cell lines and primary patient samples. Our findings provide new insights into overcoming a mechanism of intrinsic ABT-199 resistance in AML cells and support the clinical development of combined ABT-199 and CHK1 inhibition.

Xanthatin triggers Chk1-mediated DNA damage response and destabilizes Cdc25C via lysosomal degradation in lung cancer cells

Previous studies had shown that xanthatin, a natural xanthanolide sesquiterpene lactone, could induce mitotic arrest and apoptosis in non-small cell lung cancer (NSCLC) cells. Here, we examined whether the DNA damage response (DDR) could be a primary cytotoxic event underlying xanthatin-mediated anti-tumor activity. Using EdU incorporation assay in combination with novel imaging flow cytometry, our data indicated that xanthatin suppressed DNA replication, prevented cells from G2/M entry and increased the spot count of γH2AX nuclear foci. Given that checkpoint kinase 1 (Chk1) represents a core component in DDR-mediated cell cycle transition and the phosphorylation on Ser-345 is essential for kinase activation and function, we surprisingly found xanthatin distinctly modulated Ser-345 phosphorylation of Chk1 in A549 and H1299 cells. Further investigation on Cdc25C/CDK1/CyclinB1 signaling cascade in the absence or presence of pharmacological DDR inhibitors showed that xanthatin directly destabilized the protein levels of Cdc25C, and recovery of p53 expression in p53-deficient H1299 cells further intensified xanthatin-mediated inhibition of Cdc25C, suggesting p53-dependent regulation of Cdc25C in a DDR machinery. Moreover, exogenous expression of Cdc25C was also substantially repressed by xanthatin and partially impaired xanthatin-induced G2 arrest. In addition, xanthatin could induce accumulation of ubiquitinated Cdc25C without undergoing further proteasomal degradation. However, an alternative lysosomal proteolysis of Cdc25C was observed. Interestingly, lysosome-like vesicles were produced upon xanthatin treatment, accompanied by rapid accumulation of lysosomal associated membrane protein LAPM-1. Furthermore, vacuolar proton (V)-ATPases inhibitor bafilomycin A1 and lysosomal proteases inhibitor leupeptin could remarkably overturn the levels of Cdc25C in xanthatin-treated H1299 cells. Altogether, these data provide insight into how xanthatin can be effectively targeted DDR molecules towards certain tumors.

Directing the use of DDR kinase inhibitors in cancer treatment

INTRODUCTION

Defects in the DNA damage response (DDR) drive the development of cancer by fostering DNA mutation but also provide cancer-specific vulnerabilities that can be exploited therapeutically. The recent approval of three different PARP inhibitors for the treatment of ovarian cancer provides the impetus for further developing targeted inhibitors of many of the kinases involved in the DDR, including inhibitors of ATR, ATM, CHEK1, CHEK2, DNAPK and WEE1. Areas covered: We summarise the current stage of development of these novel DDR kinase inhibitors, and describe which predictive biomarkers might be exploited to direct their clinical use. Expert opinion: Novel DDR inhibitors present promising candidates in cancer treatment and have the potential to elicit synthetic lethal effects. In order to fully exploit their potential and maximize their utility, identifying highly penetrant predictive biomarkers of single agent and combinatorial DDR inhibitor sensitivity are critical. Identifying the optimal drug combination regimens that could used with DDR inhibitors is also a key objective.

Knockdown of REV3 synergizes with ATR inhibition to promote apoptosis induced by cisplatin in lung cancer cells

It has been demonstrated that REV3, the catalytic subunit of the translesion synthesis (TLS) polymerase ζ, play an important role in DNA damage response (DDR) induced by cisplatin, and Ataxia-telangietasia mutated and Rad-3-related (ATR) knase is a central player in activating cell cycle checkpoint, stabilizing replication forks, regulating DDR, and promoting repair of DNA damage caused by cisplatin. Cancer cells deficient in either one of REV3 and ATR are more sensitive to cisplatin. However, whether co-inhibition of REV3 and ATR can further increase sensitivity of non-small cell lung cancer (NSCLC) cells to cisplatin is not clear. In this study, we show that REV3 knockdown combined with ATR inhibition further enhance cytotoxicity of cisplatin in NSCLC cells, including cisplatin-sensitive and -resistant cell lines, compared to individual knockdown of REV3 or ATR, which are accompanied by markedly caspase-dependent apoptosis response, pronounced DNA damage accumulation and severe impediment of interstrand crosslink (ICL), and double strand break (DSB) repair. Our results suggest that REV3 knockdown synergize strongly with ATR inhibition to significantly increase sensitivity of cisplatin in NSCLC cells by inhibiting ICL and DSB repair. Thus simultaneously targeting REV3 and ATR may represent one approach to overcome cisplatin resistance and improve chemotherapeutic efficacy in NSCLC treatment.

