Search

Four groups of 30 dorsally shaved opossums (Monodelphis domestica) were exposed to graded doses of ultraviolet radiation A (UVA) (320-400 nm) three times per week for 90 weeks. Animals were monitored for the appearance of focal melanocytic hyperplasia (FMH) and nonmelanoma skin tumors (NMST) during the course of the exposures and for an additional 20 weeks following termination of exposures. FMH is the putative precursor for melanoma in the opossum. The lowest dose of UVA (2.5 x 10(3) J/m2) used in this study was selected based on the action spectrum for the induction of melanoma in a fish model. The prediction was that 2.5 x 10(3) J/m2 would induce FMH in the opossum if the action spectra for the induction of FMH in the opossum and melanoma in the fish were the same. The highest UVA dose was 2.5 x 10(5) J/m2. Only the highest dose of UVA gave a statistically significant induction of FMH and NMST in the opossum. As in previous studies, the FMH appeared earlier than the NMST during the course of exposures and the final prevalence of FMH was lower than the final prevalence of NMST. Overall, the results of this study indicate that the efficacy of UVA to induce FMH in the opossum is not as great as would be predicted from the action spectrum for melanoma induction in a fish model.

Chronic ultraviolet radiation (UVR) exposure to the eyes of Monodelphis domestica causes corneal opacification, neovascularization, and fibrosarcoma induction. By immunohistochemistry and Western blotting, we have shown that one to four exposures of the eyes of this opossum to UVR enhances basic fibroblast growth factor (bFGF) expression by the corneal epithelium. Treatment with photoreactivating light, which selectively removes UVR-induced pyrimidine dimers, suppresses bFGF induction, indicating that UVR induction of bFGF is ultimately due to DNA damage. Furthermore, UVR-induced corneal tumors derived from corneal keratocytes express bFGF mRNA and protein, as determined by immunohistochemistry and in situ hybridization. Taken together, these findings suggest that bFGF acts in both an autocrine and a paracrine manner to stimulate corneal fibroplasia, neovascularization, and tumor development., (Copyright 2001 Wiley-Liss, Inc.)

The induction of edema and pyrimidine dimers in epidermal DNA was determined in the skin of SKH:HR1 mice exposed to graded doses of ultraviolet radiation AI (UVAI; 340-400 nm). Exposure to UVAI induced 1.6 +/- 0.08 x 10(-6) (mean +/- standard error of mean) pyrimidine dimers per 10(8) Da of DNA per J/m2. Edema in irradiated animals was determined as an increase in skinfold thickness. A dose of 1.8 x 10(6) J/m2 of UVAI that resulted in a 50% increase in skinfold thickness (SFT50%) would have induced 1.0 x 10(5) dimers per basal cell genome. A similar increase in SFT induced by full spectrum solar ultraviolet radiation (290-400 nm) would accompany the induction of 11.0 x 10(5) pyrimidine dimers per basal cell genome. These results support a hypothesis that UVAI-induced pathological changes of the skin are mediated through the formation of nondimer photoproducts.

The gray, short-tailed opossum, Monodelphis domestica, has been used for photobiologic studies since 1984. The presence of a light-activated DNA repair pathway in the tissues of Monodelphis has been used to identify pyrimidine dimers in DNA as initiating events for a number of ultraviolet radiation (UVR)-induced pathologies of the skin and cornea. Furthermore, Monodelphis, unlike common laboratory rodents, is susceptible to the induction of melanoma by UVR alone.

