Activation of non-expressed bovine papilloma virus genomes by tumour promoters

12-O-tetradecanoyl-phorbol-13-acetate-induced rat embryo malformations in vitro are associated with an increased relative abundance of embryonic E-cadherin mRNA

Epithelial-cadherin (E-cadherin) is a member of a family of Ca(2+)-dependent cell adhesion molecules which are localized in zonulae adherens and play an important role during development. E-cadherin is abundant in rat embryos and their yolk sacs during organogenesis. The phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), has been reported to disrupt the morphology and functional development of the rat embryonic visceral yolk sac. The present study investigated the possibility that the effect of TPA on yolk sac development may be due to the altered expression of E-cadherin. Rat embryos, with their yolk sacs intact, were cultured on day 10 of gestation for 1 hr. At this time the vehicle, dimethyl sulfoxide (DMSO), or TPA (at different concentrations) was added to the culture medium; the cultures were continued for up to 24 hr. Embryos and yolk sacs were collected separately at the end of each culture period. The relative abundances of E-cadherin mRNA and protein were analyzed with Northern and Western blot analyses. Despite the TPA-induced abnormalities in yolk sac development, the relative abundance of E-cadherin mRNA or protein in the yolk sac was not altered by TPA exposure. However, in embryos exposed to dysmorphogenic concentrations of TPA, the relative abundance of E-cadherin mRNA was significantly increased after 24 hr in culture, compared to either controls or embryos exposed to non-dysmorphogenic concentrations of TPA. The magnitude of the increase in embryonic E-cadherin mRNA appeared to correlate with the severity of the embryo malformations.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose depletion enhances the anti-tumor effect of TNF

Treatment of human carcinoma xenotransplants in athymic mice with recombinant human tumor necrosis factor (rh TNF) causes necrosis mainly in the central parts of the tumors, while peripheral sections remain mitotically active. As tumors are known to be supplied with adequate glucose exclusively in their periphery, the influence of the lack of glucose on the cytotoxic activity of rh TNF was studied. The absence of glucose enhanced the killing of tumor cell lines by rh TNF in tissue culture. Meth-A, a cell line known to be resistant to TNF in vitro but highly sensitive to it in vivo, was readily killed in tissue-culture medium lacking glucose. All non-transformed cell lines tested were found to be resistant to rh TNF, regardless of the presence or absence of glucose. In tumor-bearing mice a reduction of the blood glucose content augmented by insulin led to increased anti-tumor efficiency of rh TNF. The enhanced anti-tumor activity was reflected both in histological sections of the tumor xenotransplants, by extensive central necroses, and by reduction of the tumor volumes.

Multistage carcinogenesis in mouse skin

The mouse skin model of multistage carcinogenesis has for many years provided a conceptual framework for studying carcinogenesis mechanisms and potential means for inhibiting specific stages of carcinogenesis. The process of skin carcinogenesis involves the stepwise accumulation of genetic change ultimately leading to malignancy. Initiation, the first step in multistage skin carcinogenesis involves carcinogen-induced genetic changes. A target gene identified for some skin tumor initiators is c-Ha-ras. The second step, the promotion stage, involves processes whereby initiated cells undergo selective clonal expansion to form visible premalignant lesions termed papillomas. The process of tumor promotion involves the production and maintenance of a specific and chronic hyperplasia characterized by a sustained cellular proliferation of epidermal cells. These changes are believed to result from epigenetic mechanisms such as activation of the cellular receptor, protein kinase C, by some classes of tumor promoters. The progression stage involves the conversion of papillomas to malignant tumors, squamous cell carcinomas. The accumulation of additional genetic changes in cells comprising papillomas has been correlated with tumor progression, including trisomies of chromosomes 6 and 7 and loss of heterozygosity. The current review focuses on the mechanisms involved in multistage skin carcinogenesis, a summary of known inhibitors of specific stages and their proposed mechanisms of action, and the relevance of this model system to human cancer.

