Chylomicron-remnant uptake by freshly isolated hepatocytes. Effect of heparin and of hepatic triacylglycerol lipase

Differential Effects of Low-Density Lipoprotein and Chylomicron Remnants on Lipid Accumulation in Human Macrophages

The effects of low-density lipoprotein (LDL) and chylomicron remnants on lipid accumulation in human monocyte–derived macrophages (HMDMs) and in macrophages derived from the human monocyte cell line THP-1 were compared. The HMDMs or THP-1 macrophages were incubated with LDL, oxidized LDL (oxLDL), chylomicron remnant–like particles (CMR-LPs), or oxidized CMR-LPs (oxCMR-LPs), and the amount and type of lipid accumulated were determined. As expected, the lipid content of both cell types was increased markedly by oxLDL but not LDL, and this was due to a rise in cholesterol, cholesteryl ester (CE), and triacylglycerol (TG) levels. In contrast, both CMR-LPs and oxCMR-LPs caused a considerable increase in cellular lipid in HMDMs and THP-1 macrophages, but in this case there was a greater rise in the TG than in the cholesterol or CE content. Lipid accumulation in response to oxLDL, CMR-LPs, and oxCMR-LPs was prevented by the ACAT inhibitor CI976 in HMDMs but not in THP-1 macrophages, where TG levels remained markedly elevated. The rate of incorporation of [3H]oleate into CE and TG in THP-1 macrophages was increased by oxLDL, CMR-LPs, and oxCMR-LPs, but incorporation into TG was increased to a greater extent with CMR-LPs and oxCMR-LPs compared with oxLDL. These results demonstrate that both CMR-LPs and oxCMR-LPs cause lipid accumulation in human macrophages comparable to that seen with oxLDL and that oxidation of the remnant particles does not enhance this effect. They also demonstrate that a greater proportion of the lipid accumulated in response to CMR-LPs compared with oxLDL is TG rather than cholesterol or CE and that this is associated with a higher rate of TG synthesis. This study, therefore, provides further evidence to suggest that chylomicron remnants have a role in foam cell formation that is distinct from that of oxLDL.

Efflux of lipid from macrophages after induction of lipid accumulation by chylomicron remnants

The fate of cholesterol and triacylglycerol taken up and accumulated by macrophages after exposure to chylomicron remnants was investigated using macrophages derived from the human monocyte cell line THP-1 and chylomicron remnant-like particles containing human apolipoprotein (apo) E (CRLPs) as the experimental model. In THP-1 macrophages lipid loaded with CRLPs and incubated with various cholesterol acceptors for 24 h, the mass of cholesterol and cholesteryl ester found in the cells was not changed by HDL, HDL3 or lipid-free ApoA-I, although it was decreased by 38% by ApoA-I-phosphatidylcholine vesicles (ApoA-I-PC). After loading of the macrophages with [3H]cholesterol-labelled CRLPs, only about 5% of the label was effluxed in 24 h in the absence of cholesterol acceptors, and this increased to about 10% with ApoA-I or PC only, and to about 30% with apoA-I-PC. In similar experiments with [3H]triolein, only about 4% of the labelled triacylglycerol taken up by the cells was released into the medium in 24 h, and a large (>60%) and consistent proportion of the intracellular radioactivity remained associated with the triacylglycerol throughout this period. These results suggest that cholesterol and triacylglycerol derived from chylomicron remnants are not readily cleared from macrophages, and this is likely to contribute to the atherogenicity of the remnant lipoproteins.

Protection of chylomicron remnants from oxidation by incorporation of probucol into the particles enhances their uptake by human macrophages and increases lipid accumulation in the cells

