Myb: an old oncoprotein with new roles

Gene Expression Aberrations in Alcohol-Associated Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the most prevalent primary liver cancer, ranking as the sixth most common cancer worldwide and the fourth leading cause of cancer-related deaths. Most HCC cases originate from cirrhotic livers, typically due to chronic liver diseases, such as hepatitis B (HBV) and hepatitis C (HCV) infections, and alcoholism. HCC cells often harbor numerous somatic mutations that are implicated in HCC development, but epigenetic factors, such as miRNA interference, can also affect HCC initiation and progress. miRNA-221 has been explored as a factor affecting HCC development in HCC of viral etiology, but little is known about its effects on gene expression in alcohol-associated HCC. This study aimed to explore potentially similar gene expression aberrations underlying viral and alcohol-induced HCC. We analyzed available transcriptome data from non-tumor hepatocytes and viral-induced HCC tissues. The most notable differences in gene expression associated with miRNA-221 between non-tumor hepatocytes and viral-induced HCC involved NTF-3 and MYBL1 genes. To assess these data in alcohol-induced HCC, we examined 111 tissue samples: tumor tissue and cirrhotic tissue samples from 37 HCC patients and 37 samples from non-tumor liver tissue using RT-Q PCR. We found no significant difference in NTF-3 expression, but MYBL1 expression was significantly lower in HCC tissue compared to non-tumor hepatocytes and cirrhotic tissue. Our findings highlight the importance of the MYBL1 gene in HCC development and emphasize the need for diverse approaches in evaluating tumor mechanisms.

The R2R3 MYB gene TaMYB305 positively regulates anther and pollen development in thermo-sensitive male-sterility wheat with Aegilops kotschyi cytoplasm

MAIN CONCLUSION

The loss of TaMYB305 function down-regulated the expression of jasmonic acid synthesis pathway genes, which may disturb the jasmonic acid synthesis, resulting in abnormal pollen development and reduced fertility. The MYB family, as one of the largest transcription factor families found in plants, regulates plant development, especially the development of anthers. Therefore, it is important to identify potential MYB transcription factors associated with pollen development and to study its role in pollen development. Here, the transcripts of an R2R3 MYB gene TaMYB305 from KTM3315A, a thermo-sensitive cytoplasmic male-sterility line with Aegilops kotschyi cytoplasm (K-TCMS) wheat, was isolated. Quantitative real-time PCR (qRT-PCR) and promoter activity analysis revealed that TaMYB305 was primarily expressed in anthers. The TaMYB305 protein was localized in the nucleus, as determined by subcellular localization analysis. Our data demonstrated that silencing of TaMYB305 was related to abnormal development of stamen, including anther indehiscence and pollen abortion in KAM3315A plants. In addition, TaMYB305-silenced plants exhibited alterations in the transcriptional levels of genes involved in the synthesis of jasmonic acid (JA), indicating that TaMYB305 may regulate the expression of genes related to JA synthesis and play an important role during anther and pollen development of KTM3315A. These results provide novel insight into the function and molecular mechanism of R2R3-MYB genes in pollen development.

Transcription factors as key molecular target to strengthen the drought stress tolerance in plants

Amid apprehension of global climate change, crop plants are inevitably confronted with a myriad of abiotic stress factors during their growth that inflicts a serious threat to their development and overall productivity. These abiotic stresses comprise extreme temperature, pH, high saline soil, and drought stress. Among different abiotic stresses, drought is considered the most calamitous stressor with its serious impact on the crops' yield stability. The development of climate-resilient crops that withstands reduced water availability is a major focus of the scientific fraternity to ensure the food security of the sharply increasing population. Numerous studies aim to recognize the key regulators of molecular and biochemical processes associated with drought stress tolerance response. A few potential candidates are now considered as promising targets for crop improvement. Transcription factors act as a key regulatory switch controlling the gene expression of diverse biological processes and, eventually, the metabolic processes. Understanding the role and regulation of the transcription factors will facilitate the crop improvement strategies intending to develop and deliver agronomically-superior crops. Therefore, in this review, we have emphasized the molecular avenues of the transcription factors that can be exploited to engineer drought tolerance potential in crop plants. We have discussed the molecular role of several transcription factors, such as basic leucine zipper (bZIP), dehydration responsive element binding (DREB), DNA binding with one finger (DOF), heat shock factor (HSF), MYB, NAC, TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP), and WRKY. We have also highlighted candidate transcription factors that can be used for the development of drought-tolerant crops.

AcoMYB4, an Ananas comosus L. MYB Transcription Factor, Functions in Osmotic Stress through Negative Regulation of ABA Signaling

Drought and salt stress are the main environmental cues affecting the survival, development, distribution, and yield of crops worldwide. MYB transcription factors play a crucial role in plants’ biological processes, but the function of pineapple MYB genes is still obscure. In this study, one of the pineapple MYB transcription factors, AcoMYB4, was isolated and characterized. The results showed that AcoMYB4 is localized in the cell nucleus, and its expression is induced by low temperature, drought, salt stress, and hormonal stimulation, especially by abscisic acid (ABA). Overexpression of AcoMYB4 in rice and Arabidopsis enhanced plant sensitivity to osmotic stress; it led to an increase in the number stomata on leaf surfaces and lower germination rate under salt and drought stress. Furthermore, in AcoMYB4 OE lines, the membrane oxidation index, free proline, and soluble sugar contents were decreased. In contrast, electrolyte leakage and malondialdehyde (MDA) content increased significantly due to membrane injury, indicating higher sensitivity to drought and salinity stresses. Besides the above, both the expression level and activities of several antioxidant enzymes were decreased, indicating lower antioxidant activity in AcoMYB4 transgenic plants. Moreover, under osmotic stress, overexpression of AcoMYB4 inhibited ABA biosynthesis through a decrease in the transcription of genes responsible for ABA synthesis (ABA1 and ABA2) and ABA signal transduction factor ABI5. These results suggest that AcoMYB4 negatively regulates osmotic stress by attenuating cellular ABA biosynthesis and signal transduction pathways.

MYB43 in Oilseed Rape (Brassica napus) Positively Regulates Vascular Lignification, Plant Morphology and Yield Potential but Negatively Affects Resistance to Sclerotinia sclerotiorum

Arabidopsis thaliana MYB43 (AtMYB43) is suggested to be involved in cell wall lignification. PtrMYB152, the Populus orthologue of AtMYB43, is a transcriptional activator of lignin biosynthesis and vessel wall deposition. In this research, MYB43 genes from Brassica napus (rapeseed) and its parental species B. rapa and B. oleracea were molecularly characterized, which were dominantly expressed in stem and other vascular organs and showed responsiveness to Sclerotinia sclerotiorum infection. The BnMYB43 family was silenced by RNAi, and the transgenic rapeseed lines showed retardation in growth and development with smaller organs, reduced lodging resistance, fewer silique number and lower yield potential. The thickness of the xylem layer decreased by 28%; the numbers of sclerenchymatous cells, vessels, interfascicular fibers, sieve tubes and pith cells in the whole cross section of the stem decreased by 28%, 59%, 48%, 34% and 21% in these lines, respectively. The contents of cellulose and lignin decreased by 17.49% and 16.21% respectively, while the pectin content increased by 71.92% in stems of RNAi lines. When inoculated with S. sclerotiorum, the lesion length was drastically decreased by 52.10% in the stems of transgenic plants compared with WT, implying great increase in disease resistance. Correspondingly, changes in the gene expression patterns of lignin biosynthesis, cellulose biosynthesis, pectin biosynthesis, cell cycle, SA- and JA-signals, and defensive pathways were in accordance with above phenotypic modifications. These results show that BnMYB43, being a growth-defense trade-off participant, positively regulates vascular lignification, plant morphology and yield potential, but negatively affects resistance to S. sclerotiorum. Moreover, this lignification activator influences cell biogenesis of both lignified and non-lignified tissues of the whole vascular organ.