Arsenic: opportunity for risk assessment

Arsenic is a human carcinogen that in small amounts is widely distributed in food and water. It has been regulated for almost 100 years worldwide and in the United States, on the judgment of the Royal Commission on Arsenic that a classical threshold of toxicity exists and that a daily intake of 400 micrograms/day is safe. Modern regulatory thinking in the United States has not accepted safe levels for carcinogens and is thus in conflict with the arsenic standard. Recent epidemics of arsenicism have quantitatively confirmed that threshold not only for the non-cancerous arsenical skin lesions but also for arsenical skin and internal cancers. Research shows that arsenic is a general gene inducer. Genes induced are involved in proliferation, recombination, amplification and the activation of viruses. This characterizes arsenic as an indirect carcinogen and provides a molecular basis for risk assessment and the observed threshold dose response. In the United States at present, about 300 cases of occupational arsenical cancer, declining in numbers, are known. Background arsenic below the drinking water standard is not known to have produced disease. The conspicuous nature of arsenical skin disease presents an unusual opportunity for a simplified survey of arsenical skin disease to support regulatory standards for arsenic.

Low-level beta 1 protein kinase C expression in cloned rat embryo fibroblast cells enhances transformation induced by the adenovirus type 5 E1A gene

Expression of the E1A gene of adenovirus type 5 (Ad5) in a cloned rat embryo fibroblast (CREF) cell line results in morphological transformation. The efficiency of E1A-mediated transformation of CREF cells is increased if a wild-type Ad5 E1A gene is cotransfected with a rat beta 1 protein kinase C (beta 1 PKC) gene. A direct demonstration of complementation between a functional-transforming Ad5 E1A gene and beta 1 PK in inducing transformation was demonstrated using Ad5 E1A cold-sensitive mutant (E1Acs) genes. The E1Acs gene enhanced transformation only at the transformation-permissive temperature of 37 degrees C and not at the nonpermissive transforming temperature of 32 degrees C. CREF cells constitutively expressing low levels of beta 1 PKC mRNA were transformed at a higher frequency than parental CREF cells after transfection with an Ad5 E1A gene or infection with wild-type Ad5 or the Ad5 host-range cold-sensitive mutant H5hr1. There was no enhancement of transformation in low-level beta 1 PKC-expressing CREF cells when cultures were grown continuously in the presence of the PKC-inhibitor 1-(5-isoquinolynsulfonyl)-2-methylpiperazine dihydrochloride. Transfected CREF cells expressing low levels of beta 1 PKC mRNA displayed CREF-like morphology and did not form colonies when grown in agar. In contrast, retroviral vector-transformed CREF cells expressing high levels of beta 1 PKC mRNA and beta 1 PKC enzyme activity were morphologically transformed and grew efficiently in agar. These findings indicate that the beta 1 PKC gene, when expressed at low levels, can cooperate with the Ad5 E1A gene in the initiation of viral oncogene-mediated transformation.

Transforming growth factor-beta expression in fibropapillomas induced by bovine papillomavirus type 1, in normal bovine skin, and in BPV-1-transformed cells

There is substantial evidence to suggest that transforming growth factor-beta (TGF-beta) plays an important role in wound healing and tissue repair as well as in carcinogenesis. It has also been observed that naturally occurring bovine papillomavirus type 1 (BPV-1)-induced bovine fibropapillomas occur predominantly at traumatized sites of the body, suggesting that humoral factors released in wounds might be important for papillomavirus infection. We have therefore investigated the possible role of TGF-beta 1 in BPV-1 infections. Two antipeptide antibodies which recognize different epitopes in the N-terminus of TGF-beta 1 were used to localize TGF-beta 1 in bovine fibropapillomas and normal bovine skin using immunohistochemical methods. Staining by anti-LC(1-30) is intracellular in suprabasal keratinocytes of the epidermis as well as the hair follicles and sebaceous glands and correlates with known sites of TGF-beta 1 mRNA synthesis. Anti-CC(1-30) staining is extracellular in the immediately underlying dermis. Neither the pattern nor intensity of TGF-beta 1 staining was affected by BPV-1 infection. C127 cells and BPV-1-transformed C127 cells were compared for TGF-beta 1 mRNA expression and secretion of TGF-beta 1 peptide. Although the levels of messenger RNA and secreted TGF-beta 1 peptide were similar in both cell types, five- to six-fold greater amounts of TGF-beta-like activity per cell was detected in media conditioned by the uninfected cells. TGF-beta 1 treatment had no effect on the growth rate of either cell type or on BPV-1 gene expression in the transformed cells.

