The retinoblastoma protein physically associates with the human cdc2 kinase

Proteomics and phosphoproteomics reveal key regulators associated with cytostatic effect of amino acid transporter LAT1 inhibitor

L‐type amino acid transporter 1 (LAT1) is highly expressed in various cancers and plays important roles not only in the amino acid uptake necessary for cancer growth but also in cellular signaling. Recent research studies have reported anticancer effects of LAT1 inhibitors and demonstrated their potential for cancer therapy. Here, we characterized the proteome and phosphoproteome in LAT1‐inhibited cancer cells. We used JPH203, a selective LAT1 inhibitor, and performed tandem mass tag–based quantitative proteomics and phosphoproteomics on four biliary tract cancer cell lines sensitive to JPH203. Our analysis identified hundreds to thousands of differentially expressed proteins and phosphorylated sites, demonstrating the broad influence of LAT1 inhibition. Our findings showed various functional pathways altered by LAT1 inhibition, and provided possible regulators and key kinases in LAT1‐inhibited cells. Comparison of these changes among cell lines provides insights into general pathways and regulators associated with LAT1 inhibition and particularly suggests the importance of cell cycle–related pathways and kinases. Moreover, we evaluated the anticancer effects of the combinations of JPH203 with cell cycle–related kinase inhibitors and demonstrated their potential for cancer therapy. This is the first study providing the proteome‐wide scope of both protein expression and phosphorylation signaling perturbed by LAT1 inhibition in cancer cells.

Gene network landscape of the ciliate Tetrahymena thermophila

Background

Genome-wide expression data of gene microarrays can be used to infer gene networks. At a cellular level, a gene network provides a picture of the modules in which genes are densely connected, and of the hub genes, which are highly connected with other genes. A gene network is useful to identify the genes involved in the same pathway, in a protein complex or that are co-regulated. In this study, we used different methods to find gene networks in the ciliate Tetrahymena thermophila, and describe some important properties of this network, such as modules and hubs.

Methodology/Principal Findings

Using 67 single channel microarrays, we constructed the Tetrahymena gene network (TGN) using three methods: the Pearson correlation coefficient (PCC), the Spearman correlation coefficient (SCC) and the context likelihood of relatedness (CLR) algorithm. The accuracy and coverage of the three networks were evaluated using four conserved protein complexes in yeast. The CLR network with a Z-score threshold 3.49 was determined to be the most robust. The TGN was partitioned, and 55 modules were found. In addition, analysis of the arbitrarily determined 1200 hubs showed that these hubs could be sorted into six groups according to their expression profiles. We also investigated human disease orthologs in Tetrahymena that are missing in yeast and provide evidence indicating that some of these are involved in the same process in Tetrahymena as in human.

Conclusions/Significance

This study constructed a Tetrahymena gene network, provided new insights to the properties of this biological network, and presents an important resource to study Tetrahymena genes at the pathway level.

Cell-context specific role of the E2F/Rb pathway in development and disease

Development of the central nervous system (CNS) requires the generation of neuronal and glial cell subtypes in appropriate numbers, and this demands the careful coordination of cell-cycle exit, survival, and differentiation. The E2F/Rb pathway is critical for cell-cycle regulation and also modulates survival and differentiation of distinct cell types in the developing and adult CNS. In this review, we first present the specific temporal patterns of expression of the E2F and Rb family members during CNS development and then discuss the genetic ablation of single or multiple members of these two families. Overall, the available data suggest a time-dependent and cell-context specific role of E2F and Rb family members in the developing and adult CNS.

A gene highly expressed in tumor cells encodes novel structure proteins

We isolated several related but distinct cDNA clones encoding novel structure proteins (NSP) when screening a cDNA library. Analysis revealed that these cDNAs and several similar ESTs in the public databases are derived from a single gene of 17 exons that span a minimum of 227-kb region. This gene is located at chromosome 17p11.2, a region frequently amplified in human gliomas and osteosarcomas, and involved in Birt-Hogg-Dube syndrome, a tumor-prone syndrome. The major coding sequences shared by all isolated transcripts are predicted to encode SMC (structural maintenance of chromosome)/SbcC ATPase motifs and coiled-coil domains commonly seen in motor or structure proteins. Two 5'-end and two 3'-end variants (type 5alpha/beta and 3alpha/beta, respectively) were identified, making a total of four possible transcripts. Both 5alpha and 5beta variants were detected in human testis mRNA, but only type 5alpha was detectable in RNA samples extracted from HeLa cells. The unique carboxyl-terminus of 3beta contains a Ca(2+)-dependent actin-binding domain. Immunohistochemistry studies revealed that NSPs were mostly localized to nuclei. Northern blot analysis demonstrated two major bands and the expression levels are tremendously high in testis while barely detectable in other normal tissues examined. Interestingly, NSP5alpha3alpha is highly expressed in some tumor cell lines. These results suggest that NSPs represent a new family of structure proteins with a possible role in nuclear dynamics during cell division, and that NSP5alpha3alpha may serve as a tumor marker.

