Modulations in hepatic branch-point enzymes involved in isoprenoid biosynthesis upon dietary and drug treatments of rats

Metabolism and function of coenzyme Q

Coenzyme Q (CoQ) is present in all cells and membranes and in addition to be a member of the mitochondrial respiratory chain it has also several other functions of great importance for the cellular metabolism. This review summarizes the findings available to day concerning CoQ distribution, biosynthesis, regulatory modifications and its participation in cellular metabolism. There are a number of indications that this lipid is not always functioning by its direct presence at the site of action but also using e.g. receptor expression modifications, signal transduction mechanisms and action through its metabolites. The biosynthesis of CoQ is studied in great detail in bacteria and yeast but only to a limited extent in animal tissues and therefore the informations available is restricted. However, it is known that the CoQ is compartmentalized in the cell with multiple sites of biosynthesis, breakdown and regulation which is the basis of functional specialization. Some regulatory mechanisms concerning amount and biosynthesis are established and nuclear transcription factors are partly identified in this process. Using appropriate ligands of nuclear receptors the biosynthetic rate can be increased in experimental system which raises the possibility of drug-induced upregulation of the lipid in deficiency. During aging and pathophysiological conditions the tissue concentration of CoQ is modified which influences cellular functions. In this case the extent of disturbances is dependent on the localization and the modified distribution of the lipid at cellular and membrane levels.

Mitochondrial production of oxygen radical species and the role of Coenzyme Q as an antioxidant

The mitochondrial respiratory chain is a powerful source of reactive oxygen species (ROS), which is considered as the pathogenic agent of many diseases and of aging. We have investigated the role of complex I in superoxide radical production and found by the combined use of specific inhibitors of complex I that the one-electron donor to oxygen in the complex is a redox center located prior to the sites where three different types of Coenzyme Q (CoQ) competitors bind, to be identified with an Fe-S cluster, most probably N2, or possibly an ubisemiquinone intermediate insensitive to all the above inhibitors. Short-chain Coenzyme Q analogs enhance superoxide formation, presumably by mediating electron transfer from N2 to oxygen. The clinically used CoQ analog, idebenone, is particularly effective, raising doubts on its safety as a drug. Cells counteract oxidative stress by antioxidants. CoQ is the only lipophilic antioxidant to be biosynthesized. Exogenous CoQ, however, protects cells from oxidative stress by conversion into its reduced antioxidant form by cellular reductases. The plasma membrane oxidoreductase and DT-diaphorase are two such systems, likewise, they are overexpressed under oxidative stress conditions.

Cholesterol and bile acids regulate xenosensor signaling in drug-mediated induction of cytochromes P450

Cytochromes P450 (CYP) constitute the major enzymatic system for metabolism of xenobiotics. Here we demonstrate that transcriptional activation of CYPs by the drug-sensing nuclear receptors pregnane X receptor, constitutive androstane receptor, and the chicken xenobiotic receptor (CXR) can be modulated by endogenous cholesterol and bile acids. Bile acids induce the chicken drug-activated CYP2H1 via CXR, whereas the hydroxylated metabolites of bile acids and oxysterols inhibit drug induction. The cholesterol-sensing liver X receptor competes with CXR, pregnane X receptor, or constitutive androstane receptor for regulation of drug-responsive enhancers from chicken CYP2H1, human CYP3A4, or human CYP2B6, respectively. Thus, not only cholesterol 7 alpha-hydroxylase (CYP7A1), but also drug-inducible CYPs, are diametrically affected by these receptors. Our findings reveal new insights into the increasingly complex network of nuclear receptors regulating lipid homeostasis and drug metabolism.

