Regulation Of Intestinal Iron Absorption Research Articles

Association between obesity and iron deficiency (Review).

Obesity is a risk factor for several comorbidities and complications, including iron deficiency anemia. Iron deficiency anemia is a serious global public health problem, with a worldwide prevalence. The high prevalence of obesity in combination with iron deficiency incidence observed in different age and sex categories suggests an association between obesity and iron status. Obesity may disrupt iron homeostasis, resulting in iron deficiency anemia. The association between obesity and iron deficiency may be due to increased hepcidin levels mediated by chronic inflammation. Hepcidin is a small peptide hormone that functions as a negative regulator of intestinal iron absorption. Significant body weight loss in overweight and obese individuals decreases chronic inflammation and serum hepcidin levels, resulting in improved iron status due to increased iron absorption. However, further randomized controlled trials are required to confirm this effect.

Duodenal Ferroportin Expression Is Unresponsive to Dietary Iron Excess in Nursing Piglets (FS04-01-19)

ObjectivesIn the US, prophylactic iron supplementation without thorough screening for iron deficiency is commonly practiced in both normal weight and low birth weight infants. In a nursing pig model, we previously found that excess dietary iron results in tissue iron overload, suggesting ineffective regulation of intestinal iron absorption. Herein, we aimed to determine if hepcidin-mediated ferroportin (FPN1) degradation is functionally immature in early life. MethodsTwelve nursing piglets of normal birth weight (2.1 ± 0.4 kg, PD2) were supplemented with either low (AGAL, 1 mg/d·kg body weight) or high iron (AGAH, 15 mg/d·kg BW)) in form of ferrous sulfate solution. Eight low birth weight (1.2 ± 0.4 kg on PD2) piglets were supplemented with high iron (SGAH, 15 mg/d·kg BW) from PD2 to 21. All piglets were raised by suckling their sows. Blood samples were collected weekly for analysis of hemoglobin, hematocrit, and plasma iron. Duodenal mucosa (DM) and hepatic mRNA and protein expression of iron transporters and regulators were analyzed using RT-qPCR and Western blot. Data were analyzed using PROC MIXED of SAS with CONTRAST statement for planned comparison between AGAL and high iron groups. ResultsIn comparison with AGAL, high iron, regardless of birth weight, significantly (P < 0.05) increased hemoglobin, transferrin saturation (74% vs. 49%), and plasma iron, and resulted in iron overload in DM and liver at PD21. Hepatic mRNA expression of HAMP and FTL in DM increased 275- and 3-fold, respectively, in response to high iron, whereas DMT1 in DM and TFRC in both liver and DM decreased by 7.4-, 3.8- and 5.3-fold, respectively. Consistently, protein expression of DMT1 in DM was lower in SGAH than that in AGAL; However, both mRNA and protein expression of FPN1 in DM and liver remained unaffected by iron provision or birth weight. ConclusionsHepcidin-induced ferroportin degradation is hypo-responsive to iron excess in a nursing pig model. Funding SourcesUC Davis, NIFA.

BMP-6 MEDIATES IRON-INDUCED HEPCIDIN EXPRESSION IN VIVO VIA AN HFE-DEPENDENT MECHANISM

Hepcidin is the key regulator of iron homeostasis acting as a negative regulator of intestinal iron absorption. Several proteins have recently been identified to act as upstream regulators of hepcidin expression, such as HFE and hemojuvelin (HJV). Although hepcidin is regulated by iron, the molecules involved in this regulation and whether HFE is involved in this regulation remain to be clarified. Therefore, the aim of this study was to investigate the molecules involved in hepcidin regulation by iron and the role played by HFE in this regulation in vivo. To induce iron loading, wild-type mice were injected with iron, and to investigate whether HFE is involved in hepcidin regulation by iron and BMP-Smad signalling pathway, HFE KO mice were also examined in the setting of iron loading. The obtained results showed that iron regulates the expression of BMP-6, for which HJV acts as a co-receptor, and also induces phosphorylation of SMADs 1/5/8 in the liver which in turn may regulate hepcidin gene expression in response to iron. Moreover, HFE seems to be involved in the regulation of downstream signalling of BMP-6 that regulates hepcidin expression in response to iron loading. In conclusion, the obtained results suggest that hepcidin regulation by iron is modulated by the BMP-6/Smad signalling pathway in vivo. Moreover, HFE seems to be involved in the regulation of downstream signalling of BMP-6 that regulates hepcidin expression in response to iron loading in vivo. ,

Maternal hemochromatosis gene H63D single-nucleotide polymorphism and lead levels of placental tissue, maternal and umbilical cord blood

