The iron content of human liver and spleen isoferritins correlates with their isoelectric point and subunit composition

Myeloperoxidase-induced fibrinogen unfolding and clotting

Due to its unique properties and high biomedical relevance fibrinogen is a promising protein for the development of various matrixes and scaffolds for biotechnological applications. Fibrinogen molecules may form extensive clots either upon specific cleavage by thrombin or in thrombin-free environment, for example, in the presence of different salts. Here, we report the novel type of non-conventional fibrinogen clot formation, which is mediated by myeloperoxidase and takes place even at low fibrinogen concentrations (<0.1 mg/ml). We have revealed fibrillar nature of myeloperoxidase-mediated fibrinogen clots, which differ morphologically from fibrin clots. We have shown that fibrinogen clotting is mediated by direct interaction of myeloperoxidase molecules with the outer globular regions of fibrinogen molecules followed by fibrinogen unfolding from its natural trinodular to a fibrillar structure. We have demonstrated a major role of the Debye screening effect in regulating of myeloperoxidase-induced fibrinogen clotting, which is facilitated by small ionic strength. While fibrinogen in an aqueous solution with myeloperoxidase undergoes changes, the enzymatic activity of myeloperoxidase is not inhibited in excess of fibrinogen. The obtained results open new insights into fibrinogen clotting, give new possibilities for the development of fibrinogen-based functional biomaterials, and provide the novel concepts of protein unfolding.

The Loss of Myocardial Benefit following Ischemic Preconditioning Is Associated with Dysregulation of Iron Homeostasis in Diet-Induced Diabetes

Whether the diabetic heart benefits from ischemic preconditioning (IPC), similar to the non-diabetic heart, is a subject of controversy. We recently proposed new roles for iron and ferritin in IPC-protection in Type 1-like streptozotocin-induced diabetic rat heart. Here, we investigated iron homeostasis in Cohen diabetic sensitive rat (CDs) that develop hyperglycemia when fed on a high-sucrose/low-copper diet (HSD), but maintain normoglycemia on regular-diet (RD). Control Cohen-resistant rats (CDr) maintain normoglycemia on either diet. The IPC procedure improved the post-ischemic recovery of normoglycemic hearts (CDr-RD, CDr-HSD and CDs-RD). CDs-HSD hearts failed to show IPC-associated protection. The recovery of these CDs-HSD hearts following I/R (without prior IPC) was better than their RD controls. During IPC ferritin levels increased in normoglycemic hearts, and its level was maintained nearly constant during the subsequent prolonged ischemia, but decayed to its baseline level during the reperfusion phase. In CDs-HSD hearts the baseline levels of ferritin and ferritin-saturation with iron were notably higher than in the controls, and remained unchanged during the entire experiment. This unique and abnormal pattern of post-ischemic recovery of CDs-HSD hearts is associated with marked changes in myocardial iron homeostasis, and suggests that iron and iron-proteins play a causative role/s in the etiology of diabetes-associated cardiovascular disorders.

Oxidative stress and the homeodynamics of iron metabolism

Iron and oxygen share a delicate partnership since both are indispensable for survival, but if the partnership becomes inadequate, this may rapidly terminate life. Virtually all cell components are directly or indirectly affected by cellular iron metabolism, which represents a complex, redox-based machinery that is controlled by, and essential to, metabolic requirements. Under conditions of increased oxidative stress—i.e., enhanced formation of reactive oxygen species (ROS)—however, this machinery may turn into a potential threat, the continued requirement for iron promoting adverse reactions such as the iron/H2O2-based formation of hydroxyl radicals, which exacerbate the initial pro-oxidant condition. This review will discuss the multifaceted homeodynamics of cellular iron management under normal conditions as well as in the context of oxidative stress.

PAMAM G4 dendrimers as inhibitors of the iron storage properties of human L-chain ferritin

Cationic dendrimers, such as PAMAM, are known to be positively charged at neutral pH allowing their unspecific interaction with proteins and other cellular components.

