ROS mediate the hypoxic repression of the hepcidin gene by inhibiting C/EBPα and STAT-3

Hepcidin: A Critical Regulator of Iron Metabolism during Hypoxia

Iron status affects cognitive and physical performance in humans. Recent evidence indicates that iron balance is a tightly regulated process affected by a series of factors other than diet, to include hypoxia. Hypoxia has profound effects on iron absorption and results in increased iron acquisition and erythropoiesis when humans move from sea level to altitude. The effects of hypoxia on iron balance have been attributed to hepcidin, a central regulator of iron homeostasis. This paper will focus on the molecular mechanisms by which hypoxia affects hepcidin expression, to include a review of the hypoxia inducible factor (HIF)/hypoxia response element (HRE) system, as well as recent evidence indicating that localized adipose hypoxia due to obesity may affect hepcidin signaling and organismal iron metabolism.

Synthetic and natural iron chelators: therapeutic potential and clinical use

Iron-chelation therapy has its origins in the treatment of iron-overload syndromes. For many years, the standard for this purpose has been deferoxamine. Recently, considerable progress has been made in identifying synthetic chelators with improved pharmacologic properties relative to deferoxamine. Most notable are deferasirox (Exjade(®)) and deferiprone (Ferriprox(®)), which are now available clinically. In addition to treatment of iron overload, there is an emerging role for iron chelators in the treatment of diseases characterized by oxidative stress, including cardiovascular disease, atherosclerosis, neurodegenerative diseases and cancer. While iron is not regarded as the underlying cause of these diseases, it does play an important role in disease progression, either through promotion of cellular growth and proliferation or through participation in redox reactions that catalyze the formation of reactive oxygen species and increase oxidative stress. Thus, iron chelators may be of therapeutic benefit in many of these conditions. Phytochemicals, many of which bind iron, may also owe some of their beneficial properties to iron chelation. This review will focus on the advances in iron-chelation therapy for the treatment of iron-overload disease and cancer, as well as neurodegenerative and chronic inflammatory diseases. Established and novel iron chelators will be discussed, as well as the emerging role of dietary plant polyphenols that effectively modulate iron biochemistry.

Regulation of cellular iron metabolism

Iron is an essential but potentially hazardous biometal. Mammalian cells require sufficient amounts of iron to satisfy metabolic needs or to accomplish specialized functions. Iron is delivered to tissues by circulating transferrin, a transporter that captures iron released into the plasma mainly from intestinal enterocytes or reticuloendothelial macrophages. The binding of iron-laden transferrin to the cell-surface transferrin receptor 1 results in endocytosis and uptake of the metal cargo. Internalized iron is transported to mitochondria for the synthesis of haem or iron–sulfur clusters, which are integral parts of several metalloproteins, and excess iron is stored and detoxified in cytosolic ferritin. Iron metabolism is controlled at different levels and by diverse mechanisms. The present review summarizes basic concepts of iron transport, use and storage and focuses on the IRE (iron-responsive element)/IRP (iron-regulatory protein) system, a well known post-transcriptional regulatory circuit that not only maintains iron homoeostasis in various cell types, but also contributes to systemic iron balance.

Regulation of iron pathways in response to hypoxia

Constituting an integral part of a heme's porphyrin ring, iron is essential for supplying cells and tissues with oxygen. Given tight links between oxygen delivery and iron availability, it is not surprising that iron deprivation and oxygen deprivation (hypoxia) have very similar consequences at the molecular level. Under hypoxia, the expression of major iron homeostasis genes including transferrin, transferrin receptor, ceruloplasmin, and heme oxygenase-1 is activated by hypoxia-inducible factors to provide increased iron availability for erythropoiesis in an attempt to enhance oxygen uptake and delivery to hypoxic cells. Iron-response proteins (IRP1 and IRP2) and "cap-n-collar" bZIP transcriptional factors (NE-F2 p45; Nrf1, 2, and 3; Bach1 and 2) also control gene and protein expression of the key iron homeostasis proteins. In this article, we give an overview of the mechanisms by which iron pathways are regulated by hypoxia at multiple levels. In addition, potential clinical benefits of manipulating iron pathways in the hypoxia-related conditions anemia and ischemia are discussed.

Increased serum hepcidin levels during treatment with deferasirox in iron-overloaded patients with myelodysplastic syndrome

Hepcidin is a major regulator of iron metabolism. We evaluated changes in serum hepcidin during 3 months of therapy with the iron-chelator deferasirox in patients with low-risk myelodysplastic syndrome and iron overload. Serum hepcidin was found to be high in these patients, correlated with their iron and oxidative status, and further increased by treatment with deferasirox. These findings support the concept that the hepcidin level represents a balance between the stimulating effect of iron overload and the inhibitory effects of erythropoietic activity and oxidative stress. These preliminary findings favour the rationale for iron chelation therapy in such patients.

Hypoxia inhibits hepcidin expression in HuH7 hepatoma cells via decreased SMAD4 signaling

Hepcidin negatively regulates systemic iron homeostasis in response to inflammation and elevated serum iron. Conversely, hepcidin expression is diminished in response to hypoxia, oxidative stress, and increased erythropoietic demand, though the molecular intermediates involved are incompletely understood. To address this, we have investigated hypoxic hepcidin regulation in HuH7 hepatoma cells either cultured alone or cocultured with activated THP-1 macrophages. HuH7 hepcidin mRNA expression was determined using quantitative polymerase chain reaction (Q-PCR). Hepcidin promoter activity was measured using luciferase reporter constructs containing a 0.9 kb fragment of the wild-type human hepcidin promoter, and constructs containing mutations in bone morphogenetic protein (BMP)/SMAD4, signal transducer and activator of transcription 3 (STAT3), CCAAT/enhancer-binding protein (C/EBP), and E-box-responsive elements. Hepatic expression of bone morphogenetic proteins BMP2 and BMP6 and the BMP inhibitor noggin was determined using Q-PCR, and the protein expression of hemojuvelin (HJV), pSMAD 1/5/8, and SMAD4 was determined by western blotting. Following exposure to hypoxia or H(2)O(2), hepcidin mRNA expression and promoter activity increased in HuH7 cells monocultures but were decreased in HuH7 cells cocultured with THP-1 macrophages. This repression was attenuated by mutation of the BMP/SMAD4-response element, suggesting that modulation of SMAD signaling mediated the response to hypoxia. No changes in hepatocyte BMP2, BMP6 or noggin mRNA, or protein expression of HJV or pSMAD 1/5/8 were detected. However, treatment with hypoxia caused a marked decrease in nuclear and cytosolic SMAD4 protein and SMAD4 mRNA expression in cocultured HuH7 cells. Together these data indicate that hypoxia represses hepcidin expression through inhibition of BMP/SMAD signaling.

