DNA-damaged cells can either repair the DNA or be eliminated through a homeostatic control mechanism termed "cellular proofreading." Elimination of DNA-damaged cells after ultraviolet radiation (UVR) through sunburn cell (apoptotic keratinocyte) formation is thought to be pivotal for the removal of precancerous skin cells. Sunburn cell formation was found to be dependent on Fas ligand (FasL), a pro-apoptotic protein induced by DNA damage. Chronic exposure to UVR caused 14 of 20 (70 percent) FasL-deficient mice and 1 of 20 (5 percent) wild-type mice to accumulate p53 mutations in the epidermis. Thus, FasL-mediated apoptosis is important for skin homeostasis, suggesting that the dysregulation of Fas-FasL interactions may be central to the development of skin cancer.

UV-induced mutations in the p53 tumor suppressor gene play an essential role in skin cancer development. We report here that such mutations can be detected in UV-irradiated mouse skin months before the gross appearance of skin tumors. Application of SPF-15 sunscreens to mouse skin before each UV irradiation nearly abolished the frequency of p53 mutations. These results indicate that p53 mutation is an early event in UV skin carcinogenesis and that inhibition of this event may serve as an early end point for assessing protective measures against skin cancer development.

Tumor necrosis factor (TNF) exerts cytotoxicity on many types of tumor cells but not on normal cells. The molecular events leading to cell death triggered by TNF are still poorly understood. Our previous studies have shown that enforced expression of an activated H‐ras oncogene converted non‐tumorigenic, TNF‐resistant C3H 10T1/2 fibroblasts into tumorigenic cells that also became very sensitive to TNF‐induced apoptosis. This finding suggested that Ras activation may play a role in TNF‐induced apoptosis. In this study we investigated whether Ras activation is an obligatory step in TNF‐induced apoptosis. Introduction of two different molecular antagonists of Ras, the rap1A tumor suppressor gene or the dominant‐negative rasN17 gene, into H‐ras‐transformed 10TEJ cells inhibited TNF‐induced apoptosis. Similar results were obtained with L929 cells, a fibroblast cell line sensitive to TNF‐induced apoptosis, which does not have a ras mutation. While Ras is constitutively activated in TNF‐sensitive 10TEJ cells, TNF treatment increased Ras‐bound GTP in TNF‐sensitive L929 cells but not in TNF‐resistant 10T1/2 cells. Moreover, RasN17 expression blocked TNF‐induced Ras‐GTP formation in L929 cells. These results demonstrate that Ras activation is required for TNF‐induced apoptosis in mouse fibroblasts.

We demonstrated previously that p53 mutations can be detected in ultraviolet B-irradiated mouse skin months before the gross appearance of skin tumors and that applying sun protection factor 15 sunscreens to mouse skin before each Kodacel-filtered FS40 sunlamp irradiation resulted in the reduction of such mutations. To determine whether there is an association between reduction of ultraviolet-induced p53 mutations by sunscreens and protection against skin cancer using an environmentally relevant light source, we applied sunscreens (sun protection factors 15-22) on to the shaved dorsal skin of C3H mice 30 min before each exposure to 4.54 kJ ultraviolet B (290-400 nm) radiation per m2 from a solar simulator. Control mice were treated 5 d per wk with ultraviolet only or vehicle plus ultraviolet. p53 mutation analysis indicated that mice exposed to ultraviolet only or vehicle plus ultraviolet for 16 wk (cumulative exposure to 359 kJ ultraviolet B per m2) developed p53 mutations at a frequency of 56%-69%, respectively, but less than 5% of mice treated with sunscreens plus ultraviolet showed evidence of p53 mutations. More importantly, 100% of mice that received a cumulative dose of 1000 kJ ultraviolet B per m2 only, or vehicle plus ultraviolet B developed skin tumors, whereas, the probability of tumor development in all the mice treated with the sunscreens plus 1000 kJ ultraviolet B per m2 was 2% and mice treated with sunscreens plus 1500 kJ ultraviolet B per m2 was 15%. These results demonstrate that the sunscreens used in this study not only protect mice against ultraviolet-induced p53 mutations, but also against skin cancers induced with solar-simulated ultraviolet. Because of this association, we conclude that inhibition of p53 mutations is a useful early biologic endpoint of photoprotection against an important initiating event in ultraviolet carcinogenesis.

The v-Ha-ras transgenic Tg.AC mouse line has proven to be a useful model for the study of chemical carcinogenic potential. We undertook experiments designed to study the effect of the physical carcinogen, UV radiation, on tumorigenesis in this mouse strain. Following a total of three exposures on alternating days to a radiation source covering a cumulative UVR exposure range of 2.6-42.6 kJ per m2, squamous papillomas developed by 4 wk after initial exposure in a dose-dependent manner. Malignancies developed within 18-30 wk following the initial UVR exposure and were all diagnosed as squamous cell carcinoma or spindle cell tumors. In contrast to other mouse stains used in photocarcinogenesis studies, few p53 mutations were found in Tg.AC malignancies upon polymerase chain reaction-single stranded conformational polymorphism analysis of exons 4-8 followed by sequencing of suspicious bands; however, all tumors analyzed by in situ hybridization expressed the v-Ha-ras transgene. Immunohistochemical analysis of UVR-exposed skin taken 24 h after the last of three exposures (13.1 kJ per m2 total UVR) showed expression of p53 in hair follicles and in interfollicular epidermis, which indicates that the gene was functional. Thus, although there are some differences between the Tg.AC and other mouse models, these results suggest that the Tg.AC mouse may be a useful model for the study of acute exposure photocarcinogenesis.