Triggering the all-or-nothing switch into mitosis

RNAi screen in the Drosophila wing of genes encoding proteins related to cytoskeleton organization and cell division

Cell division and cytoskeleton organization are fundamental processes participating in the development of Drosophila imaginal discs. In this manuscript we describe the phenotypes in the adult fly wing generated by knockdowns of 85% of Drosophila genes encoding proteins likely related to the regulation of cell division and cytoskeleton organization. We also compile a molecular classification of these proteins into classes that describe their expected or known main biochemical characteristics, as well as mRNA expression in the wing disc and likely protein subcellular localization for a subset of these genes. Finally, we analyze in more detail one protein family of cytoskeleton genes (Arp2/3 complex), and define the consequences of interfering with cell division for wing growth and patterning.

Anti-cancer drug molecules targeting cancer cell cycle and proliferation

Cancer, a vicious clinical burden that potentiates maximum fatality for humankind, arises due to unregulated excessive cell division and proliferation through an eccentric expression of cell cycle regulator proteins. A set of evolutionarily conserved machinery controls the cell cycle in an extremely precise manner so that a cell that went through the cycle can produce a genetically identical copy. To achieve perfection, several checkpoints were placed in the cycle for surveillance; so, errors during the division were rectified by the repair strategies. However, irreparable damage leads to exit from the cell cycle and induces programmed cell death. In comparison to a normal cell, cancer cells facilitate the constitutive activation of many dormant proteins and impede negative regulators of the checkpoint. Extensive studies in the last few decades on cell division and proliferation of cancer cells elucidate the molecular mechanism of the cell-cycle regulators that are often targeted for the development of anti-cancer therapy. Each phase of the cell cycle has been regulated by a unique set of proteins including master regulators Cyclins, and CDKs, along with the accessory proteins such as CKI, Cdc25, error-responsive proteins, and various kinase proteins mainly WEE1 kinases, Polo-like kinases, and Aurora kinases that control cell division. Here in this chapter, we have analytically discussed the role of cell cycle regulators and proliferation factors in cancer progression and the rationale of using various cell cycle-targeting drug molecules as anti-cancer therapy.

Wee1 promotes cell proliferation and imatinib resistance in chronic myeloid leukemia via regulating DNA damage repair dependent on ATM-γH2AX-MDC1

BACKGROUND

The treatment of chronic myeloid leukemia (CML) is facing the dilemma of tyrosine kinase inhibitors (TKIs) resistance and disease recurrence. The dysfunctional DNA damage repair mechanism plays an essential role not only in the initiation and progression of hematological malignancies but also links to the development of TKI resistance. Deciphering the abnormally regulated DNA damage repair and proteins involved brings new insights into the therapy of leukemias. As a G2/M phase checkpoint kinase and a DNA damage repair checkpoint kinase engaged in the DNA damage response (DDR), along with an oncogenic driver present in various cancers, the particular involvement of Wee1 in DNA damage is far from clear. Deciphering its function and targeting it via modulating DNA repair pathways is important for improving our understanding of cancer treatment.

METHODS

Wee1 expression was assessed in cell lines using RT-qPCR and western blot, and Wee1 knockdown efficacy was validated using RT-qPCR, western blot, and immunofluorescence. Wee1 function was investigated by CCK-8, colony formation, and flow cytometry assay in vitro. Wee1 role in DNA repair and its interactions with other proteins were then studied using western blot, immunofluorescence, and double plasmid-repair studies. Finally, the CCK-8 and flow cytometry assay was utilized to investigate Wee1 and imatinib's synergistic effect, and a CML mouse model was constructed to study Wee1's role in carcinogenesis in vivo.

RESULTS

Wee1 was reported to respond quickly to DDR in an ATM-γH2AX-MDC1-dependent way upon DNA double-strand breaks (DSBs) occurrence, and it regulated homologous recombination by stimulating the recruitment of critical proteins RAD51/BRCA1 upon DSB sites. Wee1 was also revealed to be abnormally upregulated in CML cells. Further suppression of Wee1 not only causes cell cycle arrest and inhibits the proliferation of cancer cells but also enhances CML cell sensitivity to Imatinib in vitro and in vivo, possibly through an excessive accumulation of overall DSBs.

CONCLUSION

Wee1 is extensively involved in the DRR signaling and DSB repair pathway. Inhibiting abnormally elevated Wee1 benefits CML therapy in both IM-resistant and IM-sensitive cells. Our data demonstrated that Wee1 participated in promoting cell proliferation and imatinib resistance in chronic myeloid leukemia via regulating DNA damage repair dependent on ATM-γH2AX-MDC1. In the fight against CML, Wee1's dysregulation in the DNA damage repair mechanism of CML pathogenesis makes it a viable therapeutic target in clinical applications.

Coordination between cell proliferation and apoptosis after DNA damage in Drosophila

Exposure to genotoxic stress promotes cell cycle arrest and DNA repair or apoptosis. These "life" or "death" cell fate decisions often rely on the activity of the tumor suppressor gene p53. Therefore, the precise regulation of p53 is essential to maintain tissue homeostasis and to prevent cancer development. However, how cell cycle progression has an impact on p53 cell fate decision-making is mostly unknown. In this work, we demonstrate that Drosophila p53 proapoptotic activity can be impacted by the G2/M kinase Cdk1. We find that cell cycle arrested or endocycle-induced cells are refractory to ionizing radiation-induced apoptosis. We show that p53 binding to the regulatory elements of the proapoptotic genes and its ability to activate their expression is compromised in experimentally arrested cells. Our results indicate that p53 genetically and physically interacts with Cdk1 and that p53 proapoptotic role is regulated by the cell cycle status of the cell. We propose a model in which cell cycle progression and p53 proapoptotic activity are molecularly connected to coordinate the appropriate response after DNA damage.

B1-type cyclins control microtubule organization during cell division in Arabidopsis

Flowering plants contain a large number of cyclin families, each containing multiple members, most of which have not been characterized to date. Here, we analyzed the role of the B1 subclass of mitotic cyclins in cell cycle control during Arabidopsis development. While we reveal CYCB1;5 to be a pseudogene, the remaining four members were found to be expressed in dividing cells. Mutant analyses showed a complex pattern of overlapping, development-specific requirements of B1-type cyclins with CYCB1;2 playing a central role. The double mutant cycb1;1 cycb1;2 is severely compromised in growth, yet viable beyond the seedling stage, hence representing a unique opportunity to study the function of B1-type cyclin activity at the organismic level. Immunolocalization of microtubules in cycb1;1 cycb1;2 and treating mutants with the microtubule drug oryzalin revealed a key role of B1-type cyclins in orchestrating mitotic microtubule networks. Subsequently, we identified the GAMMA-TUBULIN COMPLEX PROTEIN 3-INTERACTING PROTEIN 1 (GIP1/MOZART) as an in vitro substrate of B1-type cyclin complexes and further genetic analyses support a potential role in the regulation of GIP1 by CYCB1s.

