Relationship between lesions photoinduced by mono- and bi-functional furocoumarins in DNA and genotoxic effects in diploid yeast

4,6,4'-trimethylangelicin shows high anti-proliferative activity on DU145 cells under both UVA and blue light

OBJECTIVES

Furocoumarins (psoralens and angelicins) have been already used under ultraviolet A light (UVA) for the treatment of skin diseases and cutaneous T-cell lymphoma. Besides their high anti-proliferative activity, some severe long-term side effects have been observed, for example genotoxicity and mutagenicity, likely strictly related to the formation of crosslinks. It has been demonstrated that blue light (BL) activation of 8-methoxypsoralen, an FDA-approved drug, leads to less mutagenic monoadducts in the DNA. So far, in this work the less toxic and more penetrating BL is proposed to activate 4,6,4'-trimethylangelicin (TMA), an already known UVA photoactivatable compound.

MATERIALS AND METHODS

Photocleavage, crosslink formation and oxidative damage were detected in pBR322 plasmid DNA treated with 300.0 μmol/L TMA activated with various exposures of BL. Anti-proliferative activity, reactive oxygen species (ROS) formation and activation status of some signalling pathways involved in cell growth and apoptosis were verified on DU145 cells treated with 5.0 μmol/L TMA plus 2.0 J/cm² of BL.

RESULTS

Under BL-TMA, no mutagenic crosslinks, no photocleavage and neither photooxidative lesions were detected on isolated plasmid DNA. TMA showed high anti-proliferative activity on DU145 cells through induction of apoptosis. Besides ROS generation, the proapoptotic effect seemed to be related to activation of p38 and inhibition of p44/42 phosphorylation. Interestingly, the decrease in nuclear β-catenin was coupled with a significant dropping of CD44-positive cells.

CONCLUSION

Overall, our results indicate that TMA can be activated by BL and may be considered for targeted phototherapy of prostate cancer lesions.

Phototoxicity of herbal plants and herbal products

Plants are used by humans in daily life in many different ways, including as food, herbal medicines, and cosmetics. Unfortunately, many natural plants and their chemical constituents are photocytotoxic and photogenotoxic, and these phototoxic phytochemicals are widely present in many different plant families. To date, information concerning the phototoxicity and photogenotoxicity of many plants and their chemical constituents is limited. In this review, we discuss phototoxic plants and their major phototoxic constituents; routes of human exposure; phototoxicity of these plants and their constituents; general mechanisms of phototoxicity of plants and phototoxic components; and several representative phototoxic plants and their photoactive chemical constituents.

Targeting and processing of site-specific DNA interstrand crosslinks

DNA interstrand crosslinks (ICLs) are among the most cytotoxic types of DNA damage, and thus ICL-inducing agents such as cyclophosphamide, melphalan, cisplatin, psoralen, and mitomycin C have been used clinically as anticancer drugs for decades. ICLs can also be formed endogenously as a consequence of cellular metabolic processes. ICL-inducing agents continue to be among the most effective chemotherapeutic treatments for many cancers; however, treatment with these agents can lead to secondary malignancies, in part due to mutagenic processing of the DNA lesions. The mechanisms of ICL repair have been characterized more thoroughly in bacteria and yeast than in mammalian cells. Thus, a better understanding of the molecular mechanisms of ICL processing offers the potential to improve the efficacy of these drugs in cancer therapy. In mammalian cells, it is thought that ICLs are repaired by the coordination of proteins from several pathways, including nucleotide excision repair (NER), base excision repair (BER), mismatch repair (MMR), homologous recombination (HR), translesion synthesis (TLS), and proteins involved in Fanconi anemia (FA). In this review, we focus on the potential functions of NER, MMR, and HR proteins in the repair of and response to ICLs in human cells and in mice. We will also discuss a unique approach, using psoralen covalently linked to triplex-forming oligonucleotides to direct ICLs to specific sites in the mammalian genome.

Application of the concept of relative photomutagenic potencies to selected furocoumarins in V79 cells

Furocoumarins are phototoxic and photomutagenic natural plant constituents found in many medicinal plants and food items. Since plants contain mixtures of several furocoumarins, there is a need for a comparative risk assessment of a large number of furocoumarins. Previously, we have introduced the concept of relative Photomutagenicity Equivalency Factors (PMEFs) derived from the slope of the concentration-response curve of photomutagenicity of individual furocoumarins in V79 cells using the HPRT mutation assay in the presence of UVA irradiation at 125mJ/cm(2). Here we have applied this method to the furocoumarins bergamottin, isopimpinellin and psoralen using 5-methoxypsoralen (5-MOP) as a reference compound with a PMEF of 1.0. We found that neither bergamottin nor isopimpinellin, two furocoumarins abundant in plants, food etc., exerted any significant photomutagenicity while psoralen was clearly photomutagenic with a PMEF of 0.36. Similarly, isopimpinellin was not phototoxic in V79 cells, while bergamottin showed some cytotoxicity which, however, was completely independent of UVA irradiation. Only psoralen was photocytotoxic showing a similar concentration-response relationship for photomutagenicity, and for photocytotoxicity (at 72h after irradiation). Data from the micronucleus assay for DNA damage at 20h after irradiation were in complete agreement with the HPRT mutation data. Our findings indicate that individual furocoumarins differ enormously in their photomutagenic potency, and that a specific toxicological risk assessment is required for each furocoumarin instead of an assessment based on the sum of furocoumarins in a given sample.

