The F box protein Fbx6 regulates Chk1 stability and cellular sensitivity to replication stress

Roles of F-box proteins in human digestive system tumors (Review)

F-box proteins (FBPs), the substrate-recognition subunit of E3 ubiquitin (Ub) ligase, are the important components of Ub proteasome system (UPS). FBPs are involved in multiple cellular processes through ubiquitylation and subsequent degradation of their target proteins. Many studies have described the roles of FBPs in human cancers. Digestive system tumors account for a large proportion of all the tumors, and their mortality is very high. This review summarizes for the first time the roles of FBPs in digestive system tumorige-nesis and tumor progression, aiming at finding new routes for the rational design of targeted anticancer therapies in digestive system tumors.

CHK1 overexpression in T-cell acute lymphoblastic leukemia is essential for proliferation and survival by preventing excessive replication stress

Checkpoint kinase 1 (CHK1) is a key component of the ATR (ataxia telangiectasia-mutated and Rad3-related)-dependent DNA damage response pathway that protect cells from replication stress, a cell intrinsic phenomenon enhanced by oncogenic transformation. Here, we show that CHK1 is overexpressed and hyperactivated in T-cell acute lymphoblastic leukemia (T-ALL). CHEK1 mRNA is highly abundant in patients of the proliferative T-ALL subgroup and leukemia cells exhibit constitutively elevated levels of the replication stress marker phospho-RPA32 and the DNA damage marker γH2AX. Importantly, pharmacologic inhibition of CHK1 using PF-004777736 or CHK1 short hairpin RNA-mediated silencing impairs T-ALL cell proliferation and viability. CHK1 inactivation results in the accumulation of cells with incompletely replicated DNA, ensuing DNA damage, ATM/CHK2 activation and subsequent ATM- and caspase-3-dependent apoptosis. In contrast to normal thymocytes, primary T-ALL cells are sensitive to therapeutic doses of PF-004777736, even in the presence of stromal or interleukin-7 survival signals. Moreover, CHK1 inhibition significantly delays in vivo growth of xenotransplanted T-ALL tumors. We conclude that CHK1 is critical for T-ALL proliferation and viability by downmodulating replication stress and preventing ATM/caspase-3-dependent cell death. Pharmacologic inhibition of CHK1 may be a promising therapeutic alternative for T-ALL treatment.

Chk1 inhibition as a novel therapeutic strategy for treating triple-negative breast and ovarian cancers

Background

Chk1 inhibitors are currently in clinical trials as putative potentiators of cytotoxic chemotherapy drugs. Chk1 inhibitors may exhibit single agent anti-tumor activity in cancers with underlying DNA repair, DNA damage response or DNA replication defects.

Methods

Here we describe the cellular effects of the pharmacological inhibition of the checkpoint kinase Chk1 by the novel inhibitor V158411 in triple-negative breast cancer and ovarian cancer. Cytotoxicity, the effect on DNA damage response and cell cycle along with the ability to potentiate gemcitabine and cisplatin cytotoxicity in cultured cells was investigated. Western blotting of proteins involved in DNA repair, checkpoint activation, cell cycle and apoptosis was used to identify potential predictive biomarkers of Chk1 inhibitor sensitivity.

Results

The Chk1 inhibitors V158411, PF-477736 and AZD7762 potently inhibited the proliferation of triple-negative breast cancer cells as well as ovarian cancer cells, and these cell lines were sensitive compared to ER positive breast and other solid cancer cells lines. Inhibition of Chk1 in these sensitive cell lines induced DNA damage and caspase-3/7 dependent apoptosis. Western blot profiling identified pChk1 (S296) as a predictive biomarker of Chk1 inhibitor sensitivity in ovarian and triple-negative breast cancer and pH2AX (S139) in luminal breast cancer.

Conclusions

This finding suggests that Chk1 inhibitors either as single agents or in combination chemotherapy represents a viable therapeutic option for the treatment of triple-negative breast cancer. pChk1 (S296) tumor expression levels could serve as a useful biomarker to stratify patients who might benefit from Chk1 inhibitor therapy.

The interaction between checkpoint kinase 1 (Chk1) and the minichromosome maintenance (MCM) complex is required for DNA damage-induced Chk1 phosphorylation

Chk1 is an essential mediator of the DNA damage response and cell cycle checkpoint. However, how exactly Chk1 transduces the checkpoint signaling is not fully understood. Here we report the identification of the heterohexamic minichromosome maintenance (MCM) complex that interacts with Chk1 by mass spectrometry. The interaction between Chk1 and the MCM complex was reduced by DNA damage treatment. We show that the MCM complex, at least partially, contributes to the chromatin association of Chk1, allowing for immediate phosphorylation of Chk1 by ataxia telangiectasia mutated and Rad3-related (ATR) in the presence of DNA damage. Further, phosphorylation of Chk1 at ATR sites reduces the interaction between Chk1 and the MCM complex, facilitating chromatin release of phosphorylated Chk1, a critical step in the initiation and amplification of cell cycle checkpoint. Together, these data provide novel insights into the activation of Chk1 in response to DNA damage.

Differential response of normal and malignant urothelial cells to CHK1 and ATM inhibitors

While DNA damage response pathways are well characterized in cancer cells, much less is known about their status in normal cells. These pathways protect tumour cells from DNA damage and replication stress and consequently present potential therapeutic targets. Here we characterize the response of human telomerase reverse transcriptase (hTERT)-immortalized normal human urothelial (NHU) and bladder cancer cell lines to agents that disrupt the DNA damage response. Effects of replication and DNA damage response inhibitors on cell cycle progression, checkpoint induction and apoptosis were analysed in hTERT-NHU and bladder cancer cell lines. The primary signalling cascade responding to replication stress in malignant cells (ataxia telangiectasia-mutated (ATM) and Rad3-related-checkpoint kinase 1 (ATR-CHK1)) is not activated in hTERT-NHU cells after treatment with a replication inhibitor and these cells do not depend upon CHK1 for protection from apoptosis during replication stress. Instead, ATM signalling is rapidly activated under these conditions. Intriguingly, an ATM inhibitor suppressed S-phase checkpoint activation after exposure to replication inhibitors and stopped entry of cells into S-phase indicating G1 checkpoint activation. Consistent with this, hTERT-NHU cells treated with the ATM inhibitor showed increased levels of cyclin-dependent kinase inhibitor p19(INK4D), reduced levels of cyclin D1 and CDK4, and reduced phosphorylation of the retinoblastoma protein. In contrast, a bladder cancer cell line cotreated with ATM and replication inhibitors progressed more slowly through S phase and showed a marked increase in apoptosis. Taken together, our findings suggest that ATM and CHK1 signalling cascades have different roles in tumour and normal epithelial cells, confirming these as promising therapeutic targets.

Inhibition of Cell Death and Induction of G2 Arrest Accumulation in Human Ovarian Clear Cells by HNF-1β Transcription Factor: Chemosensitivity Is Regulated by Checkpoint Kinase CHK1

Objective

Appropriate cell cycle checkpoints are essential for the maintenance of normal cells and chemosensitivity of cancer cells. Clear cell adenocarcinoma (CCA) of the ovary is highly resistant to chemotherapy. Hepatocyte nuclear factor-1β (HNF-1β) is known to be overexpressed in CCA, but its role and clinical significance is unclear. We investigated the role of HNF-1β in regulation of the cell cycle in CCA.

