Lipoprotein lipase-mediated uptake and degradation of low density lipoproteins by fibroblasts and macrophages.

Lipoprotein lipase (LPL), the rate limiting enzyme for hydrolysis of lipoprotein triglyceride, also mediates nonenzymatic interactions between lipoproteins and heparan sulfate proteoglycans. To determine whether cell surface LPL increases LDL binding to cells, bovine milk LPL was added to upregulated and nonupregulated human fibroblasts along with media containing LDL. LDL binding to cells was increased 2-10-fold, in a dose-dependent manner, by the addition of 0.5-10 micrograms/ml of LPL. The amount of LDL bound to the cells in the presence of LPL far exceeded the capacity for LDL binding via the LDL receptor. Treatment of fibroblasts with heparinase and heparitinase resulted in a 64% decrease in LPL-mediated LDL binding. Compared to studies performed without LPL, more LDL was internalized and degraded in the presence of LPL, but the time course was slower than that of classical lipoprotein receptor mediated pathways. In LDL receptor negative fibroblasts, LPL increased surface bound LDL > 140-fold, intracellular LDL > 40-fold, and LDL degradation > 6-fold. These effects were almost completely inhibited by heparin and anti-LPL monoclonal antibody. LPL also increased the binding and uptake by fibroblasts of apolipoprotein-free triglyceride emulsions; binding was increased > 8-fold and cellular uptake was increased > 40-fold with LPL. LPL increased LDL binding to THP-1 monocytes, and increased LDL uptake (4.5-fold) and LDL degradation (2.5-fold) by THP-1 macrophages. In the absence of added LPL, heparin and anti-LPL monoclonal antibodies decreased LDL degradation by > 40%, and triglyceride emulsion uptake by > 50%, suggesting that endogenously produced LPL mediated lipid particle uptake and degradation. We conclude that LPL increases lipid and lipoprotein uptake by cells via a pathway not involving the LDL receptor. This pathway may be important for lipid accumulation in LPL synthesizing cells.     

Lipolysis Produces Changes in the Immunoreactivity and Cell Reactivity of Very Low Density Lipoproteins

Smaller very low density lipoprotein (VLDL) remnants interact more readily with tissues than do larger "intact" VLDL. This may be related to changes in the availability of VLDL apoproteins on the surface of the lipoproteins. To test this hypothesis VLDL were incubated at 37 degrees C with bovine milk lipase (LPL), and the abilities of LPL-treated VLDL preparations to compete with (125)I-low density lipoproteins (LDL) for interaction with cultured normal human fibroblasts were measured. At the same time, the immunologic activities of these preparations were also tested by double antibody radioimmunoassay. Triglyceride (TG) contents of VLDL fell by 30-90% during incubation with LPL and, on zonal ultracentrifugation, VLDL of faster Svedberg unit of flotation (S(f1.063)) rates (>150) were gradually converted to smaller VLDL with lower S(f) rates (21-60). LPL-treated VLDL competed two to five times more effectively with (125)I-LDL for binding to cellular receptors than did control VLDL. Control VLDL incubated with heat-inactivated LPL at 37 degrees C, or with active LPL at 4 degrees C had unaltered cell reactivities and TG contents compared with VLDL incubated without any enzyme. The direct uptake and degradation of LPL-treated VLDL was also assessed by using VLDL (125)I-labeled in apoprotein (Apo)B. LPL-treated VLDL-(125)I-ApoB were taken up and degraded by fibroblast at greater rates than were control VLDL-(125)I-ApoB. Thus, hydrolysis of VLDL lipids was accompanied by an increased ability of VLDL to interact with fibroblasts. The immunoreactivity of ApoB in the same VLDL preparations, expressed as the "apparent ApoB contents" of LPL-treated VLDL, increased by 10-50% (P < 0.02) in those assays that contained anti-LDL antisera, but the ApoB of control VLDL remained constant. However, assays that contained antisera directed against ApoB isolated from VLDL did not distinguish between LPL-treated and control VLDL. Thus, VLDL lipid hydrolysis was accompanied by changes in the immunoreactivity of VLDL-ApoB, which probably reflect changes in the disposition of ApoB on the surface of VLDL. The altered disposition of ApoB on VLDL "remnants" may be related to their enhanced interaction with cells.     