Loss of bovine papillomavirus DNA replication control in growth-arrested transformed cells

The bovine papillomavirus type 1 (BPV-1) genome replicates as a plasmid within the nuclei of BPV-1-transformed murine C127 cells at a constant multiple copy number, and spontaneous amplification of the viral DNA is rarely observed. We report here that a mutant BPV-1 plasmid within a contact-inhibited C127 cell line replicated as a stable multicopy plasmid in exponentially growing cells but amplified to a high level in confluent cell culture. In situ hybridization analysis revealed that most of the mutant viral DNA amplification occurred in a minor subpopulation of cells within the culture. These consisted of giant nondividing cells with greatly enlarged nuclei, a cell form which was specifically induced in stationary-phase cultures. These observations indicated that expression of a viral DNA replication factor was cell growth stage specific. Consistent with this hypothesis, considerable amplification of wild-type BPV-1 DNA associated with characteristic giant cell formation was observed in typical wild-type virus-transformed C127 cultures following a period of growth arrest achieved by serum deprivation. Further observations indicated that induction of the giant-cell phenotype was dependent on BPV-1 gene expression and implicated a viral E1 replication factor in this process. Moreover, heterogeneity in virus genome copy numbers within the giant-cell population suggested a complex regulation of induction of DNA synthesis in these cells. It appears that this process represents a mechanism employed by the virus to ensure maximal viral DNA synthesis within a growth-arrested cell. Fundamental questions concerning the integration of the virus-cell control circuitry in proliferating and resting cells are discussed.

Enhancement of bovine papillomavirus-induced cell transformation by tumour promoters

Cultured C3H/10T1/2 cells transfected with the plasmid pdBPV-1 were used as targets, and the frequency of transformed colonies as the endpoint to test the enhancing capacity of four promoters: 12-O-tetradecanoylphorbol-13-acetate (TPA), 4-O-methyl-tetradecanoylphorbol-13-acetate (4-O-methyl-TPA), mezerein and phorbol-12-retinoate-13-acetate (PRA). The frequency of the transfected C3H/10T1/2 cells to form transformed colonies was enhanced in the following order: mezerein greater than PRA greater than TPA greater than 4-O-methyl-TPA. The amount of promoters required to promote a tenfold increase in transformed cells was 0.24, 0.81, 30 and 100 ng/ml mezerein, PRA, TPA and 4-O-methyl-TPA, respectively. A significant promoting effect was obtained by a 3.5-day exposure to mezerein regardless of whether it was added at different time intervals after transfection with BPV-DNA. The examined promoters lacked genotoxic activity, as tested on Chinese hamster ovary cells, using chromatid aberrations and exchanges, frequency of macronuclei, unscheduled DNA synthesis (UDS) and inhibition of UDS as endpoints. The usefulness of BPV-1-induced transformation as a bioassay for detecting chemicals with promoting activities is discussed.

Chromosome aberrations in cattle raised on bracken fern pasture

Thirteen cows maintained on natural bracken fern (Pteridium aquilinum) were analyzed cytogenetically. The frequency of structural chromosome aberrations detected in peripheral blood cells was significantly higher when compared to that detected in animals raised on pasture containing no bracken fern. We discuss the clastogenic action of fern and its synergistic action with infection by type 2 and 4 papilloma virus in the same animals.

