Fas ligand (FasL), an apoptosis-inducing cytokine, is constitutively expressed on endothelial cells (EC). Here, we report that the soluble form of FasL (sFasL) is released from EC and inhibits hypoxia-induced EC apoptosis. For hypoxia experiments, human EC were exposed to low oxygen tension in airtight chambers flushed with preanalyzed gas mixtures (1% oxygen, 5% CO2, 94% N2) at 37 degrees C. Exposure of cultured EC to hypoxia transiently increased FasL mRNA and protein levels. The maximum increase was observed at 3 and 6 hours after exposure to hypoxia, respectively. Although sFasL protein was not detected in the supernatant from EC without hypoxia, sFasL protein level in the supernatant was transiently increased from 6 hours and disappeared again at 24 hours after the exposure to hypoxia. Interestingly, the supernatant from hypoxia-exposed EC inhibited EC apoptosis induced by hypoxia, which was abolished by a neutralizing antibody against FasL. In addition, incubation with KB8301, an inhibitor of metalloproteinase, suppressed the release of sFasL from EC and enhanced hypoxia-induced apoptosis in EC. Furthermore, exogenously added recombinant sFasL inhibited hypoxia-induced apoptosis. These findings indicate that sFasL released from EC may inhibit hypoxia-induced EC apoptosis. Therefore, the shedding of FasL could be a new therapeutic target in regulating hypoxia-induced EC injury.