Lipoprotein lipase enhances binding of lipoproteins to heparan sulfate on cell surfaces and extracellular matrix.

Lipoprotein lipase enhances binding at 4 degrees C of human plasma lipoproteins (chylomicrons, VLDL, intermediate density lipoprotein, LDL, and HDL3) to cultured fibroblasts and hepG-2 cells and to extracellular matrix. Heparinase treatment of cells and matrix reduces the lipoprotein lipase enhanced binding by 90-95%. Lipoprotein lipase causes only a minimal effect on the binding of lipoproteins to heparan sulfate deficient mutant Chinese hamster ovary cells while it promotes binding to wild type cells that is abolished after heparinase treatment. With 125I-LDL, lipoprotein lipase also enhances uptake and proteolytic degradation at 37 degrees C by normal human skin fibroblasts but has no effect in heparinase-treated normal cells or in LDL receptor-negative fibroblasts. These observations prove that lipoprotein lipase causes, predominantly, binding of lipoproteins to heparan sulfate at cell surfaces and in extracellular matrix rather than to receptors. This interaction brings the lipoproteins into close proximity with cell surfaces and may promote metabolic events that occur at the cell surface, including facilitated transfer to cellular receptors.     

Lipoprotein metabolism during acute inhibition of lipoprotein lipase in the cynomolgus monkey.

To clarify the role of lipoprotein lipase (LPL) in the catabolism of nascent and circulating very low density lipoproteins (VLDL) and in the conversion of VLDL to low density lipoproteins (LDL), studies were performed in which LPL activity was inhibited in the cynomolgus monkey by intravenous infusion of inhibitory polyclonal or monoclonal antibodies. Inhibition of LPL activity resulted in a three- to fivefold increase in plasma triglyceride levels within 3 h. Analytical ultracentrifugation and gradient gel electrophoresis demonstrated an increase predominantly in more buoyant, larger VLDL (Sf 400-60). LDL and high density lipoprotein (HDL) cholesterol levels fell during this same time period, whereas triglyceride in LDL and HDL increased. Kinetic studies, utilizing radiolabeled human VLDL, demonstrated that LPL inhibition resulted in a marked decrease in the catabolism of large (Sf 400-100) VLDL apolipoprotein B (apoB). The catabolism of more dense VLDL (Sf 60-20) was also inhibited, although to a lesser extent. However, there was a complete block in the conversion of tracer in both Sf 400-100 and 60-20 VLDL apoB into LDL during LPL inhibition. Similarly, endogenous labeling of VLDL using [3H]leucine demonstrated that in the absence of LPL, no radiolabeled apoB appeared in LDL. We conclude that although catabolism of dense VLDL continues in the absence of LPL, this enzyme is required for the generation of LDL.     

Apolipoprotein B metabolism in subjects with deficiency of apolipoproteins CIII and AI. Evidence that apolipoprotein CIII inhibits catabolism of triglyceride-rich lipoproteins by lipoprotein lipase in vivo.

Previous data suggest that apolipoprotein (apo) CIII may inhibit both triglyceride hydrolysis by lipoprotein lipase (LPL) and apo E-mediated uptake of triglyceride-rich lipoproteins by the liver. We studied apo B metabolism in very low density (VLDL), intermediate density (IDL), and low density lipoproteins (LDL) in two sisters with apo CIII-apo AI deficiency. The subjects had reduced levels of VLDL triglyceride, normal LDL cholesterol, and near absence of high density lipoprotein (HDL) cholesterol. Compartmental analysis of the kinetics of apo B metabolism after injection of 125I-VLDL and 131I-LDL revealed fractional catabolic rates (FCR) for VLDL apo B that were six to seven times faster than normal. Simultaneous injection of [3H]glycerol demonstrated rapid catabolism of VLDL triglyceride. VLDL apo B was rapidly and efficiently converted to IDL and LDL. The FCR for LDL apo B was normal. In vitro experiments indicated that, although sera from the apo CIII-apo-AI deficient patients were able to normally activate purified LPL, increasing volumes of these sera did not result in the progressive inhibition of LPL activity demonstrable with normal sera. Addition of purified apo CIII to the deficient sera resulted in 20-50% reductions in maximal LPL activity compared with levels of activity attained with the same volumes of the native, deficient sera. These in vitro studies, together with the in vivo results, indicate that in normal subjects apo CIII can inhibit the catabolism of triglyceride-rich lipoproteins by lipoprotein lipase.     

