Metabolism of apoprotein B of plasma very low density lipoproteins in the rat

High-fructose feeding promotes accelerated degradation of hepatic LDL receptor and hypercholesterolemia in hamsters via elevated circulating PCSK9 levels

BACKGROUND

High fructose diet (HFD) induces dyslipidemia and insulin resistance in experimental animals and humans with incomplete mechanistic understanding. By utilizing mice and hamsters as in vivo models, we investigated whether high fructose consumption affects serum PCSK9 and liver LDL receptor (LDLR) protein levels.

RESULTS

Feeding mice with an HFD increased serum cholesterol and reduced serum PCSK9 levels as compared with the mice fed a normal chow diet (NCD). In contrast to the inverse relationship in mice, serum PCSK9 and cholesterol levels were co-elevated in HFD-fed hamsters. Liver tissue analysis revealed that PCSK9 mRNA and protein levels were both reduced in mice and hamsters by HFD feeding, however, liver LDLR protein levels were markedly reduced by HFD in hamsters but not in mice. We further showed that circulating PCSK9 clearance rates were significantly lower in hamsters fed an HFD as compared with the hamsters fed NCD, providing additional evidence for the reduced hepatic LDLR function by HFD consumption. The majority of PCSK9 in hamster serum was detected as a 53 kDa N-terminus cleaved protein. By conducting in vitro studies, we demonstrate that this 53 kDa truncated hamster PCSK9 is functionally active in promoting hepatic LDLR degradation.

CONCLUSION

Our studies for the first time demonstrate that high fructose consumption increases serum PCSK9 concentrations and reduces liver LDLR protein levels in hyperlipidemic hamsters. The positive correlation between circulating cholesterol and PCSK9 and the reduction of liver LDLR protein in HFD-fed hamsters suggest that hamster is a better animal model than mouse to study the modulation of PCSK9/LDLR pathway by atherogenic diets.

Triglyceride-rich lipoproteins and plasma lipid transport

This memoir provides a history of the triglyceride-rich lipoproteins of blood plasma over the last half-century. As precursors of low-density lipoproteins and in their own right, triglyceride-rich lipoproteins are essential to the formation of atherosclerotic plaques and to consequent ischemic vascular disease. The author recounts research at the National Heart Institute during 1953 to 1956 and continuing thereafter at the University of California San Francisco. Emphasis is placed on key insights arising from investigations of human disease, the interplay of fatty acid and triglyceride-transport involving the liver, small intestine, adipose tissue and muscle, and the role of the liver in the synthesis and catabolism of atherogenic lipoproteins.

VLDL-resembling phospholipid-submicron emulsion for cholesterol-based drug targeting

The objective of the current study was to develop and evaluate VLDL-resembling phospholipid-submicron emulsion (PSME) as a carrier system for new cholesterol-based compounds for targeted delivery to cancer cells. BCH, a boronated cholesterol compound, was originally developed in our laboratory to mimic the cholesterol esters present in the LDL and to follow a similar pathway of cholesterol transport into the rapidly dividing cancer cells. The VLDL-resembling system was then designed to solubilize BCH, facilitate the interaction with LDL, and thus assist the BCH delivery to cancer cells. BCH-containing PSME was prepared by sonication. Chemical compositions and particle sizes of different PSME fractions were determined. The lipid structure of PSME and location of BCH in the formulation were assessed based on experimental results. Density gradient ultracentrifugation fractionated the emulsion into three particle-size populations with structures and compositions resembling native VLDL. In vitro interaction between PSME and LDL was evident by agarose electrophoresis, as both formed a single band with an intermediate mobility. The transfer of BCH from PSME to LDL was also observed in the presence of other serum components including serum proteins. Cell culture data showed sufficient uptake of BCH in rat 9L glioma cells (> 50 microg boron/g cells). In conclusion, this system has the capability to incorporate the cholesterol-based compound, interact with native LDL, and assist the delivery of this compound into cancer cells in vitro.

The rise of the plasma lipid concentration elicited by dietary sodium chloride restriction in Wistar rats is due to an impairment of the plasma triacylglycerol removal rate

Studies in humans have indicated that dietary salt restriction raises plasma levels of total cholesterol (TC) and triacylglycerols (TAG). In order to explain the mechanisms involved, a rat experimental model was developed consisting of chronic feeding ad libitum isocaloric diets with variable sodium chloride contents. Rates of synthesis of plasma TAG were measured either as the increase of plasma TAG after blocking its removal from plasma by the intra-arterial pulse infusion of Triton-WR 1339, or as the plasma rate of incorporation of [(14)C]-oleic acid [(14)C]-TAG. Plasma TAG removal rate was determined by the intra-arterial pulse infusion of a lipid emulsion. Severe salt restriction increased the plasma concentrations of TAG (71%) and of TC (10%). This result was not due to modification of the rate of synthesis of plasma TAG but was attributed to a 55% slower rate of removal of the TAG-containing lipoproteins. An increased plasma non-esterified fatty acid concentration, probably due to a salt restriction-related insulin resistance, may have impaired the activity of the enzyme lipoprotein lipase.

