Protein phosphatase 2A and protein kinase Calpha are physically associated and are involved in Pseudomonas aeruginosa-induced interleukin 6 production by mast cells

Inhibitory effects of rubratoxin A, a potent inhibitor of protein phosphatase 2, on the Ca2+-dependent contraction of skinned carotid artery from guinea pig

Rubratoxin A, a potent inhibitor of PP2A, is known to suppress smooth muscle contraction. The inhibitory role of PP2A in smooth muscle contraction is still unclear. In order to clarify the regulatory mechanisms of PP2A on vascular smooth muscle contractility, we examined the effects of rubratoxin A on the Ca²⁺-induced contraction of β-escin skinned carotid artery preparations from guinea pigs. Rubratoxin A at 1 µM and 10 µM significantly inhibited skinned carotid artery contraction at any Ca²⁺ concentration. The data fitting to the Hill equation in [Ca²⁺]-contraction relationship indicated that rubratoxin A decreased F_max-Ca2+ and increased [Ca²⁺]₅₀, indices of Ca²⁺ sensitivity for the force and myosin-actin interaction, respectively. These results suggest that PP2A inhibition causes downregulation of the myosin light chain phosphorylation and direct interference with myosin-actin interaction.

The controversial role of mast cells in fibrosis

Fibrosis is a medical condition characterized by an excessive deposition of extracellular matrix compounds such as collagen in tissues. Fibrotic lesions are present in many diseases and can affect all organs. The excessive extracellular matrix accumulation in these conditions can often have serious consequences and in many cases be life-threatening. A typical event seen in many fibrotic conditions is a profound accumulation of mast cells (MCs), suggesting that these cells can contribute to the pathology. Indeed, there is now substantialv evidence pointing to an important role of MCs in fibrotic disease. However, investigations from various clinical settings and different animal models have arrived at partly contradictory conclusions as to how MCs affect fibrosis, with many studies suggesting a detrimental role of MCs whereas others suggest that MCs can be protective. Here, we review the current knowledge of how MCs can affect fibrosis.

Saxagliptin but Not Sitagliptin Inhibits CaMKII and PKC via DPP9 Inhibition in Cardiomyocytes

Some oral anti-hyperglycemic drugs, including gliptins that inhibit dipeptidyl peptidase 4 (DPP4), have been linked to the increased risk of heart failure (HF) in type-2 diabetic patients. While the cardiovascular safety trial, TECOS, revealed no link between sitagliptin and the risk of HF, a substantial 27% increase in the hospitalization for HF was observed in type-2 diabetic patients treated with saxagliptin within the SAVOR-TIMI 53 trial. A previous in vitro study revealed that saxagliptin impairs the Ca2+/calmodulin-dependent protein kinase II (CaMKII)-phospholamban (PLB)-sarcoplasmic reticulum Ca2+-ATPase 2a axis and protein kinase C (PKC) activity in cardiomyocytes leading to impaired cardiac contractility and electrophysiological function. However, the link between saxagliptin and its target proteins (CaMKII and PKC) remains to be explored. Since DPP8 and DPP9 (but not DPP4) are expressed by cardiomyocytes and saxagliptin is internalized by cardiomyocytes, we investigated whether DPP8/9 contribute to saxagliptin-mediated inhibition of CaMKII and PKC activity. Structural analysis revealed that the DPP4-saxagliptin interaction motif (S630, Y547) for the cyanopyrrolidine group is conserved in DPP8 (S755, Y669) and DPP9 (S730, Y644). Conversely, F357 that facilitates binding of the anchor lock domain of sitagliptin in the S2 extensive subsite of DPP4 is not conserved in DPP8/9. In parallel, unlike saxagliptin, sitagliptin did not affect phosphorylation of CaMKII/PLB or activity of PKC in HL-1 cardiomyocytes. These findings were recapitulated by pharmacological inhibition (TC-E-5007, a DPP8/9 antagonist) and knock-down of DPP9 (but not DPP8). In primary mouse ventricular cardiomyocytes, saxagliptin (but not sitagliptin) impaired Ca2+ transient relaxation and prolonged action potential duration (APD). These results suggest that saxagliptin-DPP9 interaction impairs the CaMKII-PLB and PKC signaling in cardiomyocytes. We reveal a novel and potential role of DPP9 in cardiac signaling. The interaction of saxagliptin with DPP9 may represent an underlying mechanism for the link between saxagliptin and HF. Elucidation of saxagliptin-DPP9 interaction and downstream events may foster a better understanding of the role of gliptins as modulators of cardiac signaling.

CXCL4 contributes to host defense against acute Pseudomonas aeruginosa lung infection

Platelets have been implicated in pulmonary inflammation following exposure to bacterial stimuli. The mechanisms involved in the platelet-mediated host response to respiratory bacterial infection remain incompletely understood. In this study, we demonstrate that platelet-derived chemokine (C-X-C motif) ligand 4 (CXCL4) plays critical roles in a mouse model of acute bacterial pneumonia using Pseudomonas aeruginosa. Platelets are activated during P. aeruginosa infection, and mice depleted of platelets display markedly increased mortality and impaired bacterial clearance. CXCL4 deficiency impairs bacterial clearance and lung epithelial permeability, which correlate with decreased neutrophil recruitment to BALF. Interestingly, CXCL4 deficiency selectively regulates chemokine production, suggesting that CXCL4 has an impact on other chemokine expression. In addition, CXCL4 deficiency reduces platelet-neutrophil interactions in blood following P. aeruginosa infection. Further studies revealed that platelet-derived CXCL4 contributes to the P. aeruginosa-killing of neutrophils. Altogether, these findings demonstrate that CXCL4 is a vital chemokine that plays critical roles in bacterial clearance during P. aeruginosa infection through recruiting neutrophils to the lungs and intracellular bacterial killing.

The siRNA-mediated knockdown of GluN3A in 46C-derived neural stem cells affects mRNA expression levels of neural genes, including known iGluR interactors

For years, GluN3A was solely considered to be a dominant-negative modulator of NMDARs, since its incorporation into receptors alters hallmark features of conventional NMDARs composed of GluN1/GluN2 subunits. Only recently, increasing evidence has accumulated that GluN3A plays a more diversified role. It is considered to be critically involved in the maturation of glutamatergic synapses, and it might act as a molecular brake to prevent premature synaptic strengthening. Its expression pattern supports a putative role during neural development, since GluN3A is predominantly expressed in early pre- and postnatal stages. In this study, we used RNA interference to efficiently knock down GluN3A in 46C-derived neural stem cells (NSCs) both at the mRNA and at the protein level. Global gene expression profiling upon GluN3A knockdown revealed significantly altered expression of a multitude of neural genes, including genes encoding small GTPases, retinal proteins, and cytoskeletal proteins, some of which have been previously shown to interact with GluN3A or other iGluR subunits. Canonical pathway enrichment studies point at important roles of GluN3A affecting key cellular pathways involved in cell growth, proliferation, motility, and survival, such as the mTOR pathway. This study for the first time provides insights into transcriptome changes upon the specific knockdown of an NMDAR subunit in NSCs, which may help to identify additional functions and downstream pathways of GluN3A and GluN3A-containing NMDARs.

