Growth and differentiation signals of the erythropoietin receptor

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To identify the novel substrate of c-kit which is important for hematopoietic stem cell self-renewal or differentiation, CD34-negative, Sca-1 positive, c-KIT-positive, and Lineage marker-negative (CD34-Sca-1+c-Kit-Lin- )cells were sorted by a fluorescence-activated cell sorter from mouse and two hybrid cDNA library was constructed. By the screening using c-kit as bait, we cloned a novel cDNA, designed STAP-1, encoding an adaptor protein with a Pleckstrin homology domain, Src homology 2 domain, and a number of tyrosine phosphorylation sites. RT-PCR analysis revealed that STAP-1 expression is restricted in bone marrow cell fraction expressing c-kit, and the highest expression was observed in CD34-Sca-1+c-Kit+Lin- hematopietic stem cell enriched fraction. Murine myeloid cell line, M1 expressed high level of STAP-1. However, the expression was strongly repressed in response to leukemia inhibitory factor which induced monocytic differentiation of M1 cells, suggesting that STAP-1 is associat … More ed with undifferentiated cell type. In 293 cells, STAP-1 was tyrosine phosphorylated by activated c-kit. In vitro binding assay suggested that STAP-1 SH2 domain interacted with several tyrosine phosphorylated proteins including c-kit and STAT5.There suggest that STAP-1 functions as an adaptor molecule downstream of c-kit in hematopoietic stem cells.CIS3/SOC53 are small SH2 containing proteins that interact with and inhibit JAK tyrosine kinases. During embryonic development, CIS53/SOCS-3 is highly expressed in some but no all erythroid lineage cells of the fetal liver and this expression is independent of erythropoietin (EPO) signaling. Transgene mediated constitutive expression blocks fetal erythropoiesis resulting in an embryonic lethality. Deletion of CIS3/SOCS-3 results in an embryonic lethality at 12-16 days that is associated with a marked erythrocytosis. Moreover, the individual in vitro proliferative capacity of fetal liver progenitors is greatly increased. The results demonstrate that CIS3/SOCS-3 play a critical role in negatively regulating fetal liver hematopoiesis. We also demonstrated that CIS3-SOCS3 bound to the EPO receptor as well as JAK2 in erythroid progenitors from spleen and Ba/F3 cells expressing the EPOR (BF-ER), suggesting a specific negative regulatory effect of CIS3/SOCS3 on EPO signaling. Less