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

Cloning and characterization of E2F-2, a novel protein with the biochemical properties of transcription factor E2F

E2F is a mammalian transcription factor that appears to play an important role in cell cycle regulation. While at least two proteins (E2F-1 and DP-1) with E2F-like activity have been cloned, studies from several laboratories suggest that additional homologs may exist. A novel protein with E2F-like properties, designated E2F-2, was cloned by screening a HeLa cDNA library with a DNA probe derived from the DNA binding domain of E2F-1 (K. Helin, J. A. Lees, M. Vidal, N. Dyson, E. Harlow, and A. Fattaey, Cell 70:337-350, 1992). E2F-2 exhibits overall 46% amino acid identity to E2F-1. Both the sequence and the function of the DNA and retinoblastoma gene product binding domains of E2F-1 are conserved in E2F-2. The DNA binding activity of E2F-2 is dramatically enhanced by complementation with particular sodium dodecyl sulfate-polyacrylamide gel electrophoresis-purified components of HeLa cell E2F, and anti-E2F-2 antibodies cross-react with components of purified HeLa cell E2F. These observations are consistent with a model in which E2F binds DNA as a heterodimer of two distinct proteins, and E2F-2 is functionally and immunologically related to one of these proteins.

The retinoblastoma protein binds to a family of E2F transcription factors

E2F is a transcription factor that helps regulate the expression of a number of genes that are important in cell proliferation. Recently, several laboratories have isolated a cDNA clone that encodes an E2F-like protein, known as E2F-1. Subsequent characterization of this protein showed that it had the properties of E2F, but it was difficult to account for all of the suggested E2F activities through the function of this one protein. Using low-stringency hybridization, we have isolated cDNA clones that encode two additional E2F-like proteins, called E2F-2 and E2F-3. The chromosomal locations of the genes for E2F-2 and E2F-3 were mapped to 1p36 and 6q22, respectfully, confirming their independence from E2F-1. However, the E2F-2 and E2F-3 proteins are closely related to E2F-1. Both E2F-2 and E2F-3 bound to wild-type but not mutant E2F recognition sites, and they bound specifically to the retinoblastoma protein in vivo. Finally, E2F-2 and E2F-3 were able to activate transcription of E2F-responsive genes in a manner that was dependent upon the presence of at least one functional E2F binding site. These observations suggest that the E2F activities described previously result from the combined action of a family of proteins.

Overproduction of Rb protein after the G1/S boundary causes G2 arrest

The Rb protein is known to exert its activity at decision points in the G1 phase of the cell cycle. To investigate whether it may also play some role(s) at later points in the cell cycle, we used a system of rapid inducible gene amplification to conditionally overexpress Rb protein during G2 phase. A cell line expressing a temperature-sensitive simian virus 40 large T antigen (T-Ag) was stably transfected with plasmids containing the Rb cDNA linked to the simian virus 40 origin of replication: pRB-wt, pRB-fs, and pRB-Dra, carrying wild-type murine Rb cDNA, a frameshift mutation close to the beginning of the Rb coding region, and a single-amino-acid deletion in the E1A/T-Ag binding pocket, respectively. Numerous independent cell lines were isolated at the nonpermissive temperature; cell lines displaying a high level of episomal amplification of an intact Rb expression cassette following shiftdown to the permissive temperature were chosen for further analysis. Plasmid pRB-fs did not express detectable Rb antigen, while pRB-Dra expressed full-length Rb protein. The Dra mutation has previously been shown to abrogate phosphorylation as well as T-Ag binding. Fluorescence-activated cell sorting (FACS) analysis revealed that cultures induced to overexpress either wild-type or Dra mutant Rb proteins were significantly enriched for cells with a G2 DNA content. Cultures that amplified pRB-fs or rearranged pRB-wt and did not express Rb protein had normal cell cycle profiles. Double-label FACS analysis showed that cells overexpressing Rb or Rb-Dra proteins were uniformly accumulating in G2, whereas cells expressing endogenous levels of Rb were found throughout the cell cycle. These results indicate that Rb protein is interacting with some component(s) of the cell cycle-regulatory machinery during G2 phase.

The retinoblastoma protein binds E2F residues required for activation in vivo and TBP binding in vitro

The retinoblastoma (RB) tumour suppressor protein is capable of repressing the activity of promoters containing DNA binding sites for the transcription factor E2F. Recently a protein which binds RB and possesses the DNA binding characteristics of E2F has been cloned. Here we show that the E2F activation domain is the target for RB-induced repression. RB can silence the 57 residue E2F activation domain but cannot effectively repress an E2F mutant which has reduced RB binding capacity. Extensive mutagenesis of E2F shows residues involved in RB binding are required for transcription activation. Mutations which affect both functions most dramatically lie within the minimal RB binding region. A further subset of sensitive residues lies within a new repeat motif E/DF XX L X P which flanks the minimum RB binding site. These data show that RB can mask E2F residues involved in the activation process, possibly by mimicking a component of the transcriptional machinery. Consistent with this model, we find that the TATA box binding protein TBP can bind to the E2F activation domain in vitro in a manner indistinguishable from that of RB.

Overproduction of Rb protein after the G1/S boundary causes G2 arrest

The Rb protein is known to exert its activity at decision points in the G1 phase of the cell cycle. To investigate whether it may also play some role(s) at later points in the cell cycle, we used a system of rapid inducible gene amplification to conditionally overexpress Rb protein during G2 phase. A cell line expressing a temperature-sensitive simian virus 40 large T antigen (T-Ag) was stably transfected with plasmids containing the Rb cDNA linked to the simian virus 40 origin of replication: pRB-wt, pRB-fs, and pRB-Dra, carrying wild-type murine Rb cDNA, a frameshift mutation close to the beginning of the Rb coding region, and a single-amino-acid deletion in the E1A/T-Ag binding pocket, respectively. Numerous independent cell lines were isolated at the nonpermissive temperature; cell lines displaying a high level of episomal amplification of an intact Rb expression cassette following shiftdown to the permissive temperature were chosen for further analysis. Plasmid pRB-fs did not express detectable Rb antigen, while pRB-Dra expressed full-length Rb protein. The Dra mutation has previously been shown to abrogate phosphorylation as well as T-Ag binding. Fluorescence-activated cell sorting (FACS) analysis revealed that cultures induced to overexpress either wild-type or Dra mutant Rb proteins were significantly enriched for cells with a G2 DNA content. Cultures that amplified pRB-fs or rearranged pRB-wt and did not express Rb protein had normal cell cycle profiles. Double-label FACS analysis showed that cells overexpressing Rb or Rb-Dra proteins were uniformly accumulating in G2, whereas cells expressing endogenous levels of Rb were found throughout the cell cycle. These results indicate that Rb protein is interacting with some component(s) of the cell cycle-regulatory machinery during G2 phase.

