Direct transcriptional repression by pRB and its reversal by specific cyclins

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

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

RBP1 recruits the mSIN3-histone deacetylase complex to the pocket of retinoblastoma tumor suppressor family proteins found in limited discrete regions of the nucleus at growth arrest

Retinoblastoma (RB) tumor suppressor family pocket proteins induce cell cycle arrest by repressing transcription of E2F-regulated genes through both histone deacetylase (HDAC)-dependent and -independent mechanisms. In this study we have identified a stable complex that accounts for the recruitment of both repression activities to the pocket. One component of this complex is RBP1, a known pocket-binding protein that exhibits both HDAC-dependent and -independent repression functions. RB family proteins were shown to associate via the pocket with previously identified mSIN3-SAP30-HDAC complexes containing exclusively class I HDACs. Such enzymes do not interact directly with RB family proteins but rather utilize RBP1 to target the pocket. This mechanism was shown to account for the majority of RB-associated HDAC activity. We also show that in quiescent normal human cells this entire RBP1-mSIN3-SAP30-HDAC complex colocalizes with both RB family members and E2F4 in a limited number of discrete regions of the nucleus that in other studies have been shown to represent the initial origins of DNA replication following growth stimulation. These results suggest that RB family members, at least in part, drive exit from the cell cycle by recruitment of this HDAC complex via RBP1 to repress transcription from E2F-dependent promoters and possibly to alter chromatin structure at DNA origins.

E2F-1 induces the stabilization of p53 but blocks p53-mediated transactivation

E2F-1 induces p53 accumulation and E2F-1 and p53 form a physical complex, which affects the ability of E2F-1 to activate transcription. We mapped the domains on E2F-1 that interact with p53 and found two p53-binding domains. To understand the functional consequences of the E2F-1/p53 association on p53 activities we identified the domains of E2F-1 that were responsible for the accumulation of p53. Unexpectedly, we found that the E2F-1 transactivation domain was dispensable for p53 induction. By contrast, further deletion of the DP-1 interaction/'marked' box domain eliminated p53 accumulation. Radiolabeling pulse/chase analysis demonstrated that E2F-1 caused post-translational stabilization of p53. Although E2F-1 caused the stabilization of p53, E2F-1 expression impaired p53-dependent transactivation. Thus, the E2F-1 : p53 interaction may provide a checkpoint function to inactivate overactive E2F-1, but the association may also inactivate p53 transactivation to allow cell cycle progression.

Active repression and E2F inhibition by pRB are biochemically distinguishable

To understand mechanistically how pRB represses transcription, we used a reconstituted transcription assay and compared pRB activity on naked versus chromatin templates. Surprisingly, when pRB was directly recruited to a naked template, no transcriptional repression was observed. However, we observed active repression when the same promoter was assembled into chromatin. Histone deacetylases do not appear to play a role in this observed repression. Further experiments showed repression could occur after preinitiation complex assembly, in contrast with pRB inhibition of E2F, suggesting discrete mechanisms by which pRB directly inhibits an activator such as E2F or actively represses proximally bound transcription factors.

Integration of the pRB and p53 cell cycle control pathways

Two fundamental molecular pathways, the pRB and p53 pathways, regulate cell growth and cell death. The importance of these pathways in cellular growth control is underscored by the observation that members of these pathways are found mutated in all human cancers. These two pathways have typically been studied and described independently. However, as we discuss here, recent data have revealed an intimate molecular and genetic interaction between the p53 and pRB pathways.

Opposite functions for E2F1 and E2F4 in human epidermal keratinocyte differentiation

Proteins of the retinoblastoma family (pRb, p107, and p130) modulate cell proliferation, a function related to their capacity to control the activity of the E2F transcription factor family. The Rb proteins also control cell differentiation in different tissues. We have recently described their involvement in human epidermal keratinocyte differentiation (Paramio, J. M., Lain, S., Segrelles, C., Lane, E. B. , and Jorcano, J. L. (1998) Oncogene 17, 949-957). Here we show that E2F proteins are also involved in this process. We found that E2F1 and E2F4 are expressed differentially during the in vitro differentiation of human epidermal keratinocytes, with the former uniformly present throughout the process, whereas the second is predominantly expressed at the onset of differentiation. This pattern is also observed in human skin by confocal microscopy. Electrophoretic mobility shift assays and immunoprecipitation experiments demonstrated that the complexes formed by E2F1 and E2F4 and Rb family proteins vary throughout in vitro keratinocyte differentiation. In agreement with this observation, several E2F-responsive genes are differentially regulated during this process. To test the functional implications of these observations, we transfected HaCaT keratinocytes with plasmids coding for E2F1 and E2F4. Transfected cells display opposite in vitro differentiation properties. Although E2F1-transfected cells are unable to differentiate, E2F4-transfected cells show an increased differentiation rate compared with Neo-transfected control cells. Our data demonstrate that the differential and coordinated expression and interaction of E2F and Rb proteins modulate the process of epidermal differentiation and provide clear evidence that members of the E2F family of transcription factors play specific and opposite roles during cell differentiation.

Chromatin remodeling and Rb activity

Progression of cells through the cell cycle is central to normal cell proliferation, and checkpoints that regulate this cycle are targets of tumorigenic mutations. One of these checkpoints is the Rb family of proteins that seems to regulate exit of cells from both G(1) and S phase of the cell cycle. Recent studies have linked the function of the Rb family to chromatin remodeling enzymes.

