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Distribution of branch point prenyltransferases in regions of bovine brain.

Bovine brains contain large amounts of isoprenoid compounds and the enzymes involved in their biosynthesis were investigated. Ten different regions were dissected from fresh bovine brains and, in addition, fractions from cerebellum, spinal cord, and hypophysis were obtained. The cholesterol concentration was found to be approximately 8 mg/g in the cortex regions and three times higher in the pons, medulla oblongata, and white matter. Dolichol concentration varied between 8 and 40 micrograms/g in the different tissues, and ubiquinone was found at a lower level, which varied between 3 and 25 micrograms/g. Farnesylpyrophosphate synthase activity in cytosolic fractions from various regions exhibited only a twofold variation, whereas geranylgeranyl pyrophosphate synthase displayed larger differences, being particularly rich in the pons, medulla oblongata, white matter, and spinal cord. Squalene synthase activity was lowest in the thalamus and threefold higher in the pons. Determination of specific activity based on cholesterol content revealed that enzyme activities in various regions are not related to the actual lipid amount present. Both cis- and trans-prenyltransferases exhibited similarities in their regional distribution showing up to 20-fold differences in activity. Thus, it appears that the mevalonate pathway lipids and the various branch point enzymes involved in their syntheses vary greatly in different brain regions and are subjected to separate regulation.

Animals↗

Peroxisome proliferators induce apoptosis in hepatoma cells.

In the AH-130 hepatoma, a poorly differentiated tumor, maintained by weekly transplantations in rats, a low percentage of cells spontaneously underwent apoptosis, mainly during the transition from logarithmic- to stationary-growth phase. It was possible to induce massive apoptosis of cells by treating them with clofibrate, a peroxisome proliferator and hypolipidemic drug. Similar results were obtained with HepG2 cells. With 1 mM clofibrate, apoptosis began to manifest itself after 1 h of treatment in vitro, and was assessed by morphological analysis, by DNA fragmentation carried out with agarose gel electrophoresis, and with flow cytometric determination of terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling. The mechanisms whereby clofibrate induces apoptosis are still unclear. Since the peroxisome proliferator-activated receptor was expressed at a very low level and was not stimulated by clofibrate in the AH-130 hepatoma cells, its involvement seems unlikely. Moreover, lipid peroxidation was not increased after clofibrate treatment. Phospholipids and cholesterol were significantly decreased. The decreased cholesterol content might suggest an inhibition of the mevalonate pathway and, therefore, of isoprenylation of proteins involved in cell proliferation.

Animals↗

Lipid oxidative stress and the anti-inflammatory properties of statins and ACE inhibitors.

Observational studies in different populations indicate a positive relationship between dyslipidemia, cardiovascular events, and kidney disease progression. Patients with chronic renal diseases usually present with conditions (inflammation, oxidative stress) that can induce quantitative and qualitative changes in lipoprotein composition, making these particles more atherogenic. Several large randomized trials have shown that lowering cholesterol with statins reduces coronary mortality and morbidity across a wide range of cholesterol levels. Moreover, statins ameliorated renal function and structure in experimental models of progressive renal disease as well as in patients with proteinuric disease. It has been hypothesized that statins may directly modulate intracellular signaling systems involved in cellular proliferation, inflammatory, and fibrogenic responses. In addition, studies in animals have shown that the mevalonate pathway also may be involved in the regulation of the renin-angiotensin system, providing the rationale for the use of statins in combination with angiotensin-converting enzyme inhibitors and/or angiotensin II receptor antagonists, 2 classes of drugs that have evidenced beneficial effects in different forms of renal diseases. Preliminary studies have shown that a multidrug approach may induce remission of proteinuria, lessen renal injury, and protect from loss of function in those patients whose condition does not fully respond to a single treatment.

Angiotensin-Converting Enzyme Inhibitors↗

The molecular mechanism of nitrogen-containing bisphosphonates as antiosteoporosis drugs.

