The E2F transcription factor is a cellular target for the RB protein

E7 proteins of four groups of human papillomaviruses, irrespective of their tissue tropism or cancer association, possess the ability to transactivate transcriptional promoters E2F site dependently

In an experimental system in which an expression vector including the E7 gene of a given human papillomavirus (HPV), together with a luciferase reporter plasmid including the adenovirus E2 (Ad E2) promoter, was transiently transfected into cultured mouse NIH3T3 fibroblastic cells, we obtained the signal indicating that E7 proteins of HPV type 5, 12, 14, 20, 21, 25, and 47, which are associated with epidermodysplasia verruciformis (EV), can transactivate the Ad E2 promoter, as previously reported for E7 proteins of other HPVs. Because the underlying mechanism of the transactivation had not been analyzed, except for transactivation by E7 gene of cervical cancer-associated HPV-16, we compared the E7 genes of representatives of three other groups of HPVs (HPV-1, -11, and -47) with that of HPV-16 with regard to their transactivating activity toward artificially constructed promoters. The experiment with a shortened AdE2 promoter carrying only the E2F sites and TATA box provided evidence that all four E7 proteins can transactivate the shortened promoter and that this phenomenon is E2F site dependent. Further experiments with the reporter gene constructs carrying basal promoters or more complex forms with or without linked E2F sites, (a) confirmed previous finding by others that in cells producing no transactivator, the transcriptional level from promoters linked to E2F sites is rather repressed in comparison with the level of the corresponding promoters that are not linked to the E2F sites, and (b) demonstrated, for the first time, that in cells expected to produce the E7 protein of any one of the four HPVs, transcription from the promoter linked to the E2F sites was released from repression. In other words, the present results reveal that E7 proteins of any of the four HPVs can remove the E2F site-dependent repression, probably by modulating E2F complexes from repressing forms to activating ones.

Constitutive expression of the E2F1 transcription factor in fibroblasts alters G0 and S phase transit following serum stimulation

The E2F1 transcription factor was constitutively expressed in NIH3T3 fibroblasts to determine its effect on the cell cycle. These E2F1 cell lines were not tightly synchronized in G0 phase of the cell cycle following serum starvation, as are normal fibroblasts. Instead, the cells are spread throughout G0 and G1 phase with a portion of the population initiating DNA synthesis. Upon serum stimulation, the remaining cells in G0/G1 begin to enter S phase immediately but with a reduced rate. Constitutive expression of E2F1 appears to primarily affect the G0 phase, since transit of proliferating E2F1 cell lines through G1 phase is the same as control cells. Consistent with a shortened G0 phase, the E2F1 cell lines have a significantly reduced cellular volume. Additionally, the first S phase after serum stimulation, but not subsequent S phases, is nearly doubled in the E2F1 cell lines compared with control cells. Cell lines expressing a deletion mutant of E2F1 (termed E2F1d87), known to significantly affect cell shape, have cell cycle and volume characteristics similar to the E2F1 expressing cells. However, all S phase durations are considerably lengthened and the cells demonstrate delayed growth after plating.

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.

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].

Apoptosis and the cell cycle

Apoptosis is an evolutionarily conserved 'suicide' programme present in all metazoan cells. Despite its highly conserved nature, it is only recently that any of the molecular mechanisms underlying apoptosis have been identified. Several lines of reasoning indicate that apoptosis and cell proliferation coincide to some degree: many oncogenes that promote cell cycle progression also induce apoptosis; damage to the cell cycle or to DNA integrity is a potent trigger of apoptosis; and the key tumour suppressor proteins, p105rb and p53, exert direct effects both on cell viability and on cell cycle progression. There is less evidence, however, to indicate that apoptosis and the cell cycle share common molecular mechanisms. Moreover, the interleukin-1 beta converting enzyme (ICE) family of cysteine proteases is now known to play a key role in apoptosis but has no discernible role in the cell cycle, arguing that the two processes are discrete.

