Regulation of E2F transcription by cyclin E-Cdk2 kinase mediated through p300/CBP co-activators

Dysregulation of CREB binding protein triggers thrombin-induced proliferation of vascular smooth muscle cells

Thrombin is a potent mitogen for vascular smooth muscle cells (VSMCs). CBP has been regarded as a potential therapeutic target on the basis of its ability to affect cell growth. Therefore we hypothesized that CBP mediates thrombin-induced proliferation of VSMCs. We constructed recombinant adenoviral vector that expresses four short hairpin RNA (shRNA) targeting rat CBP mRNA (CBP-shRNA/Ad). VSMCs were infected with CBP-shRNA/Ad and treated with thrombin. CBP level were analyzed by quantitative real-time PCR and Western blot. To evaluate VSMC proliferation, the cell cycle and DNA synthesis were analyzed by flow cytometry and (3)H-thymidine incorporation, respectively. CBP-shRNA/Ad infection inhibited thrombin-induced CBP expression in a dose-dependent manner concomitant with a decrease in the percentage of cells in the S phase and in DNA synthesis. These findings suggest that CBP plays a pivotal role in the S phase progression of VSMCs.

p300 expression repression by hypermethylation associated with tumour invasion and metastasis in oesophageal squamous cell carcinoma

BACKGROUND

Aberrant promoter methylation is an important mechanism for gene silencing.

AIMS

To evaluate the promoter methylation status of p300 gene in patients with oesophageal squamous cell carcinoma (OSCC).

METHODS

The methylation status of p300 promoter was analysed by methylation-specific PCR (MSP) in 50 OSCC tissues and the matching non-cancerous tissues. Oesophageal cancer cell lines (ECa-109 and TE-10) were treated with the demethylation agent 5-aza-2'-deoxycytidine (5-Aza-CdR), and p300 mRNA expression was detected by RT-PCR.

RESULTS

p300 methylation was found in 42% (21/50) of the OSCC tissues, but in only 20% (10/50) of the corresponding non-cancerous tissues (p = 0.017). In OSCC samples, 65% of those with deep tumour invasion (adventitia) and 63% samples with metastasis revealed p300 promoter methylation (p<0.05). p300 mRNA expression was observed in 19.0% (4/21) of methylated tumours and 58.6% (17/29) of unmethylated tumours (p = 0.005). In addition, p300 mRNA expression was observed in 40% (4/10) of methylated non-neoplastic tissues and 87.5% (35/40) of unmethylated non-tumours (p = 0.001). The demethylation caused by 5-Aza-CdR increased the p300 mRNA expression levels in oesophageal cancer cell lines.

CONCLUSIONS

p300 transcription silenced by promoter hypermethylation could play a role in the pathogenesis of oesophageal squamous cell carcinoma.

C/EBPbeta activates E2F-regulated genes in vivo via recruitment of the coactivator CREB-binding protein/P300

The E2F transcription factors play an essential role in regulating the G(1)- to S-phase transition of the cell cycle. Previous studies have identified the importance of interactions between E2Fs and other transcription factors as a mechanism for transcriptional control of a subset of E2F regulated target genes. However, the mechanisms responsible for E2F target gene specificity remain incompletely understood. Here we report that in a mammalian in vivo model of synchronized proliferation, C/EBPbeta occupancy on the promoters of E2F-regulated growth-related genes increases as a function of cell cycle progression. C/EPBbeta binding to these promoters is associated with recruitment of the coactivator CBP/p300, histone H4 acetylation, and maximal activation of E2F target genes. Moreover, binding of CBP/p300 to E2F targets is markedly reduced in C/EBPbeta null mice, resulting in reduced expression of E2F regulated genes. These findings identify C/EBPbeta as a direct activator of E2F target genes in mammalian cell cycle progression through a mechanism that involves recruitment of CBP/p300. The demonstration of a functional link between C/EBPbeta and CBP/p300 for E2F target gene activation provides a potential mechanism for how coactivators such as CBP/p300 can be selectively recruited to E2F target genes in response to tissue-specific growth stimuli.

Reprogramming of the SWI/SNF complex for co-activation or co-repression in prohibitin-mediated estrogen receptor regulation

The SWI/SNF complex participates as a co-activator in the transcriptional regulation of certain genes. Conversely, we and others have recently established that Brg1 and Brm, the central components of SWI/SNF, act instead as co-repressors for E2F-mediated transcriptional repression, and for the transcription of certain other promoters. We report here that Brg-1 and Brm can switch their mode of function at same promoter between activation and repression by ligand-directed differential coordination with BAF155, BAF170, HDAC1, p300 and prohibitin. This ligand and context-dependent reprogramming of the SWI/SNF complex allows it to differentially serve as either a co-repressor or a co-activator of transcription at the same promoter.

Differential expression of members of the E2F family of transcription factors in rodent testes

BACKGROUND

The E2F family of transcription factors is required for the activation or repression of differentially expressed gene programs during the cell cycle in normal and abnormal development of tissues. We previously determined that members of the retinoblastoma protein family that interacts with the E2F family are differentially expressed and localized in almost all the different cell types and tissues of the testis and in response to known endocrine disruptors. In this study, the cell-specific and stage-specific expression of members of the E2F proteins has been elucidated.

METHODS

We used immunohistochemical (IHC) analysis of tissue sections and Western blot analysis of proteins, from whole testis and microdissected stages of seminiferous tubules to study the differential expression of the E2F proteins.

RESULTS

For most of the five E2F family members studied, the localizations appear conserved in the two most commonly studied rodent models, mice and rats, with some notable differences. Comparisons between wild type and E2F-1 knockout mice revealed that the level of E2F-1 protein is stage-specific and most abundant in leptotene to early pachytene spermatocytes of stages IX to XI of mouse while strong staining of E2F-1 in some cells close to the basal lamina of rat tubules suggest that it may also be expressed in undifferentiated spermatogonia. The age-dependent development of a Sertoli-cell-only phenotype in seminiferous tubules of E2F-1 knockout males corroborates this, and indicates that E2F-1 is required for spermatogonial stem cell renewal. Interestingly, E2F-3 appears in both terminally differentiated Sertoli cells, as well as spermatogonial cells in the differentiative pathway, while the remaining member of the activating E2Fs, E2F-2 is most concentrated in spermatocytes of mid to late prophase of meiosis. Comparisons between wildtype and E2F-4 knockout mice demonstrated that the level of E2F-4 protein displays a distinct profile of stage-specificity compared to E2F-1, which is probably related to its prevalence and role in Sertoli cells. IHC of rat testis indicates that localization of E2F-5 is distinct from that of E2F-4 and overlaps those of E2F-1 and E2F-2.

CONCLUSION

The E2F-1 represents the subfamily of transcription factors required during stages of DNA replication and gene expression for development of germ cells and the E2F-4 represents the subfamily of transcription factors that help maintain gene expression for a terminally differentiated state within the testis.

