The pathway of interferon gamma (IFN-gamma-induced suppression in tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated endothelial cell apoptosis was investigated. rTRAIL triggered apoptosis of human umbilical vein endothelial cells (HUVECs) in a type II cell death manner. IFN-gamma pretreatment significantly suppressed the expression of death receptor 4 (DR4) and DR5 on HUVECs, and inhibited apoptosis in response to TRAIL. IFN-gamma rapidly phosphorylated signal transducers and activators of transcription 1 (STAT1) and STAT6 but did not enhance phosphorylation of STAT3, Akt and extracellular signal-regulated kinase (ERK) and nuclear translocation of NF-kappaB p65. Janus kinase (JAK)-induced phosphorylation of STAT1/6 appeared to be crucial since chemical inhibition of JAK abolished phosphorylation of STAT1/6, down-regulation of DR4/DR5 expression and IFN-gamma-induced inhibition of TRAIL-mediated apoptosis. IFN-gamma/JAK/STAT-induced suppression was regulated by cycloheximide (CHX)-sensitive mechanism since the use of CHX mimicked the action of chemical inhibition of JAK in regard to DR4/DR5 expression as well as TRAIL-mediated endothelial cell apoptosis. We have not yet clarified precise mechanism, however, the present data provide a novel finding that IFN-gamma/JAK/STAT pathway elicits inhibition of TRAIL-mediated endothelial cell apoptosis through CHX-sensitive suppression of DR4/DR5.