ATM/ATR checkpoint activation downregulates CDC25C to prevent mitotic entry with uncapped telomeres

Broussoflavonol B induces S-phase arrest and apoptosis in pancreatic cancer cells by modulating the cell cycle checkpoint through inhibition of the AURKA/PLK1 pathway

BACKGROUND

Broussoflavonol B (Bf-B), a flavonoid compound identified in the roots of Daphne giraldii Nitsche, has been extensively investigated for its potential anti-inflammatory, antioxidant, and anticancer properties. However, the precise mechanism underlying the regulation of AURKA/PLK1 pathway-mediated cell cycle arrest by Bf-B in pancreatic cancer remains poorly understood.

PURPOSE

The objective of this study was to investigate the inhibitory effect of Bf-B on pancreatic ductal adenocarcinoma (PDAC) and its underlying mechanism.

METHODS

A CCK8 assay was conducted to identify the flavonoids with the highest inhibitory activity against PANC-1, the pancreatic cancer cell line among the 25 flavonoids. Through bioinformatics analysis and molecular docking, the pathogenic targets of pancreatic cancer and flavonoid-related targets were explored, and the key targets and signaling pathways of drug intervention in pancreatic cancer were analyzed. The viability and migration ability of pancreatic cancer cells were assessed following treatment with Bf-B via the CCK8, colony formation, and wound healing assays. The cell cycle distribution and cell apoptosis were analyzed through flow cytometry and Hoechst staining. Western blotting and qPCR were employed to investigate the expression of relevant proteins and genes. For in vivo experiments, we employed a xenograft mouse model to evaluate the anticancer efficacy of Bf-B. Immunohistochemistry and immunofluorescence assays were employed to investigate the expression of relevant proteins.

RESULTS

In this study, the structure‒activity relationships of 25 flavonoids were evaluated. The results demonstrated that Bf-B with diisopentenyl has potent cytotoxic effects on PANC-1 cancer cells. AURKA, PLK1, and MET might serve as key targets for Bf-B inhibition of disease progression in PDAC patients. The results demonstrated that Bf-B inhibits the proliferation and migration of PANC-1 and BXPC-3 cells and induces cell cycle S-phase arrest, apoptosis, and DNA damage. Moreover, the results of western blot and qPCR experiments indicated that Bf-B exerts anticancer effects by downregulating the expression of the genes encoding AURKA/PLK1, the cell cycle checkpoint kinase ATR/CHK1/CDC25C, and Cyclin B1/CDK1 signaling pathway-related proteins and upregulating the expression of PP53, P21, and histone H2A. XS139ph expression. In xenograft-bearing mice, AURKA/PLK1 expression was reduced in a dose-dependent manner, accompanied by an increase in histone H2A. XS139ph expression.

CONCLUSION

Bf-B might be a potent therapeutic agent for pancreatic cancer because of its ability to suppress the expression of AURKA/PLK1.

A NRAS mRNA G-quadruplex structure-targeting small-molecule ligand reactivating DNA damage response in human cancer cells for combination therapy with clinical PI3K inhibitors

The Neuroblastoma RAS (NRAS) oncogene homologue plays crucial roles in diverse cellular processes such as cell proliferation, survival, and differentiation. Several strategies have been developed to inhibit NRAS or its downstream effectors; however, there is no effective drug available to treat NRAS-driven cancers and thus new approaches are needed to be established. The mRNA sequence expressing NRAS containing several guanine(G)-rich regions may form quadruplex structures (G4s) and regulate NRAS translation. Therefore, targeting NRAS mRNA G4s to repress NRAS expression at translational level with ligands may be a feasible strategy against NRAS-driven cancers but it is underexplored. We reported herein a NRAS mRNA G4-targeting ligand, B3C, specifically localized in cytoplasm in HeLa cells. It effectively downregulates NRAS proteins, reactivates the DNA damage response (DDR), causes cell cycle arrest in G2/M phase, and induces apoptosis and senescence. Moreover, combination therapy with NARS mRNA G4-targeting ligands and clinical PI3K inhibitors for cancer cells inhibition treatment is unexplored, and we demonstrated that B3C combining with PI3Ki (pictilisib (GDC-0941)) showed potent antiproliferation activity against HeLa cells (IC50 = 1.03 μM (combined with 10 μM PI3Ki) and 0.42 μM (combined with 20 μM PI3Ki)) and exhibited strong synergistic effects in inhibiting cell proliferation. This study provides new insights into drug discovery against RAS-driven cancers using this conceptually new combination therapy strategy.

Small Molecular Inhibitors That Target ATM for Drug Discovery: Current Research and Potential Prospective

The protein kinase ataxia telangiectasia mutated (ATM) is a constituent of the phosphatidylinositol 3-kinase-related kinase (PIKK) family, exerting a pivotal influence on diverse cellular processes, notably the signaling of double-strand DNA breaks (DSB) and stress response. The dysregulation of ATM is implicated in the pathogenesis of cancer and other diseases such as neurodegeneration. Hence, ATM is deemed a promising candidate for potential therapeutic interventions across a spectrum of diseases. Presently, while ATM small molecule inhibitors are not commercially available, various selective inhibitors have progressed to the clinical research phase. Specifically, AZD1390, WSD0628, SYH2051, and ZN-B-2262 are under investigation in clinical studies pertaining to glioblastoma multiforme and advanced solid tumors, respectively. In this Perspective, we encapsulate the structure, biological functions, and disease relevance of ATM. Subsequently, we concentrate on the design concepts and structure-activity relationships (SAR) of ATM inhibitors, delineating potential avenues for the development of more efficacious ATM-targeted inhibitors.

Polyploid giant cancer cells: origin, possible pathways of formation, characteristics, and mechanisms of regulation

Polyploid giant cancer cells (PGCCs) are characterized by the presence of either a single enlarged nucleus or multiple nuclei and are closely associated with tumor progression and treatment resistance. These cells contribute significantly to cellular heterogeneity and can arise from various stressors, including radiation, chemotherapy, hypoxia, and environmental factors. The formation of PGCCs can occur through mechanisms such as endoreplication, cell fusion, cytokinesis failure, mitotic slippage, or cell cannibalism. Notably, PGCCs exhibit traits similar to cancer stem cells (CSCs) and generate highly invasive progeny through asymmetric division. The presence of PGCCs and their progeny is pivotal in conferring resistance to chemotherapy and radiation, as well as facilitating tumor recurrence and metastasis. This review provides a comprehensive analysis of the origins, potential formation mechanisms, stressors, unique characteristics, and regulatory pathways of PGCCs, alongside therapeutic strategies targeting these cells. The objective is to enhance the understanding of PGCC initiation and progression, offering novel insights into tumor biology.

Cannabinol Regulates the Expression of Cell Cycle-Associated Genes in Motor Neuron-like NSC-34: A Transcriptomic Analysis

Cannabinoids are reported to have neuroprotective properties and play a role in neurogenesis and neuroplasticity in in vitro and in vivo models. Cannabinol (CBN) is a minor cannabinoid produced by the degradation of Δ9-tetrahydrocannabinol in Cannabis sativa L. and exhibits anti-oxidant, analgesic, anti-bacterial, and anti-inflammatory effects. In this study, we explored the biological effects of 20 µM CBN (6.20 µg/mL) on differentiated NSC-34 cells by MTT assay and next-generation sequencing analysis on the transcriptome. KEGG and Gene Ontology enrichment analyses have been performed to evaluate potential CBN-associated processes. Our results highlighted the absence of any cytotoxic effect of CBN. The comparative transcriptomic analysis pointed out the downregulation of Cdkn2a, Cdkn2c and Cdkn2d genes, which are known to suppress the cell cycle. Ccne2, Cdk2, Cdk7, Anapc11, Anapc10, Cdc23, Cdc16, Anapc4, Cdc27, Stag1, Smc3, Smc1a, Nipbl, Pds5a, Pds5b, and Wapl genes, renowned for their role as cell cycle progression activators, were instead upregulated. Our work suggests that CBN regulates the expression of many genes related to the cell cycle, which are required for axonal maturation, migration, and synaptic plasticity, while not affecting the expression of genes involved in cell death or tumorigenesis.

