Role of ceruloplasmin in cellular iron uptake

Serum ceruloplasmin and ferroxidase activity are decreased in HFE C282Y homozygote male iron-overloaded patients

BACKGROUND/AIMS

A body of evidence suggests that ceruloplasmin (Cp), the major serum copper-containing protein, acts in iron metabolism due to its ferroxidase activity which appears essential for iron movements and exchanges.

METHODS

The present study investigated the serum levels of Cp and its ferroxidase activity in 53 C282Y homozygote genetic hemochromatosis (38 iron overloaded, 15 iron depleted) patients as compared to age and sex-matched healthy volunteers.

RESULTS

Serum levels of Cp were significantly decreased in iron-overloaded male hemochromatotic patients vs. the control group (P=0.02). Furthermore, serum ferroxidase activity was strongly and significantly lower in iron-overloaded male hemochromatotic patients (P<0.001). In contrast, in iron-depleted male hemochromatotic patients, who were under maintenance therapy by regular phlebotomies, serum levels of Cp and ferroxidase activity were not statistically different from those observed in controls.

CONCLUSIONS

These data: (i) show that serum Cp and ferroxidase activity are decreased when C282Y homozygote men are iron overloaded and normal when iron depleted; (ii) suggest that iron may modulate the Cp gene expression; and (iii) raise the issue of the putative role of decreased serum ferroxidase activity in the phenotypic expression of HFE-1 hereditary hemochromatosis.

Erythropoietin response is inadequate in cancer patients receiving chemotherapy

The level of serum erythropoietin (EPO) is inappropriately decreased in cancer patients and has been advocated as the main cause of their anemia. In cancer patients, chemotherapy results in a cumulative anemia severe enough to require transfusion. We investigated the changes in serum EPO, hemoglobin, ceruloplasmin, and copper levels in cancer patients receiving chemotherapy. There was a weak but significant inverse relationship between hemoglobin and log[EPO] (r = -0.41; P < .001). Observed/expected serum EPO ratios decline with repeated chemotherapy indicating inadequate EPO response for the degree of anemia. There was no difference in the severity of anemia and in the degree of EPO response between platinum- and non-platinum-treated patients. Ceruloplasmin, copper, and ferritin levels did not change during chemotherapy. Our results suggest that the EPO response is inadequate for the degree of anemia and justifies the use of recombinant human EPO in cancer patients receiving chemotherapy.

Brain iron pathways and their relevance to Parkinson's disease

A central role of iron in the pathogenesis of Parkinson's disease (PD), due to its increase in substantia nigra pars compacta dopaminergic neurons and reactive microglia and its capacity to enhance production of toxic reactive oxygen radicals, has been discussed for many years. Recent transcranial ultrasound findings and the observation of the ability of iron to induce aggregation and toxicity of alpha-synuclein have reinforced the critical role of iron in the pathogenesis of nigrostriatal injury. Presently the mechanisms involved in the disturbances of iron metabolism in PD remain obscure. In this review we summarize evidence from recent studies suggesting disturbances of iron metabolism in PD at possibly different levels including iron uptake, storage, intracellular metabolism, release and post-transcriptional control. Moreover we outline that the interaction of iron with other molecules, especially alpha-synuclein, may contribute to the process of neurodegeneration. Because many neurodegenerative diseases show increased accumulation of iron at the site of neurodegeneration, it is believed that maintenance of cellular iron homeostasis is crucial for the viability of neurons.

Rethinking the role of ceruloplasmin in brain iron metabolism

For more than three decades, it has been widely accepted that ceruloplasmin plays an important role in iron efflux from mammalian cells, including brain cells, via the activity of ferroxidase. However, in light of recent findings, this view might not be completely accurate and the role of ceruloplasmin in brain iron metabolism may need to be re-evaluated. Based on recent studies, we propose in this article that the role of ceruloplasmin in iron uptake by brain neuronal cells might be more important than its role in iron release from the cells. A possible explanation of why the absence of ceruloplasmin induces excessive iron accumulation in neurons in aceruloplasminemia (ceruloplasmin gene mutations) was also discussed.