Methods

To clarify the effects of HNF-1β on cell cycle checkpoints, we compared the cell cycle distribution and the expression of key proteins involved in CCA cells in which HNF-1β had been stably knocked down and in vector-control cell lines after treatment with bleomycin. HNF-1β (+) cells were arrested in G2 phase because of DNA damage.

Results

HNF-1β (−) cells died because of a checkpoint mechanism. G2 arrest of HNF-1β (+) cells resulted from sustained CHK1 activation, a protein that plays a major role in the checkpoint mechanism. HNF-1β (+) cells were treated with a CHK1 inhibitor after bleomycin treatment. Flow cytometric analysis of the cell cycle demonstrated that DNA damage–induced G2-arrested cells were released from the checkpoint and killed by a CHK1 inhibitor.

Conclusions

The chemoresistance of CCA may be due to aberrant retention of the G2 checkpoint through overexpression of HNF-1β. This is the first study demonstrating cell cycle regulation and chemosensitization by a CHK1 inhibitor in CCA.

LRIG2 expression and prognosis in non-small cell lung cancer

The human leucine-rich repeats and immunoglobulin-like domains 2 (LRIG2) protein has been shown to be of prognostic value in several types of human cancer, however, the expression profiles of LRIG2 have not been described in non-small cell lung cancer (NSCLC). The present study evaluated the mRNA expression of LRIG2 in tumor specimens obtained from 39 NSCLC patients by SYBR Green quantitative polymerase chain reaction and the protein expression of LRIG2 in formalin-fixed paraffin sections obtained from 116 NSCLC patients by immunohistochemistry. The correlations between LRIG2 expression and clinicopathological data were analyzed. The patient survival data were collected retrospectively and the possible prognostic value of LRIG2 protein expression was investigated. The results showed that the mRNA expression of LRIG2 was decreased in NSCLC cancer tissues, which was associated with histological subtypes and tumor differentiation status. The protein expression of LRIG2 was only observed in the cytoplasm of the tumor tissue, which conformed to the mRNA expression results. Furthermore, the patients with high LRIG2 cytoplasmic expression showed poor survival times, and the five-year survival rate for patients with high LRIG2 expression was 27.8%, compared with 38.8% for patients with low expression (P=0.034), indicating that LRIG2 expression levels may have a potential role in the pathogenesis of NSCLC, and also a significant prognostic value. Further studies are required to fully elucidate the exact function of LRIG2 in NSCLC.

A dual-fluorescent reporter facilitates identification of thiol compounds that suppress microsatellite instability induced by oxidative stress

The DNA mismatch-repair (MMR) system corrects replicative errors and minimizes mutations that occur at a high rate in microsatellites. Patients with chronic inflammation or inflammation-associated cancer display microsatellite instability (MSI), indicating a possible MMR inactivation. In fact, H2O2-generated oxidative stress inactivates the MMR function and increases mutation accumulation in a reporter microsatellite. However, it remains unclear whether MSI induced by oxidative stress is preventable because of the lack of a sufficiently sensitive detection assay. Here, we developed and characterized a dual-fluorescent system, utilizing DsRed harboring the (CA)13 microsatellite as a reporter and GFP for normalization, in near-isogenic human colorectal cancer cell lines. Via flow cytometry, this reporter sensitively detected H2O2-generated oxidative microsatellite mutations in a dose-dependent manner. The reporter further revealed that glutathione or N-acetylcysteine was better than aspirin and ascorbic acid for suppressing oxidative microsatellite mutations. These two thiol compounds also partially suppressed oxidative frameshift mutations in the coding microsatellites of the hMSH6 and CHK1 genes based on a fluoresceinated PCR-based assay. MSI suppression by N-acetylcysteine appears to be mediated through reduction of oxidative frameshift mutations in the coding microsatellite of hMSH6 and protection of hMSH6 and other MMR protein levels from being decreased by H2O2. Our findings suggest a linkage between oxidative damage, MMR deficiency, and MSI. The two thiol compounds are potentially valuable for preventing inflammation-associated MSI. The dual-fluorescent reporter with improved features will facilitate identification of additional compounds that modulate MSI, which is relevant to cancer initiation and progression.

Overcoming inherent resistance to histone deacetylase inhibitors in multiple myeloma cells by targeting pathways integral to the actin cytoskeleton

Histone deacetylase inhibitors (HDACi) are novel chemotherapeutics undergoing evaluation in clinical trials for the potential treatment of patients with multiple myeloma (MM). Although HDACi have demonstrable synergy when combined with proteasome inhibitors (PIs), recent evidence indicates that combination of HDACi and PI is beneficial only in a subset of patients with advanced MM, clearly indicating that other rational combinations should be explored. In this context we hypothesized that understanding the molecular signature associated with inherent resistance to HDACi would provide a basis for the identification of therapeutic combinations with improved clinical efficacy. Using human myeloma cell lines (HMCL) categorized as sensitive, intermediate or resistant to HDACi, gene expression profiling (GEP) and gene ontology enrichment analyses were performed to determine if a genetic signature associated with inherent resistance to HDACi-resistance could be identified. Correlation of GEP to increasing or decreasing sensitivity to HDACi indicated a unique 35-gene signature that was significantly enriched for two pathways – regulation of actin cytoskeleton and protein processing in endoplasmic reticulum. When HMCL and primary MM samples were treated with a combination of HDACi and agents targeting the signaling pathways integral to the actin cytoskeleton, synergistic cell death was observed in all instances, thus providing a rationale for combining these agents with HDACi for the treatment of MM to overcome resistance. This report validates a molecular approach for the identification of HDACi partner drugs and provides an experimental framework for the identification of novel therapeutic combinations for anti-MM treatment.

Src family kinases promote silencing of ATR-Chk1 signaling in termination of DNA damage checkpoint

The DNA damage checkpoint arrests cell cycle progression to allow time for repair. Once DNA repair is completed, checkpoint signaling is terminated. Currently little is known about the mechanism by which checkpoint signaling is terminated, and the disappearance of DNA lesions is considered to induce the end of checkpoint signaling; however, here we show that the termination of checkpoint signaling is an active process promoted by Src family tyrosine kinases. Inhibition of Src activity delays recovery from the G2 phase DNA damage checkpoint following DNA repair. Src activity is required for the termination of checkpoint signaling, and inhibition of Src activity induces persistent activation of ataxia telangiectasia mutated (ATM)- and Rad3-related (ATR) and Chk1 kinases. Src-dependent nuclear protein tyrosine phosphorylation and v-Src expression suppress the ATR-mediated Chk1 and Rad17 phosphorylation induced by DNA double strand breaks or DNA replication stress. Thus, Src family kinases promote checkpoint recovery through termination of ATR- and Chk1-dependent G2 DNA damage checkpoint. These results suggest a model according to which Src family kinases send a termination signal between the completion of DNA repair and the initiation of checkpoint termination.

The dual roles of geminin during trophoblast proliferation and differentiation

Geminin is a protein involved in both DNA replication and cell fate acquisition. Although it is essential for mammalian preimplantation development, its role remains unclear. In one study, ablation of the geminin gene (Gmnn) in mouse preimplantation embryos resulted in apoptosis, suggesting that geminin prevents DNA re-replication, whereas in another study it resulted in differentiation of blastomeres into trophoblast giant cells (TGCs), suggesting that geminin regulates trophoblast specification and differentiation. Other studies concluded that trophoblast differentiation into TGCs is regulated by fibroblast growth factor-4 (FGF4), and that geminin is required to maintain endocycles. Here we show that ablation of Gmnn in trophoblast stem cells (TSCs) proliferating in the presence of FGF4 closely mimics the events triggered by FGF4 deprivation: arrest of cell proliferation, formation of giant cells, excessive DNA replication in the absence of DNA damage and apoptosis, and changes in gene expression that include loss of Chk1 with up-regulation of p57 and p21. Moreover, FGF4 deprivation of TSCs reduces geminin to a basal level that is required for maintaining endocycles in TGCs. Thus, geminin acts both like a component of the FGF4 signal transduction pathway that governs trophoblast proliferation and differentiation, and geminin is required to maintain endocycles.

