Singlet oxygen: a major reactive species in the furocoumarin photosensitized inactivation of E. coli ribosomes

The effects of singlet oxygen produced by photodynamic action on the mitochondrial permeability transition differ in accordance with the localization of the sensitizer

We have examined whether the effects of singlet oxygen (1O2) produced by photodynamic action on the mitochondrial permeability transition (PT) can be modulated by the localization of photosensitizers in irradiated mitochondria. We have previously shown that oxidation due to 1O2 photogenerated in hematoporphyrin (HP)-loaded mitochondria can prevent opening of the PT pores, likely after degradation of some critical histidines (Salet et al, 1997, J. Biol. Chem. 272, 21938-21943). Equally, in the present study we have irradiated mitochondria in the presence of a structurally different photosensitizer producing 1O2, namely 4,5',8-trimethylpsoralen (TMP). Fluorescence studies show that TMP binds to protein sites which differ from those of HP. In sharp contrast with HP, TMP-driven photodynamic action triggers per se pore opening. Interestingly, this inducing effect is inhibited when TMP-treated mitochondria are irradiated after addition of mersalyl, a specific reagent protecting thiol groups of the inner mitochondrial membrane that are oriented toward the external hydrophilic phase. This fact suggests that 1O2-mediated thiol oxidation is responsible for TMP-photoinduced pore opening. Taken together, these findings suggest that 1O2 can activate or inactivate a cellular function such as mitochondrial PT depending on the site where it is produced in the mitochondrial membrane.

Photophysical properties of the lowest excited singlet and triplet states of thio- and seleno-psoralens

The decay processes of the lowest excited singlet and triplet states of five heteropsoralens (HPS) were investigated by steady-state and shift-phase fluorometry and by laser-flash photolysis in different solvents. The emission spectra of HPS are detectable only in trifluoroethanol (TFE), where fluorescence lifetimes (tau F) and quantum yields (phi F) were measured. The triplet lifetimes (tau T), triplet (phi T) and singlet-oxygen production (phi delta) quantum yields were determined in benzene, ethanol and TFE by laser-flash photolysis. Semiempirical (INDO/1-CI) calculations allowed the nature of the lowest excited singlet and triplet states and transition probabilities to be obtained. Theoretical and experimental results indicate that the two lowest excited singlet states S1 and S2 of HPS are close-lying and different in nature (pi,pi* and n,pi*). The "proximity effect" between these two states controls the photophysical properties of HPS as it does for the other furocoumarins. However, HPS have a peculiar behavior with respect to the related compounds because they are fluorescent and have, in three cases, detectable intersystem crossing only in TFE. This behavior can be tentatively explained by a different energy gap and/or order between the S1 and S2 states.

Proposed mechanism for the photodynamic generation of 8-oxo-7,8-dihydro-2'-deoxyguanosine produced in cultured cells by exposure to lomefloxacin

In this study, lomefloxacin (LMX), a widely used quinolone antibiotic with a high frequency of clinical phototoxicity, was investigated by measuring the effects of several antioxidants on its ability to form of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) in cultured adult rat liver cells after exposure to UVA. In the current study the observed DNA damage, reflected by the formation of 8-oxo-dG, was almost completely inhibited by co-incubation of LMX and cultured cells with sodium azide (NaN3) that specifically quenches singlet oxygen. Vitamin E (alpha-tocopherol), known to quench both superoxide and singlet oxygen, inhibited 8-oxo-dG formation by approximately 54%. Mannitol, a hydroxyl radical scavenger, inhibited 8-oxo-dG formation by 64%. Butylated hydroxyanisole (BHA), a scavenger of hydroxyl, peroxy and alkoxy radicals, showed no inhibition of 8-oxo-dG formation but in fact enhanced levels of 8-oxo-dG by 169%. The results of this study suggest that the mechanism for the photodynamic generation of 8-oxo-dG by LMX is mediated, at least in part, by both singlet oxygen and hydroxyl radical and involves both type I and type II photosensitization.

