Transferrin receptor induction by hypoxia - HIF-1-mediated transcriptionalactivation and cell-specific post-transcriptional regulation

The tight relationship between oxygen and iron prompted us to investigate w hether the expression of transferrin receptor (TfR), which mediates cellula r iron uptake, is regulated by hypoxia. In Hep3B human hepatoma cells incub ated in 1% O-2 or treated with CoCl2, which mimics hypoxia, we detected a 3 -fold increase of TfR mRNA despite a decrease of iron regulatory proteins a ctivity. Increased expression resulted from a 4-fold stimulation of the nuc lear transcription rate of the TfR gene by both hypoxia and CoCl2. A role f or hypoxia-inducible factor (HIF-1), which activates transcription by bindi ng to hypoxia-responsive elements in the activation of TfR, stems from the following observations. (a) Hypoxia and CoCl2-dependent expression of lucif erase reporter gene in transiently transfected Hep3B cells was mediated by a fragment of the human TfR promoter containing a putative hypoxia-responsi ve element sequence, (b) mutation of this sequence prevented hypoxic stimul ation of luciferase activity, (c) binding to this sequence of HIF-1 alpha, identified by competition experiments and supershift assays, was induced in Hep3B cells by hypoxia and CoCl2. In erythroid K562 cells, the same treatm ents did not affect iron regulatory proteins activity, thus resulting in a stimulation of TfR gene expression higher than in hepatoma cells.