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

Transcriptional repression of the E2-containing promoters EIIaE, c-myc, and RB1 by the product of the RB1 gene

The protein product of the retinoblastoma susceptibility gene, p110RB1, is a nuclear phosphoprotein [W.H. Lee, J.Y. Shew, F.D. Hong, T.W. Sery, L.A. Donoso, L.J. Young, R. Bookstein, and E.Y. Lee, Nature (London) 329:642-645, 1987] with properties of a cell cycle regulator (K. Buchkovich, L.A. Duffy, and E. Harlow, Cell 58:1097-1105, 1989; P.L. Chen, P. Scully, J.Y. Shew, J.Y. Wang, and W.H. Lee, Cell 58:1193-1198, 1989; J.A. DeCaprio, J.W. Ludlow, D. Lynch, Y. Furukawa, J. Griffin, H. Piwnica-Worms, C.M. Huang, and D.M. Livingston, Cell 58:1085-1095, 1989; and K. Mihara, X.R. Cao, A. Yen, S. Chandler, B. Driscoll, A.L. Murphree, A. TAng, and Y.K. Fung, Science 246:1300-1303, 1989). Although the mechanism of action of p110RB1 remains unknown, several lines of evidence suggest that it plays a role in the regulation of transcription. We now show that overexpression of p110RB1 causes repression of the adenovirus early promoter EIIaE and the promoters of two cellular genes, c-myc and RB1, both of which contain E2F-binding motifs. Mutation of the E2 element in the c-myc promoter abolishes p110RB1 repression. We also demonstrate that a p110RB1 mutant, which is refractory to cell cycle phosphorylation but intact in E1a/large T antigen-binding properties, represses EIIaE with 50- to 80-fold greater efficiency than wild-type p110RB1. These data provide evidence that hypophosphorylated p110RB1 actively represses expression of genes with promoters containing the E2F-binding motif (E2 element).

The retinoblastoma protein and the regulation of cell cycling

Increasing attention has been focused on how the retinoblastoma (RB) protein regulates cell growth. Recent evidence indicates that it is a substrate for phosphorylation by cyclin-dependent kinase-cyclin complexes and suggests that this phosphorylation modulates the ability of this protein to regulate transit through the cell cycle, perhaps in its G1 phase.

Mutations of the retinoblastoma gene in human lymphoid neoplasms

The inactivation or loss of tumor suppressor genes (anti-oncogenes) has been implicated as a mechanism central to the pathogenesis of many solid tumors. More recently, we and others have identified a role of one rumor suppressor gene, the retinoblastoma gene, in the development of human lymphoid lymphoma and leukemia. Here we review the involvement of the retinoblastoma gene in the control of normal lymphocyte cell division and the consequences of inactivation of the retinoblastoma gene for the development of lymphoid neoplasia. Our survey has disclosed a broad involvement of retinoblastoma gene inactivation in a wide variety of non-Hodgkin's lymphomas and lymphocytic leukemia. Based on these early findings, it appears likely that tumor suppressor genes may well be involved in many hematopoietic neoplasma.

Multiple cis-acting DNA elements that regulate transcription of the adenovirus 12 E1A gene

To delineate cis-acting elements for adenovirus (Ad) 12 E1A gene transcription, we transfected HeLa and NIH3T3 cells with DNAs having various deletions in the 5'-upstream region linked to the chloramphenicol acetyltransferase gene. Deletions in the regions between nucleotide (nt) positions 54 and 166, and 167 and 200, with respect to the left end of the viral genome at nt position 1, caused a two- to three-fold reduction in transcription. Transcription decreased to an almost undetectable level with loss of the region between nt positions 201 and 282. The effect of these mutations was almost consistent between both cell lines. The region between nt positions 77 and 94 stimulated transcription when situated upstream of the simian virus 40 early promoter in either orientation. Transcription was stimulated about ninefold in the presence of the DNA that encodes the product of the 13S, but not the 12S mRNA of the Ad12 E1A gene. These results indicate that transcription of the Ad12 E1A gene is regulated by multiple cis-acting elements and is stimulated by its own gene product.

Transcriptional repression of the E2-containing promoters EIIaE, c-myc, and RB1 by the product of the RB1 gene

The protein product of the retinoblastoma susceptibility gene, p110RB1, is a nuclear phosphoprotein [W.H. Lee, J.Y. Shew, F.D. Hong, T.W. Sery, L.A. Donoso, L.J. Young, R. Bookstein, and E.Y. Lee, Nature (London) 329:642-645, 1987] with properties of a cell cycle regulator (K. Buchkovich, L.A. Duffy, and E. Harlow, Cell 58:1097-1105, 1989; P.L. Chen, P. Scully, J.Y. Shew, J.Y. Wang, and W.H. Lee, Cell 58:1193-1198, 1989; J.A. DeCaprio, J.W. Ludlow, D. Lynch, Y. Furukawa, J. Griffin, H. Piwnica-Worms, C.M. Huang, and D.M. Livingston, Cell 58:1085-1095, 1989; and K. Mihara, X.R. Cao, A. Yen, S. Chandler, B. Driscoll, A.L. Murphree, A. TAng, and Y.K. Fung, Science 246:1300-1303, 1989). Although the mechanism of action of p110RB1 remains unknown, several lines of evidence suggest that it plays a role in the regulation of transcription. We now show that overexpression of p110RB1 causes repression of the adenovirus early promoter EIIaE and the promoters of two cellular genes, c-myc and RB1, both of which contain E2F-binding motifs. Mutation of the E2 element in the c-myc promoter abolishes p110RB1 repression. We also demonstrate that a p110RB1 mutant, which is refractory to cell cycle phosphorylation but intact in E1a/large T antigen-binding properties, represses EIIaE with 50- to 80-fold greater efficiency than wild-type p110RB1. These data provide evidence that hypophosphorylated p110RB1 actively represses expression of genes with promoters containing the E2F-binding motif (E2 element).

