Role of H2O2and heme-containing O2 sensors in hypoxic regulation of tyrosine hydroxylase gene expression.

In the current study, we investigated links between O2-regulated H2O2formation and the hypoxic induction of mRNA for tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis, in O2-sensitive PC-12 cells. During exposure of PC-12 cells to 5% O2, H2O2concentration decreased by 40% as measured with 2',7'-dichlorofluorescein (DCF). Treatment with H2O2reduced TH mRNA during normoxia and prevented the induction of TH mRNA during hypoxia. Treatment with catalase or N-(2-mercaptopropionyl)-glycine, a reducing antioxidant agent that decreases H2O2concentration, also induced TH mRNA. Deferoxamine (DF), an iron chelator, failed to affect H2O2formation but induced TH mRNA in normoxia and hypoxia. CoCl2 led to a decrease in H2O2at 20 h of treatment but induced TH mRNA during normoxia and hypoxia before it affected H2O2. In conclusion, TH gene expression correlates inversely with H2O2formation. DF and Co2+ seem to affect TH gene expression in the mechanism downstream from the H2O2formation rather than by interfering with the H2O2-generating activity of the O2 sensor.