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

Unraveling the interplay between iron homeostasis, ferroptosis and extramedullary hematopoiesis

Iron participates in myriad processes necessary to sustain life. During the past decades, great efforts have been made to understand iron regulation and function in health and disease. Indeed, iron is associated with both physiological (e.g., immune cell biology and function and hematopoiesis) and pathological (e.g., inflammatory and infectious diseases, ferroptosis and ferritinophagy) processes, yet few studies have addressed the potential functional link between iron, the aforementioned processes and extramedullary hematopoiesis, despite the obvious benefits that this could bring to clinical practice. Further investigation in this direction will shape the future development of individualized treatments for iron-linked diseases and chronic inflammatory disorders, including extramedullary hematopoiesis, metabolic syndrome, cardiovascular diseases and cancer.

Assessment of serum ferritin as a biomarker in COVID-19: bystander or participant? Insights by comparison with other infectious and non-infectious diseases

BACKGROUND

The 2019 coronavirus disease (COVID-19) caused by the SARS-CoV-2 virus has an impact on all aspects of patient care. Serum ferritin generally represents a biomarker of choice when iron deficiency is suspected. However, ferritin is also an acute-phase-protein exhibiting elevated serum concentration in various inflammatory diseases. Here we focus on the role of serum ferritin for diagnostic and clinical management of patients with COVID-19 in comparison with other infectious and non-infectious diseases.

METHODS

We examined scientific articles listed in PubMed reporting on ferritin in various infectious and non-infectious diseases. We then compared these results with nine current COVID-19 ferritin reports published in 2020.

RESULTS

Several non-infectious, as well as non-COVID-19 infectious diseases, are characterised by a partly dramatic elevation of serum ferritin levels. All COVID-19 studies published between February and May 2020, which documented laboratory serum ferritin, indicate ferritin as a biomarker of COVID-19 severity in hospitalised patients.

CONCLUSIONS

Serum ferritin may be considered both a prognostic and stratifying biomarker that can also contribute to therapeutic decision-making concerning patients with COVID-19. It should be emphasised, however, that most scientific reports refer to cohorts in the Asian region. Further validation in other cohorts is urgently required.

Proanthocyanidin Encapsulated in Ferritin Enhances Its Cellular Absorption and Antioxidant Activity

Proanthocyanidins (PAs) possess superior antioxidant properties and nutritious value, however, low bioavailability and stability limit their applications. Here, we developed a novel method to encapsulate PA dimers successfully into horse spleen apoferritin (apoHSF) using a disassembly/reassembly method based on pH change. The PA-HSF nanoparticles were characterized using fluorescence spectroscopy, transmission electron microscopy, circular dichroism, and high-performance liquid chromatography. One apoferritin cage could approximately encapsulate 25.6 molecules of the PA dimer. The results showed that the encapsulation of the PA dimers protected it from the damage of oxidants and temperature below room temperature would be an appropriate condition for HSF-578 solution storage. Moreover, HepG2 cell monolayer absorption and adhesion analyses indicated that the PA dimers encapsulated within apoHSF cages were more efficient in transport. In addition, it was indicated that the PA-HSF nanoparticles had higher cellular antioxidant activity. The novel strategy provided in this study indicates that the protein cage structures like ferritin have potential to be applied in the field of food nutrition.

Nanocage-Therapeutics Prevailing Phagocytosis and Immunogenic Cell Death Awakens Immunity against Cancer

A growing appreciation of the relationship between the immune system and the tumorigenesis has led to the development of strategies aimed at "re-editing" the immune system to kill tumors. Here, a novel tactic is reported for overcoming the activation-energy threshold of the immunosuppressive tumor microenvironment and mediating the delivery and presentation of tumor neoantigens to the host's immune system. This nature-derived nanocage not only efficiently presents ligands that enhance cancer cell phagocytosis, but also delivers drugs that induce immunogenic cancer cell death. The designed nanocage-therapeutics induce the release of neoantigens and danger signals in dying tumor cells, and leads to enhancement of tumor cell phagocytosis and cross-priming of tumor specific T cells by neoantigen peptide-loaded antigen-presenting cells. Potent inhibition of tumor growth and complete eradication of tumors is observed through systemic tumor-specific T cell responses in tumor draining lymph nodes and the spleen and further, infiltration of CD8+ T cells into the tumor site. Remarkably, after removal of the primary tumor, all mice treated with this nanocage-therapeutics are protected against subsequent challenge with the same tumor cells, suggesting development of lasting, tumor-specific responses. This designed nanocage-therapeutics "awakens" the host's immune system and provokes a durable systemic immune response against cancer.

Recent advances to accelerate re-endothelialization for vascular stents

Cardiovascular diseases are considered as one of the serious diseases that leads to the death of millions of people all over the world. Stent implantation has been approved as an easy and promising way to treat cardiovascular diseases. However, in-stent restenosis and thrombosis remain serious problems after stent implantation. It was demonstrated in a large body of previously published literature that endothelium impairment represents a major factor for restenosis. This discovery became the driving force for many studies trying to achieve an optimized methodology for accelerated re-endothelialization to prevent restenosis. Thus, in this review, we summarize the different methodologies opted to achieve re-endothelialization, such as, but not limited to, manipulation of surface chemistry and surface topography.

Cisplatin encapsulation within a ferritin nanocage: a high-resolution crystallographic study

Cisplatin (CDDP) can be encapsulated within the central cavity of reconstituted (apo)ferritin, (A)Ft, to form a drug-loaded protein of potential great interest for targeted cancer treatments. In this study, the interactions occurring between cisplatin and native horse spleen Ft in CDDP-encapsulated AFt are investigated by high-resolution X-ray crystallography. A protein bound Pt center is unambiguously identified in AFt subunits by comparative analysis of difference Fourier electron density maps and of anomalous dispersion data. Indeed, a [Pt(NH3)2H2O](2+) fragment is found coordinated to the His132 residue located on the inner surface of the large AFt spherical cage. Remarkably, Pt binding does not alter the overall physicochemical features (shape, volume, polarity/hydrophobicity and electrostatic potential) of the outer surface of the AFt nanocage. CDDP-encapsulated AFt appears to be an ideal nanocarrier for CDDP delivery to target sites, as it possesses high biocompatibility and can be internalized by receptor mediated endocytosis, thus carrying the drug to tumor tissue with higher selectivity than free CDDP.