HOX gene expression in normal and neoplastic human kidney

As a consequence of transformation, cancer cells generally lose some of their differentiative properties. Thus, alterations interfering with the genetic mechanisms required to maintain embryonic determination could lead to tumorigenesis. Homeobox genes are a network of genes encoding nuclear proteins containing DNA-binding homeodomains that are highly conserved throughout evolution. They are expressed in a stage-related fashion in the developing embryo and, in adult life, in normal tissues. In mice and humans, homeobox genes of the HOX family are organized in 4 clusters on different chromosomes which have presumably evolved by duplication of a primordial gene cluster. Strikingly, the order of genes within each cluster is also highly conserved throughout evolution, suggesting that the physical organization of HOX genes might be essential for their expression. Recent reports indicate that homeobox mutant mice display morphological abnormalities or show neoplastic alterations, and that growth factors can turn on homeobox genes. We have studied the expression of the Antennapedia-like HOX genes in normal human kidney and in renal carcinomas. The great majority of the HOX genes analyzed are expressed in a peculiar manner in normal kidney: blocks of genes, even entire HOX loci, are coordinately regulated. Alterations in HOX gene expression in renal carcinoma can be observed in 2 genes of the HOX-2 locus, HOX-2A and HOX-2E, which are actively expressed in normal kidney and silent in cancer biopsies. The HOX-3H gene is not expressed in normal kidney whereas the HOX-3H transcripts are present in renal carcinomas. Homeobox genes within the 4 HOX loci can be aligned on the basis of the maximal sequence homology of their homeodomains: this alignment defines 13 paralogous gene groups. In renal carcinomas, genes of group 10 (HOX-1D, 2F, 3E, 4B) display a marked difference in their transcript classes when compared to those of normal kidney. Our findings suggest an association between altered HOX gene expression and kidney cancer.

A cDNA encoding a pRB-binding protein with properties of the transcription factor E2F

The retinoblastoma protein (pRB) plays an important role in the control of cell proliferation, apparently by binding to and regulating cellular transcription factors such as E2F. Here we describe the characterization of a cDNA clone that encodes a protein with properties of E2F. This clone, RBP3, was identified by the ability of its gene product to interact with pRB. RBP3 bound to pRB both in vitro and in vivo, and this binding was competed by viral proteins known to disrupt pRB-E2F association. RBP3 bound to E2F recognition sequences in a sequence-specific manner. Furthermore, transient expression of RBP3 caused a 10-fold transactivation of the adenovirus E2 promoter, and this transactivation was dependent on the E2F recognition sequences. These properties suggest that RBP3 encodes E2F, or an E2F-like protein.

Expression cloning of a cDNA encoding a retinoblastoma-binding protein with E2F-like properties

An expression vector was modified to permit the rapid synthesis of purified, 32P-labeled, glutathione S-transferase (GST)-retinoblastoma (RB) fusion proteins. The products were used to screen lambda gt11 expression libraries, from which we cloned a cDNA encoding a polypeptide (RBAP-1) capable of binding directly to a putative functional domain (the pocket) of the retinoblastoma gene product (RB). The RB "pocket" is known to bind, directly or indirectly, to the cellular transcription factor, E2F, implicated in cell growth control. We have found that RBAP-1 copurifies with E2F, interacts specifically with the adenovirus E4 ORF 6/7 protein, binds specifically and directly to a known E2F DNA recognition sequence, and contains a functional tranasactivation domain. Therefore, RBAP-1 is a species of E2F and can bind specifically to the RB pocket.

Retinoblastoma gene product activates expression of the human TGF-beta 2 gene through transcription factor ATF-2

The retinoblastoma susceptibility gene product (pRb) plays an important role in constraining cellular proliferation and in regulating the cell cycle. The pRb inhibits transcription of genes involved in growth control (reviewed in ref. 3) and can regulate transforming growth factor beta 1 (TGF-beta 1) gene expression. TGF-beta isoforms also down-regulate cellular proliferation. To determine whether pRb also regulates expression of other TGF-beta isoforms, we examined the effect of pRb on the expression of the human TGF-beta 2 gene. The human TGF-beta 2 promoter contains multiple elements including an ATF site, which is essential for basal promoter activity. Here we report that pRb activates transcription of the human TGF-beta 2 gene. The promoter element responsible for pRb-mediated transcriptional regulation is a binding site for ATF proteins, an extensive transcription factor family. We provide evidence that implicates ATF-2 in pRb-responsiveness. First, the ATF promoter element in the TGF-beta 2 gene is a high-affinity ATF-2-binding site. Second, a GAL4-ATF2 fusion protein can support pRb-mediated transcriptional activation of a promoter containing GAL4-binding sites. Third, ATF-2 in nuclear extracts can interact with pRb. Our results reveal a new mechanism by which pRb constrains cellular proliferation: by activating expression of the inhibitory growth factor, TGF-beta 2.