Osteoporosis and low bone mass are currently estimated to be a major public health risk affecting >50% of the female population over the age of 50. Because of their bone-selective pharmacokinetics, nitrogen-containing bisphosphonates (N-BPs), currently used as clinical inhibitors of bone-resorption diseases, target osteoclast farnesyl pyrophosphate synthase (FPPS) and inhibit protein prenylation. FPPS, a key branchpoint of the mevalonate pathway, catalyzes the successive condensation of isopentenyl pyrophosphate with dimethylallyl pyrophosphate and geranyl pyrophosphate. To understand the molecular events involved in inhibition of FPPS by N-BPs, we used protein crystallography, enzyme kinetics, and isothermal titration calorimetry. We report here high-resolution x-ray structures of the human enzyme in complexes with risedronate and zoledronate, two of the leading N-BPs in clinical use. These agents bind to the dimethylallyl/geranyl pyrophosphate ligand pocket and induce a conformational change. The interactions of the N-BP cyclic nitrogen with Thr-201 and Lys-200 suggest that these inhibitors achieve potency by positioning their nitrogen in the proposed carbocation-binding site. Kinetic analyses reveal that inhibition is competitive with geranyl pyrophosphate and is of a slow, tight binding character, indicating that isomerization of an initial enzyme-inhibitor complex occurs with inhibitor binding. Isothermal titration calorimetry indicates that binding of N-BPs to the apoenzyme is entropy-driven, presumably through desolvation entropy effects. These experiments reveal the molecular binding characteristics of an important pharmacological target and provide a route for further optimization of these important drugs.

Animals↗

Studies on the biosynthesis of taxol: the taxane carbon skeleton is not of mevalonoid origin.

A cell culture of Taxus chinensis was established to produce the diterpene 2alpha,5alpha,10beta,14beta-tetra-acetoxy4 ++ +(20),11-taxadiene (taxuyunnanine C) in 2.6% (dry weight) yield. The incorporation of [U-13C6]glucose, [1-13C]glucose, and [1,2-13C2]acetate into this diterpene was analyzed by NMR spectroscopy. Label from [1,2-13C2]acetate was diverted to the four acetyl groups of taxuyunnanine C, but not to the taxane ring system. Label from [1-13C]glucose and [U-13C6]glucose was efficiently incorporated into both the taxane ring system and the acetyl groups. The four isoprenoid moieties of the diterpene showed identical labeling patterns. The analysis of long-range 13C13C couplings in taxuyunnanine C obtained from an experiment with [U-13C6]glucose documents the involvement of an intramolecular rearrangement in the biosynthesis of the isoprenoid precursor. The labeling patterns are inconsistent with the mevalonate pathway. The taxoid data share important features with the alternative pathway of isoprenoid biosynthesis operating in certain eubacteria Rohmer, M., Knani, M., Simonin, P., Sutter, B. & Sahm, H. (1993) Biochem. J. 295, 517-524].

Cells, Cultured↗

3-Formyl-1-butyl pyrophosphate A novel mycobacterial metabolite-activating human gammadelta T cells.

Most human blood gammadelta T cells react without major histocompatibility complex restriction to small phosphorylated nonpeptide antigens (phosphoantigens) that are abundantly produced by mycobacteria and several other microbial pathogens. Although isopentenyl pyrophosphate has been identified as a mycobacterial antigen for gammadelta T cells, the structure of several other stimulating compounds with bioactivities around 1000-fold higher than isopentenyl pyrophosphate remains to be elucidated. This paper describes the structural identification of 3-formyl-1-butyl-pyrophosphate as the core of several non-prenyl mycobacterial phosphoantigens bioactive at the nM range. Recognition of this molecule by gammadelta T cells is very selective and relies on its aldehyde and pyrophosphate groups. This novel pyrophosphorylated aldehyde most probably corresponds to a metabolic intermediate of the non-mevalonate pathway of prenyl phosphate biosynthesis in eubacteria and algae. The reactivity to 3-formyl-1-butyl-pyrophosphate supports the view that human gammadelta T cells are physiologically devoted to antimicrobial surveillance.

Antigens, Bacterial↗

Biosynthesis of a neo-epi-verrucosane diterpene in the liverwort Fossombronia alaskana. A retrobiosynthetic NMR study.