Targeting Wee1 kinase as a therapeutic approach in Hematological Malignancies

Hematologic malignancies include various diseases that develop from hematopoietic stem cells of bone marrow or lymphatic organs. Currently, conventional DNA-damage-based chemotherapy drugs are approved as standard therapeutic regimens for these malignancies. Although many improvements have been made, patients with relapsed or refractory hematological malignancies have a poor prognosis. Therefore, novel and practical therapeutic approaches are required for the treatment of these diseases. Interestingly several studies have shown that targeting Wee1 kinase in the Hematological malignancies, including AML, ALL, CML, CLL, DLBCL, BL, MCL, etc., can be an effective therapeutic strategy. It plays an essential role in regulating the cell cycle process by abrogating the G2-M cell-cycle checkpoint, which provides time for DNA damage repair before mitotic entry. Consistently, Wee1 overexpression is observed in various Hematological malignancies. Also, in healthy normal cells, repairing DNA damages occurs due to G1-S checkpoint function; however, in the cancer cells, which have an impaired G1-S checkpoint, the damaged DNA repair process depends on the G2-M checkpoint function. Thus, Wee1 inhibition could be a promising target in the presence of DNA damage in order to potentiate multiple therapeutic drugs. This review summarized the potentials and challenges of Wee1 inhibition combined with other therapies as a novel effective therapeutic strategy in Hematological malignancies.

Lomeguatrib Increases the Radiosensitivity of MGMT Unmethylated Human Glioblastoma Multiforme Cell Lines

Background: Treatment resistance of glioblastoma multiforme to chemo- and radiotherapy remains a challenge yet to overcome. In particular, the O⁶-methylguanine-DNA-methyltransferase (MGMT) promoter unmethylated patients have only little benefit from chemotherapy treatment using temozolomide since MGMT counteracts its therapeutic efficacy. Therefore, new treatment options in radiotherapy need to be developed to inhibit MGMT and increase radiotherapy response. Methods: Lomeguatrib, a highly specific MGMT inhibitor, was used to inactivate MGMT protein in vitro. Radiosensitivity of established human glioblastoma multiforme cell lines in combination with lomeguatrib was investigated using the clonogenic survival assay. Inhibition of MGMT was analyzed using Western Blot. Cell cycle distribution and apoptosis were investigated to determine the effects of lomeguatrib alone as well as in combination with ionizing radiation. Results: Lomeguatrib significantly decreased MGMT protein and reduced radiation-induced G2/M arrest. A radiosensitizing effect of lomeguatrib was observed when administered at 1 µM and increased radioresistance at 20 µM. Conclusion: Low concentrations of lomeguatrib elicit radiosensitization, while high concentrations mediate a radioprotective effect.

Evolution of opposing regulatory interactions underlies the emergence of eukaryotic cell cycle checkpoints

In eukaryotes the entry into mitosis is initiated by activation of cyclin-dependent kinases (CDKs), which in turn activate a large number of protein kinases to induce all mitotic processes. The general view is that kinases are active in mitosis and phosphatases turn them off in interphase. Kinases activate each other by cross- and self-phosphorylation, while phosphatases remove these phosphate groups to inactivate kinases. Crucial exceptions to this general rule are the interphase kinase Wee1 and the mitotic phosphatase Cdc25. Together they directly control CDK in an opposite way of the general rule of mitotic phosphorylation and interphase dephosphorylation. Here we investigate why this opposite system emerged and got fixed in almost all eukaryotes. Our results show that this reversed action of a kinase-phosphatase pair, Wee1 and Cdc25, on CDK is particularly suited to establish a stable G2 phase and to add checkpoints to the cell cycle. We show that all these regulators appeared together in LECA (Last Eukaryote Common Ancestor) and co-evolved in eukaryotes, suggesting that this twist in kinase-phosphatase regulation was a crucial step happening at the emergence of eukaryotes.

CDK1 inhibitor controls G2/M phase transition and reverses DNA damage sensitivity

CDK1 plays key roles in cell cycle progression through the G2/M phase transition and activation of homologous recombination (HR) DNA repair pathway. Accordingly, various CDK1 inhibitors have been developed for cancer therapy that induce prolonged G2 arrest and/or sensitize cells to DNA damaging agents in tumor cells, resulting in cell death. However, CDK1 inhibition can induce resistance to DNA damage in certain conditions. The mechanism of different DNA damage sensitivity is not completely understood. We performed immunofluorescence and flow cytometry analysis to investigate DNA damage responses in human tumor cells during low and high dose treatments with RO-3306, a selective CDK1 inhibitor. This comparative investigation demonstrated that RO-3306-induced G2 arrest prevented cells with DNA double-strand breaks from transitioning into the M-phase and that the cells maintained their DNA repair capacity in G2-phase, even under RO-3306 dose-dependent DNA repair inhibition. These findings reveal that CDK1 inhibitor-induced DNA repair inhibition and cell cycle control, which regulate each other during the G2/M phase transition determine the cellular sensitivity to DNA damage, providing insight useful for developing clinical strategies targeting CDK1 inhibition in tumor cells.

Collective Migrations of Drosophila Embryonic Trunk and Caudal Mesoderm-Derived Muscle Precursor Cells

Mesoderm migration in the Drosophila embryo is a highly conserved, complex process that is required for the formation of specialized tissues and organs, including the somatic and visceral musculature. In this FlyBook chapter, we will compare and contrast the specification and migration of cells originating from the trunk and caudal mesoderm. Both cell types engage in collective migrations that enable cells to achieve new positions within developing embryos and form distinct tissues. To start, we will discuss specification and early morphogenetic movements of the presumptive mesoderm, then focus on the coordinate movements of the two subtypes trunk mesoderm and caudal visceral mesoderm, ending with a comparison of these processes including general insights gained through study.

Regulation of Cell Cycle Entry and Exit: A Single Cell Perspective

The transition between proliferating and quiescent states must be carefully regulated to ensure that cells divide to create the cells an organism needs only at the appropriate time and place. Cyclin-dependent kinases (CDKs) are critical for both transitioning cells from one cell cycle state to the next, and for regulating whether cells are proliferating or quiescent. CDKs are regulated by association with cognate cyclins, activating and inhibitory phosphorylation events, and proteins that bind to them and inhibit their activity. The substrates of these kinases, including the retinoblastoma protein, enforce the changes in cell cycle status. Single cell analysis has clarified that competition among factors that activate and inhibit CDK activity leads to the cell's decision to enter the cell cycle, a decision the cell makes before S phase. Signaling pathways that control the activity of CDKs regulate the transition between quiescence and proliferation in stem cells, including stem cells that generate muscle and neurons. © 2020 American Physiological Society. Compr Physiol 10:317-344, 2020.

Triggering mitosis

Entry into mitosis is triggered by the activation of cyclin-dependent kinase 1 (Cdk1). This simple reaction rapidly and irreversibly sets the cell up for division. Even though the core step in triggering mitosis is so simple, the regulation of this cellular switch is highly complex, involving a large number of interconnected signalling cascades. We do have a detailed knowledge of most of the components of this network, but only a poor understanding of how they work together to create a precise and robust system that ensures that mitosis is triggered at the right time and in an orderly fashion. In this review, we will give an overview of the literature that describes the Cdk1 activation network and then address questions relating to the systems biology of this switch. How is the timing of the trigger controlled? How is mitosis insulated from interphase? What determines the sequence of events, following the initial trigger of Cdk1 activation? Which elements ensure robustness in the timing and execution of the switch? How has this system been adapted to the high levels of replication stress in cancer cells?