Construction of homo- and heteropolymers of plant ferritin subunits using an in vitro protein expression system

Ferritin is a class of iron storage protein composed of 24 subunits. Although many studies on gene expression analyses of plant ferritin have been conducted, the functions and oligomeric assembly of plant ferritin subunits are still largely unknown. In order to characterize the ability to form multimeric protein shells and determine the iron incorporating activity, we produced ferritin homo- and heteropolymers by expressing four cDNAs of ferritin subunits from soybean, sfer1, sfer2, sfer3, and sfer4, using an in vitro protein expression system. Using SDS-PAGE analysis followed by Prussian blue stain, homopolymers of SFER1, SFER2, and SFER3, and heteropolymers of SFER1/SFER2 and SFER1/SFER3 were detected as assembled polymers with iron incorporating activity, whereas only a small amount of SFER4 related homo- and heteropolymer was detected, suggesting that the SFER4 was not competent for oligomeric assembly, unlike every other ferritin. We conclude that certain combinations of plant ferritin subunits can form heteropolymers and that their iron incorporating activities depend on the formation of multimeric protein.

Modification of ferritin during iron loading

Recombinant human ferritin loaded with iron via its own ferroxidase activity did not sediment through a sucrose-density gradient as a function of iron content. Analysis of the recombinant ferritin by native PAGE demonstrated an increase in altered migration pattern of the ferritins with increasing sedimentation, indicating an alteration of the overall charge of ferritin. Additionally, analysis of the ferritin by SDS-PAGE under nonreducing conditions demonstrated that the ferritin had formed large aggregates, which suggests disulfide bonds are involved in the aggregation. The hydroxyl radical was detected by electron spin resonance spectroscopy during iron loading into recombinant ferritin by its own ferroxidase activity. However, recombinant human ferritin loaded with iron in the presence of ceruloplasmin sedimented through a sucrose-density gradient similar to native ferritin. This ferritin was shown to sediment as a function of iron content. The addition of ceruloplasmin to the iron loading assay eliminated the detection of the DMPO-*OH adduct observed during loading using the ferroxidase activity of ferritin. The elimination of the DMPO-*OH adduct was determined to be due to the ability of ceruloplasmin to completely reduce oxygen to water during the oxidation of the ferrous iron. The implications of these data for the present models for iron uptake into ferritin are discussed.

Production of recombinant human apoferritin heteromers

We are interested in learning how iron is safely inserted and stored in ferritin. Recombinant DNA technology has considerable potential in determining the functional roles of the two ferritin subunits (H and L). In previous studies, we have observed that recombinant rat H ferritin was repressive to cell growth in both prokaryotic and eukaryotic expression systems (Guo et al., Biochem. Biophys. Res. Commun. 242, 39-45 (1998)). This results in the protein being expressed at very low levels. This problem was partially bypassed by the use of an inducible expression system, which utilizes T7 RNA polymerase dependent expression of the gene, induced by isopropyl beta-D-thiogalactopyranoside (IPTG). Simultaneously expressing the H and L ferritin genes in this system resulted in only a narrow range of ferritin heteromers, which predominantly consisted of the L subunit. Addition of rifampicin to cultures, 1 h following the induction of protein synthesis by IPTG, increased the production of the H subunit and thus increased the range of ferritin H:L subunit ratios. Simultaneous expression of the H and L ferritin genes in Escherichia coli grown in a deficient medium with minimal iron and with the addition of rifampicin resulted in the production of a range of recombinant human apoferritin heteromers that could be separated based on their subunit composition.

Iron and phosphate content of rat ferritin heteropolymers

An attempt was made to relate the iron and phosphate content of ferritin to its subunit composition. Ferritins from various tissues were separated according to their subunit composition by anion exchange chromatography and according to their iron content by density-gradient centrifugation. Iron and phosphate contents were not related to subunit composition. Recombinant rat liver ferritin heteropolymers of different subunit composition (1, 4, 6, 10, 15, and 17 H chains per 24 mer) were maximally loaded with iron, using ceruloplasmin and phosphate. All loaded approximately the same amount of iron and phosphate (2250 and 380 atoms, respectively). The iron and phosphate content of all ferritin, including the maximally loaded recombinant ferritin heteropolymers, fit an equation we previously reported: [Fe] = 4404 - 5.61 [Pi] (D. deSilva et al., 1993, Arch. Biochem. Biophys. 303, 451-455). These results suggest that the amount of iron and apparently the space within the core of ferritin were not related to different subunit composition.