Development of atherosclerosis in alloxan diabetic rats

Rats with alloxan-induced diabetes developed severe atherosclerotic lesions when they were maintained on a 0.25% cholesterol diet for one year. The atheromatous changes developed at the aortic arch, appeared as early as 3 months after the start of the experiment, and increased thereafter. The diabetic rats also developed atherosclerosis when they were fed standard rat chow, but the area of the atheromatous lesion was about one tenth of that in rats fed the high-cholesterol diet. Normal rats did not develop atherosclerosis even when fed the high-cholesterol diet for one year. The alloxan diabetic rats showed no increase in body weight, but developed serum glucose levels as high as 600-800 mg/dl as well as high serum cholesterol levels and lower serum HDL-cholesterol levels. The development of atherosclerosis in these rats was significantly related to an increase in the serum cholesterol/phospholipid ratio, the atherogenic index (TC-HDLC/HDLC), and the serum total cholesterol level, but was not related to the serum glucose, HDL-cholesterol, triglyceride, or lipid peroxide levels. These relationships were found as early as B-16 weeks after the start of the experiment. These data suggest that the serum cholesterol/phospholipid ratio, the atherogenic index, and the total cholesterol level are important risk factors for the development of atherosclerosis in rats with alloxan diabetes.

Plasma lipoproteins after triglyceride clearance in cholesterol-fed rats

The clearance of particulate triglyceride from the plasma of cholesterol-fed rats with appreciable stores of hepatic cholesterol ester produces a substantial increment in plasma cholesterol. Most of this plasma cholesterol increment arises from existing tissue sources. The increment begins from 4 to 6 h after clearance and is due to the appearance of larger cholesterol-rich, triglyceride-poor, beta migrating lipoproteins, which are isolated in the d < 1.063 fraction with an apoprotein (Apo) content consisting primarily of Apo E and smaller amounts of Apo B. A concurrent decrease in alpha lipoproteins occurs with the beta lipoprotein increment. Within 1 d of clearance the beta lipoproteins fall and alpha lipoproteins increase. The increase in total plasma Apo E and Apo B initially parallels that of the cholesterol, but it persists even when cholesterol falls. A modest decrease in plasma Apo A1 was observed during the time alpha lipoproteins declined. A significant increase in plasma lecithin cholesterol acyl transferase preceded the increase in beta lipoprotein cholesterol. This enzyme increment was absent in rats with little lipoprotein response despite increased hepatic cholesterol. In vivo inhibition of this enzyme with dithionitrobenzoic acid virtually eliminated the postclearance hypercholesterolemia. Plasma particulate triglyceride clearance induces an increase in beta lipoproteins. Coupling of this clearance and hepatic lipoprotein secretion occurs by an unknown mechanism modulated by lecithin cholesterol acyl transferase.

Metabolism of very low density lipoproteins in the pig. An in vivo study

1. The metabolism of apolipoprotein B (apoB) was investigated in pigs injected with [125I]very low density lipoproteins (VLDL) to determine to which extent the two distinct low density lipoprotein subclasses (LDL1 and LDL2) derive from VLDL. 2. The lipoproteins were isolated by density gradient ultracentrifugation and the transfer of radioactivity from VLDL into LDL1 and LDL2 apoB was measured. 3. Only a minor portion of VLDL apoB was converted to LDL1 (7.7 +/- 3.2%) and LDL2 (3.6 +/- 1.5%), respectively. Thus, we conclude that the major portion of LDL, especially LDL2, is synthesized independently from VLDL catabolism.

Liver metabolism of cholesterol taken up from lipoproteins in Wistar rats. An in vivo comparison between rat and human lipoproteins

1. This study compares liver uptake, biliary secretion and blood decay of VLDL, LDL, HDL2 and HDL3 lipoprotein fractions isolated from both rat and human plasma and labelled with [14C]-cholesterol following the i.v. administration to the bile-fistulated rat model. 2. The present results demonstrate that the use of heterologous lipoproteins in bile-fistulated rat can be helpful in administering in a small volume large amounts of free and esterified cholesterol and in evaluating specific aspects of lipoprotein cholesterol metabolism by liver.

The preferential uptake of very-low-density lipoprotein cholesteryl ester by rat liver in vivo

