For elucidation of the role of repair mechanisms for carcinogenesis by chemical agents, cell transformation by 4-nitroquinoline 1-oxide (4NQO) and excision repair of 4NQO lesions were studied with mouse A31-714, a subclone derived from BALB/3T3, with posttreatment incubation in the presence or absence of hydroxyurea (a known inhibitor of normal semiconservative DNA replication). When hydroxyurea was present in the medium for 12 and 24 hr of incubation after 4NQO treatment, the transformation frequency was reduced to 28 and 6%, respectively, of the frequency observed for cells incubated in a growing state without hydroxyurea. The observed effect of hydroxyurea resembles the effect of the posttreatment placement of cells in a density-dependent nongrowing state, which also prevented the production of transformed foci by 4NQO. Excision repair of 4NQO damage was measured by autoradiography of unscheduled [3H]thymidine incorporation or by chromatographic analysis of four kinds of 4NQO-purine adduct (the major DNA lesions produced by 4NQO) remaining in DNA after posttreatment incubation of cells. The excision repair was very rapid in 4NQO-treated cells kept in the nongrowing state, and it was not affected by the presence of hydroxyurea in the incubation medium. Furthermore, the fractions of repaired lesions estimated from the excision and repair synthesis experiments were about 50% and 65 to 90% for the cells incubated for 12 and 24 hr, respectively, paralleling the decrease in the transformation frequency (assuming that the decrease was due to repair of 4NQO lesions).
From a combination of these results with others, we suggest that the four kinds of 4NQO-purine adduct are excisable in this mouse cell line by an excision repair system similar to, but possibly not identical with, that which works on UV-induced pyrimidine dimers in human cells and that unexcised fractions of these adducts are the major cause of cell transformation by 4NQO. The unique characteristics of the 4NQO-purine adducts are discussed in relation to excision repair, which may have a vital role in chemical carcinogenesis.
This work was supported in part by grants for cancer research from the Ministry of Education, Science, and Culture, Japan, and from Toray Science Foundation, Japan.