Ascorbic acid enhances ferritin mRNA translation by an IRP/aconitase switch

Ferritin: Significance in viral infections

As an indispensable trace element, iron is essential for many biological processes. Increasing evidence has shown that virus infection can perturb iron metabolism and play a role in the occurrence and development of viral infection-related diseases. Ferritin plays a crucial role in maintaining the body's iron homoeostasis. It is an important protein to stabilise the iron balance in cells. Ferritin is a 24-mer hollow iron storage protein composed of two subunits: ferritin heavy chain and ferritin light chain. It was reported that ferritin is not only an intra-cellular iron storage protein, but also a pathogenic mediator that enhances the inflammatory process and stimulates the further inflammatory pathway, which is a key member of the vicious pathogenic cycle to perpetuate. Ferritin exerts immuno-suppressive and pro-inflammatory functions during viral infection. In this review, we describe in detail the basic information of ferritin in the first section, including its structural features, the regulation of ferritin. In the second part, we focus on the role of ferritin in viral infection-related diseases and the molecular mechanisms by which viral infection regulates ferritin. The last section briefly outlines the potential of ferritin in antiviral therapy. Given the importance of iron and viral infection, understanding the role of ferritin during viral infection helps us understand the relationship between iron metabolic dysfunction and viral infection, which provides a new direction for the development of antiviral therapeutic drugs.

Mitochondrial ferritin expression in human macrophages is facilitated by thrombin-mediated cleavage under hypoxia

Ferritins are iron storage proteins, which maintain cellular iron homeostasis. Among these proteins, the ferritin heavy chain is well characterized, but the regulatory principles of mitochondrial ferritin (FTMT) remain elusive. FTMT appears to be cleaved from a 27 kDa to a 22 kDa form. In human macrophages, FTMT increased under hypoxia in a hypoxia-inducible factor 2-dependent manner. Occurrence of FTMT resulted from cleavage by thrombin, which was supplied by serum. Inhibition of thrombin as well as serum removal decreased FTMT, while supplementation of thrombin under serum-deprived conditions restored its expression. Besides hypoxia, thrombin facilitated FTMT expression after treatment with the ferroptosis inducer RSL3 and the pro-inflammatory stimulus lipopolysaccharide. This study provides insights into the regulation of FTMT under hypoxia and identifies thrombin as a FTMT maturation-associated peptidase.

Crosstalk between regulated necrosis and micronutrition, bridged by reactive oxygen species

The discovery of regulated necrosis revitalizes the understanding of necrosis from a passive and accidental cell death to a highly coordinated and genetically regulated cell death routine. Since the emergence of RIPK1 (receptor-interacting protein kinase 1)-RIPK3-MLKL (mixed lineage kinase domain-like) axis-mediated necroptosis, various other forms of regulated necrosis, including ferroptosis and pyroptosis, have been described, which enrich the understanding of pathophysiological nature of diseases and provide novel therapeutics. Micronutrients, vitamins, and minerals, position centrally in metabolism, which are required to maintain cellular homeostasis and functions. A steady supply of micronutrients benefits health, whereas either deficiency or excessive amounts of micronutrients are considered harmful and clinically associated with certain diseases, such as cardiovascular disease and neurodegenerative disease. Recent advance reveals that micronutrients are actively involved in the signaling pathways of regulated necrosis. For example, iron-mediated oxidative stress leads to lipid peroxidation, which triggers ferroptotic cell death in cancer cells. In this review, we illustrate the crosstalk between micronutrients and regulated necrosis, and unravel the important roles of micronutrients in the process of regulated necrosis. Meanwhile, we analyze the perspective mechanism of each micronutrient in regulated necrosis, with a particular focus on reactive oxygen species (ROS).

Ferroptosis is Involved in Hyperoxic Lung Injury in Neonatal Rats

Purpose

To evaluate whether ferroptosis is involved in hyperoxic acute lung injury (HALI) and its mechanisms through the HALI model.

Methods

HE staining was used to assess lung injury pathology after the establishment of neonatal rat HALI model. ELISA was used to detect ROS, GPX4, and GSH expression. Prussian blue staining and Western Blot were used to detect iron deposition and the expression of ferroptosis-related proteins, respectively.

Results

The HALI group showed pathological changes with larger and fewer alveoli and thicker alveolar septa after HE staining. Prussian blue staining detected significant iron deposition in the lung tissue of the HALI group. GPX4, GSH, GSS, and SLC7A11 expressions were significantly decreased in the HALI group than in the normal control group. In contrast, ROS, TFRC, FHC, and FLC expressions showed opposite results (p<0.05).

Conclusion

Ferroptosis may be involved in the pathological process of hyperoxic lung injury in neonatal rats.

Physiology and Inflammation Driven Pathophysiology of Iron Homeostasis-Mechanistic Insights into Anemia of Inflammation and Its Treatment

Anemia is very common in patients with inflammatory disorders. Its prevalence is associated with severity of the underlying disease, and it negatively affects quality of life and cardio-vascular performance of patients. Anemia of inflammation (AI) is caused by disturbances of iron metabolism resulting in iron retention within macrophages, a reduced erythrocyte half-life, and cytokine mediated inhibition of erythropoietin function and erythroid progenitor cell differentiation. AI is mostly mild to moderate, normochromic and normocytic, and characterized by low circulating iron, but normal and increased levels of the storage protein ferritin and the iron hormone hepcidin. The primary therapeutic approach for AI is treatment of the underlying inflammatory disease which mostly results in normalization of hemoglobin levels over time unless other pathologies such as vitamin deficiencies, true iron deficiency on the basis of bleeding episodes, or renal insufficiency are present. If the underlying disease and/or anemia are not resolved, iron supplementation therapy and/or treatment with erythropoietin stimulating agents may be considered whereas blood transfusions are an emergency treatment for life-threatening anemia. New treatments with hepcidin-modifying strategies and stabilizers of hypoxia inducible factors emerge but their therapeutic efficacy for treatment of AI in ill patients needs to be evaluated in clinical trials.

A case series highlighting structured hematological, biochemical and molecular approach to clinical oral iron refractoriness in children: A pressing need for a 3-tier system for classification of variants in TMPRSS6 gene

In current study, we discuss clinical oral iron refractoriness cases and highlight need for a classification system to define TMPRSS6 gene variants. Out of 231 cases of microcytic hypochromic anemia screened (Sept 2019-Dec 2020), 17 cases (7.35%) with unexplained iron refractoriness (URIDA) phenotype were enrolled after ruling out secondary causes and compliance related issues. 11 (65%) had absent/negligible response (0-0.4 g/dl Hb rise) while 6 (35%) partial (0.5-0.9 g/dl Hb rise) response to initial iron trial at 4-8 weeks. Of these 17 cases, inappropriate hepcidin levels (normal-high) were noted in 11/15 (73%) tested. TSAT/Hepcidin ratio was low in 13/15 (87%). Genetic analysis of TMPRSS6 gene by NGS revealed variations in 15/17 (88%) cases. 10/15 cases with variations harbored a common splice site INDEL that was noted to be pathogenic SNP (MAF-0.19) on case-control association study in combination with other known missense SNPs with an odds ratio of 6.38 and relative risk 2.66 (p- < 0.01).

