Metabolism of chromatographically separated rat serum lipoproteins specifically labeled with 125I-apolipoprotein E

Apolipoprotein E in lipid metabolism and neurodegenerative disease

Dysregulation of lipid metabolism has emerged as a central component of many neurodegenerative diseases. Variants of the lipid transport protein, apolipoprotein E (APOE), modulate risk and resilience in several neurodegenerative diseases including late-onset Alzheimer's disease (LOAD). Allelic variants of the gene, APOE, alter the lipid metabolism of cells and tissues and have been broadly associated with several other cellular and systemic phenotypes. Targeting APOE-associated metabolic pathways may offer opportunities to alter disease-related phenotypes and consequently, attenuate disease risk and impart resilience to multiple neurodegenerative diseases. We review the molecular, cellular, and tissue-level alterations to lipid metabolism that arise from different APOE isoforms. These changes in lipid metabolism could help to elucidate disease mechanisms and tune neurodegenerative disease risk and resilience.

Molecular mechanism of apolipoprotein E binding to lipoprotein particles

The exchangeability of apolipoprotein (apo) E between lipoprotein particles such as very low-density lipoprotein (VLDL) and high-density lipoprotein (HDL) is critical for lipoprotein metabolism, but despite its importance, the kinetics and mechanism of apoE-lipoprotein interaction are not known. We have used surface plasmon resonance (SPR) to monitor in real time the reversible binding of apoE to human VLDL and HDL(3); biotinylated lipoproteins were immobilized on a streptavidin-coated SPR sensor chip, and solutions containing various human apoE molecules at different concentrations were passed across the surface. Analysis of the resultant sensorgrams indicated that the apoE3-lipoprotein interaction is a two-step process. After an initial interaction, the second slower step involves opening of the N-terminal helix bundle domain of the apoE molecule. Destabilization of this domain leads to more rapid interfacial rearrangement which is seen when the lipoprotein binding of apoE4 is compared to that of apoE3. The resultant differences in interfacial packing seem to underlie the differing abilities of apoE4 and apoE3 to bind to VLDL and HDL(3). The measured dissociation constants for apoE binding to these lipoprotein particles are in the micromolar range. C-Terminal truncations of apoE to remove the lipid binding region spanning residues 250-299 reduce the level of binding to both types of lipoprotein, but the effect is weaker with HDL(3); this suggests that protein-protein interactions are important for apoE binding to this lipoprotein while apoE-lipid interactions are more significant for VLDL binding. The two-step mechanism of lipoprotein binding exhibited by apoE is likely to apply to other members of the exchangeable apolipoprotein family.

Lipoprotein metabolism in chronic renal insufficiency

Chronic renal insufficiency (CRI) is associated with a characteristic dyslipidemia. Findings in children with CRI largely parallel those in adults. Moderate hypertriglyceridemia, increased triglyceride-rich lipoproteins (TRL) and reduced high-density lipoproteins (HDL) are the most usual findings, whereas total and low-density lipoprotein cholesterol (LDL-C) remain normal or modestly increased. Qualitative abnormalities in lipoproteins are common, including small dense LDL, oxidized LDL, and cholesterol-enriched TRL. Measures of lipoprotein lipase and hepatic lipase activity are reduced, and concentrations of apolipoprotein C-III are markedly elevated. Still an active area of research, major pathophysiological mechanisms leading to the dyslipidemia of CRI include insulin resistance and nonnephrotic proteinuria. Sources of variability in the severity of this dyslipidemia include the degree of renal impairment and the modality of dialysis. The benefits of maintaining normal body weight and physical activity extend to those with CRI. In addition to multiple hypolipidemic pharmaceuticals, fish oils are also effective as a triglyceride-lowering agent, and the phosphorous binding agent sevelamer also lowers LDL-C. Emerging classes of hypolipidemic agents and drugs affecting sensitivity to insulin may impact future treatment. Unfortunately, cardiovascular benefit has not been convincingly demonstrated by any trial designed to study adults or children with renal disease. Therefore, it is not possible at this time to endorse general recommendations for the use of any agent to treat dyslipidemia in children with chronic kidney disease.

Apolipoprotein E kinetics: influence of insulin resistance and type 2 diabetes

BACKGROUNDS AND AIMS

Insulin resistance related to obesity and diabetes is characterized by an increase in plasma TG-rich lipoprotein concentrations. Apolipoprotein (apo) E plays a crucial role in the metabolism of these lipoproteins and particularly in the hepatic clearance of their remnants. The aim of this study was to explore apoE kinetics of obese subjects and to determine what parameters could influence its metabolism.