The Anti-allergic Cromones: Past, Present, and Future

The anti-allergic cromones were originally synthesized in the 1960s by Fisons Plc, and the first drug to emerge from this program, disodium cromoglycate was subsequently marketed for the treatment of asthma and other allergic conditions. Whilst early studies demonstrated that the ability of the cromones to prevent allergic reactions was due to their 'mast cell stabilizing' properties, the exact pharmacological mechanism by which this occurred, remained a mystery. Here, we briefly review the history of these drugs, recount some aspects of their pharmacology, and discuss two new explanations for their unique actions. We further suggest how these findings could be used to predict further uses for the cromones.

Protein Phosphatase 2A: a Double-Faced Phosphatase of Cellular System and Its Role in Neurodegenerative Disorders

Protein phosphatase 2A (PP2A), a ubiquitously expressed serine/threonine phosphatase, is a vitally important phosphatase for the cellular system. Structurally, it is constituted of three different subunits, namely catalytic subunit (PP2Ac), structural scaffold subunit (PP2A-A), and regulatory subunit (PP2A-B). All subunits have various isoforms, and catalytic and scaffold subunits are ubiquitously expressed, whereas regulatory subunits are more specific to tissue and cell type. It is the numerous possibilities of PP2A holoenzyme assembly with varying isoform components that make it possess a dual nature of activator or the inhibitory character in different signaling pathways, namely neural developmental pathways, Akt/protein kinase B pathway, NF-kB pathway, MAPK pathway, apoptosis pathway, and cell cycle progression to name a few. Importantly, the expression of PP2A in the brain is highest among the serine phosphatases and is known to actively participate in the neural development process. However, the exact mechanism of action of PP2A is still debated and enunciating the holoenzyme components, especially the regulatory subunit of PP2A involved in regulating neural developmental process is still poorly understood. In this review, we try to throw some light on the involvement of various PP2A holoenzyme forms in the process of neurogenesis and progression of neurodegenerative diseases.

Silver Nanoparticle-Directed Mast Cell Degranulation Is Mediated through Calcium and PI3K Signaling Independent of the High Affinity IgE Receptor

Engineered nanomaterial (ENM)-mediated toxicity often involves triggering immune responses. Mast cells can regulate both innate and adaptive immune responses and are key effectors in allergic diseases and inflammation. Silver nanoparticles (AgNPs) are one of the most prevalent nanomaterials used in consumer products due to their antimicrobial properties. We have previously shown that AgNPs induce mast cell degranulation that was dependent on nanoparticle physicochemical properties. Furthermore, we identified a role for scavenger receptor B1 (SR-B1) in AgNP-mediated mast cell degranulation. However, it is completely unknown how SR-B1 mediates mast cell degranulation and the intracellular signaling pathways involved. In the current study, we hypothesized that SR-B1 interaction with AgNPs directs mast cell degranulation through activation of signal transduction pathways that culminate in an increase in intracellular calcium signal leading to mast cell degranulation. For these studies, we utilized bone marrow-derived mast cells (BMMC) isolated from C57Bl/6 mice and RBL-2H3 cells (rat basophilic leukemia cell line). Our data support our hypothesis and show that AgNP-directed mast cell degranulation involves activation of PI3K, PLCγ and an increase in intracellular calcium levels. Moreover, we found that influx of extracellular calcium is required for the cells to degranulate in response to AgNP exposure and is mediated at least partially via the CRAC channels. Taken together, our results provide new insights into AgNP-induced mast cell activation that are key for designing novel ENMs that are devoid of immune system activation.

Mast cells in asthma--state of the art

Mast cells (MCs) play a central role in tissue homoeostasis, sensing the local environment through numerous innate cell surface receptors. This enables them to respond rapidly to perceived tissue insults with a view to initiating a co-ordinated programme of inflammation and repair. However, when the tissue insult is chronic, the ongoing release of multiple pro-inflammatory mediators, proteases, cytokines and chemokines leads to tissue damage and remodelling. In asthma, there is strong evidence of ongoing MC activation, and their mediators and cell-cell signals are capable of regulating many facets of asthma pathophysiology. This article reviews the evidence behind this.

Protein tyrosine phosphatase 1B (PTP1B) is dispensable for IgE-mediated cutaneous reaction in vivo

Mast cells play a critical role in allergic reactions. The cross-linking of FcεRI-bound IgE with multivalent antigen initiates a cascade of signaling events leading to mast cell activation. It has been well-recognized that cross linking of FcεRI mediates tyrosine phosphorylation. However, the mechanism involved in tyrosine dephosphorylation in mast cells is less clear. Here we demonstrated that protein tyrosine phosphatase 1B (PTP1B)-deficient mast cells showed increased IgE-mediated phosphorylation of the signal transducer and activator of transcription 5 (STAT5) and enhanced production of CCL9 (MIP-1γ) and IL-6 in IgE-mediated mast cells activation in vitro. However, IgE-mediated calcium mobilization, β-hexaosaminidase release (degranulation), and phosphorylation of IκB and MAP kinases were not affected by PTP1B deficiency. Furthermore, PTP1B deficient mice showed normal IgE-dependent passive cutaneous anaphylaxis and late phase cutaneous reactions in vivo. Thus, PTP1B specifically regulates IgE-mediated STAT5 pathway, but is redundant in influencing mast cell function in vivo.

Protein Tyrosine Kinase Fyn Regulates TLR4-Elicited Responses on Mast Cells Controlling the Function of a PP2A-PKCα/β Signaling Node Leading to TNF Secretion

Mast cells produce proinflammatory cytokines in response to TLR4 ligands, but the signaling pathways involved are not fully described. In this study, the participation of the Src family kinase Fyn in the production of TNF after stimulation with LPS was evaluated using bone marrow-derived mast cells from wild-type and Fyn-deficient mice. Fyn(-/-) cells showed higher LPS-induced secretion of preformed and de novo-synthesized TNF. In both cell types, TNF colocalized with vesicle-associated membrane protein (VAMP)3-positive compartments. Addition of LPS provoked coalescence of VAMP3 and its interaction with synaptosomal-associated protein 23; those events were increased in the absence of Fyn. Higher TNF mRNA levels were also observed in Fyn-deficient cells as a result of increased transcription and greater mRNA stability after LPS treatment. Fyn(-/-) cells also showed higher LPS-induced activation of TAK-1 and ERK1/2, whereas IκB kinase and IκB were phosphorylated, even in basal conditions. Increased responsiveness in Fyn(-/-) cells was associated with a lower activity of protein phosphatase 2A (PP2A) and augmented activity of protein kinase C (PKC)α/β, which was dissociated from PP2A and increased its association with the adapter protein neuroblast differentiation-associated protein (AHNAK, desmoyokin). LPS-induced PKCα/β activity was associated with VAMP3 coalescence in WT and Fyn-deficient cells. Reconstitution of MC-deficient Wsh mice with Fyn(-/-) MCs produced greater LPS-dependent production of TNF in the peritoneal cavity. Our data show that Fyn kinase is activated after TLR4 triggering and exerts an important negative control on LPS-dependent TNF production in MCs controlling the inactivation of PP2Ac and activation of PKCα/β necessary for the secretion of TNF by VAMP3(+) carriers.

