DNA synthesis in cultures of calf aortic endothelial cells treated with corpus luteum extract: Its inhibition by 3H-thymidine

Abstract

In cultures of calf aorta endothelial cells treated with extracts from corpora lutea a paradoxical effect occurred: Cell proliferation was regularly stimulated, but DNA synthesis was more or less inhibited. This inhibition was related to the presence of 3H-thymidine and could be reduced by lowering te 3H-thymidine concentration in the incubation medium. To decide whether the molar concentration or the activity of the 3H-thymidine was responsible for this effect, unlabelled thymidine was added to increase the molar thymidine concentration. A mixture of unlabelled thymidine and 3H-thymidine (ratio 200:1) added to the medium at various concentrations stimulated the DNA synthesis in a dose-dependent was up to 4.1 x 10(-6) M. Furthermore, nonradioactive thymidine slightly promoted the proliferation of calf aorta endothelial cells up to 10(-6) M independently of the presence or absence of corpus luteum extract. It is concluded that the radioactivity of 3H-thymidine is responsible for this effect. Internal 3H-beta radiation may perturb cell cycle progression after incorporation of this radioactive precursor into the cells. This view is supported by the fact that the inhibitory effect can be reduced by adding unlabelled thymidine, i.e. by lowering the specific activity of 3H-thymidine. On the other hand, increasing concentrations of growth promoting factors which stimulate the uptake of 3H-thymidine render the cells more sensitive to this effect.