Role of interleukin-6 in hypoxic regulation of intestinal iron absorption

The regulation of intestinal iron absorption is not fully understood. Hepcidin, a liver-produced peptide, has recently been identified as a negative regulator of iron absorption in various conditions associated with altered iron metabolism (e.g. inflammation, anaemia, hypoxia). It is not clear whether these perturbants share a common signalling pathway. In this study, the importance of the cytokine interleukin-6 (IL-6) was investigated in the hypoxic mouse model. Hypoxia was associated with increased levels of circulating IL-6, decreased liver hepcidin mRNA and increased iron absorption (especially MT). A significant positive correlation existed between the total iron uptake and IL-6 levels in circulation. IL-6 per se, though inducing hepcidin mRNA, failed to affect basal iron absorption. The adaptive response to absorption following the hypoxic exposure was, however, more prominent if mice had been treated concurrently with IL-6. This enhancement in absorption occurred even though hepcidin mRNA was not significantly changed. Similar prominent responses were seen with both human and mouse IL-6. Anti-IL-6 antiserum normalised iron absorption in mice exposed to hypoxia, because of a reduction in the MT. These data indicate that IL-6 can influence iron absorption (especially MT) during the hypoxic exposure, but via a mechanism independent of hepcidin.     

Iron transferrin regulates hepcidin synthesis in primary hepatocyte culture through hemojuvelin and BMP2/4

The peptide hormone hepcidin is the principal regulator of systemic iron homeostasis. We examined the pathway by which iron stimulates the production of hepcidin. In humans who ingested 65 mg of iron, the increase in transferrin saturation preceded by hours the increase in urinary hepcidin excretion. Increases in urinary hepcidin concentrations were proportional to the increment in transferrin saturation. Paradoxically, in previous studies in primary hepatocytes and cell lines, hepcidin response to iron or iron transferrin was not observed. We now report that freshly isolated murine primary hepatocytes responded to holotransferrin but not apotransferrin by increasing hepcidin mRNA. Hepcidin increase was not due to contamination of the transferrin preparations by endotoxin, a potent pathologic stimulus of hepcidin synthesis. Using this culture system, we showed that holotransferrin concentrations regulate hepcidin mRNA concentrations through a hemojuvelin/BMP2/4-dependent pathway. Although BMP9 is known to be expressed in the liver and potently increased the basal concentrations of hepcidin mRNA, it did not interact with hemojuvelin, and interference with its signaling pathway did not affect iron regulation. Fresh primary hepatocytes constitute a sufficient system for the regulation of hepcidin by physiologic iron stimuli and will greatly facilitate studies of major disorders of iron homeostasis.     

Increased adipose tissue expression of hepcidin in severe obesity is independent from diabetes and NASH

BACKGROUNDS & AIMS: Hepcidin is an acute-phase response peptide. We have investigated the possible involvement of hepcidin in massive obesity, a state of chronic low-grade inflammation. Three groups of severely obese patients with or without diabetes or nonalcoholic steatohepatitis were investigated. METHODS: Hepcidin expression was studied in liver and adipose tissue of these patients. Hepcidin regulation was investigated in vitro by adipose tissue explant stimulation studies. RESULTS: Hepcidin was expressed not only in the liver but also at the messenger RNA (mRNA) and the protein levels in adipose tissue. Moreover, mRNA expression was increased in adipose tissue of obese patients. The presence of diabetes or NASH did not modify the hepcidin expression levels in liver and adipose tissue. In adipose tissue, mRNA expression correlated with indexes of inflammation, interleukin-6, and C-reactive protein. Interleukin-6 also promoted in vitro hepcidin expression. A low transferrin saturation ratio was observed in 68% of the obese patients; moreover, 24% of these patients presented with anemia. The observed changes in iron status could be due to the role of hepcidin as a negative regulator of intestinal iron absorption and macrophage iron efflux. Interestingly, a feedback control mechanism on hepcidin expression related to low transferrin saturation occurred in the liver but not in the adipose tissue. CONCLUSIONS: Hepcidin is a proinflammatory adipokine and may play an important role in hypoferremia of inflammation in obese condition.     

Iron- and inflammation-induced hepcidin gene expression in mice is not mediated by Kupffer cells in vivo

Hepcidin, a recently discovered iron regulatory peptide, is believed to inhibit the release of iron from absorptive enterocytes and macrophages. Liver hepcidin synthesis is induced in vivo by iron stores and inflammation. The molecular basis of the regulation of hepcidin gene expression by these effectors in hepatocytes is currently unknown, although there is strong evidence that indirect mechanisms are involved. The aims of this study were to gain insight into these mechanisms and to determine to what extent other liver cell types are responsible for transducing the signal by which hepcidin expression is regulated in mouse hepatocytes. For this, we depleted Kupffer cells by injection of liposome-encapsulated clodronate and then studied iron- and inflammation-induced hepcidin gene expression. In addition, we directly evaluated the role of the inflammatory cytokine interleukin 6 (IL-6) by using IL-6-deficient mice. Our results show that iron is able to induce hepcidin gene expression independently of Kupffer cells in the liver and circulating IL-6. In contrast, we show that hepcidin gene induction by inflammation is also independent of Kupffer cells, but involves, at least partly, IL-6. In conclusion, these results show that two independent regulatory pathways control hepcidin gene expression and suggest that hepatocytes play a key role in the regulation of hepcidin gene expression by sensing iron and inflammatory signals.     

