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

Apoptotic cell death and regeneration in the newborn retina after irradiation prior to bone marrow transplantation

PURPOSE

We studied the contribution made by circulating bone marrow (BM)-derived cells to the newborn and mature retinas of BM-transplanted mice.

METHODS

Newborn and adult C57BL/6J mice were administered a lethal dose of total-body irradiation, after which pathologic changes to the retinas were periodically assessed. In addition, mice received BM cells from 8-week-old green fluorescent protein (GFP) transgenic mice, and the subsequent differentiation of GFP+ cells was studied.

RESULTS

Within 5 hr after irradiation of newborn mice, retinal cells began to die due to apoptosis. By contrast, irradiation of adult mice elicited no histologic changes in the retina. BM cells generally did not differentiate in adult mice, but numerous GFP+ BM cells were integrated into the retinal tissue of newborn mice, where they expressed various cell type-specific markers. Finally, examination of whole retina mounts showed that GFP+ cells also contributed to retinal vascularization.

CONCLUSIONS

Our findings underscore the importance of careful evaluation of the biological effects of irradiation in models making use of BM transplantation.

Hyperacute neuropathological findings after proton beam radiosurgery of the rat hippocampus

OBJECTIVE

To study the hyperacute histological and immunohistochemical effects of stereotactic proton beam irradiation of the rat hippocampus.

METHODS

Nine rats underwent proton beam radiosurgery of one hippocampus with nominal doses of cobalt-2, -12, and -60 Gray equivalents (n = 3 each). Control animals (n = 3) were not irradiated. Animals were killed 5 hours after irradiation and brain sections were stained for Nissl, silver degeneration, deoxyribonucleic acid (DNA) fragmentation (DNAF), and the activated form of two mitogen-activated protein kinases (MAPKs), phospho-Erk1/2 (P-Erk1/2) and p38. Stained cells in the hippocampus expressing DNAF and/or P-Erk1/2 were counted. Confocal microscopy with double immunofluorescent staining was used to examine cellular colocalization of DNAF and P-Erk1/2.

RESULTS

Both DNAF and P-Erk1/2 showed quantitative dose-dependent increases in staining in the targeted hippocampus compared with the contralateral side and controls. This finding was restricted to the subgranular proliferative zone of the hippocampus. Both markers also were up-regulated on the contralateral side when compared with controls in a dose-dependent fashion. Simultaneous staining for DNAF and P-Erk1/2 was found in fewer than half of all cells. p38 was unchanged compared with controls. Although Nissl staining appeared normal, silver stain confirmed dose-dependent cellular degeneration.

CONCLUSION

DNAF, a marker of cell death, was present in rat hippocampi within 5 hours of delivery of cobalt-2 Gray equivalents stereotactically focused irradiation, suggesting that even low-dose radiosurgery has hyperacute neurotoxic effects. Activated mitogen-activated protein kinase was incompletely colocalized with DNAF, suggesting that activation of this cascade is neither necessary nor sufficient to initiate acute cell death after irradiation.

How radiosensitive are the developing embryo and fetus?

In its 1990 recommendations, the ICRP considered the radiation risks after exposure during prenatal development. This report is a critical review of new experimental animal data on biological effects and evaluations of human studies after prenatal radiation published since the 1990 recommendations.

Thus, the report discusses the effects after radiation exposure during pre-implantation, organogenesis, and fetogenesis. The aetiology of long-term effects on brain development is discussed, as well as evidence from studies in man on the effects of in-utero radiation exposure on neurological and mental processes. Animal studies of carcinogenic risk from in-utero radiation and the epidemiology of childhood cancer are discussed, and the carcinogenic risk to man from in-utero radiation is assessed. Open questions and needs for future research are elaborated.

The report reiterates that the mammalian embryo and fetus are highly radiosensitive. The nature and sensitivity of induced biological effects depend upon dose and developmental stage at irradiation. The various effects, as studied in experimental systems and in man, are discussed in detail. It is concluded that the findings in the report strengthen and supplement the 1990 recommendations of the ICRP.

The effect of the timing of prenatal X-irradiation on Purkinje cell numbers in rat cerebellum

Exposure of the developing brain to X-irradiation in utero is known to cause various deleterious consequences. We have previously reported the effects of prenatal X-irradiation on the development of the cerebral cortex in rats. We have now extended this study to examine the effects of such X-irradiation on the development of the cerebellum. Wistar rats were exposed to 1.5 Gy X-irradiation either on days 14, 15 or 16 of gestation (E14, E15, E16). Sham-irradiated animals were used as controls. At seven postnatal weeks of age, male rats from each group were deeply anesthetized and killed by intracardiac perfusion with 2.5% glutaraldehyde in 0.1 M phosphate buffer. The unbiased stereological procedure known as the fractionator method was used to estimate the total number of Purkinje cells in the cerebellum of each animal. Body and cerebellar weights from E14 and E15, but not E16 irradiated rats showed significant deficits compared to control animals. Rats irradiated on E16 and control rats had about 285100-304800 Purkinje cells in the cerebellum. There was no significant difference between these values. However, E14 and E15 irradiated animals had about 117500 and 196300 Purkinje cells, respectively. These estimates were significantly different from those observed in both control and E16 irradiated rats. Given that the phase of division of Purkinje cell progenitors is mainly between E14-E15 and the phase of differentiation and migration is between E16-E20, it is concluded that the vulnerable period of the Purkinje cells to X-irradiation closely overlaps the phase of division of progenitors.

Cycloheximide induces apoptosis of astrocytes

Cultured rat astrocytes were incubated in the presence of cycloheximide (CHX; 20 microg/mL), a potent neuroprotective agent. Then cells were subjected to DNA gel electrophoresis. Electrophoresis showed DNA ladder formation, which is characteristic of apoptosis. Inhibitors of interleukin-1beta-converting enzyme (ICE) and caspase 32(CPP32), which play critical roles in certain apoptotic pathways, did not block the cycloheximide-induced apoptosis of cultured astrocytes. This observation indicates that the role of ICE and CPP32 is not significant in the CHX-induced astrocyte apoptosis process. When the blood-brain barrier was disrupted in the rat, the number of brain cells undergoing apoptosis was significantly higher after cycloheximide administration, in contrast to controls. Of the cells that produced glial fibrillary acidic protein, some were observed to undergo apoptosis. Although CHX has been shown to be useful as a neuroprotective agent against ischemic neuronal death, astroglial toxicity may be problematic, depending on CHX concentration. Therefore, a prudent use of this compound is recommended.

