Although interleukin (IL)-33, a member of the IL-1 cytokine family, plays pro-inflammatory roles in immune cells as an "alarmin", little is known regarding the biological actions of IL-33 on vascular endothelial cells. To investigate the effects of IL-33 on vascular endothelial cells, we first screened the IL-33-regulated proteins in human umbilical vein endothelial cells (HUVECs) using a dot blot array, and observed that IL-33 markedly increased growth-regulated oncogene (GRO)-α, a chemokine that is also known as CXCL1. Real time reverse transcription-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA) demonstrated that IL-33 induced the GRO-α expression and secretion in HUVECs in a dose- and a time-dependent manner. Western immunoblot assay revealed that IL-33 activated the phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 and c-Jun N-terminal kinase (JNK). In addition, translocation of nuclear factor-kappa B (NF-B) p65 to the nucleus of HUVECs was observed by IL-33 stimulation. Further, treatment with pharmacological inhibitors against ERK1/2 (PD98059), JNK (SP600125), or NF-B (BAY11-7085) significantly suppressed IL-33-induced GRO-α gene expression and secretion from HUVECs. Moreover, immunohistochemical staining demonstrated that IL-33 and GRO-α co-expressed in the endothelium of human carotid atherosclerotic plaque. Taken together, the present study indicates that IL-33 localized in the human atherosclerotic plaque increases GRO-α mRNA expression and protein secretion via activation of ERK1/2, JNK, and NF-B in HUVECs, suggesting that IL-33 plays an important role in the pathophysiology and development of atherosclerosis.