Isolation and characterization of apolipoprotein B-48 and B-100 very low density lipoproteins from type III hyperlipoproteinemic subjects

Development and partial characterisation of an antiserum against apolipoprotein B of the short-finned eel, Anguilla australis

Despite its key role in transportation of triacylglycerides in blood, the distribution, localisation and molecular weight variants of apolipoprotein B (Apob) in teleost fish have essentially escaped study. To address this, a specific short-finned eel (Anguilla australis) Apob antiserum was produced by an immunised rabbit, purified and partially characterised. Localisation of Apob at both the mRNA (in situ hybridisation) and protein (immunohistochemistry) levels mirrored that of mammals; thus immunostaining was confined to the interstitial spaces of the liver and the vascular core of the intestinal villi. Immunostaining of proteins by Western blotting, followed by high-resolution LC-MS, indicated that peptide sequence coverage of Apob in low-density lipoproteins spanned the full-length protein. We conclude that only full-length Apob is produced by eels and that both liver and intestine are key sites for its synthesis.

Intravascular transfer contributes to postprandial increase in numbers of very-low-density hepatitis C virus particles

BACKGROUND & AIMS

The physical association of hepatitis C virus (HCV) particles with lipoproteins in plasma results in distribution of HCV in a broad range of buoyant densities. This association is thought to increase virion infectivity by mediating cell entry via lipoprotein receptors. We sought to determine if factors that affect triglyceride-rich lipoprotein (TRL) metabolism alter the density and dynamics of HCV particles in the plasma of patients with chronic HCV infection.

METHODS

Fasting patients (n = 10) consumed a high-fat milkshake; plasma was collected and fractionated by density gradients. HCV- RNA was measured in the very-low-density fraction (VLDF, d < 1.025 g/mL) before and at 7 serial time points postprandially.

RESULTS

The amount of HCV RNA in the VLDF (HCV(VLDF)) increased a mean of 26-fold, peaking 180 minutes after the meal (P < .01). Quantification of HCV RNA throughout the density gradient fractions revealed that HCV(VLDF) rapidly disappeared, rather than migrating into the adjacent density fraction. Immuno-affinity separation of the VLDF, using antibodies that recognize apolipoprotein B-100 and not apolipoprotein B-48, showed that HCV(VLDF) is composed of chylomicron- and VLDL-associated HCV particles; peaking 120 and 180 minutes after the meal, respectively. Plasma from fasting HCV-infected patients mixed with uninfected plasma increased the quantity of HCV(VLDF), compared with that mixed with phosphate-buffered saline, showing extracellular assembly of HCV(VLDF).

CONCLUSIONS

Dietary triglyceride alters the density and dynamics of HCV in plasma. The rapid clearance rate of HCV(VLDF) indicates that association with TRL is important for HCV infectivity. HCV particles, such as exchangeable apolipoproteins, appear to reassociate with TRLs in the vascular compartment.

Parallel activation of de novo lipogenesis and stearoyl-CoA desaturase activity after 3 d of high-carbohydrate feeding

BACKGROUND

High-carbohydrate (HC) diets increase de novo lipogenesis (DNL), but effects on stearoyl-CoA desaturase (SCD) are not so well studied.

OBJECTIVE

The objective was to investigate DNL and SCD in liver and adipose tissue by using fatty acid ratios after short-term dietary intervention.

DESIGN

Eight subjects consumed isoenergetic 3-d HC (10% fat; 75% carbohydrates) or higher fat (HF; 40% fat; 45% carbohydrates) diets (sugar to starch ratio: 60:40 for both) in a crossover study. Blood was taken from an artery and a vein draining subcutaneous adipose tissue. DNL and SCD activity were investigated by using the ratios of 16:0 to 18:2n-6 and of 16:1n-7 to 16:0, respectively. A test meal, including [U-(13)C]palmitate was given to trace dietary fatty acid incorporation into VLDL-triacylglycerol (TG). The conversion of intravenously infused [(2)H(2)]palmitic acid to [(2)H(2)]palmitoleic acid in VLDL-TG was quantified as a specific marker of hepatic SCD activity.

RESULTS

The VLDL-TG 16:0/18:2n-6 ratio, which reflects hepatic DNL, was greater after the HC diet than after the HF diet (P = 0.02). With the HC diet, increased plasma TG concentrations correlated with 16:0/18:2n-6 ratios (r = 0.76, P = 0.028). Plasma VLDL-TG and adipose venous nonesterified fatty acid (NEFA) 16:1n-7/16:0 ratios were higher after the HC diet (fasting: P = 0.01 and P = 0.05, respectively; postprandial: P = 0.03 and P = 0.05, respectively). Changes in fasting VLDL-TG 16:0/18:2n-6 and 16:1n-7/16:0 ratios were associated (P = 0.06). The contribution of total fatty acids from splanchnic sources (including DNL) was higher after the HC diet (P = 0.02). Expression of lipogenic genes in subcutaneous adipose tissue was not significantly affected by diet.

CONCLUSION

Parallel activation of DNL and SCD was found after a short period of HC feeding.

Apolipoprotein E enrichment of immuno-separated chylomicron and chylomicron remnants following saturated fatty acids

AIM

We examined the effect of meal fatty acids on lipid and apolipoprotein concentrations of very low density lipoprotein (VLDL) and chylomicron/chylomicron remnants in lipid fractions with a Svedberg flotation rate (Sf) 60-400 and Sf 20-60.

