Two Distinct Cdc2 Pools Regulate Cell Cycle Progression and the DNA Damage Response in the Fission Yeast S.pombe

LAMMER Kinase Modulates Cell Cycle by Phosphorylating the MBF Repressor, Yox1, in Schizosaccharomyces pombe

Lkh1, a LAMMER kinase homolog in the fission yeast Schizosaccharomyces pombe, acts as a negative regulator of filamentous growth and flocculation. It is also involved in the response to oxidative stress. The lkh1-deletion mutant displays slower cell growth, shorter cell size, and abnormal DNA content compared to the wild type. These phenotypes suggest that Lkh1 controls cell size and cell cycle progression. When we performed microarray analysis using the lkh1-deletion mutant, we found that only four of the up-regulated genes in the lkh1-deletion were associated with the cell cycle. Interestingly, all of these genes are regulated by the Mlu1 cell cycle box binding factor (MBF), which is a transcription complex responsible for regulating the expression of cell cycle genes during the G1/S phase. Transcription analyses of the MBF-dependent cell-cycle genes, including negative feedback regulators, confirmed the up-regulation of these genes by the deletion of lkh1. Pull-down assay confirmed the interaction between Lkh1 and Yox1, which is a negative feedback regulator of MBF. This result supports the involvement of LAMMER kinase in cell cycle regulation by modulating MBF activity. In vitro kinase assay and NetPhosK 2.0 analysis with the Yox1T40,41A mutant allele revealed that T40 and T41 residues are the phosphorylation sites mediated by Lkh1. These sites affect the G1/S cell cycle progression of fission yeast by modulating the activity of the MBF complex.

Cell cycle involvement in cancer therapy; WEE1 kinase, a potential target as therapeutic strategy

Mitosis is the process of cell division and is regulated by checkpoints in the cell cycle. G1-S, S, and G2-M are the three main checkpoints that prevent initiation of the next phase of the cell cycle phase until previous phase has completed. DNA damage leads to activation of the G2-M checkpoint, which can trigger a downstream DNA damage response (DDR) pathway to induce cell cycle arrest while the damage is repaired. If the DNA damage cannot be repaired, the replication stress response (RSR) pathway finally leads to cell death by apoptosis, in this case called mitotic catastrophe. Many cancer treatments (chemotherapy and radiotherapy) cause DNA damages based on SSBs (single strand breaks) or DSBs (double strand breaks), which cause cell death through mitotic catastrophe. However, damaged cells can activate WEE1 kinase (as a part of the DDR and RSR pathways), which prevents apoptosis and cell death by inducing cell cycle arrest at G2 phase. Therefore, inhibition of WEE1 kinase could sensitize cancer cells to chemotherapeutic drugs. This review focuses on the role of WEE1 kinase (as a biological macromolecule which has a molecular mass of 96 kDa) in the cell cycle, and its interactions with other regulatory pathways. In addition, we discuss the potential of WEE1 inhibition as a new therapeutic approach in the treatment of various cancers, such as melanoma, breast cancer, pancreatic cancer, cervical cancer, etc.

Caffeine Stabilises Fission Yeast Wee1 in a Rad24-Dependent Manner but Attenuates Its Expression in Response to DNA Damage

The widely consumed neuroactive compound caffeine has generated much interest due to its ability to override the DNA damage and replication checkpoints. Previously Rad3 and its homologues was thought to be the target of caffeine’s inhibitory activity. Later findings indicate that the Target of Rapamycin Complex 1 (TORC1) is the preferred target of caffeine. Effective Cdc2 inhibition requires both the activation of the Wee1 kinase and inhibition of the Cdc25 phosphatase. The TORC1, DNA damage, and environmental stress response pathways all converge on Cdc25 and Wee1. We previously demonstrated that caffeine overrides DNA damage checkpoints by modulating Cdc25 stability. The effect of caffeine on cell cycle progression resembles that of TORC1 inhibition. Furthermore, caffeine activates the Sty1 regulated environmental stress response. Caffeine may thus modulate multiple signalling pathways that regulate Cdc25 and Wee1 levels, localisation and activity. Here we show that the activity of caffeine stabilises both Cdc25 and Wee1. The stabilising effect of caffeine and genotoxic agents on Wee1 was dependent on the Rad24 chaperone. Interestingly, caffeine inhibited the accumulation of Wee1 in response to DNA damage. Caffeine may modulate cell cycle progression through increased Cdc25 activity and Wee1 repression following DNA damage via TORC1 inhibition, as TORC1 inhibition increased DNA damage sensitivity.

