Vimentin dephosphorylation by protein phosphatase 2A is modulated by the targeting subunit B55

Mapping the substrate landscape of protein phosphatase 2A catalytic subunit PPP2CA

Protein phosphatase 2A (PP2A) is an essential Ser/Thr phosphatase. The PP2A holoenzyme complex comprises a scaffolding (A), regulatory (B), and catalytic (C) subunit, with PPP2CA being the principal catalytic subunit. The full scope of PP2A substrates in cells remains to be defined. To address this, we employed dTAG proteolysis-targeting chimeras to efficiently and selectively degrade dTAG-PPP2CA in homozygous knock-in HEK293 cells. Unbiased global phospho-proteomics identified 2,204 proteins with significantly increased phosphorylation upon dTAG-PPP2CA degradation, implicating them as potential PPP2CA substrates. A vast majority of these are novel. Bioinformatic analyses revealed involvement of the potential PPP2CA substrates in spliceosome function, cell cycle, RNA transport, and ubiquitin-mediated proteolysis. We identify a pSP/pTP motif as a predominant target for PPP2CA and confirm some of our phospho-proteomic data with immunoblotting. We provide an in-depth atlas of potential PPP2CA substrates and establish targeted degradation as a robust tool to unveil phosphatase substrates in cells.

Circulating Vimentin Over-Expression in Patients with Oral Sub Mucosal Fibrosis and Oral Squamous Cell Carcinoma

Oral squamous cell carcinoma (OSCC) is one of the most common (90%) types of oral carcinomas in the world. It is the 2nd most common and 3rd deadliest cancer in India. The lack of early detection marker is one of the major causes of worst prognosis. The vimentin belongs to intermediate filament family proteins which plays significant role in maintaining cellular integrity. Over-expression of vimentin has been widely reported in many epithelial cancers however the information regarding its prevalence in the oral cancers still needs further scientific intervention. The expression level of circulating vimentin protein in serum samples (n = 30) of oral submucous fibrosis (OSMF), OSCC patients and healthy controls were measured by performing ELISA. The serum level of vimentin was significantly higher in OSMF (p < 0.01) and OSCC (p < 0.003) patients as compared to healthy subjects. The circulating vimentin levels showed a gradual increase with increasing disease status (normal < OSMF < OSCC). Circulatory levels of vimentin may ba useful indicator of disease progression and as a suitable target for therapeutic intervention of oral submucous fibrosis and oral carcinoma.

Vimentin and cytokeratin: Good alone, bad together

The cytoskeleton plays an integral role in maintaining the integrity of epithelial cells. Epithelial cells primarily employ cytokeratin in their cytoskeleton, whereas mesenchymal cells use vimentin. During the epithelial-mesenchymal transition (EMT), cytokeratin-positive epithelial cells begin to express vimentin. EMT induces stem cell properties and drives metastasis, chemoresistance, and tumor relapse. Most studies of the functions of cytokeratin and vimentin have relied on the use of either epithelial or mesenchymal cell types. However, it is important to understand how these two cytoskeleton intermediate filaments function when co-expressed in cells undergoing EMT. Here, we discuss the individual and shared functions of cytokeratin and vimentin that coalesce during EMT and how alterations in intermediate filament expression influence carcinoma progression.

PP2A-B55: substrates and regulators in the control of cellular functions

PP2A is a major serine/threonine phosphatase class involved in the regulation of cell signaling through the removal of protein phosphorylation. This class of phosphatases is comprised of different heterotrimeric complexes displaying distinct substrate specificities. The present review will focus on one specific heterocomplex, the phosphatase PP2A-B55. Herein, we will report the direct substrates of this phosphatase identified to date, and its impact on different cell signaling cascades. We will additionally describe its negative regulation by its inhibitors Arpp19 and ENSA and their upstream kinase Greatwall. Finally, we will describe the essential molecular features defining PP2A-B55 substrate specificity that confer the correct temporal pattern of substrate dephosphorylation. The main objective of this review is to provide the reader with a unique source compiling all the knowledge of this particular holoenzyme that has evolved as a key enzyme for cell homeostasis and cancer development.

CCL4 induces inflammatory signalling and barrier disruption in the neurovascular endothelium

Background

During neuroinflammation many chemokines alter the function of the blood-brain barrier (BBB) that regulates the entry of macromolecules and immune cells into the brain. As the milieu of the brain is altered, biochemical and structural changes contribute to the pathogenesis of neuroinflammation and may impact on neurogenesis. The chemokine CCL4, previously known as MIP-1β, is upregulated in a wide variety of central nervous system disorders, including multiple sclerosis, where it is thought to play a key role in the neuroinflammatory process. However, the effect of CCL4 on BBB endothelial cells (ECs) is unknown.

Materials and methods

Expression and distribution of CCR5, phosphorylated p38, F-actin, zonula occludens-1 (ZO-1) and vascular endothelial cadherin (VE-cadherin) were analysed in the human BBB EC line hCMEC/D3 by Western blot and/or immunofluorescence in the presence and absence of CCL4. Barrier modulation in response to CCL4 using hCMEC/D3 monolayers was assessed by measuring molecular flux of 70 ​kDa RITC-dextran and transendothelial lymphocyte migration. Permeability changes in response to CCL4 in vivo were measured by an occlusion technique in pial microvessels of Wistar rats and by fluorescein angiography in mouse retinae.

Results

CCR5, the receptor for CCL4, was expressed in hCMEC/D3 cells. CCL4 stimulation led to phosphorylation of p38 and the formation of actin stress fibres, both indicative of intracellular chemokine signalling. The distribution of junctional proteins was also altered in response to CCL4: junctional ZO-1 was reduced by circa 60% within 60 ​min. In addition, surface VE-cadherin was redistributed through internalisation. Consistent with these changes, CCL4 induced hyperpermeability in vitro and in vivo and increased transmigration of lymphocytes across monolayers of hCMEC/D3 cells.

Conclusion

These results show that CCL4 can modify BBB function and may contribute to disease pathogenesis.

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The chemokine CCL4 induced phosphorylation of P38 in an in vitro model of the blood-brain barrier (BBB).

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CCL4 treatment resulted in reduction of plasma membrane VE-cadherin and junctional ZO-1.

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CCL4 induced neurovascular barrier breakdown in vitro and in vivo.

Protein phosphatase 2A - structure, function and role in neurodevelopmental disorders

Neurodevelopmental disorders (NDDs), including intellectual disability (ID), autism and schizophrenia, have high socioeconomic impact, yet poorly understood etiologies. A recent surge of large-scale genome or exome sequencing studies has identified a multitude of mostly de novo mutations in subunits of the protein phosphatase 2A (PP2A) holoenzyme that are strongly associated with NDDs. PP2A is responsible for at least 50% of total Ser/Thr dephosphorylation in most cell types and is predominantly found as trimeric holoenzymes composed of catalytic (C), scaffolding (A) and variable regulatory (B) subunits. PP2A can exist in nearly 100 different subunit combinations in mammalian cells, dictating distinct localizations, substrates and regulatory mechanisms. PP2A is well established as a regulator of cell division, growth, and differentiation, and the roles of PP2A in cancer and various neurodegenerative disorders, such as Alzheimer's disease, have been reviewed in detail. This Review summarizes and discusses recent reports on NDDs associated with mutations of PP2A subunits and PP2A-associated proteins. We also discuss the potential impact of these mutations on the structure and function of the PP2A holoenzymes and the etiology of NDDs.