T cell-mediated immune function, here measured as the contact hypersensitivity reaction, is readily suppressed by moderate exposure of mice to ultraviolet B (UVB) or solar-simulated radiation (SSUV), or by topical application of cis-urocanic acid. The effect of ultraviolet A (UVA) radiation on immune function has been unclear. Here we have demonstrated that when UVA radiation from a fluorescent tube source was rigorously filtered to remove contaminating UVB radiation, it was immunologically innocuous at physiologically relevant doses. Furthermore, we have found that mice exposed to UVA radiation, either immediately after, or up to 24 h before, immunosuppressive treatment with either UVB radiation, SSUV or cis-urocanic acid, became refractory to the immunosuppression and retained more normal contact hypersensitivity. A greater UVA exposure reversed the immunosuppression more effectively. The results suggest that there are immunologically significant interactions between UV wavebands, and that UVA exposure may induce a relatively long-lived immunoprotective photoproduct, as yet unidentified, that can inhibit the activity of epidermal cis-urocanic acid and thus provide protection from photoimmunosuppression.

Chronic exposure to ultraviolet radiation (UVR) induces corneal sarcomas in the South American opossum Monodelphis domestica. Cell lines are readily established from these tumors. Northern blotting of mRNA from six such cell lines revealed high expression of the H-ras oncogene. H-ras cDNA from an eye tumor cell line was cloned and characterized; the germline sequence of codons 12, 13, and 61 was confirmed by examination of H-ras sequences amplified from liver DNA by the polymerase chain reaction. The Monodelphis H-ras coding sequence is 84-89% identical to that of other vertebrates at the nucleotide level, and the predicted 189-amino-acid sequence differs by 2-12 amino acids from that of other vertebrates. Analysis of 12 primary invasive corneal sarcomas induced by chronic UVR exposure revealed no evidence of H-ras gene amplification or rearrangement. One tumor was heterozygous for an activating point mutation in codon 61 of the H-ras gene; the tumor was also homozygous for a point mutation at an adjacent site in codon 62. These results provide additional evidence for the functional importance and consequent evolutionary conservation of the ras oncogenes.

Two groups of 30 dorsally shaved opossums (Monodelphis domestica) were exposed three times per week for 81 weeks to 250 J/m2 of UV radiation from FS40 sunlamps (approximately 150 J/m2 of UV radiation B; UV-B), or to 2.5 x 10(4) J/m2 of UV radiation A (UV-A) from filtered F40BLB fluorescent lamps (black lights). Animals were monitored for the appearance of nonmelanoma skin tumors (NMSTs) and melanocytic hyperplasia (MH). After 81 weeks of exposures, the prevalence of NMSTs was 71% and 4% for animals exposed to UV-B and UV-A, respectively. The difference between the treatment groups was statistically significant (P < 0.001). However, the prevalence of MH in the treatment groups, 31% for UV-B-exposed animals and 22% for UV-A-exposed animals, was not significantly different (P > 0.05). Thus, a dose of UV-A that was relatively ineffective in producing NMSTs, compared to UV-B, was as effective as UV-B in the induction of MH. If, as shown previously, MH is the precursor lesion for melanoma in this model, these results suggest that the action spectra for the induction of melanoma and NMSTs in the opossum are different.

Pyrimidine dimers were measured in epidermal DNA of SKH:HR1 mice following exposure to solar-simulated UV radiation (SSUV, 290-400 nm) or to UVA (320-400 nm). Mice were exposed to SSUV or UVA after topical application (2 mg/cm2) of vehicle, a UVB absorber (5% 2-ethylhexyl p-methoxycinnamate [2-EHMC]), or a broad-spectrum UVA absorber (5% Mexoryl SX). The rates of induction of pyrimidine dimers in untreated animals were 5.4 +/- 0.57 x 10(-4) (mean +/- SEM) and 7.6 +/- 0.95 x 10(-6) dimers per 10(8) Da of epidermal DNA per J/m2 of SSUV and UVA, respectively. Topical application of Mexoryl SX reduced the rate of induction of pyrimidine dimers in SSUV-exposed animals to 4.7 +/- 0.44 x 10(-5) dimers per 10(8) Da per J/m2 for a dimer induction protection factor (PF) of 11.5 (5.4 x 10(-4)/4.7 x 10(-5). The rate of dimer induction in Mexoryl SX-treated, UVA-exposed mice was 0.95 +/- 0.2 x 10(-6) dimers per 10(8) Da per J/m2 (PF = 8.0). The 2-EHMC at a concentration of 5% (wt/wt) was significantly less effective than Mexoryl SX in preventing the induction of pyrimidine dimers in animals exposed to either SSUV or UVA. The rates of dimer induction in 2-EHMC-treated mice were 8.2 +/- 1.1 x 10(-5) and 3.8 +/- 0.33 x 10(-6) dimers per Da per J/m2 of SSUV (PF = 6.6) and UVA (PF = 2.0), respectively. Upon normalizing to the efficacy for edema induction, UVA induced approximately one-fourth the number of pyrimidine dimers per equivalent edematous response when compared to SSUV.