E2F-1:DP-1 induces p53 and overrides survival factors to trigger apoptosis

The E2F DNA binding activity consists of a heterodimer between E2F and DP family proteins, and these interactions are required for association of E2F proteins with pRb and the pRb-related proteins p107 and p130, which modulate E2F transcriptional activities. E2F-1 expression is sufficient to release fibroblasts from G0 and induce entry into S phase, yet it also initiates apoptosis. To investigate the mechanisms of E2F-induced apoptosis, we utilized interleukin-3 (IL-3)-dependent 32D.3 myeloid cells, a model of hematopoietic progenitor programmed cell death. In the absence of IL-3, E2F-1 alone was sufficient to induce apoptosis, and p53 levels were diminished. DP-1 alone was not sufficient to induce cell cycle progression or alter rates of death following IL-3 withdrawal. However, overexpression of both E2F-1 and DP-1 led to the rapid death of cells even in the presence of survival factors. In the presence of IL-3, levels of endogenous wild-type p53 increased in response to E2F-1, and coexpression of DP-1 further augmented p53 levels. These results provide evidence that E2F is a functional link between the tumor suppressors p53 and pRb. However, induction of p53 alone was not sufficient to trigger apoptosis, suggesting that the ability of E2F to override survival factors involves additional effectors.

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.

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.

In vivo structure of the human cdc2 promoter: release of a p130-E2F-4 complex from sequences immediately upstream of the transcription initiation site coincides with induction of cdc2 expression

In quiescent cells, cdc2 mRNA is almost undetectable. Stimulation of cells to reenter the cell cycle results in induction of cdc2 expression, beginning at the G1-to-S transition and reaching maximum levels during late S and G2 phases. To investigate cdc2 transcriptional regulation throughout cell cycle progression, we monitored protein-DNA interactions by in vivo footprinting along 800 bp of the human cdc2 promoter in quiescent fibroblasts and at different time points following serum stimulation. We found 11 in vivo protein-binding sites, but no protein binding was observed at a high-affinity E2F site that had previously been implicated in cdc2 regulation. Nine of the identified in vivo binding sites (among them were two inverted CCAAT boxes, two Sp1 sites, and one ets-2 site) bind transcription factors constitutively throughout the cell cycle. However, at two elements located at positions -60 and -20 relative to the transcription start site, the binding pattern changes significantly as the cells are entering S phase. A G0- and G1-specific protein complex disappears at the -20 element at the beginning of S phase. This sequence deviates at one base position from known E2F consensus binding sites. We found that the major E2F activity in human fibroblasts contains E2F-4 and p130. The -20 element of the cdc2 gene specifically interacts with a subset of E2F-4-p130 complexes present in G0 cells but does not interact with S-phase-specific E2F complexes. Transient-transfection experiments with wild-type and mutant cdc2 promoter constructs indicate that the -20 element is involved in suppressing cdc2 activity in quiescent cells. We suggest that the presence of the p130-E2F-4 complex in G0/G1 blocks access of components of the basal transcription machinery or prevents transaction by the constitutively bound upstream activator proteins.

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.

The RB2/p130 gene product is a nuclear protein whose phosphorylation is cell cycle regulated

The Rb2/p130 protein has been shown to have a high sequence homology with the retinoblastoma gene product (pRb), one of the most well-characterized tumor suppressor genes, and with pRB-related p107, especially in their conserved pocket domains, which display a primary role in the function of these proteins. In this study, we report on the biochemical and immunocytochemical characterization of the Rb2/p130 protein, using a polyclonal antibody developed against its "spacer" region included in the pocket domain of the whole protein. We show that pRb2/p130 is a phosphoprotein located at the nuclear level and that its phosphorylation pathway can be dramatically reduced by phosphatase treatment. Moreover pRb2/p130 with p107, is one of the major targets of the E1A viral oncoprotein-associated kinase activity, showing a phosphorylation pattern which is modulated during the cell cycle, reaching a peak of activation at the onset of S-phase.

Evidence that a cell cycle regulator, E2F1, down-regulates transcriptional activity of the human immunodeficiency virus type 1 promoter

Proliferation of eukaryotic cells is orchestrated by a series of cellular proteins which participate in various stages of the cell cycle to guide the cell through mitosis. Some of these proteins, including E2F1, play a critical role in G1 and S phases by coordinately regulating expression of several important cell cycle-associated genes. On the basis of recent observations indicating a block in human immunodeficiency virus type 1 (HIV-1) replication in cells arrested in G1/S phase of the cell cycle, we sought to evaluate the regulatory action of E2F1 on transcription from the HIV-1 long terminal repeat (LTR). Results from transient transfection of cells with an E2F1 expression plasmid indicated that E2F1 has the ability to suppress basal transcriptional activity of the LTR and to diminish the extent of the Tat-induced activation of the viral promoter. Deletion analysis of the HIV-1 LTR in transfection studies revealed the presence of two major elements responsive to E2F1 repression located distally (-454 to -381) and proximally (-117 to -80) with respect to the +1 transcription start site. E2F1-mediated suppression of LTR activity was observed in a wide range of human cell lines. Expression of E2F1 by a transgene showed an inhibitory effect on the levels of reverse transcriptase activity obtained upon introduction of the proviral genome into cells. The data presented in this study suggest that cellular regulatory proteins involved in the progression of cells through the mitotic cycle could play crucial roles in determining the efficiency of HIV-1 replication during the various stages of infection. The possible roles of these factors in viral latency and activation are discussed.

