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E Dikomey

Publications and source records attributed to E Dikomey.

17 recordsLinked to original sources
PubMed1992

Effect of heat on induction and repair of DNA strand breaks in X-irradiated CHO cells.

Chinese hamster ovary cells were exposed to various heat treatments followed by X-irradiation, and the induction and repair of DNA strand breaks was studied using the alkaline unwinding technique. Heat treatments alone were found to cause DNA strand breakage only for temperatures greater than or equal to 43 degrees C, whereas the number of radiation-induced strand breaks was unaffected by additional heating. Strand break repair was studied for irradiated cells preheated at temperatures ranging from 42 degrees C to 45 degrees C. The total repair curve could be separated into three phases, a fast (t = 0-15 min), an intermediate (t = 15-120 min) and a slow (t greater than or equal to 120 min) phase. All phases were altered when cells were heated either prior to or after irradiation. The fast and the intermediate phase could be well interpreted by the assumption that irradiation leads to both primary and secondary single-strand breaks, the latter being generated by enzymatic incision at sites of damaged bases. For irradiation alone, the ratio of all secondary strand breaks to all primary breaks was fsec = 1.5 +/- 0.5. This ratio was not altered by preceding heat treatments (mean fsec = 1.7 +/- 0.2). The main effect of heating on the repair kinetics of single-strand breaks was an increase in the repair half-time of primary and secondary breaks (maximum increase by a factor of 3.4), whereas the generation of secondary breaks was only slightly retarded (factor 1.3). The slow repair phase, which is assumed to represent the repair of DNA double-strand breaks, was best described by a single exponential component. The half-time of this component was found to increase from tau slow = 170 +/- 70 min for non-heated cells to tau slow = 345 +/- 80 min for cells heated at 45 degrees C for 20 min, indicating that heat inhibited the repair of double-strand breaks. For irradiation alone, the initial fraction of the slow component was fslow = 0.065 +/- 0.004. This fraction was enhanced by additional heating, with a maximum increase by a factor of 2.7 for cells heated at 45 degrees C for 20 min. This elevation cannot be the result of an enhanced induction of double-strand breaks, but must be associated with an additional formation of slowly repaired strand breaks during repair incubation. These additional strand breaks must arise from strand breaks which in non-heated cells are repaired during the fast or intermediate phase.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Effect of thermotolerance and step-down heating on thermal radiosensitization in CHO cells.

Chinese hamster ovary cells were exposed to single or fractionated heat treatments followed by irradiation on ice with graded doses of X-rays. The dose-response curves obtained were fitted by the linear-quadratic equation -ln(S/S0) = alpha D + beta D2 and analysed in terms of TER10%, alpha and beta. Thermal enhancement ratio, TER10%, was reduced when heat sensitivity was lowered either by chronic (pretreatment 40 degrees C, 16 h) or acute (43 degrees C, 45 min-37 degrees C, 10 h) thermotolerance, but was enhanced after step-down heating (43-40 degrees C or 45-40 degrees C). It could be shown that thermal radiosensitization, as expressed by TER10%, is modified by thermotolerance or step-down heating only to the extent to which cellular survival is modified by the corresponding pretreatments. However, the relative change of alpha and beta was found to be different for thermotolerance and step-down heating. For thermotolerant cells the values for alpha and beta were reduced by about the same factor, whereas step-down heating caused an increase in both parameters, which was greater for alpha than for beta. Data analysis showed that the modification of thermal radiosensitization by thermotolerance can be interpreted as if the cells were heated at the given temperature for a shorter time, whereas after step-down heating the cells responded as if they were exposed to a higher temperature prior to irradiation.

Animals↗
PubMed1991

Thermal radiosensitization in CHO cells by prior heating at 41-46 degrees C.

CHO cells were exposed to heat at temperatures ranging from 41 degrees C to 46 degrees C followed by irradiation on ice with graded doses of X-rays. The dose-response curves obtained were analysed in terms of D10%, D0 and Dq and thermal enhancement was expressed by the corresponding values TER10%, TEF and TEQ, respectively. TER10% and TEF were shown to increase linearly with heating time, the increase being steeper at higher temperatures. The dose-response curves were also analysed using the equation -ln(S/S0) = alpha D + beta D2; the values of alpha and beta obtained from curve-fitting were found to increase with heating time. For temperatures below 43 degrees C the relative increase in alpha was greater than that in beta; the Arrhenius activation energies were Ea = 890 kJ mol-1 for alpha and Ea = 1830 kJ mol-1 for beta. At temperatures exceeding 43 degrees C the relative increase of alpha and beta was similar and the corresponding activation energies were about the same (Ea approximately 700 kJ mol-1). The increase in the alpha-term was attributed to a depressed repair of double-strand breaks, whereas the increase of beta was assumed to be a consequence of an insufficient repair of base damage.