Ankylosis progressive homolog upregulation inhibits cell viability and mineralization during fibroblast ossification by regulating the Wnt/β‑catenin signaling pathway

Ankylosis progressive homolog (ANKH) is associated with fibroblast ossification in ankylosing spondylitis (AS). As the human ANKH gene is poorly characterized relative to its murine counterpart, the aim of the present study was to examine ANKH expression in ligament tissue isolated from patients with AS and the role played by this gene in AS‑associated fibroblast ossification. Fibroblasts were isolated from ligament tissue collected from patients with AS and ligament tissue from individuals with spinal cord fractures, then cultured. Fibroblasts from patients with AS were subsequently transfected with an ANKH overexpression vector, while those collected from individuals with spinal cord fractures were transfected with small interfering RNA specific for ANKH. Cell viability, apoptosis and mineralization were analyzed using MTT assays, flow cytometry and Alizarin Red staining, respectively. Furthermore, ANKH mRNA and protein expression levels were analyzed using reverse transcription‑quantitative PCR and western blotting analysis, respectively. The expression levels of osteogenesis markers, including alkaline phosphatase, osteocalcin, Runt‑related transcription factor 2, c‑Myc, as well as the β‑catenin signaling protein, were also determined using western blotting. The results of the present study revealed that ANKH protein expression levels were downregulated in AS total ligament tissue extract, compared with spinal fracture ligament. Moreover, the fibroblasts derived from patients with AS exhibited an increased viability and reduced apoptosis rates, compared with the fibroblasts from patients with spinal fracture. Notably, ANKH overexpression inhibited viability, mineralization and ossification, increased the phosphorylation of β‑catenin and downregulated β‑catenin and c‑Myc protein expression levels in fibroblasts from patients with AS. In addition, ANKH overexpression increased the ratio of p‑β‑catenin/β‑catenin in fibroblasts from patients with AS. By contrast, ANKH silencing in fibroblasts from patients with spinal fracture resulted in the opposite effect. In conclusion, the findings of the present study suggested that ANKH may inhibit fibroblast viability, mineralization and ossification, possibly by regulating the Wnt/β‑catenin signaling pathway.

A Journey through the Cytoskeleton with Protein Kinase CK2

Substrate pleiotropicity, a very acidic phosphorylation consensus sequence, and an apparent uncontrolled activity, are the main features of CK2, a Ser/Thr protein kinase that is required for a plethora of cell functions. Not surprisingly, CK2 appears to affect cytoskeletal structures and correlated functions such as cell shape, mechanical integrity, cell movement and division. This review outlines our current knowledge of how CK2 regulates cytoskeletal structures, and discusses involved pathways and molecular mechanisms.

Redox inhibition of protein phosphatase PP2A: Potential implications in oncogenesis and its progression

Cellular processes are dictated by the active signaling of proteins relaying messages to regulate cell proliferation, apoptosis, signal transduction and cell communications. An intricate web of protein kinases and phosphatases are critical to the proper transmission of signals across such cascades. By governing 30–50% of all protein dephosphorylation in the cell, with prominent substrate proteins being key regulators of signaling cascades, the phosphatase PP2A has emerged as a celebrated player in various developmental and tumorigenic pathways, thereby posing as an attractive target for therapeutic intervention in various pathologies wherein its activity is deregulated. This review is mainly focused on refreshing our understanding of the structural and functional complexity that cocoons the PP2A phosphatase, and its expression in cancers. Additionally, we focus on its physiological regulation as well as into recent advents and strategies that have shown promise in countering the deregulation of the phosphatase through its targeted reactivation. Finally, we dwell upon one of the key regulators of PP2A in cancer cells-cellular redox status-its multifarious nature, and its integration into the reactome of PP2A, highlighting some of the significant impacts that ROS can inflict on the structural modifications and functional aspect of PP2A.

Altered Cytoskeleton as a Mitochondrial Decay Signature in the Retinal Pigment Epithelium

Mitochondria mediate energy metabolism, apoptosis, and aging, while mitochondrial disruption leads to age-related diseases that include age-related macular degeneration. Descriptions of mitochondrial morphology have been non-systematic and qualitative, due to lack of knowledge on the molecular mechanism of mitochondrial dynamics. The current study analyzed mitochondrial size, shape, and position quantitatively in retinal pigment epithelial cells (RPE) using a systematic computational model to suggest mitochondrial trafficking under oxidative environment. Our previous proteomic study suggested that prohibitin is a mitochondrial decay biomarker in the RPE. The current study examined the prohibitin interactome map using immunoprecipitation data to determine the indirect signaling on cytoskeletal changes and transcriptional regulation by prohibitin. Immunocytochemistry and immunoprecipitation demonstrated that there is a positive correlation between mitochondrial changes and altered filaments as well as prohibitin interactions with kinesin and unknown proteins in the RPE. Specific cytoskeletal and nuclear protein-binding mechanisms may exist to regulate prohibitin-mediated reactions as key elements, including vimentin and p53, to control apoptosis in mitochondria and the nucleus. Prohibitin may regulate mitochondrial trafficking through unknown proteins that include 110 kDa protein with myosin head domain and 88 kDa protein with cadherin repeat domain. Altered cytoskeleton may represent a mitochondrial decay signature in the RPE. The current study suggests that mitochondrial dynamics and cytoskeletal changes are critical for controlling mitochondrial distribution and function. Further, imbalance of retrograde versus anterograde mitochondrial trafficking may initiate the pathogenic reaction in adult-onset neurodegenerative diseases.

Protein Phosphatase 2A: a Double-Faced Phosphatase of Cellular System and Its Role in Neurodegenerative Disorders

Protein phosphatase 2A (PP2A), a ubiquitously expressed serine/threonine phosphatase, is a vitally important phosphatase for the cellular system. Structurally, it is constituted of three different subunits, namely catalytic subunit (PP2Ac), structural scaffold subunit (PP2A-A), and regulatory subunit (PP2A-B). All subunits have various isoforms, and catalytic and scaffold subunits are ubiquitously expressed, whereas regulatory subunits are more specific to tissue and cell type. It is the numerous possibilities of PP2A holoenzyme assembly with varying isoform components that make it possess a dual nature of activator or the inhibitory character in different signaling pathways, namely neural developmental pathways, Akt/protein kinase B pathway, NF-kB pathway, MAPK pathway, apoptosis pathway, and cell cycle progression to name a few. Importantly, the expression of PP2A in the brain is highest among the serine phosphatases and is known to actively participate in the neural development process. However, the exact mechanism of action of PP2A is still debated and enunciating the holoenzyme components, especially the regulatory subunit of PP2A involved in regulating neural developmental process is still poorly understood. In this review, we try to throw some light on the involvement of various PP2A holoenzyme forms in the process of neurogenesis and progression of neurodegenerative diseases.

HEK293 cells exposed to microcystin-LR show reduced protein phosphatase 2A activity and more stable cytoskeletal structure when overexpressing α4 protein

Microcystin-LR (MC-LR) is one of the most toxic members of microcystins released by freshwater cyanobacterial. The major mechanism of MC-LR toxicity has been attributed to its inhibition of protein phosphatases 1 (PP1) and 2A (PP2A). In our prior research, α4 protein, a regulator of PP2A, was found not only crucial for PP2A regulation but also for the overall response of HEK 293 cells encountering MC-LR. To explore the role of α4 in MC-LR toxicity via PP2A regulation, here, HEK 293 cells overexpressing α4 protein were exposed to MC-LR and PP2A, cytoskeletal organization, and cytoskeleton-related proteins were investigated. The results showed that PP2A activity decreased and PP2A/C subunit expression and phosphorylation at Tyr307 increased significantly in the group exposed to high MC-LR. Vimentin IF became concentrated and formed perinuclear bundles. However, the assembly of actin filament and microtubules remained unchanged and the expression and phosphorylation of the cytoskeleton-related proteins HSP27 and VASP did not increase significantly. Some of these results differ from those of our previous study in which normal HEK293 cells were exposed to MC-LR. Our results indicate that elevated α4 expression confers some resistance to MC-LR-induced cytoskeletal change These new findings provide helpful insights into the mechanism of MC-LR toxicity and the role of α4 in regulating PP2A function. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 255-264, 2017.

Cancer cell identification by bi-color ZnO and TiO2 nanowires

Semiconductor nanocomposites provide advantages beyond the capability of typical fluorescent materials for cancer detection. In this work, nanowire-based probes with dual color channels are employed to demonstrate the capacity of cancer cell detection. Purple emitting ZnO/antibody probes are applied to detect cancer cells and meanwhile TiO₂ /antibody probes with green light emission are applied to identify normal fibroblast cells. A series of quantitative analyses are conducted to verify the correlation between the concentrations of ZnO and TiO₂ probes, cell numbers, and peak intensities of the PL spectra. The results provide a quantitative reference for developing nanowire-based cancel cell probes.

