Transcriptional regulation of Bim by FoxO3A mediates hepatocyte lipoapoptosis

Hepatocyte lipoapoptosis, a critical feature of nonalcoholic steatohepatitis, can be replicated in vitro by incubating hepatocytes with saturated free fatty acids (FFA). These toxic FFA induce Bim expression, which is requisite for their cytotoxicity. Because the FoxO3a transcription factor has been implicated in Bim expression, our aim was to determine if FFA induce Bim by a FoxO3a-dependent mechanism. In Huh-7 cells, the saturated FFA, palmitic and stearic acid, increased Bim mRNA 16-fold. Treatment of cells with the saturated FFA induced FoxO3a dephosphorylation (activation) and nuclear translocation and stimulated a FoxO luciferase-based reporter assay; direct binding of FoxO3a to the Bim promoter was also confirmed by a chromatin immunoprecipitation assay. A small interfering RNA-targeted knockdown of FoxO3a abrogated FFA-mediated Bim induction and apoptosis. FoxO3a was activated by a phosphatase 2A-dependent mechanism, since okadaic acid- and small interfering RNA-targeted knockdown of this phosphatase blocked FoxO3a dephosphorylation, Bim expression, and apoptosis. Consistent with these data, phosphatase 2A activity was also stimulated 3-fold by saturated FFA. Immunoprecpitation studies revealed an FFA-dependent association between FoxO3a and protein phosphatase 2A. FFA-mediated FoxO3a activation by protein phosphatase 2A was also observed in HepG2 cells and murine hepatocytes. In conclusion, saturated FFA stimulate protein phosphatase 2A activity, which activates FoxO3a, inducing expression of the intracellular death mediator Bim.

The structure of Tap42/alpha4 reveals a tetratricopeptide repeat-like fold and provides insights into PP2A regulation

Physiological functions of protein phosphatase 2A (PP2A) are determined via the association of its catalytic subunit (PP2Ac) with diverse regulatory subunits. The predominant form of PP2Ac assembles into a heterotrimer comprising the scaffolding PR65/A subunit together with a variable regulatory B subunit. A distinct population of PP2Ac associates with the Tap42/alpha4 subunit, an interaction mutually exclusive with that of PR65/A. Tap42/alpha4 is also an interacting subunit of the PP2Ac-related phosphatases, PP4 and PP6. Tap42/alpha4, an essential protein in yeast and suppressor of apoptosis in mammals, contributes to critical cellular functions including the Tor signaling pathway. Here, we describe the crystal structure of the PP2Ac-interaction domain of Saccharomyces cerevisiae Tap42. The structure reveals an all alpha-helical protein with striking similarity to 14-3-3 and tetratricopeptide repeat (TPR) proteins. Mutational analyses of structurally conserved regions of Tap42/alpha4 identified a positively charged region critical for its interactions with PP2Ac. We propose a scaffolding function for Tap42/alpha4 whereby the interaction of PP2Ac at its N-terminus promotes the dephosphorylation of substrates recruited to the C-terminal region of the molecule.

p21-Activated kinase-1 and its role in integrated regulation of cardiac contractility

We review here a novel concept in the regulation of cardiac contractility involving variations in the activity of the multifunctional enzyme, p21-activated kinase 1 (Pak1), a member of a family of proteins in the small G protein-signaling pathway that is activated by Cdc42 and Rac1. There is a large body of evidence from studies in noncardiac tissue that Pak1 activity is key in regulation of a number of cellular functions, including cytoskeletal dynamics, cell motility, growth, and proliferation. Although of significant potential impact, the role of Pak1 in regulation of the heart has been investigated in only a few laboratories. In this review, we discuss the structure of Pak1 and its sites of posttranslational modification and molecular interactions. We assemble an overview of the current data on Pak1 signaling in noncardiac tissues relative to similar signaling pathways in the heart, and we identify potential roles of Pak1 in cardiac regulation. Finally, we discuss the current state of Pak1 research in the heart in regard to regulation of contractility through functional myofilament and Ca(2+)-flux modification. An important aspect of this regulation is the modulation of kinase and phosphatase activity. We have focused on Pak1 regulation of protein phosphatase 2A (PP2A), which is abundant in cardiac muscle, thereby mediating dephosphorylation of sarcomeric proteins and sensitizing the myofilaments to Ca(2+). We present a model for Pak1 signaling that provides a mechanism for specifically affecting cardiac cellular processes in which regulation of protein phosphorylation states by PP2A dephosphorylation predominates.

Differential expression of protein phosphatase type 1 isotypes and nucleolin during cell cycle arrest

In the present study, we examined the expression and cytolocalization of protein phosphatase type 1 (PP1) isoforms and nucleolin in human osteoblastic cell line MG63 cells at two boundaries in the cell cycle. We treated MG63 cells with hydroxyurea and nocodazole to arrest the cells at the G(1)/S and G(2)/M boundaries, respectively. As judged from the results of Western blot analysis, PP1 isoforms were expressed differently at each boundary of the cell cycle. Nucleolin was also shown to have a different expression pattern at each boundary. In the hydroxyurea-treated cells, nucleolus-like bodies were bigger in size and decreased in number compared with those in asynchronized cells. However, the subcellular localization of PP1s and nucleolin was not changed. Anti-nucleolin antibody interacted with 110-kDa and 95-kDa proteins present in asynchronized cells and in the cells treated with hydroxyurea. Treatment of the cells with nocodazole decreased the level of the 95-kDa form of nucleolin. In the nocodazole-treated cells, it was impossible to distinguish the distribution of each protein. The phosphorylation status of nucleolin in the cell cycle arrested samples was examined by 2D-IEF-PAGE followed by Western blot analysis. In the case of asynchronized cells or hydroxyurea-treated ones, nucleolin was located at a basic isoelectric point (dephosphorylated status); whereas in the G(2)/M arrest cells, the isoelectric point of nucleolin shifted to an acidic status, indicating that nucleolin was phosphorylated. The present results indicate that PP1 and nucleolin were differently expressed at G(1)/S and G(2)/M boundaries of the cell cycle and acted in a different fashion during cell-cycle progression.

Truncated form of the Epstein-Barr virus protein EBNA-LP protects against caspase-dependent apoptosis by inhibiting protein phosphatase 2A

The Epstein-Barr virus (EBV)-encoded leader protein, EBNA-LP, strongly activates the EBNA2-mediated transcriptional activation of cellular and viral genes and is therefore important for EBV-induced B-cell transformation. However, a truncated form of EBNA-LP is produced in cells infected with variant EBV strains lacking EBNA2 due to a genetic deletion. The function of this truncated form is unknown. We show here that some Burkitt's lymphoma cells harboring defective EBV strains are specifically resistant to the caspase-dependent apoptosis induced by verotoxin 1 (VT-1) or staurosporine. These cells produced low-molecular-weight Y1Y2-truncated isoforms of EBNA-LP, which were partly localized in the cytoplasm. The transfection of sensitive cells with constructs encoding truncated EBNA-LP isoforms, but not full-length EBNA-LP, induced resistance to caspase-mediated apoptosis. Furthermore, VT-1 induced protein phosphatase 2A (PP2A) activation in sensitive cells but not in resistant cells, in which the truncated EBNA-LP interacted with this protein. Thus, the resistance to apoptosis observed in cells harboring defective EBV strains most probably results from the inactivation of PP2A via interactions with low-molecular-weight Y1Y2-truncated EBNA-LP isoforms.

Phosphorylation of Galphas influences its association with the micro-opioid receptor and is modulated by long-term morphine exposure

The recent biochemical demonstration of the association of the mu-opioid receptor (MOR) with Galpha(s) that increases after long-term morphine treatment (Mol Brain Res 135:217-224, 2005) provides a new imperative for studying MOR-Galpha(s) interactions and the mechanisms that modulate it. A persisting challenge is to elucidate those neurochemical parameters modulated by long-term morphine treatment that facilitate MOR-Galpha(s) association. This study demonstrates that 1) Galpha(s) exists as a phosphoprotein, 2) the stoichiometry of Galpha(s) phosphorylation decreases after long-term morphine treatment, and 3) in vitro dephosphorylation of Galpha(s) increases its association with MOR. Furthermore, our data suggest that increased association of Galpha(s) with protein phosphatase 2A is functionally linked to the long-term morphine treatment-induced reduction in Galpha(s) phosphorylation. These findings are observed in MOR-Chinese hamster ovary and F11 cells as well as spinal cord, indicating that they are not idiosyncratic to the particular cell line used or a "culture" phenomenon and generalize to complex neural tissue. Taken together, these results indicate that the phosphorylation state of Galpha(s) is a critical determinant of its interaction with MOR. Long-term morphine treatment decreases Galpha(s) phosphorylation, which is a key mechanism underlying the previously demonstrated increased association of MOR and Galpha(s) in opioid tolerant tissue.

