The mechanism of radiation-induced interphase death of lymphoid cells: a new hypothesis

The interphase death of irradiated rat thymocytes depends on their concentration during postirradiation incubation. The kinetics of pycnosis and cell death determined with the trypan blue exclusion test in the samples with the highest cell concentration (1-2 x 10(7) cells/ml) is consistent with the data available in the literature, whereas the samples with the lowest concentration (2 x 10(5) cells/ml) undergo almost no pycnosis and death after irradiation with doses up to 50 Gy. On the basis of these results, we suggest a new mechanism of interphase death involving an interaction between irradiated thymocytes and the fraction of thymus cells possessing cytocidal activity. The observed correlation between the cytocidal activity and interphase death of thymocytes from animals of different ages favors our mechanism. It was found that the inhibitors which prevent the conjugation of killer cells and their targets do not influence interphase death, while the substances which block the secretion of cytotoxic factors or their action on the target membrane do protect from interphase death. Thus we suggest that the irradiation activates the killer cells to secrete some cytotoxic factors which induce pycnosis and interphase death of thymocytes.     

Modification of the interphase death of thymocytes: the effect of agents inhibiting the killing process

The influence of different killing inhibiting agents on the interphase death of irradiated rat thymocytes has been investigated. The results confirm the previously proposed hypothesis of the interphase death mechanism involving activation of killing potency in the irradiated population of thymus cells.     

The interphase death of dividing cells. The effect of oxygen and the pH of the medium on the interphase death of Ehrlich ascites carcinoma cells under the action of ionizing radiation

Interphase death of in vitro irradiated (200 Gy) Ehrlich ascites tumor cells was studied as a function of oxygenation level and medium pH. The presence of oxygen both during and after irradiation as well as the increase in pH from 7.4 to 8.1 were shown to increase interphase death rate. The pronounced threshold dependence of interphase death of cells upon their concentration may be attributed to hypoxia occurring in a pericellular medium when cells concentration exceeds the threshold.     

Prostaglandins and interphase death of irradiated cells

The contribution of the post-irradiation changes in prostaglandin transformation to the biochemical mechanism of interphase death of irradiated cells is estimated. It is supposed that prostaglandins are secondary trigger-effectors which initiate the development of primary biochemical reactions giving rise to radiation sickness. It is suggested that the biochemical mechanism of interphase death is complex and involves several concurrent trigger mechanisms including prostaglandin regulation system.     

The interphase death of dividing cells. The relation of interphase death to cell energy requirements

In experiments with irradiated cells of Chinese hamster and Ehrlich ascites tumor a study was made of the influence of energy provision on their interphase death rate. The presence of the uncoupler of respiration and oxidative phosphorylation--carbonyl cyanide-3-chlorophenylhydrazone--in a medium without glucose was shown to drastically increase the interphase death rate of cells of both types, whereas this effect was not observed in a medium with glucose.     

Mathematical model of interphase cell death. Biophysical justification and generalization

The authors propose a biophysical justification of a radiation-induced injury and interphase death of cells. The injury to certain units of the microtrabecular network and cytoskeleton is considered to be a primary biological effect of radiation on cells. The role of these structural changes in the development of the specific radiation response is discussed. It is found possible to describe formally, by the defined parameters of the proposed model, the survival curves for not only interphase but also reproductive cell death.     

Prevention of adriamycin-induced interphase death by 3-aminobenzamide and nicotinamide in a human promyelocytic leukemia cell line

Adriamycin caused significant interphase death in HL-60 cells during six hours of incubation, which was abolished by the poly(ADP-ribose) polymerase inhibitors, 3-aminobenzamide or nicotinamide. Neither agent changed adriamycin uptake by HL-60 cells. Although 3-aminobenzamide did not alter the number of DNA strand breaks caused by adriamycin, it prevented adriamycin-induced depletion of intracellular NAD+ and ATP, and maintained energy charge. These findings suggest that the activation of poly(ADP-ribose) synthesis plays an important role in the adriamycin-induced interphase death of proliferating HL-60 cells.     

Quantitative evaluation of radiation damage to thrombocytopoiesis

From the comparison of the results of model and experimental kinetics of thrombocytopoiesis the following parameters have been quantitatively estimated: (1) the percentage of cells killed during the interphase (interphase death) and (2) the duration of the mitosis delay and the abortive rise. There was a 1.3-1.5-fold and 2-3-fold nonspecific and postirradiation, respectively, decrease in the megacaryocyte maturation time. The transit time of the committed megacaryocyte precursors was reduced due to death of premature precursors and survival of mature forms.     

The interphase death of dividing cells. The kinetics of the death of cultured Chinese hamster fibroblasts after irradiation at various doses

In studying the kinetics of interphase death (ID) of cultured Chinese hamster cells after irradiation with doses of 100 to 800 Gy the authors showed an increase in the ID rate with increasing radiation dose; the presence of serum in the medium both during and after irradiation prevents the cell death.     

pH changes in Chinese hamster cells after gamma-irradiation

Intracellular pH (pHin) changes after gamma-irradiation of Chinese hamster fibroblasts have been studied by a fluorescence method using the ratio of fluorescence intensities after excitation at 488 and 458 nm and measurement at emission wavelength of 515 nm. Irradiation with doses inducing reproductive death (2.5-20 Gy) causes a pHin shift towards the alkaline region by 0.4-0.5 pH units, but this shift is transient. Irradiation with a 500 Gy dose, inducing interphase death, causes a more pronounced (pHin greater than or equal to 8.0) alkalization of the intracellular medium which is retained for more than 1.5 hours post-irradiation. It is proposed that the observed alkalization of the internal medium of irradiated cells is possibly due to a change in the functional state of mitochondria. These changes are probably one of the causes of interphase cell death after irradiation with high doses.     