Vimentin dephosphorylation at ser-56 is regulated by type 1 protein phosphatase in smooth muscle

Background

The intermediate filament protein vimentin undergoes reversible phosphorylation and dephosphorylation at Ser-56, which plays an important role in regulating the contraction-relaxation cycles of smooth muscle. The protein phosphatases that mediate vimentin dephosphorylation in smooth muscle have not been previously investigated.

Methods

The associations of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A) with vimentin in mouse tracheal rings was evaluated by co-immunoprecipitation. Lentivirus-mediated shRNA against PP1 was used to assess the role of PP1 in vimentin dephosphorylation and the vimentin-associated process in smooth muscle.

Results

Co-immunoprecipitation analysis showed that vimentin interacted with PP1, but barely with PP2A, in airway smooth muscle. Knockdown of PP1 by lentivirus-mediated shRNA increased the acetylcholine-induced vimentin phosphorylation and smooth muscle contraction. Because vimentin phosphorylation is able to modulate p130 Crk-associated substrate (p130CAS) and actin polymerization, we also evaluated the role of PP1 in the biological processes. Silencing of PP1 also enhanced the agonist-induced the dissociation of p130CAS from vimentin and F/G-actin ratios (an index of actin polymerization). However, PP1 knockdown did not affect c-Abl tyrosine phosphorylation, an important molecule that controls actin dynamics.

Conclusions

Taken together, these findings suggest that PP1 is a key protein serine/threonine phosphatase that controls vimentin Ser-56 dephosphorylation in smooth muscle. PP1 regulates actin polymerization by modulating the dissociation of p130CAS from vimentin, but not by affecting c-Abl tyrosine kinase.

Protein Phosphatase 2A as a Therapeutic Target in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a heterogeneous malignant disorder of hematopoietic progenitor cells in which several genetic and epigenetic aberrations have been described. Despite progressive advances in our understanding of the molecular biology of this disease, the outcome for most patients is poor. It is, therefore, necessary to develop more effective treatment strategies. Genetic aberrations affecting kinases have been widely studied in AML; however, the role of phosphatases remains underexplored. Inactivation of the tumor-suppressor protein phosphatase 2A (PP2A) is frequent in AML patients, making it a promising target for therapy. There are several PP2A inactivating mechanisms reported in this disease. Deregulation or specific post-translational modifications of PP2A subunits have been identified as a cause of PP2A malfunction, which lead to deregulation of proliferation or apoptosis pathways, depending on the subunit affected. Likewise, overexpression of either SET or cancerous inhibitor of protein phosphatase 2A, endogenous inhibitors of PP2A, is a recurrent event in AML that impairs PP2A activity, contributing to leukemogenesis progression. Interestingly, the anticancer activity of several PP2A-activating drugs (PADs) depends on interaction/sequestration of SET. Preclinical studies show that pharmacological restoration of PP2A activity by PADs effectively antagonizes leukemogenesis, and that these drugs have synergistic cytotoxic effects with conventional chemotherapy and kinase inhibitors, opening new possibilities for personalized treatment in AML patients, especially in cases with SET-dependent inactivation of PP2A. Here, we review the role of PP2A as a druggable tumor suppressor in AML.

PP2A as a master regulator of the cell cycle

Protein phosphatase 2A (PP2A) plays a critical multi-faceted role in the regulation of the cell cycle. It is known to dephosphorylate over 300 substrates involved in the cell cycle, regulating almost all major pathways and cell cycle checkpoints. PP2A is involved in such diverse processes by the formation of structurally distinct families of holoenzymes, which are regulated spatially and temporally by specific regulators. Here, we review the involvement of PP2A in the regulation of three cell signaling pathways: wnt, mTOR and MAP kinase, as well as the G1→S transition, DNA synthesis and mitotic initiation. These processes are all crucial for proper cell survival and proliferation and are often deregulated in cancer and other diseases.

Inactivation of protein-phosphatase 2A causing hyperphosphorylation of autoantigenic paraprotein targets in MGUS/MM is due to an exchange of its regulatory subunits

Hyperphosphorylated paratarg-7 (pP-7) carrier state is the strongest molecularly defined risk factor for monoclonal gammopathy of undetermined significance (MGUS), multiple myeloma (MM) and Waldenstrom's macroglobulinemia (WM). pP-7 is inherited as autosomal-dominant trait and depending on the ethnic background is found in over one-third of MGUS/MM patients. P-7, which is the antigenic paraprotein target in these patients, is hyperphosphorylated at serine17. P-7 hyperphosphorylation can be induced in wild-type P-7 (wtP-7) carriers by PKCζ and reverted by protein-phosphatase 2A (PP2A). Here we show that dephosphorylation of pP-7 is defective in pP-7 carriers due to inactivation of the PP2A by substitution of the regulatory B55δ subunit with B56γ3. In lymphoblastoid cell lines from pP-7 carriers, transfection of recombinant B55δ or treatment with ceramide led to a partial reconstitution of PP2A activity and dephosphorylation of pP-7 to wtP7. Similar results were observed with other previously reported autoantigenic paraproteins targets. In conclusion, the mechanisms responsible for the defective dephosphorylation and maintaining the hyperphosphorylated state of P-7 and other autoantigenic paraprotein targets have been elucidated, facilitating the identification of the genetic basis underlying this phenomenon which is obviously common in the pathogenesis of MGUS/MM/WM and not restricted to pP-7 cases.

F-box protein FBXL16 binds PP2A-B55α and regulates differentiation of embryonic stem cells along the FLK1+ lineage

The programmed formation of specific tissues from embryonic stem cells is a major goal of regenerative medicine. To identify points of intervention in cardiac tissue formation, we performed an siRNA screen in murine embryonic stem cells to identify ubiquitin system genes that repress cardiovascular tissue formation. Our screen uncovered an F-box protein, Fbxl16, as a repressor of one of the earliest steps in the cardiogenic lineage: FLK1+ progenitor formation. Whereas F-box proteins typically form SCF ubiquitin ligases, shotgun mass spectrometry revealed that FBXL16 instead binds protein phosphatase 2A (PP2A) containing a B55 specificity subunit (PP2A(B55)). Phosphoproteomic analyses indicate that FBXL16 negatively regulates phosphorylation of the established PP2A(B55) substrate, vimentin. We suggest that FBXL16 negatively regulates the activity of B55α-PP2A to modulate the genesis of FLK1+ progenitor cells.

Genetic amplification of PPME1 in gastric and lung cancer and its potential as a novel therapeutic target

Protein phosphatase methylesterase 1 (PPME1) is a protein phosphatase 2A (PP2A)-specific methyl esterase that negatively regulates PP2A through demethylation at its carboxy terminal leucine 309 residue. Emerging evidence shows that the upregulation of PPME1 is associated with poor prognosis in glioblastoma patients. By performing an array comparative genomic hybridization analysis to detect copy number changes, we have been the first to identify PPME1 gene amplification in 3.8% (5/131) of Chinese gastric cancer (GC) samples and 3.1% (4/124) of Chinese lung cancer (LC) samples. This PPME1 gene amplification was confirmed by fluorescence in situ hybridization analysis and is correlated with elevated protein expression, as determined by immunohistochemistry analysis. To further investigate the role of PPME1 amplification in tumor growth, short-hairpin RNA-mediated gene silencing was employed. A knockdown of PPME1 expression resulted in a significant inhibition of cell proliferation and induction of cell apoptosis in PPME1-amplified human cancer cell lines SNU668 (GC) and Oka-C1 (LC), but not in nonamplified MKN1 (GC) and HCC95 (LC) cells. The PPME1 gene knockdown also led to a consistent decrease in PP2A demethylation at leucine 309, which was correlated with the downregulation of cellular Erk and AKT phosphorylation. Our data indicate that PPME1 could be an attractive therapeutic target for a subset of GCs and LCs.

