The 89-kDa STAT3 protein is a latent transcription factor which is activated in response to cytokines (interleukin (IL)-5 and -6) and growth factors (epidermal growth factor). Binding of IL-5 to its specific receptor activates JAK2 which leads to the tyrosine phosphorylation of STAT3 proteins. Here we report the cloning of a cDNA encoding a variant of the transcription factor STAT3 (named STAT3beta) which was isolated by screening an eosinophil cDNA library. Compared to wild-type STAT3, STAT3beta lacks an internal domain of 50 base pairs located near the C terminus. This splice product is a naturally occurring isoform of STAT3 and encodes a 80-kDa protein. We found by reconstitution of the human IL-5R in COS cells that like STAT3, STAT3beta is phosphorylated on tyrosine and binds to the pIRE from the ICAM-1 promoter after IL-5 stimulation. However, STAT3beta fails to activate a pIRE containing promoter in transient transfection assays. Instead, co-expression of STAT3beta inhibits the transactivation potential of STAT3. These results suggests that STAT3beta functions as a negative regulator of transcription.

The TREX complex couples nuclear pre-mRNA processing with mRNA export and contains multiple protein components, including Uap56, Alyref, Cip29 and the multi-subunit THO complex. Here, we have identified Chtop as a novel TREX component. We show that both Chtop and Alyref activate the ATPase and RNA helicase activities of Uap56 and that Uap56 functions to recruit both Alyref and Chtop onto mRNA. As observed with the THO complex subunit Thoc5, Chtop binds to the NTF2-like domain of Nxf1, and this interaction requires arginine methylation of Chtop. Using RNAi, we show that co-knockdown of Alyref and Chtop results in a potent mRNA export block. Chtop binds to Uap56 in a mutually exclusive manner with Alyref, and Chtop binds to Nxf1 in a mutually exclusive manner with Thoc5. However, Chtop, Thoc5 and Nxf1 exist in a single complex in vivo. Together, our data indicate that TREX and Nxf1 undergo dynamic remodelling, driven by the ATPase cycle of Uap56 and post-translational modifications of Chtop.

Erythropoiesis requires tight control of expansion, maturation, and survival of erythroid progenitors. Because activation of phosphatidylinositol-3-kinase (PI3K) is required for erythropoietin/stem cell factor–induced expansion of erythroid progenitors, we examined the role of the PI3K-controlled Forkhead box, class O (FoxO) subfamily of Forkhead transcription factors. FoxO3a expression and nuclear accumulation increased during erythroid differentiation, whereas untimely induction of FoxO3a activity accelerated differentiation of erythroid progenitors to erythrocytes. We identified B cell translocation gene 1 (BTG1)/antiproliferative protein 2 as a FoxO3a target gene in erythroid progenitors. Promoter studies indicated BTG1 as a direct target of FoxO3a. Expression of BTG1 in primary mouse bone marrow cells blocked the outgrowth of erythroid colonies, which required a domain of BTG1 that binds protein arginine methyl transferase 1. During erythroid differentiation, increased arginine methylation coincided with BTG1 expression. Concordantly, inhibition of methyl transferase activity blocked erythroid maturation without affecting expansion of progenitor cells. We propose FoxO3a-controlled expression of BTG1 and subsequent regulation of protein arginine methyl transferase activity as a novel mechanism controlling erythroid expansion and differentiation.

IntroductionDendritic cells (DCs) are key initiators and regulators of the immune response. DCs were originally described by the exclusive capacity to stimulate naive T cells. 1 Later studies showed that DCs are also involved in the stimulation of regulatory responses through the propagation of regulatory T cells. 2 Two types of DCs have been defined in mice and humans based on differential surface marker expression, origin, or function in the immune response: myeloid DCs (mDCs) and plasmacytoid DCs (pDCs). 3 Differential expression of toll-like receptors (TLRs) on mDCs and pDCs defines the recognition of distinct microbial agents (ie, mDCs predominantly express TLR4 recognizing endotoxin and pDCs express TLR3, TLR7, and TLR9). 4 DCs are distributed in lymphoid (spleen, lymph nodes, thymus, bone marrow) and nonlymphoid tissues. DCs are renewed from local DC precursors or circulating monocytes, which primarily derive from bone marrow (BM) progenitors. 5-8 mDCs and pDCs originate from either common lymphoid progenitors (CLPs) or common myeloid progenitors (CMPs), showing a redundant potential of the hematopoietic system to generate DCs. 9 The potential of CLPs or CMPs to differentiate into DCs is terminated upon definitive B-cell or megakaryocyte/erythrocyte commitment, respectively. 10 Myeloid-derived DCs share precursors with macrophages (Mfs) until late stages of differentiation. 11 The mechanisms that determine DC versus Mf differentiation remain largely unclear, and transcription factors play a central role in their commitment. Reconstitution assays and genetically modified mice facilitated understanding the role of transcription factors such as Gfi1, Id2, Ikaros, Relb, Mafb, and PU.1 in DC versus Mf differentiation. 12 Transcription factors Gata1, Gata2, and Gata3 are essential in the hematopoietic system. Gata3 is necessary for the specialization of naive T cells into T-helper 2 (Th2) lymphocytes. 13 Gata2 is important for the maintenance of hematopoietic multipotent progenitor cells and remains expressed in mast cells and megakaryocytes. 14,15 Gata1 is expressed at basal levels in hematopoietic progenitors 16 and regulates the differentiation of the megakaryocyte/erythroid lineage. 17 Gata1 is also required for mast-cell and eosinophil (Eo) development. 18,19 In contrast, Gata1 is not required for Mf differentiation and, furthermore, forced expression of Gata1 in a myeloid progenitor line prevents normal Mf differentiation in vitro. 20,21 Thus far, the expression and role of Gata1 in DC differentiation has not been reported. In this study, we show that Gata1 is expressed in DCs and that Gata1 is required for the survival of DC precursors in vitro and in vivo. In addition, Gata1 is required in both mDCs and pDCs but is irrelevant for the closely related Mfs. Our findings demonstrate that Gata1 is a transcriptional regulator of DC differentiation and suggest that Gata1 is involved in the final DC and Mf separation. Materials and methods AnimalsMice bearing a modified Gata1 allele flanked with loxP sites (Gat...