Mitotic phosphorylation of Cdc25B Ser321 disrupts 14-3-3 binding to the high affinity Ser323 site

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.

Single-cell imaging of the cell cycle reveals CDC25B-induced heterogeneity of G1 phase length in neural progenitor cells

Although lengthening of the cell cycle and G1 phase is a generic feature of tissue maturation during development, the underlying mechanism remains poorly understood. Here, we develop a time-lapse imaging strategy to measure the four cell cycle phases in single chick neural progenitor cells in their endogenous environment. We show that neural progenitors are widely heterogeneous with respect to cell cycle length. This variability in duration is distributed over all phases of the cell cycle, with the G1 phase contributing the most. Within one cell cycle, each phase duration appears stochastic and independent except for a correlation between S and M phase duration. Lineage analysis indicates that the majority of daughter cells may have a longer G1 phase than mother cells, suggesting that, at each cell cycle, a mechanism lengthens the G1 phase. We identify that the CDC25B phosphatase known to regulate the G2/M transition indirectly increases the duration of the G1 phase, partly through delaying passage through the restriction point. We propose that CDC25B increases the heterogeneity of G1 phase length, revealing a previously undescribed mechanism of G1 lengthening that is associated with tissue development.

Principal Postulates of Centrosomal Biology. Version 2020

The centrosome, which consists of two centrioles surrounded by pericentriolar material, is a unique structure that has retained its main features in organisms of various taxonomic groups from unicellular algae to mammals over one billion years of evolution. In addition to the most noticeable function of organizing the microtubule system in mitosis and interphase, the centrosome performs many other cell functions. In particular, centrioles are the basis for the formation of sensitive primary cilia and motile cilia and flagella. Another principal function of centrosomes is the concentration in one place of regulatory proteins responsible for the cell's progression along the cell cycle. Despite the existing exceptions, the functioning of the centrosome is subject to general principles, which are discussed in this review.

Design, expression, purification and crystallization of human 14-3-3ζ protein chimera with phosphopeptide from proapoptotic protein BAD

14-3-3 protein isoforms regulate multiple processes in eukaryotes, including apoptosis and cell division. 14-3-3 proteins preferentially recognize phosphorylated unstructured motifs, justifying the protein-peptide binding approach to study 14-3-3/phosphotarget complexes. Tethering of human 14-3-3σ with partner phosphopeptides via a short linker has provided structural information equivalent to the use of synthetic phosphopeptides, simultaneously facilitating purification and crystallization. Nevertheless, the broader applicability to other 14-3-3 isoforms and phosphopeptides was unclear. Here, we designed a novel 14-3-3ζ chimera with a conserved phosphopeptide from BAD, whose complex with 14-3-3 is a gatekeeper of apoptosis regulation. The chimera could be bacterially expressed and purified without affinity tags. Co-expressed PKA efficiently phosphorylates BAD within the chimera and blocks its interaction with a known 14-3-3 phosphotarget, suggesting occupation of the 14-3-3 grooves by the tethered BAD phosphopeptide. Efficient crystallization of the engineered protein suggests suitability of the "chimeric" approach for studies of other relevant 14-3-3 complexes.

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?

Regulation of the stability and activity of CDC25A and CDC25B by protein phosphatase PP2A and 14-3-3 binding

Cyclin-dependent kinase (CDK)-activating phosphatases, CDC25A and CDC25B, are labile proteins, and their levels vary in a cell cycle-dependent manner. Immediate-early response IER5 protein negatively regulates the cellular CDC25B levels, and stress-induced IER5 expression potentiates G2/M arrest. IER5 binds to protein phosphatase PP2A and regulates the PP2A substrate specificity. We show that IER5 binds to CDC25B and assists PP2A to convert CDC25B to hypophosphorylated forms. Hypophosphorylation at Ser323 results in the dissociation of CDC25B from 14-3-3 phospho-binding proteins. In IER5 expressing cells, CDC25B dissociated from 14-3-3 is unstable but slightly activated, because 14-3-3 inhibits CDC25B polyubiquitination and CDC25B binding to CDK1. The 14-3-3 binding to CDC25A also impedes CDC25A degradation and CDC25A-CDK2 interaction. We propose that 14-3-3 is an important regulator of CDC25A and CDC25B and that PP2A/IER5 controls the stability and activity of CDC25B through regulating the interaction of CDC25B and 14-3-3.

