Protein phosphatase 2A: the Trojan Horse of cellular signaling

PP2Ac Regulates Autophagy via Mediating mTORC1 and ULK1 During Osteoclastogenesis in the Subchondral Bone of Osteoarthritis

The molecular mechanism underlying abnormal osteoclastogenesis triggering subchondral bone remodeling in osteoarthritis (OA) is still unclear. Here, single-cell and bulk transcriptomics sequencing analyses are performed on GEO datasets to identify key molecules and validate them using knee joint tissues from OA patients and rat OA models. It is found that the catalytic subunit of protein phosphatase 2A (PP2Ac) is highly expressed during osteoclastogenesis in the early stage of OA and is correlated with autophagy. Knockdown or inhibition of PP2Ac weakened autophagy during osteoclastogenesis. Furthermore, the ULK1 expression of the downstream genes is significantly increased when PP2Ac is knocked down. PP2Ac-mediated autophagy is dependent on ULK1 phosphorylation activity during osteoclastogenesis, which is associated with enhanced dephosphorylation of ULK1 Ser637 residue regulating at the post-translational level. Additionally, mTORC1 inhibition facilitated the expression level of PP2Ac during osteoclastogenesis. In animal OA models, decreasing the expression of PP2Ac ameliorated early OA progression. The findings suggest that PP2Ac is also a promising therapeutic target in early OA.

A53T α-synuclein mutation increases susceptibility to postoperative delayed neurocognitive recovery via hippocampal Ang-(1-7)/MasR axis

Delayed neurocognitive recovery (dNCR) is a common complication in geriatric surgical patients. The impact of anesthesia and surgery on patients with neurodegenerative diseases, such as Parkinson's disease (PD) or prion disease, has not yet been reported. In this study, we aimed to determine the association between a pre-existing A53T genetic background, which involves a PD-related point mutation, and the development of postoperative dNCR. We observed that partial hepatectomy induced hippocampus-dependent cognitive deficits in 5-month-old A53T transgenic mice, a model of early-stage PD without cognitive deficits, unlike in age-matched wild-type (WT) mice. We respectively examined molecular changes at 6 h, 1 day, and 2 days after partial hepatectomy and observed that cognitive changes were accompanied by weakened angiotensin-(1-7)/Mas receptor [Ang-(1-7)/MasR] axis, increased alpha-synuclein (α-syn) expression and phosphorylation, decreased methylated protein phosphatase-2A (Me-PP2A), and prompted microglia M1 polarization and neuronal apoptosis in the hippocampus at 1 day after surgery. Nevertheless, no changes in blood-brain barrier (BBB) integrity or plasma α-syn levels in either A53T or WT mice. Furthermore, intranasal administration of selective MasR agonist AVE 0991, reversed the mentioned cognitive deficits in A53T mice, enhanced MasR expression, reduced α-syn accumulation and phosphorylation, and attenuated microglia activation and apoptotic response. Our findings suggest that individuals with the A53T genetic background may be more susceptible to developing postoperative dNCR. This susceptibility could be linked to central α-syn accumulation mediated by the weakened Ang-(1-7)/MasR/methyl-PP2A signaling pathway in the hippocampus following surgery, independent of plasma α-syn level and BBB.

Molecular Role of Protein Phosphatases in Alzheimer's and Other Neurodegenerative Diseases

Alzheimer's disease (AD) is distinguished by the gradual loss of cognitive function, which is associated with neuronal loss and death. Accumulating evidence supports that protein phosphatases (PPs; PP1, PP2A, PP2B, PP4, PP5, PP6, and PP7) are directly linked with amyloid beta (Aβ) as well as the formation of the neurofibrillary tangles (NFTs) causing AD. Published data reported lower PP1 and PP2A activity in both gray and white matters in AD brains than in the controls, which clearly shows that dysfunctional phosphatases play a significant role in AD. Moreover, PP2A is also a major causing factor of AD through the deregulation of the tau protein. Here, we review recent advances on the role of protein phosphatases in the pathology of AD and other neurodegenerative diseases. A better understanding of this problem may lead to the development of phosphatase-targeted therapies for neurodegenerative disorders in the near future.

Molecular Pathways Governing the Termination of Liver Regeneration

The liver has the unique capacity to regenerate, and up to 70% of the liver can be removed without detrimental consequences to the organism. Liver regeneration is a complex process involving multiple signaling networks and organs. Liver regeneration proceeds through three phases: the initiation phase, the growth phase, and the termination phase. Termination of liver regeneration occurs when the liver reaches a liver-to-body weight that is required for homeostasis, the so-called "hepatostat." The initiation and growth phases have been the subject of many studies. The molecular pathways that govern the termination phase, however, remain to be fully elucidated. This review summarizes the pathways and molecules that signal the cessation of liver regrowth after partial hepatectomy and answers the question, "What factors drive the hepatostat?" SIGNIFICANCE STATEMENT: Unraveling the pathways underlying the cessation of liver regeneration enables the identification of druggable targets that will allow us to gain pharmacological control over liver regeneration. For these purposes, it would be useful to understand why the regenerative capacity of the liver is hampered under certain pathological circumstances so as to artificially modulate the regenerative processes (e.g., by blocking the cessation pathways) to improve clinical outcomes and safeguard the patient's life.

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

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

Protective effect of Huanglianjiedu Decoction on microcystin-LR induced nerve injury

Microcystin-LR (MC-LR) presented in eutrophic water has been identified as having the capacity to induce damage to the mammalian nervous system by crossing the blood-brain barrier through organic anion transporting polypeptides. However, the lack of effective preventive and protective strategies remains a concern. Huanglianjiedu Decoction (HLJD), a classical Chinese traditional formula originating from the Tang Dynasty and comprising Rhizoma Coptidis, Radix Scutellariae, Cortex Phellodendri, and Fructus Gardeniae, has exhibited neuroprotective effects attributed to its antioxidant properties. In this study, we investigated the potential of HLJD in counteracting the neurotoxic effects induced by MC-LR. Our findings revealed that MC-LR dose-dependently inhibited the activity of the PP2A enzyme in PC 12 cells and significantly elevated the phosphorylation levels of JNK, ERK1/2, and p38. Moreover, MC-LR administration resulted in synaptic damage in mouse neurons, hyperphosphorylation of the microtubule-related protein Tau, cognitive impairment, and deficits in learning and memory in C57BL/6J mice. Notably, HLJD effectively reversed the cytotoxicity caused by MC-LR in PC 12 cells, and attenuated MC-LR-induced neuronal damage while improving learning ability in mice. These results highlight the potential of HLJD as a promising protective strategy against MC-LR-induced neurological injury.

PP1, PP2A and PP2B Interplay in the Regulation of Sperm Motility: Lessons from Protein Phosphatase Inhibitors

Male fertility relies on the ability of spermatozoa to fertilize the egg in the female reproductive tract (FRT). Spermatozoa acquire activated motility during epididymal maturation; however, to be capable of fertilization, they must achieve hyperactivated motility in the FRT. Extensive research found that three protein phosphatases (PPs) are crucial to sperm motility regulation, the sperm-specific protein phosphatase type 1 (PP1) isoform gamma 2 (PP1γ2), protein phosphatase type 2A (PP2A) and protein phosphatase type 2B (PP2B). Studies have reported that PP activity decreases during epididymal maturation, whereas protein kinase activity increases, which appears to be a requirement for motility acquisition. An interplay between these PPs has been extensively investigated; however, many specific interactions and some inconsistencies remain to be elucidated. The study of PPs significantly advanced following the identification of naturally occurring toxins, including calyculin A, okadaic acid, cyclosporin, endothall and deltamethrin, which are powerful and specific PP inhibitors. This review aims to overview the protein phosphorylation-dependent biochemical pathways underlying sperm motility acquisition and hyperactivation, followed by a discussion of the PP inhibitors that allowed advances in the current knowledge of these pathways. Since male infertility cases still attain alarming numbers, additional research on the topic is required, particularly using other PP inhibitors.

