Centrosome-associated Chk1 prevents premature activation of cyclin-B-Cdk1 kinase

Centrosome amplification primes ovarian cancer cells for apoptosis and potentiates the response to chemotherapy

Centrosome amplification is a feature of cancer cells associated with chromosome instability and invasiveness. Enhancing chromosome instability and subsequent cancer cell death via centrosome unclustering and multipolar divisions is an aimed-for therapeutic approach. Here, we show that centrosome amplification potentiates responses to conventional chemotherapy in addition to its effect on multipolar divisions and chromosome instability. We perform single-cell live imaging of chemotherapy responses in epithelial ovarian cancer cell lines and observe increased cell death when centrosome amplification is induced. By correlating cell fate with mitotic behaviors, we show that enhanced cell death can occur independently of chromosome instability. We identify that cells with centrosome amplification are primed for apoptosis. We show they are dependent on the apoptotic inhibitor BCL-XL and that this is not a consequence of mitotic stresses associated with centrosome amplification. Given the multiple mechanisms that promote chemotherapy responses in cells with centrosome amplification, we assess such a relationship in an epithelial ovarian cancer patient cohort. We show that high centrosome numbers associate with improved treatment responses and longer overall survival. Our work identifies apoptotic priming as a clinically relevant consequence of centrosome amplification, expanding our understanding of this pleiotropic cancer cell feature.

Cryo-EM structure of the CDK2-cyclin A-CDC25A complex

The cell division cycle 25 phosphatases CDC25A, B and C regulate cell cycle transitions by dephosphorylating residues in the conserved glycine-rich loop of CDKs to activate their activity. Here, we present the cryo-EM structure of CDK2-cyclin A in complex with CDC25A at 2.7 Å resolution, providing a detailed structural analysis of the overall complex architecture and key protein-protein interactions that underpin this 86 kDa complex. We further identify a CDC25A C-terminal helix that is critical for complex formation. Sequence conservation analysis suggests CDK1/2-cyclin A, CDK1-cyclin B and CDK2/3-cyclin E are suitable binding partners for CDC25A, whilst CDK4/6-cyclin D complexes appear unlikely substrates. A comparative structural analysis of CDK-containing complexes also confirms the functional importance of the conserved CDK1/2 GDSEID motif. This structure improves our understanding of the roles of CDC25 phosphatases in CDK regulation and may inform the development of CDC25-targeting anticancer strategies.

Centrosomes and associated proteins in pathogenesis and treatment of breast cancer

Breast cancer is the most prevalent malignancy among women worldwide. Despite significant advances in treatment, it remains one of the leading causes of female mortality. The inability to effectively treat advanced and/or treatment-resistant breast cancer demonstrates the need to develop novel treatment strategies and targeted therapies. Centrosomes and their associated proteins have been shown to play key roles in the pathogenesis of breast cancer and thus represent promising targets for drug and biomarker development. Centrosomes are fundamental cellular structures in the mammalian cell that are responsible for error-free execution of cell division. Centrosome amplification and aberrant expression of its associated proteins such as Polo-like kinases (PLKs), Aurora kinases (AURKs) and Cyclin-dependent kinases (CDKs) have been observed in various cancers, including breast cancer. These aberrations in breast cancer are thought to cause improper chromosomal segregation during mitosis, leading to chromosomal instability and uncontrolled cell division, allowing cancer cells to acquire new genetic changes that result in evasion of cell death and the promotion of tumor formation. Various chemical compounds developed against PLKs and AURKs have shown meaningful antitumorigenic effects in breast cancer cells in vitro and in vivo. The mechanism of action of these inhibitors is likely related to exacerbation of numerical genomic instability, such as aneuploidy or polyploidy. Furthermore, growing evidence demonstrates enhanced antitumorigenic effects when inhibitors specific to centrosome-associated proteins are used in combination with either radiation or chemotherapy drugs in breast cancer. This review focuses on the current knowledge regarding the roles of centrosome and centrosome-associated proteins in breast cancer pathogenesis and their utility as novel targets for breast cancer treatment.

Vinculin is required for interkinetic nuclear migration (INM) and cell cycle progression

Vinculin is an actin-binding protein (ABP) that strengthens the connection between the actin cytoskeleton and adhesion complexes. It binds to β-catenin/N-cadherin complexes in apical adherens junctions (AJs), which maintain cell-to-cell adhesions, and to talin/integrins in the focal adhesions (FAs) that attach cells to the basal membrane. Here, we demonstrate that β-catenin targets vinculin to the apical AJs and the centrosome in the embryonic neural tube (NT). Suppression of vinculin slows down the basal-to-apical part of interkinetic nuclear migration (BAINM), arrests neural stem cells (NSCs) in the G2 phase of the cell cycle, and ultimately dismantles the apical actin cytoskeleton. In the NSCs, mitosis initiates when an internalized centrosome gathers with the nucleus during BAINM. Notably, our results show that the first centrosome to be internalized is the daughter centrosome, where β-catenin and vinculin accumulate, and that vinculin suppression prevents centrosome internalization. Thus, we propose that vinculin links AJs, the centrosome, and the actin cytoskeleton where actomyosin contraction forces are required.

Combined Aurora Kinase A and CHK1 Inhibition Enhances Radiosensitivity of Triple-Negative Breast Cancer Through Induction of Apoptosis and Mitotic Catastrophe Associated With Excessive DNA Damage

PURPOSE

There is an urgent need for biomarkers and new actionable targets to improve radiosensitivity of triple-negative breast cancer (TNBC) tumors. We characterized the radiosensitizing effects and underlying mechanisms of combined Aurora kinase A (AURKA) and CHK1 inhibition in TNBC.

METHODS AND MATERIALS

Different TNBC cell lines were treated with AURKA inhibitor (AURKAi, MLN8237) and CHK1 inhibitor (CHK1i, MK8776). Cell responses to irradiation (IR) were then evaluated. Cell apoptosis, DNA damage, cell cycle distribution, and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) and Phosphoinositide 3-Kinase (PI3K) pathways were evaluated in vitro. Transcriptomic analysis was performed to facilitate the identification of potential biomarkers. Xenograft and immunohistochemistry were carried out to investigate the radiosensitizing effects of dual inhibition in vivo. Finally, the prognostic effect of CHEK1/AURKA in TNBC samples in the The Cancer Genome Atlas (TCGA) database and our center were analyzed.

RESULTS

AURKAi (MLN8237) induced overexpression of phospho-CHK1 in TNBC cells. The addition of MK8776 (CHK1i) to MLN8237 greatly reduced cell viability and increased radiosensitivity compared with either the control or MLN8237 alone in vitro. Mechanistically, dual inhibition resulted in inducing excessive DNA damage by prompting G2/M transition to cells with defective spindles, leading to mitotic catastrophe and induction of apoptosis after IR. We also observed that dual inhibition suppressed the phosphorylation of ERK, while activation of ERK with its agonist or overexpression of active ERK1/2 allele could attenuate the apoptosis induced by dual inhibition with IR. Additionally, dual inhibition of AURKA and CHK1 synergistically enhanced radiosensitivity in MDA-MB-231 xenografts. Moreover, we detected that both CHEK1 and AURKA were overexpressed in patients with TNBC and negatively correlated with patient survival.