Oxidative stress-induced DNA damage of mouse zygotes triggers G2/M checkpoint and phosphorylates Cdc25 and Cdc2

In vitro fertilized (IVF) embryos show both cell cycle and developmental arrest. We previously showed oxidative damage activates the ATM → Chk1 → Cdc25B/Cdc25C cascade to mediate G2/M cell cycle arrest for repair of hydrogen peroxide (H2O2)-induced oxidative damage in sperm. However, the mechanisms underlying the developmental delay of zygotes are unknown. To develop a model of oxidative-damaged zygotes, we treated mouse zygotes with different concentrations of H2O2 (0, 0.01, 0.02, 0.03, 0.04, 0.05 mM), and evaluated in vitro zygote development, BrdU incorporation to detect the duration of S phase. We also examined reactive oxygen species level and used immunofluorescence to detect activation of γH2AX, Cdc2, and Cdc25. Oxidatively damaged zygotes showed a delay in G2/M phase and produced a higher level of ROS. At the same time, γH2AX was detected in oxidatively damaged zygotes as well as phospho-Cdc25B (Ser323), phospho-Cdc25C (Ser216), and phospho-Cdc2 (Tyr15). Our study indicates that oxidative stress-induced DNA damage of mouse zygotes triggers the cell cycle checkpoint, which results in G2/M cell cycle arrest, and that phospho-Cdc25B (Ser323), phospho-Cdc25C (Ser216), and phospho-Cdc2 (Tyr15) participate in activating the G2/M checkpoint.

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.

Cyclin A/Cdk2 regulates Cdh1 and claspin during late S/G2 phase of the cell cycle

Whereas many components regulating the progression from S phase through G2 phase into mitosis have been identified, the mechanism by which these components control this critical cell cycle progression is still not fully elucidated. Cyclin A/Cdk2 has been shown to regulate the timing of Cyclin B/Cdk1 activation and progression into mitosis although the mechanism by which this occurs is only poorly understood. Here we show that depletion of Cyclin A or inhibition of Cdk2 during late S/early G2 phase maintains the G2 phase arrest by reducing Cdh1 transcript and protein levels, thereby stabilizing Claspin and maintaining elevated levels of activated Chk1 which contributes to the G2 phase observed. Interestingly, the Cyclin A/Cdk2 regulated APC/C(Cdh1) activity is selective for only a subset of Cdh1 targets including Claspin. Thus, a normal role for Cyclin A/Cdk2 during early G2 phase is to increase the level of Cdh1 which destabilises Claspin which in turn down regulates Chk1 activation to allow progression into mitosis. This mechanism links S phase exit with G2 phase transit into mitosis, provides a novel insight into the roles of Cyclin A/Cdk2 in G2 phase progression, and identifies a novel role for APC/C(Cdh1) in late S/G2 phase cell cycle progression.

14-3-3 epsilon prevents G2/M transition of fertilized mouse eggs by binding with CDC25B

Background

The 14-3-3 (YWHA) proteins are highly conserved in higher eukaryotes, participate in various cellular signaling pathways including cell cycle regulation, development and growth. Our previous studies demonstrated that 14-3-3ε (YWHAE) is responsible for maintaining prophase | arrest in mouse oocyte. However, roles of 14-3-3ε in the mitosis of fertilized mouse eggs have remained unclear. Here, we showed that 14-3-3ε interacts and cooperates with CDC25B phosphorylated at Ser321 regulating G2/M transition of mitotic progress of fertilized mouse eggs.

Results

Disruption of 14-3-3ε expression by RNAi prevented normal G2/M transition by inhibition of MPF activity and leaded to the translocation of CDC25B into the nucleus from the cytoplasm. Overexpression of 14-3-3ε-WT and unphosphorylatable CDC25B mutant (CDC25B-S321A) induced mitotic resumption in dbcAMP-arrested eggs. In addition, we examined endogenous and exogenous distribution of 14-3-3ε and CDC25B. Endogenous 14-3-3ε and CDC25B were co-localized primarily in the cytoplasm at the G1, S, early G2 and M phases whereas CDC25B was found to accumulate in the nucleus at the late G2 phase. Upon coexpression with RFP–14-3-3ε, GFP–CDC25B–WT and GFP–CDC25B–S321A were predominantly cytoplasmic at early G2 phase and then GFP–CDC25B–S321A moved to the nucleus whereas CDC25B-WT signals were observed in the cytoplasm without nucleus accumulation at late G2 phase at presence of dbcAMP.

Conclusions

Our data indicate that 14-3-3ε is required for the mitotic entry in the fertilized mouse eggs. 14-3-3ε is primarily responsible for sequestering the CDC25B in cytoplasm and 14-3-3ε binding to CDC25B-S321 phosphorylated by PKA induces mitotic arrest at one-cell stage by inactivation of MPF in fertilized mouse eggs.

