Monoclonal antibodies to low density lipoprotein used for the study of low- and very-low-density lipoproteins, in "ELISA" and immunoprecipitation technics

Molecular analysis and intestinal expression of SAR1 genes and proteins in Anderson's disease (Chylomicron retention disease)

BACKGROUND

Anderson's disease (AD) or chylomicron retention disease (CMRD) is a very rare hereditary lipid malabsorption syndrome. In order to discover novel mutations in the SAR1B gene and to evaluate the expression, as compared to healthy subjects, of the Sar1 gene and protein paralogues in the intestine, we investigated three previously undescribed individuals with the disease.

METHODS

The SAR1B, SAR1A and PCSK9 genes were sequenced. The expression of the SAR1B and SAR1A genes in intestinal biopsies of both normal individuals and patients was measured by RTqPCR. Immunohistochemistry using antibodies to recombinant Sar1 protein was used to evaluate the expression and localization of the Sar1 paralogues in the duodenal biopsies.

RESULTS

Two patients had a novel SAR1B mutation (p.Asp48ThrfsX17). The third patient, who had a previously described SAR1B mutation (p.Leu28ArgfsX7), also had a p.Leu21dup variant of the PCSK9 gene. The expression of the SAR1B gene in duodenal biopsies from an AD/CMRD patient was significantly decreased whereas the expression of the SAR1A gene was significantly increased, as compared to healthy individuals. The Sar1 proteins were present in decreased amounts in enterocytes in duodenal biopsies from the patients as compared to those from healthy subjects.

CONCLUSIONS

Although the proteins encoded by the SAR1A and SAR1B genes are 90% identical, the increased expression of the SAR1A gene in AD/CMRD does not appear to compensate for the lack of the SAR1B protein. The PCSK9 variant, although reported to be associated with low levels of cholesterol, does not appear to exert any additional effect in this patient. The results provide further insight into the tissue-specific nature of AD/CMRD.

Expression of apolipoprotein B epitopes in low density lipoproteins of hemodialyzed patients

Serum and isolated low-density lipoprotein (LDL) composition abnormalities were investigated in 20 hemodialyzed patients with chronic renal failure and 15 healthy normolipidemic subjects for comparison. LDL apolipoprotein B (apo B) epitope accessibility was determined by the use of seven monoclonal antibodies (Mabs). These Mabs recognize fragments on the N-terminal part of apo B (Mabs B1, B4), on the middle part (Mab BL7), on the C-terminal (Mabs BA11, BL3), and the two remaining Mabs recognize conformational epitopes of apo B (BL5, DA7). Mab BA11 recognizes a fragment of apo B which interacts with the B/E receptor. In hemodialyzed patients, LDL content of triglycerides (P < 0.001) and apo CIII (P < 0.005) was increased, while cholesteryl esters (P < 0.005) were decreased. The accessibility of BL5 epitopes of LDL apo B was enhanced (P < 0.05), while BA11 epitope expression was decreased (P < 0.01). The conformation of patients' LDL (CRF-LDL) was probably abnormal and seemed to be related to some modification of the lipidic environment. It is important to consider a structural modification as it alters the B/E receptor recognition domain of apo B. These results confirm LDL abnormalities in hemodialyzed patients and suggest a possible modification of the recognition of the LDL by cells.

Modulation of the expression of human LDL-Apo B-100 epitopes by lipids and apolipoproteins

The purpose of the present study was to determine the immunochemical properties of apolipoprotein (apo) B-100 associated with low density lipoprotein (LDL) in relation to lipid and apolipoprotein composition. LDLs were isolated by sequential ultracentrifugation (1.019 < d < 1.050 g/mL) from two healthy volunteers and 21 dyslipidemic patients to obtain heterogeneous samples of LDL. Lipid (free cholesterol, cholesteryl esters, triglycerides, and phospholipids) and apolipoprotein contents (apo B, apo C-III, apo E) were determined in each LDL sample. Immunoreactivities of apo B were tested in solid-phase competitive-binding radioimmunoassays using seven monoclonal anti-LDL antibodies that reacted with defined epitopes of apo B-100. The relation between lipid and/or protein variables and the immunoreactivity of apo B was evaluated by successive use of Spearman's rank simple correlation, partial correlation, and canonical correlation analyses. The canonical correlation analysis showed that apo B-100 immunoreactivity on LDL is highly dependent on lipid and apolipoprotein composition simultaneously. The results confirmed the influence of surface and core lipids on the expression of the apo B-100 epitopes, independent of their location on the molecule. However, the lipid requirement of LDL strongly influences the expression of epitopes mapped in the LDL receptor-recognition domain. In contrast to apo E, apo C-III does not seem to influence the expression of the apo B-100 epitopes in the LDL range studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Relation of arteriographically defined coronary artery disease to serum lipoprotein particles mapped with monoclonal antibodies

BACKGROUND

This study was designed to investigate the relation of a molecular analysis of apolipoprotein B (apoB)-containing atherogenic lipoprotein particles to coronary artery disease (CAD) in middle-aged men.

