Autophosphorylation-activated protein kinase inactivates the protein tyrosine phosphatase activity of protein phosphatase 2A

Low-dose benzo[a]pyrene exposure induces hepatic lipid deposition through LCMT1/PP2Ac-mediated autophagy inhibition

Non-alcoholic fatty liver disease (NAFLD) is a progressive disorder of liver metabolism and has become the most common chronic liver disease worldwide. Benzo[a]pyrene (BaP) is recognized as a potent carcinogen, but the effect of low-dose BaP on the development of NAFLD has not been well-studied, and its molecular mechanism is still unknown. In this study, we demonstrated that low-dose BaP induced hepatic steatosis in a mouse model with a notable increase in hepatic lipid content. Interestingly, mRNA expression of genes related to fatty acids uptake or synthesis was not significantly altered after BaP exposure. Instead, we found that low-dose BaP promoted lipid deposition in primary mouse hepatocytes by inhibiting autophagy, which was regulated through Leucine carboxyl methyltransferase-1 (LCMT1) mediated Protein Phosphatases 2A subunit C (PP2Ac) methylation. The role of LCMT1 in BaP-induced steatosis was further validated in a liver-specific lcmt1 knockout (L-LCMT1 KO) mouse model. In this study, we provided evidence to support a novel mechanism by which BaP induces the development of hepatic steatosis through PP2Ac mediated autophagy inhibition. These findings provided new insight into the pathogenesis of NAFLD induced by environmental exposure to low-dose BaP.

Protein Phosphatase 2A: Role in T Cells and Diseases

Protein phosphatase 2A (PP2A) is a serine-threonine phosphatase that plays an important role in the regulation of cell proliferation and signal transduction. The catalytic activity of PP2A is integral in the maintenance of physiological functions which gets severely impaired in its absence. PP2A plays an essential role in the activation, differentiation, and functions of T cells. PP2A suppresses Th1 cell differentiation while promoting Th2 cell differentiation. PP2A fosters Th17 cell differentiation which contributes to the pathogenesis of systemic lupus erythematosus (SLE) by enhancing the transactivation of the Il17 gene. Genetic deletion of PP2A in Tregs disrupts Foxp3 expression due to hyperactivation of mTORC1 signaling which impairs the development and immunosuppressive functions of Tregs. PP2A is important in the induction of Th9 cells and promotes their antitumor functions. PP2A activation has shown to reduce neuroinflammation in a mouse model of experimental autoimmune encephalomyelitis (EAE) and is now used to treat multiple sclerosis (MS) clinically. In this review, we will discuss the structure and functions of PP2A in T cell differentiation and diseases and therapeutic applications of PP2A-mediated immunotherapy.

Inhibition of Sphingosine Kinase 2 Results in PARK2-Mediated Mitophagy and Induces Apoptosis in Multiple Myeloma

Mitophagy plays an important role in maintaining mitochondrial homeostasis by clearing damaged mitochondria. Sphingosine kinase 2 (SK2), a type of sphingosine kinase, is an important metabolic enzyme involved in generating sphingosine-1-phosphate. Its expression level is elevated in many cancers and is associated with poor clinical outcomes. However, the relationship between SK2 and mitochondrial dysfunction remains unclear. We found that the genetic downregulation of SK2 or treatment with ABC294640, a specific inhibitor of SK2, induced mitophagy and apoptosis in multiple myeloma cell lines. We showed that mitophagy correlates with apoptosis induction and likely occurs through the SET/PP2AC/PARK2 pathway, where inhibiting PP2AC activity may rescue this process. Furthermore, we found that PP2AC and PARK2 form a complex, suggesting that they might regulate mitophagy through protein-protein interactions. Our study demonstrates the important role of SK2 in regulating mitophagy and provides new insights into the mechanism of mitophagy in multiple myeloma.

