The regulation of fatty acid synthesis in human skin fibroblasts. Inhibition of fatty acid synthesis by free fatty acids.

The short term regulation of fatty acid biosynthesis has been studied using primary cultures of human shin fibroblasts. These cells show rapid changes in rates of acetate incorporation into fatty acids depending upon the lipid content of the medium. When cells are incubated in medium containing lipid-deficient fetal calf serum overnight and then are switched to Eagle’s minimum essential medium, the rate of acetate incorporation into fatty acids is unaffected. Addition of 5 PM albumin-bound palmitic, stearic, oleic, linoleic, or arachadonic acids to the minimum essential medium results in a 26 to 67% inhibition in the rate of acetate incorporation depending upon the fatty acid used. These results are obtained when cells are incubated with [lJ4C]acetate from 0.5 to 10.5 min after addition of fatty acid to the medium. When cells are labeled from 5 to 15 min after addition of fatty acid, there was a greater inhibition of acetate incorporation to 70 to 85%. Several lines of evidence suggest that the site of action of fatty acids is intracellular. The effect of added fatty acids on the intracellular concentration of citrate, long chain acyl-CoA compounds, and free fatty acids were measured in these cells. Long chain acyl-CoA levels and citrate levels of cells incubated in Eagle’s minimum essential medium for 10.5 min are 73.3 f 2.5 pmoles per mg of total cellular protein and 2.1 i 0.3 nmoles per mg of total cellular protein, respectively. When albumin-bound palmitic acid at a concentration of 2.1 pM is included in the incubation medium the levels of long chain acyl-CoA compounds (69.0 & 3.0 pmoles per mg of total cellular protein) and of citrate (1.9 f 0.1 nmoles per mg of total cellular protein) remain unchanged despite inhibition of acetate incorporation into fatty acid by this time. Furthermore, incubation of cells in medium containing albumin-bound palmitic acid results in only slight increases in the intracellular free fatty acid levels compared to cells incubated in fatty acid-free medium. These results suggest that total cellular levels of these metabolites are not directly responsible for regulation of the rate of acetate incorporation into fatty acids.