Serum prohepcidin concentrations at birth and 1 month after birth in premature infants

BACKGROUND

Premature newborns are vulnerable to iron imbalance, although the iron homeostasis during the perinatal period remains unclear. To clarify the iron metabolism of premature infants, we measured serum prohepcidin concentrations of preterm infants, and analyzed the association with iron parameters.

METHODS

Seventy-one (61 preterm and 10 term) infants were enrolled for the study, that had no underlying diseases including asphyxia, bleedings, infection, and anomalies. Serum concentrations of prohepcidin at birth and 1 month after birth were determined by enzyme-linked immunosorbent assay.

RESULTS

Prohepcidin levels at birth but not 1 month postnatal age positively correlated with gestational age (correlation coefficient [CC]:0.334, P = 0.005) and birth weight (CC: 0.367, P = 0.002). The levels at birth of preterm infants (median: 29.93 ng/ml, range: 4.0-110.6) were lower than those of full-term infants, and increased thereafter. On the other hand, the levels in small-for-gestational age infants were not associated with gestational age or birth weight. Prohepcidin levels at birth correlated positively with red cell counts (CC = 0.487, P = 0.025), unsaturated iron binding capacity (CC = 0.755, P = 0.001), total protein (CC = 0.624, P = 0.005), and serum albumin levels (CC = 0.500, P = 0.025), and negatively with serum iron levels (CC = -0.688, P = 0.003), but not ferritin levels. Multivariate analyses indicated that prohepcidin levels at birth were lower in infants with pregnancy-induced hypertension (P = 0.03) or premature rupture of membrane (P = 0.01).

CONCLUSIONS

Prohepcidin production was physiologically low at birth of preterm infants according to the gestational age, and the levels might be susceptible to the in utero stress. The postnatal increase might reflect the maturation and/or adaptation of iron homeostasis.

Hypoxia-inducible factor-1alpha enhances haptoglobin gene expression by improving binding of STAT3 to the promoter

Haptoglobin (Hp) is known to play a role in angiogenesis as well as in anti-inflammation. STAT3 is a major transcription factor for expression of human Hp. We investigated whether hypoxia-inducible factor-1α (HIF-1α), a key mediator of angiogenesis, participates in Hp gene expression. HIF-1α overexpression by gene transfection or hypoxia augmented Hp transcription in HepG2 human hepatoma cells. Conversely, knockdown of HIF-1α by specific siRNA transfection diminished Hp expression, although the level of STAT3 phosphorylation remained unchanged. A luciferase reporter assay using mutant Hp promoters demonstrated that two adjacent DNA elements, a STAT3-binding element (SBE) and a cAMP-response element (CRE)-like site in human Hp promoter -120/-97, were required for HIF-1α-stimulated transactivation of the Hp gene. HIF-1α, STAT3, and p300/CBP were simultaneously bound to the SBE/CRE as a complex form. When HIF-1α was knocked down, STAT3 binding to the SBE in the Hp promoter was attenuated. Our findings suggest that HIF-1α assists STAT3 in strong binding to the proximal SBE in the Hp promoter. The CRE-like site located near the SBE may contribute to the formation of a stable complex of STAT3, HIF-1α, and p300/CBP, which leads to maximum transcription of the Hp gene.

Modulation of hepcidin production during hypoxia-induced erythropoiesis in humans in vivo: data from the HIGHCARE project

Iron is tightly connected to oxygen homeostasis and erythropoiesis. Our aim was to better understand how hypoxia regulates iron acquisition for erythropoiesis in humans, a topic relevant to common hypoxia-related disorders. Forty-seven healthy volunteers participated in the HIGHCARE project. Blood samples were collected at sea level and after acute and chronic exposure to high altitude (3400-5400 m above sea level). We investigated the modifications in hematocrit, serum iron indices, erythropoietin, markers of erythropoietic activity, interleukin-6, and serum hepcidin. Hepcidin decreased within 40 hours after acute hypoxia exposure (P < .05) at 3400 m, reaching the lowest level at 5400 m (80% reduction). Erythropoietin significantly increased (P < .001) within 16 hours after hypoxia exposure followed by a marked erythropoietic response supported by the increased iron supply. Growth differentiation factor-15 progressively increased during the study period. Serum ferritin showed a very rapid decrease, suggesting the existence of hypoxia-dependent mechanism(s) regulating storage iron mobilization. The strong correlation between serum ferritin and hepcidin at each point during the study indicates that iron itself or the kinetics of iron use in response to hypoxia may signal hepcidin down-regulation. The combined and significant changes in other variables probably contribute to the suppression of hepcidin in this setting.

