s-nitro-n-acetylpenicillamine and ferric-ammonium-citrate

Recent studies have demonstrated that the protein product (natural resistance associated macrophage protein 2, Nramp2) encoded by the gene Nramp2 acts as an Fe transporter involved in the uptake of Fe from transferrin (Tf) and low Mr Fe complexes. Interestingly, there are two splice variants of Nramp2, one with a putative iron-responsive element (IRE) in its 3' untranslated 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 effect of Fe-deprivation, Fe-loading, and nitrogen monoxide on the expression of Nramp2 mRNA. These results were compared to the expression of transferrin 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 lysates from a mouse fibroblast cell line (LMTK-). Moreover, reverse transcription-PCR (RT-PCR) demonstrated that both the IRE and non-IRE-containing transcripts were present within these cells. However, there was no change in Nramp2 mRNA expression in LMTK- cells after a 20-h incubation with either the Fe chelator, desferrioxamine (DFO), the Fe donor, ferric ammonium citrate (FAC), or the NO generator, S-nitroso-N-acetylpenicillamine (SNAP). In contrast, 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 caused an appropriate change in [59Fe] uptake from [59Fe]Tf, viz., an increase in Fe uptake after exposure to DFO and a decrease after treatment with FAC. As Nramp2 can transport Fe from non-Tf-bound Fe, the effect of preincubation with DFO and FAC was also examined on Fe uptake from [59Fe]nitrilotriacetate and [59Fe]citrate. However, in contrast to the results found for [59Fe]Tf, incubation with DFO and FAC did not result in appropriate regulation of Fe uptake from [59Fe]nitrilotriacetate or [59Fe]citrate. These data demonstrate that non-Tf-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.

Topics: 3' Untranslated Regions; Animals; Base Sequence; Biological Transport, Active; Carrier Proteins; Cation Transport Proteins; Cell Line; Chelating Agents; Deferoxamine; DNA Primers; Ferric Compounds; Gene Expression; Intracellular Fluid; Iron; Iron-Binding Proteins; Iron-Regulatory Proteins; Iron-Sulfur Proteins; Kinetics; Membrane Proteins; Mice; Molecular Sequence Data; Nitric Oxide; Nitric Oxide Donors; Nucleic Acid Conformation; Penicillamine; Quaternary Ammonium Compounds; Receptors, Transferrin; RNA-Binding Proteins; RNA, Messenger; Transferrin

Cellular iron storage and uptake are coordinately regulated post-transcriptionally by cytoplasmic factors, iron-regulatory proteins 1 and 2 (IRP-1 and IRP-2). When iron in the intracellular transit pool is scarce, IRPs bind to iron-responsive elements (IREs) in the 5'-untranslated region of the ferritin mRNA and 3'-untranslated region of the transferrin receptor (TfR) mRNA. Such binding inhibits translation of ferritin mRNA and stabilizes the mRNA for TfR, whereas the opposite scenario develops when iron in the transit pool is plentiful. However, we (Richardson, D. R., Neumannova, V., Nagy, E., and Ponka, P. (1995) Blood 86, 3211-3219) and others reported that the binding of IRPs to IREs can also be modulated by nitric oxide (NO). In this study, we showed that a short exposure of RAW 264.7 cells (a murine macrophage cell line) to the NO(+) donor, sodium nitroprusside (SNP), caused a significant decrease in IRP-2 binding to the IREs followed by IRP-2 degradation and that these changes occurred without affecting IRP-1 binding. The SNP-mediated degradation of IRP-2 in RAW 264.7 cells could be prevented by MG-132 or lactacystin, known inhibitors of proteasome-dependent protein degradation. A SNP-mediated decrease in IRP-2 binding and levels was associated with a dramatic decrease in TfR mRNA levels and an increase in ferritin synthesis. Importantly, the proteasome inhibitor MG-132 prevented the SNP-mediated decrease in TfR mRNA levels. These observations suggest that IRP-2 can play an important role in controlling transferrin receptor expression.

Topics: Animals; Cell Line; Cysteine Endopeptidases; Deferoxamine; Ferric Compounds; Gene Expression Regulation; Iron; Iron Regulatory Protein 1; Iron Regulatory Protein 2; Iron-Regulatory Proteins; Iron-Sulfur Proteins; Macrophages; Mice; Multienzyme Complexes; Nitric Oxide; Nitroprusside; Penicillamine; Protease Inhibitors; Proteasome Endopeptidase Complex; Protein Binding; Proto-Oncogene Proteins; Quaternary Ammonium Compounds; Receptors, Transferrin; RNA-Binding Proteins; RNA, Messenger; Time Factors; Wnt2 Protein