Effect of HAPO on proliferation of endothelial cells. (A) The rhHAPO stimulated the proliferation of human umbilical vein endothelial cells (HUVECs) in a concentration-dependent manner. HUVECs were seeded in 96-well plates at 5 × 10⁴ cells per well in 100 μL RPMI 1640 medium with various concentrations of HAPO (0, 5, 50, 100, and 1000 ng/mL). The viability of HUVECs was analyzed by the MTT method after 72 hours. (B) Effect of rhHAPO on the growth of human fetal bone marrow endothelial cells. Human fetal bone marrow MNCs were seeded in culture system C for 72 hours, and the adherent cells were further incubated in culture system B for 7 days and then detached for FACS analysis. (C) Effect of HAPO on KDR⁺ human fetal bone marrow cells. Purified KDR⁺ cells were plated at a cell density of 1 × 10³/mL in culture system B containing a cocktail of cytokines as described, with or without rhHAPO. All cultures were performed in quadruplicate and scored after 21 days of culture. The rhHAPO significantly stimulated the formation of hematopoietic (CFU-Mix) and endothelial colonies (endothelial progenitor cell CFUs [CFU-EPCs]). Data are represented as the mean ± SD. ^*P < .05 versus control.