Cannabinoid Receptor Agonist-induced Apoptosis of Human Prostate Cancer Cells LNCaP Proceeds through Sustained Activation of ERK1/2 Leading to G1 Cell Cycle Arrest

We have recently shown that the expression levels of both cannabinoid receptors CB(1) and CB(2) are higher in human prostate cancer cells than in normal prostate epithelial cells, and treatment of LNCaP cells with WIN-55,212-2 (a mixed CB(1)/CB(2) agonist) resulted in inhibition of cell growth and induction of apoptosis (Sarfaraz, S., Afaq, F., Adhami, V. M., and Mukhtar, H. (2005) Cancer Res. 65, 1635-1641). This study was conducted to understand the mechanistic basis of these effects. Treatment of LNCaP cells with WIN-55,212-2 (1-10 microm; 24 h) resulted in: (i) an arrest of the cells in the G(0)/G(1) phase of the cell cycle; (ii) an induction of p53 and p27/KIP1; (iii) down-regulation of cyclins D1, D2, E; (iii) decrease in the expression of cdk-2, -4, and -6; (iv) decrease in protein expression of pRb; (v) down-regulation of E2F (1-4); and (vi) decrease in the protein expression of DP1 and DP2. Similar effects were also observed when androgen-independent PC3 cells were treated with WIN-55,212-2 (5-30 microm). We further observed sustained up-regulation of ERK1/2 and inhibition of PI3k/Akt pathways in WIN-55,212-2-treated cells. Inhibition of ERK1/2 abrogated WIN-55,212-2-indued cell death suggesting that sustained activation of ERK1/2 leads to cell cycle dysregulation and arrest of cells in G(0)/G(1) phase subsequently leading to an induction of apoptosis. Further, WIN-55,212-2 treatment of cells resulted in a dose-dependent increase in Bax/Bcl-2 ratio in such a way that favors apoptosis. The induction of apoptosis proceeded through down-regulation of caspases 3, 6, 7, and 9 and cleavage of poly (ADP-ribose) polymerases. Based on these data we suggest that cannabinoid receptor agonists should be considered as novel agents for the management of prostate cancer.

CDCA4 is an E2F transcription factor family-induced nuclear factor that regulates E2F-dependent transcriptional activation and cell proliferation

The TRIP-Br1/p34(SEI-1) family proteins participate in cell cycle progression by coactivating E2F1- or p53-dependent transcriptional activation. Here, we report the identification of human CDCA4 (also know as SEI-3/Hepp) as a novel target gene of transcription factor E2F and as a repressor of E2F-dependent transcriptional activation. Analysis of CDCA4 promoter constructs showed that an E2F-responsive sequence in the vicinity of the transcription initiation site is necessary for the E2F1-4-induced activation of CDCA4 gene transcription. Chromatin immunoprecipitation analysis demonstrated that E2F1 and E2F4 bound to an E2F-responsive sequence of the human CDCA4 gene. Like TRIP-Br1/p34(SEI-1) and TRIP-Br2 (SEI-2), the transactivation domain of CDCA4 was mapped within C-terminal acidic region 175-241. The transactivation function of the CDCA4 protein was inhibited by E2F1-4 and DP2, but not by E2F5-8. Inhibition of CDCA4 transactivation activity by E2F1 partially interfered with retinoblastoma protein overexpression. Conversely, CDCA4 suppressed E2F1-3-induced reporter activity. CDCA4 (but not acidic region-deleted CDCA4) suppressed E2F1-regulated gene promoter activity. These findings suggest that the CDCA4 protein functions as a suppressor at the E2F-responsive promoter. Small interfering RNA-mediated knockdown of CDCA4 expression in cancer cells resulted in up-regulation of cell growth rates and DNA synthesis. The CDCA4 protein was detected in several human cells and was induced as cells entered the G1/S phase of the cell cycle. Taken together, our results suggest that CDCA4 participates in the regulation of cell proliferation, mainly through the E2F/retinoblastoma protein pathway.

The transcriptional regulation role of BRD7 by binding to acetylated histone through bromodomain

Studies showed that the bromodomain binds to acetyl-lysines on histone tails, which is involved in deciphering the histone codes. BRD7, a novel bromodomain gene, is the first described bromodomain gene involved in nasopharyngeal carcinoma (NPC). Previous studies showed that ectopic expression of BRD7 inhibited cell growth and cell cycle progression from G1 to S phase in HNE1 cells (a NPC cell line) by transcriptionally regulating some cell cycle related genes including E2F3 gene. In the present study, we revealed the co-localization between acetylated H3 and BRD7 and found that the bromodomain of BRD7 is required for this co-localization. More importantly, wild-type BRD7 interacted with H3 peptide acetylated at Lys14, while the bromodomain deleted mutant lost this ability. We also found that the mutant BRD7 failed to regulate E2F3 promoter activity and inhibit cell cycle progression. These results indicated that the transcriptional regulation role of BRD7 was achieved by binding to acetylated histone H3 and that the bromodomain was essential for this role. In addition, no obvious changes were observed in the acetylated level of histone H3 after transfection with BRD7, indicating that chromatin remodeling, not chromatin modification, is the major mechanism of BRD7 mediated gene transcription. Taken together, the present work shed light on the fact that a novel bromodomain gene, BRD7, is of importance in transcriptional regulation and cellular events including cell cycle.

Conditional knockout mice reveal distinct functions for the global transcriptional coactivators CBP and p300 in T-cell development

The global transcriptional coactivators CREB-binding protein (CBP) and the closely related p300 interact with over 312 proteins, making them among the most heavily connected hubs in the known mammalian protein-protein interactome. It is largely uncertain, however, if these interactions are important in specific cell lineages of adult animals, as homozygous null mutations in either CBP or p300 result in early embryonic lethality in mice. Here we describe a Cre/LoxP conditional p300 null allele (p300flox) that allows for the temporal and tissue-specific inactivation of p300. We used mice carrying p300flox and a CBP conditional knockout allele (CBPflox) in conjunction with an Lck-Cre transgene to delete CBP and p300 starting at the CD4- CD8- double-negative thymocyte stage of T-cell development. Loss of either p300 or CBP led to a decrease in CD4+ CD8+ double-positive thymocytes, but an increase in the percentage of CD8+ single-positive thymocytes seen in CBP mutant mice was not observed in p300 mutants. T cells completely lacking both CBP and p300 did not develop normally and were nonexistent or very rare in the periphery, however. T cells lacking CBP or p300 had reduced tumor necrosis factor alpha gene expression in response to phorbol ester and ionophore, while signal-responsive gene expression in CBP- or p300-deficient macrophages was largely intact. Thus, CBP and p300 each supply a surprising degree of redundant coactivation capacity in T cells and macrophages, although each gene has also unique properties in thymocyte development.

