Hypoxia-inducible factor-1 (HIF-1), which consists of oxygen-sensitive HIF-1 alpha and constitutively expressed HIF-1 beta subunits, activates transcription of genes encoding proteins that mediate adaptive responses to reduced oxygen availability. The mouse inhibitory PAS (Per/Arnt/Sim) domain protein (IPAS) functions as a negative regulator in HIF-mediated gene expression. In this report, we cloned the human orthologs of the mouse WAS gene, IPASH1 and IPASH2, to further study the regulatory mechanism of HIF-1 by the WAS proteins. The human WAS proteins inhibited the transactivation function of HIF-1 alpha under hypoxic conditions. In addition, human IPAS proteins blocked the hypoxia-induced VEGF expression and inhibited cell migration and tube formation of human umbilical vein endothelial cells. Interestingly, both HIF-1 alpha and HIF-1 beta interacted with the WAS proteins. Collectively, these results suggest that human WAS proteins inhibit angiogenesis by binding to and inhibiting HIF-1 alpha and HIF-1 beta. (c) 2005 Elsevier Inc. All rights reserved.