Role of innate immunity in the pathogenesis of otitis media

Otitis media (OM) is a public health problem in both developed and developing countries. It is the leading cause of hearing loss and represents a significant healthcare burden. In some cases, acute OM progresses to chronic suppurative OM (CSOM), characterized by effusion and discharge, despite antimicrobial therapy. The emergence of antibiotic resistance and potential ototoxicity of antibiotics has created an urgent need to design non-conventional therapeutic strategies against OM based on modern insights into its pathophysiology. In this article, we review the role of innate immunity as it pertains to OM and discuss recent advances in understanding the role of innate immune cells in protecting the middle ear. We also discuss the mechanisms utilized by pathogens to subvert innate immunity and thereby overcome defensive responses. A better knowledge about bacterial virulence and host resistance promises to reveal novel targets to design effective treatment strategies against OM. The identification and characterization of small natural compounds that can boost innate immunity may provide new avenues for the treatment of OM. There is also a need to design novel methods for targeted delivery of these compounds into the middle ear, allowing higher therapeutic doses and minimizing systemic side effects.

Calpain-1 contributes to IgE-mediated mast cell activation

Mast cells play a central role in allergy through secretion of both preformed and newly synthesized mediators. Mast cell mediator secretion is controlled by a complex network of signaling events. Despite intensive studies, signaling pathways in the regulation of mast cell mediator secretion remain incompletely defined. In this study, we examined the role of calpain in IgE-dependent mast cell activation. IgE-mediated activation of mouse bone marrow-derived mast cells enhanced calpain activity. Inhibition of calpain activity by a number of calpain inhibitors reduced IgE-mediated mast cell degranulation both in vitro and in vivo. Calpain inhibitors blocked IgE-mediated TNF and IL-6 production in vitro and reduced late-phase allergic response in vivo. Importantly, mouse calpain-1 null bone marrow-derived mast cells showed reduced IgE-mediated mast cell degranulation in vitro and in vivo, diminished cytokine and chemokine production in vitro, and impaired late-phase allergic response in vivo. Further studies revealed that calpain-1 deficiency led to specific attenuation of IκB-NF-κB pathway and IKK-SNAP23 pathway, whereas calcium flux, MAPK, Akt, and NFAT pathway proceed normally in IgE-activated calpain-1 null mast cells. Thus, calpain-1 is identified as a novel regulator in IgE-mediated mast cell activation and could serve as a potential therapeutic target for the management of allergic inflammation.

Protein phosphatase 2A is regulated by protein kinase Cα (PKCα)-dependent phosphorylation of its targeting subunit B56α at Ser41

Protein phosphatase 2A (PP2A) is a family of multifunctional serine/threonine phosphatases consisting of a catalytic C, a structural A, and a regulatory B subunit. The substrate and therefore the functional specificity of PP2A are determined by the assembly of the enzyme complex with the appropriate regulatory B subunit families, namely B55, B56, PR72, or PR93/PR110. It has been suggested that additional levels of regulating PP2A function may result from the phosphorylation of B56 isoforms. In this study, we identified a novel phosphorylation site at Ser(41) of B56α. This phosphoamino acid residue was efficiently phosphorylated in vitro by PKCα. We detected a 7-fold higher phosphorylation of B56α in failing human hearts compared with nonfailing hearts. Purified PP2A dimeric holoenzyme (subunits C and A) was able to dephosphorylate PKCα-phosphorylated B56α. The potency of B56α for PP2A inhibition was markedly increased by PKCα phosphorylation. PP2A activity was also reduced in HEK293 cells transfected with a B56α mutant, where serine 41 was replaced by aspartic acid, which mimics phosphorylation. More evidence for a functional role of PKCα-dependent phosphorylation of B56α was derived from Fluo-4 fluorescence measurements in phenylephrine-stimulated Flp293 cells. The endoplasmic reticulum Ca(2+) release was increased by 23% by expression of the pseudophosphorylated form compared with wild-type B56α. Taken together, our results suggest that PKCα can modify PP2A activity by phosphorylation of B56α at Ser(41). This interplay between PKCα and PP2A represents a new mechanism to regulate important cellular functions like cellular Ca(2+) homeostasis.

Vitamin E in the prevention of cardiovascular disease: the importance of proper patient selection

Vitamin E is a naturally occurring fat-soluble antioxidant which has been proposed as a treatment for both primary and secondary protection against cardiovascular (CV) events. Promising data from observational epidemiological studies associating higher vitamin E dietary intake with lower risk of CV events have not been validated in randomized controlled clinical trials assessing the effect of vitamin E on CV outcomes. While the pendulum of medical opinion has swung to suggest that high dose vitamin E supplements have no place in the treatment and prevention of CV disease, new data is emerging that allows identification of a specific target population for this treatment, namely patients with diabetes mellitus and the haptoglobin genotype 2-2. This review details the scientific basis and clinical evidence related to the effect of vitamin E on CV outcomes, and the importance of proper patient selection in gaining therapeutic benefit from this intervention.

Regulation of protein kinase C inactivation by Fas-associated protein with death domain

Protein kinase C (PKC) plays important roles in diverse cellular processes. PKC has been implicated in regulating Fas-associated protein with death domain (FADD), an important adaptor protein involved in regulating death receptor-mediated apoptosis. FADD also plays an important role in non-apoptosis processes. The functional interaction of PKC and FADD in non-apoptotic processes has not been examined. In this study, we show that FADD is involved in maintaining the phosphorylation of the turn motif and hydrophobic motif in the activated conventional PKC (cPKC). A phosphoryl-mimicking mutation (S191D) in FADD (FADD-D) abolished the function of FADD in the facilitation of the turn motif and hydrophobic motif dephosphorylation of cPKC, suggesting that phosphorylation of Ser-191 negatively regulates FADD. We show that FADD interacts with PP2A, which is a major phosphatase involved in dephosphorylation of activated cPKC and FADD deficiency abolished PP2A mediated dephosphorylation of cPKC. We show that FADD deficiency leads to increased stability and activity of cPKC, which, in turn, promotes cytoskeleton reorganization, cell motility, and chemotaxis. Collectively, these results reveal a novel function of FADD in a non-apoptotic process by modulating cPKC dephosphorylation, stability, and signaling termination.

