The molecular pharmacology of a human platelet-activating factor receptor

Abstract

Platelet-activating factor (PAF) is a broadly bioactive family of phospholipids which contribute to the pathogenesis of numerous diseases as well as to many normal physiologic processes. The pleiotropic nature of PAFs actions may be due to the activation of several intracellular signaling pathways or the presence of PAF receptor subtypes. Therefore, to begin to understand the complex mechanisms by which PAF molecules induce cellular responses, a molecular approach was initiated to provide tools to investigate many of the issues surrounding PAF receptors. Using a strategy based upon homology cross hybridization, a coding sequence homologous to that of the guinea pig PAF receptor cDNA was identified in a 20 kb insert obtained from human genomic DNA. A portion of the insert was sequenced and appears to be the human homolog of the cloned guinea pig receptor. Although the sequence identity shows that the gene for the human PAF receptor does not contain introns in the coding region, the 5'-untranslated sequence deviates from previously reported cDNA sequences suggesting that at least one intron is present in the untranslated region and represents evidence for alternative mRNA splicing. The 20 kb human genomic fragment also allowed for regional mapping of the PAF receptor gene by fluorescence in situ hybridization and found to localize to chromosome 1 (1p35-> p34.3). Specific localization of the PAF receptor gene to the distal portion of chromosome 1 may assist in understanding the genetic predisposition of certain patients to inflammatory diseases. To examine the second messenger coupling of the cloned PAF receptor and adenylyl cyclase, a cAMP-responsive reporter gene has been used in transiently transfected human choriocarcinoma cells. Preliminary data suggests that PAF receptor signal transduction does result in inhibition of basal and agonist-stimulated adenylyl cyclase activity. PAF receptor specific antibodies could assist in tissue localization of the cloned PAF receptor as well as provide evidence for PAF receptor subtypes. Antibodies were produced against fusion protein consisting of glutathione-S-transferase and a peptide from the purported 2nd extracellular region of the cloned PAF receptor which recognized the native protein in transfected COS-7 cells.