Characterization of human ferritin H chain synthetized in Escherichia coli

Ferritin microheterogeneity, subunit composition, functional, and physiological implications

Ferritin is a ubiquitous intracellular iron storage protein that plays a crucial role in iron homeostasis. Animal tissue ferritins consist of multiple isoforms (or isoferritins) with different proportions of H and L subunits that contribute to their structural and compositional heterogeneity, and thus physiological functions. Using size exclusion and anion exchange chromatography, capillary isoelectric focusing (cIEF), and SDS-capillary gel electrophoresis (SDS-CGE), we reveal for the first time a significant variation in ferritin subunit composition and isoelectric points, in both recombinant and native ferritins extracted from animal organs. Our results indicate that subunits composition is the main determinant of the mean pI of recombinant ferritin heteropolymers, and that ferritin microheterogeneity is a common property of both natural and recombinant proteins and appears to be an intrinsic feature of the cellular machinery during ferritin expression, regulation, post-translational modifications, and post-subunits assembly. The functional significance and physiological implications of ferritin heterogeneity in terms of iron metabolism, response to oxidative stress, tissue-specific functions, and pathological processes are discussed.

Newly uncovered biochemical and functional aspects of ferritin

Iron homeostasis is strictly regulated at both the systemic and cellular levels by complex mechanisms because of its indispensability and toxicity. Among the various iron-regulatory proteins, ferritin is the earliest discovered regulator of iron metabolism and is a molecule that safely retains excess intracellular iron in the cores of its shells. Two types of ferritin, cytosolic ferritin and mitochondrial ferritin (FTMT), have been identified in a range of organisms from plants to humans. FTMT was identified approximately 60 years after the discovery of cytosolic ferritin. Cytosolic ferritin expression is regulated in an iron-responsive manner. Recently, the molecular mechanisms of iron-dependent degradation of cytosolic ferritin or its secretion into serum have been clarified. FTMT, which shares a high degree of sequence homology with cytosolic ferritin, has distinct functions and is regulated in different ways from cytosolic ferritin. Although knowledge of the physiological role of FTMT is still incomplete, recent studies have shed light on the function and regulation of FTMT. The accumulating biological evidence of both ferritins has made it possible to deepen our knowledge about iron metabolism and its significance in diseases. In this review, we discuss the biological properties of both ferritins, focusing on their newly uncovered behaviors.

Cellular binding analysis of recombinant hybrid heteropolymer of camel hepcidin and human ferritin H chain. The unexpected human H-ferritin binding to J774 murine macrophage cells

Ferritin is a molecule with enormous potentiality in biotechnology that have been already used to encapsulate molecules, as contrast in magnetic resonance imaging and to carry epitopes. We proposed to use it to carry another key protein of iron metabolism, hepcidin that is a small hormone peptide that control systemic iron homeostasis. In this work, we purified the previously produced camel hepcidin and human H-ferritin heteropolymer (HepcH-FTH) and to monitor its binding capability toward J744 cell line in presence or absence of ferric ammonium citrate. Fused camel hepcidin and human H-ferritin monomer (HepcH) as well as the assembled HepcH-FTH heteropolymer (ratio 1:5) was easily purified by a one-step purification using size exclusion chromatography. SDS-PAGE electrophoresis of HepcH, purified from soluble and insoluble fractions, showed a single band of 24 kDa with an estimated purity of at least 90%. The purification yields of HepcH from the soluble and insoluble fractions was, respectively, of about 6.80 and 2 mg/L of bacterial culture. Time curse cellular binding assays of HepcH-FTH revealed its great potential to bind the J774 cells after 15 min of incubation. Furthermore, HepcH-FTH was able to degrade ferroportin, the unique hepcidin receptor, even after 30 min of incubation with J774 cells treated with 100 µM ferric ammonium citrate. In conclusion, we proposed ferritin as a peptide carrier to promote the association of the hybrid HepcH-FTH nanoparticle with a particular type of cell for therapeutic or diagnostic.

Molecular cloning, expression and characterization of secreted ferritin in the silkworm, Bombyx mori

Ferritin is a ubiquitous iron storage protein which plays key role in regulating iron homeostasis and metabolism. In this paper, the ferritin heavy chain homologs (HCH) and light chain homologs (LCH) from Bombyx mori (BmFerHCH and BmFerLCH) were amplified through PCR and cloned into the expression vector pET-30a(+). The recombinant BmFerHCH and BmFerLCH expressed in Escherichia coli were in the form of insoluble inclusion bodies, indicating that the two proteins were not in their natural structural conformation. In order to obtain refolded ferritin in vitro, the inclusion bodies (BmFerHCH and/or BmFerLCH) were dissolved in denaturing buffer (100 mM Tris, 50 mM Glycine, 8 M urea, 5 mM DTT, pH 8.0) and then refolded in refolding buffer (100 mM Tris, 400 mM L-arginine, 0.2 mM PMSF, 0.5 mM DTT). The result showed that it was only when both BmFerHCH and BmFerLCH were present together in the denaturing buffer that refolding was successful and resulted in the formation of heteropolymers (H-L chain dimers) over homopolymers (H-H chain or L-L chain dimers). Moreover, the molecules (NaCl, Triton and glycerol) were found to enhance protein refolding. The optimum temperature, pH and ratios of BmFerHCH/BmFerLCH required for refolding were found to be 10 °C, pH 7, 1:1 or 1:2, respectively. Finally, the refolded ferritin had the ability to store iron, exhibited ferroxidase activity, and could withstand high temperatures and pH treatment, which is consistent with ferritin in other species.

The extension peptide of plant ferritin from sea lettuce contributes to shell stability and surface hydrophobicity

Plant ferritins have some unique structural and functional features. Most of these features can be related to the plant-specific "extension peptide" (EP), which exists in the N-terminus of the mature region of a plant ferritin. Recent crystallographic analysis of a plant ferritin revealed the structure of the EP, however, two points remain unclear: (i) whether the structures of well-conserved EP of plant ferritins are common in all plants, and (ii) whether the EP truly contributes to the shell stability of the plant ferritin oligomer. To clarify these matters, we have cloned a green-plant-type ferritin cDNA from a green alga, Ulva pertusa, and investigated its crystal structure. Ulva pertusa ferritin (UpFER) has a plant-ferritin-specific extension peptide composed of 28 amino acid residues. In the crystal structure of UpFER, the EP lay on and interacted with the neighboring threefold symmetry-related subunit. The amino acid residues involved in the interaction were very highly conserved among plant ferritins. The EPs masked the hydrophobic pockets on the ferritin shell surface by lying on them, and this made the ferritin oligomer more hydrophilic. Furthermore, differential scanning calorimetric analysis of the native and its EP-deletion mutant suggested that the EP contributed to the thermal stability of the plant ferritin shell. Thus, the shell stability and surface hydrophobicity of plant ferritin were controlled by the presence or absence of the plant-ferritin-specific EP. This regulation can account for those processes such as shell stability, degradation, and association of plant ferritin, which are significantly related to iron utilization in plants.