Mast cells mediate Pseudomonas aeruginosa lipopolysaccharide-induced lung inflammation in rat

Activated mast cells have been demonstrated to play a pivotal role in Pseudomonas aeruginosa lung infections. However, there is no report about the involvement of mast cells in P. aeruginosa lipopolysaccharide (LPS)-induced lung inflammation. This study aimed at evaluating the role of mast cells in P. aeruginosa LPS-induced lung inflammation in rats. Mast cells stabilization was carried out by intraperitoneal injections of cromolyn. Lung inflammation was induced by the intratracheal instillation of P. aeruginosa LPS (5 μg/kg bw) and inflammatory status was evaluated 4 h post-LPS instillation. We found that activated mast cells could constitute a pivotal source of several inflammatory cytokines, including TNF-α, IL-1β, and IL-6. These cells might regulate polymorphonuclear neutrophil (PMN) recruitment and be implicated in the alteration of alveolar-capillary permeability via the release of TNF-α and IL-1β. We also detected that activated mast cells could be involved in the alteration of the expression of two epithelial tight junction proteins (claudin-1 and occludin) during the acute phase of inflammation. Our results suggest that activated mast cells might play a critical role in P. aeruginosa LPS-induced lung inflammation. Therefore, mast cell stabilization may be a potential novel approach for the prevention and treatment of P. aeruginosa-induced lung infections.

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

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

PP2A targeting by viral proteins: a widespread biological strategy from DNA/RNA tumor viruses to HIV-1

Protein phosphatase 2A (PP2A) is a large family of holoenzymes that comprises 1% of total cellular proteins and accounts for the majority of Ser/Thr phosphatase activity in eukaryotic cells. Although initially viewed as constitutive housekeeping enzymes, it is now well established that PP2A proteins represent a family of highly and sophistically regulated phosphatases. The past decade, multiple complementary studies have improved our knowledge about structural and functional regulation of PP2A holoenzymes. In this regard, after summarizing major cellular regulation, this review will mainly focus on discussing a particulate biological strategy, used by various viruses, which is based on the targeting of PP2A enzymes by viral proteins in order to specifically deregulate, for their own benefit, cellular pathways of their hosts. The impact of such PP2A targeting for research in human diseases, and in further therapeutic developments, is also discussed.

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

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

Antioxidant vitamins and their use in preventing cardiovascular disease

Atherosclerosis remains one of the leading causes of death in Western populations. Subsequent to the discovery that oxidative stress plays a pivotal role in the development and progression of atherosclerosis, vitamins C and E, along with other antioxidants, were studied as potential therapies for the disease. However, while in vitro and in vivo studies showed promising antiatherogenic effects for vitamins C and E, clinical trials in which patients were given high doses of vitamin E or C showed no benefit and even possible harm. This review will attempt to summarize the known mechanistic data regarding the biochemical effects of vitamins C and E and their relevance to atherosclerosis, and offer an explanation for the failure of clinical trials to show that supplementation with these vitamins provides any benefit when given indiscriminately. We provide one example of how pharmacogenomics may be used to identify a sub-population which may indeed benefit from antioxidant supplementation.

Vitamin E transport, membrane incorporation and cell metabolism: Is alpha-tocopherol in lipid rafts an oar in the lifeboat?

Vitamin E is composed of closely related compounds, including tocopherols and tocotrienols. Studies of the last decade provide strong support for a specific role of alpha-tocopherol in cell signalling and the regulation of gene expression. It produces significant effects on inflammation, cell proliferation and apoptosis that are not shared by other vitamin E isomers with similar antioxidant properties. The different behaviours of vitamin E isomers might relate, at least in part, to the specific effects they exert at the plasma membrane. alpha-Tocopherol is not randomly distributed throughout the phospholipid bilayer of biological membranes, and as compared with other isomers, it shows a propensity to associate with lipid rafts. Distinct aspects of vitamin E transport and metabolism is discussed with emphasis on the interaction between alpha-tocopherol and lipid rafts and the consequences of these interactions on cell metabolism.

Cromoglycate drugs suppress eicosanoid generation in U937 cells by promoting the release of Anx-A1

Using biochemical, epifluorescence and electron microscopic techniques in a U937 model system, we investigated the effect of anti-allergic drugs di-sodium cromoglycate and sodium nedocromil on the trafficking and release of the anti-inflammatory protein Annexin-A1 (Anx-A1) when this was triggered by glucocorticoid (GC) treatment. GCs alone produced a rapid (within 5 min) concentration-dependent activation of PKCα/β (Protein Kinase C; EC 2.7.11.13) and phosphorylation of Anx-A1 on Ser²⁷. Both phosphoproteins accumulated at the plasma membrane and Anx-A1 was subsequently externalised thereby inhibiting thromboxane (Tx) B₂ generation. When administered alone, cromoglycate or nedocromil had little effect on this pathway however, in the presence of a fixed sub-maximal concentration of GCs, increasing amounts of the cromoglycate-like drugs caused a striking concentration-dependent enhancement of Anx-A1 and PKCα/β phosphorylation, membrane recruitment and Anx-A1 release from cells resulting in greatly enhanced inhibition of TxB₂ generation. GCs also stimulated phosphatase accumulation at the plasma membrane of U937 cells. Both cromoglycate and nedocromil inhibited this enzymatic activity as well as that of a highly purified PP2A phosphatase preparation. We conclude that stimulation by the cromoglycate-like drugs of intracellular Anx-A1 trafficking and release (hence inhibition of eicosanoid release) is secondary to inhibition of a phosphatase PP2A (phosphoprotein phosphatase; EC 3.1.3.16), which probably forms part of a control loop to limit Anx-A1 release. These experiments provide a basis for a novel mechanism of action for the cromolyns, a group of drugs that have long puzzled investigators.

The relaxant effect of okadaic acid on canine basilar artery involves activation of PKCalpha and phosphorylation of the myosin light chain at Thr-9

Vasodilator responses induced by okadaic acid were investigated in canine basilar artery precontracted with 80 mM KCl. Okadaic acid (1 microM) relaxed the artery and this relaxant effect was partially inhibited by Gö6976, a conventional protein kinase C inhibitor, and calphostin C, an inhibitor of conventional and novel PKCs. Rottlerin, a specific inhibitor of PKCdelta, did not influence okadaic acid's effect. KCl increased phosphorylation of 20,000-Dalton myosin light chain (MLC(20)) at Ser-19. Okadaic acid additionally increased MLC(20) phosphorylation at Thr-18 and Thr-9, resulting in triphosphorylation of MLC(20). This phosphorylation was inhibited by Gö6976. Okadaic acid stimulated phosphorylation of PKCalpha and 17,000-Dalton PKC-potentiated inhibitory phosphoprotein (CPI-17), and Gö6976 inhibited these phosphorylations. These results suggest that okadaic acid's relaxant effect involves MLC(20) triphosphorylation through a direct phosphorylation by PKCalpha and an indirect phosphorylation by inhibition of myosin light chain phosphatase through PKCalpha-mediated CPI-17 phosphorylation.

