Microbial sensors for small molecules: development of a mevalonate biosensor

We describe a novel biosensor strain for detection and quantification of a small molecule, mevalonate. The biosensor strain is an Escherichia coli mevalonate auxotroph that expresses the green fluorescent protein and reports on the mevalonate concentration in the growth medium through a change in growth rate. A model describing the growth rate dependence on mevalonate was developed in order to use the biosensor strain for high-throughput screening (HTS) and quantitative measurement of mevalonate in the extracellular environment. In general, this method should be applicable to the quantification of any small molecule for which an auxotroph can be developed and will be useful for HTS of evolved metabolic pathways for which there is no readily available screen or selection.

HMG-CoA reductase inhibition reduces monocyte CC chemokine receptor 2 expression and monocyte chemoattractant protein-1-mediated monocyte recruitment in vivo

BACKGROUND

The migration of circulating monocytes to the arterial wall during atherogenesis is largely modulated by activation of the CC chemokine receptor 2 (CCR2), a dominant monocyte chemotaxis receptor. The present study investigated whether 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibition affects CCR2 gene expression and CCR2-dependent monocyte recruitment.

METHODS AND RESULTS

Competitive reverse transcription-polymerase chain reaction analysis and flow cytometry showed that simvastatin, an HMG-CoA reductase inhibitor, dose-dependently reduced monocyte CCR2 mRNA and protein expression. Treatment of 21 normocholesterolemic men with simvastatin (20 mg/d for 2 weeks) decreased CCR2 protein and mRNA expression in circulating monocytes. Promoter and electrophoretic mobility shift assays showed that simvastatin activated a peroxisome proliferator response element in THP-1 monocytes. Moreover, simvastatin-induced CCR2 downregulation was completely reversed by the synthetic peroxisome proliferator-activated receptor-gamma antagonist GW9662. Simvastatin-treated monocytes showed little chemotaxis movement in response to monocyte chemoattractant protein-1 (MCP-1), a specific CCR2 ligand. Treatment of C57/BL6 mice with simvastatin (0.2 microg/g body weight IP, daily for 1 week) inhibited transmigration of CD80+ monocytes to the MCP-1-injected intraperitoneal space. Moreover, few circulating inflammatory cells from simvastatin-treated Sprague-Dawley rats (0.2 microg/g body weight IP, daily for 2 weeks) were recruited to the aortic wall of hypercholesterolemic littermates.

CONCLUSIONS

The inhibition of CCR2/MCP-1-dependent monocyte recruitment by simvastatin may prevent excessive accumulation of monocytes in the arterial wall during atherogenesis.

Neonatal mortality from in utero exposure to perfluorooctanesulfonate (PFOS) in Sprague-Dawley rats: dose-response, and biochemical and pharamacokinetic parameters

Perfluorooctanesulfonate (PFOS) is a widely distributed, environmentally persistent acid found at low levels in human, wildlife, and environmental media samples. Neonatal mortality has been observed following PFOS exposure in a two-generation reproduction study in rats and after dosing pregnant rats and mice during gestation. Objectives of the current study were to better define the dose-response curve for neonatal mortality in rat pups born to PFOS-exposed dams and to investigate biochemical and pharmacokinetic parameters potentially related to the etiology of effects observed in neonatal rat pups. In the current study, additional doses of 0.8, 1.0, 1.2, and 2.0 mg/kg/day were included with original doses used in the two-generation study of 0.4 and 1.6 mg/kg/day in order to obtain data in the critical range of the dose-response curve. Biochemical parameters investigated in dams and litters included: (1) serum lipids, glucose, mevalonic acid, and thyroid hormones; (2) milk cholesterol; and (3) liver lipids. Pharmacokinetic parameters investigated included the interrelationship of administered oral dose of PFOS to maternal body burden of PFOS and the transfer of maternal body burden to the fetus in utero and pup during lactation, as these factors may affect neonatal toxicity. Dosing of dams occurred for 6 weeks prior to mating with untreated breeder males, through confirmed mating, gestation, and day four of lactation. Dose levels for the dose-response and etiological investigation were 0.0, 0.4, 0.8, 1.0, 1.2, 1.6, and 2.0 mg/kg/day PFOS. Statistically significant decreases in gestation length were observed in the 0.8 mg/kg and higher dose groups. Decreases in viability through lactation day 5 were observed in the 0.8 mg/kg and higher dose groups, becoming statistically significant in the 1.6 and 2.0 mg/kg dose groups. Reduced neonatal survival did not appear to be the result of reductions in lipids, glucose utilization, or thyroid hormones. The endpoints of gestation length and decreased viability were positively correlated, suggesting that late-stage fetal development may be affected in pups exposed to PFOS in utero and may contribute to the observed mortality. Benchmark dose (BMD) estimates for decreased gestation length, birth weight, pup weight on lactation day 5, pup weight gain through lactation day 5, and viability resulted in values ranging from 0.27 to 0.89mg/kg/day for the lower 95% confidence limit of the BMD5 (BMDL5). Results of analyses for PFOS in biological matrices indicate a linear proportionality of mean serum PFOS concentration to maternal administered dose prior to mating and through the first two trimesters of gestation. However, at 21 days of gestation, mean serum PFOS concentrations were notably reduced from values measured earlier in gestation. Urinary and fecal elimination was low as expected from prior observations in adult rats. Significant transfer of PFOS from dam to fetus in utero was confirmed, and results suggest that dam and corresponding fetal body burdens, as indicated by serum and liver PFOS levels, correlate with neonatal survival.