The p53/ZEB1-PLD3 feedback loop regulates cell proliferation in breast cancer

Breast cancer is the most prevalent cancer globally, endangering women's physical and mental health. Phospholipase D3 (PLD3) belongs to the phosphodiesterase family (PLD). PLD3 is related to insulin-mediated phosphorylation of the AKT pathway, suggesting that it may play a role in the occurrence and development of malignant tumors. This study may further explore the molecular mechanism of PLD3 inhibiting breast cancer cell proliferation. In this study, we demonstrated that PLD3 and miR-6796 are co-expressed in breast cancer. PLD3 can bind with CDK1 and inhibit its expression, leading to mitotic arrest and inhibiting breast cancer proliferation. Wild-type p53 regulates PLD3 and miR-6796 expression by competitively binding to the PLD3 promoter with ZEB1. DNMT3B, as the target gene of miR-6796, is recruited into the PLD3 promoter by combining with ZEB1 to regulate the DNA methylation of the PLD3 promoter and ultimately affect PLD3 and miR-6796 expression. In conclusion, we revealed the role and molecular mechanism of PLD3 and its embedded miR-6796 in breast cancer proliferation, providing clues and a theoretical foundation for future research and development of therapeutic targets and prognostic markers for breast cancer.

Loss of TDP-43 function contributes to genomic instability in amyotrophic lateral sclerosis

A common pathological hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) is the cytoplasmic mislocalization and aggregation of the DNA/RNA-binding protein TDP-43, but how loss of nuclear TDP-43 function contributes to ALS and FTD pathogenesis remains largely unknown. Here, using large-scale RNAi screening, we identify TARDBP, which encodes TDP-43, as a gene whose loss-of-function results in elevated DNA mutation rate and genomic instability. Consistent with this finding, we observe increased DNA damage in induced pluripotent stem cells (iPSCs) and iPSC-derived post-mitotic neurons generated from ALS patients harboring TARDBP mutations. We find that the increase in DNA damage in ALS iPSC-derived neurons is due to defects in two major pathways for DNA double-strand break repair: non-homologous end joining and homologous recombination. Cells with defects in DNA repair are sensitive to DNA damaging agents and, accordingly, we find that ALS iPSC-derived neurons show a marked reduction in survival following treatment with a DNA damaging agent. Importantly, we find that increased DNA damage is also observed in neurons with nuclear TDP-43 depletion from ALS/FTD patient brain tissues. Collectively, our results demonstrate that ALS neurons with loss of nuclear TDP-43 function have elevated levels of DNA damage and contribute to the idea that genomic instability is a defining pathological feature of ALS/FTD patients with TDP-43 pathology.

USP8 inhibitor-induced DNA damage activates cell cycle arrest, apoptosis, and autophagy in esophageal squamous cell carcinoma

Increasing evidence suggests that targeting ubiquitin-specific peptidase 8 (USP8) serves as an attractive anti-cancer strategy. However, the role of USP8 inhibitor, DUB-IN-1, in esophageal squamous cell carcinoma (ESCC) cells still needs to be explored. Here, immunohistochemistry was employed to examine the expression of USP8 in ESCC tissues. Cell Counting Kit-8 (CCK-8) was used to evaluate cell proliferation ability, and propidium iodide (PI) was selected to test the effect of DUB-IN-1 on cell cycle. AnnexinV-FITC/PI staining and the activity of caspase 3 were detedcted to evaluate apoptosis. Transmission electron microscope, microtubule-associated protein 1 light-chain 3 (LC3) expression, and acridine orange (AO) staining were selected to check if there was autophagy. Comet assay and γ-H2AX immunofluorescence was used to monitor DNA damage. Rescue experiment was used to determine the key role of of p53 in cell cycle, apoptosis, and autophagy. Results revealed that the leve of USP8 was higher in ESCC tissues than that in tissues adjacent to carcinoma. DUB-IN-1, an USP8 inhibitor, caused DNA damage, led to G2/M phase block by p53-p21 axis, and triggered apoptosis by regulating the p53 target proteins including Bax, Noxa, and Puma. Besides, DUB-IN-1 could stimulate autophagy through p53-dependent adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) activation. Taken together, this study revealed the cytotoxic effects and the mechanism of DUB-IN-1, which indicated that DUB-IN-1 may be a novel inhibitor targeting USP8 that can kill ESCC cells. USP8 inhibitor, DUB-IN-1, treatment could inhibit esophageal squamous cell carcinoma cell growth and induce G2/M cell cycle arrest, apoptosis, and autophagy by DNA damage-induced p53 activation. DUB-IN-1 treatment led to G2/M cell cycle arrest by upregulating the protein level of p21 and triggered apoptosis by modulating the p53 target proteins including Bax, Noxa, and Puma. Meanwhile, DUB-IN-1 treatment stimulated protective autophagy through p53-dependent AMPK activation. Collectively, these findings suggested that DNA damage-triggered p53 activation, p53-Puma/Noxa/Bax, p53-p21, and p53-AMPK pathways were all involved in the effect of DUB-IN-1.

Review of possible mechanisms of radiotherapy resistance in cervical cancer

Radiotherapy is one of the main treatments for cervical cancer. Early cervical cancer is usually considered postoperative radiotherapy alone. Radiotherapy combined with cisplatin is the standard treatment for locally advanced cervical cancer (LACC), but sometimes the disease will relapse within a short time after the end of treatment. Tumor recurrence is usually related to the inherent radiation resistance of the tumor, mainly involving cell proliferation, apoptosis, DNA repair, tumor microenvironment, tumor metabolism, and stem cells. In the past few decades, the mechanism of radiotherapy resistance of cervical cancer has been extensively studied, but due to its complex process, the specific mechanism of radiotherapy resistance of cervical cancer is still not fully understood. In this review, we discuss the current status of radiotherapy resistance in cervical cancer and the possible mechanisms of radiotherapy resistance, and provide favorable therapeutic targets for improving radiotherapy sensitivity. In conclusion, this article describes the importance of understanding the pathway and target of radioresistance for cervical cancer to promote the development of effective radiotherapy sensitizers.

Phloretin-induced STAT3 inhibition suppresses pancreatic cancer growth and progression via enhancing Nrf2 activity

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is a malignant pancreatic tumor charactered by a rapid progression and high lethal rate. Hyperactivation of STAT3 signaling exerts a vital effect on the growth and progression of PDAC. While dietary flavonoid phloretin has anti-inflammatory and antioxidant activities, it remains unclear whether phloretin has anti-tumor effects on PDAC.

PURPOSE

The focus of the present study is to elucidate the effects of phloretin on PDAC and investigate its underlying molecular mechanisms.

STUDY DESIGN AND METHODS

Effect of phloretin were assessed in the pancreatic cancer cells (PCCs) by colony formation assay, real-time cell analysis, flow cytometry, Immunofluorescence staining, and cell migration assay. The expressions of mRNA and protein were respectively analyzed by quantitative PCR and Western blotting. A xenograft model was used to appraise the antitumor efficacy of phloretin.

RESULTS

Phloretin treatment significantly restrained cell viability and metastasis, induced DNA injury and ROS accumulation, and triggered mitochondrial-dependent apoptosis in PCCs. Mechanistically, phloretin exhibits anti-tumor potential via inactivating STAT3 signaling and enhancing Nrf2 activity. STAT3 overexpression and Nrf2 silencing partially relieved phloretin-induced inhibition on cell growth and metastasis in PCCs. Phloretin remarkably blocked pancreatic tumor growth and metastasis in vivo.

CONCLUSIONS

Phloretin suppresses pancreatic cancer growth and progression through inhibition of STAT3 mediated by enhancing Nrf2 activity. Phloretin may serve as a promising therapeutic agent for PDAC.

