Inflammatory ER stress responses dictate the immunopathogenic progression of systemic candidiasis

Recognition of pathogen-associated molecular patterns can trigger the inositol-requiring enzyme 1 α (IRE1α) arm of the endoplasmic reticulum (ER) stress response in innate immune cells. This process maintains ER homeostasis and also coordinates diverse immunomodulatory programs during bacterial and viral infections. However, the role of innate IRE1α signaling in response to fungal pathogens remains elusive. Here, we report that systemic infection with the human opportunistic fungal pathogen Candida albicans induced proinflammatory IRE1α hyperactivation in myeloid cells that led to fatal kidney immunopathology. Mechanistically, simultaneous activation of the TLR/IL-1R adaptor protein MyD88 and the C-type lectin receptor dectin-1 by C. albicans induced NADPH oxidase-driven generation of ROS, which caused ER stress and IRE1α-dependent overexpression of key inflammatory mediators such as IL-1β, IL-6, chemokine (C-C motif) ligand 5 (CCL5), prostaglandin E2 (PGE2), and TNF-α. Selective ablation of IRE1α in leukocytes, or treatment with an IRE1α pharmacological inhibitor, mitigated kidney inflammation and prolonged the survival of mice with systemic C. albicans infection. Therefore, controlling IRE1α hyperactivation may be useful for impeding the immunopathogenic progression of disseminated candidiasis.

IRE1α-XBP1 signaling in leukocytes controls prostaglandin biosynthesis and pain

Inositol-requiring enzyme 1[α] (IRE1[α])-X-box binding protein spliced (XBP1) signaling maintains endoplasmic reticulum (ER) homeostasis while controlling immunometabolic processes. Yet, the physiological consequences of IRE1α-XBP1 activation in leukocytes remain unexplored. We found that induction of prostaglandin-endoperoxide synthase 2 (Ptgs2/Cox-2) and prostaglandin E synthase (Ptges/mPGES-1) was compromised in IRE1α-deficient myeloid cells undergoing ER stress or stimulated through pattern recognition receptors. Inducible biosynthesis of prostaglandins, including the pro-algesic mediator prostaglandin E2 (PGE₂), was decreased in myeloid cells that lack IRE1α or XBP1 but not other ER stress sensors. Functional XBP1 transactivated the human PTGS2 and PTGES genes to enable optimal PGE₂ production. Mice that lack IRE1α-XBP1 in leukocytes, or that were treated with IRE1α inhibitors, demonstrated reduced pain behaviors in PGE₂-dependent models of pain. Thus, IRE1α-XBP1 is a mediator of prostaglandin biosynthesis and a potential target to control pain.

Nitric oxide decreases intestinal haemorrhagic lesions in rat anaphylaxis independently of mast cell activation

The purpose of this study is to assess the role of nitric oxide (NO) in the intestinal lesions of passive anaphylaxis, since this experimental model resembles necrotizing enterocolitis. Sprague-Dawley rats were sensitized with IgE anti-dinitrophenol monoclonal antibody. Extravasation of protein-rich plasma and haemorrhagia were measured in the small intestine. Plasma histamine was measured to assess mast cell activation. The effect of exogenous NO on the lesions was assessed by using two structurally unrelated NO-donors: sodium nitroprusside and S-nitroso-Nacetyl-penicillamine (SNAP). An increased basal production of NO was observed in cells taken after anaphylaxis, associated with a reduced response to platelet-activating factor, interleukin 1beta, and IgE/DNP-bovine serum albumin complexes. The response to bacterial lipopolysaccharide and dibutyryl cyclic adenosine monophosphate (AMP) was enhanced 24 h after challenge, but at earlier times was not significantly different from that observed in controls. Treatment with either sodium nitroprusside or SNAP produced a significant reduction of the haemorrhagic lesions, which are a hallmark of rat anaphylaxis. The extravasation of protein-rich plasma was not influenced by NO-donors. The increase of plasma histamine elicited by the anaphylactic challenge was not influenced by SNAP treatment. NO-donors protect intestinal haemorrhagic lesions of rat anaphylaxis by a mechanism apparently independent of mast cell histamine release.

