Photosensitizing effects of 8-methoxypsoralen on the skin of hairless mice--I. Formation of interstrand cross-links in epidermal DNA

Skin tumor development and keratin expression in different experimental models. Relation to inducing agent and target tissue structure

The applicability of the experimental skin carcinogenesis model for studies of tumor development was examined by exposing the skin of various mouse strains to different chemical carcinogens and UV radiation regimens, in order to analyze the development and progression of the neoplastic process and the role of differentiation markers such as keratins. In tumor-sensitive hairy NMRI mouse skin, the chemical carcinogen, 7,12-dimethylbenz(a)-anthracene (DMBA) induced an abnormal epidermal cell differentiation and structural irregularities associated with an altered keratin expression, as well as numerous papillomas and squamous cell carcinomas. A suboptimal dose of UVB irradiation increased the number of DMBA-induced benign squamous neoplasms. Low doses of benzo(a)pyrene resulted in mild epidermal alterations, but only in one tumor. High doses of UVB induced a large number of undifferentiated spindle cell tumors with few keratinpositive cells in NMRI mice, similar though fewer tumors in hairy, heavily pigmented C57BL/6 mice, numerous papillomas and squamous cell carcinomas in hairless hr/hr mice but only two papillomas in hairy, moderately pigmented DBA/2 mice while UVA exposure produced only two papillomas in hairless SKH-1 mice. In conclusion, the extent and type of skin tumor development depended upon the induction regimen: physical, chemical, dose and duration, as well as on the skin structure: pigmentation and adnexal development, all of which have to be taken into account when relating experimental results to human conditions.

Enhanced pyrimidine dimer repair in cultured murine epithelial cells transfected with the denV gene of bacteriophage T4

The patch size for excision repair of ultraviolet radiation (UV)-induced pyrimidine dimers was determined in cultured murine epithelial cells with normal and enhanced pyrimidine dimer repair capabilities. Cells with enhanced pyrimidine dimer repair were produced by transfecting 308 cells with the denV gene of bacteriophage T4; this gene encodes the enzyme endonuclease V. Pyrimidine dimer repair following exposure to UV from an FS-40 sunlamp was determined by micrococcal dimer-specific nuclease digestion and alkaline sucrose ultracentrifugation. Patch size ws estimated based on the photolytic lability of bromodeoxyuridine-substituted DNA. Excision repair of UV-induced pyrimidine dimers in denV-transfected 308 cells was enhanced two- to threefold. Production of mRNA from the denV gene in cell lines with enhanced repair was confirmed by RNA blotting. In control cells, the patch size for excision repair of DNA photoproducts was estimated to be 34 nucleotides per photoproduct removed; in denV-transfected cells, a smaller average patch size of 10-16 nucleotides per photoproduct removed was calculated. Thus, endonuclease V activity appears to alter not only the extent, but also the nature of excision repair in UV-exposed mammalian epithelial cells.

Cell cycle progression in denV-transfected murine fibroblasts exposed to ultraviolet radiation

Repair-proficient murine fibroblasts transfected with the denV gene of bacteriophage T4 repaired 70-80% of pyrimidine dimers within 24 h after exposure to 150 J/m2 ultraviolet radiation (UVR) from an FS-40 sunlamp. Under the same conditions, control cells repaired only about 20% of UVR-induced pyrimidine dimers. After UVR exposure, both control and denV-transfected cells exhibited some degree of DNA-synthesis inhibition, as determined by flow cytometric analysis of cell-cycle kinetics in propidium iodide-stained cells. DenV-transfected cells had a longer and more profound S phase arrest than control cells, but both control and denV-transfected cells had largely recovered from UVR effects on cell-cycle kinetics by 48 h after UVR exposure. Inhibition of DNA synthesis by UVR was also measured by determining post-UVR incorporation of bromodeoxyuridine (BrdU). The amount of BrdU incorporated was quantitated by determining with flow cytometry the quenching of Hoechst dye 33342 by BrdU incorporated in cellular DNA. DenV-transfected cells showed more marked inhibition of BrdU incorporation after low fluences of UVR than control cells. Differences between denV-transfected and control cells in cell-cycle kinetics following UVR exposure may be related to differences in mechanisms of repair when excision repair of pyrimidine dimers is initiated by endonuclease V instead of cellular repair enzymes.