Treatment of human bone marrow with recombinant placenta immunoregulator ferritin results in myelopoiesis and T-cell suppression through modulation of the cytokine-chemokine networks

OBJECTIVE

Placenta immunomodulator ferritin (PLIF) is a cloned human chimeric ferritin H chain with a novel non-ferritin C-terminal 48 amino acid sequence (C48). Recombinant PLIF-C48 exhibited cell-mediated immunosuppression. The aim of the current study was to investigate the regulatory effects of native placental ferritin (PLF), recombinant PLIF, and C48 on hematopoiesis of human bone marrow (BM).

METHODS

BM mononuclear cells (BM-MNCs) and CD34(+) selected cells were treated in vitro with either PLF, PLIF, or C48 without and in combination with granulocyte (G)-monocyte (M) colony-stimulating factor (GM-CSF) and subjected to hematopoietic progenitor cell assay. Cytokines and chemokines secreted by the treated cells were evaluated in culture supernatant using antibody array assays to determine mechanism of action.

RESULTS

In vitro treatment of BM-MNCs with PLF, PLIF, or C48 induced significant growth of myeloid colonies and when mixed with GM-CSF or Granulocyte-Colony Stimulating Factor (G-CSF) exhibited additive enhanced colony forming units-granulocyte monocyte growth. Yet, C48 treatment of selected CD34(+) cells did not yield colony formation and did not affect their response to GM-CSF. Treatment of BM-MNCs with C48 for 48 hours induced secretion of marked levels of GM-CSF, interleukin (IL)-6, IL-1, and IL-10. These cytokines were secreted primarily by C48-treated BM adherent cells and partly by nonadherent cells, whereas the CD34(+) selected cells secreted IL-6 only.

CONCLUSION

C48-PLIF enhancement of myelopoiesis resulted from cross talk between BM accessory cells and progenitor cells. The differential PLIF-C48 effects (i.e., myeloid progenitor cell growth and T-cell suppression) are due to their effect on the cytokine-chemokine networks.

A highly thermostable ferritin from the hyperthermophilic archaeal anaerobe Pyrococcus furiosus

A ferritin from the obligate anaerobe and hyperthermophilic archaeon Pyrococcus furiosus (optimal growth at 100 degrees C) has been cloned and overproduced in Escherichia coli to one-fourth of total cell-free extract protein, and has been purified in one step to homogeneity. The ferritin (PfFtn) is structurally similar to known bacterial and eukaryal ferritins; it is a 24-mer of 20 kDa subunits, which add up to a total Mr 480 kDa. The protein belongs to the non-heme type of ferritins. The 24-mer contains approximately 17 Fe (as isolated), 2,700 Fe (fully loaded), or <1 Fe (apoprotein). Fe-loaded protein exhibits an EPR spectrum characteristic for superparamagnetic core formation. At 25 degrees C V(max) = 25 micromole core Fe(3+) formed per min per mg protein when measured at 315 nm, and the K(0.5) = 5 mM Fe(II). At 0.3 mM Fe(II) activity increases 100-fold from 25 to 85 degrees C. The wild-type ferritin is detected in P. furiosus grown on starch. PfFtn is extremely thermostable; its activity has a half-life of 48 h at 100 degrees C and 85 min at 120 degrees C. No apparent melting temperature was found up to 120 degrees C. The extreme thermostability of PfFtn has potential value for biotechnological applications.

Kinetic studies of iron deposition catalyzed by recombinant human liver heavy, and light ferritins and Azotobacter vinelandii bacterioferritin using O2 and H2O2 as oxidants

The discrepancy between predicted and measured H(2)O(2) formation during iron deposition with recombinant heavy human liver ferritin (rHF) was attributed to reaction with the iron protein complex [Biochemistry 40 (2001) 10832-10838]. This proposal was examined by stopped-flow kinetic studies and analysis for H(2)O(2) production using (1) rHF, and Azotobacter vinelandii bacterial ferritin (AvBF), each containing 24 identical subunits with ferroxidase centers; (2) site-altered rHF mutants with functional and dysfunctional ferroxidase centers; and (3) recombinant human liver light ferritin (rLF), containing no ferroxidase center. For rHF, nearly identical pseudo-first-order rate constants of 0.18 s(-1) at pH 7.5 were measured for Fe(2+) oxidation by both O(2) and H(2)O(2), but for rLF, the rate with O(2) was 200-fold slower than that for H(2)O(2) (k = 0.22 s(-1)). A Fe(2+)/O(2) stoichiometry near 2.4 was measured for rHF and its site altered forms, suggesting formation of H(2)O(2). Direct measurements revealed no H(2)O(2) free in solution 0.5-10 min after all Fe(2+) was oxidized at pH 6.5 or 7.5. These results are consistent with initial H(2)O(2) formation, which rapidly reacts in a secondary reaction with unidentified solution components. Using measured rate constants for rHF, simulations showed that steady-state H(2)O(2) concentrations peaked at 14 muM at approximately 600 ms and decreased to zero at 10-30 s. rLF did not produce measurable H(2)O(2) but apparently conducted the secondary reaction with H(2)O(2). Fe(2+)/O(2) values of 4.0 were measured for AvBF. Stopped-flow measurements with AvBF showed that both H(2)O(2) and O(2) react at the same rate (k = 0.34 s(-1)), that is faster than the reactions with rHF. Simulations suggest that AvBF reduces O(2) directly to H(2)O without intermediate H(2)O(2) formation.

