An improved procedure for identifying and quantitating protein phosphatases in mammalian tissues

c-Myc modulation: a key role in melanoma drug response

Understanding molecular mechanisms involved in melanoma resistance to drugs is a big challenge. Experimental evidences suggested a correlation between mutational status in B-RAF and melanoma cell susceptibility to drugs, such as paclitaxel, doxorubicin and temozolomide, which generate an accumulation of hydrogen peroxide (H2O2) in the cells. We investigated the survival phenotype and the protein level of c-myc, a B-RAF target molecule, in melanoma cells, carrying a different mutational status in B-RAF, upon paclitaxel, doxorubicin and H2O2 treatment. For the first time, we reported c-myc modulation is critical for melanoma drug response. It appeared drug-specific and post-transcriptionally driven through PP2A; in correlation, cell pre-treatment with okadaic acid (OA), a specific PP2A inhibitor, as well as PP2A silencing of melanoma cells, was able to increase melanoma cell drug-sensitivity and c-myc protein level. This is relevant for designing efficacious therapeutic strategies in melanoma.

Serine/Threonine phosphatases: classification, roles and pharmacological regulation

Phosphatases are important enzymes in a variety of biochemical pathways in different cells which they catalyze opposing reactions of phosphorylation and dephosphorylation, which may modulate the function of crucial signaling proteins in different cells. This is an important mechanism in the regulation of intracellular signal transduction pathways in many cells. Phosphatases play a key role in regulating signal transduction. It is known that phosphatases are specific for cleavage of either serine-threonine or tyrosine phosphate groups. To date, numerous compounds have been identified. This paper reviews the classification, roles and pharmacological of protein serine/threonine phosphates.

Ciliary transport regulates PDGF-AA/αα signaling via elevated mammalian target of rapamycin signaling and diminished PP2A activity

Primary cilia are built and maintained by intraflagellar transport (IFT), whereby the two IFT complexes, IFTA and IFTB, carry cargo via kinesin and dynein motors for anterograde and retrograde transport, respectively. Many signaling pathways, including platelet- derived growth factor (PDGF)-AA/αα, are linked to primary cilia. Active PDGF-AA/αα signaling results in phosphorylation of Akt at two residues: P-Akt(T308) and P-Akt(S473), and previous work showed decreased P-Akt(S473) in response to PDGF-AA upon anterograde transport disruption. In this study, we investigated PDGF-AA/αα signaling via P-Akt(T308) and P-Akt(S473) in distinct ciliary transport mutants. We found increased Akt phosphorylation in the absence of PDGF-AA stimulation, which we show is due to impaired dephosphorylation resulting from diminished PP2A activity toward P-Akt(T308). Anterograde transport mutants display low platelet-derived growth factor receptor (PDGFR)α levels, whereas retrograde mutants exhibit normal PDGFRα levels. Despite this, neither shows an increase in P-Akt(S473) or P-Akt(T308) upon PDGF-AA stimulation. Because mammalian target of rapamycin complex 1 (mTORC1) signaling is increased in ciliary transport mutant cells and mTOR signaling inhibits PDGFRα levels, we demonstrate that inhibition of mTORC1 rescues PDGFRα levels as well as PDGF-AA-dependent phosphorylation of Akt(S473) and Akt(T308) in ciliary transport mutant MEFs. Taken together, our data indicate that the regulation of mTORC1 signaling and PP2A activity by ciliary transport plays key roles in PDGF-AA/αα signaling.

Protein phosphatase 2A and Cdc7 kinase regulate the DNA unwinding element-binding protein in replication initiation

The DNA unwinding element (DUE)-binding protein (DUE-B) binds to replication origins coordinately with the minichromosome maintenance (MCM) helicase and the helicase activator Cdc45 in vivo, and loads Cdc45 onto chromatin in Xenopus egg extracts. Human DUE-B also retains the aminoacyl-tRNA proofreading function of its shorter orthologs in lower organisms. Here we report that phosphorylation of the DUE-B unstructured C-terminal domain unique to higher organisms regulates DUE-B intermolecular binding. Gel filtration analyses show that unphosphorylated DUE-B forms multiple high molecular weight (HMW) complexes. Several aminoacyl-tRNA synthetases and Mcm2-7 proteins were identified by mass spectrometry of the HMW complexes. Aminoacyl-tRNA synthetase binding is RNase A sensitive, whereas interaction with Mcm2-7 is nuclease resistant. Unphosphorylated DUE-B HMW complex formation is decreased by PP2A inhibition or direct DUE-B phosphorylation, and increased by inhibition of Cdc7. These results indicate that the state of DUE-B phosphorylation is maintained by the equilibrium between Cdc7-dependent phosphorylation and PP2A-dependent dephosphorylation, each previously shown to regulate replication initiation. Alanine mutation of the DUE-B C-terminal phosphorylation target sites increases MCM binding but blocks Cdc45 loading in vivo and inhibits cell division. In egg extracts alanine mutation of the DUE-B C-terminal phosphorylation sites blocks Cdc45 loading and inhibits DNA replication. The effects of DUE-B C-terminal phosphorylation reveal a novel S phase kinase regulatory mechanism for Cdc45 loading and MCM helicase activation.

Mastl is required for timely activation of APC/C in meiosis I and Cdk1 reactivation in meiosis II

The Greatwall kinase orthologue Mastl regulates timely activation of APC/C to allow meiosis I exit and suppresses PP2A activity and thereby allows the rapid rise of Cdk1 activity that is necessary for meiosis II entry in mouse oocytes.

In mitosis, the Greatwall kinase (called microtubule-associated serine/threonine kinase like [Mastl] in mammals) is essential for prometaphase entry or progression by suppressing protein phosphatase 2A (PP2A) activity. PP2A suppression in turn leads to high levels of Cdk1 substrate phosphorylation. We have used a mouse model with an oocyte-specific deletion of Mastl to show that Mastl-null oocytes resume meiosis I and reach metaphase I normally but that the onset and completion of anaphase I are delayed. Moreover, after the completion of meiosis I, Mastl-null oocytes failed to enter meiosis II (MII) because they reassembled a nuclear structure containing decondensed chromatin. Our results show that Mastl is required for the timely activation of anaphase-promoting complex/cyclosome to allow meiosis I exit and for the rapid rise of Cdk1 activity that is needed for the entry into MII in mouse oocytes.

SET-mediated NDRG1 inhibition is involved in acquisition of epithelial-to-mesenchymal transition phenotype and cisplatin resistance in human lung cancer cell

Development of resistance to therapy continues to be a serious clinical problem in lung cancer management. Cancer cells undergoing epithelial-to-mesenchymal transition (EMT) have been shown to play roles in resistance to chemotherapy. Here, we utilized a proteomics-based method and identified a significant downregulation of the metastasis suppressor NDRG1 in drug resistant lung cancer cells. We showed that downregulation of DNRG1 constitutes a mechanism for acquisition of EMT phenotype and endows lung cancer cells with an increased resistance to cisplatin. We also identified a signal cascade, namely, SET--| PP2A--| c-myc--| NDRG1, in which upregulation of SET is critical for inhibition of NDRG1. We also found that blockade of SET (or reactivation of PP2A) by FTY720 reverted EMT, restored drug sensitivity, and inhibited invasiveness and growth of lung tumor xenografts. Together, our results indicated a functional link between SET-mediated NDRG1 regulation and acquisition of EMT phenotype and drug resistance, and provided an evidence that blockade of SET-driven EMT can overcome drug resistance and inhibit tumor progression.