Stress-induced senescence in mesenchymal stem cells: Triggers, hallmarks, and current rejuvenation approaches

Mesenchymal stem cells (MSCs) have emerged as promising cell-based therapies in the treatment of degenerative and inflammatory conditions. However, despite accumulating evidence of the breadth of MSC functional potency, their broad clinical translation is hampered by inconsistencies in therapeutic efficacy, which is at least partly due to the phenotypic and functional heterogeneity of MSC populations as they progress towards senescence in vitro. MSC senescence, a natural response to aging and stress, gives rise to altered cellular responses and functional decline. This review describes the key regenerative properties of MSCs; summarises the main triggers, mechanisms, and consequences of MSC senescence; and discusses current cellular and extracellular strategies to delay the onset or progression of senescence, or to rejuvenate biological functions lost to senescence.

Effectiveness of Nonfunctionalized Graphene Oxide Nanolayers as Nanomedicine against Colon, Cervical, and Breast Cancer Cells

Recent studies in nanomedicine have intensively explored the prospective applications of surface-tailored graphene oxide (GO) as anticancer entity. However, the efficacy of nonfunctionalized graphene oxide nanolayers (GRO-NLs) as an anticancer agent is less explored. In this study, we report the synthesis of GRO-NLs and their in vitro anticancer potential in breast (MCF-7), colon (HT-29), and cervical (HeLa) cancer cells. GRO-NLs-treated HT-29, HeLa, and MCF-7 cells showed cytotoxicity in the MTT and NRU assays via defects in mitochondrial functions and lysosomal activity. HT-29, HeLa, and MCF-7 cells treated with GRO-NLs exhibited substantial elevations in ROS, disturbances of the mitochondrial membrane potential, an influx of Ca2+, and apoptosis. The qPCR quantification showed the upregulation of caspase 3, caspase 9, bax, and SOD1 genes in GRO-NLs-treated cells. Western blotting showed the depletion of P21, P53, and CDC25C proteins in the above cancer cell lines after GRO-NLs treatment, indicating its function as a mutagen to induce mutation in the P53 gene, thereby affecting P53 protein and downstream effectors P21 and CDC25C. In addition, there may be a mechanism other than P53 mutation that controls P53 dysfunction. We conclude that nonfunctionalized GRO-NLs exhibit prospective biomedical application as a putative anticancer entity against colon, cervical, and breast cancers.

Different impacts of TP53 mutations on cell cycle-related gene expression among cancer types

Functional properties caused by TP53 mutations are involved in cancer development and progression. Although most of the mutations lose normal p53 functions, some of them, gain-of-function (GOF) mutations, exhibiting novel oncogenic functions. No reports have analyzed the impact of TP53 mutations on the gene expression profile of the p53 signaling pathway across cancer types. This study is a cross-cancer type analysis of the effects of TP53 mutations on gene expression. A hierarchical cluster analysis of the expression profile of the p53 signaling pathway classified 21 cancer types into two clusters (A1 and A2). Changes in the expression of cell cycle-related genes and MKI67 by TP53 mutations were greater in cluster A1 than in cluster A2. There was no distinct difference in the effects between GOF and non-GOF mutations on the gene expression profile of the p53 signaling pathway.

Cytotoxic Effects of Darinaparsin, a Novel Organic Arsenical, against Human Leukemia Cells

To explore the molecular mechanisms of action underlying the antileukemia activities of darinaparsin, an organic arsenical approved for the treatment of peripheral T-cell lymphoma in Japan, cytotoxicity of darinaparsin was evaluated in leukemia cell lines NB4, U-937, MOLT-4 and HL-60. Darinaparsin was a more potent cytotoxic than sodium arsenite, and induced apoptosis/necrosis in NB4 and HL-60 cells. In NB4 cells exhibiting the highest susceptibility to darinaparsin, apoptosis induction was accompanied by the activation of caspase-8/-9/-3, a substantial decrease in Bid expression, and was suppressed by Boc-D-FMK, a pancaspase inhibitor, suggesting that darinaparsin triggered a convergence of the extrinsic and intrinsic pathways of apoptosis via Bid truncation. A dramatic increase in the expression level of γH2AX, a DNA damage marker, occurred in parallel with G2/M arrest. Activation of p53 and the inhibition of cdc25C/cyclin B1/cdc2 were concomitantly observed in treated cells. Downregulation of c-Myc, along with inactivation of E2F1 associated with the activation of Rb, was observed, suggesting the critical roles of p53 and c-Myc in darinaparsin-mediated G2/M arrest. Trolox, an antioxidative reagent, suppressed the apoptosis induction but failed to correct G2/M arrest, suggesting that oxidative stress primarily contributed to apoptosis induction. Suppression of Notch1 signaling was also confirmed. Our findings provide novel insights into molecular mechanisms underlying the cytotoxicity of darinaparsin and strong rationale for its new clinical application for patients with different types of cancer.

p53 at the crossroad of DNA replication and ribosome biogenesis stress pathways

Despite several decades of intense research focused on understanding function(s) and disease-associated malfunction of p53, there is no sign of any “mid-life crisis” in this rapidly advancing area of biomedicine. Firmly established as the hub of cellular stress responses and tumor suppressor targeted in most malignancies, p53’s many talents continue to surprise us, providing not only fresh insights into cell and organismal biology, but also new avenues to cancer treatment. Among the most fruitful lines of p53 research in recent years have been the discoveries revealing the multifaceted roles of p53-centered pathways in the fundamental processes of DNA replication and ribosome biogenesis (RiBi), along with cellular responses to replication and RiBi stresses, two intertwined areas of cell (patho)physiology that we discuss in this review. Here, we first provide concise introductory notes on the canonical roles of p53, the key interacting proteins, downstream targets and post-translational modifications involved in p53 regulation. We then highlight the emerging involvement of p53 as a key component of the DNA replication Fork Speed Regulatory Network and the mechanistic links of p53 with cellular checkpoint responses to replication stress (RS), the driving force of cancer-associated genomic instability. Next, the tantalizing, yet still rather foggy functional crosstalk between replication and RiBi (nucleolar) stresses is considered, followed by the more defined involvement of p53-mediated monitoring of the multistep process of RiBi, including the latest updates on the RPL5/RPL11/5 S rRNA-MDM2-p53-mediated Impaired Ribosome Biogenesis Checkpoint (IRBC) pathway and its involvement in tumorigenesis. The diverse defects of RiBi and IRBC that predispose and/or contribute to severe human pathologies including developmental syndromes and cancer are then outlined, along with examples of promising small-molecule-based strategies to therapeutically target the RS- and particularly RiBi- stress-tolerance mechanisms to which cancer cells are addicted due to their aberrant DNA replication, repair, and proteo-synthesis demands.

Crosstalk between Plk1, p53, cell cycle, and G2/M DNA damage checkpoint regulation in cancer: computational modeling and analysis

Different cancer cell lines can have varying responses to the same perturbations or stressful conditions. Cancer cells that have DNA damage checkpoint-related mutations are often more sensitive to gene perturbations including altered Plk1 and p53 activities than cancer cells without these mutations. The perturbations often induce a cell cycle arrest in the former cancer, whereas they only delay the cell cycle progression in the latter cancer. To study crosstalk between Plk1, p53, and G2/M DNA damage checkpoint leading to differential cell cycle regulations, we developed a computational model by extending our recently developed model of mitotic cell cycle and including these key interactions. We have used the model to analyze the cancer cell cycle progression under various gene perturbations including Plk1-depletion conditions. We also analyzed mutations and perturbations in approximately 1800 different cell lines available in the Cancer Dependency Map and grouped lines by genes that are represented in our model. Our model successfully explained phenotypes of various cancer cell lines under different gene perturbations. Several sensitivity analysis approaches were used to identify the range of key parameter values that lead to the cell cycle arrest in cancer cells. Our resulting model can be used to predict the effect of potential treatments targeting key mitotic and DNA damage checkpoint regulators on cell cycle progression of different types of cancer cells.