Replication checkpoint: tuning and coordination of replication forks in s phase

Checkpoints monitor critical cell cycle events such as chromosome duplication and segregation. They are highly conserved mechanisms that prevent progression into the next phase of the cell cycle when cells are unable to accomplish the previous event properly. During S phase, cells also provide a surveillance mechanism called the DNA replication checkpoint, which consists of a conserved kinase cascade that is provoked by insults that block or slow down replication forks. The DNA replication checkpoint is crucial for maintaining genome stability, because replication forks become vulnerable to collapse when they encounter obstacles such as nucleotide adducts, nicks, RNA-DNA hybrids, or stable protein-DNA complexes. These can be exogenously induced or can arise from endogenous cellular activity. Here, we summarize the initiation and transduction of the replication checkpoint as well as its targets, which coordinate cell cycle events and DNA replication fork stability.

DNA polymerase κ-dependent DNA synthesis at stalled replication forks is important for CHK1 activation

Formation of primed single-stranded DNA at stalled replication forks triggers activation of the replication checkpoint signalling cascade resulting in the ATR-mediated phosphorylation of the Chk1 protein kinase, thus preventing genomic instability. By using siRNA-mediated depletion in human cells and immunodepletion and reconstitution experiments in Xenopus egg extracts, we report that the Y-family translesion (TLS) DNA polymerase kappa (Pol κ) contributes to the replication checkpoint response and is required for recovery after replication stress. We found that Pol κ is implicated in the synthesis of short DNA intermediates at stalled forks, facilitating the recruitment of the 9-1-1 checkpoint clamp. Furthermore, we show that Pol κ interacts with the Rad9 subunit of the 9-1-1 complex. Finally, we show that this novel checkpoint function of Pol κ is required for the maintenance of genomic stability and cell proliferation in unstressed human cells.

A vertebrate translesion synthesis DNA polymerase broadly contributes to checkpoint-activating primer synthesis at stalled replication forks, a role previously ascribed only to replicative polymerases.

It takes two to tango: Ubiquitin and SUMO in the DNA damage response

The complexity of living cells is primarily determined by the genetic information encoded in DNA and gets fully disclosed upon translation. A major determinant of complexity is the reversible post-translational modification (PTM) of proteins, which generates variants displaying distinct biological properties such as subcellular localization, enzymatic activity and the ability to assemble in complexes. Decades of work on phosphorylation have unambiguously proven this concept. In recent years, the covalent attachment of Ubiquitin or Small Ubiquitin-like Modifiers (SUMO) to amino acid residues of target proteins has been recognized as another crucial PTM, re-directing protein fate and protein-protein interactions. This review focuses on the role of ubiquitylation and sumoylation in the control of DNA damage response proteins. To lay the ground, we begin with a description of ubiquitylation and sumoylation, providing established examples of DNA damage response elements that are controlled through these PTMs. We then examine in detail the role of PTMs in the cellular response to DNA double-strand breaks illustrating hierarchy, cross-talk, synergism or antagonism between phosphorylation, ubiquitylation and sumoylation. We conclude offering a perspective on Ubiquitin and SUMO pathways as targets in cancer therapy.

The novel Chk1 inhibitor MK-8776 sensitizes human leukemia cells to HDAC inhibitors by targeting the intra-S checkpoint and DNA replication and repair

Interactions between the novel Chk1 inhibitor MK-8776 and the histone deacetylase (HDAC) inhibitor (HDACI) vorinostat were examined in human leukemia cells harboring wild-type (wt) or deficient p53. MK-8776 synergistically potentiated vorinostat-mediated apoptosis in various p53-wt or -deficient leukemia cell lines, whereas p53 knockdown by short hairpin RNA (shRNA) sensitized p53-wt cells to lethality of this regimen. Leukemia cell lines carrying FLT3-ITD were also sensitive to the MK-8776/vorinostat regimen. Synergistic interactions were associated with inhibition of Chk1 activity, interference with the intra-S-phase checkpoint, disruption of DNA replication, and downregulation of proteins involved in DNA replication (e.g., Cdt1) and repair (e.g., CtIP and BRCA1), resulting in sharp increases in DNA damage, reflected by enhanced γ-H2A.X formation, and apoptosis. Moreover, leukemia cells expressing kinase-dead Chk1 (D130A) or Chk1 shRNA were significantly more sensitive to HDACIs compared with their wt counterparts and displayed downregulation of CtIP and BRCA1 phosphorylation following HDACI exposure. Finally, the MK-8776/vorinostat regimen was active in primary acute myelogenous leukemia (AML) blasts, particularly against the CD34(+)/CD38(-)/CD123(+) population enriched for leukemia-initiating cells. In contrast, identical regimens were relatively sparing toward normal cord blood CD34(+) cells. Together, these findings indicate that the novel Chk1 inhibitor MK-8776 markedly potentiates HDACI lethality in leukemia cells displaying various genetic backgrounds through mechanisms involving disruption of the intra-S checkpoint, DNA replication, and DNA repair. They also argue that leukemic cells, including those bearing oncogenic mutations associated with poor prognosis, for example, p53 deletion/mutation or FLT3-ITD, may also be susceptible to this strategy.

Roles of Chk1 in cell biology and cancer therapy

The evolutionally conserved DNA damage response (DDR) and cell cycle checkpoints preserve genome integrity. Central to these genome surveillance pathways is a protein kinase, Chk1. DNA damage induces activation of Chk1, which then transduces the checkpoint signal and facilitates cell cycle arrest and DNA damage repair. Significant progress has been made recently toward our understanding of Chk1 regulation and its implications in cancer etiology and therapy. Specifically, a model that involves both spatiotemporal and conformational changes of proteins has been proposed for Chk1 activation. Further, emerging evidence suggests that Chk1 does not appear to be a tumor suppressor; instead, it promotes tumor growth and may contribute to anticancer therapy resistance. Recent data from our laboratory suggest that activating, but not inhibiting, Chk1 in the absence of chemotherapy might represent an innovative approach to suppress tumor growth. These findings suggest unique regulation of Chk1 in cell biology and cancer etiology, pointing to novel strategies for targeting Chk1 in cancer therapy.

Mechanisms and function of substrate recruitment by F-box proteins

S phase kinase-associated protein 1 (SKP1)-cullin 1 (CUL1)-F-box protein (SCF) ubiquitin ligase complexes use a family of F-box proteins as substrate adaptors to mediate the degradation of a large number of regulatory proteins involved in diverse processes. The dysregulation of SCF complexes and their substrates contributes to multiple pathologies. In the 14 years since the identification and annotation of the F-box protein family, the continued identification and characterization of novel substrates has greatly expanded our knowledge of the regulation of substrate targeting and the roles of F-box proteins in biological processes. Here, we focus on the evolution of our understanding of substrate recruitment by F-box proteins, the dysregulation of substrate recruitment in disease and potential avenues for F-box protein-directed disease therapies.