Greatwall-phosphorylated Endosulfine is both an inhibitor and a substrate of PP2A-B55 heterotrimers

During M phase, Endosulfine (Endos) family proteins are phosphorylated by Greatwall kinase (Gwl), and the resultant pEndos inhibits the phosphatase PP2A-B55, which would otherwise prematurely reverse many CDK-driven phosphorylations. We show here that PP2A-B55 is the enzyme responsible for dephosphorylating pEndos during M phase exit. The kinetic parameters for PP2A-B55's action on pEndos are orders of magnitude lower than those for CDK-phosphorylated substrates, suggesting a simple model for PP2A-B55 regulation that we call inhibition by unfair competition. As the name suggests, during M phase PP2A-B55's attention is diverted to pEndos, which binds much more avidly and is dephosphorylated more slowly than other substrates. When Gwl is inactivated during the M phase-to-interphase transition, the dynamic balance changes: pEndos dephosphorylated by PP2A-B55 cannot be replaced, so the phosphatase can refocus its attention on CDK-phosphorylated substrates. This mechanism explains simultaneously how PP2A-B55 and Gwl together regulate pEndos, and how pEndos controls PP2A-B55. DOI: http://dx.doi.org/10.7554/eLife.01695.001.

TNF-α downregulates inhibitory neurotransmission through protein phosphatase 1-dependent trafficking of GABA(A) receptors

Inflammation has been implicated in the progression of neurological disease, yet precisely how inflammation affects neuronal function remains unclear. Tumor necrosis factor-α (TNFα) is a proinflammatory cytokine that regulates synapse function by controlling neurotransmitter receptor trafficking and homeostatic synaptic plasticity. Here we characterize the mechanisms through which TNFα regulates inhibitory synapse function in mature rat and mouse hippocampal neurons. Acute application of TNFα induces a rapid and persistent decrease of inhibitory synaptic strength and downregulation of cell-surface levels of GABA(A)Rs containing α1, α2, β2/3, and γ2 subunits. We show that trafficking of GABA(A)Rs in response to TNFα is mediated by neuronally expressed TNF receptor 1 and requires activation of p38 MAPK, phosphatidylinositol 3-kinase, protein phosphatase 1 (PP1), and dynamin GTPase. Furthermore, TNFα enhances the association of PP1 with GABA(A)R β3 subunits and dephosphorylates a site on β3 known to regulate phospho-dependent interactions with the endocytic machinery. Conversely, we find that calcineurin and PP2A are not essential components of the signaling pathway and that clustering of the scaffolding protein gephyrin is only reduced after the initial receptor endocytosis. Together, these findings demonstrate a distinct mechanism of regulated GABA(A)R endocytosis that may contribute to the disruption of circuit homeostasis under neuroinflammatory conditions.

A superfolding Spinach2 reveals the dynamic nature of trinucleotide repeat-containing RNA

Imaging RNA in living cells is a challenging problem in cell biology. One strategy for genetically encoding fluorescent RNAs is to express them as fusions with Spinach, an 'RNA mimic of GFP'. We found that Spinach was dimmer than expected when used to tag constructs in living cells owing to a combination of thermal instability and a propensity for misfolding. Using systematic mutagenesis, we generated Spinach2 that overcomes these issues and can be used to image diverse RNAs. Using Spinach2, we detailed the dynamics of the CGG trinucleotide repeat-containing 'toxic RNA' associated with Fragile X-associated tremor/ataxia syndrome, and show that these RNAs form nuclear foci with unexpected morphological plasticity that is regulated by the cell cycle and by small molecules. Together, these data demonstrate that Spinach2 exhibits improved versatility for fluorescently labeling RNAs in living cells.

Whole genome sequencing identifies a deletion in protein phosphatase 2A that affects its stability and localization in Chlamydomonas reinhardtii

Whole genome sequencing is a powerful tool in the discovery of single nucleotide polymorphisms (SNPs) and small insertions/deletions (indels) among mutant strains, which simplifies forward genetics approaches. However, identification of the causative mutation among a large number of non-causative SNPs in a mutant strain remains a big challenge. In the unicellular biflagellate green alga Chlamydomonas reinhardtii, we generated a SNP/indel library that contains over 2 million polymorphisms from four wild-type strains, one highly polymorphic strain that is frequently used in meiotic mapping, ten mutant strains that have flagellar assembly or motility defects, and one mutant strain, imp3, which has a mating defect. A comparison of polymorphisms in the imp3 strain and the other 15 strains allowed us to identify a deletion of the last three amino acids, Y313F314L315, in a protein phosphatase 2A catalytic subunit (PP2A3) in the imp3 strain. Introduction of a wild-type HA-tagged PP2A3 rescues the mutant phenotype, but mutant HA-PP2A3 at Y313 or L315 fail to rescue. Our immunoprecipitation results indicate that the Y313, L315, or YFLΔ mutations do not affect the binding of PP2A3 to the scaffold subunit, PP2A-2r. In contrast, the Y313, L315, or YFLΔ mutations affect both the stability and the localization of PP2A3. The PP2A3 protein is less abundant in these mutants and fails to accumulate in the basal body area as observed in transformants with either wild-type HA-PP2A3 or a HA-PP2A3 with a V310T change. The accumulation of HA-PP2A3 in the basal body region disappears in mated dikaryons, which suggests that the localization of PP2A3 may be essential to the mating process. Overall, our results demonstrate that the terminal YFL tail of PP2A3 is important in the regulation on Chlamydomonas mating.

SET/PP2A system regulates androgen production in ovarian follicles in vitro

SET has multiple cell functions including nucleosome assembly, histone binding, transcription control, and cell apoptosis. In ovaries SET is predominantly expressed in theca cells and oocytes. In our study, SET overexpression in theca cells stimulated testosterone production whereas SET knockdown decreased testosterone production. Moreover, SET negatively regulated PP2A activity. Treatment with PP2A inhibitor okadaic acid (OA) led to increased testosterone synthesis, while treatment with PP2A activators resulted in the decreased testosterone synthesis. Furthermore, PP2A knockdown confirmed the key role of PP2A in the testosterone synthesis, and OA was able to block the AdH1-SiRNA/SET-mediated inhibition of testosterone production. The central role of PP2A in SET-mediated regulation of testosterone production was confirmed by the finding that SET promoted the lyase activity of P450c17 and that PP2A inhibited its lyase activity. Taken together, these results reveal a specific, SET-initiated, PP2A-mediated, pathway that leads to the increased lyase activity of P450c17 and testosterone biosynthesis.

A role for protein phosphatase 2A in regulating p38 mitogen activated protein kinase activation and tumor necrosis factor-alpha expression during influenza virus infection

Influenza viruses of avian origin continue to pose pandemic threats to human health. Some of the H5N1 and H9N2 virus subtypes induce markedly elevated cytokine levels when compared with the seasonal H1N1 virus. We previously showed that H5N1/97 hyperinduces tumor necrosis factor (TNF)-alpha through p38 mitogen activated protein kinase (MAPK). However, the detailed mechanisms of p38MAPK activation and TNF-alpha hyperinduction following influenza virus infections are not known. Negative feedback regulations of cytokine expression play important roles in avoiding overwhelming production of proinflammatory cytokines. Here we hypothesize that protein phosphatases are involved in the regulation of cytokine expressions during influenza virus infection. We investigated the roles of protein phosphatases including MAPK phosphatase-1 (MKP-1) and protein phosphatase type 2A (PP2A) in modulating p38MAPK activation and downstream TNF-alpha expressions in primary human monocyte-derived macrophages (PBMac) infected with H9N2/G1 or H1N1 influenza virus. We demonstrate that H9N2/G1 virus activated p38MAPK and hyperinduced TNF-alpha production in PBMac when compared with H1N1 virus. H9N2/G1 induced PP2A activity in PBMac and, with the treatment of a PP2A inhibitor, p38MAPK phosphorylation and TNF-alpha production were further increased in the virus-infected macrophages. However, H9N2/G1 did not induce the expression of PP2A indicating that the activation of PP2A is not mediated by p38MAPK in virus-infected PBMac. On the other hand, PP2A may not be the targets of H9N2/G1 in the upstream of p38MAPK signaling pathways since H1N1 also induced PP2A activation in primary macrophages. Our results may provide new insights into the control of cytokine dysregulation.

