Hongfan Peng

Bone morphogenetic proteins 2, 4, and 9 stimulate murine hepcidin 1 expression independently of Hfe, transferrin receptor 2 (Tfr2), and IL-6

Recently, it has been suggested that hepcidin, a peptide involved in iron homeostasis, is regulated by bone morphogenetic proteins (BMPs), apparently by binding to hemojuvelin (Hjv) as a coreceptor and signaling through Smad4. We investigate the role of Hfe, Tfr2 (transferrin receptor 2), and IL-6 in BMP2-, BMP4-, and BMP9-stimulated up-regulation of murine hepcidin, because these molecules, like Hjv, are known to be involved in hepcidin signaling. We show that the BMP signaling pathway acts independently of Hfe, Tfr2, and IL-6: The response to BMP2, BMP4, and BMP9 is similar in isolated hepatocytes of wild-type, Hfe−/−, IL-6−/−, and Tfr2m mutant mice. The potency of different human BMPs in stimulating hepcidin transcription by murine primary hepatocytes is BMP9 > BMP4 > BMP2. However, in human HepG2 cells, BMP4 and BMP9 are equally potent, whereas BMP2 requires a higher dose to become an effective hepcidin activator. Moreover, all of the tested BMPs are more potent regulators of hepcidin than IL-6 and thus are the most potent known stimulators of hepcidin transcription.

Different regulatory elements are required for response of hepcidin to interleukin-6 and bone morphogenetic proteins 4 and 9

Hepcidin is a major regulator of iron homeostasis. Hepcidin expression is upregulated by inflammatory cytokines, particularly interleukin (IL)‐6 and even more potently by the bone morphogenetic proteins 2, 4 and 9 (BMP‐2, BMP‐4 and BMP‐9). This study showed that the regulation of hepcidin expression by IL‐6 and BMPs occurs through distinct regulatory elements. The induction of hepcidin by BMPs requires at least two regions of the Hamp1 promoter, one between 140–260 bp and the other between 1·6–2·0 kb upstream of the start of translation. Reporter constructs including 1·6–2·0 kb of the Hamp1 promoter were induced >16‐fold by BMPs whereas a 260 bp reporter Hamp1 promoter construct was induced only two‐ to threefold. The distal 1·6–2·0 kb region appeared to contain several different BMP‐responsive elements, as incremental lengthening of the promoter construct in this region produced gradual escalation of BMP‐responsiveness. In contrast, the IL‐6 response required only the proximal 260 bp Hamp1 promoter region. Furthermore, there were no regulatory elements located in the non‐coding or coding regions of Hamp1 and activation of the Hamp1 promoter was absent or markedly reduced in cells of non‐hepatic origin.

Suppression of the hepcidin-encoding gene Hamp permits iron overload in mice lacking both hemojuvelin and matriptase-2/TMPRSS6

Hepcidin, the master regulator of enteric iron absorption, is controlled by the opposing effects of pathways activated in response to iron excess or iron attenuation. Iron excess is regulated through a pathway involving the cell surface receptor hemojuvelin (HFE2) that stimulates expression of the hepcidin encoding gene (HAMP). Iron attenuation is countered through a pathway involving the hepatocyte‐specific plasma membrane protease matriptase‐2 encoded by TMPRSS6, leading to suppression of HAMP expression. The non‐redundant function of hemojuvelin and matriptase‐2 has been deduced from the phenotype imparted by mutations of HFE2 and TMPRSS6, which cause iron excess and iron deficiency respectively. Hemojuvelin is positioned to be the ideal substrate for matriptase‐2. To examine the relationship between hemojuvelin and matriptase‐2 in vivo, we crossed mice lacking the protease domain of matriptase‐2 with mice lacking hemojuvelin. Mice lacking functional matriptase‐2 and hemojuvelin exhibited low Hamp (Hamp1) expression, high serum and liver iron, and high transferrin saturation. Surprisingly, the double mutant mice also exhibited lower levels of iron in the heart compared to hemojuvelin‐deficient mice, demonstrating a possible cardioprotective effect resulting from the loss of matriptase‐2. This phenotype is consistent with hemojuvelin being a major substrate for matriptase‐2/TMPRSS6 protease activity.

Soluble Transferrin Receptor-1 Levels in Mice Do Not Affect Iron Absorption

Soluble transferrin receptor-1 (sTfR1) concentrations are increased in the plasma under two conditions that are associated with increased iron absorption, i.e. iron deficiency and increased erythropoiesis. To determine the possible role of sTfR1 as a signaling mechanism for iron absorption, a hydrodynamic gene transfer technique was established to express transfected plasmid constructs of human sTfR1 (hsTfR1) and murine sTfR1 (msTfR1) from the livers of C57BL/6 mice. Iron absorption, serum iron levels and hepcidin expression were then measured. The hydrodynamic gene transfer technique proved to be an effective approach to achieving sustained expression of sTfR1 in mice. Although expression of high levels of sTfR1 significantly increased serum iron levels, repeated experiments showed that neither hsTfR1 nor msTfR1 had any effect on iron absorption or hepcidin mRNA expression levels. Thus, despite its attractiveness as a potential modifier of iron absorption, sTfR1 levels do not exert a regulatory effect on iron absorption.

Severe Microcytic Anemia but Increased Erythropoiesis in Mice Lacking Hfe or Tfr2 and Tmprss6

Cell surface proteins Hfe, Tfr2, hemojuvelin and Tmprss6 play key roles in iron homeostasis. Hfe and Tfr2 induce transcription of hepcidin, a small peptide that promotes the degradation of the iron transporter ferroportin. Hemojuvelin, a co-receptor for bone morphogenic proteins, induces hepcidin transcription through a Smad signaling pathway. Tmprss6 (also known as matriptase-2), a membrane serine protease that has been found to bind and degrade hemojuvelin in vitro, is a potent suppressor of hepcidin expression. In order to examine if Hfe and Tfr2 are substrates for Tmprss6, we generated mice lacking functional Hfe or Tfr2 and Tmprss6. We found that double mutant mice lacking functional Hfe or Tfr2 and Tmprss6 exhibited a severe iron deficiency microcytic anemia phenotype mimicking the phenotype of single mutant mice lacking functional Tmprss6 (Tmprss6msk/msk mutant) demonstrating that Hfe and Tfr2 are not substrates for Tmprss6. Nevertheless, the phenotype of the mice lacking Hfe or Tfr2 and Tmprss6 differed from Tmprss6 deficient mice alone, in that the double mutant mice exhibited much greater erythropoiesis. Hfe and Tfr2 have been shown to play important roles in the erythron, independent of their role in regulating liver hepcidin transcription. We demonstrate that lack of functional Tfr2 and Hfe allows for increased erythropoiesis even in the presence of high hepcidin expression, but the high levels of hepcidin levels significantly limit the availability of iron to the erythron, resulting in ineffective erythropoiesis. Furthermore, repression of hepcidin expression by hypoxia was unaffected by the loss of functional Hfe, Tfr2 and Tmprss6.

EPO‐mediated reduction in Hamp expression in vivo corrects iron deficiency anaemia in TMPRSS6 deficiency

Hepcidin, a 25 amino acid liver-derived peptide, has been recognized as a regulator of iron homeostasis. Hepcidin levels are negatively correlated with erythropoietic expansion, consistent with erythrocytes representing the major compartment that utilizes iron and hepcidin the major regulator for limiting iron availability (Nemeth, 2008). However, the molecular pathway transmitting the signal from proliferating erythrocytes to the regulation of hepcidin expression in hepatocytes has not been clearly defined.