Design and implementation of the quality improvement process at a community teaching hospital

Iron regulatory elements (IREs) are a family of 28 nucleotide, non-coding elements which regulate the translation of ferritin mRNA (iron storage), erythroid delta-aminolevulinic acid synthase mRNA (heme synthesis) and the stability of the transferrin receptor (TfR) mRNA (iron uptake). IREs in the 5' end control translation (ribosome binding) and IREs in the 3' end control turnover (degradation). The specific regulator protein, the IRE-BP, is a member of the aconitase family but binds RNA only in the apo form without the Fe-S cluster. Cellular iron alters the IRE/IRE-BP interaction leading to translation of ferritin and eALAS mRNAs but degradation of the TfR mRNA. IRE function requires proximity to the 5' cap, achieved either by a short leader (eALAS) or a long, base-pairing flanking region (FL) (ferritin); a conserved triplet of FL base pairs enhances repression of ferritin mRNA. TfR mRNA has five AU-rich IREs which can also form an alternate structure with inter-IRE base pairs, in the absence of the IRE-BP. Ferritin IREs regulate both translation repression (negative control-IRE-BP dependent) and enhancement (positive control-initiation factor dependent); IRE-BP binding induces conformational changes in the FL. IREs use CAGUGU/C to form a hairpin loop with specific variations in the stem such as internal or bulge loops. A current structural model obtained using metallonucleases (1,10-phenanthroline-Cu, Fe-EDTA, Fe-bleomycin) and a preliminary analysis of the NMR spectrum, is a distorted helix with folds. The effect of cellular iron, Fe-S clusters and heme on the IRE-BP/RNA is not completely understood.(ABSTRACT TRUNCATED AT 250 WORDS)     

Iron regulatory proteins, iron responsive elements and iron homeostasis

The discovery of iron regulatory proteins (IRPs) has provided a molecular framework from which to more fully understand the coordinate regulation of vertebrate iron metabolism. IRPs bind to iron responsive elements (IREs) in specific mRNAs and regulate their utilization. The targets of IRP action now appear to extend beyond proteins that function in the storage (ferritin) or cellular uptake (transferrin receptor) of iron to include those involved in other aspects of iron metabolism as well as in the tricarboxylic acid cycle. To date, it appears that IRPs modulate the utilization of six mammalian mRNAs. Current studies are aimed at defining the mechanisms responsible for the hierarchical regulation of these mRNAs by IRPs. In addition, much interest continues to focus on the signaling pathways through which IRP function is regulated. Multiple factors modulate the RNA binding activity of IRP1 and/or IRP2 including iron, nitric oxide, phosphorylation by protein kinase C, oxidative stress and hypoxia/reoxygenation. Because IRPs are key modulators of the uptake and metabolic fate of iron in cells, they are focal points for the modulation of cellular iron homeostasis in response to a variety of agents and circumstances.     

The transferrin receptor

Transferrin receptor (TfR) is a membrane receptor involved in the control of iron supply to the cell through the binding of transferrin, the major iron-carrier protein. This receptor plays a key role in the control of cell proliferation because iron is essential for sustaining ribonucleotide reductase activity, and is the only enzyme that catalyzes the conversion of ribonucleotides to deoxyribonucleotides. Furthermore, TfR is overexpressed in several tumors. The synthesis of the receptor is controlled through an iron-dependent negative feedback in cells where the receptor is involved in the control of cell proliferation, but through a positive feedback in cell types involved in iron storage (monocytes-macrophages). Recent studies have shed light on the molecular basis by which iron regulates TfR expression. Thus, the 3' untranslated region of TfR mRNA contains a cis-acting RNA element, termed the iron-regulatory element (IRE), that interacts with an IRE-binding protein (IRE-BP). The high-affinity interaction between IRE-BP and IRE in the TfR mRNA leads to repression of mRNA degradation and thus to higher synthesis of TfR. Hence, the study of IRE-BP activity will elucidate the mechanisms that modulate TfR expression in normal and malignant cells.     

