B-Myb: a key regulator of the cell cycle

Transcription regulation of MYB: a potential and novel therapeutic target in cancer

Basal transcription factors have never been considered as a priority target in the field of drug discovery. However, their unparalleled roles in decoding the genetic information in response to the appropriate signal and their association with the disease progression are very well-established phenomena. Instead of considering transcription factors as such a target, in this review, we discuss about the potential of the regulatory mechanisms that control their gene expression. Based on our recent understanding about the critical roles of c-MYB at the cellular and molecular level in several types of cancers, we discuss here how MLL-fusion protein centred SEC in leukaemia, ligand-estrogen receptor (ER) complex in breast cancer (BC) and NF-κB and associated factors in colorectal cancer regulate the transcription of this gene. We further discuss plausible strategies, specific to each cancer type, to target those bona fide activators/co-activators, which control the regulation of this gene and therefore to shed fresh light in targeting the transcriptional regulation as a novel approach to the future drug discovery in cancer.

Zebrafish cancer: the state of the art and the path forward

The zebrafish is a recent addition to animal models of human cancer, and studies using this model are rapidly contributing major insights. Zebrafish develop cancer spontaneously, after mutagen exposure and through transgenesis. The tumours resemble human cancers at the histological, gene expression and genomic levels. The ability to carry out in vivo imaging, chemical and genetic screens, and high-throughput transgenesis offers a unique opportunity to functionally characterize the cancer genome. Moreover, increasingly sophisticated modelling of combinations of genetic and epigenetic alterations will allow the zebrafish to complement what can be achieved in other models, such as mouse and human cell culture systems.

Mammalian Mip/LIN-9 interacts with either the p107, p130/E2F4 repressor complex or B-Myb in a cell cycle-phase-dependent context distinct from the Drosophila dREAM complex

Mammalian Mip/LIN-9 is a cell cycle regulatory protein that is negatively regulated by CDK4/cyclin D. It has been demonstrated that Mip/LIN-9 collaborates with B-Myb during S and G(2)/M in the induction of cyclins A and B, and CDK1. The ortholog of Mip/LIN-9 in Drosophila, Mip130, is part of a large multisubunit protein complex that includes RBF, repressor E2Fs and Myb, in what was termed the dREAM complex. A similar complex, although lacking B-Myb, was also described in Caenorhabditis elegans. Here, we demonstrate that unlike Drosophila, Mip/LIN-9 has mutually exclusive and cell cycle-phase-specific interactions with the mammalian orthologs of the dREAM complex. In G(0)/early G(1), Mip/LIN-9 forms a complex with E2F4 and p107 or p130, while in late G(1)/S phase, it associates with B-Myb. The separation of Mip/LIN-9 from p107,p130/E2F4 is likely driven by phosphorylation of the pocket proteins by CDK4 since Mip/LIN-9 fails to interact with phosphorylated forms of p107,p130. Importantly, the repressor complex that Mip/LIN-9 forms with p107 takes functional precedence over the transcriptional activation linked to the Mip/LIN-9 and B-Myb interaction since expression of p107 blocks the activation of the cyclin B promoter triggered by B-Myb and Mip/LIN-9.

Mip/LIN-9 regulates the expression of B-Myb and the induction of cyclin A, cyclin B, and CDK1

Members of the novel family of proteins that include Drosophila Mip130, Caenorhabditis elegans LIN-9, and mammalian LIN-9 intervene in different cellular functions such as regulation of transcription, differentiation, transformation, and cell cycle progression. Here we demonstrate that LIN-9, designated as Mip/LIN-9, interacts with B-Myb but not with c-Myb or A-Myb. Mip/LIN-9 regulates the expression of B-Myb in a post-transcriptional manner, and its depletion not only decreases the level of the B-Myb protein but also affects the expression of S phase and mitotic genes (i.e. cyclin A, CDK1, and cyclin B). The critical role of Mip/LIN-9 on the expression of S and G(2)/M genes is further supported by the finding that coexpression of Mip/LIN-9 and B-Myb results in the activation of cyclin A and cyclin B promoter-luciferase reporters, and both proteins are detected on the cyclin A and B promoters. Interestingly, although Mip/LIN-9 promoter occupancy peaks earlier than B-Myb, the highest levels of expression of cyclins A and B correlate with the maximum binding of B-Myb to these promoters. These data support the concept that Mip/LIN-9 is required for the expression of B-Myb, and both proteins collaborate in the control of the cell cycle progression via the regulation of S phase and mitotic cyclins.

