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Mast cells, critical mediators of inflammation and anaphylaxis, are poised as one of the first lines of defense against external assault. Mast cells release several classes of preformed and de novo synthesized mediators. Cross-linking of the high-affinity FcεRI results in degranulation and the release of preformed, proinflammatory mediators including histamine and serotonin. We previously demonstrated that mast cell activation by Listeria monocytogenes requires the α2β1 integrin for rapid IL-6 secretion both in vivo and in vitro. However, the mechanism of IL-6 release is unknown. Here, we demonstrate the Listeria- and α2β1 integrin-mediated mast cell release of preformed IL-6 without the concomitant release of histamine or β-hexosaminidase. α2β1 integrin-dependent mast cell activation and IL-6 release is calcium independent. In contrast, IgE cross-linking-mediated degranulation is calcium dependent and does not result in IL-6 release, demonstrating that distinct stimuli result in the release of specific mediator pools. These studies demonstrate that IL-6 is presynthesized and stored in connective tissue mast cells and can be released from mast cells in response to distinct, α2β1 integrin-dependent stimulation, providing the host with a specific innate immune response without stimulating an allergic reaction.
Copyright © 2011 S. Karger AG, Basel.
A biophysical approach tested the hypothesis that tirofiban, like eptifibatide, perturbs GpIIb/IIIa structure. Tirofiban bound tightly to platelet GpIIb/IIIa (EC50 approximately 24 nmol/L) and effectively inhibited platelet aggregation (IC50 approximately 37 nmol/L) but blocked platelet adhesion to clotted fibrin only at much higher doses (IC50 approximately 580 nmol/L). Electrophoretic analyses demonstrated that tirofiban protected GpIIb/IIIa from SDS-induced subunit dissociation. However, saturating tirofiban concentrations had little or no effect on GpIIb/IIIa secondary or tertiary structure, as determined by circular dichroic spectroscopy, dynamic light scattering, and sedimentation velocity measurements performed with purified receptors in octyl glucoside. Moderate dose-dependent effects on GpIIb/IIIa quaternary structure were detected by sedimentation equilibrium. Transmission electron microscopy showed minimal tirofiban-induced receptor activation or oligomerization. Thus, even at the increased concentrations needed to block platelet:fibrin adhesive interactions, tirofiban exhibited only limited effects on GpIIb/IIIa conformation and clustering. Our results provide new insights into the mechanisms and potential prothrombotic complications of integrin antagonists.
PSD-95 is a neuronal PDZ protein that associates with receptors and cytoskeletal elements at synapses, but whose function is uncertain. We found that overexpression of PSD-95 in hippocampal neurons can drive maturation of glutamatergic synapses. PSD-95 expression enhanced postsynaptic clustering and activity of glutamate receptors. Postsynaptic expression of PSD-95 also enhanced maturation of the presynaptic terminal. These effects required synaptic clustering of PSD-95 but did not rely on its guanylate kinase domain. PSD-95 expression also increased the number and size of dendritic spines. These results demonstrate that PSD-95 can orchestrate synaptic development and are suggestive of roles for PSD-95 in synapse stabilization and plasticity.
Although the transcriptional basis for states of unresponsiveness in primary T cells is unclear, tolerant B lymphocytes exhibit inhibition of both c-Jun N-terminal kinase induction and IkappaBalpha (inhibitor of NF-kappaBalpha) degradation, leading to lower levels of both nuclear AP-1 and NF-kappaB. Expression of an IkappaBalpha mutant resistant to signal-induced degradation in transgenic T cells caused markedly deficient effector cytokine (IL-4, IFN-gamma) production after primary TCR stimulation despite a detectable level of nuclear NF-kappaB. A TCR response element from the IFN-gamma promoter, despite lacking detectable NF-kappaB/Rel sites, was also unresponsive to TCR ligation. Nuclear induction of AP-1 proteins in response to T cell activation was diminished in transgenic T cells. Costimulation induced by anti-CD28 mAb increased IL-2 production, but failed to reverse the defects in effector cytokine production. Taken together, these data indicate that impaired NF-kappaB/Rel signaling in T cells interferes with the signal transduction pathways required for efficient induction of effector cytokine production.
The functional and structural interactions of two androgen receptor-binding sites in the 5'-flanking DNA of the rat probasin gene were determined. Deletion mapping and DNase I footprinting analysis had previously identified two androgen receptor-binding sites (ARBS) necessary for androgen induction of the probasin gene: ARBS-1, which resembled a glucocorticoid-responsive element, and ARBS-2, which had a unique sequence. In this study, maximal androgen induction in transient transfection studies only occurred when both sites were present. Neither binding site functioned independently, and deletion of the DNA sequence between the sites resulted in a 60% loss of androgen inducibility. Moreover, point mutations in either ARBS-1 or ARBS-2 led to > 90% loss in activity. Scatchard analysis indicated that ARBS-1 and ARBS-2 bound a synthetic androgen receptor, AR2, with Kd values of 20.0 and 6.7 nM, respectively. Consistent with the higher affinity, ARBS-2 bound AR2 at half the threshold concentration (200 ng) of that required in reciprocal DNase I footprinting experiments with ARBS-1. By comparison, protection occurred at a much lower threshold concentration of AR2 (60 ng) and to the same extent over each site when both sites were present, suggesting a cooperative interaction between the two sites. The cooperative effect was further substantiated when a point mutation in ARBS-1 blocked AR2 binding not only to ARBS-1, but also to ARBS-2. Similarly, a point mutation in ARBS-2 also prevented receptor binding to both sites. Androgen-specific regulation of probasin gene transcription therefore required an androgen-responsive region (positions -286 and +28) containing two androgen receptor-binding sites, where the binding of the androgen receptor to both sites occurred in a cooperative, mutually dependent manner.
Selective cell death plays a critical role in the development of the immune repertoire and in the elimination of target cells expressing foreign Ags. The apoptosis induced by ligation of the Fas Ag, a member of the TNFR/nerve growth factor receptor superfamily, contributes to both of these modes of cell loss. However, in spite of the molecular cloning of the Fas Ag and the identification of a specific cytoplasmic domain required for its function, it remains unclear as to which Fas-induced second messengers mediate the development of programmed cell death. We, therefore, evaluated Fas-initiated signal transduction in susceptible cell types. We determined that Fas ligation induces the rapid tyrosine phosphorylation of multiple cellular proteins. These phosphorylation events occur within 1 min and decline toward baseline by 30 min. In addition, Fas ligation increases the in vitro protein kinase activity of the tyrosine phosphorylated proteins. Pharmacologic inhibitors of protein tyrosine kinases block, in a concentration-dependent manner, Fas-induced DNA fragmentation and prolong cell survival. These results suggest that protein tyrosine kinase activation is an early and obligatory signal in Fas-induced apoptosis.