Molecular Synergism between Hormonal Signaling and Iron Homeostasis
Final Report Abstract
Systemic iron homeostasis is disrupted in the common, potentially fatal genetic iron overload disorder caused by the mutation in the HFE gene. We previously demonstrated that Hfe acts in hepatocytes to maintain systemic iron levels by controlling the expression of the iron hormone hepcidin, clarifying a long-standing question in medicine. While systemic iron levels and some of the associated liver problems are successfully treated by phlebotomy, it still remained unsolved what are the actions of Hfe in extra-hepatocytic cells, if any. To address this question we generated unique mice with Hfe deficiency in macrophages and enterocytes. Our previous studies excluded the role of Hfe in macrophages and enterocytes for the control of iron homeostasis, however the drawback of these studies was that they have addressed iron regulation in young mice (8-10 weeks of age) leaving us still with the questions whether we might unveil novel functions of Hfe by exposing mice to certain challenge(s). Here we present two unexpected findings: 1. We demonstrate that mice with selective Hfe deficiency in macrophage develop iron-deficient phenotype in an agedependent manner. We show that isolated Hfe-deficient macrophages display significantly reduced intracellular iron levels. On systemic levels, iron deficiency was measured in the liver, spleen and duodenum of these mice. In line with this, the expression of hepcidin and several other iron genes is decreased. Likewise, the activity of hepatic Bmp/Smad signaling is accordingly affected in the liver of mutant mice. We further demonstrate that dysbalanced cellular and systemic iron homeostasis in these mutant mice is due to enhanced iron export via increased expression of ferroportin. Overexpression of Hfe in macrophages prevented ferroportin induction and normalized cellular iron levels. We next investigated the role of macrophage-Hfe in pathogenic septic conditions. During the septic shock conditions a significantly increased mortality rate was scored in hepatocyte-specific mutant mice, a model which phenocopies the constitutive Hfe deficiency. By contrast, macrophage-specific mutant mice show improved survival suggesting that mild iron deficiency, and the direct actions of Hfe in macrophages, may be beneficial under these conditions. Our study provides for the fist time evidences for the role of Hfe in macrophages and innate immunity. 2. We now reveal that Hfe in enterocytes functions to control iron uptake and iron export in the duodenum. Intriguingly, this phenotype developed in age-dependent manner since young mutant mice displayed no overt changes in iron homeostasis. Consequently, aged enterocyte-specific Hfe mutant mice developed iron overload and presented increased hepatic hepcidin expression. Thus, Hfe acts in the duodenum i) in hepcidin-independent manner and ii) to control iron uptake/export thereby alleviating iron loading in Hfe-HH disorder. Based on our data, one can envision that lowering nutritional iron consumption may at least partially restrict iron uptake and export by the enterocyte-Hfe and provide advantage in the treatment of Hfe-HH. Finally, we present evidences for a novel hormonal synergism between iron and steroid hormone metabolism. We show that glucocorticoids act as novel regulators of iron homeostasis. Molecularly, this regulation is achieved by direct binding of the glucocorticoid receptor (GR) within hepcidin promoter. Furthermore, our in vivo studies show that treatment with glucocorticoids increase the expression of hepcidin and promote iron sequestration in the macrophages, employing a well-established hepcidin-ferroportin regulatory mechanism. Whether GR actions in hepatocytes, macrophages or other cells are involved in glucocorticoid-mediated control of iron homeostasis is currently under investigation. The results obtained during this funding period have greatly fostered our understanding of fundamental molecular mechanisms involved in the regulation of iron homeostasis in Hfe-HH disorder and have paved the way towards novel hormonal cross-talks discoveries.
Publications
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(2012) Smad6 and Smad7 are coregulated with hepcidin in hemochromatosis mouse models. BBA Molecular Basis of Disease, Aug 31
Vujic Spasic, M. et al.
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(2014) Molecular basis of HFE-hemochromatosis. Front Pharmacol Mar 11;5:42
Vujic Spasic, M.
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(2016) Transforming Growth Factor beta (TGF-beta) Activates Hepcidin mRNA Expression in Hepatocytes.J. Biol. Chem., 291(25):13160-13174
Chen, S.; Feng, T.; Vujić Spasić, M.; Altamura, S.; Breitkopf-Heinlein, K.; Altenoder, J.; Weiss, T. S.; Dooley, S.; Muckenthaler, M. U.
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(2017). Heme Activates Macrophage Hepcidin Expression via Toll like Receptor 4 and Extracellular Signal-Regulated Kinases Signaling Pathway. Clinical Pharmacology & Biopharmaceutics, 6:166
Tangudu, Naveen Kumar; Vujić Spasić, Maja
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(2017). Hepcidin knockout mice spontaneously develop chronic pancreatitis owing to cytoplasmic iron overload in acinar cells. J Pathol., 241(1):104-114
Lunova, M.; Schwarz, P.; Nuraldeen, R.; Levada, K.; Kuscuoglu, D.; Stützle, M.; Vujić Spasić, M.; Haybaeck, J.; Ruchala, P.; Jirsa, M.; Deschemin, JC.; Vaulont, S.; Trautwein, C.; Strnad, P.