The biosynthesis of the diterpene 8alpha-acetoxy-13alpha-hydroxy-5-oxo-13-epi- neoverrucosane in the arctic liverwort Fossombronia alaskana was studied by incorporation experiments using [1-(13)C]- and [U-(13)C(6)]glucose as precursors. The (13)C-labeling patterns of acetyl-CoA, pyruvate, and phosphoenolpyruvate in intermediary metabolism were reconstructed from the (13)C NMR data of biosynthetic amino acids (leucine, alanine, phenylalanine) and were used to predict hypothetical labeling patterns for isopentenyl pyrophosphate formed via the mevalonate pathway and the deoxyxylulose pathway. The labeling patterns observed for the neoverrucosane diterpene were consistent with the intermediate formation of geranyllinaloyl pyrophosphate assembled from dimethylallyl pyrophosphate and three molecules of isopentenyl pyrophosphate generated predominantly or entirely via 1-deoxyxylulose 5-phosphate. The experimental data can be integrated into a detailed biosynthetic scheme involving a 1,5-hydride shift. The postulated involvement of the 1,5-hydride shift was confirmed by an incorporation experiment with [6,6-(2)H(2)]glucose.

Diterpenes↗

The kinetic mechanism of phosphomevalonate kinase.

Phosphomevalonate kinase catalyzes an essential step in the so-called mevalonate pathway, which appears to be the sole pathway for the biosynthesis of sterols and other isoprenoids in mammals and archea. Despite the well documented importance of this pathway in the cause and prevention of human disease and that it is the biosynthetic root of an enormous diverse class of metabolites, the mechanism of phosphomevalonate kinase from any organism is not yet well characterized. The first structure of a phosphomevalonate kinase from Streptococcus pneumoniae was solved recently. The enzyme exhibits an atypical P-loop that is a conserved defining feature of the GHMP kinase superfamily. In this study, the kinetic mechanism of the S. pneumoniae enzyme is characterized in the forward and reverse directions using a combination of classical initial-rate methods including alternate substrate inhibition using ADPbetaS. The inhibition patterns strongly support that in either direction the substrates bind randomly to the enzyme prior to chemistry, a random sequential bi-bi mechanism. The kinetic constants are as follows: k(cat(forward)) = 3.4 s(-1), K(i(ATP)) = 137 microm, K(m(ATP)) = 74 microm, K(i(pmev)) = 7.7 microm, K(m(pmev)) = 4.2 microm; k(cat(reverse)) = 3.9 s(-1), K(i(ADP)) = 410 microm, K(m(ADP)) = 350 microm, K(i(ppmev)) = 14 microm, K(m(ppmev)) = 12 microm, where pmev and ppmev represent phosphomevalonate and diphosphomevalonate, respectively.

Bacterial Proteins↗

Staphylococcus aureus 3-hydroxy-3-methylglutaryl-CoA synthase: crystal structure and mechanism.

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase, a member of the family of acyl-condensing enzymes, catalyzes the first committed step in the mevalonate pathway and is a potential target for novel antibiotics and cholesterol-lowering agents. The Staphylococcus aureus mvaS gene product (43.2 kDa) was overexpressed in Escherichia coli, purified to homogeneity, and shown biochemically to be an HMG-CoA synthase. The crystal structure of the full-length enzyme was determined at 2.0-A resolution, representing the first structure of an HMG-CoA synthase from any organism. HMG-CoA synthase forms a homodimer. The monomer, however, contains an important core structure of two similar alpha/beta motifs, a fold that is completely conserved among acyl-condensing enzymes. This common fold provides a scaffold for a catalytic triad made up of Cys, His, and Asn required by these enzymes. In addition, a crystal structure of HMG-CoA synthase with acetoacetyl-CoA was determined at 2.5-A resolution. Together, these structures provide the structural basis for an understanding of the mechanism of HMG-CoA synthase.

Amino Acid Motifs↗

Statin use and cancer risk: an epidemiologic review.

