Increase In Ferritin Synthesis Research Articles

Iron and zinc bioavailability in Caco-2 cells: Influence of caseinophosphopeptides

A study has been made of the influence of two pools of caseinophosphopeptides (CPPs) obtained from αs- and β-casein (CN) fractions, and of three specific CPPs (β-CN(1–25)4P, αs1-CN(64–74)4P and αs2-CN(1–19)4P), on iron bioavailability (ferritin synthesis) and zinc bioavailability (retention, transport and uptake of zinc) in Caco-2 cells.α-CPP and β-CPP pools did not improve ferritin synthesis, but the three specific CPPs showed an increase in ferritin synthesis in Caco-2 cells versus iron sulphate, β-CN(1–25)4P being the most effective. In relation to zinc bioavailability, α-CPPs, β-CPPs, αs1-CN(64–74)4P and β-CN(1–25)4P increased zinc uptake. However, this increase was of the same order as the increase due to the presence of zinc sulphate.

Hyperferritinaemia; laboratory implications

In this edition, two articles describe cases of hyperferritinaemia. Thurlow et al. describe two novel mutations in the L-ferritin coding sequence associated with benign hyperferritinaemia showing abnormal ferritin glycosylation. Marshall et al. discuss causes of extreme hyperferritinaemia (plasma ferritin concentration .10,000 mg/L). The body’s iron content is controlled through absorption in the duodenum by active processes: free Fe2þ via the divalent metal transporter 1 and haem-bound iron via the haem carrier protein 1. Within the enterocyte, some of the iron combines with the protein apoferritin to form ferritin. Ferritin is the main intracellular iron storage protein, present in all cells but mainly in hepatocytes and reticuloendothelial cells. Free iron is exported from enterocytes via ferroportin into the circulation where it binds to transferrin for storage and transport. Transferrin-bound iron then enters target cells via receptor-mediated endocytosis. Hepcidin is an iron regulatory protein (IRP), and inhibits ferroportin. The human haemochromatosis protein (HFE) regulates the binding of transferrin to the transferrin receptor as well as regulating hepcidin. Caeruloplasmin also possesses ferroxidase activity that oxidizes Fe2þ into Fe3þ ions. Ferritin is a 450 kDa protein that possesses a spherical shell with a central cavity where up to 4500 iron atoms can be oxidized and stored. Ferritin is a multimer protein composed of H (heavy; 21 kDa) and L (light; 19 kDa) subunits in variable proportions depending upon the particular tissue. The ferritin H subunit expresses ferroxidase activity which oxidizes iron from the ferrous (Fe2þ) to the less toxic ferric (Fe3þ) form. The H-ferritin gene resides on chromosome 11 and the L-ferritin gene on chromosome 19. Hepatic ferritin synthesis can occur as part of an acute phase response being stimulated by cytokines such as tumour necrosis factor a and interleukin 1a. Ferritin synthesis also involves a relationship between an iron binding protein or IRP and the ferritin mRNA. In the presence of iron overload, the IRP inhibitory system is suppressed, resulting in an increase in ferritin synthesis. In the steady-state, plasma ferritin correlates with total body iron stores. Hyperferritinaemia occurs in iron excess syndromes such as certain forms of haemochromatosis and secondary iron overload conditions such as chronic haemolysis, iron poisoning, porphyria cutanea tarda, multiple blood transfusions and iron loading anaemias, e.g. betathalassaemia, congenital sideroblastic or dyserythropoietic anaemias. Hyperferritinaemia is also observed in acute and chronic inflammation, carcinoma, chronic infection and autoimmune disease. Cytolysis from the hepatocytes such as occurs in acute or chronic liver diseases also results in hyperferritinaemia. Hyperferritinaemia can occur in familial combined hyperlipidaemia and in familial hypertriglyceridaemia. Plasma ferritin concentration correlates also with diastolic blood pressure and plasma HDL-cholesterol concentration and may be a ‘marker’ of metabolic syndrome. Plasma ferritin may additionally be an independent indicator of poor glycaemic control in type 2 diabetes mellitus. It has also been suggested that in liver steatosis, hyperferritinaemia is associated with raised plasma C-peptide concentration and insulin resistance although there is not usually iron overload. A metabolic disorder associated with hyperferritinaemia and abnormal liver function but no iron overload has been described where patients also had one or more of the following: increased body mass index, dyslipidaemia, abnormal glucose tolerance or hypertension. Benign hyperferritinaemia (BH) is associated with a missense ferritin L-subunit mutation (p.Thr30Ile) without iron overload and usually showing a normal transferrin saturation. Hereditary hyperferritinaemia cataract syndrome (HHCS) is also a genetic condition that displays hyperferritinaemia without iron overload, usually with normal transferrin saturation and bilateral nuclear cataracts. Here, mutations of the ferritin L-subunit gene (19q13.1) as part of the iron regulatory element are responsible for the upregulation and hence overexpression of ferritin. Iron overload is not found in BH or HHCS because the raised plasma ferritin concentration is due to increased ferritin L-subunit production which is not involved in the storage of iron. It is important to differentiate such conditions from true iron overload as the BH or HHCS patients may develop iron deficiency anaemia if they undergo venesection. Other causes of hyperferritinaemia that may not present witha raised transferrin saturation includehaemochromatosis type 4a, or ferroportin (SLC40A1) disease which displays macrophage iron accumulation (African iron overload mutation) and also acaeruloplasminaemia. This is a genetic disorder of parenchymal iron overload, which manifests neurological abnormalities such as those of the

