Cell cycle analysis of E2F in primary human T cells reveals novel E2F complexes and biochemically distinct forms of free E2F

Expression of cell cycle-regulated proteins pRB2/p130, p107, E2F4, p27, and pCNA in salivary gland tumors: prognostic and diagnostic implications

The retinoblastoma family consists of the tumor suppressor nuclear phosphoprotein pRb/p105 and related proteins p107 and pRb2/p130. Recent immunohistochemical studies of the retinoblastoma family of proteins in lung and endometrial cancer and choroidal melanomas show a tight inverse correlation between the histologic grading in the most aggressive tumor types and pRb2/p130 expression. This led us to investigate the role of pRb2/p130 in salivary tumors. We studied the expression of pRb2/p130, p107, E2F4, p27, and PcNA by immunohistochemistry in a panel of 44 salivary gland tumors. We found a direct correlation between the cytoplasmic expression of pRb2/p130 and tumor grading and the presence of metastasis that was highly statistically significant (P < 0.001). Additionally, increased cytoplasmic pRb2/p130 expression was significantly correlated with a decreased probability of survival (P < 0.001). Interestingly, p107 nuclear expression showed a strong direct correlation when compared with the same variables. pRb2/p130 showed the highest percentage of undetectable nuclear levels in the specimens examined and the tightest inverse correlation (P < 0.0001) with both the histologic grading and pCNA expression in malignant salivary tumors. Additionally, E2F4 showed an identical localization pattern as to that of pRb2/p130. These data suggests an important role for pRb2/p130 in the pathogenesis and progression of certain salivary gland cancers.

The p107 tumor suppressor induces stable E2F DNA binding to repress target promoters

E2F transcription factors are key players in the regulation of proliferation, apoptosis, and differentiation in mammalian cells. E2Fs are negatively regulated by members of the retinoblastoma protein family, Rb, p107 and p130. During adenovirus infection, viral proteins are expressed that displace Rb family members from E2Fs and recruit E2F complexes to viral and cellular promoter regions. This recruitment of E2F involves the induction of stable E2F binding to inverted E2F binding sites in the Ad E2a and cellular E2F-1 promoters and induces both viral and cellular gene expression. The cellular p107 tumor suppressor also displays such regulation of E2F DNA binding activity. p107 induces stable E2F-4/DP binding to inverted E2F binding sites in the Ad E2a and cellular E2F-1 promoters. The induction of E2F DNA binding by p107 minimally requires the sequences in p107 that mediate E2F interaction. The related tumor suppressor, p130, also effects this function. p107 levels increase substantially as cells progress through S phase. p107 induction of E2F DNA binding was observed primarily in S phase cells coincident with the increase in p107 protein levels. The results of promoter activity assays directly correlate the induction of E2F DNA binding by p107 with effective transcriptional repression. These results support a model in which p107 and p130 induce the stable binding of E2F complexes to promoters that drive expression of critical regulatory proteins such as E2F-1. Since p107 and p130 bind histone deacetylase complexes (HDACs) which repress promoter activity, p107-E2F and p130-E2F would stably recruit repressor complexes to effect efficient promoter repression.

Cell cycle arrest and morphological alterations following microinjection of NIH3T3 cells with Pur alpha

Levels of Pur alpha, a protein implicated in control of both DNA replication and gene transcription, fluctuate during the cell cycle, being lowest in early S phase and highest just after mitosis. Here we have employed a new video time-lapse technique enabling us to determine the cell cycle position of each cell in an asynchronous culture at a given time and to ask whether introduction of Pur alpha protein at specific times can affect cell cycle progression. Approximately 80% of all NIH3T3 cells injected with Pur alpha were inhibited from passing through mitosis. Cells injected with Pur alpha during S or G2 phases were efficiently blocked with a 4N (G2 phase) DNA level, as determined by quantitative DNA photometry of individual cells. Of the cells injected with Pur alpha during G1 phase, 40% experienced a rapid cell death characterized by extreme cellular fragmentation. Of those G1 injected cells which remained viable, approximately equal numbers were arrested with either 2N or 4N DNA levels. Cells arrested by Pur alpha in G2 phase grew to cover a large surface area. These results link fluctuations in Pur alpha levels to aspects of cell cycle control.

RBP1 induces growth arrest by repression of E2F-dependent transcription

Growth arrest and cell cycle progression are regulated by the retinoblastoma tumour suppressor pRB and related proteins p130 and p107 that bind to and inhibit the E2F family of transcription factors. Although the precise mechanism of this inhibition remains to be established, previous studies indicated the presence of transcriptional repression activity in the 'pocket' of RB family members. We show here that RBP1, a known pRB pocket-binding protein, possesses transcriptional repression activity and associates with p130-E2F and pRB-E2F complexes specifically during growth arrest. Overexpression of RBP1 both inhibited E2F-dependent gene expression and suppressed cell growth. Thus repression of E2F-dependent transcription by RBP1 via RB family members may play a central role in inducing growth arrest.

