The retinoblastoma protein copurifies with E2F-I, an E1A-regulated inhibitor of the transcription factor E2F

Stage-specific expression and phosphorylation of retinoblastoma protein (pRb) in the rat seminiferous epithelium

To assess the potential role of retinoblastoma protein (pRb) in the regulation of cell cycle during spermatogenesis, the expression of retinoblastoma (Rb) mRNA and protein, as well as the phosphorylation states of pRb, in the rat seminiferous epithelial cycle, were studied. Two transcripts, 5.4 kb and 3.4 kb long, were detected in total RNA from the adult rat testis and only the 5.4 kb transcript was detected in poly (A)+-RNA from 8, 14 and 23-day old rat testes by Northern hybridization. Polysome analysis revealed that only a small portion of both Rb transcripts could be efficiently translated. By in situ hybridization, Rb mRNA was localized to germ cells from stage V pachytene spermatocytes to step 13 spermatids along the epithelial cycle. pRb immunoreactivity was detected in Sertoli cells and spermatogonia at all stages, as well as in the elongated steps 14-19 spermatids by immunohistochemistry. The amount of pRb and the phosphorylation status varied in a stage-specific manner in Western blots. These results show that pRb is expressed in the rat seminiferous epithelium in a cyclic fashion and suggest that it is involved in the regulation of proliferation of spermatogonia and maintenance of the differentiation status of Sertoli cells and spermatids.

The retinoblastoma protein: a master regulator of cell cycle, differentiation and apoptosis

The retinoblastoma susceptibility gene is a tumour suppressor and its product retinoblastoma protein (pRb) has been known for 10 years as a repressor of progression towards S phase. Its major activity was supposed to be sequestration or inactivation of the transcription factor E2F which is required for activation of S phase genes. However, within recent years growing evidence has been accumulating for a more general function of pRb at both the transcriptional level and the cellular level. pRb not only regulates the activity of certain protein-encoding genes but also the activity of RNA polymerase pol I and pol III transcription. This protein appears to be the major player in a regulatory circuit in the late G1 phase, the so-called restriction point. Moreover, it is involved in regulating an elusive switch point between cell cycle, differentiation and apoptosis. Here, it seems to cooperate with another major tumour suppressor, p53. Thus, pRb sits at the interface of the most important cell-regulatory processes and therefore deserves close attention by specialists from different fields of research. This review provides an introduction to the complex functions of pRb.

Rapamycin (sirolimus) inhibits proliferating cell nuclear antigen expression and blocks cell cycle in the G1 phase in human keratinocyte stem cells

Because the immunosuppressant rapamycin (sirolimus) blocks T cell proliferation in G1 phase, it has been proposed as a potential treatment for psoriasis, a skin disease characterized by T cell activation and keratinocyte stem cell hyperproliferation. To determine another potentially important mechanism through which rapamycin can act as an antipsoriatic agent, we tested its direct effect on keratinocyte stem cell proliferation in vitro as well as in vivo. In vivo cell cycle quiescent (G0 phase) stem cell keratinocytes in primary culture sequentially express de novo cyclin D1 and proliferating cell nuclear antigen (PCNA), prior to S phase entry, and upregulate beta1 integrin. Rapamycin inhibited the growth of keratinocytes that were leaving quiescence as well as those already in cell cycle without affecting cell viability. Although beta1 integrin(bright) expression was not affected, the number of beta1 integrin(bright) cells entering S/G2/M was significantly lowered by rapamycin. Cells treated with rapamycin exhibited decreased PCNA expression while cyclin D1 expression, which precedes PCNA expression in the cell cycle, was not affected. We found similar effects on stem cell keratinocytes in patients with psoriasis treated systemically with rapamycin. Because PCNA is required for cell cycle progression from G1 to S phase, our data indicate that inhibition of PCNA protein synthesis may be an important regulatory element in the ability of rapamycin to exert a G1 block.

