Stabilization of Reversed Replication Forks by Telomerase Drives Telomere Catastrophe.

Telomere maintenance critically depends on the distinct activities of telomerase, which adds telomeric repeats to solve the end replication problem, and RTEL1, which dismantles DNA secondary structures at telomeres to facilitate replisome progression. Here, we establish that reversed replication forks are a pathological substrate for telomerase and the source of telomere catastrophe in Rtel1-/- cells. Inhibiting telomerase recruitment to telomeres, but not its activity, or blocking replication fork reversal through PARP1 inhibition or depleting UBC13 or ZRANB3 prevents the rapid accumulation of dysfunctional telomeres in RTEL1-deficient cells. In this context, we establish that telomerase binding to reversed replication forks inhibits telomere replication, which can be mimicked by preventing replication fork restart through depletion of RECQ1 or PARG. Our results lead us to propose that telomerase inappropriately binds to and inhibits restart of reversed replication forks within telomeres, which compromises replication and leads to critically short telomeres.

CDK phosphorylation of TRF2 controls t-loop dynamics during the cell cycle.

The protection of telomere ends by the shelterin complex prevents DNA damage signalling and promiscuous repair at chromosome ends. Evidence suggests that the 3' single-stranded telomere end can assemble into a lasso-like t-loop configuration1,2, which has been proposed to safeguard chromosome ends from being recognized as DNA double-strand breaks2. Mechanisms must also exist to transiently disassemble t-loops to allow accurate telomere replication and to permit telomerase access to the 3' end to solve the end-replication problem. However, the regulation and physiological importance of t-loops in the protection of telomere ends remains unknown. Here we identify a CDK phosphorylation site in the shelterin subunit at Ser365 of TRF2, whose dephosphorylation in S phase by the PP6R3 phosphatase provides a narrow window during which the RTEL1 helicase can transiently access and unwind t-loops to facilitate telomere replication. Re-phosphorylation of TRF2 at Ser365 outside of S phase is required to release RTEL1 from telomeres, which not only protects t-loops from promiscuous unwinding and inappropriate activation of ATM, but also counteracts replication conflicts at DNA secondary structures that arise within telomeres and across the genome. Hence, a phospho-switch in TRF2 coordinates the assembly and disassembly of t-loops during the cell cycle, which protects telomeres from replication stress and an unscheduled DNA damage response.

Inference of protein kinetics by stochastic modeling and simulation of fluorescence recovery after photobleaching experiments.

MOTIVATION: Fluorescence recovery after photobleaching (FRAP) is a functional live cell imaging technique that permits the exploration of protein dynamics in living cells. To extract kinetic parameters from FRAP data, a number of analytical models have been developed. Simplifications are inherent in these models, which may lead to inexhaustive or inaccurate exploitation of the experimental data. An appealing alternative is offered by the simulation of biological processes in realistic environments at a particle level. However, inference of kinetic parameters using simulation-based models is still limited. RESULTS: We introduce and demonstrate a new method for the inference of kinetic parameter values from FRAP data. A small number of in silico FRAP experiments is used to construct a mapping from FRAP recovery curves to the parameters of the underlying protein kinetics. Parameter estimates from experimental data can then be computed by applying the mapping to the observed recovery curves. A bootstrap process is used to investigate identifiability of the physical parameters and determine confidence regions for their estimates. Our method circumvents the computational burden of seeking the best-fitting parameters via iterative simulation. After validation on synthetic data, the method is applied to the analysis of the nuclear proteins Cdt1, PCNA and GFPnls. Parameter estimation results from several experimental samples are in accordance with previous findings, but also allow us to discuss identifiability issues as well as cell-to-cell variability of the protein kinetics. IMPLEMENTATION: All methods were implemented in MATLAB R2011b. Monte Carlo simulations were run on the HPC cluster Brutus of ETH Zurich. CONTACT: lygeros@control.ee.ethz.ch or lygerou@med.upatras.gr SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

Multi-step loading of human minichromosome maintenance proteins in live human cells.

