INDUCTION OF ENDOTHELIAL PAS DOMAIN PROTEIN-1 BY HYPOXIA - CHARACTERIZATION AND COMPARISON WITH HYPOXIA-INDUCIBLE FACTOR-1-ALPHA

Hypoxia results in adaptive changes in the transcription of a range of genes including erythropoietin. An important mediator is hypoxia-indu cible factor-1 (HIF-1), a DNA binding complex shown to contain at leas t two basic helix-loop-helix PAS-domain (bHLH-PAS) proteins, HIF-1 alp ha and aryl hydrocarbon nuclear receptor translocator (ARNT), In respo nse to hypoxia, HIF-1 alpha is activated and accumulates rapidly in th e cell. Endothelial PAS domain protein 1 (EPAS-1) is a recently identi fied bHLH-PAS protein with 48% identity to HIF-1 alpha, raising the qu estion of its role in responses to hypoxia. We developed specific anti bodies and studied expression and regulation of EPAS-1 mRNA and protei n across a range of human cell lines. EPAS-1 was widely expressed, and strongly induced by hypoxia at the level of protein but not mRNA. Com parison of the effect of a range of activating and inhibitory stimuli showed striking similarities in the EPAS-1 and HIF-1 alpha responses. Although major differences were observed in the abundance of EPAS-1 an d HIF-1 alpha in different cell types, differences in the inducible re sponse were subtle with EPAS-1 protein being slightly more evident in normoxic and mildly hypoxic cells. Functional studies in a mutant cell line (Ka13) expressing neither HIF-1 alpha nor EPAS-1 confirmed that both proteins interact with hypoxically responsive targets, but sugges t target specificity with greater EPAS-1 transactivation (relative to HIF-1 alpha transactivation) of the VEGF promoter than the LDH-A promo ter. (C) 1998 by The American Society of Hematology.