Gaining New Insights Into Regulation of Systemic Iron Homeostasis.
Final Report Abstract
Iron is required for erythropoiesis. Precise control of the hepatic hormone hepcidin, the central iron regulator in the body, is important for regulation of systemic iron homeostasis in order to maintain a balance between iron demand and supply. Hepcidin expression is stimulated, besides others, by iron, and leads to degradation and internalization of the iron export channel. As a consequence, iron cannot be absorbed in the intestine and remains trapped in the iron stores, such as in hepatocytes and macrophages. The overall goal of the research grant was to increase our knowledge in systemic iron regulation in order to be able to develop treatments for diseases caused by imbalances of hepcidin regulation. Diseases caused by an imbalance of systemic iron homeostasis include the most common form of anemia, iron deficiency anemia and the second most common form, anemia of chronic disease (ACD), also called anemia of inflammation. ACD occurs in patients with inflammatory disorders and is caused by an inadequate induction of hepcidin. The bone morphogenetic protein (BMP) signaling pathway is important in iron homeostasis, as inhibition of the BMP pathway inhibits hepcidin induction by inflammation. Inhibition of the BMP receptor was able to cure anemia of chronic disease in mice. In addition, mice with hepatocyte-specific deficiency of a BMP receptor show iron overload. These murine models represent hemochromatosis, the most common inherited iron overload disease in humans. With the help of this grant, 1. ) the role of intravenous iron (IVI) substitution as a potential treatment for ACD in a murine ACD model, 2.) the BMP receptor critical for the pathogenesis of ACD, 3.) the protein-protein interactions required for BMP type I receptors ALK2- and ALK3-dependent iron regulation were determined in vitro and 4.) the role of ALK2 and ALK3 in regulation of hepatic iron metabolism and iron overload were characterized. The results revealed the following: 1) IVI substitution in WT mice did not enhance the inflammatory response provoked by administration of Brucella abortus (BA). In addition, IVI was able to cure the anemia. Hence, IVI might be a potential therapeutic in ACD. 2.) The role of the BMP type I receptor ALK3 as an essential factor in the pathophysiology of ACD was investigated in the next experimental setup. The development of hypoferremia was dependent on ALK3 and mice with hepatocyte-specific Alk3 deficiency showed better iron mobilization in states of ACD. Thus, the ALK3/BMP/hepcidin axis may serve as a possible therapeutic target to attenuate ACD. 3.) Co-lmmunoprecipitation assays in Huh7 cells were performed to analyze the composition of the BMP type I receptor complex in vitro. ALK3 formed homodimeric complexes as well as ligand-dependent heterodimeric complexes with ALK2. ALK2 itself did not assemble into homodimers in the absence and presence of exogenous ligands. Both receptor complexes were associated with increased hepcidin levels in a human hepatocyte cell line. 4.) To elucidate whether hepatic ALK2 and ALK3 also have different functions in vivo, iron overload of mice with hepatocyte-specific deficiency for Aik2 and Alk3 was compared to the iron overload phenotype of mice with hepatocyte-specific Alk3 deficiency. The results revealed the development of a more severe iron overload phenotype in mice with hepatocyte-specific Alk2/3 deficiency than with Alk3 deficiency alone, which was indicated by increased liver iron content, distinctive liver Iron accumulation pattern and extrahepatic iron accumulation in female mice. The work with the grant has been very successful and investigated IVI supplementation as a potential therapeutic in ACD, elucidated the role of the BMP type I receptor in development of ACD, and determined the BMP type I receptor complex in vitro. Characterization of hepatocyte-specific Alk2/3 deficient mice implies additive roles for ALK2 and ALK3 in systemic iron homeostasis. The results revealed new insights into the regulation of iron homeostasis, which are of high impact to the research society and all the physicians caring for patients with diseases caused by imbalances of the iron regulatory system and critical for development of novel therapeutic strategies.
Publications
- A novel treatment for anemia of inflammation. Blood. 2014. Oct 23, 124:2618-2619
Steinbicker AU
(See online at https://doi.org/10.1182/blood-2014-09-599282) - Intravenous Iron Carboxymaltose as a Potential Therapeutic in Anemia of Inflammation. PLoS One. 2016 Jul 12;11(7)
Lofruthe N, Gallitz I, Traeger L, Bäumer N, Schulze I, Kuhlmann K, Müller-Tidow C, Steinbicker AU
(See online at https://doi.org/10.1371/journal.pone.0158599) - Deficiency of the BMP Type I receptor ALK3 partly protects mice from anemia of inflammation. BMC Physiol. 2018 Feb 27; 18(1)
Gallitz I, Lofruthe N, Traeger L, Bäumer N, Hoerr V, Faber C, Kuhlmann T, Müller-Tidow C, Steinbicker AU
(See online at https://doi.org/10.1186/s12899-018-0037-z)
