Restriction isotyping of human apolipoprotein A-IV: rapid typing of known isoforms and detection of a new isoform that deletes a conserved repeat.

Genetic polymorphisms of apolipoprotein A-IV (apoA-IV) have been detected by isoelectric focusing of serum proteins. Because genetic variation in apoA-IV has significant effects on lipid risk factors, we used restriction enzyme isoform genotyping (restriction isotyping) to determine apoA-IV isoform genotypes at the DNA level for a large population (n = 509). In contrast to isoelectric focusing methods, restriction isotyping relies on nucleotide differences, enabling unambiguous typing of known isoforms and detection of new alleles that mimic other isoforms with shared charge properties. To determine genotypes for the common A-IV-1 isoform (Gln at aa position 360) and A-IV-2 isoform (360His), we used a mismatched primer for polymerase chain reaction (PCR) to introduce a restriction site (PvuII) that distinguishes each isoform. Using a portion of the same PCR reaction, we used HinfI to distinguish isoforms with Thr at position 347 (347Thr) versus Ser (347Ser). In surveys for these common genotypes, we detected heterozygotes for an allele with an insertion of 12 bp. Nucleotide sequencing showed that this allele is identical to the A-IV-0 isoform that inserts a hydrophilic repeat (Glu Gln Gln Gln) in a conserved region near the carboxy terminus. In addition, we discovered a new allele with a 12 bp deletion that removes a repeat (Glu Gln Gln Gln) from the same region. Nucleotide sequencing showed that this allele removes an acidic charge relative to A-IV-1, so we have named this isoform A-IV-2*. This isoform has not been discovered at the protein level, perhaps due to shared charge properties with A-IV-2 isoforms.     

Apolipoprotein A-IV polymorphism in the Finnish population: gene frequencies and description of a rare allele

The apolipoprotein A-IV (apoA-IV) allele frequencies were determined in 387 adult Finns by immunoblotting after isoelectric focusing of serum. The gene frequencies were: A-IV1 = 0.942 and A-IV2 = 0.058. The phenotypes of 147 mother-child pairs studied were in accordance with the two allelic modes of inheritance. In 2 subjects, a rare apoA-IV variant was found.     

Effect of apolipoprotein A-IV genotype and dietary fat on cholesterol absorption in humans

We investigated the effect of the A-IV-2 allele, which encodes a Q360H substitution in apolipoprotein (apo) A-IV, and dietary fat on cholesterol absorption in humans. In three separate studies we compared fractional intestinal cholesterol absorption between groups of subjects heterozygous for the A-IV-2 allele (1/2) and homozygous for the common allele (1/1) receiving high cholesterol ( approximately 800 mg/day) diets with different fatty acid compositions. All subjects had the apoE 3/3 genotype. There was no difference in cholesterol absorption between the two genotype groups receiving a high saturated fat diet (33% of total energy as fat; 18% saturated, 3% polyunsaturated, 12% monounsaturated) or a low fat diet (22% of total energy as fat; 7% saturated, 7% polyunsaturated, 8% monounsaturated) diet. However, on a high polyunsaturated fat diet (32% of total energy as fat; 7% saturated, 13% polyunsaturated, 12% monounsaturated) mean fractional cholesterol absorption was 56. 7% +/- 1.9 in 1/1 subjects versus 47.5% +/- 2.1 in 1/2 subjects (P = 0.004). A post hoc analysis of the effect of the apoA-IV T347S polymorphism across all diets revealed a Q360H x T347S interaction on cholesterol absorption, and suggested that the A-IV-2 allele lowers cholesterol only in subjects with the 347 T/T genotype.We conclude that a complex interaction between apoA-IV genotype and dietary fatty acid composition modulates fractional intestinal cholesterol absorption in humans.     

A method to screen apolipoprotein polymorphisms in whole plasma: description of apolipoprotein A-IV variants in dyslipidemias and a reassessment of apolipoprotein A-I in Tangier disease

A sensitive and rapid immunological detection method was used to screen for apolipoprotein A-IV variants. Antibodies to human lymph chylomicron or plasma apolipoprotein A-IV, and plasma apolipoprotein A-I were raised in rabbits. Antibodies to apolipoprotein A-I or apolipoprotein A-IV were shown to be monospecific to their respective antigens by reactivity against human chylomicron apolipoproteins by immunoblot analysis. Plasma samples were obtained from dyslipidemic subjects from the Lipid Research Clinic of Columbia University. The plasma samples were isoelectrically focused (pH 4-6) on slab gels. Plasma proteins were then transferred to nitrocellulose paper for immunoblotting. Apolipoprotein A-IV polymorphism was determined by specific immunological detection of apolipoprotein A-IV. Identical apolipoprotein A-IV isoprotein patterns were observed when either antibodies to lymph or plasma apolipoprotein A-IV were used for immunoblotting. All the dyslipidemic plasma samples screened contained the two major and one or two minor isoproteins of normal plasma. In two instances, new apolipoprotein A-IV variants having an additional isoform were detected. One subject was hypertriglyceridemic (triacylglycerols = 342 mg/dl, cholesterol = 251 mg/dl) and had an additional major acidic apolipoprotein A-IV isoform. Another subject with mild hypocholesterolemia (triacylglycerols = 209 mg/dl, cholesterol = 120 mg/dl) was found to have additional major and minor basic apolipoprotein A-IV isoforms. The specificity of this technique allows detection of polymorphism of apolipoproteins of similar isoelectric points by use of a single dimension isoelectric focusing gel. This technique also demonstrated the presence of altered apolipoprotein A-I isoforms in the plasma of a patient with Tangier disease. These isoforms were previously identified as isoforms 2 and 4 of normal plasma by use of two-dimensional gel electrophoresis. However, by use of this new technique and careful evaluation of previously published two-dimensional gels, we now identify these apolipoprotein A-I isoforms as being more acidic than those of normal plasma.     

