HIF-2α regulates murine hematopoietic development in an erythropoietin-dependent manner

Abstract

Erythropoiesis in the adult mammal depends critically on erythropoietin, an inducible cytokine with pluripotent effects. Erythropoietin gene expression increases under conditions associated with lowered oxygen content such as anemia and hypoxia. HIF-1␣, the founding member of the hypoxia-inducible factor (HIF) alpha class, was identified by its ability to bind and activate the hypoxia-responsive enhancer in the erythropoietin regulatory region in vitro. The existence of multiple HIF alpha members raises the question of which HIF alpha member or members regulates erythropoietin expression in vivo. We previously reported that mice lacking wild-type HIF-2␣, encoded by the EPAS1 gene, exhibit pancytopenia. In this study, we have characterized the etiology of this hematopoietic phenotype. Molecular studies of EPAS1-null kidneys reveal dramatically decreased erythropoietin gene expression. EPAS1-null as well as heterozygous mice have impaired renal erythropoietin induction in response to hypoxia. Treatment of EPAS1-null mice with exogenous erythropoietin reverses the hematopoietic and other defects. We propose that HIF-2␣ is an essential regulator of murine erythropoietin production. IntroductionHematopoiesis involves complex interactions of specialized cell types and molecular signaling events that vary according to development. 1 The major sites of murine hematopoietic development depend on the particular developmental or postnatal stage. 2,3 Erythropoiesis, that aspect of hematopoiesis concerned with generation of erythrocytes, occurs in 2 distinct developmental phases characterized by primitive erythropoiesis in the yolk sac blood islands and definitive erythropoiesis in the fetal liver or bone marrow later in development. 1,4,5 Besides location, primitive and definitive erythropoiesis also differ in their growth factor or cytokine requirements.Erythropoietin was first described as an endocrine regulator of erythropoiesis produced in the kidneys; later studies revealed a paracrine role of this cytokine in global hematopoiesis and other aspects of mammalian physiology. [6][7][8][9] Primitive erythropoiesis in the yolk sac is erythropoietin-independent, but requires other cytokines such as vascular endothelial growth factor (VEGF) and c-Kit. [10][11][12] In contrast, definitive erythropoiesis in the fetal liver and in the adult bone marrow is erythropoietin-dependent as evident by gene disruption studies of the erythropoietin 13 or erythropoietin receptor 14 gene. The temporal distinction of primitive and definitive erythropoiesis is in part due to developmental timing of erythropoietin and erythropoietin receptor gene expression. 15 The sites of definitive erythropoiesis are also sites of erythropoietin production. 16 These sites include resident macrophages in the adult bone marrow as well as in the fetal liver blood islands, 17 the predominant sites of definitive erythropoiesis in the adult and neonate, respectively. Erythropoietin produced in these locations may function as a paracrine growth factor for global ...

The HIF-2α-Driven Pseudo-Hypoxic Phenotype in Tumor Aggressiveness, Differentiation, and Vascularization

“…Administration of exogenous EPO reverts this phenotype as well as some of the other defects associated with Hif2a elimination (Scortegagna et al 2005). Further supporting a role for HIF2A in EPO production, a gain-of-function mutation in the HIF2A gene has been found associated with familial erythrocytosis (Percy et al 2008a;Percy et al 2008b).…”

“…In addition, Hif2a -/-mice display defects in hematopoietic development due to greatly reduced EPO levels in the kidney (Scortegagna et al 2003;Scortegagna et al 2005;Rankin et al 2007). Administration of exogenous EPO reverts this phenotype as well as some of the other defects associated with Hif2a elimination (Scortegagna et al 2005).…”

Biology of hypoxia-inducible factor-2α in development and disease

“…Notably, administration of exogenous Epo corrected the pancytopenic phenotype. 44 Given these disparate phenotypes for global HIF-2a deletion, the differential requirements for the a subunits during development remained unclear. As a genetic test of the ability of HIF-2a to compensate for HIF-1a function, we 'knocked-in' the HIF-2a coding sequence into the HIF-1a locus, thereby expanding HIF-2a expression to all tissues in which HIF-1a is normally expressed.…”

Erythropoietin gene expression in renal carcinoma is considerably more frequent than paraneoplastic polycythemia

“…8 This has been confirmed in vivo in HIF2a knock-out animals. [9][10][11] Furthermore, the cells physiologically producing EPO in liver and kidneys also express HIF-2a rather than HIF-1a. 5,6 In our analysis of the tumours both the tumour tissues and the derived cell lines showed approximately equal distribution of expression for one or the other HIFa isoform, or both.…”

“…[5][6][7] Interestingly, there does seem to be a target gene specificity in that HIF-2a seems to be responsible for EPO induction in the kidney, which has been indicated in vitro and in vivo. 6,[8][9][10][11] In the presence of oxygen, HIFa is targeted by a specific E3 ubiquitin ligase, with the von Hippel Lindau (VHL) tumour suppressor protein as recognition component. Binding of the E3 ligase leads to ubiquitination and rapid proteasomal destruction.…”