Evaluation of the roles of lipoprotein lipase and hepatic lipase in lipoprotein metabolism: in vivo and in vitro studies in man

Abstract. The roles of lipoprotein lipase (LPL) and hepatic lipase in very low density lipoprotein (VLDL) and VLDL remnant metabolism were investigated by (1) in vivo studies where the kinetics of VLDL-apo B removal were measured in patients with non-functioning lipoprotein lipase systems, and (2) in vitro studies where the relative capacities of hepatic lipase and LPL to hydrolyse the triglyceride (TG) of different lipoprotein substrates was measured. The results indicated that VLDL-apo B removal was not impaired in patients with non-functional LPL, nor was there any apparent abnormality in the conversion of VLDL-apo B to intermediate- (IDL) and low (LDL) density lipoprotein-apo B. Post-heparin plasma hepatic lipase activity against VLDL was normal in these subjects. Purified normal hepatic lipase had a similar K_m for VLDL-TG hydrolysis (1.57 mmol/l) to that of LPL (1.49 mmol/l). However, at equal lipoprotein TG concentration, hepatic lipase had increasing activity with lipoproteins of decreasing particle size, in the order chylomicrons ≪ VLDL of Sf 100–400 < VLDL of Sf 60–100 < VLDL of Sf 20–60 < IDL. The mean contribution of hepatic lipase to VLDL-TG hydrolysis by post-heparin plasma was 35% in normal controls, but the contribution to IDL-TG hydrolysis was significantly higher (mean = 58%). It is concluded that hepatic lipase plays a significant role in VLDL and, especially, IDL metabolism, at least in patients with non-functioning lipoprotein lipase.     

Rabbit beta-migrating very low density lipoprotein increases endothelial macromolecular transport without altering electrical resistance.

Rabbit aortic endothelial cells (RAEC) were grown on micropore filters in a new device. This system allowed in situ measurement of transendothelial electrical resistance (TEER). The monolayers demonstrated a TEER of 14 +/- 1 omega X cm2 at confluence. No difference was seen in the transport of low density lipoproteins (LDL) across endothelial cell monolayers obtained from normal or Watanabe heritable hyperlipidemic rabbits, indicating that the LDL receptor was not involved in the LDL transport. TEER was inversely correlated with 22Na transport (r2 = 0.93, P = less than 0.001) but not with 125I-LDL transport. The amount of LDL transported at 15 degrees C or across glutaraldehyde-fixed monolayers was half that of the controls at 37 degrees C. Preincubation of the monolayers with rabbit beta-migrating very low density lipoproteins (beta-VLDL) increased cholesterol content by 65%, and the transport of albumin and LDL doubled without a change in TEER. Removal of beta-VLDL from the culture medium resulted in the return of cellular cholesterol content and LDL transport to control values. We conclude that preincubation of RAEC with beta-VLDL resulted in an increased permeability to LDL and albumin, and that beta-VLDL may promote increased transendothelial transport of macromolecules in cholesterol-fed rabbits.     

Subendothelial retention of lipoprotein (a). Evidence that reduced heparan sulfate promotes lipoprotein binding to subendothelial matrix.