Expression of a thioredoxin-related protein-1 is induced by prostaglandin E(2)

Prostaglandin E(2) (PGE(2)) plays an important role in protection of the gastric mucosa against various damaging agents and growth-inhibitory activity on tumor cells. However, the precise regulation mechanism of PGE(2) in gastric cancer cells is still unclear. In this study, we isolated a gene, which is regulated by PGE(2) in SNU-1, human gastric adenocarcinoma cells, using differential display RT-PCR (DD RT-PCR) and characterized the function of the gene induced by PGE(2). The full-length cDNA of the gene was cloned by the rapid amplification of cDNA ends method. The 1659 base pair cDNA consists of a 30-nt 5'-noncoding region, an 891-nt open reading frame and a 738-nt 3'noncoding region that includes a poly (A) signal. As a result of protein motif search, we found that it has a conserved thioredoxin-active site, Cys-Gly-Pro-Cys and a Myb-DNA binding domain repeat signature. Thus, we designated this gene product as thioredoxin-related protein-1, TRP-1. TRP-1 was expressed in a lower extent in renal, gastric and colon cancer tissues and is translated into 33 kDa protein in nuclear and cytoplasmic fractions. TRP-1 has a thioredoxin activity, which was detected using the insulin disulfide reduction assay. Another potential role of TRP-1 is repression of B-Myb activity through direct binding to B-Myb, a transcriptional factor induced at G1-S transition. Finally, TRP-1 overexpression inhibits mammalian cell proliferation and specifically predispose to G0/G1 phase arrest. In conclusion, these results imply that TRP-1 is a mammalian thioredoxin and plays as a transcriptional repressor through direct binding to the transcription factor B-Myb.

Generation of a conditional allele of the B-myb gene

B-Myb is an essential transcription factor involved in control of the cell cycle and the regulation of tissue-specific gene expression in a wide range of cell types. Loss of both alleles results in early embryonic lethality at E4.5-6.5. To address the function of B-Myb in later stages of embryogenesis and in specific adult tissues, a floxed B-myb allele (B-mybF) was generated. Cre-mediated deletion in vivo was demonstrated by breeding with a transgenic GATA-Cre mouse line. An intermediate allele produced in the creation of the floxed allele, in which the PGK-neo(R) cassette is present in intron 3 (B-myb(loxneo)), was deduced to be a weak hypomorph based on the later embryonic death of homozygotes compared to B-myb(-/-) embryos. To demonstrate the efficiency and possible consequences of B-myb inactivation, we performed conditional deletion in cultured MEFs and observed decreased growth that correlated with aberrant nuclear DNA replication.

Regulation of the cyclin D1 and cyclin A1 promoters by B-Myb is mediated by Sp1 binding sites

B-Myb is a highly conserved member of the Myb family of transcription factors which plays an important role during the cell cycle. Previous work has shown that B-Myb is phosphorylated at several sites by cyclin A/Cdk2 in the early S-phase. These phosphorylations increase the transactivation potential of B-Myb by counteracting the repressive function of an inhibitory domain located at the carboxyl-terminus of B-Myb. As yet, only a few genes have been identified as B-Myb target genes. Previous work has suggested that the cyclin D1 gene might be regulated by B-Myb. Here, we have studied the effect of B-Myb on the promoter of the cyclin D1 gene. We show that B-Myb is a potent activator of the cyclin D1 promoter and that this activation is not mediated by Myb binding sites but rather by a group of Sp1 binding sites which have previously been shown to be crucial for cyclin D1 promoter activity. Our data show that the C-terminal domain of B-Myb is required for the activation of the cyclin D1 promoter and that this part of B-Myb interacts with Sp1. Finally, we have found that the promoter of the cyclin A1 gene is also activated by B-Myb by a Sp1 binding site-dependent mechanism. The effect of B-Myb on the promoters of the cyclin A1 and D1 genes is reminiscent of the mechanism that has been proposed for the autoregulation of the B-myb promoter by B-Myb, which also involves Sp1 binding sites. Taken together, our identification of two novel B-Myb responsive promoters whose activation by B-Myb does not involve Myb binding sites extends previous evidence for the existence of a distinct mechanism of transactivation by B-Myb which is dependent on Sp1 binding sites. The observation that this mechanism is not subject to the inhibitory effect of the C-terminal domain of B-Myb but rather requires this domain supports the notion that the Sp1 site-dependent mechanism is already active in the G1-phase prior to the phosphorylation of B-Myb by cyclin A/Cdk2.