METHODS AND RESULTS

Two groups of men were studied. The first consisted of 97 patients with angiographically documented CAD (greater than 50% stenosis of at least one coronary artery). The second group consisted of 145 subjects without symptomatic CAD, who served as controls. In both groups, measurements were obtained for total cholesterol level, triglyceride level, cholesterol contents in apoB- and nonapoB-containing particles (LpB, LpnonB), total apoB and apolipoprotein AI (apoAI levels), lipoprotein particles recognized by monoclonal antibodies anti-apoB (LpBL3, LpBL5, LpBL7) and anti-apoAI (LpAI-2GII). Taking into account age, body mass index, hypertension, diabetes, smoking habits, and drug consumption, the analysis showed that the mean levels of cholesterol were identical in both groups but differed when cholesterol content in LpB and LpnonB subfractions were assessed, thus reflecting an increase in the low density fraction and a decrease in the high density fraction, respectively. This was confirmed by an increase in total apoB and a decrease in total apoAI. Measurements of LpBL3, LpBL5, LpBL7, and LpAI-2GII particles also discriminated between the two groups. After adjustment for cholesterol content in LpnonB particles, a difference in total apoB was no longer significant between groups, whereas LpBL3, LpBL5, and LpBL7 levels remained significantly higher in CAD patients.

CONCLUSIONS

The measurement of separate concentrations of apoB in different particles may permit a more-accurate assessment of CAD risk than measurements of total apoB levels.

Resistance to LDL oxidative modifications of an N-terminal apolipoprotein B epitope

The immunoreactivity of human apolipoprotein B (apo B) towards 5 monoclonal antibodies was studied by enzyme immunoassay in native and in vitro oxidized low density lipoproteins (LDL). LDL oxidative modifications were obtained by incubation with either copper ions or an association of lipoxygenase and phospholipase A2. The monoclonal antibodies used in the inhibition analysis were directed to epitopes located in the amino-terminal region (1D1), in the middle part (2D8, L7, 4G3) and in the carboxy-terminal region (L3) of the apo B molecule. The results demonstrated that the immuno-reactivity of 1D1 epitope was little affected by LDL oxidation with copper ions or lipoxygenase plus phospholipase A2, whereas the immunoreactivity of the other epitopes were markedly decreased by these LDL modifications. Immunoreactivity changes were more important in L3 and L7 epitopes than in 2D8 and 4G3 epitopes. Since it is known that L3 and L7 epitopes are located in apo B domains rich in lipid-associated peptides whereas 1D1 is in a domain poor in such peptides, these results suggest a relationship between the lipid environment of an apo B epitope and its susceptibility to alteration by LDL oxidation.

Effect of simvastatin on plasma lipids, apolipoproteins and lipoprotein particles in patients with primary hypercholesterolaemia

The effect of treatment with simvastatin, a new HMG-CoA reductase inhibitor, has been investigated in 27 patients with primary hypercholesterolaemia. It produced a significant decrease of cholesterol and phospholipids in plasma, LDL and apolipoprotein B-containing lipoproteins. Plasma apolipoproteins B, C-III and E were also significantly lowered. The concentration of lipoprotein particles recognized by monoclonal antibodies (BL3, BL5 and BL7), associated with atherosclerotic disease, was also lowered by the treatment. Lipoproteins LpA-II:A-I were not changed, while LpA-I, which has been suggested to be the protective fraction of the apo A-I-containing lipoproteins, was slightly and inconsistently increased.

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.

Association between coagulation factors VII and X with triglyceride rich lipoproteins

The association between the concentration of different plasma lipoproteins and plasma factor VII (F VII) was analysed by isolating plasma very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL) lipoproteins and assessing their in vitro interaction with F VII by immunoenzyme assay using peroxidase labelled anti-factor VII immunoglobulins to determine whether F VII coagulant activity is prognostic for cardiovascular mortality. F VII bound to triglyceride rich lipoproteins, the fixation being stronger on chylomicrons and VLDL fractions than on LDL fractions. In our experiments HDL did not bind to F VII. The fixation of coagulation factor X (FX) tested by the same method is comparable with that of F VII. The nature of this fixation seemed to arise from hydrophobic interaction as calcium was not necessary and the use of Tween 20 inhibited the interaction. The binding of factors VII and X was increased when lipids were previously treated by phospholipase C and the interaction seemed to be completely dependent on the lipid part of the lipoproteins. Hyrophobic fixation is a possible mechanism of interaction of plasma lipoproteins and F VII and X, and it may be of importance in the covariance of triglyceride concentrations and the activity of vitamin K dependent coagulation factors.