CONCLUSIONS

Our findings suggested that AURKAi in combination with CHK1i enhanced TNBC radiosensitivity in preclinical models, potentially providing a novel strategy of precision treatment for patients with TNBC.

CHK1-CDC25A-CDK1 regulate cell cycle progression and protect genome integrity in early mouse embryos

After fertilization, remodeling of the oocyte and sperm genomes is essential to convert these highly differentiated and transcriptionally quiescent cells into early cleavage-stage blastomeres that are transcriptionally active and totipotent. This developmental transition is accompanied by cell cycle adaptation, such as lengthening or shortening of the gap phases G1 and G2. However, regulation of these cell cycle changes is poorly understood, especially in mammals. Checkpoint kinase 1 (CHK1) is a protein kinase that regulates cell cycle progression in somatic cells. Here, we show that CHK1 regulates cell cycle progression in early mouse embryos by restraining CDK1 kinase activity due to CDC25A phosphatase degradation. CHK1 kinase also ensures the long G2 phase needed for genome activation and reprogramming gene expression in two-cell stage mouse embryos. Finally, Chk1 depletion leads to DNA damage and chromosome segregation errors that result in aneuploidy and infertility.

The fate of extra centrosomes in newly formed tetraploid cells: should I stay, or should I go?

An increase in centrosome number is commonly observed in cancer cells, but the role centrosome amplification plays along with how and when it occurs during cancer development is unclear. One mechanism for generating cancer cells with extra centrosomes is whole genome doubling (WGD), an event that occurs in over 30% of human cancers and is associated with poor survival. Newly formed tetraploid cells can acquire extra centrosomes during WGD, and a generally accepted model proposes that centrosome amplification in tetraploid cells promotes cancer progression by generating aneuploidy and chromosomal instability. Recent findings, however, indicate that newly formed tetraploid cells in vitro lose their extra centrosomes to prevent multipolar cell divisions. Rather than persistent centrosome amplification, this evidence raises the possibility that it may be advantageous for tetraploid cells to initially restore centrosome number homeostasis and for a fraction of the population to reacquire additional centrosomes in the later stages of cancer evolution. In this review, we explore the different evolutionary paths available to newly formed tetraploid cells, their effects on centrosome and chromosome number distribution in daughter cells, and their probabilities of long-term survival. We then discuss the mechanisms that may alter centrosome and chromosome numbers in tetraploid cells and their relevance to cancer progression following WGD.

The Adaptive Mechanisms and Checkpoint Responses to a Stressed DNA Replication Fork

DNA replication is a tightly controlled process that ensures the faithful duplication of the genome. However, DNA damage arising from both endogenous and exogenous assaults gives rise to DNA replication stress associated with replication fork slowing or stalling. Therefore, protecting the stressed fork while prompting its recovery to complete DNA replication is critical for safeguarding genomic integrity and cell survival. Specifically, the plasticity of the replication fork in engaging distinct DNA damage tolerance mechanisms, including fork reversal, repriming, and translesion DNA synthesis, enables cells to overcome a variety of replication obstacles. Furthermore, stretches of single-stranded DNA generated upon fork stalling trigger the activation of the ATR kinase, which coordinates the cellular responses to replication stress by stabilizing the replication fork, promoting DNA repair, and controlling cell cycle and replication origin firing. Deregulation of the ATR checkpoint and aberrant levels of chronic replication stress is a common characteristic of cancer and a point of vulnerability being exploited in cancer therapy. Here, we discuss the various adaptive responses of a replication fork to replication stress and the roles of ATR signaling that bring fork stabilization mechanisms together. We also review how this knowledge is being harnessed for the development of checkpoint inhibitors to trigger the replication catastrophe of cancer cells.

CDC6 as a Key Inhibitory Regulator of CDK1 Activation Dynamics and the Timing of Mitotic Entry and Progression

Timely mitosis is critically important for early embryo development. It is regulated by the activity of the conserved protein kinase CDK1. The dynamics of CDK1 activation must be precisely controlled to assure physiologic and timely entry into mitosis. Recently, a known S-phase regulator CDC6 emerged as a key player in mitotic CDK1 activation cascade in early embryonic divisions, operating together with Xic1 as a CDK1 inhibitor upstream of the Aurora A and PLK1, both CDK1 activators. Herein, we review the molecular mechanisms that underlie the control of mitotic timing, with special emphasis on how CDC6/Xic1 function impacts CDK1 regulatory network in the Xenopus system. We focus on the presence of two independent mechanisms inhibiting the dynamics of CDK1 activation, namely Wee1/Myt1- and CDC6/Xic1-dependent, and how they cooperate with CDK1-activating mechanisms. As a result, we propose a comprehensive model integrating CDC6/Xic1-dependent inhibition into the CDK1-activation cascade. The physiological dynamics of CDK1 activation appear to be controlled by the system of multiple inhibitors and activators, and their integrated modulation ensures concomitantly both the robustness and certain flexibility of the control of this process. Identification of multiple activators and inhibitors of CDK1 upon M-phase entry allows for a better understanding of why cells divide at a specific time and how the pathways involved in the timely regulation of cell division are all integrated to precisely tune the control of mitotic events.

On the assembly of the mitotic spindle, bistability and hysteresis

During cell division, the transition from interphase to mitosis is dictated by activation of the cyclin B-cdk1 (Cdk1) complex, master mitotic kinase. During interphase, Cdk1 accumulates in an inactive state (pre-Cdk1). When Cdk1 overcomes a certain threshold of activity upon initial activation of pre-Cdk1, then the stockpiled pre-Cdk1 is rapidly converted into overshooting active Cdk1, and mitosis is established irreversibly in a switch-like fashion. This is granted by positive Cdk1 activation loops and the concomitant inactivation of Cdk1 counteracting phosphatases, empowering Cdk1 activity and favoring the Cdk1-dependent phosphorylations that are required to establish mitosis. These circuitries prevent backtracking and ensure unidirectionality so that interphase and mitosis are considered bistable states. Mitosis also shows hysteresis, meaning that the levels of Cdk1 activity needed to establish mitosis are higher than those required to maintain it; therefore, once in mitosis cells can tolerate moderate drops in Cdk1 activity without exiting mitosis. Whether these features have other functional implications in addition to the general action of preventing backtracking is unknown. Here, we contextualize these concepts in the view of recent evidence indicating that loss of activity of small and compartmentalized amounts of Cdk1 within mitosis is necessary to assemble the mitotic spindle, the structure required to segregate replicated chromosomes. We further propose that, in addition to prevent backtracking, the stability and hysteresis properties of mitosis are also essential to move forward in mitosis by allowing cells to bear small, localized, drops in Cdk1 activity that are necessary to build the mitotic spindle.

ATM- and ATR-induced primary ciliogenesis promotes cisplatin resistance in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal cancers because of its late diagnosis and chemoresistance. Primary cilia, the cellular antennae, are observed in most human cells to maintain development and differentiation. Primary cilia are gradually lost during the progression of pancreatic cancer and are eventually absent in PDAC. Here, we showed that cisplatin-resistant PDAC regrew primary cilia. Additionally, genetic or pharmacological disruption of primary cilia sensitized PDAC to cisplatin treatment. Mechanistically, ataxia telangiectasia mutated (ATM) and ATM and RAD3-related (ATR), tumor suppressors that initiate DNA damage responses, promoted the excessive formation of centriolar satellites (EFoCS) and autophagy activation. Disruption of EFoCS and autophagy inhibited primary ciliogenesis, sensitizing PDAC cells to cisplatin treatment. Collectively, our findings revealed an unexpected interplay among the DNA damage response, primary cilia, and chemoresistance in PDAC and deciphered the molecular mechanism by which ATM/ATR-mediated EFoCS and autophagy cooperatively regulate primary ciliogenesis.

