OXYGEN-REGULATED AND DIOXIN-REGULATED GENE-EXPRESSION IN MOUSE HEPATOMA-CELLS

The discovery that the oxygen-regulated transcription factor HIF-1 alp ha and the dioxin receptor AhR share the common helerodimerization par tner ARNT (HIF-1 beta) raised the question whether a cross-talk betwee n oxygen and dioxin signal transduction pathways exists. To answer thi s question we investigated an ARNT-deficient mutant cell line (Hepa1C4 ), which has lost its capability of responding to dioxin. The results demonstrate that the presence of ARNT is indispensable for hypoxia-ind ucible HIF-1 DNA binding as well as for oxygen-regulated reporter gene activity mediated by the EPO 3' hypoxia response element (HRE). Hypox ic induction of the vascular endothelial growth factor (VEGF) gene, ho wever, was only partially abrogated in Hepa1C4 cells, suggesting that HIF-1-independent oxygen signaling pathways might exist. We further st udied HIF-1 and AhR/ARNT DNA binding activity as well as the regulatio n of oxygen- and xenobiotic-responsive genes by treating mouse Hepa1 h epatoma cells with hypoxia and/or the dioxin analogue ICZ. Hypoxia-ind ucible VEGF expression was found to be independent of ICZ-treatment, w hereas ICZ-inducible cytochrome P-450IA1 expression was slightly reduc ed by hypoxic treatment of the cells. Interestingly; the enhancer func tion of a xenobiotic response element (XRE) linked to a reporter gene was induced by hypoxia, but expression of a HRE-containing reporter ge ne was not affected by ICZ treatment.