Binding of lipoprotein lipase to apolipoprotein B-containing lipoproteins

Kinetic and Related Determinants of Plasma Triglyceride Concentration in Abdominal Obesity: Multicenter Tracer Kinetic Study

OBJECTIVES

Patients with obesity and diabetes mellitus have increased risk of cardiovascular disease. A major cause is an atherogenic dyslipidemia related primarily to elevated plasma concentrations of triglyceride-rich lipoproteins. The aim of this study was to clarify determinants of plasma triglyceride concentration. We focused on factors that predict the kinetics of very-low density lipoprotein 1 (VLDL1) triglycerides.

APPROACH AND RESULTS

A multicenter study using dual stable isotopes (deuterated leucine and glycerol) and multicompartmental modeling was performed to elucidate the kinetics of triglycerides and apoB in VLDL1 in 46 subjects with abdominal obesity and additional cardiometabolic risk factors. Results showed that plasma triglyceride concentrations were dependent on both the secretion rate (r=0.44, P<0.01; r=0.45, P<0.01) and fractional catabolism (r=0.49, P<0.001; r=0.55, P<0.001) of VLDL1-triglycerides and VLDL1-apoB. Liver fat mass was independently and directly associated with secretion rates of VLDL1-triglycerides (r=0.56, P<0.001) and VLDL1-apoB (r=0.53, P<0.001). Plasma apoC-III concentration was independently and inversely associated with the fractional catabolisms of VLDL1-triglycerides (r=0.48, P<0.001) and VLDL1-apoB (r=0.51, P<0.001).

CONCLUSIONS

Plasma triglyceride concentrations in abdominal obesity are determined by the kinetics of VLDL1 subspecies, catabolism being mainly dependent on apoC-III concentration and secretion on liver fat content. Reduction in liver fat and targeting apoC-III may be an effective approach for correcting triglyceride metabolism atherogenic dyslipidemia in obesity.

Differences between group X and group V secretory phospholipase A(2) in lipolytic modification of lipoproteins

Secretory phospholipases A(2) (sPLA(2)s) are a diverse family of low molecular mass enzymes (13-18 kDa) that hydrolyze the sn-2 fatty acid ester bond of glycerophospholipids to produce free fatty acids and lysophospholipids. We have previously shown that group X sPLA(2) (sPLA(2)-X) had a strong hydrolyzing activity toward phosphatidylcholine in low-density lipoprotein (LDL) linked to the formation of lipid droplets in the cytoplasm of macrophages. Here, we show that group V sPLA(2) (sPLA(2)-V) can also cause the lipolysis of LDL, but its action differs remarkably from that of sPLA(2)-X in several respects. Although sPLA(2)-V released almost the same amount of fatty acids from LDL, it released more linoleic acid and less arachidonic acid than sPLA(2)-X. In addition, the requirement of Ca(2+) for the lipolysis of LDL was about 10-fold higher for sPLA(2)-V than sPLA(2)-X. In fact, the release of fatty acids from human serum was hardly detectable upon incubation with sPLA(2)-V in the presence of sodium citrate, which contrasted with the potent response to sPLA(2)-X. Moreover, sPLA(2)-X, but not sPLA(2)-V, was found to specifically interact with LDL among the serum proteins, as assessed by gel-filtration chromatography as well as sandwich enzyme-immunosorbent assay using anti-sPLA(2)-X and anti-apoB antibodies. Surface plasmon resonance studies have revealed that sPLA2-X can bind to LDL with high-affinity (K(d) = 3.1 nM) in the presence of Ca(2+). Selective interaction of sPLA(2)-X with LDL might be involved in the efficient hydrolysis of cell surface or intracellular phospholipids during foam cell formation.

VLDL-induced triglyceride accumulation in human macrophages is mediated by modulation of LPL lipolytic activity in the absence of change in LPL mass

