Stimulation of the in vivo production of very low density lipoproteins by apolipoprotein E is independent of the presence of the low density lipoprotein receptor

Apolipoprotein (apo) E stimulates the secretion of very low density lipoproteins (VLDLs) by an as yet unknown mechanism. Recently, a working mechanism for apoE was proposed (Twisk, J., Gillian-Daniel, D. L., Tebon, A., Wang, L., Barrett, P. H., and Attie, A. D. (2000) J. Clin. Invest. 105, 521-532) in which apoE prevents the inhibitory action of the low density lipoprotein receptor (LDLr) by binding to it. We have first tested whether this newly described effect of the LDLr on VLDL secretion, obtained in vitro, is also observed in vivo. In LDLr knockout mice (LDLr-/-), the production of VLDL triglycerides and apoB was 30% higher than that in controls. Also the ratio of apoB100:apoB48 secretion was increased in the LDLr-/- mice. The composition of nascent VLDL was similar in both strains. To test whether the action of apoE depends on the presence of the LDLr, VLDL production was measured in LDLr-/- and apoE-/- LDLr-/- mice. Deletion of apoE on a LDLr-/- background still caused a 50% decrease of VLDL triglycerides and apoB production. The composition of nascent VLDL was again similar for both strains. We conclude that the effect of apoE on hepatic VLDL production is independent of the presence of the LDLr.

Apolipoprotein C-III deficiency accelerates triglyceride hydrolysis by lipoprotein lipase in wild-type and apoE knockout mice

Previous studies with hypertriglyceridemic APOC3 transgenic mice have suggested that apolipoprotein C-III (apoC-III) may inhibit either the apoE-mediated hepatic uptake of TG-rich lipoproteins and/or the lipoprotein lipase (LPL)-mediated hydrolysis of TG. Accordingly, apoC3 knockout (apoC3(-/-)) mice are hypotriglyceridemic. In the present study, we attempted to elucidate the mechanism(s) underlying these phenomena by intercrossing apoC3(-/-) mice with apoE(-/-) mice to study the effects of apoC-III deficiency against a hyperlipidemic background. Similar to apoE(+/+) apoC3(-/-) mice, apoE(-/-)apoC3(-/-) mice exhibited a marked reduction in VLDL cholesterol and TG, indicating that the mechanism(s) by which apoC-III deficiency exerts its lipid-lowering effect act independent of apoE. On both backgrounds, apoC3(-/-) mice showed normal intestinal lipid absorption and hepatic VLDL TG secretion. However, turnover studies showed that TG-labeled emulsion particles were cleared much more rapidly in apoC3(-/-) mice, whereas the clearance of VLDL apoB, as a marker for whole particle uptake by the liver, was not affected. Furthermore, it was shown that cholesteryl oleate-labeled particles were also cleared faster in apoC3(-/-) mice. Thus the mechanisms underlying the hypolipidemia in apoC3(-/-) mice involve both a more efficient hydrolysis of VLDL TG as well as an enhanced selective clearance of VLDL cholesteryl esters from plasma. In summary, our studies of apoC3(-/-) mice support the concept that apoC-III is an effective inhibitor of VLDL TG hydrolysis and reveal a potential regulating role for apoC-III with respect to the selective uptake of cholesteryl esters.

Dietary plant stanol esters reduce VLDL cholesterol secretion and bile saturation in apolipoprotein E*3-Leiden transgenic mice