Human apolipoprotein B. Evidence for its immunochemical heterogeneity using monoclonal antibodies and an immunoenzymometric assay

Predefined monoclonal antibodies (Mabs) were used in an immunoenzymometric assay to study the immunochemical heterogeneity of lipoproteins and to search for potential epitopes with pathological importance. By measuring apolipoprotein B (apo B) epitopes in patients with and without angiographically documented coronary artery disease and in patients with type IIa hyperlipoproteinemia, we have found that both types of patients have a significant increase in Apo B-containing particles specifically recognized by one Mab (BL3). We have also observed that the effects of fenofibrate on type IIa patients vary greatly depending on the plasma concentrations of various Apo B-containing lipoproteins. The greatest effects occurred in patients with epitopes recognized by BL3. Lastly, by sequential precipitation of specific epitopes by BL3, we have obtained evidence that the residual epitope(s) may be related to one or more lipoprotein particles.

Synthesis and secretion of lipoproteins by human hepatocytes in culture

Confluent monolayers of normal human hepatocytes obtained by collagenase perfusion of liver fragments were incubated in a serum-free medium. Intracellular apolipoproteins apo AI, apo C, apo B, and apo E were detected between Day 1 and Day 6 of the culture by immunoenzymatic staining using polyclonal antibodies directed against these apoproteins and monoclonal antibodies directed against both forms of apo B (B100 and B48). Translation of mRNA isolated from these hepatocytes in an acellular system revealed that apo AI and apo E were synthesized as the precursor forms of mature plasma apo AI and apo E. Three lipoprotein fractions corresponding to the density of very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL) were isolated from the medium at Day 5 of culture and examined by electron microscopy after negative staining. VLDL and LDL particles are similar in size and shape to plasma lipoproteins; spherical HDL are larger than normal plasma particles isolated at the same density. Their protein represented 44, 19.5, and 36.5% respectively, of the total lipoprotein protein. The secretion rate of VLDL protein corresponded to that measured in primary cultures of rat hepatocytes. After incorporation of [3H]glycerol, more than 92% of the [3H]triglyceride secreted into the medium was recovered in the VLDL fraction. These results demonstrate that primary cultures of normal human hepatocytes are able to synthesize and secrete lipoproteins and thus could be a useful model to study lipoprotein metabolism in human liver.

Effect of fenofibrate treatment on type III hyperlipoproteinemia

The effect of treatment with fenofibrate was investigated in nine patients (seven men) presenting with type III hyperlipoproteinemia. This therapy produced a statistically significant decrease in serum triglyceride, cholesterol, apolipoproteins B, CII, CIII, and E levels. Particular attention was focused on lipoproteins containing apolipoprotein B. The concentration of particles recognized by monoclonal antibodies (BL3, BL5, and BL7), associated with atherosclerotic disease, was lowered by the treatment. The most dramatic decrease was observed for lipoproteins LpE:B and LpCIII:B, which are particularly enhanced in type III hyperlipoproteinemia.

Immunological detection of low-density lipoproteins modified by malondialdehyde in vitro or in vivo

We describe an ELISA technique able to recognize malondialdehyde-modified low-density lipoproteins (LDL). For this purpose we produced antibodies to malondialdehyde-LDL, specific for the malondialdehyde modification of LDL; these antibodies recognized essentially malondialdehyde-LDL. Coating ELISA plates with the antibodies to malondialdehyde-LDL and using peroxidase-labelled antibodies to LDL, which reveal only apolipoprotein B, we obtained an accurate method of detecting malondialdehyde-modified apolipoprotein B. Preliminary studies demonstrated that this method allows the detection of lipoproteins containing malondialdehyde-modified apolipoprotein B in the serum of patients with cardiovascular diseases.

