Translational enhancement of H-ferritin mRNA by interleukin-1 beta acts through 5' leader sequences distinct from the iron responsive element

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.

Elevated ferritin, mediated by IL-18 is associated with systemic inflammation and mortality in acute respiratory distress syndrome (ARDS)

BACKGROUND

Inflammatory subphenotypes have been identified in acute respiratory distress syndrome (ARDS). Hyperferritinaemia in sepsis is associated with hyperinflammation, worse clinical outcomes, and may predict benefit with immunomodulation. Our aim was to determine if raised ferritin identified a subphenotype in patients with ARDS.

METHODS

Baseline plasma ferritin concentrations were measured in patients with ARDS from two randomised controlled trials of simvastatin (Hydroxymethylglutaryl-CoA Reductase Inhibition with Simvastatin in Acute Lung Injury to Reduce Pulmonary Dysfunction-2 (HARP-2); discovery cohort, UK) and neuromuscular blockade (ROSE; validation cohort, USA). Results were analysed using a logistic regression model with restricted cubic splines, to determine the ferritin threshold associated with 28-day mortality.

RESULTS

Ferritin was measured in 511 patients from HARP-2 (95% of patients enrolled) and 847 patients (84% of patients enrolled) from ROSE. Ferritin was consistently associated with 28-day mortality in both studies and following a meta-analysis, a log-fold increase in ferritin was associated with an OR 1.71 (95% CI 1.01 to 2.90) for 28-day mortality. Patients with ferritin >1380 ng/mL (HARP-2 28%, ROSE 24%) had a significantly higher 28-day mortality and fewer ventilator-free days in both studies. Mediation analysis, including confounders (acute physiology and chronic health evaluation-II score and ARDS aetiology) demonstrated a statistically significant contribution of interleukin (IL)-18 as an intermediate pathway between ferritin and mortality.

CONCLUSIONS

Ferritin is a clinically useful biomarker in ARDS and is associated with worse patient outcomes. These results provide support for prospective interventional trials of immunomodulatory agents targeting IL-18 in this hyperferritinaemic subgroup of patients with ARDS.

Advances in Ferritin Physiology and Possible Implications in Bacterial Infection

Due to its advantageous redox properties, iron plays an important role in the metabolism of nearly all life. However, these properties are not only a boon but also the bane of such life forms. Since labile iron results in the generation of reactive oxygen species by Fenton chemistry, iron is stored in a relatively safe form inside of ferritin. Despite the fact that the iron storage protein ferritin has been extensively researched, many of its physiological functions are hitherto unresolved. However, research regarding ferritin's functions is gaining momentum. For example, recent major discoveries on its secretion and distribution mechanisms have been made as well as the paradigm-changing finding of intracellular compartmentalization of ferritin via interaction with nuclear receptor coactivator 4 (NCOA4). In this review, we discuss established knowledge as well as these new findings and the implications they may have for host-pathogen interaction during bacterial infection.

Interactive effects of serum ferritin and high sensitivity C-reactive protein on diabetes in hypertensive patients

BACKGROUND

Hypertensive patients, often characterized by chronic inflammation, are susceptible to diabetes. Evidence suggests that the positive association between serum ferritin (SF) and diabetes was affected by high-sensitivity C-reactive protein (hs-CRP), an inflammation marker. We investigate whether there was an interaction between SF and hs-CRP on diabetes in hypertensive patients.

METHODS

We analysed data of 1,735 hypertensive people in this cross-sectional study. Diabetes was diagnosed when fasting blood glucose ≥ 7.0 mmol/L and/or a previous clinical diagnosis of diabetes. Logistic regression models were used to estimate the association of the SF and hs-CRP with diabetes. Multiplicative interaction was evaluated by incorporating a cross-product term for SF and hs-CRP to the logistic regression model. Additive interaction was assessed by calculating the relative excess risk of interaction (RERI) and attributed proportion due to interaction (AP).

RESULTS

In the adjusted analysis, SF (highest vs lowest tertile: odds ratio [OR], 1.61; 95 % confidence interval [CI], 1.20-2.16) was positively associated with diabetes. There was no multiplicative interaction between SF and hs-CRP, but evidence of additive interaction in regard to diabetes (RERI: 0.86; 95 % CI: 0.06-1.67). Compared to the patients with low SF (lower two thirds) and low hs-CRP (≤ 2 mg/L), those with high SF (upper one third) and high hs-CRP (> 2 mg/L) had increased OR for diabetes (adjusted OR: 2.33 [1.65-3.30]), with 37.0 % of the effects attributed to the additive interaction (AP: 0.37; 95 % CI: 0.09-0.65).

CONCLUSIONS

Within a cross-sectional study consisting of hypertensive patients, co-exposure to high SF and high hs-CRP was synergistically associated with diabetes. Dietary intervention or pharmacological treatment to lowering SF concentration may help to reduce diabetes morbidity in hypertensive patient with chronic inflammation.

Expression Patterns in Reductive Iron Assimilation and Functional Consequences during Phagocytosis of Lichtheimia corymbifera, an Emerging Cause of Mucormycosis

Iron is an essential micronutrient for most organisms and fungi are no exception. Iron uptake by fungi is facilitated by receptor-mediated internalization of siderophores, heme and reductive iron assimilation (RIA). The RIA employs three protein groups: (i) the ferric reductases (Fre5 proteins), (ii) the multicopper ferroxidases (Fet3) and (iii) the high-affinity iron permeases (Ftr1). Phenotyping under different iron concentrations revealed detrimental effects on spore swelling and hyphal formation under iron depletion, but yeast-like morphology under iron excess. Since access to iron is limited during pathogenesis, pathogens are placed under stress due to nutrient limitations. To combat this, gene duplication and differential gene expression of key iron uptake genes are utilized to acquire iron against the deleterious effects of iron depletion. In the genome of the human pathogenic fungus L. corymbifera, three, four and three copies were identified for FRE5, FTR1 and FET3 genes, respectively. As in other fungi, FET3 and FTR1 are syntenic and co-expressed in L. corymbifera. Expression of FRE5, FTR1 and FET3 genes is highly up-regulated during iron limitation (Fe-), but lower during iron excess (Fe+). Fe- dependent upregulation of gene expression takes place in LcFRE5 II and III, LcFTR1 I and II, as well as LcFET3 I and II suggesting a functional role in pathogenesis. The syntenic LcFTR1 I–LcFET3 I gene pair is co-expressed during germination, whereas LcFTR1 II- LcFET3 II is co-expressed during hyphal proliferation. LcFTR1 I, II and IV were overexpressed in Saccharomyces cerevisiae to represent high and moderate expression of intracellular transport of Fe3+, respectively. Challenge of macrophages with the yeast mutants revealed no obvious role for LcFTR1 I, but possible functions of LcFTR1 II and IVs in recognition by macrophages. RIA expression pattern was used for a new model of interaction between L. corymbifera and macrophages.

Glycyrrhizin prevents SARS-CoV-2 S1 and Orf3a induced high mobility group box 1 (HMGB1) release and inhibits viral replication

Efforts to understand host factors critical for COVID-19 pathogenesis have identified high mobility group box 1 (HMGB1) to be crucial for regulating susceptibility to SARS-CoV-2. COVID-19 disease severity is correlated with heightened inflammatory responses, and HMGB1 is an important extracellular mediator in inflammation processes. In this study, we evaluated the effect of HMGB1 inhibitor Glycyrrhizin on the cellular perturbations in lung cells expressing SARS-CoV-2 viral proteins. Pyroptosis in lung cells transfected with SARS-CoV-2 S-RBD and Orf3a, was accompanied by elevation of IL-1β and extracellular HMGB1 levels. Glycyrrhizin mitigated viral proteins-induced lung cell pyroptosis and activation of macrophages. Heightened release of proinflammatory cytokines IL-1β, IL-6 and IL-8, as well as ferritin from macrophages cultured in conditioned media from lung cells expressing SARS-CoV-2 S-RBD and Orf3a was attenuated by glycyrrhizin. Importantly, Glycyrrhizin inhibited SARS-CoV-2 replication in Vero E6 cells without exhibiting cytotoxicity at high doses. The dual ability of Glycyrrhizin to concomitantly halt virus replication and dampen proinflammatory mediators might constitute a viable therapeutic option in patients with SARS-CoV-2 infection.

