Chylomicron metabolism in rats: kinetic modeling indicates that the particles remain at endothelial sites for minutes

Chylomicrons labeled in vivo with ¹⁴C-oleic acid (primarily in triglycerides, providing a tracer for lipolysis) and ³H-retinol (primarily in ester form, providing a tracer for the core lipids) were injected into rats. Radioactivity in tissues was followed at a series of times up to 40 min and the data were analyzed by compartmental modeling. For heart-like tissues it was necessary to allow the chylomicrons to enter into a compartment where lipolysis is rapid and then transfer to a second compartment where lipolysis is slower. The particles remained in these compartments for minutes and when they returned to blood they had reduced affinity for binding in the tissue. In contrast, the data for liver could readily be fitted with a single compartment for native and lipolyzed chylomicrons in blood, and there was no need for a pathway back to blood. A composite model was built from the individual tissue models. This whole-body model could simultaneously fit all data for both fed and fasted rats and allowed estimation of fluxes and residence times in the four compartments; native and lipolyzed chylomicrons (“remnants”) in blood, and particles in the tissue compartments where lipolysis is rapid and slow, respectively.     

Chylomicron metabolism in an animal model for hyperlipoproteinemia type I.

Mink homozygous for the mutation Pro214Leu in lipoprotein lipase (LPL) had only traces of LPL activity but amounts of LPL protein in their tissues similar to those of normal mink. In normal mink, lymph chylomicrons from rats given [3H]retinol (incorporated into retinyl esters, providing a core label) and [14C]oleic acid (incorporated mainly in triglycerides (TG)) were rapidly cleared from the circulation. In the homozygous mink, clearance was much retarded. The ratio of TG to core label in plasma did not decrease and much less [14C]oleic acid appeared in plasma. Still, half of the labeled material disappeared from the circulating blood within 30;-40 min and the calculated total turnover of TG in the hypertriglyceridemic mink was almost as large as in normal mink. The core label was distributed to the same tissues in hypertriglyceridemic mink as in normal mink. Half to two-thirds of the cleared core label was in the liver. The large difference was that in the hypertriglyceridemic mink, TG label (about 40% of the total amount removed) followed the core label to the liver and there was no preferential uptake of TG over core label in adipose or muscle tissue. In normal mink, only small amounts of TG label (<10%) appeared in the liver, while most was in adipose and muscle tissues. Apolipoprotein B-48 dominated in the accumulated TG-rich lipoproteins in blood of hypertriglyceridemic mink, even in fasted animals.     

The acidic domain of GPIHBP1 is important for the binding of lipoprotein lipase and chylomicrons

GPIHBP1, a glycosylphosphatidylinositol-anchored endothelial cell protein of the lymphocyte antigen 6 (Ly6) family, plays a key role in the lipolysis of triglyceride-rich lipoproteins (e.g. chylomicrons). GPIHBP1 is expressed along the luminal surface of endothelial cells of heart, skeletal muscle, and adipose tissue, and GPIHBP1-expressing cells bind lipoprotein lipase (LPL) and chylomicrons avidly. GPIHBP1 contains an amino-terminal acidic domain (amino acids 24-48) that is enriched in aspartate and glutamate residues, and we previously speculated that this domain might be important in binding ligands. To explore the functional importance of the acidic domain, we tested the ability of polyaspartate or polyglutamate peptides to block the binding of ligands to pgsA-745 Chinese hamster ovary cells that overexpress GPIHBP1. Both polyaspartate and polyglutamate blocked LPL and chylomicron binding to GPIHBP1. Also, a rabbit antiserum against the acidic domain of GPIHBP1 blocked LPL and chylomicron binding to GPIHBP1-expressing cells. Replacing the acidic amino acids within GPIHBP1 residues 38-48 with alanine eliminated the ability of GPIHBP1 to bind LPL and chylomicrons. Finally, mutation of the positively charged heparin-binding domains within LPL and apolipoprotein AV abolished the ability of these proteins to bind to GPIHBP1. These studies indicate that the acidic domain of GPIHBP1 is important and that electrostatic interactions play a key role in ligand binding.     

