The bovine papillomavirus genome and its uses as a eukaryotic vector

Pteridium spp. and Bovine Papillomavirus: Partners in Cancer

Bovine papillomavirus (BPV) are a cause for global concern due to their wide distribution and the wide range of benign and malignant diseases they are able to induce. Those lesions include cutaneous and upper digestive papillomas, multiple histological types of urinary bladder cancers—most often associated with BPV1 and BPV2—and squamous cell carcinomas of the upper digestive system, associated with BPV4. Clinical, epidemiological and experimental evidence shows that exposure to bracken fern (Pteridium spp.) and other related ferns plays an important role in allowing viral persistence and promoting the malignant transformation of early viral lesions. This carcinogenic potential has been attributed to bracken illudane glycoside compounds with immune suppressive and mutagenic properties, such as ptaquiloside. This review addresses the role of BPV in tumorigenesis and its interactions with bracken illudane glycosides. Current data indicates that inactivation of cytotoxic T lymphocytes and natural killer cells by bracken fern illudanes plays a significant role in allowing viral persistence and lesion progression, while BPV drives unchecked cell proliferation and allows the accumulation of genetic damage caused by chemical mutagens. Despite limited progress in controlling bracken infestation in pasturelands, bracken toxins remain a threat to animal health. The number of recognized BPV types has steadily increased over the years and now reaches 24 genotypes with different pathogenic properties. It remains essential to widen the available knowledge concerning BPV and its synergistic interactions with bracken chemical carcinogens, in order to achieve satisfactory control of the livestock losses they induce worldwide.

Chromatin structure and dynamics: functional implications

In eucaryotes, DNA packaging into nucleosomes and its organization in a chromatin fiber generate constraints for all processes involving DNA, such as DNA-replication, -repair, -recombination, and -transcription. Transient changes in chromatin structure allow overcoming these constraints with different requirements in regions where processes described above are initiated. Mechanisms involved in chromatin dynamics are complex. Multiprotein complexes which can contain histone-acetyltransferase, -deacetylase, -methyltransferase or -kinase activities are targeted by regulatory factors to precise regions of the genome. These enzymes have been shown to modify histone-tails within specific nucleosomes. Post-translational modifications of histone-tails constitute a code that is thought to contribute to the nucleosome or to the chromatin fiber remodeling, either directly, or through the recruitment of other proteins. Other multiprotein complexes, such as ATP-dependent remodeling complexes, play an essential role in chromatin fiber dynamics allowing nucleosome sliding and redistribution on the DNA. We will focus here on the chromatin structure and its consequences for DNA damaging, replication, repair, and transcription and we will discuss the mechanisms of chromatin remodeling.

Development of human papillomavirus plasmids capable of episomal replication in human cell lines

Gene delivery into human cells has been facilitated by the development of viral vector systems. These vectors have shown great potential for the efficient delivery of therapeutic genes into human cells. A problem with many of the existing systems, however, is the integration of these vectors into the chromosome which affects the length of gene expression and may promote oncogenic transformation. In an effort to develop viral vectors that can replicate extrachromosomally in human cells, we have generated human papillomavirus (HPV) plasmids containing all the elements required for replication on a single DNA molecule. HPV plasmids containing the viral E1 and E2 genes (or the E1 gene alone) and an origin of replication were shown to replicate to significant levels in the transfected human cervical carcinoma C-33A cell line. Since approaches towards the possible gene therapy of cystic fibrosis (CF) are currently under intensive investigation, we have also tested short-term replication of HPV plasmids in the IB3 cell line derived from a CF patient. Our results demonstrate that HPV plasmids are capable of extrachromosomal replication in these cell lines and may potentially be important vectors for the delivery of therapeutic genes into human cells.

