Correlations between subunit distribution, microheterogeneity, and iron content of human liver ferritin

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.

Nuclear ferritin in corneal epithelial cells: tissue-specific nuclear transport and protection from UV-damage

We have identified the heavy chain of ferritin as a developmentally regulated nuclear protein of embryonic chicken corneal epithelial cells. The nuclear ferritin is assembled into a supramolecular form that is indistinguishable from the cytoplasmic form of ferritin found in other cell types. Thus it most likely has iron-sequestering capabilities. Free iron, via the Fenton reaction, is known to exacerbate UV-induced and other oxidative damage to cellular components, including DNA. Since corneal epithelial cells are constantly exposed to UV light, we hypothesized that the nuclear ferritin might protect the DNA of these cells from free radical damage. To test this possibility, primary cultures of cells from corneal epithelium and other tissues were UV irradiated, and damage to DNA was detected by an in situ 3'-end labeling assay. Consistent with the hypothesis, corneal epithelial cells with nuclear ferritin had significantly less DNA breakage than the other cells types examined. However, when the expression of nuclear ferritin was inhibited the cells now became much more susceptible to UV-induced DNA damage. Since ferritin is normally cytoplasmic, corneal epithelial cells must have a mechanism that effects its nuclear localization. We have determined that this involves a nuclear transport molecule which binds to ferritin and carries it into the nucleus. This transporter, which we have termed ferritoid for its similarity to ferritin, has at least two domains. One domain is ferritin-like and is responsible for binding the ferritin; the other domain contains a nuclear localization signal that is responsible for effecting the nuclear transport. Therefore, it seems that corneal epithelial cells have evolved a novel, nuclear ferritin-based mechanism for protecting their DNA against UV damage. In addition, since ferritoid is structurally similar to ferritin, it may represent an example of a nuclear transporter that evolved from the molecule it transports (i.e., ferritin).

Iron, ferritin, and nutrition

Ferritin, a major form of endogenous iron in food legumes such as soybeans, is a novel and natural alternative for iron supplementation strategies where effectiveness is limited by acceptability, cost, or undesirable side effects. A member of the nonheme iron group of dietary iron sources, ferritin is a complex with Fe3+ iron in a mineral (thousands of iron atoms inside a protein cage) protected from complexation. Ferritin illustrates the wide range of chemical and biological properties among nonheme iron sources. The wide range of nonheme iron receptors matched to the structure of the iron complexes that occurs in microorganisms may, by analogy, exist in humans. An understanding of the chemistry and biology of each type of dietary iron source (ferritin, heme, Fe2+ ion, etc.), and of the interactions dependent on food sources, genes, and gender, is required to design diets that will eradicate global iron deficiency in the twenty-first century.

Ferritoid, a tissue-specific nuclear transport protein for ferritin in corneal epithelial cells

Previously we reported that ferritin in corneal epithelial (CE) cells is a nuclear protein that protects DNA from UV damage. Since ferritin is normally cytoplasmic, in CE cells, a mechanism must exist that effects its nuclear localization. We have now determined that this involves a nuclear transport molecule we have termed ferritoid. Ferritoid is specific for CE cells and is developmentally regulated. Structurally, ferritoid contains multiple domains, including a functional SV40-type nuclear localization signal and a ferritin-like region of approximately 50% similarity to ferritin itself. This latter domain is likely responsible for the interaction between ferritoid and ferritin detected by co-immunoprecipitation analysis. To test functionally whether ferritoid is capable of transporting ferritin into the nucleus, we performed cotransfections of COS-1 cells with constructs for ferritoid and ferritin. Consistent with the proposed nuclear transport function for ferritoid, co-transfections with full-length constructs for ferritoid and ferritin resulted in a preferential nuclear localization of both molecules; this was not observed when the nuclear localization signal of ferritoid was deleted. Moreover, since ferritoid is structurally similar to ferritin, it may be an example of a nuclear transporter that evolved from the molecule it transports (ferritin).

Crosslinks between intramolecular pairs of ferritin subunits: effects on both H and L subunits and on immunoreactivity of sheep spleen ferritin