Chylomicron-chylomicron remnant clearance by liver and bone marrow in rabbits. Factors that modify tissue-specific uptake

The metabolism of [14C]cholesterol- and [3H]retinol-labeled chylomicrons obtained from canine thoracic duct or rabbit mesenteric lymph was investigated in normal fasted rabbits. Typically, 70-80% of the chylomicrons injected into the rabbits were cleared from the plasma in 20 min, and their uptake was accounted for principally by the liver and the bone marrow. Surprisingly, the bone marrow was a major site of uptake; the uptake ranged from about half that of the liver to a nearly equal amount. The importance and specificity of chylomicron-chylomicron remnant uptake by the bone marrow were established by demonstrating that (a) bone marrow throughout the body accumulated these lipoproteins, (b) the level of uptake was consistent regardless of how the values were calculated or how the chylomicrons were prepared, (c) the uptake represented specific binding, and (d) radiolabeled intestinal lipoproteins induced in vivo delivered cholesterol and retinol to the marrow. Electron microscopic examination of the rabbit bone marrow established that perisinusoidal macrophages uniquely accounted for the uptake of the chylomicrons. Whereas liver cleared a variety of both triglyceride-rich lipoproteins (chylomicrons, chylomicron remnants, and very low density lipoproteins) and cholesterol-rich lipoproteins (beta-very low density lipoproteins and high density lipoproteins containing apolipoprotein E), bone marrow uptake appeared to be restricted to the triglyceride-rich lipoproteins. More chylomicron remnants (generated in a hepatectomized rabbit) were cleared by the liver than by the bone marrow, and the addition of excess apolipoprotein E to chylomicrons resulted in their preferential uptake by the liver. The role of chylomicron-chylomicron remnant delivery of lipids or lipid-soluble vitamins to rabbit bone marrow is open to speculation, and whether triglyceride-rich lipoprotein uptake occurs to a significant extent in the bone marrow of humans remains to be determined.     

Differential binding of triglyceride-rich lipoproteins to lipoprotein lipase.

In comparison to very low density lipoprotein (VLDL), chylomicrons are cleared quickly from plasma. However, small changes in fasting plasma VLDL concentration substantially delay postprandial chylomicron triglyceride clearance. We hypothesized that differential binding to lipoprotein lipase (LPL), the first step in the lipolytic pathway, might explain these otherwise paradoxical relationships. Competition binding assays of different lipoproteins were performed in a solid phase assay with purified bovine LPL at 4 degrees C. The results showed that chylomicrons, VLDL, and low density lipoprotein (LDL) were able to inhibit specific binding of (125)I-labeled VLDL to the same extent (85.1% +/- 13.1, 100% +/- 6.8, 90.7% +/- 23.2% inhibition, P = NS), but with markedly different efficiencies. The rank order of inhibition (K(i)) was chylomicrons (0.27 +/- 0.02 nm apoB) > VLDL (12.6 +/- 3.11 nm apoB) > LDL (34.8 +/- 11.1 nm apoB). By contrast, neither triglyceride (TG) liposomes, high density lipoprotein (HDL), nor LDL from patients with familial hypercholesterolemia were efficient at displacing the specific binding of (125)I-labeled VLDL to LPL (30%, 39%, and no displacement, respectively). Importantly, smaller hydrolyzed chylomicrons had less affinity than the larger chylomicrons (K(i) = 2.34 +/- 0.85 nm vs. 0.27 +/- 0.02 nm apoB respectively, P < 0.01). This was also true for hydrolyzed VLDL, although to a lesser extent. Chylomicrons from patients with LPL deficiency and VLDL from hypertriglyceridemic subjects were also studied. Taken together, our results indicate an inverse linear relationship between chylomicron size and K(i) whereas none was present for VLDL. We hypothesize that the differences in binding affinity demonstrated in vitro when considered with the differences in particle number observed in vivo may largely explain the paradoxes we set out to study.     

Distribution of chylomicron degradation products in the isolated,perfused rat heart

Chylomicron degradation by hearts from fed and fasted rats was studied using a perfusion technique, which allows the separate collection of coronary (Q_rv) and interstitial effluent (Q_i). Upon perfusion with [³H]-cholesterol-containing chylomicrons the tissue recovery of label was highest in the fasted state, while label recovered in Q_i was highest in the fed state. Density gradient centrifugation of Q_i indicated that the label was recovered in lipoproteins with higher densities: low density lipoproteins (1.019<d<1.050), high density lipoproteins (1.050<d<1.21) and a fraction of d>1.21. These particles probably represent chylomicron degradation products (remnants and “surface fragments”). Our results indicate that tissue cholesterol uptake during chylomicron degradation may be inhibited in the fed state. Furthermore, the role of the myocyte (or interstitial) lipoprotein lipase in chylomicron degradation is discussed.     