Cyclin D1 suppresses retinoblastoma protein-mediated inhibition of TAFII250 kinase activity

The retinoblastoma tumor suppressor protein has been shown to bind directly and inhibit a transcriptionally-important amino-terminal kinase domain of TATA-binding protein-associated factor TAFII250. Cyclin D1 also is able to associate with the amino terminus of TAFII250 in a region very similar to or overlapping the Rb-binding site. In this study, we have examined whether cyclin D1 affects the functional interaction between Rb and TAFII250. We observed that when cyclin D1 is coincubated with Rb and TAFII250, the ability of Rb to inhibit TAFII250 kinase activity is effectively blocked. However, cyclin D1 by itself has no apparent effect on TAFII250 kinase activity. We further found that the Rb-related protein p107 can inhibit TAFII250 kinase activity, and this inhibition is likewise alleviated by cyclin D1. Cyclin D1 prevents the kinase-inhibitory effect of an Rb mutant unable to bind to D-type cyclins, indicating that it is acting through its association with TAFII250 and not with Rb. However, we found no evidence of TAFII250-binding competition between Rb and cyclin D1 in vitro. The adenovirus E1A protein, which also binds to both Rb and TAFII250, exhibited a suppressive effect on Rb-mediated kinase inhibition similar to that seen with cyclin D1. Our results suggest a novel means by which cyclin D1 may be able to independently regulate the activity of Rb.

Cdk2-dependent phosphorylation and functional inactivation of the pRB-related p130 protein in pRB(-), p16INK4A(+) tumor cells

The retinoblastoma family proteins pRB, p107, and p130 are phosphorylated and released from E2Fs in the late G(1) phase of the cell cycle. This phosphorylation is thought to contribute to the derepression of E2F-responsive genes and to be mediated, in part, by Cdk4 and Cdk6. Evidence that Cdk4/6 activity is inhibited by p16(INK4A) in most pRB(-) cells suggests that p107 and p130 may be underphosphorylated and remain associated with E2Fs during G(1)-S progression in cells that lack pRB. To examine this, we evaluated the cell cycle-dependent phosphorylation and E2F binding abilities of p107 and p130 in pRB(-), p16(+) Saos-2 osteosarcoma cells. p130, but not p107, was phosphorylated and released from E2F-4 in late G(1) and S phase cells, although p130 phosphorylation differed qualitatively in these and other pRB(-), p16(+) cells as compared with pRB(+), p16(-) cell types. p130 phosphorylation occurred in the absence of cyclin D-Cdk4/6 complexes, coincided with cyclin E- and Cdk2-associated kinase activity, and was prevented by expression of dominant negative Cdk2. Moreover, dominant negative Cdk2 prevented the dissociation of endogenous p130-E2F-4 complexes and inhibited E2F-4-dependent transcription. These findings show that p130 can be phosphorylated and functionally inactivated in a Cdk2-dependent process, and they highlight the involvement of distinct Cdks in the regulation of different pRB family proteins.

Caspase-dependent apoptosis by ectopic expression of E2F-4

E2F is a family of transcription factors which regulates cell cycle and apoptosis of mammalian cells. E2F-1-3 localize in the nucleus, and preferentially bind pRb, while E2F-4 and 5 have no nuclear localization signal and preferentially bind p107/p130. E2F-6 suppresses the transcriptional activity of other E2F proteins. DP-1 and 2 are heterodimeric partners of each E2F protein. Using tetracycline-responsive promoters, here we compared the effects of ectopic expression of E2F-1, DP-1 and E2F-4 on cell cycle progression and apoptosis in Chinese hamster cell lines. We found that E2F-4, as well as DP-1 and E2F-1, induced growth arrest and caspase-dependent apoptosis. E2F-4 did not have a marked effect on cell cycle progression, while E2F-1 induced DNA synthesis of resting cells and DP-1 arrested cells in G1. Ectopic expression of E2F-4 did not activate E2F-dependent transcription. Our results suggest that expression of E2F-4 at elevated levels induces growth arrest and apoptosis of mammalian cells through a mechanism distinct from E2F-1 and DP-1.

Retinoblastoma: the disease, gene and protein provide critical leads to understand cancer

Retinoblastoma has contributed much to the understanding of cancer. The protein product of the RB gene, pRB, is a multifaceted regulator of transcription which controls the cell cycle, differentiation and apoptosis in normal development of specific tissues. Elucidating the mechanisms in which pRB plays a critical role will enable novel therapies and strategies for prevention, not only for retinoblastoma, but for cancer in general.

Histone acetylation and the control of the cell cycle

The critical steps of the cell cycle are generally controlled through the transcriptional regulation of specific subsets of genes. Transcriptional regulation has been recently linked to acetylation or deacetylation of core histone tails: acetylated histone tails are generally associated with active chromatin, whereas deacetylated histone tails are associated with silent parts of the genome. A number of transcriptional co-regulators are histone acetyl-transferases or histone deacetylases. Here, we discuss some of the critical cell cycle steps in which these enzymes are involved.

Restoration of retinoblastoma mediated signaling to Cdk2 results in cell cycle arrest