METHODS

Using stable-isotope labelling technique ([(2)H(3)]-leucine-primed constant infusion) and monocompartmental model (SAAM II computer software), we have studied the plasma kinetics of very-low-density lipoprotein (VLDL) and high-density lipoprotein (HDL) apoE in 12 obese subjects (body mass index (BMI) 27.4-36.6 kg/m(2)): Seven were type 2 diabetics (age 47-65 y; HbA1c 7.1-10.2%) and five were non-diabetics (age 40-51 y, HbA1c: 4.9-5.3%). Six of the diabetic subjects were insulin resistant as assessed by insulin sensitivity index (HOMA 2.6-10.0), while non-diabetic subjects were all insulin sensitive (HOMA 1.2-2.1).

RESULTS

Plasma VLDL and HDL apoE concentrations were significantly higher in diabetic than in non-diabetic subjects (5.74+/-1.60 vs 1.46+/-1.74 mg/l, P<0.01 and 17.81+/-6.67 vs 9.97+/-3.32 mg/l, P<0.05). These increased levels were associated with significantly higher absolute production rate (APR) of VLDL and HDL apoE (0.714+/-0.343 vs 0.130+/-0.200 mg/kg/day, P<0.01, and 0.197+/-0.087 vs 0.080+/-0.060 mg/kg/day, P<0.05, respectively) while no significant difference was found for fractional catabolic rate (FCR) of VLDL and HDL apoE (3.44+/-1.64 vs 1.97+/-0.84/day and 0.30+/-0.12 vs 0.19+/-0.09/day, respectively). In the whole population, BMI was not correlated with any of apoE kinetic data. HOMA was positively correlated with FCR of VLDL apoE (r=0.64, P<0.05) and tended to be correlated with APR of VLDL apoE (r=0.58, P=0.06). HbA1c was positively correlated with APR and FCR of both VLDL apoE (r=0.91 and 0.78, P<0.01, respectively) and HDL apoE (r=0.66 and 0.69, P<0.05, respectively).

CONCLUSION

Obese diabetics are characterized by elevated VLDL and HDL apoE levels associated with enhancement of VLDL and HDL apoE production rates. Whereas obesity did not influence apoE kinetic parameters in itself, insulin resistance may lead to an increase in VLDL apoE production and fractional catabolic rates. Diabetes and the glycemic control may also specifically influence the kinetics of both VLDL and HDL apoE. All together, these disorders should explain at least part of the increase in VLDL and HDL apoE observed in diabetes.

Lamellar lipoproteins uniquely contribute to hyperlipidemia in mice doubly deficient in apolipoprotein E and hepatic lipase

Remnants of triglyceride-rich lipoproteins containing apolipoprotein (apo) B-48 accumulate in apo E-deficient mice, causing pronounced hypercholesterolemia. Mice doubly deficient in apo E and hepatic lipase have more pronounced hypercholesterolemia, even though remnants do not accumulate appreciably in mice deficient in hepatic lipase alone. Here we show that the doubly deficient mice manifest a unique lamellar hyperlipoproteinemia, characterized by vesicular particles 600 A-1,300 A in diameter. As seen by negative-staining electron microscopy, these lipoproteins also contain an electron-lucent region adjacent to the vesicle wall, similar to the core of typical lipoproteins. Correlative chemical analysis indicates that the vesicle wall is composed of a 1:1 molar mixture of cholesterol and phospholipids, whereas the electron-lucent region appears to be composed of cholesteryl esters (about 12% of the particle mass). Like the spherical lipoproteins of doubly deficient mice, the vesicular particles contain apo B-48, but they are particularly rich in apo A-IV. We propose that cholesteryl esters are removed from spherical lipoproteins of these mice by scavenger receptor B1, leaving behind polar lipid-rich particles that fuse to form vesicular lipoproteins. Hepatic lipase may prevent such vesicular lipoproteins from accumulating in apo E-deficient mice by hydrolyzing phosphatidyl choline as scavenger receptor B1 removes the cholesteryl esters and by gradual endocytosis of lipoproteins bound to hepatic lipase on the surface of hepatocytes.

