Haem homeostasis is regulated by the conserved and concerted functions of HRG-1 proteins

Department of Animal & Avian Sciences, University of Maryland, College Park, Maryland 20742, USA.
Nature (Impact Factor: 41.46). 07/2008; 453(7198):1127-31. DOI: 10.1038/nature06934
Source: PubMed


Haems are metalloporphyrins that serve as prosthetic groups for various biological processes including respiration, gas sensing, xenobiotic detoxification, cell differentiation, circadian clock control, metabolic reprogramming and microRNA processing. With a few exceptions, haem is synthesized by a multistep biosynthetic pathway comprising defined intermediates that are highly conserved throughout evolution. Despite our extensive knowledge of haem biosynthesis and degradation, the cellular pathways and molecules that mediate intracellular haem trafficking are unknown. The experimental setback in identifying haem trafficking pathways has been the inability to dissociate the highly regulated cellular synthesis and degradation of haem from intracellular trafficking events. Caenorhabditis elegans and related helminths are natural haem auxotrophs that acquire environmental haem for incorporation into haemoproteins, which have vertebrate orthologues. Here we show, by exploiting this auxotrophy to identify HRG-1 proteins in C. elegans, that these proteins are essential for haem homeostasis and normal development in worms and vertebrates. Depletion of hrg-1, or its paralogue hrg-4, in worms results in the disruption of organismal haem sensing and an abnormal response to haem analogues. HRG-1 and HRG-4 are previously unknown transmembrane proteins, which reside in distinct intracellular compartments. Transient knockdown of hrg-1 in zebrafish leads to hydrocephalus, yolk tube malformations and, most strikingly, profound defects in erythropoiesis-phenotypes that are fully rescued by worm HRG-1. Human and worm proteins localize together, and bind and transport haem, thus establishing an evolutionarily conserved function for HRG-1. These findings reveal conserved pathways for cellular haem trafficking in animals that define the model for eukaryotic haem transport. Thus, uncovering the mechanisms of haem transport in C. elegans may provide insights into human disorders of haem metabolism and reveal new drug targets for developing anthelminthics to combat worm infestations.

