Regulation of G(2)/M events by Cdc25A through phosphorylation-dependent modulation of its stability

The effect of replication protein A inhibition and post-translational modification on ATR kinase signaling

The ATR kinase responds to elevated levels of single-stranded DNA (ssDNA) to activate the G2/M checkpoint, regulate origin utilization, preserve fork stability, and allow DNA repair to ensure genome integrity. The intrinsic replication stress in cancer cells makes this pathway an attractive therapeutic target. The ssDNA that drives ATR signaling is sensed by the ssDNA-binding protein replication protein A (RPA), which acts as a platform for ATRIP recruitment and subsequent ATR activation by TopBP1. We have developed chemical RPA inhibitors (RPAi) that block RPA-ssDNA interactions (RPA-DBi) and RPA protein-protein interactions (RPA-PPIi); both activities are required for ATR activation. Here, we biochemically reconstitute the ATR kinase signaling pathway and demonstrate that RPA-DBi and RPA-PPIi abrogate ATR-dependent phosphorylation of target proteins with selectivity advantages over active site ATR inhibitors. We demonstrate that RPA post-translational modifications (PTMs) impact ATR kinase activation but do not alter sensitivity to RPAi. Specifically, phosphorylation of RPA32 and TopBP1 stimulate, while RPA70 acetylation does not affect ATR phosphorylation of target proteins. Collectively, this work reveals the RPAi mechanism of action to inhibit ATR signaling that can be regulated by RPA PTMs and offers insight into the anti-cancer activity of ATR pathway-targeted cancer therapeutics.

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.

The Effect of Replication Protein A Inhibition and Post-Translational Modification on ATR Kinase Signaling

The ATR kinase responds to elevated levels of single-stranded DNA (ssDNA) to activate the G2/M checkpoint, regulate origin utilization, preserve fork stability, and allow DNA repair towards ensuring genome integrity. The intrinsic replication stress in cancer cells makes this pathway an attractive therapeutic target. The ssDNA that drives ATR signaling is sensed by the ssDNA-binding protein replication protein A (RPA), which acts as a platform for ATRIP recruitment and subsequent ATR activation by TopBP1. We have developed chemical RPA inhibitors (RPAi) that block RPA-ssDNA interactions, termed RPA-DBi, and RPA protein-protein interactions, termed RPA-PPIi; both activities are required for ATR activation. Here, we employ a biochemically reconstituted ATR kinase signaling pathway and demonstrate that both RPA-DBi and RPA-PPIi abrogate ATR-dependent phosphorylation of downstream target proteins. We demonstrate that RPA post-translational modifications (PTMs) impact ATR kinase activation but do not alter sensitivity to RPAi. Specifically, phosphorylation of RPA32 and TopBP1 stimulate, while RPA70 acetylation has no effect on ATR phosphorylation of target proteins. Collectively, this work reveals the RPAi mechanism of action to inhibit ATR signaling that can be regulated by RPA PTMs and offers insight into the anti-cancer activity of ATR pathway targeted cancer therapeutics.

Cone photoreceptor phosphodiesterase PDE6H inhibition regulates cancer cell growth and metabolism, replicating the dark retina response

BACKGROUND

PDE6H encodes PDE6γ', the inhibitory subunit of the cGMP-specific phosphodiesterase 6 in cone photoreceptors. Inhibition of PDE6, which has been widely studied for its role in light transduction, increases cGMP levels. The purpose of this study is to characterise the role of PDE6H in cancer cell growth.

METHODS

From an siRNA screen for 487 genes involved in metabolism, PDE6H was identified as a controller of cell cycle progression in HCT116 cells. Role of PDE6H in cancer cell growth and metabolism was studied through the effects of its depletion on levels of cell cycle controllers, mTOR effectors, metabolite levels, and metabolic energy assays. Effect of PDE6H deletion on tumour growth was also studied in a xenograft model.

RESULTS

PDE6H knockout resulted in an increase of intracellular cGMP levels, as well as changes to the levels of nucleotides and key energy metabolism intermediates. PDE6H knockdown induced G1 cell cycle arrest and cell death and reduced mTORC1 signalling in cancer cell lines. Both knockdown and knockout of PDE6H resulted in the suppression of mitochondrial function. HCT116 xenografts revealed that PDE6H deletion, as well as treatment with the PDE5/6 inhibitor sildenafil, slowed down tumour growth and improved survival, while sildenafil treatment did not have an additive effect on slowing the growth of PDE6γ'-deficient tumours.

CONCLUSIONS

Our results indicate that the changes in cGMP and purine pools, as well as mitochondrial function which is observed upon PDE6γ' depletion, are independent of the PKG pathway. We show that in HCT116, PDE6H deletion replicates many effects of the dark retina response and identify PDE6H as a new target in preventing cancer cell proliferation and tumour growth.

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.

Maintaining Genome Integrity: Protein Kinases and Phosphatases Orchestrate the Balancing Act of DNA Double-Strand Breaks Repair in Cancer

DNA double-strand breaks (DSBs) are the most lethal DNA damages which lead to severe genome instability. Phosphorylation is one of the most important protein post-translation modifications involved in DSBs repair regulation. Kinases and phosphatases play coordinating roles in DSB repair by phosphorylating and dephosphorylating various proteins. Recent research has shed light on the importance of maintaining a balance between kinase and phosphatase activities in DSB repair. The interplay between kinases and phosphatases plays an important role in regulating DNA-repair processes, and alterations in their activity can lead to genomic instability and disease. Therefore, study on the function of kinases and phosphatases in DSBs repair is essential for understanding their roles in cancer development and therapeutics. In this review, we summarize the current knowledge of kinases and phosphatases in DSBs repair regulation and highlight the advancements in the development of cancer therapies targeting kinases or phosphatases in DSBs repair pathways. In conclusion, understanding the balance of kinase and phosphatase activities in DSBs repair provides opportunities for the development of novel cancer therapeutics.

Targeting CDK1 in cancer: mechanisms and implications

Cyclin dependent kinases (CDKs) are serine/threonine kinases that are proposed as promising candidate targets for cancer treatment. These proteins complexed with cyclins play a critical role in cell cycle progression. Most CDKs demonstrate substantially higher expression in cancer tissues compared with normal tissues and, according to the TCGA database, correlate with survival rate in multiple cancer types. Deregulation of CDK1 has been shown to be closely associated with tumorigenesis. CDK1 activation plays a critical role in a wide range of cancer types; and CDK1 phosphorylation of its many substrates greatly influences their function in tumorigenesis. Enrichment of CDK1 interacting proteins with Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis was conducted to demonstrate that the associated proteins participate in multiple oncogenic pathways. This abundance of evidence clearly supports CDK1 as a promising target for cancer therapy. A number of small molecules targeting CDK1 or multiple CDKs have been developed and evaluated in preclinical studies. Notably, some of these small molecules have also been subjected to human clinical trials. This review evaluates the mechanisms and implications of targeting CDK1 in tumorigenesis and cancer therapy.

