Accelerated heavy particles and the lens. III. Cataract enhancement by dose fractionation

For a number of biological end points it has been shown that, in contrast to low linear energy transfer (LET) radiation, dose fractionation of high-LET radiation does not result in a reduction in overall effectiveness. Studies were conducted to determine the effect of fractionating the exposures to heavy ion doses on the development of cataracts. Rat eyes were exposed to single doses of 1, 5, and 25 cGy of 570 MeV/amu40Ar ions and to 2, 4, and 10 Gy of 250 kVp X rays. These were compared to unirradiated controls and eyes which were exposed to the same total dose delivered in four fractions over 12 h. While in all cases fractionation of the exposure to X rays produced significant reduction in cataractogenic potential, fractionating doses of 40Ar ions caused a dose- and stage-dependent enhancement in the development of cataracts.     

On the question of RBE reversal at high doses.

We present theoretical arguments to explain observations of a "reversal" of the RBE at relatively large doses; that is, the RBE of high-LET vs low-LET radiation is less than one. Numerical examples are given and the results of Bogo et al. (Radiat. Res. 118, 341-352, 1989) are discussed qualitatively.     

Accelerated postirradiation recovery of hematopoietic marrow following priming with low doses of vincristine

The present investigation is a continuation of efforts to characterize the radioprotective potential of priming with vincristine (VcR). In this study, the postirradiation recovery kinetics of the marrow's hematopoietic stem cell, progenitor cell, and stromal cell compartments were monitored following exposure to a range of sublethal radiation doses to determine (a) the optimal VcR/radiation intertreatment interval for achieving maximal hematopoietic protection, (b) whether this optimal interval is influenced by the dose of radiation administered, and (c) whether the radioprotection observed involves the hematopoietic stroma. The results demonstrate that the degree of radioprotection observed was significantly influenced by the scheduling of the VcR priming dose with respect to the radiation exposure. An intertreatment interval of 24 h provided maximal radioprotective benefit irrespective of the radiation dose administered. Additionally, the radioprotection following VcR priming appeared to be more the result of an accelerated recovery in the hematopoietic stem cell and progenitor cell compartments than a change in their intrinsic radiosensitivity. The data also suggest that this accelerated recovery was not a consequence of greater radioprotection of marrow stroma. Finally, the radioprotection observed following VcR priming did not appear to involve a selective lineage response by either the erythroid or the granulomonocytic progenitor compartments.     

DNA damage responses at low radiation doses

Increased cell killing after exposure to low acute doses of X rays (0-0.5 Gy) has been demonstrated in cells of a number of human tumor cell lines. The mechanisms underlying this effect have been assumed to be related to a threshold dose above which DNA repair efficiency or fidelity increases. We have used cells of two radioresistant human tumor cell lines, one that shows increased sensitivity to low radiation doses (T98G) and one that does not (U373), to investigate the DNA damage response at low doses in detail and to establish whether there is a discontinuous dose response or threshold in activation of any important mediators of this response. In the two cell lines studied, we found a sensitive, linear dose response in early signaling and transduction pathways between doses of 0.1 and 2 Gy with no evidence of a threshold dose. We demonstrate that ATM-dependent signaling events to downstream targets including TP53, CHK1 and CHK2 occur after doses as low as 0.2 Gy and that these events promote an effective damage response. Using chemical inhibition of specific DNA repair enzymes, we show that inhibition of DNA-PK-dependent end joining has relatively little effect at low (<1 Gy) doses in hyper-radiosensitive cells and that at these doses the influence of RAD51-mediated repair events may increase, based on high levels of RAD51/BRCA2 repair foci. These data do not support a threshold model for activation of DNA repair in hyper-radiosensitive cells but do suggest that the balance of repair enzyme activity may change at low doses.     

Transformation of C3H 10T1/2 cells with 4.3 MeV alpha particles at low doses: effects of single and fractionated doses.

