Biological activity of tumor virus DNA

Epstein-Barr virus nuclear antigen 3C interacts with and enhances the stability of the c-Myc oncoprotein

Epstein-Barr virus (EBV) was the first human DNA virus to be associated with cancer. Its oncogenic potential was further demonstrated by its ability to transform primary B lymphocytes in vitro. EBV nuclear antigen 3C (EBNA3C) is one of a small subset of latent antigens critical for the transformation of human primary B lymphocytes. Although EBNA3C has been shown to modulate several cellular functions, additional targets involved in cellular transformation remain to be explored. EBNA3C can recruit key components of the SCF(Skp2) ubiquitin ligase complex. In this report, we show that EBNA3C residues 130 to 190, previously shown to bind to the SCF(Skp2) complex, also can strongly associate with the c-Myc oncoprotein. Additionally, the interaction of EBNA3C with c-Myc was mapped to the region of c-Myc that includes the highly conserved Skp2 binding domain. Skp2 has been shown to regulate c-Myc stability and also has been shown to function as a coactivator of transcription for c-Myc target genes. We now show that the EBV latent oncoprotein EBNA3C can stabilize c-Myc and that the recruitment of both c-Myc and its cofactor Skp2 to c-Myc-dependent promoters can enhance c-Myc-dependent transcription. This same region of EBNA3C also recruits and modulates the activity of retinoblastoma and p27, both major regulators of the mammalian cell cycle. The inclusion of c-Myc in the group of cellular targets modulated by this domain further accentuates the importance of these critical residues of EBNA3C in bypassing the cell cycle checkpoints.

Rapid transformant selection by human complement using HRF20 (CD59) cDNA as a selection marker

The 20 kDa homologous restriction factor (HRF20, CD59) prevents formation of membrane attack complexes of human complement. Transfection of the cDNA of HRF20 to heterologous cells, which are sensitive to human complement, thereby renders these cells resistant to human complement attack. This property can then be used as a selection marker of transfection. In addition, the procedure is short and can be performed within 8 h. Since only 1 h is required for incubation with complement, the damage to the transfected cells is limited compared with damage resulting from selection for drug resistance which requires 10 days or more of incubation in the presence of harmful agents.

Development of a helper-independent human adenovirus vector and its use in the transfer of the herpes simplex virus thymidine kinase gene

Approximately 2 kilobases (kb) of additional DNA can be packaged into wild-type virions of human adenovirus type 5 (Ad5). To extend this limit, a helper independent Ad5 cloning vector was constructed by deleting most of early region 3 (E3) from map coordinates 78.5 to 84.7 and essentially all of early region 1 (E1) from coordinates 1.0 to 10.6. E3 is nonessential for adenovirus replication in cultured cells, and E1 is nonessential when the virus is propagated in 293 cells which constitutively express the E1 gene products. The resulting new virus, dlE1,3 is about 5.5 kb shorter than wild-type Ad5 and therefore should be able to accept up to 7.5 kb in foreign DNA. To test the usefulness of this vector, the herpes simplex virus type 1 (HSV-1) thymidine kinase gene (tk) along with its regulatory sequences was inserted into the unique XbaI site of dlE1,3 (at map position 78.5/84.7). The resulting recombinant virus, Adtk, expressed the HSV tk at a low level (as compared with HSV-1) in infected cells; however, tk expression was markedly enhanced when Adtk-infected cells were superinfected with a tk- mutant of HSV. Furthermore, the Adtk virus efficiently transformed tk- mouse cells (line LTA) to the tk+ phenotype. At a low efficiency, it was also possible to transform tk- human cells (line 143), and tk+ transformants of both mouse and human origin have been established as permanent lines.

Expression of ki-ras oncogene in tumor cell variants exhibiting different metastatic capabilities

The expression of oncogenes in well-characterized B16 melanoma and UV-2237 fibrosarcoma cell variants that exhibit distinct metastatic properties was explored. The search for the expression of 11 different oncogenes revealed that the major oncogene in those two tumor systems is the Kirsten-ras (ki-ras). The results indicate that the amounts of specific ki-ras mRNA and the p21 protein are similar in both low- and high-metastatic counterparts. These results suggest that in these systems there is no apparent direct correlation between the amount and expression of the major cellular oncogene so far identified and the metastatic potential of these tumor cells.

Microinjection of somatic cells with micropipettes: comparison with other transfer techniques

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Construction and use of a dominant, selectable marker: a Harvey sarcoma virus-dihydrofolate reductase chimera

The transcriptional promoter of the Harvey sarcoma virus long terminal repeat has been used to construct a biologically active dihydrofolate reductase chimera. The construction placed the long terminal repeat at the 5' end of a dihydrofolate reductase cDNA. This chimera mediated methotrexate resistance when introduced into wild-type NIH3T3 mouse cells by transfection. The chimeric sequences were expressed in the form of polyadenylated RNA and dihydrofolate reductase protein and were amplified when the methotrexate-resistant transfectants were selected to grow in increasing methotrexate concentrations. This chimera was dominant acting and able to confer a methotrexate-resistant phenotype on wild-type NIH3T3 cells. It has been used in cotransfection experiments with DNA from human tumor cells to obtain foci of methotrexate-resistant transformed NIH3T3 cells resulting from uptake of exogenous DNA. The transfected methotrexate-resistant cells carried double minute chromosomes that appeared to contain DNA acquired during transfection.