The metaphorical swiss army knife: The multitude and diverse roles of HEAT domains in eukaryotic translation initiation

Biomolecular associations forged by specific interaction among structural scaffolds are fundamental to the control and regulation of cell processes. One such structural architecture, characterized by HEAT repeats, is involved in a multitude of cellular processes, including intracellular transport, signaling, and protein synthesis. Here, we review the multitude and versatility of HEAT domains in the regulation of mRNA translation initiation. Structural and cellular biology approaches, as well as several biophysical studies, have revealed that a number of HEAT domain-mediated interactions with a host of protein factors and RNAs coordinate translation initiation. We describe the basic structural architecture of HEAT domains and briefly introduce examples of the cellular processes they dictate, including nuclear transport by importin and RNA degradation. We then focus on proteins in the translation initiation system featuring HEAT domains, specifically the HEAT domains of eIF4G, DAP5, eIF5, and eIF2Bϵ. Comparative analysis of their remarkably versatile interactions, including protein-protein and protein-RNA recognition, reveal the functional importance of flexible regions within these HEAT domains. Here we outline how HEAT domains orchestrate fundamental aspects of translation initiation and highlight open mechanistic questions in the area.

Liver PP2A-Cα Protects From Parenteral Nutrition-associated Hepatic Steatosis

BACKGROUND & AIMS

Parenteral nutrition (PN) is a lifesaving therapy for patients with intestinal failure. Hepatic steatosis is a potentially fatal complication of long-term PN, but the involved pathological mechanisms are incompletely unclarified. Herein, we identify the role of protein phosphatase 2A (PP2A) in the pathogenesis of parenteral nutrition-associated hepatic steatosis (PNAHS).

METHODS

Proteomic/phosphoproteomic analyses of liver samples from patients with PNAHS were applied to identify the mechanism of PNAHS. Total parenteral nutrition (TPN) mice model, in vivo, and in vitro experiments were used to assess the effect of PP2A-Cα on liver fatty acid metabolism.

RESULTS

Reduced expression of PP2A-Cα (catalytic subunit) enhanced activation of serine/threonine kinase Akt2 and decreased activation of adenosine monophosphate-activated protein kinase (AMPK) were associated with hepatic steatosis in patients with PNAHS. Mice given PN for 14 days developed hepatic steatosis, down-regulation of PP2A-Cα, activation of Akt2, and inhibition of AMPK. Hepatocyte-specific deletion of PP2A-Cα in mice given PN exacerbated Akt2 activation, AMPK inhibition, and hepatic steatosis through an effect on fatty acid degradation, whereas hepatocyte-specific PP2A-Cα overexpression significantly ameliorated hepatic steatosis accompanied with Akt2 suppression and AMPK activation. Additionally, pharmacological activation of Akt2 in mice overexpressing PP2A-Cα led to the aggravation of hepatic steatosis.

CONCLUSIONS

Our findings demonstrate that hepatic PP2A-Cα serves as a protective factor of PNAHS due to ameliorating hepatic steatosis and improving liver function. Our study provides a strong rationale that PP2A-Cα may be involved in the pathogenesis of PNAHS.

Protein Extract of Tobacco Expressing Solanum torvum PP5-Encoding Gene Inhibits Verticillium dahliae Proliferation

Verticillium wilt, a soilborne disease caused by Verticillium dahliae (V. dahliae), can severely affect the yields of Solanaceae crops. In a previous study, it was observed in Solanum torvum (S. torvum) that protein phosphatase 5 (PP5) was induced by V. dahliae infection. To elucidate the function of PP5 more clearly, this study cloned an StPP5 cDNA from S. torvum by PCR. The cDNA contained an ORF of 1458 bp long encoding a putative protein of 485 amino acid residues with a predicted molecular mass of 54.63 kDa and a theoretical isoelectric point of 5.66. StPP5 protein contained a conserved PP domain and showed high similarity to other homologous members of the PP5 family from various plant species. The expression of StPP5 gene was upregulated after V. infection and reached its maximum value at 24 h in leaves. In order to clarify the role of StPP5, four transgenic tobacco plants expressing StPP5 were generated through Agrobacterium-mediated transformation and identified by PCR. In vitro culture assay showed that the growth of V. dahliae in PDA medium containing proteins extracted from the leaves of transgenic tobacco line P6 was inhibited, whose inhibition rate was 55.1%, higher than the non-transgenic control. These results indicated that StPP5 might be involved in plant defense against V. dahliae infection.

Microcystin-Induced Immunotoxicity in Fishes: A Scoping Review

Cyanobacteria (blue-green algae) have been present on Earth for over 2 billion years, and can produce a variety of bioactive molecules, such as cyanotoxins. Microcystins (MCs), the most frequently detected cyanotoxins, pose a threat to the aquatic environment and to human health. The classic toxic mechanism of MCs is the inhibition of the protein phosphatases 1 and 2A (PP1 and PP2A). Immunity is known as one of the most important physiological functions in the neuroendocrine-immune network to prevent infections and maintain internal homoeostasis in fish. The present review aimed to summarize existing papers, elaborate on the MC-induced immunotoxicity in fish, and put forward some suggestions for future research. The immunomodulatory effects of MCs in fish depend on the exposure concentrations, doses, time, and routes of exposure. Previous field and laboratory studies provided strong evidence of the associations between MC-induced immunotoxicity and fish death. In our review, we summarized that the immunotoxicity of MCs is primarily characterized by the inhibition of PP1 and PP2A, oxidative stress, immune cell damage, and inflammation, as well as apoptosis. The advances in fish immunoreaction upon encountering MCs will benefit the monitoring and prediction of fish health, helping to achieve an ecotoxicological goal and to ensure the sustainability of species. Future studies concerning MC-induced immunotoxicity should focus on adaptive immunity, the hormesis phenomenon and the synergistic effects of aquatic microbial pathogens.

Role of protein phosphatase 2A in kidney disease (Review)

Kidney disease affects millions of people worldwide and is a financial burden on the healthcare system. Protein phosphatase 2A (PP2A), which is involved in renal development and the function of ion-transport proteins, aquaporin-2 and podocytes, is likely to serve an important role in renal processes. PP2A is associated with the pathogenesis of a variety of different kidney diseases including podocyte injury, inflammation, tumors and chronic kidney disease. The current review aimed to discuss the structure and function of PP2A subunits in the context of kidney diseases. How dysregulation of PP2A in the kidneys causes podocyte death and the inactivation of PP2A in renal carcinoma tissues is discussed. Inhibition of PP2A activity prevents epithelial-mesenchymal transition and attenuates renal fibrosis, creating a favorable inflammatory microenvironment and promoting the initiation and progression of tumor pathogenesis. The current review also indicates that PP2A serves an important role in protection against renal inflammation. Understanding the detailed mechanisms of PP2A provides information that can be utilized in the design and application of novel therapeutics for the treatment and prevention of renal diseases.

Protein Phosphatase 2A Activation Via ApoER2 in Trophoblasts Drives Preeclampsia in a Mouse Model of the Antiphospholipid Syndrome

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Tautomycetin Synthetic Analogues: Selective Inhibitors of Protein Phosphatase I

Ser/Thr protein phosphatases (PPs) regulate a substantial range of cellular processes with protein phosphatases 1 (PP1) and 2 A (PP2A) accounting for over 90 % of the activity within cells. Nevertheless, tools to study PPs are limited as PPs inhibitors, particularly those selective for PP1 inhibition, are relatively scarce. Two examples of PP1-selective inhibitors, which share structural similarities, are tautomycin (TTM) and tautomycetin (TTN). This work describes the development of PP1/PP2A inhibitors that incorporate key structural features of TTM and TTN and are designed to conserve regions known to bind the active site of PP1/PP2A but vary regions that differentially contact the hydrophobic groove of PP1/PP2A. In all 28 TTN analogues were synthetically generated that inhibit PP1/PP2A activity at <250 mM; seven possessed inhibition activity at 100 nM. The IC50 values were determined for the seven most active analogues, which ranged from 34 to 1500 nM (PP1) and 70 to 6800 nM (PP2A). Four of the seven analogues possessed PP1 selectivity, and one demonstrated eightfold selectivity in the nanomolar range (PP1 IC50 =34 nM, PP2A IC50 =270 nM). A rationale is given for the observed differences in selectivity.

