Analysis of p107-associated proteins: p107 associates with a form of E2F that differs from pRB-associated E2F-1

Introduction to the E2F family: protein structure and gene regulation

E2F is a heterodimer composed of two partners, such as E2F1 and DP1. Although E2F1 can bind DNA as a homodimer and increase promoter activity, optimal DNA-binding and transcriptional activity occurs in the heterodimeric form. A model (Fig. 3) for the involvement of E2F activity in cell growth control that incorporates viral oncoproteins, positive regulators of cell growth (cyclins) and negative regulators of cell growth (tumor suppressor proteins) can now be advanced. Each aspect of this model is addressed in subsequent chapters of this book. It is likely that binding of growth-suppressing proteins, such as Rb, can inhibit the transactivation potential of E2F1, either by blocking the interaction of E2F1 with a separate component of the transcription complex or by bringing a repressor domain to the transcription complex (Flemington et al. 1993; Helin et al. 1993; Weintraub et al. 1992; Zamanian and La Thangue 1993; Zhu et al. 1993). Phosphorylation or sequestration of Rb by viral oncoproteins can free E2F. The influence of viral oncoproteins on E2F activity and the regulation of the different E2F complexes is the focus of the contributions by Cobrinik and by Cress and Nevens. The interaction of the free E2F induces a bend in the DNA that may also play a role in transactivation, perhaps by bringing proteins (such as an Sp1 or CCAAT family member) separated by distance on the promoter DNA into contact (Huber et al. 1994). Because E2F target genes encode proteins critical for cell growth, deregulation of E2F activity can have severe consequences, such as apoptosis or uncontrolled proliferation. The effect of altered expression of E2F activity on the cell cycle and on tumorigenicity is the focus of the contribution by Adams and Kaelin. Finally, a comparison of E2F to the genetically well-characterized factors that regulate G1/S phase transcription in yeast is the subject of the chapter by Breeden. This volume concludes with Farnham's summary of the rapid gains in knowledge concerning the E2F gene family that have been made in the past several years and provides a series of questions and lines of investigation that will be the focus of future studies.

Immunohistochemical expression of retinoblastoma protein in cutaneous melanomas

Retinoblastoma protein (pRB) is the product of a tumour-suppressor gene (rb) mapped to chromosome 13q14. pRB acts as a control checkpoint at the G1 phase of the cell cycle, preventing cells from entering into the S phase. Mutational inactivation of both normal alleles leads to loss of pRB expression and the development of malignant neoplasms. Absence of pRB occurs in retinoblastomas, sarcomas and several other types of tumours. The potential role of pRB in the pathogenesis of cutaneous melanoma is unknown, and was the subject of this investigation. Formalin-fixed, paraffin-embedded sections of four cutaneous melanoma metastases, 17 primary invasive melanomas and 10 predominantly intradermal melanocytic naevi were studied. Monoclonal antibodies directed against pRB and Ki-67 antigen were used after microwave heating of sections to restore antigenicity. pRB was not detected in morphologically normal epidermal melanocytes. In five naevi, only scattered cells (1%) expressed pRB, whereas in the other five naevi, pRB expression was undetectable. In contrast, pRB was detected in all primary and metastatic melanomas (5-70% of cells). Expression was always localized to nuclei. Ki-67 expression was detected only in the melanomas, with both cellular staining and regional localization similar to that shown by pRB in 13 of the 20 melanomas studied with both antibodies. pRB appears to be expressed at higher levels in melanomas than in benign naevi. It therefore seems unlikely that loss of rb expression is an important factor in the pathogenesis of melanoma.

E2F-1 accumulation bypasses a G1 arrest resulting from the inhibition of G1 cyclin-dependent kinase activity

Numerous experiments have defined a critical role for the G1 cyclins and associated kinases in allowing a normal progression of cells from a quiescent state, through G1, and into S phase. We now demonstrate that G1 cyclin-dependent kinase activity is critical for the accumulation of E2F activity late in G1. Moreover, E2F-1 overexpression can overcome a G1 arrest caused by the inhibition of G1 cyclin-dependent kinase activity, consistent with E2F activation being an important consequence of the action of G1 cyclins. E2F-1 also overcomes a G1 block caused by gamma irradiation and leads to an apparent complete replication of the cellular genome and entry into mitosis. This E2F-1-mediated induction of S phase and mitosis is not accompanied by the rise in either cyclin D-associated kinase activity or cdk2 activity that is normally observed during the G1 phase of the cell cycle. We conclude that one key function for G1 cyclin-dependent kinase activity is the activation of E2F-1, that the accumulation of E2F activity may be sufficient to allow initiation and completion of S phase, but that additional events, including G1 cyclin kinase activity, are likely necessary for a normal proliferative event.

