The Biochemical Effects of Ionizing Irradiation on Nuclear Function in Animal Cells

This discussion of the biochemical lesions produced in animal cells by ionizing radiation will to all intents and purposes be restricted to a consideration of the immediate response to doses of 1000 rads or less in two rat tissues only, namely the thymus gland and the regenerating liver. The experiments we have carried out with these two tissues have led us to the view that in these organs the ultimate damage to the cell stems from an initial upset of the nuclear thiol-disulphide equilibrium.

Effect of α-particle and X-ray irradiation onDNAsynthesis in tissue cultures

The effects of both α-particle and X-ray in radiation, on theDNAsynthesis rate in mouse fibroblast and Hela cells in tissue culture is described. Tritiated thymidine autoradiography was used to indicate the rate ofDNAsynthesis in the single-layer cultures used. The results of these experiments show that: (1) The fraction of cells in a culture synthesizingDNAis not markedly affected by α-particles and X-rays in the dose used in the experiment. (2) The effect of either type of radiation is to reduce the rate of synthesis ofDNAof the irradiated cells in synthesis. (3) The effect of a given dose of either type of radiation is to reduce the rate of synthesis of all the cells to a constant fraction of what it was in the unirradiated cells. (4) The rate ofDNAsynthesis is reduced to 37 % (1/e) by a dose ofca. 25 α/μ2or an X-ray dose of 14000 rads for mouse fibroblast cultures. In Hela cell cultures a dose ofca. 90000 rads is needed to reduce the rate ofDNAsynthesis to 37 % of the initial value. (5) The reduction in synthesis occurs not more than half an hour after irradiation and may be an immediate effect. From (4) above the target shape can be roughly calculated and if it is assumed to be cylindrical it appears to have dimensions 16 Å in one direction and 160000 Å in the other, i. e. a long thin thread with a mol. wt. ofca. 5 x 107in the case of the mouse fibroblast experiments. In the case of the Hela cell experiments the target volume gives a mol. wt. ofca. 107. These results are consistent with the view that the target may possibly be theDNAtemplate (or maybeDNPbecause of the high value for the molecular weight in one case). If the effects described reflect damage to theDNA(orDNP) template during the exponented phase of synthesis then observations, (1) (2), and (3) above follow as obvious corollaries.

The short-term effects of acute X-irradiation on oogonia and oocyctes

Foetal rats were exposed to 100 r (acute X-irradiation) at 15.5, 16.5 and 17.5 days of intrauterine life, and killed at daily intervals up to 20.5 days p.c . The populations of normal and degenerating germ cells in irradiated ovaries were estimated by the volumetric method of Beaumont & Mandl (1962). The chromosomal configurations of germ cells were examined in both histological and squash preparations. Irradiation appears to disturb the close developmental synchronization of germ cells. Many irradiated cells also show transient changes in chromosomal morphology, and an increase in cell and nuclear volumes. The treatment induces both immediate cell death (i.e. within 24 h of exposure) and delayed cell death. The latter becomes manifest by an increase in the incidence of degenerating cells of the same type as arise spontaneously during normal development (‘atretic divisions’; ‘ Z ’ cells). The most severe depletion in the total population of germ cells occurs following exposure at 15.5 days, only some 13 000 germ cells being present 24 h later (cf. 37 000 in coeval controls). Their number is further reduced to about 5000 at 20.5 days (cf. 56 000 in controls). In contrast, the mean populations at 20.5 days following irradiation at 16.5 and 17.5 days are ca . 16 000 and 42 500 respectively. Most degenerating germ cells are eliminated from the ovary by 20.5 days. The subsequent loss of oocytes with age (up to 25 and 100 days p.p. ) occurs at a lower rate in irradiated than in normal animals.

Relationship between gamma-ray-induced G2/M delay and cellular radiosensitivity

Seven human and rodent cell lines with markedly differing cellular radiosensitivities were examined by the anti-Br-dUrd antibody and flow cytometric method in order to measure the progression of S phase cells and their accumulation in the G2 phase of the cell cycle after gamma-irradiation. Exponentially growing cells were labelled with 10 microM BrdUrd for 2 h, gamma-irradiated, then washed and cultured at 37 degrees C. At 2-h intervals postirradiation, the cells were harvested and fixed for flow cytometric analysis. Two parameter distributions of BrdUrd content and DNA content were analysed. The time intervals for unirradiated labelled cells to progress from S to G2/M phase were about 450 min for the human squamous cell carcinoma cell lines SCC-12B.2 (D0 = 2.66 Gy), SQ-20B (D0 = 2.39 Gy) and SCC-61 (D0 = 1.07 Gy) as well as for wild-type CHO cells (D0 = 2.62 Gy). After irradiation with 2 Gy, SCC-12B.2, SQ-20B, CHO and human diploid AG1521 cells showed similar small G2/M delays (about 1 h), whereas, a G2/M delay of about 2.2 h occurred in radiosensitive SCC-61 cells and delays of 5.0-7.7 h were observed in two extremely radiosensitive mutant cell strains (human AT homozygote and CHO xrs-5 respectively). When the cells were irradiated with doses yielding similar levels of survival (about 10%), however, the duration of the G2/M delay was generally similar (2-4 h) in all seven cell lines indicating a parallel relationship between radiation-induced G2/M delay and cellular radiosensitivity. These results suggest that the delay time may be related to the level of unrepaired damage present in the cell as it approaches mitosis.