Proposal of a multicompartmental model for use in the study of apolipoprotein E metabolism

Apolipoprotein (apo) E is a 299-amino acid glycoprotein that serves a number of functions in lipoprotein metabolism. Apo E binds to the triglyceride-rich lipoproteins (TRL), very-low-density lipoprotein (VLDL), and chylomicrons, as they are lipolyzed, mediating their removal from plasma via lipoprotein receptors. Apo E is also found associated with high-density lipoprotein (HDL) and has been suggested to play a role in reverse cholesterol transport. Studies on the kinetic behavior of apo E from the TRL and HDL fractions provide insights into the metabolic relationships between TRL and HDL in vivo. We sought to develop a compartmental model that can be used for analysis of kinetic data in studies on the metabolism of TRL and HDL apo E. Using radioactive tracers, it has been previously observed that, in some instances, a portion of VLDL apo E that is removed from plasma subsequently reappears in VLDL. Four multicompartmental models were considered that could account for this type of behavior: model A, in which there is transfer of apo E from HDL to VLDL; model B, in which there is a bidirectional extravascular exchange; model C, in which there is removal and subsequent reintroduction of TRL apo E into plasma; and model D, in which there is secretion of TRL apo E into plasma directly and via an extravascular pathway. Models C and D provided the best fit to the experimental data. While no physiologically plausible analog to model C could be found, an extravascular delay, analogous to newly secreted apo E that enters the lymphatic system before appearing in plasma, was postulated for model D. It was this model that was used to analyze kinetic data from metabolic studies of apo E. The model was able to provide a satisfactory fit to kinetic data in studies in which subjects were given a primed-constant infusion of 2H3-leucine. It was determined that TRL apo E from the six subjects studied had a mean residence time of 0.11 +/- 0.05 days and a mean production rate of 10.6 +/- 7.2 mg/kg/d, while HDL apo E had a mean residence time of 2.96 +/- 0.99 days and a mean production rate of 0.07 +/- 0.07 mg/kg/d. We conclude that this model describes a potential pathway for the metabolism of a portion of apo E in plasma and can be used to calculate the residence time and production rate of TRL and HDL apo E under a variety of conditions.

Regulation of very low density lipoprotein apo B metabolism by dietary fat saturation and chain length in the guinea pig

Studies investigated the effects of dietary fatty acid composition and saturation on the regulation of very low density lipoprotein (VLDL) apo B flux, clearance, and conversion to low density lipoprotein (LDL) in guinea pigs fed semipurified diets containing 15% (w/w) corn oil (CO), lard (LA), or palm kernel oil (PK). Plasma cholesterol levels were highest with dietary PK (3.1 +/- 1.0 mmol/L) followed by LA (2.4 +/- 0.4 mmol/L) and CO (1.6 +/- 0.4 mmol/L) intake. VLDL particles were larger (P < 0.05) in the LA (78 +/- 7 nm) and PK (69 +/- 10 nm) groups compared to animals fed CO (49 +/- 5 nm). VLDL-apo B fractional catabolic rates (FCR) were highest in guinea pigs fed the LA diet (P < 0.05) and VLDL apo B flux, estimated from VLDL 125I-apo B turnover kinetics, were higher in LA compared to PK or CO fed guinea pigs. In the case of PK consumption, the kinetic estimates of VLDL apo B flux significantly underestimated rates compared to direct VLDL apo B secretion measurements and LDL turnover analyses. These data demonstrate that differences in the composition and amount of saturated fatty acids have differential effects on VLDL apo B flux, catabolism, and conversion to LDL which, together with changes in LDL receptor-mediated catabolism, determine plasma LDL cholesterol levels in guinea pigs. The data also indicate that kinetic analysis of VLDL metabolism in PK fed animals is inaccurate possibly due to the presence of a small, nonequilibrating pool of newly synthesized VLDL which is rapidly converted to LDL.

Influence of macrophage-derived apolipoprotein E on plasma lipoprotein distribution of apolipoprotein A-I in apolipoprotein E-deficient mice

High density lipoprotein (HDL) cholesterol in apolipoprotein (apo) E-deficient mice is decreased. It has been suggested that apoA-I is lost from HDL in these mice because it must substitute for apoE as a structural protein for the abnormal cholesterol-rich lipoproteins. Therefore, we examined in vivo the influence of selective apoE expression on plasma HDL cholesterol in apoE-deficient mice. Bone marrow transplantation was used to establish macrophage-specific expression of apoE. Bone marrow transplantation normalized plasma triglycerides and significantly reduced total plasma cholesterol, but it did not increase hepatic apoA-I mRNA levels or total plasma apoA-I. Although total plasma apoA-I was not increased, HDL cholesterol measured following chromatographic separation was elevated twofold. Furthermore, plasma apoA-I was recovered from this HDL in animals expressing macrophage apoE. Compared to HDL of wildtype mice, this HDL had a similar chromatographic size distribution, but it lacked apoE and was more negatively charged. These studies indicated that plasma apoA-I distribution and HDL composition are influenced by apoE and that the abnormal apoA-I lipoprotein distribution of apoE-deficient mice can be altered in vivo by macrophage-derived apoE.

