Modulation of integrin signal transduction by ILKAP, a protein phosphatase 2C associating with the integrin-linked kinase, ILK1

ILKAP, a protein serine/threonine (S/T) phosphatase of the PP2C family, was isolated in a yeast two-hybrid screen baited with integrin-linked kinase, ILK1. Association of ILK1 and ILKAP was independent of the catalytic activity of either partner, as assayed in co-precipitation and two-hybrid experiments. Condi tional expression of ILKAP in HEK 293 cells resulted in selective inhibition of ECM- and growth factor-stimulated ILK1 activity, but did not inhibit Raf-1 kinase activity. A catalytic mutant of ILKAP, H154D, did not inhibit ILK1 kinase activity. Two cellular targets of ILK1, glycogen synthase kinase 3 beta (GSK3beta) and protein kinase B (PKB)/AKT, were differentially affected by ILKAP-mediated inhibition of ILK1. Catalytically active, but not mutant ILKAP, strongly inhibited insulin-like growth factor-1-stimulated GSK3beta phosphorylation on Ser9, but did not affect phosphorylation of PKB on Ser473, suggesting that ILKAP selectively affects ILK-mediated GSK3beta signalling. Consistent with this, active, but not H154D mutant or the related PP2Calpha, selectively inhibited transactivation of a Tcf/Lef reporter gene, TOPFlash, in 293 cells. We propose that ILKAP regulates ILK1 activity, targeting ILK1 signalling of Wnt pathway components via modulation of GSK3beta phosphorylation.

Antisense inhibition of protein phosphatase 2C accelerates cold acclimation in Arabidopsis thaliana

Two related protein phosphatases 2C, ABI1 and AtPP2CA have been implicated as negative regulators of ABA signalling. In this study we characterized the role of AtPP2CA in cold acclimation. The pattern of expression of AtPP2CA and ABI1 was studied in different tissues and in response to abiotic stresses. The expression of both AtPP2CA and ABI1 was induced by low temperature, drought, high salt and ABA. The cold and drought-induced expression of these genes was ABA-dependent, but divergent in various ABA signalling mutants. In addition, the two PP2C genes exhibited differences in their tissue-specific expression as well as in temporal induction in response to low temperature. To elucidate the function of AtPP2CA in cold acclimation further, the corresponding gene was silenced by antisense inhibition. Transgenic antisense plants exhibited clearly accelerated development of freezing tolerance. Both exposure to low temperature and application of ABA resulted in enhanced freezing tolerance in antisense plants. These plants displayed increased sensitivity to ABA both during development of frost tolerance and during seed germination, but not in their drought responses. Furthermore, the expression of cold-and ABA-induced genes was enhanced in transgenic antisense plants. Our results suggest that AtPP2CA is a negative regulator of ABA responses during cold acclimation.

Selection of apoptosis-deficient adenovirus E4orf4 mutants in Saccharomyces cerevisiae

Adenovirus E4orf4 protein has been shown to induce p53-independent, protein phosphatase 2A (PP2A)-dependent apoptosis in transformed cells. Furthermore, E4orf4 also induces toxicity in Saccharomyces cerevisiae in a PP2A-dependent manner (D. Kornitzer and T. Kleinberger, submitted for publication). In this work, we utilized yeast cells to select for nonapoptotic E4orf4 mutants which, in turn, were shown to possess a diminished ability to bind PP2A. The success of this selection system will provide additional apoptosis-relevant mutants for E4orf4 research and strongly supports the relevance of E4orf4-induced toxicity in S. cerevisiae to E4orf4-induced apoptosis in mammalian cells.

Fission yeast homologues of the B' subunit of protein phosphatase 2A: multiple roles in mitotic cell division and functional interaction with calcineurin

BACKGROUND

Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase distributed in eukaryotes from yeast to human, and plays pivotal roles in diverse cellular functions such as metabolism, cell cycle progression, gene expression and development. PP2A holoenzyme is a heterodimer of a catalytic subunit C and a regulatory subunit A, or a heterotrimer of C, A and a variable regulatory subunit consisting of three families; B, B', and PR72. Specific functions for each variable subunit are not well understood.

