Bone marrow (BM)-derived endothelial progenitor cells (EPCs) play a critical role in tumor vasculogenesis because they provide both instructive (release of pro-angiogenic cytokines, such as VEGF) and structural (vessel incorporation and stabilization) functions. Celastrol, derived from Trypterygium wilfordii Hook F., a traditional Chinese medicine plant, has been studied for its antitumorigenic properties, but its mechanism of action is not well understood. The aims of this study are to investigate the effects of Celastrol on VEGF-induced functional activity of BM-EPCs and to identify any mechanisms associated with this process. Here, we show that Celastrol attenuates VEGF secretion in BM-EPCs in vitro. This attenuation, in turn, inhibits the in vitro VEGF-induced cell viability, cell-cell adhesion, cell-ECM adhesion, migration response and vascular tube formation of BM-EPCs. Additionally, Celastrol inhibits the phosphorylation of VEGFR2, endothelial nitric oxide synthase (eNOS), and Akt to attenuate cell functions. Taken together, the present study demonstrates that Celastrol decreases Akt/eNOS signaling in BM-EPCs in vitro. These findings identify novel mechanisms that regulate EPC function and may provide new insights for the medicinal use of Celastrol. (C) 2012 Elsevier Inc. All rights reserved.