RNA-protein cross-links induced by sensitization with a pyrroloquinolinone derivative, a furocoumarin analogue

The capacity of 2,6-dimethyl-9-methoxy-4H-pyrrolo [3,2,1-ij] quinolin-4-one (PQ), a furocoumarin analogue, of inhibiting protein synthesis in Ehrlich cells upon UVA irradiation was investigated. Using 8-methoxypsoralen (8-MOP) as a reference, we observed that in our short-term test the block of RNA synthesis do not affect protein synthesis, which is driven by pre-synthesised molecules of m-RNA; actually 8-MOP, studied at 100 microM concentration, practically abolished RNA synthesis without affecting significantly protein synthesis. Studying PQ sensitization in HL60 cells by alkaline elution and protein precipitation, the formation of covalent RNA-protein cross-links was observed. 8-MOP, assayed in severe experimental conditions, induced only moderate amounts of such lesion. On the basis of the data obtained in experiments carried out using various scavengers or exposing cells to UVA light in a nitrogen atmosphere, this damage appeared to be due to singlet oxygen formation, which is generated by PQ to a large extent. These results are consistent with the data obtained by H. Singh and J.A. Vadasz (Singlet oxygen: a major reactive species in the furocoumarin photosensitized inactivation of E.coli ribosomes, Photochem. Photobiol., 28 (1978) 539-545) on E.coli ribosomes. The lower activity we observed with 8-MOP might be attributed to a different sensitivity of whole mammalian cells in comparison with isolated ribosomes.

Photoreaction of new psoralen analogs with DNA: sequence and mutation specificity in the Escherichia coli lacZ gene

New thio- and seleno-analogs of psoralen were synthesized and analyzed for their photoreactivity toward DNA. Using oligonucleotides of defined sequence, we first showed that these derivatives predominantly generated interstrand crosslinks at 5'-TpA sites. We also observed a surprisingly high reactivity of 7H-thiopyrano[3,2-f][1]benzofuran-7-one (PSO[O-S]) with the BamHI and PstI oligomers, giving rise to the formation of crosslinks at 5'-ApT sites and of the thymidine-psoralen-cytosine type. Next, the sequence specificity in the photochemical binding of all the compounds was investigated in two DNA fragments encompassing the lacZ gene of Escherichia coli, using the T4 DNA polymerase sequencing methodology. Resulting maps demonstrated that thio- and seleno-analogs of psoralen preferentially photoreacted with thymine and cytosine residues. The AT-rich sequences proved to be particularly reactive sites as did adjacent thymines, especially at C-surrounding residues. Likewise, photoaddition at cytosines in CA/AC context was observed. It was highly significant that all of the derivatives exhibited similar sequence specificities with only minor differences. However, PSO(O-S) differed from the other heteropsoralens. Photoadducts occurred with a higher frequency at AC and CA dinucleotides, and new sites were detected. A comparison with 8-methoxypsoralen photobinding is also reported. Finally, the mutagenic consequences of photoadducts induced in M13mp19 DNA by PSO(O-S) were determined in a forward system that detects all classes of mutagenic events. The high phototoxicity exhibited by PSO(O-S) could be attributed to crosslinks, and the comparison of the observed mutational specificity with the photoadduct distribution within the same gene showed that mutations were targeted at potential monoadduct sites where photolesions were detected in our footprinting experiments.

Isolation and partial characterization of a novel psoralen-tyrosine photoconjugate from a photoreaction of psoralen with a natural protein

The photoreaction of psoralens with DNA is a well-characterized reaction. However, the photoreactions of psoralens with proteins is not very well understood. Our objective was to isolate an amino acid-psoralen photochemical adduct. We photoreacted 8-methoxypsoralen (8-MOP) with T7 RNA polymerase, a protein that carries out the fundamental biological process of transcription. Amino acid composition analysis of the photoreacted polymerase revealed that tyrosines quantitatively reacted with 8-MOP. From the acid hydrolysates of the photoconjugated T7 RNA polymerase, an 8-MOP-tyr adduct was partially purified by HPLC. The purified 8-MOP-tyr adduct and related parent compounds were analyzed by UV-visible absorption, fluorescence and mass spectroscopy. Excitation/absorption spectra suggested that the pyrone of the original 8-MOP was modified in the isolated photoadduct, and that the adduct probably contained a benzofuran. Chemical ionization mass spectrometry was consistent with the photoaddition of tyr to 8-MOP with a conservation of the overall mass (+/-1 atomic mass units). As far as we know, this work represents the first instance of isolation and partial characterization of an amino acid-psoralen photoadduct.