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

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

Association of human Pur alpha with the retinoblastoma protein, Rb, regulates binding to the single-stranded DNA Pur alpha recognition element

The retinoblastoma protein, Rb, is detected in extracts of monkey CV-1 cells complexed with Pur alpha, a sequence-specific single-stranded DNA-binding protein implicated in control of gene transcription and DNA replication. These complexes can be immunoextracted from cell lysates using monoclonal antibodies to either Pur alpha or Rb. The Pur alpha-Rb complexes contain a form of Pur alpha with extensive post-synthetic modification, as demonstrated following expression of Pur alpha cDNA fused to a 9-amino acid epitope tag. Human Pur alpha, expressed as a glutathione S-transferase fusion protein, specifically binds to the hypophosphorylated form of Rb with an affinity as high as that of SV40 large T-antigen. In the absence of DNA, glutathione S-transferase-Pur alpha binds to p56RB, an NH2-terminal-truncated Rb protein purified from Escherichia coli, containing the T-antigen binding domain, to form multimeric complexes. The single-stranded DNA Pur alpha recognition element disrupts these complexes. Conversely, high concentrations of p56RB prevent Pur alpha binding to DNA. Through use of a series of deletion mutants, the DNA binding activity of Pur alpha is localized to a series of modular amino acid repeats. Rb binding involves a Pur alpha region with limited homology to the Rb-binding region of SV40 large T-antigen. Binding of Pur alpha to p56RB, the COOH-terminal portion of Rb, is inhibited by a synthetic peptide containing the T-antigen Rb-binding motif.

Cervical cancer: is herpes simplex virus type II a cofactor?

In many ways, cervical cancer behaves as a sexually transmitted disease. The major risk factors are multiple sexual partners and early onset of sexual activity. Although high-risk types of human papillomaviruses (HPV) play an important role in the development of nearly all cases of cervical cancer, other sexually transmitted infectious agents may be cofactors. Herpes simplex virus type 2 (HSV-2) is transmitted primarily by sexual contact and therefore has been implicated as a risk factor. Several independent studies suggest that HSV-2 infections correlate with a higher than normal incidence of cervical cancer. In contrast, other epidemiological studies have concluded that infection with HSV-2 is not a major risk factor. Two separate transforming domains have been identified within the HSV-2 genome, but continued viral gene expression apparently is not necessary for neoplastic transformation. HSV infections lead to unscheduled cellular DNA synthesis, chromosomal amplifications, and mutations. These observations suggest that HSV-2 is not a typical DNA tumor virus. It is hypothesized that persistent or abortive infections induce permanent genetic alterations that interfere with differentiation of cervical epithelium and subsequently induce abnormal proliferation. Thus, HSV-2 may be a cofactor in some but not all cases of cervical cancer.

Association of p107 with Sp1: genetically separable regions of p107 are involved in regulation of E2F- and Sp1-dependent transcription

The retinoblastoma-related protein p107 has been shown to be a regulator of the transcription factor E2F. p107 associates with E2F via its pocket region and represses E2F-dependent transcription. In this study, we provide evidence for a novel interaction between p107 and the transcription factor Sp1. We show that p107 can be found endogenously associated with Sp1 in the extracts of several different cell lines. Moreover, in transient transfection assays, expression of p107 represses Sp1-dependent transcription. This repression of Sp1-dependent transcription does not require the DNA-binding domain of Sp1. Transcription driven by a chimeric protein containing the Ga14 DNA-binding domain and the Sp1 activation domains is inhibited by p107. Interestingly, unlike the repression of E2F-dependent transcription, the repression of Sp1-dependent transcription does not depend on an intact pocket region. We show that distinct regions of p107 are involved in the control of Sp1 and E2F.

The p21 cyclin-dependent kinase inhibitor suppresses tumorigenicity in vivo

The p21 gene encodes a cyclin-dependent kinase inhibitor that affects cell-cycle progression, but the potential of this gene product to serve as a tumour suppressor in vivo has not been established. In this report, we show that the growth of malignant cells in vitro and in vivo is inhibited by expression of p21. Expression of p21 resulted in an accumulation of cells in G0/G1, altered morphology, and cell differentiation, but apoptosis was not induced. Introduction of p21 with adenoviral vectors into malignant cells completely suppressed their growth in vivo and also reduced the growth of established pre-existing tumours. Gene transfer of p21 may provide a molecular genetic approach to arresting cancer cell growth by committing malignant cells irreversibly to a pathway of terminal differentiation.