SGK phosphorylates Cdc25 and Myt1 to trigger cyclin B-Cdk1 activation at the meiotic G2/M transition

The kinase cyclin B-Cdk1 complex is a master regulator of M-phase in both mitosis and meiosis. At the G2/M transition, cyclin B-Cdk1 activation is initiated by a trigger that reverses the balance of activities between Cdc25 and Wee1/Myt1 and is further accelerated by autoregulatory loops. In somatic cell mitosis, this trigger was recently proposed to be the cyclin A-Cdk1/Plk1 axis. However, in the oocyte meiotic G2/M transition, in which hormonal stimuli induce cyclin B-Cdk1 activation, cyclin A-Cdk1 is nonessential and hence the trigger remains elusive. Here, we show that SGK directly phosphorylates Cdc25 and Myt1 to trigger cyclin B-Cdk1 activation in starfish oocytes. Upon hormonal stimulation of the meiotic G2/M transition, SGK is activated by cooperation between the Gβγ-PI3K pathway and an unidentified pathway downstream of Gβγ, called the atypical Gβγ pathway. These findings identify the trigger in oocyte meiosis and provide insights into the role and activation of SGK.

C/EBPβ Is a Transcriptional Regulator of Wee1 at the G₂/M Phase of the Cell Cycle

The CCAAT/enhancer-binding protein β (C/EBPβ) is a transcription factor that regulates cellular proliferation, differentiation, apoptosis and tumorigenesis. Although the pro-oncogenic roles of C/EBPβ have been implicated in various human cancers, how it contributes to tumorigenesis or tumor progression has not been determined. Immunohistochemistry with human non-small cell lung cancer (NSCLC) tissues revealed that higher levels of C/EBPβ protein were expressed compared to normal lung tissues. Knockdown of C/EBPβ by siRNA reduced the proliferative capacity of NSCLC cells by delaying the G₂/M transition in the cell cycle. In C/EBPβ-knockdown cells, a prolonged increase in phosphorylation of cyclin dependent kinase 1 at tyrosine 15 (Y15-pCDK1) was displayed with simultaneously increased Wee1 and decreased Cdc25B expression. Chromatin immunoprecipitation (ChIP) analysis showed that C/EBPβ bound to distal promoter regions of WEE1 and repressed WEE1 transcription through its interaction with histone deacetylase 2. Treatment of C/EBPβ-knockdown cells with a Wee1 inhibitor induced a decrease in Y15-pCDK1 and recovered cells from G₂/M arrest. In the xenograft tumors, the depletion of C/EBPβ significantly reduced tumor growth. Taken together, these results indicate that Wee1 is a novel transcription target of C/EBPβ that is required for the G₂/M phase of cell cycle progression, ultimately regulating proliferation of NSCLC cells.

Phosphatases in Mitosis: Roles and Regulation

Mitosis requires extensive rearrangement of cellular architecture and of subcellular structures so that replicated chromosomes can bind correctly to spindle microtubules and segregate towards opposite poles. This process originates two new daughter nuclei with equal genetic content and relies on highly-dynamic and tightly regulated phosphorylation of numerous cell cycle proteins. A burst in protein phosphorylation orchestrated by several conserved kinases occurs as cells go into and progress through mitosis. The opposing dephosphorylation events are catalyzed by a small set of protein phosphatases, whose importance for the accuracy of mitosis is becoming increasingly appreciated. This review will focus on the established and emerging roles of mitotic phosphatases, describe their structural and biochemical properties, and discuss recent advances in understanding the regulation of phosphatase activity and function.

Evidence for a role of spindle matrix formation in cell cycle progression by antibody perturbation

In Drosophila it has recently been demonstrated that a spindle matrix in the form of a membrane-less macromolecular assembly embeds the microtubule-based spindle apparatus. In addition, two of its constituents, Megator and Chromator, were shown to function as spatial regulators of spindle checkpoint proteins. However, whether the spindle matrix plays a wider functional role in spatially regulating cell cycle progression factors was unknown. Here using a live imaging approach we provide evidence that a number of key cell cycle proteins such as Cyclin B, Polo, and Ran co-localize with the spindle matrix during mitosis. Furthermore, prevention of spindle matrix formation by injection of a function blocking antibody against the spindle matrix protein Chromator results in cell cycle arrest prior to nuclear envelope breakdown. In such embryos the spatial dynamics of Polo and Cyclin B enrichment at the nuclear rim and kinetochores is abrogated and Polo is not imported into the nucleus. This is in contrast to colchicine-arrested embryos where the wild-type dynamics of these proteins are maintained. Taken together these results suggest that spindle matrix formation may be a general requirement for the localization and proper dynamics of cell cycle factors promoting signaling events leading to cell cycle progression.

Zebrafish blastomere screen identifies retinoic acid suppression of MYB in adenoid cystic carcinoma

Adenoid cystic carcinoma (ACC) is a salivary gland malignancy that has no effective therapy and is caused by translocations involving MYB. A zebrafish chemical genetic screen identifies the retinoic acid class of compounds as potential MYB-inhibitory agents. Preclinical ACC mouse xenotransplantation models confirm the in vivo efficacy of retinoic acid, which represents a potential therapy for ACC.

Pluripotent cells have been used to probe developmental pathways that are involved in genetic diseases and oncogenic events. To find new therapies that would target MYB-driven tumors, we developed a pluripotent zebrafish blastomere culture system. We performed a chemical genetic screen and identified retinoic acid agonists as suppressors of c-myb expression. Retinoic acid treatment also decreased c-myb gene expression in human leukemia cells. Translocations that drive overexpression of the oncogenic transcription factor MYB are molecular hallmarks of adenoid cystic carcinoma (ACC), a malignant salivary gland tumor with no effective therapy. Retinoic acid agonists inhibited tumor growth in vivo in ACC patient–derived xenograft models and decreased MYB binding at translocated enhancers, thereby potentially diminishing the MYB positive feedback loop driving ACC. Our findings establish the zebrafish pluripotent cell culture system as a method to identify modulators of tumor formation, particularly establishing retinoic acid as a potential new effective therapy for ACC.

Multiple Mechanisms Inactivate the LIN-41 RNA-Binding Protein To Ensure a Robust Oocyte-to-Embryo Transition in Caenorhabditis elegans

In the nematode Caenorhabditis elegans, the conserved LIN-41 RNA-binding protein is a translational repressor that coordinately controls oocyte growth and meiotic maturation. LIN-41 exerts these effects, at least in part, by preventing the premature activation of the cyclin-dependent kinase CDK-1. Here we investigate the mechanism by which LIN-41 is rapidly eliminated upon the onset of meiotic maturation. Elimination of LIN-41 requires the activities of CDK-1 and multiple SCF (Skp1, Cul1, and F-box protein)-type E3 ubiquitin ligase subunits, including the conserved substrate adaptor protein SEL-10/Fbw7/Cdc4, suggesting that LIN-41 is a target of ubiquitin-mediated protein degradation. Within the LIN-41 protein, two nonoverlapping regions, Deg-A and Deg-B, are individually necessary for LIN-41 degradation; both contain several potential phosphodegron sequences, and at least one of these sequences is required for LIN-41 degradation. Finally, Deg-A and Deg-B are sufficient, in combination, to mediate SEL-10-dependent degradation when transplanted into a different oocyte protein. Although LIN-41 is a potent inhibitor of protein translation and M phase entry, the failure to eliminate LIN-41 from early embryos does not result in the continued translational repression of LIN-41 oocyte messenger RNA targets. Based on these observations, we propose a model for the elimination of LIN-41 by the SEL-10 E3 ubiquitin ligase and suggest that LIN-41 is inactivated before it is degraded. Furthermore, we provide evidence that another RNA-binding protein, the GLD-1 tumor suppressor, is regulated similarly. Redundant mechanisms to extinguish translational repression by RNA-binding proteins may both control and provide robustness to irreversible developmental transitions, including meiotic maturation and the oocyte-to-embryo transition.