In Vivo and In Vitro Toxicity Testing of Cyanobacterial Toxins: A Mini-Review

Harmful cyanobacterial blooms are increasing and becoming a worldwide concern as many bloom-forming cyanobacterial species can produce toxic metabolites named cyanotoxins. These include microcystins, saxitoxins, anatoxins, nodularins, and cylindrospermopsins, which can adversely affect humans, animals, and the environment. Different methods to assess these classes of compounds in vitro and in vivo include biological, biochemical, molecular, and physicochemical techniques. Furthermore, toxic effects not attributable to known cyanotoxins can be observed when assessing bloom material. In order to determine exposures to cyanotoxins and to monitor compliance with drinking and bathing water guidelines, it is necessary to have reliable and effective methods for the analysis of these compounds. Many relatively simple low-cost methods can be employed to rapidly evaluate the potential hazard. The main objective of this mini-review is to describe the assessment of toxic cyanobacterial samples using in vitro and in vivo bioassays. Newly emerging cyanotoxins, the toxicity of analogs, or the interaction of cyanobacteria and cyanotoxins with other toxicants, among others, still requires bioassay assessment. This review focuses on some biological and biochemical assays (MTT assay, Immunohistochemistry, Micronucleus Assay, Artemia salina assay, Daphnia magna test, Radionuclide recovery, Neutral red cytotoxicity and Comet assay, Enzyme-Linked Immunosorbent Assay (ELISA), Annexin V-FITC assay and Protein Phosphatase Inhibition Assay (PPIA)) for the detection and measurement of cyanotoxins including microcystins, cylindrospermopsins, anatoxin-a, saxitoxins, and nodularins. Although most bioassay analyses often confirm the presence of cyanotoxins at low concentrations, such bioassays can be used to determine whether some strains or blooms of cyanobacteria may produce other, as yet unknown toxic metabolites. This review also aims to identify research needs and data gaps concerning the toxicity assessment of cyanobacteria.

Chronic Sodium Selenate Treatment Restores Deficits in Cognition and Synaptic Plasticity in a Murine Model of Tauopathy

A major goal in diseases is identifying a potential therapeutic agent that is cost-effective and can remedy some, if not all, disease symptoms. In Alzheimer’s disease (AD), aggregation of hyperphosphorylated tau protein is one of the neuropathological hallmarks, and Tau pathology correlates better with cognitive impairments in AD patients than amyloid-β load, supporting a key role of tau-related mechanisms. Selenium is a non-metallic trace element that is incorporated in the brain into selenoproteins. Chronic treatment with sodium selenate, a non-toxic selenium compound, was recently reported to rescue behavioral phenotypes in tau mouse models. Here, we focused on the effects of chronic selenate application on synaptic transmission and synaptic plasticity in THY-Tau22 mice, a transgenic animal model of tauopathies. Three months with a supplement of sodium selenate in the drinking water (12 μg/ml) restored not only impaired neurocognitive functions but also rescued long-term depression (LTD), a major form of synaptic plasticity. Furthermore, selenate reduced the inactive demethylated catalytic subunit of protein phosphatase 2A (PP2A) in THY-Tau22 without affecting total PP2A.Our study provides evidence that chronic dietary selenate rescues functional synaptic deficits of tauopathy and identifies activation of PP2A as the putative mechanism.

Decrease of protein phosphatase 2A subunit B by glutamate exposure in the cerebral cortex of neonatal rats

Glutamate induces neurotoxicity during brain development, causing nerve damage. Protein phosphatase 2A (PP2A) is a type of serine/threonine phosphatase that regulates various biological functions. Among the PP2A subunit types, subunit B is abundant in brain tissue and plays an essential role in the nervous system. This study investigated changes in PP2A subunit B expression through glutamate exposure in the cerebral cortex of newborn rats. Sprague-Dawley rat pups (7 days after birth) were injected intraperitoneally with vehicle or glutamate (10 mg/kg). After 4 h of drug treatment, the brain tissue was isolated and fixed for morphological study. In addition, the cerebral cortex was collected for RNA and protein works. We observed severe histopathological changes including swollen neuron and atrophied dendrite in the glutamate exposed cerebral cortex. Glutamate exposure leads to a decrease in PP2A subunit B. Reverse-transcription PCR and Western blot analyses confirmed that glutamate induces a decrease of PP2A subunit B in the cerebral cortex of newborn rats. Moreover, immunohistochemical study showed a decrease in PP2A subunit B positive cells. The reduction of PP2A subunit B expression is considered an indicator of neurodegenerative damage. These results suggest that glutamate exposure causes neuronal damage in the cerebral cortex of new born rats through a decrease in PP2A subunit B.

The MID1 gene product in physiology and disease

MID1 is an E3 ubiquitin ligase of the Tripartite Motif (TRIM) subfamily of RING-containing proteins, hence also known as TRIM18. MID1 is a microtubule-binding protein found in complex with the catalytic subunit of PP2A (PP2Ac) and its regulatory subunit alpha 4 (α4). To date, several substrates and interactors of MID1 have been described, providing evidence for the involvement of MID1 in a plethora of essential biological processes, especially during embryonic development. Mutations in the MID1 gene are responsible of the X-linked form of Opitz syndrome (XLOS), a multiple congenital disease characterised by defects in the development of midline structures during embryogenesis. Here, we review MID1-related physiological mechanisms as well as the pathological implication of the MID1 gene in XLOS and in other clinical conditions.

Direct Interaction of PP2A Phosphatase with GABAB Receptors Alters Functional Signaling

Addictive drugs usurp the brain's intrinsic mechanism for reward, leading to compulsive and destructive behaviors. In the ventral tegmental area (VTA), the center of the brain's reward circuit, GABAergic neurons control the excitability of dopamine (DA) projection neurons and are the site of initial psychostimulant-dependent changes in signaling. Previous work established that cocaine/methamphetamine exposure increases protein phosphatase 2A (PP2A) activity, which dephosphorylates the GABABR2 subunit, promotes internalization of the GABAB receptor (GABABR) and leads to smaller GABABR-activated G-protein-gated inwardly rectifying potassium (GIRK) currents in VTA GABA neurons. How the actions of PP2A become selective for a particular signaling pathway is poorly understood. Here, we demonstrate that PP2A can associate directly with a short peptide sequence in the C terminal domain of the GABABR1 subunit, and that GABABRs and PP2A are in close proximity in rodent neurons (mouse/rat; mixed sexes). We show that this PP2A-GABABR interaction can be regulated by intracellular Ca2+ Finally, a peptide that potentially reduces recruitment of PP2A to GABABRs and thereby limits receptor dephosphorylation increases the magnitude of baclofen-induced GIRK currents. Thus, limiting PP2A-dependent dephosphorylation of GABABRs may be a useful strategy to increase receptor signaling for treating diseases.SIGNIFICANCE STATEMENT Dysregulation of GABAB receptors (GABABRs) underlies altered neurotransmission in many neurological disorders. Protein phosphatase 2A (PP2A) is involved in dephosphorylating and subsequent internalization of GABABRs in models of addiction and depression. Here, we provide new evidence that PP2A B55 regulatory subunit interacts directly with a small region of the C-terminal domain of the GABABR1 subunit, and that this interaction is sensitive to intracellular Ca2+ We demonstrate that a short peptide corresponding to the PP2A interaction site on GABABR1 competes for PP2A binding, enhances phosphorylation GABABR2 S783, and affects functional signaling through GIRK channels. Our study highlights how targeting PP2A dependent dephosphorylation of GABABRs may provide a specific strategy to modulate GABABR signaling in disease conditions.

Effect of the Large and Small T-Antigens of Human Polyomaviruses on Signaling Pathways

Viruses are intracellular parasites that require a permissive host cell to express the viral genome and to produce new progeny virus particles. However, not all viral infections are productive and some viruses can induce carcinogenesis. Irrespective of the type of infection (productive or neoplastic), viruses hijack the host cell machinery to permit optimal viral replication or to transform the infected cell into a tumor cell. One mechanism viruses employ to reprogram the host cell is through interference with signaling pathways. Polyomaviruses are naked, double-stranded DNA viruses whose genome encodes the regulatory proteins large T-antigen and small t-antigen, and structural proteins that form the capsid. The large T-antigens and small t-antigens can interfere with several host signaling pathways. In this case, we review the interplay between the large T-antigens and small t-antigens with host signaling pathways and the biological consequences of these interactions.

