The fission yeast ferric reductase gene frp1+ is required for ferric iron uptake and encodes a protein that is homologous to the gp91-phox subunit of the human NADPH phagocyte oxidoreductase.

We have identified a cell surface ferric reductase activity in the fission yeast Schizosaccharomyces pombe. A mutant strain deficient in this activity was also deficient in ferric iron uptake, while ferrous iron uptake was not impaired. Therefore, reduction is a required step in cellular ferric iron acquisition. We have cloned frp1+, the wild-type allele of the mutant gene. frp1+ mRNA levels were repressed by iron addition to the growth medium. Fusion of 138 nucleotides of frp1+ promoter sequences to a reporter gene, the bacterial chloramphenicol acetyltransferase gene, conferred iron-dependent regulation upon the latter when introduced into S. pombe. The predicted amino acid sequence of the frp1+ gene exhibits hydrophobic regions compatible with transmembrane domains. It shows similarity to the Saccharomyces cerevisiae FRE1 gene product and the gp91-phox protein, a component of the human NADPH phagocyte oxidoreductase that is deficient in X-linked chronic granulomatous disease.     

Azo reductase activity of intact saccharomyces cerevisiae cells is dependent on the Fre1p component of plasma membrane ferric reductase

Unspecific bacterial reduction of azo dyes is a process widely studied in correlation with the biological treatment of colored wastewaters, but the enzyme system associated with this bacterial capability has never been positively identified. Several ascomycete yeast strains display similar decolorizing behaviors. The yeast-mediated process requires an alternative carbon and energy source and is independent of previous exposure to the dyes. When substrate dyes are polar, their reduction is extracellular, strongly suggesting the involvement of an externally directed plasma membrane redox system. The present work demonstrates that, in Saccharomyces cerevisiae, the ferric reductase system participates in the extracellular reduction of azo dyes. The S. cerevisiae Deltafre1 and Deltafre1 Deltafre2 mutant strains, but not the Deltafre2 strain, showed much-reduced decolorizing capabilities. The FRE1 gene complemented the phenotype of S. cerevisiae Deltafre1 cells, restoring the ability to grow in medium without externally added iron and to decolorize the dye, following a pattern similar to the one observed in the wild-type strain. These results suggest that under the conditions tested, Fre1p is a major component of the azo reductase activity.     

Identification of a vacuole-associated metalloreductase and its role in Ctr2-mediated intracellular copper mobilization

Copper is an essential trace metal whose biological utility is derived from its ability to cycle between oxidized Cu(II) and reduced Cu(I). Ctr1 is a high affinity plasma membrane copper permease, conserved from yeast to humans, that mediates the physiological uptake of Cu(I) from the extracellular environment. In the baker's yeast Saccharomyces cerevisiae, extracellular Cu(II) is reduced to Cu(I) via the action of the cell surface metalloreductase Fre1, similar to the human gp91(phox) subunit of the NADPH oxidase complex, which utilizes heme and flavins to catalyze electron transfer. The S. cerevisiae Ctr2 protein is structurally similar to Ctr1, localizes to the vacuole membrane, and mobilizes vacuolar copper stores to the cytosol via a mechanism that is not well understood. Here we show that Ctr2-1, a mutant form of Ctr2 that mislocalizes to the plasma membrane, requires the Fre1 plasma membrane metalloreductase for Cu(I) import. The conserved methionine residues that are essential for Ctr1 function at the plasma membrane are also essential for Ctr2-1-mediated Cu(I) uptake. We demonstrate that Fre6, a member of the yeast Fre1 metalloreductase protein family, resides on the vacuole membrane and functions in Ctr2-mediated vacuolar copper export, and cells lacking Fre6 phenocopy the Cu-deficient growth defect of ctr2Delta cells. Furthermore, both CTR2 and FRE6 mRNA levels are regulated by iron availability. Taken together these studies suggest that copper movement across intracellular membranes is mechanistically similar to that at the plasma membrane. This work provides a model for communication between the extracellular Cu(I) uptake and the intracellular Cu(I) mobilization machinery.     

Plasma membrane electron transport in Saccharomyces cerevisiae depends on the presence of mitochondrial respiratory subunits

Most investigations into plasma membrane electron transport (PMET) in Saccharomyces cerevisiae have focused on the inducible ferric reductase responsible for iron uptake under iron/copper-limiting conditions. In this paper, we describe a PMET system, distinct from ferric reductase, which reduces the cell-impermeable water-soluble tetrazolium dye, 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulphophenyl)-2H-tetrazolium monosodium salt (WST-1), under normal iron/copper conditions. WST-1/1-methoxy-phenazine methosulphate reduction was unaffected by anoxia and relatively insensitive to diphenyleneiodonium. Dye reduction was increased when intracellular NADH levels were high, which, in S. cerevisiae , required deletion of numerous genes associated with NADH recycling. Genome-wide screening of all viable nuclear gene-deletion mutants of S. cerevisiae revealed that, although mitochondrial electron transport per se was not required, the presence of several nuclear and mitochondrially encoded subunits of respiratory complexes III and IV was mandatory for PMET. This suggests some form of interaction between components of mitochondrial and plasma membrane electron transport. In support of this, mitochondrial tubular networks in S. cerevisiae were shown to be located in close proximity to the plasma membrane using confocal microscopy.     