Polo-like kinase 1 (PLK1) signaling in cancer and beyond

PLK1 is an evolutionary conserved Ser/Thr kinase that is best known for its role in cell cycle regulation and is expressed predominantly during the G2/S and M phase of the cell cycle. PLK1-mediated phosphorylation of specific substrates controls cell entry into mitosis, centrosome maturation, spindle assembly, sister chromatid cohesion and cytokinesis. In addition, a growing body of evidence describes additional roles of PLK1 beyond the cell cycle, more specifically in the DNA damage response, autophagy, apoptosis and cytokine signaling. PLK1 has an indisputable role in cancer as it controls several key transcription factors and promotes cell proliferation, transformation and epithelial-to-mesenchymal transition. Furthermore, deregulation of PLK1 results in chromosome instability and aneuploidy. PLK1 is overexpressed in many cancers, which is associated with poor prognosis, making PLK1 an attractive target for cancer treatment. Additionally, PLK1 is involved in immune and neurological disorders including Graft versus Host Disease, Huntington's disease and Alzheimer's disease. Unfortunately, newly developed small compound PLK1 inhibitors have only had limited success so far, due to low therapeutic response rates and toxicity. In this review we will highlight the current knowledge about the established roles of PLK1 in mitosis regulation and beyond. In addition, we will discuss its tumor promoting but also tumor suppressing capacities, as well as the available PLK1 inhibitors, elaborating on their efficacy and limitations.

Roles of ATM and ATR in DNA double strand breaks and replication stress

The maintenance of genome integrity is critical for the faithful replication of the genome during cell division and for protecting cells from accumulation of DNA damage, which if left unrepaired leads to a loss of genetic information, a breakdown in cell function and ultimately cell death and cancer. ATM and ATR are master kinases that are integral to homologous recombination-mediated repair of double strand breaks and preventing accumulation of dangerous DNA structures and genome instability during replication stress. While the roles of ATM and ATR are heavily intertwined in response to double strand breaks, their roles diverge in the response to replication stress. This review summarises our understanding of the players and their mode of actions in recruitment, activation and activity of ATM and ATR in response to DNA damage and replication stress and discusses how controlling localisation of these kinases and their activators allows them to orchestrate a stress-specific response.

The Connection Between Cell Fate and Telomere

Abolition of telomerase activity results in telomere shortening, a process that eventually destabilizes the ends of chromosomes, leading to genomic instability and cell growth arrest or death. Telomere shortening leads to the attainment of the "Hayflick limit", and the transition of cells to state of senescence. If senescence is bypassed, cells undergo crisis through loss of checkpoints. This process causes massive cell death concomitant with further telomere shortening and spontaneous telomere fusions. In functional telomere of mammalian cells, DNA contains double-stranded tandem repeats of TTAGGG. The Shelterin complex, which is composed of six different proteins, is required for the regulation of telomere length and stability in cells. Telomere protection by telomeric repeat binding protein 2 (TRF2) is dependent on DNA damage response (DDR) inhibition via formation of T-loop structures. Many protein kinases contribute to the DDR activated cell cycle checkpoint pathways, and prevent DNA replication until damaged DNA is repaired. Thereby, the connection between cell fate and telomere length-associated telomerase activity is regulated by multiple protein kinase activities. Contrarily, inactivation of DNA damage checkpoint protein kinases in senescent cells can restore cell-cycle progression into S phase. Therefore, telomere-initiated senescence is a DNA damage checkpoint response that is activated with a direct contribution from dysfunctional telomeres. In this review, in addition to the above mentioned, the choice of main repair pathways, which comprise non-homologous end joining and homologous recombination in telomere uncapping telomere dysfunctions, are discussed.

Inhibition of ATR-Chk1 signaling blocks DNA double-strand-break repair and induces cytoplasmic vacuolization in metastatic osteosarcoma

BACKGROUND

Ataxia-telangiectasia and Rad3 related protein kinase (ATR) is an essential regulator of the DNA damage response in various cancers; however, its expression and roles in osteosarcoma are unclear. We therefore chose to evaluate the significance and mechanism of ATR in metastatic osteosarcoma, as well as its potential to be a therapeutic target.

METHODS

The osteosarcoma tissue microarrays constructed from 70 patient specimens underwent immunohistochemistry to quantify ATR and activated phospho-ATR (pATR) expression and their correlation with clinical outcomes. ATR sublocalization within the metastatic osteosarcoma cells was confirmed by immunofluorescence assay. Cell proliferation, apoptosis, and migration were evaluated following treatment with ATR siRNA or the selective inhibitor Berzosertib. Antitumor effects were determined with ex vivo three-dimensional (3D) culture models, and the impacts on the DNA damage repair pathways were measured with Western blotting.

RESULTS

Elevated ATR and activated pATR expression correlated with shorter patient survival and less necrosis following neoadjuvant chemotherapy. Intranuclear sublocalization of ATR and pATR suggested a mechanism related to DNA replication. ATR knockdown with siRNA or inhibition with Berzosertib suppressed cell proliferation in a time- and dose-dependent manner and induced apoptosis. In addition, ATR inhibition decreased Chk1 phosphorylation while increasing γH2AX expression and PARP cleavage, consistent with the interference of DNA damage repair. The ATR inhibitor Berzosertib also produced the characteristic cytoplasmic vacuolization preceding cell death, and suppressed ex vivo 3D spheroid formation and cell motility.

CONCLUSION

The faithful dependence of cells on ATR signaling for survival and progression makes it an emerging therapeutic target in metastatic osteosarcoma.

Telomerase and telomeres in aging theory and chronographic aging theory (Review)

The current review focuses on the connection of telomerase and telomeres with aging. In this review, we describe the changes in telomerase and telomere length (TEL) during development, their role in carcinogenesis processes, and the consequences of reduced telomerase activity. More specifically, the connection of TEL in peripheral blood cells with the development of aging‑associated diseases is discussed. The review provides systematic data on the role of telomerase in mitochondria, the biology of telomeres in stem cells, as well as the consequences of the forced expression of telomerase (telomerization) in human cells. Additionally, it presents the effects of chronic stress exposure on telomerase activity, the effect of TEL on fertility, and the effect of nutraceutical supplements on TEL. Finally, a comparative review of the chronographic theory of aging, presented by Olovnikov is provided based on currently available scientific research on telomere, telomerase activity, and the nature of aging by multicellular organisms.

PIBF1 suppresses the ATR/CHK1 signaling pathway and promotes proliferation and motility of triple-negative breast cancer cells

PURPOSE

This study evaluates the oncogenic role of PIBF1 in triple-negative breast cancer (TNBC). TNBC is considered to have a poorer prognosis than other types of breast cancer and is associated with high risk of recurrence and distant metastasis. Currently, there are no effective therapies for the TNBC patients with distant metastasis due to the lack of targeted therapeutic options.

METHODS

The effects of PIBF1 knockdown on the cell viability and motility of TNBC cell lines were investigated. Effects of PIBF1 overexpression on tumorigenicity and cell motility were confirmed using Ba/F3 cell line and xenograft study on BALB/c nude mice.

RESULTS

In TNBC cell lines that highly express PIBF1, knockdown of PIBF1 induces apoptosis and suppresses cell viability and motility with activation of the ATR/CHK1 signaling pathway. Moreover, the oncogenic function of PIBF1 was confirmed using the Ba/F3 cell line.

CONCLUSION

For the first time, these findings clarify the role of PIBF1 in regulating ATR/CHK1 signaling pathway and inhibiting the proliferation and migration of TNBC cell lines. These results demonstrate the oncogenic roles of PIBF1 and provide new insights into the function and the molecular mechanism of PIBF1 in malignant TNBC.

The role of CDC25C in cell cycle regulation and clinical cancer therapy: a systematic review

One of the most prominent features of tumor cells is uncontrolled cell proliferation caused by an abnormal cell cycle, and the abnormal expression of cell cycle-related proteins gives tumor cells their invasive, metastatic, drug-resistance, and anti-apoptotic abilities. Recently, an increasing number of cell cycle-associated proteins have become the candidate biomarkers for early diagnosis of malignant tumors and potential targets for cancer therapies. As an important cell cycle regulatory protein, Cell Division Cycle 25C (CDC25C) participates in regulating G2/M progression and in mediating DNA damage repair. CDC25C is a cyclin of the specific phosphatase family that activates the cyclin B1/CDK1 complex in cells for entering mitosis and regulates G2/M progression and plays an important role in checkpoint protein regulation in case of DNA damage, which can ensure accurate DNA information transmission to the daughter cells. The regulation of CDC25C in the cell cycle is affected by multiple signaling pathways, such as cyclin B1/CDK1, PLK1/Aurora A, ATR/CHK1, ATM/CHK2, CHK2/ERK, Wee1/Myt1, p53/Pin1, and ASK1/JNK-/38. Recently, it has evident that changes in the expression of CDC25C are closely related to tumorigenesis and tumor development and can be used as a potential target for cancer treatment. This review summarizes the role of CDC25C phosphatase in regulating cell cycle. Based on the role of CDC25 family proteins in the development of tumors, it will become a hot target for a new generation of cancer treatments.

