3-Carbethoxypsoralen-DNA photolesions: identification and quantitative detection in yeast and mammalian cells of the two cis-syn diastereoisomers formed with thymidine

Photosensitization Reactions of Biomolecules: Definition, Targets and Mechanisms

Photosensitization reactions have been demonstrated to be largely responsible for the deleterious biological effects of UV and visible radiation, as well as for the curative actions of photomedicine. A large number of endogenous and exogenous photosensitizers, biological targets and mechanisms have been reported in the past few decades. Evolving from the original definitions of the type I and type II photosensitized oxidations, we now provide physicochemical frameworks, classifications and key examples of these mechanisms in order to organize, interpret and understand the vast information available in the literature and the new reports, which are in vigorous growth. This review surveys in an extended manner all identified photosensitization mechanisms of the major biomolecule groups such as nucleic acids, proteins, lipids bridging the gap with the subsequent biological processes. Also described are the effects of photosensitization in cells in which UVA and UVB irradiation triggers enzyme activation with the subsequent delayed generation of superoxide anion radical and nitric oxide. Definitions of photosensitized reactions are identified in biomolecules with key insights into cells and tissues.

Photoinduced damage to cellular DNA: direct and photosensitized reactions

The survey focuses on recent aspects of photochemical reactions to cellular DNA that are implicated through the predominant formation of mostly bipyrimidine photoproducts in deleterious effects of human exposure to sunlight. Recent developments in analytical methods have allowed accurate and quantitative measurements of the main DNA photoproducts in cells and human skin. Highly mutagenic CC and CT bipyrimidine photoproducts, including cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) are generated in low yields with respect to TT and TC photoproducts. Another striking finding deals with the formation of Dewar valence isomers, the third class of bipyrimidine photoproducts that is accounted for by UVA-mediated isomerization of initially UVB generated 6-4PPs. Cyclobutadithymine (T<>T) has been unambiguously shown to be involved in the genotoxicity of UVA radiation. Thus, T<>T is formed in UVA-irradiated cellular DNA according to a direct excitation mechanism with a higher efficiency than oxidatively generated DNA damage that arises mostly through the Type II photosensitization mechanism. C<>C and C<>T are repaired at rates intermediate between those of T<>T and 6-4TT. Evidence has been also provided for the occurrence of photosensitized reactions mediated by exogenous agents that act either in an independent way or through photodynamic effects.

Photoaddition of 4,6-dimethyltetrahydrobenzoangelicin to thymine in DNA: X-ray studies and experiments with model oligonucleotides

The crystal structures of 4,6-dimethyltetrahydrobenzoangelicin (THBA), a furocoumarin analog, and of its furan-side cis-syn cycloadduct with thymine formed in the photoreaction with DNA, have been determined. The crystal structure of the latter compound contained only one enantiomeric form corresponding to the addition to a 5'-XpT site. Contrary to most psoralen derivatives studied, THBA showed higher photoreactivity toward synthetic oligonucleotides containing that sequence than toward those with the 5'-TpX sequence.

Photoreaction of 5-methoxypsoralen with thymidine and the thymine moiety of isolated and Saccharomyces cerevisiae DNA. Characterization and measurement of the two cis-syn furan-side monocycloadducts

The photoreaction of the furan-side moiety of 5-methoxypsoralen (5-MOP) with thymidine used as a DNA model compound was investigated in the dry state. Under these conditions, two main fluorescent photoadducts were formed and isolated by HPLC. The two modified nucleosides were characterized as the two cis-syn diastereoisomers of furan-side monoadducts of 5-MOP to thymidine on the basis of spectroscopic measurements including UV, fluorescence, 1H-NMR and circular dichroism analysis. The identification and quantification of the latter photoproducts within naked DNA exposed to photoexcited 5-MOP were achieved by enzymatic digestion completed by HPLC separation and fluorescence detection. Similarly, the two cis-syn furan-side monoadducts were found to be formed in the DNA of Saccharomyces cerevisiae cells after incubation with 5-MOP and subsequent exposure to 365 nm at an incident dose of 38.4 kJ m-2. Under these conditions, the rate of induction of two diastereoisomeric photoadducts was as low as one modification per 10(6) and 2 x 10(5) bases, respectively.

Detection of pyrimidine dimers and monoadducts induced by 7-methylpyrido(3,4-c) psoralen and UVA in Chinese hamster V79 cells by enzymatic cleavage and high-pressure liquid chromatography

The photochemotherapeutically active psoralen derivative 7-methylpyrido(3,4-c) psoralen (MePyPs) has been recently shown to be able to photoinduce monoadducts of the C4-cycloaddition type as well as pyrimidine dimers in DNA in vitro. In the present study, we report on the induction of these two types of photolesions in mammalian cells in culture. The MePyPs photocycloadducts were quantified in V79 Chinese hamster cells after treatment with MePyPs plus UVA following enzymatic hydrolysis of the DNA by DNase I, S1 nuclease and acidic phosphatase treatments. Concomitantly induced pyrimidine dimers were determined by two methods, high-pressure liquid chromatography and alkaline gel electrophoresis after dimer-specific endonucleolytic cleavage. The results show that, in Chinese hamster cells treated with MePyPs plus UVA, the yield of pyrimidine dimers is approximately 5-10% that of MePyPs-DNA photocycloadducts. Because psoralen monoadditions to DNA alone are generally not considered as being very phototoxic, a synergistic interaction of monoadditions with pyrimidine dimers may be expected to occur in order to explain the high photobiological effectiveness of this psoralen derivative.