An autonomous mathematical model for the mammalian cell cycle

A mathematical model for the mammalian cell cycle is developed as a system of 13 coupled nonlinear ordinary differential equations. The variables and interactions included in the model are based on detailed consideration of available experimental data. A novel feature of the model is inclusion of cycle tasks such as origin licensing and initiation, nuclear envelope breakdown and kinetochore attachment, and their interactions with controllers (molecular complexes involved in cycle control). Other key features are that the model is autonomous, except for a dependence on external growth factors; the variables are continuous in time, without instantaneous resets at phase boundaries; mechanisms to prevent rereplication are included; and cycle progression is independent of cell size. Eight variables represent cell cycle controllers: the Cyclin D1-Cdk4/6 complex, APC^Cdh1, SCF^βTrCP, Cdc25A, MPF, NuMA, the securin-separase complex, and separase. Five variables represent task completion, with four for the status of origins and one for kinetochore attachment. The model predicts distinct behaviors corresponding to the main phases of the cell cycle, showing that the principal features of the mammalian cell cycle, including restriction point behavior, can be accounted for in a quantitative mechanistic way based on known interactions among cycle controllers and their coupling to tasks. The model is robust to parameter changes, in that cycling is maintained over at least a five-fold range of each parameter when varied individually. The model is suitable for exploring how extracellular factors affect cell cycle progression, including responses to metabolic conditions and to anti-cancer therapies.

The p38/MK2 Pathway Functions as Chk1-Backup Downstream of ATM/ATR in G2-Checkpoint Activation in Cells Exposed to Ionizing Radiation

We have recently reported that in G₂-phase cells (but not S-phase cells) sustaining low loads of DNA double-strand break (DSBs), ATM and ATR regulate the G₂-checkpoint epistatically, with ATR at the output-node, interfacing with the cell cycle through Chk1. However, although inhibition of ATR nearly completely abrogated the checkpoint, inhibition of Chk1 using UCN-01 generated only partial responses. This suggested that additional kinases downstream of ATR were involved in the transmission of the signal to the cell cycle engine. Additionally, the broad spectrum of kinases inhibited by UCN-01 pointed to uncertainties in the interpretation that warranted further investigations. Here, we show that more specific Chk1 inhibitors exert an even weaker effect on G₂-checkpoint, as compared to ATR inhibitors and UCN-01, and identify the MAPK p38α and its downstream target MK2 as checkpoint effectors operating as backup to Chk1. These observations further expand the spectrum of p38/MK2 signaling to G₂-checkpoint activation, extend similar studies in cells exposed to other DNA damaging agents and consolidate a role of p38/MK2 as a backup kinase module, adding to similar backup functions exerted in p53 deficient cells. The results extend the spectrum of actionable strategies and targets in current efforts to enhance the radiosensitivity in tumor cells.

Cyclin A-CDK1 suppresses the expression of the CDK1 activator CDC25A to safeguard timely mitotic entry

Cyclin A and CDC25A are both activators of cyclin-dependent kinases (CDKs): cyclin A acts as an activating subunit of CDKs and CDC25A a phosphatase of the inhibitory phosphorylation sites of the CDKs. In this study, we uncovered an inverse relationship between the two CDK activators. As cyclin A is an essential gene, we generated a conditional silencing cell line using a combination of CRISPR-Cas9 and degron-tagged cyclin A. Destruction of cyclin A promoted an acute accumulation of CDC25A. The increase of CDC25A after cyclin A depletion occurred throughout the cell cycle and was independent on cell cycle delay caused by cyclin A deficiency. Moreover, we determined that the inverse relationship with cyclin A was specific for CDC25A and not for other CDC25 family members or kinases that regulate the same sites in CDKs. Unexpectedly, the upregulation of CDC25A was mainly caused by an increase in transcriptional activity instead of a change in the stability of the protein. Reversing the accumulation of CDC25A severely delayed G2-M in cyclin A-depleted cells. Taken together, these data provide evidence of a compensatory mechanism involving CDC25A that ensures timely mitotic entry at different levels of cyclin A.

Aurora Kinase A Regulation by Cysteine Oxidative Modification

Aurora kinase A (AURKA), which is a member of serine/threonine kinase family, plays a critical role in regulating mitosis. AURKA has drawn much attention as its dysregulation is critically associated with various cancers, leading to the development of AURKA inhibitors, a new class of anticancer drugs. As the spatiotemporal activity of AURKA critically depends on diverse intra- and inter-molecular factors, including its interaction with various protein cofactors and post-translational modifications, each of these pathways should be exploited for the development of a novel class of AURKA inhibitors other than ATP-competitive inhibitors. Several lines of evidence have recently shown that redox-active molecules can modify the cysteine residues located on the kinase domain of AURKA, thereby regulating its activity. In this review, we present the current understanding of how oxidative modifications of cysteine residues of AURKA, induced by redox-active molecules, structurally and functionally regulate AURKA and discuss their implications in the discovery of novel AURKA inhibitors.

A novel CDC25A/DYRK2 regulatory switch modulates cell cycle and survival

The cell division cycle 25A (CDC25A) phosphatase is a key regulator of cell cycle progression that acts on the phosphorylation status of Cyclin-Cyclin-dependent kinase complexes, with an emergent role in the DNA damage response and cell survival control. The regulation of CDC25A activity and its protein level is essential to control the cell cycle and maintain genomic integrity. Here we describe a novel ubiquitin/proteasome-mediated pathway negatively regulating CDC25A stability, dependent on its phosphorylation by the serine/threonine kinase DYRK2. DYRK2 phosphorylates CDC25A on at least 7 residues, resulting in its degradation independent of the known CDC25A E3 ubiquitin ligases. CDC25A in turn is able to control the phosphorylation of DYRK2 at several residues outside from its activation loop, thus affecting DYRK2 localization and activity. An inverse correlation between DYRK2 and CDC25A protein amounts was observed during cell cycle progression and in response to DNA damage, with CDC25A accumulation responding to the manipulation of DYRK2 levels or activity in either physiological scenario. Functional data show that the pro-survival activity of CDC25A and the pro-apoptotic activity of DYRK2 could be partly explained by the mutual regulation between both proteins. Moreover, DYRK2 modulation of CDC25A expression and/or activity contributes to the DYRK2 role in cell cycle regulation. Altogether, we provide evidence suggesting that DYRK2 and CDC25A mutually control their activity and stability by a feedback regulatory loop, with a relevant effect on the genotoxic stress pathway, apoptosis, and cell cycle regulation.