Upregulation of hepcidin by interleukin-1beta in human hepatoma cell lines.

Anemia of chronic disease (ACD) is commonly observed in chronic inflammation, although its pathogenesis is poorly understood. Hepcidin is thought to be a key regulator in iron metabolism and has been implicated in ACD. Although the induction of hepcidin by an inflammatory cytokine interleukin-6 (IL-6) seems to have been confirmed, it is still controversial whether interleukin-1beta (IL-1beta), also known as an inflammatory cytokine, regulates hepcidin expression. We demonstrated that hepcidin mRNA was upregulated by IL-1beta in human hepatoma-derived HuH-7 cells, particularly at low concentrations of IL-1beta, while high concentrations of IL-6 were needed for the upregulation of hepcidin mRNA. Therefore, IL-1beta might be more important for the upregulation of hepcidin in physiological conditions than IL-6. Although IL-1beta induces IL-6 production in hepatocytes, our data indicate that the effect of IL-1beta on hepcidin expression is independent from that of IL-6. In conclusion, IL-1beta might have an important role in ACD.     

Lipopolysaccharide induces a significant increase in expression of iron regulatory hormone hepcidin in the cortex and substantia nigra in rat brain

Hepcidin plays an essential role in maintaining normal iron homeostasis outside the brain. This recently discovered iron regulation hormone is predominantly expressed in the liver, and regulated by iron and hypoxia. As an antimicrobial peptide, this hormone is also elevated during infections and inflammation. In this study we investigated the expression of hepcidin mRNA and protein in different brain regions, including the cortex, hippocampus, striatum, and substantia nigra, and the effects of lipopolysaccharide (LPS) on the expression of hepcidin using quantitative real-time RT-PCR and immunofluorescence analysis. Our data provided further evidence for the existence of hepcidin in all the regions we examined. We also demonstrated for the first time that LPS administration by iv injection can regulate the expression of hepcidin mRNA and protein not only in peripheral organs such as the liver, but also in the brain. LPS induced a significant increase in the expression of hepcidin mRNA and protein in the cortex and substantia nigra, but not in the hippocampus and striatum, indicating a regionally specific regulation of LPS on hepcidin in the brain. The relevant mechanisms and the functions of hepcidin in the brain remain to be elucidated.     

Autocrine formation of hepcidin induces iron retention in human monocytes

Hepcidin, a master regulator of iron homeostasis, is produced in small amounts by inflammatory monocytes/macrophages. Chronic immune activation leads to iron retention within monocytes/macrophages and the development of anemia of chronic disease (ACD). We questioned whether monocyte-derived hepcidin exerts autocrine regulation toward cellular iron metabolism. Monocyte hepcidin mRNA expression was significantly induced within 3 hours after stimulation with LPS or IL-6, and hepcidin mRNA expression was significantly higher in monocytes of ACD patients than in controls. In ACD patients, monocyte hepcidin mRNA levels were significantly correlated to serum IL-6 concentrations, and increased monocyte hepcidin mRNA levels were associated with decreased expression of the iron exporter ferroportin and iron retention in these cells. Transient transfection experiments using a ferroportin/EmGFP fusion protein construct demonstrated that LPS inducible hepcidin expression in THP-1 monocytes resulted in internalization and degradation of ferroportin. Transfection of monocytes with siRNA directed against hepcidin almost fully reversed this lipopolysaccharide-mediated effect. Using ferroportin mutation constructs, we found that ferroportin is mainly targeted by hepcidin when expressed on the cell surface. Our results suggest that ferroportin expression in inflammatory monocytes is negatively affected by autocrine formation of hepcidin, thus contributing to iron sequestration within monocytes as found in ACD.     

Hepcidin: from discovery to differential diagnosis

Although iron is essential for living organisms to survive, its reactive properties require strict regulation in order to prevent toxic effects. Hepcidin, a liver produced peptide hormone, is thought to be the central regulator of body iron metabolism. Its production is mainly controlled by the erythropoietic activity of the bone-marrow, the amount of circulating and stored body iron, and inflammation. Recent reports, however, provide new hypotheses on how hepcidin might exert its regulatory function. Although hepcidin was first discovered in human urine and serum, most of our understanding of hepcidin regulation and action comes from in vitro and mice studies that often use hepcidin mRNA expression as a read out. The difficulties in carrying out studies in humans have mostly been due to the lack of suitable hepcidin assay. The recent development of assays to measure hepcidin in serum and urine has offered new opportunities to study hepcidin regulation in humans. However, for the moment, only a small number of laboratories are able to perform these assays. The aim of this review is to discuss insights into hepcidin regulation obtained from recent clinical studies in the light of findings from in vitro and mice studies. Ongoing studies in humans should provide us with more information on the etiology of iron metabolism disorders in order to create new therapeutic strategies and improve differential diagnosis protocols for these diseases.     

