In vitro transfer of chylomicron retinol and retinyl esters

The redistribution of rat chylomicron retinoids following incubation with fasting- or postheparin human plasma was investigated. With fasting plasma, chylomicron retinol appeared among higher density lipoprotein acceptors and density greater than 1.21 gm/ml plasma proteins; only small amounts of retinyl ester were found therein. With postheparin plasma, retinyl ester-containing chylomicron remnants with densities spanning the low- and high density lipoprotein distributions were generated; appreciable quantities of retinyl esters appeared among rho greater than 1.019 lipoproteins only in the presence of postheparin plasma. These observations are consistent with the conservation of retinyl esters, but not retinol, among chylomicrons and their catabolic products.     

Dietary vitamins A and E influence retinyl ester composition and content of the retinal pigment epithelium

Experiments were conducted to determine the influence of dietary levels of vitamin A and alpha-tocopherol on the amounts and composition of retinyl esters in the retinal pigment epithelium of light-adapted albino rats. Groups of rats were fed diets containing alpha-tocopherol and either no retinyl palmitate, adequate retinyl palmitate, or excessive retinyl palmitate. Other groups of rats received diets lacking alpha-tocopherol and containing the same three levels of retinyl palmitate. Retinoic acid was added to diets lacking retinyl palmitate. After 27 weeks, the animals were light-adapted to achieve essentially total visual pigment bleaches, and the neural retinas and retinal pigment epithelium-eyecups were then dissected from each eye for vitamin A ester determinations. Almost all of the retinyl esters were found in the retinal pigment epithelium-eyecup portions of the eyes, mainly as retinyl palmitate and retinyl stearate. Maintaining rats on a vitamin A-deficient, retinoic acid-containing diet led to significant reductions in retinal pigment epithelial retinyl ester levels in rats fed both the vitamin E-supplemented and vitamin E-deficient diets; contrary to expectations, the effect of dietary vitamin A deficiency was more pronounced in the vitamin E-supplemented rats. Vitamin A deficiency in retinoic acid-maintained animals also led to significant reductions in retinyl palmitate-to-stearate ester ratios in the retinal pigment epithelia of both vitamin E-supplemented and vitamin E-deficient rats. Excessive dietary intake of vitamin A had little, if any, effect on retinal pigment epithelial retinyl ester content or composition. Vitamin E deficiency resulted in significant increases in retinal pigment epithelial retinyl palmitate content and in palmitate-to-stearate ester ratios in rats fed all three levels of vitamin A, but had little effect on retinal pigment epithelial retinyl stearate content. In other tissues, vitamin E deficiency has been shown to lower vitamin A levels, and it is widely accepted that this effect is due to autoxidative destruction of vitamin A. The increase in retinal pigment epithelial vitamin A ester levels in response to vitamin E deficiency indicates that vitamin E does not regulate vitamin A levels in this tissue primarily by acting as an antioxidant, but rather may act as an inhibitor of vitamin A uptake and/or storage. The effect of vitamin E on pigment epithelial vitamin A levels may be mediated by the vitamin E-induced change in retinyl palmitate-to-stearate ratios.     

The majority of vitamin A is transported as retinyl esters in the blood of most carnivores

1. In canines and mustelides total vitamin A was 10-50 times higher compared to other species due to a high amount of retinyl esters (40-99% of total vitamin A) in blood plasma. The dominant vitamin A ester was in most species retinyl stearate. 2. In Ursidae, Procyonidae, Viveridae and Felidae, total vitamin A was much lower. When present, however, retinyl esters also represented 10-65% of total vitamin A in plasma. 3. Only retinol was detected in plasma of the family, Hyaenidae, and the suborder, Pinnipedia. 4. In maned wolf cubs it was found that retinol, retinyl esters and alpha-tocopherol increased with the age of the animals, reaching values comparable to adult animals at the age of 5 months.     