While the beneficial effects of hydroxy-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) on cardiovascular disease are well established, much uncertainty remains about their effects on cancer. The statins inhibit the rate-limiting step in the mevalonate pathway, leading to reduced levels of cholesterol and other molecules of importance for critical cellular processes. A growing body of preclinical data indicates that statins may have antineoplastic properties, but some studies raise the possibility that statins may possess a carcinogenic potential. Clinical and observational studies of the association between statin use and cancer have been inconclusive with regard to any chemopreventive or therapeutic effect, but they do provide reassuring evidence that statins do not appear to be carcinogenic. The reasons for the varying results are unclear but they may relate to methodological issues. Additional studies, including Phase II randomized trials and epidemiological studies with accurate measures of statin use and comprehensive control for confounding factors, are needed to determine the potentially beneficially effects of statins on cancer development and progression.

Adult↗

Differences in radiation-induced micronuclei yields of human cells: influence of ras gene expression and protein localization.

Expression of ras has been correlated with increased intrinsic resistance to ionizing radiation. In this study we show that increased EJras expression in human cells is associated with a decrease in the frequency of radiation-induced micronuclei. The experimental system consisted of human osteosarcoma-derived cell lines which quantitatively vary in their EJras expression. There was a dose-dependent relationship between radiation dose and micronuclei formation in all cell lines tested. Human osteosarcoma cells, in which the ras level was undetectable, had the highest frequency of micronuclei production at all radiation doses tested. At 4 Gy the most radioresistant cells exhibited a 41.5 +/- 5% decrease in the production of micronuclei concomitant with high ras expression in comparison with the relatively radiosensitive parental cell line. Cells expressing a low amount of EJras demonstrated a 23 +/- 3% decrease in micronuclei induction compared with parental cells. Treatment of cells with lovastatin, an inhibitor of ras-encoded p21ras post-translational processing via the mevalonate pathway, markedly decreased the yield of micronuclei formation in cells transfected with ras; the drug had no effect on radiation-induced micronuclei formation in parental cells. The use of the in vitro micronuclei assay has provided a convenient way to visualize differences in the genotoxic damage induced by ionizing radiation in cells which express different amount of EJras. The results indicate that elevation of ras expression in human cells can lead to a decrease in the number of radiation-induced micronuclei formed and that this relationship is dependent on membrane association of ras-encoded p21.

Cell Line↗

Anti-lymphoma effect of gammadelta T cells.

During the last few years, our knowledge about the activation and control of non-major histocompatibility complex (MHC)-restricted innate effector lymphocytes (such as natural killer (NK) cells, NK T cells and gammadelta T cells) has advanced enormously and immunotherapeutic strategies based on these cell types receive more and more attention. Apart from NK cells, several lines of evidence indicate that T cells, which express an alternative T cell receptor (TCR) composed of a CD3-associated gammadelta heterodimer, also contribute to the innate immune defense against tumors. Human gammadelta T cells represent a small subset of T cells (1-10% of peripheral blood T cells) and differ from conventional MHC-restricted ass T cells in recognition of a unique set of antigens ("phosphoantigens") and the lack of requirement of classical antigen-presenting molecules. Besides their role in the innate immune response against pathogens based on the recognition of distinctive microbial metabolic products (metabolites of the non-mevalonate pathway of isoprenoid synthesis), Vgamma9Vdelta2 T cells that constitute the dominant fraction of gammadelta T cells in humans exert potent cytotoxic activity, especially against lymphoid malignancies, mediated by as yet only partially determined pathway(s) of tumor recognition. This article will review available evidence from pre-clinical and early clinical studies regarding the contribution of gammadelta T cells in the defense against lymphoid malignancies and highlights some important issues that need to be addressed in the future.

Animals↗

In vivo requirement of protein prenylation for maintenance of retinal cytoarchitecture and photoreceptor structure.

