Protein phosphatase 4 is involved in tumor necrosis factor-alpha-induced activation of c-Jun N-terminal kinase

Protein phosphatase 4 controls circadian clock dynamics by modulating CLOCK/BMAL1 activity

In all organisms with circadian clocks, post-translational modifications of clock proteins control the dynamics of circadian rhythms, with phosphorylation playing a dominant role. All major clock proteins are highly phosphorylated, and many kinases have been described to be responsible. In contrast, it is largely unclear whether and to what extent their counterparts, the phosphatases, play an equally crucial role. To investigate this, we performed a systematic RNAi screen in human cells and identified protein phosphatase 4 (PPP4) with its regulatory subunit PPP4R2 as critical components of the circadian system in both mammals and Drosophila Genetic depletion of PPP4 shortens the circadian period, whereas overexpression lengthens it. PPP4 inhibits CLOCK/BMAL1 transactivation activity by binding to BMAL1 and counteracting its phosphorylation. This leads to increased CLOCK/BMAL1 DNA occupancy and decreased transcriptional activity, which counteracts the "kamikaze" properties of CLOCK/BMAL1. Through this mechanism, PPP4 contributes to the critical delay of negative feedback by retarding PER/CRY/CK1δ-mediated inhibition of CLOCK/BMAL1.

Functional roles of protein phosphatase 4 in multiple aspects of cellular physiology: a friend and a foe

Protein phosphatase 4 (PP4), one of serine/threonine phosphatases, is involved in many critical cellular pathways, including DNA damage response (DNA repair, cell cycle regulation, and apoptosis), tumorigenesis, cell migration, immune response, stem cell development, glucose metabolism, and diabetes. PP4 has been steadily studied over the past decade about wide spectrum of physiological activities in cells. Given the many vital functions in cells, PP4 has great potential to develop into the finding of key working mechanisms and effective treatments for related diseases such as cancer and diabetes. In this review, we provide an overview of the cellular and molecular mechanisms by which PP4 impacts and also discuss the functional significance of it in cell health.

Antibodies recognizing the C terminus of PP2A catalytic subunit are unsuitable for evaluating PP2A activity and holoenzyme composition

The methyl-esterification of the C-terminal leucine of the protein phosphatase 2A (PP2A) catalytic (C) subunit is essential for the assembly of specific trimeric PP2A holoenzymes, and this region of the C subunit also contains two threonine and tyrosine phosphorylation sites. Most commercial antibodies-including the monoclonal antibody 1D6 that is part of a frequently used, commercial phosphatase assay kit-are directed toward the C terminus of the C subunit, raising questions as to their ability to recognize methylated and phosphorylated forms of the enzyme. Here, we tested several PP2A C antibodies, including monoclonal antibodies 1D6, 7A6, G-4, and 52F8 and the polyclonal antibody 2038 for their ability to specifically detect PP2A in its various modified forms, as well as to coprecipitate regulatory subunits. The tested antibodies preferentially recognized the nonmethylated form of the enzyme, and they did not coimmunoprecipitate trimeric holoenzymes containing the regulatory subunits B or B', an issue that precludes their use to monitor PP2A holoenzyme activity. Furthermore, some of the antibodies also recognized the phosphatase PP4, demonstrating a lack of specificity for PP2A. Together, these findings suggest that reinterpretation of the data generated by using these reagents is required.

DAF-16/FOXO requires Protein Phosphatase 4 to initiate transcription of stress resistance and longevity promoting genes

In C. elegans, the conserved transcription factor DAF-16/FOXO is a powerful aging regulator, relaying dire conditions into expression of stress resistance and longevity promoting genes. For some of these functions, including low insulin/IGF signaling (IIS), DAF-16 depends on the protein SMK-1/SMEK, but how SMK-1 exerts this role has remained unknown. We show that SMK-1 functions as part of a specific Protein Phosphatase 4 complex (PP4^SMK-1). Loss of PP4^SMK-1 hinders transcriptional initiation at several DAF-16-activated genes, predominantly by impairing RNA polymerase II recruitment to their promoters. Search for the relevant substrate of PP4^SMK-1 by phosphoproteomics identified the conserved transcriptional regulator SPT-5/SUPT5H, whose knockdown phenocopies the loss of PP4^SMK-1. Phosphoregulation of SPT-5 is known to control transcriptional events such as elongation and termination. Here we also show that transcription initiating events are influenced by the phosphorylation status of SPT-5, particularly at DAF-16 target genes where transcriptional initiation appears rate limiting, rendering PP4^SMK-1 crucial for many of DAF-16’s physiological roles.

The transcription factor DAF-16/FOXO mediates a wide variety of aging-preventive responses by driving the expression of stress resistance and longevity promoting genes. Here the authors show that transcriptional initiation at many DAF-16/FOXO target genes requires the dephosphorylation of SPT-5 by Protein Phosphatase 4.

Temporal Splicing Switches in Elements of the TNF-Pathway Identified by Computational Analysis of Transcriptome Data for Human Cell Lines

Alternative splicing plays an important role in numerous cellular processes and aberrant splice decisions are associated with cancer. Although some studies point to a regulation of alternative splicing and its effector mechanisms in a time-dependent manner, the extent and consequences of such a regulation remains poorly understood. In the present work, we investigated the time-dependent production of isoforms in two Hodgkin lymphoma cell lines of different progression stages (HD-MY-Z, stage IIIb and L-1236, stage IV) compared to a B lymphoblastoid cell line (LCL-HO) with a focus on tumour necrosis factor (TNF) pathway-related elements. For this, we used newly generated time-course RNA-sequencing data from the mentioned cell lines and applied a computational pipeline to identify genes with isoform-switching behaviour in time. We analysed the temporal profiles of the identified events and evaluated in detail the potential functional implications of alterations in isoform expression for the selected top-switching genes. Our data indicate that elements within the TNF pathway undergo a time-dependent variation in isoform production with a putative impact on cell migration, proliferation and apoptosis. These include the genes TRAF1, TNFRSF12A and NFKB2. Our results point to a role of temporal alternative splicing in isoform production, which may alter the outcome of the TNF pathway and impact on tumorigenesis.

A Bioinformatics Approach to Explore MicroRNAs as Tools to Bridge Pathways Between Plants and Animals. Is DNA Damage Response (DDR) a Potential Target Process?

MicroRNAs, highly-conserved small RNAs, act as key regulators of many biological functions in both plants and animals by post-transcriptionally regulating gene expression through interactions with their target mRNAs. The microRNA research is a dynamic field, in which new and unconventional aspects are emerging alongside well-established roles in development and stress adaptation. A recent hypothesis states that miRNAs can be transferred from one species to another and potentially target genes across distant species. Here, we propose to look into the trans-kingdom potential of miRNAs as a tool to bridge conserved pathways between plant and human cells. To this aim, a novel multi-faceted bioinformatic analysis pipeline was developed, enabling the investigation of common biological processes and genes targeted in plant and human transcriptome by a set of publicly available Medicago truncatula miRNAs. Multiple datasets, including miRNA, gene, transcript and protein sequences, expression profiles and genetic interactions, were used. Three different strategies were employed, namely a network-based pipeline, an alignment-based pipeline, and a M. truncatula network reconstruction approach, to study functional modules and to evaluate gene/protein similarities among miRNA targets. The results were compared in order to find common features, e.g., microRNAs targeting similar processes. Biological processes like exocytosis and response to viruses were common denominators in the investigated species. Since the involvement of miRNAs in the regulation of DNA damage response (DDR)-associated pathways is barely explored, especially in the plant kingdom, a special attention is given to this aspect. Hereby, miRNAs predicted to target genes involved in DNA repair, recombination and replication, chromatin remodeling, cell cycle and cell death were identified in both plants and humans, paving the way for future interdisciplinary advancements.

Merkel Cell Polyomavirus Small T Antigen Drives Cell Motility via Rho-GTPase-Induced Filopodium Formation

Cell motility and migration is a complex, multistep, and multicomponent process intrinsic to progression and metastasis. Motility is dependent on the activities of integrin receptors and Rho family GTPases, resulting in the remodeling of the actin cytoskeleton and formation of various motile actin-based protrusions. Merkel cell carcinoma (MCC) is an aggressive skin cancer with a high likelihood of recurrence and metastasis. Merkel cell polyomavirus (MCPyV) is associated with the majority of MCC cases, and MCPyV-induced tumorigenesis largely depends on the expression of the small tumor antigen (ST). Since the discovery of MCPyV, a number of mechanisms have been suggested to account for replication and tumorigenesis, but to date, little is known about potential links between MCPyV T antigen expression and the metastatic nature of MCC. Previously, we described the action of MCPyV ST on the microtubule network and how it impacts cell motility and migration. Here, we demonstrate that MCPyV ST affects the actin cytoskeleton to promote the formation of filopodia through a mechanism involving the catalytic subunit of protein phosphatase 4 (PP4C). We also show that MCPyV ST-induced cell motility is dependent upon the activities of the Rho family GTPases Cdc42 and RhoA. In addition, our results indicate that the MCPyV ST-PP4C interaction results in the dephosphorylation of β₁ integrin, likely driving the cell motility pathway. These findings describe a novel mechanism by which a tumor virus induces cell motility, which may ultimately lead to cancer metastasis, and provides opportunities and strategies for targeted interventions for disseminated MCC.