The tyrosine kinase v-Src causes mitotic slippage by phosphorylating an inhibitory tyrosine residue of Cdk1

The nonreceptor tyrosine kinase v-Src is an oncogene first identified in Rous sarcoma virus. The oncogenic effects of v-Src have been intensively studied; however, its effects on chromosomal integrity are not fully understood. Here, using HeLa S3/v-Src cells having inducible v-Src expression, we found that v-Src causes mitotic slippage in addition to cytokinesis failure, even when the spindle assembly checkpoint is not satisfied because of the presence of microtubule-targeting agents. v-Src's effect on mitotic slippage was also observed in cells after a knockdown of C-terminal Src kinase (Csk), a protein-tyrosine kinase that inhibits Src-family kinases and was partially inhibited by PP2, an Src-family kinase inhibitor. Proteomic analysis and in vitro kinase assay revealed that v-Src phosphorylates cyclin-dependent kinase 1 (Cdk1) at Tyr-15. This phosphorylation attenuated Cdk1 kinase activity, resulting in a decrease in the phosphorylation of Cdk1 substrates. Furthermore, v-Src-induced mitotic slippage reduced the sensitivity of the cells to microtubule-targeting agents, and cells that survived the microtubule-targeting agents exhibited polyploidy. These results suggest that v-Src causes mitotic slippage by attenuating Cdk1 kinase activity via direct phosphorylation of Cdk1 at Tyr-15. On the basis of these findings, we propose a model for v-Src-induced oncogenesis, in which v-Src-promoted mitotic slippage due to Cdk1 phosphorylation generates genetic diversity via abnormal cell division of polyploid cells and also increases the tolerance of cancer cells to microtubule-targeting agents.

Development of Potent Pyrazolopyrimidinone-Based WEE1 Inhibitors with Limited Single-Agent Cytotoxicity for Cancer Therapy

WEE1 kinase regulates the G₂ /M cell-cycle checkpoint, a critical mechanism for DNA repair in cancer cells that can confer resistance to DNA-damaging agents. We previously reported a series of pyrazolopyrimidinones based on AZD1775, a known WEE1 inhibitor, as an initial investigation into the structural requirements for WEE1 inhibition. Our lead inhibitor demonstrated WEE1 inhibition in the same nanomolar range as AZD1775, and potentiated the effects of cisplatin in medulloblastoma cells, but had reduced single-agent cytotoxicity. These results prompted the development of a more comprehensive series of WEE1 inhibitors. Herein we report a series of pyrazolopyrimidinones and identify a more potent WEE1 inhibitor than AZD1775 and additional compounds that demonstrate that WEE1 inhibition can be achieved with reduced single-agent cytotoxicity. These studies support that WEE1 inhibition can be uncoupled from the potent cytotoxic effects observed with AZD1775, and this may have important ramifications in the clinical setting where WEE1 inhibitors are used as chemosensitizers for DNA-targeted chemotherapy.

Coordination of Protein Kinase and Phosphoprotein Phosphatase Activities in Mitosis

Dynamic changes in protein phosphorylation govern the transitions between different phases of the cell division cycle. A "tug of war" between highly conserved protein kinases and the family of phosphoprotein phosphatases (PPP) establishes the phosphorylation state of proteins, which controls their function. More than three-quarters of all proteins are phosphorylated at one or more sites in human cells, with the highest occupancy of phosphorylation sites seen in mitosis. Spatial and temporal regulation of opposing kinase and PPP activities is crucial for accurate execution of the mitotic program. The role of mitotic kinases has been the focus of many studies, while the contribution of PPPs was for a long time underappreciated and is just emerging. Misconceptions regarding the specificity and activity of protein phosphatases led to the belief that protein kinases are the primary determinants of mitotic regulation, leaving PPPs out of the limelight. Recent studies have shown that protein phosphatases are specific and selective enzymes, and that their activity is tightly regulated. In this review, we discuss the emerging roles of PPPs in mitosis and their regulation of and by mitotic kinases, as well as mechanisms that determine PPP substrate recognition and specificity.

MPF-based meiotic cell cycle control: Half a century of lessons from starfish oocytes

In metazoans that undergo sexual reproduction, genomic inheritance is ensured by two distinct types of cell cycle, mitosis and meiosis. Mitosis maintains the genomic ploidy in somatic cells reproducing within a generation, whereas meiosis reduces by half the ploidy in germ cells to prepare for successive generations. The meiotic cell cycle is believed to be a derived form of the mitotic cell cycle; however, the molecular mechanisms underlying both of these processes remain elusive. My laboratory has long studied the meiotic cell cycle in starfish oocytes, particularly the control of meiotic M-phase by maturation- or M phase-promoting factor (MPF) and the kinase cyclin B-associated Cdk1 (cyclin B-Cdk1). Using this system, we have unraveled the molecular principles conserved in metazoans that modify M-phase progression from the mitotic type to the meiotic type needed to produce a haploid genome. Furthermore, we have solved a long-standing enigma concerning the molecular identity of MPF, a universal inducer of M-phase both in mitosis and meiosis of eukaryotic cells.

Regulation of G2/M Transition by Inhibition of WEE1 and PKMYT1 Kinases

In the cell cycle, there are two checkpoint arrests that allow cells to repair damaged DNA in order to maintain genomic integrity. Many cancer cells have defective G1 checkpoint mechanisms, thus depending on the G2 checkpoint far more than normal cells. G2 checkpoint abrogation is therefore a promising concept to preferably damage cancerous cells over normal cells. The main factor influencing the decision to enter mitosis is a complex composed of Cdk1 and cyclin B. Cdk1/CycB is regulated by various feedback mechanisms, in particular inhibitory phosphorylations at Thr14 and Tyr15 of Cdk1. In fact, Cdk1/CycB activity is restricted by the balance between WEE family kinases and Cdc25 phosphatases. The WEE kinase family consists of three proteins: WEE1, PKMYT1, and the less important WEE1B. WEE1 exclusively mediates phosphorylation at Tyr15, whereas PKMYT1 is dual-specific for Tyr15 as well as Thr14. Inhibition by a small molecule inhibitor is therefore proposed to be a promising option since WEE kinases bind Cdk1, altering equilibria and thus affecting G2/M transition.

