LG3, a C-terminal fragment of perlecan bioactive on vascular cells, has been implicated in aggravation of renal allograft rejection and vascular injury. Recently, we have shown that LG3 is released by apoptotic endothelial cells within apoptotic Exosome-Like vesicles (ApoExo) that in turn induce anti-LG3 production and accelerate rejection. Caspase-3 (Casp-3) activation has been implicated in extracellular export of autophagic vacuoles in endothelial cells. Here, we aim at characterizing further the molecular pathways leading to the export of LG3.

Primary human umbilical vein endothelial cells (EC) were exposed to serum starvation (SS) and H₂O₂, two stimuli mimicking clinically relevant stressors of importance during harvesting, preservation and ischemia-reperfusion of transplanted organs. SS triggers apoptosis while exposure to H₂O₂ induces endothelial necrosis, as evaluated by fluorescence microscopy with Hoescht/Propidium iodide (HO/PI), FACS analysis with annexin V and PI staining and Casp-3 activation. LG3 release was detected in medium conditioned by apoptotic EC whereas medium conditioned by necrotic EC showed reduced levels of LG3. Small particle flow cytometry showed that both necrotic and apoptotic EC secrete extracellular vesicles (EV). While EV purified from necrotic EC showed increased levels of the exosomal markers TCTP and MFGE8, LG3 was specifically enriched in EV purified from apoptotic EC along with the lysosomal marker, LAMP2. Enzymatic activity of the lysosomal protease cathepsin L was present in ApoExo released by apoptotic but not necrotic EC. Pan-caspase inhibition with ZVAD-FMK or EC from Casp-3 -/- mice led to reduced EV release in association with decreased LG3. Consistent with a role for lysosomes in LG3 export, bafilomycin, an inhibitor of lysosome acidification, also inhibited the release of LG3.

These results highlight the central roles of caspase-activation and lysosomal activity in regulating the release of apoptotic Exosome-Like vesicles that differ from classic exosomes by their enrichment in LG3 and lysosomal markers. These results point lysosome activity as a novel target of intervention for preventing the production of immunogenic LG3 at sites of endothelial injury.

CITATION INFORMATION: Beillevaire D, Turgeon J, Boilard E, Dieudé M, Hébert M.-J. Lysosome Ativity Regulates LG3 Export by Apoptotic Cells. Am J Transplant. 2016;16 (suppl 3).

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