Early phosphorylation of the retinoblastoma gene product regulates protein binding to the c-fos retinoblastoma control element during T cell activation

Function of the retinoblastoma tumor suppressor protein [pRb] is regulated by phosphorylation during the G1 and S phases of the cell cycle. pRb regulates transcription of several genes, including c-fos. However, since c-fos is regulated during exit from G0, it has remained unclear how pRb participates in c-fos regulation. We have identified a protein complex, the retinoblastoma control factor A [RCF-A] which binds to the c-fos retinoblastoma control element [RCE] and is regulated by pRb within 10 min after T cell activation. We demonstrate that pRb control of RCF-A is dependent upon the state of phosphorylation of pRb. pRb becomes hyperphosphorylated on specific peptides at 10 min after mitogenic stimulation and pRb is dephosphorylated by 30 min. This time course coincides with RCF-A DNA binding. RCF-A binds RCE DNA longer when cells are treated with okadaic acid, and okadaic acid prevents pRb dephosphorylation. Dephosphorylated pRb inhibits RCF-A binding in vitro but phosphorylated pRb does not. Thus, in addition to the described G1/S regulation of pRb, transient inactivation by phosphorylation of pRb in T cells may also be important as resting cells leave G0.

Regulation and expression of retinoblastoma proteins p107 and p130 during 3T3-L1 adipocyte differentiation

During 3T3-L1 adipocyte differentiation, growth-arrested, postconfluent preadipocytes are required to reenter the cell cycle and proceed through a mitotic clonal expansion phase prior to terminal differentiation. The retinoblastoma proteins (pRB, p107, and p130) are thought to be critical in controlling cell cycle progression by binding to and regulating the activity of the E2F transcription factors. We show here that p130/p107 protein levels, p107 mRNA levels, and E2F DNA binding complexes are regulated during 3T3-L1 adipogenesis. The predominant E2F binding complex in day 0 preadipocytes was p130-E2F with no detectable free E2F or p107. On Day 1, during mitotic clonal expansion, there was a distinct switch to free E2F and p107-E2F complexes associated with increased p107 mRNA and protein along with decreased p130 protein levels. Following differentiation, the day 0 pattern is reestablished. The switch is not just a consequence of reentry into the cell cycle, in that p107 protein levels are both detectable and unchanged in dividing, serum-restricted, or serum restimulated preconfluent cells. Interestingly, hormonal stimulation of 3T3-C2 cells, a related nondifferentiating cell line, also induces a mitotic clonal expansion phase that is associated with the p130:p107 switch in a pattern very similar to 3T3-L1 cells, suggesting the block in differentiation observed in 3T3-C2 cells occurs after clonal expansion. Combined, these findings suggest that the regulatory mechanisms of the p130:p107 switch are not specific to differentiation but may play a key role in regulating the mitotic clonal expansion necessary for adipocyte differentiation in 3T3-L1 cells.

The p120-v-Abl protein interacts with E2F-1 and regulates E2F-1 transcriptional activity

The E2F family of transcription factors regulates cell cycle progression, and deregulated expression of E2F-1 can lead to neoplastic transformation. In myeloid cells, introduction and expression of the Abelson leukemia virus causes growth factor independence. Here, the p120 v-Abl protein activates E2F-1-mediated transcription through a physical interaction with the E2F-1 transcription factor. BCR-Abl and c-Abl also stimulate E2F-1-mediated transcription. Our results suggest a new mechanism by which v-Abl leads to factor-independent myeloid cell proliferation: the activation of E2F-1-mediated transcription.

Accumulation of E2F-4.DP-1 DNA binding complexes correlates with induction of dhfr gene expression during the G1 to S phase transition

Previously genomic DNase I footprinting showed changes in protein binding to two overlapping E2F sites correlates with activation of dhfr gene expression at the G1/S boundary of the Chinese hamster cell cycle (Wells, J., Held, P., Illenye, S., and Heintz, N. H. (1996) Mol. Cell. Biol. 16, 634-647). Here gel mobility and antibody supershift assays were used to relate changes in the components of E2F DNA binding complexes in cell extracts to repression and induction of dhfr gene expression. In extracts from log phase cells, E2F complexes contained predominantly E2F-4 and E2F-2 in association with DP-1, and DNA binding assays showed complexes containing E2F-2 preferentially interact with only one of the two overlapping E2F sites. In serum starvation-stimulation experiments, arrest in G1 by low serum was accompanied by decreased levels of dhfr mRNA and the appearance of an E2F-4.DP-1.p130 complex. After serum stimulation, induction of dhfr gene expression was preceded by loss of the p130 complex in mid G1 and coincided with marked increases in two free E2F.DP-1 complexes in late G1, one of which contained E2F-4 and a second which contained an unidentified E2F. We suggest activation of dhfr gene expression after serum stimulation requires at least two temporally distinct processes, relief of p130-mediated repression and subsequent activation of transcription by free E2F.