BRG1 controls the activity of the retinoblastoma protein via regulation of p21CIP1/WAF1/SDI

The ubiquitous mammalian chromatin-remodeling SWI/SNF-like BAF complexes play critical roles in tumorigenesis. It was suggested that the direct interaction of BRG1 with the retinoblastoma protein pRB is required for regulation of cell cycle progression by pRB. We present evidence that the BRG1-containing complexes regulate the expression of the cdk inhibitor p21(CIP1/WAF1/SDI). Furthermore, we show that the physical interaction between BRG1 and pRB is not required for induction of cell growth arrest and transcriptional repression of E2F target genes by pRB. Instead, BRG1 activates pRB by inducing its hypophosphorylation through up-regulation of the cdk inhibitor p21. The hypophosphorylation of pRB is reinforced by down-regulation of critical components, including cdk2, cyclin E, and cyclin D, in the pRB regulatory network. We demonstrate that up-regulation of p21 by BRG1 is necessary to induce formation of flat cells, growth arrest, and finally, cell senescence. Our results suggest that the BRG1-containing complexes control cellular proliferation and senescence by modulating the pRB pathway via multiple mechanisms.

Changes in retinoblastoma gene expression during cervical cancer progression

The role of tumour suppressor genes in the development of human cancers has been studied extensively. In viral carcinogenesis, the inactivation of suppressor proteins such as retinoblastoma (pRb) and p53, and cellular oncogenes overexpression, such as c-myc, has been the subject of a number of investigations. In uterine-cervix carcinomas, where high-risk human papillomavirus (HPV) plays an important role, pRb and p53 are inactivated by E7 and E6 viral oncoproteins, respectively. However, little is known about the in situ expression of some of these proteins in pre-malignant and malignant cervical tissues. On the other hand, it has also been demonstrated that c-myc is involved in cervical carcinogenesis, and that pRb participates in the control of c-myc gene expression. By using immunostaining techniques, we investigated pRb immunodetection pattern in normal tissues, squamous intraepithelial lesions (SILs) and invasive carcinomas from the uterine cervix. Our data show low pRb detection in both normal cervical tissue and invasive lesions, but a higher expression in SILs. C-Myc protein was observed in most of the cellular nuclei of the invasive lesions, while in SILs was low. These findings indicate a heterogeneous pRb immunostaining during the different stages of cervical carcinogenesis, and suggest that this staining pattern could be a common feature implicated in the pathogenesis of uterine-cervix carcinoma.

The B-domain lysine patch of pRB is required for binding to large T antigen and release of E2F by phosphorylation

Cell cycle-dependent, site-specific phosphorylation of the retinoblastoma protein, pRB, is mediated by cyclin-dependent kinases (CDKs) and regulates the binding of pRB to many proteins. We previously showed that the interaction of pRB with E2F on DNA was regulated by the accumulation of phosphate groups on pRB. Here we show that positively charged lysine residues in the B domain of pRB are necessary for the release of pRB from E2F on DNA following phosphorylation by cyclin E-cdk2 kinase. These lysine residues are also important in the binding of the simian virus 40 large T antigen (TAg) to pRB, and mutation of these lysines to arginines alters the dependency of the pRB-TAg interaction on phosphorylation of pRB.

Integration of the pRB and p53 cell cycle control pathways

Two fundamental molecular pathways, the pRB and p53 pathways, regulate cell growth and cell death. The importance of these pathways in cellular growth control is underscored by the observation that members of these pathways are found mutated in all human cancers. These two pathways have typically been studied and described independently. However, as we discuss here, recent data have revealed an intimate molecular and genetic interaction between the p53 and pRB pathways.

