Stimulation Of Lipoprotein Lipase Activity Research Articles

Effects of Gemfibrozil and Other Fibric Acid Derivatives on Blood Lipids and Lipoproteins

Fibric acid derivatives (FADs) are a class of drugs that have been shown to reduce the production of very low-density lipoprotein (VLDL) while enhancing VLDL clearance due to the stimulation of lipoprotein lipase activity. The drugs can reduce plasma triglyceride levels while raising high-density lipoprotein (HDL) cholesterol levels. Their effects on low-density lipoprotein (LDL) cholesterol levels are less marked and more variable. There is evidence that oral gemfibrozil (Lopid, Parke-Davis, Morris Plains, NJ) can reduce the risk of serious coronary events, specifically in those patients who had elevations of both LDL cholesterol levels and total plasma triglyceride levels with lower HDL cholesterol levels. Newer FADs (bezafibrate, ciprofibrate, fenofibrate) have been shown to have greater efficacy in reducing LDL cholesterol than gemfibrozil but, in general, these drugs are not as effective as the other primary drugs used to lower LDL levels. The FADs are also used to treat adult patients with very high levels of triglycerides who have pancreatitis and whose disease cannot be managed with dietary therapy. The FADs are well tolerated, with dyspepsia and abdominal pain the most common adverse effects. A small risk of cholelithiasis exists with these drugs, and caution should be used when combining these drugs with HMG-CoA reductase inhibitors because the combination increases the incidence of hyperlipidemic myositis and rhabdomyolysis.

Pharmacological Intervention for Altering Lipid Metabolism

Pharmacological intervention for altering plasma levels of lipoproteins is usually aimed at reducing atherogenesis and preventing coronary heart disease (CHD). Drug therapy should be attempted only after other nonpharmacological methods (such as elimination of smoking, weight reduction and exercise) have been tried. An overview of the metabolism of low density lipoprotein (LDL) and high density lipoprotein (HDL) particles is the basis of this paper. Various sites suitable for pharmacological intervention are identified. LDL metabolism can be altered at 2 potential sites, with a consequent reduction in the plasma level of this atherogenic lipoprotein. Hydroxymethylglutaryl coenzyme A (HMG CoA) reductase inhibitors (such as lovastatin) and cation-exchange resins (e.g. cholestyramine) reduce LDL levels by stimulating the hepatic synthesis of apolipoprotein (apo) B,E receptors. Very low density lipoprotein (VLDL) secretion is inhibited by nicotinic acid (niacin) and gemfibrozil, leading to a secondary decrease in LDL production from VLDL. Probucol also reduces the LDL concentration and inhibits the oxidative modification of LDL. Gemfibrozil and other fibrates stimulate lipoprotein lipase activity, thereby decreasing VLDL concentration. Reduction of the LDL concentration is effective in reducing CHD incidence, whether this is achieved by stimulation of catabolism or inhibition of production of the lipoprotein. In contrast, the mechanism of raising plasma HDL-cholesterol levels is probably relevant to the potential clinical benefits associated with drug therapy. Gemfibrozil and cholestyramine stimulate synthesis of apoprotein A1, the major protein constituent of HDL particles. Both drugs have been shown to reduce the incidence of CHD in clinical trials, via mechanisms that are related in part to their HDL-raising activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in lipoprotein kinetics during therapy with fenofibrate and other fibric acid derivatives

The fibric acid derivatives, including fenofibrate, significantly reduce very low-density lipoprotein triglyceride concentrations by stimulating lipoprotein lipase activity, thereby increasing very low-density lipoprotein catabolism. These agents may also reduce the hepatic secretion of nascent very low-density lipoprotein, but this effect is less consistent. Effects on low-density lipoprotein metabolism appear to depend upon the lipid disorder present before therapy. If hypertriglyceridemia and normal or low low-density lipoprotein levels are present, fibrate therapy is associated with a rise in low-density lipoprotein levels. This is due to a decreased fractional catabolism of low-density lipoprotein from an unusually high clearance to a more normal value. Treating pre-existing hypercholesterolemia usually results in a significant decrease in low-density lipoprotein levels. In this disorder, there is a demonstrable increase in low-density lipoprotein receptor-mediated clearance. It is not known at which site these drugs act to increase low-density lipoprotein receptor function in the latter patients. Some studies suggest that fibrate therapy increases high-density lipoprotein apolipoprotein AI production, but how this occurs has not been defined.

