The retinoblastoma protein as a transcriptional repressor

Expression of p16INK4a suppresses the unbounded and anchorage-independent growth of a glioblastoma cell line that lacks p16INK4a

p16INK4a is a inhibitory protein of Cyclin-dependent kinase 4(Cdk4).p16 negatively regulates the cell cycle progression from G1 to S phase. Functional p16 is absent from many human cancers, as well as from many established lines of tumor cells. However, it is not clear whether expression of p16 in p16-deficient tumor cells can suppress their anchorage-independent growth. Therefore, we introduced a cDNA for p16INK4a into the human glioblastoma cell line T98G, which lacks a gene for p16INK4a. We isolated several clones that stably expressed various amounts of p16 protein. The doubling time of the various clones was generally prolonged. Clones with high-level expression of p16 protein had characteristics of restricted growth, such as contact inhibition, while the parental T98G cells had no such characteristics. Furthermore, the efficiency of colony formation in soft agar was dramatically decreased in the case of cells that expressed exogenous p16. Our observations suggest that the expression of p16 protein restricts the unbounded growth and the anchorage-independent growth of tumor cells.

HBP1: a HMG box transcriptional repressor that is targeted by the retinoblastoma family

A prominent feature of cell differentiation is the initiation and maintenance of an irreversible cell cycle arrest with the complex involvement of the retinoblastoma (RB) family (RB, p130, p107). We have isolated the HBP1 transcriptional repressor as a potential target of the RB family in differentiated cells. By homology, HBP1 is a sequence-specific HMG transcription factor, of which LEF-1 is the best-characterized family member. Several features of HBP1 suggest an intriguing role as a transcriptional and cell cycle regulator in differentiated cells. First, inspection of the HBP1 protein sequence revealed two consensus RB interaction motifs (LXCXE and IXCXE). Second, HBP1 interaction was selective for RB and p130, but not p107. HBP1, RB, and p130 levels are all up-regulated with differentiation; in contrast, p107 levels decline. Third, HBP1 can function as a transcriptional repressor of the promoter for N-MYC, which is a critical cell cycle and developmental gene. Fourth, because the activation of the N-MYC promoter in cycling cells required the E2F transcription factor, we show that E2F-1 and HBP1 represent opposite transcriptional signals that can be integrated within the N-MYC promoter. Fifth, the expression of HBP1 lead to efficient cell cycle arrest. The arrest phenotype was manifested in the presence of optimal proliferation signals, suggesting that HBP1 exerted a dominant regulatory role. Taken together, the results suggest that HBP1 may represent a unique transcriptional repressor with a role in initiation and establishment of cell cycle arrest during differentiation.

RB kinases and RB-binding proteins: new points of view

The retinoblastoma protein (RB) binds to a variety of cellular proteins and suppresses cellular growth. Such interactions are regulated by phosphorylation during the cell cycle by several cyclin-dependent kinases, known as RB kinases. Clues to the specific physiological roles of different RB kinases have been obtained. Moreover, interesting functions of the RB protein, other than control of E2F activity, have been found.

Vitamin D3 reduces the apoptotic effect of IFN-gamma but does not facilitate HLA class II inducibility in RB-defective cells

The retinoblastoma protein (RB) regulates the cell cycle by binding and inactivating the E2F transcription factors, which prevents transcription of genes required for DNA synthesis. RB has been shown to inhibit IFN-gamma-mediated apoptosis, possibly by regulating premature entry into S phase. RB is also required for high level IFN-gamma induction of HLA class II genes, which encode antigen presenting molecules, but not for other IFN-gamma inducible genes as demonstrated in previous reports describing the analysis of RB-transformants of the RB-defective cell lines, MDA-468-S4 (S4) and H2009. The IFN-gamma response of the HLA class II genes takes much longer than does the response of the other IFN-gamma inducible genes, raising the question of whether RB facilitates HLA class II inducibility by maintaining cell viability over the long time course required for HLA class II induction. Thus, we sought to learn whether IFN-gamma induced apoptosis in an RB-defective cell line could be prevented independently of RB and whether doing so would facilitate HLA class II inducibility in the RB-defective line. Our results indicated that cotreating the RB-defective S4 cells with IFN-gamma and Vitamin D3 decreased the number of cells containing subdiploid DNA compared to cells treated with IFN-gamma alone, suggesting that Vitamin D3 reduced IFN-gamma-mediated apoptosis. S4 cells cotreated with Vitamin D3 and IFN-gamma also had decreased cell detachment, further indicating that Vitamin D3 decreased IFN-gamma induced apoptosis. However, Vitamin D3 cotreatment resulted in no detectable increase in HLA-DR, the most prominent HLA class II molecule, indicating that the effect of RB on HLA class II induction is not exclusively due to its ability to inhibit IFN-gamma induced apoptosis.

Cancer cell cycles

Uncontrolled cell proliferation is the hallmark of cancer, and tumor cells have typically acquired damage to genes that directly regulate their cell cycles. Genetic alterations affecting p16(INK4a) and cyclin D1, proteins that govern phosphorylation of the retinoblastoma protein (RB) and control exit from the G1 phase of the cell cycle, are so frequent in human cancers that inactivation of this pathway may well be necessary for tumor development. Like the tumor suppressor protein p53, components of this "RB pathway," although not essential for the cell cycle per se, may participate in checkpoint functions that regulate homeostatic tissue renewal throughout life.