METHODS AND RESULTS

Six healthy middle-aged men received in random order mixed meals enriched with saturated (SFA), polyunsaturated (PUFA) or monounsaturated (MUFA) fatty acids on 3 occasions. VLDL and chylomicron/chylomicron remnants in the lipid fractions were separated by immunoaffinity chromatography against apo B-100. In the Sf 60-400 chylomicron/chylomicron remnants, triacylglycerol and cholesterol concentrations were significantly lower following PUFA compared with SFA and MUFA (P < or = 0.05). Apolipoprotein (apo) E responses were significantly higher after SFA in chylomicron/chylomicron remnants and VLDL compared with PUFA and MUFA (P < 0.007). However, apo B responses (particle number) were higher following MUFA than SFA (P = 0.039 for chylomicron/chylomicron remnants). Composition of the chylomicron/chylomicron remnants (expressed per particle) revealed differences in their triacylglycerol and apo E contents; in the Sf 60-400 fraction, SFA-rich chylomicron/chylomicron remnants contained significantly more triacylglycerol than MUFA (P = 0.028), more apo E than PUFA- and MUFA-rich particles (P < 0.05) and in the Sf 20-60 fraction, more apo E than MUFA (P = 0.009).

CONCLUSION

There are specific differences in the composition of chylomicron/chylomicron remnants formed after saturated compared with unsaturated fatty acid-rich meals which could determine their metabolic fate in the circulation and subsequent atherogenicity.

Selective partitioning of dietary fatty acids into the VLDL TG pool in the early postprandial period

Circulating triacylglycerol (TG) arises mainly from dietary fat. However, little is known about the entry of dietary fat into the major TG pool, very low-density lipoprotein (VLDL) TG. We used a novel method to study the specific incorporation of dietary fatty acids into postprandial VLDL TG in humans. Eight healthy volunteers (age 25.4 +/- 2.2 years, body mass index 22.1 +/- 2.3 kg/m2) were fed a mixed meal containing 30 g fish oil and 600 mg [1-13C]palmitic acid. Chylomicrons and VLDL were separated using immunoaffinity against apolipoprotein B-100. The fatty acid composition of lipoproteins was analyzed by gas chromatography/mass spectrometry. [1-13C]palmitic acid started to appear in VLDL TG 3 h after meal intake, and a similar delay was observed for eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Approximately 20% of dietary fatty acids entered the VLDL TG pool 6 h after meal intake. DHA was clearly overincorporated into this pool compared with [1-13C]palmitic acid and EPA. This seemed to depend on a marked elevation of this fatty acid in the nonesterified fatty acid pool. In summary, the contribution of dietary fatty acids to early postprandial VLDL TG is substantial. The role of DHA in VLDL TG production will require further investigation.

Astroglial regulation of apolipoprotein E expression in neuronal cells. Implications for Alzheimer's disease

Although apolipoprotein (apo) E is synthesized in the brain primarily by astrocytes, neurons in the central nervous system express apoE, albeit at lower levels than astrocytes, in response to various physiological and pathological conditions, including excitotoxic stress. To investigate how apoE expression is regulated in neurons, we transfected Neuro-2a cells with a 17-kilobase human apoE genomic DNA construct encoding apoE3 or apoE4 along with upstream and downstream regulatory elements. The baseline expression of apoE was low. However, conditioned medium from an astrocytic cell line (C6) or from apoE-null mouse primary astrocytes increased the expression of both isoforms by 3-4-fold at the mRNA level and by 4-10-fold at the protein level. These findings suggest that astrocytes secrete a factor or factors that regulate apoE expression in neuronal cells. The increased expression of apoE was almost completely abolished by incubating neurons with U0126, an inhibitor of extracellular signal-regulated kinase (Erk), suggesting that the Erk pathway controls astroglial regulation of apoE expression in neuronal cells. Human neuronal precursor NT2/D1 cells expressed apoE constitutively; however, after treatment of these cells with retinoic acid to induce differentiation, apoE expression diminished. Cultured mouse primary cortical and hippocampal neurons also expressed low levels of apoE. Astrocyte-conditioned medium rapidly up-regulated apoE expression in fully differentiated NT2 neurons and in cultured mouse primary cortical and hippocampal neurons. Thus, neuronal expression of apoE is regulated by a diffusible factor or factors released from astrocytes, and this regulation depends on the activity of the Erk kinase pathway in neurons.

Postprandial enrichment of remnant lipoproteins with apoC-I in healthy normolipidemic men with early asymptomatic atherosclerosis

OBJECTIVES

Recently, we reported that exaggerated postprandial triglyceridemia in normolipidemic patients with coronary artery disease is associated with enrichment of remnant lipoproteins with apolipoprotein C-I (apoC-I). In this study, the number and composition of chylomicron remnants and very low density lipoproteins (VLDLs) were examined in 30 asymptomatic normolipidemic 50-year-old men with and without early carotid atherosclerotic lesions.

METHODS AND RESULTS

Intima-media thickness of the far wall of the common carotid artery was determined by B-mode ultrasound. Triglyceride-rich lipoproteins were subfractionated by density gradient ultracentrifugation and separated into VLDL and chylomicron remnant fractions by immunoaffinity chromatography. The postprandial triglyceridemia and increase in triglyceride-rich lipoprotein particle number (ie, apolipoprotein B concentrations) were not exaggerated in men with early atherosclerosis. In contrast, their large (Svedberg flotation rate 60 to 400) and small (Svedberg flotation rate 20 to 60) chylomicron remnants and VLDL were greatly enriched with apoC-I, and their small chylomicron remnants and VLDL particles were relatively enriched with cholesterol. Moreover, the number of apoC-I molecules on small chylomicron remnants was strongly associated with the degree of atherosclerosis.

