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Our data (Reddy et al., Radiat. Res. 141, 252-258, 1995) on the kinetics of the repair of potentially lethal damage in log-phase Chinese hamster V79 cells are used to test some predictions which arise from the different assumptions of the repair-misrepair (RMR) (C. A. Tobias, Radiat. Res. 104, S77-S95, 1985), lethal-potentially lethal (LPL) (S. B. Curtis, Radiat. Res. 106, 252-270, 1986) and double-strand break (DSB) (J. Y. Ostashevsky, Radiat. Res. 118, 437-466, 1989) models. The LPL model defines the time available for repair of PLD ( $t_{{\rm rep}}$ ) as the time taken to reach maximal survival in a delayed-plating recovery experiment. Those data show that after this time has elapsed, contrary to the expectation of the LPL model, survival can be increased by changing the medium used for delayed plating from fresh growth medium to conditioned medium. According to the RMR model, all potentially lethal lesions should also be committed by that time and be unavailable for repair in the ne...

Previous observations have shown that cells cultured in standard growth medium (100%) demonstrated similarly enhanced survival when incubated postirradiation either in non-growth-promoting conditioned medium or in growth-promoting 40% growth medium (Reddy and Lange, Radiat, Res. 119, 338-347, 1989). From these results, it was suggested that nutrient dilution altered radiosensitivity by a mechanism independent of progression of cells through the cell cycle. In this study, we have examined the effects on radiosensitivity of incubation in 40% growth medium prior to irradiation on both log- and plateau-phase Chinese hamster V79 cells and the effects on the distribution of cells in the cell cycle of incubation in 40% or 100% growth medium before and after irradiation. Radioresistance increased by a factor of 1.5-1.6 compared to 100% growth medium for both log-phase and plateau-phase cells cultured in 40% growth medium prior to X irradiation and incubated in either 40% growth medium or conditioned medium after X irradiation. The cell cycle distributions of log-phase cells in 100% and 40% growth medium before irradiation were identical. The change in cell cycle distribution induced by 10 Gy did not differ among log-phase cells incubated for 3 h postirradiation in 100% growth medium, 40% growth medium or conditioned medium. These results, in addition to supporting our previous conclusions, demonstrate that culturing prior to irradiation in 40% growth medium alone increases cell survival and that incubation in 40% growth medium before and after irradiation maximizes the survival of V79 cells.

The kinetics of the repair and fixation of potentially lethal damage (PLD) was studied in log-phase Chinese hamster V79 cells. The postirradiation (10 Gy) survival of cells treated with hypertonic saline increased when these cells were incubated further in conditioned medium but not in growth medium, indicating that damage which is neither fixed by hypertonic saline nor amenable to repair in growth medium is nonetheless repaired in conditioned medium. Recovery of X-irradiated cells incubated in growth medium or in conditioned medium was maximal by about 70 min and was two times higher in conditioned medium than in growth medium. Cells incubated in growth medium for 70-120 min postirradiation continued to repair damage when subsequently shifted to conditioned medium only. Thus PLD is not fixed by the time the recovery plateau has been attained in growth medium, and this unfixed PLD can still be repaired when cells are shifted to conditioned medium. To study the kinetics of fixation of PLD (without hypertonic saline), the survival of cells incubated in growth medium for up to 9 h postirradiation was compared with that for cells incubated in conditioned medium. These results show that the damage was neither fixed nor misrepaired in growth mediummore » but rather remained unrepaired for up to 2 h, and that damage fixation in growth medium does not begin until after 2 h and is completed by 6 h postirradiation. 21 refs., 4 figs., 1 tab.« less