The effects of protection of chylomicron remnants from oxidation on their uptake and induction of lipid accumulation in macrophages were investigated using chylomicron remnant-like particles (CRLPs) containing the lipophilic antioxidant drug, probucol, and macrophages derived from the human monocyte cell line, THP-1. The total lipid content of THP-1 macrophages was markedly higher (x2.2) after 48 h of incubation of THP-1 macrophages with CRLPs containing probucol (pCRLPs) when compared to CRLPs without probucol, and this was because of increases in triacylglycerol (x2.3) and cholesterol (x1.8) levels, while cholesteryl ester concentrations were not significantly changed. Determination of the uptake of CRLPs and pCRLPs by the cells using particles labelled with the fluorescent probe 1,1'-dioctadecyl-3,3,3'3'-tetramethylindo-carbocyanine perchlorate showed that pCRLPs are taken up at a faster rate than CRLPs. The synthesis of triacylglycerol, as measured by the incorporation of [(3)H]oleate and [(3)H]glycerol, was also increased in macrophages incubated with pCRLPs as compared to CRLPs without probucol, but phospholipid and cholesteryl ester formation from [(3)H]oleate was unaffected. In addition, no differences between the effects of CRLPs and pCRLPs on the expression of mRNA for a range of genes believed to be involved in lipoprotein uptake, intracellular lipid metabolism and the efflux of cholesterol from macrophages was detected. These results suggest that antioxidants carried in chylomicron remnants enhance lipid accumulation in macrophages by increasing the rate of uptake of the particles and raising the intracellular synthesis of triacylglycerol, but not cholesteryl ester, and that these effects are brought about by changes at the post-transcriptional level. Antioxidants carried in chylomicron remnants therefore may promote the development of atherosclerosis, and this is likely to be particularly important in conditions where clearance of remnants from the circulation is delayed.

Incorporation of lycopene into chylomicron remnant-like particles enhances their induction of lipid accumulation in macrophages

Lipid accumulation in macrophages exposed to chylomicron remnant-like particles containing the dietary antioxidant lycopene was investigated. After incubation with THP-1 macrophages (48h), chylomicron remnant-like particles containing lycopene (lycCRLPs) as compared to those without (CRLPs) caused significantly more lipid accumulation in the cells, and this was due to increases in both the triacylglycerol (+100%) and cholesterol (+62%) content. In addition, expression of mRNA for diacylglycerol acyltransferase (DGAT), a key enzyme in triacylglycerol synthesis, was significantly decreased by lycCRLPs, but not CRLPs. These findings suggest that lycopene from the diet may promote, rather than retard, lipid accumulation in macrophages during its transport in the blood in chylomicron remnants.

The influence of chylomicron remnants on cholesteryl ester metabolism in cultured rat hepatocytes: comparison of the effects of particles enriched in n-3 or n-6 polyunsaturated fatty acids

The effect of chylomicron remnants derived from fish oil (rich in n-3 polyunsaturated fatty acids) or corn oil (rich in n-6 polyunsaturated fatty acids) on the formation and hydrolysis of cholesteryl esters in cultured rat hepatocytes was investigated. Hepatocytes were incubated with or without fish or corn oil chylomicron remnants (0.25-0.75 mM triacylglycerol), and the activity of acyl-CoA:cholesterol acyltranferase (ACAT) and cholesteryl ester hydrolases in the cytosol (cCEH) and endoplasmic reticulum (erCEH), and the expression of mRNA for ACAT1, ACAT2 and cCEH, and of enzyme protein for erCEH was determined. Addition of either type of remnants to hepatocyte cultures resulted in a decreased activity of erCEH, cCEH (after 6 and 19 h incubation), and of ACAT (after 6 h only). Hepatocyte levels of mRNA encoding ACAT1 and ACAT2 were not affected by either type of chylomicron remnants after 6 h of incubation, while ACAT2 mRNA levels were down-regulated by fish oil remnants as compared with corn oil remnants, and also with control cells in the long term (19 h). In contrast, cCEH mRNA levels were down-regulated by chylomicron remnants derived from corn oil but not fish oil. The expression of erCEH protein was induced in response to the inhibitory effect of both types of remnants on the activity of the enzyme, with corn oil remnants having a significantly greater effect. These findings demonstrate that dietary polyunsaturated fatty acids when delivered to hepatocytes in chylomicron remnants regulate the activity of the enzymes governing the intracellular cholesteryl ester balance, and suggest that dietary n-3 and n-6 polyunsaturated fatty acids or a metabolite thereof have differential effects on the expression of their genes at the mRNA and post-transcriptional levels.