Ectopic expression of a R2R3 MYB transcription factor of dove tree (Davidia involucrata) aggravates seed abortion in Arabidopsis thaliana

Serious seed abortion of dove tree (Davidia involucrate Baill.) is one of the critical factors leading to the low fecundity of this species. Seed abortion is a complicated process and various factors have been verified to synergistically determine the fate of seeds. To reveal the mechanism of seed abortion in D. involucrata, we performed transcriptome analysis in normal and abortive seeds of D. involucrata. According to the transcriptome data, we noticed that most of the genes encoding a MYB transcription factor were predominantly expressed in abortive seeds. Among these, a gene named DiMYB1 was selected and its function was validated in this study. Overexpression of DiMYB1 resulted in obviously reduced viability of transgenic seeds and seedlings, and caused a significantly higher seed abortion rate. The vegetative growth of transgenic plants was hindered, resulting in an earlier flowering time. In addition, colour changes occurred in transgenic plants. Some transgenic sprouts, stems and pods appeared purple instead of green in colour. Our finding demonstrated that DiMYB1 participates in multiple plant developmental processes, especially in seed development in Arabidopsis thaliana (L.) Heynh., which indicated the similar role of this gene in D. involucrata.

ZmMYB31, a R2R3-MYB transcription factor in maize, positively regulates the expression of CBF genes and enhances resistance to chilling and oxidative stress

Maize (Zea mays L.) is an important model plant with an important role in agriculture and national economies all over the world. The optimum growth temperature of maize is between 25 and 28 °C. At temperatures below 12 °C, maize is vulnerable to damage by chilling stress. MYB transcription factors play important roles in plants' response to low temperature stress. Maize ZmMYB31 encodes a R2R3-MYB transcription factor, ZmMYB31, which localized in the nucleus. ZmMYB31 expression was induced by chilling stress and the highest expression level was detected with the 24 h chilling treatment. ZmMYB31 expression also increased in overexpressing Arabidopsis lines. The minimal fluorescence (Fo) with all photosystem II reaction centers open increased in wild type (WT) and transgenic plants under chilling stress, with the highest increase in WT. The maximal photochemical efficiency of photosystem II (Fv/Fm) decreased more in WT than in transgenic plants during chilling stress. Furthermore, the ZmMYB31-overexpressing lines showed higher superoxide dismutase and ascorbate peroxidase activity and lower reactive oxygen species (ROS) content than the WT. The expression of genes related to chilling stress was higher in transgenic plants than in WT. These results suggest that ZmMYB31 plays a positive regulatory role in chilling and peroxide stress by regulating the expression of chilling stress-related genes to reduce ion extravasation, ROS content, and low-temperature photoinhibition, thereby improving low temperature resistance.

Zebrafish blastomere screen identifies retinoic acid suppression of MYB in adenoid cystic carcinoma

Pluripotent cells have been used to probe developmental pathways that are involved in genetic diseases and oncogenic events. To find new therapies that would target MYB-driven tumors, we developed a pluripotent zebrafish blastomere culture system. We performed a chemical genetic screen and identified retinoic acid agonists as suppressors of c-myb expression. Retinoic acid treatment also decreased c-myb gene expression in human leukemia cells. Translocations that drive overexpression of the oncogenic transcription factor MYB are molecular hallmarks of adenoid cystic carcinoma (ACC), a malignant salivary gland tumor with no effective therapy. Retinoic acid agonists inhibited tumor growth in vivo in ACC patient-derived xenograft models and decreased MYB binding at translocated enhancers, thereby potentially diminishing the MYB positive feedback loop driving ACC. Our findings establish the zebrafish pluripotent cell culture system as a method to identify modulators of tumor formation, particularly establishing retinoic acid as a potential new effective therapy for ACC.

Conformation Dynamics of the Intrinsically Disordered Protein c-Myb with the ff99IDPs Force Field

The intrinsically disordered protein c-Myb plays a critical role in cellular proliferation and differentiation. Loss of c-myb function results in embryonic lethality due to failure of fetal hepatic hematopoiesis. The conformation dynamics of the intrinsically disordered c-Myb are still unknown. Here, molecular dynamics (MD) simulations with the intrinsically disordered protein force field ff99IDPs were used to study the conformation dynamics. In comparison with ff99SBildn, ff99IDPs can reproduce more diverse disordered conformers of c-Myb. The predicted secondary chemical shift under ff99IDPs is more close to that of experiment data than that under ff99SBildn. Therefore, ff99IDPs can sample native molten globule, native pre-molten globule and native coil conformers for c-Myb. These results are consistent with those of other intrinsically disordered proteins. Kinetic analysis of MD simulations shows that c-Myb folds via a two-state process and indicates that c-Myb folds in the order of tertiary folding and helical folding. The folding nucleus of KEL plays an essential role in stabilizing the folding state with dynamic correlation networks. The influences of solvent models for TIP3P, TIP4P-EW and TIP5P were also investigated and it was found that TIP3P and ff99IDPs are the best combination to research the conformer sampling of c-Myb. These results reveal the conformation dynamics of c-Myb and confirm that the ff99IDPs force field can be used to research the relationship between structure and function of other intrinsically disordered proteins.

New Insights on COX-2 in Chronic Inflammation Driving Breast Cancer Growth and Metastasis

The medicinal use of aspirin stretches back to ancient times, before it was manufactured in its pure form in the late 19th century. Its accepted mechanistic target, cyclooxygenase (COX), was discovered in the 1970s and since this landmark discovery, the therapeutic application of aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) has increased dramatically. The most significant benefits of NSAIDs are in conditions involving chronic inflammation (CI). Given the recognized role of CI in cancer development, the use of long-term NSAID treatment in the prevention of cancer is an enticing possibility. COX-2 is a key driver of CI, and here we review COX-2 expression as a predictor of survival in various cancer types, including breast. Obesity and post-partum involution are natural inflammatory states that are associated with increased breast cancer risk. We outline the COX-2 mediated mechanisms contributing to the growth of cancers. We dissect the cellular mechanism of epithelial-mesenchymal transition (EMT) and how COX-2 may induce this to facilitate tumor progression. Finally we examine the potential regulation of COX-2 by c-Myb, and the possible interplay between c-Myb/COX-2 in proliferation, and hypoxia inducible factor-1 alpha (HIF1α)/COX-2 in invasive pathways in breast cancer.

MoMyb1 is required for asexual development and tissue-specific infection in the rice blast fungus Magnaporthe oryzae

Background

The Myb super-family of proteins contain a group of functionally diverse transcriptional activators found in plant, animal and fungus. Myb proteins are involved in cell proliferation, differentiation and apoptosis, and have crucial roles in telomeres. The purpose of this study was to characterize the biological function of Myb1 protein in the rice blast fungus Magnaporthe oryzae.

