Activation of betagamma subunits of G(i2) and G(i3) proteins by basic secretagogues induces exocytosis through phospholipase Cbeta and arachidonate release through phospholipase Cgamma in mast cells

P17 induces chemotaxis and differentiation of monocytes via MRGPRX2-mediated mast cell-line activation

BACKGROUND

P17, a peptide isolated from Tetramorium bicarinatum ant venom, is known to induce an alternative phenotype of human monocyte-derived macrophages via activation of an unknown G protein-coupled receptor (GPCR).

OBJECTIVE

We sought to investigate the mechanism of action and the immunomodulatory effects of P17 mediated through MRGPRX2 (Mas-related G protein-coupled receptor X2).

METHODS

To identify the GPCR for P17, we screened 314 GPCRs. Upon identification of MRGPRX2, a battery of in silico, in vitro, ex vivo, and in vivo assays along with the receptor mutation studies were performed. In particular, to investigate the immunomodulatory actions, we used β-hexosaminidase release assay, cytokine releases, quantification of mRNA expression, cell migration and differentiation assays, immunohistochemical labeling, hematoxylin and eosin, and immunofluorescence staining.

RESULTS

P17 activated MRGPRX2 in a dose-dependent manner in β-arrestin recruitment assay. In LAD2 cells, P17 induced calcium and β-hexosaminidase release. Quercetin- and short hairpin RNA-mediated knockdown of MRGPRX2 reduced P17-evoked β-hexosaminidase release. In silico and in vitro mutagenesis studies showed that residue Lys8 of P17 formed a cation-π interaction with the Phe172 of MRGPRX2 and [Ala8]P17 lost its activity partially. P17 activated LAD2 cells to recruit THP-1 and human monocytes in Transwell migration assay, whereas MRGPRX2-impaired LAD2 cells cannot. In addition, P17-treated LAD2 cells stimulated differentiation of THP-1 and human monocytes, as indicated by the enhanced expression of macrophage markers cluster of differentiation 11b and TNF-α by quantitative RT-PCR. Immunohistochemical and immunofluorescent staining suggested monocyte recruitment in mice ears injected with P17.

CONCLUSIONS

Our data provide novel structural information regarding the interaction of P17 with MRGPRX2 and intracellular pathways for its immunomodulatory action.

Therapeutic Potential of MRGPRX2 Inhibitors on Mast Cells

Mast cells (MCs) act as primary effectors in inflammatory and allergic reactions by releasing intracellularly-stored inflammatory mediators in diseases. The two major pathways for MC activation are known to be immunoglobulin E (IgE)-dependent and -independent. Although IgE-dependent signaling is the main pathway to MC activation, IgE-independent pathways have also been found to serve pivotal roles in the pathophysiology of various inflammatory conditions. Recent studies have shown that human and mouse MCs express several regulatory receptors such as toll-like receptors (TLRs), CD48, C300a, and GPCRs, including mas-related GPCR-X2 (MRGPRX2). MRGPRX2 has been reported as a novel GPCR that is expressed in MCs activated by basic secretagogues, neurokinin peptides, host defense antimicrobial peptides, and small molecule compounds (e.g., neuromuscular blocking agents) and leads to MC degranulation and eicosanoids release under in vitro experimental condition. Functional analyses of MRGPRX2 and Mrgprb2 (mouse ortholog) indicate that MRGPRX2 is involved in MC hypersensitivity reactions causing neuroinflammation such as postoperative pain, type 2 inflammation, non-histaminergic itch, and drug-induced anaphylactic-like reactions. In this review, we discuss the roles in innate immunity through functional studies on MRGPRX2-mediated IgE-independent MC activation and also the therapeutic potential of MRGPRX2 inhibitors on allergic and inflammatory diseases.

Unlocking the Non-IgE-Mediated Pseudo-Allergic Reaction Puzzle with Mas-Related G-Protein Coupled Receptor Member X2 (MRGPRX2)

Mas-related G-protein coupled receptor member X2 (MRGPRX2) is a class A GPCR expressed on mast cells. Mast cells are granulated tissue-resident cells known for host cell response, allergic response, and vascular homeostasis. Immunoglobulin E receptor (FcεRI)-mediated mast cell activation is a well-studied and recognized mechanism of allergy and hypersensitivity reactions. However, non-IgE-mediated mast cell activation is less explored and is not well recognized. After decades of uncertainty, MRGPRX2 was discovered as the receptor responsible for non-IgE-mediated mast cells activation. The puzzle of non-IgE-mediated pseudo-allergic reaction is unlocked by MRGPRX2, evidenced by a plethora of reported endogenous and exogenous MRGPRX2 agonists. MRGPRX2 is exclusively expressed on mast cells and exhibits varying affinity for many molecules such as antimicrobial host defense peptides, neuropeptides, and even US Food and Drug Administration-approved drugs. The discovery of MRGPRX2 has changed our understanding of mast cell biology and filled the missing link of the underlying mechanism of drug-induced MC degranulation and pseudo-allergic reactions. These non-canonical characteristics render MRGPRX2 an intriguing player in allergic diseases. In the present article, we reviewed the emerging role of MRGPRX2 as a non-IgE-mediated mechanism of mast cell activation in pseudo-allergic reactions. We have presented an overview of mast cells, their receptors, structural insight into MRGPRX2, MRGPRX2 agonists and antagonists, the crucial role of MRGPRX2 in pseudo-allergic reactions, current challenges, and the future research direction.

Mas-related G protein-coupled receptor X2 and its activators in dermatologic allergies

The Mas-related G protein-coupled receptor X2 (MRGPRX2) is a multiligand receptor responding to various exogenous and endogenous stimuli. Being highly expressed on skin mast cells, MRGPRX2 triggers their degranulation and release of proinflammatory mediators, and it promotes multicellular signaling cascades, such as itch induction and transmission in sensory neurons. The expression of MRGPRX2 by skin mast cells and the levels of the MRGPRX2 agonists (eg, substance P, major basic protein, eosinophil peroxidase) are upregulated in the serum and/or skin of patients with inflammatory and pruritic skin diseases, such as chronic spontaneous urticaria or atopic dermatitis. Therefore, MRGPRX2 and its agonists might be potential biomarkers for the progression of cutaneous inflammatory diseases and the response to treatment. In addition, they may represent promising targets for prevention and treatment of signs and symptoms in patients with skin diseases or drug reactions. To assess this possibility, this review explores the role and relevance of MRGPRX2 and its activators in cutaneous inflammatory disorders and chronic pruritus.