The microarchitecture of DNA replication domains

Most DNA synthesis in HeLa cell nucleus is concentrated in discrete foci. These synthetic sites can be identified by electron microscopy after allowing permeabilized cells to elongate nascent DNA in the presence of biotin-dUTP. Biotin incorporated into nascent DNA can be then immunolabeled with gold particles. Two types of DNA synthetic sites/replication factories can be distinguished at ultrastructural level: (1) electron-dense structures--replication bodies (RB), and (2) focal replication sites with no distinct underlying structure--replication foci (RF). The protein composition of these synthetic sites was studied using double immunogold labeling. We have found that both structures contain (a) proteins involved in DNA replication (DNA polymerase alpha, PCNA), (b) regulators of the cell cycle (cyclin A, cdk2), and (c) RNA processing components like Sm and SS-B/La auto antigens, p80-coilin, hnRNPs A1 and C1/C2. However, at least four regulatory and structural proteins (Cdk1, cyclin B1, PML and lamin B1) differ in their presence in RB and RF. Moreover, in contrast to RF, RB have structural organization. For example, while DNA polymerase alpha, PCNA and hnRNP A1 were diffusely spread throughout RB, hnRNP C1/C2 was found only at the very outside. Surprisingly, RB contained only small amounts of DNA. In conclusion, synthetic sites of both types contain similar but not the same sets of proteins. RB, however, have more developed microarchitecture, apparently with specific functional zones. This data suggest possible differences in genome regions replicated by these two types of replication factories.

Tumour suppressor retinoblastoma protein Rb: a transcriptional regulator

Rb was the first tumour suppressor identified through human genetic studies. The most significant achievement after almost twenty years since its cloning is the revelation that Rb possesses functions of a transcription regulator. Rb serves as a transducer between the cell cycle machinery and promoter-specific transcription factors. In this capacity, Rb is best known as a repressor of the E2F/DP family of transcription factors, which regulate expression of genes involved in cell proliferation and survival. An equally important aspect of Rb as a transcription regulator is that Rb also activates certain differentiation transcription factors to promote cellular differentiation. The molecular mechanisms behind the repressive effects of Rb on E2Fs have come to light in significant details, while those relating to Rb activation of differentiation transcription factors are much less understood. Finally, it has become clear that there are other aspects to Rb function that are not immediately related to transcription regulation.

Probing lysine acetylation in proteins: strategies, limitations, and pitfalls of in vitro acetyltransferase assays

The acetylation of proteins at specific lysine residues by acetyltransferase enzymes has emerged as a posttranslational modification of high biological impact. Although lysine acetylation in histone proteins is an integral part of the histone code the acetylation of a multitude of non-histone proteins was recently recognized as a regulatory signal in many cellular processes. New substrates of acetyltransferase enzymes are continuously identified, and the analysis of acetylation sites in proteins is increasingly performed by mass spectrometry. However, the characterization of lysine acetylation in proteins using mass spectrometric techniques has some limitations and pitfalls. The non-enzymatic cysteine acetylation especially can result in false-positive identification of acetylated proteins. Here we demonstrate the application of various mass spectrometric techniques such as matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry for the analysis of protein acetylation. We describe diverse combinations of biochemical methods useful to map the acetylation sites in proteins and discuss their advantages and limitations. As an example, we present a detailed analysis of the acetylation of the HIV-1 transactivator of transcription (Tat) protein, which is known to be acetylated in vivo by the acetyltransferases p300 and p300/CBP-associated factor (PCAF).

RNA silencing of checkpoint regulators sensitizes p53-defective prostate cancer cells to chemotherapy while sparing normal cells

p53 is frequently mutated in patients with prostate cancer, especially in those with advanced disease. Therefore, the selective elimination of p53 mutant cells will likely have an impact in the treatment of prostate cancer. Because p53 has important roles in cell cycle checkpoints, it has been anticipated that modulation of checkpoint pathways should sensitize p53-defective cells to chemotherapy while sparing normal cells. To test this idea, we knocked down ataxia telangiectasia mutated (ATM) gene by RNA interference in prostate cancer cell lines and in normal human diploid fibroblasts IMR90. ATM knockdown in p53-defective PC3 prostate cancer cells accelerated their cell cycle transition, increased both E2F activity and proliferating cell nuclear antigen expression, and compromised cell cycle checkpoints, which are normally induced by DNA damage. Consequently, PC3 cells were sensitized to the killing effects of the DNA-damaging drug doxorubicin. Combining ATM knockdown with the Chk1 inhibitor UCN-01 further increased doxorubicin sensitivity in these cells. In contrast, the same strategy did not sensitize either IMR90 or LNCaP prostate cancer cells, both of which have normal p53. However, IMR90 and LNCaP cells became more sensitive to doxorubicin or doxorubicin plus UCN-01 when both p53 and ATM functions were suppressed. In addition, knockdown of the G(2) checkpoint regulators ATR and Chk1 also sensitized PC3 cells to doxorubicin and increased the expression of the E2F target gene PCNA. Together, our data support the concept of selective elimination of p53 mutant cells by combining DNA damage with checkpoint inhibitors and suggest a novel mechanistic insight into how such treatment may selectively kill tumor cells.

Cyclin E in normal and neoplastic cell cycles

Cyclin E-Cdk2 has long been considered an essential and master regulator of progression through G1 phase of the cell cycle. Although recent mouse models have prompted a rethinking of cyclin E function in mammals, it remains clear that cyclin E impacts upon many processes central to cell division. Normal cells maintain strict control of cyclin E activity, and this is commonly disrupted in cancer cells. Moreover, cyclin E deregulation is thought to play a fundamental role in tumorigenesis. In this review, we discuss the regulation and functions of cyclin E in normal and neoplastic mammalian cells.

Cell cycle in mouse development

Mice likely represent the most-studied mammalian organism, except for humans. Genetic engineering in embryonic stem cells has allowed derivation of mouse strains lacking particular cell cycle proteins. Analyses of these mutant mice, and cells derived from them, facilitated the studies of the functions of cell cycle apparatus at the organismal and cellular levels. In this review, we give some background about the cell cycle progression during mouse development. We next discuss some insights about in vivo functions of the cell cycle proteins, gleaned from mouse knockout experiments. Our text is meant to provide examples of the recent experiments, rather than to supply an extensive and complete list.

Cyclin E

E-type cyclins (cyclin E1 and cyclin E2) are expressed during the late G1 phase of the cell cycle until the end of the S-phase. The activity of cyclin E is limiting for the passage of cells through the restriction point "R" which marks a "point of no return" for cells entering the division cycle from a resting state or passing from G1 into S-phase. Expression of cyclin E is regulated on the level of gene transcription mainly by members of the E2F trrnscription factor family and by its degradation via the proteasome pathway. Cyclin E binds and activates the kinase Cdk2 and by phosphorylating its substrates, the so-called "pocket proteins", the cyclic/Cdk2 complexes initiate a cascade of events that leads to the expression of S-phase specific genes. Aside from this specific function as a regulator of S-phase-entry, cyclin E plays a direct role in the initiation of DNA replication, the control of genomic stability, and the centrosome cycle. Surprisingly, recent studies have shown that the once thought essential cyclin E is dispensable for the development of higher eukaryotes and for the mitotic division of eukaryotic cells. Nevertheless, high level cyclin E expression has been associated with the initiation or progression of different human cancers, in particular breast cancer but also leukemia, lymphoma and others. Transgenic mouse models in which cyclin E is constitutively expressed develop malignant diseases, supporting the notion of cyclin E as a dominant onco-protein.