Results

We identified the Saccharomyces cerevisiae BAS1 homolog MYB1 in M. oryzae, named MoMyb1. MoMyb1 encodes a protein of 322 amino acids and has two SANT domains and is well conserved in various organisms. Targeted gene deletion of MoMYB1 resulted in a significant reduction in vegetative growth and showed defects in conidiation and conidiophore development. Quantitative RT-PCR analysis revealed that the transcription levels of several conidiophore-related genes were apparently decreased in the ΔMomyb1 mutant. Inoculation with mycelia mats displayed that the virulence of the ΔMomyb1 mutant was not changed on rice leaves but was non-pathogenic on rice roots in comparison to the wild type Guy11. In addition, ∆Momyb1 mutants showed increased resistance to osmotic stresses but more sensitive to cell wall stressor calcofluor white (CFW). Further analysis revealed that MoMyb1 has an important role in the cell wall biosynthesis pathway.

Conclusion

This study provides the evidence that MoMyb1 is a key regulator involved in conidiogenesis, stress response, cell wall integrity and pathogenesis on rice roots in the filamentous phytopathogen M. oryzae.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0375-y) contains supplementary material, which is available to authorized users.

Development of fruit color in Solanaceae: a story of two biosynthetic pathways

This review highlights the major differences between the regulation of two important pathways namely anthocyanin and carotenoid pathways, responsible for fruit color generation in Solanaceae mediated by transcription factors (TFs). The anthocyanin pathway is regulated by a common set of TFs (MYB, MYC and WD40) belonging to specific families of DNA-binding proteins. Their regulation is aimed at controlling the type and amount of pigments produced and the physiological conditions (like pH) at which they are finally stored. In the carotenoid pathway, the color diversity depends on the quantity of pigment produced and the point where the pathway is arrested. TFs in the latter case are accordingly found to influence the sequestration and degradation of these pigments, which determines their final concentration in the tissue. TFs (phytochrome interacting factors, MADS-BOX, HB-ZIP and B-ZIP) also regulate important rate-determining steps, which decide the direction in which the pathway proceeds and the point at which it is terminated. In the absence of a clear pattern of TF-mediated regulation, it is suggested that the carotenoid pathway is more significantly influenced by other regulatory methods which need to be explored. It is expected that common factors affecting these pathways are the ones acting much before the initiation of the biosynthesis of respective pigments.

GSK3β inactivation induces apoptosis of leukemia cells by repressing the function of c-Myb

The level of c-Myb is a determining factor in the response of leukemia cells to GSK3β kinase inhibiton, which is of particular interest for the therapy of leukemia and cancers that have c-Myb amplifications.

Glycogen synthase kinase 3β (GSK3β) regulates diverse physiological processes, including metabolism, development, oncogenesis, and neuroprotection. GSK3β kinase activity has been reported to be critical for various types of cancer cells, but the mechanism has remained elusive. In this study we examine the mechanism by which GSK3β regulates the survival of leukemia cells. We demonstrate that upon GSK3β kinase inhibition different types of leukemia cells show severe proliferation defects as a result of apoptosis. The transcription factor c-Myb is found to be the main target of GSK3β inhibition in cell survival. GSK3β inactivation reduces the expression of c-Myb by promoting its ubiquitination-mediated degradation, thereby inhibiting the expression of c-Myb–dependent antiapoptotic genes Bcl2 and survivin. Coimmunoprecipitation, reporter assays, chromatin immunoprecipitation, and knockdown studies show that c-Myb needs to interact and cooperate with transcription factor LEF-1 in the activation of Bcl2 and survivin and that both transcription factors are required for cell survival. These data reveal an as-yet-unknown mechanism by which GSK3β controls cell survival.

MYB suppresses differentiation and apoptosis of human breast cancer cells

INTRODUCTION

MYB is highly expressed in estrogen receptor positive (ER + ve) breast tumours and tumour cell lines. We recently demonstrated that MYB is essential for the proliferation of ER + ve breast cancer cells, and have now investigated its role in mammary epithelial differentiation.

METHODS

MCF-7 breast cancer cells were treated with sodium butyrate, vitamin E succinate or 12-O-tetradecanoylphorbol-13-acetate to induce differentiation as measured by Nile Red staining of lipid droplets and β-casein expression. The non-tumorigenic murine mammary epithelial cell (MEC) line, HC11, was induced to differentiate with lactogenic hormones. MYB levels were manipulated by inducible lentiviral shRNA-mediated knockdown and retroviral overexpression.

RESULTS

We found that MYB expression decreases following chemically-induced differentiation of the human breast cancer cell line MCF-7, and hormonally-induced differentiation of a non-tumorigenic murine mammary epithelial cell (MEC) line, HC11. We also found that shRNA-mediated MYB knockdown initiated differentiation of breast cancer cells, and greatly sensitised them to the differentiative and pro-apoptotic effects of differentiation-inducing agents (DIAs). Sensitisation to the pro-apoptotic effects DIAs is mediated by decreased expression of BCL2, which we show here is a direct MYB target in breast cancer cells. Conversely, enforced expression of MYB resulted in the cells remaining in an undifferentiated state, with concomitant suppression of apoptosis, in the presence of DIAs.

CONCLUSIONS

Taken together, these data imply that MYB function is critical in regulating the balance between proliferation, differentiation, and apoptosis in MECs. Moreover, our findings suggest MYB may be a viable therapeutic target in breast cancer and suggest specific approaches for exploiting this possibility.

c-myb activates CXCL12 transcription in T47D and MCF7 breast cancer cells

Chemokine C-X-C motif ligand 12 (CXCL12) is a potent chemotactic and angiogenic factor that has been proposed to play a role in organ-specific metastasis and angiogenic activity in several malignancies. In this study, we found that the overexpression of c-myb could elevate CXCL12 mRNA level and CXCL12 promoter activity in human T47D and MCF-7 breast cancer cells. Chromatin immunoprecipitation assay demonstrated that c-myb could bind to the CXCL12 promoter in the cells transfected with cmyb expression vector. c-myb siRNA attenuated CXCL12 promoter activity and the binding of c-myb to the CXCL12 promoter in T47D and MCF-7 cells. These results indicated that c-myb could activate CXCL12 promoter transcription.

Pax9 mediated cell survival in oral squamous carcinoma cell enhanced by c-myb

Paired box gene 9 (Pax9) and c-myb are transcription factors that regulate the expression of the genes involved in mediating cell proliferation, resistance to apoptosis, and migration. However, the function of Pax9 in oral squamous cell carcinoma (OSCC) is virtually unknown. This study examined the anti-apoptotic roles of Pax9 and c-myb, and clarified interaction between the two genes in KB cells. Inhibition of Pax9 caused the induction of apoptosis with enhanced cleavage of caspase-3 and PARP, accelerated Bax, and reduced Bcl-2 expression. Transducing c-myb cells with adenovirus c-myb (Ad/c-myb) were induced cell growth and inhibited apoptosis, but dominant-negative myb cells (Ad/DN-myb) were not affected. Pax9 was upregulated in the Ad/c-myb cells with simultaneous decrease in the Ad/DN-myb infection. However, c-myb remained unaffected in the Pax9 small interfering RNA (siRNA) transfected cells. Moreover, the Pax9 siRNA transfected cells and Ad/DN-myb infected cells were able to arrest the cell cycle at the G(0) phase. This suggests that Pax9 and c-myb expression in KB cells is essential for cell growth, and survival is enhanced by c-myb. Disrupting the function of c-myb and Pax9 could be a potential target for cancer treatment.