Cumulative effect of phosphorylation of pRB on regulation of E2F activity

The product of the retinoblastoma susceptibility gene, pRB, is a nuclear phosphoprotein that controls cell growth by binding to and suppressing the activities of transcription factors such as the E2F family. Transactivation activity is inhibited when E2F is bound to hypophosphorylated pRB and released when pRB is phosphorylated by cyclin-dependent kinases (CDKs). To determine which of 16 potential CDK phosphorylation sites regulated the pRB-E2F interaction, mutant pRB proteins produced by site-directed mutagenesis were tested for the ability to suppress E2F-mediated transcription in a reporter chloramphenicol acetyltransferase assay. Surprisingly, no one CDK site regulated the interaction of pRB with E2F when E2F was bound to DNA. Instead, disruption of transcriptional repression resulted from accumulation of phosphate groups on the RB molecule.

Functions of cyclin A1 in the cell cycle and its interactions with transcription factor E2F-1 and the Rb family of proteins

Human cyclin A1, a newly discovered cyclin, is expressed in testis and is thought to function in the meiotic cell cycle. Here, we show that the expression of human cyclin A1 and cyclin A1-associated kinase activities was regulated during the mitotic cell cycle. In the osteosarcoma cell line MG63, cyclin A1 mRNA and protein were present at very low levels in cells at the G0 phase. They increased during the progression of the cell cycle and reached the highest levels in the S and G2/M phases. Furthermore, the cyclin A1-associated histone H1 kinase activity peaked at the G2/M phase. We report that cyclin A1 could bind to important cell cycle regulators: the Rb family of proteins, the transcription factor E2F-1, and the p21 family of proteins. The in vitro interaction of cyclin A1 with E2F-1 was greatly enhanced when cyclin A1 was complexed with CDK2. Associations of cyclin A1 with Rb and E2F-1 were observed in vivo in several cell lines. When cyclin A1 was coexpressed with CDK2 in sf9 insect cells, the CDK2-cyclin A1 complex had kinase activities for histone H1, E2F-1, and the Rb family of proteins. Our results suggest that the Rb family of proteins and E2F-1 may be important targets for phosphorylation by the cyclin A1-associated kinase. Cyclin A1 may function in the mitotic cell cycle in certain cells.

E2F activity is regulated by cell cycle-dependent changes in subcellular localization

E2F directs the cell cycle-dependent expression of genes that induce or regulate the cell division process. In mammalian cells, this transcriptional activity arises from the combined properties of multiple E2F-DP heterodimers. In this study, we show that the transcriptional potential of individual E2F species is dependent upon their nuclear localization. This is a constitutive property of E2F-1, -2, and -3, whereas the nuclear localization of E2F-4 is dependent upon its association with other nuclear factors. We previously showed that E2F-4 accounts for the majority of endogenous E2F species. We now show that the subcellular localization of E2F-4 is regulated in a cell cycle-dependent manner that results in the differential compartmentalization of the various E2F complexes. Consequently, in cycling cells, the majority of the p107-E2F, p130-E2F, and free E2F complexes remain in the cytoplasm. In contrast, almost all of the nuclear E2F activity is generated by pRB-E2F. This complex is present at high levels during G1 but disappears once the cells have passed the restriction point. Surprisingly, dissociation of this complex causes little increase in the levels of nuclear free E2F activity. This observation suggests that the repressive properties of the pRB-E2F complex play a critical role in establishing the temporal regulation of E2F-responsive genes. How the differential subcellular localization of pRB, p107, and p130 contributes to their different biological properties is also discussed.

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

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

The Rb family contains a conserved cyclin-dependent-kinase-regulated transcriptional repressor motif

Progression through the cell cycle is dependent on the sequential expression of cyclins, which combine with cyclin-dependent kinases (cdks) to form active kinases. The transition from G1 to S phase is dependent on D cyclins in complex with cdk4 or cdk6 and cyclin E complexed with cdk2. One target of G1 cyclins is the retinoblastoma susceptibility protein (Rb). Rb is a transcriptional repressor that is selectively targeted to genes through interaction with the E2F family of cell cycle transcription factors. Rb is a member of a family of proteins that include p107 and p130. The three proteins share a region known as the pocket that is important for binding E2F and is also the binding site for oncoproteins from DNA tumor viruses that inactivate Rb. We have found that two conserved domains within the Rb pocket (A and B) interact to form a transcriptional repressor motif (K. N. B. Chow and D. C. Dean, Mol. Cell. Biol. 16:4862-4868, 1996). Here we demonstrate that p107 also has an A-B repressor motif, and using domain swapping and coimmunoprecipitation assays, we compare A and B from Rb and p107. Finally and most importantly, we demonstrate that the A-B interaction which forms the repressor motif is blocked by G1 cdk phosphorylation, thereby blocking repressor activity. This A-B repressor motif is then the first example of a cdk-regulated transcriptional repressor.

