Insulin‐like growth factor I regulation of transcription and replicating enzyme induction necessary for DNA synthesis

Abstract

Insulin-like growth factor I (IGF-I) regulation of the sequence of transcriptional, translational, and postranslational events during G₁ that are necessary for DNA synthesis, and the induction of thymidine kinase and in vivo thymidylate synthase activities was studied in synchronized confluent BALB/c 3T3 mouse fibroblasts. IGF-I was the only growth factor necessary from 6 to 2½ hours before S phase for onset of DNA synthesis. 5,6 dichlororibofuranosylbenzimidazole (DRB), an inhibitor of transcription, was ineffective in blocking DNA synthesis and increase of both enzyme activities during an interval of about 2½ hours before S phase. Cycloheximide, an inhibitor of translation, was ineffective in blocking DNA synthesis and the increase of in vivo thymidylate synthase activity during an interval of about 1½ hours before S phase but was effective in blocking the increase of thymidine kinase activity up to the G₁/S boundary. The results demonstrate that IGF-I is dispensable beyond the R-point (2–3 hours before S phase), the time when serum factors are no longer necessary for the initiation of DNA synthesis, and that IGF-I regulates transcriptional events necessary for both DNA synthesis and the induction of thymidine kinase and in vivo thymidylate synthase. The results also demonstrate a posttranslational interval for in vivo thymidylate synthase, suggesting posttranslational modification of this enzyme.