The fatty acid composition of chylomicron remnants influences their binding and internalization by isolated hepatocytes

The binding and internalization of (125)I-labelled chylomicron remnants derived from palm, olive, corn, or fish oil (rich in saturated, monounsaturated, n-6, or n-3 polyunsaturated fatty acids, respectively) by hepatocytes from rats fed a low-fat diet or a diet supplemented with the corresponding fat for 21 days was investigated. In hepatocytes from rats fed the low-fat diet, the association of radioactivity with the cells at 4 degrees C (a measure of initial binding only) was similar with all types of remnants tested, but was more rapid at 37 degrees C (a measure of binding plus internalization) when fish oil, as compared to olive, corn or palm oil remnants, was used, and similar differences in the internalization of the particles were observed. In contrast, when hepatocytes from rats fed the fat-supplemented diets were used, the rate of association at 37 degrees C of remnants with cells from rats fed palm, corn or fish oil was similar, and higher than that found with cells from animals fed olive oil, and in this case these differences were mainly due to changes in the binding of the particles to the cells at 4 degrees C. Both excess low-density lipoprotein (LDL), which inhibits remnant uptake by the LDL receptor, and lactoferrin, which blocks the LDL receptor-related protein (LRP), were found to decrease the association of the remnants with cells from rats fed the low-fat and high-fat diets. However, in hepatocytes from animals given the low-fat diet, most of the differences between the various types of particle were retained in the presence of lactoferrin, but abolished in the presence of LDL. In contrast, in cells from rats fed the high-fat diets, the differences were reduced by both lactoferrin and LDL. These findings demonstrate that the hepatic uptake of chylomicron remnants is influenced both by the fatty acid composition of the particles, and by longer-term adaptive changes in liver tissue, and suggest that the former effects are mediated mainly by the LDL receptor, while the latter may involve both the LDL receptor and the LRP.

Receptor-mediated mechanisms of lipoprotein remnant catabolism

Chylomicron and VLDL are triglyceride-rich lipoprotein particles assembled by the intestine and liver respectively. These particles are not metabolized by the liver in their native form. However, upon entry into the plasma, their triglyceride component is rapidly hydrolyzed by lipoprotein lipase and they are converted to cholesterol-rich remnant particles. The remnant particles are recognized by the liver and rapidly cleared from the plasma. This process is believed to occur in two steps. (i) An initial sequestration of remnant particles on hepatic cell surface proteoglycans, and (ii) receptor-mediated endocytosis of remnants by hepatic parenchymal cells. The initial binding to proteoglycans may be facilitated by lipoprotein lipase and hepatic lipase which possess both lipid- and heparin-binding domains. The subsequent endocytic process may be mediated by LDL receptors and/or LRP. Both receptors have a high affinity for apoE, a major apolipoprotein component of remnant particles. The lipases may also serve as ligands for these receptors. An impairment of any component of this complex process may result in an accumulation of remnant particles in the plasma leading to atherosclerosis and coronary heart disease.