Characteristics of trapping copper ions with scrolled ferritin reactor in the flowing seawater

Native liver ferritin of Dasyatis akajei (DALF), apoDALF, and reconstituted DALF were employed to construct a ferritin reactor, respectively. An apparatus consisting of a mixer, a ferritin reactor, and a magnetic stirrer was constructed to study capacity and feasibility of trapping Cu2+ in the flowing seawater. The experimental results showed that the numbers of trapping Cu2+ with DALF reactor were higher than these with the reactors of apoDALF and reconstituted DALF, respectively, giving the maximal numbers of 98 +/- 5 Cu2+ per molecular DALF in 120 h. We found that the iron layer with a high ratio of phosphate to ion on the surface of the ferritin core played an important role in increasing numbers of trapping Cu2+. In addition, we found two positive relations of dependence of trapping Cu2+ numbers with the reactor on the incubation time and on the Cu2+ concentration in the flowing seawater. Another apparatus consisting of a buoyage, an isolation basket equipped with griddling, and a scrolled ferritin reactor was constructed to study the feasibility of trapping Cu2+ in the sea area. Moreover, the present studies indicated that this apparatus had been used to not only analyze and evaluate the concentration variety of various heavy metal ions such as Cu2+ and Pb2+ diluting by the seawater but also monitor the formation of pollution degree by various small organic molecules during the climax and the neap.

Human mitochondrial ferritin expressed in HeLa cells incorporates iron and affects cellular iron metabolism

Mitochondrial ferritin (MtF) is a newly identified ferritin encoded by an intronless gene on chromosome 5q23.1. The mature recombinant MtF has a ferroxidase center and binds iron in vitro similarly to H-ferritin. To explore the structural and functional aspects of MtF, we expressed the following forms in HeLa cells: the MtF precursor (approximately 28 kDa), a mutant MtF precursor with a mutated ferroxidase center, a truncated MtF lacking the approximately 6-kDa mitochondrial leader sequence, and a chimeric H-ferritin with this leader sequence. The experiments show that all constructs with the leader sequence were processed into approximately 22-kDa subunits that assembled into multimeric shells electrophoretically distinct from the cytosolic ferritins. Mature MtF was found in the matrix of mitochondria, where it is a homopolymer. The wild type MtF and the mitochondrially targeted H-ferritin both incorporated the (55)Fe label in vivo. The mutant MtF with an inactivated ferroxidase center did not take up iron, nor did the truncated MtF expressed transiently in cytoplasm. Increased levels of MtF both in transient and in stable transfectants resulted in a greater retention of iron as MtF in mitochondria, a decrease in the levels of cytosolic ferritins, and up-regulation of transferrin receptor. Neither effect occurred with the mutant MtF with the inactivated ferroxidase center. Our results indicate that exogenous iron is as available to mitochondrial ferritin as it is to cytosolic ferritins and that the level of MtF expression may have profound consequences for cellular iron homeostasis.

Regulation, mechanisms and proposed function of ferritin translocation to cell nuclei

Ferritin is traditionally considered a cytoplasmic iron-storage protein,but recent reports indicate that it is also found in cell nuclei. Nuclear ferritin has been proposed to be involved in both the protection of DNA and the exacerbation of iron-induced oxidative damage to DNA. We demonstrate that H-rich ferritin is present in the nucleus of human astrocytoma tumor cells. To study the mechanism and regulation of ferritin translocation to the nucleus,we developed a cell culture model using SW1088 human astrocytoma cells. Changes in cellular iron levels, cytokine treatments and hydrogen peroxide exposure affected the distribution of ferritin between the cytosol and the nucleus. Ferritin enters the nucleus via active transport through the nuclear pore and does not require NLS-bearing cytosolic factors for transport. Furthermore, H-rich ferritin is preferred over L-rich ferritin for uptake into the nucleus. Whole cell crosslinking studies revealed that ferritin is associated with DNA. Ferritin protected DNA from iron-induced oxidative damage in both in vitro and in cell culture models. These results strongly suggest a novel role for ferritin in nuclear protection. This work should lead to novel characterization of ferritin functions in the context of genomic stability and may have unparalleled biological significance in terms of the accessibility of metals to DNA. The knowledge generated as a result of these studies will also improve our understanding of iron-induced damage of nuclear constituents.

Heavy chain ferritin activates regulatory T cells by induction of changes in dendritic cells

Heavy chain ferritin (H-ferritin) is a component of the iron-binding protein, ferritin. We have previously shown that H-ferritin inhibits anti-CD3-stimulated lymphocyte proliferation and that this was due to increased production of interleukin-10 (IL-10). In the present study we have shown that induction of IL-10 production was due to effects of H-ferritin on adherent antigen-presenting cells (APCs) in blood and monocyte-derived dendritic cells (MoDCs). IL-10 was produced by a subpopulation of CD4 T cells, which expressed the CD25 component of the IL-2 receptor and the CTLA-4 receptor characteristic of regulatory T cells. The changes induced in MoDCs were compared with those induced by CD40L and their significance tested by inhibition with monoclonal antibodies. These studies indicated that H-ferritin induced relatively greater expression of CD86 and B7-H1 on MoDCs and that monoclonal antibodies against their receptors, CTLA-4 and programmed death receptor-1 (PD-1), inhibited IL-10 production from the regulatory T cells. H-ferritin did not appear to induce direct production of the cytokines IL-2, IL-4, IL-6, IL-10, IL-12, or interferon-gamma from the DCs. These results are consistent with the thesis that H-ferritin induces B7-H1 and CD86 (B7-2) on APCs, which in turn induce IL-10 production from regulatory T cells. This is possibly one mechanism by which melanoma cells may induce changes in APCs in the vicinity of the tumor and result in suppression of immune responses by induction of regulatory T cells.

Characterization of the H- and L-subunit ratios of ferritins by sodium dodecyl sulfate-capillary gel electrophoresis

Sodium dodecyl sulfate-capillary gel electrophoresis (SDS-CGE) was used to characterize the H- and L-subunit ratios of several mammalian ferritins and one bacterioferritin. Traditionally, SDS-PAGE has been used to characterize the H- and L-subunit ratios in ferritin; however, this technique is relatively slow and requires staining, destaining, and scanning before the data can be processed. In addition, the H- and L-subunits of ferritin are fairly close in molecular weight (approximately 21,000 and approximately 20,000, respectively) and are often difficult to resolve in SDS-PAGE slab gels. In contrast, SDS-CGE requires no staining or destaining procedures and the peak quantitation is superior to SDS-PAGE. SDS-CGE is effective in quickly resolving the H- and L-subunits of ferritins from horse spleen, human liver, recombinant human H and L homopolymers, and mixtures of the two- and the single-subunit of a bacterioferritin from Escherichia coli. The technique has also proven useful in assaying the quality of the protein sample from both commercial and recombinant sources. Significant amounts of low-molecular-weight degradation products were detected in all commercial sources of horse spleen ferritin. Most commercial horse spleen ferritins lacked intact H-subunits under denaturing conditions.