FcgammaRIIA exogenously expressed in HeLa cells activates the mitogen-activated protein kinase cascade by a mechanism dependent on the endogenous expression of the protein tyrosine kinase Syk

HeLa cells transfected to express the human Fc receptor FcgammaRIIA were stimulated with aggregates of IgG, IgG-ovalbumin equivalence immune complexes and monoclonal antibody reacting with FcgammaRIIA. All of these stimuli activated the cells as judged from the band-shift characteristic of the activation of the p42-MAP/ERK kinase. Since this response is currently associated with the activation of the protein tyrosine kinase Syk, the expression of which is currently thought to be restricted to hemopoietic cells, the results were considered as an indirect evidence of the expression in HeLa cells of either Syk or another protein tyrosine kinase accounting for the same function. Transfection with a dominant negative Syk mutant abrogated the response to FcgammaRIIA cross-linking, whereas overexpression of Syk did not increase the extent of the response. Further evidence of the expression of syk was obtained by the reverse transcription PCR approach and sequencing of the DNA bands. Moreover, immunoprecipitation with anti-Syk antibody of the cell lysates obtained after cross-linking of FcgammaRIIA followed by immunoblotting with anti-phosphotyrosine antibody showed the phosphorylation of a protein band migrating as Syk. These data indicate that expression of FcgammaRIIA on epithelial HeLa cells conveys signals to the p42-MAP/ERK kinase by a mechanism involving the recruitment of Syk. In contrast, cross-linking of this receptor does not yield productive signals coupled to other responses associated to the FcgammaR system such as triggering of the arachidonic acid cascade, activation of the NF-kappaB system and production of chemotactic cytokines.

Lysophosphatidic acid inhibits Ca2+ signaling in response to epidermal growth factor receptor stimulation in human astrocytoma cells by a mechanism involving phospholipase C(gamma) and a G(alphai) protein

The effect of the lysophospholipid mediators lysophosphatidic acid (LPA) and sphingosine 1-phosphate and the polypeptide growth factor epidermal growth factor (EGF) on the human astrocytoma cell line 1321N1 was assessed. These agonists produced a rapid and transient increase of the intracellular Ca(2+) concentration. When LPA was perfused before addition of EGF, the EGF-dependent Ca(2+) transient was abrogated, whereas this was not observed when EGF preceded LPA addition. This inhibitory effect was not found for other EGF-mediated responses, e.g., activation of the mitogen-activated protein kinase cascade and cell proliferation, thus pointing to the existence of cross-talk between LPA and EGF for only a branch of EGF-induced responses. As 1321N1 cells expressed mRNA encoding the LPA receptors endothelial differentiation gene (Edg)-2, Edg-4, and Edg-7 and as sphingosine 1-phosphate did not interfere with LPA signaling, Edg-2, Edg-4, and/or Edg-7 could be considered as the LPA receptors mediating the aforementioned cross-talk. Attempts to address the biochemical mechanism involved in the cross-talk between the receptors were conducted by the immunoprecipitation approach using antibodies reacting with the EGF receptor (EGFR), phosphotyrosine, phospholipase Cgamma (PLCgamma)-1, and G(alphai) protein. LPA was found to induce coupling of PLCgamma-1 to the EGFR by a mechanism involving a G(alphai) protein, in the absence of tyrosine phosphorylation of both PLCgamma and the EGFR. These data show a cross-talk between LPA and EGF limited to a branch of EGFR-mediated signaling, which may be explained by a LPA-induced, G(alphai)-protein-mediated translocation of PLCgamma-1 to EGFR in the absence of detectable tyrosine phosphorylation of both proteins.

