Control of cellular iron homeostasis by iron‐responsive elements in vivo

Abstract

It has recently been proposed that cellular iron homeostasis in mammalian cells is regulated at the post-transcriptional level by the reciprocal control of transferrin receptor and fenitin mRNA expression via an iron-regulatory factor. This iron-regulatory factor has been shown to be a cytoplasmic aconitase which can bind to iron-responsive elements in the corresponding mRNAs with greater or lesser affinity as a function of the iron status of the cell. In the present study, we show that in vivo the affinity of iron-regulatory factor for iron-responsive elements in liver reflects the long-term iron status of the tissue in animal models for iron overloading and iron deficiency, when combined with altered transferrin saturation and serum iron levels. In contrast hepatic iron overload achieved without altering such haematopoeitic indices, had a less pronounced effect. In both spleen and heart, the affinities of iron-regulatory factor changed in parallel with both altered iron status and haematological markers. In brain and duodenum, there were no consistent changes in ironregulatory-factor activity with iron loading or depletion. Iron-regulatory-factor activity in kidney responded in an as yet unexplained manner.Iron is an essential element for the growth, differentiation and well-being of most living organisms [l]. It is therefore of paramount importance to understand at the molecular, cellular and whole-organism level how iron homeostasis is regulated. Cellular iron homeostasis can be defined as the mechanisms whereby the intracellular concentration of the metal ion is maintained at a level adequate for the cell's requirements, but not sufficiently high to cause toxic effects. This is achieved by regulating the uptake of the metal ion in concert with its intracellular utilisation, storage and eventual externalization. It has been suggested that the expression of several key proteins of iron metabolism is regulated by intracellular iron levels [2-61. The post-transcriptional regulation of the iron-storage protein ferritin, the erythroid 5-aminolaevulinate synthase (AlS) and the transfenin receptor operate at the level of mRNA. All H-chain and L-chain ferritin mRNAs [3, 7, binds tightly to the IRE of transferrin-receptor mRNA, thereby ensuring its protection against nuclease digestion [20] and translation of the transferrin receptor, thus enhancing the uptake of iron by the cell. The same iron-depleted form of IRF binds strongly to the femtin and A1S mRNAs and thus blocks the biosynthesis of ferritin erythroid AlS. When cells are replete in iron the IRF binds poorly to femtin and erythroid AIS mRNAs ensuring their expression, and dissociates from the transfenin-receptor mRNA ensuring that it will be degraded by cellular nucleases. Thus, the model implies a coordinate regulation at the level of translation of mRNAs for ferritin and erythroid AlS, on the one hand, and transferrin receptor on the other.The IRF has been clearly shown to be a cytosolic aconitase which, in its active 4Fe-4S form, has a turnover number s...

Effects of marginal iron overload on iron homeostasis and immune function in alveolar macrophages isolated from pregnant and normal rats

“…No significant changes in IRP activity were evident in the macrophages isolated from either groups of pregnant rats. In contrast, the liver samples showed an enhancement in IRP activity after incubation with b-mercaptoethanol, as previously reported (Ward et al 1994). …”

“…The cells were recovered after centrifugation at 1,200 rpm and lysed in 8.3 ll/10 6 cells of Munro buffer, (10 mM Hepes pH 7.6, 3 mM MgCl 2 , 40 mM KCl, 5% glycerol, 1 mM DTT), supplemented with 0.2% NP-40. The plasmid pST-fer, containing an oligonucleotide corresponding to bases 31-58 of the 5 0 untranslated region of human ferritin heavy chain mRNA (kindly provided by Professor L. Kühn) was amplified and prepared as previously published (Ward et al 1994). Aliquots of the cytosolic fractions of liver (10 lg protein), and macrophage (20 lg protein) samples were incubated with the T7 transcribed RNA in the presence or absence of 2% b-mercaptoethanol.…”

Complex Regulation of Transferrin Receptors during Erythropoietin‐Induced Differentiation of J2E Erythroid Cells

“…protein complexes were separated from unbound probe on 6 % non-denaturing polyacrylamide gels. Total IRP present in each extract was determined by incubation of extracts in the presence of 2-mercaptoethanol, whereas affinity of IRP for the IRE was demonstrated in the absence of 2-mercaptoethanol [40]. This experiment was performed on three occasions.…”

Astrocytes Induce Hemeoxygenase-1 Expression in Microglia: A Feasible Mechanism for Preventing Excessive Brain Inflammation

“…Carbon monoxide attenuates bronchial inflammation induced by aeroallergen and prevents liver inflammation (Chapman et al, 2001;Ott et al, 2005). And finally, ferrous iron released by HO activity rapidly induces expression of ferritin, protecting cells under oxidizing conditions by sequestering free cytosolic iron (Ward et al, 1994).…”