Dietary plant stanols lower serum cholesterol levels in humans and in hyperlipidemic rodents, mainly by inhibition of the intestinal cholesterol absorption. We used female apolipoprotein E*3-Leiden transgenic mice to investigate the consequences of this effect on serum lipid levels and hepatic lipid metabolism. Five groups of 6 or 7 mice received for 9 weeks a diet containing 0.25% cholesterol and 0.0%, 0.25%, 0.5%, 0.75%, or 1.0% (wt/wt) plant stanols (sitostanol 88% [wt/wt], campestanol 10% [wt/wt]) esterified to fatty acids. Compared with the control diet, plant stanol ester treatment dose-dependently reduced serum cholesterol levels by 10% to 33% (P<0.05), mainly in very low density lipoproteins (VLDLs), intermediate density lipoproteins, and low density lipoproteins. Furthermore, 1.0% of the dietary plant stanols significantly decreased the liver contents of cholesteryl esters (-62%), free cholesterol (-31%), and triglycerides (-38%) but did not change the hepatic VLDL-triglyceride and VLDL-apolipoprotein B production rates. However, plant stanol ester feeding significantly decreased the amounts of cholesteryl esters and free cholesterol incorporated in nascent VLDLs by 72% and 30%, respectively, resulting in a net 2-fold decreased VLDL cholesterol output. Liver mRNA levels of low density lipoprotein receptors, 3-hydroxy-3-methylglutaryl coenzyme A synthase, cholesterol 7alpha-hydroxylase, and sterol 27-hydroxylase were not changed by plant stanol ester feeding. Nevertheless, the serum lathosterol-to-cholesterol ratio was significantly increased by 23%, indicating that dietary plant stanol esters increased whole-body cholesterol synthesis. Plant stanol esters also significantly decreased the cholesterol saturation index in bile by 55%. In conclusion, in apolipoprotein E*3-Leiden transgenic mice, plant stanol ester feeding dose-dependently lowered serum cholesterol levels as a result of a reduced secretion of VLDL cholesterol. This was caused by a decreased hepatic cholesterol content that also resulted in a lowered biliary cholesterol output, indicative of a reduced lithogenicity of bile in these mice.

Apolipoprotein E is resistant to intracellular degradation in vitro and in vivo. Evidence for retroendocytosis

Apolipoprotein E (apoE) is an important determinant for the uptake of triglyceride-rich lipoproteins and emulsions by the liver, but the intracellular pathway of apoE following particle internalization is poorly defined. In the present study, we investigated whether retroendocytosis is a unique feature of apoE as compared with apoB by studying the intracellular fate of very low density lipoprotein-sized apoE-containing triglyceride-rich emulsion particles and LDL after LDLr-mediated uptake. Incubation of HepG2 cells with [(3)H]cholesteryl oleate-labeled particles at 37 degrees C led to a rapid release of [(3)H]cholesterol within 30 min for both LDL and emulsion particles. In contrast, emulsion-derived (125)I-apoE was more resistant to degradation (>/=120 min) than LDL-derived (125)I-apoB (30 min). Incubation at 18 degrees C, which allows endosomal uptake but prevents lysosomal degradation, with subsequent incubation at 37 degrees C resulted in a time-dependent release of intact apoE from the cells (up to 14% of the endocytosed apoE at 4 h). The release of apoE was accelerated by the presence of protein-free emulsion (20%) or high density lipoprotein (26%). Retroendocytosis of intact particles could be excluded since little intact [(3)H]cholesteryl oleate was released (<3%). In contrast, the degradation of LDL was complete with virtually no secretion of intact apoB into the medium. The intracellular stability of apoE was also demonstrated after hepatic uptake in C57Bl/6 mice. Intravenous injection of (125)I-apoE and [(3)H]cholesteryl oleate-labeled emulsions resulted in efficient LDLr-mediated uptake of both components by the liver (45-50% of the injected dose after 20 min). At 1 h after injection, only 15-20% of the hepatic (125)I-apoE was degraded, whereas 75% of the [(3)H]cholesteryl oleate was hydrolyzed. From these data we conclude that following LDLr-mediated internalization by liver cells, apoE can escape degradation and can be resecreted. This sequence of events may allow apoE to participate in its hypothesized intracellular functions such as mediator of the post-lysosomal trafficking of lipids and very low density lipoprotein assembly.

Hyperlipidemia of ApoE2(Arg(158)-Cys) and ApoE3-Leiden transgenic mice is modulated predominantly by LDL receptor expression

