Thapsigargin-induced degranulation of mast cells is dependent on transient activation of phosphatidylinositol-3 kinase

Impaired Proliferation of CD8+ T Cells Stimulated with Monocyte-Derived Dendritic Cells Previously Matured with Thapsigargin-Stimulated LAD2 Human Mast Cells

CD8+ T cells are essential for adaptive immunity against infection and tumors. Their ability to proliferate after stimulation is crucial to their functionality. Dendritic cells (DCs) are professional antigen-presenting cells that induce their proliferation. Here, we show that thapsigargin-induced LAD2 mast cell (MC) line-released products can impair the ability of monocyte-derived DCs to induce CD8+ T-cell proliferation and the generation of Th1 cytokine-producing T cells. We found that culture medium conditioned with LAD2 MCs previously stimulated with thapsigargin (thapsLAD2) induces maturation of DCs as determined by the maturation markers CD80, CD83, CD86, and HLA-DR. However, thapsLAD2-matured DCs produced no detectable TNFα or IL-12 during the maturation. In addition, although their surface expression of PD-L1 was comparable with the immature or TLR7/8-agonist (R848)-matured DCs, their TIM-3 expression was significantly higher than in immature DCs and even much higher than in R848-matured DCs. In addition, contrary to R848-matured DCs, the thapsLAD2-matured DCs only tended to induce enhanced proliferation of CD4+ T cells than immature DCs. For CD8+ T cells, this tendency was not even detected because thapsLAD2-matured and immature DCs comparably induced their proliferation, which contrasted with the significantly enhanced proliferation induced by R848-matured DCs. Furthermore, these differences were comparably recapitulated in the ability of the tested DCs to induce IFNγ- and IFNγ/TNFα-producing T cells. These findings show a novel mechanism of MC-mediated regulation of adaptive immune responses.

Betulinic Acid Potentiates Mast Cell Degranulation by Compromising Cell Membrane Integrity and Without Involving Fcεri Receptors

Mast cells play important role in acquired and natural immunity making these favorable therapeutic targets in various inflammatory diseases. Here we observed that, pentacyclic tri terpenoid betulinic acid (BA) treatment resulted in a significantly high number (9%) of cells positive for Hoechst and negative for annexin-V indicating that BA could interfere with plasma membrane integrity. The degranulation of both activated and non-activated mast cells was enhanced upon treatment with BA. The pre-treatment of BA had remarkable effect on calcium response in activated mast cells which showed increased calcium influx relative compared to untreated cells. The results also showed potentially less migration of BA treated mast cells signifying the possible effect of BA on cell membrane. BA treatment resulted in a significant increase in mRNA levels of IL-13 while as mRNA levels of other target cytokines, IL-6 and TNF-α seem to be not affected. Moreover, there was global Increase in phosphorylation of signaling proteins and no significant change in phosphorylation of FcεRI receptors indicating that the effect of BA was independent of signaling cascade or FcεRI receptor mediated mast cell aggregation. Overall, these results portray BA potentiates mast cell effector functions by compromising the membrane integrity and independent of FcεRI involvement.

Anaphylactic degranulation by mast cells requires the mobilization of inflammasome components

The inflammasome components NLRP3 and ASC are cytosolic proteins, which upon sensing endotoxins or danger cues, form multimeric complexes to process interleukin (IL)-1β for secretion. Here we found that antigen (Ag)-triggered degranulation of IgE-sensitized mast cells (MCs) was mediated by NLRP3 and ASC. IgE-Ag stimulated NEK7 and Pyk2 kinases in MCs to induce the deposition of NLRP3 and ASC on granules and form a distinct protein complex (granulosome) that chaperoned the granules to the cell surface. MCs deficient in NLRP3 or ASC did not form granulosomes, degranulated poorly in vitro and did not evoke systemic anaphylaxis in mice. IgE-Ag-triggered anaphylaxis was prevented by an NLRP3 inhibitor. In endotoxin-primed MCs, pro-IL-1β was rapidly packaged into granules after IgE-Ag stimulation and processed within granule remnants by proteases after degranulation, causing lethal anaphylaxis in mice. During IgE-Ag-mediated degranulation of endotoxin-primed MCs, granulosomes promoted degranulation, combined with exteriorization and processing of IL-1β, resulting in severe inflammation.

Acute activation of SERCA with CDN1163 attenuates IgE-mediated mast cell activation through selective impairment of ROS and p38 signaling

Mast cells are granulocytic immune sentinels present in vascularized tissues that drive chronic inflammatory mechanisms characteristic of allergic pathologies. IgE-mediated mast cell activation leads to a rapid mobilization of Ca²⁺ from intracellular stores, which is essential for the release of preformed mediators via degranulation and de novo synthesized proinflammatory cytokines and chemokines. Given its potent signaling capacity, the dynamics of Ca²⁺ localization are highly regulated by various pumps and channels controlling cytosolic Ca²⁺ concentrations. Among these is sarco/endoplasmic reticulum Ca²⁺ -ATPase (SERCA), which functions to maintain low cytosolic Ca²⁺ concentrations by actively transporting cytosolic Ca²⁺ ions into the endoplasmic reticulum. In this study, we characterized the role of SERCA in allergen-activated mast cells using IgE-sensitized bone marrow-derived mast cells (BMMCs) treated with the SERCA activating compound, CDN1163, and simultaneously stimulated with allergen through FcεRI under stem cell factor (SCF) potentiation. Acute treatment with CDN1163 was found to attenuate early phase mast cell degranulation along with reactive oxygen species (ROS) production. Additionally, treatment with CDN1163 significantly reduced secretion of IL-6, IL-13, and CCL3, suggesting a role for SERCA in the late phase mast cell response. The protective effects of SERCA activation via CDN1163 treatment on the early and late phase mast cell response may be driven by the selective suppression of p38 MAPK signaling. Together, these findings implicate SERCA as an important regulator of the mast cell response to allergen and suggest SERCA activity may offer therapeutic potential targeting allergic pathologies, warranting further investigation.

Perfluorooctane sulfonate and bisphenol A induce a similar level of mast cell activation via a common signaling pathway, Fyn-Lyn-Syk activation

Perfluoroalkyl compounds (PFCs) as food contaminants are widely distributed persistent organic pollutants (POPs) and have been suggested to induce immune dysfunction. However, their effects on immune function are not conclusive. Mast cells play a central role in allergic and non-allergic inflammatory responses. Therefore, we have examined the effects of PFCs (PFHxS, PFOA, PFOS) on mast cell-mediated inflammatory responses using in vitro mouse bone marrow-derived mast cells (BMMCs) and human mast cells (HMC-1) and in vivo mice model. The effects of PFCs were compared with those of bisphenol A (BPA), a well-studied environmental pollutant. Among PFCs tested, PFOS had the highest effects. Both PFOS and BPA increased degranulation and production of inflammatory eicosanoids in mast cells at a similar level, which subsequently led to increased skin edema and serum LTC₄ and PGD₂ in mice. Both PFOS and BPA increased not only downstream signaling (PLCγ1, AKT, ERK), but also upstream signaling (Fyn, Lyn, Syk/LAT) in mast cells. Taken together, PFOS and BPA induce mast cell-mediated inflammatory responses via a common signaling pathways. Our results may help establish the scientific basis for understanding the etiology of mast cell-mediated inflammatory responses and improve the immune dysfunction risk assessment for emerging POPs such as PFCs.

Molecular Mechanisms of Mast Cell Activation by Cholesterol-Dependent Cytolysins

Mast cells are potent immune sensors of the tissue microenvironment. Within seconds of activation, they release various preformed biologically active products and initiate the process of de novo synthesis of cytokines, chemokines, and other inflammatory mediators. This process is regulated at multiple levels. Besides the extensively studied IgE and IgG receptors, toll-like receptors, MRGPR, and other protein receptor signaling pathways, there is a critical activation pathway based on cholesterol-dependent, pore-forming cytolytic exotoxins produced by Gram-positive bacterial pathogens. This pathway is initiated by binding the exotoxins to the cholesterol-rich membrane, followed by their dimerization, multimerization, pre-pore formation, and pore formation. At low sublytic concentrations, the exotoxins induce mast cell activation, including degranulation, intracellular calcium concentration changes, and transcriptional activation, resulting in production of cytokines and other inflammatory mediators. Higher toxin concentrations lead to cell death. Similar activation events are observed when mast cells are exposed to sublytic concentrations of saponins or some other compounds interfering with the membrane integrity. We review the molecular mechanisms of mast cell activation by pore-forming bacterial exotoxins, and other compounds inducing cholesterol-dependent plasma membrane perturbations. We discuss the importance of these signaling pathways in innate and acquired immunity.

