Farnesol is utilized for protein isoprenylation and the biosynthesis of cholesterol in mammalian cells

Evidence has been obtained indicating that free farnesol (F-OH) can be utilized for isoprenoid biosynthesis in mammalian cells. When rat C6 glial cells and an African green monkey kidney cell line (CV-1) were incubated with [³H]F-OH, radioactivity was incorporated into cholesterol, ubiquinone (CoQ) and isoprenylated proteins. The incorporation of label from [³H]F-OH into cholesterol in C6 and CV-1 cells was blocked by squalestatin 1 (SQ) which specifically inhibits the conversion of farnesyl pyrophosphate (F-P-P) to squalene. This result strongly suggests that cholesterol, and probably CoQ and protein, is metabolically labeled via F-P-P. SDS-PAGE analysis of the delipidated protein fractions from C6 and CV-1 cells revealed several labeled polypeptides. Consistent with these proteins being modified by isoprenylation of cysteine residues, Pronase E digestion released a major labeled product with the chromatographic mobility of [³H]farnesyl-cysteine (F-Cys). A different set of polypeptides was labeled when C6 and CV-1 cells were incubated with [³H]geranylgeraniol (GG-OH). Both sets of proteins appear to be metabolically labeled by [³H]mevalonolactone, and [³H]-labeled F-Cys and geranylgeranyl-cysteine (GG-Cys) were liberated from these proteins by Pronase E treatment. These cellular and biochemical studies indicate that F-OH can be used for isoprenoid biosynthesis and protein isoprenylation in mammalian cells after being converted to F-P-P by phosphorylation reactions that remain to be elucidated.