Specialized functions of the PP2A subfamily II catalytic subunits PP2A-C3 and PP2A-C4 in the distribution of auxin fluxes and development in Arabidopsis

Protein phosphorylation is a key molecular switch used to transmit information in biological signalling networks. The output of these signalling circuits is governed by the counteracting activities of protein kinases and phosphatases that determine the direction of the switch. Whereas many kinases have been functionally characterized, it has been difficult to ascribe precise cellular roles to plant phosphatases, which are encoded by enlarged gene families that may provide a high degree of genetic redundancy. In this work we have analysed the role in planta of catalytic subunits of protein phosphatase 2A (PP2A), a family encoded by five genes in Arabidopsis. Our results indicate that the two members of subfamily II, PP2A-C3 and PP2A-C4, have redundant functions in controlling embryo patterning and root development, processes that depend on auxin fluxes. Moreover, polarity of the auxin efflux carrier PIN1 and auxin distribution, determined with the DR5(pro) :GFP proxy, are affected by mutations in PP2A-C3 and PP2A-C4. Previous characterization of mutants in putative PP2A regulatory subunits had established a link between this class of phosphatases and PIN dephosphorylation and subcellular distribution. Building on those findings, the results presented here suggest that PP2A-C3 and PP2A-C4 catalyse this reaction and contribute critically to the establishment of auxin gradients for proper plant development.

Differential effect of adenosine receptors on growth of human colon cancer HCT 116 and HT-29 cell lines

The study aimed to evaluate the impact of adenosine receptors (ARs) on human colon tumor cells (HCT 116, HT-29) growth and sensitivity to 5-Fluorouracil (5-FU) an anticancer chemotherapeutic drug. The exposure of cancer cells to a selective A(3)-AR agonist (IB-MECA) resulted in an increase in HT-29 cells number, whereas the number of HCT 116 cells decreased significantly. In the presence of IB-MECA (1 μM) the percentage of apoptotic HT-29 cells significantly decreased, whereas the number of apoptotic and necrotic HCT 116 cells increased by 3- and 2,5-fold, respectively. The application of a selective A(2A)-AR agonist resulted in the increased survival of HCT 116 cells, but not HT-29 cells. The blockade of A(2A)-AR with ZM 241385 (0.1 μM) significantly increased the cytotoxicity of 5-FU (1 μM) in HCT 116 cells but not in HT-29 cells. The suppression of A(3)-AR with MRS 1523 (1 μM) increased the sensitivity of HT-29 cells to 5-FU while response of HCT 116 cells to 5-FU decreased. The growth promoting effect of IB-MECA in HT-29 cells was associated with the decreased intracellular cAMP level, whereas IB-MECA growth inhibitory effect in HCT 116 cells was abolished by okadaic acid (2 nM) indicating the involvement of protein phosphatase PP2A.

Protein phosphatase 2A dephosphorylates CaBP4 and regulates CaBP4 function

PURPOSE

CaBP4 is a neuronal Ca(2+)-binding protein that is expressed in the retina and in the cochlea, and is essential for normal photoreceptor synaptic function. CaBP4 is phosphorylated by protein kinase C zeta (PKCζ) in the retina at serine 37, which affects its interaction with and modulation of voltage-gated Ca(v)1 Ca(2+) channels. In this study, we investigated the potential role and functional significance of protein phosphatase 2A (PP2A) in CaBP4 dephosphorylation.

METHODS

The effect of protein phosphatase inhibitors, light, and overexpression of PP2A subunits on CaBP4 dephosphorylation was measured in in vitro assays. Pull-down experiments using retinal or transfected HEK293 cell lysates were used to investigate the association between CaBP4 and PP2A subunits. Electrophysiologic recordings of cotransfected HEK293 cells were performed to analyze the effect of CaBP4 dephosphorylation in modulating Ca(v)1.3 currents.

RESULTS

PP2A inhibitors, okadaic acid (OA), and fostriecin, but not PP1 selective inhibitors, NIPP-1, and inhibitor 2, block CaBP4 dephosphorylation in retinal lysates. Increased phosphatase activity in light-dependent conditions reverses phosphorylation of CaBP4 by PKCζ. In HEK293 cells, overexpression of PP2A enhances the rate of dephosphorylation of CaBP4. In addition, inhibition of protein phosphatase activity by OA increases CaBP4 phosphorylation and potentiates the modulatory effect of CaBP4 on Ca(v)1.3 Ca(2+) channels in HEK293T cells.

CONCLUSIONS

This study provides evidence that CaBP4 is dephosphorylated by PP2A in the retina. Our findings reveal a novel role for protein phosphatases in regulating CaBP4 function in the retina, which may fine tune presynaptic Ca(2+) signals at the photoreceptor synapse.

Regulation of neurite outgrowth mediated by localized phosphorylation of protein translational factor eEF2 in growth cones

Nerve growth cones contain mRNA and its translational machinery and thereby synthesize protein locally. The regulatory mechanisms in the growth cone, however, remain largely unknown. We previously found that the calcium entry-induced increase of phosphorylation of eukaryotic elongation factor-2 (eEF2), a key component of mRNA translation, within growth cones showed growth arrest of neurites. Because dephosphorylated eEF2 and phosphorylated eEF2 are known to promote and inhibit mRNA translation, respectively, the data led to the hypothesis that eEF2-mediating mRNA translation may regulate neurite outgrowth. Here, we validated the hypothesis by using a chromophore-assisted light inactivation (CALI) technique to examine the roles of localized eEF2 and eEF2 kinase (EF2K), a specific calcium calmodulin-dependent enzyme for eEF2 phosphorylation, in advancing growth cones of cultured chick dorsal root ganglion (DRG) neurons. The phosphorylated eEF2 was weakly distributed in advancing growth cones, whereas eEF2 phosphorylation was increased by extracellular adenosine triphosphate (ATP)-evoked calcium transient through P2 purinoceptors in growth cones and resulted in growth arrest of neurites. The increase of eEF2 phosphorylation within growth cones by inhibition of protein phosphatase 2A known to dephosphorylate eEF2 also showed growth arrest of neurites. CALI of eEF2 within growth cones resulted in retardation of neurite outgrowth, whereas CALI of EF2K enhanced neurite outgrowth temporally. Moreover, CALI of EF2K abolished the ATP-induced retardation of neurite outgrowth. These findings suggest that an eEF2 phosphorylation state localized to the growth cone regulates neurite outgrowth.

Fcp1-dependent dephosphorylation is required for M-phase-promoting factor inactivation at mitosis exit

Correct execution of mitosis in eukaryotes relies on timely activation and inactivation of cyclin B-dependent kinase 1 (cdk1), the M-phase-promoting factor (MPF). Once activated, MPF is sustained until mitotic spindle assembly by phosphorylation-dependent feedback loops that prevent inhibitory phosphorylation of cdk1 and ubiquitin-dependent degradation of cyclin B. Whether subsequent MPF inactivation and anaphase onset require a specific phosphatase(s) to reverse these feedback loops is not known. Here we show through biochemical and genetic evidence that timely MPF inactivation requires activity of the essential RNA polymerase II-carboxy-terminal domain phosphatase Fcp1, in a transcription-independent manner. We identify Cdc20, a coactivator of the ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C) required for cyclin degradation and anaphase onset, USP44, a deubiquitinating peptidase that opposes APC/C action, and Wee1, a cdk1 inhibitory kinase, as relevant Fcp1 targets. We propose that Fcp1 has a crucial role in the liaison between dephosphorylation and ubiquitination that drives mitosis exit.

Cyclin B-dependent kinase 1, the M-phase-promoting factor, is precisely activated and inactivated to control mitosis. In this study, Fcp1—the RNA polymerase II-carboxy-terminal domain phosphatase—is identified as a phosphatase required to inactivate the M-phase-promoting factor and promote mitosis exit.