The use of chemical and physical sunscreening agents has increased dramatically during the last two to three decades as an effective means of preventing sunbum. The use of high sunprotection factor sunscreens has also been widely promoted for the prevention of skin cancer, including melanoma. Whereas sunscreens are undoubtedly effective in preventing sunbum, their efficacy in preventing skin cancer, especially melanoma, is currently under considerable debate. Sunscreens have been shown to prevent the induction of DNA damage that presumably results from the direct effects of ultraviolet radiation (UVR) on DNA. DNA damage has been identified as an initiator of skin cancer formation. However, both laboratory and epidemiological studies indicate that sunscreens may not block the initiation or promotion of melanoma formation. These studies suggest that the action spectrum for erythema induction is different than the action spectrum for the induction of melanoma. Indeed, recent reports on the wavelength dependency for the induction of melanoma in a fish model indicate that the efficacy of ultraviolet A wavelengths (320-400 nm) to induce melanoma is orders of magnitude higher than would be predicted from the induction of erythema in man or nonmelanoma skin tumors in mice. Other strategies for the chemoprevention of skin cancer have also been reported. Low levels and degree of unsaturation of dietary fats protect against UVR-induced skin cancer in mice humens. Compounds with antioxidant activity, including green tea extracts (polyphenols), have been reported to inhibit UVR-induced skin carcinogenesis.

The suppression of contact hypersensitivity by UVB (280-320 nm) radiation can be prevented by photoreactivating light (PRL; 320-400 nm) in the opossum Monodelphis domestica, implicating epidermal DNA lesions as the immunosuppressive impairment. However, contact hypersensitivity can also be suppressed in the opossum with exogenous cis-urocanic acid, a molecule which is produced in UVB-irradiated epidermis and is a second potential mediator of photo-immunosuppression apparently independent of UVB-induced DNA damage. Here we demonstrate that irradiation of opossums with PRL either before or following treatment with exogenous cis-urocanic acid, significantly reduced the degree of immunosuppression. This suggests that, in addition to its capacity to initiate post-UVB-exposure epidermal DNA repair, the PRL waveband can induce an immunoprotective product, as yet unidentified, in opossum epidermis.

A single specific epidermal photoreceptor for the immunosuppressive action of UV radiation has not been defined, although separate evidence is accruing in favour of each of two candidates, trans-urocanic acid and DNA. In Monodelphis domestica, specific photoreactivation repair of UV radiation-induced pyrimidine dimers has been shown to abrogate the suppression of contact hypersensitivity (CHS), thus suggesting that DNA is the target for this impairment. However, the both haired and hairless mice, immunosuppressive effects of UV radiation have been reproduced by the exogenous administration of the UV photoproduct of urocanic acid, cis-urocanic acid. We show here that the epidermis of M. domestica contains urocanic acid, that UV irradiation of the shaved dorsal skin has resulted in an increase in epidermal cis-urocanic acid and that the topical application of a cis-urocanic acid-containing lotion significantly depressed the capacity of Monodelphis to respond to contact sensitisers, in a manner analogous to these responses in the hairless mouse. Therefore in Monodelphis, suppression of CHS by UV irradiation appears to involve both urocanic acid photo-isomerisation and epidermal DNA damage.