Hepatic lipase deficiency

Hepatic lipase (HL) is an enzyme that is made primarily by hepatocytes (and also found in adrenal gland and ovary) and hydrolyzes phospholipids and triglycerides of plasma lipoproteins. It is secreted and bound to the hepatocyte surface and readily released by heparin. It is a member of the lipase superfamily and is homologous to lipoprotein lipase and pancreatic lipase. The enzyme can be divided into an NH2-terminal domain containing the catalytic site joined by a short spanning region to a smaller COOH-terminal domain. The NH2-terminal portion contains an active site serine in a pentapeptide consensus sequence, Gly-Xaa-Ser-Xaa-Gly, as part of a classic Ser-Asp-His catalytic triad, and a putative hinged loop structure covering the active site. The COOH-terminal domain contains a putative lipoprotein-binding site. The heparin-binding sites may be distributed throughout the molecule, with the characteristic elution pattern from heparin-sepharose determined by the COOH-terminal domain. Of the three N-linked glycosylation sites, Asn-56 is required for efficient secretion and enzymatic activity. HL is hypothesized to directly couple HDL lipid metabolism to tissue/cellular lipid metabolism. The potential significance of the HL pathway is that it provides the hepatocyte with a mechanism for the uptake of a subset of phospholipids enriched in unsaturated fatty acids and may allow the uptake of cholesteryl ester, free cholesterol, and phospholipid without catabolism of HDL apolipoproteins. HL can hydrolyze triglyceride and phospholipid in all lipoproteins, but is predominant in the conversion of intermediate density lipoproteins to LDL and the conversion of post-prandial triglyceride-rich HDL into the postabsorptive triglyceride-poor HDL. HL plays a secondary role in the clearance of chylomicron remnants by the liver. Human post-heparin HL activity is inversely correlated with intermediate density lipoprotein cholesterol concentration only in subjects with a hyperlipidemia involving VLDL. This is consistent with intermediate-density lipoproteins being a substrate for HL. HDL cholesterol has been reported to be inversely correlated to HL activity, and on this basis it has been suggested that lowering HL would increase HDL cholesterol. However, the correlation could also be due to a common hormonal factor such as estrogen, which has been shown to up-regulate apoAI and HDL cholesterol and lower HL. A striking feature of severe deficiency of HL is the increase in HDL cholesterol and apolipoprotein AI and an approximately 10-fold increase in HDL triglyceride. Hyper-alpha-triglyceridemia is not a feature of antiatherogenic HDL. HL binds not only to heparan, but also to the LDL receptor-related protein. It has been suggested that enzymatically inactive HL can play a role in hepatic lipoprotein uptake, forming a "bridge" by binding to the lipoprotein and to the cell surface. This raises the interesting possibility that production and secretion of mutant inactive HL could promote clearance of VLDL remnants. We have described a rare family with HL deficiency. Affected patients are compound heterozygotes for a mutation of Ser267 to Phe that results in an inactive enzyme and a mutation of Thr383 to Met that results in impaired secretion and reduced specific activity. Human HL deficiency in the context of a second factor causing hyperlipidemia is strongly associated with premature coronary artery disease. Recently, it has been reported that mutations affecting the structure of HL (e.g., T383M) are relatively frequent in the Finnish population. A C-to-T polymorphism in the promotor region of the HL gene is associated with lowered HL activity and less strongly with increased HDL cholesterol. In summary, there is a good understanding of what HL does in lipoprotein metabolism; however, there is little understanding of its physiological importance, that is, why HL does what it does. (ABSTRACT TRUNCATED)

Heterozygous hepatic lipase deficiency, due to two missense mutations R186H and L334F, in the HL gene

Hepatic lipase (HL) is an endothelial enzyme involved in the metabolism of intermediate density lipoproteins (IDL) and high density lipoproteins (HDL) in plasma. In a Finnish pedigree consisting of 18 members belonging to three generations two missense mutations RI86H and L334F in exons 5 and 7 of the HL gene co-segregated with low post-heparin HL activity. Haplotype analysis of the HL gene in family members revealed a high degree of genetic variation and demonstrated that the two missense mutations reside on the same chromosome. In vitro site-directed mutagenesis and expression of the cDNA constructs in COS-1 cells revealed that the R186H mutation leads to a protein that is not secreted while the L334F mutation results in the production of a HL protein that is secreted but has only about 30% of wild type HL activity. Carriers of the mutated HL gene exhibited clearly reduced HL activity and mass in post-heparin plasma. Probably due to their heterozygous carrier status they had only moderate elevation of total triglycerides, IDL, and LDL-triglycerides. The LDL-particles were enriched in triglycerides and depleted of cholesterol. Also their HDL2- and HDL3-particles were enriched in triglycerides.

The role of lipoprotein lipase and apoprotein E in the recognition of chylomicrons and chylomicron remnants by cultured isolated mouse hepatocytes

