The two iron regulatory proteins IRP1 and IRP2 bind to transcripts of ferritin, transferrin receptor and other target genes to control the expression of iron metabolism proteins at the post-transcriptional level. Here we compare the effects of genetic ablation of IRP1 to IRP2 in mice. IRP1À/À mice misregulate iron metabolism only in the kidney and brown fat, two tissues in which the endogenous expression level of IRP1 greatly exceeds that of IRP2, whereas IRP2À/À mice misregulate the expression of target proteins in all tissues. Surprisingly, the RNA-binding activity of IRP1 does not increase in animals on a lowiron diet that is sufficient to activate IRP2. In animal tissues, most of the bifunctional IRP1 is in the form of cytosolic aconitase rather than an RNA-binding protein.Our findings indicate that the small RNA-binding fraction of IRP1, which is insensitive to cellular iron status, contributes to basal mammalian iron homeostasis, whereas IRP2 is sensitive to iron status and can compensate for the loss of IRP1 by increasing its binding activity. Thus, IRP2 dominates post-transcriptional regulation of iron metabolism in mammals.

To better understand the tissue iron overload and anemia previously reported in a human patient and mice that lack heme oxygenase-1 (HO-1), we studied iron distribution and pathology in HO-1(Hmox1) ؊/؊ mice. We found that resident splenic and liver macrophages were mostly absent in HO-1 ؊/؊ mice. Erythrophagocytosis caused the death of HO-1 ؊/؊ macrophages in in vitro experiments, supporting the hypothesis that HO-1 ؊/؊ macrophages died of exposure to heme released on erythrophagocytosis. Rupture of HO-1 ؊/؊ macrophages in vivo and release of nonmetabolized heme probably caused tissue inflammation. In the spleen, initial splenic enlargement progressed to red pulp fibrosis, atrophy, and functional hyposplenism in older mice, recapitulating the asplenia of an HO-1-deficient patient. We postulate that the failure of tissue macrophages to remove senescent erythrocytes led to intravascular hemolysis and increased expression of the heme and hemoglobin scavenger proteins, hemopexin and haptoglobin. Lack of macrophages expressing the haptoglobin receptor, CD163, diminished the ability of haptoglobin to neutralize circulating hemoglobin, and iron overload occurred in kidney proximal tubules, which were able to catabolize heme with HO-2. Thus, in HO-1 ؊/؊ mammals, the reduced function and viability of erythrophagocytosing macrophages are the main causes of tissue damage and iron redistribution. IntroductionHumans and mice contain 2 well-characterized heme oxygenase (HO) enzymes: HO-1, which is inducible, and HO-2, which is constitutively expressed in most tissues. 1,2 HO metabolizes heme and releases free iron, carbon monoxide, and biliverdin, which quickly undergoes conversion to bilirubin. Red blood cells (RBCs) contain very high concentrations of hemoglobin (Hb), 3,4 but HO allows efficient recycling of the iron that is bound to Hb molecules in RBCs. On phagocytosis of senescent RBCs, macrophages increase their expression of HO-1 to efficiently degrade heme, and iron returns to the circulation through the iron exporter ferroportin. 5 Excess free heme is highly toxic in the circulation, 4 and protective systems exist that enable animals to avoid toxicity caused by free heme and free Hb. Hemopexin (Hpx) is a heme-binding serum protein that scavenges free heme in the circulation, 6,7 and haptoglobin (Hp) binds free Hb,8,9 whereupon the Hb-Hp complex is endocytosed through the CD163 receptor 10 and is metabolized by macrophages.Previous work in mouse models has shown that the lack of both HOs is embryonically lethal, whereas work on an HO-1 Ϫ/Ϫ mouse model 11,12 and a single HO-1-deficient human patient revealed that both the HO-1 Ϫ/Ϫ mouse and the human patient were anemic. 13 However, splenomegaly was described in the mouse model, 11,12 whereas hyposplenia was present in the human patient. 14 Hepatic and renal iron overload was observed in the patient and in mouse models, but the mechanism for iron redistribution was not clear. HO-1 deficiency was discovered in a single patient with hemolytic anemia when his physicians ...

Summary Ferroportin (FPN1), the sole characterized mammalian iron exporter, has an iron responsive element (IRE) in its 5'UTR, which ensures that its translation is repressed by iron regulatory proteins in iron-deficient conditions to maintain cellular iron content. However, here we demonstrate that duodenal epithelial and erythroid precursor cells utilize an alternative upstream promoter to express a FPN1 transcript, FPN1B, which lacks the IRE and is not repressed in iron-deficient conditions. The FPN1B transcript encodes ferroportin with an identical open reading frame, and contributes significantly to ferroportin protein expression in erythroid precursors, and likely also in the duodenum of iron-starved animals. The identification of FPN1B reveals how FPN1 expression can bypass IRP-dependent repression in intestinal iron uptake, even when cells throughout the body are iron-deficient. In erythroid precursor cells, we hypothesize that FPN1B expression enhances real-time sensing of systemic iron status and facilitates restriction of erythropoiesis in response to low systemic iron.

SUMMARY Iron regulatory proteins 1 and 2 (Irps) post-transcriptionally control the expression of transcripts that contain iron responsive element (IRE) sequences, including ferritin, ferroportin, transferrin receptor and hypoxia inducible factor 2α (HIF2α). We report here that mice with targeted deletion of Irp1 developed pulmonary hypertension and polycythemia that was exacerbated by a low iron diet. Hematocrits increased to 65% in iron-starved mice, and many polycythemic mice died of abdominal hemorrhages. Irp1 deletion enhanced HIF2α protein expression in kidneys of Irp1−/− mice, which led to increased erythropoietin (EPO) expression, polycythemia and concomitant tissue iron deficiency. Increased HIF2α expression in pulmonary endothelial cells induced high expression of endothelin-1, likely contributing to the pulmonary hypertension of Irp1−/− mice. Our results reveal why anemia is an early physiological consequence of iron deficiency, highlight the physiological significance of Irp1 in regulating erythropoiesis and iron distribution, and provide important insights into the molecular pathogenesis of pulmonary hypertension.