FER-1, an enhancer of the ferritin H gene and a target of E1A-mediated transcriptional repression

Ferritin, the major intracellular iron storage protein of eucaryotic cells, is regulated during inflammation and malignancy. We previously reported that transcription of the H subunit of ferritin (ferritin H) is negatively regulated by the adenovirus E1A oncogene in mouse NIH 3T3 fibroblasts (Y. Tsuji, E. Kwak, T. Saika, S. V. Torti, and F. M. Torti, J. Biol. Chem. 268:7270-7275, 1993). To elucidate the mechanism of transcriptional repression of the ferritin H gene by E1A, a series of deletions in the 5' flanking region of the mouse ferritin H gene were constructed, fused to the chloramphenicol acetyltransferase (CAT) gene, and transiently cotransfected into NIH 3T3 cells with an E1A expression plasmid. The results indicate that the E1A-responsive region is located approximately 4.1 kb 5' to the transcription initiation site of the ferritin H gene. Further analyses revealed that a 37-bp region, termed FER-1, is the target of E1A-mediated repression. This region also serves as an enhancer, augmenting ferritin H transcription independently of position and orientation. FER-1 was dissected into two component elements, i.e., a 22-bp dyad symmetry element and a 7-bp AP1-like sequence. Insertion of these DNA sequences into a ferritin H-CAT chimeric gene lacking an E1A-responsive region indicated that (i) the 22-bp dyad symmetry sequence by itself has no enhancer activity, (ii) the AP1-like sequence has moderate enhancer activity which is repressed by E1A, and (iii) the combination of the dyad symmetry element and the AP1-like sequence is required for maximal enhancer activity and repression by E1A. Gel retardation assays and cotransfection experiments with c-fos and c-jun expression vectors suggested that members of the Fos and Jun families bind to the AP1-like element of FER-1 and contribute to its regulation. In addition, gel retardation assays showed that E1A reduces the ability of nuclear proteins to bind to the AP1-like sequence without affecting the levels of nuclear factors that recognize the 22-bp dyad symmetry element. Taken together, these results demonstrate that FER-1 serves as both an enhancer of ferritin H transcription and a target for E1A-mediated repression.

Iron-independent induction of ferritin H chain by tumor necrosis factor

Iron increases the synthesis of the iron-storage protein, ferritin, largely by promoting translation of preexisting mRNAs for both the H and L ferritin isoforms (H, heavy, heart, acidic; L, light, liver, basic). We have recently cloned and sequenced a full-length cDNA to murine ferritin H and identified ferritin H as a gene induced by tumor necrosis factor alpha (TNF-alpha, cachectin). Using primary human myoblasts, we have now examined the relationship between TNF-alpha and iron in regulating ferritin. Four lines of evidence suggest that TNF-alpha regulates ferritin independently of iron. First, evaluation of mRNA showed that TNF-alpha increased ferritin H chain specifically, provoking no change in steady-state levels of ferritin L mRNA; iron, in contrast, increased the mRNA of both isoforms. Second, the increase in ferritin H protein synthesis observed during TNF-alpha treatment was dependent on an increase in ferritin H mRNA: actinomycin D blocked the TNF-alpha-induced changes in ferritin H but did not inhibit the translational induction of ferritin seen with iron treatment. Third, equal ferritin mRNA induction was observed in iron-loaded cells and in cells depleted of iron by a permeant chelator, 2,2'-dipyridyl. Fourth, ferritin H induction by TNF-alpha and iron was additive over the entire range of iron concentrations, even at TNF-alpha doses known to maximally stimulate ferritin H mRNA levels. Nonetheless, the role of iron in translational regulation of ferritin was retained in TNF-alpha-treated cells; effective biosynthesis of TNF-alpha-induced, H-subunit-predominant ferritin protein required iron and could be enhanced by treatment of the cells with additional iron or blocked by 2,2'-dipyridyl. Finally, we observed that the TNF-alpha-mediated increase in ferritin synthesis peaked at 8 hr and was followed by a decrease in both H and L isoferritin synthesis; the addition of iron, however, reversed the late-occurring depression in ferritin synthesis. This suggests that TNF-alpha-induced synthesis of H-rich ferritin may reduce the regulatory pool of intracellular iron, secondarily inhibiting iron-mediated translation of ferritin mRNA. We conclude that TNF-alpha acts independently of iron in its induction of ferritin H mRNA but requires the presence of iron for this effect to be fully expressed at the protein level.

