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

Functional Domains of the Early Proteins and Experimental and Epidemiological Studies Suggest a Role for the Novel Human Polyomaviruses in Cancer

As their name indicates, polyomaviruses (PyVs) can induce tumors. Mouse PyV, hamster PyV and raccoon PyV have been shown to cause tumors in their natural host. During the last 30 years, 15 PyVs have been isolated from humans. From these, Merkel cell PyV is classified as a Group 2A carcinogenic pathogen (probably carcinogenic to humans), whereas BKPyV and JCPyV are class 2B (possibly carcinogenic to humans) by the International Agency for Research on Cancer. Although the other PyVs recently detected in humans (referred to here as novel HPyV; nHPyV) share many common features with PyVs, including the viral oncoproteins large tumor antigen and small tumor antigen, as their role in cancer is questioned. This review discusses whether the nHPyVs may play a role in cancer based on predicted and experimentally proven functions of their early proteins in oncogenic processes. The functional domains that mediate the oncogenic properties of early proteins of known PyVs, that can cause cancer in their natural host or animal models, have been well characterized and we examined whether these functional domains are conserved in the early proteins of the nHPyVs and presented experimental evidence that these conserved domains are functional. Furthermore, we reviewed the literature describing the detection of nHPyV in human tumors.

The functional analysis of Cullin 7 E3 ubiquitin ligases in cancer

Cullin (CUL) proteins have critical roles in development and cancer, however few studies on CUL7 have been reported due to its characteristic molecular structure. CUL7 forms a complex with the ROC1 ring finger protein, and only two F-box proteins Fbxw8 and Fbxw11 have been shown to bind to CUL7. Interestingly, CUL7 can interact with its substrates by forming a novel complex that is independent of these two F-box proteins. The biological implications of CUL-ring ligase 7 (CRL7) suggest that the CRL7 may not only perform a proteolytic function but may also play a non-proteolytic role. Among the existing studied CRL7-based E3 ligases, CUL7 exerts both tumor promotion and suppression in a context-dependent manner. Currently, the mechanism of CUL7 in cancer remains unclear, and no studies have addressed potential therapies targeting CUL7. Consistent with the roles of the various CRL7 adaptors exhibit, targeting CRL7 might be an effective strategy for cancer prevention and treatment. We systematically describe the recent major advances in understanding the role of the CUL7 E3 ligase in cancer and further summarize its potential use in clinical therapy.

Cullin-RING E3 Ubiquitin Ligase 7 in Growth Control and Cancer

CRL7^Fbxw8 is an E3 ubiquitin ligase complex, containing cullin7 (CUL7) as a scaffold, the F-box protein Fbxw8 as a substrate receptor, the Skp1 adaptor, and the ROC1/Rbx1 RING finger protein for working with E2 enzyme to facilitate ubiquitin transfer. This chapter provides an update on studies linking CRL7^Fbxw8 to hereditary human growth retardation disease, as at least 64 cul7 germ line mutations were found in patients with autosomal recessive 3-M syndrome. CRL7^Fbxw8 interacts with two additional 3-M associated proteins OBSL1 and CCDC8, leading to subcellular localization of the E3 complex to regions including plasma membrane, centrosome, and Golgi. At least ten mammalian cellular proteins were identified or implicated as CRL7^Fbxw8 substrates. Discussion focuses on the possible impact of CRL7^Fbxw8-mediated proteolytic or non-proteolytic pathways in growth control and cancer.

Inhibition of Cullin-RING E3 ubiquitin ligase 7 by simian virus 40 large T antigen

Simian virus 40 (SV40) large tumor antigen (LT) triggers oncogenic transformation by inhibition of key tumor suppressor proteins, including p53 and members of the retinoblastoma family. In addition, SV40 transformation requires binding of LT to Cullin 7 (CUL7), a core component of Cullin-RING E3 ubiquitin ligase 7 (CRL7). However, the pathomechanistic effects of LT-CUL7 interaction are mostly unknown. Here we report both in vitro and in vivo experimental evidence that SV40 LT suppresses the ubiquitin ligase function of CRL7. We show that SV40 LT, but not a CUL7 binding-deficient mutant (LT(Δ69-83)), impaired 26S proteasome-dependent proteolysis of the CRL7 target protein insulin receptor substrate 1 (IRS1), a component of the insulin and insulin-like growth factor 1 signaling pathway. SV40 LT expression resulted in the accumulation and prolonged half-life of IRS1. In vitro, purified SV40 LT reduced CRL7-dependent IRS1 ubiquitination in a concentration-dependent manner. Expression of SV40 LT, or depletion of CUL7 by RNA interference, resulted in the enhanced activation of IRS1 downstream signaling pathways phosphatidylinositol-3-kinase/AKT and Erk mitogen-activated pathway kinase, as well as up-regulation of the downstream target gene c-fos. Finally, SV40 LT-positive carcinoma of carcinoembryonic antigen 424/SV40 LT transgenic mice displayed elevated IRS1 protein levels and activation of downstream signaling. Taken together, these data suggest that SV40 LT protects IRS1 from CRL7-mediated degradation, thereby sustaining high levels of promitogenic IRS1 downstream signaling pathways.

The 3M syndrome

3M syndrome (MIM 273750) is an autosomal recessive disorder characterized by pre- and post-natal growth retardation (<-4 SD), facial dysmorphism, large head circumference, normal intelligence and endocrine function. Skeletal changes include long slender tubular bones and tall vertebral bodies. There is no specific treatment. Up till now, mutations in either CUL7 or OBSL1 genes have been identified in this rare disorder. There are no clinical or radiological differences between patients with CUL7 or OBSL1 mutations. CUL7 appears to be the major gene responsible for 3M syndrome accounting for 77.5% of cases while OBSL1 mutations accounts for 16.3%. A few patients have no mutations in these genes suggesting the involvement of a third gene.

