Haem synthesis and iron uptake by reticulocytes

Mitocentricity

Worldwide, interest in mitochondria is constantly growing, as evidenced by scientific statistics, and studies of the functioning of these organelles are becoming more prevalent than studies of other cellular structures. In this analytical review, mitochondria are conditionally placed in a certain cellular center, which is responsible for both energy production and other non-energetic functions, without which the existence of not only the eukaryotic cell itself, but also the entire organism is impossible. Taking into account the high multifunctionality of mitochondria, such a fundamentally new scheme of cell functioning organization, including mitochondrial management of processes that determine cell survival and death, may be justified. Considering that this issue is dedicated to the memory of V. P. Skulachev, who can be called mitocentric, due to the history of his scientific activity almost entirely aimed at studying mitochondria, this work examines those aspects of mitochondrial functioning that were directly or indirectly the focus of attention of this outstanding scientist. We list all possible known mitochondrial functions, including membrane potential generation, synthesis of Fe-S clusters, steroid hormones, heme, fatty acids, and CO2. Special attention is paid to the participation of mitochondria in the formation and transport of water, as a powerful biochemical cellular and mitochondrial regulator. The history of research on reactive oxygen species that generate mitochondria is subject to significant analysis. In the section "Mitochondria in the center of death", special emphasis is placed on the analysis of what role and how mitochondria can play and determine the program of death of an organism (phenoptosis) and the contribution made to these studies by V. P. Skulachev.

Serum transferrin receptors are decreased in the presence of iron overload

To test the hypothesis that the quantities of circulating transferrin receptors are reduced in iron overload, we studied serum transferrin receptors and indirect measures of iron status in 150 subjects from rural Zimbabwe. We found significant inverse correlations between serum concentrations of transferrin receptors and ferritin, the ratio of ferritin to aspartate aminotransferase, and transferrin saturation (r ≥0.44; P <0.001). The mean ± SD concentration of serum transferrin receptors in 23 subjects classified as having iron overload (ferritin >300 μg/L and transferrin saturation >60%) was 1.55 ± 0.61 mg/L, significantly lower than the 2.50 ± 0.62 mg/L in 75 subjects with normal iron stores (ferritin 20–300 μg/L and transferrin saturation 15–55%; P <0.0005) and the 2.83 ± 1.14 mg/L in 8 subjects with iron deficiency (ferritin <20 μg/L; P = 0.001). In keeping with the regulation of transferrin receptor expression at the cellular level, our findings suggest that serum transferrin receptors are decreased in the presence of iron overload.

Regulation of transferrin receptor mRNA expression. Distinct regulatory features in erythroid cells

In proliferating non-erythroid cells, the expression of transferrin receptors (TfR) is negatively regulated by the amount of intracellular iron. Fe-dependent regulation of TfR occurs post-transcriptionally and is mediated by iron-responsive elements (IRE) located in the 3' untranslated region of the TfR mRNA. IREs are recognized by a specific cytoplasmic binding protein (IRE-BP) that, in the absence of Fe, binds with high affinity to TfR mRNA, preventing its degradation. While TfR numbers are positively correlated with proliferation in non-erythroid cells, in hemoglobin-synthesizing cells, their numbers increase during differentiation and are, therefore, negatively correlated with proliferation. This suggests a distinct regulation of erythroid TfR expression and evidence, as follows, for this was found in the present study. (a) With nuclear run-on assays, our experiments show increased TfR mRNA transcription following induction of erythroid differentiation of murine erythroleukemia (MEL) with Me2SO. (b) Me2SO treatment of MEL cells does not increase IRE-BP activity which is, however, increased in uninduced MEL cells by Fe chelators. (c) Following induction of MEL cells, there is an increase in the stability of TfR mRNA, whose level is only slightly affected by iron excess. (d) Heme-synthesis inhibitors, such as succinylacetone and isonicotinic acid hydrazide, which inhibit numerous aspects of erythroid differentiation, also inhibit TfR mRNA expression in induced MEL cells. However, heme-synthesis inhibition does not lead to a decrease in TfR mRNA levels in uninduced MEL cells. Thus, these studies indicate that TfR gene expression is regulated differently in hemoglobin synthesizing as compared to uninduced MEL cells.

