EPO receptor circuits for primary erythroblast survival

Abstract

EPO functions primarily as an erythroblast survival factor, and its antiapoptotic actions have been proposed to involve predominantly PI3-kinase and BCL-X pathways. Presently, the nature of EPOregulated survival genes has been investigated through transcriptome analyses of highly responsive, primary bone marrow erythroblasts. Two proapoptotic factors, Bim and FoxO3a, were rapidly repressed not only via the wild-type EPOR, but also by PY-deficient knocked-in EPOR alleles. In parallel, Pim1 and Pim3 kinases and Irs2 were induced. For this survival gene set, induction failed via a PY-null EPOR-HM allele, but was restored upon reconstitution of a PY343 STAT5-binding site within a related EPOR-H allele. Notably, EPOR-HM supports erythropoiesis at steady state but not during anemia, while EPOR-H exhibits near wildtype EPOR activities. EPOR-H and the wild-type EPOR (but not EPOR-HM) also markedly stimulated the expression of Trb3 pseudokinase, and intracellular serpin, Serpina-3G. For SERPINA-3G and TRB3, ectopic expression in EPOdependent progenitors furthermore significantly inhibited apoptosis due to cytokine withdrawal. BCL-XL and BCL2 also were studied, but in highly responsive Kit pos CD71 high Ter119 neg erythroblasts, neither was EPO modulated. EPOR survival circuits therefore include the repression of Bim plus IntroductionIn response to anemia, erythropoietin (EPO) is expressed by interstitial kidney and fetal liver cells via hypoxia-inducible transcription factor pathways. 1,2 As a secreted monomeric sialoglycoprotein, EPO then targets developing erythroblasts, and is essential for red cell formation during definitive bone marrow and fetal liver erythropoiesis. [3][4][5][6][7][8] Prospective roles for EPO in promoting primitive red cell formation in yolk sac also have recently been described. 9 Beyond this, recombinant EPO has been demonstrated in ischemia and other cell damage models to provide cytoprotective effects for injured renal, cardiac, retinal, and neuronal tissues. 10,11 Taken together, these considerations have heightened interest in the specific nature of key EPO action mechanisms, especially those associated with progenitor cell survival.EPO's prime effects are mediated via interactions with its dimeric single-transmembrane receptor (EPOR). [3][4][5][6][7][8]12 These interactions appear to evoke EPOR conformational events, 13 which are relayed to an upstream Janus kinase, JAK2 14 (and JAK2 likewise may preassemble with EPOR dimers at a juxtamembrane box1 domain). 15,16 JAK2, as activated via a Y1007 phosphorylation loop, 17 next stimulates 2 separable signal transduction pathways. First, JAK2 interestingly can support steady-state erythropoiesis via EPOR-PY-independent routes that, in part, may involve MEK1,2 and ERK1,2 stimulation. 18 Second, JAK2 also mediates the phosphorylation of 8 conserved EPOR cytoplasmic PY sites, which can then form a scaffold for the binding of up to 20 SH2-or PTB-domain encoding signal transduction factors and molecular adaptors. [6][7][8][19][20][21] Among conserve...

Erythropoietin-driven signaling ameliorates the survival defect of DMT1-mutant erythroid progenitors and erythroblasts

“…These data suggest that the improved survival of EPO-treated erythroblasts was likely mediated by STAT5 signaling. 33 In this regard; the condition of mk/mk mice treated with EPO resembles that of β-thalassemia mice with dramatically elevated endogenous EPO and increased BCL-X L expression, which is responsible for the prevention of apoptosis of thalassemic erythroid cells in this model. 19 The possible involvement of any substantial increase in iron utilization by individual erythroid cells as a contributing factor to the positive effects of EPO therapy can be ruled out by the fact that the patients' erythrocyte characteristics (MCV and MCH) remained unchanged with EPO supplementation.…”

Kit transduced signals counteract erythroid maturation by MAPK-dependent modulation of erythropoietin signaling and apoptosis induction in mouse fetal liver

“…55 However, studies using EpoR tyrosine-mutants demonstrated that PI3K/Akt activation is also indirectly mediated via Y343, involving the adaptor Gab2. 37,55,56 Stat5 and Akt pathways are thus coupled in EpoR signaling and might cooperate in target gene activation. Interestingly, we could show that KIT murine embryonic stem cells as previously described.…”

“…Moreover, anti-apoptotic Bcl2l1/Bcl-xL, Mcl1, Trib3 and pro-apoptotic Nix (coexpressed with Bcl-xL during erythroid maturation) were not upregulated in KIT D816V cells (Figures 4a and b, Supplementary Figure 4A). 21,[37][38][39][40] Alongside, downregulation of Bad and Bid was impaired (Figure 4a). These data show that continuous Kit signaling perturbs induction of the transcriptional program associated with terminal erythroid maturation and interferes with regulation of key mediators of cell cycle control, cell death and survival, thereby leading to continued proliferation and elevated apoptosis during terminal differentiation.…”

Developmental roles of tribbles protein family members

“…In a similar manner, mammalian Tribs appear to regulate differentiation linked to cell division during (1) hematopoiesis (Lin et al, 2007;Eder et al, 2008;Sathyanarayana et al, 2008), (2) myogenesis (Kato and Du, 2007;Sung et al, 2007), (3) vascular smooth muscle formation (Chan et al, 2010), (4) lymphocyte differentiation (Selim et al, 2007), and (5) adipogenesis (described below).…”