8-Methoxypsoralen-photoinduced DNA cross-links as determined in yeast by alkaline step elution under different reirradiation conditions. Relation with genetic effects

Single strand breaks and mutagenesis in yeast induced by photodynamic treatment with chloroaluminum phthalocyanine

Photodynamic treatment of the yeast Kluyveromyces marxianus with the sensitizer aluminum phthalocyanine results in loss of clonogenicity. In this paper the effect of this treatment on DNA of this yeast was investigated by searching for single strand breaks and forward mutations. Using the alkaline step elution technique it was found that illumination of the yeast in the presence of aluminum phthalocyanine resulted in an increase in single strand breaks. These could, partially, be repaired by post-incubating illuminated cells in growth medium. At comparable survival levels, photodynamic treatment with aluminum phthalocyanine induced fewer single strand breaks than X-ray treatment. By using a medium containing 5-fluoroorotic acid, mutants in the uracil biosynthetic pathway were selected. Photodynamic treatment resulted in a light dose dependent increase of the mutation frequency. The observed mutagenicity of photodynamic treatment of the yeast with phthalocyanine was lower than the mutagenicity of UVC and X-ray treatment at equal colony forming capacity, indicating that photodynamic treatment is the least mutagenic of those treatments. It is concluded that photodynamic treatment of K. marxianus results in DNA damage. Saccharomyces cerevisiae rad14 and rad52 mutants were used to determine the effect of the nucleotide excision repair and recombinational repair pathways, respectively, on survival after photodynamic treatment. Our data indicate that DNA damage is not the main determinant for cell killing by photodynamic treatment and that the type of damage induced is apparently not subject to RAD14- or RAD52 controlled repair.

PUVA (5-methoxypsoralen plus UVA) enhances melanogenesis and modulates expression of melanogenic proteins in cultured melanocytes

PUVA (combination of psoralens and ultraviolet A radiation) is a potent inducer of melanogenesis in normal human skin. The molecular mechanisms underlying this effect are poorly characterized. This study was undertaken to investigate the action of PUVA on melanogenesis in S91 murine melanoma cells and in cultured normal human melanocytes. Tyrosinase and DOPAchrome tautomerase (DCT) activities as well as melanin neosynthesis were measured in PUVA-treated pigment cells. To determine whether a correlation exists between PUVA-induced melanogenesis and expression of melanogenic enzymes, we analyzed the levels of tyrosinase, DCT, and tyrosinase-related protein-1 (TRP-1 or gp75) by western blotting in PUVA-treated cells. We demonstrate that UVA upregulates tyrosinase activity and melanin content with 5-methoxypsoralen at 1 microM. This phenomenon depends on the energy delivered during phototreatment. In both human and mouse cells, stimulation of melanogenesis correlated with an increase of the amount of tyrosinase. In PUVA-treated S91 cells, tyrosinase mRNA was increased, but no stimulation of DCT activity occurred in these cells, in agreement with the unchanged amount of DCT protein in cell extracts. On the contrary, in melanocytes treated with PUVA, a decrease in DCT protein was observed. Finally, the amount of TRP-1 protein was not affected by PUVA in either S91 cells or melanocytes. These results show that melanogenesis induced by PUVA is related to an increase in expression of tyrosinase. In melanocytes, melanogenesis and DCT are negatively correlated, which suggests that PUVA favors the metabolic pathway of dark-eumelanins with high UV-protective properties. This study also suggests that PUVA regulates tyrosinase, DCT, and TRP-1 expression in a noncoordinate manner.