Ascorbate and Tumor Cell Iron Metabolism: The Evolving Story and Its Link to Pathology

Significance: Vitamin C or ascorbate (Asc) is a water-soluble vitamin and an antioxidant that is involved in many crucial biological functions. Asc's ability to reduce metals makes it an essential enzyme cofactor. Recent Advances: The ability of Asc to act as a reductant also plays an important part in its overall role in iron metabolism, where Asc induces both nontransferrin-bound iron and transferrin-bound iron uptake at physiological concentrations (∼50 μM). Moreover, Asc has emerged to play an important role in multiple diseases and its effects at pharmacological doses could be important for their treatment. Critical Issues: Asc's role as a regulator of cellular iron metabolism, along with its cytotoxic effects and different roles at pharmacological concentrations, makes it a candidate as an anticancer agent. Ever since the controversy regarding the studies from the Mayo Clinic was finally explained, there has been a renewed interest in using Asc as a therapeutic approach toward cancer due to its minimal side effects. Numerous studies have been able to demonstrate the anticancer activity of Asc through selective oxidative stress toward cancer cells via H₂O₂ generation at pharmacological concentrations. Studies have demonstrated that Asc's cytotoxic mechanism at concentrations (>1 mM) has been associated with decreased cellular iron uptake. Future Directions: Recent studies have also suggested other mechanisms, such as Asc's effects on autophagy, polyamine metabolism, and the cell cycle. Clearly, more has yet to be discovered about Asc's mechanism of action to facilitate safe and effective treatment options for cancer and other diseases.

An iron-dependent metabolic vulnerability underlies VPS34-dependence in RKO cancer cells

VPS34 is a key regulator of endomembrane dynamics and cargo trafficking, and is essential in cultured cell lines and in mice. To better characterize the role of VPS34 in cell growth, we performed unbiased cell line profiling studies with the selective VPS34 inhibitor PIK-III and identified RKO as a VPS34-dependent cellular model. Pooled CRISPR screen in the presence of PIK-III revealed endolysosomal genes as genetic suppressors. Dissecting VPS34-dependent alterations with transcriptional profiling, we found the induction of hypoxia response and cholesterol biosynthesis as key signatures. Mechanistically, acute VPS34 inhibition enhanced lysosomal degradation of transferrin and low-density lipoprotein receptors leading to impaired iron and cholesterol uptake. Excess soluble iron, but not cholesterol, was sufficient to partially rescue the effects of VPS34 inhibition on mitochondrial respiration and cell growth, indicating that iron limitation is the primary driver of VPS34-dependency in RKO cells. Loss of RAB7A, an endolysosomal marker and top suppressor in our genetic screen, blocked transferrin receptor degradation, restored iron homeostasis and reversed the growth defect as well as metabolic alterations due to VPS34 inhibition. Altogether, our findings suggest that impaired iron mobilization via the VPS34-RAB7A axis drive VPS34-dependence in certain cancer cells.

Hypoxia inhibits ferritinophagy, increases mitochondrial ferritin, and protects from ferroptosis

Cellular iron, at the physiological level, is essential to maintain several metabolic pathways, while an excess of free iron may cause oxidative damage and/or provoke cell death. Consequently, iron homeostasis has to be tightly controlled. Under hypoxia these regulatory mechanisms for human macrophages are not well understood. Hypoxic primary human macrophages reduced intracellular free iron and increased ferritin expression, including mitochondrial ferritin (FTMT), to store iron. In parallel, nuclear receptor coactivator 4 (NCOA4), a master regulator of ferritinophagy, decreased and was proven to directly regulate FTMT expression. Reduced NCOA4 expression resulted from a lower rate of hypoxic NCOA4 transcription combined with a micro RNA 6862-5p-dependent degradation of NCOA4 mRNA, the latter being regulated by c-jun N-terminal kinase (JNK). Pharmacological inhibition of JNK under hypoxia increased NCOA4 and prevented FTMT induction. FTMT and ferritin heavy chain (FTH) cooperated to protect macrophages from RSL-3-induced ferroptosis under hypoxia as this form of cell death is linked to iron metabolism. In contrast, in HT1080 fibrosarcome cells, which are sensitive to ferroptosis, NCOA4 and FTMT are not regulated. Our study helps to understand mechanisms of hypoxic FTMT regulation and to link ferritinophagy and macrophage sensitivity to ferroptosis.

•

Hypoxia decreases NCOA4 transcription in primary human macrophages.

•

NCOA4 mRNA is a target of miR-6862-5p.

•

Lowering NCOA4 increases FTMT abundance under hypoxia.

•

FTMT and FTH protect from ferroptosis.

•

Tumor cells lack the hypoxic decrease of NCOA4 and fail to stabilize FTMT.

Comparative metabolomics analysis of donkey colostrum and mature milk using ultra-high-performance liquid tandem chromatography quadrupole time-of-flight mass spectrometry

Donkey milk has been widely shown to be an ideal substitute for human milk because of its similar composition. However, alterations to the composition of donkey milk during lactation have not been well studied. In this study, untargeted metabolomics with ultra-high-performance liquid tandem chromatography quadrupole time-of-flight mass spectrometry were used to analyze and compare the metabolites in donkey colostrum (DC) and mature milk (DMM). Two hundred seventy metabolites were characterized in both DC and DMM. Fifty-two of the metabolites in the DC were significantly different from those in the DMM; 8 were downregulated and 44 were upregulated. This demonstrated that the composition of the donkey milk changed with lactation. Additionally, the interactions and metabolic pathways were further analyzed to explore the mechanisms that altered the milk during lactation. Our results provide comprehensive insights into the alterations in donkey milk during lactation. The results will aid in future investigations into the nutrition of donkey milk and provide practical information for the dairy industry.

Favourable improvement in haematological parameters in response to oral iron and vitamin C combination in children with Iron Refractory Iron Deficiency Anemia (IRIDA) phenotype

Treatment in IRIDA focuses on use of intravenous iron preparations to circumvent oral absorptive defect resulting from high levels of hepcidin due to TMPRSS6 gene variations. However, recent case reports and recommendations on atypical microcytic hypochromic anemias advocate use of oral iron and vitamin c trial before parenteral iron, as the same results in comparable improvement in haemoglobin. We prospectively evaluated our IRIDA cohort (n = 7) with oral iron and vitamin c dose over a period of 10 weeks and noted complete response in majority (6/7 = 86%) with >2 g/dL rise in Hb along with significant improvement of other iron related indices.