Antigenotoxic effects of three essential oils in diploid yeast (Saccharomyces cerevisiae) after treatments with UVC radiation, 8-MOP plus UVA and MMS

Essential oils (EOs) extracted from medicinal plants such as Origanum compactum, Artemisia herba alba and Cinnamomum camphora are known for their beneficial effects in humans. The present study was undertaken to investigate their possible antigenotoxic effects in an eukaryotic cell system, the yeast Saccharomyces cerevisiae. The EOs alone showed some cytotoxicity and cytoplasmic petite mutations, i.e. mitochondrial damage, but they were unable to induce nuclear genetic events. In combination with exposures to nuclear mutagens such as 254-nm UVC radiation, 8-methoxypsoralen (8-MOP) plus UVA radiation and methylmethane sulfonate (MMS), treatments with these EOs produced a striking increase in the amount of cytoplasmic petite mutations but caused a significant reduction in revertants and mitotic gene convertants induced among survivors of the diploid tester strain D7. In a corresponding rho0 strain, the level of nuclear genetic events induced by the nuclear mutagens UVC and 8-MOP plus UVA resulted in the same reduced level as the combined treatments with the EOs. This clearly suggests a close relationship between the enhancement of cytoplasmic petites (mitochondrial damage) in the presence of the EOs and the reduction of nuclear genetic events induced by UVC or 8-MOP plus UVA. After MMS plus EO treatment, induction of these latter events was comparable at least per surviving fraction in wildtype and rho0 cells, and apparently less dependent on cytoplasmic petite induction. Combined treatments with MMS and EOs clearly triggered switching towards late apoptosis/necrosis indicating an involvement of this phenomenon in EO-induced cell killing and concomitant decreases in nuclear genetic events. After UVC and 8-MOP plus UVA plus EO treatments, little apoptosis and necrosis were observed. The antigenotoxic effects of the EOs appeared to be predominantly linked to the induction of mitochondrial dysfunction.

Cytotoxicity and gene induction by some essential oils in the yeast Saccharomyces cerevisiae

In order to get an insight into the possible genotoxicity of essential oils (EOs) used in traditional pharmacological applications we tested five different oils extracted from the medicinal plants Origanum compactum, Coriandrum sativum, Artemisia herba alba, Cinnamomum camphora (Ravintsara aromatica) and Helichrysum italicum (Calendula officinalis) for genotoxic effects using the yeast Saccharomyces cerevisiae. Clear cytotoxic effects were observed in the diploid yeast strain D7, with the cells being more sensitive to EOs in exponential than in stationary growth phase. The cytotoxicity decreased in the following order: Origanum compactum>Coriandrum sativum>Artemisia herba alba>Cinnamomum camphora>Helichrysum italicum. In the same order, all EOs, except that derived from Helichrysum italicum, clearly induced cytoplasmic petite mutations indicating damage to mitochondrial DNA. However, no nuclear genetic events such as point mutations or mitotic intragenic or intergenic recombination were induced. The capacity of EOs to induce nuclear DNA damage-responsive genes was tested using suitable Lac-Z fusion strains for RNR3 and RAD51, which are genes involved in DNA metabolism and DNA repair, respectively. At equitoxic doses, all EOs demonstrated significant gene induction, approximately the same as that caused by hydrogen peroxide, but much lower than that caused by methyl methanesulfonate (MMS). EOs affect mitochondrial structure and function and can stimulate the transcriptional expression of DNA damage-responsive genes. The induction of mitochondrial damage by EOs appears to be closely linked to overall cellular cytotoxicity and appears to mask the occurrence of nuclear genetic events. EO-induced cytotoxicity involves oxidative stress, as is evident from the protection observed in the presence of ROS inhibitors such as glutathione, catalase or the iron-chelating agent deferoxamine.

Disposition of 4,6,4'-trimethylangelicin in mice maintained in the dark and after UVA irradiation

The disposition of the furocoumarin 4,6,4'-trimethylangelicin (4,6,4'-TMA) was studied in mice. After oral administration of (3)H 4,6,4'-TMA, radioactivity measured in serum shows fast absorption and slow elimination. Serum protein binding is higher as compared to 8-methoxypsoralen (8-MOP), currently used in photochemotherapy (PUVA) and linearly declines from 30 min to 6 h after administration. Distribution in the various organs was similar to that of 8-MOP and was relatively uninfluenced by UVA radiation, required for the biological effects of 4,6,4'-TMA. Mice eliminate (3)H 4,6,4'-TMA mostly through the urine, but also through the faeces. Two metabolites were identified in the urine and serum of the treated mice, one of which proved to be a derivative of 4,6,4'-TMA, formed by hydrogenation of the double 4',5' bond of the furocoumarin nucleus.