Activation of CDC25A phosphatase is limited by CDK2/cyclin A-mediated feedback inhibition

Cyclin-dependent kinase (CDK) 1 complexed with cyclin B is a driver of mitosis, while CDK2 drives S phase entry and replicon initiation. CDK2 activity increases as cells progress through S phase, and its cyclin partner switches from cyclin E to cyclin A. Activation of CDK2 requires dephosphorylation of tyrosine-15 by CDC25A. DNA damage activates the checkpoint protein CHK1, which phosphorylates and degrades CDC25A to prevent activation of CDK2 and protect from cell cycle progression before damage is repaired. CHK1 inhibitors were developed to circumvent this arrest and enhance the efficacy of many cancer chemotherapeutic agents. CHK1 inhibition results in the accumulation of CDC25A and activation of CDK2. We demonstrate that inhibition of CDK2 or suppression of cyclin A also results in accumulation of CDC25A suggesting a feedback loop that prevents over activation of this pathway. The feedback inhibition of CDC25A targets phosphorylation of S88-CDC25A, which resides within a CDK consensus sequence. In contrast, it appears that CDK complexes with cyclin B (and possibly cyclin E) stabilize CDC25A in a feed-forward activation loop. While CDK2/cyclin A would normally be active at late S/G2, we propose that this feedback inhibitory loop prevents over activation of CDK2 in early S phase, while still leaving CDK2/cyclin E to catalyze replicon initiation. One importance of this observation is that a subset of cancer cell lines are very sensitive to CHK1 inhibition, which is mediated by CDK2/cyclin A activity in S phase cells. Hence, dysregulation of this feedback loop might facilitate sensitivity of the cells.

Inhibition mechanism of naphthylphenylamine derivatives acting on the CDC25B dual phosphatase and analysis of the molecular processes involved in the high cytotoxicity exerted by one selected derivative in melanoma cells

The dual phosphatases CDC25 are involved in cell cycle regulation and overexpressed in many tumours, including melanoma. CDC25 is a promising target for discovering anticancer drugs, and several studies focussed on characterisation of quinonoid CDC25 inhibitors, frequently causing undesired side toxic effects. Previous work described an optimisation of the inhibition properties by naphthylphenylamine (NPA) derivatives of NSC28620, a nonquinonoid CDC25 inhibitor. Now, the CDC25B•inhibitor interaction was investigated through fluorescence studies, shedding light on the different inhibition mechanism exerted by NPA derivatives. Among the molecular processes, mediating the specific and high cytotoxicity of one NPA derivative in melanoma cells, we observed decrease of phosphoAkt, increase of p53, reduction of CDC25 forms, cytochrome c cytosolic translocation and increase of caspase activity, that lead to the activation of an apoptotic programme. A basic knowledge on CDC25 inhibitors is relevant for discovering potent bioactive molecules, to be used as anticancer agents against the highly aggressive melanoma.

The PHLPP1 N-Terminal Extension Is a Mitotic Cdk1 Substrate and Controls an Interactome Switch

PH domain leucine-rich repeat protein phosphatase 1 (PHLPP1) is a tumor suppressor that directly dephosphorylates a wide array of substrates, most notably the prosurvival kinase Akt. However, little is known about the molecular mechanisms governing PHLPP1 itself. Here, we report that PHLPP1 is dynamically regulated in a cell cycle-dependent manner and deletion of PHLPP1 results in mitotic delays and increased rates of chromosomal segregation errors. We show that PHLPP1 is hyperphosphorylated during mitosis by Cdk1 in a functionally uncharacterized region known as the PHLPP1 N-terminal extension (NTE). A proximity-dependent biotin identification (BioID) interaction screen revealed that during mitosis, PHLPP1 dissociates from plasma membrane scaffolds, such as Scribble, by a mechanism that depends on its NTE and gains proximity to kinetochore and mitotic spindle proteins such as KNL1 and TPX2. Our data are consistent with a model in which phosphorylation of PHLPP1 during mitosis regulates binding to its mitotic partners and allows accurate progression through mitosis. The finding that PHLPP1 binds mitotic proteins in a cell cycle- and phosphorylation-dependent manner may have relevance to its tumor-suppressive function.

LINC00662 contributes to the progression and the radioresistance of cervical cancer by regulating miR-497-5p and CDC25A

It is reported that long intergenic non-coding RNA 00662 (LINC00662) plays an oncogenic role in tumours. However, the mechanism of LINC00662 in regulating the progression and radiosensitivity of cervical cancer (CC) is not clear. In this study, quantitative real-time polymerase chain reaction (qRT-PCR) was adopted to detect LINC00662 and miR-497-5p expressions in CC tissues and cells. The expression of cell division cycle 25 A (CDC25A) in CC cells was examined by Western blot. CC cell proliferation was determined by cell counting kit-8 (CCK-8) and BrdU assays. The survival rate of CC cells was evaluated by colony formation assay under different doses of X-ray irradiation. CC cell migration and invasion were probed by Transwell assay. Besides, the interactions between miR-497-5p and LINC00662, and miR-497-5p and the 3'UTR of CDC25A were verified by dual-luciferase reporter assay, RIP assay, and RNA pull-down experiments. We demonstrated that, LINC00662 expression was remarkably raised in CC tissues and cell lines. LINC00662 overexpression promoted proliferation, migration, invasion and radioresistance of CC cells, and LINC00662 knockdown inhibited the above malignant phenotypes of CC cells. In terms of mechanism, LINC00662 facilitated CC progression and radioresistance by adsorbing miR-497-5p and indirectly up-regulating CDC25A expression. In a word, the LINC00662/miR-497-5p/CDC25A axis boosts proliferation and metastasis of CC cells and enhances the radioresistance of cancer cells. SIGNIFICANCE OF THE STUDY: CC poses a threat to the health of women all over the world. In this study, we demonstrated for the first time that LINC00662 expression was remarkably raised in CC tissues and cells. Cellular experiments confirmed that LINC00662 facilitated cell proliferation, migration, invasion and radiation resistance through the miR-497-5p/CDC25A axis, which might be a promising target for CC treatments.

TAF15 contributes to the radiation-inducible stress response in cancer

Resistance to radiation therapy is a significant problem in the treatment of non-small cell lung cancer (NSCLC). There is an unmet need to discover new molecular targets for drug development in combination with standard of care cancer therapy. We found that TAF15 was radiation-inducible using phage-displayed peptide libraries. In this study, we report that overexpression of TAF15 is correlated with worsened survival in NSCLC patients. Radiation treatment led to surface induction of TAF15 in vitro and in vivo. We genetically silenced TAF15 which led to a significant reduction in proliferation of NSCLC cells. Cells depleted of TAF15 exhibited cell cycle arrest and enhanced apoptosis through activation and accumulation of p53. In combination with radiation, TAF15 knockdown led to a significant reduction in the surviving fraction of NSCLC cell lines. To determine the importance of TAF15 surface expression, we targeted TAF15 with an antibody. In combination with radiation, the anti-TAF15 antibody led to a reduction in the surviving fraction of cancer cells. These studies show that TAF15 is a radiation-inducible molecular target that is accessible to anti-cancer antibodies and enhances cell viability in response to radiation.

Less-well known functions of cyclin/CDK complexes

Cyclin-dependent kinases (CDKs) are activated by cyclins, which play important roles in dictating the actions of CDK/cyclin complexes. Cyclin binding influences the substrate specificity of these complexes in addition to their susceptibility to inhibition or degradation. CDK/cyclin complexes are best known to promote cell cycle progression in the mitotic cell cycle but are also crucial for important cellular processes not strictly associated with cellular division. This chapter primarily explores the understudied topic of CDK/cyclin complex functionality during the DNA damage response. We detail how CDK/cyclin complexes perform dual roles both as targets of DNA damage checkpoint signaling as well as effectors of DNA repair. Additionally, we discuss the potential CDK-independent roles of cyclins in these processes and the impact of such roles in human diseases such as cancer. Our goal is to place the spotlight on these important functions of cyclins either acting as independent entities or within CDK/cyclin complexes which have attracted less attention in the past. We consider that this will be important for a more complete understanding of the intricate functions of cell cycle proteins in the DNA damage response.