PP2AC Phospho-Tyr307 Antibodies Are Not Specific for this Modification but Are Sensitive to Other PP2AC Modifications Including Leu309 Methylation

Protein phosphatase 2A (PP2A) is an important regulator of signal transduction pathways and a tumor suppressor. Phosphorylation of the PP2A catalytic subunit (PP2A_C) at tyrosine 307 has been claimed to inactivate PP2A and was examined in more than 180 studies using commercial antibodies, but this modification was never identified using mass spectrometry. Here we show that the most cited pTyr³⁰⁷ monoclonal antibodies, E155 and F-8, are not specific for phosphorylated Tyr³⁰⁷ but instead are hampered by PP2A_C methylation at leucine 309 or phosphorylation at threonine 304. Other pTyr³⁰⁷ antibodies are sensitive to PP2A_C methylation as well, and some cross-react with pTyr residues in general, including phosphorylated hemagglutinin tags. We identify pTyr³⁰⁷ using targeted mass spectrometry after transient overexpression of PP2A_C and Src kinase. Yet under such conditions, none of the tested antibodies show exclusive pTyr³⁰⁷ specificity. Thus, data generated using these antibodies need to be revisited, and the mechanism of PP2A inactivation needs to be redefined.

(DEAD)-box RNA helicase 3 modulates NF-κB signal pathway by controlling the phosphorylation of PP2A-C subunit

Asp-Glu-Ala-Asp (DEAD)-box RNA helicase 3 (DDX3), an ATP-dependent RNA helicase, is associated with RNA splicing, mRNA export, transcription, translation, and RNA decay. Recent studies revealed that DDX3 participates in innate immune response during virus infection by interacting with TBK1 and regulating the production of IFN-β. In our studies, we demonstrated that DDX3 regulated NF-κB signal pathway. We found that DDX3 knockdown reduced the phosphorylation of p65 and IKK-β and ultimately attenuated the production of inflammatory cytokines induced by poly(I:C) or TNF-α stimulation. The regulatory effect of DDX3 on NF-κB signal pathway was not affected by the loss of its ATPase or helicase activity. We further identified PP2A C subunit (PP2A-C) as an interaction partner of DDX3 by co-immunoprecipitation and mass spectrum analysis. We confirmed that DDX3 formed the complex with PP2A-C/IKK-β and regulated the interaction between IKK-β and PP2A-C. Furthermore, we demonstrated that DDX3 modulated the activity of PP2A by controlling the phosphorylation of PP2A-C, which might enable PP2A-C to regulate NF-κB signal pathway by dephosphorylating IKK-β. All these findings suggested DDX3 plays multiple roles in modulating innate immune system.

Ketamine, a Clinically Used Anesthetic, Inhibits Vascular Smooth Muscle Cell Proliferation via PP2A-Activated PI3K/Akt/ERK Inhibition

Abnormal proliferation of vascular smooth muscle cells (VSMCs) gives rise to major pathological processes involved in the development of cardiovascular diseases. The use of anti-proliferative agents for VSMCs offers potential for the treatment of vascular disorders. Intravenous anesthetics are firmly established to have direct effects on VSMCs, resulting in modulation of blood pressure. Ketamine has been used for many years in the intensive care unit (ICU) for sedation, and has recently been considered for adjunctive therapy. In the present study, we investigated the effects of ketamine on platelet-derived growth factor BB (PDGF-BB)-induced VSMC proliferation and the associated mechanism. Ketamine concentration-dependently inhibited PDGF-BB-induced VSMC proliferation without cytotoxicity, and phosphatidylinositol 3-kinase (PI3K) and extracellular signal-regulated protein kinase (ERK) inhibitors, LY294002 and PD98059, respectively, have similar inhibitory effects. Ketamine was shown to attenuate PI3K, Akt, and ERK1/2 phosphorylation induced by PDGF-BB. Okadaic acid, a selective protein phosphatase 2A (PP2A) inhibitor, significantly reversed ketamine-mediated PDGF-BB-induced PI3K, Akt, and ERK1/2 phosphorylation; a transfected protein phosphatse 2a (pp2a) siRNA reversed Akt and ERK1/2 phosphorylation; and 3-O-Methyl-sphingomyeline (3-OME), an inhibitor of sphingomyelinase, also significantly reversed ERK1/2 phosphorylation. Moreover, ketamine alone significantly inhibited tyrosine phosphorylation and demethylation of PP2A in a concentration-dependent manner. In addition, the pp2a siRNA potently reversed the ketamine-activated catalytic subunit (PP2A-C) of PP2A. These results provide evidence of an anti-proliferating effect of ketamine in VSMCs, showing activation of PP2A blocks PI3K, Akt, and ERK phosphorylation that subsequently inhibits the proliferation of VSMCs. Thus, ketamine may be considered a potential effective therapeutic agent for reducing atherosclerotic process by blocking the proliferation of VSMCs.