Role of mitochondria in oxidative stress and aging

The mitochondrial respiratory chain is a powerful source of reactive oxygen species (ROS), considered as the pathogenic agent of many diseases and of aging. We have investigated the role of Complex I in superoxide radical production and found by combined use of specific inhibitors of Complex I that the one-electron donor in the Complex to oxygen is a redox center located prior to the sites where three different types of coenzyme Q (CoQ) competitors bind, to be identified with an Fe-S cluster, most probably N2, or possibly an ubisemiquinone intermediate insensitive to all the above inhibitors. Short-chain coenzyme Q analogues enhance superoxide formation, presumably by mediating electron transfer from N2 to oxygen. The clinically used CoQ analogue idebenone is particularly effective, raising doubts about its safety as a drug. The mitochondrial theory of aging considers somatic mutations of mitochondrial DNA induced by ROS as the primary cause of energy decline; in rat liver mitochondria, Complex I appears to be most affected by aging and to become strongly rate limiting for electron transfer. Mitochondrial energetics is also deranged in human platelets upon aging, as demonstrated by the decreased Pasteur effect (enhancement of lactate production by respiratory inhibitors). Cells counteract oxidative stress by antioxidants: CoQ is the only lipophilic antioxidant to be biosynthesized. Exogenous CoQ, however, protects cells from oxidative stress by conversion into its reduced antioxidant form by cellular reductases. The plasma membrane oxidoreductase and DT-diaphorase are two such systems: likewise, they are overexpressed under oxidative stress conditions.

A novel family of longer chain length dolichols present in oleate-induced yeast Saccharomyces cerevisiae

The typical size of the yeast dolichol family ranges from 14 to 19 isoprene units D((14-19)) with dolichol(16) being the dominating species. Induction of peroxisome proliferation by growing the cells in medium containing oleate as carbon source induces the synthesis of an additional family of longer dolichols D((19-24)) with D(21) being the most prominent. This phenomenon is abolished in the peroxisome biogenesis deficient strain in which the PEX1 gene (encoding Pex1p peroxin) has been disrupted. The total amount of dolichols in pex1Delta cells is lower than in the wild-type cells, as is the amount of phosphatidylcholine. Moreover, the levels of 3-hydroxy-3-methylglutaryl CoA reductase and farnesyl diphosphate synthase, two key enzymes in dolichol biosynthesis, are decreased in the absence of a functional PEX1 gene. The presence of longer dolichols in oleate-induced Saccharomyces cerevisiae cells, the absence of this additional family in peroxisome deficient cells, and a decrease of the total amount of dolichols in these cells indicate the involvement of peroxisomes in the biosynthesis of dolichols in this organism.

Synthesis of mevalonate pathway lipids in fibroblasts from Zellweger and X-linked ALD patients

Fibroblasts were cultured to determine the involvement of peroxisomes in cholesterol and dolichol synthesis. For this purpose, the behavior of cells from patients with Zellweger syndrome, with X-linked adrenoleukodystrophy, and from nondiseased control subjects was studied. Cells both after pretreatment with mevinolin and without pretreatment were incubated in a medium containing [3H]-mevalonate. In fibroblasts from patients with peroxisomal defects, the cholesterol content and mevalonate incorporation into cholesterol were decreased by 10-20% in comparison with control cells. Mevinolin pretreatment decreased the incorporation rate of [3H]-mevalonate into cholesterol but increased the labeling of ubiquinone and dolichol both in diseased and control cells. Squalene synthase activity was unchanged, whereas the activity of farnesyl-pyrophosphate synthase was increased in the diseased states. The results show that in patients with peroxisomal deficiency neither the amount nor the rate of synthesis of cholesterol and dolichol is reduced to any greater extent.