A mitochondria-targeted derivative of ascorbate: MitoC

Mitochondrial oxidative damage contributes to a wide range of pathologies. One therapeutic strategy to treat these disorders is targeting antioxidants to mitochondria by conjugation to the lipophilic triphenylphosphonium (TPP) cation. To date only hydrophobic antioxidants have been targeted to mitochondria; however, extending this approach to hydrophilic antioxidants offers new therapeutic and research opportunities. Here we report the development and characterization of MitoC, a mitochondria-targeted version of the hydrophilic antioxidant ascorbate. We show that MitoC can be taken up by mitochondria, despite the polarity and acidity of ascorbate, by using a sufficiently hydrophobic link to the TPP moiety. MitoC reacts with a range of reactive species, and within mitochondria is rapidly recycled back to the active ascorbate moiety by the glutathione and thioredoxin systems. Because of this accumulation and recycling MitoC is an effective antioxidant against mitochondrial lipid peroxidation and also decreases aconitase inactivation by superoxide. These findings show that the incorporation of TPP function can be used to target polar and acidic compounds to mitochondria, opening up the delivery of a wide range of bioactive compounds. Furthermore, MitoC has therapeutic potential as a new mitochondria-targeted antioxidant, and is a useful tool to explore the role(s) of ascorbate within mitochondria.

Duodenal cytochrome b (DCYTB) in iron metabolism: an update on function and regulation

Iron and ascorbate are vital cellular constituents in mammalian systems. The bulk-requirement for iron is during erythropoiesis leading to the generation of hemoglobin-containing erythrocytes. Additionally, both iron and ascorbate are required as co-factors in numerous metabolic reactions. Iron homeostasis is controlled at the level of uptake, rather than excretion. Accumulating evidence strongly suggests that in addition to the known ability of dietary ascorbate to enhance non-heme iron absorption in the gut, ascorbate regulates iron homeostasis. The involvement of ascorbate in dietary iron absorption extends beyond the direct chemical reduction of non-heme iron by dietary ascorbate. Among other activities, intra-enterocyte ascorbate appears to be involved in the provision of electrons to a family of trans-membrane redox enzymes, namely those of the cytochrome b₅₆₁ class. These hemoproteins oxidize a pool of ascorbate on one side of the membrane in order to reduce an electron acceptor (e.g., non-heme iron) on the opposite side of the membrane. One member of this family, duodenal cytochrome b (DCYTB), may play an important role in ascorbate-dependent reduction of non-heme iron in the gut prior to uptake by ferrous-iron transporters. This review discusses the emerging relationship between cellular iron homeostasis, the emergent “IRP1-HIF2α axis”, DCYTB and ascorbate in relation to iron metabolism.

The active role of vitamin C in mammalian iron metabolism: much more than just enhanced iron absorption!

Ascorbate is a cofactor in numerous metabolic reactions. Humans cannot synthesize ascorbate owing to inactivation of the gene encoding the enzyme l-gulono-γ-lactone oxidase, which is essential for ascorbate synthesis. Accumulating evidence strongly suggests that in addition to the known ability of dietary ascorbate to enhance nonheme iron absorption in the gut, ascorbate within mammalian systems can regulate cellular iron uptake and metabolism. Ascorbate modulates iron metabolism by stimulating ferritin synthesis, inhibiting lysosomal ferritin degradation, and decreasing cellular iron efflux. Furthermore, ascorbate cycling across the plasma membrane is responsible for ascorbate-stimulated iron uptake from low-molecular-weight iron-citrate complexes, which are prominent in the plasma of individuals with iron-overload disorders. Importantly, this iron-uptake pathway is of particular relevance to astrocyte brain iron metabolism and tissue iron loading in disorders such as hereditary hemochromatosis and β-thalassemia. Recent evidence also indicates that ascorbate is a novel modulator of the classical transferrin-iron uptake pathway, which provides almost all iron for cellular demands and erythropoiesis under physiological conditions. Ascorbate acts to stimulate transferrin-dependent iron uptake by an intracellular reductive mechanism, strongly suggesting that it may act to stimulate iron mobilization from the endosome. The ability of ascorbate to regulate transferrin iron uptake could help explain the metabolic defect that contributes to ascorbate-deficiency-induced anemia.

Mammalian iron homeostasis in health and disease: uptake, storage, transport, and molecular mechanisms of action

Iron is a crucial factor for life. However, it also has the potential to cause the formation of noxious free radicals. These double-edged sword characteristics demand a tight regulation of cellular iron metabolism. In this review, we discuss the various pathways of cellular iron uptake, cellular iron storage, and transport. Recent advances in understanding the reduction and uptake of non-transferrin-bound iron are discussed. We also discuss the recent progress in the understanding of transcriptional and translational regulation by iron. Furthermore, we discuss recent advances in the understanding of the regulation of cellular and systemic iron homeostasis and several key diseases resulting from iron deficiency and overload. We also discuss the knockout mice available for studying iron metabolism and the related human conditions.

Transferrin iron uptake is stimulated by ascorbate via an intracellular reductive mechanism

Although ascorbate has long been known to stimulate dietary iron (Fe) absorption and non-transferrin Fe uptake, the role of ascorbate in transferrin Fe uptake is unknown. Transferrin is a serum Fe transport protein supplying almost all cellular Fe under physiological conditions. We sought to examine ascorbate's role in this process, particularly as cultured cells are typically ascorbate-deficient. At typical plasma concentrations, ascorbate significantly increased (59)Fe uptake from transferrin by 1.5-2-fold in a range of cells. Moreover, ascorbate enhanced ferritin expression and increased (59)Fe accumulation in ferritin. The lack of effect of cycloheximide or the cytosolic aconitase inhibitor, oxalomalate, on ascorbate-mediated (59)Fe uptake from transferrin indicate increased ferritin synthesis or cytosolic aconitase activity was not responsible for ascorbate's activity. Experiments with membrane-permeant and -impermeant ascorbate-oxidizing reagents indicate that while extracellular ascorbate is required for stimulation of (59)Fe uptake from (59)Fe-citrate, only intracellular ascorbate is needed for transferrin (59)Fe uptake. Additionally, experiments with l-ascorbate analogs indicate ascorbate's reducing ene-diol moiety is necessary for its stimulatory activity. Importantly, neither N-acetylcysteine nor buthionine sulfoximine, which increase or decrease intracellular glutathione, respectively, affected transferrin-dependent (59)Fe uptake. Thus, ascorbate's stimulatory effect is not due to a general increase in cellular reducing capacity. Ascorbate also did not affect expression of transferrin receptor 1 or (125)I-transferrin cellular flux. However, transferrin receptors, endocytosis, vacuolar-type ATPase activity and endosomal acidification were required for ascorbate's stimulatory activity. Therefore, ascorbate is a novel modulator of the classical transferrin Fe uptake pathway, acting via an intracellular reductive mechanism.

