Regulation of the distribution of tissue iron. Lessons learned from the hypotransferrinemic mouse

The (Bio)Chemistry of Non-Transferrin-Bound Iron

In healthy individuals, virtually all blood plasma iron is bound by transferrin. However, in several diseases and clinical conditions, hazardous non-transferrin-bound iron (NTBI) species occur. NTBI represents a potentially toxic iron form, being a direct cause of oxidative stress in the circulating compartment and tissue iron loading. The accumulation of these species can cause cellular damage in several organs, namely, the liver, spleen, and heart. Despite its pathophysiological relevance, the chemical nature of NTBI remains elusive. This has precluded its use as a clinical biochemical marker and the development of targeted therapies. Herein, we make a critical assessment of the current knowledge of NTBI speciation. The currently accepted hypotheses suggest that NTBI is mostly iron bound to citric acid and iron bound to serum albumin, but the chemistry of this system remains fuzzy. We explore the complex chemistry of iron complexation by citric acid and its implications towards NTBI reactivity. Further, the ability of albumin to bind iron is revised and the role of protein post-translational modifications on iron binding is discussed. The characterization of the NTBI species structure may be the starting point for the development of a standardized analytical assay, the better understanding of these species’ reactivity or the identification of NTBI uptake mechanisms by different cell types, and finally, to the development of new therapies.

Non-transferrin-bound iron transporters

Most cells in the body acquire iron via receptor-mediated endocytosis of transferrin, the circulating iron transport protein. When cellular iron levels are sufficient, the uptake of transferrin decreases to limit further iron assimilation and prevent excessive iron accumulation. In iron overload conditions, such as hereditary hemochromatosis and thalassemia major, unregulated iron entry into the plasma overwhelms the carrying capacity of transferrin, resulting in non-transferrin-bound iron (NTBI), a redox-active, potentially toxic form of iron. Plasma NTBI is rapidly cleared from the circulation primarily by the liver and other organs (e.g., pancreas, heart, and pituitary) where it contributes significantly to tissue iron overload and related pathology. While NTBI is usually not detectable in the plasma of healthy individuals, it does appear to be a normal constituent of brain interstitial fluid and therefore likely serves as an important source of iron for most cell types in the CNS. A growing body of literature indicates that NTBI uptake is mediated by non-transferrin-bound iron transporters such as ZIP14, L-type and T-type calcium channels, DMT1, ZIP8, and TRPC6. This review provides an overview of NTBI uptake by various tissues and cells and summarizes the evidence for and against the roles of individual transporters in this process.

Transferrin as a predictor of survival in cirrhosis

Patients with cirrhosis frequently present with high serum ferritin and low transferrin concentrations, reflecting impaired liver function and inflammation. Recent studies have shown that transferrin and its saturation with iron are Model for End-Stage Liver Disease-independent predictors of mortality in patients with acute-on-chronic liver failure or decompensated cirrhosis. The aim of this study was to evaluate the prognostic utility of serum iron parameters in relation to markers of liver function and immune activation. Clinical, demographic, and biochemical data were retrospectively analyzed from a cohort of 1255 consecutive patients with cirrhosis (age ≥ 18 years) who presented from August 1, 2004 until December 31, 2014 at the University Hospital of Innsbruck. Patients with malignancies at diagnosis including hepatocellular carcinoma were excluded. Survival analysis was carried out by Cox regression by using baseline laboratory parameters, and findings were validated in an independent patient cohort. During a median follow-up of 2.4 years, 193 deaths occurred and 254 patients underwent liver transplantation. In patients with transferrin < 180 mg/dL, 3-month, 1-year, and 5-year transplant-free survival estimates were significantly lower (91.7%, 79.0%, and 30.5%) when compared with the group of patients with transferrin ≥ 180 mg/dL (98.9%, 95.5%, and 68.0%, P < 0.001). Transferrin predicted transplant-free survival independently of Model for End-Stage Liver Disease-sodium (MELD-Na) and C-reactive protein (CRP) in multivariate regression analysis including all patients. When patients with alcoholic or nonalcoholic fatty liver disease were excluded, transferrin was in addition an albumin-independent predictor of transplant-free survival. In conclusion, the association of transferrin with transplant-free survival is independent of MELD-Na score and CRP. In patients without fatty liver disease, transferrin also predicts survival independently of albumin. Liver Transplantation 24 343-351 2018 AASLD.