Vitamin C protective role for alcoholic liver disease in mice through regulating iron metabolism

Alcoholic liver disease (ALD) is a major medical complication of drinking alcohol, and commonly accompanied with hepatic iron overload and liver injuries. Oxidative stress plays an important role in pathogenesis of ALD and also leads to iron-metabolic disorders. In this study, the effects of vitamin C (Vc) on iron metabolism-related genes expression and liver protection from drinking in mice were investigated. Twenty-four male kunming mice were divided into four groups (six mice per group): control (water drinking); alcohol group (20% alcohol drinking), alcohol + low Vc group (adding 50 mg/kg Vc daily) and alcohol + high Vc group (adding 100 mg/kg Vc daily). All these mice were sacrificed after 7 days. Vc can ameliorate the increase of sera alanine aminotransferase (ALT) activity and hepatic iron overload of drinking alcohol in mice. Vc increases the expression of the iron-regulated hormone hepcidin and decreases transferrin receptor 1 (TfR1) expression in liver. Vc also down-regulates the expression of ferroportin 1 (Fpn1) in the intestine and decreases the iron release to blood. In conclusion, Vc ameliorated the alcoholic liver injuries through regulating the iron metabolism-related genes expression.

Iron overload in MDS-pathophysiology, diagnosis, and complications

Many patients with myelodysplastic syndromes (MDS) become dependent on blood transfusions and develop transfusional iron overload, which is exacerbated by increased absorption of dietary iron in response to ineffective erythropoiesis. However, it is uncertain whether there is an association among iron accumulation, clinical complications, and decreased likelihood of survival in MDS patients. Here, we discuss our current understanding of the effects of transfusion dependency and iron overload in MDS, indicate our knowledge gaps, and suggest that particular emphasis should be placed on further characterizing the role of redox-active forms of labile iron, which may be as important as the total iron burden.

Hepcidin in beta-thalassemia

Iron overload is the principal cause of morbidity and mortality in beta-thalassemia with or without transfusion dependence. Iron homeostasis is regulated by the hepatic peptide hormone hepcidin. Hepcidin controls dietary iron absorption, plasma iron concentrations, and tissue iron distribution. A deficiency in this hormone is the main or contributing factor of iron overload in iron-loading anemias such as beta-thalassemia. Hepcidin deficiency results from a strong suppressive effect of the high erythropoietic activity on hepcidin expression. Although in thalassemia major patients iron absorption contributes less to the total iron load than transfusions, in non-transfused thalassemia, low hepcidin, and the consequent hyperabsorption of dietary iron is the major cause of systemic iron overload. Hepcidin diagnostics and future therapeutic agonists may help in management of patients with beta-thalassemia.

Successful antiviral therapy is associated with a decrease of serum prohepcidin in chronic hepatitis C

AIM: To assess serum concentrations of prohepcidin in chronic hepatitis C individuals and evaluate their associations with disease activity and efficacy of pegylated interferon (PEG-IFN)/ribavirin (RBV) therapy.

METHODS: Prohepcidin was measured in sera of 53 chronic hepatitis C patients. Concentrations of prohepcidin and other iron metabolism markers were analyzed at 9 time points before, during and after the end of antiviral therapy.

RESULTS: In hepatitis C virus (HCV) genotype 1-infected individuals, a gradual decrease of prohepcidin during antiviral therapy was observed in responders (88.8 ± 14.7 ng/mL before therapy vs 60.6 ± 0.3 ng/mL in the 48th wk, P = 0.04). In contrast, no decrease was observed in non-responders. A similar association was observed in HCV genotype 3a individuals, with a statistically significant decline in serum prohepcidin only in the responder group (99.5 ± 5.2 ng/mL at baseline vs 72.7 ± 6.1 ng/mL in the 24th wk, P = 0.01). Moreover, HCV-RNA at week 12 of therapy was positively correlated with baseline (R = 0.63, P < 0.005) and week 12 (R = 0.60, P = 0.01) serum prohepcidin concentrations in HCV genotype 1 infection.

CONCLUSION: Successful PEG-IFN/RBV therapy results in a decline of serum prohepcidin concentration in chronic hepatitis C, which may suggest a direct effect of HCV on iron metabolism at the prohormonal level of hepcidin.

Iron chelation therapy in MDS: what have we learnt recently?

Patients with myelodysplastic syndromes (MDS) who receive chronic blood transfusions for anaemia are at risk of developing iron overload, which can negatively affect organ function and survival. Evidence suggests that iron chelation therapy can restore iron balance in these patients and may improve their chances of survival. Recently, several guidelines on the management of patients with MDS have been published that address iron overload and the use of iron chelation therapy. While these guidelines differ in some specific details, they generally agree that patients with lower-risk MDS are most likely to develop iron overload and therefore benefit from iron chelation therapy. The oral iron chelator, deferasirox, has been shown to reduce serum ferritin levels and labile plasma iron in patients with MDS, and has an acceptable safety profile. Unlike other iron chelators, deferasirox also appears to inhibit the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kappaB) pathway in MDS blast cells, which may lead to additional beneficial effects.

Molecular mechanisms of hepcidin regulation: implications for the anemia of CKD

Anemia is prevalent in patients with chronic kidney disease (CKD) and is associated with lower quality of life and higher risk of adverse outcomes, including cardiovascular disease and death. Anemia management in patients with CKD currently revolves around the use of erythropoiesis-stimulating agents and supplemental iron. However, many patients do not respond adequately and/or require high doses of these medications. Furthermore, recent clinical trials have shown that targeting higher hemoglobin levels with conventional therapies leads to increased cardiovascular morbidity and mortality, particularly when higher doses of erythropoiesis-stimulating agents are used and in patients who are poorly responsive to therapy. One explanation for the poor response to conventional therapies in some patients is that these treatments do not fully address the underlying cause of the anemia. In many patients with CKD, as with patients with other chronic inflammatory diseases, poor absorption of dietary iron and the inability to use the body's iron stores contribute to the anemia. Recent research suggests that these abnormalities in iron balance may be caused by increased levels of the key iron regulatory hormone hepcidin. This article reviews the pathogenesis of anemia in CKD, the role and regulation of hepcidin in systemic iron homeostasis and the anemia of CKD, and the potential diagnostic and therapeutic implications of these findings.

