Role of non-transferrin-bound iron in the pathogenesis of iron overload and toxicity

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.

Analyses of hemorrhagic diathesis in high-iron diet-fed rats

Iron overload has been well recognized to cause oxidant-mediated cellular/tissue injury; however, little is known about the effects of iron overload on the blood coagulation system. We encountered an unexpected bleeding tendency in rats fed a high-iron diet in a set of studies using iron-modified diets. In this study, we investigated the mechanism of hemorrhagic diathesis induced by dietary iron overload in rats. Six-week-old F344/DuCrlCrlj male rats were fed a standard (containing 0.02% iron) or a high-iron diet (containing 1% iron) for 6 weeks and were then sampled for hematological, blood biochemical, coagulation, and pathological examinations. Serum and liver iron levels increased in rats fed the high-iron diet (Fe group) and serum transferrin was almost saturated with iron. However, serum transaminase levels did not increase. Moreover, plasma prothrombin time and activated partial thromboplastin time were significantly prolonged, regardless of the presence of hemorrhage. The activity of clotting factors II and VII (vitamin K-dependent coagulation factors) decreased significantly, whereas that of factor VIII was unaltered. Blood platelet levels were not influenced by dietary iron overload, suggesting that the bleeding tendency in iron-overloaded rats is caused by secondary hemostasis impairment. In addition, hemorrhage was observed in multiple organs in rats fed diets containing more than 0.8% iron. Our results suggest that iron overload can increase the susceptibility of coagulation abnormalities caused by latent vitamin K insufficiency.

Effects of long-term administration of Senna occidentalis seeds on the hematopoietic tissue of rats

Senna occidentalis (S. occidentalis) is a toxic leguminous plant that contaminates crops and has been shown to be toxic to several animal species. All parts of the plant are toxic, but most of the plant's toxicity is due to its seeds. Despite its toxicity, S. occidentalis is widely used for therapeutic purposes in humans. The aim of the present work was to investigate, for the first time, the effects of the chronic administration of S. occidentalis seeds on hematopoietic organs, including the bone marrow and spleen. Fifty male Wistar rats were divided into five groups of 10 animals. Rats were treated with diets containing 0% (control), 0.5% (So0.5), 1% (So1), or 2% (So2) S. occidentalis seeds for a period of 90 days. Food and water were provided ad libitum, except to pair-fed (PF) group which received the same amount of ration to those of So2 group, however free of S. occidentalis seeds. It was verified that rats treated with 2% S. occidentalis seeds presented changes in hematological parameters. The blood evaluation also showed a significant decrease of the Myeloid/Erythroid (M/E) ratio. Chronic treatment with S. occidentalis promoted a reduction in the cellularity of both the bone marrow and spleen. Additionally, we observed changes in bone marrow smears, iron stores and spleen hemosiderin accumulation. Histological analyses of bone marrow revealed erythroid hyperplasia which was consistent with the increased reticulocyte count. These findings suggest that the long-term administration of S. occidentalis seeds can promote blood toxicity.

Liver steatosis correlates with iron overload but not with HFE gene mutations in chronic hepatitis C

BACKGROUND

Liver steatosis and iron overload, which are frequently observed in chronic hepatitis C (CHC), may contribute to the progression of liver injury. This study aimed to evaluate the correlation between liver steatosis and iron overload in Polish patients with CHC compared to non-alcoholic fatty liver disease (NAFLD) and HFE-hereditary hemochromatosis (HH) patients.

METHODS

A total of 191 CHC patients were compared with 67 NAFLD and 21 HH patients. Liver function tests, serum markers of iron metabolism, cholesterol and triglycerides were assayed. The inflammatory activity, fibrosis, iron deposits and steatosis stages were assessed in liver specimens. HFE gene polymorphisms were investigated by PCR-RFLP.