Once-per-cell cycle replication is regulated through the assembly onto chromatin of multisubunit protein complexes that license DNA for a further round of replication. Licensing consists of the loading of the hexameric MCM2-7 complex onto chromatin during G1 phase and is dependent on the licensing factor Cdt1. In vitro experiments have suggested a two-step binding mode for minichromosome maintenance (MCM) proteins, with transient initial interactions converted to stable chromatin loading. Here, we assess MCM loading in live human cells using an in vivo licensing assay on the basis of fluorescence recovery after photobleaching of GFP-tagged MCM protein subunits through the cell cycle. We show that, in telophase, MCM2 and MCM4 maintain transient interactions with chromatin, exhibiting kinetics similar to Cdt1. These are converted to stable interactions from early G1 phase. The immobile fraction of MCM2 and MCM4 increases during G1 phase, suggestive of reiterative licensing. In late G1 phase, a large fraction of MCM proteins are loaded onto chromatin, with maximal licensing observed just prior to S phase onset. Fluorescence loss in photobleaching experiments show subnuclear concentrations of MCM-chromatin interactions that differ as G1 phase progresses and do not colocalize with sites of DNA synthesis in S phase.

Evergene: an interactive webtool for large-scale gene-centric analysis of primary tumours.

Motivation: The data sharing of large comprehensive cancer research projects, such as The Cancer Genome Atlas (TCGA), has improved the availability of high-quality data to research labs around the world. However, due to the volume and inherent complexity of high-throughput omics data, analysis of this is limited by the capacity for performing data processing through programming languages such as R or Python. Existing webtools lack functionality that supports large-scale analysis; typically, users can only input one gene, or a gene list condensed into a gene set, instead of individual gene-level analysis. Furthermore, analysis results are usually displayed without other sample-level molecular or clinical annotations. To address these gaps in the existing webtools, we have developed Evergene using R and Shiny. Results: Evergene is a user-friendly webtool that utilizes RNA-sequencing data, alongside other sample and clinical annotation, for large-scale gene-centric analysis, including principal component analysis (PCA), survival analysis (SA), and correlation analysis (CA). Moreover, Evergene achieves in-depth analysis of cancer transcriptomic data which can be explored through dimensional reduction methods, relating gene expression with clinical events or other sample information, such as ethnicity, histological classification, and molecular indices. Lastly, users can upload custom data to Evergene for analysis. Availability and implementation: Evergene webtool is available at https://bshihlab.shinyapps.io/evergene/. The source code and example user input dataset are available at https://github.com/bshihlab/evergene.

Dynamic recruitment of licensing factor Cdt1 to sites of DNA damage.

For genomic integrity to be maintained, the cell cycle and DNA damage responses must be linked. Cdt1, a G1-specific cell-cycle factor, is targeted for proteolysis by the Cul4-Ddb1(Cdt2) ubiquitin ligase following DNA damage. Using a laser nanosurgery microscope to generate spatially restricted DNA damage within the living cell nucleus, we show that Cdt1 is recruited onto damaged sites in G1 phase cells, within seconds of DNA damage induction. PCNA, Cdt2, Cul4, DDB1 and p21(Cip1) also accumulate rapidly to damaged sites. Cdt1 recruitment is PCNA-dependent, whereas PCNA and Cdt2 recruitment are independent of Cdt1. Fitting of fluorescence recovery after photobleaching profiles to an analytic reaction-diffusion model shows that Cdt1 and p21(Cip1) exhibit highly dynamic binding at the site of damage, whereas PCNA appears immobile. Cdt2 exhibits both a rapidly exchanging and an apparently immobile subpopulation. Our data suggest that PCNA provides an immobile binding interface for dynamic Cdt1 interactions at the site of damage, which leads to rapid Cdt1 recruitment to damaged DNA, preceding Cdt1 degradation.

easyFRAP: an interactive, easy-to-use tool for qualitative and quantitative analysis of FRAP data.

SUMMARY: We present easyFRAP, a versatile tool that assists quantitative and qualitative analysis of fluorescence recovery after photobleaching (FRAP) data. The user can handle simultaneously large data sets of raw data, visualize fluorescence recovery curves, exclude low quality data, perform data normalization, extract quantitative parameters, perform batch analysis and save the resulting data and figures for further use. Our tool is implemented as a single-screen Graphical User Interface (GUI) and is highly interactive, as it permits parameterization and visual data quality assessment at various points during the analysis. AVAILABILITY: easyFRAP is free software, available under the General Public License (GPL). Executable and source files, supplementary material and sample data sets can be downloaded at: ccl.med.upatras.gr/easyfrap.html.

Interaction of endothelin-1 and nitric oxide pathways in human tubular epithelial cells under the influence of cyclosporine-A.