Fluorescence fluctuation analysis reveals PpV dependent Cdc25 protein dynamics in living embryos

The protein phosphatase Cdc25 is a key regulator of the cell cycle by activating Cdk-cyclin complexes. Cdc25 is regulated by its expression levels and post-translational mechanisms. In early Drosophila embryogenesis, Cdc25/Twine drives the fast and synchronous nuclear cycles. A pause in the cell cycle and the remodeling to a more generic cell cycle mode with a gap phase are determined by Twine inactivation and destruction in early interphase 14, in response to zygotic genome activation. Although the pseudokinase Tribbles contributes to the timely degradation of Twine, Twine levels are controlled by additional yet unknown post-translational mechanisms. Here, we apply a non-invasive method based on fluorescence fluctuation analysis (FFA) to record the absolute concentration profiles of Twine with minute-scale resolution in single living embryos. Employing this assay, we found that Protein phosphatase V (PpV), the homologue of the catalytic subunit of human PP6, ensures appropriately low Twine protein levels at the onset of interphase 14. PpV controls directly or indirectly the phosphorylation of Twine at multiple serine and threonine residues as revealed by phosphosite mapping. Mutational analysis confirmed that these sites are involved in control of Twine protein dynamics, and cell cycle remodeling is delayed in a fraction of the phosphosite mutant embryos. Our data reveal a novel mechanism for control of Twine protein levels and their significance for embryonic cell cycle remodeling.

Cell Cycle and DNA Repair Regulation in the Damage Response: Protein Phosphatases Take Over the Reins

Cells are constantly suffering genotoxic stresses that affect the integrity of our genetic material. Genotoxic insults must be repaired to avoid the loss or inappropriate transmission of the genetic information, a situation that could lead to the appearance of developmental abnormalities and tumorigenesis. To combat this threat, eukaryotic cells have evolved a set of sophisticated molecular mechanisms that are collectively known as the DNA damage response (DDR). This surveillance system controls several aspects of the cellular response, including the detection of lesions, a temporary cell cycle arrest, and the repair of the broken DNA. While the regulation of the DDR by numerous kinases has been well documented over the last decade, the complex roles of protein dephosphorylation have only recently begun to be investigated. Here, we review recent progress in the characterization of DDR-related protein phosphatases during the response to a DNA lesion, focusing mainly on their ability to modulate the DNA damage checkpoint and the repair of the damaged DNA. We also discuss their protein composition and structure, target specificity, and biochemical regulation along the different stages encompassed in the DDR. The compilation of this information will allow us to better comprehend the physiological significance of protein dephosphorylation in the maintenance of genome integrity and cell viability in response to genotoxic stress.

Streptomyces sp. MUM256: A Source for Apoptosis Inducing and Cell Cycle-Arresting Bioactive Compounds against Colon Cancer Cells

New and effective anticancer compounds are much needed as the incidence of cancer continues to rise. Microorganisms from a variety of environments are promising sources of new drugs; Streptomyces sp. MUM256, which was isolated from mangrove soil in Malaysia as part of our ongoing efforts to study mangrove resources, was shown to produce bioactive metabolites with chemopreventive potential. This present study is a continuation of our previous efforts and aimed to investigate the underlying mechanisms of the ethyl acetate fraction of MUM256 crude extract (MUM256 EA) in inhibiting the proliferation of HCT116 cells. Our data showed that MUM256 EA reduced proliferation of HCT116 cells via induction of cell-cycle arrest. Molecular studies revealed that MUM256 EA regulated the expression level of several important cell-cycle regulatory proteins. The results also demonstrated that MUM256 EA induced apoptosis in HCT116 cells mediated through the intrinsic pathway. Gas chromatography-mass spectrometry (GC-MS) analysis detected several chemical compounds present in MUM256 EA, including cyclic dipeptides which previous literature has reported to demonstrate various pharmacological properties. The cyclic dipeptides were further shown to inhibit HCT116 cells while exerting little to no toxicity on normal colon cells in this study. Taken together, the findings of this project highlight the important role of exploring the mangrove microorganisms as a bioresource which hold tremendous promise for the development of chemopreventive drugs against colorectal cancer.

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?

Photoirradiation after aminolevulinic acid treatment suppresses cancer cell proliferation through the HO-1/p21 pathway

BACKGROUND

Photodynamic therapy (PDT) and diagnosis (PDD) using 5-aminolevulinic acid (ALA) to control the production of an intracellular photosensitizer, protoporphyrin IX (PpIX), are in common clinical use. Although various studies have been published regarding cell death analysis after photoirradiation by ALA-PDT, the changes in gene expressions induced by it are yet unclear. Here, we focused on studying gene expression and cell proliferation changes in cancer cells that survive photoirradiation.

METHODS

HEK293 human embryonic kidney cells, MKN45 human gastric cells, and PC-3 human prostate cancer cells were selected for this research. Cell viability was measured using trypan blue and MTT assays. ALA-PDT experiments were performed using a calibrated LED irradiation module. Furthermore, mRNA and protein gene expression analysis were performed using our previously reported methods.

RESULTS

mRNAs of PAI-1, HO-1, and p21 were upregulated after photoirradiation of HEK293, which was suppressed by N-acetyl-L-cysteine, a reactive oxygen species (ROS) scavenger. Primer array results in PC-3 cells and p21 and Ki-67 expression results in both PC-3 and MKN45 cells suggested that photoirradiation suppressed cell proliferation. Cell numbers post-photoirradiation revealed that the proliferation of surviving cells was suppressed in PC-3 and MKN45 cells.

CONCLUSION

ALA-PDD or ALA-PDT can result in rapid ROS-induced cell death and may decrease long-term recurrence rates through several pathways including the HO-1/p21 pathway.

WIP1 Contributes to the Adaptation of Fanconi Anemia Cells to DNA Damage as Determined by the Regulatory Network of the Fanconi Anemia and Checkpoint Recovery Pathways

DNA damage adaptation (DDA) allows the division of cells with unrepaired DNA damage. DNA repair deficient cells might take advantage of DDA to survive. The Fanconi anemia (FA) pathway repairs DNA interstrand crosslinks (ICLs), and deficiencies in this pathway cause a fraction of breast and ovarian cancers as well as FA, a chromosome instability syndrome characterized by bone marrow failure and cancer predisposition. FA cells are hypersensitive to ICLs; however, DDA might promote their survival. We present the FA-CHKREC Boolean Network Model, which explores how FA cells might use DDA. The model integrates the FA pathway with the G2 checkpoint and the checkpoint recovery (CHKREC) processes. The G2 checkpoint mediates cell-cycle arrest (CCA) and the CHKREC activates cell-cycle progression (CCP) after resolution of DNA damage. Analysis of the FA-CHKREC network indicates that CHKREC drives DDA in FA cells, ignoring the presence of unrepaired DNA damage and allowing their division. Experimental inhibition of WIP1, a CHKREC component, in FA lymphoblast and cancer cell lines prevented division of FA cells, in agreement with the prediction of the model.