Ferritin-supported lipid bilayers for triggering the endothelial cell response

Hybrid nanoassemblies of ferritin and silica-supported lipid bilayers (ferritin-SLBs) have been prepared and tested for the adhesion, spreading and proliferation of retinal microvascular endothelial cells (ECs). Lipid membranes with varying surface charge were obtained by mixing cationic 1-palmitoyl-2-oleoyl-sn-glycero-3-ethylphosphocholine (POEPC) with zwitterionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) at increasing POPC/POEPC ratios. The supported bilayer formation and their subsequent interaction processes with ferritin were studied at the pH of 7.4 at different protein concentrations, by using the quartz crystal microbalance with dissipation monitoring and by atomic force microscopy. Both kinetics and viscoelastic parameters of the protein-lipid membrane interface were scrutinized, as well as surface coverage. Phase-contrast optical microscopy analyses of the ferritin-SLBs substrates after their interaction with endothelial cells evidenced the highest cell adhesion (2-4h of incubation time) and proliferation (from 24h to 5 days) for the membranes of POPC/POEPC (75:25 ratio). Moreover, ferritin increased both cell adhesion and proliferation in comparison to control glass (respectively 1.5- and 1.75-fold) as well as proliferation in comparison to bare POPC/POEPC (95:5 ratio) (2 fold). Results are very promising in the goal of modulating the endothelial cell response through the interplay of viscoelastic/charge properties of the solid-supported membranes and the SLB-conditioned ferritin activity.

Hemopexin and haptoglobin: allies against heme toxicity from hemoglobin not contenders

The goal here is to describe our current understanding of heme metabolism and the deleterious effects of "free" heme on immunological processes, endothelial function, systemic inflammation, and various end-organ tissues (e.g., kidney, lung, liver, etc.), with particular attention paid to the role of hemopexin (HPX). Because heme toxicity is the impetus for much of the pathology in sepsis, sickle cell disease (SCD), and other hemolytic conditions, the biological importance and clinical relevance of HPX, the predominant heme binding protein, is reinforced. A perspective on the function of HPX and haptoglobin (Hp) is presented, updating how these two proteins and their respective receptors act simultaneously to protect the body in clinical conditions that entail hemolysis and/or systemic intravascular (IVH) inflammation. Evidence from longitudinal studies in patients supports that HPX plays a Hp-independent role in genetic and non-genetic hemolytic diseases without the need for global Hp depletion. Evidence also supports that HPX has an important role in the prognosis of complex illnesses characterized predominantly by the presence of hemolysis, such as SCD, sepsis, hemolytic-uremic syndrome, and conditions involving IVH and extravascular hemolysis (EVH), such as that generated by extracorporeal circulation during cardiopulmonary bypass (CPB) and from blood transfusions. We propose that quantitating the amounts of plasma heme, HPX, Hb-Hp, heme-HPX, and heme-albumin levels in various disease states may aid in the diagnosis and treatment of the above-mentioned conditions, which is crucial to developing targeted plasma protein supplementation (i.e., "replenishment") therapies for patients with heme toxicity due to HPX depletion.

Ferritins as nanoplatforms for imaging and drug delivery

INTRODUCTION

Due to unique architecture and surface properties, ferritin has emerged as an important class of biomaterial. Many studies suggest that ferritin and its derivatives hold great potential in a wide range of bio-applications.

AREAS COVERED

In this review, we summarize recent progress on employing ferritins as a platform to construct functional nanoparticles for applications in MRI, optical imaging, cell tracking, and drug delivery.

EXPERT OPINION

As a natural polymer, ferritins afford advantages such as high biocompatibility, good biodegradability, and a relatively long plasma half-life. These attributes put ferritins ahead of conventional materials in clinical translation for imaging and drug delivery purposes.

The hyperferritinemic syndrome: macrophage activation syndrome, Still's disease, septic shock and catastrophic antiphospholipid syndrome

BACKGROUND

Over the last few years, accumulating data have implicated a role for ferritin as a signaling molecule and direct mediator of the immune system. Hyperferritinemia is associated with a multitude of clinical conditions and with worse prognosis in critically ill patients.

DISCUSSION

There are four uncommon medical conditions characterized by high levels of ferritin, namely the macrophage activation syndrome (MAS), adult onset Still's disease (AOSD), catastrophic antiphospholipid syndrome (cAPS) and septic shock, that share a similar clinical and laboratory features, and also respond to similar treatments, suggesting a common pathogenic mechanism. Ferritin is known to be a pro-inflammatory mediator inducing expression of pro-inflammatory molecules, yet it has opposing actions as a pro-inflammatory and as an immunosuppressant. We propose that the exceptionally high ferritin levels observed in these uncommon clinical conditions are not just the product of the inflammation but rather may contribute to the development of a cytokine storm.

SUMMARY

Here we review and compare four clinical conditions and the role of ferritin as an immunomodulator. We would like to propose including these four conditions under a common syndrome entity termed "Hyperferritinemic Syndrome".

Bio-inspired protein-gold nanoconstruct with core-void-shell structure: beyond a chemo drug carrier

Chemotherapy has been widely used in clinical practice for cancer treatment. A major challenge for a successful chemotherapy is to potentiate the anticancer activity, whilst reducing the severe side effects. In this context, we design a bio-inspired protein-gold nanoconstruct (denoted as AFt-Au hereafter) with a core-void-shell structure which exhibits a high selectivity towards carcinoma cells. Anticancer drug 5-fluorouracil (5-FU) can be sequestered into the void space of the construct to produce an integrated nanoscale hybrid AFt-AuFU that exhibits an increased cellular uptake of 5-FU. More importantly, AFt-Au, serving as a bio-nano-chemosensitizer, renders carcinoma cells more susceptible to 5-FU by cell-cycle regulation, and thus, leads to a dramatic decrease of the IC₅₀ value (i.e. the drug concentration required to kill 50% of the cell population) of 5-FU in HepG2 cells from 138.3 μM to 9.2 μM. Besides HepG2 cells, a remarkably enhanced anticancer efficacy and potentially reduced side effects are also achieved in other cell lines. Our further work reveals that the drug 5-FU is internalized into cells with AFt-Au primarily via receptor-mediated endocytosis (RME). After internalization, AFt-AuFU colocalizes with lysosomes which trigger the release of 5-FU under acidic conditions. Overall, our approach provides a novel procedure in nanoscience that promises an optimal chemotherapeutic outcome.