Luteinizing hormone-releasing hormone quantified in tissues and slice explant cultures of postnatal rat hypothalami

LH-releasing hormone (LHRH) peptide from postnatal rat preoptic area (POA)/hypothalamic tissues in vivo and slice explant cultures maintained in vitro was quantitated using an enzyme-linked immunosorbant assay. Moreover, messenger RNA (mRNA) copy number was calculated in LHRH neurons maintained in culture using in situ hybridization histochemistry with autoradiographic film analysis. POA/hypothalami from postnatal day 5-6 pups averaged 1250 pg of LHRH, with approximately 28% of peptide residing within rostral tissues where most LHRH perikarya reside. Explant cultures maintained 18 days in vitro contained 30.4-92.0 pg/slice with a whole animal total of 244.8 pg. Considering cell numbers in vivo and in vitro, LHRH neurons in whole animal produce 1.0 pg of LHRH/cell, whereas those in culture average 2.0 pg/cell. Furthermore, LHRH mRNA copies/cell in organotypic culture was estimated conservatively at 1410 copies/cell, a relatively high number. This work shows that, compared with whole animal, cultures have substantial LHRH stores, indicating maturation of synthetic activity and/or formation of new terminals in vitro. High LHRH mRNA copy number also suggests a high rate of peptide biosynthesis. Our analysis, demonstrating the dynamic potential of LHRH neurons, suggests that subtle changes in LHRH mRNA expression in all cells or a subpopulation can dramatically alter the LHRH system biosynthetic capacity.

Expression of human parathyroid hormone in mammalian cells, Escherichia coli and Saccharomyces cerevisiae

The entire human parathyroid hormone (hPTH) cDNA gene with its natural signal and pro-region is expressed in transfected mouse mammary tumor cells (C127I cells) and Chinese hamster lung cells (DON cells) under control of the murine metallothioneine-1 promoter in a vector in which replication functions are provided by the entire genome of bovine papilloma virus type I (BPV-1). Authentic hPTH is efficiently produced by the non-endocrine cells and secreted to the growth medium without any abberant processing. Immunoblots from SDS-PAGE gels of concentrated growth medium reveal one band corresponding to intact, undegraded hPTH. Purification by reversed-phase HPLC results in a peptide with an amino acid content and N-terminal sequence identical to hPTH. For comparison, hPTH cDNA with deleted prepro-region is also expressed as secretory proteins in Escherichia coli and in Saccharomyces cerevisiae. In E. coli the vector construct is based on the staphylococcal protein A promoter employing protein A signal sequence. In S. cerevisiae a mating factor alpha expression system containing the factor alpha-signal sequence is employed. The results show that intact hPTH is secreted in addition to proteolytically cleaved fragments in both microorganisms. Thus, the signal sequences promote efficient secretion, and correct N-terminal processing of hPTH in both mammalian, bacterial and yeast cells. However, the folding characteristics of hPTH make it susceptible to internal proteolytical cleavage which appears to be species specific in yeast and E. coli.

High level, stable production of recombinant proteins in mammalian cell culture using the herpesvirus VP16 transactivator

We have engineered mammalian cell lines to produce high levels of heterologous proteins by constructing a cell line that expresses the herpesvirus transactivator, VP16. Subsequent stable transfection with a gene of interest under control of a herpesvirus immediate early promoter led to a rapid isolation of cell lines producing between 1 and 20 micrograms of protein/million cells/24 hours. This high level expression is stable for at least five months.

Random-choice replication of extrachromosomal bovine papillomavirus (BPV) molecules in heterogeneous, clonally derived BPV-infected cell lines

Using fluorescence in situ hybridization and Southern blot analysis, we show that three clonally derived cell lines transformed with bovine papillomavirus (BPV), including ID13, the cell line commonly employed for BPV replication studies, are heterogeneous populations having extensive cell-to-cell variation in both the distribution and amount of BPV DNA. Different subclones of ID13 were found to differ in the form and amount of BPV DNA they contain. Most subclones showed no detectable BPV sequences; some contained either extrachromosomal BPV molecules distributed throughout the nucleus or BPV sequences integrated at discrete chromosomal sites, while others contained both integrated and plasmid forms. The results of density gradient analysis of BPV DNA from individual homogeneous subclones showed replication of the extrachromosomal BPV plasmids in a random-choice mode. In all cell lines studied, the presence after one round of chromosomal DNA replication of unreplicated BPV DNA and of BPV DNA having two postreplicative strands was independent of the presence of high-BPV-copy-number ("jackpot") cells. Our results substantiate the earlier conclusion that extrachromosomal BPV molecules replicate randomly and not according to a once-per-cell-cycle mechanism.

