Bone morphogenetic protein (BMP)-responsive elements located in the proximal and distal hepcidin promoter are critical for its response to HJV/BMP/SMAD

Abstract

The hemochromatosis proteins HFE, transferrin receptor 2 (TfR2) and hemojuvelin (HJV, HFE2) positively control expression of the major iron regulatory hormone hepcidin. HJV is a bone morphogenetic protein (BMP) co-receptor that enhances the cellular response to BMP cytokines via the phosphorylation of SMAD proteins. In this study, we show that two highly conserved and sequence-identical BMP-responsive elements located at positions -84/-79 (BMP-RE1) and -2,255/-2,250 (BMP-RE2) of the human hepcidin promoter are critical for both the basal hepcidin mRNA expression and the hepcidin response to BMP-2 and BMP-6. While BMP-RE1 and BMP-RE2 show additive effects in responding to HJV-mediated BMP signals, only BMP-RE1 that is located in close proximity to a previously identified STAT-binding site is important for the hepcidin response to IL-6. These data identify a missing link between the HJV/BMP signaling pathways and hepcidin transcription, and further define the connection between inflammation and BMP-dependent hepcidin promoter activation. As such, they provide important new information furthering our understanding of disorders of iron metabolism and the anemia of inflammation.

Disordered hepcidin–ferroportin signaling promotes breast cancer growth

“…Under the normal condition, hepatic hepcidin expression is prominently induced by erythropoietic demand, iron burden and inflammatory stimuli [66][67][68], of which the BMP family members and the IL-6 family cytokines are the essential upstream regulators [68,69]. Recent studies suggest that BMP6, rather than other BMP family members, is the key endogenous regulator of hepatic hepcidin expression at least in mice [70], and BMP6 likely governs the basal expression of hepatic hepcidin [71]. We here uncovered increased concentrations of BMP-6, IL-6 and iron in sera from breast cancer patients, and verified the simulating effects of serum IL-6, BMP-6 and iron on hepatic hepcidin expression.…”

Low hepcidin accounts for the proinflammatory status associated with iron deficiency

“…The hepcidin promoter has a STAT3-responsive site (STAT3-RS) at position Ϫ72/Ϫ64 in close proximity of a BMP-responsive element (BMP-RE1) at position Ϫ87/Ϫ79. 15 Although STAT3-RS inactivation does not alter the BMP-mediated response of hepcidin, mutations of BMP-RE1 strongly downregulate IL6-mediated hepcidin activation, suggesting that the integrity of the BMP-SMAD pathway is required to activate hepcidin. 15 This is confirmed by the lack of an increase in hepcidin in response to IL6 in liver-specific conditional Smad4-deficient mice.…”

BMPS and Liver: More Questions than Answers

“…Since then, many papers have provided increasing evidence supporting a central role for BMP6 in the control of iron homeostasis. The main findings of some of these studies are briefly summarized in here (for a more extensive review see [92,93]]: i) HJV knock out mouse presents impaired BMP signaling [91]; ii) HJV knock out mice livers have low phospho Smad1,5, 8 activity [91], iii) HJV binds directly to BMP ligands, mainly BMP2, BMP4 and BMP6, being BMP6 the ligand with higher affinity, as demonstrated by both in vitro and ex vivo experiments [55,94]; iv) BMP ligands enhance hepcidin expression at the transcriptional level as hepcidin promoter have two BMP responsive elements [26,91,[94][95][96]; v) BMP2 and BMP6 administration in mice increases hepcidin expression levels in liver and decreases plasma iron [94,97]; vi) Conversely, administration of dorsomorphin (an ALK2, ALK3 and ALK6 inhibitor) or HJV.fc (acting as ligand trap) in mice decreases the expression of hepatic hepcidin and increases plasma iron [94,98]; vii) Hepatic BMP6 levels are regulated by iron levels in vivo [99]. An increase in BMP6 serum levels is observed after short-and longterm iron overload [55]; viii) Iron overload phenotype observed in BMP6 null mice resembles the phenotype of HJV knock out mice [97,100]; ix) BMP6 is produced and secreted into portal circulation by enterocytes of small intestinal villus tips in response to iron absorption [55]; x) ALK2 and ALK3 BMP type I receptors appear to be involved in mediating BMP6 signaling in liver [101].…”