Retinoblastoma protein switches the E2F site from positive to negative element

Originally E2F sites were identified as elements in the promoters of adenovirus early genes that are necessary for activation of these genes by the early protein E1a (ref. 1). E2F promoter elements have been shown to be important for transcriptional activation of several genes critical for progression through the cell cycle. During the G1 phase of the cell cycle, the E2F protein forms a complex with the cell-cycle protein Rb (ref. 5) and it has been suggested that this binding of Rb to E2F inactivates E2F (ref. 5). Here we show that Rb-E2F is an active complex that, when bound to the E2F site, inhibits the activity of other promoter elements and thus silences transcription. We propose that the ability of this complex to inhibit transcription is integral to the function of Rb and provide evidence that E2F is a positive element in the absence of an active form of Rb. It has been shown that binding of Rb to E2F depends on the phosphorylation state of Rb (only the underphosphorylated form binds) and that the phosphorylation state of Rb changes during progression through the cell cycle. We therefore suggest that the E2F site alternates between a positive and negative element with the phosphorylation/dephosphorylation cycle of Rb. This cyclic activity may be responsible for activating and then inhibiting genes during the cell cycle.

Adenovirus E1a prevents the retinoblastoma gene product from repressing the activity of a cellular transcription factor

The retinoblastoma (Rb) gene product forms a complex with the cellular transcription factor DRTF1, a property assumed to be important for mediating negative growth control because certain viral oncogenes, such as adenovirus E1a, prevent this interaction and mutant Rb alleles, which have lost the capacity to regulate growth, encode proteins that fail to associate with DRTF1. In this study, we show that the wild-type Rb protein can specifically repress transcription from promoters driven by DRTF1 whereas a naturally occurring mutant Rb protein cannot. Furthermore, Rb-mediated transcriptional repression can be overridden by adenovirus E1a; this requires regions in E1a necessary for cellular transformation. The Rb protein therefore acts in trans to repress the transcriptional activity of DRTF1 whereas adenovirus E1a prevents this interaction and thus maintains DRTF1 in a constitutively active state. The Rb protein and adenovirus E1a therefore have opposite effects on the activity of a common molecular target. Transcriptional repression mediated by the Rb protein and inactivation of repression by the E1a protein are likely to play an important role in mediating their biological effects.

Production of human papillomavirus and modulation of the infectious program in epithelial raft cultures. OFF

Human papillomaviruses trophic for anogenital epithelia cause benign warts, and certain genotypes are closely associated with cervical neoplasia. By using our modifications of the epithelial raft culture system, we were able to recapitulate and modulate the infectious program of a papillomavirus in vitro for the first time. Small pieces of a condyloma containing human papillomavirus type 11 were explanted onto a dermal equivalent consisting of a collagen matrix with fibroblasts and were cultured at the medium-air interface. The infected stem cells proliferated rapidly across the matrix, stratified, and differentiated, as judged by histology. The results correlated with the state of epithelial differentiation, which, in turn, was dependent on the type of fibroblast in the matrix. Under conditions where the epithelial outgrowth underwent terminal differentiation, the entire productive program took place, leading to virion assembly. In contrast, using an alternative condition where the outgrowth failed to achieve terminal differentiation, only the E-region RNAs from the E1 promoter accumulated to any appreciable extent. The proliferating cell nuclear antigen was induced in the differentiated suprabasal cells in the productive cyst growth, which also exhibited high copy viral DNA and abundant E6-E7 RNAs. Comparable cells in the nonproductive cyst outgrowth were negative for all three. These results suggest that the E6 and E7 proteins may play a role in establishing a cellular environment conducive to vegetative viral replication. The culture conditions described should be useful for genetic analysis of this family of important human pathogens and for testing potential pharmacological agents.

Adenovirus E1A makes two distinct contacts with the retinoblastoma protein

Two regions near the amino terminus of the adenovirus E1A protein, which were first identified by sequence conservation among various adenovirus serotypes, have been shown by genetic studies to be essential for E1A-mediated transformation. These same regions are also required for interaction with a number of cellular proteins, including the retinoblastoma protein (pRB). Using synthetic peptides corresponding to portions of these conserved regions, we show that each region can bind independently to pRB. These interactions were observed in both competition and binding assays. In both types of assay, region 2 peptides (E1A amino acids 115 to 132) bound pRB with higher affinity than did region 1 peptides (E1A amino acids 37 to 54), while a peptide combining region 1 and 2 sequences consistently provided the highest-affinity interaction. Cross-blocking experiments using region 1 peptides and region 2 peptides suggested that these two regions of E1A make distinct contacts with pRB. These data support the notion that the pRB-binding domain of E1A contains at least two functional elements.

The retinoblastoma gene product regulates Sp1-mediated transcription

We have demonstrated that the retinoblastoma gene product (Rb) can positively regulate transcription from the fourth promoter of the insulinlike growth factor II gene. Two copies of a motif (the retinoblastoma control element) similar to that found in the human c-fos, transforming growth factor beta 1, and c-myc promoters are responsible for conferring Rb regulation to the fourth promoter of the insulinlike growth factor II gene. We have shown that the transcription factor Sp1 can bind to and stimulate transcription from the retinoblastoma control element motif. Moreover, by using a GAL4-Sp1 fusion protein, we have directly demonstrated that Rb positively regulates Sp1 transcriptional activity in vivo. These results indicate that Rb can function as a positive regulator of transcription and that Sp1 is one potential target, either directly or indirectly, for transcriptional regulation by Rb.

DNA-binding properties of the E1A-associated 300-kilodalton protein

One of the major E1A-associated cellular proteins is a 300-kDa product (p300) that binds to the N-terminal region of the E1A products. The p300 binding site is distinct from sequences involved in binding the retinoblastoma product and other E1A-associated cellular products such as p60-cyclin A and p107. p300 binding to E1A is linked genetically to the enhancer repression function of E1A and the other E1A-mediated gene-regulating functions as well as to the transforming functions of E1A. However, the biochemical properties of p300 have not yet been characterized. We report here that p300 has an intrinsic DNA-binding activity and shows a preferential affinity for specific DNA sequences. The sequences selectively bound by p300 are related to those of a series of enhancer elements that are recognized by NF-kappa B. The direct physical interaction of p300 with enhancer elements provides a biochemical basis for the genetic evidence linking the E1A-mediated enhancer repression function with the p300-binding activity of E1A.

