Thyroid Hormone Modulates the Interaction between Iron Regulatory Proteins and the Ferritin mRNA Iron-responsive Element

Abstract

The cytoplasmic iron regulatory protein (IRP) modulates iron homeostasis by binding to iron-responsive elements (IREs) in the transferrin receptor and ferritin mRNAs to coordinately regulate transferrin receptor mRNA stability and ferritin mRNA translational efficiency, respectively. These studies demonstrate that thyroid hormone (T 3 ) can modulate the binding activity of the IRP to an IRE in vitro and in vivo. T 3 augmented an iron-induced reduction in IRP binding activity to a ferritin IRE in RNA electrophoretic mobility shift assays using cytoplasmic extracts from human liver hepatoma (HepG2) cells. Hepatic IRP binding to the ferritin IRE also diminished after in vivo administration of T 3 with iron to rats. In transient transfection studies using HepG2 cells and a human ferritin IRE-chloramphenicol acetyltransferase (H-IRE-CAT) construct, T 3 augmented an iron-induced increase in CAT activity by ϳ45%. RNase protection analysis showed that this increase in CAT activity was not due to a change in the steady state level of CAT mRNA. Nuclear T 3 -receptors may be necessary for this T 3 -induced response, because the effect could not be reproduced by the addition of T 3 directly to cytoplasmic extracts and was absent in CV-1 cells which lack T 3 -receptors. We conclude that T 3 can functionally regulate the IRE binding activity of the IRP. These observations provide evidence of a novel mechanism for T 3 to up-regulate hepatic ferritin expression, which may in part contribute to the elevated serum ferritin levels seen in hyperthyroidism.The iron regulatory protein (IRP, 1 previously known as the iron-responsive element-binding protein, IRE-BP, and iron responsive factor, IRF) is a trans-acting RNA-binding protein which binds with high affinity to conserved stem-loop structures, iron-responsive elements (IREs), present in the ferritin, transferrin receptor (TfR), and erythroid 5-aminolevulinate synthase mRNAs (1-3). The IRP serves a central role in the regulation of iron (Fe) homeostasis (1). In the absence of iron, the IRP binds to the IRE in the 5Ј-untranslated region (5Ј-UTR) of ferritin and erythroid 5-aminolevulinate synthase mRNAs and represses translation (4 -6). Binding of the IRP to IREs in the 3Ј-untranslated region (3Ј-UTR) of TfR mRNA stabilizes the mRNA and prevents its degradation (7-9). In iron-replete states, the reverse holds, which results in increased ferritin translation and decreased TfR mRNA stability. This reciprocal regulation is achieved at the post-translational level and is independent of new protein synthesis (10).Two IRPs have been defined in various human and rat tissues (3, 11, 12). The most widely expressed and abundant IRP in human tissues is IRP1 (1, 3). A second human IRP (IRP2) has been described recently. IRP2 is 57% identical with IRP1 at the amino acid level and 2-10 times less abundant than IRP1 in most tissues, except in the brain (3). In contrast to IRP1, cellular concentrations of IRP2 are inversely regulated by iron levels due to iron-dependent regulation of the half-lif...

Ovariectomy and estrogen treatment modulate iron metabolism in rat adipose tissue

“…. In fact, it has been reported that hormonal signals may regulate post-transcriptionally the synthesis of proteins involved in iron metabolism by affecting IRPs ability to bind to IRE, possibly through induction of signal transduction cascades that result in IRPs phosphorylation [25,47]. In adipose tissue estrogen may also have rapid, non-genomic biological effects, believed to be mediated through a small fraction of estrogen receptors localized at or near the cell membrane [45].…”

“…Among hormones, thyroid hormones regulate post-transcriptionally the synthesis of proteins involved in iron metabolism by affecting IRPs ability to bind to IRE [25]. Insulin and IGF1 have also been implicated in the regulation of ferritin at the mRNA level [26].…”

Hormetic responses of thyroid hormone calorigenesis in the liver: Association with oxidative stress

“…In addition to TNF-a/IKK/NF-jB pathway (69-71), T 3 induced the Kupffer-cell-dependent release of IL-6, and activation of hepatic STAT3 may control both type I (haptoglobin) and type II (bfibrinogen) APP genes (67). Thyroid hormones also activate transcriptionally the hepatic synthesis of the APPs a 1 -acid glycoprotein, a 2 -(acute phase) globulin (72), ceruloplasmin (73), angiotensinogen, complement component 4A, and serum amyloid A protein precursor (74), as well as that of ferritin posttranscriptionally (75). The latter acute-phase response (APR) of the liver is a major pathophysiologic reaction in which normal homeostatic mechanisms are replaced by new set points contributing to defensive or adaptive capabilities against inflammation and oxidative stress (76).…”

Identification of genes differentially expressed in T cells following stimulation with the chemokines CXCL12 and CXCL10

“…TNFa regulation of FTH1 is through its binding to the p50 and p65 subunits of NFκB [73]. In addition, expression of FTH1 appears to be regulated by insulin, IGF-1 and thyroid hormone [74,75]. Cytokines also regulate the post-transcriptional modification of ferritin possibly through their ability to induce iNOS [74,75].…”

“…In addition, expression of FTH1 appears to be regulated by insulin, IGF-1 and thyroid hormone [74,75]. Cytokines also regulate the post-transcriptional modification of ferritin possibly through their ability to induce iNOS [74,75]. As noted by Torti & Torti [70], the pathways that link ferritin gene expression with cell stress and altered growth regulation are just beginning to be explored and based on present knowledge are very complex and multifaceted.…”