Effects of regulatory subunits on the kinetics of protein phosphatase 2A

p66Shc in Cardiovascular Pathology

p66Shc is a widely expressed protein that governs a variety of cardiovascular pathologies by generating, and exacerbating, pro-apoptotic ROS signals. Here, we review p66Shc’s connections to reactive oxygen species, expression, localization, and discuss p66Shc signaling and mitochondrial functions. Emphasis is placed on recent p66Shc mitochondrial function discoveries including structure/function relationships, ROS identity and regulation, mechanistic insights, and how p66Shc-cyt c interactions can influence p66Shc mitochondrial function. Based on recent findings, a new p66Shc mitochondrial function model is also put forth wherein p66Shc acts as a rheostat that can promote or antagonize apoptosis. A discussion of how the revised p66Shc model fits previous findings in p66Shc-mediated cardiovascular pathology follows.

PPP2R1B is modulated by ubiquitination and is essential for spermatogenesis

The serine-threonine protein phosphatase 2A (PP2A) is a heterotrimeric enzyme complex that regulates many fundamental cellular processes. PP2A is involved in tumorigenesis because mutations in the scaffold subunit, PPP2R1B, were found in several types of cancers. However, the biological function of PPP2R1B remains largely unknown. We report here that homozygous deletion of Ppp2r1b in Mus musculus impairs meiotic recombination and causes meiotic arrest in spermatocytes. Consistently, male mice lacking Ppp2r1b are characterized with infertility. Furthermore, heterozygous missense mutations in the Homo sapiens PPP2R1B gene, which encodes PPP2R1B, are identified in azoospermia patients with meiotic arrest. We found that PPP2R1B mutants are susceptible to degradation by an E3 ligase CRL4A^DCAF6 , and resistant to de-polyubiquitylation by ubiquitin-specific protease 5 (USP5). In addition, heterozygous mutations in PPP2R1B reduce stability of the wild-type PPP2R1B. Our results demonstrate an essential role of PPP2R1B in spermatogenesis and identify upstream regulators of PPP2R1B.

Targeting protein phosphatase PP2A for cancer therapy: development of allosteric pharmaceutical agents

Tumor initiation is driven by oncogenes that activate signaling networks for cell proliferation and survival involving protein phosphorylation. Protein kinases in these pathways have proven to be effective targets for pharmaceutical inhibitors that have progressed to the clinic to treat various cancers. Here, we offer a narrative about the development of small molecule modulators of the protein Ser/Thr phosphatase 2A (PP2A) to reduce the activation of cell proliferation and survival pathways. These novel drugs promote the assembly of select heterotrimeric forms of PP2A that act to limit cell proliferation. We discuss the potential for the near-term translation of this approach to the clinic for cancer and other human diseases.

Critical Functions of PP2A-Like Protein Phosphotases in Regulating Meiotic Progression

Meiosis is essential to the continuity of life in sexually-reproducing organisms through the formation of haploid gametes. Unlike somatic cells, the germ cells undergo two successive rounds of meiotic divisions after a single cycle of DNA replication, resulting in the decrease in ploidy. In humans, errors in meiotic progression can cause infertility and birth defects. Post-translational modifications, such as phosphorylation, ubiquitylation and sumoylation have emerged as important regulatory events in meiosis. There are dynamic equilibrium of protein phosphorylation and protein dephosphorylation in meiotic cell cycle process, regulated by a conservative series of protein kinases and protein phosphatases. Among these protein phosphatases, PP2A, PP4, and PP6 constitute the PP2A-like subfamily within the serine/threonine protein phosphatase family. Herein, we review recent discoveries and explore the role of PP2A-like protein phosphatases during meiotic progression.

Plant protein phosphatases: What do we know about their mechanism of action?

Protein phosphorylation is a major reversible post-translational modification. Protein phosphatases function as 'critical regulators' in signaling networks through dephosphorylation of proteins, which have been phosphorylated by protein kinases. A large understanding of their working has been sourced from animal systems rather than the plant or the prokaryotic systems. The eukaryotic protein phosphatases include phosphoprotein phosphatases (PPP), metallo-dependent protein phosphatases (PPM), protein tyrosine (Tyr) phosphatases (PTP), and aspartate (Asp)-dependent phosphatases. The PPP and PPM families are serine(Ser)/threonine(Thr)-specific phosphatases (STPs), while PTP family is Tyr specific. Dual-specificity phosphatases (DsPTPs/DSPs) dephosphorylate Ser, Thr, and Tyr residues. PTPs lack sequence homology with STPs, indicating a difference in catalytic mechanisms, while the PPP and PPM families share a similar structural fold indicating a common catalytic mechanism. The catalytic cysteine (Cys) residue in the conserved HCX₅ R active site motif of the PTPs acts as a nucleophile during hydrolysis. The PPP members require metal ions, which coordinate the phosphate group of the substrate, followed by a nucleophilic attack by a water molecule and hydrolysis. The variable holoenzyme assembly of protein phosphatase(s) and the overlap with other post-translational modifications like acetylation and ubiquitination add to their complexity. Though their functional characterization is extensively reported in plants, the mechanistic nature of their action is still being explored by researchers. In this review, we exclusively overview the plant protein phosphatases with an emphasis on their mechanistic action as well as structural characteristics.

The PP2A regulatory subunits, Cdc55 and Rts1, play distinct roles in Candida albicans' growth, morphogenesis, and virulence

Protein phosphatase-2A (PP2A) is a heterotrimeric enzyme composed of a catalytic subunit, a regulatory subunit, and a structural subunit. In Candida albicans, Cdc55 and Rts1 have been identified as possible regulatory subunits of PP2A containing the catalytic subunit Pph21 and structural subunit Tpd3. The Tpd3-Pph21 phosphatase regulates cell morphogenesis and division. However, the functions of Cdc55 and Rts1 remain unclear. Here, we constructed cdc55Δ/Δ and rts1Δ/Δ mutants and found that they exhibit different defects in multiple phenotypes although both show similar hyperphosphorylation of the septin Sep7 and aberrant septin organization. Under yeast growth conditions, the cdc55Δ/Δ mutant grows slowly as pseudohyphae with some cells lacking the nucleus, while rts1Δ/Δ cells are round and enlarged and seem to undergo incomplete cell separation producing multinucleated cells. Strong chitin deposition occurs at the septum of cdc55Δ/Δ cells and on the surface of rts1Δ/Δ cells, which likely contributes to increased susceptibility to caspofungin. Also, cdc55Δ/Δ exhibits severe defects in hyphal and biofilm formation, while rts1Δ/Δ is partially defective. Both mutants show reduced virulence in mice, suggesting that PP2A-B subunits could serve as potential antifungal targets.

