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

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.

Association of cdk2 kinase with the transcription factor E2F during S phase

The transcription factor E2F controls the expression of several proliferation-related genes and is a target of the adenovirus E1A oncogene. In human cells, both cyclin A and the cdk2 protein kinase were found in complexes with E2F. Although the total amounts of cdk2 were constant in the cell cycle, binding to E2F was detected only when cells entered S phase, a time when the cdk2 kinase is activated. These data suggest that the interaction between cdk2 and E2F requires an active kinase that has cyclin A as a targeting component.

Nuclear binding of purified retinoblastoma gene product is determined by cell cycle-regulated phosphorylation

The retinoblastoma tumor suppressor gene product (pRb) is a nuclear protein subject to cell cycle-regulated hyperphosphorylation. I constructed a recombinant vaccinia virus vector that expresses both the underphosphorylated and hyperphosphorylated forms of pRb and purified the recombinant protein by using immunoaffinity chromatography directed toward a synthetic carboxy-terminal epitope. To investigate the hypothesis that hyperphosphorylation of pRb is a means of controlling its growth-regulating activity, I tested purified pRb for the ability to be reincorporated into pRb-deficient nuclei in vitro. The underphosphorylated form of pRb efficiently reassociated with nuclei, but the hyperphosphorylated form remained soluble in this assay. Nuclear binding of pRb was enhanced by phosphatase treatment and reduced by phosphorylation of pRb effected by using a preparation of the cell cycle-regulatory kinase p34cdc2. Mutant-encoded proteins with altered E1A-binding domains failed to bind to nuclei. Pretreatment of target nuclei with nucleases and high-salt extraction did not alter the specificity of binding for underphosphorylated pRb. These observations demonstrate that hyperphosphorylation of pRb can regulate its interaction with nuclei, supporting the hypothesis that hyperphosphorylation controls the growth-regulatory activities of pRb. Further, at least one target of pRb binding appears to be an integral component of the nuclear envelope.

Interferons and interleukin 6 suppress phosphorylation of the retinoblastoma protein in growth-sensitive hematopoietic cells

One approach to identify postreceptor molecular events that transduce the negative-growth signals of inhibitory cytokines is to analyze the cytokine-induced modifications in the expression of cell-cycle-controlling genes. Here we report that suppression of phosphorylation of the retinoblastoma gene product (pRb) is a receptor-generated event triggered by interferons and interleukin 6 (IL-6) in hematopoietic cell lines. The conversion of pRb to the underphosphorylated forms occurs concomitantly with the decline in c-myc protein expression and both events precede the G0/G1-phase arrest induced by the cytokines. Loss of IL-6-induced c-myc responses in cells that have been stably transfected with constitutive versions of the c-myc gene abrogates the typical G0/G1-phase arrest but does not prevent the specific dephosphorylation of pRb. Conversely, depletion of protein kinase C from cells interferes with part of the interferon-induced suppression of pRb phosphorylation and relieves the G0/G1-phase cell-cycle block without affecting the extent of c-myc inhibition. None of the cytokines, including transforming growth factor beta, reduce the phosphorylation of pRb in S-phase-blocked cells. In contrast, the other IL-6-induced molecular responses, including the decline in c-myc mRNA levels, are not phase-specific and develop normally in S-phase-blocked cells that are depleted of the underphosphorylated functional forms of pRb. These and the suppression of pRb phosphorylation, which occur independently of each other, and suggest that the development of the interferon- or IL-6-induced G0/G1-specific arrest requires at least these two receptor-generated events.