Phospholipid transfer protein deficiency protects circulating lipoproteins from oxidation due to the enhanced accumulation of vitamin E

Vitamin E is a lipophilic anti-oxidant that can prevent the oxidative damage of atherogenic lipoproteins. However, human trials with vitamin E have been disappointing, perhaps related to ineffective levels of vitamin E in atherogenic apoB-containing lipoproteins. Phospholipid transfer protein (PLTP) promotes vitamin E removal from atherogenic lipoproteins in vitro, and PLTP deficiency has recently been recognized as an anti-atherogenic state. To determine whether PLTP regulates lipoprotein vitamin E content in vivo, we measured alpha-tocopherol content and oxidation parameters of lipoproteins from PLTP-deficient mice in wild type, apoE-deficient, low density lipoprotein (LDL) receptor-deficient, or apoB/cholesteryl ester transfer protein transgenic backgrounds. In all four backgrounds, the vitamin E content of very low density lipoprotein (VLDL) and/or LDL was significantly increased in PLTP-deficient mice, compared with controls with normal plasma PLTP activity. Moreover, PLTP deficiency produced a dramatic delay in generation of conjugated dienes in oxidized apoB-containing lipoproteins as well as markedly lower titers of plasma IgG autoantibodies to oxidized LDL. The addition of purified PLTP to deficient plasma lowered the vitamin E content of VLDL plus LDL and normalized the generation of conjugated dienes. The data show that PLTP regulates the bioavailability of vitamin E in atherogenic lipoproteins and suggest a novel strategy for achieving more effective concentrations of anti-oxidants in lipoproteins, independent of dietary supplementation.     

Effects of retinoic acid on the concentrations of radioactive metabolites of retinol in tissues of rats maintained on a retinol-deficient diet

The effects of feeding retinoic acid for 2 and 6 days on the metabolism of labeled retinol in tissues of rats maintained on a vitamin A deficient diet was studied. The metabolites of retinol were analyzed by high performance liquid chromatography. Feeding retinoic acid for 2 days significantly reduced the blood retinol and retinyl ester levels without affecting the vitamin A content of the liver. In intestine and testis the content of labeled retinoic acid was decreased significantly by dietary retinoic acid. Addition of retinoic acid to the diet for 6 days resulted, in addition to decreased blood retinol and retinyl ester values, in an increase in the retinyl ester values in the liver. The accumulation of retinyl ester in the retinoic acid fed rat liver was accompanied by an absence of labeled retinoic acid. Kidney tissue was found to contain the highest levels of labeled retinoic acid, retinol, and retinyl esters; dietary retinoic acid did not alter the concentrations of these retinoids in the kidney during the experimental period. Since kidney retained more vitamin A when the liver vitamin A was low and also dietary retinoic acid did not affect the concentrations of radioactive retinoic acid in the kidney, it is suggested that the kidney may play a major role in the production of retinoic acid from retinol in the body.     

Concerted actions of cholesteryl ester transfer protein and phospholipid transfer protein in type 2 diabetes: effects of apolipoproteins

PURPOSE OF REVIEW: Type 2 diabetes frequently coincides with dyslipidemia, characterized by elevated plasma triglycerides, low high-density lipoprotein cholesterol levels and the presence of small dense low-density lipoprotein particles. Plasma lipid transfer proteins play an essential role in lipoprotein metabolism. It is thus vital to understand their pathophysiology and determine which factors influence their functioning in type 2 diabetes. RECENT FINDINGS: Cholesteryl ester transfer protein-mediated transfer is increased in diabetic patients and contributes to low plasma high-density lipoprotein cholesterol levels. Apolipoproteins A-I, A-II and E are components of the donor lipoprotein particles that participate in the transfer of cholesteryl esters from high-density lipoprotein to apolipoprotein B-containing lipoproteins. Current evidence for functional roles of apolipoproteins C-I, F and A-IV as modulators of cholesteryl ester transfer is discussed. Phospholipid transfer protein activity is increased in diabetic patients and may contribute to hepatic very low-density lipoprotein synthesis and secretion and vitamin E transfer. Apolipoprotein E could stimulate the phospholipid transfer protein-mediated transfer of surface fragments of triglyceride-rich lipoproteins to high-density lipoprotein, and promote high-density lipoprotein remodelling. SUMMARY: Both phospholipid and cholesteryl ester transfer proteins are important in very low and high-density lipoprotein metabolism and display concerted actions in patients with type 2 diabetes.     