IMPORTANCE Merkel cell polyomavirus (MCPyV) is the most recently discovered human tumor virus. It causes the majority of cases of Merkel cell carcinoma (MCC), an aggressive skin cancer. However, the molecular mechanisms implicating MCPyV-encoded proteins in cancer development are yet to be fully elucidated. This study builds upon our previous observations, which demonstrated that the MCPyV ST antigen enhances cell motility, providing a potential link between MCPyV protein expression and the highly metastatic nature of MCC. Here, we show that MCPyV ST remodels the actin cytoskeleton, promoting the formation of filopodia, which is essential for MCPyV ST-induced cell motility, and we also implicate the activity of specific Rho family GTPases, Cdc42 and RhoA, in these processes. Moreover, we describe a novel mechanism for the activation of Rho-GTPases and the cell motility pathway due to the interaction between MCPyV ST and the cellular phosphatase catalytic subunit PP4C, which leads to the specific dephosphorylation of β1 integrin. These findings may therefore provide novel strategies for therapeutic intervention for disseminated MCC.

Screening and identification of host proteins interacting with Theileria annulata cysteine proteinase (TaCP) by yeast-two-hybrid system

BACKGROUND

Theileria annulata can infect monocytes/macrophages and B lymphocytes and causes severe lymphoproliferative disease in ruminants. Meanwhile, infection by T. annulata leads to the permanent proliferation of cell population through regulating signaling pathways of host cells. Cysteine proteinases (CPs) are one kind of protein hydrolase and usually play critical roles in parasite virulence, host invasion, nutrition and host immune response. However, the biological function of T. annulata CP (TaCP) is still unclear. In this study, a yeast-two-hybrid assay was performed to screen host proteins interacting with TaCP, to provide information to help our understanding of the molecular mechanisms between T. annulata and host cells.

METHODS

The cDNA from purified bovine B cells was inserted into pGADT7-SfiI vector (pGADT7-SfiI-BcDNA, Prey plasmid) for constructing the yeast two-hybrid cDNA library. TaCP was cloned into the pGBKT7 vector (pGBKT7-TaCP) and was considered as bait plasmid after evaluating the expression, auto-activation and toxicity tests in the yeast strain Y2HGold. The yeast two-hybrid screening was carried out via co-transforming bait and prey plasmids into yeast strain Y2HGold. Sequences of positive preys were analyzed using BLAST, Gene Ontology, UniProt and STRING.

RESULTS

Two host proteins, CRBN (Bos taurus cereblon transcript variant X2) and Ppp4C (Bos indicus protein phosphatase 4 catalytic subunit) were identified to interact with TaCP. The results of functional analysis showed that the two proteins were involved in many cellular processes, such as ubiquitylation regulation, microtubule organization, DNA repair, cell apoptosis and maturation of spliceosomal snRNPs.

CONCLUSIONS

This study is the first to screen the host proteins of bovine B cells interacting with TaCP, and 2 proteins, CRBN and Ppp4C, were identified using yeast two-hybrid technique. The results of functional analysis suggest that the two proteins are involved in many cellular processes, such as ubiquitylation regulating, microtubule organization, DNA repair, cell apoptosis and maturation of spliceosomal snRNPs. The interaction with CRBN and Ppp4C indicate that TaCP possibly is involved in regulating signaling pathways and cell proliferation, which is helpful for understanding the interaction between T. annulata and host cells.

The protein phosphatase 4 complex promotes the Notch pathway and wingless transcription

The Wnt/Wingless (Wg) pathway controls cell fate specification, tissue differentiation and organ development across organisms. Using an in vivo RNAi screen to identify novel kinase and phosphatase regulators of the Wg pathway, we identified subunits of the serine threonine phosphatase Protein Phosphatase 4 (PP4). Knockdown of the catalytic and regulatory subunits of PP4 cause reductions in the Wg pathway targets Senseless and Distal-less. We find that PP4 regulates the Wg pathway by controlling Notch-driven wg transcription. Genetic interaction experiments identified that PP4 likely promotes Notch signaling within the nucleus of the Notch-receiving cell. Although the PP4 complex is implicated in various cellular processes, its role in the regulation of Wg and Notch pathways was previously uncharacterized. Our study identifies a novel role of PP4 in regulating Notch pathway, resulting in aberrations in Notch-mediated transcriptional regulation of the Wingless ligand. Furthermore, we show that PP4 regulates proliferation independent of its interaction with Notch.

Summary: The protein phosphatase 4 complex promotes Notch signaling and target gene expression during Drosophila wing development.

Protein phosphatase 4 plays dual roles during cell proliferation

OBJECTIVES

Protein phosphatase 4 (PP4) has been reported to be indispensable for cell proliferation and survival. Deletion of PP4 has been shown to induce abnormal and even lethal events in growth and development both in lower eukaryotes and in mammals. However, until now, effects of PP4 up-regulation have remained unclear.

MATERIALS AND METHODS

To test effects of PP4 on cell proliferation, cell cycle and morphology in HepG2 cells, it was down-regulated using PP4 siRNA or its activity was inhibited using PP4RL (a PP4 phosphatase-dead mutant) adenoviruses. Alternatively, PP4 was up-regulated using PP4 adenoviruses. Next, we used a functional proteomic approach to identify proteins that may interact with PP4. Furthermore, we performed rescue experiments to verify the possible mechanisms.

RESULTS

To our surprise, we found that both up-regulation and inhibition of PP4 inhibited cell proliferation. Unlike PP4 inhibition, PP4 up-regulation induced prominent arrest at the prometaphase/metaphase transition by causing defects in chromosome alignment and spindle assembly. Moreover, we identified scaffold attachment factor A (SAF-A) (an important protein required for kinetochore-microtubule attachment that participates in the prometaphase/metaphase transition), to be a novel protein that interacts with PP4, using a proteomic approach. Thus, mutual regulatory mechanisms exist between PP4 and SAF-A. Interactions between PP4 and SAF-A played a role in prometaphase/metaphase transition.

CONCLUSIONS

Our data demonstrate a novel regulatory mechanism involving PP4 in cell proliferation.

Protein phosphatase 4 (PP4) functions as a critical regulator in tumor necrosis factor (TNF)-α-induced hepatic insulin resistance

Protein phosphatase 4 (PP4) was shown to participate in multiple cellular processes, including DNA damage response, cell cycle and embryo development. Recent studies demonstrated a looming role of PP4 in glucose metabolism. However, whether PP4 is involved in hepatic insulin resistance remains poorly understood. The objective of this study was to estimate the role of PP4 in tumor necrosis factor (TNF)-α-induced hepatic insulin resistance. db/db mice and TNF-α-treated C57BL/6J mice were used as hepatic insulin resistance animal models. In vitro models were established in both HepG2 cells and primary hepatocytes by TNF-α treatment. We found that increased expression and activity of PP4 occurred in the livers of db/db mice and TNF-α-induced hepatic insulin resistance both in vitro and in vivo. Actually, PP4 silencing and suppression of PP4 activity ameliorated TNF-α-induced hepatic insulin resistance, whereas over-expression of PP4 caused insulin resistance. We then further investigated the prodiabetic mechanism of PP4 in TNF-α-induced insulin resistance. We found that PP4 formed a complex with IRS-1 to promote phosphorylation of IRS-1 on serine 307 via JNK activation and reduce the expression of IRS-1. Thus, PP4 is an important regulator in inflammatory related insulin resistance.

Loss of flfl Triggers JNK-Dependent Cell Death in Drosophila

falafel (flfl) encodes a Drosophila homolog of human SMEK whose in vivo functions remain elusive. In this study, we performed gain-of-function and loss-of-function analysis in Drosophila and identified flfl as a negative regulator of JNK pathway-mediated cell death. While ectopic expression of flfl suppresses TNF-triggered JNK-dependent cell death, loss of flfl promotes JNK activation and cell death in the developing eye and wing. These data report for the first time an essential physiological function of flfl in maintaining tissue homeostasis and organ development. As the JNK signaling pathway has been evolutionary conserved from fly to human, a similar role of PP4R3 in JNK-mediated physiological process is speculated.

