Coordinate expression of heme and globin is essential for effective erythropoiesis
Raymond T. Doty, … , Siobán B. Keel, Janis L. Abkowitz
Raymond T. Doty, … , Siobán B. Keel, Janis L. Abkowitz
Published November 9, 2015
Citation Information: J Clin Invest. 2015;125(12):4681-4691. https://doi.org/10.1172/JCI83054.
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Coordinate expression of heme and globin is essential for effective erythropoiesis

Abstract

Erythropoiesis requires rapid and extensive hemoglobin production. Heme activates globin transcription and translation; therefore, heme synthesis must precede globin synthesis. As free heme is a potent inducer of oxidative damage, its levels within cellular compartments require stringent regulation. Mice lacking the heme exporter FLVCR1 have a severe macrocytic anemia; however, the mechanisms that underlie erythropoiesis dysfunction in these animals are unclear. Here, we determined that erythropoiesis failure occurs in these animals at the CFU-E/proerythroblast stage, a point at which the transferrin receptor (CD71) is upregulated, iron is imported, and heme is synthesized — before ample globin is produced. From the CFU-E/proerythroblast (CD71+ Ter119– cells) stage onward, erythroid progenitors exhibited excess heme content, increased cytoplasmic ROS, and increased apoptosis. Reducing heme synthesis in FLVCR1-defient animals via genetic and biochemical approaches improved the anemia, implying that heme excess causes, and is not just associated with, the erythroid marrow failure. Expression of the cell surface FLVCR1 isoform, but not the mitochondrial FLVCR1 isoform, restored normal rbc production, demonstrating that cellular heme export is essential. Together, these studies provide insight into how heme is regulated to allow effective erythropoiesis, show that erythropoiesis fails when heme is excessive, and emphasize the importance of evaluating Ter119– erythroid cells when studying erythroid marrow failure in murine models.

Authors

Raymond T. Doty, Susan R. Phelps, Christina Shadle, Marilyn Sanchez-Bonilla, Siobán B. Keel, Janis L. Abkowitz

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Figure 2

CFU-E/proerythroblasts upregulate heme biosynthesis more than globin synthesis.

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CFU-E/proerythroblasts upregulate heme biosynthesis more than globin syn...
Relative expression of Flvcr1, Flvcr1b, Hmox1 and 2, Alas1 and 2, β-globin (Hbb-bs), and β-actin (Act-b) in sorted marrow erythroblast populations (I–V) quantified by real-time PCR. Mean ± SD of 3 independently sorted samples are presented from control (solid bars) or Flvcr1-deleted (open bars) mice relative to expression levels in the LNPC from control mice. Of note, the Flvcr1 probe detects total Flvcr1, which includes functional Flvcr1a and Flvcr1b transcripts containing exon 3, while the Flvcr1b probe detects all Flvcr1b transcripts, regardless of whether they encode functional proteins. Normalization of expression levels is described in the Methods. Each sorted sample was derived from a pool of marrow from 3 mice; thus, each bar includes data from 9 mice. Student’s t test: *P < 0.05, **P < 0.01.