Metal Ions Protect DNA Against Strand Breakage Induced by Fast Neutrons

In situ monitoring of the influence of water on DNA radiation damage by near-ambient pressure X-ray photoelectron spectroscopy

Ionizing radiation damage to DNA plays a fundamental role in cancer therapy. X-ray photoelectron-spectroscopy (XPS) allows simultaneous irradiation and damage monitoring. Although water radiolysis is essential for radiation damage, all previous XPS studies were performed in vacuum. Here we present near-ambient-pressure XPS experiments to directly measure DNA damage under water atmosphere. They permit in-situ monitoring of the effects of radicals on fully hydrated double-stranded DNA. The results allow us to distinguish direct damage, by photons and secondary low-energy electrons (LEE), from damage by hydroxyl radicals or hydration induced modifications of damage pathways. The exposure of dry DNA to x-rays leads to strand-breaks at the sugar-phosphate backbone, while deoxyribose and nucleobases are less affected. In contrast, a strong increase of DNA damage is observed in water, where OH-radicals are produced. In consequence, base damage and base release become predominant, even though the number of strand-breaks increases further.

Thermal Energy Electrons and OH-Radicals Induce Strand Breaks in DNA in an Aqueous Environment: Some Salts Offer Protection Against Strand Breaks

Electrons and ^•OH-radicals have been generated by using low-energy laser pulses of 6 ns duration (1064 nm wavelength) to create plasma in a suspension of plasmid DNA (pUC19) in water. Upon thermalization, these particles induce single and double strand breakages in DNA along with possible base oxidation/base degradation. The time-evolution of the ensuing structural modifications has been measured; damage to DNA is seen to occur within 30 s of laser irradiation. The time-evolution is also measured upon addition of physiologically relevant concentrations of salts containing monovalent, divalent, or trivalent alkali ions. It is shown that some alkali ions can significantly inhibit strand breakages while some do not. The inhibition is due to electrostatic shielding of DNA, but significantly, the extent of such shielding is seen to depend on how each alkali ion binds to DNA. Results of experiments on strand breakages induced by thermalized particles produced upon plasma-induced photolysis of water, and their inhibition, suggest implications beyond studies of DNA; they open new vistas for utilizing simple nanosecond lasers to explore the effect of ultralow energy radiation on living matter under physiologically relevant conditions.

Studies on the Protective Effects of Boerhaavia diffusa L. Against Gamma Radiation—Induced Damage in Mice

The radioprotective effect of the hydro-alcoholic extract of Boerhaavia diffusa was studied using the in vivo mice model. The sublethally irradiated mice (600 rads, single dose) were treated intraperitoneally with 20 mg/kg of the extract. The animals were sacrificed at different time periods after the whole-body radiation. The most affected tissues—bone marrow and intestine—were considerably protected by the intraperitoneal administration of B. diffusa as estimated by bone marrow cellularity, maturing monocytes, and intestinal glutathione. Total white blood cell count was lowered drastically after radiation exposure (ninth day, 1500 ± 500 cells/ mm3). When the animals were exposed to radiation and treated with B. diffusa, the total white blood cell count was lowered only to 4000 ± 400 cells/mm3 on the third day, and it reached an almost normal level (6250 ± 470 cells/mm 3) by the ninth day. The elevated level of serum and liver alkaline phosphatase after radiation exposure was reduced in the B. diffusa—treated group. The serum and liver glutamate pyruvate transferase, which were elevated after radiation exposure, were also reduced by treatment with B. diffusa compared to the control. The lipid peroxidation level also increased in the irradiated animals both in the liver and serum, but in B. diffusa —treated animals, there was a significant reduction in lipid peroxidation levels. The agarose gel electrophoresis of DNA isolated from bone marrow of mice exposed to gamma radiation showed heavy damage that was reduced by treatment with B. diffusa. These results are indicative of the radioprotective effect of the whole-plant extract of B. diffusa.

