Effect of ascorbate in the reduction of transferrin-associated iron in endocytic vesicles

In defence of ferroptosis

Rampant phospholipid peroxidation initiated by iron causes ferroptosis unless this is restrained by cellular defences. Ferroptosis is increasingly implicated in a host of diseases, and unlike other cell death programs the physiological initiation of ferroptosis is conceived to occur not by an endogenous executioner, but by the withdrawal of cellular guardians that otherwise constantly oppose ferroptosis induction. Here, we profile key ferroptotic defence strategies including iron regulation, phospholipid modulation and enzymes and metabolite systems: glutathione reductase (GR), Ferroptosis suppressor protein 1 (FSP1), NAD(P)H Quinone Dehydrogenase 1 (NQO1), Dihydrofolate reductase (DHFR), retinal reductases and retinal dehydrogenases (RDH) and thioredoxin reductases (TR). A common thread uniting all key enzymes and metabolites that combat lipid peroxidation during ferroptosis is a dependence on a key cellular reductant, nicotinamide adenine dinucleotide phosphate (NADPH). We will outline how cells control central carbon metabolism to produce NADPH and necessary precursors to defend against ferroptosis. Subsequently we will discuss evidence for ferroptosis and NADPH dysregulation in different disease contexts including glucose-6-phosphate dehydrogenase deficiency, cancer and neurodegeneration. Finally, we discuss several anti-ferroptosis therapeutic strategies spanning the use of radical trapping agents, iron modulation and glutathione dependent redox support and highlight the current landscape of clinical trials focusing on ferroptosis.

Redox cycling metals: Pedaling their roles in metabolism and their use in the development of novel therapeutics

Essential metals, such as iron and copper, play a critical role in a plethora of cellular processes including cell growth and proliferation. However, concomitantly, excess of these metal ions in the body can have deleterious effects due to their ability to generate cytotoxic reactive oxygen species (ROS). Thus, the human body has evolved a very well-orchestrated metabolic system that keeps tight control on the levels of these metal ions. Considering their very high proliferation rate, cancer cells require a high abundance of these metals compared to their normal counterparts. Interestingly, new anti-cancer agents that take advantage of the sensitivity of cancer cells to metal sequestration and their susceptibility to ROS have been developed. These ligands can avidly bind metal ions to form redox active metal complexes, which lead to generation of cytotoxic ROS. Furthermore, these agents also act as potent metastasis suppressors due to their ability to up-regulate the metastasis suppressor gene, N-myc downstream regulated gene 1. This review discusses the importance of iron and copper in the metabolism and progression of cancer, how they can be exploited to target tumors and the clinical translation of novel anti-cancer chemotherapeutics.

Cellular iron uptake, trafficking and metabolism: Key molecules and mechanisms and their roles in disease

Iron is a crucial transition metal for virtually all life. Two major destinations of iron within mammalian cells are the cytosolic iron-storage protein, ferritin, and mitochondria. In mitochondria, iron is utilized in critical anabolic pathways, including: iron-storage in mitochondrial ferritin, heme synthesis, and iron-sulfur cluster (ISC) biogenesis. Although the pathways involved in ISC synthesis in the mitochondria and cytosol have begun to be characterized, many crucial details remain unknown. In this review, we discuss major aspects of the journey of iron from its initial cellular uptake, its modes of trafficking within cells, to an overview of its downstream utilization in the cytoplasm and within mitochondria. The understanding of mitochondrial iron processing and its communication with other organelles/subcellular locations, such as the cytosol, has been elucidated by the analysis of certain diseases e.g., Friedreich's ataxia. Increased knowledge of the molecules and their mechanisms of action in iron processing pathways (e.g., ISC biogenesis) will shape the investigation of iron metabolism in human health and disease.

The active role of vitamin C in mammalian iron metabolism: much more than just enhanced iron absorption!