The combination of CHK1 inhibitor with G-CSF overrides cytarabine resistance in human acute myeloid leukemia

Cytarabine (AraC) represents the most effective single agent treatment for AML. Nevertheless, overriding AraC resistance in AML remains an unmet medical need. Here we show that the CHK1 inhibitor (CHK1i) GDC-0575 enhances AraC-mediated killing of AML cells both in vitro and in vivo, thus abrogating any potential chemoresistance mechanisms involving DNA repair. Importantly, this combination of drugs does not affect normal long-term hematopoietic stem/progenitors. Moreover, the addition of CHK1i to AraC does not generate de novo mutations and in patients' samples where AraC is mutagenic, addition of CHK1i appears to eliminate the generation of mutant clones. Finally, we observe that persistent residual leukemic cells are quiescent and can become responsive to the treatment when forced into cycle via granulocyte colony-stimulating factor (G-CSF) administration. This drug combination (AraC+CHK1i+G-CSF) will open the doors for a more efficient treatment of AML in the clinic.

Suppression of the FA pathway combined with CHK1 inhibitor hypersensitize lung cancer cells to gemcitabine

The combination of platinum and gemcitabine is one of the standard regimens in the treatment of advanced lung squamous carcinoma (LSC). Resistance to gemcitabine is main barrier to the successful treatment of LSC. In this study, we showed that suppression of the Fanconi anemia (FA) pathway increased the sensitivity of two LSC cell lines SK-MES-1 and KLN205 to gemcitabine. Moreover, we found that the CHK1 pathway and the FA pathway are functionally compensatory in the repair of DNA damage in the LSC cell lines. Inactivation of one of the two pathways led to DNA damage, triggering compensatory activation of other pathway. Furthermore, we demonstrated that FANCD2 depletion combined with CHK1 inhibitor MK-8776 significantly potentiated the cytotoxicity of gemcitabine to the two LSC cell lines, compared to individual FANCD2 depletion or MK-8776 treatment. The enhanced effect of gemcitabine-chemosensitization was accompanied by loss of DNA repair function and accumulation of DNA single strand breaks and double strand breaks, in parallel with obvious increase of caspase-3 dependent apoptosis. Our results indicate that the enhancement effect of FANCD2 depletion combined with CHK1 inhibitor in sensitizing the LCS cells to gemcitabine supports the FA pathway and CHK1 as two therapeutic targets for improvement of anti-tumor regimens in treatment of LSC.

ATR/CHK1 inhibitors and cancer therapy

The cell cycle checkpoint proteins ataxia-telangiectasia-mutated-and-Rad3-related kinase (ATR) and its major downstream effector checkpoint kinase 1 (CHK1) prevent the entry of cells with damaged or incompletely replicated DNA into mitosis when the cells are challenged by DNA damaging agents, such as radiation therapy (RT) or chemotherapeutic drugs, that are the major modalities to treat cancer. This regulation is particularly evident in cells with a defective G1 checkpoint, a common feature of cancer cells, due to p53 mutations. In addition, ATR and/or CHK1 suppress replication stress (RS) by inhibiting excess origin firing, particularly in cells with activated oncogenes. Those functions of ATR/CHK1 make them ideal therapeutic targets. ATR/CHK1 inhibitors have been developed and are currently used either as single agents or paired with radiotherapy or a variety of genotoxic chemotherapies in preclinical and clinical studies. Here, we review the status of the development of ATR and CHK1 inhibitors. We also discuss the potential mechanisms by which ATR and CHK1 inhibition induces cell killing in the presence or absence of exogenous DNA damaging agents, such as RT and chemotherapeutic agents. Lastly, we discuss synthetic lethality interactions between the inhibition of ATR/CHK1 and defects in other DNA damage response (DDR) pathways/genes.

