Deciphering the mechanism of Transferrin Receptor mRNA degradation under iron replete conditions

Abstract

Iron is an essential co-factor required for many biochemical reactions in our body, however iron overload can be detrimental to the cells. Thus, intra-cellular iron concentration is meticulously controlled. For most cells in the body, the transferrin receptor (TFRC-1) is the major means through which iron is imported into the cell. Influx of transferrin bound iron is regulated by changes in the stability of TFRC-1 mRNA. Under iron deplete conditions, iron regulatory proteins 1 and 2 (IRP-1 & 2) bind to specific sequence elements within the 3’ untranslated region (3’UTR) of the TFRC-1 mRNA and prevent its degradation. Whereas under iron replete conditions there is minimal binding between the IRPs and TFRC-1 mRNA, resulting in TFRC-1 mRNA degradation, the precise mechanism of which is yet unknown. This study was based on three hypotheses that could lead to identification of the mechanism of TFRC-1 mRNA degradation. First, there could be a microRNA mediated silencing of the TFRC-1 mRNA. Secondly, the TFRC-1 mRNA could catalyse its own degradation or lastly an iron responsive endonuclease could degrade the TFRC-1 mRNA. Using a luciferase reporter system, we have performed a detailed mutagenesis study of a previously minimized TFRC-1 construct and identified three critical elements for degradation. The three elements impart a graded response to stability and a similar graded response is also observed in endogenous TFRC-1 mRNA under certain conditions. Results from the mutagenesis study strongly suggest that an unidentified endonuclease is responsible for degradation. Identifying this endonuclease will provide a novel therapeutic target to manipulate iron concentration in pathological cells.