Relationship between TNF-alpha and iron metabolism in differentiating human monocytic THP-1 cells

The human monocytic cell line THP-1 differentiates along the macrophage line after phorbol-12-myristate-13-acetate (PMA) supplementation and can be stimulated to secrete tumour necrosis factor alpha (TNF-alpha) by interferon gamma (IFN-gamma) addition. We found that, in the early stage of differentiation (1-48 h), PMA induction elicited an upregulation of intracellular H ferritin and H ferritin binding sites and a downregulation of transferrin receptor. In addition, we found that iron administration to PMA-differentiating cells induced the expression of TNF-alpha mRNA and TNF-alpha secretion to levels even higher than those induced by IFN-gamma alone. The iron chelator desferrioxamine showed the opposite effect and reduced TNF-alpha release. In contrast, preincubation of the cells with iron before PMA induction resulted in a decrease of the TNF-alpha secretion induced by IFN-gamma, whereas the opposite was true after preincubation with desferrioxamine. The data support a co-ordinate interaction between iron and TNF-alpha in monocyte macrophages, with an iron-mediated upregulation of TNF-alpha in the early phase of differentiation and an iron-mediated inhibition at later stages. This complex relationship has to be considered in evaluating the effects of iron on inflammation.

Oligodendrocyte progenitor cells internalize ferritin via clathrin-dependent receptor mediated endocytosis

We previously demonstrated ferritin binding is specific to white matter in mouse and human brain tissue and is not found within Multiple Sclerotic plaques. These results suggest that ferritin receptors are selectively expressed on oligodendrocytes. The present studies were designed to test the hypothesis that oligodendrocyte progenitor cells selectively bind ferritin and internalize it by methods consistent with receptor-mediated endocytosis. Using a cell culture system enriched for oligodendrocyte progenitor cells, we determined, that oligodendrocyte progenitor cells bind ferritin in a saturable and competitive manner with a K(d) of 5 nM and a receptor density of 0.06 fmol bound/20,000 cells. FITC tagged ferritin is internalized by A2B5, O4 or CNPase expressing cells in the culture, but not by GFAP+ cells. The uptake of ferritin into the oligodendrocyte progenitors was inhibited by treating the cells with inhibitors of receptor mediated endocytosis (hypertonic medium, potassium deficient medium, ATP depletion, sulfhydryl reagents). In addition exogenous ferritin decreased iron responsive element/iron regulatory protein binding indicating that the iron within the internalized ferritin is released and contributes to the intracellular iron pool. Given the relatively high amount of iron that can be delivered via ferritin, and the selective distribution of ferritin receptors in the white matter tracts in vivo, we propose that ferritin is a major source of iron for oligodendrocytes.

Molecular diffusion into ferritin: pathways, temperature dependence, incubation time, and concentration effects

The detailed kinetics of permeation and effusion of small nitroxide spin probe radicals with the protein shells of horse spleen ferritin (HoSF) and human H-chain ferritin (HuHF) and a 3-fold channel variant D131H+E134H of HuHF were studied by electron paramagnetic resonance spectroscopy and gel permeation chromatography under a variety of experimental conditions. The results confirm that the permeation of molecular species of 7-9-A diameter into ferritin is a charge selective process and that the threefold channels are the likely pathways for entry into the protein. Studies with holoHoSF show that increased temperature increases the rates of penetration and effusion and also increases the concentration of positively charged spin probe accumulated within the protein in excess of that in the external solution. The interior of HoSF is much more accessible to small molecules at physiological temperature of approximately 40 degrees C than at room temperature. The large activation energy of 63-67 kJ/mol measured for the effusion/penetration and the small diffusion coefficient, D approximately 5 x 10(-22) m(2)/s at 20 degrees C, corresponding to a time of approximately 60 min for traversing the protein shell, is consistent with the kinetics of diffusion being largely controlled by the restrictive porosity of the protein itself. An inverse dependence of the first-order rate constant for effusion out of the protein channel on the incubation time used for radical penetration into the protein is attributed to increased binding of the radical within the funnel-shaped channel.

Distribution of transferrin and ferritin binding in normal and multiple sclerotic human brains

Delivery of iron to the brain traditionally has been considered the responsibility of transferrin. However, transferrin receptors in brain are located primarily within gray matter areas rather than in the iron rich white matter tracts. In this report we present the first demonstration of ferritin binding sites in human brain and provide evidence that these binding sites are primarily in white matter tracts. This distribution of ferritin binding is opposite of that seen for the distribution of the transferrin receptor in normal adult human brain. Ferritin binds to human brain tissue in a competitive and saturable manner with a dissociation constant of 0.35 nM and a binding site density of 116.7 fmol/mg protein. In brain tissue from multiple sclerotic (MS) patients the normal pattern of transferrin and ferritin binding distributions is disrupted. Ferritin binding is absent in the lesion itself and in the immediate periplaque region within the white matter but returns to normal as the distance from the lesion becomes greater. In direct contrast to ferritin binding, transferrin binding in the MS tissue is present in the white matter tracts, but only in the periplaque region. The periplaque region also contains transferrin receptor positive cells (as determined by immunocytochemistry) morphologically consistent with oligodendrocytes. Gray matter binding of transferrin in MS patients appears normal. These data provide the initial evidence of ferritin binding in human brain, address the enigma of the apparent absence of an iron delivery system to the iron-rich white matter, and suggest loss of ferritin binding is involved in or is a consequence of demyelination associated with MS.