Vitamin C: Overview and Update

Vitamin C functions in enzyme activation, oxidative stress reduction, and immune function. There is considerable evidence that vitamin C protects against respiratory tract infections and reduces risk for cardiovascular disease and some cancers. Current trials are examining the efficacy of intravenous vitamin C as cancer therapy. Many experts believe that the recommended intakes for vitamin C (45 to 90 mg daily) are several orders of magnitude too low to support optimal vitamin C functionality. Also, there is a misperception that vitamin C deficiency disease (scurvy) is largely historical and rarely observed in developed nations. Physical symptoms of scurvy include swelling of the lower extremities, bleeding gums, fatigue, and hemorrhaging, as well as psychological problems, including depression, hysteria, and social introversion. The long-term safety of vitamin C supplementation seems evident as large investigations have noted reduced risk of mortality in vitamin C supplementing populations and in those with elevated plasma vitamin C concentrations.

Iron chelator deferoxamine alters iron-regulatory genes and proteins and suppresses osteoblast phenotype in fetal rat calvaria cells

There are few studies describing the extent to which low iron status affects osteoblastogenesis, despite evidence that iron deficiency produces adverse effects on bone density. The purpose of this study was to evaluate alterations in intracellular iron status by measuring iron-regulated gene and protein expression and to describe development of osteoblast phenotype in primary cells treated with iron chelator deferoxamine (DFOM) during differentiation. Using the well-described fetal rat calvaria model, cells were incubated with 0-8 microM DFOM throughout differentiation (confluence to day (D) 21), or only during early differentiation (confluence to D13-15) or late differentiation (D13-15 to D21). Changes in intracellular iron status were determined by measuring alterations in gene and protein expression of transferrin receptor and ferritin light chain and heavy chain. Development of osteoblast phenotype was monitored by measuring expression of genes that are known to be up-regulated during differentiation, analyzing the percentage of mineralized surface area, and counting the number of multi-layered bone nodules at the end of culture. Results indicate that treatment throughout differentiation with 8 microM DFOM alters iron-regulated genes and proteins by mid-differentiation (D13-15) in a pattern consistent with iron deficiency with concomitant down-regulation of osteoblast phenotype genes, especially osteocalcin. Additionally, alkaline phosphatase staining was lower and there was about 70% less mineralized surface area (p<0.05) by D21 in wells treated throughout differentiation with 8 microM DFOM compared to control. Down-regulation of osteocalcin and alkaline phosphatase mRNA (p<0.05) and suppressed mineralization (p<0.05) was also evident at D21 in cells treated only during early differentiation. In contrast, treatment during late differentiation did not alter osteoblastic outcomes by D21. In conclusion, it appears that iron is required for normal osteoblast phenotype development, and that early rather than late differentiation events may be more sensitive to iron availability.

Differential induction of renal heme oxygenase and ferritin in ascorbate and nonascorbate producing species transfused with modified cell-free hemoglobin

Abstract Heme catabolism and iron sequestration systems play an important role in regulating the response to extracellular hemoglobin (Hb). We previously reported that extracellular Hb oxidizes more readily in the circulation of guinea pigs, a nonascorbate (AA)-producing species with similar plasma and tissue antioxidant status to humans, compared to rats, an AA-producing species. To determine whether these two species exhibit differences in heme catabolism and iron sequestration at the level of the kidney, we examined heme oxygenase (HO), H- and L-ferritin expression, nonheme iron deposition, and renal AA content following transfusion with polymerized bovine hemoglobin (HbG). Both species showed similar rates of hemoglobinuria but urinary HbG was significantly more oxidized in guinea pigs. HbG enhanced HO activity in both species but appeared greater and more sustained in guinea pigs. Conversely, rats showed a greater and more rapid induction of H- and L-ferritin as well as greater iron accumulation and AA content. Furthermore, ferrous and ferric iron deposits were detected in rats while only ferric iron was observed in guinea pigs. These findings suggest significant differences in the renal handling of HbG which may be important for understanding how endogenous antioxidant defenses may modulate the renal response to extracellular Hb.

Iron overload alters iron-regulatory genes and proteins, down-regulates osteoblastic phenotype, and is associated with apoptosis in fetal rat calvaria cultures

Iron overload has been implicated in decreased bone mineral density. However, the effect of iron overload on osteoblast lineage cells remains poorly understood. The purpose of this study was to examine osteoblast differentiation, function, and apoptosis in iron-loaded cells from fetal rat calvaria. Cells were incubated with media supplemented with 0-10 microM ferrous sulfate (FeSO(4)) during differentiation (days 6-20). Intracellular iron status was assessed by measuring iron content in cell layers and changes in transferrin receptor (TrfR) and ferritin gene and protein expression. Osteoblast differentiation and function were evaluated by measuring osteoblast phenotypic gene markers and capacity of cultures to form mineralized bone nodules. Apoptotic hallmarks were evaluated by microscopy. A 2.3-fold increase in media iron concentration resulted in saturable accumulation of iron in the cell layer 20-fold higher than control (p<0.05) by mid-differentiation (day 15, D15). Iron accumulation resulted in rapid and sustained down-regulation of TrfR gene and protein levels (within 24 h) and up-regulation of light and heavy chain ferritin protein levels at late differentiation (day 20, D20). Concurrently, osteoblast phenotype gene markers were suppressed by D15 and a decreased number of mineralized nodules at D20 were observed. Apoptotic events were observed within 24 h of iron loading. These results provide evidence that iron overload alters iron metabolism and suppresses differentiation and function of cells in the osteoblast lineage associated with increased apoptosis.

Effect of Intravenous Ascorbic Acid in Hemodialysis Patients With EPO-Hyporesponsive Anemia and Hyperferritinemia

BACKGROUND

Although erythropoietin (EPO)-hyporesponsive anemia in hemodialysis patients most commonly results from iron deficiency, the contributory role of chronic inflammation and oxidative stress in its pathogenesis is poorly understood. We conducted an open-label prospective study to assess the effect of vitamin C, an antioxidant, on EPO-hyporesponsive anemia in hemodialysis patients with unexplained hyperferritinemia.

METHODS

Forty-six of 262 patients in an inner-city hemodialysis center met the inclusion criteria (administration of intravenous iron and EPO for > or = 6 months at a dose > or = 450 U/kg/wk, average 3-month hemoglobin [Hb] level < or = 11.0 g/dL [< or = 110 g/L], ferritin level > or = 500 ng/mL (microg/L), and transferrin saturation [TSAT] < or = 50%). Patients were excluded if they had a clear explanation for the EPO hyporesponsiveness. Four patients refused to participate. The remaining patients were randomly assigned; 20 patients to receive standard care and 300 mg of intravenous vitamin C with each dialysis session (group 1) and 22 patients to receive standard care only (group 2). Study duration was 6 months. During the study, 1 patient from group 1 was removed (upper gastrointestinal bleeding) from final analysis. Monthly assessment included Hb level, mean corpuscular volume, iron level, iron-binding capacity, ferritin level, TSAT, and Hb content in reticulocytes. In addition, biointact parathyroid hormone, aluminum, C-reactive protein (CRP), and liver enzymes were measured every 3 months.