The use of hypotransferrinemic mice in studies of iron biology

The hypotransferrinemic (hpx) mouse is a model of inherited transferrin deficiency that originated several decades ago in the BALB/cJ mouse strain. Also known as the hpx mouse, this line is almost completely devoid of transferrin, an abundant serum iron-binding protein. Two of the most prominent phenotypes of the hpx mouse are severe anemia and tissue iron overload. These phenotypes reflect the essential role of transferrin in iron delivery to bone marrow and regulation of iron homeostasis. Over the years, the hpx mouse has been utilized in studies on the role of transferrin, iron and other metals in a variety of organ systems and biological processes. This review summarizes the lessons learned from these studies and suggests possible areas of future exploration using this versatile yet complex mouse model.

The long history of iron in the Universe and in health and disease

BACKGROUND

Not long after the Big Bang, iron began to play a central role in the Universe and soon became mired in the tangle of biochemistry that is the prima essentia of life. Since life's addiction to iron transcends the oxygenation of the Earth's atmosphere, living things must be protected from the potentially dangerous mix of iron and oxygen. The human being possesses grams of this potentially toxic transition metal, which is shuttling through his oxygen-rich humor. Since long before the birth of modern medicine, the blood-vibrant red from a massive abundance of hemoglobin iron-has been a focus for health experts.

SCOPE OF REVIEW

We describe the current understanding of iron metabolism, highlight the many important discoveries that accreted this knowledge, and describe the perils of dysfunctional iron handling.

GENERAL SIGNIFICANCE

Isaac Newton famously penned, "If I have seen further than others, it is by standing upon the shoulders of giants". We hope that this review will inspire future scientists to develop intellectual pursuits by understanding the research and ideas from many remarkable thinkers of the past.

MAJOR CONCLUSIONS

The history of iron research is a long, rich story with early beginnings, and is far from being finished. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Steap proteins: implications for iron and copper metabolism

Erythroid cells of the bone marrow, the most avid consumers of iron in the body, acquire ferric (Fe3+) iron exclusively via the transferrin cycling pathway. A long-standing fundamental molecular question of how ferric iron is handled in this pathway has been recently resolved by the identification of Steap3 (sixtransmembrane epithelial antigen of the prostate 3) as an endosomal ferrireductase needed for efficient utilization of transferrin-delivered iron. Further characterization of Steap3 and other Steap proteins reveals a possible greater role of Steap proteins in iron and copper metabolism.

Animal models with enhanced erythropoiesis and iron absorption

The regulation of iron absorption is of considerable interest in mammals since excretion is minimal. Recent advances in iron metabolism have expounded the molecular mechanisms by which iron absorption is attuned to the physiological demands of the body. The pinnacle was the discovery and identification of hepcidin, a hepatic antimicrobial peptide that regulates absorption to maintain iron homeostasis. While the intricacies of its expression and regulation by HFE, transferrin receptor 2 and hemojuvelin are still speculative, hepcidin responsiveness has correlated negatively with iron absorption in different models and disorders of iron metabolism. Consequently, hepcidin expression is repressed to enhance iron absorption during stimulated erythropoiesis even in situations of elevated iron stores. Animal models have been crucial to the advances in understanding iron metabolism and the present review focuses on phenylhydrazine treated and hypotransferrinaemic rodents. These, respectively, experimental and genetic models of enhanced erythropoiesis highlight the shifting focus of iron absorption regulation from the marrow to the liver.

nm1054: a spontaneous, recessive, hypochromic, microcytic anemia mutation in the mouse

Hypochromic, microcytic anemias are typically the result of inadequate hemoglobin production because of globin defects or iron deficiency. Here, we describe the phenotypic characteristics and pathogenesis of a new recessive, hypochromic, microcytic anemia mouse mutant, nm1054. Although the mutation nm1054 is pleiotropic, also resulting in sparse hair, male infertility, failure to thrive, and hydrocephaly, the anemia is the focus of this study. Hematologic analysis reveals a moderately severe, congenital, hypochromic, microcytic anemia, with an elevated red cell zinc protoporphyrin, consistent with functional erythroid iron deficiency. However, serum and tissue iron analyses show that nm1054 animals are not systemically iron deficient. From hematopoietic stem cell transplantation and iron uptake studies in nm1054 reticulocytes, we provide evidence that the nm1054 anemia is due to an intrinsic hematopoietic defect resulting in inefficient transferrin-dependent iron uptake by erythroid precursors. Linkage studies demonstrate that nm1054 maps to a genetic locus not previously implicated in microcytic anemia or iron phenotypes.