Molecular Link between DNA Damage Response and Microtubule Dynamics

Microtubules are major components of the cytoskeleton that play important roles in cellular processes such as intracellular transport and cell division. In recent years, it has become evident that microtubule networks play a role in genome maintenance during interphase. In this review, we highlight recent advances in understanding the role of microtubule dynamics in DNA damage response and repair. We first describe how DNA damage checkpoints regulate microtubule organization and stability. We then highlight how microtubule networks are involved in the nuclear remodeling following DNA damage, which leads to changes in chromosome organization. Lastly, we discuss how microtubule dynamics participate in the mobility of damaged DNA and promote consequent DNA repair. Together, the literature indicates the importance of microtubule dynamics in genome organization and stability during interphase.

An updated view on the centrosome as a cell cycle regulator

The centrosome is a multifunctional organelle that is known primarily for its microtubule organising function. Centrosomal defects caused by changes in centrosomal structure or number have been associated with human diseases ranging from congenital defects to cancer. We are only beginning to appreciate how the non-microtubule organising roles of the centrosome are related to these clinical conditions. In this review, we will discuss the historical evidence that led to the proposal that the centrosome participates in cell cycle regulation. We then summarize the body of work that describes the involvement of the mammalian centrosome in triggering cell cycle progression and checkpoint signalling. Then we will highlight work from the fission yeast model organism, revealing the molecular details that explain how the spindle pole body (SPB, the yeast functional equivalent of the centrosome), participates in these cell cycle transitions. Importantly, we will discuss some of the emerging questions from recent discoveries related to the role of the centrosome as a cell cycle regulator.

Tools used to assay genomic instability in cancers and cancer meiomitosis

Genomic instability is a defining characteristic of cancer and the analysis of DNA damage at the chromosome level is a crucial part of the study of carcinogenesis and genotoxicity. Chromosomal instability (CIN), the most common level of genomic instability in cancers, is defined as the rate of loss or gain of chromosomes through successive divisions. As such, DNA in cancer cells is highly unstable. However, the underlying mechanisms remain elusive. There is a debate as to whether instability succeeds transformation, or if it is a by-product of cancer, and therefore, studying potential molecular and cellular contributors of genomic instability is of high importance. Recent work has suggested an important role for ectopic expression of meiosis genes in driving genomic instability via a process called meiomitosis. Improving understanding of these mechanisms can contribute to the development of targeted therapies that exploit DNA damage and repair mechanisms. Here, we discuss a workflow of novel and established techniques used to assess chromosomal instability as well as the nature of genomic instability such as double strand breaks, micronuclei, and chromatin bridges. For each technique, we discuss their advantages and limitations in a lab setting. Lastly, we provide detailed protocols for the discussed techniques.

Targeting cell cycle regulation via the G2-M checkpoint for synthetic lethality in melanoma

Disruption of cell cycle checkpoints has been well established as a hallmark of cancer. In particular, the G1-S transition mediated by the cyclin D-cyclin-dependent kinase 4/6 (CDK4/6) pathway is dysregulated in more than 90% of melanoma cases. Therefore, tumor cells mainly rely on the G2-M checkpoint to halt the cell cycle in order to repair DNA damage. Here, we review the promising method of cell cycle-mediated synthetic lethality for melanoma treatment, which entails exploiting somatically acquired mutations in the G1-S transition with inhibitors of the G2-M transition in order to specifically kill melanoma cells. The idea stems from the theory that melanoma cells lacking G1-S checkpoints are particularly vulnerable to mitotic catastrophe when presented with G2-M checkpoint inhibition in addition to DNA damage, whereas normal cells with intact G1-S checkpoints should theoretically be spared. This review explores the link between cell cycle dysregulation and synthetic lethality in melanoma cells and discusses potential future applications for this treatment.

Wip1 controls the translocation of the chromosomal passenger complex to the central spindle for faithful mitotic exit

Dramatic cellular reorganization in mitosis critically depends on the timely and temporal phosphorylation of a broad range of proteins, which is mediated by the activation of the mitotic kinases and repression of counteracting phosphatases. The mitosis-to-interphase transition, which is termed mitotic exit, involves the removal of mitotic phosphorylation by protein phosphatases. Although protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A) drive this reversal in animal cells, the phosphatase network associated with ordered bulk dephosphorylation in mitotic exit is not fully understood. Here, we describe a new mitotic phosphatase relay in which Wip1/PPM1D phosphatase activity is essential for chromosomal passenger complex (CPC) translocation to the anaphase central spindle after release from the chromosome via PP1-mediated dephosphorylation of histone H3T3. Depletion of endogenous Wip1 and overexpression of the phosphatase-dead mutant disturbed CPC translocation to the central spindle, leading to failure of cytokinesis. While Wip1 was degraded in early mitosis, its levels recovered in anaphase and the protein functioned as a Cdk1-counteracting phosphatase at the anaphase central spindle and midbody. Mechanistically, Wip1 dephosphorylated Thr-59 in inner centromere protein (INCENP), which, subsequently bound to MKLP2 and recruited other components to the central spindle. Furthermore, Wip1 overexpression is associated with the overall survival rate of patients with breast cancer, suggesting that Wip1 not only functions as a weak oncogene in the DNA damage network but also as a tumor suppressor in mitotic exit. Altogether, our findings reveal that sequential dephosphorylation of mitotic phosphatases provides spatiotemporal regulation of mitotic exit to prevent tumor initiation and progression.

Reality CHEK: Understanding the biology and clinical potential of CHK1

The DNA damage response enables cells to cope with various stresses that threaten genomic integrity. A critical component of this response is the serine/threonine kinase CHK1 which is encoded by the CHEK1 gene. Originally identified as a regulator of the G2/M checkpoint, CHK1 has since been shown to play important roles in DNA replication, mitotic progression, DNA repair, and overall cell cycle regulation. However, the potential of CHK1 as a cancer therapy has not been realized clinically. Herein we expound our current understanding of the principal roles of CHK1 and highlight different avenues for CHK1 targeting in cancer therapy.

Functions of cyclins and CDKs in mammalian gametogenesis†

Cyclins and cyclin-dependent kinases (CDKs) are key regulators of the cell cycle. Most of our understanding of their functions has been obtained from studies in single-cell organisms and mitotically proliferating cultured cells. In mammals, there are more than 20 cyclins and 20 CDKs. Although genetic ablation studies in mice have shown that most of these factors are dispensable for viability and fertility, uncovering their functional redundancy, CCNA2, CCNB1, and CDK1 are essential for embryonic development. Cyclin/CDK complexes are known to regulate both mitotic and meiotic cell cycles. While some mechanisms are common to both types of cell divisions, meiosis has unique characteristics and requirements. During meiosis, DNA replication is followed by two successive rounds of cell division. In addition, mammalian germ cells experience a prolonged prophase I in males or a long period of arrest in prophase I in females. Therefore, cyclins and CDKs may have functions in meiosis distinct from their mitotic functions and indeed, meiosis-specific cyclins, CCNA1 and CCNB3, have been identified. Here, we describe recent advances in the field of cyclins and CDKs with a focus on meiosis and early embryogenesis.

