A cis-acting element is necessary and sufficient for translational regulation of human ferritin expression in response to iron.

Abstract

Ferritin plays a key role in determining the intracellular fate of iron and is highly regulated by the iron status of the cell. We have identified a cis-acting element in the transcribed but nontranslated 5' leader sequence of human ferritin heavy-chain mRNA. In transiently transfected murine fibroblasts, the presence of a 157-nucleotide region of the 5' leader sequence was found to be necessary for iron-dependent regulation of ferritin biosynthesis. Further, this 5' leader region is sufficient to transfer iron-mediated translational control to the expression of a heterologous gene product, chloramphenicol acetyltransferase.Ferritin is the major intracellular repository of iron and exists as a heteropolymer of 24 subunits containing both heavy and light chains (1). It serves to sequester and thereby detoxify iron not otherwise utilized for cellular metabolism (2). A more active role of ferritin has been suggested (3), in which iron sequestration may limit and regulate iron availability for cellular metabolism. Ferritin's key role is highlighted by the following observations. (i) All eukaryotic cells investigated thus far have been shown to express ferritin (4). (ii) Cellular proliferation is dependent on the availability of iron, and ferritin levels are closely regulated during normal proliferation and have been found to be abnormal in various malignancies (5, 6). (iii) Ferritin in the cells lining the duodenal mucosa is likely to regulate intestinal iron absorption. Furthermore, since one ofthe most common autosomal recessive disorders, hereditary hemochromatosis, is characterized by pathologically increased absorption of dietary iron, abnormal regulation of ferritin expression or abnormal ferritin function could play a major role in this disorder (7). Thus, the regulation of ferritin levels by iron is the means by which ferritin can both influence and respond to iron homeostasis.It is therefore important to elucidate the mechanisms by which ferritin gene expression is regulated. Munro and his colleagues (8, 9) suggested a translational regulation of ferritin expression in rat liver, based on two observations: (i) actinomycin D does not inhibit the stimulatory effect of iron on ferritin expression and (ii) cytoplasmic ferritin mRNA shifts from an inactive messenger-ribonucleoprotein pool to translationally active polysomes upon iron induction. Similar results were obtained for reticulocytes and liver of bullfrog tadpoles (10, 11). In further support of translational control of ferritin expression, cytoplasmic mRNA levels were shown to remain constant over a wide range of biosynthetic rates induced by iron (9). The recent cloning of the human ferritin heavy-chain cDNA and gene (12)(13)(14)(15) has allowed us to directly address this question of the mechanism of biosynthetic regulation at a molecular level.Previous work showed that a 7.2-kilobase (kb) HindIII genomic DNA fragment (pUCM11) is actively transcribed in stably transformed and transiently transfected murine B6 fibroblasts (13), giving rise to a...

Translational repression in eukaryotes: partial purification and characterization of a repressor of ferritin mRNA translation.

“…Early evidence suggested that the primary site of this regulation may be translational (4)(5)(6)(7)(8)(9). This has been confirmed recently by the discovery of the "iron-responsive element" (IRE), a highly conserved sequence of 28 nucleotides, in the 5' untranslated region (5' UTR) of all ferritin mRNAs (10)(11)(12)(13). The presence of this sequence renders the translation of downstream open reading frames (ORFs) responsive to the iron status of the cell.…”

Post-Transcriptional Control of Sucrose Synthase Expression in Anaerobic Seedlings of Maize

“…Ferritin transcripts are stored in an untranslated ribonuclear pool and shifted to polyribosomes as the protein is needed. The 5' untranslated leader sequence of ferritin mRNA confers the translational regulatory property to a heterologous gene in a transformation assay (13,14). Similarly, it may be possible to identify regions in the well-characterized Sh gene (27) that are responsible for the translational control of SS I transcripts during anaerobic stress.…”

Ovariectomy and estrogen treatment modulate iron metabolism in rat adipose tissue

“…Simultaneously, the binding of IRPs to IRE cis-element in the 5'-UTR of ferritin mRNA prevents its synthesis [14,15]. On the other hand, high iron levels reduce IRPs affinity to IREs (i.d.…”