Transcriptional regulation of the dihydrofolate reductase gene

Characterization of the 5'-flanking region of the gene encoding the 50 kDa subunit of human DNA polymerase delta

DNA polymerase delta consists of at least four subunits: p125, p68, p50, and p12 [Liu et al., J. Biol. Chem. 275 (2000) 18739-18744]. We have isolated genomic DNA clones covering the gene for the human DNA polymerase delta 50 kDa subunit (POLD2) and its 5'-flanking sequence. The POLD2 gene is composed of 11 exons and is distributed over 10 kb of genomic DNA. All exon-intron splice junctions conformed to the GT/AG consensus sequence. The 5'-flanking region of human POLD2 is G+C-rich and does not have a typical TATA box. A computer-based search for potential transcription factor binding sites revealed the existence of a number of motifs including those for AP1, AP2, Sp1, NF-1 and CREB. The functional activity of the regulatory region of the human POLD2 gene was demonstrated by its ability to drive the expression of a chloramphenicol acetyltransferase reporter gene in COS-7 cells.

Mutagenesis of the pRB pocket reveals that cell cycle arrest functions are separable from binding to viral oncoproteins

The pocket domain of pRB is required for pRB to arrest the cell cycle. This domain was originally defined as the region of the protein that is necessary and sufficient for pRB's interaction with adenovirus E1A and simian virus s40 large T antigen. These oncoproteins, and other pRB-binding proteins that are encoded by a variety of plant and animal viruses, use a conserved LXCXE motif to interact with pRB. Similar sequences have been identified in multiple cellular pRB-binding proteins, suggesting that the viruses have evolved to target a highly conserved binding site of pRB that is critical for its function. Here we have constructed a panel of pRB mutants in which conserved amino acids that are predicted to make close contacts with an LXCXE peptide were altered. Despite the conservation of the LXCXE binding site throughout evolution, pRB mutants that lack this site are able to induce a cell cycle arrest in a pRB-deficient tumor cell line. This G(1) arrest is overcome by cyclin D-cdk4 complexes but is resistant to inactivation by E7. Consequently, mutants lacking the LXCXE binding site were able to induce a G(1) arrest in HeLa cells despite the expression of HPV-18 E7. pRB mutants lacking the LXCXE binding site are defective in binding to adenovirus E1A and human papillomavirus type 16 E7 protein but exhibit wild-type binding to E2F or DP, and they retain the ability to interact with CtIP and HDAC1, two transcriptional corepressors that contain LXCXE-like sequences. Consistent with these observations, the pRB mutants are able to actively repress transcription. These observations suggest that viral oncoproteins depend on the LXCXE-binding site of pRB for interaction to a far greater extent than cellular proteins that are critical for cell cycle arrest or transcriptional repression. Mutation of this binding site allows pRB to function as a cell cycle regulator while being resistant to inactivation by viral oncoproteins.

Dysregulation of cyclin E gene expression in human cytomegalovirus-infected cells requires viral early gene expression and is associated with changes in the Rb-related protein p130

We have previously shown that many cell cycle regulatory gene products are markedly affected by infection of primary fibroblasts with human cytomegalovirus (HCMV) (F. M. Jault, J. M. Jault, F. Ruchti, E. A. Fortunato, C. Clark, J. Corbeil, D. D. Richman, and D. H. Spector, J. Virol. 69:6697-6704, 1995). One of these proteins, cyclin E, is a key determinant of cell cycle progression during G(1), and its mRNA levels are significantly increased in HCMV-infected fibroblasts (B. S. Salvant, E. A. Fortunato, and D. H. Spector, J. Virol. 72:3729-3741, 1998). To determine the molecular basis of this effect, we have examined the events that occur at the endogenous cyclin E promoter during the course of infection. In vivo dimethyl sulfate footprinting of the cyclin E promoter revealed several regions of protection and hypersensitivity that were unique to infected cells. In accord with this observation, we find that the virus-induced cyclin E transcripts initiate downstream of the start site identified in mock-infected cells, in regions where these newly appearing protected and hypersensitive sites occur. Viral gene expression is required for this induction. However, the viral immediate-early proteins IE1-72 and IE2-86, either alone or in combination, cannot induce expression of the endogenous cyclin E. The virus must progress past the immediate-early phase and express an early gene product(s) for activation of cyclin E expression. Moreover, IE1-72 does not appear to be required, as infection of cells with an HCMV mutant containing a deletion in the IE1-72 gene leads to full upregulation of cyclin E expression. Using electrophoretic mobility shift assays with infected cell extracts and a region of the cyclin E promoter that includes two previously defined E2F sites as the probe, we detected the appearance of an infection-specific banding pattern. One of the infection-specific bands contained the proteins E2F-4, DP-1, and p130, which were maintained in the infected cells as uniquely phosphorylated species. These results suggest that an altered E2F-4-DP-1-p130 complex along with viral early gene expression may play a role in the transcriptional regulation of cyclin E mRNA during HCMV infection.