Aurora kinases and DNA damage response

It is well established that Aurora kinases perform critical functions during mitosis. It has become increasingly clear that the Aurora kinases also perform a myriad of non-mitotic functions including DNA damage response. The available evidence indicates that inhibition Aurora kinase A (AURKA) may contribute to the G₂ DNA damage checkpoint through AURKA's functions in PLK1 and CDC25B activation. Both AURKA and Aurora kinase B (AURKB) are also essential in mitotic DNA damage response that guard against DNA damage-induced chromosome segregation errors, including the control of abscission checkpoint and prevention of micronuclei formation. Dysregulation of Aurora kinases can trigger DNA damage in mitosis that is sensed in the subsequent G₁ by a p53-dependent postmitotic checkpoint. Aurora kinases are themselves linked to the G₁ DNA damage checkpoint through p53 and p73 pathways. Finally, several lines of evidence provide a connection between Aurora kinases and DNA repair and apoptotic pathways. Although more studies are required to provide a comprehensive picture of how cells respond to DNA damage, these findings indicate that both AURKA and AURKB are inextricably linked to pathways guarding against DNA damage. They also provide a rationale to support more detailed studies on the synergism between small-molecule inhibitors against Aurora kinases and DNA-damaging agents in cancer therapies.

DNA Replication Stress and Chromosomal Instability: Dangerous Liaisons

Chromosomal instability (CIN) is associated with many human diseases, including neurodevelopmental or neurodegenerative conditions, age-related disorders and cancer, and is a key driver for disease initiation and progression. A major source of structural chromosome instability (s-CIN) leading to structural chromosome aberrations is "replication stress", a condition in which stalled or slowly progressing replication forks interfere with timely and error-free completion of the S phase. On the other hand, mitotic errors that result in chromosome mis-segregation are the cause of numerical chromosome instability (n-CIN) and aneuploidy. In this review, we will discuss recent evidence showing that these two forms of chromosomal instability can be mechanistically interlinked. We first summarize how replication stress causes structural and numerical CIN, focusing on mechanisms such as mitotic rescue of replication stress (MRRS) and centriole disengagement, which prevent or contribute to specific types of structural chromosome aberrations and segregation errors. We describe the main outcomes of segregation errors and how micronucleation and aneuploidy can be the key stimuli promoting inflammation, senescence, or chromothripsis. At the end, we discuss how CIN can reduce cellular fitness and may behave as an anticancer barrier in noncancerous cells or precancerous lesions, whereas it fuels genomic instability in the context of cancer, and how our current knowledge may be exploited for developing cancer therapies.

A guiding torch at the poles: the multiple roles of spindle microtubule-organizing centers during cell division

The spindle constitutes the cellular machinery that enables the segregation of the chromosomes during eukaryotic cell division. The microtubules that form this fascinating and complex genome distribution system emanate from specialized structures located at both its poles and known as microtubule-organizing centers (MTOCs). Beyond their structural function, the spindle MTOCs play fundamental roles in cell cycle control, the activation and functionality of the mitotic checkpoints and during cellular aging. This review highlights the pivotal importance of spindle-associated MTOCs in multiple cellular processes and their central role as key regulatory hubs where diverse intracellular signals are integrated and coordinated to ensure the successful completion of cell division and the maintenance of the replicative lifespan.

The Role of Hydrogen Peroxide and Peroxiredoxins throughout the Cell Cycle

Hydrogen peroxide (H₂O₂) is an oxidizing agent that induces cellular damage at inappropriate concentrations and gives rise to an arrest during cell cycle progression, causing cell death. Recent evidence indicates that H₂O₂ also acts as a promoter for cell cycle progression by oxidizing specific thiol proteins. The intracellular concentration of H₂O₂ is regulated tightly, enabling its use as a cellular signaling molecule while minimizing its potential to cause cellular damage. Peroxiredoxins (Prxs) have peroxidase activity toward H₂O₂, organic hydroperoxides, and peroxynitrite for protecting cells from oxidative stress. They are suggested to work as signaling mediators, allowing the local accumulation of H₂O₂ by inactivating their peroxidase activity uniquely compared with other antioxidant proteins such as catalase and glutathione peroxidase. Given that Prxs are highly sensitive to oxidation by H₂O₂, they act as sensors and transducers of H₂O₂ signaling via transferring their oxidation state to effector proteins. The concentrations of intracellular H₂O₂ increase as the cell cycle progresses from G₁ to mitosis. Here, we summarize the roles of Prxs with regard to the regulation of cell cycle-dependent kinase activity and anaphase-promoting complex/cyclosome in terms of changes in H₂O₂ levels. Protection of the cell from unwanted progression of the cell cycle is suggested to be a role of Prx. We discuss the possible roles of Prxs to control H₂O₂ levels.

DNA damage response proteins regulating mitotic cell division: double agents preserving genome stability

The DNA damage response recognizes DNA lesions and coordinates a cell cycle arrest with the repair of the damaged DNA, or removal of the affected cells to prevent the passage of genetic alterations to the next generation. The mitotic cell division, on the other hand, is a series of processes that aims to accurately segregate the genomic material from the maternal to the two daughter cells. Despite their great importance in safeguarding genomic integrity, the DNA damage response and the mitotic cell division were long viewed as unrelated processes, mainly because animal cells that are irradiated during mitosis continue cell division without repairing the broken chromosomes. However, recent studies have demonstrated that DNA damage proteins play an important role in mitotic cell division. This is performed through regulation of the onset of mitosis, mitotic spindle formation, correction of misattached kinetochore-microtubules, spindle checkpoint signaling, or completion of cytokinesis (abscission), in the absence of DNA damage. In this review, we summarize the roles of DNA damage proteins in unperturbed mitosis, analyze the molecular mechanisms involved, and discuss the potential implications of these findings in cancer therapy.

Cytostatic effects of plant essential oils on human skin and lung cells

Essential oils are volatile compounds extracted from various plants by distillation, hydrodiffusion or compression. In recent years, the use of essential oils has gained popularity. Many pharmaceutical, cosmetic, sanitary, food industry and agriculture studies have revealed that essential oils exert antibacterial, antiviral, antifungal, antiparasitic, insecticidal, anticancer, neuroprotective, psychophysiological and anti-aging effects. Despite their reported uses, recent studies of eukaryotic cells have demonstrated that essential oils exert prooxidant and cytotoxic effects. Therefore, for the effective clinical use of essential oils, an evaluation of their cytotoxicity and the identification of the mechanisms affecting cell viability are required. To evaluate cytotoxicity, the present study determined the IC₅₀ values of 15 essential oils provided by the Korea Forest Research Institute (Pinus densiflora for. multicaulis Uyeki, Trifolium repens, Ligularia fischeri, Abies nephrolepis, Illicium anisatum, Zanthoxylum coreanum, Abies koreana, Lindera obtusiloba, Chamaecyparis obtuse, Pinus densiflora, Magnolia kobus, Picea koraiensis, Picea abies, Abies holophylla and Platycladus orientalis). Their effect was then assessed in human lung cells (A549) and human skin cells (Detroit 551) by performing cell counting kit-8 assays. To identify the mechanism associated with each oil's cytotoxicity, expressions of cytotoxicity-associated marker genes (cyclin A, cyclin B, cyclin D and cyclin E) involved in the cell cycle and caspase-3 (involved in cell death) were examined by performing reverse transcription-quantitative PCR and western blotting. In conclusion, plant essential oils can be used as a good source of medicine. However, without examining the safety of essential oils, they cannot be used in clinics. The results included estimates of the degree of cytotoxicity and the mechanism of cell death for each oil. It is expected that the data obtained from the current study will form guidelines for the clinically appropriate and safe use of these tested essential oils.