Retinyl ester hydrolases and their roles in vitamin A homeostasis

In mammals, dietary vitamin A intake is essential for the maintenance of adequate retinoid (vitamin A and metabolites) supply of tissues and organs. Retinoids are taken up from animal or plant sources and subsequently stored in form of hydrophobic, biologically inactive retinyl esters (REs). Accessibility of these REs in the intestine, the circulation, and their mobilization from intracellular lipid droplets depends on the hydrolytic action of RE hydrolases (REHs). In particular, the mobilization of hepatic RE stores requires REHs to maintain steady plasma retinol levels thereby assuring constant vitamin A supply in times of food deprivation or inadequate vitamin A intake. In this review, we focus on the roles of extracellular and intracellular REHs in vitamin A metabolism. Furthermore, we will discuss the tissue-specific function of REHs and highlight major gaps in the understanding of RE catabolism. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.     

Transplacental delivery of retinoid: the role of retinol-binding protein and lipoprotein retinyl ester

Retinoids are required for normal embryonic development. Both embryonic retinoid deficiency and excess result in congenital malformations. There is little understanding of the physiology underlying retinoid transfer from the maternal circulation to the embryo. We now report studies that explore this process using retinol-binding protein-deficient (RBP-/-) mice and mice that express human RBP on the RBP-/-) background. Our studies establish that dietary retinoid, bound to lipoproteins, can serve as an important source for meeting tissue retinoid requirements during embryogenesis. Indeed, retinyl ester concentrations in the circulations of pregnant RBP-/- mice are significantly elevated over those observed in wild-type mice, suggesting that lipoprotein retinyl esters may compensate for the absence of retinol-RBP during pregnancy. We also demonstrate, contrary to earlier proposals, that maternal RBP does not cross the placenta and cannot enter the fetal circulation. Overall, our data indicate that both retinol-RBP and retinyl esters bound to lipoproteins are able to provide sufficient retinoid to the embryo to allow for normal embryonic development.     

Uptake of vitamin A in macrophages from physiologic transport proteins: role of retinol-binding protein and chylomicron remnants

Vitamin A plays an important role in reducing infectious disease morbidity and mortality by enhancing immunity, an effect that is partly mediated by macrophages. Thus, knowing how these cells take up vitamin A is important. The results in the present study demonstrate that J774 macrophages efficiently take up chylomicron remnant retinyl esters and retinol-binding protein (retinol-RBP) bound retinol by specific and saturable mechanisms. The binding of (125)I-RBP to plasma membrane vesicles demonstrated that the macrophage receptor had a similar binding affinity, as was discovered previously for other cells. The B(max) for the macrophages was smaller than the values reported for placenta, bone marrow, and kidney, but larger than that reported for liver. The J774 cells also bound and took up [(3)H]retinol-RBP. Approximately 50 to 60% of the uptake may compete with excess unlabeled retinol-RBP and approximately 30 to 40% with excess transtyrethin. Following the uptake of [(3)H]retinol-RBP, an extensive esterification occurred: After 5 hours of incubation, 77.8 +/- 3.9% (SD; n = 3) of the cellular radioactivity was recovered as retinyl esters. The J774 cells also demonstrated saturable binding of chylomicron remnant [(3)H]retinyl esters, and a continuous uptake at 37 degrees C followed by an extensive hydrolysis of the retinyl esters. Binding could be inhibited by approximately 50% by excess unlabeled low density lipoprotein (LDL). In addition, lipoprotein lipase increased the binding of chylomicron remnant [(3)H]retinyl esters by approximately 30% and the uptake of chylomicron remnant [(3)H]retinyl ester by more than 300%. Furthermore, because sodium chlorate reduced binding with 40% and uptake with 55%, the results suggest that proteoglycans are involved in the uptake. Thus, the results suggest that both LDL receptor and LDL-related protein are involved in the uptake of chylomicron remnant [(3)H]retinyl ester in macrophages.     