The subcellular location of cyclin B1 and CDC25 associated with the formation of polyploid giant cancer cells and their clinicopathological significance

Polyploid giant cancer cells (PGCCs) are key contributors to cancer heterogeneity, and the formation of PGCCs is associated with changes in the expression of cell-cycle-related proteins. This study investigated the intracellular localization and expression level of multiple cell-cycle-related proteins in PGCCs derived from BT-549 and HEY cells. In addition, the formation of PGCCs and the clinicopathological significance of cell-cycle-related proteins in human breast and ovarian cancer were examined. The expression levels of cell-cycle-related proteins, including cyclin B1, CDC25B, CDC25C, and other cell cycle phosphoproteins, including Chk2, and Aurora-A kinase, were determined using immunostaining and western blotting both in vitro and in vivo. Migration, invasion, and proliferation in control cells, cyclin B1 knockdown cells and their PGCCs following CoCl₂ treatment were compared. In addition, human breast and ovarian cancer samples were collected to determine the correlation of number of PGCCs, expression of cell-cycle-related proteins, and tumor pathologic grade and metastasis. Our results confirm that cyclin B1 was localized in the cytoplasm of PGCCs and in the nuclei of their budding daughter cells. The phosphorylated proteins Chk2 and Aurora-A kinase regulated the expression and subcellular localization of cyclin B1, CDC25B, and CDC25C. The rate of positive cytoplasmic staining of cyclin B1 and positive nuclear staining of both CDC25B and CDC25C increased with increase in tumor grade and lymph node metastasis. Cell-cycle-related proteins, including cyclin B1, CDC25B, and CDC25C play an important role in regulating the formation of PGCCs. The inhibition of cyclinB1 and CoCl₂ treatment significantly promoted cell proliferation, invasion, and migration abilities. The subcellular localization of these cell-cycle-related proteins was regulated by other cell cycle phosphoproteins, and was associated with pathologic grade and metastasis of tumors in cases of human breast and ovarian cancer.

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.

The Emerging Role of Cyclin-Dependent Kinases (CDKs) in Pancreatic Ductal Adenocarcinoma

The family of cyclin-dependent kinases (CDKs) has critical functions in cell cycle regulation and controlling of transcriptional elongation. Moreover, dysregulated CDKs have been linked to cancer initiation and progression. Pharmacological CDK inhibition has recently emerged as a novel and promising approach in cancer therapy. This idea is of particular interest to combat pancreatic ductal adenocarcinoma (PDAC), a cancer entity with a dismal prognosis which is owed mainly to PDAC's resistance to conventional therapies. Here, we review the current knowledge of CDK biology, its role in cancer and the therapeutic potential to target CDKs as a novel treatment strategy for PDAC.

Phosphatase Cdc25A Negatively Regulates the Antiviral Immune Response by Inhibiting TBK1 Activity

The phosphatase Cdc25A plays an important role in cell cycle regulation by dephosphorylating its substrates, such as cyclin-dependent kinases. In this study, we demonstrate that Cdc25A negatively regulates RIG-I-mediated antiviral signaling. We found that ectopic expression of Cdc25A in 293T cells inhibits the activation of beta interferon (IFN-β) induced by Sendai virus and poly(I·C), while knockdown of Cdc25A enhances the transcription of IFN-β stimulated by RNA virus infection. The inhibitory effect of Cdc25A on the antiviral immune response is mainly dependent on its phosphatase activity. Data from a luciferase assay indicated that Cdc25A can inhibit TBK1-mediated activation of IFN-β. Further analysis indicated that Cdc25A can interact with TBK1 and reduce the phosphorylation of TBK1 at S172, which in turn decreases the phosphorylation of its downstream substrate IRF3. Consistently, knockdown of Cdc25A upregulates the phosphorylation of both TBK1-S172 and IRF3 in Sendai virus-infected or TBK1-transfected 293T cells. In addition, we confirmed that Cdc25A can directly dephosphorylate TBK1-S172-p. These results demonstrate that Cdc25A inhibits the antiviral immune response by reducing the active form of TBK1. Using herpes simplex virus 1 (HSV-1) infection, an IFN-β reporter assay, and reverse transcription-quantitative PCR (RT-qPCR), we demonstrated that Cdc25A can also inhibit DNA virus-induced activation of IFN-β. Using a vesicular stomatitis virus (VSV) infection assay, we confirmed that Cdc25A can repress the RIG-I-like receptor (RLR)-mediated antiviral immune response and influence the antiviral status of cells. In conclusion, we demonstrate that Cdc25A negatively regulates the antiviral immune response by inhibiting TBK1 activity.IMPORTANCE The RLR-mediated antiviral immune response is critical for host defense against RNA virus infection. However, the detailed mechanism for balancing the RLR signaling pathway in host cells is not well understood. We found that the phosphatase Cdc25A negatively regulates the RNA virus-induced innate immune response. Our studies indicate that Cdc25A inhibits the RLR signaling pathway via its phosphatase activity. We demonstrated that Cdc25A reduces TBK1 activity and consequently restrains the activation of IFN-β transcription as well as the antiviral status of nearby cells. We showed that Cdc25A can also inhibit DNA virus-induced activation of IFN-β. Taken together, our findings uncover a novel function and mechanism for Cdc25A in regulating antiviral immune signaling. These findings reveal Cdc25A as an important negative regulator of antiviral immunity and demonstrate its role in maintaining host cell homeostasis following viral infection.

CENP-W inhibits CDC25A degradation by destabilizing the SCFβ-TrCP-1 complex at G2/M

Skp, Cullin, F-box (SCF)β-TrCP-1 ubiquitin ligases play a central role in cell cycle regulation and tumorigenesis via proteolytic cleavage of many essential cell cycle regulators. In this study, we propose that centromere protein (CENP)-W, a newly identified kinetochore component, is a novel negative regulator of the SCFβ-TrCP-1 complex. CENP-W interacts with Cullin (CUL)-1 and β-Transducin repeat-containing protein (β-TrCP)-1 through highly overlapped binding sites with S-phase kinase-associated protein (SKP)-1. CENP-W is incorporated into the SCFβ-TrCP-1 complex to promote complex disassembly. Unlike other known regulators that increase SCFβ-TrCP-1 ubiquitin ligase activity by promoting complex reassociation, CENP-W-mediated complex disorganization induced β-TrCP1 degradation and consequently decreased its activity. The association between CENP-W and the SCFβ-TrCP-1 complex was prominent during the G2/M transition in the nucleus. Especially, CENP-W knockdown decreased the cell division cycle-25A protein level, leading to a delay in mitotic progression. We propose that CENP-W participates in cell cycle regulation by modulating SCFβ-TrCP-1 ubiquitin ligase activity.-Cheon, Y., Lee, S. CENP-W inhibits CDC25A degradation by destabilizing the SCFβ-TrCP-1 complex at G2/M.