Transcription factors and the control of DNA replication

Initiation of DNA replication is mediated by the assembly of nucleoprotein complexes at cis-acting DNA sequences known as origins of replication. Recent studies in several systems show that accessory transcription factors accentuate origin utilization by multiple mechanisms. The remarkable similarities in the activities of accessory transcription factors at promoters and origins of replication suggest that transcription factors play a pivotal role in the regulation of chromosomal DNA synthesis in eukaryotic organisms.

TGF-beta regulation of epithelial cell proliferation

The predominant effect of TGF-beta 1 on cell proliferation is inhibition. Earlier studies demonstrated that TGF-beta 1 inhibition of skin keratinocyte proliferation involves suppression of c-myc transcription and indirect evidence suggested that the protein product of the retinoblastoma gene (pRB) may be involved in this process. Skin keratinocytes transformed by SV40 and human papilloma virus-16 (HPV-16) or HPV-18 resisted growth inhibition and suppression of c-myc mRNA by TGF-beta. Transient expression of HPV-16 E7 gene, adenovirus E1A, and SV40 large T antigen (TAg) blocked the TGF-beta 1 suppression of c-myc transcription. Studies with transformation-defective mutants of E1A and TAg suggested that a cellular protein(s) that interacts with a conserved domain of the DNA tumor virus oncoproteins mediates TGF-beta 1 suppression of c-myc transcription and keratinocyte growth. Transient expression of pRB in skin keratinocytes repressed human c-myc promoter/CAT transcription as effectively as TGF-beta 1. The same c-myc promoter region, termed the TGF-beta Control Element (TCE), was required for regulation by both TGF-beta 1 and pRB. TCE bound a cellular protein of approximately 106 kDa and this binding was decreased by TGF-beta 1 treatment. Our data indicate that pRB can inhibit c-myc transcription and suggest the involvement of cellular factor(s) in addition to pRB in the TGF-beta 1 pathway for the suppression of c-myc transcription and growth inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a growth suppression domain within the retinoblastoma gene product

To date, all naturally occurring retinoblastoma susceptibility gene (RB) mutations known to be compatible with stable protein expression map to the T/E1A and cellular protein-binding region (the "pocket" domain). This domain extends from residue 379 to 792. When full-length RB and certain truncated forms were synthesized in human RB -/- cells, we found that the minimal region necessary for overt growth suppression extended from residue 379 to 928. A functional pocket domain and sequences extending from the carboxy-terminal boundary of the pocket to the carboxyl terminus of the protein were both necessary for growth suppression. Both sets of sequences were also required for E2F binding; hence, the two functions may be linked.

Transforming growth factor beta 1 (TGF beta 1) reduces cellular levels of p34cdc2, and this effect is abrogated by adenovirus independently of the E1A-associated pRB binding activity

We have used E1A probes to study the roles of the p34cdc2 kinase and the retinoblastoma tumor susceptibility gene product (pRB) in transforming growth factor beta 1 (TGF beta 1)-mediated growth suppression in mink lung epithelial (Mv1Lu) cells. In agreement with previous reports, we see a decline in p34cdc2 kinase activity and a loss of pRB phosphorylation after TGF beta 1 treatment. We report here that TGF beta 1 induces not only a change in p34cdc2 kinase activity but a strong repression of p34cdc2 synthesis. Loss of p34cdc2 kinase activity is not seen until the steady-state level of p34cdc2 declines, suggesting that the intra-cellular signals induced by TGF beta 1 affect p34cdc2 at the level of expression, rather than by altering the posttranslational modifications of p34cdc2 that regulate its kinase activity. Infection with adenovirus expressing either wild-type E1A or a mutant E1A (pm928) defective for pRB binding alleviated TGF beta 1-mediated suppression of DNA synthesis, indicating that E1A does not need to bind pRB physically to keep cell growth-suppressing functions from being activated by TGF beta 1. The E1A.928 mutant virus is able to maintain p34cdc2 expression and kinase activity, as well as pRB phosphorylation in the presence of TGF beta 1, which may account for its ability to maintain cell cycle activity without directly sequestering pRB. Overall our results suggest that TGF beta 1 acts by signaling changes at the level of control of G1 gene expression, not at the level of posttranslational modification of p34cdc2 or its substrates.

DNA-binding properties of the E1A-associated 300-kilodalton protein

One of the major E1A-associated cellular proteins is a 300-kDa product (p300) that binds to the N-terminal region of the E1A products. The p300 binding site is distinct from sequences involved in binding the retinoblastoma product and other E1A-associated cellular products such as p60-cyclin A and p107. p300 binding to E1A is linked genetically to the enhancer repression function of E1A and the other E1A-mediated gene-regulating functions as well as to the transforming functions of E1A. However, the biochemical properties of p300 have not yet been characterized. We report here that p300 has an intrinsic DNA-binding activity and shows a preferential affinity for specific DNA sequences. The sequences selectively bound by p300 are related to those of a series of enhancer elements that are recognized by NF-kappa B. The direct physical interaction of p300 with enhancer elements provides a biochemical basis for the genetic evidence linking the E1A-mediated enhancer repression function with the p300-binding activity of E1A.

