Use of the E2F transcription factor by DNA tumor virus regulatory proteins

Distinct Oncogenic Transcriptomes in Human Mammary Epithelial Cells Infected With Cytomegalovirus

Human cytomegalovirus is being recognized as a potential oncovirus beside its oncomodulation role. We previously isolated two clinical isolates, HCMV-DB (KT959235) and HCMV-BL (MW980585), which in primary human mammary epithelial cells promoted oncogenic molecular pathways, established anchorage-independent growth in vitro, and produced tumorigenicity in mice models, therefore named high-risk oncogenic strains. In contrast, other clinical HCMV strains such as HCMV-FS, KM, and SC did not trigger such traits, therefore named low-risk oncogenic strains. In this study, we compared high-risk oncogenic HCMV-DB and BL strains (high-risk) with low-risk oncogenic strains HCMV-FS, KM, and SC (low-risk) additionally to the prototypic HCMV-TB40/E, knowing that all strains infect HMECs in vitro. Numerous pro-oncogenic features including enhanced expression of oncogenes, cell survival, proliferation, and epithelial-mesenchymal transition genes were observed with HCMV-BL. In vitro, mammosphere formation was observed only in high-risk strains. HCMV-TB40/E showed an intermediate transcriptome landscape with limited mammosphere formation. Since we observed that Ki67 gene expression allows us to discriminate between high and low-risk HCMV strains in vitro, we further tested its expression in vivo. Among HCMV-positive breast cancer biopsies, we only detected high expression of the Ki67 gene in basal tumors which may correspond to the presence of high-risk HCMV strains within tumors. Altogether, the transcriptome of HMECs infected with HCMV clinical isolates displays an “oncogenic gradient” where high-risk strains specifically induce a prooncogenic environment which might participate in breast cancer development.

The Role of the JC Virus in Central Nervous System Tumorigenesis

Cancer is the second leading cause of mortality worldwide. The study of DNA tumor-inducing viruses and their oncoproteins as a causative agent in cancer initiation and tumor progression has greatly enhanced our understanding of cancer cell biology. The initiation of oncogenesis is a complex process. Specific gene mutations cause functional changes in the cell that ultimately result in the inability to regulate cell differentiation and proliferation effectively. The human neurotropic Polyomavirus JC (JCV) belongs to the family Polyomaviridae and it is the causative agent of progressive multifocal leukoencephalopathy (PML), which is a fatal neurodegenerative disease in an immunosuppressed state. Sero-epidemiological studies have indicated JCV infection is prevalent in the population (85%) and that initial infection usually occurs during childhood. The JC virus has small circular, double-stranded DNA that includes coding sequences for viral early and late proteins. Persistence of the virus in the brain and other tissues, as well as its potential to transform cells, has made it a subject of study for its role in brain tumor development. Earlier observation of malignant astrocytes and oligodendrocytes in PML, as well as glioblastoma formation in non-human primates inoculated with JCV, led to the hypothesis that JCV plays a role in central nervous system (CNS) tumorigenesis. Some studies have reported the presence of both JC viral DNA and its proteins in several primary brain tumor specimens. The discovery of new Polyomaviruses such as the Merkel cell Polyomavirus, which is associated with Merkel cell carcinomas in humans, ignited our interest in the role of the JC virus in CNS tumors. The current evidence known about JCV and its effects, which are sufficient to produce tumors in animal models, suggest it can be a causative factor in central nervous system tumorigenesis. However, there is no clear association between JCV presence in CNS and its ability to initiate CNS cancer and tumor formation in humans. In this review, we will discuss the correlation between JCV and tumorigenesis of CNS in animal models, and we will give an overview of the current evidence for the JC virus’s role in brain tumor formation.

Cell transformation by the adenovirus oncogenes E1 and E4

Extensive studies on viral-mediated oncogenic transformation by human adenoviruses have revealed much of our current understanding on the molecular mechanisms that are involved in the process. To date, these studies have shown that cell transformation is a multistep process regulated by the cooperation of several adenoviral gene products encoded in the early regions 1 (E1) and 4 (E4). Early region 1A immortalizes primary rodent cells, whereas co-expression of early region protein 1B induces full manifestation of the transformed phenotype. Beside E1 proteins, also some E4 proteins have partial transforming activities through regulating many cellular pathways. Here, we summarize recent data of how adenoviral oncoproteins may contribute to viral transformation and discuss the challenge of pinpointing the underlying mechanisms.