Candidate tumor suppressor BTG3 maintains genomic stability by promoting Lys63-linked ubiquitination and activation of the checkpoint kinase CHK1

B-cell translocation gene 3 (BTG3) is a member of the antiproliferative BTG/ Transducer of ErbB2 gene family and is induced by genotoxic stress in a p53- and Checkpoint kinase 1 (CHK1)-dependent manner. Down-regulation of BTG3 has been observed in human cancers, suggesting that it plays an important role in tumor suppression, although the underlying mechanisms are unclear. Here, we report that BTG3 interacts with CHK1, a key effector kinase in the cell cycle checkpoint response, and regulates its phosphorylation and activation. Upon interaction, BTG3 mediates K63-linked ubiquitination of CHK1 at Lys132 through the cullin-RING ligase 4(Cdt2) E3 complex, thus facilitating CHK1 chromatin association. We show that BTG3-depleted cells phenocopy those CHK1-deficient cells, exhibiting increased cell death after replication block and impaired chromosome alignment and segregation. These defects could be corrected by wild-type BTG3 but not by a mutant impaired in CHK1 interaction. We propose that BTG3-dependent CHK1 ubiquitination contributes to its chromatin localization and activation and that a defect in this regulation may increase genome instability and promote tumorigenesis.

Checkpoint kinase 1 in DNA damage response and cell cycle regulation

Originally identified as a mediator of DNA damage response (DDR), checkpoint kinase 1 (Chk1) has a broader role in checkpoint activation in DDR and normal cell cycle regulation. Chk1 activation involves phosphorylation at conserved sites. However, recent work has identified a splice variant of Chk1, which may regulate Chk1 in both DDR and normal cell cycle via molecular interaction. Upon activation, Chk1 phosphorylates a variety of substrate proteins, resulting in the activation of DNA damage checkpoints, cell cycle arrest, DNA repair, and/or cell death. Chk1 and its related signaling may be an effective therapeutic target in diseases such as cancer.

An SCF complex containing Fbxl12 mediates DNA damage-induced Ku80 ubiquitylation

The Ku heterodimer, composed of Ku70 and Ku80, is the initiating factor of the nonhomologous end joining (NHEJ) double-strand break (DSB) repair pathway. Ku is also thought to impede the homologous recombination (HR) repair pathway via inhibition of DNA end resection. Using the cell-free Xenopus laevis egg extract system, we had previously discovered that Ku80 becomes polyubiquitylated upon binding to DSBs, leading to its removal from DNA and subsequent proteasomal degradation. Here we show that the Skp1-Cul1-F box (SCF) E3 ubiquitin ligase complex is required for Ku80 ubiquitylation and removal from DNA. A screen for DSB-binding F box proteins revealed that the F box protein Fbxl12 was recruited to DNA in a DSB- and Ku-sensitive manner. Immunodepletion of Fbxl12 prevented Cul1 and Skp1 binding to DSBs and Ku80 ubiquitylation, indicating that Fbxl12 is the F box protein responsible for Ku80 substrate recognition. Unlike typical F box proteins, the F box of Fbxl12 was essential for binding to both Skp1 and its substrate Ku80. Besides Fbxl12, six other chromatin-binding F box proteins were identified in our screen of a subset of Xenopus F box proteins: β-TrCP, Fbh1, Fbxl19, Fbxo24, Fbxo28 and Kdm2b. Our study unveils a novel function for the SCF ubiquitin ligase in regulating the dynamic interaction between DNA repair machineries and DSBs.

Endoreplication

Developmentally programmed polyploidy occurs by at least four different mechanisms, two of which (endoreduplication and endomitosis) involve switching from mitotic cell cycles to endocycles by the selective loss of mitotic cyclin-dependent kinase (CDK) activity and bypassing many of the processes of mitosis. Here we review the mechanisms of endoreplication, focusing on recent results from Drosophila and mice.

A phase II study of UCN-01 in combination with irinotecan in patients with metastatic triple negative breast cancer

Mutations in TP53 lead to a defective G1 checkpoint and the dependence on checkpoint kinase 1 (Chk1) for G2 or S phase arrest in response to DNA damage. In preclinical studies, Chk1 inhibition resulted in enhanced cytotoxicity of several chemotherapeutic agents. The high frequency of TP53 mutations in triple negative breast cancer (TNBC: negative for estrogen receptor, progesterone receptor, and HER2) make Chk1 an attractive therapeutic target. UCN-01, a non-selective Chk1 inhibitor, combined with irinotecan demonstrated activity in advanced TNBC in our Phase I study. The goal of this trial was to further evaluate this treatment in women with TNBC. Patients with metastatic TNBC previously treated with anthracyclines and taxanes received irinotecan (100–125 mg/m² IV days 1, 8, 15, 22) and UCN-01 (70 mg/m² IV day 2, 35 mg/m² day 23 and subsequent doses) every 42-day cycle. Peripheral blood mononuclear cells (PBMC) and tumor specimens were collected. Twenty five patients were enrolled. The overall response (complete response (CR) + partial response (PR)) rate was 4 %. The clinical benefit rate (CR + PR + stable disease ≥6 months) was 12 %. Since UCN-01 inhibits PDK1, phosphorylated ribosomal protein S6 (pS6) in PBMC was assessed. Although reduced 24 h post UCN-01, pS6 levels rose to baseline by day 8, indicating loss of UCN-01 bioavailability. Immunostains of γH2AX and pChk1^S296 on serial tumor biopsies from four patients demonstrated an induction of DNA damage and Chk1 activation following irinotecan. However, Chk1 inhibition by UCN-01 was not observed in all tumors. Most tumors were basal-like (69 %), and carried mutations in TP53 (53 %). Median overall survival in patients with TP53 mutant tumors was poor compared to wild type (5.5 vs. 20.3 months, p = 0.004). This regimen had limited activity in TNBC. Inconsistent Chk1 inhibition was likely due to the pharmacokinetics of UCN-01. TP53 mutations were associated with a poor prognosis in metastatic TNBC.

Bioinformatic analysis of patient-derived ASPS gene expressions and ASPL-TFE3 fusion transcript levels identify potential therapeutic targets

Gene expression data, collected from ASPS tumors of seven different patients and from one immortalized ASPS cell line (ASPS-1), was analyzed jointly with patient ASPL-TFE3 (t(X;17)(p11;q25)) fusion transcript data to identify disease-specific pathways and their component genes. Data analysis of the pooled patient and ASPS-1 gene expression data, using conventional clustering methods, revealed a relatively small set of pathways and genes characterizing the biology of ASPS. These results could be largely recapitulated using only the gene expression data collected from patient tumor samples. The concordance between expression measures derived from ASPS-1 and both pooled and individual patient tumor data provided a rationale for extending the analysis to include patient ASPL-TFE3 fusion transcript data. A novel linear model was exploited to link gene expressions to fusion transcript data and used to identify a small set of ASPS-specific pathways and their gene expression. Cellular pathways that appear aberrantly regulated in response to the t(X;17)(p11;q25) translocation include the cell cycle and cell adhesion. The identification of pathways and gene subsets characteristic of ASPS support current therapeutic strategies that target the FLT1 and MET, while also proposing additional targeting of genes found in pathways involved in the cell cycle (CHK1), cell adhesion (ARHGD1A), cell division (CDC6), control of meiosis (RAD51L3) and mitosis (BIRC5), and chemokine-related protein tyrosine kinase activity (CCL4).