Endocytic pathway of exogenous iron-loaded ferritin in intestinal epithelial (Caco-2) cells

Ferritin, a food constituent of animal and vegetal origin, is a source of dietary iron. Its hollow central cavity has the capacity to store up to 4,500 atoms of iron, so its potential as an iron donor is advantageous to heme iron, present in animal meats and inorganic iron of mineral or vegetal origin. In intestinal cells, ferritin internalization by endocytosis results in the release of its iron into the cytosolic labile iron pool. The aim of this study was to characterize the endocytic pathway of exogenous ferritin absorbed from the apical membrane of intestinal epithelium Caco-2 cells, using both transmission electron microscopy and fluorescence confocal microscopy. Confocal microscopy revealed that endocytosis of exogenous AlexaFluor 488-labeled ferritin was initiated by its engulfment by clathrin-coated pits and internalization into early endosomes, as determined by codistribution with clathrin and early endosome antigen 1 (EEA1). AlexaFluor 488-labeled ferritin also codistributed with the autophagosome marker microtubule-associated protein 1 light chain 3 (LC3) and the lysosome marker lysosomal-associated membrane protein 2 (LAMP2). Transmission electron microscopy revealed that exogenously added ferritin was captured in plasmalemmal pits, double-membrane compartments, and multivesicular bodies considered as autophagosomes and lysosomes. Biochemical experiments revealed that the lysosome inhibitor chloroquine and the autophagosome inhibitor 3-methyladenine (3-MA) inhibited degradation of exogenously added (131)I-labeled ferritin. This evidence is consistent with a model in which exogenous ferritin is internalized from the apical membrane through clathrin-coated pits, and then follows a degradation pathway consisting of the passage through early endosomes, autophagosomes, and autolysosomes.

The many faces of the octahedral ferritin protein

Iron is an essential trace nutrient required for the active sites of many enzymes, electron transfer and oxygen transport proteins. In contrast, to its important biological roles, iron is a catalyst for reactive oxygen species (ROS). Organisms must acquire iron but must protect against oxidative damage. Biology has evolved siderophores, hormones, membrane transporters, and iron transport and storage proteins to acquire sufficient iron but maintain iron levels at safe concentrations that prevent iron from catalyzing the formation of ROS. Ferritin is an important hub for iron metabolism because it sequesters iron during times of iron excess and releases iron during iron paucity. Ferritin is expressed in response to oxidative stress and is secreted into the extracellular matrix and into the serum. The iron sequestering ability of ferritin is believed to be the source of the anti-oxidant properties of ferritin. In fact, ferritin has been used as a biomarker for disease because it is synthesized in response to oxidative damage and inflammation. The function of serum ferritin is poorly understood, however serum ferritin concentrations seem to correlate with total iron stores. Under certain conditions, ferritin is also associated with pro-oxidant activity. The source of this switch from anti-oxidant to pro-oxidant has not been established but may be associated with unregulated iron release from ferritin. Recent reports demonstrate that ferritin is involved in other aspects of biology such as cell activation, development, immunity and angiogenesis. This review examines ferritin expression and secretion in correlation with anti-oxidant activity and with respect to these new functions. In addition, conditions that lead to pro-oxidant conditions are considered.

Serum ferritin: Past, present and future

BACKGROUND

Serum ferritin was discovered in the 1930s, and was developed as a clinical test in the 1970s. Many diseases are associated with iron overload or iron deficiency. Serum ferritin is widely used in diagnosing and monitoring these diseases.

SCOPE OF REVIEW

In this chapter, we discuss the role of serum ferritin in physiological and pathological processes and its use as a clinical tool.

MAJOR CONCLUSIONS

Although many aspects of the fundamental biology of serum ferritin remain surprisingly unclear, a growing number of roles have been attributed to extracellular ferritin, including newly described roles in iron delivery, angiogenesis, inflammation, immunity, signaling and cancer.

GENERAL SIGNIFICANCE

Serum ferritin remains a clinically useful tool. Further studies on the biology of this protein may provide new biological insights.

Binding and uptake of H-ferritin are mediated by human transferrin receptor-1

Ferritin is a spherical molecule composed of 24 subunits of two types, ferritin H chain (FHC) and ferritin L chain (FLC). Ferritin stores iron within cells, but it also circulates and binds specifically and saturably to a variety of cell types. For most cell types, this binding can be mediated by ferritin composed only of FHC (HFt) but not by ferritin composed only of FLC (LFt), indicating that binding of ferritin to cells is mediated by FHC but not FLC. By using expression cloning, we identified human transferrin receptor-1 (TfR1) as an important receptor for HFt with little or no binding to LFt. In vitro, HFt can be precipitated by soluble TfR1, showing that this interaction is not dependent on other proteins. Binding of HFt to TfR1 is partially inhibited by diferric transferrin, but it is hindered little, if at all, by HFE. After binding of HFt to TfR1 on the cell surface, HFt enters both endosomes and lysosomes. TfR1 accounts for most, if not all, of the binding of HFt to mitogen-activated T and B cells, circulating reticulocytes, and all cell lines that we have studied. The demonstration that TfR1 can bind HFt as well as Tf raises the possibility that this dual receptor function may coordinate the processing and use of iron by these iron-binding molecules.

Soybean ferritin: implications for iron status of vegetarians

Meeting the requirement for absorbed iron is difficult for vegetarians, and their iron status often is lower than that of nonvegetarians. Beans contain ferritin in low concentrations, but it is possible to enhance this content by plant breeding or by inserting the gene for ferritin into plants, eg, soybeans. Because each ferritin molecule can bind to thousands of iron atoms, this may be a sustainable means to increase the iron contents of plants. Before such efforts are launched, it is important to determine whether iron in ferritin is bioavailable. This has been assessed in vitro by using human intestinal (Caco-2) cells and in vivo by using radiolabeled ferritin and whole-body counting in human subjects. Dietary factors affecting iron absorption, eg, ascorbic acid, phytate, and calcium, had limited effect on iron uptake from intact ferritin by Caco-2 cells, which suggests that ferritin-bound iron is absorbed via a mechanism different from that of nonheme iron. In an in vitro digestion system, ferritin was shown to be relatively resistant to proteolytic enzymes. Binding of ferritin to Caco-2 cells was shown to be saturable, and the kinetics for binding were characteristic of a receptor-mediated process. In human subjects, iron from purified soybean ferritin given in a meal was as well absorbed as iron from ferrous sulfate. In conclusion, iron is well absorbed from ferritin and may represent a means of biofortification of staple foods such as soybeans.

Iron distribution in the liver and duodenum during seasonal iron overload in Svalbard reindeer

Seasonal iron overload in Svalbard reindeer was studied by light and electron microscopy and by X-ray microanalysis. The hepatic iron overload was of two types. The first type was characterized by massive siderosis of both parenchymal and non-parenchymal cells caused by a diet very rich in iron but low in energy and protein. Hepatocytes contained a moderate amount of free ferritin particles in the cytosol together with numerous siderosomes. This pattern is similar to that seen in primary haemochromatosis and thalassaemia. Kupffer cells contained large quantities of cytosolic ferritin, siderosomes and lysosomes with disintegrating red blood cells as seen in thalassaemia. The second type was characterized by massive non-parenchymal siderosis caused by an energy- and protein-poor diet with normal iron concentration. Hepatocytes contained little cytosolic ferritin and few siderosomes, but there were abundant electron-dense bodies without iron (i.e., autophagosomes). Kupffer cells were as described above. Ferritin was also present within the duodenal mucosa of these animals, located within enterocytes and lamina propria macrophages, as well as in the extracellular space and capillary and lacteal lumina. Ferritin was also present in the acinar cells of submucosal Brunner's glands. Changes consistent with exchange of ferritin particles between different cell types were observed. The role of ferritin as a possible iron transporter in this condition is discussed.

