Regulation of ferritin genes and protein

Role of iron and ferritin in TNFalpha-induced apoptosis in HeLa cells

We found that tumor necrosis factor alpha (TNFalpha)-induced apoptosis in HeLa cells was accompanied by a approximately 2-fold increase in H- and L-ferritin and a decrease in transferrin receptor, two indices of increased iron availability. Iron supplementation and overexpression of H-ferritin or its mutant with an inactivated ferroxidase center reduced by about approximately 50% the number of apoptotic cells after TNFalpha-treatment, while overexpression of L-ferritin was ineffective. The data indicate that H-ferritin has an anti-apoptotic activity unrelated to its ferroxidase activity and to its capacity to modify cellular iron metabolism.

Interactions between doxorubicin and the human iron regulatory system

Anthracyclines are included in clinical treatments against various malignancies, but severe cardiotoxic side-effects and the development of resistance mechanisms limit their usefulness. Many aspects of the cellular response to anthracyclines remain debated. The status of the main regulator of iron homeostasis, namely the RNA-binding activity of iron regulatory proteins (IRPs), has been assessed herein for two types of human tumor cells and their derived doxorubicin-resistant sublines. IRPs were always fully activated in the latter, whereas only partial activation occurred in the former. Doxorubicin exposure reversibly inactivated IRP1 in small cell lung carcinoma (GLC(4)) and myelogenous leukemia (K562) cell lines, but was without effect in their derived doxorubicin-resistant sublines. In contrast, adding doxorubicin to cytosolic fractions of untreated cells or to purified IRPs led to the irreversible alteration of the RNA-binding activity of IRP1. In these different conditions, interaction between doxorubicin and the iron regulatory system disturbs iron metabolism, and cells having developed a resistance mechanism are tuned to maximize the iron supply. The results reported herein may lead the path toward a better therapeutic management of cancer patients receiving doxorubicin by discriminating between the antiproliferative and cardiotoxic properties of this anthracycline.

Cytosolic aconitase and ferritin are regulated by iron in Caenorhabditis elegans

Iron regulatory protein-1 (IRP-1) is a cytosolic RNA-binding protein that is a regulator of iron homeostasis in mammalian cells. IRP-1 binds to RNA structures, known as iron-responsive elements, located in the untranslated regions of specific mRNAs, and it regulates the translation or stability of these mRNAs. Iron regulates IRP-1 activity by converting it from an RNA-binding apoprotein into a [4Fe-4S] cluster protein exhibiting aconitase activity. IRP-1 is widely found in prokaryotes and eukaryotes. Here, we report the biochemical characterization and regulation of an IRP-1 homolog in Caenorhabditis elegans (GEI-22/ACO-1). GEI-22/ACO-1 is expressed in the cytosol of cells of the hypodermis and the intestine. Like mammalian IRP-1/aconitases, GEI-22/ACO-1 exhibits aconitase activity and is post-translationally regulated by iron. Although GEI-22/ACO-1 shares striking resemblance to mammalian IRP-1, it fails to bind RNA. This is consistent with the lack of iron-responsive elements in the C. elegans ferritin genes, ftn-1 and ftn-2. While mammalian ferritin H and L mRNAs are translationally regulated by iron, the amounts of C. elegans ftn-1 and ftn-2 mRNAs are increased by iron and decreased by iron chelation. Excess iron did not significantly alter worm development but did shorten their life span. These studies indicated that iron homeostasis in C. elegans shares some similarities with those of vertebrates.

Nrf2 mediates the induction of ferritin H in response to xenobiotics and cancer chemopreventive dithiolethiones

Ferritin is a ubiquitous intracellular iron storage protein that consists of 24 subunits of the H and L type. The ability to sequester iron from participation in oxygen free radical formation is consistent with a cytoprotective role for ferritin. Here we demonstrate that ferritins H and L are induced in cells treated with beta-napthoflavone (beta-NF) and chemopreventive dithiolethiones. Induction of ferritin H by beta-NF and the dithiolethiones oltipraz and 1,2-dithiole-3-thione (D3T) occurs via a transcriptional mechanism that is mediated by the ferritin H electrophile/antioxidant-responsive element (EpRE/ARE). The murine ferritin H gene contains five potential xenobiotic-responsive element (XRE) sequences in its 5'-promoter region. However, deletion analysis demonstrates that these XRE sequences are not functional in inducing ferritin H in response to beta-NF. Electrophoretic mobility shift assays demonstrate that the ferritin H EpRE/ARE binds Nrf2. Transfection of chimeric ferritin H reporter genes with Nrf2 expression vectors and Nrf2 dominant-negative mutants indicate that Nrf2 functions at the EpRE/ARE to mediate transcriptional activation of ferritin H. Induction of ferritin H and L was not seen in Nrf2 knockout cells, demonstrating that this transcription factor is required for the induction of ferritin in response to polycyclic aromatic xenobiotics and chemopreventive agents. Nrf2 may also play a role in basal transcription of both ferritin H and L. These results provide a mechanistic link between regulation of the iron storage protein ferritin and the cancer chemopreventive response.