Enzymatic recognition and biological effects of DNA damage induced by 3-carbethoxypsoralen plus UVA

The specific recognition of DNA modifications by repair endonucleases was used to characterize damage induced by 3-carbethoxypsoralen (3-CPs) plus UvA in M13mp8 replicative form I (RF-I) DNA. Under the conditions used, 3-CPs plus UVA generates DNA base modifications which are recognized by the UvrABC complex and the Fpg protein of E. coli. The rate of formation of UvrABC sensitive sites is 3-4-fold higher than that of Fpg sensitive sites. In addition a small number of sites of base loss (sensitive to Nfo protein) were observed. M13mp8 RF-I DNA treated with 3-CPs plus UVA was tested for transfection efficiency in E. coli mutants defective in either Fpg protein and/or UvrABC complex. The survival of 3-CPs plus UVA damaged M13mp8 RF-I DNA was significantly reduced when transfected into uvrA mutants compared to that in the wild-type strain. On the other hand, the survival of 3-CPs plus UVA damaged RF-I DNA was not altered in fpg-1 mutants. These results show that nucleotide excision repair mediated by the UvrABC complex is the major repair pathway involved in the elimination of lethal lesions induced in DNA by 3-CPs plus UVA. Our data suggest that in vitro exposure of M13mp8 RF-I DNA to 3-CPs plus UVA produces predominantly thymine photoaddition and to a lesser extent guanine photooxidation partially due to singlet oxygen generated during photoreaction. The photoaddition products are primarly responsible for the observed lethal effect.

Furan-side pyridopsoralens monoadducts to the thymine moiety of DNA

Isolation of the main cycloadducts formed in DNA by the UV-A (ultraviolet light of class A) photoreaction of 7-methyl-pyrido[3,4-c]psoralen (MePyPs) and 7-methyl-pyrido[4,3- c]psoralen (2N-MePyPs) was achieved by HPLC separation subsequent to enzymatic hydrolysis of DNA. The photoadducts have been quantified and their chemical structure assigned on the basis of spectroscopic measurements, including absorption and fluorescence spectroscopy, and circular dichroism as well as mass spectrometry analysis. They all present characteristics which are consistent with furan-side monoadducts resulting from a C4-cycloaddition of the psoralens to thymidine. The two major MePyPs-thymidine monoadducts formed in DNA exhibit a diastereoisomeric relationship and are likely to have a cis-syn stereochemistry.

Repair of the two diastereoisomer photoadducts formed between 7-methylpyrido(3,4-c)psoralen (MePyPs) and thymidine in yeast cells: a chemical approach

When analysing the repair of psoralen plus UVA-induced photoadducts in DNA, it must be realized that, in most cases, different isomers are formed. The monofunctional psoralen derivative 7-methylpyrido(3,4-c)psoralen (MePyPs) is known for its high antiproliferative activity at the cellular level and interesting photochemotherapeutic properties. To understand its photobiological efficiency in more detail, the induction of specific photoadducts in DNA and their repair were analysed in a eukaryotic cell system, the yeast Saccharomyces cerevisiae. After photoaddition of MePyPs, two main diastereoisomers were characterized after enzymatic hydrolysis of the DNA and analysis by high performance liquid chromatography. One diastereoisomer was more effectively repaired in yeast than the other during post-treatment incubation, suggesting that the two diastereoisomers may be recognized differently by cellular enzymatic repair systems.

Photochemistry of nucleic acids in cells

A survey of the recent aspects of the main photoreactions induced by far-UV radiation in cellular DNA is reported. This mostly includes the formation of cyclobutadipyrimidines, pyrimidine(6-4)pyrimidone photoadducts and related Dewar valence isomers in various eukaryotic and prokaryotic cells, as monitored by using either specific or more general assays. Information is also provided on mechanistic aspects regarding the formation of 5,6-dihydro-5-(alpha-thyminyl) thymine, the so-called "spore photoproduct" within far-UV-irradiated bacterial spores. The second major topic of the review deals with the effects of near-UV radiation and visible light on cellular DNA which are mostly mediated by photosensitizers. The main photoreactions of furocoumarins with DNA, one major class of photosensitizers used in the phototherapy of skin diseases, involve a [2 + 2] cycloaddition to the thymine bases according to an oxygen-independent mechanism. In contrast a second type of photosensitized reaction which appears to play a major role in the genotoxic effects of both near-UV and visible light requires the presence of oxygen. The photodynamic effects which are mediated by either still unidentified endogenous photosensitizers or defined exogenous photosensitizers lead to the formation of a wide spectrum of DNA modifications including base damage, oligonucleotide strand breaks and DNA-protein cross-links.