Pulsed-field gel electrophoresis analysis of the repair of psoralen plus UVA induced DNA photoadducts in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, double-strand breaks (DSB) have been observed during the DNA repair of psoralen plus UVA induced lesions. In the present paper, we analyzed this repair step in some detail using pulsed-field gel electrophoresis (CHEF) to get a better understanding of this phenomenon with regard to the type of lesions induced and the repair pathways involved. The results confirm that, during post-treatment incubation of Saccharomyces cerevisiae cells, DSB are formed. Their appearance is dose-dependent and the rate of induction is comparable in large (chromosome IV) and small (chromosome III) chromosomes. The formation of DSB is evidenced by the breakage of linear chromosomes III and IV, but also, after high doses, by the linearization of a circular form of chromosome III. The induction of DSB appears to be highly dependent on the induction of interstrand cross-links since they are clearly present after treatments with 8-MOP plus 365 nm radiation (inducing monoadducts and cross-linking in DNA), but practically absent after treatment with 8-MOP plus 405 nm radiation (inducing predominantly monoadducts) at comparable levels of photoadducts. The occurrence of DSB is dependent on the RAD2 and RAD52, but not on the RAD6 gene. It is likely that the specific processing of DNA lesions involving DSB is related to the genotoxic consequences observed.

Inhibition of smooth muscle cell proliferation by visible light-activated psoralen

The present study was designed to evaluate the effect of 8-methoxypsoralen (8-MOP) activated with visible light (419 nm) on the suppression of smooth muscle cell (SMC) proliferation in vitro. We hypothesize that if visible light (VL) instead of UVA is used to photoactivate 8-MOP, cytotoxic 8-MOP-DNA cross-link formation can be minimized. Bovine aorta SMCs (2 x 10(4)/cm2) were incubated with 8-MOP (1 micrograms/mL) for 30 minutes (in the dark) and exposed to a range of VL (2 to 69 J/cm2) to determine the dose of VL that inhibits SMC proliferation with minimal toxicity. The results show that 8-MOP in combination with 2 to 12 J/cm2 VL reversibly inhibited SMC proliferation for up to 5 days after treatment. SMC viability was confirmed by trypan blue exclusion. 8-MOP in combination with 23- or 69-J/cm2 VL irreversibly inhibited SMC proliferation. In cell cycle studies, 12-J/cm2 VL was used to activate 8-MOP. A phase-specific G2 blockade that correlated temporally with recovery of SMC replication was observed. Photoadduct repair studies showed that cell proliferation rates recovered when 60% of the adducts had been removed. These results demonstrate for the first time the possibility of using VL to activate 8-MOP to inhibit cell proliferation and suggest that 8-MOP/VL photochemotherapy can be used to control SMC growth.

Regulation of melanogenesis induced by 5-methoxypsoralen without ultraviolet light in murine melanoma cells

Melanogenesis in melanoma cells can be enhanced by psoralens in the absence of UV light. Melanin biosynthesis is regulated by a number of melanocyte-specific proteins, including tyrosinase, DOPAchrome tautomerase (DCT), and tyrosinase-related protein-1 (TRP-1, gp75). To get more insight on the molecular mechanisms involved in psoralens-induced melanogenesis, we determined tyrosinase and DCT activities as well as mRNA and protein levels of tyrosinase, DCT, and TRP-1 in S91 mouse melanoma cells treated by 5-MOP. High concentration of 5-MOP (5 x 10(-5) M) induced a time-dependent increase of tyrosinase activity and melanin content, which was correlated to an increase of both mRNA and protein levels of tyrosinase. These results demonstrate that the 5-MOP stimulation of melanogenesis is related to increased tyrosinase synthesis. In addition, 5-MOP stimulated TRP-1 synthesis and induced a dose-dependent decrease of DCT activity without any modification in the expression of the protein. We explored then the signalling pathways involved in 5-MOP-induced melanogenesis and, particularly, the role of cyclic AMP and protein kinase C (PKC). A small stimulation of cyclic AMP production was observed in presence of 5-MOP. Furthermore, 1-oleoyl-2-acetylglycerol (OAG), a PKC activator, potentiated the 5-MOP stimulation of tyrosinase activity, while calphostin, a specific PKC inhibitor, inhibited the 5-MOP induction of tyrosinase activity. Phorbol-myristate acetate (PMA), described as a strong activator of PKC, inhibited also the effect of 5-MOP when used at long term. Taken together, these results demonstrate that in murine melanoma cells 5-MOP stimulates melanogenesis by increasing activity and synthesis of tyrosinase. Tyrosinase and TRP-1 expression are coordinately regulated by 5-MOP. Furthermore, a negative correlation between melanogenesis and DCT activity was observed under 5-MOP stimulation. At least, PKA and PKC systems appear to play an important role in the melanogenic effect of 5-MOP.