The removal from the blood and the uptake by the liver of injected very-low-density lipoprotein (VLDL) preparations that had been radiolabelled in their apoprotein and cholesteryl ester moieties was studied in lactating rats. Radiolabelled cholesteryl ester was removed from the blood and taken up by the liver more rapidly than sucrose-radiolabelled apoprotein. Near-maximum cholesteryl ester uptake by the liver occurred within 5 min of the injection of the VLDL. At this time, apoprotein B uptake by the liver was only about 25% of the maximum. Maximum uptake of the injected VLDL apoprotein B label was not achieved until at least 15 min after injection, by which time the total uptakes of cholesteryl ester and apoprotein B label were very similar. The results suggest that preferential uptake of the lipoprotein cholesteryl ester by the liver occurred before endocytosis of the entire lipoprotein complex. The fate of the injected VLDL cholesteryl ester after its uptake by the liver was also monitored. Radiolabel associated with the hepatic cholesteryl ester fraction fell steadily from its early maximum level, the rate of fall being faster and more extensive when the fatty acid, rather than the cholesterol, moiety of the ester was labelled. By 30 min after the injection of VLDL containing [3H]cholesteryl ester, over one-third of the injected label was already present as [3H]cholesterol in the liver. When VLDL containing cholesteryl [14C]oleate was injected, a substantial proportion (about 25%) of the injected radiolabelled fatty acid appeared in the hepatic triacylglycerol fraction within 60 min: very little was present in the plasma triacylglycerol fraction at this time.

Changes in the composition of plasma lipoproteins in the chronic uremic rat

The effect of chronic renal failure on the lipid and apolipoprotein concentrations of plasma, very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), low density lipoproteins (LDL) and high density lipoproteins (HDL) was studied in an experimental uremic rat model. Control rats were sham-operated and were divided into adlibitum-fed and pair-fed groups. The rats were studied (after an overnight fast) 32 days after the onset of uremia. The uremic rats had a 4-fold increase in plasma urea nitrogen and creatinine. The pair-fed and ad-lib-fed controls had similar levels of plasma urea nitrogen and lipid profiles. In the uremic rats, plasma triglyceride (TG) levels were increased 3.8-fold due to increased TG in the VLDL, IDL and HDL fractions. Their 2-3-fold increase in plasma free cholesterol (FC), esterified cholesterol (EC) and phospholipids (PL) were due to FC, EC and PL increases in VLDL, IDL, LDL and HDL. Their increase in plasma apo B (x 2.4) and apo E (x 1.5) were due to increases in VLDL, IDL and LDL. Their plasma apo A-I increased 2.4 fold due to increases in the LDL and HDL fractions. Uremic rats also had increases in the FC/PL molar ratio in VLDL, IDL and LDL. In their LDL, the apo B/total cholesterol (TC), apo B/PL and apo B/apo E molar ratios were decreased. In their HDL, the apo E/TC and apo E/PL molar ratios were decreased and the apo A-I/apo E molar ratio was increased. In conclusion, chronic uremia causes both quantitative changes in the levels and qualitative changes in the composition of the plasma lipoprotein particles. These results are compatible with the decreased hepatic lipase activities and impairment of remnant clearance observed in human chronic renal failure.

Cellular metabolism of human plasma intermediate-density lipoprotein (IDL)

The cellular metabolism of human plasma intermediate-density lipoprotein (IDL) was investigated in cultured human skin fibroblasts and hepG-2 cell in the absence and presence of exogenous recombinant or plasmatic apo E-3. IDL (d 1.006-1.019 g/ml) and LDL (d 1.019-1.063 g/ml) were prepared by centrifugation from the plasma of apo E-3/3 or 4/3 normolipidemic human subjects. Without added apo E-3, IDL binding and cell association are similar or slightly reduced while their degradation is one third to one half. This results in degradation to binding ratios for IDL that are half those for LDL. Exogenous apo E-3 enhances binding, association and degradation of IDL by 50-150%, but the degradation to binding ratio remains low. Exogenous apo E-3 also increased the ability of IDL but not LDL, to down-regulate the incorporation of [14C]acetate to sterol by the cells. The optimal concentration of apo E-3 is 4 micrograms protein/10 micrograms IDL protein and at that concentration appreciable amounts of the apo E are found associated with the lipoprotein. Apo E-2 has no effect on the cellular metabolism of IDL and apo E-3 is not effective in receptor-negative human fibroblasts. Monoclonal antibodies that block apo E binding to B,E (LDL) receptor (1D7) abolish the cellular metabolism of IDL while antibodies against B-100 (4G3) are ineffective. In competitive binding experiments, IDL is slightly more effective than LDL in displacing 125I-LDL from receptors in hepG-2 cells and appreciably more effective than LDL when tested against 125I-IDL. Apo E-3 increases the capacity of IDL to compete with either 125I-LDL or 125I-IDL. Addition of apo E-3 also increases the binding affinity of IDL to hepG-2 receptors, with Kd values of 2.50, 0.93 micrograms protein/ml, respectively. The study demonstrates the essential role that functional apo E molecules play in the interaction of human IDL with cellular receptors. Yet, in spite of presence of apo E in IDL (2-3 molecules/particle) and enrichment of IDL with apo E-3 (to 4-5 molecules/particle) the proteolytic degradation of the lipoprotein by specific cellular receptor is similar to LDL.