Hypoxia modulates protein phosphatase 2A through HIF-1α dependent and independent mechanisms in human aortic smooth muscle cells and ventricular cardiomyocytes

BACKGROUND AND PURPOSE

Although protein phosphatases regulate multiple cellular functions, their modulation under hypoxia remains unclear. We investigated expression of the protein phosphatase system under normoxic/hypoxic conditions and the mechanism by which hypoxia alters protein phosphatase 2A (PP2A) activity.

EXPERIMENTAL APPROACH

Human cardiovascular cells were cultured in cell type specific media under normoxic or hypoxic conditions (1% O₂ ). Effects on mRNA expression, phosphatase activity, post-translational modification, and involvement of hypoxia inducible factor 1α (HIF-1α) were assessed using RT-PCR, immunoblotting, an activity assay, and siRNA silencing.

KEY RESULTS

All components of the protein phosphatase system studied were expressed in each cell line. Hypoxia attenuated mRNA expression of the transcripts in a cell line- and time-dependent manner. In human aortic smooth muscle cells (HASMC) and AC16 cells, hypoxia decreased PP2Ac activity and mRNA expression without altering PP2Ac abundance. Hypoxia increased demethylated PP2Ac (DPP2Ac) and phosphatase methylesterase 1 (PME-1) abundance but decreased leucine carboxyl methyltransferase 1 (LCMT-1) abundance. HIF-1α siRNA prevented the hypoxia-mediated decrease in phosphatase activity and expression of the catalytic subunit of protein phosphatase 2A (PPP2CA), independently of altering pPP2Ac, DPP2Ac, LCMT-1, or PME-1 abundance.

CONCLUSION AND IMPLICATIONS

Cardiovascular cells express multiple components of the PP2A system. In HASMC and AC16 cells, hypoxia inhibits PP2A activity through HIF-1α-dependent and -independent mechanisms, with the latter being consistent with altered PP2A holoenzyme assembly. This indicates a complex inhibitory effect of hypoxia on the PP2A system, and highlights PP2A as a therapeutic target for diseases associated with dysregulated protein phosphorylation.

Regulation of the phosphoprotein phosphatase 2A system and its modulation during oxidative stress: A potential therapeutic target?

Phosphoprotein phosphatases are of growing interest in the pathophysiology of many diseases and are often the neglected partner of protein kinases. One family member, PP2A, accounts for dephosphorylation of ~55-70% of all serine/threonine phosphosites. Interestingly, dysregulation of kinase signalling is a hallmark of many diseases in which an increase in oxidative stress is also noted. With this in mind, we assess the evidence to support oxidative stress-mediated regulation of the PP2A system In this article, we first present an overview of the PP2A system before providing an analysis of the regulation of PP2A by endogenous inhibitors, post translational modification, and miRNA. Next, a detailed critique of data implicating reactive oxygen species, ischaemia, ischaemia-reperfusion, and hypoxia in regulating the PP2A holoenzyme and associated regulators is presented. Finally, the pharmacological targeting of PP2A, its endogenous inhibitors, and enzymes responsible for its post-translational modification are covered. There is extensive evidence that oxidative stress modulates multiple components of the PP2A system, however, most of the data pertains to the catalytic subunit of PP2A. Irrespective of the underlying aetiology, free radical-mediated attenuation of PP2A activity is an emerging theme. However, in many instances, a dichotomy exists, which requires clarification and mechanistic insight. Nevertheless, this raises the possibility that pharmacological activation of PP2A, either through small molecule activators of PP2A or CIP2A/SET antagonists may be beneficial in modulating the cellular response to oxidative stress. A better understanding of which, will have wide ranging implications for cancer, heart disease and inflammatory conditions.

Unfair competition governs the interaction of pCPI-17 with myosin phosphatase (PP1-MYPT1)

The small phosphoprotein pCPI-17 inhibits myosin light-chain phosphatase (MLCP). Current models postulate that during muscle relaxation, phosphatases other than MLCP dephosphorylate and inactivate pCPI-17 to restore MLCP activity. We show here that such hypotheses are insufficient to account for the observed rapidity of pCPI-17 inactivation in mammalian smooth muscles. Instead, MLCP itself is the critical enzyme for pCPI-17 dephosphorylation. We call the mutual sequestration mechanism through which pCPI-17 and MLCP interact inhibition by unfair competition: MLCP protects pCPI-17 from other phosphatases, while pCPI-17 blocks other substrates from MLCP’s active site. MLCP dephosphorylates pCPI-17 at a slow rate that is, nonetheless, both sufficient and necessary to explain the speed of pCPI-17 dephosphorylation and the consequent MLCP activation during muscle relaxation.

DOI:http://dx.doi.org/10.7554/eLife.24665.001

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

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

Regulatory roles of phosphorylation in model and pathogenic fungi

Over the past 20 years, considerable advances have been made toward our understanding of how post-translational modifications affect a wide variety of biological processes, including morphology and virulence, in medically important fungi. Phosphorylation stands out as a key molecular switch and regulatory modification that plays a critical role in controlling these processes. In this article, we first provide a comprehensive and up-to-date overview of the regulatory roles that both Ser/Thr and non-Ser/Thr kinases and phosphatases play in model and pathogenic fungi. Next, we discuss the impact of current global approaches that are being used to define the complete set of phosphorylation targets (phosphoproteome) in medically important fungi. Finally, we provide new insights and perspectives into the potential use of key regulatory kinases and phosphatases as targets for the development of novel and more effective antifungal strategies.

Overexpression of protein phosphatase 2A in a murine model of chronic myocardial infarction leads to increased adverse remodeling but restores the regulation of β-catenin by glycogen synthase kinase 3β

BACKGROUND/OBJECTIVES

Increased activity of cardiac protein phosphatases is an important feature in human heart failure. Several different protein phosphatases (PP) are involved in the regulation of excitation-contraction-coupling of the myocardium. Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase consisting of a dimeric core enzyme and tissue-specific subunits. In this study we used transgenic mice overexpressing PP2A to further investigate the role of PP2A in cardiac remodeling after myocardial infarction.

