Heme and erythropoieis: more than a structural role

Abstract

Erythropoiesis is the biological process that consumes the highest amount of body iron for heme synthesis. Heme synthesis in erythroid cells is finely coordinated with that of alpha (α) and beta (β)-globin, resulting in the production of hemoglobin, a tetramer of 2α- and 2β-globin chains, and heme as the prosthetic group. Heme is not only the structural component of hemoglobin, but it plays multiple regulatory roles during the differentiation of erythroid precursors since it controls its own synthesis and regulates the expression of several erythroid-specific genes. Heme is synthesized in developing erythroid progenitors by the stage of proerythroblast, through a series of eight enzymatic reactions divided between mitochondria and cytosol. Defects of heme synthesis in the erythroid lineage result in sideroblastic anemias, characterized by microcytic anemia associated to mitochondrial iron overload, or in erythropoietic porphyrias, characterized by porphyrin deposition in erythroid cells. Here, we focus on the heme biosynthetic pathway and on human erythroid disorders due to defective heme synthesis. The regulatory role of heme during erythroid differentiation is discussed as well as the heme-mediated regulatory mechanisms that allow the orchestration of the adaptive cell response to heme deficiency.

Figure 1.

Schematic representation of erythroid differentiation. The timing of the processes leading to hemoglobin production during erythroid differentiation is illustrated. High amount of iron is required during erythroid differentiation to sustain heme biosynthesis. Pro-erythroblast increases iron uptake through the upregulation of TfR1. At the same time, the activity of ALAS2 also increases to provide the huge amounts of heme needed for hemoglobin production. Soon after its synthesis, heme activates the transcription and translation of globin chains, thus allowing hemoglobin synthesis.

Figure 2.

Heme biosynthesis. Schematic representation of the heme biosynthetic pathway. Heme synthesis starts with the condensation of Succynil-CoA and glycine to form ALA. ALA is then transported through the two mitochondrial membranes in the cytosol where it is converted to CPgenIII through a series of enzymatic reactions. Briefly, the aminolevulinate dehydratase (ALAD) catalyzes the condensation of two molecules of ALA to form one molecule of the monopyrrole porphobilinogen. Then, the hydroxymethylbilane synthase (HMBS) catalyzes the head-to-tail synthesis of four porphobilinogen molecules to form the linear tetrapyrrole hydroxymethylbilane which is converted to uroporphyrinogen III by uroporphyrinogen synthase (UROS). The last cytoplasmic step, the synthesis of CPgenIII, is catalyzed by uroporphyrinogen decarboxylase (UROD). CPOX is a homodimer weakly associated with the outside of the inner mitochondrial membrane and it converts CPgenIII to protoporphyrinogen IX. The following oxidation of protoporphyrinogen IX to PPIX is catalyzed by PPOX, located on the outer surface of the inner mitochondrial membrane. Finally, ferrous iron is incorporated into PPIX to form heme in the mitochondrial matrix, a reaction catalyzed by FECH. In hematopoietic tissue, iron is imported into mitochondria by MFRN1. FECH is localized in the inner mitochondrial membrane in association to MFRN1 and ABCB10. SLC25A38 and ABCB10 have been proposed as mitochondrial ALA exporters on the inner mitochondrial membrane. The ALA transporter located on the outer mitochondrial membrane has not been identified yet. ABCB6 has been proposed as a putative mitochondrial CPgenIII importer. However, this role is still controversial. Finally, several data suggest that FLVCR1b is a mitochondrial heme exporter.

Figure 3.

FLVCR1 isoforms. (A) Schematic representation of the Flvcr1 gene. Flvcr1a and Flvcr1b originate from two alternative transcription start sites (arrows). Flvcr1btranscript lacks the first exon of the Flvcr1gene. (B) Role of the two FLVCR1 isoforms. FLVCR1a, a 12-transmembrane domain protein, is a heme exporter localized at the plasma membrane. FLVCR1b has 6-transmembrane domains, it is supposed to homo/eterodimerize and it is expressed in mitochondria. There is much evidence to indicate that FLVCR1b is a mitochondrial heme exporter.

Figure 4.

Heme controls the transcription of α- and β-globin genes in differentiating erythroid progenitors. (A) In early erythroid progenitors the transcription of α- and β-globin is inhibited by the transcriptional repressor BACH1 which antagonizes the activity of sMaf proteins that bind MAREs in the regulatory region of globin genes. (B) In late erythroid progenitors, when heme biosynthesis starts, heme binds to BACH1 in the nucleus and mediates its export in the cytosol. Finally, heme stabilizes the transcription factor NRF2 that accumulates in the nucleus. NFR2 associated with sMaf proteins activates the transcription of globin chains.

Figure 5.

Heme controls the translation of α- and β-globin in differentiating erythroid progenitors. The translation of globin mRNAs is regulated by heme through the heme regulated kinase HRI. When heme binds HRI, HRI is inactivated and the translation initiation factor eIF2α is not phosphorylated allowing protein synthesis to occur.