Increased corticosteroid sensitivity by a long acting β2 agonist formoterol via β2 adrenoceptor independent protein phosphatase 2A activation

Long-acting β2-adrenoceptor agonists (LABAs) are reported to enhance anti-inflammatory effects of corticosteroids in vitro and in vivo, although the molecular mechanisms have not yet been elucidated. We investigated the role of serine/threonine protein phosphatase 2A (PP2A) on regulation of corticosteroid sensitivity via inhibition of glucocorticoid receptor (GR) phosphorylation as the target of formoterol, an LABA. Corticosteroid sensitivity was determined as IC50 to dexamethasone (Dex) on TNFα-induced IL-8 release in a U937 monocytic cell line (Dex-IC50). Phosphorylation levels of GR-Ser226 and c-Jun N-terminal kinase (JNK) were determined by western-blotting. Phosphatase activity of immunopurified PP2A was measured by fluorescence-based assay. Exposure to IL-2/IL-4 for 48 h decreased Dex sensitivity with a concomitant increase of GR phosphorylation at Ser226 with JNK1 activation. Formoterol restored Dex sensitivity by inhibiting phosphorylation of GR-Ser226 and JNK1. PP2A inhibition by okadaic acid, a phosphatase inhibitor, abrogated formoterol-mediated effects. In addition, formoterol enhanced PP2A activity in intact or IL-2/IL-4 treated U937 cells and human peripheral blood mononuclear cells. In addition, PP2A activation by formoterol was not antagonized by ICI-118551, and formoterol could activate PP2A directly in cell free system. Taken together, formoterol increases corticosteroid sensitivity via activation of PP2A in receptor independent manner, explaining its benefits as add-on therapy for the treatment of corticosteroid-insensitive diseases, such as severe asthma.

Rho kinase signaling pathways during stretch in primary alveolar epithelia

Alveolar epithelial cells (AECs) maintain integrity of the blood-gas barrier with actin-anchored intercellular tight junctions. Stretched type I-like AECs undergo magnitude- and frequency-dependent actin cytoskeletal remodeling into perijunctional actin rings. On the basis of published studies in human pulmonary artery endothelial cells (HPAECs), we hypothesize that RhoA activity, Rho kinase (ROCK) activity, and phosphorylation of myosin light chain II (MLC2) increase in stretched type I-like AECs in a manner that is dependent on stretch magnitude, and that RhoA, ROCK, or MLC2 activity inhibition will attenuate stretch-induced actin remodeling and preserve barrier properties. Primary type I-like AEC monolayers were stretched biaxially to create a change in surface area (ΔSA) of 12%, 25%, or 37% in a cyclic manner at 0.25 Hz for up to 60 min or left unstretched. Type I-like AECs were also treated with Rho pathway inhibitors (ML-7, Y-27632, or blebbistatin) and stained for F-actin or treated with the myosin phosphatase inhibitor calyculin-A and quantified for monolayer permeability. Counter to our hypothesis, ROCK activity and MLC2 phosphorylation decreased in type I-like AECs stretched to 25% and 37% ΔSA and did not change in monolayers stretched to 12% ΔSA. Furthermore, RhoA activity decreased in type I-like AECs stretched to 37% ΔSA. In contrast, MLC2 phosphorylation in HPAECs increased when HPAECs were stretched to 12% ΔSA but then decreased when they were stretched to 37% ΔSA, similar to type I-like AECs. Perijunctional actin rings were observed in unstretched type I-like AECs treated with the Rho pathway inhibitor blebbistatin. Myosin phosphatase inhibition increased MLC2 phosphorylation in stretched type I-like AECs but had no effect on monolayer permeability. In summary, stretch alters RhoA activity, ROCK activity, and MLC2 phosphorylation in a manner dependent on stretch magnitude and cell type.

Lewy-like aggregation of α-synuclein reduces protein phosphatase 2A activity in vitro and in vivo

α-synuclein (α-Syn) is a chaperone-like protein that is highly implicated in Parkinson's disease (PD) as well as in dementia with Lewy bodies (DLB). Rare forms of PD occur in individuals with mutations of α-Syn or triplication of wild type α-Syn, and in both PD and DLB the intraneuronal inclusions known as Lewy bodies contain aggregated α-Syn that is highly phosphorylated on serine 129. In neuronal cells and in the brains of α-Syn overexpressing transgenic mice, soluble α-Syn stimulates the activity of protein phosphatase 2A (PP2A), a major serine/threonine phosphatase. Serine 129 phosphorylation of α-Syn attenuates its stimulatory effects on PP2A and also accelerates α-Syn aggregation; however, it is unknown if aggregation of α-Syn into Lewy bodies impairs PP2A activity. To assess for this, we measured the impact of α-Syn aggregation on PP2A activity in vitro and in vivo. In cell-free assays, aggregated α-Syn had ∼50% less PP2A stimulatory effects than soluble recombinant α-Syn. Similarly in DLB and α-Syn triplication brains, which contain robust α-Syn aggregation with high levels of serine 129 phosphorylation, PP2A activity was also ∼50% attenuated. As α-Syn normally stimulates PP2A activity, our data suggest that overexpression of α-Syn or sequestration of α-Syn into Lewy bodies has the potential to alter the phosphorylation state of key PP2A substrates; raising the possibility that all forms of synucleinopathy will benefit from treatments aimed at optimizing PP2A activity.

Defects of protein phosphatase 2A causes corticosteroid insensitivity in severe asthma

Background

Corticosteroid insensitivity is a major barrier of treatment for some chronic inflammatory diseases, such as severe asthma, but the molecular mechanism of the insensitivity has not been fully elucidated. The object of this study is to investigate the role of protein phosphate 2A (PP2A), a serine/threonine phosphatase, on corticosteroid sensitivity in severe asthma.

Methodology/Principal Findings

Corticosteroid sensitivity was determined by the dexamethasone ability to inhibit TNFα-induced IL-8 or LPS-induced TNFα production. PP2A expression, glucocorticoid receptor (GR) nuclear translocation defined as the nuclear/cytoplasmic GR ratio and phosphorylation of GR-Ser²²⁶, c-Jun N-terminal kinase 1 (JNK1) and PP2A were analysed by Western-blotting. Phosphatase activity was measured by fluorescence-based assay. Okadaic acid (OA), a PP2A inhibitor, reduced corticosteroid sensitivity with reduced GR nuclear translocation and increased GR phosphorylation in U937 monocytic cells. PP2A knockdown by RNA interference showed similar effects. IL-2/IL-4 treatment to U937 reduced corticosteroid sensitivity, and PP2A expression/activity. In peripheral blood mononuclear cells (PBMCs) from severe asthma, the PP2A expression and activity were significantly reduced with concomitant enhancement of PP2A_C-Tyr³⁰⁷ phosphorylation compared with those in healthy volunteers. As the results, GR-Ser²²⁶ and JNK1 phosphorylation were increased. The expression and activity of PP2A were negatively correlated with phosphorylation levels of GR-Ser²²⁶. Furthermore, co-immunoprecipitation assay in U937 cells revealed that PP2A associated with GR and JNK1 and IL-2/IL-4 exposure caused dissociation of each molecule. Lastly, PP2A overexpression increased corticosteroid sensitivity in U937 cells.

Conclusions/Significance

PP2A regulates GR nuclear translocation and corticosteroid sensitivity possibly by dephosphorylation of GR-Ser²²⁶ via dephosphorylation of upstream JNK1. This novel mechanism will provide new insight for the development of new therapy for severe asthma.

Protein phosphatase 2A mediates resensitization of the neurokinin 1 receptor

Activated G protein-coupled receptors (GPCRs) are phosphorylated and interact with β-arrestins, which mediate desensitization and endocytosis. Endothelin-converting enzyme-1 (ECE-1) degrades neuropeptides in endosomes and can promote recycling. Although endocytosis, dephosphorylation, and recycling are accepted mechanisms of receptor resensitization, a large proportion of desensitized receptors can remain at the cell surface. We investigated whether reactivation of noninternalized, desensitized (phosphorylated) receptors mediates resensitization of the substance P (SP) neurokinin 1 receptor (NK(1)R). Herein, we report a novel mechanism of resensitization by which protein phosphatase 2A (PP2A) is recruited to dephosphorylate noninternalized NK(1)R. A desensitizing concentration of SP reduced cell-surface SP binding sites by only 25%, and SP-induced Ca(2+) signals were fully resensitized before cell-surface binding sites started to recover, suggesting resensitization of cell-surface-retained NK(1)R. SP induced association of β-arrestin1 and PP2A with noninternalized NK(1)R. β-Arrestin1 small interfering RNA knockdown prevented SP-induced association of cell-surface NK(1)R with PP2A, indicating that β-arrestin1 mediates this interaction. ECE-1 inhibition, by trapping β-arrestin1 in endosomes, also impeded SP-induced association of cell-surface NK(1)R with PP2A. Resensitization of NK(1)R signaling required both PP2A and ECE-1 activity. Thus, after stimulation with SP, PP2A interacts with noninternalized NK(1)R and mediates resensitization. PP2A interaction with NK(1)R requires β-arrestin1. ECE-1 promotes this process by releasing β-arrestin1 from NK(1)R in endosomes. These findings represent a novel mechanism of PP2A- and ECE-1-dependent resensitization of GPCRs.