Regulation of resistin by cyclic mechanical stretch in cultured rat vascular smooth muscle cells

Resistin has a potential role in atherosclerosis; however, the molecular mechanism underlying the increase in resistin expression in atherosclerosis remains unclear. As mechanical stretch plays an important role in atherosclerosis, in the present study we sought to investigate the cellular and molecular mechanisms underlying the regulation of resistin by cyclic mechanical stretch in VSMCs (vascular smooth muscle cells). VSMCs from thoracic aorta of adult Wistar rats were cultured and subjected to cyclic stretch. Cyclic mechanical stretch significantly increased resistin protein and mRNA expression as compared with control cells without stretch. The specific p38 MAPK (mitogen-activated protein kinase) inhibitor SB203580, the antioxidant N-acetylcysteine and p38 MAPK siRNA (small interfering RNA) attenuated the induction of resistin protein by cyclic stretch. Cyclic stretch significantly increased the phosphorylation of p38 MAPK, whereas pre-treatment with SB203580 and N-acetylcysteine significantly inhibited this effect. Cyclic stretch significantly increased ROS (reactive oxygen species) production, and pre-treatment with N-acetylcysteine significantly inhibited stretch-induced ROS production. Cyclic stretch also increased STAT3 (signal transducer and activator of transcription 3)-binding activity and resistin promoter activity, and resistin promoter activity was abolished when STAT3 in the promoter area was mutated. Pre-treatment with SB203580 and N-acetylcysteine significantly attenuated resistin promoter activity induced by cyclic stretch. Cyclic stretch increased the secretion of AngII (angiotensin II) and resistin from cultured VSMCs. Exogenous AngII increased resistin expression, and AngII receptor inhibition attenuated this effect. In conclusion, cyclic mechanical stretch increases resistin expression in cultured rat VSMCs. Stretch-induced resistin expression is mediated through ROS, and the p38 MAPK and STAT3 pathways. Therefore resistin induced by cyclic stretch may contribute to the pathogenesis of atherosclerosis under haemodynamic overload.

Evidence for a lack of a direct transcriptional suppression of the iron regulatory peptide hepcidin by hypoxia-inducible factors

Background

Hepcidin is a major regulator of iron metabolism and plays a key role in anemia of chronic disease, reducing intestinal iron uptake and release from body iron stores. Hypoxia and chemical stabilizers of the hypoxia-inducible transcription factor (HIF) have been shown to suppress hepcidin expression. We therefore investigated the role of HIF in hepcidin regulation.

Methodology/Principal Findings

Hepcidin mRNA was down-regulated in hepatoma cells by chemical HIF stabilizers and iron chelators, respectively. In contrast, the response to hypoxia was variable. The decrease in hepcidin mRNA was not reversed by HIF-1α or HIF-2α knock-down or by depletion of the HIF and iron regulatory protein (IRP) target transferrin receptor 1 (TfR1). However, the response of hepcidin to hypoxia and chemical HIF inducers paralleled the regulation of transferrin receptor 2 (TfR2), one of the genes critical to hepcidin expression. Hepcidin expression was also markedly and rapidly decreased by serum deprivation, independent of transferrin-bound iron, and by the phosphatidylinositol 3 (PI3) kinase inhibitor LY294002, indicating that growth factors are required for hepcidin expression in vitro. Hepcidin promoter constructs mirrored the response of mRNA levels to interleukin-6 and bone morphogenetic proteins, but not consistently to hypoxia or HIF stabilizers, and deletion of the putative HIF binding motifs did not alter the response to different hypoxic stimuli. In mice exposed to carbon monoxide, hypoxia or the chemical HIF inducer N-oxalylglycine, liver hepcidin 1 mRNA was elevated rather than decreased.

Conclusions/Significance

Taken together, these data indicate that hepcidin is neither a direct target of HIF, nor indirectly regulated by HIF through induction of TfR1 expression. Hepcidin mRNA expression in vitro is highly sensitive to the presence of serum factors and PI3 kinase inhibition and parallels TfR2 expression.

Iron metabolism in thalassemia and sickle cell disease

THERE ARE TWO MAIN MECHANISMS BY WHICH IRON OVERLOAD DEVELOPS IN THALASSEMIAS: increased iron absorption due to ineffective erythropoiesis and blood transfusions. In nontransfused patients with severe thalassemia, abnormal dietary iron absorption increases body iron burden between 2 and 5 g per year. If regular transfusions are required, this doubles the rate of iron accumulation leading to earlier massive iron overload and iron-related damage. Iron metabolism largely differs between thalassemias and sickle cell disease, but chronic transfusion therapy partially normalize many of the disparities between the diseases, making iron overload an important issue to be considered in the management of patients with sickle cell disease too. The present review summarizes the actual knowledge on the regulatory pathways of iron homeostasis. In particular, the data presented indicate the inextricably link between erythropoiesis and iron metabolism and the key role of hepcidin in coordinating iron procurement according to erythropoietic requirement. The role of erythropoietin, hypoxia, erythroid-dependent soluble factors and iron in regulating hepcidin transcription are discussed as well as differences and similarities in iron homeostasis between thalassemia syndromes and sickle cell disease.