Expression of the HsOrc1 gene, a human ORC1 homolog, is regulated by cell proliferation via the E2F transcription factor

The initiation of DNA replication in Saccharomyces cerevisiae requires the action of a multisubunit complex of six proteins known as the origin recognition complex (ORC). The identification of higher eukaryotic homologs of several ORC components suggests a universal role for this complex in DNA replication. We now demonstrate that the expression of one of these homologs is regulated by cell proliferation. Expression of the human Orc1 gene (HsOrc1) is low in quiescent cells, and it is then dramatically induced upon stimulation of cell growth. In contrast, expression of the HsOrc2 gene does not appear to be similarly regulated. We have isolated the promoter that regulates HsOrc1 transcription, and we show that the promoter confers cell growth-dependent expression. We also demonstrate that the cell growth control is largely the consequence of E2F-dependent negative transcription control in quiescent cells. Activation of HsOrc1 transcription following growth stimulation requires G1 cyclin-dependent kinase activity, and forced E2F1 expression can bypass this requirement. These results thus provide a direct link between the initiation of DNA replication and the cell growth regulatory pathway involving G1 cyclin-dependent kinases, the Rb tumor suppressor, and E2F.

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.

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.

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

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

E2F-induced S phase requires cyclin E

Both the heterodimeric transcription factor, E2F, and the G1 cyclin, cyclin E, are required for the G1-S transition at the start of the metazoan cell cycle. It has been established that cyclin E can act as an upstream activator of E2F. In addition to this action, we show here that cyclin E has an essential role in DNA replication distinct from activating E2F. We have created transgenic Drosophila capable of inducible, ectopic production of E2F activity. Simultaneous overexpression of both Drosophila E2F subunits, dE2F and dDP, in embryos stimulated the expression of multiple E2F-target genes including cyclin E, and also caused the initiation of S phase. Mutation of cyclin E prevented the initiation of S phase after overexpression of dE2F/dDP without affecting induction of target gene expression. Thus, E2F-directed transcription cannot bypass loss of cyclin E in Drosophila embryos.

Granulocyte colony-stimulating factor-stimulated proliferation of myeloid cells: mode of cell cycle control by a range of inhibitors

The myeloid cell line, NFS-60, is dependent on granulocyte colony-stimulating factor (G-CSF) or interleukin-3 (IL-3) for survival and growth. Long-term G-CSF-dependent proliferation was found to be completely inhibited by interferon-gamma (IFN-gamma), cyclic AMP, and dimethylamiloride and partially inhibited by IFN-alpha and lipopolysaccharide. With the exception of IFN-gamma, these agents exhibited a corresponding pattern of inhibition of DNA synthesis in quiescent NFS-60 cells stimulated with G-CSF. IFN-gamma was only a weak inhibitor of DNA synthesis, suggesting that it may act at a later stage to block proliferation. The addition of G-CSF to NFS-60 cells resulted in phosphorylation of the retinoblastoma protein (pRB) and activation of E2F DNA binding activity. The inhibitors were found to suppress the phosphorylation of pRB, lead to the production of higher order E2F complexes, and suppress the expression of c-myc and proliferating cell nuclear antigen (PCNA) to an extent that correlated with their ability to block DNA synthesis. These findings are consistent with the notion that the ratio of free/bound E2F binding activity is critical in controlling cell cycle progression through G1 to S-phase in these cells.

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.

Cell cycle-regulated repression of B-myb transcription: cooperation of an E2F site with a contiguous corepressor element

B-myb belongs to a group of cell cycle genes whose transcription is repressed in G0/early G1 through a binding site for the transcription factor E2F. Here, we show that the B-myb repressor element is specifically recognised by heterodimers consisting of DP-1 and E2F-1, E2F-3 or E2F-4. Surprisingly, E2F-mediated repression is dependent on a contiguous corepressor element that resembles the CHR previously established as a corepressor of the CDE in cell cycle genes derepressed in S/G2, such as cyclin A, cdc2 and cdc25C. A factor binding to the B-myb CHR was identified in fractionated HeLa nuclear extract and found to interact with the minor groove, as previously shown by in vivo footprinting for the cyclin A CHR. The B-myb and cdc25C CHRs are related with respect to protein binding but are functionally clearly distinct. Our results support a model where both E2F- and CDE-mediated repression, acting at different stages in the cell cycle, are dependent on promoter-specific CHR elements.

Expression of dominant-negative mutant DP-1 blocks cell cycle progression in G1

Unregulated expression of the transcription factor E2F promotes the G1-to-S phase transition in cultured mammalian cells. However, there has been no direct evidence for an E2F requirement in this process. To demonstrate that E2F is obligatory for cell cycle progression, we attempted to inactivate E2F by overexpressing dominant-negative forms of one of its heterodimeric partners, DP-1. We dissected the functional domains of DP-1 and separated the region that facilitate heterodimer DNA binding from the E2F dimerization domain. Various DP-1 mutants were introduced into cells via transfection, and the cell cycle profile of the transfected cells was analyzed by flow cytometry. Expression of wild-type DP-1 or DP-1 mutants that bind to both DNA and E2F drove cells into S phase. In contrast, DP-1 mutants that retained E2F binding but lost DNA binding arrested cells in the G1 phase of the cell cycle. The DP-1 mutants that were unable to bind DNA resulted in transcriptionally inactive E2F complexes, suggesting that the G1 arrest is caused by formation of defective E2F heterodimers. Furthermore, the G1 arrest instigated by these DP-1 mutants could be rescued by coexpression of wild-type E2F or DP protein. These experiments define functional domains of DP and demonstrate a requirement for active E2F complexes in cell cycle progression.