Cross-linking of DNA-binding proteins to DNA with psoralen and psoralen furan-side monoadducts. Comparison of action spectra with DNA-DNA cross-linking

We have developed a novel photocross-linking technique using free 8-methoxypsoralen and DNA furan-side monoadducts plus long wave ultraviolet light (UVA). Both sequence-specific (Max) and nonspecific (RecA and T7 RNA polymerase) DNA-binding proteins were cross-linked. The macroscopic equilibrium binding constant ( approximately 10(9) M-1) and DNase I footprinting indicated that binding of Max to its cognate sequence (E-box) was unimpaired by 8-methoxypsoralen and that cross-linking occurred in normal complexes. RecA protein and T7 RNA polymerase were cross-linked to a 12-mer DNA furan-side monoadduct with UVA. Cross-link yields were directly proportional to the UVA dose. Cross-links were stable to 8 M urea, 1-10% SDS, commonly used alcohols, and mild acids (5% trichloroacetic acid). The DNA in cross-links was reversed with 254 nm UV (photoreversal) or with hot base (base-catalyzed reversal), consistent with (2 + 2) cycloaddition via the 4',5'-furan of the psoralen. Comparative action spectra for DNA-DNA cross-linking and DNA-protein cross-linking revealed that the latter occurred maximally at 300 nm, while the former occurred maximally at 320 nm. This 20-nm blue shift suggested a higher potential energy surface for an excited psoralen participating in protein-DNA cross-linking as compared with DNA-DNA cross-linking. As with DNA-DNA cross-linking, DNA-protein cross-linking is a two-photon process. Absorption of the first photon formed a 4',5'-adduct with DNA, which then absorbed a second photon, leading to cross-linking to protein. Based on the action spectra and the known excited states of psoralen, it is suggested that the triplet n,pi* transition localized in the C-2=O of psoralen may be involved in protein-psoralen photoreactions.

Transformation of Escherichia coli cells by pH 2.3 plasmid DNA treated with psoralens plus near-UV light

The effect of two psoralens, 8-methoxypsoralen (8MOP) and angelicin (ANG), plus near-UV (PUVA) on the transformation capacity of pH 2.3 plasmid DNA on Escherichia coli was studied. Under identical experimental conditions the 8MOP linking to plasmid DNA drastically decreased its transformation capacity compared with the ANG linking. In the case of 8MOP, the decrease depends on the UV dose, as well as on the molar ratios of psoralen and DNA nucleotides. When the effect of short-wavelength UV (UVB) was tested, the higher the molar ratios, the more the combined effects of PUVA and UVB were negative.

Impairment of phagocytic cell respiratory burst by UVA in the presence of fluoroquinolones: an oxygen-dependent phototoxic damage to cell surface microvilli

Fluoroquinolones are widely used clinically as broad-spectrum antimicrobial agents. One of their side effects is UVA-dependent photosensitivity, observed after the skin is exposed to sunlight. We have investigated five fluoroquinolones and have found that their phototoxicity is oxygen dependent. Human phagocytic leucocytes were stimulated with serum opsonized zymosan to produce superoxide radical (O2-) (respiratory burst) in the presence of a sensitive O2(-)-specific cypridina luciferin analogue, 2-methyl-6-(p-methoxyphenyl)-3,7-dihydroimidazol (1,2-alpha) pyrazin-one hydrochloride (MCLA), as chemiluminescence reagent with which O2- can react to induce photon emission. The photon count was used as a measure of respiratory burst activity. When leucocytes were irradiated with UVA for 10 min in the presence of 3 micrograms ml-1 lomefloxacin, ciprofloxacin or norfloxacin, a marked decrease in respiratory burst activity was observed; in this respect, ofloxacin and tosufloxacin were weak. Scanning electron microscopy revealed that the cell surface microvilli were destroyed. The phototoxicity of fluoroquinolones could be abolished if oxygen in the tests was replaced by nitrogen or if the aminothiol DL-cysteine (1.5 mg ml-1) was added prior to irradiation. It is suggested that an oxygen species derived from UVA-excited drug molecules and oxygen mediates the phototoxicity of these fluoroquinolones.

Effects of hydroxyl radical scavengers on relaxation of supercoiled DNA by aminomethyl-trimethyl-psoralen and monochromatic UVA photons

The ability of scavengers of hydroxyl radical (OH radical) to modulate the photosensitized relaxation (induction of the first single-strand break) of supercoiled plasmid DNA with UVA photoactivated 4'-aminomethyl-4,5',8-trimethylpsoralen was examined by comparing the dose reduction factor (DRF: the ratio of fluence required to induce the same degree of relaxation in the absence to the presence of OH radical scavengers). The addition of mannitol, azide, acetate, or formate at concentrations inversely proportional to the value of the rate constants for the scavenging of OH radicals partially attenuated the supercoiled DNA relaxation. The degrees of protection afforded by the four scavengers in the presence of AMT photoactivated by either 334 nm or 365 nm monochromatic photons were similar, giving an average DRF of about 0.25 in all cases. Given the diverse chemical nature of the scavengers and their wide range of concentrations utilized, these findings are evidence for the involvement of a Type I photosensitization in the induction of DNA single-strand breaks by photoactivated AMT.

