Carbon monoxide (CO) plays a significant role in vascular functions. We here examined the molecular mechanism by which CO regulates HIF-1 (hypoxia-inducible transcription factor-1)-dependent expression of vascular endothelial growth factor (VEGF), which is an important angiogenic factor. We found that astrocytes stimulated with CORM-2 (CO-releasing molecule) promoted angiogenesis by increasing VEGF expression and secretion. CORM-2 also induced HO-1 (hemeoxygenase-1) expression and increased nuclear HIF-1␣ protein level, without altering its promoter activity and mRNA level. VEGF expression was inhibited by treatment with HIF-1␣ siRNA and a hemeoxygenase inhibitor, indicating that CO stimulates VEGF expression via up-regulation of HIF-1␣ protein level, which is partially associated with HO-1 induction. CORM-2 activated the translational regulatory proteins p70S6k and eIF-4E as well as phosphorylating their upstream signal mediators Akt and ERK. These translational signal events and HIF-1␣ protein level were suppressed by inhibitors of phosphatidylinositol 3-kinase (PI3K), MEK, and mTOR, suggesting that the PI3K/Akt/mTOR and MEK/ERK pathways are involved in a translational increase in HIF-1␣. In addition, CORM-2 also increased stability of the HIF-1␣ protein by suppressing its ubiquitination, without altering the proline hydroxylase-dependent HIF-1␣ degradation pathway. CORM-2 increased HIF-1␣/HSP90␣ interaction, which is responsible for HIF-1␣ stabilization, and HSP90-specific inhibitors decreased this interaction, HIF-1␣ protein level, and VEGF expression. Furthermore, HSP90␣ knockdown suppressed CORM-2-induced increases in HIF-1␣ and VEGF protein levels. These results suggest that CO stimulates VEGF production by increasing HIF-1␣ protein level via two distinct mechanisms, translational stimulation and protein stabilization of HIF-1␣.Carbon monoxide (CO) is a diffusible gas that has recently been found to play an important role in several biological processes, including angiogenesis and cytoprotection (1-4). CO is the product of the breakdown of heme by heme oxygenase (HO) 2 enzymes (5). There are two heme oxygenase functional isoforms: an inducible form, HO-1, and a constitutive form, HO-2. CO produced from heme by the catalytic reaction of HO induces the synthesis of angiogenic mediators, such as vascular endothelial growth factor (VEGF), IL-8, and stromal cell-derived factor-1, as well as decreasing the anti-angiogenic factors, namely soluble VEGF receptor-1 and soluble endoglin, resulting in the promotion of endothelial cell proliferation, migration, and anti-apoptotic responses (6 -9). In addition, glutamate-induced CO in astrocytes can act as a signal or regulatory molecule that contributes to the homeostatic regulation of vascular functions such as vasodilation (10, 11).HIF-1 (hypoxia-inducible factor) is a transcriptional complex involved in the regulation of crucial aspects of cellular functions, such as cell proliferation, survival, invasion, and glucose metabolism (12, 13). HIF-1 is composed of two s...