CONCLUSIONS

Early asymptomatic atherosclerosis in normolipidemic men without exaggerated postprandial triglyceridemia is associated with the enrichment of postprandial chylomicron and VLDL particles with apoC-I. Therefore, it is conceivable that the apoC-I content of lipoprotein remnants may serve as an early marker of coronary artery disease risk.

Sources and properties of triglyceride-rich lipoproteins containing apoB-48 and apoB-100 in postprandial blood plasma of patients with primary combined hyperlipidemia

Although editing of apolipoprotein (apo)B in the small intestine, yielding apoB-48, is thought to be nearly complete in adult humans, small amounts of intestinal apoB-100 may also be produced. We have evaluated the fraction of unedited apoB secreted from the intestine postprandially in subjects with primary combined hyperlipidemia, a disorder in which secretion of apoB-100 into the blood is increased. Three hours after these subjects and healthy controls were fed a fat-rich meal containing retinol, the distribution of retinyl esters (RE) between plasma triglyceride-rich lipoprotein (TRL) fractions containing apoB-100 and apoB-48 was measured under conditions minimizing transfer of RE between lipoprotein particles. The estimated maximal percentage of unedited intestinal apoB-100 (approximately 3%) was not increased in subjects with primary combined hyperlipidemia, suggesting that reduced editing of intestinal mRNA does not contribute to the pathogenesis of this disorder. Postprandially, the triglyceride content of TRL containing apoB-48 more than doubled, leading to a 20% increase in mean diameter, yet the surface concentration of phospholipids and soluble apolipoproteins (apoE and total apoC) was unchanged. Furthermore, the surface concentrations of these components did not differ among TRL containing apoB-48 and two smaller fractions of apoB-100 TRL with distinct immunoreactivities. These findings suggest that available surface area is a major determinant of the particle content of each of these surface components of TRL species of differing size and origin.

Apolipoprotein synthesis in nonalcoholic steatohepatitis

The pathophysiology of hepatic steatosis, a prerequisite of nonalcoholic fatty liver disease, is poorly understood. Because very-low-density lipoprotein (VLDL) formation is the chief route of hepatic lipid export, we hypothesized that the synthesis of apoB-100, a rate-determining step in hepatic VLDL formation, may be altered in patients with nonalcoholic steatohepatitis (NASH). This study evaluated the relative synthesis rates of apolipoprotein B-100 (apoB-100) in patients with NASH and in lean and body mass index (BMI)-matched (obese) controls without NASH. A primed continuous infusion of L-[1-(13)C] leucine was used to measure the absolute synthesis rates (ASR) of apoB-100 and fibrinogen in 7 patients with NASH and compared them with 7 lean and 7 obese (BMI-matched) controls without NASH. The ASRs of fibrinogen and albumin also were measured. The mean ASR of apoB-100 in patients with NASH was lower (31.5 +/- 3.4 mg/kg/d) than that of obese (115.2 +/- 7.2 mg/kg/d, P <.001) and lean controls (82.4 +/- 4.1 mg/kg/d, P =.002). In contrast, the mean ASR of fibrinogen was greater in subjects with NASH than in both control groups. These data indicate that NASH is associated with markedly altered hepatic synthesis of apoB-100. The finding that albumin synthesis was not similarly decreased in patients with NASH shows that the attenuation of apoB-100 synthesis is not on the basis of globally impaired hepatic protein synthesis. In conclusion, because apoB-100 synthesis is a rate-determining step in hepatocyte lipid export, decreased synthesis of this protein may be an important factor in the development of hepatic steatosis, a prerequisite for NASH.

Lipoprotein clearance mechanisms in LDL receptor-deficient "Apo-B48-only" and "Apo-B100-only" mice

The role of the low density lipoprotein receptor (LDLR) in the clearance of apo-B48-containing lipoproteins and the role of the LDLR-related protein (LRP) in the removal of apo-B100-containing lipoproteins have not been clearly defined. To address these issues, we characterized LDLR-deficient mice homozygous for an "apo-B48-only" allele, an "apo-B100-only" allele, or a wild-type apo-B allele (Ldlr-/- Apob48/48, Ldlr-/-Apob100/100, and Ldlr-/-Apob+/+, respectively). The plasma apo-B48 and LDL cholesterol levels were higher in Ldlr-/-Apob48/48 mice than in Apob48/48 mice, indicating that the LDL receptor plays a significant role in the removal of apo-B48-containing lipoproteins. To examine the role of the LRP in the clearance of apo-B100-containing lipoproteins, we blocked hepatic LRP function in Ldlr-/-Apob100/100 mice by adenoviral-mediated expression of the receptor-associated protein (RAP). RAP expression did not change apo-B100 levels in Ldlr-/-Apob100/100 mice. In contrast, RAP expression caused a striking increase in plasma apo-B48 levels in Apob48/48 and Ldlr-/-Apob48/48 mice. These data imply that LRP is important for the clearance of apo-B48-containing lipoproteins but plays no significant role in the clearance of apo-B100-containing lipoproteins.