The molecular basis for cell cycle delays following ionizing radiation: a review

Exposure of a wide variety of cells to ionizing (X- or gamma-) irradiation results in a division delay which may have several components including a G1 block, a G2 arrest or an S phase delay. The G1 arrest is absent in many cell lines, and the S phase delay is typically seen following relatively high doses (> 5 Gy). In contrast, the G2 arrest is seen in virtually all eukaryotic cells and occurs following high and low doses, even under 1 Gy. The mechanism underlying the G2 arrest may involve suppression of cyclin B1 mRNA and/or protein in some cell lines and tyrosine phosphorylation of p34cdc2 in others. Similar mechanisms are likely to be operative in the G2 arrest induced by various chemotherapeutic agents including nitrogen mustard and etoposide. The upstream signal transduction pathways involved in the G2 arrest following ionizing radiation remain obscure in mammalian cells; however, in the budding yeast the rad9 gene and in the fission yeast the chk1/rad27 gene are involved. There is evidence indicating that shortening of the G2 arrest results in decreased survival which has led to the hypothesis that during this block, cells repair damaged DNA following exposure to genotoxic agents. In cell lines examined to date, wildtype p53 is required for the G1 arrest following ionizing radiation. The gadd45 gene may also have a role in this arrest. Elimination of the G1 arrest leads to no change in survival following radiation in some cell lines and increased radioresistance in others. It has been suggested that this induction of radioresistance in certain cell lines is due to loss of the ability to undergo apoptosis. Relatively little is known about the mechanism underlying the S phase delay. This delay is due to a depression in the rate of DNA synthesis and has both a slow and a fast component. In some cells the S phase delay can be abolished by staurosporine, suggesting involvement of a protein kinase. Understanding the molecular mechanisms behind these delays may lead to improvement in the efficacy of radiotherapy and/or chemotherapy if they can be exploited to decrease repair or increase apoptosis following exposure to those agents.

The effect of fractionated irradiation on cell kinetics

The effects of single and split-dose irradiation were compared by in vitro experiments on HeLa cells. Changes in rate of cell proliferation were detected by flow cytometry, simultaneously determining the DNA content and the bromodeoxyuridine incorporation of individual cells. Cell cultures were irradiated with either a single dose of 1-6 Gy or with a corresponding dose divided into multiple fractions given at 1-6-h intervals. A dose-dependent accumulation of cells in G2/M phase was observed. The method was sensitive enough for the detection of G2/M block even after 1 Gy. The block disappeared completely within a 24-h follow-up time at dose levels up to 3 Gy. Interestingly, no differences in cell kinetics were observed between the single and split-dose regiments. This approach proves to be valuable in evaluating novel fractionation models and the effects of radiation on the cell kinetics of human tumor cells.

The effect of 60Co gamma-radiation and hydroxyurea on the in vivo chain growth of DNA in crypt cells of the small intestine of the mouse

DNA chain growth has been studied in small intestinal crypt cells of the mouse in vivo using a sensitive method. The method is designed primarily to study radiation-induced DNA-breaks and their repair; but since there are breaks in DNA at the replicating fork, it is also possible to study DNA chain growth after a 3H-thymidine pulse. We found that DNA chain growth is not depressed by 200 rad of 60Co gamma radiation. This finding supports the hypothesis that irradiation interfers mainly with the initiation of new replicons in mammalian cells affecting DNA chain growth only at higher doses. Hydroxyurea at sufficient dosage, however, depresses or even stops DNA chain growth in mouse crypt cells in vivo.

Action of x-ray irradiation on DNA synthesis and mitotic activity in uterine epithelial cells at different stages of the sex cycle

The action of x-ray irradiation on DNA synthesis and mitotic activity of the uterine epithelium was studied by autoradiography with thymidine-H-3 at different stages of the sex cycle. In response to local irradiation in a dose of 400 R the decrease in the index of labeled nuclei and in the mitotic index differed depending on the stage of the sex cycle at which irradiation was given.