Galpha12 stimulates apoptosis in epithelial cells through JNK1-mediated Bcl-2 degradation and up-regulation of IkappaBalpha

Apoptosis is an essential mechanism for the maintenance of somatic tissues, and when dysregulated can lead to numerous pathological conditions. G proteins regulate apoptosis in addition to other cellular functions, but the roles of specific G proteins in apoptosis signaling are not well characterized. Galpha12 stimulates protein phosphatase 2A (PP2A), a serine/threonine phosphatase that modulates essential signaling pathways, including apoptosis. Herein, we examined whether Galpha12 regulates apoptosis in epithelial cells. Inducible expression of Galpha12 or constitutively active (QL)alpha12 in Madin-Darby canine kidney cells led to increased apoptosis with expression of QLalpha12, but not Galpha12. Inducing QLalpha12 led to degradation of the anti-apoptotic protein Bcl-2 (via the proteasome pathway), increased JNK activity, and up-regulated IkappaBalpha protein levels, a potent stimulator of apoptosis. Furthermore, the QLalpha12-stimulated activation of JNK was blocked by inhibiting PP2A. To characterize endogenous Galpha12 signaling pathways, non-transfected MDCK-II and HEK293 cells were stimulated with thrombin. Thrombin activated endogenous Galpha12 (confirmed by GST-tetratricopeptide repeat (TPR) pull-downs) and stimulated apoptosis in both cell types. The mechanisms of thrombin-stimulated apoptosis through endogenous Galpha12 were nearly identical to the mechanisms identified in QLalpha12-MDCK cells and included loss of Bcl-2, JNK activation, and up-regulation of IkappaBalpha. Knockdown of the PP2A catalytic subunit in HEK293 cells inhibited thrombin-stimulated apoptosis, prevented JNK activation, and blocked Bcl-2 degradation. In summary, Galpha12 has a major role in regulating epithelial cell apoptosis through PP2A and JNK activation leading to loss of Bcl-2 protein expression. Targeting these pathways in vivo may lead to new therapeutic strategies for a variety of disease processes.

Protein phosphatase PP6 N terminal domain restricts G1 to S phase progression in human cancer cells

Yeast SIT4 is an essential gene encoding a protein Ser/Thr phosphatase conserved throughout eukaryotic evolution, known as PPV in Drosophila and PP6 in vertebrates. Sit4 promotes transcription of G1 cyclins and a sit4(ts) strain exhibits a G1 arrest at the restrictive temperature. The yeast sit4(ts) was rescued by expression of PPV or a chimeric phosphatase containing the first fifty-three residues of PPV fused to Drosophila PP1. The results suggested that the N terminus of the Sit4/PPV protein exerts a specific function in the yeast cell cycle. Here we tested whether the N terminus of human PP6 exerts specific effects on G1-S progression in human cells. The N terminus of PP6 or PP2A was fused to GFP and the proteins transiently expressed in prostate cancer PC-3 cells. Expression of the PP6 fusion protein was restricted to lower levels than either the PP2A fusion protein or GFP. However, the PP6 fusion protein blocked entry into S phase and increased by >20% the proportion of cells in G1 phase. Expression of the PP6 fusion protein did not significantly change the levels of transcripts for cyclins or ca. eighty other cell cycle genes, but did suppress the levels of cyclin D1 protein in cells in G1 phase and reduce the phosphorylation of RB1 at Ser807/811. Thus, our results provide evidence that PP6 regulates cell cycle progression in human cells at least in part through control of cyclin D1 and the function of PP6 is distinct from its homolog Sit4 in yeast.

Cytokine activation of p38 mitogen-activated protein kinase and apoptosis is opposed by alpha-4 targeting of protein phosphatase 2A for site-specific dephosphorylation of MEK3

alpha-4 is an essential gene and is a dominant antiapoptotic factor in various tissues that is a regulatory subunit for type 2A protein phosphatases. A multiplexed phosphorylation site screen revealed that knockdown of alpha-4 by small interfering RNA (siRNA) increased p38 mitogen-activated protein kinase (MAPK) and c-Jun phosphorylation without changes in JNK or ERK. FLAG-alpha-4 coprecipitated hemagglutinin-MEK3 plus endogenous protein phosphatase 2A (PP2A) and selectively enhanced dephosphorylation of Thr193, but not Ser189, in the activation loop of MEK3. Overexpression of alpha-4 suppressed p38 MAPK activation in response to tumor necrosis factor alpha (TNF-alpha). The alpha-4 dominant-negative domain (DND) (residues 220 to 340) associated with MEK3, but not PP2A, and its overexpression sensitized cells to activation of p38 MAPK by TNF-alpha and interleukin-1beta, but not by ansiomycin or sorbitol. The response was diminished by nocodazole or by siRNA knockdown of the Opitz syndrome protein Mid1 that binds alpha-4 to microtubules. Interference by alpha-4 DND or alpha-4 siRNA increased caspase 3/7 activation in response to TNF-alpha. Growth of transformed cells in soft agar was enhanced by alpha-4 and suppressed by alpha-4 DND. The results show that alpha-4 targets PP2A activity to MEK3 to suppress p38 MAPK activation by cytokines, thereby inhibiting apoptosis and anoikis.

A Molecular Link between E2F-1 and the MAPK Cascade

Transcription factor E2F-1 mediates apoptosis and suppresses tumorigenesis. The mechanisms by which E2F-1 functions in these processes are largely unclear. We report here that E2F-1 acts as a transcriptional regulator of MKP-2 (MAPK phosphatase-2), a dual specificity protein phosphatase (DUSP4) with stringent substrate specificity for MAPKs. We show that E2F-1 is required for the cellular apoptotic response to oxidative damage. MKP-2 is greatly increased following oxidative stress, and E2F-1 is necessary for that induction. We found that E2F-1 is physically associated with the MKP-2 promoter and can transactivate the promoter of the MKP-2 gene. Specifically, E2F-1 binds to a perfect palindromic motif in the MKP-2 promoter. Finally, we show that this E2F-1/MKP-2 pathway mediates apoptosis under oxidative stress and that MKP-2 suppresses tumor formation in nude mice. Our findings demonstrate that E2F-1 is a transcriptional activator of MKP-2 and that MKP-2 is an essential cell death mediator in the E2F-1 pathway. Characterization of MKP-2 as a cell death mediator may lead to the development of new strategies for cancer treatment.

Protein kinase C delta negatively regulates tyrosine hydroxylase activity and dopamine synthesis by enhancing protein phosphatase-2A activity in dopaminergic neurons

Tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine synthesis, can be regulated by phosphorylation at multiple serine residues, including serine-40. In the present study, we report a novel interaction between a key member of the novel PKC family, protein kinase Cdelta (PKCdelta), and TH, in which the kinase modulates dopamine synthesis by negatively regulating TH activity via protein phosphatase 2A (PP2A). We observed that PKCdelta is highly expressed in nigral dopaminergic neurons and colocalizes with TH. Interestingly, suppression of PKCdelta activity with the kinase inhibitor rottlerin, PKCdelta-small interfering RNA, or with PKCdelta dominant-negative mutant effectively increased a number of key biochemical events in the dopamine pathway, including TH-ser40 phosphorylation, TH enzymatic activity, and dopamine synthesis in neuronal cell culture models. Additionally, we found that PKCdelta not only physically associates with the PP2A catalytic subunit (PP2Ac) but also phosphorylates the phosphatase to increase its activity. Notably, inhibition of PKCdelta reduced the dephosphorylation activity of PP2A and thereby increased TH-ser40 phosphorylation, TH activity, and dopamine synthesis. To further validate our findings, we used the PKCdelta knock-out (PKCdelta-/-) mouse model. Consistent with other results, we found greater TH-ser40 phosphorylation and reduced PP2A activity in the substantia nigra of PKCdelta-/- mice than in wild-type mice. Importantly, this was accompanied by an increased dopamine level in the striatum of PKCdelta-/- mice. Collectively, these results suggest that PKCdelta phosphorylates PP2Ac to enhance its activity and thereby reduces TH-ser40 phosphorylation and TH activity and ultimately dopamine synthesis.

The budding yeast PP2ACdc55 protein phosphatase prevents the onset of anaphase in response to morphogenetic defects

Faithful chromosome transmission requires establishment of sister chromatid cohesion during S phase, followed by its removal at anaphase onset. Sister chromatids are tethered together by cohesin, which is displaced from chromosomes through cleavage of its Mcd1 subunit by the separase protease. Separase is in turn inhibited, up to this moment, by securin. Budding yeast cells respond to morphogenetic defects by a transient arrest in G2 with high securin levels and unseparated chromatids. We show that neither securin elimination nor forced cohesin cleavage is sufficient for anaphase in these conditions, suggesting that other factors contribute to cohesion maintainance in G2. We find that the protein phosphatase PP2A bound to its regulatory subunit Cdc55 plays a key role in this process, uncovering a new function for PP2A(Cdc55) in controlling a noncanonical pathway of chromatid cohesion removal.