DMT1 expression is increased in the lungs of hypotransferrinemic mice

Despite a lack of transferrin, hypotransferrinemic (Hp) mice demonstrate an accumulation of iron in peripheral organs including the lungs. One potential candidate for such transferrin-independent uptake of iron is divalent metal transporter-1 (DMT1), an established iron transporter. We tested the hypothesis that increased concentrations of iron in the lungs of Hp mice are associated with elevations in DMT1 expression. With the use of inductively coupled plasma emission spectroscopy, measurements of nonheme iron confirmed significantly elevated concentrations in the lung tissue of Hp mice relative to the wild-type mice. Western blot analyses for the expression of two isoforms of DMT1 in the Hp mice relative to the wild-type animals demonstrated an elevation for the isoform that lacks an iron-responsive element (IRE) with significant decrements in the expression of +IRE DMT1. With the use of immunohistochemistry, -IRE DMT1 was localized to both airway epithelial cells and alveolar macrophages in wild-type mice. Staining appeared increased in both types of cells in the Hp mice. Elevated concentrations of both tissue nonheme iron and expression of -IRE DMT1 in the Hp mice were associated with increased quantities of -IRE mRNA. There was no difference between wild-type and homozygotic Hp mice in the amount of mRNA for DMT1 +IRE. We conclude that differences between Hp and wild-type mice in nonheme iron concentrations were accompanied by increases in the expression of -IRE DMT1. Increased expression of -IRE DMT1 in the lungs of the Hp mice could be responsible for elevated concentrations of the metal in these tissues.

Expression of stimulator of Fe transport is not enhanced in Hfe knockout mice

Hfe knockout (-/-) mice recapitulate many of the biochemical abnormalities of hereditary hemochromatosis (HH), but the molecular mechanisms involved in the etiology of iron overload in HH remain poorly understood. It was found previously that livers of patients with HH contained 5-fold higher SFT (stimulator of Fe transport) mRNA levels relative to subjects without HH. Because this observation suggests a possible role for SFT in HH, we investigated SFT mRNA expression in Hfe(-/-) mice. The 4- and 10-wk-old Hfe(-/-) mice do not have elevated levels of hepatic SFT transcripts relative to age-matched Hfe(+/+) mice, despite having 2.2- and 3.3-fold greater hepatic nonheme iron concentrations, respectively. Northern blot analyses of various mouse tissues revealed that SFT is widely expressed. The novel observation that SFT transcripts are abundant in brain prompted a comparison of SFT transcript levels and nonheme iron levels in the brains of Hfe(+/+) and Hfe(-/-) mice. Neither SFT mRNA levels nor nonheme iron levels differed between groups. Further comparisons of Hfe(-/-) and Hfe(+/+) mouse tissues revealed no significant differences in SFT mRNA levels in duodenum, the site of increased iron absorption in HH. Important distinctions between Hfe(-/-) mice and HH patients include not only differences in the relative rate and magnitude of iron loading but also the lack of fibrosis and phlebotomy treatment in the knockout animals.

Iron homeostasis: insights from genetics and animal models

Disorders that perturb iron balance are among the most prevalent human diseases, but until recently iron transport remained poorly understood. Over the past five years, genetic studies of patients with inherited iron homeostasis disorders and the analysis of mutant mice, rats and zebrafish have helped to identify several important iron-transport proteins. With information being mined from the genomes of four species, the study of iron metabolism has benefited enormously from positional-cloning efforts. Complementing the genomic strategy, targeted mutagenesis in mice has produced new models of human iron diseases. The animal models described in this review offer valuable tools for investigating iron homeostasis in vivo.