Effect of sodium and acetate ions on 8-hydroxyguanine formation in irradiated aqueous solutions of DNA and 2'-deoxyguanosine 5'-monophosphate

PURPOSE

The aim of this work was to study the combined effect of sodium and acetate ions on the radiation yield of 8-hydroxyguanine (8-OHG), one of the major DNA base lesions induced by free radicals.

MATERIALS AND METHODS

Aqueous solutions of DNA and 2'-deoxyguanosine 5'-monophosphate (dGMP) with various concentrations of sodium acetate and sodium perchlorate were γ-irradiated, enzymatically digested and analyzed by high-performance liquid chromatography (HPLC) methods.

RESULTS

It was found that both salts decrease the 8-OHG radiation yield in the concentration range studied for both DNA and dGMP, except in the case of dGMP wherein an increase in yield occurs in the concentration range from 0.1-1 mM. The dependence of the 8-hydroxy-2'-deoxyguanosine radiation yield on the concentration of both sodium acetate and sodium perchlorate have different shapes and have steeper slopes for the DNA compared with the dGMP solutions.

CONCLUSIONS

The observed decrease in the radiation yield of 8-OHG with increasing concentrations of sodium acetate is consistent with the hypothesis that sodium acetate produces two concentration-dependent effects in the DNA solutions: (1) A conformational change in the DNA caused by Na(+) counterions; and (2) free radical reactions related to the radiolysis of acetate ion.

Modulation of gamma ray-induced genotoxic effect by thymol, a monoterpene phenol derivative of cymene

The radioprotective effect of thymol (TOH), a monoterpene phenol, on radiation-induced DNA damage was analyzed in vitro. Chinese hamster lung fibroblast cells (V79) were treated with different concentrations of TOH (0-100 µg/mL) for 1 hour before exposure to 3 Gy gamma irradiation, and then cytokinesis-blocked micronucleus and single-cell gel electrophoresis (comet assay) assays were used to evaluate the radiation-induced cytogenetic damage and genotoxic effects. Furthermore, the modulating effect of TOH on radiation-induced cell death was assessed by apoptotic and necrotic cell detection by staining with ethidium bromide/acridine orange using fluorescence microscopy. To understand the mechanism of TOH-imparted cytoprotection, mitochondrial membrane potential (MMP) was detected by flow cytometry after staining the cells with Rhodamine 123. Pretreatment of V79 cells with various concentrations of TOH (0-100 µg/mL) for 1 hour reduced the radiation-induced micronuclei as well as percent tail DNA and mean Olive tail moment with a maximum protective effect observed at TOH (25 µg/mL). Apoptosis by microscopic, MMP measurements indicated that the V79 cells exposed to gamma radiation alone showed a maximal increase in the number of early and late apoptotic and necrotic cell death associated with a significant loss of the MMP. Pretreatment with TOH (25 µg/mL) showed a significant (P < .01) decrease in the level of apoptotic fraction as well as necrotic cells and suppressed the radiation-induced collapse of MMP when compared with the radiation alone group. These results suggest that TOH suppresses radiation-induced genotoxicity, apoptosis, and necrosis primarily by the free radical scavenging and modulation of oxidative stress.

Polyelectrolyte condensation in bulk, at surfaces, and under confinement

In this review we discuss recent results from computer simulations based on coarse-grained polyion models representing aqueous solutions of polyelectrolytes. The focus will be directed to the conformation of the polyions and, in particular, their condensation in bulk, induced by multivalent ions and oppositely charged polyelectrolytes, at responsive surfaces and under confinement.

