Tyrosine kinase receptor RON functions downstream of the erythropoietin receptor to induce expansion of erythroid progenitors

Abstract

Erythropoietin (EPO) is required for cell survival during differentiation and for progenitor expansion during stress erythropoiesis. Although signaling pathways may couple directly to docking sites on the EPO receptor (EpoR), additional docking molecules expand the signaling platform of the receptor. We studied the roles of the docking molecules Grb2-associated binder-1 (Gab1) and Gab2 in EPO-induced signal transduction and erythropoi- IntroductionErythropoietin (EPO) is required for the survival, proliferation, and differentiation of erythroblasts. It acts with other growth factors and hormones to balance the expansion and terminal differentiation of the erythroid compartment. For instance, stem cell factor (SCF), a ligand of c-Kit, cooperates with EPO to delay differentiation and to enhance progenitor numbers. 1,2 The EPO receptor (EpoR) is a homodimer constitutively associated with Janus tyrosine kinase 2 (Jak2). Ligand binding induces a conformational change of the EpoR dimer, 3,4 resulting in the activation of Jak2, the phosphorylation of the EpoR on 8 tyrosine residues, and the recruitment of multiple signaling molecules. [5][6][7] Mice deficient in EPO, EpoR, or Jak2 lack definitive erythropoiesis. [8][9][10][11] Although this demonstrates a crucial role for EPO-induced Jak2 activation in erythropoiesis, the contribution of individual downstream signaling pathways to erythroblast proliferation, differentiation, and survival is less clear. Notably, mice expressing a truncated EpoR, lacking all tyrosine residues, are not anemic. 12 This may implicate that additional proteins, such as docking molecules, can form a complex with the EpoR to create a signaling platform with sufficient specificity to regulate the balance of erythroid expansion and differentiation.Candidates for this function are the Grb2-associated binder (Gab) family members. At present, 3 Gab family members have been identified-Gab1, Gab2, and Gab3. 13,14 Gab1 is ubiquitously expressed, whereas Gab2 and Gab3 expression are tissue specific. In the hematopoietic system, Gab3 is expressed in B cells and T cells and in the myeloid compartment, whereas Gab2 is expressed in all hematopoietic cells except T cells. Gab1 deficiency is embryonically lethal because of placental defects; Gab1Ϫ/Ϫ embryos are also characterized by reduced liver size and migration defects of muscle precursor cells. 15,16 In contrast, Gab2 deficiency results only in mild defects in mast cell and macrophage responses, 17,18 and Gab3-deficient mice show normal hematopoiesis. 19 Gab2 and Gab3 lack the c-Met binding sequence (MBS) of Gab1 and, therefore, may not complement Gab1 in c-Met signaling. Furthermore, the high homology between family members suggests that complementation by the ubiquitously expressed Gab1 may occur in Gab2/3 knockouts. On tyrosine phosphorylation, Gab proteins recruit signaling intermediates, among them Shc (Src homology-containing protein), Shp2 (Src-homology domain containing phosphatase), p85 subunit of phosphatidylinositol 3-kinase (PI3K), and phosp...

EPO receptor circuits for primary erythroblast survival

“…Interestingly, this response (GAB2 PY452 phosphorylation) also was efficiently supported in EPOR-H, but not EPOR-HM proerythroblasts ( Figure 4A). These analyses were consistent with a prior report of EPO-induced GAB2 phosphorylation, 57 and further link this pathway to STAT5 signals. In addition, and following EPO exposure, coimmunoprecipitation of phospho-GAB2 with STAT5 also could be demonstrated in wt-EPOR erythroblasts (data not shown).…”

Differential Regulation of Foxo3a Target Genes in Erythropoiesis

“…Second, although Epo and SCF have overlapping functions (i.e., activation of the PI3K and Ras-mitogen-activated protein kinase pathways), there are also differences. Epo specifically activates the Jak-Stat pathway and the tyrosine kinase receptor Ron, which recruits the adaptor Gab1 (21,67). Other targets are induced by both Epo and SCF but may have stimulus-specific effects.…”

Core erythropoietin receptor signals for late erythroblast development

“…2 Via a cytoplasmic juxtamembrane box-1 domain, the EpoR also preassembles with Jak2 kinase. 9 Epo-EpoR interactions stimulate Jak2 phosphorylation at Y1007/Y1008 sites, 10 and Jak2 (potentially in concert with Src, Btk, STK, and/or Kit tyrosine kinases) [11][12][13][14][15] then mediates the phosphorylation of multiple EpoR cytoplasmic tyrosine motifs. In particular, within the EpoR of mouse, human, and zebra fish, 16 8 distal phosphotyrosine motifs are conserved that possess established binding specificities for SH2-domain-encoding effectors.…”