B-Myb Represses Elastin Gene Expression in Aortic Smooth Muscle Cells

B-Myb represses collagen gene transcription in vascular smooth muscle cells (SMCs) in vitro and in vivo. Here we sought to determine whether elastin is similarly repressed by B-Myb. Levels of tropoelastin mRNA and protein were lower in aortas and isolated SMCs of adult transgenic mice expressing the human B-myb gene, driven by the basal cytomegalovirus promoter, compared with age-matched wild type (WT) animals. However, the vessel wall architecture and levels of insoluble elastin revealed no differences. Since elastin deposition occurs early in development, microarray analysis was performed using nontransgenic mice. Aortic levels of tropoelastin mRNA were low during embryonal growth and increased substantially in neonates, whereas B-myb levels varied inversely. Tropoelastin mRNA expression in aortas of 6-day-old neonatal transgenic and WT animals was comparable. Recently, we demonstrated that cyclin A-Cdk2 prevents B-Myb-mediated repression of collagen promoter activity. Cyclin A2 levels were higher in neonatal versus adult WT or transgenic mouse aortas. Ectopic cyclin A expression reversed the ability of B-Myb to repress elastin gene promoter activity in adult SMCs. These results demonstrate for the first time that B-Myb represses SMC elastin gene expression and that cyclin A plays a role in the developmental regulation of elastin gene expression in the aorta. Furthermore, the findings provide additional insight into the mechanism of B-myb-mediated resistance to femoral artery injury.

Molecular phenotyping of human chondrocyte cell lines T/C-28a2, T/C-28a4, and C-28/I2

OBJECTIVE

Because the immortalized chondrocyte cell lines C-28/I2, T/C-28a2, and T/C-28a4 have become a common tool in cartilage research, permitting investigations in a largely unlimited and standardized manner, we investigated the molecular phenotype of these cell lines by gene expression profiling.

METHODS

Complementary DNA-array analysis as well as online quantitative polymerase chain reaction were used to identify the gene expression profiles of the 3 cell lines cultured in monolayer and alginate beads, as compared with the expression profiles of cultured human adult primary chondrocytes.

RESULTS

A similar, but not identical, gene expression profile was established for all 3 cell lines. SOX9 was expressed at a significant level in all 3 cell lines. Extracellular matrix proteins and matrix-degrading proteases were rarely expressed. In contrast, genes involved in the cell cycle were strongly up-regulated, as compared with the expression levels in physiologic chondrocytes.

CONCLUSION

The expression of SOX9, the master gene of chondrocytic cell differentiation, reflects the basically chondrocytic phenotype of these cells. However, the major issue appears to be that these cell lines mainly proliferate and show less expression of genes involved in matrix synthesis and turnover. In this respect, C-28/I2 cells display the highest levels of matrix-anabolic and matrix-catabolic genes and thus are presumably preferable for use in investigating chondrocyte anabolic and catabolic activity and its regulation. None of the 3 cell lines appears to be a direct substitute for primary chondrocytes. A successful approach will have to validate the findings obtained with chondrocyte cell lines by using primary chondrocytes or cartilage-tissue cultures. This would permit the establishment of reproducible in vitro models and subsequently allow investigators to relate the findings to the physiologic situation.

Differential gene expression underlying the functional distinctions of primary human CD34+ hematopoietic stem and progenitor cells from peripheral blood and bone marrow

The restorative capacity of human CD34(+) hematopoietic cells is clinically used in the autologous and allogeneic transplant setting to support cytotoxic therapy. We examined gene expression patterns of highly enriched bone marrow CD34(+) (BM-CD34(+)) or G-CSF-mobilized peripheral blood CD34(+) (PB-CD34(+)) cells by cDNA array technology, quantitative real-time RT-PCR, and flow cytometry, to identify molecular causes underlying the functional differences between circulating and sedentary hematopoietic stem and progenitor cells. The greater cell cycle and DNA synthesis activity of BM-CD34(+) compared to PB-CD34(+) cells was reflected by the 2- to 5-fold higher expression of 9 genes involved in cell cycle, 11 genes regulating DNA synthesis, and the cell cycle-initiating transcription factor E2F-1. The 2- to 3-fold greater expression of 5 pro-apoptotic genes in PB-CD34(+) cells indicated a higher apoptotic activity, which could functionally be corroborated by apoptosis assays. Thrombin receptor (PAR1), known to play a role in trafficking of malignant cells, was 3.6-fold higher expressed in circulating CD34(+) cells than in BM-CD34(+) cells. Guidance via thrombin receptor might molecularly mediate stem cell migration. In summary, our study provides gene expression profiles of primary human CD34(+) hematopoietic cells of blood and marrow. Our data molecularly confirm and explain the finding that CD34(+) cells residing in the bone marrow are cycling more rapidly, whereas circulating CD34(+) cells consist of a higher number of quiescent stem and progenitor cells. Moreover, our data give novel molecular insights into stem cell migration and differentiation.

DNA damage or growth factor withdrawal does not evoke activation of MYB transcription factors in neuronal cancer cell lines

It has been recently observed that MYB proteins are induced in neuronal cell cultures exposed to DNA damaging agents or growth factor deprivation. While it has been proposed that MYB proteins activity is required to bring about death in neuronal cells subjected to apoptotic stimuli, here we show that MYB is not activated in differentiated pheochromocytoma-12 cells (PC12) or LAN5 cells exposed to the DNA damaging agent campothecin or in differentiated PC12 cells subjected to nerve growth factor withdrawal. We conclude that MYB activation cannot be generalised in terminally differentiated neuronal cancer cell lines.