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.

Improved high-performance liquid chromatographic analysis of 8-methoxypsoralen monoadducts and cross-links in polynucleotide, DNA, and cellular systems: analysis of split-dose protocols

The distribution of 8-methoxypsoralen-thymidine photoadducts from polynucleotides, calf thymus DNA and mammalian cells treated with [3H]8-methoxypsoralen under a variety of irradiation conditions was determined using high-performance liquid chromatography and scintillation analysis. The split-dose protocol, with samples treated with 8-methoxypsoralen and low doses of long-wavelength UV radiation to generate monoadducts, washed to remove unreacted 8-methoxypsoralen, then irradiated further to convert the monoadducts to cross-links, was examined. The photoadduct distribution in the first step is dependent upon the UVA dose and the wavelength of the radiation, but it is relatively independent of 8-methoxypsoralen concentration. Low fluence and longer wavelengths generate mainly 4',5'-monoadducts, whereas higher fluences and shorter wavelengths yield more cross-links. The second irradiation step converts the 4',5'-monoadducts to cross-links as well as to 3,4-monoadducts. The overall yield of cross-links after the second irradiation step is not dependent upon the wavelength used in the first step. Cellular studies demonstrated that the split-dose protocol is applicable to mammalian systems. These results may affect the interpretation of mutagenesis studies based on the split-dose protocol, because the second step can convert 4',5'-monoadducts to both 3,4-monoadducts, the expected cross-links. Therefore, interpretations that link increases in mutagenicity after the second step in a split-dose study solely to cross-link formation may need re-examination.

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.

Comparison of the effect of 8-methoxypsoralen (8-MOP) plus UVA (PUVA) on human melanocytes in vitiligo vulgaris and in vitro

In this study, we examined the various effects of PUVA treatment on cultured human melanocytes, and it revealed that 1) the higher the dose of PUVA treatment, the more significant the inhibition of cell DNA and protein synthesis; 2) the higher the dose of PUVA treatment, the more significant the depletion of epidermal growth factor receptor (EGFR) expression; 3) PUVA treatment at 124 mJoule/cm2 depleted the vitiligo-associated melanocyte antigens (VAMA) immediately after irradiation, and both the VAMA and EGFR expression progressively recovered at 24 or 72 h after PUVA; 4) PUVA treatment stimulated tyrosinase activity, but not in a dose-dependent fashion. In vitiligo vulgaris, PUVA treatment may stimulate the regrowth of melanocytes from hair follicles, but deplete the epidermal Langerhans cells in depigmented lesion of patients with stable vitiligo. Comparing the above results obtained from in vivo and in vitro studies, it reveals significantly different biologic responses. Although the precise therapeutic mechanism of PUVA treatment in vitiligo is still not well known, it is proposed that 1) PUVA treatment may stimulate the other components of skin, such as keratinocytes, to release inflammatory mediators and some of them may act as melanocyte growth-stimulatory factors (MGSF), which further enhance the proliferation of remaining melanocytes in hair follicle; and 2) PUVA treatment may deplete the VAMA expression on cell membrane of melanocytes and also deplete epidermal Langerhans cells, which may result in blocking the progressing of antibody-dependent cell-mediated cytotoxicity to melanocytes in vitiligo.

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.

