Mechanisms of Heme and Non-heme Iron Absorption in Murine Models of Iron Overload

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Project Summary Iron is an essential nutrient for rats, mice, and humans, yet iron in excess is toxic. Iron deficiency (ID) and iron overload (IO) thus result in severe homeostatic perturbations. Since iron excretion is inefficient and unregulated, intestinal iron absorption dictates overall body iron levels. Dietary iron exists mainly as non-heme and heme iron, and both contribute significantly to iron nutriture. Non-heme iron absorption by duodenal enterocytes critically relies on the iron importer divalent metal-ion transporter 1 (DMT1) and the iron exporter ferroportin (FPN). Mechanisms of heme-iron absorption have not been clarified. Inappropriately elevated absorption of heme and non-heme iron underlies iron loading in the genetic disorders hereditary hemochromatosis (HH) and β- thalassemia (βthal). Iron overload can cause liver disease, arthropathies, osteoporosis, cardiomyopathy, diabetes mellitus and impotence. HH is caused by mutations in genes that regulate transcription of the Hamp gene in hepatocytes. Hamp encodes the iron-regulatory hormone hepcidin, which regulates serum iron content by modulating FPN levels in duodenal enterocytes and reticuloendothelial macrophages (which store iron). The most common gene mutated in HH is homeostatic iron regulator (HFE). In the U.S., ~1:300 adults of Northern European descent is homozygous for HFE mutations1, significantly increasing risk for severe liver disease2. HH can also be caused by mutations in Hamp which impair production of the functional hormone. βthal is an iron- loading anemia, typified by ineffective erythropoiesis, due to β-globin gene mutations, and disordered iron metabolism. Elevated iron absorption (due to low hepcidin) and frequent blood transfusion both contribute to iron loading. In a transfusion-independent form of the disease, βthal intermedia (βTI), excessive iron absorption is the main contributor to iron loading. In HH and βTI, increased FPN activity (due to low hepcidin) depletes intracellular iron from duodenal enterocytes, which upregulates DMT1. The mechanism of DMT1 induction involves a transcript variant that contains an iron-responsive element (+IRE) in the 3’ UTR. When iron is low, this +IRE variant is stabilized by binding of an iron-regulatory protein (IRP) to the IRE motif thus stabilizing the transcript and increasing translation. High DMT1 and FPN thus precipitate excessive iron absorption. In Aim 1, we seek to mitigate iron loading in Hamp KO and Th3/+ mice by targeting the +IRE DMT1 mRNA using our Folic Acid-coupled Ginger Nanoparticle-derived Lipid Vector (FA-GDLV) siRNA delivery system. We further seek to ascertain if non-heme and heme-iron absorption are coordinately regulated to maintain iron homeostasis under physiological conditions and to clarify whether this regulation goes awry in ID and IO (in Aim 2). Additional experimentation will elucidate whether DMT1 and FPN modulate heme-iron absorption and contribute to iron loading in HH (in Aim 3). Ambiguities in mechanisms and regulation of heme-iron absorption cannot be resolved using mouse models, since mice inefficiently absorb heme iron. We thus developed Sprague-Dawley (SD) rat models of heme-iron absorption and HH (Hamp KO), thus allowing this investigation to proceed in this direction.