Radiation-induced interphase death of rat thymocytes is internally programmed (apoptosis)

Thymocytes are highly radiosensitive and show 'interphase death' within a few hours after low doses of irradiation. However, the mechanisms responsible for this type of death remain ill-defined. Separation of the dead thymocyte fraction from irradiated thymocyte suspensions by centrifugation on Percoll gradients provided homogeneous populations of dead cells suitable for detailed study. Using this method, radiation-induced interphase death of thymocytes was found to involve a sharp but transient increase in buoyant density, concomitant with the appearance of distinctive morphologic changes which included disappearance of microvilli and blistering of the cell surface. The chromatin in the dead cells had a molecular weight sufficiently low to resist sedimentation, and consisted of short oligonucleosome chains. We were unable to detect populations of cells intermediate between the dead and normal in the above characteristics. Interphase death thus involves a discrete, abrupt transition from the normal state and is not merely the consequence of progressive and degenerative changes. Furthermore, immediate cessation of development of interphase death by cycloheximide suggested a possible involvement of protein synthesis on this transition step.     

A model of the stochastic dynamics of interphase lymphocyte death

Dynamics of interphase peripheral blood lymphocyte death is studied in terms of a general model based on a random damage distribution among cells and stochastic time of their death. Some particular cases of this model are analysed. Possible causes of shaping "biphase" dose-dependence curves for lymphoid cell survival after irradiation are discussed.     

Radiation disturbance of RNA metabolism during interphase death of lymphoid cells

The data are reviewed concerning the radiation--induced disturbances of RNA metabolism in lymphoid cells. The role of the observed disorders in activation and realization of the interphase cell death program is discussed.     

Modification of the interphase death of thymocytes by using colcemid

The pretreatment of thymocytes by colcemid (0.02 to 0.5 mu g/ml) induces a change in the plasma membrane state, registered by a pyrene fluorescent probe, and a decrease in the interphase cell death after 4 Gy X-irradiation. The authors discuss the role of the cell surface as a trigger initiating death program in irradiated lymphoid cells.     

Degree of chromatin fragmentation and frequency of nuclear pyknosis in Percoll-fractionated thymocytes of irradiated rats

The relationship between nuclear chromatin degradation to polydeoxyribonucleotides (PDN) and other features of interphase death were studied using thymocytes of normal and X-irradiated rats. Fractionation of the thymic cells in Percoll gradients was performed in order to separate dead from intact cells. The degree of radiation-induced chromatin fragmentation, as assessed by electrophoresis, was similar for PDN from all Percoll bands. Following irradiation 87-98 per cent of 'heavy' thymocytes were pyknotic and almost devoid of receptors to autologous erythrocytes thus comprising a dead cell population. A direct relationship between PDN content and nuclear pyknosis was noted throughout the gradient. The loss of autologous rosette-forming ability was directly related to other indices of interphase death. The possibility of PDN originating from pyknosis-prone cells and the capacity of radiosensitive thymocytes to form autologous rosettes are discussed.     

Abscopal action of gamma radiation in the death of lymphoid cells

In the in vitro experiments with lymphoid cells Raji and X63-Ag8.653 gamma-irradiated in a synthetic medium, it was shown that a cessation of division (reproductive death) and lysis (interphase death) of cells were evoked by the abscopal effect of long-lived quinoid radiotoxins enhancing the direct effect of radiation to make it not additive but synergistic: a synergism coefficient was 1.3.     

Increasing the concentration of free calcium in thymocytes during gamma-irradiation may induce their death

Free Ca2+ concentration in thymocytes increased 0.5-1.5h after gamma-irradiation (10 Gy) as was measured by Quin-2AM fluorescent probe. Cycloheximide, a protein synthesis inhibitor, suppressed Ca2+ increase and inhibited radiation-induced thymocyte death. EL-4 thymoma cells did not exhibit any changes in free Ca2+ concentration and interphase death after gamma-irradiation. It is believed that the radiation-induced increase of free Ca2+ concentration in thymocytes may induce their death.     

Lipid metabolism in rat tissues after irradiation at doses causing interphase cell death

Irradiation of animals with doses eliciting interphase death of 50-80% of cells activates lipogenesis and decreases the cholesterol content of cells. Cholesterol synthesis is activated after irradiation with doses causing death of 50% of cells: a further increase in radiation dose decreases the cholesterol synthesis. It is assumed that as membranes are destructed by radiation the adaptive lipogenesis activated to restore them.     

The role of adenine nucleotide catabolism in the interphase death of thymocytes, caused by papaverine and dipyridamole

Papaverine and dipyridamole induce the interphase death of thymocytes rapidly growing four hours later and reaching its maximum by the seventh-eighth hour of the cell incubation. To induce death of thymocytes no constant presence of these preparations in the incubation medium is needed, a definite (for each of preparations) time of the contact with cells being enough. The interphase death of thymocytes induced by papaverine and dipyridamole is preceded by acceleration of the release of adenine nucleotide catabolism products from cells mainly as hypoxanthine and inosine, respectively. These both processes are induced by papaverine for a shorter period of its incubation with cells than by dipyridamole and the joined use of these substances intensifies the above processes. The analysis of the data obtained indicates that thymocytes under the effect of papaverine die rather from the exhaustion of the adenine nucleotide pool, than from a decrease in the adenylate charge of cells. Exogenous adenosine essentially removes the toxic effect of papaverine but not of dipyridamole. Addition of adenine and inosine to thymocytes does not affect their survival rate in the presence of the preparations under study.