Differential temporal and spatial regulation of somatostatin receptor phosphorylation and dephosphorylation

The G(i)-coupled somatostatin 2A receptor (sst2A) mediates many of the neuromodulatory and neuroendocrine actions of somatostatin (SS) and is targeted by the SS analogs used to treat neuroendocrine tumors. As for other G protein-coupled receptors, agonists stimulate sst2A receptor phosphorylation on multiple residues, and phosphorylation at different sites has distinct effects on receptor internalization and uncoupling. To elucidate the spatial and temporal regulation of sst2A receptor phosphorylation, we examined agonist-stimulated phosphorylation of multiple receptor GPCR kinase sites using phospho-site-specific antibodies. SS increased receptor phosphorylation sequentially, first on Ser-341/343 and then on Thr-353/354, followed by receptor internalization. Reversal of receptor phosphorylation was determined by the duration of prior agonist exposure. In acutely stimulated cells, in which most receptors remained on the cell surface, dephosphorylation occurred only on Thr-353/354. In contrast, both Ser-341/343 and Thr-353/354 were rapidly dephosphorylated when cells were stimulated long enough to allow receptor internalization before agonist removal. Consistent with these observations, dephosphorylation of Thr-353/354 was not affected by either hypertonic sucrose or dynasore, which prevent receptor internalization, whereas dephosphorylation of Ser-341/343 was completely blocked. An okadaic acid- and fostriecin-sensitive phosphatase catalyzed the dephosphorylation of Thr-353/354 both intracellularly and at the cell surface. In contrast, dephosphorylation of Ser-341/343 was insensitive to these inhibitors. Our results show that the phosphorylation and dephosphorylation of neighboring GPCR kinase sites in the sst2A receptor are subject to differential spatial and temporal regulation. Thus, the pattern of receptor phosphorylation is determined by the duration of agonist stimulation and compartment-specific enzymatic activity.

Amino-terminal phosphorylation of activation-induced cytidine deaminase suppresses c-myc/IgH translocation

Activation-induced cytidine deaminase (AID) is a mutator enzyme that initiates class switch recombination and somatic hypermutation of immunoglobulin genes (Ig) in B lymphocytes. However, AID also produces off-target DNA damage, including mutations in oncogenes and double-stranded breaks that can serve as substrates for oncogenic chromosomal translocations. AID is strictly regulated by a number of mechanisms, including phosphorylation at serine 38 and threonine 140, which increase activity. Here we show that phosphorylation can also suppress AID activity in vivo. Serine 3 is a novel phospho-acceptor which, when mutated to alanine, leads to increased class switching and c-myc/IgH translocations without affecting AID levels or catalytic activity. Conversely, increasing AID phosphorylation specifically on serine 3 by interfering with serine/threonine protein phosphatase 2A (PP2A) leads to decreased class switching. We conclude that AID activity and its oncogenic potential can be downregulated by phosphorylation of serine 3 and that this process is controlled by PP2A.

Developmental regulation of neuronal survival by adenosine in the in vitro and in vivo avian retina depends on a shift of signaling pathways leading to CREB phosphorylation or dephosphorylation

Previous studies have shown a cAMP/protein kinase A-dependent neuroprotective effect of adenosine on glutamate or re-feeding-induced apoptosis in chick retina neuronal cultures. In the present work, we have studied the effect of adenosine on the survival of retinal progenitor cells. Cultures obtained from 6-day-old (E6) or from 8-day-old (E8) chick embryos were challenged 2 h (C0) or 1 day (C1) after seeding and analyzed after 3-4 days in vitro. Surprisingly, treatment with the selective A2a adenosine receptor agonists N(6) -[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA) or 3-[4-[2-[[6-amino-9-[(2R,3R,4S,5S)-5-(ethylcarbamoyl)-3,4-dihydroxy-oxolan-2-yl]purin-2-yl]amino]ethyl]phenyl]propanoic acid (CGS21680) promoted cell death when added at E6C0 but not at E6C1 or E8C0. DPMA-induced cell death involved activation of A2a receptors and the phospholipase C/protein kinase C but not the cAMP/protein kinase A pathway, and was not correlated with early modulation of precursor cells proliferation. Regarding cyclic nucleotide responsive element binding protein (CREB) phosphorylation, cultures from E6 embryos behave in an opposite manner from that from E8 embryos, both in vitro and in vivo. While the phospho-CREB level was high at E6C0 cultures and could be diminished by DPMA, it was lower at E8C0 and could be increased by DPMA. Similar to what was observed in cell survival studies, CREB dephosphorylation induced by DPMA in E6C0 cultures was dependent on the Phospholipase C/protein kinase C pathway. Accordingly, cell death induced by DPMA was inhibited by okadaic acid, a phosphatase blocker. Moreover, DPMA as well as the adenosine uptake blocker nitrobenzyl mercaptopurine riboside (NBMPR) modulate cell survival and CREB phosphorylation in a population of cells in the ganglion cell layer in vivo. These data suggest that A2a adenosine receptors as well as CREB may display a novel and important function by controlling the repertoire of developing retinal neurons.

Decreased Activity of the Na + /H + Exchanger by Phosphodiesterase 5A Inhibition Is Attributed to an Increase in Protein Phosphatase Activity

The beneficial effect of phosphodiesterase 5A inhibition in ischemia/reperfusion injury and cardiac hypertrophy is well established. Inhibition of the cardiac Na + /H + exchanger (NHE-1) exerts beneficial effects on these same conditions, and a possible link between these therapeutic strategies was suggested. Experiments were performed in isolated cat cardiomyocytes to gain insight into the intracellular pathway involved in the reduction of NHE-1 activity by phosphodiesterase 5A inhibition. NHE-1 activity was assessed by the rate of intracellular pH recovery from a sustained acidic load in the absence of bicarbonate. Phosphodiesterase 5A inhibition with sildenafil (1 μmol/L) did not affect basal intracellular pH; yet, it did decrease proton efflux (J H ; in millimoles per liter per minute) after the acidic load (proton efflux: 6.97±0.43 in control versus 3.31±0.58 with sildenafil; P <0.05). The blockade of both protein phosphatase 1 and 2A with 100 nmol/L of okadaic acid reverted the sildenafil effect (proton efflux: 6.77±0.82). In contrast, selective inhibition of protein phosphatase 2A (1 nmol/L of okadaic acid or 100 μmol/L of endothall) did not (3.86±1.0 and 2.61±1.2), suggesting that only protein phosphatase 1 was involved in sildenafil-induced NHE-1 inhibition. Moreover, sildenafil prevented the acidosis-induced increase in NHE-1 phosphorylation without affecting activation of the extracellular signal–regulated kinase 1/2-p90 RSK pathway. Our results suggest that phosphodiesterase 5A inhibition decreases NHE-1 activity, during intracellular pH recovery after an acidic load, by a protein phosphatase 1–dependent reduction in NHE-1 phosphorylation.