Lipoprotein lipase activity in the flight muscle of Locusta migratoria and its specificity for haemolymph lipoproteins

Lipoprotein lipase activity in flight muscle homogenates of Locusta migratoria was measured, using natural radiolabelled lipoproteins as substrates. The flight specific lipoprotein A + (or low density lipophorin) stimulated lipoprotein lipase activity several-fold compared to the resting lipoprotein A y (or high density lipophorin). However, with the high mol. wt lipoprotein fraction O AKH as a substrate, lipase activity was even doubled compared to lipoprotein A +. Lipase activity was not increased in flight muscle homogenates of insects which had flown. Neither adipokinetic hormone, nor octopamine had any direct effect on lipoprotein lipase activity. Aspects of hormonal regulation and apoprotein activation of the locust flight muscle lipoprotein lipase are discussed and compared with the model for vertebrate lipoprotein lipase.

Effects of prolactin, progesterone and ovariectomy on metabolic activities and insulin receptors in the mammary gland and adipose tissue during extended lactation in the rat.

Prolactin implants prevented the decline in milk yield and the resumption of oestrous cycles which occurred between days 22 and 28 in untreated lactating rats. Ovariectomy and progesterone implants only partially prevented the decline in milk yield despite preventing the occurrence of oestrous cycles. All three treatments increased total RNA content of the mammary gland compared with controls. In untreated rats there were no changes in mammary DNA content or the number of insulin receptors whereas lipoprotein lipase (LPL) activity decreased significantly during the declining phase of lactation. In contrast, the number of insulin receptors, LPL activity and glucose incorporation into lipid increased in adipose tissue. Prolactin prevented the increase in insulin receptors and lipid synthesis and significantly decreased LPL activity in adipose tissue. Progesterone stimulated LPL activity in the mammary gland and also prevented the increase in lipid synthesis and insulin receptors in adipose tissue but was without effect on LPL activity whereas ovariectomy stimulated LPL activity in the mammary gland but prevented only the increase in the number of insulin receptors in adipose tissue. The results show that raising the serum prolactin concentration can prevent the decline in milk yield during extended lactation and whilst part of this effect may be due to a direct effect on the mammary gland and an indirect effect due to inhibition of oestrous cycles, prolactin may also produce part of its effect on milk synthesis by inhibiting competitive metabolic processes in tissues such as adipose tissue.

Influence of bezafibrate, fenofibrate, nicotinic acid and etofibrate on plasma high-density lipoprotein levels

Increased levels of plasma high-density lipoprotein (HDL) cholesterol have been described after exposure to different dietary (such as ethanol) and environmental (such as chlorinated pesticides) factors as well as intense physical training. Several lipid-lowering drugs have also been shown to elevate HDL cholesterol. These findings are of particular interest, because adult levels of the putatively protective lipoprotein fraction appear not to be regulated by genetic factors. Four hypolipidemic drugs are herein considered, all of which reportedly elevate HDL cholesterolemia. Two of them, bezafibrate and fenofibrate, belong to the class of “fibrate” derivatives; nicotinic acid (NA) and etofibrate (E) show some similarities, E being the chemical association between nicotinic acid and clofibrate. In most studies, bezafibrate (B), with a very short half-life (2.1 hours), has induced an HDL cholesterol elevation of about 20%, both in types II and IV hyperlipidemic patients; it is controversial whether HDL 2 rather than HDL 3 levels are elevated by B. Fenofibrate (F) shows prolonged plasma half-life in man (>21 hours) and markedly increases HDL cholesterol levels, particularly in type IV patients; data are less homogeneous in type IIA and IIB cohorts. It appears that F may increase apoprotein C-II levels in very low density lipoproteins, possibly an index of stimulated lipoprotein lipase activity. Both B and F may behave as “catabolic agents,” stimulating the breakdown of triglyceride-rich lipoproteins and the transfer of surface components (free cholesterol) to HDL. A different case may be that of NA, which prevents the decrease of HDL cholesterol induced by carbohydrate-rich diets in volunteers. NA reduces the catabolism of HDL and markedly increases (300% or more) the HDL 2 HDL 3 ratio in both volunteers and type IV patients, but not in type IIB patients. Whether, in addition to a “sparing” action on HDL, NA may also increase HDL levels by stimulating lipoprotein lipase activity is being disrupted. Finally, E may increase HDL cholesterol by 10% or more while exerting significant hypocholesterolemic activity. This compound is of interest because it may stimulate the release of prostacyclin from the arterial wall; prostacyclin may be derived from HDL phospholipids, unusually rich in arachidonic acid. This observation may offer another clue to the understanding of the mode of arterial protection exerted by HDL.