Hepcidin-to-Ferritin Ratio Is Decreased in Astrocytes With Extracellular Alpha-Synuclein and Iron Exposure

Astrocytes are the most abundant glial cells in the central nervous system (CNS). As indispensable elements of the neurovascular unit, they are involved in the inflammatory response and disease-associated processes. Alpha-synuclein (α-syn) is released into the extracellular space by neurons and can be internalized by adjacent astrocytes, which activates glial cells to induce neuroinflammation. We were interested in whether astrocyte-mediated neuroinflammation is modulated by intracellular iron status and extracellular α-syn. Our results showed that recombinant α-syn (1 μg/ml and 5 μg/ml) treatment for 24 h did not affect the expression of the iron transporters divalent metal transporter 1 (DMT1) and ferroportin 1 (FPN1), nor those of iron regulatory protein (IRP) 1 or IRP2. Several proinflammatory cytokines, including tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, and IL-6 exhibited up-regulated mRNA levels in 5 μg/ml α-syn-treated astrocytes. TNF-α release was increased, indicating that inflammatory responses were triggered in these cells. Pretreatment with the iron-overload reagent ferric ammonium citrate (FAC, 100 μmol/L) for 24 h had no effects on mRNA levels and release of proinflammatory cytokines. Inflammatory responses triggered by α-syn were not affected by iron overload. The iron chelator desferrioxamine (DFO, 100 μmol/L) exerted suppressive effects on TNF-α mRNA levels, although no change was observed for TNF-α release. Hepcidin mRNA levels were down-regulated significantly in astrocytes co-treated with FAC and α-syn, although independent treatment with either FAC or α-syn did not alter hepcidin levels. In contrast, hepcidin mRNA levels were up-regulated in DFO and α-syn co-treated cells. As expected, ferritin protein levels were up-regulated or down-regulated with FAC or DFO treatment, respectively. Following the up-regulation of ferritin mediated by α-syn, hepcidin-to-ferritin levels were indicative of modulatory effects in α-syn-treated astrocytes with altered iron status. Therefore, we propose that the hepcidin-to-ferritin ratio is indicative of a detrimental response in primary cultured astrocytes experiencing iron and extracellular α-syn.

Iron Deficiency in Chronic Kidney Disease: Updates on Pathophysiology, Diagnosis, and Treatment

Anemia is a complication that affects a majority of individuals with advanced CKD. Although relative deficiency of erythropoietin production is the major driver of anemia in CKD, iron deficiency stands out among the mechanisms contributing to the impaired erythropoiesis in the setting of reduced kidney function. Iron deficiency plays a significant role in anemia in CKD. This may be due to a true paucity of iron stores (absolute iron deficiency) or a relative (functional) deficiency which prevents the use of available iron stores. Several risk factors contribute to absolute and functional iron deficiency in CKD, including blood losses, impaired iron absorption, and chronic inflammation. The traditional biomarkers used for the diagnosis of iron-deficiency anemia (IDA) in patients with CKD have limitations, leading to persistent challenges in the detection and monitoring of IDA in these patients. Here, we review the pathophysiology and available diagnostic tests for IDA in CKD, we discuss the literature that has informed the current practice guidelines for the treatment of IDA in CKD, and we summarize the available oral and intravenous (IV) iron formulations for the treatment of IDA in CKD. Two important issues are addressed, including the potential risks of a more liberal approach to iron supplementation as well as the potential risks and benefits of IV versus oral iron supplementation in patients with CKD.

Hyperferritinemia and inflammation

Understanding of ferritin biology has traditionally centered on its role in iron storage and homeostasis, with low ferritin levels indicative of deficiency and high levels indicative of primary or secondary hemochromatosis. However, further work has shown that iron, redox biology and inflammation are inexorably linked. During infection, increased ferritin levels represent an important host defense mechanism that deprives bacterial growth of iron and protects immune cell function. It may also be protective, limiting the production of free radicals and mediating immunomodulation. Additionally, hyperferritinemia is a key acute-phase reactants, used by clinicians as an indication for therapeutic intervention, aimed at controlling inflammation in high-risk patients. One school of thought maintains that hyperferritinemia is an 'innocent bystander' biomarker of uncontrolled inflammation that can be used to gauge effectiveness of intervention. Other schools of thought maintain that ferritin induction could be a protective negative regulatory loop. Others maintain that ferritin is a key mediator of immune dysregulation, especially in extreme hyperferritinemia, via direct immune-suppressive and pro-inflammatory effects. There is a clear need for further investigation of the role of ferritin in uncontrolled inflammatory conditions both as a biomarker and mediator of disease because its occurrence identifies patients with high mortality risk and its resolution predicts their improved survival.

Induction of Interleukin-1β by Human Immunodeficiency Virus-1 Viral Proteins Leads to Increased Levels of Neuronal Ferritin Heavy Chain, Synaptic Injury, and Deficits in Flexible Attention

Synaptodendritic pruning and alterations in neurotransmission are the main underlying causes of HIV-associated neurocognitive disorders (HAND). Our studies in humans and nonhuman primates indicated that the protein ferritin heavy chain (FHC) is a critical player in neuronal changes and ensuing cognitive deficit observed in these patients. Here we focus on the effect of HIV proteins and inflammatory cytokines implicated in HAND on neuronal FHC levels, dendritic changes, and neurocognitive behavior. In two well characterized models of HAND (HIV transgenic and gp120-treated rats), we report reductions in spine density and dendritic branches in prefrontal cortex pyramidal neurons compared with age-matched controls. FHC brain levels are elevated in these animals, which also show deficits in reversal learning. Moreover, IL-1β, TNF-α, and HIV gp120 upregulate FHC in rat cortical neurons. However, although the inflammatory cytokines directly altered neuronal FHC, gp120 only caused significant FHC upregulation in neuronal/glial cocultures, suggesting that glia are necessary for sustained elevation of neuronal FHC by the viral protein. Although the envelope protein induced secretion of IL-1β and TNF-α in cocultures, TNF-α blockade did not affect gp120-mediated induction of FHC. Conversely, studies with an IL-1β neutralizing antibody or specific IL-1 receptor antagonist revealed the primary involvement of IL-1β in gp120-induced FHC changes. Furthermore, silencing of neuronal FHC abrogates the effect of gp120 on spines, and spine density correlates negatively with FHC levels or cognitive deficit. These results demonstrate that viral and host components of HIV infection increase brain expression of FHC, leading to cellular and functional changes, and point to IL-1β-targeted strategies for prevention of these alterations. Significance statement: This work demonstrates the key role of the cytokine IL-1β in the regulation of a novel intracellular mediator [i.e., the protein ferritin heavy chain (FHC)] of HIV-induced dendritic damage and the resulting neurocognitive impairment. This is also the first study that systematically investigates dendritic damage in layer II/III prefrontal cortex neurons of two different non-infectious models of HIV-associated neurocognitive disorders (HAND) and reveals a precise correlation of these structural changes with specific biochemical and functional alterations also reported in HIV patients. Overall, these data suggest that targeting the IL-1β-dependent FHC increase may represent a valid strategy for neuroprotective adjuvant therapies in HAND.

Clinical features and correct diagnosis of macrophage activation syndrome

Macrophage activation syndrome (MAS) is increasingly recognized among febrile hospitalized patients. Clinically, MAS resembles multiorgan dysfunction and shock. Laboratory features include hepatobiliary dysfunction, coagulopathy, pancytopenia, hyperferritinemia and markers of immune activation. Pathologically, hemophagocytosis is commonly seen but is only present in 60% of MAS patients. MAS, or secondary hemophagocytic lymphohistiocytosis (HLH), is triggered by infectious (e.g., herpes family viruses), rheumatologic (e.g., systemic lupus erythematosus [SLE]) and oncologic (e.g., T-cell leukemia) conditions. Formal HLH criteria, while specific, are frequently insensitive for MAS diagnosis. Thus, disease-specific (e.g., SLE) and generic MAS criteria have been published. Recently, novel criteria for MAS in children with systemic juvenile idiopathic arthritis (sJIA) were developed and are a key focus of this review.