Cytoskeletal Protein 4.1R Is a Positive Regulator of the FcεRI Signaling and Chemotaxis in Mast Cells

Protein 4.1R, a member of the 4.1 family, functions as a bridge between cytoskeletal and plasma membrane proteins. It is expressed in T cells, where it binds to a linker for activation of T cell (LAT) family member 1 and inhibits its phosphorylation and downstream signaling events after T cell receptor triggering. The role of the 4.1R protein in cell activation through other immunoreceptors is not known. In this study, we used 4.1R-deficient (4.1R-KO) and 4.1R wild-type (WT) mice and explored the role of the 4.1R protein in the high-affinity IgE receptor (FcεRI) signaling in mast cells. We found that bone marrow mast cells (BMMCs) derived from 4.1R-KO mice showed normal growth in vitro and expressed FcεRI and c-KIT at levels comparable to WT cells. However, 4.1R-KO cells exhibited reduced antigen-induced degranulation, calcium response, and secretion of tumor necrosis factor-α. Chemotaxis toward antigen and stem cell factor (SCF) and spreading on fibronectin were also reduced in 4.1R-KO BMMCs, whereas prostaglandin E₂-mediated chemotaxis was not affected. Antibody-induced aggregation of tetraspanin CD9 inhibited chemotaxis toward antigen in WT but not 4.1R-KO BMMCs, implying a CD9-4.1R protein cross-talk. Further studies documented that in the absence of 4.1R, antigen-mediated phosphorylation of FcεRI β and γ subunits was not affected, but phosphorylation of SYK and subsequent signaling events such as phosphorylation of LAT1, phospholipase Cγ1, phosphatases (SHP1 and SHIP), MAP family kinases (p38, ERK, JNK), STAT5, CBL, and mTOR were reduced. Immunoprecipitation studies showed the presence of both LAT1 and LAT2 (LAT, family member 2) in 4.1R immunocomplexes. The positive regulatory role of 4.1R protein in FcεRI-triggered activation was supported by in vivo experiments in which 4.1R-KO mice showed the normal presence of mast cells in the ears and peritoneum, but exhibited impaired passive cutaneous anaphylaxis. The combined data indicate that the 4.1R protein functions as a positive regulator in the early activation events after FcεRI triggering in mast cells.

Differential use of BTK and PLC in FcεRI- and KIT-mediated mast cell activation: A marginal role of BTK upon KIT activation

In mast cells (MCs), the TEC family kinase (TFK) BTK constitutes a central regulator of antigen (Ag)-triggered, FcεRI-mediated PLCγ phosphorylation, Ca²⁺ mobilization, degranulation, and pro-inflammatory cytokine production. Less is known about the function of BTK in the context of stem cell factor (SCF)-induced KIT signaling. In bone marrow-derived MCs (BMMCs), Ag stimulation caused intense phosphorylation of BTK at Y551 in its active center and at Y223 in its SH3-domain, whereas in response to SCF only Y223 was significantly phosphorylated. Further data using the TFK inhibitor Ibrutinib indicated that BTK Y223 is phosphorylated by a non-BTK TFK upon SCF stimulation. In line, SCF-induced PLCγ1 phosphorylation was stronger attenuated by Ibrutinib than by BTK deficiency. Subsequent pharmacological analysis of PLCγ function revealed a total block of SCF-induced Ca²⁺ mobilization by PLC inhibition, whereas only the sustained phase of Ca²⁺ flux was curtailed in Ag-stimulated BMMCs. Despite this severe stimulus-dependent difference in inducing Ca²⁺ mobilization, PLCγ inhibition suppressed Ag- and SCF-induced degranulation and pro-inflammatory cytokine production to comparable extents, suggesting involvement of additional TFK(s) or PLCγ-dependent signaling components. In addition to PLCγ, the MAPKs p38 and JNK were activated by Ag in a BTK-dependent manner; this was not observed upon SCF stimulation. Hence, FcεRI and KIT employ different mechanisms for activating PLCγ, p38, and JNK, which might strengthen their cooperation regarding pro-inflammatory MC effector functions. Importantly, our data clearly demonstrate that analyzing BTK Y223 phosphorylation is not sufficient to prove BTK activation.

SHIP negatively regulates type II immune responses in mast cells and macrophages

SHIP is a hematopoietic-specific lipid phosphatase that dephosphorylates PI3K-generated PI(3,4,5)-trisphosphate. SHIP removes this second messenger from the cell membrane blunting PI3K activity in immune cells. Thus, SHIP negatively regulates mast cell activation downstream of multiple receptors. SHIP has been referred to as the "gatekeeper" of mast cell degranulation as loss of SHIP dramatically increases degranulation or permits degranulation in response to normally inert stimuli. SHIP also negatively regulates Mϕ activation, including both pro-inflammatory cytokine production downstream of pattern recognition receptors, and alternative Mϕ activation by the type II cytokines, IL-4, and IL-13. In the SHIP-deficient (SHIP-/- ) mouse, increased mast cell and Mϕ activation leads to spontaneous inflammatory pathology at mucosal sites, which is characterized by high levels of type II inflammatory cytokines. SHIP-/- mast cells and Mϕs have both been implicated in driving inflammation in the SHIP-/- mouse lung. SHIP-/- Mϕs drive Crohn's disease-like intestinal inflammation and fibrosis, which is dependent on heightened responses to innate immune stimuli generating IL-1, and IL-4 inducing abundant arginase I. Both lung and gut pathology translate to human disease as low SHIP levels and activity have been associated with allergy and with Crohn's disease in people. In this review, we summarize seminal literature and recent advances that provide insight into SHIP's role in mast cells and Mϕs, the contribution of these cell types to pathology in the SHIP-/- mouse, and describe how these findings translate to human disease and potential therapies.

Mipsagargin, a novel thapsigargin-based PSMA-activated prodrug: results of a first-in-man phase I clinical trial in patients with refractory, advanced or metastatic solid tumours

Background:

Mipsagargin (G-202; (8-O-(12-aminododecanoyl)-8-O-debutanoyl thapsigargin)-Asp-γ-Glu-γ-Glu-γ-GluGluOH)) is a novel thapsigargin-based targeted prodrug that is activated by PSMA-mediated cleavage of an inert masking peptide. The active moiety is an inhibitor of the sarcoplasmic/endoplasmic reticulum calcium adenosine triphosphatase (SERCA) pump protein that is necessary for cellular viability. We evaluated the safety of mipsagargin in patients with advanced solid tumours and established a recommended phase II dosing (RP2D) regimen.

Methods:

Patients with advanced solid tumours received mipsagargin by intravenous infusion on days 1, 2 and 3 of 28-day cycles and were allowed to continue participation in the absence of disease progression or unacceptable toxicity. The dosing began at 1.2 mg m⁻² and was escalated using a modified Fibonacci schema to determine maximally tolerated dose (MTD) with an expansion cohort at the RP2D. Plasma was analysed for mipsagargin pharmacokinetics and response was assessed using RECIST criteria.

Results:

A total of 44 patients were treated at doses ranging from 1.2 to 88 mg m⁻², including 28 patients in the dose escalation phase and 16 patients in an expansion cohort. One dose-limiting toxicity (DLT; Grade 3 rash) was observed in the dose escalation portion of the study. At 88 mg m⁻², observations of Grade 2 infusion-related reaction (IRR, 2 patients) and Grade 2 creatinine elevation (1 patient) led to declaration of 66.8 mg m⁻² as the recommended phase II dose (RP2D). Across the study, the most common treatment-related adverse events (AEs) were fatigue, rash, nausea, pyrexia and IRR. Two patients developed treatment-related Grade 3 acute renal failure that was reversible during the treatment-free portion of the cycle. To help ameliorate the IRR and creatinine elevations, a RP2D of 40 mg m⁻² on day 1 and 66.8 mg m⁻² on days 2 and 3 with prophylactic premedications and hydration on each day of infusion was established. Clinical response was not observed, but prolonged disease stabilisation was observed in a subset of patients.

Conclusions:

Mipsagargin demonstrated an acceptable tolerability and favourable pharmacokinetic profile in patients with solid tumours.

Src Homology 2 Domain-Containing Inositol 5'-Phosphatase Ameliorates High Glucose-Induced Extracellular Matrix Deposition via the Phosphatidylinositol 3-Kinase/Protein Kinase B Pathway in Renal Tubular Epithelial Cells

A typical hallmark of diabetic kidney disease (DKD) is an excessive deposition of extracellular matrix (ECM) in the glomerulus and renal tubulointerstitium, leading to glomerulosclerosis and tubular interstitial fibrosis. Src homology 2 domain-containing inositol 5'-phosphatase (SHIP) is a negative regulator of the phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signaling. Here, we investigated the effect of SHIP on ECM deposition in diabetic mice and high glucose-stimulated human renal tubular epithelial cells (HK2 cells). The decreased SHIP and increased phospho-Akt (Ser 473, Thr 308) were found in the renal tubular cells of diabetic mice, which were accompanied by overexpression of transforming growth factor-β1 (TGF-β1), α-smooth muscle actin (α-SMA), and secreted collagen type 3 (Col 3) and a low expression of E-cadherin compared to that in normal mice. In vitro research revealed that high glucose-attenuated SHIP expression accompanied the activation of the PI3K/Akt signaling and ECM production. Knocking down SHIP in HK2 cells caused an increase in the levels of phospho-Akt (Ser 473), phospho-Akt (Thr 308), TGF-β1, α-SMA, and secreted Col 3 and a decrease in E-cadherin. Again, either the M90-SHIP plasmid or the PI3K/Akt pathway inhibitor LY294002 could significantly prevent the high glucose-induced increase in TGF-β1, α-SMA, and secreted Col 3 and decreased E-cadherin. Furthermore, we confirmed that inhibition of the TGF-β1 pathway with SB431542 blocked the effect of SHIP knockdown on ECM production in HK2 cells. In summary, our study suggests that decreased SHIP mediates high glucose-induced TGF-β1 upregulation and ECM deposition through activation of the PI3K/Akt pathway in renal tubular cells. J. Cell. Biochem. 118: 2271-2284, 2017. © 2017 Wiley Periodicals, Inc.