Beta-thalassaemia/haemoglobin E tissue ferritins. I: Purification and partial characterization of liver and spleen ferritins

Ferritins from liver and spleen of both beta-thalassaemia/haemoglobin E (HbE) and non-thalassaemic patients were purified by heating a methanol-treated homogenate, followed by molecular exclusion chromatography. The concentrations of ferritins in the beta-thalassaemia/HbE liver and spleen were calculated as 3.8 and 2.0 mg/g wet tissue. The beta-thalassaemia/HbE ferritin iron/protein ratios were higher than those of normal ferritins. On PAGE, all ferritins gave a single major monomeric band with only very small differences in their mobility. Ferritins from thalassaemic patients also possessed bands corresponding to oligomers. On SDS/PAGE, all ferritins were resolved into two major subunits: H and L with L subunit predominating. While the isoferritin profiles of ferritins from beta-thalassaemia/HbE liver and spleen were similar to each other and to those of normal liver and spleen, some extra bands were present in the acidic region. The microstructure of these pathological ferritins appears to result, to a large degree, from the particular nature and amount of iron loading present.

Structure and expression of ferritin genes in a human promyelocytic cell line that differentiates in vitro

HL-60 is a human promyelocytic cell line with the capability of differentiating in vitro to give neutrophils, macrophages, or eosinophils. We screened libraries of HL-60 cDNA clones representing different time points during these differentiation processes to isolate clones corresponding to mRNAs whose expression is regulated during terminal differentiation. Upon sequencing this group of regulated clones, one clone encoding the heavy subunit and two clones encoding the light subunit of human ferritin were identified by reference to published amino acid sequences. Southern blot analyses showed that these clones are encoded by distinct multigene families. These clones identify two mRNAs whose ratios vary in a complex manner during both neutrophil and macrophage differentiation.

mRNA species regulated during the differentiation of HL-60 cells to macrophages and neutrophils

Using cDNA clone banks from differentiated and undifferentiated HL-60 promyelocytic leukemia cells, we have selected clones for genes which are regulated during this differentiation. Regulation of the corresponding mRNAs in HL-60 cells during both monocytic and neutrophilic differentiation was measured for 21 of these clones. The levels of mRNA hybridizing to some of these clones changed by more than 100-fold during differentiation. Unlike erythropoiesis or myogenesis, in which the synthesis of a few new proteins is synchronously regulated, mRNAs in differentiating HL-60 cells are asynchronously regulated, suggesting a complex series of regulatory events. About half of these regulation-selected clones contained repeat sequences, including both Alu and novel repeat families. Most of the regulated genes are members of extensive gene families.

Changes in sample isoferritin composition as a possible cause of dilutional discrepancies in ferritin 2-site immunoradiometric assay results

The effect of changes in sample isoferritin composition on the behaviour of ferritin 2-site immunoradiometric assays was investigated using a computer model and experimental studies. Modelling studies predicted that under conditions where a sample has a different isoferritin composition from the assay standards, progressively higher values will be generated for the apparent ferritin content of the sample when it is analysed at increasing dilutions. In addition, the assay results will underestimate the actual ferritin content of such samples at all dilutions. Dilutional discrepancies in assay results and underestimation of sample ferritin concentrations were found in practical assays when synthetic samples with demonstrably different isoferritin profiles were analysed. Differences in isoferritin composition between assay standards and assayed samples may therefore be a cause of dilutional discrepancies in some results from ferritin 2-site immunoradiometric assays.