Sequential purification of human apolipoprotein B-100, albumin, and fibrinogen by immunoaffinity chromatography for measurement of protein synthesis

A determinant of the accuracy of protein synthesis measurement using stable isotope is the purity of the protein under study. An Immunoaffinity chromatographic technique to sequentially purify human plasma albumin, fibrinogen, and apolipoprotein B-100 (ApoB-100) was developed to measure isotopic enrichment in these proteins. The technique, utilizing immobilized mouse monoclonal antibodies specific to human plasma ApoB-100, albumin, and fibrinogen onto an affinity matrix, allowed purification of very low density lipoprotein (VLDL) ApoB-100, albumin, and fibrinogen from 1- to 2-ml plasma samples. Analytical sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by silver staining demonstrated consistent purity of the three purified proteins. The identity and the purity of the proteins separated by this technique were also confirmed by amino acid sequence analysis. This technique was applied to sequentially purify and measure the isotopic enrichment in those proteins by mass spectrometry from human plasma samples collected after orally ingesting L[1-13C]-leucine. Reproducibility of the enrichment measurements is within 5% of the coefficient of variation. Measurements [13C]leucine in these proteins purified from plasma samples collected during a 10-h primed continuous intravenous infusion of L-[1-13C]leucine confirmed that this technique provides an efficient way to purify plasma VLDL ApoB-100, albumin, and fibrinogen for measuring their synthetic rates in human metabolism studies.

Phenotypic analysis of mice expressing exclusively apolipoprotein B48 or apolipoprotein B100

Apolipoprotein (apo)-B is found in two forms in mammals: apo-B100, which is made in the liver and the yolk sac, and apo-B48, a truncated protein made in the intestine. To provide models for understanding the physiologic purpose for the two forms of apo-B, we used targeted mutagenesis of the apo-B gene to generate mice that synthesize exclusively apo-B48 (apo-B48-only mice) and mice that synthesize exclusively apo-B100 (apo-B100-only mice). Both the apo-B48-only mice and apo-B100-only mice developed normally, were healthy, and were fertile. Thus, apo-B48 synthesis was sufficient for normal embryonic development, and the synthesis of apo-B100 in the intestines of adult mice caused no readily apparent adverse effects on intestinal function or nutrition. Compared with wild-type mice fed a chow diet, the levels of low density lipoprotein (LDL)-cholesterol and very low density lipoprotein- and LDL-triacylglycerols were lower in apo-B48-only mice and higher in the apo-B100-only mice. In the setting of apo-E-deficiency, the apo-B100-only mutation lowered cholesterol levels, consistent with the fact that apo-B100-lipoproteins can be cleared from the plasma via the LDL receptor, whereas apo-B48-lipoproteins lacking apo-E cannot. The apo-B48-only and apo-B100-only mice should prove to be valuable models for experiments designed to understand the purpose for the two forms of apo-B in mammalian metabolism.

Effect of chylomicron remnants on cholesterol metabolism in cultured rabbit hepatocytes: very low density lipoprotein and bile acid production

The interrelationship between very low density lipoprotein (VLDL) secretion and bile acid production was studied in primary culture of rabbit hepatocytes. Chylomicron remnants (CR) were added to the cultures to study their effect on VLDL secretion and bile acid production. After 24 hr preincubation of cells with CR (10-50 micrograms protein/mL), intercellular neutral lipid content was increased 1.5-4-fold in a dose-dependent manner. Neutral lipid accumulation was accompanied by a 70-90% reduction of [14C]acetate incorporation into cholesterol, while no stimulation of [14C]oleate incorporation into cholesteryl esters was observed. Incubation of cells with CR increased secretion of free cholesterol, triacylglycerol and apoproteins B and E in VLDL. Stimulation of VLDL cholesterol secretion was accompanied by a reduction of taurocholic acid synthesis. These data demonstrate the existence of an inverse relationship between secretion of VLDL cholesterol and bile acid production under conditions of effective uptake of triacylglycerol-rich CR by hepatocytes.

High receptor binding affinity of lipoproteins in atypical dysbetalipoproteinemia (type III hyperlipoproteinemia)

Familial dysbetalipoproteinemia (or type III hyperlipoproteinemia) is characterized by the presence of abnormal, cholesteryl ester-rich beta-very low density lipoproteins (beta-VLDL) in the plasma. Subjects with typical dysbetalipoproteinemia are homozygous for an amino acid substitution in apolipoprotein (apo-) E at residue 158 and have defective apo-E-mediated binding of both pre-beta-VLDL and beta-VLDL to apo-B,E(LDL) (or LDL) receptors (1988. Chappell, D.A., J. Clin. Invest. 82:628-639). To understand the effect of substitutions in apo-E at sites other than residue 158, nine dysbetalipoproteinemic (dys-beta) subjects who were either homozygous or heterozygous for substitutions in apo-E at atypical sites were studied. These substitutions occurred at residue 142 (n = 6), 145 (n = 2), or 146 (n = 1) and are known to cause less defective binding than does the 158 substitution. The chemical composition and electrophoretic mobility of pre-beta-VLDL and beta-VLDL from atypical and typical dys-beta subjects were indistinguishable. However, lipoproteins from atypical and typical dys-beta subjects differed in their affinity for the apo-B,E(LDL) receptor on cultured human fibroblasts. The pre-beta-VLDL and beta-VLDL from atypical dys-beta subjects had 640- or 17-fold higher affinity, respectively, than did corresponding lipoproteins from typical dys-beta subjects. The higher binding affinity of lipoproteins from atypical dys-beta subjects was associated with a higher ratio of apo-E to total apo-C. Since higher binding affinity should cause more rapid receptor-mediated clearance of beta-VLDL in atypical than in typical dys-beta subjects in vivo, the mechanism of beta-VLDL accumulation may differ in these two groups.

