Grieco D, Beg Z H, Romano A, Bifulco M, Aloj S M
Centre di Endocrinologie e Oncologia Sperimentale/Consiglio Nazionale delle Ricerche, University of Naples, Italy.
J Biol Chem. 1990 Nov 5;265(31):19343-50.
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.
[14C]乙酸盐掺入胆固醇的实验表明，FRTL-5细胞拥有活跃的胆固醇生物合成途径。当这些细胞进入静止期，并通过促甲状腺激素（TSH）饥饿使其同步化，在低血清（0.2%）存在的情况下，添加这种激素可使乙酸盐转化为胆固醇的量最多增加8倍。外源性胆固醇对FRTL-5细胞中甾醇合成的反馈抑制作用存在，因为在较高血清浓度（5%）存在的情况下，乙酸盐转化为胆固醇的量显著降低。即使在高血清中，TSH也能增加甾醇合成，尽管程度较小。TSH对胆固醇合成的影响的时间进程强烈表明，这个过程对于静止的FRTL-5细胞进入细胞周期是必要的。因此，在TSH刺激后12 - 16小时胆固醇合成速率最高，此后下降，在48小时时恢复到略高于基础水平。在相同的TSH饥饿/刺激条件下测量的胸苷掺入DNA的量，在20小时后增加，在36小时时最高，并在70小时时恢复到TSH刺激前的水平。TSH对胆固醇合成的影响并非FRTL-5细胞中脂质合成的普遍特征，因为TSH处理后[14C]乙酸盐掺入甘油三酯的情况具有不同的幅度和时间进程。TSH通过诱导3-羟基-3-甲基戊二酰辅酶A还原酶来增加胆固醇合成。这是由于基因转录速率成比例增加导致3-羟基-3-甲基戊二酰辅酶A还原酶信使RNA水平增加高达8倍，这是通过细胞核“连续转录”实验评估得出的。TSH对胆固醇合成和还原酶基因表达的影响可能是由环磷酸腺苷（cAMP）介导的，因为8-溴-cAMP模拟了该激素的作用。所呈现的数据表明，DNA合成的发生需要活跃的胆固醇生物合成途径。