The immunological consequences of exposure to UVA (320-400 nm) radiation are unclear. This study describes the relationship between the generation of epidermal cis-urocanic acid and the ability to respond to a contact-sensitizing agent, in hairless mice exposed to different UV radiation sources, which incorporate successively greater short-wavelength cutoff by filtration of the radiation from fluorescent UV tubes. Mice were exposed to these radiation sources at doses systematically varying in UVB radiation content but supplying increasing proportions of UVA radiation. All radiation sources were found to generate approximately 35% cis-urocanic acid in the epidermis, thus normalizing the sources for cis-urocanic acid production. However, only those sources richest in short-wavelength UVB resulted in suppression of the systemic contact hypersensitivity response. These sources also induced the greatest erythema reaction, measured as its edema component, in the exposed skin. A strong correlation was thus demonstrated between the induction of edema and the suppression of contact hypersensitivity, but there appeared to be no correlation between the generation of epidermal cis-urocanic acid and suppression of contact hypersensitivity. The sources richest in UVA content did not result in suppression of contact hypersensitivity; furthermore mice previously irradiated with such UVA-rich sources were refractory to the immunosuppressive action of exogenous cis-urocanic acid. A protective effect of the increased UVA content thus appeared to be inhibiting immunosuppression by the available endogenously generated or exogenously applied cis-urocanic acid.

Monodelphis domestica, the laboratory opossum, develops hyperplasia and neoplasia of shaved skin after repeated exposure to ultraviolet radiation (UVR). We exposed Monodelphis from genetically diverse families within our colony to determine whether there are any heritable components to the risk of two distinct skin lesion phenotypes-melanocytic nevus (MN) and advanced hyperkeratosis (HK). From about 5 months of age, animals were shaved and exposed three times a week to a dose of about 125 J/m2 of UVR (spectral peak, 302 nm; range, 280-400 nm). Of 33 sibships (151 individuals) that completed at least 30 weeks of the protocol, 137 completed 45 weeks. For genetic analyses, each animal was classified at 30 and 45 weeks as affected with MN and HK or not affected. Heritabilities were estimated using a variance decomposition approach. Susceptibility to MN showed no significant evidence for a genetic component at 30 or 45 weeks. In sharp contrast, susceptibility to HK was under virtually complete genetic control (heritability, 0.999; P < 0.001) at 30 weeks, and had a moderately high heritability (0.702; P < 0.001) at 45 weeks. We conclude that this model has great potential for identifying genes that confer susceptibility to UVR-induced skin lesions and for investigating environmental factors that may contribute to the increasing incidence of skin cancer in human populations.

The frequency of spontaneous and ultraviolet radiation (UVR)-induced mutation at the hprt locus was determined in control and denV-transfected, repair-proficient murine fibroblasts. Control cells removed an average of 25% of pyrimidine dimers induced by exposure to 150 J/m2 UVR from an FS40 sunlamp within 24 h; under the same conditions of induction and repair, denV-transfected cells removed an average of 71% of pyrimidine dimers. Control cells were somewhat more resistant than denV-transfected cells to killing by UVR. The average frequency of spontaneous mutation at the hprt locus for control and denV-transfected cells was 3 and 15 6-thioguanine (6-TG)-resistant colonies per 10(6) surviving cells, respectively; there was no statistically significant difference between control and denV-transfected cells. However, after exposure to 75 or 150 J/m2 UVR, denV-transfected cells had a significantly lower frequency of mutation to 6-TG resistance. After exposure to a fluence of 75 J/m2, the average frequency of UVR-induced mutation at the hprt locus was 166 mutant colonies per 10(6) surviving cells for control cells and 92 mutant colonies for denV-transfected cells; after 150 J/m2, control cells had 205 6-TG-resistant colonies per 10(6) cells, while denV-transfected cells had 61 mutant colonies. These results demonstrate that UVR-induced pyrimidine dimers are mutagenic photoproducts in mammalian cells.