2,6-Dimethyl-9-methoxy-4H-pyrrolo[3,2,1-ij]quinolin-4-one, a new compound with unusual photosensitizing properties

Some photobiological properties of 2,6-dimethyl-9-methoxy-4H-pyrrolo[3,2,1-ij]quinolin-4-one (PQ) have been studied in comparison with 8-methoxypsoralen (8-MOP). In Ehrlich cells, PQ induced a moderate inhibition in DNA and RNA syntheses in the dark, which appeared to be more pronounced upon UVA irradiation. In contrast to 8-MOP, in the presence of UVA, PQ also affected protein synthesis. Likewise marked antiproliferative effects were also observed in the study of the clonal growth of CHO cells cultivated in vitro. Using alkaline elution and CHO cells, a moderate formation of single-strand breaks (SSBs) and of DNA-protein cross-links (DPCs) was observed by incubation in the dark; upon UVA irradiation the amount of both lesions increased greatly, whereas no inter-strand cross-links (ISCs) were formed. As expected, 8-MOP did not damage DNA in the dark, but induced SSBs, ISCs and DPCs in the presence of UVA. The induction of SSBs by both compounds seems to be directly related to a photochemical event rather than to incisions during DNA repair. As the induction of ISCs, and also the formation of DPCs by 8-MOP and UVA, appears to be based on a two-step reaction involving photo-bound 8-MOP-DNA moieties. In contrast, the formation of DPCs by PQ and UVA seems to involve photosensitization by free PQ molecules connected with SSB and DPC formation rather than with a DNA photo-binding activity. The PQ activity observed in the dark could probably be ascribed to a moderate inhibition of topoisomerases.

Photobiological activity of sulphur and selenium analogues of psoralen

Eight psoralen analogues, in which sulphur or selenium replaces one or both intracyclic oxygen atoms, were synthesized. Photoreaction with M13mp19 RF DNA in the presence and absence of oxygen (wavelength, greater than 320 nm) was studied. The damaged viral DNA was transfected into Escherichia coli and scored for infectivity towards Ca-treated wild-type E. coli. This allowed a comparative evaluation to be made of the heteropsoralens in terms of the photoreaction with DNA and the photodynamic effect. Most of the seleno- and thio-psoralens show very high photoactivity towards DNA compared with psoralen and 8-methoxypsoralen (8-MOP). Their photoreactivity is due mainly to a [2 + 2] photoreaction, since only a minor influence of molecular oxygen could be detected. Some of the studied seleno- and thio-psoralens are very efficient DNA photoinactivating agents and show great promise in photochemotherapy.

The phototoxicity of 8-methoxythionepsoralen and 6-methylthionecoumarin

The phototoxicity of 8-methoxythionepsoralen (8-MOTP) and 6-methylthione coumarin (6-MTC) when activated by UV-A has been investigated using a variety of Escherichia coli strains, Haemophilus influenzae transforming DNA and Escherichia coli pBR322 plasmid DNA. The results demonstrate that 8-MOTP is a strictly oxygen independent photosensitizer that is about 500-fold less efficient in forming lesions leading to equivalent lethality than is the parent compound from which it is derived (8-MOP). As is true for 8-MOP, 8-MOTP is capable of inducing histidine independent mutations in E. coli and inactivating transforming DNA consistent with DNA being a target for lesions induced by this molecule in the presence of UV-A. 6-MTC is a strongly oxygen dependent photosensitizer activated by UV-A when tested with either E. coli cells or transforming DNA in contrast to the parent compound (6-methylcoumarin; 6-MC) which is not phototoxic when treated with UV-A. These results imply that the membrane may be an important target leading to lethality. 6-MTC in the presence of UV-A can inactivate pBR322 plasmid and Haemophilus influenzae transforming DNA activity in vitro suggesting that DNA is a potential target for this molecule when activated by UV-A.