Inhibiting DNA-PKcs in a non-homologous end-joining pathway in response to DNA double-strand breaks

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is a distinct factor in the non-homologous end-joining (NHEJ) pathway involved in DNA double-strand break (DSB) repair. We examined the crosstalk between key proteins in the DSB NHEJ repair pathway and cell cycle regulation and found that mouse embryonic fibroblast (MEF) cells deficient in DNA-PKcs or Ku70 were more vulnerable to ionizing radiation (IR) compared with wild-type cells and that DSB repair was delayed. γH2AX was associated with phospho-Ataxia-telangiectasia mutated kinase (Ser1987) and phospho-checkpoint effector kinase 1 (Ser345) foci for the arrest of cell cycle through the G2/M phase. Inhibition of DNA-PKcs prolonged IR-induced G2/M phase arrest because of sequential activation of cell cycle checkpoints. DSBs were introduced, and cell cycle checkpoints were recruited after exposure to IR in nasopharyngeal carcinoma SUNE-1 cells. NU7441 radiosensitized MEF cells and SUNE-1 cells by interfering with DSB repair. Together, these results reveal a mechanism in which coupling of DSB repair with the cell cycle radiosensitizes NHEJ repair-deficient cells, justifying further development of DNA-PK inhibitors in cancer therapy.

Randomized phase II trial of cytosine arabinoside with and without the CHK1 inhibitor MK-8776 in relapsed and refractory acute myeloid leukemia

PURPOSE

Cytosine arabinoside (AraC) remains the backbone of most treatment regimens for acute myeloid leukemia (AML). Incorporation of AraC into DNA activates checkpoint kinase 1 (Chk1), leading to cell-cycle arrest and diminished AraC cytotoxicity, which can be reversed by the selective Chk1 inhibitor MK-8776. Building on a Phase I trial, we conducted a phase II trial comparing timed sequential AraC with or without MK-8776.

METHODS

Patients with relapsed or primary refractory AML were randomized 1:1 to receive either AraC with MK-8776 (Arm A); or AraC alone (Arm B).

RESULTS

32 patients were treated: 14 assigned to Arm A and 18 to Arm B. There were 5 (36%) complete responses (CR/CRi) and 1 (7%) partial response (PR) in Arm A, and 8 (44%) CR/CRis and 1 (6%) PR in Arm B. Median survival did not differ significantly between the two groups (5.9months in Arm A vs. 4.5 months in Arm B). MK-8776 led to a robust increase in DNA damage in circulating leukemic blasts as measured by increased γ-H2AX (16.9%±6.1% prior and 36.4%±6.8% at one hour after MK-8776 infusion, p=0.016).

CONCLUSION

Response rates and survival were similar between the two groups in spite of evidence that MK-8776 augmented DNA damage in circulating leukemic blasts. Better than expected results in the control arm using timed sequential AraC and truncated patient enrollment may have limited the ability to detect clinical benefit from the combination.

DNA damage response inhibitors: Mechanisms and potential applications in cancer therapy

Over the last decade the unravelling of the molecular mechanisms of the DNA damage response pathways and of the genomic landscape of human tumors have paved the road to new therapeutic approaches in oncology. It is now clear that tumors harbour defects in different DNA damage response steps, mainly signalling and repair, rendering them more dependent on the remaining pathways. We here focus on the proteins ATM, ATR, CHK1 and WEE1, reviewing their roles in the DNA damage response and as targets in cancer therapy. In the last decade specific inhibitors of these proteins have been designed, and their potential antineoplastic activity has been explored both in monotherapy strategies against tumors with specific defects (synthetic lethality approach) and in combination with radiotherapy or chemotherapeutic or molecular targeted agents. The preclinical and clinical evidence of antitumor activity of these inhibitors emanating from these research efforts will be critically reviewed. Lastly, the potential therapeutic feasibility of combining together such inhibitors with the aim to target particular subsets of tumors will be also discussed.