Phospholipids as modulators of hepatic recognition of chylomicron remnants. Observations with emulsified lipoprotein lipids.

The lipids extracted from chylomicrons, chylomicron remnants generated in vivo and hepatic-lipase-treated chylomicrons were emulsified by sonication. These emulsified particles retained the capacity of the native lipoproteins to be differentiated by the liver in vivo, i.e. only the particles derived from remnant and hepatic-lipase-treated chylomicron lipids were efficiently taken up by the liver. To investigate the role of phospholipids in this differentiation process, the phospholipids of all three lipoprotein preparations were separated from the remaining lipids by silicic acid chromatography. The phospholipid-free lipid fraction of chylomicrons was then emulsified with the phospholipids derived from each of the three lipoprotein preparations. Only the particles emulsified with phospholipids derived from remnants and hepatic-lipase-treated chylomicrons were efficiently taken up by the liver in vivo. These results support the proposition that phospholipids modulate the hepatic differentiation between chylomicrons and remnants in vivo.     

Chylomicron metabolism. Chylomicron uptake by bone marrow in different animal species

Previously it was shown in rabbits that 20-40% of the injected dose of chylomicrons was cleared from the plasma by perisinusoidal bone marrow macrophages. The present study was undertaken to determine whether the bone marrow of other species also cleared significant amounts of chylomicrons. Canine chylomicrons, labeled in vivo with [14C]cholesterol and [3H] retinol, were injected into marmosets (a small, New World primate), rats, guinea pigs, and dogs. Plasma clearance and tissue uptake of chylomicrons in these species were contrasted with results obtained in rabbits in parallel studies. The chylomicrons were cleared rapidly from the plasma in all animals; the plasma clearance of chylomicrons was faster in rats, guinea pigs, and dogs compared with their clearance from the plasma of rabbits and marmosets. The liver was a major site responsible for the uptake of these lipoproteins in all species. However, as in rabbits, the bone marrow of marmosets accounted for significant levels of chylomicron uptake. The uptake by the marmoset bone marrow ranged from one-fifth to one-half the levels seen in the liver. The marmoset bone marrow also took up chylomicron remnants. Perisinusoidal macrophages protruding through the endothelial cells into the marrow sinuses were responsible for the accumulation of the chylomicrons in the marmoset bone marrow, as determined by electron microscopy. In contrast to marmosets, chylomicron clearance by the bone marrow of rats, guinea pigs, and dogs was much less, and the spleen in rats and guinea pigs took up a large fraction of chylomicrons. The uptake of chylomicrons by the non-human primate (the marmoset), in association with the observation that triglyceride-rich lipoproteins accumulate in bone marrow macrophages in patients with type I, III, or V hyperlipoproteinemia, suggests that in humans the bone marrow may clear chylomicrons from the circulation. It is reasonable to speculate that chylomicrons have a role in the delivery of lipids to the bone marrow as a source of energy and for membrane biosynthesis or in the delivery of fat-soluble vitamins.     

Lipoprotein lipase expression level influences tissue clearance of chylomicron retinyl ester

Approximately 25% of postprandial retinoid is cleared from the circulation by extrahepatic tissues. Little is known about physiologic factors important to this uptake. We hypothesized that lipoprotein lipase (LpL) contributes to extrahepatic clearance of chylomicron vitamin A. To investigate this, [3H]retinyl ester-containing rat mesenteric chylomicrons were injected intravenously into induced mutant mice and nutritionally manipulated rats. The tissue sites of uptake of 3H label by wild type mice and LpL-null mice overexpressing human LpL in muscle indicate that LpL expression does influence accumulation of chylomicron retinoid. Skeletal muscle from mice overexpressing human LpL accumulated 1.7- to 2.4-fold more 3H label than wild type. Moreover, heart tissue from mice overexpresssing human LpL, but lacking mouse LpL, accumulated less than half of the 3H-label taken up by wild type heart. Fasting and heparin injection, two factors that increase LpL activity in skeletal muscle, increased uptake of chylomicron [3H] retinoid by rat skeletal muscle. Using [3H]retinyl palmitate and its non-hydrolyzable analog retinyl [14C]hexadecyl ether incorporated into Intralipid emulsions, the importance of retinyl ester hydrolysis in this process was assessed. We observed that 3H label was taken up to a greater extent than 14C label by rat skeletal muscle, suggesting that retinoid uptake requires hydrolysis.In summary, for each of our experiments, the level of lipoprotein lipase expression in skeletal muscle, heart, and/or adipose tissue influenced the amount of [3H]retinoid taken up from chylomicrons and/or their remnants.     