Synthetic lethality of retinoblastoma mutant cells in the Drosophila eye by mutation of a novel peptidyl prolyl isomerase gene

Mutations that inactivate the retinoblastoma (Rb) pathway are common in human tumors. Such mutations promote tumor growth by deregulating the G1 cell cycle checkpoint. However, uncontrolled cell cycle progression can also produce new liabilities for cell survival. To uncover such liabilities in Rb mutant cells, we performed a clonal screen in the Drosophila eye to identify second-site mutations that eliminate Rbf(-) cells, but allow Rbf(+) cells to survive. Here we report the identification of a mutation in a novel highly conserved peptidyl prolyl isomerase (PPIase) that selectively eliminates Rbf(-) cells from the Drosophila eye.

Dynamic interaction of p220(NPAT) and CBP/p300 promotes S-phase entry

Cajal bodies contain cyclin E/cdk2 and the substrate p220(NPAT) to regulate the transcription of histones, which is essential for cell proliferation, however, recent mouse knockout studies indicate that cyclin E and cdk2 are dispensable for these events. Because the CBP/p300 histone acetyltransferase are also known to be involved in cell proliferation, we examined the molecular and functional interactions of p220(NPAT) with the CBP/p300 at the G1/S boundary as cell cycle regulators. The subnuclear localization of p220(NPAT) and CBP/p300 proteins showed that their foci partially overlapped in a cell cycle dependent manner. Overexpression of p220(NPAT) and CBP/p300 cooperatively enhanced G1/S transition and DNA synthesis even without cdk2 phosphorylation site. Finally, molecular alignment analysis indicated that p220(NPAT) contains several potential substrate sites for CBP/p300. Overall, our findings demonstrate that p220(NPAT) and CBP/p300 form a transient complex at the G1/S boundary to play cooperative roles to promote the S-phase entry.

Molecular mechanisms of E2F-dependent activation and pRB-mediated repression

Alterations in transcription of genes regulated by members of the E2F family of transcription factors can be viewed as a measure of the ebb and flow in a constantly evolving battle between repressor and activator complexes. Various chromatin regulatory complexes have been linked to Rb/E2F proteins, and changes in histone modifications correlate with states of E2F-dependent transcription. E2F has traditionally been viewed in the context of cell-cycle control. However, several recent studies have revealed a new aspect of E2F function in which pRB/E2F-family proteins confer stable repression of transcription. Such repression is evident in both actively proliferating cells and in cells that have withdrawn from the cell cycle.

Ras/mitogen-activated protein kinase signaling activates Ets-1 and Ets-2 by CBP/p300 recruitment

Cell signaling affects gene expression by regulating the activity of transcription factors. Here, we report that mitogen-activated protein kinase (MAPK) phosphorylation of Ets-1 and Ets-2, at a conserved site N terminal to their Pointed (PNT) domains, resulted in enhanced transactivation by preferential recruitment of the coactivators CREB binding protein (CBP) and p300. We discovered this phosphorylation-augmented interaction in an unbiased affinity chromatography screen of HeLa nuclear extracts by using either mock-treated or ERK2-phosphorylated ETS proteins as ligands. Binding between purified proteins demonstrated a direct interaction. Both the phosphoacceptor site, which lies in an unstructured region, and the PNT domain were required for the interaction. Minimal regions that were competent for induced CBP/p300 binding in vitro also supported MAPK-enhanced transcription in vivo. CBP coexpression potentiated MEK1-stimulated Ets-2 transactivation of promoters with Ras-responsive elements. Furthermore, CBP and Ets-2 interacted in a phosphorylation-enhanced manner in vivo. This study describes a distinctive interface for a transcription factor-coactivator complex and demonstrates a functional role for inducible CBP/p300 binding. In addition, our findings decipher the mechanistic link between Ras/MAPK signaling and two specific transcription factors that are relevant to both normal development and tumorigenesis.

Transactivation of E2F-regulated genes by polyomavirus large T antigen: evidence for a two-step mechanism

Polyomavirus large T antigen transactivates a variety of genes whose products are involved in S phase induction. These genes are regulated by the E2F family of transcription factors, which are under the control of the pocket protein retinoblastoma protein and its relatives p130 and p107. The viral protein causes a dissociation of E2F-pocket protein complexes that results in transactivation of the genes. This reaction requires the N-terminal binding site for pocket proteins and the J domain that binds chaperones. We found earlier that a mutation of the zinc finger located within the C-terminal domain, a region assumed to function mainly in the replication of viral DNA, also interferes with transactivation. Here we show that binding of the histone acetyltransferase coactivator complex CBP/p300-PCAF to the C terminus correlates with the ability of large T antigen to transactivate genes. This interaction results in promoter-specific acetylation of histones. Inactive mutant proteins with changes within the C-terminal domain were nevertheless able to dissociate the E2F pocket protein complexes, indicating that this dissociation is a necessary but insufficient step in the T antigen-induced transactivation of genes. It has to be accompanied by a second step involving the T antigen-mediated recruitment of a histone acetyltransferase complex.

E2F-7: a distinctive E2F family member with an unusual organization of DNA-binding domains

The E2F family of transcription factors play an important role in regulating cell cycle progression. We report here the characterization and functional properties of a new member of the human E2F family, referred to as E2F-7. E2F-7 has two separate DNA-binding domains, a feature that distinguishes E2F-7 from other mammalian E2F proteins, but resembling the organization of recently isolated E2F-like proteins from Arabidopsis. E2F-7 binds to DNA independently of a DP partner and delays cell cycle progression. Interestingly, E2F-7 modulates the transcription properties of other E2F proteins. A mutational analysis indicates that the integrity of both DNA-binding domains is required for cell cycle delay and transcriptional modulation. Biochemical results and protein modelling studies suggest that in binding to DNA interactions occur between the two DNA-binding domains, most probably as a homodimer, thereby mimicking the organization of an E2F/DP heterodimer. These structural and functional properties of E2F-7 imply a unique role in regulating cellular proliferation.