Turnover of 125I-VLDL and 131I-LDL apolipoprotein B in rabbits fed diets containing casein or soy protein

Rabbits fed low-fat, cholesterol-free, semi-purified diets containing casein developed a marked hypercholesterolemia compared to rabbits fed a similar diet containing soy protein (plasma cholesterol 281 +/- 31 vs. 86 +/- 9 mg/dl; P less than 0.05). Turnover studies (three per dietary group) were carried out in which homologous 125I-labeled VLDL and 131I-labeled LDL were injected simultaneously into casein- (n = 8) or soy protein- (n = 9) fed rabbits. ApoB-specific activities were determined in VLDL, IDL and LDL isolated from the pooled plasma of two or three rabbits per dietary group. The production rate of VLDL apoB (1.20 +/- 0.3 vs. 1.09 +/- 0.1 mg/h per kg) was similar for the two dietary groups. The fractional catabolic rate of VLDL apoB was lower for the casein group (0.15 +/- 0.03 vs. 0.23 +/- 0.01.h-1; 0.05 less than P less than 0.10). Although the pool size of VLDL apoB was higher in the casein group (8 +/- 2 vs. 5 +/- 0.3 mg/kg), this value did not reach statistical significance. For LDL apoB, the increased pool size in casein-fed rabbits (30 +/- 5 vs. 5 +/- 1 mg/kg; P less than 0.01) was associated with a decreased fractional catabolic rate (0.03 +/- 0.005 vs. 0.08 +/- 0.008.h-1; P less than 0.01) and a 2-fold increase in the production rate of LDL apoB (1 +/- 0.3 vs. 0.4 +/- 0.06 mg/kg per h; 0.05 less than P less than 0.10) compared to rabbits fed soy protein. Analysis of precursor-product relationships between the various lipoprotein fractions showed that casein-fed rabbits synthesized a higher proportion of LDL apoB (95% +/- 2 vs. 67% +/- 2; P less than 0.001) independent of VLDL catabolism. These results support the concept that the hypercholesterolemia in casein-fed rabbits is associated with impaired LDL removal consistent with a down-regulation of LDL receptors. These changes do not occur when the casein is replaced by soy protein.

Effect of lipid transfer protein on plasma lipids, apolipoproteins and metabolism of high-density lipoprotein cholesteryl ester in the rat

The role of human plasma lipid transfer protein (LTP) in lipoprotein metabolism was studied in the rat, a species without endogenous cholesteryl ester and triacylglycerol transfer activity. Partially purified human LTP was injected intravenously into rats. The plasma activity was between 1.5- and 4-fold that of human plasma during the experiments. 6 h after the injection of LTP, a significant increase in serum apoB, and no significant changes in serum total cholesterol, free cholesterol, triacylglycerols, apoA-I, apoE, or apoA-IV were noted. Cholesterol was increased in very-low density and low-density lipoproteins (VLDL and LDL) and decreased in large-sized apoE-rich HDL. ApoA-I-containing particles with a size smaller than in normal rats were present in serum of LTP-treated rats. The mean diameter of HDL particles decreased and apoE, normally present on large-sized HDL, was present on smaller sized particles. The metabolic fate of cholesteryl ester, originally associated with HDL, was studied by injection of [3H]cholesteryl linoleyl ether-labelled apoA-I-rich HDL in the absence and in the presence of LTP. The disappearance of [3H]cholesteryl linoleyl ether, injected as part of apoA-I-rich HDL, from serum was increased in the LTP-treated rats; the t1/2 changed from 3.9 to 2.2 h, resulting in an increased accumulation of [3H]cholesteryl linoleyl ether in the liver. This can be explained by the redistribution of HDL [3H]cholesteryl linoleyl ether to VLDL and LDL in the presence of LTP, leading to the combined contribution of VLDL, LDL and HDL to the hepatic uptake. The present findings show profound effects of LTP on the chemical composition of HDL subspecies, the size of HDL and on the plasma turnover and hepatic uptake of cholesteryl esters originally present in apo A-I-rich HDL.

Enhanced metabolism of normolipidemic human plasma very low density lipoprotein in cultured cells by exogenous apolipoprotein E-3

In this investigation in cultured human fibroblasts, an attempt was made to determine the optimal metabolism of apolipoprotein (apo) B-100 lipoproteins from normolipidemic human subjects. We supplemented culture systems containing 125I-lipoproteins with exogenous recombinant or plasmatic apo E-3. Very low density lipoprotein (VLDL) fractions I, II, and III, and low density lipoproteins (LDL) were prepared from one E 4/3 and four E 3/3 subjects. Without added apo E-3, cellular metabolism (binding, cell association, and degradation) of VLDL-I, II, and III was negligible. Exogenous apo E-3 caused a many-fold enhancement of the metabolism of the three VLDL fractions, but LDL was not affected. The effects of apo E-3 were specific, not observed with apo E-2, and not observed on receptor-negative cells. Exogenous apo E-3 also enhanced down-regulation of cellular sterol synthesis by the VLDLs, but not LDL, indicating increased particle catabolism by the cells. The optimal concentrations of exogenous apo E-3 were 4 to 6 micrograms protein/15 micrograms VLDL-protein, when most of the added apo E-3 became associated with the VLDL particles. Apo E-3 failed to associate with LDL. These results demonstrate that availability and association of adequate amounts of apo E-3 are crucial for optimal cellular metabolism of apo B-100 lipoproteins along the VLDL----LDL cascade.

