Transcriptional Responses Mediated by Hypoxia-Inducible Factor 1

Hypoxia-inducible factor 1 (HIF-1) is a basic helix-loop-helix protein that activates transcription of hypoxia-inducible genes, including those encoding erythropoietin, vascular endothelial growth factor, heme oxygenase-1, inducible nitric oxide synthase, and the glycolytic enzymes aldolase A, enolase 1, lactate dehydrogenase A, phosphofructokinase L, and phosphoglycerate kinase 1. Hypoxia response elements from these genes consist of a HIF-1-binding site as well as additional DNA sequences that are required for function, which in some elements include a second HIF-1-binding site. HIF-1 is a heterodimer: the HIF-lα subunit is unique to HIF-1, while HIF-1β (ARNT) can dimerize with other proteins. Cotransfection of HIF-1α and HIF-1β (ARNT) expression vectors and a reporter gene containing a wild-type hypoxia response element resulted in increased transcription in nonhypoxic cells and a superinduction of transcription in hypoxic cells, whereas HIF-1 expression vectors had no effect on transcription of reporter genes containing a HIF-1 binding-site mutation. In HeLa cells, HIF-lα and HIF-1β protein levels and HIF-1 DNA-binding activity increased exponentially as oxygen tension decreased, with maximum values at 0.5% oxygen and half-maximal values at 1.5%–2% oxygen. HIF-lα and HIF-1β (ARNT) mRNAs were detected in all human and rodent organs assayed. HIF-lα protein levels were induced in vivo when animals were subjected to anemia or hypoxia.