Oncogenic transformation of C3H 10T1/2 cells was determined after exposure to graded doses of 4.3-MeV alpha particles LET = 101 keV/microns. The source of alpha particles was 244Cm and the irradiation was done in an irradiation chamber built for the purpose. Graded doses in the range of 0.2 to 300 cGy were studied with special emphasis on the low-dose region, with as many as seven points in the interval up to 10 cGy. The dose-effect relationship was a complex function. Transformation frequency increased with dose up to 2 cGy; it seemed to flatten at doses between 2 and 20 cGy but increased again at higher doses. A total of 21 cGy was delivered in a single dose or in 3 or 10 equal fractions at an interval of 1.5 h. An inverse dose-protraction effect of 1.4 was found with both fractionation schemes. Measurements of the mitotic index of the population immediately before the various fractions revealed a strong effect on the rate of cell division even after very low doses of radiation. Mitotic yield decreased markedly with the total dose delivered, and it was as low as 50% of the control value after 4.2 cGy and 20% after 14 cGy with both fractionation schemes.     

Neutron dose effect relationships at low doses

Summary Stimulated by recent observations of non-linearity in the dose effect relationship for the transformation of mammalian cells in vitro by fission neutron irradiation and the reverse dose rate effect in this system, the data for mutation induction in the stamen hairs ofTradescantia occidentalis has been re-examined. The non-linear dose effect relationships suggested in the original reports are confirmed both by the dose effect relationships themselves and by an examination of the statistics of the results. This non-linearity also appears to be present in the more recent observations of other workers. It is suggested that the non-linearity in the system may be due to a sub-population of cells in a particularly sensitive phase of the cell cycle at the time of irradiation. There is a possibility that the data also indicate a qualitative difference in the underlying biophysical actions of neutron and photon radiations.Dedicated to Prof. W. Jacobi on the occasion of his 60th birthday     

Microbeams of heavy charged particles.

We have established a single cell irradiation system, which allows selected cells to be individually hit with defined number of heavy charged particles, using a collimated heavy-ion microbeam apparatus at JAERI-Takasaki. This system has been developed to study radiobiological processes in hit cells and bystander cells exposed to low dose and low dose-rate high-LET radiations, in ways that cannot be achieved using conventional broad-field exposures. Individual cultured cells grown in special dishes were irradiated in the atmosphere with a single or defined numbers of 18.3 MeV/amu 12C, 13.0 MeV/amu 20Ne, and 11.5 MeV/amu 40Ar ions. Targeting and irradiation of the cells were performed automatically at the on-line microscope of the microbeam apparatus according to the positional data of the target cells obtained at the off-line microscope before irradiation. The actual number of particle tracks that pass through cell nuclei was detected with prompt etching of the bottom of the cell dish made of ion track detector TNF-1 (modified CR-39), with alkaline-ethanol solution at 37 degrees C for 15-30 minutes. Using this system, separately inoculated Chinese hamster ovary cells, confluent normal human fibroblasts, and single plant cells (tobacco protoplasts) have been irradiated. These are the first studies in which single-ion direct hit effect and the bystander effect have been investigated using a high-LET heavy particle microbeam.     

Cellular signal adaptation with damage control at low doses versus the predominance of DNA damage at high doses

Ionizing radiation is known to potentially interfere with cellular functions at all levels of cell organization and induces DNA lesions apparently with an incidence linearly related to D, also at low doses. On the other hand, low doses have also been observed to initiate a slowly appearing temporary protection against causation and accumulation of DNA lesions, involving the radical detoxification system, DNA repair and removal of DNA damage. This protection apparently does not operate at high doses; it has been described to be nonlinear, increasing initially with D, beginning to decrease when D exceeds approximately 0.1-0.2 Gy, and eventually disappearing at higher D. The various adaptive responses have been shown to last individually from hours to weeks in different cell types and resemble responses to oxidative stress. Damage to DNA is continuously and endogenously produced mainly by reactive oxygen species (ROS) generated in a normal oxidative metabolism. This endogenous DNA damage quantitatively exceeds DNA damage from low-dose irradiation, by several orders of magnitude. Thus, the protective responses following acute low-dose irradiation may be presumed to mainly counteract the endogenous DNA damage. Accordingly, the model described here uses two dose-effect functions, a linear one for causing and a nonlinear one for protecting against DNA damage from whatever cause in the irradiated cells and tissues. The resulting net dose-risk function strongly suggests that the incidence of cancer versus dose in the irradiated tissues is much less likely to be linear than to exhibit a threshold. The observed cancer incidence may even fall below the spontaneous incidence, when D to cells is below approximately 0.2 Gy. However incomplete, these data support a reexamination of the LNT hypothesis.     