Lipoprotein composition changes induced by fenofibrate in dysbetalipoproteinemia type III

Fenofibrate (300 mg daily) was given to 9 subjects (7 men, 2 women) with dysbetalipoproteinemia type III. The treatment brought about important plasma level reductions in cholesterol (-35%), triglycerides (-56%), VLDL-cholesterol (-63%) and VLDL-triglycerides (-59%). The VLDL-C/TG ratio, which was 0.40 before treatment, was 0.30 after 4 weeks of fenofibrate, still suggestive of type III. LDL-C, when measured by conventional methods, was unchanged but isolation of the IDL (1.006-1.019 g/ml) fraction from the 1.006 g/ml infranatant revealed that true LDL-C levels actually increased in 6 individuals while IDL-C decreased considerably. The total HDL-C increase was mostly due to a 33% HDL3-C change. Apolipoprotein levels were considerably modified, notably apo B, C-III and E which were decreased, as well as the lipoprotein particles containing combinations of these apolipoproteins, namely LpE:B and LpC-III:B. Apo A-I was slightly modified as LpA-I: A-II particle levels increased and LpA-I decreased. There were marked compositional modifications of apo B-containing lipoproteins which corresponded to changes of the whole lipoprotein profile. Some abnormal classes of lipoproteins (e.g., beta-VLDL, dense LDL), characteristic of this disease, tended to disappear and were in some cases replaced by material of different size and density.

Pre-beta-very low density lipoproteins as precursors of beta-very low density lipoproteins. A model for the pathogenesis of familial dysbetalipoproteinemia (type III hyperlipoproteinemia)

The physical, chemical, and receptor binding properties of very low density lipoprotein (VLDL) fractions from familial dysbetalipoproteinemic (dys-beta) subjects, homozygous for apolipoprotein (apo-) E2 (E2/2 phenotype), and subjects with the E3/3 phenotype were studied to gain insights into the pathogenesis of dysbetalipoproteinemia, a disorder characterized by the presence of beta-VLDL in the plasma. Pre-beta-VLDL from dys-beta subjects were larger (27 vs. 17 x 10(6) D) and more triglyceride rich (68 vs. 43% dry weight) than beta-VLDL. Pre-beta-VLDL predominated in the Sf greater than 100 flotation fraction, whereas beta-VLDL predominated in the Sf 20-60 fraction. Because lipolysis converts large VLDL (Sf greater than 100) in vivo to smaller, more cholesteryl ester-rich VLDL (Sf 20-60), it is likely that pre-beta-VLDL are precursors of beta-VLDL. Although beta-VLDL were not found in type V hyperlipidemic E3/3 subjects, they were induced by intravenous heparinization, suggesting that lipolysis of pre-beta-VLDL in vivo can result in beta-VLDL formation. Similarly, heparinization of a dys-beta subject produced more beta-VLDL, at the expense of pre-beta-VLDL. The pre-beta-VLDL from normolipidemic and type V hyperlipidemic E3/3 subjects, respectively, had 90 and 280 times the affinity for the apo-B,E(LDL) receptor than did the pre-beta-VLDL from dys-beta subjects. Heparin-induced beta-VLDL from type V hyperlipidemic subjects had a sixfold higher binding affinity than did heparin-induced beta-VLDL from dys-beta subjects. These data suggest that pre-beta-VLDL from E2/2 subjects interact poorly with lipoprotein receptors in vivo, decreasing their receptor-mediated clearance and increasing their conversion to beta-VLDL during lipolytic processing.

Structural relationship of human apolipoprotein B48 to apolipoprotein B100

Although the complete amino acid sequence of human apolipoprotein (apo) B100 is known (4536 amino acids), the structure of apo B48 has not been defined. The objective of our study was to define the structure of apo B48 and its relationship to apo B100. Antibodies were produced against 22 synthetic peptides corresponding to sequences in human apo B100. The levels of immunoreactivity of the antipeptides to apo B100 and apo B48 were used to define the structural relationship between these two species of apo B. Six antibodies from sequences in the amino-terminal half of apo B100, including antipeptide 2110-2129, bound to both apo B100 and apo B48. 15 other apo B-specific antipeptides from sequences carboxyl-terminal to residue 2152 bound to apo B100, but not to apo B48. Immunoblots of cyanogen bromide digests of apo B100 and apo B48 with antipeptides 2068-2091 and 2110-2129 detected a 16-KD fragment (residues 2016-2151) in the apo B100 digest and a fragment of identical size in the apo B48 digest. Because apo B48 appears to contain the apo B100 cyanogen bromide fragment 2016-2151 and because an antiserum specific for the peptide 2152-2168 does not bind to apo B48, we conclude that apo B48 represents the amino-terminal 47% of apo B100 and that the carboxyl terminus of apo B48 is in the vicinity of residue 2151 of apo B100.

Characterization of an abnormal species of apolipoprotein B, apolipoprotein B-37, associated with familial hypobetalipoproteinemia

Steinberg and colleagues have previously described a unique kindred with normotriglyceridemic hypobetalipoproteinemia (1979. J. Clin. Invest. 64:292-301). In a reexamination of this kindred, we found an abnormal apolipoprotein (apo) B species, apo B-37 (203,000 mol wt), in the plasma lipoproteins of multiple members of the kindred. In affected individuals apo B-37 was found in very low density lipoproteins, along with the normal apo B species, apo B-100 and apo B-48. High density lipoproteins (HDL) also contained apo B-37, but no other apo B species. The first 13 amino-terminal amino acids of apo B-37 were identical to those of normal apo B-100. We utilized a panel of 18 different apo B-specific monoclonal antibodies and polyclonal antisera specific for apo B-37 and the thrombin cleavage products of apo B-100 to map apo B-37 in relation to apo B-100, apo B-48, and the thrombin cleavage products of apo B-100. The results of those immunochemical studies indicated that apo B-37 contains only amino-terminal domains of apo B-100. In affected individuals, the majority of apo B-37 in plasma was contained in the HDL density fraction. Within that fraction apo B-37 was found on discrete lipoprotein particles, termed Lp-B37, that had properties distinct from normal HDL particles containing apo A-I. This report documents for the first time the existence of an abnormal apo B species in humans. Further study of apo B-37 and lipoprotein particles containing apo B-37 should lead to an improved understanding of apo B structure and function.