The retinoblastoma gene product regulates Sp1-mediated transcription

We have demonstrated that the retinoblastoma gene product (Rb) can positively regulate transcription from the fourth promoter of the insulinlike growth factor II gene. Two copies of a motif (the retinoblastoma control element) similar to that found in the human c-fos, transforming growth factor beta 1, and c-myc promoters are responsible for conferring Rb regulation to the fourth promoter of the insulinlike growth factor II gene. We have shown that the transcription factor Sp1 can bind to and stimulate transcription from the retinoblastoma control element motif. Moreover, by using a GAL4-Sp1 fusion protein, we have directly demonstrated that Rb positively regulates Sp1 transcriptional activity in vivo. These results indicate that Rb can function as a positive regulator of transcription and that Sp1 is one potential target, either directly or indirectly, for transcriptional regulation by Rb.

Myc and Max: a putative transcriptional complex in search of a cellular target

Biochemical and genetic observations have supported the hypothesis that Myc family proteins function to regulate genes important in cellular growth and differentiation. The recent findings that Myc proteins can associate with other cellular proteins, possess sequence-specific DNA-binding activity and may directly transactivate transcription of several candidate genes have provided an experimental framework in which to test the transcription factor model. Based on principles established for several well characterized viral oncoproteins, a model is presented in which the regulation of Myc function is controlled by specific cellular protein interactions that serve to activate or repress transactivation activity or deny access of the Myc complex to its target sequences.

Wild-type murine p53 represses transcription from the murine c-myc promoter in a human glial cell line

Here we analyzed the effect of the suppressor proto-oncogene p53 on transcription from the P2 promoter of the murine c-myc gene. c-myc promoter constructs were coupled to the chloramphenicol acetyl-transferase (CAT) gene and were transiently transfected into a human glial cell along with plasmids overexpressing wild-type or mutant p53. It was found that significant repression of c-myc transcription took place following cotransfection with wild-type but not mutant p53. However wild-type p53 did not suppress transcription from the SV40 early promoter or from the MHC promoter. Promoter-CAT constructs containing only the ME1a2 or E2F elements, from the P2 promoter, were repressed by p53, indicating that p53 may exert its effect at these two sites within the P2 promoter. Finally, when the SV40 T antigen and wild-type p53 were expressed together in glial cells the repressive effect of p53 was abolished.

Chromatin changes during the cell cycle

Considerable progress has recently been made in elucidating the biochemical mechanisms regulating changes in chromatin structure during all stages of the cell cycle. Although anticipated, the apparently ubiquitous role played by phosphorylation/dephosphorylation reactions in modulating these changes is, nonetheless, remarkable.

Transforming growth factor beta induces myoblast differentiation in the presence of mitogens

Transforming growth factor beta 1 (TGF-beta 1) added to L6E9 rat skeletal myoblasts in mitogen-rich medium induces a rapid decrease in c-myc expression and delays progression through the G1 phase of the cell cycle. This growth inhibitory response is followed by cell commitment to terminal differentiation with elevated expression of myogenin muscle determination genes, induction of muscle-specific proteins, and formation of multinucleated myotubes. These results suggest that TGF-beta 1 may act as a physiological inducer of myogenic differentiation in mitogen-rich environments, and its otherwise reversible growth inhibitory effect may become permanent if coupled to induction of terminal differentiation.

Human papillomavirus type 16 E7 protein inhibits DNA binding by the retinoblastoma gene product

The human papillomavirus E7 gene can transform murine fibroblasts and cooperate with other viral oncogenes in transforming primary cell cultures. One biochemical property associated with the E7 protein is binding to the retinoblastoma tumor suppressor gene product (pRB). Biochemical properties associated with pRB include binding to viral transforming proteins (E1A, large T, and E7), binding to cellular proteins (E2F and Myc), and binding to DNA. The mechanism by which E7 stimulates cell growth is uncertain. However, E7 binding to pRB inhibits binding of cellular proteins to pRB and appears to block the growth-suppressive activity of pRB. We have found that E7 also inhibits binding of pRB to DNA. A 60-kDa version of pRB (pRB60) produced in reticulocyte translation reactions or in bacteria bound quantitatively to DNA-cellulose. Recombinant E7 protein used at a 1:1 or 10:1 molar ratio with pRB60 blocked 50 or greater than 95% of pRB60 DNA-binding activity, respectively. A mutant E7 protein (E7-Ala-24) with reduced pRB60-binding activity exhibited a parallel reduction in its blocking of pRB60 binding to DNA. An E7(20-29) peptide that blocks binding of E7 protein to pRB60 restored the DNA-binding activity of pRB60 in the presence of E7. Peptide E7(2-32) did not block pRB60 binding to DNA, while peptide E7(20-57) and an E7 fragment containing residues 1 to 60 partially blocked DNA binding. E7 species containing residues 3 to 75 were fully effective at blocking pRB60 binding to DNA. These studies indicate that E7 protein specifically blocks pRB60 binding to DNA and suggest that the E7 region responsible for this property lies between residues 32 and 75. The functional significance of these observations is unclear. However, we have found that a point mutation in pRB60 that impairs DNA-binding activity also blocks the ability of pRB60 to inhibit cell growth. This correlation suggests that the DNA-binding activity of retinoblastoma proteins contributes to their biological properties.

