Uncoupling farnesol-induced apoptosis from its inhibition of phosphatidylcholine synthesis

Genetic inactivation of the synthesis of phosphatidylcholine, the most abun dant membrane lipid in eukaryotic cells, induces apoptosis, Administration of farnesol, a catabolite within the isoprenoid/cholesterol pathway, also i nduces apoptosis, The mechanism by which farnesol induces apoptosis is curr ently believed to be by direct competitive inhibition with diacylglycerol f or cholinephosphotransferase, the final step in the phosphatidylcholine bio synthetic pathway. Our recent isolation of the first mammalian cholinephosp hotransferase cDNA has enabled us to more precisely assess how farnesol aff ects phosphatidylcholine synthesis and the induction of apoptosis, Induced over-expression of cholinephosphotransferase in Chinese hamster ovary cells prevented the block in phosphatidylcholine biosynthesis associated with ex posure to farnesol, However, induced over-expression of cholinephosphotrans ferase was not sufficient for the prevention of farnesol-induced apoptosis, In addition, exogenous administration of diacylglycerol prevented farnesol -induced apoptosis but did not relieve the farnesol-induced block in phosph atidylcholine synthesis. We also developed an in vitro lipid mixed micelle cholinephosphotransferase enzyme assay, as opposed to the delivery of the d iacylglycerol substrate in a detergent emulsion, and demonstrated that ther e was no direct inhibition of cholinephosphotransferase by farnesol or its phosphorylated metabolites. The execution of apoptosis by farnesol appears to be a separate and distinct event from farnesol-induced inhibition of pho sphatidylcholine biosynthesis and instead likely occurs through a diacylgly cerol-mediated process that is downstream of phosphatidylcholine synthesis.