Enhanced survival by photoreactivation and liquid holding following UV damage of TN-368 insect cells

Heat Resistance and Thermotolerance in a Radiation-resistant Cell Line

Exponentially growing TN-368 lepidopteran insect cells have a normal growth temperature of 28 degrees C. These cells were heated in water baths at various temperatures between 33 and 44 degrees C under conditions of constant or fractionated heating. Determinations of cell survival using colony formation as well as measurements of DNA and protein synthesis were performed to assess relative heat resistance and development of thermotolerance. The results demonstrate a marked heat resistance over previously reported findings from the same laboratory for dipteran Drosophila cells in culture. The degree of heat resistance is remarkable, especially when compared to the heat resistance of mammalian cells, i.e. TN-368 cell survival at 41.5 and 44 degrees C was somewhat similar to mammalian cell survival, even though these temperatures are 13.5 and 16 degrees C above the normal growth temperature for TN-368 cells and 4.5 and 7 degrees C above the growth temperature of mammalian cells. Furthermore, the lepidopteran cells maintain the ability to develop a notable amount of thermotolerance in addition to this heat resistance. Thermotolerance development alone is capable of enhancing survival by an additional 10,000-fold. Thermotolerance could also be detected at the level of protein synthesis as a more rapid recovery following heat treatment. In contrast, DNA synthesis inhibition was prolonged even further in cells receiving a prior heat treatment to induce thermotolerance. In summary, it appears that, in addition to their pronounced radiation resistance, the TN-368 cells are also quite resistant to heat. It remains to be seen whether a single mechanism could be responsible for resistance to these agents which act very differently.

Multiple factors conferring high radioresistance in insect Sf9 cells

Sf9, a lepidopteran cell line isolated from the fall armyworm, Spodoptera frugiperda, was shown to be significantly more resistant to growth inhibition and apoptosis induction effects of x-ray irradiation than several human cell lines of different origins. The single-cell electrophoresis technique revealed that Sf9 cells showed lower x-ray irradiation-induced DNA damage as well as better efficiency at repairing these damages. In addition, Sf9 cells were lower in both background and x-ray irradiation-induced intracellular oxidative stress, in which the higher intracellular level of reduced glutathione seemed to play a major role. The significance of oxidative stress in determining the radioresistance of Sf9 cells was confirmed by their being more resistant to hydrogen peroxide while equally susceptible to other non-reactive oxygen species of N-nitroso alkylating agents when compared with a human cell line. Although the Sf9 and human cell lines were equally susceptible to the lethal effects of N-nitroso alkylating agents, the components of DNA damage-induced and the repair enzymes involved significantly differ. This phenomenon is also discussed in this report.

Insect cells and their potential as stabilization barriers for DNA of multiple and single nucleopolyhedroviruses against ultraviolet-B-simulated sunlight inactivation

A cell line from Trichoplusia ni (TN-CL1) infected with the Autographa californica multiple nucleopolyhedrovirus (AcMNPV-HPP) and a cell line from Helicoverpa zea (BCIRL-HZ-AM1) infected with the Helicoverpa zea single nucleopolyhedrovirus (HzSNPV/BrCL2) were subjected to ultraviolet-B (UV-B) irradiation at a predetermined level of exposure that would inactivate greater than 95% of the virus suspended in the liquid. The working hypothesis was that the homologous insect cells would utilize their inherent deoxyribonucleic acid (DNA) repair mechanism(s) to prevent, repair, or at least mitigate the damaging effects of UV-B light on viral DNA synthesis. We attempted to determine this by using infected cells that were subjected to UV-B irradiation at different postinoculation periods under two experimental conditions of exposure: (1) shielded, and (2) nonshielded. Of the two cell lines infected with their respective homologous viruses, the virus from TN-CL1 cells was the least sensitive to UV-B light because the extracellular virus (ECV) and occlusion body (OB) levels of virus-infected TN-CL1 cells were higher than those of the virus-infected BCIRL-HZ-AM1 cells. Production of ECV and OB from both cell lines was lower in the exposed, nonshielded treatment than in the exposed, shielded treatment. However, AcMNPV-HPP was produced in enough quantity to indicate that TN-CL1 might impart a level of protection to the virus against UV light.

Recovery from exposure to DNA-damaging chemicals in radiation-resistant insect cells

Radioresistant TN-368 lepidopteran insect cells were examined with respect to their sensitivity to the chemical agents methyl methanesulfonate (MMS), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), propane sultone (PS), mitomycin C (MMC), and 4-nitroquinoline 1-oxide (4NQO). Based on survival ability, the TN-368 cells were more resistant than most mammalian cells to each of these agents. Concentrations of these agents which reduce survival to about 10% were used to assess recovery ability assayed by colony formation in liquid-holding and split-dose experiments. Liquid-holding experiments were performed by exposing cells in the plateau phase of growth for 1 h to 8 mM MMS, 50 microM MNNG, 9 mM PS, 110 microM MMC, or 175 microM 4NQO, removing the drug and incubating cells in spent medium for 6 h, and plating for colony formation. Split-dose experiments were performed by exposing exponentially growing cells to the above drug concentrations for 1 h, incubating in fresh medium for 6 h, exposing the cells to the agent for an additional hour, and plating. The TN-368 cells were able to significantly recover from MMS, MNNG and PS in both types of experiment. Recovery from 4NQO was observed in liquid-holding experiments and not assessed in split-dose experiments. In all cases where recovery was observed, survival enhancement was approximately 2-fold. Recovery from MMC (a cross-linking agent) exposure was not observed in either type of experiment. In addition, recovery from 8-methoxypsoralen plus UVA light (PUVA), another cross-linking treatment, was not observed. These studies indicate that DNA-DNA and/or DNA-protein crosslinking may be important molecular lesions causing death in the lepidopteran cells and that these cells may have some difficulty in repairing such damage.

Factors to consider in performing survival studies with insect cells

Insect cell lines are not well-suited to colony formation in liquid medium following low-density cell plating. The present studies demonstrate that the time of addition of fetal bovine serum to the culture medium and the number of gamma-irradiated feeder cells added to each plate are important factors in developing a useful colony formation assay. TN-368 lepidopteran and WR69-DM-1 dipteran cell lines were used for these experiments. Both cell types display increased plating efficiencies if serum is added to the medium one or more days prior to plating as compared to adding serum immediately before plating. Growth curves obtained by seeding cells at higher densities also indicate that cell growth is slightly better if serum is added one or more days before seeding. These findings are especially important for survival and toxicity studies because the results demonstrate that even seemingly minor factors involved in cell survival assays may benefit treated cells to a greater degree than untreated control cells, thus providing an erroneous assessment of cell survival.

Photoreactivation of UV damage in cultured Drosophila cells

Cell survival and photoreactivation of 254 nm ultraviolet (UV) light damage in a wild type Drosophila cell line was assayed by colony formation in liquid medium. Fo, Fq, and extrapolation number for the exponential portion of survival curves are 21 J/m2, 3.6 J/m2, and 1.5 for non-photoreactivated cells and 110 J/m2, 11.2 J/m2, and 1.3 for those exposed to photoreactivating light. Maximal photoreactivation occurs at the 100 J/m2 region of the curve. At 10 and 50% survival, 75-80% of the UV damage was photoreactivable.