METHODS AND RESULTS

Adult male CD-1 mice overexpressing the catalytic subunit α of PP2A (αMHC-PP2A; TG) underwent chronic LAD-ligation or sham surgery, respectively; wildtype littermates (WT) were used as controls. Cardiac function was determined by echocardiography before and 28 days after LAD-ligation. 28 days after MI, the animals were sacrificed and cardiac remodeling was analyzed in histological sections and by Western blots. PP2A overexpression leads to dilated cardiomyopathy in mice, and increased cardiomyocyte hypertrophy and fibrosis of the remote myocardium can be seen after myocardial infarction. However, we found an improved survival of TG in the subacute phase after MI in comparison to WT. On the molecular level, TG shows reduced expression of SERCA and CaMKII alpha both under basal condition as well 28 days after MI. Additionally, the regulation of the Akt/GSK3β/β-catenin pathway is severely disturbed in TG at baseline where a significant activation of Akt is found that coincides with the typical phosphorylation of GSK3β. However, this does not lead to the accumulation of β-catenin - on the contrary: phosphorylation-induced degradation of β-catenin is significantly enhanced.

CONCLUSION

Transgenic overexpression of myocardial PP2A causes adverse remodeling which coincides with a disruption of the classical Akt/GSK3/β-catenin pathway under baseline conditions that is restored to normal values in chronic myocardial infarction. Even so overall survival of TG after myocardial infarction was not constrained and survival after day 2 post MI was improved.

Scaffold subunit Aalpha of PP2A is essential for female meiosis and fertility in mice

Ppp2r1a encodes the scaffold subunit Aalpha of protein phosphatase 2A (PP2A), which is an important and ubiquitously expressed serine threonine phosphatase family and plays a critical role in many fundamental cellular processes. To identify the physiological role of PP2A in female germ cell meiosis, we selectively disrupted Ppp2r1a expression in oocytes by using the Cre-Loxp conditional knockout system. Here we report for the first time that oocyte-specific deletion of Ppp2r1a led to severe female subfertility without affecting follicle survival, growth, and ovulation. PP2A-Aalpha was essential for regulating oocyte meiotic maturation because depletion of PP2A-Aalpha facilitated germinal vesicle breakdown, causing elongation of the MII spindle and precocious separation of sister chromatids. The resulting eggs had high risk of aneuploidy, though they could be fertilized, leading to defective embryonic development and thus subfertility. Our findings provide strong evidence that PP2A-Aalpha within the oocyte plays an indispensable role in oocyte meiotic maturation, though it is dispensable for folliculogenesis in the mouse ovary.

CREB/TRH pathway in the central nervous system regulates energy expenditure in response to deprivation of an essential amino acid

BACKGROUND

In the central nervous system (CNS), thyrotropin-releasing hormone (TRH) has an important role in regulating energy balance. We previously showed that dietary deprivation of leucine in mice increases energy expenditure through CNS-dependent regulation. However, the involvement of central TRH in this regulation has not been reported.

METHODS

Male C57J/B6 mice were maintained on a control or leucine-deficient diet for 7 days. Leucine-deprived mice were either third intracerebroventricular (i.c.v.) injected with a TRH antibody followed by intraperitoneal (i.p.) injection of triiodothyronine (T3) or i.c.v. administrated with an adenovirus of shCREB (cAMP-response element binding protein) followed by i.c.v. injection of TRH. Food intake and body weight were monitored daily. Oxygen consumption, physical activity and rectal temperature were assessed after the treatment. After being killed, the hypothalamus and the brown adipose tissue were collected and the expression of related genes and proteins related was analyzed. In other experiments, control or leucine-deficient medium incubated primary cultured neurons were either infected with adenovirus-mediated short hairpin RNA targeting extracellular signal-regulated kinases 1 and 2 (Ad-shERK1/2) or transfected with plasmid-overexpressing protein phosphatase 1 regulatory subunit 3C (PPP1R3C).

RESULTS

I.c.v. administration of anti-TRH antibodies significantly reduced leucine deprivation-stimulated energy expenditure. Furthermore, the effects of i.c.v. TRH antibodies were reversed by i.p. injection of T3 during leucine deprivation. Moreover, i.c.v. injection of Ad-shCREB (adenovirus-mediated short hairpin RNA targeting CREB) significantly suppressed leucine deprivation-stimulated energy expenditure via modulation of TRH expression. Lastly, TRH expression was regulated by CREB, which was phosphorylated by ERK1/2 and dephosphorylated by PPP1R3C-containing protein Ser/Thr phosphatase type 1 (PP1) under leucine deprivation in vitro.

CONCLUSIONS

Our data indicate a novel role for TRH in regulating energy expenditure via T3 during leucine deprivation. Furthermore, our findings reveal that TRH expression is activated by CREB, which is phosphorylated by ERK1/2 and dephosphorylated by PPP1R3C-containing PP1. Collectively, our studies provide novel insights into the regulation of energy homeostasis by the CNS in response to an essential amino-acid deprivation.

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

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

Protein kinase C-dependent dephosphorylation of tyrosine hydroxylase requires the B56δ heterotrimeric form of protein phosphatase 2A

Tyrosine hydroxylase, which plays a critical role in regulation of dopamine synthesis, is known to be controlled by phosphorylation at several critical sites. One of these sites, Ser40, is phosphorylated by a number of protein kinases, including protein kinase A. The major protein phosphatase that dephosphorylates Ser40 is protein phosphatase-2A (PP2A). A recent study has also linked protein kinase C to the dephosphorylation of Ser40 [1], but the mechanism is unclear. PP2A isoforms are comprised of catalytic, scaffold, and regulatory subunits, the regulatory B subunits being able to influence cellular localization and substrate selection. In the current study, we find that protein kinase C is able to phosphorylate a key regulatory site in the B56δ subunit leading to activation of PP2A. In turn, activation of the B56δ-containing heterotrimeric form of PP2A is responsible for enhanced dephosphorylation of Ser40 of tyrosine hydroylase in response to stimulation of PKC. In support of this mechanism, down-regulation of B56δ expression in N27 cells using RNAi was found to increase dopamine synthesis. Together these studies reveal molecular details of how protein kinase C is linked to reduced tyrosine hydroxylase activity via control of PP2A, and also add to the complexity of protein kinase/protein phosphatase interactions.