RESULTS

Two fission yeast genes pbp1+ and pbp2+ homologous to the regulatory subunit B' were isolated. Physical in vivo interaction of the gene products with the catalytic subunit was demonstrated. A double disruption haploid mutant (Deltapbp1Deltapbp2) showed growth defect, cell shape and size abnormality, multiseptation and anucleated cell formation due to abnormality in septum positioning. These phenotypes were suppressed by human B' cDNA, indicating the striking conservation of the B' function from yeast to human. Over-expression of fission yeast B' led to growth defects, a loss of cell shape polarity, septal abnormality and anucleated cell formation. Deltapbp1Deltapbp2 and pbp1 null haploids were hypersensitive to calcineurin inhibitors, cyclosporin A and FK506, with which the mutants underwent arrest at post-anaphase and cell lysis. Double disruption of calcineurin and pbp1+, but not pbp2+, genes led to synthetic lethality.

CONCLUSION

The fission yeast B' subunit of PP2A plays critical roles in cell shape control and septum formation, and shares essential functions with calcineurin for viability, possibly through their roles in cytokinesis and cell wall integrity.

Antisense inhibition of protein phosphatase 2C accelerates cold acclimation in Arabidopsis thaliana

Two related protein phosphatases 2C, ABI1 and AtPP2CA have been implicated as negative regulators of ABA signalling. In this study we characterized the role of AtPP2CA in cold acclimation. The pattern of expression of AtPP2CA and ABI1 was studied in different tissues and in response to abiotic stresses. The expression of both AtPP2CA and ABI1 was induced by low temperature, drought, high salt and ABA. The cold and drought-induced expression of these genes was ABA-dependent, but divergent in various ABA signalling mutants. In addition, the two PP2C genes exhibited differences in their tissue-specific expression as well as in temporal induction in response to low temperature. To elucidate the function of AtPP2CA in cold acclimation further, the corresponding gene was silenced by antisense inhibition. Transgenic antisense plants exhibited clearly accelerated development of freezing tolerance. Both exposure to low temperature and application of ABA resulted in enhanced freezing tolerance in antisense plants. These plants displayed increased sensitivity to ABA both during development of frost tolerance and during seed germination, but not in their drought responses. Furthermore, the expression of cold-and ABA-induced genes was enhanced in transgenic antisense plants. Our results suggest that AtPP2CA is a negative regulator of ABA responses during cold acclimation.

Multiple effects of protein phosphatase 2A on nutrient-induced signalling in the yeast Saccharomyces cerevisiae

The trehalose-degrading enzyme trehalase is activated upon addition of glucose to derepressed cells or in response to nitrogen source addition to nitrogen-starved glucose-repressed yeast (Saccharomyces cerevisiae) cells. Trehalase activation is mediated by phosphorylation. Inactivation involves dephosphorylation, as trehalase protein levels do not change upon multiple activation/inactivation cycles. Purified trehalase can be inactivated by incubation with protein phosphatase 2A (PP2A) in vitro. To test whether PP2A was involved in trehalase inactivation in vivo, we overexpressed the yeast PP2A isoform Pph22. Unexpectedly, the moderate (approximately threefold) overexpression of Pph22 that we obtained increased basal trehalase activity and rendered this activity unresponsive to the addition of glucose or a nitrogen source. Concomitant with higher basal trehalase activity, cells overexpressing Pph22 did not store trehalose efficiently and were heat sensitive. After the addition of glucose or of a nitrogen source to starved cells, Pph22-overexpressing cells showed a delayed exit from stationary phase, a delayed induction of ribosomal gene expression and constitutive repression of stress-regulated element-controlled genes. Deletion of the SCH9 gene encoding a protein kinase involved in nutrient-induced signal transduction restored glucose-induced trehalase activation in Pph22-overexpressing cells. Taken together, our results indicate that yeast PP2A overexpression leads to the activation of nutrient-induced signal transduction pathways in the absence of nutrients.