Phosphorylation/inactivation of RB is typically required for cell cycle progression. However, we have identified a tumor cell line, C33A, which progresses through the cell cycle in the presence of an active allele of RB (PSM-RB). To determine how C33A cells evade RB-mediated arrest, we compared RB signaling to downstream effectors in this resistant cell line to that of the RB-sensitive SAOS-2 cell line. Although introduction of PSM-RB repressed E2F-mediated transcription in both C33A and SAOS-2 cells, PSM-RB failed to repress Cyclin A promoter activity in C33A. Ectopic expression of PSM-RB in SAOS-2 cells resulted in a decrease in both Cyclin A and Cdk2 protein levels without affecting Cyclin E or Cdk4. In contrast, over-expression of PSM-RB in C33A cells did not alter endogenous Cyclin A, Cyclin E, or Cdk2 protein levels or impact Cdk2 kinase activity, indicating that signaling from RB to down-stream targets is abrogated in this cell line. The importance of Cdk2 activity was demonstrated by p27Kip1, which attenuated Cdk2 activity and inhibited cell cycle progression in C33A cells. Since RB signaling to Cdk2 is disrupted in these tumor cells, we co-expressed two proteins that cooperate with RB in transcriptional repression, AHR and BRG-1, in an attempt to correct this signaling dysfunction. Co-expression of AHR/BRG-1 with PSM-RB attenuated Cyclin A and Cdk2 expression as well as Cdk2-associated kinase activity, resulting in cell cycle inhibition of C33A cells. Importantly, ectopic expression of Cyclin A was able to reverse the arrest mediated by co-expression of AHR/BRG-1 with PSM-RB. These results indicate that down-regulation of Cdk2 activity is requisite for RB-mediated cell cycle arrest. Thus, this study reveals a new mechanism through which tumor cells evade anti-proliferative signals, and provides insight into how RB-signaling is mediated.

Rb function in cell-cycle regulation and apoptosis

Loss of cell-cycle control is a hallmark of neoplastic cells. One regulator of the critical G1 to S-phase transition in the cell cycle is the retinoblastoma tumour suppressor protein Rb, which interacts with the E2F family of cell-cycle transcription factors to repress gene transcription required for this transition. Through its interaction with E2F, Rb also regulates genes that control apoptosis. Here we review the roles of Rb in regulating the cell cycle and apoptosis and discuss recent results linking these Rb functions to chromatin-remodelling enzymes.

Cloning and characterization of a novel Kruppel-associated box family transcriptional repressor that interacts with the retinoblastoma gene product, RB

The retinoblastoma gene product, RB, seems to function as a key tumor suppressor by repressing the expression of genes activated by members of the E2F family of transcription factors. In order to accomplish this, RB has been proposed to interact with a transcriptional repressor. However, no genuine transcriptional repressors have been identified by virtue of interaction with RB. By using the yeast two-hybrid system, we have identified a novel member of a known family of transcriptional repressors that contain zinc fingers of the Kruppel type and a portable transcriptional repressor motif known as the Kruppel-associated box (KRAB). The mouse and human forms of the novel RB-associated KRAB protein (RBaK) are widely expressed. The amino acid motif that links the KRAB domain and zinc fingers appears to be required for interaction with RB in vitro. Human RBaK ectopically expressed in fibroblasts is an 80-kDa protein that is localized to the nucleus. The expression of either RB or RBaK in 10T1/2 fibroblasts represses the activation of an E2F-dependent promoter and decreases DNA synthesis to a similar degree. However, a mutant form of RBaK that cannot interact with RB in vitro is unable to prevent DNA synthesis. We present a model in which RB physically interacts with the novel transcriptional repressor RBaK to repress E2F-dependent genes and prevent DNA synthesis.

Induction of the cellular E2F-1 promoter by the adenovirus E4-6/7 protein

The adenovirus type 5 (Ad5) E4-6/7 protein interacts directly with different members of the E2F family and mediates the cooperative and stable binding of E2F to a unique pair of binding sites in the Ad5 E2a promoter region. This induction of E2F DNA binding activity strongly correlates with increased E2a transcription when analyzed using virus infection and transient expression assays. Here we show that while different adenovirus isolates express an E4-6/7 protein that is capable of induction of E2F dimerization and stable DNA binding to the Ad5 E2a promoter region, not all of these viruses carry the inverted E2F binding site targets in their E2a promoter regions. The Ad12 and Ad40 E2a promoter regions bind E2F via a single binding site. However, these promoters bind adenovirus-induced (dimerized) E2F very weakly. The Ad3 E2a promoter region binds E2F very poorly, even via a single binding site. A possible explanation of these results is that the Ad E4-6/7 protein evolved to induce cellular gene expression. Consistent with this notion, we show that infection with different adenovirus isolates induces the binding of E2F to an inverted configuration of binding sites present in the cellular E2F-1 promoter. Transient expression of the E4-6/7 protein alone in uninfected cells is sufficient to induce transactivation of the E2F-1 promoter linked to chloramphenicol acetyltransferase or green fluorescent protein reporter genes. Further, expression of the E4-6/7 protein in the context of adenovirus infection induces E2F-1 protein accumulation. Thus, the induction of E2F binding to the E2F-1 promoter by the E4-6/7 protein observed in vitro correlates with transactivation of E2F-1 promoter activity in vivo. These results suggest that adenovirus has evolved two distinct mechanisms to induce the expression of the E2F-1 gene. The E1A proteins displace repressors of E2F activity (the Rb family members) and thus relieve E2F-1 promoter repression; the E4-6/7 protein complements this function by stably recruiting active E2F to the E2F-1 promoter to transactivate expression.

The paradox of E2F1: oncogene and tumor suppressor gene

Cancer cells often contain mutations that lead to the loss of retinoblastoma tumor suppressor (Rb) function and the activation of E2F-dependent transcription. As a result, proliferation is deregulated, and sensitivity to apoptotic stimuli is increased. In cell culture studies, the transcription factor E2F1 has been shown to be equally adept at inducing proliferation and apoptosis. Several groups using mouse models have been examining how these E2F1-regulated processes impact the development of cancer. The conclusion from these studies is that E2F1 can function as both oncogene and tumor suppressor gene and that both positive and negative effects on tumorigenesis can be observed whether E2F1 is absent or overexpressed. These findings are discussed in the context of a model in which pathways controlling cell-cycle progression and apoptosis are intimately linked.