Elevated endoplasmic reticulum stress reinforced immunosuppression in the tumor microenvironment via myeloid-derived suppressor cells

The role of endoplasmic reticulum (ER) stress in cancer has been studied in detail, and ER stress is known to increase tumor cell apoptosis, and thus, reduce tumor growth. However, in our study, persistent ER stress induced by multiple administrations of low-dose thapsigargin (Tg) accelerated tumor growth in mice. Tg-mediated ER stress increased the generation of Ly6G⁺CD11b⁺ myeloid cells, but did not alter anti-tumor effector T cells. 4-Phenylbutyric acid (4-PBA), a chemical chaperone widely used as an ER stress reducer, attenuated Tg-induced myeloid-derived suppressor cell (MDSC) expansion and tumor growth. Tg-mediated ER stress enhanced the immunosuppressive capacity of tumor-infiltrating MDSCs by increasing expression of ARG1, iNOS, and NOX2, although splenic MDSCs were not affected. Consistent with these results, 4-PBA restored the anti-tumor immune response by regulating inflammatory cytokines such as TNF-α and CXCL1/KC, and activated tumor-infiltrating CD8⁺ T cells that were inhibited by Tg-mediated ER stress. These results suggest that significant ER stress in a tumor-bearing host might induce tumor growth mediated by enhancement of MDSC-mediated suppression. Therefore, ER stress reducers such as 4-PBA could restore anti-tumor immunity by inhibiting suppressive MDSCs that are exacerbated by ER stress.

Anti-allergic properties of curine, a bisbenzylisoquinoline alkaloid

Curine is a bisbenzylisoquinoline alkaloid isolated from Chondrodendron platyphyllum (Menispermaceae). Recent findings have shed light on the actions of curine in different models of allergy and inflammation. Here we review the properties and mechanisms of action of curine focusing on its anti-allergic effects. Curine pre-treatment significantly inhibited the scratching behavior, paw edema and systemic anaphylaxis induced by either ovalbumin (OVA) in sensitized animals or compound 48/80, through mechanisms of mast cell stabilization and inhibition of mast cell activation to generate lipid mediators. In addition, oral administration of curine significantly inhibited eosinophil recruitment and activation, as well as, OVA-induced airway hyper-responsiveness in a mouse model of asthma, through inhibition of the production of IL-13 and eotaxin, and of Ca²⁺ influx. In conclusion, curine exhibit anti-allergic effects in models of lung, skin and systemic allergy in the absence of significant toxicity, and as such has the potential for anti-allergic drug development.

Adenine suppresses IgE-mediated mast cell activation

Nucleobase adenine is produced by dividing human lymphoblasts mainly from polyamine synthesis and inhibits immunological functions of lymphocytes. We investigated the anti-allergic effect of adenine on IgE-mediated mast cell activation in vitro and passive cutaneous anaphylaxis (PCA) in mice. Intraperitoneal injection of adenine to IgE-sensitized mice attenuated IgE-mediated PCA reaction in a dose dependent manner, resulting in a median effective concentration of 4.21 mg/kg. In mast cell cultures, only adenine among cytosine, adenine, adenosine, ADP and ATP dose-dependently suppressed FcɛRI (a high affinity receptor for IgE)-mediated degranulation with a median inhibitory concentration of 1.6mM. It also blocked the production of LTB4, an inflammatory lipid mediator, and inflammatory cytokines TNF-α and IL-4. In addition, adenine blocked thapsigargin-induced degranulation which is FcɛRI-independent but shares FcɛRI-dependent signaling events. Adenine inhibited the phosphorylation of signaling molecules important to FcɛRI-mediated allergic reactions such as Syk, PLCγ2, Gab2, Akt, and mitogen activated protein kinases ERK and JNK. From this result, we report for the first time that adenine inhibits PCA in mice and allergic reaction by inhibiting FcɛRI-mediated signaling events in mast cells. Therefore, adenine may be useful for the treatment of mast cell-mediated allergic diseases. Also, the upregulation of adenine production may provide another mechanism for suppressing mast cell activity especially at inflammatory sites.

Small molecule inhibition of PAX3-FOXO1 through AKT activation suppresses malignant phenotypes of alveolar rhabdomyosarcoma

Alveolar rhabdomyosarcoma comprises a rare highly malignant tumor presumed to be associated with skeletal muscle lineage in children. The hallmark of the majority of alveolar rhabdomyosarcoma is a chromosomal translocation that generates the PAX3-FOXO1 fusion protein, which is an oncogenic transcription factor responsible for the development of the malignant phenotype of this tumor. Alveolar rhabdomyosarcoma cells are dependent on the oncogenic activity of PAX3-FOXO1, and its expression status in alveolar rhabdomyosarcoma tumors correlates with worst patient outcome, suggesting that blocking this activity of PAX3-FOXO1 may be an attractive therapeutic strategy against this fusion-positive disease. In this study, we screened small molecule chemical libraries for inhibitors of PAX3-FOXO1 transcriptional activity using a cell-based readout system. We identified the Sarco/endoplasmic reticulum Ca(2+)-ATPases (SERCA) inhibitor thapsigargin as an effective inhibitor of PAX3-FOXO1. Subsequent experiments in alveolar rhabdomyosarcoma cells showed that activation of AKT by thapsigargin inhibited PAX3-FOXO1 activity via phosphorylation. Moreover, this AKT activation appears to be associated with the effects of thapsigargin on intracellular calcium levels. Furthermore, thapsigargin inhibited the binding of PAX3-FOXO1 to target genes and subsequently promoted its proteasomal degradation. In addition, thapsigargin treatment decreases the growth and invasive capacity of alveolar rhabdomyosarcoma cells while inducing apoptosis in vitro. Finally, thapsigargin can suppress the growth of an alveolar rhabdomyosarcoma xenograft tumor in vivo. These data reveal that thapsigargin-induced activation of AKT is an effective mechanism to inhibit PAX3-FOXO1 and a potential agent for targeted therapy against alveolar rhabdomyosarcoma.

PKCβ phosphorylates PI3Kγ to activate it and release it from GPCR control

All class I phosphoinositide 3-kinases (PI3Ks) associate tightly with regulatory subunits through interactions that have been thought to be constitutive. PI3Kγ is key to the regulation of immune cell responses activated by G protein-coupled receptors (GPCRs). Remarkably we find that PKCβ phosphorylates Ser582 in the helical domain of the PI3Kγ catalytic subunit p110γ in response to clustering of the high-affinity IgE receptor (FcεRI) and/or store-operated Ca²⁺- influx in mast cells. Phosphorylation of p110γ correlates with the release of the p84 PI3Kγ adapter subunit from the p84-p110γ complex. Ser582 phospho-mimicking mutants show increased p110γ activity and a reduced binding to the p84 adapter subunit. As functional p84-p110γ is key to GPCR-mediated p110γ signaling, this suggests that PKCβ-mediated p110γ phosphorylation disconnects PI3Kγ from its canonical inputs from trimeric G proteins, and enables p110γ to operate downstream of Ca²⁺ and PKCβ. Hydrogen deuterium exchange mass spectrometry shows that the p84 adaptor subunit interacts with the p110γ helical domain, and reveals an unexpected mechanism of PI3Kγ regulation. Our data show that the interaction of p110γ with its adapter subunit is vulnerable to phosphorylation, and outline a novel level of PI3K control.

Role of yessotoxin in calcium and cAMP-crosstalks in primary and K-562 human lymphocytes: the effect is mediated by anchor kinase A mitochondrial proteins

Yessotoxin (YTX) is a marine polyether toxin previously described as a phosphodiesterase (PDE) activator in fresh human lymphocytes. This toxin induces a decrease of adenosine 3',5'-cyclic monophosphate (cAMP) levels in fresh human lymphocytes in a medium with calcium (Ca(2+) ), whereas the contrary effect has been observed in a Ca(2+) -free medium. In the present article, the effect of YTX in K-562 lymphocytes cell line has been analysed. Surprisingly, results obtained in K-562 cell line are completely opposite than in fresh human lymphocytes, since in K-562 cells YTX induces an increase of cAMP levels. YTX cytotoxicity was also studied in both K-562 cell line and fresh human lymphocytes. Results demonstrate that YTX does not modify fresh human lymphocytes viability, whereas in K-562 cells, YTX has a highly cytotoxic effect. It has been described in a previous study that YTX induces a small cytosolic Ca(2+) increase in fresh human lymphocytes but no effect was observed on Ca(2+) pools depletion in these cells. However, our results show that, in K-562 cells, YTX has no effect on cytosolic Ca(2+) levels in a medium with Ca(2+) and induces an increase on Ca(2+) pools depletion followed by a Ca(2+) influx. As far as Ca(2+) modulation is concerned these results demonstrate that YTX has a clear opposite effect in tumoural and fresh human lymphocytes. In addition, intracellular Ca(2+) reservoirs affected by YTX are different than thapsigargin-sensible pools. Furthermore, YTX-dependent Ca(2+) pools depletion was abolished by cAMP analogue (dibutyryl cAMP), phosphodiesterase-4 (PDE4) inhibitor (rolipram), protein kinase A inhibitor (H89) and oxidative phosphorylation uncoupler carbonyl cyanide p-(trifluoromethoxy) (FCCP) treatments. This evidences the crosstalks between Ca(2+) , YTX and cAMP pathways. Also, results obtain demonstrate that YTX-dependent Ca(2+) influx was only abolished by FCCP pre-treatment, which indicates a link between YTX and mitochondria in K-562 cell line. Cytosolic expression of A-kinase anchor proteins (AKAPs), the proteins which integrates phosphodiesterases (PDEs) and PKA to the mitochondria, was determined in both cell models. On the one hand, in human fresh lymphocytes, YTX increases AKAP149 cytosolic expression. This fact is accompanied with a decrease in cAMP levels, and therefore PDEs activation, which finally leads to cell survival. On the other hand, in tumoural lymphocytes, YTX has an opposite effect since decreases AKAP149 cytosolic expression and increase cAMP levels which leads to cell death. This is the first time that YTX and mitochondrial AKAPs proteins relationship is characterised.

