The effect of added monoacylglycerols on the removal from plasma of chylomicron-like emulsions injected intravenously in rats

Lipid emulsions were prepared with compositions similar to the triacylglycerol-rich plasma lipoproteins, but also incorporating added small amounts of monoacylglycerols. Control emulsions without monoacylglycerol were metabolized similarly to natural chylomicrons or very-low-density lipoproteins when injected intravenously in rats. The emulsion triacylglycerols and cholesteryl esters were both removed rapidly from the bloodstream, with the removal rates of triacylglycerols faster than those of cholesteryl esters. Much of the removed cholesteryl ester was found in the liver, but only a small fraction of the triacylglycerol, consistent with hepatic uptake of the triacylglycerol-depleted remnants of the injected emulsion. Emulsions incorporating added monooleoylglycerol or stearic acid were metabolized similarly. Added 1- or 2-monostearoylglycerol had no effect on triacylglycerol removal from plasma, but the removal rate of cholesteryl esters was decreased and less cholesteryl ester was found in the liver. These effects are similar to those recently described when emulsions and chylomicrons contained triacylglycerols with a saturated acyl chain at the glycerol 2-position, suggesting that saturated monoacylglycerol produced by the action of lipoprotein lipase may cause triacylglycerol-depleted remnant particles to remain in the plasma instead of being rapidly taken up by the liver.     

The effects of Triton WR-1339, protamine sulfate and heparin on the plasma removal of emulsion models of chylomicrons and remnants in rats

Protein-free lipid emulsions with compositions modelling chylomicrons (chylomicron-like emulsion) or chylomicron remnants (remnant-like emulsion) were injected intra-arterially into nonanesthetized rats. Compared with control untreated rats, treatment with Triton WR-1339, protamine sulfate or heparin strongly modified the plasma removal of triacylglycerols and cholesteryl ester moieties of chylomicron-like emulsions, but had little effect on removal rates of triacylglycerols or cholesteryl esters of remnant-like emulsions. The effects on chylomicron-like removal were similar to those on natural lymph chylomicrons. The relative lack of effects on remnant-like emulsion removal provides additional evidence that remnant-like emulsions are a metabolic model for natural chylomicron remnants.     

Influence of the fatty acid composition of lipids in chylomicron remnants derived from fish or corn oil on the lipid profile of cultured rat hepatocytes

The aim of this work was to characterise the lipid and fatty acid composition of chylomicron remnants enriched in n-3 or n-6 polyunsaturated fatty acids (PUFA) and to investigate their influence on the fatty acid profiles of the lipids of rat hepatocytes cultured in monolayers. Chylomicrons were prepared from the lymph collected from the thoracic duct of rats given an oral dose of fish or corn oil (high in n-3 and n-6 PUFA, respectively), and remnants were prepared in vitro from such chylomicrons using rat plasma containing lipoprotein lipase. The fatty acids predominating in the oils abounded also in their respective chylomicrons and remnants, especially in triacylglycerols. Chylomicrons as well as remnants contained small amounts of phospholipids and long-chain PUFA that were minor in, or absent from, the dietary oils, evidently provided by the intestinal epithelium. The incubation of hepatocytes for 6 h, with either n-3 or n-6 PUFA-rich remnants (0.25-0.75 mM triacylglycerol) resulted in a dose-dependent increase in the amount of triacylglycerols and phospholipids in the cells, which was not affected further by increasing the incubation time to 19 h. Whereas hepatocyte triacylglycerols mostly incorporated the PUFA predominating in each remnant type, the fatty acid profile of cell phospholipids was virtually unchanged. In addition, irrespective of whether they were enriched in n-3 or n-6 PUFA, remnants promoted a relative decrease in the amount of cholesteryl esters, a minor hepatocyte lipid class poor in PUFA. The results demonstrate that the hepatocyte fatty acid profile is modulated in a lipid-class specific way by the amount and type of dietary PUFA delivered to cells in chylomicron remnants.     