Enhanced pyrimidine dimer removal in repair-proficient murine fibroblasts transformed with the denV gene of bacteriophage T4

The denV gene of bacteriophage T4, which encodes the pyrimidine dimer-specific repair enzyme endonuclease V, was introduced into murine fibroblasts with normal rodent pyrimidine dimer repair capabilities. Endonuclease V recognizes ultraviolet radiation (UVR)-induced pyrimidine dimers and produces single-strand breaks adjacent to the dimers. These nicks may serve as substrates to initiate excision repair of pyrimidine dimers by endogenous enzymes. In the present study, murine fibroblasts stably transfected with denV were able to remove 50-80% of UVR-induced pyrimidine dimers, while control cells removed only about 20% of dimers under the same conditions of pyrimidine dimer induction and repair. For both control and denV-transfected cells, repair continued for at least 24 h after exposure. When removal of UVR-induced photoproducts was initiated by endogenous excision repair mechanisms, an average of 38 nucleotides were replaced per dimer removed, as determined by bromouracil photolysis; denV-initiated excision repair, on the other hand, resulted in removal of an average of 6 nucleotides per dimer repaired. The enhanced pyrimidine dimer repair capabilities conferred by denV gene expression did not appear to improve post-UVR survival.

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.

Excision repair characteristics of denV-transformed xeroderma pigmentosum cells

Introduction of the denV gene of phage T4, encoding the pyrimidine dimer-specific endonuclease V, into xeroderma pigmentosum cells XP12RO(M1) was reported to result in partial restoration of colony-forming ability and excision repair synthesis. We have further characterized 3 denV-transformed XP clones in terms of rates of excision of pyrimidine dimers and size of the resulting resynthesized regions following exposure to 100 J/m2 from an FS-40 sunlamp. In the denV-transformed XP cells we observed 50% dimer removal within 3-6 h after UV exposure as compared to no measurable removal in the XP12RO(M1) line and 50% dimer excision after 18 h in the GM637A human, control cells. Dimer removal was assayed with Micrococcus luteus UV-endonuclease in conjunction with sedimentation of treated DNA in alkaline sucrose gradients. The size of the resulting repaired regions was determined by the bromouracil photolysis technique. Based on the photolytic sensitivity of DNA repaired in the presence of bromodeoxyuridine, we calculated that the excision of a dimer in the GM637A cells appears to be accompanied by the resynthesis of a region approximately 95 nucleotides in length. Conversely, the resynthesized regions in the denV-transformed clones were considerably smaller and were estimated to be between 13 and 18 nucleotides in length. These results may indicate that either the endonuclease that initiated dimer repair dictated the size of the resynthesized region or that the long-patch repair observed in the normal cells resulted from the repair of non-dimer DNA lesions.

A first approach to an action spectrum for 8-MOP phototumorigenesis in mice

Groups of hairless albino mice were treated with 8-MOP and exposed to a fixed daily dose of UVR obtained from one of four different emission spectra. Each spectrum was obtained by filtering a xenon arc source with a different WG filter. The resultant spectra showed differences in their proportions of shorter UVA wavelengths. A decreased proportion of the shorter wave bands systematically increased the time to first tumor. These data suggest that the action spectrum for 8-MOP sensitized phototumorigenesis is similar to that of acute 8-MOP action spectra in mammalian skin, which usually show maximal activity in the 320-335 nm region.

Neoplastic transformation of C3H mouse embryo 10T1/2 cells by 8-methoxypsoralen plus UVA radiation

The effect of 8-methoxypsoralen plus UVA radiation (PUVA) on cell killing and induction of transformation was studied in the C3H mouse embryo 10T1/2 cell line. Dose-response data for both survival and transformation were obtained as a function of 8-methoxypsoralen (8-MOP) concentration and UVA dose. PUVA treatment caused cell death and induced transformation in a dose-dependent manner. Treatment of cells with 8-MOP alone (10 micrograms/ml) or UVA alone (90 J/m2) had no effect on either cell killing or transformation. The product of 8-MOP concentration and UVA dose calculated at 10% survival and 10(-3) transformation frequency levels were quite similar regardless of 8-MOP concentration or UVA dose. This suggests that there exists a simple reciprocal relationship between 8-MOP concentration and UVA dose. Both type II and type III foci induced by PUVA treatment were tumorigenic in vivo. These data provide further evidence for the carcinogenicity of PUVA treatment. In addition, the system described here could serve as a valuable model for studying the relationships between transformation and the specific cellular and molecular lesions induced by PUVA treatment.