Ascites fluid lipoproteins in experimental nephrotic syndrome

Lipoprotein content and composition were studied in ascites fluid of puromycin aminonucleoside-nephrotic rats. All of the lipoprotein density classes were found in ascites fluid. Protein levels compared to plasma were: very low density lipoprotein (VLDL, d less than 1.006), 1.2%; intermediate density lipoprotein (IDL, 1.006 less than d less than 1.02), 2.6%; low density lipoprotein (LDL, 1.02 less than d less than 1.063), 1.0%; and high density lipoprotein (HDL, 1.063 less than d less than 1.21), 1.1%. The predominant protein in ascites fluid was albumin, present at 1.9% of the plasma level. Radioiodinated VLDL and HDL injected intravenously into nephrotic rats appeared in lipoprotein fractions of the ascites fluid. VLDL and IDL triacylglycerol content and particle diameter were low compared with plasma particles, suggesting peritoneal triacylglycerol lipase activity; such lipase activity could account for the increased proportion of LDL in the ascites fluid. Ascites fluid LDL and HDL phospholipid and free cholesterol were high and cholesteryl ester was low. Ascites lipoproteins contained the same apolipoproteins as plasma, but in different proportions. Ascites VLDL had higher apolipoprotein B and lower apolipoprotein E, while LDL and HDL had higher apolipoprotein E. Ascites HDL could be separated by heparin-Sepharose affinity column chromatography into a retained and a non-retained fraction, while nearly all nephrotic plasma HDL was non-retained. These data suggest that modification of ascites fluid lipoproteins occurs prior to their entry into the lymph and return to the blood, perhaps mediated by peritoneal macrophages.

Metabolism of human intermediate and very low density lipoprotein subfractions from normal and dysbetalipoproteinemic plasma. In vivo studies in rat

Subfractions of radioiodinated d less than 1.019 g/ml lipoproteins were isolated by nonequilibrium density gradient ultracentrifugation (DGU) from normal and dysbetalipoproteinemic human plasma and were injected into rats. Size and density (d) of lipoprotein products formed over 8 hours were assessed by gradient gel electrophoresis and equilibrium DGU, respectively. Subfractions containing a subspecies of very low density lipoproteins (VLDL) of particle diameter greater than 350 A were cleared rapidly from the plasma and formed only small amounts of low density lipoproteins (LDL). Fractions containing VLDL subspecies of smaller diameter (300 to 350 A) were cleared much more slowly, and formed greater amounts of a discrete LDL product with the characteristics of human LDL-II (peak particle diameter 255 to 265 A, d = 1.030 to 1.040 g/ml). A similar LDL product was formed from subfractions containing intermediate density lipoproteins (IDL). Cholesterol-enriched subspecies within the smaller, denser portion of the IDL spectrum, however, yielded two additional products. One had size and density characteristic of the major human LDL-I subclass reported previously (265 to 275 A, d = 1.025 to 1.030 g/ml), while the other was yet larger (275 to 285 A) and overlapped normal IDL in size and density. In dysbetalipoproteinemic plasma, the metabolic precursors of the largest product were shifted from the IDL to the small VLDL (beta-VLDL) particle distribution. Since beta-VLDL are known to predispose to accelerated atherosclerosis in dysbetalipoproteinemia, it may be that metabolically homologous cholesterol-enriched IDL subspecies in other subjects have similar atherogenic properties.

Generation of plasma free cholesterol from circulating lipoprotein-associated cholesteryl ester

Studies were performed to investigate the contribution of lipoprotein-associated cholesteryl ester (CE) in the monkey to circulating free cholesterol (FC). Monkey plasma was incubated with [14C]- or [3H]cholesteryl ester, and radiolabeled low-density lipoproteins (LDL) and high-density lipoproteins (HDL) were isolated by ultracentrifugation. Animals received labeled LDL or HDL. A rapid transfer of CE between lipoproteins was observed, consistent with an active CE transfer protein activity in the monkey. Within 4 h the percent of plasma radioactivity in FC after injection of CE-labeled LDL or HDL was, respectively, 30 and 7% of that of the ester. To determine whether the generation of FC was due to a circulating plasma cholesteryl ester hydrolase, monkey plasma was incubated with CE-labeled lipoproteins with and without 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB). A small amount of FC (less than 3% of the radioactivity) was generated during this incubation but most of the FC production was inhibited by DTNB. Although a small amount of FC can be produced by a plasma cholesteryl esterase (perhaps via reverse action of lecithin-cholesterol acyltransferase), most of the FC in plasma derived from lipoprotein-associated CE is probably due to tissue uptake of lipoproteins and subsequent intracellular hydrolysis of the CE to produce FC.