Extracellular annexin-A1 promotes myeloid/granulocytic differentiation of hematopoietic stem/progenitor cells via the Ca2+/MAPK signalling transduction pathway

Annexin A1 (AnxA1) modulates neutrophil life span and bone marrow/blood cell trafficking thorough activation of formyl-peptide receptors (FPRs). Here, we investigated the effect of exogenous AnxA1 on haematopoiesis in the mouse. Treatment of C57BL/6 mice with recombinant AnxA1 (rAnxA1) reduced the granulocyte–macrophage progenitor (GMP) population in the bone marrow, enhanced the number of mature granulocytes Gr-1⁺Mac-1⁺ in the bone marrow as well as peripheral granulocytic neutrophils and increased expression of mitotic cyclin B1 on hematopoietic stem cells (HSCs)/progenitor cells (Lin⁻Sca-1⁺c-Kit⁺: LSK). These effects were abolished by simultaneous treatment with Boc-2, an FPR pan-antagonist. In in vitro studies, rAnxA1 reduced both HSC (LSKCD90^lowFLK-2⁻) and GMP populations while enhancing mature cells (Gr1⁺Mac1⁺). Moreover, rAnxA1 induced LSK cell proliferation (Ki67⁺), increasing the percentage of cells in the S/G2/M cell cycle phases and reducing Notch-1 expression. Simultaneous treatment with WRW4, a selective FPR2 antagonist, reversed the in vitro effects elicited by rAnxA1. Treatment of LSK cells with rAnxA1 led to phosphorylation of PCLγ2, PKC, RAS, MEK, and ERK1/2 with increased expression of NFAT2. In long-term bone marrow cultures, rAnxA1 did not alter the percentage of LSK cells but enhanced the Gr-1⁺Mac-1⁺ population; treatment with a PLC (U73122), but not with a PKC (GF109203), inhibitor reduced rAnxA1-induced phosphorylation of ERK1/2 and Elk1. Therefore, we identify here rAnxA1 as an inducer of HSC/progenitor cell differentiation, favouring differentiation of the myeloid/granulocytic lineage, via Ca²⁺/MAPK signalling transduction pathways.

Current Advances in Immunological Studies on the Vespidae Venom Antigen 5: Therapeutic and Prophylaxis to Hypersensitivity Responses

Although systemic reactions caused by allergenic proteins present in venoms affect a small part of the world population, Hymenoptera stings are among the main causes of immediate hypersensitivity responses, with risk of anaphylactic shock. In the attempt to obtain therapeutic treatments and prophylaxis to hypersensitivity responses, interest in the molecular characterization of these allergens has grown in the scientific community due to the promising results obtained in immunological and clinical studies. The present review provides an update on the knowledge regarding the immune response and the therapeutic potential of Antigen 5 derived from Hymenoptera venom. The results confirm that the identification and topology of epitopes, associated with molecular regions that interact with antibodies, are crucial to the improvement of hypersensitivity diagnostic methods.

Differential Ca2+ mobilization and mast cell degranulation by FcεRI- and GPCR-mediated signaling

Mast cells play a primary role in allergic diseases. During an allergic reaction, mast cell activation is initiated by cross-linking IgE-FcεRI complex by multivalent antigen resulting in degranulation. Additionally, G protein-coupled receptors also induce degranulation upon activation. However, the spatio-temporal relationship between Ca²⁺ mobilization and mast cell degranulation is not well understood. We investigated the relationship between oscillations in Ca²⁺ level and mast cell degranulation upon stimulation in rat RBL-2H3 cells. Nile red and Fluo-4 were used as probes for monitoring histamine and intracellular Ca²⁺ levels, respectively. Histamine release and Ca²⁺ oscillations in real-time were monitored using total internal reflection fluorescence microscopy (TIRFM). Mast cell degranulation followed immediately after FcεRI and GPCR-mediated Ca²⁺ increase. FcεRI-induced Ca²⁺ increase was higher and more sustained than that induced by GPCRs. However, no significant difference in mast cell degranulation rates was observed. Although intracellular Ca²⁺ release was both necessary and sufficient for mast cell degranulation, extracellular Ca²⁺ influx enhanced the process. Furthermore, cytosolic Ca²⁺ levels and mast cell degranulation were significantly decreased by downregulation of store-operated Ca²⁺ entry (SOCE) via Orai1 knockdown, 2-aminoethyl diphenylborinate (2-APB) or tubastatin A (TSA) treatment. Collectively, this study has demonstrated the role of Ca²⁺ signaling in regulating histamine degranulation.

Emerging Roles for MAS-Related G Protein-Coupled Receptor-X2 in Host Defense Peptide, Opioid, and Neuropeptide-Mediated Inflammatory Reactions

Mast cells (MCs) are tissue-resident immune cells that contribute to host defense but are best known for their roles in allergic and inflammatory diseases. In humans, MCs are divided into two subtypes based on the protease content of their secretory granules. Thus, human lung MCs contain only tryptase and are known as MC_T, whereas skin MCs contain both tryptase and chymase and are known as MC_TC. Patients with severe asthma display elevated MCs in the lung, which undergo phenotypic change from MC_T to MC_TC. Although the human genome contains four Mas related G protein coupled receptor X (MRGPRX) genes, an important feature of MC_TC is that they selectively express MRGPRX2. It is activated by antimicrobial host defense peptides such as human β-defensins and the cathelicidin LL-37 and likely contributes to host defense. MRGPRX2 is also a receptor for the neuropeptide substance P, major basic protein, eosinophil peroxidase, opioids, and many FDA-approved cationic drugs. Increased expression of MRGPRX2 or enhanced downstream signaling likely contributes to chronic inflammatory diseases such as rosacea, atopic dermatitis, chronic urticaria, and severe asthma. In this chapter, I will discuss the expression profile and function of MRGPRX1-4 and review the emerging roles of MRGPRX2 on host defense, chronic inflammatory diseases, and drug-induced pseudoallergic reactions. I will also examine the novel aspects of MRGPRX2 signaling in MCs as it related to degranulation and review the mechanisms of its regulation.

Tissue Factor Prothrombotic Activity Is Regulated by Integrin-arf6 Trafficking

OBJECTIVE

Coagulation initiation by tissue factor (TF) is regulated by cellular inhibitors, cell surface availability of procoagulant phosphatidylserine, and thiol-disulfide exchange. How these mechanisms contribute to keeping TF in a noncoagulant state and to generating prothrombotic TF remain incompletely understood.

APPROACH AND RESULTS

Here, we study the activation of TF in primary macrophages by a combination of pharmacological, genetic, and biochemical approaches. We demonstrate that primed macrophages effectively control TF cell surface activity by receptor internalization. After cell injury, ATP signals through the purinergic receptor P2rx7 induce release of TF⁺ microvesicles. TF cell surface availability for release onto microvesicles is regulated by the GTPase arf6 associated with integrin α4β1. Furthermore, microvesicles proteome analysis identifies activation of Gα_i2 as a participating factor in the release of microvesicles with prothrombotic activity in flowing blood. ATP not only prevents TF and phosphatidylserine internalization but also induces TF conversion to a conformation with high affinity for its ligand, coagulation factor VII. Although inhibition of dynamin-dependent internalization also exposes outer membrane procoagulant phosphatidylserine, the resulting TF⁺ microvesicles distinctly lack protein disulfide isomerase and high affinity TF and fail to produce fibrin strands typical for microvesicles generated by thrombo-inflammatory P2rx7 activation.

CONCLUSIONS

These data show that procoagulant phospholipid exposure is not sufficient and that TF affinity maturation is required to generate prothrombotic microvesicles from a variety of cell types. These findings are significant for understanding TF-initiated thrombosis and should be considered in designing functional microvesicles-based diagnostic approaches.