FOCAD loss impacts microtubule assembly, G2/M progression and patient survival in astrocytic gliomas

In search of novel genes associated with glioma pathogenesis, we have previously shown frequent deletions of the KIAA1797/FOCAD gene in malignant gliomas, and a tumor suppressor function of the encoded focadhesin impacting proliferation and migration of glioma cells in vitro and in vivo. Here, we examined an association of reduced FOCAD gene copy number with overall survival of patients with astrocytic gliomas, and addressed the molecular mechanisms that govern the suppressive effect of focadhesin on glioma growth. FOCAD loss was associated with inferior outcome in patients with isocitrate dehydrogenase 1 or 2 (IDH)-mutant astrocytic gliomas of WHO grades II-IV. Multivariate analysis considering age at diagnosis as well as IDH mutation, MGMT promoter methylation, and CDKN2A/B homozygous deletion status confirmed reduced FOCAD gene copy number as a prognostic factor for overall survival. Using a yeast two-hybrid screen and pull-down assays, tubulin beta-6 and other tubulin family members were identified as novel focadhesin-interacting partners. Tubulins and focadhesin co-localized to centrosomes where focadhesin was enriched in proximity to centrioles. Focadhesin was recruited to microtubules via its interaction partner SLAIN motif family member 2 and reduced microtubule assembly rates, possibly explaining the focadhesin-dependent decrease in cell migration. During the cell cycle, focadhesin levels peaked in G2/M phase and influenced time-dependent G2/M progression potentially via polo like kinase 1 phosphorylation, providing a possible explanation for focadhesin-dependent cell growth reduction. We conclude that FOCAD loss may promote biological aggressiveness and worsen clinical outcome of diffuse astrocytic gliomas by enhancing microtubule assembly and accelerating G2/M phase progression.

Inhibition of the ATR-CHK1 Pathway in Ewing Sarcoma Cells Causes DNA Damage and Apoptosis via the CDK2-Mediated Degradation of RRM2

Inhibition of ribonucleotide reductase (RNR), the rate-limiting enzyme in the synthesis of deoxyribonucleotides, causes DNA replication stress and activates the ataxia telangiectasia and rad3-related protein (ATR)-checkpoint kinase 1 (CHK1) pathway. Notably, a number of different cancers, including Ewing sarcoma tumors, are sensitive to the combination of RNR and ATR-CHK1 inhibitors. However, multiple, overlapping mechanisms are reported to underlie the toxicity of ATR-CHK1 inhibitors, both as single agents and in combination with RNR inhibitors, toward cancer cells. Here, we identified a feedback loop in Ewing sarcoma cells in which inhibition of the ATR-CHK1 pathway depletes RRM2, the small subunit of RNR, and exacerbates the DNA replication stress and DNA damage caused by RNR inhibitors. Mechanistically, we identified that the inhibition of ATR-CHK1 activates CDK2, which targets RRM2 for degradation via the proteasome. Similarly, activation of CDK2 by inhibition or knockdown of the WEE1 kinase also depletes RRM2 and causes DNA damage and apoptosis. Moreover, we show that the concurrent inhibition of ATR and WEE1 has a synergistic effect in Ewing sarcoma cells. Overall, our results provide novel insight into the response to DNA replication stress, as well as a rationale for targeting the ATR, CHK1, and WEE1 pathways, in Ewing sarcoma tumors. IMPLICATIONS: Targeting the ATR, CHK1, and WEE1 kinases in Ewing sarcoma cells activates CDK2 and increases DNA replication stress by promoting the proteasome-mediated degradation of RRM2.

The Cytotoxicity Effect of Resveratrol: Cell Cycle Arrest and Induced Apoptosis of Breast Cancer 4T1 Cells

Resveratrol, a natural polyterpenoid, can scavenge reactive oxygen species in vivo to carry out the functions of antioxidation and antiaging. Resveratrol’s anti-cancer capability has attracted widespread attention, but its molecular mechanism has not been systematically explained. In this study, by comparing the activity of normal cell lines and cancer cell lines after treating with resveratrol, it was found that resveratrol has more significant cytotoxicity in cancer cell lines. Resveratrol could play a toxic role through inducing apoptosis of the cancer cell in a time- and concentration-dependent manner. A total of 330 significantly differential genes were identified through large-scale transcriptome sequencing, among which 103 genes were upregulated and 227 genes were downregulated. Transcriptome and qRT-PCR data proved that a large number of genes related to cell cycle were differentially expressed after the treatment of resveratrol. The changes of cell cycle phases at different time points after treating with resveratrol were further detected, and it was found that the cells were arrested in the S phase because of the percentage of cells in S phase increased and cells in G1/G0 phase decreased. In conclusion, resveratrol can inhibit the proliferation of 4T1 cancer cells by inhibiting cell cycle and inducing apoptosis.

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.

Mild replication stress causes chromosome mis-segregation via premature centriole disengagement

Replication stress, a hallmark of cancerous and pre-cancerous lesions, is linked to structural chromosomal aberrations. Recent studies demonstrated that it could also lead to numerical chromosomal instability (CIN). The mechanism, however, remains elusive. Here, we show that inducing replication stress in non-cancerous cells stabilizes spindle microtubules and favours premature centriole disengagement, causing transient multipolar spindles that lead to lagging chromosomes and micronuclei. Premature centriole disengagement depends on the G2 activity of the Cdk, Plk1 and ATR kinases, implying a DNA-damage induced deregulation of the centrosome cycle. Premature centriole disengagement also occurs spontaneously in some CIN+ cancer cell lines and can be suppressed by attenuating replication stress. Finally, we show that replication stress potentiates the effect of the chemotherapeutic agent taxol, by increasing the incidence of multipolar cell divisions. We postulate that replication stress in cancer cells induces numerical CIN via transient multipolar spindles caused by premature centriole disengagement.

Chromosome instability can be caused by replication stress, although the mechanism is unclear. Here, the authors show that inducing mild replication stress in cancerous and non-cancerous cell lines leads to centriole disengagement and the subsequent formation of lagging chromosomes and micronuclei.