Notch1 receptor regulates AKT protein activation loop (Thr308) dephosphorylation through modulation of the PP2A phosphatase in phosphatase and tensin homolog (PTEN)-null T-cell acute lymphoblastic leukemia cells

Notch1 activating mutations occur in more than 50% of T-cell acute lymphoblastic leukemia (T-ALL) cases and increase expression of Notch1 target genes, some of which activate AKT. HES1 transcriptionally silences phosphatase and tensin homolog (PTEN), resulting in AKT activation, which is reversed by Notch1 inhibition with γ-secretase inhibitors (GSIs). Mutational loss of PTEN is frequent in T-ALL and promotes resistance to GSIs due to AKT activation. GSI treatments increased AKT-Thr(308) phosphorylation and signaling in PTEN-deficient, GSI-resistant T-ALL cell lines (Jurkat, CCRF-CEM, and MOLT3), suggesting that Notch1 represses AKT independent of its PTEN transcriptional effects. AKT-Thr(308) phosphorylation and downstream signaling were also increased by knocking down Notch1 in Jurkat (N1KD) cells. This was blocked by treatment with the AKT inhibitor perifosine. The PI3K inhibitor wortmannin and the protein phosphatase type 2A (PP2A) inhibitor okadaic acid both impacted AKT-Thr(308) phosphorylation to a greater extent in nontargeted control than N1KD cells, suggesting decreased dephosphorylation of AKT-Thr(308) by PP2A in the latter. Phosphorylations of AMP-activated protein kinaseα (AMPKα)-Thr(172) and p70S6K-Thr(389), both PP2A substrates, were also increased in both N1KD and GSI-treated cells and responded to okadaic acid treatment. A transcriptional regulatory mechanism was implied because ectopic expression of dominant-negative mastermind-like protein 1 increased and wild-type HES1 decreased phosphorylation of these PP2A targets. This was independent of changes in PP2A subunit levels or in vitro PP2A activity, but was accompanied by decreased association of PP2A with AKT in N1KD cells. These results suggest that Notch1 can regulate PP2A dephosphorylation of critical cellular regulators including AKT, AMPKα, and p70S6K.

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.

Cellular serine/threonine phosphatase activity during human cytomegalovirus infection

While the importance of cellular and viral kinases in HCMV replication has been demonstrated, relatively little is known about the activity of cellular phosphatases. We conducted a series of experiments designed to investigate the effect of HCMV infection on cellular serine/threonine phosphatase activity. We found that the abundance of two major cellular serine/threonine phosphatases, PP1 and PP2A, increases during HCMV infection. This was associated with an increase in threonine phosphatase activity in HCMV-infected cells. HCMV infection conferred resistance to the effects of the phosphatase inhibitors calyculin A (CA) and okadaic acid with regards to global protein hyperphosphorylation and the shutoff of protein synthesis. The protective effect of HCMV infection could be overcome at a high concentration of CA, suggesting that cellular phosphatase activity is required for critical cellular processes during HCMV infection. Specifically, phosphatase activity was required to limit the accumulation of phospho-eIF2alpha, but not phospho-PKR, during HCMV infection.

An inactive protein phosphatase 2A population is associated with methylesterase and can be re-activated by the phosphotyrosyl phosphatase activator

We have described recently the purification and cloning of PP2A (protein phosphatase 2A) leucine carboxylmethyltransferase. We studied the purification of a PP2A-specific methylesterase that co-purifies with PP2A and found that it is tightly associated with an inactive dimeric or trimeric form of PP2A. These inactive enzyme forms could be reactivated as Ser/Thr phosphatase by PTPA (phosphotyrosyl phosphatase activator of PP2A). PTPA was described previously by our group as a protein that stimulates the in vitro phosphotyrosyl phosphatase activity of PP2A; however, PP2A-specific methyltransferase could not bring about the activation. The PTPA activation could be distinguished from the Mn2+ stimulation observed with some inactive forms of PP2A, also found associated with PME-1 (phosphatase methylesterase 1). We discuss a potential new function for PME-1 as an enzyme that stabilizes an inactivated pool of PP2A.