Adhesion- and stress-related adaptation of glioma radiochemoresistance is circumvented by β1 integrin/JNK co-targeting

Resistance of cancer stem-like and cancer tumor bulk cells to radiochemotherapy and destructive infiltration of the brain fundamentally influence the treatment efficiency to cure of patients suffering from Glioblastoma (GBM). The interplay of adhesion and stress-related signaling and activation of bypass cascades that counteract therapeutic approaches remain to be identified in GBM cells. We here show that combined inhibition of the adhesion receptor β1 integrin and the stress-mediator c-Jun N-terminal kinase (JNK) induces radiosensitization and blocks invasion in stem-like and patient-derived GBM cultures as well as in GBM cell lines. In vivo, this treatment approach not only significantly delays tumor growth but also increases median survival of orthotopic, radiochemotherapy-treated GBM mice. Both, in vitro and in vivo, effects seen with β1 integrin/JNK co-inhibition are superior to the monotherapy. Mechanistically, the in vitro radiosensitization provoked by β1 integrin/JNK targeting is caused by defective DNA repair associated with chromatin changes, enhanced ATM phosphorylation and prolonged G2/M cell cycle arrest. Our findings identify a β1 integrin/JNK co-dependent bypass signaling for GBM therapy resistance, which might be therapeutically exploitable.

A mitosis-specific and R loop-driven ATR pathway promotes faithful chromosome segregation

The ataxia telangiectasia mutated and Rad3-related (ATR) kinase is crucial for DNA damage and replication stress responses. Here, we describe an unexpected role of ATR in mitosis. Acute inhibition or degradation of ATR in mitosis induces whole-chromosome missegregation. The effect of ATR ablation is not due to altered cyclin-dependent kinase 1 (CDK1) activity, DNA damage responses, or unscheduled DNA synthesis but to loss of an ATR function at centromeres. In mitosis, ATR localizes to centromeres through Aurora A-regulated association with centromere protein F (CENP-F), allowing ATR to engage replication protein A (RPA)-coated centromeric R loops. As ATR is activated at centromeres, it stimulates Aurora B through Chk1, preventing formation of lagging chromosomes. Thus, a mitosis-specific and R loop-driven ATR pathway acts at centromeres to promote faithful chromosome segregation, revealing functions of R loops and ATR in suppressing chromosome instability.

Calcium signaling and cell cycle: Progression or death

Cytosolic Ca²⁺ concentration levels fluctuate in an ordered manner along the cell cycle, in line with the fact that Ca²⁺ is involved in the regulation of cell proliferation. Cell proliferation should be an error-free process, yet is endangered by mistakes. In fact, a complex network of proteins ensures that cell cycle does not progress until the previous phase has been successfully completed. Occasionally, errors occur during the cell cycle leading to cell cycle arrest. If the error is severe, and the cell cycle checkpoints work perfectly, this results into cellular demise by activation of apoptotic or non-apoptotic cell death programs. Cancer is characterized by deregulated proliferation and resistance against cell death. Ca²⁺ is a central key to these phenomena as it modulates signaling pathways that control oncogenesis and cancer progression. Here, we discuss how Ca²⁺ participates in the exogenous and endogenous signals controlling cell proliferation, as well as in the mechanisms by which cells die if irreparable cell cycle damage occurs. Moreover, we summarize how Ca²⁺ homeostasis remodeling observed in cancer cells contributes to deregulated cell proliferation and resistance to cell death. Finally, we discuss the possibility to target specific components of Ca²⁺ signal pathways to obtain cytostatic or cytotoxic effects.

Endosulfan induces cell dysfunction through cycle arrest resulting from DNA damage and DNA damage response signaling pathways

Our previous study showed that endosulfan increases the risk of cardiovascular disease. To identify toxic mechanism of endosulfan, we conducted an animal study for which 32 male Wistar rats were randomly and equally divided into four groups: Control group (corn oil only) and three treatment groups (1, 5 and 10mgkg^-1·d^-1). The results showed that exposure to endosulfan resulted in injury of cardiac tissue with impaired mitochondria integrity and elevated 8-OHdG expression in myocardial cells. Moreover, endosulfan increased the expressions of Fas, FasL, Caspase-8, Cleaved Caspase-8, Caspase-3 and Cleaved Caspase-3 in cardiac tissue. In vitro, human umbilical vein endothelial cells (HUVECs) were treated with different concentrations of endosulfan (1, 6 and 12μgmL^-1) for 24h. An inhibitor for Ataxia Telangiectasia Mutated Protein (ATM) (Ku-55933, 10μM) was added in 12μgmL^-1 group for 2h before exposure to endosulfan. Results showed that endosulfan induced DNA damage and activated DNA damage response signaling pathway (ATM/Chk2 and ATR/Chk1) and consequent cell cycle checkpoint. Furthermore, endosulfan promoted the cell apoptosis through death receptor pathway resulting from oxidative stress. The results provide a new insight for mechanism of endosulfan-induced cardiovascular toxicity which will be helpful in future prevention of cardiovascular diseases induced by endosulfan.

CyclinD-CDK4/6 complexes phosphorylate CDC25A and regulate its stability

The phosphatase CDC25A is a key regulator of cell cycle progression by dephosphorylating and activating cyclin-CDK complexes. CDC25A is an unstable protein expressed from G1 until mitosis. CDC25A overexpression, which can be caused by stabilization of the protein, accelerates the G1/S and G2/M transitions, leading to genomic instability and promoting tumorigenesis. Thus, controlling CDC25A protein levels by regulating its stability is a critical mechanism for timing cell cycle progression and to maintain genomic integrity. Herein, we show that CDC25A is phosphorylated on Ser40 throughout the cell cycle and that this phosphorylation is established during the progression from G1 to S phase. We demonstrate that CyclinD-CDK4/CDK6 complexes mediate the phosphorylation of CDC25A on Ser40 during G1 and that these complexes directly phosphorylate this residue in vitro. Importantly, we also find that CyclinD1-CDK4 decreases CDC25A stability in a ßTrCP-dependent manner and that Ser40 and Ser88 phosphorylations contribute to this regulation. Thus our results identify cyclinD-CDK4/6 complexes as novel regulators of CDC25A stability during G1 phase, generating a negative feedback loop allowing control of the G1/S transition.