v-Abl activates c-myc transcription through the E2F site

The v-abl oncogene of Abelson murine leukemia virus encodes a deregulated form of the cellular nonreceptor tyrosine kinase. v-Abl activates c-myc transcription, and c-Myc is an essential downstream component in the v-Abl transformation program. To explore the mechanism by which v-Abl activates c-myc transcription, a cotransfection assay was developed. We show that transactivation of a c-myc promoter by v-Abl requires the SH1 (tyrosine kinase) and SH2 domains of v-Abl; the C-terminal domains are not required for transactivation. The assay also identified the E2F site in the c-myc promoter as a v-Abl-responsive element. In addition, multimerized E2F sites were shown to be sufficient to confer v-Abl-dependent activation on a minimal promoter. This is the first identification of a v-Abl response element for transcriptional activation. v-Abl tyrosine kinase-dependent changes in proteins binding the c-myc E2F site were also demonstrated, including induction of a complex containing DP1, p107, cyclin A, and cdk2. Identification of v-Abl-dependent changes in E2F-binding proteins provides an important link between v-Abl, transcription, cell cycle regulation, and control of cellular growth.

Cell cycle regulation of the cyclin A gene promoter is mediated by a variant E2F site

Cyclin A is involved in the control of S phase and mitosis in mammalian cells. Expression of the cyclin A gene in nontransformed cells is characterized by repression of its promoter during the G1 phase of the cell cycle and its induction at S-phase entry. We show that this mode of regulation is mediated by the transcription factor E2F, which binds to a specific site in the cyclin A promoter. It differs from the prototype E2F site in nucleotide sequence and protein binding; it is bound by E2F complexes containing cyclin E and p107 but not pRB. Ectopic expression of cyclin D1 triggers premature activation of the cyclin A promoter by E2F, and this effect is blocked by the tumor suppressor protein p16INK4.

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.

Temporally distinct patterns of p53-dependent and p53-independent apoptosis during mouse lens development

Programmed cell death, or apoptosis, is a critical event in the development of multicellular organisms, and its perturbation is implicated in many diseases including cancer. The tumor suppressor protein p53 is known to mediate apoptosis induced by the DNA tumor virus oncoproteins, adenovirus E1A (AdE1A) and SV40 T antigen (SV40 Tag). We have recently demonstrated that the E6 and E7 oncoproteins of human papillomavirus type 16 (HPV-16) modulate apoptosis when expressed in the lens of transgenic mice. In this study we have identified the pathways that mediate E7 induction and E6 inhibition of apoptosis during different stages in the development of the lens. E7 transgenic mice made p53-null were only partially rescued in their apoptotic phenotype, indicating that both p53-dependent and -independent pathways mediate E7-induced apoptosis in the lens. The E6 transgene and p53-null genotype acted additively to reduce levels of apoptosis induced by E7 in neonatal lenses, indicating that E6 modulates apoptosis at least in part through p53-independent mechanisms. The partial reduction in E7-induced apoptosis by the p53-null genotype correlated with an increased incidence of lens tumors in adult E7 transgenic mice. Analyses of embryonic lenses at E13.5, E15.5, and E17.5 revealed a temporally distinct activation of p53-dependent and -independent apoptosis in the E7 lens. During the early stages of lens development, apoptosis was highly p53-dependent, whereas at later stages, apoptosis occurred through both p53-independent and -dependent pathways. This later time correlates temporally with the time of normal fiber cell denucleation, which can be inhibited by E6 through a p53-independent mechanism. These data suggest a similarity between the mechanism regulating E7-induced, p53-independent apoptosis and the apoptotic-like developmental process of fiber cell denucleation, and the mechanisms through which E6 suppresses both processes.