BACKGROUND: The exact mechanism of cyclosporine (CsA) nephrotoxicity has not been clarified. In this study, we investigated the effect of pharmacological doses of CsA on the production of nitric oxide synthases (NOSs) and endothelin (ET) receptors (ETR-A, ETR-B), in human tubular cells [human kidney (HK)-2], to identify any implication of these pathways in CsA nephrotoxicity. METHODS: Human tubular epithelial cells (HK-2) were cultured in the presence of CsA at various concentrations (0-1000 ng/mL). Reverse transcriptase-polymerase chain reaction (RT-PCR) was used to determine mRNA synthesis of NOSs (eNOS, iNOS) and ET receptors (ETR-A, ETR-B) and western blot analysis for the subsequent proteins. RESULTS: A dose-dependent induction of synthesis of NO synthases eNOS and iNOS and ET receptors ETR-A and ETR-B was observed, even at therapeutic doses of CsA. An interaction between NO and ET-1 systems under the influence of CsA was also observed. Blockage of NO production was followed by down-regulation of ETR-B whereas blockade of ET pathway with ET receptor antagonists was followed by down-regulation of eNOS expression. CONCLUSION: CsA induces NOSs as well as ET receptor mRNA and protein synthesis in tubular epithelial cells. The up-regulation of NO and ET-1 pathways is probably implicated in the nephrotoxic action of CsA, whereas an interplay between ETR-B and eNOS seems to be involved.

RTEL1 Regulates G4/R-Loops to Avert Replication-Transcription Collisions.

Regulator of telomere length 1 (RTEL1) is an essential helicase that maintains telomere integrity and facilitates DNA replication. The source of replication stress in Rtel1-deficient cells remains unclear. Here, we report that loss of RTEL1 confers extensive transcriptional changes independent of its roles at telomeres. The majority of affected genes in Rtel1-/- cells possess G-quadruplex (G4)-DNA-forming sequences in their promoters and are similarly altered at a transcriptional level in wild-type cells treated with the G4-DNA stabilizer TMPyP4 (5,10,15,20-Tetrakis-(N-methyl-4-pyridyl)porphine). Failure to resolve G4-DNAs formed in the displaced strand of RNA-DNA hybrids in Rtel1-/- cells is suggested by increased R-loops and elevated transcription-replication collisions (TRCs). Moreover, removal of R-loops by RNaseH1 overexpression suppresses TRCs and alleviates the global replication defects observed in Rtel1-/- and Rtel1PIP_box knockin cells and in wild-type cells treated with TMPyP4. We propose that RTEL1 unwinds G4-DNA/R-loops to avert TRCs, which is important to prevent global deregulation in both transcription and DNA replication.

Cyclosporine induces endothelin-1 mRNA synthesis and nitric oxide production in human proximal tubular epithelial cell cultures.

BACKGROUND: Cyclosporine (CsA) is implicated in the development of chronic allograft nephropathy, which is related to reduced long-term allograft survival. The activation of tubular epithelial cells is involved in the renal scarring process via stimulation of factors such as endothelin-1 (ET-1) and nitric oxide (NO). The effect of CsA on the activation of tubular epithelial cells towards increased production of ET-1 and NO was investigated in this study. METHODS: Human tubular epithelial cells (HK-2) were cultured in the presence of CsA at different concentrations (125, 250, 500, and 1,000 ng/mL). ET-1 m-RNA and NO production were measured using RT-PCR and Griess method, respectively. The cytotoxic effect of CsA was examined by the MTT method and cell count. RESULTS: A statistically significant and dose-dependent cytotoxic effect of cyclosporine on HK-2 cells was observed. A dose-dependent up-regulation of ET-1 mRNA production and NO accumulation was observed under the influence of CsA. CONCLUSION: Increased synthesis of endothelin-1 mRNA and nitric oxide as well as a significant cytotoxic effect on tubular epithelial cells under the influence of CsA might be related to the development of CsA nephrotoxicity.

Elk-1 associates with FAK, regulates the expression of FAK and MAP kinases as well as apoptosis in HK-2 cells.