Communication between Cyclin-dependent kinase Cdc2 and the Wis1-Spc1 MAPK pathway determines mitotic timing in Schizosaccharomyces pombe

Checkpoint activation and gene expression modulation represent key determinants of cellular survival in adverse conditions. The former is regulated by cyclin-dependent kinases (CDKs) while the latter can be controlled by mitogen-activated protein kinases (MAPKs). Association between cell-cycle progression and MAPK-dependent gene expression exists in cells growing in optimal environments. While MAPK-mediated regulation of the cell cycle is well characterised, the reciprocal influence of mitotic CDK on stress response is not well studied. We present evidence that CDK activity can regulate the extent of MAPK activation in Schizosaccharomyces pombe cells. We show that increasing or decreasing mitotic CDK (Cdc2) activity in S. pombe cells can affect the activation of stress responsive MAPK (Spc1) even in the absence of stress stimuli. Our results indicate that the strong correlation between Cdc2 activity and Spc1 MAPK-activity in S. pombe is important in regulating mitotic timing.This article has an associated First Person interview with the first author of the paper.

Therapeutic effect of Arthrocnemum machrostachyum methanolic extract on Ehrlich solid tumor in mice

Background

The anti-cancer effect of the halophyte Arthrocnemum indicum, a member of Arthrocnemum family of salt-tolerant plants, was evaluated against colorectal cancer cell, CaCo2. However, the anti-cancer effect of another halophyte Arthrocnemum machrostachyum was not investigated yet. Herein, the anticancer effect of A. machrostachyum methanolic extract (AME) was evaluated against Ehrlich solid tumor (EST) in mice and the potential mechanism of action was also studied.

Methods

Male Swiss albino mice (n = 28) were randomly divided into 4 groups (n = 7/group). Group 1 (negative control group); group 2 (EST) injected intramuscularly by 0.2 mL Ehrlich ascitic carcinoma (2 × 10⁶ cells); and groups 3 and 4 injected intratumorally with AME (180 and 360 mg/kg body weight, respectively) at D12 trice weekly for 2 weeks. Gene expression, protein expression, DNA damage, and TNFa level in tumors were determined by real-time PCR, western blot, comet assay, and Elisa, respectively.

Results

Treatment with AME induced anti-tumor effects against EST as indicated by 1) notable reduction in tumor size; 2) elevation in tissue necrosis and apoptosis, as confirmed histologically; 3) increased DNA fragmentation; 4) decreased expression of the apoptotic genes (p53, Bax and caspase 3), and increased expression of the anti-apoptotic marker Bcl2; 5) significantly upregulated cell cycle regulatory genes Cdc2 and connexin26, and; 6) decreased TNFa levels in tumor tissues. Interestingly, a high dose of AME exhibited a more potent anti-tumor effect against EST.

Conclusion

These findings indicate that AME has a potent antitumor effect against EST and could be used as an adjuvant to anticancer drugs to combat tumor, but after application of further confirmatory clinical trials.

Preclinical Efficacy and Involvement of AKT, mTOR, and ERK Kinases in the Mechanism of Sulforaphane against Endometrial Cancer

Sulforaphane exerts anti-cancer activity against multiple cancer types. Our objective was to evaluate utility of sulforaphane for endometrial cancer therapy. Sulforaphane reduced viability of endometrial cancer cell lines in association with the G2/M cell cycle arrest and cell division cycle protein 2 (Cdc2) phosphorylation, and intrinsic apoptosis. Inhibition of anchorage-independent growth, invasion, and migration of the cell lines was associated with sulforaphane-induced alterations in epithelial-to-mesenchymal transition (EMT) markers of increased E-cadherin and decreased N-cadherin and vimentin expression. Proteomic analysis identified alterations in AKT, mTOR, and ERK kinases in the networks of sulforaphane effects in the Ishikawa endometrial cancer cell line. Western blots confirmed sulforaphane inhibition of AKT, mTOR, and induction of ERK with alterations in downstream signaling. AKT and mTOR inhibitors reduced endometrial cancer cell line viability and prevented further reduction by sulforaphane. Accumulation of nuclear phosphorylated ERK was associated with reduced sensitivity to the ERK inhibitor and its interference with sulforaphane activity. Sulforaphane induced apoptosis-associated growth inhibition of Ishikawa xenograft tumors to a greater extent than paclitaxel, with no evidence of toxicity. These results verify sulforaphane's potential as a non-toxic treatment candidate for endometrial cancer and identify AKT, mTOR, and ERK kinases in the mechanism of action with interference in the mechanism by nuclear phosphorylated ERK.

Kinome Expansion in the Fusarium oxysporum Species Complex Driven by Accessory Chromosomes

Isolates of Fusarium oxysporum are adapted to survive a wide range of host and nonhost conditions. In addition, F. oxysporum was recently recognized as the top emerging opportunistic fungal pathogen infecting immunocompromised humans. The sensory and response networks of these fungi undoubtedly play a fundamental role in establishing the adaptability of this group. We have examined the kinomes of 12 F. oxysporum isolates and highlighted kinase families that distinguish F. oxysporum from other fungi, as well as different isolates from one another. The amplification of kinases involved in environmental signal relay and regulating downstream cellular responses clearly sets Fusarium apart from other Ascomycetes. Although the functions of many of these kinases are still unclear, their specific proliferation highlights them as a result of the evolutionary forces that have shaped this species complex and clearly marks them as targets for exploitation in order to combat disease.