The 1,4-benzodiazepine Ro5-4864 (4-chlorodiazepam) suppresses multiple pro-inflammatory mast cell effector functions

Activation of mast cells (MCs) can be achieved by the high-affinity receptor for IgE (FcεRI) as well as by additional receptors such as the lipopolysaccharide (LPS) receptor and the receptor tyrosine kinase Kit (stem cell factor [SCF] receptor). Thus, pharmacological interventions which stabilize MCs in response to different receptors would be preferable in diseases with pathological systemic MC activation such as systemic mastocytosis. 1,4-Benzodiazepines (BDZs) have been reported to suppress MC effector functions. In the present study, our aim was to analyze molecularly the effects of BDZs on MC activation by comparison of the effects of the two BDZs Ro5-4864 and clonazepam, which markedly differ in their affinities for the archetypical BDZ recognition sites, i.e., the GABA_A receptor and TSPO (previously termed peripheral-type BDZ receptor). Ro5-4864 is a selective agonist at TSPO, whereas clonazepam is a selective agonist at the GABA_A receptor. Ro5-4864 suppressed pro-inflammatory MC effector functions in response to antigen (Ag) (degranulation/cytokine production) and LPS and SCF (cytokine production), whereas clonazepam was inactive. Signaling pathway analyses revealed inhibitory effects of Ro5-4864 on Ag-triggered production of reactive oxygen species, calcium mobilization and activation of different downstream kinases. The initial activation of Src family kinases was attenuated by Ro5-4864 offering a molecular explanation for the observed impacts on various downstream signaling elements. In conclusion, BDZs structurally related to Ro5-4864 might serve as multifunctional MC stabilizers without the sedative effect of GABA_A receptor-interacting BDZs.

MEK/Erk-based negative feedback mechanism involved in control of Steel Factor-triggered production of Krüppel-like factor 2 in mast cells

The receptor tyrosine kinase, c-kit (Steel Factor (SF) receptor) controls survival, proliferation, chemotaxis, and secretion of proinflammatory cytokines in mast cells (MCs). Activation of c-kit results, amongst others, in induction of the PI3K and MEK/Erk pathways. Comparison of two MEK inhibitors, the specific, widely used U0126 and the more selective PD0325901, in different MC models revealed severe differences on SF-induced expression of proinflammatory cytokines IL-6 and TNF-α as well as the transcription factor Krüppel-like factor 2 (KLF2). Expression of the latter in MCs was not investigated so far. Whereas SF-induced expression of IL-6, TNF-α, and KLF2 was unaltered by U0126, it was significantly augmented by PD0325901. The effect of PD0325901 was corroborated by a second selective MEK inhibitor, PD184352 (Cl-1040), indicating the presence of MEK/Erk-based negative feedback mechanism(s) downstream of c-kit activation. Further analysis of KLF2 production revealed a positive function of PI3K. Depending on additional stimuli (e.g. antigen, IGF-1, LPS, thapsigargin), SF-triggered KLF2 expression was differentially modified, most likely controlled by the respective ratio between MEK/Erk and PI3K pathway activation. Moreover, the statin, simvastatin, was demonstrated to upregulate expression of KLF2 in MCs. In conclusion, data obtained by solely using the MEK inhibitor U0126 have to be carefully corroborated by using more selective inhibitors, such as PD0325901 or PD184352. SF-induced expression of the transcription factor KLF2 and its regulation by the MEK/Erk and PI3K pathways could impact on physiological as well as pathophysiological MC functions.

CGRP1 receptor activation induces piecemeal release of protease-1 from mouse bone marrow-derived mucosal mast cells

BACKGROUND

The parasitized or inflamed gastrointestinal mucosa shows an increase in the number of mucosal mast cells (MMC) and the density of extrinsic primary afferent nerve fibers containing the neuropeptide, calcitonin gene-related peptide (CGRP). Currently, the mode of action of CGRP on MMC is unknown.

METHODS

The effects of CGRP on mouse bone marrow-derived mucosal mast cells (BMMC) were investigated by measurements of intracellular Ca(2+)[Ca(2+)](i) and release of mMCP-1.

KEY RESULTS

Bone marrow-derived mucosal mast cells responded to the application of CGRP with a single transient rise in [Ca(2+)](i). The proportion of responding cells increased concentration-dependently to a maximum of 19 ± 4% at 10(-5)mol L(-1) (mean ±SEM; C48/80 100%; EC(50)10(-8) mol L(-1) ). Preincubation with the CGRP receptor antagonist BIBN4096BS (10(-5) mol L(-1)) completely inhibited BMMC activation by CGRP [range 10(-5) to 10(-11) mol L(-1); analysis of variance (ANOVA) P < 0.001], while preincubation with LaCl(3) to block Ca(2+) entry did not affect the response (P = 0.18). The presence of the CGRP1 receptor on BMMC was confirmed by simultaneous immunofluorescent detection of RAMP1 or CRLR, the two components of the CGRP1 receptor, and mMCP-1. Application of CGRP for 1 h evoked a concentration-dependent release of mMCP-1 (at EC(50) 10% of content) but not of β-hexosaminidase and alterations in granular density indicative of piecemeal release.

CONCLUSIONS & INFERENCES

We demonstrate that BMMC express functional CGRP1 receptors and that their activation causes mobilization of Ca(2+) from intracellular stores and piecemeal release of mMCP-1. These findings support the hypothesis that the CGRP signaling from afferent nerves to MMC in the gastrointestinal wall is receptor-mediated.

L-type Ca2+ channels: a new player in the regulation of Ca2+ signaling, cell activation and cell survival in immune cells

Ca(2+) is a highly versatile intracellular second messenger in many cell types, and regulates many complicated cellular processes, including cell activation, proliferation and apoptosis. Influx of Ca(2+) from the extracellular fluid is required for sustained elevation of the cytosolic Ca(2+) concentration and full activation of Ca(2+)-dependent processes. It is widely accepted that Ca(2+) release-activated Ca(2+) channels are the major routes of Ca(2+) influx in electrically non-excitable cells, including hematopoietic cells, whereas voltage-gated Ca(2+) channels such as L-type Ca(2+) channels (LTCCs) serve as the principal routes of Ca(2+) entry into electrically excitable cells such as neurons and myocytes. However, recent pharmacological and molecular genetic studies have revealed the existence of functional LTCCs and/or LTCC-like channels in a variety of immune cells including mast cells. In this article, we review recent advances in our understanding of Ca(2+) signaling in immune cells with a special interest in mast cells. We highlight roles for LTCCs in antigen receptor-mediated mast cell activation and survival.

L-type Ca2+ channels in mast cells: activation by membrane depolarization and distinct roles in regulating mediator release from store-operated Ca2+ channels

Store-operated Ca(2+) channels (SOCs) are considered to be the principal route of Ca(2+) influx in non-excitable cells. We have previously shown that in mast cells IgE+antigen (Ag) induces a dihydropyridine (DHP)-sensitive Ca(2+) influx independently of Ca(2+) store depletion. Since the DHP receptor is the alpha subunit of L-type Ca(2+) channels (LTCCs), we examined the possible role of LTCCs in mast cell activation. Mast cells exhibited substantial expression of the alpha(1C) (Ca(V)1.2) subunit mRNA and protein on their cell surface. IgE+Ag-induced Ca(2+) influx was substantially reduced by the LTCC inhibitor nifedipine, and enhanced by the LTCC activator (S)-BayK8644, whereas these agents had minimal effects on thapsigargin (TG)-induced Ca(2+) influx. These LTCC-modulating agents regulated IgE+Ag-induced cell activation but not TG-induced cell activation. Inhibition of SOCs by 2-aminoethoxydiphenyl borate reduced both degranulation and production of cytokines, including interleukin-13 and tumor necrosis factor-alpha, whereas LTCC modulation reciprocally regulated degranulation and cytokine production. IgE+Ag, but not TG, induced substantial plasma membrane depolarization, which stimulated a DHP-sensitive Ca(2+) response. Moreover, IgE+Ag-, but not TG-induced mitochondrial Ca(2+) increase was regulated by LTCC modulators. Finally, gene silencing analyses using small interfering RNA revealed that the alpha(1C) (Ca(V)1.2) LTCC mediated the pharmacological effects of the LTCC-modulating agents. These results demonstrate that mast cells express LTCCs, which becomes activated by membrane depolarization to regulate cytosolic and mitochondrial Ca(2+), thereby controlling mast cell activation in a distinct manner from SOCs.