Cytochrome P-450 reductase is responsible for the ferrireductase activity associated with isolated plasma membranes of Saccharomyces cerevisiae

Cytochrome P-450 reductase (encoded by the NCP1 gene) was found to catalyse all the NADPH-dependent ferrireductase activities associated with isolated plasma membranes of the yeast Saccharomyces cerevisiae. We therefore examined the contribution of this enzyme to the ferrireductase activity of cells in vivo. Cytochrome P-450 reductase was shown to be not essential for the cell ferrireductase activity, but it influenced this activity, with different effects on the Fre1- and the Fre2-dependent reductase systems. Overexpression of FRE1 did not lead to an increased ferrireductase activity of the cells when NCP1 was repressed. In contrast, cells that overexpressed FRE2 had maximal ferrireductase activity when NCP1 was repressed. The degree of NCP1 expression also affected the amount of iron and copper accumulated by the cells during growth. The biochemical implications and the physiological significance of these observations are discussed.     

Possible role of membrane gamma-glutamyltransferase activity in the facilitation of transferrin-dependent and -independent iron uptake by cancer cells.

BACKGROUND: The molecular mechanisms by which iron is physiologically transported trough the cellular membranes are still only partially understood. Several studies indicate that a reduction step of ferric iron to ferrous is necessary, both in the case of transferrin-mediated and transferrin-independent iron uptake. Recent studies from our laboratory described gamma-glutamyltransferase activity (GGT) as a factor capable to effect iron reduction in the cell microenvironment. GGT is located on the outer aspect of plasma membrane of most cell types, and is often expressed at high levels in malignant tumors and their metastases. The present study was aimed at verifying the possibility that GGT-mediated iron reduction may participate in the process of cellular iron uptake. RESULTS: Four distinct human tumor cell lines, exhibiting different levels of GGT activity, were studied. The uptake of transferrin-bound iron was investigated by using 55Fe-loaded transferrin, as well as by monitoring fluorimetrically the intracellular iron levels in calcein-preloaded cells. Transferrin-independent iron uptake was investigated using 55Fe complexed by nitrilotriacetic acid (55Fe-NTA complex).The stimulation of GGT activity, by administration to cells of the substrates glutathione and glycyl-glycine, was generally reflected in a facilitation of transferrin-bound iron uptake. The extent of such facilitation was correlated with the intrinsic levels of the enzyme present in each cell line. Accordingly, inhibition of GGT activity by means of two independent inhibitors, acivicin and serine/boric acid complex, resulted in a decreased uptake of transferrin-bound iron. With Fe-NTA complex, the inhibitory effect - but not the stimulatory one - was also observed. CONCLUSION: It is concluded that membrane GGT can represent a facilitating factor in iron uptake by GGT-expressing cancer cells, thus providing them with a selective growth advantage over clones that do not possess the enzyme.     

Cloning of Pichia pastoris Fet3: insights into the high affinity iron uptake system

High-affinity iron uptake by yeast cells appears to require the presence of a complex formed on the plasma membrane by the multicopper oxidase Fet3 and the permease Ftr1 which work together to allow iron to enter safely inside the cell. The Pichia pastoris ferroxidase Fet3 has been cloned and it has been found to display high sequence similarity to other yeast multicopper oxidases, including all the predicted ligands for the catalytic copper atoms and for the iron substrate. P. pastoris appears to possess a high-affinity iron uptake system similar to that of S. cerevisiae, as far as regulation of expression is concerned. However, the P. pastoris high-affinity iron uptake system presents a K(m) value for iron almost ten times higher than that of S. cerevisiae, possibly to control iron fluxes over a wider range of concentrations of this metal, in order to avoid toxic iron overloading.     

Modulation of Dcytb (Cybrd 1) expression and function by iron,dehydroascorbate and Hif-2α in cultured cells

Background

Duodenal cytochrome b (Dcytb) is a mammalian plasma ferric reductase enzyme that catalyses the reduction of ferric to ferrous ion in the process of iron absorption. The current study investigates the relationship between Dcytb, iron, dehydroascorbate (DHA) and Hif-2α in cultured cell lines.

Methods

Dcytb and Hif-2α protein expression was analysed by Western blot technique while gene regulation was determined by quantitative PCR. Functional analyses were carried out by ferric reductase and ⁵⁹Fe uptake assays.