Diminished injury in hypotransferrinemic mice after exposure to a metal-rich particle

Using the hypotransferrinemic (Hp) mouse model, we studied the effect of altered iron homeostasis on the defense of the lung against a catalytically active metal. The homozygotic (hpx/hpx) Hp mice had greatly diminished concentrations of both serum and lavage fluid transferrin relative to wild-type mice and heterozygotes. Fifty micrograms of a particle containing abundant concentrations of metals (a residual oil fly ash) was instilled into wild-type mice and heterozygotic and homozygotic Hp animals. There was an oxidative stress associated with particle exposure as manifested by decreased lavage fluid concentrations of ascorbate. However, rather than an increase in lung injury, diminished transferrin concentrations in homozygotic Hp mice were associated with decreased indexes of damage, including concentrations of relevant cytokines, inflammatory cell influx, lavage fluid protein, and lavage fluid lactate dehydrogenase. Comparable to other organs in the homozygotic Hp mouse, siderosis of the lung was evident, with elevated concentrations of lavage fluid and tissue iron. Consequent to these increased concentrations of iron, proteins to store and transport iron, ferritin, and lactoferrin, respectively, were increased when assayed by immunoprecipitation and immunohistochemistry. We conclude that the lack of transferrin in Hp mice did not predispose the animals to lung injury after exposure to a particle abundant in metals. Rather, these mice demonstrated a diminished injury that was associated with an increase in the metal storage and transport proteins.

Importance of Anemia and Transferrin Levels in the Regulation of Intestinal Iron Absorption in Hypotransferrinemic Mice

The hypotransferrinemic mouse (trf hpx) is a mutant strain exhibiting transferrin deficiency, marked anemia, hyperabsorption of iron, and elevated hepatic iron stores. We set out to investigate the relative roles of anemia and of transferrin in the malregulation of intestinal iron absorption in these animals. Transfusion of erythrocytes obtained from littermate controls increased hemoglobin levels and reduced reticulocyte counts in recipient animals. Although mucosal to carcass 59Fe transfer was reduced, total duodenal iron uptake was not significantly affected. Iron absorption in homozygotes, in contrast to littermate controls, was not reduced by hyperoxia. Mouse transferrin injections, in the short term, increased delivery of iron to the marrow and raised hemoglobin levels. Although mucosal transfer and total iron uptake were reduced at the higher transferrin doses, total uptake was still higher than in controls. Daily injections of mouse/human transferrin for 3 weeks from weaning, normalized hemoglobin values, and markedly reduced liver iron and intestinal iron absorption values in trf hpxanimals. When such daily-injected mice were left for a week to allow transferrin clearance, iron absorption values were significantly enhanced; hemoglobin or hepatic iron levels were, however, not significantly altered. These data indicate that hyperabsorption of iron in trf hpx mice is not solely because of the anemia; transferrin levels per se do affect iron absorption, possibly via a direct effect on the intestinal mucosa.

Importance of Anemia and Transferrin Levels in the Regulation of Intestinal Iron Absorption in Hypotransferrinemic Mice

The hypotransferrinemic mouse (trf hpx) is a mutant strain exhibiting transferrin deficiency, marked anemia, hyperabsorption of iron, and elevated hepatic iron stores. We set out to investigate the relative roles of anemia and of transferrin in the malregulation of intestinal iron absorption in these animals. Transfusion of erythrocytes obtained from littermate controls increased hemoglobin levels and reduced reticulocyte counts in recipient animals. Although mucosal to carcass 59Fe transfer was reduced, total duodenal iron uptake was not significantly affected. Iron absorption in homozygotes, in contrast to littermate controls, was not reduced by hyperoxia. Mouse transferrin injections, in the short term, increased delivery of iron to the marrow and raised hemoglobin levels. Although mucosal transfer and total iron uptake were reduced at the higher transferrin doses, total uptake was still higher than in controls. Daily injections of mouse/human transferrin for 3 weeks from weaning, normalized hemoglobin values, and markedly reduced liver iron and intestinal iron absorption values in trf hpxanimals. When such daily-injected mice were left for a week to allow transferrin clearance, iron absorption values were significantly enhanced; hemoglobin or hepatic iron levels were, however, not significantly altered. These data indicate that hyperabsorption of iron in trf hpx mice is not solely because of the anemia; transferrin levels per se do affect iron absorption, possibly via a direct effect on the intestinal mucosa.