Protein phosphatase 2A regulatory subunits and cancer

The serine/threonine protein phosphatase (PP2A) is a trimeric holoenzyme that plays an integral role in the regulation of a number of major signaling pathways whose deregulation can contribute to cancer. The specificity and activity of PP2A are highly regulated through the interaction of a family of regulatory B subunits with the substrates. Accumulating evidence indicates that PP2A acts as a tumor suppressor. In this review we summarize the known effects of specific PP2A holoenzymes and their roles in cancer relevant pathways. In particular we highlight PP2A function in the regulation of MAPK and Wnt signaling.

A G-tract element in apoptotic agents-induced alternative splicing

Alternative splicing of a single pre-mRNA transcript can produce protein isoforms that promote either cell growth or death. Here we show that Ro-31-8220 (Ro), an apoptotic agent that inhibits protein kinase C and activates the c-Jun N terminal kinase, decreased the proportion of the cell growth-promoting Bcl-xL splice variant. Targeted mutagenesis analyses narrowed down a critical sequence to a 16-nt G-tract element (Gt16). Transferring this element to a heterologous gene conferred Ro response on an otherwise constitutive exon. The Ro effect was reduced by okadaic acid, an inhibitor of protein phosphatases PP1 and PP2A, in a concentration-dependent manner. Search in the human genome followed by RT–PCR identified a group of genes that contain similar exonic G-tract elements and are responsive to Ro. Moreover, the Gt16 element also mediates the regulation of alternative splicing by other cell apoptosis-inducers particularly retinoic acid. Therefore, the G-tract element likely plays a role in the apoptotic agents-induced alternative splicing of a group of genes. The functions of these genes imply that this regulation will have impact on cell growth/death.

Pin1 regulates TGF-beta1 production by activated human and murine eosinophils and contributes to allergic lung fibrosis

Eosinophilic inflammation is a cornerstone of chronic asthma that often culminates in subepithelial fibrosis with variable airway obstruction. Pulmonary eosinophils (Eos) are a predominant source of TGF-beta1, which drives fibroblast proliferation and extracellular matrix deposition. We investigated the regulation of TGF-beta1 and show here that the peptidyl-prolyl isomerase (PPIase) Pin1 promoted the stability of TGF-beta1 mRNA in human Eos. In addition, Pin1 regulated cytokine production by both in vitro and in vivo activated human Eos. We found that Pin1 interacted with both PKC-alpha and protein phosphatase 2A, which together control Pin1 isomerase activity. Pharmacologic blockade of Pin1 in a rat asthma model selectively reduced eosinophilic pulmonary inflammation, TGF-beta1 and collagen expression, and airway remodeling. Furthermore, chronically challenged Pin1(-/-) mice showed reduced peribronchiolar collagen deposition compared with wild-type controls. These data suggest that pharmacologic suppression of Pin1 may be a novel therapeutic option to prevent airway fibrosis in individuals with chronic asthma.

Ahnak protein activates protein kinase C (PKC) through dissociation of the PKC-protein phosphatase 2A complex

We have previously reported that central repeated units (CRUs) of Ahnak act as a scaffolding protein networking phospholipase Cgamma and protein kinase C (PKC). Here, we demonstrate that an Ahnak derivative consisting of four central repeated units binds and activates PKC-alpha in a phosphatidylserine/1,2-dioleoyl-sn-glycerol-independent manner. Moreover, NIH3T3 cells expressing the 4 CRUs of Ahnak showed enhanced c-Raf, MEK, and Erk phosphorylation in response to phorbol 12-myristate 13-acetate (PMA) compared with parental cells. To evaluate the effect of loss-of-function of Ahnak in cell signaling, we investigated PKC activation and Raf phosphorylation in embryonic fibroblast cells (MEFs) of the Ahnak knock-out (Ahnak(-/-)) mouse. Membrane translocation of PKC-alpha and phosphorylation of Raf in response to PMA or platelet-derived growth factor were decreased in Ahnak null MEF cells compared with wild type MEFs. Several lines of evidence suggest that PKC-alpha activity is regulated through association with protein phosphatase 2A (PP2A). A co-immunoprecipitation assay indicated that the association of PKC-alpha with PP2A was disrupted in NIH3T3 cells expressing 4 CRUs of Ahnak in response to PMA. Consistently, Ahnak null MEF cells stimulated by PMA showed enhanced PKC-PP2A complex formation, and add-back expression of Ahnak into Ahnak null MEF cells abolished the PKC-PP2A complex formation in response to PMA. These data indicate that Ahnak potentiates PKC activation through inhibiting the interaction of PKC with PP2A.

Tumor necrosis factor-alpha-induced anterior pituitary folliculostellate TtT/GF cell uncoupling is mediated by connexin 43 dephosphorylation

The anterior pituitary folliculostellate (FS) cells are key elements of the paracrine control of the pituitary function. These cells are the source and the target of growth factors and cytokines, and are connected to other pituitary cells via Cx43-mediated gap junctions. Here, we show that acute treatment of the FS TtT/GF cell line with TNF-alpha caused a transient cell uncoupling that was accompanied by the dephosphorylation of Cx43 in Ser368. These TNF-alpha-evoked effects were dependent on protein phosphatase 2A (PP2A) and protein kinase C (PKC) activities. TNF-alpha did not affect total cell Cx43-PP2A catalytic subunit interaction, but it did induce PP2A catalytic subunit recruitment to the Triton X-100 insoluble subcellular fraction, in which Cx43-gap junction plaques are recovered. This recruitment temporally coincided with Cx43 phosphorylated in Ser368-Cx43 dephosphorylation. Cx43 did not interact with the conventional PKC-alpha, but it did interact with the atypical PKC-zeta. Moreover, this interaction was weakened by TNF-alpha. Cx43 dephosphorylation in Ser368 was followed by the tyrosine phosphorylation of the protein. The temporary closure of gap junctions during acute TNF-alpha challenge may constitute a protective mechanism to limit or confine the spread of inflammatory signals among the FS cells.

Modulation of signal transduction by vitamin E

The ability of vitamin E to modulate signal transduction and gene expression has been observed in numerous studies; however, the detailed molecular mechanisms involved are often not clear. The eight natural vitamin E analogues and synthetic derivatives affect signal transduction with different potency, possibly reflecting their different ability to interact with specific proteins. Vitamin E modulates the activity of several enzymes involved in signal transduction, such as protein kinase C, protein kinase B, protein tyrosine kinases, 5-, 12-, and 15-lipoxygenases, cyclooxygenase-2, phospholipase A2, protein phosphatase 2A, protein tyrosine phosphatase, and diacylglycerol kinase. Activation of some these enzymes after stimulation of cell surface receptors with growth factors or cytokines can be normalized by vitamin E. At the molecular level, the translocation of several of these enzymes to the plasma membrane is affected by vitamin E, suggesting that the modulation of protein-membrane interactions may be a common theme for vitamin E action. In this review the main effects of vitamin E on enzymes involved in signal transduction are summarized and the possible mechanisms leading to enzyme modulation evaluated. The elucidation of the molecular and cellular events affected by vitamin E could reveal novel strategies and molecular targets for developing similarly acting compounds.