Regulation of L-Type Calcium Channel and Delayed Rectifier Potassium Channel Activity by p 21 -Activated Kinase-1 in Guinea Pig Sinoatrial Node Pacemaker Cells

Phosphorylation of ion channels plays an important role in the regulation of cardiac function, but signaling mechanisms controlling dephosphorylation are not well understood. We have tested the hypothesis that p 21 -activated kinase-1 (Pak1), a serine–threonine protein kinase regulated by Ras-related small G proteins, regulates sinoatrial node (SAN) ion channel activity through a mechanism involving protein phosphatase 2A. We report a novel role of Pak1-mediated signaling in attenuating isoproterenol-induced enhancement of L-type Ca 2+ current ( I CaL ) and delayed rectifier potassium current ( I K ) in guinea pig SAN pacemaker cells. We demonstrate that in guinea pig SAN: (1) there is abundant expression of endogenous Pak1 in pacemaker cells; (2) expression of constitutively active Pak1 depresses isoproterenol-induced upregulation of I CaL and I K ; (3) inhibition of protein phosphatase 2A increases the enhancement of I K and I CaL by isoproterenol in Ad-Pak1–infected cells; (4) protein phosphatase 2A coimmunoprecipitates with endogenous Pak1 in SAN tissue; and (5) expression of constitutively active Pak1 suppresses the chronotropic action of isoproterenol on pacemaker activity of intact SAN preparations. In conclusion, our data demonstrate that a Pak1 signaling pathway exists in cardiac pacemaker cells and that this novel pathway plays a role in the regulation of ion channel activity.

Disruption of microtubule network by Alzheimer abnormally hyperphosphorylated tau

Hyperphosphorylated tau has long been proposed as the key molecule disrupting normal neuronal microtubule dynamics and leading to neurofibrillary degeneration in Alzheimer disease. Here we provide a direct evidence of hyperphosphorylated tau-induced disruption of microtubule network. Using Nocodozole-treated and detergent-extracted cells, we created a neuronal environment in mouse embryonic fibroblasts, 3T3 cells, by replacing their cytoplasm with adult rat brain cytosol. By recreating neuronal microtubule network in these cells, we were able to follow the effects of hyperphosphorylated tau on microtubule dynamics in real time. Whereas recombinant human brain tau promoted assembly and bundling of microtubules, abnormally hyperphosphorylated tau isolated from Alzheimer disease brain cytosol (AD P-tau) inhibited the assembly and disrupted preformed microtubule network by sequestering normal brain tau and MAP2. This breakdown of the microtubule network was reversed by treatment of the extracted cells with protein phosphatase-2A. This study, for the first time, provides direct mechanistic insights into the molecular basis of both axonal and dendritic neurodegeneration seen in Alzheimer disease.

Ligand binding to the androgen receptor induces conformational changes that regulate phosphatase interactions

We describe a mechanism for protein phosphatase 2A (PP2A) targeting to the androgen receptor (AR) and provide insight into the more general issue of kinase and phosphatase interactions with AR. Simian virus 40 (SV40) small t antigen (ST) binding to N-terminal HEAT repeats in the PP2A A subunit induces structural changes transduced to C-terminal HEAT repeats. This enables the C-terminal HEAT repeats in the PP2A A subunit, including HEAT repeat 13, to discriminate between androgen- and androgen antagonist-induced AR conformations. The PP2A-AR interaction was used to show that an AR mutant in prostate cancer cells (T877A) is activated by multiple ligands without acquiring the same conformation as that induced by androgen. The correlation between androgen binding to AR and increased phosphorylation of the activation function 1 (AF-1) region implies that changes in AR conformation or chaperone composition are causal to kinase access to phosphorylation sites. However, AF-1 phosphorylation sites are kinase accessible prior to androgen binding. This suggests that androgens can enhance the phosphorylation state of AR either by negatively regulating the ability of the ligand-binding domain to bind phosphatases or by inducing an AR conformation that is resistant to phosphatase action. SV40 ST subverts this mechanism by promoting the direct transfer of PP2A onto androgen-bound AR, resulting in multisite dephosphorylation.

Clarification of the C-35 stereochemistries of dinophysistoxin-1 and dinophysistoxin-2 and its consequences for binding to protein phosphatase

Okadaic acid analogues are well known as protein phosphatase inhibitors and occur naturally in marine shellfish feeding on dinoflagellates of the genus Dinophysis, leading to diarrhetic shellfish poisoning of shellfish consumers. Knowledge of the correct structures for these toxins is important in understanding their toxicology, biochemistry, and biosynthesis. We have performed extensive NMR analyses on okadaic acid (1), dinophysistoxin-1 (DTX-1), and dinophysistoxin-2 (DTX-2) obtained from natural sources. Consequently, we were able to unambiguously deduce the stereochemistries at C-35 for DTX-1 and DTX-2 based on analysis of NMR coupling constants and NOE interactions. Our results revealed that DTX-2 (3) has a stereochemistry opposite to that of DTX-1 (2) at C-35. Molecular modeling of the docking of 1-3 with protein phosphatase-1 and protein phosphatase 2A (PP2A) suggested that the reduced affinity of DTX-2 for PP2A may be due to the newly defined stereochemistry at the 35-methyl group. The implications of these findings for biosynthesis and toxicology are discussed.

Analysis of 11q22-q23 deletion target genes in B-cell chronic lymphocytic leukaemia: evidence for a pathogenic role of NPAT, CUL5, and PPP2R1B

Deletion of 11q22-q23 is associated with an aggressive course of B-cell chronic lymphocytic leukaemia (B-CLL). Since only in a subset of these cases biallelic inactivation of ATM was observed, we sought to identify other disease-associated genes within 11q22-q23 by analysing NPAT (cell-cycle regulation), CUL5 (ubiquitin-dependent apoptosis regulation) and PPP2R1B (component of the cell-cycle and apoptosis regulating PP2A) for point mutations and their expression in B-CLL by single-strand conformation polymorphism/sequence analysis of the transcripts and real-time polymerase chain reaction. Though none of the genes were affected by deleterious mutations, we observed a significant down-regulation of NPAT in B-CLL versus CD19+ B cells and of CUL5 in 11q deletion versus non-deletion B-CLL samples and measured reduced PPP2R1B transcript levels in a subset of B-CLL cases. Alternative splicing of PPP2R1B transcripts (skipping of exons 2/3, 3, 9) was associated with a reduced activity of protein phosphatase 2A. Together, these results implicate deregulation of the cell-cycle and apoptosis regulators NPAT, CUL5 and PPP2R1B and a role for these genes in the pathogenesis of B-CLL.

Mitotic phosphorylation: breaking the balance of power by a tactical retreat

Profound changes in the phosphorylation state of many proteins occur during mitosis. It is well established that many of these mitotic phosphorylations are carried out by archetypal mitotic kinases that are activated only during mitosis, shifting the equilibrium of kinases and phosphatases towards phosphorylation. However, many studies have also detailed the phosphorylation of proteins at mitosis by kinases that are constitutively active throughout the cell cycle. In most cases, it is uncertain how kinases and phosphatases that appear to be constitutively active can induce phosphorylations specifically at mitosis. In this issue of the Biochemical Journal, Escargueil and Larsen provide evidence of an interesting alternative mechanism to attain specific mitotic phosphorylation. A mitosis-specific phosphorylation site in DNA topoisomerase IIalpha, which is recognized by the MPM-2 antibody, is phosphorylated by protein kinase CK2. The authors found that phosphorylation of this site is suppressed during interphase due to competing dephosphorylation by protein phosphatase 2A. Interestingly, protein phosphatase 2A is excluded from the nucleus during early mitosis, allowing CK2 to phosphorylate topoisomerase IIalpha. It is possible that similar mechanisms are used to regulate the phosphorylation of other proteins.

Mitosis-specific MPM-2 phosphorylation of DNA topoisomerase IIalpha is regulated directly by protein phosphatase 2A

Recent results suggest a role for topoIIalpha (topoisomerase IIalpha) in the fine-tuning of mitotic entry. Mitotic entry is accompanied by the formation of specific phosphoepitopes such as MPM-2 (mitotic protein monoclonal 2) that are believed to control mitotic processes. Surprisingly, the MPM-2 kinase of topoIIalpha was identified as protein kinase CK2, otherwise known as a constitutive interphase kinase. This suggested the existence of alternative pathways for the creation of mitotic phosphoepitopes, different from the classical pathway where the substrate is phosphorylated by a mitotic kinase. In the present paper, we report that topoIIalpha is co-localized with both CK2 and PP2A (protein phosphatase 2A) during interphase. Simultaneous incubation of purified topoIIalpha with CK2 and PP2A had minimal influence on the total phosphorylation levels of topoIIalpha, but resulted in complete disappearance of the MPM-2 phosphoepitope owing to opposite sequence preferences of CK2 and PP2A. Accordingly, short-term exposure of interphase cells to okadaic acid, a selective PP2A inhibitor, was accompanied by the specific appearance of the MPM-2 phosphoepitope on topoIIalpha. During early mitosis, PP2A was translocated from the nucleus, while CK2 remained in the nucleus until pro-metaphase thus permitting the formation of the MPM-2 phosphoepitope. These results underline the importance of protein phosphatases as an alternative way of creating cell-cycle-specific phosphoepitopes.