Activation of tomato PR and wound-related genes by a mutagenized tomato MAP kinase kinase through divergent pathways

A mitogen-activated protein kinase kinase (MAPKK) gene, tMEK2, was isolated from tomato cv. Bonny Best. By mutagenesis, a permanently active variant, tMEK2MUT, was created. Both wild-type tMEK2 and mutant tMEK2MUT were driven by a newly described strong plant constitutive promoter, tCUP, in a tomato protoplast transient gene expression system. Pathogenesis-related genes, PRlb1, PR3 and Twi1, and a wound-inducible gene, ER5, were activated by tMEK2MUT. Specific inhibitors of p38 class MAPK inhibited tMEK2MUT-induced activation of PR3 and ER5 genes but not that of the PRlb1 or Twi1 gene. Arabidopsis dual-specificity protein tyrosine phosphatase 1 (DsPTP1) and maize protein phosphatase 1 (PP1) inhibited tMEK2MUT-induced activation of the ER5 gene and the Twi1 gene, respectively, whereas PRlb1 and PR3 were not affected by either AtDsPTP1, or maize PP1, or Arabidopsis protein phosphatase 2A (PP2A). We have demonstrated for the first time that a single MAPKK activates an array of PR and wound-related genes. Our observation indicates that the activation of the genes downstream of tMEK2 occurs through divergent pathways and that tMEK2 may play an important role in the interaction of signal transduction pathways that mediate responses to both biotic (e.g. disease) and abiotic stresses (e.g. wound responsiveness).

Modulation of insulin-stimulated glycogen synthesis by Src Homology Phosphatase 2

We have examined the requirement of the protein tyrosine phosphatase Src Homology Phosphatase 2 (SHP2) for insulin-stimulated glycogen synthesis. To this end, 3T3L1 fibroblasts were stably transfected with either wild type or a catalytically inactive C463A-mutant of SHP2, and analysed for insulin-induced glycogen synthesis, tyrosine phosphorylation of the insulin receptor and IRS-1, and activation of phosphatidylinositol 3'-kinase (PI 3'-kinase). Glycogen synthesis was stimulated 9.1+/-0.9-fold by insulin in untransfected cells. In cells expressing the dominant-negative C463A-SHP2 mutant, the stimulation of glycogen synthesis by insulin was strongly enhanced (18.7+/-2.7-fold stimulation), while this response was impaired in cells overexpressing wild-type SHP2 (6.6+/-1.1-fold stimulation). When exploring the early post-receptor signalling pathways that contribute to glycogen synthesis, we found that insulin stimulated the tyrosine phosphorylation of IRS-1, and the activation of IRS-1-associated PI 3'-kinase more strongly in C463A-SHP2 expressing 3T3L1-cells (18.1+/-4.7-fold) than in parental 3T3L1 cells (6.8+/-0.5-fold). In 3T3L1 cells overexpressing wild-type SHP2, the insulin stimulation of IRS-1 tyrosine phosphorylation and the activation of PI 3'-kinase (4.5+/-1.0-fold) were impaired. An enhanced activity of SHP2 leads to negative modulation of insulin signalling by reducing the tyrosine phosphorylation of IRS-1 and the concomitant activation of PI 3'-kinase. This results in an impaired ability of insulin to stimulate glycogen synthesis.

A new role for protein methylation: switching partners at the phosphatase ball

Reversible protein methylation may be another posttranslational modification that serves an important role in modulating signal transduction pathways. Not only does protein phosphatase 2A (PP2A), a key regulator of many signal transduction cascades, undergo this modification, but the methylation process itself may be regulated by various cellular stimuli or states. Mumby describes how PP2A is regulated and how methylation may alter its subcellular targeting and substrate specificity by influencing its interaction with regulatory subunits.