Impaired mitochondrial function protects against free radical-mediated cell death

Free radical damage can have fatal consequences. Mitochondria carry out essential cellular functions and produce high levels of reactive oxygen species (ROS). Many agents also generate ROS. Using the yeast Saccharomyces cerevisiae as a eukaryotic model, the role of functional mitochondria in surviving free radical damage was investigated. Respiratory-deficient cells lacking mitochondrial DNA (rho(0)) were up to 100-fold more resistant than isogenic rho(+) cells to killing by ROS generated by the bleomycin-phleomycin family of oxidative agents. Up to approximately 90% of the survivors of high oxidative stress lost mitochondrial function and became "petites." The selective advantage of respiratory deficiency was studied in several strains, including DNA repair-deficient rad52/rad52 and blm5/blm5 diploid strains. These mutant strains are hypersensitive to lethal effects of free radicals and accumulate more DNA damage than related wild-type strains. Losses in mitochondrial function were dose-dependent, and mutational alteration of the RAD52 or BLM5 gene did not affect the resistance of surviving cells lacking mitochondrial function. The results indicate that inactivation of mitochondrial function protects cells against lethal effects of oxygen free radicals.

Manipulating the mammalian genome by homologous recombination

Gene targeting in mammalian cells has proven invaluable in biotechnology, in studies of gene structure and function, and in understanding chromosome dynamics. It also offers a potential tool for gene-therapeutic applications. Two limitations constrain the current technology: the low rate of homologous recombination in mammalian cells and the high rate of random (nontargeted) integration of the vector DNA. Here we consider possible ways to overcome these limitations within the framework of our present understanding of recombination mechanisms and machinery. Several studies suggest that transient alteration of the levels of recombination proteins, by overexpression or interference with expression, may be able to increase homologous recombination or decrease random integration, and we present a list of candidate genes. We consider potentially beneficial modifications to the vector DNA and discuss the effects of methods of DNA delivery on targeting efficiency. Finally, we present work showing that gene-specific DNA damage can stimulate local homologous recombination, and we discuss recent results with two general methodologies--chimeric nucleases and triplex-forming oligonucleotides--for stimulating recombination in cells.

Synthesis and biological activity of (hydroxymethyl)- and (diethylaminomethyl)benzopsoralens

Some benzopsoralens, carrying a hydroxymethyl or a diethylaminomethyl group at the 3, 5, 8, and 11 positions, were prepared, and their biological activity was compared with that of 4-(hydroxymethyl)benzopsoralen (BP). 5-(Hydroxymethyl)benzopsoralen (7b), 11-(hydroxymethyl)benzopsoralen (7c), and 11-(diethylaminomethyl)benzopsoralen (8c) induced marked antiproliferative effects in mammalian cells by simple incubation in the dark; this activity appeared to be related to their ability to inhibit topoisomerase II. Benzopsoralens appeared to be more active, especially BP and 7c, upon UVA activation. Compounds carrying a methyl group at the 4 position together with a hydroxymethyl or diethylaminomethyl at the 8 position (7d and 8d, respectively) were also effective, although to a lower extent; instead, a substituent at the 3 position canceled all activity. Benzopsoralens did not induce interstrand cross-links in DNA in vitro, as seen in the induction of cytoplasmic <<petite>> mutations and double-strand breaks in yeast. This behavior is also compatible with their low mutagenic activity in E. coli WP2 and with the absence of any phototoxicity on the skin. For these features, benzopsoralens seem to be interesting potential drugs for PUVA photochemotherapy and photopheresis. The activity shown in the dark is not sufficient for their possible use as antitumor drugs, but it does offer a new model for the study of topoisomerase inhibitors.

Effects of beta-carotene and alpha-tocopherol on photogenotoxicity induced by 8-methoxypsoralen: the role of oxygen