To investigate the relative roles of the LDL receptor- and non-LDL receptor-mediated pathways in the clearance of apolipoprotein E (apoE) variants in vivo, we have generated apoE2(Arg(158)-Cys) (apoE2) and apoE3-Leiden transgenic mice deficient for the endogenous mouse Apoe and Ldl receptor genes (Apoe-/-.Ldlr-/- mice). Unexpectedly, on the Apoe-/-.Ldlr-/- background, expression of neither apoE2 nor apoE3-Leiden results in a decrease of the hyperlipidemia. In contrast, serum cholesterol levels are increased by the introduction of apoE2 and apoE3-Leiden in Apoe-/-.Ldlr-/- mice (to 39.1+/-7.1 and 37.6+/-7.6 mmol/L, respectively, from 25. 9+/-6.5 mmol/L). In addition, in these transgenic mice, the serum triglyceride levels are substantially increased (to 9.6+/-7.0 and 5. 8+/-2.8 mmol/L, respectively, from 0.7+/-0.5 mmol/L), which is associated with a decreased efficiency of in vitro LPL-mediated lipolysis of circulating VLDL. The VLDL-triglyceride secretion rate is not affected by the expression of apoE2 or apoE3-Leiden on the Apoe-/-.Ldlr-/- background. These results indicate that in the absence of the LDL receptor, clearance of triglyceride-rich apoE2 and apoE3-Leiden-containing lipoproteins via alternative hepatic receptors, such as the LDL receptor-related protein (LRP) is inefficient. Although apoE2 and apoE3-Leiden are disturbed in binding to the LDL receptor in vitro, expression of 1 or 2 mouse Ldlr alleles in an apoE2.Apoe-/- or apoE3-Leiden.Apoe-/- background results in a gene dose-dependent decrease of the hyperlipidemia. Furthermore, overexpression of the LDL receptor via adenovirus-mediated gene transfer rescues the hyperlipidemia associated with apoE2 and apoE3-Leiden expression. These data indicate that in apoE2 and apoE3-Leiden transgenic mice, the LDL receptor constitutes the predominant route for clearance of VLDL remnants, carrying even poorly binding apoE variants, and that this pathway is functional despite an apoE-mediated disturbance in VLDL triglyceride lipolysis.

Reversal of hypercholesterolemia in apolipoprotein E2 and apolipoprotein E3-Leiden transgenic mice by adenovirus-mediated gene transfer of the VLDL receptor

We have investigated the interaction of apolipoprotein E2(Arg158-Cys) (apoE2) and apolipoprotein E3-Leiden (apoE3-Leiden) with the very low density lipoprotein (VLDL) receptor in vivo and in vitro to define the possible role of this receptor in lipoprotein metabolism and atherosclerosis. The in vivo binding specificity of the VLDL receptor for apoE2 and apoE3-Leiden was investigated by adenovirus-mediated gene transfer of the VLDL receptor in apoE2 and apoE3-Leiden transgenic mice lacking endogenous mouse apoE (Apoe-/-). Ectopic overexpression of the VLDL receptor gene in the liver resulted in a >50% decrease of plasma cholesterol levels in both apoE2 and apoE3-Leiden transgenic mice compared with liver expression of the beta-galactosidase gene. This reduction in plasma cholesterol was mainly due to a reduction in the VLDL level. Overexpression of the VLDL receptor did not affect the hepatic VLDL triglyceride production, indicating that the hypocholesterolemic effect is due to an increased level of plasma clearance mediated by the VLDL receptor. In vitro binding analysis showed that both apoE2 and apoE3-Leiden VLDL compete efficiently with rabbit beta-VLDL for binding to the VLDL receptor expressed on LDL receptor-deficient Chinese hamster ovary cells. We conclude from these data that both apoE2 and apoE3-Leiden function as proper ligands for the VLDL receptor in vitro and in vivo. This finding substantiates a possible role for the VLDL receptor in atherosclerosis in hyperlipidemic subjects homozygous for apoE2 or carrying apoE3-Leiden and indicates that the VLDL receptor expressed on the liver has therapeutic potential as an alternative route for clearance of binding-defective lipoproteins.

In the absence of endogenous mouse apolipoprotein E, apolipoprotein E*2(Arg-158 --> Cys) transgenic mice develop more severe hyperlipoproteinemia than apolipoprotein E*3-Leiden transgenic mice