Cyclin A expression is under negative transcriptional control during the cell cycle

Transcription of the gene coding for cyclin A, a protein required for S-phase transit, is cell cycle regulated and is restricted to proliferating cells. To further explore transcriptional regulation linked to cell division cycle control, a genomic clone containing 5' flanking sequences of the murine cyclin A gene was isolated. When it was fused to a luciferase reporter gene, it was shown to function as a proliferation-regulated promoter in NIH 3T3 cells. Transcription of the mouse cyclin A gene is negatively regulated by arrest of cell proliferation. A mutation of a GC-rich sequence conserved between mice and humans is sufficient to relieve transcriptional repression, resulting in a promoter with constitutively high activity. In agreement with this result, in vivo footprinting reveals a protection of the cell cycle-responsive element in G0/early G1 cells which is not observed at later stages of the cell cycle. Moreover, the footprint is present in dimethyl sulfoxide-induced differentiating and not in proliferating Friend erythroleukemia cells. Conversely, two other sites, which in vitro bind ATF-1 and NF-Y, respectively, are constitutively occupied throughout cell cycle progression.

Ectopic E2F expression induces S phase and apoptosis in Drosophila imaginal discs

Previous experiments suggest that a key event in the commitment of cultured mammalian cells to entering S phase is a rise in activity of the transcription factor E2F. In this report, we study the role of Drosophila E2F in imaginal disc cells in vivo, by examining the distribution of the endogenous protein and studying the consequences of ectopic E2F expression. First, we find that endogenous E217 falls from high to very low levels as cells initiate DNA synthesis during a developmentally regulated G1-S-transition in the eye disc. Second, we find that ectopic E2F expression drives many otherwise quiescent cells to enter S phase. Subsequently, cells throughout the discs express reaper (a regulator of apoptosis) and then die. Third, we find that ectopic E2F expression during S phase in normally cycling cells blocks their re-entry into S phase in the following cell cycle. Although we do not know the fate of these cells, we suspect that ultimately they are killed by ectopic E2F. Taken together, our results show that an elevation in the level of E2F is sufficient to induce imaginal disc cells to enter S phase. Furthermore, they suggest that the downregulation of E2F upon entry into S phase may be essential to prevent the induction of apoptosis.

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.

The E2F transcription factor activates a replication-dependent human H2A gene in early S phase of the cell cycle

Histone gene expression is restricted to the S phase of the cell cycle. Control is mediated by a complex network of sequence-specific DNA-binding factors and protein-protein interactions in response to cell cycle progression. To further investigate the regulatory functions that are associated at the transcriptional level, we analyzed the regulation of a replication-dependent human H2A.1-H2B.2 gene pair. We found that transcription factor E2F binds specifically to an E2F recognition motif in the H2A.1 promoter region. Activation of the H2A.1 promoter by E2F-1 was shown by use of luciferase reporter constructs of the intergenic promoter region. Overexpression of the human retinoblastoma suppressor gene product RB suppressed E2F-1 mediated transcriptional activation, indicating an E2F-dependent regulation of promoter activity during the G1-to-S-phase transition. Furthermore, the activity of the H2A.1 promoter was also downregulated by overexpression of the RB-related p107, a protein that has been detected in S-phase-specific protein complexes of cyclin A, E2F, and cdk2. In synchronized HeLa cells, expression of luciferase activity was induced at the beginning of DNA synthesis and was dependent on the presence of an E2F-binding site in the H2A.1 promoter. Together with the finding that E2F-binding motifs are highly conserved in H2A promoters of other species, our results suggest that E2F plays an important role in the coordinate regulation of S-phase-specific histone gene expression.

Inhibition of cyclin D-CDK4/CDK6 activity is associated with an E2F-mediated induction of cyclin kinase inhibitor activity

Alterations of various components of the cell cycle regulatory machinery that controls the progression of cells from a quiescent to a growing state contribute to the development of many human cancers. Such alterations include the deregulated expression of G1 cyclins, the loss of function of activities such as those of protein p16INK4a that control G1 cyclin-dependent kinase activity, and the loss of function of the retinoblastoma protein (RB), which is normally regulated by the G1 cyclin-dependent kinases. Various studies have revealed an inverse relationship in the expression of p16INK4a protein and the presence of functional RB in many cell lines. In this study we show that p16INK4a is expressed in cervical cancer cell lines in which the RB gene, Rb, is not functional, either as a consequence of Rb mutation or expression of the human papillomavirus E7 protein. We also demonstrate that p16INK4a levels are increased in primary cells in which RB has been inactivated by DNA tumor virus proteins. Given the role of RB in controlling E2F transcription factor activity, we investigated the role of E2F in controlling p16INK4a expression. We found that E2F1 overexpression leads to an inhibition of cyclin D1-dependent kinase activity and induces the expression of a p16-related transcript. We conclude that the accumulation of G1 cyclin-dependent kinase activity during normal G1 progression leads to E2F accumulation through the inactivation of RB, and that this then leads to the induction of cyclin kinase inhibitor activity and a shutdown of G1 kinase activity.

A unique role for the Rb protein in controlling E2F accumulation during cell growth and differentiation

Examination of the interactions involving transcription factor E2F activity during cell growth and terminal differentiation suggests distinct roles for Rb family members in the regulation of E2F accumulation. The major species of E2F in quiescent cells is a complex containing the E2F4 product in association with the Rb-related p130 protein. As cells enter the cell cycle, this complex disappears, and there is a concomitant accumulation of free E2F activity of which E2F4 is a major component. E2F4 then associates with the Rb-related p107 protein as cells enter S phase. Rb can be found in interactions with each E2F species, including E2F4, during G1, but there appears to be a limited amount of Rb with respect to E2F, likely due to the maintenance of most Rb protein in an inactive state by phosphorylation. A contrasting circumstance can be found during the induction of HL60 cell differentiation. As these cells exit the cell cycle, active Rb protein appears to exceed E2F, as there is a marked accumulation of E2F-Rb interactions, involving all E2F species, including E2F4, which is paralleled by the conversion of Rb from a hyperphosphorylated state to a hypophosphorylated state. These results suggest that the specific ability of Rb protein to interact with each E2F species, dependent on concentration of active Rb relative to accumulation of E2F, may be critical in cell-growth decisions.