Interaction and co-localization of JC virus large T antigen and the F-box protein β-transducin-repeat containing protein

Lytic infection and transformation of cultured cells by JC virus (JCV) require five tumor proteins, which interact with factors regulating critical cellular processes. We demonstrate that JCV large T antigen (TAg) binds the F-box proteins β-transducin-repeat containing protein-1 and 2 (βTrCP1/2). These interactions involve a phosphodegron (DpSGX(2-4)pS) found in βTrCP substrates. TAg stability is unaltered, suggesting TAg is a pseudo-substrate. βTrCP and TAg co-localize in the cytoplasm, and a functional SCF complex is required. We examined whether TAg influences the levels of β-catenin, a βTrCP substrate. We were unable to demonstrate that TAg elevates β-catenin as previously reported, and a mutant TAg unable to bind βTrCP also had no detectable effect on β-catenin stability. Results presented in this study link JCV TAg to the cellular degradation complex, SCF(βTrCP1/2). Proteasomal degradation is essential for proper regulation of cellular functions, and interference with proteasomal pathways highlights possible JCV pathogenic and oncogenic mechanisms.

Update on human polyomaviruses and cancer

Over 50 years of polyomavirus research has produced a wealth of insights into not only general biologic processes in mammalian cells, but also, how conditions can be altered and signaling systems tweaked to produce transformation phenotypes. In the past few years three new members (KIV, WUV, and MCV) have joined two previously known (JCV and BKV) human polyomaviruses. In this review, we present updated information on general virologic features of these polyomaviruses in their natural host, concentrating on the association of MCV with human Merkel cell carcinoma. We further present a discussion on advances made in SV40 as the prototypic model, which has and will continue to inform our understanding about viruses and cancer.

Cellular transformation by Simian Virus 40 and Murine Polyoma Virus T antigens

Simian Virus 40 (SV40) and Mouse Polyoma Virus (PY) are small DNA tumor viruses that have been used extensively to study cellular transformation. The SV40 early region encodes three tumor antigens, large T (LT), small T (ST) and 17KT that contribute to cellular transformation. While PY also encodes LT and ST, the unique middle T (MT) generates most of the transforming activity. SV40 LT mediated transformation requires binding to the tumor suppressor proteins Rb and p53 in the nucleus and ST binding to the protein phosphatase PP2A in the cytoplasm. SV40 LT also binds to several additional cellular proteins including p300, CBP, Cul7, IRS1, Bub1, Nbs1 and Fbxw7 that contribute to viral transformation. PY MT transformation is dependent on binding to PP2A and the Src family protein tyrosine kinases (PTK) and assembly of a signaling complex on cell membranes that leads to transformation in a manner similar to Her2/neu. Phosphorylation of MT tyrosine residues activates key signaling molecules including Shc/Grb2, PI3K and PLCgamma1. The unique contributions of SV40 LT and ST and PY MT to cellular transformation have provided significant insights into our understanding of tumor suppressors, oncogenes and the process of oncogenesis.

A truncated T antigen expressed from an alternatively spliced BK virus early mRNA

The early region of BK virus (BKV) is known to encode two well-characterized tumour (T) antigens, large T antigen (TAg) and small T antigen (tAg). In this study, we provide evidence of a third early BKV mRNA that codes for an additional early region product with an apparent molecular mass of 17-20 kDa. This truncated form of TAg (truncTAg) is expressed from an alternatively spliced mRNA that is derived from the excision of a second intron from the mRNA encoding TAg. The first 133 aa of truncTAg are identical to those of TAg but the additional splice results in translation from a different reading frame, adding three new amino acids before reaching a stop codon. TruncTAg is expressed in both BKV-transformed and lytically infected cells and it is found to be primarily localized to the nucleus. The function of BKV truncTAg is likely to be relevant to transformation, similar to the additional T antigens of simian virus 40, JC virus and mouse polyomavirus.

T antigen transgenic mouse models

The study of polyomavirus has benefited immensely from two scientific methodologies, cell culture and in vitro studies on one side and the use of transgenic mice as experimental models on the other. Both approaches allowed us to identify cellular products targeted by the viruses, the consequences of these interactions at the phenotypic and molecular level, and thus the potential roles of the targets within their normal cellular context. In particular, cell culture and in vitro reports suggest a model explaining partially how SV40 large T antigen contributes to oncogenic transformation. In most cases, T antigen induces cell cycle entry by inactivation of the Rb proteins (pRb, p130, and p107), thus activating E2F-dependent transcription and subsequent S-phase entry. Simultaneously, T antigen blocks p53 activity and therefore prevents the ensuing cell-cycle arrest and apoptosis. For the most part, studies of T antigen expression in transgenic mice support this model, but the use of T antigen mutants and their expression in different tissue and cell type settings have expanded our knowledge of the model system and raised important questions regarding tumorigenic mechanisms functioning in vivo.

Manipulation of the ubiquitin-proteasome pathway by small DNA tumor viruses

Viruses have evolved to use cellular pathways to their advantage, including the ubiquitin-proteasome pathway of protein degradation. In several cases, viruses produce proteins that highjack cellular E3 ligases to modify their substrate specificity in order to eliminate unwanted cellular proteins, in particular inhibitors of the cell cycle. They can also inhibit E3 ligase to prevent specific protein degradation or even use the system to control the level of expression of their own proteins. In this review we explore the specific ways that small DNA tumor viruses exploit the ubiquitin-proteasome pathway for their own benefit.

A large-scale mutation search reveals genetic heterogeneity in 3M syndrome

The 3M syndrome is a rare autosomal recessive disorder recently ascribed to mutations in the CUL7 gene and characterized by severe pre- and postnatal growth retardation. Studying a series of 33 novel cases of 3M syndrome, we have identified deleterious CUL7 mutations in 23/33 patients, including 19 novel mutations and one paternal isodisomy of chromosome 6 encompassing a CUL7 mutation. Lack of mutations in 10/33 cases and exclusion of the CUL7 locus on chromosome 6p21.1 in six consanguineous families strongly support the genetic heterogeneity of the 3M syndrome.