Analysis of Ferritins in Lymphoblastoid Cell Lines and in the Lens of Subjects With Hereditary Hyperferritinemia-Cataract Syndrome

Hereditary hyperferritinemia-cataract syndrome (HHCS) is an autosomal and dominant disease caused by heterogeneous mutations in the iron responsive element (IRE) of the 5′ untranslated flanking region of ferritin L-chain mRNA, which reduce the binding to the trans iron regulatory proteins and make L-chain synthesis constitutively upregulated. In the several families identified so far, the serum and tissue L-ferritin levels are fivefold to 20-fold higher than in nonaffected control subjects, iron metabolism is apparently normal, and the only relevant clinical symptom is early onset, bilateral cataract. Some pathogenetic aspects of HHCS remain obscure, with particular reference to the isoferritins produced by HHCS cells, as well as the mechanism of cataract formation. We analyzed lymphoblastoid cell lines obtained from two nonaffected control subjects and from HHCS patients carrying the substitution A40G (Paris-1), G41C (Verona-1), and the deletion of the residues 10-38 (Verona-2) in the IRE structure. Enzyme-linked immunosorbent assays specific for the H- and L-type ferritins showed that L-ferritin levels were up to 20-fold higher in HHCS than in control cells and were not affected by iron supplementation or chelation. Sequential immunoprecipitation experiments of metabolically-labeled cells with specific antibodies indicated that in HHCS cells about half of the L-chain was assembled in L-chain homopolymers, which did not incorporate iron, and the other half was assembled in isoferritins with a high proportion of L-chain. In control cells, all ferritin was assembled in functional heteropolymers with equivalent proportion of H- and L-chains. Cellular and ferritin iron uptake was slightly higher in HHCS than control cells. In addition, we analyzed the lens recovered from cataract surgery of a HHCS patient. We found it to contain about 10-fold more L-ferritin than control lens. The ferritin was fully soluble with a low iron content. It was purified and partially characterized. Our data indicate that: (1) in HHCS cells a large proportion of L-ferritin accumulates as nonfunctional L-chain 24 homopolymers; (2) the concomitant fivefold to 10-fold expansion of ferritin heteropolymers, with a shift to L-chain–rich isoferritins, does not have major effects on cellular iron metabolism; (3) L-chain accumulation occurs also in the lens, where it may induce cataract formation by altering the delicate equilibrium between other water-soluble proteins (ie, crystallins) and/or the antioxidant properties.

Analysis of Ferritins in Lymphoblastoid Cell Lines and in the Lens of Subjects With Hereditary Hyperferritinemia-Cataract Syndrome

Hereditary hyperferritinemia-cataract syndrome (HHCS) is an autosomal and dominant disease caused by heterogeneous mutations in the iron responsive element (IRE) of the 5′ untranslated flanking region of ferritin L-chain mRNA, which reduce the binding to the trans iron regulatory proteins and make L-chain synthesis constitutively upregulated. In the several families identified so far, the serum and tissue L-ferritin levels are fivefold to 20-fold higher than in nonaffected control subjects, iron metabolism is apparently normal, and the only relevant clinical symptom is early onset, bilateral cataract. Some pathogenetic aspects of HHCS remain obscure, with particular reference to the isoferritins produced by HHCS cells, as well as the mechanism of cataract formation. We analyzed lymphoblastoid cell lines obtained from two nonaffected control subjects and from HHCS patients carrying the substitution A40G (Paris-1), G41C (Verona-1), and the deletion of the residues 10-38 (Verona-2) in the IRE structure. Enzyme-linked immunosorbent assays specific for the H- and L-type ferritins showed that L-ferritin levels were up to 20-fold higher in HHCS than in control cells and were not affected by iron supplementation or chelation. Sequential immunoprecipitation experiments of metabolically-labeled cells with specific antibodies indicated that in HHCS cells about half of the L-chain was assembled in L-chain homopolymers, which did not incorporate iron, and the other half was assembled in isoferritins with a high proportion of L-chain. In control cells, all ferritin was assembled in functional heteropolymers with equivalent proportion of H- and L-chains. Cellular and ferritin iron uptake was slightly higher in HHCS than control cells. In addition, we analyzed the lens recovered from cataract surgery of a HHCS patient. We found it to contain about 10-fold more L-ferritin than control lens. The ferritin was fully soluble with a low iron content. It was purified and partially characterized. Our data indicate that: (1) in HHCS cells a large proportion of L-ferritin accumulates as nonfunctional L-chain 24 homopolymers; (2) the concomitant fivefold to 10-fold expansion of ferritin heteropolymers, with a shift to L-chain–rich isoferritins, does not have major effects on cellular iron metabolism; (3) L-chain accumulation occurs also in the lens, where it may induce cataract formation by altering the delicate equilibrium between other water-soluble proteins (ie, crystallins) and/or the antioxidant properties.

An in vitro model for analysis of oxidative death in primary mouse astrocytes

Astrocytes provide a vital protective function in the brain. These cells are also vulnerable to oxidative stress, thus their loss of function could contribute to neurodegeneration. The goal of this study is to develop a cell culture model to study oxidative stress in astrocytes. Enriched astrocytic cultures were generated from neonatal mice. tertiary-butyl hydroperoxide (t-bOOH) was used as an exogenous peroxide and lactate dehydrogenase (LDH) release as a measure of loss of viability. Exposure to t-bOOH resulted in a linear increase in astrocytic death reaching 91.2% after 4 h exposure. That cell death was due to oxidative injury, was shown by the ability of the antioxidant N,N'-diphenyl-1,4-phenylenediamine (DPPD) to protect the t-bOOH treated cells. The involvement of iron in cell toxicity was demonstrated by the ability of the iron specific chelator desferal (DF) to completely prevent t-bOOH induced LDH release. Cells treated with a lipid soluble iron compound 3,5, 5-trimethyl (hexanoyl) ferrocene (TMH-Ferrocene), were more vulnerable to t-bOOH whereas neither ferrous ammonium sulfate (FAS) nor ferric ammonium citrate (FAC) had an effect. The increased sensitivity of the cells exposed to TMHF was reversible with the iron chelator desferal. Addition of recombinant human heavy chain ferritin or human apo-transferrin (Tf) did not alter LDH release. Electron microscopic analysis indicated astrocytes exposed to t-bOOH exhibited mitochondrial swelling prior to cell death (lactate dehydrogenase release). Additional increases in mitochondrial swelling were seen when the astrocytes were exposed to the lipophilic iron compound TMH-ferrocene and t-bOOH. These studies show that astrocytes are exquisitely sensitive to oxidative stress and that their vulnerability is related to and enhanced by iron. Decreased mitochondrial function in response to oxidative stress may precede cell death.