Non-heme Induction of Heme Oxygenase-1 Does Not Alter Cellular Iron Metabolism

The catabolism of heme is carried out by members of the heme oxygenase (HO) family. The products of heme catabolism by HO-1 are ferrous iron, biliverdin (subsequently converted to bilirubin), and carbon monoxide. In addition to its function in the recycling of hemoglobin iron, this microsomal enzyme has been shown to protect cells in various stress models. Implicit in the reports of HO-1 cytoprotection to date are its effects on the cellular handling of heme/iron. However, the limited amount of uncommitted heme in non-erythroid cells brings to question the source of substrate for this enzyme in non-hemolytic circumstances. In the present study, HO-1 was induced by either sodium arsenite (reactive oxygen species producer) or hemin or overexpressed in the murine macrophage-like cell line, RAW 264.7. Both of the inducers elicited an increase in active HO-1; however, only hemin exposure caused an increase in the synthesis rate of the iron storage protein, ferritin. This effect of hemin was the direct result of the liberation of iron from heme by HO. Cells stably overexpressing HO-1, although protected from oxidative stress, did not display elevated basal ferritin synthesis. However, these cells did exhibit an increase in ferritin synthesis, compared with untransfected controls, in response to hemin treatment, suggesting that heme levels, and not HO-1, limit cellular heme catabolism. Our results suggest that the protection of cells from oxidative insult afforded by HO-1 is not due to the catabolism of significant amounts of cellular heme as thought previously.

CD163-Mediated Hemoglobin-Heme Uptake Activates Macrophage HO-1, Providing an Antiinflammatory Function

See related article, pages 943–950 A study by Schaer et al,1 which appears in this issue of Circulation Research , provides evidence suggesting that CD163 acts as a hemoglobin (Hb) transporter and, via activation of macrophages, heme oxygenase-1 (HO-1) provides antiinflammatory activity, presumably by an increase in ferritin. Induction of HO-1 was observed to occur when the cultured macrophages, endothelial cells or tubular cells were exposed to heme or hemoglobin (for a review see Abraham and Kappas2). Heme and denatured hemoglobin toxicity play an important role in a broad spectrum of pathological circumstances such as myocardial ischemia, hypertension, cardiomyopathy, reperfusion, organ transplantation, pulmonary disorders, and inflammation among others.3 In the cell free system, hemoglobin is bound to haptoglobin before clearance by the macrophage hemoglobin scavenger receptor CD163. Schaer et al1 demonstrated that CD163-hemoglobin transport is regarded as a critical step in the hemoglobin clearance pathway in the macrophage, especially under conditions of extreme hemoglobin release resulting from hemolysis. Binding hemoglobin-haptoglobin to CD163 cells also elicits IL-10 secretion, which contributes to the induction of HO-1.4 Thus, CD163 has a functional role as an antioxidant by enhancing HO-1, the major cytoprotective response. This links heme transport directly to a known receptor with well characterized endocytic properties and signaling functions.5 Heme-hemopexin is also taken up by LRP/CD91 and, as such, may inhibit inflammatory functions in phagocytic and antiinflammatory macrophages; however, a link to functional HO-1 was not described. Schaer et al1 demonstrated that induction of HO-1 by Hb is functionally linked to CD163 and, using gene array analysis, were able to show that Hb elicits a noninflammatory transcriptional response in macrophages, presumably by the coordinated increase in ferritin synthesis following the activation of HO-1. This response was mediated by CD163 and did not involve the …