Evidence for the involvement of protein kinase activity in transforming growth factor-beta signal transduction

Transforming growth factor-beta 1 (TGF-beta 1) rapidly increases the expression of junB transcription factor and plasminogen activator inhibitor-1 (PAI-1) and prevents the cell cycle-dependent phosphorylation of the RB retinoblastoma susceptibility gene product during late G1 phase in Mv1Lu lung epithelial cells. These responses are shown in this report to be blocked by the potent serine/threonine protein kinase inhibitor, H7, added with TGF-beta 1. Added alone, H7 does not alter the basal junB or PAI-1 mRNA levels, the deposition of PAI-1 into the extracellular matrix, or the phosphorylation of RB in late G1 phase, suggesting that this inhibitor does not have a general nonspecific effect on the cell. The analogs H8 and H9, which are preferential inhibitors of cyclic nucleotide-dependent protein kinases, are fivefold less potent than H7 as inhibitors of the TGF-beta response. The PAI-1 response to TGF-beta 1 is not affected by the simultaneous addition of staurosporine, which is a protein kinase C inhibitor, or by the prolonged preincubation of cells with phorbol 12-myristate 13-acetate, which down-regulates protein kinase C. The results suggest the possibility that H7 and its analogs block various early TGF-beta responses by inhibiting a protein serine/threonine kinase(s).

Factors involved in specific transcription by mammalian RNA polymerase II: purification and analysis of transcription factor IIA and identification of transcription factor IIJ

The previously described transcription factor IIA (TFIIA) protein fraction was separated into two factors that affect transcription, TFIIA and TFIIJ. TFIIA was found to have a stimulatory effect, and TFIIJ was found to be required for transcription. The requirement of TFIIJ was observed when bacterially produced purified human or yeast (Saccharomyces cerevisiae) TATA-binding protein (TBP) was used in lieu of the endogenous HeLa cell TFIID complex, suggesting that TFIIJ may be part of the TFIID complex. The stimulatory activity of TFIIA was found also to be dependent on the source of the TBP. Transcription reactions reconstituted with TFIID were stimulated by TFIIA; however, when human or yeast TBP was used instead of TFIID, TFIIA had no effect. TFIIA was found to interact with the TBP and was extensively purified by the use of affinity chromatography on columns containing immobilized recombinant yeast TBP. TFIIA is a heterotrimer composed of polypeptides of 34, 19, and 14 kDa. These three polypeptides were required to isolate, by using the gel mobility shift assay, a stable complex between TBP and the TATA box sequence.

The interaction of RB with E2F coincides with an inhibition of the transcriptional activity of E2F

Recent experiments have shown that the E2F transcription factor is in a complex with the RB1 gene product. The E2F-pRB complex can be reconstituted in an in vitro assay using a GST-RB fusion protein isolated from Escherichia coli. This interaction is dependent on pRB sequences involved in E1A/T-antigen binding as well as carboxy-terminal pRB sequences that are not necessary for E1A/T binding. Moreover, reconstitution assays reveal a requirement for an accessory factor, in addition to E2F and pRB, for formation of the E2F-pRB complex. Assays of transcription from the adenovirus E2 promoter in transfection experiments demonstrate that formation of the complex containing pRB and E2F coincides with an inhibition of E2F-dependent transcriptional activity. A mutant pRB protein that does not associate with E2F does not inhibit transcription. We conclude that as a consequence of its interaction with E2F, pRB may regulate the transcriptional function of the E2F factor.

Nuclear binding of purified retinoblastoma gene product is determined by cell cycle-regulated phosphorylation

The retinoblastoma tumor suppressor gene product (pRb) is a nuclear protein subject to cell cycle-regulated hyperphosphorylation. I constructed a recombinant vaccinia virus vector that expresses both the underphosphorylated and hyperphosphorylated forms of pRb and purified the recombinant protein by using immunoaffinity chromatography directed toward a synthetic carboxy-terminal epitope. To investigate the hypothesis that hyperphosphorylation of pRb is a means of controlling its growth-regulating activity, I tested purified pRb for the ability to be reincorporated into pRb-deficient nuclei in vitro. The underphosphorylated form of pRb efficiently reassociated with nuclei, but the hyperphosphorylated form remained soluble in this assay. Nuclear binding of pRb was enhanced by phosphatase treatment and reduced by phosphorylation of pRb effected by using a preparation of the cell cycle-regulatory kinase p34cdc2. Mutant-encoded proteins with altered E1A-binding domains failed to bind to nuclei. Pretreatment of target nuclei with nucleases and high-salt extraction did not alter the specificity of binding for underphosphorylated pRb. These observations demonstrate that hyperphosphorylation of pRb can regulate its interaction with nuclei, supporting the hypothesis that hyperphosphorylation controls the growth-regulatory activities of pRb. Further, at least one target of pRb binding appears to be an integral component of the nuclear envelope.