Cholestasis downregulate hepcidin expression through inhibiting IL-6-induced phosphorylation of signal transducer and activator of transcription 3 signaling

Hepcidin is downregulated during progressive cholestasis in biliary atresia, but the mechanism is unknown. To verify whether downregulation of hepcidin is specific to cholestasis irrespective of the patient's age, we first analyzed liver hepcidin mRNA and protein expression in adults with primary biliary cirrhosis (PBC) (n=4), non-cholestatic cirrhosis (n=19) and in controls (n=9). We evaluated the tyrosine phosphorylation of signal transducer and activator of transcription 3 (pSTAT3) expressions in the liver sections. A rat model of cholestasis by ligation of the extrahepatic bile duct (BDL) was created, and lipopolysaccharide (LPS)-induced cholangitis in cholestatic rats 2 weeks after BDL was also established to study the modulation of hepcidin by interleukin-6 (IL-6) and STAT3 signaling pathway in these models, using real-time quantitative reverse transcription-PCR, immunohistochemistry, western blotting and enzyme-linked immunosorbent assay (ELISA). An in vitro study of the effect of bile acids on hepcidin expression was carried out to re-confirm the in vivo findings. There was significantly lower hepcidin mRNA and pSTAT3 protein expression in cholestatic cirrhosis compared with non-cholestatic cirrhosis in adults. BDL caused significant decrease in hepcidin and gp130 mRNA expression compared with sham-operated group and normal control. Furthermore, there was significantly lower pSTAT3 protein expression and nuclear translocation in the cholestatic liver from the patients and the BDL rats, which was comparable to lower liver hepcidin mRNA and plasma hepcidin expression. Furthermore, BDL for 2 weeks attenuated the upregulation of hepcidin expression induced by LPS. Hydrophobic bile acid glycochenodeoxycholate inhibited IL-6-induced pSTAT3 expression in primary hepatocytes and resulted in the downregulation of hepcidin mRNA expression. In conclusion, the study shows that cholestasis or its important component-hydrophobic bile acids-can downregulate hepcidin expression through inhibiting IL-6-induced STAT3 phosphorylation and pSTAT3 protein nuclear translocation.

Hypoxia induces chemoresistance in ovarian cancer cells by activation of signal transducer and activator of transcription 3

Signal transducer and activator of transcription 3 (STAT3) is activated in a variety of human cancers, including ovarian cancer. The molecular mechanism by which the STAT3 is activated in cancer cells is poorly understood. We observed that human ovarian xenograft tumors (A2780) in mice were severely hypoxic (pO(2) approximately 2 mmHg). We further observed that hypoxic exposure significantly increased the phosphorylation of STAT3 (pSTAT3) at the Tyr705 residue in A2780 cell line. The pSTAT3 (Tyr705) level was highly dependent on cellular oxygenation levels, with a significant increase at <2% O(2), and without any change in the pSTAT3 (Ser727) or total STAT3 levels. The pSTAT3 (Tyr705) elevation following hypoxic exposure could be reversed within 12 hr after returning the cells to normoxia. The increased level of pSTAT3 was partly mediated by increased levels of reactive oxygen species generation in the hypoxic cancer cells. Conventional chemotherapeutic drugs cisplatin and taxol were far less effective in eliminating the hypoxic ovarian cancer cells suggesting a role for pSTAT3 in cellular resistance to chemotherapy. Inhibition of STAT3 by AG490 followed by treatment with cisplatin or taxol resulted in a significant increase in apoptosis suggesting that hypoxia-induced STAT3 activation is responsible for chemoresistance. The results have important clinical implications for the treatment of hypoxic ovarian tumors using STAT3-specific inhibitors.

Recent developments in liver pathology

CONTEXT

Hepatocellular carcinoma is the sixth most common malignancy and the third leading cause of cancer deaths worldwide, making pathologic identification of precursor lesions essential. Recent molecular genetic, pathologic, and clinical data have led to the stratification of hepatic adenomas into subgroups with unique molecular profiles and varying potential for malignant transformation, as well as to the reclassification of telangiectatic focal nodular hyperplasia as telangiectatic adenoma. Clinical, morphologic, and molecular genetic studies have also established juvenile hemochromatosis and pediatric nonalcoholic steatohepatitis as entities distinct from their adult counterparts.

OBJECTIVE

To review the recent molecular genetic characterization of telangiectatic hepatic adenomas and juvenile hemochromatosis, as well as the recent clinicopathologic characterization of pediatric nonalcoholic steatohepatitis.

DATA SOURCES

Literature review, personal experience, and material from the University of Chicago.

CONCLUSIONS

Basic science and translational research have led to the classification of many pathologic entities of the liver according to molecular genetic and protein expression profiles that correspond to traditional morphologic categories. Insights into signal transduction pathways that are activated in, and protein expression patterns unique to, an individual disease may lead to the development of new therapeutic agents and novel diagnostic biomarkers.

Regulation of hepcidin and iron-overload disease

Hepcidin, a 25-amino-acid antimicrobial peptide, is the central regulator of iron homeostasis. Hepcidin transcription is upregulated by inflammatory cytokines, iron, and bone morphogenetic proteins and is downregulated by iron deficiency, ineffective erythropoiesis, and hypoxia. The iron transporter ferroportin is the cognate receptor of hepcidin and is destroyed as a result of interaction with the peptide. Except for inherited defects of ferroportin and hepcidin itself, all forms of iron-storage disease appear to arise from hepcidin dysregulation. Studies using multiple approaches have begun to delineate the molecular mechanisms that regulate hepcidin expression, particularly at the transcriptional level. Knowledge of the regulation of hepcidin by inflammation, iron, erythropoiesis, and hypoxia will lead to an understanding of the pathogenesis of primary hemochromatosis, secondary iron overload, and anemia of inflammatory disease.

Iron homeostasis in the neonate

The regulation of the availability of micronutrients is particularly critical during periods of rapid growth and differentiation such as the fetal and neonatal stages. Both iron deficiency and excess during the early weeks of life can have severe effects on neurodevelopment that may persist into adulthood and may not be corrected by restoration of normal iron levels. This article provides a succinct overview of our current understanding of the extent to which newborns, particularly premature newborns, are able (or not able) to regulate their iron status according to physiologic need. Postnatal development of factors important to iron homeostasis such as intestinal transport, extracellular transport, cellular uptake and storage, intracellular regulation, and systemic control are examined. Also reviewed are how factors peculiar to the sick and premature neonate can further adversely influence iron homeostasis and exacerbate iron-induced oxidative stress, predispose the infant to bacterial infections, and, thus, compromise his or her clinical situation further. The article concludes with a discussion of the areas of relative ignorance that require urgent investigation to rectify our lack of understanding of iron homeostasis in what is a critical stage of development.

