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Biomedical subjects

G Assmann

Publications and source records attributed to G Assmann.

At least 73 records · Page 4Linked to original sources
PubMed1999

Aberrant oxidation of the cholesterol side chain in bile acid synthesis of sterol carrier protein-2/sterol carrier protein-x knockout mice.

Peroxisomal beta-oxidation plays an important role in the metabolism of a wide range of substrates, including various fatty acids and the steroid side chain in bile acid synthesis. Two distinct thiolases have been implicated to function in peroxisomal beta-oxidation: the long known 41-kDa beta-ketothiolase identified by Hashimoto and co-workers (Hijikata, M., Ishii, N., Kagamiyama, H., Osumi, T., and Hashimoto, T. (1987) J. Biol. Chem. 262, 8151-8158) and the recently discovered 60-kDa SCPx thiolase, that consists of an N-terminal domain with beta-ketothiolase activity and a C-terminal moiety of sterol carrier protein-2 (SCP2, a lipid carrier or transfer protein). Recently, gene targeting of the SCP2/SCPx gene has shown in mice that the SCPx beta-ketothiolase is involved in peroxisomal beta-oxidation of 2-methyl-branched chain fatty acids like pristanic acid. In our present work we have investigated bile acid synthesis in the SCP2/SCPx knockout mice. Specific inhibition of beta-oxidation at the thiolytic cleavage step in bile acid synthesis is supported by our finding of pronounced accumulation in bile and serum from the knockout mice of 3alpha,7alpha, 12alpha-trihydroxy-27-nor-5beta-cholestane-24-one (which is a known bile alcohol derivative of the cholic acid synthetic intermediate 3alpha,7alpha,12alpha-trihydroxy-24-keto-cholestano yl-coenzyme A). Moreover, these mice have elevated concentrations of bile acids with shortened side chains (i.e. 23-norcholic acid and 23-norchenodeoxycholic acid), which may be produced via alpha- rather than beta-oxidation. Our results demonstrate that the SCPx thiolase is critical for beta-oxidation of the steroid side chain in conversion of cholesterol into bile acids.

Acetyl-CoA C-Acetyltransferase↗
PubMed1999

Increased lipoprotein(a) is an important risk factor for venous thromboembolism in childhood.

BACKGROUND: Serum levels of lipoprotein(a) [Lp(a)] are determined largely by genetic variation in the gene encoding for apolipoprotein(a) [apo(a)], the specific protein component of Lp(a) that is very homologous to plasminogen. High plasma levels of Lp(a) increase the risk for premature atherosclerotic vessel diseases. We investigated the little-characterized role of Lp(a) as a risk factor for venous thromboembolic diseases, alone and in conjunction with established thrombophilic risk factors of proteins regulating blood coagulation and fibrinolysis. METHODS AND RESULTS: Serum levels of Lp(a) and lipids, protein C, protein S, and antithrombin, as well as the size of apo(a) isoforms and the presence of the factor V:Q(506) mutation, were determined in 186 consecutively admitted children from neonates to 18 years old with a history of venous thrombosis and in 186 age- and disease-matched control subjects. Children with a history of venous thrombosis had a significantly higher median Lp(a) level (19 versus 4.4 mg/dL) than control subjects. The risk for thromboembolic events in children with Lp(a) levels in the upper quartile, ie, >30 mg/dL, was 7.2 (95% CI, 3.7 to 14.5). The size of apo(a) isoforms was inversely related to Lp(a) levels and to the risk for thromboembolic events. Compared with the highest quartile of the apo(a) size distribution, the lowest quartile was associated with a risk of 8.2. In addition, multivariate statistical analysis gives evidence that the factor V:Q(506) mutation (OR/CI, 2.8/1.6 to 4.9), protein C (OR/CI, 6.5/2.1 to 19), and antithrombin deficiency (OR/CI, 10.4/1.2 to 90) were independent risk factors of childhood venous thrombosis. Coincidence of elevated Lp(a) with factor V:Q(506) mutation or deficiencies of protein C or antithrombin further increased the risk for thromboembolic events to 8.4. CONCLUSIONS: Lp(a) >30 mg/dL is a risk factor for venous thromboembolism in childhood. Lp(a) measurements should be included in the screening of causal factors in children with venous thromboembolic events.

Activated Protein C Resistance↗
PubMed1999

Stability and binding properties of wild-type and c17s mutated human sterol carrier protein 2.