Coupling heme and iron metabolism via ferritin H chain

SIGNIFICANCE

Inflammation and immunity can be associated with varying degrees of heme release from hemoproteins, eventually leading to cellular and tissue iron (Fe) overload, oxidative stress, and tissue damage. Presumably, these deleterious effects contribute to the pathogenesis of systemic infections.

RECENT ADVANCES

Heme release from hemoglobin sensitizes parenchyma cells to undergo programmed cell death in response to proinflammatory cytokines, such as tumor necrosis factor. This cytotoxic effect is driven by a mechanism involving intracellular accumulation of free radicals, which sustain the activation of the c-Jun N-terminal kinase (JNK) signaling transduction pathway. While heme catabolism by heme oxygenase-1 (HO-1) prevents programmed cell death, this cytoprotective effect requires the co-expression of ferritin H (heart/heavy) chain (FTH), which controls the pro-oxidant effect of labile Fe released from the protoporphyrin IX ring of heme. This antioxidant effect of FTH restrains JNK activation, whereas JNK activation inhibits FTH expression, a cross talk that controls metabolic adaptation to cellular Fe overload associated with systemic infections.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Identification and characterization of the mechanisms via which FTH provides metabolic adaptation to tissue Fe overload should provide valuable information to our current understanding of the pathogenesis of systemic infections as well as other immune-mediated inflammatory diseases.

Iron therapy in chronic kidney disease: current controversies

Anaemia in chronic kidney disease (CKD) is a complex disease that requires an integrated approach to incorporate both diagnostic and therapeutic interventions and to address the different facets of its aetiology and pathophysiology. The advent of erythropoiesis stimulating agents (ESA) has revolutionised the therapy of anaemia of CKD, and has resulted in a significant decline in the need for blood transfusions in CKD patients. The routine application of ESA has also led to the need for concomitant iron supplementation. ESA and iron therapy now form the cornerstone of anaemia management in CKD. Intravenous iron administration is effective with acceptable safety, and may improve ESA responsiveness. However, less is known about the long-term safety of iron supplementation in CKD patients. Whereas maintenance (weekly to monthly) intravenous iron has been routinely used in maintenance dialysis patients, iron replacement in patients with non-dialysis-dependent CKD is less well studied, in spite of the much larger number of patients affected. This review discusses iron supplementation in CKD with an emphasis toward controversial issues that continue to pose dilemmas in clinical practice. Concerns related to both the optimal amount of iron supplementation and to the safety of various agents available in clinical practice are presented.

Relationship between iron-deficiency anemia and periodontal status in female patients

BACKGROUND

The aim of this study was to evaluate the periodontal status of patients with iron-deficiency anemia (IDA) and the correlation of changes in serum and gingival crevicular fluid (GCF) ferritin levels after periodontal therapy.

METHODS

Nineteen female patients with anemic hematologic values were classified as group A, and 20 healthy females with normal hematologic values were classified as group B. After group A was recruited, group B was enrolled with patients who had similar gingival indices as group A. At baseline and the 3-month follow-up visit, clinical periodontal indices and hematologic parameters were recorded, and GCF samples were taken. All patients received an oral hygiene-improvement session followed by scaling, and sites with >4-mm probing depths received root planing. At the 3-month follow-up visit, all measurements and analyses were repeated.

RESULTS

During the follow-up period, all clinical indices decreased in both groups (P <0.05), but the gingival index in group A did not change. The GCF ferritin concentration showed statistically significant decreases (P <0.05), but total amounts of ferritin in GCF did not change. No significant correlation was found between serum and GCF ferritin levels.

CONCLUSION

The findings of this study showed that changes in serum ferritin levels did not correlate with the GCF ferritin levels, and IDA was not a direct risk factor for periodontal diseases.

Amyloid precursor protein and alpha synuclein translation, implications for iron and inflammation in neurodegenerative diseases

Recent studies that alleles in the hemochromatosis gene may accelerate the onset of Alzheimer's disease by five years have validated interest in the model in which metals (particularly iron) accelerate disease course. Biochemical and biophysical measurements demonstrated the presence of elevated levels of neurotoxic copper zinc and iron in the brains of AD patients. Intracellular levels of APP holoprotein were shown to be modulated by iron by a mechanism that is similar to the translation control of the ferritin L- and H mRNAs by iron-responsive element (IRE) RNA stem loops in their 5' untranslated regions (5'UTRs). More recently a putative IRE-like sequence was hypothesized present in the Parkinsons's alpha synuclein (ASYN) transcript (see [A.L. Friedlich, R.E. Tanzi, J.T. Rogers, The 5'-untranslated region of Parkinson's disease alpha-synuclein messenger RNA contains a predicted iron responsive element, Mol. Psychiatry 12 (2007) 222-223. [6]]). Together with the demonstration of metal dependent translation of APP mRNA, the involvement of metals in the plaque of AD patients and of increased iron in striatal neurons in the substantia nigra (SN) of Parkinson's disease patients have stimulated the development of metal attenuating agents and iron chelators as a major new therapeutic strategy for the treatment of these neurodegenerative diseases. In the case of AD, metal based therapeutics may ultimately prove more cost effective than the use of an amyloid vaccine as the preferred anti-amyloid therapeutic strategy to ameliorate the cognitive decline of AD patients.

Ferritin heavy chain-mediated iron homeostasis and subsequent increased reactive oxygen species production are essential for epithelial-mesenchymal transition

The epithelial-mesenchymal transition (EMT) plays a critical role in tumor progression. To obtain a broad view of the molecules involved in EMT, we carried out a comparative proteomic analysis of transforming growth factor-beta1 (TGF-beta1)-induced EMT in AML-12 murine hepatocytes. A total of 36 proteins with significant alterations in abundance were identified. Among these proteins, ferritin heavy chain (FHC), a cellular iron storage protein, was characterized as a novel modulator in TGF-beta1-induced EMT. In response to TGF-beta1, there was a dramatic decrease in the FHC levels, which caused iron release from FHC and, therefore, increased the intracellular labile iron pool (LIP). Abolishing the increase in LIP blocked TGF-beta1-induced EMT. In addition, increased LIP levels promoted the production of reactive oxygen species (ROS), which in turn activated p38 mitogen-activated protein kinase. The elimination of ROS inhibited EMT, whereas H2O2 treatment rescued TGF-beta1-induced EMT in cells in which the LIP increase was abrogated. Overexpression of exogenous FHC attenuated the increases in LIP and ROS production, leading to a suppression of EMT. We also showed that TGF-beta1-mediated down-regulation of FHC occurs via 3' untranslated region-dependent repression of the translation of FHC mRNA. Moreover, we found that FHC down-regulation is an event that occurs between the early and highly invasive advanced stages in esophageal adenocarcinoma and that depletion of LIP or ROS suppresses the migration of tumor cells. Our data show that cellular iron homeostasis regulated by FHC plays a critical role in TGF-beta1-induced EMT.

Serum Ferritin: Deceptively Simple or Simply Deceptive? Lessons Learned From Iron Therapy in Patients With Chronic Kidney Disease

Iron is an essential micronutrient that is indispensable for erythropoesis. Correct assessment of iron stores is needed both for the diagnosis of iron deficiency and to direct iron replacement therapies. Serum ferritin is a commonly employed measure to assess iron stores, yet there are caveats that influence its accuracy as a diagnostic tool. While low ferritin levels are specific for iron deficiency, high levels can be the result of inflammation, liver disease, or malignancies and could be independent of iron stores. Optimal anemia management involves administration of adequate amounts of iron. The right dose of iron that allows optimal erythropoesis yet avoids oxidative stress is a matter of ongoing debate, especially when using imperfect diagnostic tools such as serum ferritin to direct therapy. Data from hemodialysis patients are presented to illustrate the challenges one faces when trying to achieve the best possible therapeutic benefit from iron-replacement therapy.

The fascinating but deceptive ferritin: to measure it or not to measure it in chronic kidney disease?