Inhibitors of protein phosphatase-2A: topography and subcellular localization

The mRNA and protein expressions of I1(PP2A) and I2(PP2A), the two inhibitors of protein phosphatase 2A (PP2A) were investigated in adult rat brain. The rat brain and human brain inhibitors showed similar molecular weights by Western blots. The cDNA probes for human brain I1(PP2A) and I2(PP2A) readily hybridized with the corresponding mRNAs of rat brain inhibitors in Northern blots. We detected 3.7 and 2.1 kb transcripts of I1(PP2A) and 2.9 and 2.0 kb transcripts of I2(PP2A) in rat brain. In situ hybridization revealed that the mRNAs of the two inhibitors were mainly localized in neurons. Strong expression of both I1(PP2A) and I2(PP2A) mRNAs were observed in the olfactory bulb, hippocampal pyramidal and dentate granule cell layers, and cerebellar Purkinje cell, granular and molecular layers. Moderate expression of I1(PP2A) and I2(PP2A) mRNAs were observed in the cerebral cortex, caudate putamen, thalamus, hypothalamus, amygdala and pontine nucleus. The expression of I1(PP2A) and I2(PP2A) and as well as of PP-2A was also investigated by immunohistochemistry using antibodies to each protein. The distribution patterns of the two inhibitor proteins were similar to those of their corresponding mRNAs and to the expression of PP-2A. While PP-2A was localized to neuronal perikarya, I1(PP2A) was observed both in the neuronal cytoplasm and the nucleus. I2(PP2A) had mainly nuclear localization but it could also be seen in the neuronal cytoplasm. All three proteins were also expressed in the neuropil. These studies suggest that PP-2A activity is probably regulated by I1(PP2A) and I2(PP2A) in the adult mammalian central nervous system, and that these inhibitors are conserved between rat and human brains.

Conversion of CTLA-4 from inhibitor to activator of T cells with a bispecific tandem single-chain Fv ligand

Abs or their recombinant fragments against surface receptors of the Ig superfamily can induce or block the receptors' native function depending on whether they induce or prevent the assembly of signalosomes on their cytoplasmic tails. In this study, we introduce a novel paradigm based on the observation that a bispecific tandem single-chain variable region fragment ligand of CTLA-4 by itself converts this inhibitory receptor into an activating receptor for primary human T lymphocytes. This reversal of function results from increased recruitment of the serine/threonine phosphatase 2A to the cytoplasmic tail of CTLA-4, consistent with a role of this phosphatase in the regulation of CTLA-4 function, and assembly of a distinct signalosome that activates an lck-dependent signaling cascade and induces IL-2 production. Our data demonstrate that the cytoplasmic domain of CTLA-4 has an inherent plasticity for signaling that can be exploited therapeutically with recombinant ligands for this receptor.

Activation of Adenovirus Type 2 Early Region 4 ORF4 Cytoplasmic Death Function by Direct Binding to Src Kinase Domain

Adenovirus type 2 (Ad2) early region 4 ORF4 (E4orf4) triggers a major death pathway that requires its accumulation in cellular membranes and its tyrosine phosphorylation. This program is regulated by Src family kinases and triggers a potent ZVAD (benzyloxycarbonyl-VAD)- and Bcl2-resistant cell death response in human-transformed cells. How E4orf4 deregulates Src-dependent signaling is unknown. Here we provide strong evidence that a physical interaction requiring the kinase domain of Src and the arginine-rich motif of E4orf4 is involved. The Src binding domain of E4orf4 overlaps with, but is distinct from that of the Balpha subunit of protein phosphatase 2A (PP2A-Balpha) and some E4orf4 complexes contain both PP2A and Src. Functional assays using mutant E4orf4 revealed that deregulation of Src signaling, activation of the Jun kinase pathway, and cell blebbing were all critically dependent on Src binding. In contrast, PP2A-Balpha binding per se was not required to engage the Src-dependent death pathway but was more critical for triggering a distinct death activity. Both E4orf4 death activities were manifested within a given cell population, were typified by distinct morphological features, and contributed to overall cell killing, although to different extents in various cell types. We conclude that E4orf4 binding to the Src kinase domain leads to deregulation of Src signaling and plays a crucial role in induction of the cytoplasmic death pathway. Nonetheless, both Src and PP2A enzymes are critical targets of E4orf4 that likely cooperate to trigger E4orf4-induced tumor cell killing and whose relative contributions may vary in function of the cellular background.