Alterations in the biosynthesis of cholesterol, dolichol and dolichyl-P in the genetic cholesterol homeostasis disorder, Niemann-Pick type C disease

The biosynthesis of cholesterol, dolichol and dolichyl-P were investigated in a murine model of Niemann-Pick type C disease using both in vitro and in vivo systems. In vivo incorporation of [3H]mevalonate into squalene, dolichol and dolichyl-P decreased. The amount of dolichyl-P was elevated due to a decrease in the rate of degradation. Labeling of squalene and cholesterol of liver homogenates in vitro was decreased in the diseased mice and a lowering of microsomal activities of both HMG-CoA reductase and squalene synthase were also observed. In experiments with brain homogenate, decreased [3H]mevalonate labeling of squalene, cholesterol and dolichol was found in vitro. The decreases in cis-prenyltransferase and squalene synthase activities were observed at a very early phase of the disease. In contrast to the decreased biosynthesis of cholesterol observed in vitro, the labeling of total liver cholesterol was found to be increased in Niemann-Pick type C liver upon in vivo investigation, possibly due to the accumulation of this lipid as a result of a deficient transport process. In the brain, where in vivo labeling reflects only biosynthesis, a decreased rate of cholesterol synthesis was demonstrated.

Gemfibrozil-induced decrease in serum ubiquinone and alpha- and gamma-tocopherol levels in men with combined hyperlipidaemia

BACKGROUND

Low blood levels of antioxidants are associated with an increased risk of developing coronary artery disease. Lipophilic antioxidants are transported in lipoproteins, and hypolipidaemic therapy may therefore alter their blood concentrations.

METHODS

The present randomized, placebo-controlled cross-over study of 21 men with combined hyperlipidaemia examines whether 10-12 weeks of gemfibrozil treatment affects the serum concentrations of the antioxidants ubiquinone-10 or alpha- or gamma-tocopherol.

RESULTS

Gemfibrozil treatment lowered plasma triglycerides and both total and very low-density lipoprotein (VLDL)-cholesterol (P < 0.001 for all by ANOVA), whereas high-density lipoprotein (HDL)-cholesterol increased (P < 0.001). The median serum levels of ubiquinone-10 decreased from 1.30 mumol L-1 (interquartile range 0.87-1.71 mumol L-1) with placebo to 0.76 mumol L-1 (0.66-0.95) with gemfibrozil treatment (P < 0.001). Corresponding levels for alpha- and gamma-tocopherol were: 68.5 mumol L-1 (51.1-84.7) vs. 40.8 mumol L-1 (30.3-55.0) and 8.6 mumol L-1 (5.2-16.7) vs. 4.3 mumol L-1 (3.5-7.0) respectively (P < 0.001 for both). The decrease in serum antioxidants was also evident when standardized for total cholesterol (P < 0.05) or LDL-cholesterol (P < 0.001). Normolipaemic control subjects had significantly lower antioxidant levels than placebo-treated patients: ubiquinone 0.63 mumol L-1 (0.41-1.05), alpha-tocopherol 34.3 mumol L-1 (27.3-45.6) and gamma-tocopherol 3.2 mumol L-1 (2.5-4.2) (P < 0.001 for all). The association of antioxidants with lipoprotein lipids was further established by positive correlations between the levels of antioxidants and those of total cholesterol (r = 0.64, P < 0.001) or total triglycerides (r = 0.71, P < 0.001).

CONCLUSION

Gemfibrozil treatment of men with combined hyperlipidaemia reduces serum antioxidant levels to the levels seen in healthy normolipidaemic men. The mechanisms and the relevance of this finding remain unclear and need to be addressed in further studies.

Biochemistry of peroxisomes in health and disease

The ubiquitous distribution of peroxisomes and the identification of a number of inherited diseases associated with peroxisomal dysfunction indicate that peroxisomes play an essential part in cellular metabolism. Some of the most important metabolic functions of peroxisomes include the synthesis of plasmalogens, bile acids, cholesterol and dolichol, and the oxidation of fatty acids (very long chain fatty acids > C22, branched chain fatty acids (e.g. phytanic acid), dicarboxylic acids, unsaturated fatty acids, prostaglandins, pipecolic acid and glutaric acid). Peroxisomes are also responsible for the metabolism of purines, polyamines, amino acids, glyoxylate and reactive oxygen species (e.g. O-2 and H2O2). Peroxisomal diseases result from the dysfunction of one or more peroxisomal metabolic functions, the majority of which manifest as neurological abnormalities. The quantitation of peroxisomal metabolic functions (e.g. levels of specific metabolites and/or enzyme activity) has become the basis of clinical diagnosis of diseases associated with the organelle. The study of peroxisomal diseases has also contributed towards the further elucidation of a number of metabolic functions of peroxisomes.

