Multiphasic Survival Response of a Radioresistant Lepidopteran Insect Cell Line

Abstract

TN-368 lepidopteran insect cells display a multiphasic survival response in both air and nitrogen. In each case the survival curve is characterized by an initial small- shouldered component having a steep slope, a plateau or broad- shouldered region near the 0.1 survival level, and finally a shallow slope component. The D0, Dq, and n values for the initial steep slope component in air and nitrogen are, respectively, 65.7 Gy, 9.0 Gy, and 1.2, and 104.4 Gy, 28.8 Gy, and 1.3. The oxygen enhancement ratio (OER) for this portion of the curve is 1.6. The D0, Dq, and n values for the shallow slope component in air and nitrogen are, respectively, 130.2 Gy,--36.1 Gy, and 0.8, and 226.8 Gy, 121.0 Gy, and 1.7. The OER for this portion of the curve is 1.7. The D0 values for each slope and the width of the plateau region all increase proportionally for the nitrogen curve over that of air, the OER being approximately the same for both curve components. A similar multiphasic response was observed at dose rates of 202, 49.6, and 9.1 Gy/min. In addition, the survival of cells which had previously been irradiated with a dose well into the logarithmic region of the more resistant shallow slope portion of the curve retained a multiphasic response. Although cell cycle variations in radiosensitivity may contribute slightly to the response, an inducible or activated repair process would be consistent with the results.

Anhydrobiosis in invertebrates

“…The LD 50 values for such anhydrobiotic invertebrates are extremely high in comparison with those for vertebrates such as humans and mice (less than 7 Gy) (Hirano, 1964;National Astronomical Observatory, 2004). Furthermore, cell lines from lepidopteran insects that do not have desiccation tolerance express much higher resistance to radioactivity than those from mammals: Sf9 and TN368 lepidopteran cell lines can proliferate after 200 Gy of gamma irradiation and can survive 18 to 20 days after 800 Gy of X-ray irradiation, respectively (Koval, 1984;Chandna et al, 2004). The physiological reasons for large difference of the resistance between invertebrates and vertebrates remains unclear, although several hypotheses have been proposed, for example, small sizes of body, cells, chromosomes, and genome, composition and structure of membranes, a protective role of cuticle and large amounts of radio-protective chemicals such as cystein and glutathione in the hemolymph (Hirano, 1964;Koyama, 2001).…”

Increased radioresistance at clinical doses: Investigations from the survival perspective

“…In a more detailed analysis we were unfortunately unable to clarify whether the initial hypersensitive region is modified by the presence of oxygen [64] owing to the difficulty associated with accurately measuring hypoxic survival at doses below 0.1 Gy. However, a comparable structure observed in studies using more radioresistant insect cells at similar survival levels suggests that hypersensitivity is modified by oxygen [30].…”

“…1B). It has been shown mathematically that the survival curve of a mixed population cannot exhibit increased survival with dose [30,37], as in Figure lB, and also in HT29 [9,35,38].…”

“…Thus, applying a concept of an "OER for the trigger dose" (0.7/ 0.5 -1.4), it is still quite low if ssbs are the trigger. Note that for the lepidopteran cells, the OER for the sensitive portion of the curve appears to be -1.6 [30]. If ssbs are the trigger, one might anticipate that a much higher dose (2-3 Gy) would be required in hypoxia to produce an IRR response, with the result that the HRS would have a high OER.…”

“…For example, a response analogous to IRR is evident at high survival levels in a radioresistant lepidopteran cell line [30,31], and Calkins et al [32, and references therein] suggested induced repair to explain their results in various organisms, including mufine cells. Although the pollen data in Figure 1B were attributed to mixed populations, this does not fully explain the data (see next section).…”