Altered patterns of retinoblastoma gene product expression in adult soft-tissue sarcomas

Altered expression of the retinoblastoma (RB) tumour-suppressor gene product (pRB) has been detected in sporadic bone and soft-tissue sarcomas. Earlier studies, analysing small cohorts of sarcoma patients, have suggested that these alterations are more commonly associated with high-grade tumours, metastatic lesions and poorer survival. This study was designed to re-examine the prevalence and clinical significance of altered pRB expression in a large and selected group of soft-tissue sarcomas from 174 adult patients. Representative tissue sections from these sarcomas were analysed by immunohistochemistry using a well-characterised anti-pRB monoclonal antibody. Tumours were considered to have a positive pRB phenotype only when pure nuclear staining was demonstrated, and cases were segregated into one of three groups. Group 1 (n = 36) were patients whose tumours have minimal or undetectable pRB nuclear staining (< 20% of tumour cells) and were considered pRB negative. Patients with tumours staining in a heterogeneous pattern (20-79% of tumour cells) were classified as group 2 (n = 99). The staining of group 3 (n = 39) was strongly positive with a homogeneous pRB nuclear immunoreactivity (80-100% of tumour cells). pRB alterations were frequently observed in both low- and high-grade lesions. Altered pRB expression did not correlate with known predictors of survival and was not itself an independent predictor of outcome in the long-term follow-up. These findings support earlier observations that alterations of pRB expression are common events in soft-tissue sarcomas; nevertheless, long-term follow-up results indicate that altered patterns of pRB expression do not influence clinical outcome of patients affected with soft-tissue sarcomas. It is postulated that RB alterations are primary events in human sarcomas and may be involved in tumorigenesis or early phases of tumour progression in these neoplasias.

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.

Activation of a retinoblastoma-protein-dependent pathway by sphingosine

The retinoblastoma protein (Rb) is a tumour suppressor that is activated by dephosphorylation the function of which appears to be mediated, at least partly, through the inhibition of several transcription factors, such as E2F. We have recently described sphingosine, a sphingolipid-breakdown product, as a potent and specific inducer of Rb dephosphorylation resulting in inhibition of cell growth and a specific arrest in the G0/G1 phase of the cell cycle. Here we examine the role of Rb and its interaction with E2F in mediating the effects of sphingosine on cell growth. Sphingosine potently inhibited growth of lymphoblastic leukaemic cells, Molt-4, at submicromolar concentrations but showed a 10-fold reduced potency in inhibiting growth of retinoblastoma cells, WERI-Rb-1, which lack functional Rb. In addition, sphingosine's ability to inhibit growth of mink lung epithelial cells was significantly attenuated in cells overexpressing simian virus 40 large T antigen which binds Rb and related proteins. Sphingosine treatment of Molt-4 cells, but not WERI-Rb-1 cells, resulted in the loss of the specific E2F bands produced by the interaction of E2F and its specific DNA sequence element on gel-shift assays. The concentration (submicromolar) and kinetics (4 h) of sphingosine treatment were identical with those required to induce Rb dephosphorylation. In addition, at similar concentrations, sphingosine caused c-myc down-regulation in Molt-4 cells starting at 6 h after treatment. These results demonstrate that activation of Rb by sphingosine leads to sequestration of E2F by the active (hypophosphorylated) form of Rb with the resultant loss of its DNA-binding and genetranscribing abilities. A functional Rb is required to mediate the specific effects of sphingosine on growth arrest.

Cell cycle regulation of a novel DNA binding complex in Saccharomyces cerevisiae with E2F-like properties