Increases in tissue levels of ubiquinone in association with peroxisome proliferation

Rats were treated with various peroxisome proliferators and concomitant changes in ubiquinone levels were monitored. In addition to clofibrate and di(2-ethylhexyl)phthalate, acetylsalicylic acid, 2-ethylhexanoic acid, thyroxine and dehydroepiandrosterone were used as proliferators. Administration of these compounds increased the contents of ubiquinone in liver and, to some extent, in kidney and muscle. No change in corresponding valued for heart or brain were observed. The treatments did not influence cholesterol levels, but increased the amounts of dolichol in the liver to various extents. Treatment of rats with the catalase inhibitor aminotriazole increased the ubiquinone levels in kidney, heart and muscle but not in liver. Comparison of peroxisomal fatty acid beta-oxidation with ubiquinone amounts in liver homogenates after treatment with a number of peroxisome proliferators demonstrated a direct correlation between these two parameters. Subcellular fractionation of liver after peroxisome proliferation revealed that the ubiquinone level was increased in mitochondria and lysosomes which are the main compartments for this lipid, but an increase was also observed in both peroxisomes and microsomes. The increase in hepatic ubiquinone after treatment with various types of proliferators was related to the decrease in blood cholesterol level. These results show that the volume of the peroxisomal compartment and the ubiquinone content in animal tissues are interrelated.

Age-dependent modifications in the metabolism of mevalonate pathway lipids in rat brain

The levels and rates of biosynthesis of mevalonate pathway lipids in rat brain were investigated during development and aging. Between birth and 18 months of age there are only moderate decreases in the phospholipid and cholesterol contents but an increase in the levels of dolichyl-P and, particularly of dolichol. The amount of ubiquinone is unchanged. The rate of incorporation of [3H]leucine into protein decreases by 10% during the first year, while the incorporation of [3H]glycerol into phospholipids decreases by 20%. The high rates of [3H]mevalonate incorporation into cholesterol and dolichol after birth decreases rapidly. In contrast, the rate of incorporation into ubiquinone is constant. Squalene synthase activity decreases rapidly in the early postnatal period and at 18 months of age this activity is 10-fold lower than immediately after birth. cis-Prenyltransferase activity is also high during the first postnatal month and reaches a constant level at 4 months of age. Significantly, nonaprenyl 4-hydroxybenzoate transferase activity is high during the entire period investigated. The rate of lipid peroxidation does not change during aging. These results demonstrate that brain cholesterol and dolichol exhibit a low rate of turnover during aging, whereas ubiquinone is synthesized at a high rate and exhibits rapid turnover throughout the entire lifespan.

Regulation of coenzyme Q biosynthesis

The side-chain moiety of coenzyme Q is synthesized by a trans-prenyltransferase present in microsomes. Condensation of this moiety with the precursor ring takes place in the Golgi system. The enzymes involved, as well as the cytosolic geranylgeranyl-PP synthase, are regulated in an independent fashion. When the size of the farnesyl-PP pool is decreased or increased by employing appropriate inhibitors, the rate of CoQ synthesis is modified accordingly, indicating the dependence of trans-prenyltransferase activity on the level of intracellular substrate concentrations. Administration of peroxisome proliferators elevates CoQ concentrations not only in blood, but also in various tissues. Thus, it may be possible in the future to selectively increase CoQ concentrations in certain organs, without increasing the level of cholesterol.