Interruption of growth signal transduction by an antiviral and antitumoral xanthate compound

The binding of growth factors to the cellular receptors elicits the phosphorylation of proteins which transmit growth signals to the nucleus [E. Rozengurt (1986) Science 234, 161-166]. Both the tyrosine-specific kinase (growth factor receptor) and the threonine-serine phosphorylating protein kinase C (pkC) become activated upon binding of the epidermal growth factor (EGF) to its receptor. Here we describe the selective inhibition of the pkC activation by tricyclodecane-9-yl-xanthogenate (D609) in the presence of unsuppressed receptor tyrosine autophosphorylation. As a consequence the affinity of EGF to the receptor was not down-regulated and the complex failed to be internalized.

DNA viruses and human cancer

This review examines some of the evidence which aetiologically implicates various DNA viruses (primarily papillomavirus, hepatitis B virus and Epstein-Barr virus) in certain human cancers (cervical carcinoma, primary liver cell carcinoma, Burkitt's lymphoma and nasopharyngeal carcinoma, respectively). The evidence includes: presence of viral DNA, RNA and proteins in tumours (and cell lines derived from them); occurrence of viruses with apparently different oncogenic potential; their ability to transform cell lines in vitro or cause tumours in animals; epidemiological and serological data. Factors which affect the progression to cancer are briefly considered as they illustrate that there are several stages in tumorigenesis. These factors include the immune system, irradiation, presence of other viruses or carcinogens and treatment. The lack of a single unique characteristic which defines a transformed cell would be expected from the multistep hypothesis and is related to the possible virus-cell interactions that can occur. These form a continuous spectrum ranging from productive infection of a permissive cell, through infection of a non-permissive cell, to the inability of a virus to infect a cell. This spectrum may reflect the absence of increasing numbers of cellular functions necessary for productive virus infection, with cell transformation occurring as a rare type of abortive infection. The evidence, especially for human papillomavirus, indicates that it is quite probable that particular DNA viruses are the causative agents for certain human cancers. Even so other factors can play decisive roles in tumorigenesis. Final aetiological proof will only be obtained when an anti-virus vaccine eradicates one form of human cancer.

Plasmodium falciparum exoprotein stimulation of human T-lymphocytes unsensitized to malaria

The effects of Plasmodium falciparum proteins released in asexual blood stage culture supernatants on human T-lymphocytes from malaria non-immune donors were examined. Supernatants from several plasmodial strains stimulated both CD+4 and CD+8 T-lymphocytes to proliferate and secrete interferon gamma in vitro. Active moieties were predominantly released during the final stages of the parasite cycle. They were enriched by gel filtration and were further purified by anion-exchange and Superose 12 column fast protein liquid chromatography. Three active fractions of apparent 250, 70 and 18 kilodaltons were identified. The parasitic origin of the predominant 70-kilodaltons protein(s) was shown by biosynthesis experiments with radioactive amino acid precursors and was also demonstrated by in vitro translation of parasitic mRNA species. Interestingly, antibodies to the 70-kilodalton exoprotein(s) also reacted to a schizont protein of similar molecular weight.

"Hit and run" transformation of mouse C127 cells by bovine papillomavirus type 4: the viral DNA is required for the initiation but not for maintenance of the transformed phenotype

