Cloning and nucleotide sequence of the 5' part of v-myb cDNA

GATA-1 and c-myb crosstalk during red blood cell differentiation through GATA-1 binding sites in the c-myb promoter

GATA-1 and c-Myb transcription factors represent key regulators of red blood cell development. GATA-1 is upregulated and c-myb proto-oncogene expression is downregulated when red cell progenitors differentiate into erythrocytes. Here we have employed a culture system, that faithfully recapitulates red blood cell differentiation in vitro, to follow the kinetics of GATA-1 and c-myb expression. We show that c-myb proto-oncogene expression is high in progenitors and effectively downregulated at the time when nuclear GATA-1 accumulates and cells differentiate into erythrocytes. Additionally, we identified two GATA-1 binding sites within the c-myb promoter and demonstrate that GATA-1 protein binds to these sites in vitro. Furthermore, GATA-1 represses c-myb expression through one of the GATA-1 binding sites in transient transfection experiments and this requires FOG-1. Thus, our study provides evidence for a direct molecular link between GATA-1 activity and c-myb proto-oncogene expression during terminal red cell differentiation.

The leucine zipper region of Myb oncoprotein regulates the commitment of hematopoietic progenitors

The development of blood cells proceeds from pluripotent stem cells through multipotent progenitors into mature elements belonging to at least 8 different lineages. The lineage choice process during which stem cells and progenitors commit to a particular lineage is regulated by a coordinated action of extracellular signals and transcription factors. Molecular mechanisms controlling commitment are largely unknown. Here, the transcription factor v-Myb and its leucine zipper region (LZR) are identified as regulators of the commitment of a common myeloid progenitor and progenitors restricted to the myeloid lineage. It is demonstrated that wild-type v-Myb with the intact LZR directs development of progenitors into the macrophage lineage. Mutations in this region compromise commitment toward myeloid cells and cause v-Myb to also support the development of erythroid cells, thrombocytes, and granulocytes, similar to the c-Myb protein. In agreement with that, the wild-type v-Myb induces high expression of myeloid factors C/EBP beta, PU.1, and Egr-1 in its target cells, whereas SCL, GATA-1, and c-Myb are more abundant in cells expressing the v-Myb LZR mutant. It is proposed that Myb LZR can function as a molecular switch, affecting expression of lineage-specifying transcription factors and directing the development of hematopoietic progenitors into either myeloid or erythroid lineages.

An ex vivo model to study v-Myb-induced leukemogenicity

The v-myb(AMV) oncogene transforms myelomonocytic cells in vitro and induces acute monoblastic leukemia in chickens. We analyzed the activity of the evolutionarily conserved PEST-like domain (P1 domain) for biochemical and biological activities of v-Myb in ex vivo cultures and in vivo. Deletion of the P1 domain did not affect v-Myb transcriptional activity, intracellular stability, or subcellular localization. However, it resulted in subtle yet important changes in biological activities. Although the mutant DeltaP1 v-Myb protein blocked the terminal differentiation of the monocyte/macrophage lineage as efficiently as the wild type (wt) in ex vivo cultures, it failed to induce the acute phase of monoblastic leukemia, with its fatal consequences, in vivo. Interestingly, in DeltaP1 v-myb-infected animals large numbers of monoblasts, comparable to those induced by wt v-myb, were present in the bone marrow but very few were found in the peripheral blood. The comparison of ex vivo wt- and DeltaP v-Myb bone marrow cells revealed several important features of v-Myb transformation: (i) the proliferation of transformed monoblasts is not an apparent consequence of the differentiation block with these processes being at least in part independent; (ii) the P1 domain is required for proliferation of v-Myb-mediated transformed monoblasts; (iii) the mechanism which renders transformed cells growth factor independent does not involve activation of an autocrine growth factor loop; and (iv) deletion of the P1 domain affects self-adhesion properties of v-myb-transformed monoblasts as well as their interaction with bone marrow stromal cells. These data indicate that the DeltaP1 v-myb mutant and ex vivo bone marrow cell cultures represent a valuable tool for studies on the mechanisms of leukemia formation.

Myb-interacting protein, ATBF1, represses transcriptional activity of Myb oncoprotein

Using the yeast two-hybrid system, the transcription factor ATBF1 was identified as v-Myb- and c-Myb-binding protein. Deletion mutagenesis revealed amino acids 2484-2520 in human ATBF1 and 279-300 in v-Myb as regions required for in vitro binding of both proteins. Further experiments identified leucines Leu325 and Leu332 of the Myb leucine zipper motif as additional amino acid residues important for efficient ATBF1-Myb interaction in vitro. In co-transfection experiments, the full-length ATBF1 was found to form in vivo complexes with v-Myb and inhibit v-Myb transcriptional activity. Both ATBF1 2484-2520 and Myb 279-300 regions were required for the inhibitory effect. Finally, the chicken ATBF1 was identified, showing high degree of amino acid sequence homology with human and murine proteins. Our data reveal Myb proteins as the first ATBF1 partners detected so far and identify amino acids 279-300 in v-Myb as a novel protein-protein interaction interface through which Myb transcriptional activity can be regulated.