IONUT CAZAN, SIDONIA SUSANU, OTILIA BOISTEANU, DIANA TATARCIUC, VICTOR VLAD COSTAN, MARCEL COSTULEANU BIOCHEMICAL EFFECTS OF COLLAGEN-CULTURED MESENCHYMAL STEM CELLS ON ENDOTHELIAL PROGENITOR CELLS IN VITRO We tempted to explore the biochemical effects, represented by apoptosis, triggered by conditioned medium, obtained as a result of culturing mesenchymal stem cells for 24 h, on endothelial progenitor cells. The mesenchymal stem cells were developed on collagen fibers in the presence of thapsigargin, concanavalin A, leptin and ghrelin. Meanwhile, were used the following inhibitors: AG490, a JAK2 inhibitor; PD98059, a MEK/ERK inhibitor; and LY294002, a PI3K/Akt signaling inhibitor. When we analyzed the results, we observed that the conditioned medium, obtained from mesenchymal stem cells cultivation, applied in a 10% concentration for 24 H , induced apoptosis of endothelial progenitor cells in different degrees: 10 µM leptin> ghrelin>>concanavalin A @ apelin. Furthermore, LY294002 and AG490-conditioned medium of mesenchymal stem cells reduced the apoptotic effects induced by leptin on developed endothelial progenitor cells. The very first conclusion is that the inhibition of PI3K/Akt signaling pathway is blocking the apoptotic effects of leptin stimulation of mesenchymal stem cells population. The inhibition of JAK/STAT pathways seems to be less effective.