Glutamic acid mutagenesis of retinoblastoma protein phosphorylation sites has diverse effects on function

The retinoblastoma tumor suppressor gene (Rb) has many functions within the cell including regulation of transcription, differentiation, apoptosis, and the cell cycle. Regulation of these functions is mediated by phosphorylation at as many as 16 cyclin-dependent kinase (CDK) phosphorylation sites in vivo. The contribution of these sites to the regulation of the various Rb functions is not well understood. To characterize the effect of phosphorylation at these sites, we systematically mutagenized the serines or threonines to glutamic acid. Thirty-five mutants with different combinations of modified phosphorylation sites were assayed for their ability to arrest the cell cycle and for their potential to induce differentiation. Only the most highly substituted mutants failed to arrest cell cycle progression. However, mutants with as few as four modified phosphorylation sites were unable to promote differentiation. Other mutants had increased activity in this assay. We conclude that modification of Rb phosphorylation sites can increase or decrease protein activity, that different Rb functions can be regulated independently by distinct combinations of sites, and that the effects of modification at any one site are context dependent.

Rb and prohibitin target distinct regions of E2F1 for repression and respond to different upstream signals

E2F transcription factor is subject to stringent regulation by a variety of molecules. We recently observed that prohibitin, a potential tumor suppressor protein, binds to the retinoblastoma (Rb) protein and represses E2F transcriptional activity. Here we demonstrate that prohibitin requires the marked box region of E2F for repression; further, prohibitin can effectively inhibit colony formation induced by overexpression of E2F1 in T47D cells. Prohibitin was also found to interact with the signaling kinase c-Raf-1, and Raf-1 could effectively reverse prohibitin-mediated repression of E2F activity. Agents such as E1A, p38 kinase, and cyclins D and E had no effect on prohibitin-mediated repression of E2F1, but all of these molecules could reverse Rb function. Similarly, stimulation of the immunoglobulin M signaling pathway in Ramos cells could inactivate prohibitin, but this had no effect on Rb function. Serum stimulation of quiescent Ramos cells inactivated Rb and prohibitin with different kinetics; further, while the serum-dependent inactivation of Rb was dependent on cyclin-dependent kinase activity, the inactivation of prohibitin was not. We believe that prohibitin is a novel regulator of E2F function which channels specific signaling cascades to the cell cycle regulatory machinery.

Repression of Epstein-Barr virus EBNA-1 gene transcription by pRb during restricted latency

During the restricted programs of Epstein-Barr virus (EBV) latency in EBV-associated tumors and a subpopulation of latently infected B cells in healthy EBV carriers, transcription of the EBV nuclear antigen 1 (EBNA-1) gene is mediated by the promoter Qp. Previously, two noncanonical E2F binding sites were identified within Qp. The role of E2F in the regulation of Qp, however, has been controversial and is undefined. Here we demonstrate that an E2F factor(s) within Burkitt lymphoma (BL) cells binds to a G/C-rich element [GGCG(C/G)] within the previously identified binding sites in Qp and prototypical E2F response elements. Furthermore, Qp-driven reporter gene expression could be efficiently repressed through either E2F binding site by the tumor suppressor pRb, a potent transcriptional repressor targeted to promoters during G(0) and the early G(1) phase of the cell cycle via its interaction with E2F; a mutant pRb (pRb(706)) lacking E2F binding capability was unable to repress Qp. However, we did not observe cell cycle variation in the expression of either EBNA-1 mRNA or protein in exponentially growing BL cells, consistent with previous predictions that Qp is constitutively active in these cells and with the extremely long t(1/2) of EBNA-1. By contrast, within G(0)/G(1) in growth-arrested BL cells, EBNA-1 mRNA levels were twofold lower than in S phase, similar to the two- to eightfold differences in cell cycle expression of some cyclin mRNAs. Thus, although regulation of Qp is coupled to the cell cycle, this clearly has no impact on the level of EBNA-1 expressed in proliferating cells. We conclude, therefore, that the most important contribution of E2F to the regulation of Qp is to direct the pRb-mediated suppression of EBNA-1 expression within resting B cells, the principal reservoir of latent EBV. This would provide a means to restrict unneeded and potentially deleterious expression of EBNA-1 in a nonproliferating cell and to coordinate the activation of EBNA-1 expression necessary for EBV genome replication and maintenance upon reentry of the cell cycle in response to proliferative signals.

RBP1 recruits both histone deacetylase-dependent and -independent repression activities to retinoblastoma family proteins

Retinoblastoma (RB) tumor suppressor family proteins block cell proliferation in part by repressing certain E2F-specific promoters. Both histone deacetylase (HDAC)-dependent and -independent repression activities are associated with the RB "pocket." The mechanism by which these two repression functions occupy the pocket is unknown. A known RB-binding protein, RBP1, was previously found by our group to be an active corepressor which, if overexpressed, represses E2F-mediated transcription via its association with the pocket. We show here that RBP1 contains two repression domains, one of which binds all three known HDACs and represses them in an HDAC-dependent manner while the other domain functions independently of the HDACs. Thus, RB family members repress transcription by recruiting RBP1 to the pocket. RBP1, in turn, serves as a bridging molecule to recruit HDACs and, in addition, provides a second HDAC-independent repression function.

