A genetic analysis of the E2F1 gene distinguishes regulation by Rb, p107, and adenovirus E4

RB1 loss triggers dependence on ESRRG in retinoblastoma

Retinoblastoma (Rb) is a deadly childhood eye cancer that is classically initiated by inactivation of the RB1 tumor suppressor. Clinical management continues to rely on nonspecific chemotherapeutic agents that are associated with treatment resistance and toxicity. Here, we analyzed 103 whole exomes, 20 whole transcriptomes, 5 single-cell transcriptomes, and 4 whole genomes from primary Rb tumors to identify previously unknown Rb dependencies. Several recurrent genomic aberrations implicate estrogen-related receptor gamma (ESRRG) in Rb pathogenesis. RB1 directly interacts with and inhibits ESRRG, and RB1 loss uncouples ESRRG from negative regulation. ESRRG regulates genes involved in retinogenesis and oxygen metabolism in Rb cells. ESRRG is preferentially expressed in hypoxic Rb cells in vivo. Depletion or inhibition of ESRRG causes marked Rb cell death, which is exacerbated in hypoxia. These findings reveal a previously unidentified dependency of Rb cells on ESRRG, and they implicate ESRRG as a potential therapeutic vulnerability in Rb.

Loss of dE2F compromises mitochondrial function

E2F/DP transcription factors regulate cell proliferation and apoptosis. Here, we investigated the mechanism of the resistance of Drosophila dDP mutants to irradiation-induced apoptosis. Contrary to the prevailing view, this is not due to an inability to induce the apoptotic transcriptional program, because we show that this program is induced; rather, this is due to a mitochondrial dysfunction of dDP mutants. We attribute this defect to E2F/DP-dependent control of expression of mitochondria-associated genes. Genetic attenuation of several of these E2F/DP targets mimics the dDP mutant mitochondrial phenotype and protects against irradiation-induced apoptosis. Significantly, the role of E2F/DP in the regulation of mitochondrial function is conserved between flies and humans. Thus, our results uncover a role of E2F/DP in the regulation of mitochondrial function and demonstrate that this aspect of E2F regulation is critical for the normal induction of apoptosis in response to irradiation.

E2F1 effects on osteoblast differentiation and mineralization are mediated through up-regulation of frizzled-1

Frizzled homolog 1 (FZD1) is a transmembrane receptor that mediates Wnt signaling. The transcriptional regulation of FZD1 and the role of FZD1 in osteoblast biology are not well understood. We examined the role of E2F1 in FZD1 promoter activation and osteoblast differentiation and mineralization. A putative E2F1 binding site in the FZD1 promoter region was initially identified in silico and characterized further in Saos2 cells in vitro by chromatin immunoprecipitation (ChIP), electrophoretic mobility shift (EMSA) and promoter reporter assays. Over-expression of E2F1 transactivated the FZD1 promoter and increased endogenous FZD1 mRNA and protein levels in Saos2 cells. Over-expression of E2F1 in Saos2 cells up-regulated osteoblast differentiation markers alkaline phosphatase (ALP), type I collagen α (COL1A), and osteocalcin (OCN). Furthermore, E2F1 over-expression enhanced mineralization of differentiated Saos2 cells, whereas siRNA knockdown of FZD1 diminished the effects of E2F1 on osteoblast mineralization. The effects of E2F1 on FZD1 expression and osteoblast mineralization were further confirmed in normal human FOB osteoblasts. Taken together, our experiments demonstrate a role of E2F1 in osteoblast differentiation and mineralization and suggest that FZD1 is required, in part, for E2F1 regulation of osteoblast mineralization.

Repression of androgen receptor transcription through the E2F1/DNMT1 axis

Although androgen receptor (AR) function has been extensively studied, regulation of the AR gene itself has been much less characterized. In this study, we observed a dramatic reduction in the expression of androgen receptor mRNA and protein in hyperproliferative prostate epithelium of keratin 5 promoter driven E2F1 transgenic mice. To confirm an inhibitory function for E2F1 on AR transcription, we showed that E2F1 inhibited the transcription of endogenous AR mRNA, subsequent AR protein, and AR promoter activity in both human and mouse epithelial cells. E2F1 also inhibited androgen-stimulated activation of two AR target gene promoters. To elucidate the molecular mechanism of E2F-mediated inhibition of AR, we evaluated the effects of two functional E2F1 mutants on AR promoter activity and found that the transactivation domain appears to mediate E2F1 repression of the AR promoter. Because DNMT1 is a functional intermediate of E2F1 we examined DNMT1 function in AR repression. Repression of endogenous AR in normal human prostate epithelial cells was relieved by DNMT1 shRNA knock down. DNMT1 was shown to be physically associated within the AR minimal promoter located 22 bps from the transcription start site; however, methylation remained unchanged at the promoter regardless of DNMT1 expression. Taken together, our results suggest that DNMT1 operates either as a functional intermediary or in cooperation with E2F1 inhibiting AR gene expression in a methylation independent manner.

A genetic reporter system to gauge cell proliferation rate

In higher eukaryotes, E2F transcription factors often drive expression of genes necessary for the cell cycle, notably the G1/S phase transition. With conventional transcriptional reporter systems, expression of a reporter gene from an E2F-responsive promoter would allow one to identify the fraction of cells making this transition. Here, we have engineered an E2F-responsive genetic reporter system that outputs the proliferation rate. The system takes advantage of the long half-lives of fluorescent protein reporters and output signal normalization. By doing so, it converts dynamic pulses of E2F activity into an analog output proportional to the proliferation rate. Such a system should be useful for applications involving high-throughput drug or genetic screens, investigation of cellular environment, and biological engineering.

CDKN1C negatively regulates RNA polymerase II C-terminal domain phosphorylation in an E2F1-dependent manner

CDKN1C is a cyclin-dependent kinase inhibitor and is a candidate tumor suppressor gene. We previously found that the CDKN1C protein represses E2F1-driven transcription in an apparent negative feedback loop. Herein, we explore the mechanism by which CDKN1C represses transcription. We find that adenoviral-mediated overexpression of CDKN1C leads to a dramatic reduction in phosphorylation of the RNA polymerase II (pol II) C-terminal domain (CTD). RNA interference studies demonstrate that this activity is not an artifact of CDKN1C overexpression, because endogenous CDKN1C mediates an inhibition of RNA pol II CTD phosphorylation in HeLa cells upon treatment with dexamethasone. Surprisingly, we find that CDKN1C-mediated repression of RNA pol II phosphorylation is E2F1-dependent, suggesting that E2F1 may direct CDKN1C to chromatin. Chromatin immunoprecipitation assays demonstrate that CDKN1C is associated with E2F1-regulated promoters in vivo and that this association can dramatically reduce the level of RNA pol II CTD phosphorylation at both Ser-2 and Ser-5 of the C-terminal domain repeat. In addition, we show that CDKN1C interacts with both CDK7 and CDK9 (putative RNA pol II CTD kinases) and that CDKN1C blocks their ability to phosphorylate a glutathione S-transferase-CTD fusion protein in vitro. E2F1 and CDKN1C are found to form stable complexes both in vivo and in vitro. Molecular studies demonstrate that the E2F1-CDKN1C interaction is mediated by two E2F domains. A central E2F1 domain interacts directly with CDKN1C, whereas a C-terminal E2F1 domain interacts with CDKN1C via interaction with Rb. The results presented in this report highlight a novel mechanism of tumor suppression by CDKN1C.