Structure and expression of ferritin genes in a human promyelocytic cell line that differentiates in vitro

HL-60 is a human promyelocytic cell line with the capability of differentiating in vitro to give neutrophils, macrophages, or eosinophils. We screened libraries of HL-60 cDNA clones representing different time points during these differentiation processes to isolate clones corresponding to mRNAs whose expression is regulated during terminal differentiation. Upon sequencing this group of regulated clones, one clone encoding the heavy subunit and two clones encoding the light subunit of human ferritin were identified by reference to published amino acid sequences. Southern blot analyses showed that these clones are encoded by distinct multigene families. These clones identify two mRNAs whose ratios vary in a complex manner during both neutrophil and macrophage differentiation.

Structure of human ferritin light subunit messenger RNA: comparison with heavy subunit message and functional implications

Ferritin has a protein shell of 5 X 10(6) Da consisting of 24 subunits of two types, a heavier (H) chain of 21,000 Da and a lighter (L) chain of 19,000 Da. A cDNA clone of the messenger for the L subunit has been isolated from a human monocyte-like leukemia cell line. The clone contains an open reading frame of 522 nucleotides coding for an amino acid sequence matching 97% of the published sequence of human liver ferritin L subunit determined by sequenator, but it corresponds to only 55% of the reported amino acid sequence of a human liver H-subunit clone. Nevertheless, computer analysis of the subunit conformations predicted from the open reading frames of the L and H clones shows that most of the amino acid differences are conservative and would allow both subunits to form the five alpha-helices and beta-turns established by x-ray crystallography for horse spleen ferritin subunits. This suggests that L and H subunits are structurally interchangeable in forming an apoferritin shell. The 5' untranslated region of our human ferritin L clone has considerable homology with that of the rat liver ferritin L clone in the region immediately upstream from the initiator codon, notably showing an identical sequence of 10 nucleotides at the same position in both subunit clones that may participate in regulating the known activation of ferritin mRNA after iron administration. Extensive homology, including several blocks of nucleotides, was identified between the 3' untranslated regions of the human and rat L clones. The common structural features of the H and L subunits lead us to conclude that they have diverged from a single ancestral gene.

Isolation and fractionation of ferritin from human term placenta--a source for human isoferritins

A method for isolating ferritin from human term placenta was described. The placenta was homogenized in water containing protease inhibitor and heated at 70 degrees C. The ferritin was precipitated with ammonium sulfate at pH 5.2 and purified by repeated cycles of ultracentrifugation and molecular sieve chromatography through Sepharose 4B columns. Isoelectric focusing revealed a broad spectrum of isoferritins. These isoferritins were separated by ion-exchange chromatography on Sephadex A-25 at pH 7.5 and stepwise elution with increasing concentrations of NaCl. By this method "basic," "intermediate," and "acid" isoferritins were separated. The most basic placental isoferritin was shown to be identical to splenic ferritin by isoelectric focusing, subunit analysis, and fluorescent ELISA. The acid placental isoferritin had similar characteristics to heart-type ferritin. It was suggested that the easily available placental tissue could serve as a source for human isoferritins in research and in clinical assays.

A comparison by HPLC of ferritin subunit types in human tissues

Ferritin was isolated from human liver and spleen. Reversed phase high performance liquid chromatography of the ferritin subunits from each tissue yielded the same three chromatographic fractions. Physical and chemical characterization of the three fractions indicated that they represented at least two, perhaps three, chemically distinct subunits.

Iron and phosphorus content of rabbit liver ferritin fractions with different subunit composition

After isolation of subtypes of rabbit liver-ferritin, the phosphorus to iron ratio (P/Fe-ratio) in the samples was found to parallel the shift in subunit composition of the two types of subunits of which ferritins generally consist. No relation was found with the amount of iron per ferritin molecule. The increase of the P/Fe-ratio, in relation to subunit composition, is postulated to be a result of the change of total surface area of the iron microcrystallites inside the ferritin molecule. This surface area depends on the number of nucleation points and thereby may be dependent on the subunit composition.