Antioxidant potential of sesamol and its role on radiation-induced DNA damage in whole-body irradiated Swiss albino mice

Sesamol (SM) is a dietary phytochemical present in the processed sesame oil. In this present study we have evaluated the antioxidant potential of SM and its role in the protection of radiation-induced DNA damage in γ-irradiated mice. The antioxidant properties of SM were evaluated by using different in vitro antioxidant assays. SM shows scavenging effect against hydroxyl (OH), superoxide anion (O(2)(-)), nitric oxide, 2,2-azino-bis-3-ethylbenzothiazoline-6-sulphonic acid radical cation (ABTS(+)) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals. Our results demonstrate that SM exhibits strong antioxidant property in all the in vitro assays. When mice were exposed to 7 Gy γ-radiations there was an increase in % tail DNA, tail length, tail moment and Olive tail moment in blood lymphocytes. SM (100mg/kg b.wt) pretreatment significantly decreased the % tail DNA, tail length, tail moment and Olive tail moment in irradiated mice lymphocytes. These results suggest that SM protects γ-radiation-induced DNA damage in mice lymphocytes, which may be attributed to its antioxidant property.

Strand breaks induced in plasmid DNA by ultrasoft X-rays: Influence of hydration and packing

PURPOSE

To study the effect of hydration level and plasmid packing on strand break induction in DNA by ultrasoft X-ray.

MATERIALS AND METHODS

Bluescript (pBS, tight packing) and pSP189 (pSP, loose packing) plasmids were irradiated by 250, 380, and 760 eV ultrasoft X-rays at the Laboratoire pour l'Utilisation du Rayonnement Electromagnétique synchrotron facility (Orsay, France). Single and double strand breaks (SSB and DSB) were quantified by gel electrophoresis.

RESULTS

The number of DSB per Gray and per Dalton in pBS plasmids were (5.6 +/- 0.1), (6.3 +/- 0.1) and (8.5 +/- 0.4)x10(-12) at 250, 380 and 760 eV, respectively. They were respectively 1.4 +/- 0.1, 1.1 +/- 0.1 and 1.9 +/- 0.2 times larger for pSP plasmids. SSB/DSB ratios varied between 4.4 and 6.4.

CONCLUSION

The observed dependency of strand break induction by ultrasoft X-rays on the hydration level of DNA in plasmids films may be associated with: (i) Damage transfer from the water shell to the DNA and/or (ii) change in packing. 760 eV photons which are more often absorbed in the hydration shell and yield longer range electrons than 250 and 380 eV photons, induce more DSB per Gray and per Dalton, especially for the looser plasmid (pSP).

Effect of curcumin analog on gamma-radiation-induced cellular changes in primary culture of isolated rat hepatocytes in vitro

The present study was aimed to evaluate the radioprotective effect of curcumin analog, on gamma-radiation-induced toxicity in primary cultures of isolated rat hepatocytes. Hepatocytes were isolated from the liver of rats by collagenase perfusion. The DNA damage was analysed by single cell gel electrophoresis (comet assay). An increase in the severity of DNA damage was observed with the increase in gamma-radiation dose at 1-4 Gy in cultured rat hepatocytes. The levels of lipid peroxidative indices like thiobarbituric acid reactive substances (TBARSs) were increased significantly, whereas the levels of reduced glutathione (GSH) and antioxidant enzymes were significantly decreased in gamma-irradiated groups. The maximum damage to hepatocytes was observed at 4Gy gamma-irradiation. Pretreatment with different concentrations of curcumin analog (1.38, 6.91 and 13.82 microM) shows a significant decrease in the levels of TBARS and DNA damage. Pretreatment with curcumin analog prevents the loss of enzymic and non-enzymic antioxidants like GSH upon gamma-irradiation. The maximum protection of hepatocytes was observed at 6.91 microM of curcumin analog pretreatment. Thus, our result shows that pretreatment with curcumin analog protects the hepatocytes against gamma-radiation-induced cellular damage.