Lipoprotein lipase (LPL) has been proposed to play a role in the uptake of chylomicron remnants by hepatocytes by mediating the binding of these lipoproteins to cell-surface glycosaminoglycans and to the low-density-lipoprotein receptor-related protein (LRP). This proposal is based on studies that examined the binding of chylomicrons to HepG2 cells, fibroblasts and Chinese hamster ovary cells in culture, in the presence of large amounts of LPL [Beisiegel (1995) Curr. Opin. Lipidol. 6, 117-122]. We have investigated whether LPL attached to the surface of chylomicrons enhances the binding and uptake of these lipoproteins to isolated hepatocytes maintained in culture. Bovine milk LPL was bound to mouse chylomicrons, double-labelled in vivo with [3H]retinol (in retinyl esters) and with [14C]palmitic acid (in triacylglycerols), collected from the mesenteric lymph of normal mice and from mice lacking the apoprotein E (apo E) gene. Normal chylomicrons (containing apo E) and apo E-free chylomicrons, with or without bound LPL, were incubated with cultured hepatocytes isolated from mice lacking the apo E gene. At 0 degree C LPL did not enhance the binding of the normal or apo E-free chylomicrons by the hepatocytes. When incubations were performed at 37 degrees C the triacylglycerols of normal and apo E-free chylomicrons were hydrolysed by LPL and there was a significant uptake of [14C]fatty acids and [3H]retinol by the hepatocytes. The addition of heparin or lactoferrin, a known inhibitor of hepatic uptake of chylomicron remnants, to the incubation medium inhibited the uptake of [3H]retinol, present in the lipoprotein core, but not the uptake of the [14C]fatty acids. We conclude that: (1) LPL attached to chylomicrons in amounts sufficient to effectively hydrolyse their core triacylglycerols does not enhance the binding of these lipoproteins to the surface of isolated hepatocytes; (2) the recognition and uptake of chylomicrons by hepatocytes requires that these lipoproteins be first hydrolysed by LPL; and (3) the uptake of lipolysed chylomicrons (remnants) by hepatocytes does not require the mediation of apo E.

Chylomicron assembly and catabolism: role of apolipoproteins and receptors

Chylomicrons are lipoproteins synthesized exclusively by the intestine to transport dietary fat and fat-soluble vitamins. Synthesis of apoB48, a translational product of the apob gene, is required for the assembly of chylomicrons. The apob gene transcription in the intestine results in 14 and 7 kb mRNAs. These mRNAs are post-transcriptionally edited creating a stop codon. The edited mRNAs chylomicrons from the shorter apoB48 peptide remains to be elucidated. In addition, the roles of proteins involved in the assembly pathway, e.g. apobec-1, MTP and apoA-IV, needs to be studied. Cloning of enzymes involved in the intestinal biosynthesis of triglycerides will be crucial to fully appreciate the assembly of chylomicrons. There is a need for cell culture and transgenic animal models that can be used for intestinal lipoprotein assembly. The catabolism of chylomicrons is far more complex and efficient than the catabolism of VLDL. Even though the major steps involved in the catabolism of chylomicrons are now known, the determinants for apolipoprotein exchange, processing of remnants in the space of Disse, as well as the mechanism of uptake of these particles by extra-hepatic tissue needs further exploration.

Intracellular localization and metabolism of chylomicron remnants in the livers of low density lipoprotein receptor-deficient mice and apoE-deficient mice. Evidence for slow metabolism via an alternative apoE-dependent pathway

The metabolism of chylomicron remnants in mice deficient in low density lipoprotein receptor (LDLr) or apolipoprotein E (apoE) was compared with that of control C57BL/6J mice. Mice were injected intravenously with chylomicron-like emulsions labeled with radioactive lipids. Blood samples were taken at fixed time intervals from the retro-orbital sinus, and clearance rates of the lipoproteins were assessed from the decline in plasma radioactivities. To follow the intracellular pathway of remnants in the liver, emulsions labeled with a fluorescent cholesteryl ester (BODIPY) were injected, and liver sections were processed and assayed by laser confocal microscopy. Catabolism of remnant cholesteryl esters was assessed by injecting emulsions labeled with cholesteryl[1-14C]oleate and measuring the expired CO2 from each animal. In apoE-deficient mice, remnant removal from plasma was totally impeded, while the clearance of remnants in LDLr-deficient mice was similar to that in C57BL/6J control mice. The confocal micrographs of livers 20 min after injection of fluorescent chylomicron-like emulsions showed evenly distributed fluorescent particles in the hepatocytes from control mice. In contrast, the fluorescent particles were mainly located in sinusoidal spaces in LDLr-deficient mice. Three hours after injection the livers from control mice showed few fluorescent particles, indicating that remnants have been catabolized, while the sections from LDLr-deficient mice were still highly fluorescent. Micrographs from apoE-deficient mice showed no fluorescent particles in the liver at any time after injection. Measurement of expired radioactive CO2 after injection of emulsions labeled in the fatty acid moiety of cholesteryl oleate indicated that remnant metabolism was slower in the LDLr-deficient mice and essentially nil in the apoE-deficient mice. Control mice had expired 50% of the injected label by 3 h after injection. We conclude that under normal circumstances, chylomicron remnants are rapidly internalized by LDLr and catabolized in hepatocytes, with a critical requirement for apoE. When LDLr is absent, remnants are taken up by a second apoE-dependent pathway, first to the sinusoidal space of the liver, with subsequent slow endocytosis and slow catabolism. Hepatic clearance via this second pathway is increased by heparin, inhibited by lactoferrin, heparinase, and suramin, and down-regulated by feeding a high fat diet.