Effects of phospholipid composition on the metabolism of triacylglycerol, cholesteryl ester and phosphatidylcholine from lipid emulsions injected intravenously in rats

Lipid emulsions were prepared with a similar size and lipid composition to natural lymph chylomicrons, but in which the surface phospholipid was either egg phosphatidylcholine, dioleoyl-, dimyristoyl-, dipalmitoyl- or 1-palmitoyl-2-oleoylphosphatidylcholine (EYPC, DOPC, DMPC, DPPC or POPC). When injected into the bloodstream of conscious rats, the emulsions containing EYPC or POPC were metabolized similarly to natural chylomicrons, consistent with rapid lipoprotein lipase-mediated hydrolysis of triacylglycerols, followed by hepatic uptake of the remnants derived from the emulsions. Phospholipids from the injected emulsions were removed more slowly and became associated with the high-density lipoprotein fractions of the plasma. Emulsions containing DPPC were metabolized differently. Triacylglycerols disappeared very slowly from plasma, indicating lack of hydrolysis by lipoprotein lipase, and phospholipid radioactivity did not transfer to high-density lipoprotein. With emulsions containing DMPC, the plasma removal rates for emulsion triacylglycerols and cholesteryl esters were fast, but phospholipid radioactivity failed to transfer to the high-density lipoprotein fractions of plasma. With DOPC emulsions, clearances were slower than EYPC or POPC emulsions, but transfer to high-density lipoproteins was efficient. Therefore, an unsaturated chain at the glycerol 2-position was necessary for rapid hydrolysis by lipoprotein lipase and for efficient transfer of phospholipids to high-density lipoproteins. With an unsaturated chain at the glycerol 2-position, a saturated chain at the glycerol 1-position optimized the rate of remnant removal from the plasma.     

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.     

Metabolism of chylomicrons by the isolated rat liver

Isolated livers perfused with washed corn oil chylomicrons labeled in vivo with palmitic acid-1-(14)C removed a large proportion of the chylomicrons. Slices from these livers oxidized chylomicron fatty acid esters to both carbon dioxide and acetoacetate. The liver slices also generated free fatty acids from chylomicron lipids and converted chylomicron triglycerides to phospholipids. Similar activities were observed in rat liver slices prepared shortly after the intravenous administration of chylomicrons to intact rats. The observed chylomicron uptake and lipid conversions were similar in livers from both fed and fasted rats. Fasting increased the oxidation of chylomicron fatty acid esters by livers labeled in vivo and by perfusion. In livers removed from intact rats given labeled chylomicrons, the triglyceride-(14)C to phospholipid-(14)C ratio was high, a finding unexpected if the liver had acquired this (14)C by removal of circulating fatty acids formed by extrahepatic lipolysis. These results demonstrate the ability of the liver to remove and utilize chylomicrons directly and suggest that direct removal accounts for a significant portion of the chylomicron fatty acids utilized by the liver of intact rats.     

Apparent convergence (at 2-monoacylglycerol level) of phosphatidic acid and 2-monoacylglycerol pathways of synthesis of chylomicron triacylglycerols

Dietary fats are converted into chylomicron triacylglycerols via the 2-monoacylglycerol and phosphatidic acid pathways of acylglycerol formation. In view of the known positional and fatty acid specificity of the acyltransferases, the triacylglycerol structures resulting from the two pathways would be expected to differ, but this has not been demonstrated. We have performed stereospecific analyses on the chylomicron triacylglycerols from rats fed menhaden oil and the corresponding fatty acid alkyl esters, which would be expected to be assimilated via the monoacylglycerol and the phosphatidic acid pathways, respectively. The results show a remarkable similarity between the two triacylglycerol types in the fatty acid composition of the sn-1 and sn-3 positions, along with marked differences in the composition of the sn-2 positions. The triacylglycerols from rats fed oil retained about 85% of the original fatty acids in the sn-2 position, including a high proportion of the long chain polyunsaturates (e.g., 5-7% 20:5 and 4-5% 22:6). The triacylglycerols from rats fed the alkyl ester contained large amounts of endogenous fatty acids in the sn-2 position (e.g., 18% 16:1, 14% 18:1, 14% 18:2, and 2.5% 20:4), which approximated the composition of the sn-2 position of the presumed phosphatidic acid intermediates. The sn-1 position contained a much higher proportion of polyunsatured fatty acids (e.g., 12-13% 20:5, 5-6% 22:6) than the sn-2 position (e.g. 2-3% 20:5, 0-0.6% 22:6) of triacylglycerols from rats fed the ester. We conclude that the chylomicron triacylglycerols arising via the 2-monoacylglycerol and the phosphatidic acid pathways differ mainly in the composition of the fatty acids in the sn-2 position. The similarity in the acids of the sn-1 and sn-3 positions of the chylomicron triacylglycerols from rats fed oil or ester is consistent with a hydrolysis of the acylglycerol products of the phosphatidic acid pathway to 2-monoacylglycerols prior to reconversion to triacylglycerols via the monoacylglycerol pathway and secretion as chylomicrons.     