Inherited cancer genes

The identification of several genes associated with inherited cancer syndromes has opened a door to understanding mechanisms of carcinogenesis in common, non-inherited forms of cancer. Each of these genes appears to play a role in the control of cell growth and differentiation.

A cyclin A-protein kinase complex possesses sequence-specific DNA binding activity: p33cdk2 is a component of the E2F-cyclin A complex

The E2F transcription factor has been found in association with the cyclin A protein, and this complex accumulates during the S phase of the cell cycle, suggesting that E2F may play a role in cell cycle control. In independent studies, cyclin A has been shown to be associated with two other proteins, the Rb-related p107 protein and the cdc2-related p33 cdk2 protein kinase. Through an analysis of the E2F-cyclin A complex, we now find that both the p107 protein and the cdc2-related p33cdk2 kinase are components of the previously described complex. Moreover, the complex possesses H1 kinase activity. These results thus define a cyclin A-cdk2 kinase complex that possesses sequence-specific DNA binding activity. This suggests that the cdk2 kinase may phosphorylate other DNA-bound substrates, and that one role of the E2F factor may be to localize this protein kinase to the DNA.

The transcription factor E2F interacts with the retinoblastoma product and a p107-cyclin A complex in a cell cycle-regulated manner

E2F is a transcription factor believed to play a role in the activation of genes required for cellular proliferation. Its regulation is likely important for maintenance of G0 and for the initiation of cell growth. The retinoblastoma product (RB) forms a complex with E2F in G1 in primary and established human cells. As these cells enter S, a second E2F-containing complex appears. It contains p107, a nuclear "pocket" protein with similarities in structure and protein-binding properties to RB, and cyclin A, a cyclin believed to play a role in facilitating DNA replication. Hence, the regulation of E2F is carried out differently in G1 or S. The presence of cyclin A and a pocket protein, a possible cell growth regulator, in the same S phase-associated complex suggests a link between the function of E2F and the regulation of the DNA replication process.

Independent binding of the retinoblastoma protein and p107 to the transcription factor E2F

The cellular protein p107 and the retinoblastoma protein (pRB) have many features in common. Most strikingly, they contain homologous protein domains that mediate interaction with the oncoproteins of several small DNA tumour viruses, including adenovirus E1A and SV40 large-T antigen. In cells that do not contain these viral oncoproteins, pRB interacts with the cellular transcription factor E2F or a related protein termed DRTF1. E2F associates with a form of pRB that is found primarily in G1 cells. It seems that the E2F-pRB complex dissociates near the G1-S boundary before the initiation of S phase, releasing free E2F and apparently stimulating the ability of E2F to activate transcription. Cells that express E1A have no or little pRB-E2F complex, presumably because of the association of E1A with pRB. During S phase, E2F forms a second complex that contains cyclin A but apparently lacks pRB. Here, we report that p107 is found in the cyclin A/E2F complex and that this complex also contains p33cdk2. These observations suggest that p107 and pRB cooperate in the regulation of E2F activity, each affecting different stages of the cell cycle. Thus, by binding to pRB and p107, E1A and large-T antigen target two distinct aspects of E2F regulation.

Interaction of p107 with cyclin A independent of complex formation with viral oncoproteins

The p107 protein and the retinoblastoma protein (RB) both bind specifically to two viral oncoproteins, the SV40 T antigen (T) and adenoviral protein E1A (E1A). Like RB, p107 contains a segment (the pocket) that, alone, can bind specifically to T, E1A, and multiple cellular proteins. Cyclin A bound to the p107 pocket, but not the RB pocket. Although both pockets contain two, related collinear subsegments (A and B), the unique sequence in the p107 pocket that occupies the space between A and B is required for the interaction with cyclin A.