Differential post-transcriptional regulation of p21WAF1/Cip1 levels in the developing nervous system following gamma-irradiation

Radiation-induced death in the developing brain is p53-dependent. However, genetic studies indicate that the signalling pathways that couple irradiation to p53 expression can vary between different developing neural populations [Herzog et al. (1998) Science, 280, 1089-1091]. Here we establish that signalling downstream of p53 also exhibits brain region-specific differences that are associated with the relative vulnerability of some cell populations to radiation-induced killing in the mouse. Following gamma-irradiation, p53 and p21WAF1/cip1, but not Bax, protein levels increased in the developing cerebellum. In contrast, neither p21WAF1/cip1 nor Bax protein levels were elevated in the retina following irradiation, despite increased p53 expression. In the retina, p53 expression was associated with cells destined to die, whereas in the cerebellum, p53 was expressed in both radiation-sensitive and radiation-resistant neuroblasts of the external granule cell layer. Although p21WAF1/cip1 mRNA was expressed in all p53-positive neuroblasts after irradiation, p21WAF1/cip1 protein was only detected in radiation-resistant neuroblasts of the cerebellum. Thus, p21WAF1/cip1 was subject to post-transcriptional regulation with p21WAF1/cip1 protein only accumulating in cells destined to survive irradiation. Nevertheless, p21WAF1/cip1 function was not essential for radiation resistance, as postmitotic neuroblasts in the external granule cell layer were spared in p21WAF1/cip1 knockout mice.

DNA damage and apoptosis in the immature mouse cerebellum after acute exposure to a 1 mT, 60 Hz magnetic field

Several recent studies have reported that whole-body exposure of rodents to power frequency magnetic fields (MFs) can result in DNA single- and double-strand breaks in the brains of these animals. The current study was undertaken to investigate whether an acute 2h exposure of a 1 mT, 60 Hz MF could elicit DNA damage, and subsequently apoptosis, in the brains of immature (10-day-old) mice. DNA damage was quantitated at 0, 2, 4, and 24h after exposure using the alkaline comet assay. Apoptosis was quantitated in the external granule cell layer (EGCL) of the immature mouse cerebellum at 0 and 24h after exposure to MF by the TdT-mediated dUTP nick-end labeling (TUNEL) assay. Four parameters (tail ratio, tail moment, comet length and tail length) were used to assess DNA damage for each comet. While increased DNA damage was detected by tail ratio at 2h after MF exposure, no supporting evidence of increased DNA damage was detected by the other parameters. In addition, no similar differences were observed using these parameters at any of the other post-exposure times. No increase in apoptosis was observed in the EGCL of MF-exposed mice, when compared to sham mice. Taken together, these results do not support the hypothesis that acute MF exposure causes DNA damage in the cerebellums of immature mice.

Radiation-induced diffuse brain injury in the neonatal rat model--radiation-induced apoptosis of oligodendrocytes

The mechanism of radiation-induced diffuse brain injury was investigated using a model of delayed myelination in the irradiated neonatal rat brain in which the number of oligodendrocytes decreases without associated necrosis of the cerebral white matter. Immunohistochemical analysis using antibody against the large myelin-associated glycoprotein, a specific marker of oligodendrocytes at an early stage of development, showed that the number of the oligodendrocytes associated with myelination decreased in the irradiated hemisphere 1 day after irradiation and remained low until 5 days after irradiation. In situ terminal deoxynucleotidyl transferase-mediated nick end-labeling assay revealed that apoptosis mainly occurred in the cerebral white matter of the irradiated hemisphere. Three hours after irradiation, apoptotic cells were found in the subcortical white matter and the periventricular white matter. Six hours after irradiation, apoptotic cells were found in the internal capsule, and the numbers of apoptotic cells in the periventricular white matter and subcortical white matter increased. One day after irradiation, the number of apoptotic cells in the periventricular white matter decreased. Three days after irradiation, apoptotic cells were not observed in the cerebral white matter. These results suggest that the oligodendrocytes associated with myelination may be damaged via radiation-induced apoptosis, and depletion of the oligodendrocytes may cause delay of myelination.

Differential effect of cycloheximide on neuronal and glioma cells treated with chemotherapy and radiation

Dividing cells and non-dividing cells are distinct in their cell cycle kinetics, and react differently when facing cytotoxic stimuli. A protein synthesis inhibitor, cycloheximide (CHX), has recently been found to protect neuronal cells from oxidative stress. We investigated whether CHX exerts differential effects on dividing and non-dividing cells in the brain under cytotoxic stimuli. Mitotic C6 rat glioma cells and postmitotic neuronal cells were studied with a cytotoxic regimen combining gamma-irradiation (RT) and 1,3-bis,2-chloroethyl-1-nitrosourea (BCNU). Cells were exposed to BCNU (1 g/ml) for 15 h before gamma-irradiation and incubated with CHX (1 g/ml) from 30 min before and until 5 h after irradiation. Clonogenic assay was used to assess cytotoxic effects on C6 glioma cells. LDH assay was used for the viability of H19-7 postmitotic neuronal cells. A 2.27-3.75 fold enhancement of cytotoxicity was noticed with the addition of CHX to BCNU and 2-10 Gy of radiation. Our data demonstrated that CHX enhanced cytotoxicity of RT plus BCNU, while no additional toxicity was incurred to the postmitotic neuronal cells when CHX was added. We further studied whether the inhibition of DNA repair, assayed by single-cell DNA electrophoresis (comet assay), is a contributing factor for the enhanced cytotoxicity on C6 glioma cells. Interestingly, the initial DNA damage after RT plus BCNU was equivalent; whereas DNA repair was significantly less at 5 h after radiation in CHX-treated C6 glioma cells. Protecting non-dividing neuronal cells to avoid excessive functional deficit is an integral part of a successful brain tumor treatment regimen. Taking advantage of the differential effect of CHX on glioma and neuronal cells may improve tumor control without excessive neural toxicity.