Cell-based screen of HMG-CoA reductase inhibitors and expression regulators using LC–MS

We present an integrated consolidation of previously reported methods for screening hydroxymethylglutaryl-coenzyme A reductase (HMGR) inhibitors in 96-well microtiter plates with rapid workup using established mammalian cell lines and liquid chromatography-mass spectrometry analysis. Inhibitors as well as expression regulators of HMGR (inducers or repressors) can be screened. To validate the method, three competitive inhibitors of HMGR (lovastatin, simvastatin, and atorvastatin), as well as a potent sterol repressor of HMGR synthesis (25-hydroxycholesterol), were assayed on two cell lines: HepG2, a human hepatic derived cell line, and L cells, a subline of NCTC clone 929 mouse fibroblasts. The direct inhibition of HMGR by statins, induction of HMGR synthesis by the same statins following incubation with the cells, and repression of HMGR synthesis by 25-hydroxycholesterol were confirmed.

Analysis of 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors using liquid chromatography-electrospray mass spectrometry

Employing high-performance liquid chromatography-electrospray mass spectrometry, we describe a new assay for monitoring 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase activity. Incubations were carried out with HMG-CoA reductase (rat liver), HMG-CoA and NADPH, and terminated by the addition of HCl. The reaction product, mevalonolactone, and internal standard, were extracted with ethyl acetate, dissolved in methanol, and analyzed by LC-MS. Using an isocratic mobile phase of 10% acetonitrile and 0.1% formic acid (flow-rate, 0.2 ml/min), the protonated molecules of mevalonolactone at m/z 131 and internal standard, beta,beta-dimethyl-gamma-(hydroxymethyl)-gamma-butyrolactone, at m/z 145, were detected using selected ion monitoring. The limit of detection was approximately 6.5 pg, and the limit of quantitation was approximately 16.3 pg. Extraction recovery was >90%. The relative standard deviations for intra- and inter-day assays were approximately 4.1+/-2.7 and 9.4+/-3.4%, respectively. Mevalonolactone was examined over a period of 3 days and found to be stable. Using this assay, lovastatin and mevastatin inhibited HMG-CoA reductase activity with IC50 values 0.24+/-0.02 and 2.16+/-0.31 microM, respectively. These methods offer some advantages over those reported previously which employ radiolabeled substrate and products, and should be useful in searching for compounds that could lower serum cholesterol or alter cell growth and differentiation.

Feasibility of an immunoassay for mevalonolactone

Mevalonic acid is a key intermediate in a broad spectrum of cellular biological processes and their regulation. Availability of a rapid, sensitive and accurate method for its assay would be highly useful. Therefore, the feasibility of developing an immunoassay for mevalonic acid in biological samples was explored. The strategy employed was to synthesize several racemic haptens structurally resembling R-mevalonolactone, the cyclic form of mevalonic acid present at lower pH and presumed to be more antigenic. Two of these haptens were coupled to keyhole limpet hemocyanin, and the resulting conjugates were used successfully to generate antibodies in rabbits. The first antiserum bound to R,S-mevalonolactone much more effectively at pH 4.0 than at pH 6.0, consistent with the structural resemblance of the haptens to the lactone form. This antiserum also bound the free hapten from which it was generated and two others of different structure with comparable effectiveness; and slightly better than it bound R,S-mevalonolactone at pH 4.0. Similar results were obtained with the antiserum to the second hapten. The binding of either antiserum to the natural enantiomer, R-mevalonolactone, was 20 times weaker than to R,S-mevalonolactone, suggesting that the nonbiological enantiomer was more antigenic. Nevertheless, the results demonstrate that an immunochemical approach to accurate quantitation of mevalonic acid in biological samples is feasible.