Control of E2F activity by p21Waf1/Cip1

Cyclin-dependent kinase inhibitors (cdkis), such as p21, are believed to control proliferation through an ability to function as stoichiometric antagonists of cyclin-dependent kinases (cdks). The p21 gene is a direct transcriptional target for the p53 protein, and its activation is likely to be important in effecting the p53 response. It is widely accepted that p21 can influence cell cycle progression by controlling the activity of cdks that act on the retinoblastoma tumour suppressor protein (pRb) which, in a hypophosphorylated state, associates with E2F transcription factors to prevent the activation of genes required for progression into S phase. Phosphorylation of pRb by G1 cdk complexes releases E2F and thereby enables progress through the cell cycle. Here, we describe results which suggest a p21-dependent mechanism that facilitates the regulation of E2F through a pathway that is independent of the cdk control of pRb activity. As p21 can associate with E2F subunits, it is possible that these effects are exerted through a complex with E2F. Furthermore, we find that p21 can regulate transcription in vitro. The results suggest that p21 may control E2F activity through a pathway that acts independently of pRb.

Nuclear localization conferred by the pocket domain of the retinoblastoma gene product

The tumor suppressor Rb is a nuclear phosphoprotein that controls cell growth and differentiation by modulating the activity of certain transcription factors. Transport of Rb to the nucleus is affected by both a bipartite nuclear localization signal (NLS) in the C-terminus of the protein and a central domain, termed A/B or pocket, through which Rb interacts with transcription factors and viral oncoproteins. Mutations in either the A or B subdomains of the pocket render a NLS-deficient Rb completely cytoplasmic. Fusing the A/B domain of Rb to the Escherichia coli beta-galactosidase, to create betagal-A/B, confers nuclear localization upon this bacterial protein. Moreover, co-expression with the adenovirus oncoprotein, E1A, further augments nuclear localization of betagal-A/B. These findings provide direct evidence that the pocket domain of Rb is not only required but also sufficient to induce nuclear transport by a 'piggyback' mechanism. Thus, nuclear localization of Rb is dictated by two independent and autonomous domains: (i) the bipartite NLS and (ii) the pocket domain. We suggest that via these domains, Rb chaperons and co-compartmentalizes with its associated factors and preempts their activity prior to nuclear transport.

Regulation of E2F: a family of transcription factors involved in proliferation control

Members of the E2F family of transcription factors are key participants in orchestration of the cell cycle, cell growth arrest and apoptosis. Therefore, an understanding of the regulation of E2F activity is essential for an understanding of the control of cellular proliferation. E2F activity is regulated by the retinoblastoma family of tumor suppressors and by multiple other mechanisms. This review will describe our current knowledge of these mechanisms which together constitute a highly complex network by which the cell cycle and cellular proliferation can be controlled.

pRB is required for interferon-gamma-induction of the MHC class II abeta gene

pRB is required for IFN-gamma-induction of MHC class II in human tumor cell lines, providing a potential link between tumor suppressors and the immune system. However, other genes, such as cyclin D1, show pRB-dependency only in tumor cells, so by analogy, pRB may not be necessary for cII-regulation in normal cells. Here, we demonstrate that induction of the mouse MHC class II I-A heterodimer is normal in RB+/+ mouse embryonic fibroblasts (MEFs), but deficient in RB-/- MEFs. Inducibility is restored in RB-/- MEFs stably transfected with wild type RB cDNA or infected with an adenovirus expressing pRB. Thus, involvement of pRB in MHC class II expression is conserved in the mouse and is not an aberrant feature of tumorigenic, aneuploid, human tumor cells. Although cII genes are generally induced in a coordinate fashion, suggesting a common mechanism, we found that pRB was specifically required for induction of the Abeta, but not Aalpha or other MHC cII genes including Ebeta, Ii and H2-Malpha. Finally, IFN-gamma-induction of class II transactivator (CIITA), was pRB-independent, suggesting that pRB works downstream of this master-regulator of MHC class II expression.

A mechanism for Rb/p130-mediated transcription repression involving recruitment of the CtBP corepressor

Previous work has demonstrated the critical role for transcription repression in quiescent cells through the action of E2F-Rb or E2F-p130 complexes. Recent studies have shown that at least one mechanism for this repression involves the recruitment of histone deacetylase. Nevertheless, these studies also suggest that other events likely contribute to E2F/Rb-mediated repression. Using a yeast two-hybrid screen to identify proteins that specifically interact with the Rb-related p130 protein, we demonstrate that p130, as well as Rb, interacts with a protein known as CtIP. This interaction depends on the p130 pocket domain, which is important for repression activity, as well as an LXCXE sequence within CtIP, a motif previously shown to mediate interactions of viral proteins with Rb. CtIP interacts with CtBP, a protein named for its ability to interact with the C-terminal sequences of adenovirus E1A. Recent work has demonstrated that the Drosophila homologue of CtBP is a transcriptional corepressor for Hairy, Knirps, and Snail. We now show that both CtIP and CtBP can efficiently repress transcription when recruited to a promoter by the Gal4 DNA binding domain, thereby identifying them as corepressor proteins. Moreover, the full repression activity of CtIP requires a PLDLS domain that is also necessary for the interaction with CtBP. We propose that E2F-mediated repression involves at least two events, either the recruitment of a histone deacetylase or the recruitment of the CtIP/CtBP corepressor complex.

DP1 phosphorylation in multimeric complexes: weaker interaction with cyclin A through the E2F1 cyclin A binding domain leads to more efficient phosphorylation than stronger interaction through the p107 cyclin A binding domain

Stable enzyme-substrate interaction has been recognized as a major mechanism underlying the substrate preferences of cyclin-dependent kinases (Cdks). To learn the relationship between stability of physical association and efficiency of phosphorylation, we studied DP1 phosphorylation by cyclin A-Cdk2 in multiprotein complexes. When DP1 was connected to cyclin A-Cdk2 through E2F4 and p107, its phosphorylation was very inefficient, although its association with cyclin A-Cdk2 was stable. In contrast, DP1 was efficiently phosphorylated when weakly connected to cyclin A-Cdk2 via E2F1 or E2F4 with a fused cyclin A binding domain of E2F1. The transactivation activity of E2F4-DP1 heterodimers was reduced when DP1 was phosphorylated, while a phosphorylation deficient mutant of DP1 resisted this down-regulation. Phosphorylation and functional regulation of DP1 were not due to nuclear localization. Thus, stronger physical association between the kinase and the substrate does not necessarily lead to more efficient phosphorylation than weaker interaction does.

