Novel mechanism for translational control in regulation of ferritin synthesis by iron.

Abstract

Poly(A)containing RNA was isolated from the polyribosomal and post-ribosomal fractions of the livers of normal and iron-treated rats. These RNA fractions were then translated in a wheat germ system to provide a measure of the amount of ferritin mRNA present in each fraction. Following iron administration, there was a 2-fold increase in the amount of ferritin mRNA in the polyribosomal fraction. This increase was not inhibited by prior treatment of the rats with actinomycin D or cordycepin, suggesting a cytoplasmic control mechanism. In normal rats, the post-ribosomal fraction contained an amount of ferritin mRNA equal to that in the polyribosomes. When iron was administered, this untranslated ferritin mRNA became reduced to negligible quantities, thus accounting for the doubling of the ferritin mRNA content of the polyribosomal fraction. A scheme is proposed in which translation of the ferritin mRNA in the post-ribosomal fraction is prevented by adhering ferritin subunits. Iron administration removes this inhibition of the translation of ferritin mRNA by promoting aggregation of these subunits into ferritin.Evidence from many tissues shows that not all cytoplasmic mRNA is associated with polyribosomes (1-19). This pool of free untranslated cytoplasmic mRNA constitutes 10-30% of the total cytoplasmic mRNA content of a number of tissues (8,10,15,(17)(18)(19) and in some others even accounts for 50-80% (1,3,4,11). It is generally believed to represent a storage of precursor form of the polyribosomal mRNA. Recently, several individual mRNAs, coding for specific proteins, have been identified in the post-ribosomal supernatant of a variety of tissues, amongst them the mRNAs coding for actin (10), ferritin (18), the a-chain of hemoglobin (14, 15), myosin (9, 11), and vesicular stomatitis virus proteins (8). Nevertheless, no precise function for any one of these post-ribosomal mRNAs has been demonstrated so far.Since the early observations of Granick et al. (20), it has been shown by many authors that iron increases the synthesis of ferritin in many different tissues (21)(22)(23)(24)(25)(26)(27)(28)(29)(30) Liver Polyribosomal and Post-Ribosomal RNA. Twenty pooled rat livers were homogenized in 2 volume of 0.1 M Tris-HCl, pH 7.6, 0.1 M NaCl, 0.1 mM EDTA, 5 mM MgCl2, 150 Ag/ml of heparin, 0.25 M sucrose, and centrifuged at 10,000 X g for 20 min. The resulting post-mitochondrial supernatant was filtered through glass wool and centrifuged at 42,000 rpm for 50-70 min in the Beckman Ti 50.1 rotor. The post-ribosomal supernatant was filtered through glass wool, diluted with 0.5 volume of 0.1 M Tris-HC1, pH 7.4, 0.1 M NaCl, 1 mM EDTA, and 50 ,ug/ml of heparin and made 1% in sodium dodecyl sulfate and 1 mM in EDTA. The pH was adjusted to 9.0 with 1 M NaOH prior to phenol extraction (33). Analysis on sucrose density gradients showed ribosome subunits to be nearly absent (18), and this was confirmed by sucrose gradient analysis of post-ribosomal RNA.Total polyribosomal RNA was isolated from the microsomal pellets essential...

Serum transferrin receptor in children with respiratory infections

“…This is supported by the observation that SF levels are raised during both mild and severe infections, similar to earlier observations (Konijn et al, 1981). Zahringer et al (1976) reported an increase in synthesis of apoferritin by the membrane-bound polysomes with subsequent release into circulation as an acute phase response to infection. Being apoferritin in nature, SF fails to reflect the true iron status of the individual and thus has limited use as a marker of iron status during infection.…”

The up-regulation of ferritin expression using a small-molecule ligand to the native mRNA

“…The yohimbine effect, therefore, is more than simply displacing the IRP. One explanation could be that unwinding of the IRE structure by yohimbine is more complete than by endogenous initiation factors, a conjecture supported by the observations that during iron overload, when the IRP is presumed to be displaced (51), 40 -50% of endogenous ferritin mRNA remains untranslated (52,53). The complexity of the interactions among the IRE RNA, IRP repressor, and initiation factor proteins is illustrated by the binding of the IRP repressor to both the IRE and the large initiation scaffold protein, eIF-4G (54).…”

Fe 2+ binds iron responsive element-RNA, selectively changing protein-binding affinities and regulating mRNA repression and activation

“…4B) shows Fe 2þ metabolite-regulated IRE-mRNA-IRP inhibition of protein synthesis, where ribosome binding is prevented (7,15,28), combined with Fe 2þ -metabolite regulated IRE-mRNA-eIF4F enhancement of protein synthesis (Fig. 4A), where ribosome binding is facilitated.…”

“…Deletion of the 30 nt IRE-RNA not only removes IRP regulation but also decreases the rate of IRP-independent protein synthesis (14). A number of current models of IRE-RNA/IRP regulation feature iron-dependent degradation/modification of the IRP proteins as the main control point (8,9,15,16). Such models do not answer two important questions: (i) How does iron increase release of IRP protein for [4Fe-4S]-modification and/or degradation?…”