Increased IRP1 and IRP2 RNA binding activity accompanies a reduction of the labile iron pool in HFE-expressing cells

Iron regulatory proteins (IRPs), the cytosolic proteins involved in the maintenance of cellular iron homeostasis, bind to stem loop structures found in the mRNA of key proteins involved iron uptake, storage, and metabolism and regulate the expression of these proteins in response to changes in cellular iron needs. We have shown previously that HFE-expressing fWTHFE/tTA HeLa cells have slightly increased transferrin receptor levels and dramatically reduced ferritin levels when compared to the same clonal cell line without HFE (Gross et al., 1998, J Biol Chem 273:22068-22074). While HFE does not alter transferrin receptor trafficking or non-transferrin mediated iron uptake, it does specifically reduce ⁵⁵Fe uptake from transferrin (Roy et al., 1999, J Biol Chem 274:9022-9028). In this report, we show that IRP RNA binding activity is increased by up to 5-fold in HFE-expressing cells through the activation of both IRP isoforms. Calcein measurements show a 45% decrease in the intracellular labile iron pool in HFE-expressing cells, which is in keeping with the IRP activation. These results all point to the direct effect of the interaction of HFE with transferrin receptor in lowering the intracellular labile iron pool and establishing a new set point for iron regulation within the cell.