Brain iron transport and neurodegeneration

Despite years of investigation, it is still not known why iron levels are abnormally high in some regions of the brain in neurodegenerative disorders. Also, it is not clear whether iron accumulation in the brain is an initial event that causes neuronal death or is a consequence of the disease process. Here, we propose that iron and iron-induced oxidative stress constitute a common mechanism that is involved in the development of neurodegeneration. Also, we suggest that, at least in some neurodegenerative disorders, brain iron misregulation is an initial cause of neuronal death and that this misregulation might be the result of either genetic or non-genetic factors.

High levels of ceruloplasmin in the serum of transgenic mice developing hepatocellular carcinoma

Transgenic mice expressing the Simian virus 40 large T antigen under the control of the liver-specific human antithrombin-III promoter all develop well-differentiated hepatocellular carcinoma. During tumour development serum ceruloplasmin (Cp) increases gradually until it reaches 30 times control levels in all transgenic mice at 6 months of age. The accumulation of Cp in the serum is due to the increased transcription of the Cp gene as well as to the increase in Cp mRNA stability in the livers of the transgenic mice. One-half of the overproduced Cp is charged with copper and Cp-associated serum oxidase activity increases in parallel with the holo-Cp concentration. Through its ferroxidase activity Cp is involved prominently in iron metabolism. Analysis of copper and iron in serum and liver revealed increased copper levels in the serum of tumour-bearing animals and which increased in parallel with Cp concentration; the amounts of copper in the liver were unchanged. In contrast, serum iron remained constant during tumour development whereas the iron concentration in the livers of the transgenic mice decreased.

Iron transport

Iron homeostasis is maintained by regulating its absorption: Under conditions of deficiency, assimilation is enhanced but iron uptake is otherwise limited to prevent toxicity due to overload. Iron deficiency remains the most important micronutrient deficiency worldwide, but increasing awareness of the genetic basis for iron-loading diseases points to iron overload as a major public health issue as well. Recent identification of mutant alleles causing iron uptake disorders in mice and humans provides new insights into the mechanisms involved in iron transport and its regulation. This article summarizes these discoveries and discusses their impact on our current understanding of iron transport and its regulation.

The roles of iron in health and disease

Iron is vital for almost all living organisms by participating in a wide variety of metabolic processes, including oxygen transport, DNA synthesis, and electron transport. However, iron concentrations in body tissues must be tightly regulated because excessive iron leads to tissue damage, as a result of formation of free radicals. Disorders of iron metabolism are among the most common diseases of humans and encompass a broad spectrum of diseases with diverse clinical manifestations, ranging from anemia to iron overload and, possibly, to neurodegenerative diseases. The molecular understanding of iron regulation in the body is critical in identifying the underlying causes for each disease and in providing proper diagnosis and treatments. Recent advances in genetics, molecular biology and biochemistry of iron metabolism have assisted in elucidating the molecular mechanisms of iron homeostasis. The coordinate control of iron uptake and storage is tightly regulated by the feedback system of iron responsive element-containing gene products and iron regulatory proteins that modulate the expression levels of the genes involved in iron metabolism. Recent identification and characterization of the hemochromatosis protein HFE, the iron importer Nramp2, the iron exporter ferroportin1, and the second transferrin-binding and -transport protein transferrin receptor 2, have demonstrated their important roles in maintaining body's iron homeostasis. Functional studies of these gene products have expanded our knowledge at the molecular level about the pathways of iron metabolism and have provided valuable insight into the defects of iron metabolism disorders. In addition, a variety of animal models have implemented the identification of many genetic defects that lead to abnormal iron homeostasis and have provided crucial clinical information about the pathophysiology of iron disorders. In this review, we discuss the latest progress in studies of iron metabolism and our current understanding of the molecular mechanisms of iron absorption, transport, utilization, and storage. Finally, we will discuss the clinical presentations of iron metabolism disorders, including secondary iron disorders that are either associated with or the result of abnormal iron accumulation.

Iron homeostasis: new tales from the crypt

The enterocyte is a highly specialized cell of the duodenal epithelium that coordinates iron uptake and transport into the body. Until recently, the molecular mechanisms underlying iron absorption and iron homeostasis have remained a mystery. This review focuses on the proteins and regulatory mechanisms known to be present in the enterocyte precursor cell and in the mature enterocyte. The recent cloning of a basolateral iron transporter and investigations into its regulation provide new insights into possible mechanisms for iron transport and homeostasis. The roles of proteins such as iron regulatory proteins, the hereditary hemochromatosis protein (HFE)–transferrin receptor complex, and hephaestin in regulating this transporter and in regulating iron transport across the intestinal epithelium are discussed. A speculative, but testable, model for the maintenance of iron homeostasis, which incorporates the changes in the iron-related proteins associated with the life cycle of the enterocyte as it journeys from the crypt to the tip of the villous is proposed.