Role of H and L subunits in mouse ferritin

Ferritin is an iron-binding protein composed of two subunits, H and L. Twenty-four of these subunits assemble to form apoferritins whose subunit composition varies in a characteristic way in different tissues. Using recombinant proteins, we have assessed the role of H and L subunits in mouse ferritin function and compared these to human ferritin subunits. We report that mouse ferritin subunits exhibit considerable functional similarity to their human counterparts, including a prominent role of the H subunit in the facilitation of rapid iron uptake, and a key role of amino acid residues Glu-62 and His-65 in this process. In addition, amino acid residues important to assembly of the protein are conserved between mouse and human, permitting the formation of fully functional hybrid proteins containing both mouse and human subunits. However, murine and human ferritin H subunits also evidenced substantial functional differences; murine ferritin H showed a consistent reduction in iron uptake activity relative to human ferritin H. Creation of chimeric human/mouse ferritin H subunits by "helix swapping" mapped the domain of the protein critical to this activity difference to the DE helix. These findings suggest a novel functional role for carboxyl-terminal domains of the ferritin H subunit.

Loop mutations affect ferritin solubility causing non-native aggregation of subunits or precipitation of fully assembled polymers

As a consequence of elevated expression rates, the intracellular aggregation of polypeptide chains is commonly observed in E. coli. Although wild-type human ferritin, a polymeric iron storage protein, accumulates in the soluble form at high level in the bacterial cytoplasmic fraction, some amino acid substitutions in an exposed loop direct the synthesis of a highly insoluble product. We found that two mechanisms can lead to the aggregation of ferritin. While some mutations prevent ferritin polymerisation, others cause the precipitation of molecules in the assembled state.

Modelling antibody-antigen interactions: ferritin as a case study

In this work, we propose a model for the structure of the antigen-antibody complex formed by human H-ferritin and an antibody that specifically recognizes it. We cloned and sequenced the antibody gene, predicted the antibody three-dimensional structure, and reconstructed the H-ferritin-antibody complex using an automated docking procedure previously validated on known complexes. This procedure allowed us to identify one putative complex which we carefully analysed, in order both to evaluate its likelihood, in light of a set of experimental results described in the literature, and to predict precisely which are the sites of interaction between the two molecules. Our model is compatible with the experimentally determined characteristics of the complex. Some of the residues that form the predicted antigenic site of ferritin can be found in the amino acid sequence of peptides selected from a random peptide library because of their affinity for the ferritin monoclonal antibody. Furthermore, the structural difference between the antigenic site in human H-ferritin and the corresponding region in other species permits us to rationalize the inability of the antibody to recognize human L-ferritin and rat, chicken and mouse H-ferritin. Through the analysis of our model complex, we identify a number of other residues putatively involved in the interaction. This multidisciplinary approach shows that synergy between computational and experimental methods may bring further insight into the understanding of antibody-antigen recognition rules.

An isoform of ferritin as a component of protein yolk platelets in Schistosoma mansoni

Schistosoma mansoni possesses two isoforms of the iron storage protein ferritin, Fer1 and Fer2. At the mRNA level as well as at the protein level, Fer1 is much more abundant than Fer2; females contain an about 15-fold excess of Fer1 compared with males. In contrast, nearly equal amounts of Fer2 occur in both sexes. By electron microscopy we identified ferritin as a component of electron dense membrane-bound bodies in cells of the vitellarium. The mode of formation of these inclusions (as inferred from electron microscopy) and the abundance of phospholipid multilayered membranes suggest that these bodies are of a lysosomal nature. Here we interpret these ferritin-containing inclusions as protein yolk platelets. To date, most of the literature does not contain any hints of the existence of protein yolk in trematodes. The possible function of ferritin in embryonic development is discussed.

A general strategy to identify mimotopes of pathological antigens using only random peptide libraries and human sera

A strategy to identify disease-specific epitopes from phage-displayed random peptide libraries using human sera is described. Peptides on phage (phagotopes) that react with antibodies present in patient sera are purified from > 10(7) different sequences by affinity selection and immunological screening of plaques. Disease-specific phagotopes can be identified out of this pool through an 'antigen independent' procedure which avails itself only of patient and normal human sera. Using this strategy, we have selected antigenic mimics (mimotopes) of two different epitopes from the human hepatitis B virus envelope protein (HBsAg). We could show that a humoral response to these mimotopes is widespread in the immunized population, suggesting that the strategy identifies phagotopes that have a potential role as diagnostic reagents. Immunization of mice with the selected phagotopes elicited a strong specific response against the HBsAg. These results open new inroads into disease-related epitope discovery and provide the potential for vaccine development without a requirement for the use of, or even information about, the aetiological agent or its antigens.

Magnetoferritin: characterization of a novel superparamagnetic MR contrast agent

A protein-encaged superparamagnetic iron oxide has been developed and characterized by using horse spleen apoferritin as a novel bioreactive environment. The roughly spherical magnetoferritin molecules, 120 A in diameter, are composed of a monocrystalline maghemite or magnetite core 73 A +/- 14 in diameter. Except for the additional presence of iron-rich molecules of higher molecular weight, the appearance and molecular weight (450 kd) of magnetoferritin are identical to that of natural ferritin; the molecules are externally indistinguishable from their precursor, with a pI (isoelectric point) in the range 4.3-4.6. The measured magnetic moment of the superparamagnetic cores is 13,200 Bohr magnetons per molecule, with T1 and T2 relaxivities (r1 and r2) of 8 and 175 L.mmol-1 (Fe).sec-1, respectively, at body temperature and clinical field strengths. The unusually high r2/r1 ratio of 22 is thought to arise from ideal core composition, with no evidence of crystalline paramagnetic inclusions. T2 relaxation enhancement can be well correlated to the field-dependent molecular magnetization, as given by the Langevin magnetization function, raised to a power in the range 1.4-1.6. With its nanodimensional biomimetic protein cage as a rigid, convenient matrix for complexing a plethora of bioactive substances, magnetoferritin may provide a novel template for specific targeting of selected cellular sites.

Human ferritin H-chains can be obtained in non-assembled stable forms which have ferroxidase activity

We found conditions to obtain the Leu-169-->Arg mutant of human ferritin H chain in a stable and non-assembled state. The protein obtained is an oligomer of subunits with a high degree of structured conformation, and when concentrated it re-assembles into ferritin cages. Functional studies showed that (i) it promotes iron oxidation like the assembled ferritin, but at slower rate, (ii) it is readily precipitated by the oxidised iron unless apotransferrin or L-chain ferritin are added to sequester Fe(III). The results confirm that ferroxidase activity is located within the H-chain, and indicate that the cages of the fully assembled ferritins are important not only in maintaining iron in a soluble form, but also in eliciting the activity of the ferroxidase centres.