Differential autophagic cell death under stress with ectopic cytoplasmic and mitochondrial-specific PPP2R2B in human neuroblastoma cells

Protein phosphatase 2A is one of four major classes of serine/threonine phosphatases. Overexpression of brain-specific regulatory subunit PPP2R2 in neuron cells is implicated in pathogenesis. The alternative splicing of PPP2R2B encodes two isoforms. They are subunit of cytoplasmic specific Bβ1 and mitochondria-targeted Bβ2. The two constructs were transfected into human neuroblastoma cells, SK-N-SH, respectively, and the stable clones overexpressing either Bβ1 or Bβ2 established. We have reported that Bβ2 clones are sensitive to reactive oxygen species (ROS) treatment by inducing autophagic cell death. To study more on the onset of neuropathogenesis under strain, both clones were exposed to different environmental stress, e.g. starvation and endoplasmic reticulum (ER) stress. To learn how PPP2R2B overexpression responds to starvation, cells were incubated in Hank's buffered salt solution of deprived nutrient. Cell death was induced in Bβ1 clones after 6 h starvation, but not in Bβ2 clones. The pharmacological inhibitor, Bafilomycin A1, rescued the cell death while suppressing autophagy. On the other hand, to assess how cells respond to ER stress, the cells were treated with 0.1 μM of N-glycosylation inhibitor, tunicamycin (TM). In contrast with Bβ1, the apoptotic cell death appeared in Bβ2 after 48 h treatment. The formation of autophagolysosome was detected in Bβ2 following 12 h treatment with TM as evidenced by lysotracker and GFP-LC3 staining for fluorescence microscopy analysis. The autophagy inhibitor, 3-methyladenine, salvaged the final apoptosis. The stable cell lines with ectopically transfected PPP2R2B genes encoding isoforms of brain-specific regulatory subunit exhibit distinct apoptosis under different stressors. The induced autophagic apoptotic cell death is related to mitochondrial membrane potential drop and ROS generation. Disturbance of autophagy alleviates the induced cell death. The results promised a good model for understanding the onset in pathogenesis under stress in neuron cells with aberrant PPP2R2B expression.

Phosphorylation of serine 4,642 in the C-terminus of plectin by MNK2 and PKA modulates its interaction with intermediate filaments

Plectin is a versatile cytolinker of the plakin family conferring cell resilience to mechanical stress in stratified epithelia and muscles. It acts as a critical organizer of the cytoskeletal system by tethering various intermediate filament (IF) networks through its C-terminal IF-binding domain (IFBD). Mutations affecting the IFBD cause devastating human diseases. Here, we show that serine 4642, which is located in the extreme C-terminus of plectin, is phosphorylated in different cell lines. Phosphorylation of S4642 decreased the ability of plectin IFBD to associate with various IFs, as assessed by immunofluorescence microscopy and cell fractionation studies, as well as in yeast two-hybrid assays. Plectin phosphorylated at S4642 was reduced at sites of IF network anchorage along cell-substrate contacts in both skin and cultured keratinocytes. Treatment of SK-MEL-2 and HeLa cells with okadaic acid increased plectin S4642 phosphorylation, suggesting that protein phosphatase 2A dephosphorylates this residue. Moreover, plectin S4642 phosphorylation was enhanced after cell treatment with EGF, phorbol ester, sorbitol and 8-bromo-cyclic AMP, as well as during wound healing and protease-mediated cell detachment. Using selective protein kinase inhibitors, we identified two different kinases that modulate the phosphorylation of plectin S4642 in HeLa cells: MNK2, which is downstream of the ERK1/2-dependent MAPK cascade, and PKA. Our study indicates that phosphorylation of S4642 has an important regulatory role in the interaction of plectin with IFs and identifies a novel link between MNK2 and the cytoskeleton.

Phosphatase: PP2A structural importance, regulation and its aberrant expression in cancer

Protein Phosphatase 2A (PP2A) is an important and ubiquitously expressed serine threonine phosphatase and regulates the function by dephosphorylating many critical cellular molecules like Akt, p53, c-Myc and β-catenin. It plays a critical role in cellular processes, such as cell proliferation, signal transduction and apoptosis. Structurally, it is multifarious as it is composed of catalytic, scaffold and regulatory subunits. The catalytic and scaffold subunits have two isoforms and the regulatory subunit has four different families containing different isoforms. The regulatory subunit is the most diverse with temporal and spatial specificity. PP2A undergoes post-translational modifications (i.e. phosphorylation and methylation), which in turn, regulates its enzymatic activity. Aberrant expression, mutations and somatic alterations of the PP2A scaffold and regulatory subunits have been observed in various human malignancies, including lung, breast, skin and colon cancer, highlighting its role as a 'tumor suppressor'. This review is focused on the structural complexity of serine/threonine phosphatase PP2A and summarizes its expression pattern in cancer. Additionally, the PP2A interacting and regulatory proteins and substrates are also discussed. Finally, the mouse models developed to understand the biological role of PP2A subunits in an in vivo model system are also reviewed in this article.

Structure, regulation, and pharmacological modulation of PP2A phosphatases

Protein phosphatases of the type 2A family (PP2A) represent a major fraction of cellular Ser/Thr phosphatase activity in any given human tissue. In this review, we describe how the holoenzymic nature of PP2A and the existence of several distinct PP2A composing subunits allow for the generation of multiple structurally and functionally different PP2A complexes, explaining why PP2A is involved in the regulation of so many diverse cell biological and physiological processes. Moreover, in human disease, most notably in several cancers and Alzheimer's Disease, PP2A expression and/or activity have been found significantly decreased, underscoring its important functions as a major tumor suppressor and tau phosphatase. Hence, several recent preclinical studies have demonstrated that pharmacological restoration of PP2A activity, as well as pharmacological PP2A inhibition, under certain conditions, may be of significant future therapeutic value.

Identification and functional analysis of variant haplotypes in the 5'-flanking region of protein phosphatase 2A-Bδ gene

Serine-threonine protein phosphatase 2A (PP2A) is a trimeric holoenzyme that plays an integral role in the regulation of cell growth, differentiation, and apoptosis. The substrate specificity and (sub)cellular localization of the PP2A holoenzymes are highly regulated by interaction with a family of regulatory B subunits (PP2A-Bs). The regulatory subunit PP2A-B/PR55δ (PP2A-Bδ) is involving in the dephosphorylation of PP2A substrates and is crucial for controlling entry into and exit from mitosis. The molecular mechanisms involved in the regulation of expression of PP2A-Bδ gene (PPP2R2D) remain largely unknown. To explore genetic variations in the 5′-flanking region of PPP2R2D gene as well as their frequent haplotypes in the Han Chinese population and determine whether such variations have an impact on transcriptional activity, DNA samples were collected from 70 healthy Chinese donors and sequenced for identifying genetic variants in the 5′-flanking region of PPP2R2D. Four genetic variants were identified in the 1836 bp 5′-flanking region of PPP2R2D. Linkage disequilibrium (LD) patterns and haplotype profiles were constructed for the genetic variants. Using serially truncated human PPP2R2D promoter luciferase constructs, we found that a 601 bp (−540 nt to +61 nt) fragment constitutes the core promoter region. The subcloning of individual 5′-flanking fragment revealed the existence of three haplotypes in the distal promoter of PPP2R2D. The luciferase reporter assay showed that different haplotypes exhibited distinct promoter activities. The EMSA revealed that the −462 G>A variant influences DNA-protein interactions involving the nuclear factor 1 (NF1). In vitro reporter gene assay indicated that cotransfection of NF1/B expression plasmid could positively regulate the activity of PPP2R2D proximal promoter. Introduction of exogenous NF1/B expression plasmid further confirmed that the NF1 involves in the regulation of PPP2R2D gene expression. Our findings suggest that functional genetic variants and their haplotypes in the 5′-flanking region of PPP2R2D are critical for transcriptional regulation of PP2A-Bδ.

Synemin promotes AKT-dependent glioblastoma cell proliferation by antagonizing PP2A

Synemin is an intermediate filament protein present in glioblastomas (GBMs) but not in normal brain. In GBM cells synemin interacts with and antagonizes PP2A, which is the phosphatase dephosphorylating Akt. This maintains the phosphorylation status of Akt sites that are substrates for PDPK1 and mTORc2, thereby fostering proliferation.