A stress survival response in retinal cells mediated through inhibition of the serine/threonine phosphatase PP2A

Cell survival signalling involving the PI3K/Akt survival pathway can be negatively regulated by several phosphatases including PP2A. When retinal-derived 661W cells were subjected to trophic factor deprivation this initiated a survival response through inhibition of the activity of PP2A and subsequent upregulation of the Erk and Akt survival pathways. We show this survival response via inhibition of PP2A activity was due in part to increased reactive oxygen species production when retinal cells were deprived of trophic factors. Inhibition of PP2A activity was mediated by a rapid and transient increase in phosphorylation at Tyr307, accompanied by an increase in demethylation and a decrease in the methylated form. Pre-treatment with N-acetyl-L-cysteine, which is involved in scavenging reactive oxygen species, prevented PP2A inhibition and subsequent upregulation of survival pathways. Pre-treatment with the Src family kinase inhibitor PP2 resulted in approximately 50% reduction in cellular levels of phospho-PP2A in trophic factor-deprived 661W cells, suggesting an Src tyrosine kinase had a role to play in this redox regulation of cell survival. We observed similar events in the rd10 mouse retina where there was an increased survival response prior to retinal cell death mediated through an increase in both phospho-PP2A and phospho-Gsk. Together, these results demonstrate that when retinal cells are stressed there is an initial struggle to survive, mediated through inhibition of PP2A and subsequent upregulation of survival pathways, and that these events occur simultaneously with production of reactive oxygen species, thus suggesting an important cell-signalling role for reactive oxygen species.

Essential requirement for PP2A inhibition by the oncogenic receptor c-KIT suggests PP2A reactivation as a strategy to treat c-KIT+ cancers

Oncogenic mutations of the receptor tyrosine kinase c-KIT play an important role in the pathogenesis of gastrointestinal stromal tumors, systemic mastocytosis, and some acute myeloid leukemias (AML). Although juxtamembrane mutations commonly detected in gastrointestinal stromal tumor are sensitive to tyrosine kinase inhibitors, the kinase domain mutations frequently encountered in systemic mastocytosis and AML confer resistance and are largely unresponsive to targeted inhibition by the existing agent imatinib. In this study, we show that myeloid cells expressing activated c-KIT mutants that are imatinib sensitive (V560G) or imatinib resistant (D816V) can inhibit the tumor suppressor activity of protein phosphatase 2A (PP2A). This effect was associated with the reduced expression of PP2A structural (A) and regulatory subunits (B55alpha, B56alpha, B56gamma, and B56delta). Overexpression of PP2A-Aalpha in D816V c-KIT cells induced apoptosis and inhibited proliferation. In addition, pharmacologic activation of PP2A by FTY720 reduced proliferation, inhibited clonogenic potential, and induced apoptosis of mutant c-KIT(+) cells, while having no effect on wild-type c-KIT cells or empty vector controls. FTY720 treatment caused the dephosphorylation of the D816V c-KIT receptor and its downstream signaling targets pAkt, pSTAT5, and pERK1/2. Additionally, in vivo administration of FTY720 delayed the growth of V560G and D816V c-KIT tumors, inhibited splenic and bone marrow infiltration, and prolonged survival. Our findings show that PP2A inhibition is essential for c-KIT-mediated tumorigenesis, and that reactivating PP2A may offer an attractive strategy to treat drug-resistant c-KIT(+) cancers.

Protein phosphatase 2A has an essential role in the activation of gamma-irradiation-induced G2/M checkpoint response

G2/M checkpoint activation after DNA damage results in G2/M cell cycle arrest that allows time for DNA repair before the entry of cells into mitosis. Activation of G2/M checkpoint involves a series of signaling events, which include activation of ataxia telangiectecia-mutated and Rad3-related (ATR) and Chk1 kinases and inhibition of Cdc2/Cyclin B activity. Studies presented in this report show that serine (Ser)/threonine (Thr) protein phosphatase 2A (PP2A) has an important role in G2/M checkpoint activation in response to gamma-irradiation (IR) exposure. Using PP2A inhibitors, as well as siRNA targeting various forms of Ser/Thr protein phosphatases, results presented in this report show that specific PP2A inhibition abrogates IR-induced activation of ATR and Chk1 kinases, as well as phosphorylation of Cdc2-Tyr15, and attenuates IR-induced G2/M arrest. These results suggest an important regulation of PP2A on IR-induced G2/M checkpoint signaling response.

Apoptosis signal-regulating kinase 1 in peptidoglycan-induced COX-2 expression in macrophages

In this study, we investigated the role of ASK1 in PGN-induced C/EBPbeta activation and COX-2 expression in RAW 264.7 macrophages. The PGN-induced COX-2 expression was attenuated by the DNs of ASK1, JNK1, JNK2, a JNK inhibitor (SP600125), and an AP-1 inhibitor (curcumin). PGN caused ASK1 dephosphorylation time-dependently at Ser967, dissociation from the ASK1-14-3-3 complex, and subsequent ASK1 activation. In addition, PGN activated PP2A and suppression of PP2A by okadaic acid markedly inhibited PGN-induced ASK1 Ser967 dephosphorylation and COX-2 expression. PGN induced the activation of the JNK-AP-1 signaling cascade downstream of ASK1. PGN-increased C/EBPbeta expression and DNA-binding activity were inhibited by the ASK1-JNK-AP-1 signaling blockade. COX-2 promoter luciferase activity induced by PGN was attenuated in cells transfected with the COX-2 reporter construct possessing the C/EBP-binding site mutation. In addition, the ASK1-JNK-AP-1-C/EBPbeta cascade was activated in human peripheral mononuclear cells exposure to PGN. The TLR2 agonist Pam(3)CSK(4) was also shown to induce ASK1 Ser967 dephosphorylation, JNK and c-jun phosphorylation, C/EBPbeta activation, and COX-2 expression in RAW 264.7 macrophages. PGN-induced COX-2 promoter luciferase activity was prevented by selective inhibition of TLR2 and c-Jun in RAW 264.7 macrophages. Our data demonstrate that PGN might activate the TLR2-mediated PP2A-ASK1-JNK-AP-1-C/EBPbeta cascade and subsequent COX-2 expression in RAW 264.7 macrophages.

Passive mechanical forces upregulate the fast myosin heavy chain IId/x via integrin and p38 MAP kinase activation in a primary muscle cell culture

We have studied the mechanism by which a previously described primary muscle culture growing on microcarriers predominantly expresses fast myosin heavy chain (MHC) IId/x. We have measured MHC IId/x mRNA and protein levels, mRNA of MHC I and markers of muscle metabolism, insulin-like growth factor (IGF)-1 and mechano-growth factor (MGF) transcripts, indicators of the activation of the Akt-mammalian target of rapamycin (mTOR) axis, the p38-, ERK1/2-, and JNK-mitogen-activated protein kinase (MAP) kinase pathways, and of protein phosphatase PP2A, and we have assessed the involvement of integrin. By placing the culture flasks on a rotary shaker, we induce a continuous motion of the culture medium in which the carrier-myotube aggregates are suspended. This motion exerts passive forces on the myotubes that are decisive for the predominance of MHC II expression. These forces act via integrin, which transduces the mechanical signal into activation of PP2A and of p38 MAP-Kinase. The latter presumably is directly responsible for a drastic upregulation of MHC IId/x, whereas MHC I and metabolic markers remain unaffected. At the same time, despite an elevated level of IGF-1 transcription under passive forces, the IGF-1 receptor-Akt-mTOR axis is switched off as evident from the lack of an effect of inhibition of the IGF-1 receptor and from the PP2A-mediated low degree of phosphorylation of Akt and 4E-BP1. Similarly, the ERK1/2- and JNK-MAP kinase pathways are repressed. We conclude that passive stretch exerted on the myotubes by the rotary fluid motion induces a rather selective upregulation of fast MHC II, which goes along with a mild muscle hypertrophy as judged from the amount of protein per cell and is caused by p38 MAP kinase activity elevated via integrin sensing. The direct link between passive stretch and MHC II expression constitutes a novel mechanism, which is expected to become effective physiologically under passive stretch and eccentric contractions of skeletal muscles.