Alterations in lipolytic activity at hepatic subcellular sites of fed and fasted rats

This study investigates the relationship between the nutritional state of rats and lipid metabolism in distinct hepatic intracellular sites. Hepatic uptake of both protein and triacylglycerol (TG) moieties of injected very low-density lipoprotein (VLDL) is increased in fasted rats compared with fed controls. The VLDL-TG hydrolysis rate is increased in the plasma of fasted rats. This is shown by a higher ratio of labeled free fatty acid (FFA) to TG (FFA/TG). In both fed and fasted rats, a much greater increase of the labeled FFA/TG ratio in endosomes, compared with that in plasma, shows that further TG hydrolysis occurs in prelysosomal compartments. However, in fasted rats, this increase (18-fold) is much less than that in fed rats (69-fold). This observation is supported by the finding of significantly lower TG-lipase activity at pH 5, 7, and 8.6 in the endosomes of fasted rats. In contrast, during fasting, TG-lipase activity in whole liver homogenate and in isolated lysosomes is increased at pH 5. These observations suggest that after feeding there is a shift in intracellular lipolytic activity from lysosomes to prelysosomal organelles.

Metabolic regulation of plasma apolipoprotein E by estrogen and progesterone in the baboon (Papio sp)

Apolipoprotein (apo) E plays an important role in the metabolism of lipoproteins. To determine the effects of estrogen and progesterone on plasma levels and metabolism of apo E, we used 12 ovariectomized baboons fed a cholesterol- and fat-enriched diet. These baboons were divided into four groups and treated with estrogen, progesterone, estrogen + progesterone, and a placebo control. After 10 months, although the lipid levels were not different among the treatment groups, low-density lipoprotein (LDL)/high-density lipoprotein (HDL) ratios in the estrogen + progesterone group were significantly lower than those in the control and progesterone groups. Estrogen alone or in combination with progesterone decreased plasma apo E levels significantly compared with those in the control group. Plasma apo E levels in the progesterone group were similar to those in the control group. In all groups, most (greater than 60%) of the apo E was present in HDL. HDL apo E concentrations in the estrogen and estrogen + progesterone groups were significantly lower than those in the control and progesterone groups. To determine the metabolic mechanisms of these changes in apo E levels, turnover studies were conducted by injection of iodinated apo E-labeled very-low-density lipoprotein (VLDL) and HDL. Residence times were calculated using multicompartment modeling. Progesterone alone and in combination with estrogen decreased residence times of apo E injected in both HDL and VLDL compared with estrogen alone and control groups. Progesterone alone also increased the apo E production rate compared with other groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic fate of sphingomyelin of high-density lipoprotein in rat plasma

The metabolic fate of high density lipoprotein (HDL) sphingomyelin in plasma was studied in rats over a 24-hr period after injection of HDL containing sphingomyelin which was 14C-labeled in the stearic (18:0) or lignoceric acid (24:0) moiety and 3H-labeled in the choline methyl groups. Decay of label in plasma followed three phases. The first two phases were similar for both isotopes and both types of sphingomyelin (t1/2 approximately 10 and 110 min). However, during the third phase (from 10 hr after injection), 3H label disappeared more slowly than 14C label from 18:0 sphingomyelin, whereas the 3H/14C ratio remained relatively constant when 24:0 sphingomyelin was used. Intact, doubly-labeled 18:0 sphingomyelin disappeared from HDL rapidly (t1/2 = 38 min) by tissue uptake and by transfer to very low density lipoprotein (VLDL). VLDL contained up to 12% of the sphingomyelin 1 hr after injection. This is the first demonstration of a transfer in vivo of sphingomyelin from HDL to VLDL. A similarly rapid transfer was also observed in vitro. Some nontritiated, [14C]18:0 or [14C]24:0 sphingomyelin was redistributed more slowly into HDL. Doubly-labeled phosphatidylcholine appeared in VLDL and HDL within 1 hr after injection and reached 1.8 and 2.1% of the injected 14C and 3H in VLDL at 1 hr, and 4.8 and 6.9% in HDL at 3 hr, respectively.

