Abstract
The frequency of ultraviolet light (UV)-induced neoplastic transformation of Syrian hamster embryo cells (HEC) is enhanced 3- to 10-fold when the cells are first treated with either X-irradiation or with methyl methanesulfonate. Maximum enhancement occurs when the interval between the two treatments is 48 hr. The relevance of UV-induced transformation to neoplasia is confirmed because the transformants produce tumors when injected into nude mice. Excision and postreplication DNA repair were studied to determine whether the enhanced transformations were associated with either of these repair mechanisms. Independent of X-ray or of methyl methanesulfonate pretreatment, approximately 25% of the pyrimidine dimers are excised within 24 hr in cells irradiated with UV with 3 J/sq m. During this period, more than 70% of the genome of cells irradiated with UV has been replicated. Postreplication repair is measured by the time required to chase pulse-labeled nascent DNA strands to parental-sized DNA. Regardless of pretreatment, 1 and 3 hr are required for pulselabeled DNA in control and irradiated (10 J/sq m) cells, respectively, to reach parental size. Therefore, no correlation is found between a change in the rate of excision or postreplication repair and enhancement of transformation. Relative to control cloning efficiency, the survival of HEC irradiated with 3 J/sq m is higher than 70% even though HEC contain more than 105 pyrimidine dimers/genome. The level of survival is similar to the survival of human skin fibroblasts which excise pyrimidine dimers four to five times as efficiently. Moreover, postreplication repair cannot account for the ability of these cells to survive because it is three times slower than in human fibroblasts. Therefore, other repair mechanisms must be responsible for HEC survival and transformation.
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