Different p53 genotypes regulating different phosphorylation sites and subcellular location of CDC25C associated with the formation of polyploid giant cancer cells

BACKGROUND

Our previous studies have confirmed that cobalt chloride (CoCl2) can induce the formation of polyploid giant cancer cells (PGCCs), which is the key to the heterogeneity of solid tumors. PGCC formation is closely related to the abnormal expression of cell cycle-related proteins and cell fusion. In this study, we investigated the molecular mechanism of PGCCs formation by detecting the expression of cell cycle-related proteins in mutant and wild-type p53 cancer cell lines.

METHODS

HEY, BT-549, SKOv3 and MDA-MB-231 cells were treated with CoCl2 and the cell cycle was detected by flow cytometry. The expression and subcellular localization of cell cycle-related proteins, kinases, and P53 were compared before and after CoCl2 treatment. Immunoprecipitation was used to analyze the interacting proteins of pCDC25C-Ser216 and pCDC25C-Ser198. The clinicopathologic significances of these cell cycle-related proteins and protein kinases expression were studied.

RESULTS

CoCl2 induced the formation of PGCCs and G2/M arrest. CDC25C, cyclin B1, and CDK1 expressions after CoCl2 treatment were lower than that in control cells. Cytoplasmic CDC25C was degraded by ubiquitin-dependent proteasome. The expression of P53 and phosphokinases including CHK1, CHK2, PLK1, and Aurora A increased after CoCl2 treatment. The expression of pCDC25C-Ser216 and pCDC25C-Ser198 depended upon the genotype of p53. The expressions of cell cycle-related proteins and kinases gradually increased with the development of ovarian cancer and breast cancer.

CONCLUSION

CHK1, CHK2-pCDC25C-Ser216-cyclin B1-CDK1, and Aurora A-PLK1-pCDC25C-Ser198-cyclin B1-CDK1 signaling pathways may participate in the formation of PGCCs and different phosphorylation sites of CDC25C may be associated with the genotype of p53.

DNA damage response signaling pathways and targets for radiotherapy sensitization in cancer

Radiotherapy is one of the most common countermeasures for treating a wide range of tumors. However, the radioresistance of cancer cells is still a major limitation for radiotherapy applications. Efforts are continuously ongoing to explore sensitizing targets and develop radiosensitizers for improving the outcomes of radiotherapy. DNA double-strand breaks are the most lethal lesions induced by ionizing radiation and can trigger a series of cellular DNA damage responses (DDRs), including those helping cells recover from radiation injuries, such as the activation of DNA damage sensing and early transduction pathways, cell cycle arrest, and DNA repair. Obviously, these protective DDRs confer tumor radioresistance. Targeting DDR signaling pathways has become an attractive strategy for overcoming tumor radioresistance, and some important advances and breakthroughs have already been achieved in recent years. On the basis of comprehensively reviewing the DDR signal pathways, we provide an update on the novel and promising druggable targets emerging from DDR pathways that can be exploited for radiosensitization. We further discuss recent advances identified from preclinical studies, current clinical trials, and clinical application of chemical inhibitors targeting key DDR proteins, including DNA-PKcs (DNA-dependent protein kinase, catalytic subunit), ATM/ATR (ataxia-telangiectasia mutated and Rad3-related), the MRN (MRE11-RAD50-NBS1) complex, the PARP (poly[ADP-ribose] polymerase) family, MDC1, Wee1, LIG4 (ligase IV), CDK1, BRCA1 (BRCA1 C terminal), CHK1, and HIF-1 (hypoxia-inducible factor-1). Challenges for ionizing radiation-induced signal transduction and targeted therapy are also discussed based on recent achievements in the biological field of radiotherapy.

Telomere length and telomerase activity in osteoporosis and osteoarthritis

Osteoarthritis (OA) and osteoporosis (OP) are associated skeletal pathologies and have as a distinct feature the abnormal reconstruction of the subchondral bone. OA and OP have been characterized as age-related diseases and have been associated with telomere shortening and altered telomerase activity (TA). This review discusses the role of telomeres and telomerase in OA and OP pathologies and focuses on the usability of telomere length (TL) and the rate of telomere shortening as potential disease biomarkers. A number of studies have demonstrated that telomere shortening may contribute to OA and OP as an epigenetic factor. Therefore, it has been claimed that the measurement of TL of chondrocytes and/or peripheral blood cells may be an appropriate marker for the evaluation of the progression of these diseases. However, there is a need to be perform further studies with larger cohorts, with the aim of obtaining objective results and a better understanding of the association between TL, inflammation and aging, in order to provide further insight into the pathophysiology of degenerative joint diseases.

DNA damage response manages cell cycle restriction of senile multipotent mesenchymal stromal cells

Multipotent mesenchymal stromal cells (MMSCs) are promising to treat a variety of traumatic and degenerative diseases. However, in vitro-passage aging induces cell cycle arrest and a series of genetic and biological changes, which greatly limits ex vivo cell number expansion and further clinical application of MMSCs. In most cases, DNA damage and DNA damage response (DDR) act as the main cause and executor of cellular senescence respectively. Mechanistically, DNA damage signals induce cell cycle arrest and DNA damage repair via DDR. If the DNA damage is indelible, MMSCs would entry into a permanent cell cycle arrest. It should be noted that apart from DDR signaling, certain proliferation or metabolism pathways are also occupied in DNA damage related cell cycle arrest. New findings of these aspects will also be summarized in this study. In summary, we aim to provide a comprehensive review of DDR associated cell cycle regulation and other major molecular signaling in the senescence of MMSCs. Above knowledge could contribute to improve the limited capacity of in vitro expansion of MMSCs, and then promote their clinical applications.

CD98hc (SLC3A2) sustains amino acid and nucleotide availability for cell cycle progression

CD98 heavy chain (CD98hc) forms heteromeric amino acid (AA) transporters by interacting with different light chains. Cancer cells overexpress CD98hc-transporters in order to meet their increased nutritional and antioxidant demands, since they provide branched-chain AA (BCAA) and aromatic AA (AAA) availability while protecting cells from oxidative stress. Here we show that BCAA and AAA shortage phenocopies the inhibition of mTORC1 signalling, protein synthesis and cell proliferation caused by CD98hc ablation. Furthermore, our data indicate that CD98hc sustains glucose uptake and glycolysis, and, as a consequence, the pentose phosphate pathway (PPP). Thus, loss of CD98hc triggers a dramatic reduction in the nucleotide pool, which leads to replicative stress in these cells, as evidenced by the enhanced DNA Damage Response (DDR), S-phase delay and diminished rate of mitosis, all recovered by nucleoside supplementation. In addition, proper BCAA and AAA availability sustains the expression of the enzyme ribonucleotide reductase. In this regard, BCAA and AAA shortage results in decreased content of deoxynucleotides that triggers replicative stress, also recovered by nucleoside supplementation. On the basis of our findings, we conclude that CD98hc plays a central role in AA and glucose cellular nutrition, redox homeostasis and nucleotide availability, all key for cell proliferation.