β-Arrestin 1 has an essential role in neurokinin-1 receptor-mediated glioblastoma cell proliferation and G2/M phase transition

Glioblastoma is the most common malignant brain tumor and has a poor prognosis. Tachykinin receptor neurokinin-1 (NK1R) is a promising target in glioblastoma therapy because of its overexpression in human glioblastoma. NK1R agonists promote glioblastoma cell growth, whereas NK1R antagonists efficiently inhibit cell growth both in vitro and in vivo However, the molecular mechanisms involved in these effects are incompletely understood. β-Arrestins (ARRBs) serve as scaffold proteins and adapters to mediate intracellular signal transduction. Here we show that the ARRB1-mediated signaling pathway is essential for NK1-mediated glioblastoma cell proliferation. ARRB1 knockdown significantly inhibited NK1-mediated glioblastoma cell proliferation and induced G₂/M phase cell cycle arrest. ARRB1 knockdown cells showed remarkable down-regulation of CDC25C/CDK1/cyclin B1 activity. We also demonstrated that ARRB1 mediated prolonged phosphorylation of ERK1/2 and Akt in glioblastoma cells induced by NK1R activation. ERK1/2 and Akt phosphorylation are involved in regulating CDC25C/CDK1/cyclin B1 activity. The lack of long-term ERK1/2 and Akt activation in ARRB1 knockdown cells was at least partly responsible for the delayed cell cycle progression and proliferation. Moreover, we found that ARRB1-mediated ERK1/2 and Akt phosphorylation regulated the transcriptional activity of both NF-κB and AP-1, which were involved in cyclin B1 expression. ARRB1 deficiency increased the sensitivity of glioblastoma cells to the treatment of NK1R antagonists. Taken together, our results suggest that ARRB1 plays an essential role in NK1R-mediated cell proliferation and G₂/M transition in glioblastoma cells. Interference with ARRB1-mediated signaling via NK1R may have potential significance for therapeutic strategies targeting glioblastoma.

Methylation of DACT2 accelerates esophageal cancer development by activating Wnt signaling

Esophageal cancer is one of the most common malignancies worldwide. DACT2 is frequently methylated in human lung, hepatic, gastric and thyroid cancers. The methylation status and function of DACT2 remain to be elucidated in human esophageal cancer. Ten esophageal cancer cell lines, 42 cases of dysplasia and 126 cases of primary esophageal cancer samples were analyzed in this study. The expression of DACT2 was detected in YES2 cells, while reduced DACT2 expression levels were found in TE8 and KYSE70 cells, and complete loss of DACT2 expression was found in KYSE30, KYSE140, KYSE150, KYSE410, KYSE450, TE3 and TE7 cells. Loss of expression or reduced expression of DACT2 correlated with promoter region hypermethylation in esophageal cancer cells. Restoration of DACT2 expression was induced by 5-aza-2′-deoxycytidine. In human primary esophageal squamous carcinoma, 69% (87/126) of samples were methylated. Methylation of DACT2 was significantly associated with tumor stage and metastasis (P < 0.01, P < 0.05). DACT2 suppressed colony formation, cell migration and invasion in esophageal cancer cells, and it also suppressed esophageal cancer cell xenograft growth. DACT2 inhibited Wnt signaling in human esophageal cancer cells. In conclusion, DACT2 is frequently methylated in human esophageal cancer and its expression is regulated by promoter region methylation. DACT2 suppresses esophageal cancer growth by inhibiting Wnt signaling.

Protein Phosphatases Involved in Regulating Mitosis: Facts and Hypotheses

Almost all eukaryotic proteins are subject to post-translational modifications during mitosis and cell cycle, and in particular, reversible phosphorylation being a key event. The recent use of high-throughput experimental analyses has revealed that more than 70% of all eukaryotic proteins are regulated by phosphorylation; however, the mechanism of dephosphorylation, counteracting phosphorylation, is relatively unknown. Recent discoveries have shown that many of the protein phosphatases are involved in the temporal and spatial control of mitotic events, such as mitotic entry, mitotic spindle assembly, chromosome architecture changes and cohesion, and mitotic exit. This implies that certain phosphatases are tightly regulated for timely dephosphorylation of key mitotic phosphoproteins and are essential for control of various mitotic processes. This review describes the physiological and pathological roles of mitotic phosphatases, as well as the versatile role of various protein phosphatases in several mitotic events.

Targeting WEE1 Kinase in Cancer

WEE1 kinase plays a crucial role in the G2-M cell-cycle checkpoint arrest for DNA repair before mitotic entry. Normal cells repair damaged DNA during G1 arrest; however, cancer cells often have a deficient G1-S checkpoint and depend on a functional G2-M checkpoint for DNA repair. WEE1 is expressed at high levels in various cancer types including breast cancers, leukemia, melanoma, and adult and pediatric brain tumors. Many of these cancers are treated with DNA-damaging agents; therefore, targeting WEE1 for inhibition and compromising the G2-M checkpoint presents an opportunity to potentiate therapy. In this review we summarize the current WEE1 inhibitors, the potential for further inhibitor development, and the challenges in the clinic for the WEE1 inhibitor strategy.

Timing the Drosophila Mid-Blastula Transition: A Cell Cycle-Centered View

At the mid-blastula transition (MBT), externally developing embryos refocus from increasing cell number to elaboration of the body plan. Studies in Drosophila reveal a sequence of changes in regulators of Cyclin:Cdk1 that increasingly restricts the activity of this cell cycle kinase to slow cell cycles during early embryogenesis. By reviewing these events, we provide an outline of the mechanisms slowing the cell cycle at and around the time of MBT. The perspectives developed should provide a guiding paradigm for the study of other MBT changes as the embryo transits from maternal control to a regulatory program centered on the expression of zygotic genes.

Identification of Candidate Cyclin-dependent kinase 1 (Cdk1) Substrates in Mitosis by Quantitative Phosphoproteomics

Cyclin-dependent kinase 1 (Cdk1) is an essential regulator of many mitotic processes including the reorganization of the cytoskeleton, chromosome segregation, and formation and separation of daughter cells. Deregulation of Cdk1 activity results in severe defects in these processes. Although the role of Cdk1 in mitosis is well established, only a limited number of Cdk1 substrates have been identified in mammalian cells. To increase our understanding of Cdk1-dependent phosphorylation pathways in mitosis, we conducted a quantitative phosphoproteomics analysis in mitotic HeLa cells using two small molecule inhibitors of Cdk1, Flavopiridol and RO-3306. In these analyses, we identified a total of 24,840 phosphopeptides on 4,273 proteins, of which 1,215 phosphopeptides on 551 proteins were significantly reduced by 2.5-fold or more upon Cdk1 inhibitor addition. Comparison of phosphopeptide quantification upon either inhibitor treatment revealed a high degree of correlation (R(2) value of 0.87) between the different datasets. Motif enrichment analysis of significantly regulated phosphopeptides revealed enrichment of canonical Cdk1 kinase motifs. Interestingly, the majority of proteins identified in this analysis contained two or more Cdk1 inhibitor-sensitive phosphorylation sites, were highly connected with other candidate Cdk1 substrates, were enriched at specific subcellular structures, or were part of protein complexes as identified by the CORUM database. Furthermore, candidate Cdk1 substrates were enriched in G2 and M phase-specific genes. Finally, we validated a subset of candidate Cdk1 substrates by in vitro kinase assays. Our findings provide a valuable resource for the cell signaling and mitosis research communities and greatly increase our knowledge of Cdk1 substrates and Cdk1-dependent signaling pathways.