Contribution of endosomes to intrahepatic distribution of apolipoprotein B and apolipoprotein E

The endosomal contribution of apolipoprotein B and apolipoprotein E to the intrahepatic distribution of these 2 apolipoproteins was studied. The endosomal contribution of the apolipoproteins was estimated by comparing the specific radioactivities of the injected apolipoproteins with that of apolipoproteins in the liver. The endosomal contribution is further validated by using a least-squares linear combination method that has been described previously (Wong and Pino: European Journal of Biochemistry 164:357-367, 1987). The results using these two independent analyses were similar. There was no difference in endosomal contribution of the apolipoproteins between the fasted and fed rats. In both cases, total endosomal contribution in the microsomal fraction of apolipoprotein B was estimated at 7-10% and that for apolipoprotein E was estimated at 62-66%. The molecular weights of the two apolipoproteins in the endosomes closely resembles those of the injected apolipoproteins in the very low density lipoproteins, indicating little or no apolipoprotein loss during receptor-mediated endocytosis. It is concluded that while there was no substantial contribution of apolipoprotein B to the intrahepatic distribution of that protein, the contribution of extrahepatic apolipoprotein E to intrahepatic apolipoprotein E distribution is substantial.

Differences in uptake of high-density lipoproteins by rat adrenals using in vivo vs. in situ perfusion techniques

This study describes the effect of the delivery route of high-density lipoproteins (HDL) on the ultimate fate of the lipoprotein in the intact rat adrenal. Equal amounts of human (h)-derived affinity-purified apoE-free 125I-labeled HDL3 was given to ethinyl estradiol-treated (i.e., lipoprotein-deficient) rats either intravenously (in vivo route) or by non-recycling perfusion (in situ perfusion route). After 60-90 min, the adrenals were either excised and assessed for uptake of radioactivity, or perfusion-fixed with glutaraldehyde and prepared for autoradiograms at the electron microscope level. The results show that hHDL3 circulated in vivo binds 9-times more readily to adrenal tissues than the same quantity of ligand delivered by perfusion. Also, when the lipoprotein is administered in vivo, it is 5-times more likely to be interiorized as an intact particle by zona fasciculata (corticosterone-secreting) cells via an endocytic pathway than when delivered by perfusion. Similar differences between the in vivo and in situ routes were not seen when 125I-labeled rat HDL was the ligand delivered. Whereas the starting hHDL3 ligand was free of apoE, there was a substantial (7-fold) conversion of the HDL3 to apoE-containing HDL3 following in vivo circulation of the ligand, as shown by sodium phosphotungstate-MgCl2 precipitation or heparin-Sepharose column chromatography. These results show that the route of lipoprotein delivery to specific tissues can play a major role in determining both the binding and the processing of the ligand by the tissue in question. With hHDL3, acquisition of apoE during only 1 h of recirculation in lipoprotein-deficient rats was sufficient to totally alter the fate of the ligand in the adrenal cortex.

Changes of apolipoprotein A-IV in the human neonate: evidence for different inductions of apolipoproteins A-IV and A-I in the postpartum period

The levels, isoforms and distribution of apolipoprotein A-IV (apo A-IV) were investigated in 127 term human umbilical cord sera. In addition, apo A-IV levels and isoforms were determined on the 3rd (n = 82) and 6th (n = 68) day following parturition and compared to apo A-I concentrations. Levels of apo A-IV were low in umbilical cord serum (5.7 +/- 1.9 mg/dl) as compared to adult serum (17.6 +/- 4.8 mg/dl). No difference was found between male and female neonates. The serum distribution of apo A-IV closely resembled that seen in the adult human. Apo A-IV concentrations dramatically increased during the first week of life reaching levels of 13.4 +/- 4.1 mg/dl on day 3 and 16.7 +/- 3.4 mg/dl on day 6 post-partum. During this time apo A-I levels did not change significantly (81.0 +/- 16.5 mg/dl in cord serum, 75.3 +/- 10.6 mg/dl and 84.2 +/- 14.5 mg/dl on day 3 and 6, respectively). Cord serum already exhibited the major serum apo A-IV isoforms seen in the adult. Isofocusing of apo A-IV also identified the known genetic polymorphism of apo A-IV. Among 127 cord sera studied we identified 109 homozygote normal patterns, apo A-IV (1-1), 16 heterozygotes, apo A-IV (1-2) and 2 individuals homozygote for the variant peptide, apo A-IV (2-2). We provide evidence that apo A-IV and apo A-I are differently induced in the human neonate during the beginning of the feeding period.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Metabolism of high-density lipoprotein in the hyperlipidemic, diabetic SHR/N-corpulent rat