MYB function in normal and cancer cells

The transcription factor MYB has a key role as a regulator of stem and progenitor cells in the bone marrow, colonic crypts and a neurogenic region of the adult brain. It is in these compartments that a deficit in MYB activity leads to severe or lethal phenotypes. As was predicted from its leukaemogenicity in several animal species, MYB has now been identified as an oncogene that is involved in some human leukaemias. Moreover, recent evidence has strengthened the case that MYB is activated in colon and breast cancer: a block to MYB expression is overcome by mutation of the regulatory machinery in the former disease and by oestrogen receptor-alpha (ERalpha) in the latter.

c-Myb is required for neural progenitor cell proliferation and maintenance of the neural stem cell niche in adult brain

Ongoing production of neurons in adult brain is restricted to specialized neurogenic niches. Deregulated expression of genes controlling homeostasis of neural progenitor cell division and/or their microenvironment underpins a spectrum of brain pathologies. Using conditional gene deletion, we show that the proto-oncogene c-myb regulates neural progenitor cell proliferation and maintains ependymal cell integrity in mice. These two cellular compartments constitute the neurogenic niche in the adult brain. Brains devoid of c-Myb showed enlarged ventricular spaces, ependymal cell abnormalities, and reduced neurogenesis. Neural progenitor cells lacking c-Myb showed a reduced intrinsic proliferative capacity and reduction of Sox-2 and Pax-6 expression. These data point to an important role for c-Myb in the neurogenic niche of the adult brain.

Adenovirus-mediated expression of dominant negative c-myb induces apoptosis in head and neck cancer cells and inhibits tumor growth in animal model

The recent demonstration of aberrant expression of the c-myb proto-oncogene in various cancers suggests that c-myb plays an important role in the development of cancer. On this basis, it has been proposed that ablation of c-myb function might be an effective approach for therapy of c-myb dependent malignancies. We previously used a dominant negative c-myb (DN-myb) construct to induce apoptosis in K562 cells. In this study, DN-myb was expressed in an adenovirus-mediated gene delivery system and introduced into head and neck squamous cell carcinoma cells (HNSCC) in vitro and in vivo to examine its tumor suppressive function and its potential in HNSCC gene therapy. Over expression of DN-myb in HNSCC cells inhibited in vitro cell proliferation, expression of growth factors such as IGF-I, -II, IGF-1R, and VEGF, inhibited Akt/PKB pathway activation, and enhanced induction of apoptosis. Similarly, in vivo administration of DN-myb retarded tumor-growth. Our results support a role for DN-myb in inducing apoptosis and tumor suppression, and, furthermore, suggest that DN-myb gene therapy might provide a powerful tool for treatment of c-myb dependent malignancies such as HNSCC.

Mutations in the MYB intron I regulatory sequence increase transcription in colon cancers

Although MYB overexpression in colorectal cancer (CRC) is known to be a prognostic indicator for poor survival, the basis for this overexpression is unclear. Among multiple levels of MYB regulation, the most dynamic is the control of transcriptional elongation by sequences within intron 1. The authors have proposed that this regulatory sequence is transcribed into an RNA stem-loop and 19-residue polyuridine tract, and is subject to mutation in CRC. When this region was examined in colorectal and breast carcinoma cell lines and tissues, the authors found frequent mutations only in CRC. It was determined that these mutations allowed increased transcription compared with the wild type sequence. These data suggest that this MYB regulatory region within intron 1 is subject to mutations in CRC but not breast cancer, perhaps consistent with the mutagenic insult that occurs within the colon and not mammary tissue. In CRC, these mutations may contribute to MYB overexpression, highlighting the importance of noncoding sequences in the regulation of key cancer genes.

DNA binding-independent transcriptional activation of the vascular endothelial growth factor gene (VEGF) by the Myb oncoprotein

Myb is a key transcription factor that can regulate proliferation, differentiation, and apoptosis, predominantly in the haemopoietic system. Abnormal expression of Myb is associated with a number of cancers, both haemopoietic and non-haemopoietic. In order to better understand the role of Myb in normal and tumorigenic processes, we undertook a cDNA array screen to identify genes that are regulated by this factor. In this way, we identified the gene encoding vascular endothelial growth factor (VEGF) as being potentially regulated by the Myb oncoprotein in myeloid cells. To determine whether this was a direct effect on VEGF gene transcription, we examined the activity of the murine VEGF promoter in the presence of either wild-type (WT) or mutant forms of Myb. It was found that WT Myb was able to activate the VEGF promoter and that a minimal promoter region of 120 bp was sufficient to confer Myb responsiveness. Surprisingly, activation of the VEGF promoter was independent of DNA binding by Myb. This was shown by the use of DNA binding-defective Myb mutants and by mutagenesis of a potential Myb-binding site in the minimal promoter. Mutation of Sp1 sites within this region abolished Myb-mediated regulation of a reporter construct, suggesting that Myb DNA binding-independent activation of VEGF expression occurs via these Sp1 binding elements. Regulation of VEGF production by Myb has implications for the potential role of Myb in myeloid leukaemias and in solid tumours where VEGF may be functioning as an autocrine growth factor.

c-myb Heterozygous Mice Are Hypersensitive to 5-Fluorouracil and Ionizing Radiation

Hypersensitivity to chemo- and radiotherapy employed during cancer treatment complicates patient management. Identifying mutations in genes that compromise tissue recovery would rationalize treatment and may spare hypersensitive patients undue tissue damage. Genes that govern stem cell homeostasis, survival, and progenitor cell maintenance are of particular interest in this regard. We used wild-type and c-myb knock-out mice as model systems to explore stem and progenitor cell numbers and sensitivity to cytotoxic damage in two radiosensitive tissue compartments, the bone marrow and colon. Because c-myb null mice are not viable, we used c-myb heterozygous mice to test for defects in stem-progenitor cell pool recovery following γ-radiation and 5-fluorouracil treatment, showing that c-myb+/− mice are hypersensitive to both agents. While apoptosis is comparable in mutant and wild-type mice following radiation exposure, the crypt beds of c-myb+/− mice are markedly depleted of proliferating cells. Extrapolating from these data, we speculate that acute responses to cytotoxic damage in some patients may also be attributed to compromised c-myb function.

Regulation of the gene encoding glutathione S-transferase M1 (GSTM1) by the Myb oncoprotein

The identification of Myb 'target' genes will not only aid in the understanding of how overexpression of Myb, or expression of activated forms of Myb, leads to cellular transformation but will also shed light on its role in normal cells. Using a combination of an estrogen-regulated Myb-transformed cell line (ERMYB) and PCR-based subtractive hybridization, we have identified the gene (GSTM1) encoding the detoxification enzyme glutathione S-transferase M1 as being transcriptionally upregulated by Myb. Functional analysis of the GSTM1 promoter using reporter assays indicated that both the DNA binding and transactivation domains of Myb were required for transcriptional activation. Mutational ana-lysis of consensus Myb-binding sites (MBS) in the promoter and electrophoretic mobility gel shift analysis indicated that one of the three potential MBS can bind Myb protein, and is the primary site involved in the regulation of this promoter by Myb.