THE RETINOBLASTOMA PROTEIN: A MASTER TUMOR SUPPRESSOR ACTS AS A LINK BETWEEN CELL CYCLE AND CELL ADHESION

RB1 was the first tumor suppressor gene discovered. Over four decades of work have revealed that the Rb protein (pRb) is a master regulator of biological pathways influencing virtually every aspect of intrinsic cell fate including cell growth, cell-cycle checkpoints, differentiation, senescence, self-renewal, replication, genomic stability and apoptosis. While these many processes may account for a significant portion of RB1's potency as a tumor suppressor, a small, but growing stream of evidence suggests that RB1 also significantly influences how a cell interacts with its environment, including cell-to-cell and cell-to-extracellular matrix interactions. This review will highlight pRb's role in the control of cell adhesion and how alterations in the adhesive properties of tumor cells may drive the deadly process of metastasis.

Long Range Force between Pre-Replication Complexes (Pre-RC) in DNA Controls Replication and Cell Cycle Progression

A nonstationary interaction, that controls DNA replication and the cell cycle, is derived from a manybody physics model in a chemically open T cell. The model predicts a long range force F'(ξ)=-(κ/2) ξ(1-ξ)(2-ξ) between the pre-replication complexes (pre-RCs) bound by DNA, ξ=ϕ/N being the relative displacement of preRCs, ϕ the number of pre-RCs, N the threshold for initiation, and κ the compressibility modulus in thelattice of pre-RCs which behaves like an elastically braced string. Initiation of DNA replication is induced by a switch of sign of F'(ξ), from attraction (-)and assembly in the G(1) phase (0 < ϕ < N), to repulsion (+) and partialdisassembly in the S phase (N < ϕ < 2N), with release of licensing factors from the pre-RCs, thus explaining prevention of re-replication. Replication is terminated by a switch of sign of F at ϕ = 2N, when all primed replicons are duplicated once, and F(0)=0 corresponds to a resting cell in absence of driving force at ϕ = 0. The switch of sign of force at ϕ = N also explains the dynamic instability in growing microtubules (MTs), as well as switch in the interleukin-2 (IL2) interaction with its receptor in late G(1), at the restriction point. Shape, slope and scale of the response curves derived agree well with experimental data from dividing T cells and polymerizing MTs, the variable length of which is due to anonlinear dependence of the growth amplitude on the initial concentrations of tubulin dimers and guanosine-tri-phosphate (GTP).

LONG RANGE INTERACTION BETWEEN PROTEIN COMPLEXES IN DNA CONTROLS REPLICATION AND CELL CYCLE PROGRESSION

A model that controls DNA replication and the cell cycle is derived in terms of manybody physics. It predicts a long range force F(φ) =-(κ/2)φ(1-φ/N) (2-φ/N) in the lattice of pre-replication complexes (pre-RCs) bound by the DNA duplex, φ being the number of pre-RCS, N the threshold number at which replication is initiated, and κ the compressibility modulus in the lattice which behaves like an elastically braced string. Initiation of replication is explained by a switch of sign of F, from attraction (-) and assembly in the G1 phase (0 <φ<N), to repulsion (+) and partial disassembly in the S phase (N<φ<2N), with concomitant release of licensing factors from pre-RCs and prevention of re-replication. Termination of replication is due to a vanishing of F at φ=2N, at which all primed replicons in DNA have been duplicated once, and F(0)=0 corresponds to a resting cell in absence of a driving force at φ=0. The switch of sign of F at φ=N similarly explains the dynamic instability in growing microtubules (MTs), as well as the switch mechanism in the interaction of interleukin-2 (IL2) with its receptor in late G1, at the R-point, after which a T cell proceeds to replication without further exposure to IL2. Shape, slope and scale of the response curves derived agree well with experimental data from dividing T cells and polymerizing MTs, the variable length of which is explained here by a nonlinear dependence of the growth amplitude on the initial concentrations of guanosine-triphosphate (GTP) and tubulin dimers.

