Biological effects and repair of damage photoinduced by a derivative of psoralen substituted at the 3,4 reaction site: photoreactivity of this compound and lethal effect in yeast

The reaction of the psoralens with deoxyribonucleic acid

Psoralen photochemistry is specific for nucleic acids and is better understood at the molecular level than are all other methods of chemical modification of nucleic acids. These compounds are used both for in vivo structure analysis and for photochemotherapy since they easily penetrate both cells and virus particles. Apparently, natural selection has selected for membrane and virus penetrability during the evolution of these natural products. Most cells are unaffected by relatively high concentrations of psoralens in the absence of ultraviolet light, and the metabolites of the psoralens have thus far not created a problem. Finally, psoralens form both monoadduct and cross-links in nucleic acid helices, the yield of each being easily controlled by the conditions used during the photochemistry.

Fate of photo-induced 8-methoxypsoralen mono-adducts in yeast. Evidence for bypass of these lesions in the absence of excision repair

A fraction of UVA-induced 8-methoxypsoralen (8-MOP) mono-adducts can be transformed by a second UVA (365 nm) irradiation procedure into lethal cross-links in Saccharomyces cerevisiae. To follow the fate of cross-linkable mono-adducts, cells were incubated in complete medium between the two UVA doses and survival was measured. The killing effect of the second UVA dose decreases rapidly in haploid wild-type as well as in strains blocked in mutagenic (RAD6+ type) or in recombinogenic (RAD52+ type) repair pathways. This is also true in the pso1-1 and pso2-1 strains selected for sensitivity to 8-MOP plus UVA treatment. In contrast, persistence of mono-adducts is observed in strains blocked in the excision-resynthesis repair pathway. In other words, cross-linkable mono-adducts are repaired by the excision process. The use of the cell-cycle conditional mutant strain (cdc14-1) permitted us to apply the second dose at a specific cell-cycle stage (post-G2 phase) after a 'priming' UVA treatment on stationary (G1) phase cells. Such experiments showed a bypass of mono-adducts in an excision-deficient context for at least one round of DNA replication.

Photobiological activity of 5,7-dimethoxycoumarin

Some biological properties of 5,7-dimethoxycoumarin (DMC) include dark induced frameshift mutagenesis in bacteria, lethal photosensitization and the formation of sister chromatid exchanges in Chinese hamster cells. The number of sister chromatid exchanges per unit of cell lethality produced by DMC is almost the same as observed with 5-methoxypsoralen.

The sunburn cell in hairless mouse epidermis: quantitative studies with UV-A radiation and mono- and bifunctional psoralens

The production of the sunburn cell by UV-A radiation and topical psoralens in hairless mouse epidermis has been studied. It has been shown that the appearance of this cell is dependent on the dose of both UV-A radiation and of the psoralen. The time-course with 8-methoxypsoralen has peak sunburn cell numbers at 28 hr postirradiation. A comparison of 2 bifunctional (8-methoxypsoralen and 5-methoxypsoralen) and 2 monofunctional (angelicin and 3-carbethoxypsoralen) psoralens showed the former are more potent. This suggests that DNA crosslink lesions may play a rle in sunburn cell production.

Action spectra for 8-methoxypsoralen and 3-carbethoxypsoralen in skin fibroblasts in vitro

8-methoxypsoralen (8-MOP) and 3-Carbethoxypsoralen (3-CPs) are known photoreagents which have been employed in the treatment of psoriasis. Using skin fibroblasts grown in vitro we have determined the action spectra of both compounds in the long ultraviolet range. Narrow-band interference filters were applied to a Xenon lamp. Reduction in methyl-3H-thymidine (3H-TdR) uptake served as a parameter for photoinhibited cells. The results show a linear increate of 8-MOP-mediated photoinhibition with increasing wavelengths; 3-CPs showed comparatively weak inhibitory rates at the lower wavelength range (349-365 nm). Possibly this is due to the formation of 3-CPs photoproducts which are unreactive with DNA. Compared to 8-MOP, the monofunctional photoreagent 3-CPs is a weak photoinhibitor.

