Lipolysis of emulsion models of triglyceride-rich lipoproteins is altered in male patients with abdominal aorta aneurysm

Disorders of the lipid metabolism may play a role in the genesis of abdominal aorta aneurysm. The present study examined the intravascular catabolism of chylomicrons, the lipoproteins that carry the dietary lipids absorbed by the intestine in the circulation in patients with abdominal aorta aneurysm. Thirteen male patients (72 +/- 5 years) with abdominal aorta aneurysm with normal plasma lipid profile and 13 healthy male control subjects (73 +/- 5 years) participated in the study. The method of chylomicron-like emulsions was used to evaluate this metabolism. The emulsion labeled with 14C-cholesteryl oleate and (3)H-triolein was injected intravenously in both groups. Blood samples were taken at regular intervals over 60 min to determine the decay curves. The fractional clearance rate (FCR) of the radioactive labels was calculated by compartmental analysis. The FCR of the emulsion with (3)H-triolein was smaller in the aortic aneurysm patients than in controls (0.025 +/- 0.017 vs 0.039 +/- 0.019 min-1; P < 0.05), but the FCR of 14C-cholesteryl oleate of both groups did not differ. In conclusion, as indicated by the triglyceride FCR, chylomicron lipolysis is diminished in male patients with aortic aneurysm, whereas the remnant removal which is traced by the cholesteryl oleate FCR is not altered. The results suggest that defects in the chylomicron metabolism may represent a risk factor for development of abdominal aortic aneurysm.

Lipid metabolism and TNF-alpha secretion in response to dietary sterols in human monocyte derived macrophages

BACKGROUND

The postprandial phase is characterized by the circulation of atherogenic dietary-triacylglycerol rich lipoproteins. Little is known about the modulation of lipid and immune functions in macrophages by these particles or of the role of the oxysterols found in food such as 7beta-hydroxycholesterol and 7-ketocholesterol.

MATERIALS AND METHODS

Human macrophages were tested with different concentrations of chylomicron remnant-like particles (CRLP) with or without incorporated oxysterols to study their uptake by the cells, and their effects on cholesteryl ester and triacylglycerol synthesis and the secretion of inflammatory mediators, including tumour necrosis factor-alpha (TNF-alpha), interleukin 6 (IL-6) and interleukin 10 (IL-10).

RESULTS

Independently of the presence of oxysterols, CRLP caused cholesterol accumulation. However, the dose-dependent increase in [3H]cholesterol internalization by macrophages after incubation with [3H]cholesteryl ester-labelled CRLP was abolished by the presence of oxysterols in the particles. TNF-alpha secretion was decreased and that of IL-10 unaffected by CRLP independently of the presence of oxysterol. Exposure to CRLP containing 7beta-hydroxysterol, but not to CRLP or 7-ketosterol-containing CRLP, reduced IL-6 secretion with respect to cells not exposed to any particles. Because TNF-alpha levels have been related to scavenger receptor expression, we tested the uptake of modified LDL in macrophages exposed to human postprandial triacylglycerol-rich lipoproteins and found it to be markedly increased.

CONCLUSIONS

Cholesterol loading as a result of dietary lipids depresses the inflammatory response of macrophages and the presence of 7beta-hydroxysterol may exacerbate this effect. In addition, exposure to dietary lipids enhances scavenger receptor activity in macrophages. These results suggest that changes induced by dietary lipids in human macrophage function are related to an increased propensity of the cells to accumulate lipids during the postprandial phase.

Rapid clearance of hexachlorobenzene from chylomicrons

Toxic organochlorines that are present in food are lipophilic and carried by chylomicrons. We have studied the clearance of an organochlorine, hexachlorobenzene, from chylomicrons. Chylomicrons were obtained from mesenteric lymph of rats that were intraduodenally given 14C-hexachlorobenzene and 3H-triolein. The labeled chylomicrons were injected intravenously into recipient rats, and the clearance of isotopes was followed. Surprisingly, the hexachlorobenzene disappeared from the plasma more rapidly than the triolein. This unexpected result raises questions about the manner in which hexachlorobenzene is delivered to tissues. The tissue distribution of the hexachlorobenzene is consistent with its rapid uptake.