The intermediate filament protein synemin is present in astrocyte progenitors and glioblastoma cells but not in mature astrocytes. Here we demonstrate a role for synemin in enhancing glioblastoma cell proliferation and clonogenic survival, as synemin RNA interference decreased both behaviors by inducing G1 arrest along with Rb hypophosphorylation and increased protein levels of the G1/S inhibitors p21^Cip1 and p27^Kip1. Akt involvement was demonstrated by decreased phosphorylation of its substrate, p21^Cip1, and reduced Akt catalytic activity and phosphorylation at essential activation sites. Synemin silencing, however, did not affect the activities of PDPK1 and mTOR complex 2, which directly phosphorylate Akt activation sites, but instead enhanced the activity of the major regulator of Akt dephosphorylation, protein phosphatase type 2A (PP2A). This was accompanied by changes in PP2A subcellular distribution resulting in increased physical interactions between PP2A and Akt, as shown by proximity ligation assays (PLAs). PLAs and immunoprecipitation experiments further revealed that synemin and PP2A form a protein complex. In addition, treatment of synemin-silenced cells with the PP2A inhibitor cantharidic acid resulted in proliferation and pAkt and pRb levels similar to those of controls. Collectively these results indicate that synemin positively regulates glioblastoma cell proliferation by helping sequester PP2A away from Akt, thereby favoring Akt activation.

Spinocerebellar ataxia type 12

SCA12 is a late-onset, autosomal dominant, slowly progressive disorder. Action tremor is the usual presenting sign. Subsequent development of ataxia and hyperreflexia suggests spinocerebellar ataxia. In the index SCA12 kindred, which resides in North America and is of German ancestry, parkinsonism, anxiety, depression, and cognitive dysfunction are not uncommon. SCA12 is linked to a CAG repeat expansion mutation in exon 7 of PPP2R2B, a gene that encodes Bβ, a regulatory subunit of protein phosphatase 2A (PP2A). CAG repeats number 7-28 in normal individuals and 55-78 in SCA12 patients. The mechanism by which this mutation leads to SCA12 has not been determined. The CAG expansion in PPP2R2B has promoter function in vitro. CAG length correlates with increased Bβ expression. There is no evidence that this CAG expansion results in polyglutamine production. In addition to the North. American SCA12 kindred, multiple SCA12 families have been found in Northern India that are not related to the index SCA12 kindred. SCA12 has been reported, rarely, in Singapore and China. Action tremor, anxiety, and depression in SCA12 have responded to usual treatments for these disorders. SCA12 may be considered in patients who present with action tremor and later develop signs of cerebellar and cortical dysfunction.

Evaluation of PPP2R2A as a prostate cancer susceptibility gene: a comprehensive germline and somatic study

PPP2R2A, mapped to 8p21.2, codes for the α isoform of the regulatory B55 subfamily of protein phosphatase 2 (PP2A). PP2A is one of the four major serine/threonine phosphatases and is implicated in the negative control of cell growth and division. Because of its known functions and location within a chromosomal region where evidence for linkage and somatic loss of heterozygosity was found, we hypothesized that either somatic copy number changes or germline sequence variants in PPP2R2A may increase prostate cancer (PCa) risk. We examined PPP2R2A deletion status in 141 PCa samples using Affymetrix SNP arrays. It was found that PPP2R2A was commonly (67.1%) deleted in tumor samples, including a homozygous deletion in three tumors (2.1%). We performed a mutation screen for PPP2R2A in 96 probands of hereditary prostate cancer families. No high risk mutations were identified. In addition, we re-analyzed 10 SNPs of PPP2R2A in sporadic PCa cases and controls. No significant differences in the allele and genotype frequencies were observed among either PCa cases and controls or PCa aggressive and non-aggressive cases. Taken together, these results suggest that a somatic deletion rather than germline sequence variants of PPP2R2A may play a more important role in PCa susceptibility.

Altered phosphorylation and distribution status of vimentin in rat seminiferous epithelium following 17β-estradiol treatment

Vimentin, type III intermediate filament, has stage-specific localization in the Sertoli cell. In the rat, during stages I-V and XI-XIV of the seminiferous epithelium, vimentin is localized in the perinuclear area with filaments projecting into the apical region toward the developing germ cells. These filaments decrease in length at stages VI-VII with perinuclear staining in stages VIII-IX, when spermiation occurs. Our earlier studies following 17β-estradiol treatment to adult male rats demonstrated an increase in germ cell apoptosis, spermiation failure and disruption of Sertoli cell microfilaments and microtubules. The present study was undertaken to determine the stage-specific distribution of vimentin and its involvement in spermiation failure and germ cell apoptosis. Immunofluorescence studies revealed that in contrast to the perinuclear localization with small extensions in control stages VII-IX, long extensions radiating apically to the spermatids in deep recess were observed in the treated group. Immunoprecipitation studies showed marked absence of phosphorylated vimentin in stages VII-VIII in the treated group. Further, localization of plectin, cytoskeletal linker protein, showed decrease in all the stages of spermatogenesis following estradiol treatment. Interestingly, for the first time the localization of plectin in the tubulobulbar complex was observed. In conclusion, the study suggests that estradiol treatment leads to an effect on vimentin phosphorylation, which could have inhibited the disassembly of vimentin leading to retention of apical projection in stages VII-VIII. These effects could be presumably due to a decrease in plectin, affecting the reorganization of vimentin and therefore the apical movement of spermatids, leading to spermiation failure.

Vimentin in cancer and its potential as a molecular target for cancer therapy

Vimentin, a major constituent of the intermediate filament family of proteins, is ubiquitously expressed in normal mesenchymal cells and is known to maintain cellular integrity and provide resistance against stress. Vimentin is overexpressed in various epithelial cancers, including prostate cancer, gastrointestinal tumors, tumors of the central nervous system, breast cancer, malignant melanoma, and lung cancer. Vimentin's overexpression in cancer correlates well with accelerated tumor growth, invasion, and poor prognosis; however, the role of vimentin in cancer progression remains obscure. In recent years, vimentin has been recognized as a marker for epithelial-mesenchymal transition (EMT). Although EMT is associated with several tumorigenic events, vimentin's role in the underlying events mediating these processes remains unknown. By virtue of its overexpression in cancer and its association with tumor growth and metastasis, vimentin serves as an attractive potential target for cancer therapy; however, more research would be crucial to evaluate its specific role in cancer. Our recent discovery of a vimentin-binding mini-peptide has generated further impetus for vimentin-targeted tumor-specific therapy. Furthermore, research directed toward elucidating the role of vimentin in various signaling pathways would reveal new approaches for the development of therapeutic agents. This review summarizes the expression and functions of vimentin in various types of cancer and suggests some directions toward future cancer therapy utilizing vimentin as a potential molecular target.

PP2A targeting by viral proteins: a widespread biological strategy from DNA/RNA tumor viruses to HIV-1

Protein phosphatase 2A (PP2A) is a large family of holoenzymes that comprises 1% of total cellular proteins and accounts for the majority of Ser/Thr phosphatase activity in eukaryotic cells. Although initially viewed as constitutive housekeeping enzymes, it is now well established that PP2A proteins represent a family of highly and sophistically regulated phosphatases. The past decade, multiple complementary studies have improved our knowledge about structural and functional regulation of PP2A holoenzymes. In this regard, after summarizing major cellular regulation, this review will mainly focus on discussing a particulate biological strategy, used by various viruses, which is based on the targeting of PP2A enzymes by viral proteins in order to specifically deregulate, for their own benefit, cellular pathways of their hosts. The impact of such PP2A targeting for research in human diseases, and in further therapeutic developments, is also discussed.