Stimulation of lipoprotein lipase activity of rat adipose tissue and post-heparin plasma by N6-(phenylisopropyl)adenosine

Adenosine has profound effects on adipose tissue metabolism. It is an inhibitor of 13-adrenergic stimulation of cyclic AMP accumulation [1,2] and lipolysis [2-101 in tire isolated rat fat cell. It also opposes the effects of adrenaline on phosphate uptake [ 11 ]. In vivo, tile nucleoside has been reported to lower plasma free fatty acid and glycerol concentrations [8]. The effects of adenosine are thought to be mediated by two membrane receptors, one requiring an intact ribose (R-site) and tile other one an intact purine moiety [ 12-14] (P-site). Adenosine is rapidly metabolized but its effects on the R-site [12-14] are shared by purine-substituted analogs such as N6-(phenylisopro pyl)adenosine that are not deaminated by the enzyme adenosine deaminase (EC 3.5.4.4) [15]. Lipoprotein lipase (EC 3.1.1.3.4) is the enzyme responsible for the hydrolysis and uptake of triglycerides from circulating chylomicrons and very low density lipoproteins [16-19] . The enzyme is thought to be located on the endothelial surface of the capillary bed of extrahepatic tissues and it can be released into the circulation by intravenous injection of heparin [ 16-20] . Another heparin-releasable ectoenzyme is probably located on the outer surface of the endothelial cells of the liver and it is therefore called hepatic lipase [20]. This enzyme has been suggested to hydrolyze high density lipoprotein phospholipids and triglycerides [21 ]. This work was undertaken to find out ifN6-(phenyl isopropyl)adenosine has effects on lipoprotein lipase activity and, consequently, on the uptake of circulating triglycerides into tissues. Male Lewis albino rats ( 150 -250 g body wt) were used. They were fed ad libitum with a standard chow until injected intraperitoneally with 50-300/al isotonic NaC1 with/without N6-(phenylisopropyl)adenosine, at which time food was withheld. At the times indicated, the animals were anaesthetized with ether and blood was drawn by cardiac puncture into heparinized syringes. For post-heparin plasma lipase assays, blood was drawn 2 min after intravenous injection ofheparin (2000 units/kg body wt). The blood was centrifuged and the resulting plasma was stored at 2 0 ° C until assayed. Free glycerol was estimated enzymatically by using the commercial kit no. 125 032 of BoehringerMannheim GmbH. Tile assays for porst-heparin plasma lipases and the antibody to rat hepatic lipase have been described in [22]. The method used to measure heparin-releasable lipoprotein lipase activity of adipose tissue will be presented in detail in the text. N6-(phenylisopropyl)Adenosine was a gift from Dr Harald Stork of Boehringer-Mannheim GmbH. Glyceroltri-[14C] oleate (49 mCi/mmol) was from the Radiochemical Centre (Amersham). Heparin was from Medica (ttelsinki). All other reagents were from Sigma (St Louis MO).

Guinea pig very low density lipoproteins are a good substrate for lipoprotein lipase

In contrast to plasma from most other animals, guinea pig plasma causes little or no stimulation of lipoprotein lipase activity. Very low density lipoproteins (VLDL) isolated by ultracentrifugation of guinea pig serum caused a definite stimulation of lipase activity, whereas the infranatant inhibited the activity. Gel filtration in 5 M guanidinium hydrochloride of delipidated VLDL demonstrated that the activation was caused by a low molecular weight protein. The VLDL themselves were hydro lized at similar rates as human VLDL both by guinea pig and by bovine lipoprotein lipases. Thus, guinea pig VLDL contain an activator for lipoprotein lipase analogous to that in other animals and there is enough of the activator to support rapid hydrolysis of the VLDL lipids by the lipase.

