Myb proteins: angels and demons in normal and transformed cells

c-Myb Binding Sites in Haematopoietic Chromatin Landscapes

Strict control of tissue-specific gene expression plays a pivotal role during lineage commitment. The transcription factor c-Myb has an essential role in adult haematopoiesis and functions as an oncogene when rearranged in human cancers. Here we have exploited digital genomic footprinting analysis to obtain a global picture of c-Myb occupancy in the genome of six different haematopoietic cell-types. We have biologically validated several c-Myb footprints using c-Myb knockdown data, reporter assays and DamID analysis. We show that our predicted conserved c-Myb footprints are highly dependent on the haematopoietic cell type, but that there is a group of gene targets common to all cell-types analysed. Furthermore, we find that c-Myb footprints co-localise with active histone mark H3K4me3 and are significantly enriched at exons. We analysed co-localisation of c-Myb footprints with 104 chromatin regulatory factors in K562 cells, and identified nine proteins that are enriched together with c-Myb footprints on genes positively regulated by c-Myb and one protein enriched on negatively regulated genes. Our data suggest that c-Myb is a transcription factor with multifaceted target regulation depending on cell type.

Situational awareness: regulation of the myb transcription factor in differentiation, the cell cycle and oncogenesis

This review summarizes the mechanisms that control the activity of the c-Myb transcription factor in normal cells and tumors, and discusses how c-Myb plays a role in the regulation of the cell cycle. Oncogenic versions of c-Myb contribute to the development of leukemias and solid tumors such as adenoid cystic carcinoma, breast cancer and colon cancer. The activity and specificity of the c-Myb protein seems to be controlled through changes in protein-protein interactions, so understanding how it is regulated could lead to the development of novel therapeutic strategies.

Heterologous expression of AtMYB12 in kale (Brassica oleracea var. acephala) leads to high flavonol accumulation

Overexpression of Arabidopsis AtMYB12 transcription factor greatly increases the total phenolic and flavonol content in transgenic kale leaves. Flavonoids are a diverse group of plant secondary metabolites exhibiting a number of health-promoting effects. There has been a growing interest to develop biotechnological methods for the enhanced production of flavonoids in crop plants. AtMYB12 is an Arabidopsis transcription factor which specifically activates flavonol synthesis and its overexpression has led to increased flavonol accumulation in several transgenic plants. In the present study, AtMYB12 was overexpressed in a commercial cultivar of kale and the transgenic plants were tested both in in vitro and in semi-field conditions in cages under natural light. Using this method, a severalfold increase in both total phenolics content and flavonol accumulation was achieved. This study provides a reliable and efficient transformation protocol for kale and suggests the potential of this flavonol-enriched vegetable for the production of kaempferol.

Mybs in mouse hair follicle development

The Myb transcription factors are involved in essential cellular processes, such as cell proliferation, differentiation and cell death. Biological functions carried out by specific Myb proteins are distinct. Hair follicles are ectodermal-derived organs with cycling character of the growth resulting from the presence of somatic stem cells. In this study, we followed the expression of the Myb proteins in developing hair follicles and in the hair follicle stem cell niche by immunofluorescence staining. During hair follicle development, B-Myb was present in a few cells located in the area of cell division; c-Myb was abundant postanally in dividing cells but also in keratinizing zone. In addition, c-Myb was also detected in cells under the hair follicle bulge. These findings indicate possible involvement of c-Myb in regulation of activated stem cells leaving the niche.

Direct repression of MYB by ZEB1 suppresses proliferation and epithelial gene expression during epithelial-to-mesenchymal transition of breast cancer cells

INTRODUCTION

Epithelial-to-mesenchymal transition (EMT) promotes cell migration and is important in metastasis. Cellular proliferation is often downregulated during EMT, and the reverse transition (MET) in metastases appears to be required for restoration of proliferation in secondary tumors. We studied the interplay between EMT and proliferation control by MYB in breast cancer cells.

METHODS

MYB, ZEB1, and CDH1 expression levels were manipulated by lentiviral small-hairpin RNA (shRNA)-mediated knockdown/overexpression, and verified with Western blotting, immunocytochemistry, and qRT-PCR. Proliferation was assessed with bromodeoxyuridine pulse labeling and flow cytometry, and sulforhodamine B assays. EMT was induced with epidermal growth factor for 9 days or by exposure to hypoxia (1% oxygen) for up to 5 days, and assessed with qRT-PCR, cell morphology, and colony morphology. Protein expression in human breast cancers was assessed with immunohistochemistry. ZEB1-MYB promoter binding and repression were determined with Chromatin Immunoprecipitation Assay and a luciferase reporter assay, respectively. Student paired t tests, Mann-Whitney, and repeated measures two-way ANOVA tests determined statistical significance (P < 0.05).