The incorporation and metabolism of amyloid-beta into chylomicron-like lipid emulsions

The aggregation and deposition of amyloid-beta (Abeta) in the brain is thought to be an early event in the pathology of Alzheimer's disease (AD). Many studies have reported the association of Abeta with lipoproteins from plasma suggesting an involvement of lipoprotein particles in Abeta transport. Chylomicron-like lipid emulsions, resembling chylomicrons in composition, size and metabolism were prepared in the presence of [125I]Abeta1-40. Abeta was found to associate significantly with these lipid emulsions during their preparation. The chylomicron-like emulsions containing Abeta were then injected into a lateral ear vein of conscious rabbits and blood sampled at regular intervals up to 30 mins. It was observed that there was no difference in the plasma clearance of [125I]Abeta and that of the 3H-cholesteryl ester, a marker of the emulsion particles, demonstrating that Abeta remains associated with these particles throughout both their lipolysis and tissue uptake. Our results show that Abeta can be metabolised in association with triglyceride rich lipoproteins (TRLs). In addition we report the presence of specific markers of TRLs of hepatic and intestinal origin in human CSF thus suggesting a potential means of cerebral Abeta delivery.

The effects of dietary n-3 polyunsaturated fatty acids delivered in chylomicron remnants on the transcription of genes regulating synthesis and secretion of very-low-density lipoprotein by the liver: modulation by cellular oxidative state

The influence of chylomicron remnants enriched in n-3 or n-6 polyunsaturated fatty acids (PUFA) (derived from fish or corn oil, respectively) on the expression of mRNA for four genes involved in the regulation of the synthesis, assembly, and secretion of very-low-density lipoprotein (VLDL) in the liver was investigated in normal rat hepatocytes and after manipulation of the cellular oxidative state by incubation with N-acetyl cysteine (NAC) or CuSO(4). The four genes investigated were those encoding apolipoprotein B (apoB), the microsomal triacylglycerol transfer protein (MTP), and the enzymes acyl coenzyme A:diacylglycerol acyltransferase (DGAT) and acyl coenzyme A:cholesterol acyltransferase 2 (ACAT2), which play a role in the regulation of triacylglycerol and cholesteryl ester synthesis, respectively. mRNA levels for apoB, MTP, and DGAT were unaffected by either fish or corn oil chylomicron remnants, but the amount of ACAT2 mRNA was significantly reduced after incubation of the hepatocytes with fish oil remnants as compared with corn oil remnants or without remnants. These findings indicate that the delivery of dietary n-3 PUFA to hepatocytes in chylomicron remnants downregulates the expression of mRNA for ACAT2, and this may play a role in their inhibition of VLDL secretion. However, when the cells were shifted into a pro-oxidizing or pro-reducing state by pretreatment with CuSO(4) (1 mM) or NAC (5 mM) for 24 hr, levels of mRNA for MTP were increased by about 2- or 4-fold, respectively, by fish oil remnants, whereas corn oil remnants had no significant effect. Fish oil remnants also caused a smaller increase in apoB mRNA in comparison with corn oil remnants in NAC-treated cells (+38%). These changes would be expected to lead to increased VLDL secretion rather than the decrease associated with dietary n-3 PUFA in normal conditions. These findings suggest that relatively minor changes in cellular redox levels can have a major influence on important liver functions such as VLDL synthesis and secretion.

Recombinant lipoproteins: lipoprotein-like lipid particles for drug targeting

Lipoproteins are endogenous particles that transport lipids through the blood to various cell types, where they are recognised and taken up via specific receptors. These particles are, therefore, excellent candidates for the targeted delivery of drugs to various tissues. For example, the remnant receptor and the asialoglycoprotein receptor (ASGPr), which are uniquely localised on hepatocytes, recognise chylomicrons and lactosylated high density lipopoteins (HDL), respectively. In addition, tumour cells of various origins overexpress the low density lipoprotein (LDL) receptor that recognises apolipoprotein E (apoE) on small triglyceride-rich particles and apoB-100 on LDL. Being endogenous, lipoproteins are biodegradable, do not trigger immune reactions, and are not recognised by the reticuloendothelial system (RES). However, their endogenous nature also hampers large-scale pharmaceutical application. In the past two decades, various research groups have successfully synthesised recombinant lipoproteins from commercially available natural and synthetic lipids and serum-derived or recombinant apolipoproteins, which closely mimic the metabolic behaviour of their native counterparts in animal models as well as humans. In this paper, we will summarise the studies that led to the development of these recombinant lipoproteins, and we will address the possibility of using these lipidic particles to selectively deliver a wide range of lipophilic, amphiphilic, and polyanionic compounds to hepatocytes and tumour cells. In addition, the intrinsic therapeutic activities of recombinant chylomicrons and HDL in sepsis and atherosclerosis will be discussed.