The temperature- and solvent-induced denaturation of both the SCP2 wild-type and the mutated protein c71s were studied by CD measurements at 222 nm. The temperature-induced transition curves were deconvoluted according to a two-state mechanism resulting in a transition temperature of 70.5 degrees C and 59.9 degrees C for the wild-type and the c71s, respectively, with corresponding values of the van't Hoff enthalpies of 183 and 164 kJ/mol. Stability parameters characterizing the guanidine hydrochloride denaturation curves were also calculated on the basis of a two-state transition. The transitions of the wild-type occurs at 0.82 M GdnHCl and that of the c71s mutant at 0.55 M GdnHCl. These differences in the half denaturation concentration of GdnHCl reflect already the significant stability differences between the two proteins. A quantitative measure are the Gibbs energies DeltaG(0)(D)(buffer) at 25 degrees C of 15.5 kJ/mol for the wild-type and 8.0 kJ/mol for the mutant. We characterized also the alkyl chain binding properties of the two proteins by measuring the interaction parameters for the complex formation with 1-O-Decanyl-beta-D-glucoside using isothermal titration microcalorimetry. The dissociation constants, K(d), for wild-type SCP2 are 335 microM at 25 degrees C and 1.3 mM at 35 degrees C. The corresponding binding enthalpies, DeltaH(b), are -21. 5 kJ/mol at 25 degrees C and 72.2 kJ/mol at 35 degrees C. The parameters for the c71s mutant at 25 degrees C are K(d)=413 microM and DeltaH(b)=16.6 kJ/mol. These results suggest that both SCP2 wild-type and the c71s mutant bind the hydrophobic compound with moderate affinity.

Calorimetry↗
PubMed1999

Phytanic acid activates the peroxisome proliferator-activated receptor alpha (PPARalpha) in sterol carrier protein 2-/ sterol carrier protein x-deficient mice.

We showed recently that a targeted null mutation in the murine sterol carrier protein 2-/sterol carrier protein x-gene (Scp2) leads to defective peroxisomal catabolism of 3,7,11, 15-tetramethylhexadecanoic acid (phytanic acid), peroxisome proliferation, hypolipidemia, and enhanced hepatic expression of several genes that have been demonstrated to be transcriptionally regulated by the peroxisome proliferator-activated receptor alpha (PPARalpha). As a broad range of fatty acids activates PPARalpha in vitro, we examined whether the latter effects could be because of phytanic acid-induced activation of this transcription factor. Dietary phytol supplementation was used to modulate the concentration of phytanic acid in C57Bl/6 and Scp2 (-/-) mice. We found that the serum concentrations of phytanic acid correlated well with the expression of genes encoding peroxisomal beta-oxidation enzymes and liver fatty acid-binding protein, which have all been demonstrated to contain functionally active peroxisome proliferator response elements in their promoter regions. In accordance with these findings, a stimulating effect on acyl-CoA oxidase gene expression was also observed after incubation of the rat hepatoma cell line MH1C1 with phytanic acid. Moreover, reporter gene studies revealed that phytanic acid induces the expression of a peroxisome proliferator response element-driven chloramphenicol transferase reporter gene comparable with strong peroxisome proliferators. In addition, the ability of phytanic acid to act as an inductor of PPARalpha-dependent gene expression corresponded with high affinity binding of this dietary branched chain fatty acid to recombinant PPARalpha. We conclude that phytanic acid can be considered as a bona fide physiological ligand of murine PPARalpha.

Acyl-CoA Oxidase↗
PubMed1999

Apolipoprotein A-I (R151C)Paris is defective in activation of lecithin: cholesterol acyltransferase but not in initial lipid binding, formation of reconstituted lipoproteins, or promotion of cholesterol efflux.