Differential roles of two tandem E2F sites in repression of the human p107 promoter by retinoblastoma and p107 proteins

Although many lines of evidence indicate that the cellular protein p107 is closely related to the retinoblastoma protein, the exact function of the p107 gene and its regulation are presently not known. To investigate the molecular mechanism controlling expression of the human p107 gene, a 5' flanking sequence of this gene was isolated and shown to promote high-level expression of a luciferase reporter gene in cycling human 293 and Saos-2 cells. Sequencing and transcription mapping analyses showed that the human p107 promoter is TATA-less and contains a tandem, direct repeat of E2F-binding sites, with the 3' copy overlapping the major transcription initiation site. Deletion analysis of the p107 promoter showed that a promoter DNA fragment containing only the two E2F sites together with the leader sequence could direct relatively efficient expression in 293 cells. Site-directed mutagenesis of these E2F sites revealed that although both sites were important for p107 promoter activity, mutation on the proximal, initiation site copy of the E2F site showed a stronger effect. The human p107 promoter could be repressed by the retinoblastoma protein and its own gene product. Interestingly, the repression was found to be mediated through the 5' copy of the E2F site. These studies demonstrate for the first time differential roles of two tandem E2F sites in promoter regulation.

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.

Relief of YY1 transcriptional repression by adenovirus E1A is mediated by E1A-associated protein p300

YY1 represses transcription when bound upstream of transcriptional initiation sites. This repression can be relieved by adenovirus E1A. Here, we present genetic evidence that the ability of E1A to relieve YY1 repression was impaired by mutations that affect E1A binding to its associated protein p300. This suggests that E1A may modulate the repressor activity of YY1 by binding to p300, which may be physically complexed with YY1. A YY1/p300 protein complex in vivo was demonstrated by several independent approaches, and the YY1-interacting domain was mapped to the carboxy-terminal region of p300, distinct from the E1A-binding site. Unlike E2F/RB, the YY1/p300 complex is not disrupted by E1A. Functional studies using recombinant p300 demonstrated unequivocally that p300 is capable of mediating E1A-induced transcriptional activation through YY1. Taken together, these results reveal, for the first time, a YY1/p300 complex that is targeted by E1A and demonstrate a function for p300 in mediating interactions between YY1 and E1A. Our data thus identify YY1 as a partner protein for p300 and uncover a molecular mechanism for the relief of YY1-mediated repression by E1A.

In vivo association of E2F and DP family proteins

The mammalian transcription factor E2F plays an important role in regulating the expression of genes that are required for passage through the cell cycle. This transcriptional activity is inhibited by association with the retinoblastoma tumor suppressor protein (pRB) or its relatives p107 and p103. The first cDNA from the E2F family to be cloned was designated E2F-1, and multiple E2F family members have now been identified. They bind to DNA as heterodimers, interacting with proteins known as DP. Here we demonstrate that DP is also a family of polypeptides with at least two members (hDP-1 and hDP-2). Both hDP-1 and hDP-2 bind to all E2F family members in vivo, and each complex is capable of activating transcription. However, the various E2F/DP complexes display strong differences in the ability to bind to either pRB or p107 in vivo, and the specificity of pRB or p107 binding is mediated by the E2F subunit.

An inverted repeat motif stabilizes binding of E2F and enhances transcription of the dihydrofolate reductase gene

An overlapping inverted repeat sequence that binds the eukaryotic transcription factor E2F is 100% conserved near the major transcription start sites in the promoters of three mammalian genes encoding dihydrofolate reductase, and is also found in the promoters of several other important cellular and viral genes. This element, 5'-TTTCGCGCCAAA-3', is comprised of two overlapping, oppositely oriented sites which match the consensus E2F site (5'-TTT(C/G)(C/G)CGC-3'). Recent work has shown that E2F binding activity is composed of at least six related cellular polypeptides which are capable of forming DNA-binding homo- and heterodimers. We have investigated the binding of cellular E2F activity and of homo- and heterodimers of cloned E2F proteins to the inverted repeat E2F element. We have demonstrated that mutations in this element that abolish its inverted repeat nature, while preserving a single consensus E2F site, significantly decrease the binding stability of all of the forms of E2F tested. The rate of association of E2F-1/DP-1 heterodimers with the inverted repeat wild type site was not significantly different from those with the two single site mutated probes. Furthermore, the mutations decrease in vitro transcription and transient reporter gene expression 2-5-fold, an effect equivalent to that of abolishing E2F binding altogether. These data suggest a functional role that may explain the conservation of inverted repeat E2F elements among the DHFR promoters and several other cellular and viral promoters.

