DNA repair by articular chondrocytes. III. Unscheduled DNA synthesis following ultraviolet light irradiation of resting cartilage.

Unscheduled DNA synthesis (UDS) was demonstrated both in fresh slices of rabbit articular cartilage and in resting chondrocytes dissociated from it. Uptake of [3H]thymidine by chondrocytes treated with 10 mM hydroxyurea and irradiated with 254 nm ultraviolet light (20 J/m2) was measured by autoradiography. The UDS in the resting chondrocytes was minuscule compared with that of chondrocytes growing in monolayer culture. It was greater in resting cells from 3- than in 18-month old rabbits but persisted in animals up to 52 months of age. In both resting and cultured chondrocytes, UDS was greatly inhibited by aphidicolin (5 micrograms/ml). The finding of age-related decrease in UDS of resting chondrocytes, contrasted with previously demonstrated lack of it in dividing ones, raises questions about in vitro artefacts in studies of cellular senescence.