The adenoviral E1B 55-kilodalton protein controls expression of immune response genes but not p53-dependent transcription

The human adenovirus type 5 (Ad5) E1B 55-kDa protein modulates several cellular processes, including activation of the tumor suppressor p53. Binding of the E1B protein to the activation domain of p53 inhibits p53-dependent transcription. This activity has been correlated with the transforming activity of the E1B protein, but its contribution to viral replication is not well understood. To address this issue, we used microarray hybridization methods to examine cellular gene expression in normal human fibroblasts (HFFs) infected by Ad5, the E1B 55-kDa-protein-null mutant Hr6, or a mutant carrying substitutions that impair repression of p53-dependent transcription. Comparison of the changes in cellular gene expression observed in these and our previous experiments (D. L. Miller et al., Genome Biol. 8:R58, 2007) by significance analysis of microarrays indicated excellent reproducibility. Furthermore, we again observed that Ad5 infection led to efficient reversal of the p53-dependent transcriptional program. As this same response was also induced in cells infected by the two mutants, we conclude that the E1B 55-kDa protein is not necessary to block activation of p53 in Ad5-infected cells. However, groups of cellular genes that were altered in expression specifically in the absence of the E1B protein were identified by consensus k-means clustering of the hybridization data. Statistical analysis of the enrichment of genes associated with specific functions in these clusters established that the E1B 55-kDa protein is necessary for repression of genes encoding proteins that mediate antiviral and immune defenses.

Transcriptional upregulation of p57 (Kip2) by the cyclin-dependent kinase inhibitor BMS-387032 is E2F dependent and serves as a negative feedback loop limiting cytotoxicity

In spite of the fact that cyclin-dependent kinase (cdk) inhibiting drugs are potent transcriptional repressors, we discover that p57 (Kip2, CDKN1C) transcription is significantly upregulated by three small molecule cdk inhibitors, including BMS-387032. Treatment of MDA-MB-231 breast cancer cells with BMS-387032 led to a stabilization of the E2F1 protein that was accompanied by significant increases in the p57 mRNA and protein. This increase did not occur in an E2F1-deficient cell line. An E2F1-estrogen receptor fusion protein activated the endogenous p57 promoter in response to hydroxytamoxifen treatment in the presence of cycloheximide. Luciferase constructs driven by the p57 promoter verified that upregulation of p57 mRNA by BMS-387032 is transcriptional and dependent on E2F-binding sites in the promoter. Expression of exogenous p57 significantly decreased the fraction of cells in S phase. Furthermore, p57-deficient MDA-MB-231 cell lines were significantly more sensitive to BMS-387032-induced apoptosis than controls. The results presented in this manuscript demonstrate that small molecule cdk inhibitors transcriptionally activate p57 dependent upon E2F1 and that this activation in turn serves to limit E2F1's death-inducing activity.

Expression of JC virus regulatory proteins in human cancer: potential mechanisms for tumourigenesis

JC virus (JCV) is a human polyomavirus that is the etiologic agent of the fatal demyelinating disease of the central nervous system known as progressive multifocal leukoencephalopathy (PML). JCV is also linked to some tumours of the brain and other organs as evidenced by the presence of JCV DNA sequences and the expression of viral proteins in clinical samples. Since JCV is highly oncogenic in experimental animals and transforms cells in culture, it is possible that JCV contributes to the malignant phenotype of human tumours with which it is associated. JCV encodes three non-capsid regulatory proteins: large T-antigen, small t-antigen and agnoprotein that interact with a number of cellular target proteins and interfere with certain normal cellular functions. In this review, we discuss how JCV proteins deregulate signalling pathways especially ones pertaining to transcriptional regulation and cell cycle control. These effects may be involved in the progression of JCV-associated tumours and may represent potential therapeutic targets.

Cell transformation by human adenoviruses

The last 40 years of molecular biological investigations into human adenoviruses have contributed enormously to our understanding of the basic principles of normal and malignant cell growth. Much of this knowledge stems from analyses of their productive infection cycle in permissive host cells. Also, initial observations concerning the carcinogenic potential of human adenoviruses subsequently revealed decisive insights into the molecular mechanisms of the origins of cancer, and established adenoviruses as a model system for explaining virus-mediated transformation processes. Today it is well established that cell transformation by human adenoviruses is a multistep process involving several gene products encoded in early transcription units 1A (E1A) and 1B (E1B). Moreover, a large body of evidence now indicates that alternative or additional mechanisms are engaged in adenovirus-mediated oncogenic transformation involving gene products encoded in early region 4 (E4) as well as epigenetic changes resulting from viral DNA integration. In particular, detailed studies on the tumorigenic potential of subgroup D adenovirus type 9 (Ad9) E4 have now revealed a new pathway that points to a novel, general mechanism of virus-mediated oncogenesis. In this chapter, we summarize the current state of knowledge about the oncogenes and oncogene products of human adenoviruses, focusing particularly on recent findings concerning the transforming and oncogenic properties of viral proteins encoded in the E1B and E4 transcription units.

