Human umbilical cord vein endothelial cells (HUVECs) were obtained as previously described with minor modifications[14 (link)]. Briefly, umbilical cord veins were washed with phosphate buffered saline (PBS) and subjected to collagenase digestion. Resultant HUVECs were seeded onto T-25 culture flasks and maintained in endothelial basal medium EBM-Plus (Lonza, MD). All of the cells used in this study were from passages 1 to 4.