Phosphatase PP4 Negatively Regulates Type I IFN Production and Antiviral Innate Immunity by Dephosphorylating and Deactivating TBK1

The effective recognition of viral infection and subsequent type I IFN production is essential for the host antiviral innate immune responses. The phosphorylation and activation of kinase TANK-binding kinase 1 (TBK1) plays crucial roles in the production of type I IFN mediated by TLR and retinoic acid-inducible gene I-like receptors. Type I IFN expression must be tightly regulated to prevent the development of immunopathological disorders. However, how the activated TBK1 is negatively regulated by phosphatases remains poorly understood. In this study, we identified a previously unknown role of protein phosphatase (PP)4 by acting as a TBK1 phosphatase. PP4 expression was upregulated in macrophages infected with RNA virus, vesicular stomatitis virus, and Sendai virus in vitro and in vivo. Knockdown of PP4C, the catalytic subunit of PP4, significantly increased type I IFN production in macrophages and dentritic cells triggered by TLR3/4 ligands, vesicular stomatitis virus, and Sendai virus, and thus inhibited virus replication. Similar results were also found in peritoneal macrophages with PP4C silencing in vivo and i.p. infection of RNA virus. Accordingly, ectopic expression of PP4C inhibited virus-induced type I IFN production and promoted virus replication. However, overexpression of a phosphatase-dead PP4C mutant abolished the inhibitory effects of wild-type PP4C on type I IFN production. Mechanistically, PP4 directly bound TBK1 upon virus infection, then dephosphorylated TBK1 at Ser(172) and inhibited TBK1 activation, and subsequently restrained IFN regulatory factor 3 activation, resulting in suppressed production of type I IFN and IFN-stimulated genes. Thus, serine/threonine phosphatase PP4 functions as a novel feedback negative regulator of RNA virus-triggered innate immunity.

PP4R1 accelerates cell growth and proliferation in HepG2 hepatocellular carcinoma

Hepatocellular carcinoma (HCC), as the fifth most common cancer worldwide, has become the third leading cause of cancer-related deaths. It is reported that protein phosphatase 4 (PP4) is an essential protein for nucleation, growth, and stabilization of microtubules in centrosomes/spindle bodies during cell division. Besides, previous studies have identified protein phosphatase 4 regulatory subunit 1 (PP4R1) as a constitutive interaction partner of PP4 catalytic subunit PP4C. The PP4C-PP4R1 PP4 complex plays a role in dephosphorylation, regulation of histone acetylation, and NF-κB activation. However, little is known about the pathological functions of PP4R1 in human cancers. Thus, in order to investigate how PP4R1 functions in human HCC, two common hepatocarcinogenesis HCC cell lines HepG2 and SMMC-7721 were employed, transduced with recombinant lentivirus expressing PP4R1 short hairpin RNA. Compared with the controls, the cells treated with Lv-shPP4R1 showed a significant decrease in cell proliferation and colony formation. The results of flow cytometry showed that the knockdown of PP4R1 caused HepG2 cells arrest at G2/M phase in the cell cycle. Furthermore, the transduction of Lv-shPP4R1 into HepG2 cells led to the inactivation of two major mitogen-activated protein kinase signaling cascades: p38 and c-Jun N-terminal kinase (JNK), indicating that PP4R1 could promote cell proliferation, which might be regulated by p38 and c-Jun N-terminal kinase pathways. In a word, this study highlights the crucial role of PP4R1 in promoting HCC cell growth, which might elucidate the pathological mechanism of HCC.

Protein phosphatase 4 promotes chromosome pairing and synapsis, and contributes to maintaining crossover competence with increasing age

Prior to the meiotic divisions, dynamic chromosome reorganizations including pairing, synapsis, and recombination of maternal and paternal chromosome pairs must occur in a highly regulated fashion during meiotic prophase. How chromosomes identify each other's homology and exclusively pair and synapse with their homologous partners, while rejecting illegitimate synapsis with non-homologous chromosomes, remains obscure. In addition, how the levels of recombination initiation and crossover formation are regulated so that sufficient, but not deleterious, levels of DNA breaks are made and processed into crossovers is not understood well. We show that in Caenorhabditis elegans, the highly conserved Serine/Threonine protein phosphatase PP4 homolog, PPH-4.1, is required independently to carry out four separate functions involving meiotic chromosome dynamics: (1) synapsis-independent chromosome pairing, (2) restriction of synapsis to homologous chromosomes, (3) programmed DNA double-strand break initiation, and (4) crossover formation. Using quantitative imaging of mutant strains, including super-resolution (3D-SIM) microscopy of chromosomes and the synaptonemal complex, we show that independently-arising defects in each of these processes in the absence of PPH-4.1 activity ultimately lead to meiotic nondisjunction and embryonic lethality. Interestingly, we find that defects in double-strand break initiation and crossover formation, but not pairing or synapsis, become even more severe in the germlines of older mutant animals, indicating an increased dependence on PPH-4.1 with increasing maternal age. Our results demonstrate that PPH-4.1 plays multiple, independent roles in meiotic prophase chromosome dynamics and maintaining meiotic competence in aging germlines. PP4's high degree of conservation suggests it may be a universal regulator of meiotic prophase chromosome dynamics.

Meiosis creates gametes by distributing diploid genomes containing homologous chromosome pairs into daughter cells that receive only one of each chromosome. To segregate correctly at the first meiotic division, chromosomes must pair and synapse with their homologous partners, and undergo crossover recombination, which requires breaking and repairing the DNA strands of all chromosomes. How chromosomes recognize their partners, and how a cell controls the amount of DNA breakage and recombination that occurs, are open questions. In this study, we observed meiosis in the nematode Caenorhabditis elegans to examine the role of Protein Phosphatase 4 (PP4). We found that in the absence of PP4, chromosomes often paired and synapsed with non-homologous chromosomes, or synapsed with themselves by folding in half. Additionally, without PP4 activity, the number of DNA breaks and of crossover recombination events were both independently reduced. The latter two defects became even worse with increasing age, indicating that older animals require PP4 to a greater extent. These findings shed light on how protein phosphorylation controls meiotic events, and demonstrate unanticipated, important roles for PP4.

Merkel cell polyomavirus small T antigen mediates microtubule destabilization to promote cell motility and migration

Merkel cell carcinoma (MCC) is an aggressive skin cancer of neuroendocrine origin with a high propensity for recurrence and metastasis. Merkel cell polyomavirus (MCPyV) causes the majority of MCC cases due to the expression of the MCPyV small and large tumor antigens (ST and LT, respectively). Although a number of molecular mechanisms have been attributed to MCPyV tumor antigen-mediated cellular transformation or replication, to date, no studies have investigated any potential link between MCPyV T antigen expression and the highly metastatic nature of MCC. Here we use a quantitative proteomic approach to show that MCPyV ST promotes differential expression of cellular proteins implicated in microtubule-associated cytoskeletal organization and dynamics. Intriguingly, we demonstrate that MCPyV ST expression promotes microtubule destabilization, leading to a motile and migratory phenotype. We further highlight the essential role of the microtubule-associated protein stathmin in MCPyV ST-mediated microtubule destabilization and cell motility and implicate the cellular phosphatase catalytic subunit protein phosphatase 4C (PP4C) in the regulation of this process. These findings suggest a possible molecular mechanism for the highly metastatic phenotype associated with MCC.

IMPORTANCE

Merkel cell polyomavirus (MCPyV) causes the majority of cases of Merkel cell carcinoma (MCC), an aggressive skin cancer with a high metastatic potential. However, the molecular mechanisms leading to virally induced cancer development have yet to be fully elucidated. In particular, no studies have investigated any potential link between the virus and the highly metastatic nature of MCC. We demonstrate that the MCPyV small tumor antigen (ST) promotes the destabilization of the host cell microtubule network, which leads to a more motile and migratory cell phenotype. We further show that MCPyV ST induces this process by regulating the phosphorylation status of the cellular microtubule-associated protein stathmin by its known association with the cellular phosphatase catalytic subunit PP4C. These findings highlight stathmin as a possible biomarker of MCC and as a target for novel antitumoral therapies.