Declining phosphatases underlie aging-related hyperphosphorylation of neurofilaments

Cytoskeletal protein phosphorylation is frequently altered in neuropathologic states but little is known about changes during normal aging. Here we report that declining protein phosphatase activity, rather than activation of kinases, underlies aging-related neurofilament hyperphosphorylation. Purified PP2A or PP2B dephosphorylated the heavy neurofilament (NFH) subunit or its extensively phorphorylated carboxyl-terminal domain in vitro. In cultured primary hippocampal neurons, inhibiting either phosphatase induced NFH phosphorylation without activating known neurofilament kinases. Neurofilament phosphorylation in the mouse CNS, as reflected by levels of the RT-97 phosphoepitope associated with late axon maturation, more than doubled during the 12-month period after NFH expression plateaued at p21. This was accompanied by declines in levels and activity of PP2A but not PP2B, and no rise in activities of neurofilament kinases (Erk1,2, cdk5 and JNK1,2). Inhibiting PP2A in mice in vivo restored brain RT-97 to levels seen in young mice. Declining PP2A activity, therefore, can account for rising neurofilament phosphorylation in maturing brain, potentially compounding similar changes associated with adult-onset neurodegenerative diseases.

Dual engagement of 14-3-3 proteins controls signal relay from ASK2 to the ASK1 signalosome

Faithful and efficient transmission of biological signals through mitogen-activated protein kinase (MAPK) pathways requires engagement of highly regulated cellular machinery in response to diverse environmental cues. Here, we report a novel mechanism controlling signal relay between two MAP3Ks, apoptosis signal-regulating kinase (ASK) 1 and ASK2. We show that ASK2 specifically interacts with 14-3-3 proteins through phosphorylated S964. Although a 14-3-3-binding defective mutant of ASK1 (S967A) has no effect on the ASK2/14-3-3 interaction, both overexpression of the analogous ASK2 (S964A) mutant and knockdown of ASK2 dramatically reduced the amount of ASK1 complexed with 14-3-3. These data suggest a dominant role of ASK2 in 14-3-3 control of ASK1 function. Indeed, ASK2 S964A-induced dissociation of 14-3-3 from ASK1 correlated with enhanced phosphorylation of ASK1 at T838 and increased c-Jun N-terminal kinase phosphorylation, the two biological readouts of ASK1 activation. Our results suggest a model in which upstream signals couple ASK2 S964 phosphorylation to the ASK1 signalosome through dual engagement of 14-3-3.

Peroxynitrite-dependent activation of protein phosphatase type 2A mediates microvascular endothelial barrier dysfunction

Aims

We investigated the mechanism by which proinflammatory stimulation induces microvascular endothelial barrier dysfunction. Since protein phosphatase type 2A (PP2A) can mediate paracellular leak and can be inactivated by tyrosine phosphorylation in its catalytic subunit (PP2Ac), we hypothesized that microvascular endothelial cells exposed to proinflammatory stimulation produce peroxynitrite that nitrates PP2Ac, and this nitration inhibits tyrosine phosphorylation of PP2Ac and thereby increases PP2A activity to mediate endothelial barrier dysfunction.

Methods and results

Exposure of mouse skeletal muscle microvascular endothelial cell monolayers to a proinflammatory stimulus [lipopolysaccharide (LPS) + interferon (IFN)γ] increased permeability to albumin, and this barrier dysfunction was attenuated by PP2A inhibitor okadaic acid or by siRNA (small interfering ribonucleic acid) against PP2Ac. LPS + IFNγ increased synthesis of peroxynitrite precursors nitric oxide (NO) and superoxide by inducible NO synthase (iNOS) and NADPH oxidase, respectively. PP2Ac immunoprecipitates isolated from LPS + IFNγ- or peroxynitrite-treated cells showed increased tyrosine nitration, decreased tyrosine phosphorylation and increased phosphatase activity. 3-Nitrotyrosine immunoprecipitates from LPS + IFNγ-stimulated cells also exhibited increased PP2A activity. Further, iNOS inhibitor 1400W, iNOS deficiency, NADPH oxidase inhibitor apocynin, or p47phox deficiency prevented the increase in PP2A activity and preserved barrier function.

Conclusion

LPS + IFNγ stimulates endothelial cells to produce iNOS-derived NO and NADPH oxidase-derived superoxide, which form peroxynitrite that nitrates tyrosine residues in PP2Ac and inhibits their phosphorylation. This nitration in PP2Ac is correlated with PP2A activation that mediates endothelial barrier dysfunction.

Use of protein phosphatase inhibitors

Reversible protein phosphorylation is recognized as a major mechanism regulating the physiology of plant and animal cells. Virtually every biochemical process within eukaryotic cells is controlled by the covalent modification of key regulatory proteins. This in turn dictates the cellular response to a variety of physiological and environmental stimuli; errors in signals transduced by phosphoproteins contribute to many human diseases. Thus, defining protein phosphorylation events, and specifically, the phosphoproteins involved, is crucial for obtaining a better understanding of the physiological events that distinguish normal and diseased states. Protein phosphatase inhibitors are useful when deciphering physiological events regulated by reversible protein phosphorylation but the hormonal stimuli or signaling pathways involved are not known. They are also useful in analyzing the impact of hormones and other physiological stimuli on the function of a specific phosphoprotein. This unit describes protocols for inhibiting the cellular PP1/PP2A activity with okadaic acid, microcystin-LR, and PP2B/calcineurin and a widely utilized strategy for inhibiting protein tyrosine phosphatases.

Phosphoprotein profiling by PA-GeLC-MS/MS

A significant consequence of protein phosphorylation is to alter protein-protein interactions, leading to dynamic regulation of the components of protein complexes that direct many core biological processes. Recent proteomic studies have populated databases with extensive compilations of cellular phosphoproteins and phosphorylation sites and a similarly deep coverage of the subunit compositions and interactions in multiprotein complexes. However, considerably less data are available on the dynamics of phosphorylation, composition of multiprotein complexes or that define their interdependence. We describe a method to identify candidate phosphoprotein complexes by combining phosphoprotein affinity chromatography, separation by size, denaturing gel electrophoresis, protein identification by tandem mass spectrometry, and informatics analysis. Toward developing phosphoproteome profiling, we have isolated native phosphoproteins using a phosphoprotein affinity matrix, Pro-Q Diamond resin (Molecular Probes-Invitrogen). This resin quantitatively retains phosphoproteins and associated proteins from cell extracts. Pro-Q Diamond purification of a yeast whole cell extract followed by 1-D PAGE separation, proteolysis and ESI LC-MS/MS, a method we term PA-GeLC-MS/MS, yielded 108 proteins, a majority of which were known phosphoproteins. To identify proteins that were purified as parts of phosphoprotein complexes, the Pro-Q eluate was separated into two fractions by size, <100 kDa and >100 kDa, before analysis by PAGE and ESI LC-MS/MS and the component proteins queried against databases to identify protein-protein interactions. The <100 kDa fraction was enriched in phosphoproteins indicating the presence of monomeric phosphoproteins. The >100 kDa fraction contained 171 proteins of 20-80 kDa, nearly all of which participate in known protein-protein interactions. Of these 171, few are known phosphoproteins, consistent with their purification by participation in protein complexes. By comparing the results of our phosphoprotein profiling with the informational databases on phosphoproteomics, protein-protein interactions and protein complexes, we have developed an approach to examining the correlation between protein interactions and protein phosphorylation.

Use of protein phosphatase inhibitors

Reversible protein phosphorylation is recognized as a major mechanism regulating the physiology of plant and animal cells. Virtually every biochemical process within eukaryotic cells is controlled by the covalent modification of key regulatory proteins. This in turn dictates the cellular response to a variety of physiological and environmental stimuli; errors in signals transduced by phosphoproteins contribute to many human diseases. Thus, defining protein phosphorylation events, and specifically, the phosphoproteins involved, is crucial for obtaining a better understanding of the physiological events that distinguish normal and diseased states. Protein phosphatase inhibitors are useful when deciphering physiological events regulated by reversible protein phosphorylation but the hormonal stimuli or signaling pathways involved are not known. They are also useful in analyzing the impact of hormones and other physiological stimuli on the function of a specific phosphoprotein. This unit describes protocols for inhibiting cellular phosphorylation activity with okadaic acid, microcystin-LR, and PP2B/calcineurin and a widely utilized strategy for inhibiting protein tyrosine phosphatases.