Animals↗
PubMed1990

Induction and repair of DNA strand breaks in X-irradiated proliferating and quiescent CHO cells.

Induction and repair of DNA strand breaks was studied in X-irradiated proliferating and quiescent CHO cells using the alkaline unwinding technique. The results showed that induction of strand breaks is identical for both states of proliferation, whereas repair is different. The decrease in the number of DNA strand breaks with incubation time at 37 degrees C is best described by a sum of three exponential components I, II and III. The half-times of component I were similar (tau I,p = 1.73 min versus tau I,q = 1.66 min) whereas strand breaks comprising component II were repaired slightly faster (tau II,p = 17.0 min versus tau II,q = 14.2 min) and those comprising component III were repaired significantly faster (tau III,p = 218 min versus tau III,q = 113 min) in quiescent as compared with proliferating cells. In contrast, the initial fractions, f, of the three components were closely similar for both states of growth (proliferating cells: fI = 0.69, fII = 0.25, fIII = 0.06; quiescent cells: fI = 0.65, fII = 0.29, fIII = 0.06). Radiosensitivity as assayed by colony formation was found to be lower for quiescent cells than for proliferating cells. By fitting the survival data to the linear-quadratic equation, -ln(S/S0) = alpha D + beta D2, the ratios alpha p/alpha q = 1.7 and beta p/beta q = 1.2 were obtained, which are similar to the ratios of the half-times of components III and II (tau III,p/tau III,q = 1.9; tau II,p/tau II,q = 1.2). This result indicates that the DNA damage represented by components II and III might be involved in cell killing.

Animals↗
PubMed1988

Correlation between polymerase beta activity and thermal radiosensitization in Chinese hamster ovary cells.

The enhancement of the radiosensitivity of CHO cells was determined for various single and multiple heat treatments followed by X-irradiation at 0 degrees C to prevent repair during exposure. Furthermore, the reduction in the activity of DNA polymerase beta was determined after the same heat treatments. The thermal enhancement ratios for cell killing, calculated on the 10% survival level for the various experimental conditions, showed a clear-cut correlation with the activity of DNA polymerase beta measured by the end of the various heat treatments. These results indicate that the thermal radiosensitization observed in many biological systems might be associated with the transient loss of polymerase beta activity.

Animals↗
PubMed1988

DNA denaturation kinetics in CHO cells exposed to different X-ray doses and after different repair intervals using the alkaline unwinding technique.

The kinetics of DNA denaturation in alkaline solution (pH 12.2) was studied in CHO cells using the alkaline unwinding technique. After X-ray doses of 0, 3, 5 and 9 Gy, the kinetics of alkaline denaturation was found to be independent of the number of induced strand breaks confirming earlier studies on this subject. In addition, the denaturation kinetics measured in cells exposed to 9 Gy were found to be identical for different repair intervals. This result shows that for the three different classes of DNA strand breaks described previously (Dikomey and Franzke 1986a) strand separation in alkaline solution occurs at the same kinetics. As a consequence, the relationship between the numbers of strand breaks and the fraction of remaining double-stranded DNA is considered the same for the three different classes.

Animals↗
PubMed1988

Effect of dose rate on cell killing and DNA strand break repair in CHO cells exposed to internal beta-rays from incorporated [3H]thymidine.

Survival as well as repair of DNA strand breaks were studied in CHO cells after exposure to internal beta-rays from incorporated [3H]thymidine at 4 degrees C (equivalent to an exposure at 'infinitely high' dose rate) and at 37 degrees C (low dose rate). DNA strand breaks were determined by the alkaline unwinding technique. In cells exposed at 4 degrees C cell killing was five times higher (Do = 250 decays per cell) than in cells exposed at 37 degrees C (Do = 1280 decays per cell). Strand breaks induced by 3H decay at 37 degrees C were repaired with the same kinetics as those generated at 4 degrees C. Therefore the different degrees of cell killing at 4 degrees C and 37 degrees C cannot be attributed to a difference in the repair kinetics for DNA strand breaks.

Animals↗
PubMed1987

Reduction of DNA-polymerase beta activity of CHO cells by single and combined heat treatments.