Ferritin is a multisubunit protein, controlling iron storage, with a protein coat composed of 24 subunits (up to three distinct types) in different proportions depending on cell type. Little is known about the subunit interactions in ferritin protein coats composed of heterologous subunits, despite the relevance to ferritin structure and ferritin function (iron uptake and release). Synthetic crosslinking is a convenient way to probe subunit contacts. Crosslinks between subunit pairs in ferritin protein coats are also a natural post-translational modification which coincides with different iron content in ferritin from sheep spleen; ferritin from sheep spleen also contains H and L subunits. Crosslinks synthesized by the reaction of ferritin low in natural crosslinks with difluorodinitrobenzene (F2DNB) reproduced the effects of the natural crosslinks on iron uptake and release. We now extend our observations on the structural effects of natural and synthetic crosslinks to include immunoreactivity of the assembled protein, with monoclonal antibodies as a probe. We also demonstrate, for the first time, ferritin peptides involved in an apparent H- and L-subunit contact: two peptides decreased 4X in cyanogen bromide peptide maps after F2DNB crosslinking were residues L-96-138 and H-66-96; the major DNP-dipeptide was Lys-DNP-Lys. Using the structure of an all L-subunit ferritin as a model, the most likely site for the H-L DNP crosslink is L-Lys 104 (C helix) and H-Lys 67 (B helix). The B helix forms the internal subunit dimer interface, a putative site of iron core nucleation. Alteration by crosslinks of the B helix could, therefore, explain the effect of crosslinks on ferritin iron uptake, release, and iron content.

Dissociation of ferritins

Apoferritins prepared from horse spleen and heart and rat heart and liver were dissociated by treatment with acetic acid (pH 1.3-3.0). Sedimentation velocity studies showed that apoferritins of spleen and liver (16-17 S) and heart (18-19 S) dissociated into material sedimenting near 3.2 S. Sedimentation equilibrium measurements determined that most of the material had a molecular weight of 38,000-43,000, corresponding to subunit dimers. Failure to dissociate into subunit monomers was confirmed by gel chromatography on Sephadex G-75 and G-150. With the exception of boiling in sodium dodecyl sulfate, further treatments with 0.1-0.4 M KCl, NaCl, 4-9 M urea, 0.01-0.5 M KSCN, 0.1-0.5% Triton X-100, 5-52% dimethylsulfoxide, 10% ethylene glycol, or 0.1% trifluoroacetic acid all failed to cause dissociation into individual subunits, as did exposure to 6 M guanidine-HCl or formic acid, or prior succinylation and/or nitration of the protein. Reassociation occurred between pH 4 and 7 but was not aided by the addition of Fe(II) or reducing agents. It is concluded that ferritins readily dissociate to subunit dimer units and that further dissociation does not occur without full denaturation of the protein.

Immunoreactivities of human isoferritins

We have examined the immunoreactivities of antisera prepared against ferritins from human liver and HeLa cells to tissue ferritins and to individual isoferritins. In a radioimmune assay for HeLa ferritin the cross-reactivity of liver ferritin was about 2.5%. However, the apparent recovery of liver ferritin in the presence of different levels of HeLa ferritin was very much greater than that predicted from the measured cross-reactivity. This anomalous behaviour was eliminated by absorption of the HeLa antiserum with L-rich ferritins, suggesting that it represented interaction with common determinants in H and L subunits. The relative levels of H and L determinants measured by radioimmunoassay in individual isoferritins correlated with their relative contents of H and L subunits. However, in some parts of the isoferritin spectrum, the radioimmunoassay underestimated the H subunit content of L-rich isoferritins and overestimated the H subunit content of L-rich isoferritins and overestimated the H subunit content of H-rich isoferritins. This finding suggests differential expression of determinants in the various heteropolymers. These could arise from conformational changes leading to exposure or internalization of different determinants, or to recognition of determinants from certain subunit interactions.

Structural homology between mouse liver and horse spleen ferritins

Mouse liver ferritin is composed almost exclusively of polypeptide chains similar in molecular mass (22 kDa) to that characteristic of the major chain (H) found in heart ferritin isolated from human, horse or rat. In these species the predominant polypeptide of liver (L) is smaller (about 20 kDa). Here we show that mouse liver and horse spleen ferritins and apoferritins exhibit extensive structural homology as judged by the similarity in the diffraction patterns of their crystals grown from cadmium sulphate solutions. Implications of this finding are discussed.

A chemical and physical comparison of ferritin subunit species fractionated by high-performance liquid chromatography

Selected chemical and physical properties were measured for different forms of ferritin subunits which had been separated by reverse-phase high-performance liquid chromatography. Ferritin subunits from porcine spleen behaved, on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, as though they were approximately Mr 2000 larger than equine spleen ferritin, whereas no difference in size was observed by gel chromatography in 6 M guanidinium chloride. All subunit species exhibited similar isoelectric focusing properties. In contrast to previous reports, no carbohydrate could be found associated with any of the isolated subunit species. Thus, the aberrant behavior of the porcine ferritin subunits between the two empirical molecular weight estimation methods appears to be the result of factor(s) other than protein intrinsic charge or covalently attached carbohydrate.