Cell cycle regulation of E2F site occupation in vivo

DNA-binding E2F complexes have been identified throughout the mammalian cell cycle, including the transcriptionally inactive complexes with pocket proteins, which occur early in the prereplicative G1 phase of the cycle, and the transactivating free E2F, which increases in late G1. Here, a regulatory B-myb promoter site was shown to bind with high affinity to free E2F and to E2F-pocket protein complexes in an indistinguishable way in vitro. In contrast, in vivo footprinting with NIH 3T3 cells demonstrated E2F site occupation specifically in early G1, when the B-myb promoter is inactive. These observations indicate that a novel mechanism governs E2F-DNA interactions during the cell cycle and emphasize the relevance of E2F site-directed transcriptional repression.

Intramolecular inhibition of activating transcription factor-2 function by its DNA-binding domain

ATF-2 is a cellular basic region-leucine zipper (bZIP) transcription factor that can mediate diverse transcriptional responses, including activation by the adenovirus Ela protein. ATF-2 contains an activation domain, required for transcriptional activity, but in the absence of an appropriate inducer, full-length ATF-2 is transcriptionally inactive. Here we have investigated the mechanism underlying this regulated inhibition of ATF-2 transcriptional activity. We show that the region of ATF-2 that suppresses the activation region is the bZIP DNA-binding domain and that maximal inhibition requires both the basic region and leucine zipper subdomains. Inhibition is activation domain specific: The ATF-2 bZIP suppresses the ATF-2 and the related Ela activation domains but not acidic- and glutamine-rich activation domains. In vitro protein interaction assays demonstrate that the ATF-2 activation domain and bZIP specifically bind to one another. Finally, we show that bZIP-mediated inhibition can be modulated in a cell-type-specific manner by another sequence element within ATF-2. On the basis of these and other data, we propose that the ATF-2 bZIP and activation domain are engaged in an inhibitory intramolecular interaction and that inducers of ATF-2 disrupt this interaction to activate transcription.

Deregulated expression of E2F family members induces S-phase entry and overcomes p16INK4A-mediated growth suppression

The E2F family of transcription factors regulate genes, whose products are essential for progression through the mammalian cell cycle. The transcriptional activity of the E2Fs is inhibited through the specific binding of the retinoblastoma protein, pRB, and the pRB homologs p107 and p130 to their transactivation domains. Seven members of the E2F transcription factor family have been isolated so far, and we were interested in investigating the possible contribution of the various E2Fs to cell cycle control. By presenting the results of the generation of cell lines with tetracycline-controlled expression of E2F-1 and E2F-4 and microinjection of expression plasmids for all members of the E2F family, we demonstrate here that the pRB-associated ED2Fs (E2F-1, E2F-2, and E2F-3) all induce S phase in quiescent rate fibroblasts when expressed alone. In contrast, the p107/p130-associated E2Fs require the coexpression of the heterodimeric partner DP-1 to promote S-phase entry and accelerate G1 progression. Furthermore, the pRB-associated E2Fs were all able to overcome a G1 arrest mediated by the p16INK4 tumor suppressor protein, and E2F-1 was shown to override a G1 block mediated by a neutralizing antibody to cyclin D1. The p16INK4-induced G1 arrest was not affected by expression of E2F-4, E2F-5, or DP-1 alone, but simulataneous expression of E2F-4 and DP-1 could overcome this block. Our results demonstrate that the generation of E2F activity is rate limiting for G1 progession, is sufficient to induce S-phase entry, and overcomes a p16-mediated G1 block, and since E12F-1, E2F-2, and E2F-3 are associated with pRB, they are the most likely downstream effectors in the p126-cyclin D-pRB pathway. Furthermore, our date suggest that the two subsets of E2Fs are regulated by distinct mechanisms and/or that they have distinct functions in cell cycle control. Since E2F-4 and E2F-5 cannot promote S-phase entry by themselves, our results may provide an explanation for the apparent lack of aberrations in p107 or p130 in human cancer.

Blocking the transcription factor E2F/DP by dominant-negative mutants in a normal breast epithelial cell line efficiently inhibits apoptosis and induces tumor growth in SCID mice

The transcription factor E2F is regulated during the cell cycle through interactions with the product of the retinoblastoma susceptibility gene and related proteins. It is thought that E2F-mediated gene regulation at the G1/S boundary and during S phase may be one of the rate-limiting steps in cell proliferation. It was reported that in vivo overexpression of E2F-1 in fibroblasts induces S phase entry and leads to apoptosis. This observation suggests that E2F plays a role in both cell cycle regulation and apoptosis. To further understand the role of E2F in cell cycle progression, cell death, and tumor development, we have blocked endogenous E2F activity in HBL-100 cells, derived from nonmalignant human breast epithelium, using dominant-negative mutants under the control of a tetracycline-dependent expression system. We have shown here that induction of dominant-negative mutants led to strong downregulation of transiently transfected E2F-dependent chloramphenicol acetyl transferase reporter constructs and of endogenous c-myc, which has been described as a target gene of the transcription factor E2F/DP. In addition, we have shown that blocking of E2F could efficiently protect from apoptosis induced by serum starvation within a period of 10 d, whereas control cells started to die after 24 h. Surprisingly, blocking of E2F did not alter the rate of proliferation or of DNA synthesis of these cells; this finding indicates that cell-cycle progression could be driven in an E2F-independent manner. In addition, we have been able to show that blocking of endogenous E2F in HBL-100 cells led to rapid induction of tumor growth in severe combined immunodeficiency mice. No tumor growth could be observed in mice that received mock-transfected clones or tetracycline to block expression of the E2F mutant constructs in vivo. Thus, it appears that E2F has a potential tumor-suppressive function under certain circumstances. Furthermore, we provide evidence that dysregulation of apoptosis may be an important step in tumorigenesis.