Potential involvement of serine/threonine protein phosphatases in apoptosis of HepG2 cells during selenite treatment

Selenium, an essential biological trace element present in both prokaryotic and eukaryotic cells, exerts its regulatory effect in a variety of cellular events, including cell growth, survival, and death. Selenium compounds have been shown in different cell lines to inhibit apoptosis by several mechanisms. Serine/threonine phosphatases (STPs) are potentially important in selenite-induced apoptosis because of their role in regulation of diverse set of cellular processes. In this study, the regulatory role of STPs in selenite-induced apoptosis has been implied by the use of two specific inhibitors: ocadaic acid and calyculin A. Our results show a decrease in cell density in HepG2 cells under selenite treatment. Resulting specific enzyme activities showed a concentration-dependent increase in all three phosphatase activities after 24 h in cells treated with 5 microM selenite and these activities decreased at 48 and 72 h. However, in cells treated with 10 microM selenite, PP2A and PP2B decreased at 48 h, whereas PP2C activity did not change at this dose. In cells treated with 25 microM, there was not a significant change in PP2C activity. These data suggest that the most specific response to selenite treatment was in PP2A and PP2B activities in a dose-dependent manner. Our results with OA and Cal-A further support the view that PP1 and PP2A might act as negative regulators of growth. With these data, we have first demonstrated the role of serine/threonine protein phosphatases in the signaling pathway of selenite-induced apoptosis and resulting cytotoxicity.

Pseudomonas aeruginosa-Induced Human Mast Cell Apoptosis Is Associated with Up-Regulation of Endogenous Bcl-xSand Down-Regulation of Bcl-xL

Mast cells play a critical role in the host defense against bacterial infection. Recently, apoptosis has been demonstrated to be essential in the regulation of host response to Pseudomonas aeruginosa. In this study we show that human mast cell line HMC-1 and human cord blood-derived mast cells undergo apoptosis as determined by the ssDNA formation after infection with P. aeruginosa. P. aeruginosa induced activation of caspase-3 in mast cells as evidenced by the cleavage of D4-GDI, an endogenous caspase-3 substrate and the generation of an active form of caspase-3. Interestingly, P. aeruginosa treatment induced up-regulation of Bcl-x(S) and down-regulation of Bcl-x(L). Bcl-x(S), and Bcl-x(L) are alternative variants produced from the same Bcl-x pre-mRNA. The former is proapoptotic and the latter is antiapoptotic likely through regulating mitochondrial membrane integrity. Treatment of mast cells with P. aeruginosa induced release of cytochrome c from mitochondria and loss of mitochondrial membrane potentials. Moreover, P. aeruginosa treatment reduced levels of Fas-associated death domain protein-like IL-1beta-converting enzyme-inhibitory proteins (FLIPs) that are endogenous apoptosis inhibitors through counteraction with caspase-8. Thus, human mast cells undergo apoptosis after encountering P. aeruginosa through a mechanism that likely involves both the Bcl family protein mitochondrial-dependent and the FLIP-associated caspase-8 pathways.

Insulin-like growth factor I controls a mutually exclusive association of RACK1 with protein phosphatase 2A and beta1 integrin to promote cell migration

The WD repeat scaffolding protein RACK1 can mediate integration of the insulin-like growth factor I receptor (IGF-IR) and integrin signaling in transformed cells. To address the mechanism of RACK1 function, we searched for regulatory proteins that associate with RACK1 in an IGF-I-dependent manner. The serine threonine phosphatase protein phosphatase 2A (PP2A) was found associated with RACK1 in serum-starved cells, and it dissociated immediately upon stimulation with IGF-I. This dissociation of PP2A from RACK1 and an IGF-I-mediated decrease in cellular PP2A activity did not occur in cells expressing either the serine 1248 or tyrosine 1250/1251 mutants of the IGF-IR that do not interact with RACK1. Recombinant RACK1 could bind to PP2A in vitro and restore phosphatase activity to PP2A from IGF-I-stimulated cells. Ligation of integrins with fibronectin or Matrigel was sufficient to facilitate IGF-I-mediated dissociation of PP2A from RACK1 and also to recruit beta1 integrin as PP2A dissociated. By using TAT-fused N-terminal and C-terminal deletion mutants of RACK1, we determined that both PP2A and beta1 integrin interact in the C terminus of RACK1 within WD repeats 4 to 7. This suggests that integrin ligation displaces PP2A from RACK1. MCF-7 cells overexpressing RACK1 exhibited enhanced motility, which could be reversed by the PP2A inhibitor okadaic acid. Small interfering RNA-mediated suppression of RACK1 also decreased the migratory capacity of DU145 cells. Taken together, our findings indicate that RACK1 enhances IGF-I-mediated cell migration through its ability to exclusively associate with either beta1 integrin or PP2A in a complex at the IGF-IR.

Mast cell function: regulation of degranulation by serine/threonine phosphatases

Mast cells play both effector and modulatory roles in a range of allergic and immune responses. The principal function of these cells is the release of inflammatory mediators from mast cells by degranulation, which involves a complex interplay of signalling molecules. Understanding the molecular architecture underlying mast cell signalling has attracted renewed interest as the capacity for therapeutic intervention through controlling mast cell degranulation is now accepted as a viable proposition. The dynamic regulation of signalling by protein phosphorylation is a well-established phenomenon and many of the early events involved in mast cell activation are well understood. Less well understood however are the events further downstream of receptor activation that allow movement of granules through the cytoskeletal barrier and docking and fusion of granules with the plasma membrane. Whilst a potential role for the protein phosphatase family of signalling enzymes in mast cell function has been accepted for some time, the evidence has largely been derived from the use of broad specificity pharmacological inhibitors and results often depend upon the experimental conditions, leading to conflicting views. In this review, we present and discuss the pharmacological and recent molecular evidence that protein phosphatases, and in particular the protein phosphatase serine/threonine phosphatase type 2A (PP2A), have major regulatory roles to play and may be potential targets for the design of new therapeutic agents.

Identification of Pseudomonas aeruginosa-induced genes in human mast cells using suppression subtractive hybridization: up-regulation of IL-8 and CCL4 production

Mast cells have recently been found to be a major player in the host defence against bacterial infection through secretion of potent mediators. Identification of bacteria-induced mast cell mediators and intracellular signalling molecules involved during bacterial infection remains a major area of investigation. Recently we found an active interaction between mast cells and Pseudomonas aeruginosa bacteria. To further characterize specific genes in mast cells modulated by P. aeruginosa, we used a new approach for the study of mast cell-bacteria interaction; the suppression subtractive hybridization (SSH). SSH approach does not require a prerequisite knowledge of target genes and does not rely on the availability of the assay reagents for the specific genes. Using SSH, 94 clones were randomly selected from the subtracted cDNA library for differential screening leading to the identification of 14 P. aeruginosa-up-regulated transcripts. Sequence analysis revealed that expression of IL-1, IL-8 and CCL4 was increased by human mast cells after P. aeruginosa infection. Increased production of IL-1, IL-8 and CCL4 was confirmed at the protein levels. In addition, sequence analysis of the clones also suggests that ribosomal protein S3 and cytochrome b as well as additional 4 uncharacterized genes may potentially be involved in P. aeruginosa pathogenesis. Thus, SSH is an effective approach by identifying potential molecular targets for the study of mechanisms involved in P. aeruginosa and mast cell interaction.

