Screening and Development of New Immnomodulatory Drugs using PAF Receptor Gene as a Probe

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Platelet-activating factor (PAF) is a potent mediator of inflammation, immunological disorders, and endotoxin shock. We have recently isolated cDNA clones of human and guinea pig PAF receptors. Using these clones as probe, we have obtained following results.1. Isolation and structure of PAF receptor gene.PAF receptor gene is consisted of three exons which are separated by >20kb introns. All exons are localized on chromosome 1. By alternative splicing, the leukocyte- and heart-type transcripts are produced. They have differentially regulated by distinct promoters.2. Elucidation of signal transduction through a cloned PAF receptor.A stable transfectant cell (CHO) was established carrying a cloned PAF receptor. By the application of PAF, phospholipase C and A_2, and MAP (mitogen-associated protein) are all activated, while adenylate cyclase was inhibited. These cellular events appear to be regulated by different types of G-proteins. Furthermore, we found that endotoxin. LPS, stimulated PAF receptor and yielded a Ca increase in human platelets as well as CHO transfectants. Thus, a part of LPS signaling, especially that related to platelet-aggregation might be due to the direct activation of PAF receptor.3. Development of Screening System of PAF Receptor Agonists and Antagonists.PAF receptor was expressed in Xenopus oocytes and a sensitive assay system of PAF receptor agonists/antagonists was made. Furthermore, COS cells/CHO cells carrying an excess amount of PAF receptors were constructed. By using membrane fractions of these cells, the highly specific screening system of PAF receptor modulator (either receptor antagonists or uncoupler of receptor-G-protein) was established. Through screening of naturally-occurring and synthetic compounds, several candidate chemicals have been obtained, which have potentiality for immunomodulatory drugs.