Strain-dependent myeloid hyperplasia, growth deficiency, and accelerated cell cycle in mice lacking the Rb-related p107 gene

To investigate the function of the Rb-related p107 gene, a null mutation in p107 was introduced into the germ line of mice and bred into a BALB/cJ genetic background. Mice lacking p107 were viable and fertile but displayed impaired growth, reaching about 50% of normal weight by 21 days of age. Mutant mice exhibited a diathetic myeloproliferative disorder characterized by ectopic myeloid hyperplasia in the spleen and liver. Embryonic p107(-/-) fibroblasts and primary myoblasts isolated from adult p107(-/-) mice displayed a striking twofold acceleration in doubling time. However, cell sort analysis indicated that the fraction of cells in G1, S, and G2 was unaltered, suggesting that the different phases of the cell cycle in p107(-/-) cells was uniformly reduced by a factor of 2. Western analysis of cyclin expression in synchronized p107(-/-) fibroblasts revealed that expression of cyclins E and A preceded that of D1. Mutant embryos expressed approximately twice the normal level of Rb, whereas p130 levels were unaltered. Lastly, mutant mice reverted to a wild-type phenotype following a single backcross with C57BL/6J mice, suggesting the existence of modifier genes that have potentially epistatic relationships with p107. Therefore, we conclude that p107 is an important player in negatively regulating the rate of progression of the cell cycle, but in a strain-dependent manner.

Novel mechanisms of E2F induction by BK virus large-T antigen: requirement of both the pRb-binding and the J domains

E2F activity is regulated in part by the retinoblastoma family of tumor suppressor proteins. Viral oncoproteins, such as simian virus 40 (SV40) large-T antigen (TAg), adenovirus E1A, and human papillomavirus E7, can disrupt the regulation of cellular proliferation by binding to pRb family members and dissociating E2F-pRb family protein complexes. BK virus (BKV), which infects a large percentage of the human population and has been associated with a variety of human tumors, encodes a TAg homologous to SV40 TAg. It has been shown that BKV TAg, when expressed at low levels, does not detectably bind to pRb family members, yet it induces a serum-independent phenotype and causes a decrease in the overall levels of pRb family proteins. The experiments presented in this report show that, despite the lack of TAg-pRb interactions, BKV TAg can induce transcriptionally active E2F and that this induction does in fact require an intact pRb-binding domain as well as an intact J domain. In addition, E2F-pRb family member complexes can be detected in both BKV and SV40 TAg-expressing cells. These results suggest the presence of alternate cellular mechanisms for the release of E2F in addition to the well-established model for TAg-pRb interactions. These results also emphasize a role for BKV TAg in the deregulation of cellular proliferation, which may ultimately contribute to neoplasia.

p130 is dispensable in peripheral T lymphocytes: evidence for functional compensation by p107 and pRB

The proteins encoded by the retinoblastoma gene family, pRB, p107, and p130, have been implicated in the regulation of cellular proliferation, differentiation, and transformation. Because interactions between p130 and E2F transcription factors have been proposed to play a role in the establishment and/or maintenance of quiescence in human peripheral T lymphocytes, we examined lymphoid differentiation and proliferation in p130-deficient mice. We show that p130-/- T cells proliferate normally in culture and exhibit normal cell-mediated immune function in vivo. However, p130-/- T lymphocytes expressed elevated levels of p107, and the characteristic p130-E2F DNA binding complex was replaced by a p107-E2F complex. Adoptive transfer of fetal liver lymphoid progenitors allowed us to circumvent the neonatal lethality associated with loss of p130 and p107 and to analyze the phenotype of p130-/-;p107-/- peripheral T lymphocytes. These cells achieved a quiescent state, exhibited derepression of a subset of E2F target genes, and were hypersensitive to concanavalin A stimulation. Interestingly, a significant portion of the E2F-4 in p130-/-;p107-/- T cells was detected in a complex with pRB and an as-yet-unidentified protein. These findings provide a biochemical basis for functional compensation between pRB family proteins.

E2F activity is regulated by cell cycle-dependent changes in subcellular localization

E2F directs the cell cycle-dependent expression of genes that induce or regulate the cell division process. In mammalian cells, this transcriptional activity arises from the combined properties of multiple E2F-DP heterodimers. In this study, we show that the transcriptional potential of individual E2F species is dependent upon their nuclear localization. This is a constitutive property of E2F-1, -2, and -3, whereas the nuclear localization of E2F-4 is dependent upon its association with other nuclear factors. We previously showed that E2F-4 accounts for the majority of endogenous E2F species. We now show that the subcellular localization of E2F-4 is regulated in a cell cycle-dependent manner that results in the differential compartmentalization of the various E2F complexes. Consequently, in cycling cells, the majority of the p107-E2F, p130-E2F, and free E2F complexes remain in the cytoplasm. In contrast, almost all of the nuclear E2F activity is generated by pRB-E2F. This complex is present at high levels during G1 but disappears once the cells have passed the restriction point. Surprisingly, dissociation of this complex causes little increase in the levels of nuclear free E2F activity. This observation suggests that the repressive properties of the pRB-E2F complex play a critical role in establishing the temporal regulation of E2F-responsive genes. How the differential subcellular localization of pRB, p107, and p130 contributes to their different biological properties is also discussed.