Chronic exposure of the South American opossum, Monodelphis domestica, to ultraviolet radiation (UVR) induced 154 primary tumours of the cornea in 152 eyes. Tumours developed gradually; frank neoplasia was preceded by non-neoplastic proliferation of corneal stromal fibroblasts (keratocytes) and extensive neovascularization. Histologically, the majority of tumours (134 of 154) appeared to be fibrosarcomas arising from keratocytes, but about 12 per cent of the tumours (18 of 154) had a highly vascular appearance, suggesting haemangiosarcoma. In two eyes, squamous cell carcinomas overlay mesenchymal tumours. Ultrastructural features of UVR-induced corneal tumours were consistent with tumours, and cultured skin fibroblasts expressed high content of messenger RNA for the intermediate filament vimentin; no cytokeratin messenger RNA was detected in these cells and cell lines. Based upon their light microscopic, ultrastructural, and intermediate filament biosynthetic characteristics, the majority of UVR-induced corneal tumours in M. domestica appeared to be fibrosarcomas. Haemangiosarcomas constituted a smaller proportion of the tumours, and squamous cell carcinomas were very rare.

LP-BM5 murine leukemia virus (MuLV) induces an immunodeficiency syndrome (MAIDS) in C57BL/6 mice which resembles immunological abnormalities observed in early stages of human AIDS. In our study, MAIDS virus-infected mice were exposed to low doses of ultraviolet radiation (UVR) before and after virus inoculation and compared with MAIDS-infected but not UVR-exposed mice. In all tested parameters (blood IgM levels; mitogenic responses to PHA, ConA, LPS and anti-mu; MLR; antigenic response to SRBC; enlargement and histopathologic changes of the spleen) we observed the same trend: changes due to MAIDS infection were more pronounced in the UVR-exposed group than in the unexposed group. Statistically significant differences between these two groups were seen for mitogenic responses at two different time points after virus inoculation. These results demonstrate that in vivo UVR exposure enhances the immunosuppressive effects of a retroviral infection. UVR exposure may affect the progression of AIDS in a similar manner.

Exposure of an established marsupial cell line, PtK2 (Potorous tridactylus), to ultraviolet radiation (UVR) from an FS-40 sunlamp (280-400 nm) resulted in a fluence-dependent release of radiolabeled arachidonic acid (AA) from cell membranes. Post-UVR, but not pre-UVR, exposure to photoreactivating light reversed UVR-induced pyrimidine dimers in DNA and suppressed the UVR-induced release of AA. These data indicate that DNA damage contributes to the release of AA from membrane phospholipids.

A controversy has arisen concerning the ability of sunscreens to protect mice from the immunosuppressive effects of UV radiation. We have assessed the photoprotection in hairless mice of two sun protection factor (SPF)15 sunscreens containing different UVB (280-320-nm) absorbers, namely, octyl-N-dimethyl-p-aminobenzoate (o-PABA) or 2-ethylhexyl-p-methoxycinnamate (2-EHMC). Following three minimum erythemal exposures to UV radiation, both systemic suppression of contact hypersensitivity to 2,4-dinitrofluorobenzene and induction of susceptibility to transplanted UV radiation-induced tumor cells was established. Topically applied 2-EHMC sunscreen protected totally from both forms of immunosuppression, but the o-PABA sunscreen failed to protect, although both sunscreens were equally effective in protection from UV radiation-induced erythema and edema.