Effects of etofibrate upon the metabolism of chylomicron-like emulsions in patients with coronary artery disease

Slow chylomicron intravascular catabolism has been associated with coronary artery disease and screening for drugs that can speed-up this process can be important. In this study, the effects of etofibrate upon chylomicron metabolism was tested by determination of the plasma kinetics of a chylomicron-like emulsion model in 12 patients with coronary artery disease, aged 59+/-11 years, (total cholesterol: 240+/-41 mg/dl; triglycerides: 188+/-42 mg/dl) submitted to a randomized, crossover, double-blind, placebo-controlled study with administration of 1 g per day etofibrate or placebo for 1-month. A 1-month washout period was inserted between the treatment periods. Patients were intravenously injected a chylomicron-like emulsion doubly labeled with 14C-cholesteryl oleate and 3H-triolein at baseline and after treatments. After etofibrate treatment, there was decrease of total cholesterol and triglyceride plasma levels and a trend to increase high-density lipoprotein cholesterol plasma levels. Etofibrate elicited 62% enhancement of post-heparin lipolytic activity and 100% increase of 3H-triglyceride fractional clearance rate compared with placebo treatment. 14C-cholesterol ester fractional clearance rate was 260% greater after etofibrate than after placebo. Therefore, a potent effect of etofibrate on both chylomicron lipolysis and remnant removal was achieved, indicating that this drug can be used to improve this metabolism in future prospective studies.

Plasma kinetics of an artificial emulsion resembling chylomicrons in patients with chronic lymphocytic leukemia

Chylomicrons are the lipoproteins that transport dietary lipids in the blood. Although neoplastic diseases are often accompanied by alterations in lipid metabolism, chylomicrons are scarcely explored in cancer, despite their importance for the body's energy supply. Moreover, no data are available regarding chylomicron metabolism in chronic lymphocytic leukemia (CLL). Chylomicron metabolism in the bloodstream consists of lipolysis by lipoprotein lipase and uptake of remnants by the liver and is difficult to assess in the human body. Among the methods to evaluate this pathway, the determination of the plasma kinetics of triglyceride-rich emulsions that mimic chylomicrons is a practical and straightforward approach. A double-labeled chylomicron-resembling emulsion was injected into 10 patients with CLL and into 11 normolipidemic healthy subjects. The plasma kinetic curves of the emulsion 3H-triglyceride and 14Ccholesteryl ester were determined in plasma samples collected over 30 min. The fractional clearance rate (FCR) of triglycerides in CLL was not changed compared with controls. The FCR of cholesteryl esters was also no different from controls. These results indicate that chylomicron lipolysis and remnant removal are not affected in CLL.

Transient triglyceridemia in healthy normolipidemic men increases cellular processing of large very low density lipoproteins by fibroblasts in vitro

Exaggerated and prolonged postprandial triglyceridemia is a characteristic of patients with precocious coronary heart disease. Although large very low density lipoprotein (VLDL) particles accumulate during alimentary lipemia, the biological properties of the postprandial VLDL remain unknown. In the present study, an intravenous infusion of a chylomicron-like emulsion was given to healthy normolipidemic men to examine the effects of transient triglyceridemia in vivo on compositional and cell biological characteristics of VLDL. The postinfusion large(Svedberg flotation rate (Sf) (60-400) VLDL was found to have increased capacity to inhibit low density lipoprotein (LDL) binding to the LDL-receptor and a greater ability to suppress the 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase activity of cultured fibroblasts compared to VLDL isolated from fasting plasma. These alterations in cellular interactions were accompanied by increases in the number of apolipoprotein (apo) E, C-I, and C-III molecules per large VLDL particle and loss of apoC-II, compositional changes similar to those observed after an oral fat load. The increase in number of apoE molecules per large VLDL particle correlated positively and significantly with the increase in the capacity of large VLDL to inhibit LDL binding to the LDL receptor (r = 0.76, P = 0.01, n = 10). In contrast, the composition of the small (Sf 20-60) VLDL particles did not change significantly, nor was the LDL receptor-mediated processing of these particles altered consistently. These observations indicate that large VLDL particles that accumulate during alimentary lipemia undergo compositional changes that render them more prone to cellular binding and uptake.

