Abnormal G1 arrest in the cell lines from LEC strain rats after X-irradiation

Inhibition of replication induces non-apoptotic cell death in fibroblast cell lines derived from LEC rats

Hydroxyurea (HU), an anticancer drug, inhibits ribonucleoside diphosphate reductase and reduces pool sizes of deoxyribonucleoside triphosphate (dNTP). The reduction of dNTP results in inhibition of DNA replication. The cytotoxic effect of HU was investigated using fibroblast cell lines from LEC rats. LEC rat cells showed significantly higher sensitivity to HU than did cell lines from control WKAH rats. No significant differences were observed between the percentages of apoptotic cells in either LEC or WKAH rat cells that had been treated with HU and those that had not been treated with HU. LEC rat cells also showed significantly higher sensitivity to aphidicolin, which blocks DNA synthesis by inhibiting DNA polymerase alpha, than did WKAH rat cells. In both LEC and WKAH rat cells, intensified bands of p53 protein were observed immediately after treatment with HU. Although the high level of p53 protein persisted in WKAH rat cells until 6 hr post-incubation time after treatment with HU, the level of p53 protein had decreased at 6 hr post-incubation time in LEC rat cells. When the cells were X-irradiated in the absence or presence of HU, the ratio of the surviving fraction without HU to that with HU only slightly increased after X-irradiation in WKAH rat cells. In contrast, the ratio in LEC rat cells significantly increased after X-irradiation in a dose-dependent manner.

High sensitivity of thymocytes of LEC strain rats to induction of apoptosis by X-irradiation

It is known that physical disruption of cell contacts induces apoptosis of thymocytes. When thymocytes from LEC and WKAH rats were incubated in vitro at 37 degrees C for 0-6 hr and then the proportion of apoptotic cells was determined using a flow cytometer, it was found that the percentages of apoptotic thymocytes from both LEC and WKAH rats increased with incubation time and that the proportion of apoptotic cells from LEC rats was significantly higher than that from WKAH rats at each incubation time. The fact that cycloheximide, an inhibitor of protein synthesis, did not show significant inhibitory effects on induction of apoptosis of thymocytes indicates that induction of apoptosis during in vitro cultivation did not require de novo protein synthesis. When thymocytes from LEC and WKAH rats were X-irradiated in vitro at 4 and 8 Gy, the percentages of radiation-induced apoptotic cells increased with post-incubation time after X-irradiation in both LEC and WKAH rat thymocytes and the proportions of apoptotic cells from LEC rats were significantly higher than those from WKAH rat cells at 2 and 4 hr post-incubation after X-irradiation. When thymocytes from LEC and WKAH rats were X-irradiated in the presence of cycloheximide, the induction of apoptosis was substantially inhibited, indicating that radiation-induced apoptosis of thymocytes from LEC and WKAH rats required de novo protein synthesis. The present results showed high sensitivities of thymocytes of LEC rats to induction of apoptosis during in vitro cultivation and by X-irradiation.

No significant differences were observed in the amounts of DNA strand breaks produced by copper between male and female liver cells of long-evans cinnamon (LEC) strain rats

The amounts of DNA single strand breaks that are oxidative damage produced by copper were examined by comet assay in the liver cells of an inbred strain of Long-Evans Cinnamon (LEC) rats that spontaneously develops fulminant hepatitis. At 4 weeks of age, copper contents in the liver of LEC rats were approximately 30-fold higher than those of WKAH rats that are control rats used in the present study. Copper accumulated in the liver of LEC rats in an age-dependent manner and no significant differences were observed between copper contents in the livers of males and females at each week of age from 4 to 15 weeks. No significant amounts of DNA strand breaks were found in the liver cells of both male and female WKAH rats from 4 to 15 weeks of age. DNA strand breaks were produced in the substantial population of LEC rat liver cells at 10 weeks of age and induced in an age-dependent manner from 10 to 15 weeks of age. The amounts of DNA strand breaks produced by copper accumulation in the liver cells of female LEC rats are not more abundant than those in the cells of male rats, although it has been reported that hepatitis in female rats is more serious than that in male rats.

Higher sensitivity in induction of apoptosis in fibroblast cell lines derived from LEC strain rats to ultraviolet B radiation

When lung fibroblast cell lines from LEC and WKAH rats were irradiated with ultraviolet B (UVB) and assayed for colony formation, LEC rat cells showed a higher sensitivity than did WKAH rat cells. The LEC rat cells were approximately 1.5-fold more sensitive to UVB radiation than were the WKAH rat cells in terms of D37 values, which are the doses of UVB required to reduce cell survival to 37%. When the rat cells were irradiated with UVB in the presence of 0.5 M dimethyl sulfoxide (DMSO), which efficiently scavenges free radicals such as hydroxyl radicals, no significant difference was observed between the survival curves of either LEC or WKAH rat cells irradiated with UVB in the presence of 0.5 M DMSO and those irradiated with UVB in the absence of DMSO. Therefore, formation of free radicals may not be involved in cell death induced by UVB radiation. Flow cytometry showed that the percentage of apoptotic cells in the LEC rat cell population increased with post-incubation time after UVB radiation. The proportion of apoptotic cells in the UVB-irradiated LEC rat cell population increased as the dose of UVB was increased. In contrast, no significant proportion of apoptotic cells was observed in the UVB-irradiated WKAH rat cell population. These results showed a higher sensitivity in induction of apoptosis by UVB radiation in LEC rat cells than in WKAH rat cells.