Recent studies have demonstrated that inhibition of mevalonate synthesis in cultured cells leads to altered cell morphology due to inhibition of protein prenylation. To investigate the effects in vivo of mevalonate deprivation in nondividing, terminally differentiated neural cells, we have analyzed the effects on retinal tissue of intravitreal injection of lovastatin, a potent inhibitor of the mevalonate-producing enzyme, HMG-CoA reductase. A single injection of lovastatin (0.25 mumol) produced profound dysplastic-like changes in adult rat retinas primarily involving the photoreceptor layer. Within 2 d after injection, photoreceptor nuclei migrated in a circular pattern resulting in the formation of rosette-like structures by 4 d. Also during this period, photoreceptor inner and outer segment degeneration was evident. By 21 d, intact photoreceptor nuclei with remnants of inner and outer segments were dispersed throughout all retinal layers. To investigate the biochemical specificity of the lovastatin-induced alterations, and to distinguish the relative importance of the various branches of the mevalonate pathway, the incorporation of [3H]acetate into retinal lipids was examined in the presence and absence of metabolic inhibitors. HPLC analysis of lovastatin-treated retinas revealed a dramatic reduction in the incorporation of intravitreally injected [3H]acetate into nonsaponifiable lipids, compared with controls. In contrast, intravitreal injection of NB-598, a specific inhibitor of squalene epoxidase, eliminated the conversion of newly synthesized squalene to sterols without obvious pathology. Hence, involvement to the sterol branch of isoprenoid metabolism in the lovastatin-induced morphologic disruption was obviated. Intravitreal injection of 0.27 mumol of N-acetyl-S-trans,trans-farnesyl-L-cysteine (AFC), an inhibitor of carboxyl methyltransferase activity and prenylated protein function, produced morphologic changes that were virtually indistinguishable from those induced with lovastatin. These results implicate a defect in protein prenylation in the lovastatin-induced retinal degeneration, and suggest the presence of a dynamic pathway in the retina that requires isoprenylated proteins to maintain retinal cytoarchitecture.

Acetates↗

Fractures, calcium, and the modern diet.

Although high calcium intakes have long been recommended to prevent osteoporosis, there is little evidence that high calcium intakes effectively prevent fractures. Osteoporotic fractures are, like coronary artery disease, largely a disease of Western societies. Recent evidence that the statins that block the mevalonate pathway, lower serum cholesterol concentrations, and improve cardiovascular disease risk also prevent fractures, together with the increasing evidence that diets high in fruit and vegetables are beneficial in preventing fractures, suggest common dietary etiologic factors. Further research in this area should answer the long-standing question: Why do populations who consume low-calcium diets have fewer fractures than do Western societies who consume high-calcium diets?

Adaptation, Physiological↗

Mice with the mono-allelic p.R37H Dhdds variant show aberrant glycosylation and interneuron deficits.

Developmental delay and seizures with or without movement abnormalities (OMIM 617836) caused by heterozygous pathogenic variants in the DHDDS gene (DHDDS-CDG) is a rare genetic disease that belongs to the progressive encephalopathy spectrum. It results in cognitive delay in affected children, accompanied by myoclonus, seizures, ataxia and tremor, which worsens over time. DHDDS encodes a subunit of a DHDDS/NUS1 cis-prenyltransferase (cis-PTase), a branch point enzyme of the mevalonate pathway essential for N-linked glycosylation. We describe the first mouse model of this disease, DhddsR37H+/- strain, heterozygous for the human recurrent de novo c.110G>A:p.R37H pathogenic variant. DhddsR37H+/- mice present with seizures, myoclonus and memory deficits associated with reduced density or/and maturity of inhibitory interneurons in the cortex. Multiomics analyses of mouse CNS tissues, together with the enzymatic/structural characterization of the R37H DHDDS mutant protein, reveal that the variant produces a catalytically inactive enzyme and results in a brain dolichol deficit, aberrant glycosylation of brain glycoproteins, including those involved in synaptic transmission and major perturbations in the CNS proteome and lipidome. Acetazolamide, a carbonic anhydrase inhibitor clinically approved for treatment of glaucoma, epilepsy, and intracranial hypertension, and successfully used "off-label" to treat genetic movement disorders, reduces seizure susceptibility to pentylenetetrazol in DhddsR37H+/- mice, suggesting potential therapeutic value of using this drug in human DHDDS-CDG patients. Together, our results define cis-PTase as a master regulator of CNS development and function and establish that its monoallelic debilitating variants cause a novel congenital disorder of glycosylation associated with aberrant levels of neuronal proteins and lipids.

DHDDS↗

Inhibition of azoxymethane-induced neoplasia of the large bowel by 3-hydroxy-3,7,11-trimethyl-1,6,10-dodecatriene (nerolidol).