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

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

Polyomavirus large T-antigen binds the "pRb related' protein p130 through sequences in conserved region 2

The transforming potential of DNA tumor viruses derives mainly from the ability of their encoded oncogene products to interact with cellular proteins. Many of these viral oncoproteins share regions of sequence similarity, designated conserved region 1 and 2, which have been implicated in complex formation with pRb, the product of the retinoblastoma tumor suppressor gene, and related p107 and p130 species. It has now been shown that the EIA protein of adenovirus is able to bind to all three pRb-related proteins through sequences in conserved region 1 and 2. We have shown previously that polyomavirus large T-antigen also interacts with pRb and p107 in vitro. The pRb and p107 binding domains reside between residues 141, 158 which include conserved region 2. In the present study, we demonstrate that polyomavirus large T-antigen also interacted with p130 in vitro through the same sequences in conserved region 2.

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

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

Evasion of apoptosis by DNA viruses

Apoptosis is a form of cell death distinct from necrosis which plays an important role in processes such as homoeostasis and the elimination of damaged cells. It can be triggered by a variety of stimuli including DNA damage and cytotoxic T lymphocyte activity, both of which may be induced in the course of a viral infection. Initially, induction of apoptosis may occur through pathways which have also been shown to be activated on disturbance of the cell cycle or damage to cellular DNA. At later time points during the course of infection, apoptosis can also be triggered by cytokines and immune effector cells. Apoptosis of the host cell before the completion of the viral replication cycle may limit the number of progeny and the spread of infection. The importance of apoptosis as an antiviral defence is illustrated by the presence of multiple pathways for apoptosis induction and inhibition in both the host and virus. In this review, the inhibition of apoptosis is described in adenovirus and poxvirus infection. These examples illustrate two of the divergent paths by which viruses may avoid the apoptotic response.

XAP2, a novel hepatitis B virus X-associated protein that inhibits X transactivation

The hepatitis B virus X protein is a promiscuous transcriptional transactivator. Transactivation by the X protein is most likely mediated through binding to different cellular factors. Using the yeast two-hybrid method, we have isolated a clone that encodes a novel X-associated cellular protein: XAP2. X and XAP2 interactions also occur in vitro. Antiserum raised against XAP2 recognizes a cytoplasmic protein with an apparent molecular mass of 36 kDa. The interaction between X and XAP2 requires a small region on X containing amino acids 13-26. From Northern blot analyses, XAP2 is ubiquitously expressed in both liver-derived and non-liver-derived cell lines as well as in normal non-liver tissues. In contrast, XAP2 is expressed in very low level in the normal human liver. In transfection assays, overexpression of XAP2 abolishes transactivation by the X protein. Based on these results, we suggest that XAP2 is an important cellular negative regulator of the X protein, and that X-XAP2 interaction may play a role in HBV pathology.

Functional characterization of the transcription silencer element located within the human Pi class glutathione S-transferase promoter