Discovery of potent telomerase activators: Unfolding new therapeutic and anti-aging perspectives

Telomere length, a marker of cellular aging, decreases with age and it has been associated with aging‑related diseases. Environmental factors, including diet and lifestyle factors, affect the rate of telomere shortening which can be reversed by telomerase. Telomerase activation by natural molecules has been suggested to be an anti‑aging modulator that can play a role in the treatment of aging‑related diseases. This study aimed to investigate the effect of natural compounds on telomerase activity in human peripheral blood mononuclear cells (PBMCs). The tested compounds included Centella asiatica extract formulation (08AGTLF), Astragalus extract formulation (Nutrient 4), TA‑65 (containing Astragalus membranaceus extract), oleanolic acid (OA), maslinic acid (MA), and 3 multi‑nutrient formulas (Nutrients 1, 2 and 3) at various concentrations. The mean absorbance values of telomerase activity measured following treatment with some of the above‑mentioned formulations were statistically significantly higher compared to those of the untreated cells. In particular, in order of importance with respect to telomerase activation from highest to lowest, 08AGTLF, OA, Nutrient 4, TA‑65, MA, Nutrient 3 and Nutrient 2, triggered statistically significant increase in telomerase activity compared to the untreated cells. 08AGTLF reached the highest levels of telomerase activity reported to date, at least to our knowledge, increasing telomerase activity by 8.8 folds compared to untreated cells, while Nutrient 4 and OA were also potent activators (4.3‑fold and 5.9‑fold increase, respectively). On the whole, this study indicates that the synergistic effect of nutrients and natural compounds can activate telomerase and produce more potent formulations. Human clinical studies using these formulations are required to evaluate their mode of action. This would reveal the health benefits of telomerase activation through natural molecules and would shed new light onto the treatment of aging‑related diseases.

Breaking the DNA Damage Response via Serine/Threonine Kinase Inhibitors to Improve Cancer Treatment

Multiple, both endogenous and exogenous, sources may induce DNA damage and DNA replication stress. Cells have developed DNA damage response (DDR) signaling pathways to maintain genomic stability and effectively detect and repair DNA lesions. Serine/ threonine kinases such as Ataxia-telangiectasia mutated (ATM) and Ataxia-telangiectasia and Rad3-Related (ATR) are the major regulators of DDR, since after sensing stalled DNA replication forks, DNA double- or single-strand breaks, may directly phosphorylate and activate their downstream targets, that play a key role in DNA repair, cell cycle arrest and apoptotic cell death. Interestingly, key components of DDR signaling networks may constitute an attractive target for anti-cancer therapy through two distinct potential approaches: as chemoand radiosensitizers to enhance the effectiveness of currently used genotoxic treatment or as single agents to exploit defects in DDR in cancer cells via synthetic lethal approach. Moreover, the newest data reported that serine/threonine protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) is also closely associated with cancer development and progression. Thereby, utilization of small-molecule, serine/threonine kinase inhibitors may provide a novel, groundbreaking, anti-cancer treatment strategy. Currently, a range of potent, highlyselective toward ATM, ATR and PERK inhibitors has been discovered, but after foregoing study, additional investigations are necessary for their future clinical use.

Anticancer activity of synthetic (±)-kusunokinin and its derivative (±)-bursehernin on human cancer cell lines

Kusunokinin is a potent lignan compound with a several biological properties including antitrypanosomal and anticancer. In this study, (±)-kusunokinin and its derivative, (±)-bursehernin, were synthesized and investigated for their anticancer activities on cell viability, cell cycle arrest and apoptosis in cancer cell lines including breast cancer (MCF-7, MDA-MB-468 and MDA-MB-231), colon cancer (HT-29) and cholangiocarcinoma (KKU-K100, KKU-M213 and KKU-M055) cells. The result showed that (±)-kusunokinin and (±)-bursehernin represented the strongest growth inhibition against breast cancer (MCF-7) and cholangiocarcinoma (KKU-M213) cells with the IC₅₀ values of 4.30 ± 0.65 μM and 3.70 ± 0.79 μM, respectively, both of which were lower than IC₅₀ of normal fibroblast cells (L929). Etoposide was used as a positive control since this chemotherapeutic drug is in the lignan group same as (±)-kusunokinin. Surprisingly, etoposide showed less cytotoxicity than (±)-kusunokinin and its derivative on MCF-7, HT-29, KKU-M213 and KKU-K100. Moreover, (±)-bursehernin induced cell cycle arrest at G2/M phase, meanwhile (±)-kusunokinin tended to increased cell population at G2/M phase but did not show the significant difference compared with non-treated cells. Interestingly, protein levels related to cell proliferation pathway (topoisomerase II, STAT3, cyclin D1, and p21) were significantly decreased at 72 h. Both compounds induced apoptotic cell in time-dependent manner as confirmed by MultiCaspase assay. In conclusion, synthetic compound, (±)-kusunokinin and (±)-bursehernin, showed anticancer effects via the reduction of cell proliferation proteins and induction of apoptosis.

Temozolomide Induces Senescence and Repression of DNA Repair Pathways in Glioblastoma Cells via Activation of ATR-CHK1, p21, and NF-κB

The DNA-methylating drug temozolomide, which induces cell death through apoptosis, is used for the treatment of malignant glioma. Here, we investigate the mechanisms underlying the ability of temozolomide to induce senescence in glioblastoma cells. Temozolomide-induced senescence was triggered by the specific DNA lesion O⁶-methylguanine (O⁶MeG) and characterized by arrest of cells in the G₂-M phase. Inhibitor experiments revealed that temozolomide-induced senescence was initiated by damage recognition through the MRN complex, activation of the ATR/CHK1 axis of the DNA damage response pathway, and mediated by degradation of CDC25c. Temozolomide-induced senescence required functional p53 and was dependent on sustained p21 induction. p53-deficient cells, not expressing p21, failed to induce senescence, but were still able to induce a G₂-M arrest. p14 and p16, targets of p53, were silenced in our cell system and did not seem to play a role in temozolomide-induced senescence. In addition to p21, the NF-κB pathway was required for senescence, which was accompanied by induction of the senescence-associated secretory phenotype. Upon temozolomide exposure, we found a strong repression of the mismatch repair proteins MSH2, MSH6, and EXO1 as well as the homologous recombination protein RAD51, which was downregulated by disruption of the E2F1/DP1 complex. Repression of these repair factors was not observed in G₂-M arrested p53-deficient cells and, therefore, it seems to represent a specific trait of temozolomide-induced senescence. SIGNIFICANCE: These findings reveal a mechanism by which the anticancer drug temozolomide induces senescence and downregulation of DNA repair pathways in glioma cells.

ATR inhibition controls aggressive prostate tumors deficient in Y-linked histone demethylase KDM5D

Epigenetic modifications control cancer development and clonal evolution in various cancer types. Here, we show that loss of the male-specific histone demethylase lysine-specific demethylase 5D (KDM5D) encoded on the Y chromosome epigenetically modifies histone methylation marks and alters gene expression, resulting in aggressive prostate cancer. Fluorescent in situ hybridization demonstrated that segmental or total deletion of the Y chromosome in prostate cancer cells is one of the causes of decreased KDM5D mRNA expression. The result of ChIP-sequencing analysis revealed that KDM5D preferably binds to promoter regions with coenrichment of the motifs of crucial transcription factors that regulate the cell cycle. Loss of KDM5D expression with dysregulated H3K4me3 transcriptional marks was associated with acceleration of the cell cycle and mitotic entry, leading to increased DNA-replication stress. Analysis of multiple clinical data sets reproducibly showed that loss of expression of KDM5D confers a poorer prognosis. Notably, we also found stress-induced DNA damage on the serine/threonine protein kinase ATR with loss of KDM5D. In KDM5D-deficient cells, blocking ATR activity with an ATR inhibitor enhanced DNA damage, which led to subsequent apoptosis. These data start to elucidate the biological characteristics resulting from loss of KDM5D and also provide clues for a potential novel therapeutic approach for this subset of aggressive prostate cancer.