Results

Iron and dehydroascorbic acid treatment of cells inhibited Dcytb mRNA and protein expression. Desferrioxamine also enhanced Dcytb mRNA level after cells were treated overnight. Dcytb knockdown in HuTu cells resulted in reduced mRNA expression and lowered reductase activity. Preloading cells with DHA (to enhance intracellular ascorbate levels) did not stimulate reductase activity fully in Dcytb-silenced cells, implying a Dcytb-dependence of ascorbate-mediated ferrireduction. Moreover, Hif-2α knockdown in HuTu cells led to a reduction in reductase activity and iron uptake.

Conclusions

Taken together, this study shows the functional regulation of Dcytb reductase activity by DHA and Hif-2α.

General significance

Dcytb is a plasma membrane protein that accepts electrons intracellularly from DHA/ascorbic acid for ferrireduction at the apical surface of cultured cells and enterocytes.     

Evidence for iron channeling in the Fet3p-Ftr1p high-affinity iron uptake complex in the yeast plasma membrane

In high-affinity iron uptake in the yeast Saccharomyces cerevisiae, Fe(II) is oxidized to Fe(III) by the multicopper oxidase, Fet3p, and the Fe(III) produced is transported into the cell via the iron permease, Ftr1p. These two proteins are likely part of a heterodimeric or higher order complex in the yeast plasma membrane. We provide kinetic evidence that the Fet3p-produced Fe(III) is trafficked to Ftr1p for permeation by a classic metabolite channeling mechanism. We examine the (59)Fe uptake kinetics for a number of complexes containing mutant forms of both Fet3p and Ftr1p and demonstrate that a residue in one protein interacts with one in the other protein along the iron trafficking pathway as would be expected in a channeling process. We show that, as a result of some of these mutations, iron trafficking becomes sensitive to an added Fe(III) chelator that inhibits uptake in a strictly competitive manner. This inhibition is not strongly dependent on the chelator strength, however, suggesting that Fe(III) dissociation from the iron uptake complex, if it occurs, is kinetically slow relative to iron permeation. Metabolite channeling is a common feature of multifunctional enzymes. We constructed the analogous ferroxidase, permease chimera and demonstrate that it supports iron uptake with a kinetic pattern consistent with a channeling mechanism. By analogy to the Fe(III) trafficking that leads to the mineralization of the ferritin core, we propose that ferric iron channeling is a conserved feature of iron homeostasis in aerobic organisms.     

Adrenodoxin reductase homolog (Arh1p) of yeast mitochondria required for iron homeostasis

Arh1p is an essential mitochondrial protein of yeast with reductase activity. Here we show that this protein is involved in iron metabolism. A yeast strain was constructed in which the open reading frame was placed under the control of a galactose-regulated promoter. Protein expression was induced by galactose and repressed to undetectable levels in the absence of galactose, although cells grew quite well in the absence of inducer. Under noninducing conditions, cellular iron uptake was dysregulated, exhibiting a failure to repress in response to medium iron. Iron trafficking within the cell was also disturbed. Exposure of Arh1p-depleted cells to increasing iron concentrations during growth led to drastic increases in mitochondrial iron, indicating a loss of homeostatic control. Activity of aconitase, a prototype Fe-S protein, was deficient at all concentrations of mitochondrial iron, although the protein level was unaltered. Heme protein deficiencies were exacerbated in the iron-loaded mitochondria, suggesting a toxic side effect of accumulated iron. Finally, a time course correlated the cellular depletion of Arh1p with the coordinated appearance of various mutant phenotypes including dysregulated cellular iron uptake, deficiency of Fe-S protein activities in mitochondria and cytoplasm, and deficiency of hemoproteins. Thus, Arh1p is required for control of cellular and mitochondrial iron levels and for the activities of Fe-S cluster proteins.     

Ferric reductase activity of low molecular weight human milk fraction is associated with enhanced iron solubility and uptake in Caco-2 cells

It is known that the fractional absorption of extrinsic iron from human milk is higher in infants and adults. A low molecular weight milk fraction has been proposed to increase the bioavailability of iron from human milk. Nevertheless, the mechanisms remained elusive. Here in we demonstrate ferric reductase activity (Km 7.73 × 10⁻⁶ M) in low molecular weight human milk fraction (10kF, filtrate derived from ultra filtration of milk whey through 10 kDa cutoff membrane), which increased ferric iron solubility and iron uptake in Caco-2 cells. The 10kF fraction was as effective as ascorbic acid (1:20 iron to ascorbic acid) in increasing the ferric iron solubility and uptake in Caco-2 cells. Further, gel filtration chromatography on peptide column led to co-elution of ferric reductase and iron solubilization activities at an apparent molecular mass of <1500 Da. Interestingly, only these fractions containing ferric reductase activity also stimulated the uptake of iron in Caco-2 cells. Thus, it is concluded that human milk possesses ferric reductase activity and is associated with ferric iron solubilization and enhanced absorption.