Uptake of chylomicron remnants by the liver: further evidence for the modulating role of phospholipids

Rat lymph chylomicrons were treated with rat heparin-releasable hepatic lipase (HL) or with bovine milk lipoprotein lipase (LPL). The ability of the resulting particles to be taken up by the liver in vivo was assessed following their infusion into the portal vein of partially hepatectomized animals. The following observations were made: a) the rate of phospholipid depletion, relative to the rate of triglyceride hydrolysis, induced by HL was two- to threefold higher than that observed for LPL; b) the depletion of at least 57% of phospholipids from the surface of HL-treated chylomicrons caused no major alterations in the apoprotein profile of the particles; c) for the same extent of triglyceride hydrolysis, HL-treated chylomicrons were taken up by liver at a rate significantly higher (P less than 0.005) than LPL-treated particles; d) the liver uptake of HL-treated chylomicrons was competitively inhibited by endogenously generated chylomicron remnants, indicating that these two types of lipoproteins share the same process of recognition and uptake by liver cells. It is concluded that the in vivo changes in phospholipid content, or composition, on the surface of chylomicrons during their transformation into remnants, modulate the differentiation of these two particles by the hepatic remnant receptor.     

An extrahepatic receptor-associated protein-sensitive mechanism is involved in the metabolism of triglyceride-rich lipoproteins

We have used adenovirus-mediated gene transfer in mice to investigate low density lipoprotein receptor (LDLR) and LDLR-related protein (LRP)-independent mechanisms that control the metabolism of chylomicron and very low density lipoprotein (VLDL) remnants in vivo. Overexpression of receptor-associated protein (RAP) in mice that lack both LRP and LDLR (MX1cre(+)LRP(flox/flox)LDLR(-/-)) in their livers elicited a marked hypertriglyceridemia in addition to the pre-existing hypercholesterolemia in these animals, resulting in a shift in the distribution of plasma lipids from LDL-sized lipoproteins to large VLDL-sized particles. This dramatic increase in plasma lipids was not due to a RAP-mediated inhibition of a unknown hepatic high affinity binding site involved in lipoprotein metabolism, because no RAP binding could be detected in livers of MX1cre(+)LRP(flox/flox)LDLR(-/-) mice using both membrane binding studies and ligand blotting experiments. Remarkably, RAP overexpression also resulted in a 7-fold increase (from 13.6 to 95.6 ng/ml) of circulating, but largely inactive, lipoprotein lipase (LPL). In contrast, plasma hepatic lipase levels and activity were unaffected. In vitro studies showed that RAP binds to LPL with high affinity (K(d) = 5 nM) but does not affect its catalytic activity, in vitro or in vivo. Our findings suggest that an extrahepatic RAP-sensitive process that is independent of the LDLR or LRP is involved in metabolism of triglyceride-rich lipoproteins. There, RAP may affect the functional maturation of LPL, thus causing the accumulation of triglyceride-rich lipoproteins in the circulation.     

Defective Chylomicron Synthesis as a Cause of Delayed Particle Clearance in Diabetes?