Systematic Investigation of Expression of G2/M Transition Genes Reveals CDC25 Alteration in Nonfunctioning Pituitary Adenomas

Dysregulation of G1/S checkpoint of cell cycle has been reported in pituitary adenomas. In addition, our previous finding showing that deregulation of Wee1 kinase by microRNAs together with other studies demonstrating alteration of G2/M transition in nonfunctioning pituitary adenomas (NFPAs) suggest that G2/M transition may also be important in pituitary tumorigenesis. To systematically study the expression of members of the G2/M transition in NFPAs and to investigate potential microRNA (miRNA) involvement. Totally, 80 NFPA and 14 normal pituitary (NP) tissues were examined. Expression of 46 genes encoding members of the G2/M transition was profiled on 34 NFPA and 10 NP samples on TaqMan Low Density Array. Expression of CDC25A and two miRNAs targeting CDC25A were validated by individual quantitative real time PCR using TaqMan assays. Protein expression of CDC25A, CDC25C, CDK1 and phospho-CDK1 (Tyr-15) was investigated on tissue microarray and immunohistochemistry. Several genes' expression alteration were observed in NFPA compared to normal tissues by transcription profiling. On protein level CDC25A and both the total and the phospho-CDK1 were overexpressed in adenoma tissues. CDC25A correlated with nuclear localized CDK1 (nCDK1) and with tumor size and nCDK1 with Ki-67 index. Comparing primary vs. recurrent adenomas we found that Ki-67 proliferation index was higher and phospho-CDK1 (inactive form) was downregulated in recurrent tumors compared to primary adenomas. Investigating the potential causes behind CDC25A overexpression we could not find copy number variation at the coding region nor expression alteration of CDC25A regulating transcription factors however CDC25A targeting miRNAs were downregulated in NFPA and negatively correlated with CDC25A expression. Our results suggest that among alterations of G2/M transition of the cell cycle, overexpression of the CDK1 and CDC25A may have a role in the pathogenesis of the NFPA and that CDC25A is potentially regulated by miRNAs.

Human TRIB2 Oscillates during the Cell Cycle and Promotes Ubiquitination and Degradation of CDC25C

Tribbles homolog 2 (TRIB2) is a member of the mammalian Tribbles family of serine/threonine pseudokinases (TRIB1-3). Studies of TRIB2 indicate that many of the molecular interactions between the single Drosophila Tribbles (Trbl) protein and interacting partners are evolutionary conserved. In this study, we examined the relationship between TRIB2 and cell division cycle 25 (CDC25) family of dual-specificity protein phosphatases (mammalian homologues of Drosophila String), which are key physiological cell cycle regulators. Using co-immunoprecipitation we demonstrate that TRIB2 interacts with CDC25B and CDC25C selectively. Forced overexpression of TRIB2 caused a marked decrease in total CDC25C protein levels. Following inhibition of the proteasome, CDC25C was stabilized in the nuclear compartment. This implicates TRIB2 as a regulator of nuclear CDC25C turnover. In complementary ubiquitination assays, we show that TRIB2-mediated degradation of CDC25C is associated with lysine-48-linked CDC25C polyubiquitination driven by the TRIB2 kinase-like domain. A cell cycle associated role for TRIB2 is further supported by the cell cycle regulated expression of TRIB2 protein levels. Our findings reveal mitotic CDC25C as a new target of TRIB2 that is degraded via the ubiquitin proteasome system. Inappropriate CDC25C regulation could mechanistically underlie TRIB2 mediated regulation of cellular proliferation in neoplastic cells.

Phosphorylation of CDC25A on SER283 in late S/G2 by CDK/cyclin complexes accelerates mitotic entry

The Cdc25A phosphatase is an essential activator of CDK-cyclin complexes at all steps of the eukaryotic cell cycle. The activity of Cdc25A is itself regulated in part by positive and negative feedback regulatory loops performed by its CDK-cyclin substrates that occur in G1 as well as during the G1/S and G2/M transitions. However, the regulation of Cdc25A during G2 phase progression before mitotic entry has not been intensively characterized. Here, we identify by mass spectrometry analysis a new phosphorylation event of Cdc25A on Serine283. Phospho-specific antibodies revealed that the phosphorylation of this residue appears in late S/G2 phase of an unperturbed cell cycle and is performed by CDK-cyclin complexes. Overexpression studies of wild-type and non-phosphorylatable mutant forms of Cdc25A indicated that Ser283 phosphorylation increases the G2/M-promoting activity of the phosphatase without impacting its stability or subcellular localization. Our results therefore identify a new positive regulatory loop between Cdc25A and its CDK-cyclin substrates which contributes to accelerate entry into mitosis through the regulation of Cdc25A activity in G2.

PAXIP1 Potentiates the Combination of WEE1 Inhibitor AZD1775 and Platinum Agents in Lung Cancer

The DNA damage response (DDR) involves a complex network of signaling events mediated by modular protein domains such as the BRCA1 C-terminal (BRCT) domain. Thus, proteins that interact with BRCT domains and are a part of the DDR constitute potential targets for sensitization to DNA-damaging chemotherapy agents. We performed a pharmacologic screen to evaluate 17 kinases, identified in a BRCT-mediated interaction network as targets to enhance platinum-based chemotherapy in lung cancer. Inhibition of mitotic kinase WEE1 was found to have the most effective response in combination with platinum compounds in lung cancer cell lines. In the BRCT-mediated interaction network, WEE1 was found in complex with PAXIP1, a protein containing six BRCT domains involved in transcription and in the cellular response to DNA damage. We show that PAXIP1 BRCT domains regulate WEE1-mediated phosphorylation of CDK1. Furthermore, ectopic expression of PAXIP1 promotes enhanced caspase-3-mediated apoptosis in cells treated with WEE1 inhibitor AZD1775 (formerly, MK-1775) and cisplatin compared with cells treated with AZD1775 alone. Cell lines and patient-derived xenograft models expressing both PAXIP1 and WEE1 exhibited synergistic effects of AZD1775 and cisplatin. In summary, PAXIP1 is involved in sensitizing lung cancer cells to the WEE1 inhibitor AZD1775 in combination with platinum-based treatment. We propose that WEE1 and PAXIP1 levels may be used as mechanism-based biomarkers of response when WEE1 inhibitor AZD1775 is combined with DNA-damaging agents. Mol Cancer Ther; 15(7); 1669-81. ©2016 AACR.

14-3-3γ Prevents Centrosome Amplification and Neoplastic Progression

More than 80% of malignant tumors show centrosome amplification and clustering. Centrosome amplification results from aberrations in the centrosome duplication cycle, which is strictly coordinated with DNA-replication-cycle. However, the relationship between cell-cycle regulators and centrosome duplicating factors is not well understood. This report demonstrates that 14-3-3γ localizes to the centrosome and 14-3-3γ loss leads to centrosome amplification. Loss of 14-3-3γ results in the phosphorylation of NPM1 at Thr-199, causing early centriole disjunction and centrosome hyper-duplication. The centrosome amplification led to aneuploidy and increased tumor formation in mice. Importantly, an increase in passage of the 14-3-3γ-knockdown cells led to an increase in the number of cells containing clustered centrosomes leading to the generation of pseudo-bipolar spindles. The increase in pseudo-bipolar spindles was reversed and an increase in the number of multi-polar spindles was observed upon expression of a constitutively active 14-3-3-binding-defective-mutant of cdc25C (S216A) in the 14-3-3γ knockdown cells. The increase in multi-polar spindle formation was associated with decreased cell viability and a decrease in tumor growth. Our findings uncover the molecular basis of regulation of centrosome duplication by 14-3-3γ and inhibition of tumor growth by premature activation of the mitotic program and the disruption of centrosome clustering.

