Radioprotection of mammalian cells by procaine

Inhibition of radiation-induced changes of glyoxalase I activity in mouse spleen and liver by phenothiazines

Swiss albino mice (male) were irradiated with gamma-rays at a dose-rate of 0.05 Gy s-1, and the activities of glyoxalase I (GI) and glyoxalase II (GII) were determined after 24 h in the spleen and liver. Radiation up to 4 Gy increased the activity of GI and decreased that of GII. It was possible that the radiation-induced changes in the activity of the glyoxalase system, particularly that of GI, were suggestive of the regeneration status of the tissue. Phenothiazines such as chlorpromazine (CPZ), promethazine (PMZ) and trimeprazine (TMZ) inhibited the radiation-enhanced activity of GI in a concentration-dependent manner. On the other hand, almost no change in the activity of GII was observed using phenothiazines. The effect of phenothiazines on radiation-induced changes of glyoxalase activity were reversed in the presence of ferrous (Fe2+) ions. However, phenothiazines inhibited the radiation effect in the presence of ferric (Fe3+) ions. This combined effect was predominant in the liver. A possible mechanism for the modifying effect of phenothiazines is suggested.

Effects of calmodulin antagonists on radiation-induced lipid peroxidation in microsomes

Rat liver microsomes were irradiated with gamma-rays at a dose rate of 1.31 Gys-1. The extent of lipid peroxidation, measured in terms of malondialdehyde (MDA) formed, increased with radiation dose. The presence of calmodulin antagonists during irradiation decreased lipid peroxidation. The order of their protective efficiency was: chlorpromazine (CPZ) greater than promethazine (PMZ) greater than trimeprazine (TMZ). Their protective effect was diminished in the presence of ferrous (Fe2+) ions and was restored on addition of EDTA. However, calmodulin antagonists considerably inhibited radiation-induced lipid peroxidation in the presence of ferric (Fe3+) ions. Calmodulin antagonists also decreased the cytochrome P-450 content of microsomes. These results are discussed with respect to their applicability to radiotherapy. A possible mechanism for the inhibition of radiation-induced lipid peroxidation is suggested.

Modification of effects of radiation on thymidine kinase

Thymidine kinase (TdR-K) and the incorporation of iododeoxyuridine (IUdR) into DNA of murine bone marrow cells are acutely and temporarily inhibited by low doses (0.01 Gy) of whole-body gamma-radiation with a maximal effect at 4 h after exposure and full recovery at 10 h. The inhibitory effect was totally abolished by whole-body exposure to a strong static magnetic field of 1.4T. The present investigation was designed to gain insight into the mechanism(s) underlying the inhibition of TdR-K activity and the incorporation of 125I-UdR by challenging the system with various pharmacological and biochemical means. To this end the response of TdR-K and 125I-UdR incorporation into DNA to the administration of actinomycin-D, cycloheximide, cysteamine, misonidazole and procaine hydrochloride as well as to dietary manipulations, i.e. vitamin E deficiency and enrichment of the diet by soya oil, and to changes in the glutathione levels were investigated in bone marrow cells of irradiated and sham-irradiated mice. Furthermore, the effect of various NaHCO3 concentrations on optimizing the radiation-induced inhibition of TdR-K was investigated under conditions of radiation, vitamin E deficiency and enrichment of the diet by soya oil. The data point to the involvement of the cellular radical-detoxification system in changing the activity of TdR-K, especially on the basis of the concurrent increase of glutathione concentration and decrease in TdR-K activity.

Radiobiologic response of CHO-KI cells treated with vitamin A

Treatment of CHO-KI cells with vitamin A altered their response to subsequent gamma irradiation. In general longterm preincubation with low doses of the vitamin caused a relative increase in the number of cells surviving a given radiation dose. The effect resulted in an increase in the D0 of the survival curve. Long or short term exposure to high concentrations of the vitamin caused a decrease in the number of surviving cells leading to a decrease in the extrapolation number of the survival curve. Recovery of cells from radiation damage, assessed using the split dose technique, was also impaired by vitamin A pretreatment. A mechanism involving repair of potentially lethal damage may explain the protective effect of vitamin A since this was highly dependent on the cell density of cultures at the time of irradiation. However, in view of the data showing that the vitamin A concentrations necessary to alter the radiation survival curve shoulder caused a significant release of sialic acid into the medium, a mechanism involving membrane stability may account for both the reduction in repair/recovery capacity of the treated cells and the radioprotective effect.

Membrane effects of ionizing radiation and hyperthermia

Results of numerous studies demonstrate that membranes are important sites of cell damage by both ionizing radiation and hyperthermia. Modification of membrane properties (mainly lipid fluidity) affects the cellular responses to radiation and hyperthermia but former concepts that membrane rigidification sensitizes cells to radiation while membrane fluidization potentiates hyperthermic damage have now been seriously challenged. It seems that the effects of membrane fluidity on cell responses to hyperthermia and radiation are due to an indirect influence on functional membrane proteins. The major role of lipid peroxidation in radiation damage to membranes has also been questioned. The existing evidence makes it unlikely that the interaction between radiation and hyperthermia is determined by the action of both agents on the same membrane components.

