Hypoxia-inducible factor 1 levels vary exponentially over a physiologically relevant range of O-2 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-1 alpha and HIF-1 beta subunits in human HeLa cells exposed to O-2 concentrations ranging from 0 to 20% in the absence or presence of 1 mM KCN to inhibit oxidative phosphorylation and cellular O-2 consumption. HIF-1 DNA-binding activity, HIF-1 alpha protein, and HIF-1 beta protein each increased exponentially as cells were subjected to decreasing O-2 concentrations, with a half-maximal response between 1.5 and 2% O-2 and a maximal response at 0.5% O-2, both in the presence and absence of KCN. The HIF-1 response was greatest over O-2 concentra tions associated with ischemic/hypoxic events in vivo. These results provide evidence for the involvement of HIF-1 in O-2 homeostasis and represent a functional characterization of the putative O-2 sensor that initiates hypoxia signal transduction leading to HIF-1 expression.