Characterization of a MYBR2R3 gene from black spruce (Picea mariana) that shares functional conservation with maize C1

PCR amplification with degenerate primers targeted to highly conserved amino acid motifs within the MYB domain was used to demonstrate that black spruce (Picea mariana) possesses a diverse MYB gene family. Amino acid sequence comparisons revealed three broad MYB subfamilies, one of which shares extensive similarity with maize C1, a central regulator of anthocyanin biosynthesis. A cDNA clone encoding a MYBR2R3 protein from P. mariana with high levels of sequence homology to maize C1 was shown to transactivate the Bz2 promoter in combination with maize R in embryonal tissues of both black spruce and larch. Functional dependence on the maize R protein, and the presence of a conserved C-terminal GIDPxTH motif, support the conservation of MYBR2R3 function in conifers, and demonstrate that the basic components of MYBR2R3-dependent transcriptional regulation have been conserved between angiosperms and gymnosperms.

The human neuroendocrine thyrotropin-releasing hormone receptor promoter is activated by the haematopoietic transcription factor c-Myb

Thyrotropin-releasing hormone (TRH) receptor (TRHR) is a G-protein-coupled receptor playing a crucial role in the anterior pituitary where it controls the synthesis and secretion of thyroid-stimulating hormone and prolactin. Its widespread presence not only in the central nervous system, but also in peripheral tissues, including thymus, indicates other important, but unknown, functions. One hypothesis is that the neuropeptide TRH could play a role in the immune system. We report here that the human TRHR promoter contains 11 putative response elements for the haematopoietic transcription factor c-Myb and is highly Myb-responsive in transfection assays. Analysis of Myb binding to putative response elements revealed one preferred binding site in intron 1 of the receptor gene. Transfection studies of promoter deletions confirmed that this high-affinity element is necessary for efficient Myb-dependent transactivation of reporter plasmids in CV-1 cells. The Myb-dependent activation of the TRHR promoter was strongly suppressed by expression of a dominant negative Myb-Engrailed fusion. In line with these observations, reverse transcriptase PCR analysis of rat tissues showed that the TRHR gene is expressed both in thymocytes and bone marrow. Furthermore, specific, high-affinity TRH agonist binding to cell-surface receptors was demonstrated in thymocytes and a haematopoietic cell line. Our findings imply a novel functional link between the neuroendocrine and the immune systems at the level of promoter regulation.

Targeting c-Myb expression in human disease

c-Myb is a transcription factor employed in the haematopoietic system and gastrointestinal tract to regulate the exquisite balance between cell division, differentiation and survival. In its absence, these tissues either fail to form, or show aberrant biology. Mice lacking a functional c-myb gene die in utero by day 15 of development. When inappropriately expressed, as is common in leukaemia and epithelial cancers of the breast, colon and gastro-oesophagus, c-Myb appears to activate gene targets of key importance to cancer progression and metastasis. These genes include cyclooxygenase-2 (COX-2), Bcl-2, BclX(L) and c-Myc, which influence diverse processes such as angiogenesis, proliferation and apoptosis. The clinical potential for blocking c-Myb expression in malignancies is based upon strong preclinical data and some trial-based evidence. The modest clinical experience to date has been with haematopoietic malignancies, but other disease classes may be amenable to similar interventions. The frontline agents to achieve this are nuclease-resistant oligodeoxynucleotides (ODNs), which are proving to be acceptable therapeutic reagents in terms of tolerable toxicities and delivery. Nevertheless, further effort must be focused on improving their efficacy, eliminating non-specific toxicity and optimising delivery. Optimisation issues aside, it would appear that anti-c-Myb therapies will be used with most success when combined with other agents, some of which will be established cytotoxic and differentiation-inducing drugs. This review will explore the future strategic use of ODNs in vivo, focusing on a wide spectrum of diseases, including several beyond the haematopoietic malignancies, in which c-Myb appears to play a role.

Alteration of C-MYB DNA binding to cognate responsive elements in HL-60 variant cells

AIMS

To establish whether the MYB protein expressed in HL-60 variant cells, which are cells resistant to 12-O-tetradecanoylphorbol-13-acetate (TPA) induced differentiation, is able to bind MYB recognition elements (MREs) involved in the transcriptional regulation of myb target genes. In addition, to determine whether alterations in the binding of the MYB protein to MREs affects HL-60 cell proliferation and differentiation.

METHODS

Nuclear extracts of HL-60 variant cells exhibiting different degrees of resistance to TPA induced monocytic differentiation were used in electrophoretic mobility shift experiments (EMSAs), bandshift experiments performed with labelled oliogonucleotides containing the MYB consensus binding sequences.

RESULTS

The MYB protein contained in nuclear extracts from HL-60 variant cells did not bind efficiently to the MYB recognition elements identified in the mim-1 and PR264 promoters. Molecular cloning of the myb gene and analysis of the MYB protein expressed in the HL-60 variant cells established that the lack of binding did not result from a structural alteration of MYB in these cells. The lack of MRE binding did not abrogate the ability of variant HL-60s to proliferate and to undergo differentiation. Furthermore, the expression of the PR264/SC35 splicing factor was not affected as a result of the altered MYB DNA binding activity.

CONCLUSIONS

Because the MYB protein expressed in HL-60 variant cells did not appear to be structurally different from the MYB protein expressed in parental HL-60 cells, it is possible that the HL-60 variant cells contain a MYB binding inhibitory factor (MBIF) that interferes with MYB binding on MREs. The increased proliferation rate of HL-60 variant cells and their reduced serum requirement argues against the need for direct MYB binding in the regulation of cell growth.

Characterization of interactions between transcription factors and a regulatory region spanning nt -320 to -281 of the HIV-1 LTR in T-lymphoid and non-T-lymphoid cells

HIV-1 gene expression is regulated by the interplay of transcription factors with multiple binding motifs present within the U3, R and U5 regions of the long terminal repeat (LTR). Here we report novel DNA binding complexes (termed 9a, 9b and 9c) between nuclear proteins from T-lymphoid and non-T-lymphoid cells and a region of the U3 LTR between nucleotides (nts) -320 to -281 in the HIV strain HXB2. Complex 9b bound a motif predicted to bind E-box or c-Myb proteins and a partially overlapping dyad symmetrical motif, and included basic helix-loop-helix proteins (E12, E47 or ITF-1) but surprisingly not c-Myb. Complex 9c, which bound to a pair of GATA sites, included GATA-3 and GATA-4 in Jurkat and MT-2 cells, respectively. We also demonstrate that the c-Myb/E-box and GATA sites form a bipartite motif required for the formation of complex 9a. Transient transfection experiments with T cells revealed that in the context of a minichromosome assembled full-length LTR, mutation of region -320 to -281 increased basal and PMA-stimulated LTR activity. These findings suggest that this region is an important component of the HIV-1 LTR required for response to different cellular transcription factors.