RESULTS

Parental PMC42-ET cells displayed higher expression of ZEB1 and lower expression of MYB than did the PMC42-LA epithelial variant. Knockdown of ZEB1 in PMC42-ET and MDA-MB-231 cells caused increased expression of MYB and a transition to a more epithelial phenotype, which in PMC42-ET cells was coupled with increased proliferation. Indeed, we observed an inverse relation between MYB and ZEB1 expression in two in vitro EMT cell models, in matched human breast tumors and lymph node metastases, and in human breast cancer cell lines. Knockdown of MYB in PMC42-LA cells (MYBsh-LA) led to morphologic changes and protein expression consistent with an EMT. ZEB1 expression was raised in MYBsh-LA cells and significantly repressed in MYB-overexpressing MDA-MB-231 cells, which also showed reduced random migration and a shift from mesenchymal to epithelial colony morphology in two dimensional monolayer cultures. Finally, we detected binding of ZEB1 to MYB promoter in PMC42-ET cells, and ZEB1 overexpression repressed MYB promoter activity.

CONCLUSIONS

This work identifies ZEB1 as a transcriptional repressor of MYB and suggests a reciprocal MYB-ZEB1 repressive relation, providing a mechanism through which proliferation and the epithelial phenotype may be coordinately modulated in breast cancer cells.

Stress-induced phosphorylation of Thr486 in c-Myb by p38 mitogen-activated protein kinases attenuates conjugation of SUMO-2/3

c-Myb plays an essential role in regulation of properly balanced hematopoiesis through transcriptional regulation of genes directly controlling cellular processes such as proliferation, differentiation, and apoptosis. The transcriptional activity and protein levels of c-Myb are strictly controlled through post-translational modifications such as phosphorylation, acetylation, ubiquitination, and SUMOylation. Conjugation of small ubiquitin-like modifier (SUMO) proteins has been shown to suppress the transcriptional activity of c-Myb. SUMO-1 modifies c-Myb under physiological conditions, whereas SUMO-2/3 conjugation was reported in cells under stress. Because stress also activates several cellular protein kinases, we investigated whether phosphorylation of c-Myb changes in stressed cells and whether a mutual interplay exists between phosphorylation and SUMOylation of c-Myb. Here we show that several types of environmental stress induce a rapid change in c-Myb phosphorylation. Interestingly, the phosphorylation of Thr(486), located in close proximity to SUMOylation site Lys(499) of c-Myb, is detected preferentially in nonSUMOylated protein and has a negative effect on stress-induced SUMOylation of c-Myb. Stress-activated p38 MAPKs phosphorylate Thr(486) in c-Myb, attenuate its SUMOylation, and increase its proteolytic turnover. Stressed cells expressing a phosphorylation-deficient T486A mutant demonstrate decreased expression of c-Myb target genes Bcl-2 and Bcl-xL and accelerated apoptosis because of increased SUMOylation of the mutant protein. These results suggest that phosphorylation-dependent modulation of c-Myb SUMOylation may be important for proper response of cells to stress. In summary, we have identified a novel regulatory interplay between phosphorylation and SUMOylation of c-Myb that regulates its activity in stressed cells.

Characterization of the transcriptional activity of the basic helix-loop-helix (bHLH) transcription factor Atoh8

The atonal-related Neurogenin/NeuroD family of basic helix-loop-helix (bHLH) transcription factors comprises potent inducers of neuronal and endocrine differentiation programs in the nervous and digestive system. Atonal homolog 8 (Atoh8) displays high similarity in the bHLH domain with NeuroD proteins. Yet, available evidences indicate that Atoh8 has distinctive features including a ubiquitous expression pattern in embryonic tissues and the ability to inhibit differentiation. To gain insights into Atoh8 function, we aimed at identifying Atoh8 targets and investigated the effects of Atoh8 on global gene expression patterns in pancreatic mPAC cells, a model of bHLH-dependent endocrine differentiation. Our data reveal that Atoh8 is a weak transcriptional activator and does not exhibit proendocrine activity. Conversely, it blocks the induction of a reduced group of gene targets of the atonal-related proendocrine factor Neurogenin3. We show that Atoh8 lacks a transactivation domain and possesses intrinsic repressor activity that depends on a conserved Proline-rich domain. Atoh8 binds the ubiquitous E protein E47 and its ability to repress transcription may partly result from its ability to inhibit E47/E47 and Neurogenin3/E47 dimer activities. These results reveal distinctive transcriptional properties of Atoh8 within the atonal-related bHLH family that may be associated with the acquisition of new biological functions.

