Structure and regulation of platelet activating factor receptor gene

Clarithromycin suppresses human respiratory syncytial virus infection-induced Streptococcus pneumoniae adhesion and cytokine production in a pulmonary epithelial cell line

Human respiratory syncytial virus (RSV) sometimes causes acute and severe lower respiratory tract illness in infants and young children. RSV strongly upregulates proinflammatory cytokines and the platelet-activating factor (PAF) receptor, which is a receptor for Streptococcus pneumoniae, in the pulmonary epithelial cell line A549. Clarithromycin (CAM), which is an antimicrobial agent and is also known as an immunomodulator, significantly suppressed RSV-induced production of interleukin-6, interleukin-8, and regulated on activation, normal T-cell expressed and secreted (RANTES). CAM also suppressed RSV-induced PAF receptor expression and adhesion of fluorescein-labeled S. pneumoniae cells to A549 cells. The RSV-induced S. pneumoniae adhesion was thought to be mediated by the host cell's PAF receptor. CAM, which exhibits antimicrobial and immunomodulatory activities, was found in this study to suppress the RSV-induced adhesion of respiratory disease-causing bacteria, S. pneumoniae, to host cells. Thus, CAM might suppress immunological disorders and prevent secondary bacterial infections during RSV infection.

Fosfomycin suppresses RS-virus-induced Streptococcus pneumoniae and Haemophilus influenzae adhesion to respiratory epithelial cells via the platelet-activating factor receptor

Human respiratory syncytial virus (RSV) sometimes causes acute and severe lower respiratory tract illness in infants and young children. The platelet-activating factor (PAF) receptor, which is a receptor for Streptococcus pneumoniae and Haemophilus influenzae, is upregulated by RSV infection in the pulmonary epithelial cell line A549. Fosfomycin, an antimicrobial agent, significantly suppressed PAF receptor induction by RSV infection at the mRNA and cell surface expression levels. Fosfomycin also suppressed RSV-induced adhesion of fluorescence-labeled S. pneumoniae and H. influenzae cells, as determined by flow cytometry and fluorescence microscopy. The RSV-induced bacterial adhesion was suggested to be host-PAF-receptor and bacterial-phosphocholine mediated. Fosfomycin, which has been shown to exhibit antimicrobial and immunomodulatory activities, was found here to suppress adhesion by disease-causing bacteria. Thus, fosfomycin might prevent secondary bacterial infection during RSV infection.

Understanding the regulation mechanisms of PAF receptor by agonists and antagonists: Molecular modeling and molecular dynamics simulation studies

Platelet-activating factor receptor (PAFR) is a member of G-protein coupled receptor (GPCR) superfamily. Understanding the regulation mechanisms of PAFR by its agonists and antagonists at the atomic level is essential for designing PAFR antagonists as drug candidates for treating PAF-mediated diseases. In this study, a 3D model of PAFR was constructed by a hierarchical approach integrating homology modeling, molecular docking and molecular dynamics (MD) simulations. Based on the 3D model, regulation mechanisms of PAFR by agonists and antagonists were investigated via three 8-ns MD simulations on the systems of apo-PAFR, PAFR-PAF and PAFR-GB. The simulations revealed that binding of PAF to PAFR triggers the straightening process of the kinked helix VI, leading to its activated state. In contrast, binding of GB to PAFR locks PAFR in its inactive state.

PAF, a putative mediator of oral inflammation

PAF, or platelet-activating factor, is a family of structurally related phospholipids (1-O-alkyl/acyl/alkenyl-2-acetyl-sn-glycero-3-phosphocholine) which possesses a wide spectrum of potent pro-inflammatory actions. These phospholipids are synthesized by a diverse array of cells, including neutrophilic polymorphonuclear leukocytes (PMN), platelets, mast cells, monocytes/macrophages, vascular endothelial cells, and lymphocytes. PAF targets these and other cells via specific, G-protein-coupled receptors to initiate intracrine, autocrine, paracrine, and juxtacrine cell activation. Of importance, these unique acetylated phospholipids are frequently synthesized in concert with pro-inflammatory lipid mediators derived from arachidonic acid. Since PAF synergizes with these and other mediators to amplify the inflammatory response, it seems likely that PAF plays an integral, perhaps pivotal, role in acute and chronic inflammatory processes. PAF is present in the mixed saliva of dentate, but not edentulous, human subjects. The levels of PAF in mixed saliva or in gingival crevicular fluid and tissues are significantly increased during oral inflammatory conditions such as periodontitis and mucositis. Interestingly, the levels of salivary PAF correlate with the extent/severity of these oral diseases. These observations suggest that PAF may participate in pathophysiologic events during the course of oral inflammation. The availability of specific PAF receptor antagonists and human recombinant PAF-acetylhydrolase (PAF-AH), a plasma enzyme which rapidly destroys PAF, should provide clinical tools for the investigation of the role of PAF in these and other inflammatory disorders; and perhaps, ultimately, some of these reagents may prove to be therapeutically useful in the treatment and management of these conditions.