Morphological transformation of C127 mouse fibroblasts by bovine papillomavirus type 4 (BPV-4) DNA depends on additional factors, including cell density, the presence of TPA, the concentration of fetal calf serum, and the physical state of the input DNA. Low cell density or the presence of TPA allows the achievement of full transformation, suggesting that disturbance of cell-to-cell contact may be necessary for the expression of the malignant phenotype. TPA also induces a burst of viral DNA synthesis and transcription. The frequency of focus formation is fivefold higher in the presence of high serum than in low serum, indicating that growth factors are required during the early stages of transformation. Although the same number of foci is obtained with either circular or linear DNA, the cells transfected with linear DNA are at a more advanced stage of transformation, being capable of anchorage-independent growth and tumorigenic in nude mice; only 5 out of 21 of these cell lines contain viral DNA in integrated and rearranged forms. In contrast cells transfected with circular DNA are anchorage-dependent but tumorigenic in mice, and do not contain any viral DNA. The linear DNA has an interrupted E1 ORF and fully transformed cells are obtained also with a subgenomic viral DNA fragment containing part of the L1 ORF, the complete E6, E7, and E8 ORFs, and the truncated E1 ORF under the control of the BPV-4 regulatory region; viral DNA is not maintained in any of these cell lines. In total, only 15% of the transformed cell lines contains viral DNA. These results are discussed in the light of the "hit and run" model of cell transformation.

Chemical carcinogenesis: from animal models to molecular models in one decade

During the last decade, progress in chemical carcinogenesis research has been substantial, and understanding the cellular changes and molecular causes of initiation, promotion, and malignant conversion appears to be within reach. Cancer begins as a carcinogen-induced genetic change in a single cell. The interaction of a particular carcinogen with specific genetic sites results, in part, from selectivity of metabolically activated carcinogens for particular nucleosides or gene sequences. In turn, modification of the molecular structure at specific genetic loci will have tissue-specific and species-specific consequences dependent on the expression of a particular gene, its sequence, and the function of the gene product in the target cell. It is likely that inactivation of regulatory regions, genomic rearrangements, and point mutations in coding sequences all can result in an altered cell phenotype. The rasH gene (and perhaps other members of the ras gene family) appears to be a common target for coding sequence mutations in the initiation of carcinogenesis in several organ sites and species by specific carcinogens. Whatever genetic mechanisms are involved, an initiated cell phenotype common to many epithelial cell types is observed. Initiated cells have an altered program of terminal differentiation, are resistant to cytotoxic substances or show altered requirements for specific growth factors or nutrients. These cells would have a selective growth advantage in cytostatic or cytotoxic situations or under conditions favoring terminal differentiation. Tumor promoters, some acting through specific cellular receptors, produce a tissue environment conductive to the selective clonal outgrowth of the initiated cell population resulting in a clinically evident premalignant lesion. The tissue specificity for most promoters depends on the ability of a particular agent to produce the selective conditions required for the initiated phenotype of that organ. At the molecular level, phorbol ester tumor promoters bind to and activate protein kinase C and transduce signals through this second-messenger pathway. Heterogeneity in the species of protein kinase C molecule expressed by normal and initiated epidermal cells could account for the differential response pattern observed in these cell types during skin tumor promotion. Malignant conversion of benign tumors requires further genetic changes in the tumor cell. Such changes could result from inherent instability in the genome of initiated cells, from spontaneous mutations more likely to occur in the expanding population of proliferating benign tumor cells, or by additional exposure to exogenous genotoxic agents.(ABSTRACT TRUNCATED AT 400 WORDS)

cis activation of the c-myc gene in bovine papilloma virus type 1/human c-myc hybrid plasmids

The c-myc gene amplification observed in human tumors is likely to represent an activation mechanism aiming at an increased transcription level. In order to evaluate the biological significance of this amplification in the malignant transformation we have designed an experimental model that could possibly mimic this situation in vitro. We have constructed a series of plasmids which physically link the human c-myc gene to the bovine papilloma virus type 1 genome (BPV1) and therefore should be maintained as amplified episomes upon transformation of rodent cells. Anticipating that the high copy number will bring about the immortalizing capacity of the c-myc gene, the constructions were introduced into primary rat embryo cells. Immortal cell lines were established by transfection of the hybrid plasmids carrying either the complete BPV1 genome or the transforming region of the viral genome. The BPV1 DNA alone or the c-myc gene alone has no activity in this assay. The analysis of the established cell lines demonstrates that the transfected plasmids are present not as free copies as anticipated but rather integrated as tandem repeats. We present data which strongly suggest that the immortalization capacity of the hybrid plasmids reflects the activation of the c-myc gene by the transactivable BPV1 enhancer. Although both the BPV1 early genes and the c-myc gene are actively transcribed, most of the cell lines do not display a transformed phenotype.