Functional interaction between E2F-4 and p130: evidence for distinct mechanisms underlying growth suppression by different retinoblastoma protein family members

Little is known of the mechanisms controlling the G0/G1 transition of the cell cycle. The induction of immediate early gene expression, thought to be important for this process, suggests that the key factors controlling this transition preexist in quiescent cells. The E2F family of transcription factors likely play an important role in this process, because E2F DNA-binding activity exists in quiescent cells, and the induction of at least some immediate early genes requires intact E2F regulatory promoter sites. Here, we show that the major G0 E2F activity of primary human T cells, E2F-4, is stably bound to the p130 pocket protein in association with a DP heterodimerization partner. p130-E2F-4 binding has functional implications because p130 effectively suppressed E2F-4-mediated trans-activation, and coexpression of E2F4 overcame p130-mediated G1 arrest more efficiently than RB-induced G1 blockade. Conversely, E2F-1 overrode an RB-induced G1 block more efficiently than E2F-4. Thus, p130 and RB appear to induce cell cycle arrest via biochemically distinct mechanisms that involve different E2F family members.

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.

E2F-4 and E2F-5, two members of the E2F family, are expressed in the early phases of the cell cycle

The E2F transcription factors play a role in regulating the expression of genes required for cell proliferation. Their activity appears to be regulated by association with the retinoblastoma protein (pRb) and the pRb-related proteins p107 and p130. In vivo, pRb is found in complex with a subset of E2F components--namely, E2F-1, E2F-2, and E2F-3. Here we describe the characterization of cDNAs encoding two unusual E2Fs, E2F-4 and E2F-5, each identified by the ability of their gene product to interact with p130 in a yeast two-hybrid system. E2F-4 and -5 share common sequences with E2F-1, E2F-2, and E2F-3 and, like these other E2Fs, the ability to heterodimerize with DP-1, thereby acquiring the ability to bind an E2F DNA recognition sequence with high affinity. However, in contrast to E2F-1, E2F-4 and E2F-5 fail to bind pRb in a two-hybrid assay. Moreover, they show a unique pattern of expression in synchronized human keratinocytes: E2F-4 and E2F-5 mRNA expression is maximal in mid-G1 phase before E2F-1 expression is detectable. These findings suggest that E2F-4 and E2F-5 may contribute to the regulation of early G1 events including the G0/G1 transition.

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.

Viral oncoprotein binding to pRB, p107, p130, and p300

The purpose of this review is to bring attention to some additional work in the tumor virus/tumor suppressor field which may have been overshadowed by reports describing adenovirus, SV40, and HPV oncoprotein binding to pRB and p53. The data reviewed herein provide further support for the model that a common mechanism by which DNA tumor viruses transform cells involves inactivation of cellular proteins which function as negative regulators of cell growth.

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.

The Rb-related p107 protein can suppress E2F function independently of binding to cyclin A/cdk2

The interaction of the retinoblastoma susceptibility gene product (Rb)-related p107 protein with the E2F transcription factor in S-phase cells facilitates the formation of a multicomponent complex also containing cyclin A and the p33cdk2 kinase. We have created a series of p107 mutants to assess the ability of p107 to inhibit E2F function and the role of the cyclin A/cdk2 complex in this process. We find that p107 mutants that do not bind to E2F also fail to repress E2F-dependent transcription. Moreover, we find that the ability of p107 to suppress E2F-dependent transcription is not dependent on the ability of p107 to associate with cyclin A/cdk2. Finally, an analysis of the ability of the p107 mutant proteins to suppress cell growth suggests that both E2F-dependent and E2F-independent events correlate with this activity.

Positive and negative regulation of cell proliferation by E2F-1: influence of protein level and human papillomavirus oncoproteins

E2F-1 is a member of a family of transcription factors implicated in the activation of genes required for the progression through the S phase of the cell cycle. We have examined the biological activities of E2F-1 with short-term colony-forming assays and long-term immortalization assays. High levels of E2F-1, produced by transfection of the E2F-1 cDNA under the control of a strong promoter, reduced colony formation in normal human foreskin keratinocytes (NHFKs). This inhibition could not be overcome by wild-type human papillomavirus type 16 (HPV16) E6 and E7, two proteins which cooperate to immortalize NHFKs, or by a transdominant p53 mutant. High levels of E2F-1 also inhibited growth of primary and established fibroblasts. The growth-inhibitory activity required the DNA binding function of E2F-1 but not its transactivation or pRB binding activities. A positive role for lower levels of E2F-1 in NHFK immortalization was established by examining the ability of E2F-1 to complement HPV16 E7 mutants that were unable to cooperate with HPV16 E6 to immortalize NHFKs. Although E2F-1 was unable by itself to cooperate with E6, it did, in conjunction with E6, complement a p24GLY mutant of E7 that is defective for immortalization and binding of pRB and pRB-related proteins. By contrast, E2F-1 was unable to complement two other E7 mutants, p2PRO and p31/32ARG/PRO, which are also defective in the immortalization assay, although their proteins display wild-type binding of pRB in vitro. Since the binding of E7 to pRB results in disruption of pRB-E2F interaction and release of transcriptionally active E2F, the data support the hypothesis that binding of pRB by E7 and the consequence increase in E2F, the data support the hypothesis that binding of pRB by E7 and the consequence increase in E3F activity are important but not sufficient for E7-induced keratinocyte immortalization.