Palmitate action to inhibit glycogen synthase and stimulate protein phosphatase 2A increases with risk factors for type 2 diabetes

Recent studies have suggested that abnormal regulation of protein phosphatase 2A (PP2A) is associated with Type 2 diabetes in rodent and human tissues. Results with cultured mouse myotubes support a mechanism for palmitate activation of PP2A, leading to activation of glycogen synthase kinase 3. Phosphorylation and inactivation of glycogen synthase by glycogen synthase kinase 3 could be the mechanism for long-chain fatty acid inhibition of insulin-mediated carbohydrate storage in insulin-resistant subjects. Here, we test the effects of palmitic acid on cultured muscle glycogen synthase and PP2A activities. Palmitate inhibition of glycogen synthase fractional activity is increased in subjects with high body mass index compared with subjects with lower body mass index (r = -0.43, P = 0.03). Palmitate action on PP2A varies from inhibition in subjects with decreased 2-h plasma glucose concentration to activation in subjects with increased 2-h plasma glucose concentration (r = 0.45, P < 0.03) during oral glucose tolerance tests. The results do not show an association between palmitate effects on PP2A and glycogen synthase fractional activity. We conclude that subjects at risk for Type 2 diabetes have intrinsic differences in palmitate regulation of at least two enzymes (PP2A and glycogen synthase), contributing to abnormal insulin regulation of glucose metabolism.

Cytoplasmic linker protein-170 enhances spreading and phagocytosis in activated macrophages by stabilizing microtubules

Activation of macrophages causes increased cell spreading, increased secretion of cytokines and matrix metalloproteinases, and enhanced phagocytosis. The intracellular mechanisms driving the up-regulation of these activities have not been completely clarified. We observe that classical activation of murine resident peritoneal or RAW 264.7 macrophages with a combination of IFN-gamma and LPS induces an increase in stabilized cytoplasmic microtubules (MTs), measured with an anti-acetylated alpha-tubulin Ab. We examined the mechanism of this MT stabilization and find that macrophage activation causes redistribution of the MT plus-end tracking protein, cytoplasmic linker protein-170 (CLIP-170). CLIP-170 is localized at the distal plus-ends of MTs in resting macrophages, but accumulates along the length of MTs in IFN-gamma/LPS-activated cells. A direct involvement of CLIP-170 in MT stabilization has not been thoroughly established. In this study, we show that expression of a mutant CLIP-170 chimeric protein (dominant-negative CLIP-170-GFP), lacking the MT-binding domain, prevents MT stabilization in activated RAW 264.7 macrophages. Furthermore, we find enhanced CLIP-170 association with MTs and MT stabilization by treating resting macrophages with okadaic acid, implicating the protein phosphatase 2A in CLIP-170 binding and MT stabilization in RAW 264.7 cells. Finally, we observed enhanced cell spreading and phagocytosis in both IFN-gamma/LPS-activated and okadaic acid-treated resting RAW 264.7 cells, which are markedly reduced in activated cells expressing dominant-negative CLIP-170-GFP. These results identify CLIP-170 as a key regulator of MT stabilization and establish a prominent role for stabilized MTs in cell spreading and phagocytosis in activated macrophages.

Scd5p mediates phosphoregulation of actin and endocytosis by the type 1 phosphatase Glc7p in yeast

Pan1p plays essential roles in both actin and endocytosis in yeast. It interacts with, and regulates the function of, multiple endocytic proteins and actin assembly machinery. Phosphorylation of Pan1p by the kinase Prk1p down-regulates its activity, resulting in disassembly of the endocytic vesicle coat complex and termination of vesicle-associated actin polymerization. In this study, we focus on the mechanism that acts to release Pan1p from phosphorylation inhibition. We show that Pan1p is dephosphorylated by the phosphatase Glc7p, and the dephosphorylation is dependent on the Glc7p-targeting protein Scd5p, which itself is a phosphorylation target of Prk1p. Scd5p links Glc7p to Pan1p in two ways: directly by interacting with Pan1p and indirectly by interacting with the Pan1p-binding protein End3p. Depletion of Glc7p from the cells causes defects in cell growth, actin organization, and endocytosis, all of which can be partially suppressed by deletion of the PRK1 gene. These results suggest that Glc7p antagonizes the activity of the Prk1p kinase in regulating the functions of Pan1p and possibly other actin- and endocytosis-related proteins.

Lymph node cells from BALB/c mice with chronic visceral leishmaniasis exhibiting cellular anergy and apoptosis: involvement of Ser/Thr phosphatase

Visceral leishmaniasis (VL) produced in BALB/c mice through intracardial administration of Leishmania donovani amastigotes was accompanied by hepatosplenomegaly with high organ parasite load and lymphadenopathy when followed up to 4-months or so. To elucidate the mechanism of immunosuppression associated with VL, we report here progressive impairment of the proliferative response of lymph node cells (lymphocytes) from infected animals (I-LNC) to in vitro stimulation with the combination of phorbol 12-myristate 13-acetate (PMA) and ionomycin (Io) that could be related to the downregulation of PKC and MAP kinase (ERK 1/2) activation process. Further, pretreatment of I-LNC with the protein phosphatase inhibitor okadaic acid (OA), but not with calyculin A or sodium orthovanadate, significantly restored their proliferative response as well as PMA-induced activation of PKC. A population of LNC (primarily T-lymphocytes) from chronically infected animals was shown to undergo apoptosis, the number of which increased considerably following PMA+ Io stimulation. The apoptotic pathway, which was followed through binding of cells to Annexin V, activation of caspase-3 and fragmentation of DNA, involved destabilization of mitochondria, probably as a result of downregulation of PKC and Bcl-2. Interestingly, prior incubation of I-LNC with OA reversed the state of cell cycle arrest (anergy) and apoptosis through progression of cells from G0/G1 to S and G2/M phases with transcriptional activation of IL-2 and IL-2R genes. Our results suggest that the cellular (immune) dysfunction in VL could be attributed to dephosphorylation of key molecules in the T-lymphocyte signaling pathway by Ser/Thr phosphatase leading to their inactivation.

Insulin-inhibited and stimulated cultured vascular smooth muscle cell migration are related to divergent effects on protein phosphatase-2A and autonomous calcium/calmodulin-dependent protein kinase II

Insulin, in the permissive presence of nitric oxide (NO), stimulates cGMP production which inhibits autonomous calcium/calmodulin-dependent protein kinase II (CaM kinase II) thereby inhibiting cultured vascular smooth muscle cell (VSMC) migration. In the presence of angiotensin II (Ang II), insulin stimulates NAD(P)H oxidase activity leading to increased VSMC migration. We wished to see whether insulin-stimulated cGMP stimulates protein phosphatase-2A (PP-2A) thereby inhibiting autonomous CaM kinase II and migration, and whether insulin, in the presence of Ang II, inhibits PP-2A and stimulates autonomous CaM kinase II in a NAD(P)H oxidase-dependent manner. One nanomole per litre of insulin in the presence of NO, or 50 micromol/L 8-Br-cGMP stimulated PP-2A activity by 46+/-6 and 247+/-23%, respectively (both P<0.05), and 8-Br-cGMP inhibited autonomous CaM kinase II activity by 67+/-9% (P<0.05) by a 10 nmol/L okadaic acid-sensitive pathway. Insulin plus Ang II inhibited PP-2A activity by 57+/-7% (P<0.05) and stimulated autonomous CaM kinase II activity by 120+/-14% (P<0.05), both by an apocynin-sensitive pathway. 8-Br-cGMP-inhibited VSMC migration was blocked by okadaic acid. It is concluded that insulin in the presence of NO stimulates cGMP which stimulates PP-2A activity causing inhibition of autonomous CaM kinase II activity and thus VSMC migration, and that insulin in the presence of Ang II inhibits PP-2A and stimulates autonomous CaM kinase II activities by a NAD(P)H oxidase-dependent mechanism which are associated with insulin-stimulated NAD(P)H oxidase-dependent migration.