Mechanisms of Intestinal Heme-iron Absorption in Rat Models of Iron Deficiency and Iron Overload - Project Summary/Abstract Iron deficiency (ID) is the predominant cause of anemia globally. Children, menstruating and pregnant women, and the elderly are at highest risk for iron depletion. ID complicates pregnancies, impairs cognitive development in infants, and decreases work output in adults. Iron overload (IO) occurs most frequently in hereditary hemochromatosis (HH). Up to ~1:300 individuals of Northern European descent carry the most frequent mutation; 10% of these individuals are likely to develop pathological liver iron overload. HH causes arthralgia, osteoporosis, cirrhosis, cardiomyopathy, diabetes, and hypogonadism. HH results from mutations ultimately affecting production of the iron-regulatory hormone, hepcidin. Humans cannot efficiently excrete excess iron, so body iron content is maintained by modulation of intestinal iron absorption. Dysregulation of intestinal iron absorption underlies perturbations of iron homeostasis in ID and IO. Dietary iron exists mainly as heme iron (HI) and nonheme iron (NHI). Absorption of dietary NHI critically involves an iron importer (DMT1) and an iron exporter (FPN). Mechanisms of HI absorption have remained elusive. Most research in iron biology has focused on mouse models; however, mice are thought to inefficiently absorb dietary heme. To overcome this hurdle, we established a new model of HI absorption, the Sprague-Dawley (SD) rat. SD rats efficiently utilized dietary HI to normally support pregnancy, lactation, and postnatal pup development. Moreover, hepcidin (Hamp) KO rats (modeling HH) developed iron overload when fed a HI diet, thus reflecting elevated HI absorption. In this investigation, we will leverage unique dietary and genetic SD rat models of ID and IO to test novel hypotheses related to HI absorption. Our specific goals are to test: 1) Whether NHI and HI absorption is coordinately regulated. This seems likely given that intestinal iron absorption must be tightly controlled to maintain overall body iron homeostasis; 2) Whether DMT1 and FPN influence HI absorption. Heme is likely absorbed by endocytosis, followed by export from endosomes by a heme transporter. Cytosolic heme has two possible fates: a) Degradation by heme-oxygenase 1 (HMOX1), which liberates iron that then mixes with the dietary NHI pool; or b) Export out of the enterocyte via a heme exporter, followed by degradation in the liver. We hypothesize that HI and NHI transporters co-localize on plasma and vesicular membranes, thus facilitating functional interactions; and 3) Whether HRG1 is an intestinal heme transporter. HRG1 transports heme with high affinity, it is expressed on the apical surface of human duodenal enterocytes, and it is regulated by iron and heme. HRG1 is thus a plausible candidate for the long sought intestinal heme iron importer. Studies proposed herein will utilize the HRG1 KO SD rat. Collectively, the use of new dietary and genetic SD rat models of iron adequacy, ID, and IO uniquely position us to be able to address unresolved, fundamental questions relating to mechanisms of intestinal HI absorption, and to elucidate whether pathways of HI and NHI absorption intersect. Clinical and translational potential is high, as dietary HI and NHI both contribute to systemic iron homeostasis in humans.