Involvement of nitric oxide in aminoglycoside vestibulotoxicity in guinea pigs

Involvement of nitric oxide (NO) has been reported in physiological and pathological conditions in the inner ear. Recently, the presence of nitric oxide synthase (NOS) was demonstrated in the vestibular epithelium. In this study we used nicotinamide adenine dinucleotide phosphate-diapholase staining to monitor NOS activity during degeneration of guinea pig vestibular epithelia affected by streptomycin. Increased NOS activity was observed in affected epithelia in a dose- and time-dependent manner and a NOS inhibitor could protect hair cells from apoptosis. Additionally, cycloheximide significantly reduced NOS activity and the occurrence of apoptosis. These findings suggest that NO is involved in the degenerative process of vestibular epithelia caused by aminoglycosides.

Lack of correlation between apoptosis and DNA single-strand breaks in X-irradiated human peripheral blood mononuclear cells in the course of ageing

The dependence on age of both the basal and the X-radiation-induced levels of apoptosis was examined in human peripheral blood mononuclear cells (PBMC). In the same samples, the base value and the extent of induced DNA single-strand breaks were determined, using a sensitive and fast microplate assay. PBMC were isolated from blood of donors of various age groups (20-30, 40-60 and > 70 years of age) and X-irradiated ex vivo using a 6 MV linear accelerator to give a total exposure of 4 Gy. The mean basal levels of apoptosis in PBMC from donors in the 40-60 year age group and the > 70 year age group were found to be only slightly higher (by 20-10%) compared to that of the 20-30 year age group, whereas the extent of DNA damage strongly and significantly (P < 0.01) increased with age by up to 2-fold. In contrast to the extent of induced DNA damage, which steadily increased in the course of ageing by up to 1.8-fold, there was only a transient increase in the level of induced apoptosis to 1.5-fold in PBMC from X-irradiated blood (4 Gy photons) from donors aged 40-60 followed by a decrease to 0.9-fold in PBMC from old donors (>70), compared to age group 20-30. The results show that X-ray-induced apoptosis and DNA damage in PBMC are not correlated during ageing.

[Significance of apoptotic processes in radiotherapy. II]

Apoptosis is known as an active process of cell death forced by radio- and chemotherapy. Therefore, established concepts (terms, therapy schemes) will reflect a picture different from that usually seen, when examined under the apoptotic point of view. Furthermore, the development of new concepts for innovative diagnosis, prognosis and therapy could be accomplished. This is an attempt to reveal actual features of both aspects.

Accumulation of cyclin B1, activation of cyclin B1-dependent kinase and induction of programmed cell death in human epidermoid carcinoma KB cells treated with taxol

Cyclin B1 plays a critical role in regulating cell-cycle progression from G2 through M phase (including exit from M phase). In this study, we investigated the relationship between taxol-induced M-phase arrest, disruption of the cyclin B1-regulation pathway and apoptosis in KB cells. Continuous exposure of KB cells to 0.5 microg/ml taxol caused mitotic arrest and >90% cell death at 48 hr. Mitotic blockade peaked at 24 hr, with 68% of cells in mitosis at that time compared with 3% at baseline, and decreased thereafter. Apoptosis assessed by morphological changes and DNA ladder fragmentation was a later event, peaking at 48 hr (later time points were not studied). Taxol also caused an increase in cyclin B1 accumulation, as assessed by Western blot analysis, and stimulated cyclin B1-dependent kinase. Cyclin B1 accumulation and kinase stimulation peaked at 12 and 24 hr, respectively, at which times they were 5-fold and 90-fold higher than in control untreated cells. These effects decreased thereafter. All taxol-induced cellular effects were abrogated by the protein and RNA synthesis inhibitors cycloheximide and actinomycin D. In contrast, the endonuclease inhibitors aurintricarboxilic acid and zinc markedly inhibited taxol-induced DNA ladder fragmentation without altering taxol-induced cell-cycle arrest, cyclin B1 accumulation, activation of cyclin B1 kinase activity and cytotoxicity. We conclude that taxol-induced stimulation of cyclin B1-dependent kinase activity parallels mitotic arrest, is more pronounced than mitotic arrest and precedes the induction of programmed cell death.

Difference in sensitivity of inner cell mass and trophectoderm to X-irradiation in mouse blastocysts

Pregnant B6C3F1 mice were exposed to a single whole body X-irradiation on day 4 (73-74 hr postconception) of gestation. In experiment 1, they were sacrificed at 2, 4, 6, or 9 hr after a dose of 2 Gy, and their embryos were removed and examined with light and electron microscopy. In experiment 2, dose-response effects of irradiation on the embryos were examined 4 hr after doses of 0-4 Gy. In experiment 3, DNA fragmentation (a marker of apoptosis) was observed by 3'-OH nick-end labeling technique. In inner cell mass (ICM) and trophectoderm (TE) of blastocysts exposed to 2 Gy, cells with cytoplasmic degeneration, or dead cells phagocytosed by their neighboring cells, were found. Although morphological features of these dying cells did not reveal typical characteristics of apoptosis such as nuclear condensation and membrane blebbing, DNA fragmentation was detected by nick-end labeling technique. The degenerated cytoplasm consisted of aggregating ribosomes. Degenerated cells began to increase from 2 hr after irradiation and reached maximal at 4 hr in both ICM and TE. The incidences of degenerated cells in ICM were higher than those in TE at any time point. These findings provide evidence that cell death observed in blastocysts after X-irradiation is apoptotic and sensitivity of the two groups of cells (ICM and TE) to X-rays is different.

HIV expression is induced in dying cells

Using HeLa cells stably transfected with an HIV-LTR-CAT construct, we demonstrated a peak in CAT induction that occurs in viable (but not necessarily cell-division-competent) cells 24 h following exposure to some cell-killing agents. gamma rays were the only cell-killing agent which did not induce HIV transcription; this can be attributed to the fact that gamma-ray-induced apoptotic death requires functional p53, which is not present in HeLa cells. For all other agents, HIV-LTR induction was dose-dependent and correlated with the amount of cell killing that occurred in the culture. Doses which caused over 99% cell killing induced HIV-LTR transcription maximally, demonstrating that cells that will go on to die by 14 days are the cells expressing HIV-LTR-CAT.

