Comparison of mechanisms for DNA strand break formation by the direct and indirect effect of radiation

Hematopoietic Stem Cells and Mesenchymal Stromal Cells in Acute Radiation Syndrome

With the extensive utilization of radioactive materials for medical, industrial, agricultural, military, and research purposes, medical researchers are trying to identify new methods to treat acute radiation syndrome (ARS). Radiation may cause injury to different tissues and organs, but no single drug has been proven to be effective in all circumstances. Radioprotective agents are always effective if given before irradiation, but many nuclear accidents are unpredictable. Medical countermeasures that can be beneficial to different organ and tissue injuries caused by radiation are urgently needed. Cellular therapy, especially stem cell therapy, has been a promising approach in ARS. Hematopoietic stem cells (HSCs) and mesenchymal stromal cells (MSCs) are the two main kinds of stem cells which show good efficacy in ARS and have attracted great attention from researchers. There are also some limitations that need to be investigated in future studies. In recent years, there are also some novel methods of stem cells that could possibly be applied on ARS, like "drug" stem cell banks obtained from clinical grade human induced pluripotent stem cells (hiPSCs), MSC-derived products, and infusion of HSCs without preconditioning treatment, which make us confident in the future treatment of ARS. This review focuses on major scientific and clinical advances of hematopoietic stem cells and mesenchymal stromal cells on ARS.

Monoglucosyl-rutin as a potential radioprotector in mammalian cells

In the present study, the role of monoglucosyl-rutin as a potential radioprotector was investigated using mammalian cell culture models. Cell survival and DNA damage were assessed using colony formation, sister chromatid exchange and γH2AX assays. It was demonstrated that monoglucosyl-rutin was able to increase cell survival when exposed to ionizing radiation, possibly by decreasing the amount of base damage experienced by the cell. However, the present study also demonstrated that, despite monoglucosyl-rutin exhibiting radioprotective effects at low doses, high doses of monoglucosyl-rutin led to a decrease in plating efficiency and an increased doubling time. This effect may be due to double-strand breaks caused by high concentrations of monoglucosyl-rutin.

Reaction of Dithiothreitol and Para-nitroacetophenone with Different Radical Precursors of .OH Radical-induced Strand Break Formation of Single-stranded DNA in Anoxic Aqueous Solution

The yields of single-strand breakage (ssb) in single-stranded calf thymus DNA (ssDNA) have been determined after 60Co gamma-irradiation of aqueous anoxic solutions in the presence of different concentrations of dithiothreitol (DTT), ascorbate or trans-4,5-dihydroxy-1,2-dithiane, using low-angle laser light scattering. The influence of DTT on the kinetics of ssb formation has been determined by conductivity measurements in pulse radiolysis. The results suggest that strand breakage in ssDNA proceeds via two modes of about equal contribution and with half-lives of about 7 ms and 0.8s, respectively. Both modes reflect reactions of at least two DNA radicals, which react with DTT by hydrogen-atom transfer reactions with similar rate constants of about 5-9 x 10(5) dm3 mol-1 s-1. These hydrogen-atom transfer reactions inhibit strand break formation. The slow mode is shown to represent the decay of base-radicals to generate sugar radicals. The involvement of the oxidizing .OH adduct radical of guanine in the formation of strand breaks can be ruled out and there is no evidence for a contribution from the anion or radical anion of DTT to the inhibition of strand breaks via electron transfer reactions to DNA radicals.

Inhibition of OH radical-induced strand break formation of poly(U) by Ru(bpy)32+ or Ru(phen)32+ attached to the polynucleotide

Reactions of OH radicals with poly(U) (polyuridylic acid) in the presence of Ru(bpy)32+ or Ru(phen)32+ in aqueous solutions were studied. OH radicals were produced by pulse radiolysis and their reactions with ruthenium complexes were measured spectrophotometrically under conditions were the complexes are attached to the polynucleotide. The OH radical adds to either the uracil moiety or the ruthenium complexes. The ratio of the radicals produced depends only on the ratio of their rate constants and the concentrations of poly(U) and ruthenium complexes. Similar results were obtained with uridine-5'-monosphosphate, where the ruthenium complexes are not attached to the nucleotide. Surprisingly, the yield of single-strand break formation from the OH adducts of uracil in poly(U) is much smaller than that expected on the basis of the yield measured in the absence of ruthenium complexes. Possible reasons for this behaviour are discussed.

Can .OH scavengers protect against direct UV-C damage in vivo?

It has previously been shown that ethanol and tert-butanol protect poly(U) against strand breaks from pulsed laser ultraviolet (UV) photons (lambda = 248 nm). This protection is believed to be based on a modification of direct DNA damage, not on scavenging hydroxyl radicals. In this paper, several .OH scavengers were tested in vivo with Escherichia coli DNA repair-deficient mutants to see if protection could also be demonstrated with non-laser UV photons (lambda approximately 254 nm). No protection was observed.