The chromogenicity and quantitation of apoB-100 and apoB-48 of human plasma lipoproteins on analytical SDS gel electrophoresis

ApoB-100 and apoB-48 may be readily resolved in 3.3% sodium dodecyl sulphate-polyacrylamide gels. This study has characterized the relative chromogenicities (staining intensity/micrograms protein) of human apoB-100 and apoB-48 in various lipoprotein classes with Coomassie Brilliant Blue (R250) upon SDS-PAGE. The relation between dye uptake and the mass of each apoB species in any lipoprotein preparation, was linear at least within the concentration range of total apoprotein B which is optimally resolved in these gels (20-50 micrograms total apoprotein B), and was a function of the density of the particular lipoprotein fraction under investigation. There was a constant and characteristic difference between the chromogenicity for apoB-100 and that for apoB-48 as determined from the slopes of their respective chromogenicity curves. The slope of the lines describing staining intensity vs. protein mass for both apoB-100 and apoB-48 decreased as the density of the lipoprotein fraction increased. The slope of the line for apoB-100 was steeper than that for apoB-48 (i.e. chromogenicity apoB-100 greater than apoB-48) in all lipoprotein fractions where both were present. The relationship between the slopes of the lines for apoB-100 and apoB-48 was constant regardless of the density of the lipoprotein fraction. The chromogenicity curves for apoB-100 and for apoB-48 obtained when lipoprotein samples were applied to gels in concentrations conventionally used for this technique (i.e. 20-100 micrograms total apoB/gel) did not extrapolate to the same point on the ordinate, which precludes the use of a simple ratio or "chromogenicity factor" to describe their relative chromogenicities over this concentration range, Hence, a novel approach was developed to determine the relative mass of apoB-100/apoB-48 in lipoprotein samples, based on their staining characteristics in SDS-PAGE.

Immunological heterogeneity of rat apolipoprotein B epitope expression. A study using monoclonal antibodies to rat apolipoprotein B

Epitope expression of rat apolipoprotein B on lipoproteins was investigated with the help of six monoclonal antibodies produced from mice. Through a variety of techniques, which include cotitrations, ELISAs and quantitative immunoadsorption precipitation, we concluded that the six monoclonal antibodies recognize five different epitopes. LRB 110 and LRB 260 recognize epitopes that may be overlapping. LRB 240 and LRB 250 recognize epitopes that are preferentially expressed in triacylglycerol-rich particles. LRB 220 recognizes an epitope that is expressed by all apolipoprotein-B-containing lipoproteins. We have also determined that apolipoprotein B epitope expression in rat lipoproteins is very similar to its human counterpart. Both rat and human apolipoprotein B epitope expression on lipoproteins showed heterogeneities even in homologous lipoprotein preparations. We concluded that a variety of techniques are necessary to fully characterize monoclonal antibodies to apolipoproteins. The possible implications of epitope expression in pathophysiology are also discussed.

Mapping of human apolipoprotein B antigenic determinants

A minimum of 16 epitopes which provide a group of topographical markers to study the conformation of apolipoprotein (apo) B have been mapped in relation to elements of the sequence of apo B-100. Six of these epitopes are identified by monoclonal antibodies (Mabs) directed against low density lipoprotein (LDL) apo B, while at least 10 others react with Mabs obtained by immunization with delipidated and solubilized apo B. Five epitopes which are also expressed on apo B-48 have been assigned to the thrombolytic fragment T4 on the N-terminal side of apo B-100. None of these five epitopes requires the presence of lipids for its expression, suggesting that the conformation of the T4 region of apo B is more dependent on peptide-chain interactions than on peptide-lipid interactions. Four distinct epitopes have been assigned to the median thrombolytic fragment T3 of apo B-100, all of which require the presence of lipids for their expression; those epitopes closer to the C-terminus of T3 require specific interaction with cholesteryl esters. The same lipid dependence also characterizes a cluster of epitopes mapped to the N-terminal region of fragment T2. The epitopes that are close to the T2/T3 cleavage site and depend on the presence of cholesteryl esters for their expression are also those that react with the Mabs that inhibit the binding of LDL to its receptor. Therefore this region, which in addition contains two sequences with structural homology to the apo E receptor binding domain, probably constitutes a physiologically important receptor binding site for apo B. Finally, four other distinct epitopes which do not require the presence of lipids for their expression have been mapped on T2. In conclusion, the present report presents evidence that the immunochemical analogy of apo B-48 and apo B-100 is on the N-terminal half of apo B-100, whereas the apo B receptor binding domain is localized on the C-terminal half of apo B-100 close to the T2/T3 cleavage site.

