Thermal Burn Injury Generates Bioactive Microvesicles: Evidence for a Novel Transport Mechanism for the Lipid Mediator Platelet-Activating Factor (PAF) That Involves Subcellular Particles and the PAF Receptor

Abstract

Thermal burn injuries are an important environmental stressor that can result in considerable morbidity and mortality. The exact mechanism by which an environmental stimulus to skin results in local and systemic effects is an area of active research. One potential mechanism to allow skin keratinocytes to disperse bioactive substances is via microvesicle particles, which are subcellular bodies released directly from cellular membranes. Our previous studies have indicated that thermal burn injury of the skin keratinocyte in vitro results in the production of the lipid mediator platelet-activating factor (PAF). The present studies demonstrate that thermal burn injury to keratinocytes in vitro and human skin explants ex vivo, and mice in vivo generate microvesicle particles. Use of pharmacologic and genetic tools indicates that the optimal release of microvesicles is dependent upon the PAF receptor. Of note, burn injury-stimulated microvesicle particles do not carry appreciable protein cytokines yet contain high levels of PAF. These studies describe a novel mechanism involving microvesicle particles by which a metabolically labile bioactive lipid can travel from cells in response to environmental stimuli.

Platelet-Activating Factor-Receptor Signaling Mediates Targeted Therapies-Induced Microvesicle Particles Release in Lung Cancer Cells

“…The biogenesis of MVP involves activation of the aSMase pathway, and previous reports including ours have shown that the inhibition of aSMase blocks MVP release regardless of the stimuli used [ 34 , 35 , 36 , 45 , 46 , 47 ]. To that end, our next studies tested the effect of a specific pharmacologic inhibitor of aSMase (i.e., imipramine) on targeted therapies-induced MVP release.…”

“…Given that gefitinib and erlotinib generate ROS in a process blocked by antioxidant and PAFR antagonist ( Figure 1 ), and our previous reports demonstrating that pro-oxidative stressors induce MVP release in a PAFR-dependent manner [ 34 , 35 , 36 ], we tested our working hypothesis if these targeted therapies can induce MVP release in lung cancer cells. It should be noted that depending upon the nature of the stimuli or the cell types, the secretion of MVPs has been shown to be time and/or concentration-dependent [ 44 ].…”

“…It should be noted that depending upon the nature of the stimuli or the cell types, the secretion of MVPs has been shown to be time and/or concentration-dependent [ 44 ]. Thus, our first studies evaluated the time-dependent response of gefitinib (used as a drug model) on MVP release from the A549 cell line using CPAF and PMA as positive controls, as per our previous reports [ 34 , 35 , 36 ]. We observed that gefitinib induces MVP release in a time-dependent manner that significantly peaks between 4 to 8 h, yet no differences in MVP release were noted between these time points ( Figure 2 A).…”

“…However, the role of the PAFR signaling in modulating targeted therapy effects in lung cancer models has not been studied. Given that these PAF agonists are metabolically labile and upon generation get readily metabolized by PAF-AH, our recent studies have defined the critical roles of a subpopulation of extracellular vesicles known as microvesicles or microvesicle particles as a novel mechanism by which these potent lipids are not only protected but circulated to exert local as well as delayed systemic effects [ 34 , 35 , 36 ].…”

“…Extracellular vesicles (EVs) are released from a wide variety of cell types and serve to mediate cell-to-cell communications, both in physiological and pathological conditions. These EVs mainly comprise of three types, which differ particularly in their sizes, for example, exosomes (size ~30–100 nm), microvesicle particles (MVP, size ~100–1000 nm), and apoptotic bodies (size >1000 nm) [ 35 , 36 ], their contents, and mechanism of formation. In as much as PAFR signaling plays critical roles ranging from acute pro-inflammatory to delayed systemic immunosuppression and augmentation of tumor growth as well as limiting the efficacy of therapeutic agents [ 29 , 30 , 31 , 32 , 33 ], the current studies were sought to determine its relevance in targeted therapies-mediated MVP release and define the underlying mechanisms in lung cancer cells.…”