Photobiology and photomedicine: the future is bright

Important events since 1966 that have helped to advance photobiology in general and photomedicine in particular are reviewed. More formal courses on photobiology are needed so that future photobiologists and photodermatologists will not have to be self-taught about the properties and action of light. The effectiveness of current phototherapies and their future improvement are discussed. Some of the areas of photobiology what will impact on photomedicine in the years to come are ultraviolet (UV) radiation effects on the immune system, the light activation of enzymes as a potential new type of photothoerapy, the development of new photosensitizers for phototherapy, the effects of near-UV radiation on cellular membranes, and, of course, the role of DNA damage and repair in mutagenesis and carcinogenesis. The future is bright for photomedicine.

Repair of interstrand cross-links in DNA of Saccharomyces cerevisiae requires two systems for DNA repair: the RAD3 system and the RAD51 system

We have studied the role of the excision-repair system and the recombination-repair system in the removal of cross-links and monoadducts caused by furocoumarins plus 360 nm radiation in yeast DNA by neutral and alkaline sucrose gradients and by a fluorometric procedure which detects cross-linked DNA molecules. We found that the excision-repair system, represented by the rad3 mutations, is required both for the removal of monoadducts, causing single-strand break formation, and for the removal of cross-links, causing double-strand break formation. The recombination-repair system, represented by the rad51 mutation, is necessary for double-strand break repair following cross-link removal, but it has no role in the repair of monoadducts. It can be concluded, that at least some of the same enzymes are used in yeast for both the excision of pyrimidine dimers and the excision of cross-links or monoadducts caused by furocoumarins plus light. The RAD3 and RAD51 repair systems, which act independently in the repair of UV-induced lesions, are part of a single system for the repair of cross-links.

On the involvement of singlet oxygen in mutation induction by 8-methoxypsoralen and UVA irradiation in Escherichia coli K-12

The possible mutagenic effects induced by single oxygen, which is formed during UVA irradiation of bacterial cells pretreated with 8-methoxypsoralen (8-MOP), were investigated. As genetic endpoint, black mutation from arg-56 to arg+ was assayed in strain Escherichia coli K-12/343/113/uvrB; this system, in preliminary experiments, was rather sensitive to 8-MOP-induced photodynamic effects. To assess the involvement of singlet oxygen (1O2) in the mutation induction process, 2 tests were applied, namely, comparative mutation induction in D2O and H2O media (pH 7.0) and quenching of 1O2 with 1,4-diazabicyclo[2.2.2]octane (DABCO). When photodynamy was performed with the indicator cells suspended in D2O buffer, the mutagenic effect was substantially higher than that obtained with cells suspended in H2O buffer; this increase was even more pronounced when the incubation mixtures were thoroughly oxygenated before irradiation. D2O itself was not mutagenic under th present experimental conditions. Addition of DABCO in concentrations of 0.1--10 mM to the irradiation mixtures effectively reduced the number of 8-MOP-induced mutant yields by about 40%. DABCO itself had no effect on cell viability or on spontaneous mutation frequency under our experimental conditions. From these 2 sets of results, and from the preliminary findings that the photomutagenic effect of 8-MOP is higher in the uvrB derivative than in the corresponding excision-repair-proficient parent strain, which is in concordance with previous observations in other E coli strains, it can be concluded that 1O2 generated upon UVA irradiation of 8-MOP solutions is probably responsible for part of the observed genetic effects.

Proliferative response patterns of human fibroblasts after photoinjury with 4,5',8-trimethylpsoralen

The extent of growth suppression and recovery following exposure to 4,5',8-trimethylpsoralen (TMP) plus UV-A irradiation was studied in 3 diploid human fibroblast strains. Inhibition of cellular proliferation was dose-dependent within the concentration range of TMP that was tested (1-4 X 10(-7) M), using a constant level of UV-A (0.24 J/cm2). The population generation (doubling) times for all cell strains were progressively lengthened under these conditions while maximal cell densities were reduced. At 2-4 X 10(-7) M TMP in the presence of UV-A, there was a triphasic pattern of growth which consisted of (1) proliferative activity during the first 24-36 hr, followed by; (2) complete growth inhibition for variable periods of time and; (3) a recovery period of log phase proliferation that was not as vigorous as measured for untreated cells. There were also declines in the percentage of cells labeled with 3H-Tdr at various times after TMP-UV-A treatment. These measurements were essentially identical for the three fibroblast strains evaluated. In that the cells employed for these investigations were derived from embryonic pulmonary tissue, neonatal foreskin and the buttock skin of an adult male, it seems unlikely that donor age and tissue source were important variables in determining growth response patterns after TMP-UV-A exposure. Because proliferative recovery was attenuated after this photochemical injury, we conclude that the biologic effect(s) of TMP-UV-A extend beyond the known period of psoralen-DNA cross-link removal.