The SHR/N-corpulent rat is a new genetically obese strain that is both hyperlipidemic and diabetic. The high density lipoprotein (HDL) fraction from 12-week-old obese males contained significantly greater amounts of protein (+83%), free (+72%) and esterified (+76%) cholesterol, phospholipid (+94%), and triglyceride (+78%). HDL from obese rats were also enriched in C apolipoproteins (apo C-III0 and apo C-III3) but had similar relative amounts of both apo A-I and apo E compared to HDL from their lean littermates. HDL protein turnover, measured with 125I-labeled HDL, showed that obese rats had a smaller fractional catabolic rate (FCR) than lean rats, but due to their much larger HDL pool size, they had a significantly higher rate of HDL protein catabolism (obese, 1.98 +/- 0.07 mg/whole animal/h v lean, 1.32 +/- 0.05 mg/whole animal/h). Therefore, under steady-state conditions, HDL protein production must also have been increased in the obese animals. To determine whether the increased catabolism of HDL protein was associated with increased catabolism of cholesteryl ester (CE), tissue uptake of HDL CE was measured using the nonhydrolyzable ether analogue [3H]cholesteryl linoleyl ether. After four hours 41.6 +/- 1.6% of the injected dose was cleared from the plasma of lean rats compared with 37.0 +/- 1.1% from the plasma of obese rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Complete protein sequence and identification of structural domains of human apolipoprotein B

Epidemiological, pathological and genetic studies show a strong positive correlation between elevated plasma concentrations of low-density lipoprotein (LDL) cholesterol and the risk of premature coronary heart disease. Apolipoprotein (apo) B-100 is the sole protein component of LDL and is the ligand responsible for the receptor-mediated uptake and clearance of LDL from the circulation. Apo B-100 is made by the liver and is essential for the assembly of triglyceride-rich very low-density lipoproteins (VLDL) in the cisternae of the endoplasmic reticulum and for their secretion into the plasma. VLDL transports triglyceride to peripheral muscle and adipose tissue, where the triglyceride is hydrolysed by lipoprotein lipase. The resultant particle, relatively enriched in cholesteryl ester, constitutes LDL. LDL delivers cholesterol to peripheral tissues where it is used for membrane and steroid hormone biosynthesis and to the liver, the only organ which can catabolize and excrete cholesterol. Plasma LDL levels are therefore determined by the balance between their rate of production from VLDL and clearance by the hepatic LDL (apo B/E) receptor pathway. Here we report the complete 4,563-amino-acid sequence of apo B-100 precursor (relative molecular mass (Mr) 514,000 (514K] determined from complementary DNA clones. Numerous lipid-binding structures are distributed throughout the extraordinary length of apo B-100 and must underlie its special functions as a nucleus for lipoprotein assembly and maintenance of plasma lipoprotein integrity. A domain enriched in basic amino-acid residues has been identified as important for the cellular uptake of cholesterol by the LDL receptor pathway.

Plasma catabolism of human apolipoprotein E isoproteins: lack of conversion of the doubly sialylated form to the asialo form in plasma

Apolipoprotein E (apoE) circulates as a mixture of sialylated and asialylated forms. In this study the catabolic fate and the plasma turnover rate of the different apoE forms have been investigated in vivo in humans. Asialo apoE (E) and doubly sialylated apoE (Ess) were isolated by preparative isoelectrofocusing from the VLDL of subjects homozygous for the E3 allele. 131E3 and 125E3ss were injected simultaneously into three hypertriglyceridemic subjects, and plasma samples were collected up to the sixth day. VLDL were isolated by ultracentrifugation, and the apoE forms were separated by isoelectrofocusing. Gel bands corresponding to E3 and E3ss were cut out and counted for the associated radioactivity. Residence times in plasma for 131E3 and 125E3ss were 0.95 +/- 0.16 and 0.74 +/- 0.16 days, respectively. As determined from the gel count distribution up to 24 hours, no conversion of the injected sialylated form to the correspondent asialylated form was detected.

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.

Discrepancies in the catabolic pathways of human and rat high-density lipoprotein apolipoprotein A-I in the rat

The in vivo metabolism in the rat of radioiodinated human and rat high-density lipoprotein was compared with a double-label procedure using 125I and 131I. While rat high-density lipoprotein showed a biphasic serum decay, human high-density lipoprotein was characterized by a monoexponential serum decay. No differences were observed between the serum decay of human high-density lipoprotein-2 and -3 subfractions, isolated by rate zonal ultracentrifugation. The catabolic sites of human and rat high-density lipoprotein were analysed using the lysosomal cathepsin inhibitor leupeptin. Radioiodinated rat high-density lipoprotein was catabolized by the kidneys and by the liver. In contrast, radioiodinated human high-density lipoprotein was catabolized almost exclusively in the liver. No difference in the catabolic sites of human high-density lipoprotein-2 and -3 subfractions was observed. The catabolic sites of human high-density lipoprotein apolipoprotein A-I in the rat were further analysed using the O-(4-diazo-3-[125I]iodobenzoyl) sucrose label. Compared with rat high-density lipoprotein apolipoprotein A-I, the kidneys played a minor role in the catabolism of human high-density lipoprotein apolipoprotein A-I. It is concluded that in the rat the catabolic pathways of the apolipoprotein A-I moieties of rat and human high-density lipoproteins are different, indicating that homologous high-density lipoproteins should be used for the investigation of in vivo metabolism.

