Proteases are known to be involved in the apoptotic pathway. We report here that benzyloxycarbonyl (Z)-Leu-Leu-leucinal(ZLLLal), a leupeptin analogue, can induce apoptosis in MOLT-4 and L5178Y cells. ZLLLal is a cell-permeant inhibitor of proteasome. Among the protease inhibitors tested, only calpain inhibitor I (acetyl-Leu-Leu-norleucinal) and ZLLLal caused a marked induction of apoptosis in MOLT-4 cells. In contrast Z-Leu-leucinal, a specific inhibitor of calpain, did not induce apoptosis. When MOLT-4 cells were incubated in the presence of ZLLLal, p53 accumulated in the cells. These results strongly suggest that inhibition of proteasome induces p53-dependent apoptosis and that proteasome can protect cell from apoptosis.

The contribution of indirect action mediated by OH radicals to cell inactivation by ionizing radiations was evaluated for photons over the energy range from 12.4 keV to 1.25 MeV and for heavy ions over the linear energy transfer (LET) range from 20 keV/microm to 440 keV/microm by applying competition kinetics analysis using the OH radical scavenger DMSO. The maximum level of protection provided by DMSO (the protectable fraction) decreased with decreasing photon energy down to 63% at 12.4 keV. For heavy ions, a protectable fraction of 65% was found for an LET of around 200 keV/microm; above that LET, the value stayed the same. The reaction rate of OH radicals with intracellular molecules responsible for cell inactivation was nearly constant for photon inactivation, while for the heavy ions, the rate increased with increasing LET, suggesting a reaction with the densely produced OH radicals by high-LET ions. Using the protectable fraction, the cell killing was separated into two components, one due to indirect action and the other due to direct action. The inactivation efficiency for indirect action was greater than that for direct action over the photon energy range and the ion LET range tested. A significant contribution of direct action was also found for the increased RBE in the low photon energy region.

The killing effects of ionizing radiation on MOLT-4 cells were studied using colony formation assays, dye exclusion tests, and cell growth analysis. When MOLT-4 cells were exposed to X rays, the fraction of cells stained with erythrosin B increased markedly during the incubation after irradiation at 37 degrees C and reached a maximum within 24 h. In contrast, no such increase was observed for L5178Y cells or their derivative, radiosensitive mutant M10 cells, at a dose resulting in the same level of cell survival as measured by the colony formation assay. Detailed analysis of cell survival as measured by the dye exclusion test in comparison with that measured by the colony formation assay in MOLT-4 cells indicated that MOLT-4 cells are subject to interphase death as well as reproductive death. Analysis of subclones showed a wide variety of differences in the level of stained cells using the dye exclusion test. The results confirmed the population heterogeneity of MOLT-4 cells. Moreover, none of the subclones showed exactly the same cell survival when this was measured with the two assays. The present results strongly suggest that both modes of cell death may occur in single MOLT-4 cells. Possibly apoptosis is the mechanism accounting for both modes of death in these cells.

X-ray-induced cell death in MOLT-4N1, a subclone of MOLT-4 cells, and M10 cells was studied with respect to their modes of cell death, apoptosis and necrosis. MOLT-4N1 cells showed radiosensitivity similar to that of M10 cells, a radiosensitive mutant of L5178Y, as determined by the colony formation assay. Analysis of cell size demonstrated that MOLT-4N1 cells increased in size at an early stage after irradiation and then decreased to a size smaller than that of control cells, whereas the size of irradiated M10 cells increased continuously. Apoptosis detected by morphological changes and DNA ladder formation (the cleavage of DNA into oligonucleosomal fragments) occurred in X-irradiated MOLT-4N1 cells but not in M10 cells. Pulsed-field gel electrophoresis showed that the ladder formation involved an intermediate-sized DNA (about 20 kbp). Most of the DNA was detected at the origin in both methods of electrophoresis in the case of M10 cells, though a trace amount of ladder formation was observed. Heat treatment of M10 cells induced apoptosis within 30 min after treatment, in contrast to MOLT-4N1 cells. The results suggest that apoptosis and necrosis are induced by X rays in a manner which is dependent on the cell line irrespective of the capability of the cells to develop apoptosis. DNA fragmentation was the earliest change observed in the development of apoptosis.

The effects of low linear energy transfer (LET) radiation on mammalian cells have been studied at dose-rates as high as 10(9) Gy/sec delivered as a single 3-nanosecond pulse, and no increase in cytotoxicity was shown compared with delivery at a conventional dose-rate. There have been no observations on the effects of radiation delivered at even higher dose-rates on the picosecond time-scale. Here we examined, for the first time, the effects on cultured mouse L5178Y cells and its radiosensitive XRCC4-deficient mutant M10 cells of sub-picosecond X-rays emitted from laser-produced plasmas at the ultrahigh dose-rate of 10(12)-10(13) Gy/sec. No increase in the sensitivity to the X-rays was observed compared with gamma-rays at a conventional dose-rate. The increase in the sensitivity of L5178Y cells by labeling with 5-iododeoxyuridine was smaller than those irradiated with gamma-rays at a conventional dose-rate, while the difference was apparently the reverse in M10 cells. The D10 ratio between L5178Y cells and M10 cells produced by the X-rays at temporally dense ionization was the same as that produced by X(gamma)-rays at the conventional dose-rate, while the ratio is greatly reduced in the case of particle radiation. These results suggest that there is no increase in the cytotoxic effects of X-rays at dose-rates as high as 10(13) Gy/sec, and that the increased cytotoxicity of particle radiation is not attributable to temporally dense ionization. It is discussed that the mechanism for the induction of radiation damage responsible for cytotoxicity may be slightly modified at ultrahigh dose-rates.