Expression of a constitutive mutant of iron regulatory protein 1 abolishes iron homeostasis in mammalian cells

Iron regulatory proteins (IRPs) are iron-sensing proteins that bind to RNA stem-loop sequences known as iron-responsive elements (IREs) when cells are depleted of iron. Although IRPs have been shown to bind to IREs derived from ferritin and transferrin receptor (TfR) mRNAs in vitro, there has not been a direct demonstration of the impact of a recombinant IRP on the expression of endogenous IRE-containing transcripts. In this study, we evaluate the impact of expression of C437S, a mutant of IRP1 that binds IREs regardless of cellular iron status, on the regulation of biosynthesis of ferritin and TfR. Despite being made iron-replete, cells expressing C437S continue to synthesize and express high amounts of TfR, while the synthesis of ferritin is repressed. Thus, a single mutant IRP can prevent the usual homeostatic changes in ferritin and TfR biosynthesis. Cells expressing the mutant protein would therefore be predicted to be unable to defend against iron overload. Preliminary results show that cells treated with iron have diminished cell survival when C437S is expressed, and we have thus created a tissue culture model system for the study of iron toxicity.