Modulatory effects of curcumin on γ-radiation-induced cellular damage in primary culture of isolated rat hepatocytes

Ionizing radiation is known to induce oxidative stress through generation of reactive oxygen species (ROS) resulting in imbalance of the pro-oxidant and antioxidant in the cells, which is suggested to culminate in cell death. The present work was aimed to evaluate the radioprotective effect of curcumin, a yellow pigment of turmeric on γ-radiation-induced toxicity in primary cultures of isolated rat hepatocytes. Hepatocytes were isolated from the liver of rats by collagenase perfusion. The cellular changes were estimated using lipid peroxidative indices like thiobarbituric acid reactive substances (TBARS), the antioxidants superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and reduced glutathione (GSH), ceruloplasmin, vitamins A, E and C and uric acid. The comet assay is a sensitive and rapid technique for quantifying and analyzing DNA damage in individual cells was exposed under γ-radiation. The increase in the severity of DNA damage was observed with the increase dose (1, 2 and 4Gy) of γ-radiation in cultured hepatocytes. TBARS were increased significantly, whereas the levels of GSH, vitamins C, E and A, ceruloplasmin, uric acid and antioxidant enzymes were significantly decreased in γ-irradiated hepatocytes. The maximum damage to hepatocytes was observed at 4Gy irradiation. On pretreatment with curcumin (1, 5 and 10μg/ml) showed a significant decrease in the levels of TBARS and DNA damage. The antioxidant enzymes were increased significantly along with the levels of GSH, vitamins A, E and C, uric acid and ceruloplamin. The maximum protection of hepatocytes was observed at 10μg/ml of curcumin pretreatment. Thus, pretreatment with curcumin helps in protecting the hepatocytes against γ-radiation-induced cellular damage and can be developed as an effective radioprotector during radiotherapy in near future.

Lycopene as a natural protector against gamma-radiation induced DNA damage, lipid peroxidation and antioxidant status in primary culture of isolated rat hepatocytes in vitro

The present study was designed to evaluate the radioprotective effect of lycopene, a naturally occurring dietary carotenoid, on gamma-radiation induced toxicity in cultured rat hepatocytes. The cellular changes were estimated using lipid peroxidative indices like thiobarbituric acid reactive substances (TBARS), superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), reduced glutathione (GSH), ceruloplasmin, vitamins A, E, C and uric acid. The DNA damage was analysed by single cell gel electrophoresis (comet assay). The increase in the severity of DNA damage was observed with the increase in gamma-radiation dose (1, 2 and 4 Gy) in cultured rat hepatocytes. TBARS were increased significantly whereas the levels of GSH, vitamins C, E and A, ceruloplasmin, uric acid and antioxidant enzymes were significantly decreased in gamma-irradiated groups. The maximum damage to hepatocytes was observed at 4 Gy irradiation. Pretreatment with lycopene (1.86, 9.31 and 18.62 microM) showed a significant decrease in the levels of TBARS and DNA damage. The antioxidant enzymes increased significantly along with the levels of GSH, vitamins A, E, C, uric acid and ceruloplasmin. The maximum protection of hepatocytes was observed at 9.31 muM of lycopene pretreatment. Thus, our results show that pretreatment with lycopene offers protection against gamma-radiation induced cellular damage and can be developed as an effective radioprotector during radiotherapy.

Radioprotective effect of sesamol on γ-radiation induced DNA damage, lipid peroxidation and antioxidants levels in cultured human lymphocytes

Sesamol pretreated (1, 5 and 10 microg/ml) lymphocytes were exposed to different doses of gamma-radiation, i.e., 1, 2 and 4 Gray (Gy) and the cellular changes were estimated by using cytokinesis blocked micronucleus assay (MN), dicentric aberration (DC), thiobarbituric acid reactive substances (TBARS), reduced glutathione (GSH) and the activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx). Radiation significantly increased MN, DC frequencies, TBARS levels and decreased GSH and antioxidant enzyme levels in a dose dependent manner. The highest damage to lymphocytes was observed at 4 Gy irradiation. On the other hand, sesamol pretreatment significantly decreased MN, DC frequencies, TBARS levels and increased GSH levels and SOD, CAT and GPx activities in a concentration dependent manner. At 1 Gy irradiation all concentrations of sesamol (1, 5 and 10 microg/ml) significantly protects the lymphocytes from radiation damage. At 2 Gy irradiation 5 and 10 microg/ml of sesamol shows significant radioprotection. Since the highest damage was observed at 4 Gy irradiation both 1 and 5 microg/ml of sesamol pretreatment were not sufficient to protect the lymphocytes from radiation damage but 10 microg/ml of sesamol significantly (p<0.05) protects the lymphocytes from radiation effect. Thus, sesamol pretreatment gives significant protection to cultured human lymphocytes against gamma-radiation induced cellular damage. The possible mechanism involved in the radioprotective influence of sesamol is discussed.