The Fusarium oxysporum species complex (FOSC) is a group of soilborne pathogens causing severe disease in more than 100 plant hosts, while individual strains exhibit strong host specificity. Both chromosome transfer and comparative genomics experiments have demonstrated that lineage-specific (LS) chromosomes contribute to the host-specific pathogenicity. However, little is known about the functional importance of genes encoded in these LS chromosomes. Focusing on signaling transduction, this study compared the kinomes of 12 F. oxysporum isolates, including both plant and human pathogens and 1 nonpathogenic biocontrol strain, with 7 additional publicly available ascomycete genomes. Overall, F. oxysporum kinomes are the largest, facilitated in part by the acquisitions of the LS chromosomes. The comparative study identified 99 kinases that are present in almost all examined fungal genomes, forming the core signaling network of ascomycete fungi. Compared to the conserved ascomycete kinome, the expansion of the F. oxysporum kinome occurs in several kinase families such as histidine kinases that are involved in environmental signal sensing and target of rapamycin (TOR) kinase that mediates cellular responses. Comparative kinome analysis suggests a convergent evolution that shapes individual F. oxysporum isolates with an enhanced and unique capacity for environmental perception and associated downstream responses.

IMPORTANCE Isolates of Fusarium oxysporum are adapted to survive a wide range of host and nonhost conditions. In addition, F. oxysporum was recently recognized as the top emerging opportunistic fungal pathogen infecting immunocompromised humans. The sensory and response networks of these fungi undoubtedly play a fundamental role in establishing the adaptability of this group. We have examined the kinomes of 12 F. oxysporum isolates and highlighted kinase families that distinguish F. oxysporum from other fungi, as well as different isolates from one another. The amplification of kinases involved in environmental signal relay and regulating downstream cellular responses clearly sets Fusarium apart from other Ascomycetes. Although the functions of many of these kinases are still unclear, their specific proliferation highlights them as a result of the evolutionary forces that have shaped this species complex and clearly marks them as targets for exploitation in order to combat disease.

The cyclin-dependent kinase G group defines a thermo-sensitive alternative splicing circuit modulating the expression of Arabidopsis ATU2AF65A

The ability to adapt growth and development to temperature variations is crucial to generate plant varieties resilient to predicted temperature changes. However, the mechanisms underlying plant response to progressive increases in temperature have just started to be elucidated. Here, we report that the cyclin-dependent kinase G1 (CDKG1) is a central element in a thermo-sensitive mRNA splicing cascade that transduces changes in ambient temperature into differential expression of the fundamental spliceosome component, ATU2AF65A. CDKG1 is alternatively spliced in a temperature-dependent manner. We found that this process is partly dependent on both the cyclin-dependent kinase G2 (CDKG2) and the interacting co-factor CYCLIN L1 (CYCL1), resulting in two distinct messenger RNAs. The relative abundance of both CDKG1 transcripts correlates with ambient temperature and possibly with different expression levels of the associated protein isoforms. Both CDKG1 alternative transcripts are necessary to fully complement the expression of ATU2AF65A across the temperature range. Our data support a previously unidentified temperature-dependent mechanism based on the alternative splicing (AS) of CDKG1 and regulated by CDKG2 and CYCL1. We propose that changes in ambient temperature affect the relative abundance of CDKG1 transcripts, and this in turn translates into differential CDKG1 protein expression coordinating the AS of ATU2AF65A.

Nutrient Limitation Inactivates Mrc1-to-Cds1 Checkpoint Signalling in Schizosaccharomyces pombe

The S. pombe checkpoint kinase, Cds1, protects the integrity of stalled DNA replication forks after its phosphorylation at threonine-11 by Rad3 (ATR). Modified Cds1 associates through its N-terminal forkhead-associated domain (FHA)-domain with Mrc1 (Claspin) at stalled forks. We report here that nutrient starvation results in post-translational changes to Cds1 and the loss of Mrc1. A drop in glucose after a down-shift from 3% to 0.1–0.3%, or when cells enter the stationary phase, triggers a sharp decline in Mrc1 and the accumulation of insoluble Cds1. Before this transition, Cds1 is transiently activated and phosphorylated by Rad3 when glucose levels fall. Because this coincides with the phosphorylation of histone 2AX at S129 by Rad3, an event that occurs towards the end of every unperturbed S phase, we suggest that a glucose limitation promotes the exit from the S phase. Since nitrogen starvation also depletes Mrc1 while Cds1 is post-translationally modified, we suggest that nutrient limitation is the general signal that promotes exit from S phase before it inactivates the Mrc1–Cds1 signalling component. Why Cds1 accumulates in resting cells while its activator Mrc1 declines is, as yet, unclear but suggests a novel function of Cds1 in non-replicating cells.