Prolactin, interleukin-2 and FGF-2 stimulate expression, nuclear distribution and DNA-binding of rat homolog of pombe Cdc5 in Nb2 T lymphoma cells

Pombe and human Cdc5 have been implicated in G2/M progression, but recently Cdc5 was identified as a component of a multiprotein complex essential for pre-mRNA splicing. We have previously isolated a prolactin (PRL)-inducible partial cDNA (1907 bp) encoding rat Cdc5. In the present study, the full length rCdc5 sequence (2847 bp) was obtained by 5'-RACE and cytokine regulation of Cdc5 expression was examined. PRL and interleukin-2 (IL2) act as mitogens in Nb2 T-lymphoma cells. Fibroblast growth factor (FGF-2) is not mitogenic in Nb2 cells but inhibits apoptosis of PRL-deprived cells. This study showed that PRL, IL-2 and FGF-2 rapidly increased Nb2 Cdc5 expression (3.4 kb mRNA) to reach 2-3-fold above controls at 4 h, and Cdc5 mRNA levels remained elevated at 24 h. There was a corresponding 2-3-fold increase in Cdc5 protein (105 kDa) levels at 24 h. Immunoblotting and fluorescent confocal microscopy showed predominant nuclear/perinuclear Cdc5 in quiescent Nb2 cells. PRL or FGF-2 treatment transiently increased nuclear Cdc5-specific immunofluorescence at 4 h but IL-2 gave maximal nuclear accumulation of Cdc5 at 24 h. The deduced rCdc5 protein has approximately 98% amino acid identity with human Cdc5. Like other Cdc5 family members, the N-terminus of rCdc5 contains two repeats of a DNA-binding domain found in a-, b- and c-Myb. Gel shift assays using (32)P-labeled Myb consensus oligonucleotides revealed two Myb-specific DNA-protein complexes in Nb2 nuclear extracts. Formation of both complexes was increased by PRL or FGF-2 at 1-5 and at 20 h and was partially inhibited by anti-Myb or anti-Cdc5 antibodies. In summary, rapid activation of Cdc5 in response to mitogenic and non-mitogenic stimuli suggests a complex role for Cdc5 in cellular regulation and this may not be restricted to mitotic entry or G2/M progression as previously supposed.

c-Myb influences HIV type 1 gene expression and virus production

c-Myb is expressed in proliferating T cells. Fifteen c-Myb-binding sites can be identified in the HIV-1 long terminal repeat (LTR), suggesting that c-Myb may regulate HIV-1 gene expression and virus replication. Increasing the cellular levels of c-Myb by transient transfection of CEM cells resulted in a 10- to 20-fold activation of HIV-1 LTR-driven gene expression and mutation of one high-affinity Myb-binding site within the LTR reduced this activation by 60 to 70%. Conversely, inhibition of c-Myb expression in MT-2 cells by treatment with c-myb antisense oligonucleotides decreased HIV-1 replication by 85%, as measured by reverse transcriptase activity and cytopathic effects. The effect of c-myb antisense oligonucleotides on HIV-1 gene expression and virus particle production appeared to be independent of cell proliferation, but dependent on the presence of c-Myb activity mediated through the HIV-1 LTR. These data show that c-myb expression affects HIV-1 replication in CD4(+) T cells.

LAF1, a MYB transcription activator for phytochrome A signaling

The photoreceptor phytochrome (phy) A has a well-defined role in regulating gene expression in response to specific light signals. Here, we describe a new Arabidopsis mutant, laf1 (long after far-red light 1) that has an elongated hypocotyl specifically under far-red light. Gene expression studies showed that laf1 has reduced responsiveness to continuous far-red light but retains wild-type responses to other light wavelengths. As far-red light is only perceived by phyA, our results suggest that LAF1 is specifically involved in phyA signal transduction. Further analyses revealed that laf1 is affected in a subset of phyA-dependent responses and the phenotype is more severe at low far-red fluence rates. LAF1 encodes a nuclear protein with strong homology with the R2R3-MYB family of DNA-binding proteins. Experiments using yeast cells identified a transactivation domain in the C-terminal portion of the protein. LAF1 is constitutively targeted to the nucleus by signals in its N-terminal portion, and the full-length protein accumulates in distinct nuclear speckles. This accumulation in speckles is abolished by a point mutation in a lysine residue (K258R), which might serve as a modification site by a small ubiquitin-like protein (SUMO).

Increased c-myb mRNA expression in Barrett's esophagus and Barrett's-associated adenocarcinoma

BACKGROUND

Esophageal adenocarcinoma develops through a multistage process which is characterized histopathologically by progression from Barrett's intestinal metaplasia to Barrett's esophagus with dysplasia and ultimately to adenocarcinoma. The genetic basis of this process is increasingly well understood, but no studies have examined the role of the transcription factor c-myb in this disease.

MATERIALS AND METHODS

c-myb mRNA expression levels were measured using a quantitative reverse transcription-polymerase chain reaction (RT-PCR) method in specimens of Barrett's intestinal metaplasia (n = 16), adenocarcinoma (n = 22), matching normal squamous esophagus tissues (n = 38), and normal squamous esophagus tissues from patients without Barrett's esophagus or chronic gastroesophageal reflux disease (n = 10).

RESULTS

The median c-myb mRNA expression levels were significantly increased in Barrett's intestinal metaplasia tissues compared to normal esophagus tissues (P = 0.013) and in Barrett's-associated adenocarcinoma tissues compared to normal squamous esophagus tissues (P = 0.001). The c-myb expression levels increased progressively and significantly in histopathologically worse tissue types, with an increase from normal squamous esophagus mucosa to Barrett's intestinal metaplasia, and from Barrett's intestinal metaplasia to adenocarcinoma of the esophagus (P = 0.002). Median c-myb expression levels were also significantly higher in histologically normal squamous esophagus tissues from cancer patients compared to normal esophagus tissues from patients without cancer (P < 0.001) and a control group without evidence of Barrett's esophagus or gastroesophageal reflux disease (P = 0.003). Very high c-myb mRNA expression levels were found only in patients with cancer.

CONCLUSION

These findings suggest that upregulation of c-myb mRNA expression is an early event in the development of Barrett's esophagus and associated adenocarcinoma, that high c-myb mRNA expression levels may be a clinically useful biomarker for the detection of occult adenocarcinoma, and that a widespread cancer "field" effect is present in the esophagus of patients with Barrett's-associated adenocarcinoma.

Developmentally distinct MYB genes encode functionally equivalent proteins in Arabidopsis

The duplication and divergence of developmental control genes is thought to have driven morphological diversification during the evolution of multicellular organisms. To examine the molecular basis of this process, we analyzed the functional relationship between two paralogous MYB transcription factor genes, WEREWOLF (WER) and GLABROUS1 (GL1), in Arabidopsis. The WER and GL1 genes specify distinct cell types and exhibit non-overlapping expression patterns during Arabidopsis development. Nevertheless, reciprocal complementation experiments with a series of gene fusions showed that WER and GL1 encode functionally equivalent proteins, and their unique roles in plant development are entirely due to differences in their cis-regulatory sequences. Similar experiments with a distantly related MYB gene (MYB2) showed that its product cannot functionally substitute for WER or GL1. Furthermore, an analysis of the WER and GL1 proteins shows that conserved sequences correspond to specific functional domains. These results provide new insights into the evolution of the MYB gene family in Arabidopsis, and, more generally, they demonstrate that novel developmental gene function may arise solely by the modification of cis-regulatory sequences.

c-Myb and Bcl-x overexpression predicts poor prognosis in colorectal cancer: clinical and experimental findings

The aim of this study was twofold: to assess the relationship between c-Myb and Bcl-x expression and to evaluate the prognostic significance of their expression in colorectal carcinoma (CRC) patients. Analysis of tumors from 91 CRC patients for expression of c-Myb and Bcl-x revealed a significant relationship between these two proteins. Kaplan-Meier's analysis showed an increased risk of relapse and death in patients whose tumor specimens displayed high c-Myb levels and Bcl-x positivity. Similar results were also observed excluding Dukes' D patients. Molecular analysis using three c-Myb-overexpressing LoVo clones indicated that c-Myb overexpression was accompanied by up-regulation of Bcl-x(L) protein and mRNA. Tumors originating from these clones injected in nude mice were significantly larger than those formed in mice injected with parental or vector-transfected LoVo cells. Moreover, tumors derived from parental and control vector-transfected but not from c-Myb-overexpressing LoVo cells showed high frequency of apoptotic cells. These results provide direct evidence of an association between c-Myb and Bcl-x expression and suggest that expression of both molecules might be a useful prognostic marker in CRC.