HoxA10 activates transcription of the gene encoding mitogen-activated protein kinase phosphatase 2 (Mkp2) in myeloid cells

HoxA10 is a homeodomain transcription factor that is frequently overexpressed in human acute myeloid leukemia. In murine bone marrow transplantation studies, HoxA10 overexpression induces a myeloproliferative disorder with accumulation of mature phagocytes in the peripheral blood and tissues. Over time, differentiation block develops in these animals, resulting in acute myeloid leukemia. In immature myeloid cells, HoxA10 represses transcription of some genes that confer the mature phagocyte phenotype. Therefore, overexpressed HoxA10 blocks differentiation by repressing myeloid-specific gene transcription in differentiating myeloid cells. In contrast, target genes involved in myeloproliferation due to HoxA10 overexpression have not been identified. To identify such genes, we screened a CpG island microarray with HoxA10 co-immunoprecipitating chromatin. We identified the DUSP4 gene, which encodes mitogen-activated protein kinase phosphatase 2 (Mkp2), as a HoxA10 target gene. We analyzed the DUSP4 5'-flank and identified two proximal-promoter cis elements that are activated by HoxA10. We find that DUSP4 transcription and Mkp2 expression decrease during normal myelopoiesis. However, this down-regulation is impaired in myeloid cells overexpressing HoxA10. In hematopoietic cells, c-Jun N-terminal kinases (Jnk) are the preferred substrates for Mkp2. Therefore, Mkp2 inhibits apoptosis by dephosphorylating (inactivating) Jnk. Consistent with this, HoxA10 overexpression decreases apoptosis in differentiating myeloid cells. Therefore, our studies identify a mechanism by which overexpressed HoxA10 contributes to inappropriate cell survival during myelopoiesis.

Anesthesia leads to tau hyperphosphorylation through inhibition of phosphatase activity by hypothermia

Postoperative cognitive dysfunction, confusion, and delirium are common after general anesthesia in the elderly, with symptoms persisting for months or years in some patients. Even middle-aged patients are likely to have postoperative cognitive dysfunction for months after surgery, and Alzheimer's disease (AD) patients appear to be particularly at risk of deterioration after anesthesia. Several investigators have thus examined whether general anesthesia is associated with AD, with some studies suggesting that exposure to anesthetics may increase the risk of AD. However, little is known on the biochemical consequences of anesthesia on pathogenic pathways in vivo. Here, we investigated the effect of anesthesia on tau phosphorylation and amyloid precursor protein (APP) metabolism in mouse brain. We found that, regardless of the anesthetic used, anesthesia induced rapid and massive hyperphosphorylation of tau, rapid and prolonged hypothermia, inhibition of Ser/Thr PP2A (protein phosphatase 2A), but no changes in APP metabolism or Abeta (beta-amyloid peptide) accumulation. Reestablishing normothermia during anesthesia completely rescued tau phosphorylation to normal levels. Our results indicate that changes in tau phosphorylation were not a result of anesthesia per se, but a consequence of anesthesia-induced hypothermia, which led to inhibition of phosphatase activity and subsequent hyperphosphorylation of tau. These findings call for careful monitoring of core temperature during anesthesia in laboratory animals to avoid artifactual elevation of protein phosphorylation. Furthermore, a thorough examination of the effect of anesthesia-induced hypothermia on the risk and progression of AD is warranted.

Molecular basis for PP2A regulatory subunit B56alpha targeting in cardiomyocytes

Protein phosphatase 2A (PP2A) is a multifunctional protein phosphatase with critical roles in excitable cell signaling. In the heart, PP2A function is linked with modulation of beta-adrenergic signaling and has been suggested to regulate key ion channels and transporters including Na/Ca exchanger, ryanodine receptor, inositol 1,4,5-trisphosphate receptor, and Na/K ATPase. Although many of the functional roles and molecular targets for PP2A in heart are known, little is established regarding the cellular pathways that localize specific PP2A isoform activities to subcellular sites. We report that the PP2A regulatory subunit B56alpha is an in vivo binding partner for ankyrin-B, an adapter protein required for normal subcellular localization of the Na/Ca exchanger, Na/K ATPase, and inositol 1,4,5-trisphosphate receptor. Ankyrin-B and B56alpha are colocalized and coimmunoprecipitate in primary cardiomyocytes. Using multiple strategies, we identified the structural requirements on B56alpha for ankyrin-B association as a 13 residue motif in the B56alpha COOH terminus not present in other B56 family polypeptides. Finally, we report that reduced ankyrin-B expression in primary ankyrin-B(+/-) cardiomyocytes results in disorganized distribution of B56alpha that can be rescued by exogenous expression of ankyrin-B. These new data implicate ankyrin-B as a critical targeting component for PP2A in heart and identify a new class of signaling proteins targeted by ankyrin polypeptides.

Protein phosphatase 2A methyltransferase links homocysteine metabolism with tau and amyloid precursor protein regulation

Alzheimer's disease (AD) neuropathology is characterized by the accumulation of phosphorylated tau and amyloid-beta peptides derived from the amyloid precursor protein (APP). Elevated blood levels of homocysteine are a significant risk factor for many age-related diseases, including AD. Impaired homocysteine metabolism favors the formation of S-adenosylhomocysteine, leading to inhibition of methyltransferase-dependent reactions. Here, we show that incubation of neuroblastoma cells with S-adenosylhomocysteine results in reduced methylation of protein phosphatase 2A (PP2A), a major brain Ser/Thr phosphatase, most likely by inhibiting PP2A methyltransferase (PPMT). PP2A methylation levels are also decreased after ectopic expression of PP2A methylesterase in Neuro-2a (N2a) cells. Reduced PP2A methylation promotes the downregulation of B alpha-containing holoenzymes, thereby affecting PP2A substrate specificity. It is associated with the accumulation of both phosphorylated tau and APP isoforms and increased secretion of beta-secretase-cleaved APP fragments and amyloid-beta peptides. Conversely, incubation of N2a cells with S-adenosylmethionine and expression of PPMT enhance PP2A methylation. This leads to the accumulation of dephosphorylated tau and APP species and increased secretion of neuroprotective alpha-secretase-cleaved APP fragments. Remarkably, hyperhomocysteinemia induced in wild-type and cystathionine-beta-synthase +/- mice by feeding a high-methionine, low-folate diet is associated with increased brain S-adenosylhomocysteine levels, PPMT downregulation, reduced PP2A methylation levels, and tau and APP phosphorylation. We reported previously that downregulation of neuronal PPMT and PP2A methylation occur in affected brain regions from AD patients. The link between homocysteine, PPMT, PP2A methylation, and key CNS proteins involved in AD pathogenesis provides new mechanistic insights into this disorder.

Beta-adrenergic receptor agonists delay while antagonists accelerate epithelial wound healing: evidence of an endogenous adrenergic network within the corneal epithelium

Wound healing is a complex and well-orchestrated biological process. Corneal epithelial cells (CECs) must respond quickly to trauma to rapidly restore barrier function and protect the eye from noxious agents. They express a high level of beta2-adrenergic receptors but their function is unknown. Here, we report the novel finding that they form part of a regulatory network in the corneal epithelium, capable of modulating corneal epithelial wound repair. Beta-adrenergic receptor agonists delay CEC migration via a protein phosphatase 2A-mediated mechanism and decrease both electric field-directed migration and corneal wound healing. Conversely, beta-adrenergic receptor antagonists accelerate CEC migration, enhance electric field-mediated directional migration, and promote corneal wound repair. We demonstrate that CECs express key enzymes required for epinephrine (beta-adrenergic receptor agonist) synthesis in the cytoplasm and can detect epinephrine in cell extracts. We propose that the mechanism for the pro-motogenic effect of the beta-adrenergic antagonist is blockade of the beta2-adrenergic receptor preventing autocrine catecholamine binding. Further investigation of this network will improve our understanding of one of the most frequently prescribed class of drugs.

The Lipoxin A4 receptor is coupled to SHP-2 activation: implications for regulation of receptor tyrosine kinases

Mesangial cell proliferation is pivotal to the pathology of glomerular injury in inflammation. We have previously reported that lipoxins, endogenously produced eicosanoids with anti-inflammatory and pro-resolution bioactions, can inhibit mesangial cell proliferation in response to several agents. This process is associated with elaborate receptor cross-talk involving modification receptor tyrosine kinase phosphorylation (McMahon, B., Mitchell, D., Shattock, R., Martin, F., Brady, H. R., and Godson, C. (2002) FASEB J. 16, 1817-1819). Here we demonstrate that the lipoxin A(4) (LXA(4)) receptor is coupled to activation and recruitment of the SHP-2 (SH2 domain-containing tyrosine phosphatase-2) within a lipid raft microdomain. Using site-directed mutagenesis of the cytosolic domain of the platelet-derived growth factor receptor beta (PDGFRbeta), we report that mutation of the sites for phosphatidylinositol 3-kinase (Tyr(740) and Tyr(751)) and SHP-2 (Tyr(763) and Tyr(1009)) recruitment specifically inhibit the effect of LXA(4) on the PDGFRbeta signaling; furthermore inhibition of SHP-2 expression with short interfering RNA constructs blocked the effect of LXA(4) on PDGFRbeta phosphorylation. We demonstrate that association of the PDGFRbeta with lipid raft microdomains renders it susceptible to LXA(4)-mediated dephosphorylation by possible reactivation of oxidatively inactivated SHP-2. These data further elaborate on the potential mechanisms underlying the anti-inflammatory, proresolution, and anti-fibrotic bioactions of lipoxins.