We have read the paper by Olive and MacPhail (1) with great interest. Their data indicate that: (a) "trypsin did not influence cell killing by ionizing radiation" (i.e., the radiation sensitivity of round cells in spheroids with and without trypsin treatment was the same), and (b) the increase in the radiation resistance and the limited DNA unwinding ability (as measured by the alkaline unwinding assay) of V79 spheroid cells appear to be contingent upon a change in cell shape. Their results and their interpretation of our data are pertinent to the observations on the relationship between radiosensitivity, trypsin, cell shape, and chromatin structure published by us for V79 (S-171) cells (2-5). Our data show that: (a) trypsin can radiosensitize spread monolayer cells (2-5); (b) trypsin cannot radiosensitize round mitotic cells (2), round cells in spheroids (4), or round mouse lymphoma cells (L5178Y-S) in suspension (5); (c) radiosensitization by trypsin is related to cell shapedependent chromatin structure (2-5); and (d) the degree of unwinding ofchromatin in round cells is higher than that of spread monolayer cells (5). There has also been sporadic mention of a possible radiosensitizing effect by trypsin [(6, 7), and see Ref. (2) for additional references on trypsin]. The observation by Olive and MacPhail (1) that trypsin did not radiosensitize round cells in spheroids is essentially the same as our results for round mitotic cells (2) or round cells in spheroids (4) or for round L5178Y-S cells (5). However, their other observations that spheroid cells were more radioresistant than monolayer cells and that the degree of DNA unwinding in the former cells was lower than that of the latter cells are not in agreement with our results. Our results showed that the radiosensitivity and the degree of DNA unwinding (as measured by the nucleoid halo technique) of monolayer and spheroid cells treated with trypsin was nearly the same (5). The reason for this difference in results is not clear. Lest any confusion arise from their misquotation of our observations (2-5) in their paper (1) pertaining to the effects of trypsin on radiosensitivity, we note the following: In the last sentence of the first paragraph of the Discussion, they state: "Recent results suggest that treatment of V79-171S cells with trypsin increases their resistance to radiation damage, an effect which appears to be related to differences in cell shape between monolayers and spheroids (18)". [Their Ref. (18) is Ref. (4) here]. What we have repeatedly shown is that: (a) spread cells in monolayers treated with trypsin either immediately before or immediately after irradiation are more radiosensitive than those cells which have been treated with trypsin 2-3 h after irradiation and repair incubation, and (b) the radiosensitization under immediate plating conditions is correlated with the rounding of spread cells by trypsin (2-5). Reference to mouse lymphoma cells (L5178Y) as monolayers in Table I appears to be a typographical error.

Evidence is presented for the interaction of X irradiation, slightly toxic levels of chloroquine, and mild hyperthermia in the inactivation of colony-forming ability in asynchronous HeLa cells. A three-way interaction was observed which resulted in the potentiation of radiation-induced lethality. There was little evidence of toxicity in unirradiated cells incubated for 3 h with 0.1 mM chloroquine at either 37 or 41 degrees C. The radiopotentiation factor, which is similar to the dose modification factor, was determined from dose-response curves by relating the reciprocal of the slope (D0) of the reference survival curve to that of the survival curve of cells receiving the combined postirradiation treatment with chloroquine and mild hyperthermia. Radiopotentiation factors larger than 1.7 were obtained irrespective of whether the reference D0's were obtained from survival curves for cells irradiated at 37 degrees C without drug or from cells receiving postirradiation treatment with heat or drug only.

Nutrient concentration in the growth medium and trypsin affect cellular radiosensitivity in a manner that is related to cell shape (Reddy, Stevenson, and Lange, Int. J. Radiat. Biol. 55, 105-117 (1989); Reddy and Lange, Radiat. Res. 119, 338-347 (1989]. Hence we hypothesized that the concentration of serum in the medium could influence the X-ray sensitivity of cells and that the spread cells in monolayers and round cells in spheroids may differ in their response to the radiosensitizing effect of trypsin. We compared the X-ray sensitivity of monolayer and spheroid cells grown for 19 +/- 1 h in MEM supplemented with 5 or 15% serum. Cells were trypsinized and plated either immediately before, or 2.5 +/- 0.5 h after, irradiation and incubation for repair in situ. Survival of cells in monolayers and in spheroids was higher in MEM with 5% serum than with 15% serum. Trypsin treatment affected the shape and radiosensitivity of cells in monolayers but not in spheroids. When all cells were grown in the same serum concentration and a 2.5-h postirradiation incubation was allowed prior to trypsinization, the X-ray sensitivity of cells in spheroids was greater than that of cells in monolayers. The survival of cells in spheroids became equal to that of monolayer cells when cells in spheroids were converted to monolayers by placing them in 25-cm2 flasks and allowing them 3 h to attach and spread. Cell cycle distributions were nearly the same in monolayers and spheroids cultured in MEM with 5 or 15% serum. We conclude that: (1) serum concentration in the growth medium and trypsin do appear to contribute to the differences in the radiosensitivity of spheroids and monolayer V79 cells; (2) these differences are associated with changes in cell morphology.