Roles for non-TATA core promoter sequences in transcription and factor binding

Sequence blocks within the core region were swapped among RNA polymerase II promoters to explore effects on transcription in vitro. The pair of blocks flanking TATA strongly influenced general transcription, with an additional effect on promoter activation. These flanking elements induced a change in the ratio of activated to basal transcription, whereas swapping TATA and initiator sequences only altered general transcription levels. Swapping the flanking blocks influenced binding by general transcription factors TBP and TFIIB. The results suggest that the architecture of the extended core sequence is important in determining promoter-specific effects on both general transcription levels and the tightness of regulation.

Oxidative stress and cell cycle checkpoint function

Oxidative stress and the damage that results from it have been implicated in a wide number of disease processes including atherosclerosis, autoimmune disorders, neuronal degeneration, and cancer. Reactive oxygen species (ROS) are ubiquitous and occur naturally in all aerobic species, coming from both exogenous and endogenous sources. ROS are quite reactive and readily damage biological molecules, including DNA. While the damaging effects of ROS on DNA have been intensively studied, the effects of oxidative damage on cell cycle checkpoint function have not. Here will we review several biologically important ROS and their sources, the cell cycle, checkpoints, and current knowledge about the effects of ROS on initiating checkpoint responses.

Cloning and characterization of the 5'-upstream sequence governing the cell cycle-dependent transcription of mouse DNA polymerase alpha 68 kDa subunit gene

We have isolated the genomic DNA fragment spanning the 5-end and the first four exons encoding the 68 kDa subunit (p68) of the mouse DNA polymerase alpha-primase complex [corrected]. The p68 promoter region lacks TATA and CAAT boxes, but contains a GC-rich sequence, two palindrome sequences and two putative E2F-binding sites [corrected]. A series of transient expression assays using a luciferase reporter gene indicated that a region from nucleotide position -89 to -30 (-89/-30) with respect to the transcription initiation site is crucial for basal transcription of the p68 gene in proliferating NIH 3T3 cells. In particular, part of the GC-rich sequence (-57/-46) and the palindrome (-81/-62) elements were necessary for promoter activity, both of which share homology with the E-box sequence. Gel mobility shift assays using NIH 3T3 nuclear extracts revealed that the upstream stimulatory factor, known as an E-box-binding protein, binds to these sites. Moreover, we observed binding of E2F to two sites near the transcription initiation site (-11/-3 and +9/+16). A transient luciferase expression assay using synchronized NIH 3T3 cells in G(0)phase revealed that these E2F sites are essential for transcription induction of the p68 gene after serum stimulation, but are dispensable for basal transcription. These results indicate that growth-dependent regulation of transcription of the mouse p68 and p180 genes is mediated by a common factor, E2F; however, basal transcription of the genes, interestingly, is regulated by different transcription factors.

E2F is required to prevent inappropriate S-phase entry of mammalian cells

E2F is a family of transcription factors that regulates the cell cycle. It is widely accepted that E2F-mediated transactivation of a set of genes is the critical activity that governs cellular progression through G(1) into S phase. In contrast to this hypothesis, we demonstrate that E2F actually suppresses the onset of S phase in two cell types when the cells are arrested by gamma irradiation. Our findings indicate that in these cells, the critical event triggering progression from G(0)/G(1) arrest into S phase is the release of E2F-mediated transrepression of cell cycle genes, not transactivation by E2F. Furthermore, our data suggest that E2F-mediated transactivation is not necessary for the G(1)/S-phase transition in these cells.

