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Biomedical subjects

G Olivecrona

Publications and source records attributed to G Olivecrona.

67 records · Page 4Linked to original sources
PubMed1995

Triglyceride lipases and atherosclerosis.

Lipoprotein lipase is at the center of lipoprotein metabolism, unloading triglycerides for metabolic use in extrahepatic tissues, regulated in concert with energy metabolism by insulin and other factors, and converting the input of large triglyceride-rich lipoproteins to smaller, potentially atherogenic remnants that are either remodeled into LDL and HDL, or quickly cleared. In addition, lipoprotein lipase has turned out to be a multifunctional protein that also acts as a ligand for binding of lipoproteins to proteoglycans and receptors. Hepatic lipase has evolved more recently, and its main role appears to be in the remodeling processes.

Arteriosclerosis↗
PubMed1995

Lipoprotein lipase mass and activity in plasma and their increase after heparin are separate parameters with different relations to plasma lipoproteins.

Lipoprotein lipase (LPL) activity and mass in plasma and their increase after heparin administration were measured in 61 men who had suffered myocardial infarction before the age of 45 years and in 69 population-based age- and sex-matched control subjects without coronary heart disease to study the relations between these parameters in plasma and their correlations with plasma lipoproteins in subjects with a wide range of lipoprotein and LPL levels. There was a relatively large amount of LPL protein compared with LPL activity in preheparin plasma, indicating that the majority of circulating LPL is catalytically inactive. LPL mass and activity in postheparin plasma (postheparin minus preheparin values) were highly correlated, and the calculated mean specific activity (0.35 mU/ng) was in the range expected for catalytically active LPL. Hence, heparin releases mainly active LPL. The four LPL parameters (mass and activity in plasma and their increase after heparin administration) were not related to each other, except for postheparin plasma LPL mass and activity, and they showed different correlations with plasma lipoprotein lipid concentrations. There was a strong positive correlation between LPL mass in preheparin plasma and the HDL cholesterol level as well as weak negative relations to VLDL triglyceride and cholesterol concentrations in the patients. In contrast, preheparin LPL activity showed no correlation with the HDL cholesterol level but weak positive relations to VLDL triglyceride and cholesterol concentrations in the control subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1995

Nutritional regulation of lipoprotein lipase.

Lipoprotein lipase (LPL) is needed for normal catabolism of triglyceride-rich lipoproteins. In some tissues, notably the adipose tissue, the local LPL activity is an important determinant for how much lipid is taken up. There is regulation of gene expression, but the rapid changes that occur in response to the nutritional state are mediated mainly by post-transcriptional mechanisms. In the fed state, the adipose tissue expresses its full potential for LPL production, as set by the mRNA levels and the rate of protein synthesis. During fasting, LPL activity is suppressed by an unknown post-translational mechanism. In heart, regulation is primarily exerted on the equilibrium between LPL at endothelial sites and LPL in blood, with more endothelial LPL in the fasted state. LPL forms complexes with fatty acids which results in shut-down of lipolysis and detachment of both lipase and lipoproteins from the endothelial site. This provides a molecular coupling device between the cellular metabolic state and the rate of lipoprotein catabolism. There is growing evidence that LPL is a ligand for binding of lipoprotein particles such as chylomicron remnants to cell surfaces and receptors.

Adipose Tissue↗
PubMed1995

Lipoprotein lipase enhances removal of chylomicrons and chylomicron remnants by the perfused rat liver.