Using a biochemical approach, we have detected an activity in Saccharomyces cerevisiae extract that displays the same DNA binding specificity as the mammalian E2F transcription factor and interacts with TTTCGCGC promoter elements. Additional studies revealed that this factor, termed SCELA (S. cerevisiae E2F-like activity), also binds to the closely related SCB promoter sequences. SCB sites (consensus: TTTCGTG) are involved in the cell cycle regulation of several S. cerevisiae cyclin genes and have been shown to interact with the heterodimeric yeast Swi4-Swi6 complex. However, genetic studies clearly demonstrate that SCELA is not related to Swi4 or Swi6. These experiments imply that SCB sites are able to interact with at least two activities: Swi4-Swi6 and SCELA. Because SCB sites are critical for the periodic activation of cell cycle genes, we asked whether SCELA is regulated during yeast cell cycle. Employing a temperature-sensitive strain, we were able to demonstrate that the DNA binding activity of SCELA oscillates during the cell cycle and reaches its maximum at the transition between the G1 and S phases. Preliminary studies suggest that this fluctuation is mediated by phosphorylation/dephosphorylation events. Further characterization of SCELA by UV cross-linking experiments indicate a molecular mass of 47 kDa for this activity. In addition, we present evidence strongly suggesting that SCELA is actually the DNA binding moiety of a large 300-kDa protein complex. Together, these studies firmly indicate that SCELA (as part of a larger complex) plays a critical role in cell cycle regulation of SCB-containing genes, such as CLN cyclins and HO endonuclease. This hypothesis is consistent with other studies that conclude that the SCB-mediated cell cycle oscillation of CLN cyclins and HO requires activities that are distinct from Swi4-Swi6. Finally, it is worth mentioning that the similarities between SCELA and E2F, which is a crucial component in mammalian cell cycle regulation, extend well beyond the DNA binding specificity. In analogy to E2F, SCELA oscillates during the cell cycle, interacts with other cellular activities, and binds to promoter elements that are known mediators of cell cycle control. We will discuss possible functions for SCELA in yeast cell cycle regulation and its relationship to E2F.

Stimulation of human insulin receptor gene expression by retinoblastoma gene product

Multiple cis-acting elements have been defined to be important for the transcriptional regulation of the human insulin receptor (hIR) gene expression. We report here that one of these elements also mediated the stimulation of hIR promoter activity by the retinoblastoma gene product (Rb). The cis-element responsible for Rb stimulation was localized to the GA and GC boxes situated between -643 to -607 of the hIR gene. We have previously demonstrated that these GA and GC boxes bind Sp1 with high affinity and are responsible for E1a activation of hIR promoter activity. Mutation of these sequences completely abolished Rb-dependent enhancement of hIR promoter activity. In addition, we localized three regions in the N-terminal domain of Rb to be involved in stimulation of hIR promoter activity. Our results represent one of the first studies to demonstrate a functional importance assigned to the multiple phosphorylation sites in the N terminus of Rb. Finally, the mechanism by which Rb activates the hIR promoter are presented.

Isolation and sequence polymorphism of a rat retinoblastoma (RB) cDNA

A cDNA of the rat retinoblastoma gene (RB) was prepared from total RNA of rat liver using reverse transcription-polymerase chain reaction (RT-PCR). The 4432-nt sequence isolated contained 2700-nt translated and 1732-nt 3'-untranslated regions (UTR). The isolated cDNA detected poly(A)+RNAs of 5.4 and 3.4 kb in rat liver and kidney by Northern blot hybridization. The nt sequence of the isolated cDNA had 85% homology with that of mouse and 73% with human. The 899-amino-acid (aa) sequence was 95% homologous to that of mouse and 90% to human. The aa sequences of two functional domains of oncoprotein-binding and ten putative phosphorylation sites regulating RB function were conserved in the three species. However, the 3'-UTR were less homologous among the three, and had polymorphism in three portions, even in rats. These polymorphisms were strain-specific and genetically segregated. Thus, the rat RB cDNA and its sequence information may be useful for clarifying the role of the RB protein and genetic linkage analysis in basic biomedical research using rats, especially in experimental carcinogenesis.

The retinoblastoma protein and cell cycle regulation

Although the precise function of the retinoblastoma gene product, p110RB1, remains unknown, recent data suggest that it plays a role in the control of cellular proliferation by regulating transcription of genes required for a cell to enter or stay in a quiescent or G0 state, or for progression through the G1 phase of the cell cycle. However, it is difficult to rationalize the expression of p110RB1 in a wide range of tissues with the fact that mutations in the RB1 gene initiate cancers in a limited number of tissues.

Alterations of the retinoblastoma a susceptibility gene in chronic lymphocytic leukemia

Research in recent years has shown that malignant transformation is a genetic multistep process. This holds true not only for in-vitro model systems, but has also been elegantly shown in-vivo, as in colorectal cancer. Chronic lymphocytic leukemia (CLL) is the most frequent leukemia in Western countries and occurs mainly in elderly patients, suggesting that in this form of leukemia, cumulative molecular lesions may be necessary for transformation. However, the molecular background is unknown in most cases. Cytogenetic aberrations may be used as markers for genes involved in the process of malignant transformation. In CLL, the most frequently observed structural cytogenetic lesion is a deletion/translocation involving the long arm of chromosome 13, a region where the retinoblastoma susceptibility gene (Rb-gene) has been mapped (13q14). Many groups have studied the question as to whether alterations of the Rb-gene play a causal role in the pathogenesis of CLL. This review deals with recent data indicating that i) the Rb-gene may be altered in a minority of CLL cases, and ii) there may be another gene localized on chromosome 13q14 that may be important in the molecular biology of CLL.