We have previously demonstrated enhanced transcriptional activity of the human Pi class glutathione S-transferase (GSTP1) promoter in a multidrug-resistant derivative (VCREMS) of the human mammary carcinoma cell line, MCF7 (Moffat, G. J., McLaren, A. W., and Wolf, C. R. (1994) J. Biol. Chem. 269, 16397-16402). Furthermore, we have identified an essential sequence (C1; -70 to -59) within the GSTP1 promoter that bound a Jun-Fos heterodimer in VCREMS but not in MCF7 cells. These present studies have examined the negative regulatory element (-105 to -86), which acted to suppress GSTP1 transcription in MCF7 cells. Mutational analysis of this silencer element further defined the repressor binding site to be located between nucleotides -97 and -90. In vitro DNA binding assays suggested that the repressor exerted its action by causing displacement of the essential non-AP-1-like MCF7 C1 complex. However, the addition of MCF7 nuclear extract did not disrupt binding of the VCREMS Jun-Fos C1 complex to the GSTP1 promoter. Furthermore, upstream insertion of the GSTP1 silencer element failed to inhibit activity of a heterologous promoter in MCF7 cells. These results highlighted the cell and promoter specificity of the GSTP1 transcriptional repressor and implicated a functional requirement for contact between the repressor and C1 complex. In this regard, the introduction of half-helical turns between the silencer and the C1 element abrogated repressor activity, thus leading to the hypothesis that a direct interaction between the repressor and C1 complex was required to suppress GSTP1 transcription. Moreover, these findings suggest that cell-specific differences in the composition of the C1 nuclear complex may dictate repressor activity.

Early stage of human adenovirus type 12-inoculated retinal tissue of F344 newborn rat

To elucidate the pathogenesis of adenovirus type 12 (Ad12)-induced rat retinal tumor, an experimental animal model of human retinoblastoma (RB), DNA analysis, in situ hybridization and immunohistochemistry were performed. The adenovirus oncogene E1A was detected in the host genome by Southern blot hybridization. Examined retinal tissues did not show any histological changes, but the number of retinal cells immunoreactive with an antibody to proliferating cell nuclear antigen (PCNA) increased with the course of study. In in situ hybridization, E1A gene expression was recognized at the inner granular layer of the retina at an early stage after virus inoculation, and subsequently, N-myc gene expression was recognized at the same region. No alteration was found in the retinoblastoma susceptibility gene (Rb gene) expression. The product of the virus oncogene integrated into the host genome could induce an increase in N-myc expression, without any abnormality of the Rb gene itself. Results from the present study could be useful in clarifying the tumorigenesis of this experimental model.

DP-2, a heterodimeric partner of E2F: identification and characterization of DP-2 proteins expressed in vivo

E2F is a heterodimeric transcription factor that regulates the expression of genes at the G1/S boundary and is composed of two related but distinct families of proteins, E2F and DP. E2F/DP heterodimers form complexes with the retinoblastoma (Rb) protein, the Rb-related proteins p107 and p130, and cyclins/cdks in a cell cycle-dependent fashion in vivo. E2F is encoded by at least five closely related genes, E2F-1 through -5. Here we report studies of DP-2, the second member of the DP family of genes. Our results indicate that (i) DP-2 encodes at least five distinct mRNAs, (ii) a site of alternative splicing occurs within the 5' untranslated region of DP-2 mRNA, (iii) at least three DP-2-related proteins (of 55, 48, and 43 kDa) are expressed in vivo, (iv) each of these proteins is phosphorylated, and (v) one DP-2 protein (43 kDa) carries a truncated amino terminus. Our data also strongly suggest that the 55-kDa DP-2-related protein is a novel DP-2 isoform that results from alternative splicing. Thus, we conclude that DP-2 encodes a set of structurally, and perhaps functionally, distinct proteins in vivo.

Functional implications of mutations within polyomavirus large T antigen Rb-binding domain: effects on pRb and p107 binding in vitro and immortalization activity in vivo

In this study, we have extensively modified the Rb-binding domain of polyomavirus large T antigen. Mutant polyomavirus large T antigens were tested for their ability to bind pRb and p107 in vitro and assayed for their capacity to immortalize primary rat embryo fibroblasts in vivo. Polyomavirus large T antigen bound pRb and p107 through a common region located between amino acids 141 to 158, containing the consensus Rb-binding sequence D/N-L-X-C-X-E. Substitution of any amino acid within the core Rb-binding sequence abolished pRb and p107 binding in vitro and immortalization activity in vivo. Substitution of amino acids outside the core Rb-binding sequence reduced pRb and p107 binding in vitro and decreased or abolished immortalization of rat embryo fibroblasts in vivo. Although duplication of the Rb-binding domain within the polyomavirus large T antigen results in a molecule that can bind at least twice as much pRb and p107 in vitro, this mutant displayed an essentially wild-type level of immortalization activity. More importantly, we found that the addition of acidic residues within the casein kinase II consensus phosphorylation region flanking the Rb-binding domain, or the deletion of amino acids 256 to 272, increased the immortalizing activity of the mutant polyomavirus large T antigen. These two mutants displayed a greater than wild-type level of pRb binding in vitro, while in contrast, a decreased affinity for p107 binding in vitro was observed. Together, these results indicate that while pRb binding appears to be an essential event for immortalization, there is no tight correlation between the frequency of immortalization and the absolute level of pRb binding in vitro, indicating that other large T antigen functions are important for cellular immortalization.

