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.