RESULTS

Liver steatosis was associated with obesity and diabetes mellitus. This disease was confirmed in 76/174 (44%) CHC patients, most of whom were infected with genotype 1. The average grade of steatosis was higher in NAFLD patients. CHC patients had significantly higher iron concentrations and transferrin saturations than NAFLD patients. Compared with CHC patients, HH patients had higher values of serum iron parameters and more intensive hepatocyte iron deposits without differences in the prevalence and intensity of liver steatosis. In the CHC group, lipids accumulation in hepatocytes was significantly associated with the presence of serum markers of iron overload. No correlation between the HFE gene polymorphism and liver steatosis in CHC patients was found.

CONCLUSIONS

Liver steatosis was diagnosed in nearly half of CHC patients, most of whom were infected with genotype 1. The intensity of steatosis was lower in CHC patients than that in NAFLD patients because of a less frequent diagnosis of metabolic syndrome. Only in CHC patients were biochemical markers of iron accumulation positively correlated with liver steatosis; these findings were independent of HFE gene mutations.

ZIP8 is an iron and zinc transporter whose cell-surface expression is up-regulated by cellular iron loading

ZIP8 (SLC39A8) belongs to the ZIP family of metal-ion transporters. Among the ZIP proteins, ZIP8 is most closely related to ZIP14, which can transport iron, zinc, manganese, and cadmium. Here we investigated the iron transport ability of ZIP8, its subcellular localization, pH dependence, and regulation by iron. Transfection of HEK 293T cells with ZIP8 cDNA enhanced the uptake of (59)Fe and (65)Zn by 200 and 40%, respectively, compared with controls. Excess iron inhibited the uptake of zinc and vice versa. In RNA-injected Xenopus oocytes, ZIP8-mediated (55)Fe(2+) transport was saturable (K(0.5) of ∼0.7 μm) and inhibited by zinc. ZIP8 also mediated the uptake of (109)Cd(2+), (57)Co(2+), (65)Zn(2+) > (54)Mn(2+), but not (64)Cu (I or II). By using immunofluorescence analysis, we found that ZIP8 expressed in HEK 293T cells localized to the plasma membrane and partially in early endosomes. Iron loading increased total and cell-surface levels of ZIP8 in H4IIE rat hepatoma cells. We also determined by using site-directed mutagenesis that asparagine residues 40, 88, and 96 of rat ZIP8 are glycosylated and that N-glycosylation is not required for iron or zinc transport. Analysis of 20 different human tissues revealed abundant ZIP8 expression in lung and placenta and showed that its expression profile differs markedly from ZIP14, suggesting nonredundant functions. Suppression of endogenous ZIP8 expression in BeWo cells, a placental cell line, reduced iron uptake by ∼40%, suggesting that ZIP8 participates in placental iron transport. Collectively, these data identify ZIP8 as an iron transport protein that may function in iron metabolism.

How I treat hemochromatosis

Hemochromatosis is a common genetic disorder in which iron may progressively accumulate in the liver, heart, and other organs. The primary goal of therapy is iron depletion to normalize body iron stores and to prevent or decrease organ dysfunction. The primary therapy to normalize iron stores is phlebotomy. In this opinion article, we discuss the indications for and monitoring of phlebotomy therapy to achieve iron depletion, maintenance therapy, dietary and pharmacologic maneuvers that could reduce iron absorption, and the role of voluntary blood donation.

