HYPOXIA-INDUCIBLE FACTOR-I MEDIATES TRANSCRIPTIONAL ACTIVATION OF THEHEME OXYGENASE-1 GENE IN RESPONSE TO HYPOXIA

Exposure of rats to hypoxia (7% O-2) markedly increased the level of h eme oxygenase-1 (HO-1) mRNA in several tissues. Accumulation of HO-1 t ranscripts was also observed after exposure of rat aortic vascular smo oth muscle (VSM) cells to 1% O-2, and this induction was dependent on gene transcription, Activation of the mouse HO-1 gene by all agents th us far tested is mediated by two 5'-enhancer sequences, SX2 and AB1, b ut neither fragment was responsive to hypoxia in VSM cells. Hypoxia-de pendent induction of the chloramphenicol acetyltransferase (CAT) repor ter gene was mediated by a 163-bp fragment located approximately 9.5 k ilobases upstream of the transcription start site. This fragment conta ins 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 foll owing 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) hypo xia increased HIF-1 alpha and HIF-1 beta protein levels in VSM cells, and 4) hypoxia-dependent HO-1 mRNA accumulation was not observed in mu tant hepatoma cells lacking HIF-1 DNA-binding activity, Taken together , these data demonstrate that hypoxia induces HO-1 expression in anima l tissues and cell cultures and implicate HIF-1 in this response.