Comparison of the uptake and processing of cholesterol from chylomicrons of different fatty acid composition in rats fed high-fat and low-fat diets

The fate of [3H]cholesterol carried in chylomicrons prepared from rats given a meal of palm oil (rich in long-chain saturated fatty acids), olive oil (rich in monounsaturated fatty acids) or corn oil (rich in n-6 polyunsaturated fatty acids) was investigated in vivo in rats fed a low-fat diet or a diet supplemented with the corresponding oil (to provide 40% of the calories) for 21 days. In the low-fat-fed groups, radioactivity was removed from the blood and secreted into bile over 180 min more rapidly when the chylomicrons were derived from corn oil as compared to palm or olive oil. After feeding the corresponding high-fat diets, however, both parameters were decreased in rats fed palm and corn oil, but not olive oil. As a result of these changes, the rates of removal of radioactivity from the blood and secretion into bile were similar in animals given the olive oil and corn oil diets, and higher than those in rats fed the palm oil diet. All the high-fat diets tended to increase the proportion of the radioactivity in the plasma found in the 1.006-1.050-g/ml fraction (low-density lipoprotein) and decrease that in the 1.050-1.25-g/ml (high-density lipoprotein) fraction in comparison to the respective low-fat diet groups, but the transfer of radioactivity to the plasma high-density lipoprotein fraction was particularly slow in palm-oil-fed rats. These findings indicate that diets high in saturated or n-6 polyunsaturated fat retard the metabolism of chylomicron cholesterol in comparison to diets low in fat, while those high in monounsaturated fat do not have this effect. As a consequence of this, the rate of removal of cholesterol of dietary origin from the body is slower in animals fed saturated as compared to monounsaturated or n-6 polyunsaturated fat. Thus, differential metabolism of chylomicron cholesterol clearly plays an important role in the hyper- and hypo-cholesterolaemic effects of these dietary fats.

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.

Chylomicron metabolism in experimental cirrhosis and cholestasis

Recently it has been demonstrated that artificial emulsions made of lecithin, cholesterol, cholesteryl-oleate and triolein simulate the metabolism of the natural chylomicra. Artificial-chylomicron delipidation and remnant disappearance from plasma were investigated in rats with carbon tetrachloride-induced hepatic cirrhosis or with cholestasis due to bile-duct ligation. Artificial chylomicra were labelled simultaneously with glyceryl tri [9, 10 (N)-3H] oleate and cholesteryl [1-14C] oleate and injected intra-arterially. Simultaneous chylomicron delipidation and remnant removal by the liver were calculated from the plasma radioactivity decay curves: that of glyceryl tri [9, 10 (N)-3H] oleate signifying the combined delipidation and particle-removal processes, whereas that of cholesteryl [1-14C] oleate representing the particle disappearance rate from plasma. Particle delipidation was increased in cirrhosis and decreased in cholestasis, implying faster and slower lipolysis rates respectively. On the other hand, the remnant removal rate by the liver slowed down in both experimental pathologies.

Infusion of atherogenic lipoprotein particles increases hepatic lipase activity in the rabbit

Hepatic lipase plays a key role in the turnover of potentially atherogenic lipoprotein remnants and in determining the relative distribution of high density lipoprotein (HDL) particle size subclasses. Rabbits fed a cholesterol-enriched diet have been found to accumulate potentially atherogenic chylomicron remnants and beta-very low density lipoprotein (beta-VLDL) and show a rapid increase in liver and postheparin plasma hepatic lipase activity. To determine whether the particles that accumulate during cholesterol feeding are a stimulus for this increase in hepatic lipase activity, we infused normal chow-fed rabbits with a chylomicron remnant plus beta-VLDL-enriched plasma fraction isolated from rabbits fed 0.5% cholesterol-supplemented chow. The infusion of this plasma fraction for 4 h increased hepatic lipase activity up to 2.9-fold over control rabbits and resulted in a loss of larger sized HDL particles consistent with the action of hepatic lipase. The increase in activity was significantly correlated with the concentration of infusate phospholipid, unesterified cholesterol, and esterified cholesterol, but not with the infusate triglyceride concentration. The change in the plasma cholesterol concentration of recipient rabbits, which reflects the degree of lipoprotein accumulation in these rabbits, was also significantly correlated with the change in hepatic lipase activity. However, a chylomicron remnant and beta-VLDL-depleted fraction of plasma from cholesterol-fed rabbits did not increase hepatic lipase activity. Furthermore, triglyceride presented as an artificial lipid emulsion (Intralipid) was not able to stimulate hepatic lipase activity, although triglyceride is a substrate for hepatic lipase.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular transport and degradation of chylomicron remnants in rat liver cells after in vivo endocytosis

The intracellular transport and degradation of in vivo endocytosed chylomicron remnants labelled with 125I in the protein moiety was studied in rat liver cells by means of subcellular fractionation in Nycodenz and sucrose density gradients. Initially, the radioactivity was located in low-density endosomes and was sequentially transferred to light and dense lysosomes. Data from gel filtration of the light and dense lysosomal fractions showed radioactive material with a molecular weight of about 1000-2000, representing short peptide fragments or amino acids which remain attached to iodinated tyramine cellobiose. In addition, undegraded apoproteins accumulated in both types of lysosome. Our data suggest that endocytosed chylomicron remnant apoproteins are first located in low-density endosomes and are sequentially transferred to light and dense lysosomes. Furthermore, the degradation process starts in the light lysosomes.

