Hypoxia-inducible Factor-1 Mediates Transcriptional Activation of the Heme Oxygenase-1 Gene in Response to Hypoxia

Exposure of rats to hypoxia (7% O2) markedly increased the level of heme oxygenase-1 (HO-1) mRNA in several tissues. Accumulation of HO-1 transcripts was also observed after exposure of rat aortic vascular smooth muscle (VSM) cells to 1% O2, and this induction was dependent on gene transcription. Activation of the mouse HO-1 gene by all agents thus far tested is mediated by two 5'-enhancer sequences, SX2 and AB1, but neither fragment was responsive to hypoxia in VSM cells. Hypoxia-dependent induction of the chloramphenicol acetyltransferase (CAT) reporter gene was mediated by a 163-bp fragment located approximately 9.5 kilobases upstream of the transcription start site. This fragment contains two potential binding sites for hypoxia-inducible factor 1 (HIF-1). A role for HIF-1 in HO-1 gene regulation was established by the following observations: 1) HIF-1 specifically bound to an oligonucleotide spanning these sequences, 2) mutation of these sequences abolished HIF-1 binding and hypoxia-dependent gene activation in VSM cells, 3) hypoxia increased HIF-1alpha and HIF-1beta protein levels in VSM cells, and 4) hypoxia-dependent HO-1 mRNA accumulation was not observed in mutant hepatoma cells lacking HIF-1 DNA-binding activity. Taken together, these data demonstrate that hypoxia induces HO-1 expression in animal tissues and cell cultures and implicate HIF-1 in this response.