Positive and negative regulation of cell proliferation by E2F-1: influence of protein level and human papillomavirus oncoproteins

E2F-1 is a member of a family of transcription factors implicated in the activation of genes required for the progression through the S phase of the cell cycle. We have examined the biological activities of E2F-1 with short-term colony-forming assays and long-term immortalization assays. High levels of E2F-1, produced by transfection of the E2F-1 cDNA under the control of a strong promoter, reduced colony formation in normal human foreskin keratinocytes (NHFKs). This inhibition could not be overcome by wild-type human papillomavirus type 16 (HPV16) E6 and E7, two proteins which cooperate to immortalize NHFKs, or by a transdominant p53 mutant. High levels of E2F-1 also inhibited growth of primary and established fibroblasts. The growth-inhibitory activity required the DNA binding function of E2F-1 but not its transactivation or pRB binding activities. A positive role for lower levels of E2F-1 in NHFK immortalization was established by examining the ability of E2F-1 to complement HPV16 E7 mutants that were unable to cooperate with HPV16 E6 to immortalize NHFKs. Although E2F-1 was unable by itself to cooperate with E6, it did, in conjunction with E6, complement a p24GLY mutant of E7 that is defective for immortalization and binding of pRB and pRB-related proteins. By contrast, E2F-1 was unable to complement two other E7 mutants, p2PRO and p31/32ARG/PRO, which are also defective in the immortalization assay, although their proteins display wild-type binding of pRB in vitro. Since the binding of E7 to pRB results in disruption of pRB-E2F interaction and release of transcriptionally active E2F, the data support the hypothesis that binding of pRB by E7 and the consequence increase in E2F, the data support the hypothesis that binding of pRB by E7 and the consequence increase in E3F activity are important but not sufficient for E7-induced keratinocyte immortalization.

Differential specificity for binding of retinoblastoma binding protein 2 to RB, p107, and TATA-binding protein

The growth suppressor activities of the RB and p107 products are believed to be mediated by the reversible binding of a heterogeneous family of cellular proteins to a conserved T/E1A pocket domain that is present within both proteins. To study the functional role of these interactions, we examined the properties of cellular retinoblastoma binding protein 2 (RBP2) binding to RB, p107, and the related TATA-binding protein (TBP) product. We observed that although RBP2 bound exclusively to the T/E1A pocket of p107, it could interact with RB through independent T/E1A and non-T/E1A domains and with TBP only through the non-T/E1A domain. Consistent with this observation, we found that a mutation within the Leu-X-Cys-X-Glu motif of RBP2 resulted in loss of ability to precipitate p107, while RB- and TBP-binding activities were retained. We located the non-T/E1A binding site of RBP2 on a 15-kDa fragment that is independent from the Leu-X-Cys-X-Glu motif and encodes binding activity for RB and TBP but does not interact with p107. Despite the presence of a non-T/E1A binding site, however, recombinant RBP2 retained the ability to preferentially precipitate active hypophosphorylated RB from whole-cell lysates. In addition, we found that cotransfection of RBP2 can reverse in vivo RB-mediated suppression of E2F activity. These findings confirm the differential binding specificities of the related RB, p107, and TBP proteins and support the presence of multifunctional domains on the nuclear RBP2 product which may allow complex interactions with the cellular transcription machinery.

E2F-4, a new member of the E2F gene family, has oncogenic activity and associates with p107 in vivo

The E2F family of transcription factors controls the expression of genes that are involved in cell cycle regulation. E2F DNA-binding activity is found in complex with the retinoblastoma protein, pRb, and with the pRb-related p107 and p130. To date, cDNAs for three members of the E2F gene family have been isolated. However, all three E2Fs associate in vivo exclusively with pRb. We report here the cloning and functional analysis of a fourth E2F family member. E2F-4 encodes a 413-amino-acid protein with significant homology to E2F-1. E2F-4 antibodies recognize a 60-kD protein in anti-p107 immunoprecipitates, indicating that E2F-4 associates with p107 in vivo. Like the other E2Fs, E2F-4 requires DP-1 for efficient DNA binding and transcriptional activation of E2F site-containing promoters. Increased expression of E2F-4 and DP-1 in SaoS-2 osteosarcoma cells causes a shift from G1-phase cells to S and G2/M-phase cells, suggesting a role for E2F-4 in regulation of cell-cycle progression. We show that expression of E2F-4 and DP-1 together with an activated ras oncogene in rat embryo fibroblasts, causes transformation, indicating that E2F-4 has oncogenic activity.

