Acetyl Coenzyme A Synthetase and the Regulation of Lipid Synthesis from Acetate in Cultured Cells

The regulation of lipid synthesis from acetate in cultures of L cells in response to changes in exogenous lipid supply has been studied, and the enzyme acetyl-CoA synthetase has been investigated in reference to its possible role in the regulation of lipid biosynthesis from acetate. When serum lipid was removed from monolayers of L cells, a relatively rapid stimulation of [14C]acetate incorporation into lipid was observed within 2 hours. This stimulation of incorporation was observed in both sterol and fatty acid fractions. Conversely, an inhibition of [14C]acetate incorporation was observed within 1 to 2 hours when cells cultured in lipid-free medium were transferred to serum-supplemented medium. As with the stimulation, this inhibition was observed both in sterol and fatty acid fractions of the cell lipid. The data indicated a coordination of fatty acid and cholesterol metabolism in the cells. Inhibition of [14C]acetate incorporation was observed in sterol as well as free fatty acid and glycerolipid fractions in cultures grown in lipid-free medium and transferred to fatty acid-supplemented medium; and similarly, cultures transferred to medium supplemented with cholesterol showed inhibition not only of acetate incorporation into sterol but also into fatty acid and glycerolipid fractions. Cycloheximide, actinomycin D and mitomycin C seemed to have no influence on the early stimulation or inhibition of [14C]acetate incorporation into total lipid. Acetyl-CoA synthetase activity was assayed in homogenates of L cells cultured in the presence of serum-supplemented or lipid-free medium and a 5-fold decrease in enzyme activity was observed in homogenates of cultures grown in the presence of exogenous lipid. When cells grown in serum-supplemented medium were transferred to serum-free medium, a stimulation of enzyme activity occurred within 2 to 3 hours which reached a maximum by 6 hours. Conversely, when cells were cultured in lipid-free medium and transferred to serum-supplemented medium, inhibition of enzyme activity occurred within the same time course. Cycloheximide, actinomycin D, and mitomycin C had no influence on the stimulation or inhibition of enzyme activity observed in response to changes in exogenous lipids. Inhibition of enzyme activity also was observed in cells cultured in lipid-free medium which were transferred to medium containing either fatty acid or cholesterol, indicating fatty acid and cholesterol might be part of the components of serum lipid that can influence the activity of acetyl-CoA synthetase. However, when various concentrations of serum lipid were added to the reaction mixture in vitro, there was no influence on enzyme activity. The data suggest that the enzyme acetyl-CoA synthetase may play a role in the regulation of lipid biosynthesis from acetate in cultured cells.