Comparison of the hepatic uptake and processing of cholesterol from chylomicrons of different fatty acid composition in the rat in vivo

The effect of the fatty acid composition of chylomicrons on the uptake and processing of the cholesterol they carry was investigated in the rat in vivo. Rats kept on a standard low fat pellet diet and tube fed a single dose of palm, olive, corn or fish oil (rich in saturated, n-9 monounsaturated, n-6 polyunsaturated and n-3 polyunsaturated fatty acids, respectively) were used to prepare [3H]cholesterol-labelled chylomicrons of different fatty acid composition. These were then injected intravenously into rats (kept on the standard diet), and the clearance of radioactivity from the blood, distribution in the plasma lipoprotein density fractions, uptake by the liver and appearance in the bile were studied. [3H]Cholesterol from fish and corn oil chylomicrons was cleared from the blood more rapidly than that from palm and olive oil chylomicrons. After 180 min the proportion of the radioactivity present in the plasma in high density lipoprotein (HDL) was less when the chylomicrons were derived from palm oil as compared to any of the other oils. Approx. 40% of the administered label was recovered in the liver after 180 min in all experiments. The percentage of the injected radioactivity secreted into bile during 180 min was significantly higher with corn and fish oil chylomicrons than with palm oil chylomicrons, with chylomicrons from olive oil in an intermediate position, and these differences were most pronounced between 60 and 120 min after administration of the label. These studies clearly demonstrate that the fatty acid composition of chylomicrons has important effects on the hepatic uptake and processing of the cholesterol they carry, with enrichment with polyunsaturated fatty acids leading to an increased rate of uptake and more rapid removal from the body via the bile as compared to enrichment with saturated or monounsaturated fatty acids.

Uptake of chylomicron remnants and hepatic lipase-treated chylomicrons by a non-transformed murine hepatocyte cell line in culture

AML 12 is a recently established differentiated, non-transformed hepatocyte cell line derived from mice transgenic for transforming growth factor alpha (Wu et al. (1994) Proc. Natl. Acad. Sci. 91, 674-678). The ability of these cells to take up [3H]cholesterol-labeled in vivo-generated chylomicron remnants, as well as [3H]cholesterol-labeled chylomicrons treated with hepatic lipase in vitro was investigated. Both types of lipoprotein particles were taken up by the AML hepatocytes at a much faster rate than intact chylomicrons, and in a saturable and specific manner. Chylomicrons treated with hepatic lipase in vitro competed with in vivo-generated chylomicron remnants for uptake by the AML hepatocytes, and the uptake of both types of lipoproteins was inhibited by lactoferrin, suggesting that they share the same process of cellular recognition and uptake. It is suggested that hepatic lipase-treated chylomicrons may be valuable in studies aimed at gaining a better understanding of the processes involved in the hepatic recognition and uptake of chylomicron remnants. AML hepatocytes, which can be maintained as replicating, untransformed, and differentiated under standard culture conditions, may be useful and practical for such studies.