Efficiency of binding the retinoblastoma protein correlates with the transforming capacity of the E7 oncoproteins of the human papillomaviruses

The human papillomaviruses (HPVs) associated with genital tract lesions can be classified as either "high risk" or "low risk" based on their association with human anogenital cancer. The E7 proteins of the high-risk and the low-risk viruses are quite similar in their amino acid composition and structural organization yet differ in their transforming potential and in a number of biochemical properties. A series of chimeric proteins consisting of segments of the high-risk HPV-16 and the low-risk HPV-6 E7 proteins were constructed in order to define which domains within the amino-terminal half of E7 were responsible for the different biological and biochemical properties. The E7 oncogenic capacity, which was determined by assaying transformation of baby rat kidney cells in cooperation with an activated ras oncogene, segregated with the retinoblastoma tumor suppressor protein (pRB) binding domain of the HPV-16 E7 protein. A comparison of the pRB binding sites of the sequenced genital tract HPVs revealed a consistent amino acid difference (aspartic acid/glycine) between the high-risk and low-risk viruses. Single amino acid substitution mutations were generated at this position in the HPV-6 and HPV-16 E7 proteins, and this single amino acid residue was shown to be the principal determinant responsible for the differences in the apparent pRB binding affinity and transformation capacity distinguishing the HPV E7 proteins of the high-risk and low-risk HPVs.

Adenovirus E1A, simian virus 40 tumor antigen, and human papillomavirus E7 protein share the capacity to disrupt the interaction between transcription factor E2F and the retinoblastoma gene product

The adenovirus E1A gene product, the simian virus 40 large tumor antigen, and the human papillomavirus E7 protein share a short amino acid sequence that constitutes a domain required for the transforming activity of these proteins. These sequences are also required for these proteins to bind to the retinoblastoma gene product (pRb). Recent experiments have shown that E1A can dissociate complexes containing the transcription factor E2F bound to pRb, dependent on this conserved sequence element. We now show that the E7 protein and the simian virus 40 large tumor antigen can dissociate the E2F-pRb complex, dependent on this conserved sequence element. We also find that the E2F-pRb complex is absent in various human cervical carcinoma cell lines that either express the E7 protein or harbor an RB1 mutation, suggesting that the loss of the E2F-pRb interaction may be an important aspect in human cervical carcinogenesis. We suggest that the ability of E1A, the simian virus 40 large tumor antigen, and E7 to dissociate the E2F-pRb complex may be a common activity of these viral proteins that has evolved to stimulate quiescent cells into a proliferating state so that viral replication can proceed efficiently. In circumstances in which a lytic infection does not proceed, the consequence of this action may be to initiate the oncogenic process in a manner analogous to the mutation of the RB1 gene.

Identification of critical cis elements involved in mediating Epstein-Barr virus nuclear antigen 2-dependent activity of an enhancer located upstream of the viral BamHI C promoter

The six genes encoding the Epstein-Barr virus nuclear antigens (EBNAs) are transcribed from one of two promoters, BamHI C promoter (Cp) or BamHI W promoter (Wp), located near the left end of the viral genome. During the establishment of viral latency in B lymphocytes, Wp is used exclusively before a switch to Cp usage. We and others have previously identified an enhancer in the region upstream of Cp which requires EBNA 2 for activity (M. Woisetschlaeger, X. W. Jin, C. N. Yandava, L. A. Furmanski, J. L. Strominger, and S. H. Speck, Proc. Natl. Acad. Sci. USA 88:3942-3946, 1991; N. S. Sung, S. Kenney, D. Gutsch, and J. S. Pagano, J. Virol. 65:2164-2169, 1991). Infection of B lymphocytes with a mutant virus lacking the EBNA 2 gene results in prolonged usage of Wp and failure to switch to Cp usage, indicating that EBNA 2 transactivation of the enhancer upstream of Cp may be critical for promoter switching. In this study, we have defined the minimal EBNA 2-dependent enhancer by using a series of deletion mutants. The results of site-directed mutagenesis revealed that there are three regions of the enhancer that are important for activity, two of which appear to bind B-lymphocyte-specific factors.

Human papillomavirus type 16 E7 protein inhibits DNA binding by the retinoblastoma gene product

The human papillomavirus E7 gene can transform murine fibroblasts and cooperate with other viral oncogenes in transforming primary cell cultures. One biochemical property associated with the E7 protein is binding to the retinoblastoma tumor suppressor gene product (pRB). Biochemical properties associated with pRB include binding to viral transforming proteins (E1A, large T, and E7), binding to cellular proteins (E2F and Myc), and binding to DNA. The mechanism by which E7 stimulates cell growth is uncertain. However, E7 binding to pRB inhibits binding of cellular proteins to pRB and appears to block the growth-suppressive activity of pRB. We have found that E7 also inhibits binding of pRB to DNA. A 60-kDa version of pRB (pRB60) produced in reticulocyte translation reactions or in bacteria bound quantitatively to DNA-cellulose. Recombinant E7 protein used at a 1:1 or 10:1 molar ratio with pRB60 blocked 50 or greater than 95% of pRB60 DNA-binding activity, respectively. A mutant E7 protein (E7-Ala-24) with reduced pRB60-binding activity exhibited a parallel reduction in its blocking of pRB60 binding to DNA. An E7(20-29) peptide that blocks binding of E7 protein to pRB60 restored the DNA-binding activity of pRB60 in the presence of E7. Peptide E7(2-32) did not block pRB60 binding to DNA, while peptide E7(20-57) and an E7 fragment containing residues 1 to 60 partially blocked DNA binding. E7 species containing residues 3 to 75 were fully effective at blocking pRB60 binding to DNA. These studies indicate that E7 protein specifically blocks pRB60 binding to DNA and suggest that the E7 region responsible for this property lies between residues 32 and 75. The functional significance of these observations is unclear. However, we have found that a point mutation in pRB60 that impairs DNA-binding activity also blocks the ability of pRB60 to inhibit cell growth. This correlation suggests that the DNA-binding activity of retinoblastoma proteins contributes to their biological properties.

