Cross-talk between the aryl hydrocarbon receptor and hypoxia inducible factor signaling pathways: Demonstration of competition and compensation

The aryl hydrocarbon receptor (AHR) and the α-class hypoxia inducible factors (HIF1α, HIF2α, and HIF3α) are basic helix-loop-helix PAS (bHLH- PAS) proteins that heterodimerize with ARNT. In response to 2,3,7,8-tetra- chlorodibenzo-p-dioxin, the AHR · ARNT complex binds to 'dioxin responsive enhancers' (DREs) and activates genes involved in the metabolism of xenobiotics, e.g. cytochrome P4501A1 (Cyp1a1). The HIF1 · ARNT complex binds to 'hypoxia responsive enhancers' and activates the transcription of genes that regulate adaptation to low oxygen, e.g. erythropoietin (Epo). We postulated that activation of one pathway would inhibit the other due to competition for ARNT or other limiting cellular factors. Using pathway specific reporters in transient transfection assays, we observed that DRE driven transcription was markedly inhibited by hypoxia and that hypoxia responsive enhancer driven transcription was inhibited by AHR agonists. When we attempted to support this cross-talk model using endogenous loci, we observed that activation of the hypoxia pathway inhibited Cyp1a1 up- regulation, but that activation of the AHR actually enhanced the induction of Epo by hypoxia. To explain this unexpected additivity, we examined the Epo gene and found that its promoter harbors DREs immediately upstream of its transcriptional start site. These experiments outline conditions where inhibitory and additive cross-talk occur between the hypoxia and dioxin signal transduction pathways and identify Epo as an AHR-regulated gene.