Clearance of chylomicron remnants in normolipidaemic patients with coronary artery disease: case control study over three years.

OBJECTIVE--To test the hypothesis that subjects who clear chylomicron remnants slowly from plasma may be at higher risk of coronary artery disease than indicated by their fasting plasma lipid concentrations. DESIGN--Case control study over three years. SETTING--An 800 bed general municipal hospital. SUBJECTS--85 normolipidaemic patients with coronary artery disease selected prospectively and matched with 85 normolipidaemic subjects with normal coronary arteries on angiography. INTERVENTIONS--All subjects were given a vitamin A fat loading test which specifically labels intestinal lipoproteins with retinyl palmitate. MAIN OUTCOME MEASURE--Postprandial lipoprotein metabolism. RESULTS--The area below the chylomicron remnant retinyl palmitate curve was significantly increased in the coronary artery disease group as compared with the controls (mean 23.4 (SD 15.0) v 15.3 (8.9) mumol/l.h; 95% confidence interval of difference 4.37 to 11.82). CONCLUSION--Normolipidaemic patients with coronary artery disease had significantly higher concentrations of chylomicron remnants in plasma than normolipidaemic subjects with normal coronary vessels. This may explain the mechanism underlying the susceptibility to atherosclerosis of coronary artery disease patients with normal fasting lipid values. As diet and drugs can ameliorate the accumulation of postprandial lipoproteins in plasma, the concentration of chylomicron remnants should be measured in patients at high risk of coronary artery disease.     

Fatty acid, carotenoid and vitamin A composition of tissues of free living gulls

The aim of this study was to investigate fatty acid and carotenoid profile as well as vitamin A (retinol and retinol esters) content in gull (Larus fucus) tissues. Palmitic (16:0) and stearic (18:0) fatty acids were major saturates in all the tissues studied. Oleic acid (18:1n-9) was the major monounsaturate in the tissue phospholipids varying from 11.9% (liver) up to 18.2% (lung). Arachidonic acid (20:4n-6) was the major unsaturate in the phospholipid fraction in all the tissues. Liver contained the highest total carotenoid concentration which was 5 and 7 fold higher compared to kidney and pancreas. In the liver beta-carotene was major carotenoid. In contrast, in all other tissues beta-carotene was minor fraction with lutein being major carotenoid. Zeaxanthin, canthaxanthin, beta-cryptoxanthin and echinenone were also identified in the gull tissues. Liver and kidney were characterised by the highest vitamin A concentrations (1067.5 and 867.5 microg/g, respectively). Retinol comprised from 55.3% (pancreas) down to 8% (kidney) of the total vitamin A but was not detected in the abdominal fat. Retinyl palmitate was the major retinyl ester in the liver, kidney and heart (44.2; 38.1 and 46.0% of total retinyl esters). In muscles and abdominal fat retinyl stearate was the major retinyl ester fraction. Therefore high proportions of beta-carotene were found in gull liver and peripheral tissues were enriched by lutein and zeaxanthin compared to the liver, a very high concentration of retinyl esters in the kidney was observed and tissue-specificity in retinyl ester proportions in peripheral tissues was found.     