BK Polyomavirus Activates the DNA Damage Response To Prolong S Phase

BK polyomavirus (PyV) is a major source of kidney failure in transplant recipients. The standard treatment for patients with lytic BKPyV infection is to reduce immunosuppressive therapy, which increases the risk of graft rejection. PyVs are DNA viruses that rely upon host replication proteins for viral genome replication. A hallmark of PyV infection is activation of the DNA damage response (DDR) to prevent severe host and viral DNA damage that impairs viral production by an unknown mechanism. Therefore, we sought to better understand why BKPyV activates the DDR through the ATR and ATM pathways and how this prevents DNA damage and leads to increased viral production. When ATR was inhibited in BKPyV-infected primary kidney cells, severe DNA damage occurred due to premature Cdk1 activation, which resulted in mitosis of cells that were actively replicating host DNA in S phase. Conversely, ATM was required for efficient entry into S phase and to prevent normal mitotic entry after G₂ phase. The synergistic activation of these DDR kinases promoted and maintained BKPyV-mediated S phase to enhance viral production. In contrast to BKPyV infection, DDR inhibition did not disrupt cell cycle control in uninfected cells. This suggests that DDR inhibitors may be used to specifically target BKPyV-infected cells.IMPORTANCE BK polyomavirus (BKPyV) is an emerging pathogen that reactivates in immunosuppressed organ transplant patients. We wanted to understand why BKPyV-induced activation of the DNA damage response (DDR) enhances viral titers and prevents host DNA damage. Here, we show that the virus activates the DNA damage response in order to keep the infected cells in S phase to replicate the viral DNA. The source of DNA damage was due to actively replicating cells with uncondensed chromosomes entering directly into mitosis when the DDR was inhibited in BKPyV-infected cells. This study clarifies the previously enigmatic role of the DDR during BKPyV infection by demonstrating that the virus activates the DDR to maintain the cells in S phase in order to promote viral replication and that disruption of this cell cycle arrest can lead to catastrophic DNA damage for the host.

Motile ciliogenesis and the mitotic prism

Motile cilia of epithelial multiciliated cells transport vital fluids along organ lumens to promote essential respiratory, reproductive and brain functions. Progenitors of multiciliated cells undergo massive and coordinated organelle remodelling during their differentiation for subsequent motile ciliogenesis. Defects in multiciliated cell differentiation lead to severe cilia-related diseases by perturbing cilia-based flows. Recent work designated the machinery of mitosis as the orchestrator of the orderly progression of differentiation associated with multiple motile cilia formation. By examining the events leading to motile ciliogenesis with a methodological prism of mitosis, we contextualise and discuss the recent findings to broaden the spectrum of questions related to the differentiation of mammalian multiciliated cells.

Pharmacological Inhibition of PARP6 Triggers Multipolar Spindle Formation and Elicits Therapeutic Effects in Breast Cancer

: PARP proteins represent a class of post-translational modification enzymes with diverse cellular functions. Targeting PARPs has proven to be efficacious clinically, but exploration of the therapeutic potential of PARP inhibition has been limited to targeting poly(ADP-ribose) generating PARP, including PARP1/2/3 and tankyrases. The cancer-related functions of mono(ADP-ribose) generating PARP, including PARP6, remain largely uncharacterized. Here, we report a novel therapeutic strategy targeting PARP6 using the first reported PARP6 inhibitors. By screening a collection of PARP compounds for their ability to induce mitotic defects, we uncovered a robust correlation between PARP6 inhibition and induction of multipolar spindle (MPS) formation, which was phenocopied by PARP6 knockdown. Treatment with AZ0108, a PARP6 inhibitor with a favorable pharmacokinetic profile, potently induced the MPS phenotype, leading to apoptosis in a subset of breast cancer cells in vitro and antitumor effects in vivo. In addition, Chk1 was identified as a specific substrate of PARP6 and was further confirmed by enzymatic assays and by mass spectrometry. Furthermore, when modification of Chk1 was inhibited with AZ0108 in breast cancer cells, we observed marked upregulation of p-S345 Chk1 accompanied by defects in mitotic signaling. Together, these results establish proof-of-concept antitumor efficacy through PARP6 inhibition and highlight a novel function of PARP6 in maintaining centrosome integrity via direct ADP-ribosylation of Chk1 and modulation of its activity. SIGNIFICANCE: These findings describe a new inhibitor of PARP6 and identify a novel function of PARP6 in regulating activation of Chk1 in breast cancer cells.

Cell adhesion is regulated by CDK1 during the cell cycle

In most tissues, anchorage-dependent growth and cell cycle progression are dependent on cells engaging extracellular matrices (ECMs) via integrin-receptor adhesion complexes. In a highly conserved manner, cells disassemble adhesion complexes, round up, and retract from their surroundings before division, suggestive of a primordial link between the cell cycle machinery and the regulation of cell adhesion to the ECM. In this study, we demonstrate that cyclin-dependent kinase 1 (CDK1) mediates this link. CDK1, in complex with cyclin A2, promotes adhesion complex and actin cytoskeleton organization during interphase and mediates a large increase in adhesion complex area as cells transition from G1 into S. Adhesion complex area decreases in G2, and disassembly occurs several hours before mitosis. This loss requires elevated cyclin B1 levels and is caused by inhibitory phosphorylation of CDK1-cyclin complexes. The inactivation of CDK1 is therefore the trigger that initiates remodeling of adhesion complexes and the actin cytoskeleton in preparation for rapid entry into mitosis.

A PIM-CHK1 signaling pathway regulates PLK1 phosphorylation and function during mitosis

Although the kinase CHK1 is a key player in the DNA damage response (DDR), several studies have recently provided evidence of DDR-independent roles of CHK1, in particular following phosphorylation of its S280 residue. Here, we demonstrate that CHK1 S280 phosphorylation is cell cycle-dependent and peaks during mitosis. We found that this phosphorylation was catalyzed by the kinase PIM2, whose protein expression was also increased during mitosis. Importantly, we identified polo-like kinase 1 (PLK1) as a direct target of CHK1 during mitosis. Genetic or pharmacological inhibition of CHK1 reduced the activating phosphorylation of PLK1 on T210, and recombinant CHK1 was able to phosphorylate T210 of PLK1 in vitro Accordingly, S280-phosphorylated CHK1 and PLK1 exhibited similar specific mitotic localizations, and PLK1 was co-immunoprecipitated with S280-phosphorylated CHK1 from mitotic cell extracts. Moreover, CHK1-mediated phosphorylation of PLK1 was dependent on S280 phosphorylation by PIM2. Inhibition of PIM proteins reduced cell proliferation and mitotic entry, which was rescued by expressing a T210D phosphomimetic mutant of PLK1. Altogether, these data identify a new PIM-CHK1-PLK1 phosphorylation cascade that regulates different mitotic steps independently of the CHK1 DDR function.This article has an associated First Person interview with the first author of the paper.

Centrosome aberrations and chromosome instability contribute to tumorigenesis and intra-tumor heterogeneity

Centrosomes serve as the major microtubule organizing centers in cells and thereby contribute to cell shape, polarity, and motility. Also, centrosomes ensure equal chromosome segregation during mitosis. Centrosome aberrations arise when the centrosome cycle is deregulated, or as a result of cytokinesis failure. A long-standing postulate is that centrosome aberrations are involved in the initiation and progression of cancer. However, this notion has been a subject of controversy because until recently the relationship has been correlative. Recently, it was shown that numerical or structural centrosome aberrations can initiate tumors in certain tissues in mice, as well as invasion. Particularly, we will focus on centrosome amplification and chromosome instability as drivers of intra-tumor heterogeneity and their consequences in cancer. We will also discuss briefly the controversies surrounding this theory to highlight the fact that the role of both centrosome amplification and chromosome instability in cancer is highly context-dependent. Further, we will discuss single-cell sequencing as a novel technique to understand intra-tumor heterogeneity and some therapeutic approaches to target chromosome instability.