Quercetin alleviates the injury-induced decrease of protein phosphatase 2A subunit B in cerebral ischemic animal model and glutamate-exposed HT22 cells

Quercetin is a plant flavonoid that has anti-oxidant, anti-inflammatory, anti-cancer, and anti-ischemic properties. Moreover, quercetin exerts neuroprotective effects against focal cerebral ischemia. Protein phosphatase 2A (PP2A) is a form of serine/threonine phosphatase that modulates various biological functions. Among PP2A subunit types, subunit B exists abundantly in brain tissue and plays an essential function in nervous system. We previously reported the decrease of PP2A subunit B in focal cerebral animal model. This study explored the change of PP2A subunit B expression by quercetin treatment in cerebral ischemic animal model and glutamate-treated hippocampal-derived (HT22) cell culture. Quercetin (10 mg/kg) or vehicle was injected intraperitoneally into male rats before 30 min of middle cerebral artery occlusion (MCAO), and cerebral cortices were isolated 24 hr after MCAO. MCAO induced the neurological behavioral deficit and increased infarct volume. However, quercetin treatment attenuated the increase of neurological deficit and infarction. We detected the alleviation of MCAO-induced the decrease in PP2A subunit B by quercetin treatment using a proteomic approach. Reverse-transcription PCR and Western blot analyses confirmed lower PP2A subunit B expression levels in MCAO group with vehicle. However, quercetin treatment attenuated MCAO-induced this reduction. We also observed the neuroprotective effect of quercetin and the change of PP2A subunit B expression in glutamate-exposed HT22 cells. Glutamate exposure dramatically reduced cell viability and PP2A subunit B expression, and quercetin treatment significantly improved these decreases. We clearly showed that quercetin performs a neuroprotective function and modulates down-regulation of PP2A subunit B against MCAO injury and glutamate toxicity. Thus, our finding suggests that the regulation of PP2A subunit B by quercetin contributes to neuroprotective function in ischemic brain injury.

CD25 and Protein Phosphatase 2A Cooperate to Enhance IL-2R Signaling in Human Regulatory T Cells

Low-dose IL-2 therapy is a direct approach to boost regulatory T cells (Tregs) and promote immune tolerance in autoimmune patients. However, the mechanisms responsible for selective response of Tregs to low-dose IL-2 is not fully understood. In this study we directly assessed the contribution of CD25 and protein phosphatase 2A (PP2A) in promoting IL-2R signaling in Tregs. IL-2-induced tyrosine phosphorylation of STAT5 (pSTAT5) was proportional to CD25 levels on human CD4⁺ T cells and YT human NK cell line, directly demonstrating that CD25 promotes IL-2R signaling. Overexpression of the PP2A catalytic subunit (PP2Ac) by lentiviral transduction in human Tregs increased the level of IL-2R subunits and promoted tyrosine phosphorylation of Jak3 and STAT5. Interestingly, increased expression of CD25 only partially accounted for this enhanced activation of pSTAT5, indicating that PP2A promotes IL-2R signaling through multiple mechanisms. Consistent with these findings, knockdown of PP2Ac in human Tregs and impaired PP2Ac activity in mouse Tregs significantly reduced IL-2-dependent STAT5 activation. In contrast, overexpression or knockdown of PP2Ac in human T effector cells did not affect IL-2-dependent pSTAT5 activation. Overexpression of PP2Ac in human Tregs also increased the expressions of proteins related to survival, activation, and immunosuppressive function, and upregulated several IL-2-regulated genes. Collectively, these findings suggest that CD25 and PP2A cooperatively enhance the responsiveness of Tregs to IL-2, which provide potential therapeutic targets for low-dose IL-2 therapy.

Protein Phosphatase 2A: More Than a Passenger in the Regulation of Epithelial Cell-Cell Junctions

Cell–cell adhesion plays a key role in the maintenance of the epithelial barrier and apicobasal cell polarity, which is crucial for homeostasis. Disruption of cell–cell adhesion is a hallmark of numerous pathological conditions, including invasive carcinomas. Adhesion between apposing cells is primarily regulated by three types of junctional structures: desmosomes, adherens junctions, and tight junctions. Cell junctional structures are highly regulated multiprotein complexes that also serve as signaling platforms to control epithelial cell function. The biogenesis, integrity, and stability of cell junctions is controlled by complex regulatory interactions with cytoskeletal and polarity proteins, as well as modulation of key component proteins by phosphorylation/dephosphorylation processes. Not surprisingly, many essential signaling molecules, including protein Ser/Thr phosphatase 2A (PP2A) are associated with intercellular junctions. Here, we examine how major PP2A enzymes regulate epithelial cell–cell junctions, either directly by associating with and dephosphorylating component proteins, or indirectly by affecting signaling pathways that control junctional integrity and cytoskeletal dynamics. PP2A deregulation has severe consequences on the stability and functionality of these structures, and disruption of cell–cell adhesion and cell polarity likely contribute to the link between PP2A dysfunction and human carcinomas.

The Anti-Tumor Agent Sodium Selenate Decreases Methylated PP2A, Increases GSK3βY216 Phosphorylation, Including Tau Disease Epitopes and Reduces Neuronal Excitability in SHSY-5Y Neurons

Selenium application as sodium selenate was repeatedly shown to have anti-carcinogenic properties by increasing levels of the serine/ threonine protein phosphatase 2A (PP2A) in cancer cells. PP2A has a prominent role in cell development, homeostasis, and in neurons regulates excitability. PP2A, GSK3β and Tau reside together in a complex, which facilitates their interaction and (dys)-function as has been reported for several neurological disorders. In this study we recorded maximum increase in total PP2A at 3 µM sodium selenate in a neuron cell line. In conjunction with these data, whole-cell electrophysiological studies revealed that this concentration had maximum effect on membrane potentials, conductance and currents. Somewhat surprisingly, the catalytically active form, methylated PP2A (mePP2A) was significantly decreased. In close correlation to these data, the phosphorylation state of two substrate proteins, sensitive to PP2A activity, GSK3β and Tau were found to be increased. In summary, our data reveal that sodium selenate enhances PP2A levels, but reduces catalytic activity of PP2A in a dose dependent manner, which fails to reduce Tau and GSK3β phosphorylation under physiological conditions, indicating an alternative route in the rescue of cell pathology in neurological disorders.

FcαRI Dynamics Are Regulated by GSK-3 and PKCζ During Cytokine Mediated Inside-Out Signaling

IgA binding to FcαRI (CD89) is rapidly enhanced by cytokine induced inside-out signaling. Dephosphorylation of serine 263 in the intracellular tail of FcαRI by PP2A and PI3K activation are instrumental in this process. To further investigate these signaling pathways, we targeted downstream kinases of PI3K. Our experiments revealed that PI3K activates PKCζ, which subsequently inhibits GSK-3, a constitutively active kinase in resting cells and found here to be associated with FcαRI. We propose that GSK-3 maintains FcαRI in an inactive state at homeostatic conditions. Upon cytokine stimulation, GSK-3 is inactivated through a PI3K-PKCζ pathway, preventing the maintenance of phosphorylated inactive FcαRI. The concomitantly activated PP2A is then able to dephosphorylate and activate FcαRI. Moreover, FRAP and FLIP studies showed that FcαRI activation coincides with an increased mobile fraction of the receptor. This can enhance FcαRI valency and contribute to stronger avidity for IgA immune complexes. This tightly regulated inside-out signaling pathway allows leukocytes to respond rapidly and efficiently to their environment and could be exploited to enhance the efficacy of future IgA therapeutics.

Architecture, substructures, and dynamic assembly of STRIPAK complexes in Hippo signaling

Striatin-interacting phosphatases and kinases (STRIPAKs) are evolutionarily conserved supramolecular complexes, which have been implicated in the Hippo signaling pathway. Yet the topological structure and dynamic assembly of STRIPAK complexes remain elusive. Here, we report the overall architecture and substructures of a Hippo kinase-containing STRIPAK complex. PP2Aa/c-bound STRN3 directly contacts the Hippo kinase MST2 and also controls the loading of MST2 via two “arms” in a phosphorylation-dependent manner, one arm being STRIP1 and the other SIKE1-SLMAP. A decreased cell density triggered the dissociation of the STRIP1 arm from STRIPAK, reflecting the dynamic assembly of the complex upon sensing upstream signals. Crystallographic studies defined at atomic resolution the interface between STRN3 and SIKE1, and that between SIKE1 and SLMAP. Disrupting the complex assembly abrogated the regulatory effect of STRIPAK towards Hippo signaling. Collectively, our study revealed a “two-arm” assembly of STRIPAK with context-dependent dynamics, offering a framework for further studies on Hippo signaling and biological processes involving MST kinases.

Protein phosphatase 2A as therapeutic targets in various disease models

There are a large number of signalling pathways responsible for transmitting information within the cell. Although cellular signalling is thought to be majorly governed by protein kinases 'cascade effects'; their antagonists protein phosphatases also play a crucial dual role in signal transduction. By dephosphorylating the proteins involved in signalling pathways, phosphatases may lead to their activation and sometimes they may terminate a signal generated by kinases activity. Due to counterbalancing the function of phosphorylation, the protein phosphatases are very important to signal transduction processes and thus the control of phosphatase activity is as significant as kinases, in the regulation of a plethora of cellular processes. In general, the protein phosphatases are comprised of a catalytic subunit with one or more regulatory and/or targeting subunits associated with it. The Protein Phosphatase 2A (PP2A), a member of serine/threonine phosphatases family, is ubiquitously expressed a remarkably conserved enzyme in the cell. Its catalytic activity has been highly regulated and may have enormous therapeutic potential which is still untapped. It has specificities for a number of substrates which witnessed its involvement in various signalling modules of cell cycle regulation, cell morphology and development. Thus it can be an appropriate target for studying different diseases associated with abnormal signal transduction pathways such as neurodegenerative diseases and malignancies. This review will focus on the structure and regulatory pathways of PP2A. The de-regulation of PP2A in some specific pathology such as Cancer, Heart diseases, Neurodegenerative disorders and Diabetes will also be touched upon.