Metabolism of chylomicron arachidonic and linoleic acid in the rat

Chyle and chylomicrons, obtained after feeding thoracic duct cannulated rats [3H]arachidonic (20:4) and [14C]linoleic acid (18:2) in cream, were injected i.v. into recipient animals. 7.5-15 min after injection, the 14C/3H ratio of the triacylglycerols remaining in plasma was about half of that in the injected chylomicrons, indicating that the chylomicron remnants formed retained relatively more [3H]20:4 than [14C]18:2. The 14C/3H ratio of plasma diacylglycerols was about 6-fold lower than that of plasma free fatty acids. The proportion of [3H]20:4 found in plasma cholesteryl esters was several-fold higher than that of [14C]18:2. Inhibition of hepatic lipase by a specific antiserum did not significantly influence the clearance of triacylglycerols, but increased the amount of 3H in plasma diacylglycerols. It also prevented the rapid clearance of phosphatidylethanolamine from plasma. The liver uptake of [3H]20:4 exceeded that of [14C]18:2. Antiserum against hepatic lipase diminished the difference. In contrast, the 14C/3H ratio of adipose tissue was higher than that of the injected chyle lipoproteins.     

Comparison of the metabolism of chylomicrons and chylomicron remnants by the perfused liver

1. The hepatic metabolism of chylomicrons and chylomicron remnants was compared after adding approximately equal numbers of each lipoprotein particle to the perfusate of isolated livers. 2. At least 40% of the added remnants were metabolized by the liver compared with less than 3% for chylomicrons. 3. There was significantly more net removal of labelled remnants than of chylomicrons by the liver. 4. A greater proportion of labelled cholesterol than of labelled triacylglycerol fatty acids was transferred to the liver from each lipoprotein. 5. Cholesteryl esters of remnants were hydrolysed to triacylglycerol fatty lipoprotein. 5. Cholesteryl esters of remnants were hydrolysed to triacylglycerol fatty acids of remnants were oxidized to CO2 more extensively than those of chylomicrons. 6. There was greater oxidation of remnant glycerolipic [(1(-14)C]oleate than of glycerolipid [1(-14)C]palmitate. 7. A large fraction of the fatty acids of remnants, but not of chylomicrons, was transferred to phospholipids, which were released by the liver in a lipoprotein of relative density less than 1.006. 8. Label from remnants, but not from chylomicrons, was found in lipoproteins of relative density greater than 1.006, which were not released during perfusion but could be flushed out from the liver at the end of perfusion.     