The cullin7 E3 ubiquitin ligase: a novel player in growth control

Cullin7 (CUL7) is a molecular scaffold that organizes an E3 ubiquitin ligase containing the F-box protein Fbw8, Skp1 and the ROC1 RING finger protein. Dysregulation of the CUL7 E3 Ligase has been directly linked to hereditary human diseases as cul7 germline mutations were found in patients with autosomal-recessive 3-M and Yakuts short stature syndromes, which are characterized by profound pre- and postnatal growth retardation. In addition, genetic ablation of CUL7 in mice resulted in intrauterine growth retardation and perinatal lethality, underscoring its importance for growth regulation. The recent identification of insulin receptor substrate 1, a critical mediator of insulin and insulin-like growth factor-1 signaling, as the proteolytic target of the CUL7 E3 ligase, provided a molecular link between CUL7 and a well-established growth regulatory pathway. This result, coupled with other studies demonstrating interactions between CUL7 and the p53 tumor suppressor protein, as well as the simian virus 40 large T antigen oncoprotein, further implicated CUL7 as a novel player in growth control and suggested pathomechanistic insights into CUL7-linked growth retardation syndromes.

The CUL7 E3 ubiquitin ligase targets insulin receptor substrate 1 for ubiquitin-dependent degradation

Recent genetic studies have documented a pivotal growth-regulatory role played by the Cullin 7 (CUL7) E3 ubiquitin ligase complex containing the Fbw8-substrate-targeting subunit, Skp1, and the ROC1 RING finger protein. In this report, we identified insulin receptor substrate 1 (IRS-1), a critical mediator of the insulin/insulin-like growth factor 1 signaling, as a proteolytic target of the CUL7 E3 ligase in a manner that depends on mammalian target of rapamycin and the p70 S6 kinase activities. Interestingly, while embryonic fibroblasts of Cul7-/- mice were found to accumulate IRS-1 and exhibit increased activation of IRS-1's downstream Akt and MEK/ERK pathways, these null cells grew poorly and displayed phenotypes reminiscent of those associated with oncogene-induced senescence. Taken together, our findings demonstrate a key role for the CUL7 E3 in targeting IRS-1 for degradation, a process that may contribute to the regulation of cellular senescence.

Ataxia telangiectasia-mutated damage-signaling kinase- and proteasome-dependent destruction of Mre11-Rad50-Nbs1 subunits in Simian virus 40-infected primate cells

Although the mechanism of simian virus 40 (SV40) DNA replication has been extensively investigated with cell extracts, viral DNA replication in productively infected cells utilizes additional viral and host functions whose interplay remains poorly understood. We show here that in SV40-infected primate cells, the activated ataxia telangiectasia-mutated (ATM) damage-signaling kinase, gamma-H2AX, and Mre11-Rad50-Nbs1 (MRN) assemble with T antigen and other viral DNA replication proteins in large nuclear foci. During infection, steady-state levels of MRN subunits decline, although the corresponding mRNA levels remain unchanged. A proteasome inhibitor stabilizes the MRN complex, suggesting that MRN may undergo proteasome-dependent degradation. Analysis of mutant T antigens with disrupted binding to the ubiquitin ligase CUL7 revealed that MRN subunits are stable in cells infected with mutant virus or transfected with mutant viral DNA, implicating CUL7 association with T antigen in MRN proteolysis. The mutant genomes produce fewer virus progeny than the wild type, suggesting that T antigen-CUL7-directed proteolysis facilitates virus propagation. Use of a specific ATM kinase inhibitor showed that ATM kinase signaling is a prerequisite for proteasome-dependent degradation of MRN subunits as well as for the localization of T antigen and damage-signaling proteins to viral replication foci and optimal viral DNA replication. Taken together, the results indicate that SV40 infection manipulates host DNA damage-signaling to reprogram the cell for viral replication, perhaps through mechanisms related to host recovery from DNA damage.

Induction of cullin 7 by DNA damage attenuates p53 function

The p53 tumor suppressor gene encodes a transcription factor, which is translationally and posttranslationally activated after DNA damage. In a proteomic screen for p53 interactors, we found that the cullin protein Cul7 efficiently associates with p53. After DNA damage, the level of Cul7 protein increased in a caffeine-sensitive, but p53-independent, manner. Down-regulation of Cul7 by conditional microRNA expression augmented p53-mediated inhibition of cell cycle progression. Ectopic expression of Cul7 inhibited activation of p53 by DNA damaging agents and sensitized cells to adriamycin. Although Cul7 recruited the F-box protein FBX29 to p53, the combined expression of Cul7/FBX29 did not promote ubiquitination and degradation of p53 in vivo. Therefore, the inhibition of p53 activity by Cul7 is presumably mediated by alternative mechanisms. The interplay between p53 and Cul7 resembles the negative feedback loop described for p53 and Mdm2. Pharmacological modulation of Cul7 function may allow the sensitization of cancer cells expressing wild-type p53 to genotoxic agents used in cancer therapy.

JC virus T'135, T'136 and T'165 proteins interact with cellular p107 and p130 in vivo and influence viral transformation potential

The JC virus (JCV) regulatory proteins, large T antigen, small t antigen, T'135, T'136, and T'165, are encoded by five transcripts alternatively spliced from the viral early precursor mRNA. T antigen and the T' proteins share N-terminal amino acid sequences that include the L x CxE and J domains, motifs in SV40 T antigen known to mediate binding to the retinoblastoma (Rb) proteins and Hsc70, respectively. In this study, G418-resistant cell lines were created that express wild-type or mutant JCV T antigen and T' proteins individually or in combination. These cell lines were used to evaluate the ability of each viral protein to bind p107 and p130 in vivo, and to influence cellular growth characteristics. Differences were observed in the abilities of individual T' proteins to bind p107 and p130 and to alter their phosphorylation status. The T' proteins were also found to localize to the cell's nucleus and to be phosphorylated in a cell cycle-dependent manner. JCV T antigen and T' proteins expressed from a cytomegalovirus promoter failed to induce dense focus formation in Rat2 cells, but they did cooperate with a mutant Ras protein to overcome cellular senescence and immortalize rat embryo fibroblasts. These data indicate that, despite their sequence similarities, JCV early proteins exhibit unique activities that, in combination, effect the inactivation of cell cycle regulators, a requirement for polyomavirus-induced transformation.