Forkhead box M1B transcriptional activity requires binding of Cdk-cyclin complexes for phosphorylation-dependent recruitment of p300/CBP coactivators

Previous liver regeneration studies demonstrated that the mouse forkhead box M1B (FoxM1B) transcription factor regulates hepatocyte proliferation through expression of cell cycle genes that stimulate cyclin-dependent kinase 2 (Cdk2) and Cdk1 activity. In this study, we demonstrated that disruption of the FoxM1B Cdk1/2 phosphorylation site at Thr residue 596 significantly reduced both FoxM1B transcriptional activity and Cdk phosphorylation of the FoxM1B T596A mutant protein in vivo. Retention of this FoxM1B 596 Cdk phosphorylation site was found to be essential for recruiting the histone acetyltransferase CREB binding protein (CBP) to the FoxM1B transcriptional activation domain. Consistent with these findings, dominant negative Cdk1 protein significantly reduced FoxM1B transcriptional activity and inhibited FoxM1B recruitment of the CBP coactivator protein. Likewise, Cdc25B-mediated stimulation of Cdk activity together with elevated levels of the CBP coactivator protein provided a 6.2-fold synergistic increase in FoxM1B transcriptional activity. Furthermore, mutation of the FoxM1B Leu 641 residue within an LXL motif (residues 639 to 641) inhibited recruitment of Cdk-cyclin complexes and caused significant reduction in both FoxM1B transcriptional activity and in vivo Cdk phosphorylation of the FoxM1B Thr 596 residue. We demonstrated that FoxM1B transcriptional activity requires binding of either S-phase or M-phase Cdk-cyclin complexes to mediate efficient Cdk phosphorylation of the FoxM1B Thr 596 residue, which is essential for recruitment of p300/CBP coactivator proteins.

Strategic attack on host cell gene expression during adenovirus infection

To understand the interaction between the virus and its host, we used three sources of cDNA microarrays to examine the expression of 12,309 unique genes at 6 h postinfection of HeLa cells with high multiplicities of adenovirus type 2. Seventy-six genes with significantly changed expression ratios were identified, suggesting that adenovirus only modulates expression of a limited set of cellular genes. Quantitative real-time PCR analyses on selected genes were performed to confirm the microarray results. Significantly, a pronounced transcriptional activation by the promiscuous E1A-289R transcriptional activator was not apparent. Instead, promoter sequences in 45% of the upregulated genes harbored a potential E2F binding site, suggesting that the ability of the amino-terminal domain of E1A to regulate E2F-dependent transcription may be a major pathway for regulation of cellular gene expression. CDC25A was the only upregulated gene directly involved in cell cycle control. In contrast, several genes implicated in cell growth arrest were repressed. The transforming growth factor beta superfamily was specifically affected in the expression of both the upstream ligand and an intracellular regulator. In agreement with previous reports, adenovirus also targeted the innate immune response by downregulating several cytokines, including CLL2, CXCL1, and interleukin-6. Finally, stress response genes encoding GADD45B, ATF3, and TP53AP1 were upregulated. Importantly, we also found a novel countermeasure-activation of the apoptosis inhibitor survivin.

Direct activation of cyclin-dependent kinase 2 by human papillomavirus E7

Addition of human papillomavirus (HPV) E7 CDK2/cyclin A or CDK2/cyclin E, purified from either insect cells or bacteria, dramatically upregulates histone H1 kinase activity. Activation is substrate specific, with a smaller effect noted for retinoblastoma protein (Rb). The CDK2 stimulatory activity is equivalent in high-risk (HPV type 16 [HPV16] and HPV31) and low-risk (HPV6b) E7. Mutational analyses of HPV16 E7 indicate that the major activity resides in amino acids 9 to 38, spanning CR1 and CR2, and does not require casein kinase II or Rb-binding domain functions. Synthetic peptides spanning HPV16 amino acid residues 9 to 38 also activate CDK2. Peptides containing this sequence that carry biotin on the carboxy terminus, as well as a photoactivated cross-linking group (benzophenone), also activate the complex and covalently associate with the CDK2/cyclin A complex in a specific manner requiring UV. Cross-linking studies that use protein monomers detect association of the E7 peptides with cyclin A but not CDK2. Together, our results indicate a novel mechanism whereby E7 promotes HPV replication by directly altering CDK2 activity and substrate specificity.

Cyclin-dependent kinase 2 is essential for meiosis but not for mitotic cell division in mice

We targeted the locus encoding the cyclin-dependent kinase 2 (CDK2) by homologous recombination in mouse embryonic stem (ES) cells. Embryonic fibroblasts lacking CDK2 proliferate normally and become immortal after continuous passage in culture. Elimination of a conditional Cdk2 allele in immortal cells does not have a significant effect on proliferation. Cdk2-/- mice are viable and survive for up to two years, indicating that CDK2 is also dispensable for proliferation and survival of most cell types. But CDK2 is essential for completion of prophase I during meiotic cell division in male and female germ cells, an unforeseen role for this cell cycle kinase.

Gene and protein expression profiling during differentiation of neuroblastoma cells triggered by 13-cis retinoic acid

PURPOSE

The precise changes in RNA and protein expression that accompany neuroblastoma differentiation remain unknown. The authors used microarray technologies to screen molecules associated with the differentiation of neuroblastoma (NB) cells induced by 13-cis retinoic acid.

METHODS

The authors quantified the expression of 2,061 RNA transcripts related to oncogenesis and of 380 proteins expressed in SK-N-SH and CHP-134 NB cell lines in the presence or absence of 13-cis retinoic acid.

RESULTS

Hierarchical clustering captured gene expression altered during neuroblastoma differentiation induced by 13-cis retinoic acid. Several genes were further abstracted based on P values below 0.0017 or protein chips observed in both NB cell lines. The altered expressions of gene products revealed by both DNA and protein chips were in agreement. The expressions of N-myc, cyclin D3, and Wnt10B were downregulated, whereas those of retinoblastoma (RB) and related genes (p107, RB2/p130, p300/CBP, E2F-1, DP-1) as well as others were upregulated.

CONCLUSIONS

These results suggest that microarray technology can screen for genes that are important in neuroblastoma differentiation.

Dynamic recruitment of NF-Y and histone acetyltransferases on cell-cycle promoters

Regulation of transcription during the cell-cycle is under the control of E2 factors (E2Fs), often in cooperation with nuclear factor Y (NF-Y), a histone-like CCAAT-binding trimer. NF-Y is paradigmatic of a constitutive, ubiquitous factor that pre-sets the promoter architecture for other regulatory proteins to access it. We analyzed the recruitment of NF-Y, E2F1/4/6, histone acetyltransferases, and histone deacetylase (HDAC) 1/3/4 to several cell-cycle promoters by chromatin immunoprecipitation assays in serum-starved and restimulated NIH3T3 cells. NF-Y binding is not constitutive but timely regulated in all promoters tested, being displaced when promoters are repressed. p300 association correlates with activation, and it is never found in the absence of NF-Y, whereas PCAF/hGCN5 is often found before NF-Y association. E2F4 and E2F6, together with HDACs, are bound to repressed promoters, including the G2/M Cyclin B2. As expected, an inverse relationship between HDACs association and histones H3/H4 acetylation is observed. Blocking cells in G1 with the cyclin-dependent kinase 2 inhibitor R-roscovitine confirms that NF-Y is bound to G1/S but not to G2/M promoters in G1. These data indicate that following the release of E2Fs/HDACs, a hierarchy of PCAF-NF-Y-p300 interactions and H3-H4 acetylations are required for activation of cell-cycle promoters.

