Inhibition of retrovirus DNA supercoiling in interferon-treated cells

Mechanisms of viral inhibition by interferons

Interferons (IFNs) are a family of related proteins grouped in four species (alpha, beta, gamma and omega) according to their cellular origin, inducing agents and antigenic and functional properties. Their binding to specific receptors leads to the activation of signal transduction pathways that stimulate a defined set of genes, whose products are eventually responsible for the IFN antiviral effects. Their action against viruses is a complex phenomenon. It has been reported that IFNs restrict virus growth at the levels of penetration, uncoating, synthesis of mRNA, protein synthesis and assembly. This review will attempt to evaluate evidence of the involvement of the IFN-inducible proteins in the expression of the antiviral state against RNA or DNA viruses.

Acidic phospholipids directly inhibit DNA binding of mammalian DNA topoisomerase I

Inhibition of mammalian DNA topoisomerase I by phospholipids was investigated using purified enzyme. Acidic phospholipids inhibited the DNA relaxation activity of topoisomerase I whereas neutral phospholipid, phosphatidylethanolamine, did not. Accumulation of a protein-DNA cleavable complex, an intermediate which is known to accumulate upon inhibition by a specific inhibitor camptothecin, did not occur. The filter binding assay revealed that the DNA binding activity of the enzyme was inhibited by acidic phospholipids. Moreover, direct binding of phosphatidylglycerol to topoisomerase I was demonstrated. These results indicated that the inhibitory effect of acidic phospholipids on topoisomerase I was due to the loss of the DNA binding of the enzyme as a result of direct interaction between phospholipids and the enzyme.

Interferon-mediated inhibition of retroviral infection: use of a defective retrovirus carrying a drug-resistance gene

We here report the results of an investigation of the effect of interferon on the establishment of new infections by a retrovirus. For this study, we used an infectious but replication-incompetent retrovirus carrying a drug-resistance gene and assayed for infectivity by measuring drug-resistant colony formation. Mouse interferon-beta inhibited retroviral infection of mouse CG1 cells in a dose-dependent manner. However, a higher dose of interferon was needed for eliciting the antiretroviral effects than for action against vesicular stomatitis virus. The degree of antiretroviral effect was comparable over at least a 100-fold range of multiplicity of infection and the effect was most pronounced when the cells were continuously treated with interferon before infections and during infection and drug-selection.

Depression of early phase of HTLV-I infection in vitro mediated by human beta-interferon

Natural human interferon beta (beta-IFN) was tested during the early phase of in vitro infection with HTLV-I virus of human cord blood mononuclear cells (CBL), to evaluate whether its antiviral and immunomodulating effects might prevent spreading of infection in the host. beta-IFN was found to reduce HTLV-I transmission and integration in CBL cultures. Moreover, beta-IFN had no effect in preventing virus transmission and integration in K562 and a very limited effect in HL60 and Molt-4 human tumour lines, suggesting a cell-type specific mode of action. beta-IFN induced a 'priming' response on CBL, since overnight pretreatment of recipient cells or one single treatment at the onset of the coculture were almost equally effective in protecting against HTLV-I infection. During the early days post infection (p.i.), IFN-treated CBL showed a pattern of phenotypic markers that was closer to that of non-infected CBL. In contrast, untreated CBL exposed to HTLV-I showed a percent increase of Tac+, M3+ and Leu 11+ subpopulations. Cell-mediated immune responses of CBL were depressed after coculturing with HTLV-I producer MT-2 cells. beta-IFN was able to boost the cell-mediated cytotoxicity of fresh and infected CBL against both K562 and MT-2 target cells. Leukocyte blastogenesis in mixed lymphocyte/tumour cell cultures, evaluated in terms of 3H-thymidine incorporation during the first week p.i., was also enhanced by IFN when macrophages and lymphocytes were reconstituted at an optimal 1:20 ratio. It is conceivable that this overall enhancement of the immune response induced by beta-IFN could contribute to reduce HTLV-I infection in vitro.

Persistent expression of v-mos oncogene in transformed cells that revert to nonmalignancy after prolonged treatment with interferon

BALB/c embryonic fibroblasts productively transformed by Moloney sarcoma virus and cultivated for over 600 generations in the presence of mouse alpha/beta interferon reverted to an apparently normal phenotype and were unable to produce tumors in nude mice. Nevertheless, the presence of an integrated Moloney sarcoma virus genome in the nonmalignant Moloney sarcoma virus-transformed interferon-treated cell DNA could be shown by focus formation upon transfection and by hybridization with a v-mos probe. After digestion with various restriction endonucleases, similar hybridization patterns of v-mos sequences were obtained with DNAs from both reverted and transformed cells. However, additional integration sites and at least twice as many copies of the oncogene were found in the nonmalignant Moloney sarcoma virus-transformed interferon-treated cell DNA. Polyadenylylated RNA extracted from reverted and control cells contained two mos-specific transcripts. Interestingly, the nonmalignant Moloney sarcoma virus-transformed interferon-treated cells produced helper virus, but no detectable mos-containing virions, suggesting that a posttranscriptional block in the v-mos gene expression had occurred in these cells. It should be stressed that, after up to 100 additional passages, cells cultured in the absence of interferon maintained their nontumorigenic character in spite of the persistent transcription of the mos oncogene.