Chylomicrons promote intestinal absorption of lipopolysaccharides

Recent data suggest that dietary fat promotes intestinal absorption of lipopolysaccharides (LPS) from the gut microflora, which might contribute to various inflammatory disorders. The mechanism of fat-induced LPS absorption is unclear, however. Intestinal-epithelial cells can internalize LPS from the apical surface and transport LPS to the Golgi. The Golgi complex also contains newly formed chylomicrons, the lipoproteins that transport dietary long-chain fat through mesenteric lymph and blood. Because LPS has affinity for chylomicrons, we hypothesized that chylomicron formation promotes LPS absorption. In agreement with our hypothesis, we found that CaCo-2 cells released more cell-associated LPS after incubation with oleic-acid (OA), a long-chain fatty acid that induces chylomicron formation, than with butyric acid (BA), a short-chain fatty acid that does not induce chylomicron formation. Moreover, the effect of OA was blocked by the inhibitor of chylomicron formation, Pluronic L-81. We also observed that intragastric triolein (TO) gavage was followed by increased plasma LPS, whereas gavage with tributyrin (TB), or TO plus Pluronic L-81, was not. Most intestinally absorbed LPS was present on chylomicron remnants (CM-R) in the blood. Chylomicron formation also promoted transport of LPS through mesenteric lymph nodes (MLN) and the production of TNFalpha mRNA in the MLN. Together, our data suggest that intestinal epithelial cells may release LPS on chylomicrons from cell-associated pools. Chylomicron-associated LPS may contribute to postprandial inflammatory responses or chronic diet-induced inflammation in chylomicron target tissues.     

Competition between chylomicrons and their remnants for plasma removal: a study with artificial emulsion models of chylomicrons

In previous studies, protein-free emulsions of defined lipid composition were shown capable of simulating either the metabolism of chylomicrons (chylomicron-like emulsion) or their remnants (remnant-like emulsion), depending on the content of free, unesterified cholesterol. To validate further the assumption that remnant-like and chylomicron-like emulsion have metabolic pathways in common with their natural counterparts, studies of competition for plasma removal were undertaken: the remnant-like emulsion labeled with [3H]triolein was injected sequentially twice in the carotid arteries of rats to compare the clearance of remnant-like emulsion of the second injection with the first (control). Prior to the second injection, a large bolus of the chylomicron-like emulsion or rat lymph chylomicron was injected, to check the hypothesis that remnant generated from chylomicron-like emulsion or natural chylomicrons could compete with and displace remnant-like emulsion particles from their tissue receptor sites. Experiments were also performed in rats treated with Triton WR-1339, to block the generation of remnants. Results showed that remnants derived from either natural chylomicrons or chylomicron-like emulsion both strongly competed with the remnant-like emulsion. In contrast, when transformation of remnants was prevented by Triton, the undegraded particles of chylomicron-like emulsion or natural chylomicron were unable to compete with or displace remnant-like emulsion from its sites of removal from the plasma. In agreement with plasma clearance data, the hepatic uptake of the remnant-like emulsion was inhibited by the surplus dose of natural chylomicrons. In contrast, the spleen uptake was unaffected by it.     

Chylomicron remnant cholesteryl esters as the major constituent of very low density lipoproteins in plasma of cholesterol-fed rabbits.

Feeding rabbits 500 mg of cholesterol daily for 4 to 15 days greatly increased the concentration of esterified cholesterol in lipoproteins of d less than 1.006 g/ml. The origin of hypercholesterolemic very low density lipoproteins was investigated by monitoring the degradation of labeled lymph chyomicrons administered to normal and cholesterol-fed rabbits. Chylomicrons were labeled in vivo by feeding either 1) [3H]cholesterol and [14C]oleic acid or 2) [14C]cholesterol and [3H]retinyl acetate. After intravenous injection of labeled chylomicrons to recipient rabbits, [14C]triglyceride hydrolysis was equally rapid in normal and cholesterol-fed animals. Normal rabbits rapidly removed from plasma both labeled cholesteryl and retinyl esters, whereas cholesterol-fed rabbits retained nearly 50% of doubly labeled remnants in plasma 25 min after chylomicron injection. Ultracentrifugal separation of plasma into subfractions of very low density lipoproteins showed that chylomicron remnants in cholesterol-fed animals are found among all subclasses of very low density lipoproteins. Analysis of cholesteryl ester specific activity-time curves for the very low density lipoproteins subfraction from hypercholesterolemic plasma showed that nearly all esterified cholesterol in large very low density lipoproteins and approximately 30% of esterified cholesterol in small very low density lipoproteins was derived from chylomicron degradation. Apparently, nearly two-thirds of the esterified cholesterol in total very low density lipoproteins from moderately hypercholesterolemic rabbits is of dietary origin.     