Cell death in the normal developing brain, and following ionizing radiation, methyl-azoxymethanol acetate, and hypoxia-ischaemia in the rat

Naturally occurring (programmed) cell death in the developing brain has morphological characteristics of apoptosis and is associated with internucleosomal DNA fragmentation. Apoptosis also plays a role in cell death following hypoxia-ischaemia in the developing rat brain. Ionizing radiation-induced cell death in the brain of the young rat has morphological characteristics of apoptosis, is mediated by protein synthesis and is associated with internucleosomal DNA fragmentation. Methyl-azoxymethanol (MAM) acetate injection in the young rat produces apoptotic cell death in the external granule cell layer of the cerebellum. In addition, strong c-Jun immunore-activity is observed in apoptotic cells during normal development and following experimentally induced cell death. Moreover, c-Jun mRNA induction and de novo c-Jun protein synthesis, together with activation of c-Jun/AP-1, as revealed with gel mobility shift assay, occurs in irradiated animals. Western blotting of total brain homogenates shows a c-Jun-immunoreactive band at p39, which corresponds to the molecular weight of c-Jun, in control rats. However, a thick c-Jun-immunoreactive band at about p62, accompanied by a decrease of the p39 band, occurs in irradiated and MAM-treated rats. A thin band immediately above the thick p62 band, suggestive of c-Jun phosphorylation, is also observed in treated rats. Taken together, these observations indicate that c-Jun expression is associated with apoptotic cell death in the developing central nervous system.

Naturally occurring (programmed) and radiation-induced apoptosis are associated with selective c-Jun expression in the developing rat brain

Expression of the different members of transcription factors Fos and Jun was examined in the developing rat brain. Constitutive expression of c-Fos, Fos-related antigens, Jun B and Jun D, as revealed with immunohistochemistry, is higher and more widely distributed in the developing rat brain than in the adult. Selective strong c-Jun expression is observed in the cytoplasm and nuclei of apoptotic cells during the whole process of naturally occurring (programmed) cell death. Cells expressing strong c-Jun immunoreactivity are undetermined cells, neurons and astrocytes. Selective c-Jun expression is also observed following ionizing radiation in rats aged 3 days. Induction of c-jun mRNA, as revealed with in situ hybridization, occurs between 5 and 15 min following gamma-irradiation. Strong c-Jun protein expression appears at 2 h, peaks at 6 h and decreases thereafter to reach normal levels 48 h after gamma-ray exposure. Strong c-Jun protein expression is coincidental with endonuclease activation, as revealed with the method of in situ labelling of nuclear DNA fragmentation, and is restricted to apoptotic cells. Cycloheximide injection at the time of irradiation blocks c-Jun expression, indicating that c-Jun immunoreactivity is attributable to de novo protein synthesis. These observations demonstrate in vivo selective strong c-Jun expression associated with programmed cell death and ionizing radiation-induced apoptosis in the developing rat brain.

Selective c-Jun overexpression is associated with ionizing radiation-induced apoptosis in the developing cerebellum of the rat

Immunohistochemistry to Bcl-2, Bax, c-Myc, c-Fos, Fos-related, c-Jun, Jun B and Jun D was used to study the involvement of these factors in ionizing radiation-induced apoptosis in the cerebellum of the developing rat. Selective c-Jun overexpression was observed during the whole process of radiation-induced cell death. Furthermore, c-Jun overexpression was restricted to apoptotic cells, as shown by double labeling with the method of in situ labeling of nuclear DNA fragmentation and c-Jun immunohistochemistry. This is the first in vivo evidence that selective c-Jun overexpression is associated with apoptotic cell death in the developing nervous system following ionizing radiation.

Neuronal cell death in the mammalian nervous system: the calmortin hypothesis

1. This review is concerned with the calcium-dependent mechanisms involved in neuronal cell death. To this end, it provides definitions of the major types of cell death and then describes what is known of their occurrence during development and degeneration of the mammalian nervous system. 2. An analysis is presented of the different sources and compartments of calcium in neurons and of how these are related to the known calcium-dependent enzymes whose excess activation will lead to cell death. 3. The review uses the relatively large amount of pertinent information now available for other cell types, especially thymocytes, to reveal our limited knowledge of how calcium controls neuronal cell death. 4. In the final section, consideration is given to the identification of those factors that may mitigate against the calcium-dependent pathways leading to neuronal degeneration.

Expression of enhanced spontaneous and gamma-ray-induced apoptosis by lymphocytes of the wasted mouse

Mice bearing the autosomal recessive mutation wasted (wst/wst) display a disease pattern including increased sensitivity of lymphocytes to ionizing radiation, neurologic dysfunction, and immunodeficiency. Many of the features of this mouse model have suggested a premature or increased spontaneous frequency of apoptosis in thymocytes. Past work has documented an inability to establish cultured T cell lines, and abnormally high death rate of stimulated T cells in culture, and an increased sensitivity of T cells to the killing effects of ionizing radiations in the wst/wst mouse relative to controls. The experiments reported here were designed to examine splenic and thymic lymphocytes from the wasted and control mouse for signs of early apoptosis. Our results revealed enhanced expression of Rp-8 mRNA (which has been associated with apoptosis) in thymic lymphocytes and to a lesser extent in spinal cord in the wst/wst mouse relative to controls; expression of Rp-2 and Tcl-30 mRNA (also reported to be induced during apoptosis) were not detectable in spleen or thymus. Expression of Rp-2, Rp-8, and Tcl-30 mRNA in other affected tissues of the wasted mouse (brain and liver) were similar in the wasted mouse and controls. Thymus and spleen from the wasted mouse have reduced numbers of viable cells relative to controls. Higher spontaneous DNA fragmentation was observed in lymphocytes from the wasted mouse than in controls; however, gamma-ray-induced DNA fragmentation peaked at a lower dose and occurred to a greater extent in lymphocytes derived from the wasted mouse relative to controls. These results suggest that high spontaneous and gamma-ray-induced apoptosis in T cells of the wasted mouse may contribute to the mechanism underlying the observed lymphocyte and DNA repair abnormalities.