The sites of degradation of purified rat low density lipoprotein and high density lipoprotein in the rat

Low density lipoprotein and high density lipoprotein were isolated from rat serum by sequential ultracentrifugation in the density intervals 1.025-1.050 g/ml and 1.125-1.21 g/ml, respectively. The isolated lipoproteins were radioiodinated using ICl. Low density lipoprotein was further purified by concanavalin A affinity chromatography and concentrated by ultracentrifugation. 95% of the purified low density lipoprotein radioactivity was precipitable by tetramethylurea, while only 4% was associated with lipids. The radioiodinated high density lipoprotein was incubated for 1 h at 4 degrees C with unlabelled very low density lipoprotein, followed by reisolation by sequential ultracentrifugation. Only 3% of the radioactivity was associated with lipids and 90% was present on apolipoprotein A-I. The serum decay curves of labelled and subsequently purified rat low and high density lipoprotein, measured over a period of 28 h, clearly exhibited more than one component, in contrast to the monoexponential decay curves of iodinated human low density lipoprotein. The decay curves were not affected by the methods used to purify the LDL and HDL preparations. The catabolic sites of the labelled rat lipoproteins were analyzed in vivo using leupeptin-treated rats. In vivo treatment of rats with leupeptin did not affect the rate of disappearance from serum of intravenously injected labelled rat low density lipoprotein and high density lipoprotein. Leupeptin-dependent accumulation of radioiodine occurred almost exclusively in the liver after intravenous injection of iodinated low density lipoprotein, while both the liver and the kidneys showed leupeptin-dependent accumulation of radioactivity after injection of iodinated high density lipoprotein.

Effect of chronic ethanol feeding upon the catabolism of protein and lipid moieties of chylomicrons and very low density lipoproteins in vivo and in the perfused heart system

Effect of chronic ethanol feeding for 6 weeks to male Wistar rats upon the catabolism of rat chylomicrons and very low density lipoproteins was investigated both in vivo and in the perfused heart system. The exponential decay curves in the plasma compartment or in the perfused heart system of these lipoproteins labeled in the protein or triacylglycerol or cholesterol moieties were determined. It was found that chronic ethanol feeding inhibited the catabolism of both protein and triacylglycerol moieties by 26-35%, whereas that of the cholesterol moiety was inhibited by 67-71%. Since the catabolism of the triacylglycerol moiety takes place essentially in the extrahepatic tissues while that of the cholesterol moiety occurs in the liver, it is concluded that ethanol affects the catabolism of triacylglycerol-rich lipoproteins in the liver more than in the extrahepatic tissues.

Leupeptin as a tool for the detection of the sites of catabolism of rat high-density lipoprotein apolipoproteins A-I and E

Leupeptin, an inhibitor of lysosomal cathepsin activity, was injected intravenously into male rats. Tissues obtained from leupeptin-treated animals showed a depressed cathepsin activity when compared with tissues from saline-treated control animals. Leupeptin treatment did not change the hepatic activities and subcellular distribution of marker enzymes for mitochondria, microsomes and plasma membranes. Hepatic lysosomal cathepsin activity was specifically inhibited, but the subcellular distribution of all lysosomal marker enzymes tested was changed, indicating the occurrence of enlarged lysosomes in the leupeptin-treated animals. No significant differences were observed in the serum concentrations of protein, cholesterol, cholesteryl esters, phospholipids and apolipoproteins A-I, A-IV and E between leupeptin-treated rats and control animals. When radioiodinated asialofetuin was injected intravenously, the radiolabel was retained for an extended period of time in the liver of leupeptin-treated animals, indicating diminished catabolism of this protein in the liver. When rat high-density lipoprotein, labelled specifically in the apolipoprotein A-I or E moiety was injected intravenously, only the kidneys and the liver showed a leupeptin-induced accumulation of radioactivity. These studies provide evidence for an important contribution of the kidneys and the liver to the in vivo catabolism of high-density lipoprotein apolipoproteins, using a method completely different from sugar-containing labelling compounds.