The accumulation of an E2F-p130 transcriptional repressor distinguishes a G0 cell state from a G1 cell state

Previous studies have demonstrated cell cycle-dependent specificities in the interactions of E2F proteins with Rb family members. We now show that the formation of an E2F-p130 complex is unique to cells in a quiescent, G0 state. The E2F-p130 complex does not reform when cells reenter a proliferative state and cycle through G1. The presence of an E2F-p130 complex in quiescent cells coincides with the E2F-mediated repression of transcription of the E2F1 gene, and we show that the E2F sites in the E2F1 promoter are important as cells enter quiescence but play no apparent role in cycling cells. In addition, the decay of the E2F-p130 complex as cells reenter the cell cycle requires the action of G1 cyclin-dependent kinase activity. We conclude that the accumulation of the E2F-p130 complex in quiescent cells provides a negative control of certain key target genes and defines a functional distinction between these G0 cells and cells that exist transiently in G1.

E2F-4 switches from p130 to p107 and pRB in response to cell cycle reentry

The E2F transcription factor couples the coordinate expression of cell cycle proteins to their appropriate transition points. Its activity is controlled by the cell cycle regulators pRB, p107, and p130. These bind to E2F at defined but distinct stages of the cell cycle. Using specific antisera, we have identified the DP and E2F components of each of these species. Although present at very different levels, DP-1 and DP-2 are evenly distributed among each of these complexes. In contrast, the individual E2Fs have distinctly different binding profiles. Consistent with previous studies, E2F-1, E2F-2, and E2F-3 bind specifically to the retinoblastoma protein. In each case, their expression and DNA binding activity are restricted to post-G1/S fractions. Surprisingly, E2F-1 and E2F-3 make unequal contributions to the pRB-associated and free E2F activity, suggesting that these proteins perform different cell cycle functions. Most significantly, this study showed E2F-4 accounts for the vast majority of the endogenous E2F activity. In arrested cells, E2F-4 is sequestered by the p130 protein. However, as the cells pass the G1-to-S transition, the levels of pRB and p107 increase and E2F-4 now associates with both of these regulators. Despite this, a considerable amount of E2F-4 exists as free E2F. In G1 cells, this accounts for almost all of the free activity. Once the cells enter S phase, free E2F is composed of an equal mixture of E2F-4 and E2F-1.

A potent transrepression domain in the retinoblastoma protein induces a cell cycle arrest when bound to E2F sites

An intact T/E1A-binding domain (the pocket) is necessary, but not sufficient, for the retinoblastoma protein (RB) to bind to DNA-protein complexes containing E2F and for RB to induce a G1/S block. Indirect evidence suggests that the binding of RB to E2F may, in addition to inhibiting E2F transactivation function, generate a complex capable of functioning as a transrepressor. Here we show that a chimera in which the E2F1 transactivation domain was replaced with the RB pocket could, in a DNA-binding and pocket-dependent manner, mimic the ability of RB to repress transcription and induce a cell cycle arrest. In contrast, a transdominant negative E2F1 mutant that is capable of blocking E2F-dependent transactivation did not. Fusion of the RB pocket to a heterologous DNA-binding domain unrelated to E2F likewise generated a transrepressor protein when scored against a suitable reporter. These results suggest that growth suppression by RB is due, at least in part, to transrepression mediated by the pocket domain bound to certain promoters via E2F.

Lack of functional retinoblastoma protein mediates increased resistance to antimetabolites in human sarcoma cell lines

Growth inhibition assays indicated that the IC50 values for methotrexate (MTX) and 5-fluorodeoxyuridine (FdUrd) in HS-18, a liposarcoma cell line lacking retinoblastoma protein (pRB), and SaOS-2, an osteosarcoma cell line with a truncated and nonfunctional pRB, were 10- to 12-fold and 4- to 11-fold higher, respectively, than for the HT-1080 (fibrosarcoma) cell line, which has wild-type pRB. These Rb-/- cell lines exhibited a 2- to 4-fold increase in both dihydrofolate reductase (DHFR) and thymidylate synthase (TS) enzyme activities as well as a 3- to 4-fold increase in mRNA levels for these enzymes compared to the HT-1080 (Rb+/+) cells. This increase in expression was not due to amplification of the DHFR and TS genes. Growth inhibition by MTX and FdUrd was increased and DHFR and TS activities and expression were correspondingly decreased in Rb transfectants of SaOS-2 cells. In contrast, there was no significant difference in growth inhibition among these cell lines for the nonantimetabolites VP-16, cisplatin, and doxorubicin. A gel mobility-shift assay showed that parental SaOS-2 cells had increased levels of free E2F compared to the Rb-reconstituted SaOS-2 cells. These results indicate that pRB defective cells may have decreased sensitivity to growth inhibition by target enzymes encoded by genes whose transcription is enhanced by E2F proteins and suggest mechanisms of interaction between cytotoxic agents and genes involved in cell cycle progression.

