Neither miR-7-5p nor miR-141-3p is a major mediator of iron-responsive transferrin receptor-1 mRNA degradation

The transferrin receptor (TfR1) is the principal means of iron importation for most mammalian cells, and regulation of mRNA stability is a major mechanism through which TfR1 expression is controlled in response to changing intracellular iron levels. An endonuclease activity degrades the TfR1 mRNA during iron-repletion, which reduces iron importation and contributes to the restoration of homeostasis. Correct identification of the TfR1 mRNA endonuclease activity is important as it has the potential to be a pharmacological target for the treatment of several pathologies in which iron homeostasis is perturbed. A recent RNA article identified both miR-7-5p and miR-141-3p as mediators of TfR1 mRNA degradation during iron-repletion. However, the proposed TfR1 microRNA binding sites are inconsistent with several earlier studies. To better understand the discrepancy, we tested the proposed sites within an assay developed to detect changes to TfR1 mRNA stability. The complete disruption of both proposed binding sites failed to impact the assay in all cell lines tested, which include cell lines derived from mouse connective tissue (L-M), a human colon adenocarcinoma (SW480), and a human ovarian carcinoma (A2780). The overexpression of a miR-7-5p mimic also failed to decrease expression of both the endogenous TfR1 mRNA and a luciferase-TfR1 reporter under conditions in which the expression of a previously identified mir-7- 5p target is attenuated. As a result, it is unlikely that the microRNAs are directly mediating iron-responsive degradation of the TfR1 mRNA as recently proposed. Instead, three short hairpin loops within the TfR1 3'-UTR are shown to be more consistent as endonuclease recognition elements.