Identification of distinct and common gene expression changes after oxidative stress and gamma and ultraviolet radiation

The human genome is exposed to many different kinds of DNA-damaging agents. While most damage is detected and repaired through complex damage recognition and repair machineries, some damage has the potential to escape these mechanisms. Unrepaired DNA damage can give rise to alterations and mutations in the genome in an individual cell, which can result in malignant transformation, especially when critical genes are deregulated. In this study, we investigated gene expression changes in response to oxidative stress, gamma (gamma) radiation, and ultraviolet (UV) radiation and their potential implications in cancer development. Doses were selected for each of the three treatments, based on their ability to cause a similar G(1) checkpoint induction and slow down in early S-phase progression, as reflected by a comparable reduction in cyclin E-associated kinase activity of at least 75% in logarithmically growing human dermal diploid fibroblasts. To investigate gene expression changes, logarithmically growing dermal diploid fibroblasts were exposed to either gamma radiation (5 Gy), oxidative stress (75 microM of tert-butyl hydroperoxide (t-butyl-OOH)), or UV radiation (UVC) (7.5 J/m(2)) and RNA was harvested 6 h after treatment. Gene expression was analyzed using the NIEHS Human ToxChip 2.0 with approximately 1901 cDNA clones representing known genes and expressed sequence tags (ESTs). We were able to identify common and distinct responses in dermal diploid fibroblasts to the three different stimuli used. Within our analysis, gene expression profiles in response to gamma radiation and oxidative stress appeared to be more similar than profiles expressed after UV radiation. Interestingly, equivalent cyclin E-associated kinase activity reduction with all the three treatments was associated with greater transcriptional changes after UV radiation than after gamma radiation and oxidative stress. While samples treated with UV radiation displayed modulations of their mitogen activated protein kinase (MAPK) pathway, gamma radiation had its major influence on cell-cycle progression in S-phase and mitosis. In addition, cell cultures from different individuals displayed significant differences in their gene expression responses to DNA damage.

Identification of E-box factor TFE3 as a functional partner for the E2F3 transcription factor

Various studies have demonstrated a role for E2F proteins in the control of transcription of genes involved in DNA replication, cell cycle progression, and cell fate determination. Although it is clear that the functions of the E2F proteins overlap, there is also evidence for specific roles for individual E2F proteins in the control of apoptosis and cell proliferation. Investigating protein interactions that might provide a mechanistic basis for the specificity of E2F function, we identified the E-box binding factor TFE3 as an E2F3-specific partner. We also show that this interaction is dependent on the marked box domain of E2F3. We provide evidence for a role for TFE3 in the synergistic activation of the p68 subunit gene of DNA polymerase alpha together with E2F3, again dependent on the E2F3 marked box domain. Chromatin immunoprecipitation assays showed that TFE3 and E2F3 were bound to the p68 promoter in vivo and that the interaction of either E2F3 or TFE3 with the promoter was facilitated by the presence of both proteins. In contrast, neither E2F1 nor E2F2 interacted with the p68 promoter under these conditions. We propose that the physical interaction of TFE3 and E2F3 facilitates transcriptional activation of the p68 gene and provides strong evidence for the specificity of E2F function.

Chk2 activates E2F-1 in response to DNA damage

The E2F-1 transcription factor is regulated during cell cycle progression and induced by cellular stress, such as DNA damage. We report that checkpoint kinase 2 (Chk2) regulates E2F-1 activity in response to the DNA-damaging agent etoposide. A Chk2 consensus phosphorylation site in E2F-1 is phosphorylated in response to DNA damage, resulting in protein stabilization, increased half-life, transcriptional activation and localization of phosphorylated E2F-1 to discrete nuclear structures. Expression of a dominant-negative Chk2 mutant blocks induction of E2F-1 and prevents E2F-1-dependent apoptosis. Moreover, E2F-1 is resistant to induction by etoposide in tumour cells expressing mutant chk2. Therefore, Chk2 phosphorylates and activates E2F-1 in response to DNA damage, resulting in apoptosis. These results suggest a role for E2F-1 in checkpoint control and provide a plausible explanation for the tumour suppressor activity of E2F-1.

Telomerase accelerates osteogenesis of bone marrow stromal stem cells by upregulation of CBFA1, osterix, and osteocalcin

Telomerase activity can prevent telomere shortening and replicative senescence in human somatic cells. We and others have previously demonstrated that forced expression of telomerase in human bone marrow stromal stem cells (BMSSCs) was able to extend their life-span and enhance their bone-forming capability, without inducing malignant transformation. In this study, we determined that telomerase was able to accelerate calcium accumulation of human BMSSCs under osteogenic inductive conditions. Similarly, xenogeneic transplantation of telomerase-expressing BMSSCs (BMSSC-Ts) yielded ectopic bone formation at 2 weeks post-transplantation, 2-4 weeks earlier than typically seen with BMSSCs transfected with empty vector (BMSSC-Cs). Low-density DNA array analysis revealed that telomerase activity increases the expression of G1 regulating genes including cyclin D3, cyclin E1, E2F-4, and DP2, associated with hyperphosphorylation of retinoblastoma (pRb), leading to the extended proliferative capacity of BMSSC-Ts. Importantly, BMSSC-T transplants showed a higher number of human osteogenic cells at 8 weeks post transplantation compared with the BMSSC-C transplants, coupled with a significantly increased osteogenic capacity. One possible mechanism leading to accelerated osteogenesis by BMSSC-Ts may be attributed, at least in part, to the upregulation of the important osteogenic genes such as CBFA1, osterix, and osteocalcin in vitro. Taken together, these findings show that telomerase can accelerate cell cycle progression from G1-to-S phase and enhance osteogenic differentiation of BMSSCs, because of the upregulation of CBFA1, osterix, and osteocalcin.

Dominant Negative E2F Inhibits Progression of the Cell Cycle after the Midblastula Transition in Xenopus

The cleavage cycle, which is initiated by fertilization, consists of only S and M phases, and the gap phases (G1 and G2) appear after the midblastula transition (MBT) in the African clawed frog, Xenopus laevis. During early development in Xenopus, we examined the E2F activity, which controls transition from the G1 to S phase in the somatic cell cycle. Gel retardation and transactivation assays revealed that, although the E2F protein was constantly present throughout early development, the E2F transactivation activity was induced in a stage-specific manner, that is, low before MBT and rapidly increased after MBT. Introduction of the recombinant dominant negative E2F (dnE2F), but not the control, protein into the 2-cell stage embryos specifically suppressed E2F activation after MBT. Cells in dnE2F-injected embryos appeared normal before MBT, but ceased to proliferate and eventually died at the gastrula. These cells contained decreased cdk activity with enhanced inhibitory phosphorylation of Cdc2 at Tyr15. Thus, E2F activity is required for cell cycle progression and cell viability after MBT, but not essential for MBT transition and developmental progression during the cleavage stage.