Iron regulatory protein‐independent regulation of ferritin synthesis by nitrogen monoxide

The discovery of iron-responsive elements (IREs), along with the identification of iron regulatory proteins (IRP1, IRP2), has provided a molecular basis for our current understanding of the remarkable post-transcriptional regulation of intracellular iron homeostasis. In iron-depleted conditions, IRPs bind to IREs present in the 5'-UTR of ferritin mRNA and the 3'-UTR of transferrin receptor (TfR) mRNA. Such binding blocks the translation of ferritin, the iron storage protein, and stabilizes TfR mRNA, whereas the opposite scenario develops when iron in the intracellular transit pool is plentiful. Nitrogen monoxide (commonly designated nitric oxide; NO), a gaseous molecule involved in numerous functions, is known to affect cellular iron metabolism via the IRP/IRE system. We previously demonstrated that the oxidized form of NO, NO(+), causes IRP2 degradation that is associated with an increase in ferritin synthesis [Kim, S & Ponka, P (2002) Proc Natl Acad Sci USA99, 12214-12219]. Here we report that sodium nitroprusside (SNP), an NO(+) donor, causes a dramatic and rapid increase in ferritin synthesis that initially occurs without changes in the RNA-binding activities of IRPs. Moreover, we demonstrate that the translational efficiency of ferritin mRNA is significantly higher in cells treated with SNP compared with those incubated with ferric ammonium citrate, an iron donor. Importantly, we also provide definitive evidence that the iron moiety of SNP is not responsible for such changes. These results indicate that SNP-mediated increase in ferritin synthesis is, in part, due to an IRP-independent and NO(+)-dependent post-transcriptional, regulatory mechanism.

Hemodialysis reduces inhibitory effect of plasma ultrafiltrate on LDL oxidation and subsequent endothelial reactions

Oxidative modification of low-density lipoprotein (LDL) and its deleterious effect on endothelium is implicated in the pathogenesis of atherosclerosis. Endothelium responds to such an insult by upregulating the synthesis of heme oxygenase-1 (HO-1) and ferritin. Endothelial cell damage and dysfunction have been observed in patients with chronic kidney disease (CKD) on maintenance hemodialysis (HD). We studied the effect of low-molecular-weight components of uremic plasma on LDL oxidation and LDL-oxidation-provoked endothelial cell reactions, such as the induction of cytotoxicity and the upregulation of cell-protective HO-1 and ferritin. Plasma ultrafiltrate (molecular weight<5000 Da) from CKD patients on HD or when treated conservatively exhibited a pronounced inhibition on heme-mediated oxidative modification of LDL. Endothelial cell cytotoxicity provoked by LDL oxidation was also attenuated by plasma ultrafiltrate from CKD patients. During HD treatment, a dramatic drop occurred in the retardation of oxidative reactions, and a loss of endothelial cytoprotection exerted by plasma ultrafiltrate was noted. The upregulation of HO-1 and ferritin in response to oxidative stress of LDL was blunted by uremic plasma ultrafiltrate that was released by the end of HD. The decreased antioxidant capacity of ultrafiltrate after HD occurred as a consequence of the intradialytic removal of L-ascorbic acid, uric acid, bilirubin, 3-indoxyl sulfate, indoxyl-beta-D-glucuronide, p-cresol, and phenol. Intradialytic removal of L-ascorbic acid, uric acid, bilirubin, 3-indoxyl sulfate, indoxyl-beta-D-glucuronide, p-cresol, and phenol increases the risk of LDL oxidation and subsequent endothelial cell damage, which underlines the importance of activation of cytoprotective HO-1 and ferritin in endothelium.

Carbon monoxide in biology and medicine.