Psoralen preparation of antigenically intact noninfectious rotavirus particles

The use of the synthetic psoralen 4'-aminomethyl-4,5',8-trimethylpsoralen hydrochloride (AMT) is described for the inactivation of infectious rotavirus, a member of the viral family Reoviradae with a double-stranded RNA genome. This method not only provides complete inactivation of the virus but leaves antigenically intact particles. The lack of viral replication following inactivation was determined with an immunohistochemical focus assay. The antigenic authenticity of the particles was determined by monoclonal antibody ELISA and a viral hemagglutination assay.

Angelicins, angular analogs of psoralens: chemistry, photochemical, photobiological and phototherapeutic properties

Angelicin and some of its derivatives are naturally occuring compounds which show interesting photobiological properties. In this review various aspects of angelicin and its derivatives have been reported. The natural occurrence and the chemical synthesis both of naturally occurring and synthetic angelicins have been reviewed. Photochemical and photophysical properties of angelicins have been considered with particular reference to the capacity to generate active forms of oxygen, photoreactions with nucleic acids, proteins and unsaturated fatty acids. Photobiological effects have been considered: skin phototoxicity, antiproliferative effects, genotoxicity, ability to induce hemolysis in erythrocytes, inactivation of prokaryotic and eukaryotic microorganism and of viruses. The ability of some angelicins to induce photocarcinogenesis has been reviewed as well as in the activity as photochemotherapeutic agents.

Mechanisms of photodynamic effects of furocoumarins

The photosensitizing action of furocoumarins on biological systems occurs by both an oxygen-independent pathway, which involves the photoaddition of the sensitizer to nucleic acids, proteins and lipids, and an oxygen-dependent pathway, which includes furocoumarins in the category of photodynamic sensitizers. The photodynamic action of furocoumarins, as studied using isolated biomolecules, human erythrocytes and human skin, appears to involve both activated oxygen species (singlet oxygen, superoxide anion, and hydroxyl radicals) and radical species formed by electron transfer from or to photoexcited furocoumarins. Another oxygen-dependent process involves the formation of photo-oxidized furocoumarin derivatives, which can react in the dark with several substrates (in particular, membrane components), causing an irreversible damage of cells. The latter type of process is temperature dependent. The relative importance of the different photosensitization mechanisms under various experimental conditions is discussed.

Synergism between aflatoxins in covalent binding to DNA and in mutagenesis in the photoactivation system

Aflatoxins (AFs) produce singlet oxygen upon their exposure to UV (365-nm) light. Singlet oxygen in turn activates them to mutagens and DNA-binding species. DNA binding and mutagenesis by AFs were enhanced in D2O as compared to reactions in H2O, and a singlet oxygen scavenger inhibited mutagenesis. DNA photobinding of 3H-AFB1 increased in the presence of unlabeled AFB2, and the addition of AFB2 enhanced mutagenesis by AFB1 in a synergistic manner. These results are compatible with the notion that singlet oxygen, formed by one aflatoxin molecule, can readily activate another aflatoxin molecule. This may bear an environmental implication in that the weakly carcinogenic AFB2, which is often produced in nature together with AFB1, may be important in enhancing the activation of AFB1 by sunlight.

Cancer chemoprevention by supplemental carotenoids and synergism with retinol in mastodynia treatment

Cancer chemoprevention with beta-carotene (BC), canthaxanthin (CX) and retinol-BC is reported with respect to skin, breast, gastric, colon carcinogeneses induced by benzo(a)pyrene (BP) with or without ultra violet radiation (UV-A, UV-B), dimethylbenzathracene (DMBA) +/- UVB, P-UVA, N-methyl-N'-N-nitro-nitrosoguanidine (MNNG), dimethylhydrazine (DMH), and with respect to transplanted tumours. When animals were loaded with carotenoid supplementation one month before the carcinogenic induction (continued throughout the experiment), cancer prevention was observed up to 60-100%. The absence of provitamin A-activity in CX shows the carotenoid antioxidant property. Fifteen patients given BC + CX to prevent recurrences after radical removal of the primary neoplasia in organs like lung, breast, colon, urinary bladder, head and neck were studied in 1980-89. A longer than expected disease-free interval was preliminarily found. Supplementation of BC +/- retinol was also reported to prevent and treat oral leucoplakia. Supplementation and intermittent retinol administration was also tested in benign cyclical mastalgia with clear cut side effect free therapeutic results.