Ferritin functions as a proinflammatory cytokine via iron-independent protein kinase C zeta/nuclear factor kappaB-regulated signaling in rat hepatic stellate cells

Circulating ferritin levels reflect body iron stores and are elevated with inflammation in chronic liver injury. H-ferritin exhibits a number of extrahepatic immunomodulatory properties, although its role in hepatic inflammation and fibrogenesis is unknown. Hepatic stellate cells respond to liver injury through production of proinflammatory mediators that drive fibrogenesis. A specific receptor for ferritin has been demonstrated on activated hepatic stellate cells, although its identity and its role in stellate cell activation is unclear. We propose that ferritin acts as a cytokine regulating proinflammatory function via nuclear factor kappaB (NF-kappaB)-regulated signaling in hepatic stellate cell biology. Hepatic stellate cells were treated with tissue ferritin and iron-free apoferritin, recombinant H-ferritins and L-ferritins, to assess the role of ferritin versus ferritin-bound iron in the production of proinflammatory mediators of fibrogenesis, and to determine whether signaling pathways act via a proposed H-ferritin endocytosis receptor, T cell immunoglobulin-domain and mucin-domain 2 (Tim-2). This study demonstrated that ferritin activates an iron-independent signaling cascade, involving Tim-2 independent phosphoinositide 3 (PI3)-kinase phosphorylation, protein kinase C zeta (PKCzeta) and p44/p42-mitogen-activated protein kinase, resulting in p50/p65-NF-kappaB activation and markedly enhanced expression of hepatic proinflammatory mediators interleukin-1beta (IL-1beta), inducible nitric oxide synthase (iNOS), regulated on activation normal T cell expressed and secreted (RANTES), inhibitor of kappa Balpha (IkappaBalpha), and intercellular adhesion molecule 1 (ICAM1).

CONCLUSIONS

This study has defined the role of ferritin as a proinflammatory mediator of hepatic stellate cell biology acting through the NF-kappaB signaling pathway, and suggests a potential role in the inflammatory processes associated with hepatic fibrogenesis.

Effects of ascorbic acid, phytic acid and tannic acid on iron bioavailability from reconstituted ferritin measured by an in vitro digestion-Caco-2 cell model

The effects of ascorbic acid (AA), phytate and tannic acid (TA) on Fe bioavailability from Fe supplied as reconstituted ferritin were compared with FeSO4 using an in vitro digestion-Caco-2 cell model. Horse spleen apoferritin was chemically reconstituted into an animal-type ferritin (HSF) and a plant-type ferritin (P-HSF) according to the typical ratios of Fe:P found in these molecules. In the presence of AA (Fe:AA molar ratio of 1:20), significantly more Fe was absorbed from FeSO4 (about 303 %), HSF (about 454 %) and P-HSF (about 371 %) when compared with ferrous sulfate or ferritin without AA. Phytic acid (PA; Fe:PA molar ratio of 1:20) significantly reduced Fe bioavailability from FeSO4 (about 86 %), HSF (about 82 %) and P-HSF (about 93 %) relative to FeSO4 and the ferritin controls. Treatment with TA (Fe:TA molar ratio of 1:1) significantly decreased Fe bioavailability (about 97 %) from both FeSO4 and the ferritin samples. AA was able to partially reverse the negative effect of PA (Fe:PA:AA molar ratio of 1:20:20) on Fe bioavailability but did not reverse the inhibiting effect of TA (Fe:TA:AA molar ratio of 1:1:20) on Fe bioavailability from ferritin and FeSO4. Overall, there were no significant differences in bioavailable Fe between P-HSF, HSF or FeSO4. Furthermore, the addition of AA (a known promoter) or the inhibitors, PA and TA, or both, did not result in significant differences in bioavailable Fe from ferritin relative to FeSO4. The results suggest that Fe in the reconstituted ferritin molecule is easily released during in vitro digestion and interacts with known promoters and inhibitors.

Intracellular iron transport and storage: from molecular mechanisms to health implications

Maintenance of proper "labile iron" levels is a critical component in preserving homeostasis. Iron is a vital element that is a constituent of a number of important macromolecules, including those involved in energy production, respiration, DNA synthesis, and metabolism; however, excess "labile iron" is potentially detrimental to the cell or organism or both because of its propensity to participate in oxidation-reduction reactions that generate harmful free radicals. Because of this dual nature, elaborate systems tightly control the concentration of available iron. Perturbation of normal physiologic iron concentrations may be both a cause and a consequence of cellular damage and disease states. This review highlights the molecular mechanisms responsible for regulation of iron absorption, transport, and storage through the roles of key regulatory proteins, including ferroportin, hepcidin, ferritin, and frataxin. In addition, we present an overview of the relation between iron regulation and oxidative stress and we discuss the role of functional iron overload in the pathogenesis of hemochromatosis, neurodegeneration, and inflammation.

New functions for an iron storage protein: the role of ferritin in immunity and autoimmunity

Ferritin is a ubiquitous and specialised protein involved in the intracellular storage of iron; it is also present in serum and other biological fluids, although its secretion processes are still unclear. We here review evidence supporting the hypothesis that macrophages play a role in the production and secretion of extracellular ferritin, as well as evidence supporting a novel function as a signalling molecule and immune regulator. In particular, H-ferritin, which inhibits the proliferation of lymphoid and myeloid cells, may be regarded as a negative regulator of human and murine hematopoiesis. The idea that it also acts as a signalling protein has been supported by the cloning and characterisation of the specific H-ferritin receptor TIM-2, a member of the TIM gene family. A number of studies of the mouse TIM gene family indicate that this protein plays an important role in immune-mediated diseases. This last finding, together with the fact that ferritin acts as an immuno-suppressor, has allowed us to formulate hypotheses regarding the possible role of alterations of H-ferritin/TIM-2 binding/signalling in the pathogenesis of autoimmune diseases.