The hypotransferrinaemic mouse: ultrastructural and laser microprobe analysis observations

Homozygote hypotransferrinaemic mice (hpx/hpx) have cytopathological features similar to those of human congenital atransferrinaemia, genetic haemochromatosis, and neonatal haemochromatosis. These conditions all have in common high levels of cytotoxic non-transferrin-bound serum iron. This study describes the ultrastructural features of iron overload in liver, pancreas, heart, and small intestine of 2- and 12-month-old hypotransferrinaemic mice. Electron microscopic studies of unstained sections showed early parenchymal cell siderosis, with accumulation of numerous ferritin particles and clusters in the cytosol, as well as ferritin and haemosiderin in lysosomes (siderosomes). In the 12-month-old animals, iron was also found in Kupffer cells and macrophages in other tissues. In addition, there were conspicuous iron-containing compounds in the bile canaliculi, and marked iron deposition in the pancreas and heart. Laser microprobe mass analysis (LAMMA) enabled localization and relative quantitation of iron deposition in subcellular compartments providing in situ documentation of iron accumulation in siderosomes and contributed in assessing total cytosolic iron in various cell types. Moreover, it demonstrated the importance and magnitude of the biliary route for iron excretion in these animals.

Cellular distribution of iron, transferrin, and ferritin in the hypotransferrinemic (Hp) mouse brain

Hypotransferrinemic (Hp) mice have a point mutation or small deletion in the transferrin (Tf) gene, resulting in defective splicing of precursor Tf mRNA. Hp animals produce < 1% of normal Tf levels and require supplemental serum or purified Tf for survival. Because of the lack of endogenous brain Tf, we examined regional and cellular distributions of iron and iron regulatory proteins (Tf and ferritin) in selected brain regions of Hp mice. The regional distribution of iron, Tf, and ferritin in Hp brain was similar to normal except for the pattern of iron staining in hippocampus. The cellular distribution of iron, ferritin, and Tf was similar between Hp and normal animals. The predominant cell type staining for Tf and iron was oligodendrocytes. Qualitative observations suggest that the number of cells staining for iron was similar between Hp and normal mice, whereas the number of Hp Tf-positive cells was reduced. Ferritin immunostaining was similar in both cases. However, ferritin-positive cells were predominantly astrocytes, an observation unique to mice among species studied previously. Western blot analysis revealed that Tf present in Hp brain was of exogenous origin (from supplemental injections). Presumably, Tf transports the iron found in Hp oligodendrocytes. These data demonstrate that, despite reduced endogenous Hp brain Tf, iron and plasma Tf migrate or are transported to the appropriate cells (oligodendrocytes), bringing into question the role of endogenous brain Tf in extracellular iron transport.

Tissue iron loading and histopathological changes in hypotransferrinaemic mice

Tissue iron loading in hypotransferrinaemic (hpx/hpx) mice was investigated as a model for genetic (primary) haemochromatosis. Iron loading of liver preceded that in the pancreas and heart. One-year-old hpx/hpx mice showed iron staining in exocrine pancreas, liver parenchymal cells, and cardiac and intestinal smooth muscle cells. Iron-loaded macrophages were observed in all these tissues. Islets of Langerhans, biliary epithelial cells, and spleen were iron-free. The pancreas was fibrotic with massive macrophage infiltration and loss of secretory epithelium. Liver showed evidence of chronic inflammatory infiltration with increased collagen fibres in the parenchymal region but no cirrhosis. Serum aspartate aminotransferase activity and plasma glucose were increased in hpx/hpx compared with wild-type mice. Heavy iron loading with haemosiderin deposition in the liver could be demonstrated in hpx/hpx mice from 6 weeks of age. Heterozygous hypotransferrinaemic mice showed minor increases in liver iron stores at 6-12 weeks, but not at 1 year of age. Serum ferritin levels in heterozygous mice were also increased at 6-8 weeks of age. It was concluded that 1-year-old hpx/hpx mice showed evidence of liver and pancreatic damage secondary to tissue iron overload. The iron loading pattern and tissue damage showed some features which were distinct from those observed in haemochromatosis.

