Activation of the p38 mitogen-activated protein kinase mediates the suppressive effects of type I interferons and transforming growth factor-beta on normal hematopoiesis.

Type I interferons (IFNs) are potent regulators of normal hematopoiesis in vitro and in vivo, but the mechanisms by which they suppress hematopoietic progenitor cell growth and differentiation are not known. In the present study we provide evidence that IFN alpha and IFN beta induce phosphorylation of the p38 mitogen-activated protein (Map) kinase in CD34+-derived primitive human hematopoietic progenitors. Such type I IFN-inducible phosphorylation of p38 results in activation of the catalytic domain of the kinase and sequential activation of the MAPK-activated protein kinase-2 (MapKapK-2 kinase), indicating the existence of a signaling cascade, activated downstream of p38 in hematopoietic progenitors. Our data indicate that activation of this signaling cascade by the type I IFN receptor is essential for the generation of the suppressive effects of type I IFNs on normal hematopoiesis. This is shown by studies demonstrating that pharmacological inhibitors of p38 reverse the growth inhibitory effects of IFN alpha and IFN beta on myeloid (colony-forming granulocytic-macrophage) and erythroid (burst-forming unit-erythroid) progenitor colony formation. In a similar manner, transforming growth factor beta, which also exhibits inhibitory effects on normal hematopoiesis, activates p38 and MapKapK-2 in human hematopoietic progenitors, whereas pharmacological inhibitors of p38 reverse its suppressive activities on both myeloid and erythroid colony formation. In further studies, we demonstrate that the primary mechanism by which the p38 Map kinase pathway mediates hematopoietic suppression is regulation of cell cycle progression and is unrelated to induction of apoptosis. Altogether, these findings establish that the p38 Map kinase pathway is a common effector for type I IFN and transforming growth factor beta signaling in human hematopoietic progenitors and plays a critical role in the induction of the suppressive effects of these cytokines on normal hematopoiesis.     

A novel mechanism of cooperation between c-Kit and erythropoietin receptor. Stem cell factor induces the expression of Stat5 and erythropoietin receptor, resulting in efficient proliferation and survival by erythropoietin

Optimal production of red cells in vivo requires collaboration between c-Kit, erythropoietin receptor (Epo-R), and GATA-1. However, the mechanism(s) of collaboration remain unclear. Utilizing an embryonic stem cell-derived erythroid progenitor cell line from mice deficient in GATA-1, we have examined the role of c-Kit and Epo-R in erythroid cell proliferation, survival, and differentiation. In the absence of GATA-1, we demonstrate an essential role for c-Kit in survival and proliferation of erythroid progenitors via the regulation of Bcl-2 expression. In addition, we demonstrate that Epo-R and Stat5 are regulated by a second, novel mechanism. We demonstrate that c-Kit stimulation by stem cell factor is essential for the maintenance of Epo-R and Stat5 protein expression, which results in significantly enhanced Bcl-x(L) induction and survival of erythroid progenitors in response to Epo stimulation. Restoration of GATA-1 function results in terminal erythroid maturation and up-regulation of Epo-R and Bcl-x(L) expression, leading also to significantly enhanced survival of terminally differentiating erythroid progenitors in the presence of only Epo. These results demonstrate that c-Kit and Epo-R have unique role(s) during distinct phases of erythroid maturation, and both stem cell factor and Epo contribute to the regulation of the Epo-R-Stat5-Bcl-x(L) pathway to ensure optimal survival, proliferation, and differentiation of erythroid progenitors.     

A hematopoietic-specific transmembrane protein, Art-1, is possibly regulated by AML1

The functions of AML1 in hematopoietic differentiation are repressed by AML1-mutants including the AML1/ETO chimeric protein, which is seen in t(8;21) acute myeloid leukemia. Erythroid progenitors of the patients with t(8;21) AML expressed AML1/ETO. To investigate the effect of AML1/ETO in erythroid cells, we made a tetracycline-regulated AML1/ETO overexpression system in mouse erythroleukemic (MEL) cells. Enforced AML1/ETO repressed the terminal erythroid differentiation. Furthermore, we performed representational difference analysis using this MEL cell system to clone the downstream targets of AML1 in erythroid cell differentiation. We cloned a novel transmembrane protein, Art-1 (AML1-regulated transmembrane protein 1), which is a member of tetramembrane spanning superfamily. Art-1 expression was restricted in hematopoietic cells. It was upregulated by AML1 and downregulated by AML1/ETO in both erythroid and myeloid cells, and increased during erythroid cell differentiation. Art-1 may play an important role in the differentiation of erythroid cells, possibly as a direct downstream target of AML1.     

新型GATA-1相互作用蛋白质及其在造血调控中的作用

GATA-1在造血干细胞的谱系分化中起关键的调控作用,为红系和巨核系发育成熟必不可少,对肥大细胞系及嗜酸性粒细胞系发育也有一定的调控作用。GATA-1的转录活性受到多层次的精确调节,其调控不同谱系分化的功能大多通过与不同的蛋白质相互作用来实施。近年来,多种新的GTAT-1相互作用蛋白质被确定,特别是GATA-1复合体的研究,揭示了GATA-1转录活性及其调控造血分化的新机制。