Ascorbate is a cofactor in numerous metabolic reactions. Humans cannot synthesize ascorbate owing to inactivation of the gene encoding the enzyme l-gulono-γ-lactone oxidase, which is essential for ascorbate synthesis. Accumulating evidence strongly suggests that in addition to the known ability of dietary ascorbate to enhance nonheme iron absorption in the gut, ascorbate within mammalian systems can regulate cellular iron uptake and metabolism. Ascorbate modulates iron metabolism by stimulating ferritin synthesis, inhibiting lysosomal ferritin degradation, and decreasing cellular iron efflux. Furthermore, ascorbate cycling across the plasma membrane is responsible for ascorbate-stimulated iron uptake from low-molecular-weight iron-citrate complexes, which are prominent in the plasma of individuals with iron-overload disorders. Importantly, this iron-uptake pathway is of particular relevance to astrocyte brain iron metabolism and tissue iron loading in disorders such as hereditary hemochromatosis and β-thalassemia. Recent evidence also indicates that ascorbate is a novel modulator of the classical transferrin-iron uptake pathway, which provides almost all iron for cellular demands and erythropoiesis under physiological conditions. Ascorbate acts to stimulate transferrin-dependent iron uptake by an intracellular reductive mechanism, strongly suggesting that it may act to stimulate iron mobilization from the endosome. The ability of ascorbate to regulate transferrin iron uptake could help explain the metabolic defect that contributes to ascorbate-deficiency-induced anemia.

Transferrin iron uptake is stimulated by ascorbate via an intracellular reductive mechanism

Although ascorbate has long been known to stimulate dietary iron (Fe) absorption and non-transferrin Fe uptake, the role of ascorbate in transferrin Fe uptake is unknown. Transferrin is a serum Fe transport protein supplying almost all cellular Fe under physiological conditions. We sought to examine ascorbate's role in this process, particularly as cultured cells are typically ascorbate-deficient. At typical plasma concentrations, ascorbate significantly increased (59)Fe uptake from transferrin by 1.5-2-fold in a range of cells. Moreover, ascorbate enhanced ferritin expression and increased (59)Fe accumulation in ferritin. The lack of effect of cycloheximide or the cytosolic aconitase inhibitor, oxalomalate, on ascorbate-mediated (59)Fe uptake from transferrin indicate increased ferritin synthesis or cytosolic aconitase activity was not responsible for ascorbate's activity. Experiments with membrane-permeant and -impermeant ascorbate-oxidizing reagents indicate that while extracellular ascorbate is required for stimulation of (59)Fe uptake from (59)Fe-citrate, only intracellular ascorbate is needed for transferrin (59)Fe uptake. Additionally, experiments with l-ascorbate analogs indicate ascorbate's reducing ene-diol moiety is necessary for its stimulatory activity. Importantly, neither N-acetylcysteine nor buthionine sulfoximine, which increase or decrease intracellular glutathione, respectively, affected transferrin-dependent (59)Fe uptake. Thus, ascorbate's stimulatory effect is not due to a general increase in cellular reducing capacity. Ascorbate also did not affect expression of transferrin receptor 1 or (125)I-transferrin cellular flux. However, transferrin receptors, endocytosis, vacuolar-type ATPase activity and endosomal acidification were required for ascorbate's stimulatory activity. Therefore, ascorbate is a novel modulator of the classical transferrin Fe uptake pathway, acting via an intracellular reductive mechanism.

Intraphagosomal Mycobacterium tuberculosis acquires iron from both extracellular transferrin and intracellular iron pools. Impact of interferon-gamma and hemochromatosis

Mycobacterium tuberculosis multiplies within the macrophage phagosome and requires iron for growth. We examined the route(s) by which intracellular M. tuberculosis acquires iron. During intracellular growth of the virulent Erdman M. tuberculosis strain in human monocyte-derived macrophages (MDM), M. tuberculosis acquisition of (59)Fe from transferrin (TF) provided extracellularly (exogenous source) was compared with acquisition when MDM were loaded with (59)Fe from TF prior to M. tuberculosis infection (endogenous sources). M. tuberculosis (59)Fe acquisition required viable bacteria and was similar from exogenous and endogenous sources at 24 h and greater from exogenous iron at 48 h. Interferon-gamma treatment of MDM reduced (59)Fe uptake from TF 51% and TF receptor expression by 34%. Despite this, intraphagosomal M. tuberculosis iron acquisition in IFN-gamma-treated cells was decreased by only 30%. Macrophages from hereditary hemochromatosis patients have altered iron metabolism. Intracellular M. tuberculosis acquired markedly less iron in MDM from these individuals than in MDM from healthy donors, regardless of the iron source (exogenous and endogenous): 36 +/- 3.8% and 17 +/- 9.6% of control, respectively. Thus, intraphagosomal M. tuberculosis can acquire iron from both extracellular TF and endogenous macrophage sources. Acquisition of iron from macrophage cytoplasmic iron pools may be critical for the intracellular growth of M. tuberculosis. This acquisition is altered by IFN-gamma treatment to a small extent, but is markedly reduced in macrophages from hemochromatosis patients.