E2F-4, a new member of the E2F transcription factor family, interacts with p107

The E2F family of transcription factors has been implicated in the regulation of cell proliferation, and E2F-binding sites are present in the promoters of several growth-regulating genes. E2F family members are functionally regulated, in part, by complex formation with one or more members of the nuclear pocket protein family, RB, p107, and p130. Pocket protein regulation of E2F likely contributes to normal cellular growth control. While the three cloned species of E2F, E2F-1, E2F-2, and E2F-3, are known to be targets of RB interaction, no E2F species has yet been shown to be a specific p107 or p130 target. Here, we describe the cloning of a new member of the E2F family, E2F-4, which forms heterodimers with a member(s) of the DP family and, unlike some family members, is present throughout the cell cycle and appears to be a differentially phosphorylated p107-binding partner. p107 binding not only can be linked to the regulation of E2F-4 transcriptional activity, but also to suppression of the ability of E2F-4 to transform an immortalized rodent cell line.

Nerve growth factor regulates the expression and activity of p33cdk2 and p34cdc2 kinases in PC12 pheochromocytoma cells

In the absence of serum, nerve growth factor (NGF) promotes the survival and differentiation of the PC12 pheochromocytoma cell line. In the presence of serum, NGF acts primarily as a differentiation factor and negative regulator of cell cycling. To investigate NGF control of cell cycling, we have analyzed the regulation of cyclin dependent kinases during PC12 cell differentiation. NGF treatment leads to a reduction in the steady-state protein levels of p33cdk2 and p34cdc2, two key regulators of cell cycle progression. The decrease in p33cdk2 and p34cdc2 coincides with a decrease in the enzymatic activity of cyclinA-p34cdc2, cyclinB-p34cdc2, cyclinE-p33cdk2, and cyclinA-p33cdk2 kinases. The decline in p33cdk2 and p34cdc2 kinase activity in response to NGF is accelerated in cells that over-express the p140trk NGF receptor, suggesting that the timing of the down- regulation is dependent on the level of p140trk and the strength of the NGF signal. The level of cyclin A, a regulatory subunit of p33cdk2 and p34cdc2, is relatively constant during PC12 differentiation. Nevertheless, the DNA binding activity of the cyclinA-associated transcription factor E2F/DP decreases. Thus, NGF down-regulates the activity of cyclin dependent kinases and cyclin-transcription factor complexes during PC12 differentiation.

Differential specificity for binding of retinoblastoma binding protein 2 to RB, p107, and TATA-binding protein

The growth suppressor activities of the RB and p107 products are believed to be mediated by the reversible binding of a heterogeneous family of cellular proteins to a conserved T/E1A pocket domain that is present within both proteins. To study the functional role of these interactions, we examined the properties of cellular retinoblastoma binding protein 2 (RBP2) binding to RB, p107, and the related TATA-binding protein (TBP) product. We observed that although RBP2 bound exclusively to the T/E1A pocket of p107, it could interact with RB through independent T/E1A and non-T/E1A domains and with TBP only through the non-T/E1A domain. Consistent with this observation, we found that a mutation within the Leu-X-Cys-X-Glu motif of RBP2 resulted in loss of ability to precipitate p107, while RB- and TBP-binding activities were retained. We located the non-T/E1A binding site of RBP2 on a 15-kDa fragment that is independent from the Leu-X-Cys-X-Glu motif and encodes binding activity for RB and TBP but does not interact with p107. Despite the presence of a non-T/E1A binding site, however, recombinant RBP2 retained the ability to preferentially precipitate active hypophosphorylated RB from whole-cell lysates. In addition, we found that cotransfection of RBP2 can reverse in vivo RB-mediated suppression of E2F activity. These findings confirm the differential binding specificities of the related RB, p107, and TBP proteins and support the presence of multifunctional domains on the nuclear RBP2 product which may allow complex interactions with the cellular transcription machinery.