APC +/- alters colonic fibroblast proteome in FAP

Here we compared the proteomes of primary fibroblast cultures derived from morphologically normal colonic mucosa of familial adenomatous polyposis (FAP) patients with those obtained from unaffected controls. The expression signature of about 19% of total fibroblast proteins separates FAP mutation carriers from unaffected controls (P < 0.01). More than 4,000 protein spots were quantified by 2D PAGE analysis, identifying 368 non-redundant proteins and 400 of their isoforms. Specifically, all three classes of cytoskeletal filaments and their regulatory proteins were altered as were oxidative stress response proteins. Given that FAP fibroblasts showed heightened sensitivity to transformation by KiMSV and SV40 including elevated levels of the p53 protein, events controlled in large measure by the Ras suppressor protein-1 (RSU-1) and oncogenic DJ-1, here we show decreased RSU1 and augmented DJ-1 expression in both fibroblasts and crypt-derived epithelial cells from morphologically normal colonic mucosa of FAP gene-carriers. The results indicate that heterozygosity for a mutant APC tumor suppressor gene alters the proteomes of both colon-derived normal fibroblasts in a gene-specific manner, consistent with a "one-hit" effect.

The substrate of Greatwall kinase, Arpp19, controls mitosis by inhibiting protein phosphatase 2A

Initiation and maintenance of mitosis require the activation of protein kinase cyclin B-Cdc2 and the inhibition of protein phosphatase 2A (PP2A), which, respectively, phosphorylate and dephosphorylate mitotic substrates. The protein kinase Greatwall (Gwl) is required to maintain mitosis through PP2A inhibition. We describe how Gwl activation results in PP2A inhibition. We identified cyclic adenosine monophosphate-regulated phosphoprotein 19 (Arpp19) and α-Endosulfine as two substrates of Gwl that, when phosphorylated by this kinase, associate with and inhibit PP2A, thus promoting mitotic entry. Conversely, in the absence of Gwl activity, Arpp19 and α-Endosulfine are dephosphorylated and lose their capacity to bind and inhibit PP2A. Although both proteins can inhibit PP2A, endogenous Arpp19, but not α-Endosulfine, is responsible for PP2A inhibition at mitotic entry in Xenopus egg extracts.

Oxidative stress promotes autophagic cell death in human neuroblastoma cells with ectopic transfer of mitochondrial PPP2R2B (Bbeta2)

Background

The multifunctional protein phosphatase 2A (PP2A) is a heterotrimeric serine/threonine protein phosphatase composed of a scaffolding, catalytic and regulatory subunits. By modifying various downstream signal transducers, the aberrant expression of the brain-targeted regulatory subunit PPP2R2B is associated with the onset of a panel of neuronal disorders. The alternatively splicing of PPP2R2B encodes two regulatory subunit isoforms that determine cellular distribution of the neuron-specific holoenzyme to mitochondria (Bβ2) and cytoplasm (Bβ1), respectively.

Results

Human neuroblastoma cells were transfected with PPP2R2B constructs encoding the complete sequences of Bβ2 and Bβ1, respectively. The colonies with antibiotic resistance were selected as stable cell lines. Both ectopic Bβ1 and Bβ2 clones exhibited characteristics of autophagy. To test how cells respond to reactive oxygen species generators, the cells were treated with either hydrogen peroxide or t-butyl hydroperoxide and Bβ2 clones induced cell death. Suppression of autophagy using either RNA interference of the essential autophagy gene or pharmacological inhibitor rescued cell death caused by oxidative stress.

Conclusions

Cells with ectopically expressed mitochondria-targeted regulatory subunit PPP2R2B of the holoenzyme PP2A were shown predisposed to autophagy and oxidative stress induced cell death that is related to apoptosis. The results promised a model for studying the mechanism and function of aberrant PPP2R2B expression in neuronal cells. The work provided a new target for understanding and prevention of neuropathogenesis.

Protein phosphatase 2A contributes to the cardiac dysfunction induced by endotoxemia

Aims

Sepsis-associated cardiac dysfunction represents an intrinsic impairment of cardiomyocyte function due in part to a decrease in myofilament Ca²⁺ sensitivity associated with a sustained increase in cardiac troponin I (cTnI) phosphorylation at Ser23/24. Dephosphorylation of cTnI is under regulatory control. Thus, muscarinic and adenosine A₁-receptor agonists antagonize β-adrenergic stimulation via activation of protein phosphatase 2A (PP2A). The aim of this study was to determine whether modulation of PP2A and thus cTnI phosphorylation could improve sepsis-induced contractile dysfunction.

Methods and results

Cardiomyocytes were isolated from control or septic mice 16–18 h after an injection of vehicle or lipopolysaccharide (LPS; 9 mg/kg ip) respectively. Protein expression and phosphatase activity were determined in homogenates of control and septic hearts. Our data showed that LPS significantly increased cTnI phosphorylation at Ser23/24 in cardiomyocytes and reduced contraction amplitude without affecting Ca²⁺-transients. Treatment of cardiomyocytes with the A₁ agonist cyclopentyladenosine (CPA) or the protein kinase A inhibitor H89 significantly attenuated the LPS-induced contractile dysfunction without effect on Ca²⁺-transients. Co-treatment with CPA and H89 completely reversed the contractile dysfunction. Increased cTnI phosphorylation in septic hearts was associated with a significant reduction in the protein expression of both the catalytic and regulatory subunits (B56α) of PP2A and a decrease in PP2A activity. CPA treatment of septic hearts increased PP2A activity. An increase in the protein expression of demethylated PP2A and a decrease in the PP2A-methyltransferase (PPMT; the methyltransferase that catalyses this reaction) were also observed.

Conclusion

These data support the hypothesis that sustained cTnI phosphorylation underlies the contractile dysfunction seen in sepsis.

Protein phosphatase 2A regulatory subunits and cancer

The serine/threonine protein phosphatase (PP2A) is a trimeric holoenzyme that plays an integral role in the regulation of a number of major signaling pathways whose deregulation can contribute to cancer. The specificity and activity of PP2A are highly regulated through the interaction of a family of regulatory B subunits with the substrates. Accumulating evidence indicates that PP2A acts as a tumor suppressor. In this review we summarize the known effects of specific PP2A holoenzymes and their roles in cancer relevant pathways. In particular we highlight PP2A function in the regulation of MAPK and Wnt signaling.

Molecular mechanism of mitotic Golgi disassembly and reassembly revealed by a defined reconstitution assay

In mammalian cells, flat Golgi cisternae closely arrange together to form stacks. During mitosis, the stacked structure undergoes a continuous fragmentation process. The generated mitotic Golgi fragments are distributed into the daughter cells, where they are reassembled into new Golgi stacks. In this study, an in vitro assay has been developed using purified proteins and Golgi membranes to reconstitute the Golgi disassembly and reassembly processes. This technique provides a useful tool to delineate the mechanisms underlying the morphological change. There are two processes during Golgi disassembly: unstacking and vesiculation. Unstacking is mediated by two mitotic kinases, cdc2 and plk, which phosphorylate the Golgi stacking protein GRASP65 and thus disrupt the oligomer of this protein. Vesiculation is mediated by the COPI budding machinery ARF1 and the coatomer complex. When treated with a combination of purified kinases, ARF1 and coatomer, the Golgi membranes were completely fragmented into vesicles. After mitosis, there are also two processes in Golgi reassembly: formation of single cisternae by membrane fusion, and restacking. Cisternal membrane fusion requires two AAA ATPases, p97 and NSF (N-ethylmaleimide-sensitive fusion protein), each of which functions together with specific adaptor proteins. Restacking of the newly formed Golgi cisternae requires dephosphorylation of Golgi stacking proteins by the protein phosphatase PP2A. This systematic study revealed the minimal machinery that controls the mitotic Golgi disassembly and reassembly processes.