Identification and validation of candidate Myb target genes

While a considerable number of candidate Myb target genes have been reported to date, most of these are likely to play little or no role in transformation by myb oncogenes. Here we have used a conditionally myb-transformed myeloid cell line (ERMYB) to further examine Myb regulation of one candidate target gene--c-myc--that has the potential to affect cell proliferation. It was found that the major influence on c-myc expression was the presence of cytokine (GM-CSF) rather than Myb activity. We also describe the application of PCR-based subtractive hybridization and low-density cDNA array screening, in conjunction with the ERMYB line, to the identification of additional Myb target genes. Preliminary identification of a number of candidates is reported; these include myeloperoxidase, which is known to have essential Myb-binding sites in its regulatory region.

A novel silencer element repressing expression of the GLP-1 receptor gene in fibroblasts and pancreatic A-cells, but not in pancreatic B- and D-cells

The effects of the incretin hormone glucagon-like peptide 1 (7-36)amide (GLP-1) are mediated by the GLP-1 receptor (GLP-1R). This is expressed in a cell- and tissue-specific manner. Recently, we have cloned the 5'-flanking region of the human GLP-1R gene. The basal promoter activity is driven by the ubiquitous transcription factor Sp1. The tissue- and cell-specific expression of the gene requires several negatively acting cis-regulatory elements. We have now characterized one so far unknown distal cell-specific silencer element (DCS), repressing gene transcription of the human GLP-1R gene in fibroblasts and pancreatic A-cells, but not in pancreatic B- and D-cells. Our data suggests that the basal activity of the GLP-1R promoter is repressed in a tissue- and cell-specific manner by this novel silencer element.

Acetylation of histones and transcription-related factors

The state of chromatin (the packaging of DNA in eukaryotes) has long been recognized to have major effects on levels of gene expression, and numerous chromatin-altering strategies-including ATP-dependent remodeling and histone modification-are employed in the cell to bring about transcriptional regulation. Of these, histone acetylation is one of the best characterized, as recent years have seen the identification and further study of many histone acetyltransferase (HAT) proteins and their associated complexes. Interestingly, most of these proteins were previously shown to have coactivator or other transcription-related functions. Confirmed and putative HAT proteins have been identified from various organisms from yeast to humans, and they include Gcn5-related N-acetyltransferase (GNAT) superfamily members Gcn5, PCAF, Elp3, Hpa2, and Hat1: MYST proteins Sas2, Sas3, Esa1, MOF, Tip60, MOZ, MORF, and HBO1; global coactivators p300 and CREB-binding protein; nuclear receptor coactivators SRC-1, ACTR, and TIF2; TATA-binding protein-associated factor TAF(II)250 and its homologs; and subunits of RNA polymerase III general factor TFIIIC. The acetylation and transcriptional functions of these HATs and the native complexes containing them (such as yeast SAGA, NuA4, and possibly analogous human complexes) are discussed. In addition, some of these HATs are also known to modify certain nonhistone transcription-related proteins, including high-mobility-group chromatin proteins, activators such as p53, coactivators, and general factors. Thus, we also detail these known factor acetyltransferase (FAT) substrates and the demonstrated or potential roles of their acetylation in transcriptional processes.

HTLV-I Tax transrepresses the human c-Myb promoter independently of its interaction with CBP or p300

The c-Myb proto-oncogene is preferentially expressed in hematopoietic lineages, and highly expressed in several leukemia types. The Human T-cell Leukemia Virus Type I (HTLV-I) is the etiological agent of Adult T-cell Leukemia/Lymphoma (ATLL). A previous report suggested that Tax, the viral transactivator, is able to suppress the transactivation potential of c-Myb protein by competing for recruitment of CBP. We tested whether such a competition could affect transcription from the c-Myb promoter in Tax expressing T-cells. Using several c-Myb promoter reporter constructs carrying mutations in various regions, we demonstrate that Tax suppression of c-Myb transactivation results in transrepression of the c-Myb promoter through the Myb responsive elements in Jurkat T-cells. The ability of Tax mutants M22, M47 and V89A to interact with the full-length CBP and p300 proteins in vitro, and their ability to repress the c-Myb promoter, was then evaluated. Although both M47 and M22 bind to CBP and p300 to a similar extent, only M47 was able to repress the c-Myb promoter, suggesting that competition for CBP/p300 binding was not the mechanism underlying Tax's effect. This concept was further supported by the fact that the Tax mutant V89A transrepresses the c-Myb promoter efficiently in spite of an impaired binding to CBP and p300. Therefore, Tax-mediated repression of the c-Myb promoter appears to be independent from a direct competition between c-Myb and Tax for recruitment of CBP/p300. Interestingly, a decreased transcription from the endogenous c-Myb promoter was observed in several HTLV-I transformed T-cell lines. Finally, the ability of Tax to directly repress the endogenous c-Myb promoter was demonstrated in a Jurkat cell line stably transfected with a tax gene driven by a cadmium-inducible promoter.

Molecular cloning and chromosomal mapping of the human homologue of MYB binding protein (P160) 1A (MYBBP1A) to 17p13.3

We have previously isolated and characterized murine MYB binding protein (p160) 1a, a protein that specifically interacts with the leucine zipper motif within the negative regulatory domain of the c-Myb proto-oncoprotein. We now describe the molecular cloning of the human MYBBP1A cDNA and chromosomal localization to 17p13.3 by fluorescence in situ hybridization analysis. Given the likely presence of a tumor suppressor gene (or genes) within this region of chromosome 17, the position of MYBBP1A was further mapped by radiation hybrid analysis and was found to lie between markers D17S1828 and D17S938. A P1 artificial chromosome clone containing the 5' region of MYBBP1A was isolated and indicates a physical linkage between MYBBP1A and the 15-lipoxygenase gene (ALOX15). A novel, polymorphic (CA)(25) dinucleotide repeat was also isolated from this PAC and may serve as a useful marker for MYBBP1A and this region of chromosome 17.

v-Myb can transform and regulate the differentiation of melanocyte precursors

The activity of the c-Myb transcription factor is essential for the development of definitive multi- and uni-lineage progenitors of the haemopoietic system. Reflecting this requirement, c-Myb has been oncogenically activated by transduction in the E26 avian retrovirus which elicits an acute leukaemia by transforming haemopoietic progenitors. Here, we report the novel finding that Myb in cooperation with EGF receptor signalling can be used to generate clonally expanded populations of transformed cells which have the phenotype of melanocyte precursors. Through the use of a conditional temperature sensitive mutant of Myb, we show that in the transformed cells Myb regulates commitment to melanocyte differentiation and possibly proliferation. These results add to our understanding of the roles of c-Myb beyond the haemopoietic system and to our knowledge and means of investigating the importance of transcription factors in the melanocyte lineage.