Viral replication in HeLa/fibroblast hybrid cells infected with human cytomegalovirus

Three human hybrid cell lines were generated by the fusion of D 98OR, a HeLa cell variant, and TIG human diploid fibroblasts. Chromosome numbers of the hybrid cells fell between that of D 98OR cells and the combined chromosome number of the two cell lines, with three marker chromosomes identical to those of D 98OR cells. One hybrid, B-3, produced large amounts of human fibronectin as analyzed by immunohistochemistry. Only human cytomegalovirus (HCMV) infected B-3 cells showed positive fluorescence as detected by human antiserum to HCMV. Further cloning of B-3 by limiting dilution resulted in two cloned hybrids with markedly enhanced virus production as compared with B-3 cells. Treatment of these two clones with phorbol ester further enhanced virus production. These cloned hybrids may provide a tool to analyze host cell factors controlling the transcription and replication of HCMV.

Characterization of Fischer rat embryo (CREF) cells transformed by bovine papillomavirus type 1

Transformation of an established Fischer rat embryo (CREF) cell line by bovine papillomavirus type 1 (BPV-1), in contrast to transformation by type 5 adenovirus or the T24 (Ha-ras) oncogene, resulted in transformants which did not exhibit a major increase in saturation density or a decrease in 125I-epidermal growth factor binding. BPV-1-transformed CREF clones did, however, grow in agar suspension culture and were tumorigenic in both nude mice and Fischer rats. The majority of transformed clones contained multiple extrachromosomal copies of BPV DNA. One transformed CREF clone contained integrated BPV-1 DNA which underwent sequence rearrangements following tumor formation in a Fischer rat and reestablishment in cell culture. BPV copy number varied in subclones of transformants isolated from the same monolayer focus, in agar-derived subclones of the same transformed focus, in tumors and in tumor-derived BPV-transformed CREF subclones. The degree of expression of specific transformation-related phenotypes, i.e., saturation density, growth in agar, and tumorigenicity, was not correlated with BPV copy number in transformed clones. Analysis of the biological properties of tumor-derived BPV-1-transformed CREF subclones indicated that certain transformants developed a stable increase in expression of transformation-related properties, a process termed "progression." TPA did not enhance the frequency of BPV-1 transformation or BPV DNA copy number in transformed CREF cells. The present study demonstrates that the CREF- transformation system can be utilized to study the molecular basis of BPV-1 transformation and should prove useful in studying the role of specific BPV-1 transforming proteins in regulating expression of the transformed phenotype and the mechanism by which transformed cells undergo progression of the transformed state.

Phorbol ester induces the transcriptional stimulatory activity of the SV40 enhancer

Phorbol ester tumour promoters can induce the transcription of a number of genes, including c-myc and c-fos. These genes are part of a group referred to as 'competence' genes because they are expressed very early after quiescent cells are stimulated to enter the cell cycle. The 'competence' genes are coordinately induced by serum and by factors such as platelet-derived growth factor and fibroblast growth factor. These factors, as well as the tumour promoter, 12-O-tetradecanoyl phorbol-13-acetate (TPA), are thought to exert their action by a mechanism involving the activation of protein kinase C. It is likely that these factors induce the transcription of the 'competence' genes either by activating specific transcription factors or by increasing their intracellular concentration; either mechanism may be mediated by protein kinase C. One approach to identifying such a putative transcription factor is to characterize the cis-acting transcriptional control elements that serve as a target site for the factor. Here we report that, in a human hepatoma cell line, TPA can specifically induce the activity of the simian virus 40 (SV40) transcriptional enhancer element. Since the SV40 enhancer is a thoroughly characterized cis-acting element, this system may facilitate the eventual identification of the trans-acting factor(s) whose activity is modified by TPA treatment.