Targeting Sphingosine Kinase Isoforms Effectively Reduces Growth and Survival of Neoplastic Mast Cells With D816V-KIT

Mastocytosis is a disorder resulting from an abnormal mast cell (MC) accumulation in tissues that is often associated with the D816V mutation in KIT, the tyrosine kinase receptor for stem cell factor. Therapies available to treat aggressive presentations of mastocytosis are limited, thus exploration of novel pharmacological targets that reduce MC burden is desirable. Since increased generation of the lipid mediator sphingosine-1-phosphate (S1P) by sphingosine kinase (SPHK) has been linked to oncogenesis, we studied the involvement of the two SPHK isoforms (SPHK1 and SPHK2) in the regulation of neoplastic human MC growth. While SPHK2 inhibition prevented entry into the cell cycle in normal and neoplastic human MCs with minimal effect on cell survival, SPHK1 inhibition caused cell cycle arrest in G2/M and apoptosis, particularly in D816V-KIT MCs. This was mediated via activation of the DNA damage response (DDR) cascade, including phosphorylation of the checkpoint kinase 2 (CHK2), CHK2-mediated M-phase inducer phosphatase 3 depletion, and p53 activation. Combination treatment of SPHK inhibitors with KIT inhibitors showed greater growth inhibition of D816V-KIT MCs than either inhibitor alone. Furthermore, inhibition of SPHK isoforms reduced the number of malignant bone marrow MCs from patients with mastocytosis and the growth of D816V-KIT MCs in a xenograft mouse model. Our results reveal a role for SPHK isoforms in the regulation of growth and survival in normal and neoplastic MCs and suggest a regulatory function for SPHK1 in the DDR in MCs with KIT mutations. The findings also suggest that targeting the SPHK/S1P axis may provide an alternative to tyrosine kinase inhibitors, alone or in combination, for the treatment of aggressive mastocytosis and other hematological malignancies associated with the D816V-KIT mutation.

Mdm2 promotes Cdc25C protein degradation and delays cell cycle progression through the G2/M phase

Upon different types of stress, the gene encoding the mitosis-promoting phosphatase Cdc25C is transcriptionally repressed by p53, contributing to p53's enforcement of a G2 cell cycle arrest. In addition, Cdc25C protein stability is also decreased following DNA damage. Mdm2, another p53 target gene, encodes a ubiquitin ligase that negatively regulates p53 levels by ubiquitination. Ablation of Mdm2 by siRNA led to an increase in p53 protein and repression of Cdc25C gene expression. However, Cdc25C protein levels were actually increased following Mdm2 depletion. Mdm2 is shown to negatively regulate Cdc25C protein levels by reducing its half-life independently of the presence of p53. Further, Mdm2 physically interacts with Cdc25C and promotes its degradation through the proteasome in a ubiquitin-independent manner. Either Mdm2 overexpression or Cdc25C downregulation delays cell cycle progression through the G2/M phase. Thus, the repression of the Cdc25C promoter by p53, together with p53-dependent induction of Mdm2 and subsequent degradation of Cdc25C, could provide a dual mechanism by which p53 can enforce and maintain a G2/M cell cycle arrest.

High Mobility Group A2 protects cancer cells against telomere dysfunction

The non-histone chromatin binding protein High Mobility Group AT-hook protein 2 (HMGA2) plays important roles in the repair and protection of genomic DNA in embryonic stem cells and cancer cells. Here we show that HMGA2 localizes to mammalian telomeres and enhances telomere stability in cancer cells. We present a novel interaction of HMGA2 with the key shelterin protein TRF2. We found that the linker (L1) region of HMGA2 contributes to this interaction but the ATI-L1-ATII molecular region of HMGA2 is required for strong interaction with TRF2. This interaction was independent of HMGA2 DNA-binding and did not require the TRF2 interacting partner RAP1 but involved the homodimerization and hinge regions of TRF2. HMGA2 retained TRF2 at telomeres and reduced telomere-dysfunction despite induced telomere stress. Silencing of HMGA2 resulted in (i) reduced binding of TRF2 to telomere DNA as observed by ChIP, (ii) increased telomere instability and (iii) the formation of telomere dysfunction-induced foci (TIF). This resulted in increased telomere aggregation, anaphase bridges and micronuclei. HMGA2 prevented ATM-dependent pTRF2T188 phosphorylation and attenuated signaling via the telomere specific ATM-CHK2-CDC25C DNA damage signaling axis. In summary, our data demonstrate a unique and novel role of HMGA2 in telomere protection and promoting telomere stability in cancer cells. This identifies HMGA2 as a new therapeutic target for the destabilization of telomeres in HMGA2+ cancer cells.

The G2 checkpoint-a node-based molecular switch

Tight regulation of the eukaryotic cell cycle is paramount to ensure genomic integrity throughout life. Cell cycle checkpoints are present in each phase of the cell cycle and prevent cell cycle progression when genomic integrity is compromised. The G2 checkpoint is an intricate signaling network that regulates the progression of G2 to mitosis (M). We propose here a node-based model of G2 checkpoint regulation, in which the action of the central CDK1-cyclin B1 node is determined by the concerted but opposing activities of the Wee1 and cell division control protein 25C (CDC25C) nodes. Phosphorylation of both Wee1 and CDC25C at specific sites determines their subcellular localization, driving them either toward activity within the nucleus or to the cytoplasm and subsequent ubiquitin-mediated proteasomal degradation. In turn, this subcellular balance of the Wee1 and CDC25C nodes is directed by the action of the PLK1 and CHK1 nodes via what we have termed the 'nuclear and cytoplasmic decision states' of Wee1 and CDC25C. The proposed node-based model provides an intelligible structure of the complex interactions that govern the decision to delay or continue G2/M progression. The model may also aid in predicting the effects of agents that target these G2 checkpoint nodes.

ATM participates in the regulation of viability and cell cycle via ellipticine in bladder cancer

Ellipticine, an alkaloid isolated from Apocyanaceae plants, has been demonstrated to exhibit antitumor activity in several cancers. However, the effect and the mechanisms underlying its action have not been investigated in human bladder cancer cells. The aim of the present study was to investigate the effect and mechanism of ellipticine on the behavior of T-24 bladder cancer cells. T-24 cells were treated with varying concentrations and durations of ellipticine. Cell viability was evaluated by Cell Counting Kit-8 assay. Cell motility was analyzed by Transwell migration assay. Flow cytometry, reverse transcription-quantitative polymerase chain reaction and western blot analyses were performed to detect the cell cycle and signaling pathways involved. The results demonstrated that ellipticine suppressed proliferation and inhibited the migration ability of T-24 bladder cancer cells in a dose- and time-dependent manner, and resulted in G2/M cell cycle arrest. The mechanism of this action was demonstrated to be due to ellipticine-triggered activation of the ATM serine/threonine kinase pathway. These data therefore suggest that ellipticine may be effective towards treating human bladder cancer.

Pot1 OB-fold mutations unleash telomere instability to initiate tumorigenesis

Chromosomal aberrations are a hallmark of human cancers, with complex cytogenetic rearrangements leading to genetic changes permissive for cancer initiation and progression. Protection of Telomere 1 (POT1) is an essential component of the shelterin complex and functions to maintain chromosome stability by repressing the activation of aberrant DNA damage and repair responses at telomeres. Sporadic and familial mutations in the oligosaccharide-oligonucleotide (OB) folds of POT1 have been identified in many human cancers, but the mechanism underlying how hPOT1 mutations initiate tumorigenesis has remained unclear. Here we show that the human POT1’s OB-folds are essential for the protection of newly replicated telomeres. Oncogenic mutations in hPOT1 OB-fold fail to bind to ss telomeric DNA, eliciting a DNA damage response at telomeres that promote inappropriate chromosome fusions via the mutagenic alternative non-homologous end joining (A-NHEJ) pathway. hPOT1 mutations also result in telomere elongation and the formation of transplantable hematopoietic malignancies. Strikingly, conditional deletion of both mPot1a and p53 in mouse mammary epithelium resulted in development of highly invasive breast carcinomas and the formation of whole chromosomes containing massive arrays of telomeric fusions reminiscent of chromothripsis. Our results reveal that hPOT1 OB-folds are required to protect and prevent newly replicated telomeres from engaging in A-NHEJ mediated fusions that would otherwise promote genome instability to fuel tumorigenesis.