ApoA-I(R151)Paris is a natural apolipoprotein (apo) A-I variant that is associated with low levels of high-density lipoprotein cholesterol (HDL-cholesterol) and the partial deficiency of lecithin:cholesterol acyl-transferase (LCAT) in the plasma of heterozygous carriers. We compared the abilities of recombinant normal apoA-I and recombinant apoA-I(R151C)Paris to clear an emulsion of dimyristoylphosphatidylcholine (DMPC), to form reconstituted lipoproteins with dipalmitoylphosphatidylcholine (DPPC), to activate LCAT, and to promote efflux of biosynthetic cholesterol from porcine aortic smooth muscle cells (SMCs) or of exogenous cholesterol from lipid-loaded mouse peritoneal macrophages. Recombinant apoA-I(R151C)Paris occurred in monomeric and dimeric forms at a ratio of 60:40. Normal apoA-I and apoA-I(R151C)Paris cleared DMPC emulsions at equal rates. Both isoforms associated completely with DPPC during cholate dialysis. Normal apoA-I formed one single particle with a mean diameter of 9.3 nm, whereas apoA-I(R151)Paris gave rise to three particles with mean diameters of 9.3 nm (containing 74% of apoA-I), 10.6 nm, and 12.1 nm, respectively. Compared to normal apoA-I, apoA-I(R151C)Paris had a reduced LCAT-cofactor activity with a 60% lower Vmax/Km ratio due to a 50% higher affinity constant, Km. During incubations for 10 min and 360 min, normal apoA-I/DPPC complexes and apoA-I(R151C)Paris/DPPC complexes were equally efficient in releasing biosynthetic cholesterol from SMCs. In the lipid-free form, apoA-I(R151C)Paris induced normal hydrolysis of cholesteryl esters and normal cholesterol efflux from lipid-loaded mouse-peritoneal macrophages. In conclusion, in addition to its ability to form homo- and heterodimers, apoA-I(R151C)Paris is characterized by defective LCAT-cofactor activity but by normal lipid binding and cholesterol-efflux-promoting abilities.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1999

The LQT syndromes--current status of molecular mechanisms.

Our knowledge on the molecular genetics of inherited cardiac arrhythmias is very recent in comparison to the advances of genetics achieved in other inherited cardiac disorders. This is related to the high mortality and early disease onset of these arrhythmias resulting in mostly small nucleus families. Thus, traditional genetic linkage studies that are based on the genetic information obtained from large multi-generation families were made difficult. In 1991, the first chromosomal locus for congenital long-QT (LQT) syndrome was identified on chromosome 11p15.5 (LQT1 locus) by linkage analysis. Meanwhile, the disease-causing gene at the LQT1 locus (KCNQ1), a gene encoding a K+ channel subunit of the IKs channel, and three other, major genes, all encoding cardiac ion channel components, have been identified. Taken together, LQT syndrome turned out to be a heterogeneous channelopathy. Moreover, the power of linkage studies to reveal the genetic causes of the LQT syndrome was also important to identify unknown but fundamental channel components that contribute to the ion currents tuning ventricular repolarization. In-vitro expression of the altered ion channel genes demonstrated in each case that the altered ion channel function produces prolongation of the action potential and thus the increasing propensity to ventricular tachyarrhythmias. Since these ion channels are pharmacological targets of many antiarrhythmic (and other) drugs, individual and potentially deleterious drug responses may be related to genetic variation in ion channel genes. Very recently, also in acquired LQT syndrome, which is a frequent clinical disorder in cardiology a genetic basis has been proposed in part since mutations in LQT genes have been specifically found. The discovery of ion channel defects in LQT syndrome represents the major achievement in our understanding and implies potential therapeutic options. The knowledge of the genomic structure of the LQT genes now offers the possibility to detect the underlying genetic defect in 80-90% of all patients. With this specific information, containing the type of ion channel (Na+ versus K+ channel) and electrophysiological alteration by the mutation (loss-of-function versus change-of-function mutation), gene-directed, elective drug therapies have been initiated in genotyped LQT patients. Based on preliminary data, that were supported by in vitro studies, this approach may be useful in recompensating the characteristic phenotypes in some LQT patients. Mutation detection is a new diagnostic tool which may become of more increasing importance in patients with a normal QTc or just a borderline prolongation of the QTc interval at presentation. These patients represent approximately 40% of all familial cases. Moreover, LQT3 syndrome and idiopathic ventricular fibrillation are allelic disorders and genetically overlap. In both mutations in the LQT3 gene SCN5A encoding the Na+ channel alpha-subunit for INa have been reported. Thus, the clinical nosology of inherited arrhythmias may be reconsidered after elucidation of the underlying molecular bases. Meanwhile, genotype-phenotype correlations in large families are on the way to evaluate intergene, interfamilial, and intrafamilial differences in the clinical phenotype reflecting gene specific, gene-site specific, and individual consequences of a given mutation. LQT syndrome is phenotypically heterogeneous due to the reduced penetrance and variable expressivity associated with the mutations. This paper discusses the current data on molecular genetics and genotype-phenotype correlations and the implications for diagnosis and treatment.

Chromosome Mapping↗
PubMed1999

Na(+)/Ca(2+) exchange inhibitors modulate thapsigargin-induced Ca(2+) and Na(+) influx in human lymphocytes.