Cellular targets for activation by the E2F1 transcription factor include DNA synthesis- and G1/S-regulatory genes

Although a number of transfection experiments have suggested potential targets for the action of the E2F1 transcription factor, as is the case for many transcriptional regulatory proteins, the actual targets in their normal chromosomal environment have not been demonstrated. We have made use of a recombinant adenovirus containing the E2F1 cDNA to infect quiescent cells and then measure the activation of endogenous cellular genes as a consequence of E2F1 production. We find that many of the genes encoding S-phase-acting proteins previously suspected to be E2F targets, including DNA polymerase alpha, thymidylate synthase, proliferating cell nuclear antigen, and ribonucleotide reductase, are indeed induced by E2F1. Several other candidates, including the dihydrofolate reductase and thymidine kinase genes, were only minimally induced by E2F1. In addition to the S-phase genes, we also find that several genes believed to play regulatory roles in cell cycle progression, such as the cdc2, cyclin A, and B-myb genes, are also induced by E2F1. Moreover, the cyclin E gene is strongly induced by E2F1, thus defining an autoregulatory circuit since cyclin E-dependent kinase activity can stimulate E2F1 transcription, likely through the phosphorylation and inactivation of Rb and Rb family members. Finally, we also demonstrate that a G1 arrest brought about by gamma irradiation is overcome by the overexpression of E2F1 and that this coincides with the enhanced activation of key target genes, including the cyclin A and cyclin E genes.

Identification of NAB1, a repressor of NGFI-A- and Krox20-mediated transcription

NGFI-A (also called Egr1, Zif268, or Krox24) and the closely related proteins Krox20, NGFI-C, and Egr3 are zinc-finger transcription factors encoded by immediate-early genes which are induced by a wide variety of extracellular stimuli. NGFI-A has been implicated in cell proliferation, macrophage differentiation, synaptic activation, and long-term potentiation, whereas Krox20 is critical for proper hindbrain segmentation and peripheral nerve myelination. In previous work, a structure/function analysis of NGFI-A revealed a 34-aa inhibitory domain that was hypothesized to be the target of a cellular factor that represses NGFI-A transcriptional activity. Using the yeast two-hybrid system, we have isolated a cDNA clone which encodes a protein that interacts with this inhibitory domain and inhibits the ability of NGFI-A to activate transcription. This NGFI-A-binding protein, NAB1, is a 570-aa nuclear protein that bears no obvious sequence homology to known proteins. NAB1 also represses Krox20 activity, but it does not influence Egr3 or NGFI-G, thus providing a mechanism for the differential regulation of this family of immediate-early transcription factors.

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.

Interaction of the CCAAT displacement protein with shared regulatory elements required for transcription of paired histone genes

The H2A and H2B genes of the Xenopus xlh3 histone gene cluster are transcribed in opposite directions from initiation points located approximately 235 bp apart. The close proximity of these genes to one another suggests that their expression may be controlled by either a single bidirectional promoter or by separate promoters. Our analysis of the transcription of histone gene pairs containing deletions and site-specific mutations of intergenic DNA revealed that both promoters are distinct but that they overlap physically and share multiple regulatory elements, providing a possible basis for the coordinate regulation of their in vivo activities. Using the intergenic DNA fragment as a probe and extracts from mammalian and amphibian cells, we observed the formation of a specific complex containing the CCAAT displacement protein (CDP). The formation of the CDP-containing complex was not strictly dependent on any single element in the intergenic region but instead required the presence of at least two of the three CCAAT motifs. Interestingly, similar CDP-containing complexes were formed on the promoters from the three other histone genes. The binding of CDP to histone gene promoters may contribute to the coordination of their activities during the cell cycle and early development.