An aromatic amino acid within intracellular loop 2 of the prostaglandin EP2 receptor is a prerequisite for selective association and activation of Gαs

We previously demonstrated that the aromatic moiety of Tyr¹⁴³ within the intracellular loop 2 (ICL2) region of the prostaglandin EP2 receptor plays a crucial role in Gs coupling. Here we investigated whether the ICL2 of the EP2 receptor directly binds to Gαs and whether an aromatic moiety affects this interaction. In Chinese hamster ovary cells, mutations of Tyr¹⁴³ reduced the ability of the EP2 receptor to interact with G proteins as demonstrated by GTPγS sensitivity, as well as the ability of agonist-induced cAMP formation, with the rank order of Phe>Tyr (wild-type)=Trp>Leu>Ala (=0). We found that the wild-type ICL2 peptide (i2Y) and its mutant with Phe at Tyr¹⁴³ (i2F) inhibited receptor-G protein complex formation of wild-type EP2 in membranes, whereas the Ala-substituted mutant (i2A) did not. Specific interactions between these peptides and the Gαs protein were detected by surface plasmon resonance, but Gαs showed different association rates, with a rank order of i2F>i2Y≫i2A, with similar dissociation rates. Moreover, i2F and i2Y, but not i2A activated membrane adenylyl cyclase. These results indicate that the ICL2 region of the EP2 receptor is its potential interaction site with Gαs, and that the aromatic side chain moiety at position 143 is a determinant for the accessibility of the ICL2 to the Gαs protein.

Roles of Mas-related G protein-coupled receptor X2 on mast cell-mediated host defense, pseudoallergic drug reactions, and chronic inflammatory diseases

Mast cells (MCs), which are granulated tissue-resident cells of hematopoietic lineage, contribute to vascular homeostasis, innate/adaptive immunity, and wound healing. However, MCs are best known for their roles in allergic and inflammatory diseases, such as anaphylaxis, food allergy, rhinitis, itch, urticaria, atopic dermatitis, and asthma. In addition to the high-affinity IgE receptor (FcεRI), MCs express numerous G protein-coupled receptors (GPCRs), which are the largest group of membrane receptor proteins and the most common targets of drug therapy. Antimicrobial host defense peptides, neuropeptides, major basic protein, eosinophil peroxidase, and many US Food and Drug Administration-approved peptidergic drugs activate human MCs through a novel GPCR known as Mas-related G protein-coupled receptor X2 (MRGPRX2; formerly known as MrgX2). Unique features of MRGPRX2 that distinguish it from other GPCRs include their presence both on the plasma membrane and intracellular sites and their selective expression in MCs. In this article we review the possible roles of MRGPRX2 on host defense, drug-induced anaphylactoid reactions, neurogenic inflammation, pain, itch, and chronic inflammatory diseases, such as urticaria and asthma. We propose that host defense peptides that kill microbes directly and activate MCs through MRGPRX2 could serve as novel GPCR targets to modulate host defense against microbial infection. Furthermore, mAbs or small-molecule inhibitors of MRGPRX2 could be developed for the treatment of MC-dependent allergic and inflammatory disorders.

Tachykinin peptide, substance P, and its receptor NK-1R play an important role in alimentary tract mucosal inflammation during cytotoxic therapy

The alimentary tract mucosal inflammation has been a topic of concern in oncology; though many modalities of treatment have been proposed for mucosal inflammation, the contributing adverse effects have severely affected the quality of life of patients. This review focuses on the importance of neurogenic peptide, Substance P and its receptor NK-1R in modulating the cascades of events in mucosal inflammation during cytotoxic therapy. There are various preclinical and clinical models showing increased expression of Substance P/NK-1R in ionizing radiation and chemotherapy, but only very few preclinical studies to our knowledge have highlighted or examined its role in mucosal inflammation. Hence, the importance of neuropeptide involved in the inflammatory events in mucosal inflammation in cytotoxic therapy could be a major breakthrough for future research purposes and treatment. The factors contributing to the severity of tissue reactions have been multietiogenic; thus, resultant treatment also has to be directed toward multiple contributing factors. This review also focuses on the significance of care strategy to be adopted in alimentary tract mucositis when multietiogenic factors are taken into consideration.

Phorbol ester stimulates ethanolamine release from the metastatic basal prostate cancer cell line PC3 but not from prostate epithelial cell lines LNCaP and P4E6

Background:

Malignancy alters cellular complex lipid metabolism and membrane lipid composition and turnover. Here, we investigated whether tumorigenesis in cancer-derived prostate epithelial cell lines influences protein kinase C-linked turnover of ethanolamine phosphoglycerides (EtnPGs) and alters the pattern of ethanolamine (Etn) metabolites released to the medium.

Methods:

Prostate epithelial cell lines P4E6, LNCaP and PC3 were models of prostate cancer (PCa). PNT2C2 and PNT1A were models of benign prostate epithelia. Cellular EtnPGs were labelled with [1-³H]-Etn hydrochloride. PKC was activated with phorbol ester (TPA) and inhibited with Ro31-8220 and GF109203X. D609 was used to inhibit PLD (phospholipase D). [³H]-labelled Etn metabolites were resolved by ion-exchange chromatography. Sodium oleate and mastoparan were tested as activators of PLD2. Phospholipase D activity was measured by a transphosphatidylation reaction. Cells were treated with ionomycin to raise intracellular Ca²⁺ levels.

Results:

Unstimulated cell lines release mainly Etn and glycerylphosphorylEtn (GPEtn) to the medium. Phorbol ester treatment over 3h increased Etn metabolite release from the metastatic PC3 cell line and the benign cell lines PNT2C2 and PNT1A but not from the tumour-derived cell lines P4E6 and LNCaP; this effect was blocked by Ro31-8220 and GF109203X as well as by D609, which inhibited PLD in a transphosphatidylation reaction. Only metastatic PC3 cells specifically upregulated Etn release in response to TPA treatment. Oleate and mastoparan increased GPEtn release from all cell lines at the expense of Etn. Ionomycin stimulated GPEtn release from benign PNT2C2 cells but not from cancer-derived cell lines P4E6 or PC3. Ethanolamine did not stimulate the proliferation of LNCaP or PC3 cell lines but decreased the uptake of choline (Cho).

Conclusions:

Only the metastatic basal PC3 cell line specifically increased the release of Etn on TPA treatment most probably by PKC activation of PLD1 and increased turnover of EtnPGs. The phosphatidic acid formed will maintain a cancer phenotype through the regulation of mTOR. Ethanolamine released from cells may reduce Cho uptake, regulating the membrane PtdEtn:PtdCho ratio and influencing the action of PtdEtn-binding proteins such as RKIP and the anti-apoptotic hPEBP4. The work highlights a difference between LNCaP cells used as a model of androgen-dependent early stage PCa and androgen-independent PC3 cells used to model later refractory stage disease.