Oct-1 potentiates CREB-driven cyclin D1 promoter activation via a phospho-CREB- and CREB binding protein-independent mechanism

Cyclin D1, the regulatory subunit for mid-G(1) cyclin-dependent kinases, controls the expression of numerous cell cycle genes. A cyclic AMP-responsive element (CRE), located upstream of the cyclin D1 mRNA start site, integrates mitogenic signals that target the CRE-binding factor CREB, which can recruit the transcriptional coactivator CREB-binding protein (CBP). We describe an alternative mechanism for CREB-driven cyclin D1 induction that involves the ubiquitous POU domain protein Oct-1. In the breast cancer cell line MCF-7, overexpression of Oct-1 or its POU domain strongly increases transcriptional activation of cyclin D1 and GAL4 reporter genes that is specifically dependent upon CREB but independent of Oct-1 DNA binding. Gel retardation and chromatin immunoprecipitation assays confirm that POU forms a complex with CREB bound to the cyclin D1 CRE. In solution, CREB interaction with POU requires the CREB Q2 domain and, notably, occurs with CREB that is not phosphorylated on Ser 133. Accordingly, Oct-1 also potently enhances transcriptional activation mediated by a Ser133Ala CREB mutant. Oct-1/CREB synergy is not diminished by the adenovirus E1A 12S protein, a repressor of CBP coactivator function. In contrast, E1A strongly represses CBP-enhanced transactivation by CREB phosphorylated on Ser 133. Our observation that Oct-1 potentiates CREB-dependent cyclin D1 transcriptional activity independently of Ser 133 phosphorylation and E1A-sensitive coactivator function offers a new paradigm for the regulation of cyclin D1 induction by proliferative signals.

p14(ARF) regulates E2F activity

The ARF protein product of the ink4a/arf locus is induced by a variety of oncogenic signals. ARF facilitates growth arrest through the p53 pathway by hindering the down-regulation of p53 activity mediated by MDM2, through the formation of a protein complex with MDM2. Here we have explored the possibility that human p14(ARF) activity is integrated with growth regulating pathways other than p53, and report our results that p14(ARF) can control the activity of the E2F transcription factor. p14(ARF) regulates E2F activity in different cell-types, including p53(-/-)/mdm(-/-) MEFs, thus excluding that the effects of p14(ARF) are indirectly caused through MDM2 modulation. p14(ARF) down-regulates E2F-dependent transcription, and in cells undergoing E2F-dependent apoptosis prompts cell cycle arrest. p14(ARF) possesses multiple binding domains for E2F-1, one of which resides within the N-terminal region and coincides with the regulation of E2F activity. A mutational analysis of p14(ARF) indicates that the E2F-1 and MDM2 binding domains can be distinguished. These results highlight the potential interplay between p14(ARF) and E2F, and establish p14(ARF) as a pleiotrophic regulator of cell growth that acts by targetting at least two key pathways in the control of proliferation, namely E2F and p53.

Alternative fates of keratinocytes transduced by human papillomavirus type 18 E7 during squamous differentiation

The human papillomavirus type 18 (HPV-18) E7 protein promotes S-phase reentry in postmitotic, differentiated keratinocytes in squamous epithelium to facilitate vegetative viral DNA amplification. To examine the nature and fate of the differentiated cells that reenter S phase, organotypic cultures of primary human keratinocytes transduced with HPV-18 E7 were pulse-chase-pulse-labeled with (3)H-thymidine ((3)H-TdR) and bromodeoxyuridine (BrdU). The kinetics of the appearance of doubly labeled suprabasal cells demonstrate that E7 expression did not promote prolonged S phase. Rather, there was a considerable lag before a small percentage of the cells reentered another round of S phase. Fluorescence in situ hybridization analysis, indeed, revealed a small fraction of the cells with more than 4n chromosomes in the differentiated strata. Differentiated cells positive for (3)H-TdR, BrdU, or both often had enlarged nuclei or were binucleated. These results suggest that S phase is not followed by cell division, although nuclear division may occur. Interestingly, a significant fraction of differentiated cells that entered S phase subsequently accumulated p27kip1 protein with a kinetics preceding the accumulation of cyclin E. We conclude that E7-transduced, differentiated keratinocytes that enter S phase have two alternative fates: (i) a low percentage of cells undergoes endoreduplication, achieving higher than 4n ploidy, and (ii) a high percentage of cells accumulates the p27kip1, cyclin E, and p21cip1 proteins, resulting in arrest and preventing further S-phase reentry.

E2F transcriptional activation requires TRRAP and GCN5 cofactors

The E2F family of transcription factors regulates the temporal transcription of genes involved in cell cycle progression and DNA synthesis. E2F transactivation is antagonized by retinoblastoma protein (pRb), which recruits chromatin-remodeling proteins such as histone deacetylases and SWI.SNF complexes to the promoter to repress transcription. We hypothesized that E2F proteins must reverse the pRb-imposed chromatin structure to stimulate transcription. If this is true, E2F proteins should recruit proteins capable of histone acetylation. Here we map the E2F-4 transactivation domain and show that E2F-1 and E2F-4 transactivation domains bind the acetyltransferase GCN5 and cofactor TRRAP in vivo. TRRAP and GCN5 co-expression stimulated E2F-mediated transactivation, and c-Myc repressed E2F transactivation dependent on an intact TRRAP/GCN5 binding motif. The transactivation domain of E2F-4 recruited proteins with significant histone acetyltransferase activity in vivo, and this activity required catalytically active GCN5. E2F-4 proteins with subtle mutations in the transactivation domain exhibited a positive correlation among transcriptional activation and GCN5 and TRRAP binding capacity and associated acetyltransferase activity. We conclude that E2F stimulates transcription by recruiting acetyltransferase activity and the essential cofactors GCN5 and TRRAP. These results provide a mechanism for E2F transcription factors to overcome pRb-mediated dominant repression of transcription.