G Protein-Coupled Receptor Resensitization Paradigms

Cellular responses to extracellular milieu/environment are driven by cell surface receptors that transmit the signal into the cells resulting in a synchronized and measured response. The ability to provide such exquisite responses to changes in external environment is mediated by the tight and yet, deliberate regulation of cell surface receptor function. In this regard, the seven transmembrane G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors that regulate responses like cardiac contractility, vision, and olfaction including platelet activation. GPCRs regulate these plethora of events through GPCR-activation, -desensitization, and -resensitization. External stimuli (ligands or agonists) activate GPCR initiating downstream signals. The activated GPCR undergoes inactivation or desensitization by phosphorylation and binding of β-arrestin resulting in diminution of downstream signals. The desensitized GPCRs are internalized into endosomes, wherein they undergo dephosphorylation or resensitization by protein phosphatase to be recycled back to the cell membrane as naïve GPCR ready for the next wave of stimuli. Despite the knowledge that activation, desensitization, and resensitization shoulder an equal role in maintaining GPCR function, major advances have been made in understanding activation and desensitization compared to resensitization. However, increasing evidence shows that resensitization is exquisitely regulated process, thereby contributing to the dynamic regulation of GPCR function. In recognition of these observations, in this chapter we discuss the key advances on the mechanistic underpinning that drive and regulate GPCR function with a focus on resensitization.

PPP2R2D, a regulatory subunit of protein phosphatase 2A, promotes gastric cancer growth and metastasis via mechanistic target of rapamycin activation

Protein phosphatase 2A (PP2A) is an essential serine/threonine protein phosphatase that regulates the basic activities of eukaryotes by dephosphorylating its substrates. The function and substrate specificity of PP2A are generally determined by its regulatory subunits. In the present study, the clinical significance and roles of PPP2R2D, one of the regulatory subunits of PP2A, were demonstrated in gastric cancer (GC) carcinogenesis. Through a tissue microarray and quantitative polymerase chain reaction analysis, it was demonstrated that PPP2R2D was commonly upregulated in GC samples. This upregulation was positively correlated with the patients' tumor stage (P<0.01), T classification (P<0.01) and N classification (P=0.01). Furthermore, a high expression of PPP2R2D was closely associated with poor prognosis of patients. Knockdown of PPP2R2D significantly inhibited the proliferation and migration of GC cells in vitro, as well as the tumorigenicity and metastasis in vivo in an animal GC model. By contrast, overexpression of PPP2R2D promoted GC cell proliferation and migration in vitro. The analysis of underlying mechanisms indicated that PPP2R2D silencing decreased the phosphorylation level of mechanistic target of rapamycin (mTOR), thereby implicating that PPP2R2D is involved in the regulation of mTOR activity during tumorigenesis. Thus, the findings of the present study suggested that PPP2R2D may serve as a potential oncogene in GC and as a novel target for therapeutic strategies against this disease.

Antiphospholipid antibodies induce thrombosis by PP2A activation via apoER2-Dab2-SHC1 complex formation in endothelium

In the antiphospholipid syndrome (APS), antiphospholipid antibody (aPL) recognition of β2 glycoprotein I promotes thrombosis, and preclinical studies indicate that this is due to endothelial nitric oxide synthase (eNOS) antagonism via apolipoprotein E receptor 2 (apoER2)-dependent processes. How apoER2 molecularly links these events is unknown. Here, we show that, in endothelial cells, the apoER2 cytoplasmic tail serves as a scaffold for aPL-induced assembly and activation of the heterotrimeric protein phosphatase 2A (PP2A). Disabled-2 (Dab2) recruitment to the apoER2 NPXY motif promotes the activating L309 methylation of the PP2A catalytic subunit by leucine methyl transferase-1. Concurrently, Src homology domain-containing transforming protein 1 (SHC1) recruits the PP2A scaffolding subunit to the proline-rich apoER2 C terminus along with 2 distinct regulatory PP2A subunits that mediate inhibitory dephosphorylation of Akt and eNOS. In mice, the coupling of these processes in endothelium is demonstrated to underlie aPL-invoked thrombosis. By elucidating these intricacies in the pathogenesis of APS-related thrombosis, numerous potential new therapeutic targets have been identified.

The tumor suppressor phosphatase PP2A-B56α regulates stemness and promotes the initiation of malignancies in a novel murine model

Protein phosphatase 2A (PP2A) is a ubiquitously expressed Serine-Threonine phosphatase mediating 30–50% of protein phosphatase activity. PP2A functions as a heterotrimeric complex, with the B subunits directing target specificity to regulate the activity of many key pathways that control cellular phenotypes. PP2A-B56α has been shown to play a tumor suppressor role and to negatively control c-MYC stability and activity. Loss of B56α promotes cellular transformation, likely at least in part through its regulation of c-MYC. Here we report generation of a B56α hypomorph mouse with very low B56α expression that we used to study the physiologic activity of the PP2A-B56α phosphatase. The predominant phenotype we observed in mice with B56α deficiency in the whole body was spontaneous skin lesion formation with hyperproliferation of the epidermis, hair follicles and sebaceous glands. Increased levels of c-MYC phosphorylation on Serine62 and c-MYC activity were observed in the skin lesions of the B56α^hm/hm mice. B56α deficiency was found to increase the number of skin stem cells, and consistent with this, papilloma initiation was accelerated in a carcinogenesis model. Further analysis of additional tissues revealed increased inflammation in spleen, liver, lung, and intestinal lymph nodes as well as in the skin lesions, resembling elevated extramedullary hematopoiesis phenotypes in the B56α^hm/hm mice. We also observed an increase in the clonogenicity of bone marrow stem cells in B56α^hm/hm mice. Overall, this model suggests that B56α is important for stem cells to maintain homeostasis and that B56α loss leading to increased activity of important oncogenes, including c-MYC, can result in aberrant cell growth and increased stem cells that can contribute to the initiation of malignancy.

PHD2 Targeting Overcomes Breast Cancer Cell Death upon Glucose Starvation in a PP2A/B55α-Mediated Manner

B55α is a regulatory subunit of the PP2A phosphatase. We have recently found that B55α-associated PP2A promotes partial deactivation of the HIF-prolyl-hydroxylase enzyme PHD2. Here, we show that, in turn, PHD2 triggers degradation of B55α by hydroxylating it at proline 319. In the context of glucose starvation, PHD2 reduces B55α protein levels, which correlates with MDA-MB231 and MCF7 breast cancer cell death. Under these conditions, PHD2 silencing rescues B55α degradation, overcoming apoptosis, whereas in SKBR3 breast cancer cells showing resistance to glucose starvation, B55α knockdown restores cell death and prevents neoplastic growth in vitro. Treatment of MDA-MB231-derived xenografts with the glucose competitor 2-deoxy-glucose leads to tumor regression in the presence of PHD2. Knockdown of PHD2 induces B55α accumulation and treatment resistance by preventing cell apoptosis. Overall, our data unravel B55α as a PHD2 substrate and highlight a role for PHD2-B55α in the response to nutrient deprivation.

Protein phosphatase 2A and tau: an orchestrated 'Pas de Deux'

The neuronal microtubule-associated protein tau serves a critical role in regulating axonal microtubule dynamics to support neuronal and synaptic functions. Furthermore, it contributes to glutamatergic regulation and synaptic plasticity. Emerging evidence also suggests that tau serves as a signaling scaffold. Tau function and subcellular localization are tightly regulated, in part, by the orchestrated interplay between phosphorylation and dephosphorylation events. Significantly, protein phosphatase type 2A (PP2A), encompassing the regulatory PPP2R2A (or Bα) subunit, is a major brain heterotrimeric enzyme and the primary tau Ser/Thr phosphatase in vivo. Herein, we closely examine how the intimate and compartmentalized interactions between PP2A and tau regulate tau phosphorylation and function, and play an essential role in neuronal homeostasis. We also review evidence supporting a strong link between deregulation of tau-PP2A functional interactions and the molecular underpinnings of various neurodegenerative diseases collectively called tauopathies. Lastly, we discuss the opportunities and associated challenges in more specifically targeting PP2A-tau interactions for drug development for tauopathies.