Non-transferrin-bound iron species in the serum of hypotransferrinaemic mice

Serum from homozygous hypotransferrinaemic mice (a mixed group of males and females, aged 6-8 wk) was found to contain low levels of iron (mean 0.9 +/- 0.5 microM (SEM, n = 4), as assayed by conventional serum iron assays. Similarly, low levels of non-transferrin-bound iron were determined with a nitrilotriacetate chelation assay (1.3 +/- 0.4 microM, n = 4) (Singh, S., Hider, R.C. and Porter, J.B. (1990) Analytical Biochemistry 186, 320-323). Mononuclear Fe (citrate) was undectable by electron paramagnetic resonance spectroscopy (EPR). Significantly larger quantities of iron (16 +/- 5 microM, n = 8) were detected by the bleomycin assay (Gutteridge, J.M.C., Rowley, D.A. and Halliwell, B. (1981) Biochemical Journal 199, 263-265), while non-haem iron assay or atomic absorption spectrophotometry revealed up to 96 microM iron. Haemoglobin iron was detectable at approximately 10 microM by spectrophotometry. Ferri-haem was undetectable by EPR spectroscopy. Serum ferritin levels of 641 +/- 128 micrograms/l (n = 14) in hypotransferrinaemic mice (wild-types 44 +/- 6 micrograms/l, n = 14) were observed and these cannot account for the non-transferrin-bound iron. Hypotransferrinaemic mouse serum therefore contains large quantities of non-transferrin-bound iron which is unreactive in some assays used to detect such iron in human iron overload. Fractionation by Sephadex G200 chromatography revealed three distinct species with apparent molecular weights of > or = 150 kDa, 40-80 kDa and 1-5 kDa. The iron may be distinguished from known extracellular iron proteins and haem-proteins by its availability to hot acid extractions.

Effect of hypoxic exposure on iron absorption in heterozygous hypotransferrinaemic mice

Heterozygous hypotransferrinaemic mice show serum iron, haemoglobin and reticulocyte levels similar to those of normal (+/+) controls but the plasma apotransferrin level is significantly reduced. Male heterozygous hypotransferrinaemic Balb/cJ mice and control (+/+) littermates were exposed to 1-3 days' hypoxia at 0.5 atmospheres. Controls increased their haemoglobin and iron absorption significantly during the first day of exposure; serum transferrin levels were increased and iron absorption had returned to normal levels by the third day or serum transferrin above normal levels and the elevated iron absorption persisted for longer than in controls. Serum iron levels were lower following hypoxic exposure than in controls. Liver iron loading was significantly enhanced by hypoxia in heterozygotes, but not in controls. These observations demonstrate the importance of the reservoir of plasma apotransferrin in the initial phase of enhanced erythropoiesis, and that the ability to enhance plasma levels of iron-bound transferrin is required in order to increase haemoglobin levels under the hypoxic stress.

Iron absorption by hypotransferrinaemic mice

Iron absorption rates by homozygous and wild-type mice from a hypotransferrinaemic mouse colony were examined with in vivo tied-off duodenal segments and in vitro incubated duodenal fragments. Enhanced initial rates of mucosal uptake and carcass transfer by homozygotes, compared to wild-types, were observed. The changes in vivo and in in vitro uptake kinetics resemble changes seen in iron deficient or hypoxic mice, suggesting that the liver iron loading shown by homozygotes is due to a failure of the normal mechanism for regulation of iron absorption. In vivo mucosal uptake and carcass transfer of radioiron showed an inverse correlation with liver non-haem iron content in homozygous hypotransferrinaemic mice, suggesting that some degree of control of absorption, albeit at markedly reduced sensitivity, can operate in these mice. No correlation between haemoglobin level and iron absorption was observed in homozygous hypotransferrinaemic mice, suggesting that this regulator of iron absorption does not function in these mice. The precise pathogenic mechanism of the enhanced iron absorption in hypotransferrinaemia remains to be determined. Mucosal transferrin levels were found to parallel serum transferrin levels in homozygotes, heterozygotes and wild-type mice. This supports previous suggestions that mucosal transferrin is derived from plasma transferrin and that the enhancement of iron absorption, by physiological mechanisms, does not require the presence of mucosal transferrin.