Slower catabolism of apo B48 than of apo B100 in triglyceride-rich lipoproteins during heparin infusions in type 5 hyperlipoproteinemic subjects

The catabolism of chylomicrons plus remnants and of VLDL was studied in four subjects with type 5 hyperlipoproteinemia. Large triglyceride-rich lipoproteins Sf greater than 60 were labeled with 125I, reinjected, and the removal of radioactivity in apo B48 and in apo B100 was measured before and during intravenous heparin. The constant infusion of heparin stimulated lipolytic activity and accelerated the disappearance of apo B100 radioactivity (VLDL and their remnants) from the Svedberg flotation unit (Sf) greater than 60 fraction by 27%, 33%, 51%, and 55% in the four subjects. However, the removal rate of radioactivity in apo B48 (chylomicrons and their remnants) was unaffected by heparin in three subjects; in the fourth the fall in radioactivity was less in apo B48 than in apo B100. There were corresponding rises with heparin in Sf 12 to 60 apo B48 radioactivity in only one subject. Thus, increased plasma lipolytic activity accelerated chylomicron (remnant) catabolism very much less than of VLDL (remnant) removal of type 5 hyperlipoproteinemic subjects. It suggests that a major defect leading to type 5 hyperlipoproteinemia is failure of chylomicrons to become degraded to the point where the particles can interact optimally with the hepatic chylomicron remnant receptor.

Metabolism of apolipoproteins B-48 and B-100 of triglyceride-rich lipoproteins in patients with familial dysbetalipoproteinemia

The metabolism of apolipoproteins B-48 and B-100 (apo B-48 and B-100) in large triglyceride-rich lipoproteins was studied in three adults with familial dysbetalipoproteinemia (F. dys.) and compared to that of normolipidemic subjects. One Caucasian F. dys. subject was apparently homozygous for the common form of apo E-2, (Arg158----Cys), whereas the two Black subjects were homozygous for a different apo E-2 mutant (Arg145----Cys), which displays much less defective binding to cells than apo E-2 (Arg158----Cys). The lipoproteins were labeled with 125I and injected intravenously into fasted recipients. The results indicate that the terminal catabolism of triglyceride-rich lipoproteins of intestinal and hepatic origin is markedly impaired in apo E2/2 homozygotes with alleles Arg158----Cys and Arg145----Cys; despite long residence times, apo B-48 of chylomicrons and apo B-100 of large very low density lipoproteins are not converted appreciably to intermediate or low density lipoproteins in apo E2/2 homozygotes.

Characterization of a monoclonal antibody that binds equally to all apolipoprotein and lipoprotein forms of human plasma apolipoprotein B. I. Specificity and binding studies

A stable mouse hybridoma cell line has been developed that produces monoclonal antibody to human plasma apolipoprotein B. This antibody was proven to be specific for apolipoprotein B immunoblotting and an enzyme immunoassay using apolipoprotein B and other apolipoproteins. The antibody bound with comparable affinities to soluble apolipoprotein B, chylomicrons, very-low-density (VLDL) and low-density lipoproteins (LDL). Coupled to agarose, this antibody allowed complete removal of apolipoprotein B-containing lipoproteins from normolipidemic, hypertriglyceridemic and hypercholesterolemic plasma. Desialyzation and deglycosylation had no effect on its binding to LDL. The described antibody had no effect on the receptor-mediated binding of radiolabeled LDL to the human hepatoma cells (HepG2) in culture. Analysis of 25 different samples of human plasma indicated identical expression of the corresponding epitope in these individuals. The described monoclonal antibody, most likely, binds to a rather stable domain of apolipoprotein B that is not altered by the interaction with lipids or polymorphism of the apolipoprotein B. We propose that this antibody be called 'Pan B' antibody.

Characterization of a monoclonal antibody that binds equally to all apolipoprotein and lipoprotein forms of human plasma apolipoprotein B. II. Isolation of apolipoprotein B-containing lipoproteins from human plasma

Monoclonal antibody ('Pan B' antibody) that binds equally to all major forms of human plasma apolipoprotein B was used in an immunoaffinity chromatography procedure to isolate apolipoprotein B-containing lipoproteins from hyperlipidemic human plasma. These lipoproteins were compared with lipoproteins in native plasma, with lipoproteins isolated by polyclonal antibodies and with lipoproteins isolated by the conventional ultracentrifugational method. Judged by the apolipoprotein and lipid composition, lipoproteins isolated with 'Pan B' antibody were virtually identical to those isolated by ultracentrifugation or polyclonal antibodies. Lipoproteins isolated by 'Pan B' antibody were comparable in size and shape to the lipoproteins in native plasma and to the lipoproteins isolated by polyclonal antibodies or ultracentrifugation. The immunoaffinity column with monoclonal 'Pan B' antibody retained all apolipoprotein B-containing lipoproteins and showed significantly higher capacity than polyclonal immunoaffinity column. The column with the highest capacity allowed the isolation from whole plasma of 0.144 mg of apolipoprotein B per ml of gel in less than 2 h.

Metabolism of the apolipoprotein B-containing lipoproteins

This chapter was designed to describe the approaches one can take to study the metabolism of the apoB-containing particles in vivo. The focus has been to blend (1) what is the current tracer kinetics analysis methodology and (2) what are the current experimental protocols being used into a total picture so that the experimentalist wishing to perform such studies may have a better perspective of the strong points and pitfalls of this important experimental tool. Hence, these points have been summarized from the point of view of what caveats are associated with each methodology. Recognition of these is essential to avoid reaching potentially erroneous conclusions. More important, attention has been focused on the realization that certain methodologies can be chosen depending upon what questions are being asked. Finally, areas where future development is needed in order to proceed to the next level of understanding are pointed out in the context of using tracer kinetic analysis as an integral part of a total experimental design.