Molecular cloning of mouse glycolate oxidase. High evolutionary conservation and presence of an iron-responsive element-like sequence in the mRNA

Iron regulatory proteins (IRPs) control the synthesis of several proteins in iron metabolism by binding to iron-responsive elements (IREs), a hairpin structure in the untranslated region (UTR) of corresponding mRNAs. Binding of IRPs to IREs in the 5' UTR inhibits translation of ferritin heavy and light chain, erythroid aminolevulinic acid synthase, mitochondrial aconitase, and Drosophila succinate dehydrogenase b, whereas IRP binding to IREs in the 3' UTR of transferrin receptor mRNA prolongs mRNA half-life. To identify new targets of IRPs, we devised a method to enrich IRE-containing mRNAs by using recombinant IRP-1 as an affinity matrix. A cDNA library established from enriched mRNA was screened by an RNA-protein band shift assay. This revealed a novel IRE-like sequence in the 3' UTR of a liver-specific mouse mRNA. The newly identified cDNA codes for a protein with high homology to plant glycolate oxidase (GOX). Recombinant protein expressed in bacteria displayed enzymatic GOX activity. Therefore, this cDNA represents the first vertebrate GOX homologue. The IRE-like sequence in mouse GOX exhibited strong binding to IRPs at room temperature. However, it differs from functional IREs by a mismatch in the middle of its upper stem and did not confer iron-dependent regulation in cells.     

Diacerhein blocks iron regulatory protein activation in inflamed human monocytes

Iron Regulatory Proteins (IRPs), by modulating expression of ferritin, which stores excess iron in a non toxic form, and transferrin receptor, which controls iron uptake, are the main controller of cellular iron metabolism. During inflammation, modification of IRP activity may affect iron availability, free radical generation and cytokine gene response in inflammatory cells. In the present study we tested the effect of inflammatory stimuli on IRP function in a human monocytic-macrophagic cell line and the possibility of interfering with these pathways by using an antiinflammatory compound, diacerhein (DAR). IRP activity was enhanced by interferon gamma/lipopolysaccarhide (IFN/LPS), and this effect was consistently counteracted by increasing concentrations of DAR. No direct effect of DAR on IRP activity was found in vitro. However, in vivo, similar IRP activation was achieved by exposing cells to nitric oxide (NO) donors and the LPS/IFN-induced activation of IRP was reversed by NO inhibitors. Interestingly, NO-induced IRP activation was efficiently blocked by DAR. These data show for the first time that a clinically useful antiinflammatory compound, DAR, interferes with IRP activation by NO in inflammed human cells.     

Overexpression of the ferritin H subunit in cultured erythroid cells changes the intracellular iron distribution

To test the hypothesis that variations in H- and L-subunit composition in the ferritin shell affect intracellular iron metabolism, we established stable transfectants of mouse erythroleukemia cells overexpressing the H-ferritin subunit. Analyses were performed on individual clones of transfected cells induced to differentiate with hexamethylenbisacetamide (HMBA). The results showed that there was a reduction in the amount of hemoglobin produced, in inverse relationship with the level of H-subunit overexpression. Incorporation of [2-14C]glycine into heme was reduced by 20% t0 30% in the clones overexpressing H-ferritin subunit compared with control clone. However, the reduction in hemoglobin production was not reversed by addition of heme precursors (delta-aminolevulinic acid or iron) or by hemin itself. A reduced accumulation of beta-globin mRNA was also observed, which could account for the impaired hemoglobin synthesis. Furthermore, synthesis of the endogenous L-ferritin subunit was greatly repressed. Gel retardation assays performed on cytoplasmic extracts of transfected cells using an iron-responsive element (IRE) as a probe revealed that in overexpressing cells, the iron-regulatory protein (IRP) had a conformation with a high RNA-binding affinity, thus leading to translational repression of the endogenous L-ferritin synthesis. These data suggest that an increased formation of H-rich isoferritins leads to a rapid chelation of the regulatory iron pool. While the mechanism underlying the reduction in beta-globin mRNA remains to be elucidated, this study provides direct evidence for the role of IRP-mediated regulation of ferritin expression in erythroid cell metabolism.     