Transcriptional control by cell-cycle regulators: a review

In eukaryotes, progression of the cell cycle is associated with periodic transcription activation/repression of growth-regulatory genes. We summarize here current knowledge and views on the role of critical cell-cycle regulators such as the retinoblastoma pocket family members and cyclin-dependent kinases in the regulation of gene transcription. In particular, we discuss here the role of specific cyclin-dependent kinase complexes in the regulation of basal transcription. Although the functional connections between transcription and cell-cycle regulators is far from being understood, recent progress has been made in connecting cell-cycle progression to dedicated components of the RNA polymerase II transcription apparatus complex.

Cumulative effect of phosphorylation of pRB on regulation of E2F activity

The product of the retinoblastoma susceptibility gene, pRB, is a nuclear phosphoprotein that controls cell growth by binding to and suppressing the activities of transcription factors such as the E2F family. Transactivation activity is inhibited when E2F is bound to hypophosphorylated pRB and released when pRB is phosphorylated by cyclin-dependent kinases (CDKs). To determine which of 16 potential CDK phosphorylation sites regulated the pRB-E2F interaction, mutant pRB proteins produced by site-directed mutagenesis were tested for the ability to suppress E2F-mediated transcription in a reporter chloramphenicol acetyltransferase assay. Surprisingly, no one CDK site regulated the interaction of pRB with E2F when E2F was bound to DNA. Instead, disruption of transcriptional repression resulted from accumulation of phosphate groups on the RB molecule.

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

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

Retinoblastoma protein meets chromatin

The retinoblastoma (RB) protein exerts its tumour-suppressor function by repressing the transcription of cellular genes required for DNA replication and cell division. Recent investigations into the mechanism of RB repression have revealed that RB can regulate transcription by effecting changes in chromatin structure. These findings point towards a link between chromatin regulation and cancer.

Balancing proliferation and apoptosis in vivo: the Goldilocks theory of E2F/DP action

Stimulation of both proliferation and apoptosis by E2F-1 provides a mechanistic basis for how E2F-1 functions as an oncogene and a tumor suppressor in vivo. In each normal tissue, a precise balance of proliferation versus apoptosis must be maintained, and in many tissues this appears to be controlled by E2F-1 levels. Presumably, variable expression of all E2F family members in each tissue dictates a tissue-specific sensitivity to loss or overexpression of any one family member. At sites where E2F-1 contributes mainly to proliferation and p53 levels remain low, loss of E2F-1 expression may lead to tissue atrophy and overexpression may lead to hyperplasia or tumors. Hence, E2F-1 would act as an oncogene. At other sites where E2F-1 levels induce p19, which stabilizes p53 leading to apoptosis, E2F-1 overexpression may lead to tissue atrophy and loss of expression may lead to hyperplasia or tumors. And thus, E2F-1 would act as a tumor suppressor. Perhaps it is the unique property of E2F-1 within the E2F family to stimulate both proliferation and apoptosis which makes it a bimodal switch that pRB must control so carefully. It is a delicate equilibrium that must be maintained throughout embryonic development and adult life. However, it is easy to envision that mutations which deregulate other E2F family members and which ultimately lead to changes in E2F-1 levels could lead to similar growth aberrations. In summary, although pRB interacts with numerous transcription factors, pRB minimally must restrain the E2F/DP transcription factor family to prevent the cell cycle from whirling onwards out of control.

Structural basis of DNA recognition by the heterodimeric cell cycle transcription factor E2F-DP

The E2F and DP protein families form heterodimeric transcription factors that play a central role in the expression of cell cycle-regulated genes. The crystal structure of an E2F4-DP2-DNA complex shows that the DNA-binding domains of the E2F and DP proteins both have a fold related to the winged-helix DNA-binding motif. Recognition of the central c/gGCGCg/c sequence of the consensus DNA-binding site is symmetric, and amino acids that contact these bases are conserved among all known E2F and DP proteins. The asymmetry in the extended binding site TTTc/gGCGCc/g is associated with an amino-terminal extension of E2F4, in which an arginine binds in the minor groove near the TTT stretch. This arginine is invariant among E2Fs but not present in DPs. E2F4 and DP2 interact through an extensive protein-protein interface, and structural features of this interface suggest it contributes to the preference for heterodimers over homodimers in DNA binding.

Inhibition of cyclin-dependent kinase 2 by p21 is necessary for retinoblastoma protein-mediated G1 arrest after gamma-irradiation

In mammalian cells, activation of certain checkpoint pathways as a result of exposure to genotoxic agents results in cell cycle arrest. The integrity of these arrest pathways is critical to the ability of the cell to repair mutations that otherwise might compromise viability or contribute to deregulation of cellular growth and proliferation. Here we examine the mechanism through which DNA damaging agents result in a G1 arrest that depends on the tumor suppressor p53 and its transcriptional target p21. By using primary cell lines lacking specific cell cycle regulators, we demonstrate that this pathway functions through the growth suppressive properties of the retinoblastoma protein (pRB) tumor suppressor. Specifically, gamma-irradiation inhibits the phosphorylation of pRB at cyclin-dependent kinase 2-specific, but not cyclin-dependent kinase 4-specific, sites in a p21-dependent manner. Most importantly, we show that pRB is a critical component of this DNA damage checkpoint. These data indicate that the p53 --> p21 checkpoint pathway uses the normal cell cycle regulatory machinery to induce the accumulation of the growth suppressive form of pRB and suggest that loss of pRB during the course of tumorigenesis disrupts the function of an important DNA damage checkpoint.