RESULTS

Age, sex, race, and time on dialysis therapy were similar in both groups. At 6 months, Hb levels significantly increased from 9.3 to 10.5 g/dL (93.0 to 105.0 g/L) in group 1, but not group 2 (9.3 to 9.6 g/dL [93.0 to 96.0 g/L]; P = 0.0001). Similarly, TSAT increased from 28.9% to 37.3% in group 1, but not group 2 (28.7% to 29.3%; P = 0.0001). EPO dose (477 to 429 versus 474 to 447 U/kg/wk), iron-binding capacity (216 to 194 versus 218 to 257 microg/dL [38.7 to 34.7 versus 39 to 46 micromol/L]), and CRP level (2.8 to 0.9 versus 2.8 to 2.2 mg/dL) decreased significantly in group 1, but not in controls. Changes in Hb content in reticulocytes and ferritin level also were statistically significant in group 1. There was no change in biointact parathyroid hormone levels. Although serum iron levels and intravenous iron doses changed within each group, changes were equal between the 2 groups.

CONCLUSION

In hemodialysis patients with refractory anemia and hyperferritinemia, vitamin C improved responsiveness to EPO, either by augmenting iron mobilization from its tissue stores or through antioxidant effects.

Soluble nickel interferes with cellular iron homeostasis

Soluble nickel compounds are likely human carcinogens. The mechanism by which soluble nickel may contribute to carcinogenesis is unclear, though several hypotheses have been proposed. Here we verify the ability of nickel to enter the cell via the divalent metal ion transporter 1 (DMT1) and disturb cellular iron homeostasis. Nickel may interfere with iron at both an extracellular level, by preventing iron from being transported into the cell, and at an intracellular level, by competing for iron sites on enzymes like the prolyl hydroxylases that modify hypoxia inducible factor-1alpha (HIF-1alpha). Nickel was able to decrease the binding of the Von Hippel-Lindau (VHL) protein to HIF-1alpha, indicating a decrease in prolyl hydroxylase activity. The ability of nickel to affect various iron dependent processes may be an important step in nickel dependent carcinogenesis. In addition, understanding the mechanisms by which nickel activates the HIF-1alpha pathway may lead to new molecular targets in fighting cancer.

Modulation of iron on mitochondrial aconitase expression in human prostatic carcinoma cells

The mitochondrial aconitase (mACON) containing a [4Fe-4S] cluster is regarded as the key enzyme for citrate oxidation in the epithelial cells of human prostate. In vitro studies using the human prostatic carcinoma cells, PC-3 cells, found that both hemin and ferric ammonium citrate (FAC) significantly increased mACON enzymatic activity and gene expression. The effect of FAC on mACON was enhanced 2-fold by co-treating with ascorbic acid but blocked by co-treating with iron chelator, deferoxamine mesylate. Hemin treatments blocked 30% of citrate secretion from PC-3 cells but upregualted 2-fold of intracellular ATP biosynthesis. Results from reporter assay by using a cytomegalovirus enhance/promoter driven luciferase mRNA ligated to the iron response element (IRE) of mACON as a reporter construct demonstrated that modulation of FAC on gene translation of mACON gene is dependent on the IRE. Transient gene expression assays indicated that upregulation of mACON gene transcription by FAC may through the putative antioxidant response element (ARE) signal pathway. This study provides the first evidence of the biologic mechanism of human mACON gene translation/transcription and suggests a regulatory link between the energy utilization and the iron metabolism in human prostatic carcinoma cells.

Decreased transferrin receptor expression by neuromelanin cells in restless legs syndrome

BACKGROUND

Restless legs syndrome (RLS) is a sensory-movement disorder affecting 5 to 10% of the population. Its etiology is unknown, but MRI analyses and immunohistochemical studies on autopsy tissue suggest the substantia nigra (SN) of patients with RLS has subnormal amounts of iron.

METHODS

Neuromelanin cells from the SN of four RLS and four control brains were isolated by laser capture microdissection, and a profile of iron-management protein expression was obtained by immunoblot analysis. Binding assays for iron regulatory protein activity were performed on cell homogenates.

RESULTS

Ferritin, divalent metal transporter 1, ferroportin, and transferrin receptor (TfR) were decreased in RLS neuromelanin cells compared with control. Transferrin was increased in RLS neuromelanin cells. This protein profile in RLS neuromelanin cells is consistent with iron deficiency with the exception that TfR expression was decreased rather than increased. The concentration and activity of the iron regulatory proteins (IRP1 and IRP2) were analyzed to determine whether there was a functional deficit in the post-transcriptional regulatory mechanism for TfR expression. Total IRP activity, IRP1 activity, and IRP1 protein levels were decreased in RLS, but total IRP2 protein levels were not decreased in RLS.

CONCLUSION

Restless legs syndrome may result from a defect in iron regulatory protein 1 in neuromelanin cells that promotes destabilization of the transferrin receptor mRNA, leading to cellular iron deficiency.

Iron-mediated degradation of IRP2, an unexpected pathway involving a 2-oxoglutarate-dependent oxygenase activity

Iron regulatory protein 2 (IRP2), a central posttranscriptional regulator of cellular and systemic iron metabolism, undergoes proteasomal degradation in iron-replete cells. The prevailing model postulates that the mechanism involves site-specific oxidation of 3 cysteine residues (C168, C174, and C178) within a 73-amino-acid (73-aa) degradation domain. By expressing wild-type and mutated versions of IRP2 in H1299 cells, we find that a C168S C174S C178S triple mutant, or a deletion mutant lacking the entire "73-aa domain," is sensitive to iron-mediated degradation, like wild-type IRP2. The antioxidants N-acetylcysteine, ascorbate, and alpha-tocopherol not only fail to stabilize IRP2 but, furthermore, promote its proteasomal degradation. The pathway for IRP2 degradation is saturable, which may explain earlier data supporting the "cysteine oxidation model," and shows remarkable similarities with the degradation of the hypoxia-inducible factor 1 alpha (HIF-1 alpha): dimethyl-oxalylglycine, a specific inhibitor of 2-oxoglutarate-dependent oxygenases, stabilizes IRP2 following the administration of iron to iron-deficient cells. Our results challenge the current model for IRP2 regulation and provide direct pharmacological evidence for the involvement of 2-oxoglutarate-dependent oxygenases in a pathway for IRP2 degradation.

