The ecotropic viral integration site-1 (Evi-1) gene was first identified as a common locus of retroviral integration in myeloid tumors in mice. Evi-1 is highly expressed in cases with human acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS) as a consequence of chromosomal rearrangements involving 3q26, where Evi-1 is mapped. Mice deficient in Evi-1 die during embryogenesis with widespread hypocellularity, hemorrhaging, and disruption in the development of the heart, somite, and neural crest-derived cells. It was recently reported that Evi-1 is expressed at a high level in the para-aortic splanchnopleural (P-Sp) region, from which definitive hematopoiesis originates. Hematopoietic stem cells (HSCs) in Evi-1-deficient embryos are decreased in number with defective proliferation capacity, and this defective proliferation of Evi-1-deficient P-Sp cells is successfully rescued in vitro by retroviral transfer of GATA-2. However, detailed molecular mechanisms underlying Evi-1-mediated hematopoiesis remain to be elucidated. In the present study, we used a coculture system of cells derived from the P-Sp region with a layer of a stromal cell line OP9, on which hematopoietic cell development is efficiently induced. In this culture system, Evi-1-deficient P-Sp-derived cells showed severely decreased colony forming capacity. This defect was overcome by reactivating Evi-1 retrovirally. Using this assay, we examined a hematopoietic potential of a series of Evi-1 mutants and found that the first zinc finger domain and the acidic domain are required for the hematopoietic rescue of Evi-1-deficient P-Sp cells. These two domains were also related to some extent for GATA-2 up-regulation, suggesting that GATA-2 is one of the key molecules in Evi-1-mediated hematopoiesis. Furthermore, we found that blocking of TGF-β signaling is also able to recover the hematopoietic defect of Evi-1-deficient P-Sp cells. These findings suggest that Evi-1 promotes hematopoietic stem/progenitor expansion during embryogenesis through at least two pathways: up-regulation of GATA-2 and inhibition of TGF-β signaling.

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