Angiotensin II stimulates fibronectin protein synthesis via a Gβγ/arachidonic acid-dependent pathway

In rabbit proximal tubular cells, ANG II type 2-receptor (AT2)-induced arachidonic acid release is PLA2 coupled and dependent of G protein βγ (Gβγ) subunits. Moreover, ANG II activates ERK1/2 and transactivates EGFR via a c-Src-dependent mechanism. Arachidonic acid has been shown to mimic this effect, at least in part, by an undetermined mechanism. In this study, we determined the effects of ANG II on fibronectin expression in cultured rabbit proximal tubule cells and elucidated the signaling pathways associated with such expression. We found that ANG II and transfection of Gβγ subunits directly increased fibronectin protein expression, and this increase was inhibited by overexpression of β-adrenergic receptor kinase (βARK)-ct or DN-Src. Moreover, ANG II-induced fibronectin protein expression was significantly abrogated by the AT2 receptor antagonist PD123319. In addition, inhibition of cystolic PLA2 diminished ANG II-induced fibronectin expression. Endogenous arachidonic acid mimicked ANG II-induced fibronectin expression. We also found that overexpression of Gβγ subunits induced c-Src, ERK1/2, and EGFR tyrosine phosphorylation, which can be inhibited by overexpression of βARK-ct or DN-Src. Gβγ also induced c-Src SH2 domain association with the EGFR. Supporting these findings, in rabbit proximal tubular epithelium, immunoblot analysis indicated that βγ expression was significant. Interestingly, arachidonic acid- and eicosatetraenoic acid-induced responses were preserved in the presence of βARK-ct. This is the first report demonstrating the regulation of EGFR, ERK1/2, c-Src, and fibronectin by Gβγ subunits in renal epithelial cells. Moreover, this work demonstrates a role for Gβγ heterotrimeric proteins in ANG II, but not arachidonic acid, signaling in renal epithelial cells.

Role of meningeal mast cells in intrathecal morphine-evoked granuloma formation

BACKGROUND

Intrathecal morphine forms granulomas that arise from the adjacent arachnoid membrane. The authors propose that these inflammatory cells exit the meningeal vasculature secondary to meningeal mast cell degranulation.

METHODS

Three sets of experiments were accomplished in dogs: (1) ex vivo meningeal mast cell degranulation (histamine release was measured ex vivo from canine dura incubated with opiates); (2) in vivo cutaneous mast cell degranulation (flare areas on the dog abdomen were measured after subcutaneous opiates); and (3) in vivo granuloma pharmacology. Dogs with lumbar intrathecal catheters received infusion of intrathecal saline or intrathecal morphine. Intrathecal morphine dogs received (1) no other treatment (control); (2) twice-daily subcutaneous naltrexone; (3) intrathecal co-infusion of cromolyn; or (4) twice-daily subcutaneous cromolyn for the 24- to 28-day study course.

RESULTS

Morphine but not fentanyl evoked dural histamine release, which was blocked by cromolyn but not naloxone. Wheal/flare was produced by subcutaneous morphine, methadone, hydromorphone, but not fentanyl, and was unaffected by naltrexone but prevented by cromolyn. Granulomas occurred in all dogs receiving intrathecal morphine (15 of 15); subcutaneous naltrexone had no effect on granulomas (six of six) but was reduced by concurrent intrathecal cromolyn (zero of five) or twice-daily subcutaneous cromolyn (one of five).

CONCLUSIONS

The pharmacology of cutaneous/dural mast cell degranulation and intrathecal granulomas are comparable, not mediated by opioid receptors, and reduced by agents preventing mast cell degranulation. If an agent produces cutaneous mast cell degranulation at concentrations produced by intrathecal delivery, the agent may initiate granulomas.

G Protein βγ-subunit signaling mediates airway hyperresponsiveness and inflammation in allergic asthma

Since the Gβγ subunit of Gi protein has been importantly implicated in regulating immune and inflammatory responses, this study investigated the potential role and mechanism of action of Gβγ signaling in regulating the induction of airway hyperresponsiveness (AHR) in a rabbit model of allergic asthma. Relative to non-sensitized animals, OVA-sensitized rabbits challenged with inhaled OVA exhibited AHR, lung inflammation, elevated BAL levels of IL-13, and increased airway phosphodiesterase-4 (PDE4) activity. These proasthmatic responses were suppressed by pretreatment with an inhaled membrane-permeable anti-Gβγ blocking peptide, similar to the suppressive effect of glucocorticoid pretreatment. Extended mechanistic studies demonstrated that: 1) corresponding proasthmatic changes in contractility exhibited in isolated airway smooth muscle (ASM) sensitized with serum from OVA-sensitized+challenged rabbits or IL-13 were also Gβγ-dependent and mediated by MAPK-upregulated PDE4 activity; and 2) the latter was attributed to Gβγ-induced direct stimulation of the non-receptor tyrosine kinase, c-Src, resulting in downstream activation of ERK1/2 and its consequent transcriptional upregulation of PDE4. Collectively, these data are the first to identify that a mechanism involving Gβγ-induced direct activation of c-Src, leading to ERK1/2-mediated upregulation of PDE4 activity, plays a decisive role in regulating the induction of AHR and inflammation in a rabbit model of allergic airway disease.

Cdk5 mediates vimentin Ser56 phosphorylation during GTP-induced secretion by neutrophils

Secretion by neutrophils contributes to acute inflammation following injury or infection. Vimentin has been shown to be important for secretion by neutrophils but little is known about its dynamics during secretion, which is regulated by cyclin-dependent kinase 5 (Cdk5). In this study, we sought to examine the vimentin dynamics and its potential regulation by Cdk5 during neutrophil secretion. We show that vimentin is a Cdk5 substrate that is specifically phosphorylated at Ser56. In response to neutrophil stimulation with GTP, vimentin Ser56 was phosphorylated and colocalized with Cdk5 in the cytoplasmic compartment. Vimentin pSer56 and Cdk5 colocalization was consistent with coimmunoprecipitation from stimulated cells. Vimentin Ser56 phosphorylation occurred immediately after stimulation, and a remarkable increase in phosphorylation was noted later in the secretory process. Decreased GTP-induced vimentin Ser56 phosphorylation and secretion resulted from inhibition of Cdk5 activity by roscovitine or olomoucine or by depletion of Cdk5 by siRNA, suggesting that GTP-induced Cdk5-mediated vimentin Ser56 phosphorylation may be related to GTP-induced Cdk5-mediated secretion by neutrophils. Indeed, inhibition of vimentin Ser56 phosphorylation led to a corresponding inhibition of GTP-induced secretion, indicating a link between these two events. While fMLP also induced vimentin Ser56 phosphorylation, such phosphorylation was unaffected by roscovitine, which nonetheless, inhibited secretion, suggesting that Cdk5 regulates fMLP-induced secretion via a mechanism independent of Cdk5-mediated vimentin Ser56 phosphorylation. These findings demonstrate the distinct involvement of Cdk5 in GTP- and fMLP-induced secretion by neutrophils, and support the notion that specific targeting of Cdk5 may serve to inhibit the neutrophil secretory process.