Monoclonal antibodies to human apolipoprotein A-I: characterization and application as structural probes for apolipoprotein A-I and high density lipoprotein

Three mouse monoclonal antibodies (Mabs) to human apo A-I were produced using apolipoprotein A-I or HDL3 as immunogens. These monoclonal antibodies, 2G11, 4A12 and 4B11, were characterized for their reactivity with isolated apolipoprotein A-I and HDL in solution. The immunoblotting patterns of the HDL3 two-dimensional electrophoresis show that these three monoclonal antibodies reacted with all the polymorphic forms of apolipoprotein A-I. Cotitration experiments indicated that they correspond to three distinct epitopes. In order to locate these three antigenic determinants on the isolated apolipoprotein A-I, the reactivity of the three monoclonal antibodies has been studied on CNBr-cleaved apolipoprotein A-I. The monoclonal antibodies 2G11 and 4A12 addressed to the amino (CNBr 1) and carboxy (CNBr 4) terminal segments, respectively. In comparison with the monoclonal antibodies characterized by Weech et al. ((1985) Biochim. Biophys. Acta 835, 390-401), monoclonal antibody 4A12 is the only one described in the literature which is specific of the carboxy terminal segment of apolipoprotein A-I. Monoclonal antibody 4B11 does not react with any CNBr fragment, its binding is temperature dependent, it could be directed to a conformational epitope. Relative differences were demonstrated in the expression of the three epitopes in HDL subfractions isolated by density gradient ultracentrifugation. According to Curtiss and Edgington ((1985) J. Biol. Chem. 260, 2982-2993) our results indicate the existence of an immunochemical heterogeneity in the organization of apolipoprotein A-I at the surface of HDL particles as well as in the soluble form of apolipoprotein A-I.

Hypobetalipoproteinemia with accumulation of an apoprotein B-like protein in intestinal cells. Immunoenzymatic and biochemical characterization of seven cases of Anderson's disease

We describe here seven cases (from five kindreds) of Anderson's disease, which is characterized by diarrhea, steatorrhea, hypobetalipoproteinemia with low levels of cholesterol, triglycerides, and phospholipids, and failure to secrete chylomicrons after a fat meal. Enterocytes isolated from intestinal biopsies of patients after overnight fast showed numerous fat droplets, a histological picture resembling that of abetalipoproteinemia. Immunoenzymatic staining of the enterocytes demonstrated large amounts of material that reacted with a polyclonal antiserum to apolipoprotein B. Further, the immunoreactive material was found to react with several different monoclonal antibodies capable of recognizing both the B100 and B48 forms of apoprotein B, but not with any of several monoclonal antibodies capable of recognizing only B100. This suggests that the material in the enterocytes is the B48 form of apoprotein B or a fragment thereof. Additional findings included decreased low density lipoprotein levels with an abnormal chemical composition, abnormal high density lipoprotein2 (HDL2) and HDL3 particle size distributions, and an abnormal HDL apoprotein composition. Increased amounts of proteins having electrophoretic mobilities similar to apo E and the E-AII complex were present. Finally, some cases exhibited additional protein components of apparent molecular weights between 17,000 and 28,000, which was similar to some cases of abetalipoproteinemia. These findings demonstrate that Anderson's disease is not due to the absence of synthesis of intestinal apo B and suggest that it is more complex than previously thought, affecting all the lipoprotein classes.

Isolation and characterization of three monoclonal antibodies to human serum low density lipoprotein apoprotein B

Human serum low density lipoprotein (LDL) is a large (Mr = 2-3 X 10(6), complex particle composed of lipid, protein and carbohydrate. We obtained about 40 mouse spleen-myeloma hybrid cell lines which produce antibodies against LDL. Three of them, SC2, SC3 and SC10, have been cloned and subcloned and their antibody products characterized. They recognize three non-overlapping epitopes in native LDL. Two of them, SC3 and SC10, also are capable of recognizing very low density lipoprotein, (VLDL), whereas SC2 reacts only weakly with VLDL. All three antigenic determinants remain intact, and accessible to antibodies on the LDL protein apo B, prepared by delipidation in a 'non-denaturing' detergent, sodium deoxycholate. However, apo B prepared by organic solvent, ether-ethanol, or sodium dodecyl sulfate (SDS) delipidation, while reacting strongly with SC10, is only poorly recognized by SC2 or SC3. Proteolysis of LDL with trypsin, chymotrypsin, Staphylococcus aureus protease, papain or thermolysin gives, in each case, several non-identical protein fragments which are separable by SDS-polyacrylamide gel electrophoresis. Upon immunoblotting, some of these fragments are now recognized by either SC3 or SC10 but not SC2, some are recognized by both SC3 and SC10, and others are immunologically unreactive. The protein bands that are separated by SDS gel electrophoresis are composed of several non-identical fragments and contain the antigenic sites to differing degrees. Some of the immunologically reactive fragments do not appear to contain carbohydrate. Reduction and carboxymethylation do not destroy the immunoreactivity of LDL toward any of the antibodies; however, modification of lysine residues by citraconic anhydride markedly diminishes the reactivity of LDL toward SC3. It is likely that the two antibodies SC3 and SC10 are directed against different linear amino acid sequences or very stable domains, whereas the third, SC2, is directed against a more fragile conformational domain of apo B.