Psoralen/UV inactivation of HIV-1-infected cells for use in cytologic and immunologic procedures

A rapid procedure for the inactivation of HIV-1-infected cells using psoralen and ultraviolet (UV) light is described. Exposure of HIV-1-infected cells to 5 micrograms/ml psoralen followed by UV irradiation (320-380 nm) for 5 minutes yields cells that are noninfectious as assessed by extended infectivity assays. The psoralen/UV inactivation procedure described is effective with cells chronically or acutely infected with HIV-1 and is unaffected by cell densities up to 12 x 10(6)/ml. At 5 micrograms/ml psoralen does little damage to cellular permeability as shown by the ability of treated cells to exclude trypan blue and propidium iodide. Psoralen/UV treatment of HIV-1-infected cells does not cause a significant decrease in the reactivity of HIV-1 core and envelope antigens or cellular antigens to monoclonal antibodies. Experiments are presented demonstrating the use of these cells for flow cytometry studies and for cell surface labeling using the lactoperoxidase 125I iodination procedure.

Cancer chemoprevention by supplemental carotenoids in animals and humans

Experiments were carried out in mice demonstrating that dietary carotenoids (beta-carotene or canthaxanthin), starting before cancer initiation and continuing throughout the experiment, have a protective effect against indirect skin carcinogenesis induced by benzo[a]pyrene +/- UVA and breast cancer induced by 8-methoxypsoralen + UVA. Experiments in rats demonstrated that carotenoids also prevent the direct gastric carcinogenesis induced by N-methyl-N'-nitro-nitroso-guanidine. Recently, prevention by beta-carotene against colon cancer induced in mice by dimethylhydrazine, another indirect carcinogen, was confirmed by others. The prospects for carotenoid intervention with humans were based on their antitumorigenic effect, which is quite independent of pro-vitamin A activity, their lack of toxicity even after prolonged administration, and their immunostimulating activity. These facts helped to build up a rationale predicting that any epithelial cancer, after radical surgery, can be chemoprevented with supplemental carotenoids. Thus, it is expected that the remaining initiated epithelial tissue will be protected by quenching oxygen radical formation, against the onset of a second primary malignancy. This type of prevention can be envisaged in organs like the lung, urinary bladder, breast, stomach, and colon-rectum. At present, human intervention protocols with a randomized drug/placebo method are underway under the supervision of the Centro Tumori of Pavia to chemoprevent with beta-carotene second primary lung or bladder cancer after radical surgery. Preliminary observations regarding findings in humans without randomization (1980-1988) in Pavia are also reported here. This consisted of chemoprevention with beta-carotene plus canthaxanthin against recurrence of different epithelial malignancies after radical treatment (surgery +/- chemoradiotherapy). None of the 11 cases recruited, on the basis of radical nature of treatment and patient adherence, have shown any recurrence beyond their expected disease-free intervals.

Lysosomes: a possible target for psoralen photodamage

Treatment in vitro of Ehrlich ascites tumor cells or human fibroblasts with 8-methoxypsoralen (8-MOP, 2.4 microM) and UVA irradiation results in a 30% and 60% respectively reduction in lysosomal beta-galactosidase activity in situ. Under identical conditions one 8-MOP adduct was formed per 2 X 10(4) bases of DNA, one 8-MOP adduct was formed per approximately 10(4) tRNA molecules and one per approximately 100 ribosomes. It is suggested that the decrease in lysosomal beta-galactosidase activity is a result of leakage through the lysosomal membrane caused by psoralen-UVA damage of the lipids in the membrane, since no effect was found on beta-galactosidase in vitro. These results indicate that the lysosomes may also be a target for cellular photodamage by 8-methoxy-psoralen.

Chemoprevention of indirect and direct chemical carcinogenesis by carotenoids as oxygen radical quenchers