Molecular mechanisms involved in intestinal iron absorption

Iron is an essential trace metal in the human diet due to its obligate role in a number of metabolic processes. In the diet, iron is present in a number of different forms, generally described as haem (from haemoglobin and myoglobin in animal tissue) and non-haem iron (including ferric oxides and salts, ferritin and lactoferrin). This review describes the molecular mechanisms that co-ordinate the absorption of iron from the diet and its release into the circulation. While many components of the iron transport pathway have been elucidated, a number of key issues still remain to be resolved. Future work in this area will provide a clearer picture regarding the transcellular flux of iron and its regulation by dietary and humoral factors.

Effects of dietary factors on iron uptake from ferritin by Caco-2 cells

Biofortification of staple foods with iron (Fe) in the form of ferritin (Ft) is now possible, both by conventional plant breeding methods and transgenic approaches. Ft-Fe from plants and animals is absorbed well (25-30%) by human subjects, but little is known about dietary factors affecting its absorption. We used human intestinal Caco-2 cells and compared Fe absorption from animal Ft and FeSO4 to determine the effects of inhibitors and enhancers, such as phytic acid, ascorbic acid, tannic acid, calcium and heme. When postconfluent cells were coincubated with 59Fe-labeled (1 microM) FeSO4 and dietary factors, at different molar ratios of dietary factor to Fe (phytic acid:Fe, 10:1; ascorbic acid:Fe, 50:1; tannic acid:Fe, 50:1; calcium:Fe, 10:1 and hemin:Fe, 10:1), all inhibited uptake from FeSO4, except ascorbate, confirming earlier studies. In contrast, these dietary factors had little or no effect on Fe uptake from undigested Ft or Ft digested in vitro at pH 4, except tannins. However, results after in vitro digestion of Ft at pH 2 were similar to those obtained for FeSO4. These results suggest that Fe uptake occurs from both undigested as well as digested Ft but, possibly, via different mechanisms. The Fe-Ft stability shown here could minimize Fe-induced oxidation of Fe-supplemented food products.

TIM-2 is expressed on B cells and in liver and kidney and is a receptor for H-ferritin endocytosis

T cell immunoglobulin-domain and mucin-domain (TIM) proteins constitute a receptor family that was identified first on kidney and liver cells; recently it was also shown to be expressed on T cells. TIM-1 and -3 receptors denote different subsets of T cells and have distinct regulatory effects on T cell function. Ferritin is a spherical protein complex that is formed by 24 subunits of H- and L-ferritin. Ferritin stores iron atoms intracellularly, but it also circulates. H-ferritin, but not L-ferritin, shows saturable binding to subsets of human T and B cells, and its expression is increased in response to inflammation. We demonstrate that mouse TIM-2 is expressed on all splenic B cells, with increased levels on germinal center B cells. TIM-2 also is expressed in the liver, especially in bile duct epithelial cells, and in renal tubule cells. We further demonstrate that TIM-2 is a receptor for H-ferritin, but not for L-ferritin, and expression of TIM-2 permits the cellular uptake of H-ferritin into endosomes. This is the first identification of a receptor for ferritin and reveals a new role for TIM-2.

Iron absorption from soybean ferritin in nonanemic women

BACKGROUND

Dietary ferritin, a protein cage around an iron mineral, is an underestimated source of bioavailable iron. Plant ferritin, the most common dietary ferritin, has not been studied. Iron from animal ferritin is absorbed as well as is iron from FeSO4 in women.

OBJECTIVE

The objective was to examine iron absorption from purified soybean ferritin.

DESIGN

Healthy, nonanemic women (n = 16) were fed a standardized meal (bagel, cream cheese, and apple juice) containing 1 microCi 59Fe/meal as FeSO4 or (extrinsically labeled) as iron-free soybean ferritin reconstituted with the high phosphate characteristic of plant ferritin (iron:phosphorus = 4:1). Iron-free, apo-soybean ferritin was prepared (with the use of thioglycolic acid and extensive dialysis) from purified ferritin. In a randomized crossover design, the other labeled meal, which contained FeSO4 or ferritin, was given after 4 wk. The subjects received 140 microg Fe as ferritin (2.5 mg) or as FeSO4. After 28 d, whole-body 59Fe and 59Fe in red blood cells were measured before and after dosing.

RESULTS

There was no significant difference in whole-body iron absorption from soybean ferritin (29.9 +/- 19.8%) and that from FeSO4 (34.3 +/- 23.6%) or in iron absorption calculated from red blood cell incorporation (33.0 +/- 20.1% for soybean ferritin and 35.3 +/- 23.4% for FeSO4), which confirmed previous results with animal ferritin that was mineralized and labeled similarly. An inverse relation was observed between serum ferritin and iron absorption from both ferritin and FeSO4, which suggested that sensors regulating iron absorption respond similarly to iron provided as ferrous salts or as ferritin mineral.

CONCLUSION

Iron from soybean ferritin is well absorbed and may provide a model for novel, utilizable, plant-based forms of iron for populations with a low iron status.

Ferritin induction protects cortical astrocytes from heme-mediated oxidative injury

Hemin is released from hemoglobin after CNS hemorrhage and may contribute to its cytotoxic effect. In a prior study, we demonstrated that heme oxygenase-1 induction protected murine cortical astrocytes from hemoglobin toxicity. Since heme metabolism releases iron, this observation suggested that these cells are able to effectively sequester and detoxify free iron. In this study, we tested the hypotheses that astrocytes increased ferritin synthesis after exposure to heme-bound iron, and that this induction protected cells from subsequent exposure to toxic concentrations of hemin. Incubation with low micromolar concentrations of hemin, hemoglobin, or ferrous sulfate increased ferritin expression, as detected on immunoblots stained with a polyclonal antibody that was raised against horse spleen ferritin. Time course studies demonstrated an increase in ferritin levels within 2 h. Weak and scattered cellular staining was detected by immunohistochemistry in control, untreated cultures, while diffuse immunoreactivity was observed in cultures exposed to heme-bound iron. An enhanced ferritin band was detected on immunoblots from cultures that were treated with purified apoferritin, consistent with astrocytic ferritin uptake. Immunoreactivity after apoferritin treatment was not altered by concomitant treatment with cycloheximide. Pretreatment with apoferritin protected astrocytes from hemin toxicity in a concentration-dependent fashion between 1 and 4 mg/ml. At the highest concentration, cell death due to a 6-h exposure to 30 microM hemin was decreased by about 85%. A protective effect was also produced by induction of endogenous ferritin with nontoxic concentrations of ferrous sulfate, hemoglobin, or hemin. These results suggest that cortical astrocytes respond to exogenous heme-bound or free iron by rapidly increasing ferritin synthesis. The combined action of heme oxygenase-1 and ferritin may be a primary astrocytic defense against heme-mediated injury.