The physiopathological significance of ceruloplasmin. A possible therapeutic approach

This article reviews and comments on the physiological roles of ceruloplasmin (Cp). We show that, in addition to its ascertained involvement in iron homeostasis, the protein, by virtue of its unique structure among multicopper oxidases, is likely involved in other processes of both an enzymatic and a nonenzymatic nature. In particular, based on the analysis of the kinetic parameters, on the one hand, and of the side-products of the oxidation, on the other, we propose that the long-recognized ability of Cp to interact with and oxidize non-iron substrates may be of physiological relevance. The striking example of 6-hydroxydopamine oxidation is presented, where we show that the catalytic action is carried out readily under physiological conditions, without release of potentially toxic oxygen intermediates.

Iron homeostasis: new tales from the crypt

The enterocyte is a highly specialized cell of the duodenal epithelium that coordinates iron uptake and transport into the body. Until recently, the molecular mechanisms underlying iron absorption and iron homeostasis have remained a mystery. This review focuses on the proteins and regulatory mechanisms known to be present in the enterocyte precursor cell and in the mature enterocyte. The recent cloning of a basolateral iron transporter and investigations into its regulation provide new insights into possible mechanisms for iron transport and homeostasis. The roles of proteins such as iron regulatory proteins, the hereditary hemochromatosis protein (HFE)–transferrin receptor complex, and hephaestin in regulating this transporter and in regulating iron transport across the intestinal epithelium are discussed. A speculative, but testable, model for the maintenance of iron homeostasis, which incorporates the changes in the iron-related proteins associated with the life cycle of the enterocyte as it journeys from the crypt to the tip of the villous is proposed.

Mechanisms of biological S-nitrosation and its measurement

Nitric oxide (NO) exhibits multiple biological actions through formation of various oxidized intermediates derived from NO. Among them, nitrosothiol adducts (RS-NOs) with the sulfhydryl moiety of proteins and amino acids appears to be an important species in view of its unique chemical reactivity. Understanding of the biologically relevant S-nitrosation mechanism is essential because RS-NOs seem to be critically involved in modulation of intracellular and intercellular signal transduction, including gene transcription, cell apoptosis, and oxidative stress. RS-NOs have been recently found to be formed efficiently via one-electron oxidation of NO catalyzed by ceruloplasmin, a major copper-containing protein in mammalian plasma. Ceruloplasmin is synthesized mainly by hepatocytes, but it is also expressed by other cells such as macrophages and astrocytes. Once RS-NOs are formed, they function as NO transporters in biological systems, the NO being transferred to different sulfhydryls of various biomolecules. This transfer may be mediated by transnitrosation reactions occurring chemically or enzymatically by a means of specific enzymes such as protein disulfide isomerase. The molecular mechanism of biological S-nitrosation is discussed as related to the important physiological and pathophysiological functions of RS-NOs. Also, RS-NO assays that are being successfully used for detection of biological S-nitrosation are briefly reviewed.

The effect of ceruloplasmin on iron release from placental (BeWo) cells; evidence for an endogenous Cu oxidase

The mechanism of iron release from the placenta into the fetal circulation is not well understood. Ceruloplasmin, a plasma ferroxidase, has been implicated in iron efflux from a variety of cell types. The hypothesis is that circulating ceruloplasmin facilitates iron efflux by oxidizing the released Fe(II) to Fe(III) for incorporation into transferrin. We tested whether this mechanism mediates iron release from placental cells into the fetal circulation, using the BeWo cell line, a choriocarcinoma which can differentiate into a syncytium.(59)Fe release from undifferentiated or differentiated cells and from cells grown on porous filters was not stimulated by extracellular ceruloplasmin. Instead, we found that BeWo cells express an endogenous ferroxidase. The protein is membrane bound and cross-reacts with an anti-ceruloplasmin antibody, but has a different size; 100 and 140 kDa. Similar immunoreactivity was identified in first- and third-trimester human placentae. In BeWo cells, the protein has a perinuclear localization but does not entirely co-localize with markers for the endoplasmic reticulum or Golgi apparatus. We propose that this oxidase performs the same function as serum ceruloplasmin and is involved in iron release into the fetal circulation.