Efficient transcriptional activation of many simple modular promoters by simian virus 40 large T antigen

Simian virus 40 (SV40) large T antigen is a multifunctional protein which plays central roles during both lytic and transforming infections by SV40. It is a potent transcriptional activator and increases expression from the SV40 late promoter and from several cellular promoters. To understand better the transcriptional activation activity of large T antigen, we examined its ability to transactivate a set of simple modular promoters containing one of four upstream activation sequences coupled with one of three different TATA box sequences originally constructed and studied by Taylor and Kingston (Mol. Cell. Biol. 10:165-175, 1990). Large T antigen activated transcription from all of these simple promoters. The identity of the TATA box was a more important determinant of the final level of gene expression than was the identity of the upstream activating sequence element. We also determined the ability of a set of mutant SV40 large T antigens to activate a subset of these promoters. Several mutant SV40 large T antigens which had reduced ability to activate the complex SV40 late and Rous sarcoma virus long terminal repeat promoters showed reduced transcriptional activation activity on all of the modular promoters tested. We used a set of promoter derivatives of the human U6 small nuclear RNA promoter containing different TATA boxes and found that wild-type large T antigen could activate transcription from all of them, although to widely different levels of expression.

A genetic analysis of the E2F1 gene distinguishes regulation by Rb, p107, and adenovirus E4

The cellular transcription factor E2F appears to be a target for the regulatory action of the retinoblastoma tumor suppressor gene product. The recent isolation of the E2F1 cDNA clone, which encodes a polypeptide with properties characteristic of E2F, has now allowed a more detailed analysis of the regulation of E2F function by Rb as well as the Rb-related p107 protein and the adenovirus 19-kDa E4 gene product. Previous experiments have shown that each of these regulatory proteins can modulate the activity of cellular E2F. We find that each of these regulatory events can be mediated through the E2F1 product. Moreover, an examination of various E2F1 mutations reveals distinct specificities for these regulatory proteins. For instance, the ability of E4 to alter E2F1 function is dependent upon sequences within a putative leucine repeat of E2F1 as well as within the C-terminal acidic domain. In contrast, the leucine repeat element was not important for Rb- or p107-mediated inhibition of E2F1 activity. Although the C-terminal acidic domain of E2F1, previously shown to be important for Rb binding, appears to be a site for regulation of E2F1 by Rb and p107, point mutations within this region distinguish recognition by Rb and p107. These results underscore the complexity of E2F regulatory interactions and also demonstrate a qualitative distinction in the interactions of Rb and p107 with E2F1, perhaps reflective of functional differences.

Inhibition of E2F-1 transactivation by direct binding of the retinoblastoma protein

Loss of a functional retinoblastoma tumor suppressor gene product, pRB, is a key step in the development of many human tumors. pRB is a negative regulator of cell proliferation and appears to participate in control of entry into the S phase of the cell cycle. The recent demonstration that pRB binds to transcription factor E2F has provided a model for the mechanism of pRB-mediated growth regulation. Since adenovirus E1A proteins dissociate the pRB-E2F complexes and stimulate E2F-dependent transcription, it has been suggested that pRB inhibits E2F transactivation. Although some evidence for this hypothesis has been provided, it has not been possible to determine the mechanism of pRB-mediated inhibition of E2F transactivation. In this study, we constructed mutants of E2F-1 that do not bind to pRB yet retain the ability to transactivate the adenovirus E2 promoter through E2F DNA-binding sites. We demonstrated that transactivation mediated by the wild-type E2F-1 protein was inhibited by overexpression of wild-type pRB but not by a naturally occurring mutant of pRB. Transactivation mediated by mutants of E2F-1 which do not bind to pRB was not affected by overexpression of wild-type pRB. Furthermore, when the E2F-1 transactivation domain was fused to the GAL4 DNA-binding domain, pRB inhibited GAL4-E2F-1 transactivation through GAL4 sites. Expression of pRB did not inhibit transactivation mediated by GAL4-E2F-1 mutant constructs which were devoid of pRB binding. In conclusion, these data demonstrate that pRB inhibits E2F-dependent transactivation by direct protein-protein interaction.

Association of the human papillomavirus type 16 E7 protein with the S-phase-specific E2F-cyclin A complex

The transcription factor E2F has been shown to be involved in the expression of several cell cycle-regulated genes, and the activity of this factor is controlled by cellular proteins such as pRB and p107. E2F is also a target of the DNA virus oncoproteins (adenovirus E1A, simian virus 40 T antigen, and human papillomavirus [HPV] E7) (see the review by J. R. Nevins [Science 258: 424-429, 1992]). These viral oncoproteins dissociate an inactive complex between E2F and the retinoblastoma tumor suppressor protein (pRB), and this dissociation of the E2F-pRB complex correlates with a stimulation of the E2F-dependent transcription. In the S phase of the cell cycle, E2F forms a complex with p107, cyclin A, and the cdk2 kinase (E2F-cyclin A complex). The cellular function of this S-phase-specific complex is unclear. The adenovirus E1A protein dissociates the E2F-cyclin A complex. The HPV type 16 (HPV-16) E7 protein, which possesses significant sequence homology with E1A, does not dissociate the E2F-cyclin A complex. We find that the HPV-16 E7 protein associates very efficiently with the E2F-cyclin A complex. This association is dependent on the sequences that are also necessary for the transforming activity of E7. Moreover, the E7 protein of a low-risk HPV (type 6b) is much less efficient in binding to the E2F-cyclin A complex compared with that of the high-risk type. We also find that the E2F-cyclin A complex remains endogenously associated with the E7 protein in extracts of Caski cells, which express high levels of HPV-16 E7 protein. Finally, we have extensively purified the E2F-cyclin A complex from mouse L-cell extracts and show that, in cell extracts, the E2F-cyclin A complex remains associated with other cellular proteins.