Bovine papillomavirus genome elicits skin tumours in transgenic mice

Transmission of the bovine papillomavirus-1 (BPV-1) genome through the mouse germ line results in the heritable formation of fibropapillomas of the skin, a tissue-specific phenotype analogous to that observed in natural BPV-1 infection of cattle. Oncogenesis is slow, with tumours first arising at 8-9 months of age, usually in areas prone to wounding. Extrachromosomal BPV-1 DNA is detected in all tumours, whereas normal tissues show only integrated DNA.

Reversion of bovine papillomavirus-induced transformation and immortalization by a xanthate compound

Bovine papilloma virus-transformed hamster embryo fibroblasts (HEF-BPV) reacted to exposure to tricyclodecan-9-yl-xanthogenate (D609) with immediate reversion to the growth kinetics and the flat morphology of the untransformed parental cells. After six population doublings in the presence of D609, clones which displayed an untransformed morphology in the absence of D609 arose with a high frequency (90%). Such clones had reacquired a limited in vitro lifetime and had lost the ability to induce tumors in athymic nude mice. At the molecular level the revertant clones had lost all extrachromosomal monomeric BPV-1 DNA molecules. Only high molecular weight (HMW) oligomeric BPV-1 DNA that was probably integrated into the cellular genome was still detectable in a methylated transcriptionally inactive state. In contrast to transformed cells, the revertant clones no longer transcribed BPV-1-specific mRNA molecules, but were stimulated by a tumor promoter to transient viral gene expression. This article provides direct evidence for the complete reversibility of the property of "immortality".

The search for a culture system for papillomavirus

Papillomaviruses induce tumors of keratinocytes. Vegetative viral DNA replication and virion assembly are seen in those cells which are in the process of keratinizing or are keratinized. To date, no cell culture system has been developed that permits expression of the complete viral life cycle. Keratinocytes infected in culture may harbor the virus as a stable, replicating episome, but they do not support vegetative viral growth, nor do they become immortalized or transformed. The major obstacle in using keratinocyte cultures may be related to a dual need for transformation and full differentiation. Some animal papillomaviruses have been shown to be capable of transforming cultured murine fibroblasts. The fibroblast model is useful for identifying the viral-transforming gene(s) and their products.

Accelerated onset of viral transcription in adenovirus-infected HeLa cells treated with the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate

When adenovirus type 5-infected HeLa cells were exposed to the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate, short pulse-labeling with [3H]uridine in vivo and [3H]UTP incorporation by isolated nuclei in vitro were both consistent with a decreased latent period before initiation by RNA polymerase at early viral promoters. Acceleration was not dependent upon concurrent protein synthesis and could not be attributed to rapid entry of virus into the cell nucleus. 12-O-tetradecanoyl-phorbol-13-acetate suppressed the transcription-delay phenotype of the E1a mutant, hr1, without restoring its ability to replicate.

The transformed phenotype in culture and tumorigenicity of Fischer rat fibroblast cells (FR3T3) transformed with bovine papilloma virus type 1

Unlike cell lines transformed with Polyomaviruses, transformants derived by focus formation or colony formation in agarose medium after transfer into rat fibroblast cells (FR3T3 line) of Bovine Papilloma Type 1 (BPV1) DNA were consistently observed to grow poorly in suspension and to remain highly serum dependent for growth in culture. These cells did not produce detectable amounts of plasminogen activator, and kept the flat morphology and organized cytoskeleton characteristic of the normal fibroblast. However, they induced in syngeneic animals the development of tumors with a greater invasive potential than tumors induced by Polyomaviruses. By contrast with the original transformants, cells recovered from the tumors grew efficiently in suspension and produced high levels of plasminogen activator. They still had, however, extended cytoskeletal structures and remained completely dependent on high serum concentrations for growth in culture. The stepwise transformation process induced by BPV1 thus appears strikingly different from that previously observed with polyoma and SV40 viruses. The observed changes in transformation phenotype between transformed line and tumor cells do not correlate with any important modification of the number of autonomous copies of the viral genome, nor with any rearrangement of viral sequences detectable at the level of the blot analysis.

