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.

This study was conducted to explore the involvement of DNA damage in the suppression of contact hypersensitivity (CHS) by UV irradiation. The opossum, Monodelphis domestica, was used because cells of these marsupials have an enzyme that is activated by visible light (photoreactivating enzyme) and repairs ultraviolet radiation (UVR)-induced pyrimidine dimers in DNA. A single dose of 1,500 J/m2 of UVB (280-320 nm) radiation, representing 2 minimal erythema doses, was administered to the dorsal skin of opossums. This treatment prevented the opossums from developing a CHS response to dinitrofluorobenze (DNFB) applied either at the site of irradiation or an unirradiated site. In addition, this dose of UVR decreased the number of ATPase+ epidermal Langerhans cells in the dorsal epidermis to approximately 3% of that in unirradiated skin at the time of DNFB application. Treatment of the animals with wavelengths that activate the repair enzyme (320-500 nm, photoreactivating light, PRL) for 120 min immediately after UV irradiation inhibited the UVR-induced suppression of CHS almost completely. Exposure to PRL before UVR did not prevent UVR-induced suppression of CHS. PRL treatment after UV irradiation also prevented the decrease in the number of ATPase+ Langerhans cells. Measurements of lesions in DNA indicated that PRL treatment removed around 85% of the UVR-induced pyrimidine dimers. These data provide direct evidence that DNA, and most likely, the pyrimidine dimer, is the primary molecular target for the UVB-induced suppression of contact hypersensitivity to haptens applied to irradiated or unexposed skin.

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.

Controversy continues both as to which wavelengths of sunlight cause melanoma and the mechanisms by which these different wavelengths act. Direct absorption of UVB by DNA is central in albino animal models, but melanin-pigmented models have shown major contributions by wavelengths longer than UVB that are thought to be mediated by photosensitized oxidant production. The only model for which the action spectrum of melanoma causation is known is a genetically melanoma-susceptible specific cross of Xiphophorus fish. We used electron paramagnetic resonance to quantitatively detect the UV induction of reactive melanin radicals in situ in the melanin-containing cells in the skin of this model and derived the action spectrum for melanin-photosensitized oxidant production (⌽ ox). This action spectrum was identical to that for melanoma induction (⌽ mel). These results confirm the hypothesis that melanin-photosensitized radical production is the major causative step of melanoma in this model and demonstrate that the wavelengths and mechanisms of melanoma causation in different models are dependent on the presence of melanin. This approach should be applicable to humans, thus providing an accurate surrogate for ⌽ mel for prevention studies.action spectrum ͉ free radical C utaneous malignant melanoma incidence continues to increase (1), yet prevention strategies are hindered by a lack of knowledge of which wavelengths of sunlight cause melanoma, and the mechanisms by which these different wavelengths cause melanoma are not understood. Although nonmelanoma skin cancers are predominantly caused by UVB wavelengths, melanoma causation has efficiently been observed by wavelengths longer than UVB, such as UVA, with this observation supported by both experimental animal (2-4) and human epidemiological evidence (5-7). However, the role of UVA in melanoma causation still remains controversial (8, 9). Similarly, there is controversy over the mechanisms by which these different wavelengths act. The prevailing view is that UVA leads to DNA photooxidation, with melanin thought to be the important photosensitizer when present, whereas UVB leads to pyrimidine dimer formation through direct absorption by DNA (6, 10). The relative importance of UVB and UVA in these processes is, however, currently under scrutiny, because some have found that UVA appears able to produce pyrimidine dimers in cultured cells (11,12), whereas melanin can act as an efficient UVB sensitizer in vivo (13), and so these questions need further investigation.The question of which wavelengths cause a biological effect, in this case melanoma, is best evaluated through measuring the wavelength dependence of that effect to generate its action spectrum (14). Although melanoma has been observed in many species (15), there is only one model in which the action spectrum of melanoma causation has been determined, namely select interspecies crosses of Xiphophorus fish (4, 16). Despite the phylogenetic distance between Xiphophorus and humans, and the differences in skin structure...

A new technique has been developed for studying the extent of repair of UV-radiation damage to DNA in human cells. It is easy to use, has excellent sensitivity, and provides rapid quantitative estimates of repair. UV-irradiated cells whose DNA has been previously labeled with a radioisotope are grown after irradiation in nonradioactive bromodeoxyuridine, which is incorporated at the breaks induced by repair enzymes. After a period of growth in the thymidine analog the cells are exposed to a large flux of 313 nm radiation and then lysed on top of an alkaline sucrose gradient. Bromodeoxyuridine-containing sections of the DNA are thus selectively photolysed. Sedimentation in the alkaline gradient reveals the average molecular weight of disrupted segments and gives a measure of the number of breaks induced by repair enzymes over the whole period allowed for repair. The large change in average molecular weight observed upon exposure of normal repairing cells to 313 nm radiation is not observed in the repair-deficient cells from patients with xeroderma pigmentosum. The quantitative aspects of this assay for repair and its sensitivity should make it applicable to the study of repair induced by agents other than UV radiation.Xeroderma pigmentosum (XP) is an ultraviolet-sensitive, hereditary disease of the skin caused by an autosomal recessive mutation (1). The mutation results in a defect in the enzyme mechanism responsible for the repair of UV-induced lesions in DNA. Cleaver (2) first called attention to XP as possibly due to a repair-deficient mutation by his demonstration of decreased or absent unscheduled synthesis and repair replication in UV-irradiated "XP cells" (fibroblasts cultured from skin biopsy specimens of patients with XP). We demonstrated that XP cells do not excise UV-induced pyrimidine dimers (3), in contrast to normal cells, which can excise dimers (4). The excision process reflects early steps in repair; Cleaver (5) presented indirect evidence and we (3) provided direct evidence suggesting that a UV-specific endonuclease, the enzyme presumed to initiate the repair process, is nonfunctional in XP cells.The steps in repair are believed to be the following (6-8): (a) an ultraviolet-specific nuclease makes a single-strand break near a lesion (usually a pyrimidine dimer); (b) an exonuclease, perhaps associated with the DNA polymerase molecule, excises the dimer and other nucleotides as well; (c) DNA polymerase inserts new nucleotides into the gap produced by excision; and (d) DNA ligase closes the gap. Repair may be measured in many ways: for example, as loss of dimers from the DNA. This is a difficult measurement because of the small amounts of radioactive thymine associated with dimerized pyrimidines and the fact that the excision of dimers is only about 50% efficient in human cells (3,4). Repair may also be measured by centrifugation in alkali of DNA from normal and XP cells. After irradiation of normal cells a shift to lower molecular weight is observed, followed by the return to higher molecu...