Prevalence of tumor prevention rather than tumor enhancement when repetitive UV radiation treatments precede initiation and promotion.

UV irradiation can act as a tumor initiator in mouse skin, yet repetitive UV irradiation can systemically prevent chemically induced two-stage skin tumorigenesis. The present study addressed the question of whether repetitive dorsal UV irradiation would enhance or inhibit subsequent initiation and promotion applied dorsally. Approximately 4.25 x 10(5) J/m2 was applied intermittently to 30 shaved CDF1 mice over an 8 week period. Mice were then initiated dorsally with 100 micrograms of 7,12-dimethylbenz[a]anthracene and subsequently promoted with 7.5 micrograms 12-O-tetradecanoylphorbol-13-acetate (TPA) applied twice weekly for 19 weeks. Initiated and promoted mice that had been treated repetitively with 1.06 x 10(4) J/m2 showed a decrease in tumor incidence from 92 to 28%, and a reduction in tumor yield per mouse from 5.35 to 0.58, at 19 weeks after the first TPA treatment. Histological analysis revealed that the UVB radiation treatments used in these experiments did not produce permanent loss of epidermal cells or sebaceous gland atrophy. When the same dose of UVB irradiation was applied after initiation and promotion, no increased conversion of papillomas to carcinomas was found, within a 48 weeks experimental duration. Thus, repetitive UV irradiation prevented rather than enhanced subsequent two-stage tumorigenesis. Repetitive UVB irradiation at late stages of promotion failed to enhance conversion of papillomas to carcinomas within the time frame in which chemical initiators mediate conversion to malignancy.