A developmental transition in definitive erythropoiesis: erythropoietin expression is sequentially regulated by retinoic acid receptors and HNF4

Abstract

The cytokine erythropoietin (Epo) promotes erythropoietic progenitor cell proliferation and is required for erythropoietic differentiation. We have found that the Epo gene is a direct transcriptional target gene of retinoic acid signaling during early erythropoiesis (prior to embryonic day E12.5) in the fetal liver. Mouse embryos lacking the retinoic acid receptor gene RXR␣ have a morphological and histological phenotype that is comparable with embryos in which the Epo gene itself has been mutated, and flow cytometric analysis indicates that RXR␣-deficient embryos are deficient in erythroid differentiation. Epo mRNA levels are reduced substantially in the fetal livers of RXR␣ −/− embryos at E10.25 and E11.25, and genetic analysis shows that the RXR␣ and Epo genes are coupled in the same pathway. We furthermore show that the Epo gene is retinoic acid inducible in embryos, and that the Epo gene enhancer contains a DR2 sequence that represents a retinoic acid receptor-binding site and a retinoic acid receptor transcriptional response element. However, unlike Epo-deficient embryos that die from anemia, the erythropoietic deficiency in RXR␣ −/− embryos is transient; Epo mRNA is expressed at normal levels by E12.5, and erythropoiesis and liver morphology are normal by E14.5. We show that HNF4, like RXR␣ a member of the nuclear receptor family, is abundantly expressed in fetal liver hepatocytes, and is competitive with retinoic acid receptors for occupancy of the Epo gene enhancer DR2 element. We propose that Epo expression is regulated during the E9.5-E11.5 phase of fetal liver erythropoiesis by RXR␣ and retinoic acid, and that expression then becomes dominated by HNF4 activity from E11.5 onward. This transition may be responsible for switching regulation of Epo expression from retinoic acid control to hypoxic control, as is found throughout the remainder of life.

Erythropoietin

“…These elements in the Epo enhancer interact with an orphan nuclear receptor, i.e., a DNA-binding protein that shares structural homology with hormone-binding nuclear receptors but lacks a known ligand (Blanchard et al 1992). The orphan receptor HNF-4 plays a critical role in Epo gene regulation (Galson et al 1995;Makita et al 2001). The expression of HNF-4 is limited to renal cortex and liver, sites of Epo production, and also intestine.…”

Pleiotropic effects and compensation mechanisms determine tissue specificity in mitochondrial myopathy and sideroblastic anemia (MLASA)

“…SRA (Steroid Receptor Activator) related pathway may play a particularly important role in the sideroblastic anemia phenotype of MLASA. Pus1p and SRA cooperatively enhance mRAR (retinoic acid receptor) gamma-mediated transcription of a number of target genes [8], including erythropoietin gene (EPO) [32]. Erythropoietin regulates cellular iron homeostasis in erythroid cells through the direct effect on the interaction of RNA stem loop structures, called iron-responsive elements (IREs), with iron-regulatory protein (IRP)-1 and IRP-2 [33][34][35].…”

Endodermal and mesenchymal cross talk: a crossroad for the maturation of foregut organs

“…Hnf4α may be critical to reinduce a proper epithelial morphology on the endodermal cells after migration through the STM and to reduce EMT by stimulating high levels of cell adhesion genes (33). In addition to its potential role in regulating epithelial morphology in HE cells, HNF4α has also been shown to control erythropoietin (EPO)-stimulated induction of definitive erythropoiesis in the late stages of embryonic hematopoiesis (51). EPO is a protein released by hepatocytes (52) that signals via its receptor (EPOR) on primitive erythrocytes within the hematopoietic liver to induce definitive erythropoiesis.…”

“…EPO is a protein released by hepatocytes (52) that signals via its receptor (EPOR) on primitive erythrocytes within the hematopoietic liver to induce definitive erythropoiesis. EPO directly stimulates the proliferation and survival of definitive erythrocytes in the fetal liver (51). However, it is not required for primitive erythropoiesis within the yolk sac (site for erythropoiesis before E11.5) or liver (site for erythropoiesis after E11.5).…”