Autoregulatory mechanisms of phosphorylation of checkpoint kinase 1

Checkpoint kinase 1 (Chk1), a serine/threonine protein kinase, is centrally involved in cell-cycle checkpoints and cellular response to DNA damage. Phosphorylation of Chk1 at 2 Ser/Gln (SQ) sites, Ser-317 and Ser-345, by the upstream kinase ATR is critical for checkpoint activation. However, the precise molecular mechanisms controlling Chk1 phosphorylation and subsequent checkpoint activation are not well understood. Here, we report unique autoregulatory mechanisms that control protein phosphorylation of human Chk1, as well as checkpoint activation and cell viability. Phosphorylation of Ser-317 is required, but not sufficient, for maximal phosphorylation at Ser-345. The N-terminal kinase domain of Chk1 prevents Chk1 phosphorylation at the C-terminus by ATR in the absence of DNA damage. Loss of the inhibitory effect imposed by the N-terminus causes constitutive phosphorylation of Chk1 by ATR under normal growth conditions, which in turn triggers artificial checkpoints that suppress the S-phase progression. Furthermore, two point mutations were identified that rendered Chk1 constitutively active, and expression of the constitutively active mutant form of Chk1 inhibited cancer cell proliferation. Our findings therefore reveal unique regulatory mechanisms of Chk1 phosphorylation and suggest that expression of constitutively active Chk1 may represent a novel strategy to suppress tumor growth. Cancer Res; 72(15); 3786-94. ©2012 AACR.

CRL4(CDT2) targets CHK1 for PCNA-independent destruction

CDT2 targets proteins involved in replication licensing (CDT1), cell cycle control (p21), and chromatin modification (SET8) for destruction by the CUL4-based E3 ligase (CRL4). CRL4(CDT2) recruits these substrates through interactions with chromatin-bound PCNA and ubiquitinates them exclusively on chromatin. Rereplication and G(2) cell cycle arrest are observed in CDT2-depleted cells. The rereplication phenotype has been attributed to an inability to destroy CDT1, but the molecular target important for G(2) cell cycle arrest in CDT2-depleted cells has not been identified. Here we identify CHK1 as a novel CRL4(CDT2) substrate and demonstrate that CHK1 activity is required for maintaining G(2) arrest in CDT2-depleted cells. We demonstrate that CRL4(CDT2) targets the activated form of CHK1 for destruction in the nucleoplasm rather than on chromatin and that this occurs in a PCNA-independent manner. Although both CRL1 and CRL4 ubiquitinate CHK1, we report that they bind CHK1 in distinct cellular compartments. Our study provides insight into how elevated CDT2 expression levels may provide tumors with a proliferative advantage.

CHK1 cleavage in programmed cell death is intricately regulated by both caspase and non-caspase family proteases

BACKGROUND

CHK1 is an important effector kinase that regulates the cell cycle checkpoint. Previously, we showed that CHK1 is cleaved in a caspase (CASP)-dependent manner during DNA damage-induced programmed cell death (PCD) and have examined its physiological roles.

METHODS AND RESULTS

In this study, we investigated the behavior of CHK1 in PCD. Firstly, we found that CHK1 is cleaved at three sites in PCD, and all cleavages were inhibited by the co-treatment of a pan-CASP inhibitor or serine protease inhibitors. We also showed that CHK1 is cleaved by CASP3 and/or CASP7 recognizing at (296)SNLD(299) and (348)TCPD(351), and that the cleavage results in the enhancement of CHK1 kinase activity. Furthermore, as a result of the characterization of cleavage sites by site-directed mutagenesis and an analysis performed using deletion mutants, we identified (320)EPRT(323) as an additional cleavage recognition sequence. Considering the consensus sequence cleaved by CASP, it is likely that CHK1 is cleaved by non-CASP family protease(s) recognizing at (320)EPRT(323). Additionally, the cleavage catalyzed by the (320)EPRT(323) protease(s) markedly and specifically increased when U2OS cells synchronized into G1 phase were induced to PCD by cisplatin treatment.

CONCLUSION

CHK1 cleavage is directly and indirectly regulated by CASP and non-CASP family proteases including serine protease(s) and the "(320)EPRT(323) protease(s)." Furthermore, (320)EPRT(323) cleavage of CHK1 occurs efficiently in PCD which is induced at the G1 phase by DNA damage.

GENERAL SIGNIFICANCE

CASP and non-CASP family proteases intricately regulate cleavage for up-regulation of CHK1 kinase activity during PCD.

DNA damage-induced CHK1 autophosphorylation at Ser296 is regulated by an intramolecular mechanism

CHK1 regulates the DNA damage-induced checkpoint involving an ATR- or ATM- dependent pathway. In this paper, we focused on the autophosphorylation of Ser296, one of the DNA damage-induced phosphorylation sites. First, we demonstrated that the Ser296 autophosphorylation of CHK1 is mainly regulated by an intramolecular mechanism in response to DNA damage. In examining the relationship between Ser296 and Ser317/Ser345, the other ATR dependent phosphorylation sites, we found that the Ser296 cis-autophosphorylation was dependent on both Ser317 and Ser345 phosphorylation. Our findings suggest that CHK1 mediates cell cycle checkpoint signals by both cis-autophosphorylation and trans-phosphorylation of downstream factors.

"The Octet": Eight Protein Kinases that Control Mammalian DNA Replication

Development of a fertilized human egg into an average sized adult requires about 29 trillion cell divisions, thereby producing enough DNA to stretch to the Sun and back 200 times (DePamphilis and Bell, 2011)! Even more amazing is the fact that throughout these mitotic cell cycles, the human genome is duplicated once and only once each time a cell divides. If a cell accidentally begins to re-replicate its nuclear DNA prior to cell division, checkpoint pathways trigger apoptosis. And yet, some cells are developmentally programmed to respond to environmental cues by switching from mitotic cell cycles to endocycles, a process in which multiple S phases occur in the absence of either mitosis or cytokinesis. Endocycles allow production of viable, differentiated, polyploid cells that no longer proliferate. What is surprising is that among the 516 (Manning et al., 2002) to 557 (BioMart web site) protein kinases encoded by the human genome, only eight regulate nuclear DNA replication directly. These are Cdk1, Cdk2, Cdk4, Cdk6, Cdk7, Cdc7, Checkpoint kinase-1 (Chk1), and Checkpoint kinase-2. Even more remarkable is the fact that only four of these enzymes (Cdk1, Cdk7, Cdc7, and Chk1) are essential for mammalian development. Here we describe how these protein kinases determine when DNA replication occurs during mitotic cell cycles, how mammalian cells switch from mitotic cell cycles to endocycles, and how cancer cells can be selectively targeted for destruction by inducing them to begin a second S phase before mitosis is complete.

Human cells enter mitosis with damaged DNA after treatment with pharmacological concentrations of genotoxic agents

In the present paper, we report that mitosis is a key step in the cellular response to genotoxic agents in human cells. Cells with damaged DNA recruit γH2AX (phosphorylated histone H2AX), phosphorylate Chk1 (checkpoint kinase 1) and arrest in the G2-phase of the cell cycle. Strikingly, nearly all cells escape the DNA damage checkpoint and become rounded, by a mechanism that correlates with Chk1 dephosphorylation. The rounded cells are alive and in mitosis as measured by low phospho-Tyr(15) Cdk1 (cyclin-dependent kinase 1), high Cdk activity, active Plk1 (Polo-like kinase 1) and high phospho-histone H3 signals. This phenomenon is independent of the type of DNA damage, but is dependent on pharmacologically relevant doses of genotoxicity. Entry into mitosis is likely to be caused by checkpoint adaptation, and the HT-29 cell-based model provides a powerful experimental system in which to explore its molecular basis. We propose that mitosis with damaged DNA is a biologically significant event because it may cause genomic rearrangement in cells that survive genotoxic damage.