ATR inhibition facilitates targeting of leukemia dependence on convergent nucleotide biosynthetic pathways

Leukemia cells rely on two nucleotide biosynthetic pathways, de novo and salvage, to produce dNTPs for DNA replication. Here, using metabolomic, proteomic, and phosphoproteomic approaches, we show that inhibition of the replication stress sensing kinase ataxia telangiectasia and Rad3-related protein (ATR) reduces the output of both de novo and salvage pathways by regulating the activity of their respective rate-limiting enzymes, ribonucleotide reductase (RNR) and deoxycytidine kinase (dCK), via distinct molecular mechanisms. Quantification of nucleotide biosynthesis in ATR-inhibited acute lymphoblastic leukemia (ALL) cells reveals substantial remaining de novo and salvage activities, and could not eliminate the disease in vivo. However, targeting these remaining activities with RNR and dCK inhibitors triggers lethal replication stress in vitro and long-term disease-free survival in mice with B-ALL, without detectable toxicity. Thus the functional interplay between alternative nucleotide biosynthetic routes and ATR provides therapeutic opportunities in leukemia and potentially other cancers.

Leukemic cells depend on the nucleotide synthesis pathway to proliferate. Here the authors use metabolomics and proteomics to show that inhibition of ATR reduced the activity of these pathways thus providing a valuable therapeutic target in leukemia.

Melatonin: A pleiotropic molecule that modulates DNA damage response and repair pathways

DNA repair is responsible for maintaining the integrity of the genome. Perturbations in the DNA repair pathways have been identified in several human cancers. Thus, compounds targeting DNA damage response (DDR) hold great promise in cancer therapy. A great deal of effort, in pursuit of new anticancer drugs, has been devoted to understanding the basic mechanisms and functions of the cellular DNA repair machinery. Melatonin, a widely produced indoleamine in all organisms, is associated with a reduced risk of cancer and has multiple regulatory roles on the different aspects of the DDR and DNA repair. Herein, we have mainly discussed how defective components in different DNA repair machineries, including homologous recombination (HR), nonhomologous end-joining (NHEJ), base excision repair (BER), nucleotide excision repair (NER), and finally DNA mismatch repair (MMR), can contribute to the risk of cancer. Melatonin biosynthesis, mode of action, and antioxidant effects are reviewed along with the means by which the indoleamine regulates DDR at the transduction, mediation, and functional levels. Finally, we summarize recent studies that illustrate how melatonin can be combined with DNA-damaging agents to improve their efficacy in cancer therapy.

Changes in gene expression variability reveal a stable synthetic lethal interaction network in BRCA2-ovarian cancers

Synthetic lethal interactions (SLIs) are robust mechanisms that provide cells with the ability to remain viable despite having mutations in genes critical to the DNA damage response, a core cellular process. Studies in model organisms such as S. cerevisiae showed that thousands of genes important in maintaining DNA integrity cooperated in a SLI network. Two genes participate in a SLI when a mutation in one gene has no effect on the cell, but mutations in both interacting genes are lethal. Furthermore in C. elegans, a mutation in a critical gene that is important for development induced a change in expression variability in the synthetic lethal interactor. In cancer, targeting SLIs shows promise in selectively killing cancer cells. For example, targeting PARP1 is an effective treatment for BRCA1/2- breast and ovarian cancers. Although PARP1 is already identified as having a SLI with BRCA1/2-, computationally searching for other genes that cooperate in the SLI network could highlight genes that may have promise for being a cancer-specific drug target. Using RNA sequencing data for ovarian cancer patients with BRCA2 mutations and the R Bioconductor package pathVar, we showed that genes whose expression changes to an invariant, stable expression state are likely candidates for SLIs with BRCA2. Our results highlight the interactions between the genes with predicted SLIs and protein-coding genes that are functionally important in the DNA damage response. The method of analyzing expression variability to computationally identify genes with SLIs can be applied to query SLIs in other tumor types.