Hepatic lipase gene expression is upregulated by a cystine-rich diet in male but not in female rats

Male and female rats fed a cystine-rich diet (5% L-cystine) became hypercholesterolemic after 2 months, with 2-fold higher cholesterol levels carried mainly by the HDL1 and HDL2 lipoprotein fractions. Post-heparin lipoprotein lipase activity was increased in male rats only (60%, P < 0.01), while hepatic lipase (HL) activity was increased in both males and females (48%, P < 0.001 and 27%, P < 0.01, respectively). In the liver, HL activity and mRNA levels were increased in males (30%, P < 0.01, and 70%, P < 0.001, respectively), but not in females. A higher correlation between HDL1-cholesterol and liver HL activity was found in male rats than in female rats. In the latter, although the cystine diet induced a virtually identical increase in HDL1-cholesterol, HL gene expression was not promoted. It is suggested that HL gene expression may be triggered by the uptake of HDL1-cholesterol in male rats, while oestrogens in female rats would counteract this effect.

Effects of a fish oil-lard diet on rat plasma lipoproteins, liver FAS, and lipolytic enzymes

The effects of a fish oil concentrate on blood lipids and lipoproteins were examined in relation to their effects on liver fatty acid synthase (FAS), 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, adipose tissue lipoprotein lipase (LPL), and hepatic triglyceride lipase (H-TGL). For 15 days, 2-mo-old rats were fed a control diet (10% of calories from fat, 4% fat by weight) or diets with 50% of calories (25% wt/wt) provided by lard, lard and fish oil calories (35%/15%), or lard and corn oil (35%/15%). The high-lard diet increased plasma chylomicron and liver triglycerides. The high-lard diet greatly decreased FAS, HMG-CoA reductase, and LPL activities; it also reduced H-TGL activity. Compared with the lard diet, the lard-fish oil diet decreased plasma TG by drastically lowering chylomicron (4-fold, P < 0.001) and very-low-density lipoprotein levels (P < 0.001). It also reduced high-density lipoprotein levels. The lard-fish oil diet prevented hepatic triglyceride accumulation and decreased FAS activity and mass by 3.5-fold (P < 0.001) but did not further decrease HMG-CoA reductase activity. Adipose tissue LPL activity was 2.5-fold (P < 0.001) higher with the lard-fish oil diet than with the lard diet, and H-TGL activity decreased significantly (-32%, P < 0.01), despite unaltered levels of H-TGL mRNA. These effects were significant with only 10% fish oil concentrate in the lard diet. They were not observed with the lard-corn oil diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of dietary cholesterol on activity and mRNA levels of hepatic lipase in rat

Female lean Zucker rats were fed for four weeks with either a control diet or the same diet enriched with 2% (w/w) cholesterol and cholic acid (0.5%, w/w). This treatment resulted in a 6-fold increase in plasma total cholesterol. A 30% decrease was observed in plasma post-heparin HL activity, in contrast with lipoprotein lipase, which was unmodified in the cholesterol/cholate-fed rats. HL activity measured in liver homogenate from these rats was also decreased (-30%, p < 0.05), as was its protein mass, quantified by immunoblot analysis (-57%, (p < 0.01), whereas HL mRNA levels were 3-fold lower in the cholesterol/cholate-fed rats. We conclude that the cholesterol/cholate-enriched diet decreases the HL gene expression by acting at the transcriptional level and/or by affecting HL mRNA stability, or both.

Hepatic lipase may act as a ligand in the uptake of artificial chylomicron remnant-like particles by isolated rat hepatocytes

Active and heat-inactivated hepatic lipase stimulated to a statistically comparable extent the uptake of chylomicron remnant-like particles by isolated rat hepatocytes by 3-fold and 2.3-fold respectively and, likewise, their binding to hepatic plasma membranes by 5-fold and 4-fold respectively. Hepatic lipase may facilitate uptake of these particles, not only as a lipolytic enzyme, but also as a ligand anchored to extracellular glycosaminoglycans.