Interferon-gamma enhances radiation-induced cell death via downregulation of Chk1

Interferon-gamma (IFNγ) is a cytokine with roles in immune responses as well as in tumor control. Interferon is often used in cancer treatment together with other therapies. Here we report a novel approach to enhancement of cancer cell killing by combined treatment of IFNγ with ionizing radiation. We found that IFNγ treatment alone in HeLa cells induced phosphorylation of Chk1 in a time- and dose-dependent manner, and resulted in cell arrest. Moreover IFNγ treatment was correlated with attenuation of Chk1 as the treatment shortened protein half-life of Chk1. As Chk1 is an essential cell cycle regulator for viability after DNA damage, attenuation of Chk1 by IFNγ pre-treatment in HeLa cells resulted in increased cell death following ionizing radiation about 2-folds than ionizing radiation treatment alone whereas IFNγ treatment alone had little effect on cell death. X-linked inhibitor of apoptosis-associated factor 1 (XAF1), an IFN-induced gene, seems to partly regulate IFNγ-induced Chk1 destabilization and radiation sensitivity because transient depletion of XAF1 by siRNA prevented IFNγ-induced Chk1 attenuation and partly protected cells from IFNγ-enhanced radiation cell killing. Therefore the results provide a novel rationale to combine IFNγ pretreatment and DNA-damaging anti-cancer drugs such as ionizing radiation to enhance cancer cell killing.

Coupling cellular localization and function of checkpoint kinase 1 (Chk1) in checkpoints and cell viability

Chk1 plays a key role in regulating the replication checkpoint and DNA damage response. Recent evidence suggests that mammalian Chk1 regulates both the nuclear and cytoplasmic checkpoint events. However, mechanisms regulating cellular mobilization of Chk1 were not well understood. Here, we report the identification of regions of human Chk1 that regulate its protein cellular localization and checkpoint function. We demonstrate that the two highly conserved motifs (CM1 and CM2) at the C terminus of Chk1 function as a nuclear export signal and nuclear localization signal, respectively. Mutating five highly conserved residues within these two motifs of Chk1 resulted in its accumulation mainly in the cytoplasm. These cytoplasmic Chk1 mutants were less stable and exhibited significantly reduced phosphorylation by DNA damage treatment, yet they retained, at least partially, checkpoint function. Using an adenovirus-mediated gene targeting technique, we attempted to create an HCT116 cell line in which endogenous Chk1 is mutated so that it is expressed exclusively in the cytoplasm. However, we failed to obtain homozygous mutant cell lines. We found that even the heterozygous mutant cell lines showed cell survival defects accompanied by spontaneous cell death. Together, these results reveal novel regulatory mechanisms that couple protein cellular localization with the checkpoint response and cell viability of Chk1.

ATP depletion triggers acute myeloid leukemia differentiation through an ATR/Chk1 protein-dependent and p53 protein-independent pathway

Despite advances in oncology drug development, most commonly used cancer therapeutics exhibit serious adverse effects. Often the toxicities of chemotherapeutics are due to the induction of significant DNA damage that is necessary for their ability to kill cancer cells. In some clinical situations, the direct induction of significant cytotoxicity is not a requirement to meet clinical goals. For example, differentiation, growth arrest, and/or senescence is a valuable outcome in some cases. In fact, in the case of acute myeloid leukemia (AML), the use of the differentiation agent all-trans-retinoic acid (ATRA) has revolutionized the therapy for a subset of leukemia patients and led to a dramatic survival improvement. Remarkably, this therapeutic approach is possible even in many elderly patients, who would not be able to tolerate therapy with traditional cytotoxic chemotherapy. Because of the success of ATRA, there is widespread interest in identifying differentiation strategies that may be effective for the 90-95% of AML patients who do not clinically respond to ATRA. Utilizing an AML differentiation agent that is in development, we found that AML differentiation can be induced through ATP depletion and the subsequent activation of DNA damage signaling through an ATR/Chk1-dependent and p53-independent pathway. This study not only reveals mechanisms of AML differentiation but also suggests that further investigation is warranted to investigate the potential clinical use of low dose chemotherapeutics to induce differentiation instead of cytotoxicity. This therapeutic approach may be of particular benefit to patients, such as elderly AML patients, who often cannot tolerate traditional AML chemotherapy.

Proteomic identification of common SCF ubiquitin ligase FBXO6-interacting glycoproteins in three kinds of cells

FBOX6 ubiquitin ligase complex is involved in the endoplasmic reticulum-associated degradation pathway by mediating the ubiquitination of glycoproteins. FBXO6 interacts with the chitobiose in unfolded N-glycoprotein, pointing glycoproteins toward E2 for ubiquitination. Although the glycoprotein-recognizing mechanism of FBXO6 is well documented, its bona fide interacting glycoproteins are largely unknown. Here we utilized a protein purification approach combined with LC-MS to systematically identify the FBXO6-interacting glycoproteins. Following identification of 39 proteins that specifically interact with FBXO6 in all three different cell lines, 293T, HeLa and Jurkat cells, we compared the protein complex organization between wild-type FBXO6 and its mutant, which fails to recognize glycoproteins. Combining these databases, 29 highly confident glycoproteins that interact with FBXO6 in an N-glycan dependent manner are identified. Our data provide valuable information for the discovery of the interacting glycoproteins of FBXO6 and also demonstrate the potential of these approaches as general platforms for the global discovery of interacting glycoproteins of other FBAs (F-box associated regions) containing F-box proteins.

SCF ubiquitin ligases in the maintenance of genome stability

In response to genotoxic stress, eukaryotic cells activate the DNA damage response (DDR), a series of pathways that coordinate cell cycle arrest and DNA repair to prevent deleterious mutations. In addition, cells possess checkpoint mechanisms that prevent aneuploidy by regulating the number of centrosomes and spindle assembly. Among these mechanisms, ubiquitin-mediated degradation of key proteins has an important role in the regulation of the DDR, centrosome duplication and chromosome segregation. This review discusses the functions of a group of ubiquitin ligases, the SCF (SKP1-CUL1-F-box protein) family, in the maintenance of genome stability. Given that general proteasome inhibitors are currently used as anticancer agents, a better understanding of the ubiquitylation of specific targets by specific ubiquitin ligases may result in improved cancer therapeutics.

Therapeutic implications for the induced levels of Chk1 in Myc-expressing cancer cells

PURPOSE

The transcription factor c-Myc (or "Myc") is a master regulator of pathways driving cell growth and proliferation. MYC is deregulated in many human cancers, making its downstream target genes attractive candidates for drug development. We report the unexpected finding that B-cell lymphomas from mice and patients exhibit a striking correlation between high levels of Myc and checkpoint kinase 1 (Chk1).

EXPERIMENTAL DESIGN

By in vitro cell biology studies as well as preclinical studies using a genetically engineered mouse model, we evaluated the role of Chk1 in Myc-overexpressing cells.

RESULTS

We show that Myc indirectly induces Chek1 transcript and protein expression, independently of DNA damage response proteins such as ATM and p53. Importantly, we show that inhibition of Chk1, by either RNA interference or a novel highly selective small molecule inhibitor, results in caspase-dependent apoptosis that affects Myc-overexpressing cells in both in vitro and in vivo mouse models of B-cell lymphoma.