Paths from DNA damage and signaling to genome rearrangements via homologous recombination

DNA damage is a constant threat to genome integrity. DNA repair and damage signaling networks play a central role maintaining genome stability, suppressing tumorigenesis, and determining tumor response to common cancer chemotherapeutic agents and radiotherapy. DNA double-strand breaks (DSBs) are critical lesions induced by ionizing radiation and when replication forks encounter damage. DSBs can result in mutations and large-scale genome rearrangements reflecting mis-repair by non-homologous end joining or homologous recombination. Ionizing radiation induces genetic change immediately, and it also triggers delayed events weeks or even years after exposure, long after the initial damage has been repaired or diluted through cell division. This review covers DNA damage signaling and repair pathways and cell fate following genotoxic insult, including immediate and delayed genome instability and cell survival/cell death pathways.

DNA damage response and autophagy in the degeneration of retinal pigment epithelial cells-Implications for age-related macular degeneration (AMD)

In this review we will discuss the links between autophagy, a mechanism involved in the maintenance of cellular homeostasis and controlling cellular waste management, and the DNA damage response (DDR), comprising various mechanisms preserving the integrity and stability of the genome. A reduced autophagy capacity in retinal pigment epithelium has been shown to be connected in the pathogenesis of age-related macular degeneration (AMD), an eye disease. This degenerative disease is a major and increasing cause of vision loss in the elderly in developed countries, primarily due to the profound accumulation of intra- and extracellular waste: lipofuscin and drusen. An abundance of reactive oxygen species is produced in the retina since this tissue has a high oxygen demand and contains mitochondria-rich cells. The retina is exposed to light and it also houses many photoactive molecules. These factors are clearly reflected in both the autophagy and DNA damage rates, and in both nuclear and mitochondrial genomes. It remains to be revealed whether DNA damage and DDR capacity have a more direct role in the development of AMD.

Inhibition of CHK1 sensitizes Ewing sarcoma cells to the ribonucleotide reductase inhibitor gemcitabine

Ewing sarcoma is a bone and soft tissue sarcoma that occurs in children and young adults. The EWS-FLI1 gene fusion is the driver mutation in most Ewing sarcoma tumors and functions, in part, as an aberrant transcription factor. We recently identified that Ewing sarcoma cells are sensitive to inhibition of ribonucleotide reductase (RNR), which catalyzes the formation of deoxyribonucleotides from ribonucleotides. In this report, we show that Ewing sarcoma cells are sensitive to treatment with clofarabine, which is a nucleoside analogue and allosteric inhibitor of RNR. However, clofarabine is a reversible inhibitor of RNR and we found that the effect of clofarabine is limited when using a short (6-hour) drug treatment. Gemcitabine, on the other hand, is an irreversible inhibitor of the RRM1 subunit of RNR and this drug induces apoptosis in Ewing sarcoma cells when used in both 6-hour and longer drug treatments. Treatment of Ewing sarcoma cells with gemcitabine also results in activation of checkpoint kinase 1 (CHK1), which is a critical mediator of cell survival in the setting of impaired DNA replication. Notably, inhibition of CHK1 function in Ewing sarcoma cells using a small-molecule CHK1 inhibitor, or siRNA knockdown, in combination with gemcitabine results in increased toxicity both in vitro and in vivo in a mouse xenograft experiment. Overall, our results provide insight into Ewing sarcoma biology and identify a candidate therapeutic target, and drug combination, in Ewing sarcoma.

Synthesis and Profiling of a Novel Potent Selective Inhibitor of CHK1 Kinase Possessing Unusual N-trifluoromethylpyrazole Pharmacophore Resistant to Metabolic N-dealkylation

Checkpoint-mediated dependency of tumor cells can be deployed to selectively kill them without substantial toxicity to normal cells. Specifically, loss of CHK1, a serine threonine kinase involved in the surveillance of the G₂-M checkpoint in the presence of replication stress inflicted by DNA-damaging drugs, has been reported to dramatically influence the viability of tumor cells. CHK1's pivotal role in maintaining genomic stability offers attractive opportunity for increasing the selectivity, effectivity, and reduced toxicity of chemotherapy. Some recently identified CHK1 inhibitors entered clinical trials in combination with DNA antimetabolites. Herein, we report synthesis and profiling of MU380, a nontrivial analogue of clinically profiled compound SCH900776 possessing the highly unusual N-trifluoromethylpyrazole motif, which was envisioned not to undergo metabolic oxidative dealkylation and thereby provide greater robustness to the compound. MU380 is a selective and potent inhibitor of CHK1 which sensitizes a variety of tumor cell lines to hydroxyurea or gemcitabine up to 10 times. MU380 shows extended inhibitory effects in cells, and unlike SCH900776, does not undergo in vivo N-dealkylation to the significantly less selective metabolite. Compared with SCH900776, MU380 in combination with GEM causes higher accumulation of DNA damage in tumor cells and subsequent enhanced cell death, and is more efficacious in the A2780 xenograft mouse model. Overall, MU380 represents a novel state-of-the-art CHK1 inhibitor with high potency, selectivity, and improved metabolic robustness to oxidative N-dealkylation. Mol Cancer Ther; 16(9); 1831-42. ©2017 AACR.

