Mast cell-derived exosomes activate endothelial cells to secrete plasminogen activator inhibitor type 1

Objective - Previous studies supported the contribution of exosomes to an acellular mode of communication, leading to intercellular transfer of molecules. In this study we provide evidence that mast cell- derived exosomes induce plasminogen activator inhibitor type 1 ( PAI- 1) expression in endothelial cells, detectable at the level of PAI- 1 mRNA and protein synthesis. The stimulating effect was also measured at the level of PAI- 1 promoter activity. Methods and Results - To identify components responsible for this activity, exosome proteins were separated by 2- dimensional PAGE, and protein spots were identified by microsequencing using electrospray ( ISI- Q- TOF- Micromass) spectrometer. Components of 3 independent systems that can be involved in activation of endothelial cells, namely the prothrombinase complex, tumor necrosis factor-alpha, and angiotensinogen precursors were identified. Procoagulant activity of exosomes was confirmed by a thrombin generation assay using a specific chromogenic substrate. Because the potential of mast cell- derived exosomes to induce PAI- 1 expression was completely abolished by hirudin, thrombin generated on exosomes seems to be responsible for this activity. Conclusions - It can be concluded that mast cell- derived exosomes via significant upregulation of PAI- 1 secretion from endothelial cells may provide feedback between the characteristically increased PAI- 1 levels and procoagulant states, both observed in diverse syndromes manifesting as endothelial cell dysfunction.