A genetic analysis of the E2F1 gene distinguishes regulation by Rb, p107, and adenovirus E4

The cellular transcription factor E2F appears to be a target for the regulatory action of the retinoblastoma tumor suppressor gene product. The recent isolation of the E2F1 cDNA clone, which encodes a polypeptide with properties characteristic of E2F, has now allowed a more detailed analysis of the regulation of E2F function by Rb as well as the Rb-related p107 protein and the adenovirus 19-kDa E4 gene product. Previous experiments have shown that each of these regulatory proteins can modulate the activity of cellular E2F. We find that each of these regulatory events can be mediated through the E2F1 product. Moreover, an examination of various E2F1 mutations reveals distinct specificities for these regulatory proteins. For instance, the ability of E4 to alter E2F1 function is dependent upon sequences within a putative leucine repeat of E2F1 as well as within the C-terminal acidic domain. In contrast, the leucine repeat element was not important for Rb- or p107-mediated inhibition of E2F1 activity. Although the C-terminal acidic domain of E2F1, previously shown to be important for Rb binding, appears to be a site for regulation of E2F1 by Rb and p107, point mutations within this region distinguish recognition by Rb and p107. These results underscore the complexity of E2F regulatory interactions and also demonstrate a qualitative distinction in the interactions of Rb and p107 with E2F1, perhaps reflective of functional differences.

Association of the human papillomavirus type 16 E7 protein with the S-phase-specific E2F-cyclin A complex

The transcription factor E2F has been shown to be involved in the expression of several cell cycle-regulated genes, and the activity of this factor is controlled by cellular proteins such as pRB and p107. E2F is also a target of the DNA virus oncoproteins (adenovirus E1A, simian virus 40 T antigen, and human papillomavirus [HPV] E7) (see the review by J. R. Nevins [Science 258: 424-429, 1992]). These viral oncoproteins dissociate an inactive complex between E2F and the retinoblastoma tumor suppressor protein (pRB), and this dissociation of the E2F-pRB complex correlates with a stimulation of the E2F-dependent transcription. In the S phase of the cell cycle, E2F forms a complex with p107, cyclin A, and the cdk2 kinase (E2F-cyclin A complex). The cellular function of this S-phase-specific complex is unclear. The adenovirus E1A protein dissociates the E2F-cyclin A complex. The HPV type 16 (HPV-16) E7 protein, which possesses significant sequence homology with E1A, does not dissociate the E2F-cyclin A complex. We find that the HPV-16 E7 protein associates very efficiently with the E2F-cyclin A complex. This association is dependent on the sequences that are also necessary for the transforming activity of E7. Moreover, the E7 protein of a low-risk HPV (type 6b) is much less efficient in binding to the E2F-cyclin A complex compared with that of the high-risk type. We also find that the E2F-cyclin A complex remains endogenously associated with the E7 protein in extracts of Caski cells, which express high levels of HPV-16 E7 protein. Finally, we have extensively purified the E2F-cyclin A complex from mouse L-cell extracts and show that, in cell extracts, the E2F-cyclin A complex remains associated with other cellular proteins.

Inhibition of E2F-1 transactivation by direct binding of the retinoblastoma protein

Loss of a functional retinoblastoma tumor suppressor gene product, pRB, is a key step in the development of many human tumors. pRB is a negative regulator of cell proliferation and appears to participate in control of entry into the S phase of the cell cycle. The recent demonstration that pRB binds to transcription factor E2F has provided a model for the mechanism of pRB-mediated growth regulation. Since adenovirus E1A proteins dissociate the pRB-E2F complexes and stimulate E2F-dependent transcription, it has been suggested that pRB inhibits E2F transactivation. Although some evidence for this hypothesis has been provided, it has not been possible to determine the mechanism of pRB-mediated inhibition of E2F transactivation. In this study, we constructed mutants of E2F-1 that do not bind to pRB yet retain the ability to transactivate the adenovirus E2 promoter through E2F DNA-binding sites. We demonstrated that transactivation mediated by the wild-type E2F-1 protein was inhibited by overexpression of wild-type pRB but not by a naturally occurring mutant of pRB. Transactivation mediated by mutants of E2F-1 which do not bind to pRB was not affected by overexpression of wild-type pRB. Furthermore, when the E2F-1 transactivation domain was fused to the GAL4 DNA-binding domain, pRB inhibited GAL4-E2F-1 transactivation through GAL4 sites. Expression of pRB did not inhibit transactivation mediated by GAL4-E2F-1 mutant constructs which were devoid of pRB binding. In conclusion, these data demonstrate that pRB inhibits E2F-dependent transactivation by direct protein-protein interaction.

Heterodimerization of the transcription factors E2F-1 and DP-1 leads to cooperative trans-activation

The E2F transcription factor has been implicated in the regulation of genes whose products are involved in cell proliferation. Two proteins have recently been identified with E2F-like properties. One of these proteins, E2F-1, has been shown to mediate E2F-dependent trans-activation and to bind the hypophosphorylated form of the retinoblastoma protein (pRB). The other protein, murine DP-1, was purified from an E2F DNA-affinity column, and it was subsequently shown to bind the consensus E2F DNA-binding site. To study a possible interaction between E2F-1 and DP-1, we have now isolated a cDNA for the human homolog of DP-1. Human DP-1 and E2F-1 associate both in vivo and in vitro, and this interaction leads to enhanced binding to E2F DNA-binding sites. The association of E2F-1 and DP-1 leads to cooperative activation of an E2F-responsive promoter. Finally, we demonstrate that E2F-1 and DP-1 association is required for stable interaction with pRB in vivo and that trans-activation by E2F-1/DP-1 heterodimers is inhibited by pRB. We suggest that "E2F" is the activity that is formed when an E2F-1-related protein and a DP-1-related protein dimerize.

Expression of transcription factor E2F1 induces quiescent cells to enter S phase

Several lines of evidence implicate the E2F transcription factor as an important component of cell proliferation control. First, E2F binding sites are found in the promoters of genes responsive to proliferation signals and the level of E2F binding activity increases at a time when many of these genes are activated. Second, the tumour suppressor protein Rb, as well as the related p107 protein, complexes with E2F, resulting in an inhibition of E2F transcriptional activity. Third, oncogenic products of the DNA tumour viruses can dissociate these E2F complexes. We provide here direct evidence that E2F is involved in cellular proliferation control. Specifically, we demonstrate that overexpression of the E2F1 complementary DNA can activate DNA synthesis in cells that would otherwise growth-arrest, with an efficiency that is similar to that achieved by the expression of the adenovirus E1A gene. Moreover, microinjection of the E2F1 cDNA into quiescent cells can induce S-phase entry, whereas two E2F1 mutants, which are unable to transactivate the DHFR and TK promoters, are unable to induce S phase. We conclude that the E2F transcription factor plays an important role in progression into S phase and that this probably coincides with its capacity to stimulate transcription.