Physical and chemical changes in isolated chylomicrons: prevention by EDTA

When washed chylomicrons obtained from the lymph of rats, which had been fed corn oil, were maintained at 0-4 degrees C in 0.9% NaCl, the following physical and chemical changes were observed to occur: increased polarity of chylomicron lipids, increased ultraviolet absorption of chylomicron lipids at 232 nm, decreased linoleic acid content, and aggregation. These alterations occurred more rapidly at room temperature and were found to be associated with an increased lipid peroxide content. The behavior of isolated olive oil chylomicrons was qualitatively similar. All observed changes in the properties of washed chylomicrons were prevented by 0.25 mm EDTA; when tested, 25 micro m EDTA was equally effective. The changes were also prevented by 0.02% hydroquinone. The alterations in chylomicron lipids appear to result from the autoxidation of esterified linoleic acid. Studies with linoleic acid-U-(14)C-labeled chylomicrons indicated that cleavage of the linoleic acid carbon chain did not accompany the physicochemical changes. These results demonstrate the usefulness of EDTA in preventing these specific and progressive alterations in chylomicrons dispersed in aqueous systems.     

Effect of copper deficiency on the lymphatic absorption of cholesterol, plasma chylomicron clearance, and postheparin lipase activities

The lymphatic absorption of cholesterol and plasma clearance of chylomicrons were investigated in Cu-deficient rats (CuD) fed 0.5 mg Cu/kg diet, as compared with Cu-adequate control rats (CuA) fed 7.5 mg/kg diet. Cholesterol absorption was measured by the 14C-radioactivity appearing in the mesenteric lymph at hourly intervals for 8 hr after an intraduodenal dose of [14C]cholesterol. The plasma clearance of chylomicrons was measured at 3, 6, and 10 min after an intravenous dose of chylomicrons labeled in vivo with [3H]retinyl ester. Cumulative [14C]cholesterol absorption and total lymphatic output of cholesterol were significantly decreased in CuD at 4 hr and thereafter, with no change in percentage distribution of free and esterified cholesterol. Over an 8-hr period, 7.3% of the dose was absorbed by CuD and 9.2% by CuA. When [3H]chylomicrons, obtained from a CuD or CuA donor rat, were injected into CuD and CuA recipient rats, the label was cleared faster in CuD during the first 3 min. At 6 and 10 min, however, no significant difference in percentage clearance of the dose was observed between the groups. The half-life (t1/2) of [3H]chylomicrons and the total 3H-radioactivity taken up by the liver during the entire 10-min period did not differ between the groups, regardless of the source of chylomicrons. The activities of both endothelial lipoprotein lipase (LPL) and hepatic lipase (HL) in postheparin plasma were markedly lower in CuD. As expressed in micromoles fatty acid released/hr/ml plasma, the activities of LPL in CuD and CuA were 32.6 +/- 1.9 and 45.6 +/- 1.3, respectively. A similar magnitude of difference was also observed in HL activity. The data provide evidence that copper deficiency impairs the intestinal transport of cholesterol and the peripheral lipolysis of chylomicrons. The data, however, strongly suggest that the hepatic uptake of chylomicron remnants via the apo-E-dependent mechanism may not be impaired in Cu deficiency.     

Composition, removal and metabolic fate of chylomicrons derived from diabetic rats