Disruption of mutually negative regulatory feedback loop between interferon-inducible p202 protein and the E2F family of transcription factors in lupus-prone mice

Studies have identified IFN-inducible Ifi202 gene as a lupus susceptibility gene (encoding p202 protein) in mouse models of lupus disease. However, signaling pathways that regulate the Ifi202 expression in cells remain to be elucidated. We found that steady-state levels of Ifi202 mRNA and protein were high in mouse embryonic fibroblasts (MEFs) from E2F1 knockout (E2F1(-/-)) and E2F1 and E2F2 double knockout (E2F1(-/-)E2F2(-/-)) mice than isogenic wild-type MEFs. Moreover, overexpression of E2F1 in mouse fibroblasts decreased expression of p202. Furthermore, expression of E2F1, but not E2F4, transcription factor in mouse fibroblasts repressed the activity of 202-luc-reporter in promoter-reporter assays. Interestingly, the E2F1-mediated transcriptional repression of the 202-luc-reporter was independent of p53 and pRb expression. However, the repression was dependent on the ability of E2F1 to bind DNA. We have identified a potential E2F DNA-binding site in the 5'-regulatory region of the Ifi202 gene, and mutations in this E2F DNA-binding site reduced the E2F1-mediated transcriptional repression of 202-luc-reporter. Because p202 inhibits the E2F1-mediated transcriptional activation of genes, we compared the expression of E2F1 and its target genes in splenic cells from lupus-prone B6.Nba2 congenic mice, which express increased levels of p202, with age-matched C57BL/6 mice. We found that increased expression of Ifi202 in the congenic mice was associated with inhibition of E2F1-mediated transcription and decreased expression of E2F1 and its target genes that encode proapoptotic proteins. Our observations support the idea that increased Ifi202 expression in certain strains of mice contributes to lupus susceptibility in part by inhibiting E2F1-mediated functions.

Transcriptional regulation of subtilisin-like proprotein convertase PACE4 by E2F: Possible role of E2F-mediated upregulation of PACE4 in tumor progression

PACE4, a member of the subtilisin-like proprotein convertase (SPC) family, is expressed at high levels in certain tumor cells and plays a role in metastatic progression through activation of matrix metalloproteinases. The mechanism leading to overexpression of PACE4 in tumor cells remains unclear. In this study, we show that the E2F1 transcription factor, which is implicated in carcinoma invasiveness, upregulates the expression of PACE4. HT1080 (highly tumorigenic and invasive) cells expressed much higher levels of PACE4 and E2F family (E2F1 and E2F2) transcripts than IMR90 (normal fibroblast) cells. Expression levels of other SPCs (furin and PC6) remained unchanged in these cells. Promoter analysis indicated that two E2F consensus binding sites (-117/-110 and -86/-79) in the 5'-flanking region of the human PACE4 gene function as positive regulatory elements. Mutation of these sites abolished PACE4 promoter response to E2F1 as well as binding of E2F1 in electrophoretic mobility-shift assays. Other E2F members, E2F2 and E2F3, also activated PACE4 expression, as in the case of E2F1. These results indicate a novel mechanism for E2F family-mediated promotion of carcinoma invasiveness through PACE4.

Genetic ablation of CCAAT/enhancer binding protein alpha in epidermis reveals its role in suppression of epithelial tumorigenesis

CCAAT/enhancer binding protein alpha (C/EBPalpha) is a basic leucine zipper transcription factor that inhibits cell cycle progression and regulates differentiation in various cell types. C/EBPalpha is inactivated by mutation in acute myeloid leukemia (AML) and is considered a human tumor suppressor in AML. Although C/EBPalpha mutations have not been observed in malignancies other than AML, greatly diminished expression of C/EBPalpha occurs in numerous human epithelial cancers including lung, liver, endometrial, skin, and breast, suggesting a possible tumor suppressor function. However, direct evidence for C/EBPalpha as an epithelial tumor suppressor is lacking due to the absence of C/EBPalpha mutations in epithelial tumors and the lethal effect of C/EBPalpha deletion in mouse model systems. To examine the function of C/EBPalpha in epithelial tumor development, an epidermal-specific C/EBPalpha knockout mouse was generated. The epidermal-specific C/EBPalpha knockout mice survived and displayed no detectable abnormalities in epidermal keratinocyte proliferation, differentiation, or apoptosis, showing that C/EBPalpha is dispensable for normal epidermal homeostasis. In spite of this, the epidermal-specific C/EBPalpha knockout mice were highly susceptible to skin tumor development involving oncogenic Ras. These mice displayed decreased tumor latency and striking increases in tumor incidence, multiplicity, growth rate, and the rate of malignant progression. Mice hemizygous for C/EBPalpha displayed an intermediate-enhanced tumor phenotype. Our results suggest that decreased expression of C/EBPalpha contributes to deregulation of tumor cell proliferation. C/EBPalpha had been proposed to block cell cycle progression through inhibition of E2F activity. We observed that C/EBPalpha blocked Ras-induced and epidermal growth factor-induced E2F activity in keratinocytes and also blocked Ras-induced cell transformation and cell cycle progression. Our study shows that C/EBPalpha is dispensable for epidermal homeostasis and provides genetic evidence that C/EBPalpha is a suppressor of epithelial tumorigenesis.

Proapoptotic role of novel gene-expression factors

The mechanisms that control cellular proliferation, as well as those related with programmed cell death or apoptosis, require precise regulation systems to prevent diseases such as cancer. Events related to cellular proliferation as well as those associated with apoptosis involve the regulation of gene expression carried out by three basic genetic expression regulation mechanisms: transcription, splicing of the primary transcript for mature mRNA formation, and RNA translation, a ribosomal machinery-dependent process for protein synthesis. While development of each one of these processes requires energy for recognition and assembly of a number of molecular complexes, it has been reported that an increased expression of several members of these protein complexes promotes apoptosis in distinct cell types. The question of how these factors interact with other proteins in order to incorporate themselves into the different transduction cascades and stimulate the development of programmed cell death, although nowadays actively studied, is still waiting for a clear-cut answer. This review focuses on the interactions established between different families of transcription, elongation, translation and splicing factors associated to the progression of apoptosis.

Elevated E2F1 inhibits transcription of the androgen receptor in metastatic hormone-resistant prostate cancer

Activation of E2F transcription factors, through disruption of the retinoblastoma (Rb) tumor-suppressor gene, is a key event in the development of many human cancers. Previously, we showed that homozygous deletion of Rb in a prostate tissue recombination model exhibits increased E2F activity, activation of E2F-target genes, and increased susceptibility to hormonal carcinogenesis. In this study, we examined the expression of E2F1 in 667 prostate tissue cores and compared it with the expression of the androgen receptor (AR), a marker of prostate epithelial differentiation, using tissue microarray analysis. We show that E2F1 expression is low in benign and localized prostate cancer, modestly elevated in metastatic lymph nodes from hormone-naïve patients, and significantly elevated in metastatic tissues from hormone-resistant prostate cancer patients (P = 0.0006). In contrast, strong AR expression was detected in benign prostate (83%), localized prostate cancer (100%), and lymph node metastasis (80%), but decreased to 40% in metastatic hormone-resistant prostate cancer (P = 0.004). Semiquantitative reverse transcription-PCR analysis showed elevated E2F1 mRNA levels and increased levels of the E2F-target genes dihyrofolate reductase and proliferating cell nuclear antigen in metastatic hormone-independent prostate cancer cases compared with benign tissues. To identify a role of E2F1 in hormone-independent prostate cancer, we examined whether E2F1 can regulate AR expression. We show that exogenous expression of E2F1 significantly inhibited AR mRNA and AR protein levels in prostate epithelial cells. E2F1 also inhibited an AR promoter-luciferase construct that was dependent on the transactivation domain of E2F1. Furthermore, using chromatin immunoprecipitation assays, we show that E2F1 and the pocket protein family members p107 and p130 bind to the AR promoter in vivo. Taken together, these results show that elevated E2F1, through its ability to repress AR transcription, may contribute to the progression of hormone-independent prostate cancer.