Interferons and interleukin-6 suppress the DNA-binding activity of E2F in growth-sensitive hematopoietic cells

Transcription factor E2F binds to cellular promoters of certain growth- and cell cycle-controlling genes and forms distinct heteromeric complexes with other nuclear proteins. We show here that alpha and beta interferons (alpha, beta) and interleukin-6 abolished the E2F-containing DNA-binding complexes in Daudi Burkitt lymphoma cells and in M1 myeloblastic cells, which responded to the cytokines by suppression of c-myc transcription. Time kinetics studies showed that the abolishment of E2F complexes coincided with reduction of c-myc expression and that both molecular events preceded the cell cycle block in G0/G1 phase. In contrast, the pattern of E2F complexes remained unchanged in an interferon-treated growth-resistant Daudi cell mutant that displayed relaxed regulation of c-myc. All of the DNA-binding E2F complexes, including those containing the retinoblastoma protein (pRB), cyclin A-p33cdk2, and the free forms of E2F, were reduced by interferons or interleukin-6. Their abolishment was unperturbed by pharmacological treatments that alleviated the cyclin A and pRB responses to interferon. Thus, changes in cyclin A expression and pRB phosphorylation are not primary events that influence the pattern of E2F responses to cytokines. Addition of EDTA to cell extracts of interferon-treated Daudi cells restored the DNA-binding activity of E2F, resulting in the appearance of a single E2F complex that exclusively contained pRB. It is suggested that the regulation of E2F by growth-inhibitory cytokines that induce cell cycle exit takes place at the level of the DNA-binding activity, and by that mean it differs basically from the phase-specific regulation of E2F in cycling cells.

Interferons and interleukin-6 suppress the DNA-binding activity of E2F in growth-sensitive hematopoietic cells

Transcription factor E2F binds to cellular promoters of certain growth- and cell cycle-controlling genes and forms distinct heteromeric complexes with other nuclear proteins. We show here that alpha and beta interferons (alpha, beta) and interleukin-6 abolished the E2F-containing DNA-binding complexes in Daudi Burkitt lymphoma cells and in M1 myeloblastic cells, which responded to the cytokines by suppression of c-myc transcription. Time kinetics studies showed that the abolishment of E2F complexes coincided with reduction of c-myc expression and that both molecular events preceded the cell cycle block in G0/G1 phase. In contrast, the pattern of E2F complexes remained unchanged in an interferon-treated growth-resistant Daudi cell mutant that displayed relaxed regulation of c-myc. All of the DNA-binding E2F complexes, including those containing the retinoblastoma protein (pRB), cyclin A-p33cdk2, and the free forms of E2F, were reduced by interferons or interleukin-6. Their abolishment was unperturbed by pharmacological treatments that alleviated the cyclin A and pRB responses to interferon. Thus, changes in cyclin A expression and pRB phosphorylation are not primary events that influence the pattern of E2F responses to cytokines. Addition of EDTA to cell extracts of interferon-treated Daudi cells restored the DNA-binding activity of E2F, resulting in the appearance of a single E2F complex that exclusively contained pRB. It is suggested that the regulation of E2F by growth-inhibitory cytokines that induce cell cycle exit takes place at the level of the DNA-binding activity, and by that mean it differs basically from the phase-specific regulation of E2F in cycling cells.

E2F-1-mediated transactivation is inhibited by complex formation with the retinoblastoma susceptibility gene product

Previous studies have shown that the carboxyl-terminal region of E2F-1 (residues 368-437) can support transcriptional activation when linked to the DNA-binding domain of the yeast transcription factor GAL4. This region also contains an 18-residue retinoblastoma (RB)-binding sequence, raising the possibility that RB binding might inhibit the ability of E2F-1 to form protein-protein contacts required for activation. Here we report a further analysis of the E2F-1 activation domain. In addition, we show that overexpression of RB, but not the RB mutant, RBd22, can inhibit GAL4/E2F-1 activity in vivo. Moreover, expression of the simian virus 40 large tumor antigen (T antigen), but not the RB-binding defective T antigen point mutant, K1, can overcome this repression. Three different GAL4/E2F-1 mutants that activate transcription, but fail to bind to RB, are not significantly affected by overexpression of RB. These findings support a model wherein RB suppresses E2F-1-mediated transcriptional activation through direct physical association.

Cell cycle analysis of E2F in primary human T cells reveals novel E2F complexes and biochemically distinct forms of free E2F

The transcription factor E2F activates the expression of multiple genes involved in cell proliferation, such as c-myc and the dihydrofolate reductase gene. Regulation of E2F involves its interactions with other cellular proteins, including the retinoblastoma protein (Rb), the Rb-related protein p107, cyclin A, and cdk2. We undertook a detailed analysis of E2F DNA-binding activities and their cell cycle behavior in primary human T cells. Three E2F DNA-binding activities were identified in resting (G0) T cells with mobilities in gel shift assays distinct from those of previously defined E2F complexes. One of these activities was found to be a novel, less abundant, Rb-E2F complex. The most prominent E2F activity in resting T cells (termed complex X) was abundant in both G0 and G1 but disappeared as cells entered S phase, suggesting a possible role in negatively regulating E2F function. Complex X could be dissociated by adenovirus E1A with a requirement for an intact E1A conserved region 2. However, X failed to react with a variety of antibodies against Rb or p107, implicating the involvement of an E1A-binding protein other than Rb or p107. In addition to these novel E2F complexes, three distinct forms of unbound (free) E2F were resolved in gel shift experiments. These species showed different cell cycle kinetics. UV cross-linking experiments suggested that a distinct E2F DNA-binding protein is uniquely associated with the S-phase p107 complex and is not associated with Rb. Together, these results suggest that E2F consists of multiple, biochemically distinct DNA-binding proteins which function at different points in the cell cycle.

Cell cycle analysis of E2F in primary human T cells reveals novel E2F complexes and biochemically distinct forms of free E2F

The transcription factor E2F activates the expression of multiple genes involved in cell proliferation, such as c-myc and the dihydrofolate reductase gene. Regulation of E2F involves its interactions with other cellular proteins, including the retinoblastoma protein (Rb), the Rb-related protein p107, cyclin A, and cdk2. We undertook a detailed analysis of E2F DNA-binding activities and their cell cycle behavior in primary human T cells. Three E2F DNA-binding activities were identified in resting (G0) T cells with mobilities in gel shift assays distinct from those of previously defined E2F complexes. One of these activities was found to be a novel, less abundant, Rb-E2F complex. The most prominent E2F activity in resting T cells (termed complex X) was abundant in both G0 and G1 but disappeared as cells entered S phase, suggesting a possible role in negatively regulating E2F function. Complex X could be dissociated by adenovirus E1A with a requirement for an intact E1A conserved region 2. However, X failed to react with a variety of antibodies against Rb or p107, implicating the involvement of an E1A-binding protein other than Rb or p107. In addition to these novel E2F complexes, three distinct forms of unbound (free) E2F were resolved in gel shift experiments. These species showed different cell cycle kinetics. UV cross-linking experiments suggested that a distinct E2F DNA-binding protein is uniquely associated with the S-phase p107 complex and is not associated with Rb. Together, these results suggest that E2F consists of multiple, biochemically distinct DNA-binding proteins which function at different points in the cell cycle.