Carbon monoxide (CO), a product of organic oxidation processes, arises in vivo during cellular metabolism, most notably heme degradation. CO binds to the heme iron of most hemoproteins. Tissue hypoxia following hemoglobin saturation represents a principle cause of CO-induced mortality in higher organisms, though cellular targets cannot be excluded. Despite extreme toxicity at high concentrations, low concentrations of CO can confer cytoprotection during ischemia/reperfusion or inflammation-induced tissue injury. Likewise, heme oxygenase, an enzyme that produces CO, biliverdin and iron, as well as a secondary increase in ferritin synthesis, from the oxidation of heme, can confer protection in vivo and in vitro. CO has been shown to affect several intracellular signaling pathways, including guanylate cyclase, which generates guanosine 3':5' cyclic monophosphate and the mitogen-activated protein kinases (MAPK). Such pathways mediate, in part, the known vasoregulatory, anti-inflammatory, anti-apoptotic and anti-proliferative effects of this gas. Exogenous CO delivered at low concentrations is showing therapeutic potential as an anti-inflammatory agent and as such can modulate numerous pathophysiological states. This review will delve into the biological significance and medical applications of this gas molecule.

Nitric Oxide-Mediated Modulation of Iron Regulatory Proteins: Implication for Cellular Iron Homeostasis

ABSTRACT Iron regulatory proteins (IRP1 and IRP2) control the synthesis of transferrin receptors (TfR) and ferritin by binding to iron-responsive elements (IREs) that are located in the 3′ untranslated region (UTR) and the 5′ UTR of their respective mRNAs. Cellular iron levels affect binding of IRPs to IREs and consequently expression of TfR and ferritin. Moreover, NO •, a redox species of nitric oxide that interacts primarily with iron, can activate IRP1 RNA-binding activity resulting in an increase in TfR mRNA levels and a decrease in ferritin synthesis. We have shown that treatment of RAW 264.7 cells (a murine macrophage cell line) with NO + (nitrosonium ion, which causes S-nitrosylation of thiol groups) resulted in a rapid decrease in RNA-binding of IRP2, followed by IRP2 degradation, and these changes were associated with a decrease in TfR mRNA levels and a dramatic increase in ferritin synthesis. Moreover, we demonstrated that stimulation of RAW 264.7 cells with lipopolysaccharide (LPS) and interferon-γ (IFN-γ) increased IRP1 binding activity, whereas RNA-binding of IRP2 decreased and was followed by a degradation of this protein. Furthermore, the decrease of IRP2 binding/protein levels was associated with a decrease in TfR mRNA levels and an increase in ferritin synthesis in LPS/IFN-γ-treated cells, and these changes were prevented by inhibitors of inducible nitric oxide synthase. These results suggest that NO +-mediated degradation of IRP2 plays a major role in iron metabolism during inflammation.

Nitrogen monoxide-mediated control of ferritin synthesis: implications for macrophage iron homeostasis.

Intracellular iron homeostasis is regulated posttranscriptionally by iron regulatory proteins 1 and 2 (IRP1 and IRP2). In the absence of iron in the labile pool, IRPs bind to specific nucleotide sequences called iron responsive elements (IREs), which are located in the 5' untranslated region of ferritin mRNA and the 3' untranslated region of transferrin receptor mRNA. IRP binding to the IREs suppresses ferritin translation and stabilizes transferrin receptor mRNA, whereas the opposite scenario develops in iron-replete cells. Binding of IRPs to the IREs is also affected by nitrogen monoxide (NO), but there are conflicting reports regarding the effect of NO on ferritin synthesis. In this study, we demonstrated that a short exposure of RAW 264.7 cells (a macrophage cell line) to the NO+ donor, sodium nitroprusside (SNP), resulted in a dramatic increase in ferritin synthesis. The SNP-mediated increase of ferritin synthesis could be blocked by MG132, an inhibitor of proteasome-dependent protein degradation, which also prevented the degradation of IRP2 caused by SNP treatment. Moreover, treatment of RAW 264.7 cells with IFN-gamma and lipopolysaccharide caused IRP2 degradation and stimulated ferritin synthesis, changes that could be prevented by specific inhibitors of inducible nitric oxide synthase. Furthermore, the SNP-mediated increase in ferritin synthesis was associated with a significant enhancement of iron incorporation into ferritin. These observations indicate that NO+-mediated modulation of IRP2 plays an important role in controlling ferritin synthesis and iron metabolism in murine macrophages.