Noise Reduction in Complex Biological Switches

Cells operate in noisy molecular environments via complex regulatory networks. It is possible to understand how molecular counts are related to noise in specific networks, but it is not generally clear how noise relates to network complexity, because different levels of complexity also imply different overall number of molecules. For a fixed function, does increased network complexity reduce noise, beyond the mere increase of overall molecular counts? If so, complexity could provide an advantage counteracting the costs involved in maintaining larger networks. For that purpose, we investigate how noise affects multistable systems, where a small amount of noise could lead to very different outcomes; thus we turn to biochemical switches. Our method for comparing networks of different structure and complexity is to place them in conditions where they produce exactly the same deterministic function. We are then in a good position to compare their noise characteristics relatively to their identical deterministic traces. We show that more complex networks are better at coping with both intrinsic and extrinsic noise. Intrinsic noise tends to decrease with complexity, and extrinsic noise tends to have less impact. Our findings suggest a new role for increased complexity in biological networks, at parity of function.

JNK-associated Leucine Zipper Protein Functions as a Docking Platform for Polo-like Kinase 1 and Regulation of the Associating Transcription Factor Forkhead Box Protein K1

JLP (JNK-associated leucine zipper protein) is a scaffolding protein that interacts with various signaling proteins associated with coordinated regulation of cellular process such as endocytosis, motility, neurite outgrowth, cell proliferation, and apoptosis. Here we identified PLK1 (Polo-like kinase 1) as a novel interaction partner of JLP through mass spectrometric approaches. Our results indicate that JLP is phospho-primed by PLK1 on Thr-351, which is recognized by the Polo box domain of PLK1 leading to phosphorylation of JLP at additional sites. Stable isotope labeling by amino acids in cell culture and quantitative LC-MS/MS analysis was performed to identify PLK1-dependent JLP-interacting proteins. Treatment of cells with the PLK1 kinase inhibitor BI2536 suppressed binding of the Forkhead box protein K1 (FOXK1) transcriptional repressor to JLP. JLP was found to interact with PLK1 and FOXK1 during mitosis. Moreover, knockdown of PLK1 affected the interaction between JLP and FOXK1. FOXK1 is a known transcriptional repressor of the CDK inhibitor p21/WAF1, and knockdown of JLP resulted in increased FOXK1 protein levels and a reduction of p21 transcript levels. Our results suggest a novel mechanism by which FOXK1 protein levels and activity are regulated by associating with JLP and PLK1.

Entry into mitosis: a solution to the decades-long enigma of MPF

Maturation or M phase-promoting factor (MPF) is the universal inducer of M phase common to eukaryotic cells. MPF was originally defined as a transferable activity that can induce the G2/M phase transition in recipient cells. Today, however, MPF is assumed to describe an activity that exhibits its effect in donor cells, and furthermore, MPF is consistently equated with the kinase cyclin B-Cdk1. In some conditions, however, MPF, as originally defined, is undetectable even though cyclin B-Cdk1 is fully active. For over three decades, this inconsistency has remained a long-standing puzzle. The enigma is now resolved through the elucidation that MPF, defined as an activity that exhibits its effect in recipient cells, consists of at least two separate kinases, cyclin B-Cdk1 and Greatwall (Gwl). Involvement of Gwl in MPF can be explained by its contribution to the autoregulatory activation of cyclin B-Cdk1 and by its stabilization of phosphorylations on cyclin B-Cdk1 substrates, both of which are essential when MPF induces the G2/M phase transition in recipient cells. To accomplish these tasks, Gwl helps cyclin B-Cdk1 by suppressing protein phosphatase 2A (PP2A)-B55 that counteracts cyclin B-Cdk1. MPF, as originally defined, is thus not synonymous with cyclin B-Cdk1, but is instead a system consisting of both cyclin B-Cdk1 that directs mitotic entry and Gwl that suppresses the anti-cyclin B-Cdk1 phosphatase. The current view that MPF is a synonym for cyclin B-Cdk1 in donor cells is thus imprecise; instead, MPF is best regarded as the entire pathway involved in the autoregulatory activation of cyclin B-Cdk1, with specifics depending on the experimental system.

"Ready, set, go": checkpoint regulation by Cdk1 inhibitory phosphorylation

ABSTRACT Cell cycle checkpoints prevent mitosis from occurring before DNA replication and repair are completed during S and G2 phases. The checkpoint mechanism involves inhibitory phosphorylation of Cdk1, a conserved kinase that regulates the onset of mitosis. Metazoans have two distinct Cdk1 inhibitory kinases with specialized developmental functions: Wee1 and Myt1. Ayeni et al used transgenic Cdk1 phospho-acceptor mutants to analyze how the distinct biochemical properties of these kinases affected their functions. They concluded from their results that phosphorylation of Cdk1 on Y15 was necessary and sufficient for G2/M checkpoint arrest in imaginal wing discs, whereas phosphorylation on T14 promoted chromosome stability by a different mechanism. A curious relationship was also noted between Y15 inhibitory phosphorylation and T161 activating phosphorylation. These unexpected complexities in Cdk1 inhibitory phosphorylation demonstrate that the checkpoint mechanism is not a simple binary "off/on" switch, but has at least three distinct states: "Ready", to prevent chromosome damage and apoptosis, "Set", for developmentally regulated G2 phase arrest, and "Go", when Cdc25 phosphatases remove inhibitory phosphates to trigger Cdk1 activation at the G2/M transition.

A germ cell determinant reveals parallel pathways for germ line development in Caenorhabditis elegans

Despite the central importance of germ cells for transmission of genetic material, our understanding of the molecular programs that control primordial germ cell (PGC) specification and differentiation are limited. Here, we present findings that X chromosome NonDisjunction factor-1 (XND-1), known for its role in regulating meiotic crossover formation, is an early determinant of germ cell fates in Caenorhabditis elegans. xnd-1 mutant embryos display a novel 'one PGC' phenotype as a result of G2 cell cycle arrest of the P4 blastomere. Larvae and adults display smaller germ lines and reduced brood size consistent with a role for XND-1 in germ cell proliferation. Maternal XND-1 proteins are found in the P4 lineage and are exclusively localized to the nucleus in PGCs, Z2 and Z3. Zygotic XND-1 turns on shortly thereafter, at the ∼300-cell stage, making XND-1 the earliest zygotically expressed gene in worm PGCs. Strikingly, a subset of xnd-1 mutants lack germ cells, a phenotype shared with nos-2, a member of the conserved Nanos family of germline determinants. We generated a nos-2 null allele and show that nos-2; xnd-1 double mutants display synthetic sterility. Further removal of nos-1 leads to almost complete sterility, with the vast majority of animals without germ cells. Sterility in xnd-1 mutants is correlated with an increase in transcriptional activation-associated histone modification and aberrant expression of somatic transgenes. Together, these data strongly suggest that xnd-1 defines a new branch for PGC development that functions redundantly with nos-2 and nos-1 to promote germline fates by maintaining transcriptional quiescence and regulating germ cell proliferation.