Multiple regulatory elements ensure accurate transcription of a human ribosomal protein gene

Previously we have shown that expression of a cloned human ribosomal protein gene, RPS14, depends upon regulatory sites located within the gene's proximal upstream DNA plus its first intron. In order to identify cis-active sequence motifs within the RPS14 promoter-enhancer complex, we transiently expressed a set of informative deletion clones in cultured Chinese hamster ovary cells. These experiments revealed three DNA sequence motifs that surround the S14 mRNA initiation site and are necessary for accurate transcription. Electrophoretic mobility shift, DNase I footprint, and methylation interference assays resolved two nuclear proteins, NF alpha-1 and NF beta-1, which bind specifically to these regulatory motifs. NF-alpha 1 recognizes a pair of 6-bp target motifs (5'-TTCCGG-3') that flank the 5' end of RPS14 exon I; and NF-beta 1 binds to a 10-bp target sequence (5'-CCGTGGGAAC-3') within the gene's first intron. Site-directed deletion mutations within the NF-alpha 1 and -beta 1 binding sites markedly inhibit S14 mRNA transcription.

Transcriptional inhibition by the retinoblastoma protein

The retinoblastoma protein, pRB, appears to play a key role in coordinating the regulation of cell cycle position and transcriptional events. pRB undergoes specific cell-cycle-dependent phosphorylation, being underphosphorylated in G1 and heavily phosphorylated in S, G2, and M. The underphosphorylated form is able to interact with the E2F transcription factor. Recently, we have cloned a cDNA for E2F-1. By using this clone and a series of non-pRB binding mutants, we have been able to show that the binding of pRB to E2F-1 causes inhibition of E2F-mediated transactivation. pRB's inhibition of E2F-mediated transcription would be lost by mutation in the retinoblastoma gene in human tumours, by pRB's interaction with DNA tumour virus oncoproteins, or by phosphorylation during the cell cycle.

Transacting activities of the E7 genes of several types of human papillomavirus

In accordance with previous reports by others, the E7 gene of human papillomavirus (HPV) 16, considered to be etiologically associated with cervical cancer, transactivated the E2 promoter of adenovirus. This promoter, however, was equally stimulated by the E7 gene of HPV1, a skin type HPV never associated with human malignancy. A variety of promoters were tested to see the effect of the E7 genes of low-risk and high-risk type HPVs. The result indicated that there was no obvious relationship in the levels of transactivation or transrepression by the E7 gene between low-risk and high-risk types. It has been suggested that the binding of the E7 protein to the product of retinoblastoma susceptibility gene (pRb) is the underlying mechanism by which the E7 protein transactivates the E2 promoter. Therefore, the association of the E7 protein and pRb alone did not seem to fully explain the mechanism by which this protein participates in the activation of transcription and induction of human malignancies.

Regulation of the Ets-related transcription factor Elf-1 by binding to the retinoblastoma protein

The retinoblastoma gene product (Rb) is a nuclear phosphoprotein that regulates cell cycle progression. Elf-1 is a lymphoid-specific Ets transcription factor that regulates inducible gene expression during T cell activation. In this report, it is demonstrated that Elf-1 contains a sequence motif that is highly related to the Rb binding sites of several viral oncoproteins and binds to the pocket region of Rb both in vitro and in vivo. Elf-1 binds exclusively to the underphosphorylated form of Rb and fails to bind to Rb mutants derived from patients with retinoblastoma. Co-immunoprecipitation experiments demonstrated an association between Elf-1 and Rb in resting normal human T cells. After T cell activation, the phosphorylation of Rb results in the release of Elf-1, which is correlated temporally with the activation of Elf-1-mediated transcription. Overexpression of a phosphorylation-defective form of Rb inhibited Elf-1-dependent transcription during T cell activation. These results demonstrate that Rb interacts specifically with a lineage-restricted Ets transcription factor. This regulated interaction may be important for the coordination of lineage-specific effector functions such as lymphokine production with cell cycle progression in activated T cells.

The human papillomavirus E7 protein as a transforming and transactivating factor

The HPV proteins encoded by the early viral genes, including E6 and E7, are thought to subvert the normal regulatory pathways of infected cells to accommodate viral replication. Mechanistically some of this is accomplished by protein-protein interactions between viral proteins and a number of key cellular regulatory proteins that include tumor suppressor gene products. By undermining cellular regulatory pathways the HPV oncogenes cause hyperproliferation and the perturbation of normal cellular differentiation pathways. Although expression of the high-risk HPV-encoded E6 and E7 oncoproteins may be important prerequisites for cellular transformation, it is very likely that additional cellular changes are necessary for carcinogenic progression. The elucidation of the role of the early HPV genes in the initiation and/or maintenance of carcinogenic progression will continue to be a fascinating area of investigation and may reveal new opportunities for antiviral therapy and antitumor intervention.

Identification of a protein from Saccharomyces cerevisiae with E2F-like DNA-binding and transactivating properties

The promoter of the human proto-oncogene MYC has been the first cellular target shown to be subject to regulation by the E2F transcription factor. E2F also has binding sites in other promoters regulated by cell proliferation and during the cell cycle. We have analyzed Saccharomyces cerevisiae for the presence of an E2F-analogous protein. GAL1-based promoter constructs carrying the E2F binding site of the MYC or the adenovirus E2 promoter showed transcriptional activity in yeast cells. A DNA-binding factor, designated YE2F, binds specifically to the E2F consensus sequence and was partially purified from yeast extracts. YE2F showed identical contact points within the MYC binding site as authentic E2F protein from mammalian cells. The results suggest that the existence of an E2F-like protein in the yeast S. cerevisiae.