Although the emergence of erythropoiesis-stimulating agents has revolutionized the anemia management of chronic kidney disease (CKD) in the past two decades, strategies to assess iron (Fe) status and to provide Fe supplementation have remained indistinct. The reported cases of hemochromatosis in dialysis patients from the pre-erythropoiesis-stimulating agent era along with the possible associations of Fe with infection and oxidative stress have fueled the "iron apprehension." To date, no reliable marker of Fe stores in CKD has been agreed on. Serum ferritin continues to be the focus of attention. Almost half of all maintenance hemodialysis patients have a serum ferritin >500 ng/ml. In this ferritin range, Fe supplementation currently is not encouraged, although most reported hemochromatosis cases had a serum ferritin >2000 ng/ml. The moderate-range hyperferritinemia (500 to 2000 ng/ml) seems to be due mostly to non-Fe-related conditions, including inflammation, malnutrition, liver disease, infection, and malignancy. Recent epidemiologic studies have shown that a low, rather than a high, serum Fe is associated with a poor survival in maintenance hemodialysis patients. In multivariate adjusted models that mitigate the confounding effect of malnutrition-inflammation, serum ferritin <1200 ng/ml and Fe saturation ratio in 30 to 50% range are associated with the greatest survival in maintenance hemodialysis patients. Although ferritin is a fascinating molecule, moderate hyperferritinemia is a misleading marker of Fe stores in patients with CKD. It may be time to revisit the utility of serum ferritin in CKD and ask ourselves whether its measurement has helped us or has caused more confusion and controversy.

Melatonin corrects reticuloendothelial blockade and iron status in haemodialysed patients

AIM

Treatment of anaemia in haemodialysed patients in the setting of inflammation usually displays high levels of serum ferritin (>800 ng/mL) and low transferrin saturation (TSAT) (<20%) despite i.v. iron supplementation, thus proving iron trapping in the reticuloendothelial system. Melatonin has been reported to reduce cytokine production and, in dialysis patients, to prevent oxidative stress resulting from iron and erythropoietin treatment.

METHOD

In this study, we evaluated a group of 10 patients undergoing haemodialysis who displayed elevated serum ferritin (981 +/- 44.6 ng/mL) and TSAT <20% (15.6 +/- 3.8%) after having received 1.2 g of i.v. iron dextran over a period of 8 weeks. These patients received oral melatonin, 6 mg/day at night for 30 days.

RESULTS

After this treatment, all of them markedly increased TSAT values, reaching 35.5 +/- 6.7% (P < 0.0001 vs basal values). In addition, ferritin values decreased to 754.4 +/- 263.7 ng/mL (P < 0.05), and serum iron dramatically increased in all of the patients under study (42.4 +/- 9.4 vs 109.7 +/- 24.3 microg/dL; P < 0.0001). Values for haematocrit (28.6 +/- 2.7 vs 31.9 +/- 3.57%; P < 0.05) and haemoglobin (9.19 +/- 0.97 vs 10.04 +/- 1.29 g/dL; P < 0.05) were also improved. Measurements were then repeated 2 weeks after melatonin withdrawal, showing an impressive decrease in TSAT (16.4 +/- 5.3%; P < 0.00001) and serum iron (48 +/- 14.7 microg/dL; P < 0.0001) values and an almost significant increase in ferritin values (954.4 +/- 86 ng/mL; P < 0.054).

CONCLUSION

The present study demonstrates that melatonin may strongly correct the reticuloendothelial blockade seen in dialysis patients under an inflammatory status, thus allowing a better management of iron derangements and renal anaemia.

The acute box cis-element in human heavy ferritin mRNA 5'-untranslated region is a unique translation enhancer that binds poly(C)-binding proteins

Intracellular levels of the light (L) and heavy (H) ferritin subunits are regulated by iron at the level of message translation via a modulated interaction between the iron regulatory proteins (IRP1 and IRP2) and a 5'-untranslated region. Iron-responsive element (IRE). Here we show that iron and interleukin-1beta (IL-1beta) act synergistically to increase H- and L-ferritin expression in hepatoma cells. A GC-rich cis-element, the acute box (AB), located downstream of the IRE in the H-ferritin mRNA 5'-untranslated region, conferred a substantial increase in basal and IL-1beta-stimulated translation over a similar time course to the induction of endogenous ferritin. A scrambled version of the AB was unresponsive to IL-1. Targeted mutation of the AB altered translation; reverse orientation and a deletion of the AB abolished the wild-type stem-loop structure and abrogated translational enhancement, whereas a conservative structural mutant had little effect. Labeled AB transcripts formed specific complexes with hepatoma cell extracts that contained the poly(C)-binding proteins, iso-alphaCP1 and -alphaCP2, which have well defined roles as translation regulators. Iron influx increased the association of alphaCP1 with ferritin mRNA and decreased the alphaCP2-ferritin mRNA interaction, whereas IL-1beta reduced the association of alphaCP1 and alphaCP2 with H-ferritin mRNA. In summary, the H-ferritin mRNA AB is a key cis-acting translation enhancer that augments H-subunit expression in Hep3B and HepG2 hepatoma cells, in concert with the IRE. The regulated association of H-ferritin mRNA with the poly(C)-binding proteins suggests a novel role for these proteins in ferritin translation and iron homeostasis in human liver.

ANP-induced decrease of iron regulatory protein activity is independent of HO-1 induction

Atrial natriuretic peptide (ANP)-preconditioned livers are protected from ischemia-reperfusion injury. ANP-treated organs show increased expression of heme oxygenase (HO)-1. Because HO-1 liberates bound iron, the aim of our study was to determine whether ANP affects iron regulatory protein (IRP) activity and, thus, the levels of ferritin. Rat livers were perfused with Krebs-Henseleit buffer [+/-ANP, 8-bromo-cGMP (8-Br-cGMP), and tin protoporphyrin, 20 min], stored in University of Wisconsin solution (4 degrees C, 24 h), and reperfused (120 min). IRP activity was assessed by gel-shift assays, and ferritin, IRP phosphorylation, and PKC localization were assessed by Western blot. Control livers displayed decreased IRP activity at the end of ischemia but no change in ferritin content during ischemia and reperfusion. ANP-pretreated livers showed reduced IRP activity, an effect mimicked by 8-Br-cGMP. Ferritin levels were increased in ANP-pretreated organs. Simultaneous perfusion of livers with ANP and tin protoporphyrin did not reduce ANP-induced action, arguing against a role for HO-1 in changes in IRP activity. ANP and 8-Br-cGMP decreased membrane localization of PKC-alpha and PKC-epsilon, but this modulation of PKC seems unrelated to inhibition of IRP binding. This work shows the cGMP-mediated attenuation of IRP binding activity by ANP, which results in increased hepatic ferritin levels. This change in IRPs is independent of ANP-induced HO-1 and reduced PKC activation.

Alterations in cellular IRP-dependent iron regulation by in vitro manganese exposure in undifferentiated PC12 cells

Manganese (Mn) may interfere with iron regulation by altering the binding of iron regulatory proteins (IRPs) to their response elements found on the mRNA encoding proteins critical to iron homeostasis. To explore this, the effects of 24-h in vitro manganese exposure (1, 10, 50, and 200 microM Mn) on: (i) total intracellular and labile iron concentrations; (ii) the cellular abundance of transferrin receptor (TfR), H- and L-ferritin, and mitochondrial aconitase proteins; and (iii) IRP binding to a [32P](-) labeled mRNA sequence of L-ferritin were evaluated in undifferentiated PC12 cells. In vitro manganese exposure altered the cellular abundance of TfR, H-/L-ferritin, and m-aconitase, resulting in an increase in labile iron. This latter effect led to a decrease in IRP binding activity at the lower (10 and 50 microM) manganese exposures. In contrast, 200 microM manganese exposure increased IRP binding, in spite of the significant increase in labile iron. These data indicate that at lower exposures, manganese directly interfered with IRP-dependent translational events, producing an increase in labile iron, which in turn signaled a decrease in IRP binding at 24 h. At higher exposures, the intracellular burden of manganese resulted in overt cytotoxicity and appeared to compromise the normal compensatory response to increased labile iron, producing increased IRP binding. We conclude that low to moderate manganese exposure interferes with cellular iron regulation, and thus may serve as a contributory mechanism underlying manganese neurotoxicity.