Role of pRb-related proteins in simian virus 40 large-T-antigen-mediated transformation

Simian virus 40 large T-antigen (TAg) transformation is thought to be mediated, at least in part, by binding to and modulating the function of certain cellular proteins, including the retinoblastoma tumor suppressor gene product, pRb. TAg can disrupt the inhibitory complexes formed by pRb with the oncogenic transcription factor E2F, and this mechanism has been suggested to be important for TAg-mediated transformation. Residues 102 to 114 of TAg (including the LXCXE motif) are required for binding to pRb. Mutations within this LXCXE motif abolish the ability of TAg to bind to pRb as well as to transform certain cell types. TAg can also bind to at least two other cellular proteins, p107 and p130, that are related to pRb by sequence homology and share the ability to bind E2F. However, whether p107 and p130 are also targets in TAg-mediated transformation is less clear. To assess the relative contribution of the inactivation of pRb, p107, and p130 to transformation by TAg, fibroblasts were prepared from embryos derived from matings of mice heterozygous for an Rb knockout allele. The ability of TAg to transform fibroblasts homozygous for either wild-type or knockout Rb alleles was evaluated. It is demonstrated that the integrity of the LXCXE motif provides a growth advantage in Rb+/+ and Rb-/- cells. Furthermore, wild-type TAg, but not the LXCXE mutants, could bind to p107 and p130 and disrupt p107-E2F and p130-E2F binding complexes. These results suggest that p107 and p130 participate in TAg-mediated transformation and that they may behave as tumor suppressors.

Characterization of cis-regulatory elements of the c-myc promoter responding to human GM-CSF or mouse interleukin 3 in mouse proB cell line BA/F3 cells expressing the human GM-CSF receptor

Interleukin 3 (IL-3) or granulocyte macrophage colony-stimulating factor (GM-CSF) activates c-fos, c-jun, and c-myc genes and proliferation in both hematopoietic and nonhematopoietic cells. Using a series of deletion mutants of the beta subunit of human GM-CSF receptor (hGMR) and inhibitors of tyrosine kinase, two distinct signaling pathways, one for activation of c-fos and c-jun genes, and the other for cell proliferation and activation of c-myc gene have been elucidated. In contrast to wealth of information on the pathway leading to activation of c-fos/c-jun genes, knowledge of the latter is scanty. To clarify the mechanisms of activation of c-myc gene by cytokines, we established a transient transfection assay in mouse proB cell line BA/F3 cells expressing hGMR. Analyses of hGMR beta subunit mutants revealed two cytoplasmic regions involved in activation of the c-myc promoter, one is essential and the other is dispensable but enhances the activity. These regions are located at the membrane proximal and the distal regions covering amino acid positions 455-544 and 544-589, respectively. Characterization of cis-acting regulatory elements of the c-myc gene showed that the region containing the P2 promoter initiation site is sufficient to mediate the response to mIL-3 or hGM-CSF. Electrophoretic mobility shift assay using an oligonucleotide corresponding to the distal putative E2F binding site revealed that p107/E2F complex, the negative regulator of E2F, decreased, and free E2F increased after mIL-3 stimulation. These results support the thesis that mIL-3 or hGM-CSF regulates the c-myc promoter by altering composition of the E2F complexes at E2F binding site.

E2F-5, a new E2F family member that interacts with p130 in vivo

E2F DNA binding sites are found in a number of genes whose expression is tightly regulated during the cell cycle. The activity of E2F transcription factors is regulated by association with specific repressor molecules that can bind and inhibit the E2F transactivation domain. For E2F-1, E2F-2, and E2F-3, the repressor is the product of the retinoblastoma gene, pRb. E2f-4 interacts with pRb-related p107 and not with pRb itself. Recently, a cDNA encoding a third member of the retinoblastoma gene family, p130, was isolated. p130 also interacts with E2F DNA binding activity, primarily in the G0 phase of the cell cycle. We report here the cloning of a fifth member of the E2F gene family. The human E2F-5 cDNA encodes a 346-amino-acid protein with a predicted molecular mass of 38 kDa. E2F-5 is more closely related to E2F-4 (78% similarity) than to E2F-1 (57% similarity). E2F-5 resembles the other E2Fs in that it binds to a consensus E2F site in a cooperative fashion with DP-1. By using a specific E2F-5 antiserum, we found that under physiological conditions, E2F-5 interacts preferentially with p130.