Vessel wall subendothelial extracellular matrix, a dense mesh formed of collagens, fibronectin, laminin, and proteoglycans, has important roles in lipid and lipoprotein retention and cell adhesion. In atherosclerosis, vessel wall heparan sulfate proteoglycans (HSPG) are decreased and we therefore tested whether selective loss of HSPG affects lipoprotein retention. A matrix synthesized by aortic endothelial cells and a commercially available matrix (Matrigel; , Rutherford, NJ) were used. Treatment of matrix with heparinase/heparitinase (1 U/ml each) increased LDL binding by approximately 1.5-fold. Binding of lipoprotein (a) [Lp(a)] to both subendothelial matrix and Matrigel(R) increased 2-10-fold when the HSPG were removed by heparinase treatment. Incubation of endothelial cells with oxidized LDL (OxLDL) or lysolecithin resulted in decreased matrix proteoglycans and increased Lp(a) retention by matrix. The effect of OxLDL or lysolecithin on endothelial PG was abolished in the presence of HDL. The decrease in matrix HSPG was associated with production of a heparanase-like activity by OxLDL-stimulated endothelial cells. To test whether removal of HSPG exposes fibronectin, a candidate Lp(a) binding protein in the matrix, antifibronectin antibodies were used. The increased Lp(a) binding after HSPG removal was inhibited 60% by antifibronectin antibodies. Similarly, the increased Lp(a) binding to matrix from OxLDL-treated endothelial cells was inhibited by antifibronectin antibodies. We hypothesize that atherogenic lipoproteins stimulate endothelial cell production of heparanase. This enzyme reduces HSPG which in turn promotes Lp(a) retention.     

Lipoprotein lipase regulates Fc receptor-mediated phagocytosis by macrophages maintained in glucose-deficient medium.

During periods of intense activity such as phagocytosis, macrophages are thought to derive most of their energy from glucose metabolism under both aerobic and anaerobic conditions. To determine whether fatty acids released from lipoproteins by macrophage lipoprotein lipase (LPL) could substitute for glucose as a source of energy for phagocytosis, we cultured peritoneal macrophages from normal and LPL knockout (LPL-KO) mice that had been rescued from neonatal demise by expression of human LPL via the muscle creatine kinase promoter. Normal and LPL-KO macrophages were cultured in medium containing normal (5 mM) or low (1 mM) glucose, and were tested for their capacity to phagocytose IgG-opsonized sheep erythrocytes. LPL-KO macrophages maintained in 1 and 5 mM glucose phagocytosed 67 and 79% fewer IgG-opsonized erythrocytes, respectively, than macrophages from normal mice. Addition of VLDL to LPL-expressing macrophages maintained in 1 mM glucose enhanced the macrophages' phagocytosis of IgG-opsonized erythrocytes, but did not stimulate phagocytosis by LPL-KO macrophages. Inhibition of secreted LPL with a monoclonal anti-LPL antibody or with tetrahydrolipstatin blocked the ability of VLDL to enhance phagocytosis by LPL-expressing macrophages maintained in 1 mM glucose. Addition of oleic acid significantly enhanced phagocytosis by both LPL-expressing and LPL-KO macrophages maintained in 1 mM glucose. Moreover, oleic acid stimulated phagocytosis in cells cultured in non-glucose-containing medium, and increased the intracellular stores of creatine phosphate. Inhibition of oxidative phosphorylation, but not of glycolysis, blocked the capacity of oleic acid to stimulate phagocytosis. Receptor-mediated endocytosis of acetyl LDL by macrophages from LPL-expressing and LPL-KO mice was similar whether the cells were maintained in 5 or 1 mM glucose, and was not augmented by VLDL. We postulate that fatty acids derived from macrophage LPL-catalyzed hydrolysis of triglycerides and phospholipids provide energy for macrophages in areas that have limited amounts of ambient glucose, and during periods of intense metabolic activity.     