Mechanism of transcriptional repression of E2F by the retinoblastoma tumor suppressor protein

The retinoblastoma tumor suppressor protein (pRB) is a transcriptional repressor, critical for normal cell cycle progression. We have undertaken studies using a highly purified reconstituted in vitro transcription system to demonstrate how pRB can repress transcriptional activation mediated by the E2F transcription factor. Remarkably, E2F activation became resistant to pRB-mediated repression after the establishment of a partial (TFIIA/TFIID) preinitiation complex (PIC). DNase I footprinting studies suggest that E2F recruits TFIID to the promoter in a step that also requires TFIIA and confirm that recruitment of the PIC by E2F is blocked by pRB. These studies suggest a detailed mechanism by which E2F activates and pRB represses transcription without the requirement of histone-modifying enzymes.

Integration of a growth-suppressing BTB/POZ domain protein with the DP component of the E2F transcription factor

Transcription factor E2F plays an important role in orchestrating early cell cycle progression through its ability to co-ordinate and integrate the cell cycle with the transcription apparatus. Physiological E2F arises when members of two distinct families of proteins interact as E2F-DP heterodimers, in which the E2F component mediates transcriptional activation and the physical interaction with pocket proteins, such as the tumour suppressor protein pRb. In contrast, a discrete role for the DP subunit has not been defined. We report the identification and characterization of DIP, a novel mammalian protein that can interact with the DP component of E2F. DIP was found to contain a BTB/POZ domain and shows significant identity with the Drosophila melanogaster germ cell-less gene product. In mammalian cells, DIP is distributed in a speckled pattern at the nuclear envelope region, and can direct certain DP subunits and the associated heterodimeric E2F partner into a similar pattern. DIP-dependent growth arrest is modulated by the expression of DP proteins, and mutant derivatives of DIP that are compromised in cell cycle arrest exhibit reduced binding to the DP subunit. Our study defines a new pathway of growth control that is integrated with the E2F pathway through the DP subunit of the heterodimer.

Rb inhibits the intrinsic kinase activity of TATA-binding protein-associated factor TAFII250

The retinoblastoma tumor suppressor protein, Rb, interacts directly with the largest TATA-binding protein-associated factor, TAFII250, through multiple regions in each protein. To define the potential role(s) of this interaction, we examined whether Rb could regulate the intrinsic, bipartite kinase activity of TAFII250. Here, we report that Rb is able to inhibit the kinase activity of immunopurified and gel-purified recombinant TAFII250. Rb inhibits the autophosphorylation of TAFII250 as well as its phosphorylation of the RAP74 subunit of TFIIF in a dose-responsive manner. Inhibition of TAFII250 kinase activity involves the Rb pocket (amino acids 379 to 928) but not its amino terminus. In addition, Rb appears to specifically inhibit the amino-terminal kinase domain of TAFII250 through a direct protein-protein interaction. We further demonstrate that two different tumor-derived Rb pocket mutants, C706F and Deltaex22, are functionally defective for kinase inhibition, even though they are able to bind the amino terminus of TAFII250. Our results suggest a novel mechanism of transcriptional regulation by Rb, involving direct interaction with TAFII250 and inhibition of its ability to phosphorylate itself, RAP74, and possibly other targets.

Endogenous E2F-1 promotes timely G0 exit of resting mouse embryo fibroblasts

Much evidence strongly suggests a positive role for one or more E2F species in the control of exit from G0/G1. Results described here provide direct evidence that endogenous E2F-1, as predicted, contributes to progression from G0 to S. By contrast, cycling cells lacking an intact E2F-1 gene demonstrated normal cell cycle distribution. Therefore, E2F-1 exerts a unique function leading to timely G0 exit of resting cultured primary cells, while at the same time being unnecessary for normal G1 to S phase progression of cycling cells.

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

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

Cell cycle-regulated expression of mammalian CDC6 is dependent on E2F

The E2F transcription factors are essential regulators of cell growth in multicellular organisms, controlling the expression of a number of genes whose products are involved in DNA replication and cell proliferation. In Saccharomyces cerevisiae, the MBF and SBF transcription complexes have functions similar to those of E2F proteins in higher eukaryotes, by regulating the timed expression of genes implicated in cell cycle progression and DNA synthesis. The CDC6 gene is a target for MBF and SBF-regulated transcription. S. cerevisiae Cdc6p induces the formation of the prereplication complex and is essential for initiation of DNA replication. Interestingly, the Cdc6p homolog in Schizosaccharomyces pombe, Cdc18p, is regulated by DSC1, the S. pombe homolog of MBF. By cloning the promoter for the human homolog of Cdc6p and Cdc18p, we demonstrate here that the cell cycle-regulated transcription of this gene is dependent on E2F. In vivo footprinting data demonstrate that the identified E2F sites are occupied in resting cells and in exponentially growing cells, suggesting that E2F is responsible for downregulating the promoter in early phases of the cell cycle and the subsequent upregulation when cells enter S phase. Our data also demonstrate that the human CDC6 protein (hCDC6) is essential and limiting for DNA synthesis, since microinjection of an anti-CDC6 rabbit antiserum blocks DNA synthesis and CDC6 cooperates with cyclin E to induce entry into S phase in cotransfection experiments. Furthermore, E2F is sufficient to induce expression of the endogenous CDC6 gene even in the absence of de novo protein synthesis. In conclusion, our results provide a direct link between regulated progression through G1 controlled by the pRB pathway and the expression of proteins essential for the initiation of DNA replication.