Transcriptional upregulation of p57 (Kip2) by the cyclin-dependent kinase inhibitor BMS-387032 is E2F dependent and serves as a negative feedback loop limiting cytotoxicity

In spite of the fact that cyclin-dependent kinase (cdk) inhibiting drugs are potent transcriptional repressors, we discover that p57 (Kip2, CDKN1C) transcription is significantly upregulated by three small molecule cdk inhibitors, including BMS-387032. Treatment of MDA-MB-231 breast cancer cells with BMS-387032 led to a stabilization of the E2F1 protein that was accompanied by significant increases in the p57 mRNA and protein. This increase did not occur in an E2F1-deficient cell line. An E2F1-estrogen receptor fusion protein activated the endogenous p57 promoter in response to hydroxytamoxifen treatment in the presence of cycloheximide. Luciferase constructs driven by the p57 promoter verified that upregulation of p57 mRNA by BMS-387032 is transcriptional and dependent on E2F-binding sites in the promoter. Expression of exogenous p57 significantly decreased the fraction of cells in S phase. Furthermore, p57-deficient MDA-MB-231 cell lines were significantly more sensitive to BMS-387032-induced apoptosis than controls. The results presented in this manuscript demonstrate that small molecule cdk inhibitors transcriptionally activate p57 dependent upon E2F1 and that this activation in turn serves to limit E2F1's death-inducing activity.

17beta-estradiol (E2) induces cdc25A gene expression in breast cancer cells by genomic and non-genomic pathways

Cdc25A is a potent tyrosine phosphatase that catalyzes specific dephosphorylation of cyclin/cyclin-dependent kinase (cdk) complexes to regulate G1 to S-phase cell cycle progression. Cdc25A mRNA levels are induced by 17beta-estradiol (E2) in ZR-75 breast cancer cells, and deletion analysis of the cdc25A promoter identified the -151 to -12 region as the minimal E2-responsive sequence. Subsequent mutation/deletion analysis showed that at least three different cis-elements were involved in activation of cdc25A by E2, namely, GC-rich Sp1 binding sites, CCAAT motifs that bind NF-Y, and E2F sites that bind DP/E2F1 proteins. Studies with inhibitors and dominant negative expression plasmids show that E2 activates cdc25A expression through activation of genomic ERalpha/Sp1 and E2F1 and cAMP-dependent activation of NF-YA. Thus, both genomic and non-genomic pathways of estrogen action are involved in induction of cdc25A in breast cancer cells.

Compensation and specificity of function within the E2F family

Functions encoded by single genes in lower organisms are often represented by multiple related genes in the mammalian genome. An example is the retinoblastoma and E2F families of proteins that regulate transcription during the cell cycle. Analysis of gene function using germline mutations is often confounded by overlapping function resulting in compensation. Indeed, in cells deleted of the E2F1 or E2F3 genes, there is an increase in the expression of the other family member. To avoid complications of compensatory effects, we have used small-interfering RNAs that target individual E2F proteins to generate a temporary loss of E2F function. We find that both E2F1 and E2F3 are required for cells to enter the S phase from a quiescent state, whereas only E2F3 is necessary for the S phase in growing cells. We also find that the acute loss of E2F3 activity affects the expression of genes encoding DNA replication and mitotic activities, whereas loss of E2F1 affects a limited number of genes that are distinct from those regulated by E2F3. We conclude that the long-term loss of E2F activity does lead to compensation by other family members and that the analysis of acute loss of function reveals specific and distinct roles for these proteins.

DNA-binding independent cell death from a minimal proapoptotic region of E2F-1

The ability to induce cell cycle progression while evading cell death is a defining characteristic of cancer. Deregulation of E2F is a common event in most human cancers. Paradoxically, this can lead to both cell cycle progression and apoptosis. Although the way in which E2F causes cell cycle progression is well characterized, the pathways by which E2F induces cell death are less well defined. Many of the known mechanisms through which E2F induces apoptosis occur through regulation of E2F target genes. However, mutants of E2F-1 that lack the transactivation domain are still able to induce cell death. To further investigate this activity, we refined a transactivation independent mutant to identify a minimal apoptotic domain. This revealed that only 75 amino acids from within the DNA-binding domain of E2F-1 is sufficient for cell death and that this activity is also present in the DNA-binding domains of E2F-2 and E2F-3. However, analysis of this domain from E2F-1 revealed it does not bind DNA and is consequently unable to transactivate, repress or de-repress E2F target genes. This provocative observation therefore defines a potential new mechanism of death from E2F and opens up new opportunities for inducing cell death in tumours for therapeutic gain.

Inactivating E2f1 reverts apoptosis resistance and cancer sensitivity in Trp53-deficient mice

The E2f1 transcription factor, which regulates genes required for S-phase entry, also induces apoptosis by transcriptional and post-translational mechanisms. As E2f1 is inducible by DNA damage we investigated its importance in vivo in ultraviolet (UV)-induced apoptosis, a protective mechanism that prevents the epidermis from accumulating UV-induced mutations. Contrary to expectation, E2f1-/- mice demonstrated enhanced keratinocyte apoptosis after UVB exposure, whereas apoptosis was suppressed by epidermis-specific overexpression of human E2F1. Apoptosis induced by -radiation was also repressed by E2f1. E2f1-/-;Trp53-/- double knockout mice exhibited the elevated UVB-induced apoptosis of E2f1-/- alone, rather than the profound apoptosis defect seen in Trp53-/- mice, indicating that Trp53 (p53) lies functionally upstream of E2f1. Transfecting E2F1 into E2f1-/-;Trp53-/- primary fibroblasts suppressed UVB-induced apoptosis and this suppression was relieved by Trp53. The double knockout also reverted the abnormal sex ratio and early-onset tumours of Trp53-/- mice. These results imply that E2f1 functions as a suppressor of an apoptosis pathway that is initiated by DNA photoproducts and perhaps genetic abnormalities; p53 relieves this suppression.

Regulation of the cyclin D3 promoter by E2F1

We have previously demonstrated that ectopic expression of E2F1 is sufficient to drive quiescent cells into S phase and that E2F1 expression can contribute to oncogenic transformation. Key target genes in this process include master regulators of the cell cycle, such as cyclin E, which regulates G(1) progression, and cyclin A, which is required for the initiation of DNA synthesis. In the present work, we present novel evidence that a second G(1) cyclin, cyclin D3, is also potently activated by E2F1. First, an estrogen receptor-E2F1 fusion protein (ER-E2F1) potently activates the endogenous cyclin D3 mRNA upon treatment with 4-hydroxytamoxifen, which induces nuclear accumulation of the otherwise cytosolic fusion protein. Furthermore, trans-activation of cyclin D3 by ER-E2F1 occurs even in the presence of the protein synthesis inhibitor cycloheximide and thus appears direct. Second, all of the growth-stimulatory members of the E2F family (E2F1, -2, and -3A) potently activate a cyclin D3 promoter reporter, whereas growth-restraining members of the family (E2F4, -5, and -6) have little effect. Third, recombinant E2F1 binds with high affinity to the cyclin D3 promoter in vitro. Fourth, chromatin immunoprecipitation assays demonstrate that endogenous E2F1 is associated with the cyclin D3 promoter in vivo. Finally, mapping experiments localize the essential E2F regulatory element of the cyclin D3 promoter to a noncanonical E2F site in the promoter between nucleotides -143 and -135 relative to the initiating methionine codon. We conclude that in addition to cyclins E and A, E2F family members can also activate one member of the D-type cyclins, further contributing to the ability of the stimulatory E2F family members to drive cellular proliferation.