Facts and artefacts in mevalonic aciduria: development of a stable isotope dilution GCMS assay for mevalonic acid and its application to physiological fluids, tissue samples, prenatal diagnosis and carrier detection

A stable isotope dilution assay using D3-mevalonic acid was developed and applied to the study of mevalonic aciduria. The method also appears to be suitable for the evaluation of different therapeutic regimens in patients with hypercholesterolemia. Mevalonic acid was isolated by liquid partition chromatography and quantified as the underivatized lactone by means of ammonia chemical ionization selected ion monitoring capillary gas chromatography-mass spectrometry. In heterozygotes there was significantly greater urinary excretion of mevalonic acid, while the range of enzymatic activity of mevalonate kinase showed an overlap with that of controls. The analysis of amniotic fluids of two pregnancies at risk for mevalonic aciduria showed a 3277-fold elevation as compared to controls in the first case, diagnostic of an affected fetus, and a normal value in the second one. Mevalonic acid concentration was much increased in tissues of the affected and aborted fetus. Concentrations ranged from 840 to 1120 mumol/kg in various tissues and were as high as 1810 mumol/kg in brain. Concentrations in control fetal tissues were approximately 1 mumol/kg.

Determination of mevalonolactone in capsules by capillary gas-liquid chromatography

A wide bore capillary gas chromatographic method was developed to determine mevalonolactone in capsule formulations. The method uses beta,beta-dimethyl-gamma-hydroxymethyl-gamma- butyrolactone as an internal standard and has been validated for its accuracy, precision and linearity. The method has been applied for stability testing of the capsule formulation. High-performance liquid chromatographic and gas chromatographic studies demonstrated cyclization of mevalonic acid (open-chain form) to mevalonolactone (cyclic form) under the described gas chromatography conditions. Mass spectrometric analysis indicated that mevalonolactone prepared in water or an organic solvent emerged from the gas chromatographic column as the intact cyclic lactone.

An improved chemical ionization assay for mevalonic acid

The determination of mevalonic acid by gas chromatography/mass spectrometry (GC/MS) is complicated by poor extractability and multiple product formation. Reliable quantification down to physiological levels was achieved by isolating mevalonic acid by liquid partition chromatography and quantifying the underivatized lactone by means of ammonia chemical ionization, selected ion monitoring, capillary GC/MS. (2H3)mevalonic acid served as the internal standard. The method appears to be suitable for the study of mevalonic aciduria as well as the evaluation of different therapeutic regimens in patients with hypercholesterolaemia.

Hydroxymethylglutaryl coenzyme A reductase activity of adult Hymenolepis diminuta

The occurrence of hydroxymethylglutaryl coenzyme A (HMG-CoA) reductase in adult Hymenolepis diminuta was demonstrated. This activity was negligible in the cestode's cytosolic fraction but was noted when the mitochondrial or microsomal fraction served as the enzyme source. The predominant localization of HMG-CoA reductase activity was with the microsomal fraction. This fraction did not contain appreciable mitochondrial contamination based on the distribution of marker enzymes. The enzymatic nature of HMG-CoA conversion to mevalonic acid by either fraction was apparent because the reaction was heat labile and responded linearly to time of assay and protein content. The enzymatic reduction of HMG-CoA absolutely required NADPH when either fraction was assayed. The lesser activity of the mitochondrial fraction was membrane-associated. The predominant localization of HMG-CoA reductase activity with microsomal membranes and its separation with the membranous component of the mitochondrial fraction suggest that mitochondrial activity reflects the presence of microsomal membranes. In its predominant localization and pyridine nucleotide requirement, the cestode's HMG-CoA reductase activity resembles that of mammalian systems. The finding of HMG-CoA reductase provides an enzymatic mechanism for the intermediate conversion of HMG-CoA to mevalonic acid that would be needed for acetate-dependent isoprenoid lipid synthesis by adult H. diminuta.

Measurement of mevalonate in human plasma and urine by multiple selected ion monitoring

Plasma levels and urinary excretion of mevalonate were reported to be correlated with cholesterol biosynthesis. Evaluation of mevalonate concentration in plasma and urine represents therefore a non-invasive method for studying the modifications of cholesterol synthesis. A method is described here by which mevalonate in plasma and urine is determined by the selected ion monitoring technique after extraction as mevalonolactone and conversion into the trimethylsilyl ether. Linear responses were obtained in the evaluation of mevalonate added to plasma in the 10-100 ng/ml (r greater than 0.995) and to urine in the 50-1000 ng/ml concentration ranges, respectively. Identity of mevalonate in plasma and urine was confirmed by high-resolution mass spectrometry.