Biological functions of ceruloplasmin and their deficiency caused by mutation in genes regulating copper and iron metabolism

Ceruloplasmin, a multicopper ferroxidase, is involved in iron and copper homeostasis and integrates these metabolic pathways. Impaired biosynthesis of ceruloplasmin caused by gene mutations disturbs iron metabolism with iron deposition in different organs, especially in the basal ganglia, and severe neuronal damage. Dysfunction of ATP7B, a copper-transporting ATPase leads to the development of Wilson's disease, i.e., multiple abnormalities in copper metabolism associated with reduced synthesis of holoceruloplasmin and biliary copper excretion controlled by both proteins. The lowest content of serum ceruloplasmin is observed in the most grave early neurological form of Wilson's disease (according to N. V. Konovalov's classification), which confirms the important role of ceruloplasmin in the striatal metabolism of catecholamines.

Role of hypoxia-inducible factor-1 in transcriptional activation of ceruloplasmin by iron deficiency

A role of the copper protein ceruloplasmin (Cp) in iron metabolism is suggested by its ferroxidase activity and by the tissue iron overload in hereditary Cp deficiency patients. In addition, plasma Cp increases markedly in several conditions of anemia, e.g. iron deficiency, hemorrhage, renal failure, sickle cell disease, pregnancy, and inflammation. However, little is known about the cellular and molecular mechanism(s) involved. We have reported that iron chelators increase Cp mRNA expression and protein synthesis in human hepatocarcinoma HepG2 cells. Furthermore, we have shown that the increase in Cp mRNA is due to increased rate of transcription. We here report the results of new studies designed to elucidate the molecular mechanism underlying transcriptional activation of Cp by iron deficiency. The 5'-flanking region of the Cp gene was cloned from a human genomic library. A 4774-base pair segment of the Cp promoter/enhancer driving a luciferase reporter was transfected into HepG2 or Hep3B cells. Iron deficiency or hypoxia increased luciferase activity by 5-10-fold compared with untreated cells. Examination of the sequence showed three pairs of consensus hypoxia-responsive elements (HREs). Deletion and mutation analysis showed that a single HRE was necessary and sufficient for gene activation. The involvement of hypoxia-inducible factor-1 (HIF-1) was shown by gel-shift and supershift experiments that showed HIF-1alpha and HIF-1beta binding to a radiolabeled oligonucleotide containing the Cp promoter HRE. Furthermore, iron deficiency (and hypoxia) did not activate Cp gene expression in Hepa c4 hepatoma cells deficient in HIF-1beta, as shown functionally by the inactivity of a transfected Cp promoter-luciferase construct and by the failure of HIF-1 to bind the Cp HRE in nuclear extracts from these cells. These results are consistent with in vivo findings that iron deficiency increases plasma Cp and provides a molecular mechanism that may help to understand these observations.

Genetic disorders affecting proteins of iron metabolism: clinical implications

Remarkable progress is being made in understanding the molecular basis of disorders of human iron metabolism. Recent work has uncovered unanticipated relationships with the immune and nervous systems, intricate interconnections with copper metabolism, and striking homologies between yeast and human genes involved in the transport of transition metals. This review examines the clinical consequences of new insights into the pathophysiology of genetic abnormalities affecting iron metabolism. The proteins recently found to be involved in the absorption, transport, utilization, and storage of iron are briefly described, and the clinical manifestations of genetic disorders that affect these proteins are discussed. This chapter considers the most common inherited disorder in individuals of European ancestry (hereditary hemochromatosis), a widespread disease in sub-Saharan populations for which the genetic basis is still uncertain (African dietary iron overload), and several less frequent or rare disorders (juvenile hemochromatosis, atransferrinemia, aceruloplasminemia, hyperferritinemia with autosomal dominant congenital cataract, Friedreich's ataxia, and X-linked sideroblastic anemia with ataxia).

