Hypoxia-inducible factor 1 levels vary exponentially over a physiologically relevant range of O₂ tension

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric basic helix-loop- helix protein implicated in the transcriptional activation of genes encoding erythropoietin, glycolytic enzymes, and vascular endothelial growth factor in hypoxic mammalian cells. In this study, we have quantitated HIF-1 DNA- binding activity and protein levels of the HIF-1α and HIF-1β subunits in human HeLa cells exposed to O₂ concentrations ranging from 0 to 20% in the absence or presence of 1 mM KCN to inhibit oxidative phosphorylation and cellular O₂ consumption. HIF-1 DNA-binding activity, HIF-1α protein, and HIF-1β protein each increased exponentially as cells were subjected to decreasing O₂ concentrations, with a half-maximal response between 1.5 and 2% O₂ and a maximal response at 0.5% O₂, both in the presence and absence of KCN. The HIF-1 response was greatest over O₂ concentrations associated with ischemic/hypoxic events in vivo. These results provide evidence for the involvement of HIF-1 in O₂ homeostasis and represent a functional characterization of the putative O₂ sensor that initiates hypoxia signal transduction leading to HIF-1 expression.