Apolipoprotein E*2(Arg-158 --> Cys) (APOE*2) transgenic mice were generated and compared to the previously generated apolipoprotein E*3-Leiden (APOE*3-Leiden) transgenic mice to study the variable expression of hyperlipoproteinemia associated with these two APOE variants. In the presence of the endogenous mouse Apoe gene, the expression of the APOE*3-Leiden gene resulted in slightly elevated levels of serum cholesterol as compared with control mice (2.7 +/- 0. 5 versus 2.1 +/- 0.2 mmol/liter, respectively), whereas the expression of the APOE*2(Arg-158 --> Cys) gene did not affect serum cholesterol levels, even after high/fat cholesterol feeding. The extreme cholesterol level usually found in apoE-deficient mice (Apoe-/- mice; 23.6 +/- 5.0 mmol/liter) could be rescued by introducing the APOE*3-Leiden gene (APOE*3-Leiden.Apoe-/-; 3.6 +/- 1. 5 mmol/liter), whereas the expression of the APOE*2(Arg-158 --> Cys) gene in Apoe-/- mice minimally reduced serum cholesterol levels (APOE*2.Apoe-/-; 16.6 +/- 2.9 mmol/liter). In vivo very low density lipoprotein (VLDL) turnover studies revealed that APOE*2.Apoe-/- VLDL and APOE*3-Leiden.Apoe-/- VLDL display strongly reduced fractional catabolic rates as compared with control mouse VLDL (4.0 and 6.1 versus 22.1 pools/h). In vitro low density lipoprotein (LDL) receptor binding studies using HepG2 and J774 cells showed that APOE*2. Apoe-/- VLDL is completely defective in binding to the LDL receptor, whereas APOE*3-Leiden.Apoe-/- VLDL still displayed a considerable binding activity to the LDL receptor. After transfection of APOE*2.Apoe-/- and APOE*3-Leiden.Apoe-/- mice with adenovirus carrying the gene for the receptor-associated protein (AdCMV-RAP), serum lipid levels strongly increased (15.3 to 42.8 and 1.4 to 15.3 mmol/liter for cholesterol and 5.0 to 35.7 and 0.3 to 20. 7 mmol/liter for triglycerides, respectively). This indicates that RAP-sensitive receptors, possibly the LDL receptor-related protein (LRP), mediate the plasma clearance of both APOE*2.Apoe-/- and APOE*3-Leiden. Apoe-/- VLDL. We conclude that in vivo the APOE*2 variant is completely defective in LDL receptor binding but not in binding to LRP, whereas for the APOE*3-Leiden mutant both LRP and LDL receptor binding activity are only mildly affected. As a consequence of this difference, APOE*2.Apoe-/- develop more severe hypercholesterolemia than APOE*3-Leiden.Apoe-/- mice.

Modulation of very low density lipoprotein production and clearance contributes to age- and gender- dependent hyperlipoproteinemia in apolipoprotein E3-Leiden transgenic mice

Apolipoprotein E3-Leiden (APOE*3-Leiden) transgenic mice have been studied to identify factors modulating chylomicron and VLDL remnant lipoprotein metabolism. Transient elevated levels of VLDL/LDL-sized lipoproteins occurred in these mice with maximal levels during the period of rapid growth (optimum at 45 d of age). After about 100 d of age, serum cholesterol and triglyceride levels stabilized to slightly elevated levels as compared to control mice. The expression of the APOE*3-Leiden transgene was not age-dependent. In young mice the in vivo hepatic production of VLDL-triglycerides was 50% increased as compared to older mice. This is sustained by in vivo VLDL-apo B turnover studies showing increased (75%) VLDL-apo B secretion rates in young mice, whereas the VLDL-apo B clearance rate appeared not to be age dependent. On a high fat/cholesterol diet, females displayed significantly higher cholesterol levels than males (10 versus 7.0 mmol/liter, respectively). Serum levels of VLDL/LDL sized lipoproteins increased upon administration of estrogens, whereas administration of testosterone gave the opposite result. As compared to male mice, in female mice the hepatic VLDL-triglyceride production rate was significantly elevated. Injection of estrogen in males also resulted in increased VLDL-triglyceride production, although not statistically significant. In vivo VLDL-apo B turnover experiments showed that the VLDL secretion rate tended to be higher in females. Although, the fractional catabolic rate of VLDL-apo B is not different between males and females, administration of estrogens in males resulted in a decreased clearance rate of VLDL, whereas administration of testosterone in females resulted in an increased clearance rate of VLDL. The latter presumably due to an inhibiting effect of testosterone on the expression of the APOE*3-Leiden transgene. We conclude that hyperlipidemia in APOE*3-Leiden transgenic mice is strongly affected by age via its effect on hepatic VLDL production rate, whereas gender influences hyperlipidemia by modulating both hepatic VLDL production and clearance rate.