Control of Transferrin Receptor Expression via Nitric Oxide-mediated Modulation of Iron-regulatory Protein 2

Cellular iron storage and uptake are coordinately regulated post-transcriptionally by cytoplasmic factors, iron-regulatory proteins 1 and 2 (IRP-1 and IRP-2). When iron in the intracellular transit pool is scarce, IRPs bind to iron-responsive elements (IREs) in the 5'-untranslated region of the ferritin mRNA and 3'-untranslated region of the transferrin receptor (TfR) mRNA. Such binding inhibits translation of ferritin mRNA and stabilizes the mRNA for TfR, whereas the opposite scenario develops when iron in the transit pool is plentiful. However, we (Richardson, D. R., Neumannova, V., Nagy, E., and Ponka, P. (1995) Blood 86, 3211-3219) and others reported that the binding of IRPs to IREs can also be modulated by nitric oxide (NO). In this study, we showed that a short exposure of RAW 264.7 cells (a murine macrophage cell line) to the NO(+) donor, sodium nitroprusside (SNP), caused a significant decrease in IRP-2 binding to the IREs followed by IRP-2 degradation and that these changes occurred without affecting IRP-1 binding. The SNP-mediated degradation of IRP-2 in RAW 264.7 cells could be prevented by MG-132 or lactacystin, known inhibitors of proteasome-dependent protein degradation. A SNP-mediated decrease in IRP-2 binding and levels was associated with a dramatic decrease in TfR mRNA levels and an increase in ferritin synthesis. Importantly, the proteasome inhibitor MG-132 prevented the SNP-mediated decrease in TfR mRNA levels. These observations suggest that IRP-2 can play an important role in controlling transferrin receptor expression.

Resistance to the Antitumor Agent Gallium Nitrate in Human Leukemic Cells Is Associated with Decreased Gallium/Iron Uptake, Increased Activity of Iron Regulatory Protein-1, and Decreased Ferritin Production

The mechanism of drug resistance to gallium nitrate is not known. Since gallium can be incorporated into ferritin, an iron storage protein that protects cells from iron toxicity, we investigated whether ferritin expression was altered in gallium-resistant (R) CCRF-CEM cells. We found that the ferritin content of R cells was decreased, while heavy chain ferritin mRNA levels and iron regulatory protein-1 (IRP-1) RNA binding activity were increased. IRP-1 protein levels were similar in gallium-sensitive (S) and R cells, indicating that R cells contain a greater proportion of IRP-1 in a high affinity mRNA binding state. 59Fe uptake and transferrin receptor expression were decreased in R cells. In both S and R cells, gallium inhibited cellular 59Fe uptake, increased ferritin mRNA and protein, and decreased IRP-1 binding activity. Gallium uptake by R cells was markedly diminished; however, the sensitivity of R cells to gallium could be restored by increasing their uptake of gallium with excess transferrin. Our results suggest that R cells have developed resistance to gallium by down-regulating their uptake of gallium. In parallel, iron uptake by R cells is also decreased, leading to changes in iron homeostasis. Furthermore, since gallium has divergent effects on iron uptake and ferritin synthesis, its action may also include a direct effect on ferritin mRNA induction and IRP-1 activity.

Mechanisms by which Ascorbic Acid Increases Ferritin levels in Cultured Lens Epithelial Cells

A previous study demonstrated that ascorbic acid increased the concentration of the iron storage protein, ferritin, in cultured lens epithelial cells. The current study was designed to determine the mechanism by which ascorbic acid exerts this effect. Ascorbic acid increased both ferritin mRNA levels (by about 30%) and translation of ferritin (de novo synthesis was increased up to 15-fold) within 6 hr. Cycloheximide completely abolished the ability of ascorbic acid to increase ferritin levels, whereas actinomycin D only decreased it by about 30%. Therefore, the ascorbic-acid induced increase in ferritin concentration is due mainly to an increase in ferritin synthesis at the translational level. This is a novel role for ascorbic acid. Addition of iron with ascorbic acid further increased de novo synthesis of ferritin, but this additive effect was only noted at a later time point (20 hr). Factors which decrease ferritin mRNA translation, such as the reducing agent dithiothreitol or the iron chelator desferrioxamine, reduced the ascorbic acid effect on de novo ferritin synthesis. The effects of ascorbic acid on ferritin mRNA levels may be mediated by its oxidation product, H2O2, since, like ascorbic acid, H2O2increased ferritin mRNA levels by 30%. However, in contrast to the ascorbic acid-induced increase in translation of ferritin, H2O2substantially decreased de novo ferritin synthesis. This effect of H2O2could have physiological significance in eyes where concentrations of H2O2in the aqueous humor are elevated. High levels of H2O2could decrease the concentration of ferritin within the lens. Since ferritin sequesters iron and has been shown to decrease oxidative damage by limiting the availability of iron to catalyse free radical reactions, H2O2-induced reduction in ferritin concentration in the lens could have deleterious effects. The ability of ascorbic acid to increase ferritin concentration in lens epithelial cells could provide an additional protective mechanism for this antioxidant vitamin. The importance of ferritin to normal lens functioning is underscored by the recent finding that humans with a dominantly inherited abnormality in ferritin synthesis exhibit early bilateral cataracts.