In vitro clearance of chylomicron triglycerides containing (omega-3) eicosapentaenoate

Rat mesenteric lymph chylomicrons, containing triglycerides enriched with either [14C]oleic acid or [14C]eicosapentaenoic acid, were prepared by ultracentrifugation of lymph samples collected for 6 h after a single duodenal infusion of an emulsion containing 0.3 mmol of either fatty acid. After determination of protein and of total fatty acid content and composition, enriched chylomicrons were suspended in Krebs-bicarbonate buffer. Non-working hearts were perfused in a recirculating system for 45 min using the enriched chylomicron preparations. At 15 min intervals during perfusion, the media were assayed for total radioactivity, 14CO2 and 14C-labeled fatty acids associated with triglycerides, unesterified fatty acids, phospholipids, mono- and diglycerides. After perfusion, the hearts were extracted and assayed for total lipid radioactivity and isotope distribution among heart lipid fractions. With this membrane-supported lipoprotein lipase system, clearances of chylomicron triglycerides containing either fatty acid were identical, as were the myocardial uptakes of the fatty acids and oxidations to 14CO2. Furthermore, except for a significantly greater incorporation of eicosapentaenoate into myocardial phospholipids, tissue isotope distributions of the two labeled fatty acids were also the same. These studies suggest that at least the initial phases of peripheral clearance of chylomicrons enriched in omega-3 fatty acids is as efficient as with those containing oleate.

Potential tumor- or organ-imaging agents XXIV: chylomicron remnants as carriers for hepatographic agents

This paper describes the possible utility of plasma lipoproteins for the site-specific delivery of diagnostic agents. The class of lipoproteins known as chylomicrons was selected for this preliminary study, since they are known to be rapidly metabolized and taken up by the liver. Cholesteryl iopanoate (II), an iodinated analogue of a normal constituent of the hydrophobic core of chylomicrons, was synthesized from cholesterol and iopanoic acid (I) and subsequently radiolabeled with ioidine-125. Whereas intravenous administration of II in physiological saline resulted in the appearance of approximately 31% of the dose in the liver at 0.5 hr, prior incorporation of II into chylomicrons resulted in an almost threefold (87%) increase in the liver accumulation of II in the same time period. A more gradual appearance of II in steroid-secreting tissues was consistent with the association of II with high-density lipoproteins following administration.

Cholesterol content as a possible determinant in the removal of chylomicron triglyceride by the liver

The nature of chylomicron removal from the circulation and the role of the liver in this process have been investigated by measuring the removal of chylomicron triglyceride and cholesterol from the plasma of normal and functionally hepatectomized rats. The rate of removal of chylomicron triglyceride exceeded that of both free and esterified cholesterol in all animals. Whereas chylomicrons were completely removed from the circulation of normal rats soon after intravenous injection, chylomicron remnant particles accumulated in the plasma very low density lipoprotein and chylomicron fractions of hepatectomized rats. The rate of removal of chylomicron triglyceride from the blood of the hepatectomized rats decreased when the cholesterol content of the chylomicrons was elevated. In addition, more chylomicron triglyceride remained in the circulation of the hepatectomized rats when the chylomicrons were enriched with cholesterol and the remnant particles derived from these chylomicrons were of lower average density. These observations indicate that the cholesteryl ester content of the chylomicrons influences the degree of triglyceride removal in extrahepatic tissue capillaries and thereby the amount of residual triglyceride in the remnant particles subsequently removed by the liver.

Serum and tissue distribution of benzol[a]pyrene from intravenously injected chylomicrons in rat in vivo

Rats were injected intravenously with chylomicrons (CM) containing benzo[a]pyrene (B[a]P), a carcinogenic aromatic hydrocarbon. The disappearance of B[a]P paralleled the removal of CM, both having a rapid initial decay and a secondary slow decay. After 5 min, at the end of the rapid phase, blood contained 19% of the total injected B[a]P, with 6% in blood cells and 13% in serum. At 60 min, serum contained 5% and blood cells 6% of the total B[a]P. During the slow phase, further distribution of B[a]P within serum albumin and various lipoproteins occurred, most of the label being in low density and very low density lipoproteins. Highest tissue specific activities of [3H]B[a]P were in the lung, liver and kidney tissues. These results suggest that in the rat, distribution of ingested polycyclic aromatic hydrocarbons in vivo tends to be primarily determined by the catabolism of chylomicrons.