Characterization of the translation-dependent step during iron-regulated decay of transferrin receptor mRNA

Roquin is a major mediator of iron-regulated changes to transferrin receptor-1 mRNA stability

Transferrin receptor-1 (TfR1) has essential iron transport and proposed signal transduction functions. Proper TfR1 regulation is a requirement for hematopoiesis, neurological development, and the homeostasis of tissues including the intestine and muscle, while dysregulation is associated with cancers and immunodeficiency. TfR1 mRNA degradation is highly regulated, but the identity of the degradation activity remains uncertain. Here, we show with gene knockouts and siRNA knockdowns that two Roquin paralogs are major mediators of iron-regulated changes to the steady-state TfR1 mRNA level within four different cell types (HAP1, HUVEC, L-M, and MEF). Roquin is demonstrated to destabilize the TfR1 mRNA, and its activity is fully dependent on three hairpin loops within the TfR1 mRNA 3′-UTR that are essential for iron-regulated instability. We further show in L-M cells that TfR1 mRNA degradation does not require ongoing translation, consistent with Roquin-mediated instability. We conclude that Roquin is a major effector of TfR1 mRNA abundance.

•

Roquin is a major mediator of iron-regulated TfR1 mRNA instability

•

Roquin-mediated instability requires three stem loops within the TfR1 3′-UTR

•

Iron-regulated TfR1 mRNA instability can occur in the absence of Regnase-1

Iron-metabolic function and potential antibacterial role of Hepcidin and its correlated genes (Ferroportin 1 and Transferrin Receptor) in turbot (Scophthalmus maximus)

Antimicrobial peptide plays an important role in fish immunity. The small molecular antimicrobial peptide Hepcidin in turbot was studied and reported in this paper. The Ferroportin 1 (FPN1) and Transferrin Receptor (TFR) genes, which are related to Hepcidin, were cloned in turbot. The characteristics of Hepcidin and its related genes were studied, including an analysis of the expression patterns and cloning of the Hepcidin promoter, the relationship between Hepcidin and NF-κB and the regulation of iron-metabolism. The results showed that the promoter of SmHepcidin contains the binding sites of NF-κB, and NF-κB may directly or indirectly receive feedback signals from SmHepcidin. In the liver, spleen and kidney, in which there was an increased SmHepcidin expression level, SmFPN1 dramatically decreased and SmTFR was also either decreased or exhibited no obvious change after bacterial/viral infection and an injection of exogenous Hepcidin protein. RNAi experiments in turbot kidney cells confirmed the expression changes of these gene patterns. Furthermore, the administration of exogenous Hepcidin protein, which regulates the level of chelatable iron in cells, further confirmed the function of Hepcidin in iron metabolism. It is speculated that the rapidly increased expression of SmHepcidin may induce changes in the expression of related genes, and that the in vivo chelatable iron concentration which participates in the antibacterial process was also changed when exogenous pathogens are present in turbot. It is suggested that SmHepcidin plays a defensive role against pathogenic infection.

Requirement of new protein synthesis of a transcription factor for memory consolidation: paradoxical changes in mRNA and protein levels of C/EBP

Some specific transcription factors are essential for memory consolidation across species. However, it is still unclear whether only the activation of constitutively expressed forms of these conserved transcription factors is involved in memory consolidation or their de novo synthesis also occurs after learning. This question has remained unanswered partly because of the lack of an efficient method for the determination of copy numbers of particular mRNAs in single neurons, which allows the detection of new transcription at the cellular level. Here we applied a newly developed protocol of single-cell quantitative real-time polymerase chain reaction (qRT-PCR) to single neurons playing an important role in associative learning. Specifically, we examined the changes in the mRNA and protein expression levels of a highly conserved transcription factor, CCAAT/enhancer binding protein (C/EBP), in the paired B2 motoneurons of the pond snail Lymnaea stagnalis. These buccal neurons are involved in the motor control of feeding behavior, with a potentially important role in conditioned taste aversion (CTA). Single-cell qRT-PCR revealed a significant decrease in LymC/EBP mRNA copy numbers in the B2 motoneurons during memory consolidation after CTA training. By contrast, isoelectric focusing and immunoblotting of extracts of the buccal ganglia showed that translation and phosphorylation levels of LymC/EBP significantly increased during memory consolidation. The C/EBP-like immunoreactivity in the B2 motoneurons, which are the major immunopositive component in the buccal ganglia, also significantly increased during memory consolidation, suggesting that the main source of increase in the level of protein in the buccal ganglia are the B2 motoneurons. Thus, early memory consolidation after CTA learning in L.stagnalis involves both the rapid synthesis and phosphorylation of LymC/EBP as well as the rapid breakdown of LymC/EBP mRNA in the neural network controlling feeding, suggesting that all of these processes play a role in the function of C/EBP in memory consolidation.

Post-transcriptional regulation of plasminogen activator inhibitor-1 by intracellular iron in cultured human lung fibroblasts--interaction of an 81-kDa nuclear protein with the 3'-UTR

The proteinase inhibitor, type-1 plasminogen activator inhibitor (PAI-1), is a major regulator of the plasminogen activator system involved in plasmin formation and fibrinolysis. The present study explores the effects of intracellular iron on the expression of PAI-1 and associated cell-surface plasmin activity in human lung fibroblasts; and reports the presence of a novel iron-responsive protein. ELISA revealed a dose-dependent increase in PAI-1 antigen levels expressed in the conditioned medium of cells treated with deferoxamine, in the three cell lines studied. A concomitant increase in mRNA levels was also observed by Northern analyses. Presaturation with ferric citrate quenched the effect of deferoxamine. Experiments with transcription and translation inhibitors on TIG 3-20 cells demonstrated that intracellular iron modulated PAI-1 expression at the post-transcriptional level with the requirement of de-novo protein synthesis. Electrophoretic mobility shift assay and UV crosslinking assays revealed the presence of an approximately 81-kDa nuclear protein that interacted with the 3'-UTR of PAI-1 mRNA in an iron-sensitive manner. Finally, we demonstrated that the increased PAI-1 is functional in suppressing cell-surface plasmin activity, a process that can affect wound healing and tissue remodeling.