Mutations of the LIM protein AJUBA mediate sensitivity of head and neck squamous cell carcinoma to treatment with cell-cycle inhibitors

The genomic alterations identified in head and neck squamous cell carcinoma (HNSCC) tumors have not resulted in any changes in clinical care, making the development of biomarker-driven targeted therapy for HNSCC a major translational gap in knowledge. To fill this gap, we used 59 molecularly characterized HNSCC cell lines and found that mutations of AJUBA, SMAD4 and RAS predicted sensitivity and resistance to treatment with inhibitors of polo-like kinase 1 (PLK1), checkpoint kinases 1 and 2, and WEE1. Inhibition or knockdown of PLK1 led to cell-cycle arrest at the G2/M transition and apoptosis in sensitive cell lines and decreased tumor growth in an orthotopic AJUBA-mutant HNSCC mouse model. AJUBA protein expression was undetectable in most AJUBA-mutant HNSCC cell lines, and total PLK1 and Bora protein expression were decreased. Exogenous expression of wild-type AJUBA in an AJUBA-mutant cell line partially rescued the phenotype of PLK1 inhibitor-induced apoptosis and decreased PLK1 substrate inhibition, suggesting a threshold effect in which higher drug doses are required to affect PLK1 substrate inhibition. PLK1 inhibition was an effective therapy for HNSCC in vitro and in vivo. However, biomarkers to guide such therapy are lacking. We identified AJUBA, SMAD4 and RAS mutations as potential candidate biomarkers of response of HNSCC to treatment with these mitotic inhibitors.

Targeting the ATR-CHK1 Axis in Cancer Therapy

Targeting the DNA damage response (DDR) is a new therapeutic approach in cancer that shows great promise for tumour selectivity. Key components of the DDR are the ataxia telangiectasia mutated and Rad3 related (ATR) and checkpoint kinase 1 (CHK1) kinases. This review article describes the role of ATR and its major downstream target, CHK1, in the DDR and why cancer cells are particularly reliant on the ATR-CHK1 pathway, providing the rationale for targeting these kinases, and validation of this hypothesis by genetic manipulation. The recent development of specific inhibitors and preclinical data using these inhibitors not only as chemosensitisers and radiosensitisers but also as single agents to exploit specific pathologies of tumour cells is described. These potent and specific inhibitors have now entered clinical trial and early results are presented.

Melatonin, a Full Service Anti-Cancer Agent: Inhibition of Initiation, Progression and Metastasis

There is highly credible evidence that melatonin mitigates cancer at the initiation, progression and metastasis phases. In many cases, the molecular mechanisms underpinning these inhibitory actions have been proposed. What is rather perplexing, however, is the large number of processes by which melatonin reportedly restrains cancer development and growth. These diverse actions suggest that what is being observed are merely epiphenomena of an underlying more fundamental action of melatonin that remains to be disclosed. Some of the arresting actions of melatonin on cancer are clearly membrane receptor-mediated while others are membrane receptor-independent and involve direct intracellular actions of this ubiquitously-distributed molecule. While the emphasis of melatonin/cancer research has been on the role of the indoleamine in restraining breast cancer, this is changing quickly with many cancer types having been shown to be susceptible to inhibition by melatonin. There are several facets of this research which could have immediate applications at the clinical level. Many studies have shown that melatonin's co-administration improves the sensitivity of cancers to inhibition by conventional drugs. Even more important are the findings that melatonin renders cancers previously totally resistant to treatment sensitive to these same therapies. Melatonin also inhibits molecular processes associated with metastasis by limiting the entrance of cancer cells into the vascular system and preventing them from establishing secondary growths at distant sites. This is of particular importance since cancer metastasis often significantly contributes to death of the patient. Another area that deserves additional consideration is related to the capacity of melatonin in reducing the toxic consequences of anti-cancer drugs while increasing their efficacy. Although this information has been available for more than a decade, it has not been adequately exploited at the clinical level. Even if the only beneficial actions of melatonin in cancer patients are its ability to attenuate acute and long-term drug toxicity, melatonin should be used to improve the physical wellbeing of the patients. The experimental findings, however, suggest that the advantages of using melatonin as a co-treatment with conventional cancer therapies would far exceed improvements in the wellbeing of the patients.