Gamma-radiation-induced cell death in the fetal rat brain possesses molecular characteristics of apoptosis and is associated with specific messenger RNA elevations

Low-dose ionizing irradiation of 16-18-day pregnant rats rapidly kills stem cells in the fetal forebrain. We have examined gamma-irradiated 17-day fetal rat brain tissue for molecular characteristics of apoptosis and changes in levels of mRNAs relevant to apoptosis. In many forebrain cells radiation elicits within 5 h nuclear condensation and fragmentation consistent with apoptosis. An electrophoretic DNA ladder indicative of internucleosomal chromatin cleavage was prominent within 3 h after irradiation. Pretreatment of pregnant rats with cycloheximide, or pretreatment of dissociated fetal brain cells in culture with actinomycin D, abolished the radiation-induced internucleosomal DNA fragmentation, demonstrating requirements for protein and RNA synthesis. Irradiation dramatically increased the level of the p53 transcription factor and the abundances of mRNAs coding for the cell-cycle inhibitor p21/Waf-1/Cip-1 and the AP-1-associated transcription factors Fos and JunB. Irradiation moderately increased the level of mRNA for the positive apoptosis regulator Bax. In contrast, irradiation reduced by 50-70% the abundances of most other mRNAs tested, including those for housekeeping proteins, p53, Jun, Myc, interleukin-1-beta-converting enzyme, and the negative apoptosis regulators Bcl-2 and Bcl-xL. These results indicate that radiation-elicited apoptosis of fetal brain cells is associated with activation of the p53 system, probable increases in AP-1 Fos/JunB heterodimers, and an increased ratio of Bax to Bcl-2 + Bcl-xL.

Ionizing radiation-induced apoptosis is associated with c-Jun expression and c-Jun/AP-1 activation in the developing cerebellum of the rat

Previous studies have shown that ionizing radiation-induced cell death in the developing brain has morphological characteristics of apoptosis and is associated with internucleosomal DNA fragmentation. In the present study, we have observed c-Jun induction in cells sensitive to ionizing radiation during the whole process of radiation-induced cell death, and that this expression is accompanied by modifications in the composition of AP-1 complexes: c-Jun/AP-1 activity is highly increased whereas Jun D/AP-1 is slightly decreased. These results show that c-Jun expression and c-Jun/AP-1 activity are induced in the developing brain following ionizing radiation.

Cycloheximide-induced apoptosis in Burkitt lymphoma (BJA-B) cells with and without Epstein-Barr virus infection

Epstein-Barr virus (EBV) has been shown, in many instances, to protect B cells from apoptosis via expression of select EBV proteins and up-regulation of bcl-2 or its homologues. However, at present little is known about the influence of EBV infection against cancer therapy-induced apoptosis in EBV-associated cancers. Many anti-cancer treatments act via inhibition of protein synthesis and so could influence the reported protein-dependent mechanisms involved in EBV inhibition of apoptosis. In the present study, Burkitt lymphoma (BJA-B) cells were treated with a potent protein synthesis inhibitor, cycloheximide (CHX). Two variants of BJA-B cells were used, one with EBV infection (EBV(+)), and one free of infection (EBV(-)). Cells were collected 0,3,6,12, 24 and 48 h after addition of either 1 or 50 micrograms/mL of CHX. Control cultures were untreated. Apoptosis was quantified using established morphological and biochemical characteristics, and protein concentrations assessed. CHX treatment of EBV(-) BJA-B cells induced massive levels of apoptosis. Apoptosis was inhibited, but remained significantly higher than that found in control cultures, in similarly treated EBV(+) cells. The study demonstrates that induction of apoptosis in EBV(-) and EBV(+) cells is not dependent on new protein synthesis and so may be indicative of a bcl-2 independent mechanism in this instance. The results have important implications for devising and assessing treatment of EBV-associated malignancies.

In vivo induction of apoptosis in human lymphocytes by therapeutic fractionated total body irradiation

Ionizing radiations have been reported as an in vitro apoptosis initiating stimulus in human lymphocytes. As the cytotoxicity of ionizing radiations and chemotherapeutic agents appears to be dependent on the efficacy of cell death induction, the manipulation of apoptosis initiation might be used as a means to supress some pathological process. In the present study the in vivo induction of gamma-ray mediated programmed cell death in humans is reported. The in vivo induction of apoptosis in peripheral blood lymphocytes (PBL) by ionizing radiations was investigated in 33 patients after each of two sessions (2 Gy and 4 Gy) of fractionated total body irradiation (FTBI) as part of their conditioning regimen before bone marrow transplantation. PBL committed to apoptosis were scored before irradiation (S1), 4 h (S2) and 24 h after 2 Gy (S3, 14-17 h after the second 2 Gy fraction). Nuclear morphology and chromatin-DNA were analysed by fluorescence microscopy immediately after blood sample withdrawal (I) and after 24 h in cell culture medium (II). When scored immediately after withdrawal, no circulating PBL with the apoptotic nuclear morphology were observed in S1 and S2 blood samples whereas S3 disclosed 21.9 +/- 11.7% of circulating lymphocytes with an apoptotic nuclear morphology. After 24 h in culture, S1 samples (before irradiation) generally contained less than 20% of apoptotic lymphocytes. A higher percentage of apoptotic cells was noted in some cases in relation with recent chemotherapy and possibly with pathology. After 24 h in culture, S2 and S3 samples contained 51.7 +/- 17.9% and 60.4 +/- 16.4% of apoptotic lymphocytes, respectively. These results confirm that ionizing radiations induce apoptosis in vivo in human lymphocytes and that the commitment to apoptosis can be determined after low doses (2 Gy) of therapeutic whole body irradiation. The results suggest that susceptibility to apoptosis induction by ionizing radiations could be related to previous therapy by cytotoxic drugs and possibly to the type of haematological malignancy.

Apoptosis overview emphasizing the role of oxidative stress, DNA damage and signal-transduction pathways

Apoptosis (programmed cell death) is a central protective response to excess oxidative damage (especially DNA damage), and is also essential to embryogenesis, morphogenesis and normal immune function. An understanding of the cellular events leading to apoptosis is important for the design of new chemotherapeutic agents directed against the types of leukemias and lymphomas that are resistant to currently used chemotherapeutic protocols. We present here a review of the characteristic features of apoptosis, the cell types and situations in which it occurs, the types of oxidative stress that induce apoptosis, the signal-transduction pathways that either induce or prevent apoptosis, the biologic significance of apoptosis, the role of apoptosis in cancer, and an evaluation of the methodologies used to identify apoptotic cells. Two accompanying articles, demonstrating classic apoptosis and non-classic apoptosis in the same Epstein-Barr virus-transformed lymphoid cell line, are used to illustrate the value of employing multiple criteria to determine the type of cell death occurring in a given experimental system. Aspects of apoptosis and programmed cell death that are not covered in this review include histochemistry, details of cell deletion processes in the sculpting of tissues and organs in embryogenesis and morphogenesis, and the specific pathways leading to apoptosis in specific cell types. The readers should refer to the excellent books and reviews on the morphology, biochemistry and molecular biology of apoptosis already published on these topics. Emphasis is placed, in this review, on a proposed common pathway of apoptosis that may be relevant to all cell types.