Interaction between human cyclin A and adenovirus E1A-associated p107 protein

The products of the adenovirus early region 1A (E1A) gene are potent oncoproteins when tested in standard transformation and immortalization assays. Many of the changes induced by E1A may be due to its interaction with cellular proteins. Four of these cellular proteins are the retinoblastoma protein (pRB), p107, cyclin A, and p33cdk2. The pRB and p107 proteins are structurally related and have several characteristics in common, including that they both bind to the SV40 large T oncoprotein as well as to E1A. Cyclin A and p33cdk2 are thought to function in the control of the cell cycle. They bind to one another, forming a kinase that closely resembles the cell cycle-regulating complexes containing p34cdc2. Cyclin A is now shown to bind to p107 in the absence of E1A. The association of p107 with cyclin A suggests a direct link between cell cycle control and the function of p107.

Evidence for the involvement of protein kinase activity in transforming growth factor-beta signal transduction

Transforming growth factor-beta 1 (TGF-beta 1) rapidly increases the expression of junB transcription factor and plasminogen activator inhibitor-1 (PAI-1) and prevents the cell cycle-dependent phosphorylation of the RB retinoblastoma susceptibility gene product during late G1 phase in Mv1Lu lung epithelial cells. These responses are shown in this report to be blocked by the potent serine/threonine protein kinase inhibitor, H7, added with TGF-beta 1. Added alone, H7 does not alter the basal junB or PAI-1 mRNA levels, the deposition of PAI-1 into the extracellular matrix, or the phosphorylation of RB in late G1 phase, suggesting that this inhibitor does not have a general nonspecific effect on the cell. The analogs H8 and H9, which are preferential inhibitors of cyclic nucleotide-dependent protein kinases, are fivefold less potent than H7 as inhibitors of the TGF-beta response. The PAI-1 response to TGF-beta 1 is not affected by the simultaneous addition of staurosporine, which is a protein kinase C inhibitor, or by the prolonged preincubation of cells with phorbol 12-myristate 13-acetate, which down-regulates protein kinase C. The results suggest the possibility that H7 and its analogs block various early TGF-beta responses by inhibiting a protein serine/threonine kinase(s).

Factors involved in specific transcription by mammalian RNA polymerase II: purification and analysis of transcription factor IIA and identification of transcription factor IIJ

The previously described transcription factor IIA (TFIIA) protein fraction was separated into two factors that affect transcription, TFIIA and TFIIJ. TFIIA was found to have a stimulatory effect, and TFIIJ was found to be required for transcription. The requirement of TFIIJ was observed when bacterially produced purified human or yeast (Saccharomyces cerevisiae) TATA-binding protein (TBP) was used in lieu of the endogenous HeLa cell TFIID complex, suggesting that TFIIJ may be part of the TFIID complex. The stimulatory activity of TFIIA was found also to be dependent on the source of the TBP. Transcription reactions reconstituted with TFIID were stimulated by TFIIA; however, when human or yeast TBP was used instead of TFIID, TFIIA had no effect. TFIIA was found to interact with the TBP and was extensively purified by the use of affinity chromatography on columns containing immobilized recombinant yeast TBP. TFIIA is a heterotrimer composed of polypeptides of 34, 19, and 14 kDa. These three polypeptides were required to isolate, by using the gel mobility shift assay, a stable complex between TBP and the TATA box sequence.