Role of myosin regulatory light chain and Rac1 in the migration of polyamine-depleted intestinal epithelial cells

We have previously shown that polyamine depletion decreased migration, Rac activation, and protein serine threonine phosphatase 2A activity. We have also shown that polyamine depletion increased cortical F-actin and decreased lamellipodia and stress fibers. In this study, we used staurosporine (STS), a potent, cell-permeable, and broad-spectrum serine/threonine kinase inhibitor, and studied migration. STS concentrations above 100 nM induced apoptosis. However, in polyamine-depleted cells, a lower concentration of STS (5 nM) increased attachment, spreading, Rac1 activation, and, subsequently, migration without causing apoptosis. STS-induced migration was completely prevented by a Rac1 inhibitor (NSC-23766) and dominant negative Rac1. These results imply that STS restores migration in polyamine-depleted cells through Rac1. The most important finding in this study was that polyamine depletion increased the association of phosphorylated myosin regulatory light chain (pThr(18)/Ser(19)-MRLC) at the cell periphery, which colocalized with thick cortical F-actin. Localization of pThr(18)- and pSer(19)-MRLC was found with stress fibers and nuclei, respectively. STS decreased the phosphorylation of cellular and peripheral pThr(18)-MRLC without any effect on nuclear pSer(19)-MRLC, dissolved thick cortical F-actin, and increased lamellipodia and stress fiber formation in polyamine-depleted cells. In control and polyamine-depleted cells, focal adhesion kinase (FAK) colocalized with stress fibers and the actin cortex, respectively. STS reorganized FAK, paxillin, and the cytoskeleton. These results suggest that polyamine depletion prevents the dephosphorylation of MRLC and thereby prevents the dynamic reorganization of the actin cytoskeleton and decreases lamellipodia formation resulting in the inhibition of migration.

HER-2/neu raises SHP-2, stops IFN-γ anti-proliferation in bladder cancer

Gene amplification or HER-2/neu protein overexpression signals a poor outcome for bladder cancer patients. We investigated the anti-proliferative effect of IFN-gamma in HER-2/neu-transfected human bladder cancer cells (TCC-N5 and TCC-N10). The cells continued growing after IFN-gamma stimulation but did not activate the Janus kinase (Jak)/Stat pathway. We found Jak/Stat protein phosphatase in TCC-N5 and TCC-N10 cells with upregulated Src homology 2-containing protein tyrosine phosphatase-2 (SHP-2). After the cells had been treated with AG825, a HER-2/neu-specific inhibitor, SHP-2 expression declined, and Jak2/Stat1 reactivated. Similar results were reported in a mouse bladder cancer cell line, MBT2, with constitutive HER-2/neu overexpression. Further, AG825 pretreatment restored the anti-proliferation activity of IFN-gamma in TCC-N5 and TCC-N10 cells. Therefore, the suppression of IFN-gamma signaling in HER-2/neu-overexpressing bladder cancer cells might be due to SHP-2 upregulation. The regulation of SHP-2 by HER-2/neu provides a new target for blocking the HER-2/neu oncogenic pathway.

Antagonistic functions of Par-1 kinase and protein phosphatase 2A are required for localization of Bazooka and photoreceptor morphogenesis in Drosophila

Establishment and maintenance of apical basal cell polarity are essential for epithelial morphogenesis and have been studied extensively using the Drosophila eye as a model system. Bazooka (Baz), a component of the Par-6 complex, plays important roles in cell polarity in diverse cell types including the photoreceptor cells. In ovarian follicle cells, localization of Baz at the apical region is regulated by Par-1 protein kinase. In contrast, Baz in photoreceptor cells is targeted to adherens junctions (AJs). To examine the regulatory pathways responsible for Baz localization in photoreceptor cells, we studied the effects of Par-1 on Baz localization in the pupal retina. Loss of Par-1 impairs the maintenance of AJ markers including Baz and apical polarity proteins of photoreceptor cells but not the establishment of cell polarity. In contrast, overexpression of Par-1 or Baz causes severe mislocalization of junctional and apical markers, resulting in abnormal cell polarity. However, flies with similar overexpression of kinase-inactive mutant Par-1 or unphosphorylatable mutant Baz protein show relatively normal photoreceptor development. These results suggest that dephosphorylation of Baz at the Par-1 phosphorylation sites is essential for proper Baz localization. We also show that the inhibition of protein phosphatase 2A (PP2A) mimics the polarity defects caused by Par-1 overexpression. Furthermore, Par-1 gain-of-function phenotypes are strongly enhanced by reduced PP2A function. Thus, we propose that antagonism between PP2A and Par-1 plays a key role in Baz localization at AJ in photoreceptor morphogenesis.

Proper Chromatin Condensation and Maintenance of Histone H3 Phosphorylation During Mouse Oocyte Meiosis Requires Protein Phosphatase Activity1

We have shown okadaic acid (OA) and calyculin-A (CLA) inhibition of mouse oocyte phosphoprotein phosphatase 1 (PPP1C) and/or phosphoprotein phosphatase 2A (PPP2CA) results in aberrant chromatin condensation, as evidenced by the inability to resolve bivalents. Phosphorylation of histone H3 at specific residues is thought to regulate chromatin condensation. Therefore, we examined changes in histone H3 phosphorylation during oocyte meiosis and the potential regulation by protein PPPs. Western blot and immunocytochemical analysis revealed histone H3 phosphorylation changed during mouse oocyte meiosis, with changes in chromatin condensation. Germinal vesicle-intact (GV-intact; 0 h) oocytes had no phospho-Ser10 but did have phospho-Ser28 histone H3. Oocytes that had undergone germinal vesicle breakdown (GVBD; 2 h) and progressed to metaphase I (MI; 7 h) and MII (16 h) had phosphorylated Ser10 and Ser28 histone H3 associated with condensed chromatin. To determine whether OA-induced aberrations in chromatin condensation were due to alterations in levels of histone H3 phosphorylation, we assessed phosphorylation of Ser10 and Ser28 residues following PPP inhibition. Oocytes treated with OA (1 microM) displayed increased phosphorylation of histone H3 at both Ser10 and Ser28 compared with controls. To begin to elucidate which OA-sensitive PPP is responsible for regulating chromatin condensation and histone H3 phosphorylation, we examined spatial and temporal localization of OA-sensitive PPPs, PPP1C, and PPP2CA. PPPC2A did not localize to condensed chromatin, whereas PPP1beta (PPP1CB) associated with condensing chromatin in GVBD, MI, and MII oocytes. Additionally, Western blot and immunocytochemistry confirmed presence of the PPP1C regulatory inhibitor subunit 2 (PPP1R2) in oocytes at condensed chromatin during meiosis and indicated a change in PPP1R2 phosphorylation. Inhibition of oocyte glycogen synthase kinase 3 (GSK3) appeared to regulate phosphorylation of PPP1R2. Furthermore, inhibition of GSK3 resulted in aberrant oocyte bivalent formation similar to that observed following PPP inhibition. These data suggest that PPP1CB is the OA/CLA-sensitive PPP that regulates oocyte chromatin condensation through regulation of histone H3 phosphorylation. Furthermore, GSK3 inhibition results in aberrant chromatin condensation and appears to regulate phosphorylation of PPP1R2.

Integrin-mediated cell adhesion and spreading engage different sources of reactive oxygen species

The tightly regulated production of intracellular reactive oxygen species (ROS) participates in several biologic processes such as cellular growth, programmed cell death, senescence, and adhesion. It is increasingly evident that the same enzymatic processes that were originally linked to ROS generation during host defence or apoptosis execution are also involved in redox-mediated signal transduction. We investigated in murine NIH3T3 fibroblasts the contribution of a variety of redox-dependent events during signal transduction initiated by integrin engagement due to fibronectin stimulation and report that a mitochondrial ROS release occurs, strictly confined to the early phase of extracellular matrix (ECM) contact (10 min). Besides, 5-lipoxygenase (5-LOX) is engaged by integrin receptor ligation as another ROS source, contributing to the more-intense, second ROS burst (45 min), possibly orchestrating the spreading of cells in response to ECM contact. To define a potential mechanism for ROS signaling, we demonstrate that on integrin recruitment, the Src homology-2 domain-containing phosphatase 2 (SHP-2) undergoes a reversible oxidization/inactivation to which mitochondrial and 5-lipoxygenase ROS contribute differentially. In keeping with a key role of oxidants during integrin signaling, the inactivation of SHP-2 prevents the dephosphorylation and inactivation of SHP-2 substrates (p125FAK and SHPS-1), thus enabling the continued propagation of the signal arising by integrin engagement.

A specific PP2A regulatory subunit, B56gamma, mediates DNA damage-induced dephosphorylation of p53 at Thr55

Protein phosphatase 2A (PP2A) has been implicated to exert its tumor suppressive function via a small subset of regulatory subunits. In this study, we reported that the specific B regulatory subunits of PP2A B56gamma1 and B56gamma3 mediate dephosphorylation of p53 at Thr55. Ablation of the B56gamma protein by RNAi, which abolishes the Thr55 dephosphorylation in response to DNA damage, reduces p53 stabilization, Bax expression and cell apoptosis. To investigate the molecular mechanisms, we have shown that the endogenous B56gamma protein level and association with p53 increase after DNA damage. Finally, we demonstrate that Thr55 dephosphorylation is required for B56gamma3-mediated inhibition of cell proliferation and cell transformation. These results suggest a molecular mechanism for B56gamma-mediated tumor suppression and provide a potential route for regulation of B56gamma-specific PP2A complex function.

