Overexpression of mitochondrial ferritin causes cytosolic iron depletion and changes cellular iron homeostasis

Abstract

Cytosolic ferritin sequesters and stores iron and, consequently, protects cells against iron-mediated free radical damage. However, the function of the newly discovered mitochondrial ferritin (MtFt) is unknown. To examine the role of MtFt in cellular iron metabolism, we established a cell line that stably overexpresses mouse MtFt under the control of a tetracycline-responsive promoter. The overexpression of MtFt caused a dose-dependent iron deficiency in the cytosol that was revealed by increased RNA-binding activity of iron regulatory proteins (IRPs) along with an increase in transferrin receptor levels and decrease in cytosolic ferritin. Consequently, the induction of MtFt resulted in a dramatic increase in cellular iron uptake from transferrin, most of which was incorporated into MtFt. The induction of MtFt caused a shift of iron from cytosolic ferritin to MtFt. In addition, iron inserted into MtFt was less available for chelation than that in cytosolic ferritin and the expression of MtFt was associated with decreased mitochondrial and cytosolic aconitase activities, the latter being consistent with the increase in IRPbinding activity. In conclusion, our results indicate that overexpression of MtFt causes a dramatic change in intracellular iron homeostasis and that shunting iron to MtFt likely limits its availability for active iron proteins. IntroductionFerritins are ubiquitous, highly conserved iron storage proteins that play a critical role in cellular and organismal iron homeostasis. In mammalian cells, cytosolic ferritin consists of 2 subunits, the H and L chains. Twenty-four subunits assemble to form a spherical shell that can contain up to 4500 atoms of iron in a mineral and compact form. 1,2 In ferritin shells, the H subunit has ferroxidase activity that converts soluble ferrous ions into inert ferric hydroxides. The L subunit lacks ferroxidase activity but is more efficient than the H subunit in inducing iron nucleation and mineralization within the shells. 3,4 Ferritin H and L chain mRNAs have identical ironresponsive elements (IREs) at their 5Ј-untranslated regions (UTRs) that bind to cytosolic iron regulatory proteins IRP1 and IRP2 by an iron-dependent mechanism. 2,5 When the level of iron in the so-called "labile iron pool" (LIP) is low, translation of ferritin mRNA is suppressed by the binding of IRPs to IREs, whereas iron supplementation up-regulates ferritin synthesis through the conversion of IRP1 to cytosolic aconitase and the degradation of IRP2. [5][6][7][8] Mitochondria have a key role in iron metabolism because heme and various iron-sulfur (Fe-S) cluster-containing proteins are synthesized in them. 9,10 The last step in heme biosynthesis, the insertion of Fe 2ϩ into protoporphyrin IX by ferrochelatase, occurs in the mitochondrial matrix. 9 Fe-S clusters are synthesized mainly, if not entirely, in mitochondria and are combined with mitochondrial apo-proteins to form mature proteins or are exported from mitochondria for use by cytosolic and nuclear proteins. 10 Mitochondrial iron level mu...

Flow cytometry evaluation of erythroid dysplasia in patients with myelodysplastic syndrome

“…This observation may indicate a link between MtF and transferrin receptor expression: might be related to the cytosolic iron deprivation induced by the presence of MtF. 25,26 In an erythroid culture model we found that aberrant MtF expression, as well as induction of HF, occur at very early stage of differentiation, confirming a strong relationship with the myelodysplastic phenotype. 20 A distinctive sign of ineffective erythropoiesis is a high proportion of immature erythroblasts.…”

Biology of ferritin in mammals: an update on iron storage, oxidative damage and neurodegeneration

“…Since the mRNA lacks an IRE hairpin structure its expression is not regulated by iron at post-translational level and the protein is absent in iron storage organs, such as the liver and spleen. When overexpressed in cell lines, the protein causes a shift of cellular iron to mitochondria, with consequent iron deficiency in the cytosol (low ferritin and high TfR) and iron accumulation in the organelles (Corsi et al 2002;Nie et al 2005;Wu et al 2013). This suggests that FtMt can cooperate with cytosolic ferritin in the maintenance of intracellular iron balance.…”

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“…The defective ALAS2 activity in bone-marrow erythroblasts diminishes heme biosynthesis, leading to insufficient protoporphyrin IX to use all the available Fe. Most of the Fe deposited in perinuclear mitochondria of ringed sideroblasts is stored in the recently identified mitochondrial ferritin (Cazzola et al, 2003;Nie et al, 2005). Iron overload disturbs cellular reduction-oxidation state and results in the shortening of the cellular lifetime.…”