The Checkpoint Kinase 1 Inhibitor Prexasertib Induces Regression of Preclinical Models of Human Neuroblastoma

Purpose: Checkpoint kinase 1 (CHK1) is a key regulator of the DNA damage response and a mediator of replication stress through modulation of replication fork licensing and activation of S and G₂-M cell-cycle checkpoints. We evaluated prexasertib (LY2606368), a small-molecule CHK1 inhibitor currently in clinical testing, in multiple preclinical models of pediatric cancer. Following an initial assessment of prexasertib activity, this study focused on the preclinical models of neuroblastoma.Experimental Design: We evaluated the antiproliferative activity of prexasertib in a panel of cancer cell lines; neuroblastoma cell lines were among the most sensitive. Subsequent Western blot and immunofluorescence analyses measured DNA damage and DNA repair protein activation. Prexasertib was investigated in several cell line-derived xenograft mouse models of neuroblastoma.Results: Within 24 hours, single-agent prexasertib promoted γH2AX-positive double-strand DNA breaks and phosphorylation of DNA damage sensors ATM and DNA-PKcs, leading to neuroblastoma cell death. Knockdown of CHK1 and/or CHK2 by siRNA verified that the double-strand DNA breaks and cell death elicited by prexasertib were due to specific CHK1 inhibition. Neuroblastoma xenografts rapidly regressed following prexasertib administration, independent of starting tumor volume. Decreased Ki67 and increased immunostaining of endothelial and pericyte markers were observed in xenografts after only 6 days of exposure to prexasertib, potentially indicating a swift reduction in tumor volume and/or a direct effect on tumor vasculature.Conclusions: Overall, these data demonstrate that prexasertib is a specific inhibitor of CHK1 in neuroblastoma and leads to DNA damage and cell death in preclinical models of this devastating pediatric malignancy. Clin Cancer Res; 23(15); 4354-63. ©2017 AACR.

Mechanisms responsible for the synergistic antileukemic interactions between ATR inhibition and cytarabine in acute myeloid leukemia cells

Acute myeloid leukemia (AML) continues to be a challenging disease to treat, thus new treatment strategies are needed. In this study, we investigated the antileukemic effects of ATR inhibition alone or combined with cytarabine in AML cells. Treatment with the ATR-selective inhibitor AZ20 caused proliferation inhibition in AML cell lines and primary patient samples. It partially abolished the G2 cell cycle checkpoint and caused DNA replication stress and damage, accompanied by CDK1-independent apoptosis and downregulation of RRM1 and RRM2. AZ20 synergistically enhanced cytarabine-induced proliferation inhibition and apoptosis, abolished cytarabine-induced S and G2/M cell cycle arrest, and cooperated with cytarabine in inducing DNA replication stress and damage in AML cell lines. These key findings were confirmed with another ATR-selective inhibitor AZD6738. Therefore, the cooperative induction of DNA replication stress and damage by ATR inhibition and cytarabine, and the ability of ATR inhibition to abrogate the G2 cell cycle checkpoint both contributed to the synergistic induction of apoptosis and proliferation inhibition in AML cell lines. Synergistic antileukemic interactions between AZ20 and cytarabine were confirmed in primary AML patient samples. Our findings provide insight into the mechanism of action underlying the synergistic antileukemic activity of ATR inhibition in combination with cytarabine in AML.