Bone morphogenic proteins in iron homeostasis

Abstract

The bone morphogenetic protein (BMP)-SMAD signaling pathway plays a central role in regulating hepcidin, which is the master hormone governing systemic iron homeostasis. Hepcidin is produced by the liver and acts on the iron exporter ferroportin to control iron absorption from the diet and iron release from body stores, thereby providing adequate iron for red blood cell production, while limiting the toxic effects of excess iron. BMP6 and BMP2 ligands produced by liver endothelial cells bind to BMP receptors and the coreceptor hemojuvelin (HJV) on hepatocytes to activate SMAD1/5/8 signaling, which directly upregulates hepcidin transcription. Most major signals that influence hepcidin production, including iron, erythropoietic drive, and inflammation, intersect with the BMP-SMAD pathway to regulate hepcidin transcription. Mutation or inactivation of BMP ligands, BMP receptors, HJV, SMADs or other proteins that modulate the BMP-SMAD pathway result in hepcidin dysregulation, leading to iron-related disorders, such as hemochromatosis and iron refractory iron deficiency anemia. Pharmacologic modulators of the BMP-SMAD pathway have shown efficacy in pre-clinical models to regulate hepcidin expression and treat iron-related disorders. This review will discuss recent insights into the role of the BMP-SMAD pathway in regulating hepcidin to control systemic iron homeostasis.

Keywords: Anemia; Bone morphogenetic protein; Hemochromatosis; Hemojuvelin; Hepcidin; Iron; SMAD.

Figure 1.. Systemic iron homeostasis is regulated by the hepcidin-ferroportin axis.

Iron circulates in the bloodstream mainly bound to transferrin. The majority of the iron is delivered to the bone marrow for the production of red blood cells, with lesser amounts delivered to all other tissues, and excess iron delivered mainly to the liver for storage. The major sources of iron are reticuloendothelial macrophages that recycle iron from phagocytosed senescent red blood cells and, to a lesser extent, duodenal enterocytes that absorb dietary iron. Iron is also released directly into the circulation from liver stores, red blood cells, and the kidney, which can reabsorb filtered iron from the urine. The liver iron hormone hepcidin controls systemic iron homeostasis by binding the iron exporter ferroportin (FPN) to block iron export directly and to induce FPN degradation, thereby inhibiting iron release into the circulation from all of these sources. Hepcidin expression in the liver is upregulated by iron, inflammation and endoplasmic reticulum (ER) stress to prevent iron overload and limit iron availability to pathogens. Hepcidin is downregulated by iron deficiency, erythropoietic drive and some hormones and growth factors to increase iron availability for red blood cell production and other body needs.

Figure 2.. The BMP-SMAD pathway and hepcidin regulation in different iron states.

(A) Two types of iron signals, tissue iron and circulating iron, can be sensed via distinct mechanisms to activate the bone morphogenic protein (BMP)-SMAD pathway and increase hepcidin production. Increases in tissue iron (via uptake of transferrin-bound iron [2Fe-Tf] and nontransferrin bound iron [NTBI]) are sensed by liver sinusoidal endothelial cells (LSEC), at least in part through increased mitochondrial reactive oxygen species (ROS), which activates nuclear factor erythroid–related factor 2 (NRF2) to upregulate Bmp6 transcription. Bmp2 expression is also induced by tissue iron loading, albeit to a lesser extent, although the mechanism is unknown. Secreted BMP6 and BMP2 act together, most likely as a heterodimer (BMP2/6), to bind to BMP receptor complexes on the hepatocyte membrane containing 2 type I receptors (ALK3 homodimers or ALK2/3 heterodimers), 2 type II receptors (ACVR2A and/or BMPR2), and co-receptor hemojuvelin (HJV). Neogenin (NEO) interacts with BMP/HJV receptor complex to facilitate complex formation and localization. The activated BMP/HJV receptor complex phosphorylates SMAD1, SMAD5, and SMAD8 proteins (SMAD1/5/8), which bind to SMAD4 and translocate to the nucleus to bind BMP response elements (BRE) in hepcidin (HAMP) promoter, thereby inducing transcription. Circulating 2Fe-Tf binds to transferrin receptor 2 (TFR2) and transferrin receptor 1 (TFR1) on hepatocytes, thereby stabilizing TFR2, displacing HFE from TFR1, and favoring an interaction between HFE and TFR2, which enhance SMAD1/5/8 signaling and hepcidin production, possibly via a physical interaction with the HJV/BMP receptor complex. Inhibitory SMAD7 and SMAD6 are induced by iron-mediated BMP signaling as a negative feedback inhibitor to prevent excessive increases of SMAD1/5/8 signaling and hepcidin. (B) In iron deficiency, BMP2/6 ligand production is reduced, HFE is sequestered by TFR1, TFR2 is degraded, and TMPRSS6 and furin cleave HJV from the membrane surface, thereby suppressing BMP-SMAD signaling and hepcidin production. Furin-cleaved soluble HJV (sHJV) may also inhibit this pathway by sequestering BMP ligands from activating the BMP receptor complex. FK506-Binding Protein 1A (FKBP12) interacting with ALK2 also plays an inhibitory role in BMP-SMAD signaling and hepcidin regulation.

Figure 3.. Regulation of the BMP-SMAD pathway and hepcidin by erythropoietic drive.

Conditions that increase erythropoietic drive, including anemia, erythropoietin, and ineffective erythropoiesis, increase the production and secretion of erythroferrone (ERFE) by erythroblasts. ERFE binds and sequesters BMP ligands, preventing their activation of the BMP receptor complex and thus reducing hepcidin transcription. The increased utilization of 2Fe-Tf for erythropoiesis can also reduce circulating 2Fe-Tf levels, thereby suppressing hepcidin via the iron sensing pathway described in Figure 2B.

Figure 4.. Crosstalk between inflammation and the BMP-SMAD pathway in hepcidin regulation.

During inflammation, interleukin (IL)-6 binds to its receptor to activate Janus Kinases (JAK)2, which induces the dimerization and phosphorylation of the signal transducer and activator of transcription (STAT)3. Phosphorylated STAT3 translocates to the nucleus and binds to a STAT3 response element (SRE) to upregulate hepcidin transcription. An intact BMP-SMAD1/5/8 signaling pathway is necessary for optimal hepcidin induction by inflammation by controlling basal hepcidin expression. Activin B is also generated during inflammation. In hepatocyte cell cultures, Activin B can utilize Activin type II receptors (ACVR2A and ACVR2B) and BMP type I receptors (ALK3 and ALK2) to activate SMAD1/5/8 signaling and induce hepcidin production, although a study in Inhbb−/− knockout mice (which lack Activin B) did not support a role for Activin B in hepcidin regulation by inflammation in vivo.