Opposing regulatory roles of E2F in human telomerase reverse transcriptase (hTERT) gene expression in human tumor and normal somatic cells

Telomerase activity is closely correlated with cellular proliferative activity in human tissues. Human cells with high proliferative potential, such as tumor cells or stem cells, exhibit telomerase activity, whereas most normal human somatic cells do not. Telomerase activity is tightly regulated by the expression of its catalytic subunit human telomerase reverse transcriptase (hTERT). Through an expression cloning approach, we identified E2F-1 as a repressor of the hTERT gene in human tumor cells. Ectopic expression of E2F-1 repressed hTERT promoter activity by inhibiting Sp1 activation of the hTERT promoter. In contrast to the repressor function of E2F-1 in human tumor cells, we demonstrated that E2F-1 is an activator of the hTERT gene in normal human somatic cells. Ectopically expressed E2F-1 activated the hTERT promoter through a noncanonical DNA binding site. E2F-1, E2F-2, and E2F-3 (but not E2F-4 and E2F-5) repressed hTERT promoter activity in human tumor cells, whereas they activated it in normal somatic cells. These contrasting effects of E2F transcription factors on the hTERT promoter could underlie the paradoxical biological activities of E2F, which can both promote and inhibit cellular proliferation and tumorigenesis.

E2F-3B is a physiological target of cyclin A

The E2F family of transcription factors controls the expression of numerous genes that are required for the G(1)/S transition. Among the mechanisms that modulate the activity of the E2F proteins, cyclin A has been found to be important for the down-regulation of E2F-1, -2, and -3A activity after cells have progressed through G(1)/S. Specifically, phosphorylation of these E2F proteins by cyclin A/Cdk2 ultimately results in their necessary degradation as cells progress through S phase. E2F-3B was recently identified as an alternatively spliced form of E2F-3A that was predicted to lack a functional cyclin A binding domain. In this paper, we present considerable evidence that contradicts this prediction. First, we demonstrate binding of cyclin A to E2F-3B as bacterially expressed proteins in vitro. Second, we demonstrate binding of cyclin A to E2F-3B in mammalian cells in vivo. Third, we show that co-expression of cyclin A with E2F-3B significantly reduces E2F-3B-mediated transcriptional activity. Finally, in synchronized cells, we observe down-regulation of E2F-3B protein expression coincident with the up-regulation of cyclin A. We conclude that E2F-3B is a physiological target of cyclin A.

E2F1 and c-Myc potentiate apoptosis through inhibition of NF-kappaB activity that facilitates MnSOD-mediated ROS elimination

Overexpression of c-Myc or E2F1 sensitizes host cells to various types of apoptosis. Here, we found that overexpressed c-Myc or E2F1 induces accumulation of reactive oxygen species (ROS) and thereby enhances serum-deprived apoptosis in NIH3T3 and Saos-2. During serum deprivation, MnSOD mRNA was induced by NF-kappaB in mock-transfected NIH3T3, while this induction was inhibited in NIH3T3 overexpressing c-Myc or E2F1. In these clones, E2F1 inhibited NF-kappaB activity by binding to its subunit p65 in competition with a heterodimeric partner p50. In addition to overexpressed E2F1, endogenous E2F1 released from Rb was also found to inhibit NF-kappaB activity in a cell cycle-dependent manner by using E2F1(+/+) and E2F1(-/-) murine embryonic fibroblasts. These results indicate that E2F1 promotes apoptosis by inhibiting NF-kappaB activity.

Differences in DNA binding properties between E2F1 and E2F4 specify repression of the Mcl-1 promoter

E2F1 is a potent inducer of apoptosis whereas its relative, E2F4, generally does not promote cell death. Other work from our laboratory has demonstrated that E2F1 can directly bind and represss the Mcl-1 promoter - contributing to E2F1-mediated apoptosis. Here we show that while E2F1 can repress the Mcl-1 promoter, other members of the E2F family (such as E2F4) cannot. Characterization of the Mcl-1 promoter demonstrates that the -143/+10 region is critical for E2F1-mediated downregulation. We demonstrate that the ability of E2F1 to repress the Mcl-1 promoter correlates with its ability to bind within the required -143/+10 region of this promoter. In contrast, E2F4 is unable to bind to the -143/+10 region of the Mcl-1 promoter. We propose that E2F4 is unable to repress the Mcl-1 promoter primarily as a result of insufficient binding to the essential regulatory region. This is the first evidence of DNA binding specificity among E2F family members that results in differential regulation of a naturally occurring promoter.

Direct repression of the Mcl-1 promoter by E2F1

E2F1 induces apoptosis via both p53-dependent and p53-independent mechanisms. The direct targets in the p53-independent pathway remain enigmatic; however, the induction of this pathway does not require the transactivation domain of E2F1. Using cells that are defective in p53 activation, we show that E2F1 potently represses the expression of Mcl-1--an anti-apoptotic Bcl-2 family member whose depletion results in apoptosis. We also show that this transcriptional repression is direct and dependent upon E2F1's DNA-binding domain, but does not require the transactivation domain of E2F1. Consistent with this DNA binding requirement of E2F1 to repress Mcl-1, we show that E2F1 binds to the Mcl-1 promoter both in vitro and in vivo, and have identified the DNA element (-143/-117) within this promoter that is required for E2F1 binding and repression. Additionally, cell lines constitutively expressing Mcl-1 are resistant to E2F1-mediated apoptosis--suggesting that Mcl-1 downregulation is a necessary event in the p53-independent apoptotic process. Thus, we identify a p53 family-independent mechanism of E2F1-induced apoptosis in which E2F1 directly represses Mcl-1 expression.

Retinoblastoma protein partners

Studies of the retinoblastoma gene (Rb) have shown that its protein product (pRb) acts to restrict cell proliferation, inhibit apoptosis, and promote cell differentiation. The frequent mutation of the Rb gene, and the functional inactivation of pRb in tumor cells, have spurred interest in the mechanism of pRb action. Recently, much attention has focused on pRb's role in the regulation of the E2F transcription factor. However, biochemical studies have suggested that E2F is only one of many pRb-targets and, to date, at least 110 cellular proteins have been reported to associate with pRb. The plethora of pRb-binding proteins raises several important questions. How many functions does pRb possess, which of these functions are important for development, and which contribute to tumor suppression? The goal of this review is to summarize the current literature of pRb-associated proteins.