Genetic interaction between two insulin-dependent diabetes susceptibility loci, Idd2 and Idd13, in determining immunoregulatory DN T cell proportion

Several immune regulatory cell types participate in the protection against autoimmune diseases such as autoimmune diabetes. Of these immunoregulatory cells, we and others have shown that peripheral CD4^-CD8^- double negative (DN) T cells can induce antigen-specific immune tolerance. Particularly, we have described that diabetes-prone mice exhibit a lower number of peripheral DN T cells compared to diabetes-resistant mice. Identifying the molecular pathways that influence the size of the DN T cell pool in peripheral lymphoid organs may thus be of interest for maintaining antigen-specific immune tolerance. Hence, through immunogenetic approaches, we found that two genetic loci linked to autoimmune diabetes susceptibility, namely Idd2 and Idd13, independently contribute to the partial restoration of DN T cell proportion in secondary lymphoid organs. We now extend these findings to show an interaction between the Idd2 and Idd13 loci in determining the number of DN T cells in secondary lymphoid organs. Using bioinformatics tools, we link potential biological pathways arising from interactions of genes encoded within the two loci. By focusing on cell cycle, we validate that both the Idd2 and Idd13 loci influence RAD51 expression as well as DN T cell progression through the cell cycle. Altogether, we find that genetic interactions between Idd2 and Idd13 loci modulate cell cycle progression, which contributes, at least in part, to defining the proportion of DN T cells in secondary lymphoid organs.

Autophagy is a pro-survival mechanism in ovarian cancer against the apoptotic effects of euxanthone

BACKGROUND

Ovarian cancer is one of the most prevalent gynecological malignancies and thus the development of novel therapeutic agents for managing ovarian cancer is imperative. Euxanthone, a xanthone derived from Polygala caudata, has been found to exert cytotoxic effects on cancerous cells. This study was designed to assess the role of euxanthone in ovarian cancer.

METHODS AND MATERIALS

Cell Counting Kit-8 (CCK-8) assay was utilized to assess the viability of human ovarian cancer SKOV3 and OVCAR3 cell lines. The population of apoptotic cells was measured by flow cytometry and TUNEL assay. Cell cycle analysis was carried out by flow cytometry. Autophagy was determined by western blotting to detect LC3-II, p62 degradation, and Beclin1 expression, by transfection with GFP-LC3B expressing plasmid and by flow cytometry. To examine the role of STAT3 in the induction of autophagy and apoptosis by euxanthone, STAT3 expression was suppressed using siRNA. Moreover, xenograft model was established to evaluate the therapeutic effect of euxanthone in vivo.

RESULTS

Euxanthone decreased cell viability and blocked cell cycle progression at G2/M phase. Euxanthone induced apoptotic cell death in a caspase-dependent manner in ovarian cancer cells. Euxanthone treatment also led to the accumulation of autophagosomes. We also found that inhibition of autophagy by 3-MA or Beclin1 siRNA enhanced the pro-apoptotic effect of euxanthonein ovarian cancer cells. Furthermore, our results revealed that euxanthone induced apoptosis and autophagy by modulating pSTAT3/Bcl-2 signaling. In vivo data also demonstrated that euxanthone exerted anti-tumor activities without harming healthy tissues.

CONCLUSION

Euxanthone induced cytoprotective autophagy in ovarian cancer cells, which negatively contributed to its anti-tumor activities. Our findings provide preliminary experimental data that support further investigation on the therapeutic efficacy of euxanthone in ovarian cancer.

Signaling protein signature predicts clinical outcome of non-small-cell lung cancer

BACKGROUND

Non-small-cell lung cancer (NSCLC) is characterized by abnormalities of numerous signaling proteins that play pivotal roles in cancer development and progression. Many of these proteins have been reported to be correlated with clinical outcomes of NSCLC. However, none of them could provide adequate accuracy of prognosis prediction in clinical application.

METHODS

A total of 384 resected NSCLC specimens from two hospitals in Beijing (BJ) and Chongqing (CQ) were collected. Using immunohistochemistry (IHC) staining on stored formalin-fixed paraffin-embedded (FFPE) surgical samples, we examined the expression levels of 75 critical proteins on BJ samples. Random forest algorithm (RFA) and support vector machines (SVM) computation were applied to identify protein signatures on 2/3 randomly assigned BJ samples. The identified signatures were tested on the remaining BJ samples, and were further validated with CQ independent cohort.

RESULTS

A 6-protein signature for adenocarcinoma (ADC) and a 5-protein signature for squamous cell carcinoma (SCC) were identified from training sets and tested in testing sets. In independent validation with CQ cohort, patients can also be divided into high- and low-risk groups with significantly different median overall survivals by Kaplan-Meier analysis, both in ADC (31 months vs. 87 months, HR 2.81; P <  0.001) and SCC patients (27 months vs. not reached, HR 9.97; P <  0.001). Cox regression analysis showed that both signatures are independent prognostic indicators and outperformed TNM staging (ADC: adjusted HR 3.07 vs. 2.43, SCC: adjusted HR 7.84 vs. 2.24). Particularly, we found that only the ADC patients in high-risk group significantly benefited from adjuvant chemotherapy (P = 0.018).

CONCLUSIONS

Both ADC and SCC protein signatures could effectively stratify the prognosis of NSCLC patients, and may support patient selection for adjuvant chemotherapy.

Isoliquiritigenin Induces Autophagy and Inhibits Ovarian Cancer Cell Growth

Ovarian cancer is one of the commonest gynecologic malignancies, which has a poor prognosis for patients at the advanced stage. Isoliquiritigenin (ISL), an active flavonoid component of the licorice plant, previously demonstrated antioxidant, anti-inflammatory, and tumor suppressive effects. In this study, we investigated the antitumor effect of ISL on human ovarian cancer in vitro using the human ovarian cancer cell lines, OVCAR5 and ES-2, as model systems. Our results show that ISL significantly inhibited the viability of cancer cells in a concentration- and time-dependent manner. Flow cytometry analysis indicated that ISL induced G2/M phase arrest. Furthermore, the expression of cleaved PARP, cleaved caspase-3, Bax/Bcl-2 ratio, LC3B-II, and Beclin-1 levels were increased in western blot analysis. To clarify the role of autophagy and apoptosis in the effect of ISL, we used the autophagy inhibitor-3-methyladenine (3-MA) to attenuate the punctate fluorescence staining pattern of the p62/sequestosome 1 (SQSTM1, red fluorescence) and LC3 (green fluorescence) proteins after ISL treatment, and 3-MA inhibited the cytotoxicity of ISL. These findings provide new information about the link between ISL-induced autophagy and apoptosis and suggest that ISL is a candidate agent for the treatment of human ovarian cancer.

MK-8776, a novel Chk1 inhibitor, exhibits an improved radiosensitizing effect compared to UCN-01 by exacerbating radiation-induced aberrant mitosis

Checkpoint kinase 1 (Chk1) is an evolutionarily conserved serine/threonine kinase that plays an important role in G₂/M checkpoint signaling. Here, we evaluate the radiosensitizing effects of a novel selective Chk1 inhibitor MK-8776, comparing its efficacy with a first-generation Chk1 inhibitor UCN-01, and attempt to elucidate the mechanism of radiosensitization. In a clonogenic survival assay, MK-8776 demonstrated a more pronounced radiosensitizing effect than UCN-01, with lower cytotoxicity. Importantly, radiosensitization by MK-8776 can be achieved at doses as low as 2.5 Gy, which is a clinically applicable irradiation dose. MK-8776, but not UCN-01, exacerbated mitotic catastrophe (MC) and centrosome abnormalities, without affecting repair kinetics of DNA double strand breaks. Furthermore, live-cell imaging revealed that MK-8776 significantly abrogated the radiation-induced G₂/M checkpoint, prolonged the mitotic phase, and enhanced aberrant mitosis. This suggests that Chk1 inhibition by MK-8776 activates a spindle assembly checkpoint and increases mitotic defects in irradiated EMT6 cells. In conclusion, we have shown that, at minimally toxic concentrations, MK-8776 enhances radiation-induced cell death through the enhancement of aberrant mitosis and MC, without affecting DNA damage repair.