An action spectrum for 8-methoxypsoralen-sensitized inhibition of DNA synthesis in vivo

The wavelength dependence for 8-methoxypsoralen (8-MOP)-sensitized inhibition of scheduled DNA synthesis was investigated in the epidermis of albino hairless mice. Topical (0.1%) applications of 8-MOP followed by exposure to narrow bands from a monochromator in the range of 300-380 nm produced a dose-dependent inhibition of DNA synthesis. Prior treatment with 8-MOP did not alter the dose-dependent inhibition of DNA synthesis following exposure to 300 nm and to 310 nm. By contrast, DNA synthesis inhibition following exposure to UVA wavelengths was seen only after treatment with 8-MOP. An action spectrum, constructed from the dose-response regression lines, showed peak effectiveness at 335 nm. Since the therapeutic usefulness of psoralen photochemotherapy may be related to inhibition of cell proliferation, it is suggested that light sources with peak emission in the 335-nm region would be more efficient than the commonly employed UVA blacklights.

The shape of the blacklight dose photoreactivation curve for chick embryo fibroblasts

The curves of UV (254 nm) induced pyrimidine dimers (endonuclease sensitive sites) vs. photoreactivating blacklight (365 nm) dose for cultured chick embryo fibroblasts reveal several new features. When the cells are incubated in the dark at 37 degrees following UV (254 nm) treatment, the efficiency of subsequent photorepair increases for the first few hours post-UV. The efficiency then remains approximately constant for several hours. Photorepair data obtained during this later period were plotted as the logarithm of dimer-enzyme complexes available for photoreactivation vs. blacklight (365 nm) dose. For a fixed damaging UV (254 nm) dose, the resulting curve has a shoulder of approximately 6-10 kJ/m2 followed by a straight line portion with a slope of magnitude about 1.5 X 10(-4) m2/J for UV doses up to 15 J/m2. For higher UV doses the shoulder remains about the same, but the slope decreases in magnitude. The shoulder is interpreted to indicate that a light-dependent step is necessary to activate the enzyme. The decrease in slope with increased UV dose together with some split photoreactivation dose experiments suggests that some site-to-site motion and multiple site function of the photorepair enzyme molecules may come into play at the higher levels of damage, but the evidence indicates that these complications are relatively unimportant at low UV doses.

Induction of pyrimidine dimers in epidermal DNA of hairless mice by UVB: an action spectrum

An action spectrum for the induction of pyrimidine dimers in the epidermis of hairless mice was determined between 288 and 307 nm. The presence of pyrimidine dimers in tritium-labeled DNA extracted from exposed SKH:hairless-1 mouse skin was determined using dimer-specific nucleases from Micrococcus luteus in conjunction with sedimentation of the irradiated DNA in alkaline sucrose gradients. The rate of induction of pyrimidine dimers was maximal at 293 nm. These values were used to propose a UVB transmission curve for mouse epidermis.

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.

Increased efficiency of photoreversal of UV-induced dimers in the DNA of chick embryo fibroblasts with post-UV dark time

The kinetics of photoreversal of UV-induced dimers in the DNA of early passage chick embryo fibroblasts was studied by monitoring disappearance of UV-endonuclease-sensitive sites. Photorepair was found to increase in efficiency when cells were incubated in the dark for several hours at 37 degrees C following the dimer-inducing short-wavelength (254 nm) UV treatment, but prior to the photoreactivating black light (365 nm). Following a UV dose of 10 J/m2 it took at least 4 h in the dark to saturate this effect. This UV dose inserts roughly 2.4 dimers/10(7) daltons of DNA. Dark repair removes about 0.08 dimers/h/10(7) daltons. After 6 h in the dark, exposure to black light removes an additional 1.4 dimers/10(7) daltons leaving about 0.5 dimers unaffected by this treatment. After saturation of the dark effect, the amount of photoreactivation depends only on total black light fluence and not on fluence rate for the range of rates studied. This indicates that during 30 min, the maximum time of black light exposure, no appreciable reattachment of the photorepair molecule to additional unrepaired dimer sites occurs. We estimate that the number of effective photorepair molecules per chick chick cell is at least of the order of 2 x 10(5).

Photoinactivation and recovery in skin fibroblasts after formation of mono- and bifunctional adducts by furocoumarins-plus-UVA

Cultured skin fibroblasts from young male guinea pigs were irradiated with UVA light in the presence of 8-methoxypsoralen (8-MOP) or angelicin. As compared to 8-MOP 30 times higher concentrations of angelicin were needed to obtain comparative inhibition rates of DNA-synthesis. Complete cellular recovery could be observed when the cell cultures were treated with angelicin-plus-UVA (320-400 nm) or 8-MOP-plus-395 nm. Both treatment schedules are known to cause monofunctional photoreactions. In contrast to this, bifunctional photoreactions caused by 8-MOP-plus-365 nm produced an inhibition of DNA synthesis which lasted more than four days. Also, UVA (320-400 nm) applied to cells treated with 3H-labeled 8-MOP resulted in a dose-dependent binding of 8-MOP molecules again lasting several days. Application of 8-MOP-plus-UVA (320-400 nm) to cells growing in log-phase showed a characteristic change in morphology. An increasing number of polynuclear and hyperchromatic cells appeared with time after treatment. In this subpopulation of cells DNA synthesis continued without division as revealed by DNA measurements and autoradiography. It is concluded that monofunctional adducts caused by angelicin-plus-UVA as well as 8-MOP-plus-395 nm permit cellular recovery whereas bifunctional photoadducts remained without recovery. In the latter case semiconservative DNA synthesis continued leading to hyperchromatic cells which could serve as a parameter for the presence of cross-linked nuclear DNA strands.