Studies were conducted to examine ultraviolet radiation (UVR)-induced alterations of the cornea of the gray, short-tailed opossum. Monodelphis domestica, and the effect of post-UVR illumination to photoreactivation light (PRL, 320-500 nm). As photoreactivation treatment specifically monomerizes pyrimidine dimers, an amelioration of the UVR-induced biological end-point would implicate DNA as being a primary chromophore for induction of that end-point. Corneas of anesthetized, four-month-old, opossums were exposed to 250 J m-2 (0.025 J cm-2) from a Westinghouse FS20 sunlamp either one or three times a week for up to 13 exposures. The corneas of 4-5 animals received either: (a) 90 min of PRL immediately prior to UVR; (b) PRL immediately following UVR; (c) PRL alone; or (d) UVR alone. Eyes were examined with a slit lamp microscope 24 hr following each exposure and scored for the appearance of opacification and neovascularization of the cornea. In animals exposed to UVR alone, 2-5 exposures, depending on whether the exposures were given once or three times per week, were required to obtain opacification and neovascularization in 50% of the irradiated corneas. The onset of both opacification and neovascularization in 50% of the corneas required 8-11 exposures when the UVR was immediately followed by PRL. Based on the specificity of photoreactivation repair to act solely on pyrimidine dimers, these observations suggest that UVR-induced pyrimidine dimers in corneal DNA are involved in UVR-induced opacification and neovascularization of the cornea of Monodelphis domestica.

A total of 19 male and 21 female South American opossums (Monodelphis domestica) were exposed to 250 J/m2 ultraviolet radiation from FS-40 sunlamps (280-400 nm) three times weekly for 70 weeks. The backs of the opossums were shaved as necessary to remove hair. In order to prevent photoreactivation of ultraviolet radiation-induced pyrimidine dimers by the light-dependent photolyase enzyme of the opossum, ultraviolet radiation-exposed opossums were housed under red lights (600-800 nm). The opossum photolyase requires light in the 320-450 nm range for its activity. Twenty-nine control opossums (14 males and 15 females) were irradiated by fluorescent lights with emission spectra primarily in the visible light range (320-700 nm); these control opossums were also housed under red lights, and their backs were also shaved to remove hair. No skin tumors were observed in control opossums, while ultraviolet radiation-exposed opossums developed a variety of hyperplastic and neoplastic skin lesions on the backs and on a single ear. Hyperplastic lesions included foci of epithelial hyperplasia, dermal fibroplasia, and focal proliferation of dermal melanocytes. A total of 20 ultraviolet radiation-exposed opossums (50%) developed skin tumors, and 13 opossums (32.5%) had more than a single tumor. Epithelial tumors included 25 papillomas, four keratoacanthomas, seven carcinomas in situ, three microinvasive squamous cell carcinomas, two invasive squamous cell carcinomas, and a single basal cell tumor. Ten dermal spindle cell tumors also occurred; most of these appeared to be fibrosarcomas. Two benign melanomas and one malignant melanoma were observed.

Radioimmunoassays were used to investigate the repair of cyclobutane pyrimidine dimers and pyrimidine (6-4)pyrimidone photoproducts ((6-4] photoproducts) in the epidermis of the South American opossum, Monodelphis domestica. In the absence of photoreactivating light, both types of photodamage were excised with similar kinetics, 50% of the damage remaining 8 h after UV irradiation in vivo. Exposure of UV-irradiated skin to photoreactivating light resulted in removal of most of the cyclobutane dimers and an enhanced rate of (6-4) photoproduct repair. Photoenhanced excision repair of non-dimer damage increases the range of biologically effective lesions removed by in vivo photoreactivation.

We have determined the ability of a chemical sunscreen with a sun protection factor (SPF) of 15 to protect human skin from ultraviolet (UV) radiation-induced DNA damage. The DNA damage was susceptible to cleavage by Micrococcus luteus UV endonuclease, which recognizes pyrimidine dimers in DNA. An alkaline agarose gel electrophoresis method was used to quantify the number of pyrimidine dimers in nonradioactive DNA from skin biopsies of 5 individuals irradiated with UV from a solar simulator. After exposure to an equivalent dose of UV, the number of pyrimidine dimers was 0.8 per 10(7) bases in sunscreen-treated skin as compared with 32 dimers per 10(7) bases in untreated skin. This assay provides a means of determining the efficacy of sunscreens in protecting skin from UV-induced DNA damage.