Activation of adipose tissue lipoprotein lipase by lipoprotein fractions from normals and patients with type V hyperlipoproteinemia

The effects of the main lipoprotein density classes on the human adipose tissue lipoprotein lipase activity were studied. A dose-dependent stimulation of lipoprotein lipase activity was obtained for HDL and, to a lesser extent, for VLDL on a constant weight basis. LDL exerted virtually no effect. At higher concentrations, HDL as well as VLDL inhibited the stimulated lipolytic activity. In type V hyperlipoproteinemia, the stimulating effect of VLDL and of HDL was significantly lower, whereas the inhibiting action of HDL was markedly increased.

The Role of Calcium in Insulin Action - V. Importance of Cyclic Guanosine 3':5' Monophosphate and Calcium Ions in Insulin Stimulation of Lipoprotein Lipase Activity and Protein Synthesis in Adipose Tissue

The Role of Calcium in Insulin Action - V. Importance of Cyclic Guanosine 3':5' Monophosphate and Calcium Ions in Insulin Stimulation of Lipoprotein Lipase Activity and Protein Synthesis in Adipose Tissue

Specific conditions for enhancement of lipoprotein lipase activity by platelets

Homogenates of human blood platelets, but not of red blood cells, have been found to stimulate lipoprotein lipase activity only when assayed against an emulsified triglyceride substrate sonicated for a short period of time. The degree of stimulation was inversely related to the time of sonication of the substrate. Using chylomicrons as substrate no stimulation of lipoprotein lipase activity by platelet homogenate was seen.

Activation of lipoprotein lipase. Evaluation of calcium, magnesium, and ammonium as cofactors.

Lipoprotein lipase (LPL) from rat heart acetone powders has been reported to depend on the presence of NH 4 + , calcium, or other divalent cations for optimal activity. In addition, the enzyme will not hydrolyze an artificial triglyceride emulsion unless it is converted to an active substrate by the addition of very low density lipoproteins, high density lipoproteins (HDL), or certain peptides contained in these complexes. We observed earlier that HDL that had been extensively dialyzed against 0.005M ethylenediaminetetraacetic acid (EDTA), which probably removed most of the divalent cations, produced little lipolytic activity in LPL from rat heart acetone powders. This observation led us to evaluate the cation requirements for LPL. LPL was prepared in soluble form from rat heart acetone powders. HDL was isolated from rat serum by ultracentrifugation. The assay system contained a phospholipid-stabilized triglyceride emulsion as substrate. The addition of the whole rat serum to the LPL assay system produced high LPL activity. The addition of dialyzed HDL isolated from the same volume of serum produced low LPL activity. The addition of calcium in physiological concentration to the assay containing HDL increased LPL activity markedly. The high enzyme activity produced by both serum and HDL plus calcium was inhibited by ethyleneglycol bis(β-aminoethylether)-N,N'-tetraacetic acid (EGTA), a specific calcium chelating agent. The addition of NH 4 + or magnesium at several concentrations did not replace calcium in stimulating LPL activity. We conclude that calcium has a cofactor function in the LPL lipolytic reaction and that the optimal enzyme reaction rate is attained in the presence of calcium concentrations found in serum.

A specific apoprotein activator for lipoprotein lipase

These studies were designed to determine which of the apoproteins of high density lipoprotein (HDL) function as the cofactor for lipoprotein lipase (LPL). ApoLP-gln and apoLP-thr, the major HDL apoproteins, as well as apoLP-val, a minor apoprotein constituent, are inactive as cofactors even in the presence of phospholipid. ApoLP-ala, another minor constituent, is inactive alone but in the presence of phospholipid stimulates lipase activity twofold. Only apoLP-glu is able to stimulate LPL activity in the absence of phospholipid and, in the presence of phospholipid, increases activity twelvefold over baseline levels. It is possible that apoLP-glu and perhaps apoLP-ala are obligatory “co-factors” for the hydrolytic step required for normal clearing of triglyceride from the plasma.

Activation of lipoprotein lipase. Comparative study of man and other mammals.