WEREWOLF, a MYB-related protein in Arabidopsis, is a position-dependent regulator of epidermal cell patterning

The formation of the root epidermis of Arabidopsis provides a simple and elegant model for the analysis of cell patterning. A novel gene, WEREWOLF (WER), is described here that is required for position-dependent patterning of the epidermal cell types. The WER gene encodes a MYB-type protein and is preferentially expressed within cells destined to adopt the non-hair fate. Furthermore, WER is shown to regulate the position-dependent expression of the GLABRA2 homeobox gene, to interact with a bHLH protein, and to act in opposition to the CAPRICE MYB. These results suggest a simple model to explain the specification of the two root epidermal cell types, and they provide insight into the molecular mechanisms used to control cell patterning.

c-Myb is critical for murine colon development

The mammalian colon develops from a simple tube of undifferentiated cells into a complex, highly ordered organ, with a continuously self-renewing epithelial layer. We have previously described c-Myb expression in the epithelia of murine and human colon crypts and documented increased expression in colorectal adenocarcinoma cells. To investigate the role of c-Myb in colonic epithelium development, we have used embryos with a disrupted c-myb gene. Prior to the in utero death of these embryos at E15, we excised colon tissue and transplanted it under the kidney capsule of recipient mice to allow further development and cyto-differentiation. Compared to the colons of wildtype and heterozygous littermates, the c-myb homozygous knockout colon is highly irregular with a disordered epithelium and abnormal crypts. In addition, the expression of Bcl-2, a known target of c-Myb, is reduced and apoptosis is increased, indicating a critical requirement for c-Myb in normal colon development.

Haemopoietic progenitor cell lines generated by the myeloproliferative leukaemia virus: a model system to analyse murine and human lineage-affiliated genes

Multipotential progenitor and stem cells occur with a low frequency in haemopoietic tissue. As a result, it is often difficult to obtain sufficient numbers of cells to undertake many of the assays that would be informative about the molecular events involved in the regulation of lineage-affiliated genes within these multipotent cells. To circumvent this problem, we have used the myeloproliferative leukaemia virus (MPLV) to generate a phenotypically diverse array of haemopoietic progenitors from adult mouse bone marrow and embryonic blood. These cells could be expanded to perform a variety of analyses that would not previously have been possible using analogous primary cells. The validity of these assays was supported by the observation that the phenotype of several MPLV-infected lines was very similar to previously described primary haemopoietic progenitor cells. By using mice transgenic for the human alpha and beta globin gene clusters, we have shown that human genes may also be investigated. In addition, this strategy has a wide potential applicability including the rescue of haemopoietic progenitors from mouse embryos lacking genes critical for their survival as well as the study of any haemopoietic gene for which an appropriate transgenic mouse is available.

c-Maf Induces Monocytic Differentiation and Apoptosis in Bipotent Myeloid Progenitors

The transcriptional mechanisms that drive colony-forming unit granulocyte-macrophage (CFU-GM) myeloid progenitors to differentiate into cells of either the granulocytic or monocytic lineage are not fully understood. We have shown that the c-Maf and c-Myb transcription factors physically interact in myeloid cells to form inhibitory complexes that hinder transactivation of c-Myb target genes through direct binding to Myb consensus sites. These complexes arise in a developmentally regulated pattern, peaking at the promyelocyte stage, or in cell model systems, appearing soon after the induction of monocytic differentiation. We wished to determine if this developmentally related interaction is a consequence of myeloid differentiation or an intrinsic differentiating stimulus. Because the elevated Myb:Maf status seen in differentiating cells can be recapitulated by overexpression of c-Maf in myeloid cell lines, we inducibly expressed the c-Maf cDNA in 2 bipotent human myeloid progenitor cells. Elevated levels of c-Maf protein led to marked increases in Myb:Maf complexes and the accumulation of monocyte/macrophage cells, followed by eventual programmed cell death. Analysis of targets that could mediate these phenotypic changes indicated that c-Maf likely plays a key role in myeloid cell development through dual mechanisms; inhibition of a select set of c-Myb regulated targets, such as Bcl-2 and CD13/APN, coupled with the activation of as yet undefined differentiation-promoting genes.

c-Maf Induces Monocytic Differentiation and Apoptosis in Bipotent Myeloid Progenitors

The transcriptional mechanisms that drive colony-forming unit granulocyte-macrophage (CFU-GM) myeloid progenitors to differentiate into cells of either the granulocytic or monocytic lineage are not fully understood. We have shown that the c-Maf and c-Myb transcription factors physically interact in myeloid cells to form inhibitory complexes that hinder transactivation of c-Myb target genes through direct binding to Myb consensus sites. These complexes arise in a developmentally regulated pattern, peaking at the promyelocyte stage, or in cell model systems, appearing soon after the induction of monocytic differentiation. We wished to determine if this developmentally related interaction is a consequence of myeloid differentiation or an intrinsic differentiating stimulus. Because the elevated Myb:Maf status seen in differentiating cells can be recapitulated by overexpression of c-Maf in myeloid cell lines, we inducibly expressed the c-Maf cDNA in 2 bipotent human myeloid progenitor cells. Elevated levels of c-Maf protein led to marked increases in Myb:Maf complexes and the accumulation of monocyte/macrophage cells, followed by eventual programmed cell death. Analysis of targets that could mediate these phenotypic changes indicated that c-Maf likely plays a key role in myeloid cell development through dual mechanisms; inhibition of a select set of c-Myb regulated targets, such as Bcl-2 and CD13/APN, coupled with the activation of as yet undefined differentiation-promoting genes.

Construction of an artificial tandem protein of the c-Myb DNA-binding domain and analysis of its DNA binding specificity

An artificial tandem protein was generated using the third repeat of the c-Myb DNA-binding domain, and its DNA binding affinity and specificity were analyzed by a filter binding assay, isothermal titration calorimetry, and surface plasmon resonance. Although this artificial protein had the proper secondary structure, which is similar to the third repeat by itself, it could not bind to the expected base sequences specifically. Compared with the successful results of the zinc finger fusion proteins with novel sequence specificities, the cooperativity between the adjacent repeats, observed in the c-Myb-DNA complex, should also be required for the DNA recognition by the artificial tandem protein. Using the previous analyses of the DNA binding specificities by Myb homologous proteins, the differences in the DNA recognition mechanisms between the animal and plant Myb domains are also discussed.

Molecular cloning, expression and chromosomal localisation of the mouse Rev3l gene, encoding the catalytic subunit of polymerase zeta

The REV3 gene of Saccharomyces cerevisiae encodes the catalytic subunit of DNA polymerase zeta which is involved in translesion synthesis. The mouse homolog of this gene, Rev3l, was cloned and sequenced. The gene encodes a putative protein of 3122 amino acids. The sequence conservation to its yeast counterpart is restricted to several regions. In the carboxy-terminal part of the protein all six domains are present that are characteristic for alpha-type DNA polymerases. In the amino-terminal part of the protein two regions can be identified with considerable similarity to the NT boxes of mouse polymerase delta. In addition, a region of 60 residues unique for the REV3 homologs can be found in the middle part of the protein. The mouse REV3L protein shows strong sequence conservation with the recently cloned human REV3L protein (86% identity overall). Northern blot analysis of various tissues of the mouse revealed that transcription of the Rev3l gene was highest in brain, ovaries and testis. The human REV3L gene was localised to the long arm of chromosome 6, region 21-22. The mouse equivalent maps to chromosome 10, distal to the c-myb gene, close to the Macs gene.