Studies on the binding and degradation of human very-low-density lipoproteins by human hepatoma cell line HepG2

The regulation of the hepatic catabolism of normal human very-low-density lipoproteins (VLDL) was studied in human-derived hepatoma cell line HepG2. Concentration-dependent binding, uptake and degradation of 125I-labeled VLDL demonstrated that the hepatic removal of these particles proceeds through both the saturable and non-saturable processes. In the presence of excess unlabeled VLDL, the specific binding of 125-labeled VLDL accounted for 72% of the total binding. The preincubation of cells with unlabeled VLDL had little effect on the expression of receptors, but reductive methylation of VLDL particles reduced their binding capacity. Chloroquine and colchicine inhibited the degradation of 125I-labeled VLDL and increased their accumulation in the cell, indicating the involvement of lysosomes and microtubuli in this process. Receptor-mediated degradation was associated with a slight (13%) reduction in de novo sterol synthesis and had no significant effect on the cellular cholesterol esterification. Competition studies demonstrated the ability of unlabeled VLDL, low-density lipoproteins (LDL) and high-density lipoproteins (HDL) to effectively compete with 125I-labeled VLDL for binding to cells. No correlation was observed between the concentrations of apolipoproteins A-I, A-II, C-I, C-II and C-III of unlabeled lipoproteins and their inhibitory effect on 125I-labeled VLDL binding. When unlabeled VLDL, LDL and HDL were added at equal contents of either apolipoprotein B or apolipoprotein E, their inhibitory effect on the binding and uptake of 125I-labeled VLDL only correlated with apolipoprotein E. Under similar conditions, the ability of unlabeled VLDL, LDL and HDL to compete with 125I-labeled LDL for binding was a direct function of only their apolipoprotein B. These results demonstrate that in HepG2 cells, apolipoprotein E is the main recognition signal for receptor-mediated binding and degradation of VLDL particles, while apolipoprotein B functions as the sole recognition signal for the catabolism of LDL. Furthermore, the lack of any substantial regulation of beta-hydroxy-beta-methylglutaryl-CoA reductase and acyl-CoA:cholesterol acyltransferase activities subsequent to VLDL degradation, in contrast to that observed for LDL catabolism, suggests that, in HepG2 cells, the receptor-mediated removal of VLDL proceeds through processes independent of those involved in LDL catabolism.

Interactions of endogenous lipoproteins with capillary endothelium in spontaneously hyperlipoproteinemic rats

In spontaneously hyperlipoproteinemic old Sprague-Dawley rats, endogenous lipoproteins (LP) in the size range of 15 to 40 nm were directly visualized within the blood vessels due to specimen mordanting with tannic acid. LP morphometric analysis at the level of the endothelium of diaphragm capillaries revealed that particles of the dimensions of low-density lipoproteins, high-density lipoproteins (HDL1), and very low density lipoproteins occur in endothelial structures involved in receptor-mediated endocytosis coated pits-vesicles, endosomes, lysosomes) and transcytosis (plasmalemmal vesicles and transendothelial channels). No such particles could be detected in the intercellular junctions. Intravenously injected cationized ferritin (CF) of pI 8.4 bound uniformly to LP forming an CF-LP complex. Examined at 5, 20, and 60 min after CF administration, the CF-LP complex was found to be taken up by endothelium only by endocytosis (adsorptive via coated pits-vesicles, and fluid phase through a fraction of plasmalemmal vesicles). CF-LP complexes are progressively accumulated within lysosomes. These findings reveal the importance of the net surface charge of macromolecular complexes for their intracellular sorting and fate.

Direct synthesis of low-density lipoprotein apoprotein B in the miniature pig

The metabolism of apoprotein B (apo B) was investigated in five miniature pigs following the injection of radiolabeled, very low-density lipoproteins (VLDL). The fractional catabolic rate (FCR) for VLDL apoprotein B was 0.71 +/- 0.10 h-1 (mean +/- SE), the rate of flux was 0.77 +/- 0.05 mg h-1 kg-1, and the pool size of apoprotein B averaged 1.26 +/- 0.20 mg kg-1. Examination of precursor-product relationships between VLDL and low-density lipoprotein (LDL) apoprotein B illustrated that a significant proportion (greater than 80%) of LDL apo B was derived from some source other than VLDL catabolism. In further experiments (n = 4), 125I-VLDL and 131I-LDL were simultaneously injected into miniature pigs. The fractional catabolic rate of LDL apo B averaged 0.055 +/- 0.008 h-1 and the flux rate 0.73 +/- 0.07 mg h-1 kg-1. These dual-label studies allowed us to calculate that an average of 16% of VLDL apoprotein B was converted to LDL and thus the remainder was cleared directly from the circulation. Simultaneous injection of radiolabeled homologous and human VLDL indicated that the catabolism of the two tracers was qualitatively similar. However, human VLDL apo B exhibited a slower fractional catabolic rate (0.42 v 0.71 h-1 P less than 0.05) and reduced rate of conversion to LDL. Therefore, low-density lipoproteins in the pig are largely produced by direct secretion into the circulation, independent of VLDL catabolism. Apo B metabolism in miniature pigs is similar to that of cynomologous and squirrel monkeys, and rats, but differs from normal humans in whom all LDL apo B is derived from VLDL catabolism.

