ubiquinone and squalestatin-1

2,3-Oxidosqualene is an intermediate in cholesterol biosynthesis and 2,3:22,23-dioxidosqualene act as the substrate for an alternative pathway that produces 24(S),25-epoxycholesterol which effects cholesterol homeostasis. In light of our previous findings concerning the biological effects of certain epoxidated all-trans-polyisoprenes, the effects of squalene carrying epoxy moieties on the second and third isoprene residues were investigated here. In cultures of HepG2 cells both monoepoxides of squalene and one of their hydrolytic products inhibited cholesterol synthesis and stimulated the synthesis of coenzyme Q (CoQ). Upon prolonged treatment the cholesterol content of these cells and its labeling with [(3)H]mevalonate were reduced, while the amount and labeling of CoQ increased. Injection of the squalene monoepoxides into mice once daily for 6days elevated the level of CoQ in their blood, but did not change the cholesterol level. The same effects were observed upon treatment of apoE-deficient mice and diabetic GK-rats. This treatment increased the hepatic level of CoQ10 in mice, but the amount of CoQ9, which is the major form, was unaffected. The presence of the active compounds in the blood was supported by the finding that cholesterol synthesis in the white blood cells was inhibited. Since the ratio of CoQ9/CoQ10 varies depending on the experimental conditions, the cells were titrated with substrate and inhibitors, leading to the conclusion that the intracellular isopentenyl-PP pool is a regulator of this ratio. Our present findings indicate that oxidosqualenes may be useful for stimulating both the synthesis and level of CoQ both in vitro and in vivo.

Topics: Animals; Bridged Bicyclo Compounds, Heterocyclic; Cholesterol; Diabetes Mellitus, Experimental; Etidronic Acid; Hemiterpenes; Hep G2 Cells; Humans; Lovastatin; Male; Mevalonic Acid; Mice; Mice, Inbred C57BL; Mice, Knockout; Organophosphorus Compounds; Rats; Rats, Wistar; Risedronic Acid; Squalene; Tricarboxylic Acids; Ubiquinone

Plant isoprenoids represent a large group of compounds with a wide range of physiological functions. In the cytosol, isoprenoids are synthesized via the classical acetate/mevalonate pathway. In this pathway, farnesyl diphosphate (FPP) occupies a central position, from which isoprene units are dispatched to the different classes of isoprenoids, with sterols as the major end products. The present work deals with effects of squalestatin (SQ) on the metabolism of FPP in proliferating and synchronized cultured tobacco cv. Bright Yellow-2 cells. SQ is a potent inhibitor of squalene synthase (SQS), the first committed enzyme in the sterol pathway. At nanomolar concentrations, SQ severely impaired cell growth and sterol biosynthesis, as attested by the rapid decrease in SQS activity. At the same time, it triggered a several-fold increase in both the enzymic activity and mRNA levels of 3-hydroxy-3-methylglutaryl CoA reductase. When SQ was added to cells synchronized by aphidicolin treatment, it was found to block the cell cycle at the end of G(1) phase, but no cell death was induced. Tobacco cells were also fed exogenous tritiated trans-trans farnesol, the allylic alcohol derived from FPP, in the presence and absence of SQ. Evidence is presented that this compound was incorporated into sterols and ubiquinone Q(10). In the presence of SQ, the sterol pathway was inhibited, but no increase in the radioactivity of ubiquinone was observed, suggesting that this metabolic channel was already saturated under normal conditions.

Topics: Aphidicolin; Bridged Bicyclo Compounds, Heterocyclic; Carbon Radioisotopes; Cell Cycle; Cell Division; Cell Line; Coenzymes; Farnesol; Farnesyl-Diphosphate Farnesyltransferase; G1 Phase; Hydroxymethylglutaryl CoA Reductases; Mitochondria; Nicotiana; Plants, Toxic; Polyisoprenyl Phosphates; Radioisotope Dilution Technique; Sesquiterpenes; Sodium Acetate; Sterols; Transcription, Genetic; Tricarboxylic Acids; Ubiquinone

The effects of squalestatin 1 on rat brain and liver homogenates and on Chinese hamster ovary tissue culture cells have been investigated. This compound effectively inhibits squalene biosynthesis in a highly selective manner. Cytoplasmic farnesyl pyrophosphate and geranylgeranyl pyrophosphate synthases are not affected, which is also the case for microsomal cis-prenyltransferase. In tissue culture cells, squalestatin 1 inhibits cholesterol biosynthesis completely, but does not alter dolichol synthesis or protein isoprenylation to a great extent. Incorporation of [3H]mevalonate into ubiquinone-9 and -10 increases 3-4-fold, probably as a result of increased synthesis of this lipid. Squalestatin 1 appears not only to be an effective inhibitor of cholesterol biosynthesis, but also to be more specific than other inhibitors used earlier in various in vitro and in vivo systems.

Topics: Animals; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; CHO Cells; Cricetinae; Lipids; Male; Mevalonic Acid; Polyisoprenyl Phosphates; Rats; Rats, Sprague-Dawley; Sesquiterpenes; Tricarboxylic Acids; Ubiquinone