Ferritin binds to light chain of human H-kininogen and inhibits kallikrein-mediated bradykinin release

Ferritin is an iron-storage protein that exists in both intracellular and extracellular compartments. We have previously identified H-kininogen (high-molecular-weight kininogen) as a ferritin-binding protein [Torti and Torti (1998) J. Biol. Chem. 273, 13630-13635]. H-Kininogen is a precursor of the potent pro-inflammatory peptide bradykinin, which is released from H-kininogen following cleavage of H-kininogen by the serine protease kallikrein. In this report, we demonstrate that binding of ferritin to H-kininogen occurs via the modified light chain of H-kininogen, and that ferritin binds preferentially to activated H-kininogen. We further demonstrate that binding of ferritin to H-kininogen retards the proteolytic cleavage of H-kininogen by kallikrein and its subsequent release of bradykinin from H-kininogen. Ferritin does not interfere with the ability of kallikrein to digest a synthetic substrate, suggesting that ferritin specifically impedes the ability of kallikrein to digest H-kininogen, perhaps by steric hindrance. Based on these results, we propose a model of sequential H-kininogen cleavage and ferritin binding. These results are consistent with the hypothesis that the binding of ferritin to H-kininogen may serve to modulate bradykinin release.

Expression of ferritin receptor in placental microvilli membrane in pregnant women with different iron status at mid-term gestation

OBJECTIVE

The effect of iron status in pregnant women on expression of ferritin receptor in placental microvilli membrane at mid-term gestation was investigated.

DESIGN

Ferritin receptor binding sites and dissociation constants (K(d)) were determined in specimens of placental microvilli from 30 pregnant women at mid-term gestation and six women at term-delivery.

RESULTS

The ferritin receptor binding sites in the placental microvilli membrane in pregnant women with mild iron deficiency and moderate iron deficiency anemia were significantly higher then those in pregnant women with normal iron status. However, no significant difference was found between pregnant women with severe iron deficiency anemia and with normal measurements. No significant differences of K(d) values were detected between pregnant women with normal iron status and those with iron-deficiency. Data also revealed that the ferritin receptor binding sites in placental microvilli membrane and K(d) values at mid-term gestation did not differ significantly from those at term gestation.

CONCLUSION

Lower iron status in pregnant women could lead to an increase in expression of ferritin receptor in placental microvilli membrane at mid-term gestation while the dissociation constant of ferritin receptor remains unchanged. This implies that the regulation of maternal-fetal iron homeostasis via the ferritin receptor-mediated pathway is achieved by changes in the numbers of ferritin receptors rather then binding properties.

Immunosuppressive effects of melanoma-derived heavy-chain ferritin are dependent on stimulation of IL-10 production

Cultured melanoma cells release soluble factors that influence immune responses. Screening of a cDNA library with anti-sera from a melanoma patient identified an immunoreactive plaque, which encoded heavy-chain ferritin (H-ferritin). Previous studies have drawn attention to the immunosuppressive effects of this molecule and prompted further studies on its biochemical and functional properties in human melanoma. These studies demonstrated, firstly, that H-ferritin appeared to be secreted by melanoma cells, as shown by immunoprecipitation of a 21.5 kDa band from supernatants. It was also detected in extracts of melanoma cells by Western blotting as 43 and 64 kDa dimers and trimers of the 21.5 kDa fraction. Secondly, flow-cytometric analysis of H- and light-chain ferritin (L-ferritin) expression on melanoma showed a wide variation in L-ferritin expression and consequently of the ratio of H- to L-ferritin expression. Suppression of mitogenic responses of lymphocytes to anti-CD3 showed a correlation with the ratio of H- to L-ferritin in the supernatants and was specific for H-ferritin, as shown by inhibition studies with a monoclonal antibody (MAb) against H-ferritin. Similar results were obtained with H- and L-ferritin from other sources. Suppression of mitogenic responses of lymphocytes to anti-CD3 by H-ferritin was inhibited using a MAb against IL-10, which suggested that the immunosuppressive effect of H-ferritin was mediated by IL-10. Assays of cytokine production from anti-CD3-stimulated lymphocytes showed that H-ferritin markedly increased production of IL-10 and IFN-gamma and had only slight effects on IL-2 and IL-4 production. Our results suggest that melanoma cells may be a major source of H-ferritin and that production of the latter may account for some of the immunosuppressive effects of melanoma.

Effect of chronic chloroquine administration on iron loading in the liver and reticuloendothelial system and on oxidative responses by the alveolar macrophages

The ability of chloroquine to alter iron loading in the liver, spleen, and alveolar macrophages was investigated in iron-loaded or -depleted rats. Chloroquine significantly reduced incorporation of iron into the liver, spleen, and alveolar macrophages of animals loaded in vivo with iron dextran. The ability of these macrophages to respond to oxidative stress was assayed by their capacity to release reactive nitrogen intermediates after lipopolysaccharide (LPS) stimulation. A significant reduction in nitrite release was observed in primary cultures of macrophages isolated from chloroquine/iron dextran-administered rats in comparison to macrophages lavaged from rats iron-loaded alone. Macrophages isolated from iron-deficient rats showed a significant increase in nitrite after LPS stimulation, whereas nitrite release in the macrophages lavaged from the rats which had also received chloroquine during the iron depletion regime was much lower. These results indicate that the use of agents which decrease the iron content and diminish the oxidative response of the cell to altered iron status may be of therapeutic value in patients with iron loading, particularly of the reticuloendothelial system.

Interrelationships of alcohol and iron in liver disease with particular reference to the iron-binding proteins, ferritin and transferrin

It is known that the regular consumption of alcohol is responsible for the disruption of normal iron metabolism in humans, resulting in the excess deposition of iron in the liver in approximately one-third of alcoholic subjects. The mechanisms involved are largely unknown; however, it is likely that the two major proteins of iron metabolism, ferritin and transferrin are intimately involved in the process. Tissue damage in alcoholic liver disease and the inherited iron-overload disease, haemochromatosis, are caused by excess alcohol and iron, respectively. The mechanisms of this damage are believed to be similar in both disease conditions and involve free radical-mediated toxicity. A high proportion of haemochromatosis sufferers consume excessive amounts of alcohol and synergistic hepatotoxic events may occur leading to the earlier development of liver cirrhosis. This review describes briefly the role of ferritin and transferrin in normal iron metabolism and in iron overload disease and explores the possible involvement of these proteins in the pathophysiology of excess iron deposition in alcoholic subjects.