Effects of deferasirox and deferiprone on cellular iron load in the human hepatoma cell line HepaRG

Two oral chelators, CP20 (deferiprone) and ICL670 (deferasirox), have been synthesized for the purpose of treating iron overload diseases, especially thalassemias. Given their antiproliferative effects resulting from the essential role played by iron in cell processes, such compounds might also be useful as anticancer agents. In the present study, we tested the impact of these two iron chelators on iron metabolism, in the HepaRG cell line which allowed us to study proliferating and differentiated hepatocytes. ICL670 uptake was greater than the CP20 uptake. The iron depletion induced by ICL670 in differentiated cells increased soluble transferrin receptor expression, decreased intracellular ferritin expression, inhibited (55)Fe (III) uptake, and reduced the hepatocyte concentration of the labile iron pool. In contrast, CP20 induced an unexpected slight increase in intracellular ferritin, which was amplified by iron-treated chelator exposure. CP20 also promoted Fe(III) uptake in differentiated HepaRG cells, thus leading to an increase of both the labile pool and storage forms of iron evaluated by calcein fluorescence and Perls staining, respectively. In acellular conditions, compared to CP20, iron removing ability from the calcein-Fe(III) complex was 40 times higher for ICL670. On the whole, biological responses of HepaRG cells to ICL670 treatment were characteristic of expected iron depletion. In contrast, the effects of CP20 suggest the potential involvement of this compound in the iron uptake from the external medium into the hepatocytes from the HepaRG cell line, therefore acting like a siderophore in this cell model.

Use of Nramp2-transfected Chinese hamster ovary cells and reticulocytes from mk/mk mice to study iron transport mechanisms

OBJECTIVE

We investigated mechanisms involved in iron (Fe) transport by DMT1 (endosomal Fe(II) exporter, encoded by the Nramp2 gene) using wild-type Chinese hamster ovary (CHO) cells and Nramp2-transfected CHO cells, as well as reticulocytes from normal and mk/mk mice that have a defect in DMT1.

MATERIALS AND METHODS

CHO cells and reticulocytes were incubated with 59Fe bound to various ligands. The radioiron was present in its Fe(II) or Fe(III) forms or bound to transferrin (Tf), and the internalized 59Fe measured under varying experimental conditions. Additionally, 125I-Tf interaction with reticulocytes was investigated and 59Fe incorporation into their heme was determined.

RESULTS

Hyperexpression of DMT1 in CHO cells greatly increases their capacity to acquire ferrous iron. Although CHO-Nramp2 cells showed an increase in Fe(III) uptake as compared to CHO cells, they transported Fe(III) with much lower efficacy than Fe(II). In addition to their defect in Fe uptake, mk/mk reticulocytes also showed a decrease in Tf receptor levels.

CONCLUSIONS

Given that CHO cells acquire iron from Fe(II)-ascorbate with much higher rates than from Fe(III)-Tf, Tf-receptor levels represent the rate-limiting step in their iron uptake. As Fe(III) transport by CHO-Nramp2 cells can be inhibited by the impermeable oxidant K3Fe(CN)6, a membrane ferric reductase is probably needed for reduction of Fe(III) to Fe(II), which is then transported by DMT1. DMT1 is not a limiting factor in Fe acquisition by normal reticulocytes and their heme synthesis.

Antiproliferative and apoptotic effects in rat and human hepatoma cell cultures of the orally active iron chelator ICL670 compared to CP20: a possible relationship with polyamine metabolism

OBJECTIVE

Iron loading has been observed to have a hyperproliferative effect on hepatocytes in vitro and on tumour cells in vivo; removal of this iron being required to induce antitumour activity.

MATERIAL AND METHODS

Antiproliferative effects of orally active tridentate iron chelator ICL670 (deferasirox) and bidentate iron chelator CP20 (deferiprone), mediated through the chelation of intracellular iron, were compared in rat hepatoma cell line FAO and human hepatoma cell line HUH7.

RESULTS

In FAO cell cultures, we have shown that ICL670 decreased cell viability and DNA replication and induced apoptosis more efficiently than an iron-binding equivalent concentration of CP20. Moreover, ICL670 decreased significantly the number of the cells in G(2)-M phase. In the HUH7 cell cultures, ICL670 and a four-time higher iron-binding equivalent concentration of CP20, decreased cell viability and DNA replication in the same range. CP20 increased the number of the cells in G(2)-M phase. However, ICL670 inhibited polyamine biosynthesis by decreasing ornithine decarboxylase mRNA level; in contrast, CP20 increased polyamine biosynthesis, particularly putrescine level, by stimulating spermidine-spermine N(1)-acetyl transferase activity that could activate the polyamine retro-conversion pathway. By mass spectrometry, we observed that ICL670 cellular uptake was six times higher than CP20.