Protein phosphatase 2A affects myofilament contractility in non-failing but not in failing human myocardium

Protein phosphatase (PP) type 2A is a multifunctional serine/threonine phosphatase that is involved in cardiac excitation-contraction coupling. The PP2A core enzyme is a dimer, consisting of a catalytic C and a scaffolding A subunit, which is targeted to several cardiac proteins by a regulatory B subunit. At present, it is controversial whether PP2A and its subunits play a critical role in end-stage human heart failure. Here we report that the application of purified PP2AC significantly increased the Ca2+-sensitivity (ΔpCa50=0.05±0.01) of the contractile apparatus in isolated skinned myocytes of non-failing (NF) hearts. A higher phosphorylation of troponin I (cTnI) was found at protein kinase A sites (Ser23/24) in NF compared to failing myocardium. The basal Ca2+-responsiveness of myofilaments was enhanced in myocytes of ischemic (ICM, ΔpCa50=0.10±0.03) and dilated (DCM, ΔpCa50=0.06±0.04) cardiomyopathy compared to NF. However, in contrast to NF myocytes the treatment with PP2AC did not shift force-pCa relationships in failing myocytes. The higher basal Ca2+-sensitivity in failing myocytes coincided with a reduced protein expression of PP2AC in left ventricular tissue from patients suffering from ICM and DCM (by 50 and 56% compared to NF, respectively). However, PP2A activity was unchanged in failing hearts despite an increase of both total PP and PP1 activity. The expression of PP2AB56α was also decreased by 51 and 62% in ICM and DCM compared to NF, respectively. The phosphorylation of cTnI at Ser23/24 was reduced by 66 and 49% in ICM and DCM compared to NF hearts, respectively. Our results demonstrate that PP2A increases myofilament Ca2+-sensitivity in NF human hearts, most likely via cTnI dephosphorylation. This effect is not present in failing hearts, probably due to the lower baseline cTnI phosphorylation in failing compared to non-failing hearts.

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

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

Endotoxin conditioning induces VCP/p97-mediated and inducible nitric-oxide synthase-dependent Tyr284 nitration in protein phosphatase 2A

Endotoxins activate Toll-like receptors and reprogram cells to be refractory to secondary exposure. Here we found that activation of different Toll-like receptors elicited a time- and dose-dependent increase in the levels of the protein phosphatase 2A catalytic subunit (PP2Ac) but not its partner A subunit. We purified the lipopolysaccharide-induced form of PP2A by chromatography plus immunoprecipitation and used mass spectrometry to identify VCP/p97 as a novel partner for PP2Ac. Endogenous VCP/p97 and PP2Ac were co-immunoprecipitated from primary murine macrophages and human lymphocytes. GST-VCP/p97 bound purified PP2A in pulldown assays, showing direct protein-protein interaction. Endotoxin conditioning of macrophages induced formation of 3-nitrotyrosine in the PP2Ac associated with VCP/p97, a response severely reduced in macrophages from iNOS knock-out mice. The reaction of purified PP2A with peroxynitrite dissociated the A subunit, and 3-nitro-Tyr(284) was identified in PP2Ac by mass spectrometry. Myc-PP2Ac (Y284F) expressed in cells was resistant to peroxynitrite-induced nitration and reduction of A subunit binding. Transient expression of either VCP/p97 or PP2Ac was sufficient to elevate levels of the dual specificity phosphatase DUSP1, reduce p38 MAPK activation, and suppress tumor necrosis factor-alpha release. We propose that VCP/p97-mediated Tyr nitration of PP2A increases the levels of phosphatases PP2A and DUSP1 to contribute to the refractory response of conditioned cells.

Targeting protein serine/threonine phosphatases for drug development

With the recent clinical success of drugs targeting protein kinase activity, drug discovery efforts are focusing on the role of reversible protein phosphorylation in disease states. The activity of protein phosphatases, enzymes that oppose protein kinases, can also be manipulated to alter cellular signaling for therapeutic benefits. In this review, we present protein serine/threonine phosphatases as viable therapeutic targets, discussing past successes, current challenges, and future strategies for modulating phosphatase activity.

A PP2A active site mutant impedes growth and causes misregulation of native catalytic subunit expression

Activity of protein phosphatase 2A (PP2A) is tightly regulated and performs a diverse repertoire of cellular functions. Previously we isolated a dominant-negative active site mutant of the PP2A catalytic (C) subunit using a yeast complementation assay. We have established stable fibroblastic cell lines expressing epitope-tagged versions of the wild-type and H118N mutant C subunits and have used these cells to investigate mechanisms that regulate PP2A activity. Cells expressing the mutant C subunit exhibit a decreased growth rate and a prolonged G1 cell cycle phase. The mutant protein is enzymatically inactive, but extracts made from cells expressing the H118N C subunit show normal levels of total PP2A activity in vitro. The H118N mutant shows reduced binding to the regulatory A subunit, but binds normally to the alpha4 protein, a non-canonical regulator of PP2A. Expression of the H118N mutant interferes with the normal control of C subunit abundance, causing accumulation of the endogenous wild-type protein as well as the mutant transgene product. Our results indicate that the H118N mutant isoform retards C subunit turnover and suggest that PP2A C subunit turnover may be important for normal cell cycle progression.

Specificity of RCN1-mediated protein phosphatase 2A regulation in meristem organization and stress response in roots

Protein dephosphorylation by the serine/threonine protein phosphatase 2A (PP2A) modulates a broad array of cellular functions. PP2A normally acts as a heterotrimeric holoenzyme complex comprising a catalytic subunit bound by regulatory A and B subunits. Characterization of the regulatory A subunit isoforms (ROOTS CURL IN NAPHTHYLPHTHALAMIC ACID1 [RCN1], PP2AA2, and PP2AA3) of Arabidopsis thaliana PP2A has shown that RCN1 plays a primary role in controlling root and hypocotyl PP2A activity in seedlings. Here we show that hypocotyl and root growth exhibit different requirements for RCN1-mediated regulation of PP2A activity. Roots of rcn1 mutant seedlings exhibit characteristic abnormalities in cell division patterns at the root apical meristem, as well as reduced growth under ionic, osmotic, and oxidative stress conditions. We constructed chimeric A subunit genes and found that restoration of normal root tip development in rcn1 plants requires both regulatory and coding sequences of RCN1, whereas the hypocotyl elongation defect of rcn1 plants can be complemented by either RCN1 or PP2AA3 transgenes. Furthermore, the RCN1 and PP2AA3 proteins exhibit ubiquitous subcellular localization patterns in seedlings and both associate with membrane compartments. Together, these results show that RCN1-containing PP2A has unique functions that cannot be attributed to isoform-specific expression and localization patterns. Postembryonic RCN1 function is required to maintain normal auxin distribution and stem cell function at the root apex. Our data show that RCN1-regulated phosphatase activity plays a unique role in regulating postembryonic root development and stress response.