Rotavirus replication is correlated with S/G2 interphase arrest of the host cell cycle

In infected cells rotavirus (RV) replicates in viroplasms, cytosolic structures that require a stabilized microtubule (MT) network for their assembly, maintenance of the structure and perinuclear localization. Therefore, we hypothesized that RV could interfere with the MT-breakdown that takes place in mitosis during cell division. Using synchronized RV-permissive cells, we show that RV infection arrests the cell cycle in S/G2 phase, thus favoring replication by improving viroplasms formation, viral protein translation, and viral assembly. The arrest in S/G2 phase is independent of the host or viral strain and relies on active RV replication. RV infection causes cyclin B1 down-regulation, consistent with blocking entry into mitosis. With the aid of chemical inhibitors, the cytoskeleton network was linked to specific signaling pathways of the RV-induced cell cycle arrest. We found that upon RV infection Eg5 kinesin was delocalized from the pericentriolar region to the viroplasms. We used a MA104-Fucci system to identify three RV proteins (NSP3, NSP5, and VP2) involved in cell cycle arrest in the S-phase. Our data indicate that there is a strong correlation between the cell cycle arrest and RV replication.

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.

Alternative Chk1-independent S/M checkpoint in somatic cells that prevents premature mitotic entry

Genomic instability is the hallmark of cancer. Checkpoint kinase-1 (Chk1) is required for cell cycle delay after DNA damage or blocked DNA replication. Chk1-depleted tumor cells undergo premature mitosis and apoptosis. Here we analyzed the depletion of Chk1 in normal somatic cells in the absence of DNA damage in order to investigate alternative cell cycle checkpoint mechanism(s). By means of adenoviruses, flow cytometry, immunofluorescence and Western blotting, Chk1-depleted mouse embryonic fibroblasts (MEFs) were investigated. Chk1^-/- MEFs arrested at the S/G2 boundary of the cell cycle with decreased protein levels of many cell cycle key players. Cyclin B1 was predominantly cytoplasmic. Interestingly, overexpression of nuclear dominant Cyclin B1 leads to nuclear translocation and premature mitosis. Chk1^-/- MEFs exhibited the absence of double-strand breaks, yet cells showed delayed DNA damage recovery with pan-nuclear immunostaining pattern of Histone H2AX. Activation of this checkpoint would elicit a senescent-like phenotype. Taken together, our elaborated data revealed the existence of an additional S/M checkpoint functioning via γH2AX signaling and cytoplasmic retention of Cyclin B1 in somatic cells.

Rescue from replication stress during mitosis

Genomic instability is a hallmark of cancer and a common feature of human disorders, characterized by growth defects, neurodegeneration, cancer predisposition, and aging. Recent evidence has shown that DNA replication stress is a major driver of genomic instability and tumorigenesis. Cells can undergo mitosis with under-replicated DNA or unresolved DNA structures, and specific pathways are dedicated to resolving these structures during mitosis, suggesting that mitotic rescue from replication stress (MRRS) is a key process influencing genome stability and cellular homeostasis. Deregulation of MRRS following oncogene activation or loss-of-function of caretaker genes may be the cause of chromosomal aberrations that promote cancer initiation and progression. In this review, we discuss the causes and consequences of replication stress, focusing on its persistence in mitosis as well as the mechanisms and factors involved in its resolution, and the potential impact of incomplete replication or aberrant MRRS on tumorigenesis, aging and disease.

Centrosome - a promising anti-cancer target

The centrosome, an organelle discovered >100 years ago, is the main microtubule-organizing center in mammalian organisms. The centrosome is composed of a pair of centrioles surrounded by the pericentriolar material (PMC) and plays a major role in the regulation of cell cycle transitions (G1-S, G2-M, and metaphase-anaphase), ensuring the normality of cell division. Hundreds of proteins found in the centrosome exert a variety of roles, including microtubule dynamics, nucleation, and kinetochore–microtubule attachments that allow correct chromosome alignment and segregation. Errors in these processes lead to structural (shape, size, number, position, and composition), functional (abnormal microtubule nucleation and disorganized spindles), and numerical (centrosome amplification [CA]) centrosome aberrations causing aneuploidy and genomic instability. Compelling data demonstrate that centrosomes are implicated in cancer, because there are important oncogenic and tumor suppressor proteins that are localized in this organelle and drive centrosome aberrations. Centrosome defects have been found in pre-neoplasias and tumors from breast, ovaries, prostate, head and neck, lung, liver, and bladder among many others. Several drugs/compounds against centrosomal proteins have shown promising results. Other drugs have higher toxicity with modest or no benefits, and there are more recently developed agents being tested in clinical trials. All of this emerging evidence suggests that targeting centrosome aberrations may be a future avenue for therapeutic intervention in cancer research.

The Centrosome, a Multitalented Renaissance Organelle

The centrosome acts as a microtubule-organizing center (MTOC) from the G₁ to G₂ phases of the cell cycle; it can mature into a spindle pole during mitosis and/or transition into a cilium by elongating microtubules (MTs) from the basal body on cell differentiation or cell cycle arrest. New studies hint that the centrosome functions in more than MT organization. For instance, it has recently been shown that a specific substructure of the centrosome-the mother centriole appendages-are required for the recycling of endosomes back to the plasma membrane. This alone could have important implications for a renaissance in our understanding of the development of primary cilia, endosome recycling, and the immune response. Here, we review newly identified roles for the centrosome in directing membrane traffic, the immunological synapse, and the stress response.

CDC25B is involved in the centrosomal microtubule nucleation in two-cell stage mouse embryos

CDC25B has been demonstrated to activate the complex of CDK1/Cyclin B and trigger mitosis. We have recently demonstrated that p-CDC25B-Ser351 is located at the centrosomes of mouse oocytes and contributes to the release of mouse oocytes from prophase I arrest. But much less is known about CDC25B function at the centrosome in two-cell stage mouse embryos. Here we investigate the effect of CDC25B regulating the microtubules nucleation. Microinjection of anti-CDC25B antibody caused aberrant microtubule nucleation. In addition, embryos injected with anti-CDC25B antibody showed the marked absence of microtubule repolymerization and Nek2 foci after nocodazole washout. CDC25B overexpression caused microtubule-organizing center (MTOC) overduplication. Moreover, overexpression of CDC25B-▵65 mutant resulted in the loss of CDC25B localization in the perinuclear region and made CDC25B less efficient in inducing mitosis. We additionally identified that CDC25B is responsible for the pericentrin localization to the MTOC. Our data suggest an important role of CDC25B for microtubule nucleation and organization. N-terminal of CDC25B is required for regulating the microtubule dynamics and mitotic function.