Fbxw8 is essential for Cul1-Cul7 complex formation and for placental development

Cullin-based ubiquitin ligases (E3s) constitute one of the largest E3 families. Fbxw8 (also known as Fbw6 or Fbx29) is an F-box protein that is assembled with Cul7 in an SCF-like E3 complex. Here we show that Cul7 forms a heterodimeric complex with Cul1 in a manner dependent on Fbxw8. We generated mice deficient in Fbxw8 and found that Cul7 did not associate with Cul1 in cells of these mice. Two-thirds of Fbxw8-/- embryos die in utero, whereas the remaining one-third are born alive and grow to adulthood. Fbxw8-/- embryos show intrauterine growth retardation and abnormal development of the placenta, characterized by both a reduced thickness of the spongiotrophoblast layer and abnormal vessel structure in the labyrinth layer. Although the placental phenotype of Fbxw8-/- mice resembles that of Cul7-/- mice, other abnormalities of Cul7-/- mice are not apparent in Fbxw8-/- mice. These results suggest that the Cul7-based SCF-like E3 complex has both Fbxw8-dependent and Fbxw8-independent functions.

A novel p53-binding domain in CUL7

CUL7 is a member of the cullin RING ligase family and forms an SCF-like complex with SKP1 and FBXW8. CUL7 is required for normal mouse embryonic development and cellular proliferation, and is highly homologous to PARC, a p53-associated, parkin-like cytoplasmic protein. We determined that CUL7, in a manner similar to PARC, can bind directly to p53 but does not affect p53 expression. We identified a discrete, co-linear domain in CUL7 that is conserved in PARC and HERC2, and is necessary and sufficient for p53-binding. The presence of p53 stabilized expression of this domain and we demonstrate that this p53-binding domain of CUL7 contributes to the cytoplasmic localization of CUL7. The results support the model that p53 plays a role in regulation of CUL7 activity.

SV40 large T antigen targets multiple cellular pathways to elicit cellular transformation

DNA tumor viruses such as simian virus 40 (SV40) express dominant acting oncoproteins that exert their effects by associating with key cellular targets and altering the signaling pathways they govern. Thus, tumor viruses have proved to be invaluable aids in identifying proteins that participate in tumorigenesis, and in understanding the molecular basis for the transformed phenotype. The roles played by the SV40-encoded 708 amino-acid large T antigen (T antigen), and 174 amino acid small T antigen (t antigen), in transformation have been examined extensively. These studies have firmly established that large T antigen's inhibition of the p53 and Rb-family of tumor suppressors and small T antigen's action on the pp2A phosphatase, are important for SV40-induced transformation. It is not yet clear if the Rb, p53 and pp2A proteins are the only targets through which SV40 transforms cells, or whether additional targets await discovery. Finally, expression of SV40 oncoproteins in transgenic mice results in effects ranging from hyperplasia to invasive carcinoma accompanied by metastasis, depending on the tissue in which they are expressed. Thus, the consequences of SV40 action on these targets depend on the cell type being studied. The identification of additional cellular targets important for transformation, and understanding the molecular basis for the cell type-specific action of the viral T antigens are two important areas through which SV40 will continue to contribute to our understanding of cancer.

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.

Simian virus 40 large T antigen's association with the CUL7 SCF complex contributes to cellular transformation

Simian virus 40 large T antigen (T Ag) is capable of immortalizing and transforming rodent cells. The transforming activity of T Ag is due in large part to perturbation of the tumor suppressor proteins p53 and the retinoblastoma (pRB) family members. Inactivation of these tumor suppressors may not be sufficient for T Ag-mediated cellular transformation. It has been shown that T Ag associates with an SCF-like complex that contains a member of the cullin family of E3 ubiquitin ligases, CUL7, as well as SKP1, RBX1, and an F-box protein, FBXW8. We identified T Ag residues 69 to 83 as required for T Ag binding to the CUL7 complex. We demonstrate that delta69-83 T Ag, while it lost its ability to associate with CUL7, retained binding to p53 and pRB family members. In the presence of CUL7, wild-type (WT) T Ag but not delta69-83 T Ag was able to induce proliferation of mouse embryo fibroblasts, an indication of cellular transformation. In contrast, WT and delta69-83 T Ag enabled mouse embryo fibroblasts to proliferate to similarly high densities in the absence of CUL7. Our data suggest that, in addition to p53 and the pRB family members, T Ag serves to bind to and inactivate the growth-suppressing properties of CUL7. In addition, these results imply that, at least in the presence of T Ag, CUL7 may function as a tumor suppressor.

Identification of mutations in CUL7 in 3-M syndrome

Intrauterine growth retardation is caused by maternal, fetal or placental factors that result in impaired endovascular trophoblast invasion and reduced placental perfusion. Although various causes of intrauterine growth retardation have been identified, most cases remain unexplained. Studying 29 families with 3-M syndrome (OMIM 273750), an autosomal recessive condition characterized by severe pre- and postnatal growth retardation, we first mapped the underlying gene to chromosome 6p21.1 and then identified 25 distinct mutations in the gene cullin 7 (CUL7). CUL7 assembles an E3 ubiquitin ligase complex containing Skp1, Fbx29 (also called Fbw8) and ROC1 and promotes ubiquitination. Using deletion analysis, we found that CUL7 uses its central region to interact with the Skp1-Fbx29 heterodimer. Functional studies indicated that the 3-M-associated CUL7 nonsense and missense mutations R1445X and H1464P, respectively, render CUL7 deficient in recruiting ROC1. These results suggest that impaired ubiquitination may have a role in the pathogenesis of intrauterine growth retardation in humans.