The B''/PR72 subunit mediates Ca2+-dependent dephosphorylation of DARPP-32 by protein phosphatase 2A

In dopaminoceptive neurons, dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) plays a central role in integrating the effects of dopamine and other neurotransmitters. Phosphorylation of DARPP-32 at Thr-34 by protein kinase A results in inhibition of protein phosphatase 1 (PP1), and phosphorylation at Thr-75 by Cdk5 (cyclin-dependent kinase 5) results in inhibition of protein kinase A. Dephosphorylation at Thr-34 involves primarily the Ca(2+)-dependent protein phosphatase, PP2B (calcineurin), whereas dephosphorylation of Thr-75 involves primarily PP2A, the latter being subject to control by both cAMP- and Ca(2+)-dependent regulatory mechanisms. In the present study, we have investigated the mechanism of Ca(2+)-dependent regulation of Thr-75 by PP2A. We show that the PR72 (or B'' or PPP2R3A) regulatory subunit of PP2A is highly expressed in striatum. Through the use of overexpression and down-regulation by using RNAi, we show that PP2A, in a heterotrimeric complex with the PR72 subunit, mediates Ca(2+)-dependent dephosphorylation at Thr-75 of DARPP-32. The PR72 subunit contains two Ca(2+) binding sites formed by E and F helices (EF-hands 1 and 2), and we show that the former is necessary for the ability of PP2A activity to be regulated by Ca(2+), both in vitro and in vivo. Our studies also indicate that the PR72-containing form of PP2A is necessary for the ability of glutamate acting at alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and NMDA receptors to regulate Thr-75 dephosphorylation. These studies further our understanding of the complex signal transduction pathways that regulate DARPP-32. In addition, our studies reveal an alternative intracellular mechanism whereby Ca(2+) can activate serine/threonine phosphatase activity.

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.

Functional interaction of Aurora-A and PP2A during mitosis

Entry into mitosis is a highly regulated process, promoted by the activated Cyclin B1/Cdk1 complex. Activation of this complex is controlled, in part, by the protein kinase Aurora-A, which is a member of a multigenic serine/threonine kinase family. In normal cells, Aurora-A activity is regulated, at least in part, by degradation through the APC-ubiquitin-proteasome pathway. It has recently been proposed that, in Xenopus, Aurora-A degradation can be inhibited by phosphorylation. It would thus be expected that a phosphatase activity would release this blockade at the end of mitosis. Here, we have shown that the protein phosphatase PP2A and Aurora-A are colocalized at the cell poles during mitosis in human cells and interact within the same complex. Using the PP2A inhibitor okadaic acid and an RNAi approach, we have shown that this interaction is functional within the cell. PP2A/Aurora-A interaction is promoted by an S51D mutation in Aurora-A and inhibited by a phosphomimetic peptide centered around Aurora-A S51, thereby strongly suggesting that PP2A controls Aurora-A degradation by dephosphorylating serine 51 in the A box of the human enzyme.

Role of protein phosphatase 2A in calcium-dependent chloride secretion by human colonic epithelial cells

EGF inhibits carbachol-induced chloride secretion by regulating a basolateral potassium channel via phosphatidylinositol 3-kinase (PI 3-kinase) and PKCepsilon activation. Although both EGF and carbachol cause tyrosine phosphorylation of p85 of PI 3-kinase, only EGF activates the enzyme. Serine phosphorylation of p85 is thought to suppress the lipid kinase of PI 3-kinase. Our present study examined whether the differential effects of carbachol and EGF on PI 3-kinase activity correspond to varying phosphorylation of p85, and the mechanisms and consequences. T(84) colonic epithelial cells were treated with either EGF or carbachol. Cell lysates were immunoprecipitated with p85 antibody and blotted with either phosphotyrosine or phosphoserine antibodies. Protein phosphatase (PP) 1 and 2A activities were also measured. Both tyrosine and serine residues of p85 were phosphorylated by carbachol, whereas EGF induced only tyrosine phosphorylation. Moreover, EGF abolished carbachol-induced serine phosphorylation of p85 and activated PP2A without affecting PP1. Carbachol did not affect either phosphatase. Calyculin A or okadaic acid pretreatment reversed the inhibitory action of EGF on carbachol-induced chloride secretion and restored serine phosphorylation of p85. Although carbachol recruits p85, it phosphorylates both serine and tyrosine residues so that the lipid kinase of PI 3-kinase is inhibited. EGF results in p85 tyrosine phosphorylation as well as dephosphorylation of serine residues via the activation of PP2A. This explains the differential induction of PI 3-kinase enzyme activity in response to EGF and/or carbachol and has functional implications. Our data provide further insights into negative signals that regulate chloride secretion and into the molecular basis of signaling diversification in the intestinal epithelium.

The alpha4 regulatory subunit exerts opposing allosteric effects on protein phosphatases PP6 and PP2A

The protein Ser/Thr phosphatase family contains three enzymes called PP2A, PP4, and PP6 with separate biological functions inferred from genetics of the yeast homologues Pph21/22, Pph3, and Sit4. These catalytic subunits associate with a common subunit called alpha4 (related to yeast Tap42). Here, we characterized recombinant PP6 and PP2A catalytic monomers and alpha4.phosphatase heterodimers. Monomeric PP6 and PP2A showed identical kinetics using either p-nitrophenyl phosphate (pNPP) or 32P-myelin basic protein (MBP) as substrates, with matching Km and Vmax values. Using pNPP as substrate, PP6 and PP2A gave the same IC50 with active site inhibitors okadaic acid, microcystin-LR, calyculin A, and cantharidin. However, with MBP as substrate, PP6 was inhibited at 5-fold lower concentrations of toxins relative to PP2A, suggesting PP6 might be a preferred in vivo target of toxins. Heterodimeric alpha4.PP6 and alpha4.PP2A were starkly different. With MBP as substrate the alpha4.PP2A heterodimer had a 100-fold higher Vmax than alpha4.PP6, and neither heterodimer was active with pNPP. Thus, these phosphatases are distinguished by their different responses to allosteric binding of the common regulatory subunit alpha4. Transient expression of alpha4 differentially increased or decreased phosphorylation of endogenous phosphoproteins, consistent with opposing effects on PP2A and PP6.