B-Myb overcomes a p107-mediated cell proliferation block by interacting with an N-terminal domain of p107

B-Myb is a cell-cycle regulated transcription factor which is implicated in cell proliferation and has an essential role in early embryonic development. In this study we examined the functions of B-Myb required to overcome G1 arrest in Saos-2 cells induced by the retinoblastoma-related p107 protein. Our results demonstrated that this activity was independent of B-Myb transactivation function, but correlated with its capacity to form an in vivo complex with p107. A large proportion of B-Myb formed complexes with p107 in cotransfected cells, however, B-Myb bound weakly to the related p130 protein and not at all to pRb. In contrast to the E2F transcription factors, which bind the p107 C-terminal pocket domain, B-Myb recognizes an N-terminal p107 region which overlaps the larger cyclin-binding domain. B-Myb and cyclin A2 formed mutually exclusive complexes with p107, and B-Myb enhanced the activity of co-transfected cyclin E kinase activity, implying that B-Myb affects the cell cycle by preventing sequestration of active cyclin/cdk2 complexes. This study defines a novel function of B-Myb and further suggests that the p107 N-terminus provides an interaction domain for transcription factors involved in cell cycle control.

The transcription factor B-Myb is maintained in an inhibited state in target cells through its interaction with the nuclear corepressors N-CoR and SMRT

The B-Myb transcription factor has been implicated in coordinating the expression of genes involved in cell cycle regulation. Although it is expressed in a ubiquitous manner, its transcriptional activity is repressed until the G(1)-S phase of the cell cycle by an unknown mechanism. In this study we used biochemical and cell-based assays to demonstrate that the nuclear receptor corepressors N-CoR and SMRT interact with B-Myb. The significance of these B-Myb-corepressor interactions was confirmed by the finding that B-Myb mutants, which were unable to bind N-CoR, exhibited constitutive transcriptional activity. It has been shown previously that phosphorylation of B-Myb by cdk2/cyclin A enhances its transcriptional activity. We have now determined that phosphorylation by cdk2/cyclin A blocks the interaction between B-Myb and N-CoR and that mutation of the corepressor binding site within B-Myb bypasses the requirement for this phosphorylation event. Cumulatively, these findings suggest that the nuclear corepressors N-CoR and SMRT serve a previously unappreciated role as regulators of B-Myb transcriptional activity.

Gene expression profiling identifies significant differences between the molecular phenotypes of bone marrow-derived and circulating human CD34+ hematopoietic stem cells

CD34+ hematopoietic stem cells are used clinically to support cytotoxic therapy, and recent studies raised hope that they could even serve as a cellular source for nonhematopoietic tissue engineering. Here, we examined in 18 volunteers the gene expressions of 1185 genes in highly enriched bone marrow CD34+ (BM-CD34+) or granulocyte-colony-stimulating factor-mobilized peripheral blood CD34+ (PB-CD34+) cells by means of cDNA array technology to identify molecular causes underlying the functional differences between circulating and sedentary hematopoietic stem and progenitor cells. In total, 65 genes were significantly differentially expressed. Greater cell cycle and DNA synthesis activity of BM-CD34+ than PB-CD34+ cells were reflected by the 2- to 5-fold higher expression of 9 genes involved in cell cycle progression, 11 genes regulating DNA synthesis, and cell cycle-initiating transcription factor E2F-1. Conversely, 9 other transcription factors, including the differentiation blocking GATA2 and N-myc, were expressed 2 to 3 times higher in PB-CD34+ cells than in BM-CD34+ cells. Expression of 5 apoptosis driving genes was also 2 to 3 times greater in PB-CD34+ cells, reflecting a higher apoptotic activity. In summary, our study provides a gene expression profile of primary human CD34+ hematopoietic cells of the blood and marrow. Our data molecularly confirm and explain the finding that CD34+ cells residing in the bone marrow cycle more rapidly, whereas circulating CD34+ cells consist of a higher number of quiescent stem and progenitor cells. Moreover, our data provide novel molecular insight into stem cell physiology.

A Myb dependent pathway maintains Friend murine erythroleukemia cells in an immature and proliferating state

Friend murine erythroleukemia (MEL) cells are transformed erythroid precursors that are held in an immature and proliferating state but can be induced to differentiate in vivo by treatment with a variety of chemical agents such as N, N-hexamethylene bisacetamide (HMBA). To investigate the role of Myb proteins in maintaining MEL cells in an immature and proliferating state we have produced stable transfectants in the C19 MEL cell line that contain a dominant interfering Myb allele (MEnT) under the control of an inducible mouse metallothionein I promoter. When expression of MEnT protein was induced with ZnCl2, the stable transfectants differentiated with kinetics that were similar to wild type C19 MEL cells treated with HMBA, including induction of alpha-globin mRNA expression, assembly of hemoglobin and growth arrest. Expression of endogenous c-myb and c-myc was also decreased in response to MEnT. Expression of mad-1 mRNA was rapidly increased in response to expression of MEnT resulting in a shift from predominantly c-Myc/Max complexes to predominantly Mad/Max containing complexes. These results strongly suggest that C19 MEL cells are held in an immature and proliferating state by a pathway that is dependent on Myb activity.

