Nitric Oxide Upregulates Ferritin Synthesis Independently of Iron Regulatory Protein-Iron Responsive Element Binding Activity.

The remarkable post-transcriptional regulation of intracellular iron homeostasis by iron regulatory proteins 1 and 2 (IRP1 and IRP2) is touted as a classic example of translational control. During low levels of intracellular iron, IRPs bind to specific nucleotide sequences known as iron responsive elements (IRE) present in the 5′-untranslated region (UTR) of ferritin (Ft) mRNA and the 3′UTR of transferrin receptor (TfR) mRNA. Such binding during conditions of low intracellular iron, blocks the translation of Ft, the ubiquitous and major iron storing protein, while TfR mRNA is stabilized, leading to increased TfR synthesis and iron uptake. High levels of intracellular iron will abrogate IRP-IRE binding activity, de-repressing Ft translation on the one hand, while causing TfR mRNA instability on the other. This system ensures that potentially hazardous iron is safely stored when it is in abundance and acquired when its levels are low. Nitric oxide (NO), a gaseous molecule involved in a myriad of functions, is known to affect the IRP system in many ways. The NO+ redox form of nitric oxide is involved in the reversible post-translational regulation of numerous proteins (including IRP2), a process called S-nitrosylation. We previously reported that NO+ causes upregulation of Ft synthesis along with IRP2 degradation. Here we report that sodium nitroprusside (SNP), an NO+ donor, causes a dramatic and rapid increase of ferritin synthesis that for a time occurs without interfering with IRP-IRE binding activities. We also show that NO+ increases the translational efficiency of ferritin mRNA as compared to treatment with ferric ammonium citrate, an iron donor. In addition, we also report that SNP does not function as an iron donor. These results indicate that NO+-mediated upregulation of ferritin synthesis is, in part, due to an IRP-independent post-transcriptional mechanism of regulation.