Role of cyclin A and p27 in anti-IgM induced G1 growth arrest of murine B-cell lymphomas

Cross-linking surface immunoglobulin (Ig)M on the WEHI-231 B-cell lymphoma results in decreased cell size, G1/S growth arrest, and finally DNA cleavage into oligonucleosomal fragments that are the classical features of apoptotic cells. Treatment of WEHI-231 cells with anti-IgM in early G1 phase prevents phosphorylation of the retinoblastoma gene product (pRb) and inhibits entry into S phase. Using unsynchronized cells, we previously demonstrated that cyclin A-associated and Cdk2-dependent GST-pRb kinase activity were inhibited in WEHI-231 cells treated with anti-IgM. We now show that progression of elutriated early G1 phase WEHI-231 cells from early into late G1 phase is accompanied by an increase in the abundance of cyclin A protein and cyclin A-associated kinase activity. Treatment of early G1 cells with anti-IgM prevented this increase in cyclin A-associated kinase activity at late G1, despite minimal changes in the overall level of cyclin A and Cdk2 proteins. Late G1 cells, which already possess high cyclin A-associated kinase activity, were insensitive to anti-IgM treatment and were able to complete the cell cycle. We also found that anti-IgM-treated cells contained increased amounts of the Cdk inhibitor protein p27Kip1. Essentially all of the cyclin A in treated cells was associated with p27, a result which we propose explains the lack of cyclin A/Cdk2 kinase activity. Accumulation of p27 in cyclin A kinase complexes, however, did not decrease the amount of Cdk2 bound to cyclin A. Thus, cross-linking IgM on growth-inhibitable B-cell lymphomas affects cyclin A kinase activity by increasing the levels of p27 in this complex, thus preventing productive pRb phosphorylation and leading to cell cycle arrest and subsequent apoptosis. These results are discussed in terms of the cell cycle restriction points that regulate lymphocyte function, as well as the lineage-specific differences in cell cycle control.

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

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

Different roles for cyclins D1 and E in regulation of the G1-to-S transition

Ectopic expression of cyclins D1 and E was previously shown to accelerate the G1/S-phase transition, indicating that both classes of G1 cyclin control an event(s) that is rate limiting for entry into S phase. In order to determine whether cyclins D1 and E control the same or two different rate-limiting events, we have created cell lines that express both cyclins in an inducible manner. We show here that ectopic expression of both cyclins E and D1 in the same cell has an additive effect on shortening of the G1 interval relative to expression of any single cyclin. In order to further explore the molecular basis for G1 cyclin action, we used cell lines capable of expressing cyclin D1, E, or both prematurely and measured the effect of cyclin expression in early G1 on phosphorylation of the retinoblastoma susceptibility gene product (pRb). We show here that while premature expression of either cyclin alone advances the G1/S-phase transition to the same extent, premature expression of cyclin D1 leads to immediate appearance of hyperphosphorylated pRb, while premature expression of cyclin E does not. Ectopic expression of both cyclins E and D1 in the same cell has an additive effect on shortening of the G1 interval, while the effect on pRb phosphorylation is similar to the effect of cyclin D1 alone. These results suggest that cyclins E and D1 control two different events, both rate limiting for the G1/S-phase transition, and that pRb phosphorylation might be the rate-limiting event controlled by cyclin D1.

Identification of G1 kinase activity for cdk6, a novel cyclin D partner

A family of vertebrate cdc2-related kinases has been identified, and these kinases are candidates for roles in cell cycle regulation. Here, we show that the human PLSTIRE gene product is a novel cyclin-dependent kinase, cdk6. The cdk6 kinase is associated with cyclins D1, D2, and D3 in lysates of human cells and is activated by coexpression with D-type cyclins in Sf9 insect cells. Furthermore, we demonstrate that endogenous cdk6 from human cell extracts is an active kinase which can phosphorylate pRB, the product of the retinoblastoma tumor suppressor gene. The activation of cdk6 kinase occurs during mid-G1 in phytohemagglutinin-stimulated T cells, well prior to the activation of cdk2 kinase. This timing suggests that cdk6, and by analogy its homolog cdk4, links growth factor stimulation with the onset of cell cycle progression.