Stimulation of Fc gamma R receptors induces monocyte chemoattractant protein-1 in the human monocytic cell line THP-1 by a mechanism involving I kappa B-alpha degradation and formation of p50/p65 NF-kappa B/Rel complexes

THP-1 monocytic/macrophage cells were stimulated via their FcgammaR receptors with insoluble aggregates of human IgG and the production of the C-C chemokine monocyte chemoattractant protein (MCP)-1 assayed. A dose- and time-dependent production of MCP-1 comparable to that produced by the most potent agonists could be detected in the culture medium by a sensitive ELISA assay. This was accompanied by a parallel activation of the transcription factor NF-kappaB as judged from both the appearance of kappaB-binding activity containing p50/p65 NF-kappaB/Rel complexes in the nuclear extract and the disappearance of the NF-kappaB inhibitor IkappaB-alpha in the cell lysate. In contrast, IkappaB-beta and IkappaB-epsilon expression was not modified, thus pointing to the occurrence of a selective degradation of IkappaB-alpha under those conditions. Attempts to modulate MCP-1 production with compounds that display inhibitory effects on the activation of NF-kappaB such as the proteasome inhibitor N-acetyl-leucinyl-leucinyl-norleucinal, the antioxidant pyrrolidine dithiocarbamate and the salicylate derivative 2-hydroxy-4-trifluoromethylbenzoic acid showed a parallel effect on both MCP-1 production and NF-kappaB activation, thus pointing to the involvement of kappaB-binding sites on the transcriptional regulation of MCP-1 production. Our findings suggest the existence in monocytic cells of a signaling mechanism initiated by cross-linking of low-affinity FcgammaR, most likely of the FcgammaRII family since THP-1 cells do not express FcgammaRIII receptors, that involves activation of NF-kappaB associated to the proteolytic degradation of IkappaB-alpha and leads to the transcriptional up-regulation of MCP-1.

Secretory phospholipase A2 activates the cascade of mitogen-activated protein kinases and cytosolic phospholipase A2 in the human astrocytoma cell line 1321N1

The biological effects of type IIA 14-kDa phospholipase A2 (sPLA2) on 1321N1 astrocytoma cells were studied. sPLA2 induced a release of [3H]arachidonic acid ([3H]AA) similar to that elicited by lysophosphatidic acid (LPA), a messenger acting via a G-protein-coupled receptor and a product of sPLA2 on lipid microvesicles. In contrast, no release of [1-14C]oleate could be detected in cells labeled with this fatty acid. As these findings pointed to a selective mechanism of [3H]AA release, it was hypothesized that sPLA2 could act by a signaling mechanism involving the activation of cytosolic PLA2 (cPLA2), i.e. the type of PLA2 involved in the release of [3H]AA elicited by agonists. In keeping with this view, stimulation of 1321N1 cells with sPLA2 elicited the decrease in electrophoretic mobility that is characteristic of the phosphorylation of cPLA2, as well as activation of p42 mitogen-activated protein (MAP) kinase, c-Jun kinase, and p38 MAP kinase. Incubation with sPLA2 of quiescent 1321N1 cells elicited a mitogenic response as judged from an increased incorporation of [3H]thymidine. Attempts to correlate the effect of extracellular PLA2 with the generation of LPA were negative. Incubation with pertussis toxin prior to the addition of either sPLA2 or LPA only showed abrogation of the response to LPA, thus suggesting the involvement of pertussis-sensitive Gi-proteins in the case of LPA. Treatments with inhibitors of the catalytic effect of sPLA2 such as p-bromophenacyl bromide and dithiothreitol did not prevent the effect on cPLA2 activation. In contrast, preincubation of 1321N1 cells with the antagonist of the sPLA2 receptor p-aminophenyl-alpha-D-mannopyranoside-bovine serum albumin, blocked cPLA2 activation with a EC50 similar to that described for the inhibition of binding of sPLA2 to its receptor. Moreover, treatment of 1321N1 cells with the MAP kinase kinase inhibitor PD-98059 inhibited the activation of both cPLA2 and p42 MAP kinase produced by sPLA2. In summary, these data indicate the existence in astrocytoma cells of a signaling pathway triggered by engagement of a sPLA2-binding structure, that produces the release of [3H]AA by activating the MAP kinase cascade and cPLA2, and leads to a mitogenic response after longer periods of incubation.