Estradiol suppresses phosphorylation of ERα serine 167 through upregulation of PP2A in breast cancer cells

Aromatase inhibitors (AIs) are effective endocrine therapeutics for postmenopausal women with estrogen receptor (ER)α-positive breast cancer. However, the efficacy of the treatment is often limited by the onset of AI resistance, owing to the phosphorylation of ERα serine 167 (Ser167). Previous studies have indicated that hyperactivation of the phosphoinositide-3 kinase/RAC serine/threonine-protein kinase signaling pathway occurs in AI-resistant breast cancer models, which coincides with elevated levels of ERα phosphorylation at Ser167. The tumor suppressor serine/threonine-protein phosphatase 2A (PP2A) regulates the phosphatidylinositol 3-kinase/RAC serine/threonine-protein kinase signaling pathway. A previous study indicated that PP2A inhibition decreased ERα Ser167 phosphorylation and estradiol (E₂)-independent cell growth. The present study investigated the potential relevance of PP2A in E₂ deprivation-resistant MCF-7 cells. E₂ depletion reduced the susceptibility of MCF-7 cells to inhibitors of mechanistic target of rapamycin (mTOR) and significantly increased ERα Ser167 phosphorylation and decreased expression of PP2A. Conversely, long-term E₂-deprived (LTED) MCF-7 cells, a model of AI-resistant breast cancer, exhibited decreased ERα Ser167 phosphorylation and further upregulation of PP2A in E₂-containing medium. The PP2A activator forskolin (FSK) significantly inhibited LTED cell proliferation by increasing the effect of everolimus (Eve), an mTOR inhibitor. In summary, the present study provides further evidence that PP2A represents a therapeutic target for AI-resistant breast cancer.

A novel function of twins, B subunit of protein phosphatase 2A, in regulating actin polymerization

Actin is an important component of the cytoskeleton and its polymerization is delicately regulated by several kinases and phosphatases. Heterotrimeric protein phosphatase 2A (PP2A) is a potent phosphatase that is crucial for cell proliferation, apoptosis, tumorigenesis, signal transduction, cytoskeleton arrangement, and neurodegeneration. To facilitate these varied functions, different regulators determine the different targets of PP2A. Among these regulators of PP2A, the B subunits in particular may be involved in cytoskeleton arrangement. However, little is known about the role of PP2A in actin polymerization in vivo. Using sophisticated fly genetics, we demonstrated a novel function for the fly B subunit, twins, to promote actin polymerization in varied tissue types, suggesting a broad and conserved effect. Furthermore, our genetic data suggest that twins may act upstream of the actin-polymerized-proteins, Moesin and Myosin-light-chain, and downstream of Rho to promote actin polymerization. This work opens a new avenue for exploring the biological functions of a PP2A regulator, twins, in cytoskeleton regulation.

Noncanonical regulation of insulin-mediated ERK activation by phosphoinositide 3-kinase γ

Classically, Class IB phosphoinositide 3-kinase (PI3Kγ) plays a role in ERK activation following G-protein–coupled receptor (GPCR) activation. Here we show that PI3Kγ noncanonically regulates ERK phosphorylation in a kinase-independent mechanism, irrespective of the upstream signals. PI3Kγ sequesters PP2A, allowing sustained ERK function.

Classically Class IB phosphoinositide 3-kinase (PI3Kγ) plays a role in extracellular signal–regulated kinase (ERK) activation following G-protein coupled receptor (GPCR) activation. Knock-down of PI3Kγ unexpectedly resulted in loss of ERK activation to receptor tyrosine kinase agonists such as epidermal growth factor or insulin. Mouse embryonic fibroblasts (MEFs) or primary adult cardiac fibroblasts isolated from PI3Kγ knock-out mice (PI3KγKO) showed decreased insulin-stimulated ERK activation. However, expression of kinase-dead PI3Kγ resulted in rescue of insulin-stimulated ERK activation. Mechanistically, PI3Kγ sequesters protein phosphatase 2A (PP2A), disrupting ERK–PP2A interaction, as evidenced by increased ERK–PP2A interaction and associated PP2A activity in PI3KγKO MEFs, resulting in decreased ERK activation. Furthermore, β-blocker carvedilol-mediated β-arrestin-dependent ERK activation is significantly reduced in PI3KγKO MEF, suggesting accelerated dephosphorylation. Thus, instead of classically mediating the kinase arm, PI3Kγ inhibits PP2A by scaffolding and sequestering, playing a key parallel synergistic step in sustaining the function of ERK, a nodal enzyme in multiple cellular processes.

Hyperglycemia reduces PP2A subunit B expression in a middle cerebral artery occlusion animal model

Diabetes is a metabolic disorder that worsens clinical outcome following cerebral ischemia. Protein phosphatase 2A (PP2A) is a conserved, heterotrimeric, serine/threonine phosphatase with various cellular functions. PP2A subunit B is abundant in brain tissue and modulates PP2A function. The aim of this study was to investigate PP2A subunit B protein expression in the cerebral cortex of non-diabetic and diabetic animals with middle cerebral artery occlusion (MCAO) injury. Sprague-Dawley rats were injected with streptozotocin (40 mg/kg, i.p.) to induce diabetic conditions. After 4 weeks of streptozotocin treatment, the rats underwent MCAO to induce focal cerebral ischemia. The cerebral cortex tissue was collected 24 hr after MCAO. Body weight and blood glucose were measured, and Western blot analysis was performed to elucidate the expression of PP2A subunit B. We confirmed decreased body weight and increased blood glucose in diabetic animals. Reverse transcription-PCR and Western blot analyses showed decreased PP2A subunit B expression in the cerebral cortices of MCAO-injured animals. Moreover, diabetic animals with MCAO showed more severe decreases in PP2A subunit B protein levels than non-diabetic animals following MCAO. The decline of PP2A subunit B indicates degradation of neuronal function. These findings suggest that conspicuous decreases in PP2A subunit B may exacerbate cerebral ischemia under diabetic conditions following MCAO.

miR-3941: A novel microRNA that controls IGBP1 expression and is associated with malignant progression of lung adenocarcinoma

Immunoglobulin (CD79a) binding protein 1 (IGBP1) is universally overexpressed in lung adenocarcinoma and exerts an anti‐apoptotic effect by binding to PP2Ac. However, the molecular mechanism of IGBP1 overexpression is still unclear. In the present study, we used a microRNA (miRNA) array and TargetScan Human software to detect IGBP1‐related miRNAs that regulate IGBP1 expression. The miRNA array analysis revealed more than 100 miRNAs that are dysregulated in early invasive adenocarcinoma. On the other hand, in silico analysis using TargetScan Human revealed 79 miRNAs that are associated with IGBP1 protein expression. Among the miRNAs selected by miRNA array analysis, six (miR‐34b, miR‐138, miR‐374a, miR‐374b, miR‐1909, miR‐3941) were also included among those selected by TargetScan analysis. Real‐time reverse transcription PCR (real‐time RT‐PCR) showed that the six microRNAs were downregulated in invasive adenocarcinoma (IGBP1+) relative to adjacent normal lung tissue (IGBP1−). Among these microRNAs, only miR‐34b and miR‐3941 depressed luciferase activity by targeting 3′UTR‐IGBP1 in the luciferase vector. We transfected miR‐34b and miR‐3941 into lung adenocarcinoma cell lines (A549, PC‐9), and both of them suppressed IGBP1 expression and cell proliferation. Moreover, the transfected miR‐34b and miR‐3941 induced apoptosis of a lung adenocarcinoma cell line, similarly to the effect of siIGBP1 RNA. As well as miR‐34b, we found that miR‐3941 targeted IGBP1 specifically and was able to exclusively downregulate IGBP1 expression. These findings indicate that suppression of miR‐3941 has an important role in the progression of lung adenocarcinoma at an early stage.