Sp1 and NF-Y are necessary and sufficient for growth-dependent regulation of the hamster thymidine kinase promoter

Thymidine kinase (TK) genes from different species are growth- and cell cycle-regulated in a very similar manner; still, the promoter regions of these genes show little homology to each other. It was previously shown that the murine TK gene is growth-regulated by Sp1 and E2F. Here we have characterized cis-regulatory elements in the hamster promoter that are essential and sufficient to confer efficient and serum-responsive expression. The TK promoter was isolated from baby hamster kidney cells. DNase I protection experiments revealed a protected region from positions -24 to -99 relative to the transcription start site. Within this region, binding sites for the transcription factor Sp1 and a CCAAT box, which interacts with the transcription factor NF-Y, were identified. An E2F-like sequence was found not to bind protein, and its removal did not affect promoter activity. This was supported by the observation that cotransfection of a hamster TK reporter gene construct with E2F-1 does not lead to transactivation of the promoter. A 122-base pair region that contains a single Sp1 site, a CCAAT box, and a TATA element was found to be sufficient for serum-responsive expression of a reporter gene. Mutations that inactivate any one of these three elements caused a strong reduction or a loss of promoter activity.

Genomic amplification of the human DHFR/MSH3 locus remodels mismatch recognition and repair activities

Mismatch recognition in human cells is mediated by two heterodimers, MutS alpha and MutS beta. MutS alpha appears to shoulder primary responsibility for mismatch correction during replication, based on its relative abundance and ability to recognize a broad spectrum of base-base and base-insertion mismatches. Because MutS alpha and MutS beta share a common component, MSH2, conditions that influence the expression or degradation of MSH3 or MSH6 can redistribute the profile of mismatch recognition and repair. MSH3 is linked by a shared promoter with DHFR, connecting two pathways with key roles in DNA metabolism. In a classic example of gene amplification, the DHFR (and MSH3) locus can become amplified to several hundred copies in the presence of methotrexate. Under these conditions, MutS beta forms at the expense of MutS alpha, and the mutation rate in these tumor cells rises more than 100-fold. The implications for cancer chemotherapy include a potential increase in mutability when tumors are treated with methotrexate, which could increase the frequency of subsequent mutations that influence the tumor's drug sensitivity or aggressiveness. Because processing certain types of DNA damage by the mismatch repair pathway has also been implicated in tumor sensitivity to agents such as cisplatin, changes in expression at the DHFR/MSH3 locus may have further relevance to the outcome of multi-drug treatment regimens.

The HMG protein T160 colocalizes with DNA replication foci and is down-regulated during cell differentiation

The high mobility group protein T160, the murine homolog of the human structure-specific recognition protein 1, was first supposed to be involved in the process of V-(D)-J recombination, since it could bind to recombination signal sequence probes. We have recently cloned T160 by using an unrelated DNA probe and shown that it binds to either cruciform or linear DNA with no sequence specificity. In this work, we performed a detailed analysis of T160 expression and immunolocalization. We show that T160 is a phosphoprotein broadly conserved from yeast to mammals, with a high level of expression in all the cell lines tested and in tissues containing a high degree of proliferating cells. Indirect immunofluorescence analysis by confocal laser microscopy revealed that T160 distribution in the cell nucleus is not uniform, and focus-like staining was observed. Cell cycle studies by BrdU incorporation suggest that the appearance of T160 nuclear foci is specific of mid to late S phase. Furthermore, while T160 expression does not change during the cell cycle, it is dramatically down-regulated when cells begin to differentiate, as highlighted in C2C12 myoblasts and myotubes. The disappearance of T160 nuclear staining in multinucleated myotubes is shown. Taken together, these data suggest that its function may be less specific than V-(D)-J recombination and more related to some cellular basic process, such as DNA replication or repair.

Histone deacetylase 1 can repress transcription by binding to Sp1

The members of the Sp1 transcription factor family can act as both negative and positive regulators of gene expression. Here we show that Sp1 can be a target for histone deacetylase 1 (HDAC1)-mediated transcriptional repression. The histone deacetylase inhibitor trichostatin A activates the chromosomally integrated murine thymidine kinase promoter in an Sp1-dependent manner. Coimmunoprecipitation experiments with Swiss 3T3 fibroblasts and 293 cells demonstrate that Sp1 and HDAC1 can be part of the same complex. The interaction between Sp1 and HDAC1 is direct and requires the carboxy-terminal domain of Sp1. Previously we have shown that the C terminus of Sp1 is necessary for the interaction with the transcription factor E2F1 (J. Karlseder, H. Rotheneder, and E. Wintersberger, Mol. Cell. Biol. 16:1659-1667, 1996). Coexpression of E2F1 interferes with HDAC1 binding to Sp1 and abolishes Sp1-mediated transcriptional repression. Our results indicate that one component of Sp1-dependent gene regulation involves competition between the transcriptional repressor HDAC1 and the transactivating factor E2F1.