Disruption of BRCA1 LXCXE motif alters BRCA1 functional activity and regulation of RB family but not RB protein binding

The tumor suppressor activity of the BRCA1 gene product is due, in part, to functional interactions with other tumor suppressors, including p53 and the retinoblastoma (RB) protein. RB binding sites on BRCA1 were identified in the C-terminal BRCT domain (Yarden and Brody, 1999) and in the N-terminus (aa 304-394) (Aprelikova et al., 1999). The N-terminal site contains a consensus RB binding motif, LXCXE (aa 358-362), but the role of this motif in RB binding and BRCA1 functional activity is unclear. In both in vitro and in vivo assays, we found that the BRCA1:RB interaction does not require the BRCA1 LXCXE motif, nor does it require an intact A/B binding pocket of RB. In addition, nuclear co-localization of the endogenous BRCA1 and RB proteins was observed. Over-expression of wild-type BRCA1 (wtBRCA1) did not cause cell cycle arrest but did cause down-regulation of expression of RB, p107, p130, and other proteins (e.g., p300), associated with increased sensitivity to DNA-damaging agents. In contrast, expression of a full-length BRCA1 with an LXCXE inactivating mutation (LXCXE-->RXRXH) failed to down-regulate RB, blocked the down-regulation of RB by wtBRCA1, induced chemoresistance, and abrogated the ability of BRCA1 to mediate tumor growth suppression of DU-145 prostate cancer cells. wtBRCA1-induced chemosensitivity was partially reversed by expression of either Rb or p300 and fully reversed by co-expression of Rb plus p300. Our findings suggest that: (1) disruption of the LXCXE motif within the N-terminal RB binding region alters the biologic function of BRCA1; and (2) over-expression of BRCA1 inhibits the expression of RB and RB family (p107 and p130) proteins.

The transcription factor E2F1 promotes dopamine-evoked neuronal apoptosis by a mechanism independent of transcriptional activation

The E2F1 transcription factor plays an important role in promoting neuronal apoptosis; however, it is not clear how E2F1 does this. Here we show that E2F1 is involved in dopamine (DA)-evoked apoptosis in cerebellar granule neurons (CGNs). E2F1 -/- CGNs and CGNs expressing an antisense E2F1 cDNA were significantly protected from DA-toxicity relative to controls. The neuronal protection was accompanied by significantly reduced caspase 3 activity. E2F1-mediated neuronal apoptosis did not require activation of gene transcription because: (1) ectopic expression of E2F1 or its mutants lacking the transactivation domain induced neuronal apoptosis, whereas an E2F1 mutant lacking the DNA-binding domain did not; (2) under all of these conditions, known E2F1 target genes including cyclin A, cdc2 and p19(ARF) were not induced; and (3) DA-evoked neuronal apoptosis was associated with up-regulated E2F1, but not transcription of its target genes. Finally, E2F1-mediated neuronal apoptosis was associated with reduced nuclear factor (NF)-kappaB DNA-binding activity. Taken together, these data suggest that E2F1 promotes DA-evoked caspase 3-dependent neuronal apoptosis by a mechanism independent of gene transactivation, and this may possibly occur through inhibition of anti-apoptotic genes including NF-kappaB.

Role of the p53-homologue p73 in E2F1-induced apoptosis

Most human cancers harbour aberrations of cell-cycle control, which result in deregulated activity of the E2F transcription factors with concomitant enhanced cell-cycle progression. Oncogenic signalling by E2F1 has recently been linked to stabilization and activation of the tumour suppressor p53 (refs 1,3,4). The p73 protein shares substantial sequence homology and functional similarity with p53 (refs 5-7 ). Hence, several previously considered p53-independent cellular activities may be attributable to p73. Here we provide evidence that E2F1 directly activates transcription of TP73, leading to activation of p53-responsive target genes and apoptosis. Disruption of p73 function by a tumour-derived p53 mutant reduced E2F1-mediated apoptosis. Thus, p73 activation by deregulated E2F1 activity might constitute a p53-independent, anti-tumorigenic safeguard mechanism.

Induction of cell-cycle regulators in simian immunodeficiency virus encephalitis

Neuronal degeneration associated with human immunodeficiency virus encephalitis has been attributed to neurotoxicity of signaling molecules secreted by activated, infected macrophages. We hypothesized that the barrage of signals present in the extracellular milieu of human immunodeficiency virus-infiltrated brain causes inappropriate activation of neuronal cell-cycle machinery. We examined the presence of three members of the cell-cycle control machinery: pRb, E2F1, and p53 in the simian immunodeficiency virus encephalitis (SIVE) model. Compared to noninfected and simian immunodeficiency virus-infected, nonencephalitic controls, we observed increased protein expression of E2F1 and p53 and aberrant cellular localization of E2F1 and pRb. In SIVE, E2F1 was abundant in the cytoplasm of neurons in both neurons and astrocytes proximal to SIVE pathology in the basal ganglia. pRb staining was nuclear and cytoplasmic in cortical neurons of SIVE cases. Antibodies to phosphorylated pRb also labeled the cytoplasm of cortical neurons. These data suggest that in SIVE, cell signaling results in phosphorylation of pRb which may result in subsequent alteration in E2F1 activity. As increased E2F1 and p53 activities have been linked to cell death, these data suggest that the neurodegeneration in SIVE could in part be because of changes in expression and activity of cell-cycle machinery.

Identification of E2F-3B, an alternative form of E2F-3 lacking a conserved N-terminal region

We have identified a novel form of the full-length E2F-3 protein that we term E2F-3B. In contrast to full-length E2F-3, which is expressed only at the G1/S boundary, E2F-3B is detected throughout the cell cycle with peak levels in GO where it is associated with Rb. Transfection and in vitro translation experiments demonstrate that a protein identical to E2F-3B in size and iso-electric point is produced from the E2F-3 mRNA via the use of an alternative translational start site. This alternative initiation codon was mapped by mutagenesis to codon 102, an ACG codon. Mutation of the ACG codon at position 102 abolished E2F-3B expression, whereas the conversion of ACG 102 to a consensus ATG led to the expression of a protein indistinguishable from E2F-3B. Given these results, E2F-3B is missing 101 N-terminal amino acids relative to full-length E2F-3. This region includes a moderately conserved sequence of unknown function that is present only in the growth-promoting E2F family members, including E2F-1, 2 and full-length E2F-3. These observations make E2F-3B the first example of an E2F gene giving rise to two different protein species and also suggest that E2F-3 and E2F-3B may have opposing roles in cell cycle control.

Exogenous E2F expression is growth inhibitory before, during, and after cellular transformation

To gain insight into the tumor suppressor properties of E2F1, we investigated growth inhibition by the E2F family of transcription factors using a tissue culture model system. We first show that exogenous E2F expression causes an 80% decrease in NIH3T3 colony formation and activated c-Ha-Ras-mediated focus formation. Inhibition of Ras-mediated transformation was dependent upon E2F DNA binding activity but did not require amino- or carboxy-terminal E2F1 protein interaction domains. Because E2F upregulation has been suggested to be associated with a neoplastic phenotype, it was possible that increased E2F activity would not be inhibitory to previously transformed cells. However, we found that exogenous E2F was also inhibitory to growth of NIH3T3 cells previously transformed by Ras or Neu. Further characterization revealed that exogenous E2F expression is inhibitory at very early times after transfection, causing dramatic losses in transfected cell populations. Interestingly, those few cells which do establish appear to be unaffected by the overexpressed E2F. Therefore, we propose that increased E2F activity may only be tolerated in a subset of cells which have acquired specific alterations that are dominant over E2F-mediated growth inhibition.