Influence of heat treatment on rabbit liver ferritin

Ferritin was purified from rabbit livers either by heat treatment and immunoaffinity chromatography, or by immunoaffinity chromatography alone. The immunoreactivity of ferritin with antibodies raised against heat-treated ferritin was significantly higher for heat-treated preparations than for non-heated preparations. The amount of ferritin protein could be estimated with equal reliability by the assay according to Lowry et al. and by nitrogen determination. Heat treatment favoured the L-subunit-rich ferritin fraction, as measured by densitometric scanning of SDS gradient-pore polyacrylamide gels. Amino acid analysis showed small changes in the amounts of valine, isoleucine and histidine in the heat-treated ferritin, possibly due to selective partial degradation of H-subunit-rich forms of ferritin. These results illustrate that heat treatment, which is a commonly used step in most purification procedures, induces partial denaturation of the ferritin molecules.

Changes in the characteristics and distribution of ferritin in iron-loaded cell cultures

When Chang liver cells are grown in an iron-rich medium for up to 20 weeks, iron loading up to 50 times the normal cellular iron content may be obtained, although ferritin increases only to about 10 times normal. Ferritin has been isolated from such cells, and the isoferritin pattern found on elution from DEAE-Sephadex A-50 by increasing chloride concentrations has been used as a basis for studying changes in the properties of ferritin under conditions of cellular loading. A consistent shift of peak ferritin-elution position to higher chloride concentrations (lower pI) occurs when cells are loaded with ferric nitrilotriacetate for increasing lengths of time. A change in immunoreactivity also takes place on loading, the ratio of ferritin reacting with heart and spleen ferritin antibodies increasing at any particular value of pI. Cells were pulse-labelled with [59Fe]ferric nitrilotriacetate and [3H]leucine followed by non-radioactive iron in the same form. During the 72 h after the synthesis of new protein and its incorporation of iron, there is a slight acid shift in its isoelectric point. This effect is seen in both normal and loaded cells, with the whole spectrum being shifted towards lower pI in the loaded state. These findings suggest that the shift to more acidic ferritins on iron loading and the associated changes in antigenicity may be unrelated to subunit composition.

A biochemical comparison of normal human liver and hepatocellular carcinoma ferritins

1. The iron contents, gel migration rates and isoelectric-focusing patterns of normal liver and hepatocellular carcinoma ferritins from the same patients were compared. 2. Sucrose-density-gradient centrifugation showed that the number of iron atoms per ferritin molecule was decreased to approximately half in carcinoma tissue when compared with normal liver. 3. On electrophoresis, hepatocellular carcinoma ferritin migrates faster and is therefore more negatively charged than normal liver ferritin, thus refuting the general view that the more negatively charged a ferritin molecule the greater its iron content. 4. Comparison of tumour and normal liver ferritin subunit compositions on acid/urea/polyacrylamide gels showed hepatocellular carcinoma ferritin to contain an additional, more negatively charged, subunit to normal liver ferritin. 5. Isoelectric focusing showed that hepatocellular carcinoma tissue contains isoferritins with isoelectric points intermediate between the ranges of normal liver and normal heart isoferritins.

Characterization of ferritin from human placenta. Implications for analysis of tissue specificity and microheterogeneity of ferritins

Mammalian ferritins can be resolved into multiple components by isoelectric focusing, and each tissue contains a characteristic subset of isoferritins. Ferritin isolated from human liver was compared to acidic ferritin isolated from mid-gestational human placenta to define a structural basis for ferritin heterogeneity. Placenta ferritin contained several major bands with isoelectric points in the range of pI = 4.7-5.0 which were more acidic than the predominant isoferritins of human liver. Ferritin from each tissue was resistant to denaturation by 10 M urea and appeared to be identical by electron microscopy. Circular dichroism measurements revealed that placenta ferritin had substantially less ordered secondary structure than liver ferritin. Both types of ferritin contained only two subunits when analyzed by electrophoresis in sodium dodecyl sulfate gels, but isoelectric focusing of dissociated subunits in urea revealed 6-7 different components. In this system, placenta ferritin was enriched in the more acidic subunits and it completely lacked the most basic subunits noted in liver ferritin; placental ferritin had no unique components. Differences in isoelectric points among assembled ferritins from these two tissues appear to result from different proportions of these acidic and basic subunits.