Chylomicron metabolism is abnormal in diabetes and the chylomicron particle may play a very important role in atherosclerosis. The aim of this study was to examine the effect of diabetes on the metabolism of chylomicrons in cholesterol-fed alloxan diabetic and nondiabetic rabbits. Five diabetic rabbits and 5 control rabbits were given [¹⁴C]linoleic acid and [³H]cholesterol by gavage. Lymph was collected following cannulation of the lymph duct and radiolabelled chylomicrons were isolated by ultracentrifugation. The chylomicrons from each animal were injected into paired control and diabetic recipients. Lymph apolipoprotein (apo) B48, apo B100, and apo E were measured using sodium dodecyl sulfate–polyacrylamide gradient gel electrophoresis. Mean blood sugar of the diabetic donors and diabetic recipients were 19.7 ± 2.3 and 17.2 ± 3.2 mmol/L. Diabetic rabbits had significantly raised plasma triglyceride (10.8 ± 13.9 versus 0.8 ± 0.5 mmol/L, P < 0.02). There was a large increase in apo B48 in lymph chylomicrons in the diabetic donor animals (0.19 ± 0.10 versus 0.04 ± 0.02 mg/h, P < 0.01) and apo B100 (0.22 ± 0.15 versus 0.07 ± 0.07 mg/h, P < 0.05) and a reduction in apo E on the lymph chylomicron particle (0.27 ± 0.01 versus 0.62 ± 0.07 mg/mg apo B, P < 0.001). Diabetic recipients cleared both control and diabetic chylomicron triglyceride significantly more slowly than control recipients (P < 0.05). Clearance of control chylomicron cholesterol was delayed when injected into diabetic recipients compared to when these chylomicrons were injected into control recipients (P < 0.005). Clearance of diabetic chylomicron cholesterol was significantly slower when injected into control animals compared to control chylomicron injected into control animals (P < 0.02). In this animal model of atherosclerosis, we have demonstrated that diabetes leads to the production of an increased number of lipid and apo E–deficient chylomicron particles. Chylomicron particles from the control animals were cleared more slowly by the diabetic recipient (both triglyceride and cholesterol). The chylomicron particles obtained from the diabetic animals were cleared even more slowly when injected into the diabetic recipient. Although there was an initial delay in clearance of chylomicron triglyceride from the diabetic particle when injected into the control animals, the clearance over the first 15 minutes was not significantly different when compared to the control chylomicron injected into the control animal. On the other hand, the cholesterol clearance was significantly delayed. Thus, diabetes resulted in the production of an increased number of lipid- and apo E–deficient chylomicron particles. These alterations account, in part, for the delay in clearance of these particles.     

Plasma clearance and liver uptake of chylomicron remnants generated by hepatic lipase lipolysis: evidence for a lactoferrin-sensitive and apolipoprotein E-independent pathway

Chylomicrons labeled with [3H]cholesterol and [14C]triglyceride fatty acids were lipolyzed by hepatic lipase (HL) in vitro and then injected intravenously into normal mice fed low- or high-fat diets, and into apolipoprotein (apo) E-deficient mice. In normal mice fed the high-fat diet and injected with non-lipolyzed chylomicrons, the plasma clearance and hepatic uptake of the resulting [3H]cholesterol-labeled remnants was markedly inhibited. In contrast, chylomicrons lipolyzed by HL were taken up equally rapidly by the livers of mice fed the low- and high-fat diets. The removal of non-lipolyzed chylomicrons lacking apoE from the plasma of apoE-deficient mice was inhibited, but not the removal of chylomicrons lipolyzed by HL. Pre-injection of lactoferrin into normal mice inhibited the plasma clearance of both non-lipolyzed chylomicrons and chylomicrons lipolyzed by HL. The removal of HL from the surface of the lipolyzed particles by proteolytic digestion did not affect their rapid uptake, indicating that the hepatic recognition of the lipoproteins was not mediated by HL. These observations support previous findings that phospholipolysis of chylomicrons by hepatic lipase generates remnant particles that are rapidly cleared from circulation by the liver. They also support the concept that chylomicron remnants can be taken up by the liver by an apolipoprotein E-independent mechanism. We hypothesize that this mechanism is modulated by the remnant phospholipids and that it may involve their interaction with a phospholipid-binding receptor on the surface of hepatocytes such as the class B scavenger receptor BI.     