p21 Disrupts the interaction between cdk2 and the E2F-p130 complex

In nonproliferating or growth-arrested cells, the transcription factor E2F remains bound to the retinoblastoma-related protein p130. Accumulation of this E2F-p130 complex correlates with an arrest of the cell cycle progression. Progression through G1 phase is associated with a cyclin-dependent binding of the cyclin-dependent kinase cdk2 to the E2F-p130 complex. By fractionating mouse L-cell extracts, we have obtained a partially purified preparation of the E2F-p130 complex that also contains cdk2. Incubation of this complex with recombinant p21 results in a disruption of the interaction between cdk2 and the E2F-p130 complex in extracts of a cell line that expresses a temperature-sensitive mutant of p53. Incubation at the permissive temperature (32 degrees C) results in an induction of p21 synthesis. An increase in the level of p21 in these cells correlates with a loss of cdk2 from the cdk2-containing E2F-p130 complex. We also show that the expression of a reporter gene containing E2F sites in the promoter region is reduced by the coexpression of p21. Since p21 is believed to be a mediator of p53, we speculated that the p21-mediated disruption of the cdk2-containing E2F-p130 complex plays a role in the growth suppression function of p53.

Molecular and immunochemical analyses of RB1 and cyclin D1 in human ductal pancreatic carcinomas and cell lines

Somatic mutations in the retinoblastoma-1 gene (RB1) and loss of RB1 protein function have been implicated in a number of human malignancies, but the role of RB1 gene and protein abnormalities in ductal pancreatic cancer (DPCA) is virtually unknown. We therefore analyzed expression of the RB1 protein immunohistochemically and/or by western blotting in a total of 54 sporadic and eight familial cases of archival and frozen DPCA and in 18 pancreatic carcinoma cell lines by using the antibodies RB-WL-1, 84-B3-1, and PMG3-245. Mutations in the RB1 promotor region and exons 13-21 of the RB1 gene were likewise examined by single-strand conformation polymorphism (SSCP) analyses and DNA sequencing of genomic DNA from 30 microdissected primary pancreatic tumors and the pancreatic carcinoma cell lines. Moreover, amplification and expression of a major regulatory component of RB1 function, cyclin D1, were assessed by southern and immunohistochemical analyses, respectively. The DPCAs were heterogeneous in both the intensity of RB1 nuclear staining and the percentage of immunoreactive cells. The tumors often had areas where RB1 staining was weak or absent adjacent to normal pancreatic tissue; however, only two of 32 archival cases and one of 30 frozen cases of DPCA completely lacked RB1 nuclear staining. Immunohistochemical and western blot analyses of 18 pancreatic carcinoma cell lines demonstrated the absence of RB1 expression in only two cell lines, Capan-1 and QGP-1. Analyses of the RB1 gene and promotor region by SSCP and DNA sequencing largely confirmed the immunochemical findings. Three of 30 primary carcinomas had abnormalities revealed by SSCP analyses. In one case a single base-pair deletion was confirmed in exon 18 and resulted in premature termination and the absence of detectable RB1 protein. A second case had TAC-->TTC missense mutation in exon 13. The third primary carcinoma could not be reliably sequenced because it had a low percentage of epithelial cells. The cyclin D1 gene was not amplified in any of the primary pancreatic tumors or cell lines examined. These immunochemical and molecular analyses of the RB1 tumor suppressor gene and cyclin D1 proto-oncogene in a large series of human pancreatic cancers and cell lines indicate that RB1 and cyclin D1 alterations occur during the development of some human DPCAs. Nevertheless, it is probable that alterations in cell-cycle regulation in DPCAs more frequently involve pathways other than those involving RB1 and cyclin D1.

Genetic aspects of embryonic eye development in vertebrates

The vertebrate eye comprises tissues from different embryonic origins, e.g., iris and ciliary body are derived from the wall of the diencephalon via optic vesicle and optic cup. Lens and cornea, on the other hand, come from the overlying surface ectoderm. The timely action of transcription factors and inductive signals ensure the correct development of the different eye components. Establishing the genetic basis of eye defects has been an important tool for the detailed analysis of this complex process. One of the main control genes for eye development was discovered by the analysis of the allelic series of the Small eye mouse mutants and characterized as Pax6. It is involved in the interaction between the optic cup and the overlaying ectoderm. The central role for Pax6 in eye development is conserved throughout the animal kingdom as the murine Pax6 gene induces ectopic eyes in transgenic Drosophila despite the obvious diverse organization of the eye in the fruit fly compared to vertebrates. In human, mutations in the PAX6 gene are responsible for aniridia and Peter's anomaly. In addition to Pax6, other mutations affecting the interaction of the optic cup and the lens placode have been documented in the mouse. For the differentiation of the retina from the optic cup several genes are responsible: Mi leads to microphthalmia, if mutated, and encodes for a transcription factor, which is expressed in the melanocytes of the pigmented layer of the retina. In addition, further genes are implicated in the correct development of the retina, e.g., Chx10, Dlx1, GH6, Msx1 and -2, Otx1 and -2, or Wnt7b. Mutations within the retinoblastoma gene (RB1) are responsible for retinal tumors. Knock-out mutants of RB1 exhibit a block of lens differentiation prior to the retinal defect. Besides the influence of Rb1, the lens differentiates under the influence of growth factors (e.g., FGF, IGF, PDGF, TGF), and specific genes become activated encoding cytoskeletal proteins (e.g., filensin, phakinin, vimentin), structural proteins (e.g., crystallins) or membrane proteins (e.g., Mip). The optic nerve originates from the neural retina; ganglion cells grow to the optic stalk, forming the optic nerve. Its retrograde walk to the brain through the rudiment of the optic stalk depends on the correct Pax2 expression.