Beta-carotene (BC) and canthaxanthine (CX), two carotenoids with and without pro-vitamin A activity, respectively, were found to help to prevent benzo[a]pyrene (BP)-induced skin carcinogenesis in the dark and BP photocarcinogenesis (UV 300-400 nm) when given as an oral supplement to female Swiss albino mice. The same experimental procedure was adapted to 8-methoxypsoralen (8-MOP) photoinduction of mammary carcinomas in mice. Here also, the two carotenoids were strongly antitumorigenic. Indeed, 8-MOP photomutagenesis, tested in S. typhimurium TA 102, appeared to depend on a two-step reaction, namely an oxygen-independent DNA-8-MOP photoadduct, followed by an oxygen-dependent second step, sensitive to carotenoids. This result suggests that dietary carotenoids (powerful antioxidant molecules) might prevent the carcinogenic risk caused by substances that are transformed into ultimate carcinogens by oxidative processes which are indirectly carcinogenic. Finally, to verify whether supplemental carotenoids can affect carcinogenesis where neither light excitation nor oxidative metabolic processes are involved, an experimental attempt was made on gastric carcinogenesis induced in rats by the direct carcinogen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The results demonstrate that supplemental carotenoids do not affect initiation and progression stages, but do prevent the progression stage of dysplasias to infiltrating gastric carcinomas. Thus, this provides strong presumptive evidence for oxygen radical involvement in the later stages of this neoplastic development, as recently reported in the literature. As far as mutagenicity in S. typhimurium is concerned, carotenoids do not exert, as expected, any protective effect on MNNG mutagenic activity. The above experimental data suggest that supplemental carotenoids, instead of sunscreen preparations, can be adopted by outdoor workers to prevent skin cancer. Accordingly, such natural antioxidants may be useful in human chemoprevention against neoplasias of the lung, breast, urinary bladder, and colon and rectum even after radical surgery.

Syntheses of rRNA, 5.8S, 5S and tRNA are inhibited equally by 8-methoxypsoralen phototreatment of Tetrahymena thermophila

Treatment of the ciliated protozoan Tetrahymena thermophila with 8-methoxypsoralen combined with long wavelength ultraviolet irradiation (UVA, lambda approximately 360 nm) resulted in a dose dependent equal inhibition of the synthesis of rRNA, 5.8S, 5S and tRNA. Similar results were obtained with 3-carbethoxy-8-methoxypsoralen which predominantly forms DNA mono-adducts. In contrast the synthesis of tRNA in T. thermophila was much less sensitive than that of rRNA, 5.8S and 5S RNA to treatment with short wavelength ultraviolet irradiation (UVB, lambda approximately 254 nm). These results are interpreted in favor of a mechanism by which psoralen-DNA adducts (crosslinks much greater than monoadducts) inhibit RNA transcription initiation (in contrast to UVB which causes premature chain termination). Furthermore it is argued that RNA synthesis is regulated in equally sized domains regardless of the gene-size.

Drug-induced photosensitivity: phototoxic and photoallergic reactions--a few molecular aspects

Drug-induced photosensitivity involves mainly phototoxic and photoallergic reactions. The main features of phototoxic and photoallergic reactions are presented and some molecular aspects involved in the mechanisms leading to an adverse skin response are illustrated with examples.

Photosensitization of mammalian cells by psoralens and porphyrins

Because of the ability of photosensitizers to induce specific photochemical reactions in vivo, leading to cell injury and death, many such molecules have been considered as therapeutic agents. Among them two classes of sensitizers, i.e. furocoumarins (psoralens) and porphyrins, are currently used for the photochemotherapy of various skin diseases and malignant lesions. Different types of cell responses can result according to the intracellular localization of the photosensitizer and to the nature of the photochemistry induced by the chromophore which absorbs photons. In this review, the cytological aspects of photosensitization by psoralens and porphyrins will be discussed.

Effect of primary and secondary radicals on chain breaks in ribosomal RNA in E. coli ribosomes

Chain breaks in ribosomal RNA have been observed on gamma-radiation-induced inactivation of E. coli ribosomes. These breaks are caused by .OH and .H radicals, but not by e-aq and tertiary butanol-peroxy radicals.

Production of active oxygen species (1O2 and O2-.) by psoralens and ultraviolet radiation (320-400 nm)