Mitochondrial ferritin: a new player in iron metabolism

Mitochondrial ferritin (MtF) is a novel H-type ferritin encoded by an intronless gene on chromosome 5q23.1. The protein is synthesized as a precursor of about 30 kDa that is targeted to mitochondria by a leader sequence of 60 amino acids. This leader is proteolytically removed inside the mitochondria and the resulting 22 kDa subunit forms typical ferritin shells. These shells have ferroxidase activity and are therefore likely to sequester potentially harmful free iron. However, this may be a limited function since MtF has a very restricted tissue expression. High amounts are found in testis but only very low levels are found in iron storage organs. The levels of MtF appear to correlate more with mitochondrial abundance than with iron metabolism. MtF does not seem to be an obligatory intermediate in transfer of free iron to heme and other iron compounds in mitochondria. However, its level increases dramatically in sideroblastic anemia when heme synthesis is disrupted. This increased synthesis does not appear to involve the classical translational control since MtF mRNA lacks an apparent iron response element. In transfected HeLa cells added iron is incorporated as quickly into MtF as into cytosolic ferritin. In addition, increased levels of MtF cause a redistribution of iron from cytosol to mitochondria and this effect is enhanced by iron chelation. Thus high levels of MtF result in an iron deficient phenotype in cytosol with decreased expression of ferritin and increased expression of transferrin receptor. This avidity for iron may explain why MtF levels are maintained at low levels in most normal cells. The regulation of MtF expression and possible therapeutic applications of MtF in neurological disorders involving increased iron deposition are topics for future research.

Genetic hyperferritinaemia and reticuloendothelial iron overload associated with a three base pair deletion in the coding region of the ferroportin gene (SLC11A3)

Iron overload may predominantly involve parenchymal or reticuloendothelial cells, the prototype of parenchymal iron overload being HFE-related genetic haemochromatosis. We studied a family with autosomal dominant hyperferritinaemia in whom the proband showed selective iron accumulation in the Kupffer cells on liver biopsy. Analysis of L and H ferritin genes excluded mutations responsible for hereditary hyperferritinaemia/cataract syndrome or similar translational disorders. Sequence analysis of the ferroportin gene (SLC11A3) in four individuals with hyperferritinaemia singled out a three base pair deletion in a region that contains four TTG repeats. This mutation removes a TTG unit from 780 to 791, and predicts the loss of one of three sequential valine residues 160-162. Denaturing high performance liquid chromatography can be used for its detection. SLC11A3 polymorphism analysis indicates that this probably represents a recurrent mutation due to slippage mispairing. Affected individuals may show marginally low serum iron and transferrin saturation, and young women may have marginally low haemoglobin concentration levels. Serum ferritin levels are directly related to age, but are 10-20 times higher than normal. Heterozygosity for the ferroportin Val 162 deletion represents the prototype of selective reticuloendothelial iron overload, and should be taken into account in the differential diagnosis of hereditary or congenital hyperferritinaemias.

Neuroferritinopathy: a window on the role of iron in neurodegeneration

Neuroferritinopathy is a recently recognised genetic disease resulting in a dominantly inherited movement disorder. The condition was mapped by linkage analysis to chromosome 19q13.3 and found to be due to a single adenine insertion in the ferritin light chain (FTL) gene at position 460-461 which is predicted to alter the C terminus of the FTL polypeptide. Clinical features of neuroferritinopathy are highly variable, with chorea, dystonia, and Parkinsonian features predominating in different affected individuals. The most consistent feature is a dystonic dysarthria. Symptoms and abnormal physical signs appear to be restricted to the nervous system and onset is typically in the fourth to sixth decades. Low serum ferritin also characterises this condition. Brain MR imaging of affected patients demonstrates iron deposition in the basal ganglia, progressing over years to cystic degeneration, and brain histochemistry shows abnormal aggregates of ferritin and iron. Now that the molecular basis of the condition is known, therapeutic interventions to reduce or reverse brain iron deposition are being evaluated. This rare disease provides evidence of a central role for iron metabolism in neurodegenerative disorders.