Expression of angiogenic factors such as vascular endothelial growth factor (VEGF) under conditions of cell stress involves both transcriptional and translational events, as well as an important role for inducible endoplasmic reticulum (ER) chaperones. Coexpression of VEGF and 150-kDa oxygen-regulated protein (ORP), a novel ER chaperone, in human glioblastoma suggested a link between angiogenesis and ORP150. C6 glioma cells stably transfected with ORP150 antisense displayed selectively reduced ORP150 expression. Tumors raised after inoculation of immunocompromised mice with ORP150 antisense C6 glioma transfectants demonstrated an initial phase of growth comparable to wild-type C6 glioma cells which was followed by marked regression within 8 days. Decreased density of platelet/endothelial cell adhesion molecule 1-positive structures within the tumor bed was consistent with reduced angiogenesis in C6 gliomas expressing ORP150 antisense, compared with tumors derived from C6 cells overexpressing ORP150 sense or vector controls. In vitro, inhibition of ORP150 expression decreased release of VEGF into culture supernatants; in ORP150 antisense transfectants, VEGF accumulated intracellularly within the ER. These findings demonstrate a critical role for the inducible ER chaperone ORP150 in tumor-mediated angiogenesis via processing of VEGF, and, thus, highlight a new facet of angiogenic mechanisms amenable to therapeutic manipulation in tumors.