Acetylation control of the retinoblastoma tumour-suppressor protein

The retinoblastoma tumour-suppressor protein (pRb) and p300/CBP co-activator proteins are important for control of proliferation and in tumour cells these are sequestered by viral oncoproteins such as E1A. pRb is involved in negatively regulating growth, and p300/CBP proteins have histone acetyltransferase (HAT) activity, which influences gene expression. Although it is known that phosphorylation by G1 cyclin-dependent kinases (CDKs) regulates pRb activity, the nature and role of other post-translational modifications is not understood. Here we identify acetylation as a new type of modification and level of control in pRb function. Adenovirus E1A, which binds p300/CBP through an amino-terminal transformation-sensitive domain, stimulates the acetylation of pRb by recruiting p300 and pRb into a multimeric-protein complex. Furthermore, pRb acetylation is under cell-cycle control, and acetylation hinders the phosphorylation of pRb by cyclin-dependent kinases. pRb binds more strongly when acetylated to the MDM2 oncoprotein, which indicates that acetylation may regulate protein-protein interactions in the pRb pathway. The acetylation of pRb defines a new level of cell-cycle control mediated by HAT. Furthermore, our results establish a relationship between p300, pRb and acetylation in which E1A acts to recruit and target a cellular HAT activity to pRb.

p300/CBP proteins: HATs for transcriptional bridges and scaffolds

p300/CBP transcriptional co-activator proteins play a central role in co-ordinating and integrating multiple signal-dependent events with the transcription apparatus, allowing the appropriate level of gene activity to occur in response to diverse physiological cues that influence, for example, proliferation, differentiation and apoptosis. p300/CBP activity can be under aberrant control in human disease, particularly in cancer, which may inactivate a p300/CBP tumour-suppressor-like activity. The transcription regulating-properties of p300 and CBP appear to be exerted through multiple mechanisms. They act as protein bridges, thereby connecting different sequence-specific transcription factors to the transcription apparatus. Providing a protein scaffold upon which to build a multicomponent transcriptional regulatory complex is likely to be an important feature of p300/CBP control. Another key property is the presence of histone acetyltransferase (HAT) activity, which endows p300/CBP with the capacity to influence chromatin activity by modulating nucleosomal histones. Other proteins, including the p53 tumour suppressor, are targets for acetylation by p300/CBP. With the current intense level of research activity, p300/CBP will continue to be in the limelight and, we can be confident, yield new and important information on fundamental processes involved in transcriptional control.

Ca2+ and BMP-6 signaling regulate E2F during epidermal keratinocyte differentiation

The epidermis consists of a squamous epithelium continuously replenished by committed stem cells, which can either self-renew or differentiate. We demonstrated previously that E2F genes are differentially expressed in developing epidermis (Dagnino, L., Fry, C. J., Bartley, S. M., Farnham, P., Gallie, B. L., and Phillips, R. A. (1997) Cell Growth Differ. 8, 553-563). Thus, we hypothesized that various E2F proteins likely play distinct growth regulatory roles in the undifferentiated stem cells and in terminally differentiated keratinocytes. To further understand the function of E2F genes in epidermal morphogenesis, we have examined the expression, regulation, and protein-protein interactions of E2F factors in undifferentiated cultured murine primary keratinocytes or in cells induced to differentiate with Ca(2+) or BMP-6 (bone morphogenetic protein 6). We find similar patterns of E2F regulation with both differentiating agents and demonstrate a switch in expression from E2F-1, -2, and -3 in undifferentiated, proliferating cells to E2F-5 in terminally differentiated keratinocytes. Inhibition of keratinocyte proliferation by transforming growth factor-beta1 did not enhance E2F-5 protein levels, suggesting that this response is specific to differentiation rather than reversible cell cycle withdrawal. E2F-5 up-regulation is also accompanied by formation of heteromeric nuclear complexes containing E2F5, p130, and histone deacetylase (HDAC) 1. Overexpression of E2F5 specifically inhibited DNA synthesis in undifferentiated keratinocytes in an HDAC-dependent manner, suggesting that E2F-5.p130.HDAC1 complexes are likely involved in the permanent withdrawal from the cell cycle of keratinocytes responding to differentiation stimuli.

Inhibition of cyclin A/Cdk2 phosphorylation impairs B-Myb transactivation function without affecting interactions with DNA or the CBP coactivator

Expression of the B-Myb transcription factor is directed by an E2F-dependent transcriptional mechanism to late G1 and S phases of the cell cycle, where its transactivation properties are enhanced post-translationally by cyclin A/Cdk2-mediated phosphorylation. Other experiments have shown that removal of the B-Myb C-terminus constitutively activates both transactivation and DNA-binding activities, suggesting that autoregulation by this inhibitory domain is counteracted by phosphorylation. We report here on further experiments to examine this hypothesis. The importance of this modification was first emphasized by showing that co-transfected dominant-negative Cdk2 (Cdk2DN) substantially reduced B-Myb transactivation activity. We then attempted to map the autoregulatory domain by analysing a series of progressively deleted C-terminal B-Myb mutants. Removal of just 29 C-terminal aa increased transactivation appreciably, however, maximal activity required removal of 143 amino acids (as in B-Myb + 561). Enhanced B-Myb + 561 function correlated with the acquisition of DNA binding activity to a single Myb binding site (MBS) oligonucleotide as determined by bandshift assays, however, further assays showed that even wt B-Myb could bind a DNA fragment containing three MBS. Although transactivation by B-Myb was severely dependent on hyperphosphorylation, neither inhibiting this activity by co-transfecting Cdk2DN nor augmenting it with cyclin A resulted in significant effects on DNA-binding. We also found that B-Myb could synergize with the CBP coactivator and that this cooperativity was cyclin A/Cdk2-dependent. Despite this, the physical association between these proteins was not influenced by the B-Myb phosphorylation status. We discuss these findings in relation to the autoregulation of B-Myb by the C-terminal domain.

E2F regulates growth-dependent transcription of genes encoding both catalytic and regulatory subunits of mouse primase

DNA polymerase alpha-primase is one of the principal enzymes involved in eukaryotic chromosomal DNA replication. Mouse DNA polymerase alpha-primase consists of four subunits with molecular masses of 180, 68, 54 and 46 kDa. Protein and mRNA expression levels of the four subunits are up-regulated in a coordinated manner in response to growth stimulation. We have previously analysed the transcription of the 180 kDa (p180) and 68 kDa (p68) subunits, which form the DNA polymerase catalytic complex, and found that growth-dependent regulation of transcription of the mouse p180 and p68 genes is mediated by a common factor, E2F, while the basal transcription of the genes is regulated by different transcription factors. We characterized the transcriptional regulation of the 54 kDa (p54) and 46 kDa (p46) subunits, which form the DNA primase catalytic complex. We isolated genomic clones spanning the 5'-flanking regions of the p54 and p46 genes and showed, using transient expression and gel mobility shift assays, that the basal transcription of p54 is controlled by Sp1 and GA-binding protein, as is the basal transcription of the p180 gene. The basal transcription of p46 is controlled by unknown factor(s) which were bound to the upstream sequence. The variant E2F sites close to the transcription initiation sites of the p54 and p46 genes had no basal promoter activity, but were essential for the growth-dependent transcription of both genes. The promoter regions of the four subunits of mouse DNA polymerase d-primase complex share several common features. The coordinated transcription of all four subunits in response to growth stimulation appears to be controlled by E2F.

Rb function in cell-cycle regulation and apoptosis

Loss of cell-cycle control is a hallmark of neoplastic cells. One regulator of the critical G1 to S-phase transition in the cell cycle is the retinoblastoma tumour suppressor protein Rb, which interacts with the E2F family of cell-cycle transcription factors to repress gene transcription required for this transition. Through its interaction with E2F, Rb also regulates genes that control apoptosis. Here we review the roles of Rb in regulating the cell cycle and apoptosis and discuss recent results linking these Rb functions to chromatin-remodelling enzymes.