The protective effect of beta-carotene (beta-C) and alpha-tocopherol (alpha-T), singularly and in equimolar mixtures, toward the photomutagenicity induced by 8-methoxypsoralen (8-MOP), at different oxygen partial pressure (pO2), was evaluated in two different experimental models: Salmonella typhimurium TA102 and Saccharomyces cerevisiae D7. After phototreatment with 8-MOP, the results show a lethal effect under hypoxic conditions in both experimental model systems, an increase in revertants associated to the pO2 increase in S. typhimurium TA102, and a decrease in revertants and convertants associated to the pO2 increase in S. cerevisiae D7. In S. typhimurium TA102, in atmospheric condition, beta-C and alpha-T (1.86 or 18.6 microM) show a protective effect only at the higher dosage. Alpha-T was more protective than beta-C. The equimolar mixtures show an antimutagenic effect at both dosage used with a synergistic effect at lower dosage and an additive antimutagenic activity at higher dosage. An inhibition of the spontaneous mutagenicity by mixtures at higher dosage was also observed. The results obtained in S. typhimurium TA102 show an antimutagenic effects of beta-C, alpha-T and their mixture at 190 mmHg pO2, confirming the data obtained in air condition. At 380 mmHg pO2, alpha-T and the mixture show a significant antimutagenic activity; at 570 mmHg pO2, only alpha-T is protective. At 760 mmHg pO2, no protective effect was observed by the two antioxidants, and beta-C increases the photomutagenicity induced by 8-MOP. In S. cerevisiae D7 a protective effect was only observed at 380 mmHg pO2 with the mixture. No antigenotoxic effect was found in the other experimental conditions, even if the uptake of the two antioxidants was confirmed by HPLC. Our results underline the role of oxygen in the photomutagenicity induced by 8-MOP and in the antimutagenic activity of beta-C and alpha-T. This is the first report confirming in a cellular experimental model the data obtained in some chemical systems: the protective effect of beta-C only at low pO2 and the synergistic effect of mixture of beta-C and alpha-T.

Solution structure of oligonucleotides covalently linked to a psoralen derivative

Psoralen (pso) was attached via its C-5 position to the 5'-phosphate group of an oligodeoxynucleotide d(TAAGCCG) by a hexamethylene linker (m6). Complex formation between pso-m6-d(TAAGCCG) and the complementary strands d(CGGCTTA)[7-7mer] or d(CGGCTTAT)[7-8mer] was investigated by nuclear magnetic resonance in aqueous solution. Structural informations derived from DQF-COSY and NOESY maps, revealed that the mini double helix adopts a B-form conformation and that the deoxyriboses preferentially adopt a C2'-endo conformation. The nOe connectivities observed between the protons of the bases or the sugars in each duplex, and the protons of the psoralen and the hexamethylene chain, led us to propose a model involving an equilibrium between two conformations due to different locations of the psoralen. Upon UV-irradiation, the psoralen moiety cross-linked the two DNA strands at the level of 5'TpA3' sequences. NMR studies of the single major photo-cross-linked duplex pso-m6-d(TAAGCCG) and d(CGGCTTA) were performed. The stereochemistry of the diadduct is indeed cis-syn at both cyclobutane rings. In addition, the effects of this diadduct on the helical structure are analyzed in detail.

Induction of the genes RAD54 and RNR2 by various DNA damaging agents in Saccharomyces cerevisiae

The relationship between the induction of the genes RAD54 and RNR2 and the induction and repair of specific DNA lesions was studied in the yeast Saccharomyces cerevisiae using Rad54-lacZ and RNR2-lacZ fusion strains. Gene induction was followed by measuring beta-galactosidase activity. At comparable levels of furocoumarin-DNA photoadducts, RAD54 was more effectively induced by bifunctional than by monofunctional furocoumarins indicating that mixtures of monoadducts (MA) and interstrand cross-links (CL) provide a stronger inducing signal than MA. RNR2 induction kinetics were measured in relation to cell growth and survival responses after treatment with the furocoumarins 8-methoxypsoralen (8-MOP), 5-methoxypsoralen (5-MOP), 3-carbethoxypsoralen (3-CPs), 7-methyl-pyrido[3,4-c]psoralen (MePyPs) and 4,4',6-trimethylangelicin (TMA), benzo[a]pyrene (B(a)P and 1,6-dioxapyrene (1,6-DP) plus UVA, 254 nm UV radiation and cobalt-60 gamma-radiation. Induction of RNR2 took place during the DNA repair period before resumption of cell growth and clearly increased with increasing equitoxic dose levels. Treatments with furocoumarin plus 365 nm radiation (UVA) and 254 nm (UV) radiation were effective inducers whereas gene induction was relatively weak after gamma-radiation and absent after the induction of oxidative damage by B(a)P and 1,6-DP and UVA. The results suggest that it is the specific processing of different DNA lesions that determines the potency of the induction signal. Apparently, DNA lesions such as CL, and probably also closely located MA or pyrimidine dimers in opposite DNA strands involving the formation of double-strand breaks as repair intermediates, are most effective inducers.

Biophysical and biological properties of newly synthesized dioxinocoumarin derivatives. II. Dark and photoinduced effects on T7 phage, yeast and HeLa cells

