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LE Magazine November 2001

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Radiation

Management of radiation-induced accelerated carotid atherosclerosis.

Patients with long survival following cervical irradiation are at risk for accelerated carotid atherosclerosis. The neurologic presentation in these patients mimics naturally occurring atheromatous disease, but patients often present at younger ages and with less concurrent coronary or systemic vascular disease. Hypercholesterolemia also contributes to this accelerated arteriosclerosis. Angiographic findings in this disorder include disproportionate involvement of the distal common carotid artery and unusually long carotid lesions. Pathologic findings include destruction of the internal elastic lamina and replacement of the normal intima and media with fibrous tissue. This article describes two surgical patients with radiation-induced accelerated carotid atherosclerosis who typify the presentation and characteristics of this disease.

Arch Neurol 1987 Jul;44(7):711-714

Differential radiation response of cultured endothelial cells and smooth myocytes.

In vivo observations have suggested that endothelial cells are the most radiosensitive elements of the vascular wall. To test whether this represents an intrinsic differential sensitivity, the response of bovine aortic endothelial cells and smooth myocytes was investigated in confluent cell cultures exposed to single doses of gamma radiation (250, 500, 1,000 or 2,000 rad). Both cell types showed a dose-dependent decrease in attachment efficiency when dissociated and replated at six hours after radiation. However, the attachment efficiency in both cell types was similar when a 72-hour postirradiation incubation period was used prior to dissociation of the cells. Growth inhibition was significantly greater (7- to 10-fold) in endothelial cells than in myocytes when examined four days after attachment. Confluent endothelial monolayers showed a dose-dependent, progressive cell loss during the 72-hour postirradiation period (70% after 1,000 rad); the myocyte cultures showed no radiation effect on the cell numbers. In spite of the reduction in number, the endothelial cells maintained the continuity of their monolayer by compensation with an increase in mean cell size. Endothelial cells developed multiple structural lesions, including an increase in the number and size of residual and lysosomal bodies, electron-lucent cytoplasmic defects, interruptions in the plasma membrane and irregular aggregation of chromatin, causing electron-lucent nuclei. These changes increased in severity with time and dose and were most pronounced 24 to 72 hours after 1,000 rad. No significant ultrastructural alterations were detected in myocytes four days after 2,000 rad.

Anal Quant Cytol 1982 Sep;4(3):188-198

Resonant formation of DNA strand breaks by low-energy (3 to 20 eV) electrons.

Most of the energy deposited in cells by ionizing radiation is channeled into the production of abundant free secondary electrons with ballistic energies between 1 and 20 electron volts. Here it is shown that reactions of such electrons, even at energies well below ionization thresholds, induce substantial yields of single- and double-strand breaks in DNA, which are caused by rapid decays of transient molecular resonances localized on the DNA’s basic components. This finding presents a fundamental challenge to the traditional notion that genotoxic damage by secondary electrons can only occur at energies above the onset of ionization, or upon solvation when they become a slowly reacting chemical species.

Science 2000 Mar 3;287(5458):1658-1660

Establishment of a radiation- and estrogen-induced breast cancer model.

It is well accepted that cancer arises in a multistep fashion in which exposure to environmental carcinogens is a major etiological factor. The aim of this work was to establish an experimental breast cancer model in order to understand the mechanism of neoplastic transformation induced by high LET radiation in the presence of 17beta-estradiol (E). Immortalized human breast cells (MCF-10F) were exposed to low doses of high LET alpha particles (150 keV/microm) and subsequently cultured in the presence or absence of E for periods of up to 10 months post-irradiation. MCF-10F cells irradiated with either a single 60 cGy dose or 60/60 cGy doses of alpha particles showed gradual phenotypic changes including altered morphology, increase in cell proliferation relative to the control, anchorage-independent growth and invasive capability before becoming tumorigenic in nude mice. In alpha particle-irradiated cells and in those cells subsequently cultured in the presence of E, increased BRCA1, BRCA2 and RAD51 expression were detected by immunofluorescence staining and quantified by confocal microscopy. These studies showed that high LET radiation such as that emitted by radon progeny, in the presence of estrogen, induced a cascade of events indicative of cell transformation and tumorigenicity in human breast epithelial cells.

Carcinogenesis 2000 Apr;21(4):769-776

Induction of a bystander mutagenic effect of alpha particles in mammalian cells.

Ever since the discovery of X-rays was made by Rontgen more than a hundred years ago, it has always been accepted that the deleterious effects of ionizing radiation such as mutation and carcinogenesis are attributable mainly to direct damage to DNA. Although evidence based on microdosimetric estimation in support of a bystander effect appears to be consistent, direct proof of such extranuclear/extracellular effects are limited. Using a precision charged particle microbeam, we show here that irradiation of 20% of randomly selected A(L) cells with 20 alpha particles each results in a mutant fraction that is 3-fold higher than expected, assuming no bystander modulation effect. Furthermore, analysis by multiplex PCR shows that the types of mutants induced are significantly different from those of spontaneous origin. Pretreatment of cells with the radical scavenger DMSO had no effect on the mutagenic incidence. In contrast, cells pretreated with a 40 microM dose of lindane, which inhibits cell-cell communication, significantly decreased the mutant yield. The doses of DMSO and lindane used in these experiments are nontoxic and nonmutagenic. We further examined the mutagenic yield when 5-10% of randomly selected cells were irradiated with 20 alpha particles each. Results showed, likewise, a higher mutant yield than expected assuming no bystander effects. Our studies provide clear evidence that irradiated cells can induce a bystander mutagenic response in neighboring cells not directly traversed by alpha particles and that cell-cell communication process play a critical role in mediating the bystander phenomenon.

