Cell-cycle regulation of human B-myb transcription

Distinct roles for Sp1 and E2F sites in the growth/cell cycle regulation of the DHFR promoter

Dihydrofolate reductase activity is required for many biosynthetic pathways including nucleotide synthesis. Its expression is therefore central to cellular growth, and it has become a key target for cancer chemotherapy. Transcription of the dihydrofolate reductase gene is regulated with growth, being expressed maximally in late G1/early S phase following serum stimulation of quiescent cells. This regulation is directed by a promoter which contains binding sites for only the transcription factors Sp1 and E2F. In this study, the role of these promoter elements in growth/cell cycle regulation of dihydrofolate transcription was addressed directly by transient transfection of Balb/c 3T3 cells with mutant promoter-reporter gene constructs. The E2F sites were found to repress transcription in G0 and early G1 but did not contribute to the level of transcription in late G1/S phase. In contrast, Sp1 sites were able to mediate induction of transcription from the dihydrofolate reductase promoter, as well as a heterologous promoter, following serum stimulation of quiescent cells. These findings add dihydrofolate reductase to a growing list of genes at which E2F sites are primarily repressive elements and delineate a role for Sp1 sites in the growth/cell cycle regulation of transcription.

Position-dependent transcriptional regulation of the murine dihydrofolate reductase promoter by the E2F transactivation domain

Activity of the dihydrofolate reductase (dhfr) promoter increases at the G1-S-phase boundary of the cell cycle. Mutations that abolish protein binding to an E2F element in the dhfr promoter also abolish the G1-S-phase increase in dhfr transcription, indicating that transcriptional regulation is mediated by the E2F family of proteins. To investigate the mechanism by which E2F regulates dhfr transcription, we moved the E2F element upstream and downstream of its natural position in the promoter. We found that the E2F element confers growth regulation to the dhfr promoter only when it is proximal to the transcription start site. Using a heterologous E2F element, we showed that position-dependent regulation is a property that is promoter specific, not E2F element specific. We demonstrated that E2F-mediated growth regulation of dhfr transcription requires activation of the dhfr promoter in S phase and that the C-terminal activation domains of E2F1, E2F4, and E2F5, when fused to the Gal4 DNA binding domain, are sufficient to specify position-dependent activation. To further investigate the role of activation in dhfr regulation, we tested other transactivation domains for their ability to activate the dhfr promoter. We found that the N-terminal transactivation domain of VP16 cannot activate the dhfr promoter. We propose that, unlike other E2F-regulated promoters, robust transcription from the dhfr promoter requires an E2F transactivation domain close to the transcription start site.

Activation of human B-MYB by cyclins

B-MYB expression is associated with cell proliferation and recent studies have suggested that it promotes the S phase of mammalian cells. Based on its homology to the transcription factors c-MYB and A-MYB, B-MYB is thought to be involved in transcriptional regulation; however, its activity is not detectable in several cell lines. It was postulated that B-MYB function may depend on the presence of a cofactor, and recent studies suggested that B-MYB is phosphorylated specifically during S phase in murine fibroblasts. In this report we provide evidence that the product of the human B-myb gene can be activated in vivo by coexpression with cyclin A or cyclin E. Transfection studies showed that B-MYB was a weak transcriptional activator in SAOS-2 cells and was unable to promote their proliferation. In contrast, overexpression of both B-MYB and cyclin A or cyclin E caused a drastic increase in the number of SAOS-2 cells in S phase. Also, overexpression of cyclin A and cyclin E in SAOS-2 cells enhanced the ability of B-MYB, but not c-MYB, to transactivate various promoters, including the cdc2 promoter, the HIV-1-LTR, and the simian virus 40 minimal promoter. A direct role for cyclin-dependent activation of B-MYB was demonstrated using an in vitro transcription assay. These observations suggest that one mechanism by which cyclin A and E may promote the S phase is through modification and activation of B-MYB.