Inappropriately high iron regulatory protein activity in monocytes of patients with genetic hemochromatosis

In genetic hemochromatosis (GH), excess iron is deposited in parenchymal cells, whereas little iron is found in reticuloendothelial (RE) cells until the later stages of the disease. As iron absorption is inversely related to RE cells stores, a failure of RE to retain iron has been proposed as the basic defect in GH. In RE cells of GH subjects, we examined the activity of iron regulatory protein (IRP), a reliable indicator of the elusive regulatory labile iron pool, which modulates cellular iron homeostasis through control of ferritin (Ft) and transferrin receptor gene expression. RNA-bandshift assays showed a significant increase in IRP activity in monocytes from 16 patients with untreated GH compared with 28 control subjects (1.5-fold) and five patients with secondary hemochromatosis (SH) with similar iron burden (fourfold). In 17 phlebotomy-treated GH patients, IRP activity did not differ from that of control subjects. In both GH and SH monocyte-macrophages, Ft content increased by twofold and the L subunit-rich isoferritin profile was unchanged as compared with controls. IRP activity was still upregulated in vitro in monocyte-derived macrophages of GH subjects but, following manipulations of iron levels, was modulated normally. Therefore, the sustained activity of monocyte IRP found in vivo in monocytes of GH patients is not due to an inherent defect of its control, but is rather the expression of a critical abnormality of iron metabolism, eg, a paradoxical contraction of the regulatory iron pool. By preventing Ft mRNA translation, high IRP activity in monocytes may represent a molecular mechanism contributing to the inadequate Ft accumulation and insufficient RE iron storage in GH.     

Expression and biochemical characterization of iron regulatory proteins 1 and 2 in Saccharomyces cerevisiae

Iron-regulatory proteins (IRPs) 1 and 2 are cytosolic RNA-binding proteins that bind to specific stem-loop structures, termed iron-responsive elements (IREs) that are located in the untranslated regions of specific mRNAs encoding proteins involved in iron metabolism. The binding of IRPs to IREs regulates either translation or stabilization of mRNA. Although IRP1 and IRP2 are similar proteins in that they are ubiquitously expressed and are negatively regulated by iron, they are regulated by iron by different mechanisms. IRP1, the well-characterized IRP in cells, is a dual-function protein exhibiting either aconitase activity when cellular iron is abundant or RNA-binding activity when cellular iron is scarce. In contrast, IRP2 lacks detectable aconitase activity and functions exclusively as an RNA-binding protein. To study and compare the biochemical characteristics of IRP1 and IRP2, we expressed wild-type and mutant rat IRP1 and IRP2 in the yeast Saccharomyces cerevisiae. IRP1 and IRP2 expressed in yeast bind the IRE RNA with high affinity, resulting in the inhibition of translation of an IRE-reporter mRNA. Mutant IRP2s lacking a 73 amino acid domain unique to IRP2 and a mutant IRP1 containing an insertion of this domain bound RNA, but lacked detectable aconitase activity, suggesting that the presence of this domain prevents aconitase activity. Like IRP1, the RNA-binding activity of IRP2 was sensitive to inactivation by N-ethylmaleimide (NEM) or 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), indicating IRP2 contains a cysteine(s) that is (are) necessary for RNA binding. However, unlike IRP1, where reconstitution of the 4Fe-4S cluster resulted in a loss in RNA-binding activity, the RNA-binding activity of IRP2 was unaffected using the same iron treatment. These data suggested that IRP2 does not contain a 4Fe-4S cluster similar to the cluster in IRP1, indicating that they sense iron by different mechanisms.     

Preferential activation of iron regulatory protein-2 in cell lines as a result of higher sensitivity to iron

Iron regulatory proteins (IRP)-1 and 2 are cytoplasmic mRNA-binding proteins that control intracellular iron homeostasis by regulating the translation of ferritin mRNA and stability of transferrin receptor mRNA in an iron-dependent fashion. Although structurally and functionally similar, the two IRP are different in their mode of regulation, pattern of tissue expression and modulation by multiple factors, such as bioradicals. In the present study RNA bandshift assays demonstrated that IRP-2, but not IRP-1, activity was higher in cultured cells than in tissues. Increased expression of IRP-2 in cell lines was not related to immortalization and differentiation but seemed associated to cell proliferation, although not closely dependent on cell growth rate. As a growing cell consumes more iron than its quiescent counterpart, we assessed the iron status of cell lines and found that ferritin content was lower than in tissues. Analysis of IRP activity in cell lines supplemented with heme or non-heme iron and in livers of iron-loaded and iron-deficient rats indicated that IRP-2 responds more promptly than IRP-1 to modulations of iron content. We propose that enhanced IRP-2 activity in cultured cells could be due to a proliferation-dependent, relative iron deficiency that is sensed first by IRP-2.     