The host-protein-independent iron uptake by Tritrichomonas foetus

Iron uptake from a low-molecular-weight chelate Fe(III)-nitriloacetate (Fe-NTA) by anaerobic protozoan parasite Tritrichomonas foetus was investigated and compared with that from iron-saturated lactoferrin and transferrin. The results showed that the iron uptake from Fe-NTA was saturable (Km = 2.7 microM, Vmax = 21.7 fmol. microg-1.min-1) and time, and temperature dependent, thus suggesting involvement of a membrane transport carrier. The accumulation of iron from 59Fe-NTA was inhibited by NaF and iron chelators. Amilorid and inhibitors of endosome acidification did not influence the process. Ascorbate stimulated the uptake while a membrane impermeable chelator of bivalent iron (bathophenanthroline disulfonic acid) was inhibitory, suggesting that prior to transport iron is reduced extracellularly. In accord with this assumption, the reduction of ferric to ferrous iron in the presence of intact T. foetus cells was demonstrated. Dynamics and properties of uptake of iron released from transferrin were similar to those from Fe-NTA, indicating involvement of common mechanisms. Iron uptake from lactoferrin displayed profoundly different characteristics consistent with receptor-mediated endocytosis. Metronidazole-resistant derivative of the investigated T. foetus strain showed marked deficiency in iron acquisition from Fe-NTA and transferrin while its iron uptake from lactoferrin was higher than that of the parent strain. The results presented show that T. foetus possesses at least two independent mechanisms that mediate acquisition of iron.

Regulation of mammalian iron metabolism: current state and need for further knowledge

Due to its character as an essential element for all forms of life, the biochemistry and physiology of iron has attracted very intensive interest for many decades. In more recent years, the ways that iron metabolism is regulated in mammalian and human organisms have been clarified, and many aspects of iron metabolism have been reviewed. In this article, some newer aspects concerning absorption and intracellular regulation of iron concentration are considered. These include a sorting of possible models for intestinal iron absorption, a description of ways for membrane passage of iron after release from transferrin during receptor-mediated endocytosis, a consideration of possible mechanisms for non-transferrin bound iron uptake and its regulation, and a review of recent knowledge on the properties of iron regulatory proteins and on regulation of iron metabolism by these proteins, changes of their own properties by non-iron-mediated influences, and regulatory events not mediated by these proteins. This somewhat heterogeneous collection of themes is a consequence of the intention to avoid repetition of the many aforementioned reviews already existing and to concentrate on newer findings generated within the last couple of years.

Uptake and processing of 59Fe-labelled and 125I-labelled rat transferrin by early organogenesis rat conceptuses in vitro

The delivery of iron to the early organogenesis rat embryo has been studied, using 59Fe- and 125I-labelled rat transferrin. Rat conceptuses at 9.5 days postconception were cultured for 27 or 51 h in whole rat serum. Rat transferrin labelled with 59Fe was added for the final 0.1, 0.5, 6, 24 or 48 h of culture. Radioactivity accumulated progressively in both the embryo and the visceral yolk sac. Similar results were obtained when unconjugated 59Fe3+ was added to the rat serum used as culture medium. Both acid-soluble and acid-insoluble 59Fe were substantially present in the embryo and yolk sac after all exposure periods. When conceptuses were cultured in the presence of 125I-labelled rat transferrin, acid-soluble radioactivity was progressively released into the culture medium, but accumulation into the embryo and visceral yolk sac was slight and did not change with duration of exposure to the labelled protein. Similar findings were obtained using 125I-labelled bovine serum albumin. In these experiments, there was a close correspondence between the amount of iron accumulated by the embryo and visceral yolk sac in the final 24 h of a 51-h culture and the amount of transferrin converted into acid-soluble products in the same period. Visceral yolk sacs from 17.5-day pregnant rats were explanted and cultured in the presence of 59Fe-labelled rat transferrin, 125I-labelled rat transferrin or 125I-labelled bovine serum albumin, for periods up to 3 h. Again uptake of 59Fe increased with time of incubation, and the 125I-labelled proteins were digested to acid-soluble products which were released into the culture medium. The results indicate that transferrin delivers iron for incorporation into both the embryo and the visceral yolk sac, and are consistent with a mechanism involving receptor-mediated endocytosis of iron-laden transferrin by the cells of the visceral yolk sac. The transferrin itself appears to be quantitatively degraded, following delivery of iron to the yolk sac cells, a result that differs from findings in other cell types, in which the protein is not degraded but returns to the plasma membrane to participate in further cycles of iron acquisition and delivery.

