Transferrin receptor gene expression and transferrin-bound iron uptake are increased during postischemic rat liver reperfusion.

Iron-catalyzed production of reactive oxygen species is a cause of liver injury after ischemia/reperfusion (I/R). The aim of the present study was to address the regulation of transferrin receptor (TfR), which mediates cellular iron uptake, during I/R. The molecular mechanisms controlling TfR gene expression in vivo during I/R of rat liver were investigated by molecular biology procedures. We also analyzed transferrin-bound iron uptake into surviving liver slices. Increased amounts of TfR protein and messenger RNA (mRNA) were found 2 to 6 hours after reestablishment of blood supply. RNA bandshift analysis showed that iron regulatory protein (IRP) activity was decreased in the first hours of reperfusion, thus indicating that IRP-mediated mRNA stabilization was not involved in early TfR upregulation. On the contrary, increased transcription of the TfR gene in isolated nuclei was observed during reperfusion; during the ischemic phase this was preceded by enhanced binding of hypoxia inducible factor (HIF-1) to a DNA sequence derived from the TfR promoter. TfR2 mRNA levels were also enhanced after reperfusion. The increased expression of TfR at the cell surface resulted in increased uptake of transferrin-bound-iron into surviving liver slices; however, iron was not incorporated into ferritin. In conclusion, HIF-1 mediated activation of TfR gene transcription and IRP-mediated increase of TfR mRNA stability ensure a steady induction of TfR, and hence higher iron uptake in reperfused rat liver. TfR-mediated entry of the metal into liver cells may represent a source of catalytically active iron, which may play a role in reperfusion damage.