Mevalonic aciduria--an inborn error of cholesterol and nonsterol isoprene biosynthesis

A two-year-old boy presented with severe failure to thrive, developmental delay, anemia, hepatosplenomegaly, central cataracts, and dysmorphic features. Quantitative analyses of urinary organic acids revealed massive excretion of mevalonic acid, a metabolic precursor of cholesterol and nonsterol isoprenes: 46,000 to 56,200 mmol per mole of creatinine, as compared with 0.2 to 0.3 mmol per mole in normal children. The mevalonic acid concentration in plasma was also greatly increased at 440 mumol per liter (normal, less than 0.05). The activity of mevalonate kinase, the enzyme that catalyzes the first step in mevalonate metabolism, was severely deficient in the patient's fibroblasts, lymphocytes, and lymphoblasts. In the subsequent pregnancy of the patient's mother, gas chromatography-mass spectrometry demonstrated a marked elevation of mevalonic acid in the mother's urine and a 3000-fold elevation, as compared with control levels in the amniotic fluid, suggesting that the fetus was affected. The diagnosis was confirmed by demonstration of the deficiency of mevalonate kinase in amniocytes and ultimately in liver from the abortus. Intermediate activities of the enzyme in both parents indicated an autosomal recessive mode of inheritance. These observations identify an inherited disorder of cholesterol and nonsterol isoprene biosynthesis in humans.

Separation and estimation of short-chain fatty acids and organic acids by thermodetective liquid chromatography

Short-chain fatty acids and organic acids in aqueous solution were separated quantitatively by using a liquid chromatography. JLC-2A, which posseses a thermodetector with ion-exchange resin. A peak height of thermogram showing a thermal difference caused by adsorption and desorption of substance was proportional to the concentration. Each sample was detectable at least between approximately 10(-5) and 2 X 10(-3) moles. The "inter-day" coefficient of variation obtained from two peak heights of acetic and n-butyric acids was 1.9 and 2.3%, respectively.

Indirect assessment of hydroxymethylglutaryl-CoA reductase (NADPH) activity in liver tissue

We describe an indirect method for assessing variation in 3-hydroxy-3-methylglutaryl-coenzyme A reductase (NADPH) activity in liver tissue. 3-Hydroxy-3-methyglutaryl-CoA and mevalonate concentrations in the tissue homogenate are estimated in terms of absorbances and the ratio between the two is taken as an index of activity of the enzyme, which catalyzes the conversion of 3-hydroxy-3-methylglutaryl-CoZ to mevalonate. The ratio increases under conditions in which activity of this enzyme in liver reportedly decreases (e.g., fasting, cholesterol feeding) and decreases under conditions in which the activity of this enzyme reportedly increases (e.g., Triton injection, phenobarbital treatment). We assessed the magnitude of hepatic cholesterogenesis in intact male albino rats by using [1,2-14C] acetate, in fed controls, fasted, cholesterol-fed, Triton-treated, and phenobarbital-treated. The ratio, 3-hydroxy-3-methylglutaryl-CoA/mevalonate was estimated in liver tissue for all the above groups. The method is simple, reproducible, and does not require any unusual chemicals.

Feedback regulation of cholesterol biosynthesis: studies with cholestyramine

Regulation of hepatic cholesterol biosynthesis is thought to occur at the step involving formation of mevalonate from 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA); this reaction is catalyzed by HMG-CoA reductase. Oral administration of cholestyramine, an agent that interferes with cholesterol reabsorption and is associated with a compensatory increase in hepatic cholesterol synthesis, was used to evaluate feedback regulation of cholesterol biosynthesis. Liver slices and cell-free fractions were prepared from control rats and rats fed 3% cholestyramine for 3-7 days. Cholestyramine feeding was associated with a four- to eightfold increase in incorporation of [ 14 C]acetate into cholesterol and mevalonate. However, there was no effect on incorporation of [ 14 C]acetate into CO 2 , fatty acids, or acetoacetate or on incorporation of [ 14 C]mevalonate into cholesterol, indicating stimulation at an early regulatory site prior to mevalonate formation. In fractionated liver homogenates, incorporation of [ 14 C] acetate, [ 14 C]acetyl-CoA, and [ 14 C]HMG-CoA into mevalonate increased markedly following cholestyramine administration. In the absence of microsomes, incorporation of [ 14 C]acetate and [ 14 C]acetyl-CoA into HMG was also increased. Changes in incorporation of [ 14 C]acetate into HMG, mevalonate, and cholesterol corresponded to changes in product synthesis. I concluded that (1) interruption of the enterohepatic circulation of cholesterol causes a release of feedback inhibition of cholesterol synthesis, (2) mevalonate synthesis catalyzed by HMG-CoA condensing enzyme and HMG-CoA reductase, and (3) HMG-CoA condensing enzyme, by limiting the availability of HMG-CoA, may have an important regulatory function in cholesterol biosynthesis.