The very low density lipoprotein (VLDL) receptor – a peripheral lipoprotein receptor for remnant lipoproteins into fatty acid active tissues

The VLDL (very low density lipoprotein) receptor is a member of the LDL (low density lipoprotein) receptor family. The VLDL receptor binds apolipoprotein (apo) E but not apo B, and is expressed in fatty acid active tissues (heart, muscle, adipose) and macrophages abundantly. Lipoprotein lipase (LPL) modulates the binding of triglyceride (TG)-rich lipoprotein particles to the VLDL receptor. By the unique ligand specificity, VLDL receptor practically appeared to function as IDL (intermediate density lipoprotein) and chylomicron remnant receptor in peripheral tissues in concert with LPL. In contrast to LDL receptor, the VLDL receptor expression is not down regulated by lipoproteins. Recently several possible functions of the VLDL receptor have been reported in lipoprotein metabolism, atherosclerosis, obesity/insulin resistance, cardiac fatty acid metabolism and neuronal migration. The gene therapy of VLDL receptor into the LDL receptor knockout mice liver showed a benefit effect for lipoprotein metabolism and atherosclerosis. Further researches about the VLDL receptor function will be needed in the future.     

Apolipoprotein AV accelerates plasma hydrolysis of triglyceride-rich lipoproteins by interaction with proteoglycan-bound lipoprotein lipase

Apolipoprotein A5 (APOA5) is associated with differences in triglyceride levels and familial combined hyperlipidemia. In genetically engineered mice, apoAV plasma levels are inversely correlated with plasma triglycerides. To elucidate the mechanism by which apoAV influences plasma triglycerides, metabolic studies and in vitro assays resembling physiological conditions were performed. In human APOA5 transgenic mice (hAPOA5tr), catabolism of chylomicrons and very low density lipoprotein (VLDL) was accelerated due to a faster plasma hydrolysis of triglycerides by lipoprotein lipase (LPL). Hepatic VLDL and intestinal chylomicron production were not affected. The functional interplay between apoAV and LPL was further investigated by cross-breeding a human LPL transgene with the apoa5 knock-out and the hAPOA5tr to an lpl-deficient background. Increased LPL activity completely normalized hypertriglyceridemia of apoa5-deficient mice; however, overexpression of human apoAV modulated triglyceride levels only slightly when LPL was reduced. To reflect the physiological situation in which LPL is bound to cell surface proteoglycans, we examined hydrolysis in the presence or absence of proteoglycans. Without proteoglycans, apoAV derived either from triglyceride-rich lipoproteins, hAPOA5tr high density lipoprotein, or a recombinant source did not alter the LPL hydrolysis rate. In the presence of proteoglycans, however, apoAV led to a significant and dose-dependent increase in LPL-mediated hydrolysis of VLDL triglycerides. These results were confirmed in cell culture using a proteoglycan-deficient cell line. A direct interaction between LPL and apoAV was found by ligand blotting. It is proposed, that apoAV reduces triglyceride levels by guiding VLDL and chylomicrons to proteoglycan-bound LPL for lipolysis.     

Lipoprotein lipase inhibits hepatitis C virus (HCV) infection by blocking virus cell entry

A distinctive feature of HCV is that its life cycle depends on lipoprotein metabolism. Viral morphogenesis and secretion follow the very low-density lipoprotein (VLDL) biogenesis pathway and, consequently, infectious HCV in the serum is associated with triglyceride-rich lipoproteins (TRL). Lipoprotein lipase (LPL) hydrolyzes TRL within chylomicrons and VLDL but, independently of its catalytic activity, it has a bridging activity, mediating the hepatic uptake of chylomicrons and VLDL remnants. We previously showed that exogenously added LPL increases HCV binding to hepatoma cells by acting as a bridge between virus-associated lipoproteins and cell surface heparan sulfate, while simultaneously decreasing infection levels. We show here that LPL efficiently inhibits cell infection with two HCV strains produced in hepatoma cells or in primary human hepatocytes transplanted into uPA-SCID mice with fully functional human ApoB-lipoprotein profiles. Viruses produced in vitro or in vivo were separated on iodixanol gradients into low and higher density populations, and the infection of Huh 7.5 cells by both virus populations was inhibited by LPL. The effect of LPL depended on its enzymatic activity. However, the lipase inhibitor tetrahydrolipstatin restored only a minor part of HCV infectivity, suggesting an important role of the LPL bridging function in the inhibition of infection. We followed HCV cell entry by immunoelectron microscopy with anti-envelope and anti-core antibodies. These analyses demonstrated the internalization of virus particles into hepatoma cells and their presence in intracellular vesicles and associated with lipid droplets. In the presence of LPL, HCV was retained at the cell surface. We conclude that LPL efficiently inhibits HCV infection by acting on TRL associated with HCV particles through mechanisms involving its lipolytic function, but mostly its bridging function. These mechanisms lead to immobilization of the virus at the cell surface. HCV-associated lipoproteins may therefore be a promising target for the development of new therapeutic approaches.     

