Endothelial cells rapidly respond to changes in oxygen homeostasis by regulating gene expression. Regulator of G protein signaling 5 (RGS5) is a negative regulator of G protein-mediated signaling that is strongly expressed in vessels during angiogenesis; however, the role of RGS5 in hypoxia has not been fully understood. Under hypoxic conditions, we found that the expression of RGS5, but not other RGS, was induced in human umbilical vein endothelial cells (HUVEC). RGS5 mRNA was increased when HUVEC were incubated with chemicals that stabilized hypoxiainducible factor-1␣ (HIF-1␣), whereas hypoxia-stimulated RGS5 promoter activity was absent in HIF-1␤؊/؊ cells. Vascular endothelial growth factor (VEGF), which is regulated by HIF-1, did not appear to be involved in hypoxia-induced RGS5 expression; however, VEGF-mediated activation of p38 but not ERK1/2 was increased by RGS5. Overexpression of RGS5 in HUVEC exhibited a reduced growth rate without affecting the cell proliferation. Annexin V assay revealed that RGS5 induced apoptosis with significantly increased activation of caspase-3 and the Bax/ Bcl-2 ratio. Small interfering RNA-specific for RGS5, caspase-3 inhibitor, and p38 inhibitor resulted in an attenuation of RGS5-stimulated apoptosis. Matrigel assay proved that RGS5 significantly impaired the angiogenic effect of VEGF and stimulated apoptosis in vivo. We concluded that RGS5 is a novel HIF-1-dependent, hypoxia-induced gene that is involved in the induction of endothelial apoptosis. Moreover, RGS5 antagonizes the angiogenic effect of VEGF by increasing the activation of p38 signaling, suggesting that RGS5 could be an important target for apoptotic therapy.Hypoxia is a common pathophysiological phenomenon that has a profound impact on endothelial cell properties during many cardiovascular disease processes and tumorigenesis. It has been extensively suggested that cell response to hypoxia is mainly regulated by hypoxia inducible factor-1␣ (HIF-1␣) 2 , which is rapidly ubiquitinated and degraded in normal conditions but can be stabilized by hypoxia (1, 2). Under hypoxic conditions, HIF-1 activates diverse genes involved in both cell growth and cell death (3). In endothelial cells, hypoxia stimulates the secretion of vascular endothelial growth factor (VEGF) and other angiogenic factors and receptors through transcriptional regulation by HIF-1, which leads to neovascularization and protection against ischemic injury (4 -7). Conversely, HIF-1 has been shown to be a factor mediating hypoxia-induced apoptosis, growth of tumors with HIF-1␣ Ϫ/Ϫ was not retarded but accelerated because of decreased hypoxia-induced apoptosis (3).Regulator of G protein signaling (RGS) proteins are responsible for the rapid turnoff of G protein-coupled receptor signaling pathways via the GTPase-stimulating protein activity of their RGS domain (8, 9). More than 25 RGS proteins have been identified, and there are indications that each will specifically regulate a particular G protein-coupled receptor pathway (10, 11). RGS5 belongs to the R4...