Differential modulation of cyclin gene expression by MYC

We have investigated the effects of deregulated expression of the human c-MYC protooncogene on cyclin gene expression and on the transcription factor E2F. We found that constitutive expression of MYC or activation of conditional MycER chimeras led to higher levels of cyclin A and cyclin E mRNA. Activation of cyclin A expression by MYC led to a growth factor-independent association of cyclin A and cdk2 with the transcription factor E2F and correlated with an increase in E2F transcriptional activity. In contrast, expression of the G1 phase cyclin D1 was strongly reduced in MYC-transformed cells. In synchronized cells, repression of cyclin D1 by MYC occurred very early in the G1 phase of the cell cycle.

Transcription factor E2F binds DNA as a heterodimer

E2F is a mammalian transcription factor that appears to play an important role in cell cycle control. DNA affinity column-purified E2F from HeLa cells reproducibly exhibits multiple protein bands when analyzed by SDS/PAGE. After electrophoretic purification, electroelution, and refolding of the individual protein components, the E2F DNA binding activity of the individual proteins was poor. However, upon mixing the individual components together, a dramatic (100- to 1000-fold) increase in specific DNA binding activity was observed. The five protein bands isolated can be separated into two groups based on apparent molecular mass. Optimal reconstitution of activity requires one of the two proteins found in the group of larger molecular mass (approximately 60 kDa) and one of the three proteins in the smaller-sized group (approximately 50 kDa). The reconstituted heterodimer is identical to authentic affinity-purified E2F by three criteria: DNA-binding specificity, DNA pattern, and binding to the retinoblastoma gene product. A recently cloned protein with E2F-like activity, RBP3/E2F-1, is related to the protein components of the group of larger molecular mass, as determined by Western blot analysis and reconstitution experiments. These data suggest that E2F, like many other transcription factors, binds DNA as an oligomeric complex composed of at least two distinct proteins.

Integration of cell cycle control with transcriptional regulation by the retinoblastoma protein

Rapid progress in several areas of molecular biology has led to the realization that the retinoblastoma protein may play a pivotal role in the coordination between cell cycle control and regulation of gene expression. This role is a subtle one, and is important in only certain mammalian cell types. Exploring the details of these connections, and why only some cells rely on them, is already beginning to shed light on the regulation of cell multiplication.

The human papillomavirus E7 oncoprotein and the cellular transcription factor E2F bind to separate sites on the retinoblastoma tumor suppressor protein

The ability of the high-risk and low-risk human papillomavirus E7 oncoproteins to disrupt complexes of the retinoblastoma tumor suppressor protein pRB and the cellular transcription factor E2F was studied. The ability of E7 to disrupt this transcription factor complex correlated with the different pRB binding efficiencies of the high-risk and low-risk human papillomavirus-encoded E7 proteins. The pRB binding site was the sole determinant for these observed differences. The phosphorylation status of the casein kinase II site that is immediately adjacent to the pRB binding site in E7 had no marked effect on this biochemical property of E7. Peptides consisting of the pRB binding site of E7, however, were not able to disrupt the pRB/E2F complex. These data suggest that additional carboxy-terminal sequences in E7 are also required for the efficient disruption of the pRB/E2F complex and that E7 and E2F may interact with nonidentical sites of pRB.

A protein synthesis-dependent increase in E2F1 mRNA correlates with growth regulation of the dihydrofolate reductase promoter

Enhanced expression of genes involved in nucleotide biosynthesis, such as dihydrofolate reductase (DHFR), is a hallmark of entrance into the DNA synthesis (S) phase of the mammalian cell cycle. To investigate the regulated expression of the DHFR gene, we stimulated serum-starved NIH 3T3 cells to synchronously reenter the cell cycle. Our previous results show that a cis-acting element at the site of DHFR transcription initiation is necessary for serum regulation. Recently, this element has been demonstrated to bind the cloned transcription factor E2F. In this study, we focused on the role of E2F in the growth regulation of DHFR. We demonstrated that a single E2F site, in the absence or presence of other promoter elements, was sufficient for growth-regulated promoter activity. Next, we showed that the increase in DHFR mRNA at the G1/S-phase boundary required protein synthesis, raising the possibility that a protein(s) lacking in serum-starved cells is required for DHFR transcription. We found that, similar to DHFR mRNA expression, levels of murine E2F1 mRNA were low in serum-starved cells and increased at the G1/S-phase boundary in a protein synthesis-dependent manner. Furthermore, in a cotransfection experiment, expression of human E2F1 stimulated the DHFR promoter 22-fold in serum-starved cells. We suggest that E2F1 may be the key protein required for DHFR transcription that is absent in serum-starved cells. Expression of E2F also abolished the serum-stimulated regulation of the DHFR promoter and resulted in transcription patterns similar to those seen with expression of the adenoviral oncoprotein E1A. In summary, we provide evidence for the importance of E2F in the growth regulation of DHFR and suggest that alterations in the levels of E2F may have severe consequences in the control of cellular proliferation.

Interactions of the p107 and Rb proteins with E2F during the cell proliferation response

The E2F transcription factor is found in complexes with a variety of cellular proteins including the retinoblastoma tumor suppressor protein. Various assays have demonstrated a tight correlation between the functional capacity of Rb as a growth suppressor and its ability to bind to E2F. Moreover, only the underphosphorylated form of Rb, which appears to be the active species, interacts with E2F. Despite the fact that the majority of Rb becomes hyperphosphorylated at the end of G1, we now show that the E2F-Rb interaction persists through the G1/S transition and into S phase. A distinct E2F complex does appear to be regulated in relation to the transition from G1 to S phase. We now demonstrate that this complex contains the Rb-related p107 protein. Moreover, like the Rb protein, p107 inhibits E2F-dependent transcription in a co-transfection assay. This result, together with the observation that free, uncomplexed E2F accumulates as cells leave G1 and enter S phase, suggests that the p107 protein may regulate E2F-dependent transcription during G1. In contrast, although Rb does regulate the transcriptional activity of E2F, this association does not coincide with the G1 to S phase transition.