Human H-kininogen is a ferritin-binding protein

H-kininogen is a multifunctional protein: it inhibits cysteine proteases, plays a role in contact activation of the coagulation cascade, and is the precursor of the potent proinflammatory peptide bradykinin. In the experiments described here, we identify H-kininogen as a ferritin-binding protein. Ferritin is a cellular and serum protein that is elevated in acute and chronic inflammation and many cancers. Despite numerous reports of ferritin-binding protein(s) in human serum, the nature and function of these proteins remain unclear. As a first step in characterizing the interaction between ferritin and its binding protein(s), we devised a ligand blot assay and used it to guide purification of a ferritin-binding protein from human serum. Edman degradation of the purified protein determined the sequence HNLGHGHK(H)ERDQGHG, a sequence with identity to residues 421-436 of human H-kininogen. These results were confirmed by demonstrating that commercially purified H-kininogen possessed ferritin binding activity and that ferritin binding could not be detected in plasma from kininogen-deficient individuals. Ligand blot assays mapped the ferritin binding domain to the light chain of H-kininogen chain, and revealed that both H and L recombinant ferritins possess H-kininogen binding activity. The unexpected identification of H-kininogen as a ferritin-binding protein may link ferritin in the complex chain of interactions by which H-kininogen mediates its multiple effects in contact activation and inflammation.

Comparison of the three-dimensional structures of recombinant human H and horse L ferritins at high resolution

Mammalian ferritins are 24-mers assembled from two types of polypeptide chain which provide the molecule with different functions. H(eavy) chains catalyse the first step in iron storage, the oxidation of iron(II). L(ight) chains promote the nucleation of the mineral ferrihydrite enabling storage of iron(III) inside the protein shell. We report here the comparison of the three-dimensional structures of recombinant human H chain (HuHF) and horse L chain (HoLF) ferritin homopolymers, which have been refined at 1.9 A resolution. There is 53% sequence identity between these molecules, and the two structures are very similar, the H and L subunit alpha-carbons superposing to within 0.5 A rms deviation with 41 water molecules in common. Nevertheless, there are significant important differences which can be related to differences in function. In particular, the centres of the four-helix bundles contain distinctive groups of hydrophilic residues which have been associated with ferroxidase activity in H chains and enhanced stability in L chains. L chains contain a group of glutamates associated with mineralisation within the iron storage cavity of the protein.

Iron metabolism: a comprehensive review

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

The ferritins: molecular properties, iron storage function and cellular regulation

The iron storage protein, ferritin, plays a key role in iron metabolism. Its ability to sequester the element gives ferritin the dual functions of iron detoxification and iron reserve. The importance of these functions is emphasised by ferritin's ubiquitous distribution among living species. Ferritin's three-dimensional structure is highly conserved. All ferritins have 24 protein subunits arranged in 432 symmetry to give a hollow shell with an 80 A diameter cavity capable of storing up to 4500 Fe(III) atoms as an inorganic complex. Subunits are folded as 4-helix bundles each having a fifth short helix at roughly 60 degrees to the bundle axis. Structural features of ferritins from humans, horse, bullfrog and bacteria are described: all have essentially the same architecture in spite of large variations in primary structure (amino acid sequence identities can be as low as 14%) and the presence in some bacterial ferritins of haem groups. Ferritin molecules isolated from vertebrates are composed of two types of subunit (H and L), whereas those from plants and bacteria contain only H-type chains, where 'H-type' is associated with the presence of centres catalysing the oxidation of two Fe(II) atoms. The similarity between the dinuclear iron centres of ferritin H-chains and those of ribonucleotide reductase and other proteins suggests a possible wider evolutionary linkage. A great deal of research effort is now concentrated on two aspects of ferritin: its functional mechanisms and its regulation. These form the major part of the review. Steps in iron storage within ferritin molecules consist of Fe(II) oxidation, Fe(III) migration and the nucleation and growth of the iron core mineral. H-chains are important for Fe(II) oxidation and L-chains assist in core formation. Iron mobilisation, relevant to ferritin's role as iron reserve, is also discussed. Translational regulation of mammalian ferritin synthesis in response to iron and the apparent links between iron and citrate metabolism through a single molecule with dual function are described. The molecule, when binding a [4Fe-4S] cluster, is a functioning (cytoplasmic) aconitase. When cellular iron is low, loss of the [4Fe-4S] cluster allows the molecule to bind to the 5'-untranslated region (5'-UTR) of the ferritin m-RNA and thus to repress translation. In this form it is known as the iron regulatory protein (IRP) and the stem-loop RNA structure to which it binds is the iron regulatory element (IRE). IREs are found in the 3'-UTR of the transferrin receptor and in the 5'-UTR of erythroid aminolaevulinic acid synthase, enabling tight co-ordination between cellular iron uptake and the synthesis of ferritin and haem. Degradation of ferritin could potentially lead to an increase in toxicity due to uncontrolled release of iron. Degradation within membrane-encapsulated "secondary lysosomes' may avoid this problem and this seems to be the origin of another form of storage iron known as haemosiderin. However, in certain pathological states, massive deposits of "haemosiderin' are found which do not arise directly from ferritin breakdown. Understanding the numerous inter-relationships between the various intracellular iron complexes presents a major challenge.

Red blood cell polyamines, anaemia and tumour growth in the rat

In rats with Mat Lylu prostatic carcinoma, significant changes in blood composition and red blood cell (RBC) characteristics were observed. Anaemia, characterised by a decrease in the number of RBC and the reduction of haemoglobin and the iron content in plasma, was correlated with tumour size and the accumulation of spermidine and spermine in the RBC. In large tumours, spermidine levels were increased by 8-fold over normal value. Spleen weight and splenic spermidine concentrations were enhanced in animals with tumours. After splenectomy, the rate of tumour growth decreased by 30%. It is proposed that anaemia in tumour-bearing animals is caused by enhanced RBC lysis, owing to the alteration of the rheological properties of RBC. These may be caused by the alterated surface characteristics due to polyamine accumulation. RBC lysis and high concentrations of polyamines in RBC and spleen appear, not only to favour tumour growth, but also to compromise the immunological defence mechanisms against neoplastic invasion.

Horse spleen ferritin inhibits superoxide production by equine blood monocytes in vitro

The effect of horse spleen ferritin (HFR) on the production of superoxide anion (O2.-) by equine blood monocytes was investigated. Preincubation of monocytes with HFR resulted in pronounced inhibition of O2.- production in response to phorbol 12-myristate 13-acetate (PMA), N-formyl-methionyl-leucyl-phenylalanine (FMLP), and opsonized zymosan (OZ). The inhibitory effect of HFR upon stimulation of monocytes with PMA was both dose and time dependent. Maximum inhibition (90%) was observed after preincubation of monocytes with HFR (2 mg/ml) for 18 h before stimulation with PMA. ApoHFR at the same concentration showed only about one-third of the inhibitory effect of iron-saturated HFR. Various iron complexes, such as iron dextran, hemin, or ferric ammonium sulfate, had no significant effect on O2.- production by monocytes. Neither catalase (Cat) nor desferrioxamine (DFO) changed the inhibitory effect of HFR. These findings suggest that HFR may play an important role in inhibition of superoxide generation by equine monocytes. Although the mechanism of this inhibition remains unknown, the results obtained suggest that it is not due to ferritin-dependent oxidative inactivation of the NADPH-oxidase system in stimulated monocytes.

