Matriptase-2 suppresses hepcidin expression by cleaving multiple components of the hepcidin induction pathway

Abstract

Systemic iron homeostasis is maintained by regulation of iron absorption in the duodenum, iron recycling from erythrocytes, and iron mobilization from the liver and is controlled by the hepatic hormone hepcidin. Hepcidin expression is induced via the bone morphogenetic protein (BMP) signaling pathway that preferentially uses two type I (ALK2 and ALK3) and two type II (ActRIIA and BMPR2) BMP receptors. Hemojuvelin (HJV), HFE, and transferrin receptor-2 (TfR2) facilitate this process presumably by forming a plasma membrane complex with BMP receptors. Matriptase-2 (MT2) is a protease and key suppressor of hepatic hepcidin expression and cleaves HJV. Previous studies have therefore suggested that MT2 exerts its inhibitory effect by inactivating HJV. Here, we report that MT2 suppresses hepcidin expression independently of HJV. In mice, increased expression of exogenous MT2 in the liver significantly reduced hepcidin expression similarly as observed in wild-type mice. Exogenous MT2 could fully correct abnormally high hepcidin expression and iron deficiency in mice. In contrast to MT2, increased Hjv expression caused no significant changes in wild-type mice, suggesting that Hjv is not a limiting factor for hepcidin expression. Further studies revealed that MT2 cleaves ALK2, ALK3, ActRIIA, Bmpr2, Hfe, and, to a lesser extent, Hjv and Tfr2. MT2-mediated Tfr2 cleavage was also observed in HepG2 cells endogenously expressing MT2 and TfR2. Moreover, iron-loaded transferrin blocked MT2-mediated Tfr2 cleavage, providing further insights into the mechanism of Tfr2's regulation by transferrin. Together, these observations indicate that MT2 suppresses hepcidin expression by cleaving multiple components of the hepcidin induction pathway.

The catalytic, stem, and transmembrane portions of matriptase-2 are required for suppressing the expression of the iron-regulatory hormone hepcidin

“…We sought to explore the functions of the noncatalytic domains of MT2 by using cultured cells and MT2 Ϫ/Ϫ mice as models. We previously showed that MT2 cleaves multiple components of the hepcidin-induction pathway (9). Our pilot studies on two other membrane-anchored serine proteases, matriptase and prostasin, strongly indicated the critical roles of the nonproteolytic sequence of MT2 in the specific and efficient cleavage of substrates.…”

“…Early studies suggested that MT2 suppresses hepcidin expression solely by cleaving HJV (8,25). Our recent data showed that MT2 suppresses hepcidin expression independently of Hjv in vivo and that it cleaves multiple components of the hepcidin-induction pathway, including BMP receptors (ALK2, ALK3, ActRIIA, and Bmpr2), Hfe, and to a lesser extent, Hjv and Tfr2, in vitro (9). Consistent with these latter observations, MT2 Ϫ/Ϫ mice have a lower, rather than higher, level of Hjv protein in the liver (26,27).…”

“…For the convenience of immunodetection, a FLAG/MYC epitope was added to these constructs as well as to most of other constructs used in this study ( Table 1). Addition of a C-terminal FLAG/MYC epitope to MT2 does not affect either its cleavage of substrates in transfected cells or the suppression of hepcidin expression in mice (9,16,25). All expressed proteins migrated on SDS-PAGE as predicted by their molecular masses ( Fig.…”

Hepcidin and its therapeutic potential in neurodegenerative disorders

“…In hepatocytes, it has been well‐documented that inflammation induces hepcidin gene expression via the interleukin‐6 (IL‐6)/signal transducer and activator of transcription 3 (STAT3) pathway, via iron sensing through the transferrin receptor 2 (TfR2)/human hemochromatosis protein (HFE), and the bone morphogenetic protein (BMP)/hemojuvelin (HJV)/SMAD (a small mothers of decapentaplegic protein) pathways, and downregulated by erythroferrone (ERFE) ‐dependent and ‐independent mechanisms . Recent studies have also demonstrated that hepcidin expression can be upregulated by progesterone and mifepristone, two steroid hormones, through cell surface protein progesterone receptor membrane component‐1, transforming growth factor β1, SMAD1/5/8‐independent signaling by activin B, oxidases such as NADPH‐dependent oxidase 4 or artificially overexpressed urate oxidase (UOX), IL‐1β via inducing CCAAT enhancer‐binding protein δ (C/EBPδ) expression, peroxiredoxin‐2 which is a supporter for STAT3 transcriptional activity, fibroblast growth factor‐encoding gene, and downregulated by the immunophilin FKBP12 (FK506‐binding protein 1A) via binding with BMP type I receptor ALK2, cystathionine β‐synthase, TfR1, and matriptase‐2 …”

Transferrin Receptors TfR1 and TfR2 Bind Transferrin through Differing Mechanisms

“…Whether HFE-TfR2 complex or HFE in complex with another protein initiates a signaling cascade to induce hepcidin expression is controversial. Signaling then decreases both passively with falling Tf iron saturation and actively through cleavage from the protease matriptase-2 12 . These hypothetical iron-sensing mechanisms are supported by multiple in vitro and in vivo experiments, but mechanisms remain unknown.…”