Early postirradiation chromatin degradation in thymocytes

Accumulation of CAMP in γ-irradiated Thymocytes and Internucleosomal DNA Fragmentation

Ionizing radiation, glucocorticosteroids and chemical inducers of differentiation (CID) are cytotoxic to thymocytes, and induce internucleosomal DNA fragmentation. Tissue cAMP levels in thymi of irradiated mice were significantly elevated as early as 30 min post-irradiation. In contrast, cAMP content in the liver was not changed significantly up to 1 h post-irradiation, and then some decrease occurred. Irradiation of isolated thymocytes gave essentially the same results as after irradiation of animals, and the elevation in cAMP 30 min after the irradiation, DNA fragmentation and cell death were linearly related to the dose up to 2.5 Gy. The maximal induction of cAMP level occurs in the fractions of radiosensitive cortical thymocytes. In thymocytes all CID tested also induced the increase in cAMP level with concomitant DNA fragmentation. Unlike ionizing radiation, UVC light did not induce cAMP accumulation and DNA fragmentation in thymocytes. Treatment of UV-irradiated cells with But2 cAMP did not result in an increase in DNA fragmentation. Ionizing radiation induced DNA fragmentation and cell death can be prevented by adding the protein kinases inhibitor H-7. Theophylline was shown to reduce the cAMP response, DNA fragmentation and cell death in gamma-irradiated thymocytes, suggesting that the accumulation of cAMP may be partly related to adenosine receptor sites.

A Role for Calcium in Regulating Apoptosis in Rat Thymocytes Irradiatedin Vitro

Thymus-derived lymphocytes undergo death after gamma-irradiation via a pathway termed apoptosis, or programmed cell death. An early step in this pathway is the production of nucleosome-sized fragments of DNA. DNA fragmentation was used as the endpoint in these investigations to examine apoptosis in lymphocytes extracted from the rat thymus and irradiated in vitro. In unirradiated thymocytes the level of DNA fragmentation rose to 15% by the first hour of culture, where it remained approximately constant until the fifth hour. In contrast, thymocytes irradiated with a dose of 2.5 Gy exhibited a large and dramatic increase in DNA fragmentation beginning 2 h postirradiation. DNA fragmentation measured 6 h after irradiation was detected after as little as 0.25 Gy and reached a maximum of 90% with 10 Gy. Metabolic control of DNA fragmentation after irradiation was evidenced by the suppression of DNA fragmentation when thymocytes were incubated with cyclohexamide or actinomycin D. When gamma-irradiated thymocytes were incubated with the Ca2+ chelator EGTA, DNA fragmentation was reduced significantly. BAPTA-AM, a highly specific intracellular Ca2+ chelator, essentially eliminated DNA fragmentation in cells irradiated with 2.5 Gy and, unlike EGTA, eliminated the background level of fragmentation in unirradiated samples. Therefore, our data are consistent with the possibility that Ca2+ serves as a second messenger to induce DNA fragmentation in irradiated thymocytes, suggesting a common pathway for cells prompted to enter apoptosis from seemingly dissimilar interval events.

Elevation of oleate-activated phospholipase D activity during thymic atrophy

Various phospholipases are thought to be associated with the in vitro apoptosis of thymocytes. In the present study, the in vivo phospholipase D (PLD) activity of rat thymus was studied after whole-body X-irradiation or injection of dexamethasone (DEX). Using exogenous [14C]dipalmitoyl phosphatidylcholine (PC) as the substrate, an elevation of oleate-activated PLD activity was observed during thymic atrophy. The activity increases were sevenfold at 48 hr after 5-Gy irradiation and fourfold at 72 hr after injection of 5 mg/kg DEX. The elevation of PLD activity appeared to parallel extensive thymus shrinkage. An increased level of thymic phosphatidic acid (PA), the presumed physiological product of PLD action on PC, was also detected. By comparing the acyl chains of PA with those of other phospholipids, PA appeared to originate from PC. To assess the role of PLD during thymic atrophy, thymocytes and stromal cells were isolated. Although thymocytes themselves exhibited significant PLD activation, the major elevation in PLD activity (greater than fourfold) was found in isolated stromal cells. PLD was also activated during in vitro phagocytosis of apoptotic thymocytes by the macrophage-like cell line P388D1. This in vitro phagocytosis was significantly inhibited by PLD action blockers, such as 2,3-diphosphoglycerate and 1-butanol. These observations strongly suggest that the alteration of oleate-activated PLD activity is part of an in vivo event in the progression of thymic atrophy, including phagocytic clearance of apoptotic thymocytes.