Maternal second-trimester serum ferritin concentrations and subsequent risk of preterm delivery

We studied the relationship between maternal second-trimester serum ferritin concentrations and preterm delivery. The 312 preterm delivery cases, studied in aggregate and in subgroups [spontaneous preterm labour, preterm premature rupture of membranes, medically induced preterm delivery, moderate preterm delivery (gestational age at delivery 34-36 weeks) and very preterm delivery (gestational age at delivery <34 weeks)] were compared with 424 randomly selected women who delivered at term. Maternal ferritin concentrations, measured in serum collected at 17 weeks gestation on average, was determined using a two-site chemiluminometric immunoassay. Using multiple logistic regression, we derived maximum likelihood estimates of adjusted odds ratios (OR) and 95% confidence intervals [CI]. Elevation in maternal second-trimester ferritin was weakly associated with the risk of preterm delivery overall. After adjusting for possible confounding by maternal age, race/ethnicity, parity, Medicaid payment status and smoking during the index pregnancy, the OR for extreme quartiles (>64.5 vs. <26.0 ng/mL) of ferritin was 1.3 [95% CI 0.8, 2.1]. Stratified analyses indicated that elevated maternal serum ferritin was associated with an increased risk of preterm premature rupture of membranes (OR = 2.1; 95% CI 1.1, 4.1), but not with spontaneous preterm labour (OR = 0.9; 95% CI 0.4, 1.7) or medically induced preterm delivery (OR = 1.1; 95% CI 0.6, 2.0). The relationship between elevated maternal second-trimester serum ferritin concentrations and preterm delivery was strongest for spontaneous very preterm deliveries (<34 weeks gestation). Women with ferritin concentrations in the highest decile (>96 ng/mL) experienced a 2.7-fold increased risk of delivering before 34 completed weeks, compared with women with concentrations <26.0 ng/mL. These results are consistent with some previous reports, and further underline the potential for heterogeneity in the aetiology of preterm delivery.

MRNA stability and the control of gene expression: implications for human disease

Regulation of gene expression is essential for the homeostasis of an organism, playing a pivotal role in cellular proliferation, differentiation, and response to specific stimuli. Multiple studies over the last two decades have demonstrated that the modulation of mRNA stability plays an important role in regulating gene expression. The stability of a given mRNA transcript is determined by the presence of sequences within an mRNA known as cis-elements, which can be bound by trans-acting RNA-binding proteins to inhibit or enhance mRNA decay. These cis-trans interactions are subject to a control by a wide variety of factors including hypoxia, hormones, and cytokines. In this review, we describe mRNA biosynthesis and degradation, and detail the cis-elements and RNA-binding proteins known to affect mRNA turnover. We present recent examples in which dysregulation of mRNA stability has been associated with human diseases including cancer, inflammatory disease, and Alzheimer's disease.

Ferritin, iron homeostasis, and oxidative damage

Ferritin is one of the major proteins of iron metabolism. It is almost ubiquitous and tightly regulated by the metal. Biochemical and structural properties of the ferritins are largely conserved from bacteria to man, although the role in the regulation of iron trafficking varies in the different organisms. Recent studies have clarified some of the major aspects of the reaction between iron and ferritin, which results in the formation of the iron core and production of hydrogen peroxide. The characterization of cellular models in which ferritin expression is modulated has shown that the ferroxidase catalytic site on the H-chain has a central role in regulating iron availability. In turn, this has secondary effects on a number of cellular activities, which include proliferation and resistance to oxidative damage. Moreover, the response to apoptotic stimuli is affected by H-ferritin expression. Altered ferritin L-chain expression has been found in at least two types of genetic disorders, although its role in the determination of the pathology has not been fully clarified. The recent discovery of a new ferritin specific for the mitochondria, which is functionally similar to the H-ferritin, opens new perspectives in the study of the relationships between iron, oxidative damage and free radicals.

Mechanism of interleukin-1- and tumor necrosis factor alpha-dependent regulation of the alpha 1-antichymotrypsin gene in human astrocytes

The expression of alpha(1)-antichymotrypsin (ACT) is significantly enhanced in affected brain regions in Alzheimer's disease. This serine proteinase inhibitor specifically colocalizes with filamentous beta-amyloid deposits and recently has been shown to influence both formation and destabilization of beta-amyloid fibrils. In the brain, ACT is expressed in astrocytes, and interleukin-1 (IL-1), tumor necrosis factor alpha (TNF), oncostatin M (OSM), and IL-6/soluble IL-6 receptor complexes control synthesis of this inhibitor. Here, we characterize a molecular mechanism responsible for both IL-1 and TNF-induced expression of ACT gene in astrocytes. We identify the 5' distal IL-1/TNF-responsive enhancer of the ACT gene located 13 kb upstream of the transcription start site. This 413-bp-long enhancer contains three elements, two of which bind nuclear factor kB (NF-kB) and one that binds activating protein 1 (AP-1). All of these elements contribute to the full responsiveness of the ACT gene to both cytokines, as determined by deletion and mutational analysis. The 5' NF-kB high-affinity binding site and AP-1 element contribute most to the enhancement of gene transcription in response to TNF and IL-1. In addition, we demonstrate that the 5' untranslated region of the ACT mRNA does not contribute to cytokine-mediated activation. Finally, we find that overexpression of the NF-kB inhibitor (IkB) totally inhibits any activation mediated by the newly identified IL-1/TNF enhancer of the ACT gene.

In vitro selection of an RNA sequence that interacts with high affinity with thymidylate synthase

Previous studies have shown that the repressive effect of thymidylate synthase (TS) mRNA translation is mediated by direct binding of TS itself to two cis-acting elements on its cognate mRNA. To identify the optimal RNA nucleotides that interact with TS, we in vitro synthesized a completely degenerate, linear RNA pool of 25 nt and employed in vitro selection to isolate high affinity RNA ligands that bind human TS protein. After 10 rounds of selection and amplification, a single RNA molecule was selected that bound TS protein with nearly 20-fold greater affinity than native, wild-type TS RNA sequences. Secondary structure analysis of this RNA sequence predicted it to possess a stem-loop structure. Deletion and/or modification of the UGU loop element within the RNA sequence decreased binding to TS by up to 1000-fold. In vivo transfection experiments revealed that the presence of the selected RNA sequence resulted in a significant increase in the expression of a heterologous luciferase reporter construct in human colon cancer H630 and TS-overexpressing HCT-C:His-TS+ cells, but not in HCT-C18 cells expressing a functionally inactive TS. In addition, the presence of this element in H630 cells leads to induced expression of TS protein. An immunoprecipitation method using RT-PCR confirmed a direct interaction between human TS protein and the selected RNA sequence in transfected human cancer H630 cells. This study identified a novel RNA sequence from a degenerate RNA library that specifically interacts with TS.

Mechanism of interleukin-1- and tumor necrosis factor alpha-dependent regulation of the alpha 1-antichymotrypsin gene in human astrocytes

The expression of alpha(1)-antichymotrypsin (ACT) is significantly enhanced in affected brain regions in Alzheimer's disease. This serine proteinase inhibitor specifically colocalizes with filamentous beta-amyloid deposits and recently has been shown to influence both formation and destabilization of beta-amyloid fibrils. In the brain, ACT is expressed in astrocytes, and interleukin-1 (IL-1), tumor necrosis factor alpha (TNF), oncostatin M (OSM), and IL-6/soluble IL-6 receptor complexes control synthesis of this inhibitor. Here, we characterize a molecular mechanism responsible for both IL-1 and TNF-induced expression of ACT gene in astrocytes. We identify the 5' distal IL-1/TNF-responsive enhancer of the ACT gene located 13 kb upstream of the transcription start site. This 413-bp-long enhancer contains three elements, two of which bind nuclear factor kB (NF-kB) and one that binds activating protein 1 (AP-1). All of these elements contribute to the full responsiveness of the ACT gene to both cytokines, as determined by deletion and mutational analysis. The 5' NF-kB high-affinity binding site and AP-1 element contribute most to the enhancement of gene transcription in response to TNF and IL-1. In addition, we demonstrate that the 5' untranslated region of the ACT mRNA does not contribute to cytokine-mediated activation. Finally, we find that overexpression of the NF-kB inhibitor (IkB) totally inhibits any activation mediated by the newly identified IL-1/TNF enhancer of the ACT gene.