Cell cycle regulation of the human cdc2 gene

Transcription of the human cdc2 gene is cell cycle regulated and restricted to proliferating cells. Nuclear run-on assays show that cdc2 transcription is high in S and G2 phases of the cell cycle but low in G1. To investigate transcriptional control further, genomic clones of the human cdc2 gene containing 5' flanking sequences were isolated and shown to function as a growth regulated promoter in vivo when fused to a CAT reporter gene. In primary human fibroblasts, the human cdc2 promoter is negatively regulated by arrest of cell growth in a similar fashion to the endogenous gene. This requires specific 5' flanking upstream negative control (UNC) sequences which mediate repression. The retinoblastoma susceptibility gene product (Rb) specifically represses cdc2 transcription in cycling cells via 136 bp of 5' flanking sequence located between -245 and -109 within the UNC region. E2F binding sites in this region were shown to be essential for optimal repression. A model is proposed where Rb negatively regulates the cdc2 promoter in non-cycling and cycling G1 cells.

Thymidine kinase transcription is regulated at G1/S phase by a complex that contains retinoblastoma-like protein and a cdc2 kinase

Transcription of the murine thymidine kinase gene, which is coregulated with the G1/S phase transition, is activated by changing the binding of protein complexes Yi1 and Yi2 to three upstream DNA motifs. Yi1 is replaced by Yi2 shortly before S phase. Yi1 contains a protein of 110 kDa that binds to the DNA motif sites and may be an underphosphorylated murine retinoblastoma protein, shown by its molecular mass, timing of its activity, and antibody recognition. An H1 kinase related to cdc2 cofractionates with both complexes. We propose that this kinase phosphorylates the murine retinoblastoma protein, releasing transcriptional inhibitions by Yi1 and permitting cell cycle progression. These results provide a cycle-related molecular target for such complexes. They are based on investigations of cycle control in uninfected cells. The Yi complexes are similar but not identical to complexes that include a cellular protein, E2F, that was originally found to bind to adenovirus DNA.

The retinoblastoma protein physically associates with the human cdc2 kinase

The protein product (pRB) of the retinoblastoma susceptibility gene functions as a negative regulator of cell proliferation, and its activity appears to be modulated by phosphorylation. Using a new panel of anti-human pRB monoclonal antibodies, we have investigated the biochemical properties of this protein. These antibodies have allowed us to detect a pRB-associated kinase that has been identified as the cell cycle-regulating kinase p34cdc2 or a closely related enzyme. Since this associated kinase phosphorylates pRB at most of the sites used in vivo, these results suggest that this kinase is one of the major regulators of pRB. The associated kinase activity follows the pattern of phosphorylation seen for pRB in vivo. The associated kinase activity is not seen in the G1 phase but appears in the S phase, and the levels continue to increase throughout the remainder of the cell cycle.

The HIP1 binding site is required for growth regulation of the dihydrofolate reductase gene promoter

The transcription rate of the dihydrofolate reductase (DHFR) gene increases at the G1/S boundary of the proliferative cell cycle. Through analysis of transiently and stably transfected NIH 3T3 cells, we have now demonstrated that DHFR promoter sequences extending from -270 to +20 are sufficient to confer similar regulation on a reporter gene. Mutation of a protein binding site that spans sequences from -16 to +11 in the DHFR promoter resulted in loss of the transcriptional increase at the G1/S boundary. Purification of an activity from HeLa nuclear extract that binds to this region enriched for a 180-kDa polypeptide (HIP1). Using this HIP1 preparation, we have identified specific positions within the binding site that are critical for efficient protein-DNA interactions. An analysis of association and dissociation rates suggests that bound HIP1 protein can exchange rapidly with free protein. This rapid exchange may facilitate the burst of transcriptional activity from the DHFR promoter at the G1/S boundary.

Structure-function analysis of the human papillomavirus type 16 E7 oncoprotein

The E7 gene of human papillomavirus type 16 encodes a multifunctional nuclear phosphoprotein that is functionally and structurally similar to the adenovirus (Ad) E1A proteins and the T antigens of other papovaviruses. E7 can cooperate with an activated ras oncogene to transform primary rodent cells, trans activate the Ad E2 promoter, and abrogate transforming growth factor beta-mediated repression of c-myc. Recent studies suggest that these functions may in part be a consequence of the ability of E7 to associate with the product of the retinoblastoma tumor suppressor gene (pRB). In this study, a series of site-specific mutations of the human papillomavirus type 16 E7 gene product were constructed and assessed for their effects on intracellular protein stability, ras cooperativity, transcriptional trans activation, pRB association, and phosphorylation. The results of these studies indicate that the transforming and trans-activating domains extensively overlap within a region of the protein analogous to conserved region 2 of Ad E1A, suggesting that pRB binding is necessary for both activities. Deletion of sequences in conserved region 1 abrogates cellular transformation but has only a marginal effect on trans activation. These data suggest that E7 trans activation and cellular transformation are interrelated but separable functions.

Tumor suppressor genes in the cell cycle

In the past year, two tumor suppressor genes, retinoblastoma and p53, have been established as important players in cell-cycle control, mediated by phosphorylation and by stage-specific transcription complexes. Evidence that they also participate in other transcription complexes is accumulating and searches are underway for the downstream genes under their regulation. Genes down-regulated in tumor cells are being screened by subtractive hybridization to bridge the gap between transcription factors and their targets.