PP4 is essential for germinal center formation and class switch recombination in mice

PP4 is a serine/threonine phosphatase required for immunoglobulin (Ig) VDJ recombination and pro-B/pre-B cell development in mice. To elucidate the role of PP4 in mature B cells, we ablated the catalytic subunit of murine PP4 invivo utilizing the CD23 promoter and cre-loxP recombination and generated CD23^crePP4^F/F mice. The development of follicular and marginal zone B cells was unaffected in these mutants, but the proliferation of mature PP4-deficient B cells stimulated by invitro treatment with either anti-IgM antibody (Ab) or LPS was partially impaired. Interestingly, the induction of CD80 and CD86 expression on these stimulated B cells was normal. Basal levels of serum Igs of all isotypes were strongly reduced in CD23^crePP4^F/F mice, and their B cells showed a reduced efficiency of class switch recombination (CSR) invitro upon stimulation by LPS or LPS plus IL-4. When CD23^crePP4^F/F mice were challenged with either the T cell-dependent antigen TNP-KLH or the T cell-independent antigen TNP-Ficoll, or by H1N1 virus infection, the mutant animals failed to form germinal centers (GCs) in the spleen and the draining mediastinal lymph nodes, and did not efficiently mount antigen-specific humoral responses. In the resting state, PP4-deficient B cells exhibited pre-existing DNA fragmentation. Upon stimulation by DNA-damaging drug etoposide invitro, mutant B cells showed increased cleavage of caspase 3. In addition, the mutant B cells displayed impaired CD40-mediated MAPK activation, abnormal IgM-mediated NF-κB activation, and reduced S phase entry upon IgM/CD40-stimulation. Taken together, our results establish a novel role for PP4 in CSR, and reveal crucial functions for PP4 in the maintenance of genomic stability, GC formation, and B cell-mediated immune responses.

Ser/Thr-phosphoprotein phosphatases in chondrogenesis: neglected components of a two-player game

Protein phosphorylation plays a determining role in the regulation of chondrogenesis in vitro. While signalling pathways governed by protein kinases including PKA, PKC, and mitogen-activated protein kinases (MAPK) have been mapped in great details, published data relating to the specific role of phosphoprotein phosphatases (PPs) in differentiating chondroprogenitor cells or in mature chondrocytes is relatively sparse. This review discusses the known functions of Ser/Thr-specific PPs in the molecular signalling pathways of chondrogenesis. PPs are clearly equally important as protein kinases to counterbalance the effect of reversible protein phosphorylation. Of the main Ser/Thr PPs, some of the functions of PP1, PP2A and PP2B have been characterised in the context of chondrogenesis. While PP1 and PP2A appear to negatively regulate chondrogenic differentiation and maintenance of chondrocyte phenotype, calcineurin is an important stimulatory mediator during chondrogenesis but becomes inhibitory in mature chondrocytes. Furthermore, PPs are implicated to be mediators during the pathogenesis of osteoarthritis that makes them potential therapeutic targets to be exploited in the close future. Among the many yet unexplored targets of PPs, modulation of plasma membrane ion channel function and participation in mechanotransduction pathways are emerging novel aspects of signalling during chondrogenesis that should be further elucidated. Besides the regulation of cellular ion homeostasis, other potentially significant novel roles for PPs during the regulation of in vitro chondrogenesis are discussed.

Protein phosphatase 4 is an essential positive regulator for Treg development, function, and protective gut immunity

BACKGROUND

Protein phosphates 4 (PP4), encoded by the ppp4c gene, is a ubiquitously expressed phosphatase that has been implicated in the regulation of cytokine signaling and lymphocyte survival; recent reports suggest that PP4 may be involved in pre-TCR signaling and B cell development. However, whether PP4 also modulates the functions of peripheral T cells has not been investigated due to the lack of a suitable in vivo model. Treg cells are a specialized subset of CD4 helper T cells that can suppress the proliferation of activated effector T cells. In the absence of this negative regulation, autoimmune syndromes and inflammatory diseases, such as human Crohn's disease, will arise.

RESULTS

In this report, we generated mice with T cell-specific ablation of the ppp4c gene (CD4cre:PP4(f/f)) and a Foxp3-GFP reporter gene to examine the roles of PP4 in Treg development and function. Characterizations of the CD4cre:PP4(f/f) mice showed that PP4 deficiency induced partial αβ T lymphopenia and T cell hypo-proliferation. Further analyses revealed significant reductions in the numbers of thymic and peripheral Treg cells, as well as in the efficiency of in vitro Treg polarization. In addition, PP4-deficient Treg cells exhibited reduced suppressor functions that were associated with decreased IL-10, CTLA4, GITR and CD103 expression. More interestingly, the CD4cre:PP4(f/f) mice developed spontaneous rectal prolapse and colitis with symptoms similar to human Crohn's disease. The pathogenesis of colitis required the presence of commensal bacteria, and was correlated with reduced Treg cells in the gut. Nevertheless, PP4-deficient Treg cells were still capable of suppressing experimental colitis, suggesting that multiple factors contributed to the onset of the spontaneous colitis.

CONCLUSIONS

While the molecular mechanisms remain to be investigated, our results clearly show that PP4 plays a non-redundant role for the differentiation, suppressor activity and gut homeostasis of Treg cells. The onset of spontaneous colitis in the CD4cre:PP4(f/f) mice further suggests that PP4 is essential for the maintenance of protective gut immunity. The CD4cre:PP4(f/f) mice thus may serve as a good model for studying the interactions between Treg cells and gut commensal bacteria for the regulation of mucosal immunity.

The phosphatase JKAP/DUSP22 inhibits T-cell receptor signalling and autoimmunity by inactivating Lck

JNK pathway-associated phosphatase (JKAP, also known as DUSP22 or JSP-1) is a JNK activator. The in vivo role of JKAP in immune regulation remains unclear. Here we report that JKAP directly inactivates Lck by dephosphorylating tyrosine-394 residue during T-cell receptor (TCR) signalling. JKAP-knockout T cells display enhanced cell proliferation and cytokine production. JKAP-knockout mice show enhanced T-cell-mediated immune responses and are more susceptible to experimental autoimmune encephalomyelitis (EAE). In addition, the recipient mice that are adoptively transferred with JKAP-knockout T cells show exacerbated EAE symptoms. Aged JKAP-knockout mice spontaneously develop inflammation and autoimmunity. Thus, our results indicate that JKAP is an important phosphatase that inactivates Lck in the TCR signalling turn-off stage, leading to suppression of T-cell-mediated immunity and autoimmunity.

The serine/threonine phosphatase PP4 is required for pro-B cell development through its promotion of immunoglobulin VDJ recombination

PP4 phosphatase regulates a number of crucial processes but the role of PP4 in B cells has never been reported. We generated B cell-specific pp4 knockout mice and have identified an essential role for PP4 in B cell development. Deficiency of PP4 in B lineage cells leads to a strong reduction in pre-B cell numbers, an absence in immature B cells, and a complete loss of mature B cells. In PP4-deficient pro-B cells, immunoglobulin (Ig) DJ(H) recombination is impaired and Ig µ heavy chain expression is greatly decreased. In addition, PP4-deficient pro-B cells show an increase of DNA double-strand breaks at Ig loci. Consistent with their reduced numbers, residual PP4-deficient pre-B cells accumulate in the G1 phase, exhibit excessive DNA damage, and undergo increased apoptosis. Overexpression of transgenic Ig in PP4-deficient mice rescues the defect in B cell development such that the animals have normal numbers of IgM(+) B cells. Our study therefore reveals a novel function for PP4 in pro-B cell development through its promotion of V(H)DJ(H) recombination.

TWEAK prevents TNF-α-induced insulin resistance through PP2A activation in human adipocytes

Visceral fat is strongly associated with insulin resistance. Obesity-associated adipose tissue inflammation and inflammatory cytokine production are considered key mediators of insulin signaling inhibition. TWEAK is a relatively new member of the TNF cytokine superfamily, which can exist as full length membrane-associated (mTWEAK) and soluble (sTWEAK) isoforms. Although TWEAK has been shown to have important functions in chronic inflammatory diseases its physiological role in adipose tissue remains unresolved. In this study, we explore the molecular mechanisms involved in the modulation of TNF-α-induced effects on insulin sensitivity by sTWEAK in a human visceral adipose cell line and also in primary human adipocytes obtained from visceral fat depots. Our data reveal that sTWEAK ameliorates TNF-α-induced insulin resistance on glucose uptake, GLUT4 translocation and insulin signaling without affecting other metabolic effects of TNF-α such as lipolysis or apoptotis. Co-immunoprecipitation experiments in adipose cells revealed that pretreatment with sTWEAK specifically inhibits TRAF2 association with TNFR1, but not with TNFR2, which mediates insulin resistance. However, sTWEAK does not affect other downstream molecules activated by TNF-α, such as TAK1. Rather, sTWEAK abolishes the stimulatory effect of TNF-α on JNK1/2, which is directly involved in the development of insulin resistance. This is associated with an increase in PP2A activity upon sTWEAK treatment. Silencing of the PP2A catalytic subunit gene overcomes the dephosphorylation effect of sTWEAK on JNK1/2, pointing to PP2A as a relevant mediator of sTWEAK-induced JNK inactivation. Overall, our data reveal a protective role of TWEAK in glucose homeostasis and identify PP2A as a new driver in the modulation of TNF-α signaling by sTWEAK.