Nedd8 on cullin: building an expressway to protein destruction

This review summarizes recent advances concerning the Nedd8 regulatory pathway in four areas. One, substantial progress has been made in delineating the role of cullin family proteins, the only known substrates of the Nedd8 modification system. Cullins are molecular scaffolds responsible for assembling the ROC1/Rbx1 RING-based E3 ubiquitin ligases, of which several play a direct role in tumorigenesis. Two, a large body of work has helped elucidate the molecular details underlying the Nedd8 modification reaction, which results in covalent conjugation of a Nedd8 moiety onto a conserved cullin lysine residue. Three, studies using a variety of genetic model systems have established an essential role for Nedd8 in cell cycle control and in embryogenesis by upregulating the activities of cullin-based E3 ligases. In vitro experiments have revealed a direct role for Nedd8 in activating ubiquitination. Construction of a model of the ROC1/Rbx1-CUL1-Nedd8 structure suggests a mechanism by which the cullin-linked Nedd8 may assist the neighboring ROC1/Rbx1 in landing and positioning the E2 conjugating enzyme for the ubiquitin transfer reaction. Finally, increasing evidence indicates that removal of Nedd8 from its cullin targets, by the action of COP9 Signalosome and possibly other proteases, plays a significant role in the regulation of cullin-mediated proteolysis.

Cul7/p185/p193 binding to simian virus 40 large T antigen has a role in cellular transformation

Simian virus 40 large T antigen (TAg) is a viral oncoprotein that can promote cellular transformation. TAg's transforming activity results in part by binding and inactivating key tumor suppressors, including p53 and the retinoblastoma protein (pRb). We have identified a TAg-associated 185-kDa protein that has significant homology to the cullin family of E3 ubiquitin ligases. TAg binds to an SCF-like complex that contains p185/Cul7, Rbx1, and the F box protein Fbw6. This SCF-like complex binds to an N-terminal region of TAg. Several p185/Cul7-binding-deficient mutants of TAg were generated that retained binding to pRb and p53 and were capable of overcoming Rb-mediated repression of E2F transcription. Despite binding to pRb and p53, these p185/Cul7-binding-defective mutants of TAg were unable to transform primary mouse embryo fibroblasts. Cells expressing p185/Cul7-binding-defective mutants of TAg were unable to grow to high density or grow in an anchorage-independent manner as determined by growth in soft agar. Considering the significance of other TAg-interacting proteins in regulation of the cell cycle, p185/Cul7 may also regulate an important growth control pathway.

Targeted disruption of p185/Cul7 gene results in abnormal vascular morphogenesis

Cul1, a member of the cullin ubiquitin ligase family, forms a multiprotein complex known as SCF and plays an essential role in numerous cellular and biological activities. A Cul1 homologue, p185 (Cul7), has been isolated as an simian virus 40 large T antigen-binding protein. To understand the physiological role of p185, we generated mice lacking p185. p185-/- embryos are runted and die immediately after birth because of respiratory distress. Dermal and hypodermal hemorrhage is detected in mutant embryos at late gestational stage. p185-/- placentas show defects in the differentiation of the trophoblast lineage with an abnormal vascular structure. We demonstrate that p185 forms an SCF-like complex with Skp1, Rbx1, Fbw6 (Fbx29), and FAP68 (FAP48, glomulin). FAP68 has recently been identified as a gene responsible for familial glomuvenous malformation. These results suggest that p185 forms a multiprotein complex and plays an important role in vascular morphogenesis.

Functional abrogation of p53 is required for T-Ag induced proliferation in cardiomyocytes

Targeted expression of the SV40 large T-antigen oncoprotein (T-Ag) induces cardiomyocyte proliferation in the atria and ventricles of transgenic mice. Previous studies have identified the p53 tumor suppressor, p107 (a homologue of the retinoblastoma tumor suppressor), and p193 (a novel BH3 only proapoptosis protein) as prominent TAg binding proteins in cardiomyocyte cell lines derived from these transgenic mice. To further explore the significance of these protein-protein interactions in the regulation of cardiomyocyte proliferation, a transgene comprising the human atrial natriuretic factor (ANF) promoter and sequences encoding a mutant T-Ag lacking the p53 binding domain was generated. Repeated micro-injection of this DNA gave rise to genetically mosaic animals with minimal transgene content, suggesting that widespread cardiac expression of mutant T-Ag was deleterious. This notion was supported by the observation that the transgene was selectively lost from the cardiac myocytes (but not the cardiac fibroblasts) in the mosaic animals. Crosses between the mosaic mice and animals expressing a cardiac restricted dominant negative p53 resulted in transgene transmission with ensuing overt cardiac tumorigenesis. Transfection of the mutant T-Ag in embryonic stem (ES) cell-derived cardiomyocytes resulted in wide-spread cell death with characteristics typical of apoptosis. Co-transfection with a dominant negative p53 transgene rescued mutant TAg-induced cell death in the ES-derived cardiomyocyte cultures, resulting in a marked proliferative response similar to that seen in vivo with the rescued transgenic mouse study. These results indicate that T-Ag expression in the absence of p53 functional abrogation results in cardiomyocyte death.

Cellular transformation by SV40 large T antigen: interaction with host proteins

SV40 large T antigen (TAg) is a powerful oncoprotein capable of transforming a variety of cell types. The transforming activity of TAg is due in large part to its perturbation of the retinoblastoma (pRB) and p53 tumor suppressor proteins. In addition, TAg binds to several other cellular factors, including the transcriptional co-activators p300 and CBP, which may contribute to its transformation function. Several other features of TAg that appear to contribute to its full transformation potential are yet to be completely understood. Study of TAg therefore continues to provide new insights into the mechanism of cellular transformation.