Ferritin and the response to oxidative stress

Iron is required for normal cell growth and proliferation. However, excess iron is potentially harmful, as it can catalyse the formation of toxic reactive oxygen species (ROS) via Fenton chemistry. For this reason, cells have evolved highly regulated mechanisms for controlling intracellular iron levels. Chief among these is the sequestration of iron in ferritin. Ferritin is a 24 subunit protein composed of two subunit types, termed H and L. The ferritin H subunit has a potent ferroxidase activity that catalyses the oxidation of ferrous iron, whereas ferritin L plays a role in iron nucleation and protein stability. In the present study we report that increased synthesis of both subunits of ferritin occurs in HeLa cells exposed to oxidative stress. An increase in the activity of iron responsive element binding proteins in response to oxidative stress was also observed. However, this activation was transient, allowing ferritin protein induction to subsequently proceed. To assess whether ferritin induction reduced the accumulation of ROS, and to test the relative contribution of ferritin H and L subunits in this process, we prepared stable transfectants that overexpressed either ferritin H or ferritin L cDNA under control of a tetracycline-responsive promoter. We observed that overexpression of either ferritin H or ferritin L reduced the accumulation of ROS in response to oxidant challenge.

Complexities in the neurotoxic actions of 6-hydroxydopamine in relation to the cytoprotective properties of taurine

The neurotoxin 6-hydroxydopamine was shown to cause an imbalance between the direct and indirect pathways of the striato-nigral system as evidenced by a decreased release of gamma-aminobutyric acid and taurine in the substantia nigra but not in the globus pallidus following neostriatal stimulation with kainate (100 microM). The neurotoxicity of 6-hydroxydopamine is generally believed to result from reactive-oxygen radical formation, although it is also known to inhibit mitochondrial NADH dehydrogenase. The release of Fe(II) from the unactivated form [3Fe(III)-4S] of cytoplasmic aconitase (EC(50) < 8 microM) was shown to be followed by the slower oxidation of thiol groups in the protein. Complete loss of -SH groups, and enzyme activity, was seen after incubation of glyceraldenyde-3-phosphate dehydrogenase with 200 microM 6-hydroxydopamine for 75 min at 37 degrees C (IC(50) = 70.8 +/- 0.3 microM). Thus the cellular effects of 6-hydroxydopamine are complex, involving impairment of mitochondrial function, iron- release, sulphydryl-group oxidation, and enzyme inhibition in addition to direct generation of reactive oxygen radicals. Taurine, which is known to be neuroprotective in some other systems, only affords protection against some of these effects, thereby explaining its reported ineffectiveness against 6-hydroxydopamine toxicity.

Iron regulatory proteins and the molecular control of mammalian iron metabolism

Mammalian iron homeostasis is maintained through the concerted action of sensory and regulatory networks that modulate the expression of proteins of iron metabolism at the transcriptional and/or post-transcriptional levels. Regulation of gene transcription provides critical developmental, cell cycle, and cell-type-specific controls on iron metabolism. Post-transcriptional control through the action of iron regulatory protein 1 (IRP1) and IRP2 coordinate the use of messenger RNA-encoding proteins that are involved in the uptake, storage, and use of iron in all cells of the body. IRPs may also provide a link between iron availability and cellular citrate use. Multiple factors, including iron, nitric oxide, oxidative stress, phosphorylation, and hypoxia/reoxygenation, influence IRP function. Recent evidence indicates that there is diversity in the function of the IRP system with respect to the response of specific IRPs to the same effector, as well as the selectivity with which IRPs modulate the use of specific messenger RNA.

Vitamin C transport systems of mammalian cells

Vitamin C is essential for many enzymatic reactions and also acts as a free radical scavenger. Specific non-overlapping transport proteins mediate the transport of the oxidized form of vitamin C, dehydroascorbic acid, and the reduced form, L-ascorbic acid, across biological membranes. Dehydroascorbic acid uptake is via the facilitated-diffusion glucose transporters, GLUT 1, 3 and 4, but under physiological conditions these transporters are unlikely to play a major role in the uptake of vitamin C due to the high concentrations of glucose that will effectively block influx. L-ascorbic acid enters cells via Na+-dependent systems, and two isoforms of these transporters (SVCT1 and SVCT2) have recently been cloned from humans and rats. Transport by both isoforms is stereospecific, with a pH optimum of approximately 7.5 and a Na+:ascorbic acid stoichiometry of 2:1. SVCT2 may exhibit a higher affinity for ascorbic acid than SVCT1 but with a lower maximum velocity. SVCT1 and SVCT2 are predicted to have 12 transmembrane domains, but they share no structural homology with other Na+ co-transporters. Potential sites for phosphorylation by protein kinase C exist on the cytoplasmic surface of both proteins, with an additional protein kinase A site in SVCT1. The two isoforms also differ in their tissue distribution: SVCT1 is present in epithelial tissues, whereas SVCT2 is present in most tissues with the exception of lung and skeletal muscle.

Identification and characterization of the iron regulatory element in the ferritin gene of a plant (soybean)

Iron increases ferritin synthesis, targeting plant DNA and animal mRNA. The ferritin promoter in plants has not been identified, in contrast to the ferritin promoter and mRNA iron-responsive element (IRE) in animals. The soybean leaf, a natural tissue for ferritin expression, and DNA, with promoter deletions and luciferase or glucuronidase reporters, delivered with particle bombardment, were used to show that an 86-base pair fragment (iron regulatory element (FRE)) controlled iron-mediated derepression of the ferritin gene. Mutagenesis with linkers of random sequence detected two subdomains separated by 21 base pairs. FRE has no detectable homology to the animal IRE or to known promoters in DNA and bound a trans-acting factor in leaf cell extracts. FRE/factor binding was abrogated by increased tissue iron, in analogy to mRNA (IRE)/iron regulatory protein in animals. Maximum ferritin derepression was obtained with 50 microm iron citrate (1:10) or 500 microm iron citrate (1:1) but Fe-EDTA was ineffective, although the leaf iron concentration was increased; manganese, zinc, and copper had no effect. The basis for different responses in ferritin expression to different iron complexes, as well as the significance of using DNA but not mRNA as an iron regulatory target in plants, remain unknown.