Plasma apoprotein changes after selective inhibition of hepatic triglyceride lipase in rat

Selective inhibition of hepatic triglyceride lipase (HTGL) by specific antibodies in rats led to an altered VLDL apoprotein composition. Apoprotein analysis by isoelectric focusing revealed a new protein band in VLDL and an increase in apoprotein E (apo E) content. Apoproteins in LDL and HDL remained unchanged. Electronmicroscopy showed a significant increase in particle size of VLDL from 452 to 497 A with no significant changes in LDL and HDL diameters.

Lipoprotein lipase-mediated sequestration of long-chain polyunsaturated triacylglycerols in serum LDL from normal and hypothyroid rats

Rat serum VLDL, unlike human, contains significant proportions of triacylglycerols with polyunsaturated C20 and C22 fatty acids. Hypothyroidism in this species is characterized by low levels of serum VLDL, the accumulation of LDL, elevated levels of lipoprotein lipase and depressed hepatic lipase activity. The hypothyroid rat thus represents an interesting model in which to study hepatic VLDL metabolism and the substrate specificity of lipoprotein lipase. This report shows that serum IDL and LDL in both euthyroid and hypothyroid rats contain progressively enhanced proportions of triacylglycerols with polyunsaturated C20 and C22 fatty acids when compared to VLDL. Hypothyroidism resulted in a decrease in the proportion of 22:6 fatty acid within the serum VLDL triacylglycerols when compared to euthyroid VLDL. Lipolysis of VLDL from euthyroid rats in vitro using the perfused rat heart system resulted in increases or sequestration of triacylglycerols containing long-chain polyunsaturated fatty acids within the IDL fraction similar to those seen in vivo. It is concluded that lipoprotein lipase-mediated hydrolysis of VLDL triacylglycerols and the conversion of VLDL to IDL and LDL in the rat results in a progressive sequestration of the longer-chain polyunsaturated triacylglycerol molecular species with the IDL and LDL.

The effect of dietary casein and soy protein on cholesterol and very low density lipoprotein metabolism in the rat

Rats fed a high-cholesterol semipurified diet containing casein developed higher levels of serum cholesterol than soy-fed animals. The hypercholesterolaemia of casein-fed rats was due to accumulation of very low density lipoproteins (VLDL), as measured by increased concentrations of serum VLDL cholesterol, protein and apoprotein B. High density lipoprotein (HDL) cholesterol was similar for the two dietary groups. Cholesterol absorption, as measured by the dual isotope ratio method and by direct measurement of cholesterol secretion into thoracic duct lymph, did not differ between the two groups. Cholesterol kinetics were derived from plasma cholesterol specific radioactivity curves and the casein-fed rats had a similar rate of plasma cholesterol production, but a significantly lower plasma cholesterol fractional catabolic rate (FCR) compared with the soy-fed rats. Kinetics of plasma VLDL apoprotein B, derived from analysis of reinjected 125I-labelled VLDL protein, also showed a lower fractional catabolic rate with casein feeding. This suggests that the accumulation of VLDL in the plasma of rats fed dietary casein is not due to excess VLDL production but to deficient VLDL removal. The hypercholesterolaemia appears to be a consequence of diminished VLDL catabolism.

Metabolism of apolipoproteins B-48 and B-100 of triglyceride-rich lipoproteins in normal and lipoprotein lipase-deficient humans

The metabolism of apolipoproteins B-48 and B-100 (apo B-48 and B-100) in large triglyceride-rich lipoproteins (300 to 1500 A in diameter) has been compared in three normal subjects and two subjects with genetically determined deficiency of lipoprotein lipase. The triglyceride-rich lipoproteins were obtained from a lipoprotein lipase-deficient donor 4 hr after a fat-rich meal in order to obtain chylomicrons (containing apo B-48) and very low density lipoproteins (VLDL) (containing apo B-100), whose properties had not been modified by the action of this enzyme. The triglyceride-rich lipoproteins were labeled with 125I and injected intravenously into recipients who had fasted overnight. In normal recipients, most of the apo B-48 was removed from the blood within 15 min, and most of the apo B-100 was removed within 30 min. In the lipoprotein lipase-deficient recipients, most of the injected apo B-100 remained in the blood for more than 8 hr; removal of apo B-48 was only slightly more rapid. In all subjects, only trace amounts of either protein were found in lipoproteins more dense than 1.006 g/ml. The results indicate that (i) the removal of the apo B of both chylomicrons and large VLDL from the blood is dependent upon the hydrolysis of their component triglycerides by lipoprotein lipase, and (ii) little or no apo B-48 of chylomicrons or apo B-100 of large VLDL is converted appreciably to low density lipoproteins (LDL). Our results suggest that the reported variability of the conversion of VLDL to LDL may be related to the size and composition of the particles secreted from the liver. The rapid production of remnant particles that are removed efficiently by the liver may minimize the opportunity for further reactions leading to the formation of LDL.