Mimicking of discontinuous epitopes by phage-displayed peptides, I. Epitope mapping of human H ferritin using a phage library of constrained peptides

We have constructed a random nonapeptide library in the N-terminal region of the major coat protein VIII of bacteriophage f1, with two cysteines flanking the insert, and preliminary data suggest that many of the clones display at least some of their peptides in cyclized form. This library was used to select oligopeptides binding to the monoclonal antibody (mAb) H107, recognising the assembled native conformation of recombinant human H-subunit ferritin (H Fer), whose three-dimensional structure is known. Comparison of the selected oligopeptides with one another allowed us to derive two consensus sequences characterized by conserved amino acid (aa) residues. Analysis of the distribution of the aa side chains exposed on the surface of H Fer reveals that most of the aa defining both consensus sequences are present either at the end of the big loop or at the end of the A helix. These two regions of the H Fer, though separated in the linear sequence, are very close in the folded molecule. Interestingly, each consensus sequence derived from the selected phage-displayed peptides is characterized by aa present both at the end of the big loop and at the end of the A helix. These two H Fer regions are good candidates for mimicry by the selected peptides and therefore for constituting part of the H107 epitope. To provide support to this hypothesis, we constructed several H Fer mutants carrying point mutations in different positions of these two regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Loop mutations can cause a substantial conformational change in the carboxy terminus of the ferritin protein

Although some protein folding theories sustain that the peptides (loops) that connect elements of more compact secondary structure may be important in the folding process, most of the data accumulated until now seems to contradict this notion. To approach this problem we have isolated and characterized a number of mutants in which the amino acid sequence of the peptide that connects helix D and helix E in the H-chain of human ferritin has been randomized. Our results indicate that, though no single loop residue is absolutely required for ferritin to attain the native conformation, most of the mutants that we have obtained by random regional mutagenesis, affect its folding/assembly process. This conclusion was reached utilizing a sensitive test that associates the color formed by a colony synthesizing a hybrid ferritin-beta-galactosidase protein to the ability of the ferritin domain to fold and assemble as the native protein. The characterization of the folding/assembly properties of our collection of mutants and the comparison of the mutant loop sequences, have allowed us to draw the following conclusions. Mutants that have positively charged residues at position 159, 160 or 161 fail to assemble into the native protein shell and form an insoluble aggregate. Interestingly some loop amino acid sequences cause the E-helix to reverse direction and to expose its COOH group, normally hidden inside the protein cavity, to the solvent. The propensity of a given ferritin mutant to fold into this "non-native" conformation can be attenuated by the introduction of Gly at position 159 and 164, as in the natural ferritin.

Functional roles of the ferritin receptors of human liver, hepatoma, lymphoid and erythroid cells

Ferritin receptors are present on the membranes of many normal and malignant cells. The binding specificity of these receptors for H and L subunits was examined using recombinant human ferritin homopolymers. At least two different types of ferritin receptors were found, one derived from normal rat, pig, and human liver which shows similar binding of H- and L-ferritin. The second receptor type, specific for the H-chain ferritin, has been identified on membranes of hepatic and other transformed cells, and of normal lymphoblasts and erythroid precursors. These two receptor types may have different metabolic functions: the hepatic receptor acting as a scavenger for circulating ferritin and possibly for iron exchange between hepatocytes and macrophages; the H-ferritin receptor having a regulatory role which is not directly related to iron metabolism. The expression of the H-ferritin receptor is closely related to the activation and proliferation state of the cells. Addition of H-ferritin to the culture medium of cells expressing the H-ferritin receptor resulted in inhibition of cell proliferation and of colony formation.

Binding and suppressive activity of human recombinant ferritins on erythroid cells

We studied the relation between ferritin cellular binding and suppressive activity of recombinant H- and L-ferritin on human erythroid cells at different proliferation/differentiation phases. L-ferritin failed to show any suppressive activity or detectable binding to erythroblasts at any stage of maturation. In contrast, H-ferritin demonstrated binding to erythroblasts derived from peripheral BFU-E cells which increased steadily between 7-14 days of culture up to 15,000 molecules per cell. Reticulocytes and erythrocytes failed to bind either L- or H-ferritin. H-ferritin suppressed BFU-E colony formation and reduced K562 cell proliferation at nanomolar concentrations. This suggests that the expression of H-ferritin binding sites is modulated by cellular proliferation and differentiation, that cells expressing H-ferritin binding sites are sensitive to ferritin suppressive activity and that a causal relation exists between ferritin cellular binding and suppressive activity.

Influence of site-directed modifications on the formation of iron cores in ferritin

The structure and crystal chemical properties of iron cores of reconstituted recombinant human ferritins and their site-directed variants have been studied by transmission electron microscopy and electron diffraction. The kinetics of Fe uptake have been compared spectrophotometrically. Recombinant L and H-chain ferritins, and recombinant H-chain variants incorporating modifications in the threefold (Asp131----His or Glu134----Ala) and fourfold (Leu169----Arg) channels, at the partially buried ferroxidase sites (Glu62,His65----Lys,Gly), a putative nucleation site on the inner surface (Glu61,Glu64,Glu67----Ala), and both the ferroxidase and nucleation sites (Glu62,His65----Lys,Gly and Glu61,Glu64,Glu67----Ala), were investigated. An additional H-chain variant, incorporating substitution of the last ten C-terminal residues for those of the L-chain protein, was also studied. Most of the proteins assimilated iron to give discrete electron-dense cores of the Fe(III) hydrated oxide, ferrihydrite (Fe2O3.nH2O). No differences were observed for variants modified in the three- or fourfold channels compared with the unmodified H-chain ferritin. The recombinant L-chain ferritin and H-chain variant depleted of the ferroxidase site, however, showed markedly reduced uptake kinetics and comprised cores of increased diameter and regularity. Depletion of the inner surface Glu residues, whilst maintaining the ferroxidase site, resulted in a partially reduced rate of Fe uptake and iron cores of wider particle size distribution. Modification of both ferroxidase and inner surface Glu residues resulted in complete inhibition of iron uptake and deposition. No cores were observed by electron microscopy although negative staining showed that the protein shell was intact. The general requirement of an appropriate spatial charge density across the cavity surface rather than specific amino acid residues could explain how, in spite of an almost complete lack of identity between the amino acid sequences of bacterioferritin and mammalian ferritins, ferrihydrite is deposited within the cavity of both proteins under similar reconstitution conditions.