Chemical, molecular biology, and genetic techniques for correlating DNA base damage induced by ionizing radiation with biological end points

The types of DNA base damage induced by ionizing radiation, and also relevant model system investigations on replication and mutagenesis, are reviewed in this paper. Recent advances in DNA synthesis technology and site-directed mutagenesis suggest that these methods can be profitably utilized to correlate specific types of DNA base damage with selected biological end points. A deeper insight can be obtained into the molecular origins of mutations, and the effects of base sequence surrounding the lesions on the nature and types of mutations.

Transients of uracil and thymine derivatives and the quantum yields of electron ejection and intersystem crossing upon 20 ns photolysis at 248 nm

Transients of uracil and a series of 17 correlated pyrimidines, e.g. methylated bases, nucleosides, nucleotides, and polyuridylic acid [poly(U)] were studied after 248 nm excitation by 20 ns laser pulses. The transient absorption spectra in aqueous solution at room temperature reveal the triplet state and the hydrated electron (e-aq), while the corresponding radical cation could not be observed at pH 6-7. Fast loss of the chromophore in the 260-290 nm range within 0.1 microsecond was observed in aqueous solution in some cases [e.g. poly(U), 5'-UMP, uridine, uracil] and in others (thymine, thymidine) virtually not. This photobleaching is assigned to formation of the photohydrate. The concentration of e-aq shows a quadratic dependence on the laser pulse intensity (IL) in the range (0.2-2) x 10(7) W cm-2 and the quantum yield of electron ejection (phi c-) thus depends linearly on IL. This behaviour, suggesting that the photoionization involves a two-step absorption process, was found for poly(U) and all pyrimidine monomers examined. At a constant IL value of 2 x 10(7) W cm-2, phi c- ranges from 3 x 10(-3) for 1,3-dimethylthymine to 4 x 10(-2) for poly(U). The triplet state shows a much larger transient absorbance (delta A, typically in a broad range, e.g. 290-500 nm) than that of the neutral radical resulting from the radical cation. The triplet state in organic solvents (acetonitrile and ethanol) shows generally a significantly larger delta A value than in aqueous solution. The estimated quantum yields of intersystem crossing at room temperature are compared with those of phosphorescence at -196 degrees C.

Single- and double-strand break formation in double-stranded DNA upon nanosecond laser-induced photoionization

Double-stranded (ds) calf thymus DNA (0.4 mM), excited by 20 ns laser pulses at 248 nm, was studied in deoxygenated aqueous solution at room temperature and pH 6.7 in the presence of a sodium salt (10 mM). The quantum yields for the formation of hydrated electrons (phi c-), single-strand breaks (phi ssb) and double-strand breaks (phi dsb) were determined for various laser pulse intensities (IL). phi c- and phi ssb increase linearly with increasing IL; however, phi ssb has a tendency to reach saturation at high IL (greater than 5 X 10(6) Wcm-2). The ratio phi ssb/phi c-, representing the number of ssb per radical cation, is about 0.08 at IL less than or equal to 5 X 10(6) Wcm-2. For comparison, the number of ssb per OH radical reacting with dsDNA is 0.22. On going from argon to N2O saturation, phi ssb and phi dsb become larger by factors of approximately 5 and 10-15, respectively. This enhancement is produced by attack on DNA bases by OH radicals generated by N2O-scavenging of the photoelectrons. While phi ssb is essentially independent of the dose (Etot), phi dsb depends linearly on Etot in both argon- and N2O-saturated solutions. The linear dependence of phi dsb implies a square dependence of the number of dsb on Etot. This portion of dsb formation is explained by the occurrence of two random ssb, generated within a critical distance (h) in opposite strands. For both argon- and N2O-saturated solutions h was found to be of the order of 40-70 phosphoric acid diester bonds. On addition of electron scavengers such as 2-chloroethanol (or N2O plus t-butanol), phi dsb is similar to that in neat, argon-saturated solutions. Thus, hydrated electrons are not involved in the chemical pathway leading to laser-pulse-induced dsb of DNA.

Photosensitized reactions of poly(U) with tris(2,2'-bipyridyl)ruthenium(II) and peroxydisulfate