Cell cycle control, checkpoint mechanisms, and genotoxic stress

The ability of cells to maintain genomic integrity is vital for cell survival and proliferation. Lack of fidelity in DNA replication and maintenance can result in deleterious mutations leading to cell death or, in multicellular organisms, cancer. The purpose of this review is to discuss the known signal transduction pathways that regulate cell cycle progression and the mechanisms cells employ to insure DNA stability in the face of genotoxic stress. In particular, we focus on mammalian cell cycle checkpoint functions, their role in maintaining DNA stability during the cell cycle following exposure to genotoxic agents, and the gene products that act in checkpoint function signal transduction cascades. Key transitions in the cell cycle are regulated by the activities of various protein kinase complexes composed of cyclin and cyclin-dependent kinase (Cdk) molecules. Surveillance control mechanisms that check to ensure proper completion of early events and cellular integrity before initiation of subsequent events in cell cycle progression are referred to as cell cycle checkpoints and can generate a transient delay that provides the cell more time to repair damage before progressing to the next phase of the cycle. A variety of cellular responses are elicited that function in checkpoint signaling to inhibit cyclin/Cdk activities. These responses include the p53-dependent and p53-independent induction of Cdk inhibitors and the p53-independent inhibitory phosphorylation of Cdk molecules themselves. Eliciting proper G1, S, and G2 checkpoint responses to double-strand DNA breaks requires the function of the Ataxia telangiectasia mutated gene product. Several human heritable cancer-prone syndromes known to alter DNA stability have been found to have defects in checkpoint surveillance pathways. Exposures to several common sources of genotoxic stress, including oxidative stress, ionizing radiation, UV radiation, and the genotoxic compound benzo[a]pyrene, elicit cell cycle checkpoint responses that show both similarities and differences in their molecular signaling.

The primary function of a redundant Sp1 binding site in the mouse aprt gene promoter is to block epigenetic gene inactivation

The promoter region of the mouse adenine phosphoribosyltransferase (aprt) gene contains one non-consensus Sp1 binding site at its 5' end followed by three consensus Sp1 binding sites. The two 3'-most binding sites are sufficient for maximal expression of aprt , suggesting that the non-consensus and consensus binding sites at the 5' end are redundant. However, the two 3' sites are not sufficient to block epigenetic inactivation, which led to the hypothesis that the redundant consensus and/or non-consensus 5' Sp1 binding sites are required to block inactivation events. To test this hypothesis, promoter region constructs were made in which the two 5' Sp1 binding sites were mutated alone or in tandem, and then each construct was tested for its ability to withstand epigenetic inactivation. A cis -acting methylation center that is normally located 1.2 kb upstream of the promoter was used to induce inactivation. The results demonstrate that the presence of the redundant consensus Sp1 binding site is required to block methylation-associated gene inactivation. Therefore, the Sp1 binding sites comprising the mouse aprt promoter have evolved two distinct functions, one to promote transcription and the other to block epigenetic inactivation.

The relative ability of human papillomavirus type 6 and human papillomavirus type 16 E7 proteins to transactivate E2F-responsive elements is promoter- and cell-dependent