Human hemochromatosis protein (HFE), a major histocompatibility complex class I-like integral membrane protein, participates in the down regulation of intestinal iron absorption by binding to transferrin receptor (TR). HFE competes with transferrin-bound iron for the TR and thus reduces uptake of iron into cells. On the other hand, a lack of HFE increases the intestinal absorption of iron similarly to iron deficiency associated with increasing in absorption and deposition of lead. During pregnancy, placenta cannot prevent transfer lead to the fetus; even low-level lead poisoning causes neurodevelopmental toxicity in children. The aim of this study was to determine the association between the maternal HFE H63D single-nucleotide polymorphism and lead levels in placental tissue, maternal blood and umbilical cord bloods. The study population comprised 93 mother–placenta pairs. Venous blood from mother was collected to investigate lead levels and HFE polymorphism that was detected by standard PCR–RFLP technique. Cord bloods and placentas were collected for lead levels which were analyzed by dual atomic absorption spectrometer system. The HFE H63D genotype frequencies of mothers were found as 75.3% homozygote typical (HH), 23.6% heterozygote (HD) and 1.1% homozygote atypical (DD). Our study results showed that the placental tissue, umbilical cord and maternal blood lead levels of mothers with HD+DD genotypes were significantly higher than those with HH genotype (p<0.05). The present study indicated for the first time that mothers with H63D gene variants have higher lead levels of their newborn's placentas and umbilical cord bloods.

Differentiation‐dependent Regulation of Intestinal Iron Absorption: Cell and Molecular Mechanisms

Differentiation of intestinal epithelial cells is associated with up‐ and down‐regulation of a variety of genes, but little is known about the differentiation‐dependent regulation of genes encoding proteins involved in intestinal iron absorption. We have addressed this issue using human‐derived intestinal epithelial Caco‐2 cells and crypt/villus epithelial cells isolated from wild‐type mice as models. We examined expression of divalent metal‐ion transporter‐1 (DMT‐1; brush border iron importer), ferroportin (FPN; basolateral iron exporter) and hephaestin (HEPH; basolateral iron oxidase). Differentiation of Caco‐2 cells was associated with a significant up‐regulation of iron uptake. This up‐regulation was associated with a significant increase in the expression of DMT‐1 and HEPH mRNA and protein, as well as increased expression of FPN mRNA, but not FPN protein. Similarly, iron uptake and the expression of DMT‐1, HEPH, and FPN mRNAs was higher in mouse villous cells than crypt cells. Promoter‐reporter constructs were used to show that the activity of the promoters of DMT‐1 and HEPH increased with differentiation of Caco‐2 cells. These studies demonstrate that differentiation of intestinal epithelial cells is associated with up‐regulation of the iron uptake process and increased expression of DMT‐1, HEPH, and FPN. The expression of all these proteins of iron transport can be mediated at the post‐transcriptional level according to intracellular iron availability, but our results indicate that up‐regulation of DMT‐1 and HEPH during differentiation appears to be mediated at least in part via transcriptional regulatory mechanisms. (Supported by funds from the Department of Veterans Affairs.)

The Gut: Role at Steady State and Variations in Disordered Conditions

Abstract SCI-23As the human body cannot excrete excess iron, its absorption needs to be finely regulated at the intestinal level. Ferric iron (Fe3+) is reduced to ferrous iron (Fe2+) by brush border ferric reductases, including duodenal cytochrome b (DCYTB), before being transported across the apical membrane by divalent metal transporter 1 (DMT1), which is the principal iron importer. Depending on body iron requirements, iron can be either stored bound to ferritin or exported across the basolateral enterocyte membrane into the plasma by the sole iron exporter ferroportin (FPN). Iron absorption responds to systemic signals reflecting body iron requirements and local signals in the enterocyte. At the systemic level, hepcidin is the key circulating peptide hormone maintaining iron homeostasis. Hepcidin controls plasma iron concentration by inhibiting intestinal iron absorption and iron recycling by macrophages. Hepcidin acts by inhibiting cellular iron efflux through binding to and inducing the degradation of FPN. Hepcidin transcription is upregulated by iron repletion and downregulated by iron deficiency, ineffective erythropoiesis, and hypoxia. Hypoxia-inducible factors HIF-1 and HIF-2 are heterodimeric transcriptional factors and central mediators of cellular and systemic adaptation to hypoxia. In the presence of oxygen, the HIF-α subunit is targeted to the proteasome, while in hypoxia (or iron deficiency), HIF-α is stabilized and induces the transcription of target genes. We hypothesized that HIFs, stabilized in the hypoxic intestinal epithelium, may also play critical local roles in regulating intestinal iron absorption. We generated conditional knockout mice that lacked either Hif1a or Hif2a specifically in the intestinal epithelium and found that HIF-1α was not necessary for iron absorption, whereas HIF-2α played a crucial role in maintaining iron balance in the organism by directly regulating the transcription of the genes encoding DMT1 and DCYTB. Specific deletion of Hif2a led to a decrease in serum and liver iron levels. Alterations in HIF-2 at the intestinal level can override systemic regulation via hepcidin. Interestingly, we further demonstrated that HIF-2α contributes to iron hyperabsorption in a genetic mouse model of hereditary hemochromatosis (HH). HH is a genetic disorder characterized by abnormally low hepcidin expression and excessive iron accumulation in the liver and parenchyma. These findings suggest a prominent role of HIF-2 in the physiopathological regulation of intestinal iron absorption and may provide new therapeutic perspectives for the treatment of anemias and iron overload-associated disorders. Disclosures:No relevant conflicts of interest to declare.

