The role of the cellular redox state in the oxygen dependent expression of the erythropoffitin gene and its prictpal transcription factor HIF-1

Expression of the gene encoding for erythropoietm (Epo) is stimulated by hypoxia through binding of hypoxia mducibte factor 1 (HIF-1) to the oxygen sensitive enhancer of the Epo gene. We have recently proposed a b-type cytochrome as part of the oxygen sensor that is capable of producing hydrogen peroxide as an intraceBular agniflmg molecule. Studies with the Epo-producing hepatoma cell fine HepG2 have revealed that the formation of hydrogen peroxide was strictly dependent on the periceDuUr POj. A high cellular hydrogen peroxide content inhibited Epo production while low levels - as under nypoxic conditions allowed full expression of the Epo gene. Parallel changes were observed in the HIF-lct mRNA content in HepG2 cells. The most likely mode of action of hydrogen peroxide is a change in the cellular redox state. We have therefore investigated the effect of substances that are known to cause a more reduced state of the cells. By this treatment we were able to mimic a hypoxic cellular state under expérimental conditions of a high periceOular PO₂. As a result the expression of HIF-1 a increased as well as the production of Epo. Since the iron chelator desfenioxamme (DSF) has also been widely used to mimic hypoxia and increase Epo production we furthermore studied the role of DSF as a reducing agent. When HepG2 cells were incubated with DSF (130 μM) we observed a complete restoration of Epo production that had been reduced by the addition of hydrogen peroxide. In conclusion, changes in the ceOular redox state seem to be req>onsible for hypoxia induced gene expression of the ubiquiously active transcription factor PDF-1α and, as a consequence, of genes that are under its control, e.g. Epo, VEGF and aldolase A.