The dioxinocoumarin derivatives 5H-[2]benzopyrano-[3,4-g][1,4]benzodioxin-5-one (I), 5H-[2]benzopyrano-[3,4-g][2,3]-dihydro-[1,4]benzodioxin-5-on e II, 6H-[2]benzopyrano[3,4-f]-1,4-benzodioxin-6-one (III) and 6H-[2]benzopyrano[3,4-f]-2,3-dihydro-1,4-benzodioxin-6-one (IV) were synthesized. Their biological effect was studied in the presence and absence of UVA radiation, and compared with that of 8-methoxypsoralen (8-MOP) and angelicin derivatives on T7 phage, diploid yeast (Saccharomyces cerevisiae) and HeLa cells. The photobiological activities of compounds I and III were stronger than that of 8-MOP in phage inactivation and DNA synthesis inhibition in HeLa cells, whereas compounds II and IV, with a saturated dioxin ring, showed very poor activity. The photosensitizing activity of dioxinocoumarins on phage inactivation decreased by a factor of two to three in the absence of oxygen. Treatments with compound I and UVA in the presence of oxygen modified the helical structure and stability of phage DNA and proteins. Compounds I and II were more active than IV for photoinduced cell killing in yeast, although always less active than 8-MOP. At comparable photocytotoxic levels, compounds I and III were as strong inducers of cytoplasmic "petite" mutants in yeast as angelicin, suggesting a possible monofunctional mode of action with cellular DNA.

Effects of a new psoralen, 5-geranoxypsoralen, plus UVA radiation on murine ATPase positive Langerhans cells

5-geranoxypsoralen (5-GOP), commonly called bergamottin, is a highly photoreactive psoralen, which in contrast to most furocoumarins, does not strongly interact with DNA. 5-GOP gives the opportunity to study, in a more selective way, the mechanisms of phototoxic and immunological activities induced by psoralen and UVA radiation. We investigated the effects of repetitive treatments with 5-GOP plus UVA radiation (320-400 nm) on the number of ATPase+ epidermal Langerhans cells and on the induction of photoreactivity. These effects were compared with those of 8-methoxypsoralen (8-MOP) or 5-methoxypsoralen (5-MOP) plus UVA radiation and UVA radiation alone. C3H/HeN mice were treated topically with the psoralen three times/week for 4 consecutive weeks followed each time by 1 J/cm2 of UVA radiation. At the end of the treatment, mice treated with 8-MOP or 5-MOP plus UVA radiation exhibited severe gross phototoxicity and nearly total depletion of ATPase-stained Langerhans cells. Both treatments produced severe morphological alterations of Langerhans cells. No gross but a microscopic phototoxic effect was observed after 5-GOP plus UVA radiation treatment, while the number of ATPase+ Langerhans cells was also greatly reduced. Interestingly the latter treatment induced no morphological alterations of the remaining Langerhans cells in contrast to treatment with 8-MOP or 5-MOP plus UVA radiation. We conclude that phototoxicity and decrease in the number of ATPase-stained epidermal immune cells observed after treatment with 5-GOP plus UVA radiation are not related to the DNA binding activity of the psoralen.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Photobiological activities of 1,6-dioxapyrene in pro- and eukaryotic cells

The photobiological effect of a new pyrene derivative, 1,6-dioxapyrene (1,6-DP), was studied in Salmonella typhimurium (strain TA100) and in the diploid strain D7 of the yeast Saccharomyces cerevisiae. In Salmonella, 1,6-DP shows little mutagenicity in the dark in comparison to benzo[a]pyrene (B[a]P). This mutagenic activity decreases in the presence of liver S9 homogenates from Aroclor induced XVIInc/Z mice. However, in combination with 365 nm (UVA) radiation and in the absence of S9 mix, 1,6-DP behaves as an effective photodynamic compound inducing lethal and mutagenic effects in both organisms. In yeast, its activity, like that of B[a]P, is highly dependent on the presence of oxygen. For the same incident dose of UVA, 1,6-DP is, however, at least 6 times more effective than B[a]P in inducing cytotoxic and mutagenic effects. At equitoxic doses, 1,6-DP is as photomutagenic as B[a]P, suggesting that in both cases mutagenicity is due to similar mechanisms. Spectrophotometric measurements indicate physical interaction of 1,6-DP with DNA in the dark. Laser flash photolysis experiments show that 1,6-DP generates singlet oxygen with a quantum yield of 0.17. In vitro 1,6-DP produces oxidative damage to guanine bases specific for singlet oxygen mediated reactions. Alkaline step elution analysis of 1,6-DP plus UVA treated yeast cells indicates a decrease in average molecular weights in DNA and an induction of single strand breaks (ssb) originating from alkali labile sites. This effect is enhanced by D2O and is thus likely to be due to the production of singlet oxygen. The strand breaks appear to differ from those induced by gamma-rays because little, if any, repair of these ssb occurs during 30 min of post-treatment incubation in complete growth medium. These results suggest that the photobiological effects of 1,6-DP are due to oxidative damage in DNA mostly induced by singlet oxygen.

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.

The binding of bridged bis-pyridinium oximes to DNA and its relevance to the induction of mitochondrial dysfunction in yeast

Bis-pyridium oximes and methoximes from a newly synthesized series are weak DNA binders (K = 3.10(4) M-1 under physiological conditions). From the number of binding sites per phosphate, 0.25, the ionic strength dependence of the binding constant and the negative electric dichroism, it is concluded that monointercalation is the mode of association. In contrast to methoxy compounds, the oxime derivatives are able both to induce the mutated "petite" phenotype in yeast S. cerevisiae and to cause "in vitro" extensive condensation of single stranded DNA. This reaction is postulated to be relevant to the mutational process that leads to "peptide" cells. The absence of nuclear mutation is interpreted in terms of sequestration of the drug in mitochondria under the effect of the organelle inner membrane electrochemical potential.