Analysis of trans activation by human papillomavirus type 16 E7 and adenovirus 12S E1A suggests a common mechanism

The human papillomavirus E7 gene product is an oncoprotein with properties similar to those of the adenovirus E1A proteins. The human papillomavirus E7 proteins possess substantial amino acid sequence similarity to portions of conserved regions 1 and 2 of E1A, and the human papillomavirus type 16 E7 protein trans-activates the adenovirus E2 early promoter. Analysis of point mutations in the E2 promoter indicated that the E2F recognition sites were critical to E7 stimulation. In contrast to the activation of the E2 promoter, E7 could not trans-activate various other E1A-inducible promoters. Although the promoter specificity for E7 differs from that of 13S E1A trans activation, it is very similar to activation by the E1A 12S product. Moreover, analysis of the E7 protein has suggested that amino acid sequences critical for trans activation include those shared with E1A within conserved region 2. Biochemical studies demonstrate that the E7 protein, like the 12S E1A product, can alter the interaction of cellular factors with the E2F transcription factor. We therefore conclude that E7 trans activation is functionally related to that mediated by the 12S E1A product.

The retinoblastoma protein is phosphorylated on multiple sites by human cdc2

The retinoblastoma gene product (pRB) is a nuclear phosphoprotein that is thought to play a key role in the negative regulation of cellular proliferation. pRB is phosphorylated in a cell cycle dependent manner, and studies in both actively dividing and differentiated cells suggest that this modification may be essential for cells to progress through the cell cycle. Using tryptic phosphopeptide mapping we have shown that pRB is phosphorylated on multiple serine and threonine residues in vivo and that many of these phosphorylation events can be mimicked in vitro using purified p34cdc2. Using synthetic peptides corresponding to potential cdc2 phosphorylation sites, we have developed a strategy which has allowed the identification of five sites. S249, T252, T373, S807 and S811 are phosphorylated in vivo, and in each case these sites correspond closely to the consensus sequence for phosphorylation by p34cdc2. This and the observation that pRB forms a specific complex with p34cdc2 in vivo suggests that p34cdc2 or a p34cdc2-related protein is a major pRB kinase.

High level expression and purification of herpes simplex virus type 1 immediate early polypeptide Vmw110

Herpes simplex virus type 1 (HSV-1) encodes five immediate early (IE) polypeptides. This paper reports the construction of a baculovirus vector which expresses large amounts of Vmw110, the product of IE gene 1. The expressed protein has been purified to near homogeneity and has a mobility on SDS polyacrylamide gels identical to that of Vmw110 produced during HSV-1 infection. Characterisation of its properties indicated that it forms dimers and perhaps higher order oligomers in solution and that the purified protein binds to both single stranded and double stranded calf thymus DNA cellulose columns. However, filter binding experiments were unable to detect any stable association of Vmw110 with DNA in solution.

Tumor suppressor genes

For the past decade, cellular oncogenes have attracted the attention of biologists intent on understanding the molecular origins of cancer. As the present decade unfolds, oncogenes are yielding their place at center stage to a second group of actors, the tumor suppressor genes, which promise to teach us equally important lessons about the molecular mechanisms of cancer pathogenesis.

Sequences and factors required for the F9 embryonal carcinoma stem cell E1a-like activity

F9 embryonal carcinoma (EC) stem cells contain an E1a-like activity that is absent from differentiated derivatives. We have previously characterized proteins present in F9 EC cell extracts that bind to the E1a-dependent E2A promoter and have shown that two of them, TF68 and DRTF1, are required for efficient transcription in vitro (N. B. La Thangue, B. Thimmapaya, and P. W. J. Rigby, Nucleic Acids Res. 18:2929-2938, 1990). We now show that the E1a-like activity is detectable in transient transfection assays. Deletion mutations show that a distal sequence element, which includes the ATF/CREB consensus, is required for expression in both cell types, although it does not mediate the down-regulation of promoter activity that accompanies differentiation. A series of point mutations generated by in vitro mutagenesis confirm this and show that sequences around -60 are necessary for efficient expression in stem cells but not in differentiated derivatives. These sequences bind DRTF1, the activity of which is strongly down-regulated during differentiation. Surprisingly, mutations in a previously uncharacterized region of the promoter restore activity to a promoter carrying the -60 mutation and lead to the formation of a new DNA-protein complex.