Metabolism of human plasma triacylglycerol-rich lipoproteins in rodent macrophages: capacity for interaction at beta-VLDL receptor

The capacity of human plasma triacylglycerol-rich lipoproteins to be metabolized by rat macrophages was studied with plasma triacylglycerol-rich lipoproteins obtained from subjects with fasting chylomicronemia or from normal subjects after a fat meal. Triacylglycerol-rich lipoproteins were separated by chromatography into two fractions designated TRL1 and TRL2; from their composition and changing concentration during alimentary lipemia, TRL1 contained a higher proportion of chylomicron remnants than TRL2. Degradation of 125I-labeled TRL1 was greater than that of 125I-labeled TRL2. In competition studies with 125I-labeled beta-VLDL from cholesterol-fed rabbits, unlabeled TRL1 displaced beta-VLDL as completely as did unlabeled beta-VLDL, being slightly more potent than TRL2, which contained less apolipoprotein E than TRL1. This reflected common interaction at receptors that probably included both beta-VLDL and B/E receptors, since: (1) in fresh macrophages, VLDL from hypertriglyceridemic subjects partially displaced beta-VLDL; (2) in B/E receptor-repressed macrophages, TRL1 maintained capacity to totally displace beta-VLDL. This was confirmed in experiments with J774 murine macrophages in which triacylglycerol-rich lipoproteins and beta-VLDL displaced each other equally, whereas LDL was ineffective in displacing beta-VLDL. Furthermore, monoclonal antibodies raised against apolipoprotein B48 and reacting strongly with LDL, failed to inhibit the binding of triacylglycerol-rich lipoprotein to the macrophages. This indicates an interaction through apolipoprotein E which is present in high concentration in triacylglycerol-rich lipoprotein as well as in beta-VLDL. It applies to triacylglycerol-rich particles derived from either the intestine (chylomicron remnants) or the liver (VLDL remnants from hypertriglyceridemic subjects).

Apolipoprotein B-100 deficiency. Intestinal steatosis despite apolipoprotein B-48 synthesis

We describe a child, the issue of phenotypically normal parents, who had fat malabsorption, both intestinal and hepatic steatosis, and serum cholesterol and triglyceride concentrations of 38 and 63 mg/dl, respectively. Lipoprotein electrophoresis, Ouchterlony double diffusion, and electron microscopy demonstrated that normal low density lipoproteins (LDL: 1.006 less than rho less than 1.063 g/ml) were absent. Lipoprotein particles in the rho less than 1.006-g/ml fraction were triglyceride rich, very large (93.2 +/- 35.1 nm), and contained the B-48 but not the B-100 apoprotein; both species of apolipoprotein (apo) B were found in the parents' lipoproteins. These chylomicrons and chylomicron remnants were present even in the patient's fasting plasma, which suggested prolonged dietary fat absorption. Plasma levels of high density lipoprotein lipids and proteins were low, and the phosphatidylcholine/sphingomyelin ratio was reduced as in typical abetalipoproteinemia. The monosialylated form of apo C-III was not identified on polyacrylamide gel electrophoresis, which suggested that this protein was elaborated only with very low density lipoproteins (VLDL). A radioimmunoassay for apo B employing a polyclonal antisera to plasma LDL gave apparent plasma apo B levels of 0.6, 66, and 57 mg/dl in the patient and his father and mother, respectively. The displacement curve generated by the parents' VLDL and LDL did not did not differ from control lipoproteins. The patient's chylomicron-chylomicron remnant fraction displaced normal LDL over the entire radioimmunoassay range, but the efficiency of displacement was strikingly less than with B-100 containing lipoproteins. If the patient's B-48 protein is not qualitatively abnormal, these results confirm very limited immunochemical cross-reactivity between at least one major epitope on B-100 and the epitopes expressed on B-48. The apo B defect in this patient appears to be recessive. It abolishes B-100 production and may additionally limit the formation of B-48.

Apolipoprotein B-48 and B-100 very low density lipoproteins. Comparison in dysbetalipoproteinemia (type III) and familial hypertriglyceridemia (type IV)

A protein band having the same migration as apolipoprotein (apo) B-48 was observed by SDS electrophoresis in the plasma very low density lipoprotein (VLDL) from 14 Type IV and three Type III hyperlipoproteinemic subjects and from six normal fasting subjects. The VLDL from five Type IV, three Type III, and one normal subject were separated into two subfractions, retained and nonretained, by immunoaffinity chromatography on monoclonal anti-apo B-100 Sepharose. Based on results of electrophoresis and radioimmunoassay, we have concluded that these two fractions represent apo B-48 and apo B-100 lipoproteins that we have named apo B-48 and apo B-100 VLDL. When compared to their respective apo B-100 VLDL, the apo B-48 VLDL from either Type III or Type IV was principally enriched in total lipids, in apo E, and had an electrophoretic migration similar to chylomicrons. This suggests that apo B-48 VLDL has the same origin (i.e., intestinal) in the two disorders. Both apo B-48 and apo B-100 VLDL were enriched in cholesteryl ester (CE) and depleted in triglyceride (TG) in Type III; however, both fractions were rich in TG and poor in CE in Type IV and in normal subjects. In addition, compared to their respective apo B-100 VLDL, the apo B-48 fraction was enriched in CE in Type III and in TG in Type IV. We conclude that, despite a possible similar origin for apo B-48 VLDL in Type III and in Type IV subjects, the composition of apo B-48 VLDL is variable and the CE/TG ratio is more characteristic of the type of hyperlipidemia than of the particular VLDL subfractions.