Transactivation of human cdc2 promoter by adenovirus E1A

Expression of the adenovirus oncoprotein E1A 12S induces the heterotrimeric transcription factor, NF-Y. NF-Y binds to the two CCAAT motifs upstream of the transcriptional start site of the human cdc2 promoter and is required for activation of the promoter by E1A 12S in cycling cells. The observations that a number of eukaryotic cell cycle regulatory genes also contain the CCAAT motifs and NF-Y binds to them support the notion that E1A 12S could play an important role in deregulated expression of these genes through activation of NF-Y gene in cycling cells.

Role of the E1A Rb-binding domain in repression of the NF-kappa B-dependent defense against tumor necrosis factor-alpha

The adenoviral E1A oncogene sensitizes mammalian cells to tumor necrosis factor-alpha (TNF-alpha), in part by repressing the nuclear factor-kappa B (NF-kappa B)-dependent defense against this cytokine. Other E1A activities involve binding to either p300/cyclic AMP response element-binding protein (CBP) or retinoblastoma (Rb)-family proteins, but the roles of E1A interactions with these transcriptional regulators in sensitizing cells to TNF-alpha are unclear. E1A expression did not block upstream events in TNF-alpha-induced activation of NF-kappa B in NIH 3T3 cells, including degradation of I kappa B-alpha, nuclear translocation of NF-kappa B subunits, and their dimeric binding to kappa B sequences in the nucleus. However, E1A markedly repressed NF-kappa B-dependent transcription and sensitized cells to TNF-alpha induced apoptosis. These E1A effects were selective for kappa B-dependent transcription and for the function of the NF-kappa B p65/RelA subunit. A four amino acid E1A deletion that eliminates binding to Rb-family proteins blocked both repression of TNF-alpha-induced transcription and sensitization to apoptosis. In contrast, mutations that eliminate E1A binding to p300/CBP (coactivators of p65/RelA) did not affect either E1A activity. These data suggest that E1A-Rb-binding blocks the NF-kappa B-dependent activation response to TNF-alpha by altering the function of p65/RelA at a stage after formation of the transcription factor-enhancer complex. These observations also open questions about the general role of Rb-family proteins in modulation of NF-kappa B-dependent transcription.

DNA Replication and Cell Cycle Progression Regulatedby Long Range Interaction between Protein Complexes bound to DNA

A nonstationary interaction that controlsDNA replication and the cell cycle isderived from many-body physics in achemically open T cell. The model predictsa long range force F'(ξ) =- (κ/2) ξ(1 - ξ)(2 - ξ)between thepre-replication complexes (pre-RCs) boundby the origins in DNA, ξ = ϕ/N being the relativedisplacement of pre-RCs, ϕ the number of pre-RCs, N the number of replicons to be replicated,and κ the compressibilitymodulus in the lattice of pre-RCs whichbehaves dynamically like an elasticallybraced string. Initiation of DNAreplication is induced at the thresholdϕ = N by a switch ofsign of F''(ξ), fromattraction (-) and assembly in the G(1) phase (0<ϕ Collapse

Key Words

• Cell cycle
• DNA duplex
• DNA origin
• DNA replication
• Rb protein
• cyclin
• kinases
• licensing factor
• microtubules
• origin recognition complex
• p27
• recognition complex

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Adenovirus early E4 genes in viral oncogenesis

Previous investigations into potential transforming activities of adenovirus (Ad) early genes were largely overshadowed by the more obvious roles of E1A and E1B products. One exception was an Ad9 E4 protein (ORF1) shown to enhance transformation of cultured cells and promote mammary tumors in female rats. Recently, significant advances in understanding Ad E4 gene products at the molecular level have revealed that these proteins possess an unexpectedly diverse collection of functions, which not only orchestrate many viral processes, but overlap with oncogenic transformation of primary mammalian cells. Operating through a complex network of protein interactions with key viral and cellular regulatory components, Ad E4 products are apparently involved in transcription, apoptosis, cell cycle control, DNA repair, cell signaling, posttranslational modifications and the integrity of nuclear multiprotein complexes known as PML oncogenic domains (PODs). Some of these functions directly relate to known transforming and oncogenic processes, or implicate mechanisms such as modulating the function and subcellular localization of cellular PDZ domain-containing proteins, POD reorganization, targeted proteolytic degradation, inhibition of DNA double-strand break repair and 'hit-and-run' mutagenesis. Here, we summarize the recent data and discuss how E4 gene product interactions may contribute to viral oncogenesis.

