<p><b>OBJECTIVE</b>To investigate the activation pattern of signal transducers and activators of transcription (STAT) induced by vascular endothelial growth factor (VEGF) in CD34+ hematopoietic progenitor cells, and gain an insight into the molecular mechanism and signal transduction pathway of VEGF that has an effect on CD34+ hematopoietic progenitor cells.</p><p><b>METHODS</b>After isolated from umbilical cord blood by using a high-gradient magnetically activated cell sorting system (MACS), CD34+ cells were stimulated by VEGF (50 ng/ml) for different time (0, 15, 30, 45, 60, 90 min) to detect the tyrosine phosphorylation and nuclear translocation of STAT-3 and STAT-5 with Western blot and immunocytochemistry methods. The expression of VEGF receptor-2 (VEGFR2) on the membrane of CD34+ progenitor cells was examined by immunocytochemistry. ATWLPPR, an effective peptide screened from phage epitope library by affinity for membrane-expressed VEGFR2 and blocking the binding of VEGF to VEGFR2, was used to determine whether the activation of STAT pathway induced by VEGF was blocked.</p><p><b>RESULTS</b>Tyrosine phosphorylation of STAT-3 and STAT-5 was undetectable in unstimulated CD34+ cells, but was evident at 15 min in response to VEGF stimulation. VEGF resulted in a rapid and transient tyrosine phosphorylation of STAT-3 and STAT-5. The maximal tyrosine phosphorylation was catched at 30 and 45 min, respectively (P = 0.0001), and returned to basal levels at 90 min. Immunocytochemistry confirmed that increased phosphorylated STAT-3 was translocated into the nuclei at 30 min (P = 0.0001), and mainly in cytoplasms again at 90 min after stimulation with VEGF. However, compared with unstimulated CD34+ cells, there was only increased phosphorylation of STAT-5 appeared mainly in cytoplasms, but no significant nuclear translocation was found after stimulation with VEGF (P > 0.05). The presence of VEGFR2 was confirmed using anti-VEGFR2 antibody staining by immunocytochemistry, moreover, the phosphorylation of STAT-3 and STAT-5 failed to be activated by the co-culture with ATWLPPR and VEGF, suggesting that activation of the STAT pathway be specifically mediated by VEGFR2 in CD34+ progenitor cells.</p><p><b>CONCLUSIONS</b>STAT signaling pathway participates in the signal transduction of VEGF via VEGFR2 in CD34+ hemopoietic progenitor cells.</p>