To understand better the biophysical mechanism of neutral filter elution (pH 9.6), we eluted genomes of known size and shape: coliphage T4c (Mr 1.15 x 10(8), E. coli (Mr 2.7 x 10(9)), and Chinese hamster lung fibroblasts (V79, Mr 2-4 x 10(10)). DNA eluted through 15% sucrose atop the filter in a biphasic pattern. The elution rate of the initial component correlated (r greater than 0.97) exponentially with 1/Mr for monodisperse samples of DNA eluted through pore sizes 0.1-3.0 microns. Using this relationship between elution rate and Mr, we estimated Mn of polydisperse, X-irradiated (253 Gy) samples of DNA from E. coli or V79 cells to be 3.15 +/- 1.46 and 1.42 +/- 0.33, respectively, compared to expected values of 2.93 and 3.52 (10(8) Da). The best predictor of elution rate for DNA from T4c and intact and X-irradiated V79 cells was pore density, and pore diameter for DNA from X-irradiated E. coli. The rate of elution of DNA from unirradiated E. coli was unrelated to pore density or diameter. While the mechanism of neutral filter elution remains unknown, its use for linear DNAs with Mn ca. 10(8) Da appears to be valid quantitatively.

The planarian owes its extensive powers of regeneration to the possession of a totipotential stem cell system. The survival of the animal after irradiation depends mainly upon this system. In this respect the planarian is analogous to mammalian organ systems such as bone marrow or gut epithelium. The differentiated cells control the course of stem cell mediated tissue renewal by the secretion of differentiator and/or inhibitor substances. One such inhibitor substance, present in extracts prepared from homogenized whole planarians, specifically inhibits brain formation. This substance is organ specific, but not species specific. The differentiative integrity of the stem cells after irradiation is measured by comparing the regenerated brain volumes resulting from the presence or absence of the brain inhibitory extract during the regeneration period. Our data suggest that increasing doses of x irradiation decreases the ability of the stem cells to respond to differentiative substances. The data presented also explore the possibility of altering the postirradiation recovery pattern by shifting the differentiative demands placed on the stem cells. The final proportions of animals (one-half regenerated with, and one-half without, the extract) surviving after 60 days were not significantly different.

DNA supercoiling ability was assayed following irradiation in two cell lines of differing radiosensitivity, L5178Y-S (LY-S) and L5178Y-R (LY-R). Cells treated with NaCl and Triton X-100 were exposed to increasing concentrations of the fluorescent, DNA-intercalating dye, propidium iodide (PI), and the diameter of the resulting fluorescent halo of DNA was measured. As the PI concentration was increased from 0.5 to 5 micrograms/ml, halo diameter increased from 20-25 to 45-55 microns due to the unwinding of the DNA supercoils. This process was similar for both cell lines under all conditions studied. As the PI concentration was increased to 50 micrograms/ml, the halo rewound to a diameter of 25-30 microns in unirradiated cells from both lines. However, following exposure to 3-12 Gy of 137Cs gamma rays, the ability of the DNA to be rewound was inhibited in a dose-dependent manner. Rewinding inhibition was greater in LY-S cells than in LY-R cells. Since the induction of DNA damage (e.g., single-strand DNA breaks) appears to be the same for both cell lines, this result implies that a similar extent of damage results in a greater loss of topological constraints on the DNA loops in LY-S. Such a change might be related to the protein composition of the nucleoid cores. One-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated that nucleoids from LY-S cells were missing a 55-kDa protein present in LY-R.

Potentially lethal damage (PLD) and sublethal damage (SLD) modification in L5178Y-S (LY-S) and L5178Y-R (LY-R) cells was investigated for postirradiation holding in either plateau phase or log phas...

Exposure of Chinese hamster V-79 cells to 1.1-MHz continuous wave (CW) ultrasound at intensities of 10, 20, and $30\ {\rm W}/{\rm cm}^{2}$ resulted in cell lysis and the loss of reproductive integrity (i.e., a decrease in plating efficiency) in the remaining intact cells. Sonication in the presence of 8 mM cysteamine, a free-radical scavenger, did not alter the amount of cell lysis, but did result in a smaller decrease in plating efficiency at 20 and $30\ {\rm W}/{\rm cm}^{2}$ . Hence, while free radicals do not appear responsible for ultrasonically induced cell lysis, free radicals do appear to be at least partially responsible for loss of reproductive integrity in the remaining intact cells.