The inhibitory capacities of four terpenes on azoxymethane (AOM)-induced neoplasia of the large bowel and duodenum was studied in male F344 rats. A complete course of AOM administrations was given and 3 days later the rats were fed a semipurified diet containing 5 mg/g of the test compounds, i.e. 3-hydroxy-3,7,11-trimethyl-1,6,10-dodecatriene (nerolidol), beta-citronellol, (+/-)-linalool and (1R,2S,5R)-(-)-menthol or a corresponding control diet. The experiment was terminated 22 weeks after the last dose of AOM. Under these conditions, nerolidol showed an inhibitory effect on carcinogenesis of the large bowel. The number of rats bearing large bowel neoplasms (adenomas) was reduced from 82% in the controls to 33% in rats fed nerolidol and the number of tumors/rat from 1.5 in the controls to 0.7 in the nerolidol group. A reduction in adenocarcinomas of the duodenum was found but the data are not statistically significant. The effects of nerolidol are of interest in terms of the identification of a new inhibitor of carcinogenesis of the large bowel. The chemical structure of nerolidol suggests the possibility that the compound might have an impact on protein prenylation or some other aspect of the mevalonate pathway, but this remains to be established.

Animals↗

Inhibition of protein farnesyltransferase: a possible mechanism of tumor prevention by dehydroepiandrosterone sulfate.

Dehydroepiandrosterone sulfate (DHEAS) is the most abundant adrenal steroid with apparent anticarcinogenic properties. Given our recent observation of the dehydro-epiandrosterone-mediated inhibition of protein isoprenylation and the fact that 99% of the circulating dehydro-epiandrosterone is sulfated, with less than 1% representing the free steroid, we investigated the effects of DHEAS on post-translational isoprenylation of proteins. We here report that exposure of HT-29 SF human colonic adenocarcinoma cells to DHEAS inhibited the incorporation of [3H]mevalonate into cellular proteins in a dose-dependent manner when endogenous mevalonate synthesis was blocked by lovastatin. Interestingly, significant inhibition was observed at concentrations of DHEAS which are comparable to peak serum levels of this steroid occurring in the second decade of life. Immunoprecipitation revealed that isoprenylation of p21ras was also suppressed in DHEAS-treated HT-29 SF cells. In a cell-free system, DHEAS inhibited the farnesylation of a biotinylated decapeptide corresponding to the C-terminus of K-ras by 50% at a concentration of 100 microM. This suggests that DHEAS inhibits isoprenylation of cellular proteins, including p21ras, at a point in the mevalonate pathway distal to 3-hydroxy-3-methylglutaryl-CoA reductase and that the DHEAS-mediated suppression of protein farnesylation may largely be due to inhibition at the level of protein farnesyltransferase. Thus, these findings may provide a plausible explanation for the antitumor activity of DHEAS.

Alkyl and Aryl Transferases↗

Simvastatin has deleterious effects on human first trimester placental explants.

BACKGROUND: Statins inhibit 3-hydroxy-3-methylglutaryl coenzyme-A reductase (HMG-CoA reductase), the rate-limiting enzyme of the mevalonate pathway, and have been used successfully in the treatment of hypercholesterolaemia. Animal models have provided evidence for the teratogenic effects of statins on pregnancy outcome. Thus statins are contraindicated during pregnancy. However, conflicting data are available from inadvertent use of statins in human pregnancy. Therefore we decided to explore the effects of simvastatin on the placenta in an in vitro human placental model. METHODS: Human first trimester placental explants that were grown on matrigel were exposed to medium supplemented with simvastatin. Migration of extravillous trophoblast cells was assessed by visual observation. Proliferative and apoptotic events of the trophoblast cells were assesed by immunohistochemical examination using anti-Ki67 and anti-activated caspase-3 antibodies respectively. Hormone levels were measured. RESULTS: Simvastatin sharply inhibited migration of extravillous trophoblast cells from the villi to the matrigel (P < 0.05). Moreover, simvastatin inhibited half of the proliferative events in the villi (P < 0.05) and increased apoptosis of cytotrophoblast cells compared to control. Moreover, simvastatin significantly decreased secretion of progesterone from the placental explants (P < 0.01). CONCLUSION: Simvastatin adversely affects human first trimester trophoblast.

Apoptosis↗