Wortmannin, a radiation sensitizer, enhances the radiosensitivity of WKAH rat cells but not that of LEC rat cells

No significant cytotoxic effect was observed in WKAH rat cells by the treatment of wortmannin, a radiation sensitizer, at concentrations lower than 30 microM for 24 hr. The relative surviving fractions of LEC rat cells were slightly, but significantly, lower than those of WKAH rat cells at each concentration of wortmannin. When the wortmannin-treated WKAH rat cells were X-irradiated, the relative surviving fractions decreased in a wortmannin concentration-dependent manner. On the contrary, no significant difference was observed between the survival curves of untreated and wortmannin-treated LEC rat cells after X-irradiation.

Deficiency in fast repair process of potentially lethal damage induced by X-irradiation in fibroblasts derived from LEC strain rats

The time course for the repair of PLD in LEC and WKAH rat cells irradiated at 5 Gy was examined. In the case of WKAH rat cells, the surviving fraction increased with increasing incubation times after X-irradiation. When hypertonic treatment was performed at each incubation time with 0.5 M NaCl for 20 min, increase in the surviving fractions was not shown. In contrast, no significant recovery of the surviving fraction in LEC rat cells was observed after incubation of irradiated cells with or without 0.5 M NaCl for 20 min. On dose-survival curves, hypertonic treatment with 0.5 M NaCl enhanced radiosensitivity of WKAH rat cells, but not LEC rat cells. Although the surviving fraction of the cells from backcross mice with normal radiosensitivity reduced by treatment with 0.5 M NaCl, the survival fraction was not affected in the cells from backcross mice with higher radiosensitivity by treatment with 0.5 M NaCl. When the cells were X-irradiated and incubated with or without 0.225 M NaCl, the radiosensitivities of LEC and WKAH rat cells treated with 0.225 M NaCl for 4 h were approximately two-fold higher than those of untreated cells. Treatment with caffeine also reduced the surviving fractions of both X-irradiated LEC and WKAH rat cells, compared with those of untreated cells. These results indicated that the slow repair of PLD occurred in LEC rat cells but not the fast repair of PLD.

Abnormal accumulation of G2/M-phase cells from LEC strain rats after X-irradiation at S phase

The effect of X-irradiation on the progression of the cell cycle in cell lines from LEC and WKAH rats was investigated by a flow cytometer. When the cells were exposed to 5 Gy of X-rays at S phase, the proportion of S-phase cells in both cell populations decreased with incubation time and that of G2/M-phase cells was approximately 80% at 6 hr post-irradiation. At 12 hr post-irradiation, approximately 45% of the WKAH rat cells appeared in the G1 phase. However, 80-90% of LEC rat cells remained in the G2/M phase and less than 5% in the G1 phase during 6-12 hr post-irradiation. Thus, the LEC rat cells irradiated at S phase remained in the G2/M phase for at least 6 hr longer than did the WKAH rat cells.

Higher sensitivity in induction of apoptosis in fibroblast cell lines derived from LEC strain rats to UV-irradiation

The proportion of S-phase cells in a WKAH rat cell population decreased and that of G1-phase cells increased at 8 and 18 hr post-incubation following UV-irradiation, although no significant change was observed in the ratio of the proportion of S-phase to G1-phase cells in a LEC rat cell population. Thus, UV-radiation-induced delay in the progression from the G1 to S phase was observed in WKAH rat cells but was not apparent in LEC rat cells. The fraction of LEC rat cells containing a sub-G0 DNA content increased with post-incubation time after UV-irradiation, but not that of WKAH rat cells. The proportion of the sub-G0 fraction in LEC rat cells increased with increasing doses of UV-rays. Low molecular weight DNAs extracted from UV-irradiated LEC rat cells exhibited an intense DNA ladder pattern at 18 and 24 hr post-irradiation by electrophoretic analysis, but not those from UV-irradiated WKAH rat cells. These results showed a higher sensitivity of LEC rat cells in induction of apoptosis than that of WKAH rat cells to UV-irradiation, although there was no difference in the survival curves among the cell lines from LEC and WKAH rats after UV-irradiation.