Accelerated heavy ions and the lens. IV. Biomicroscopic and cytopathological analyses of the lenses of mice irradiated with 600 MeV/amu 56Fe ions

The lenses of mice exposed to 600 MeV/amu iron ions were evaluated by slit-lamp biomicroscopy and cytopathological analyses. The doses ranged from 0.05 to 1.6 Gy, and the lenses were assessed at several intervals postirradiation. Cataract, the development of which is dependent on both time and dose, is significantly more advanced in all of the exposed mice when compared to the unirradiated controls. The great difference between the severity of the cataracts caused by 0.05 Gy (the lowest dose used) and those that developed spontaneously in the control animals is an indication that 0.05 Gy may far exceed the threshold dose for the production of cataracts by accelerated iron ions. Cytopathologically, a similar dose dependence was observed for a number of end points including micronucleation, interphase death, and meridional row disorganization. In addition the exposure to the 56Fe ions produced a long-term effect on the mitotic population and a pronounced "focal" loss of epithelial cytoarchitecture. The microscopic changes support the view that the mechanism of heavy-ion-induced cataractogenesis is the same as that for cataracts caused by low-LET radiation.     

Biophysical studies on subcellular particles VI. Photosynthetic activities in isolated spinach chloroplasts after transient warming

Isolated spinach chloroplasts were warmed for a period (typically5 min) at temperatures ranging from 15? to 55?C. Some generalfeatures of the after-effects of transient warming on photosyntheticactivities and some physico-chemical properties were surveyedunder various conditions. Fecy reduction was stimulated when chloroplasts were warmedat about 40?C and was inhibited at higher temperatures. Thetemperature for maximum stimulation shifted to a lower temperaturewith prolongation of warming time, depending on the pH of thewarmed suspension of chloroplasts, and was the same that atwhich photophosphorylation activity fell to zero. The decrease in photosynthetic activities is discussed in connectionwith possible modification of the membrane construction in thylakoidsand is ascribed to the loss of coupling efficiency. (Received August 5, 1972; )     

Flash photolysis electron spin resonance studies of the electron acceptor species at low temperatures in photosystem I of spinach subchloroplast particles.

The light-induced electron spin resonance signals of Photosystem I spinach subchloroplast particles have been studied at approximately 6 degrees K. Using the technique of flash photolysis-electron spin resonance with actinic illumination at 647 nm, a kinetic analysis of the previously observed bound ferredoxin ESR signals was carried out. Signal I (P700+) exhibits a partial light-reversible behavior at 6 degrees K so it was expected that if the bound ferredoxin is the primary acceptor of Photosystem I, it should also exhibit a partial reversible behavior. However, none of the bound ferredoxin ESR signals showed any such light reversible behavior. A search to wider fields revealed two components which did exhibit the expected kinetic behavior. These components are very broad (about 80 G) and are centered at g equals to 1.75 and g equals to 2.07. These two components exhibit the expected characteristics of the primary electron acceptor. A model is presented to account for the reversible and irreversible photochemical changes in Photosystem I. The possible identity of the primary acceptor responsible for these two new components, is discussed in terms of the available information. The primary acceptor may be an iron-sulfur protein, but not of the type characteristic of the bound or water-soluble ferredoxins found so far in chloroplasts.     

Biological effect of low doses of alkylating agents in drosophila studies

The low dose (0.05-0.1 mM) influence of alkylating agents on germ cell survival and male fertility, the level of embryonic and postembryonic lethality as well as the sex-linked recessive lethal (SLRL) frequency induced by high alkylating agent doses was studied in Drosophila melanogaster. The pretreatment of adult males with low doses of methyl and ethyl methanesulfonate (MMS and EMS) did not change or even enhanced EMS cytotoxicity and mutagenicity in both mature sperm and premeiotic cells. On the contrary, the low EMS dose pretreatment of larvae protected them against higher mutagen doses increasing male fertility, decreasing embryonic and postembryonic lethality in F1, and leading to three-fold reduction in the SLRL frequency in F2. The adaptive response was dependent on the Drosophila developmental stage exposed to challenge mutagen doses, since the protection was maximal in larvae and practically absent when the high dose was administered to adult males. The adaptive response observed does not seem to be associated with DNA repair, but it is rather due to other protective mechanisms.