The DREAM complex: master coordinator of cell cycle-dependent gene expression

The dimerization partner, RB-like, E2F and multi-vulval class B (DREAM) complex provides a previously unsuspected unifying role in the cell cycle by directly linking p130, p107, E2F, BMYB and forkhead box protein M1. DREAM mediates gene repression during the G0 phase and coordinates periodic gene expression with peaks during the G1/S and G2/M phases. Perturbations in DREAM complex regulation shift the balance from quiescence towards proliferation and contribute to the increased mitotic gene expression levels that are frequently observed in cancers with a poor prognosis.

The DREAM complex: master coordinator of cell cycle-dependent gene expression

The dimerization partner, RB-like, E2F and multi-vulval class B (DREAM) complex provides a previously unsuspected unifying role in the cell cycle by directly linking p130, p107, E2F, BMYB and forkhead box protein M1. DREAM mediates gene repression during the G0 phase and coordinates periodic gene expression with peaks during the G1/S and G2/M phases. Perturbations in DREAM complex regulation shift the balance from quiescence towards proliferation and contribute to the increased mitotic gene expression levels that are frequently observed in cancers with a poor prognosis.

Oxidative Stress and Hepatic Stellate Cells: A PARADOXICAL RELATIONSHIP

In physiology, reactive oxygen species (ROS) are produced by most cells for normal function and as a defense mechanism against foreign particles, microbes and viruses. Hepatic macrophages (Kupffer cells), sinusoidal endothelial cells, hepatocytes and hepatic stellate cells (HSCs) are all capable of generating ROS in physiology and pathology. ROS are also produced by infiltrating inflammatory cells during acute and chronic liver injury. Increased levels of ROS have been implicated in apoptotic/necrotic death of hepatocytes, and liver failure. In contrast to causing injury to hepatocytes, ROS and lipid peroxidation products induce transdifferentiation of the quiescent HSCs into an activated highly proliferative myofibroblast-like phenotype. ROS and lipid peroxidation products also stimulate the synthesis of extracellular matrix (ECM) by activated HSCs. Deposition of excessive amounts of ECM is the primary mechanism of fibrosis and cirrhosis of the liver, and interactions between ROS and HSCs appear to play a major role in this pathology. Although these findings suggest that HSCs are resistant to the injurious actions of ROS, there is compelling evidence demonstrating ROS-induced death of activated HSCs. Detailed mechanistic understanding of such paradoxical interactions between ROS and HSCs will be critical for developing therapies for chronic fibrotic liver disease.

Single molecule analysis of c-myb alternative splicing reveals novel classifiers for precursor B-ALL

The c-Myb transcription factor, a key regulator of proliferation and differentiation in hematopoietic and other cell types, has an N-terminal DNA binding domain and a large C-terminal domain responsible for transcriptional activation, negative regulation and determining target gene specificity. Overexpression and rearrangement of the c-myb gene (MYB) has been reported in some patients with leukemias and other types of cancers, implicating activated alleles of c-myb in the development of human tumors. Alternative RNA splicing can produce variants of c-myb with qualitatively distinct transcriptional activities that may be involved in transformation and leukemogenesis. Here, by performing a detailed, single molecule assay we found that c-myb alternative RNA splicing was elevated and much more complex in leukemia samples than in cell lines or CD34+ hematopoietic progenitor cells from normal donors. The results revealed that leukemia samples express more than 60 different c-myb splice variants, most of which have multiple alternative splicing events and were not detectable by conventional microarray or PCR approaches. For example, the single molecule assay detected 21 and 22 splice variants containing the 9B and 9S exons, respectively, most of which encoded unexpected variant forms of c-Myb protein. Furthermore, the detailed analysis identified some splice variants whose expression correlated with poor survival in a small cohort of precursor B-ALL samples. Our findings indicate that single molecule assays can reveal complexities in c-myb alternative splicing that have potential as novel biomarkers and could help explain the role of c-Myb variants in the development of human leukemia.