Differential effect of UV-B and UV-C on DNA damage in L-132 cells

Ultraviolet radiation is known to induce skin cancer. The induction of DNA damage caused by UV-B and UV-C was investigated using cultured L-132 cells. DNA strand breaks assayed by the alkaline elution procedure occurred in a dose-dependent manner, the extent of the strand breaks were inversely well correlated with the number of viable L-132 cells after 24 h incubation. About a 10-fold dose of UV-B irradiation was required to induce a similar degree of strand breaking to that induced by UV-C. Similarly about a 10-fold dose of UV-B was required to produce a similar amount of pyrimidine dimers, such as cyclobutane-type dimers and pyrimidine-(6-4)-pyrimidone photoproducts, which were determined by ELISA using the specific monoclonal antibody, to that produced by UV-C. Strand breaks induced by UV-B, however, were not fully repaired in viable cells remaining after incubation of cells for a longer period of time, although UV-C-induced strand breaks were repaired in a time-dependent manner. Furthermore, an experiment with a cell-free system, where the induction of strand breaks by repair enzymes did not take place, indicated that UV-B caused significantly more direct DNA strand breaks than that caused by one-tenth the dose of UV-C. The data shown here suggest that UV-B-induced DNA damage is mediated, at least in part, via a different mechanism from the UV-C induced one.