The Role of Hepatocyte Hemojuvelin in the Regulation of Bone Morphogenic Protein-6 and Hepcidin Expression in Vivo

Abstract

Both hemojuvelin (HJV) and bone morphogenic protein-6 (BMP6) are essential for hepcidin expression. Hepcidin is the key peptide hormone in iron homeostasis, and is secreted predominantly by hepatocytes. HJV expression is detected in hepatocytes, as well as in skeletal and heart muscle. HJV binds BMP6 and increases hepcidin expression presumably by acting as a BMP co-receptor. We characterized the role of hepatocyte HJV in the regulation of BMP6 and hepcidin expression. In HJV-null (Hjv Iron is an indispensable element for life. Iron homeostasis is controlled elegantly by hepcidin (1, 2). Hepcidin is a peptide hormone that is secreted predominantly by the hepatocytes in the liver. Under physiological conditions, its expression is regulated positively by bodily iron-loading (3, 4). Recent studies demonstrate that normal levels of hepatic hepcidin expression require the presence of both hemojuvelin (HJV) 2 and bone morphogenic protein-6 (BMP6) (5-7).HJV is a GPI-linked membrane protein that is encoded by the gene, HFE2, in humans (8, 9). Repulsive guidance molecule c (RGMc) is the ortholog of HJV in mice, and it is encoded by the gene, Hfe2. For simplicity, we will use Hjv for the gene and HJV for the protein in human and mice in this report. Hjv is expressed highly in skeletal and heart muscle, and at a relatively low level in hepatocytes (9, 10). Homozygous or compound heterozygous mutations of HJV in humans and disruption of both Hjv alleles (Hjv Ϫ/Ϫ ) in mice, markedly reduce hepatic hepcidin expression and cause severe iron overload (9, 11-13). Thus, HJV is a critical upstream regulator of hepcidin transcription. However, the precise role of HJV expression in different tissues in the regulation of hepcidin expression in vivo has not been addressed.Hepatic hepcidin expression is mediated via the BMP signaling cascade (14). BMP signaling is initiated upon the binding of BMP ligands to BMP receptor complexes on the cell surface. This binding triggers the phosphorylation of Smad1, Smad5, and Smad8 (Smad1/5/8) in the cytoplasm. The phosphorylated Smads (pSmad1/5/8) form heteromeric complexes with Smad4 and then translocate to the nucleus where they induce the transcription of target genes (15). Liver-specific disruption of Smad4 in mice markedly decreases hepcidin expression and causes iron accumulation in particular organs (14).Hepatocytes are the only known cell types in the body where both HJV and hepcidin are co-expressed (3,10,12,16). In hepatoma cell lines, hepcidin expression can be induced robustly by BMP2, BMP4, and BMP6 independently of HJV (10,17,18). Recent studies show that BMP6, rather than other BMP ligands, plays an essential role in iron homeostasis. BMP6 mRNA in the liver and the small intestine is positively regulated by bodily iron-loading (5, 19). Disruption of both BMP6 alleles in mice markedly suppresses hepatic hepcidin expression and causes severe iron overload (6, 7). These phenotypes resemble those reported in Hjv Ϫ/Ϫ mice (12, 13). Therefore, BMP6 and HJV are both important in the induct...

Iron overload induces BMP6 expression in the liver but not in the duodenum

“…Similar data were recently obtained with hemojuvelin-deficient mice on a 129/SvEvTac background. 15 Hemojuvelin …”

“…[16][17][18][19] Interestingly, whereas Hjv-deficient mice have Bmp6 mRNA levels 4 times higher than wild-type mice in their liver, no significant difference in Bmp6 mRNA expression was found in their small intestine. 15 Immunodetection was performed with two Santa Cruz anti-BMP6 antibodies, S-20 as in Arndt et al 11 and N-19 as in Kautz et al 10 A centrilobular pattern of Bmp6 staining was observed in the liver of iron overloaded mice. Noticeably, the centrilobular Bmp6 staining in these mice does not coincide with the periportal location of iron visualized by Perls' coloration, which excludes iron interference with immunodetection.…”

“…Although it was not specified whether Hjv-deficient mice have been fasted, 15 we wanted to eliminate this potential bias and repeated the study on a group of 5 iron-L. Kautz et al 202 haematologica | 2011; 96(2) Figure 2. Cellular localization of iron and BMP6 in the liver and the duodenum of mice with iron overload secondary to an ironenriched diet or due to Hfe-deficiency.…”

Reducing TMPRSS6 ameliorates hemochromatosis and β-thalassemia in mice

“…At the molecular level, Hamp expression is controlled by the activation of the SMAD1,5,8/SMAD4 complex, which in turn is triggered by the association between the BMP6 ligand, whose expression is proportional to intracellular hepatic iron concentrations, and the BMP-receptor complex (10)(11)(12). Hemojuvelin (HJV) acts as a coreceptor and is required to fully activate the SMAD pathway (10,(13)(14)(15). In contrast, TMPRSS6 or Matriptase-2, a transmembrane serine protease, cleaves HJV, reducing the phosphorylation of the SMAD complex and attenuating HAMP expression (16)(17)(18)(19)(20).…”

Effect of hemochromatosis on manganese neurotoxicity

“…HJV is highly expressed in the liver, heart and skeletal muscle. In the liver, only hepatocytes express HJV but no other cells [60,61]. HJV plays an important role in hepcidin expression [62].…”