Abnormal haem biosynthesis in the chronic anaemia of rheumatoid arthritis

OBJECTIVES

The chronic microcytic anaemia of rheumatoid arthritis (RA) occurs despite the presence of adequate reticulo-endothelial iron stores. The red cell microcytosis is evidence of impaired haemoglobin production. This study has examined possible abnormalities of erythroid haem biosynthesis that may contribute to the anaemia.

METHODS

5-Aminolaevulinate (ALA) synthase and ferrochelatase activities were assayed in whole bone marrow and in purified erythroblasts from patients with RA and in control subjects. All patients were iron replete with demonstrable iron in the bone marrow.

RESULTS

ALA synthase activity was significantly reduced in both whole bone marrow and purified erythroblasts from patients with the anaemia of RA. Erythrocyte protoporphyrin levels were raised in nine of 12 patients tested while ferrochelatase activity was normal.

CONCLUSION

These abnormalities provide absolute evidence of abnormal erythroblast haem biosynthesis and iron metabolism in the anaemia of RA and most likely reflect decreased ALA synthase mRNA translation and some abnormality of erythroblast iron transport. Further studies using highly purified erythroblast populations will attempt to identify the causal factors leading to this abnormal erythroblast metabolism.

Accumulation of iron in erythroblasts of patients with erythropoietic protoporphyria

We have studied the iron metabolism in nine patients with erythropoietic protoporphyria (EPP) and three patients with sideroblastic anaemia (SA). All, except one EPP patient were iron deficient. The SA patients had a secondary haemochromatosis. The bone marrow aspirates of patients with SA and also three patients with EPP had a high incidence of ring sideroblasts. Ultrastructural examination of the bone marrow consistently showed finely dispersed electron-dense deposits localized in mitochondria of erythroblasts in all patients with EPP and SA. Mitochondrial electron energy-loss spectroscopy (EELS) indicated identical iron compounds in erythroblasts of all EPP and SA patients. These findings indicate that the mitochondrial iron utilization is disturbed in EPP and SA. The observation of mitochondrial iron deposition in erythroblasts in EPP and SA suggests that this failure is not of pathognomonic value for diagnosis of SA, but is apparently the result of an inefficient haem synthesis, in EPP due to a defective ferrochelatase. The mitochondrial iron deposition does not depend on the iron status (iron overload or iron deficiency) of the EPP patient.

Control of heme synthesis during Friend cell differentiation: role of iron and transferrin

In many types of cells the synthesis of delta-aminolevulinic acid (ALA) limits the rate of heme formation. However, results from our laboratory with reticulocytes suggest that the rate of iron uptake from transferrin (Tf), rather than ALA synthase activity, limits the rate of heme synthesis in erythroid cells. To determine whether changes occur in iron metabolism and the control of heme synthesis during erythroid cell development Friend erythroleukemia cells induced to erythroid differentiation by dimethylsulfoxide (DMSO) were studied. While added ALA stimulated heme synthesis in uninduced Friend cells (suggesting ALA synthase is limiting) it did not do so in induced cells. Therefore the possibility was investigated that, in induced cells, iron uptake from Tf limits and controls heme synthesis. Several aspects of iron metabolism were investigated using the synthetic iron chelator salicylaldehyde isonicotinoyl hydrazone (SIH). Both induced and uninduced Friend cells take up and utilize Fe for heme synthesis directly from Fe-SIH without the involvement of transferrin and transferrin receptors and to a much greater extent than from saturating levels of Fe-Tf (20 microM). Furthermore, in induced Friend cells 100 microM Fe-SIH stimulated 2-14C-glycine incorporation into heme up to 3.6-fold as compared to the incorporation observed with saturating concentrations of Fe-Tf. In contrast, Fe-SIH, even when added in high concentrations, did not stimulate heme synthesis in uninduced Friend cells but was able to do so as early as 24 to 48 h following induction. In addition, contrary to previous results with rabbit reticulocytes, Fe-SIH also stimulated globin synthesis in induced Friend cells above the level seen with saturating concentrations of transferrin. These results indicate that some step(s) in the pathway of iron from extracellular Tf to protoporphyrin, rather than the activity of ALA synthase, limits and controls the overall rate of heme and possibly hemoglobin synthesis in differentiating Friend erythroleukemia cells.