Retinoblastoma gene in mouse neural development

The retinoblastoma gene (Rb) was the first tumor suppressor gene to be cloned [Dryja et al., 1986; Friend et al., 1986; Lee et al., 1987], and, as a consequence, has been studied intensively within the context of cell cycle regulation and oncogenesis. However, a number of recent findings indicate that the retinoblastoma gene product (pRb) likely plays an essential role not only in controlling entry into the cell cycle, but also in the terminal differentiation of a number of different cell types [Lee et al., 1994; Gu et al., 1993]. In particular, the phenotype of the Rb nullizygous mice, created by a number of groups using homologous recombination [Jacks et al., 1992: Clarke et al., 1992; Lee et al., 1992], indicates that pRb is essential for normal development of the nervous and hematopoietic systems and may even function to regulate apoptosis [Haas-Kogan et al., 1995]. Although this paper briefly reviews the traditional role of pRB in regulation of cellular proliferation, we focus on the role of pRB in neuronal development and apoptosis. Recent reviews have been published on the role of pRb in cell cycle and transcriptional regulation [Hamel et al., 1992; Cobrinik et al., 1992; Kouzarides, 1993; Hollingsworth et al., 1993; Helin and Harlow, 1993; Sherr, 1994], as well as the relationship between pRb and p53 [Picksley and Lane, 1994; White, 1994].

Interaction of the 72-kilodalton human cytomegalovirus IE1 gene product with E2F1 coincides with E2F-dependent activation of dihydrofolate reductase transcription

Three polypeptides are produced from the major immediate-early (IE) region of human cytomegalovirus by alternative splicing. The IE gene products regulate subsequent viral and cellular gene expression. We previously reported that cotransfection of a genomic clone of the major IE region stimulated transient expression of chloramphenicol acetyltransferase driven by the dihydrofolate reductase (DHFR) promoter and that an intact E2F site was required for the trans activation (M. Wade, T. F. Kowalik, M. Mudryj, E.-S. Huang, and J. C. Azizkhan, Mol. Cell. Biol. 12:4364-4374, 1992). With the availability of cDNA clones for the individual major IE proteins, we sought to determine which of these proteins exerted this effect and whether the IE protein(s) interacted with E2F. In this study, we use cotransfection to demonstrate that the 55- and 86-kDa major IE proteins from the IE2 region can each moderately trans activate the DHFR promoter and that the 72-kDa IE1 protein stimulates DHFR transcription to a much higher level. Furthermore, trans activation through the 72-kDa IE1 protein is in part E2F dependent, while activation by the 55- and 86-kDa IE proteins is E2F independent. We also demonstrate by in vitro pull-down assays that the 72-kDa IE1 protein can specifically interact with the DNA binding domain of E2F1 (amino acids 88 to 191) in the presence of nuclear extract. Moreover, antibodies to either E2F1 or IE72 will immunoprecipitate both E2F and IE72 from cells that stably express IE72, and antibody to E2F1 will immunoprecipitate IE72 from normal human fibroblast cells infected with human cytomegalovirus.

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.

Melanoma formation in Xiphophorus: a model system for the role of receptor tyrosine kinases in tumorigenesis

Cancer is one of the most frequent fatal human diseases. It is a genetic disease, and molecular analysis of the genes involved revealed that they belong to several distinct classes of molecules, one of which is the receptor tyrosine kinases. Neoplastic transformation is regarded as the result of a multistep process and, in most cases, it is hard to evaluate what the initial events in tumor formation are. What makes it difficult to approach this question is the paucity of animals models for tumorigenesis allowing investigation of the mechanisms leading to uncontrolled cell proliferation. Melanoma formation in Xiphophorus is one of these model systems. Here, overexpression and activation of a receptor tyrosine kinase causes neoplastic transformation of pigment cells. Xiphophorus provides all the advantages of a well-characterized genetic system. In addition, some crucial components of the transformation pathway have been identified at the molecular level. As a vertebrate, Xiphophorus might serve as a model system to aid understanding, in more general terms, of the mechanisms of tumorigenesis in human diseases.

Cytomegalovirus infection induces high levels of cyclins, phosphorylated Rb, and p53, leading to cell cycle arrest

Human cytomegalovirus (HCMV) infection stimulates cellular DNA synthesis and causes chromosomal damage. Because such events likely affect cellular proliferation, we investigated the impact of HCMV infection on key components of the cell cycle. Early after infection, HCMV induced elevated levels of cyclin E, cyclin E-associated kinase activity, and two tumor suppressor proteins, p53 and the retinoblastoma gene product (Rb). The steady-state concentration of Rb continued to rise throughout the infection, with most of the protein remaining in the highly phosphorylated form. At early times, HCMV infection also induced cyclin B accumulation, which was associated with a significant increase in mitosis-promoting factor activity as the infection progresses. In contrast, the levels of cyclin A and cyclin A-associated kinase activity increased only at late times in the infection, and the kinetics were delayed relative to those for cyclins E and B. Analysis of the cellular DNA content in the infected cells by flow cytometry showed a progressive shift of the cells from the G1 to the S and G2/M phases of the cell cycle, leading to an accumulation of aneuploid cells at late times. We propose that these HCMV-mediated perturbations result in cell cycle arrest in G2/M.