Characterization of the deoxycytidine kinase promoter in human lymphoblast cell lines

Deoxycytidine kinase (dCK) phosphorylates 2'-deoxycytidine, as well as the purine deoxyribonucleosides and a number of nucleoside analogues that are important in the chemotherapy of leukemias. The enzyme is highly expressed in the thymus relative to other tissues and may play an important role in the T cell depletion associated with adenosine deaminase and purine nucleoside phosphorylase deficiencies. To characterize the dCK promoter region and to determine whether it mediates higher levels of gene expression in T lymphoblasts, we have analyzed a 700-bp genomic fragment encompassing 548 bp of 5' flanking region for functional activity and for transcription factor binding using T and B lymphoblast cell lines and nuclear extracts. The regions of the promoter that were defined as important to its function include a 5' GC box, and E box, a 3' GC box, and an E2F site. The transcription factor Sp1 binds to both GC boxes, activating at the 5' site but repressing at the 3' site. MLTF/USF activates transcription through the E box, whereas E2F activates through the E2F site, but binds weakly to this site in vitro and does not appear to mediate cell cycle-specific expression of dCK in vivo. No significant differences in promoter activity or transcription factor binding were observed between Jurkat T and Raji B lymphoblasts. The promoter of the dCK gene is thus regulated by a number of ubiquitously expressed transcription factors. DCK expression in cultured lymphoblast cell lines is not solely a function of the T or B lineage derivation.

Multiple change in E2F function and regulation occur upon muscle differentiation

We have examined regulation of the E2F transcription factor during differentiation of muscle cells. E2F regulates many genes involved in growth control and is also the target of regulation by diverse cellular signals, including the RB family of growth suppressors (e.g., the retinoblastoma protein [RB], p107, and p130). The following aspects of E2F function and regulation during muscle differentiation were investigated: (i) protein-protein interactions, (ii) protein levels, (iii) phosphorylation of the E2F protein, and (iv) transcriptional activity. A distinct E2F complex was present in differentiated cells but not in undifferentiated cells. The p130 protein was a prominent component of the E2F complex associated with differentiation. In contrast, in undifferentiated cells, the p107 protein was the prominent component in one of three E2F complexes. In addition, use of a differentiation-defective muscle line provided genetic and biochemical evidence that quiescence and differentiation are separable events. Exclusive formation of the E2F-p130 complex did not occur in this differentiation-defective line; however, E2F complexes diagnostic of quiescence were readily apparent. Thus, sole formation of the E2F-p130 complex is a necessary event in terminal differentiation. Other changes in E2F function and regulation upon differentiation include decreased phosphorylation and increased repression by E2F. These observations suggest that the regulation of E2F function during terminal differentiation may proceed through differential interaction within the RB family and/or phosphorylation.

Multiple members of the E2F transcription factor family are the products of oncogenes

The retinoblastoma gene product (pRB) is a known tumor suppressor, capable of arresting growth in mid-to-late G1. Part of its growth suppression action arises from interaction(s) with one or more members of the E2F family of transcription factors. These proteins most likely contribute to progression from G0 to S phase in mammalian cells, and pRB binding most likely inhibits aspects of their suspected growth-promoting function. Given their growth-stimulating potential, we asked whether one or more E2F alleles can function as oncogenes. Uncloned pools of NIH 3T3 cells producing the pRB binding target E2F-1, E2F-2, or E2F-3 grew in semisolid medium. In addition, they grew to much higher saturation density than controls. From the study of cells producing selected E2F-1 mutant species, it appears that E2F DNA-binding function contributes to, and pRB/E2F binding suppresses, soft-agar growth. Thus, three E2F family members can act as oncogene products, suggesting that part of the normal role of pRB is to down-modulate this potential activity.

The transcription factor E2F-1 is a downstream target of RB action

Reintroduction of RB into SAOS2 (RB-/-) cells causes a G1 arrest and characteristic cellular swelling. Coexpression of the cellular transcription factor E2F-1 could overcome these effects. The ability of E2F-1 to bind to RB was neither necessary nor sufficient for this effect, and S-phase entry was not accompanied by RB hyperphosphorylation under these conditions. Furthermore, E2F-1 could overcome the actions of a nonphosphorylatable but otherwise intact RB mutant. These data, together with the fact that RB binds to E2F-1 in vivo, suggest that E2F-1 is a downstream target of RB action. Mutational analysis showed that the ability of E2F-1 to bind to DNA was necessary and sufficient to block the formation of large cells by RB, whereas the ability to induce S-phase entry required a functional transactivation domain as well. Thus, the induction of a G1 arrest and the formation of large cells by RB in these cells can be genetically dissociated. Furthermore, the ability of the E2F-1 DNA-binding domain alone to block one manifestation of RB action is consistent with the notion that RB-E2F complexes actively repress transcription upon binding to certain E2F-responsive promoters. In keeping with this view, we show here that coproduction of an E2F1 mutant capable of binding to DNA, yet unable to transactivate, is sufficient to block RB-mediated transcriptional repression.

