Absorption of dietary iron in the doudenum determines overall body iron levels as no active excretory systems exist. As such, this process must be tightly controlled to avoid the adverse consequences of tissue iron accumulation (e.g. in hereditary hemochromatosis) or deficiency (e.g. in anemia of chronic inflammation). We have been investigating molecular aspects of iron transport across intestinal epithelial cells (IECs) fo the past decade, with a long-term goal of developing drugs or dietary treatments to modulate iron absorption in humans. Although iron importers and exporters have been identified, a paucity of knowledge exists regarding the specific details of iron movement across IECs and export into the circulation (which is the rate-limiting step). We made the novel observation that copper-related processes are activated by iron deprivation of rodents. In enterocytes, a copper transporting ATPase (Atp7a) and a copper-binding protein (metallothionein) were upregulated in the setting of increased intracellular copper levels. These observations provided mechanistic insight into the relationship between body copper levels (which increase during iron deficiency &decrease in iron overload) and control of intestinal iron transport. Based upon these findings, identification of copper-specific mechanisms involved in control of iron flux was an imperative. Ferrous iron (Fe2+) export from enterocytes is functionally coupled to an oxidation step which is required for iron (Fe3+) binding to transferrin (Tf) in the interstitial fluid. A membrane-bound, multi-copper ferroxidase (FOX), Hephaestin (Heph), may mediate this step. However, Heph KO mice are viable and intestinal iron transport is only partially attenuated, suggesting that other FOXs exist. We recently discovered that enterocytes have two, distinct novel cyosolic ferroxidases (FOXs), one being an undiscovered, soluble form of Heph (sHeph) and the other termed cytoFOX. We postulate that cytosolic FOXs participate in transcytosis of iron across IECs. Studies in Heph KO mice demonstrated that both proteins contribute to cytosolic FOX activity (sHeph ~35-40%;cytoFOX ~60-65%). Another, circulating FOX, ceruloplasmin (Cp), may also participate in Fe export from IECs. The current proposal will elucidate specific, mechanistic details of intestinal Fe transport by testing the central hypothesis that novel multi-copper FOXs (sHeph and cytoFOX) in enterocytes and Cp in blood play integral roles in control of iron export from IECs. We will also define the mechanism(s) of Cu delivery for the biosynthesis of these proteins, likely involving intestinal Atp7a. The integrative approach outlined in this application, using unique in vivo and in vitro models of mammalian iron transport, will first decipher (in Aim 1) the role of Atp7a in delivering copper to sHeph to mediate iron efflx and in potentiating hepatic copper loading.
Aim 2 will define the role(s) of novel soluble FOXs in the transcytotic iron pathway, while Aim 3 will clarify the role of Cp in iron absorption and will determine the mechanism of enhanced production of holo-Cp during iron deficiency. Overall, this project will advance the field of iron biology by revealing new mechanistic details of iron transport and may provide opportunities to develop molecular approaches to modulate iron absorption.
A detailed understanding of the molecular mechanisms that govern intestinal iron absorption is important as this represents the rate-limiting step in body irn acquisition and no active excretory mechanisms exist. Moreover, perturbations in iron transport underlie several human pathologies, leading to the adverse physiologic consequences of tissue iron accumulation or iron deficiency. Our recent studies have identified novel cytosolic ferroxidases that may function in iron export from intestinal epithelial cells, which is the regulated step in iron transport. Successfully completion of the experiments delineated in this application may thus identify novel molecular processes that could be targeted to modulate iron absorption.
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