Inhibition of antigen-receptor signaling by Platelet Endothelial Cell Adhesion Molecule-1 (CD31) requires functional ITIMs, SHP-2, and p56(lck)

Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1, CD31) is a 130-kd member of the immunoglobulin gene superfamily that is expressed on the surface of platelets, endothelial cells, myeloid cells, and certain lymphocyte subsets. PECAM-1 has recently been shown to contain functional immunoreceptor tyrosine-based inhibitory motifs (ITIMs) within its cytoplasmic domain, and co-ligation of PECAM-1 with the T-cell antigen receptor (TCR) results in tyrosine phosphorylation of PECAM-1, recruitment of Src homology 2 domain-containing protein tyrosine phosphatase-2 (SHP-2), and attenuation of TCR-mediated cellular signaling. To determine the molecular basis of PECAM-1 inhibitory signaling in lymphocytes, the study sought to (1) establish the importance of the PECAM-1 ITIMs for its inhibitory activity, (2) determine the relative importance of SHP-2 versus SHP-1 in mediating the inhibitory effect of PECAM-1, and (3) identify the protein tyrosine kinases required for PECAM-1 tyrosine phosphorylation in T cells. Co-ligation of wild-type PECAM-1 with the B-cell antigen receptor expressed on chicken DT40 B cells resulted in a marked reduction of calcium mobilization-similar to previous observations in T cells. In contrast, co-ligation of an ITIM-less form of PECAM-1 had no inhibitory effect. Furthermore, wild-type PECAM-1 was unable to attenuate calcium mobilization in SHP-2-deficient DT40 variants despite abundant levels of SHP-1 in these cells. Finally, PECAM-1 failed to become tyrosine phosphorylated in p56(lck)-deficient Jurkat T cells. Together, these data provide important insights into the molecular requirements for PECAM-1 regulation of antigen receptor signaling.

Alterations in protein phosphatase 2A subunit interaction in human carcinomas of the lung and colon with mutations in the A beta subunit gene

Protein phosphatase 2A (PP2A) consists of three subunits, A, B and C. The A and B subunits have regulatory functions while C is the catalytic subunit. PP2A core enzyme is composed of subunits A and C, and the holoenzyme of subunits A, B and C. All subunits exist as multiple isoforms or splice variants. The A subunit exists as two isoforms, A alpha and A beta. Here we report about the properties of eight A beta mutants, which were found in human lung and colon cancer. These mutants were reconstructed by site-directed mutagenesis and assayed for their ability to bind B and C subunits. Two mutants showed decreased binding of PR72, a member of the B" family of B subunits, but normal C subunit binding; two mutants exhibited decreased binding of the C subunit and of B"/PR72; and one mutant showed increased binding of both the C subunit and B"/PR72. Of three mutants that behaved like the wild-type A beta subunit, one is a polymorphic variant and another one is altered outside the binding region for B and C subunits. Importantly, we also found that the wild-type A alpha and A beta isoforms, although 85% identical, are remarkably different in their ability to bind B and C subunits. Our findings may have important implications in regard to the role of PP2A as a tumor suppressor.

A new protein phosphatase 2C (FsPP2C1) induced by abscisic acid is specifically expressed in dormant beechnut seeds

An abscisic acid (ABA)-induced cDNA fragment encoding a putative protein phosphatase 2C (PP2C) was obtained by means of differential reverse transcriptase-polymerase chain reaction approach. The full-length clone was isolated from a cDNA library constructed using mRNA from ABA-treated beechnut (Fagus sylvatica) seeds. This clone presents all the features of plant type PP2C and exhibits homology to members of this family such as AthPP2CA (61%), ABI1 (48%), or ABI2 (47%), therefore it was named FsPP2C1. The expression of FsPP2C1 is detected in dormant seeds and increases after ABA treatment, when seeds are maintained dormant, but it decreases and tends to disappear when dormancy is being released by stratification or under gibberellic acid treatment. Moreover, drought stress seems to have no effect on FsPP2C1 transcript accumulation. The FsPP2C1 transcript expression is tissue specific and was found to accumulate in ABA-treated seeds rather than in other ABA-treated vegetative tissues examined. These results suggest that the corresponding protein could be related to ABA-induced seed dormancy. By expressing FsPP2C1 in Escherichia coli as a histidine tag fusion protein, we have obtained direct biochemical evidence supporting Mg2+-dependent phosphatase activity of this protein.