Construction and uses of cell lines containing integrated adenovirus E2 promoters

The adenovirus E1a gene encodes polypeptides which regulate the expression of adenovirus early genes as well as a variety of cellular genes. Although it is likely that the E1a encoded polypeptides regulate the expression of these genes by interaction with a variety of cellular transcription factors, the precise mechanism by which this occurs is currently unknown. This report describes the development of cell lines which contain integrated copies of the E2 promoter driving the expression of the Tn5 neo gene or the Escherichia coli beta-galactosidase gene. In each case phenotypic changes concurrent with expression of the E1a 289 amino acid polypeptide are demonstrated. The use of these cell lines to detect rare events in the activation of the E2 promoter is demonstrated in transfection experiments. These cell lines are also used to study the effects of c-myc expression on integrated E2 promoters.

Sequences and factors required for the F9 embryonal carcinoma stem cell E1a-like activity

F9 embryonal carcinoma (EC) stem cells contain an E1a-like activity that is absent from differentiated derivatives. We have previously characterized proteins present in F9 EC cell extracts that bind to the E1a-dependent E2A promoter and have shown that two of them, TF68 and DRTF1, are required for efficient transcription in vitro (N. B. La Thangue, B. Thimmapaya, and P. W. J. Rigby, Nucleic Acids Res. 18:2929-2938, 1990). We now show that the E1a-like activity is detectable in transient transfection assays. Deletion mutations show that a distal sequence element, which includes the ATF/CREB consensus, is required for expression in both cell types, although it does not mediate the down-regulation of promoter activity that accompanies differentiation. A series of point mutations generated by in vitro mutagenesis confirm this and show that sequences around -60 are necessary for efficient expression in stem cells but not in differentiated derivatives. These sequences bind DRTF1, the activity of which is strongly down-regulated during differentiation. Surprisingly, mutations in a previously uncharacterized region of the promoter restore activity to a promoter carrying the -60 mutation and lead to the formation of a new DNA-protein complex.

Transcriptional activation by viral regulatory proteins

The control of transcription involves the use of many transcriptional regulatory proteins. Viral systems and proteins have been used as models to gain insight into these control processes. These include the adenovirus E1A13S and E1A12S products and the herpes virus VP16 protein. This review examines these diverse mechanisms, but also explores the elements of commonality between them.

Complementation of transforming domains in E1a/myc chimaeras

The myc oncogene is functionally similar to adenovirus E1a in its ability to collaborate with activated ras oncogenes to transform primary fibroblasts. The transforming functions of E1a and myc have been mapped to two distinct regions in each protein. I investigated the functional similarities between E1a and myc by constructing E1a/myc chimaeras to discover whether the individual transforming domains of E1a could complement individual myc-transforming domains. Transformation assays in rat embryo fibroblasts demonstrated that the N-terminal transforming domain of E1a (CR1) could complement the C-terminal transforming domain of myc in cis, and that the reciprocal chimaera (N-terminal myc/C-terminal E1a) was also active. Chimaeras constructed using domains from transformation-defective mutants of either E1a or myc were inactive, indicating that both E1a and myc domains contribute to function. These experiments suggest that transformation by myc and E1a may involve interactions with common substrates.

Repression of the interferon signal transduction pathway by the adenovirus E1A oncogene

The signal transduction pathway initiated by type I interferon (alpha and beta interferons) is inhibited by expression of the adenovirus type 5 E1A oncogene. Cotransfection analyses with the E1A oncogene and an interferon-stimulated reporter gene show that mutations within an amino-terminal domain of the E1A oncoprotein are defective in transcriptional repression. Cotransfection experiments also revealed that the transcriptional repression is mediated through the interferon-stimulated response element (ISRE) found within the promoter of interferon-stimulated genes. Since interferon treatment activates a latent cytoplasmic DNA-binding factor that can recognize the ISRE and subsequently stimulate transcription, the appearance of this factor was analyzed in a cell line that constitutively expresses the E1A oncogene. The DNA binding activity of this transcriptional activator was found to be inhibited in the E1A-expressing cell line. In vitro cytoplasmic mixing experiments with extracts from control and E1A-expressing cells identified a specific component of this multimeric transcription factor to be defective.