Lipoprotein lipase stimulates the binding and uptake of moderately oxidized low-density lipoprotein by J774 macrophages

Lipoprotein lipase (LPL) stimulates the uptake of low-density lipoprotein (LDL) and very-low-density lipoprotein (VLDL) in different cell types, including macrophages, through bridging of LPL between lipoproteins and extracellular heparan sulphate proteoglycans (HSPG). Because macrophages produce LPL and because modified lipoproteins are present in the arterial wall in vivo, we wondered whether LPL also enhances the uptake of oxidized LDL by J774 macrophages. LDL samples with different degrees of oxidation, as evaluated by relative electrophoretic mobility (REM) as compared with native LDL are used as well as native and acetylated LDL. Addition of 5 microg/ml LPL to the J774 cell culture medium stimulated the binding of both native LDL and moderately oxidized LDL (REM < 3.5) 50-100-fold, and their uptake was stimulated approx. 20-fold. The LPL-mediated binding of native LDL and moderately oxidized LDL was dose-dependent. Preincubation of the cells with heparinase (2.4 units/ml) inhibited the stimulatory effect of LPL, indicating that this LPL-mediated stimulation was due to bridging between the lipoproteins and HSPG. The binding to J774 macrophages of severely oxidized LDL (REM=4.3) was stimulated less than 3-fold by LPL, whereas its uptake was not stimulated significantly. The binding and uptake of acetylated LDL (AcLDL) were not stimulated by LPL, although the LPL-molecule itself does bind to AcLDL. Measurements of the cellular lipid content showed that addition of LPL also stimulated the accumulation in the cells of cholesteryl ester derived from both native LDL and moderately oxidized LDL in a dose-dependent manner. We conclude that our results present experimental evidence for the hypothesis that LPL serves as an atherogenic component in the vessel wall.

Increased response to cholesterol feeding in apolipoprotein C1-deficient mice

The function of apolipoprotein (apo) C1 in vivo is not well understood. From in vitro studies it has been reported that an excess of apoC1 relative to apoE inhibits receptor-mediated uptake of remnant lipoproteins [Sehayek and Eisenberg (1991) J. Biol. Chem. 266, 22453-22459]. In order to gain a better understanding of the role of apoC1 in lipoprotein metabolism in vivo, we have generated apoC1-deficient mice by gene targeting in embryonic stem cells. Homozygous mutant mice are viable and do not show overt abnormalities. Serum triacylglycerol levels are increased by 60% on both a standard mouse diet and a mild hypercholesterolaemic diet compared with controls. Total serum cholesterol levels are similar to controls on the two diets. However, the level of high-density lipoprotein cholesterol in the apoC1-deficient mice fed on the mild hypercholesterolaemic diet is slightly decreased, which is accompanied by a 3-fold increase in very-low-density plus low-density lipoprotein (VLDL+LDL) cholesterol. On a severe atherogenic diet, the homozygous apoC1-deficient mice become hypercholesterolaemic, with a serum cholesterol level of 10.7 +/- 3.3 mM compared with 6.7 +/- 1.8 mM and 5.1 +/- 1.6 mM in heterozygous and control mice respectively. The increase in cholesterol is mainly confined to the VLDL+LDL-sized fractions. Binding experiments revealed that lipoproteins lacking apoC1 with d < 1.006 g/ml are poor competitors for 125I-labelled LDL binding to the LDL receptor on HepG2 cells. This suggests that total apoC1 deficiency leads to impaired receptor-mediated clearance of remnant lipoproteins rather than enhanced uptake, as was expected from data reported in the literature.