Serum lipoprotein lipase mass: clinical significance of its measurement

Lipoprotein lipase (LPL) is a lipolytic enzyme involved in catalyzing hydrolysis of triglycerides (TG) in chylomicrons and very low-density lipoprotein (VLDL) particles. Over the last decade, increasing attention has been paid to the clinical significance of measuring serum LPL protein mass without heparin injection to the study subjects. In earlier studies, this marker was utilized to classify LPL deficient subjects, which is an extremely rare metabolic disorder with a frequency of one in one million. Later, researchers paid more attention to the clinical significance of measuring this parameter in more common metabolic disorders. Studies have shown that pre-heparin plasma or serum LPL mass has significant relationships with serum lipids and lipoproteins, visceral fat area, insulin resistance, and even the development of coronary atherosclerosis in cross-sectional studies, although this might be a metabolic surrogate marker with almost no catalytic activities, which does not appear to be involved in catalyzing hydrolysis of TG in TG-rich lipoproteins. Recently, a prospective study has demonstrated that low serum LPL concentration predicts future coronary events. Taken together, we suggest that pre-heparin LPL mass in plasma or sera provide us with useful and important information on the development of metabolic disorders leading to atherosclerotic disease.     

Chylomicronemia due to apolipoprotein CIII overexpression in apolipoprotein E-null mice. Apolipoprotein CIII-induced hypertriglyceridemia is not mediated by effects on apolipoprotein E.

The mechanism of apolipoprotein (apo) CIII-induced hypertriglyceridemia remains uncertain. We crossed apoCIII transgenic and apoE gene knockout (apoE0) mice, and observed severe hypertriglyceridemia with plasma triglyceride levels of 4,521+/-6, 394 mg/dl vs. 423+/-106 mg/dl in apoE0 mice, P < 0.00001 for log(triglycerides [TG]). Cholesterols were 1,181+/-487 mg/dl vs. 658+/-151 mg/dl, P < 0.0001. Lipoprotein fractionation showed a marked increase in triglyceride-enriched chylomicrons+VLDL. This increase was limited to the lowest density (chylomicrons and Sf 100-400) subfractions. Intermediate density lipoproteins (IDL)+LDL increased moderately, and HDL decreased. There was no significant increase in triglyceride production in apoCIII transgenic/apoE0 mice. The clearance of VLDL triglycerides, however, was significantly decreased. Lipoprotein lipase in postheparin plasma was elevated, but activation studies suggested LPL inhibition by both apoCIII transgenic and apoCIII transgenic/apoE0 plasma. ApoCIII overexpression also produced a marked decrease in VLDL glycosaminoglycan binding which was independent of apoE. The predominant mechanism of apoCIII-induced hypertriglyceridemia appears to be decreased lipolysis at the cell surface. The altered lipoprotein profile that was produced also allowed us to address the question of the direct atherogenicity of chylomicrons and large VLDL. Quantitative arteriosclerosis studies showed identical results in both apoCIII transgenic/apoE0 and apoE0 mice, supporting the view that very large triglyceride-enriched particles are not directly atherogenic.     

Effects of dextran sulfate on stabilization of milk lipoprotein lipase and VLDL triglyceride hydrolysis in vitro

The effects of dextran sulfate (DS), which has various molecular numbers, on hydrolysis of very low density lipoprotein triglyceride (VLDL-TG) by bovine milk lipoprotein lipase (LPL) and the stability of LPL were studied. VLDL-TG hydrolysis was increased by the addition of DS; DS caused linear increase in the Vmax for VLDL-TG with increase in its sulfate content, but did not change the Km value for VLDL-TG. DS also stabilized LPL, but this effect was not dependent on its sulfate content. These results suggest that the mechanism of action of DS in LPL stabilization may be different from that in enhancement of VLDL hydrolysis.     