The essential role of Glu-185 and Tyr-354 residues in the ferroxidase activity of Saccharomyces cerevisiae Fet3

The structural determinants required for ferroxidase activity by the yeast multicopper oxidase Fet3 have been partially clarified by site-directed mutagenesis based on homology modeling. Glu-185 and Tyr-354 were substituted with Ala and Phe, respectively. Fet3 E185A retained ca. 5% residual ferroxidase catalytic efficiency, and almost 40% oxidase efficiency. On the other hand, Fet3 Y354F exhibited 50% residual efficiency as a ferroxidase and more than 70% as an oxidase. These results provide new insights in the mechanism of iron binding and oxidation by Fet3, establishing the essential role of Glu-185 and Tyr-354, and allowing to dissect ferroxidase from non-iron oxidase activity.

Discovery of the ceruloplasmin homologue hephaestin: new insight into the copper/iron connection

Approximately 75 years ago Hart and colleagues discovered that copper deficiency impaired mammalian iron metabolism. Discovery of hephaestin identifies a critical new component of the copper and iron connection in mammals. Hephaestin appears to be a multicopper oxidase required for efficient export of iron from the intestine.

Role of ceruloplasmin and ascorbate in cellular iron release

The process of iron (Fe) release from cells plays an important role in health and disease, although the mechanisms responsible remain unclear. In this study we have examined the process of Fe efflux from HepG2 cells, including the possible roles of Cp and ascorbate in this process. Recently, it has been suggested that Cp plays no role in Fe release but can increase Fe uptake by Fe-deficient HepG2 cells (Mukhopadhyay et al. Science 1998;279:714-7). However, this latter study used a nonphysiologically relevant Fe complex (iron 59-NTA) to label cells with 59Fe at a nonphysiologic temperature (25 degrees C) and Cp concentration (<100 microg/mL). Because of these problems, the experiments have been repeated by maintaining physiologic conditions and labeling cells with the physiologic Fe donor diferric Tf. When cells were labeled at 37 degrees C with 59Fe-Tf in the presence of a physiologically relevant Cp concentration (300 microg/mL), this latter protein had no effect on the uptake of 59Fe in control cells or in cells depleted of Fe by using desferrioxamine. In addition, when Fe-replete or Fe-depleted cells were incubated with 59Fe-NTA at 25 degrees C or 37 degrees C, Cp had no effect on 59Fe uptake compared with the control. When the effect of Cp (10-500 microg/mL) on 59Fe release was examined in cells prelabeled with 59Fe-Tf, a concentration-dependent increase in 59Fe efflux was observed, whereas BSA had no effect. However, in contrast to membrane-permeable Fe chelators that caused a marked increase in Fe release, the effect of Cp on Fe efflux was less impressive. To further investigate the mechanism of 59Fe mobilization, we compared 59Fe efflux among HepG2 cells, SK-Mel-28 melanoma cells, and SK-N-MC neuroblastoma cells. These studies demonstrated that 59Fe release was dependent on the incubation time with 59Fe-Tf, the cell line, and the reincubation temperature. Although 59Fe mobilization from cells was markedly temperature dependent, a range of metabolic inhibitors did not affect 59Fe release. Additional experiments showed that physiologic concentrations of ascorbate reduced 59Fe efflux, whereas glutathione had no effect. This study provides further evidence that Cp is involved in Fe mobilization but does not appear to affect Fe uptake from Tf or NTA.

Delayed translational silencing of ceruloplasmin transcript in gamma interferon-activated U937 monocytic cells: role of the 3' untranslated region

Ceruloplasmin (Cp) is an acute-phase protein with ferroxidase, amine oxidase, and pro- and antioxidant activities. The primary site of Cp synthesis in human adults is the liver, but it is also synthesized by cells of monocytic origin. We have shown that gamma interferon (IFN-gamma) induces the synthesis of Cp mRNA and protein in monocytic cells. We now report that the induced synthesis of Cp is terminated by a mechanism involving transcript-specific translational repression. Cp protein synthesis in U937 cells ceased after 16 h even in the presence of abundant Cp mRNA. RNA isolated from cells treated with IFN-gamma for 24 h exhibited a high in vitro translation rate, suggesting that the transcript was not defective. Ribosomal association of Cp mRNA was examined by sucrose centrifugation. When Cp synthesis was high, i.e., after 8 h of IFN-gamma treatment, Cp mRNA was primarily associated with polyribosomes. However, after 24 h, when Cp synthesis was low, Cp mRNA was primarily in the nonpolyribosomal fraction. Cytosolic extracts from cells treated with IFN-gamma for 24 h, but not for 8 h, contained a factor which blocked in vitro Cp translation. Inhibitor expression was cell type specific and present in extracts of human cells of myeloid origin, but not in several nonmyeloid cells. The inhibitory factor bound to the 3' untranslated region (3'-UTR) of Cp mRNA, as shown by restoration of in vitro translation by synthetic 3'-UTR added as a "decoy" and detection of a binding complex by RNA gel shift analysis. Deletion mapping of the Cp 3'-UTR indicated an internal 100-nucleotide region of the Cp 3'-UTR that was required for complex formation as well as for silencing of translation. Although transcript-specific translational control is common during development and differentiation and global translational control occurs during responses to cytokines and stress, to our knowledge, this is the first report of translational silencing of a specific transcript following cytokine activation.