Overexpression of protein phosphatase 4 correlates with poor prognosis in patients with stage II pancreatic ductal adenocarcinoma

PURPOSE

Protein phosphatase 4 (PP4) has been reported to be overexpressed in breast and lung cancers. PP4 plays an important role in the regulation of centrosome maturation, DNA repair, NF-κB, and c-jun-NH(2)-kinase (JNK) signaling pathways. However, the expression and functions of PP4 in pancreatic cancer have not been studied.

EXPERIMENTAL DESIGN

We examined the expression of PP4 catalytic subunit (PP4C) protein in 133 patients with stage II pancreatic ductal adenocarcinoma (PDAC) and their paired benign pancreatic samples (N = 113) by immunohistochemistry. To confirm the immunohistochemical results, we measured PP4C protein and mRNA levels by Western blotting and real-time reverse transcriptase PCR. Using univariate and multivariate analysis, we correlated PP4C expression with survival and other clinicopathologic features.

RESULTS

PP4C was overexpressed in 75 of 133 (56.4%) stage II PDAC samples, which was significantly higher than the paired benign pancreatic tissue (15%, 17 of 113). PP4C mRNA expression levels were also higher in PDAC samples than the paired benign pancreatic tissue. Overexpression of PP4C in PDAC samples was associated with higher frequencies of distant metastasis (P = 0.02) and poor disease-free and overall survivals in patients with stage II PDAC (P = 0.006 and 0.02) independent of tumor size, margin status, and lymph node status (stage).

CONCLUSIONS

Our study showed that PP4C is overexpressed in PDAC. Overexpression of PP4C in PDAC samples is associated with poor prognosis in patients with stage II PDAC. Therefore, targeting PP4 signaling pathway may represent a new approach for the treatment of PDAC.

IMPACT

Our study showed that PP4C is an independent prognostic factor in patients with stage II PDAC.

Loss of CCDC6, the first identified RET partner gene, affects pH2AX S139 levels and accelerates mitotic entry upon DNA damage

CCDC6 was originally identified in chimeric genes caused by chromosomal translocation involving the RET proto-oncogene in some thryoid tumors mostly upon ionizing radiation exposure. Recognised as a pro-apoptotic phosphoprotein that negatively regulates CREB1-dependent transcription, CCDC6 is an ATM substrate that is responsive to genotoxic stress. Here we report that following genotoxic stress, loss or inactivation of CCDC6 in cancers that carry the CCDC6 fusion, accelerates the dephosphorylation of pH2AX S139, resulting in defective G2 arrest and premature mitotic entry. Moreover, we show that CCDC6 depleted cells appear to repair DNA damaged in a shorter time compared to controls, based on reporter assays in cells. High-troughput proteomic screening predicted the interaction between the CCDC6 gene product and the catalytic subunit of Serin-Threonin Protein Phosphatase 4 (PP4c) recently identified as the evolutionarily conserved pH2AX S139 phosphatase that is activated upon DNA Damage. We describe the interaction between CCDC6 and PP4c and we report the modulation of PP4c enzymatic activity in CCDC6 depleted cells. We discuss the functional significance of CCDC6-PP4c interactions and hypothesize that CCDC6 may act in the DNA Damage Response by negatively modulating PP4c activity. Overall, our data suggest that in primary tumours the loss of CCDC6 function could influence genome stability and thereby contribute to carcinogenesis.

Protein Ser/Thr phosphatases--the ugly ducklings of cell signalling

This review traces the historical origins and conceptual developments leading to the current state of knowledge of the three superfamilies of protein Ser/Thr phosphatases. 'PR enzyme' was identified as an enzyme that inactivates glycogen phosphorylase, although it took 10 years before this ugly duckling was recognized for its true identity as a protein Ser/Thr phosphatase. Ethanol denaturation for purification in the 1970s yielded a phosphatase that exhibited broad specificity, which was resolved into type-1 and type-2 phosphatases in the 1980s. More recent developments show that regulation and specificity are achieved through assembly of multisubunit holoenzymes, transient phosphorylation and the action of inhibitor proteins. Still not widely appreciated, there are hundreds of discrete protein Ser/Thr phosphatases available to counteract protein kinases, offering potential therapeutic targets. Signalling networks and modelling schemes need to incorporate the full gamut of protein Ser/Thr phosphatases and their interconnections.

Loss, mutation and deregulation of L3MBTL4 in breast cancers

Background

Many alterations are involved in mammary oncogenesis, including amplifications of oncogenes and losses of tumor suppressor genes (TSG). Losses may affect almost all chromosome arms and many TSGs remain to be identified.

Results

We studied 307 primary breast tumors and 47 breast cancer cell lines by high resolution array comparative genomic hybridization (aCGH). We identified a region on 18p11.31 lost in about 20% of the tumors and 40% of the cell lines. The minimal common region of loss (Chr18:6,366,938-6,375,929 bp) targeted the L3MBTL4 gene. This gene was also targeted by breakage in one tumor and in two cell lines. We studied the exon sequence of L3MBTL4 in 180 primary tumor samples and 47 cell lines and found six missense and one nonsense heterozygous mutations. Compared with normal breast tissue, L3MBTL4 mRNA expression was downregulated in 73% of the tumors notably in luminal, ERBB2 and normal-like subtypes. Losses of the 18p11 region were associated with low L3MBTL4 expression level. Integrated analysis combining genome and gene expression profiles of the same tumors pointed to 14 other potential 18p TSG candidates. Downregulated expression of ZFP161, PPP4R1 and YES1 was correlated with luminal B molecular subtype. Low ZFP161 gene expression was associated with adverse clinical outcome.

Conclusion

We have identified L3MBTL4 as a potential TSG of chromosome arm 18p. The gene is targeted by deletion, breakage and mutations and its mRNA is downregulated in breast tumors. Additional 18p TSG candidates might explain the aggressive phenotype associated with the loss of 18p in breast tumors.

Crosstalk in inflammation: the interplay of glucocorticoid receptor-based mechanisms and kinases and phosphatases

Glucocorticoids (GCs) are steroidal ligands for the GC receptor (GR), which can function as a ligand-activated transcription factor. These steroidal ligands and derivatives thereof are the first line of treatment in a vast array of inflammatory diseases. However, due to the general surge of side effects associated with long-term use of GCs and the potential problem of GC resistance in some patients, the scientific world continues to search for a better understanding of the GC-mediated antiinflammatory mechanisms. The reversible phosphomodification of various mediators in the inflammatory process plays a key role in modulating and fine-tuning the sensitivity, longevity, and intensity of the inflammatory response. As such, the antiinflammatory GCs can modulate the activity and/or expression of various kinases and phosphatases, thus affecting the signaling efficacy toward the propagation of proinflammatory gene expression and proinflammatory gene mRNA stability. Conversely, phosphorylation of GR can affect GR ligand- and DNA-binding affinity, mobility, and cofactor recruitment, culminating in altered transactivation and transrepression capabilities of GR, and consequently leading to a modified antiinflammatory potential. Recently, new roles for kinases and phosphatases have been described in GR-based antiinflammatory mechanisms. Moreover, kinase inhibitors have become increasingly important as antiinflammatory tools, not only for research but also for therapeutic purposes. In light of these developments, we aim to illuminate the integrated interplay between GR signaling and its correlating kinases and phosphatases in the context of the clinically important combat of inflammation, giving attention to implications on GC-mediated side effects and therapy resistance.

Protein phosphatase 4 regulates apoptosis in leukemic and primary human T-cells

The control of T-cell survival is of overwhelming importance for preventing leukemia and lymphoma. The present report demonstrates that the serine/threonine protein phosphatase PP4 regulates the survival of both leukemic T-cells and untransformed human peripheral blood T-cells, particularly after treatment with anti-leukemic drugs and other cytotoxic stimuli. PP4-induced apoptosis is mediated, at least in part, through de-phosphorylation of apoptosis regulator PEA-15, previously implicated in the control of leukemic cell survival. PP4 activity significantly affects the mutation rate in leukemic T-cells, indicating that PP4 dysfunction may be important in the development and progression of leukemia.