A protein synthesis-dependent increase in E2F1 mRNA correlates with growth regulation of the dihydrofolate reductase promoter

Enhanced expression of genes involved in nucleotide biosynthesis, such as dihydrofolate reductase (DHFR), is a hallmark of entrance into the DNA synthesis (S) phase of the mammalian cell cycle. To investigate the regulated expression of the DHFR gene, we stimulated serum-starved NIH 3T3 cells to synchronously reenter the cell cycle. Our previous results show that a cis-acting element at the site of DHFR transcription initiation is necessary for serum regulation. Recently, this element has been demonstrated to bind the cloned transcription factor E2F. In this study, we focused on the role of E2F in the growth regulation of DHFR. We demonstrated that a single E2F site, in the absence or presence of other promoter elements, was sufficient for growth-regulated promoter activity. Next, we showed that the increase in DHFR mRNA at the G1/S-phase boundary required protein synthesis, raising the possibility that a protein(s) lacking in serum-starved cells is required for DHFR transcription. We found that, similar to DHFR mRNA expression, levels of murine E2F1 mRNA were low in serum-starved cells and increased at the G1/S-phase boundary in a protein synthesis-dependent manner. Furthermore, in a cotransfection experiment, expression of human E2F1 stimulated the DHFR promoter 22-fold in serum-starved cells. We suggest that E2F1 may be the key protein required for DHFR transcription that is absent in serum-starved cells. Expression of E2F also abolished the serum-stimulated regulation of the DHFR promoter and resulted in transcription patterns similar to those seen with expression of the adenoviral oncoprotein E1A. In summary, we provide evidence for the importance of E2F in the growth regulation of DHFR and suggest that alterations in the levels of E2F may have severe consequences in the control of cellular proliferation.

Protein kinase C-mediated phospholipase A2 activation, platelet-activating factor generation and prostacyclin release in spontaneously beating rat cardiomyocytes

The expression of protein kinase C (PKC) isoenzymes and the effects of PKC activation on myocardial phospholipase A2 (PLA2) activity, platelet-activating factor (PAF) generation and eicosanoid release were studied in spontaneously beating cultured rat cardiomyocytes. Western blotting analysis indicated that these cells contain PKC alpha, beta, delta and zeta, but not PKC gamma or epsilon. Stimulation of cardiomyocytes with 4 beta-phorbol 12-myristate 13-acetate (PMA) led to a rapid increase in particulate-bound PKC activity, a response attributed to the activation of alpha-, delta- and zeta- type PKCs but not beta-type PKC. Translocation of PKC alpha, delta and zeta was accompanied by simultaneous increases in cellular lysophosphatidylcholine (lyso-PC), PAF, 15(S)-hydroxy-5,8,-11,13-eicosatetraenoic acid (15-HETE), prostaglandin E2 (PGE2) and prostacyclin (PGI2) generation, suggesting that one or more of these isoenzymes directly or indirectly activates a PLA2 in these cells. Confirming this, 4 beta-phorbol 12-monoacetate and 4 alpha-phorbol had no effect on cellular eicosanoid formation, while the PMA-induced response was fully abolished both in the presence of the PKC inhibitors staurosporine and CGP 41251 and in PKC-down-regulated cells. PKC alpha, delta and/or zeta therefore appear to play an important role in the PMA-mediated activation of cardiomyocyte PLA2, an event leading to subsequent production of PGI2, PGE2, 15-HETE, lyso-PC and PAF in this tissue.

Protein domains governing interactions between E2F, the retinoblastoma gene product, and human papillomavirus type 16 E7 protein

Human papillomaviruses (HPVs) are the etiological agents for genital warts and contribute to the development of cervical cancer in humans. The HPV E7 gene product is expressed in these diseases, and the E7 genes from HPV types 16 and 18 contribute to transformation in mammalian cells. Mutation and deletion analysis of this gene suggests that the transforming activity of the protein product resides in the same domain as that which is directly involved in complex formation with the retinoblastoma gene product (pRB). This domain is one of two conserved regions (designated CRI and CRII) shared by E7 and other viral oncoproteins which bind pRB, including adenovirus E1A protein. Binding of HPV type 16 E7 protein to pRB has previously been shown to affect pRB's ability to bind DNA and to form complexes with other cellular proteins. In the current study, we map the functional interaction between E7 protein and pRB by monitoring the association between a 60-kDa version of the pRB, pRB60, and the cellular transcription factor E2F. We observe that CRII of E7 (amino acids 20 to 29), which completely blocks binding of full-length E7 protein, is necessary but not sufficient to inhibit E2F/pRB60 complex formation. While CRI of E1A (amino acids 37 to 55) appears to be sufficient to compete with E2F for binding to pRB60, the equivalent region of E7 is neither necessary nor sufficient. Only E7 fragments that contained both CRII and at least a portion of the zinc-binding domain (amino acids 60 to 98) inhibited E2F/pRB60 complex formation. These results suggest that pRB60 associates with E7 and E2F through overlapping but distinct domains.

Interaction of myogenic factors and the retinoblastoma protein mediates muscle cell commitment and differentiation

The experiments reported here document that the tumor suppressor retinoblastoma protein (pRB) plays an important role in the production and maintenance of the terminally differentiated phenotype of muscle cells. We show that pRB inactivation, through either phosphorylation, binding to T antigen, or genetic alteration, inhibits myogenesis. Moreover, inactivation of pRB in terminally differentiated cells allows them to reenter the cell cycle. In addition to its involvement in the myogenic activities of MyoD, pRB is also required for the cell growth-inhibitory activity of this myogenic factor. We also show that pRB and MyoD directly bind to each other, both in vivo and in vitro, through a region that involves the pocket and the basic-helix-loop-helix domains, respectively. All the results obtained are consistent with the proposal that the effects of MyoD on the cell cycle and of pRB on the myogenic pathway result from the direct binding of the two molecules.

Retinoblastoma protein and the cell cycle

Deregulation of the cell cycle may contribute one of the primary mechanisms through which cancer arises. Eukaryotic cell division has been found to be a strictly controlled process, involving response to both positive and negative external signals and assessment of the cell's internal state. Several recent discoveries have strengthened and refined the theory that the retinoblastoma protein is involved in the decision between cell division and differentiation, and have begun to provide an outline of the nature of this involvement.

Papillomavirus E7 protein binding to the retinoblastoma protein is not required for viral induction of warts

Human papillomaviruses (HPVs) are the etiologic agents responsible for benign epithelial proliferative disorders including genital warts and are a contributory factor in the pathogenesis of cervical cancer. HPVs demonstrate strict species and cell-type specificity, which is manifested by the inability of these viruses to induce disease in any species other than humans. The natural history of HPV infection in humans is closely mimicked by cottontail rabbit papillomavirus (CRPV) infection in domestic laboratory rabbits. The CRPV E7 gene is known to play an essential role in virus-mediated induction of papillomas. We now show by mutational analysis that the CRPV E7 protein's biochemical and biological properties, including binding to the retinoblastoma suppressor protein (pRB), transcription factor E2F transactivation of the adenovirus E2 promoter, disruption of pRB-E2F complexes, and cellular transformation as measured by growth in soft agar, mimic those of the HPV E7 protein. Intradermal injection of CRPV DNA lacking E7 gene sequences critical for the binding of the CRPV E7 protein to pRB induced papillomas in rabbits. These studies indicate that E7 protein binding to pRB is not required in the molecular pathogenesis of virally induced warts and suggest that other properties intrinsic to the E7 protein are necessary for papilloma formation.