Stress, order and survival

Much of the sophisticated chemistry of life is accomplished by multicomponent complexes, which act as molecular machines. Intrinsic to their accuracy and efficiency is the energy that is supplied by hydrolysis of nucleoside triphosphates. Conditions that deplete energy sources should therefore cause decay and death. But studies on organisms that are exposed to prolonged stress indicate that this fate could be circumvented through the formation of highly ordered intracellular assemblies. In these thermodynamically stable structures, vital components are protected by a physical sequestration that is independent of energy consumption.

Regulated phase transitions of bacterial chromatin: a non-enzymatic pathway for generic DNA protection

The enhanced stress resistance exhibited by starved bacteria represents a central facet of virulence, since nutrient depletion is regularly encountered by pathogens in their natural in vivo and ex vivo environments. Here we explore the notion that the regular stress responses, which are mediated by enzymatically catalyzed chemical transactions and promote endurance during the logarithmic growth phase, can no longer be effectively induced during starvation. We show that survival of bacteria in nutrient-depleted habitats is promoted by a novel strategy: finely tuned and fully reversible intracellular phase transitions. These non-enzymatic transactions, detected and studied in bacteria as well as in defined in vitro systems, result in DNA sequestration and generic protection within tightly packed and highly ordered assemblies. Since this physical mode of defense is uniquely independent of enzymatic activity or de novo protein synthesis, and consequently does not require energy consumption, it promotes virulence by enabling long-term bacterial endurance and enhancing antibiotic resistance in adverse habitats.

Structural specificity of polyamines and polyamine analogues in the protection of DNA from strand breaks induced by reactive oxygen species

Reactive oxygen species are known to induce strand breaks and/or base modifications in DNA. DNA strand breaks are associated with many pathologies and programmed cell death. We have examined the ability of the polyamines and their analogues to protect phi X-174 plasmid DNA from strand breakage induced by a oxygen-radical generating system. Spermine and several unsymmetrically substituted polyamine analogues reduced the amount of strand breakage at a physiologically relevant concentration of 1 mM. However, putrescine, spermidine, N1-acetylspermine, N1-acetylspermidine and symmetrically alkylated polyamine analogues were not able to reduce strand breakage at the same concentration. Thus, the unsymmetrically alkylated polyamine analogues and natural spermine can protect DNA against strand breakage induced by Cu(II)/H2O2 generated ROS similar to other more classical antioxidants.

Sequence-modulated radiosensitization of DNA by copper ions

Plasmid DNA and restriction fragments of 80 and 120 base pairs were irradiated with fast neutrons in the presence of CuCl2. The number of single and double strand breaks is higher in the presence than in the absence of Cu2+ ions. The radiosensitizing effect was lower for solutions of high compared with low ionic strength, and also lower for deoxygenated than for aerated solutions. This effect was inhibited by EDTA, catalase and Tris, but not by ethanol. Superoxide dismutase partially inhibited the effect of low copper concentrations (< 1 Cu2+/nucleotide). Saturation of the solutions with N2O removed the effect for these concentrations of copper. The sensitization occurred preferentially at pyrimidines (thymines > cytosines) situated 5' to one or several purines (guanine > adenine) or located between two purines, at runs of purines (guanine > adenine), and at combinations of such sequences. The results can be only partially explained by a Fenton-like mechanism involving radiation induced hydrated electrons and hydrogen peroxide, which produces OH. radicals at the sites of binding of copper on DNA. The regions around these binding sites may undergo conformational changes. A second path for sensitization could be the enhancement of the efficiency of cleavage by the radiolytically produced OH. radicals in these conformationally modified regions.

