Differential effect of ultraviolet light on the induction of simian virus 40 and a cellular mutator phenotype in transformed mammalian cells

Homologous recombination is not enhanced in UV-irradiated normal and repair-deficient human fibroblasts

Many mammalian cells exhibit damage-inducible phenomena that resemble the bacterial SOS functions. However, whereas RecA plays a prominent role in the prokaryotic SOS response, in mammalian cells so far no enhanced recombination as a result of treatment with DNA-damaging agents of the cells, rather than of infecting viruses, has been found. In order to study recombination as a UV-inducible cellular phenomenon we infected UV-irradiated normal and repair-deficient human fibroblasts with a mixed population of adenovirus 5 (Ad5) mutants that carried a deletion in the E1A or the E2A gene. Wild-type recombinant progeny viruses were readily obtained, but no enhanced recombination was observed at any UV dose given to the cells, nor at any time point between -6 h and +4 days between irradiation and infection. Control experiments, in which we infected unirradiated cells with UV-irradiated Ad5 deletion mutants (a test for recombination targeted at UV-damaged DNA) showed a strong increase in wild-type recombinant viruses when both deletion mutants had been irradiated compared to the additive effect of irradiation of either one of the mutants alone. Therefore, this study shows that UV irradiation results in an enhanced recombination activity in cells that is specifically targeted to damaged DNA, but it does not cause a general (untargeted) recombinational response (enhanced recombination) in the cell.

Vaccinia virus, herpes simplex virus, and carcinogens induce DNA amplification in a human cell line and support replication of a helpervirus dependent parvovirus

The SV40-transformed human kidney cell line, NB-E, amplifies integrated as well as episomal SV40 DNA upon treatment with chemical (DMBA) or physical (uv irradiation) carcinogens ("initiators") as well as after infection with herpes simplex virus (HSV) type 1 or with vaccinia virus. In addition it is shown that vaccinia virus induces SV40 DNA amplification also in the SV40-transformed Chinese hamster embryo cell line, CO631. These findings demonstrate that human cells similar to Chinese hamster cells amplify integrated DNA sequences after treatment with carcinogens or infection with specific viruses. Furthermore, a poxvirus--vaccinia virus--similar to herpes group viruses induces DNA amplification. As reported for other systems, the vaccinia virus-induced DNA amplification in NB-E cells is inhibited by coinfection with adeno-associated virus (AAV) type 5. This is in line with previous studies on inhibition of carcinogen- or HSV-induced DNA amplification in CO631 cells. The experiments also demonstrate that vaccinia virus, in addition to herpes and adenoviruses acts as a helper virus for replication and structural antigen synthesis of AAV-5 in NB-E cells.

Enhanced induction of SV40 replication from transformed rat cells by fusion with UV-irradiated normal and repair-deficient human fibroblasts

Fusion of SV40-transformed rat (BRKSV) cells which do not spontaneously produce infectious virus, with permissive monkey cells resulted in a low level of production of infectious virus in the heterokaryons. UV-Irradiation of the BRKSV cells prior to fusion did not result in increased virus production, but irradiation of the monkey cells prior to fusion did result in enhanced induction (EI) of SV40, as compared to control experiments in which neither cell type was irradiated. This indicated that rat cells lack the ability to initiate replication of integrated SV40 upon UV-irradiation and do not contain "permissiveness" factors that are required to support SV40 replication. In contrast, monkey cells do contain such permissiveness factors which seem to be temporally enhanced by UV-irradiation, and thus may be responsible for the EI phenomenon. Expression of EI was dose-dependent and reached maximum values approximately 24 h after UV-irradiation. The kinetics of EI resembled that of EI previously established for SV40 induction in semi-permissive cells, and of enhanced reactivation (ER) and enhanced mutagenesis (EM) of SV40 in monkey cells. Similar kinetics of EI were obtained when human diploid fibroblasts were used for fusion with BRKSV cells. Similar levels of EI were found with normal human cells and repair-deficient xeroderma pigmentosum (XP) cells of complementation groups A and C, and XP variant cells. This suggests that expression of EI is not related to excision repair. Since EI is also normally expressed in XP cells which display an abnormal ER of HSV and in XP variant cells which show a delayed EM of HSV, we conclude that EI may occur independently of ER and EM. Finally it was shown that treatment of human cells with N-ethyl-N-nitrosourea results in similar induction of EI as irradiation with UV-light, and that addition of TPA in fusion experiments has no effect on EI.