70-kDa heat shock cognate protein interacts directly with the N-terminal region of the retinoblastoma gene product pRb. Identification of a novel region of pRb-mediating protein interaction

Retinoblastoma protein (pRb) functions as a tumor suppressor, and certain proteins are known to bind to pRb in the C-terminal region. Although the N-terminal region of pRb may also mediate interaction with some proteins, no such protein has been identified yet. We demonstrated previously the in vivo protein association between pRb and 73-kDa heat shock cognate protein (hsc73) in certain human tumor cell lines. In this report we analyzed the interaction between these two proteins in vitro. Our data showed that hsc73 interacts with the novel N-terminal region of pRb; that is, pRb binds directly to hsc73 and dissociates from hsc73 in an ATP-dependent manner. By using deletion mutants of cDNA encoding pRb, the hsc73 binding site of pRb was determined to be located in the region (residues 301-372) outside the so-called A pocket (residues 373-579) of this tumor suppressor protein. This finding was compatible with the fact that the adenovirus E1A oncoprotein, which is known to bind to the E2F binding pocket region of pRb, could not compete with hsc73 for the binding. Furthermore, phosphorylation of pRb by cyclin-dependent kinase inhibited the binding of pRb to hsc73. These data suggest that hsc73 may act exclusively as the molecular chaperone for nonphosphorylated pRb. As a result, hsc73 may function as a molecular stabilizer of nonphosphorylated pRb.

RB and a novel E2F-1 binding protein in MHC class II deficient B-cell lines and normal IFN-gamma induction of the class IL transactivator CIITA in class II non-inducible RB-defective tumor lines

The major histocompatibility (MHC) class II genes encode cell surface proteins that bind antigenic peptide for presentation to T-cells. The class II proteins are expressed constitutively on B-cells and EBV-transformed B-cells, and are inducible by IFN-gamma on a wide variety of cell types. Retinoblastoma protein (RB) is a tumor suppressor and functions as a transcriptional repressor by binding and inactivating the transactivator E2F-I. RB-defective tumor lines are non-inducible for MHC class II by IFN-gamma, or very weakly inducible, but transfection of 2 different lines with RB expression vectors re-establishes or substantially enhances class II inducibility. Therefore, we examined the RB status of a series of B-cell mutants that are defective in class II expression, generated either in vitro or derived from Bare Lymphocyte Syndrome (BLS) patients. Nuclear matrix-bound RB was detectable in all cases, indicating that loss of RB is not responsible for decreased class II expression in these lines. A second E2F-I binding protein, most likely DP-I, was also apparently normal in both class II-positive and -negative B-cell lines. We also examined the IFN-gamma induction of CIITA in RB-defective lines. CIITA is a class II gene transactivator known to be defective in one form of BLS and to be required for the induction of MHC class II by IFN-gamma. CIITA mRNA is normally inducible by IFN-gamma in class II non-inducible, RB-defective lines, and in one line, re-expression of RB has no effect on CIITA mRNA induction levels. Thus, the block in MHC class II inducibility in RB-defective cells is not due to a block in CIITA inducibility.

p107 uses a p21CIP1-related domain to bind cyclin/cdk2 and regulate interactions with E2F

The kinase activities of the cyclin/cdk complexes can be regulated in a number of ways. The most recently discovered mechanism of regulation is the association of cdk inhibitors (CKIs), such as p21, p27, and p57, with these complexes. In this report we demonstrate that the pRB-related protein p107, like the p21 family of cdk inhibitors, can inhibit the phosphorylation of target substrates by cyclin A/cdk2 and cyclin E/cdk2 complexes, and the associations of p107 and p21 with cyclin/cdk2 rely on a structurally and functionally related interaction domain. Furthermore, interactions between p107 or p21 with cyclin/cdk2 complexes are mutually exclusive. In cells treated with DNA-damaging agents elevated levels of p21 cause a dissociation of p107/cyclin/cdk2 complexes to yield p21/cyclin/cdk2 complexes. Finally, the consequences of cyclin/cdk2 interactions with p107 have been examined. The activation of the p107-bound cyclin/cdk kinases leads to dissociation of p107 from the transcription factor E2F. Together, these results suggest that cyclin/cdk complexes can be regulated by protein molecules from different families in a mutually exclusive manner in response to certain signals and that these inhibitory proteins may have a potential role in regulating macromolecular assembly.

The transcription factor E2F is required for S phase during Drosophila embryogenesis

Overexpression of the E2F-1 cDNA in mammalian cells disrupts normal control of the cell cycle and drives cells into S phase. Whereas eliminating E2F activity would test its inferred involvement in the G1-S transition, elimination is complicated by the existence of gene families encoding mammalian E2F. Here we identify mutations in a single essential Drosophila gene, dE2F, that encodes a homolog of the mammalian E2F gene family. Embryos homozygous for null mutations of dE2F complete early cell cycles, presumably using maternal contributions of gene products, but DNA synthesis falls to virtually undetectable levels in cycle 17. Mutant embryos also lack the pulses of coordinate transcription of genes encoding replication functions that usually accompany each transition from quiescence to S phase. We conclude that in most cells dE2F is essential for a G1-S transcriptional program and for G1-S progression.