Characterization of a new endogenous vitamin A metabolite

Here, we describe the discovery of a new major endogenous vitamin A metabolite with particularly high hepatic concentrations. This metabolite was isolated from mouse livers and was characterized as 9-cis-4-oxo-13,14-dihydro-retinoic acid (RA) based on mass spectral, ultraviolet, and nuclear magnetic resonance analyses. It was also detected in one human liver. To gain further insight into endogenous retinoid metabolism, mice were fed over a period of 14 days ad libitum with diets enriched with different amounts of retinyl palmitate [15,000, 45,000 or 150,000 international units (IU)/kg diet]. Higher retinyl palmitate amounts in the diet resulted surprisingly in a dose-dependent decrease in all-trans-RA levels in serum, kidney, and brain, whereas levels of 9-cis-4-oxo-13,14-dihydro-RA, retinol, and retinyl esters were dose-dependently elevated in serum, kidney, and liver. 13-cis-RA levels could be detected in serum, liver, and kidney, but were unaffected by the dietary vitamin A status. 9-cis-RA levels were below the detection limit of 0.2 ng/ml serum or 0.4 ng/g tissue. This study indicates that the oxidation at C4 of the cyclohexenyl ring, isomerization of the C9/C10 double bond, and reduction of the C13/C14 double bond are major endogenous metabolic pathways of vitamin A.     

Retinyl ester storage particles (retinosomes) from the retinal pigmented epithelium resemble lipid droplets in other tissues

Levels of many hydrophobic cellular substances are tightly regulated because of their potential cytotoxicity. These compounds tend to self-aggregate in cytoplasmic storage depots termed lipid droplets/bodies that have well defined structures that contain additional components, including cholesterol and various proteins. Hydrophobic substances in these structures become mobilized in a specific and regulated manner as dictated by cellular requirements. Retinal pigmented epithelial cells in the eye produce retinyl ester-containing lipid droplets named retinosomes. These esters are mobilized to replenish the visual chromophore, 11-cis-retinal, and their storage ensures proper visual function despite fluctuations in dietary vitamin A intake. But it remains unclear whether retinosomes are structures specific to the eye or similar to lipid droplets in other organs/tissues that contain substances other than retinyl esters. Thus, we initially investigated the production of these lipid droplets in experimental cell lines expressing lecithin:retinol acyltransferase, a key enzyme involved in formation of retinyl ester-containing retinosomes from all-trans-retinol. We found that retinosomes and oleate-derived lipid droplets form and co-localize concomitantly, indicating their intrinsic structural similarities. Next, we isolated native retinosomes from bovine retinal pigmented epithelium and found that their protein and hydrophobic small molecular constituents were similar to those of lipid droplets reported for other experimental cell lines and tissues. These unexpected findings suggest a common mechanism for lipid droplet formation that exhibits broad chemical specificity for the hydrophobic substances being stored.     

Vitamin A status regulates hepatic lecithin: retinol acyltransferase activity in rats

The activity of lecithin:retinol acyltransferase (LRAT) was determined in microsomes from the liver and small intestine of rats with differing vitamin A status. In animals depleted of retinol, as judged by undetectable liver vitamin A stores and low plasma retinol concentrations, hepatic LRAT activity was almost undetectable, whether assayed with retinol bound to cellular retinol-binding protein or solvent-dispersed retinol. In contrast, neither the activity of intestinal LRAT nor that of acyl-CoA:retinol acyltransferase in either liver or intestine differed from that of vitamin A-adequate rats. During the course of vitamin A depletion, liver LRAT activity fell progressively, nearly in parallel to the decrease in plasma retinol concentration. Oral repletion of vitamin A-depleted rats with 0.8 mg of retinol resulted in a very rapid restoration of plasma retinol concentration and full recovery of hepatic LRAT activity within 24 h, together with deposition of retinyl ester in the liver. These data strongly implicate LRAT activity in liver as responsible for the storage of hepatic retinyl esters. Retention of the intestine's capacity to esterify retinol during vitamin A deficiency provides a mechanism for capture of dietary vitamin A, while reduced hepatic LRAT activity may function to redirect retinol in liver from storage to other metabolic pathways.     