Iron metabolism: a comprehensive review

Despite its abundance in the earth's crust, iron deficiency is a serious health issue in many parts of the world. Although fundamental observations about iron metabolism and the significance of iron nutriture were first noted some time ago, the molecular mechanisms involved in iron metabolism are just now being defined.

Effect of lipid peroxidation on transferrin-free iron uptake by rabbit reticulocytes

The relationship between lipid peroxidation and uptake of transferrin- free iron, Fe(II), by reticulocytes in an experimental system for studying membrane transport of Fe(II) was investigated by using free radical scavengers: BHA (butylated hydroxyanisole), BHT (butylated hydroxytoluene), superoxide dismutase, alpha-tocopherol, propyl gallate and DPPD (N,N-diphenyl-1,4-phenylenediamine), and producers: t-butyl hydroperoxide, cumene hydroperoxide, H2O2 and aluminium carbonate. Measurements were made of MDA (malondialdehyde) and the rate of Fe(II) uptake from a sucrose solution buffered at pH 6.5 by Pipes. Most scavengers and producers used could increase or decrease only slightly the rate of Fe(II) uptake and some of them had no effect on Fe(II) uptake and MDA could not be detected at iron concentration of lower than 10 microM and incubation time of 20 min. At iron concentration of higher than 100 microM and incubation time of 4 h, there was the production of MDA which increased with the increment of iron concentration of incubation medium and BHT could inhibit the production of MDA. In addition, no difference was found in the rates of Fe(II) uptake in three experimental groups whose incubation medium was buffered by Pipes, Mops and Mes respectively. The results suggested that iron could induce free radical reaction under experimental conditions, especially at high concentration of iron and longer incubation time; however, at low concentration of iron (<10 microM) and the usual incubation time (20 min) free radical reaction was very slight and the extent of the reaction was not enough to damage the integrity and function of the membrane of reticulocytes, and that Fe(II) uptake by reticulocytes was not the result of free radical reaction and lipid peroxidation. It was therefore concluded that iron could not initiate its own membrane transport in rabbit reticulocytes by free radical reaction and lipid peroxidation and that the experimental system we used for studying membrane transport of Fe(II) is valid.

Role of redox systems on Fe3+ uptake by transformed human intestinal epithelial (Caco-2) cells

Caco-2 cells were used as a model of human intestinal epithelium to investigate the role of redox systems in transepithelial transport of 59Fe3+. The cells reduced Fe3+ present in the apical medium; the reduction was 50% inhibited by adriamycin and p-chloromercuribenzoate. Addition of [14C]ascorbate to the basolateral medium resulted in accumulation of 14C radioactivity in both cells and apical medium; apical radioactivity increased with time and was probably caused by paracellular flux. The cells provided Fe3+ reduction capacity to the apical incubation medium. Addition of ascorbate to the basolateral medium increased this reduction capacity 2-fold and the cellular uptake of 59Fe3+ 1.8-fold. Adriamycin significantly inhibited both cellular 59Fe uptake and Fe transport into the basolateral side. The results indicate that Caco-2 cells reduce apical Fe3+ by two parallel mechanisms: by a plasma membrane ferrireductase and by the secretion of reductants of either cellular or basolateral origin. The data support a model for Fe3+ intestinal absorption in which cell-mediated Fe3+ reduction occurs before cellular Fe uptake.

Effects of acute ethanol administration on the uptake of 59Fe-labeled transferrin by rat liver and cerebellum

The uptake of iron by the liver and cerebellum was measured in rats using [59Fe]transferrin. An acute ethanol load (50 mmol/kg body wt., i.p.) elicited a significant increase in the hepatic and cerebellar non-heme iron concentration. The uptake of 59Fe by the liver and the cerebellum was significantly greater in the ethanol-treated rats than in control animals. The administration of allopurinol prior to the ethanol load prevented the changes in liver and cerebellar non-heme iron content. Moreover pretreatment with allopurinol reduced the ethanol-induced enhancement of 59Fe uptake by the liver and completely prevented the changes in 59Fe uptake by the cerebellum. These effects of allopurinol lead us to suggest that oxygen-derived free radicals are involved in the ethanol-induced disturbances of iron uptake both at the hepatic and cerebellar level.