Thapsigargin has been shown the elevate intracellular Na(+) concentration in human lymphocytes, but mechanisms underlying thapsigargin-induced Na(+) entry are little understood. In the present study we investigated thapsigargin-induced changes in cytosolic free Na(+) and Ca(2+) concentration in human lymphocytes after inhibition of the Na(+)/Ca(2+) exchange with two structurally unrelated compounds, dimethylthiourea ad bepridil. The intracellular Na(+) increase induced by 5 microM thapsigargin was significantly enhanced in the presence of 5 mM dimethylthiourea or 40 microM bepridil. In contrast, both compounds significantly decreased the thapsigargin-induced intracellular Ca(2+) elevation. No effect of dimethylthiourea or bepridil on thapsigargin-induced Ca(2+) influx was observed in the absence of extracellular Na(+). These observations are consistent with the hypothesis that thapsigargin stimulates Na(+)/Ca(2+ )exchange in human lymphocytes. However, Na(+)/Ca(2+) exchange does not mediate Na(+) influx in human lymphocytes.

Bepridil↗
PubMed1999

High risk strategies for atherosclerosis.

Calculating a person's chances of developing coronary heart disease (CHD) is not simple, as many risk factors interact in a complex fashion. Thus many markers, though significant in univariate comparisons, are no longer so when multivariate analysis is performed. Those factors contributing independently to risk can be identified only in prospective investigations such as the Münster Heart (PROCAM) or the Framingham studies. In the Münster Heart study, follow-up of middle-aged men for eight years identified the following nine independent risk variables: age, smoking history, personal history of angina pectoris, family history of myocardial infarction, systolic blood pressure, raised plasma low density lipoprotein cholesterol (LDL-C), low plasma high density lipoprotein cholesterol, raised fasting plasma triglyceride and presence of diabetes mellitus. These have been used to generate an algorithm for prediction of first coronary events which is available in interactive fashion on the internet'. Large trials have shown that lowering LDL-C reduces the risk of CHD, and diminishes CHD morbidity and mortality in persons without prior evidence of coronary atherosclerosis (primary prevention). This is even more the case in patients with such evidence (secondary prevention). It appears that lowering of LDL-C also reduces all-cause mortality in secondary prevention.

Algorithms↗
PubMed1999

Associations of obesity markers, insulin, and sex hormones with HDL-cholesterol levels in Turkish and German individuals.

Turkish men and women have about 20% lower mean levels of HDL-C and apoA-I than German individuals. To obtain some information on the metabolic basis of this difference, we compared anthropometric data as well as serum levels of leptin, insulin, testosterone (T), estradiol (E2), and sex hormone binding globuline (SHBG) in 289 German and 120 Turkish men as well as in 108 German and 182 Turkish women aged 20-60. Individuals who smoke, take hormones, have overt diabetes mellitus, BMI > 30 kg/m2, triglycerides > 400 mg/dl, or LDL-cholesterol > 200 mg/dl were excluded. In both sexes, Turks had significantly lower levels of HDL-C, apoA-I, Lp(a), and SHBG than Germans. Moreover, German men had a larger waist circumference, lower levels of E2 and a lower ratio of T/SHBG. German women also had a lower BMI, smaller waist circumference, lower insulin levels and higher T levels. Mean values of age, waist-hip-ratio (WHR), leptin, triglycerides, LDL-C, and apoB did not differ significantly among Germans and Turks. Upon univariate analysis HDL-C had inverse correlations with BMI, waist circumference, WHR, leptin, and insulin as well as positive correlations with SHBG in both sexes. Upon multivariate analysis, most of the different levels of HDL-C and apoA-I between Germans and Turks were explained by ethnicity, independently of obesity markers, insulin, and sex hormones.

Adult↗
PubMed1999

Obesity, mortality and cardiovascular disease in the Münster Heart Study (PROCAM).

In the Münster Heart Study (PROCAM), 16,288 men aged 40.6 +/- 11.3 years (mean +/- S.D.) and 7328 women aged 36.0 +/- 12.3 years were enrolled between 1979 and 1991. Mean body mass indices (BMIs) were 25.6 +/- 3.3 and 23.8 +/- 4.1 kg/m2 in men and women, respectively. There was a graded and continuous positive interaction in both men and women between BMI, age and serum total cholesterol, low density lipoprotein (LDL) cholesterol, and blood pressure (both systolic and diastolic). High density lipoprotein (HDL) cholesterol tended to increase with age, but decreased in graded fashion with increases in BMI in both sexes. Triglyceride increased with BMI in both sexes and with age in women, but decreased in the older age groups of overweight and obese men. Though fasting blood glucose increased with age and BMI in both sexes, the increase was more marked in women. Among the 10,856 men aged 36-65 years at study entry, 313 deaths occurred within a follow-up period of 7.1 +/- 2.4 years. Among these men, increased mortality was seen at high BMIs in both smokers and non-smokers and was caused by coronary heart disease (CHD). Increased mortality at low BMI was seen in smokers but not in non-smokers and was due to an increase in cancer deaths. The BMI-associated increase in CHD death was completely accounted for by the factors contained in the Münster Heart Study (PROCAM) risk algorithm, indicating that the effect of overweight and obesity on CHD is mediated via other risk factors.