Thyrotropin-releasing hormone and epidermal growth factor regulate iron-regulatory protein binding in pituitary cells via protein kinase C-dependent and -independent signaling pathways

Intracellular iron homeostasis is regulated, in part, by interactions between iron-regulatory proteins (IRP1 and IRP2) and iron-responsive elements (IREs) in ferritin and transferrin receptor mRNAs. In addition to iron, cellular oxidative stress induced by H(2)O(2), nitric oxide, and hypoxia, and hormonal activation by thyroid hormone and erythropoeitin have each been shown to regulate IRP binding to IREs. Hormonal signals, in particular mediated through protein kinase C (PKC), play a central role in the modulation of IRP/IRE interactions since phorbol esters were shown to activate IRP binding (Eisenstein, R. S., Tuazon, P. T., Schalinske, K. L., Anderson, S. A., and Traugh, J. A. (1993) J. Biol. Chem. 268, 27363-27370). In pituitary thyrotrophs (TtT97), we found that thyrotropin releasing hormone (TRH) and epidermal growth factor (EGF) increased IRP binding to a ferritin IRE, dependent on PKC and mitogen-activated protein kinase (MAPK) activity. In contrast, TRH and EGF decreased IRP binding in pituitary lactotrophs (GH3), despite activation of PKC and MAPK. IRP1 and IRP2 levels remained constant and IRP2 binding was predominant throughout. TRH and EGF markedly decreased IRP binding in MAPK kinase inhibitor-treated GH3 cells, whereas, they increased IRP binding in phosphatase inhibitor-treated GH3 cells. IRE-dependent CAT reporter translational expression closely reflected IRP binding to the ferritin IRE in both GH3 and TtT97 cells. Interestingly, ferritin protein levels were regulated similarly by TRH in both cell lines. These data link two different cell receptor systems to common signaling pathways that regulate IRP binding and ferritin expression. Remarkably, for TRH and EGF, these effects may be PKC-dependent or -independent determined by the cell type.

Interleukin-1beta increases binding of the iron regulatory protein and the synthesis of ferritin by increasing the labile iron pool

This study was undertaken to begin to elucidate the mechanisms by which cytokines influence intracellular iron homeostasis. Intracellular iron homeostasis is maintained by the coordinated regulation of ferritin and transferrin receptor synthesis. The synthesis of these proteins is coordinated by cytoplasmic iron regulatory proteins (IRP), which bind to iron responsive elements (IRE) on their mRNAs. We evaluated the effects of interleukin-1beta (IL-1beta) on iron metabolism in human astrocytoma cells (SW1088). Exposure to IL-1beta for 16 h increased binding of the IRPs to the IRE and also increased ferritin synthesis. Using the iron sensitive dye calcein, we determined that the intracellular labile iron pool increased within 4 h of IL-1beta exposure and continued to increase for 8 h, returning to normal by 16 h. We propose that the cytokine induced increase in the labile iron pool stimulates ferritin synthesis resulting in a subsequent decrease in the labile iron pool. The decrease in the labile iron pool is consistent with the increase in IRE/IRP interaction measured at 16 h. These results indicate that cytokines can influence the labile iron pool and the post-transcriptional regulatory mechanism for maintaining iron homeostasis. These results contribute to understanding the response of ferritin to inflammation by suggesting ferritin synthesis may reflect changes in the labile iron pool. The approach used in this study may provide a model system for studying relations between the labile iron pool and proteins responsible for maintaining intracellular homeostasis

Iron is a regulatory component of human IL-1beta production. Support for regional variability in the lung

The human lung accumulates iron with senescence. Smoking escalates the accumulation of iron, and we have demonstrated regional variability in the accumulation of iron in smokers' lungs. Iron has been reported to influence the production of a number of proinflammatory mediators, including human interleukin (IL)-1beta. We postulated that we could (1) demonstrate regional differences in the release of IL-1beta from human alveolar macrophages and (2) influence the production of IL-1beta in human macrophages by altering intracellular iron concentrations. To test these hypotheses, alveolar macrophages were obtained by independent lavage of the upper and lower lobes of healthy volunteers (both smokers and nonsmokers), after which the ability of each population to secrete IL-1beta was quantified, together with their ability to produce tumor necrosis factor-alpha, IL-6, and IL-8. Additionally, we established an in vitro model of "iron-loaded" cells of the human myelomonocytic cell line THP-1 in order to examine more directly the effect of iron and its chelation on the secretion of IL-1beta. We report here that an intracellular, chelatable pool of iron expands with exogenous iron-loading as well as with lipopolysaccharide (LPS) stimulation and appears to suppress transcription of IL-1beta, whereas shrinkage of this pool by early chelation augments transcription of IL-1beta beyond that induced by LPS alone. And finally, we demonstrate a regional relationship in the lung between excess alveolar iron and the production of human alveolar macrophage-derived IL-1beta, suggesting a partnership between iron and inflammation that may have clinical significance, especially in relation to lung diseases with a regional predominance.

RNA diversity has profound effects on the translation of neuronal nitric oxide synthase

A comprehensive analysis of the structure of neuronal nitric oxide synthase (nNOS; EC 1.14.13.39) mRNA species revealed NOS1 to be the most structurally diverse human gene described to date in terms of promoter usage. Nine unique exon 1 variants are variously used for transcript initiation in diverse tissues, and each is expressed from a unique 5'-flanking region. The dependence on unique genomic regions to control transcription initiation in a cell-specific fashion burdens the transcripts with complex 5'-mRNA leader sequences. Elaborate splicing patterns that involve alternatively spliced leader exons and exon skipping have been superimposed on this diversity. Highly structured nNOS mRNA 5'-untranslated regions, which have profound effects on translation both in vitro and in cells, contain cis RNA elements that modulate translational efficiency in response to changes in cellular phenotype.

Iron-regulatory proteins, iron-responsive elements and ferritin mRNA translation

Iron plays a central role in the metabolism of all cells. This is evident by its major contribution to many diverse functions, such as DNA replication, bacterial pathogenicity, photosynthesis, oxidative stress control and cell proliferation. In mammalian systems, control of intracellular iron homeostasis is largely due to posttranscriptional regulation of binding by iron-regulatory RNA-binding proteins (IRPs) to iron-responsive elements (IREs) within ferritin and transferrin receptor (TfR) mRNAs. the TfR transports iron into cells and the iron is subsequently stored within ferritin. IRP binding is under tight control so that it responds to changes in intracellular iron requirements in a coordinate manner by differentially regulating ferritin mRNA translational efficiency and TfR mRNA stability. Several different stimuli, as well as intracellular iron levels and oxidative stress, are capable of regulating these RNA-protein interactions. In this mini-review, we shall concentrate on the mechanisms underlying modulation of the interaction of IRPs and the ferritin IRE and its role in regulating ferritin gene expression.

Translation of the alzheimer amyloid precursor protein mRNA is up-regulated by interleukin-1 through 5'-untranslated region sequences

The amyloid precursor protein (APP) has been associated with Alzheimer's disease (AD) because APP is processed into the beta-peptide that accumulates in amyloid plaques, and APP gene mutations can cause early onset AD. Inflammation is also associated with AD as exemplified by increased expression of interleukin-1 (IL-1) in microglia in affected areas of the AD brain. Here we demonstrate that IL-1alpha and IL-1beta increase APP synthesis by up to 6-fold in primary human astrocytes and by 15-fold in human astrocytoma cells without changing the steady-state levels of APP mRNA. A 90-nucleotide sequence in the APP gene 5'-untranslated region (5'-UTR) conferred translational regulation by IL-1alpha and IL-1beta to a chloramphenicol acetyltransferase (CAT) reporter gene. Steady-state levels of transfected APP(5'-UTR)/CAT mRNAs were unchanged, whereas both base-line and IL-1-dependent CAT protein synthesis were increased. This APP mRNA translational enhancer maps from +55 to +144 nucleotides from the 5'-cap site and is homologous to related translational control elements in the 5'-UTR of the light and and heavy ferritin genes. Enhanced translation of APP mRNA provides a mechanism by which IL-1 influences the pathogenesis of AD.

