Regulation of Lipid Synthesis in Cultured Animal Cells

Fatty acid and sterol biosynthesis were studied in several cell lines in culture. Two of these cell lines, L-M and NCTC 1469, demonstrated an 8.6- and 7.2-fold increase, respectively, in acetate incorporation into fatty acids and a 5.2- and 2.4-fold increase, respectively, in acetate incorporation into sterols within 24 hours after serum-containing medium was replaced with serum-free medium. Three other cell lines, Chang liver, HeLa, and NCTC 2544, demonstrated little change in fatty acid or sterol synthesis 48 hours after serum removal from the medium, but they demonstrated up to an 8-fold increase in fatty acid synthesis and a 3-fold increase in sterol synthesis 48 hours after serum removal when insulin was present in the serum-free medium. The increased fatty acid synthesis following transfer of L-M and NCTC 1469 cells to serum-free medium could be attributed to increased activities of both acetyl-coenzyme A carboxylase and fatty acid synthetase, whereas increased synthesis in Chang liver, HeLa, and NCTC 2544 cells was associated primarily with increased fatty acid synthetase activity. In Chang liver cells fatty acid synthetase activity did not change during the first 24 hours after replacement of serum-containing medium with serum-free medium. During the subsequent 24 hours activity increased approximately 160 % in the absence of insulin and approximately 450 % in the presence of insulin. Insulin at a concentration of 2 x 10-9m produced 50 % of the maximal increase in synthetase activity, while proinsulin at 1.2 x 10-7m was required for 50 % of maximal increase. The continued presence of insulin was necessary for maintenance of elevated fatty acid synthetase activity as demonstrated by the decrease observed when insulin was removed from the medium. Immunotitration experiments, using antiserum produced against pure fatty acid synthetase, indicated that the increased fatty acid synthetase activity observed after the removal of serum from the medium or the addition of insulin to serum-free medium was due to increased levels of enzyme in the cells. An increase in fatty acid synthetase synthesis presumably accounted for the increased levels of the enzyme since the effects of serum removal and insulin addition were blocked by cycloheximide, an inhibitor of protein synthesis. In Chang liver cells low levels of actinomycin D and camptothecin, inhibitors of RNA synthesis, prevented the induction of fatty acid synthetase by serum removal or insulin addition. A c