p202, an interferon-inducible negative regulator of cell growth, is a target of the adenovirus E1A protein

Studies have revealed that human adenovirus-encoded E1A protein promotes cell proliferation through the targeted interaction with cellular proteins that act as key negative regulators of cell growth. The targets of E1A protein include the retinoblastoma tumor suppressor protein (pRb). Because p202, an interferon (IFN)-inducible murine protein (52-kDa), negatively regulates cell growth in part through the pRb/E2F pathway, we tested whether the p202 is a target of the adenovirus-encoded E1A protein for functional inactivation. Here we report that the expression of E1A protein overcame p202-mediated inhibition of cell growth and this correlated with an alleviation of p202-mediated inhibition of the transcriptional activity of E2F. Furthermore, E1A protein relieved p202-mediated inhibition of the specific DNA-binding activity of E2F complexes, including those containing the pocket proteins. Additionally, the E1A protein bound to p202 both in vitro and in vivo and a deletion of four amino acids in the conserved region 2 (CR2) of E1A protein significantly reduced the binding of E1A to p202. Interestingly, ectopic expression of p202 under reduced serum conditions significantly reduced E1A-mediated apoptosis. Taken together, our observations provide support to the idea that the p202 and adenovirus E1A protein functionally counteract each other and E1A protein targets p202 to promote cell proliferation.

The E4-6/7 protein functionally compensates for the loss of E1A expression in adenovirus infection

The E1A gene products are required and sufficient for activation of adenovirus gene expression in cultured cells. The E4-6/7 gene product induces the binding of the cellular transcription factor E2F to the viral E2a promoter region. The induction of E2F binding to the E2a promoter in vitro is directly correlated with transcriptional activation of the E2a promoter in vivo. The E2 region encodes the viral replication proteins, yet adenoviruses lacking E4-6/7 function demonstrate no defective phenotype in infected cells. Here we show that the E4-6/7 protein can functionally compensate for E1A expression in virus infection. In the absence of the E1A gene products, expression of the E4-6/7 protein is sufficient to displace retinoblastoma protein family members from E2Fs, activate expression of early region 2 via induction of E2F DNA binding to the E2a promoter region, and significantly enhance replication of an E1A-defective adenovirus. These results have implications in the regulation of viral gene expression and for the development of recombinant adenovirus vectors.

Induction of the cellular E2F-1 promoter by the adenovirus E4-6/7 protein

The adenovirus type 5 (Ad5) E4-6/7 protein interacts directly with different members of the E2F family and mediates the cooperative and stable binding of E2F to a unique pair of binding sites in the Ad5 E2a promoter region. This induction of E2F DNA binding activity strongly correlates with increased E2a transcription when analyzed using virus infection and transient expression assays. Here we show that while different adenovirus isolates express an E4-6/7 protein that is capable of induction of E2F dimerization and stable DNA binding to the Ad5 E2a promoter region, not all of these viruses carry the inverted E2F binding site targets in their E2a promoter regions. The Ad12 and Ad40 E2a promoter regions bind E2F via a single binding site. However, these promoters bind adenovirus-induced (dimerized) E2F very weakly. The Ad3 E2a promoter region binds E2F very poorly, even via a single binding site. A possible explanation of these results is that the Ad E4-6/7 protein evolved to induce cellular gene expression. Consistent with this notion, we show that infection with different adenovirus isolates induces the binding of E2F to an inverted configuration of binding sites present in the cellular E2F-1 promoter. Transient expression of the E4-6/7 protein alone in uninfected cells is sufficient to induce transactivation of the E2F-1 promoter linked to chloramphenicol acetyltransferase or green fluorescent protein reporter genes. Further, expression of the E4-6/7 protein in the context of adenovirus infection induces E2F-1 protein accumulation. Thus, the induction of E2F binding to the E2F-1 promoter by the E4-6/7 protein observed in vitro correlates with transactivation of E2F-1 promoter activity in vivo. These results suggest that adenovirus has evolved two distinct mechanisms to induce the expression of the E2F-1 gene. The E1A proteins displace repressors of E2F activity (the Rb family members) and thus relieve E2F-1 promoter repression; the E4-6/7 protein complements this function by stably recruiting active E2F to the E2F-1 promoter to transactivate expression.