Coated pit formation: a membrane function involved in the regulation of cellular iron uptake

Diferric-transferrin induces a marked increase in the number of coated pits on the reticulocyte membrane. This increase is followed by a decline, often to below the initial number. Since a good correlation was found between the rate of iron uptake and the number of coated pits, but not between the rate of transferrin recycling and the coated pit count, it is likely that coated pit formation is necessary for the removal of iron from transferrin. The decline in the number of transferrin-induced coated pits was observed only when haem synthesis was undisturbed, indicating that the accumulation of intracellular haem inhibits coated pit formation. Based on these results we suggest that haem regulates the rate of iron uptake by inhibiting iron removal rather than receptor recycling.

Transferrin receptors and iron utilization in DMSO-inducible and -uninducible Friend erythroleukemia cells

Dimethylsulfoxide (DMSO) induces hemoglobin synthesis and erythroid differentiation of Friend erythroleukemia cells in vitro. Induction is accompanied by increased transferrin-binding activity which is necessary for the cellular acquisition of iron from transferrin for hemoglobin synthesis. There are Friend cell variants in which hemoglobin synthesis is not induced by DMSO unless exogenous hemin is also present. In this study we have compared the inducibility of transferrin receptors and iron incorporation in DMSO-inducible (745) and -uninducible (M-18 and TG-13) Friend cell lines. Cellular transferrin-binding sites were estimated by Scatchard analysis of data obtained from specific binding of [125I]transferrin by the cells. Our results show that unlike 745, DMSO treatment of the variant cell lines M-18 and TG-13 does not result in increased transferrin-binding activity. The number of transferrin-binding sites and the rate of iron uptake is similar in uninduced 745 and DMSO-treated M-18 and TG-13 cells. Although exposure of M-18 cells to DMSO and hemin induces hemoglobinization, this treatment does not cause induction of transferrin receptors. These results indicate that the primary defect in M-18 cells may be the uninducibility of transferrin receptors. We have also shown that exposure of 745 cells to hemin during DMSO treatment prevents the induction of transferrin receptors, suggesting that hemin may control the expression of transferrin receptors in erythroid cells.

Iron granules in plasma cells

Resistance of cancer cells to chemotherapy is the first cause of cancer-associated death. Thus, new strategies to deal with the evasion of drug response and to improve clinical outcomes are needed. Genetic and epigenetic mechanisms associated with uncontrolled cell growth result in metabolism reprogramming. Cancer cells enhance anabolic pathways and acquire the ability to use different carbon sources besides glucose. An oxygen and nutrient-poor tumor microenvironment determines metabolic interactions among normal cells, cancer cells and the immune system giving rise to metabolically heterogeneous tumors which will partially respond to metabolic therapy. Here we go into the best-known cancer metabolic profiles and discuss several studies that reported tumors sensitization to chemotherapy by modulating metabolic pathways. Uncovering metabolic dependencies across different chemotherapy treatments could help to rationalize the use of metabolic modulators to overcome therapy resistance.

Iron granules in plasma cells

The curious and unusual finding of coarse iron granules in marrow plasma cells is reported in 13 patients, in whom the finding was incidental. In 10 of these patients there was known alcohol abuse and serious medical complications of that abuse. Previous reports of the finding are reviewed. Haematological data of the 13 patients are presented. A hypothesis is outlined which may account for the finding.

The activities of delta-aminolaevulinic acid synthase and haem synthase in experimental sideroblastic anaemia. Effect of mitochondrial iron excess on the enzyme activity in peripheral red blood cells

Experimental sideroblastic anaemia was produced in normal and in iron loaded guinea pigs by intraperitoneal (i.p.) administration of isoniazid and cycloserine. Subsequently, the activities of delta-aminolaevulinic acid synthase (ALA-S) and of haem synthase in peripheral red blood cells were measured and in particular the relationships of enzyme activities to the iron status were examined. The ALA-S activity showed a similar decrease in all animals with sideroblastic anaemia. The haem synthase activity was increased probably due to secondary induction, but it was significantly less increased in animals with the highest values for iron status. This finding indicates that mitochondrial iron accumulation may have limited the compensatory increase of haem synthase activity. It is likely that also in human sideroblastic anaemia mitochondrial iron overload may have a secondary limiting effect on the haem synthase activity in erythroid cells.

