Cell cycle progression and 3-hydroxy-3-methylglutaryl coenzyme A reductase are regulated by thyrotropin in FRTL-5 rat thyroid cells.

The incorporation of [14C]acetate into cholesterol shows that FRTL-5 cells possess an active cholesterol biosynthetic pathway. When these cells were made quiescent, and synchronized by thyrotropin (TSH) starvation, in the presence of low serum (0.2%), addition of this hormone increased acetate conversion into cholesterol up to a maximum of 8-fold. Feedback inhibition of sterol synthesis by exogenous cholesterol occurs in FRTL-5 cells since, in the presence of higher serum concentration (5%), acetate conversion into cholesterol was significantly depressed. Even in high serum TSH increased sterol synthesis, albeit to a lesser extent. The time course of the TSH effect on cholesterol synthesis, strongly suggests that this process is necessary for quiescent FRTL-5 cells to enter the cell cycle. Thus, the rate of cholesterol synthesis was maximal 12-16 h after TSH challenge and declined thereafter, returning to levels slightly above the basal at 48 h. Thymidine incorporation into DNA, measured under identical conditions of TSH starvation/challenge, increased after 20 h, was maximal at 36 h, and returned to pre-TSH level at 70 h. The effect of TSH on cholesterol synthesis is not a general feature of lipid synthesis in FRTL-5 since [14C]acetate incorporation into triglycerides after TSH treatment has a different magnitude and time course. TSH increases cholesterol synthesis through the induction of the enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase. This is due to an increase in the level of 3-hydroxy-3-methylglutaryl-CoA reductase messenger RNA up to 8-fold caused by a proportional increase in the rate of gene transcription, as assessed by nuclear "run on" experiments. The effect of TSH on cholesterol synthesis and reductase gene expression is likely to be mediated by cAMP since 8-bromo-cAMP mimicked the effect of the hormone. The data presented suggest that an active cholesterol biosynthetic pathway is required for DNA synthesis to occur.