A cellular repressor of E1A-stimulated genes that inhibits activation by E2F

The adenovirus E1A protein both activates and represses gene expression to promote cellular proliferation and inhibit differentiation. Here we report the identification and characterization of a cellular protein that antagonizes transcriptional activation and cellular transformation by E1A. This protein, termed CREG for cellular repressor of E1A-stimulated genes, shares limited sequence similarity with E1A and binds both the general transcription factor TBP and the tumor suppressor pRb in vitro. In transfection assays, CREG represses transcription and antagonizes 12SE1A-mediated activation of both the adenovirus E2 and cellular hsp70 promoters. CREG also antagonizes E1A-mediated transformation, as expression of CREG reduces the efficiency with which E1A and the oncogene ras cooperate to transform primary cells. Binding sites for E2F, a key transcriptional regulator of cell cycle progression, were found to be required for repression of the adenovirus E2 promoter by CREG, and CREG was shown to inhibit activation by E2F. Since both the adenovirus E1A protein and transcriptional activation by E2F function to promote cellular proliferation, the results presented here suggest that CREG activity may contribute to the transcriptional control of cell growth and differentiation.

Unusual proliferation arrest and transcriptional control properties of a newly discovered E2F family member, E2F-6

E2F transcription factors play an important role in the regulation of cell cycle progression. We report here the cloning and characterization of an additional member of this family, E2F-6. E2F-6 lacks pocket protein binding and transactivation domains, and it is a potent transcriptional repressor that contains a modular repression domain at its carboxyl terminus. Overproduction of E2F-6 had no specific effect on cell cycle progression in asynchronously growing Saos2 and NIH 3T3 cells, but it inhibited entry into S phase of NIH 3T3 cells stimulated to exit G0. Taken together, these data suggest that E2F-6 can regulate a subset of E2F-dependent genes whose products are required for entry into the cell cycle but not for normal cell cycle progression.

Retinoblastoma protein tethered to promoter DNA represses TBP-mediated transcription

The retinoblastoma (RB) tumour suppressor protein negatively regulates cell proliferation by modulating transcription of growth-regulatory genes. Recruitment of Rb to promoters, by association with E2F complex or by fusion with heterologous DNA-binding domains, demonstrated that Rb represses directly transcription. Recent studies also suggest that the RB protein is able to repress gene transcription mediated by the RNA polymerase I and III. Since the TATA-binding protein (TBP) is an important component for transcription mediated by all three RNA polymerases, we have analysed the functional interaction between Rb and TBP in vivo in the context of RNA pol II-driven transcription. We demonstrated that in mammalian cells Rb tethered to promoter represses TBP-mediated activation in vivo, and Rb-mediated repression is reversed in the presence of the inhibition of histone deacetylase activity by trichostatin A (TSA).

Increased E2F1 activity induces skin tumors in mice heterozygous and nullizygous for p53

The p16(INK4a)-cyclin D-retinoblastoma tumor suppressor pathway is disrupted in most human cancers, and it has been suggested that the subsequent release of E2F transcription factors from inhibitory complexes may be a key event in tumor development. We described recently the generation of transgenic mice with E2F1 gene expression targeted to squamous epithelial tissues by a keratin 5 (K5) promoter. In the present study, K5 E2F1 transgenic mice were crossed with p53 null mice to examine functional interactions between E2F1 and p53 in vivo. We find that E2F1-induced apoptosis of epidermal keratinocytes is reduced in K5 E2F1 transgenic mice lacking p53, whereas E2F1-induced hyperproliferation is unaffected by p53 status. We also find that K5 E2F1 transgenic mice heterozygous or nullizygous for p53 develop spontaneous skin carcinomas, which normally are rare in p53-deficient mice. The timing of tumor development correlates with the level of E2F1 transgene expression and the status of p53. In primary transgenic keratinocytes, the major change in E2F1 DNA-binding activity is the generation of a complex also containing the retinoblastoma tumor suppressor protein. Nevertheless, the expression and associated kinase activity of cyclin E, a known target for E2F transcriptional activity, is elevated significantly in K5 E2F1 transgenic keratinocytes. These findings firmly establish that increased E2F1 expression can contribute to tumor development and suggest that p53 plays an important role in eliminating cells with deregulated E2F1 activity.

Rapid, high level protein production using DNA-based Semliki Forest virus vectors

Semliki Forest virus (SFV) vectors can be produced faster, and have a wider host range, than baculovirus vectors. However, the original SFV system requires in vitro manipulation of RNA. We have generated a system that is wholly DNA-based. Both the replicon vector, encoding SFV polymerase and the protein of interest, and the helper vector, encoding viral structural proteins, were modified so that expression was RNA polymerase II-dependent. Transfection of the modified replicon plasmid alone generated 20-30-fold more protein than obtained from a simple expression vector. Expression required the SFV replicase, which amplifies replicon RNA. The SFV-based vector generated 10-20-fold more protein than a plasmid based on Sindbis virus. Cotransfection of SFV replicon and helper vectors generated viral titers of around 10(6) infectious particles/ml. A single electroporation, plated on one 10-cm plate, generated enough virus (10(7) particles) to produce >500 microg of protein. Wild type, replication proficient virus was not detected in three tests utilizing almost 10(8) viral particles, a distinct advantage over a DNA Sindbis-based system in which over half the virus particles generated are fully infectious. The new SFV vectors significantly enhance the utility of this expression system.