Developmental control of the G1 to S transition in Drosophila: cyclin Eis a limiting downstream target of E2F

The E2F transcription factor is required for S phase in Drosophila. While it also triggers expression of replication genes at the G1-S transition, the relevance of this transcription is not clear because many of the induced gene products are sufficiently stable that new expression is not required for S phase. However, one unstable product could couple S phase to E2F activation. Here we show that cyclin E expression at G1-S requires E2F, that activation of E2F without cyclin E is not sufficient for S phase, and that early in G1 ectopic expression of cyclin E alone can bypass E2F and induce S phase. We conclude that cyclin E is the downstream gene that couples E2F activity to G1 control. Not all embryonic cycles are similarly coupled to E2F activation, however. The rapidly proliferating CNS cells, which exhibit no obvious G1, express cyclin E constitutively and independently to E2F. Instead, cyclin E expression activates E2F in the CNS. Thus, this tissue-specific E2F-independent transcription of cyclin E reverses the hierarchical relationship between cyclin E and E2F. Both hierarchies activate expression of the full complement of replication functions controlled by E2F; however, whereas inactivation of E2F can produce a G1 when cyclin E is downstream of E2F, we propose that an E2F-independent source of E eliminates G1.

Tumor suppressor genes and clonal evolution in B-CLL

This review highlights the genetic alterations that have been detailed in the malignant B-cell clones of patients with B-chronic lymphocytic leukemia (CLL). In particular, the alterations seen in p53 and the retinoblastoma (Rb) genes are reviewed. In addition, the multiplicity of cytogenetic alterations observed at baseline and on sequential analysis are summarized. The cytogenetic and molecular biologic analysis of B-CLL clones has revealed that there is a dynamic array of genetic events which occur within a B-cell clone. This latter data strongly suggests that clonal evolution may occur in B-CLL patients. However the relationship of the clonal instability to the patient's clinical course is still unclear. The relatively frequent detection of multiple tumor suppressor gene alterations in the B-CLL clones offer several interesting clues regarding the transformation event within B-CLL. A model is proposed which attempts to explain the potential contribution and interaction of p53 and Rb gene alterations in a malignant B-cell transformation.

Direct transcriptional repression by pRB and its reversal by specific cyclins

It was recently shown that the E2F-pRB complex is a negative transcriptional regulator. However, it was not determined whether the whole complex or pRB alone is required for repression. Here we show that pRB and the related protein p107 are capable of direct transcriptional repression independent of E2F. When fused to the DNA binding domain of GAL4, pRB or p107 represses transcription of promoters with GAL4 binding sites. Thus, E2F acts as a tether for pRB or p107 but is not actively involved in repression of other enhancers. This function of pRB maps to the pocket and is abrogated by mutation of this domain. This result suggests an intriguing model in which the pocket has a dual function, first to bind E2F and second to repress transcription directly, possibly through interaction with other proteins. We also show that direct transcriptional repression by pRB is regulated by phosphorylation. Mutations which render pRB constitutively hypophosphorylated potentiate repression, while phosphorylation induced by cyclin A or E reduces repression ninefold.

Inactivation of the retinoblastoma susceptibility protein is not sufficient for the transforming function of the conserved region 2-like domain of simian virus 40 large T antigen

Simian virus 40 large T antigen interacts with three cellular proteins, pRb, p107, and p130, through a common binding site on the T antigen protein called the E1A conserved region 2-like (CR2-like) domain. Mutations in this domain inactivate the transforming activity of large T antigen. Since these mutations have been demonstrated to abolish binding to pRb and p107, and presumably therefore affect binding to p130, assessment of the relative roles of these three proteins in transformation of rodent fibroblasts by T antigen has been difficult. We have examined the role of T antigen-pRb interactions in transformation. We have introduced a mutant T antigen, which is unable to bind any of these three proteins, into primary mouse fibroblasts derived from the embryos of mice in which the Rb gene encoding the retinoblastoma protein had been disrupted. This mutant is unable to transform the Rb-negative fibroblasts, indicating that inactivation of pRb is not the sole function of the CR2-like domain in the induction of transformation of mouse fibroblasts by simian virus 40.

Regulation of the retinoblastoma protein-related p107 by G1 cyclin complexes

The orderly progression through the cell cycle is mediated by the sequential activation of several cyclin/cyclin-dependent kinase (cdk) complexes. These kinases phosphorylate a number of cellular substrates, among which is the product of the retinoblastoma gene, pRb. Phosphorylation of pRb in late G1 causes the release of the transcription factor E2F from pRb, resulting in the transcriptional activation of E2F-responsive genes. We show here that phosphorylation of the pRb-related p107 is also cell cycle regulated. p107 is first phosphorylated at 8 hr following serum stimulation of quiescent fibroblasts, which coincides with an increase in cyclin D1 protein levels. Consistent with this, we show that a cyclin D1/cdk4 complex, but not a cyclin E/cdk2 complex, can phosphorylate p107 in vivo. Furthermore, phosphorylation of p107 can be abolished by the overexpression of a dominant-negative form of cdk4. Phosphorylation of p107 results in the loss of the ability to associate with E2F-4, a transcription factor with growth-promoting and oncogenic activity. A p107-induced cell cycle block can be released by cyclin D1/cdk4 but not by cyclin E/cdk2. These data indicate that the activity of p107 is regulated by phosphorylation through D-type cyclins.