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

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

Cell cycle regulation by the retinoblastoma family of growth inhibitory proteins

The retinoblastoma family of growth-inhibitory proteins act by binding and inhibiting several proteins with growth-stimulatory activity, the most prominent of which is the cellular transcription factor E2F. In higher organisms, progression through the cell division cycle is accompanied by the cyclical activation of a number of protein kinases, the cyclin-dependent kinases. Phosphorylation of retinoblastoma family proteins by these cyclin-dependent kinases leads to release of the associated growth-stimulatory proteins which in turn mediate progression through the cell division cycle.

Activity of the retinoblastoma family proteins, pRB, p107, and p130, during cellular proliferation and differentiation

Genetic evidence from retinoblastoma patients and experiments describing the mechanism of cellular transformation by the DNA tumor viruses have defined a central role for the retinoblastoma protein (pRB) family of tumor suppressors in the normal regulation of the eukaryotic cell cycle. These proteins, pRB, p107, and p130, act in a cell cycle-dependent manner to regulate the activity of a number of important cellular transcription factors, such as the E2F-family, which in turn regulate expression of genes whose products are important for cell cycle progression. In addition, inhibition of E2F activity by the pRB family proteins is required for cell cycle exit after terminal differentiation or nutrient depletion. The loss of functional pRB, due to mutation of both RB1 alleles, results in deregulated E2F activity and a predisposition to specific malignancies. Similarly, inactivation of the pRB family by the transforming proteins of the DNA tumor viruses overcomes cellular quiescence and prevents terminal differentiation by blocking the interaction of pRB, p107, and p130 with the E2F proteins, leading to cell cycle progression and, ultimately, cellular transformation. Together these two lines of evidence implicate the pRB family of negative cell cycle regulators and the E2F family of transcription factors as central components in the cell cycle machinery.

E2F-1 functions in mice to promote apoptosis and suppress proliferation

Members of the E2F transcription factor family (E2F-1-E2F-5) are believed to be critical positive regulators of cell cycle progression in eukaryotes although the in vivo functions of the individual E2Fs have not been elucidated. Mice were generated that lack E2F-1 and, surprisingly, these mice develop and reproduce normally. However, E2F-1-/- mice exhibit a defect in T lymphocyte development leading to an excess of mature T cells due to a maturation stage-specific defect in thymocyte apoptosis. As E2F-1-/- mice age they exhibit a second phenotype marked by aberrant cell proliferation. These findings suggest that while certain members of the E2F family may positively regulate cell cycle progression, E2F-1 functions to regulate apoptosis and to suppress cell proliferation.

Simian virus 40 large T antigen alters the phosphorylation state of the RB-related proteins p130 and p107

p130 and p107 are nuclear phosphoproteins related to the retinoblastoma gene product (pRb). pRb, p107, and p130 each undergo cell cycle-dependent phosphorylation, form complexes with the E2F family of transcription factors, and associate with oncoproteins of DNA tumor viruses, including simian virus 40 (SV40) large T antigen (TAg) and adenovirus ElA protein. The results of recent studies with mouse embryo fibroblasts (MEFs) lacking the retinoblastoma gene (Rb-1) have suggested that p130 and p107 may be important targets for SV40 large TAg-mediated transformation (J.B. Christensen and M.J. Imperiale, J. Virol. 65:3945-3948, 1995; J. Zalvide and J.A. DeCaprio, Mol. Cell. Biol. 15:5800-5810, 1995). In this report, we demonstrate that the expression of TAg affects the phosphorylation state of p130 and p107. In cells expressing wild-type TAg, only un(der)phosphorylated p130 and p107 were detected. To determine the domains within TAg that contribute to this effect on the phosphorylation of p130, we performed transient expression assays. While transiently expressed p130 was apparently phosphorylated normally, only un(der)phosphorylated p130 was detected when p130 was coexpressed with TAg. Using this assay, we found that the first 147 amino acids of TAg were sufficient to alter the phosphorylation state of p130. Within this region, the LXCXE domain of TAg, required for binding to the retinoblastoma family of proteins, was necessary but not sufficient to affect p130 phosphorylation. Residues within the first 82 amino acids of TAg were also required. TAg with mutations in the N terminus retained the ability to efficiently associate with p130 but did not affect its phosphorylation state. This demonstrates that the effect of SV40 TAg on p130 is not simply the result of binding and suggests that TAg has a novel effect on p130 and p107 that differs from its effect on pRb.