Isolation and characterization of a cDNA clone for human ferritin heavy chain

Ferritin, the main iron-storage protein, is composed of two partially homologous subunits, heavy (H) and light (L), with MrS of 21,000 and 19,000, respectively. We have isolated a cDNA clone for human ferritin H chains by screening a human lymphocyte cDNA library with synthetic oligodeoxyribonucleotides. The oligonucleotide sequences were derived from two pentapeptides found in human spleen ferritin. The selected clone hybridized to both probes and selected H-chain mRNA, but not L-chain mRNA, when hybridized to HeLa cell mRNA. These results indicate that the cloned DNA codes for a H chain of human ferritin. Since the amino acid sequence derived from the cloned DNA was almost identical to the partial amino acid sequence of a minor component found in human spleen ferritin, we conclude that the minor sequence found in human spleen ferritin must be a H subunit. Genomic analysis gives a complex pattern that suggests that ferritin H chains are encoded by a multigene family or have an unusually large number of exons.

Functional studies on rat-liver isoferritins

Rat-liver and horse-spleen isoferritins were obtained by preparative isoelectric focussing and several of these were fractionated further by sucrose density gradient centrifugation. H- and L-subunit compositions were also measured. Isoferritins were found to have neither a fixed iron content nor a unique subunit composition. In both species within a single isoferritin a small increase in the percentage of H subunit paralleled increasing iron content. Although in horse-spleen ferritin a similar correlation was found over the isoferritin profile as a whole, this was not generally true of rat-liver isoferritins, since iron distributions varied with the iron status of the animals. Rates of iron incorporation into isoapoferritins were measured in vitro and the distribution of 59Fe among rat liver isoferritins was measured at various times after injection of 59Fe. The data do not support the proposal that, in rat liver, L-rich isoferritins are the preferred iron-storage form.

Preparative isolation of distinct molecular forms of rabbit liver ferritin using high-performance liquid chromatography

Rabbit liver ferritin was separated to fractions of distinct molecular form using a chromatofocusing column coupled to high-performance liquid chromatography equipment. This purification method was fast, less than 1 h, and enabled preparation of fractions, highly enriched in particular subtypes of ferritin. Analytical isoelectrofocusing of these fractions demonstrated a gradual shift in the range of isoelectric points of these subtypes of ferritin. Gradient-pore polyacrylamide slab-gel electrophoresis showed a distinct shift in the subunit ratio of the ferritins, ranging from 87% low molecular weight (L) subunit in the first fraction eluting at a pH 5.4, to 28% L-subunit in the fraction eluting in the trailing edge of the protein peak at pH 4.0. The pI range of the fractions covered the complete range, from pH 4.9 to pH 5.4, of isoelectric points of the whole rabbit liver ferritin preparation.

Biosynthesis of ferritin subunits from different cell lines of HL-60 human promyelocytic leukaemia cells and the release of acidic isoferritin-inhibitory activity against normal granulocyte-macrophage progenitor cells

Biosynthesis of acidic isoferritins was investigated in human promyelocytic HL-60 cells, characterized by diploid (2C), tetraploid (4C) and mixed diploid--tetraploid (2C-4C) DNA cell lines. The three cell lines were studied for the biosynthesis of ferritin and its subunits and for the release of acidic isoferritin-inhibitory activity against normal CFU-GM before and after addition of DMSO. While the tetraploid and mixed diploid--tetraploid cell lines synthesized more H-(Mr = 21) than L-subunits (Mr = 19) after induction, the tetraploid line synthesized more H-subunit before and after induction, compared to the diploid line. The release of acidic isoferritin-inhibitory activity was greater before than after induction in both cell lines, but the tetraploid cell line released more acidic isoferritin-inhibitory activity consistent with its greater production of Mr = 21 subunit. However, after induction no inhibitory activity could be detected from the diploid cells and much less activity was detected with the tetraploid cells, suggesting that differentiation caused a decrease in production of acidic isoferritin-inhibitory activity.

Studies on heterogeneity in ferritin subunits

Subunits prepared by dissociating rat and human ferritins by acid/urea or SDS can be resolved by isoelectric focusing in urea/Triton gels into many discrete forms. Most of these are not true isosubunits but aggregation artefacts formed during electrofocusing. The distribution of H and L subunit classes in these aggregates indicates that HeLa and heart ferritins contain similar classes of H and L subunits but that one or both of these classes is different in liver and kidney ferritins. To avoid aggregation artefacts, we examined subunits synthesised in vitro from exogenous mRNA. Our results indicate that HeLa and rat liver cells synthesise only one class of L subunit but two classes of H subunit.