PP4 and PP2A regulate Hedgehog signaling by controlling Smo and Ci phosphorylation

The seven-transmembrane protein Smoothened (Smo) and Zn-finger transcription factor Ci/Gli are crucial components in Hedgehog (Hh) signal transduction that mediates a variety of processes in animal development. In Drosophila, multiple kinases have been identified to regulate Hh signaling by phosphorylating Smo and Ci; however, the phosphatase(s) involved remain obscured. Using an in vivo RNAi screen, we identified PP4 and PP2A as phosphatases that influence Hh signaling by regulating Smo and Ci, respectively. RNAi knockdown of PP4, but not of PP2A, elevates Smo phosphorylation and accumulation, leading to increased Hh signaling activity. Deletion of a PP4-interaction domain (amino acids 626-678) in Smo promotes Smo phosphorylation and signaling activity. We further find that PP4 regulates the Hh-induced Smo cell-surface accumulation. Mechanistically, we show that Hh downregulates Smo-PP4 interaction that is mediated by Cos2. We also provide evidence that PP2A is a Ci phosphatase. Inactivating PP2A regulatory subunit (Wdb) by RNAi or by loss-of-function mutation downregulates, whereas overexpressing regulatory subunit upregulates, the level and thus signaling activity of full-length Ci. Furthermore, we find that Wdb counteracts kinases to prevent Ci phosphorylation. Finally, we have obtained evidence that Wdb attenuates Ci processing probably by dephosphorylating Ci. Taken together, our results suggest that PP4 and PP2A are two phosphatases that act at different positions of the Hh signaling cascade.

PP4R4/KIAA1622 forms a novel stable cytosolic complex with phosphoprotein phosphatase 4

Protein serine/threonine phosphatase 4 (PP4c) is an essential polypeptide involved in critical cellular processes such as microtubule growth and organization, DNA damage checkpoint recovery, apoptosis, and tumor necrosis factor alpha signaling. Like other phosphatases of the PP2A family, PP4c interacts with regulatory proteins, which specify substrate targeting and intracellular localization. The identification of these regulatory proteins is, therefore, key to fully understanding the function of this enzyme class. Here, using a sensitive affinity purification/mass spectrometry approach, we identify a novel, stable cytosolic PP4c interacting partner, KIAA1622, which we have renamed PP4R4. PP4R4 displays weak sequence homology with the A (scaffolding) subunit of the PP2A holoenzyme and specifically associates with PP4c (and not with the related PP2Ac or PP6c phosphatases). The PP4c.PP4R4 interaction is disrupted by mutations analogous to those abrogating the association of PP2Ac with PP2A A subunit. However, unlike the PP2A A subunit, which plays a scaffolding role, PP4R4 does not bridge PP4c with previously characterized PP4 regulatory subunits. PP4c.PP4R4 complexes exhibit phosphatase activity toward a fluorogenic substrate and gammaH2AX, but this activity is lower than that associated with the PP4c.PP4R2.PP4R3 complex, which itself is less active than the free PP4c catalytic subunit. Our data demonstrate that PP4R4 forms a novel cytosolic complex with PP4c, independent from the complexes containing PP4R1, PP4R2.PP4R3, and alpha4, and that the regulatory subunits of PP4c have evolved different modes of interaction with the catalytic subunit.

Deactivation of sphingosine kinase 1 by protein phosphatase 2A

Sphingosine kinase 1 (SK1) is an important regulator of cellular signaling that has been implicated in a broad range of cellular processes. Cell exposure to a wide array of growth factors, cytokines, and other cell agonists can result in a rapid and transient increase in SK activity via an activating phosphorylation. We have previously identified extracellular signal-regulated kinases 1 and 2 (ERK1/2) as the kinases responsible for the phosphorylation of human SK1 at Ser(225), but the corresponding phosphatase targeting this phosphorylation has remained undefined. Here, we provide data to support a role for protein phosphatase 2A (PP2A) in the deactivation of SK1 through dephosphorylation of phospho-Ser(225). The catalytic subunit of PP2A (PP2Ac) was found to interact with SK1 using both GST-pulldown and coimmunoprecipitation analyses. Coexpression of PP2Ac with SK1 resulted in reduced Ser(225) phosphorylation of SK1 in human embryonic kidney (HEK293) cells. In vitro phosphatase assays showed that PP2Ac dephosphorylated both recombinant SK1 and a phosphopeptide based on the phospho-Ser(225) region of SK1. Finally, both basal and tumor necrosis factor-alpha-stimulated cellular SK1 activity were regulated by molecular manipulation of PP2Ac activity. Thus, PP2A appears to function as an endogenous regulator of SK1 phosphorylation.

A PP4-phosphatase complex dephosphorylates gamma-H2AX generated during DNA replication

The histone H2A variant H2AX is rapidly phosphorylated in response to DNA double-stranded breaks to produce gamma-H2AX. gamma-H2AX stabilizes cell-cycle checkpoint proteins and DNA repair factors at the break site. We previously found that the protein phosphatase PP2A is required to resolve gamma-H2AX foci and complete DNA repair after exogenous DNA damage. Here we describe a three-protein PP4 phosphatase complex in mammalian cells, containing PP4C, PP4R2, and PP4R3beta, that specifically dephosphorylates ATR-mediated gamma-H2AX generated during DNA replication. PP4 efficiently dephosphorylates gamma-H2AX within mononucleosomes in vitro and does not directly alter ATR or checkpoint kinase activity, suggesting that PP4 acts directly on gamma-H2AX in cells. When the PP4 complex is silenced, repair of DNA replication-mediated breaks is inefficient, and cells are hypersensitive to DNA replication inhibitors, but not radiomimetic drugs. Therefore, gamma-H2AX elimination at DNA damage foci is required for DNA damage repair, but accomplishing this task involves distinct phosphatases with potentially overlapping roles.

Protein phosphatase 4 catalytic subunit regulates Cdk1 activity and microtubule organization via NDEL1 dephosphorylation

Protein phosphatase 4 catalytic subunit (PP4c) is a PP2A-related protein serine/threonine phosphatase with important functions in a variety of cellular processes, including microtubule (MT) growth/organization, apoptosis, and tumor necrosis factor signaling. In this study, we report that NDEL1 is a substrate of PP4c, and PP4c selectively dephosphorylates NDEL1 at Cdk1 sites. We also demonstrate that PP4c negatively regulates Cdk1 activity at the centrosome. Targeted disruption of PP4c reveals disorganization of MTs and disorganized MT array. Loss of PP4c leads to an unscheduled activation of Cdk1 in interphase, which results in the abnormal phosphorylation of NDEL1. In addition, abnormal NDEL1 phosphorylation facilitates excessive recruitment of katanin p60 to the centrosome, suggesting that MT defects may be attributed to katanin p60 in excess. Inhibition of Cdk1, NDEL1, or katanin p60 rescues the defective MT organization caused by PP4 inhibition. Our work uncovers a unique regulatory mechanism of MT organization by PP4c through its targets Cdk1 and NDEL1 via regulation of katanin p60 distribution.

Cyclic AMP stimulates SF-1-dependent CYP11A1 expression through homeodomain-interacting protein kinase 3-mediated Jun N-terminal kinase and c-Jun phosphorylation

Steroids are synthesized in adrenal glands and gonads under the control of pituitary peptides. These peptides bind to cell surface receptors to activate the cyclic AMP (cAMP) signaling pathway leading to an increase of steroidogenic gene expression. Exactly how cAMP activates steroidogenic gene expression is not clear, except for the knowledge that transcription factor SF-1 plays a key role. Investigating the factors participating in SF-1 action, we found that c-Jun and homeodomain-interacting protein kinase 3 (HIPK3) were required for basal and cAMP-stimulated expression of one major steroidogenic gene, CYP11A1. HIPK3 enhanced SF-1 activity, and c-Jun was required for the functional interaction of HIPK3 with SF-1. Furthermore, after cAMP stimulation, both c-Jun and Jun N-terminal kinase (JNK) were phosphorylated through HIPK3. These phosphorylations were important for SF-1 activity and CYP11A1 expression. Thus, we have defined HIPK3-mediated JNK activity and c-Jun phosphorylation as important events that increase SF-1 activity for CYP11A1 transcription in response to cAMP. This finding has linked three common factors, HIPK3, JNK, and c-Jun, to the cAMP signaling pathway leading to increased steroidogenic gene expression.