Rb and prohibitin target distinct regions of E2F1 for repression and respond to different upstream signals

E2F transcription factor is subject to stringent regulation by a variety of molecules. We recently observed that prohibitin, a potential tumor suppressor protein, binds to the retinoblastoma (Rb) protein and represses E2F transcriptional activity. Here we demonstrate that prohibitin requires the marked box region of E2F for repression; further, prohibitin can effectively inhibit colony formation induced by overexpression of E2F1 in T47D cells. Prohibitin was also found to interact with the signaling kinase c-Raf-1, and Raf-1 could effectively reverse prohibitin-mediated repression of E2F activity. Agents such as E1A, p38 kinase, and cyclins D and E had no effect on prohibitin-mediated repression of E2F1, but all of these molecules could reverse Rb function. Similarly, stimulation of the immunoglobulin M signaling pathway in Ramos cells could inactivate prohibitin, but this had no effect on Rb function. Serum stimulation of quiescent Ramos cells inactivated Rb and prohibitin with different kinetics; further, while the serum-dependent inactivation of Rb was dependent on cyclin-dependent kinase activity, the inactivation of prohibitin was not. We believe that prohibitin is a novel regulator of E2F function which channels specific signaling cascades to the cell cycle regulatory machinery.

Disruption of retinoblastoma protein family function by human papillomavirus type 16 E7 oncoprotein inhibits lens development in part through E2F-1

Complexes between the retinoblastoma protein (pRb) and the transcription factor E2F-1 are thought to be important for regulating cell proliferation. We have shown previously that the E7 oncoprotein from human papillomavirus type 16, dependent upon its binding to pRb proteins, induces proliferation, disrupts differentiation, and induces apoptosis when expressed in the differentiating, or fiber, cells of the ocular lenses in transgenic mice. Mice that carry a null mutation in E2F-1 do not exhibit any defects in proliferation and differentiation in the lens. By examining the lens phenotype in mice that express E7 on an E2F-1 null background, we now show genetic evidence that E7's ability to alter the fate of fiber cells is partially dependent on E2F-1. On the other hand, E2F-1 status does not affect E7-induced proliferation in the undifferentiated lens epithelium. These data provide genetic evidence that E2F-1, while dispensible for normal fiber cell differentiation, is one mediator of E7's activity in vivo and that the requirement for E2F-1 is context dependent. These data suggest that an important role for pRb-E2F-1 complex during fiber cell differentiation is to negatively regulate cell cycle progression, thereby allowing completion of the differentiation program to occur.

Regulation of differentiation by HBP1, a target of the retinoblastoma protein

Differentiation is a coordinated process of irreversible cell cycle exit and tissue-specific gene expression. To probe the functions of the retinoblastoma protein (RB) family in cell differentiation, we isolated HBP1 as a specific target of RB and p130. Our previous work showed that HBP1 was a transcriptional repressor and a cell cycle inhibitor. The induction of HBP1, RB, and p130 upon differentiation in the muscle C2C12 cells suggested a coordinated role. Here we report that the expression of HBP1 unexpectedly blocked muscle cell differentiation without interfering with cell cycle exit. Moreover, the expression of MyoD and myogenin, but not Myf5, was inhibited in HBP1-expressing cells. HBP1 inhibited transcriptional activation by the MyoD family members. The inhibition of MyoD family function by HBP1 required binding to RB and/or p130. Since Myf5 might function upstream of MyoD, our data suggested that HBP1 probably blocked differentiation by disrupting Myf5 function, thus preventing expression of MyoD and myogenin. Consistent with this, the expression of each MyoD family member could reverse the inhibition of differentiation by HBP1. Further investigation implicated the relative ratio of RB to HBP1 as a determinant of whether cell cycle exit or full differentiation occurred. At a low RB/HBP1 ratio cell cycle exit occurred but there was no tissue-specific gene expression. At elevated RB/HBP1 ratios full differentiation occurred. Similar changes in the RB/HBP1 ratio have been observed in normal C2 differentiation. Thus, we postulate that the relative ratio of RB to HBP1 may be one signal for activation of the MyoD family. We propose a model in which a checkpoint of positive and negative regulation may coordinate cell cycle exit with MyoD family activation to give fidelity and progression in differentiation.