BK virus large T antigen: interactions with the retinoblastoma family of tumor suppressor proteins and effects on cellular growth control

BK virus (BKV) is a polyomavirus which infects a large percentage of the human population. It is a potent transforming agent and is tumorigenic in rodents. BKV DNA has also been found in human brain, pancreatic islet, and urinary tract tumors, implicating this virus in neoplastic processes. BKV T antigen (TAg) is highly homologous to simian virus 40 TAg, particularly in regions required for mitogenic stimulation and binding to tumor suppressor proteins, The experiments presented in this report show that BKV TAg can bind the tumor suppressor protein p53. BKV TAg also has the ability to bind to members of the retinoblastoma (pRb) family of tumor suppressor proteins both in vivo and in vitro. However, these interactions are detected only when large amounts of total protein are used, because the levels of BKV TAg normally produced from viral promoter-enhancer elements are too low to bind a significant amount of the pRb family proteins in the cell. The low levels of BKV TAg produced by the viral promoter elements are sufficient to affect the levels and the phosphorylation patterns of these proteins and to induce serum-independent growth in these cells. Additional events, however, are required for full transformation. These data further support the notion that BKV TAg can affect cellular growth control mechanisms and may in fact be involved in neoplastic processes.

Functional interactions between the hBRM/hBRG1 transcriptional activators and the pRB family of proteins

hBRG1 and hBRM are mammalian homologs of the SNF2/SW12 yeast transcriptional activator. These proteins exist in a large multisubunit complex that likely serves to remodel chromatin and, in so doing, facilitates the function of specific transcription factors. The retinoblastoma protein (pRB) inhibits cell cycle progression by repressing transcription of specific growth-related genes. Using the yeast two-hybrid system, we demonstrate that the members of the hBRG1/hBRM family of proteins interact with the pRB family of proteins, which includes pRB, p107, and p130. Interaction between the hBRG1/hBRM family with the pRB family likely influences cellular proliferation, as both hBRG1 and hBRM, but not mutants of these proteins unable to bind to pRB family members, inhibit the formation of drug-resistant colonies when transfected into the SW13 human adenocarcinoma cell line, which lacks endogenous hBRG1 or hBRM. Further, hBRM and two isoforms of hBRG1 induce the formation of flat, growth-arrested cells in a pRB family-dependent manner when introduced into SW13 cells. This flat-cell inducing activity is severely reduced by cotransfection of the wild-type E1A protein and variably reduced by the cotransfection of mutants of E1A that lack the ability to bind to some or all members of the pRB family.

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

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

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

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

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

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

Distribution and subcellular localization of a growth inhibitory factor in hamster liver and its intracellular partner(s)