Carfilzomib induces leukaemia cell apoptosis via inhibiting ELK1/KIAA1524 (Elk-1/CIP2A) and activating PP2A not related to proteasome inhibition

Enhancing the tumour suppressive activity of protein phosphatase 2A (PP2A) has been suggested to be an anti-leukaemic strategy. KIAA1524 (also termed CIP2A), an oncoprotein inhibiting PP2A, is associated with disease progression in chronic myeloid leukaemia and may be prognostic in cytogenetically normal acute myeloid leukaemia. Here we demonstrated that the selective proteasome inhibitor, carfilzomib, induced apoptosis in sensitive primary leukaemia cells and in sensitive leukaemia cell lines, associated with KIAA1524 protein downregulation, increased PP2A activity and decreased p-Akt, but not with the proteasome inhibition effect of carfilzomib. Ectopic expression of KIAA1524, or pretreatment with the PP2A inhibitor, okadaic acid, suppressed carfilzomib-induced apoptosis and KIAA1524 downregulation in sensitive cells, whereas co-treatment with the PP2A agonist, forskolin, enhanced carfilzomib-induced apoptosis in resistant cells. Mechanistically, carfilzomib affected KIAA1524 transcription through disturbing ELK1 (Elk-1) binding to the KIAA1524 promoter. Moreover, the drug sensitivity and mechanism of carfilzomib in xenograft mouse models correlated well with the effects of carfilzomib on KIAA1524 and p-Akt expression, as well as PP2A activity. Our data disclosed a novel drug mechanism of carfilzomib in leukaemia cells and suggests the potential therapeutic implication of KIAA1524 in leukaemia treatment.

The protein serine/threonine phosphatases PP2A, PP1 and calcineurin: A triple threat in the regulation of the neuronal cytoskeleton

The microtubule, F-actin and neurofilament networks play a critical role in neuronal cell morphogenesis, polarity and synaptic plasticity. Significantly, the assembly/disassembly and stability of these cytoskeletal networks is crucially modulated by protein phosphorylation and dephosphorylation events. Herein, we aim to more closely examine the role played by three major neuronal Ser/Thr protein phosphatases, PP2A, PP1 and calcineurin, in the homeostasis of the neuronal cytoskeleton. There is strong evidence that these enzymes interact with and dephosphorylate a variety of cytoskeletal proteins, resulting in major regulation of neuronal cytoskeletal dynamics. Conversely, we also discuss how multi-protein cytoskeletal scaffolds can also influence the regulation of these phosphatases, with important implications for neuronal signalling and homeostasis. Not surprisingly, deregulation of these cytoskeletal scaffolds and phosphatase dysfunction are associated with many neurological diseases.

Phosphatase inhibition increases AQP2 accumulation in the rat IMCD apical plasma membrane

Vasopressin triggers the phosphorylation and apical plasma membrane accumulation of aquaporin 2 (AQP2), and it plays an essential role in urine concentration. Vasopressin, acting through protein kinase A, phosphorylates AQP2. However, the phosphorylation state of AQP2 could also be affected by the action of protein phosphatases (PPs). Rat inner medullas (IM) were incubated with calyculin (PP1 and PP2A inhibitor, 50 nM) or tacrolimus (PP2B inhibitor, 100 nM). Calyculin did not affect total AQP2 protein abundance (by Western blot) but did significantly increase the abundances of pS256-AQP2 and pS264-AQP2. It did not change pS261-AQP2 or pS269-AQP2. Calyculin significantly enhanced the membrane accumulation (by biotinylation) of total AQP2, pS256-AQP2, and pS264-AQP2. Likewise, immunohistochemistry showed an increase in the apical plasma membrane association of pS256-AQP2 and pS264-AQP2 in calyculin-treated rat IM. Tacrolimus also did not change total AQP2 abundance but significantly increased the abundances of pS261-AQP2 and pS264-AQP2. In contrast to calyculin, tacrolimus did not change the amount of total AQP2 in the plasma membrane (by biotinylation and immunohistochemistry). Tacrolimus did increase the expression of pS264-AQP2 in the apical plasma membrane (by immunohistochemistry). In conclusion, PP1/PP2A regulates the phosphorylation and apical plasma membrane accumulation of AQP2 differently than PP2B. Serine-264 of AQP2 is a phosphorylation site that is regulated by both PP1/PP2A and PP2B. This dual regulatory pathway may suggest a previously unappreciated role for multiple phosphatases in the regulation of urine concentration.

Dysregulation of protein phosphatase 2A in parkinson disease and dementia with lewy bodies

OBJECTIVE

Protein phosphatase 2A (PP2A) is a heterotrimeric holoenzyme composed of a catalytic C subunit, a structural A subunit, and one of several regulatory B subunits that confer substrate specificity. The assembly and activity of PP2A are regulated by reversible methylation of the C subunit. α-Synuclein, which aggregates in Parkinson disease (PD) and dementia with Lewy bodies (DLB), is phosphorylated at Ser129, and PP2A containing a B55α subunit is a major phospho-Ser129 phosphatase. The objective of this study was to investigate PP2A in α-synucleinopathies.

METHODS

We compared the state of PP2A methylation, as well as the expression of its methylating enzyme, leucine carboxyl methyltransferase (LCMT-1), and demethylating enzyme, protein phosphatase methylesterase (PME-1), in postmortem brains from PD and DLB cases as well as age-matched Controls. Immunohistochemical studies and quantitative image analysis were employed.

RESULTS

LCMT-1 was significantly reduced in the substantia nigra (SN) and frontal cortex in both PD and DLB. PME-1, on the other hand, was elevated in the PD SN. In concert with these changes, the ratio of methylated PP2A to demethylated PP2A was markedly decreased in PD and DLB brains in both SN and frontal cortex. No changes in total PP2A or total B55α subunit were detected.

INTERPRETATION

These findings support the hypothesis that PP2A dysregulation in α-synucleinopathies may contribute to the accumulation of hyperphosphorylated α-synuclein and to the disease process, raising the possibility that pharmacological means to enhance PP2A phosphatase activity may be a useful disease-modifying therapeutic approach.

Protein phosphatase 2A regulatory subunit B56β modulates erythroid differentiation

Anemia due to attenuated erythroid terminal differentiation is one of the most common hematological disorders occurring at all stages of life. We previously demonstrated that catalytic subunit α of protein phosphatase 2A (PP2Acα) modulates fetal liver erythropoiesis. However the corresponding PP2A regulatory subunit in this process remains unknown. In this study, we report that chemical inhibition of PP2A activity with okadaic acid impairs hemin-induced erythroid differentiation. Interestingly, B56 family member B56β is the only regulatory subunit whose expression is induced by both erythropoietin in fetal liver cells and hemin in erythroleukemia K562 cells. Finally, knockdown of B56β attenuates hemin-induced K562 erythroid differentiation. Collectively, our data identify B56β as the potential functional regulatory subunit of PP2A in erythroid differentiation, shedding light on new target for precise modulation of PP2A activity for treatment of anemia and related diseases.

Comparative Analysis of Salivary Gland Proteomes of Two Glossina Species that Exhibit Differential Hytrosavirus Pathologies

Glossina pallidipes salivary gland hypertrophy virus (GpSGHV; family Hytrosaviridae) is a dsDNA virus exclusively pathogenic to tsetse flies (Diptera; Glossinidae). The 190 kb GpSGHV genome contains 160 open reading frames and encodes more than 60 confirmed proteins. The asymptomatic GpSGHV infection in flies can convert to symptomatic infection that is characterized by overt salivary gland hypertrophy (SGH). Flies with SGH show reduced general fitness and reproductive dysfunction. Although the occurrence of SGH is an exception rather than the rule, G. pallidipes is thought to be the most susceptible to expression of overt SGH symptoms compared to other Glossina species that are largely asymptomatic. Although Glossina salivary glands (SGs) play an essential role in GpSGHV transmission, the functions of the salivary components during the virus infection are poorly understood. In this study, we used mass spectrometry to study SG proteomes of G. pallidipes and G. m. morsitans, two Glossina model species that exhibit differential GpSGHV pathologies (high and low incidence of SGH, respectively). A total of 540 host proteins were identified, of which 23 and 9 proteins were significantly up- and down-regulated, respectively, in G. pallidipes compared to G. m. morsitans. Whereas 58 GpSGHV proteins were detected in G. pallidipes F₁ progenies, only 5 viral proteins were detected in G. m. morsitans. Unlike in G. pallidipes, qPCR assay did not show any significant increase in virus titers in G. m. morsitans F₁ progenies, confirming that G. m. morsitans is less susceptible to GpSGHV infection and replication compared to G. pallidipes. Based on our results, we speculate that in the case of G. pallidipes, GpSGHV employs a repertoire of host intracellular signaling pathways for successful infection. In the case of G. m. morsitans, antiviral responses appeared to be dominant. These results are useful for designing additional tools to investigate the Glossina-GpSGHV interactions.