Triglyceride-rich lipoproteins of subjects heterozygous for apolipoprotein E2(Lys146-->Gln) are inefficiently converted to cholesterol-rich lipoproteins

The APOE*2(Lys146-->Gln) allele behaves like a dominant trait in the expression of familial dysbetalipoproteinemia (FD) (Smit et al., J. Lipid Res. 1990; 31: 45-53). FD patients carrying the APOE*2(Lys146-->Gln) allele exhibit less elevated cholesterol to triglyceride ratios in the d < 1.019 g/ml lipoprotein density fraction as compared to classical FD patients displaying homozygosity for the APOE*2(Arg158-->Cys) allele (0.8 vs. 1.4). Upon treatment of complete serum with lipoprotein lipase (LPL), the mean cholesterol to triglyceride molar ratio of the d < 1.019 g/ml lipoprotein fraction in these FD patients increased only marginally (from 0.8 to 1.1), as compared with that of classical FD subjects (from 1.4 to 2.6) and non-FD control subjects (from 0.7 to 1.5). In order to obtain further evidence for an inefficient lipolysis of the d < 1.019 g/ml lipoprotein fraction in APOE*2(Lys146-->Gln) carriers, possibly in combination with a less efficient cholesteryl ester transfer protein (CETP) activity, blood samples of APOE*2(Lys146-->Gln) allele carrying FD patients were analysed and compared with classical FD patients and controls. In the APOE*2(Lys149-->Gln) allele carrying FD patients were analysed and compared with classical FD patients and controls. In the APOE*2(Lys146-->Gln) FD patients, the increase in plasma cholesterol was mainly confined to the very low density lipoprotein (VLDL) fraction, whereas in classical FD patients, the levels of cholesterol in the intermediate density lipoprotein (IDL) fraction was also dramatically increased (ratios of VLDL to IDL cholesterol are 4.7 and 2.6, respectively). Family analyses of the APOE*2(Lys146-->Gln) FD subjects showed that the apo E to apo B ratio in the d < 1.019 g/ml lipoprotein fraction of allele carriers is 3.5 times as high as that found in non-carriers (2.8 vs. 0.8, by wt.). Also, in the APOE*2(Lys146-->Gln) allele carrying family members, the ratio of cholesterol to triglyceride of the d < 1.019 g/ml lipoprotein fraction is less markedly elevated upon addition of LPL when compared to that in non-carrying controls (from 1.1 to 1.8 vs 0.7 to 1.6). The efficiency of the d < 1.019 g/ml lipoprotein fraction of APOE*2(Lys146-->Gln) FD patients to compete with low density lipoprotein (LDL) for binding to the LDL receptor is intermediate to that of controls and classical APOE*2(Arg158-->Cys) homozygous FD patients. These findings suggest that in APOE*2(Lys146-->Gln) allele carriers, the conversion of VLDL into IDL is impaired due to an inefficient lipolysis, possibly in combination with a retarded CETP activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Diet-induced hyperlipoproteinemia and atherosclerosis in apolipoprotein E3-Leiden transgenic mice

Apolipoprotein E3-Leiden (APOE*3-Leiden) transgenic mice have been used to study the effect of different cholesterol-containing diets on the remnant lipoprotein levels and composition and on the possible concurrent development of atherosclerotic plaques. On high fat/cholesterol (HFC) diet, the high expressing lines 2 and 181 developed severe hypercholesterolemia (up to 40 and 60 mmol/liter, respectively), whereas triglyceride levels remained almost normal when compared with regular mouse diet. The addition of cholate increased the hypercholesterolemic effect of this diet. In lines 2 and 181, serum levels of apo E3-Leiden also increased dramatically upon cholesterol feeding (up to 107 and 300 mg/dl, respectively). In these high expressing APOE*3-Leiden transgenic mice, the increase in both serum cholesterol and apo E3-Leiden occurred mainly in the VLDL/LDL-sized fractions, whereas a considerable increase in large, apo E-rich HDL particles also occurred. In contrast to the high expressing lines, the low expressing line 195 reacted only mildly upon HFC diet. On HFC diets, the high expresser APOE*3-Leiden mice developed atherosclerotic lesions in the aortic arch, the descending aorta, and the carotid arteries, varying from fatty streaks containing foam cells to severe atherosclerotic plaques containing cholesterol crystals, fibrosis, and necrotic calcified tissue. Quantitative evaluation revealed that the atherogenesis is positively correlated with the serum level of cholesterol-rich VLDL/LDL particles. In conclusion, with APOE*3-Leiden transgenic mice, factors can be studied that influence the metabolism of remnant VLDL and the development of atherosclerosis.