Interaction of PP2A catalytic subunit with Rb2/p130 is required for all-trans retinoic acid suppression of ovarian carcinoma cell growth

All-trans retinoic acid (ATRA) treatment causes CAOV3 ovarian carcinoma cells to growth arrest in the G0/G1 phase and to elevate the level of Rb2/p130 protein. PP2A, a serine/threonine phosphatase, binds and dephosphorylates Rb2/p130, thereby increasing the half-life of Rb2/p130 in the cell. In order to further characterize the interaction between Rb2/p130 and PP2A upon ATRA treatment, we examined the posttranslational modification of PP2A. ATRA treatment leads to hypophosphorylation of PP2A catalytic subunit (PP2Ac) that correlates with increased PP2A activity. In addition, the N-terminus of PP2Ac binds directly to NLS sequences located in the C-terminus of Rb2/p130. Furthermore, CAOV3 cells transfected with a truncated Rb2/p130 construct consisting of only the wt C-terminus grew more aggressively and were less sensitive to ATRA treatment when compared to parental CAOV3 cells. In contrast, CAOV3 cells transfected with a truncated Rb2/p130 construct consisting of only the C-terminus in which the NLS sites were mutated and which could not interact with PP2A, were as sensitive to ATRA treatment as parental CAOV3 cells. These studies suggest that ATRA treatment suppresses the growth of CAOV3 cells via a novel posttranscriptional mechanism involving PP2A.

The protein phosphatases of Synechocystis sp. strain PCC 6803: open reading frames sll1033 and sll1387 encode enzymes that exhibit both protein-serine and protein-tyrosine phosphatase activity in vitro

The open reading frames (ORFs) encoding two potential protein-serine/threonine phosphatases from the cyanobacterium Synechocystis sp. strain PCC 6803 were cloned and their protein products expressed in Escherichia coli cells. The product of ORF sll1033, SynPPM3, is a homologue of the PPM family of protein-serine/threonine phosphatases found in all eukaryotes as well as many members of the Bacteria. Surprisingly, the recombinant protein phosphatase dephosphorylated phosphotyrosine- as well as phosphoserine-containing proteins in vitro. While kinetic analyses indicate that the enzyme was more efficient at dephosphorylating the latter, replacement of Asp608 by asparagine enhanced activity toward a phosphotyrosine-containing protein fourfold. The product of ORF sll1387, SynPPP1, is the sole homolog of the PPP family of protein phosphatases encoded by the genome of Synechocystis sp. strain PCC 6803. Like many other bacterial PPPs, the enzyme dephosphorylated phosphoserine- and phosphotyrosine-containing proteins with comparable efficiencies. However, while previously described PPPs from prokaryotic organisms required the addition of exogenous metal ion cofactors, such as Mg2+ or Mn2+, for activity, recombinantly produced SynPPP1 displayed near-maximal activity in the absence of added metals. Inductively coupled plasma mass spectrometry indicated that recombinant SynPPP1 contained significant quantities, 0.32 to 0.44 mol/mole total, of Mg and Mn. In this respect, the cyanobacterial enzyme resembled eukaryotic members of the PPP family, which are metalloproteins. mRNA encoding SynPPP1 or SynPPM3 could be detected in cells grown under many, but not all, environmental conditions.

Ultrasensitization: switch-like regulation of cellular signaling by transcriptional induction

Cellular signaling networks are subject to transcriptional and proteolytic regulation under both physiological and pathological conditions. For example, the expression of proteins subject to covalent modification by phosphorylation is known to be altered upon cellular differentiation or during carcinogenesis. However, it is unclear how moderate alterations in protein expression can bring about large changes in signal transmission as, for example, observed in the case of haploinsufficiency, where halving the expression of signaling proteins abrogates cellular function. By modeling a fundamental motif of signal transduction, the phosphorylation–dephosphorylation cycle, we show that minor alterations in the concentration of the protein subject to phosphorylation (or the phosphatase) can affect signal transmission in a highly ultrasensitive fashion. This “ultrasensitization” is strongly favored by substrate sequestration on the catalyzing enzymes, and can be observed with experimentally measured enzymatic rate constants. Furthermore, we show that coordinated transcription of multiple proteins (i.e., synexpression) within a protein kinase cascade results in even more pronounced all-or-none behavior with respect to signal transmission. Finally, we demonstrate that ultrasensitization can account for specificity and modularity in the regulation of cellular signal transduction. Ultrasensitization can result in all-or-none cell-fate decisions and in highly specific cellular regulation. Additionally, switch-like phenomena such as ultrasensitization are known to contribute to bistability, oscillations, noise reduction, and cellular heterogeneity.

Hormones and other external stimuli induce cellular transitions such as cell division or differentiation by regulating gene expression. Hormone-induced cellular transitions are known to occur in a switch-like fashion: while weak background stimuli are rejected, cellular transitions proceed fully as soon as a threshold hormone concentration is exceeded. Earlier studies have described several mechanisms whereby such a switch-like behavior can be realized in intracellular communication via signal transduction networks, which convert hormonal signals into alterations in gene expression.

The authors demonstrate how switch-like behavior can be further enhanced downstream of hormone-induced gene expression. They show that even minor (hormone-induced) alterations in gene expression can dramatically affect the activity of intracellular signal transduction networks, and thereby modify cellular behavior. This phenomenon has been termed “ultrasensitization.”

Ultrasensitization can explain the pronounced dosage sensitivity observed for many disease-associated signal transduction proteins: for example, the mutation of one of two alleles (gene copies), resulting in a 2-fold reduction of gene expression, can already initiate disease progression. Although such sensitivity towards mutations is potentially harmful, the fact that cells nevertheless exhibit ultrasensitization suggests that somehow cells benefit from ultrasensitization. The authors illustrate how ultrasensitization improves the specificity and efficiency of cell-to-cell communication and contributes to cellular memory.

Protein phosphatase 2A is a negative regulator of IL-2 production in patients with systemic lupus erythematosus

Decreased IL-2 production in systemic lupus erythematosus (SLE) represents a central component of the disease immunopathology. We report that the message, protein, and enzymatic activity of the catalytic subunit of protein phosphatase 2A (PP2Ac), but not PP1, are increased in patients with SLE regardless of disease activity and treatment and in a disease-specific manner. Treatment of SLE T cells with PP2Ac-siRNA decreased the protein levels and activity of PP2Ac in a specific manner and increased the levels of phosphorylated cAMP response element-binding protein and its binding to the IL2 and c-fos promoters, as well as increased activator protein 1 activity, causing normalization of IL-2 production. Our data document increased activity of PP2A as a novel SLE disease-specific abnormality and define a distinct mechanism whereby it represses IL-2 production. We propose the use of PP2Ac-siRNA as a novel tool to correct T cell IL-2 production in SLE patients.

