Tissue hypoxemia is common in several pathological diseases, including vaso-occlusion in sickle cell disease and myocardial infarction. One finds increased presence of leukocytes during lung injury and at sites of inflammation in vascular endothelium. In this study, we used human pulmonary microvascular endothelial cells and human dermal microvascular endothelial immortalized cell line to delineate the cellular signaling mechanism of hypoxia-and CoCl 2 (a mimetic of hypoxia)-induced IL-8 expression, and the latter’s role in chemotaxis of polmorphonuclear neutrophils. We show that hypoxia-and CoCl 2-induced IL-8 mRNA and protein expression involved activation of PI3K/Akt and p38 MAPK, but not MEK kinase. Analysis of some transcription factors associated with IL-8 promoter revealed that hypoxia and CoCl 2 increased DNA-binding activity of hypoxia-inducible factor-1α (HIF-1α), NF-κB, and AP-1. In addition, we show that hypoxia-and CoCl 2-induced IL-8 expression requires activation of HIF as demonstrated by the following: 1) EMSA; 2) transfection studies with IL-8 promoter reporter constructs with mutation in HIF-1α binding site; 3) attenuation of IL-8 expression by both HIF-1α small interfering RNA and R59949; 4) augmentation of IL-8 expression by either transfection with HIF-prolyl hydroxylase-2 small interfering RNA or overexpression of HIF-1α; and 5) chromatin immunoprecipitation analysis. Moreover, conditioned medium from hypoxia-treated endothelial cells augmented chemotaxis of neutrophils, due to release of IL-8. These data indicate that hypoxia-induced signaling in vascular endothelium for transcriptional activation of IL-8 involves PI3K/Akt, p38 MAPK, and HIF-1α. Pharmacological agents, which inhibit HIF-1α, may possibly ameliorate inflammation associated with hypoxia in pathological diseases.