Glycosyl phosphatidylinositol-anchored ceruloplasmin is expressed by rat Sertoli cells and is concentrated in detergent-insoluble membrane fractions

The copper-binding protein, ceruloplasmin, is both a serum component and a secretory product of Sertoli cells. Studies on serum ceruloplasmin have demonstrated it to be a ferroxidase that is essential for iron transport throughout the body. We report here that a glycosyl phosphatidylinositol (GPI)-anchored form of ceruloplasmin is expressed by Sertoli cells. Sertoli cell GPI-anchored proteins were selectively released by phosphatidylinositol-specific phospholipase C and were analyzed by Western blotting. A 135-kDa band was identified as ceruloplasmin by multiple antibody recognition and by amino acid sequence analysis. The presence of the GPI anchor on ceruloplasmin was confirmed by Triton X-114 phase partitioning experiments and by recognition with an antibody to the GPI anchor. GPI-anchored ceruloplasmin was enriched in detergent-insoluble glycolipid-enriched membrane microdomains (DIGs) of Sertoli cells. This is the first report of GPI-anchored ceruloplasmin in Sertoli cells and the first study of GPI-anchored ceruloplasmin in DIGs. We suggest that GPI-anchored ceruloplasmin may be the dominant form expressed by Sertoli cells and that Sertoli cell DIGs may play a role in iron metabolism within the seminiferous tubule.

Nitrosothiol formation catalyzed by ceruloplasmin. Implication for cytoprotective mechanism in vivo

Ceruloplasmin (CP) is a major multicopper-containing plasma protein that is not only involved in iron metabolism through its ferroxidase activity but also functions as an antioxidant. However, physiological substrates for CP have not been fully identified nor has the role of CP been fully understood. The reaction of nitric oxide (NO) with CP was investigated in view of nitrosothiol (RS-NO) formation. First, formation of heavy metal- or CP-catalyzed RS-NO was examined with physiologically relevant concentrations of NO and various thiol compounds (RSH) such as glutathione (GSH). Among the various heavy metal ions and copper-containing enzymes and proteins examined, only copper ion (Cu(2+)) and CP showed potent RS-NO (S-nitrosoglutathione)-producing activity. Also, RS-NO-forming catalytic activity was evident for CP added exogenously to RAW264 cells expressing inducible NO synthase in culture, but this was not the case for copper ion. Similarly, CP produced endogenously by HepG2 cells showed potent RS-NO-forming activity in the cell culture. One-electron oxidation of NO appears to be operative for RS-NO production via electron transfer from type 1 copper to a cluster of types 2 and 3 copper in CP. Neurological disorders are associated with aceruloplasminemia; besides RS-NO, S-nitrosoglutathione particularly has been shown to have neuroprotective effect against oxidative stress induced by iron overload. Thus, we suggest that CP plays an important catalytic role in RS-NO formation, which may contribute to its potent antioxidant and cytoprotective activities in vivo in mammalian biological systems.

Targeted gene disruption reveals an essential role for ceruloplasmin in cellular iron efflux