Ultrafiltration of lipoproteins through a synthetic membrane: Implications for the filtration theory of atherogenesis

To investigate the interaction of lipoproteins with semipermeable membranes, solutions of low density lipoproteins (LDL), very low density lipoproteins (VLDL), mixtures of the two, and diluted, normal, and hyperlipidemic serum were ultrafiltered through a synthetic membrane (500 A nominal pore diameter) using a stirred laboratory ultrafiltration cell. The pressure dependence of ultrafiltrate flux showed that a concentrated layer of lipoproteins was built up at the membrane surface (concentration polarization) and that VLDL was more subject to polarization than LDL. This phenomenon controlled the observed lipoprotein transport behavior. Whereas true membrane rejection (the fraction of the solute on the membrane surface which does not pass through the membrane) was greater than 0.95 for both LDL and VLDL, observed solute rejection varied from nearly 0 to 1.0, depending upon experimental conditions.If concentration polarization occurs in the arterial system, these results suggest that lipoprotein transport into arterial wall may be influenced not only by arterial blood pressure and the properties of the arterial wall, but also by local hemodynamic conditions and by the relative as well as absolute magnitudes of LDL and VLDL concentration.     

Effects of chronic uremia, hemodialysis, and renal transplantation on plasma lipids and lipoproteins in man.

Since quantitative and qualitative alterations in plasma lipoproteins may provide insights into mechanism(s) of altered lipid transport in renal failure, whole plasma triglyceride (TG) and cholesterol (Chol) concentrations and lipoprotein neutral lipids and composition were examined in patients with chronic renal failure (undialyzed and dialyzed) and following successful renal transplantation. Both uremic groups demonstrated increased TG (p less than 0.001) and normal Chol in whole plasma and increased total TG and Chol in the very low-density lipoprotein fraction (VLDL). All hyperlipidemic subjects showed a Type IV phenotype. The percentage triglyceride in VLDL was slightly higher than control in the dialysis patients, and significantly increased in LDL in both undialyzed (p less than 0.001) and dialyzed (p less than 0.005) uremic groups. Transplant patients had significant increases (p less than 0.001) in both TG and Chol in whole plasma, and increased total TG and Chol in both the low-density lipoproteins (LDL) and VLDL fractions. Transplant patients with hyperlipidemia showed a variety of phenotypes and an enrichment of triglyceride in VLDL and LDL. These findings indicate that abnormalities in lipoprotein metabolism in renal failure patients are not appreciably affected by chronic dialysis treatment and continue following successful transplantation. The tendency toward increased VLDL and LDL triglyceride content in these patients resembles the lipoprotein neutral lipid composition found in nonrenal patients with similarly elevated plasma lipids. These alterations could result from primary disturbances in VLDL production and/or removal.     

Apolipoprotein E and lipoprotein lipase co-ordinately enhance binding and uptake of chylomicrons by human hepatocytes

Abstract. ApoE and LpL are important in the metabolism of triglyceride rich lipoproteins, and defects in either or both may result in hyperlipidaemia. It has previously been shown that ApoE and LPL specifically enhance cellular catabolism of lipoproteins by various cell lines. The authors determine in this paper the effect of ApoE and LpL on chylomicron and LDL binding and uptake by human hepatocytes in primary culture. Separate addition of ApoE and LpL greatly enhanced binding and uptake of chylomicrons. Simultaneous addition of ApoE and LPL further increased chylomicron uptake in an additive way. For LDL a different situation was observed: neither ApoE nor LPL mediated a significant increase of lipoprotein uptake. The authors conclude that ApoE and LpL co-ordinately enhance binding and uptake of chylomicrons by primary human hepatocytes. The effect appears to be independent of LDL receptors and the co-ordinate effect of ApoE and LPL may be important for normal chylomicron catabolism.     

Delayed catabolism of apoB-48 lipoproteins due to decreased heparan sulfate proteoglycan production in diabetic mice

