Significance of prolyl hydroxylase 2 in the interference of aryl hydrocarbon receptor and hypoxia-inducible factor-1 alpha signaling.

Hypoxia-inducible factor-1 alpha (HIF-1 alpha) and the aryl hydrocarbon receptor (AhR) work as environmental sensors in human tissues. These proteins are members of the helix-loop-helix/Per-ARNT-SIM transcription factor family and form heterodimers with the aryl hydrocarbon receptor nuclear translocator. HIF-1 alpha can be activated by low oxygen concentrations and hypoxia-inducing agents. The AhR is activated by xenobiotica such as dioxins. Here, we analyze the interference between the AhR signaling, activated by 10 nM 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD), and the HIF-1 alpha pathway, induced by hypoxia (5% O2), in two human cell lines, the breast carcinoma cell line MCF-7 and the hepatocyte cell line HepG2. In both cell lines, treatment with TCDD and hypoxia clearly reduced the stabilization of HIF-1 alpha and HRE-mediated promoter activity when compared to the induction under hypoxia alone. Because these effects were not observed after alpha-naphthoflavone treatment and HIF-1 alpha mRNA was not down-regulated, HIF-1 alpha stabilization was revealed to be the target by TCDD in an AhR-depended mechanism. Under exposure to TCDD or hypoxia, the main regulator of HIF-1 alpha stability, the prolyl hydroxylase domain containing protein 2 (PHD2) showed an increase in promoter activity, transcript numbers, and protein amount. Therefore, PHD2 expression is regulated in an AhR-dependent manner under normoxia. The AhR-dependent regulation of PHD2 under normoxia, however, is overwritten by the TCDD-mediated destabilization of HIF-1 alpha. The destabilization of HIF-1 alpha is the dominant effect causing the reduced PHD2 expression after simultaneous exposure to TCDD and hypoxia. We conclude that PHD2 does not mediate the TCDD-mediated HIF-1 alpha destabilization and does not control the interference of AhR and HIF-1 alpha pathways.