Protein tyrosine phosphatase 1B dephosphorylates PITX1 and regulates p120RasGAP in hepatocellular carcinoma

The effective therapeutic targets for hepatocellular carcinoma remain limited. Pituitary homeobox 1 (PITX1) functions as a tumor suppressor in hepatocarcinogenesis by regulating the expression level of Ras guanosine triphosphatase-activating protein. Here, we report that protein tyrosine phosphatases 1B (PTP1B) directly dephosphorylated PITX1 at Y160, Y175, and Y179 to further weaken the protein stability of PITX. The PTP1B-dependent decline of PITX1 reduced its transcriptional activity for p120RasGAP (RASA1), a Ras guanosine triphosphatase-activating protein. Both silencing of PTP1B and PTP1B inhibitor up-regulated the PITX1-p120RasGAP axis through hyperphosphorylation of PITX1. Sorafenib, the first and only targeted drug approved for hepatocellular carcinoma, directly decreased PTP1B activity and promoted the expression of PITX1 and p120RasGAP by PITX1 hyperphosphorylation. Molecular docking also supported the potential interaction between PTP1B and sorafenib. PTP1B overexpression impaired the sensitivity of sorafenib in vitro and in vivo, implying that PTP1B has a significant effect on sorafenib-induced apoptosis. In sorafenib-treated tumor samples, we further found inhibition of PTP1B activity and up-regulation of the PITX1-p120RasGAP axis, suggesting that PTP1B inhibitor may be effective for the treatment of hepatocellular carcinoma. By immunohistochemical staining of hepatic tumor tissue from 155 patients, the expression of PTP1B was significantly in tumor parts higher than nontumor parts (P = 0.02). Furthermore, high expression of PTP1B was significantly associated with poor tumor differentiation (P = 0.031).

CONCLUSION

PTP1B dephosphorylates PITX1 to weaken its protein stability and the transcriptional activity for p120RasGAP gene expression and acts as a determinant of the sorafenib-mediated drug effect; targeting the PITX1-p120RasGAP axis with a PTP1B inhibitor may provide a new therapy for patients with hepatocellular carcinoma.

Phosphatases and kinases regulating CDC25 activity in the cell cycle: clinical implications of CDC25 overexpression and potential treatment strategies

Alterations in the cell-cycle regulatory genes result in uncontrolled cell proliferation leading to several disease conditions. Cyclin-dependent kinases (CDK) and their regulatory subunit, cyclins, are essential proteins in cell-cycle progression. The activity of CDK is regulated by a series of phosphorylation and dephosphorylation at different amino acid residues. Cell Division Cycle-25 (CDC25) plays an important role in transitions between cell-cycle phases by dephosphorylating and activating CDKs. CDC25B and CDC25C play a major role in G2/M progression, whereas CDC25A assists in G1/S transition. Different isomers of CDC25 expressions are upregulated in various clinicopathological situations. Overexpression of CDC25A deregulates G1/S and G2/M events, including the G2 checkpoint. CDC25B has oncogenic properties. Binding to the 14-3-3 proteins regulates the activity and localization of CDC25B. CDC25C is predominantly a nuclear protein in mammalian cells. At the G2/M transition, mitotic activation of CDC25C protein occurs by its dissociation from 14-3-3 proteins along with its phosphorylation at multiple sites within its N-terminal domain. In this article, we critically reviewed the biology of the activation/deactivation of CDC25 by kinases/phosphatases to maintain the level of CDK-cyclin activities and thus the genomic stability, clinical implications due to dysregulation of CDC25, and potential role of CDC25 inhibitors in diseases.

Oncogenic MicroRNAs: Key Players in Malignant Transformation

MicroRNAs (miRNAs) represent a class of non-coding RNAs that exert pivotal roles in the regulation of gene expression at the post-transcriptional level. MiRNAs are involved in many biological processes and slight modulations in their expression have been correlated with the occurrence of different diseases. In particular, alterations in the expression of miRNAs with oncogenic or tumor suppressor functions have been associated with carcinogenesis, malignant transformation, metastasis and response to anticancer treatments. This review will mainly focus on oncogenic miRNAs whose aberrant expression leads to malignancy.

Chk1 Activation Protects Rad9A from Degradation as Part of a Positive Feedback Loop during Checkpoint Signalling

Phosphorylation of Rad9A at S387 is critical for establishing a physical interaction with TopBP1, and to downstream activation of Chk1 for checkpoint activation. We have previously demonstrated a phosphorylation of Rad9A that occurs at late time points in cells exposed to genotoxic agents, which is eliminated by either Rad9A overexpression, or conversion of S387 to a non-phosphorylatable analogue. Based on this, we hypothesized that this late Rad9A phosphorylation is part of a feedback loop regulating the checkpoint. Here, we show that Rad9A is hyperphosphorylated and accumulates in cells exposed to bleomycin. Following the removal of bleomycin, Rad9A is polyubiquitinated, and Rad9A protein levels drop, indicating an active degradation process for Rad9A. Chk1 inhibition by UCN-01 or siRNA reduces Rad9A levels in cells synchronized in S-phase or exposed to DNA damage, indicating that Chk1 activation is required for Rad9A stabilization in S-phase and during checkpoint activation. Together, these results demonstrate a positive feedback loop involving Rad9A-dependend activation of Chk1, coupled with Chk1-dependent stabilization of Rad9A that is critical for checkpoint regulation.

ATM and ATR signaling at a glance

ATM and ATR signaling pathways are well conserved throughout evolution and are central to the maintenance of genome integrity. Although the role of both ATM and ATR in DNA repair, cell cycle regulation and apoptosis have been well studied, both still remain in the focus of current research activities owing to their role in cancer. Recent advances in the field suggest that these proteins have an additional function in maintaining cellular homeostasis under both stressed and non-stressed conditions. In this Cell Science at a Glance article and the accompanying poster, we present an overview of recent advances in ATR and ATM research with emphasis on that into the modes of ATM and ATR activation, the different signaling pathways they participate in - including those that do not involve DNA damage - and highlight their relevance in cancer.

βTrCP controls the lysosome-mediated degradation of CDK1, whose accumulation correlates with tumor malignancy

In mammals, cell cycle progression is controlled by cyclin-dependent kinases, among which CDK1 plays important roles in the regulation of the G2/M transition, G1 progression and G1/S transition. CDK1 is highly regulated by its association to cyclins, phosphorylation and dephosphorylation, changes in subcellular localization, and by direct binding of CDK inhibitor proteins. CDK1 steady-state protein levels are held constant throughout the cell cycle by a coordinated regulation of protein synthesis and degradation. We show that CDK1 is ubiquitinated by the E3 ubiquitin ligase SCF^βTrCP and degraded by the lysosome. Furthermore, we found that DNA damage not only triggers the stabilization of inhibitory phosphorylation sites on CDK1 and repression of CDK1 gene expression, but also regulates βTrCP-induced CDK1 degradation in a cell type-dependent manner. Specifically, treatment with the chemotherapeutic agent doxorubicin in certain cell lines provokes CDK1 degradation and induces apoptosis, whereas in others it inhibits destruction of the protein. These observations raise the possibility that different tumor types, depending on their pathogenic spectrum mutations, may display different sensitivity to βTrCP-induced CDK1 degradation after DNA damage. Finally, we found that CDK1 accumulation in patients’ tumors shows a negative correlation with βTrCP and a positive correlation with the degree of tumor malignancy.