Role of apoptosis in biology and pathology: resistance to apoptosis in colon carcinogenesis

The overview of apoptosis presented here emphasizes cell deletion in the immune system, with particular reference to T- and B-lymphocyte development, and the in vivo and in vitro senescence of human neutrophils. Some biochemical criteria that are used to identify apoptotic cells are described. Pitfalls in using agarose gel electrophoresis as the sole method for the identification of apoptotic cells are discussed. There are multiple modes of cell death that can be identified at the morphologic level. Thus the central role of microscopic methods, and in particular, electron microscopy, as an important tool in the study of cell death mechanisms, is presented. Apoptosis has a protective role against disease and could, a priori, have an important role in either the initiation or progression of cancer. Two paradoxes concerning the relationship of tumor aggressiveness at the clinical level to mitotic activity have been explained by an evaluation of apoptotic index. In the first case, basal cell carcinomas grow slowly but show a high rate of mitosis. Here, the apoptotic rate is quite high, but just below the mitotic rate, thereby accounting for the slow rate of growth. A second instance is follicular lymphoma, which has a low rate of mitosis that is less than that described for reactive germinal centers. However, apoptosis is markedly reduced in follicular lymphomas compared with that seen in reactive germinal centers, thus providing an explanation for the progressive growth of the follicle. We present a brief description of recent work from our laboratory that indicates that apoptosis may play an important role in colon carcinogenesis. We have shown that sodium deoxycholate, the particular bile salt present in highest concentration in the colon, induces apoptosis in the goblet cells of the human colonic mucosa in an in vitro assay. The intriguing finding is that cells of the normal-appearing mucosa of colon cancer patients are resistant to bile salt-induced apoptosis. This suggests a novel hypothesis about the etiologic role of bile salts in colon cancer. The chronic presence of bile salts that accompany a high-fat diet could select for apoptosis-resistant epithelial cells in the colon over time. Thus, a resistance-to-apoptosis bioassay may prove useful as an intermediate biomarker for determining which individuals are at high risk for colon cancer.

Amoeboid microglial response following X-ray-induced apoptosis in the neonatal rat brain

The phagocytic response following X-ray-induced apoptosis in the neonatal rat brain was examined by immunohistochemistry with the antibodies OX-6 and OX-42 which recognize MHC class II antigens and the CR3 complement receptor, respectively. Few OX-6-immunoreactive cells were observed in control rats, and in rats irradiated at postnatal day 2 and examined during the first 2 postnatal weeks. However, a transient increase in the number of OX-42-immunoreactive amoeboid microglia, containing large numbers of apoptotic cells, occurred at 6, 24 and 48 h after irradiation when compared with age-matched controls. These results show that X-ray-induced apoptosis promotes a short-lasting phagocytic response.

Evidence of internucleosomal DNA fragmentation and identification of dying cells in X-ray-induced cell death in the developing brain

Newborn Sprague-Dawley rats received a single dose of 2 Gy X-rays and were killed 6 hr later. Dying cells were characterized by extreme chromatin condensation and nuclear fragmentation. Dying cells were distributed in the primary and secondary germinal zones and in other brain regions. Among these latter, dying cells occurred in the cortical layers of the olfactory bulb, layers II-III and VIb of the neocortex, piriform and entorhinal cortex, stratum oriens and pyramidale of the hippocampus, striatum, thalamus, amygdala, brainstem, internal granular layer of the cerebellum, and cerebral and cerebellar white matter. Dying cells were immature cells, neurons and glial cells (including radial glia). In-situ labeling of nuclear DNA fragmentation identified individual cells bearing fragmented DNA. Since the number of cells stained with this method was larger than the number of dying cells, as revealed with current histological techniques, it is suggested that nuclear DNA fragmentation precedes chromatin condensation and nuclear fragmentation in X-ray-induced apoptosis. Furthermore, agarose gel electrophoresis of extracted DNA from irradiated brains showed a "ladder" pattern which is typical of internucleosomal DNA fragmentation and endonuclease activation.

Distinction of apoptotic and necrotic cell death by in situ labelling of fragmented DNA

The occurrence and spatial distribution of intracellular DNA fragmentation was investigated by in situ 3' end labelling of DNA breaks in K562 cells treated in such a way to cause either apoptotic or necrotic cell death. The localisation of DNA breaks was examined by confocal laser microscopy and compared with the electron-microscopic appearance of the cells. In addition, the number of cells with fragmented DNA was counted and compared with the number of dead cells, as determined by the nigrosin dye exclusion test. Apoptosis was induced by cultivation of the cells in the presence of actinomycin D. Cells undergoing apoptosis were characterised by massive intracellular DNA fragmentation that was highly ordered into successive steps. Cells in early stages of the apoptotic process had DNA breaks diffusely distributed in the entire nucleus, except the nucleolus, with crescent-like accumulations beyond the nuclear membrane. In the more advanced stages, the nucleus was transformed into many round bodies with intense labelling. Intracellular accumulations of fragmented DNA corresponded exactly to electron-dense chromatin seen in the electron microscope, whereas diffuse DNA breaks had no morphological correlate at the ultrastructural level. In necrosis induced by ionomycin, NaN3, or rapid freezing combined with thawing, no DNA fragmentation occurred at the onset of cell death, but appeared 24 h later. This fragmentation was not characterised by a unique morphology, but represented the breakdown of the chromatin in the configuration remaining after cell death. Therefore, apoptosis is characterised by DNA fragmentation that proceeds in a regular orderly sequence at the beginning of cell death, and can be detected by in situ 3'end labelling of DNA breaks.