Aceruloplasminemia is an autosomal recessive disorder of iron metabolism. Affected individuals evidence iron accumulation in tissue parenchyma in association with absent serum ceruloplasmin. Genetic studies of such patients reveal inherited mutations in the ceruloplasmin gene. To elucidate the role of ceruloplasmin in iron homeostasis, we created an animal model of aceruloplasminemia by disrupting the murine ceruloplasmin (Cp) gene. Although normal at birth, Cp(-/-) mice demonstrate progressive accumulation of iron such that by one year of age all animals have a prominent elevation in serum ferritin and a 3- to 6-fold increase in the iron content of the liver and spleen. Histological analysis of affected tissues in these mice shows abundant iron stores within reticuloendothelial cells and hepatocytes. Ferrokinetic studies in Cp(+/+) and Cp(-/-) mice reveal equivalent rates of iron absorption and plasma iron turnover, suggesting that iron accumulation results from altered compartmentalization within the iron cycle. Consistent with this concept, Cp(-/-) mice showed no abnormalities in cellular iron uptake but a striking impairment in the movement of iron out of reticuloendothelial cells and hepatocytes. Our findings reveal an essential physiologic role for ceruloplasmin in determining the rate of iron efflux from cells with mobilizable iron stores.

Dietary copper primarily affects antioxidant capacity and dietary iron mainly affects iron status in a surface response study of female rats fed varying concentrations of iron, zinc and copper

This study was designed to examine the interactions among dietary iron (Fe), copper (Cu), and zinc (Zn) and their effects on Fe status and oxidative stress in female rats. In a three-factor central composite response surface design, rats were assigned to 15 groups and fed modified AIN-93G basal diets with varying amounts of Fe and Zn (7.0, 15.5, 45.8, 135.6, or 300 micrograms/g diet) and Cu (0.5, 1.1, 3.2, 9.2, or 20 micrograms/g diet) for 6 wk. Variations in hemoglobin, hematocrit, and serum ferritin were mainly related to dietary Fe. Liver nonheme Fe was directly affected by dietary Fe and was slightly attenuated by interactions between Cu and Zn, and Zn and Fe. Serum ceruloplasmin activity was primarily determined by an interaction between Cu and Zn with substantial moderation by the quadratic effect of dietary Cu. Liver and heart total superoxide dismutase (SOD) and Cu/Zn SOD activities were directly affected by dietary Cu. Dietary Fe was the only significant, yet weak, predictor of liver thiobarbituric acid reactive substances (TBARS) and vitamin E content and serum triacylglycerols. Variability in serum Cu was mostly determined by the interaction between Cu and Fe, with modification from the quadratic effect of dietary Cu. Serum Zn varied with dietary Zn with a small negative influence from the interaction between Cu and Fe. In summary, Fe status was minimally influenced by dietary Zn or Cu, and Fe intakes 10-fold greater than required did not induce overt oxidative stress in female rats. In addition, measures of antioxidant capacity were primarily influenced by dietary Cu and were optimal at moderate intakes of this micronutrient.

Differential response of non-transferrin bound iron uptake in rat liver cells on long-term and short-term treatment with iron

BACKGROUND

Uptake of non-transferrin-bound iron by the liver is important as a clearance mechanism in iron overload. In contrast to physiological uptake via receptor-mediated endocytosis of transferrin, no regulatory mechanisms for this process are known. This study compares the influence of long-term and short-term depletion and loading of hepatocytes with iron on the uptake of non-transferrin bound iron, its affinity, specificity and the interaction with the transferrin-mediated pathways.

METHODS

Rats were fed iron-deficient, normal and 3,5,5-trimethylhexanoyl-ferrocene-containing diets to obtain livers with the corresponding desired status and the hepatocytes from these livers were used for transport studies. Hepatocytes from normal rats were depleted or loaded with iron by short-term treatment with desferrioxamine or ferric ammonium citrate, respectively. Uptake of non-transferrin bound iron was assayed from ferric citrate and from ferric diethylene triammine pentaacetate.

RESULTS

Uptake of non-transferrin-bound iron in hepatocytes could be seen as consisting of a high-affinity (Km=600 nM) and a low-affinity component. Whereas in normal and in iron-starved rats the high-affinity component was more prominent, it disappeared altogether in hepatocytes from rats with iron overload resulting from prolonged feeding with TMH-ferrocene-enriched diet. Overloading also led to loss of inhibition by diferric transferrin, which occured in starved as well as normal cells. In contrast, short-term iron-depletion of isolated hepatocytes with desferrioxamine had only a weak stimulatory effect, whereas treatment with ferric ammonium citrate strongly increased the uptake rates. However, the inhibition by diferric transferrin also disappeared. In both cases, uptake of non-transferrin bound iron was inhibited by apotransferrin.