The expression of cytokine-induced neutrophil chemoattractants (CINC-1 and CINC-2) in rat peritoneal macrophages is triggered by Fc gamma receptor activation: study of the signaling mechanism

The expression of cytokine-induced neutrophil chemoattractants (CINC-1 and CINC-2) mRNA was studied in rat peritoneal cells stimulated with insoluble IgG/ovalbumin immune complexes. A dose- and time-dependent induction was observed in adherent cells, which was more prominent than that induced by the lipid mediator platelet-activating factor (PAF), comparable to that observed in response to 10 micrograms endotoxin in the absence of lipopolysaccharide (LPS)-binding protein, but lower than that produced by 1 mM dibutyryl cyclic AMP, a compound which stabilized transiently expressed genes containing AU-rich sequences in the 3' untranslated region. Analysis of CINC-1 protein by specific enzyme-linked immunosorbent assay confirmed the presence of CINC-1 in the supernatants at concentrations of approximately 4 nM, 4 h after addition of 100 micrograms/ml immune complexes. CINC-2 beta protein was detectable at a lower concentration (approximately 0.3 nM) under the same conditions. Attempts to relate CINC-1 induction with the pathways for cytoplasmic signaling showed a dissociation of Ca2+ mobilization and protein kinase C activation as judged from the small effect of thapsigargin and the lack of effect of phorbol ester. In contrast, these agents produced a marked mobilization of arachidonate linked to the MAP kinase-dependent activation of cytosolic phospholipase A2. The possible dependence of CINC-1 induction on the autocrine generation of lipid mediators was ruled out by a set of experiments including the use of the PAF receptor antagonist BB823, and the analysis of the effect of free arachidonate and leukotriene B4 on CINC-1 induction. Surprisingly, the inhibitor of leukotriene synthesis MK-886 in the range of concentration 1-10 microM inhibited CINC-1 induction by a mechanism that appears to be independent of its effect on eicosanoid production. Interestingly, CINC-1 induction appeared to be related to protein tyrosine phosphorylation reactions on the basis of both the appearance of several tyrosine-phosphorylated protein bands in lysates from adherent peritoneal cells treated with immune complexes and the complete blockade of CINC-1 induction by treatment with 1 microM herbimycin A, an inhibitor of src protein tyrosine kinases.

Platelet-activating factor: the effector of protein-rich plasma extravasation and nitric oxide synthase induction in rat immune complex peritonitis

1. The involvement of platelet-activating factor (PAF) in immune complex-induced/polymorphonuclear-mediated tissue injury was studied by use of a reverse passive Arthus (RPA) model in the peritoneal cavity of rats. 2. Extravasation of protein-rich plasma, accumulation of polymorphonuclear leukocytes (PMN), and the production of nitric oxide (NO) by resident peritoneal mononuclear phagocytes were assayed. 3. Treatment of rats with either UR-12460 or BB-823, two compounds which possess different chemical structures, but elicit the same antagonistic effect on the PAF receptor, abrogated protein-rich plasma extravasation. In contrast, they did not show any effect on the accumulation of PMN. 4. Inhibition of NO production with both NG-mono methyl-L-arginine and NG-nitro-L-arginine failed to prevent protein-rich plasma extravasation. 5. The production of NO by peritoneal adherent cells following RPA was measured in cells maintained for 2 to 28 h in culture, and it was significantly increased in cells removed as early as 15 min after RPA induction, as compared to controls. 6. Addition of 10 nM PAF to the culture medium reduced the generation of NO by peritoneal cells from RPA rats, whereas this mediator enhanced NO production in cells from naive control animals. 7. Treatment with either UR-12460 or BB-823 prior to the induction of RPA produced an almost complete inhibition of NO production. 8. Assay of nitric oxide synthase activity in cell homogenates from peritoneal cells showed that the activity was due to the inducible form of the enzyme. 9. Study by Northen blotting of mRNA coding for the inducible NO synthase (iNOS) showed transcription at 6 and 18 h after the induction of RPA, which was inhibited in UR-12460-treated rats.10. These data indicate that PAF is the main mediator of the early plasma leakage observed in RPA,and also that PAF is implicated in the triggering of long-term changes via induction of specific genes, as judged from its ability to promote the expression of iNOS.