The effect of single and combined heat treatments on the activity of DNA polymerase beta was studied in CHO cells. The activity of polymerase beta was determined by measuring the amount of [3H]TTP incorporated into activated calf thymus DNA in the presence of aphidicolin, a specific inhibitor of DNA polymerase alpha. Biphasic response curves were obtained for all temperatures tested (40-46 degrees C) showing the sensitivity to decrease during heating. A constant activation energy of Ea = 120 +/- 10 kcal/mole was found for the initial heat sensitivity, whereas the Arrhenius plot for the final sensitivity is characterized by an inflection point at 43 degrees C with Ea = 360 +/- 40 kcal/mole or Ea = 130 +/- 20 kcal/mole for temperatures below or above 43 degrees C, respectively. The observed decrease of the polymerase activity is not due to a decrease in the number of active enzyme molecules but to a change in its affinity, since the inhibition is reversible when increasing concentrations of TTP are applied. When acute or chronic thermo-tolerance was induced by a priming heat treatment at 43 degrees C for 45 min followed by a time interval at 37 degrees C for 16 h or by a preincubation at 40 degrees C for 16 h, respectively, the thermal sensitivity of polymerase beta was lowered by a factor of up to 5. By contrast, pretreatment at a higher temperature followed by a lower temperature (step-down heating) did not alter the sensitivity of polymerase beta to the second treatment. The results indicate that heat-induced cell death cannot be the consequence of the reduction of the polymerase beta activity, confirming earlier studies on this subject.

Animals↗
PubMed1986

DNA repair kinetics after exposure to X-irradiation and to internal beta-rays in CHO cells.

DNA strand breaks induced by X- or internal beta-rays were measured using the alkaline unwinding technique. For either type of radiation, repair kinetics were found to be best described by three exponential components, the half-times of which are 2 min, 17 min and 200 min, respectively. These values are the same for X- and internal, beta-irradiation but the initial fractions of the components are different.

Animals↗
PubMed1986

Three classes of DNA strand breaks induced by X-irradiation and internal beta-rays.

Repair kinetics of DNA strand breaks were investigated after exposing exponentially growing CHO cells to X-radiation or to internal beta-rays from incorporated tritium, respectively. DNA strand breaks were analysed by the alkaline unwinding technique followed by chromatography on hydroxyapatite. For either type of radiation, the repair kinetics are statistically best described by a sum of three exponential components. The half-times determined are tau I approximately 2 min, tau II approximately 20 min and tau III approximately 170 min; they are identical for both types of radiation. But the initial fractions of the components are different for X- and internal beta-rays; X-rays; fI = 0.70, fII = 0.25, fIII = 0.05; internal beta-rays: fI = 0.40, fII = 0.40, fIII = 0.20. Components I and II are considered to represent the repair of two different classes of single-strand breaks and component III the repair of double-strand breaks. Two alternative interpretations for the occurrence of the two classes of single-strand breaks are discussed.

Beta Particles↗
PubMed1984

Thermotolerance and thermosensitization in CHO and R1H cells: a comparative study.

In CHO and R1H cells thermotolerance was induced by a pre-incubation at 40 degrees C, by an acute heat shock at 43 degrees C followed by a time interval at 37 degrees C, and during continuous heating at 42 degrees C. Thermotolerance, which was tested at 43 degrees C, primarily causes an increase in D0 of the heat-response curve. The degree of maximum thermotolerance was found to be generally more pronounced in CHO than in R1H cells, but the time interval at 37 degrees C, as well as at 40 degrees C, to reach this maximum level was the same in both cell lines. CHO and R1H cells could be sensitized to 40 degrees C by a pre-treatment at 43 degrees C. When compared for the same survival rate after pre-treatment at 43 degrees C alone the degree of thermosensitization was about the same in both cell lines. In either cell line thermosensitization was found to be suppressed when cells were made thermotolerant by a previous incubation at 40 degrees C for 16 hours.

Acclimatization↗
PubMed1983

Induction and repair of DNA strand breaks in CHO-cells irradiated in various phases of the cycle.

Induction and repair of DNA strand breaks was investigated in synchronized CHO cells by using the alkaline DNA-unwinding technique followed by chromatography on hydroxyapatite. The velocity of DNA denaturation in an alkaline solution (pH 12.1) was found to be independent of the position in the cycle. In unirradiated cells the number of DNA strand breaks per 3.6 x 10(12) dalton was found to vary with age in cell cycle amounting to about 250 breaks per cell for G1, 400 for S, and 500 for G2 + M cells. The number of induced DNA strand breaks per unit dose was found to be the same in all phases; an average of 340 strand breaks were induced per Gy and per 3.6 x 10(12) dalton. Within the first hour after irradiation about 90 per cent of all radiation-induced strand breaks are rejoined.

Animals↗
PubMed1982

Effect of hyperthermia at 42 and 45 degrees C on repair of radiation-induced DNA strand breaks in CHO cells.

The effect of hyperthermia on DNA strand break repair was studied in CHO cells. DNA strand breaks were analysed by the alkaline DNA-unwinding technique followed by chromatography on hydroxyapatite. Immediately after irradiation with doses ranging from 2 to 7 Gy, cells were exposed to 42 or 45 degrees C. Heat alone was found to induce DNA strand breaks only at temperatures exceeding 45 degrees C. In comparison to 37 degrees, C, the rate of single-strand break repair was increased by hyperthermia at 42 degrees C, but decreased at 45 degrees C. In contrast hyperthermia at either temperature resulted in a higher number of remaining double-strand breaks 1 hour after irradiation. For the three treatments applied, i.e. X-rays alone or combined with hyperthermia at 42 or 45 degrees C, the relation between cell survival and the number of double-strand breaks measured 1 hour after irradiation could be described by the same function.