High-dose recombinant human erythropoietin stimulates reticulocyte production in patients with multiple organ dysfunction syndrome

OBJECTIVE

To investigate erythropoietin (EPO) production and the erythropoietic potency of recombinant human EPO in the multiple organ dysfunction syndrome.

DESIGN

Randomized, prospective, controlled clinical trial.

MATERIALS AND METHODS

Patients received either 600 IU/kg intravenous EPO three times weekly (n = 9) or saline (control, n = 10).

MEASUREMENTS

EPO levels, circulating soluble receptors for tumor necrosis factor and interleukin-2, levels of interleukin-6 and intercellular adhesion molecule, and early peripheral blood cell progenitors.

RESULTS

EPO production in the control group remained low. Pharmacologic EPO blood levels were associated with increased reticulocyte counts compared with both controls (p < 0.04) and baseline (p < 0.006). Increased levels of soluble receptors for tumor necrosis factor in the treatment group compared with the controls did not prevent this effect. Interleukin 6 inhibited reticulocyte production.

CONCLUSION

Despite increased cytokine levels, pharmacologic EPO blood levels were associated with increased reticulocyte counts in patients with multiple organ dysfunction syndrome.

Unidirectional upregulation of the synthesis of the major iron proteins, transferrin-receptor and ferritin, in HepG2 cells by the acute-phase protein alpha1-antitrypsin

BACKGROUND/AIMS

We have previously shown that the hepatic acute-phase protein alpha1-antitrypsin (alpha1-AT) is an important mediator of changes in iron metabolism in the course of anaemia of chronic disease. Alpha1-AT profoundly reduces growth of erythroid cells by interfering with transferrin-mediated iron uptake. In the present work we investigate the effects of alpha1-AT on hepatic iron metabolism, as the liver plays a central role in body iron metabolism and in metabolic changes during acute-phase response.

METHODS

The human hepatoma cell line Hep G2 was cultured in RPMI 1640+10% FCS. The effect of alpha1-AT on transferrin-receptor binding was investigated in equilibrium binding assays with 125I-transferrin. Expression of transferrin receptor was determined by saturation experiments and intracellular ferritin was measured in cell lysates after incubating cells either alone or with alpha1-AT. To determine iron regulatory protein binding activity to iron responsive elements we used gel retardation assays and Northern blot analysis was carried out to investigate transferrin receptor and ferritin mRNA expression.

RESULTS

Alpha1-AT completely prevented transferrin from binding to its receptor and internalization of the transferrin-transferrin receptor complex on HepG2. In addition, alpha1-AT caused a distinct increase in iron regulatory protein binding activity to iron responsive elements, which is characteristic of iron deprivation. Normally, the synthesis of transferrin receptor and ferritin is regulated bidirectionally, but alpha1-AT promoted a unidirectional regulation. Alpha1-AT increased the synthesis of both transferrin receptor and ferritin and, moreover, increased cellular amounts of transferrin receptor mRNA and ferritin H-chain mRNA.

CONCLUSIONS

The effect of alpha1-AT on transferrin receptor synthesis appears to be mediated via activation of iron responsive element binding affinity of iron regulatory protein leading to an increased stability of transferrin receptor mRNA. Changes in ferritin, however, may be related to a transcriptionally mediated, iron-independent pathway which overrides the influence of activated iron regulatory protein. These specific changes in iron metabolism are the very ones seen in the course of anaemia of chronic disease. Our results emphasize the central role of alpha1-AT as a mediator of altered iron metabolism, characteristic of anaemia of chronic disease, not only with respect to erythroid cells but also with respect to liver cells.

Ferritin is a developmentally regulated nuclear protein of avian corneal epithelial cells

Previously, we generated monoclonal antibodies against chicken corneal cells (Zak, N. B., and Linsenmayer, T. F. (1983) Dev. Biol. 99, 373). We have now observed that one group of these antibodies reacts with a developmentally regulated component of corneal epithelial cell nuclei. This component is the heavy chain of ferritin, as determined by analyses of immunoisolated cDNA clones and immunoblotting of the protein. Immunoblotting also suggests that the nuclear ferritin may be in a supramolecular form that is similar to the iron-binding ferritin complex found in the cytoplasm of many cells. In vitro cultures and transfection studies show that the nuclear localization depends predominantly on cell type but can be altered by the in vitro environment. The appearance of nuclear ferritin is at least partially under translational regulation, as is known to be true for the cytoplasmic form of the molecule. The tissue and developmental distributions of the mRNA for the molecule are much more extensive than the protein itself, and the removal of iron from cultures of corneal epithelial cells with the iron chelator deferoxamine prevents the appearance of nuclear ferritin. At present the functional role(s) of nuclear ferritin remain unknown, but previous studies on cytoplasmic ferritin raise the possibility that it prevents damage due to free radical generation ("oxidative stress") by sequestering iron. Although it remains to be tested whether nuclear ferritin prevents oxidative damage, we find this an attractive possibility. Since the corneal epithelium is transparent and is constantly exposed to free radical-generating UV light, it is possible that the cells of this tissue have evolved a specialized mechanism to prevent oxidative damage to their nuclear components.

Interaction between iron-regulatory proteins and their RNA target sequences, iron-responsive elements

In this chapter, we have focused on the biochemistry of IRP-1 and the features which distinguish it from the related RNA-binding protein, IRP-2. IRP-1 is the cytoplasmic isoform of the enzyme aconitase, and, depending on iron status, may switch between enzymatic and RNA-binding activities. IRP-1 and IRP-2 are trans-acting regulators of mRNAs involved in iron uptake, storage and utilisation. The finding of an IRE in the citric acid cycle enzymes, mitochondrial aconitase and succinate dehydrogenase, suggests that the IRPs may also influence cellular energy production. These two proteins appear to bind RNAs with different but overlapping specificity, suggesting that they may regulate the stability or translation of as yet undefined mRNA targets, possibly extending their regulatory function beyond that of iron homeostasis. The interaction between the IRPs and the IRE represents one of the best characterised model systems for posttranscriptional gene control, and given that each IRP can also recognise its own unique set of RNAs, the search for new in vivo mRNA targets is expected to provide yet more surprises and insights into the fate of cytoplasmic mRNAs.

Iron regulatory proteins 1 and 2

Iron uptake and storage in mammalian cells is at least partly regulated at a post-transcriptional level by the iron regulatory proteins (IRP-1 and IRP-2). These cytoplasmic regulators share 79% similarity in protein sequence and bind tightly to conserved mRNA stem-loops, named iron-responsive elements (IREs). The IRP:IRE interaction underlies the regulation of translation and stability of several mRNAs central to iron metabolism. The question of why the cell requires two such closely related regulatory proteins may be resolved as we learn more about the expression and regulation of these proteins. It is evident so far that, despite similarities, the IRPs differ in several important respects. They are coordinately regulated by cellular iron, but whereas IRP-1 is inactivated by high iron levels, IRP-2 is rapidly degraded. Further differences arise in their expression and RNA-binding specificity. The two proteins each recognise a large repertoire of IRE-like sequences, including a small group of exclusive RNA targets. These findings hint that IRP-1 and IRP-2 may bind preferentially to certain mRNAs in vivo, possibly extending their known functions beyond the regulation of intracellular iron homeostasis.