Persistence of Ha-ras-induced metastatic potential of SP1 mouse mammary tumors despite loss of the Ha-ras shuttle vector

Previous studies have shown that the SP1 mouse mammary adenocarcinoma cell line, which is tumorigenic but nonmetastatic, acquires metastatic potential when transfected with the activated human Ha-ras gene. In addition, the process of calcium phosphate-mediated DNA transfection, as well as treatment with the calcium ionophore A23187 or with phorbol 12-myristate 13-acetate, can also result in heritable changes in the malignant behavior of SP1 cells. It was of interest, therefore, to determine whether the metastatic consequences of Ha-ras oncogene expression in SP1 cells are a primary effect of the transfected gene or whether heritable secondary changes are induced by Ha-ras oncogene expression. In the latter case, continued expression of the Ha-ras oncogene would not be required to maintain the metastatic phenotype. To test this hypothesis we introduced the Ha-ras oncogene into SP1 cells on a shuttle vector in which maintenance of the vector was dependent on selection for resistance to the antibiotic G418. Subclones which had lost the transfected Ha-ras gene were subsequently isolated following growth in nonselective medium. The Ha-ras-transfected clones and the revertant subclones were found to be equally metastatic, indicating that transfection with the Ha-ras gene does induce stable secondary changes in the metastatic phenotype of SP1 cells.

Replication of bovine papillomavirus vectors in murine cells

Varying capacities for autonomous replication have been obtained with bovine papillomavirus type 1 (BPV-1)-based expression vectors in mouse C127 cells. Both integration of the vector DNA into the genome of the host cell and replication as monomeric extrachromosomal elements have been observed. In this study, we have examined what features of BPV-1 vectors influence their replication potential. Transfection of the entire BPV-1 genome into C127 cells resulted in the replication of extrachromosomal monomeric BPV-1 elements. The same result was obtained when a plasmid sequence was inserted into the BPV-1 DNA. However, introduction of foreign, transcriptionally active units resulted in chromosomal integration of the expression vectors. This result was obtained with clones isolated by co-transfection followed by neomycin selection, as well as with clones isolated from neoplastic foci. Supertransfection of a BPV-1-based expression vector into cells harbouring unintegrated replicating BPV-1 genomes resulted in integration of the vector DNA, whereas replication of the resident BPV-1 genomes was unaffected. Extrachromosomal replication of such a vector was achieved when the enhancer and promoter region of the foreign gene were deleted.

Differences in human cell lines to support stable replication of Epstein-Barr virus-based vectors

Vectors carrying the origin of replication, ori-P, of the Epstein-Barr virus (EBV) are maintained extrachromosomally in human cells expressing the EBV nuclear antigen 1 (EBNA-1). We have studied the EBV vectors p201 and p292 in which both ori-P and EBNA-1 functions are present using the human cell lines A431 and HeLa. The two lines showed differences in their transfectability by the EBV vectors. Thousands of HeLa transfectants were obtained with either vector and these remained intact as episomes. A431 could only be efficiently transfected with p292 and a high ratio of chromosomal integrations and rearrangements were observed. The vector p292 expressed the EBNA-1 gene more efficiently than p201 and this was found to be associated with a harmful effect on the grown of both HeLa and A431 lines. These results indicate that EBV vectors behave differently, depending on the cell line and that over-expression of EBNA 1 from these vectors may be detrimental to the cells.

Human tissue-type plasminogen activator synthesized by using a baculovirus vector in insect cells compared with human plasminogen activator produced in mouse cells

A cDNA fragment encoding the human tissue-type plasminogen activator was inserted into the baculovirus Autographa californica nuclear polyhedrosis virus downstream from the polyhedrin promoter. The induction kinetics of t-PA was followed, after infection of Spodoptera frugiperda cells, at both mRNA and protein levels. Fibrinolytically active plasminogen activator accumulated in the culture medium and reached 2.5 micrograms/ml after 120 h. The protein was compared with recombinant plasminogen activator produced in mouse cells and was found to be slightly smaller. This difference in size was found to be caused by N-linked oligosaccharides which are shorter in the recombinant activator obtained from insect cells. The molecules produced in such cells contain at least two different types of N-linked glycans, since only one out of three oligosaccharides is sensitive to endoglycosidase H. However, all glycan structures bind strongly to concanavalin A-Sepharose.