Loss of p19(ARF) eliminates the requirement for the pRB-binding motif in simian virus 40 large T antigen-mediated transformation

At least three domains of simian virus 40 large T antigen (TAg) participate in cellular transformation. The LXCXE motif of TAg binds to all members of the retinoblastoma protein (pRB) family of tumor suppressors. The N-terminal 70 residues of TAg have significant homology to the J domain of Hsp40/DnaJ and cooperate with the LXCXE motif to inactivate the pRB family. A bipartite C-terminal domain of TAg binds to p53 and thereby disrupts the ability of p53 to act as a sequence-specific transcription factor. The contribution of these three domains of TAg to cellular transformation was evaluated in cells that contained inactivating mutations in the pRB and p53 pathways. Cells that stably expressed wild-type or selected mutant forms of TAg were generated in mouse embryo fibroblasts (MEFs) containing homozygous deletions in the RB, INK4a, and ARF loci. It was determined that the J domain, the LXCXE motif, and the p53-binding domain of TAg were required for full transformation of wild-type and RB(-/-) MEFs. In contrast, INK4a(-/-) MEFs that lacked expression of p16(INK4a) and p19(ARF) and ARF(-/-) MEFs that lacked p19(ARF) but expressed p16(INK4a) acquired anchorage-independent growth when expressing wild-type TAg or mutant derivatives that disrupted either the pRB-binding or p53-binding domain. The expression and function of the pRB family members were not overly disrupted in ARF(-/-) MEFs expressing LXCXE mutants of TAg. These results suggest that inactivating mutations of p19(ARF) can relieve the requirement for the LXCXE motif in TAg-mediated transformation and that TAg may have additional functions in transformation.

Simian virus 40 large T antigen binds a novel Bcl-2 homology domain 3-containing proapoptosis protein in the cytoplasm

A 193-kDa SV40 large T antigen (T-Ag)-binding protein, designated p193, was identified and cloned. Inspection of the deduced amino acid sequence revealed the presence of a short motif similar to the Bcl-2 homology (BH) domain 3, suggesting that p193 may be a member of a family of apoptosis promoting proteins containing only BH3 motifs. In support of this, p193 expression promoted apoptosis in NIH-3T3 cells. Deletion of the BH3 motif abolished p193 apoptosis activity. p193-induced apoptosis was antagonized by co-expression of Bcl-X(L). Immune cytologic analysis indicated that p193 is localized to the cytoplasm of transfected cells. p193-induced apoptosis was also antagonized by co-expression of T-Ag, which resulted in the cytoplasmic localization of both proteins. The p193 binding site was mapped to an N-terminal region of T-Ag previously implicated in transforming activity. These results suggest that T-Ag possesses an antiapoptosis activity, independent of p53 sequestration, which is actuated by T-Ag/p193 binding in the cytoplasm.

The initiation of simian virus 40 DNA replication in vitro

DNA replication is a complicated process that is largely regulated during stages of initiation. The Siman Virus 40 in vitro replication system has served as an excellent model for studies of the initiation of DNA replication, and its regulation, in eukaryotes. Initiation of SV40 replication requires a single viral protein termed T-antigen, all other proteins are supplied by the host. The recent determination of the solution structure of the T-antigen domain that recognizes the SV40 origin has provided significant insights into the initiation process. For example, it has afforded a clearer understanding of origin recognition, T-antigen oligomerization, and DNA unwinding. Furthermore, the Simian virus 40 in vitro replication system has been used to study nascent DNA formation in the vicinity of the viral origin of replication. Among the conclusions drawn from these experiments is that nascent DNA synthesis does not initiate in the core origin in vitro and that Okazaki fragment formation is complex. These and related studies demonstrate that significant progress has been made in understanding the initiation of DNA synthesis at the molecular level.

Adding an Rb-binding site to an N-terminally truncated simian virus 40 T antigen restores growth to high cell density, and the T common region in trans provides anchorage-independent growth and rapid growth in low serum concentrations

The simian virus 40 large T antigen is sufficient to confer on cells multiple transformed cell growth characteristics, including growth to a high cell density, rapid growth in medium containing low serum concentrations, and anchorage-independent growth. We showed previously that distinct regions of the protein were involved in conferring these properties and that removal of the first 127 amino acids of T antigen abrogated all three activities. At least three large-T-antigen transformation-related activities have been localized to that region: binding of the tumor suppressor gene product Rb and two independent activities contained within the common region shared by large T and small t antigens. The experiments described here were directed toward determining whether these were the only activities from the N terminus that were needed. To do so we reintroduced an Rb-binding region into the N-terminally truncated T antigen (T128-708) and examined the growth properties of cells immortalized by it in the presence and absence of small t antigen, which can provide the T-common-region transformation-related activities in trans. We show that an Rb-binding region consisting of amino acids 101 to 118, when introduced into a heterologous site in T128-708, is capable of physically binding Rb and that binding is sufficient for cells expressing the protein to acquire the ability to grow to a high saturation density. However, in low-serum medium, the growth rate of the cells and maximal cell density are reduced relative to those of wild-type-T-antigen-expressing cells, and the cells cannot divide without anchorage. This result suggests that although Rb binding is sufficient in the context of T128-708 to confer growth to a high density, one or more other N-terminally located T-antigen activities are needed for cells to acquire the additional growth properties. Small t antigen in trans supplied those activities. These results indicate that the T-common-region activities and Rb binding are the only activities from the T-antigen N terminus needed to restore full transforming activity to the N-terminally truncated T antigen.

Simian virus 40 small t antigen activates the carboxyl-terminal transforming p53-binding domain of large T antigen

Expression of the simian virus 40 large T antigen (large T) in F111 rat fibroblasts generated only minimal transformants (e.g., F5 cells). Interestingly, F111-derived cells expressing only an amino-terminal fragment of large T spanning amino acids 1 to 147 (e.g., FR3 cells), revealed the same minimal transformed phenotype as F111 cells expressing full-length large T. This suggested that in F5 cells the transforming domain of large T contained within the C-terminal half of the large T molecule, and spanning the p53 binding domain, was not active. Progression to a more transformed phenotype by coexpression of small t antigen (small t) could be achieved in F5 cells but not in FR3 cells. Small-t-induced progression of F5 cells correlated with metabolic stabilization of p53 in complex with large T: whereas in F5 cells the half-life of p53 in complex with large T was only slightly elevated compared with that of (uncomplexed) p53 in parental F111 cells or that in FR3 cells, coexpression of small t in F5 cells led to metabolic stabilization and to high-level accumulation of p53 complexed to large T. In contrast, coexpression of small t had no effect on p53 stabilization or accumulation in FR3 cells. This finding strongly supports the assumption that the mere physical interaction of large T with p53, and thus p53 inactivation, in F5 cells expressing large T only does not reflect the main transforming activity of the C-terminal transforming domain of large T. In contrast, we assume that the transforming potential of this domain requires activation by a cellular function(s) which is mediated by small t and correlates with metabolic stabilization of p53.

