The dose dependence of cyclobutane dimer induction and repair in UVB-irradiated human keratinocytes

UVB and UVA components of the solar spectrum or from artificial UV-sources might be important etiological factors for the induction and development of skin cancer. In particular, deficiencies In the capacity to repair UV-indu ced DNA-lesions have been linked to this phenomenon. However, until now onl y limited data are available on the biological and physical parameters gove rning repair capacity. We have, therefore, developed a flowcytometric assay using fluorescence-labeled monoclonal antibodies to study the dose-depende nce of induction and repair of UVB-induced cyclobutane pyrimidine dimers in a spontaneously immortalized keratinocytic cell line (HaCaT). Our results show that the kinetics of recognition and incision of UVB-induced DNA lesio ns slows down by a factor of about 3 in a dose range of 100-800 J m(-2), Fu rthermore, a thorough analysis of repair kinetics indicates that this reduc tion in repair capacity might not be dependent on saturation of enzymatic r epair capacity (Michalis-Menten) but may be caused by a UV-induced impairme nt of enzymes involved in DNA repair. Because this effect is evident in vit ro at doses comparable to the minimal erythemal dose in vivo, our results m ight have significant impact on risk assessment for UV-induced carcinogenes is.