Inhibitory effect of ganglioside on mastoparan-induced cytotoxicity and degranulation in lipid raft of connective tissue type mast cell

Antihistamine, the most important drug for Hymenoptera stinging, cannot attenuate cytotoxicity and mast cell direct activation by mastoparan that is the most abundant polypeptides in the venoms of social wasps. The aim of this study was to investigate whether gangliosides inhibit the effect of mastoparan on mast cells activation. The degranulation and cytotoxicity in canine cutaneous mastocytoma cells (CM-MC) were done by measurement of β-hexosaminidase release and MTT assay. Lipid raft was isolated with discontinuous sucrose gradient centrifuge for the analysis of distribution of Gα(q) and Gα(i) protein by western blotting. We found that mastoparan induced the degranulation in (CM-MC) via direct activation of Gα(i) and Gα(q) with a decrease in their amount in lipid raft. Ganglioside G(D1a) (disialoganglioside) and G(M1) (monosialoganglioside) strongly reduced the degranulation and cytotoxicity through stabilizing the structure of lipid raft domain. In addition, mastoparan generated intracellular reactive oxygen species (ROS) independently from cytotoxicity, through arachidonic cascade but not G-protein activations. Crude wasp venom showed cytotoxicity and induction of the release from CM-MC, which were potently reduced by gangliosides. We show here that mastoparan activates both Gα(i) and Gα(q) protein and that the exogenous ganglioside G(D1a) and G(M1) inhibit the degranulation and cytotoxicity through stabilizing lipid raft. Gangliosides have potentials to be therapeutic tool or clinical prophylaxis for wasp stinging.

Ghrelin and obestatin promote the allergic action in rat peritoneal mast cells as basic secretagogues

Ghrelin is an endogenous ligand of the type 1a growth hormone secretagogue receptor (GHSR1a) that regulates energy balance. Ghrelin and obestatin, derived from the post-translational processing of preproghrelin, are involved in a diverse range of biological activities, yet their effect on the immune system is not fully understood. In the present study, we investigated the roles of ghrelin and obestatin on mast cell degranulation and found that both ghrelin and obestatin induce the release of histamine from rat peritoneal mast cells. This induced histamine release was inhibited by the pertussis toxin, an inhibitor of Gα(i) protein, and extracellular Ca(2+). Rat peritoneal mast cells and rat basophilic leukemia (RBL-2H3) cells did not express the ghrelin receptor GHSR1a, suggesting that histamine release induced by ghrelin occurs via a receptor-independent mechanism. We report here that ghrelin and obestatin, belonging to the family of basic secretagogues, stimulate mast cells independent of a receptor, and this may play a crucial role at the site of allergy or inflammation.

Advanced glycation end products (AGEs) activate mast cells

BACKGROUND AND PURPOSE

Advanced glycation endproducts (AGEs) represent one of the many types of chemical modifications that occur with age in long-lived proteins. AGEs also accumulate in pathologies such as diabetes, cardiovascular diseases, neurodegeneration and cancer. Mast cells are major effectors of acute inflammatory responses that also contribute to the progression of chronic diseases. Here we investigated interactions between AGEs and mast cells.

EXPERIMENTAL APPROACHES

Histamine secretion from AGEs-stimulated mast cells was measured. Involvement of a receptor for AGEs, RAGE, was assessed by PCR, immunostaining and use of inhibitors of RAGE. Production of reactive oxygen species (ROS) and cytokines was measured.

KEY RESULTS

Advanced glycation endproducts dose-dependently induced mast cell exocytosis with maximal effects being obtained within 20 s. RAGE mRNA was detected and intact cells were immunostained by a specific anti-RAGE monoclonal antibody. AGEs-induced exocytosis was inhibited by an anti-RAGE antibody and by low molecular weight heparin, a known RAGE antagonist. RAGE expression levels were unaltered after 3 h treatment with AGEs. AGE-RAGE signalling in mast cells involves Pertussis toxin-sensitive G(i)-proteins and intracellular Ca(2+) increases as pretreatment with Pertussis toxin, caffeine, 2-APB and BAPTA-AM inhibited AGE-induced exocytosis. AGEs also rapidly stimulated ROS production. After 6 h treatment with AGEs, the pattern of cytokine secretion was unaltered compared with controls.

CONCLUSIONS AND IMPLICATIONS

Advanced glycation endproducts activated mast cells and may contribute to a vicious cycle involving generation of ROS, increased formation of AGEs, activation of RAGE and to the increased low-grade inflammation typical of chronic diseases.

Mast cells, peptides and cardioprotection - an unlikely marriage?

1 Mast cells have classically been regarded as the 'bad guys' in the setting of acute myocardial ischaemia, where their released contents are believed to contribute both to tissue injury and electrical disturbances resulting from ischaemia. Recent evidence suggests, however, that if mast cell degranulation occurs in advance of ischaemia onset, this may be cardioprotective by virtue of the depletion of mast cell contents that can no longer act as instruments of injury when the tissue becomes ischaemic. 2 Many peptides, such as ET-1, adrenomedullin, relaxin and atrial natriuretic peptide, have been demonstrated to be cardioprotective when given prior to the onset of myocardial ischaemia, although their physiological functions are varied and the mechanisms of their cardioprotective actions appear to be diverse and often ill defined. However, one common denominator that is emerging is the ability of these peptides to modulate mast cell degranulation, raising the possibility that peptide-induced mast cell degranulation or stabilization may hold the key to a common mechanism of their cardioprotection. 3 The aim of this review was to consolidate the evidence implying that mast cell degranulation could play both a detrimental and protective role in myocardial ischaemia, depending upon when it occurs, and that this may underlie the cardioprotective effects of a range of diverse peptides that exerts physiological effects within the cardiovascular system.

Amyloid beta peptides trigger CD47-dependent mast cell secretory and phagocytic responses

Mast cells are found in the brain, where they contribute to immune responses. They have been implicated in multiple sclerosis, but their potential role in Alzheimers disease (AD), another inflammatory disease of the central nervous system, remains elusive. In the present study, we examined mast cell responses to amyloid beta (Abeta) peptides 1-40 and 1-42, the major components of the Alzheimer amyloid plaques. Rat peritoneal mast cells were used as experimental model for human brain serosal mast cells. Fibrillar Abeta1-40 and Ami1-42 peptides induced concentration-dependent exocytosis, as assessed by measurement of histamine secretion; exocytosis was reduced by pre-treatment with pertussis toxin and with antibodies against the CD47 receptor and the beta1-integrin subunit. Fibrillar Abeta1-40 and Abeta1- 42 peptides coated on heat-inactivated yeast particles and soluble fibrillar Abeta1-40 and Abeta1-42 peptides were also recognized and phagocyted by mast cells. Uptake of the peptides was decreased in the presence of 4N1, a peptide agonist of the CD47 receptor, but remained unchanged in the presence of 4NGG, a peptide derived from 4N1 which does not bind to CD47. Non-fibrillar forms of Abeta1-40 and 1-42 peptides were unable to elicit mast cell responses. These results show that fibrillar Abeta peptides can trigger mast cells and elicit exocytosis and phagocytosis. The Abeta-induced activation of mast cells operates through a CD47/beta1-integrin membrane complex coupled with Gi-protein. The present data support the hypothesis that mast cells, similarly to microglial cells, could play a major role in AD pathogenesis.