Post-ultraviolet radiation (UVR) treatment of the South American opossum, Monodelphis domestica with long-wavelength radiation (320-400 nm) suppressed the induction of histopathological alterations in the skin. This study identifies DNA as a primary chromophore involved in the induction of various photobiological responses of the skin such as hyperplasia and sunburn cell formation, and also identified pyrimidine dimers as one responsible DNA lesion. The histology of skin from opossums exposed to multiple doses of UVR showed that pre-malignant changes had occurred in the skin.

Several lines of evidence support the hypothesis that ultraviolet radiation (UVR) is involved in the etiology of cutaneous melanoma in humans. However, progress in understanding the mechanisms involved in induction of melanotic tumors by UVR has been hindered by lack of a suitable animal model. During the course of multiple exposures (3 times/wk for 70 wk) of the South American opossum, Monodelphis domestica, to UVR, we first observed the appearance of areas of dermal melanocytic hyperplasia (MH) on the exposed skin. Post-UVR exposure to photoreactivating light (320-500 nm) suppressed the occurrence of MH. We also observed at 100 weeks from first exposure that 10 of 46 surviving animals had developed melanotic tumors which arose, presumably, from areas of MH. Tumors on three of the 10 animals have been classified as malignant melanomas based on metastasis to lymph nodes. We conclude from these results that UVR can act as a complete carcinogen for melanoma induction and, based on the photoreactivation of MH induction, that DNA damage is involved in melanoma formation.

The resynthesis step of the excision repair pathway has been examined in Escherichia coli K12 strains isogenic except for mutations in the uvrD cistron. Strains mutant at the uvrD3, uvrD101, uvrE156, and recL152 loci exhibited slight but distinct differences in their response to ultraviolet radiation. The repair capacity of the uvrD101 mutant was given special attention. Repair resynthesis in this mutant was saturated at fluences greater than about 20 J/m2. Isopycnic analysis of repaired deoxyribonucleic acid from this strain revealed a two-fold increase over its wild-type counterpart in the amount of repair replication performed after a dose of 15 J/m2. Sedimentation velocity analysis of DNA after selective photolysis of bromouracil-containing repaired regions showed that the uvrD101 mutation exerted its primary effect on the long-patch component of excision repair. The uvrD101 mutant performed long-patch repair at a larger number of sites than the number repaired by this mode in the wild-type strain; these patches were more extensive in length than the long-patch component in wild-type.

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.

Chronic exposure of the skin of the South American opossum (Monodelphis domestica) to ultraviolet radiation (UVR) from an FS-40 sunlamp (280-400 nm) 3 times per week for a total of 200 exposures resulted in the appearance of actinic keratoses, fibrosarcomas, squamous cell carcinomas and keratoacanthomas. At the higher doses of UVR used in this study, moderate to severe hyperplasia was also observed. The susceptibility of this animal to the induction of skin tumors by UVR in conjunction with the capacity to enzymatically photoreactive pyrimidine dimers in cutaneous DNA identifies this animal as a useful model in determining the role of pyrimidine dimers in skin tumor induction by UVR.

We present data from experiments designed to investigate the role of DNA interstrand cross-links induced by exposure to 8-MOP plus UVR and skin carcinogenesis. 8-MOP was administered topically to two strains of hairless mice, SKH:hairless-1 and HRS/J/An1, which were then exposed to UV light sources with emission in the range of 1) 300-400, 2) 320-400, and 3) predominantly 365 nm. We found no strain dependency for DNA cross-link production, but a marked strain-dependent difference in tumor susceptibility was noted. Only a small strain-dependent difference occurred in tumor incidence when TPA was administered after exposure to 8-MOP and 320-400 nm. These results suggest that the events concerned with tumor promotion are dependent on strain. Because the most effective tumorigenic wavelength spectrum was 300-400 nm, we investigated the possibility of interaction between lesions induced by the 300- to 320-nm wavelengths and the psoralen photoadducts. In the course of this experiment, we found that the tumorigenic effect was also dependent on the time interval between exposures to 8-MOP plus 365-nm light.

