During the past two decades, studies describing the chemistry and biology of PAF have been extensive. This potent phosphoacylglycerol exhibits a wide variety of physiological and pathophysiological effects in various cells and tissues. PAF acts, through specific receptors and a variety of signal transduction systems, to elicit diverse biochemical responses. Several important future directions can be enumerated for the characterization of PAF receptors and their attendant signalling mechanisms. The recent cloning and sequence analysis of the gene for the PAF receptor will allow a number of important experimental approaches for characterizing the structure and analysing the function of the various domains of the receptor. Using molecular genetic and immunological technologies, questions relating to whether there is receptor heterogeneity, the precise mechanism(s) for the regulation of the PAF receptor, and the molecular details of the signalling mechanisms in which the PAF receptor is involved can be explored. Another area of major significance is the examination of the relationship between the signalling response(s) evoked by PAF binding to its receptor and signalling mechanisms activated by a myriad of other mediators, cytokines and growth factors. A very exciting recent development in which PAF receptors undoubtedly play a role is in the regulation of the function of various cellular adhesion molecules. Finally, there remain many incompletely characterized physiological and pathophysiological situations in which PAF and its receptor play a crucial signalling role. Our laboratory has been active in the elucidation of several tissue responses in which PAF exhibits major autocoid signalling responses, e.g. hepatic injury and inflammation, acute and chronic pancreatitis, and cerebral stimulation and/or trauma. As new experimental strategies are developed for characterizing the fine structure of the molecular mechanisms involved in tissue injury and inflammation, the essential role of PAF as a primary signalling molecule will be affirmed. Doubtless the next 20 years of experimental activity will be even more interesting and productive than the past two decades.

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Platelet-activating factor: receptors and signal transduction

Cultured human endothelial cells synthesize prostacyclin (PGI2), a potent inhibitor of platelet function, when stimulated with histamine, bradykinin, or ATP. Paradoxically, we report that these agonists also induced the rapid and sustained synthesis of platelet-activating factor (PAF) by endothelial cells. In fact, the synthesis of this potent activator of platelets and neutrophils was induced by stimulation of the same receptor subtype that induced PGI2 synthesis: stimulation of a histamine H1 or a bradykinin B2 receptor induced both PAF and PGI2 synthesis. However, two physiologically important differences exist between the production of PAF and PGI2 by endothelial cells. The synthesis of PGI2 proceeded for only 7.5 min before the abrupt termination of synthesis, whereas the synthesis of PAF was clearly detectable even 45 min after stimulation. Although maximal accumulation of PAF occurred after 10-15 min of stimulation, the prolonged synthesis resulted in the presence of PAF for up to 1 h after stimulation. Secondly, whereas PGI2 was released from the cell monolayer, PAF remained cell-associated without significant release to the external medium. Endothelial cell-generated PAF, therefore, does not function as a hormone. The prolonged association of this potent activator of platelets and neutrophils with endothelial cells may mediate some of the inflammatory properties of histamine and bradykinin. It may also be a factor in the formation of a thrombogenic vascular surface, an event suggested to play a primary role in the pathogenesis of thrombosis and atherosclerosis.

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Cultured endothelial cells synthesize both platelet-activating factor and prostacyclin in response to histamine, bradykinin, and adenosine triphosphate.

Lawrence W. Byers

Papers

Antihypertensive activity of an alkyl ether analog of phosphatidylcholine.