A new platelet-derived growth factor-regulated genomic element which binds a serine/threonine phosphoprotein mediates induction of the slow immediate-early gene MCP-1

The MCP-1 chemokine gene belongs to a cohort of immediate-early genes that are induced with slower kinetics than c-fos. In this study, we identified a cluster of four platelet-derived growth factor (PDGF)-responsive elements within a 240-bp enhancer found in the distal 5' flanking MCP-1 sequences. Two of the elements bind one or more forms of the transcription factor NF-kappa B. We focused on the other two elements which are hitherto unreported, PDGF-regulated genomic motifs. One of these novel elements, detected as a 28-mer by DNase I footprinting, restores PDGF inducibility when added in two copies to a 5' truncated MCP-1 gene. A single copy of the second novel element, a 27-mer, restores PDGF inducibility to a 5' truncated MCP-1 gene. The 27-base element interacts with a PDGF-activated serine/threonine phosphoprotein that is detected only within the nucleus of PDGF-treated 3T3 cells. DNA binding of this phosphoprotein is activated by PDGF treatment with slow kinetics that match the time course of MCP-1 gene expression, and activation is not inhibited by cycloheximide. PDGF-activated binding to the 27-mer is shown to involve a single 30-kDa protein by UV-cross-linking analysis.

Cyclin A/CDK2 binds directly to E2F-1 and inhibits the DNA-binding activity of E2F-1/DP-1 by phosphorylation

E2F-1, a member of the E2F transcription factor family, contributes to the regulation of the G1-to-S phase transition in higher eukaryotic cells. E2F-1 forms a heterodimer with DP-1 and binds to several cell cycle regulatory proteins, including the retinoblastoma family (RB, p107, p130) and cyclin A/CDK2 complexes. We have analyzed E2F-1 phosphorylation and its interaction with cyclin A/CDK2 complexes both in vivo and in vitro. In vitro, E2F-1 formed a stable complex with cyclin A/CDK2 but not with either subunit alone. DP-1 did not interact with cyclin A, CDK2, or the cyclin A/CDK2 complex. While the complex of cyclin A/CDK2 was required for stable complex formation with E2F-1, the kinase-active form of CDK2 was not required. However, E2F-1 was phosphorylated by cyclin A/CDK2 in vitro and was phosphorylated in vivo in HeLa cells. Two-dimensional tryptic phosphopeptide mapping studies demonstrated an overlap in the phosphopeptides derived from E2F-1 labeled in vitro and in vivo, indicating that cyclin A/CDK2 may be responsible for the majority of E2F-1 phosphorylation in vivo. Furthermore, an active DNA-binding complex could be reconstituted from purified E2F-1/DP-1 and cyclin A/CDK2. Binding studies conducted both in vitro and in vivo demonstrated that the cyclin A/CDK2-binding region resided within the N-terminal 124 amino acids of E2F-1. Because the stable association of E2F-1 with cyclin A/CDK2 in vitro and in vivo did not require a DP-1- or RB-binding domain and because the interactions could be reconstituted from purified components in vitro, we conclude that the interactions between cyclin A/CDK2 and E2F-1 are direct. Finally, we report that the DNA-binding activity of the E2F-1/DP-1 complex is inhibited following phosphorylation by cyclin A/CDK2.

Cyclin A/CDK2 binds directly to E2F-1 and inhibits the DNA-binding activity of E2F-1/DP-1 by phosphorylation

E2F-1, a member of the E2F transcription factor family, contributes to the regulation of the G1-to-S phase transition in higher eukaryotic cells. E2F-1 forms a heterodimer with DP-1 and binds to several cell cycle regulatory proteins, including the retinoblastoma family (RB, p107, p130) and cyclin A/CDK2 complexes. We have analyzed E2F-1 phosphorylation and its interaction with cyclin A/CDK2 complexes both in vivo and in vitro. In vitro, E2F-1 formed a stable complex with cyclin A/CDK2 but not with either subunit alone. DP-1 did not interact with cyclin A, CDK2, or the cyclin A/CDK2 complex. While the complex of cyclin A/CDK2 was required for stable complex formation with E2F-1, the kinase-active form of CDK2 was not required. However, E2F-1 was phosphorylated by cyclin A/CDK2 in vitro and was phosphorylated in vivo in HeLa cells. Two-dimensional tryptic phosphopeptide mapping studies demonstrated an overlap in the phosphopeptides derived from E2F-1 labeled in vitro and in vivo, indicating that cyclin A/CDK2 may be responsible for the majority of E2F-1 phosphorylation in vivo. Furthermore, an active DNA-binding complex could be reconstituted from purified E2F-1/DP-1 and cyclin A/CDK2. Binding studies conducted both in vitro and in vivo demonstrated that the cyclin A/CDK2-binding region resided within the N-terminal 124 amino acids of E2F-1. Because the stable association of E2F-1 with cyclin A/CDK2 in vitro and in vivo did not require a DP-1- or RB-binding domain and because the interactions could be reconstituted from purified components in vitro, we conclude that the interactions between cyclin A/CDK2 and E2F-1 are direct. Finally, we report that the DNA-binding activity of the E2F-1/DP-1 complex is inhibited following phosphorylation by cyclin A/CDK2.