IgE-induced mast cell survival requires the prolonged generation of reactive oxygen species

We show in this study that the ability of five different monomeric IgEs to enhance murine bone marrow-derived mast cell (BMMC) survival correlates with their ability to stimulate extracellular calcium (Ca(2+)) entry. However, whereas IgE+Ag more potently stimulates Ca(2+) entry, it does not enhance survival under our conditions. Exploring this further, we found that whereas all five monomeric IgEs stimulate a less robust Ca(2+) entry than IgE+Ag initially, they all trigger a more prolonged Ca(2+) influx, generation of reactive oxygen species (ROS), and ERK phosphorylation. These prolonged signaling events correlate with their survival-enhancing ability and positively feedback on each other to generate the prosurvival cytokine, IL-3. Interestingly, the prolonged ERK phosphorylation induced by IgE appears to be regulated by a MAPK phosphatase rather than MEK. IgE-induced ROS generation, unlike that triggered by IgE+Ag, is not mediated by 5-lipoxygenase. Moreover, ROS inhibitors, which block both IgE-induced ROS production and Ca(2+) influx, convert the prolonged ERK phosphorylation induced by IgE into the abbreviated phosphorylation pattern observed with IgE+Ag and prevent IL-3 generation. In support of the essential role that IgE-induced ROS plays in IgE-enhanced BMMC survival, we found the addition of H(2)O(2) to IgE+Ag-stimulated BMMCs leads to IL-3 secretion.

JunB is required for IgE-mediated degranulation and cytokine release of mast cells

Mast cells are effector cells of IgE-mediated immune responses frequently found at the vicinity of blood vessels, the margins of diverse tumors and at sites of potential infection and inflammation. Upon IgE-mediated stimulation, mast cells produce and secrete a broad spectrum of cytokines and other inflammatory mediators. Recent work identified JunB, a member of the AP-1 transcription factor family, as critical regulator of basal and induced expression of inflammatory mediators in fibroblasts and T cells. To study the impact of JunB on mast cell biology, we analyzed JunB-deficient mast cells. Mast cells lacking JunB display a normal in vivo maturation, and JunB-deficient bone marrow cells in vitro differentiated to mast cells show no alterations in proliferation or apoptosis. But these cells exhibit impaired IgE-mediated degranulation most likely due to diminished expression of SWAP-70, Synaptotagmin-1, and VAMP-8, and due to impaired influx of extracellular calcium. Moreover, JunB-deficient bone marrow mast cells display an altered cytokine expression profile in response to IgE stimulation. In line with these findings, the contribution of JunB-deficient mast cells to angiogenesis, as analyzed in an in vitro tube formation assay on matrigel, is severely impaired due to limiting amounts of synthesized and secreted vascular endothelial growth factor. Thus, JunB is a critical regulator of intrinsic mast cell functions including cross-talk with endothelial cells.

Reactive oxygen species produced up- or downstream of calcium influx regulate proinflammatory mediator release from mast cells: role of NADPH oxidase and mitochondria

Earlier studies have demonstrated that mast cells produce reactive oxygen species (ROS), which play a role in regulating Ca(2+) influx, while in other cell types ROS are produced in a Ca(2+)-dependent manner. We sought to determine whether ROS are produced downstream of the extracellular Ca(2+) entry in mast cells. Thapsigargin (TG), a receptor-independent agonist, could evoke a robust burst of intracellular ROS. However, this response was distinct from the antigen-induced burst of ROS with respect to time course and dependence on Ca(2+) and phosphatidylinositol-3-kinase (PI3K). The antigen-induced ROS generation occurred immediately, while the TG-induced ROS generation occurred with a significant lag time (~2 min). Antigen but not TG caused extracellular release of superoxide (O(2)(*-))/hydrogen peroxide (H(2)O(2)), which was blocked by diphenyleneiodonium, apocynin, and wortmannin. A capacitative Ca(2+) entry resulted in the generation of O(2)(*-) in the mitochondria in a PI3K-independent manner. Blockade of ROS generation inhibited TG-induced mediator release. Finally, when used together, antigen and TG evoked the release of leukotriene C(4), tumor necrosis factor-alpha, and interleukin-13 as well as ROS generation synergistically. These results suggest that ROS produced upstream of Ca(2+) influx by NADPH oxidase and downstream of Ca(2+) influx by the mitochondria regulate the proinflammatory response of mast cells.

Insulin and insulin-like growth factor-1 promote mast cell survival via activation of the phosphatidylinositol-3-kinase pathway

OBJECTIVE

Mast cells (MCs) play central roles for the onset and development of immediate-type and inflammatory allergic reactions. Since the inverse relationship between atopic disorders and diabetes mellitus has been observed in animals and humans, we investigated the effects of insulin (Ins) on MC signaling and biological function.

METHODS

In bone marrow-derived MCs (BMMCs) from wild-type as well as SHIP-deficient mice Ins as well as insulin-like growth factor-1 (IGF-I)-triggered intracellular signaling events and MC effector functions were studied.

RESULTS

We found that the addition of either Ins or IGF-1 to BMMCs triggers the phosphorylation of protein kinase B (PKB) and p38 kinase but not extracellular signal-regulated kinase (Erk). We also found that Ins/IGF-1 stimulates the tyrosine phosphorylation of SHIP1 and, in keeping with this, Ins/IGF-1-induced PKB phosphorylation is higher in SHIP1-/- BMMCs and is inhibited in SHIP+/+ as well as SHIP1-/- BMMCs with inhibitors of phosphatidylinositol-3-kinase (PI3K). Ins/IGF-1, like antigen (Ag), also stimulates the Rac-dependent activation of PAK as well as the production of hydrogen peroxide (H2O2). To elucidate the role of Ins and IGF-1 in MC biology, we studied their effects on Ag-mediated degranulation and MC survival. Although both only slightly enhanced Ag-mediated degranulation, they significantly promoted MC survival in the absence of IL-3 in a PI3K-dependent manner.

CONCLUSION

The promotion of BMMC survival by induction of Ins/IGF-1 signaling may, in part, be responsible for the inverse correlation observed between atopic disorders and diabetes mellitus.

Vitamin E and mast cells

Mast cells play an important role in the immune system by interacting with B and T cells and by releasing several mediators involved in activating other cells. Hyperreactivity of mast cells and their uncontrolled accumulation in tissues lead to increased release of inflammatory mediators contributing to the pathogenesis of several diseases such as rheumatoid arthritis, atherosclerosis, multiple sclerosis, and allergic disorders such as asthma and allergic rhinitis. Interference with mast cell proliferation, survival, degranulation, and migration by synthetic or natural compounds may represent a preventive strategy for the management of these diseases. Natural vitamin E covers a group of eight analogues-the alpha-, beta-, gamma-, and delta-tocopherols and the alpha-, beta-, gamma-, and delta-tocotrienols, but only alpha-tocopherol is efficiently retained by the liver and distributed to peripheral tissues. Mast cells preferentially locate in the proximity of tissues that interface with the external environment (the epithelial surface of the skin, the gastrointestinal mucosa, and the respiratory system), what may render them accessible to treatments with inefficiently retained natural vitamin E analogues and synthetic derivatives. In addition to scavenging free radicals, the natural vitamin E analogues differently modulate signal transduction and gene expression in several cell lines; in mast cells, protein kinase C, protein phosphatase 2A, and protein kinase B are affected by vitamin E, leading to the modulation of proliferation, apoptosis, secretion, and migration. In this chapter, the possibility that vitamin E can prevent diseases with mast cells involvement by modulating signal transduction and gene expression is evaluated.

The inositol 5'-phosphatase SHIP-2 negatively regulates IgE-induced mast cell degranulation and cytokine production

Aggregation of the high-affinity IgE receptor (FcepsilonRI) on mast cells initiates signaling pathways leading to degranulation and cytokine release. It has been reported that SHIP-1 negatively regulates FcepsilonRI-triggered pathways but it is unknown whether its homologous protein SHIP-2 has the same function. We have used a lentiviral-based RNA interference technique to obtain SHIP-2 knockdown bone marrow-derived mast cells (BMMCs) and have found that elimination of SHIP-2 results in both increased mast cell degranulation and cytokine (IL-4 and IL-13) gene expression upon FcepsilonRI stimulation. Elimination of SHIP-2 from BMMCs has no effect on FcepsilonRI-triggered calcium flux, tyrosine phosphorylation of MAPKs or in actin depolymerization following activation. Rather, we observe that absence of SHIP-2 results in increased activation of the small GTPase Rac-1 and in enhanced microtubule polymerization upon FcepsilonRI engagement. Coimmunoprecipitation experiments in rat basophilic leukemia (RBL 2H3) cells show that SHIP-2 interacts with the FcepsilonRI beta-chain, Gab2 and Lyn and that unlike SHIP-1, it does not associate with SHC in mast cells. Our results report a negative regulatory role of SHIP-2 on mast cell activation that is calcium independent and distinct from the regulation by SHIP-1.