Natural iron isotopic composition of blood is an indicator of dietary iron absorption efficiency in humans

Persistent impairments in the regulation of intestinal iron absorption result in iron deficiency or iron accumulation in the long term. Diagnosis remains difficult unless pathological symptoms develop as iron absorption varies strongly between meals and days. Variations in the natural iron isotopic composition of whole blood have recently been suggested as a novel parameter to assess long-term differences in intestinal absorption efficiency between individuals. In this study, baseline blood samples collected in two previous conventional iron absorption studies in Swiss and Thai women using stable isotope tracers were reanalyzed by multicollector inductively coupled plasma mass spectrometry. The natural iron isotopic compositions obtained were compared with fractional absorption from the test meals observed in these earlier trials. Correlations of natural blood iron isotopic composition and fractional absorption from the test meals were found to be highly significant in both cohorts (for Swiss women, r = 0.40, P = 0.01, n = 38; for Thai women, r = 0.57, P < 0.01, n = 24), with the blood of both ethnicities clearly differing in iron isotopic composition (P < 0.001). Combining the findings of this study and those of recent animal and human studies confirms that blood iron isotopic patterns may serve as a novel compound biomarker of iron metabolism to assess impairments in regulation of intestinal iron absorption in individuals or population groups.

Duodenal cytochrome b (Cybrd 1) and HIF-2α expression during acute hypoxic exposure in mice

Recent evidence suggests that the duodenum can regulate iron absorption independently of hepcidin via the transcription factor Hif-2α acting directly on the transcription of the proteins involved in the iron transport. The current study investigates the temporal relationship between Dcytb and Hif-2α during early hypoxic stimulus in the enterocyte in vivo. Duodenal Dcytb and Hif-2α protein expression was analysed by Western blot technique while gene regulation was determined by quantitative PCR. Both Dcytb and Hif-2α protein expression were increased during the first hours of hypoxic duration. A change in hepcidin expression however, was significant only at 72 h hypoxia. Increased iron absorption reported in early hypoxia could be accounted for in part by the enhancement of Dcytb expression by Hif-2α in the duodenum. Modulation of Hif-2α predominates over hepcidin in the regulation of intestinal iron absorption during short hypoxic duration. The intestine exerts regulatory mechanisms in the dietary absorption of iron into systemic circulation.

Molecular mechanism of intestinal iron absorption

Iron is an essential trace metal in the human diet because of its role in a number of metabolic processes including oxygen transport. In the diet, iron is present in two fundamental forms, heme and non-heme iron. This article presents a brief overview of the molecular mechanisms of intestinal iron absorption and its regulation. While many proteins that orchestrate iron transport pathway have been identified, a number of key factors that control the regulation of iron absorption still remain to be elucidated. This review also summarizes new and emerging information about iron metabolic regulators that coordinate regulation of intestinal iron absorption.

Iron supply determines apical/basolateral membrane distribution of intestinal iron transporters DMT1 and ferroportin 1

