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 alpha-class hypoxia inducible f actors (HIF1 alpha, HIF2 alpha, and HIF3 alpha) are basic helix-loop-helix PAS (bHLH-PAS) proteins that heterodimerize with ARNT, In response to 2,3,7 ,8-tetrachlorodibenzo-p-dioxin, the AHR . ARNT complex binds to "dioxin res ponsive enhancers" (DREs) and activates genes involved in the metabolism of xenobiotics, e.g. cytochrome P4501A1 (Cyp1a1), The HIF1 alpha . ARNT compl ex 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 t o competition for ARNT or other limiting cellular factors. Using pathway sp ecific reporters in transient transfection assays, we observed that DRE dri ven transcription was markedly inhibited by hypoxia and that hypoxia respon sive enhancer driven transcription was inhibited by AHR agonists, When we a ttempted to support this cross-talk model using endogenous loci, we observe d that activation of the hypoxia pathway inhibited Cyp1a1 up-regulation, bu t that activation of the AHR actually enhanced the induction of Epo by hypo xia. To explain this unexpected additivity, we examined the Epo gene and fo und that its promoter harbors DREs immediately upstream of its transcriptio nal start site. These experiments outline conditions where inhibitory and a dditive cross-talk occur between the hypoxia and dioxin signal transduction pathways and identify Epo as an AHR-regulated gene.