Radiation-induced apoptosis in human myeloma cell line increases BCL-2/BAX dimer formation and does not result in BAX/BAX homodimerization

A popular model of BCL-2 and BAX involvement in apoptosis suggests that upon apoptosis induction cytosolic BAX translocates to the mitochondria, where it displays the pro-apoptotic function, which involves its homodimerization. BCL-2 exerts anti-apoptotic function by forming heterodimers with BAX, thus neutralizing the pro-apoptotic activity of the latter. We have shown that irradiation of the human myeloma cell line RPMI-8226 induced apoptosis as determined by DNA degradation, cytochrome c release into cytoplasm and BCL-2 caspase-mediated cleavage. BCL-2 protein was present only in the membrane fraction, whereas BAX was found both in cytosol and membranes isolated from non-irradiated cells. Radiation induced moderate redistribution of BAX from cytosol to membranes with a concomitant increase in BCL-2/BAX heterodimer formation. Rapid and transient BCL-2 phosphorylation in membrane fractions of irradiated cells did not affect BCL-2/BAX heterodimerization. We failed to detect any BAX/BAX homodimers in apoptotic cells. Our findings show that in irradiated RPMI-8226 cells the formation of BCL-2/BAX heterodimers correlates with apoptosis. We conclude that BCL-2/BAX heterodimers are negative regulators of death protection, and our data agree with those who propose that BCL-2 does not require BAX to exert its survival function.

Ionizing radiation-induced cell death

Selected aspects of radiation-induced cell death, connected with signal transduction pathways are reviewed. Cell death is defined as insufficiency of the cellular signal transducing system to maintain the cell's physiological functions. The insufficiency may be due to impaired signal reception and/or transduction, lack or erroneous transcription activation, and eventual cellular 'misexpression' of the signal. The molecular basis of this insufficiency would be damage to genomic (but also other cellular) structures and closing of specific signalling pathways or opening of others (like those leading to apoptosis). I describe experimental data that suggest an important role of RAS/NF1 and p53/p105 Rb proteins in cell cycle control-coupled responses to DNA damage.

Novel concepts in modification of radiation sensitivity

PURPOSE

To determine whether biological effects of radiation, such as apoptosis, that differ from classical clonogenic cell killing, can be modified with agents that would not be expected to modify classical clonogenic cell killing. This would expand the range of potential modifiers of radiation therapy.

METHODS AND MATERIALS

EL4 murine lymphoma cell apoptosis was determined by electrophoretic analysis of deoxyribonucleic acid (DNA) fragmentation. DNA was extracted 24 h after irradiation or addition of inducing agents. Modifiers of radiation-induced apoptosis were added immediately after irradiation. The effects of radiation on wounded endothelial monolayers were studied by scraping a line across the monolayer 30 min after irradiation. Cell detachment was used as an endpoint to determine the protective effect of prolonged exposure to retinol prior to irradiation.

RESULTS

EL4 cell apoptosis can be induced by tert-butyl hydroperoxide or the glutathione oxidant SR-4077. Radiation-induced EL4 cell apoptosis can be inhibited with 3-aminobenzamide, an agent that sensitizes cells to classical clonogenic cell killing. Radiation-induced endothelial cell detachment from confluent monolayers can be modified by pretreatment with retinol.

CONCLUSION

These results raise the possibility that radiation could induce apoptosis by an oxidative stress mechanism that is different from that involved in classical clonogenic cell killing. These and other recent findings encourage the notion that differential modification of classical clonogenic cell killing and other important endpoints of radiation action may be possible.

Mouse lymphoma cells that undergo interphase death show markedly increased sensitivity to radiation-induced DNA double-strand breakage as compared with cells that undergo mitotic death

The relationship between radiation-induced DNA double-strand breakage (dsb) and reproductive death (clonogenicity) for two mouse lymphoma cell lines was compared with that for the fibroblast-like hamster cell line V79. One of the lymphoma lines (STRij-4-2.2), which undergoes rapid disintegration following cytotoxic insult, showed extreme sensitivity to gamma-ray or DNA-associated 125I decay-induced DNA dsb (7 +/- 1 125I decays per clonogenic lethal event). Surprisingly, the other lymphoma line (WEHI-22.1), which does not undergo rapid disintegration, was also much more sensitive to DNA dsb than were V79 cells (17 +/- 1 versus 61 +/- 2 125I decays per clonogenic lethal event). Ultrastructure, DNA degradation, and flow cytometric cell cycle data suggested that both lymphoma cell lines may undergo interphase death, but that the induction of this process in WEHI-22.1 may depend upon blockage in the G2 phase. It is concluded that there are marked differences between the radiation responses of lymphoma and fibroblast lines, that there may be different forms of radiation-induced interphase death, and that the low number of DNA dsb required to produce a clonogenic lethal event in cells undergoing interphase death could explain the radiosensitivity of organs such as ovary, testis and thymus.