Intestinal iron absorption comprises the coordinated activity of the influx transporter divalent metal transporter 1 (DMT1) and the efflux transporter ferroportin (FPN). In this work, we studied the movement of DMT1 and FPN between cellular compartments as a function of iron supply. In rat duodenum, iron gavage resulted in the relocation of DMT1 to basal domains and the internalization of basolateral FPN. Considerable FPN was also found in apical domains. In Caco-2 cells, the apical-to-basal movement of cyan fluorescent protein-tagged DMT1 was complete 90 min after the addition of iron. Steady-state membrane localization studies in Caco-2 cells revealed that iron status determined the apical/basolateral membrane distribution of DMT1 and FPN. In agreement with the membrane distribution of the transporters, (55)Fe flux experiments revealed inward and outward iron fluxes at both membrane domains. Antisense oligonucleotides targeted to DMT1 or FPN inhibited basolateral iron uptake and apical iron efflux, respectively, indicating the participation of DMT1 and FPN in these fluxes. The fluxes were regulated by the iron supply; increased iron reduced apical uptake and basal efflux and increased basal uptake and apical efflux. These findings suggest a novel mechanism of regulation of intestinal iron absorption based on inward and outward fluxes at both membrane domains, and repositioning of DMT1 and FPN between membrane and intracellular compartments as a function of iron supply. This mechanism should be complementary to those based in the transcriptional or translational regulation of iron transport proteins.

Regulation of Intestinal Iron Absorption: The Mucosa Takes Control?

Two studies (Shah et al., 2009; Mastrogiannaki et al., 2009) show that the hypoxia inducible factor HIF-2alpha is a major player in regulating iron absorption by directly controlling the transcription of iron transporters in the intestine in response to changes in mucosal iron or oxygen levels. The HIF-2alpha mechanism has major effects on iron metabolism which can override the well-known hepcidin-ferroportin regulatory axis.

Lack of the bone morphogenetic protein BMP6 induces massive iron overload

Expression of hepcidin, a key regulator of intestinal iron absorption, can be induced in vitro by several bone morphogenetic proteins (BMPs), including BMP2, BMP4 and BMP9 (refs. 1,2). However, in contrast to BMP6, expression of other BMPs is not regulated at the mRNA level by iron in vivo, and their relevance to iron homeostasis is unclear. We show here that targeted disruption of Bmp6 in mice causes a rapid and massive accumulation of iron in the liver, the acinar cells of the exocrine pancreas, the heart and the renal convoluted tubules. Despite their severe iron overload, the livers of Bmp6-deficient mice have low levels of phosphorylated Smad1, Smad5 and Smad8, and these Smads are not significantly translocated to the nucleus. In addition, hepcidin synthesis is markedly reduced. This indicates that Bmp6 is critical for iron homeostasis and that it is functionally nonredundant with other members of the Bmp subfamily. Notably, Bmp6-deficient mice retain their capacity to induce hepcidin in response to inflammation. The iron burden in Bmp6 mutant mice is significantly greater than that in mice deficient in the gene associated with classical hemochromatosis (Hfe), suggesting that mutations in BMP6 might cause iron overload in humans with severe juvenile hemochromatosis for which the genetic basis has not yet been characterized.

New and old players in the hepcidin pathway

The identification of the liver peptide hepcidin at the beginning of the new millennium opened a new era in our understanding of iron metabolism.[1][1]–[3][2] Hepcidin is the main regulator of intestinal iron absorption and macrophage iron release, thus ultimately of the iron available for

Alte und neue Eisenparameter – Bedeutung im Eisenstoffwechsel und in der Diagnostik

Iron is an element which is essential to life but also potentially toxic. Therefore, clever mechanisms exist in the human body for uptake, transport and storage of iron. Hepcidin, which seems to be the master protein for regulation of intestinal iron absorption, is known for a short time. The expression of hepcidin is not only influenced by iron levels but also by mediators of inflammation and growth factors of erythropoiesis. Hence hepcidin plays also a crucial role in the development of anemia of chronic disease and iron overload due to ineffective erythropoiesis. Serum ferritin is a reliable parameter to estimate the storage iron. It is an acute phase protein which is elevated during infections and inflammations, though. In these situations, measurement of soluble transferrin receptors is a useful tool to differentiate between iron deficiency and anemia of chronic disease. Newer parameters as erythrocyte zink protoporphyrin or percentage of hypochromic erythrocytes (%HYPO) are suited to detect a functional iron deficiency. Early diagnosis of iron overload is essential to prevent organ damage. Serum ferritin and transferrin are useful parameters to screen for iron overload. If no clear reason for a secondary iron overload can be found, the search for a hereditary haemochromatosis is recommended. Most of these hereditary haemochromatoses are a result of mutations in the HFE gene (homozygous state for Cys282Tyr or compound heterozygosity for Cys282Tyr/ His63Asp) which can be detected by PCR technique. Liver biopsy is still the gold standard for quantification of storage iron. However, a method of increasing importance for quantification of iron overload is magnetic resonance imaging with new approaches as for example T2*.