Developmentally regulated instability of the GPI-PLC mRNA is dependent on a short-lived protein factor

The expression of the vast majority of protein coding genes in trypanosomes is regulated exclusively at the post-transcriptional level. Developmentally regulated mRNAs that vary in levels of expression have provided an insight into one mechanism of regulation; a decrease in abundance is due to a shortened mRNA half-life. The decrease in half-life involves cis-acting elements in the 3′ untranslated region of the mRNA. The trans-acting factors necessary for the increased rate of degradation remain uncharacterized. The GPI-PLC gene in Trypanosoma brucei encodes a phospholipase C expressed in mammalian bloodstream form, but not in the insect procyclic form. Here, it is reported that the differential expression of the GPI-PLC mRNA also results from a 10-fold difference in half-life. Second, the instability of the GPI-PLC mRNA in procyclic forms can be reversed by the inhibition of protein synthesis. Third, specifically blocking the translation of the GPI-PLC mRNA in procyclic forms by the inclusion of a hairpin in the 5′ untranslated region does not result in stabilization of the mRNA. Thus, the effect of protein synthesis inhibitors in stabilizing the GPI-PLC mRNA operates in trans through a short-lived factor dependent on protein synthesis.

Transferrin receptor hyperexpression in primary erythroblasts is lost on transformation by avian erythroblastosis virus

In primary chicken erythroblasts (stem cell factor [SCF] erythroblasts), transferrin receptor (TfR) messenger RNA (mRNA) and protein were hyperexpressed as compared to nonerythroid chicken cell types. This erythroid-specific hyperexpression was abolished in transformed erythroblasts (HD3E22 cells) expressing the v-ErbA and v-ErbB oncogenes of avian erythroblastosis virus. TfR expression in HD3E22 cells could be modulated by changes in exogenous iron supply, whereas expression in SCF erythroblasts was not subject to iron regulation. Measurements of TfR mRNA half-life indicated that hyperexpression in SCF erythroblasts was due to a massive stabilization of transcripts even in the presence of high iron levels. Changes in mRNA binding activity of iron regulatory protein 1 (IRP1), the primary regulator of TfR mRNA stability in these cells, correlated well with TfR mRNA expression; IRP1 activity in HD3E22 cells and other nonerythroid cell types tested was iron dependent, whereas IRP1 activity in primary SCF erythroblasts could not be modulated by iron administration. Analysis of avian erythroblasts expressing v-ErbA alone indicated that v-ErbA was responsible for these transformation-specific alterations in the regulation of iron metabolism. In SCF erythroblasts high amounts of TfR were detected on the plasma membrane, but a large fraction was also located in early and late endosomal compartments, potentially concealing temporary iron stores from the IRP regulatory system. In contrast, TfR was almost exclusively located to the plasma membrane in HD3E22 cells. In summary, stabilization of TfR mRNA and redistribution of Fe-Tf/TfR complexes to late endosomal compartments may contribute to TfR hyperexpression in primary erythroblasts, effects that are lost on leukemic transformation.

The first exon coding region of cystathionine gamma-synthase gene is necessary and sufficient for downregulation of its own mRNA accumulation in transgenic Arabidopsis thaliana

Expression of the gene for cystathionine gamma-synthase (CGS), which catalyzes the key step of methionine biosynthesis, is feedback regulated at the level of mRNA stability. The first exon polypeptide of CGS is suggested to be involved in this regulation and amino acid sequence alterations caused by mto1 mutations in that region lead to an overaccumulation of CGS mRNA [Chiba et al. (1999) Science 286: 1371-1374]. Transgenic Arabidopsis thaliana harboring chimeric constructs in which wild-type or mto1 mutant CGS exon 1 are fused in-frame to reporter genes and driven by the cauliflower mosaic virus 35S RNA promoter were constructed. Studies with these transgenic lines demonstrated that the coding region of CGS exon 1 is necessary and sufficient for downregulation of its own mRNA accumulation in response to methionine application and that this region acts in cis in this process.

Iron regulatory proteins and the molecular control of mammalian iron metabolism

Mammalian iron homeostasis is maintained through the concerted action of sensory and regulatory networks that modulate the expression of proteins of iron metabolism at the transcriptional and/or post-transcriptional levels. Regulation of gene transcription provides critical developmental, cell cycle, and cell-type-specific controls on iron metabolism. Post-transcriptional control through the action of iron regulatory protein 1 (IRP1) and IRP2 coordinate the use of messenger RNA-encoding proteins that are involved in the uptake, storage, and use of iron in all cells of the body. IRPs may also provide a link between iron availability and cellular citrate use. Multiple factors, including iron, nitric oxide, oxidative stress, phosphorylation, and hypoxia/reoxygenation, influence IRP function. Recent evidence indicates that there is diversity in the function of the IRP system with respect to the response of specific IRPs to the same effector, as well as the selectivity with which IRPs modulate the use of specific messenger RNA.