Phosphorylation of vascular endothelial cadherin controls lymphocyte emigration

Lymphocytes emigrate from the circulation to target tissues through the microvascular endothelial cell (EC) barrier. During paracellular transmigration cell-cell junctions have been proposed to disengage and provide homophilic and heterophilic interaction surfaces in a zip-like process. However, it is not known whether ECs modulate junction proteins during this process. Here we show that tyrosine phosphorylation of adherens junction vascular endothelial cadherin (VEC) is required for successful transendothelial lymphocyte migration. We found that adhesion of lymphocytes or activation of the endothelial intercellular adhesion molecule 1 (ICAM1) led to tyrosine phosphorylation of VEC. Substitution of tyrosine for phenylalanine in VEC at positions 645, 731 or 733 produced ECs that were significantly less permissive to lymphocyte migration. We also found that these same tyrosine residues are involved in ICAM1-dependent changes of VEC phosphorylation. ICAM1 activation enhanced transendothelial permeability, suggesting the occurrence of junction disassembly. In agreement, the expression of VEC mutated at Y645F, Y731F or Y733F predominantly affected lymphocyte transmigration in paracellular areas. Taken together, these results demonstrate that phosphorylation of adherens junctions constitutes a molecular endpoint of lymphocyte-induced vascular EC signaling and may be exploited as a new target of anti-inflammatory therapies.

Applying a targeted label-free approach using LC-MS AMT tags to evaluate changes in protein phosphorylation following phosphatase inhibition

To identify phosphoproteins regulated by the phosphoprotein phosphatase (PPP) family of S/T phosphatases, we performed a large-scale characterization of changes in protein phosphorylation on extracts from HeLa cells treated with or without calyculin A, a potent PPP enzyme inhibitor. A label-free comparative phosphoproteomics approach using immobilized metal ion affinity chromatography and targeted tandem mass spectrometry was employed to discover and identify signatures based upon distinctive changes in abundance. Overall, 232 proteins were identified as either direct or indirect targets for PPP enzyme regulation. Most of the present identifications represent novel PPP enzyme targets at the level of both phosphorylation site and protein. These include phosphorylation sites within signaling proteins such as p120 Catenin, A Kinase Anchoring Protein 8, JunB, and Type II Phosphatidyl Inositol 4 Kinase. These data can be used to define underlying signaling pathways and events regulated by the PPP family of S/T phosphatases.

Modulation of the host cell proteome by the intracellular apicomplexan parasite Toxoplasma gondii

To investigate how intracellular parasites manipulate their host cell environment at the molecular level, we undertook a quantitative proteomic study of cells following infection with the apicomplexan parasite Toxoplasma gondii. Using conventional two-dimensional electrophoresis, difference gel electrophoresis (DIGE), and mass spectrometry, we identified host proteins that were consistently modulated in expression following infection. We detected modification of protein expression in key metabolic pathways, including glycolysis, lipid and sterol metabolism, mitosis, apoptosis, and structural-protein expression, suggestive of global reprogramming of cell metabolism by the parasite. Many of the differentially expressed proteins had not been previously implicated in the response to the parasite, while others provide important corroborative protein evidence for previously proposed hypotheses of pathogen-cell interactions. Significantly, over one-third of all modulated proteins were mitochondrial, and this was further investigated by DIGE analysis of a mitochondrion-enriched preparation from infected cells. Comparison of our proteomic data with previous transcriptional studies suggested that a complex relationship exits between transcription and protein expression that may be partly explained by posttranslational modifications of proteins and revealed the importance of investigating protein changes when interpreting transcriptional data. To investigate this further, we used phosphatase treatment and DIGE to demonstrate changes in the phosphorylation states of several key proteins following infection. Overall, our findings indicate that the host cell proteome responds in a dramatic way to T. gondii invasion, in terms of both protein expression changes and protein modifications, and reveal a complex and intimate molecular relationship between host and parasite.

Regulation of vascular endothelial cell barrier function and cytoskeleton structure by protein phosphatases of the PPP family

Reversible phosphorylation of cytoskeletal and cytoskeleton-associated proteins is a significant element of endothelial barrier function regulation. Therefore, understanding the mechanisms of phosphorylation/dephosphorylation of endothelial cell cytoskeletal proteins is vital to the treatment of severe lung disorders such as high permeability pulmonary edema. In vivo, there is a controlled balance between the activities of protein kinases and phosphatases. Due to various external or internal signals, this balance may be shifted. The actual balances at a given time alter the phosphorylation level of certain proteins with appropriate physiological consequences. The latest information about the structure and regulation of different types of Ser/Thr protein phosphatases participating in the regulation of endothelial cytoskeletal organization and barrier function will be reviewed here.

Fusion of the tumor-suppressor gene CHEK2 and the gene for the regulatory subunit B of protein phosphatase 2 PPP2R2A in childhood teratoma

We characterized the molecular genetic consequences of a balanced chromosome translocation t(8;22)(p21;q12), which occurred as the sole cytogenetic aberration in short-term cultured cells from an intrathoracic mature teratoma in a 15-year-old girl. Fluorescence in situ hybridization and reverse transcription-polymerase chain reaction disclosed that t(8;22) resulted in the fusion of the genes PPP2R2A and CHEK2, with an inserted fragment belonging to class I endogenous retrovirus-related sequences at the junction. Sequencing of the two genes did not reveal any additional mutation. None of the three detected PPP2R2A/CHEK2 fusion transcripts resulted in an in-frame PPP2R2A/CHEK2 chimerical open reading frame; however, in all of them, the known open reading frame of CHEK2 was preserved. Thus, promoter swapping leading to deregulated CHEK2 expression would be the most likely oncogenic mechanism. Whereas inactivating mutations of CHEK2 previously have been described in a variety of sporadic tumors and in inherited cancer-predisposing syndromes, PPP2R2A, encoding a regulatory subunit of the multimeric enzyme phosphatase 2, has not been directly implicated in tumorigenesis. Our findings suggest that deregulation of CHEK2 and/or PPP2R2A is of pathogenetic importance in at least a subset of germ cell tumors.

Protein phosphatase-2A associates with and dephosphorylates keratin 8 after hyposmotic stress in a site- and cell-specific manner

Keratins 8 and 18 (K8 and K18) are regulated by site-specific phosphorylation in response to multiple stresses. We examined the effect and regulation of hyposmotic stress on keratin phosphorylation. K8 phospho-Ser431 (Ser431-P) becomes dephosphorylated in HT29 cells, but hyperphosphorylated on other K8 but not K18 sites in HRT18 and Caco2 cells and in normal human colonic ex vivo cultures. Hyposmosis-induced dephosphorylation involves K8 but not K18, K19 or K20, occurs preferentially in mitotically active cells, and peaks by 6-8 hours then returns to baseline by 12-16 hours. By contrast, hyperosmosis causes K8 Ser431 hyperphosphorylation in all tested cell lines. Hyposmosis-induced dephosphorylation of K8 Ser431-P is inhibited by okadaic acid but not by tautomycin or cyclosporine. The PP2A catalytic subunit co-immunoprecipitated with K8 and K18 after hyposmotic stress in HT29 cells, but not in HRT18 or Caco2 cells where K8 Ser431 becomes hyperphosphorylated. K8 Ser431-P dephosphorylation after hyposmosis was independent of PP2A levels but correlated with increased PP2A activity towards K8 Ser431-P. Therefore, hyposmotic stress alters K8 phosphorylation in a cell-dependent manner, and renders K8 Ser431-P a physiologic substrate for PP2A in HT29 cells as a result of PP2A activation and the physical association with K8 and K18. The divergent hyposmosis versus hyperosmosis K8 Ser431 phosphorylation changes in HT29 cells suggest that there are unique signaling responses to osmotic stress.

Protein phosphatase 2A regulatory subunit B56alpha associates with c-myc and negatively regulates c-myc accumulation

Protein phosphatase 2A (PP2A) plays a prominent role in controlling accumulation of the proto-oncoprotein c-Myc. PP2A mediates its effects on c-Myc by dephosphorylating a conserved residue that normally stabilizes c-Myc, and in this way, PP2A enhances c-Myc ubiquitin-mediated degradation. Stringent regulation of c-Myc levels is essential for normal cell function, as c-Myc overexpression can lead to cell transformation. Conversely, PP2A has tumor suppressor activity. Uncovering relevant PP2A holoenzymes for a particular target has been limited by the fact that cellular PP2A represents a large heterogeneous population of trimeric holoenzymes, composed of a conserved catalytic subunit and a structural subunit along with a variable regulatory subunit which directs the holoenzyme to a specific target. We now report the identification of a specific PP2A regulatory subunit, B56alpha, that selectively associates with the N terminus of c-Myc. B56alpha directs intact PP2A holoenzymes to c-Myc, resulting in a dramatic reduction in c-Myc levels. Inhibition of PP2A-B56alpha holoenzymes, using small hairpin RNA to knock down B56alpha, results in c-Myc overexpression, elevated levels of c-Myc serine 62 phosphorylation, and increased c-Myc function. These results uncover a new protein involved in regulating c-Myc expression and reveal a critical interconnection between a potent oncoprotein, c-Myc, and a well-documented tumor suppressor, PP2A.