Benzoangelicins: new monofunctional DNA photobinding agents

4,6-Dimethylbenzoangelicin, obtained by fusing a benzene ring at the furan side of 4,6-dimethylangelicin, was studied in terms of crystal structure and interactions with DNA in both ground and excited states. 4,6-Dimethylbenzoangelicin has a planar structure and forms a molecular complex with DNA, undergoing intercalation inside the double helix. Under UVA irradiation, it photoconjugates covalently with the macromolecule, showing a DNA photobinding rate slightly lower than that of 8-methoxypsoralen, involving however only its 3,4 double bond, i.e. behaving as a pure monofunctional agent. The parameters of dark binding and photobinding were determined, and two C4 cycloadducts with thymine were isolated and characterized.

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.

Psoralen damage-induced plasmid recombination in Saccharomyces cerevisiae: dependence on RAD1 and RAD52

Photoreaction with psoralen, a DNA-crosslinking reagent, induces mitotic recombination in the yeast Saccharomyces cerevisiae. Psoralen damage-induced recombination was studied with non-replicating plasmids, which transform yeast cells by undergoing recombination events with chromosomal DNA. When plasmid DNA was photoreacted with psoralen in vitro and transformed into yeast cells, transformation was stimulated by psoralen modification in a dose-dependent manner. The stimulation by psoralen damage requires RAD52 gene function and is partially dependent on RAD1. Analysis of transformants indicates that plasmid integration occurs at the homologous chromosomal loci. Multiple tandem integrations are common in repair-proficient cells, with more than 20 copies of integrated plasmid seen in some transformants. Multiple integration depends on RAD1 function; only 9% of rad1 transformants, compared to 80% of RAD transformants, contained multiple plasmid copies, while 52% of the rad1 transformants were produced by gene conversion.

Non-specific incision of DNA due to the presence of 8-methoxypsoralen photoinduced interstrand cross-links in Saccharomyces cerevisiae

The repair of DNA interstrand cross-links (CL) induced by 8-methoxypsoralen (8-MOP) plus UVA irradiation was analyzed by the alkaline step elution technique. A double-exposure protocol was used with 8-MOP, starting with exposure to monochromatic 405-nm radiation inducing only DNA monoadducts (MA), followed, after washing out of unbound 8-MOP molecules, by a second exposure to 365-nm radiation inducing varying relative amounts of CL at a constant level of total photoadducts. In the range of doses used for the second exposure, repair of CL took place; however, in the presence of increased relative amounts of CL induced non-specific incision of DNA occurred. This endonucleolytic cleavage appears to be related to the increased mutagenic and recombinogenic effects observed at increased levels of CL.

Repair of exogenous (plasmid) DNA damaged by photoaddition of 8-methoxypsoralen in the yeast Saccharomyces cerevisiae

The contribution of different repair pathways to the repair of 8-methoxypsoralen (8-MOP) plus UVA induced lesions on a centromeric plasmid (YCp50) was investigated in the yeast Saccharomyces cerevisiae using the lithium acetate transformation method. The pathways of excision-resynthesis (RAD1) and recombination (RAD52) were found to be involved in the repair of exogenous as well as of genomic DNA. Mutants in RAD6 and PSO2 genes showed the same transformation efficiency with 8-MOP plus UVA treated plasmid as wild-type cells suggesting that these latter pathways involved in mutagenesis are not operating on plasmid DNA although required for the repair of 8-MOP photoadducts induced in genomic DNA. These results indicate that DNA-repair gene products may be differently involved in the repair of exogenous and endogenous DNA depending on the repair system and the nature of the DNA damage considered.

Metabolism of 5-methoxypsoralen by Saccharomyces cerevisiae

Incubation of methoxypsoralen (5-MOP) in the presence of diploid yeast cells (Saccharomyces cerevisiae) before UV-A exposure leads to an incubation-time dependent decrease of photoinduced genotoxic effects. The reduction in photoinduced genotoxicity is stronger in cells grown in the presence of 20% glucose and containing high levels of cytochrome P-450 than in cells grown in the presence of 0.5% glucose and containing undetectable levels of cytochrome P-450. Inhibition of P-450 activity by specific inhibitors, such as tetrahydrofuran and metyrapone, strongly affects the observed decrease in 5-MOP genotoxicity, indicating the involvement of P-450 in 5-MOP metabolism. As demonstrated by spectrophotometric and chromatographic (HPLC) analysis during incubation of 5-MOP with P-450 containing yeast cells, 5-MOP gradually disappears from the cell supernatant of the incubation mixture. The reduction in the chromatographic peak corresponding to 5-MOP is accompanied by the appearance of a new peak that probably corresponds to a metabolite. As shown by the use of P-450 specific inhibitors, the metabolite appears to be due to P-450 mediated 5-MOP metabolisation. Its UV absorption spectrum suggests an alteration of the pyrone moiety of the 5-MOP molecule.