Identification of a PP2A-interacting protein that functions as a negative regulator of phosphatase activity in the ATM/ATR signaling pathway

Protein serine/threonine phosphatase 2A (PP2A) activity must be tightly controlled to maintain cell homeostasis. Here, we report the identification of a previously uncharacterized mammalian protein, type 2A-interacting protein (TIP), as a novel regulatory protein of PP2A and the PP2A-like enzymes PP4 and PP6. TIP is a ubiquitously expressed protein and parallels the distribution of the PP2A catalytic subunit. Unlike its role in yeast, TIP does not interact with the mammalian homolog of type 2A-associated protein of 42 kDa (Tap42), alpha4, but instead associates with PP2A, PP4 and PP6 catalytic subunits independently of mammalian target of rapamycin kinase activity. Interestingly, the 20 kDa TIP splice variant TIP_i2, which lacks amino acids 173-272 of TIP's C-terminus, does not interact with PP2A; this finding indicates that residues 173-272 are important for the assembly of the TIP.phosphatase complex. In contrast to purified PP2A holoenzymes, TIP.PP2A complexes are devoid of phosphatase activity. Furthermore, alterations in the cellular levels of TIP influence the phosphorylation state of a specific protein substrate of ataxia-telangiectasia mutated (ATM)/ATM- and Rad3-related (ATR) kinases. Elevated levels of TIP result in an increase in the phosphorylation state of this protein substrate, whereas TIP-depleted cells exhibit a significant decrease in this protein's phosphorylation state, which is reversed by treatment with the PP2A inhibitor okadaic acid. These results indicate TIP is a novel inhibitory regulator of PP2A and implicate a role for TIP.PP2A complexes within the ATM/ATR signaling pathway controlling DNA replication and repair.

Expression of protein phosphatase 2A mutants and silencing of the regulatory B alpha subunit induce a selective loss of acetylated and detyrosinated microtubules

Carboxymethylation and phosphorylation of protein phosphatase 2A (PP2A) catalytic C subunit are evolutionary conserved mechanisms that critically control PP2A holoenzyme assembly and substrate specificity. Down-regulation of PP2A methylation and PP2A enzymes containing the B alpha regulatory subunit occur in Alzheimer's disease. In this study, we show that expressed wild-type and methylation- (L309 Delta) and phosphorylation- (T304D, T304A, Y307F, and Y307E) site mutants of PP2A C subunit differentially bind to B, B', and B''-type regulatory subunits in NIH 3T3 fibroblasts and neuro-2a (N2a) neuroblastoma cells. They also display distinct binding affinity for microtubules (MTs). Relative to controls, expression of the wild-type, T304A and Y307F C subunits in N2a cells promotes the accumulation of acetylated and detyrosinated MTs. However, expression of the Y307E, L309 Delta, and T304D mutants, which are impaired in their ability to associate with the B alpha subunit, induces their loss. Silencing of B alpha subunit in N2a and NIH 3T3 cells is sufficient to induce a similar breakdown of acetylated and detyrosinated MTs. It also confers increased sensitivity to nocodazole-induced MT depolymerization. Our findings suggest that changes in intracellular PP2A subunit composition can modulate MT dynamics. They support the hypothesis that reduced amounts of neuronal B alpha-containing PP2A heterotrimers contribute to MT destabilization in Alzheimer's disease.

Mechanisms of the HRSL3 tumor suppressor function in ovarian carcinoma cells

HRSL3 (also known as H-REV107-1) belongs to a class II tumor suppressor gene family and is downregulated in several human tumors including ovarian carcinomas. To unravel the mechanism of HRSL3 tumor suppressor action, we performed a yeast two-hybrid screen and identified the alpha-isoform of the regulatory subunit A of protein phosphatase 2A (PR65alpha) as a new interaction partner of HRSL3. Interaction between HRSL3 and PR65alpha was confirmed in vitro and by co-immunoprecipitation in mammalian cells. We demonstrate that HRSL3 binds to the endogenous PR65alpha, thereby partially sequestering the catalytic subunit PR36 from the PR65 protein complex, and inhibiting PP2A catalytic activity. Furthermore, binding of HRSL3 to PR65 induces apoptosis in ovarian carcinoma cells in a caspase-dependent manner. Using several mutant HRSL3 constructs, we identified the N-terminal proline-rich region within the HRSL3 protein as the domain that is relevant for both binding of PR65alpha and induction of programmed cell death. This suggests that the negative impact of HRSL3 onto PP2A activity is important for the HRSL3 pro-apoptotic function and indicates a role of PP2A in survival of human ovarian carcinomas. The analysis of distinct PP2A target molecules revealed PKCzeta as being involved in HRSL3 action. These data implicate HRSL3 as a signaling regulatory molecule, which is functionally involved in the oncogenic network mediating growth and survival of ovarian cancer cells.

Human KIR2DL5 Is an Inhibitory Receptor Expressed on the Surface of NK and T Lymphocyte Subsets

Human NK cells, by means of a repertoire of clonally distributed killer cell Ig-like receptors (KIR), survey the expression of individual self HLA class I molecules, which is often altered in infections and tumors. KIR2DL5 (CD158f) is the last identified KIR gene and, with KIR2DL4, constitutes a structurally divergent lineage conserved in different primate species. Research on KIR2DL5 has thus far been limited to its genetic aspects due to a lack of reagents to detect its product. We report here the identification and characterization of the receptor encoded by KIR2DL5 using a newly generated specific mAb that recognizes its most commonly expressed allele, KIR2DL5A*001. KIR2DL5 displays a variegated distribution on the surface of CD56(dim) NK cells. This contrasts with the expression pattern of its structural homolog KIR2DL4 (ubiquitous transcription, surface expression restricted to CD56(bright) NK cells) and resembles the profile of KIR recognizing classical HLA class I molecules. Like other MHC class I receptors, KIR2DL5 is also found in a variable proportion of T lymphocytes. KIR2DL5 is detected on the cell surface as a monomer of approximately 60 kDa that, upon tyrosine phosphorylation, recruits the Src homology region 2-containing protein tyrosine phosphatase-2 and, to a lesser extent, Src homology region 2-containing protein tyrosine phosphatase-1. Ab-mediated cross-linking of KIR2DL5 inhibits NK cell cytotoxicity against murine FcR+ P815 cells. KIR2DL5 is thus an inhibitory receptor gathering a combination of genetic, structural, and functional features unique among KIR, which suggests that KIR2DL5 plays a specialized role in innate immunity.

p56lck, LFA-1 and PI3K but not SHP-2 interact with GM1- or GM3-enriched microdomains in a CD4-p56lck association-dependent manner

We previously showed that the association of CD4 and G(M3) ganglioside induced by CD4 ligand binding was required for the down-regulation of adhesion and that aggregation of ganglioside-enriched domains was accompanied by transient co-localization of LFA-1 (lymphocyte function-associated antigen-1), PI3K (phosphoinositide 3-kinase) and CD4. We also showed that these proteins co-localized with the G(M1) ganglioside that partially co-localized with G(M3) in these domains. In the present study, we show that CD4-p56(lck) association in CD4 signalling is required for the redistribution of p56(lck), PI3K and LFA-1 in ganglioside-enriched domains, since ganglioside aggregation and recruitment of these proteins were not observed in a T-cell line (A201) expressing the mutant form of CD4 that does not bind p56(lck). In addition, we show that although these proteins associated in different ways with G(M1) and G(M3), all of the associations were dependent on CD4-p56(lck) association. Gangliosides could associate with these proteins that differ in affinity binding and could be modified following CD4 signalling. Our results suggest that through these associations, gangliosides transiently sequestrate these proteins and consequently inhibit LFA-1-dependent adhesion. Furthermore, while structural diversity of gangliosides may allow association with distinct proteins, we show that the tyrosine phosphatase SHP-2 (Src homology 2 domain-containing protein tyrosine phosphatase 2), also required for the down-regulation of LFA-1-dependent adhesion, transiently and partially co-localized with PI3K and p56(lck) in detergent-insoluble membranes without association with G(M1) or G(M3). We propose that CD4 ligation and binding with p56(lck) and their interaction with G(M3) and/or G(M1) gangliosides induce recruitment of distinct proteins important for CD4 signalling to form a multimolecular signalling complex.

Identification of proteins interacting with the catalytic subunit of PP2A by proteomics

The protein phosphatase 2A (PP2A) is a serine/threonine phosphatase involved in the regulation of multiple signaling pathways including the Wnt/beta-catenin and the ERK pathways. To understand the complex signaling networking associated with PP2A, we searched proteins interacting with the catalytic subunit of protein phosphatase 2A (PP2Ac) by a pull-down analysis followed by 2-D gel electrophoresis and proteomic analyses. The probability of identification of the proteins interacting with PP2Ac was increased by searching proteins differently interacting with PP2Ac according to stimulation of Wnt3a, which regulates both the Wnt/beta-catenin and the ERK pathways. Around 100 proteins, pulled-down by His-tagged PP2Ac, were identified in 2-D gels stained with CBB. By MALDI-TOF-MS analyses of 45 protein spots, we identified several proteins that were previously known to interact with PP2A, such as Axin and CaMK IV. In addition, we also identified many proteins that potentially interact with PP2Ac. The interactions of several candidate proteins, such as tuberous sclerosis complex 2, RhoB, R-Ras, and Nm23H2, with PP2Ac, were confirmed by in vitro binding analyses and/or coimmunoprecipitation experiments.