Animals↗
PubMedDate not supplied

Thermotolerance kinetics and growth rate changes in the R1H tumour heated at 43 degrees C.

R1H rhabdomyosarcomas implanted into the foot of the right hind leg of female WAG/Rij rats were exposed to fractionated hyperthermia at 43 degrees C and the kinetics of thermotolerance and heat-induced growth rate changes were studied. Tumours of anaesthetized animals were exposed to heat by immersing the leg up to the thigh in a water bath. Tumour growth delay (TGD) and tumour volume doubling time were calculated from individual growth curves. After single heating, TGD increased with increasing heating time, the increase being linear for heating times exceeding 60 min. Thermotolerance was induced by a priming heat treatment at 43 degrees C for 60 min and the kinetics of development and decay was studied for fractionation intervals ranging from 4 to 144 h. After 4 h the thermal sensitivity of the tumours was enhanced by about 30 per cent, probably due to the sensitizing effect of heat-induced physiological alterations in the tumour tissue such as suboptimal environmental conditions caused by depressed blood flow. For longer time intervals thermotolerance developed and reached a maximum at 24 h where the thermotolerance ratio was 4.5 +/- 1.5. From 24 to 144 h thermotolerance decayed exponentially with a half-time of 28 +/- 8 h. Heat also affected the growth rate of the treated tumours. After single heat treatments at 43 degrees C for 15-60 min the tumours grew faster than untreated control tumours. This change was statistically significant. After prolonged single heating, growth rate was found to be reduced. Tumour volume doubling time was not detectably changed after fractionated heat treatments.

Animals↗
PubMedDate not supplied

Effect of pH on development and decay of thermotolerance in CHO cells using fractionated heating at 43 degrees C.

The development and decay of thermotolerance at pH 6.7, 7.1 and 7.7 was studied after fractionated hyperthermia at 43 degrees C using exponentially growing CHO cells. The maximum of thermotolerance and the time interval to reach this maximum were found to correlate with the survival decrement after the priming heat treatment. Both parameters were only affected by pH in so far as the pH altered survival after the priming treatment. Decay of thermotolerance was exponential. For a given priming heat treatment for the time t1, the half-time of decay, tau 1/2, increased linearly with increasing cell doubling time, tau d, measured for non-heated cells growing at different pH. On the other hand, for a given cell doubling time, tau d, the half-time, tau 1/2, increased exponentially with increasing duration of the priming heat treatment, t1. For all measured data the half-time of thermotolerance decay could be described by the equation tau 1/2 = alpha. tau d.exp(k.t1), with k = 2.2 +/- 0.2 h-1 and alpha = 0.094 +/- 0.009 for all pretreatments applied and all pH conditions tested. This relationship might indicate that the decay of thermotolerance is governed by a single mechanism.

Animals↗
PubMedDate not supplied

Effect of chronic thermotolerance on thermosensitization in Chinese hamster ovary cells studied at various temperatures.

The effect of chronic thermotolerance on the thermal responses of Chinese hamster ovary (CHO) cells to single and step-down heating was studied. Thermotolerance was induced by pre-heating exponentially growing cells at 39 degrees C for 9 h, followed by test treatments for variable times at temperatures ranging from 39 to 43 degrees C. In the temperature range studied, the heat sensitivity of thermotolerant CHO cells was characterized by an Arrhenius activation energy of Ea = 1175 +/- 40 kJ/mol. This value agreed well with Ea = 1180 +/- 45 kJ/mol measured after single heating, indicating that the induction of chronic thermotolerance did not affect the activation energy for cell killing by heat. Thermosensitization was studied after a priming treatment at 43 degrees C for 50 min followed by step-down heating at temperatures ranging from 39 to 43 degrees C. The temperature dependence of the thermal response after step-down heating was characterized by an activation energy of Ea = 490 +/- 17 kJ/mol. When the cells were pre-treated for 1-16 h at 39 degrees C prior to step-down heating (43 degrees C, 50 min, followed by graded exposure to 39-43 degrees C), the activation energy was gradually enhanced and approached Ea = 825 +/- 42 kJ/mol for 39 degrees C, 16 h. This change in Ea reflects the effect of thermotolerance on the priming treatment at 43 degrees C for 50 min, whereas the effect on the final test treatment resulted in a parallel shift of the Arrhenius curve without changing the slope, indicating that the effect of thermotolerance on the priming and the test treatment is expressed in the Arrhenius diagram in different ways.

Animals↗