Calculation of hydroxyl radical attack on different forms of DNA

A stochastic model of hydroxyl radical reaction with the DNA macromolecule in dilute aqueous solution is presented. It is based on the Monte Carlo method, the model of Smoluchowski, the model of the regular DNA atomic structure and the knowledge of reaction rate constants for .OH radical reaction with DNA constituents. The calculated respective overall probability of .OH radical reaction with sugar or base moiety dependents on DNA form (A, B or Z), base composition (AT/GC content) and strandedness (single and double). While for all three DNA forms studied (A, B and Z) as well as for both single- and double-stranded B-DNA, the specific probability of .OH radical attack on nucleobases is modulated by their sequence, the attack on deoxyriboses is only slightly sequence dependent (except for Z-DNA). The model predictions are compared to the experimental patterns of strand breaks and alkali revealed breaks for single and double stranded DNA in B-form and discussed in terms of molecular mechanisms of DNA damage induced by hydroxyl radical.

Radioprotection of DNA by polyamines

Putrescine, spermidine and spermine are natural polyamines bearing at neutral pH the net electrical charges +2, +3 and +4 respectively. We report here the radioprotective effect of these polyamines on the radiolysis of pBR322 plasmid DNA. We observe a very efficient protection against fast neutron-induced single and double-strand breakage in the presence of spermine and spermidine, and a significantly less efficient protection in the presence of putrescine. An ionic strength dependence is observed for spermidine and spermine, but not for putrescine. Circular dichroism measurements show spermidine- and spermine-induced structural modifications of DNA, i.e. the formation of tightly packaged condensates in the concentration range corresponding to radioprotection. No structural change is observed for concentrations of putrescine affording radioprotection. We explain the radioprotection by: (1) the scavenging of OH radicals in the bulk, essentially observed in the case of putrescine; (2) a local scavenging at the sites of binding of polyamines; and (3) the reduced accessibility of the attack sites in the condensed structures induced by spermine or spermidine.

Sites of strand breakage in DNA irradiated by fast neutrons

Therapeutic fast neutrons are densely ionizing particles, with a high relative biological effectiveness relative to 60Co gamma rays (RBE) and a low oxygen enhancement ratio (OER). The molecular basis of their properties is not yet entirely understood. In a previous work, we have shown that neutrons induce a different number of DNA frank strand breaks as compared to gamma photons, and we have revealed the presence of breaks due to the direct effects of neutrons. In the present work, we searched for eventual differences in the chemical nature of the attacked sites in DNA irradiated in oxygenated diluted solution. We compare our results with neutrons to those previously reported by other authors using gamma- or X-rays. Using sequencing gel electrophoresis of short natural DNA restriction fragments, or synthetic oligonucleotides, we have shown that, in the case of neutrons, the attack occurs with almost the same probability, at each nucleotide, as reported for gamma- and X-rays. The doubling of bands in the bottom of gels shows the presence of two types of termini, the 3'-phosphate and the 3'-phosphoglycolate. Upon neutron irradiation, the 3'-phosphate end appears with a higher yield than the 3'-phosphoglycolate, whereas equal amounts were obtained with gamma- or X-rays.

Radioprotection of DNA by a DNA-binding protein: MC1 chromosomal protein from the archaebacterium Methanosarcina sp. CHTI55

The archaebacterial chromosomal protein MC1 binds tightly and unspecifically to DNA; binding protects DNA against radiolysis by fast neutrons. At low covering of pBR322 plasmid DNA, one bound protein protects some 50 attack sites (phosphate-sugar moieties) against both single (ssb) and double strand breaks (dsb). At high covering of plasmid, protection against dsb becomes almost complete, although about half of the attack sites remain accessible to ssb. DNA restriction fragments were used to investigate the organization of the complexes, and its consequences on DNA radiolysis. Sequencing gel electrophoresis of the radiolytically-broken fragments are almost regular in the absence of protein, showing that breakage occurs at every base. In the presence of the protein, a periodic protection pattern is observed. The period of 11 base pairs is interpreted as the minimum distance between two adjacent MC1 proteins.