Effects of B-Myb on gene transcription: phosphorylation-dependent activity ans acetylation by p300

The transcription factor B-Myb is a cell-cycle regulated phosphoprotein involved in cell cycle progression through the transcriptional regulation of many genes. In this study, we show that the promoter of the fibroblast growth factor-4 (FGF-4) gene is strongly activated by B-Myb in HeLa cells and it can serve as a novel diagnostic tool for assessing B-Myb activity. Specifically, B-Myb deletion mutants were examined and domains of B-Myb required for activation of the FGF-4 promoter were identified. Using phosphorylation-deficient mutant forms of B-Myb, we also show that phosphorylation is essential for B-Myb activity. Moreover, a mutant form of B-Myb, which lacks all identified phosphorylation sites and which has little activity, can function as a dominant-negative and suppress wild-type B-Myb activity. Acetylation is another post-translational modification known to affect the activity of other Myb family members. We show that B-Myb is acetylated by the co-activator p300. We also show that the bromo and histone acetyltransferase domains of p300 are sufficient to interact with and acetylate B-Myb. These data indicate that phosphorylation of B-Myb is an essential modification for activity and that acetylation of B-Myb may play a role in B-Myb activity.

PARP co-activates B-MYB through enhanced phosphorylation at cyclin/cdk2 sites

PARP is a multifunctional protein that can affect genome stability, transcription control, telomere length and cell death. Recently we have reported that PARP binds to and enhances B-MYB transactivating potential. B-MYB is a potentially oncogenic transcription factor involved in mammalian cell proliferation, survival and differentiation. B-MYB gene expression is growth regulated and B-MYB protein is phosphorylated during S phase by cyclin A or E/cdk2 kinase, resulting in augmented transactivating potential. Here we show that PARP induces phosphorylation of B-MYB protein at cdk2 phosphorylation sites, since a B-MYB protein with mutated cdk2 phosphorylation sites is refractory to PARP-induced phosphorylation and co-activation in mammalian cells. We propose that PARP functions as a B-MYB co-factor by promoting cyclin/cdk2-dependent B-MYB phosphorylation. These results highlight a novel role for PARP as a factor that integrates cyclin-dependent kinases signaling with gene transcription.

The highly conserved DNA-binding domains of A-, B- and c-Myb differ with respect to DNA-binding, phosphorylation and redox properties

In the Myb family, as in other families of transcription factors sharing similar DNA-binding domains (DBDs), diversity of function is believed to rely mainly on the less conserved parts of the proteins and on their distinct patterns of expression. However, small conserved differences between DBDs of individual members could play a role in fine-tuning their function. We have compared the highly conserved DBDs of the three vertebrate Myb proteins (A-, B- and c-Myb) and found distinct functional differences. While A- and c-Myb behaved virtually identically in a variety of DNA-binding assays, B-Myb formed complexes of comparatively lower stability, rapidly dissociating under competitive conditions and showing less tolerance to binding site variations. The three protein domains also differed as substrates for protein kinases. Whereas PKA in theory should target the DBDs of A- and c-Myb, but not B-Myb, only c-Myb was phosphorylated by PKA. CK2 phosphorylated all three proteins, although on different sites in the N-terminal region. Finally, B-Myb was remarkably sensitive to cysteine-directed oxidation compared to the other Myb proteins. Our data suggest that the small differences that have evolved between individual Myb family members lead to clear differences in DBD properties even if their sequence recognition remains the same.

Cyclin A1 directly interacts with B-myb and cyclin A1/cdk2 phosphorylate B-myb at functionally important serine and threonine residues: tissue-specific regulation of B-myb function

Cyclin A1 is tissue-specifically expressed during spermatogenesis, but it is also highly expressed in acute myeloid leukemia (AML). Its pathogenetic role in AML and in the cell cycle of leukemic blasts is unknown. B-myb is essential for G1/S transition and has been shown to be phosphorylated by the cyclin A2/cdk2 complex. Here it is demonstrated that cyclin A1 interacts with the C-terminal portion of B-myb as shown by glutathione S-transferase (GST) precipitation. This interaction is confined to cyclin A1 because binding could not be detected between cyclin A2 and B-myb. Also, cdk2 was not pulled down by GST-B-myb from U937 lysates. In addition, co-immunoprecipitation of cyclin A1 and B-myb in leukemic cells evidenced protein interaction in vivo. Baculovirus-expressed cyclin A1/cdk2 complexes were able to phosphorylate human as well as murine B-myb in vitro. Tryptic phosphopeptide mapping revealed that cyclin A1/cdk2 complexes phosphorylated the C-terminal part of B-myb at several sites including threonine 447, 490, and 497 and serine 581. These phosphorylation sites have been demonstrated to be important for the enhancement of B-myb transcriptional activity. Further studies showed that cyclin A1 cooperated with B-myb to transactivate myb binding site containing promoters including the promoter of the human cyclin A1 gene. Taken together, the data suggest that cyclin A1 is a tissue-specific regulator of B-myb function and activates B-myb in leukemic blasts. (Blood. 2001;97:2091-2097)