Identification of G1 kinase activity for cdk6, a novel cyclin D partner

A family of vertebrate cdc2-related kinases has been identified, and these kinases are candidates for roles in cell cycle regulation. Here, we show that the human PLSTIRE gene product is a novel cyclin-dependent kinase, cdk6. The cdk6 kinase is associated with cyclins D1, D2, and D3 in lysates of human cells and is activated by coexpression with D-type cyclins in Sf9 insect cells. Furthermore, we demonstrate that endogenous cdk6 from human cell extracts is an active kinase which can phosphorylate pRB, the product of the retinoblastoma tumor suppressor gene. The activation of cdk6 kinase occurs during mid-G1 in phytohemagglutinin-stimulated T cells, well prior to the activation of cdk2 kinase. This timing suggests that cdk6, and by analogy its homolog cdk4, links growth factor stimulation with the onset of cell cycle progression.

The cell cycle and the retinoblastoma protein family

Tumor formation results from alterations in the control of normal cell proliferation. To further our understanding of the molecular mechanisms underlying the deregulation of cell proliferation much attention, over the past decade, has been focused on the function of proto-oncogenes. Cellular oncogenes are thought to be growth promoting. More recently, a class of genes known as tumor suppressors have come under intense study. Tumor suppressors are largely thought to restrain cell proliferation. The retinoblastoma protein (Rb) is one of a growing list of tumor suppressors. Concurrent with the study of tumor suppressor genes has been a rapid increase in our understanding of the cell cycle at the molecular level. Rb and a related protein p107 are involved in the processes of cell proliferation and differentiation. Each functionally interacts with and affects the activity of the transcription factor E2F as well as other transcription factors involved in cell proliferation and differentiation. Additionally, Rb and p107 are modified by, and/or form specific complexes with, several elements of the basic cell cycle machinery. Specifically, Rb and p107 interact with and are modified by various cyclins and cyclin dependent kinases (cdk), some of which have been shown to be essential for cell cycle progression and in some cases their deregulation has been implicated in the development of cancer. This review will attempt to convey our current functional and mechanistic understanding of the biological roles Rb and p107 play in proliferation, development and differentiation. A knowledge of the interplay between these positive and negative regulators of cell proliferation and differentiation, noted above, is central to our understanding of human cancer.

The transcription factor E2F-1 mediates the autoregulation of RB gene expression

The retinoblastoma (RB) gene is the prototype tumor suppressor gene. Mutations in this gene are often associated with the occurrence of various tumors. Several mutations have been found in the promoter region of the gene, suggesting that inappropriate transcriptional regulation of the RB gene contributes to tumorigenesis. Sequence analysis of the RB promoter has revealed a potential E2F recognition site within a region critical for RB gene transcription. By using the cloned E2F-1 gene, here we report that (i) RB expression is negatively regulated by its own gene product, (ii) E2F-1 binds specifically to an E2F recognition sequence in the RB promoter and transactivates the RB promoter, (iii) overexpression of RB suppresses E2F-1-mediated stimulation of RB promoter activity, and (iv) the expression of the RB gene is paralleled by the expression of the E2F-1 gene during cell cycle progression. These results demonstrate that expression of RB is negatively autoregulated through E2F-1.

The transcription factor E2F-1 mediates the autoregulation of RB gene expression

The retinoblastoma (RB) gene is the prototype tumor suppressor gene. Mutations in this gene are often associated with the occurrence of various tumors. Several mutations have been found in the promoter region of the gene, suggesting that inappropriate transcriptional regulation of the RB gene contributes to tumorigenesis. Sequence analysis of the RB promoter has revealed a potential E2F recognition site within a region critical for RB gene transcription. By using the cloned E2F-1 gene, here we report that (i) RB expression is negatively regulated by its own gene product, (ii) E2F-1 binds specifically to an E2F recognition sequence in the RB promoter and transactivates the RB promoter, (iii) overexpression of RB suppresses E2F-1-mediated stimulation of RB promoter activity, and (iv) the expression of the RB gene is paralleled by the expression of the E2F-1 gene during cell cycle progression. These results demonstrate that expression of RB is negatively autoregulated through E2F-1.

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.

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.

Simian virus 40 large T antigen associates with cyclin A and p33cdk2

In this paper we provide evidence that a fraction of large T antigen of simian virus 40 (SV40) interacts with cyclin A and p33cdk2 in both virus-infected and stably transformed cells. Immunoprecipitates of SV40 large T antigen from SV40-infected or SV40 large-T-antigen-transformed cells contain cyclin A, p33cdk2, and histone H1 kinase activity. Conversely, immunoprecipitates of cyclin A from these cells contain SV40 large T antigen. In this respect, SV40 large T antigen has properties similar to those of the E1A oncogene of adenoviruses and the E7 oncogene of human papillomaviruses.