Cells differentiating into neuroectoderm undergo apoptosis in the absence of functional retinoblastoma family proteins

The retinoblastoma (RB) protein is present at low levels in early mouse embryos and in pluripotent P19 embryonal carcinoma cells; however, the levels of RB rise dramatically in neuroectoderm formed both in embryos and in differentiating cultures of P19 cells. To investigate the effect of inactivating RB and related proteins p107 and p130, we transfected P19 cells with genes encoding mutated versions of the adenovirus E1A protein that bind RB and related proteins. When these E1A-expressing P19 cells were induced to differentiate into neuroectoderm, there was a striking rise in the expression of c-fos and extensive cell death. The ultrastructural and biochemical characteristics of the dying cells were indicative of apoptosis. The dying cells were those committed to the neural lineages because neurons and astrocytes were lost from differentiating cultures. Cell death was dependent on the ability of the E1A protein to bind RB and related proteins. Our results suggest that proteins of the RB family are essential for the development of the neural lineages and that the absence of functional RB activity triggers apoptosis of differentiating neuroectodermal cells.

Cyclin D1 is dispensable for G1 control in retinoblastoma gene-deficient cells independently of cdk4 activity

To elucidate the regulator-versus-target relationship in the cyclin D1/cdk4/retinoblastoma protein (pRB) pathway, we examined fibroblasts from RB-1 gene-deficient and RB-1 wild-type littermate mouse embryos (ME) and in human tumor cell lines that differed in the status of the RB-1 gene. The RB+/+ and RB-/- ME fibroblasts expressed similar protein levels of D-type cyclins, cdk4, and cdk6, showed analogous spectra and abundance of cellular proteins complexed with cdk4 and/or cyclins D1 and D2, and exhibited comparable associated kinase activities. Of the two human cell lines established from the same sarcoma biopsy, the RB-positive SKUT1B cells contained cdk4 that was mainly associated with D-type cyclins, contrary to a predominant cdk4-p16INK4 complex in the RB-deficient SKUT1A cells. Antibody-mediated neutralization of cyclin D1 arrested the RB-positive ME and SKUT1B cells in G1, whereas this cyclin appeared dispensable in the RB-deficient ME and SKUT1A cells. Lack of requirement for cyclin D1 therefore correlated with absence of functional pRB, regardless of whether active cyclin D1/cdk4 holoenzyme was present in the cells under study. Consistent with a potential role of cyclin D/cdk4 in phosphorylation of pRB, monoclonal anti-cyclin D1 antibodies supporting the associated kinase activity failed to significantly affect proliferation of RB-positive cells, whereas the antibody DCS-6, unable to coprecipitate cdk4, efficiently inhibited G1 progression and prevented pRB phosphorylation in vivo. These data provide evidence for an upstream control function of cyclin D1/cdk4, and a downstream role for pRB, in the order of events regulating transition through late G1 phase of the mammalian cell division cycle.

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.

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.

Cytostatic gene therapy for vascular proliferative disorders with a constitutively active form of the retinoblastoma gene product

Vascular smooth muscle cell (SMC) proliferation in response to injury is an important etiologic factor in vascular proliferative disorders such as atherosclerosis and restenosis after balloon angioplasty. The retinoblastoma gene product (Rb) is present in the unphosphorylated and active form in quiescent primary arterial SMCs, but is rapidly inactivated by phosphorylation in response to growth factor stimulation in vitro. A replication-defective adenovirus encoding a nonphosphorylatable, constitutively active form of Rb was constructed. Infection of cultured primary rat aortic SMCs with this virus inhibited growth factor-stimulated cell proliferation in vitro. Localized arterial infection with the virus at the time of balloon angioplasty significantly reduced SMC proliferation and neointima formation in both the rat carotid and porcine femoral artery models of restenosis. These results demonstrate the role of Rb in regulating vascular SMC proliferation and suggest a gene therapy approach for vascular proliferative disorders associated with arterial injury.

Fibroblast subpopulations as accelerators of tumor progression: the role of migration stimulating factor

Tumor progression is a relatively indolent process, with many years commonly intervening between the inception of an initiating genetic lesion and the development of overt malignant disease. We suggest that the perturbation of normal epithelial-mesenchymal interactions caused by the inappropriate presence of fibroblast subpopulations displaying various 'fetal-like' phenotypic characteristics may significantly alter the kinetics of tumor progression and hence enhance susceptibility to cancer development. In this communication, we review our own data indicating the presence of fetal-like fibroblasts in cancer patients and put these observations in the context of similar published reports. We then discuss our interpretation of these findings, emphasising the possible direct involvement of fetal-like fibroblasts in cancer pathogenesis and putting forward an epigenetic 'clonal modulation' model to account for their presence in cancer patients.

Amplification of the E2F1 transcription factor gene in the HEL erythroleukemia cell line

The E2F transcription factor plays an important regulatory role in cell proliferation, mediating the expression of genes whose products are essential for inducing resting cells to enter the cell cycle and synthesize DNA. To investigate the possible involvement of E2F in hematopoietic malignancies, we isolated genomic clones encompassing the human E2F1 gene. We then used fluorescence in situ hybridization to localize E2F1 to human chromosome 20q11, telomeric to the p107 locus, a gene whose product is related to the retinoblastoma gene product (pRb). This finding contrasts with the 1p36 and 6q22 chromosomal locations previously assigned E2F2 and E2F3, two additional members of the E2F family. Although deletions or structural rearrangements of E2F1 were not detected in 14 primary acute leukemia or myelodysplasia samples with structural abnormalities of chromosome 20q11, the gene was amplified and overexpressed in HEL erythroleukemia cells and translocated to other chromosomes in several established human leukemia cell lines. This study provides the first evidence of gene amplification involving a member of the E2F family of transcription factors. We propose that E2F1 overexpression in erythroid progenitors may stimulate abnormal cell proliferation by overriding negative regulatory signals mediated by tumor suppressor proteins such as pRb.