BS69, an adenovirus E1A-associated protein, inhibits the transcriptional activity of c-Myb

The carboxyl terminus of c-Myb contains a negative regulatory domain that is absent in the v-Myb oncoprotein, but conserved among all the known Myb proteins of animals. This domain inhibits transcriptional activation by c-Myb in animal cells, but not in budding yeast, suggesting that additional protein(s) present in animal cells but not yeast are required for this negative regulatory function. A yeast two-hybrid screen identified BS69, an adenovirus E1A-associated protein, as interacting with the carboxy-terminal region of c-Myb. BS69 contains regions of similarity to the PHD finger, the bromodomain, and the MYND domain, all of which are found in other proteins present in high molecular weight complexes that regulate transcription and/or modify chromatin structure. Further study showed that BS69 inhibited the transcriptional activity of c-Myb, that this inhibition was specific, that it mapped to the carboxyl termini of the two proteins and that it was dose-dependent. A direct interaction between these two proteins was observed in vitro. Furthermore, the 289R E1A protein could inhibit the BS69-mediated decrease in transcriptional activation by c-Myb. By analogy with the inhibition of the Rb/E2F regulatory axis by E1A, we propose that a BS69/Myb regulatory circuit may also be a target of disruption during oncogenesis. Oncogene (2001) 20, 125 - 132.

c-Myb binds to a sequence in the proximal region of the RAG-2 promoter and is essential for promoter activity in T-lineage cells

The RAG-2 gene encodes a component of the V(D)J recombinase which is essential for the assembly of antigen receptor genes in B and T lymphocytes. Previously, we reported that the transcription factor BSAP (PAX-5) regulates the murine RAG-2 promoter in B-cell lines. A partially overlapping but distinct region of the proximal RAG-2 promoter was also identified as an important element for promoter activity in T cells; however, the responsible factor was unknown. In this report, we present data demonstrating that c-Myb binds to a Myb consensus site within the proximal promoter and is critical for its activity in T-lineage cells. We show that c-Myb can transactivate a RAG-2 promoter-reporter construct in cotransfection assays and that this transactivation depends on the proximal promoter Myb consensus site. By using a chromatin immunoprecipitation (ChIP) strategy, fractionation of chromatin with anti-c-Myb antibody specifically enriched endogenous RAG-2 promoter DNA sequences. DNase I genomic footprinting revealed that the c-Myb site is occupied in a tissue-specific fashion in vivo. Furthermore, an integrated RAG-2 promoter construct with mutations at the c-Myb site was not enriched in the ChIP assay, while a wild-type integrated promoter construct was enriched. Finally, this lack of binding of c-Myb to a chromosomally integrated mutant RAG-2 promoter construct in vivo was associated with a striking decrease in promoter activity. We conclude that c-Myb regulates the RAG-2 promoter in T cells by binding to this consensus c-Myb binding site.

An alternatively spliced isoform of B-Myb is a transcriptional inhibitor

B-Myb is a highly conserved member of the Myb transcription factor family. The primary transcript of the B-myb gene is spliced alternatively in two mRNAs which either contain or lack a sequence corresponding to the so-called exon 9A of c-myb. Recent studies showed that full-length B-Myb containing the exon 9A encoded amino acids is a cell cycle regulated transcription factor whose activity is stimulated by cyclin A/Cdk 2-dependent phosphorylation at the carboxyl-terminus of B-Myb. We have now investigated in more detail the transactivation potential of the shorter isoform of B-Myb lacking exon 9A. Here, we show that B-Myb lacking exon 9A has no transactivation activity even in the presence of cyclin A. This inactivity of the shorter isoform of B-Myb is not due an improper subcelluar localization. Our work suggests that B-Myb lacking exon 9A may act as an inhibitor for full-length B-Myb mediated transactivation. Furthermore, by analysing the transactivation potential of Gal4/B-Myb fusion proteins we have identified the amino-terminal part of the exon 9A as the principal transactivation domain of full-length B-Myb. The results presented here demonstrate that B-myb encodes both an activator and an inhibitor of transcription and, thus, reveal an additional level of regulation of B-Myb activity beside the known cyclin dependent mechanisms.