Adult↗
PubMed1999

Tangier disease is caused by mutations in the gene encoding ATP-binding cassette transporter 1.

Tangier disease (TD) was first discovered nearly 40 years ago in two siblings living on Tangier Island. This autosomal co-dominant condition is characterized in the homozygous state by the absence of HDL-cholesterol (HDL-C) from plasma, hepatosplenomegaly, peripheral neuropathy and frequently premature coronary artery disease (CAD). In heterozygotes, HDL-C levels are about one-half those of normal individuals. Impaired cholesterol efflux from macrophages leads to the presence of foam cells throughout the body, which may explain the increased risk of coronary heart disease in some TD families. We report here refining of our previous linkage of the TD gene to a 1-cM region between markers D9S271 and D9S1866 on chromosome 9q31, in which we found the gene encoding human ATP cassette-binding transporter 1 (ABC1). We also found a change in ABC1 expression level on cholesterol loading of phorbol ester-treated THP1 macrophages, substantiating the role of ABC1 in cholesterol efflux. We cloned the full-length cDNA and sequenced the gene in two unrelated families with four TD homozygotes. In the first pedigree, a 1-bp deletion in exon 13, resulting in truncation of the predicted protein to approximately one-fourth of its normal size, co-segregated with the disease phenotype. An in-frame insertion-deletion in exon 12 was found in the second family. Our findings indicate that defects in ABC1, encoding a member of the ABC transporter superfamily, are the cause of TD.

ATP Binding Cassette Transporter 1↗
PubMed1999

In vivo kinetics of human apolipoprotein A-I variants in rabbits.

BACKGROUND: Genetic variants of human apolipoprotein (apo) A-I, the major protein component of high-density lipoprotein (HDL), with a single amino acid substitution have been reported, and some of these result in very low plasma HDL-cholesterol (C) levels. Examining the kinetics of radiolabelled apolipoprotein is a straightforward technique for determining its metabolism in vivo. In this study, we investigated the in vivo kinetics of several human apo A-I variants, which we had identified previously, in rabbits. MATERIALS AND METHODS: Apo A-I variants from heterozygous carriers of Lys-107-->0, Lys-107-->Met, Pro-3-->Arg, Pro-4-->Arg, Pro-165-->Arg and Glu-198-->Lys and the corresponding normal apo A-I were purified and then radioiodinated with 131I and 125I. A kinetic study of apo A-I variants was performed in normolipidaemic rabbits after simultaneous injection of the two isotopes that had been incorporated into HDL. The fractional catabolic rate (FCR) was calculated from the radioactive decay curve. RESULTS: Acidic mature (negatively charged) apo A-I variants caused by a single amino acid substitution (Lys-107-->0, and Lys-107-->Met) were catabolized faster (FCR, 1.931 +/- 0.539 per day vs. 1.636 +/- 0.460 per day, P </= 0.01 using the Wilcoxon signed-rank test) and basic mature (positively charged) apo A-I variants (Pro-3-->Arg, Pro-4-->Arg, Pro-165-->Arg and Glu-198-->Lys) were catabolized more slowly (FCR 1.470 +/- 0.380 per day vs. 1.654 +/- 0.430 per day, P </= 0.01) than the corresponding normal mature apo A-I in vivo in rabbits. In addition, an inverse linear relationship was observed between the deviation in the FCR of variant human apo A-I from that of normal human apo A-I and the number of electric charges that the apo A-I variant carried (r = -0. 90, k = -0.188, P = 0.0003), as assessed by a linear regression analysis, suggesting that the electric charge of apo A-I variants may determine, at least in part, its in vivo kinetics in rabbits. CONCLUSIONS: Genetic variants of apo A-I with a single amino acid substitution show abnormal kinetics, and the electric charge of a apo A-I variant could contribute to determining its kinetics in vivo in this xenologous model.

Animals↗