Liver hepcidin mRNA correlates with iron stores, but not inflammation, in patients with chronic hepatitis C

PURPOSE: Liver iron is frequently elevated in chronic hepatitis C and may contribute to liver injury. The pathophysiology behind this phenomenon may involve hepcidin, a gene that is up-regulated in the liver by inflammation and iron. Inappropriately low hepcidin is important to the pathophysiology of hereditary hemochromatosis. However, the role of hepcidin in the iron loading of patients with hepatitis C is unknown. SUBJECTS AND METHODS: To determine whether liver hepcidin mRNA correlates with markers of hepatic inflammation and iron status in patients with hepatitis C, we extracted total RNA from liver biopsy specimens of patients with chronic hepatitis C and quantified hepcidin mRNA. Liver hepcidin mRNA levels were then correlated with aspartate aminotransferase, alanine aminotransferase, ferritin, viral load, fibrosis, hepatic iron concentration, and Hepatic Activity Index (HAI). RESULTS: Among patients with hepatitis C, there was a significant correlation of hepcidin mRNA expression in the liver with hepatic iron concentration and serum ferritin (r = 0.72, P = 0.006, and r = 0.60, P = 0.01, respectively). Hepcidin mRNA expression in the liver did not correlate with aspartate aminotransferase, alanine aminotransferase, HAI, or viral load. No differences in hepcidin mRNA were found based on viral genotype or the presence of fibrosis. CONCLUSION: In contrast to other inflammatory states, hepcidin mRNA expression in the liver was independent of markers of inflammation in hepatitis C. Instead, our results suggest that iron stores in patients with hepatitis C regulate hepcidin expression and that iron loading in chronic hepatitis C is not due to inappropriate hepcidin expression.     

Liver-gut axis in the regulation of iron homeostasis

The human body requires about 1-2 mg of iron per day for its normal functioning, and dietary iron is the only source for this essential metal. Since humans do not possess a mechanism for the active excretion of iron,the amount of iron in the body is determined by the amount absorbed across the proximal small intestine and, consequently, intestinal iron absorption is a highly regulated process. In recent years, the liver has emerged as a central regulator of both iron absorption and iron release from other tissues. It achieves this by secreting a peptide hormone called hepcidin that acts on the small intestinal epithelium and other cells to limit iron delivery to the plasma. Hepcidin itself is regulated in response to various systemic stimuli including variations in body iron stores, the rate of erythropoiesis, inflammation and hypoxia, the same stimuli that have been known for many years to modulate iron absorption. This review will summarize recent findings on the role played by the liver and hepcidin in the regulation of body iron absorption.     

Hepcidin excess induces the sequestration of iron and exacerbates tumor-associated anemia

The iron-regulatory hormone hepcidin has been proposed as the mediator of anemia of inflammation (AI). We examined the acute and chronic effects of hepcidin in the mouse. Injections of human hepcidin (50 microg/mouse), but not of its diluent, induced hypoferremia within 4 hours. To examine the chronic effects of hepcidin, we implanted either tumor xenografts engineered to overexpress human hepcidin or control tumor xenografts into nonobese diabetic-severe combined immunodeficiency (NOD-SCID) mice. Despite abundant dietary iron, mice with hepcidin-producing tumors developed more severe anemia, lower serum iron, and increased hepatic iron compared with mice with control tumors. Hepcidin contributes to AI by shunting iron away from erythropoiesis and sequestering it in the liver, predominantly in hepatocytes.     

Hepcidin--a regulator of intestinal iron absorption and iron recycling by macrophages

Hepcidin is a recently discovered peptide made in the liver, distributed in plasma and excreted in urine. This peptide hormone is the homeostatic regulator of intestinal iron absorption, iron recycling by macrophages, and iron mobilization from hepatic stores. Hepcidin acts by inhibiting the efflux of iron through ferroportin, the sole known iron exporter of enterocytes, macrophages and hepatocytes. As befits an iron-regulatory hormone, hepcidin synthesis is increased by iron loading and decreased by anemia and hypoxia. Hepcidin is markedly induced during infections and inflammation, causing iron to be sequestered in macrophages, hepatocytes and enterocytes. The resulting decrease in plasma iron levels eventually contributes to the anemia associated with infection and inflammation. These alterations in iron metabolism probably have a role in host defense by limiting the availability of iron to invading microorganisms. At the opposite extreme, early studies indicate that hepcidin deficiency--due to the dysregulation of its synthesis or mutations in the hepcidin gene itself--is the immediate cause of most forms of hemochromatosis.