Isolation and characterization of the human A-myb promoter: regulation by NF-Y and Sp1

The A-myb transcription factor shows a restricted tissue distribution and is cell cycle regulated. Furthermore its deregulation has profound effects on the growth and/or differentiation of the cells in which it is normally expressed. We have therefore characterized its promoter. A 12 kb genomic clone was isolated that comprises the first exon, part of the first intron as well as upstream regulatory sequences. Multiple transcription start sites have been identified which operate in both B lymphocytes and epithelial cells and the upsteam region was shown to have promoter, activity. The boundaries of the minimal promoter region (-183-14), of a positive upstream (-538-183) and a negative downstream regulatory region (NRE) (+83+374) have been defined. The NRE is promoter- and orientation-independent but position specific. The A-myb minimal promoter is GC-rich, does not contain any TATA box but has a functional CCAAT box. The CCAAT box and minimal promoter is highly conserved in the corresponding murine sequence. The CCAAT box efficiently binds the NF-Y complex and its mutation decreases basal promoter activity by 50%. Two Sp1 binding sites are present upstream from the CCAAT box which can bind Spl and contribute to A-myb promoter activity by 70 and 30%, respectively. The two Sp1 sites and CCAAT box together contribute to over 80% of A-myb basal promoter activity and are therefore the major regulatory elements. Finally, we show that the promoter is cell cycle regulated and that the SP1 and CCAAT elements are required for S phase induction.

Chromosome 20 deletions in myeloid malignancies: reduction of the common deleted region, generation of a PAC/BAC contig and identification of candidate genes. UK Cancer Cytogenetics Group (UKCCG)

Deletion of the long arm of chromosome 20 represents the most common chromosomal abnormality associated with the myeloproliferative disorders (MPDs) and is also found in other myeloid malignancies including myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML). Previous studies have identified a common deleted region (CDR) spanning approximately 8 Mb. We have now used G-banding, FISH or microsatellite PCR to analyse 113 patients with a 20q deletion associated with a myeloid malignancy. Our results define a new MPD CDR of 2.7 Mb, an MDS/AML CDR of 2.6 Mb and a combined 'myeloid' CDR of 1.7 Mb. We have also constructed the most detailed physical map of this region to date--a bacterial clone map spanning 5 Mb of the chromosome which contains 456 bacterial clones and 202 DNA markers. Fifty-one expressed sequences were localized within this contig of which 37 lie within the MPD CDR and 20 within the MDS/AML CDR. Of the 16 expressed sequences (six genes and 10 unique ESTs) within the 'myeloid' CDR, five were expressed in both normal bone marrow and purified CD34 positive cells. These data identify a set of genes which are both positional and expression candidates for the target gene(s) on 20q.

Direct transactivation of the anti-apoptotic gene apolipoprotein J (clusterin) by B-MYB

B-MYB is a ubiquitously expressed transcription factor involved in the regulation of cell survival, proliferation, and differentiation. In an attempt to isolate B-MYB-regulated genes that may explain the role of B-MYB in cellular processes, representational difference analysis was performed in neuroblastoma cell lines with different levels of B-MYB expression. One of the genes, the mRNA levels of which were enhanced in B-MYB expressing cells, was ApoJ/Clusterin(SGP-2/TRMP-2) (ApoJ/Clusterin), previously implicated in regulation of apoptosis and tumor progression. Here we show that the human ApoJ/Clusterin gene contains a Myb binding site in its 5' flanking region, which interacts with bacterially synthesized B-MYB protein and mediates B-MYB-dependent transactivation of the ApoJ/Clusterin promoter in transient transfection assays. Endogenous ApoJ/Clusterin expression is induced in mammalian cell lines following transient transfection of a B-MYB cDNA. Blockage of secreted clusterin by a monoclonal antibody results in increased apoptosis of neuroblastoma cells exposed to the chemotherapeutic drug doxorubicin. Thus, activation of ApoJ/Clusterin by B-MYB may be an important step in the regulation of apoptosis in normal and diseased cells.

Nucleolin, a novel partner for the Myb transcription factor family that regulates their activity

To unravel the mechanisms of action of transcriptional regulation by the Myb family of transcription factors, we have set out to isolate their protein partners. We identify nucleolin as one of the nuclear polypeptides that interact specifically with the A-Myb and c-Myb, but not B-Myb DNA-binding domains. We show unambiguously that this interaction is direct and takes place in vivo, as demonstrated by co-immunoprecipitation of the endogenously and exogenously expressed proteins. The minimal DNA-binding domain containing only the R2R3 c-Myb repeats is sufficient for nucleolin binding. Computer analysis of the R2R3 three-dimensional structure, as well as extensive mutational analysis within this region, reveals that the Arg(161) residue, present in c-Myb and A-Myb, but not B-Myb, is crucial for this interaction. We show that the interaction of nucleolin with Myb is functional because co-transfection of nucleolin down-regulates Myb transcriptional activity. Nucleolin is a multifunctional phosphoprotein present in both nucleoplasm and more abundantly in the nucleolus and shows helicase and chromatin decondensing activities. This is the first demonstration of nucleolin binding to a transcription factor.

