Farnesol Is Not the Nonsterol Regulator Mediating Degradation of HMG-CoA Reductase in Rat Liver

A recent report, in which cultured tumor cells were used, identified farnesol as the nonsterol mevalonate-derived metabolite required for the accelerated degradation of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (C. C. Correll, L. Ng, and P. A. Edwards, 1994,J. Biol. Chem.269, 17390–17393). We examined this proposed linkage in animals by measuring hepatic farnesol levels and rates of HMG-CoA reductase degradation under conditions previously shown to alter the stability of the reductase. In normal rats, the hepatic farnesol level, quantified by high-pressure liquid chromatography, was 0.10 ± 0.08 μg/g and the half-life of HMG-CoA reductase was 2.5 h. Administration of mevalonolactone at 1 g/kg body wt to provide all nonsterol metabolites in addition to cholesterol increased farnesol levels 6-fold without significantly affecting the half-life of the reductase. Treatment of rats with zaragozic acid A, an inhibitor of squalene synthase, raised hepatic farnesol levels 10-fold and decreased the half-life of HMG-CoA reductase to 0.25 h. However, feeding lovastatin to rats did not lower hepatic farnesol levels despite a marked stabilization of HMG-CoA reductase protein. Moreover, intubation of rats with 500 mg/kg body wt of farnesol failed to decrease the half-life of HMG-CoA reductase protein, alter the levels of enzyme activity, or change of the levels of immunoreactive protein despite an increase of 1000-fold in hepatic farnesol levels. These observations indicate that farnesol per se does not induce accelerated degradation of HMG-CoA reductase in rat liver.