The role of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in the regulation of ubiquinone synthesis in human fibroblasts.

Ubiquinone biosynthesis in cultured human skin fibroblasts was studied using 4-hydroxy~-'4C]benoic acid (4-W-14C]HB) and [5-3H]mevalonolactone as markers.The incorporation of 4-w-14C]HB was maximal in cells incubated in the presence of whole human serum but was inhibited to varying degrees when incubated in lipoprotein-deficient serum or in the presence of .Further inhibition was obtained when these substances, as well as low density lipoprotein (LDL), were incubated with cells in the lipoproteindeficient medium.Comparison of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity of cells incubated in human serum and lipoprotein-deficient serum showed that it was increased 17-fold in the latter.Despite this, the incorporation of 4-HB into ubiquinone was inhibited by 356, thereby indicating that some factor in whole serum is involved in the regulation of 4-HB incorporation into ubiquinone.The level of mevalonate, on the other hand, is also important in regulating ubiquinone synthesis.7-Ketocholesterol, 25hydroxycholesterol, and LDL were inhibitory to HMG-CoA reductase activity as expected.Addition of exogenous mevalonate relieved the inhibition by LDL and ML-236B of the incorporation of 4-[U-'4C]HB into ubiquinone, thereby indicating that the effect was due to decreased levels of endogenous mevalonate related to reduced HMG-CoA reductase activity.When [3H]mevalonolactone incorporation into ubiquinone and cholesterol was assayed in the presence and absence of LDL, it was found that LDL stimulated the incorporation of mevalonate into ubiquinone; the increase was &fold at lower concentrations of MVL compared to 1%-fold at higher concentrations.The incorporation into cholesterol was reduced significantly in the presence of LDL, indicating an effect of LDL on the pathway from mevalonate to cholesterol.ML-236B had the same general effect as LDL with respect to incorporation of mevalonate into ubiquinone but had little effect on incorporation into cholesterol when mevalonate concentration was not limiting.Double labeling experiments revealed that LDL and ML-236B inhibited the incorporation 4-v-'4C]HB into ubiquinone while [3H]mevalonate incorporation was enhanced.This apparent increase in [3H]mevalonate incorporation under conditions where HMG-CoA reductase is inhibited is largely due to higher specific radioactivity of the intracellular pool of mevalonate