A B56 regulatory subunit of protein phosphatase 2A localizes to nuclear speckles in cardiomyocytes

Protein phosphatase 2A (PP2A) is widely distributed in heart tissues, yet its precise cellular functions are poorly understood. This study is based on the notion that PP2A action is governed by interactions of the core enzyme with B targeting/regulatory subunits. The subcellular localizations of two B subunits, B56α and B56γ1, were assessed using adenovirus-driven expression of epitope-tagged (hemagglutinin, HA) in cultured neonatal and adult rat ventricular myocytes. Confocal imaging revealed that HA-B56α was excluded from the nucleus and decorated striated structures, whereas HA-B56γ1 was principally found in the nucleus. Precise immunolabeling studies showed that B56γ1 was concentrated in intranuclear structures known as nuclear speckles, macromolecular structures that accumulate transcription and splicing factors. Western blot analyses revealed that overexpression of either B subunit had no effect on the levels of other PP2A subunits in cultured neonatal cardiac cells. However, overexpression of only B56γ1 increased whole cell PP2A activity by 40% when measured in cell extracts. Finally, B56γ1 did not alter global gene expression or expression of hypertrophic gene markers such as α-skeletal actin. However, morphometric analyses of confocal images revealed that B56γ1 alters the dynamic assembly/disassembly process of nuclear speckles in heart cells. These studies provide new insight into mechanisms of PP2A targeting in the subnuclear architecture in cardiomyocytes and into the role of this phosphatase in nuclear signaling.

Protein phosphatase 2A B56alpha during development in the spontaneously hypertensive rat

Mistargeting of the regulatory subunit of protein phosphatase 2A (PP2A), B56alpha is involved in the hyperphosphorylation and desensitization of the D1 dopamine receptor in renal proximal tubules of spontaneously hypertensive rats (SHRs). However, the renal expression of B56alpha before hypertension develops is not known. Therefore, we studied the expression of B56alpha and PP2A activity in the kidney during development in the SHR and its normotensive control, the Wistar-Kyoto (WKY) rat. PP2A B56alpha was expressed in proximal and distal tubules with no differences in the pattern of expression in WKY and SHRs at any age. In brush border membranes of renal proximal tubules, PP2A B56alpha protein was greatest in the immature rats and decreased with development. However, PP2A activity did not change with age. PP2A B56alpha protein and PP2A activity were similar in WKY and SHRs except at 2 weeks when both PP2A B56alpha protein and PP2A activity were higher in SHRs than in WKY rats. The PP2A catalytic subunit co-immunoprecipitated with the D1 receptor in renal proximal tubule cells. It is possible that the increased expression of PP2A B56alpha and increased basal PP2A activity in the young, especially in the SHRs, may serve as a compensatory mechanism in the increased phosphorylation and decreased renal D1 receptor function, including D1-receptor mediated stimulation in renal proximal tubules of SHRs.

Overlapping Binding Sites in Protein Phosphatase 2A for Association with Regulatory A and α-4 (mTap42) Subunits

Diverse functions of protein Ser/Thr phosphatases depend on the distribution of the catalytic subunits among multiple regulatory subunits. In cells protein phosphatase 2A catalytic subunit (PP2Ac) mostly binds to a scaffold subunit (A subunit or PR65); however, PP2Ac alternatively binds to alpha-4, a subunit related to yeast Tap42 protein, which also associates with phosphatases PP4 or PP6. We mapped alpha-4 binding to PP2Ac to the helical domain, residues 19-165. We mutated selected residues and transiently expressed epitope-tagged PP2Ac to assay for association with A and alpha-4 subunits by co-precipitation. The disabling H118N mutation at the active site or the presence of the active site inhibitor microcystin-LR did not interfere with binding of PP2Ac to either the A subunit or alpha-4, showing that these are allosteric regulators. Positively charged side chains Lys(41), Arg(49), and Lys(74) on the back surface of PP2Ac are unique to PP2Ac, compared with phosphatases PP4, PP6, and PP1. Substitution of one, two, or three of these residues with Ala produced a progressive loss of binding to the A subunit, with a corresponding increase in binding to alpha-4. Conversely, mutation of Glu(42) in PP2Ac essentially eliminated PP2Ac binding to alpha-4, with an increase in binding to the A subunit. Reciprocal changes in binding because of mutations indicate competitive distribution of PP2Ac between these regulatory subunits and demonstrate that the mutated catalytic subunits retained a native conformation. Furthermore, neither the Lys(41)-Arg(49)-Lys(74) nor Glu(42) mutations affected the phosphatase-specific activity or binding to microcystin-agarose. Binding of PP2Ac to microcystin and to alpha-4 increased with temperature, consistent with an activation energy barrier for these interactions. Our results reveal that the A subunit and alpha-4 (mTap42) require charged residues in separate but overlapping surface regions to associate with the back side of PP2Ac and modulate phosphatase activity.

Characterization of the Aalpha and Abeta subunit isoforms of protein phosphatase 2A: differences in expression, subunit interaction, and evolution

Protein phosphatase 2A (PP2A) is very versatile owing to a large number of regulatory subunits and its ability to interact with numerous other proteins. The regulatory A subunit exists as two closely related isoforms designated Aalpha and Abeta. Mutations have been found in both isoforms in a variety of human cancers. Although Aalpha has been intensely studied, little is known about Abeta. We generated Abeta-specific antibodies and determined the cell cycle expression, subcellular distribution, and metabolic stability of Abeta in comparison with Aalpha. Both forms were expressed at constant levels throughout the cell cycle, but Aalpha was expressed at a much higher level than Abeta. Both forms were found predominantly in the cytoplasm, and both had a half-life of approx. 10 h. However, Aalpha and Abeta differed substantially in their expression patterns in normal tissues and in tumour cell lines. Whereas Aalpha was expressed at similarly high levels in all tissues and cell lines, Abeta expression varied greatly. In addition, in vivo studies with epitope-tagged Aalpha and Abeta subunits demonstrated that Abeta is a markedly weaker binder of regulatory B and catalytic C subunits than Aalpha. Construction of phylogenetic trees revealed that the conservation of Aalpha during the evolution of mammals is extraordinarily high in comparison with both Abeta and cytochrome c, suggesting that Aalpha is involved in more protein-protein interactions than Abeta. We also measured the binding of polyoma virus middle tumour antigen and simian virus 40 (SV40) small tumour antigen to Aalpha and Abeta. Whereas both isoforms bound polyoma virus middle tumour antigen equally well, only Aalpha bound SV40 small tumour antigen.