Effect of hexachlorobiphenyl on vitamin A homeostasis in the rat

Vitamin A status and turnover were examined in rats that had been exposed to chronic dietary treatment of 3,4,5,3',4',5'-hexachlorobiphenyl (HCB), 1 mg/kg diet. HCB caused hepatic depletion and renal accumulation of vitamin A, and a 1.7-fold increase in the serum retinol concentration. Intravenously administered [3H]retinol bound to retinol binding protein-transthyretin complex (RBP-TTR complex) was used to study the dynamics of circulatory retinol in these rats. In HCB-treated rats, the plasma turnover rate of retinol was increased compared to vitamin A-adequate untreated controls. HCB caused a 50% reduction of total radioactivity in liver, and, except for 0.5 h after the [3H]retinol-RBP-TTR dose, the specific activity of the hepatic retinyl ester pool was greater compared to control rats. The kidneys of HCB-treated rats accumulated radioactivity in the retinyl ester fraction. HCB also caused a 50% reduction in adrenal radioactivity compared with control rats. Urinary and fecal excretion of radioactivity was 3-fold higher in HCB-treated rats as compared to controls. Our findings demonstrate that chronic HCB feeding results in expansion of plasma vitamin A mass, in changes of liver and kidney retinol and retinyl ester pool dynamics and in an increased metabolism of vitamin A.     

Randomised controlled trial of effect of fruit and vegetable consumption on plasma concentrations of lipids and antioxidants.

OBJECTIVES: To determine the extent to which plasma antioxidant concentrations in people with habitual low intake of fruit and vegetables respond to increased intakes of these foods. To examine whether advice to increase fruit and vegetables will result in reduction of concentrations of total and low density lipoprotein cholesterol. DESIGN: Randomised controlled trial in which intervention and control groups were followed up for eight weeks. The intervention group was asked to consume eight servings of fruit and vegetables a day. SETTING: Dunedin, New Zealand. SUBJECTS: Eighty seven subjects with normal lipid concentrations who ate three or fewer servings of fruit and vegetables daily. MAIN OUTCOME MEASURES: Plasma concentrations of vitamin C, retinol, alpha and beta carotene, alpha tocopherol, lipids, and lipoproteins. Dietary intake assessed with diet records over four days. RESULTS: The mean plasma vitamin C, alpha carotene, and beta carotene concentrations increased in parallel with increased dietary intake of fruit and vegetables in the intervention group. Concentrations of retinol, alpha tocopherol, lipids, and lipoproteins remained unchanged despite some increase in dietary vitamin E and a small reduction in saturated fat intake. CONCLUSIONS: Following a recommendation to increase fruit and vegetable consumption produces change in plasma concentrations of vitamin C, alpha carotene, and beta carotene likely to reduce incidence of cancer. More specific dietary advice to modify fat intake may be necessary to reduce the risk of cardiovascular disease mediated by lipoprotein and vitamin E.     

Perisinusoidal fat-storing cells are the main vitamin A storage sites in rat liver

Highly purified sinusoidal (fat-storing, Kupffer and endothelial cells) and parenchymal cells were isolated to assess the cellular distribution of vitamin A in liver of adult vitamin A-sufficient rats. A modified simple procedure was developed for the purification of fat-storing cells from rat liver. This was achieved by a single centrifugation step in a two-layer density Nycodenz gradient. Endothelial and Kupffer cells were obtained from the same gradient and further purified by centrifugal elutriation. Reverse-phase HPLC analysis showed that fat-storing cells contained about 300-fold the amount of retinyl esters present in parenchymal cells on a mg cell protein basis. In fat-storing cells, the same retinyl esters, viz. retinyl palmitate, retinyl stearate and retinyl oleate, were present as in whole liver. It was also observed that, within 12 h after intravenous injection of chylomicron [3H]retinyl ester, most of the radioactivity had accumulated in the fat-storing cells. It is concluded that fat-storing cells are the main storage sites for vitamin A in rat liver.     