Protein phosphatase 2A: a highly regulated family of serine/threonine phosphatases implicated in cell growth and signalling

Protein phosphatase 2A (PP2A) comprises a family of serine/threonine phosphatases, minimally containing a well conserved catalytic subunit, the activity of which is highly regulated. Regulation is accomplished mainly by members of a family of regulatory subunits, which determine the substrate specificity, (sub)cellular localization and catalytic activity of the PP2A holoenzymes. Moreover, the catalytic subunit is subject to two types of post-translational modification, phosphorylation and methylation, which are also thought to be important regulatory devices. The regulatory ability of PTPA (PTPase activator), originally identified as a protein stimulating the phosphotyrosine phosphatase activity of PP2A, will also be discussed, alongside the other regulatory inputs. The use of specific PP2A inhibitors and molecular genetics in yeast, Drosophila and mice has revealed roles for PP2A in cell cycle regulation, cell morphology and development. PP2A also plays a prominent role in the regulation of specific signal transduction cascades, as witnessed by its presence in a number of macromolecular signalling modules, where it is often found in association with other phosphatases and kinases. Additionally, PP2A interacts with a substantial number of other cellular and viral proteins, which are PP2A substrates, target PP2A to different subcellular compartments or affect enzyme activity. Finally, the de-regulation of PP2A in some specific pathologies will be touched upon.

Long-term depression in the hippocampus in vivo is associated with protein phosphatase-dependent alterations in extracellular signal-regulated kinase

There is growing evidence that activation of either protein kinases or protein phosphatases determines the type of plasticity observed after different patterns of hippocampal stimulation. Because activation of the extracellular signal-regulated kinase (ERK) has been shown to be necessary for long-term potentiation, we investigated the regulation of ERK in long-term depression (LTD) in the adult hippocampus in vivo. We found that ERK immunoreactivity was decreased following the induction of LTD and that this decrease required NMDA receptor activation. The LTD-associated decrease in ERK immunoreactivity could be simulated in vitro via incubation of either purified ERK2 or hippocampal homogenates with either protein phosphatase 1 or protein phosphatase 2A. The protein phosphatase-dependent decrease in ERK immunoreactivity was inhibited by microcystin. Intrahippocampal administration of the protein phosphatase inhibitor okadaic acid blocked the LTD-associated decrease in ERK2, but not ERK1, immunoreactivity. Collectively, these data demonstrate that protein phosphatases can decrease ERK immunoreactivity and that such a decrease occurs with ERK2 during LTD. These observations provide the first demonstration of a biochemical alteration of ERK in LTD.

Stimulation of phosphorylase phosphatase activity of protein phosphatase 2A1 by protamine is ionic strength dependent and involves interaction of protamine with both substrate and enzyme

The effects of protamine on the phosphorylase phosphatase activity of porcine cardiac protein phosphatase 2A1 (PP2A1) were complex and ionic strength dependent. Under ionic strength conditions that protamine activation was optimal, activation of PP2A1 by either dilution or heparin was prevented. A time-dependent deactivation of the protamine-stimulated phosphatase activity was observed when PP2A1 was preincubated with protamine. Protamine forms a very tight association with phosphorylase a, which is optimal at a 1:1 protamine:phosphorylase a monomer molar ratio. Protamine activation of PP2A1 activity, however, is not substrate-directed since the basic polypeptide did not stimulate either the activity of the catalytic subunit or trypsinolysis of [32P]phosphorylase a. The interaction of protamine with phosphorylase a does not apparently involve the phosphorylation site in the protein substrate (ser 14). The activation of PP2A1 by protamine is proposed to involve part of the basic polypeptide, not associated with phosphorylase a monomer, interacting with the regulatory and/or the catalytic subunit(s) of the phosphatase. A minimal model for the activation of PP2A1 by protamine was tested kinetically. In this model, free PP2A1 binds with decreasing affinities to the protamine:phosphorylase a complex, free phosphorylase a, and free protamine. Protamine decreases the K(m) of PP2A1 for the phosphorylase a monomer 5-fold and increases the Vmax 17-fold. Interaction of free protamine with PP2A1 inhibits the phosphatase activity.