Injection of heparin into a number of animal species releases lipoprotein lipase into the circulation. We have studied the effect of heparin added in vitro to the lipase activity in post-heparin serum from six mammalian species. With the exception of man, all animals were studied under anesthesia. Our results demonstrate that only post-heparin serum from the rat developed increasing lipoprotein lipase activity when increasing concentrations of heparin were added to the assay system. Heparin decreased activity in the other species. These results prompted us to test the effect of adding rat serum to post-heparin serum from the other species in the presence of increasing concentrations of heparin. Rat serum stimulated lipoprotein lipase activity markedly. In guinea pigs, post-heparin serum activity increased 2,700% at a heparin concentration of 1.0 U/ml in the assay system. This effect may be related to the extremely low level of high-density lipoprotein in the guinea pig and the presence of a unique high-density lipoprotein in the rat.

Influence of heparin on the removal of serum lipoprotein lipase by the perfused liver of the rat

Isolated rat livers were perfused with whole rat blood containing postheparin lipoprotein lipase (LPL) activity. LPL activity disappeared rapidly from the perfusate; the extraction ratio (portal vein-hepatic vein difference) was 0.70 for all time periods studied. Control experiments established that the disappearance of LPL was not due to non-specific inactivation in the apparatus or to the release of an inhibitory by the liver. The addition of heparin to the perfusate in suitable concentration (4 units/ml) almost completely blocked the disappearance of LPL activity from the perfusate. In addition to the perfusion experiments, we studied the effect of heparin on LPL activity when added to the LPL assay system. When heparin was added to the assay system containing fresh postheparin serum from rats, it stimulated LPL activity by about 70%. When heparin was added to postheparin serum which had been perfused through the liver, it stimulated LPL activity over 200%, but it did not restore LPL to its preperfusion value. These observations are compatible with a two-step inactivation system for LPL by the liver. The first step may involve a dissociation of a heparin-apoenzyme complex followed by destruction of the heparin. The second step may involve the removal of the apoenzyme of LPL.

Effect of Glucose and Fructose Administration on Lipid Metabolism in the Rat

The effect of glucose and fructose on serum triglycerides was studied after prolonged feeding or under acute loads. In the rat, administration of fructose caused hypertriglyceridemia, but no change in serum triglyceride was found in the guinea pig in which fructose is said to be absorbed mostly in the form of glucose. This finding, as well as the low yield of labeled triglyceride in the chyle after fructose-14C administration, and the lack of stimulation of lipogenesis in the intestine indicate that the absorption of unchanged fructose is operative in the induction of hypertriglyceridemia. In the liver more fructose than glucose was converted to triglycerides and secretion of triglycerides into the serum was higher after fructose than after glucose administration to Triton-treated rats. In addition, fructose, unlike glucose, did not stimulate lipoprotein lipase activity in adipose tissue. The sequence of events occurring in the rat after fructose feeding could be summarized as follows: When owing to low activity of glucose 6-phosphatase in the intestine, fructose is absorbed as such into the portal circulation and reaches the liver, it is converted to α-glycerophosphate. At the same time no repression of the outflow of free fatty acid from adipose tissue occurs, leading to increased triglyceride formation in the liver and its secretion into the serum. As feeding of fructose does not induce lipoprotein lipase activity in adipose tissue the egress of triglyceride from the serum and thus the homeostatic regulation of triglyceride levels is impaired leading to its accumulation in the blood stream.

Uptake of chylomicron lipid by rat liver parenchymal cells in vivo.

THERE are two theories about the uptake of chylomicron triglyceride by the liver. The widely held view that chylomicrons are removed intact from the circulation1–3 has been challenged by Felts and Mayes4. They propose that the triglycerides are not taken up directly by the liver but are hydrolysed elsewhere by lipoprotein lipase, the resulting non-esterified fatty acids (NEFA) being taken up by the liver cells. In vivo evidence that intact chylomicrons are removed by the liver has been presented earlier5 but was open to two objections4,6. First, heparin was used as an anticoagulant in the collection of the chylomicrons and sufficient could have remained after washing to stimulate lipoprotein lipase activity in the circulation. Second, studies on whole liver tissue may not give a true picture of events occurring in the parenchymal cells themselves, since trapping of chylomicrons in the tissue spaces does occur both in vivo7 and in perfusion experiments6.