Intestinal absorption of dietary cholesteryl ester is decreased but retinyl ester absorption is normal in carboxyl ester lipase knockout mice

Carboxyl ester lipase (CEL; EC 3.1.1.13) hydrolyzes cholesteryl esters and retinyl esters in vitro. In vivo, pancreatic CEL is thought to liberate cholesterol and retinol from their esters prior to absorption in the intestine. CEL is also a major lipase in the breast milk of many mammals, including humans and mice, and is thought to participate in the processing of triglycerides to provide energy for growth and development while the pancreas of the neonate matures. Other suggested roles for CEL include the direct facilitation of the intestinal absorption of free cholesterol and the modification of plasma lipoproteins. Mice with different CEL genotypes [wild type (WT), knockout (CELKO), heterozygote] were generated to study the functions of CEL in a physiological system. Mice grew and developed normally, independent of the CEL genotype of the pup or nursing mother. Consistent with this was the normal absorption of triglyceride in CELKO mice. The absorption of free cholesterol was also not significantly different between CELKO (87 +/- 26%, mean +/- SD) and WT littermates (76 +/- 10%). Compared to WT mice, however, CELKO mice absorbed only about 50% of the cholesterol provided as cholesteryl ester (CE). There was no evidence for the direct intestinal uptake of CE or for intestinal bacterial enzymes that hydrolyze it, suggesting that another enzyme besides CEL can hydrolyze dietary CE in mice. Surprisingly, CELKO and WT mice absorbed similar amounts of retinol provided as retinyl ester (RE). RE hydrolysis, however, was required for absorption, implying that CEL was not the responsible enzyme. The changes in plasma lipid and lipoprotein levels to diets with increasing lipid content were similar in mice of all three CEL genotypes. Overall, the data indicate that in the mouse, other enzymes besides CEL participate in the hydrolysis of dietary cholesteryl esters, retinyl esters, and triglycerides.     

Kinetics of chylomicron remnant clearance in normal and in hyperlipoproteinemic subjects

The kinetics of chylomicron metabolism have been studied by measuring retinyl palmitate in chylomicrons and their remnants for 10-12 hr following oral administration of vitamin A and Lipomul in three groups of adult male subjects: A) normal plasma triglyceride levels (n = 7); B) endogenous hypertriglyceridemia (n = 12); C) apolipoprotein E (apoE) phenotype E2/2, with Type 3 hyperlipoproteinemia (n = 4) or normal plasma lipids (n = 1). A multicompartmental model was developed using SAAM 27 to characterize the appearance, intravascular metabolism, and clearance from the plasma of retinyl palmitate-labeled dietary lipoproteins. The half-times for retinyl palmitate clearance from the chylomicron remnant fraction (T1/2 REMNANT) were 14.1 +/- 9.7 min in Group A; they were prolonged in Group B (50.7 +/- 20.8 min) and were extremely prolonged for Type 3 subjects in Group C (611.9 +/- 419.9 min). One subject with the apoE 2/2 phenotype and normal plasma triglycerides had a T1/2 REMNANT of 66.8 min. T1/2 REMNANT was highly correlated with fasting plasma triglycerides in Group A and B (r = 0.77, slope = 0.15), and in Group C (r = 0.97, slope = 0.85). These results support the interpretation that delayed chylomicron remnant clearance in subjects with endogenous hypertriglyceridemia may be largely secondary to overproduction of VLDL particles, whose remnants compete with chylomicron remnants for removal by the liver via apoE receptor-mediated endocytosis. The subjects with apoE 2/2 have an additional defect in the removal of chylomicron remnants presumably due to the structural abnormality in their apoE.     

Expression of carboxylesterase and lipase genes in rat liver cell-types

Approximately 80% of the body vitamin A is stored in liver stellate cells with in the lipid droplets as retinyl esters. In low vitamin A status or after liver injury, stellate cells are depleted of the stored retinyl esters by their hydrolysis to retinol. However, the identity of retinyl ester hydrolase(s) expressed in stellate cells is unknown. The expression of carboxylesterase and lipase genes in purified liver cell-types was investigated by real-time PCR. We found that six carboxylesterase and hepatic lipase genes were expressed in hepatocytes. Adipose triglyceride lipase was expressed in Kupffer cells, stellate cells and endothelial cells. Lipoprotein lipase expression was detected in Kupffer cells and stellate cells. As a function of stellate cell activation, expression of adipose triglyceride lipase decreased by twofold and lipoprotein lipase increased by 32-fold suggesting that it may play a role in retinol ester hydrolysis during stellate cell activation.