Hepcidin modulation in human diseases: From research to clinic

By modulating hepcidin production, an organism controls intestinal iron absorption, iron uptake and mobilization from stores to meet body iron need. In recent years there has been important advancement in our knowledge of hepcidin regulation that also has implications for understanding the physiopathology of some human disorders. Since the discovery of hepcidin and the demonstration of its pivotal role in iron homeostasis, there has been a substantial interest in developing a reliable assay of the hormone in biological fluids. Measurement of hepcidin in biological fluids can improve our understanding of iron diseases and be a useful tool for diagnosis and clinical management of these disorders. We reviewed the literature and our own research on hepcidin to give an updated status of the situation in this rapidly evolving field.

Iron behaving badly: inappropriate iron chelation as a major contributor to the aetiology of vascular and other progressive inflammatory and degenerative diseases

BACKGROUND

The production of peroxide and superoxide is an inevitable consequence of aerobic metabolism, and while these particular 'reactive oxygen species' (ROSs) can exhibit a number of biological effects, they are not of themselves excessively reactive and thus they are not especially damaging at physiological concentrations. However, their reactions with poorly liganded iron species can lead to the catalytic production of the very reactive and dangerous hydroxyl radical, which is exceptionally damaging, and a major cause of chronic inflammation.

REVIEW

We review the considerable and wide-ranging evidence for the involvement of this combination of (su)peroxide and poorly liganded iron in a large number of physiological and indeed pathological processes and inflammatory disorders, especially those involving the progressive degradation of cellular and organismal performance. These diseases share a great many similarities and thus might be considered to have a common cause (i.e. iron-catalysed free radical and especially hydroxyl radical generation).The studies reviewed include those focused on a series of cardiovascular, metabolic and neurological diseases, where iron can be found at the sites of plaques and lesions, as well as studies showing the significance of iron to aging and longevity. The effective chelation of iron by natural or synthetic ligands is thus of major physiological (and potentially therapeutic) importance. As systems properties, we need to recognise that physiological observables have multiple molecular causes, and studying them in isolation leads to inconsistent patterns of apparent causality when it is the simultaneous combination of multiple factors that is responsible.This explains, for instance, the decidedly mixed effects of antioxidants that have been observed, since in some circumstances (especially the presence of poorly liganded iron) molecules that are nominally antioxidants can actually act as pro-oxidants. The reduction of redox stress thus requires suitable levels of both antioxidants and effective iron chelators. Some polyphenolic antioxidants may serve both roles.Understanding the exact speciation and liganding of iron in all its states is thus crucial to separating its various pro- and anti-inflammatory activities. Redox stress, innate immunity and pro- (and some anti-)inflammatory cytokines are linked in particular via signalling pathways involving NF-kappaB and p38, with the oxidative roles of iron here seemingly involved upstream of the IkappaB kinase (IKK) reaction. In a number of cases it is possible to identify mechanisms by which ROSs and poorly liganded iron act synergistically and autocatalytically, leading to 'runaway' reactions that are hard to control unless one tackles multiple sites of action simultaneously. Some molecules such as statins and erythropoietin, not traditionally associated with anti-inflammatory activity, do indeed have 'pleiotropic' anti-inflammatory effects that may be of benefit here.

CONCLUSION

Overall we argue, by synthesising a widely dispersed literature, that the role of poorly liganded iron has been rather underappreciated in the past, and that in combination with peroxide and superoxide its activity underpins the behaviour of a great many physiological processes that degrade over time. Understanding these requires an integrative, systems-level approach that may lead to novel therapeutic targets.

Function of the hemochromatosis protein HFE: Lessons from animal models

Hereditary hemochromatosis (HH) is caused by chronic hyperabsorption of dietary iron. Progressive accumulation of excess iron within tissue parenchymal cells may lead to severe organ damage. The most prevalent type of HH is linked to mutations in the HFE gene, encoding an atypical major histocompatibility complex classImolecule. Shortly after its discovery in 1996, the hemochromatosis protein HFE was shown to physically interact with transferrin receptor 1 (TfR1) and impair the uptake of transferrin-bound iron in cells. However, these findings provided no clue why HFE mutations associate with systemic iron overload. It was later established that all forms of HH result from misregulation of hepcidin expression. This liver-derived circulating peptide hormone controls iron efflux from duodenal enterocytes and reticuloendothelial macrophages by promoting the degradation of the iron exporter ferroportin. Recent studies with animal models of HH uncover a crucial role of HFE as a hepatocyte iron sensor and upstream regulator of hepcidin. Thus, hepatocyte HFE is indispensable for signaling to hepcidin, presumably as a constituent of a larger iron-sensing complex. A working model postulates that the signaling activity of HFE is silenced when the protein is bound to TfR1. An increase in the iron saturation of plasma transferrin leads to displacement of TfR1 from HFE and assembly of the putative iron-sensing complex. In this way, iron uptake by the hepatocyte is translated into upregulation of hepcidin, reinforcing the concept that the liver is the major regulatory site for systemic iron homeostasis, and not merely an iron storage depot.