Photochemotherapy (PUVA) of psoriasis using 3-carbethoxypsoralen, a non-carcinogenic compound in mice

The carcinogenic risk of photochemotherapy (PUVA) with bi-functional furocoumarins such as 8-methoxypsoralen (8-MOP) which form cross-links in cellular DNA has initiated a search for active but less hazardous psoralens. A new compound, 3-carbethoxypsoralen (3-CPs), studied in the yeast Saccharomyces cerevisiae (eukaryote), has been shown to be very photoactive on DNA and to form only mono-additions to DNA. These lesions appear to be more easily repaired than the cross-links induced by 8-MOP. 3-CPs produces less nuclear genetic events such as nuclear mutations and mitotic crossovers, but more cytoplasmic 'petite' mutations (damage to mitochondrial DNA) than 8-MOP. In mice it was demonstrated that after local or intra-peritoneal administration, in contrast to 8-MOP, 3-CPs is non-toxic, non-erythematogenic, and non-carcinogenic. A study of ten psoriatic patients had shown that local applications of 3-CPs plus UV-A exhibit about the same therapeutic activity for the clearing of psoriatic lesions as local treatment with 8-MOP plus UV-A, but without any localized hyperpigmentation.

Preferential deletion of a specific region of mitochondrial DNA in Saccharomyces cerevisiae by ethidium bromide and 3-carbethoxy-psoralen: directional retention of DNA sequence

Grande strains of Saccharomyces cerevisiae were mutagenized either by ethidium bromide or by 3-carbethoxy-psoralen (a monofunctional furocoumarin derivative) activated by 365nm light. 973 primary rho- clones induced were randomly collected and analyzed individually for the presence or absence of fifteen mitochondrial genetic markers. 1. Under mild conditions of mutagenesis, 83% of the primary clones showed single-deletion genotypes; a unique order of 14 markers could be deduced from the patterns of the deletion. The gene order confirmed our previous map constructed from the analysis of established non-random petite clones. From the frequencies of disjunction between markers, the distance separating 14 mitochondrial markers were estimated. 2. One region, carrying oxi-3, pho-1 and mit 175 loci, was preferentially lost in rho- mutants: there is a strong constraint in the frequencies of various genotypes found in rho- clones. On each side of this particular region, a bidirectionally oriented pattern of retention of markers is observed.

Dose-rate effects of 8-methoxypsoralen plus 365-NM irradiation on cell killing in Saccharomyces cerevisiae

Tow types of dose-rate effect that alter the survival response of haploid yeast cells to 8-methoxypsoralen (8-MOP) plus treatment with irradiation at 365 nm were studied. (1) When the concentration of 8-MOP was varied between 9.2 X 10(-5) and 2.3 X 10(-8) M and the dose rate of 365-nm irradiation kept constant, the efficiency of the irradiation for killing increased relatively to that of 8-MOP whe the concentration of 8-MOP decreased. This indicated that there was no strict reciprocity between radiation dose and concentration of drug. (2) When the dose rate of radiation was varied between 0.66 X 10(3) and 108 X 10(3) J m-2 h-1 and the concentration of 8-MOP was kept constant, the survival of wild-type cells increased strikingly at low dose rates of radiation as compared with high dose rates. Cells responded more to changes at low dose rates than to equal changes a high dose rates. The high resistance of wild-type cells to 8-MOP plus radiation delivered at low dose rates absent from rad 1-3 cells defective in excision-repair. This suggests that the dose-rate effect seen in wild-type cells depended at least in part on an active excision-repair function. At low dose rates of radiation, the shoulder of the survival curve for rad1-3 cells, i.e. the ability to accumulate sub-lethal damage, was increased by a factor of about 2 when compared with that seen at a high dose rate. Thus it is likely that at low dose rates a repair function other than excision-resynthesis may operate in rad1-3 cells.