Proc Natl Acad Sci U S A 2000 Feb 29;97(5):2099-2104

Clustered DNA damages induced in isolated DNA and in human cells by low doses of ionizing radiation.

Clustered DNA damages-two or more closely spaced damages (strand breaks, abasic sites, or oxidized bases) on opposing strands-are suspects as critical lesions producing lethal and mutagenic effects of ionizing radiation. However, as a result of the lack of methods for measuring damage clusters induced by ionizing radiation in genomic DNA, neither the frequencies of their production by physiological doses of radiation, nor their repairability, nor their biological effects are known. On the basis of methods that we developed for quantitating damages in large DNAs, we have devised and validated a way of measuring ionizing radiation-induced clustered lesions in genomic DNA, including DNA from human cells. DNA is treated with an endonuclease that induces a single-strand cleavage at an oxidized base or abasic site. If there are two closely spaced damages on opposing strands, such cleavage will reduce the size of the DNA on a nondenaturing gel. We show that ionizing radiation does induce clustered DNA damages containing abasic sites, oxidized purines, or oxidized pyrimidines. Further, the frequency of each of these cluster classes is comparable to that of frank double-strand breaks; among all complex damages induced by ionizing radiation, double-strand breaks are only about 20%, with other clustered damage constituting some 80%. We also show that even low doses (0.1-1 Gy) of high linear energy transfer ionizing radiation induce clustered damages in human cells.

Proc Natl Acad Sci U S A 2000 Jan 4;97(1):103-108

Induction of a senescence-like phenotype in bovine aortic endothelial cells by ionizing radiation.

Treatment of confluent monolayers of bovine aortic endothelial cells (BAEC) with gamma rays resulted in the delayed appearance of cells with an enlarged surface area that were morphologically similar to senescent cells. The majority of these cells stained positively for senescence-associated beta-galactosidase (SA-beta-gal), indicating that these cells are biochemically similar to senescent cells. The incidence of the senescence-like phenotype increased with dose (5-15 Gy) and time after irradiation. Cells with a senescence-like phenotype began to appear in the monolayer several days after irradiation. The onset of the appearance of this phenotype was accelerated by subculturing 24 h after irradiation. This acceleration was not entirely due to stimulation of progression through the cell cycle, since a high percentage of the senescent-like cells that appeared after subculture were not labeled with BrdUrd during the period after subculture. Prolonged up-regulation of expression of CDKN1A (also known as p21(CIP1/WAF1)) after irradiation was noted by Western blot analysis, again suggesting a similarity to natural senescence. Phenotypically altered endothelial cells were present in the irradiated monolayers as long as 20 weeks after irradiation, suggesting that a subpopulation of altered endothelial cells that might be functionally deficient could persist in the vasculature of irradiated tissue for a prolonged period after irradiation.

Radiat Res 2001 Sep;156(3):232-240

Ionizing radiation accelerates aortic lesion formation in fat-fed mice via SOD-inhibitable processes.

Ionizing radiation promotes formation of reactive oxygen species, including the superoxide anion (O2-). To evaluate whether O2- or O2—mediated perturbations may contribute to the known atherogenic effects of radiation, we examined aortic lesion formation in irradiated C57BL/6 mice and evaluated the effects of CuZn-superoxide dismutase (CuZn-SOD) overexpression. Ten-week-old mice were exposed to a 2-, 4-, or 8-Gy dose of 250-keV x-rays to the upper thorax and then placed on a high-fat diet for 18 weeks. Based on quantitative lipid staining of serial sections of the proximal aorta, mean lesion area was increased with increasing radiation dose and was 3-fold greater in 8-Gy-irradiated than sham-irradiated mice (7800+/-2140 versus 2635+/-709 micrometer(2), P<0.05). These effects were absolutely dependent on a high-fat diet, which had to be introduced within 1 to 2 weeks of the radiation exposure, suggesting the early involvement of atherogenic lipoproteins that were elevated in response to the diet. The importance of radiation-induced oxidative stress was supported by the observation of a 2-fold lower mean lesion area in irradiated CuZn-SOD transgenic mice than in their irradiated, nontransgenic littermates (3026+/-1590 versus 6102+/-1834 micrometer(2), P<0.05). Lucigenin-enhanced chemiluminescence, used as an index of aortic O2- concentrations, was significantly elevated in the postradiation period, and this response was reduced in CuZn-SOD transgenics. On the basis of these results, we propose that radiation may be a useful tool for initiating oxidative or redox-regulated events that promote atherogenesis and for testing the antiatherogenic properties of antioxidants.

Arterioscler Thromb Vasc Biol 1999 Jun;19(6):1387-1392


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