New aspects of the repair and genotoxicity of psoralen photoinduced lesions in DNA

Several approaches are described aiming at a better understanding of the genotoxicity of psoralen photoinduced lesions in DNA. Psoralens can photoinduce different types of photolesions including 3,4- and 4',5'-monoadducts and interstrand cross-links, oxidative damage (in the case of 3-carbethoxypsoralen (3-CPs)) and even pyrimidine dimers (in the case of 7-methylpyrido(3,4-c)psoralen (MePyPs)). The characterization and detection of different types of lesions has been essential for the analysis of their possible contributions to genotoxicity. For example, oxidative damage photoinduced by 3-CPs can be detected by the formamidopyrimidine glycosylase (FPG) protein. Furthermore, it is shown how the presence of MePyPs induced monoadducts may interfere with the photoreactivation of concomitantly induced pyrimidine dimers, how the ratio of monoadducts and interstrand cross-links (CL) affects the occurrence of double-strand breaks during the repair of photolesions and genotoxicity. In vitro treatment of yeast plasmids, followed by transformation, also indicates that the repair of photoadducts on exogenous DNA differs for 8-methoxy-psoralen (8-MOP) induced mono- and diadducts and for monoadducts alone. The recombinational rad52 dependent pathway is not needed for the repair of 8-MOP induced monoadducts. The results obtained suggest that the genotoxic effects of psoralens are conditioned by the nature, number, ratio and sequence distribution of the photolesions induced in DNA.

Photoreactions of furocoumarins with biomolecules

Recent aspects of the photoreactions of linear and angular furocoumarins with DNA and related compounds, including [2 + 2] cycloaddition to pyrimidine bases, covalent attachment to the osidic moiety of adenine nucleosides and photodynamic effects, are surveyed. Reactions of photoexcited furocoumarins with proteins and unsaturated lipids and the possible biological roles of the resulting adducts are also presented and discussed.

Genetic effects and repair of DNA photo-adducts induced by 8-methoxypsoralen and homopsoralen (pyranocoumarin) in diploid yeast

The relationship between DNA mono- and di-adducts and genetic effects induced by the pyranocoumarin 8,8-desmethylxanthyletine (homopsoralen) HP and 365 nm radiation (UVA) was investigated in the diploid yeast strain D7 (Saccharomyces cerevisiae) taking 8-methoxypsoralen (8-MOP) as a reference compound. The number of DNA cross-links (CLs) induced was determined using alkaline step elution analysis. The induction and removal of total photo-adducts was followed using radioactively labelled compounds. HP showed the same photobinding capacity as 8-MOP. As a function of UVA dose, it was less effective than 8-MOP for the induction of CLs and genetic effects. However, as a function of CLs induced, HP was shown to be more effective for the induction of lethal effects and mitotic recombination than 8-MOP but equally effective for the induction of mutations. The results suggest that, although CLs are recognized as genetically effective lesions, at a given number of CLs, HP induced mono-adducts efficiently contribute to the induction of lethal effects and mitotic recombination but less to the induction of mutations. Using a re-irradiation protocol, HP was brought to yield the same relative amounts of CLs at the same number of total adducts as single UVA exposures with 8-MOP. In these conditions, mutation induction and the kinetics for the removal of photo-adducts were the same for both agents indicating that not only the removal of adducts but also mutation induction are highly dependent on the relative level of CLs induced.

Geometry of intercalation of psoralens in DNA approached by molecular mechanics

The results of molecular mechanical calculations on intercalation complexes of 3-carbethoxypsoralen, 5-methoxypsoralen, 8-methoxypsoralen, 7-methylpyrido[3,4-c]psoralen (MepyPs) and 7-methylpyrido[4,3-c]psoralen (2N-MePyPs) with the double stranded duodecanucleotide d(CGCGATATCGCG)2 are presented. In the energy-minimized structures, the psoralens are intercalated with their plane orthogonal to the helix axis. Stacking interactions between the furan ring of the psoralen and the adjacent bases are maximized in most derivatives studied, whereas the effect of the various substituents of the psoralen ring is to specifically push part of the molecule towards either the minor or the major groove, preventing a symmetrical intercalation (with respect to the two strands of the DNA). The relative position of the psoralen ring and of the adjacent thymine foreshadows the formation of furan-side monoadducts in 3-CPs, MePyPs and 2N-MePyPs, whereas the formation of a pyrone-side monoadduct appears as geometrically more favourable in 5-MOP and both furan- and pyrone-side monoadducts can be geometrically envisaged in 8-MOP. A good correlation therefore exists between the more or less favourable equilibrium geometries and the experimentally observed photoreactions. The present study is the first attempt to characterize the geometrical parameters as part of a complex set of geometrical, dynamical and excited state parameters governing the overall DNA-psoralen photoreaction.