G1 events and the regulation of genes for S-phase enzymes

The genes for S-phase enzymes are expressed at low levels in quiescent mammalian cells but at high levels during DNA replication. Regulation occurs at multiple levels by mechanisms that are different for each gene. Current research is focused on identifying the control elements and trans-acting factors for each gene and establishing relationships between these regulatory mechanisms and the G1 signal transduction pathway.

Simian virus 40 small t antigen trans activates the adenovirus E2A promoter by using mechanisms distinct from those used by adenovirus E1A

As reported previously for simian virus 40 small t antigen, polyomavirus small t antigen stimulates transcription directed by the adenovirus E2A and VA-I promoters during transient transfection assays. To determine whether papovaviral small t antigens might employ biochemical mechanisms during transcription activation that are either similar to or distinct from other viral trans activators, I compared the abilities of simian virus 40 small t antigen and adenovirus E1A to regulate the E2A promoter during transient transfection assays. I determined that, whereas activation of the E2A promoter by E1A involves the transcription factors ATF and EIIF, activation by small t antigen involves only EIIF. The effects of cotransfecting maximal concentrations of plasmids encoding small t antigen with E1A suggested that they activate the E2A promoter by different mechanisms. To determine whether small t antigen employs a mechanism different from that encoded in E1A domain II, domain III, or both, I compared the effects of transfecting plasmids expressing small t antigen, the 12S product of E1A, or the 13S product with a mutation in domain II on trans activation of the E2A promoter in two cellular backgrounds. On the basis of these comparisons, it appears that small t antigen does not activate transcription by a mechanism similar to either of the activities encoded in E1A. This suggests that papovavirus small t antigens belong to a distinct class of trans-acting proteins.

Cell proliferation and control

In the past year much of the focus in cell-cycle research has turned from the regulation of mitosis to the control of the initiation of DNA replication. Novel findings include the discovery of vertebrate G1 cyclins, an additional cdc2-related kinase potentially involved in G1 phase, and a positive-feedback loop regulating the start of the cell cycle in yeast.

Simian virus 40 large T antigen stably complexes with a 185-kilodalton host protein

Stable interactions between simian virus 40 large T antigen and host proteins are believed to play a major role in the ability of the viral protein to transform cells in culture and induce tumors in vivo. Two of these host proteins, the retinoblastoma susceptibility protein (pRB) and p53, are products of tumor suppressor genes, suggesting that T antigen exerts at least a portion of its transforming activity by complexing with and inactivating the function of these proteins. While analyzing T antigen-host protein complexes in mouse cells, we noted a protein of 185 kDa (p185) which specifically coimmunoprecipitates with T antigen. Coimmunoprecipitation results from the formation of stable complexes between T antigen and p185. Complex formation is independent of the interactions of T antigen with pRB, p120, and p53. Furthermore, analysis of T-antigen mutants suggests that T antigen-p185 complex formation may be important in transformation by simian virus 40.

How do retinoblastoma tumours form?

The causes of retinoblastoma (RB) can now be described with considerable accuracy, although many details are still unclear. Understanding the genetic changes leading to RB has provided an awareness of general mechanisms of cancer development and progression, previously only suspected. From the basic understanding have come new diagnostic technologies that are now ready to be applied directly to RB patients and their families, and a rational approach, based on this understanding, will help us to develop new therapies that avoid the severe complications of conventional treatment.

The retinoblastoma protein physically associates with the human cdc2 kinase

The protein product (pRB) of the retinoblastoma susceptibility gene functions as a negative regulator of cell proliferation, and its activity appears to be modulated by phosphorylation. Using a new panel of anti-human pRB monoclonal antibodies, we have investigated the biochemical properties of this protein. These antibodies have allowed us to detect a pRB-associated kinase that has been identified as the cell cycle-regulating kinase p34cdc2 or a closely related enzyme. Since this associated kinase phosphorylates pRB at most of the sites used in vivo, these results suggest that this kinase is one of the major regulators of pRB. The associated kinase activity follows the pattern of phosphorylation seen for pRB in vivo. The associated kinase activity is not seen in the G1 phase but appears in the S phase, and the levels continue to increase throughout the remainder of the cell cycle.

The retinoblastoma-susceptibility gene product becomes phosphorylated in multiple stages during cell cycle entry and progression

The retinoblastoma-susceptibility gene product (RB) undergoes cell cycle-dependent phosphorylation and dephosphorylation. We characterized RB phosphorylation after mitogenic stimulation of primary human T lymphocytes, initially arrested in the G0 state. RB is phosphorylated in at least three steps when T cells are driven into the cell cycle. The first event occurs during mid G1 phase, the second during S phase, and the third in G2/M. Tryptic phosphopeptide mapping indicates that the different phosphorylation events occur, at least in part, on different residues in RB. Given the known relationship of the RB phosphorylation state to function, it is possible that RB regulates growth at multiple points in the cell cycle.

Cyclin A is required at two points in the human cell cycle

Cyclins play a fundamental role in regulating cell cycle events in all eukaryotic cells. The human cyclin A gene was identified as the site of integration of hepatitis B virus in a hepatocarcinoma cell line; in addition, cyclin A is associated with the E2F transcription factor in a complex which is dissociated by the E1A oncogene product. Such findings suggest that cyclin A is a target for oncogenic signals. We have now found that DNA synthesis and entry into mitosis are inhibited in human cells microinjected with anti-cyclin A antibodies at distinct times. Cyclin A binds both cdk2 and cdc2, giving two distinct cyclin A kinase activities, one appearing in S phase, the other in G2. These results suggest that cyclin A defines novel control points of the human cell cycle.