Protein kinase C-mediated down-regulation of cyclin D1 involves activation of the translational repressor 4E-BP1 via a phosphoinositide 3-kinase/Akt-independent, protein phosphatase 2A-dependent mechanism in intestinal epithelial cells

We reported previously that protein kinase Calpha (PKCalpha), a negative regulator of cell growth in the intestinal epithelium, inhibits cyclin D1 translation by inducing hypophosphorylation/activation of the translational repressor 4E-BP1. The current study explores the molecular mechanisms underlying PKC/PKCalpha-induced activation of 4E-BP1 in IEC-18 nontransformed rat ileal crypt cells. PKC signaling is shown to promote dephosphorylation of Thr(45) and Ser(64) on 4E-BP1, residues directly involved in its association with eIF4E. Consistent with the known role of the phosphoinositide 3-kinase (PI3K)/Akt/mTOR pathway in regulation of 4E-BP1, PKC signaling transiently inhibited PI3K activity and Akt phosphorylation in IEC-18 cells. However, PKC/PKCalpha-induced activation of 4E-BP1 was not prevented by constitutively active mutants of PI3K or Akt, indicating that blockade of PI3K/Akt signaling is not the primary effector of 4E-BP1 activation. This idea is supported by the fact that PKC activation did not alter S6 kinase activity in these cells. Further analysis indicated that PKC-mediated 4E-BP1 hypophosphorylation is dependent on the activity of protein phosphatase 2A (PP2A). PKC signaling induced an approximately 2-fold increase in PP2A activity, and phosphatase inhibition blocked the effects of PKC agonists on 4E-BP1 phosphorylation and cyclin D1 expression. H(2)O(2) and ceramide, two naturally occurring PKCalpha agonists that promote growth arrest in intestinal cells, activate 4E-BP1 in PKC/PKCalpha-dependent manner, supporting the physiological significance of the findings. Together, our studies indicate that activation of PP2A is an important mechanism underlying PKC/PKCalpha-induced inhibition of cap-dependent translation and growth suppression in intestinal epithelial cells.

Subunit composition and developmental regulation of hepatic protein phosphatase 2A (PP2A)

The prototypical form of the Ser/Thr phosphatase PP2A is a heterotrimeric complex consisting of catalytic subunit (C), and A and B regulatory subunits. C-terminal methylation of PP2A-C influences holoenzyme assembly. Using late gestation development in the rat as an in vivo model of liver growth, we found that PP2A-C protein and activity levels were higher in fetal compared to adult liver extracts. However, unmethylated PP2A-C was much higher in the adult extracts. In MonoQ fractionation, unmethylated C eluted separately from methylated C, which was present predominantly in ABC heterotrimers. Gel filtration chromatography revealed that some unmethylated C was present as free catalytic subunit in adult liver. In addition, a significant proportion of PP2A was in inactive forms that may involve novel regulatory subunits. Our results indicate that methylation of PP2A-C appears to be a primary determinant for the biogenesis of PP2A heterotrimers.

Protein phosphatase 2A plays a crucial role in Giardia lamblia differentiation

The ability of Giardia lamblia to undergo two distinct differentiations in response to physiologic stimuli is central to its pathogenesis. The giardial cytoskeleton changes drastically during encystation and excystation. However, the signal transduction pathways mediating these transformations are poorly understood. We tested the hypothesis that PP2A, a highly conserved serine/threonine protein phosphatase, might be important in giardial differentiation. We found that in vegetatively growing trophozoites, gPP2A-C protein localizes to basal bodies/centrosomes, and to cytoskeletal structures unique to Giardia: the ventral disk, and the dense rods of the anterior, posterior-lateral, and caudal flagella. During encystation, gPP2A-C protein disappears from only the anterior flagellar dense rods. During excystation, gPP2A-C localizes to the cyst wall in excysting cysts but is not found in the wall of cysts with emerging excyzoites. Transcriptome and immunoblot analyses indicated that gPP2A-C mRNA and protein are upregulated in mature cysts and during the early stage of excystation that models passage through the host stomach. Stable expression of gPP2A-C antisense RNA did not affect vegetative growth, but strongly inhibited the formation of encystation secretory vesicles (ESV) and water-resistant cysts. Moreover, the few cysts that formed were highly defective in excystation. Thus, gPP2A-C localizes to universal cytoskeletal structures and to structures unique to Giardia. It is also important for encystation and excystation, crucial giardial transformations that entail entry into and exit from dormancy.

A wheat (Triticum aestivum) protein phosphatase 2A catalytic subunit gene provides enhanced drought tolerance in tobacco

BACKGROUND AND AIMS

Multiple copies of genes encoding the catalytic subunit (c) of protein phosphatase 2A (PP2A) are commonly found in plants. For some of these genes, expression is up-regulated under water stress. The aim of this study was to investigate expression and characterization of TaPP2Ac-1 from Triticum aestivum, and to evaluate the effects of TaPP2Ac-1 on Nicotiana benthamiana in response to water stress.

METHODS

TaPP2Ac-1 cDNA was isolated from wheat by in silico identification and RT-PCR amplification. Transcript levels of TaPP2Ac-1 were examined in wheat responding to water deficit. Copy numbers of TaPP2Ac-1 in wheat genomes and subcellular localization in onion epidermal cells were studied. Enzyme properties of the recombinant TaPP2Ac-1 protein were determined. In addition, studies were carried out in tobacco plants with pCAPE2-TaPP2Ac-1 under water-deficit conditions.

KEY RESULTS

TaPP2Ac-1 cDNA was cloned from wheat. Transcript levels of TaPP2Ac-1 in wheat seedlings were up-regulated under drought condition. One copy for this TaPP2Ac-1 was present in each of the three wheat genomes. TaPP2Ac-1 fused with GFP was located in the nucleus and cytoplasm of onion epidermis cells. The recombinant TaPP2Ac-1 gene was over-expressed in Escherichia coli and encoded a functional serine/threonine phosphatase. Transgenic tobacco plants over-expressing TaPP2Ac-1 exhibited stronger drought tolerance than non-transgenic tobacco plants.

CONCLUSIONS

Tobacco plants with pCAPE2-TaPP2Ac-1 appeared to be resistant to water deficit, as shown by their higher capacity to maintain leaf relative water content, leaf cell-membrane stability index, water-retention ability and water use efficiency under water stress. The results suggest that the physiological role of TaPP2Ac-1 is related to drought stress response, possibly through its involvement in drought-responding signal transduction pathways.

The Adenoviral E4orf6 Protein Induces Atypical Apoptosis in Response to DNA Damage

Adenoviral proteins interact with host-cell proteins to either exploit or inhibit cellular functions for the purpose of viral propagation. E4orf6, the 34-kDa gene product of the E4 gene, interacts with the double-strand break repair (DSBR) protein DNA-dependent protein kinase and cooperates with binding partner E1B-55K to degrade MRE11, preventing viral DNA concatemer formation. We previously demonstrated that E4orf6 radiosensitizes human tumor cells through the inhibition of DSBR, notably in the absence of E1B-55K. Here, we report that E4orf6 prolongs the signaling of DNA damage by inhibiting the activity of protein phosphatase 2A (PP2A), the phosphatase responsible for dephosphorylating gammaH2AX. The inhibition of PP2A occurs without significant disruption of the DNA re-ligation rate. Prolonged signaling of DNA damage in the presence of E4orf6 initiates caspase-dependent and independent cell death. This is accompanied by poly(ADP-ribose) polymerase (PARP) hyperactivation and the translocation of apoptosis-inducing factor (AIF) from the mitochondria to the nucleus. Knockdown of AIF by shRNA rescues the radiosensitization induced by E4orf6. Taken together, these data suggest that E4orf6 disrupts cellular DSBR signaling by inhibiting PP2A, leading to prolonged H2AX phosphorylation, hyperactivation of PARP, and AIF translocation to the nucleus. The function of E4orf6 as an inhibitor of PP2A and activator of PARP in the absence of other adenoviral gene products is of importance in delineating the adenovirus-host cell interplay.

Mechanisms of checkpoint kinase Rad53 inactivation after a double-strand break in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, double-strand breaks (DSBs) activate DNA checkpoint pathways that trigger several responses including a strong G(2)/M arrest. We have previously provided evidence that the phosphatases Ptc2 and Ptc3 of the protein phosphatase 2C type are required for DNA checkpoint inactivation after a DSB and probably dephosphorylate the checkpoint kinase Rad53. In this article we have investigated further the interactions between Ptc2 and Rad53. We showed that forkhead-associated domain 1 (FHA1) of Rad53 interacts with a specific threonine of Ptc2, T376, located outside its catalytic domain in a TXXD motif which constitutes an optimal FHA1 binding sequence in vitro. Mutating T376 abolishes Ptc2 interaction with the Rad53 FHA1 domain and results in adaptation and recovery defects following a DSB. We found that Ckb1 and Ckb2, the regulatory subunits of the protein kinase CK2, are necessary for the in vivo interaction between Ptc2 and the Rad53 FHA1 domain, that Ckb1 binds Ptc2 in vitro and that ckb1Delta and ckb2Delta mutants are defective in adaptation and recovery after a DSB. Our data thus strongly suggest that CK2 is the kinase responsible for the in vivo phosphorylation of Ptc2 T376.