Mixed dyslipidemia of phenotype IIB is characterized by elevated levels of very low density lipoprotein (VLDL)-1 and VLDL-2 subfractions and of low density lipoprotein (LDL), which are associated with premature formation of atherosclerotic plaques, characterized by the presence of lipid-rich macrophage foam cells. Lipoprotein lipase (LPL) is a key factor in mediating macrophage lipid accumulation and foam-cell formation from native VLDL particles. The action of macrophage-derived LPL in the induction of intracellular lipid accumulation from triglyceride-rich lipoprotein (TRL) subfractions (VLDL-1, VLDL-2) is, however, indeterminate, as is the potential role of VLDL-1 and VLDL-2 in modulating macrophage LPL expression. We evaluated the role of LPL in the interaction of type IIB VLDL-1 and VLDL-2 with human macrophages. Both VLDL-1 and VLDL-2 subfractions induced significant accumulation of triglyceride (9.8-fold, P<0.0001, and 4.8-fold, P<0.0001, respectively) and of free cholesterol content (1.4-fold, P<0.001, and 1.2-fold, P=0.02, respectively). Specific inhibition (90%) of the lipolytic activity of endogenous LPL by tetrahydrolipstatin (THL) in the presence of VLDL-1 or VLDL-2 resulted in marked reduction in cellular loading of both triglycerides (-89%, P=0.008, and -89%, P=0.015, respectively) and free cholesterol (-76%, P=0.02, and -55%, P=0.06 respectively). Furthermore, VLDL-1 and VLDL-2 induced marked increase in macrophage-derived LPL enzyme activity (+81%, P=0.002, and +45%, P=0.02), but did not modulate macrophage-derived LPL mRNA and protein expression; consequently, LPL specific activity was significantly increased from 1.6 mU/microg at baseline to 4.1 mU/microg (P=0.01) and 3.1 mU/microg (P=0.05), in the presence of VLDL-1 and VLDL-2, respectively. We conclude that type IIB VLDL-1 and VLDL-2 induce triglyceride accumulation in human monocyte-macrophages primarily via the lipolytic action of LPL, which may involve stabilization and activation of the macrophage-secreted enzyme, rather than via modulation of enzyme production.

Identification of factors regulating lipoprotein lipase catalyzed hydrolysis in rats with the aid of monoacid-rich lipoprotein preparations(1)

To identify the substrate specificity and regulatory factors in lipoprotein lipase (LPL) catalyzed hydrolysis of triacylglycerol-rich lipoprotein, monoacid-rich lipoproteins were used to study the kinetic parameters of LPL. Feeding growing rats with diets rich in palmitic acid (16:0), oleic acid (18:1) or linoleic acid (18:2) for 10 days increased the corresponding acid content in the triacylglycerols of the lipoproteins. Force-feeding the monoacid-rich triacylglycerols, particularly 16:0 or 18:1, increased the respective fatty acid content in both chylomicrons and VLDLs. Major apolipoproteins and lipid compositions were essentially similar among all lipoproteins differing in monoacid species, except for apo A-IV. The Vmax of LPL for 16:0-rich chylomicrons and VLDLs were higher than for 18:1- or 18:2-rich lipoproteins. Order parameter (S), an indicator of the surface fluidity of lipoproteins, decreased with the chain length and unsaturation of monoacid in similar manner as the Vmax. The Vmax of LPL increased linearly (P < 0.05) with an increase in either the palmitic acid content of the lipoprotein triacylglycerols or order parameter (S) of the lipoproteins. The order parameter (S) and Vmax of LPL were higher in 16:0 triacylglycerol emulsions with apo B than with 18:1 or 18:2 triacylglycerols. The apo A-IV in triacylglycerol emulsions stimulated Vmax of LPLs in the presence of apo B and apo C-II. The binding of apo A-IV to 16:0 triacylglycerol emulsions was higher than to other triacylglycerol emulsions. These findings suggest that lipoprotein catalysis by LPL is modulated by the 16:0 level in the lipoprotein triacylglycerol, which affects the surface fluidity and apo A-IV content of lipoproteins.

Lipoprotein lipase (LPL) strongly links native and oxidized low density lipoprotein particles to decorin-coated collagen. Roles for both dimeric and monomeric forms of LPL

Low density lipoprotein (LDL) and oxidized LDL are associated with collagen in the arterial intima, where the collagen is coated by the small proteoglycan decorin. When incubated in physiological ionic conditions, decorin-coated collagen bound only small amounts of native and oxidized LDL, the interaction being weak. When decorin-coated collagen was first allowed to bind lipoprotein lipase (LPL), binding of native and oxidized LDL increased dramatically (23- and 7-fold, respectively). This increase depended on strong interactions between LPL that was bound to the glycosaminoglycan chains of the collagen-bound decorin and native and oxidized LDL (kDa 12 and 5.9 nM, respectively). To distinguish between binding to monomeric (inactive) and dimeric (catalytically active) forms of LPL, affinity chromatography on heparin columns was conducted, which showed that native LDL bound to the monomeric LPL, whereas oxidized LDL, irrespective of the type of modification (Cu(2+), 2, 2'-azobis(2-amidinopropane)hydrochloride, hypochlorite, or soybean 15-lipoxygenase), bound preferably to dimeric LPL. However, catalytic activity of LPL was not required for binding to oxidized LDL. Finally, immunohistochemistry of atherosclerotic lesions of human coronary arteries revealed specific areas in which LDL, LPL, decorin, and collagen type I were present. The results suggest that LPL can retain LDL in atherosclerotic lesions along decorin-coated collagen fibers.