Lipoprotein lipase has been found to efficiently mediate binding of lipoproteins to cell surfaces and to the low density lipoprotein (LDL) receptor-related protein (LRP) under cell culture conditions (Beisiegel et al. 1991. Proc. Natl. Acad. Sci. USA. 88: 8242-8346). This supports the previously proposed idea that the lipase could have a role in receptor-mediated uptake of chylomicron remnants in the liver. We have investigated the effects of lipoprotein lipase on the clearance of chylomicrons during perfusions of rat livers. The chylomicrons were doubly labeled in vivo with [14C]retinol (in retinyl esters) and with [3H]oleic acid (in triacylglycerols) and were collected from lymph. In the absence of any lipase the clearance of chylomicron label from the perfusion medium was slow. Addition of lipoprotein lipase caused lipolysis of chylomicron triacylglycerols as evidenced by increased levels of 14C-labeled fatty acids in the perfusate. Simultaneously, the level of [14C]retinyl esters in the perfusate decreased dramatically, indicating core-particle removal. Similar effects were seen with an unrelated lipase from Pseudomonas fluorescens. To discriminate between the effects of lipolysis and a true liganding effect of the lipoprotein lipase protein, the active site inhibitors tetrahydrolipstatinR and hexadecylsulfonylfluoride were used to reduce or totally inhibit the catalytical activity. With lipase covalently inhibited by the latter inhibitor, lipolysis during perfusions was low or absent. Nonetheless, the inhibited enzyme had a clear effect on the removal of chylomicrons by the liver. With 1.2 micrograms of inhibited lipase/ml perfusate, about 70% of the core label had been removed after 15 min as compared to about 20% in perfusions without lipase. With identical amounts of active lipoprotein lipase protein, more than 90% of the label was removed. We conclude that any lipase causing lipolysis of chylomicrons can stimulate their clearance by the liver, but that lipoprotein lipase has an additional effect on the removal, which is not dependent on its catalytic activity.

Animals↗
PubMed1995

Structural features in lipoprotein lipase necessary for the mediation of lipoprotein uptake into cells.

Lipoprotein lipase (LpL) has been shown to mediate the uptake of lipoproteins into cells. The uptake is initiated by binding of LpL to cell surface proteoglycans and to the low density lipoprotein (LDL) receptor-related protein. This ability of LpL is independent of catalytic activity and depends on the intact dimeric structure of the lipase and functional residues in the C-terminal domain. The goal of this study was to identify structural features in LpL that are essential in the mediation of lipoprotein uptake. Naturally occurring variants and LpL mutants produced by site-directed mutagenesis were cloned and expressed in COS-cells. A combination of immunoassays and separation on heparin-Sepharose columns was used to determine the molar ratio of monomeric to dimeric LpL in the expression media. The mutants were tested for their ability to mediate the uptake of 125I-labeled beta-VLDL in cultured Hep3b cells in direct comparison with wild type LpL. We found that the concentration of monomer in the media correlated negatively with the effect on the uptake mediated by the dimeric form of LpL. A mutation affecting the catalytic activity (Asp 156Gly) resulted in no significant reduction in the lipase-mediated beta-VLDL uptake. Point mutations in the proposed lipid binding region Trp390Ala or Trp393Ala and the substitution of 390-393 with the homologous hepatic lipase (HL) sequence were also normal, while the deletion of 390-393 reduced the ability to mediate the uptake by about 60% in comparison to wild type. A mutation known to impair heparin binding (Arg294Ala) was also less efficient than the wild type in mediating uptake. In conclusion, it is important to determine the monomer/dimer ratio in mutant preparations as the presence of monomers inhibits the uptake mediated by the dimeric LpL. Moreover, sites involved in heparin and lipid binding between residues 390-421 are important for LpL-mediated lipoprotein uptake.

Catalysis↗
PubMed1994

A carboxyl-terminal fragment of lipoprotein lipase binds to the low density lipoprotein receptor-related protein and inhibits lipase-mediated uptake of lipoprotein in cells.