PP2Acα positively regulates the termination of liver regeneration in mice through the AKT/GSK3β/Cyclin D1 pathway

BACKGROUND & AIMS

Liver injury triggers a highly organized and ordered liver regeneration (LR) process. Once regeneration is complete, a stop signal ensures that the regenerated liver is an appropriate functional size. The inhibitors and stop signals that regulate LR are unknown, and only limited information is available about these mechanisms.

METHODS

A 70% partial hepatectomy (PH) was performed in hepatocyte-specific PP2Acα-deleted (PP2Acα(-/-)) and control (PP2Acα(+/+)) mice. LR was estimated by liver weight, serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels and cell proliferation, and the related cellular signals were analyzed.

RESULTS

We found that the catalytic subunit of PP2A was markedly upregulated during the late stage of LR. PP2Acα(-/-) mice showed prolonged LR termination, an increased liver size compared to the original mass and lower levels of serum ALT and AST compared with control mice. In these mice, cyclin D1 protein levels, but not mRNA levels, were increased. Mechanistically, AKT activated by the loss of PP2Acα inhibited glycogen synthase kinase 3β (GSK3β) activity, which led to the accumulation of cyclin D1 protein and accelerated hepatocyte proliferation at the termination stage. Treatment with the PI3K inhibitor wortmannin at the termination stage was sufficient to inhibit cyclin D1 accumulation and hepatocyte proliferation.

CONCLUSIONS

PP2Acα plays an essential role in the proper termination of LR via the AKT/GSK3β/Cyclin D1 pathway. Our findings enrich the understanding of the molecular mechanism that controls the termination of LR and provides a potential therapeutic target for treating liver injury.

[6]-Gingerol Affects Glucose Metabolism by Dual Regulation via the AMPKα2-Mediated AS160-Rab5 Pathway and AMPK-Mediated Insulin Sensitizing Effects

[6]-Gingerol has been used to control diabetes and dyslipidemia; however, its metabolic role is poorly understood. In this study, [6]-gingerol increased adenosine monophosphate (AMP)-activated protein kinase (AMPK) phosphorylation in mouse skeletal muscle C2C12 cells. Stimulation of glucose uptake by [6]-gingerol was dependent on AMPKα2. Moreover, both Inhibition and knockdown of AMPKα2 blocked [6]-gingerol-induced glucose uptake. [6]-Gingerol significantly decreased the activity of protein phosphatase 2A (PP2A). Inhibition of PP2A activity with okadaic acid enhanced the phosphorylation of AMPKα2. Moreover, the interaction between AMPKα2 and PP2A was increased by [6]-gingerol, suggesting that PP2A mediates the effect of [6]-gingerol on AMPK phosphorylation. In addition, [6]-gingerol increased the phosphorylation of Akt-substrate 160 (AS160), which is a Rab GTPase-activating protein. Inhibition of AMPKα2 blocked [6]-gingerol-induced AS160 phosphorylation. [6]-gingerol increased the Rab5, and AMPKα2 knockdown blocked [6]-gingerol-induced expression of Rab5, indicating AMPK play as an upstream of Rab5. It also increased glucose transporter 4 (GLUT4) mRNA and protein expression and stimulated GLUT4 translocation. Furthermore, insulin-mediated glucose uptake and Akt phosphorylation were further potentiated by [6]-gingerol treatment. This potentiation was not observed in the presence of AMPK inhibitor compound C. In summary, our results suggest that [6]-gingerol plays an important role in glucose metabolism via the AMPKα2-mediated AS160-Rab5 pathway and through potentiation of insulin-mediated glucose regulation.

Dopamine D2 receptors' effects on renal inflammation are mediated by regulation of PP2A function

Lack or downregulation of the dopamine D2 receptor (D2R) results in increased renal expression of injury markers and proinflammatory factors that is independent of a blood pressure increase. This study aimed to determine the mechanisms involved in the regulation of renal inflammation by D2Rs. Silencing D2Rs in mouse renal proximal tubule cells increased the expression of the proinflammatory TNF-α, monocyte chemoattractant protein-1 (MCP-1), and IL-6. D2R downregulation also increased Akt phosphorylation and activity, and glycogen synthase kinase-3β (GSK3β) phosphorylation and cyclin D1 expression, downstream targets of Akt; however. phosphatidylinositol 3-kinase (PI3K) activity was not affected. Conversely, D2R stimulation decreased Akt and GSK3β phosphorylation and cyclin D1 expression. Increased phospho-Akt, in the absence of increased PI3K activity, may result from decreased Akt dephosphorylation. Inhibition of protein phosphatase 2A (PP2A) with okadaic acid reproduced the effects of D2R downregulation on Akt, GSK3β, and cyclin D1. The PP2A catalytic subunit and regulatory subunit PPP2R2C coimmunoprecipitated with the D2R. Basal phosphatase activity and the expression of PPP2R2C were decreased by D2R silencing that also blunted the increase in phosphatase activity induced by D2R stimulation. Similarly, silencing PPP2R2C also increased the phosphorylation of Akt and GSK3β. Moreover, downregulation of PPP2R2C resulted in increased expression of TNF-α, MCP-1, and IL-6, indicating that decreased phosphatase activity may be responsible for the D2R effect on inflammatory factors. Indeed, the increase in NF-κB reporter activity induced by D2R silencing was blunted by increasing PP2A activity with protamine. Our results show that D2R controls renal inflammation, at least in part, by modulation of the Akt pathway through effects on PP2A activity/expression.

Protein phosphatase 2A plays an important role in migration of bone marrow stroma cells

Administration of bone marrow stroma cells (BMSCs) has the potential to ameliorate degenerative disorders and to repair injured sites. The homing of transplanted BMSCs to damaged tissues is a critical property of engraftment. Therefore, it is important to understand signal molecules controlling migration of BMSCs. Here, we demonstrate that serine-threonine protein phosphatase 2A (PP2A) is responsive to migration of BMSCs. Pharmacological Inhibition of PP2A, using okadaic acid (OA), leads to attenuated cell migration in rat primary BMSCs both in the absence or presence of stromal cell-derived factor-1 (SDF-1). Consistent with the above findings, knockdown of the main catalytic subunit PP2Acα using small interfering RNA also attenuates chemotaxis of BMSCs. On the other hand, cell viability of BMSCs remains unchanged with OA treatment or knockdown of PP2Acα subunit. Moreover, we observed an upregulation of PP2A-B55β in transcription level after SDF-1 treatment, indicating their potential role as the functioning regulatory subunit of PP2A phosphatase in BMSCs migration model. Collectively, these data provide first insight into the modulation of BMSCs migration by PP2A phosphatase activity and lay a foundation for exploring PP2A signaling as a modulating target for BMSCs transplantation.

Changes in Carboxy Methylation and Tyrosine Phosphorylation of Protein Phosphatase PP2A Are Associated with Epididymal Sperm Maturation and Motility

Mammalian sperm contain the serine/threonine phosphatases PP1γ2 and PP2A. The role of sperm PP1γ2 is relatively well studied. Here we confirm the presence of PP2A in sperm and show that it undergoes marked changes in methylation (leucine 309), tyrosine phosphorylation (tyrosine 307) and catalytic activity during epididymal sperm maturation. Spermatozoa isolated from proximal caput, distal caput and caudal regions of the epididymis contain equal immuno-reactive amounts of PP2A. Using demethyl sensitive antibodies we show that PP2A is methylated at its carboxy terminus in sperm from the distal caput and caudal regions but not in sperm from the proximal caput region of the epididymis. The methylation status of PP2A was confirmed by isolation of PP2A with microcystin agarose followed by alkali treatment, which causes hydrolysis of protein carboxy methyl esters. Tyrosine phosphorylation of sperm PP2A varied inversely with methylation. That is, PP2A was tyrosine phosphorylated when it was demethylated but not when methylated. PP2A demethylation and its reciprocal tyrosine phosphorylation were also affected by treatment of sperm with L-homocysteine and adenosine, which are known to elevate intracellular S-adenosylhomocysteine, a feedback inhibitor of methyltransferases. Catalytic activity of PP2A declined during epididymal sperm maturation. Inhibition of PP2A by okadaic acid or by incubation of caudal epididymal spermatozoa with L-homocysteine and adenosine resulted in increase of sperm motility parameters including percent motility, velocity, and lateral head amplitude. Demethylation or pharmacological inhibition of PP2A also leads to an increase in phosphorylation of glycogen synthase kinase-3 (GSK3). Our results show for the first time that changes in PP2A activity due to methylation and tyrosine phosphorylation occur in sperm and that these changes may play an important role in the regulation of sperm function.