Effects of X-irradiation on glial cells in the developing rat brain

Sprague-Dawley rats were given a single dose of 2 Gy X-rays when 1 or 3 days of age. Dying cells in the germinal layer of the telencephalon reached peak values 6 h after irradiation; dead cells were cleared 48 h later. These effects were almost abolished with the injection of cycloheximide (1 microgram/g body weight) given at the time of irradiation. PCNA-immunoreactive cells (cells in late G1 and S phases of the cell cycle) and PCNA-negative cells were sensitive to X-rays. Long-term effects on glial cell populations in the subcortical white matter of the cingulum were examined in irradiated rats, killed at postnatal day 30 (P30), by means of glial fibrillary acidic protein, vimentin and S-100 immunohistochemistry, as well as with anti-TGF-alpha (transformerly growth factor) antibodies that are used as putative oligodendroglial cell markers in the white matter of rat. The subcortical white matter was reduced in irradiated animals, mainly in rat irradiated at P3, as revealed with myelin basic protein immunohistochemistry. Quantitative studies showed no significant differences in the number of glial cells in animals irradiated at P1 when compared with age-matched controls. However, reduced numbers of vimentin-, S-100-, and TGF-alpha-immunoreactive cells were found in animals irradiated at P3. These features indicate a limited capacity of surviving germinal cells, in animals irradiated at P3, to give rise to normal values of glial cells in the white matter in rat aged 30 days. This limitation mainly affects putative oligodendrocytes and glial cell precursors.

Kinetics of radiation-induced apoptosis in the cerebellum of 14-day-old rats after acute or during continuous exposure

We have studied, by histological methods, cytological progression, frequency and distribution of apoptosis in the external granular layer of the cerebellum after whole-body irradiation of 14-day-old rats by gamma-rays from 60 Co. After acute exposure to 0.25, 0.5, 1.5 and 3 Gy (18 cGy/min), the duration of the apoptotic process gradually increased with dose from 6-9 h after 0.25 Gy, to > 24 h after 3 Gy. Up to 1 Gy, maximal frequency was found 6 h after exposures, and at this postirradiation time a linear increase in apoptosis with dose was observed. No effect of dose-rate on apoptosis induction could be demonstrated 6 h after delivering 1 Gy at dose-rates from 2.2 to 18 cGy/min. Continuous irradiation at 1.8 cGy/h induced a gradual increase of apoptosis that remained at a plateau value of about 3% from 15 to 29 h (controls 0.12%, SD = 0.07) and then gradually decreased to 1% at 53 h. At this time the mitotic index was similar to that measured in controls. Apoptosis occurring 3 h after acute irradiation, confined to proliferative cells, was only observed for doses of 1.5 and 3 Gy.

Oxidative stress-induced apoptosis prevented by Trolox

The ability of oxidative stress to induce apoptosis (programmed cell death), and the effect of Trolox, a water soluble vitamin E analog, on this induction were studied in vitro in mouse thymocytes. Cells were exposed to oxidative stress by treating them with 0.5-10 microM hydrogen peroxide (H2O2) for 10 min, in phosphate-buffered saline supplemented with 0.1 mM ferrous sulfate. Cells were resuspended in RPMI 1640 medium with 10% serum and incubated at 37 degrees C under 5% CO2 in air. Electron microscopic studies revealed morphological changes characteristic of apoptosis in H2O2-treated cells. H2O2 treatment fragmented the DNA in a manner typical of apoptotic cells, producing a ladder pattern of 200 base pair increments upon agarose gel electrophoresis. The percentage of DNA fragmentation (determined fluorometrically) increased with increasing doses of H2O2 and postexposure incubation times. Pre- or posttreatment of cells with Trolox reduced H2O2-induced DNA fragmentation to control levels and below. The results indicate that oxidative stress induces apoptosis in thymocytes, and this induction can be prevented by Trolox, a powerful inhibitor of membrane damage.

Radiation-induced apoptosis in F9 teratocarcinoma cells

We have found that F9 murine teratocarcinoma cells undergo morphological changes and internucleosomal DNA fragmentation characteristic of apoptosis after exposure to ionizing radiation. We studied the time course, radiation dose-response, and the effects of protein and RNA synthesis inhibitors on this process. The response is dose dependent in the range 2-12 Gy. Internucleosomal DNA fragmentation can be detected as early as 6 h postirradiation and is maximal by 48 h. Cycloheximide, a protein synthesis inhibitor, and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, an RNA synthesis inhibitor, both induced internucleosomal DNA fragmentation in the unirradiated cells and enhanced radiation-induced DNA fragmentation. F9 cells can be induced to differentiate into cells resembling endoderm with retinoic acid. After irradiation, differentiated F9 cells exhibit less DNA fragmentation than stem cells. This indicates that ionizing radiation can induce apoptosis in non-lymphoid tumours. We suggest that embryonic tumour cells may be particularly susceptible to agents that induce apoptosis.

Apoptosis in adult retinal ganglion cells after axotomy

Lesions to the mature mammalian central nervous system cause irreversible degeneration, in which neurons have been previously thought to be passive victims. In this study, axon-lesioned adult rat neurons are shown instead to actively degrade themselves through the process of apoptosis: a programmed type of cell death in which the cellular apparatus is actively involved in the degradation process. To investigate whether retinal ganglion cells of an adult mammal follow an apoptotic type of death when their axons are severed, DNA breaks in nuclei were labeled in situ, using a method that specifically incorporates biotinylated deoxynucleotides by exogenous terminal deoxynucleotidyl transferase on the 3'-OH ends of DNA. The active nature of the death mechanism was demonstrated by the reduction in biotin-labeled nuclei after administering the protein synthesis inhibitor cycloheximide. Our results suggest that retinal ganglion cells of the adult rat die through apoptosis when axotomized. This raises new possibilities in the treatment of CNS injuries, by the potential interruptibility of a program for neuronal death.

Dose-related effects of cycloheximide on delayed neuronal death in the gerbil hippocampus after bilateral transitory forebrain ischemia

Degeneration of dendrites followed by punctate chromatin condensation in the CA1 area of the hippocampus is a characteristic of delayed neuronal death following bilateral forebrain ischemia. The effects of the protein synthesis inhibitor cycloheximide on delayed neuronal death following 20 min of bilateral forebrain ischemia were examined in the gerbil hippocampus at the 4th day of reperfusion. Low doses of cycloheximide beginning 10 min after ischemia (1.0 microgram/g body weight in saline followed by 1.0 microgram/g every 24 h) reduced the number of dying cells in the CA1 area, whereas high doses (2.0 micrograms/g, followed by 1.0 microgram/g every 12 h) increased the number of dying cells. No effects were seen when a single dose of cycloheximide was injected 1 h before ischemia. These results indicate that the effects of cycloheximide are dose-dependent, low doses reduce, high doses increase cell death. These findings also indirectly suggest that protein synthesis may play a role in the extent of delayed neuronal death. Some involved proteins could be heat shock proteins, which are induced after ischemia and had been correlated with increased resistance to injury. However, changes of heat shock immunoreactivity in the postischemic hippocampus were not seen in the present study following cycloheximide injection.