Hematopoietic progenitor kinase 1 negatively regulates T cell receptor signaling and T cell-mediated immune responses

HPK1 is a Ste20-related serine-threonine kinase that inducibly associates with the adaptors SLP-76 and Gads after T cell receptor (TCR) signaling. Here, HPK1 deficiency resulted in enhanced TCR-induced phosphorylation of SLP-76, phospholipase C-gamma1 and the kinase Erk, more-persistent calcium flux, and increased production of cytokines and antigen-specific antibodies. Furthermore, HPK1-deficient mice were more susceptible to experimental autoimmune encephalomyelitis. Although the interaction between SLP-76 and Gads was unaffected, the inducible association of SLP-76 with 14-3-3tau (a phosphorylated serine-binding protein and negative regulator of TCR signaling) was reduced in HPK1-deficient T cells after TCR stimulation. HPK1 phosphorylated SLP-76 and induced the interaction of SLP-76 with 14-3-3tau. Our results indicate that HPK1 negatively regulates TCR signaling and T cell-mediated immune responses.

Conditional knockout mice reveal an essential role of protein phosphatase 4 in thymocyte development and pre-T-cell receptor signaling

Okadaic acid-sensitive serine/threonine phosphatases have been shown to regulate interleukin-2 transcription and T-cell activation. Okadaic acid inhibits protein phosphatase 4 (PP4), a novel PP2A-related serine/threonine phosphatase, at a 50% inhibitory concentration (IC(50)) comparable to that for PP2A. This raises the possibility that some cellular functions of PP2A, determined in T cells by using okadaic acid, may in fact be those of PP4. To investigate the in vivo roles of PP4 in T cells, we generated conventional and T-cell-specific PP4 conditional knockout mice. We found that the ablation of PP4 led to the embryonic lethality of mice. PP4 gene deletion in the T-cell lineage resulted in aberrant thymocyte development, including T-cell arrest at the double-negative 3 stage (CD4(-) CD8(-) CD25(+) CD44(-)), abnormal thymocyte maturation, and lower efficacy of positive selection. PP4-deficient thymocytes showed decreased proliferation and enhanced apoptosis in vivo. Analysis of pre-T-cell receptor (pre-TCR) signaling further revealed impaired calcium flux and phospholipase C-gamma1-extracellular signal-regulated kinase activation in the absence of PP4. Anti-CD3 injection in PP4-deficient mice led to enhanced thymocyte apoptosis, accompanied by increased proapoptotic Bim but decreased antiapoptotic Bcl-xL protein levels. In the periphery, antigen-specific T-cell proliferation and T-cell-mediated immune responses in PP4-deficient mice were dramatically compromised. Thus, our results indicate that PP4 is essential for thymocyte development and pre-TCR signaling.

The Saccharomyces cerevisiae orthologue of the human protein phosphatase 4 core regulatory subunit R2 confers resistance to the anticancer drug cisplatin

The anticancer agents cisplatin and oxaliplatin are widely used in the treatment of human neoplasias. A genome-wide screen in Saccharomyces cerevisiae previously identified PPH3 and PSY2 among the top 20 genes conferring resistance to these anticancer agents. The mammalian orthologue of Pph3p is the protein serine/threonine phosphatase Ppp4c, which is found in high molecular mass complexes bound to a regulatory subunit R2. We show here that the putative S. cerevisiae orthologue of R2, which is encoded by ORF YBL046w, binds to Pph3p and exhibits the same unusually high asymmetry as mammalian R2. Despite the essential function of Ppp4c-R2 in microtubule-related processes at centrosomes in higher eukaryotes, S. cerevisiae diploid strains with homozygous deletion of YBL046w and two or one functional copies of the TUB2 gene were viable and no more sensitive to microtubule-depolymerizing drugs than the control strain. The protein encoded by YBL046w exhibited a predominantly nuclear localization. These studies suggest that the centrosomal function of Ppp4c-R2 is not required or may be performed by a different phosphatase in yeast. Homozygous diploid deletion strains of S. cerevisiae, pph3Delta, ybl046wDelta and psy2Delta, were all more sensitive to cisplatin than the control strain. The YBL046w gene therefore confers resistance to cisplatin and was termed PSY4 (platinum sensitivity 4). Ppp4c, R2 and the putative orthologue of Psy2p (termed R3) are shown here to form a complex in Drosophila melanogaster and mammalian cells. By comparison with the yeast system, this complex may confer resistance to cisplatin in higher eukaryotes.

Localization of a nuclear serine/threonine protein phosphatase in insulin-secreting INS-1 cells: potential regulation by IL-1β

Emerging evidence suggests critical roles for protein phosphatase 2A (PP2A) in islet beta cell function, including survival and demise (Kowluru A: Biochemical Pharmacol 69:1681-1691, 2005). Herein, we identified an okadaic acid (OKA)-sensitive PP2A-like phosphatase in the nuclear fraction from insulin-secreting INS-1 cells. Western blot analysis indicated relatively higher abundance of the catalytic subunit of protein phosphatase 4 (PP4c) compared to PP2Ac in this fraction. Autoradiographic and vapor-phase equilibration analyses suggested that the nuclear PP4c undergoes OKA-sensitive carboxylmethylation (CML) when S-adenosyl-L-((3)H-methyl) methionine (SAM) was used as the methyl donor. Exposure of INS cells to interleukin-1beta (IL-1beta; 600 pM; 48 h) resulted in a marked increase in nitric oxide (NO) release with concomitant reduction in the degree of expression, the CML and the catalytic activity of only PP4, but not PP2A, in the nuclear fraction. Immunoprecipitation studies suggested potential complexation of PP4c with nuclear lamin-B, a key regulatory protein involved in the nuclear envelope assembly. Based on these findings, we propose that IL-1beta-mediated inhibition of PP4 activity might result in the retention of lamin-B in its phosphorylated state, which is a requisite for its degradation by caspases leading to the apoptotic demise of the beta cell (Veluthakal et al.: Am J Physiol Cell Physiol 287:C1152-C1162, 2004).

Mapping the Ca2+ -dependent binding of an invertebrate homolog of protein phosphatase 4 regulatory subunit 2 to the small EF-hand protein, calsensin

The EF-hand family of calcium-binding proteins regulates cellular signal transduction events via calcium-dependent interactions with target proteins. Here, we show that the COOH-terminal tail of the leech homolog of protein phosphatase 4 regulatory subunit 2 (PP4-R2) interacts with the small neuronal EF-hand calcium-binding protein, Calsensin, in a calcium-dependent manner. Using two-dimensional NMR spectroscopy and chemical shift perturbations we have identified and mapped the residues of Calsensin that form a binding surface for PP4-R2. We show that the binding groove is formed primarily of discontinuous hydrophobic residues located in helix 1, the hinge region, and helix 4 of the unicornate-type four helix structure of Calsensin. The findings suggest the possibility that calcium-dependent modulation of phosphatase complexes through interactions with small calcium-binding proteins may be a general mechanism for regulation of signal transduction pathways.

Protein phosphatase 4 - from obscurity to vital functions

Protein phosphatase 4 (Ppp4) is a ubiquitous serine/threonine phosphatase in the PPP family that is now recognised to regulate a variety of cellular functions independently of protein phosphatase 2A (PP2A). Regulatory subunits (R1 and R2) have been identified in mammals that interact with the catalytic subunit of Ppp4 (Ppp4c) and control its activity. Ppp4c-R2 complexes play roles in organelle assembly; not only are they essential for maturation of the centrosome, but they are also involved in spliceosomal assembly via interaction with the survival of motor neurons (SMNs) complex. Several cellular signalling routes, including NF-kappaB and the target of rapamycin (TOR) pathways appear to be regulated by Ppp4. Emerging evidence indicates that Ppp4 may play a role in the DNA damage response and that Ppp4c-R1 complexes decrease the activity of a histone deacetylase, implicating Ppp4 in the regulation of chromatin activities. Antitumour agents, cantharidin and fostriecin, potently inhibit the activity of Ppp4. Orthologues of mammalian Ppp4 subunits in Saccharomyces cerevisiae confer resistance to the anticancer, DNA-binding drugs, cisplatin and oxaliplatin.

