Regulation of thymidylate synthase enzyme synthesis in 5‐fluorodeoxyuridine‐resistant mouse fibroblasts during the transition from the resting to growing state

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Abstract
Thymidylate synthase (TS) activity is very low in resting mouse 3T6 fibroblasts but increases sharply in growth‐stimulated cells at about the same time the cells enter S phase. To study the mechanism responsible for the increase in TS level, we isolated a 5‐fluorodeoxyuridine (5‐FdUrd)‐resistant cell line (LU3‐7) that overproduces TS and its mRNA about 50–100‐fold. In this paper we show that the LU3‐7 cells were able to rest in the GO state of the cell cycle when maintained in medium containing 0.5% serum. When the serum concentration was increased to 10% the resting cells reentered the cell cycle and began DNA replication about 12 hr later. TS activity remained at the resting level until DNA replication began, then increased at later times. The increase was not affected when the cells were stimulated in the presence of DNA synthesis inhibitors. The rate of synthesis of TS (as determined in a pulselabeling experiment) remained at the resting level for the first 10 hr following stimulation, then increased 8–9‐fold by 25 hr following serum stimulation. The half‐life of TS in growing LU3‐7 cells was measured in a pulse‐chase experiment and found to be greater than 24 hr. Therefore the increase in TS activity was primarily due to an increase in the rate of synthesis of the enzyme. Since TS gene expression appears to be regulated in a similar manner in LU3–7 cells and in the parental 3T6 cells, the LU3–7 cells should be a good model system for detailed analysis of the mechanism for regulating TS gene expression in mammalian cells.