Long persistence of monofunctional 8-methoxypsoralen-DNA adducts in human skin in vivo

Human skin can be persistently photosensitized by topical application of aqueous 8-methoxypsoralen plus immediate irradiation with a non-erythemogenic dose of wavelengths above 380 nm. Re-exposure of skin thus sensitized to broadband UV-A produces phototoxic erythema 72-120 h later. The persistence of the photosensitization was demonstrated by phototoxic erythema after re-exposure up to 15 days after the first sensitizing irradiation. According to the concept that the first exposure induces primarily psoralen monoadducts, we consider this an investigation of psoralen monoadduct persistence. In contrast to several earlier studies, this sensitive method indicates that psoralen monoadducts may remain in human skin in vivo for more than 2 weeks after formation.

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.

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.

Repair of furocoumarin-plus-UVA-induced damage and mutagenic consequences in eukaryotic cells

In the presence of near-UV radiation (UVA) furocoumarins (psoralens) photoinduce defined lesions in DNA, i.e. monoadducts and interstrand crosslinks. Their use in photochemotherapy (psoralen plus UVA (PUVA) treatment) and cosmetics raises questions concerning the repairability of these lesions and their genotoxic consequences. We have analysed the repair of psoralen photoadducts in cultured eukaryotic cells, such as yeast and mammalian cells, for furocoumarins of photochemotherapeutic interest. In yeast, the interaction of repair pathways differs in exogenous (plasmid) and endogenous (chromosomal) DNA. The order of mutagenic activity is 4,5',8-trimethylpsoralen greater than 5-methoxypsoralen greater than 8-methoxypsoralen greater than 7-methylpyrido[3,4-c]psoralen greater than 3-carbethoxypsoralen. The mutagenicity is dependent on psoralen functionality, concentration and bioavailability, maximal UVA dose, wavelength, dose (fluence) rate and presence or absence of chemical filters. It probably involves an inducible component. Chromosome breakage occurs during the repair period after PUVA treatment. It appears that the genotoxic effects of psoralens are produced by a specific arrangement of induced photolesions and the interaction of different repair systems.

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.

Molecular dosimetry of 8-MOP + UVA-induced DNA photoadducts in Saccharomyces cerevisiae: correlation of lesions number with genotoxic potential

Acid hydrolysis of purified DNA extracted from cells of a haploid repair-proficient (RAD) yeast strain that had been treated with 8-MOP + UVA revealed the existence of two major and one minor thymine photoproduct. At survival levels of the RAD strain between 100% and 1% furanside monoadducts constituted the major DNA lesion, followed by diadducts that, at the lowest survival level, nearly reached 50% of the thymine photoproducts; pyrone-side monoadducts were only detectable at the highest UVA exposure dose applied and clearly constitute a minority photoproduct. The number of induced diadducts was verified by determination of interstrand cross-links via denaturation and renaturation of 8-MOP + UVA-treated DNA from RAD and rad2 yeast strain. 8-MOP + UVA was shown to induce two types of locus-specific mutations: reversion of the lys1-1 ochre allele was between 20- to 50-fold higher than that of the his4-38 frameshift allele. Mutant yield for the lys 1-1 reversion was the same in RAD and excision repair-deficient rad2-20 strains whereas frameshift mutagenesis was about eightfold higher in the rad2-20 background.

Induction and removal of DNA interstrand cross-links in V-79 Chinese hamster cells measured by hydroxylapatite chromatography after treatments with bifunctional furocoumarins

DNA interstrand crosslinks (CL) photoinduced by bifunctional furocoumarins in V-79 Chinese hamster cells were measured by alkaline denaturation and hydroxylapatite chromatography. Treatments with 5-methoxypsoralen (5-MOP), 8-methoxypsoralen (8-MOP) and 4,5',8-trimethylpsoralen (4,5',8-TMP) and 365 nm irradiation (UVA) confer a dose-dependent linear increase in the amount of double-stranded DNA indicating the induction of CL. Determination in alkaline sucrose gradients of the molecular weight of the DNA and estimation of drug-induced strand breakage allowed quantification of the CL induced. 5-MOP was found to be slightly more effective than 8-MOP whereas 4,5',8-TMP was 9 times more effective for the induction of CL. The fate of CL during post-treatment incubation was also followed. Cells in exponential growth phase were found to be efficient in the removal of CL.