Isoprene synthesis in isolated embryonic Drosophila cells. I. Sterol-deficient eukaryotic cells

Since insects are cholesterol auxotrophs, we analyzed the apparent paradox presented by an established cell line (Kc cells) from Drosophila embryos which grew in media which contained less than 0.05 micrograms/ml of sterols. Fresh Drosophila embryos contained 3.7 micrograms of 3 beta-hydroxysterols/mg of protein; however, Kc cells had maximally 0.50 micrograms of 3 beta-hydroxysterols/mg of protein. Kc cells, grown in media which contained cholesterol, showed the presence of sterol in their plasma and intracellular membranes. Kc cells did not synthesize sterols or any apparent replacement lipophilic molecule. However, two major compounds which comigrated with ubiquinone and dolichol were synthesized from radioactive mevalonate and acetate. Cholesterol incorporation into Kc cell membranes did not significantly alter total phospholipid head or acyl group composition. Similar observations were obtained with Schneider's Drosophila cell line I and a mosquito (Aedes albopictus) cell line. Our results (a) challenged current concepts that sterols or related replacement isopentenoid molecules were required for eukaryotic membrane structure, (b) demonstrated that marked alterations in eukaryotic membrane sterol composition was insufficient to change total phospholipid head and/or acyl group composition, and (c) set the stage for the use of a eukaryotic cell to examine the regulation of 3-hydroxy,3-methylglutaryl coenzyme A reductase activity independent of a requirement for sterol synthesis.