Functional importance of complex formation between the retinoblastoma tumor suppressor family and adenovirus E1A proteins as determined by mutational analysis of E1A conserved region 2

Adenovirus early region 1A (E1A) products induce DNA synthesis, transform primary rodent cells, and activate transcription factor E2F through complex formation with an array of cellular proteins via the E1A amino terminus and conserved regions 1 and 2 (CR1 and CR2). Interactions with the retinoblastoma tumor suppressor, pRb, and related proteins p107 and p130 rely somewhat on CR1 but largely on CR2, which contains a core binding sequence Leu-122-X-Cys-X-Glu. We introduced point mutations in CR2 to define such interactions more precisely. In human cells, alteration of any of the conserved residues within the binding core eliminated complex formation with pRb. Conversion of nonconserved Thr-123 to Pro (but not to either Ala or Ser) disrupted binding of pRb, presumably because of conformational changes in the binding core. No single E1A point mutant was completely defective in binding p107, suggesting that molecular interactions between E1A proteins and p107 clearly differ from those with pRb and p130. In general, the patterns of complex formation by E1A mutants in rat, monkey, and human cells were quite similar. All mutants which failed to bind significant amounts of pRb also failed to transform primary rat cells. Several mutants demonstrated selective binding to pRb, p107, and p130, but transforming activity corresponded largely with complex formation with pRb, regardless of the levels of interactions with p107 and p130. Mutants defective for binding of both pRb and p107 failed to induce the activity of transcription factor E2F; however, quite high levels were activated by E1A mutants that interacted with p107 alone. These results suggested that both pRb and p107 are important regulators of E2F activity but that complex formation with and activation of E2F by p107 are insufficient for cell transformation.

Differential regulation of E2F transactivation by cyclin/cdk2 complexes

The mammalian transcription factor E2F plays a critical role in the expression of genes required for cellular proliferation. To understand how E2F is regulated, we have developed a reconstituted in vitro transcription assay. Using this E2F-responsive assay, we can demonstrate that E2F-mediated transcription can be directly repressed by the tumor suppressor protein pRB. This inhibition is abolished by phosphorylation of pRB with either cyclin A/cdk2 or cyclin E/cdk2. However, these cyclin/kinase complexes exhibit differences in the ability to phosphorylate E2F. Only cyclin A/cdk2 can phosphorylate E2F effectively, and this phosphorylation abolishes its ability to bind DNA and mediate trans-activation. Thus, this in vitro transcriptional assay allows activation and inactivation of E2F transcription, and our findings demonstrate how transcriptional regulation of E2F can be linked to cell cycle-dependent activation of kinases.

DP and E2F proteins: components of a heterodimeric transcription factor implicated in cell cycle control

In mammalian cells, DRTF1/E2F is a transcription factor widely believed to integrate cell-cycle progression with the transcription apparatus through its cyclical interactions with important regulators of the cell cycle, such as the retinoblastoma tumour-suppressor gene product, cyclins and cyclin-dependent kinases. Recently, a number of exciting developments have uncovered the heterodimeric nature of DRTF1/E2F by defining two distinct families of proteins, DP and E2F, which comprise its activity; efficient DNA-binding activity arises when a DP protein interacts with an E2F protein. Combinatorial interactions generate a surprising array of sequence-specific heterodimers, a diversity that is probably necessary to enable different cell cycle regulating proteins to integrate their activities with transcription.

Independent regions of adenovirus E1A are required for binding to and dissociation of E2F-protein complexes

The transcription factor E2F is present in independent complexes with the product of the retinoblastoma susceptibility gene, pRB, and a related gene product, p107, in association with the cyclin A-cdk2 or the cyclin E-cdk2 kinase complex. pRB and p107 can negatively regulate E2F activity, since overexpression of pRB or p107 in cells lacking a functional pRB leads to the repression of E2F activity. The products of the adenovirus E1A gene can disrupt E2F complexes and result in free and presumably active E2F transcription factor. The regions of E1A required for this function are also essential for binding to a number of cellular proteins, including pRB and p107. Through the use of a number of glutathione S-transferase fusion proteins representing different regions of E1A, as well as in vivo expression of E1A proteins containing deletions of either conserved region 1 (CR1) or CR2, we find that CR2 of E1A can form stable complexes with E2F. E1A proteins containing both CR1 and CR2 also associate with E2F, although the presence of these proteins results in the release of free E2F from its complexes. In vitro reconstitution experiments indicate that E1A-E2F interactions are not direct and that pRB can serve to facilitate these interactions. Complexes containing E1A, p107, cyclin A, and E2F were identified in vivo, which indicates that E1A may associate with E2F through either p107 or pRB. Peptide competition experiments demonstrate that the pRB-binding domain of the human E2F-1 protein can compete with the CR1 but not CR2 domain of E1A for binding to pRB. These results indicate that E1A CR1 and E2F-1 may bind to the same or overlapping sites on pRB and that E1A CR2 binds to an independent region. On the basis of our results, we propose a two-step model for the release of E2F from pRB and p107 cellular proteins.