Preparation of chylomicrons and VLDL with monoacid-rich triacylglycerol and characterization of kinetic parameters in lipoprotein lipase-mediated hydrolysis in chickens

To identify the substrate specificity of lipoprotein lipase (LPL) for triacylglycerol-rich lipoproteins with monoacid-rich triacylglycerols, monoacid-rich lipoproteins were prepared and kinetic parameters of LPL were characterized. Male broiler chickens were fed 8 g/100 g fat diets differing only in the fat source: palm oil (tripalmitin-rich), olive oil (triolein-rich), safflower oil (trilinolein-rich) and linseed oil (trilinolenin-rich). After diets were fed for 3 d, chickens were starved for 2 d and then force-fed emulsions containing one of the monoacid-triacylglycerols: tripalmitin, triolein, trilinolein or trilinolenin. The triacylglycerols in chylomicrons and very low density lipoprotein (VLDL) of chickens force-fed tripalmitin, triolein or trilinolein contained the corresponding acid at more than 70% of total acids. Linolenic acid was incorporated into chylomicrons and VLDL to a lower extent (51.2 and 57.2%, respectively) in chickens force-fed trilinolein. Major apolipoproteins and lipid compositions were not significantly different among all lipoproteins isolated from chickens fed the different fats. Vmax of LPL was significantly higher (P < 0.05) for palmitic acid-rich chylomicrons and VLDL and decreased with increasing chain length and unsaturation of monoacid: 16:0>18:1>18:2>18:3. The electron spin resonance analysis, order parameter (S), decreased with monoacid chain length and unsaturation. In addition, the Vmax of LPL increased linearly (P < 0.01, r = 0. 912) with an increase in the palmitic acid content of the lipoprotein triacylglycerols. These findings suggest that lipoprotein catalysis by LPL is modulated by the palmitic acid content of the lipoprotein triacylglycerol, which affects the fluidity of lipoproteins.

Receptor and non-receptor mediated uptake of chylomicron remnants by the liver

The uptake of chylomicron remnants by rodent liver is mediated by proteins residing on the microvillous surface of hepatocytes and occurs in two steps. First, initial removal of the remnants from the blood occurs through binding to the low density lipoprotein (LDL) receptor via apo E and to hepatic lipase via polar lipids and proteins on the remnant surface. Second, chylomicron remnants are taken up into the cell mainly by the LDL receptor and follow the classical receptor-mediated pathway of endocytosis. The LDL receptor-related protein (LRP), which binds weakly to chylomicron remnants via apo E, does not appear to have a significant role in the initial removal process. The remnant particles can, however, be enriched with proteoglycan-bound apo E present on hepatocytic microvilli, which increases their affinity for LRP to the extent that they are subject to endocytosis by this receptor, particularly when the LDL receptor is deficient or down-regulated. Hepatic lipase can also mediate binding of remnants to LRP, for which it has high affinity. Lipolysis of remnant lipids by hepatic lipase may promote but is not required for interaction of remnants with the endocytic receptors. Proteoglycan-bound hepatic lipase may also mediate endocytosis of chylomicron remnants independent of apo E, so that hepatic catabolism of these particles is not completely dependent upon this apoprotein. Available data from experiments in vivo thus indicate redundancy of both steps of hepatic uptake of chylomicron remnants, consistent with the centrality of this process in nutrient delivery.