Tri[14C]acylglycerol-labelled chylomicrons, obtained from cannulated mesenteric lymph of streptozotocin-diabetic donor rats, when intravenously injected into non-diabetic recipient rats, disappeared from the circulation at a significantly slower rate than similarly prepared tri[14C]acylglycerol chylomicrons from non-diabetic donor rats (t1/2, 5.6 +/- 0.7 vs. 3.2 +/- 0.5 min-1, P less than 0.02). The appearance of labelled lipolysis products among plasma lipids (free fatty acid, cholesterol ester and phospholipid fractions) was delayed, indicating decreased availability for lipolysis of the chylomicron-borne triacylglycerol of diabetic origin. Tissue distribution of triacylglycerol, 15 min after the injection of chylomicrons to recipient rats, disclosed a 4-5-fold increase in uptake by muscles (heart and diaphragm) in relation to adipose tissues (epididymal and perirenal sites), in the case of chylomicrons of diabetic derivation. Since a large share of the chylomicron triacylglycerol was taken up by the liver, this tissue was perfused with chylomicron 'remnants' prepared by partial in vitro lipolysis with purified lipoprotein lipase. The 'remnants' of diabetic derivation were taken up by the liver at a 2-3-fold slower rate than those of non-diabetic origin. Chylomicrons derived from diabetic rats were found to be similar in size but markedly depleted of E apolipoproteins as determined by SDS-polyacrylamide gel electrophoresis, isoelectric focussing and a specific immunoassay. Decreases were also seen in A-I apolipoproteins by immunoassay and isoelectric focussing. Chylomicron 'remnants' were also markedly apolipoprotein E-deficient. In vitro incubation of the 'diabetic remnants' with high-density lipoproteins raised their apolipoprotein E content approx. 3-fold and considerably increased their hepatic uptake. Injection of intact chylomicrons preincubated with high-density lipoproteins likewise increased their in vivo removal rate toward the range of that of 'non-diabetic' chylomicrons. We conclude that diabetes-induced changes in the apolipoprotein composition of the chylomicrons and chylomicron remnants play an important role in their removal from the circulation. It appears that their recognition pattern is altered, reducing their ability to interact with receptor sites in the peripheral tissues and the liver, respectively.     

Dietary saturated fatty acid content affects lymph lipoproteins: studies in the rat

We examined effects on intestinal absorption of cholesterol and triglycerides and intestinal lipoprotein formation by feeding rats diets in which saturated fatty acids (palmitic plus stearic) comprised 78%, 68%, 48%, or 38% of triglyceride fatty acids. Absorption into lymph of radiolabeled cholesterol was proportional to triglyceride absorption. The rates of absorption of these lipids were related inversely to the % saturated fatty acids fed. The distribution of newly absorbed cholesterol and triglyceride into intestinal lipoproteins differed. With increasing cholesterol absorption more was recovered in very low density lipoproteins in contrast to the appearance preferentially in chylomicrons of larger quantities of fatty acid. Lymph lipid content did not reflect a consistent pattern in relation to the experimental diet fed. The fatty acid composition of triglyceride-rich lymph lipoproteins resembled the diet closely. One-quarter of the intestinal lymph particles from rats fed the highly saturated diets was flattened and polygonal as judged by electron microscopy if cooled to room temperature; whereas with the same diets, particles collected and isolated at 37 degrees C were round. Proportions of A-I and C apolipoproteins in triglyceride-rich intestinal particles varied inversely; apoA-I increased as fat/cholesterol absorption was greater. Diet-induced alterations in plasma lipoproteins and increased circulating triglycerides in this study in rats were unrelated to the variations in intestinal absorption or lymph lipoprotein formation.     

Lymphatic absorption and transport of retinol and vitamin D-3 from rat intestine. Evidence for different pathways

The lymphatic absorption and transport of retinol and vitamin D-3 from rat intestine has been studied. When rats were cannulated in the intestinal lymph duct and given an intraduodenal bolus of [3H]retinol and 14C-labelled vitamin D-3, 14C-labeled vitamin D-3 appeared later in the intestinal lymph than [3H]retinol and the rate of absorption of vitamin D-3 was still maximal at a time when that of retinol had declined. Both vitamins were absorbed via the lymphatic route in association with chylomicrons. Almost all the retinol was esterified, while vitamin D-3 appeared in the chylomicrons as free vitamin D-3. In vitro incubations and in vivo studies using hepatectomized and normal rats showed that the retinyl ester was a relatively nonexchangeable component of the chylomicrons and their remnants. Hence, all the vitamin A followed the remnants in their clearance from plasma. In contrast, significant amounts of vitamin D-3 were transferred from the chylomicrons to other plasma fractions. Therefore, only a fraction of this vitamin may be removed in association with the chylomicron remnants.     

Regulation of the hepatic removal of chylomicron remnants and beta-very low density lipoproteins in the rat.