A maternal effect rough deal mutation suggests that multiple pathways regulate Drosophila RZZ kinetochore recruitment

Proper kinetochore recruitment and regulation of dynein and the Mad1-Mad2 complex requires the Rod-Zw10-Zwilch (RZZ) complex. Here, we describe rod(Z3), a maternal-effect Drosophila mutation changing a single residue in the Rough Deal (Rod) subunit of RZZ. Although the RZZ complex containing this altered subunit (denoted R(Z3)ZZ) is present in early syncytial stage embryos laid by homozygous rod(Z3) mothers, it is not recruited to kinetochores. Consequently, the embryos have no spindle assembly checkpoint (SAC), and syncytial mitoses are profoundly perturbed. The polar body (residual meiotic products) cannot remain in its SAC-dependent metaphase-like state, and decondenses into chromatin. In neuroblasts of homozygous rod(Z3) larvae, R(Z3)ZZ recruitment is only partially reduced, the SAC is functional and mitosis is relatively normal. R(Z3)ZZ nevertheless behaves abnormally: it does not further accumulate on kinetochores when microtubules are depolymerized; it reduces the rate of Mad1 recruitment; and it dominantly interferes with the dynein-mediated streaming of RZZ from attached kinetochores. These results suggest that the mutated residue of rod(Z3) is required for normal RZZ kinetochore recruitment and function and, moreover, that the RZZ recruitment pathway might differ in syncytial stage embryos and post-embryonic somatic cells.

WEE1 is a validated target of the microRNA miR-17-92 cluster in leukemia

MicroRNAs are short single-stranded RNAs that regulate target gene expression by binding to complementary sites in the 3' untranslated region (UTR) of their mRNA targets. The polycistronic miR-17-92 cluster, which encodes miR-17, miR-18a, miR-19a, miR-20a, miR-19b, and miR-92a, was previously shown to be overexpressed in multiple types of cancer. In this study, target gene prediction algorithms were used to predict potential targets of the miR-17-92 cluster. WEE1, a kinase that inhibits cell cycle progression, was identified as a possible target of five of the six miRNAs in the cluster. Luciferase reporter assays were used to determine that miR-17, miR-20a, and miR-18a specifically target nucleotides 465-487 of the 3' UTR of WEE1, whereas miR-19a and miR-19b exert control on WEE1 by targeting nucleotides 1069-1091. A negative correlation was determined between endogenous miR-17 or miR-19a expression and endogenous WEE1 protein expression in the same panel of cell lines. We conclude that WEE1 is a valid target of the miR-17-92 cluster in leukemia.

The Eukaryotic Translation Initiation Factor 4E (eIF4E) as a Therapeutic Target for Cancer

Cancer cells depend on cap-dependent translation more than normal tissue. This explains the emergence of proteins involved in the cap-dependent translation as targets for potential anticancer drugs. Cap-dependent translation starts when eIF4E binds to mRNA cap domain. This review will present eIF4E's structure and functions. It will also expose the use of eIF4E as a therapeutic target in cancer.

6,7-di-O-acetylsinococuline (FK-3000) induces G2/M phase arrest in breast carcinomas through p38 MAPK phosphorylation and CDC25B dephosphorylation

We evaluated the cytostatic effect of 6,7-di-O-acetyl-sinococuline (FK-3000) isolated from Stephania delavayi Diels. against breast carcinoma cell lines MDA-MB‑231 and MCF-7. FK-3000 suppressed CDC25B phosphorylation directly and indirectly via p38 MAPK phosphorylation. CDC25B dephosphorylation decreased levels of cyclin B and phospho-CDC-2, and ultimately induced cell cycle arrest at the G2/M phase. The p38 MAPK inhibitor, SB 239063 blocked FK-3000-induced p38 MAPK phosphorylation and nuclear accumulation, but did not completely rescue cell death. Conclusively FK-3000 exerts its antiproliferative effect through two pathways: i) G2/M cell cycle arrest via downregulation of cyclin B and phospho-CDC2 by p38 MAPK phosphorylation and CDC25B dephosphorylation, and ii) p38 MAPK-independent induction of apoptosis.

Interplay between cell growth and cell cycle in plants

The growth of organs and whole plants depends on both cell growth and cell-cycle progression, but the interaction between both processes is poorly understood. In plants, the balance between growth and cell-cycle progression requires coordinated regulation of four different processes: macromolecular synthesis (cytoplasmic growth), turgor-driven cell-wall extension, mitotic cycle, and endocycle. Potential feedbacks between these processes include a cell-size checkpoint operating before DNA synthesis and a link between DNA contents and maximum cell size. In addition, key intercellular signals and growth regulatory genes appear to target at the same time cell-cycle and cell-growth functions. For example, auxin, gibberellin, and brassinosteroid all have parallel links to cell-cycle progression (through S-phase Cyclin D-CDK and the anaphase-promoting complex) and cell-wall functions (through cell-wall extensibility or microtubule dynamics). Another intercellular signal mediated by microtubule dynamics is the mechanical stress caused by growth of interconnected cells. Superimposed on developmental controls, sugar signalling through the TOR pathway has recently emerged as a central control point linking cytoplasmic growth, cell-cycle and cell-wall functions. Recent progress in quantitative imaging and computational modelling will facilitate analysis of the multiple interconnections between plant cell growth and cell cycle and ultimately will be required for the predictive manipulation of plant growth.

Down-regulation of tricarboxylic acid (TCA) cycle genes blocks progression through the first mitotic division in Caenorhabditis elegans embryos

The cell cycle is a highly regulated process that enables the accurate transmission of chromosomes to daughter cells. Here we uncover a previously unknown link between the tricarboxylic acid (TCA) cycle and cell cycle progression in the Caenorhabditis elegans early embryo. We found that down-regulation of TCA cycle components, including citrate synthase, malate dehydrogenase, and aconitase, resulted in a one-cell stage arrest before entry into mitosis: pronuclear meeting occurred normally, but nuclear envelope breakdown, centrosome separation, and chromosome condensation did not take place. Mitotic entry is controlled by the cyclin B-cyclin-dependent kinase 1 (Cdk1) complex, and the inhibitory phosphorylation of Cdk1 must be removed in order for the complex to be active. We found that following down-regulation of the TCA cycle, cyclin B levels were normal but CDK-1 remained inhibitory-phosphorylated in one-cell stage-arrested embryos, indicative of a G2-like arrest. Moreover, this was not due to an indirect effect caused by checkpoint activation by DNA damage or replication defects. These observations suggest that CDK-1 activation in the C. elegans one-cell embryo is sensitive to the metabolic state of the cell, and that down-regulation of the TCA cycle prevents the removal of CDK-1 inhibitory phosphorylation. The TCA cycle was previously shown to be necessary for the development of the early embryo in mammals, but the molecular processes affected were not known. Our study demonstrates a link between the TCA cycle and a specific cell cycle transition in the one-cell stage embryo.

Modelling cell division and endoreduplication in tomato fruit pericarp

In many developing plant tissues and organs, differentiating cells switch from the classical cell cycle to an alternative partial cycle. This partial cycle bypasses mitosis and allows for multiple rounds of genome duplication without cell division, giving rise to cells with high ploidy numbers. This partial cycle is referred to as endoreduplication. Cell division and endoreduplication are important processes for biomass allocation and yield in tomato. Quantitative trait loci for tomato fruit size or weight are frequently associated with variations in the pericarp cell number, and due to the tight connection between endoreduplication and cell expansion and the prevalence of polyploidy in storage tissues, a functional correlation between nuclear ploidy number and cell growth has also been implicated (karyoplasmic ratio theory). In this paper, we assess the applicability of putative mechanisms for the onset of endoreduplication in tomato pericarp cells via development of a mathematical model for the cell cycle gene regulatory network. We focus on targets for regulation of the transition to endoreduplication by the phytohormone auxin, which is known to play a vital role in the onset of cell expansion and differentiation in developing tomato fruit. We show that several putative mechanisms are capable of inducing the onset of endoreduplication. This redundancy in explanatory mechanisms is explained by analysing system behaviour as a function of their combined action. Namely, when all these routes to endoreduplication are used in a combined fashion, robustness of the regulation of the transition to endoreduplication is greatly improved.

