Role of Heme in the Regulation of Transferrin Receptor Expression in Erythroid Cells

Physiologically, iron is transported between sites of absorption, storage, and use by the plasma glycoprotein transferrin (Tf). Most cells acquire iron via the binding of Tf to the Tf receptor (TfR) on the cell membrane. In non-erythroid cells, pivotal players in the regulation of TfR are the iron regulatory proteins, IRP1 and IRP2, which "sense" iron levels in the labile iron pool. IRPs control the synthesis of TfR by binding to iron-responsive elements (IREs) in the 3'UTR region of TfR mRNA. When cellular iron is scarce, IRPs bind to the IREs stabilizing the TfR mRNA, while excess iron prevents the binding of IRPs to IREs resulting in the rapid degradation of TfR mRNA.

Developing red blood cells are the largest consumers of iron, which is delivered to them exclusively by Tf via TfR. Erythroid cells have by far the greatest need for iron required for the rapid and efficient synthesis of hemoglobin. Thus, it is not surprising that transcription plays a dominant role over the regulation via the IRE/IRP system in the control of TfR expression (Lok and Ponka (2000) J. Biol. Chem . 275: 24185-24190). The exceptionally high capacity of immature erythroid cells to synthesize heme is primarily a result of their unique control of iron metabolism as well as their distinct heme biosynthetic enzyme, erythroid-specific aminolevulinic acid synthase (ALA-S2). Here we provide evidence that TfR expression and the cellular uptake of iron from Tf is stimulated by enhanced heme synthesis. Incubation of murine eryhroleukemia cells with 5-aminolevulinic acid (ALA) increased TfR expression as well as iron incorporation into heme. This effect of ALA can be completely prevented by the inhibitors of heme biosynthesis (succinylacetone (blocks ALA dehydratase) or N-methylprotoporphyrin (blocks ferrochelatase)), indicating that the effect of ALA requires its metabolism to heme. The induction of TfR expression by ALA is mainly the result of increased mRNA synthesis, since the effect of ALA can be abolished by actinomycin D. Importantly, the TfR promoter is activated in vitro by addition of ALA and hemin to cultured murine erythroleukemia cells differentiated with DMSO. Direct-site specific mutagenesis of the erythroid active element1 in TfR promoter abolished the effects of ALA and hemin. These results suggest that heme may directly or indirectly interact with TfR promoter, consequently activating gene expression. Hence, our results indicate that in erythroid cells heme serves as a positive feedback regulator that maintains high TfR levels, thus ensuring appropriate iron availability for hemoglobin synthesis.