Furocoumarins (psoralens) are potent skin photosensitizing agents that are used in combination with long-wavelength ultraviolet radiation (320-400 nm) in the treatment of psoriasis and other skin diseases. Twelve linear and angular psoralens, capable of forming monofunctional and bifunctional adducts with DNA, were examined with a view to elucidate the role of 1O2 and O2-. in evoking skin photosensitization reactions and skin carcinogenesis. The results showed that both linear psoralens (capable of forming interstrand cross-links) and isopsoralens (angular, monofunctional type) and 3-carbethoxypsoralen (a linear and monofunctional type) produced 1O2 and O2-., although at varying degrees. Psoralen and 3-carbethoxypsoralen produced 1O2 greater than isopsoralens (angelicins). However, nonphotosensitizing angelicin, 5-methylangelicin, and 4,8-dimethyl-5'-carboxypsoralen produced 1O2 greater than 8-methoxypsoralen and 5-methoxypsoralen. The three monofunctional angelicin derivatives (isopsoralens) produced more O2-. than 8-methoxypsoralen, 5-methoxypsoralen, and 3,4'-dimethyl-8-methoxypsoralen. 3-Carbethoxypsoralen, a potent generator of 1O2 and a moderate producer of O2-., was highly photolabile. Until recently, skin photosensitization reactions (erythema, edema, damage to DNA or the membrane of cutaneous cells, the inhibition of scheduled DNA synthesis and skin carcinogenesis, etc.) were believed to involve photocyclo-addition of psoralens to DNA mediated by a type-I or anoxic reaction (a sensitizer-substrate interaction through the transfer of hydrogen atoms or electrons, but no direct involvement of molecular oxygen). Oxygen-dependent sensitized photodynamic reactions of type-II, involving the production of reactive oxygen (1O2 and O2-.), were believed not to mediate psoralen photosensitization reactions. We suggest that 1O2 and O2-. may also participate in skin photosensitization and cell membrane-damaging reactions. The fact that certain monofunctional isopsoralens produce 1O2 and O2-. at rates comparable to or better than bifunctional psoralens suggests that these reactive moieties of oxygen could play a major role in explaining their recently observed carcinogenic property and cell membrane-damaging reactions (e.g., edema or inflammation, etc.).

PUVA-induced erythema and changes in mechanoelectrical properties of skin. Inhibition by tocopherols

Influence of antioxidants on two phototoxic effects of 8-methoxypsoralen (8-MOP) was studied: erythema and changes in mechanoelectrical properties of skin. alpha-Tocopherol and its analogs with shortened lateral hydrocarbon chains at C2-atoms of chromane groups (chromanols) were used as antioxidants. alpha-Tocopherol and its analogs inhibited both phototoxic effects of 8-MOP. Inhibition was observed only if antioxidants were present in skin during irradiation. When applied after irradiation these antioxidants produce no inhibitory effect. The antioxidant antierythemal action depends greatly on their concentration. The protective effects is maximal at antioxidant concentrations 2.5 . 10(-10) - 5 . 10(-9) mol . cm-2 of skin, at concentrations higher than 5 . 10(-9) mol . cm-2 the protective action is decreased. The protective effect of antioxidants depends on the irradiation dose.

Effect of gamma radiation on E. coli ribosomes. I. Inactivation by hydrogen atoms, hydroxyl radicals, hydrated electrons and secondary radicals

E. coli ribosomes in aerated solutions were inactivated with a D37 dose of 144 Gy. In comparison, deaerated solutions bubbled with N2 or N2O showed a slight enhancement of the inactivation with D37 doses of 115 and 125 Gy, respectively. Isopropanol and tertiary-butanol provided partial protection. Results show that hydroxyl radicals, hydrated electrons and hydrogen atoms are involved in the inactivation process. Secondary radicals derived from the alcohols also inactivate ribosomes, with reduced efficiency.

Differentiated inhibition of DNA, RNA and protein synthesis in L1210 cells by 8-methoxypsoralen

The synthesis of DNA, RNA and protein was measured in L1210 cells following treatment with 8-methoxypsoralen in combination with long wavelength ultraviolet irradiation. The results show that the DNA synthesis is strongly inhibited (approximately 95%) at 200 ng/ml reaching a minimum within 2 hours while RNA synthesis is only weakly affected at this concentration (approximately 40% inhibition). At 2 micrograms/ml the RNA synthesis is inhibited approximately 90%. Even at this concentration only a moderate effect is seen on the protein synthesis. These results strongly indicate that the phototoxic action of 8-methoxypsoralen is primarily due to inhibition of DNA synthesis.

Production of singlet oxygen and superoxide radicals by psoralens and their biological significance

We have investigated a series of linear and angular furocoumarins, capable of forming either the monofunctional adducts (single strand) or bifunctional adducts (interstrand cross-links) with DNA with a view to examine the relationship of their skin photosensitizing potency, their ability to produce singlet oxygen (1O2) or superoxide radicals (O-.2 or HO.2), and their carcinogenic activity. The significance of photochemical interactions of psoralens and DNA is well known in skin photosensitization and skin carcinogenesis. Our data suggest that both monofunctional and bifunctional psoralens produce 1O2 and O-.2, and these reactive forms of oxygen may contribute to the development of skin cancer and membrane-damaging effects of these furocoumarins.