The retinoblastoma protein binds to a family of E2F transcription factors

E2F is a transcription factor that helps regulate the expression of a number of genes that are important in cell proliferation. Recently, several laboratories have isolated a cDNA clone that encodes an E2F-like protein, known as E2F-1. Subsequent characterization of this protein showed that it had the properties of E2F, but it was difficult to account for all of the suggested E2F activities through the function of this one protein. Using low-stringency hybridization, we have isolated cDNA clones that encode two additional E2F-like proteins, called E2F-2 and E2F-3. The chromosomal locations of the genes for E2F-2 and E2F-3 were mapped to 1p36 and 6q22, respectfully, confirming their independence from E2F-1. However, the E2F-2 and E2F-3 proteins are closely related to E2F-1. Both E2F-2 and E2F-3 bound to wild-type but not mutant E2F recognition sites, and they bound specifically to the retinoblastoma protein in vivo. Finally, E2F-2 and E2F-3 were able to activate transcription of E2F-responsive genes in a manner that was dependent upon the presence of at least one functional E2F binding site. These observations suggest that the E2F activities described previously result from the combined action of a family of proteins.

Independent regions of adenovirus E1A are required for binding to and dissociation of E2F-protein complexes

The transcription factor E2F is present in independent complexes with the product of the retinoblastoma susceptibility gene, pRB, and a related gene product, p107, in association with the cyclin A-cdk2 or the cyclin E-cdk2 kinase complex. pRB and p107 can negatively regulate E2F activity, since overexpression of pRB or p107 in cells lacking a functional pRB leads to the repression of E2F activity. The products of the adenovirus E1A gene can disrupt E2F complexes and result in free and presumably active E2F transcription factor. The regions of E1A required for this function are also essential for binding to a number of cellular proteins, including pRB and p107. Through the use of a number of glutathione S-transferase fusion proteins representing different regions of E1A, as well as in vivo expression of E1A proteins containing deletions of either conserved region 1 (CR1) or CR2, we find that CR2 of E1A can form stable complexes with E2F. E1A proteins containing both CR1 and CR2 also associate with E2F, although the presence of these proteins results in the release of free E2F from its complexes. In vitro reconstitution experiments indicate that E1A-E2F interactions are not direct and that pRB can serve to facilitate these interactions. Complexes containing E1A, p107, cyclin A, and E2F were identified in vivo, which indicates that E1A may associate with E2F through either p107 or pRB. Peptide competition experiments demonstrate that the pRB-binding domain of the human E2F-1 protein can compete with the CR1 but not CR2 domain of E1A for binding to pRB. These results indicate that E1A CR1 and E2F-1 may bind to the same or overlapping sites on pRB and that E1A CR2 binds to an independent region. On the basis of our results, we propose a two-step model for the release of E2F from pRB and p107 cellular proteins.

The retinoblastoma protein binds to a family of E2F transcription factors

E2F is a transcription factor that helps regulate the expression of a number of genes that are important in cell proliferation. Recently, several laboratories have isolated a cDNA clone that encodes an E2F-like protein, known as E2F-1. Subsequent characterization of this protein showed that it had the properties of E2F, but it was difficult to account for all of the suggested E2F activities through the function of this one protein. Using low-stringency hybridization, we have isolated cDNA clones that encode two additional E2F-like proteins, called E2F-2 and E2F-3. The chromosomal locations of the genes for E2F-2 and E2F-3 were mapped to 1p36 and 6q22, respectfully, confirming their independence from E2F-1. However, the E2F-2 and E2F-3 proteins are closely related to E2F-1. Both E2F-2 and E2F-3 bound to wild-type but not mutant E2F recognition sites, and they bound specifically to the retinoblastoma protein in vivo. Finally, E2F-2 and E2F-3 were able to activate transcription of E2F-responsive genes in a manner that was dependent upon the presence of at least one functional E2F binding site. These observations suggest that the E2F activities described previously result from the combined action of a family of proteins.