Subcellular compartmentalization of E2F family members is required for maintenance of the postmitotic state in terminally differentiated muscle

Maintenance of cells in a quiescent state after terminal differentiation occurs through a number of mechanisms that regulate the activity of the E2F family of transcription factors. We report here that changes in the subcellular compartmentalization of the E2F family proteins are required to prevent nuclei in terminally differentiated skeletal muscle from reentering S phase. In terminally differentiated L6 myotubes, E2F-1, E2F-3, and E2F-5 were primarily cytoplasmic, E2F-2 was nuclear, whereas E2F-4 became partitioned between both compartments. In these same cells, pRB family members, pRB, p107, and p130 were also nuclear. This compartmentalization of the E2F-1 and E2F-4 in differentiated muscle cells grown in vitro reflected their observed subcellular location in situ. We determined further that exogenous E2F-1 or E2F-4 expressed in myotubes at levels fourfold greater than endogenous proteins compartmentalized identically to their endogenous counterparts. Only when overexpressed at higher levels was inappropriate subcellular location for these proteins observed. At these levels, induction of the E2F-regulated genes, cyclins A and E, and suppression of factors associated with myogenesis, myogenin, and p21(Cip1) was observed. Only at these levels of E2F expression did nuclei in these terminally differentiated cells enter S phase. These data demonstrate that regulation of the subcellular compartmentalization of E2F-family members is required to maintain nuclei in a quiescent state in terminally differentiated cells.

The cloning of plant E2F, a retinoblastoma-binding protein, reveals unique and conserved features with animal G(1)/S regulators

Association of the retinoblastoma (Rb) protein with E2F transcription factors is central to cell cycle-specific gene expression and growth in animal cells. Whether Rb-E2F complexes are also involved in plant cell growth and differentiation is still unknown since E2F proteins have not yet been identified in plants. Here we report the isolation and characterisation of a wheat E2F (TmE2F) cDNA clone. Interestingly, the overall domain organisation of plant E2F is related to the human E2F-1/2/3 subset but its primary sequence is slightly more related to the E2F-4/5 subset. TmE2F-Rb binding depends on residues, located at the C-terminus, which are different from those of animal E2Fs. However, the acidic or hydrophobic nature of certain residues is maintained, strongly suggesting that they may have a crucial role in E2F activities. Plant E2F is expressed in proliferating cultured cells and in differentiated tissues and is up-regulated early in S phase. Our studies reinforce the idea that G(1)/S regulators in plants are unrelated to those of yeast cells but similar to those of animal cells and provide new tools to analyse the links between cell cycle regulators, plant growth and developmental signals.

E1A can provoke G1 exit that is refractory to p21 and independent of activating cdk2

E1A can evoke G1 exit in cardiac myocytes and other cell types by displacing E2F transcription factors from tumor suppressor "pocket" proteins and by a less well-characterized p300-dependent pathway. Bypassing pocket proteins (through overexpression of E2F-1) reproduces the effect of inactivating pocket proteins (through E1A binding); however, pocket proteins associate with a number of molecular targets apart from E2F. Hence, pocket protein binding by E1A might engage mechanisms for cell cycle reentry beyond those induced by E2F-1. To test this hypothesis, we used adenoviral gene transfer to express various E2F-1 and E1A proteins in neonatal rat cardiac myocytes that are already refractory to mitogenic serum, in the absence or presence of several complementary cell cycle inhibitors-p16, p21, or dominant-negative cyclin-dependent kinase-2 (Cdk2). Rb binding by E2F-1 was neither necessary nor sufficient for G1 exit, whereas DNA binding was required; thus, exogenous E2F-1 did not merely function by competing for the Rb "pocket." E2F-1-induced G1 exit was blocked by the "universal" Cdk inhibitor p21 but not by p16, a specific inhibitor of Cdk4/6; p21 was permissive for E2F-1 induction of cyclins E and A, but prevented their stimulation of Cdk2 kinase activity. In addition, E2F-1-induced G1 exit was blocked by dominant-negative Cdk2. Forced expression of cyclin E induced endogenous Cdk2 activity but not G1 exit. Thus, E2F-1-induced Cdk2 function was necessary, although not sufficient, to trigger DNA synthesis in cardiac muscle cells. In contrast, pocket protein-binding forms of E1A induced G1 exit that was resistant to inhibition by p21, whereas G1 exit via the E1A p300 pathway was sensitive to inhibition by p21. Both E1A pathways-via pocket proteins and via p300-upregulated cyclins E and A and Cdk2 activity, consistent with a role for Cdk2 in G1 exit induced by E1A. However, p21 blocked Cdk2 kinase activity induced by both E1A pathways equally. Thus, E1A can cause G1 exit without an increase in Cdk2 activity, if the pocket protein-binding domain is intact. E1A also overrides p21 in U2OS cells, provided the pocket protein-binding domain is intact; thus, this novel function of E1A is not exclusive to cardiac muscle cells. In summary, E1A binding to pocket proteins has effects beyond those produced by E2F-1 alone and can drive S-phase entry that is resistant to p21 and independent of an increase in Cdk2 function. This suggests the potential involvement of other endogenous Rb-binding proteins or of alternative E1A targets.

Activation of the murine dihydrofolate reductase promoter by E2F1. A requirement for CBP recruitment

The E2F family of heterodimeric transcription factors plays an important role in the regulation of gene expression at the G1/S phase transition of the mammalian cell cycle. Previously, we have demonstrated that cell cycle regulation of murine dihydrofolate reductase (dhfr) expression requires E2F-mediated activation of the dhfr promoter in S phase. To investigate the mechanism by which E2F activates an authentic E2F-regulated promoter, we precisely replaced the E2F binding site in the dhfr promoter with a Gal4 binding site. Using Gal4-E2F1 derivatives, we found that E2F1 amino acids 409-437 contain a potent core transactivation domain. Functional analysis of the E2F1 core domain demonstrated that replacement of phenylalanine residues 413, 425, and 429 with alanine reduces both transcriptional activation of the dhfr promoter and protein-protein interactions with CBP, transcription factor (TF) IIH, and TATA-binding protein (TBP). However, additional amino acid substitutions for phenylalanine 429 demonstrated a strong correlation between activation of the dhfr promoter and binding of CBP, but not TFIIH or TBP. Finally, transactivator bypass experiments indicated that direct recruitment of CBP is sufficient for activation of the dhfr promoter. Therefore, we suggest that recruitment of CBP is one mechanism by which E2F activates the dhfr promoter.

Active transcriptional repression by the Rb-E2F complex mediates G1 arrest triggered by p16INK4a, TGFbeta, and contact inhibition

Rb inhibits progression from G1 to S phase of the cell cycle. It associates with a number of cellular proteins; however, the nature of these interactions and their relative significance in cell cycle regulation are still unclear. We present evidence that Rb must normally interact with the E2F family of transcription factors to arrest cells in G1, and that this arrest results from active transcriptional repression by the Rb-E2F complex, not from inactivation of E2F. Thus, a major role of E2F in cell cycle regulation is assembly of this repressor complex. We demonstrate that active repression by Rb-E2F mediates the G1 arrest triggered by TGFbeta, p16INK4a, and contact inhibition.

The role of cyclin D3-dependent kinase in the phosphorylation of p130 in mouse BALB/c 3T3 fibroblasts

We have observed that cyclin D3-dependent kinase activity is increased in the late G1 phase in BALB/c 3T3 fibroblasts. The profile of cyclin D3-associated activity closely parallels that of cyclin D1, which is also induced after mitogenic stimulation of quiescent cells. These activities correlate with the appearance of hyperphosphorylated p130, an Rb family member important in regulating E2F-4 and E2F-5 activity in fibroblastic cells. We demonstrated, however, that only the cyclin D3 activity efficiently phosphorylated p130 in an in vitro kinase assay. This apparent specificity was further demonstrated by experiments which demonstrated that cyclin D3 was physically associated with p130 at the times when D3-dependent kinase activity and p130 hyperphosphorylation were observed. Examination of E2F by electrophoretic mobility shift assay revealed that E2F-4 DNA binding activity existed in a p130.E2F complex at times before D3-dependent kinase activity was apparent and in a free E2F-4 complex after D3 activity developed. Thus, our data suggest that cyclin D3 preferentially phosphorylates p130 and is thereby specifically targeted to overcoming growth-suppressive control mediated through p130 pathways.

