Evidence for distinct pathways of hepcidin regulation by acute and chronic iron loading in mice

Abstract

In response to iron loading, hepcidin synthesis is homeostatically increased to limit further absorption of dietary iron and its release from stores. Mutations in HFE, transferrin receptor 2 (Tfr2), hemojuvelin (HJV), or bone morphogenetic protein 6 (BMP6) prevent appropriate hepcidin response to iron, allowing increased absorption of dietary iron, and eventually iron overload. To understand the role each of these proteins plays in hepcidin regulation by iron, we analyzed hepcidin messenger RNA (mRNA) responsiveness to short and long-term iron challenge in iron-depleted Hfe, Tfr2, Hjv, and Bmp6 mutant mice. After 1-day (acute) iron challenge, Hfe 2/2 mice showed a smaller hepcidin increase than their wild-type strain-matched controls, Bmp6 2/2 mice showed nearly no increase, and Tfr2 and Hjv mutant mice showed no increase in hepcidin expression, indicating that all four proteins participate in hepcidin regulation by acute iron changes. After a 21-day (chronic) iron challenge, Hfe and Tfr2 mutant mice increased hepcidin expression to nearly wildtype levels, but a blunted increase of hepcidin was seen in Bmp6 2/2 and Hjv 2/2 mice. BMP6, whose expression is also regulated by iron, may mediate hepcidin regulation by iron stores. None of the mutant strains (except Bmp6 2/2 mice) had impaired BMP6 mRNA response to chronic iron loading. Conclusion: TfR2, HJV, BMP6, and, to a lesser extent, HFE are required for the hepcidin response to acute iron loading, but are partially redundant for hepcidin regulation during chronic iron loading and are not involved in the regulation of BMP6 expression. Our findings support a model in which acute increases in holotransferrin concentrations transmitted through HFE, TfR2, and HJV augment BMP receptor sensitivity to BMPs. A distinct regulatory mechanism that senses hepatic iron may modulate hepcidin response to chronic iron loading. (HEPATOLOGY 2011;53:1333-1341 S ystemic iron homeostasis is dependent on the hepatic peptide hormone hepcidin, its receptor/iron channel ferroportin, and the feedback regulation of the two molecules by iron. Hepcidin controls the delivery of dietary and recycled iron to plasma by binding to the iron exporter ferroportin and inducing its endocytosis and degradation. 1 Increases in body iron levels stimulate hepcidin production in the liver, limiting further absorption of iron. The mechanism of hepcidin regulation by iron is not completely understood. Human studies have shown that blood hepcidin concentrations rapidly increase in response to oral iron challenge, are proportional to increases in diferric transferrin (holotransferrin) concentrations, 2,3 and strongly correlate with iron stores as reflected by serum ferritin. 3 In mice, hepcidin messenger RNA (mRNA) increase within 24 hours after the switch from a lowiron diet to a standard diet 4 and in vivo imaging of hepcidin promoter-reporter constructs in mice 5 confirmed high responsiveness of hepcidin promoter Abbreviations: ANOVA, analysis of variance; apo-Tf, apotransferrin; BMP6, bone morphogenetic pr...

Estrogen regulates iron homeostasis through governing hepatic hepcidin expression via an estrogen response element

“…Considering iron metabolism upon certain chronic physiological alternation, the end-point of iron-related studies are normally set over 1 month after ovariectomy when using ovariectomized models (Ikeda et al, 2012;Liu et al, 2006;Mattace Raso et al, 2009). Moreover, a previous study by Ramos et al has demonstrated that hepcidin expression were distinct in response to long-term (3 weeks) or acute (1 day) iron loading (Ramos et al, 2011). To this end, we decided to address a long-time effect of chronic estrogen deficiency on systemic iron homeostasis.…”

Lack of hepcidin expression attenuates steatosis and causes fibrosis in the liver

“…On the other hand, most high fat diet models induce metabolic changes but not fibrosis [66,67] . Furthermore, introduction of iron in the diet can create secondary effects by up-regulating liver hepcidin synthesis and thereby inhibiting the expression of iron exporter, ferroportin [68][69][70] . This will then lead to sequestration of iron in Kupffer cells and trigger inflammation.…”

Transforming Growth Factor β1 (TGF-β1) Activates Hepcidin mRNA Expression in Hepatocytes

“…In this study, we show that TGF-␤1 mRNA levels are increased in mouse models of iron overload and that TGF-␤1 contributes to hepatocyte hepcidin activation via an ALK5 and Smad1/5-dependent signaling pathway. We speculate that TGF-␤1 may contribute to the iron-mediated hepcidin response and may be the factor that mediates increased hepcidin levels in iron-loaded BMP6 KO mice (4).…”

“…Hepcidin expression is regulated by several signals, including plasma iron levels, hepatic iron stores, inflammation, erythropoietic activity, and hypoxia (2). Plasma iron levels and hepatic iron stores activate BMP6-ALK2/3-Smad1/5/8 signaling through a yet poorly defined mechanism to transcriptionally modulate hepcidin synthesis (3)(4)(5). Notably, hepcidin can be mildly increased upon chronic iron loading of BMP6 knock-out mice, suggesting that the BMP6 pathway does not completely account for the iron response of hepcidin (4).…”

“…Plasma iron levels and hepatic iron stores activate BMP6-ALK2/3-Smad1/5/8 signaling through a yet poorly defined mechanism to transcriptionally modulate hepcidin synthesis (3)(4)(5). Notably, hepcidin can be mildly increased upon chronic iron loading of BMP6 knock-out mice, suggesting that the BMP6 pathway does not completely account for the iron response of hepcidin (4). In addition, inflammatory stimuli such as IL6 induce hepcidin expression in human (6) and mouse (7) hepatocytes via the JAK/STAT signaling pathway (8,9).…”