PUVA suppresses the proliferative stimulus produced by stripping on hairless mice

Hairless mice were fed with 8-methoxypsoralen by gastric tube and exposed to UVA light to produce threshold phototoxic reactions. 3H-thymidine autoradiography revealed no significant difference of the epidermal labelling index as compared to that of nonirradiated animals (4.8 vs 6.2). Single PUVA exposures performed 2,8,14 and 24 hr after tape stripping lead to suppression of a wave of synchronized DNA synthesis present in nonirradiated control animals. These data demonstrate different reactions of stripped and unstripped epidermis to PUVA exposures and offer indirect evidence for the suppression of DNA synthesis in hyperproliferative skin disorders by PUVA in vivo.

New studies on the interaction between 8-methoxypsoralen and DNA in vitro

Some aspects of the interactions between DNA and 8-methoxypsoralen (8-MOP) in its ground state (complex formation) or in its excited state (photobinding) have been investigated. 8-MOP shows a low affinity towards DNA in the complex formation; this fact minimizes the possible biological consequences deriving from this interaction, when it occurs in vivo. In covalent photobinding to DNA, 8-MOP forms mainly monofunctional adducts, and to a lesser extent bifunctional adducts, showing a behavior similar to that of other linearly condensed furocoumarins (psoralens); the ratio between mono- and bifunctional adducts was found to be 9:1. The covalent photobinding to DNA does not occur at random along the macromolecule, but preferentially at the level of specific receptor sites. The regions having an alternate sequence of A-T seem to be the best receptor sites for the formation of monoadducts while the regions containing an alternate sequence of A-T and C-G appeared to be the preferential sites for the cross-linkage formation.

Photoinactivation of skin fibroblasts by fractionated treatment with 8-methoxypsoralen and UVA

Guinea pig skin fibroblasts treated with low doses of 8-methoxypsoralen (8-MOP) and long-wave ultraviolet light (UVA) showed a dose-dependent inhibition of 3H-Thymidine incorporation as determined by liquid scintillation counting. The minimum incubation time necessary to obtain constant inhibition rates was 60 min. By washing the drug was removed from the reactive sites within 30 min. Repeated light exposure at a constant concentration of 8-MOP caused a cumulative inhibition of DNA synthesis. Irradiation of 8-MOP-plus-UVA treated cells, from which the drug was removed, produced a small increase in photoinhibition. Split dose treatment at various time intervals (ranging from 1--48 hr) revealed inhibitory rates, which correspond to the total amount of UVA applied. No recovery effects were seen in cultures treated by single or multiple applications of 8-MOP-plus-UVA.

Binding of 4'-aminomethyl 4,5',8-trimethyl psoralen to DNA, RNA and protein in HeLa cells and Drosophila cells

In Drosophila cells and HeLa cells treated with 4'-aminomethyl trioxsalen and ultraviolet light, this compound binds covalently to DNA and RNA. The maximum number of molecules bound to 10(3) base pairs in DNA is 60 and in RNA it is 20. In nuclei treated likewise the number of molecules bound to 10(3) base pairs in DNA can be as high as 376. When cells are irradiated in the frozen state the number of 4'-aminomethyl trioxsalen molecules bound per 10(3) base pairs in DNA is about 40 and in RNA about 20. DNA molecules from cells or nuclei treated with 4'-aminomethyl trioxsalen and ultraviolet light are highly crosslinked and appear as loops interspersed by double stranded regions when analyzed in the electron microscope under denaturing conditions. The loop sizes are heterogeneous and the fraction of double stranded regions increases to almost complete double-strandedness at high degrees of reaction. No secondary structures could be found in ribosomal RNA from Drosophila cells or HeLa cells after treatment with 4'-aminomethyl trioxsalen and ultraviolet light. In cells treated with 4'-aminomethyl trioxsalen and ultraviolet light the RNAase activity is increased considerably suggesting a release of lysosomal enzymes. 4'-aminomethyl trioxsalen and its photodecomposition products bind strongly to cellular proteins.