High expression of Cdc25B and low expression of 14-3-3σ is associated with the development and poor prognosis in urothelial carcinoma of bladder

Cdc25 dual-specicity phosphatases are essential regulators at critical stages of cell cycle. Cdc25B is overexpressed in several human tumor types. The activity of Cdc25B is regulated by 14-3-3 dimer. To investigate the roles of Cdc25B and 14-3-3σ in bladder carcinoma, we examined expressions of Cdc25B and 14-3-3σ proteins in bladder carcinoma and cell lines and analyzed their roles in the development and prognosis of urinary bladder carcinoma. Immunohistochmistry was used to detect the expressions of Cdc25B and 14-3-3σ in 105 bladder carcinomas. Moreover, expressions of Cdc25B and 14-3-3σ were analyzed by real-time PCR and Western blot in 40 bladder carcinomas and 20 normal epithelial tissues. Specific siRNA was used to knockdown the expression of Cdc25B or 14-3-3σ. Wild-type plasmid was used to overexpress 14-3-3σ. MTT assay and Flow cytometry were used to examine proliferation and cell cycle of bladder cancer cells. There were higher Cdc25B expression and lower 14-3-3σ expression in carcinomas than in the adjacent normal tissues (P < 0.05), positive and negative correlations being noted with clinical stage and histopathologic grade. Cdc25B expression was positively correlated with recurrence and poor prognosis. Downregulation of Cdc25B resulted in slower growth, more G2/M cells and 14-3-3σ increasing. However, upregulation and downregulation of 14-3-3σ did not affect cell growth and Cdc25B expression. It showed that Cdc25B upregulation and 14-3-3σ downregulation might promote development of bladder cancer and suggested a poor prognosis. Moreover, Cdc25B could play an important role on the bladder cancer cell proliferation and cell cycle progression and regulate expression of 14-3-3σ.

Phospho-Ser/Thr-binding domains: navigating the cell cycle and DNA damage response

Coordinated progression through the cell cycle is a complex challenge for eukaryotic cells. Following genotoxic stress, diverse molecular signals must be integrated to establish checkpoints specific for each cell cycle stage, allowing time for various types of DNA repair. Phospho-Ser/Thr-binding domains have emerged as crucial regulators of cell cycle progression and DNA damage signalling. Such domains include 14-3-3 proteins, WW domains, Polo-box domains (in PLK1), WD40 repeats (including those in the E3 ligase SCF(βTrCP)), BRCT domains (including those in BRCA1) and FHA domains (such as in CHK2 and MDC1). Progress has been made in our understanding of the motif (or motifs) that these phospho-Ser/Thr-binding domains connect with on their targets and how these interactions influence the cell cycle and DNA damage response.

Cyclic nucleotide dependent dephosphorylation of regulator of G-protein signaling 18 in human platelets

Regulator of G-protein signaling 18 (RGS18) is a GTPase-activating protein that turns off Gq signaling in platelets. RGS18 is regulated by binding to the adaptor protein 14-3-3 via phosphorylated serine residues S49 and S218 on RGS18. In this study we confirm that thrombin, thromboxane A2, or ADP stimulate the interaction of RGS18 and 14-3-3 by increasing the phosphorylation of S49. Cyclic AMP- and cyclic GMP-dependent kinases (PKA, PKG) inhibit the interaction of RGS18 and 14-3-3 by phosphorylating S216. To understand the effect of S216 phosphorylation we studied the phosphorylation kinetics of S49, S216, and S218 using Phos-tag gels and phosphorylation site-specific antibodies in transfected cells and in platelets. Cyclic nucleotide-induced detachment of 14-3-3 from RGS18 coincides initially with double phosphorylation of S216 and S218. This is followed by dephosphorylation of S49 and S218. Dephosphorylation of S49 and S218 might be mediated by protein phosphatase 1 (PP1) which is linked to RGS18 by the regulatory subunit PPP1R9B (spinophilin). We conclude that PKA and PKG induced S216 phosphorylation triggers the dephosphorylation of the 14-3-3 binding sites of RGS18 in platelets.

Regulator of G-protein signaling 18 integrates activating and inhibitory signaling in platelets

Regulator of G-protein signaling 18 (RGS18) is a GTPase-activating protein for the G-α-q and G-α-i subunits of heterotrimeric G-proteins that turns off signaling by G-protein coupled receptors. RGS18 is highly expressed in platelets. In the present study, we show that the 14-3-3γ protein binds to phosphorylated serines 49 and 218 of RGS18. Platelet activation by thrombin, thromboxane A2, or ADP stimulates the association of 14-3-3 and RGS18, probably by increasing the phosphorylation of serine 49. In contrast, treatment of platelets with prostacyclin and nitric oxide, which trigger inhibitory cyclic nucleotide signaling involving cyclic AMP-dependent protein kinase A (PKA) and cyclic GMP-dependent protein kinase I (PKGI), induces the phosphorylation of serine 216 of RGS18 and the detachment of 14-3-3. Serine 216 phosphorylation is able to block 14-3-3 binding to RGS18 even in the presence of thrombin, thromboxane A2, or ADP. 14-3-3–deficient RGS18 is more active compared with 14-3-3–bound RGS18, leading to a more pronounced inhibition of thrombin-induced release of calcium ions from intracellular stores. Therefore, PKA- and PKGI-mediated detachment of 14-3-3 activates RGS18 to block Gq-dependent calcium signaling. These findings indicate cross-talk between platelet activation and inhibition pathways at the level of RGS18 and Gq.