Histone deacetylase 3 (HDAC3) activity is regulated by interaction with protein serine/threonine phosphatase 4

Histone deacetylase 3 (HDAC3) is one of four members of the human class I HDACs that regulates gene expression by deacetylation of histones and nonhistone proteins. Early studies have suggested that HDAC3 activity is regulated by association with the corepressors N-CoR and SMRT. Here we demonstrate that, in addition to protein-protein interactions with NCoR/SMRT, the activity of HDAC3 is regulated by both phosphorylation and dephosphorylation. A protein kinase CK2 phosphoacceptor site in the HDAC3 protein was identified at position Ser424, which is a nonconserved residue among the class I HDACs. Mutation of this residue was found to reduce deacetylase activity. Interestingly, unlike other class I HDACs, HDAC3 uniquely copurifies with the catalytic and regulatory subunits of the protein serine/threonine phosphatase 4 complex (PP4c/PP4R1). Furthermore, HDAC3 complexes displayed protein phosphatase activity and a series of subsequent mutational analyses revealed that the N terminus of HDAC3 (residues 1-122) was both necessary and sufficient for HDAC3-PP4c interactions. Significantly, both overexpression and siRNA knock-down approaches, and analysis of cells devoid of PP4c, unequivocally show that HDAC3 activity is inversely proportional to the cellular abundance of PP4(c). These findings therefore further highlight the importance of protein-protein interactions and extend the significance of dephosphorylation in the regulation of HDAC activity, as well as present a novel alternative pathway by which HDAC3 activity is regulated.

Protein Phosphatase 2A Activity Associated with Golgi Membranes during the G2/M Phase May Regulate Phosphorylation of ERK2

The extracellular signal-regulated kinase (ERK) 1 and 2 proteins are mitogen-activated protein kinase (MAPK) members that regulate cell proliferation and differentiation. ERK proteins are activated exclusively by MAPK kinase 1 and 2 phosphorylation of threonine and tyrosine residues located within the conserved TXY MAPK activation motif. Although dual phosphorylation of Thr and Tyr residues confers full activation of ERK, in vitro studies suggest that a single phosphorylation on either Thr or Tyr may yield partial ERK activity. Previously, we have demonstrated that phosphorylation of the tyrosine residue (Tyr(P) ERK) may be involved in regulating the Golgi complex structure during the G2 and M phases of the cell cycle (Cha, H., and Shapiro, P. (2001) J. Cell Biol. 153, 1355-1368). In the present study, we examined mechanisms for generating Tyr(P) ERK by determining cell cycle-dependent changes in localized phosphatase activity. Using fractionated nuclei-free cell lysates, we find increased serine/threonine phosphatase activity associated with Golgi-enriched membranes in cells synchronized in the late G2/early M phase as compared with G1 phase cells. The addition of phosphatase inhibitors in combination with immunodepletion assays identified this activity to be related to protein phosphatase 2A (PP2A). The increased activity was accounted for by elevated PP2A association with mitotic Golgi membranes as well as increased catalytic activity after normalization of PP2A protein levels in the phosphatase assays. These data indicate that localized changes in PP2A activity may be involved in regulating proteins involved in Golgi disassembly as cells enter mitosis.

Germline transmission and efficient DNA recombination in mouse embryonic stem cells mediated by adenoviral-Cre transduction

Following gene targeting, a loxP-neo-loxP cassette was introduced into ES cells. The presence of a selectable marker such as neo in the targeted allele may result in gene interference in flox mice or unexpected phenotypes due to genetic ambiguity in direct knockout mice. Typically, the neo cassette is selectively removed by transient expression of the Cre recombinase in targeted ES cell. However, this method involves a tedious process of selecting, expanding, and screening ES cell clones which may compromise germline competency. Here, we describe a novel method of combining adenovirus-Cre mediated gene recombination with ES gene targeting to facilitate efficient loxP-neo-loxP removal in ES cells. We demonstrate that adenovirus-Cre infected ES cells can retain their germline competency. The procedures described here facilitate a rapid genetic manipulation of ES cells to obtain neo-free knockout animals, multiple gene targeting, homozygous mutant ES cells ideal for in vitro characterization, or Rag-deficient blastocyst complementation.

Protein Phosphatase 4 Is a Positive Regulator of Hematopoietic Progenitor Kinase 1

Hematopoietic progenitor kinase 1 (HPK1) is a hematopoietic specific mammalian Ste20-like protein kinase and has been implicated in many cellular signaling pathways including T cell receptor (TCR) signaling. However, little is known about the in vivo regulation of HPK1. We present evidence that HPK1 is positively regulated by protein phosphatase 4 (PP4; also called PPX and PPP4), a serine/threonine phosphatase. We found that PP4 interacted with HPK1 and that the proline-rich region of HPK1 was necessary and sufficient for this interaction. We also found that PP4 had phosphatase activity toward HPK1 in vivo and that co-transfection of PP4 with HPK1 resulted in specific kinase activation of HPK1. Moreover, we found that the PP4-induced HPK1 kinase activation was accompanied by an increase in protein expression of HPK1. Pulse-chase analysis showed that PP4 increased the half-life of HPK1. Further studies showed that HPK1 was subject to regulation by ubiquitination and ubiquitin-targeted degradation and that PP4 inhibited HPK1 ubiquitination. In addition, we found that TCR stimulation enhanced the PP4-HPK1 interaction and that wild-type PP4 enhanced, whereas a phosphatase-dead PP4 mutant inhibited, TCR-induced activation of HPK1 in Jurkat T cells. Combined with the observation that PP4 enhanced HPK1-induced JNK activation, our studies identify PP4 as a positive regulator for HPK1 and the HPK1-JNK signaling pathway.

Protein phosphatase 4 interacts with and down-regulates insulin receptor substrate 4 following tumor necrosis factor-alpha stimulation

Protein phosphatase 4 (PP4; also named PPX or PPP4) is a PP2A-related protein serine/threonine phosphatase with important roles in a variety of cellular processes such as microtubule growth/organization, apoptosis, tumor necrosis factor (TNF)-alpha signaling, and activation of c-Jun N-terminal kinase and NF-kappaB. To further investigate the cellular functions of PP4, we isolated and identified PP4-interacting proteins using a proteomic approach. We found that insulin receptor substrate 4 (IRS-4) interacted with PP4 and that this interaction was enhanced following TNF-alpha stimulation. We also found that PP4, but not PP2A, down-regulated IRS-4 in a phosphatase activity-dependent manner. Pulse-chase analysis revealed that PP4 decreased the half-life of IRS-4 from 4 to 1 h. Moreover, we found that TNF-alpha stimulated a PP4-dependent degradation of IRS-4, as indicated by the blockage of the degradation by a potent PP4 inhibitor (okadaic acid) and a phosphatase-dead PP4 mutant (PP4-RL). Taken together, our studies indicate that IRS-4 is subject to regulation by TNF-alpha and that PP4 mediates TNF-alpha-induced degradation of IRS-4.

Suppression of MEK/ERK signaling pathway enhances cisplatin-induced NF-kappaB activation by protein phosphatase 4-mediated NF-kappaB p65 Thr dephosphorylation

We previously reported that suppression of the MEK/ERK pathway increases drug resistance of SiHa cells. In this study, we further characterized the underlying mechanism of this phenomenon. Pretreatment of SiHa cells with MEK/ERK inhibitor enhanced cisplatin-induced NF-kappaB activation. However, results of immunoblotting analysis showed that neither cisplatin nor MEK/ERK inhibitors induced marked IkappaBalpha degradation, suggesting that suppression of the MEK/ERK signaling pathway may enhance cisplatin-induced NF-kappaB activation via mechanisms other than the conventional pathway. Previous findings that protein phosphatase 4 (PP4), a nuclear serine/threonine phosphatase, directly interacts with and activates NF-kappaB led us to examine the phosphorylation status of NF-kappaB p65. Coincident with activation of NF-kappaB, cisplatin induced Ser phosphorylation but decreased Thr phosphorylation of NF-kappaB p65. Suppression of the MEK/ERK pathway further enhanced cisplatin-induced Thr dephosphorylation but did not affect cisplatin-induced Ser phosphorylation of NF-kappaB p65. Further, in parallel with Thr dephosphorylation, the protein level of nuclear PP4 was increased in cisplatin-treated cells and was further increased by suppression of the MEK/ERK pathway. SiHa cells were then transfected by a sense or an antisense PP4 gene. PP4-overexpressing cells showed a decrease in Thr phosphorylation of NF-kappaB p65 to nearly undetectable levels, and both basal and cisplatin-induced NF-kappaB activities were higher than those in parental cells. By contrast, cisplatin, either alone or with MEK/ERK inhibitors, induced little NF-kappaB activation in antisense PP4-transfected cells. Coprecipitated complex kinase assay revealed a fragment of NF-kappaB p65 (amino acids 279-444) to contain potential phosphorylation sites that directly interact with PP4. Further studies by site-directed mutagenesis suggested that Thr(435) was the major phosphorylation site.