Relationship between alkaline phosphatase levels, osteopontin expression, and mineralization in differentiating MC3T3-E1 osteoblasts

We are using viral oncogene probes to study the pathways by which osteoblast-specific gene expression is induced in ascorbic acid-treated MC3T3-E1 cells. The 12S product of the adenovirus E1A gene binds directly to key cellular regulators and, as a result, represses tissue specific gene expression and blocks differentiation in a wide variety of cell types. The main cellular targets of the E1A 12S product are the pRB family and p300/CBP family. The p300 family appears to be the primary target for E1A-mediated repression of tissue-specific gene expression in a variety of cell types. We have generated MC3T3-E1 cell lines that stably express either the wild-type 12S product or a mutant that targets p300/CBP, but not the pRB family. Using these constructs to dissect osteoblast differentiation, we found that targeting of p300/CBP appears to be sufficient to repress alkaline phosphatase expression, although a low but functional level of expression can be maintained if the pRB family is not targeted as well. Induction of alkaline phosphatase expression and activity can be dissociated from expression of late-stage markers such as osteocalcin and osteopontin. Surprisingly, cell lines exhibiting severe repression of alkaline phosphatase activity differentiate to a mineral-secreting phenotype much like normal MC3T3-E1 cells. Osteopontin induction is dependent on at least a minimal level of alkaline phosphatase activity, although it is not dependent on induction of alkaline phosphatase at the RNA level. If alkaline phosphatase is supplied exogenously, osteopontin expression can be induced in conditions in which endogenous alkaline phosphatase is severely repressed.

Identification of domains within the human cytomegalovirus major immediate-early 86-kilodalton protein and the retinoblastoma protein required for physical and functional interaction with each other

The human cytomegalovirus major immediate-early 86-kDa protein (IE2 86) plays an important role in the trans activation and regulation of HCMV gene expression. Previously, we demonstrated that IE2 86 contains three regions (amino acids [aa] 86 to 135, 136 to 290, and 291 to 364) that can independently bind to in vitro-translated Rb when IE2 86 is produced as a glutathione S-transferase fusion protein (M. H. Sommer, A. L. Scully, and D. H. Spector, J. Virol. 68:6223-6231, 1994). In this report, we have elucidated the regions of Rb involved in binding to IE2 86 and have further analyzed the functional nature of the interaction between these two proteins. We find that two domains on Rb, the A/B pocket and the carboxy terminus, can each independently form a complex with IE2 86. In functional assays, we demonstrate that IE2 86 and another IE protein, IE1 72, can counter the enlarged flat cell phenotype, but not the G1/S block, which results from expression of wild-type Rb in the human osteosarcoma cell line Saos-2. Mutational analysis reveals that there are two domains on IE2 86 that can independently affect Rb function. One region (aa 241 to 369) includes the major Rb-binding domain, while the second maps to the amino-terminal region (aa 1 to 85) common to both IE2 86 and IE1 72. These data show that Rb and IE2 86 physically and functionally interact with each other via at least two separate domains and provide further support for the hypothesis that IE2 86 may exert its pleiotropic effects through the formation of multimeric protein complexes.

Position-dependent transcriptional regulation of the murine dihydrofolate reductase promoter by the E2F transactivation domain

Activity of the dihydrofolate reductase (dhfr) promoter increases at the G1-S-phase boundary of the cell cycle. Mutations that abolish protein binding to an E2F element in the dhfr promoter also abolish the G1-S-phase increase in dhfr transcription, indicating that transcriptional regulation is mediated by the E2F family of proteins. To investigate the mechanism by which E2F regulates dhfr transcription, we moved the E2F element upstream and downstream of its natural position in the promoter. We found that the E2F element confers growth regulation to the dhfr promoter only when it is proximal to the transcription start site. Using a heterologous E2F element, we showed that position-dependent regulation is a property that is promoter specific, not E2F element specific. We demonstrated that E2F-mediated growth regulation of dhfr transcription requires activation of the dhfr promoter in S phase and that the C-terminal activation domains of E2F1, E2F4, and E2F5, when fused to the Gal4 DNA binding domain, are sufficient to specify position-dependent activation. To further investigate the role of activation in dhfr regulation, we tested other transactivation domains for their ability to activate the dhfr promoter. We found that the N-terminal transactivation domain of VP16 cannot activate the dhfr promoter. We propose that, unlike other E2F-regulated promoters, robust transcription from the dhfr promoter requires an E2F transactivation domain close to the transcription start site.