PP2A mRNA expression is quantitatively decreased in Alzheimer's disease hippocampus

Since abnormal tau phosphorylation may play a role in neurofibrillary tangle (NFT) formation in aging and Alzheimer's disease (AD), we probed the distribution and abundance of protein phosphatase 2A (PP2A) catalytic (Calpha) and regulatory (PR55alpha and gamma, PR61varepsilon and delta) subunit mRNA in control and AD hippocampus using in situ hybridization. Quantitation of grain density per neuron area of PP2A subunits and beta-actin was determined for the CA3 region of hippocampus and cerebellum, while a qualitative assessment was performed for CA1, CA4, and dentate gyrus. All subunits are expressed in neurons, while PR55gamma and PR55alpha mRNA are also evident in glia. The expression levels of Calpha, all PP2A regulatory subunits studied, and beta-actin were similar in control and AD cerebellum. beta-Actin mRNA was, however, reduced in AD hippocampus. In addition to the generalized reduction of mRNA, as indicated by decreased beta-actin signal, there was a significant loss of Calpha, PR55gamma, and PR61epsilon mRNA in the CA3 hippocampus of AD. This study delineates the distribution of critical PP2A mRNAs and reveals a neuron- and subunit-specific reduction in PP2A catalytic and regulatory mRNA in AD hippocampus. This could result in decreased protein expression and phosphatase activity, leading to the hyperphosphorylation of tau and the formation of NFTs, as well as neuron degeneration in AD.

The Saccharomyces cerevisiae phosphotyrosyl phosphatase activator proteins are required for a subset of the functions disrupted by protein phosphatase 2A mutations

In Saccharomyces cerevisiae, PTPA is encoded by two genes, YPA1 and YPA2. In order to examine the biological role of PTPA as potential regulator of protein phosphatase 2A (PP2A), we compared the phenotypes of the ypaDelta mutants with these of PP2A-deficient strains. While deletion of both YPA genes is lethal, deletion of YPA1 alone results in a phenotype resembling that of PP2A-deficient strains in specific aspects such as aberrant bud morphology, abnormal actin distribution, and similar growth defects under various growth conditions. These phenotypes were even more pronounced when YPA1 was deleted in a pph21Delta genetic background. Moreover, ypaDelta mutants are hypersensitive to nocodazole and show inappropriate mitotic spindle formation as previously described for mutants in the catalytic subunit of PP2A, suggesting that Ypa, like PP2A, has a function in mitotic spindle formation. These results are consistent with an in vivo role of Ypa as a regulator of PP2A. However, unlike a PP2A-deficient strain, ypaDelta mutants do not show a G2 arrest. Therefore, Ypa does not seem to play a role in the regulation of PP2A at this stage of the cell cycle. These results imply that Ypa regulates a specific subset of PP2A functions, possibly by controlling the subunit composition of PP2A.