The BEG (PP2A-B55/ENSA/Greatwall) pathway ensures cytokinesis follows chromosome separation

Cytokinesis follows separase activation and chromosome segregation. This order is ensured in budding yeast by the mitotic exit network (MEN), where Cdc14p dephosphorylates key conserved Cdk1-substrates exemplified by the anaphase spindle-elongation protein Ase1p. However, in metazoans, MEN and Cdc14 function is not conserved. Instead, the PP2A-B55α/ENSA/Greatwall (BEG) pathway controls the human Ase1p ortholog PRC1. In this pathway, PP2A-B55 inhibition is coupled to Cdk1-cyclin B activity, whereas separase inhibition is maintained by cyclin B concentration. This creates two cyclin B thresholds during mitotic exit. Simulation and experiments using PRC1 as a model substrate show that the first threshold permits separase activation and chromosome segregation, and the second permits PP2A-B55 activation and initiation of cytokinesis. Removal of the ENSA/Greatwall (EG) timer module eliminates this second threshold, as well as associated delay in PRC1 dephosphorylation and initiation of cytokinesis, by uncoupling PP2A-B55 from Cdk1-cyclin B activity. Therefore, temporal order during mitotic exit is promoted by the metazoan BEG pathway.

Pin1 acts as a negative regulator of the G2/M transition by interacting with the Aurora-A-Bora complex

Pin1 was the first prolyl isomerase identified that is involved in cell division. The mechanism by which Pin1 acts as a negative regulator of mitotic activity in G2 phase remains unclear. Here, we found that Aurora A can interact with and phosphorylate Pin1 at Ser16, which suppresses the G2/M function of Pin1 by disrupting its binding ability and mitotic entry. Our results also show that phosphorylation of Bora at Ser274 and Ser278 is crucial for binding of Pin1. Through the interaction, Pin1 can alter the cytoplasmic translocation of Bora and promote premature degradation by β-TrCP, which results in a delay in mitotic entry. Together with the results that Pin1 protein levels do not significantly fluctuate during cell-cycle progression and Aurora A suppresses Pin1 G2/M function, our data demonstrate that a gain of Pin1 function can override the Aurora-A-mediated functional suppression of Pin1. Collectively, these results highlight the physiological significance of Aurora-A-mediated Pin1 Ser16 phosphorylation for mitotic entry and the suppression of Pin1 is functionally linked to the regulation of mitotic entry through the Aurora-A-Bora complex.

Double strand breaks and cell-cycle arrest induced by the cyanobacterial toxin cylindrospermopsin in HepG2 cells

The newly emerging cyanobacterial cytotoxin cylindrospermopsin (CYN) is increasingly found in surface freshwaters, worldwide. It poses a potential threat to humans after chronic exposure as it was shown to be genotoxic in a range of test systems and is potentially carcinogenic. However, the mechanisms of CYN toxicity and genotoxicity are not well understood. In the present study CYN induced formation of DNA double strand breaks (DSBs), after prolonged exposure (72 h), in human hepatoma cells, HepG2. CYN (0.1–0.5 µg/mL, 24–96 h) induced morphological changes and reduced cell viability in a dose and time dependent manner. No significant increase in lactate dehydrogenase (LDH) leakage could be observed after CYN exposure, indicating that the reduction in cell number was due to decreased cell proliferation and not due to cytotoxicity. This was confirmed by imunocytochemical analysis of the cell-proliferation marker Ki67. Analysis of the cell-cycle using flow-cytometry showed that CYN has an impact on the cell cycle, indicating G0/G1 arrest after 24 h and S-phase arrest after longer exposure (72 and 96 h). Our results provide new evidence that CYN is a direct acting genotoxin, causing DSBs, and these facts need to be considered in the human health risk assessment.

Sparstolonin B inhibits pro-angiogenic functions and blocks cell cycle progression in endothelial cells

Sparstolonin B (SsnB) is a novel bioactive compound isolated from Sparganium stoloniferum, an herb historically used in Traditional Chinese Medicine as an anti-tumor agent. Angiogenesis, the process of new capillary formation from existing blood vessels, is dysregulated in many pathological disorders, including diabetic retinopathy, tumor growth, and atherosclerosis. In functional assays, SsnB inhibited endothelial cell tube formation (Matrigel method) and cell migration (Transwell method) in a dose-dependent manner. Microarray experiments with human umbilical vein endothelial cells (HUVECs) and human coronary artery endothelial cells (HCAECs) demonstrated differential expression of several hundred genes in response to SsnB exposure (916 and 356 genes, respectively, with fold change ≥2, p<0.05, unpaired t-test). Microarray data from both cell types showed significant overlap, including genes associated with cell proliferation and cell cycle. Flow cytometric cell cycle analysis of HUVECs treated with SsnB showed an increase of cells in the G1 phase and a decrease of cells in the S phase. Cyclin E2 (CCNE2) and Cell division cycle 6 (CDC6) are regulatory proteins that control cell cycle progression through the G1/S checkpoint. Both CCNE2 and CDC6 were downregulated in the microarray data. Real Time quantitative PCR confirmed that gene expression of CCNE2 and CDC6 in HUVECs was downregulated after SsnB exposure, to 64% and 35% of controls, respectively. The data suggest that SsnB may exert its anti-angiogenic properties in part by downregulating CCNE2 and CDC6, halting progression through the G1/S checkpoint. In the chick chorioallantoic membrane (CAM) assay, SsnB caused significant reduction in capillary length and branching number relative to the vehicle control group. Overall, SsnB caused a significant reduction in angiogenesis (ANOVA, p<0.05), demonstrating its ex vivo efficacy.

A novel imidazopyridine PI3K inhibitor with anticancer activity in non-small cell lung cancer cells

Lung cancer is the leading cause of cancer-related mortality in the world, and non-small cell lung cancer (NSCLC) accounts for approximately 85% of all cases. Since more than 60% of NSCLC cases express the epidermal growth factor receptor (EGFR), EGFR tyrosine kinase inhibitors are used to treat NSCLC. However, due to the acquired resistance associated with EGFR-targeted therapy, other strategies for the treatment of NSCLC are urgently needed. Therefore, we investigated the anticancer effects of a novel phosphatidylinositol 3-kinase α (PI3Kα) inhibitor, HS-173, in human NSCLC cell lines. HS-173 demonstrated anti-proliferative effects in NSCLC cells and effectively inhibited the PI3K signaling pathway in a dose‑dependent manner. In addition, it induced cell cycle arrest at G2/M phase as well as apoptosis. Taken together, our results demonstrate that HS-173 exhibits anticancer activities, including the induction of apoptosis, by blocking the PI3K/Akt/mTOR pathway in human NSCLC cell lines. We, therefore, suggest that this novel drug could potentially be used for targeted NSCLC therapy.