Mitochondrial uptake of bridged bis-methylpyridinium aldoximes and induction of the "petite" phenotype in yeast

The 3,3'-[omega,omega'-alkanediylbis(oxy)]bis[2- (hydroxyimino)methyl]-1-methylpyridinium derivatives bearing a linking chain of 4, 5 and 6 methylene groups are accumulated in mitochondria with increasing efficiency under the effect of the electrical potential. Accumulation does not take place with derivatives carrying a 2 and 3 methylene-long linking chain. The uptake process is saturable. The efficiency of the various derivatives to induce the "petite" phenotype in yeast reflects the uptake rate observed with purified mitochondria.

Repair of 8-methoxypsoralen photoinduced cross-links in yeast. Analysis by alkaline step-elution and electron microscopy

The repair of interstrand cross-links induced by 8-methoxypsoralen plus UVA (365 nm) radiation DNA was analyzed in diploid strains of the yeast Saccharomyces cerevisiae. The strains employed were the wild-type D7 and derivatives homozygous for the rad18-1 or the rad3-12 mutation. Alkaline step-elution and electron microscopy were performed to follow the process of induction and removal of photoinduced cross-links. In accordance with previous reports, the D7 rad3-12 strain failed to remove the induced lesions and could not incise cross-links. The strain D7 rad18-1 was nearly as efficient in the removal of 8-MOP photoadducts after 2 h of post-treatment incubation as the D7 RAD+ wild-type strain. However, as demonstrated by alkaline step-elution and electron microscopic analysis, the first incision step at DNA cross-links was three times more effective in D7 rad18-1 than in D7 RAD+. This is consistent with the hypothesis that the RAD18 gene product is involved in the filling of gaps resulting from persistent non-informational DNA lesions generated by the endonucleolytic processing of DNA cross-links. Absence of this gene product may lead to extensive strand breakage and decreased recognition of such lesions by structural repair systems.

Sequence-specific photo-induced cross-linking of the two strands of double-helical DNA by a psoralen covalently linked to a triple helix-forming oligonucleotide

On the basis of the structure of DNA-psoralen bis adducts (formed by psoralen with two thymines on opposite strands), a psoralen-oligonucleotide conjugate was designed to photoinduce a cross-link between the two DNA strands at a specific sequence. Psoralen was attached via its C-5 position to a 5'-thiophosphate group of an 11-mer homopyrimidine oligonucleotide. The 11-mer binds to an 11-base-pair homopurine.homopyrimidine sequence of a DNA fragment, where it forms a triple helix. Upon near-UV-irradiation, the two strands of DNA are crosslinked at the TpA step present at the triplex-duplex junction. The reaction is specific for the homopurine.homopyrimidine DNA sequence and requires both oligonucleotide recognition of the DNA major groove and intercalation of psoralen at the triplex-duplex junction. The yield of the photo-induced cross-linking reaction is quite high (greater than 80%). Such psoralen-oligonucleotide conjugates are probes of sequence-specific triple-helix formation and could be used to selectively control gene expression or to induce site-directed mutations.

4,4',5'-trimethyl-8-azapsoralen, a new-photoreactive and non-skin-phototoxic bifunctional bioisoster of psoralen

Photochemical and photobiological properties of a new isoster of psoralen, 4,4',5'-trimethyl-8-azapsoralen (4,4',5'-TMAP), have been studied. This compound shows a high DNA-photobinding rate, higher than that of 8-methoxypsoralen (8-MOP), forming both monoadducts and inter-strand cross-links. The yield of cross-links, however, is markedly lower than that of 8-MOP. Antiproliferative activity of 4,4',5'-TMAP, in terms of DNA synthesis inhibition in Ehrlich ascites tumor cells, is higher than that of 8-MOP. Mutagenic activity on E. coli WP2 R46+ cells appeared similar to or even lower than that of 8-MOP. This new compound applied on depilated guinea pig skin and irradiated with UVA did not show any skin-phototoxicity. On the basis of these properties 4,4',5'-TMAP appears to be a potential photochemotherapeutic agent.

Allergic contact and photocontact dermatitis due to psoralens in patients with psoriasis treated with topical PUVA

The incidence and clinical features of allergic contact and/or photocontact dermatitis due to psoralens were examined in 371 patients with psoriasis treated with topical PUVA. The psoralen derivatives used in the study were 8-methoxypsoralen (8MOP), 3-carbethoxypsoralen (3CPs), 4,6,4'-trimethylangelicin (TMA) and 7-methyl pyridopsoralen (MPP). Of 371 patients treated with 8MOP, three (0.8%) developed an acute dermatitis in the PUVA-treated areas. This incidence was significantly lower (P less than 0.01) than that for 3CPs (four of 10 patients) or that for TMA (six of 17 patients). None of the seven patients receiving MPP on PUVA had a reaction. It was confirmed that these dermatitis reactions were due to contact and/or photocontact allergy to psoralens by several methods that include patch and photopatch tests, photopatch test mapping, determination of the minimal erythema dose (MED) and immunohistochemistry.

