Regulation of transferrin receptor-1 mRNA by the interplay between IRE-binding proteins and miR-7/miR-141 in the 3′-IRE stem–loops

Abstract

Intracellular iron is tightly regulated by coordinated expression of iron transport and storage genes, such as transferrin receptor-1 (TfR1) and ferritin. They are primarily regulated by iron through iron-induced dissociation of iron-regulatory proteins (IRPs) from iron-responsive elements (IREs) in the 3'-UTR (untranslated region) of TfR1 or 5'-UTR of ferritin mRNA, resulting in destabilization of TfR1 mRNA and release of ferritin translation block. Thus high iron decreases iron transport via TfR1 mRNA degradation and increases iron storage via ferritin translational up-regulation. However, the molecular mechanism of TfR1 mRNA destabilization in response to iron remains elusive. Here, we demonstrate that miR-7-5p and miR-141-3p target 3'-TfR1 IREs and down-regulate TfR1 mRNA and protein expression. Conversely, miR-7-5p and miR-141-3p antagomiRs partially but significantly blocked iron- or IRP knockdown-induced down-regulation of TfR1 mRNA, suggesting the interplay between these microRNAs and IRPs along with involvement of another uncharacterized mechanism in TfR1 mRNA degradation. Luciferase reporter assays using 3'-UTR TfR1 IRE mutants suggested that the IREs C and E are targets of miR-7-5p and miR-141-3p, respectively. Furthermore, miR-7 expression was inversely correlated with TfR1 mRNA in human pancreatic adenocarcinoma patient samples. These results suggest a role of microRNAs in the TfR1 regulation in the IRP-IRE system.

Altered Iron Metabolism and Impact in Cancer Biology, Metastasis, and Immunology

“…miR-7-5p is a potent tumor suppressor of HCC growth including in models of sorafenib resistance (231). miR-7-5p expression was reduced in pancreatic adenocarcinoma samples and loss of miR-7-5p was proposed to permit TfR1driven cell proliferation and metabolism (232). Interestingly, miR-7-5p and miR-141-3p were found to target IREs within 3 ′ -UTR of TfR1, thereby reducing its mRNA and protein expression by competing with the IRE-IRP system (232).…”

“…miR-7-5p expression was reduced in pancreatic adenocarcinoma samples and loss of miR-7-5p was proposed to permit TfR1driven cell proliferation and metabolism (232). Interestingly, miR-7-5p and miR-141-3p were found to target IREs within 3 ′ -UTR of TfR1, thereby reducing its mRNA and protein expression by competing with the IRE-IRP system (232). Although, this finding was later disputed by another research group and requires further clarification (233).…”

Intravenous Iron Therapy Replenishes Iron Stores without Enhancing Tumour Responses in Anaemic Patients with Colorectal Cancer: Comparison with Oral Iron Supplementation.

“…Cellular iron transporter levels are controlled at a post-transcriptional level by iron-responsive binding proteins (IRP) 1 and 2 [13]. When activated by iron-de ciency, IRPs bind to iron-responsive elements (IREs) in the untranslated regions of messenger RNA including TfR1 and ferritin and promote the translation of TfR1 and repression of ferritin which increases the labile intracellular iron pool with decreases in iron export, utilisation and storage [14,15].…”

Iron-induced transferrin receptor-1 mRNA destabilization: A response to “Neither miR-7-5p nor miR-141-3p is a major mediator of iron-responsive transferrin receptor-1 mRNA degradation”

“…However, it is plausible that this discrepancy is primarily due to their luciferase assay set-up that used only an iron-replete condition (+iron) without taking an untreated (i.e., no additional iron [−iron]) control. They compared luciferase expression only between wild-type and mutant reporters in the iron-replete condition rather than comparing ±iron within the same reporter construct like we did (Miyazawa et al 2018). This assay condition and approach were also used in their previous work to minimize potential complications from IRP binding and protection (Rupani and Connell 2016).…”

“…For this reason and more importantly to measure luciferase reporter expression under the condition of iron-induced TfR1 mRNA degradation via the IRP-IRE system, we performed comparisons between untreated and iron-replete conditions within a single construct rather than comparing across different plasmid sequences ( Fig. 3 in Miyazawa et al 2018). In addition, by forcing IRP dissociation through the addition of iron to push TfR1 mRNA into an unstable form, without also comparing to an untreated control, it is unknown what effect their mutations are having on IRP binding ability and TfR1 mRNA stability under physiological conditions.…”