Inefficient degradation of triglyceride-rich lipoprotein by HepG2 cells is due to a retarded transport to the lysosomal compartment

Binding studies at 37 degrees C showed that lipoprotein lipase-treated very low density lipoproteins (LPL-VLDL) and very low density lipoproteins (VLDL), once taken up via the low density lipoprotein (LDL) receptor, are poorly degraded by HepG2 cells as compared with LDL. Determination of the initial endocytotic rate for LPL-VLDL and VLDL as compared to LDL shows that LPL-VLDL and VLDL are internalized at a similar rate as LDL. Incubation of cells with labeled LDL, LPL-VLDL, and VLDL at 18 degrees C for 4.5 h resulted in the accumulation of these particles in the early endosomes, without subsequent transport to the lysosomes and degradation. After washing the cells and a temperature shift to 37 degrees C, the labeled LDL present in the early endosomes is transported to the lysosomal compartment almost completely within 15 min. Strikingly, for LPL-VLDL and for VLDL, only about 50% or less of the label was moved to the lysosomal compartment within 45 min. However, once present in the lysosomes, VLDL and LPL-VLDL are degraded about 1.6-fold more rapidly than LDL. Retroendocytosis accounts for less than 10% of the internalized LDL, whereas a higher rate of retroendocytosis, up to 20 and 40%, respectively, was observed for LPL-VLDL and VLDL. To evaluate the effect of the inefficient transport of VLDL and LPL-VLDL to the lysosomal compartment on cellular cholesterol homeostasis, acyl-CoA:cholesterol acyltransferase (ACAT) activity was measured. Incubation with 30 micrograms/ml of LDL induced a 2.5-fold increase in ACAT activity, whereas the incubation with similar amounts of both VLDL and LPL-VLDL failed to stimulate this enzyme. We conclude that both a slower transport to the lysosomal compartment and a higher rate of retroendocytosis, possibly as the consequence of the longer residence time in the early endosomes, are responsible for the poor degradation of VLDL and LPL-VLDL by HepG2 cells.

Transgenic mice carrying the apolipoprotein E3-Leiden gene exhibit hyperlipoproteinemia

Apolipoprotein (apo) E3-Leiden, described in a large Dutch family, is associated with a dominantly inherited form of familial dysbetalipoproteinemia. To study the effect of the APOE*3-Leiden mutation in vivo, transgenic mice were generated using a genomic 27-kilobase DNA construct isolated from the APOE*3-Leiden proband. This construct carried the APOE gene, the APOC1 gene, and all known regulatory elements including an element that mediates liver expression. Three strains were generated that showed human APOE and APOC1 expression. All strains had significantly elevated levels of total plasma cholesterol and triglycerides on a regular diet. When mice of one strain were fed a semisynthetic cholesterol-rich diet, total plasma cholesterol and triglyceride levels increased dramatically. This increase was observed mainly in the very low density lipoprotein (VLDL)- and low density lipoprotein (LDL)-sized fractions. In cholesterol-fed mice, the apoE3-Leiden protein became equally distributed between the VLDL/LDL and HDL-sized fractions, while in mice kept on a regular diet, apoE3-Leiden protein was mainly associated with HDL-sized fractions. The presence of hyperlipoproteinemia in the APOE*3-Leiden-expressing transgenic mice supports our finding that the apoE3-Leiden variant behaves like a dominant trait in the expression of familial dysbetalipoproteinemia. ApoE3-Leiden transgenic mice may serve as a model to elucidate additional factors involved in the metabolism of apoE containing remnant lipoproteins in general and the etiology of familial dysbetalipoproteinemia in particular.

Molecular aspects of age-related changes in albumin synthesis in female WAG/Rij rats

The rate of albumin gene transcription and the level of albumin RNA sequences in liver nuclei of rats of various ages were determined to investigate the regulation of age-related changes in levels of cytoplasmic albumin mRNA in rat liver. No change was observed with age in both the rate of albumin gene transcription and the amount of nuclear albumin RNA sequences, which suggests that the increase in cytoplasmic albumin mRNA content in the rat liver with age is caused by a decreased turnover of this messenger. The length of poly(A)-tails of rat liver cytoplasmic RNA with age was also examined. Although no change with age was observed in the poly(A)-tail length sedimenting at the peak fraction, a shift towards shorter poly(A)-tails was found with age in the overall poly(A)-tail length distribution.