SHIP-deficient mice provide insights into the regulation of dendritic cell development and function

OBJECTIVE

Dendritic cells (DC) play a critical role in establishment and maintenance of central and peripheral tolerance. Despite intensive research, our knowledge of the molecular mechanisms regulating DC development and function is limited, thus hindering our ability to generate appropriate DC populations for manipulating immune tolerance. We utilized mice deficient in the SH2-containing inositol-5-phosphatase (SHIP) to examine the role of cytokine signaling in DC development and function.

METHODS

We analyzed the phenotype of both primary and bone marrow (BM)-derived DC (BMDC) using flow cytometry. In addition, cytokine production was measured using cytometric bead arrays and the ability of DC to induce allogeneic T-cell proliferation was assessed using thymidine incorporation assays.

RESULTS

We demonstrated that spleen DC isolated from SHIP-deficient mice are increased in number and have an altered phenotype. In vitro analyses revealed that SHIP-deficient BM cells give rise to a higher frequency of myeloid, but not plasmacytoid, DC due to both an increased progenitor frequency and enhanced cytokine sensitivity. The BMDC exhibit an altered phenotype that correlates with a reduced capacity to induce allogeneic T-cell proliferation. Addition of interleukin-6 to WT BM cultures during DC differentiation partially induces a KO phenotype.

CONCLUSION

These studies suggest that myeloid and plasmacytoid DC progenitors are differentially sensitive to signaling pathways in which SHIP is involved. Moreover, they suggest that interleukin-6 may have an important role in regulating the phenotype and function of myeloid DC.

Oxysterol-binding protein-related protein (ORP) 9 is a PDK-2 substrate and regulates Akt phosphorylation

The oxysterol-binding protein and oxysterol-binding protein-related protein family has been implicated in lipid transport and metabolism, vesicle trafficking and cell signaling. While investigating the phosphorylation of Akt/protein kinase B in stimulated bone marrow-derived mast cells, we observed that a monoclonal antibody directed against phospho-S473 Akt cross-reacted with oxysterol-binding protein-related protein 9 (ORP9). Further analysis revealed that mast cells exclusively express ORP9S, an N-terminal truncated version of full-length ORP9L. A PDK-2 consensus phosphorylation site in ORP9L and OPR9S at S287 (VPEFS(287)Y) was confirmed by site-directed mutagenesis. In contrast to Akt, increased phosphorylation of ORP9S S287 in stimulated mast cells was independent of phosphatidylinositol 3-kinase but sensitive to inhibition of conventional PKC isotypes. PKC-beta dependence was confirmed by lack of ORP9S phosphorylation at S287 in PKC-beta-deficient, but not PKC-alpha-deficient, mast cells. Moreover, co-immunoprecipitation of PKC-beta and ORP9S, and in vitro phosphorylation of ORP9S in this complex, argued for direct phosphorylation of ORP9S by PKC-beta, introducing ORP9S as a novel PKC-beta substrate. Akt was also detected in a PKC-beta/ORP9S immune complex and phosphorylation of Akt on S473 was delayed in PKC-deficient mast cells. In HEK293 cells, RNAi experiments showed that depletion of ORP9L increased Akt S473 phosphorylation 3-fold without affecting T308 phosphorylation in the activation loop. Furthermore, mammalian target of rapamycin was implicated in ORP9L phosphorylation in HEK293 cells. These studies identify ORP9 as a PDK-2 substrate and negative regulator of Akt phosphorylation at the PDK-2 site.

A redundant role for PKC-epsilon in mast cell signaling and effector function

Protein kinase (PK) C-epsilon is strongly expressed in mast cells (MCs) and activated in response to antigen-mediated high-affinity receptor for IgE (Fc epsilonR1) engagement. A critical role of PKC-epsilon in antigen-triggered activation of various signaling pathways was observed in basophilic leukemia cells. To study the function of PKC-epsilon in MCs differentiated in vitro from murine bone marrow, we used our established PKC-epsilon null mice. Unexpectedly, we did not reveal any difference in antigen-induced activation of many central signaling molecules (PKB, mitogen-activated protein kinase, p38, Jun-N-terminal kinase, phospholipase C-gamma1, Bruton's tyrosine kinase, PKD, Fos and PKC-delta) in time-course as well as dose-response studies between PKC-epsilon-deficient and wild-type MCs. In correlation, antigen-triggered degranulation, release of arachidonic acid and secretion of IL-6 were unaltered by the loss of PKC-epsilon. Furthermore, stimulation of MCs via different receptor systems [Steel factor receptor (c-kit) and toll-like receptor 4] did not lead to differences in the measured responses between both cell types. These results strongly suggest that PKC-epsilon plays a redundant role in MCs stimulated by antigen as well as other well-known MC stimuli.

A leucine zipper in the N terminus confers membrane association to SLP-65

Membrane recruitment of adaptor proteins is crucial for coupling antigen receptors to downstream signaling events. Despite the essential function of the B cell adaptor SLP-65, the mechanism of its recruitment to the plasma membrane is not yet understood. Here we show that a highly conserved leucine zipper in the SLP-65 N terminus is responsible for membrane association. Alterations in the N terminus abolished SLP-65 membrane localization and activity, both of which were restored by replacement of the N terminus with a myristoylation signal. The N terminus is an autonomous domain that confers specific localization and function when transferred to green fluorescent protein or the adaptor protein SLP-76. Our data elucidate the mechanism of SLP-65 membrane recruitment and suggest that leucine zipper motifs are essential interaction domains of signaling proteins.

SHIP Down-Regulates FcεR1-Induced Degranulation at Supraoptimal IgE or Antigen Levels

Cross-linking of the IgE-loaded high-affinity IgE receptor (FcepsilonR1) by multivalent Ags results in mast cell activation and subsequent release of multiple proinflammatory mediators. The dose-response curve for FcepsilonR1-mediated degranulation is bell-shaped, regardless of whether the IgE or the Ag concentration is varied. Although overall calcium influx follows this bell-shaped curve, intracellular calcium release continues to increase at supraoptimal IgE or Ag concentrations. As well, overall calcium mobilization adopts more transient kinetics when stimulations are conducted with supraoptimal instead of optimal Ag concentrations. Moreover, certain early signaling events continue to increase whereas degranulation drops under supraoptimal conditions. We identified SHIP, possibly in association with the FcepsilonR1 beta-chain, as a critical negative regulator acting within the inhibitory (supraoptimal) region of the dose-response curve that shifts the kinetics of calcium mobilization from a sustained to a transient response. Consistent with this, we found that degranulation of SHIP-deficient murine bone marrow-derived mast cells was not significantly reduced at supraoptimal Ag levels. A potential mediator of SHIP action, Bruton's tyrosine kinase, did not seem to play a role within the supraoptimal suppression of degranulation. Interestingly, SHIP was found to colocalize with the actin cytoskeleton (which has been shown previously to mediate the inhibition of degranulation at supraoptimal Ag doses). These results suggest that SHIP, together with other negative regulators, restrains bone marrow-derived mast cell activation at supraoptimal IgE or Ag concentrations in concert with the actin cytoskeleton.

Antiapoptotic Activity of Akt Is Down-regulated by Ca2+ in Myocardiac H9c2 Cells

Cell survival signaling of the Akt/protein kinase B pathway was influenced by a change in the cytoplasmic free calcium concentration ([Ca2+]i) for over 2 h via the regulation of a Ser/Thr phosphatase, protein phosphatase 2Ac (PP2Ac), in rat myocardiac H9c2 cells. Akt was down-regulated when [Ca2+]i was elevated by thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, but was up-regulated when it was suppressed by 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl)ester (BAPTA-AM), a cell permeable Ca2+ chelator. The inactivation of Akt was well correlated with the susceptibility to oxidant-induced apoptosis in H9c2 cells. To investigate the mechanism of the Ca(2+)-dependent regulation of Akt via the regulation of PP2A, we examined the transcriptional regulation of PP2Acalpha in H9c2 cells with Ca2+ modulators. Transcription of the PP2Acalpha gene was increased by thapsigargin but decreased by BAPTA-AM. The promoter activity was examined and the cAMP response element (CRE) was found responsible for the Ca(2+)-dependent regulation of PP2Acalpha. Furthermore, phosphorylation of CRE-binding protein increased with thapsigargin but decreased with BAPTA-AM. A long term change of [Ca2+]i regulates PP2Acalpha gene transcription via CRE, resulting in a change in the activation status of Akt leading to an altered susceptibility to apoptosis.

Increased expression of the DNA-binding cytokine HMGB1 in human atherosclerotic lesions: role of activated macrophages and cytokines

OBJECTIVE

Atherosclerosis is a chronic inflammatory response of the arterial wall to injury. High-mobility group box 1 (HMGB1) is a DNA-binding protein, which on release from cells exhibits potent inflammatory actions. We examined its expression in atherosclerotic lesions and regulation by cytokines.