Non-nitro radiation sensitizers

A review of the literature on radiosensitization reveals that at least some of the non-nitro compounds, such as metabolic inhibitors and membrane-active drugs, could be considered as potentially valuable radiosensitizers for possible use in future cancer radiotherapy.

Radioprotection of euoxic bacteria by phenothiazine drugs

Survival studies on irradiated euoxic E. coli B/r cells in presence of various concentrations of four radioprotecting phenothiazine drugs have been carried out. Maximum radioprotection was obtained at a optimal concentration for each drug and it decreased on either side of it. The DNA strand break studies at the maximum protective and non-protective concentrations of chlorpromazine and promethazine revealed that the number of ssbs in DNA were less at the protective concentration which were efficiently repaired by the type-III repair process. On the other hand, at the non-protective concentrations, inhibition of DNA repair was noticed and a higher number of DNA ssbs were detected. We suggest that the membrane is fluidized to a greater extent at the protective concentrations allowing the chemical restitution of damaged sites by NPSH compounds. At the non-protective high concentrations of the drugs, the membrane may be too grossly disorganised to allow any repair and at the same time high concentrations of the drugs or their radicals may also react with radioprotective intracellular sulphhydryls.

Modification of radiation response of E. coli B/r cells by phenothiazines

Promethazine and trimeprazine sensitized anoxic E. coli B/r cells to 60Co gamma-rays, but both drugs showed a radioprotective effect under euoxic conditions. Their radiosensitizing effect was found to be due to the reaction of radiolytically induced hydroxyl radicals with the sensitizers. The radioprotective effect of these drugs is attributed to changes in the membrane structure conducive with chemical repair of the damaged sites in the gel region of the cellular membrane by intracellular sulphydryl compounds. Pre-irradiation depletion of sulphydryls from E. coli B/r by treatment with N-ethyl maleimide abolished the radio-protective effect of these drugs under euoxic conditions.

Effect of membrane fatty acid changes on the radiation sensitivity of human lymphoid cells

To test the influence of changes in membrane fatty acid composition on the radiation response of mammalian cells, human LDV cells were cultured in medium containing delipidated serum supplemented with either oleic or linoleic acid. Analysis of lipid extracts of the cells by gas liquid chromatography showed that, after 3 or more days growth in oleic or linoleic acid supplemented media, there were substantial overall increases in the proportion of oleate and linoleate, respectively, in the cellular lipid. Smaller absolute changes were measured in nuclear phospholipid, which was found to be low in unsaturated phospholipid compared to the rest of the cell. Fluorescence polarization measurements using diphenylhexatriene indicated an increased membrane fluidity in cells grown in the presence of excess linoleic acid and to a lesser extent for oleic acid. The clonogenic capacity of the cells after irradiation in air or nitrogen was not altered by any of these membrane compositional changes. The lack of effect on radiation sensitivity, in contrast to that reported for bacteria (E. coli K1060), is consistent with the fact that little or no change was brought about in the nuclear membrane composition, since evidence from partial cell irradiation experiments indicates that the cell nucleus is the sensitive target for cell killing by ionizing radiation. The ability of the cell to maintain a low level of unsaturated phospholipids in its nuclear membrane may be an important general defence mechanism against free radical damage to chromatin mediated by lipid peroxidation.

Local anaesthetics as hypoxic radiosensitizers, oxic radioprotectors and potentiators of hyperthermic killing in mammalian cells

Local anaesthetics with well-known pharmacology are found to radiosensitize hypoxic and radioprotect oxic murine L5178Y cells. Under the specified experimental conditions, radiation-modifying effects exerted by local anaesthetics are dose-dependent. It appears that in order that the drugs should act as oxic radioprotectors or as hypoxic radio-sensitizers, the agent must be present during the period of irradiation. Local anaesthetics are also found to potentiate the hyperthermic killing of cells. The possibility of applying compounds with such dual radiation-modifying properties plus hyperthermia-potentiating capacity to enhance the therapeutic gains in tumour therapy is briefly discussed.

Radiation killing of E. coli K1060: role of membrane fluidity, hypothermia and local anaesthetics

The enhancement of killing by gamma-irradiation, which is seen when E. coli K1060 are cooled below the transition temperature of their membrane lipids, is blocked by procaine-HCl. These data are consistent with the hypothesis that increased killing associated with irradiation at 0 degree C is the result of membrane microviscosity increases, since procaine is known to fluidize membranes. A cooling enhancement ratio (c.e.r.) is defined as the ratio of radiation D0 at 22 degrees C to its value at 0 degree C. The c.e.r. for oxygen-bubbled cells is 1.5 and for nitrogen-bubbled cells is 2.1. In the presence of 25 mM procaine the respective c.e.r. values are 1.08 and 1.29. The oxygen enhancement ratio (o.e.r.) at 22 degree C is 3.43 and at 0 degree C is 2.45. The addition of procaine does not change the o.e.r. Thus, the temperature effect on o.e.r. does not appear to be related to membrane fluidity.