Role of ceruloplasmin and ascorbate in cellular iron release

The process of iron (Fe) release from cells plays an important role in health and disease, although the mechanisms responsible remain unclear. In this study we have examined the process of Fe efflux from HepG2 cells, including the possible roles of Cp and ascorbate in this process. Recently, it has been suggested that Cp plays no role in Fe release but can increase Fe uptake by Fe-deficient HepG2 cells (Mukhopadhyay et al. Science 1998;279:714-7). However, this latter study used a nonphysiologically relevant Fe complex (iron 59-NTA) to label cells with 59Fe at a nonphysiologic temperature (25 degrees C) and Cp concentration (<100 microg/mL). Because of these problems, the experiments have been repeated by maintaining physiologic conditions and labeling cells with the physiologic Fe donor diferric Tf. When cells were labeled at 37 degrees C with 59Fe-Tf in the presence of a physiologically relevant Cp concentration (300 microg/mL), this latter protein had no effect on the uptake of 59Fe in control cells or in cells depleted of Fe by using desferrioxamine. In addition, when Fe-replete or Fe-depleted cells were incubated with 59Fe-NTA at 25 degrees C or 37 degrees C, Cp had no effect on 59Fe uptake compared with the control. When the effect of Cp (10-500 microg/mL) on 59Fe release was examined in cells prelabeled with 59Fe-Tf, a concentration-dependent increase in 59Fe efflux was observed, whereas BSA had no effect. However, in contrast to membrane-permeable Fe chelators that caused a marked increase in Fe release, the effect of Cp on Fe efflux was less impressive. To further investigate the mechanism of 59Fe mobilization, we compared 59Fe efflux among HepG2 cells, SK-Mel-28 melanoma cells, and SK-N-MC neuroblastoma cells. These studies demonstrated that 59Fe release was dependent on the incubation time with 59Fe-Tf, the cell line, and the reincubation temperature. Although 59Fe mobilization from cells was markedly temperature dependent, a range of metabolic inhibitors did not affect 59Fe release. Additional experiments showed that physiologic concentrations of ascorbate reduced 59Fe efflux, whereas glutathione had no effect. This study provides further evidence that Cp is involved in Fe mobilization but does not appear to affect Fe uptake from Tf or NTA.

Post-transcriptional control via iron-responsive elements: the impact of aberrations in hereditary disease

Tight regulation of iron metabolism is crucial to avoid formation of deleterious radicals and is mainly executed at the post-transcriptional level. The regulatory loops are exerted by trans-acting iron regulatory proteins (IRPs) and cis-acting stem-loop motifs, termed iron-responsive elements (IREs), located in the untranslated regions (UTRs) of target mRNAs. Iron scarcity induces binding of IRPs to a single IRE in the 5'-UTR of ferritin, eALAS, aconitase and SDHb mRNAs, which specifically suppresses translation initiation. Simultaneous interaction of IRPs with multiple IREs in the 3'-UTR of transferrin receptor (TfR) mRNA selectively causes its stabilization. The pattern is reverted under iron overload: IRP-mRNA binding affinity is reduced, which results in efficient protein synthesis of target transcripts harboring IREs in the 5'-UTR and rapid degradation of TfR mRNA. Although multiple evidences support this model, several studies reported massive alterations in the regulation of iron homeostasis under specific physiological conditions, raising the possibility for additional regulatory events. Intensive analysis of the palindromic IRE consensus sequence revealed the critical elements for the formation of a functional structure and demonstrated the consequences of IRE mutations in IRP binding. Recent investigations indicated the involvement of naturally occurring IRE mutations of the ferritin L subunit in the hyperferritinemia-cataract syndrome, a hereditary disorder. This review summarizes the apparent links between iron-dependent post-transcriptional control and its abnormalities, governed by the properties of a single mRNA stem-loop structure.

Vitamin C: poison, prophylactic or panacea?

Ascorbate is an essential enzyme cofactor but is often also regarded as an important antioxidant in vivo, protecting against cancer by scavenging DNA-damaging reactive oxygen species. Recent studies suggest that ascorbate sometimes increases DNA damage in humans. Although there is no evidence that any of these effects are deleterious to humans, we might need to change our thinking about the mechanisms of the antioxidant action of ascorbate in vivo.

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 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.

Hypoxia alters iron-regulatory protein-1 binding capacity and modulates cellular iron homeostasis in human hepatoma and erythroleukemia cells

Ferritin and transferrin receptor expression is post-transcriptionally regulated by a conserved mRNA sequence termed the iron-responsive element (IRE), to which a transacting protein called the iron-regulatory protein (IRP) is bound. Our data demonstrate that hypoxia powerfully enhances IRE/IRP-1 binding in human cell lines. Using the human hepatoma cell line Hep3B as a model, we found that 16 h in a 1% oxygen atmosphere markedly increases IRE/IRP-1 binding as assessed by electromobility shift assay. Hypoxia also decreased cytosolic aconitase activity. The hypoxia-enhanced IRE/IRP-1 binding stabilized the transferrin receptor message, increased the cellular mRNA content by over 10-fold, and doubled surface receptor expression. Simultaneously, hypoxia suppressed ferritin message translation. Hypoxia's effect was most strikingly depicted by the absence of ferritin synthesis in cells challenged with inorganic iron. Our results contrast with previously reported data (Hanson, E. S., and Leibold, E. A. (1998) J. Biol. Chem. 273, 7588-7593) in which a 3% oxygen atmosphere reduced IRE/IRP-1 binding in rat hepatoma cells. We discuss some possible reasons for the differences. In aggregate with other investigations involving responses to hypoxia, iron, or nitric oxide, our data indicate that cellular iron metabolic responses are complex and that IRE/IRP-1 interactions vary between cell lines and perhaps between species.

Thioredoxin activation of iron regulatory proteins. Redox regulation of RNA binding after exposure to nitric oxide

Iron regulatory proteins (IRP1 and IRP2) are redox-sensitive RNA-binding proteins that modulate the expression of several genes encoding key proteins of iron metabolism. IRP1 can also exist as an aconitase containing a [4Fe-4S] cluster bound to three cysteines at the active site. We previously showed that biosynthesis of nitric oxide (NO) induces the transition of IRP1 from aconitase to apoprotein able to bind RNA. This switch is also observed when cytosolic extracts are exposed to NO donors. However, the activation of IRP1 under these conditions is far from maximal. In this study we examined the capacity of physiological reducing systems to cooperate with NO in the activation of IRP1. Cytosolic extracts from the macrophage cell line RAW 264.7 or purified IRP1 were incubated with NO donors and subsequently exposed to glutathione or to thioredoxin (Trx), a strong protein disulfide reductase. Trx was the most effective, inducing a 2-6-fold enhancement of the RNA binding activity of NO-treated IRP1. Furthermore, the effect of NO on IRP1 from cytosolic extracts was abolished in the presence of anti-Trx antibodies. We also studied the combined effect of NO and Trx on IRP2, which exhibits constitutive RNA binding activity. We observed an inhibition of IRP2 activity following exposure to NO donors which was restored by Trx. Collectively, these results point to a crucial role of Trx as a modulator of IRP activity in situations of NO production.

Iron and gene expression: molecular mechanisms regulating cellular iron homeostasis

In recent years, specific post-transcriptional mechanisms in the cytoplasm of vertebrate cells have been elucidated that directly affect the stability and translation of mRNAs coding for central proteins in iron metabolism. This review shall focus primarily on these mechanisms. Other levels of control, either affecting gene transcription and/ or related to the function of iron-capturing substances and transmembrane transport, are also likely to exist and to influence the iron balance and utilization. They are, however, much less clear.