METHODS AND RESULTS

In atherosclerotic lesions, HMGB1 protein is expressed by endothelial cells, some intimal smooth muscle cells, and macrophages. As atherosclerosis develops and progresses from fatty streaks to fibrofatty lesion, the number of HMGB1-producing macrophages increases markedly. Studies using the THP-1 cell line indicated that HMGB1 mRNA expression could be markedly upregulated by inflammatory cytokines, interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha and also transforming growth factor (TGF)-beta. IFN-gamma, TNF-alpha, TWEAK, and TGF-beta induced an intracellular redistribution of HMGB1 and stimulated secretion by THP-1 cells and human blood monocytes. Inhibitors of MEK1/MEK2, protein kinase C, and PI-3/Akt, which inhibit lysosomal degranulation and mRNA translation, attenuated cytokine-induced HMGB1 secretion.

CONCLUSIONS

Macrophage is the major cell type responsible for HMGB1 production in human atherosclerotic lesions. Inflammatory cytokines and TGF-beta increase HMGB1 expression and secretion by monocyte/macrophages. HMGB1 appears to be a common mediator of inflammation induced by inflammatory cytokines and is likely to contribute to lesion progression and chronic inflammation.

CD69 expression induced by thapsigargin, phorbol ester and ouabain on thymocytes is dependent on external Ca2+ entry

In the present work murine thymocytes exposed to Thapsigargin (TG 10, 20 and 50 nM), Phorbol-12,13,20-triacetate (TPA16 nM) and Ouabain (OUA100 nM) exhibited an increased expression of CD69, a molecule related to cellular activation and associated to Ca(++) influx in other systems. The kinetics of CD69 appearance depended on the stimuli and dose used. TG 50 nM induced an increased expression by 6 h whereas with lower doses (10 and 20 nM) an increase was detected at 18 h. TPA maximal increase was evident at 6 h. OUA lead to an observable increase at 18 h. However, in the case of TPA or TG the presence of the stimuli was only necessary for the first 2 h of culture, whereas OUA needed to be present during the whole assay. It was also demonstrated that Ca(++) influx was an essential feature, as EGTA diminished or abolished CD69 increased expression. Nevertheless, EGTA was only capable of this effect when present at the time of the stimuli. No correlation of CD69 expression with thymocyte death was observed. Similarly, the agents under study did not promote the maturation from double-positive into single-positive thymocytes. TPA and Thapsigargin were capable of decreasing the level of CD4 molecules on the cell surface, probably due to the loss of these molecules. OUA, on the other hand, did not modify CD4/CD8 expression on these cells.

Distinct phosphoinositide 3-kinases mediate mast cell degranulation in response to G-protein-coupled versus FcepsilonRI receptors

Phosphoinositide (PI) 3-kinases are critical regulators of mast cell degranulation. The Class IA PI 3-kinases p85/p110beta and p85/p110delta but not p85/p110alpha are required for antigen-mediated calcium flux in RBL-2H3 cells (Smith, A. J., Surviladze, Z., Gaudet, E. A., Backer, J. M., Mitchell, C. A., and Wilson, B. S. et al., (2001) J. Biol. Chem. 276, 17213-17220). We now examine the role of Class IA PI 3-kinases isoforms in degranulation itself, using a single-cell degranulation assay that measures the binding of fluorescently tagged annexin V to phosphatidylserine in the outer leaflet of the plasma membrane of degranulated mast cells. Consistent with previous data, antibodies against p110delta and p110beta blocked FcepsilonR1-mediated degranulation in response to FcepsilonRI ligation. However, antigen-stimulated degranulation was also inhibited by antibodies against p110alpha, despite the fact that these antibodies have no effect on antigen-induced calcium flux. These data suggest that p110alpha mediates a calcium-independent signal during degranulation. In contrast, only p110beta was required for enhancement of antigen-stimulated degranulation by adenosine, which augments mast cell-mediated airway inflammation in asthma. Finally, we examined carbachol-stimulated degranulation in RBL2H3 cells stably expressing the M1 muscarinic receptor (RBL-2H3-M1 cells). Surprisingly, carbachol-stimulated degranulation was blocked by antibody-mediated inhibition of the Class III PI 3-kinase hVPS34 or by titration of its product with FYVE domains. Antibodies against Class IA PI 3-kinases had no effect. These data demonstrate: (a) a calcium-independent role for p110alpha in antigen-stimulated degranulation; (b) a requirement for p110beta in adenosine receptor signaling; and (c) a requirement for hVPS34 during M1 muscarinic receptor signaling. Elucidation of the intersections between these distinct pathways will lead to new insights into mast cell degranulation.

Regulation of ionomycin-mediated granule release from rat basophil leukemia cells

Cross-linking the high affinity IgE receptor on the rat basophil leukemia clone 2H3 (RBL-2H3) cell line, an vitro model for mast cell signaling, results in granule release. A great deal of research has focused on the earliest steps in this signaling cascade resulting in models which include the participation of lyn, syk, phospholipase C (PLC), protein kinase C (PKC) and intracellular calcium mobilization. In an effort to look at pathways downstream of calcium mobilization, ionomycin-mediated granule release was studied. The kinase inhibitors PP1 (src family), GF109203X (PKC), PD98059 (MEK1/2), and U0126 (MEK1/2) substantially inhibited ionomycin-mediated granule release, while the p38 kinase inhibitor SB203580 did not. Both p38 and erk were phosphorylated upon ionomycin treatment, but only extracellular regulated kinase (erk) activation was completely inhibited by PP1 treatment and partially inhibited by the MEK inhibitors, thus, correlating with the granule release data. Interestingly, while GF109203X alone had no affect on erk activation, combining it with U0126 completely blocked this response. This suggests the existence an alternate pathway for erk activation that is MEK independent and PKC dependent. Experiments in which ionomycin and PP1 were titrated (independently) demonstrated a correlation between erk phosphorylation and granule release, implicating erk in a PP1-inhibitable pathway operating downstream of calcium and controlling mast cell degranulation.

Association of the Src homology 2 domain-containing inositol 5' phosphatase (SHIP) to releasability in human basophils

During the study of the biology of the Human recombinant Histamine Releasing Factor (HrHRF), we uncovered a hyperreleasable phenotype of basophils from HrHRF-responder donors. Basophils from these donors released histamne to HrHRF, IL-3 and D(2)O. While there has been a significant amount of work elucidating signal transduction events in human basophils, the reason for this hyperreleasable phenotype remained illusive. A clue to the releasability of these highly allergic, asthmatic HrHRF-responder donor basophils was demonstrated in studies using SHIP knockout mice. Bone marrow-derived mast cells from the SHIP knockout mice demonstrated hyperreleasability to stimuli through the IgE receptor and alteration of subsequent signal transduction events. We have demonstrated a highly significant negative correlation between the amount of SHIP protein per cell equivalent and maximum histamine release to HrHRF. These results provide a clue to the hyperreleasable phenotype and implicate SHIP as an additional regulator of secretion in human basophils.

Protein kinase C-delta is a negative regulator of antigen-induced mast cell degranulation

Regulation of mast cell degranulation is dependent on the subtle interplay of cellular signaling proteins. The Src homology 2 (SH2) domain-containing inositol-5'-phosphatase (SHIP), which acts as the gatekeeper of degranulation, binds via both its SH2 domain and its phosphorylated NPXY motifs to the adapter protein Shc via the latter's phosphorylated tyrosines and phosphotyrosine-binding domain, respectively. This theoretically leaves Shc's SH2 domain available to bind proteins, which might be part of the SHIP/Shc complex. In a search for such proteins, protein kinase C-delta (PKC-delta) was found to coprecipitate in mast cells with Shc and to interact with Shc's SH2 domain following antigen or pervanadate stimulation. Phosphorylation of PKC-delta's Y(332), most likely by Lyn, was found to be responsible for PKC-delta's binding to Shc's SH2 domain. Using PKC-delta(-/-) bone marrow-derived mast cells (BMMCs), we found that the antigen-induced tyrosine phosphorylation of Shc was similar to that in wild-type (WT) BMMCs while that of SHIP was significantly increased. Moreover, increased translocation of PKC-delta to the membrane, as well as phosphorylation at T505, was observed in SHIP(-/-) BMMCs, demonstrating that while PKC-delta regulates SHIP phosphorylation, SHIP regulates PKC-delta localization and activation. Interestingly, stimulation of PKC-delta(-/-) BMMCs with suboptimal doses of antigen yielded a more sustained calcium mobilization and a significantly higher level of degranulation than that of WT cells. Altogether, our data suggest that PKC-delta is a negative regulator of antigen-induced mast cell degranulation.

Vav1 is a component of transcriptionally active complexes

The importance of the hematopoietic protooncogene Vav1 in immune cell function is widely recognized, although its regulatory mechanisms are not completely understood. Here, we examined whether Vav1 has a nuclear function, as past studies have reported its nuclear localization. Our findings provide a definitive demonstration of Vav1 nuclear localization in a receptor stimulation-dependent manner and reveal a critical role for the COOH-terminal Src homology 3 (SH3) domain and a nuclear localization sequence within the pleckstrin homology domain. Analysis of DNA-bound transcription factor complexes revealed nuclear Vav1 as an integral component of transcriptionally active nuclear factor of activated T cells (NFAT)- and nuclear factor (NF)kappaB-like complexes, and the COOH-terminal SH3 domain as being critical in their formation. Thus, we describe a novel nuclear role for Vav1 as a component and facilitator of NFAT and NFkappaB-like transcriptional activity.