Bromodeoxyuridine-containing regions approximately 1.5 X 10(4) Nucleotides in length, and at intervals equivalent to the pyrimidine dimer content of the DNA, have been observed in the parental DNA of an excision-defective strain of Escherichia coli exposed to 10 ergs mm-2 at 254 nm followed by prolonged incubation in the presence of bromodeoxyuridine.

The induction and fate of pyrimidine dimers and unscheduled DNA synthesis were measured in u.v.-irradiated primary, newborn SENCAR mouse epithelial cells. Unscheduled DNA synthesis was induced in a dose responsive manner by two u.v. sources, a germicidal lamp (254 nm) and an FS40 sunlamp (280--400 nm). Using the endonuclease-sensitive site assay to detect pyrimidine dimer production and excision, we examined the response of the newborn mouse cells to both u.v. sources. We were unable to detect the removal of pyrimidine dimers with either of the two sources of u.v. The speculation is made that primary, newborn mouse epidermal cells excise u.v.-induced pyrimidine dimers to an extent below the level of detection of the endonuclease-sensitive site assay but to an extent sufficient to induce unscheduled DNA synthesis.

The induction and photorepair of pyrimidine dimers in DNA have been measured in the ultraviolet-irradiated, corneal epithelium of the marsupial, Monodelphis domestica, using damage-specific nucleases from Micrococcus luteus in conjunction with agarose gel electrophoresis. We observed that FS-40 sunlamps (280-400 nm) induced 7.2 +/- 1.0 X 10(-5) pyrimidine dimers per kilobase (kb) of DNA per J/m2. Following 100 J/m2, 50% and greater than 90% of the dimers were photorepaired during a 10- and 30-min exposure to photoreactivating light (320-400 nm), respectively. In addition, approximately 70% and approximately 60% of the dimers induced by 300 and 500 J/m2, respectively, were repaired by a 60-min exposure to photoreactivating light. The capacity of the corneal epithelium of M. domestica to photorepair pyrimidine dimers identifies this animal as a potentially useful model with which to determine whether pyrimidine dimers are involved in pathological changes of the irradiated eye.

Hair follicles in the telogen stage of the hair cycle were induced into anagen phase after the skin of the South American opossum, Monodelphis domestica, was exposed to levels of ultraviolet B radiation (280 to 320 nm), which induced desquamation. Post-ultraviolet B exposure to long-wavelength radiation (320 to 400 nm, ultraviolet A) suppressed the induction of edema and desquamation and the induction of hair growth. Hair growth induction was also observed in skin damaged either by mechanical abrasion or by exposure to methoxsalen and ultraviolet A.

The ability to selectively photolyze bromouracil-(BrUra-)containing repaired regions in cellular DNA has allowed us to estimate the average size of repaired regions in ultraviolet (UV) light-irradiated Escherichia coli. Cells were labeled with thymidine-(3)H, irradiated at 254 nm, and incubated in nonradioactive bromodeoxyuridine (BrdUrd). After incubation the cells were exposed to 10(6) ergs.mm(-2) at 313 nm, lysed, and sedimented in alkaline sucrose gradients so as to measure the average molecular weight of single DNA strands. In strains that had excised approximately 45 cyclobutane pyrimidine dimers/10(8) daltons, the 313 nm treatment resulted in approximately 6 single-strand breaks/10(8) daltons. In an excisionless strain, the same treatment resulted in only 1.5 breaks/10(8) daltons. From the determination of the sensitivities of fully substituted DNAs to 313 nm light, we calculate that the repaired regions in excising strains of E. coli contain an average of 4-6 BrUra residues. Photoreactivation experiments indicate that the excision of pyrimidine dimers in the presence of BrdUrd is the primary source of repaired regions selectively photolyzed by 313 nm radiation.