The human papillomavirus 16 E7 protein (HPV 16 E7) transactivates the adenovirus E2 promoter (AdE2) by altering interactions between retinoblastoma (pRb) family members and the transcription factor E2F. To understand factors limiting the oncogenic potential of HPV 6, the relative ability of HPV 6 E7 as compared to HPV 16 E7 to transactivate the AdE2 promoter was determined. In primary baby rat kidney cells and human foreskin keratinocytes, HPV 16 E7 transactivated the AdE2 promoter to a greater extent than HPV 6 E7, consistent with the observation that HPV 16 E7 binds pRb with greater affinity. HPV 6 E7 gain of function correlated with increasing the affinity of the HPV 6 E7 pRb binding site of conserved region 2 (CR2). In keratinocytes, in contrast to the AdE2 promoter, the abilities of the two E7 proteins to transactivate the B-myb promoter, a promoter regulated by E2F bound to p107/p130, were comparable. Introducing a negative charge into the N-terminus (CR1) and a high affinity pRb binding site into CR2 of HPV 6 E7 resulted in a transactivator with greater activity than HPV 16 E7 for both the AdE2 and B-myb promoters. Both of the promoters were negatively regulated by E2F and transactivation by the E7 proteins required an intact E2F site. In C33-A cells, which contain a mutated pRb, the two E7 proteins had comparable transactivating activity on both the AdE2 and B-myb promoters. The data are consistent with the interpretation that HPV 16 E7 affects interactions of pRb and p107/p130 with the E2F transcription factor, whereas HPV 6 E7 only affects interactions of p107/p130.

Phosphatidylinositol 3-kinase couples the interleukin-2 receptor to the cell cycle regulator E2F

Cell cycle progression initiated by interleukin-2 (IL-2) in T cells is critical for lymphoproliferation and an immune response. Phosphatidyl inositol 3-kinase (PI3K) is activated by IL-2. However, nuclear targets for PI3K are not known. Here we identify the cell cycle regulator E2F as an IL-2 target in T lymphocytes and PI3K as the critical signaling pathway. We eliminate both Stat5 and Raf/MEK pathways from E2F regulation. Protein kinase B (PKB) is activated by IL-2 via PI3K. The expression of an active PKB is sufficient to induce E2F activity. Inhibition of PI3K inhibits phosphorylation of Rb, induction of cyclin D3, and degradation of p27kip1. These results establish a crucial PI3K/PKB-mediated link between the IL-2 teceptor and the cell cycle machinery.

Identification of new regulatory genes controlling synthesis of folate-dependent enzymes in Aspergillus nidulans

Prototrophic revertants of a meth2 strain of aspergillus nidulans which is impaired in the regulation of synthesis of folate-dependent enzymes were isolated and six of them analysed. In three of the isolates reversion was the result of an intragenic suppressor mutation in the metH locus. In the remaining strains suppressor mutations occurred in independent genes. These genes, designated folA, folB and folC, are linked and located in chromosome VI. Mutations in these genes render synthesis of some folate enzymes, particularly folylpolyglutamate synthetase, insensitive to methionine-mediated repression.

p53-dependent regulation of MDR1 gene expression causes selective resistance to chemotherapeutic agents

Loss of functional p53 paradoxically results in either increased or decreased resistance to chemotherapeutic drugs. The inconsistent relationship between p53 status and drug sensitivity may reflect p53's selective regulation of genes important to cytotoxic response of chemotherapeutic agents. We reasoned that the discrepant effects of p53 on chemotherapeutic cytotoxicity is due to p53-dependent regulation of the multidrug resistance gene (MDR1) expression in tumors that normally express MDR1. To test the hypothesis that wild-type p53 regulates the endogenous mdr1 gene we stably introduced a trans-dominant negative (TDN) p53 into rodent H35 hepatoma cells that express P-glycoprotein (Pgp) and have wild-type p53. Levels of Pgp and mdr1a mRNA were markedly elevated in cells expressing TDN p53 and were linked to impaired p53 function (both transactivation and transrepression) in these cells. Enhanced mdr1a gene expression in the TDN p53 cells was not secondary to mdr1 gene amplification and Pgp was functional as demonstrated by the decreased uptake of vinblastine. Cytotoxicity assays revealed that the TDN p53 cell lines were selectively insensitive to Pgp substrates. Sensitivity was restored by the Pgp inhibitor reserpine, demonstrating that only drug retention was the basis for loss of drug sensitivity. Similar findings were evident in human LS180 colon carcinoma cells engineered to overexpress TDN p53. Therefore, the p53 inactivation seen in cancers likely leads to selective resistance to chemotherapeutic agents because of up-regulation of MDR1 expression.