[Still's syndrome in the adult. A report of 8 cases with special reference to diagnostic value of ferritin]

BACKGROUND

Adult onset Still's disease (AOSD) is an uncommon, systemic, inflammatory disorder of unknown etiology, characterized by the triad of fever, arthritis and rash.

PATIENTS AND RESULTS

We describe 8 cases of AOSD (3 male, 5 female) diagnosed and treated in the Department of Rheumatology from 1980 to 1996. The delay in reaching a firm diagnosis was between 2 and 86 months, due to both lack of specific serum markers and the abundance of possible differential diagnoses. Our therapeutic strategies and results are presented and the value of obtaining serum ferritin levels for both diagnosis and follow-up studies is discussed. The patients data are compared to those of the world's literature on AOSD.

CONCLUSION

The differential diagnosis of fever of unknown origin should always include AOSD, because these patients could be spared from invasive and unnecessary diagnostic measures. Increased serum ferritin levels are of particular value in the diagnosis of acute AOSD and the normalization of the serum ferritin value is a reliable indicator of therapeutic success.

Redox modulation of iron regulatory proteins by peroxynitrite

Expression of several proteins of higher eukaryotes is post-transcriptionally regulated by interaction of iron-responsive elements (IREs) on their mRNAs and iron regulatory proteins (IRP1 and IRP2). IRP1 is a redox-sensitive iron-sulfur protein whose regulatory activity is modulated by iron depletion, synthesis of nitric oxide, or oxidative stress. IRP2 is closely related to IRP1, but it does not possess a [Fe-S] cluster. IRP2 is also regulated by intracellular iron level, but it is assumed that regulation is achieved by accelerated turn-over. In this report, the effect of peroxynitrite, a strong oxidant produced when nitric oxide and O-2 are biosynthesized simultaneously, on the RNA binding activity of IRP1 and IRP2 was investigated in vitro. Macrophage cytosolic extracts were exposed directly to a bolus addition of peroxynitrite or to SIN-1, which releases a continuous flux of peroxynitrite. Under these two experimental conditions, IRP1 lost its aconitase activity but did not gain increased capacity to bind IRE. However, addition of low amounts of the disulfide-reducing agent 2-ME during the binding assay revealed formation of a complex between IRP1 and IRE. Substrates of aconitase, which bind to the cluster of IRP1, prevented this effect, pointing to the [Fe-S] cluster as the target of peroxynitrite. Moreover, single mutation of the redox active Cys437 precluded oxidation of human recombinant IRP1 by SIN-1. Collectively, these results imply that peroxynitrite predisposes IRP1 to bind IREs under a suitable reducing environment. It is assumed that in addition to disrupting the cluster peroxynitrite also promotes disulfide bridge(s) between proximal cysteine residues in the vicinity of the IRE-binding domain, in particular Cys437. When exposed to peroxynitrite, IRP2 lost its spontaneous IRE binding activity, which was restored by further exposure to 2-mercaptoethanol, thus showing that peroxynitrite can also regulate IRP2 by a post-translational event.

Effect of antiproliferative flavonoids on ascorbic acid accumulation in human colon adenocarcinoma cells

Dietary flavonoids were found to be antiproliferative for human colon cancer cells, Caco-2 and HT-29, and rat nontransformed intestinal crypt cells, IEC-6. The antiproliferative potency was found to be structure-dependent. We report here a correlation between the antiproliferative potency of these flavonoids and their ability to inhibit cellular accumulation of ascorbic acid (vitamin C). Caco-2, HT-29 and IEC-6 cells were found to accumulate ascorbic acid in a sodium-dependent fashion although some ascorbic acid may also enter the cells through sodium-independent mechanisms. Flavonoids that have been found to be antiproliferative, quercetin and genistein, inhibited the accumulation of ascorbic acid. The inhibition was dose-dependent and could be observed after as short as 10-min of incubation. The degree of inhibition of accumulation was more during rapid cell division as compared to post-confluency Caco-2 cells. Flavonoids that were found to show little antiproliferative effect, naringenin and catechin, also had little effect on ascorbic acid accumulation. The antiproliferative property of flavonoids could be linked to their ascorbic acid deprivation property.

Resistance to the antitumor agent gallium nitrate in human leukemic cells is associated with decreased gallium/iron uptake, increased activity of iron regulatory protein-1, and decreased ferritin production

The mechanism of drug resistance to gallium nitrate is not known. Since gallium can be incorporated into ferritin, an iron storage protein that protects cells from iron toxicity, we investigated whether ferritin expression was altered in gallium-resistant (R) CCRF-CEM cells. We found that the ferritin content of R cells was decreased, while heavy chain ferritin mRNA levels and iron regulatory protein-1 (IRP-1) RNA binding activity were increased. IRP-1 protein levels were similar in gallium-sensitive (S) and R cells, indicating that R cells contain a greater proportion of IRP-1 in a high affinity mRNA binding state. 59Fe uptake and transferrin receptor expression were decreased in R cells. In both S and R cells, gallium inhibited cellular 59Fe uptake, increased ferritin mRNA and protein, and decreased IRP-1 binding activity. Gallium uptake by R cells was markedly diminished; however, the sensitivity of R cells to gallium could be restored by increasing their uptake of gallium with excess transferrin. Our results suggest that R cells have developed resistance to gallium by down-regulating their uptake of gallium. In parallel, iron uptake by R cells is also decreased, leading to changes in iron homeostasis. Furthermore, since gallium has divergent effects on iron uptake and ferritin synthesis, its action may also include a direct effect on ferritin mRNA induction and IRP-1 activity.

Molecular control of vertebrate iron metabolism: mRNA-based regulatory circuits operated by iron, nitric oxide, and oxidative stress

As an essential nutrient and a potential toxin, iron poses an exquisite regulatory problem in biology and medicine. At the cellular level, the basic molecular framework for the regulation of iron uptake, storage, and utilization has been defined. Two cytoplasmic RNA-binding proteins, iron-regulatory protein-1 (IRP-1) and IRP-2, respond to changes in cellular iron availability and coordinate the expression of mRNAs that harbor IRP-binding sites, iron-responsive elements (IREs). Nitric oxide (NO) and oxidative stress in the form of H2O2 also signal to IRPs and thereby influence cellular iron metabolism. The recent discovery of two IRE-regulated mRNAs encoding enzymes of the mitochondrial citric acid cycle may represent the beginnings of elucidating regulatory coupling between iron and energy metabolism. In addition to providing insights into the regulation of iron metabolism and its connections with other cellular pathways, the IRE/IRP system has emerged as a prime example for the understanding of translational regulation and mRNA stability control. Finally, IRP-1 has highlighted an unexpected role for iron sulfur clusters as post-translational regulatory switches.