The effect of intracellular iron concentration and nitrogen monoxide on Nramp2 expression and non-transferrin-bound iron uptake

Recent studies have demonstrated that the protein product (natural resistan ce associated macrophage protein 2, Nramp2) encoded by the gene Nramp2 acts as an Fe transporter involved in the uptake of Fe from transferrin (Tf) an d low M-r Fe complexes. Interestingly, there are two splice variants of Nra mp2, one with a putative iron-responsive element (IRE) in its 3' untranslat ed region (UTR) and another without. Due to the importance of Nramp2 in Fe transport, and the presence of an IRE in its 3'-UTR, we have examined the e ffect of Fe-deprivation, Fe-loading, and nitrogen monoxide on the expressio n of Nramp2 mRNA. These results were compared to the expression of transfer rin receptor (TfR) mRNA which also has IREs in its 3'-UTR and is regulated by Fe and NO via the binding of iron-regulatory proteins (IRPs) to its IREs . Our experiments show that the IRE in Nramp2 mRNA does bind the IRPs in ly sates from a mouse fibroblast cell line (LMTK-). Moreover, reverse transcri ption-PCR (RT-PCR) demonstrated that both the IRE and non-IRE-containing tr anscripts were present within these cells. However, there was no change in Nramp2 mRNA expression in LMTK- cells after a 20-h incubation with either t he Fe chelator, desferrioxamine (DFO), the Fe donor, ferric ammonium citrat e (FAC), or the NO generator, S-nitroso-N-acetylpenicillamine (SNAP). In co ntrast, these agents caused a marked change in the RNA-binding activity of the IRPs and the expression of TfR mRNA. In addition, both FAC and DFO caus ed an appropriate change in [Fe-59] uptake from [Fe-59]Tf. viz., an increas e in Fe uptake after exposure to DFO and a decrease after treatment with FA C. As Nramp2 can transport Fe from non-Tf-bound Fe, the effect of preincuba tion with DFO and FAC was also examined on Fe uptake from [Fe-59]nitrilotri acetate and [Fe-59]citrate. However, in contrast to the results found for [ Fe-59]Tf, incubation with DFO and FAC did not result in appropriate regulat ion of Fe uptake from [Fe-59]nitrilotriacetate or [Fe-59]citrate. These dat a demonstrate that non-Trf-bound Fe uptake was not under control of the IRP -IRE system in these cells. Collectively, the results indicate that in LMTK -fibroblasts Nramp2 mRNA expression was not regulated like TfR mRNA.