Fusion-mediated injection of SV40-DNA. Introduction of SV40-DNA into tissue culture cells by the use of DNA-loaded reconstituted Sendai virus envelopes

Sendai virus envelopes can be solubilized by non-ionic detergents such as Triton X-100. Removal of the detergent from a supernatant containing the solubilized viral envelope glycoproteins results in the formation of reconstituted fusogenic viral envelopes. When SV40-DNA is added to the reconstitution system, it is trapped within the viral envelope. Incubation of SV40-DNA-loaded Sendai virus envelopes with permissive cells (CV1 and TC7 cells) resulted in fusion-mediated injection of the trapped DNA, as was demonstrated by the ability of the injected cells to synthesize SV40-T-antigen. Quantitative estimation revealed that up to 20% of the injected cells were able to synthesize T-antigen. Loaded viral envelopes were able to inject SV40-DNA and to promote synthesis of T-antigen also in cells which are resistant to infection by intact SV40 viruses, such as F1' 1-4 cells. In addition, it is shown that reconstituted envelopes of Sendai virus are able to transfer membrane fragments from SV40 receptor-positive into SV40 receptor-negative cells, such as F1' 1-4 cells. After implantation of SV40 receptors, the F1' 1-4 cells became susceptible to infection by intact SV40 viruses.

Construction and use of a dominant, selectable marker: a Harvey sarcoma virus-dihydrofolate reductase chimera

The transcriptional promoter of the Harvey sarcoma virus long terminal repeat has been used to construct a biologically active dihydrofolate reductase chimera. The construction placed the long terminal repeat at the 5' end of a dihydrofolate reductase cDNA. This chimera mediated methotrexate resistance when introduced into wild-type NIH3T3 mouse cells by transfection. The chimeric sequences were expressed in the form of polyadenylated RNA and dihydrofolate reductase protein and were amplified when the methotrexate-resistant transfectants were selected to grow in increasing methotrexate concentrations. This chimera was dominant acting and able to confer a methotrexate-resistant phenotype on wild-type NIH3T3 cells. It has been used in cotransfection experiments with DNA from human tumor cells to obtain foci of methotrexate-resistant transformed NIH3T3 cells resulting from uptake of exogenous DNA. The transfected methotrexate-resistant cells carried double minute chromosomes that appeared to contain DNA acquired during transfection.

Molecular cloning of the avian myelocytomatosis virus genome and recovery of infectious virus by transfection of chicken cells

The avian retrovirus myelocytomatosis virus 19 (MCV) possesses an interesting diversity of oncogenic potentials, but the virus has proven difficult to study because of its inability to replicate without the assistance of a helper virus. We have therefore isolated and amplified the genome of MCV by molecular cloning in a procaryotic vector. The topography of the cloned DNA was explored by the use of restriction endonucleases and radioactive complementary DNAs representing specific domains in avian retrovirus genomes. The cloned DNA appeared to be an authentic representation of the MCV genome: the size and genetic topography of the DNA were comparable to those of MCV, and transfection of the cloned DNA into chicken cells (in company with the DNA of a suitable helper virus) gave rise to virus with the genome and transforming potentials of MCV. The availability of cloned MCV DNA should facilitate a variety of genetic and biochemical manipulations directed at elucidating the mechanism of oncogenesis by MCV.

Direct transfer of cloned genes from bacteria to mammalian cells

Induction of a virus infection by cloned simian virus 40 DNA was chosen as a test system to detect transfer of genes from bacteria to cultured mammalian cells. Escherichia coli cells containing a recombinant plasmid with three tandem inserts of simian virus 40 DNA were able to infect CV-1 monkey cells under various conditions. The gene transfer was resistant to DNase I and therefore seems not to occur via free DNA but most likely via uptake of whole bacteria, followed by release of plasmid DNA and generation of infectious circular simian virus 40 DNA in a recombination-excision process. Spontaneous transfer was found to be infrequent, 4 x 10(9) bacteria yielding one infection per 10(7) monkey cells. The frequency was greatly increased by adding bacteria as a calcium phosphate coprecipitate or by fusion of lysozyme-treated bacteria (protoplasts) with monkey cells in the presence of polyethylene glycol. With the latter technique, 10(4) protoplasts gave rise to one infection per 15 monkey cells. Experiments with other cell lines of human, monkey, and mouse origin, and also with bacteria harboring another recombinant plasmid, indicate that DNA transfer from bacteria to mammalian cells is a general phenomenon.