CONCLUSIONS

Non-transferrin bound iron uptake in liver cells is apparently regulated by the iron status of the liver. The mode of response to iron loading depends on the method of loading in terms of time course and the form of iron used. It cannot be explained by the behavior of the iron regulatory protein, and it is complex, seeming to involve more than one transport system.

Iron metabolism

The understanding of iron metabolism at the molecular level has been enormously expanded in recent years by new findings about the functioning of transferrin, the transferrin receptor and ferritin. Other recent developments include the discovery of the hemochromatosis gene HFE, identification of previously unknown proteins involved in iron transport, divalent metal transporter 1 and stimulator of Fe transport, and expanded insights into the regulation and expression of proteins involved in iron metabolism. Interactions among principal participants in iron transport have been uncovered, although the complexity of such interactions is still incompletely understood. Correlated efforts involving techniques and concepts of crystallography, spectroscopy and molecular biology applied to cellular processes have been, and should continue to be, particularly revealing.

Ceruloplasmin ferroxidase activity stimulates cellular iron uptake by a trivalent cation-specific transport mechanism

The balance required to maintain appropriate cellular and tissue iron levels has led to the evolution of multiple mechanisms to precisely regulate iron uptake from transferrin and low molecular weight iron chelates. A role for ceruloplasmin (Cp) in vertebrate iron metabolism is suggested by its potent ferroxidase activity catalyzing conversion of Fe2+ to Fe3+, by identification of yeast copper oxidases homologous to Cp that facilitate high affinity iron uptake, and by studies of "aceruloplasminemic" patients who have extensive iron deposits in multiple tissues. We have recently shown that Cp increases iron uptake by cultured HepG2 cells. In this report, we investigated the mechanism by which Cp stimulates cellular iron uptake. Cp stimulated the rate of non-transferrin 55Fe uptake by iron-deficient K562 cells by 2-3-fold, using a transferrin receptor-independent pathway. Induction of Cp-stimulated iron uptake by iron deficiency was blocked by actinomycin D and cycloheximide, consistent with a transcriptionally induced or regulated transporter. Cp-stimulated iron uptake was completely blocked by unlabeled Fe3+ and by other trivalent cations including Al3+, Ga3+, and Cr3+, but not by divalent cations. These results indicate that Cp utilizes a trivalent cation-specific transporter. Cp ferroxidase activity was required for iron uptake as shown by the ineffectiveness of two ferroxidase-deficient Cp preparations, copper-deficient Cp and thiomolybdate-treated Cp. We propose a model in which iron reduction and subsequent re-oxidation by Cp are essential for an iron uptake pathway with high ion specificity.

Haemochromatosis: an inherited metal and toxicity syndrome

A newly-identified major histocompatibility Class I-like gene, HFE (originally HLA-H) located approximately 3.5 Mb telomeric to the Class I cluster on chromosome 6p 21.3 harbours mutations in haemochromatosis. Two of these, Cys282Tyr (C282Y) and His63Asp (H63D, a minor determinant) have diagnostic utility as approximately 90% of adults are homozygous or compound heterozygotes for these alleles. The pathophysiological role of HFE is unclear: it is expressed as a surface molecule on many cells and the C282Y mutation disrupts interactions with beta 2-microglobulin, thus preventing surface expression. Lately, there has been experimental evidence that HFE protein interacts with the transferrin-receptor, affecting receptor turnover or its affinity for ligand.

Wilson disease

Wilson disease is a recessively inherited disorder of copper transport. Clinical features are highly variable, with any combination of neurological, hepatic or psychiatric illness. The age of onset varies from 3 to 50 years of age. Diagnosis is challenging because no specific combination of clinical or biochemical features is necessarily definitive. The genetic defect is due to a variety of abnormalities in a copper-transporting membrane ATPase. Most of the more than 80 mutations are present at a low frequency, and mutations differ between ethnic groups. At least two mutations are sufficiently common to aid in rapid diagnosis, in European and Asian populations respectively. Molecular analysis can provide a definitive diagnosis for asymptomatic sibs. Treatment, using chelating agents or zinc, is most effective when started before permanent tissue damage occurs.

Iron and copper transport in yeast and its relevance to human disease

Recent progress in the field of copper and iron metabolism has resulted from a convergence of human and yeast genetics. The mechanisms of iron and copper transport are remarkably conserved between yeast and humans. Studies of the yeast homologs of human disease genes involved in metal homeostasis have shed light on the pathophysiology of these disorders.