Protein domains governing interactions between E2F, the retinoblastoma gene product, and human papillomavirus type 16 E7 protein

Human papillomaviruses (HPVs) are the etiological agents for genital warts and contribute to the development of cervical cancer in humans. The HPV E7 gene product is expressed in these diseases, and the E7 genes from HPV types 16 and 18 contribute to transformation in mammalian cells. Mutation and deletion analysis of this gene suggests that the transforming activity of the protein product resides in the same domain as that which is directly involved in complex formation with the retinoblastoma gene product (pRB). This domain is one of two conserved regions (designated CRI and CRII) shared by E7 and other viral oncoproteins which bind pRB, including adenovirus E1A protein. Binding of HPV type 16 E7 protein to pRB has previously been shown to affect pRB's ability to bind DNA and to form complexes with other cellular proteins. In the current study, we map the functional interaction between E7 protein and pRB by monitoring the association between a 60-kDa version of the pRB, pRB60, and the cellular transcription factor E2F. We observe that CRII of E7 (amino acids 20 to 29), which completely blocks binding of full-length E7 protein, is necessary but not sufficient to inhibit E2F/pRB60 complex formation. While CRI of E1A (amino acids 37 to 55) appears to be sufficient to compete with E2F for binding to pRB60, the equivalent region of E7 is neither necessary nor sufficient. Only E7 fragments that contained both CRII and at least a portion of the zinc-binding domain (amino acids 60 to 98) inhibited E2F/pRB60 complex formation. These results suggest that pRB60 associates with E7 and E2F through overlapping but distinct domains.

Regulation of transcription by the retinoblastoma protein

The product of the retinoblastoma gene (RB1) is believed to function as a negative regulator of cell growth. Recent experimental results suggest that RB1 may exert its growth-suppressing activity by regulating the transcription of a variety of growth-related genes, including FOS, MYC, and TGFBI. A series of biochemical and molecular analyses suggest that RB1 indirectly affects gene expression via cell-cycle-regulated interactions with transcription factors, such as E2F and SPI. Determination of the mechanisms regulating such protein-protein interactions and the identification of additional targets of RB1 function will provide vital insights into the role of this tumor-suppressor gene in mammalian cell proliferation.

Transcriptional activation by the adenovirus larger E1a product is mediated by members of the cellular transcription factor ATF family which can directly associate with E1a

We recently isolated three cDNA clones encoding closely related proteins (ATFa1, ATFa2, and ATFa3) that belong to the activating transcription factor-cyclic AMP-responsive element family of cellular transcription factors. Using cotransfection experiments, we showed that these proteins mediate the transcriptional activation induced by the adenovirus E1a 13S mRNA gene product and that the zinc-binding domains present in both E1a conserved region 3 and the most N-terminal portion of the ATFa proteins play crucial roles in this activity. Reciprocal coimmunoprecipitation experiments demonstrated direct interactions between these proteins. Neither the conserved region 3 domain of E1a nor the N-terminal metal-binding element of ATFa is essential for these interactions. The simultaneous alteration of both the N-terminal and the C-terminal domains of ATFa abolished E1a binding, while either mutation alone failed to impair these interactions.

The adenovirus E1A 12S product displays functional redundancy in activating the human proliferating cell nuclear antigen promoter

The adenovirus E1A 243R oncoprotein stimulates expression from the promoter of the human proliferating cell nuclear antigen (PCNA). To gain insight into the mechanism of activation, we analyzed deletion and point mutations of the 243R protein for their abilities to activate PCNA promoter-directed reporter gene expression upon cotransfection into HeLa cells. Large deletions that in combination span the entire protein severely impaired the ability of E1A 243R to induce PCNA expression. Smaller deletions and specific point mutations that target specific E1A-binding proteins were less deleterious to PCNA induction. The data suggest that E1A activates transcription of the PCNA gene by multiple mechanisms and that, of the known 243R-associated proteins, p300 and p107-cyclin A can mediate the response while p105-RB does not appear to participate. Presumably, the functional redundancy ensures that 243R can activate expression of this essential DNA replication protein regardless of cell type and physiological conditions.

Rapamycin blocks cell cycle progression of activated T cells prior to events characteristic of the middle to late G1 phase of the cycle

The effects of rapamycin (RAP) on cell cycle progression of human T cells stimulated with PHA were examined. Cell cycle analysis showed that the RNA content of cells stimulated with PHA in the presence of RAP was similar to that of control T cells stimulated with PHA for 12-24 hr in the absence of the drug. This level was substantially higher than that seen in cells stimulated in the presence of cyclosporin A (CsA), an immunosuppressant known to block cell cycle progression at an early point in the cycle. However, the point in the cell cycle at which RAP acted appeared to be well before the G1/S transition, which occurs about 30-36 hr after stimulation with PHA. In an attempt to further localize the point in the cell cycle where arrest occurred, a set of key regulatory events leading to the G1/S boundary were examined, including p110Rb phosphorylation, which occurred at least 6 hr prior to DNA synthesis, p34cdc2 synthesis, and cyclin A synthesis. In control cultures, p110Rb phosphorylation was detected within 24 hr of PHA stimulation; p34cdc2 and cyclin A synthesis were detected within 30 hr. Addition of RAP to the cultures inhibited each of these events. In contrast, early events, including c-fos, IL-2, and IL-4 mRNAs expression, and IL-2 receptor (p55) expression, were only marginally affected, if at all, in PHA-stimulated T cells. Furthermore, the inhibition of cell proliferation by RAP could not be overcome by addition of exogenous IL-2. These results indicate that RAP blocks cell cycle progression of activated T cells after IL-2/IL-2 receptor interaction but prior to p110Rb phosphorylation and other key regulatory events signaling G1/S transition.

Diversity of human p53 mutants revealed by complex formation to SV40 T antigen

The products of the two major suppressor genes p53 and Rb interact with the oncogene products of the DNA tumour viruses. These viral-host protein interactions mimic and interfere with the normal interactions of p53 and Rb with host proteins. The Rb gene product is frequently mutated in human cancers such that it no longer binds to viral or host proteins. In contrast we find that this is not the case with p53 as some, but not all, mutant p53 proteins still bind to the SV40 T antigen. In particular the hot spot mutation found in most Chinese and African cases of hepatocellular carcinoma (HCC) retains T binding activity. The simple subdivision of different p53 mutations revealed by this analysis may have diagnostic and prognostic consequences.