SRC-dependent signalling regulates actin ruffle formation induced by glycerophosphoinositol 4-phosphate

The glycerophosphoinositols are diffusible phosphoinositide metabolites reported to modulate actin dynamics and tumour cell spreading. In particular, the membrane permeant glycerophosphoinositol 4-phosphate (GroPIns4P) has been shown to act at the level of the small GTPase Rac1, to induce the rapid formation of membrane ruffles. Here, we have investigated the signalling cascade involved in this process, and show that it is initiated by the activation of Src kinase. In NIH3T3 cells, exogenous addition of GroPIns4P induces activation and translocation of Rac1 and its exchange factor TIAM1 to the plasma membrane; in addition, in in-vitro assays, GroPIns4P favours the formation of a protein complex that includes Rac1 and TIAM1. Neither of these processes involves direct actions of GroPIns4P on these proteins. Thus, through the use of specific inhibitors of tyrosine kinases and phospholipase C (and by direct evaluation of kinase activities and inositol 1,4,5-trisphosphate production), we show that GroPIns4P activates Src, and as a consequence, phospholipase Cgamma and Ca(2+)/calmodulin kinase II, the last of which directly phosphorylates TIAM1 and leads to TIAM1/Rac1-dependent ruffle formation.

Roles of PKA, PI3K, and cPLA2 in the NO-mediated negative inotropic effect of β2-adrenoceptor agonists in guinea pig right papillary muscles

Although β2-adrenoceptors represent 15–25% of β-adrenoceptors in the guinea pig heart, their functionality is controversial. We assessed the inotropic effects of β2-adrenoceptor partial agonists in right papillary muscles. Salbutamol induced a small but significant concentration-dependent negative inotropic effect (NIE, −5% at 60 nM) followed by a moderate positive inotropic effect (+36% at 6 μM) due to activation of β1-adrenoceptors. In the presence of 4 μM atenolol, the concentration-dependent NIE (−12% at 6 μM) was biphasic, best described by a double logistic equation with respective EC50 values of 3 and ∼420 nM, and was insensitive to SR59230A. In muscles from pertussis toxin-treated guinea pigs, the salbutamol-induced positive inotropic effect was sensitive to low concentrations of ICI-118551 in an unusual manner. Experiments in reserpinized animals revealed the importance of the phosphorylation-dephosphorylation processes. PKA inhibition reduced and suppressed the effects obtained at low and high concentrations, respectively, indicating that its activation was a prerequisite to the NIE. The effect occurring at nanomolar concentrations depended upon PKA/phosphatidylinositol 3-kinase/cytosolic phospholipase A2 (cPLA2) activations leading to nitric oxide (NO) release via the arachidonic acid/cyclooxygenase pathway. NO release via PKA-dependent phosphorylation of the receptor was responsible for the inotropic effect observed at submicromolar concentrations, which is negatively controlled by cPLA2. The possibility that these effects are due to an equilibrium between different affinity states of the receptor (Gs/Gi coupled and Gi independent with different signaling pathways) that can be displaced by ICI-118551 is discussed. We conclude that β2-adrenoceptors are functional in guinea pig heart and can modulate the inotropic state.

Inhibition of Basic Secretagogue-Induced Signaling in Mast Cells by Cell Permeable Gαi-Derived Peptides

BACKGROUND

Basic secretagogues of connective tissue mast cells act as receptor mimetic agents that trigger mast cells by activating G proteins. This leads to simultaneous propagation of two signaling pathways: one that culminates in exocytosis, while the other involves protein tyrosine phosphorylation and leads to release of arachidonic acid metabolites. We have previously shown that introduction of a peptide that comprises the C-terminal end of G alpha i3 into permeabilized mast cells inhibits basic secretagogue-induced exocytosis [Aridor et al., Science 1993;262:1569-1572]. We investigated whether cell-permeable peptides, composed of the C-terminus of G alpha i3 fused with importation sequences, affect mast cell function.

METHODS

Following preincubation with the fused peptides, rat peritoneal mast cells were activated by compound 48/80 and analyzed for histamine and prostaglandin D2 release and protein tyrosine phosphorylations.

RESULTS

We demonstrate that out of three importation sequences tested only G alpha i3 peptide fused with the Kaposi fibroblast growth factor importation sequence (ALL1) inhibited release of histamine. ALL1 as well as a cell-permeable peptide that corresponds to G alpha i2 also blocked compound 48/80-stimulated protein tyrosine phosphorylation, though the latter did not block histamine release. ALL1 effect was G protein-specific, as it was incapable of blocking protein tyrosine phosphorylation stimulated by pervanadate.

CONCLUSION

ALL1, a transducible G alpha i3-corresponding peptide, blocks the two signaling pathways in mast cells: histamine release and protein tyrosine phosphorylation. Cell permeable peptides that block these two signaling cascades may constitute a novel approach for preventing the onset of the allergic reaction.

Phospholipase C beta3 is a key component in the Gbetagamma/PKCeta/PKD-mediated regulation of trans-Golgi network to plasma membrane transport

The requirement of DAG (diacylglycerol) to recruit PKD (protein kinase D) to the TGN (trans-Golgi network) for the targeting of transport carriers to the cell surface, has led us to a search for new components involved in this regulatory pathway. Previous findings reveal that the heterotrimeric Gbetagamma (GTP-binding protein betagamma subunits) act as PKD activators, leading to fission of transport vesicles at the TGN. We have recently shown that PKCeta (protein kinase Ceta) functions as an intermediate member in the vesicle generating pathway. DAG is capable of activating this kinase at the TGN, and at the same time is able to recruit PKD to this organelle in order to interact with PKCeta, allowing phosphorylation of PKD's activation loop. The most qualified candidates for the production of DAG at the TGN are PI-PLCs (phosphatidylinositol-specific phospholipases C), since some members of this family can be directly activated by Gbetagamma, utilizing PtdIns(4,5)P2 as a substrate, to produce the second messengers DAG and InsP3. In the present study we show that betagamma-dependent Golgi fragmentation, PKD1 activation and TGN to plasma membrane transport were affected by a specific PI-PLC inhibitor, U73122 [1-(6-{[17-3-methoxyestra-1,3,5(10)-trien-17-yl]amino}hexyl)-1H-pyrrole-2,5-dione]. In addition, a recently described PI-PLC activator, m-3M3FBS [2,4,6-trimethyl-N-(m-3-trifluoromethylphenyl)benzenesulfonamide], induced vesiculation of the Golgi apparatus as well as PKD1 phosphorylation at its activation loop. Finally, using siRNA (small interfering RNA) to block several PI-PLCs, we were able to identify PLCbeta3 as the sole member of this family involved in the regulation of the formation of transport carriers at the TGN. In conclusion, we demonstrate that fission of transport carriers at the TGN is dependent on PI-PLCs, specifically PLCbeta3, which is necessary to activate PKCeta and PKD in that Golgi compartment, via DAG production.

Immunoglobulin E-independent activation of mast cell is mediated by Mrg receptors

Mast cells play a central role in inflammatory and allergic reactions by releasing inflammatory mediators through two main pathways, immunoglobulin E-dependent and -independent activation. In the latter, mast cells are activated by a diverse range of basic molecules, including peptides and amines such as substance P, neuropeptide Y, and compound 48/80. These secretagogues are thought to activate the G proteins in mast cells through a receptor-independent mechanism. Here, we report that the basic molecules activate G proteins through the Mas-related gene (Mrg) receptors on mast cells, leading to mast cell degranulation. We suggest that one of the Mrg receptors, MrgX2, has an important role in regulating inflammatory responses to non-immunological activation of human mast cells.