Enhanced activity and level of protein kinase A in the spinal cord supernatant of diisopropyl phosphorofluoridate (DFP)-treated hens. Distribution of protein kinases and phosphatases in spinal cord subcellular fractions

Diisopropyl phosphorofluoridate (DFP) is a type I organophosphorus compound and produces delayed neurotoxicity (OPIDN) in adult hens. A single dose of DFP (1.7 mg/kg, s.c.) produces mild ataxia in hens in 7-14 days, which develops into severe ataxia or paralysis as the disease progresses. We have previously shown altered expression of several proteins (e.g. Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) alpha-subunit, tau, tubulin, neurofilament protein (NF), vimentin, GFAP) and an immediate early gene (e.g. c-fos) in DFP-treated hens. Here we show an increase in protein kinase A (PKA) protein level and activity in the spinal cord at 1-day and 5-days time periods after DFP administration. We also determined the protein levels of protein kinase C (PKC), CaM kinase II and several phosphatases (i.e. phosphatase 1 (PP1), phosphatase 2A (PP2A), phosphatase 2B (PP2B) in the spinal cord of DFP-treated hens after 1, 5, 10, and 20 days). There was increase in CaM kinase II alpha subunit level after 10 and 20 days of treatment, and decrease in PKC level at 1-day and 20-days time periods in spinal cord mitochondria. In contrast, the cerebrum, which is resistant to DFP-induced axonal degeneration, did not show change in PKA and CaM Kinase II levels at any time period DFP post-administration. No alteration was found in the protein levels of PP1, PP2A, and PP2B at any time period. An early induction in PKA, which is an important protein kinase in signal transduction, followed by that of CaM kinase might be contributing towards the development of OPIDN in DFP-treated hens.

Protein phosphatases regulate DNA-dependent protein kinase activity

DNA-dependent protein kinase (DNA-PK) is a complex of DNA-PK catalytic subunit (DNA-PKcs) and the DNA end-binding Ku70/Ku80 heterodimer. DNA-PK is required for DNA double strand break repair by the process of nonhomologous end joining. Nonhomologous end joining is a major mechanism for the repair of DNA double strand breaks in mammalian cells. As such, DNA-PK plays essential roles in the cellular response to ionizing radiation and in V(D)J recombination. In vitro, DNA-PK undergoes phosphorylation of all three protein subunits (DNA-PK catalytic subunit, Ku70 and Ku80) and phosphorylation correlates with inactivation of the serine/threonine protein kinase activity of DNA-PK. Here we show that phosphorylation-induced loss of the protein kinase activity of DNA-PK is restored by the addition of the purified catalytic subunit of either protein phosphatase 1 or protein phosphatase 2A (PP2A) and that this reactivation is blocked by the potent protein phosphatase inhibitor, microcystin. We also show that treating human lymphoblastoid cells with either okadaic acid or fostriecin, at PP2A-selective concentrations, causes a 50-60% decrease in DNA-PK protein kinase activity, although the protein phosphatase 1 activity in these cells was unaffected. In vivo phosphorylation of DNA-PKcs, Ku70, and Ku80 was observed when cells were labeled with [(32)P]inorganic phosphate in the presence of the protein phosphatase inhibitor, okadaic acid. Together, our data suggest that reversible protein phosphorylation is an important mechanism for the regulation of DNA-PK protein kinase activity and that the protein phosphatase responsible for reactivation in vivo is a PP2A-like enzyme.

Crystal structure of the C-terminal domain of the RAP74 subunit of human transcription factor IIF

The x-ray structure of a C-terminal fragment of the RAP74 subunit of human transcription factor (TF) IIF has been determined at 1.02-A resolution. The alpha/beta structure is strikingly similar to the globular domain of linker histone H5 and the DNA-binding domain of hepatocyte nuclear factor 3gamma (HNF-3gamma), making it a winged-helix protein. The surface electrostatic properties of this compact domain differ significantly from those of bona fide winged-helix transcription factors (HNF-3gamma and RFX1) and from the winged-helix domains found within the RAP30 subunit of TFIIF and the beta subunit of TFIIE. RAP74 has been shown to interact with the TFIIF-associated C-terminal domain phosphatase FCP1, and a putative phosphatase binding site has been identified within the RAP74 winged-helix domain.