A mutational analysis of the epitopes of recombinant human H-ferritin

Murine monoclonal antibodies were elicited by the recombinant human H-ferritin overexpressed in Escherichia coli. They had a specificity analogous to that of the antibodies elicited by natural human H-chain, and all of them showed low additivity in binding the recombinant ferritin. Four antibodies of each group were challenged with four H-ferritin mutants overexpressed in E. coli, altered in different accessible areas of the molecule. They consisted of deletions of the first 13 and last 22 amino acids, a duplication of an 18 amino acid sequence in the loop region, and a substitution of a 5 amino acid stretch in the three-fold symmetry axis region. Double diffusion, immunodot analyses and inhibition plots indicated that: (1) all the mutants were recognized by at least one antibody; (2) the deletion of the N-terminus and the duplication in the loop region had the strongest effect on antibody binding; and (3) epitope boundaries of the various antibodies could not be recognized. The antibodies were tested with H-containing ferritins from rat and hen hearts, and showed low or absent reactivities despite their high structural homology with human ferritin. Comparison of the amino acid sequences of human, mouse, rat and hen H-chains, together with mutational data, suggested that; (i) ferritin epitopes are large, probably encompassing a large portion of the subunit surface and (ii) Thr-5 and Cys-90 have a role in H-ferritin immunogenicity.

Development of an immunoassay for all human isoferritins, and its application to serum ferritin evaluation

Calibrated mixtures of anti-H and anti-L ferritin subunit monoclonal antibodies were used in a sandwich enzyme-immunoassay for the evaluation of all isoferritins. The assay was designed to have overlapping calibration plots for human liver (95% L-chain) and recombinant human H-chain ferritin (100% H-chain). It appeared to recognize all the heart isoferritins including the ones in the middle of the isoferritin spectrum. By direct comparison it was shown that these isoferritins are under-evaluated by the assays for H- and L-subunit-rich ferritins, based on the two separated antibodies. The three assays (for total, H-rich and L-rich ferritins) provide an index of the presence of the isoferritins with intermediate H/L composition. Sera from 30 tumor and non-tumor patients were analyzed. Intermediate isoferritins were found in 2 non-tumor subjects and in none of the 14 patients with Hodgkin's disease or mammary carcinoma. It is concluded that the evaluation of the total and intermediate isoferritins is possible, but does not have an evident clinical significance for tumor monitoring.

Identification of the ferroxidase centre in ferritin

Ferroxidase activity in human H-chain ferritin has been studied with the aid of site-directed mutagenesis. A site discovered by X-ray crystallography has now been identified as the ferroxidase centre. This centre is present only in H-chains and is located within the four-helix bundle of the chain fold.

Immunohistochemical evidence for a lack of ferritin in duodenal absorptive epithelial cells in idiopathic hemochromatosis

Patients with idiopathic hemochromatosis exhibit an unexplained increase in intestinal iron absorption. The aim of this work was to study immunohistochemical H- and L-ferritin distribution in duodenal mucosal cells of patients with idiopathic hemochromatosis, and of subjects with various degrees of iron loading. Biopsy sections of gastrointestinal mucosa from 24 patients with idiopathic hemochromatosis, 10 patients with secondary iron overload, 6 normal subjects, and 13 iron-deficient subjects were analyzed with monoclonal antibodies for the presence of immunohistochemical H and L ferritin types, and with Perls' stain for hemosiderin. Ferritin content of duodenal homogenates was evaluated in 5 cases. The absorptive duodenal cells were found to contain ferritin, mostly of the L type, in apical granules; these ferritin granules were present in all normal, iron-deficient, and iron-over-loaded subjects, but were absent in 21 (87%) of the patients with established idiopathic hemochromatosis. In cells other than those of the duodenal epithelium, such as lamina propria or antral mucosa, ferritin and hemosiderin contents were related to iron loading and no difference was evident between primary and secondary iron overload. These findings indicate that (a) idiopathic hemochromatosis is associated with an altered ferritin expression in the duodenal absorptive epithelial cells, (b) this alteration cannot be detected by analysis of duodenal homogenates, (c) idiopathic hemochromatosis does not affect ferritin accumulation in the other cell types analyzed, and (d) ferritin in absorptive duodenal cells may have a regulatory role in iron absorption.

Over-expression of natural and variant human H-chain ferritins in E. coli

The natural human H-chain ferritin was expressed in E. coli using a multi-copy expression vector containing the lambda pL promoter. A variant H-ferritin, having an altered N-terminus, was also produced. These proteins are overproduced (greater than 30% of the soluble protein), correctly assembled into its 24-subunit shell, and able to bind iron. The identity of the products was confirmed using an antibody specific for H-ferritin.

The production of myeloid blood cells and their regulation during health and disease

The regulation of myelopoiesis in vivo most likely entails a complex set of interactions between cell-derived biomolecules and their target cells: hematopoietic stem and progenitor cells and accessory cells. Stimulating and suppressing factors have been characterized through in vitro studies, and their mechanisms of action in vitro and in vivo have begun to be elucidated. Among those factors being studied are the hematopoietic colony-stimulating factors (CSF): interleukin-3 (multi-CSF), granulocyte-macrophage-CSF, granulocyte-CSF, and macrophage-CSF; other molecules include erythropoietin, B-cell-stimulating factor-1, interleukin-1, interleukin-2, prostaglandin E, leukotrienes, acidic ferritins, lactoferrin, transferrin, the interferons-gamma, -alpha, and -beta, and the tumor necrosis factors-alpha and -beta (lymphotoxin). These factors interact to modulate blood cell production in vitro and in vivo. The proposed review characterizes these biomolecules biochemically and functionally, including receptor-ligand interactions and the secondary messengers within the cell which mediate their functional activity. The production and action of the molecules are described under conditions of hematopoietic disorders, as well as under normal conditions. Studies in vitro are correlated with studies in vivo using animal models to give an overall view of what is known about these molecules and their relevance physiologically and pathologically.