Mast cell activation is differentially affected by heat shock

OBJECTIVE

Mast cells play pivotal roles in immediate-type and inflammatory allergic and nonallergic reactions. Cross-linking of the high-affinity receptor for IgE (FcepsilonRI) on mast cells activates a signaling pathway leading to Ca2+ mobilization and is followed by degranulation and the release of histamine and other preformed mediators, as well as de novo synthesis of arachidonic acid metabolites. In a previous study, we have demonstrated that heat shock activates heat shock transcription factor-1 (HSF-1), induces heat shock protein 70 (HSP70), and suppresses cytokine production in bone marrow-derived mast cells (BMMC). In this study, we further investigated the effects of heat shock on the activation of mast cells and the release of mast cell mediators.

METHODS

In mouse mast cells, derived from a culture of bone marrow cells of male BALB/cBy and null HSF-1(-/-)mice, responsiveness to heat shock was monitored by measuring beta-hexosaminidase and leukotriene C4 (LTC4) release.

RESULTS

Using BMMC, we found that heat shock inhibits degranulation of BMMC without affecting leukotriene production. To further elucidate the mechanism of suppression of degranulation, we studied the effects of heat shock on the regulation of signal transduction in more detail. We found that heat shock inhibits calcium mobilization and tyrosine phosphorylation of Syk and SHIP upon IgE receptor activation, but increases the phosphorylation of SHP-1 and -2. Moreover, our results revealed that suppression of tyrosine phosphorylation of Syk and SHIP coincided with an increased tyrosine phosphatase activity.

CONCLUSION

The inhibitory action of heat shock toward mast cell degranulation is likely due to shifting the balance between kinase and phosphatase activity.

CB2 cannabinoid receptor activation produces antinociception by stimulating peripheral release of endogenous opioids

CB(2) cannabinoid receptor-selective agonists are promising candidates for the treatment of pain. CB(2) receptor activation inhibits acute, inflammatory, and neuropathic pain responses but does not cause central nervous system (CNS) effects, consistent with the lack of CB(2) receptors in the normal CNS. To date, there has been virtually no information regarding the mechanism of CB(2) receptor-mediated inhibition of pain responses. Here, we test the hypothesis that CB(2) receptor activation stimulates release from keratinocytes of the endogenous opioid beta-endorphin, which then acts at opioid receptors on primary afferent neurons to inhibit nociception. The antinociceptive effects of the CB(2) receptor-selective agonist AM1241 were prevented in rats when naloxone or antiserum to beta-endorphin was injected in the hindpaw where the noxious thermal stimulus was applied, suggesting that beta-endorphin is necessary for CB(2) receptor-mediated antinociception. Further, AM1241 did not inhibit nociception in mu-opioid receptor-deficient mice. Hindpaw injection of beta-endorphin was sufficient to produce antinociception. AM1241 stimulated beta-endorphin release from rat skin tissue and from cultured human keratinocytes. This stimulation was prevented by AM630, a CB(2) cannabinoid receptor-selective antagonist and was not observed in skin from CB(2) cannabinoid receptor-deficient mice. These data suggest that CB(2) receptor activation stimulates release from keratinocytes of beta-endorphin, which acts at local neuronal mu-opioid receptors to inhibit nociception. Supporting this possibility, CB(2) immunolabeling was detected on beta-endorphin-containing keratinocytes in stratum granulosum throughout the epidermis of the hindpaw. This mechanism allows for the local release of beta-endorphin, where CB(2) receptors are present, leading to anatomical specificity of opioid effects.

Progesterone primes zona pellucida-induced activation of phospholipase A2 during acrosomal exocytosis in guinea pig spermatozoa

We investigated, using guinea-pig spermatozoa as a model, whether phospholipase A2 (PLA2) is involved in progesterone or zona pellucida (ZP)-stimulated acrosomal exocytosis, if progesterone enhances ZP-induced activation of PLA2, and mechanisms underlying PLA2 regulation. Spermatozoa were capacitated and labeled in low Ca2+ medium with [14C]choline chloride or [14C]arachidonic acid, washed, and then exposed to millimolar Ca2+ and progesterone and/or ZP. Each agonist stimulated decrease of phosphatidylcholine (PC) and release of arachidonic acid and lysoPC, indicative of PLA2 activation. Aristolochic acid (a PLA2 inhibitor) abrogated lipid changes and exocytosis, indicating that these lipid changes are essential for exocytosis. Exposure of spermatozoa to submaximal concentrations of both progesterone and ZP resulted in a synergistic increase of arachidonic acid and lysoPC releases, and exocytosis, suggesting that, under natural conditions, both agonists interact to bring about acrosomal exocytosis. Progesterone-induced PLA2 activation appears to be mediated by a GABA(A)-like receptor, because bicuculline (a GABA(A) receptor antagonist) blocked arachidonic acid release and exocytosis. In agreement with this, GABA mimicked progesterone actions. ZP-induced activation of PLA2 seemed to be transduced via G(i) proteins because pertussis toxin blocked arachidonic acid release and acrosomal exocytosis. PLA2 may be regulated by PKC because progesterone- or ZP-induced release of arachidonic acid was blocked by the PKC inhibitors staurosporine or chelerythrine chloride. PLA2 could also be regulated by the cAMP-PKA pathway; inclusion of the PKA inhibitor 14-22 amide or H-89 led to a reduction in arachidonic acid release or exocytosis after progesterone or ZP. Taken together, these results suggest that PLA2 plays an essential role in progesterone or ZP-stimulated exocytosis with progesterone priming ZP action.

The Rac/Cdc42 guanine nucleotide exchange factor beta1Pix enhances mastoparan-activated Gi-dependent pathway in mast cells

Carbachol stimulates granule exocytosis, phospholipase C (PLC), and phospholipase D (PLD) in RBL-2H3hm1 mast cells by a mechanism that involves Galphaq. However, mastoparan stimulates the same responses through Gi protein. Both Gi and Galphaq pathways are suppressed by Clostridium difficile toxin B, suggesting that Rac and Cdc42 small GTPases are also involved. Over-expression of beta1Pix, a guanine nucleotide exchange factor for Rac and Cdc42, enhances mastoparan-but not carbachol-induced hexosaminidase secretion and PLC and PLD activation. Furthermore, cells expressing beta1Pix exhibit elevated levels of mastoparan-stimulated IP3 production. Taken together, these findings implicate beta1Pix in regulating hexoasaminidase secretion and IP3 production in early stage upon mastoparan stimulation.

Substance P can stimulate prostaglandin D2 and leukotriene C4 generation without granule exocytosis in murine mast cells

Mast cells play a central role in immediate type hypersensitivity and inflammatory events. Activation of mast cells not only can result in the release of preformed granule-associated mediators generally followed by de novo synthesis of lipid-derived substances. In the present study, we show that mast cell can be activated to release lipid mediators in absence of granule exocytosis. Primary cultured murine mast cells were stimulated with substance P and produced leukotriene C4, and prostaglandin D2 without the release of the granule-associated enzyme beta-hexosaminidase. Indomethacin and nordihydroguaiaretic acid caused complete inhibition of arachidonic metabolite generation. Leukotriene C4 and prostaglandin D2 production was blocked by genistein, a specific inhibitor of tyrosine kinases, and bisindolylmaleimide, a protein kinase C inhibitor, indicating a role for both phosphorylation pathways in the substance P-stimulated lipid mediator production. We suggest that the cytokine microenvironment of the mast cell determines whether mast cell stimulation leads to only lipid mediator release or full activation. Analysis of granule-associated mediators only might underestimate the role of mast cell activation under (patho)physiological conditions.