The molecular and functional characterization of E2F-5 transcription factor

The E2F activity plays a critical role in the control of cell cycle and action of tumor suppressor proteins and is also a target of the transforming proteins of small DNA tumor viruses. We describe here molecular cloning and functional characterization of a fifth member of the E2F family of transcription factors. E2F-5 protein is more homologous to E2F-4 (72% amino acid identity) than to E2F-1, E2F-2, and E2F-3 (35% amino acid identity). Based on structural and functional criteria, the E2F family appears to comprise two distinct sub-families, one composed of E2F-1, E2F-2, and E2F-3 and the other composed of E2F-4 and E2F-5, E2F-5 mRNA is expressed in a wide variety of human tissues. The protein is expressed as multiple species ranging in size from 46 to 54 kDa as a result of differential phosphorylation. The expression of a reporter gene containing E2F binding sites in the promoter is transcriptionally activated by E2F-5 in a cooperative manner with the DP-1 protein. The interaction between E2F-5 and DP-1 is demonstrated using a two-hybrid system in mammalian cells. We have also demonstrated the presence of a strong transactivation domain at the carboxy terminus (273-346 amino acid residues) of E2F-5 protein.

Reduction in fibronectin expression and alteration in cell morphology are coincident in NIH3T3 cells expressing a mutant E2F1 transcription factor

Fibronectin within the extracellular matrix plays a role in cell attachment, spreading, and shape, while it also affects aspects of cell proliferation. Transcription factors such as E2F1 are also known to regulate cell shape and cell proliferation. Yet, to date no linkage has been established between fibronectin expression and E2F1. We show here that cells constitutively expressing a mutant E2F1 protein (E2F1d87) produce reduced amounts of fibronectin mRNA and protein. The altered expression of fibronectin seen in the E2F1d87 expressing cells is due, in part, to a reduction in transcription from the fibronectin promoter. Providing exogenous fibronectin, but not Type I collagen or laminin, as a substrate for cell adhesion is sufficient to revert the altered morphology and reestablish actin-containing microfilaments lost in the mutant cell line. An additional characteristic of the cells expressing the mutant E2F1 is that they demonstrate slow growth and a doubling in S phase duration. While providing exogenous fibronectin as an adhesion substrate did not shorten the S phase duration in the mutant line, it did significantly shorten the S phase duration in the parental NIH3T3 cell line, implicating a role for the extracellular matrix in regulating S phase transit in normal cells.

Inhibition of myogenesis by multiple cyclin-Cdk complexes. Coordinate regulation of myogenesis and cell cycle activity at the level of E2F

During skeletal myogenesis, cell cycle withdrawal accompanies the expression of the contractile phenotype. Here we show that ectopic expression of each D-type cyclin is sufficient to inhibit the transcriptional activation of the muscle-specific creatine kinase (MCK) gene. In contrast, ectopic expression of cyclin A or cyclin E inhibits MCK expression only when they are co-expressed with their catalytic partner cyclin-dependent kinase 2 (Cdk2). For each of these conditions, myogenic transcriptional inhibition is reversed by the ectopic co-expression of the general Cdk inhibitor p21. Inhibition of MCK expression by cyclins or cyclin-Cdk combinations correlates with E2F activation, suggesting that the inhibition is mediated by the overall Rb-kinase activities of the Cdk complexes. In support of this hypothesis, a hyperactive mutant of Rb was found to partially reverse the inhibition of MCK expression by cyclin D1 and by the combination of cyclin A and Cdk2. These data demonstrate that the inhibition of myogenic transcriptional activity is a general feature of overall Cdk activity which is mediated, at least in part, by an pocket protein/E2F-dependent pathway. MCK promoter activity is also inhibited by ectopic E2F1 expression, but this inhibition is not reversed by the co-expression of p21. Analyses of a series of E2F1 mutants revealed that the transcriptional activation, leucine zipper, basic, and cyclin A/Cdk2-binding domains are dispensable, but the helix-loop-helix region is essential for myogenic inhibition. These data demonstrate that myocyte proliferation and differentiation are coordinated at the level of E2F and that these opposing activities are regulated by different E2F domains.

Inhibition of cell proliferation by an RNA ligand that selectively blocks E2F function

The control of cell proliferation is of central importance to the proper development of a multicellular organism, the homeostatic maintenance of tissues, and the ability of certain cell types to respond appropriately to environmental cues. Disruption of normal cell growth control underlies many pathological conditions, including endothelial proliferative disorders in cardiovascular disease as well as the development of malignant tumors. Particularly critical for the control of cell growth is the pathway involving the G1 cyclin-dependent kinases that regulate the Rb family of proteins, which in turn control E2F transcription factor activity. Because E2F is critical for regulation of cell proliferation, we sought to identify and to develop specific inhibitors of E2F function that might also be useful in the control of cellular proliferation. Moreover, because the control of E2F activity appears to be the end result of G1 regulatory cascades, the ability to inhibit E2F may be particularly effective in impeding a wide variety of proliferative events. We have used in vitro selection to isolate several unique RNA species from high complexity RNA libraries that avidly bind to the E2F family of proteins. These RNAs also inhibit the DNA binding capacity of the E2F proteins. We also show that an E2F RNA ligand can block the induction of S phase in quiescent cells stimulated by serum addition. As such, these data demonstrate the critical role for E2F activity in cell proliferation and suggest that such RNA molecules may be effective as therapeutic entities to control cellular proliferation.

E2F1 inhibition of transcription activation by myogenic basic helix-loop-helix regulators

Cellular transcription factor E2F1 is thought to regulate the expression of genes important for cell cycle progression and cell proliferation. Deregulated E2F1 expression induces S-phase entry in quiescent cells and inhibits myogenic differentiation. We show here that E2F1 inhibits the activation of gene transcription by myogenic basic helix-loop-helix proteins myoD and myogenin. Transfection assay using different deletion constructs indicates that both the DNA binding and the transactivation domains of E2F1 are required for its inhibition of myoD transcription activation. However, the retinoblastoma protein (RB) binding domain is not required. Furthermore, co-transfection with the RB, which inhibits the transcription activity of E2F1, can also repress E2F1 inhibition of myoD transactivation. These results suggest an essential role of E2F1-mediated transcription in its inhibition of myogenesis.

RB [corrected] as a modulator of transcription

pRB interacts with a number of transcription factors and can both directly and indirectly modulate transcriptional activity. Growth suppression by pRB is tightly linked to its ability to form complexes with E2F which are capable of repressing transcription of certain genes required for S phase. The ability of pRB to enhance the activity of several non-E2F transcription factors might suggest a mechanism by which pRB could coordinately regulate sets of genes at or near the restriction point. Specifically, complexes consisting of underphosphorylated pRB and E2F, by virtue of transcriptional repression of promoters containing E2F sites, would act to block entry into S phase. At the same time, distinct complexes of underphosphorylated pRB and transcription factors such as the glucocorticoid receptor, ATF-2, or MyoD, might lead to an increase in the transcription of genes required for differentiation or for additional growth inhibitory functions (e.g. TGF-beta 1). Changes in the activities of various cyclin-dependent kinase complexes would lead to phosphorylation of pRB and thus coordinate a release of S phase genes from repression with a loss of activation of differentiation genes. While this model is speculative, the role of pRB as a transcriptional modulator, as well as its interactions with cell-cycle regulatory kinases, places it in a position to integrate extracellular and intracellular growth signals and to transduce those signals into changes in gene transcription which ultimately influence cell growth and differentiation.