Deficient heme synthesis as the cause of noninducibility of hemoglobin synthesis in a Friend erythroleukemia cell line

Friend cells of the line Fw are not induced to accumulate substantial amounts of hemoglobin and to become benzidine-positive when treated with butyric acid or other inducers, except in the presence of exogenous hemin. The cells are shown to have a deficiency in heme synthesis since they require exogenous hemin during the period of maximal hemoglobin synthesis; since endogenous heme synthesis cannot be induced to the level found in normal inducible Friend cells, even after hemoglobin synthesis has been induced by hemin and butyric acid and the hemin has then been withdrawn; since they are not inducible for ferrochelatase (heme synthetase) activity; and since they accumulate free globin chains after stimulation with butyric acid in the absence of hemlin. Comparison of globin synthesis and globin mRNA content of the cells shows that globin synthesis is not controlled by the hemin-controlled repressor of protein synthesis (HCR) nor by any specific translational control of globin synthesis by hemlin.

Use of iron from transferrin and microbial chelates as substrate for heme synthetase in transformed and primary erythroid cell cultures

The enzymatic heme production in cell-free extracts of virus-transformed Friend erythroleukemia cells and primary bone marrow cells from rabbits has been measured by determining the activity of heme synthetase after addition of iron sulfate, transferrin or microbial iron chelates. In transformed cells the amounts of heme formed did not show significant difeerences independent of which substrate was offered. In cell-free extracts of primary bone marrow cells no increase of heme production could be observed.

Control of iron delivery to haemoglobin in erythroid cells

This paper reviews and reports the results of experiments on the mechanism by which iron is delivered from extracellular transferrin to reticulocyte mitochondria in which haem is synthesized. It is suggested that transferrin donates the iron directly to mitochondria. Transferrin seems to be bound to mitochondria during the process of iron release. When the release of iron from transferrin is blocked by haem, the iron-transferrin complex remains bound to mitochondria so that the total amount of transferrin molecules associated with mitochondria increases in haem-treated reticulocytes. This also leads to an increase in the number of transferrin molecules in the cytosol. In haem-deficient reticulocytes, the rate of dissociation of iron from transferrin is accelerated and the uptake of iron by mitochondria is increased. When the synthesis of haem is inhibited, the non-haem iron in the cytosol (i.e. mainly low-molecular-weight and ferritin iron) comes from mitochondria. Greater amounts of non-haem iron can also be induced in reticulocytes incubated with highly saturated transferrin but, in this case, iron does not seem to be accumulated in mitochondria. These results represent an experimental basis for the elucidation of the excessive non-haem iron accumulation in erythroid cells observed in various clinical conditions.

Transferrin and iron uptake by isolated rat liver mitochondria

Isolated rat liver mitochondria accumulate iron from the suspending medium when [59Fe]transferrin is used as a model compound. The accumulation proceeds by two different mechanisms, i.e. by an energy-independent and an energy-dependent (uncoupler sensitive) mechanism, which have different time, pH, and temperature dependencies. The energy-dependent accumulation, which is inhibited by ruthenium red and sulphydryl reagents, reaches a saturation level of approx. 30 pmoles iron/mg protein during 30 min incubation. The energy-independent accumulation of iron-transferrin reveals no saturation kinetics, it is inhibited neither by ruthenium red nor by N-ethylmaleimide, and it proceeds linearly for at least 90 min. With [125I]transferrin as a model compound, quantitatively the energy-independent accumulation is as reported for [59Fe]transferrin. There is, however, no energy-dependent accumulation of [125I]transferrin. The results indicate that the energy-dependent accumulation of [59Fe]transferrin represents a process by which mitochondria accumulate iron from transferrin.