CONCLUSIONS

These results suggest that ICL670 has a powerful antitumoural effect and blocks cell proliferation in neoplastic cells by a pathway different from that of CP20 and may constitute a potential adjuvant drug for anticancer therapy.

Pathology of hepatic iron overload

Although progress in imaging and genetics allow for a noninvasive diagnosis of most cases of genetic iron overload, liver pathology remains often useful (1) to assess prognosis by grading fibrosis and seeking for associated lesions and (2) to guide the etiological diagnosis, especially when no molecular marker is available. Then, the type of liver siderosis (parenchymal, mesenchymal or mixed) and its distribution throughout the lobule and the liver are useful means for suggesting its etiology: HLA-linked hemochromatosis gene (HFE) hemochromatosis or other rare genetic hemochromatosis, nonhemochromatotic genetic iron overload (ferroportin disease, aceruloplasminemia), or iron overload secondary to excessive iron supply, inflammatory syndrome, noncirrhotic chronic liver diseases including dysmetabolic iron overload syndrome, cirrhosis, and blood disorders.

The new orally active iron chelator ICL670A exhibits a higher antiproliferative effect in human hepatocyte cultures than O-trensox

By comparing the antiproliferative effect of the iron chelators ICL670A and O-trensox in the human hepatoma cell line HUH7 and human hepatocyte cultures, we have shown that ICL670A decreased cell viability, inhibited DNA replication and induced DNA fragmentation more efficiently than O-trensox. O-trensox and ICL670A induced a cell cycle blockade in G0-G1 and S phases respectively. In parallel, ICL670A inhibited polyamine biosynthesis by decreasing ornithine decarboxylase and spermidine/spermine N(1)-acetyltransferase activities. O-trensox increased polyamine biosynthesis and particularly putrescine level by stimulating spermidine-spermine N(1)-acetyltransferase activity which could activate the polyamine retro-conversion pathway. Moreover, the two chelators exhibit some cytotoxic effect in the two culture models; ICL670A was more cytotoxic than O-trensox and higher concentrations of the two chelators were necessary to induce a cytotoxicity in primary cultures versus hepatoma cells. These results suggested that ICL670A has the most efficient antitumoral effect, blocks cell proliferation by a pathway different of O-trensox and may constitute a potential drug for anticancer therapy.

Redox active plasma iron in C282Y/C282Y hemochromatosis

Labile plasma iron (LPI) represents the redox active component of non-transferrin-bound iron (NTBI). Its presence in thalassemic patients has been recently reported. The aim of the present study was to quantify LPI in HFE genetic hemochromatosis (GH) and to characterize the mechanisms accounting for its appearance. We studied 159 subjects subdivided into the following groups: (1) 23 with iron overloaded GH; (2) 14 with iron-depleted GH; (3) 26 with dysmetabolic hepatosiderosis; (4) 33 with alcoholic cirrhosis; (5) 63 healthy controls. Both NTBI and LPI were substantially higher in patients with iron-overloaded GH than in those with iron-depleted GH or in healthy controls. LPI was significantly correlated with serum transaminase increase in this group. LPI was elevated in the alcoholic cirrhosis subgroup of severely affected patients. LPI was found essentially when transferrin saturation exceeded 75%, regardless of the etiologic condition. Transferrin saturation above 75% was related to iron overload in GH and to liver failure in alcoholic cirrhosis. LPI is present in C282Y/C282Y hemochromatosis and may be a marker of toxicity due to its potential for catalyzing the generation of reactive oxygen radicals in vivo.