Radiosensitive populations and recovery in X-ray-induced apoptosis in the developing cerebellum

Sprague-Dawley rats received a single dose of 2 Gy X-rays at the age of 1 or 3 days and were killed at different intervals. Dying cells with the morphological characteristics of apoptosis appeared in the external and internal granular layers (EGL and IGL) and white matter (WM) of the cerebellum, mainly 3-6 h after irradiation, and decreased thereafter to reach normal values between 48 h and 5 days later. This process was curbed by the injection of cycloheximide at a dose of 1 microgram/g body weight. In addition, the number of mitoses in EGL rapidly decreased after irradiation and did not reach normal values until a few days later. Proliferating cell nuclear antigen (PCNA)-immunoreactive cells, which were chiefly found in EGL but also in IGL and WM, dramatically decreased in number from 3 to 48 h after irradiation. PCNA-immunoreactive cells reappeared and reached age-matched values in the following days. Hu (considered as an early neuronal marker) and vimentin immunocytochemistry disclosed that Hu-nonreactive cells in the upper level of EGL, Hu-immunoreactive cells in the inner level of EGL, Bergmann glia and many astrocytes in WM, as well as many non-typified cells in WM, were radiosensitive populations, whereas Purkinje cells were not. The present results indicate that irradiation at P1 or P3 blocks mitosis in EGL and kills sensitive cells mainly in the late G1 and S phases of the cell cycle, probably by apoptosis through a protein synthesis-mediated process. Radiosensitive cells are germinal cells and neuroblasts in EGL, Bergmann glia, astrocytes in WM, and non-typified cells, probably glial cell precursors, in WM. Surviving cells in EGL and PCNA-immunoreactive cells in other cortical layers and white matter reconstitute the cerebellum following a single dose of X-rays.

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

Apoptosis (the classical type of programmed cell death) can be triggered in many cell types by widely diverse stimuli. gamma rays, at low doses, can induce apoptosis in vitro in interphase human lymphocytes. In this type of apoptosis induction, activated gene expression is necessary for the fulfillment of the death program. In this report, we present evidence for a relationship between ubiquitin gene expression or ubiquitination and gamma-irradiation-mediated apoptosis in normal circulating human lymphocytes. Using in vitro nuclear transcription assays (run-on), Northern (RNA) blot analysis, immunolocalization studies, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis after immunoprecipitation, we demonstrate that (i) the ubiquitin mRNA level is increased as a consequence of the activation of ubiquitin gene transcription 15 to 90 min after initiation of apoptosis; (ii) specifically in apoptotic cells, and not in all irradiated cells, nuclear proteins are highly ubiquitinated; and (iii) ubiquitin sequence-specific antisense oligonucleotide inhibition results in a decreased level of ubiquitinated nuclear proteins and considerably diminishes the proportion of cells exhibiting the apoptotic death pattern. Each of these results might be explained by different modifications occurring in irradiated cells. Their convergence strongly suggests that the ubiquitin gene is one of the genes with induced activity in the apoptotic death program and that ubiquitination of nuclear proteins might be involved in chromatin disorganization and oligonucleosomal fragmentation, which are among the key events occurring in apoptosis.

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

Apoptosis (the classical type of programmed cell death) can be triggered in many cell types by widely diverse stimuli. gamma rays, at low doses, can induce apoptosis in vitro in interphase human lymphocytes. In this type of apoptosis induction, activated gene expression is necessary for the fulfillment of the death program. In this report, we present evidence for a relationship between ubiquitin gene expression or ubiquitination and gamma-irradiation-mediated apoptosis in normal circulating human lymphocytes. Using in vitro nuclear transcription assays (run-on), Northern (RNA) blot analysis, immunolocalization studies, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis after immunoprecipitation, we demonstrate that (i) the ubiquitin mRNA level is increased as a consequence of the activation of ubiquitin gene transcription 15 to 90 min after initiation of apoptosis; (ii) specifically in apoptotic cells, and not in all irradiated cells, nuclear proteins are highly ubiquitinated; and (iii) ubiquitin sequence-specific antisense oligonucleotide inhibition results in a decreased level of ubiquitinated nuclear proteins and considerably diminishes the proportion of cells exhibiting the apoptotic death pattern. Each of these results might be explained by different modifications occurring in irradiated cells. Their convergence strongly suggests that the ubiquitin gene is one of the genes with induced activity in the apoptotic death program and that ubiquitination of nuclear proteins might be involved in chromatin disorganization and oligonucleosomal fragmentation, which are among the key events occurring in apoptosis.

Toward an understanding of the molecular mechanisms of physiological cell death

Cell death is a normal physiological process. Morphological studies have shown that cells that die by physiological mechanisms often undergo characteristic changes termed "apoptosis" or "programmed cell death." Recent work has begun to unravel the molecular mechanisms of these deaths and has shown that one of the primary cell-death pathways is conserved throughout much of evolution. In the nematode Caenorhabditis elegans programmed cell deaths are mediated by a mechanism controlled by the ced-9 gene; in mammals apoptosis can often be inhibited by expression of the bcl-2 gene. The ability of the human BCL2 gene to prevent cell deaths in C. elegans strongly suggests that bcl-2 and ced-9 are homologous genes. Although the process of cell death controlled by bcl-2 can occur in many cell types, there appears to be more than one physiological cell-death mechanism. Targets of cytotoxic T cells and cells deprived of growth factor both exhibit changes characteristic of apoptosis, such as DNA degradation. However, bcl-2 expression protects cells from factor withdrawal but fails to prevent cytotoxic T-cell killing. DNA degradation is, thus, not specific for any one cell-death mechanism. The ability of bcl-2 to protect cells from a wide variety of pathological, as well as physiological, stimuli indicates that many triggers can serve to activate the same suicide pathway, even some thought to cause necrosis, and not physiological cell death.

The effect of cycloheximide on natural and X-ray-induced cell death in the developing cerebral cortex

Naturally occurring cell death in the cerebral cortex and subcortical white matter is increased after X-irradiation, and this process is curbed with cycloheximide, an inhibitor of protein synthesis. However, cycloheximide alone increases cell death during development, and this effect is dose-dependent. This suggests that, in both normal and experimentally-induced cortical cell death during development, different proteins are activated or inhibited, depending on the agent, the time of its application, and the previous metabolic or functional state of the cell.