Focal adhesion kinase (FAK), MAP kinases and the nuclear transcription factor Elk-1 have been reported to be implicated in the same cellular processes, however, their direct or indirect interaction and potential function(s) has not been documented. Here, we explored the association of FAK with Elk-1, the implication of Elk-1 in the regulation of FAK and MAP kinases expression as well as apoptosis, in HK-2 cells. Biochemical and immunofluorescence approaches strongly support the association of low molecular weight protein bands, recognized by FAK antibodies, with Elk-1 or p(ser383)Elk-1. The FAK/Elk-1 complex is found, mainly, in the cytoplasm, near the nuclear membrane periphery, raising the possibility that Elk-1 may have alternative extranuclear function(s) in HK-2 cells. Furthermore, we demonstrated that Elk-1 siRNA-mediated knockdown experiments, increased apoptosis. By contrast, Elk-1 siRNA decreased significantly the expression of FAK and MAP kinases, supporting the hypothesis that Elk-1 may act as a potential physiological substrate and regulator of FAK and MAP kinases expression. These results strongly support that Elk-1 protein is a novel binding-protein partner for FAK, a finding that significantly broadens the potential functioning of FAK and Elk-1.

Mechanisms of Oncogene-Induced Replication Stress: Jigsaw Falling into Place.

Oncogene activation disturbs cellular processes and accommodates a complex landscape of changes in the genome that contribute to genomic instability, which accelerates mutation rates and promotes tumorigenesis. Part of this cellular turmoil involves deregulation of physiologic DNA replication, widely described as replication stress. Oncogene-induced replication stress is an early driver of genomic instability and is attributed to a plethora of factors, most notably aberrant origin firing, replication-transcription collisions, reactive oxygen species, and defective nucleotide metabolism.Significance: Replication stress is a fundamental step and an early driver of tumorigenesis and has been associated with many activated oncogenes. Deciphering the mechanisms that contribute to the replication stress response may provide new avenues for targeted cancer treatment. In this review, we discuss the latest findings on the DNA replication stress response and examine the various mechanisms through which activated oncogenes induce replication stress. Cancer Discov; 8(5); 537-55. ©2018 AACR.

Increased global transcription activity as a mechanism of replication stress in cancer.

Cancer is a disease associated with genomic instability that often results from oncogene activation. This in turn leads to hyperproliferation and replication stress. However, the molecular mechanisms that underlie oncogene-induced replication stress are still poorly understood. Oncogenes such as HRASV12 promote proliferation by upregulating general transcription factors to stimulate RNA synthesis. Here we investigate whether this increase in transcription underlies oncogene-induced replication stress. We show that in cells overexpressing HRASV12, elevated expression of the general transcription factor TATA-box binding protein (TBP) leads to increased RNA synthesis, which together with R-loop accumulation results in replication fork slowing and DNA damage. Furthermore, overexpression of TBP alone causes the hallmarks of oncogene-induced replication stress, including replication fork slowing, DNA damage and senescence. Consequently, we reveal that increased transcription can be a mechanism of oncogene-induced DNA damage, providing a molecular link between upregulation of the transcription machinery and genomic instability in cancer.

Cancer therapy and replication stress: forks on the road to perdition.

Deregulated DNA replication occurs in cancer where it contributes to genomic instability. This process is a target of cytotoxic therapies. Chemotherapies exploit high DNA replication in cancer cells by modifying the DNA template or by inhibiting vital enzymatic activities that lead to slowing or stalling replication fork progression. Stalled replication forks can be converted into toxic DNA double-strand breaks resulting in cell death, i.e., replication stress. While likely crucial for many cancer treatments, replication stress is poorly understood due to its complexity. While we still know relatively little about the role of replication stress in cancer therapy, technical advances in recent years have shed new light on the effect that cancer therapeutics have on replication forks and the molecular mechanisms that lead from obstructed fork progression to cell death. This chapter will give an overview of our current understanding of replication stress in the context of cancer therapy.

BRCA2 and RAD51 promote double-strand break formation and cell death in response to gemcitabine.

Replication inhibitors cause replication fork stalling and double-strand breaks (DSB) that result from processing of stalled forks. During recovery from replication blocks, the homologous recombination (HR) factor RAD51 mediates fork restart and DSB repair. HR defects therefore sensitize cells to replication inhibitors, with clear implications for cancer therapy. Gemcitabine is a potent replication inhibitor used to treat cancers with mutations in HR genes such as BRCA2. Here, we investigate why, paradoxically, mutations in HR genes protect cells from killing by gemcitabine. Using DNA replication and DNA damage assays in mammalian cells, we show that even short gemcitabine treatments cause persistent replication inhibition. BRCA2 and RAD51 are recruited to chromatin early after removal of the drug, actively inhibit replication fork progression, and promote the formation of MUS81- and XPF-dependent DSBs that remain unrepaired. Our data suggest that HR intermediates formed at gemcitabine-stalled forks are converted into DSBs and thus contribute to gemcitabine-induced cell death, which could have implications for the treatment response of HR-deficient tumors.