Ferritin as a source of iron and protection from iron-induced toxicities

The iron storage protein ferritin can contribute to or protect against toxicities which involve iron. Iron can catalyze the oxidation of lipid, protein, DNA and various biomolecules that can reduce iron. Iron can be reduced and released from ferritin by the free radical form of various toxins or Superoxide resulting from oxygen reduction by chemicals which redox cycle. Iron can also increase ferritin synthesis by an iron-binding protein which releases from an iron-responsive element in mRNA for ferritin. This increase in ferritin synthesis provides a non-reactive storage site for iron. The mechanism by which iron is placed into ferritin is unknown. We propose that it is catalyzed by ceruloplasmin, the copper-containing ferroxidase that loads iron into transferrin. We believe that the ferroxidase activity, thought to reside in the heavy chain of ferritin, is an artifact resulting from ferrous iron autoxidation. We load iron into ferritin with ceruloplasmin so ferritin plus ceruloplasmin is an effective ‘antioxidant’.

Regulation of Ferritin H-Chain Expression in Differentiating Friend Leukemia Cells

The mechanisms that regulate the expression of ferritin, the iron storage protein, have been investigated in Friend erythroleukemia cells (FLCs) induced to differentiate by several chemical compounds. In differentiating FLCs, administration of hemin increases the steady-state level of ferritin mRNA about 15-fold and the ferritin content about 20- to 25-fold. Conversely, iron salts have only mild stimulatory effects on these parameters and iron chelators only slightly inhibited the stimulatory effect of hemin. Transient transfection experiments with a construct in which the human ferritin H-chain promoter drives the expression of the indicator chloramphenicol acetyltransferase (CAT) gene show that the increase in mRNA content is mainly due to enhanced transcription. In addition to transcriptional effects, translational regulation resulting in the further increase in ferritin synthesis is shown by CAT assays from cells transiently transfected with a construct containing the coding region for the indicator CAT mRNA under the translational control of the mRNA ferritin iron-responsive element. We conclude that, in FLCs induced to differentiate, hemin acts synergistically with the differentiation inducers, increasing ferritin expression. Both transcriptional and translational mechanisms are responsible for this synergistic effect, which appears to be characteristic of differentiated erythroid cells because it is not observed in other cell types (ie, fibroblastic cell lines).

Ferritin synthesis is controlled by iron-dependent translational derepression and by changes in synthesis/transport of nuclear ferritin RNAs.

Ferritin synthesis, maintained at a very low basal rate when extracellular iron levels are low, is elevated up to 50-fold when iron levels are increased. Previous examinations of this iron-dependent activation have concluded that changes in ferritin synthesis results from selective translational activation conferred by an "iron response element" that lies near the 5' terminus of all known ferritin mRNAs. By placing an iron response element in an optimal position in other mRNAs, we find the iron response element to be a potent translational repressor whose influence can only partially be abrogated under optimal inducing conditions. Further, we show that the 25- to 50-fold iron-mediated increase in ferritin synthesis results from coupling a 5- to 6-fold iron-dependent translational derepression with a similar 5- to 6-fold nuclear-dependent increase in mRNA level.

Modulation of ferritin H-chain expression in Friend erythroleukemia cells: transcriptional and translational regulation by hemin.