Dysfunctional telomeres induce p53-dependent and independent apoptosis to compromise cellular proliferation and inhibit tumor formation

Aging is associated with progressive telomere shortening, resulting in the formation of dysfunctional telomeres that compromise tissue proliferation. However, dysfunctional telomeres can limit tumorigenesis by activating p53-dependent cellular senescence and apoptosis. While activation of both senescence and apoptosis is required for repress tumor formation, it is not clear which pathway is the major tumor suppressive pathway in vivo. In this study, we generated Eμ-myc; Pot1b(∆/∆) mouse to directly compare tumor formation under conditions in which either p53-dependent apoptosis or senescence is activated by telomeres devoid of the shelterin component Pot1b. We found that activation of p53-dependent apoptosis plays a more critical role in suppressing lymphoma formation than p53-dependent senescence. In addition, we found that telomeres in Pot1b(∆/∆) ; p53(-/-) mice activate an ATR-Chk1-dependent DNA damage response to initiate a robust p53-independent, p73-dependent apoptotic pathway that limited stem cell proliferation but suppressed B-cell lymphomagenesis. Our results demonstrate that in mouse models, both p53-dependent and p53-independent apoptosis are important to suppressing tumor formation.

Proteasomal Inhibition by Ixazomib Induces CHK1 and MYC-Dependent Cell Death in T-cell and Hodgkin Lymphoma

Proteasome-regulated NF-κB has been shown to be important for cell survival in T-cell lymphoma and Hodgkin lymphoma models. Several new small-molecule proteasome inhibitors are under various stages of active preclinical and clinical development. We completed a comprehensive preclinical examination of the efficacy and associated biologic effects of a second-generation proteasome inhibitor, ixazomib, in T-cell lymphoma and Hodgkin lymphoma cells and in vivo SCID mouse models. We demonstrated that ixazomib induced potent cell death in all cell lines at clinically achievable concentrations. In addition, it significantly inhibited tumor growth and improved survival in T-cell lymphoma and Hodgkin lymphoma human lymphoma xenograft models. Through global transcriptome analyses, proteasomal inhibition showed conserved overlap in downregulation of cell cycle, chromatin modification, and DNA repair processes in ixazomib-sensitive lymphoma cells. The predicted activity for tumor suppressors and oncogenes, the impact on "hallmarks of cancer," and the analysis of key significant genes from global transcriptome analysis for ixazomib strongly favored tumor inhibition via downregulation of MYC and CHK1, its target genes. Furthermore, in ixazomib-treated lymphoma cells, we identified that CHK1 was involved in the regulation of MYC expression through chromatin modification involving histone H3 acetylation via chromatin immunoprecipitation. Finally, using pharmacologic and RNA silencing of CHK1 or the associated MYC-related mechanism, we demonstrated synergistic cell death in combination with antiproteasome therapy. Altogether, ixazomib significantly downregulates MYC and induces potent cell death in T-cell lymphoma and Hodgkin lymphoma, and we identified that combinatorial therapy with anti-CHK1 treatment represents a rational and novel therapeutic approach. Cancer Res; 76(11); 3319-31. ©2016 AACR.

Constitutive activation of the DNA damage response pathway as a novel therapeutic target in diffuse large B-cell lymphoma

The recent finding that MYC-driven cancers are sensitive to inhibition of the DNA damage response (DDR) pathway, prompted us to investigate the role of DDR pathway as therapeutic target in diffuse large B-cell lymphoma (DLBCL), which frequently overexpresses the MYC oncogene. In a preliminary immunohistochemical study conducted on 99 consecutive DLBCL patients, we found that about half of DLBCLs showed constitutive expression of the phosphorylated forms of checkpoint kinases (CHK) and CDC25c, markers of DDR activation, and of phosphorylated histone H2AX (γH2AX), marker of DNA damage and genomic instability. Constitutive γH2AX expression correlated with c-MYC levels and DDR activation, and defined a subset of tumors characterised by poor outcome. Next, we used the CHK inhibitor PF-0477736 as a tool to investigate whether the inhibition of the DDR pathway might represent a novel therapeutic approach in DLBCL. Submicromolar concentrations of PF-0477736 hindered proliferation in DLBCL cell lines with activated DDR pathway. These results were fully recapitulated with a different CHK inhibitor (AZD-7762). Inhibition of checkpoint kinases induced rapid DNA damage accumulation and apoptosis in DLBCL cell lines and primary cells. These data suggest that pharmacologic inhibition of DDR through targeting of CHK kinases may represent a novel therapeutic strategy in DLBCL.

Checkpoint Kinase 2 Negatively Regulates Androgen Sensitivity and Prostate Cancer Cell Growth

Prostate cancer is the second leading cause of cancer death in American men, and curing metastatic disease remains a significant challenge. Nearly all patients with disseminated prostate cancer initially respond to androgen deprivation therapy (ADT), but virtually all patients will relapse and develop incurable castration-resistant prostate cancer (CRPC). A high-throughput RNAi screen to identify signaling pathways regulating prostate cancer cell growth led to our discovery that checkpoint kinase 2 (CHK2) knockdown dramatically increased prostate cancer growth and hypersensitized cells to low androgen levels. Mechanistic investigations revealed that the effects of CHK2 were dependent on the downstream signaling proteins CDC25C and CDK1. Moreover, CHK2 depletion increased androgen receptor (AR) transcriptional activity on androgen-regulated genes, substantiating the finding that CHK2 affects prostate cancer proliferation, partly, through the AR. Remarkably, we further show that CHK2 is a novel AR-repressed gene, suggestive of a negative feedback loop between CHK2 and AR. In addition, we provide evidence that CHK2 physically associates with the AR and that cell-cycle inhibition increased this association. Finally, IHC analysis of CHK2 in prostate cancer patient samples demonstrated a decrease in CHK2 expression in high-grade tumors. In conclusion, we propose that CHK2 is a negative regulator of androgen sensitivity and prostate cancer growth, and that CHK2 signaling is lost during prostate cancer progression to castration resistance. Thus, perturbing CHK2 signaling may offer a new therapeutic approach for sensitizing CRPC to ADT and radiation.

Long G2 accumulates recombination intermediates and disturbs chromosome segregation at dysfunction telomere in Schizosaccharomyces pombe

Protection of telomere (Pot1) is a single-stranded telomere binding protein which is essential for chromosome ends protection. Fission yeast Rqh1 is a member of RecQ helicases family which has essential roles in the maintenance of genomic stability and regulation of homologous recombination. Double mutant between fission yeast pot1Δ and rqh1 helicase dead (rqh1-hd) maintains telomere by homologous recombination. In pot1Δ rqh1-hd double mutant, recombination intermediates accumulate near telomere which disturb chromosome segregation and make cells sensitive to microtubule inhibitors thiabendazole (TBZ). Deletion of chk1(+) or mutation of its kinase domain shortens the G2 of pot1Δ rqh1-hd double mutant and suppresses both the accumulation of recombination intermediates and the TBZ sensitivity of that double mutant. In this study, we asked whether the long G2 is the reason for the TBZ sensitivity of pot1Δ rqh1-hd double mutant. We found that shortening the G2 of pot1Δ rqh1-hd double mutant by additional mutations of wee1 and mik1 or gain of function mutation of Cdc2 suppresses both the accumulation of recombination intermediates and the TBZ sensitivity of pot1Δ rqh1-hd double mutant. Our results suggest that long G2 of pot1Δ rqh1-hd double mutant may allow time for the accumulation of recombination intermediates which disturb chromosome segregation and make cells sensitive to TBZ.

Cell cycle checkpoints and pathogenesis of HIV-1 infection: a brief overview

To understand HIV pathogenesis or development is no simple undertaking and neither is the cell cycle which is highly complex that requires the coordination of multiple events and machinery. It is interesting that these two processes are interrelated, intersect and interact as HIV-1 infection results in cell cycle arrest at the G2 phase which is accompanied by massive CD4+ T cell death. For its own benefit, in an impressive manner and with the overabundance of tactics, HIV maneuvers DNA damage responses and cell cycle check points for viral replication at different stages from infection, to latency and to pathogenesis. Although the cell cycle is the most critical aspect involved in both viral and cellular replication, in this review, our main focus is on recent developments, including our own observations in the field of cell cycle proteins, checkpoints and strategies utilized by the viruses to manipulate these pathways to promote their own replication and survival. We will also discuss the emerging concept of targeting the replication initiation machinery for HIV therapy.