The human dUTPase gene encodes both nuclear and mitochondrial isoforms. Differential expression of the isoforms and characterization of a cDNA encoding the mitochondrial species

We have previously identified distinct nuclear and mitochondrial isoforms of dUTPase in human cells, reporting the cDNA sequence of the nuclear isoform (DUT-N). We now report a cDNA corresponding to the mitochondrial isoform (DUT-M). The DUT-M cDNA contains an 252-amino acid open reading frame, encoding a protein with a predicted Mr of 26,704. The amino-terminal region of the protein contains an arginine-rich, 69-residue mitochondrial targeting presequence that is absent in the mature protein. In vitro transcription and translation of the DUT-M cDNA results in the production of a precursor protein with an apparent molecular mass of 31 kDa as judged by SDS-polyacrylamide gel electrophoresis. The DUT-M precursor is enzymatically active and immunoreacts with a dUTPase-specific monoclonal antibody. Mitochondrial import and processing studies demonstrate that the DUT-M precursor is processed into a 23-kDa protein and imported into mitochondria in vitro. Isoelectric focusing experiments demonstrate that the DUT-N has a pI of 6.0, while the processed form of DUT-M has a more basic pI of 8.1, measurements that are in agreement with predicted values. Studies aimed at understanding the expression of these isoforms were performed utilizing quiescent and replicating 34Lu human lung fibroblasts as a model cell culture system. Northern blot analysis, employing an isoform-specific probe, demonstrates that DUT-N and DUT-M are encoded by two distinct mRNA species of 1.1 and 1.4 kilobases, respectively. Western and Northern blot analysis reveal that DUT-M protein and mRNA are expressed in a constitutive fashion, independent of cell cycle phase or proliferation status. In contrast, DUT-N protein and mRNA levels are tightly regulated to coincide with nuclear DNA replication status. Because DUT-N and DUT-M have identical amino acid and cDNA sequences in their overlapping regions, we set out to determine if they were encoded by the same gene. The 5' region of the gene encoding dUTPase was isolated and characterized by a combination of Southern hybridization and DNA sequencing. These analyses demonstrate that the dUTPase isoforms are encoded by the same gene with isoform-specific transcripts arising through the use of alternative 5' exons. This finding represents the first example in humans of alternative 5' exon usage to generate differentially expressed nuclear and mitochondrial specific protein isoforms.

E2F-mediated growth regulation requires transcription factor cooperation

Previous studies have indicated that the presence of an E2F site is not sufficient for G1/S phase transcriptional regulation. For example, the E2F sites in the E2F1 promoter are necessary, but not sufficient, to mediate differential promoter activity in G0 and S phase. We have now utilized the E2F1 minimal promoter to test several hypotheses that could account for these observations. To test the hypothesis that G1/S phase regulation is achieved via E2F-mediated repression of a strong promoter, a variety of transactivation domains were brought to the E2F1 minimal promoter. Although many of these factors caused increased promoter activity, growth regulation was not observed, suggesting that a general repression model is incorrect. However, constructs having CCAAT or YY1 sites or certain GC boxes cloned upstream of the E2F1 minimal promoter displayed E2F site-dependent regulation. Further analysis of the promoter activity suggested that E2F requires cooperation with another factor to activate transcription in S phase. However, we found that the requirement for E2F to cooperate with additional factors to achieve growth regulation could be relieved by bringing the E2F1 activation domain to the promoter via a Gal4 DNA binding domain. Our results suggest a model that explains why some, but not all, promoters that contain E2F sites display growth regulation.

Transcription reinitiation rate: a special role for the TATA box

Promoters need to specify both the timing of transcriptional induction and the amount of transcript synthesized. In order to explore each of these effects separately, in vitro assays for the level of active preinitiation complex formation and for the rate of continuous RNA production were done. The effects were found to be influenced differently by different promoter elements. A consensus TATA element had a very strong effect on the rate of continuous RNA production, whereas two types of activators were important primarily in forming active transcription preinitiation complexes. Consensus TATA promoters exhibited high rates of continuous transcription; they assembled active preinitiation transcription complexes slowly but then produced transcripts continuously at an approximately fivefold-higher rate. Initiator-containing TATA-less promoters produced continuous transcripts slowly. Point mutations in the TATA element led to lower levels of transcription by reducing the number of preinitiation complexes and amplifying this reduction by lowering the apparent reinitiation rate. The results allow understanding of the sequence diversity of promoter elements in terms of specifying separate controls over the sensitivity of gene induction and over the strength of the induced promoter.

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.