De novo synthesis of early growth response factor-1 is required for the full responsiveness of mast cells to produce TNF and IL-13 by IgE and antigen stimulation

Early growth-response factor 1 (Egr-1) is a zinc-finger transcription factor that plays a regulatory role in the expression of many genes important for inflammation. Whether Egr-1 is involved in IgE-dependent mast-cell activation was investigated. We demonstrated that IgE and antigen (TNP) stimulation induced a rapid expression of Egr-1 mRNA in mouse bone marrow-derived mast cells (BMMCs). As early as 15 to 20 minutes after IgE + TNP stimulation, Egr-1 protein was detectable in the nucleus of BMMCs by immunofluorescence or electrophoretic mobility shift assay. To examine a role for Egr-1 in IgE-dependent cytokine production by mast cells, Egr-1-deficient (Egr-1-/-) BMMCs were developed from the bone marrow cells of Egr-1 knockout mice. Egr-1-/- BMMCs express similar levels of surface c-kit and IgE receptor as compared with those on Egr-1+/+ BMMCs. Importantly, IgE + TNP-induced TNF and IL-13 expression was significantly reduced at both mRNA and protein levels in Egr-1-/- BMMCs as compared with those in Egr-1+/+ BMMCs. Thus, our results suggest that de novo synthesis of Egr-1 represents a novel mechanism in FcepsilonRI signaling and is required for the full responsiveness of IgE-dependent TNF and IL-13 production by mast cells.

A Direct Interaction between the N Terminus of Adenylyl Cyclase AC8 and the Catalytic Subunit of Protein Phosphatase 2A

Although protein scaffolding complexes compartmentalize protein kinase A (PKA) and phosphodiesterases to optimize cAMP signaling, adenylyl cyclases, the sources of cAMP, have been implicated in very few direct protein interactions. The N termini of adenylyl cyclases are highly divergent, which hints at isoform-specific interactions. Indeed, the Ca(2+)-sensitive adenylyl cyclase 8 (AC8) contains a Ca(2+)/calmodulin binding site on the N terminus that is essential for stimulation of activity by the capacitative entry of Ca(2+) in the intact cell. Here, we have used the N terminus of AC8 as a bait in a yeast two-hybrid screen of a human embryonic kidney (HEK) 293 cell cDNA library and identified the catalytic subunit of the serine/threonine protein phosphatase 2A (PP2A(C)) as a binding partner. Confirming the highly specific nature of this novel interaction, glutathione-S-transferase fusion proteins containing the full-length N terminus of AC8 affinity precipitated catalytically active PP2A(C) from both HEK293 and mouse forebrain membranes-the latter a normal source of AC8. The scaffolding subunit of PP2A (PP2A(A); 65 kDa) was also precipitated by the N terminus of AC8, indicating that AC8 may occur in a complex with the PP2A core dimer. The interaction between the N terminus of AC8 and PP2A(C) was antagonized by Ca(2+)/calmodulin. However, PP2A(C) and Ca(2+)/calmodulin did not share identical binding specificities in the N terminus of AC8. PKA-mediated phosphorylation did not influence either calmodulin or PP2A(C) association with AC8. In addition, both PP2A(C) and AC8 occurred in lipid rafts. These findings are the first demonstration of an association between adenylyl cyclase and any downstream element of cAMP signaling.

The use of okadaic acid to elucidate the intracellular role(s) of protein phosphatase 2A: Lessons from the mast cell model system

In recent years a heightened appreciation has emerged for the role(s) that phosphatases play in regulating signal transduction pathways and other cellular processes. The tumor-promoting agent okadaic acid (OA) has been an invaluable tool in efforts aimed at delineating the contributions of the most abundant mammalian serine/threonine phosphatase, protein phosphatase 2A (PP2A), to intracellular signaling and cell function. PP2A, which is ubiquitous and vital in virtually every cell system studied, continues to be the focus of much research on phosphorylation control machinery. Mast cells represent an excellent in vitro model for the study of protein phosphorylation events because they possess a number of distinct signaling pathways that lead to the production and/or release of discreet mediators in response to different stimuli. The utility of OA in analyzing PP2A function has been demonstrated in mast cells across several species. Results of these studies have contributed to the current recognition that PP2A plays a crucial role in the biology of mast cells and other cell types.

An aqueous Chlorella extract inhibits IL-5 production by mast cells in vitro and reduces ovalbumin-induced eosinophil infiltration in the airway in mice in vivo

An aqueous extract of the edible microalga (CP) (1), has recently been tested for its immunomodulatory effects in a human clinical trial. Here, the CP extract was dialyzed and fractionated using Sephadex G 100 chromatography. The effects of a dialyzed aqueous CP extract, fraction 2 , on mast cell mediator release in vitro and ovalbumin-induced allergic airway inflammation in vivo were examined. In vitro, treatment of mouse bone marrow-derived mast cells with 2 for 18 h significantly inhibited antigen (trinitrophenyl-BSA)-induced IL-5 production. In vivo, treatment of mice with 2 during ovalbumin sensitization and stimulation process significantly reduced eosinophil and neutrophil infiltration in the airways. Moreover, fractions obtained by size exclusion chromatography of 2 inhibited IgE-dependent cytokine GM-CSF production from human cord blood-derived mast cells. Taken together, these results suggest that 2 is composed of biopolymers with anti-allergic potential.

Inhibition of IKK down-regulates antigen + IgE-induced TNF production by mast cells: a role for the IKK-IκB-NF-κB pathway in IgE-dependent mast cell activation

Mast cells (MC) are major effector cells for allergic diseases. Cross-linking of immunoglobulin E (IgE) and its high-affinity receptor, FcepsilonRI, by antigen initiates a cascade of signaling events leading to nuclear factor (NF)-kappaB activation and tumor necrosis factor (TNF) production. Here, we demonstrated that inhibition of inhibitor of kappaB (IkappaB) kinase (IKK) by a peptide IKK inhibitor or by four individual chemical IKK inhibitors including 15-deoxy-prostaglandin J(2), BMS-345541, SC-514, or sulindac significantly blocked IgE + trinitrophenyl (TNP)-induced TNF production by mouse bone marrow-derived MC (BMMC). Moreover, IgE + TNP induced a rapid phosphorylation of IKKalpha but not IKKbeta in BMMC. IgE + TNP-induced phosphorylation of IKKalpha was accompanied with phosphorylation and degradation of IkappaBalpha, subsequent NF-kappaB activation, and TNF production. Inhibition of IKK by sulindac decreased IKKalpha phosphorylation, IkappaBalpha phosphorylation and degradation, NF-kappaB activation, and TNF production by BMMC. It is interesting that IgE + TNP stimulation also induced a prominent synthesis of IKKalpha and IkappaBalpha. Inhibition of NF-kappaB activity by pyrrolidine dithiocarbomate (PDTC) blocked IgE + TNP-induced IkappaBalpha synthesis. NF-kappaB activity and TNF production were also inhibited when PDTC was used even after IgE + TNP stimulation, suggesting a potential role for the newly synthesized IkappaBalpha in MC activation. In addition, IgE + TNP-induced IKKalpha and IkappaBalpha phosphorylation was inhibited by a protein kinase C (PKC) inhibitor Ro 31-8220. Taken together, our results support a role for the IKK-IkappaB-NF-kappaB pathway, which likely involves PKC in IgE-dependent TNF production by MC. Thus, IKK may serve as a new target for the regulation of MC function in allergy.