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

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

Study of the role of retinoblastoma protein in terminal differentiation of murine erythroleukemia cells

Hexamethylenebisacetamide-induced terminal differentiation of Friend virus-transformed murine erythroleukemia (MEL) cells can be inhibited by okadaic acid, an inhibitor of type 1 and type 2A protein phosphatases. The inhibition is shown to be correlated with prevention of dephosphorylation of retinoblastoma protein (pRB) in cells and bypass of G1 prolongation in the cell cycle. These results suggest that pRB-mediated G1 prolongation is necessary for MEL cells to commit to terminal differentiation. However, further experiments demonstrate that the simple cell cycle exit is not sufficient for commitment to terminal differentiation. Induction of dephosphorylation of pRB and subsequent G1 prolongation by forskolin does not lead MEL cells to differentiate. Additional pRB has been expressed in MEL cells by transfection with a neo-resistant plasmid containing RB cDNA under the control of a cytomegalovirus promoter. Exogenously expressed pRB is hyperphosphorylated in logarithmically growing MEL cells without any noticeable change in growth rate between the transfected cell line and the parental cell line. This result suggests that pRB in MEL cells is regulated by protein kinases and protein phosphatases and not by transcription.

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.

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.

Expression of Rb, E2F1, cdc2, and D, and B cyclins in developing spinal cord

The spatial and temporal distribution of transcripts for the tumor suppressor gene Rb, transcription factor E2F1, cdc2 kinase, cyclins D1, D2, B1 and B2 during neurogenesis of the spinal cord was determined by in situ hybridization. The Rb and E2F1 transcripts were detectable in proliferating and differentiating cells. By contrast, cdc2, cyclins D1, B1 and B2 are expressed in the ventricular zone where proliferating cells are localized. Cyclin D2 mRNA was detectable only in the marginal zone of the developing neural tube. Electrophoretic mobility shift analyses demonstrated a changing pattern of DNA/protein complexes that bind to E2F binding site. These observations suggest that Rb and E2F1 may be involved in the early stages of neuronal differentiation in addition to the cell cycle regulation.

The retinoblastoma susceptibility gene product represses transcription when directly bound to the promoter

Rb represses E2F-mediated transcription in part by blocking the trans-activation domain of E2F. In addition, Rb can convert an E2F binding site from a positive to a negative element. To examine the effect of a Rb-DNA-bound complex on transcription, full-length Rb was fused to the DNA binding domain of GAL4. Here, we report that GAL4-Rb can repress transcription mediated by either Sp1, AP-1, or p53, dependent upon the presence of both the GAL4 DNA binding domain and GAL4 binding sites. Moreover, GAL4-Rb inhibited the activity of the herpes simplex virus tk promoter from GAL4 binding sites located at a distance from the promoter. In contrast, GAL4-Rb was unable to repress basal transcription. Cotransfection of specific cyclins and cyclin-dependent kinases or SV40 T-antigen abolished the repressive activity of GAL4-Rb. The domains of Rb involved in mediating the repression of transcription were mapped to regions that are overlapping, but not identical, to those required for the interaction with E2F. We propose that Rb can function as a general repressor of transcription when bound to the promoter region.

The retinoblastoma-susceptibility gene product binds directly to the human TATA-binding protein-associated factor TAFII250

RB, the protein product of the retinoblastoma tumor-suppressor gene, regulates the activity of specific transcription factors. This regulation appears to be mediated either directly through interactions with specific transcription factors or through an alternative mechanism. Here we report that stimulation of Sp1-mediated transcription by RB is partially abrogated at the nonpermissive temperature in ts13 cells. These cells contain a temperature-sensitive mutation in the TATA-binding protein-associated factor TAFII250, first identified as the cell cycle regulatory protein CCG1. The stimulation of Sp1-mediated transcription by RB in ts13 cells at the nonpermissive temperature could be restored by the introduction of wild-type human TAFII250. Furthermore, we demonstrate that RB binds directly to hTAFII250 in vitro and in vivo. These results suggest that RB can confer transcriptional regulation and possibly cell cycle control and tumor suppression through an interaction with TFIID, in particular with TAFII250.

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.