Nitropyrenes are inducers of polyoma viral DNA synthesis

The evolution of small DNA viruses of eukaryotes: past and present considerations

Historically, viral evolution has often been considered from the perspective of the ability of the virus to maintain viral pathogenic fitness by causing disease. A predator-prey model has been successfully applied to explain genetically variable quasi-species of viruses, such as influenza virus and human immunodeficiency virus (HIV), which evolve much faster rates than the host. In contrast, small DNA viruses (polyomaviruses, papillomaviruses, and parvoviruses) are species specific but are stable genetically, and appear to have co-evolved with their host species. Genetic stability is attributable primarily to the ability to establish and maintain a benign persistent state in vivo and not to the host DNA proofreading mechanisms. The persistent state often involves a cell cycle-regulated episomal state and a tight linkage of DNA amplification mechanisms to cellular differentiation. This linkage requires conserved features among viral regulatory proteins, with characteristic host-interactive domains needed to recruit and utilize host machinery, thus imposing mechanistic constrains on possible evolutionary options. Sequence similarities within these domains are seen amongst all small mammalian DNA viruses and most of the parvo-like viruses, including those that span the entire spectrum of evolution of organisms from E. coli to humans that replicate via a rolling circle-like mechanism among the entire spectrum of organisms throughout evolution from E. coli to humans. To achieve benign inapparent viral persistence, small DNA viruses are proposed to circumvent the host acute phase reaction (characterized by minimal inflammation) by mechanisms that are evolutionarily adapted to the immune system and the related cytokine communication networks. A striking example of this is the relationship of hymenoptera to polydnaviruses, in which the crucial to the recognition of self, development, and maintenance of genetic identity of both the host and virus. These observations in aggregate suggest that viral replicons are not recent "escapies" of host replication, but rather provide relentless pressure in driving the evolution of the host through cospeciation.

Analysis of mutagenic activity and ability to induce replication of polyoma DNA sequences by different model metabolites of the carcinogenic tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) requires metabolic activation to express its carcinogenic activity. This activation leads to the formation of methylating and pyridyloxobutylating agents. To determine the possible biological effects mediated by each of these metabolic pathways we have studied the activities of model compounds that are metabolized to either a methylating or pyridyloxobutylating species. Each model compound was evaluated for its mutagenic activity in both prokaryotic and eukaryotic cell systems. The model compounds were also tested for their ability to induce asynchronous replication of viral DNA sequences. We demonstrate here that both the methylating model compound acetoxymethylmethylnitrosamine (AMMN) and the pyridyloxobutylating model compound 4-(acetoxymethyl)-1-(3-pyridyl)-1-butanone (NNKOAc) were mutagenic in strains TA98, TA100, and TA1535 but not TA102. While NNKOAc appeared to be 10 times more potent than AMMN in Salmonella, AMMN was found to be a more potent mutagen in mammalian G12 cells. Both chemicals could induce asynchronous replication of polyoma DNA sequences in rat fibroblast cells carrying an integrated copy of the polyoma virus with AMMN appearing to be more active. Measurement of DNA adduct levels suggest that the damage produced by NNKOAc was at least as active as that produced by AMMN when viewed on a per adduct basis. The possible implications of the biological activities exhibited by methylating and pyridyloxobutylating model compounds to NNK induced carcinogenesis are discussed.

Inducible cellular responses to ultraviolet light irradiation and other mediators of DNA damage in mammalian cells

Both naturally occurring and carcinogen-induced tumors display not only point mutations in cellular oncogenes but also more complex changes in cellular oncogenes and other cellular genes. For this and other reasons, it seems likely that DNA damage in mammalian cells can induce alterations in gene expression that may have both short and long term consequences in the target cell. The purpose of this review is to summarize current available information on inducible responses to UV-irradiation and other mediators of DNA damage in mammalian cells, and to provide some working hypotheses. We have divided these responses into three time frames, immediate (0-12 hours), early (12-48) and late (beyond 48 hours). Immediate responses include the action of DNA repair enzymes, some of which are induced as a consequence of DNA damage, and transient inhibition of DNA synthesis. Within the past few years considerable evidence has accumulated that during this immediate period there is increased expression of certain cellular oncogenes, proteases and proteins whose functions remain to be identified. It is of interest that the expression of some of these genes is also induced by certain growth factors, tumor promoters and heat shock. Alterations in gene expression during the subsequent "early" period (12-48 hrs.) have not been studied in detail, but it is during this period that one can detect increased replication of several types of viruses in cells that harbor these viruses. We have examined in detail the induction of asynchronous polyoma DNA replication (APR) in a rat fibroblast cell line carrying integrated copies of this DNA. We have obtained evidence that UV-irradiation of these cells leads to the synthesis of a 40 kd protein, within the first 1-24 hrs after irradiation, that binds to a specific sequence TGACAACA in the regulatory region of polyoma DNA. We suggest that this protein acts together with other proteins to induce APR and that this serves as a useful model for understanding the mechanisms responsible for amplification of cellular genes, a phenomenon often seen in malignant tumors. Finally, we discuss how the events occurring during the immediate and early periods following DNA damage might lead to late effects in the target cell that are stable and contribute to the genotype and phenotype of some of the progeny of these cells that are destined to become tumor cells.

Identification of a UV-induced trans-acting protein that stimulates polyomavirus DNA replication

Previous studies provided indirect evidence that the ability of a variety of DNA-damaging agents to induce asynchronous polyomavirus DNA replication in the H3 rat fibroblast cell line is mediated by a trans-acting factor. Using an erythrocyte insertion technique to introduce protein fractions from UV-irradiated cells into unirradiated H3 cells, we have now obtained evidence that this factor is a 60-kilodalton protein. These findings provide evidence that DNA damage in mammalian cells induces a factor that can alter the replication of a viral DNA.