Species differences in substrate specificity of lipoprotein lipase purified from chickens and rats

Kinetic parameters of chicken and rat lipoprotein lipase (LPL) were determined in the incubation in vitro with various monoacid triacylglycerol emulsion and plasma lipoproteins. In rat- and chicken-LPL there is an inverse relationship between the hydrolytic rate by both LPL and the increased acyl-chain unsaturation of monoacid triacylglycerol; C18:1>C18:2>C18:3. The rat LPL catalyzed hydrolysis of saturated monoacid triaclyglycerol increased with an increase of chain length as C16>C14>C12, whereas in chicken LPL hydrolytic rate of C12 was higher than C14 and C16 triaclyglycerol. Vmax of rat- and chicken-LPL for chylomicron and VLDL were higher but apparent Km for those were lower than other lipoproteins. In chicken, Vmax and apparent Km of LPL for VLDL were almost the same as those for chylomicron, whereas in rat, Vmax of LPL for VLDL was twice that of chylomicron with the same apparent Km. The chicken and rat VLDL with different particle size prepared by Bio-Gel A50 gel chromatography were similarly hydrolyzed by LPL, while the hydrolysis of small chicken-chylomicron particles was inclined to be higher than that of the large particles. These results show species differences between chickens and rats in the substrate specificity of LPL.     

The mechanism of assimilation of constituents of chylomicrons, very low density lipoproteins and remnants - a new theory.

Purified remnant lipoproteins produced from chylomicrons $\text{in vivo}$ or $\text{in vitro}$ by the action of lipoprotein lipase (LPL) contain firmly bound LPL. The perfused rat liver removes the particulate bound LPL and triglyceride-labeled remnants at exactly the same rate, while purified chylomicrons are not removed. Once remnants are removed by the liver, they are not rereleased into the perfusate. These observations have led to the theory that the LPL attached to the remnant is the signal that allows the liver to “recognize” remnants from chylomicrons. This is followed by fusion of the particle with the cell surface and may be associated with the splitting off a low density lipoprotein particle. The remaining lipids of the remnant are further metabolized by the liver triglyceridase and the cholesterol esterase.     

Intestinal lipoprotein assembly

PURPOSE OF REVIEW: The assembly of intestinal lipoproteins is critical for the transport of fat and fat-soluble vitamins. In this review we propose a nomenclature for these lipoproteins and have summarized recent data about their intracellular assembly and factors that modulate their secretion. RECENT FINDINGS: The assembly and secretion of intestinal lipoproteins increases with the augmented synthesis of apoB, apoAIV and lipids. Chylomicron assembly begins with the formation of primordial, phospholipid-rich particles in the membrane, and their conversion to large chylomicrons occurs in the lumen of the smooth endoplasmic reticulum. Chylomicrons are transported from the endoplasmic reticulum via specialized vesicles to the Golgi for secretion. The identification of genetic mutations in chylomicron retention disease indicates that Sar1b may play a critical role in this process. In addition to chylomicron assembly, intestinal cells have been shown to transport dietary cholesterol via apoB-independent pathways, such as efflux. SUMMARY: Understanding the mechanisms involved in the intracellular transport of chylomicrons and chylomicron-independent secretion pathways are expected to be the next frontiers in the field of intestinal lipoprotein assembly and secretion.     

Human chylomicron apolipoprotein metabolism.

Chylomicron apolipoprotein metabolism was studied utilizing chylomicrons isolated from the pleural fluid of a patient with a recurrent chylous pleural effusion. Chylomicrons contained apolipoproteins A-I, A-II, B, C-I, C-II, C-III, D, E, and albumin. Following intravenous injection of [¹²⁵I] chylomicrons, almost all of the A apolipoprotein radioactivity was recovered in high density lipoproteins, while only a small amount of the B apolipoprotein radioactivity was recovered in low density lipoproteins. These observations indicate that intestinal chylomicron A apolipoproteins serve as precursors for plasma high density lipoprotein A apolipoproteins and only a small fraction of chylomicron apolipoprotein B is metabolized to form low density lipoprotein apolipoprotein B.