2',5'-Oligoadenylates and related 2',5'-oligonucleotide analogues. 2. Effect on cellular proliferation, protein synthesis, and endoribonuclease activity

A number of the new enzymatically synthesized 2',5'-oligonucleotide trimers, namely, those containing the nucleosides 8-azaadenosine, toyocamycin, sangivamycin, formycin, 8-bromoadenosine, tubercidin, and guanosine, were found to inhibit protein synthesis and cellular proliferation after uptake into intact L and HeLa cells. 2',5'-Oligonucleotide trimers containing cytidine, inosine, uridine, and 1,N6-ethenoadenosine had some effect while those containing 2-chloroadenosine, 3-ribosyladenine, ribavirin, and 2-beta-D-ribofuranosylthiazole-4-carboxamide had no detectable effect on protein synthesis or cellular proliferation after uptake into L or HeLa cells. All of these 2',5'-oligonucleotide analogues inhibited protein synthesis in the in vitro rabbit reticulocyte lysate system except for the trimer containing ribavirin. Such nucleoside substitutions have further defined the substrate-specificity requirements for the endoribonuclease and/or the inhibitors for the 2',5'-phosphodiesterase. Most of the 2',5'-analogues were degraded in L-cell extracts so the endogenous nucleases are not very specific. The 2',5'-trimers containing tubercidin and 2-beta-D-ribofuranosylthiazole-4-carboxamide were quite stable in comparison to the 2',5'-A trimer. The inhibition of protein synthesis and cellular proliferation observed correlated well with the degradation of rRNA and polyadenylated mRNA observed after uptake of the 2',5'-analogues into intact L cells. The degradation of the polyadenylated mRNA appeared to be a more sensitive test than inhibition of cellular protein synthesis for determining biological activities of the 2',5'-oligonucleotide analogues.     

Nitric oxide reduces nontransferrin-bound iron transport in HepG2 cells

Nitric oxide (NO) donors S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside (SNP) modulate iron regulatory protein (IRP) activity and may, therefore, affect iron uptake through transferrin receptor expression. However, iron also enters the cell as nontransferrin-bound iron (NTBI), and the aim of this study was to evaluate the effects of NO donors on NTBI transport in HepG2 cells, a model of liver physiology. Incubation with SNP and SNAP led to a time- and concentration-dependent reduction in Fe3+ and Fe2+ uptake, thus indicating an effect on the transporter rather than on the reductase. In terms of Fe2+ uptake, no variations in the Michaelis-Menten constant (Km) and a reduction in maximum uptake (Vmax) (50, 33, and 16.6 fmol/microgram protein/min in control, SNP-, and SNAP-treated cells, respectively) were detected, which suggested a decrease in the number of putative NTBI transport protein(s). Gel shift assays showed that IRP activity was reduced by SNP and slightly increased by SNAP. Northern blot analysis of transferrin receptor messenger RNA (mRNA) levels showed variations similar to those observed for IRPs, but both NO donors increased L-ferritin mRNA levels and had no effect on the stimulator of Fe transport (SFT) mRNA. In conclusion, NO donors significantly reduce NTBI transport in HepG2 cells, an effect that seems to be IRP and SFT independent. Moreover, the reduction in NTBI uptake after NO treatment suggests that this form of iron may play a minor role in the increased hepatic iron stores observed in inflammation or that other liver cells are more involved in this pathological condition.     

Iron-dependent oxidation, ubiquitination, and degradation of iron regulatory protein 2: implications for degradation of oxidized proteins

The ability of iron to catalyze formation of reactive oxygen species significantly contributes to its toxicity in cells and animals. Iron uptake and distribution is regulated tightly in mammalian cells, in part by iron regulatory protein 2 (IRP2), a protein that is degraded efficiently by the proteasome in iron-replete cells. Here, we demonstrate that IRP2 is oxidized and ubiquitinated in cells before degradation. Moreover, iron-dependent oxidation converts IRP2 into a substrate for ubiquitination in vitro. A regulatory pathway is described in which excess iron is sensed by its ability to catalyze site-specific oxidations in IRP2, oxidized IRP2 is ubiquitinated, and ubiquitinated IRP2 subsequently is degraded by the proteasome. Selective targeting and removal of oxidatively modified proteins may contribute to the turnover of many proteins that are degraded by the proteasome.