Platelet-activating factor receptor and signal transduction mechanisms

Platelet-activating factor (PAF) is the most potent phospholipid agonist known to date. Radioligand binding studies using [3H]PAF and structurally different PAF antagonists have provided the characteristics of PAF receptor(s) and its heterogeneity. Although efforts have been made to isolate the receptor, it was not until the recent cloning of the PAF receptor that the molecular architecture of the receptor can be visualized. The receptor shows homology to the G protein-coupled receptors with seven transmembrane spanning segments. Several serine, threonine, and tyrosine residues are present at the cytoplasmic side, which could serve as sites for phosphorylation. PAF activates GTPase, causes phospholipid turnover via phospholipases C, D, and A2 pathways and also activates protein kinase C and tyrosine kinase. Further, PAF stimulates Ca2+ mobilization some of which may occur via receptor operated channel. Second messengers generated by these multiple signalling pathways play role (or roles) in PAF responses and in the PAF induced expression of primary response genes. These recent developments throw light on the PAF receptor and its signal transduction mechanisms.