Altered phosphorylation of cytoskeletal proteins in mutant protein phosphatase 2A transgenic mice

Protein phosphatase 2A (PP2A) is a family of heterotrimeric enzymes with diverse functions under physiologic and pathologic conditions such as Alzheimer's disease. All PP2A holoenzymes have in common a catalytic subunit C and a structural scaffolding subunit A. These core subunits assemble with various regulatory B subunits to form heterotrimers with distinct functions in the cell. Substrate specificity of PP2A in vitro is determined by regulatory subunits with leucine 309 of the catalytic subunit C playing a crucial role in the recruitment of regulatory subunits into the complex. Here we expressed a mutant form of Calpha, L309A, in brain and Harderian (lacrimal) gland of transgenic mice. We found an altered recruitment of regulatory subunits into the complex, demonstrating a role for the carboxyterminal leucine of Calpha in regulating holoenzyme assembly in vivo. This was associated with an increased phosphorylation of tau in brain and an impaired dephosphorylation of vimentin demonstrating that both cytoskeletal proteins are in vivo substrates of distinct PP2A holoenzyme complexes.

Defining the cellular target(s) of porcine reproductive and respiratory syndrome virus blocking monoclonal antibody 7G10

We produced a monoclonal antibody (MAb) (7G10) that has blocking activity against porcine reproductive and respiratory syndrome virus (PRRSV). In this study, we identified the components of the 7G10 MAb-bound complex as cytoskeletal filaments: vimentin, cytokeratin 8, cytokeratin 18, actin, and hair type II basic keratin. Vimentin bound to PRRSV nucleocapsid protein and anti-vimentin antibodies showed PRRSV-blocking activity. Vimentin was expressed on the surface of MARC-145, a PRRSV-susceptible cell line. Simian vimentin rendered BHK-21 and CRFK, nonsusceptible cell lines, susceptible to PRRSV infection. These results suggest that vimentin is part of the PRRSV receptor complex and that it plays an important role in PRRSV binding with the other cytoskeletal filaments that mediate transportation of the virus in the cytosol.

Positive regulation of Raf1-MEK1/2-ERK1/2 signaling by protein serine/threonine phosphatase 2A holoenzymes

Protein serine/threonine phosphatase 2A (PP2A) regulates a wide variety of cellular signal transduction pathways. The predominant form of PP2A in cells is a heterotrimeric holoenzyme consisting of a scaffolding (A) subunit, a regulatory (B) subunit, and a catalytic (C) subunit. Although PP2A is known to regulate Raf1-MEK1/2-ERK1/2 signaling at multiple steps in this pathway, the specific PP2A holoenzymes involved remain unclear. To address this question, we established tetracycline-inducible human embryonic kidney 293 cell lines for overexpression of FLAG-tagged Balpha/delta regulatory subunits by approximately 3-fold or knock-down of Balpha by greater than 70% compared with endogenous levels. The expression of functional epitope-tagged B subunits was confirmed by the detection of A and C subunits as well as phosphatase activity in FLAG immune complexes from extracts of cells overexpressing the FLAG-Balpha/delta subunit. Western analysis of the cell extracts using phosphospecific antibodies for MEK1/2 and ERK1/2 demonstrated that activation of these kinases in response to epidermal growth factor was markedly diminished in Balpha knock-down cells but elevated in Balpha- and Bdelta-overexpressing cells as compared with control cells. In parallel with the activation of MEK1/2 and ERK1/2, the inhibitory phosphorylation site of Raf1 (Ser-259) was dephosphorylated in cells overexpressing Balpha or Bdelta. Pharmacological inhibitor studies as well as reporter assays for ERK-dependent activation of the transcription factor Elk1 revealed that the PP2A holoenzymes ABalphaC and ABdeltaC act downstream of Ras and upstream of MEK1 to promote activation of this MAPK signaling cascade. Furthermore both PP2A holoenzymes were found to associate with Raf1 and catalyze dephosphorylation of inhibitory phospho-Ser-259. Together these findings indicate that PP2A ABalphaC and ABdeltaC holoenzymes function as positive regulators of Raf1-MEK1/2-ERK1/2 signaling by targeting Raf1.

Unfolding-resistant translocase targeting: a novel mechanism for outer mitochondrial membrane localization exemplified by the Bbeta2 regulatory subunit of protein phosphatase 2A

Heterotrimeric serine/threonine protein phosphatase 2A (PP2A) consists of scaffolding (A), catalytic (C), and variable (B, B', and B'') subunits. Variable subunits dictate subcellular localization and substrate specificity of the PP2A holoenzyme. The Bbeta regulatory subunit gene is mutated in spinocerebellar ataxia type 12, and one of its splice variants, Bbeta2, targets PP2A to mitochondria to promote apoptosis in PC12 cells (Dagda, R. K., Zaucha, J. A., Wadzinski, B. E., and Strack, S. (2003) J. Biol. Chem. 278, 24976-24985). Here, we report that Bbeta2 is localized to the outer mitochondrial membrane by a novel mechanism, combining a cryptic mitochondrial import signal with a structural arrest domain. Scanning mutagenesis demonstrates that basic and hydrophobic residues mediate mitochondrial association and the proapoptotic activity of Bbeta2. When fused to green fluorescent protein, the N terminus of Bbeta2 acts as a cleavable mitochondrial import signal. Surprisingly, full-length Bbeta2 is not detectably cleaved and is retained at the outer mitochondrial membrane, even though it interacts with the TOM22 import receptor, as shown by luciferase complementation in intact cells. Mutations that open the C-terminal beta-propeller of Bbeta2 facilitate mitochondrial import, indicating that this rigid fold acts as a stop-transfer domain by resisting the partial unfolding step prerequisite for matrix translocation. Because hybrids of prototypical import and beta-propeller domains recapitulate this behavior, we predict the existence of other similarly localized proteins and a selection against highly stable protein folds in the mitochondrial matrix. This unfolding-resistant targeting to the mitochondrial translocase is necessary but not sufficient for the proapoptotic activity of Bbeta2, which also requires association with the rest of the PP2A holoenzyme.

Functional characterization of two-dimensional gel-separated proteins using sequential staining

Proteins separated by two-dimensional (2-D) gel electrophoresis can be visualized using various protein staining methods. This is followed by downstream procedures, such as image analysis, gel spot cutting, protein digestion, and mass spectrometry (MS), to characterize protein expression profiles within cells, tissues, organisms, or body fluids. Characterizing specific post-translational modifications on proteins using MS of peptide fragments is difficult and labor-intensive. Recently, specific staining methods have been developed and merged into the 2-D gel platform so that not only general protein patterns but also patterns of phosphorylated and glycosylated proteins can be obtained. We used the new Pro-Q Diamond phosphoprotein dye technology for the fluorescent detection of phosphoproteins directly in 2-D gels of mouse leukocyte proteins, and Pro-Q Emerald 488 glycoprotein dye to detect glycoproteins. These two fluorescent stains are compatible with general protein stains, such as SYPRO Ruby stain. We devised a sequential procedure using Pro-Q Diamond (phosphoprotein), followed by Pro-Q Emerald 488 (glycoprotein), followed by SYPRO Ruby stain (general protein stain), and finally silver stain for total protein profile. This multiple staining of the proteins in a single gel provided parallel determination of protein expression and preliminary characterization of post-translational modifications of proteins in individual spots on 2-D gels. Although this method does not provide the same degree of certainty as traditional MS methods of characterizing post-translational modifications, it is much simpler, faster, and does not require sophisticated equipment and expertise in MS.