The ferritins: molecular properties, iron storage function and cellular regulation

The iron storage protein, ferritin, plays a key role in iron metabolism. Its ability to sequester the element gives ferritin the dual functions of iron detoxification and iron reserve. The importance of these functions is emphasised by ferritin's ubiquitous distribution among living species. Ferritin's three-dimensional structure is highly conserved. All ferritins have 24 protein subunits arranged in 432 symmetry to give a hollow shell with an 80 A diameter cavity capable of storing up to 4500 Fe(III) atoms as an inorganic complex. Subunits are folded as 4-helix bundles each having a fifth short helix at roughly 60 degrees to the bundle axis. Structural features of ferritins from humans, horse, bullfrog and bacteria are described: all have essentially the same architecture in spite of large variations in primary structure (amino acid sequence identities can be as low as 14%) and the presence in some bacterial ferritins of haem groups. Ferritin molecules isolated from vertebrates are composed of two types of subunit (H and L), whereas those from plants and bacteria contain only H-type chains, where 'H-type' is associated with the presence of centres catalysing the oxidation of two Fe(II) atoms. The similarity between the dinuclear iron centres of ferritin H-chains and those of ribonucleotide reductase and other proteins suggests a possible wider evolutionary linkage. A great deal of research effort is now concentrated on two aspects of ferritin: its functional mechanisms and its regulation. These form the major part of the review. Steps in iron storage within ferritin molecules consist of Fe(II) oxidation, Fe(III) migration and the nucleation and growth of the iron core mineral. H-chains are important for Fe(II) oxidation and L-chains assist in core formation. Iron mobilisation, relevant to ferritin's role as iron reserve, is also discussed. Translational regulation of mammalian ferritin synthesis in response to iron and the apparent links between iron and citrate metabolism through a single molecule with dual function are described. The molecule, when binding a [4Fe-4S] cluster, is a functioning (cytoplasmic) aconitase. When cellular iron is low, loss of the [4Fe-4S] cluster allows the molecule to bind to the 5'-untranslated region (5'-UTR) of the ferritin m-RNA and thus to repress translation. In this form it is known as the iron regulatory protein (IRP) and the stem-loop RNA structure to which it binds is the iron regulatory element (IRE). IREs are found in the 3'-UTR of the transferrin receptor and in the 5'-UTR of erythroid aminolaevulinic acid synthase, enabling tight co-ordination between cellular iron uptake and the synthesis of ferritin and haem. Degradation of ferritin could potentially lead to an increase in toxicity due to uncontrolled release of iron. Degradation within membrane-encapsulated "secondary lysosomes' may avoid this problem and this seems to be the origin of another form of storage iron known as haemosiderin. However, in certain pathological states, massive deposits of "haemosiderin' are found which do not arise directly from ferritin breakdown. Understanding the numerous inter-relationships between the various intracellular iron complexes presents a major challenge.

Thyroid hormone modulates the interaction between iron regulatory proteins and the ferritin mRNA iron-responsive element

The cytoplasmic iron regulatory protein (IRP) modulates iron homeostasis by binding to iron-responsive elements (IREs) in the transferrin receptor and ferritin mRNAs to coordinately regulate transferrin receptor mRNA stability and ferritin mRNA translational efficiency, respectively. These studies demonstrate that thyroid hormone (T3) can modulate the binding activity of the IRP to an IRE in vitro and in vivo. T3 augmented an iron-induced reduction in IRP binding activity to a ferritin IRE in RNA electrophoretic mobility shift assays using cytoplasmic extracts from human liver hepatoma (HepG2) cells. Hepatic IRP binding to the ferritin IRE also diminished after in vivo administration of T3 with iron to rats. In transient transfection studies using HepG2 cells and a human ferritin IRE-chloramphenicol acetyltransferase (H-IRE-CAT) construct, T3 augmented an iron-induced increase in CAT activity by approximately 45%. RNase protection analysis showed that this increase in CAT activity was not due to a change in the steady state level of CAT mRNA. Nuclear T3-receptors may be necessary for this T3-induced response, because the effect could not be reproduced by the addition of T3 directly to cytoplasmic extracts and was absent in CV-1 cells which lack T3-receptors. We conclude that T3 can functionally regulate the IRE binding activity of the IRP. These observations provide evidence of a novel mechanism for T3 to up-regulate hepatic ferritin expression, which may in part contribute to the elevated serum ferritin levels seen in hyperthyroidism.

The cellular reactions to experimental intracerebral hemorrhage

The resolution of an intracerebral hemorrhage can be measured by the occurrence of hemosiderin. Extravasation of blood elicits a cellular reaction in the adjacent surviving tissue where the lesion activates resident microglia and attracts many more phagocytes from the blood stream. The signals for this migration into the perifocal reactive zone are not fully understood but it is likely that proteins in the coagulated blood contribute to cellular activation. In order to study the role of plasma proteins in the pathogenesis of the perifocal reactive zone, intracerebral injections of either autologous whole blood (0.1 ml) or an equal volume of washed autologous red blood cells (RBC) in lactated Ringer's solution were made in adult rabbits. The amount of total iron was the same (30 micrograms). The cellular responses to the injections were studied by iron histochemistry and immunocytochemistry for ferritin, the ferritin repressor protein (FRP), the glial fibrillary acidic protein (GFAP), and the complement receptor CR3. Experimental hematomas resolved much more slowly after the injection of whole blood than after the injection of RBC. Qualitative microglial and astrocytic responses were quite similar. However, at 48 h, iron- and ferritin-reactive microglia were more numerous following the injection of whole blood. After injections of either type, ferritin-immunoreactive cells were more abundant than iron-positive cells. This observation implied that the biosynthesis of holoferritin protein and iron incorporation proceeded independently. Expression of CR3 on the surface of microglia was much more prominent after whole blood, suggesting a role of inactivated complement 3b in the attraction of additional phagocytes. Conversion to hemosiderin began at 5 days after the injection of either blood or RBC. The lesions caused initial destruction of astrocytes in the perifocal zone as judged by GFAP- and FRP-immunoreactivity. However, at 5 days, astrocytic processes reentered the perifocal zone and intermingled with microglia and macrophages. It is proposed that this contact between astrocytes and microglia reversed the uncoupling of ferritin biosynthesis and iron incorporation and initiated the storage of iron and formation of hemosiderin.

Role for NF-kappa B in the regulation of ferritin H by tumor necrosis factor-alpha

Ferritin is a ubiquitously distributed iron-binding protein that plays a key role in cellular iron homeostasis. It is composed of two subunits, termed H (heavy or heart) and L (light or liver). In fibroblasts and other cells, the cytokine tumor necrosis factor-alpha (TNF) specifically induces synthesis of the ferritin H subunit. Using nuclear run-off assays, we demonstrate that this TNF-dependent increase in ferritin H is mediated by a selective increase in ferritin H transcription. Transfection of murine fibroblasts with chimeric genes containing the 5'-flanking region of murine ferritin H fused to the human growth hormone reporter gene reveals that the cis-acting element that mediates this response is located approximately 4.8 kilobases distal to the start site of transcription. Deletion analyses delimit the TNF-responsive region to a 40-nucleotide sequence located between nucleotides -4776 and -4736, which we term FER-2. Electrophoretic mobility shift assays and site-specific mutations indicate that this region contains two independent elements: one contains a sequence that binds a member of the NF-kappa B family of transcription factors, and a second contains a novel sequence that partially conforms to the NF-kappa B consensus sequence and may bind a different member of the NF-kappa B/Rel transcription factor family. Thus, effects of an inflammatory cytokine on ferritin are mediated by a family of transcription factors responsive to oxidative stress.

Ascorbic acid enhances iron-induced ferritin translation in human leukemia and hepatoma cells

Ascorbate is an important cofactor in many cellular metabolic reactions and is intimately linked to iron homeostasis. Continuously cultured cells are ascorbate deficient due to the lability of the vitamin in solution and to the fact that daily supplementation of media with ascorbate is unusual. We found that ascorbate repletion alone did not alter ferritin synthesis. However, ascorbate-replete human hepatoma cells, Hep3B and HepG2, as well as K562 human leukemia cells achieved a substantially higher cellular ferritin content in response to a challenge with iron than did their ascorbate-deficient counterparts grown under standard culture conditions. Most of the elevation in ferritin content was due to an increase in de novo ferritin synthesis of greater than 50-fold, as shown by in vivo labeling with [35S]methionine and immunoprecipitation. RNA-blot analysis showed only minor changes in steady state levels of ferritin mRNA, suggesting that ascorbate enhances iron-induced ferritin synthesis primarily by post-transcriptional events. Transient gene expression experiments using chloramphenicol acetyltransferase reporter gene constructs showed that the ascorbate effect on ferritin translation is not mediated through the stem-loop near the translational start site that transduces ferritin synthesis in response to cytokines. The data suggest that ascorbate possibly modifies the action of the iron-responsive element on ferritin translation, although more precise structure-function studies are needed to clarify this issue. These data demonstrate a novel role of ascorbate as a signaling molecule in post-transcriptional gene regulation. The mechanism by which ascorbate modulates cellular iron metabolism is complex and requires additional detailed investigation.