Effects of Cycloheximide and Actinomycin D on Radiation-induced Apoptotic Cell Death in the Developing Mouse Cerebellum

Abstract

Effects of cycloheximide and actinomycin D on radiation-induced cell death in the external granular layer (EGL) of the cerebellum were studied in vivo. Newborn mice were exposed to 0.24 Gy gamma-radiation, and dying cells which exhibited pyknosis of nuclei in the EGL were examined at various post-irradiation periods. The number of pyknotic cells began to increase 3 h after irradiation, reached a peak incidence at 6 h, and then gradually fell to the sham-irradiated level by 18 h. When pups were injected with cycloheximide 1 h after irradiation, cell death was suppressed for 6 h, but a peak mortality as high as in the case of radiation alone was attained at 15 h after irradiation. When pups were treated with cycloheximide twice, at 1 and 6 h after irradiation, cell death did not occur for 15 h, but then the incidence rose to a level similar to that after irradiation alone. These findings showed that radiation-induced cell death in the EGL is suppressed by cycloheximide until the chemical is metabolized. Hence, death is by apoptosis which is known to require macromolecular synthesis, and the 'signal' for apoptosis in the cell persists for at least 15 h after irradiation. On the other hand, actinomycin D injected immediately before or after irradiation did not affect the initiation of cell death; actinomycin D alone induced cell death.

Toward an understanding of the molecular mechanisms of physiological cell death.

“…Evidence that inhibitors of macromolecular synthesis block cell death argue that the cell is, indeed, actively involved in its own demise (4,39,(44)(45)(46)(47)(48)(49), but in just as many cases macromolecular-synthesis inhibitors do not protect cells, so the cell-death machinery presumably already exists (50)(51)(52)(53). To add to the confusion, there are numerous reports of actinomycin D and cyloheximide themselves triggering apoptosis (54)(55)(56)(57)(58)(59)(60). Thus, the effect of macromolecular-synthesis inhibitors on cell death is of no help in determining whether it is physiological.…”

Ubiquitin pathway involvement in human lymphocyte gamma-irradiation-induced apoptosis.

“…Cell death is correlated with gene activation (16,29), but the molecular mechanisms involved remain poorly understood.…”

Inhibitory Effect of Thyrotropic Hormone on Apoptosis Induced by Actinomycin D in a Functioning Rat Thyroid Cell Line.

“…CHX, an inhibitor of protein synthesis, did not induce apoptosis in FRTL-5 cells cultured in 6H medium, 5H medium, 5H medium containing 1 mM Bt2cAMP or 5H medium containing 10 pM forskolin, because no DNA fragmentation was seen upon electrophoresis. These results suggest that the apoptotic pathway of FRTL-5 cells requires de novo protein synthesis because CHX inhibits apoptosis in some circumstances by decreasing the synthesis of protein required in order for the process to proceed [12].…”