MAV-2-O replicates at a reduced rate in cells from the osteopetrosis resistant G-B1 chicken

The replication of the avian osteopetrosis virus MAV-2-O was compared in chick embryo fibroblasts from two strains of chicken. These were G-B1 which is relatively resistant to MAV-2-O and CB which is susceptible. The production of MAV-2-O was delayed in G-B1 cells (compared with CB cells). The same result was observed after infection with Rous sarcoma viruses of subgroups B, C, and D. In addition, the transforming viruses induced foci on G-B1 fibroblasts 24 to 48 hours later than on CB fibroblasts. In G-B1 cells there was also a delayed kinetics of intracellular viral RNA production. Integrated and linear unintegrated MAV-2-O DNA species were also present in lower amounts in G-B1 than in CB fibroblasts at 3 days postinfection. In vivo studies confirmed the in vitro situation. There was a marked difference in the amount of virus present in the osteoid bone matrix and the osteocytic lacunae of osteopetrotic bones from susceptible and G-B1 chickens. In contrast to the bone lesions from susceptible animals, budding virus particles were not detectable in lesions from G-B1 chickens. There was no difference in the amount of virus in osteopetrotic and non-osteopetrotic bone of susceptible chickens suggesting that virus replication alone is not sufficient for induction of osteopetrosis and that an additional specific virus-cell interaction is required. The relative resistance of strain G-B1 may therefore, be a consequence of a reduced frequency of this interaction. Its basis may be the lower amount of integrated, as well as unintegrated, viral DNA.

Effect of Moloney murine leukemia virus on the carcinogenicity of 3-methylcholanthrene in normal rat kidney cells

Normal rat kidney (NRK) cell were found to be resistant to neoplastic transformation by diverse carcinogenic chemicals. To study chemical-retroviral co-carcinogenesis in this cells they were infected with a low multiplicity of Moloney murine leukemia virus (M-MLV). Using a single cell cloning procedure, a virus-producing clone was isolated from the infected cells, which was shown to carry only one integrated M-MLV provirus per cell. It was found that this single provirus was sufficient to render the clone susceptible to transformation by 3-methylcholanthrene (3-MC). However this clone responded differently to the carcinogen at different passages after infection. When exposed to 3-MC at a low passage postinfection (passage 5), cell transformation was evident only after 11 subsequent subcultures. On the other hand when it was chemically treated at a high passage postinfection (passage 29), cell transformation could clearly be detected already at the next subculture after the chemical treatment. It is suggested that an M-MLV-mediated cumulative effect is necessary to complement the action of the carcinogen in order to complete the carcinogenic process in these cells. This cumulative viral effect appeared to be associated with a change in the control of the virus expression, since 3-MC was found to stimulate virus replication in this clone also only at the high passage postinfection. Indeed virus release by cells of isolated transformed foci, produced by the chemical-M-MLV co-carcinogenesis, was extremely higher than by untransformed cells.

Interferon inhibits marker rescue of vaccinia virus

In this investigation we have examined the effect of human interferon (IFN) type alpha on the ability of vaccinia virus to recombine within infected African green monkey kidney cells (BSC-40). We measured by marker rescue, the extent of insertion of cloned 5-kb Hind III-J restriction fragment of wild-type vaccinia DNA into the genome of temperature-sensitive mutants. We showed that IFN at doses of 100-1000 U/ml inhibited the replication of vesicular stomatitis virus (VSV) and polio viruses but not of vaccinia virus. Vaccinia virus adsorption, penetration, uncoating, protein synthesis, and yields were not inhibited. However, marker rescue of vaccinia virus was inhibited by IFN. This inhibition was not related to IFN-mediated changes in uptake of exogenous DNA or enhanced degradation of the transfected DNA. These results suggest that IFN affects homologous vaccinia DNA recombination.

Virus- and cell-encoded tyrosine protein kinases inactivate DNA topoisomerases in vitro

Tyrosine protein kinase activity is associated with at least eight different retrovirus-encoded onc gene products and with cell receptors for epidermal growth factor, platelet-derived growth factor, tumour growth factor and insulin. Both the onc kinases and the growth factor receptors are membrane proteins whose enzymatic activity has been implicated in stimulation of growth. However, the mechanism by which a signal passes from the plasma membrane to the nucleus to initiate growth remains unknown. As DNA topoisomerases catalyse the interconversion of topological isomers of DNA and hence affect DNA replication, transcription and recombination, they may be involved also in stimulation of growth. Several DNA topoisomerases have been shown to form a covalent complex with DNA via a phosphotyrosine linkage. The DNA-protein complex is postulated to be an intermediate in breaking and rejoining of DNA. The aim of the present study was to determine whether tyrosine protein kinases modulate the activity of topoisomerases by phosphorylating the tyrosine residue involved in DNA binding. We report that incubation of Escherichia coli and calf thymus type I DNA topoisomerases with the Rous sarcoma virus transforming gene product, pp60src, and TPK75, a tyrosine protein kinase purified from normal rat liver, results in a 10-fold loss of topoisomerase activity.

Slow virus visna: reproduction in vitro of virus from extrachromosomal DNA

Under permissive conditions of growth in tissue culture, the retrovirus visna multiples over the course of a few days to high titer and kills the host cell. We show that in this lytic life cycle, viral DNA is tightly associated with, but not covalently linked to, chromosomal DNA. This finding provides explanations for a number of the unusual properties of the lentivirus subfamily of retroviruses, and suggests potential mechanisms for the block in virus gene expression in vivo responsible for the slow infection in nature.