Iron Deficiency Can Upregulate Expression of Transferrin Receptor at Both the MRNA and Protein Level

Expression of the transferrin receptor (TfR) is controlled essentially by post-transcriptional regulation, which is mediated by the interaction between iron regulatory proteins (IRPs) and iron-responsive elements (IREs) located in the 3'-untranslated region (UTR) of TfR mRNA. We examined whether additional controls of expression might occur by investigating the regulation of TfR in the absence of its' IREs by cloning 3'-UTR truncated TfR cDNA (TfRDeltaIRE) into pcDNA3.1(+) and stably expressing it in CHO-TRVb (Chinese Hamster Ovary, TfR variant cells, clone b) cells which have no endogenous TfR. Both the surface and the total cell TfR protein levels increased significantly after treating the stably transfected cells with the iron chelator desferrioxamine (DFO) compared with the control cells. In contrast, expression of the transferrin receptor 2 (TfR2), which was also cloned in the same vector and stably expressed in the TRVb-CHO cells, was not regulated by iron chelation. In addition, TfR2 fused to the 3'-UTR of TfR (TfR2-IRE) was cloned into the same vector and stably expressed in the CHO-TRVb cells. A comparison showed that expression of full-length TfR and TfR2-IRE increased by fivefold and twofold, respectively, after addition of DFO. Further experiments showed that TRVb-CHO cells, which were stably transfected with TfRDeltaIRE and exposed to either normal medium or DFO, resulted in similar levels of TfR mRNA, but the TfR protein biosynthetic rate increased with iron chelation. In summary, expression of TfR in cells placed in an iron-deficient milieu can increase by a mechanism which is independent of the IREs of TfR.