Nuclear localization of DP and E2F transcription factors by heterodimeric partners and retinoblastoma protein family members

E2F is a family of transcription factors implicated in the regulation of genes required for progression through G1 and entry into the S phase. The transcriptionally active forms of E2F are heterodimers composed of one polypeptide encoded by the E2F gene family and one polypeptide encoded by the DP gene family. The transcriptional activity of E2F/DP heterodimers is influenced by association with the members of the retinoblastoma tumor suppressor protein family (pRb, p107, and p130). Here the intracellular distribution of E2F and DP proteins was investigated in transiently transfected Chinese hamster and human cells. In transfected cells, DP-1 did not accumulate in the nucleus unless it was coexpressed with the heterodimeric partners E2F-1, E2F-2, or E2F-3. Domain mapping experiments showed that regions of E2F-1 and DP-1 that are required for stable association of the two proteins were also required for nuclear localization of DP-1. Unlike E2F-1, -2, and -3, E2F-4 did not accumulate in the nucleus unless it was coexpressed with DP-2, p107 and p130, but not pRb, stimulated nuclear localization of E2F-4, either alone or in combination with DP-2. These results indicate that DP proteins preferentially associate with specific E2F partners, and suggest that the ability of specific E2F/DP heterodimers to localize in the nucleus contributes to the regulation of E2F activity.

The N-terminal region of E2F-1 is required for transcriptional activation of a new class of target promoter

Because of its expression in numerous cells, the herpes simplex virus thymidine kinase promoter (HSV-TK) is one of the best characterized promoters. Using the HSV-TK promoter as a model system, we have defined a new mode of E2F-1 transcriptional activation which utilizes the N-terminal region of E2F-1. We demonstrate that E2F-1 strongly activated HSV-TK, but in the absence of consensus E2F DNA elements. Nonetheless, E2F-1 could bind to GC-rich elements, which were conclusively identified in classic studies of HSV-TK as SP-1 sites. Second, the transcriptional activation of HSV-TK required the entire E2F-1 protein, including the N-terminal 89 amino acids. In contrast, the N-terminal 89 amino acids of E2F-1 were dispensable for transcriptional activation through consensus E2F sites. Third, we demonstrated that S phase entry is not sufficient for activation of HSV-TK by E2F-1, while the activation through consensus E2F sites is strictly linked to the cell cycle. Taken together, the activation of HSV-TK by E2F-1 proceeds by a different mechanism directed in part through the N-terminal region of E2F-1 and may be uncoupled from the known cell cycle regulatory role.

E2F-1 functions in mice to promote apoptosis and suppress proliferation

Members of the E2F transcription factor family (E2F-1-E2F-5) are believed to be critical positive regulators of cell cycle progression in eukaryotes although the in vivo functions of the individual E2Fs have not been elucidated. Mice were generated that lack E2F-1 and, surprisingly, these mice develop and reproduce normally. However, E2F-1-/- mice exhibit a defect in T lymphocyte development leading to an excess of mature T cells due to a maturation stage-specific defect in thymocyte apoptosis. As E2F-1-/- mice age they exhibit a second phenotype marked by aberrant cell proliferation. These findings suggest that while certain members of the E2F family may positively regulate cell cycle progression, E2F-1 functions to regulate apoptosis and to suppress cell proliferation.

E2F-1 but not E2F-4 can overcome p16-induced G1 cell-cycle arrest

BACKGROUND

The transition from G1 to S phase is the key regulatory step in the mammalian cell cycle. This transition is regulated positively by G1-specific cyclin-dependent kinases (cdks) and negatively by the product of the retinoblastoma tumour suppressor gene, pRb. Hypophosphorylated pRb binds to and inactivates the E2F transcription factor, which controls the expression of genes required for S-phase progression. Hyperphosphorylation of pRb in late G1 phase results in the accumulation of active E2F, a critical event in the progression to S phase. The E2F factor is not a single entity, but rather represents a family of highly related molecules, all of which bind to pRb or the pRb-related proteins p107 and p130.

RESULTS

In this study, we have used specific inhibitors of cdks to explore the requirements for cell-cycle progression from G1 to S phase. Expression of p16Ink4, which specifically inhibits cyclin D-directed cdks, blocks cells in G1 phase; this block can be overcome by expression of the viral proteins that inactivate pRb or by E2F-1. Importantly however, the G1 arrest is not overcome by overexpression of E2F-4. By using chimeric E2F proteins, containing amino-acid sequences from E2F-1 and E2F-4, we have shown that their differential abilities to overcome a p16-imposed arrest is determined by their respective amino-terminal regions. We also demonstrate that E2F-1 can promote entry into S phase without concomitant phosphorylation of pRb. In contrast to the p16-mediated G1 block, G1 arrest mediated by the cdk inhibitors p21Cip1 or p27Kip1 cannot be bypassed either by inactivation of pRb or overexpression of E2F family members.

CONCLUSIONS

These data demonstrate that the role of the cyclin D-directed cdks in promoting the progression of cells from G1 into S phase is wholly to activate an E2F-1-like activity through phosphorylation, thus preventing the formation of the E2F-pRb complex. The cyclin E-cdk2 complex is also required for the G1/S transition but has a different and as yet undefined role. We also provide evidence for a functional difference between E2F-1 and E2F-4, dependant upon the region that contains the DNA-binding and dimerization domains. These results indicate that these two E2F family members are likely to regulate the expression of different subsets of E2F-responsive promoters.

Transforming growth factor-beta1 modulates p107 function in myeloid cells: correlation with cell cycle progression

Transforming growth factor-beta1 (TGF-beta1) is a potent inhibitor of hematopoietic cell growth. Here we report that TGF-beta1 signals inhibition of IL-3-dependent 32D-123 murine myeloid cell growth by modulating the activities of cyclin E and cyclin-dependent kinase 2 (cdk2) proteins and their complex formation in the G1 phase of the cell cycle. Whereas the cyclin E protein was hyperphosphorylated in TGF-beta1 treated cells, TGF-beta1 decreased both the phosphorylation of cdk2 and the kinase activity of the cyclin E-cdk2 complex. Decreased cyclin E-cdk2 kinase activity correlated with decreased phosphorylation of the retinoblastoma-related protein p107. In support of these observations, transient overexpression of p107 inhibited the proliferation of the myeloid cells, and expression of antisense oligodeoxynucleotides to p107 mRNA blocked TGF-beta1 inhibition of myeloid cell growth. Furthermore, as reported previously, in 32D-123 TGF-beta1 treated cells, c-Myc protein expression was decreased. TGF-beta1 increased the binding of p107 to the transcription factor E2F, leading to decreased c-Myc protein levels. p107 inhibited E2F transactivation activity and was also found to bind the c-Myc protein, suggesting p107 negative regulation of c-Myc protein function. These studies demonstrate the modulation of p107 function by TGF-beta1 and suggest a novel mechanism by which TGF-beta1 blocks cell cycle progression in myeloid cells.