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    • "However, the mechanism for heme absorption is not yet clear. In the last decade, several heme transporters have been identified, including heme carrier protein-1 (HCP1; Qiu et al., 2006; Laftah et al., 2009), HRG-1 (Rajagopal et al., 2008; White et al., 2013), and FLVCR1 and 2 (Quigley et al., 2004; Keel et al., 2008; Duffy et al., 2010), but their significance in intestinal iron absorption remains to be elucidated. For non-heme iron absorption, ferric iron [Fe(III)] in the diet must be reduced by a ferrireductase duodenal cytochrome b561 (Dcytb) to ferrous iron [Fe(II)] before the divalent metal transporter 1 (DMT1, also known as DCT1 or NRAMP2) can transport iron across the apical membrane into the cytosol of duodenal epithelial cells (so-called enterocytes; Gunshin et al., 1997; McKie et al., 2001). "
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    ABSTRACT: Iron regulatory proteins (IRPs) regulate the expression of genes involved in iron metabolism by binding to RNA stem-loop structures known as iron responsive elements (IREs) in target mRNAs. IRP binding inhibits the translation of mRNAs that contain an IRE in the 5'untranslated region of the transcripts, and increases the stability of mRNAs that contain IREs in the 3'untranslated region of transcripts. By these mechanisms, IRPs increase cellular iron absorption and decrease storage and export of iron to maintain an optimal intracellular iron balance. There are two members of the mammalian IRP protein family, IRP1 and IRP2, and they have redundant functions as evidenced by the embryonic lethality of the mice that completely lack IRP expression (Irp1 (-/-)/Irp2(-/-) mice), which contrasts with the fact that Irp1 (-/-) and Irp2 (-/-) mice are viable. In addition, Irp2 (-/-) mice also display neurodegenerative symptoms and microcytic hypochromic anemia, suggesting that IRP2 function predominates in the nervous system and erythropoietic homeostasis. Though the physiological significance of IRP1 had been unclear since Irp1 (-/-) animals were first assessed in the early 1990s, recent studies indicate that IRP1 plays an essential function in orchestrating the balance between erythropoiesis and bodily iron homeostasis. Additionally, Irp1 (-/-) mice develop pulmonary hypertension, and they experience sudden death when maintained on an iron-deficient diet, indicating that IRP1 has a critical role in the pulmonary and cardiovascular systems. This review summarizes recent progress that has been made in understanding the physiological roles of IRP1 and IRP2, and further discusses the implications for clinical research on patients with idiopathic polycythemia, pulmonary hypertension, and neurodegeneration.
    Frontiers in Pharmacology 06/2014; 5:124. DOI:10.3389/fphar.2014.00124 · 3.80 Impact Factor
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    • "Mammalian Hrg1 has been shown to be expressed widely, with the highest expression in the brain, heart, kidney, and muscle, with some expression in the placenta and intestine (Rajagopal et al., 2008). Hrg1 has been linked to a possible role in cancer progression, as its interaction with V-type ATPases is associated with changes in endocytic trafficking, extracellular acidification, altered glucose metabolism, and matrix metalloprotease activity (O’Callaghan et al., 2010; Fogarty et al., 2013). "
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    ABSTRACT: Heme is an iron-containing porphyrin ring that serves as a prosthetic group in proteins that function in diverse metabolic pathways. Heme is also a major source of bioavailable iron in the human diet. While the synthesis of heme has been well-characterized, the pathways for heme trafficking remain poorly understood. It is likely that heme transport across membranes is highly regulated, as free heme is toxic to cells. This review outlines the requirement for heme delivery to various subcellular compartments as well as possible mechanisms for the mobilization of heme to these compartments. We also discuss how these trafficking pathways might function during physiological events involving inter- and intra-cellular mobilization of heme, including erythropoiesis, erythrophagocytosis, heme absorption in the gut, as well as heme transport pathways supporting embryonic development. Lastly, we aim to question the current dogma that heme, in toto, is not mobilized from one cell or tissue to another, outlining the evidence for these pathways and drawing parallels to other well-accepted paradigms for copper, iron, and cholesterol homeostasis.
    Frontiers in Pharmacology 06/2014; 5. DOI:10.3389/fphar.2014.00126 · 3.80 Impact Factor
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    • "Therefore, a thorough interrogation of the mechanisms of how iron excess contributes to tumorigenesis and the molecular basis of the selective efficacy of iron deprivation would not only further our understanding of cancer biology but also improve the design of targeted chemotherapy for better clinical outcomes. Heme is an iron polyporphyrin that constitutes the prosthetic group for proteins functioning in a myriad of fundamental biological processes, including respiration, energetic homeostasis , signal transduction, xenobiotic detoxification, iron metabolism , mRNA processing, and control of circadian rhythm (Boon et al., 2005; Dioum et al., 2002; Faller et al., 2007; Gilles- Gonzalez and Gonzalez, 2005; Hu et al., 2008; Ishikawa et al., 2005; Rajagopal et al., 2008; Yang et al., 2010). The switch between ferrous (Fe 2+ ) and ferric (Fe 3+ ) states of iron in the metallo-polyporphyrin, termed as heme and hemin, respectively, underlies its unique roles in transducing redox and gas signaling in vivo. "
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    ABSTRACT: Iron excess is closely associated with tumorigenesis in multiple types of human cancers, with underlying mechanisms yet unclear. Recently, iron deprivation has emerged as a major strategy for chemotherapy, but it exerts tumor suppression only on select human malignancies. Here, we report that the tumor suppressor protein p53 is downregulated during iron excess. Strikingly, the iron polyporphyrin heme binds to p53 protein, interferes with p53-DNA interactions, and triggers both nuclear export and cytosolic degradation of p53. Moreover, in a tumorigenicity assay, iron deprivation suppressed wild-type p53-dependent tumor growth, suggesting that upregulation of wild-type p53 signaling underlies the selective efficacy of iron deprivation. Our findings thus identify a direct link between iron/heme homeostasis and the regulation of p53 signaling, which not only provides mechanistic insights into iron-excess-associated tumorigenesis but may also help predict and improve outcomes in iron-deprivation-based chemotherapy.
    Cell Reports 03/2014; 7(1). DOI:10.1016/j.celrep.2014.02.042 · 8.36 Impact Factor
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