Nucleotide sequence of chicken myb proto-oncogene promoter region: detection of an evolutionarily conserved element

Intricate crosstalk between MYB and noncoding RNAs in cancer

MYB is often overexpressed in malignant tumors and plays a carcinogenic role in the initiation and development of cancer. Deletion of the MYB regulatory C-terminal domain may be a driving mutation leading to tumorigenesis, therefore, different tumor mechanisms produce similar MYB proteins. As MYB is a transcription factor, priority has been given to identifying the genes that it regulates. All previous attention has been focused on protein-coding genes. However, an increasing number of studies have suggested that MYB can affect the complexity of cancer progression by regulating tumor-associated noncoding RNAs (ncRNAs), such as microRNAs, long-non-coding RNAs and circular RNAs. ncRNAs can regulate the expression of numerous downstream genes at the transcription, RNA processing and translation levels, thereby having various biological functions. Additionally, ncRNAs play important roles in regulating MYB expression. This review focuses on the intricate crosstalk between oncogenic MYB and ncRNAs, which play a pivotal role in tumorigenesis, including proliferation, apoptosis, angiogenesis, metastasis, senescence and drug resistance. In addition, we discuss therapeutic strategies for crosstalk between MYB and ncRNAs to prevent the occurrence and development of cancer.

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.

c-Myb transcription is activated by protein kinase B (PKB) following interleukin 2 stimulation of Tcells and is required for PKB-mediated protection from apoptosis

During T-cell activation, c-Myb is induced upon interleukin 2 (IL-2) stimulation and is required for correct proliferation of cells. In this paper, we provide evidence that IL-2-mediated induction of the c-myb gene occurs via the phosphoinositide 3-kinase (PI3K) signaling pathway, that protein kinase B (PKB) is the principal transducer of this signal, and that activation of the c-myb promoter can be abolished by deletion of conserved E2F and NF-kappaB binding sites. We show that Myb is required to protect activated peripheral T cells from bcl-2-independent apoptosis and that overexpression of oncogenic v-Myb is antiapoptotic. Overexpression of a Myb dominant-negative transgene abrogates PKB-mediated protection from apoptosis. Taken together, these results suggest that induction of c-myb transcription is an important downstream event for PKB-mediated protection of T cells from programmed cell death.

Regulation of c-myb oncogene expression in immature and mature murine T cells

The c-myb oncogene encodes a nuclear binding protein which may play a major role in differentiation during early T cell development. However, the functionally important transcription regions in the GC promoter site have not been defined and the significance of the regulation of this promoter site in T cell differentiation has not been determined. Therefore, the promoter strength was determined by measurement of the CAT activity in cell extracts of EL-4 cells that were transfected with a CAT expression vector that contained cloned segments of the 5' myb gene. Stepwise removal of DNA sequences between -2300 bp and -346 bp upstream from the ATG initiation codon resulted in a gradual loss of 50% of CAT activity, whereas deletion of DNA sequences from -346 to -295 and -232 to -155 bp upstream from the ATG initiation codon eliminated promoter activity. On analysis of the CAT activity after transfection of various cell lines with these same constructs, it was found that the same two promoter regions were required for high CAT activity in all the cell lines, including murine cell lines which express the alpha/beta TCR and high levels of c-myb (BW5147), the alpha/beta TCR and low levels of c-myb (Yac-1), or the gamma/delta TCR (KN 12.1 and KN 2.4 T), a murine fibroblast T cell line (NIH-3T3), and a human epithelial cell line (HeLa). However, the CAT activity did not correlate with steady state levels of expression of the c-myb gene in the murine cell lines. Our data indicate that the c-myb oncogene promoter is constitutively expressed is highly dependent on a limited region of the 5' myb gene, requires two DNA elements for optimal activity, and is functional in diverse T cell lines.

Structure and biological activity of the transcriptional initiation sequences of the murine c-myb oncogene

To study the control mechanism(s) that govern the transcription of c-myb, genomic clones corresponding to the 5' region of the murine c-myb gene have been isolated and characterized structurally and functionally. Primer extension and nuclease protection analysis have revealed the presence of multiple transcriptional initiation sites, that are utilized in several hemopoietic cell lines (WEHI3B(D+). FDC-P1 and RB22.2). Some of the sites are used in all cell lines but others are unique; all are located in a region of the c-myb gene that is G-C rich, contains a number of potential Sp1 binding sites and lacks classical promoter consensus sequences. Experiments in which well characterized promoters controlling expression of a reporter gene have been replaced by fragments of c-myb DNA (including the observed cap sites) were performed in an attempt to demonstrate promoter activity in various cell types. It was shown that a region of the c-myb gene (approximately 1.0 kbp upstream from the splice donor site of the first exon) contains a weak promoter that has a low level of transcriptional activity in hemopoietic as well as in fibroblastic cells. These results support the suggestion that c-myb expression is not regulated primarily at the level of initiation of transcription.

Transcription of the chicken myb proto-oncogene starts within a CpG island

The nucleotide sequence of an 8.2-kb DNA fragment from the 5' proximal part of the chicken myb proto-oncogene spanning 1761 nucleotides upstream and 6436 nucleotides downstream from a presumed c-myb initiation codon was determined. A 3.3-kb G + C-rich region found in this sequence had also other features characterizing CpG islands, i.e. no CpG underrepresentation and lack of CpG methylation. In haematopoietic tissues c-myb mRNA synthesis starts in two major regions of the CpG island, namely 98 to 108 and 143 to 145 nucleotides upstream from the c-myb initiation codon. These two regions are in or close to the 124-bp evolutionarily conserved element located in the middle part of the CpG island. No alternative splicing of the 5' end of c-myb mRNA suggested earlier (1,2) was observed. The c-myb promoter contains neither TATA nor CAAT box-like structures at the usual positions. Instead, numerous potential Sp1 factor binding sites were found both upstream and downstream from the transcription initiation sites. Moreover, consensus v-myb protein DNA-binding sites were revealed in the promoter region and in sequences downstream from it.