Protein phosphatase 2A isotypes regulate cell surface expression of the T cell receptor

The mechanisms underlying T cell receptor (TCR) down-regulation have been extensively studied during the last decade. Whereas the importance of phosphorylation in this process has been established, it is less certain whether dephosphorylation plays a role in TCR down-regulation. In this study, we show that inhibition of the serine/threonine protein phosphatase PP2A family had a biphasic effect on TCR expression. Thus, low concentrations of PP2A inhibitors induced TCR down-regulation, whereas higher concentrations of PP2A inhibitors induced TCR up-regulation. The effect of PP2A inhibition was independent of phosphorylation of the CD3gamma endocytosis motif. Whereas TCR down-regulation was caused by a partial inhibition of exocytosis, TCR up-regulation was caused by an inhibition of endocytosis. The effects on exocytosis and endocytosis were not restricted to the TCR, indicating a more general regulatory role for PP2A in both exocytosis and endocytosis.

PPP2R1B gene in chronic lymphocytic leukemias and mantle cell lymphomas

Deletion of chromosome bands 11q22-q23 is one of the most common structural chromosome alterations in chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL). The PPP2R1B gene is located very close to the minimal common deletion region of 11q22-q23 in CLL and MCL. Recently, the PPP2R1B gene was found to be mutated in human lung and colon cancers. To evaluate the role of the PPP2R1B gene in the pathogenesis of CLL and MCL, we performed RT-PCR analysis and cDNA sequencing on 10 CLL RNA samples and SSCP analysis on 26 CLL and 37 MCL genomic DNA samples. A deletion of exon 3 was found in one CLL sample. No mutation was detected in the SSCP analysis. To exclude the possibility of large genomic deletions we performed Southern blotting analysis. One MCL sample showed abnormal bands. Our results do not suggest that the PPP2R1B gene has a major pathogenic role in CLL and MCL.

Protein phosphatase 2A activates the proapoptotic function of BAD in interleukin- 3-dependent lymphoid cells by a mechanism requiring 14-3-3 dissociation

BAD is a proapoptotic member of the BCL-2 family of proteins, which play a major role in regulating apoptosis in cytokine-dependent hematopoietic cells. The function of BAD is regulated by reversible phosphorylation. Deprivation of survival factors induces BAD dephosphorylation, resulting in apoptosis. Serine-threonine phosphatase activity dephosphorylated BAD in interleukin-3-dependent FL5.12 lymphoid cells. Inhibition of PP2A activity by treatment of cells with PP2A-selective inhibitors, okadaic acid and fostriecin, prevented BAD dephosphorylation in these cells. Conversely, BAD dephosphorylation was not inhibited by the PP1-selective inhibitor tautomycin. In cell-free extracts, BAD phosphatase activity was also inhibited by the PP2A-selective inhibitors okadaic acid and fostriecin, but not by the PP1-specific protein inhibitor I-2. Dissociation of 14-3-3 from BAD was a prerequisite for BAD dephosphorylation in vitro, suggesting a mechanism by which 14-3-3 can regulate the activation of the proapoptotic function of BAD in vivo. Significantly, the inhibition of BAD phosphatase activity rescued cell death induced by survival factor withdrawal in FL5.12 cells expressing wild-type BAD but not phosphorylation-defective mutant BAD. These data indicate that PP2A, or a PP2A-like enzyme, dephosphorylates BAD and, in conjunction with 14-3-3, modulates cytokine-mediated survival.

Biological action of nitric oxide donor compounds on platelets from patients with sickle cell disease