The endocytic pathway for H-ferritin established in live MOLT-4 cells by laser scanning confocal microscopy

We have established the intracellular destination of the putative immunoregulatory protein, human recombinant H (heavy)-ferritin, in the transformed T-cell line MOLT-4, by laser scanning confocal microscopy of live cells. A series of confocal images was collected over a 60 min time course using indirect immunofluorescence of H-ferritin and transferrin, their respective monoclonal antibodies, and fluorescein isothiocyanate (FITC)-labelled IgG. A marked drop in FITC fluorescence after 40 min of H-ferritin internalization, indicative of an acidic environment, and co-localization with tetramethylrhodamine isothiocyanate-labelled-dextran strongly suggests that H-ferritin is transferred to the lysosome. In contrast, transferrin was observed to return to the cell surface. Electron microscopy confirmed that H-ferritin was transferred to the lysosome. The receptor-mediated endocytosis and lysosomal delivery of H-ferritin may thus potentiate its putative immunoregulatory activity.

Hyperthermia of the cancerous breast: analysis of mechanism

Cancerous breast hyperthermia is seemingly associated with non-neurological vasodilation modulated by nitric oxide (NO). NO, associated with enhanced immune response, is produced autocatalytically involving ferritin as the supplier of Fe2+, which catalyses the formation of NO. NO, in turn, releases Fe2+ from ferritin. This mechanism implies: (1) dependence of hyperthermia on the ferritin content of the neoplastic tissue; (2) oscillatory behavior of the hyperperfusion; (3) hyperthermia that extends far beyond the boundaries of the neoplastic tissue; (4) diminished neurological control of the perfusion in the affected breast; (5) limitations on the observed asymmetry between the breasts. These five effects were previously observed in numerous independent studies. Monitoring the temporal behavior of the hyperthermia is expected to substantially increase both sensitivity and specificity of cancer detection.

Identification and characterization of a receptor for tissue ferritin on activated rat lipocytes

Hepatic iron overload causes lipocyte activation with resultant fibrogenesis. This study examines whether rat lipocytes express ferritin receptors, which could be involved in paracellular iron movement and in cellular regulation. Lipocytes from normal rat liver were cultured on plastic and incubated with 125I-labeled rat liver ferritin (RLF) +/- a 100-fold excess of either unlabeled RLF or human heart ferritin, human liver ferritin, human recombinant H-ferritin, a mutant human recombinant L-ferritin, or a variety of nonspecific proteins. Specific binding sites for ferritin were demonstrated by displacement of 125I-RLF by RLF (64.5 +/- 4.3%) and by other ferritins (55-60%), but not by recombinant L-ferritin. Scatchard analysis demonstrated a single class of binding sites with a Kd of 5.1 +/- 2.9 x 10(-10) M, maximum binding capacity of 4.7 +/- 1.3 x 10(-12) M, and 5,000-10,000 receptor sites/cell. Ferritin receptor expression was observed only in activated lipocytes. Internalization of RLF was observed within 15 min using FITC-RLF and confocal microscopy. This study demonstrates that (a) activated lipocytes express a specific high affinity ferritin receptor; (b) the binding appears to be dependent on the H-ferritin subunit; and (c) lipocytes internalize ferritin. Expression of ferritin receptors in activated lipocytes suggests that the receptor may either be involved in the activation cascade or may be a marker of activation.

cDNA cloning and deduced amino acid sequence of two ferritins: soma ferritin and yolk ferritin, from the snail Lymnaea stagnalis L

Pulmonate freshwater snails contain two different ferritin types, soma ferritin and yolk ferritin. A cDNA library was constructed from midgut gland poly(A)-rich RNA of the snail Lymnaea stagnalis L. and recombinant clones encoding both ferritin types were obtained by immunoscreening. The longest cDNA inserts had a length of 859 bp (soma ferritin) and 1548 bp (yolk ferritin) and the specificity of these inserts was confirmed by immunoprecipitation of both ferritin types translated in vitro from hybrid-selected mRNAs. The 5' untranslated region (UTR) of the soma ferritin mRNA contains a 28-bp element which shows 64% sequence identity with the iron-responsive element (IRE) of vertebrate ferritin mRNAs. The soma ferritin mRNA is strongly translated in the wheat germ system but poorly translated in rabbit reticulocyte lysate. The yolk ferritin mRNA, which contains no IRE, is equally well translated in both in vitro translation systems. The deduced amino acid sequence of the soma ferritin subunit (174 amino acid residues, M(r) 20140) shows 50-70% sequence identity with subunits of vertebrate ferritins. After removal of an 18-amino-acid-residue signal sequence the deduced protein sequence of yolk ferritin contains 221 amino acids (M(r) 25438). Sequence identity of this chain with other eukaryotic ferritin chains is only 31-42%. Both snail ferritin sequences are more similar to the H-subunit type of vertebrate ferritins than to the L-type and both have the H-specific amino acid residues of the ferroxidase centre. The yolk ferritin sequence has a 42-amino-acid-residue insertion predicted to reside in the L loop of the subunit.

Pathophysiology of iron toxicity

There are several inherited and acquired disorders that can result in chronic iron overload in humans, and the major clinical consequences are hepatic fibrosis, cirrhosis, hepatocellular cancer, cardiac disease, and diabetes. It is clear that lipid peroxidation occurs in experimental iron overload if sufficiently high levels of iron within hepatocytes are achieved. Lipid peroxidation is associated with hepatic mitochondrial and microsomal dysfunction in experimental iron overload, and lipid peroxidation may underlie the increased lysosomal fragility that has been detected in liver samples from both iron-loaded human subjects and experimental animals. Reduced cellular ATP levels, impaired cellular calcium homeostasis, and damage to DNA may all contribute to hepatocellular injury in iron overload. Long-term dietary iron overload in rats can lead to increased collagen gene expression and hepatic fibrosis, perhaps due to activation of hepatic lipocytes. The mechanisms whereby lipocytes are activated in iron overload remain to be elucidated; possible mediators include aldehydic products of iron-induced lipid peroxidation produced in hepatocytes, tissue ferritin, and/or cytokines released by activated Kupffer cells.