DNA and cholesterol biosynthesis in synchronized embryonic rat fibroblasts. I. Temporal relationships between HMG-CoA reductase activity, sterol biosynthesis and thymidine incorporation into DNA

Abstract

Temporal relationships between hydroxymethylglutaryl-CoA reductase activity, biosynthesis of C27 sterols, and [3H]thymidine incorporation into DNA were studied in a rat embryo fibroblast cell line synchronized by double thymidine block and cultured in cholesterol-containing medium. Cyclic variations of HMG-CoA reductase activity and C27 sterols occurred, with two maxima in S and G2M phases; the relative shortness of the G1 phase (3 h) in these cells could be responsible for the shift of sterol synthesis in the S phase. No noticeable variation of the individual C27 sterols was observed during the entire cell cycle. In each experiment, there was a good linear correlation between HMG-CoA reductase activity and C27 sterol synthesis, but from one experiment to another, a given level of enzymatic activity led to varying levels of [2-14C]acetate incorporation into sterols. In our experimental conditions, total HMG-CoA reductase activity is measured, and the preceding observation could be explained by a varying degree of phosphorylation of the enzyme depending on the metabolic state of the cells at the start of the experiment. The cyclic variations of the enzyme activity seem to be due more to increased synthesis at given times of the cycle than to periodic dephosphorylation. We question the existence of a relationship between cell division and cyclic sterol synthesis occurring in cells cultured in cholesterol-containing medium.