The Journal of immunology : official journal of the American Association of Immunologists
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The peroxisome proliferator-activated receptor gamma (PPAR-gamma) belongs to a receptor superfamily of ligand-activated transcription factors involved in the regulation of metabolism and inflammation. Oral administration of PPAR-gamma agonists ameliorates the clinical course and histopathological features in experimental autoimmune encephalomyelitis, an animal model for multiple sclerosis (MS), and PPAR-gamma agonist treatment of PBMCs from MS patients suppresses PHA-induced cell proliferation and cytokine secretion. These effects are pronounced when cells are preincubated with the PPAR-gamma agonists and reexposed at the time of stimulation, indicating a sensitizing effect. ⋯ Additionally, preincubation decreased NF-kappaB DNA-binding activity to control levels, whereas the inhibitory protein IkappaBalpha was increased. In MS patients, pioglitazone-induced increase in PPAR-gamma DNA-binding activity and decrease in NF-kappaB DNA-binding activity was only observed in the absence of an acute MS relapse. These results suggest that the sensitizing effect observed in the preincubation experiments is mediated by prevention of inflammation-induced suppression of PPAR-gamma expression with consecutive increase in PPAR-gamma DNA-binding activity.
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Comparative Study
Molecular basis of reduced potency of underacylated endotoxins.
Potent TLR4-dependent cell activation by gram-negative bacterial endotoxins depends on sequential endotoxin-protein and protein-protein interactions with LPS-binding protein, CD14, myeloid differentiation protein 2 (MD-2), and TLR4. Previous studies have suggested that reduced agonist potency of underacylated endotoxins (i.e., tetra- or penta- vs hexa-acylated) is determined by post-CD14 interactions. To better define the molecular basis of the differences in agonist potency of endotoxins differing in fatty acid acylation, we compared endotoxins (lipooligosaccharides (LOS)) from hexa-acylated wild-type (wt), penta-acylated mutant msbB meningococcal strains as well as tetra-acylated LOS generated by treatment of wt LOS with the deacylating enzyme, acyloxyacylhydrolase. ⋯ However, msbB LOS:MD-2 and acyloxyacylhydrolase-treated LOS:MD-2 were at least 10-fold less potent in inducing TLR4-dependent cell activation than wt LOS:MD-2 and partially antagonized the action of wt LOS:MD-2. These findings suggest that underacylated endotoxins produce decreased TLR4-dependent cell activation by altering the interaction of the endotoxin:MD-2 complex with TLR4 in a way that reduces receptor activation. Differences in potency among these endotoxin species is determined not by different aggregate properties, but by different properties of monomeric endotoxin:MD-2 complexes.
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Mucus hypersecretion is a prominent manifestation in patients with chronic inflammatory airway diseases and contributes to their morbidity and mortality by plugging airways and causing recurrent infections. Human neutrophil elastase (HNE) exists in high concentrations (1-20 microM) in airway secretions of these patients and induces overproduction of MUC5AC mucin, a major component of airway mucus. Previous studies showed that HNE induces MUC5AC mucin production involving reactive oxygen species (ROS) generation and TGF-alpha-dependent epidermal growth factor receptor (EGFR) activation in human airway epithelial cells. ⋯ These responses to HNE were also reduced by pretreatment with ROS scavengers, implicating ROS. Furthermore, we show that HNE causes protein kinase C (PKC) activation and translocation from cytosol to plasma membrane; blockade of this effect by PKC inhibitors reduced HNE-induced ROS generation and other responses, implicating PKC. We conclude that HNE induces MUC5AC mucin expression via a cascade involving PKC-ROS-TACE in human airway epithelial cells.
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Excessive mucus production is an important pathological feature of asthma. The Th2 cytokines IL-4 and IL-13 have both been implicated in allergen-induced mucus production, inflammation, and airway hyperreactivity. Both of these cytokines use receptors that contain the IL-4Ralpha subunit, and these receptors are expressed on many cell types in the lung. ⋯ Clara cell-specific IL-4Ralpha-deficient mice and control mice developed similar elevations in serum IgE levels, airway inflammatory cell numbers, Th2 cytokine production, and airway reactivity following OVA sensitization and challenge. However, compared with control mice, Clara cell-specific IL-4Ralpha-deficient mice were nearly completely protected from allergen-induced mucus production. Because only IL-13 and IL-4 are thought to signal via IL-4Ralpha, we conclude that direct effects of IL-4 and/or IL-13 on Clara cells are required for allergen-induced mucus production in the airway epithelium.
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Lipoxins (LX) are arachidonic acid metabolites able to induce monocyte chemotaxis in vitro and in vivo. Nonetheless, the signaling pathways mediating this process are yet unclear. In this study, we have investigated the mechanisms associated with human monocyte activation in response to 15-epi-16-(para-fluoro)-phenoxy-LXA4 (ATL-1), a stable 15-epi-LXA4 analog. ⋯ The specific MLCK inhibitor ML-7, as well as Y-27632, abrogated monocyte chemotaxis stimulated by the analog, confirming the central role of the Rho kinase/MLCK pathway on ATL-1 action. Together, these results indicate that ATL-1 acts as a potent monocyte chemoattractant via Rho kinase and MLCK. The present study clarifies some of the mechanisms involved on the activation of monocytes by LXs and opens new avenues for investigation of these checkpoint controllers of inflammation.