Reactive oxygen species as mediators of signal transduction in ischemic preconditioning

Ischemic preconditioning (IPC) is a most powerful endogenous mechanism for myocardial protection against ischemia/reperfusion injury. It is now apparent that reactive oxygen species (ROS) generated in the mitochondrial respiratory chain act as a trigger of IPC. ROS mediate signal transduction in the early phase of IPC through the posttranslational modification of redox-sensitive proteins. ROS-mediated activation of Src tyrosine kinases serves a scaffold for interaction of proteins recruited by G protein-coupled receptors and growth factor receptors that is necessary for amplification of cardioprotective signal transduction. Protein kinase C (PKC) plays a central role in this signaling cascade. A crucial target of PKC is the mitochondrial ATP-sensitive potassium channel, which acts as a trigger and a mediator of IPC. Mitogen-activated protein (MAP) kinases (extracellular signal-regulated kinase, p38 MAP kinase, and c-Jun NH(2)-terminal kinase) are thought to exist downstream of the Src-PKC signaling module, although the role of MAP kinases in IPC remains undetermined. The late phase of IPC is mediated by cardioprotective gene expression. This mechanism involves redox-sensitive activation of transcription factors through PKC and tyrosine kinase signal transduction pathways that are in common with the early phase of IPC. The effector proteins then act against myocardial necrosis and stunning presumably through alleviation of oxidative stress and Ca(2+) overload. Elucidation of IPC-mediated complex signaling processes will help in the development of more effective pharmacological approaches for prevention of myocardial ischemia/reperfusion injury.

Substance P induces TNF-α and IL-6 production through NFκB in peritoneal mast cells

The neuropeptide Substance P (SP) is an important mediator of neuroimmunomodulatory activity. The aim of this study is to elucidate the mechanism used by SP to promote increased production of pro-inflammatory cytokines in fresh isolated rat peritoneal mast cells (rPMC). We have demonstrated that SP induces production of interleukin-6 (IL-6) in rPMC through the PI-3K, p42/44 and p38 MAP kinase pathways. SP-stimulated rPMC also exhibited an enhanced nuclear translocation of the nuclear factor kappa B (NF kappa B). The tumour necrosis factor-alpha (TNF-alpha) and IL-6 production was completely inhibited by using (E)-4-hydroxynonenal (HNE) as an inhibitor of I kappa B-alpha and -beta phosphorylation. Further, TNF-alpha and IL-6 expression was significantly inhibited by the oligonucleotides (ODNs) containing the NF kappa B element (NF kappa B decoy ODNs) but not by the scrambled control ODNs. These findings indicate that the NF kappa B pathway is involved in the transcriptional regulation of the TNF-alpha and IL-6 overexpression in primary SP-stimulated mast cells.

Age Increases Expression and Receptor-Mediated Activation of Gαi in Human Atria

Recently, we demonstrated that beta2AR and several other Galphas-coupled receptors in human atria also couple to Galphai, a G protein that inhibits adenylyl cyclase (AC). The present study was undertaken to determine whether age increases expression of Galphai in human atrium, and more specifically whether it results in an increase in receptor-mediated activation of Galphai. Right atrial appendages were obtained from 14 mature adult (40-55 years) and 14 elderly (71-79 years) patients undergoing cardiac surgery. Immunoblotting of atrial membranes indicates that elderly atria have 82 +/- 18% more Galphai2 than atria from mature adults (P < 0.002); this increase in Galphai with age is confirmed by pertussis toxin-catalyzed ADP-ribosylation as well as by photoaffinity labeling with [32P]azidoanilido-GTP. We also find that receptor-mediated activation of Galphai is greater in elderly atria and that both basal and receptor-mediated AC activities decrease in elderly atria. These decreases in AC activity can be reversed by disabling Galphai with pertussis toxin, indicating that the age-dependent increases in Galphai expression and activation have functional consequences. Because beta2ARs in human atria mediate contractility through cAMP-mediated phosphorylation of phospholamban, we conclude that an age-induced increase in Galphai may have a role in depressing cardiac function in aged human atria.

Activation of murine lung mast cells by the adenosine A3 receptor

Adenosine has been implicated to play a role in asthma in part through its ability to influence mediator release from mast cells. Most physiological roles of adenosine are mediated through adenosine receptors; however, the mechanisms by which adenosine influences mediator release from lung mast cells are not understood. We established primary murine lung mast cell cultures and used real-time RT-PCR and immunofluorescence to demonstrate that the A(2A), A(2B), and A(3) adenosine receptors are expressed on murine lung mast cells. Studies using selective adenosine receptor agonists and antagonists suggested that activation of A(3) receptors could induce mast cell histamine release in association with increases in intracellular Ca(2+) that were mediated through G(i) and phosphoinositide 3-kinase signaling pathways. The function of A(3) receptors in vivo was tested by exposing mice to the A(3) receptor agonist, IB-MECA. Nebulized IB-MECA directly induced lung mast cell degranulation in wild-type mice while having no effect in A(3) receptor knockout mice. Furthermore, studies using adenosine deaminase knockout mice suggested that elevated endogenous adenosine induced lung mast cell degranulation by engaging A(3) receptors. These results demonstrate that the A(3) adenosine receptor plays an important role in adenosine-mediated murine lung mast cell degranulation.

G protein-dependent activation of mast cell by peptides and basic secretagogues

Signaling pathways leading to exocytosis and arachidonate release from serosal mast cells by basic secretagogues, including cationic peptides, arise from the involvement of betagamma subunits from G(i2) and G(i3) GTP-binding proteins. The original concept that basic secretagogues directly interact with G proteins implicated the entry of secretagogues into mast cells. This has been demonstrated only for the neuropeptide substance P. Basic secretagogues might share a common mechanism of penetration with the newly described cell-penetrating peptides. The involvement of some membrane transporter or non-selective membrane receptor to basic secretagogues cannot be excluded.

Agmatine: a mastoparan-like activity related to direct activation of heterotrimeric G proteins

We examined agmatine and imidazoline derivatives as putative ligands of trimeric G protein in rat peritoneal mast cells. Agmatine induced a concentration-dependent and pertussis toxin-sensitive secretion of histamine (exocytosis) and arachidonate. Clonidine and idazoxan had no effect. Blockage of Gbetagamma dimers by a specific anti-Gbeta antibody inhibited exocytosis elicited by agmatine and mastoparan. The G protein antagonist [p-Glu(5),D-Trp(7,9,10)]substance P-(5-11) prevented both mastoparan- and agmatine-induced exocytosis when it was allowed to reach its intracellular targets by streptolysin-O permeabilisation. In intact cells, this response was prevented by both the removal of sialic acid residues by neuraminidase and by [D-Pro(4),D-Trp(7,9,10)]substance P-(4-11) acting at the mast cell surface. Exocytosis was restored by permeabilisation of the plasma membrane with streptolysin-O. These results suggest that agmatine might have several molecular targets, exerting its neurotransmitter function at low concentrations (i.e., with high affinity) through membrane receptors and at high concentrations (i.e., with weak affinity) through direct G protein activation.