Repression of Repulsive Guidance Molecule C during Inflammation Is Independent of Hfe and Involves Tumor Necrosis Factor-α

Genetic iron overload, or hemochromatosis, can be caused by mutations in HFE, hemojuvelin, and hepcidin genes. Hepcidin, a negative regulator of intestinal iron absorption, is found to be inappropriately low in both patients and in animal models, indicating that proper control of basal hepcidin levels requires both hemojuvelin and HFE. In mice, repulsive guidance molecule c (Rgmc, the hemojuvelin mouse ortholog) and hepcidin levels are transcriptionally regulated during inflammation. Here, we report that basal Rgmc levels in Hfe-deficient mice are normal and that these mice retain the ability to suppress Rgmc expression after lipopolysaccharide (LPS) challenge. Thus, Rgmc regulation by LPS is Hfe-independent. The response of Rgmc to LPS involves signaling through toll-like receptor 4 (Tlr4), because Tlr4-deficient mice do not show altered Rgmc expression after LPS administration. We further show that tumor necrosis factor-alpha, but not interleukin-6, is sufficient to cause Rgmc down-regulation by LPS. These results contrast with previous data demonstrating that hepcidin levels are directly regulated by interleukin-6 but not by tumor necrosis factor-alpha. The regulation of iron-related genes by different cytokines may allow for time-dependent control of iron metabolism changes during inflammation and may be relevant to chronic inflammation, infections, and cancer settings, leading to the development of anemia of chronic disease.

Increased Adipose Tissue Expression of Hepcidin in Severe Obesity Is Independent From Diabetes and NASH

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. Hepcidin expression was studied in liver and adipose tissue of these patients. Hepcidin regulation was investigated in vitro by adipose tissue explant stimulation studies. 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. Hepcidin is a proinflammatory adipokine and may play an important role in hypoferremia of inflammation in obese condition.

Iron Imports. VI. HFE and regulation of intestinal iron absorption

The majority of clinical cases of iron overload is caused by mutations in the HFE gene. However, the role that HFE plays in the physiology of intestinal iron absorption remains enigmatic. Two major models have been proposed: 1) HFE exerts its effects on iron homeostasis indirectly, by modulating the expression of hepcidin; and 2) HFE exerts its effects directly, by changing the iron status (and therefore the iron absorptive activity) of intestinal enterocytes. The first model places the primary role of HFE in the liver (hepatocytes and/or Kupffer cells). The second model places the primary role in the duodenum (crypt cells or villus enterocytes). These models are not mutually exclusive, and it is possible that HFE influences the iron status in each of these cell populations, leading to cell type-specific downstream effects on intestinal iron absorption and body iron distribution.

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.

Mechanisms of Haem and Non-Haem Iron Absorption: Lessons from Inherited Disorders of Iron Metabolism

Our current state of knowledge of the mechanism and regulation of intestinal iron absorption has been critically dependent on the analysis of inherited disorders of iron homeostasis in both humans and other animal species. Mutations in DMT1 and Ireg1 have revealed that these molecules are major mediators of iron transport across the brush border and basolateral membranes of the enterocyte, respectively. Similarly, the iron oxidase hephaestin has been shown to play an important role in basolateral iron efflux. The analysis of a range of human iron loading disorders has provided very strong evidence that the products of the HFE, TfR2, hepcidin and hemojuvelin genes comprise integral components of the machinery that regulates iron absorption and iron traffic around the body. Engineered mouse strains have already proved very effective in helping to dissect pathways of iron homeostasis, and in the future they will continue to provide important insights into the absorption of both inorganic and haem iron by the gut.

Systemic Regulation of Intestinal Iron Absorption

The intestinal absorption of the essential trace element iron and its mobilization from storage sites in the body are controlled by systemic signals that reflect tissue iron requirements. Recent advances have indicated that the liver-derived peptide hepcidin plays a central role in this process by repressing iron release from intestinal enterocytes, macrophages and other body cells. When iron requirements are increased, hepcidin levels decline and more iron enters the plasma. It has been proposed that the level of circulating diferric transferrin, which reflects tissue iron levels, acts as a signal to alter hepcidin expression. In the liver, the proteins HFE, transferrin receptor 2 and hemojuvelin may be involved in mediating this signal as disruption of each of these molecules decreases hepcidin expression. Patients carrying mutations in these molecules or in hepcidin itself develop systemic iron loading (or hemochromatosis) due to their inability to down regulate iron absorption. Hepcidin is also responsible for the decreased plasma iron or hypoferremia that accompanies inflammation and various chronic diseases as its expression is stimulated by pro-inflammatory cytokines such as interleukin 6. The mechanisms underlying the regulation of hepcidin expression and how it acts on cells to control iron release are key areas of ongoing research.