It has previously been shown that lipoprotein lipase can mediate uptake of remnant lipoprotein particles via binding to the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP). Binding of lipoprotein lipase, and of triglyceride-rich lipoproteins associated with the lipase, to LRP depends on an intact carboxyl-terminal folding domain of the lipase (Nykjaer, A., Bengtsson-Olivecrona, G., Lookene, A., Moestrup, S. K., Petersen, C. M., Weber, W., Beisiegel, W., and Gliemann, J. (1993) J. Biol. Chem. 268, 15048-15055). Here we show that the site for binding to the receptor is within residues 380-425 of the bovine and residues 378-423 of the human lipoprotein lipase. We demonstrate that a carboxyl-terminal fragment of human lipoprotein lipase (residues 378-448), expressed as fusion protein in Escherichia coli, binds to purified and cellular LRP but not to lipoproteins. Binding of the fragment to purified LRP was blocked by heparin. In addition, the fragment inhibited the binding of lipase and the lipase-mediated binding of lipoproteins to the purified receptor. The fragment exhibited reduced binding to proteoglycan-deficient cells. Moreover, the fragment inhibited the uptake of lipoproteins in cells mediated by the lipase via binding to heparan sulfate proteoglycans and LRP. We conclude that the fragment contains the site for binding to LRP and a candidate site for interaction with heparan sulfate proteoglycans, whereas binding to lipoproteins is inefficient. The fragment can therefore inhibit the lipase-mediated lipoprotein uptake, a process that may promote the development of atherosclerosis when occurring in cells of the arterial wall.

Amino Acid Sequence↗
PubMed1994

Effect of protamine on lipoprotein lipase and hepatic lipase in rats.

The polycation protamine impedes the catabolism of triglyceride-rich lipoproteins and this has been suggested to be due to intravascular inactivation of lipoprotein lipase. We have made intravenous injections of protamine to rats and found that both lipoprotein lipase and hepatic lipase activities were released to plasma. The effect of protamine was more short-lived than that obtained by injection of heparin. The release of hepatic lipase by protamine was as effective as the release by heparin, while the amount of lipoprotein lipase released by protamine was only about one-tenth of that released by heparin. This was not due to inactivation of lipoprotein lipase, since injection of an excess of heparin 10 min after injection of protamine released as much lipoprotein lipase activity to plasma as in controls. The results in vivo differed from those obtained in model experiments in vitro. Protamine was able to almost quantitatively release both lipoprotein lipase and hepatic lipase from columns of heparin-agarose. The displacement was dependent on the total amount of protamine that had passed over the column, indicating that it was due to occupation by protamine of all available binding sites. Our results in vivo showed that the binding sites for lipoprotein lipase were not blocked as efficiently as those for hepatic lipase, indicating that the binding structures were not identical. It was concluded that the impaired turnover of lipoproteins by protamine probably was due to prevention of binding of the lipoproteins to endothelial cell surfaces rather than to impaired lipase function.

Animals↗
PubMed1994

The role of alpha 2M receptor/LRP in chylomicron remnant metabolism.

A strong candidate for the long-searched CR receptor might be the alpha 2MR/LRP. Presently, we are overseeing a whole series of in vitro experiments from different laboratories that show that LRP expresses all the features for being such a receptor protein. LRP is localized on the liver cell surface, as well as on most other animal cells. It recognizes apo E-enriched lipoproteins as beta-VLDL and CR. There is evidence that CR contain LPL and it has been demonstrated that LPL binds with high affinity to LRP. This has been shown in cell binding experiments with subsequent cross-linking and in direct assays on purified receptor protein. HL, which is expressed in liver cells and localized at the liver cell surface, is also able to bind to LRP. Moreover, LRP is found in endosomes and can mediate the uptake of beta-VLDL and CR. Further studies are necessary to evaluate its role in vivo as well as its regulation. The interplay between the different ligands of this large multifunctional receptor protein needs to be clarified. It should be emphasized here that, by describing LPL as a new mediator of CR uptake in the liver and by providing evidence for a direct interaction between LPL and LRP, the role of LRP in the remnant catabolism has become even more likely.

Amino Acid Sequence↗
PubMed1994

Interactions of lipoprotein lipase with the active-site inhibitor tetrahydrolipstatin (Orlistat).