Independent regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase and chylomicron remnant receptor activities in rat liver

Treatment of rats with pharmacological doses of oestrogen resulted in a 3-fold decrease in the activity of hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase) and a 4-fold increase in saturable binding of 125I-labelled chylomicron remnants to liver membranes in vitro. Intragastric administration of mevalonolactone to rats did not affect the capacity of the liver membranes to bind to labelled chylomicron remnants even though there was a substantial decrease in the activity of HMG-CoA reductase. Similar results were obtained after cholesterol feeding. Simultaneous treatment of rats with cholestyramine and compactin increased hepatic HMG-CoA reductase activity 6-fold. However, liver membranes derived from these animals showed no change in their capacity to bind to labelled chylomicron remnants in vitro. Administration of mevalonolactone to the cholestyramine/compactin-treated animals also failed to produce a change in remnant-binding capacity. Although administration of mevalonolactone alone produced a significant 3-fold decrease in the activity of hepatic HMG-CoA reductase it was unable to suppress significantly the increase in enzyme activity caused by treatment with cholestyramine and compactin.

Secretion of lipoproteins from the liver of normal and Watanabe heritable hyperlipidemic rabbits

We compared the rate of accumulation of lipoproteins in perfusates of isolated livers from normal New Zealand White rabbits and Watanabe heritable hyperlipidemic (WHHL) rabbits, in which a gene mutation has produced a virtually complete deficiency of low density lipoprotein (LDL) receptors. The rate of accumulation of apolipoprotein B-100 did not differ in perfusates of livers from normal and mutant animals and little or no apolipoprotein B-48 was detected. In both groups, virtually all apolipoprotein B accumulated in very low density lipoprotein (VLDL). Experiments in which [3H]lysine was added to the perfusates showed that the apolipoprotein B that accumulated in VLDL was newly synthesized by the liver whereas the small amount of apolipoprotein B found in lipoproteins of higher density appeared to be washed out of extravascular spaces during perfusion. Perfusate VLDL from both groups contained more triglycerides and less cholesteryl esters than their counterparts from blood plasma. As compared with perfusate VLDL from normal livers, those from livers of WHHL rabbits were enriched in cholesteryl esters. Experiments in which Triton WR-1339 was injected into the blood of intact rabbits confirmed the observations with perfused livers. Previous studies have shown that the extent to which VLDL is converted to LDL is increased several-fold in WHHL rabbits. Taken together with our present results, which fail to provide evidence for increased secretion of apolipoprotein B or de novo secretion of lipoproteins other than VLDL that contain apolipoprotein B, it can be concluded that overproduction of LDL in rabbits lacking LDL receptors is solely the result of altered metabolism of VLDL.

Slower removal of intestinal apolipoprotein B-48 than of apolipoprotein B-100 in severely hypertriglyceridemic subjects

Apolipoprotein B, the major structural protein of triacylglycerol-rich lipoprotein, occurs in two immunologically distinct forms, termed apolipoproteins B-100 and B-48. In man, the former is associated with triacylglycerol-rich lipoproteins of hepatic origin and the latter with intestinal triacylglycerol-rich lipoproteins. We have studied the rates of removal of the two proteins when 125I-labelled triacylglycerol-rich lipoproteins were reinjected into six severely hypertriglyceridemic subjects showing hyperchylomicronemia, and into two normal subjects. The specific radioactivities of apolipoproteins B-100 and B-48 were determined over periods of up to 30 h. In all six hyperlipemic subjects the removal of apolipoprotein B-100 was either significantly faster than that of apolipoprotein B-48 (in four) or similar in the two who cleared triacylglycerol-rich lipoproteins very slowly. When triacylglycerol-rich lipoproteins from two hyperlipemic subjects (in whom apolipoprotein B-48 was cleared more slowly) were injected into two normal subjects both B apolipoproteins were cleared at similar rates. Since apolipoprotein B-48 appears to be a marker for remnants of intestinal particles, its slower removal than that of apolipoprotein B-100 in severe hypertriglyceridemia suggests that one metabolic defect associated with the hyperchylomicronemia is defective removal of chylomicron remnants.

Heterogeneity of very-low-density lipoprotein remnants bound and taken up by liver of starved rat in vivo

Very-low-density lipoprotein (VLDL), labelled in vivo with [9,10-3H]oleate, was taken up rapidly by liver after injection in vivo. Initially, radioactive lipoprotein remnants in the VLDL density range were present in liver as a bound extracellular pool that could be released by perfusion with polyphosphate or heparin. The bound remnant showed a decrease in mean diameter and an increased proportion of cholesteryl ester as a function of time after injection. When VLDL of different mean diameters was injected, it was found that: (1) total uptake by liver was independent of diameter; (2) small VLDL was not taken up more rapidly than large VLDL; and (3) Large VLDL lost no more triacylglycerol before binding than did small VLDL and larger species of mean diameter greater than 40 nm were bound. It is concluded that there is no unique VLDL remnant taken up by liver in vivo. When livers were perfused after binding radioactive VLDL in vivo, the lipoprotein was metabolized, with the production of water-soluble products, and this metabolism was inhibited by chloroquine.