Protein phosphatase 2A forms a molecular complex with Shc and regulates Shc tyrosine phosphorylation and downstream mitogenic signaling

Protein phosphatase 2A (PP2A) is a multimeric serine/threonine phosphatase that carries out multiple functions. Although numerous observations suggest that PP2A plays a major role in downregulation of the mitogen-activated protein (MAP) kinase pathway, the precise mechanisms are unknown. To clarify the role of PP2A in growth factor (insulin, epidermal growth factor [EGF], and insulin-like growth factor 1 [IGF-1]) stimulation of the Ras/MAP kinase pathway, simian virus 40 small t antigen was expressed in Rat-1 fibroblasts which overexpress insulin receptors. Small t antigen is known to specifically inhibit PP2A by binding to the A PP2A regulatory subunit, interfering with the ability of PP2A to bind to its cellular substrates. Overexpressed small t protein was coimmunoprecipitated with PP2A and inhibited cellular PP2A activity but did not inhibit protein phosphatase 1 (PP1) activity. Insulin, IGF-1, and EGF stimulation also inhibited PP2A activity. Growth factor-stimulated Ras, Raf-1, MAP kinase, and mitogen-activated extracellular-signal-regulated kinase kinase (MEK) activities were elevated in small-t-antigen-expressing cells. Furthermore, Shc tyrosine phosphorylation and its association with Grb2 were also elevated in small-t-antigen-expressing cells. Expression levels of Shc, Ras, MEK, or MAP kinase and phosphorylation of insulin, EGF, and IGF-1 receptors were not altered. Interestingly, we found that PP2A associated with Shc in the basal state and dissociated in response to insulin and EGF and that this dissociation was inhibited by 65% in small-t-antigen-expressing cells. In addition, we found that PP2A associates with the phosphotyrosine-binding domain (PTB domain) of Shc and that phosphorylation of tyrosine 317 of Shc was required for PP2A-Shc dissociation. We conclude (i) that PP2A negatively regulates the Ras/MAP kinase pathway by binding to Shc, inhibiting tyrosine phosphorylation; (ii) that the Shc-PP2A association is mediated by the Shc PTB domain but the interaction is independent of phosphotyrosine binding, indicating a new molecular function for the PTB domain; (iii) that growth factor stimulation, or small-t-antigen expression, causes dissociation of the PP2A-Shc complex, facilitating Shc phosphorylation and downstream activations of the Ras/MAP kinase pathway; and (iv) that this defines a new mechanism of small-t-antigen action to promote mitogenesis.

ATM-dependent dissociation of B55 regulatory subunit from nuclear PP2A in response to ionizing radiation

Ionizing radiation (IR) is known to activate multiple cell cycle checkpoints that are thought to enhance the ability of cells to respond to DNA damage. Protein phosphatase 2A (PP2A) has been implicated in IR-induced activation of checkpoints; therefore, Jurkat cells were exposed to an activating dose of IR or sham treatment as control, and nuclear extracts were analyzed for PP2A by Mono Q anion exchange chromatography and microcystin affinity chromatography. PP2A exists in eukaryotic cells both as a heterodimer consisting of a 65-kDa scaffolding subunit (A) plus a 36-kDa catalytic subunit (C) and as ABC heterotrimers, containing one of a variety of regulatory (B) subunits. Here we show that IR produces a transient and reversible reduction in the amount of nuclear AB55C heterotrimer without affecting the AB'C heterotrimer or AC heterodimer. In ataxia telangiectasia-mutated (ATM)-deficient cells the amount of nuclear PP2A heterotrimer relative to heterodimer was not reduced by radiation, but the radiation response was restored by transfection of these cells with plasmids encoding ATM. Wortmannin, an inhibitor of kinases such as phosphatidylinositol 3-kinase, also prevented the IR-induced reduction in nuclear PP2A heterotrimer. The changes in nuclear PP2A occurred without a noticeable difference in the carboxyl-terminal methylation of the C subunit, which is known to influence association with B subunits. We conclude a novel ATM-dependent mechanism is regulating association of B55 subunits with nuclear PP2A in response to IR.

Okadaic acid-sensitive activation of Maxi Cl(-) channels by triphenylethylene antioestrogens in C1300 mouse neuroblastoma cells

1. The regulation of Maxi Cl(-) channels by 17beta-oestradiol and non-steroidal triphenylethylene antioestrogens represents a rapid, non-classical effect of these compounds. In the present study we have investigated the signalling pathways used for the regulation of Maxi Cl(-) channel activity by oestrogens and antioestrogens in C1300 neuroblastoma cells. 2. Whole-cell Maxi Cl(-) currents were readily and reversibly activated by tamoxifen, toremifene and the membrane-impermeant ethyl-bromide tamoxifen, only when applied to the extracellular medium. 3. Pre-treatment of C1300 cells with oestrogen or cAMP prevented the antioestrogen-induced activation of Maxi Cl(-) channels. The inhibitory effect of 17beta-oestradiol and cAMP was abolished by the kinase inhibitor staurosporine. 4. Current activation was unaffected by the removal of intracellular Ca(2+) and Mg(2+), but was completely abolished in the presence of okadaic acid. These results are consistent with the participation of an okadaic acid-sensitive serine/threonine protein phosphatase in the activation of Maxi Cl(-) channels. However, neither oestrogen or antioestrogen treatment modified the total activity of the two major serine/threonine phosphatases, PP1 and PP2A, in C1300 cells. 5. Although the role of these Maxi Cl(-) channels remains unknown, our findings suggest strongly that their modulation by oestrogens and antioestrogens is linked to intracellular signalling pathways.

Protein phosphatase 2A and its B56 regulatory subunit inhibit Wnt signaling in Xenopus

Wnt signaling increases beta-catenin abundance and transcription of Wnt-responsive genes. Our previous work suggested that the B56 regulatory subunit of protein phosphatase 2A (PP2A) inhibits Wnt signaling. Okadaic acid (a phosphatase inhibitor) increases, while B56 expression reduces, beta-catenin abundance; B56 also reduces transcription of Wnt-responsive genes. Okadaic acid is a tumor promoter, and the structural A subunit of PP2A is mutated in multiple cancers. Taken together, the evidence suggests that PP2A is a tumor suppressor. However, other studies suggest that PP2A activates Wnt signaling. We now show that the B56, A and catalytic C subunits of PP2A each have ventralizing activity in Xenopus embryos. B56 was epistatically positioned downstream of GSK3beta and axin but upstream of beta-catenin, and axin co-immunoprecipitated B56, A and C subunits, suggesting that PP2A:B56 is in the beta-catenin degradation complex. PP2A appears to be essential for beta-catenin degradation, since beta-catenin degradation was reconstituted in phosphatase-depleted Xenopus egg extracts by PP2A, but not PP1. These results support the hypothesis that PP2A:B56 directly inhibits Wnt signaling and plays a role in development and carcinogenesis.

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.