Mild oxidation of lipoproteins increases their affinity for surfaces covered by heparan sulfate and lipoprotein lipase

Lipoprotein lipase (LPL) is present in cells involved in development of atherosclerosis (endothelial cells, smooth muscle cells, and macrophages). A direct involvement of LPL in atherogenesis has been suggested. Previously we used the surface plasmon resonance technique to study the interaction of lipoproteins with surfaces covered by heparan sulfate proteoglycans (HSPG) and LPL [A. Lookene et al. (1997) Biochemistry 36, 5267-5275]. The binding was much increased by the presence of LPL. Here we demonstrate that mild oxidation of low-density-lipoprotein (LDL) and very-low-density lipoprotein (VLDL) in vitro increases their binding to surfaces covered by HSPG and LPL, while extensive oxidation decreases it. Similar results were obtained with a lipid emulsion (Intralipid), indicating that oxidation-induced changes of the lipid part could explain the effects. LPL increased binding and uptake of the mildly oxidized (compared to nonoxidized) LDL by THP-I monocyte-derived macrophages. Our studies indicate that LPL has the highest affinity for mildly oxidized LDL and support its involvement in development of atherosclerosis.

Characterization of a new case of autoimmune type I hyperlipidemia: long-term remission under immunosuppressive therapy

Only a few cases of type I hyperlipidemia occurring in patients with autoimmune disease have been reported. We describe the case of a 35-yr-old woman suffering from severe type I hyperchylomicronemia. A combination of various hypolipidemic treatments, including strict hypolipidemic dietary therapy and administration of fibrates or n-3 fatty acids, was inefficient. Because of a history of familial autoimmunity, we introduced an immunosuppressive therapy that resulted in consistent long term and stable remission. Two attempts to reduce the immunosuppressor dose resulted in major relapses. To explain the defect of chylomicron hydrolysis, we investigated the postheparin plasma lipase activities. Hepatic triglyceride lipase activity was normal, whereas that of lipoprotein lipase (LPL) was reduced to about 30% of normal. Immunosuppressive therapy resulted in a complete and durable normalization of LPL activity. Using Western blot analysis, we found in the plasma of the patient a circulating IgG specifically directed against LPL, which became undetectable during immunosuppressive therapy. Western blot analysis revealed that the whole circulating anti-LPL autoantibody was bound to chylomicrons. Proteins extracted from patient's chylomicrons were able to induce a dose-related inhibition of LPL activity in vitro, whereas that of hepatic triglyceride lipase remained unchanged. These data constitute the first description of autoimmune hyperchylomicronemia due to an exclusive defect of LPL activity, and they show that a complete remission has been obtained after immunosuppressive therapy. Finally, our finding that the anti-LPL autoantibody is bound to chylomicrons emphasizes their previously unrecognized ability to transport LPL, already described for other lipoprotein fractions.

Effect of lipid composition on lipoprotein lipase activity measured by a continuous fluorescence assay: effect of cholesterol supports an interfacial surface penetration model

The breakdown of normal substrates by lipases requires an interfacial binding step prior to hydrolysis. Interfacial binding and subsequent hydrolysis will be affected by the lipid components and hence physical properties of the substrate surface. In order to investigate in detail the effect of lipid structure on the activity of lipoprotein lipase (LPL), triolein-containing emulsion particles of defined composition have been used as substrates. In addition, lipase activity has been measured using a continuous fluorescence displacement assay that monitors the release of long-chain fatty acids as an alternative to normal radiochemical assays. Using this fluorescence assay, rates of hydrolysis of triolein were the same as when using a standard radiochemical assay under identical conditions. Activation by apolipoprotein CII was very similar by both methods; however, the extent of activation (2-3-fold) was less than has been reported previously using different assay conditions. In order to investigate the effect of cholesterol on LPL activity, emulsion particles were prepared in which the cholesterol/egg-phosphatidylcholine ratio was increased up to a 1:1 molar ratio. A pronounced stimulatory effect of cholesterol was observed under these assay conditions, with up to a 5-fold increase in rate compared with emulsion particles without cholesterol. Since high molar ratios of cholesterol are reported to exclude triacylglycerol from the phospholipid surface [Spooner and Small (1987) Biochemistry 26, 5820-5825], these results are not consistent with a mechanism involving LPL hydrolysis of surface triacylglycerol. Instead, they support an interfacial penetration model, allowing the enzyme's active site direct access to triacylglycerol in the lipoprotein core. Perturbation of the surface phospholipid monolayer of the emulsion particle as a result of hydrolysis by Naja naja phospholipase A2 resulted in a 10-fold activation of LPL, providing further support for an interfacial penetration model. The stimulatory effect of apolipoprotein CII was not modulated by modification of the interface with cholesterol.