The subcellular, intralobular distributions and intracellular partner(s) of a factor which inhibits the proliferation of cell growth (Hashimoto C. et al. (1994) Biochim. Biophys. Acta 1221, 107-117) were determined in hamster livers, using a combination of immunological and biochemical techniques. The IgG fraction from an antiserum raised against the growth inhibitory factor with 37 kDa was shown to be highly specific for the antigen. The nuclear and cytosolic fractions demonstrated inhibitory effects on cell growth and Western blot analysis revealed that both fractions contained the immunoreactive 37 kDa protein with the anti-inhibitory factor IgG but microsomal and mitochondrial fractions did not. The nuclear and cytoplasmic localization of the inhibitory factor were further confirmed by immunochemical staining mediated through the immune IgG and an avidin-biotinylated horseradish peroxidase complex, the parenchymal liver cells were clearly stained, but endothelial and connective tissue cells were not. Although some staining was evident throughout the liver parenchyma, the hepatocytes with most intensively stained nuclei were located in the periportal region. In the liver from hamsters 6 days old or the regenerating hamster livers 3 days after partial hepatectomy, the staining intensity was low and the number of hepatocytes with the inhibitory factor positive nuclei was very few compared with the adult hamster livers. In primary cultures of the isolated hepatocytes from adult hamster the inhibitory factor disappeared from nuclei after incubation for 24-48 h. The extracts of hepatic nuclei from adult hamsters were immunoprecipitated with either the anti-growth inhibitory factor IgG or a monoclonal antibody to the RM protein. The growth inhibitory factor and the RB protein coprecipitated in each case, implying that the proteins were complexed with each other in the nuclei. The RB protein family is composed of two sets of species, an un- or underphosphorylated species and a hyperphosphorylated one. It was suggested that the factor bound preferentially to the un- or underphosphorylated member of the family.

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.

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.

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.

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.

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.

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.

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.

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.

Multiple change in E2F function and regulation occur upon muscle differentiation

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

Identification of a novel E1A response element in the mouse c-fos promoter

Transcriptional activation of the c-fos gene in mouse S49 cells by the adenovirus 243-amino-acid E1A protein depends on domains of E1A that are also required for transformation and that bind the cellular protein p300. Activation additionally depends on stimulation of endogenous cyclic AMP (cAMP)-dependent protein kinase by analogs or inducers of cAMP. Transient transfection assays were used to analyze the c-fos promoter for sequences that confer responsiveness to E1A. Linker substitution and point mutants revealed that transcriptional activation by E1A depended on a cAMP response element (CRE) located at -67 relative to the start site of transcription and a neighboring binding site for transcription factor YY1 located at -54. A 22-bp sequence containing the -67 CRE and the -54 YY1 site was sufficient to confer responsiveness to a minimal E1B promoter and was termed the c-fos E1A response element (ERE). Function of the c-fos ERE depended on both the CRE and the YY1 site, since mutation of either site resulted in a loss of responsiveness to E1A. These results imply a specific functional interaction between CRE-binding proteins, transcription factor YY1, and E1A in the regulation of the c-fos gene.

Genetic instability as a consequence of inappropriate entry into and progression through S-phase

The stability of the mammalian genome depends on the proper function of G1 and G2 cell cycle control mechanisms. Two tumor suppressors, p53 and retinoblastoma (Rb), play key roles in progression from G1 into S-phase. We address the mechanisms by which these proteins mediate a G1 arrest in response to DNA damage and limiting metabolic conditions. Gamma-irradiation induced a prolonged, p53-dependent G1 arrest associated with a long-term increase in the levels of the cdk-inhibitor p21WAFl/Cipl (p21). Microinjection of linear plasmid DNA also caused a G1 arrest. The p53-dependent arrest induced by inhibitors of UMP biosynthesis was reversible and occurred in the absence of detectable DNA damage. Both arrest mechanisms contribute to limiting the formation and propagation of damaged genomes. Cells containing mutant p53 but wild-type Rb do not generate methotrexate (Mtx) resistant variants. However, pre-treatment with DNA damaging agents prior to drug selection resulted in resistant clones containing amplified dihydrofolate reductase (DHFR) genes, suggesting that DNA breakage is a rate limiting step for gene amplification. The Mtx-induced arrest did not occur in cells with non-functional Rb. Rb acts as a negative regulator of the E2F transcription factors, and Rb-deficient primary mouse embryo fibroblasts (MEFs) produced elevated levels of mRNA and protein for key E2F target genes. Failure to prevent entry into S-phase in Rb-/- MEFs exposed to DNA-damaging or nutrient limiting conditions caused apoptosis and correlated with p53 induction. Taken together, these findings indicate a link between p53 and Rb function and suggest that their coordination insures correct entry into S-phase, minimizing the emergence of genetic variants.