SHIP negatively regulates IgE + antigen-induced IL-6 production in mast cells by inhibiting NF-kappa B activity

We demonstrate in this study that IgE + Ag-induced proinflammatory cytokine production is substantially higher in Src homology-2-containing inositol 5'-phosphatase (SHIP)(-/-) than in SHIP(+/+) bone marrow-derived mast cells (BMMCs). Focusing on IL-6, we found that the repression of IL-6 mRNA and protein production in SHIP(+/+) BMMCs requires the enzymatic activity of SHIP, because SHIP(-/-) BMMCs expressing wild-type, but not phosphatase-deficient (D675G), SHIP revert the IgE + Ag-induced increase in IL-6 mRNA and protein down to levels seen in SHIP(+/+) BMMCs. Comparing the activation of various signaling pathways to determine which ones might be responsible for the elevated IL-6 production in SHIP(-/-) BMMCs, we found the phosphatidylinositol 3-kinase/protein kinase B (PKB), extracellular signal-related kinase (Erk), p38, c-Jun N-terminal kinase, and protein kinase C (PKC) pathways are all elevated in IgE + Ag-induced SHIP(-/-) cells. Moreover, inhibitor studies suggested that all these pathways play an essential role in IL-6 production. Looking downstream, we found that IgE + Ag-induced IL-6 production is dependent on the activity of NF-kappa B and that I kappa B phosphorylation/degradation and NF-kappa B translocation, DNA binding and transactivation are much higher in SHIP(-/-) BMMCs. Interestingly, using various pathway inhibitors, it appears that the phosphatidylinositol 3-kinase/PKB and PKC pathways elevate IL-6 mRNA synthesis, at least in part, by enhancing the phosphorylation of I kappa B and NF-kappa B DNA binding while the Erk and p38 pathways enhance IL-6 mRNA synthesis by increasing the transactivation potential of NF-kappa B. Taken together, our data are consistent with a model in which SHIP negatively regulates NF-kappa B activity and IL-6 synthesis by reducing IgE + Ag-induced phosphatidylinositol-3,4,5-trisphosphate levels and thus PKB, PKC, Erk, and p38 activation.

Phosphatidylinositol (3,4,5)P3 is essential but not sufficient for protein kinase B (PKB) activation; phosphatidylinositol (3,4)P2 is required for PKB phosphorylation at Ser-473: studies using cells from SH2-containing inositol-5-phosphatase knockout mice

Using bone marrow derived mast cells from SH2-containing inositol-5-phosphatase (SHIP) +/+ and minus sign/minus sign mice, we found that the loss of SHIP leads to a dramatic increase in Steel Factor (SF)-stimulated phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)), a substantial reduction in PI(3,4)P(2), and no change in PI(4,5)P(2) levels. We also found that SF-induced activation of protein kinase B (PKB) is increased and prolonged in SHIP -/- cells, due in large part to more PKB associating with the plasma membrane in these cells. Pretreatment of SHIP -/- cells with 25 microm LY294002 resulted in complete inhibition of SF-induced PI(3,4)P(2), while still yielding PI(3,4,5)P(3) levels similar to those achieved in SHIP+/+ cells. This offered a unique opportunity to study the regulation of PKB by PI(3,4,5)P(3), in the absence of PI(3,4)P(2). Under these conditions, PKB activity was markedly reduced compared with that in SF-stimulated SHIP+/+ cells, even though more PKB localized to the plasma membrane. Although phosphoinositide-dependent kinase 1 mediated phosphorylation of PKB at Thr-308 was unaffected by LY294002, phosphorylation at Ser-473 was dramatically reduced. Moreover, intracellular delivery of PI(3,4)P(2) to LY294002-pretreated, SF-stimulated SHIP -/- cells increased phosphorylation of PKB at Ser-473 and increased PKB activity. These results are consistent with a model in which SHIP serves as a regulator of both activity and subcellular localization of PKB.

A view through the clouds of imprinting

The purpose of this review is to examine whether our current knowledge of the higher order control of gene expression and nuclear organization can help us understand the mechanisms of genomic imprinting. Imprinting involves the inheritance of a silenced allele of a gene through either a paternal or maternal germline. We have approached the problem of imprinting using a model based on the dynamic attachment of chromatin loops to immobilized RNA polymerases and control elements. We have combined the information from different experimental approaches, examining primarily the IGF2-H19 locus, in an attempt to simplify the complexity of the imprinting data that has accumulated. It is hoped that a unified model may generate predictions amenable to experimental testing and contribute to the interpretation of future experiments.

Rottlerin-independent attenuation of pervanadate-induced tyrosine phosphorylation events by protein kinase C-delta in hemopoietic cells

The understanding and control of many pathophysiological conditions is based on knowledge of subtly regulated intracellular signaling networks. We have found that in pervanadate (PV)-treated J558L myeloma cells, amongst other signaling proteins, protein kinase C (PKC)-delta and src homology 2-containing inositol phosphatase (SHIP) are tyrosine phosphorylated on expression of the B cell receptor, suggesting a role for these proteins in the preformed B cell receptor transducer complex. Rottlerin, a widely used PKC-delta-specific inhibitor, efficiently blocks these PV-induced tyrosine phosphorylation events. Furthermore, PV treatment of bone marrow-derived mast cells (BMMC) also results in tyrosine phosphorylation of PKC-delta, SHIP, and additional proteins. Rottlerin also inhibits these responses, indicating that PKC-delta might play an important enhancing role in the propagation of phosphotyrosine signals in B cells and mast cells and hence in the regulation of function of both cell types. Therefore, BMMC from PKC-delta -/- mice were generated by in vitro differentiation and assayed for tyrosine phosphorylation events in response to PV. Intriguingly, and opposite to the Rottlerin data, PKC-delta -/- BMMC show a stronger response to PV than wild-type cells, suggesting an attenuating role for PKC-delta. This response can be inhibited equally well by Rottlerin, indicating clearly that Rottlerin is not specific for PKC-delta in vivo. A comparison between Rottlerin and the panspecific PKC inhibitor bisindolylmaleimide suggests that Rottlerin also targets kinases beyond the PKC family. Moreover, Ser473 phosphorylation of protein kinase B (PKB) after PV treatment is blocked by Rottlerin as efficiently as by the phosphatidylinositol 3-kinase inhibitor LY294002. In this report, we provide evidence that PKC-delta constitutes a crucial attenuating factor in B cell and mast cell signal transduction and suggest that PKC-delta is important for the regulation of physiological B and mast cell functions as well as for their pathophysiology. Furthermore, dominant PKC-delta-independent effects of Rottlerin are presented, indicating restrictions of this inhibitor for use in signal transduction research.

Vav1 regulates phospholipase cgamma activation and calcium responses in mast cells

The hematopoietic cell-specific protein Vav1 is a substrate of tyrosine kinases activated following engagement of many receptors, including FcepsilonRI. Vav1-deficient mice contain normal numbers of mast cells but respond more weakly than their normal counterparts to a passive systemic anaphylaxis challenge. Vav1-deficient bone marrow-derived mast cells also exhibited reduced degranulation and cytokine production, although tyrosine phosphorylation of FcepsilonRI, Syk, and LAT (linker for activation of T cells) was normal. In contrast, tyrosine phosphorylation of phospholipase Cgamma1 (PLCgamma1) and PLCgamma2 and calcium mobilization were markedly inhibited. Reconstitution of deficient mast cells with Vav1 restored normal tyrosine phosphorylation of PLCgamma1 and PLCgamma2 and calcium responses. Thus, Vav1 is essential to FcepsilonRI-mediated activation of PLCgamma and calcium mobilization in mast cells. In addition to its known role as an activator of Rac1 GTPases, these findings demonstrate a novel function for Vav1 as a regulator of PLCgamma-activated calcium signals.

SHIP's C-terminus is essential for its hydrolysis of PIP3 and inhibition of mast cell degranulation

The SH2-containing inositol-5'-phosphatase, SHIP, restrains bone marrow-derived mast cell (BMMC) degranulation, at least in part, by hydrolyzing phosphatidylinositol (PI)-3-kinase generated PI-3,4,5-P(3) (PIP3) to PI-3,4-P(2). To determine which domains within SHIP influence its ability to hydrolyze PIP3, bone marrow from SHIP(-/-) mice was retrovirally infected with various SHIP constructs. Introduction of wild-type SHIP into SHIP(-/-) BMMCs reverted the Steel factor (SF)-induced increases in PIP3, calcium entry, and degranulation to those observed in SHIP(+/+) BMMCs. A 5'-phosphatase dead SHIP, however, could not revert the SHIP(-/-) response, whereas a SHIP mutant in which the 2 NPXY motifs were converted to NPXFs (2NPXF) could partially revert the SHIP(-/-) response. SF stimulation of BMMCs expressing the 2NPXF, which could not bind Shc, led to the same level of mitogen-activated protein kinase (MAPK) phosphorylation as that seen in BMMCs expressing the other constructs. Surprisingly, C-terminally truncated forms of SHIP, lacking different amounts of the proline rich C-terminus, could not revert the SHIP(-/-) response at all. These results suggest that the C-terminus plays a critical role in enabling SHIP to hydrolyze PIP(3) and inhibit BMMC degranulation.