Tumour induction in the rodent Mastomys natalensis by activation of endogenous papilloma virus genomes

Specific DNA sequences from human papillomavirus have recently been detected in carcinomas from epidermodysplasia verruciformis patients, and in vulvar and cervical carcinomas but the role of papilloma viruses in the aetiology of these tumours is unclear. Indeed, little is known about the mechanisms that convert benign papillomas into malignant tumours and it is not even possible in tumour induction. Here, we describe an animal system that permits an analysis of the interaction of papilloma virus genomes with carcinogenic agents at the molecular level. In our colony of Mastomys natalensis (a close relative of the rat family), we have found extrachromosomal papilloma virus genomes persisting in a variety of tissues such as skin, muscle, liver and colon. With the ageing of the animals, the average copy number of viral DNA in skin cells increases and virus-producing tumours begin to appear in Mastomys at about 1 year old. This process is drastically enhanced by chronic treatment with a tumour promoter and transcription of the viral genomes has been found to be correlated with tumour formation.

Accelerated onset of viral transcription in adenovirus-infected HeLa cells treated with the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate

When adenovirus type 5-infected HeLa cells were exposed to the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate, short pulse-labeling with [3H]uridine in vivo and [3H]UTP incorporation by isolated nuclei in vitro were both consistent with a decreased latent period before initiation by RNA polymerase at early viral promoters. Acceleration was not dependent upon concurrent protein synthesis and could not be attributed to rapid entry of virus into the cell nucleus. 12-O-tetradecanoyl-phorbol-13-acetate suppressed the transcription-delay phenotype of the E1a mutant, hr1, without restoring its ability to replicate.

The simultaneous extraction of high-molecular-weight DNA and of RNA from solid tumors

A novel method for the isolation of both macromolecular DNA and RNA from solid tissues based upon the disruption by vibration of deep-frozen material in a mechanical device termed Mikro-dismembrator, is described. This technique reveals a yield of, on the average, 1 to 3 mg of either DNA or RNA per gram of tissue. The quality of the purified nucleic acids permits the detailed analysis of integrated tumor virus DNA sequences and their mRNA transcripts. Furthermore, the efficient isolation of papilloma virions from keratinized wart tissue is facilitated by the application of the Mikro-dismembrator.

Comparative studies of the expression of linked Escherichia coli gpt gene and BPV-1 DNAs in transfected cells

A series of hybrid plasmids, containing two selective markers that can be expressed in mammalian cells have been constructed. These plasmids are derived from the pSV2gpt recombinant plasmid described by Mulligan and Berg (1980) and contain the entire BPV-1 DNA, or the HindIII-BamHI large transforming fragment (T69) or the early transforming region of polyoma virus DNA. DNA transfers into Fisher rat 3T3 cells were performed either by the calcium phosphate coprecipitation technique, or by protoplast fusion. For all plasmids, the frequency of formation of phenotypically transformed foci or of the expression of the gpt+ marker in selective medium have been scored comparatively. Both series of recombinant plasmids gave similar yield of gpt+ colonies, whereas BPV plasmids (both entire or T69 subgenome) transformed morphologically rat cells 8-50 times less frequently than their polyoma homologues. Although the pSV2gpt BPV-1 plasmids can replicate autonomously to high copy number in monkey COS cells, the rate of transcription of the BPV-1 genome in these cells is 10(2) to 10(3) times lower than that of gpt transcribed from the SV40 early promoter. This low rate of transcription may explain the low frequency of transformation by this viral DNA.