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Platelet-activating factor and oxidized phosphatidylcholines do not suppress endotoxin-induced pro-inflammatory signaling among human myeloid and endothelial cells

1 Department of Studies in Biochemistry and Molecular biology, University of Mysore, Manasagangothri, Mysuru-570006, Karnataka, India
2 Department of Cellular and Molecular Medicine (NC10), Cleveland Clinic Lerner Research Institute, 9500 Euclid Avenue, Cleveland, Ohio 44195, USA
3 Division of Allergy and Immunology, University of Utah, 81 N Mario Capecchi Drive, Salt Lake City, Utah 84113, USA
4 Department of Physiology and Pharmacology, Health Science Campus, Block health Science Building, 3000 Transverse Drive, Toledo, Ohio 43614, USA

Platelet-activating factor (PAF) and related phospholipid oxidation products termed oxidized phospholipids (OxPLs) promote inflammation. PAF is made in response to bacterial endotoxin-lipopolysaccharide (LPS) that is recognized by Toll-like receptor-4 (TLR-4) whose activation leads to translocation of transcription factor NF-ΚB to the nucleus—a key regulator of multiple pro-inflammatory genes including COX-2 and IL-8. Paradoxically, PAF and OxPLs are claimed to inhibit LPS-mediated signaling, questioning the very pro-inflammatory roles of PAF and OxPLs and anti-inflammatory nature of PAF-acetylhydrolase (PAF-AH), an enzyme that attenuates both PAF and OxPLs signaling. We investigated the effect of PAF and representative OxPLs: 1-palmitoyl-2-oxovaleroyl-sn-glycero-3-phosphocholine (POVPC), 1-palmitoyl-2-glutaroyl-sn- glycero-3-phosphocholine (PGPC) and 1-alkyl-2-butanoyl-sn-glycero-3-phosphocholine PAF (C4 PAF) on LPS-induced expression of NF-ΚB mediated inflammation in isolated human myeloid cells: polymorphonuclear leukocyte (PMNs), monocytes and human umbilical vein endothelial cells (HUVECs). Using intracellular calcium transients, we show that POVPC and PGPC dose-dependently activate the PAF-receptor (PAF-R) in PMNs, that can beblocked by the PAF-R antagonist WEB-2086 and rPAF-AH pre-treatment. All the three cell types express minute or no detectable COX-2 when stimulated with either PAF (0.1 µM) or OxPLs (0.1 µM) alone. While LPS (100 ng/mL) induced expression of COX-2 in all the cell types, pre-activation of PAF-R with PAF (0.1 µM) or OxPLs (0.1 µM) did not suppress LPS (100 ng/mL)-induced COX-2 expression and in fact we obresved incereased PGE2 levels in an NS-398 sensitive manner. In addition, pre-activation of PAF-R significantly augmented LPS (100 ng/mL)-induced IL-8 production in PMNs. Thus, PAF and OXPLs do not suppress the ability of LPS to exert its pro-inflammatory effects in isolated human vascular cells.
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