Commentary: Building Upon the History of Platelet-activating Factor Research to Garner Further Insights Involves Endogenous Acyl Platelet-activating Factor Inhibitors and Extracellular Vesicle Carriers

As outlined in detail by Demopoulos[1] in this issue of Journal of Atherosclerosis, Prevention and Treatment, Platelet-activating Factor (PAF) has a storied history from a biological activity to the most potent lipid-derived mediator yet described. However, the lack of availability of highly effective PAF synthesis or receptor (PAFR) inhibitors has contributed to our limited understanding of the role of this enigmatic family of lipid mediators in human pathophysiologic conditions. We propose that the future of PAF research lies in two separate but linked areas, namely, the role of sn-1 acyl PAF analogs as endogenous inhibitors and subcellular microvesicle particles as carriers of PAF. Soon after its initial discovery, PAF was noted to be metabolically labile, chiefly through removal of the sn-2 acetate by acetylhydrolases.[2] In addition, 1-acyl PAF analogs were also found to be generated alongside the more potent 1-alkyl species.[3-5] Although initially thought to be lesser active PAF species, several lines of evidence have indicated that these 1-acyl PAF species serve as endogenous inhibitors of PAF. First, pretreatment of PAFR-positive cells with 1-palmitoyl-2-acetyl-glycerophosphocholine (PAPC) blocked subsequent PAF-induced signaling.[4,5] Moreover, treatment of PAPC along with PAF also protected mice from the lethal effects of PAF.[5] More sophisticated studies have recently revealed that 1-acyl PAF and 1-alkyl PAF bind to the same pocket on the PAFR, yet 1-acyl PAF results in structural instability and impairs G protein signaling.[6] Hence, 1-acyl PAF species are endogenous PAFR inhibitors, and the systems in which these lipids are generated are protected from potentially harmful PAFR signaling. That the PAF system would have an endogenous inhibitor should be of no surprise, as other potent systems, such as interleukin-1 and tumor necrosis factor, also have similar regulatory mechanisms. The important lesson derived from these studies is that there is a need to define the exact levels of 1-acyl versus 1-alkyl PAF species generated in a system to assess the importance of PAFR signaling. An example of this importance comes from our group’s recent studies assessing the role of the PAF system in the downstream systemic pathologies of systemic immunosuppression and multiple organ dysfunction (MOD) associated with thermal burn injury (TBI).[7,8] Previous studies have revealed that, unlike stimuli such as an ionophore or cold injury, which generated equal amounts of PAF and PAPC in keratinocyte cells, experimental TBI generated predominantly PAF with very little 1-acyl species.[4,9,10] Thus, TBI would be expected to generate greater PAFR activity due to the lack of the concomitant production of the inhibitory 1-acyl PAF species. Indeed, our recent preclinical studies have revealed that both the immediate MOD and delayed systemic immunosuppression in response to murine TBI were PAFR dependent.[7,8] A second related area of PAF research involves how this highly potent yet metabolically labile lipid can travel systemically. Using environmental injury models involving clinically relevant environmental stressors such as ultraviolet B radiation and TBI that act upon the skin yet generate PAFR-dependent systemic effects, we have demonstrated that subcellular microvesicle particles (size ~100–1000 nm; large extracellular vesicles; MVP) are the effectors.[7,8,11,12] Our model, based upon accumulating evidence[12] is that these agents act upon the skin keratinocyte which generates PAF, which then acting upon the PAFR, then not only generates more PAF, it translocates the MVP-generating enzyme acid sphingomyelinase (aSMase) to the plasma membrane, where it causes MVP to be formed and released from the cell. The MVP formed by the plasma membrane then carries the PAF species systemically. We hypothesize that PAF traveling in an MVP is protected from degradation. In this model, PAF is allowed to travel systemically where it can interact with the mast cell PAFR resulting in systemic immunosuppression, and the intestinal PAFR, causing bacterial translocation from the gut with resultant MOD.[12] The model of PAF traveling via MVP has translational consequences, as MVP generation can be blocked by aSMase inhibitors.[7-9] There are multiple compounds currently on the market that act as potent aSMase inhibitors, including tricyclic antidepressants (imipramine and amitriptyline) and selective serotonin reuptake inhibitors (sertraline and fluoxetine).[13] These drugs appear to function as weak bases by increasing lysosomal pH levels which detach aSMase from the membranes to enhance the degradation of this MVP-generating enzyme. The future of PAF research will involve both testing the system for the presence of endogenous inhibitory 1-acyl PAF species as well as the potential use of these agents as therapeutics. The potential role of MVP as carriers in systems in which PAF generation takes place away from PAF effects, such as is the case for environmental stressors acting upon skin with systemic effects (e.g. TBI), should be explored. These directions can provide an enhanced understanding and therapeutic targets which will continue and expand the legacy of the pioneers in this field.[1]