Reciprocal regulation of the Epstein-Barr virus BamHI-F promoter by EBNA-1 and an E2F transcription factor

The Epstein-Barr virus BamHI-F promoter (Fp) is one of three used to transcribe the EBNA latency proteins, in particular, EBNA-1, the only viral gene product needed for episomal replication. Fp is distinguished by possession of the only EBNA-1 binding sites (the Q locus) in the Epstein-Barr virus genome outside oriP. Activity of Fp is negatively autoregulated by interaction of EBNA-1 at two sites in the Q locus, which is situated downstream of the RNA start site. We demonstrate in transient assays that this EBNA-1-mediated repression of Fp can be overcome by an E2F transcription factor which interacts with the DNA at a site centered between the two EBNA-1 binding sites within the Q locus. An E2F-1 fusion protein protects the sequence 5'-GGATGGCGGGTAATA-3' from DNase I digestion, and a DNA probe containing this sequence binds an E2F-specific protein complex from cell extracts, although this region is only loosely homologous with known consensus binding sites for E2F transcription factors. In mobility shift assays, E2F can displace the binding of EBNA-1 from the Q locus but not from oriP, where the E2F binding site is not present. E2F also activates expression of Fp in epithelial cells. These findings identify a potentially new binding site for members of the E2F family of transcription factors and suggest that such a factor is important for expression of EBNA-1 in lymphoid and epithelial cells by displacing EBNA-1 from the Q locus. In addition, the possibility that Fp activity is under cell cycle control is raised. Since the supply of functional E2F varies during the cell cycle and since in these assays overexpression of E2F can overcome repression of Fp by EBNA-1, control of transcription of EBNA-1 mRNA by cell cycle regulatory factors may help to bring about ordered replication of episomes.

Reciprocal regulation of the Epstein-Barr virus BamHI-F promoter by EBNA-1 and an E2F transcription factor

The Epstein-Barr virus BamHI-F promoter (Fp) is one of three used to transcribe the EBNA latency proteins, in particular, EBNA-1, the only viral gene product needed for episomal replication. Fp is distinguished by possession of the only EBNA-1 binding sites (the Q locus) in the Epstein-Barr virus genome outside oriP. Activity of Fp is negatively autoregulated by interaction of EBNA-1 at two sites in the Q locus, which is situated downstream of the RNA start site. We demonstrate in transient assays that this EBNA-1-mediated repression of Fp can be overcome by an E2F transcription factor which interacts with the DNA at a site centered between the two EBNA-1 binding sites within the Q locus. An E2F-1 fusion protein protects the sequence 5'-GGATGGCGGGTAATA-3' from DNase I digestion, and a DNA probe containing this sequence binds an E2F-specific protein complex from cell extracts, although this region is only loosely homologous with known consensus binding sites for E2F transcription factors. In mobility shift assays, E2F can displace the binding of EBNA-1 from the Q locus but not from oriP, where the E2F binding site is not present. E2F also activates expression of Fp in epithelial cells. These findings identify a potentially new binding site for members of the E2F family of transcription factors and suggest that such a factor is important for expression of EBNA-1 in lymphoid and epithelial cells by displacing EBNA-1 from the Q locus. In addition, the possibility that Fp activity is under cell cycle control is raised. Since the supply of functional E2F varies during the cell cycle and since in these assays overexpression of E2F can overcome repression of Fp by EBNA-1, control of transcription of EBNA-1 mRNA by cell cycle regulatory factors may help to bring about ordered replication of episomes.

Molecular characterization of Xenopus laevis DP proteins

It is widely believed that in mammalian cells the cellular transcription factor (DRTF1/E2F integrates cell-cycle events with the transcription apparatus by interacting with important regulators of the cell cycle, such as the retinoblastoma gene product (pRb) and related proteins, cyclins, and cyclin-dependent kinases. Here, we have defined DRTF1/E2F in Xenopus laevis that, like its mammalian counterpart, specifically binds to the E2F site, is regulated during development, and interacts with pRb and related proteins. We have isolated cDNAs that encode the functional homologue of mammalian DP-1, X1 DP-1, together with a close relative, X1 DP-2. X1 DP-1, which is highly conserved with murine DP-1, is a major DNA binding component of X1 DRTF1/E2F. Both DP-1 and DP-2 synergistically interact with members of the E2F family of proteins, E2F-1, E2F-2, and E2F-3, to generate DNA binding complexes that specifically recognize the E2F site and functionally interact with E2F-1 in E2F site-dependent transcriptional activation of cellular genes. DP-1 and DP-2 encode maternally stored transcripts that are expressed during early development. In the adult however, the expression of DP-1 and DP-2 is tissue restricted. This study therefore defines a new family of transcription factors, the DP proteins, members of which can interact combinatorially with E2F proteins to generate an array of DNA binding complexes that integrate cell-cycle progression with the transcription apparatus through the E2F binding site. The tissue-specific expression of DP family members suggests that the combination of DP/E2F heterodimers that constitute DRTF1/E2F is influenced by the phenotype of the cell.

