Induction of in vitro transformation by near-u.v. light and its interaction with β-propiolactone

The ability of u.v.-A light (320-400 nm) to induce cellular transformation in vitro and to modify chemical carcinogen-induced cellular transformation was investigated in BALB/c 3T3 cell cultures. When administered as a series of nontoxic exposures, u.v.-A alone was found to induce cellular transformation as a linear function of the numbers of u.v-A exposures. Possible interactions of u.v.-A with environmentally encountered chemical carcinogens were studied by examining the effects of u.v.-A light exposures on cellular transformation in cells exposed to the direct acting carcinogen, beta-propiolactone (BPL), an alkylating agent, with a standard initiation/promotion protocol. Twenty-four hours after a single treatment with 2.5 micrograms/ml of beta-propiolactone, cells were exposed to 3.0 kJ/m2 of u.v.-A light. U.v.-A exposures were repeated weekly for up to 5 weeks, after which cells were fixed, stained and dishes were scored for type III transformed foci. Weekly exposures to u.v.-A alone for 5 weeks induced approximately 3 foci/dish. Treatment with BPL alone induced approximately 1 focus/dish (background was 0.17 foci/dish). A combination of the two treatments resulted in a marked increase in the yield of transformed foci/dish, with the u.v.-A enhancement increasing with increasing numbers of exposures (approximately 10 foci/dish after a single exposure to BPL and five u.v.-A exposures). These results suggest a synergistic interaction between BPL and subsequent u.v.-A exposures in the induction of in vitro neoplastic transformation.