We used wild-type (WT) mice and mice engineered to express either apoB-100 only (B100 mice) or apoB-48 only (B48 mice) to examine the effects of streptozotocin-induced diabetes (DM) on apoB-100- and apoB-48-containing lipoproteins. Plasma lipids increased with DM in WT mice, and fat tolerance was markedly impaired. Lipoprotein profiles showed increased levels and cholesterol enrichment of VLDL in diabetic B48 mice but not in B100 mice. C apolipoproteins, in particular apoC-I in VLDL, were increased. To investigate the basis of the increase in apoB-48 lipoproteins in streptozotocin-treated animals, we characterized several parameters of lipoprotein metabolism. Triglyceride and apoB production rates were normal, as were plasma lipase activity, VLDL glycosaminoglycan binding, and VLDL lipolysis. However, beta-VLDL clearance decreased due to decreased trapping by the liver. Whereas LRP activity was normal, livers from treated mice incorporated significantly less sulfate into heparan sulfate proteoglycans (HSPG) than did controls. Hepatoma (HepG2) cells and endothelial cells cultured in high glucose also showed decreased sulfate and glucosamine incorporation into HSPG. Western blots of livers from diabetic mice showed a decrease in the HSPG core protein, perlecan. Delayed clearance of postprandial apoB-48-containing lipoproteins in DM appears to be due to decreased hepatic perlecan HSPG.     

The passage of apoproteins from plasma lipoproteins into the lipoproteins of peripheral lymph in man.

1. The transport of apoprotein B from the lipoprotein of plasma into the lipoproteins of lymph draining the foot has been studied in four men with type III hyperlipoproteinaemia. 2. Three subjects were given autologous 125I-labelled very-low-density lipoprotein (VLDL) and 131I-labelled low-density lipoprotein (LDL) by intravenous injection; the fourth was given autologous 125I-labelled VLDL and 131I-labelled intermediate-density lipoprotein (IDL) plus LDL. 3. The 125I/131I ratios in serum and lymph apoprotein B, and the 125I and 131I specific radioactivities of apoprotein B in VLDL, IDL and LDL from serum and lymph, indicate that apoprotein B in the circulating VLDL can reach peripherallymph without the intermediacy of circulating LDL.     

Association of plasma lipoproteins with postheparin lipase activities.

Studies were designed to explore the association of lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) activities with lipoproteins in human postheparin plasma (PHP). The major peak of LPL activity after gel filtration of PHP eluted after the triglyceride-rich lipoproteins and just before the peak of low density lipoprotein (LDL) cholesterol. When PHP contained chylomicrons, an additional peak of LPL activity eluted in the void volume of the column. Most HTGL activity eluted after the LDL and preceded the elution of high density lipoprotein cholesterol. LPL activity in preheparin plasma eluted in the same position, relative to lipoproteins, as did LPL in PHP. Gel filtration of purified human milk LPL mixed with plasma or isolated LDL produced a peak of activity eluting before LDL. During gel filtration of PHP in high salt buffer (1 M NaCl) or after isolation of lipoproteins by ultracentrifugation in high salt density solutions, most of the lipase activity was not associated with lipoproteins. LPL activity was removed from PHP by elution through immunoaffinity columns containing antibodies to apolipoprotein (apo) B and apo E. Since lipoproteins in PHP have undergone prior in vivo lipolysis, LPL activity in PHP may be bound to remnants of chylomicrons and very low density lipoproteins.     

Metabolic relationships among the plasma lipoproteins: Reciprocal changes in the concentrations of very low and low density lipoproteins in man

The changes in other plasma lipoproteins which accompany alterations in very low density lipoproteins (VLDL) were studied in 31 normal and hyperlipidemic men and women who underwent weight reduction, carbohydrate induction, or clofibrate treatment. Plasma lipids and individual lipoprotein cholesterol concentrations were measured serially during control and treatment periods. Low density lipoprotein (LDL) protein was determined by radial immunodiffusion. Oppositely directed changes in VLDL and LDL were found with each of the three metabolic perturbations. Changes in high density lipoprotein (HDL) cholesterol generally paralleled those in LDL but were less consistent. Two patients with type III hyperlipoproteinemia failed to demonstrate reciprocal increases in LDL despite more than 40% reduction in plasma glycerides or VLDL with weight reduction or clofibrate therapy. After clofibrate therapy, LDL increased in proportion to the absolute decrease in VLDL cholesterol during treatment. LDL protein changed relatively less than did LDL cholesterol. The mechanism for the interdependency of plasma VLDL and LDL concentrations over the long term is not known and may be the result of altered rates of interconversion of these lipoproteins, or to feedback inhibition by VLDL of LDL production and release.