Albumin upregulates eNOS mRNA through ETRA/B in human proximal tubular epithelial cells.

BACKGROUND: Proximal tubular cells respond to proteinuria by expressing several cytokines and inflammatory molecules that induce interstitial fibrosis. Increased attention has been drawn toward the systems of endothelin (ET) and nitric oxide (NO). This work contributes to the elucidation of the interplay between these two systems in proximal tubular epithelial cells (PTECs) after exposure in proteinuric conditions. METHODS: HK-2 cells, a human PTEC line, were incubated with albumin, simulating proteinuric conditions. Cells were then lysed and either total RNA was isolated or whole cell extracts were prepared. PreproET-1, ET receptors (ETRA and ETRB) and NO synthases (eNOS, iNOS) mRNA accumulation was estimated by RT-PCR, and proteins by Western blot analysis. NO production was assessed using Griess reaction. Furthermore, we treated HK-2 cells with NO donor sodium nitroprusside, NO inhibitor L-NAME, ETRA inhibitor BQ123, ETRB inhibitor BQ788 and purified ET-1, and investigated the potential interplay between albumin-induced stimulation of NO or ET-1 systems. RESULTS: We found that albumin upregulates preproET-1, ETRA, ETRB, eNOS and iNOS mRNA as well as protein and stimulates NO production. Additionally, we recorded an ETRA/B dependent regulation of albumin-induced eNOS expression. CONCLUSIONS: For the first time an in vitro albumin-induced ET-1 and NO interplay was revealed.

ARF triggers senescence in Brca2-deficient cells by altering the spectrum of p53 transcriptional targets.

ARF is a tumour suppressor activated by oncogenic stress, which stabilizes p53. Although p53 is a key component of the response to DNA damage, a similar function for ARF has not been ascribed. Here we show that primary mouse and human cells lacking the tumour suppressor BRCA2 accumulate DNA damage, which triggers checkpoint signalling and ARF activation. Furthermore, senescence induced by Brca2 deletion in primary mouse and human cells is reversed by the loss of ARF, a phenotype recapitulated in cells lacking RAD51. Surprisingly, ARF is not necessary for p53 accumulation per se but for altering the spectrum of genes activated by this transcription factor. Specifically, ARF enables p53 transcription of Dusp4 and Dusp7, which encode a pair of phosphatases known to inactivate the MAP kinases ERK1/2. Our results ascribe a previously unanticipated function to the ARF tumour suppressor in genome integrity, controlled by replicative stress and ATM/ATR-dependent checkpoint responses.

Cdt1 is differentially targeted for degradation by anticancer chemotherapeutic drugs.

BACKGROUND: Maintenance of genome integrity is crucial for the propagation of the genetic information. Cdt1 is a major component of the pre-replicative complex, which controls once per cell cycle DNA replication. Upon DNA damage, Cdt1 is rapidly targeted for degradation. This targeting has been suggested to safeguard genomic integrity and prevent re-replication while DNA repair is in progress. Cdt1 is deregulated in tumor specimens, while its aberrant expression is linked with aneuploidy and promotes tumorigenesis in animal models. The induction of lesions in DNA is a common mechanism by which many cytotoxic anticancer agents operate, leading to cell cycle arrest and apoptosis. METHODOLOGY/PRINCIPAL FINDING: In the present study we examine the ability of several anticancer drugs to target Cdt1 for degradation. We show that treatment of HeLa and HepG2 cells with MMS, Cisplatin and Doxorubicin lead to rapid proteolysis of Cdt1, whereas treatment with 5-Fluorouracil and Tamoxifen leave Cdt1 expression unaffected. Etoposide affects Cdt1 stability in HepG2 cells and not in HeLa cells. RNAi experiments suggest that Cdt1 proteolysis in response to MMS depends on the presence of the sliding clamp PCNA. CONCLUSION/SIGNIFICANCE: Our data suggest that treatment of tumor cells with commonly used chemotherapeutic agents induces differential responses with respect to Cdt1 proteolysis. Information on specific cellular targets in response to distinct anticancer chemotherapeutic drugs in different cancer cell types may contribute to the optimization of the efficacy of chemotherapy.