The differential activation of ornithine decarboxylase in synchronized cultures maintained with a salts/glucose medium

Asparagine and cAMP caused differential activation of ornithine decarboxylase activity that was dependent upon the cell cycle phase of Chinese Hamster Ovary cells maintained with a salts/glucose medium. Prior to incubation in the salts/glucose medium, stationary phase cultures were stimulated to proliferate by refeeding with fresh complete growth medium. Induction of ornithine decarboxylase activity was dependent upon two conditions: (1) the salts/ glucose medium required supplementation with an appropriate amino acid and cAMP; (2) cultures had to be in late G1/early S phase when placed in the salts/glucose medium. Cultures in G0 phase had moderate capacity for ornithine decarboxylase enhancement but lost this capacity as they traversed into early G1 or into mid S phase. These results demonstrate the importance of the cell cycle in modulating the activation of ornithine decarboxylase by asparagine and cAMP.