The adenovirus E1A-associated kinase consists of cyclin E-p33cdk2 and cyclin A-p33cdk2

The adenovirus E1A oncoproteins form stable complexes with several cellular proteins. Association of E1A with these proteins has been shown to be important for the oncogenic potential of E1A. Several of these proteins have been identified and include the product of the retinoblastoma gene and a key cell cycle regulatory protein, cyclin A. E1A also associates with a potent histone H1 kinase. The two major components of this activity are the cyclin E-p33cdk2 and cyclin A-p33cdk2 complexes. Both the cyclin E-p33cdk2 and cyclin A-p33cdk2 complexes have been implicated in regulatory events controlling entry into or passage through DNA synthesis. Although the architecture of such interactions remains unclear, it is likely that by targeting such complexes, adenovirus is affecting some aspect of cell cycle control.

Biological function of the retinoblastoma protein requires distinct domains for hyperphosphorylation and transcription factor binding

Despite the importance of the retinoblastoma susceptibility gene to tumor growth control, the structural features of its encoded protein (pRb) and their relationship to protein function have not been well explored. We constructed a panel of deletion mutants of pRb expression vectors and used a biological assay for pRb that measures growth inhibition and morphologic changes in pRb-transfected Saos-2 cells to correlate structural alterations of the pRb coding region with function. We tested the deleted proteins for the ability to bind to viral oncoprotein E1A and to the transcription factor E2F. We also measured the ability of the mutant proteins to become hyperphosphorylated in vivo and to be recognized as substrates in vitro by a cell cycle-regulatory kinase associated with cyclin A. We identified two regions of pRb that are required for E2F binding and for hyperphosphorylation. E1A binding domains partially overlap but are distinct from both of these other two regions. Biological function of pRb is dependent on retention of the integrity of both of these biochemically defined domains. These data support the model that pRb is a transducer of afferent signals (via the kinase that phosphorylates it) and efferent signals (through transcription factor binding), using distinct structural elements. Preservation of both of these features is essential for the ability of pRb to induce growth inhibition and morphologic changes upon reintroduction into transfected cells.

Biological function of the retinoblastoma protein requires distinct domains for hyperphosphorylation and transcription factor binding

Despite the importance of the retinoblastoma susceptibility gene to tumor growth control, the structural features of its encoded protein (pRb) and their relationship to protein function have not been well explored. We constructed a panel of deletion mutants of pRb expression vectors and used a biological assay for pRb that measures growth inhibition and morphologic changes in pRb-transfected Saos-2 cells to correlate structural alterations of the pRb coding region with function. We tested the deleted proteins for the ability to bind to viral oncoprotein E1A and to the transcription factor E2F. We also measured the ability of the mutant proteins to become hyperphosphorylated in vivo and to be recognized as substrates in vitro by a cell cycle-regulatory kinase associated with cyclin A. We identified two regions of pRb that are required for E2F binding and for hyperphosphorylation. E1A binding domains partially overlap but are distinct from both of these other two regions. Biological function of pRb is dependent on retention of the integrity of both of these biochemically defined domains. These data support the model that pRb is a transducer of afferent signals (via the kinase that phosphorylates it) and efferent signals (through transcription factor binding), using distinct structural elements. Preservation of both of these features is essential for the ability of pRb to induce growth inhibition and morphologic changes upon reintroduction into transfected cells.

Phosphorylation of the retinoblastoma protein by cdk2

The retinoblastoma gene product (the RB protein) is phosphorylated in a cell cycle-dependent manner and this modification is believed to be important for cells to progress through the cell cycle. We found that purified cdk2 (cyclin-dependent kinase/cell division kinase 2) can phosphorylate the RB protein in vitro at the sites phosphorylated in the cell. The timing of activation of cdk2 in the cell cycle was similar to that of the onset of phosphorylation of the RB protein. The kinase coprecipitated with the RB protein also exhibited a similar substrate specificity to cdk2 and a similar time course of activation during the cell cycle. We further showed that cdk2 formed a complex with the RB protein in vitro and that its formation was not competitively inhibited by the simian virus 40 large T antigen. These observations suggest that cdk2 or a cdk2-related protein is involved in the cell cycle-dependent phosphorylation of the RB protein.