Hypoxia Up-Regulates Hypoxia-Inducible Factor-1α Transcription by Involving Phosphatidylinositol 3-Kinase and Nuclear Factor κB in Pulmonary Artery Smooth Muscle Cells

The oxygen sensitive α-subunit of the hypoxia-inducible factor-1 (HIF-1) is a major trigger of the cellular response to hypoxia. Although the posttranslational regulation of HIF-1α by hypoxia is well known, its transcriptional regulation by hypoxia is still under debate. We, therefore, investigated the regulation of HIF-1α mRNA in response to hypoxia in pulmonary artery smooth muscle cells. Hypoxia rapidly enhanced HIF-1α mRNA levels and HIF-1α promoter activity. Furthermore, inhibition of the phosphatidylinositol 3-kinase (PI3K)/AKT but not extracellular signal-regulated kinase 1/2 pathway blocked the hypoxia-dependent induction of HIF-1α mRNA and HIF-1α promoter activity, suggesting involvement of a PI3K/AKT-regulated transcription factor. Interestingly, hypoxia also induced nuclear factor-κB (NFκB) nuclear translocation and activity. In line, expression of the NFκB subunits p50 and p65 enhanced HIF-1α mRNA levels, whereas blocking of NFκB by an inhibitor of nuclear factor-κB attenuated HIF-1α mRNA induction by hypoxia. Reporter gene assays revealed the presence of an NFκB site within the HIF-1α promoter, and mutation of this site abolished induction by hypoxia. In line, gel shift analysis and chromatin immunoprecipitation confirmed binding of p50 and p65 NFκB subunits to the HIF-1α promoter under hypoxia. Together, these findings provide a novel mechanism in which hypoxia induces HIF-1α mRNA expression via the PI3K/AKT pathway and activation of NFκB.