Polyamine biosynthesis and accumulation during the G1 to S phase transition

Ornithine decarboxylase, an important enzyme in growth regulation, is increased in CHO cells in G1 phase of the cell cycle and decreases as the cells progress into S phase. S‐adenosyl‐L‐methionine decarboxylase activity, which is dependent on either the presence of putrescine or spermidine for the synthesis of spermidine and spermine respectively, shows a maximal increase in late G1/early S phase which corresponds very closely with the cell cycle phase specific accumulation of spermidine and spermine during S phase. Total culture evaluation of spermidine and spermine, which included extracellular as well as intracellular concentrations, indicated that extracellular accumulations of these polyamines occurred only in G1 and that entry into S phase was concomitant with intracellular accumulation patterns. Hyperthermia (43°C for 1 hour) in mid‐G1 phase of the cell cycle resulted in rapid decreases in the activities of ornithine decarboxylase and S‐adenosyl‐L‐methionine decarboxylase. In these cells, DNA replication was also not detectable until nine hours after mitosis, a time at which there had been recovery of ornithine decarboxylase and S‐adenosyl‐L‐methionine decarboxylase activities. Previous data have further indicated a requirement for polyamine reaccumulation before control DNA replication rates are resumed. We therefore suggest that polyamine biosynthesis and intracellular accumulation are both temporal and quantitative prerequisites for transition through S phase.