Mutagenic effects photoinduced in normal human lymphoblasts by a monofunctional pyridopsoralen in comparison to 8-methoxypsoralen

The photobiological effects induced by the monofuctional 7-methylpyrido[3,4-c]psoralen (MePyPs) in comparison to the bifunctional furocoumarin 8-methoxypsoralen (8-MOP) have been studied in a human lymphoblast cell line TK6. We report that, in human lymphoblasts, the cytotoxic effect of MePyPs plus UVA (365 nm) is much higher than that of 8-MOP plus 365-nm irradiation. The dose-modifying factor at the 37% survival level between the 2 compounds equals 120. Mutation induction by photoactivated MePyPs and 8-MOP has been studied in 2 genetic loci, hypoxanthine phosphoribosyl transferase (HPRT) and Na+/K+ ATPase. For equal UVA doses, the mutagenic effectiveness of MePyPs was higher than that of 8-MOP. However at equal survival levels, the mononfuctional psoralen MePyPs was less efficient than the bifunctional 8-MOP. In other words, compared to 8-MOP, the monofunctional agent MePyPs is more cytotoxic than mutagenic. This higher phototoxic and mutagenic efficiency of MePyPs in comparison to 8-MOP is likely to be related to the chemical nature of MePyPs-induced lesions which may be responsible for a reduced recognition and/or accessibility of the repair enzymes to damaged DNA.

Genotoxicity of bergapten and bergamot oil in Saccharomyces cerevisiae

In order to determine the genotoxic potential of bergapten (5-methoxypsoralen (5-MOP] and bergamot oil (BO), the genetic effects of 5-MOP and BO (containing equivalent amounts of 5-MOP) were studied in haploid and diploid yeast (Saccharomyces cerevisiae) using solar simulated radiation (SSR). At equal doses of SSR, equal concentrations of 5-MOP alone or 5-MOP in BO have a similar influence on survival and on the induction of cytoplasmic "petite" mutations, reverse and forward mutations, mitotic gene conversion and genetically aberrant colonies including mitotic crossing over. No reciprocity is found between SSR dose and 5-MOP concentration for cytotoxic, mutagenic and recombinogenic effects. In the presence of chemical filters (Parsol 1789, a UVA filter, and Parsol MCX, a cinnamate derivative acting as a UVB filter) considerable protection is observed against the induction of genetic effects by 5-MOP and BO containing 5-MOP in haploid and diploid cells. As indicated by the lower induction kinetics, the protection is higher than expected from the light-absorbing properties, suggesting photochemical interaction. The protection is slightly higher for BO than for 5-MOP. The induction of genetic effects by 5-MOP alone or BO containing 5-MOP is independent of oxygen. Experiments on suction blister fluids taken from patients after topical treatment with BO containing 5-MOP indicate that in comparison with water the bioavailability and thus the genotoxic effects of the compounds are decreased. Moreover, in addition to the filtering effect against the photoinduced genotoxic effects of BO, the presence of chemical filters apparently reduces the penetration of BO containing 5-MOP and provides a reduction in biological effectiveness.

Treatment of a case of mycosis fungoides and one of parapsoriasis en plaque with topical PUVA using a monofunctional furocoumarin derivative, 4,6,4'-trimethylangelicin

A case of plaque stage mycosis fungoides and one of parapsoriasis en plaque were treated with topical PUVA therapy using a monofunctional furocoumarin derivative, 4,6,4'-trimethylangelicin (TMA). Both patients showed complete clearance of eruptions within 16 treatments. The therapeutic effectiveness of TMA was confirmed by the fact that those eruptions exposed to UVA alone, without TMA application, showed slower and less significant improvement. Histologically, dermal infiltrates of mycosis cells and associated epidermotrophism disappeared almost completely in response to TMA PUVA. No side effects or changes in values in laboratory examinations were observed during treatment.

An action spectrum for the elicitation of erythema in skin persistently sensitized by photobound 8-methoxypsoralen

Human skin can be rendered persistently photosensitive by topical application of aqueous 8-methoxypsoralen (8-MOP) and exposure to a suberythemogenic dose of more than 380 nm radiation. We report an action spectrum for the elicitation of phototoxic erythema induced by a second exposure of skin pretreated in this way. After correction for unsensitized skin erythema this action spectrum resembles the absorption spectrum of the 4',5'-monoadduct of 8-MOP to DNA. This suggests that the monoadduct is the chromophore for erythema elicited by the second irradiation, and supports the DNA crosslink as the crucial photoproduct causing phototoxic erythema due to 8-MOP in human skin.