Role of complement-derived peptides in thrombocytopenia elicited by soluble aggregates of immunoglobulin G in the rat

The variations in platelet counts upon intravenous challenge with soluble aggregates of IgG were assessed in normal rats. A time- and dose-dependent thrombocytopenia, followed by recovery to preinfusion values after 30 minutes was observed. Rats injected with immune aggregates showed an increase in plasma levels of immunoreactive thromboxane B2, however, this increase was delayed as compared with the peak level of the thrombocytopenia. Previous treatment of rats with either indomethacin or aspirin, inhibited thromboxane B2 release, but did not affect thrombocytopenia. Pretreatment of the animals with BN 52021, a potent antagonist of platelet-activating factor binding to its receptor, also failed to block thrombocytopenia. Complement depletion by prior treatment with cobra venom factor, caused a significant reduction of the thrombocytopenia, whereas DL-2-mercaptomethyl-3-guanidinoethylthiopropanoic acid, an inhibitor of carboxypeptidase N, potentiated the thrombocytopenia elicited by submaximal doses of either IgG aggregates or a homogeneous preparation of rat anaphylatoxin containing C5a. In addition, rats challenged with doses of IgG aggregates higher than 5 mg/kg showed a massive complement consumption coincident with the onset of thrombocytopenia. "In vitro" aggregation/secretion experiments with rat platelets showed little platelet-stimulating activity either by aggregated IgG through the Fc receptor or through the CR1 receptor. By contrast, a preparation of rat serum anaphylatoxins containing C5a, showed a high platelet-secreting activity. These data suggest that a complement-derived peptide(s), most probably C5a, is one of the effector substances for platelet activation in response to soluble aggregates of IgG.

Characteristics of the binding of platelet-activating factor to platelets of different animal species

The binding of platelet-activating factor (PAF-acether) to human, rabbit and rat platelets was investigated by incubating washed platelets at 37 degrees C with radiolabeled [3H]PAF-acether. Scatchard plot analysis of the binding showed that human and rabbit platelets possess two different types of binding sites. One of them was saturable and of high affinity (KD 1.58 +/- 0.36 nM in human and 0.9 +/- 0.5 nM in rabbits), and the other one had nearly infinite binding capacity. By contrast, rat platelets only showed non-specific binding. Addition of unlabeled PAF-acether 5 min after the addition of [3H]PAF-acether showed that internalization of 66 +/- 9% of the specific binding in human and 52 +/- 6.9% in rabbit platelets had occurred. Specific binding of [3H]PAF-acether in rabbit platelets correlated well with [3H]serotonin release in response to different doses of PAF-acether and with the uptake of calcium by the platelets. By contrast, PAF-acether did not induce [3H]serotonin release or calcium uptake by rat platelets. The following data suggest that the potential of PAF-acether to activate platelets depends on the interaction with a specific membrane receptor rather than on a non-receptor-mediated alteration of the platelet membrane: (1) platelets from animal species sensitive to PAF-acether show saturability and specificity of binding; (2) platelets from one animal species non-sensitive to PAF-acether lack specific binding; (3) PAF-acether does not induce calcium uptake by platelets from an animal species which lacks specific binding sites.