Oxidized low density lipoproteins downregulate LPS-induced platelet-activating factor receptor expression in human monocyte-derived macrophages: implications for LPS-induced nuclear factor-kappaB binding activity

Monocytes/macrophages play a key role in atherogenesis due to their inflammatory properties including formation of lipid mediators such as platelet-activating-factor (PAF). We investigated the effect of oxidized low-density lipoprotein (oxLDL) on lipopolysaccharide (LPS)-induced PAF receptor (PAF-R) expression in human macrophages and the implication of the nuclear factor (NF)-kappaB in this regulation. LPS-treatment (1 microg.mL(-1)) of macrophages increased PAF binding and PAF-R mRNA expression by 56% (P < 0.05) and twofold (P < 0.01), respectively. In contrast, highly oxidized low-density lipoprotein [ox24hLDL; 100 microg.mL(-1); thiobarbituric acid reacting substances: 31 +/- 3 nmol equiv. malondialdehyde (MDA).mg protein LDL-1] diminished PAF-R expression (-69%; P < 0.05) and mRNA level (- 45%; P < 0.01). LPS pretreatment induced the activated form of p65 in the nuclear compartment of macrophages (detected by Western blotting) and NF-kappaB binding activity (by electrophoretic mobility shift assay). Treatment of macrophages with ox24hLDL suppressed the LPS-induced binding of NF-kappaB to DNA. In addition, treatment of macrophages with lysophosphatidylcholine (2 and 10 microM), a major component of oxLDL, inhibited the LPS-induced NF-kappaB binding to DNA and reduced PAF binding by 30 and 70%, respectively. In conclusion, oxLDL may downregulate PAF-R expression in human macrophages by inhibiting LPS-induced NF-kappaB binding to DNA.

Human intestinal epithelial cells express receptors for platelet-activating factor

The intestinal epithelium produces and responds to cytokines and lipid mediators that play a key role in the induction and regulation of mucosal inflammation. The lipid mediator platelet-activating factor (PAF) can be produced and degraded by the human intestinal epithelium and is known to mediate a range of proinflammatory and other biological effects in the intestinal mucosa. In the studies herein, we assessed whether or not human intestinal epithelial cells express cell surface or intracellular PAF receptors (PAF-R), whether expression of these receptors can be regulated, and whether human intestinal epithelial cells respond to PAF. Several human colon epithelial cell lines (HT-29, Caco-2, T84, HCT-8, HCA-7, I407, and LS-174T) were shown by RT-PCR to constitutively express mRNA for PAF-R. In addition, PAF-R expression was demonstrated by immunoblot analysis and PAF-R was shown to be constitutively expressed on the cell surface of several of these cell lines, as assessed by flow cytometry. PAF-R expression by human colon epithelial cells was upregulated by stimulation with retinoic acid but not by stimulation with PAF, proinflammatory agonists (tumor necrosis factor-alpha, interleukin-1, interferon-gamma), or transforming growth factor-alpha. PAF-R on intestinal epithelial cells were functional, as PAF stimulation of the cells increased tyrosine phosphorylation of several cellular proteins, including proteins of 75 and 125 kDa, and this response was blocked by a PAF-R antagonist. Consistent with the findings using cell lines, PAF-R were also constitutively expressed by normal human colon and small intestinal epithelium in vivo, as shown by immunohistology. The constitutive and regulated expression of functional PAF-R by human intestinal epithelium suggests PAF produced by the intestinal epithelial cells or cells underlying the epithelium has autocrine or paracrine effects on intestinal epithelial cells.