Hepatitis C virus-induced oxidative stress suppresses hepcidin expression through increased histone deacetylase activity

Chronic hepatitis C is characterized by iron accumulation in the liver, and excessive iron is hepatotoxic. However, the mechanism by which hepatitis C virus (HCV) regulates iron metabolism is poorly understood. Hepcidin plays a pivotal role as a negative regulator of iron absorption. The aim of the current study was to elucidate the mechanisms that govern hepcidin expression by HCV. Huh 7 cells, Huh7.5 cells, full-length HCV replicon cells established from Huh7.5 cells, and adenoviruses expressing HCV-core or HCV nonstructural proteins 3 through 5 (NS3-5) were used. Hepcidin expression was significantly lower in HCV replicon cells and in HCV core-expressing Huh7 cells. The expression was inversely correlated with the amount of reactive oxygen species (ROS) production. Anti-oxidants restored hepcidin expression in HCV replicon cells and Huh7 cells expressing HCV core. In HCV replicon cells, histone deacetylase (HDAC) activity was elevated at baseline and after exposure to hydrogen peroxide. Anti-oxidants reduced HDAC activity in a dose-dependent manner. HDAC inhibition increased hepcidin expression without affecting ROS production in HCV replicon cells. HCV-induced ROS stabilized the expression of two negative hepcidin regulators, HIF1alpha and HIF2alpha, and its expression was decreased by a HDAC inhibitor or an anti-oxidant. HCV-induced ROS also caused hypoacetylation of histones and inhibited binding of two positive regulators, C/EBPalpha and STAT3, to the hepcidin promoter, whereas anti-oxidant treatment of cells recovered C/EBPalpha and STAT3 binding to the hepcidin promoter. In addition, an HDAC inhibitor restored their binding to the hepcidin promoter via acetylation of histones.

CONCLUSION

HCV-induced oxidative stress suppresses hepcidin expression through increased HDAC activity.

Upstream stimulatory factor 2 is implicated in the progression of biliary atresia by regulation of hepcidin expression

BACKGROUND

Hepcidin is downregulated during the progression of biliary atresia (BA), but the mechanism is still unknown.

METHODS

We analyzed single nucleotide polymorphism of rs7251432 and 916145 within hepcidin and its upstream, USF2 gene, respectively, in 52 patients of BA and 96 healthy controls. Liver tissues were obtained from 10 patients with early and late stage of BA, 10 patients with choledochal cyst, and 4 normal controls to study upstream stimulatory factor 2 (USF2) messenger RNA (mRNA) and protein expressions. Chromatin immunoprecipitation assay and USF2-specific short interference RNA (siRNA) were used in human HepG2 cells to show that USF2 can regulate hepcidin expression.

RESULTS

C and CC allele frequencies of rs916145 of USF2 were significantly higher in patients with BA than in healthy controls. There was also significantly higher USF2 protein nuclear translocation in the early stage of BA than in the late stage, which was compatible with higher hepcidin mRNA expression in the early stage of BA. Chromatin immunoprecipitation assay demonstrated physiologic bindings of USF2 to the hepcidin promoter in HepG2 cells. USF2 siRNA also significantly knocked down hepcidin mRNA expression.

CONCLUSION

The study demonstrates that C allele of rs916145 in USF2 gene has more frequency for developing BA, and decreased USF2 protein nuclear translocation might partly play a role in the decreased hepcidin expression in the cholestatic liver injury of the late stage of BA.

Reactive oxygen species-mediated cyclin D1 degradation mediates tumor growth retardation in hypoxia, independently of p21cip1 and hypoxia-inducible factor

Cell growth arrest is an adaptation process for tumor survival in hypoxic environments. As proliferation is a very complicated and dynamic process, hypoxic growth arrest is not considered to be simply determined by a few molecules. Recently, several research groups have demonstrated that hypoxia-inducible factor (HIF)-1alpha plays a crucial role in hypoxia-induced cell-cycle arrest by inhibiting c-Myc and subsequently inducing p21(cip1) expression. However, we found that hypoxic growth arrest could occur even in p21-null cancer cells, and addressed the p21-independent process of cell-cycle arrest. We show that cyclin D1 was downregulated in various cancer cell lines under hypoxic conditions, which was independent of p21 and HIF-1 and -2alpha expression. It was also found that cyclin D1 was destabilized by the ubiquitin-proteasome system and this degradation process was highly activated by hypoxia. Moreover, antioxidants prevented the hypoxic degradation of cyclin D1 and hydrogen peroxide destabilized cyclin D1 in normoxia. Finally, we demonstrated that ectopic expression of cyclin D1 rescued hypoxic growth arrest in both p21+/+ and p21-/- HCT116 cells. Given the results, we here propose that reactive oxygen species-mediated cyclin D1 degradation contributes to tumor growth retardation in hypoxic environments.

Iron regulation and erythropoiesis

PURPOSE OF REVIEW

The peptide hormone hepcidin regulates iron metabolism in response to erythropoietic demand, iron stores and inflammation. Major advances have been made in understanding the regulation of hepcidin production, and consequently the availability of iron for erythropoiesis.

RECENT FINDINGS

It is becoming clear that the bone morphogenetic protein (BMP) pathway plays a major role in setting the baseline hepcidin level and, with the assistance of BMP2/4 and hemochromatosis-related proteins hemojuvelin, HFE and transferrin receptor 2, also regulates hepcidin expression in response to iron. Regulation of hepcidin in anemias has now been linked to increased erythropoietic activity and is likely mediated by factor(s) secreted by erythroid precursors. GDF-15 was identified as a candidate for one of the erythroid factors suppressing hepcidin. Tissue hypoxia may also directly contribute to hepcidin suppression in anemias. Regulation of hepcidin by inflammation may include multiple cytokines and the Toll-like receptors pathways. Although it has not yet been shown that increased hepcidin is indispensible for the development of anemia of inflammation, transgenic overexpression of hepcidin was sufficient to replicate its key features.

SUMMARY

Regulation of hepcidin and iron availability for erythropoiesis has revealed unexpected pathways and much complexity. The renaissance of the study of iron regulation continues to reward researchers with interesting biology and potential therapeutic targets.

2-Oxoglutarate-dependent oxygenases control hepcidin gene expression

BACKGROUND/AIMS

Hepcidin is a liver-produced hormone that regulates systemic iron homeostasis. Hepcidin expression is stimulated upon iron overload or inflammation while iron deficiency, anemia and tissue hypoxia are negative regulators. We investigated the involvement of 2-oxoglutarate-dependent oxygenases, HIF-1 and other transcription factors in the hypoxic suppression of hepcidin.