The contribution of the low density lipoprotein (LDL) receptor to the removal of chylomicron remnants was determined in vitro and in vivo by using interventions that up- or down-regulate the LDL receptor but not the LDL receptor-related protein (LRP). In vitro, chylomicron remnants and beta-very low density lipoprotein (VLDL) bind to the LDL receptor on endosomal membranes; their binding can be competed by LDL and beta-VLDL and the binding capacity is greatly augmented in membranes from estradiol-treated rats. Likewise, estradiol treatment almost doubled the removal of chylomicron remnants during a single pass through perfused rat livers. However, in vivo the removal of chylomicron remnants and beta-VLDL was very rapid even in untreated rats so that the effect of the stimulation by estradiol was barely detectable when trace amounts of lipoproteins were injected. Yet, when saturating doses of either lipoprotein were injected, the effect of estradiol treatment on the removal of chylomicron remnants and beta-VLDL was readily disclosed. In rats fed a diet containing lard, cholesterol, and bile acids, removal of chylomicron remnants or beta-VLDL was significantly retarded. Likewise, perfused livers from diet-fed rats removed only a mean of 16% of chylomicron remnants during a single passage as compared to 29% in livers from control animals. Also, when large doses of beta-VLDL had been infused into rats for 4 h, in subsequent perfusions of the livers the removal of chylomicron remnants was decreased to 11%. From these results it is concluded that the LDL receptor mediates the hepatic removal of a major fraction of chylomicron remnants and beta-VLDL.     

Comparison of the metabolic effects of chylomicrons and their remnants on isolated hepatocytes

A comparison was made between the effects of chylomicrons and chylomicron remnants on metabolic processes of isolated hepatocytes. Since isolated triacylglycerol-rich lipoproteins are contaminated with nonesterified fatty acids, control incubations were conducted with an amount of fatty acid equivalent to the contaminating fatty acids present in the chylomicrons and the remnant preparations, respectively. Chylomicron remnants, produced in vitro by incubation of chylomicrons in postheparin rat plasma, caused marked inhibition of glycolysis, fatty acid synthesis, and cholesterol synthesis, along with marked stimulation of ketogenesis. These effects were traced to the release of nonesterified fatty acids from these remnant particles as a consequence of contamination with lipoprotein lipase, picked up by the particles during the incubation with rat plasma. Fatty acids inhibit glycolysis, cholesterol, and fatty acid synthesis, but enhance ketone body formation by isolated hepatocytes. Chylomicrons and remnants prepared in vivo by the injection of chylomicrons into functionally hepatectomized rats were not contaminated with lipoprotein lipase and did not inhibit glycolysis and cholesterol synthesis nor increase ketone body formation. These lipoprotein particles did, however, cause significant inhibition of fatty acid synthesis, with the chylomicrons being more effective on a protein basis than the remnants produced in vivo. The mechanism responsible for the inhibition of fatty acid synthesis by chylomicrons and remnants prepared in vivo remains to be resolved.     

Effects of exogenous apo E-3 and of cholesterol-enriched meals on the cellular metabolism of human chylomicrons and their remnants

The effects of exogenous apo E-3 and of cholesterol-enriched meals on the binding, cell association and proteolytic degradation of human chylomicrons and their remnants were determined in cultured human skin fibroblasts. Chylomicrons were prepared from plasma of normolipemic humans 4 h after a fat meal with normal or high cholesterol content. Remnants were obtained after incubation of chylomicrons with lipoprotein lipase in vitro. Cellular metabolism of chylomicrons was minimal, less than 10% that of LDL. Exogenous apo E-2 enhanced chylomicron metabolism by 3-4-fold. The cellular metabolism of remnants was 2.5-3.5-fold higher as compared to intact chylomicrons but their response to exogenous apo E-3 was considerably lower. The cellular metabolism of chylomicrons and chylomicron remnants obtained from subjects eating cholesterol-enriched fat meal was the highest either without or with added exogenous apo E-3. Yet, even in the preparation that exhibits the highest metabolic activity (apo E-3 enriched remnants from cholesterol-enriched meals) the absolute proteolytic degradation was about two-thirds that of LDL. We conclude that although LDL-receptors take up and degrade chylomicron remnants, the rate of catabolism of remnants by this route can not explain the rapid and complete remnant removal process as observed in vivo.