The reactions of polyuridylic acid [poly(U)] with Ru(bpy)3(3+) [Ru(III)] and SO4.-, following UV and visible light irradiation of Ru(bpy)3(2+) [Ru(II)] in the presence of S2O8(2-), were studied in an argon-saturated aqueous solution using time-resolved absorption and conductivity methods. The kinetics of the Ru(III) conversion to Ru(II) in the presence of poly(U) was monitored spectroscopically either in the absence of SO4.- [rapid mixing with Ru(III)] or in its presence (after laser flash excitation, lambda exc = 353 nm). The conversion of Ru(III) to Ru(II) is complete at a [nucleotide]/[sensitizer] (N/S) ratio greater than or equal to 10 (rate constant k = 12 s-1) for rapid mixing and at N/S greater than or equal to 6 (k = 15 s-1 at N/S = 10) after laser pulsing. Conductivity measurements following the laser pulse revealed a fast conductivity increase (risetime less than 10 micros), due to the formation of charged species and protons. A slower increase in the 0.1-0.5 s range was observed for poly(U) but it is considerably smaller for poly(dU) and absent in uracil containing monounits. The slow increase is unaffected by pH changes in the 3.5-7 range, markedly reduced in the 7-9 range and is replaced by a slight decrease in conductivity in buffered solutions. An explanation is that poly(U)-bound excited Ru(II) reacts with S2O8(2-) forming Ru(III) and SO4.- as oxidizing species both of which react with poly(U) bases. The resulting base radicals react with Ru(III) or the ligands in the ruthenium complex, producing protons which give rise to the slow conductivity increase (k = 15 s-1 at N/S = 10). The formation of single-strand breaks and the ensuing release of condensed counterions does not appear to contribute significantly to the slow conductivity signal. At N/S less than 10 the observed rate and extent of Ru(III)--Ru(II) conversion and of the slow proton production vary markedly with the N/S ratio.

A study of some polypyridylruthenium(II) complexes as DNA binders and photocleavage reagents

The nature of the binding of several ruthenium polypyridyl complexes containing 2,2'-bipyridine (bipy), 4,4'-dimethyl-2,2'-bipyridine (DMB), 1,10-phenanthroline (phen), 4,7-diphenyl-1,10-phenanthroline (DPP), 2,2',2"-terpyridine (terpy), 2,2'-biquinoline (biq), 1,4,5,8-tetraazaphenanthrene (TAP) and 1,4,5,8,9,12-hexaazatriphenylene (HAT), with calf thymus DNA, poly[d(A-T)] and poly[d(G-C)] were studied by absorption and emission spectroscopy, DNA melting techniques, and emission lifetime measurements. In low ionic strength phosphate buffer, spectroscopic changes and DNA stabilization depended on the polypyridyl ligands present, and indicated binding that varied from substantially electrostatic to intercalative. Ru(bipy)2(HAT)2+ and Ru(phen)3(2+), which bind by partial intercalation, also show a strong preference for poly[d(A-T)]. The emission quantum yields for most complexes were increased in the presence of DNA. An exception was Ru(TAP)3(2+) which has a markedly reduced emission quantum yield and lifetime in the presence of poly[d(G-C)] or CT-DNA, due to photoredox interaction with quanines. Emission decays of the complexes generally showed multiexponential behaviour. The ability of the ruthenium complexes to sensitise DNA cleavage was determined using pBR322 plasmid DNA. Ru(TAP)3(2+) is the most efficient sensitiser while uncharged complexes and complexes with very short-lived excited states do not cleave DNA.

Radioprotection of pyrimidines by oxygen and sensitization by phosphate: a feature of their electron adducts

The reactions of the electron adducts of thymine, uracil and 1,3-dimethylthymine in the presence of phosphate buffer and low oxygen concentrations have been investigated. Oxygen reacts with the pyrimidine electron adducts and their O(4)-protonated forms to restore the pyrimidine and produce O2-./HO2.. Thus oxygen acts as a radiation protector. In the presence of high buffer concentrations the electron adducts are irreversibly protonated at C(6). On reaction of this radical with oxygen no restitution of the original pyrimidine occurs and the pyrimidine is destroyed. Thus phosphate buffer acts as a radiation sensitizer. Some speculations are made as to the possible relevance of these reactions to biological systems.

The reverse of the 'repair' reaction of thiols: H-abstraction at carbon by thiyl radicals

Thiyl radicals (RS) formed by the reaction of radiolytically generated OH radicals with thiols, e.g. 1,4-dithiothreitol (DTT), react with cis- and trans-2,5-dimethyltetrahydrofuran by abstracting an H atom in the alpha-position to the ether function (k approximately equal to 5 X 10(3) dm3 mol-1 s-1). The so-formed planar ether radical is 'repaired' by the thiol (k = 6 X 10(8) dm3 mol-1 s-1) thereby regenerating a cis- or trans-2,5-dimethyltetrahydrofuran molecule. In this reaction a thiyl radical is reproduced. Thus trans-2,5-Me2THF from cis-2,5-Me2THF and vice versa are formed in a chain reaction: at a dose rate of 2.8 X 10(-3) Gys-1 and a trans-2,5-Me2THF concentration of 1 X 10(-2) mol dm-3 using DTT as the thiol, G(cis-2,5-Me2THF) = 160 has been found. The chain reaction is very sensitive to impurities and also to disulphides such as those radiolytically formed. 2,5-Me2THF can be regarded as a model for the sugar moiety of DNA where the C(4')-radical is known to lead to DNA strand breakage. The possible role of cellular thiols in the repair of the C(4') DNA radical, and also the conceivable role of thiyl radicals inducing DNA strand breakage, are discussed.