Alterations of tumour suppressor gene PPP2R1B in hepatocellular carcinoma

To evaluate whether the tumour suppressor gene, PPP2R1B, is involved in pathogenesis of hepatocellular carcinoma (HCC), reverse transcription-polymerase chain reaction (RT-PCR) and cDNA sequencing were performed. Eleven of 38 (29%) tumours and 1 of 34 (3%) corresponding non-tumour tissues showed coexpression of wild-type and aberrant mRNA. Various deletions were found in aberrant transcripts. Southern blot analysis did not show gene deletion in tumours, suggesting abnormal RNA splicing may be involved. These data suggest the possibility that aberrant transcripts of PPP2R1B might be associated with the development of HCC.

Regulation of cell adhesion by PP2A and SV40 small tumor antigen: an important link to cell transformation

The serine/threonine protein phosphatase 2A (PP2A) represents a large family of highly conserved heterotrimeric enzymes. Their critical importance in cell homeostasis is underlined by the fact that they are targets of natural toxins like the tumor promoter okadaic acid, and of simian virus 40 small tumor antigen (SV40 small t), a viral protein known to promote cell transformation. Furthermore, mutated or lower expression levels of PP2A subunits have been found in certain cancers. One major known event in PP2A-dependent cell transformation is the alteration of key signaling pathways that control cell growth and survival. In this review, we focus on how PP2A enzymes also affect cell adhesion and cytoskeletal dynamics, the disruption of which is linked to loss of cell polarity, increased cell motility and invasiveness. We also examine how those various pathways participate in the transforming activity of SV40 small t.

Heptad repeats regulate protein phosphatase 2a recruitment to I-kappaB kinase gamma/NF-kappaB essential modulator and are targeted by human T-lymphotropic virus type 1 tax

The switching on-and-off of I-kappaB kinase (IKK) and NF-kappaB occurs rapidly after signaling. How activated IKK becomes down-regulated is not well understood. Here we show that following tumor necrosis factor-alpha stimulation, protein phosphatase 2A (PP2A) association with IKK is increased. A heptad repeat in IKKgamma, helix 2 (HLX2), mediates PP2A recruitment. Two other heptad repeats downstream of HLX2, termed coiled-coil region 2 (CCR2) and leucine zipper (LZ), bind HLX2 and negatively regulate HLX2 interaction with PP2A. HTLV-1 transactivator Tax also binds HLX2, and this interaction is enhanced by CCR2 but reduced by LZ. In the presence of Tax, PP2A-IKKgamma binding is greatly strengthened. Interestingly, peptides spanning CCR2 and/or LZ disrupt IKKgamma-Tax and IKKgamma-PP2A interactions and potently inhibit NF-kappaB activation by Tax and tumor necrosis factor-alpha. We propose that when IKK is resting, HLX2, CCR2, and LZ form a helical bundle in which HLX2 is sequestered. The HLX2-CCR2-LZ bundle becomes unfolded by signal-induced modifications of IKKgamma or after Tax binding. In this conformation, IKK becomes activated. IKKgamma then recruits PP2A via the exposed HLX2 domain for rapid down-regulation of IKK. Tax-PP2A interaction, however, renders PP2A inactive, thus maintaining Tax-PP2A-IKK in an active state. Finally, CCR2 and LZ possibly inhibit IKK activation by stabilizing the HLX2-CCR2-LZ bundle.

The 3D structure of protein phosphatase 2A: new insights into a ubiquitous regulator of cell signaling

Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase implicated in cancer. Three new crystal structures of PP2A show how it interacts with inhibitory toxins and with one of its regulatory subunits. The structures also explain how specific site mutations may lead to cancer and suggest a novel role for PP2A methylation in the formation of PP2A holoenzymes.

Protein kinase A activates protein phosphatase 2A by phosphorylation of the B56delta subunit

Our previous studies of DARPP-32 in striatal slices have shown that activation of D1 receptors leads to cAMP-dependent dephosphorylation of Thr-75, the Cdk5 site in DARPP-32. In the current study, we have elucidated a mechanism whereby protein phosphatase 2A (PP2A) is activated by a cAMP/PKA-dependent pathway, leading to dephosphorylation of Thr-75. PP2A consists of a catalytic C subunit that associates with the scaffolding A subunit and a variety of B subunits. We have found that the A/C subunits of PP2A, in association with the B56delta (or PPP2R5D) regulatory subunit, is an active DARPP-32 phosphatase. The B56delta subunit expressed in HEK293 cells forms a heterotrimeric assembly that catalyzes PKA-mediated dephosphorylation at Thr-75 in DARPP-32 (also cotransfected into HEK293 cells). The B56delta subunit is phosphorylated by PKA, and this increases the overall activity of PP2A in vitro and in vivo. Among four PKA-phosphorylation sites identified in B56delta in vitro, Ser-566 was found to be critical for the regulation of PP2A activity. Moreover, Ser-566 was phosphorylated by PKA in response to activation of D1 receptors in striatal slices. Based on these studies, we propose that the B56delta/A/C PP2A complex regulates the dephosphorylation of DARPP-32 at Thr-75, thereby helping coordinate the efficacy of dopaminergic neurotransmission in striatal neurons. Moreover, stimulation of protein phosphatase activity by this mechanism may represent an important signaling pathway regulated by cAMP in neurons and other types of cell.

Significant association between PTPN11 polymorphism and gastric atrophy among Japanese Brazilians

BACKGROUND

Helicobacter pylori, especially the cytotoxin-associated antigen A (cagA)-positive strains, plays a crucial role in the development of gastric atrophy and gastric cancer. CagA delivered into gastric epithelial cells combines with src homology 2 domain-containing protein tyrosine phosphatase-2 (SHP-2), possibly leading to atrophy/cancer. Our previous study found that a single-nucleotide polymorphism (SNP; IMS-JST057927) of the PTPN11 gene encoding SHP-2, was associated with gastric atrophy among H. pylori-seropositive subjects. This study aimed to examine the reproducibility of the association among Japanese residing in a different circumstance.

METHODS

The subjects were 918 healthy adult Japanese Brazilians from four different areas in Brazil. Blood was sampled from March to May 2001. The target SNP in intron 3 of PTPN11 was genotyped by polymerase chain reaction with confronting two-pair primers (PCR-CTPP). Gastric atrophy was evaluated with serum pepsinogens (PGs); PG I, less than 70 ng/dl and PG I/II ratio, less than 3.

RESULTS

The genotype frequency of PTPN11 was in Hardy-Weinberg equilibrium: 65.5% for G/G, 30.4% for G/A, and 4.1% for A/A. The PTPN11 polymorphism had no significant association with H. pylori seropositivity. Among the H. pylori-seropositive subjects, the odds ratios (ORs) of gastric atrophy were 0.93 (95% confidence interval [CI], 0.59-1.47) for the G/A genotype and 0.31 (95% CI, 0.10-0.95) for the A/A genotype, compared with the G/G genotype.

CONCLUSIONS

The present study reproduced the significant association between the A/A genotype and reduced risk of gastric atrophy among Japanese outside Japan. According to the Japan Single Nucleotide Polymorphisms (JSNP) database (db)SNP data, the G allele is very frequent among Japanese and rare in Caucasians. This fact may partly explain the distribution of gastric atrophy/cancer in the world.

Domains Necessary for Gα12Binding and Stimulation of Protein Phosphatase-2A (PP2A): Is Gα12a Novel Regulatory Subunit of PP2A?

Many cellular signaling pathways share regulation by protein phosphatase-2A (PP2A), a widely expressed serine/threonine phosphatase, and the heterotrimeric G protein Galpha(12). PP2A activity is altered in carcinogenesis and in some neurodegenerative diseases. We have identified binding of Galpha(12) with the Aalpha subunit of PP2A, a trimeric enzyme composed of A (scaffolding), B (regulatory), and C (catalytic) subunits and demonstrated that Galpha(12) stimulated phosphatase activity (J Biol Chem 279: 54983-54986, 2004). We now show in substrate-velocity analysis using purified PP2A that V(max) was stimulated 3- to 4-fold by glutathione transferase (GST)-Galpha(12) with little effect on K(m) values. To identify the binding domains mediating the Aalpha-Galpha(12) interaction, an extensive mutational analysis was performed. Well-characterized mutations of Aalpha were expressed in vitro and tested for binding to GST-Galpha(12) in pull-down assays. Galpha(12) binds to Aalpha along repeats 7 to 10, and PP2A B subunits are not necessary for binding. To identify where Aalpha binds to Galpha(12), a series of 61 Galpha(12) mutants were engineered to contain the sequence Asn-Ala-Ala-Ile-Arg-Ser (NAAIRS) in place of 6 consecutive amino acids. Mutant Galpha(12) proteins were individually expressed in human embryonic kidney cells and analyzed for interaction with GST or GST-Aalpha in pull-down assays. The Aalpha binding sites were localized to regions near the N and C termini of Galpha(12). The expression of constitutively activated Galpha(12) (QLalpha(12)) in Madin Darby canine kidney cells stimulated PP2A activity as determined by decreased phosphorylation of tyrosine 307 on the catalytic subunit. Based on crystal structures of Galpha(12) and PP2A Aalpha, a model describing the binding surfaces and potential mechanisms of Galpha(12)-mediated PP2A activation is presented.