Association of p300 and CBP with simian virus 40 large T antigen

p300 and the CREB-binding protein CBP are two large nuclear phosphoproteins that are structurally highly related. Both function, in part, as transcriptional adapters and are targeted by the adenovirus E1A oncoprotein. We show here that p300 and CBP interact with another transforming protein, the simian virus 40 large T antigen (T). This interaction depends on the integrity of a region of T which is critical for its transforming and mitogenic properties and includes its LXCXE Rb-binding motif. T interferes with normal p300 and CBP function on at least two different levels. The presence of T alters the phosphorylation states of both proteins and inhibits their transcriptional activities on certain promoters. Although E1A and T show little sequence similarity, they interact with the same domain of p300 and CBP, suggesting that this region exhibits considerable flexibility in accommodating diverse protein ligands.

Hypothesis: "Rogue cell"-type chromosomal damage in lymphocytes is associated with infection with the JC human polyoma virus and has implications for oncopenesis

The hemagglutination inhibition antibody titers against the JC and BK polyoma viruses (JCV and BKV, respectively) are significantly elevated in individuals exhibiting "rogue" cells among their cultured lymphocytes. However, the elevation is so much greater with respect to JCV that the BKV elevation could readily be explained by cross reactivity to the capsid protein of these two closely related viruses. The JCV exhibits high sequence homology with the simian papovavirus, simian virus 40 (SV40), and inoculation of human fetal brain cells with JCV produces polyploidy and chromosomal damage very similar to that produced by SV40. We suggest, by analogy with the effects of SV40, that these changes are due to the action of the viral large tumor antigen, a pluripotent DNA binding protein that acts in both transcription and replication. The implications of these findings for oncogenesis are briefly discussed.

Association of insulin receptor substrate 1 with simian virus 40 large T antigen

Mouse embryo cells expressing a wild-type number of insulin-like growth factor I receptors (IGF-IR) (W cells) can be transformed either by simian virus 40 large T antigen (SV40 T) or by overexpressed insulin receptor substrate 1 (IRS-1), singly transfected. Neither SV40 T antigen nor IRS-1, individually, can transform mouse embryo cells with a targeted disruption of the IGF-IR genes (R- cells). However, cotransfection of SV40 T antigen and IRS-1 does transform R- cells. In this study, using different antibodies and different cell lines, we found that SV40 T antigen and IRS-1 are coprecipitated from cell lysates in a specific fashion, regardless of whether the lysates are immunoprecipitated with an antibody to SV40 T antigen or an antibody to IRS-1. The same antibody to SV40 T antigen, however, fails to coprecipitate another substrate of IGF-IR, the transforming protein Shc, and two other signal-transducing molecules, Grb2 and Sos. Finally, an SV40 T antigen lacking the amino-terminal 250 amino acids fails to coprecipitate IRS-1 and also fails to transform R- cells overexpressing mouse IRS-1. These experiments indicate that IRS-1 associates with SV40 T antigen and that this association plays a critical role in the combined ability of these proteins to transform R- cells. This finding is discussed in light of the crucial role of the IGF-IR in the establishment and maintenance of the transformed phenotype.

Inactivation of the retinoblastoma susceptibility protein is not sufficient for the transforming function of the conserved region 2-like domain of simian virus 40 large T antigen

Simian virus 40 large T antigen interacts with three cellular proteins, pRb, p107, and p130, through a common binding site on the T antigen protein called the E1A conserved region 2-like (CR2-like) domain. Mutations in this domain inactivate the transforming activity of large T antigen. Since these mutations have been demonstrated to abolish binding to pRb and p107, and presumably therefore affect binding to p130, assessment of the relative roles of these three proteins in transformation of rodent fibroblasts by T antigen has been difficult. We have examined the role of T antigen-pRb interactions in transformation. We have introduced a mutant T antigen, which is unable to bind any of these three proteins, into primary mouse fibroblasts derived from the embryos of mice in which the Rb gene encoding the retinoblastoma protein had been disrupted. This mutant is unable to transform the Rb-negative fibroblasts, indicating that inactivation of pRb is not the sole function of the CR2-like domain in the induction of transformation of mouse fibroblasts by simian virus 40.

The kinetics of simian virus 40-induced progression of quiescent cells into S phase depend on four independent functions of large T antigen

Microinjection of purified simian virus 40 large-T-antigen protein or DNA encoding T antigen into serum-starved cells stimulates them to re-enter the cell cycle and progress through G1 into the S phase. Genetic analysis of T antigen indicated that neither its Rb/p107-binding activity nor its p53-binding activity is essential to induce DNA synthesis in CV1P cells. However, T antigens bearing missense mutations that inactivate either activity induced slower progression of the cells into the S phase than did wild-type T antigen. Inactivation of both activities resulted in a T antigen essentially unable to induce DNA synthesis. Missense mutations in either the DNA-binding region of the N terminus also impaired the ability of full-length T antigen to stimulate DNA synthesis in CV1P cells. The wild-type kinetics of cell cycle progression were restored by genetic complementation after coinjection of plasmid DNAs encoding different mutant T antigens or coinjection of purified mutant T-antigen proteins, suggesting that the four mitogenic functions of T antigen are independent. The maximal rate of induction of DNA synthesis in secondary primate cells and established rodent cell lines required the same four functions of T antigen. A model to explain how four independent activities could cooperate to stimulate cell cycle progression is presented.