Annexin II is a novel receptor for Pseudomonas aeruginosa

Infections with Pseudomonas aeruginosa (P. aeruginosa) are critical in ventilated and poly-traumatized patients. Most important, these bacteria cause frequent and chronic pulmonary infections in patients with cystic fibrosis. Therefore, identification of molecular mechanisms that mediate the infection of mammalian cells with P. aeruginosa is urgently required. Here, we aimed to identify novel receptors that are involved in internalization of P. aeruginosa into mammalian epithelial cells. Employing SDS-PAGE purification and mass spectrometry we demonstrate that annexin II specifically binds to P. aeruginosa. The significance of the interaction of annexin II with P. aeruginosa for the infection of mammalian cells is indicated by the finding that neutralization of the ligands on P. aeruginosa by incubation of the bacteria with recombinant, soluble annexin II prevents internalization of P. aeruginosa into human epithelial cells.

Increases in intracellular calcium dephosphorylate histone H3 at serine 10 in human hepatoma cells: Potential role of protein phosphatase 2A-protein kinase CβII complex

We present evidence that increases in intracellular calcium, induced by treatment with calcium ionophore A23187 or the endoplasmic reticulum calcium-ATPase inhibitor thapsigargin, dephosphorylated histone H3 at serine10 (histone H3-Ser10) in a dose-dependent manner in human hepatoma HepG2 cells. Inhibition of p42/44MAPK, pp90RSK, or p38MAPK did not affect the ability of A23187 to dephosphorylate histone H3-Ser10. This response is significantly blocked by okadaic acid, indicating a requirement for protein phosphatase 2A (PP2A). A23187 increased the activity of PP2A towards phosphorylated histone H3-Ser10. Furthermore, pretreatment with calphostin C, a selective protein kinase C (PKC) inhibitor, blocked A23187-dependent dephosphorylation of histone H3-Ser10, and coimmunoprecipitation analysis showed PP2A association with the PKCbetaII isoform. Unlike untreated cells, coimmunoprecipitated complex from A23187-treated cells showed greater dephosphorylation of histone H3-Ser10 in a PP2A-dependent manner. Inhibition of PP2A increased phosphorylation at Ser660 that determines calcium sensitivity and activity of PKCbetaII isoform, thus supporting a role for intracomplex regulation. Finally, chromatin immunoprecipitation assays following exposure to A23187 and okadaic acid revealed regulatory role of histone H3-Ser10 phosphorylation in selective gene induction. Altogether, our findings suggest a novel role for calcium in modulating histone H3-Ser10 phosphorylation level and led us to propose a model emphasizing PP2A activation, occurring downstream following perturbations in calcium homeostasis, as key event in dephosphorylating histone H3-Ser10 in mammalian cells.

Pseudomonas aeruginosa exotoxin A induces human mast cell apoptosis by a caspase-8 and -3-dependent mechanism

Mast cells play an important role in both allergy and innate immunity. Recently, we demonstrated an active interaction between human mast cells and Pseudomonas aeruginosa leading to the production of multiple cytokines. Here, we show that both primary cultured human cord blood-derived mast cells and the human mast cell line HMC-1 undergo apoptosis as determined by single-stranded DNA (ssDNA) formation after stimulation with P. aeruginosa exotoxin A (ETA), a major toxin produced by this bacterium. ETA-induced ssDNA formation was completely inhibited by Z-VAD (where Z is benzyloxycarbonyl), which blocks multiple caspases, suggesting a role for caspases in this process. Active caspase-3 formation in mast cells after an ETA challenge was detected by both Western blotting and flow cytometry analysis. ETA-induced caspase-3 activity in human mast cells was demonstrated by the detection of a characteristic 23 kDa product of D4-GDI (where GDI is guanine nucleotide dissociation inhibitor), an endogenous caspase-3 substrate. Interestingly, a specific caspase-8 inhibitor, Z-IETD-fmk (where fmk is fluoromethyl ketone), blocked ETA-induced cleavage of D4-GDI, but a caspase-9 inhibitor (Z-LEHD-fmk) did not. Treatment of mast cells with caspase-3 inhibitor Z-DEVD-fmk or caspase-8 inhibitor Z-IETD-fmk reduced the generation of ssDNA induced by ETA, suggesting a role for caspase-8 and -3 in ETA-induced mast cell apoptosis. Furthermore, treatment of mast cells with ETA induced decreases of the short form and a long form (p43) of Fas-associated death domain protein (FADD)-like interleukin-1beta-converting enzyme (FLICE) (caspase-8)-inhibitory proteins (FLIPs), which are endogenous caspase-8 inhibitors. Taken together, these results suggest that ETA-induced mast cell apoptosis involves down-regulation of antiapoptotic proteins, FLIPs, and activation of caspase-8 and -3 pathways.

Phosphatase inhibition potentiates IL-6 production by mast cells in response to FcepsilonRI-mediated activation: involvement of p38 MAPK

Mast cells are crucial effector cells in the immune response through mediator secretion and release of cytokines. A coordinated balance between protein kinases and phosphatases plays an essential role in the regulation of mast cell mediator secretion. We have previously shown that treatment of mast cells with okadaic acid (OA), a protein phosphatase 2A (PP2A) inhibitor, results in a dose-dependent increase in interleukin (IL)-6 production. We show here for the first time a synergism between OA and immunoglobulin E (IgE)-mediated IL-6 secretion by murine bone marrow-derived mast cells (BMMC). Selective p38 mitogen-activated protein kinase (p38 MAPK) inhibition reduces OA and IgE-mediated IL-6 production. Regulation of p38 MAPK by PP2A was demonstrated, as OA treatment caused a dose-dependent increase in p38 MAPK phosphorylation. Antigen-mediated activation of murine mast cells also resulted in an increase in p38 MAPK phosphorylation, which was potentiated by cotreatment of the cells with OA. Lastly, in two mast cell lines (human mast cell-1 5C6 and murine MC/9) and primary-cultured murine BMMC, we show by coimmunoprecipitation an interaction between p38 MAPK and PP2A. These data support a role for PP2A through interaction with p38 MAPK in the regulation of IgE-dependent mast cell activation.