Lipoprotein lipase (LPL) was rapidly inactivated by low concentrations of the active-site inhibitor tetrahydrolipstatin (THL). The presence of amphiphils (e.g. long-chain fatty acids) or of lipid/water interfaces (lipid emulsions) was required for inhibition to occur. Apolipoprotein CII increased the maximal inactivation rate constant by 1.8-fold in the presence of an emulsion of long-chain triacylglycerols, but had no effect in the presence of an emulsion of tributyrylglycerol. The fully inhibited enzyme had a ratio of THL/LPL of nearly 2, indicating that both subunits of the LPL homo-dimer bound THL. The THL-LPL complex was stable below pH 7.5. At higher pH reactivation occurred indicating that THL was slowly turned over by the enzyme. The apparent reactivation rate constant was increased about threefold by the presence of lipid/water interfaces. Sucrose density gradient centrifugation revealed that THL induces tetramerisation of LPL. This aggregation was reversible on reactivation of the inhibited enzyme. Binding to heparin was not affected by THL. In contrast, binding to lipid droplets and to lipoproteins was increased, indicating exposure of hydrophobic regions in the inhibited LPL. It is suggested that THL induces local conformational changes in LPL, which may involve opening of the putative surface lid structure which covers the active-site.

Animals↗
PubMed1994

The effects of cortisol on the regulation of lipoprotein lipase activity in human adipose tissue.

The influence of cortisol, in the presence of insulin, on the regulation of lipoprotein lipase (LPL) activity was studied in human adipose tissue, using a tissue incubation technique. Tissue pieces were preincubated for 3 days in a control medium containing insulin (7175 pmol/L), then incubated for 2 additional days in the control medium with and without cortisol (1000 nmol/L). After the 5 days of incubation, the levels of LPL messenger ribonucleic acid (mRNA), relative LPL synthesis, and LPL activity (total and heparin releasable) were studied. Cortisol exposure for 2 days increased all of the variables related to LPL. The average increase was 2.5-fold for LPL mRNA, 3.0-fold for relative LPL synthesis, 5.2-fold for total LPL activity, and 9.4-fold for heparin-releasable LPL activity compared to that in controls without cortisol. The results confirm previous findings that cortisol, in the presence of insulin, has a marked stimulatory effect on LPL activity in human adipose tissue in vitro. New data have been presented on the mechanisms of cortisol regulation of LPL activity. They involve both an increased level of LPL mRNA, leading to increased relative LPL synthesis, and additional posttranslational regulation.

Adipose Tissue↗
PubMed1994

Metabolism of emulsions containing medium- and long-chain triglycerides or interesterified triglycerides.

This study compares the clearing and metabolism of three different lipid emulsions. They had the same phospholipid emulsifier and similar particle sizes. In one (LLL) the core component was long-chain triglycerides (TG), the second (MMM/LLL) contained equal molar amounts of medium- and long-chain TG, the third (MLM) contained synthetic TG with medium-chain (M) fatty acids in the 1,3-positions and a long-chain (L) fatty acid in the 2-position. In model experiments with bovine lipoprotein lipase, the MMM component was hydrolyzed preferentially in the MMM/LLL emulsion so that the initial products were M fatty acids and M monoglycerides. The MLM emulsion, in contrast, gave M fatty acids and formation of L-MG (monoglyceride) throughout hydrolysis. For in vivo studies [3H]oleic acid was incorporated into the emulsion TG as marker for the long-chain component. After bolus injection to rats, the MMM/LLL and MLM emulsions were cleared more rapidly than the LLL emulsion. This was true at all TG loads studied (4-64 mg for a 200 g rat). The labeled oleic acid was oxidized somewhat more rapidly when administered in the MLM emulsion compared to the MMM/LLL emulsion. There were only slight differences in tissue distribution of label. Hence, differences in in vivo metabolism of the long-chain fatty acids were small compared to the marked differences in TG structure and in patterns of product release during in vitro lipolysis.

Animals↗