Oncolytic viruses as novel anticancer agents: turning one scourge against another

Although the use of viruses as oncolytic agents is an historic concept, the use of genetically modified viruses to selectively target tumour cells is relatively novel and recent. The ability of viruses to efficiently infect and lyse cells, combined with the potential augmentation of this effect by progeny viruses throughout the tumour provide justification for exploitation of these agents in cancer therapy. Before application to humans, though, issues related to tumour cell selectivity, lack of toxicity to normal tissues and the effect of the antiviral immune response, will have to be clarified. The more commonly used oncolytic viruses are based on mutant strains of herpes simplex virus, adenovirus and reovirus. The tumour selectivity of each of these strains is discussed, particularly the complementation of the viral defect by cellular pathways involved in tumourigenesis. The combination of oncolytic viruses with radiation, chemotherapy and gene therapy is also reviewed. Further study of the interaction of viral proteins with cellular pathways involved in cell cycle control will provide the rationale for viral mutants with increased selectivity for tumour cells.

Phosphorylation of B-Myb regulates its transactivation potential and DNA binding

The transcription factor B-Myb is a cell cycle-regulated phosphoprotein and a potent regulator of cell cycle progression. Previous studies demonstrated that B-Myb was phosphorylated at the onset of S phase, suggesting that it could be due to cyclin-dependent kinases. We identified 10 B-Myb phosphorylation sites by automated peptide radiosequencing of tryptic phosphopeptides derived from in vivo (32)P-labeled B-Myb. Each B-Myb phosphorylation site contained a phosphoserine or phosphothreonine followed by a proline, suggesting that this phosphorylation is due to a proline-directed kinase. Cyclin A-Cdk2 and cyclin E-Cdk2 complexes each phosphorylated B-Myb in a cell-free system on the same sites as in intact cells. Furthermore, the ability of B-Myb to activate a reporter plasmid was enhanced by the cotransfection of cyclin A, whereas mutagenesis of the 10 identified phosphorylation sites from B-Myb blocked the effect of cyclin A coexpression. Additional analysis revealed that the effect of phosphorylation on B-Myb transactivation potential was enhanced by phosphorylation sites in its carboxyl-terminal half. One phosphorylation site (Ser(581)) appeared to negatively regulate DNA binding, as mutation of this site enhanced the ability of B-Myb to bind a Myb-binding sequence. These data suggest that B-Myb is a target for phosphorylation by cyclin-Cdk2 and that phosphorylation of B-Myb regulates its transcriptional activity.

Transformation by v-Myb

The v-myb oncogene of the avian myeloblastosis virus (AMV) is unique among known oncogenes in that it causes only acute leukemia in animals and transforms only hematopoietic cells in culture. AMV was discovered in the 1930s as a virus that caused a disease in chickens that is similar to acute myelogenous leukemia in humans (Hall et al., 1941). This avian retrovirus played an important role in the history of cancer research for two reasons. First, AMV was used to demonstrate that all oncogenic viruses did not contain a single cancer-causing principle. In particular, although both Rous sarcoma virus (RSV) and AMV could replicate in cultures of either embryonic fibroblasts or hematopoietic cells, RSV could transform only fibroblasts whereas AMV could transform only hematopoietic cells (Baluda, 1963; Durban and Boettiger, 1981a). Second, chickens infected with AMV develop remarkably high white counts and therefore their peripheral blood contains remarkably large quantities of viral particles (Beard, 1963). For this reason AMV was often used as a prototypic retrovirus in order to study viral assembly and later to produce large amounts of reverse transcriptase for both research and commercial purposes. Following the discovery of the v-src oncogene of RSV and the demonstration that it arose from the normal c-src proto-oncogene, a number of acute leukemia viruses were analysed by similar techniques and found to also contain viral oncogenes of cellular origin (Roussel et al., 1979). In the case of AMV, it was shown that almost the entire retroviral env gene had been replaced by a sequence of cellular origin (initially called mab or amv, but later renamed v-myb) (Duesberg et al., 1980; Souza et al., 1980). Remarkably, sequences contained in this myb oncogene were shared between AMV and the avian E26 leukemia virus, but were not contained in any other acutely transforming retroviruses. In addition, the E26 virus contained a second sequence of cellular origin (ets) that was unique. The E26 leukemia virus was first described in the 1960s and causes an acute erythroblastosis in chickens, more reminiscent of the disease caused by avian erythroblastosis virus (AEV) than by AMV (Ivanov et al., 1962).