Several lines of evidence point to the potential role of nitric oxide (NO) in the pathophysiology, as well as in the therapy, of sickle cell disease (SCD). In this study, we compared the effects of NO on platelets from normal individuals and from patients with SCD. Three NO donors were used to deliver NO to platelets: sodium 2-(N, N-diethylamino)-diazenolate-2-oxide (DEANO), S-nitrosocysteine (CysNO) and sodium trioxdintrate (OXINO or Angeli's salt). ADP-induced platelet aggregation, CD62P expression, PAC-1 binding and calcium elevation were evaluated in paired studies of normal and SCD subjects. DEANO significantly reduced aggregation in SCD platelets compared with normal platelets. DEANO similarly reduced the extent of CD62P expression in SCD platelets. All NO donors reduced PAC-1 binding, but there were no significant differences between platelets from normal or SCD subjects. Calcium elevation, as induced by ADP, was not altered by the presence of NO donors. However, when platelets were stimulated with thrombin, there was an increased initial response of SCD platelets compared with normal platelets. Taken together, these data suggest that the mode of NO delivery to platelets may produce various physiological responses and the optimization of NO delivery may contribute to reducing platelet aggregation in sickle cell disease.

Regulation of the TAK1 signaling pathway by protein phosphatase 2C

Protein phosphatase 2C (PP2C) is implicated in the negative regulation of stress-activated protein kinase cascades in yeast and mammalian cells. In this study, we determined the role of PP2Cbeta-1, a major isoform of mammalian PP2C, in the TAK1 signaling pathway, a stress-activated protein kinase cascade that is activated by interleukin-1, transforming growth factor-beta, or stress. Ectopic expression of PP2Cbeta-1 inhibited the TAK1-mediated mitogen-activated protein kinase kinase 4-c-Jun amino-terminal kinase and mitogen-activated protein kinase kinase 6-p38 signaling pathways. In vitro, PP2Cbeta-1 dephosphorylated and inactivated TAK1. Coimmunoprecipitation experiments indicated that PP2Cbeta-1 associates with the central region of TAK1. A phosphatase-negative mutant of PP2Cbeta-1, PP2Cbeta-1 (R/G), acted as a dominant negative mutant, inhibiting dephosphorylation of TAK1 by wild-type PP2Cbeta-1 in vitro. In addition, ectopic expression of PP2Cbeta-1(R/G) enhanced interleukin-1-induced activation of an AP-1 reporter gene. Collectively, these results indicate that PP2Cbeta negatively regulates the TAK1 signaling pathway by direct dephosphorylation of TAK1.

Activation of mitogen-activated protein kinase phosphatase 2 by gonadotropin-releasing hormone

The aim of these studies was to identify the signaling mechanism(s) that contribute to GnRH-induced expression of MAPK phosphatase (MKP)-2, a dual specificity phosphatase that selectively inactivates MAPKs. GnRH receptor activation induced MKP-2 expression in both clonal (alphaT3-1) and primary gonadotropes. Activation of PKC isozymes was sufficient and required for MKP-2 induction. Inhibition of the extracellular signal-regulated kinase (ERK) or c-Jun N-terminal kinase (JNK) but not the p38 MAPK cascade was sufficient to block GnRH-induced MKP-2 expression. Induction of MKP-2 by GnRH was dependent on elevation in intracellular Ca(2+). Inhibition of Ca(2+) influx through L-type voltage-gated calcium channels blocked GnRH-induced MKP-2 expression. Depletion of intracellular Ca(2+) stores with thapsigargin blocked MKP-2 activation by GnRH independent of ERK and JNK activity. These results support the conclusion that MKP-2 induction by GnRH occurs via MAPK-dependent and -independent pathways. One mechanism requires GnRH-induced ERK and JNK activation, while a second MAPK-independent pathway requires a thapsigargin-sensitive calcium signal.

Oligobodies: bench made synthetic antibodies

Using synthetic peptides and a combinatorial library of 56 mer random oligonucleotides, we have developed reagents that behave as "synthetic antibodies". The results obtained with the protein phosphatase 2A as a model system are shown here. The specificity of these reagents, named "oligobodies", has been demonstrated by Western blot analysis and immunohistochemistry. The oligobodies have enormous advantages compared to antibodies: their production is independent of the immune system, they can be prepared in a few days and there is no need for a purified target protein. These reagents can be produced even if the corresponding protein was never isolated or purified, since only a partial DNA sequence from a database provides enough information to make them.