CONCLUSION

Our data suggest that Chk1 inhibitors should be further evaluated as potential drugs against Myc-driven malignancies such as certain B-cell lymphoma/leukemia, neuroblastoma, and some breast and lung cancers.

Genetic variation and DNA replication timing, or why is there late replicating DNA?

Mutation rates vary significantly within the genome and across species. Recent studies revealed a long suspected replication-timing effect on mutation rate, but the mechanisms that regulate the increase in mutation rate as the genome is replicated remain unclear. Evidence is emerging, however, that DNA repair systems, in general, are less efficient in late replicating heterochromatic regions compared to early replicating euchromatic regions of the genome. At the same time, mutation rates in both vertebrates and invertebrates have been shown to vary with generation time (GT). GT is correlated with genome size, which suggests a possible nucleotypic effect on species-specific mutation rates. These and other observations all converge on a role for DNA replication checkpoints in modulating generation times and mutation rates during the DNA synthetic phase (S phase) of the cell cycle. The following will examine the potential role of the intra-S checkpoint in regulating cell cycle times (GT) and mutation rates in eukaryotes. This article was published online on August 5, 2011. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected October 4, 2011.

Checkpoint kinase 1 prevents cell cycle exit linked to terminal cell differentiation

Trophoblast stem (TS) cells proliferate in the presence of fibroblast growth factor 4, but in its absence, they differentiate into polyploid trophoblast giant (TG) cells that remain viable but nonproliferative. Differentiation is coincident with expression of the cyclin-dependent kinase (CDK)-specific inhibitors p21 and p57, of which p57 is essential for switching from mitotic cell cycles to endocycles. Here, we show that, in the absence of induced DNA damage, checkpoint kinase-1 (CHK1), an enzyme essential for preventing mitosis in response to DNA damage, functions as a mitogen-dependent protein kinase that prevents premature differentiation of TS cells into TG cells by suppressing expression of p21 and p57, but not p27, the CDK inhibitor that regulates mitotic cell cycles. CHK1 phosphorylates p21 and p57 proteins at specific sites, thereby targeting them for degradation by the 26S proteasome. TG cells lack CHK1, and restoring CHK1 activity in TG cells suppresses expression of p57 and restores mitosis. Thus, CHK1 is part of a "G2 restriction point" that prevents premature cell cycle exit in cells programmed for terminal differentiation, a role that CHK2 cannot play.

Glucose deprivation is associated with Chk1 degradation through the ubiquitin-proteasome pathway and effective checkpoint response to replication blocks

Chk1 plays a key role in the DNA replication checkpoint and in preserving genomic integrity. Previous studies have shown that reduced Chk1 function leads to defects in the checkpoint response and is closely associated with tumorigenesis. Here, we report that glucose deprivation caused the degradation of Chk1 protein without perturbing cell cycle progression. The induction of Chk1 degradation in response to glucose deprivation was observed in various cancer cell lines and in normal human fibroblasts. Therefore, it appears to be a universal phenomenon in mammalian cells. A specific proteasome inhibitor blocked glucose deprivation-induced Chk1 degradation. Ubiquitination of Chk1 was detected, indicating that the proteasome-ubiquitin pathway mediates Chk1 degradation upon glucose deprivation. Mechanistic studies have demonstrated that ATR-dependent phosphorylation of Chk1 at the Ser317 and Ser345 sites is not required, suggesting that the molecular mechanism for Chk1 degradation upon glucose deprivation is distinct from genotoxic stress-induced degradation. Under conditions of glucose deprivation, the cells manifested a defective checkpoint response to replication stress, camptothecin or hydroxyurea. The forced expression of Myc-Chk1 partially rescued the defective response to the replication block upon glucose deprivation. Taken together, our results indicate that glucose deprivation induces ubiquitin-mediated Chk1 degradation and defective checkpoint responses, implying its potential role in genomic instability and tumor development.

USP1 deubiquitinase maintains phosphorylated CHK1 by limiting its DDB1-dependent degradation

The maintenance of genetic stability depends on the fine-tuned initiation and termination of pathways involved in cell cycle checkpoints and DNA repair. Here, we describe a new pathway that regulates checkpoint kinase 1 (CHK1) activity, a key element controlling both checkpoints and DNA repair. We show that the ubiquitin-specific peptidase 1 (USP1) deubiquitinase participates in the maintenance of both total and phosphorylated levels of CHK1 in response to genotoxic stress. We establish that USP1 depletion stimulates the damage-specific DNA-binding protein 1-dependent degradation of phosphorylated CHK1 in both a monoubiquitinylated Fanconi anaemia, complementation group D2 (FANCD2)-dependent and -independent manner. Our data support the existence of a circuit in which CHK1 activates checkpoints, DNA repair and proliferating cell nuclear antigen and FANCD2 monoubiquitinylation. The latter two events, in turn, switch off activated CHK1 by negative feedback inhibition, which contributes to the downregulation of the DNA damage response. This pathway, which is compromised in the cancer-prone disease Fanconi anaemia (FA), likely contributes to the hypersensitivity of cells from FA patients to DNA damage and to the clinical phenotype of the syndrome; it may also represent a pharmacological target to improve patient care and develop new cancer therapies.

A new in vitro system for activating the cell cycle checkpoint

In response to DNA damage, cells launch elegant networks of genome surveillance mechanisms, called cell cycle checkpoints, to detect and repair damaged DNA to maintain the genome stability. Key components of cell cycle checkpoints are two PI3K-related protein kinases (PIKK), ATR and ATM, which participate in both sensing the DNA damage and transducing the damage signal through phosphorylating two target protein kinases, Chk1 and Chk2, respectively. However, how exactly cell cycle checkpoints are activated, maintained, and terminated are not completely understood. Given the complexity of the cell cycle checkpoint signaling and the cellular environment, systems that can faithfully mimic the cell cycle checkpoint activation in vitro, such as the Xenopus egg extracts, are of extreme value in dissecting the precise molecular mechanisms underlying DNA damage response. Here we describe that the well-established in vitro transcription and translation (IVTNT) system has the capability to induce protein phosphorylation of substrates for ATR or ATM, including Chk1, Rad17, and ATM itself. These phosphorylation events highly mimic those occurring in cells when treated with DNA damaging agents. Our results demonstrate that the IVTNT system could be developed into a novel in vitro system to facilitating the dissecting of mechanisms leading to cell cycle checkpoint activation in vivo.

Multiple functions of the S-phase checkpoint mediator

There is mounting evidence that replication defects are the major source of spontaneous genomic instability in cells, and that S-phase checkpoints are the principal defense against such instability. The S-phase checkpoint mediator protein Mrc1/Claspin mediates the checkpoint response to replication stress by facilitating phosphorylation of effector kinase by a sensor kinase. In this review, the multiple functions and the regulation of the S-phase checkpoint mediator are discussed.