Induction versus progression of brain tumor development: differential functions for the pRB- and p53-targeting domains of simian virus 40 T antigen

The ability of simian virus 40-encoded large T antigen to disrupt the growth control of a variety of cell types is related to its ability to interfere with certain cellular proteins, such as p53 and the retinoblastoma susceptibility gene product (pRB). We have used wild-type and mutant forms of T antigen in transgenic mice to dissect the roles of pRB, p53, and other cellular proteins in tumorigenesis of different cell types. In this study, using a cell-specific promoter to target expression specifically to brain epithelium (the choroid plexus) and to B and T lymphoid cells, we characterize the tumorigenic capacity of a T-antigen fragment that comprises only the amino-terminal 121 residues. This fragment (dl1137) retains the ability to interact with pRB and p107 but lacks the p53-binding domain. While loss of the p53-binding region results in loss of the capacity to induce lymphoid abnormalities, dl1137 retains the ability to induce choroid plexus tumors that are histologically indistinguishable from those induced by wild-type T antigen. Tumors induced by dl1137 develop much more slowly, however, reaching an end point at around 8 months of age rather than at 1 to 2 months. Analysis of tumor progression indicates that tumor induction by dl1137 does not require secondary genetic or epigenetic events. Rather, the tumor growth rate is significantly slowed, indicating that the T-antigen C-terminal region contributes to tumor progression in this cell type. In contrast, the pRB-binding region appears essential for tumorigenesis as mutation of residue 107, known to disrupt pRB and p107 binding to wild-type T antigen, abolishes the ability of the dl1137 protein to induce growth abnormalities in the brain.

Induction versus progression of brain tumor development: differential functions for the pRB- and p53-targeting domains of simian virus 40 T antigen

The ability of simian virus 40-encoded large T antigen to disrupt the growth control of a variety of cell types is related to its ability to interfere with certain cellular proteins, such as p53 and the retinoblastoma susceptibility gene product (pRB). We have used wild-type and mutant forms of T antigen in transgenic mice to dissect the roles of pRB, p53, and other cellular proteins in tumorigenesis of different cell types. In this study, using a cell-specific promoter to target expression specifically to brain epithelium (the choroid plexus) and to B and T lymphoid cells, we characterize the tumorigenic capacity of a T-antigen fragment that comprises only the amino-terminal 121 residues. This fragment (dl1137) retains the ability to interact with pRB and p107 but lacks the p53-binding domain. While loss of the p53-binding region results in loss of the capacity to induce lymphoid abnormalities, dl1137 retains the ability to induce choroid plexus tumors that are histologically indistinguishable from those induced by wild-type T antigen. Tumors induced by dl1137 develop much more slowly, however, reaching an end point at around 8 months of age rather than at 1 to 2 months. Analysis of tumor progression indicates that tumor induction by dl1137 does not require secondary genetic or epigenetic events. Rather, the tumor growth rate is significantly slowed, indicating that the T-antigen C-terminal region contributes to tumor progression in this cell type. In contrast, the pRB-binding region appears essential for tumorigenesis as mutation of residue 107, known to disrupt pRB and p107 binding to wild-type T antigen, abolishes the ability of the dl1137 protein to induce growth abnormalities in the brain.

Studying the interaction of polyoma virus middle T antigen with cellular proteins

The oncogenic mouse polyoma virus encodes six proteins, two of which (the large and middle T antigens), when expressed together in primary rodent cells, generate an alteration of growth patterns commonly known as cellular transformation. The transformed cells grow with an unlimited life span and when introduced into immunocompromised mice rapidly form tumours. The large T gene product confers an indefinite growth potential on primary cells; however, the middle T gene product has been identified as bringing about the changes which ultimately allow the cell to form tumours. The 55 kDa middle T antigen has been shown to associate with a number of cellular enzymes involved in regulation of growth factor signalling pathways, all of which were identified as being components of the immunocomplexes that can be isolated from transformed cells using middle T antigen specific antibodies. Two-dimensional gels have assisted the search for less prominent species present in these complexes. These methods represents one approach to investigating associating proteins, and as such, select for those interactions that are stable under the conditions used. In order to explore the possibility that middle T antigen could form complexes with other cellular proteins given different conditions, recombinant middle T antigen was used in a series of "filter overlay" experiments.

Construction of SV40 deletion mutants and delimitation of the binding domain for heat shock protein to the amino terminus of large T-antigen

SV40 large T-antigen (T-ag) mutants were generated using a cassette mutagenesis strategy and naturally occurring restriction sites. T-ag mutant constructs included internal in-frame deletions, frame-shift deletions that resulted in amino-terminal fragments, and internal initiation mutants that produced carboxy-terminal fragments; no foreign amino acids were introduced. The deletion mutants were stably expressed in BALB/c 3T3E cells and were analyzed for ability to bind heat shock cognate protein 70 using an ATP release assay of T-ag immunoprecipitates. Complex formation between heat shock protein and T-ag was independent of p53 involvement. The heat shock protein binding domain was narrowed to the amino-terminal 97 amino acids of T-ag, with the first 29 residues influencing the interaction. The amino-terminal domain of T-ag is important in both viral replication and cell transformation. We propose that the functional interactions of this highly interactive region of T-ag may be modulated by heat shock cognate protein 70.

Association of p53 binding and immortalization of primary C57BL/6 mouse embryo fibroblasts by using simian virus 40 T-antigen mutants bearing internal overlapping deletion mutations

To more precisely map the immortalization and p53 binding domains of T antigen, a large series of overlapping deletion mutations were created between codons 251 to 651 by utilizing a combination of Bal 31 deletion and oligonucleotide-directed mutagenesis. Immortalization assay results indicated that amino acids (aa) 252 to 350, 400, and 451 to 532 could be removed without seriously compromising immortalization, although the appearance of immortal colonies was delayed in some cases. Western immunoblotting experiments indicated that the p53 binding capacities of T antigen produced by mutants missing aa 252 to 300, 301 to 350, 400, or 451 to 532 were only slightly reduced relative to that of wild-type T antigen. Within the limits of this deletion analysis, the immortalization and p53 binding domains appear to be colinear and, in fact, may represent two aspects of the same domain. This deletion analysis eliminates the entire zinc finger domain (aa 302 to 320), a small portion of the leucine-rich region (aa 345 to 350), and a large portion of the ATP binding domain (aa 451 to 528) as participants in p53 binding or in the immortalization process. The results also show that removal of T antigen amino acids within the region 451 to 532 appears to alter the capacity of newly synthesized but not older T antigen and p53 molecules to form complexes.

Modified glutathione S-transferase fusion proteins for simplified analysis of protein-protein interactions

Images