Cloning, chromosomal location, and characterization of mouse E2F1

E2F has been implicated in growth control because of its association with the retinoblastoma protein and the presence of E2F binding sites in the promoters of several growth-regulated genes. Proteins that bind to an E2F site have been cloned from human and mouse cells. However, these two proteins (human E2F1 and mouse DP-1) are quite different in sequence. We have now cloned a mouse cDNA encoding a protein 86% identical to the human E2F1 protein. The mouse E2F1 cDNA encodes a 430-amino-acid protein with a predicted molecular weight of 46,322 and detects mRNAs of 2.7 and 2.2 kb. Using primers complementary to sequences in the mouse E2F1 3' untranslated region, we mapped the mouse E2F1 gene to chromosome 2, near the Agouti and c-src loci. To understand the role of the different E2F family members in the growth of mouse NIH 3T3 cells, we examined the levels of E2F1 and DP-1 mRNAs in different stages of the cell cycle. Since the levels of E2F1 but not DP-1 mRNA correlated with changes in transcription from the dhfr promoter, we examined whether E2F1 could activate various growth-regulated promoters. We found that E2F1 could activate some (dhfr, thymidine kinase, and DNA polymerase alpha) but not all (thymidylate synthase, cad, and c-myc) of these promoters. On the basis of changes in levels of E2F1 and its ability to transactivate growth-regulated promoters, we propose that E2F1 may mediate growth factor-initiated signal transduction.

Cloning, chromosomal location, and characterization of mouse E2F1

E2F has been implicated in growth control because of its association with the retinoblastoma protein and the presence of E2F binding sites in the promoters of several growth-regulated genes. Proteins that bind to an E2F site have been cloned from human and mouse cells. However, these two proteins (human E2F1 and mouse DP-1) are quite different in sequence. We have now cloned a mouse cDNA encoding a protein 86% identical to the human E2F1 protein. The mouse E2F1 cDNA encodes a 430-amino-acid protein with a predicted molecular weight of 46,322 and detects mRNAs of 2.7 and 2.2 kb. Using primers complementary to sequences in the mouse E2F1 3' untranslated region, we mapped the mouse E2F1 gene to chromosome 2, near the Agouti and c-src loci. To understand the role of the different E2F family members in the growth of mouse NIH 3T3 cells, we examined the levels of E2F1 and DP-1 mRNAs in different stages of the cell cycle. Since the levels of E2F1 but not DP-1 mRNA correlated with changes in transcription from the dhfr promoter, we examined whether E2F1 could activate various growth-regulated promoters. We found that E2F1 could activate some (dhfr, thymidine kinase, and DNA polymerase alpha) but not all (thymidylate synthase, cad, and c-myc) of these promoters. On the basis of changes in levels of E2F1 and its ability to transactivate growth-regulated promoters, we propose that E2F1 may mediate growth factor-initiated signal transduction.

The cell cycle and the retinoblastoma protein family

Tumor formation results from alterations in the control of normal cell proliferation. To further our understanding of the molecular mechanisms underlying the deregulation of cell proliferation much attention, over the past decade, has been focused on the function of proto-oncogenes. Cellular oncogenes are thought to be growth promoting. More recently, a class of genes known as tumor suppressors have come under intense study. Tumor suppressors are largely thought to restrain cell proliferation. The retinoblastoma protein (Rb) is one of a growing list of tumor suppressors. Concurrent with the study of tumor suppressor genes has been a rapid increase in our understanding of the cell cycle at the molecular level. Rb and a related protein p107 are involved in the processes of cell proliferation and differentiation. Each functionally interacts with and affects the activity of the transcription factor E2F as well as other transcription factors involved in cell proliferation and differentiation. Additionally, Rb and p107 are modified by, and/or form specific complexes with, several elements of the basic cell cycle machinery. Specifically, Rb and p107 interact with and are modified by various cyclins and cyclin dependent kinases (cdk), some of which have been shown to be essential for cell cycle progression and in some cases their deregulation has been implicated in the development of cancer. This review will attempt to convey our current functional and mechanistic understanding of the biological roles Rb and p107 play in proliferation, development and differentiation. A knowledge of the interplay between these positive and negative regulators of cell proliferation and differentiation, noted above, is central to our understanding of human cancer.