Human keratinocytes catabolize thymidine

Human neonatal foreskin keratinocytes incorporate exogenous thymidine into DNA and proliferate in vitro even after reaching confluence. Keratinocytes also catabolize thymidine, as reported for the first time below. Stratified cultures of keratinocytes reduced the amount of thymidine in the medium by more than 90% within 2 to 4 h. Consequently, the rate of incorporation of thymidine (0.2 microM, 4 microCi/ml) into DNA was linear for no more than 2 h. Linear incorporation of thymidine into DNA for at least 12 h could be achieved by continual addition of fresh radioactive thymidine to the culture medium. Different tissues have widely differing abilities to catabolize thymidine. Cutaneous catabolism of thymidine shows striking species differences. Soluble extracts from human neonatal foreskin and adult skin, as well as from cultivated human keratinocytes, actively catabolize thymidine. Soluble extracts of skin from mouse, rabbit, or guinea pig do not catabolize thymidine. Extracts from cultivated human fibroblasts and melanocytes have little or no ability to catabolize thymidine. Catabolism of thymidine by keratinocytes has important implications for the use of [3H]thymidine in studies of keratinocyte proliferation and for the use of thymidine analogs in therapy of cutaneous disease.