Analysis of kinase gene expression patterns across 5681 human tissue samples reveals functional genomic taxonomy of the kinome

Kinases play key roles in cell signaling and represent major targets for drug development, but the regulation of their activation and their associations with health and disease have not been systematically analyzed. Here, we carried out a bioinformatic analysis of the expression levels of 459 human kinase genes in 5681 samples consisting of 44 healthy and 55 malignant human tissues. Defining the tissues where the kinase genes were transcriptionally active led to a functional genomic taxonomy of the kinome and a classification of human tissues and disease types based on the similarity of their kinome gene expression. The co-expression network around each of the kinase genes was defined in order to determine the functional context, i.e. the biological processes that were active in the cells and tissues where the kinase gene was expressed. Strong associations for individual kinases were found for mitosis (69 genes, including AURKA and BUB1), cell cycle control (73 genes, including PLK1 and AURKB), DNA repair (49 genes, including CHEK1 and ATR), immune response (72 genes, including MATK), neuronal (131 genes, including PRKCE) and muscular (72 genes, including MYLK2) functions. We then analyzed which kinase genes gain or lose transcriptional activity in the development of prostate and lung cancers and elucidated the functional associations of individual cancer associated kinase genes. In summary, we report here a systematic classification of kinases based on the bioinformatic analysis of their expression in human tissues and diseases, as well as grouping of tissues and tumor types according to the similarity of their kinome transcription.

DNA adducts of decarbamoyl mitomycin C efficiently kill cells without wild-type p53 resulting from proteasome-mediated degradation of checkpoint protein 1

The mitomycin derivative 10-decarbamoyl mitomycin C (DMC) more rapidly activates a p53-independent cell death pathway than mitomycin C (MC). We recently documented that an increased proportion of mitosene1-β-adduct formation occurs in human cells treated with DMC in comparison to those treated with MC. Here, we compare the cellular and molecular response of human cancer cells treated with MC and DMC. We find the increase in mitosene 1-β-adduct formation correlates with a condensed nuclear morphology and increased cytotoxicity in human cancer cells with or without p53. DMC caused more DNA damage than MC in the nuclear and mitochondrial genomes. Checkpoint 1 protein (Chk1) was depleted following DMC, and the depletion of Chk1 by DMC was achieved through the ubiquitin proteasome pathway since chemical inhibition of the proteasome protected against Chk1 depletion. Gene silencing of Chk1 by siRNA increased the cytotoxicity of MC. DMC treatment caused a decrease in the level of total ubiquitinated proteins without increasing proteasome activity, suggesting that DMC mediated DNA adducts facilitate signal transduction to a pathway targeting cellular proteins for proteolysis. Thus, the mitosene-1-β stereoisomeric DNA adducts produced by the DMC signal for a p53-independent mode of cell death correlated with reduced nuclear size, persistent DNA damage, increased ubiquitin proteolysis and reduced Chk1 protein.

Death by releasing the breaks: CHK1 inhibitors as cancer therapeutics

Defects in p53 function, which occur frequently in human cancers due to mutations in TP53 or disruptions in the p53 regulatory pathway, render cells dependent on CHK1 (Checkpoint Kinase 1) to activate cell cycle checkpoints. In the presence of DNA damage or replication stress, inhibition of CHK1 leads to "mitotic catastrophe" and cell death in p53-deficient tumors while sparing p53-proficient cells. CHK1 inhibitors sensitize tumors to a variety of DNA-damaging agents or antimetabolites in preclinical models and are being evaluated in early phase clinical trials. In this review, we summarize recent advances and controversies in the development and application of CHK1 inhibitors as cancer therapeutics.

Phosphorylation and ubiquitination of degron proximal residues are essential for class II transactivator (CIITA) transactivation and major histocompatibility class II expression

Major histocompatibility (MHC) class II molecules are cell surface glycoproteins that present extracellular antigens to CD4(+) T cells and are essential for initiation of the adaptive immune response. MHC class II expression requires recruitment of a master regulator, the class II transactivator (CIITA), to the MHC class II promoter. Post-translational modifications to CIITA play important roles in modulating CIITA mediated transcription of various genes in different cell types. We have previously linked regulation of CIITA to the Ubiquitin Proteasome System (UPS), and we and others have demonstrated that mono-ubiquitination of CIITA dramatically increases its transactivity whereas poly-ubiquitination leads to CIITA degradation. Here we identify three degron proximal lysine residues, Lys-315, Lys-330, and Lys-333, and a phosphorylation site, Ser-280, located within the CIITA degron, that regulate CIITA ubiquitination, stability, and MHC class II expression. Together, these findings contribute to the developing post-translational modification code for CIITA.

PERK promotes cancer cell proliferation and tumor growth by limiting oxidative DNA damage

To proliferate and expand in an environment with limited nutrients, cancer cells co-opt cellular regulatory pathways that facilitate adaptation and thereby maintain tumor growth and survival potential. The endoplasmic reticulum (ER) is uniquely positioned to sense nutrient deprivation stress and subsequently engage signaling pathways that promote adaptive strategies. As such, components of the ER stress-signaling pathway represent potential antineoplastic targets. However, recent investigations into the role of the ER resident protein kinase, RNA-dependent protein kinase (PKR)-like ER kinase (PERK) have paradoxically suggested both pro- and anti-tumorigenic properties. We have used animal models of mammary carcinoma to interrogate the contribution of PERK in the neoplastic process. The ablation of PERK in tumor cells resulted in impaired regeneration of intracellular antioxidants and accumulation of reactive oxygen species triggering oxidative DNA damage. Ultimately, PERK deficiency impeded progression through the cell cycle because of the activation of the DNA damage checkpoint. Our data reveal that PERK-dependent signaling is used during both tumor initiation and expansion to maintain redox homeostasis, thereby facilitating tumor growth.

Tipin-replication protein A interaction mediates Chk1 phosphorylation by ATR in response to genotoxic stress

Mammalian Timeless is a multifunctional protein that performs essential roles in the circadian clock, chromosome cohesion, DNA replication fork protection, and DNA replication/DNA damage checkpoint pathways. The human Timeless exists in a tight complex with a smaller protein called Tipin (Timeless-interacting protein). Here we investigated the mechanism by which the Timeless-Tipin complex functions as a mediator in the ATR-Chk1 DNA damage checkpoint pathway. We find that the Timeless-Tipin complex specifically mediates Chk1 phosphorylation by ATR in response to DNA damage and replication stress through interaction of Tipin with the 34-kDa subunit of replication protein A (RPA). The Tipin-RPA interaction stabilizes Timeless-Tipin and Tipin-Claspin complexes on RPA-coated ssDNA and in doing so promotes Claspin-mediated phosphorylation of Chk1 by ATR. Our results therefore indicate that RPA-covered ssDNA not only supports recruitment and activation of ATR but also, through Tipin and Claspin, it plays an important role in the action of ATR on its critical downstream target Chk1.

Targeting the checkpoint kinase Chk1 in cancer therapy

A paramount objective of the eukaryotic cell division cycle is to overcome numerous internal and external insults to faithfully duplicate the genetic information once per every cycle. This is carried out by elaborate networks of genome surveillance signaling pathways, termed replication checkpoints. Central to replication checkpoints are two protein kinases, the upstream kinase ATR, and its downstream target kinase, Chk1. When the DNA replication process is interrupted, the ATR-Chk1 pathway transmits signals to delay cell cycle progression, and to maintain fork viability so that DNA duplication can resume after the initial damage is corrected. Previous studies showed that replicative stress not only activated Chk1, but also triggered the ubiquitin-dependent destruction of Chk1 in cultured human cells. In a recent study, we identified the F-box protein, Fbx6, as the mediator that regulates Chk1 ubiquitination and degradation in both normally cycling cells and during replication stress. We further showed that expression levels of Chk1 and Fbx6 exhibited an overall inverse correlation in both cultured cancer cell lines and in breast tumor tissues, and that defects in Chk1 degradation, for instance, due to reduced expression of Fbx6, rendered tumor cells resistant to anticancer treatment. Here we highlight those findings and their implications in the replication checkpoint and cellular sensitivity to cancer therapies.