Human ferritin gene is assigned to chromosome 19

Ferritin is the intracellular iron storage protein. Tissue ferritin stores are markedly increased in hemochromatosis, a disease of iron overload that has been linked to chromosome 6. In order to provide further information concerning the genetics of ferritin synthesis and to determine if the structural gene for ferritin was on chromosome 6, studies were performed to identify the human chromosome that contains the ferritin gene. Ferritin immunoassays were performed on extracts of Chinese hamster ovary somatic cells that were hybridized with human lymphocytes and fibroblasts and contained various human chromosomes in different combinations. None of the 13 cell lines that lacked immunoreactive human ferritin contained chromosome 19, and all 9 of the cell lines that produced human ferritin contained chromosome 19. No other human chromosome shared this association with human ferritin. In studies of subclones of ferritin-positive cell lines, immunoreactive ferritin consistently segregated only with chromosome 19. Immunoprecipitation studies performed on cells that had been incubated with 59Fe-containing transferrin indicated that chromosome 19-containing cells incorporated iron into intact and functional molecules of human ferritin. The necessary and exclusive association of chromosome 19 with human ferritin indicates that a defect in the structural gene for human ferritin cannot account for the abnormalities of hemochromatosis. Moreover, this hamster-human hybrid cell system should prove useful in further studies of regulation of ferritin concentration and composition.

A single site serum ferritin immunoradiometric assay. A comparison between serum and liver ferritin standards and two antibody preparations

A single site immunoradiometric assay (1S-IRMA) for serum ferritin was developed to investigate some standardization problems in the assay of ferritin. In comparison to this 1S-IRMA, a 2-site solid phase IRMA gave lower answers of serum ferritin in the normal reference range, as well as in serum from idiopathic hemochromatosis patients. Purified serum ferritin or liver ferritin gave different standard curves in this new immunoassay system and the differences obtained were further dependent on the antibody preparation employed. The different isoferritin profiles obtained, dependent on the antibody preparation employed for ferritin purification, indicate a different antibody specificity for different ferritin subunits. Serum ferritin purified from different serum pools from healthy persons showed similar isoferritin profiles and immunoassay reactivities.

Serum ferritin in young men during prolonged heavy physical exercise

The level of serum ferritin were investigated in 44 young men during strenuous ranger training courses lasting 4-5 d. These cadets were continuously subjected to heavy physical activities, almost without sleep and with a caloric deficit amounting to 29000-39000 kj per 24 h. Ferritin concentrations were consistently increased during the courses, and were still significantly elevated 2 d after the end of the courses. The increase of ferritin appeared to be correlated with decreased Hb values and increased total bilirubin. The results indicate that iron released from disintegrated erythrocytes probably causes increased ferritin synthesis in the reticuloendothelic cells and a subsequent release of ferritin in blood.

Ferritin synthesis by human T lymphocytes

Mononuclear cells from peripheral blood of normal humans, unselected spleen cells from patients with Hodgkin's disease, and selected T and non-T lymphoid cells from normal peripheral blood and from the spleens of Hodgkin's disease patients were examined for de novo synthesis and secretion of ferritin. After precipitation of labeled lysates and supernatants from unseparated and selected T cells with antiserum to human liver ferritin, two bands were visible on sodium dodecyl sulfate-polyacrylimide gel analysis. The two bands were detected in molecular weight regions 19,000 and 21,000, which are thought to represent the L and H subunits of the ferritin molecule, respectively. The slower band (subunit H) was more radioactive than the faster band (subunit L). The H subunit is found in greater amounts in the serum of some tumor patients, but its cellular origin has not been established. The present findings indicate that cells of the immune system contribute to the synthesis and secretion of a ferritin molecule with a high proportion of H subunits.

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.

Mouse hepatoma and liver ferritins. Comparative structural studies

Pure ferritin from male mouse liver produces a single band of monomers (RF = 0.199) with electrophoresis in polyacrylamide gels at pH 9.0. The five sub-bands within this monomeric band appear to represent charge isomers having the same molecular size. Ferritin from BH3 transplantable mouse hepatoma shows two overlapping bands of monomers (RFA = 0.208 and RFB = 0.240); further electrophoretic studies show that these bands represent two subpopulations of molecules differing both in charge and size. Sub-bands are not found in this hepatoma ferritin. The larger tumor ferritin reaches the same end migration position as all liver isoferritins on gradient gels, signifying a very similar or identical molecular size; however, the absence of sub-bands indicates that this hepatoma ferritin differs in charge from the homologous liver proteins. Liver and hepatoma ferritins both produce a single prominent subunit band corresponding to nominal molecular weights of 22 250 and 21 700, with polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and dithiothreitol. With electrophoresis on polyacrylamide gradient slabs containing sodium dodecyl sulfate and dithiothreitol, both liver and hepatoma ferritins now reveal two subunits bands situated at identical positions. The polypeptides of these two closely spaced bands have a nominal molecular weight difference of less than 1000. Neither the hepatoma nor the liver seems to produce the ferritins found in the other tissue. Nevertheless, all these ferritins are composed of the same two types of subunits, albeit in different relative amounts. Observed distinctions in the ferritins from these normal or neoplastic cells must reflect differences in assembly and processing, as well as in the regulated expression of the same ferritin genes.