The mechanisms that regulate the expression of the H chain of the iron storage protein ferritin in Friend erythroleukemia cells (FLCs) after exposure to hemin (ferric protoporphyrin IX), protoporphyrin IX, and ferric ammonium citrate (FAC) have been investigated. Administration of hemin increases the steady-state level of ferritin mRNA about 10-fold and that of ferritin protein expression 20-fold. Experiments with the transcriptional inhibitor actinomycin D and transfection studies demonstrate that the increment in cytoplasmic mRNA content results from enhanced transcription of the ferritin H-chain gene and cannot be attributed to stabilization of preexisting mRNAs. In addition to transcriptional effects, translational regulation induces the recruitment of stored mRNAs into functional polyribosomes after hemin and FAC administration, resulting in a further increase in ferritin synthesis. Administration of protoporphyrin IX to FLCs produces divergent transcriptional and translational effects. It increases transcription but appears to suppress ferritin mRNA translation. FAC treatment increases the mRNA content slightly (about twofold), and the ferritin levels rise about fivefold over the control values. We conclude that in FLCs, hemin induces ferritin H-chain biosynthesis by multiple mechanisms: a transcriptional mechanism exerted also by protoporphyrin IX and a translational one, not displayed by protoporphyrin IX but shared with FAC.

Modulation of ferritin H-chain expression in Friend erythroleukemia cells: transcriptional and translational regulation by hemin.

The mechanisms that regulate the expression of the H chain of the iron storage protein ferritin in Friend erythroleukemia cells (FLCs) after exposure to hemin (ferric protoporphyrin IX), protoporphyrin IX, and ferric ammonium citrate (FAC) have been investigated. Administration of hemin increases the steady-state level of ferritin mRNA about 10-fold and that of ferritin protein expression 20-fold. Experiments with the transcriptional inhibitor actinomycin D and transfection studies demonstrate that the increment in cytoplasmic mRNA content results from enhanced transcription of the ferritin H-chain gene and cannot be attributed to stabilization of preexisting mRNAs. In addition to transcriptional effects, translational regulation induces the recruitment of stored mRNAs into functional polyribosomes after hemin and FAC administration, resulting in a further increase in ferritin synthesis. Administration of protoporphyrin IX to FLCs produces divergent transcriptional and translational effects. It increases transcription but appears to suppress ferritin mRNA translation. FAC treatment increases the mRNA content slightly (about twofold), and the ferritin levels rise about fivefold over the control values. We conclude that in FLCs, hemin induces ferritin H-chain biosynthesis by multiple mechanisms: a transcriptional mechanism exerted also by protoporphyrin IX and a translational one, not displayed by protoporphyrin IX but shared with FAC.

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

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

Ferritin is a translationally regulated heat shock protein of avian reticulocytes.

Heat-shock avian reticulocytes exhibit enhanced synthesis of a greater than 450-kDa protein. Biochemical, immunochemical, and visual criteria were used to identify this protein as the iron storage protein ferritin. The 21-kDa ferritin subunits synthesized during heat shock are similar in size and pI to the subunits that are constitutively synthesized. The 2-6-fold heat shock-induced increase in ferritin synthesis appears to be regulated at the translational level as it is insensitive to actinomycin D. Northern and dot-blot hybridization analyses of cytoplasmic RNAs with avian H-ferritin cDNA fragments support the contention that the heat shock stimulation of ferritin synthesis is translationally regulated. These latter studies demonstrate that the heat shock-induced synthesis of ferritin does not involve a change in the amount of total cytoplasmic ferritin mRNAs, but rather appears to entail a translocation of cytoplasmic H-ferritin mRNAs from a polyribosome-free, translationally repressed state to a polyribosome-associated, translationally active state. These results suggest that thermally stressed avian reticulocytes have a critical and functional need for the synthesis of additional ferritin and that its enhanced synthesis, unlike the new and/or enhanced synthesis of the well-established avian heat shock proteins, is regulated wholly at the translational level.

Ferritin synthesis in peripheral blood lymphocytes following in vitro stimulation.

The incubation of normal peripheral blood lymphocytes with phytohaemagglutinin results in an increase in DNA synthesis accompanied by an increase in ferritin synthesis, as determined by 3H-leucine incorporation into ferritin isolated by immunoprecipitation. The increase was 5 to 8 times greater than that of non-stimulated cells. This phenomenon was associated with a 3- to 6-fold increase of intracellular ferritin when compared with the original ferritin concentration. While the significance of increased ferritin expression in proliferative cells is not known, an increase in ferritin synthesis following in vitro stimulation is probably related to increased metabolic activity of the cells.