In vivo regulation of hepatic lipase activity and mRNA levels by diets which modify cholesterol influx to the liver

The aim of this study was to assess whether diets enriched in cholesterol, sodium cholate and drugs known to modify liver cholesterol biosynthesis can modulate hepatic lipase (H-TGL) expression and activity in vivo. Female lean Zucker rats, known to be good responders to cholesterol, were fed for 7 days with a control C diet or the C diet supplemented (w/w) with either 2% cholesterol, 0.5% sodium cholate, 2% cholestyramine or simvastatin (0.1%) added to the cholestyramine diet or given by gavage (10 mg/rat) for 3 days. H-TGL activity decreased by 34% with cholesterol, and by 27% when both cholesterol and cholate were administered to the rats. Under these conditions, H-TGL mRNA decreased by 34% and 87%, respectively. The sharp decrease in H-TGL expression was associated with a strong increase in cholesteryl ester in total liver and in the liver microsome fraction. H-TGL activity decreased by 33% with cholestyramine and the mRNA level decreased by 47%. Simvastatin lowered H-TGL activity by 55% when added to the cholestyramine diet, probably because of a reduction in food intake. When administrated by gavage, simvastatin increased both the H-TGL activity (by 28%) and mRNA (by 23%). These variations may be linked to the availability of mevalonate-derived sterol and non-sterol products.

Hepatic lipase function and the accumulation of beta-very-low-density lipoproteins in the plasma of cholesterol-fed rabbits

The accumulation of cholesterol-rich beta-very-low-density lipoproteins (beta-VLDL) in the plasma of rabbits fed on a high-fat high-cholesterol diet is due to a defect in the clearance of these lipoprotein remnants from circulation by the liver. In view of the evidence that hepatic lipase participates in the process of rapid removal of remnants from circulation, and considering that rabbits are naturally deficient in hepatic lipase, we examined whether this defect in the clearance of beta-VLDL could be reversed by exogenous hepatic lipase. We report that treatment in vitro of [3H]cholesterol-labelled beta-VLDL, or rat chylomicrons, with hepatic lipase resulted in the formation of particles that were rapidly cleared from circulation by the liver when injected intravenously into hypercholesterolaemic rabbits. These results are consistent with the notion that, in addition to the well-established requirement for lipoprotein lipase activity, the generation of remnants capable of being efficiently taken up by the liver also requires the action of hepatic lipase. Lipoprotein lipase acts on triacylglycerol-rich lipoproteins to transform them into particles (remnants) which bind to the surface of liver cells, where they become accessible to hepatic lipase. Hepatocyte endocytosis of these remnants occurs only after further modification by hepatic lipase. According to this scheme, the results presented suggest that the accumulation of beta-VLDL in the circulation of rabbits fed on a high-fat high-cholesterol diet is the result of the saturation of the available hepatic lipase by abnormally high levels of lipoprotein-lipase-generated chylomicron remnants.

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.

Lipoprotein lipase and hepatic lipase activities in a hypercholesterolaemic (RICO) strain of rat. Effect of dietary cholesterol

Hepatic lipase (HL) and lipoprotein lipase (LPL) were assayed in heparinized plasma from male normocholesterolaemic (SW) and genetically hypercholesterolaemic (RICO) rats. Both strains were fed on either a semi-purified control diet or the same diet enriched with 0.5% or 1% cholesterol. HL activity was similar in both groups of rats fed on the control diet. LPL activity was found to be significantly lower in RICO rats (35% decrease, P less than 0.05). Feeding with a high-cholesterol diet led to a decrease in HL activity (15-23%) in both groups of rats but no change was detected in LPL activity, which remained consistently lower in the RICO rats. Thus, with the control diet, LPL activity is lower in RICO rats but presumably is not rate-limiting for their triacylglycerol clearance, given the normal triacylglycerol levels present. After cholesterol feeding, however, the lower LPL activity may become rate-limiting together with the decrease in HL activity, as in these circumstances hypertriacylglycerolaemia was evident and the hypercholesterolaemia of this strain was further increased.

Inhibition of hepatic lipase activity impairs chylomicron remnant-removal in rats

[4-14C]Cholesteryl oleyl ether-labeled chylomicron remnants were injected into rats which received a specific goat antibody against rat hepatic lipase or a control serum. Chylomicron remnant cholesterol ether disappeared from circulation with a significantly higher half-life (2-fold) in antibody-treated rats than in controls (P less than 0.001). Recovered radioactivity in the liver was 2-fold lower in antibody-treated rats (22.8% (n = 6) vs. 45% (n = 4) P less than 0.01). These results clearly show that hepatic lipase may strongly promote chylomicron remnant cholesterol ether uptake by the liver.