Hepcidin decreases iron transporter expression in vivo in mouse duodenum and spleen and in vitro in THP-1 macrophages and intestinal Caco-2 cells

Intestinal ferroportin expression in pediatric Crohn's disease

BACKGROUND

Anemia is a frequent complication of Crohn's disease (CD). The intestinal iron exporter ferroportin (FPN) is involved in both iron deficiency anemia and the anemia of chronic disease. To examine its role in CD, intestinal FPN expression was studied in subjects with and without CD.

METHODS

Duodenal mucosal biopsies from 29 pediatric subjects with CD (n=19) and without CD (n=10) were obtained. FPN protein was measured using Western blot analysis and mRNA was assessed using quantitative real-time polymerase chain reaction (PCR).

RESULTS

Intestinal FPN protein was higher in anemic CD subjects than in nonanemic CD subjects (P=0.01), while FPN mRNA levels were not different (P=0.66). In nonanemic CD subjects, erythrocyte sedimentation rate (ESR) (P=0.04), C-reactive protein (CRP) (P=0.03), and interleukin-6 (IL-6) (P=0.01) levels were elevated compared to controls. Nonanemic CD subjects had a lower median FPN protein than nonanemic controls, although it did not reach statistical significance (P=0.07). Median FPN mRNA was similar between groups (P=0.71). Although no correlation between FPN protein and IL-6 was noted, there was a strong negative correlation between serum iron and IL-6, both in subjects with CD (r=-0.88, P<0.0001) and those without anemia (r=-0.58, P=0.02).

CONCLUSIONS

Intestinal FPN protein is upregulated in anemic CD subjects, suggesting that iron deficiency or anemia is the driving force regulating FPN levels. A transporter distinct from FPN appears to be involved in the hypoferremia associated with the inflammatory process of CD.

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.

Advancements in anemias related to chronic conditions

Anemia of chronic disease (ACD), the most frequent anemia among hospitalized patients, occurs in chronic inflammatory disorders, such as chronic infections, cancer and autoimmune diseases. Different causes contribute to ACD including diversion of iron traffic, diminished erythropoiesis, blunted response to erythropoietin, erythrophagocytosis, hematologic malignancies and solid tumors. A particular case of ACD is represented by anemia of chronic kidney disease (CKD). ACD is characterized by hyposideremia and altered iron transport. Cytokines are implicated in the ACD by reducing erythropoiesis and increasing iron sequestration in the reticuloendothelial system. The regulation of iron absorption across the epithelium of the proximal small intestine is essential for maintaining body iron concentrations within a physiologically defined range. Hepcidin controls cellular iron efflux by binding to the iron export protein ferroportin, causing ferroportin to be phosphorylated and degraded in lysosomes. Finally, hepcidin inhibits iron release from the reticulo-endothelial system. Increased expression of hepcidin leads to decreased iron absorption and iron deficient anemia. Hepcidin, therefore, is a negative regulator of iron transport in plasma. Causes of anemia in patients with CKD are multifactorial, but the most well-known cause is inadequate erythropoietin production. In these patients, anemia increases the risk of either cardiovascular disease or renal failure.

Molecular basis of iron-loading disorders

Iron-loading disorders (haemochromatosis) represent an important class of human diseases. Primary iron loading results from inherited disturbances in the mechanisms regulating intestinal iron absorption, such that excess iron is taken up from the diet. Body iron load can also be increased by repeated blood transfusions (secondary iron loading), usually as part of the treatment for various haematological disorders. In these syndromes, an element of enhanced iron absorption is also often involved. The central regulator of body iron trafficking is the liver-derived peptide hepcidin. Hepcidin limits iron entry into the plasma from macrophages, intestinal enterocytes and other cells by binding to the sole iron-export protein ferroportin, and facilitating its removal from the plasma membrane. Mutations in hepcidin or its upstream regulators (HFE, TFR2, HFE2 and BMP6) lead to reduced or absent hepcidin expression and a concomitant increase in iron absorption. Mutations in ferroportin that prevent hepcidin binding produce a similar result. Increased ineffective erythropoiesis, which often characterises erythrocyte disorders, also leads to reduced hepcidin expression and increased absorption. Recent advances in our understanding of hepcidin and body iron homeostasis provide the potential for a range of new diagnostic and therapeutic tools for haemochromatosis and related conditions.

Regulatory mechanisms of intestinal iron absorption-uncovering of a fast-response mechanism based on DMT1 and ferroportin endocytosis

Knowledge on the intestinal iron transport process and the regulation of body iron stores has greatly increased during the last decade. The liver, through the sensing of circulating iron, is now recognized as the central organ in this regulation. High iron levels induce the synthesis of hepcidin, which in turn decreases circulating iron by inhibiting its recycling from macrophages and its absorption at the intestine. Another mechanism for the control of iron absorption by the enterocyte is an active Iron Responsive Element (IRE)/Iron Regulatory Protein (IRP) system. The IRE/IRP system regulates the expression of iron uptake and storage proteins thus regulating iron absorption. Similarly, increasing evidence points to the transcriptional regulation of both divalent metal transporter 1 (DMT1) and ferroportin expression. A new mechanism of regulation related to a phenomenon called the mucosal block is starting to be unveiled. The mucosal block describes the ability of an initial dose of ingested iron to block absorption of a second dose given 2-4 h later. Here, we review the mechanisms involved in the expression of DMT1 and ferroportin, and present recent evidence on the molecular components and cellular processes involved in the mucosal block response. Our studies indicate that mucosal block is a fast-response endocytic mechanism destined to decrease intestinal iron absorption during a high ingest of iron.

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.