METHODS

Northern blotting analysis and real time PCR were used to determine hepcidin mRNA levels in hepatoma cells and hepcidin promoter activity was measured using Huh7 cells transfected with suitable reporter constructs under various conditions.

RESULTS

Treatment of human cultured hepatoma cells with hypoxia or known inhibitors of 2-oxoglutarate-dependent oxygenases, such as the iron chelator desferrioxamine, cobalt or the 2-oxoglutarate analogue dimethyl-oxalylglycine significantly reduced hepcidin mRNA levels and down-regulated its gene promoter activity. This effect was not dependent on the HREs or other known putative response elements in the hepcidin promoter and was observed even under interleukin-6 treatment.

CONCLUSIONS

2-Oxoglutarate-dependent oxygenases are important to maintain high hepcidin mRNA expression in a HIF-1-independent manner. We suggest that modulation of oxygenase activity may be of therapeutic value in iron-related disorders.

Erythropoietin mediates hepcidin expression in hepatocytes through EPOR signaling and regulation of C/EBPalpha

Hepcidin is the principal iron regulatory hormone, controlling the systemic absorption and remobilization of iron from intracellular stores. Recent in vivo studies have shown that hepcidin is down-regulated by erythropoiesis, anemia, and hypoxia, which meets the need of iron input for erythrocyte production. Erythropoietin (EPO) is the primary signal that triggers erythropoiesis in anemic and hypoxic conditions. Therefore, a direct involvement of EPO in hepcidin regulation can be hypothesized. We report here the regulation of hepcidin expression by EPO, in a dose-dependent manner, in freshly isolated mouse hepatocytes and in the HepG2 human hepatocyte cell model. The effect is mediated through EPOR signaling, since hepcidin mRNA levels are restored by pretreatment with an EPOR-blocking antibody. The transcription factor C/EBPalpha showed a pattern of expression similar to hepcidin, at the mRNA and protein levels, following EPO and anti-EPOR treatments. Chromatin immunoprecipitation experiments showed a significant decrease of C/EBPalpha binding to the hepcidin promoter after EPO supplementation, suggesting the involvement of this transcription factor in the transcriptional response of hepcidin to EPO.

Iron homeostasis and erythropoiesis

Erythrocytes require iron to perform their duty as oxygen carriers. Mammals have evolved a mechanism to maintain systemic iron within an optimal range that fosters erythroid development and function while satisfying other body iron needs. This chapter reviews erythroid iron uptake and utilization as well as systemic factors that influence iron availability. One of these factors is hepcidin, a circulating peptide hormone that maintains iron homeostasis. Elevated levels of hepcidin in the bloodstream effectively shut off iron absorption by disabling the iron exporter ferroportin. Conversely, low levels of circulating hepcidin allow ferroportin to export iron into the bloodstream. Aberrations in hepcidin expression or responsiveness to hepcidin result in disorders of iron deficiency and iron overload. It is clear that erythroid precursors communicate their iron needs to the liver to influence the production of hepcidin and thus the amount of iron available for use. However, the mechanism by which erythroid cells accomplish this remains unclear and is an area of active investigation.

Hepcidin, a key regulator of iron metabolism, is transcriptionally activated by p53

Hepcidin is an iron-regulatory protein that is upregulated in response to increased iron or inflammatory stimuli. Hepcidin reduces serum iron and induces iron sequestration in the reticuloendothelial macrophages - the hallmark of anaemia of inflammation. Iron deprivation is used as a defense mechanism against infection, and it also has a beneficial effect on the control of cancer. The tumour-suppressor p53 transcriptionally regulates genes involved in growth arrest, apoptosis and DNA repair, and perturbation of p53 pathways is a hallmark of the majority of human cancers. This study inspected a role of p53 in the transcriptional regulation of hepcidin. Based on preliminary bioinformatics analysis, we identified a putative p53 response-element (p53RE) contained in the hepcidin gene (HAMP) promoter. Chromatin immunoprecipitation (ChIP), reporter assays and a temperature sensitive p53 cell-line system were used to demonstrate p53 binding and activation of the hepcidin promoter. p53 bound to hepcidin p53RE in vivo, andthis p53RE could confer p53-dependent transcriptional activation. Activation of p53 increased hepcidin expression, while silencing of p53 resulted in decreased hepcidin expression in human hepatoma cells. Taken together, these results define HAMP as a novel transcriptional target of p53. We hypothesise that hepcidin upregulation by p53 is part of a defence mechanism against cancer, through iron deprivation. Hepcidin induction by p53 might be involved in the pathogenesis of anaemia accompanying cancer.

Elevated basal hepcidin levels in the liver may inhibit the development of malaria infection: another piece towards solving the malaria puzzle?

BACKGROUND

Inflammatory cytokines play a crucial role in the human immune response to infection by malaria. During the initial sporozoite infection of the liver the presence of Interleukin-6 (IL-6) can be determinant. IL-6 controls systemic iron homeostasis through hepcidin, which is produced mainly by hepatocytes. An elevated basal hepcidin level in the liver can be induced by chronic inflammatory disease. Hepcidin is also a peptide with antimicrobial properties.

PRESENTATION OF THE HYPOTHESIS

We hypothesize that elevated basal hepcidin levels in the liver inhibit the development of malaria infection. When hepcidin is abundant, hepatocytes sequester iron, and this inhibits sporozoite development in liver-stage malaria infection.

TESTING THE HYPOTHESIS

The validity of our hypothesis can be proven by observing sporozoite growth in hepcidin-treated hepatocytes, or in hepatocytes, stimulated with IL-6 to increase hepcidin levels before incubation with malaria sporozoites and observing the effect the hepcidin knockout function has on the infection.

IMPLICATIONS OF THE HYPOTHESIS

Confirmation of our hypothesis could help to understand the complexity of the malaria infection.