Effect of a high carbohydrate diet on the content of apolipoproteins C-II, C-III and E in human plasma high density lipoprotein subfractions

The effect of isocaloric high and low carbohydrate (Carb) diets on the structure and apoprotein composition of plasma high density lipoproteins (HDL) was assessed in four healthy men. The high Carb diet contained 65% calories as Carb and 15% as fat; the low Carb was 15% and 65%, respectively, with protein fixed at 20% of calories in each case. Cholesterol was 400 mg/day and the P/S ratio of the fat was 0.4. Each diet was sequentially consumed for periods of 3 weeks. At the end of each 3-week study period, plasma HDL2 and HDL3 were isolated by zonal ultracentrifugation and their apoprotein and lipid compositions were determined. Compared to the low Carb diet, the high Carb diet was associated with an increase in the size of HDL2 (116.0 +/- 1.8 vs. 109.1 +/- 1.8 A) and in the content (mean weight % +/- SEM) of apoE (2.81 +/- 0.71 vs. 1.79 +/- 0.49, P less than 0.01) and of apoC-II (1.73 +/- 0.09 vs. 1.11 +/- 0.12, P less than 0.01). HDL2 apoC-III content was not significantly different on the two diets (6.49 +/- 0.50 vs. 7.42 +/- 1.21). On the two diets, HDL3 size and HDL3 apoE content were not significantly changed. HDL3 apoC-II and apoC-III, however, were higher on the high Carb diet, P less than 0.05. The ratio (by weight) of HDL2 apoE/HDL2 apoC-II + C-III increased on the high Carb diet compared to the low Carb diet (0.344 +/- 0.058 vs. 0.228 +/- 0.053, P less than 0.01). We suggest that the increased amount of apolipoprotein E in HDL2 may influence its rate of catabolic clearance and may account for the well-known decrease in plasma HDL-cholesterol in subjects on high Carb diets.

Cholesteryl ester and apolipoprotein E transfer between human high density lipoproteins and chylomicrons

The transfer of cholesteryl esters and apolipoprotein E has been studied between plasma HDL and chylomicrons isolated either from ascitic fluid or from the plasma of a patient with type V hyperlipoproteinemia. Whereas apolipoprotein E transfer was rapid and occurred at low temperature, cholesteryl ester transfer was suppressed at 4 degrees C. Apolipoprotein E transfer did not depend upon the presence of cholesteryl ester transfer protein and was in fact inhibited by the partially purified preparation of this protein. Apolipoprotein E transfer was not increased by reduction with dithiothreitol. The transfer of cholesteryl esters increased sharply at a chylomicron to HDL ratio of cholesteryl ester above 1/10, a value which may be of physiological significance at the peak of postprandial lipemia. At this ratio, the transfer of apolipoprotein E was minimal and increased only at ratios above 2/1. From these results, it is concluded that there is no connection between apolipoprotein E and cholesteryl ester transfer from HDL to chylomicrons. It is, therefore, proposed that whereas chylomicron apolipoprotein E is acquired rapidly and mostly in the lymphatic system, the concentration of chylomicron cholesteryl esters increases significantly and independently in the circulation.

Lipoproteins and lipoprotein metabolism. A dynamic evaluation of the plasma fat transport system

Data now available suggest that a dynamic equilibrium exists in the plasma lipoproteins. Chylomicrons and very low density lipoproteins (VLDL) are primary secretory products of cells and carry triglycerides through the blood stream. As intravascular triglyceride hydrolysis occurs via the action of lipoprotein lipase (LPL), the further metabolism of nontriglyceride constituents of chylomicrons and VLDL can be followed along two interrelated pathways. Along the core pathway, cholesterol ester increasingly becomes a major core lipid with resultant formation of intermediate density (IDL, or remnant particles) and eventually low density (LDL) lipoprotein. Concomitant with reduction of core volume, redundant surface lipids and proteins move along a surface pathway and either form high density (HDL) lipoprotein precursors, or become associated with existing HDL particles. Cholesterol esters are formed via the action of lecithin: cholesterol acyltransferase (LCAT) in HDL. Therefore, action of LPL and LCAT on triglyceride-rich lipoproteins and their catabolic products is sufficient and necessary for formation, in plasma, of LDL and HDL. Once formed, all plasma lipoproteins are further remodelled by the activity of exchange and transfer reactions. In humans, a major remodelling occurs through exchange of LDL and HDL cholesterol ester by VLDL (and chylomicrons) triglyceride. The reaction is the main source of cholesterol esters in triglyceride-rich lipoproteins and is responsible for the enrichment of LDL and HDL with triglycerides. When followed by triglyceride lipolysis, this cycle results in limitation of size and cholesterol content of both LDL and HDL. The physiology and pathophysiology of the plasma lipid transport system in humans can therefore be fully appreciated only when the interrelations of all these metabolic reactions is taken into account.