The retinoblastoma family member p107 binds to B-MYB and suppresses its autoregulatory activity

It was recently reported that B-MYB can overcome p107-induced growth arrest. Here we show that B-MYB autoregulation of its own transcription is specifically suppressed by p107 and transient transfection assays with p107 deletion constructs determined that the carboxyl terminus of the protein, containing the major pocket region, was associated with inhibition of B-MYB-dependent transactivation. Consistent with these results, co-immunoprecipitation studies showed that p107 interacted in vivo with B-MYB through its pocket and carboxyl terminus domain. Thus, B-MYB-dependent promotion of cell proliferation and gene transactivation might be specifically repressed by the growth suppressor p107 through direct interaction with B-MYB.

A dominant interfering Myb mutant causes apoptosis in T cells

The c-Myb transcription factor is required for the production of most hemopoietic lineages, but information is sparse about its mode of action and the key genes it regulates. We have made an inducible dominant interfering Myb protein, by creating a chimera comprising the DNA binding domain of c-Myb, the Drosophila Engrailed repressor domain, and a modified estrogen receptor hormone binding domain. When expressed in the murine thymoma cell line EL4, activation of this mutant results in a significant proportion of the cell population undergoing apoptosis, as assessed by nuclear breakdown and DNA fragmentation, but has no apparent effect on cell-cycle progression. The apoptotic phenotype is mirrored during thymopoiesis in transgenic mice expressing dominant interfering Myb mutants; their T cells are fragile both in vivo and in vitro. Induction of the Myb dominant interfering mutant in EL4 cells correlates with down-regulation of bcl-2, but does not affect transcription of other bcl-2 family members; conversely, overexpression of bcl-2 in the transgenic mouse model rescues thymocytes from death. Analysis of the bcl-2 promoter by run-on transcription, bandshifting, and transient expression assays shows that it is a direct target of Myb. These data suggest a new and important role for Myb proteins as regulators of cell survival during hemopoiesis.

CDP/cut is the DNA-binding subunit of histone gene transcription factor HiNF-D: a mechanism for gene regulation at the G1/S phase cell cycle transition point independent of transcription factor E2F

Transcription of the genes for the human histone proteins H4, H3, H2A, H2B, and H1 is activated at the G1/S phase transition of the cell cycle. We have previously shown that the promoter complex HiNF-D, which interacts with cell cycle control elements in multiple histone genes, contains the key cell cycle factors cyclin A, CDC2, and a retinoblastoma (pRB) protein-related protein. However, an intrinsic DNA-binding subunit for HiNF-D was not identified. Many genes that are up-regulated at the G1/S phase boundary are controlled by E2F, a transcription factor that associates with cyclin-, cyclin-dependent kinase-, and pRB-related proteins. Using gel-shift immunoassays, DNase I protection, and oligonucleotide competition analyses, we show that the homeodomain protein CDP/cut, not E2F, is the DNA-binding subunit of the HiNF-D complex. The HiNF-D (CDP/cut) complex with the H4 promoter is immunoreactive with antibodies against CDP/cut and pRB but not p107, whereas the CDP/cut complex with a nonhistone promoter (gp91-phox) reacts only with CDP and p107 antibodies. Thus, CDP/cut complexes at different gene promoters can associate with distinct pRB-related proteins. Transient coexpression assays show that CDP/cut modulates H4 promoter activity via the HiNF-D-binding site. Hence, DNA replication-dependent histone H4 genes are regulated by an E2F-independent mechanism involving a complex of CDP/cut with cyclin A/CDC2/ RB-related proteins.

B-myb promotes S phase and is a downstream target of the negative regulator p107 in human cells

The retinoblastoma protein family has been implicated in growth control and modulation of the activity of genes involved in cell proliferation, such as B-myb. Recent evidence indicates that the product of the B-myb gene is necessary for the growth and survival of several human and murine cell lines. Upon overexpression, B-myb induces deregulated cell growth of certain cell lines. Here we show that B-myb overexpression is able to induce DNA synthesis in p107 growth-arrested human osteosarcoma cells (SAOS2). p107 might exert its growth-suppressive activity by regulating B-myb gene transcription. Indeed, p107 down-modulated B-myb promoter activity and drastically decreased E2F-mediated transactivation. Finally, B-myb was able to stimulate DNA synthesis of both stably and transiently transfected human glioblastoma cells (T98G). Altogether, these data provide definitive evidence that the human B-myb protein is involved in growth control of human cells, and that p107 has a significant role in regulating B-myb gene activity.