The kinase domain residue serine 173 of Schizosaccharomyces pombe Chk1 kinase is critical for the response to DNA replication stress

While mammalian Chk1 kinase regulates replication origins, safeguards fork integrity and promotes fork progression, yeast Chk1 acts only in G1 and G2. We report here that the mutation of serine 173 (S173A) in the kinase domain of fission yeast Chk1 abolishes the G1-M and S-M checkpoints with little impact on the G2-M arrest. This separation-of-function mutation strongly reduces the Rad3-dependent phosphorylation of Chk1 at serine 345 during logarithmic growth, but not when cells experience exogenous DNA damage. Loss of S173 lowers the restrictive temperature of a catalytic DNA polymerase epsilon mutant (cdc20.M10) and is epistatic with a mutation in DNA polymerase delta (cdc6.23) when DNA is alkylated by methyl-methanesulfate (MMS). The chk1-S173A allele is uniquely sensitive to high MMS concentrations where it displays a partial checkpoint defect. A complete checkpoint defect occurs only when DNA replication forks break in cells without the intra-S phase checkpoint kinase Cds1. Chk1-S173A is also unable to block mitosis when the G1 transcription factor Cdc10 (cdc10.V50) is impaired. We conclude that serine 173, which is equivalent to lysine 166 in the activation loop of human Chk1, is only critical in DNA polymerase mutants or when forks collapse in the absence of Cds1.

Summary: Mutation of serine-173 in the kinase domain of Chk1 increases genomic instability as it abolishes the response to DNA lesions that arise while chromosomes are being copied.

The drinking water contaminant dibromoacetonitrile delays G1-S transition and suppresses Chk1 activation at broken replication forks

Chlorination of drinking water protects humans from water-born pathogens, but it also produces low concentrations of dibromoacetonitrile (DBAN), a common disinfectant by-product found in many water supply systems. DBAN is not mutagenic but causes DNA breaks and elevates sister chromatid exchange in mammalian cells. The WHO issued guidelines for DBAN after it was linked with cancer of the liver and stomach in rodents. How this haloacetonitrile promotes malignant cell transformation is unknown. Using fission yeast as a model, we report here that DBAN delays G1-S transition. DBAN does not hinder ongoing DNA replication, but specifically blocks the serine 345 phosphorylation of the DNA damage checkpoint kinase Chk1 by Rad3 (ATR) at broken replication forks. DBAN is particularly damaging for cells with defects in the lagging-strand DNA polymerase delta. This sensitivity can be explained by the dependency of pol delta mutants on Chk1 activation for survival. We conclude that DBAN targets a process or protein that acts at the start of S phase and is required for Chk1 phosphorylation. Taken together, DBAN may precipitate cancer by perturbing S phase and by blocking the Chk1-dependent response to replication fork damage.

Downregulation of microRNA-637 Increases Risk of Hypoxia-Induced Pulmonary Hypertension by Modulating Expression of Cyclin Dependent Kinase 6 (CDK6) in Pulmonary Smooth Muscle Cells

BACKGROUND The objective of this study was to investigate the molecular mechanism by which miR-637 interferes with the expression of CDK6, which contributes to the development of pulmonary hypertension (PH) with chronic obstructive pulmonary disease (COPD). MATERIAL AND METHODS We used an online miRNA database to identify CDK6 as a virtual target of miR-637, and validated the hypothesis using luciferase assay. Furthermore, we transfected SMCs with miR-637 mimics and inhibitor, and expression of CDK6 was determined using Western blot and real-time PCR. RESULTS In this study, we identified CDK6 as a target of miR-637 in smooth muscle cells (SMCs), and determined the expression of miR-637 in SMCs from PH patients with COPD and normal controls. We also identified the exact miR-637 binding site in the 3'UTR of CDK6 by using a luciferase reporter system. The mRNA and protein expression levels of CDK6 in SMCs from PH patients with COPD were clearly upregulated compared with the normal controls. Cells exposed to hypoxia also showed notably increased CKD6 mRNA and protein expression levels, and when treated with miR-637 or CDK6 siRNA, this increase in CKD6 expression was clearly attenuated. Additionally, cell viability and cell cycle analysis showed that hypoxia markedly increased viability of SMCs by causing an accumulation in S phase, which was relieved by the introduction of miR-637 or CDK6 siRNA. CONCLUSIONS Our study proved that the CDK6 gene is a target of miR-637, and demonstrated the regulatory association between miR-637 and CDK6, suggesting a possible therapeutic target for PH, especially in patients with COPD.