Isolation and lipid-binding properties of rat apolipoprotein A-IV

Apolipoprotein A-IV was isolated from the d less than 1.21 g/ml fraction of rat serum by gel filtration followed by heparin-Sepharose affinity chromatography; this method also facilitated the preparation of apolipoprotein A-I and apolipoprotein E. The apolipoprotein A-IV preparation was characterized by SDS-gel electrophoresis, isoelectric focusing, amino acid analysis and immunodiffusion. The lipid-binding properties of this protein were studied. Apolipoprotein A-IV associated with dimyristoylphosphatidylcholine (DMPC) to form recombinants which contained two molecules of apolipoprotein A-IV and had a lipid/protein molar ratio of 110. The density of the DMPC/apolipoprotein A-IV particles was determined to be 1.08 g/ml and the particles were visualized by electron microscopy as discs which were 5.8 nm thick and 18.0 nm in diameter. The stability of the DMPC/apolipoprotein A-IV recombinants, as determined by resistance to denaturation, was comparable to the stability of DMPC/apolipoprotein A-I complexes. However, by competition studies it was found that apolipoprotein A-I competed for the binding to DMPC more effectively than did apolipoprotein A-IV. It is concluded that, while rat apolipoprotein A-IV resembles other apolipoproteins in its lipid-binding characteristics, it may be displaced from lipid complexes by apolipoprotein A-I.     

Comparison of the metabolic behavior of rat apolipoproteins A-I and A-IV, isolated from both lymph chylomicrons and serum high density lipoproteins

Rat apolipoprotein (apo) A-I and A-IV, isolated from both lymph chylomicrons and serum high density lipoproteins (HDL) were analyzed by isoelectric focusing. Lymph chylomicron apo A-I consisted for 81 +/- 2% of the pro form and for 19 +/- 2% of the mature form, while apo A-I isolated from serum HDL was present for 36 +/- 4% in the pro form and for 64 +/- 4% in the mature form. Apo A-IV also showed two major protein bands after analysis by isoelectric focusing. The most prominent component is the more basic protein that amounts to 80 +/- 2% in apo A-IV isolated from lymph chylomicrons and to 60 +/- 3% in apo A-IV isolated from serum HDL. Apo A-I (or apo A-IV), isolated from both sources (lymph chylomicrons or serum HDL), was iodinated and the radioactive apolipoproteins were incorporated into rat serum lipoproteins. The resulting labeled HDL was isolated from serum by molecular sieve chromatography on 6% agarose columns and injected intravenously into rats. No difference in the fractional turnover rate or the tissue uptake of the two labeled HDL preparations was observed, neither for apo A-I nor for apo A-IV. It is concluded that the physiological significance of the extracellular pro apo A-I conversion or the post-translational modification of apo A-IV is not related to the fractional turnover rate in serum or to the rate of catabolism in liver and kidneys.     

Effect of the apolipoprotein A-IV Q360H polymorphism on postprandial plasma triglyceride clearance

Apolipoprotein (apo)A-IV is synthesized in the small intestine during fat absorption and is incorporated onto the surface of nascent chylomicrons. In circulation, apoA-IV is displaced from the chylomicron surface by high density lipoprotein-associated C and E apolipoproteins; this exchange is critical for activation of lipoprotein lipase and chylomicron remnant clearance. The variant allele A-IV-2 encodes a Q360H polymorphism that increases the lipid affinity of the apoA-IV-2 isoprotein. We hypothesized that this would impede the transfer of C and E apolipoproteins to chylomicrons, and thereby delay the clearance of postprandial triglyceride-rich lipoproteins. We therefore measured triglycerides in plasma, S(f) > 400 chylomicrons, and very low density lipoproteins (VLDL) in 14 subjects heterozygous for the A-IV-2 allele (1/2) and 14 subjects homozygous for the common allele (1/1) who were fed a standard meal containing 50 gm fat per m(2) body surface area. All subjects had the apoE-3/3 genotype. Postprandial triglyceride concentrations in the 1/2 subjects were significantly higher between 2;-5 h in plasma, chylomicrons, and VLDL, and peaked at 3 h versus 2 h for the 1/1 subjects. The area under the triglyceride time curves was greater in the 1/2 subjects (plasma, P = 0.045; chylomicrons, P = 0.027; VLDL, P = 0.063). A post-hoc analysis of the frequency of the apoA-IV T347S polymorphism suggested that it had an effect on triglyceride clearance antagonistic to that of the A-IV-2 allele. We conclude that individuals heterozygous for the A-IV-2 allele display delayed postprandial clearance of triglyceride-rich lipoproteins.     

Population genetics of apolipoproteins A-IV, E, and H, and the angiotensin converting enzyme (ACE): associations with lipids, and apolipoprotein levels in American Samoans

Distributions of alleles at three apolipoprotein loci (APO E, APO H, and APO A-IV) and an insertion/deletion (I/D) polymorphism at the angiotensin converting enzyme (ACE) locus among 274 American Samoans are described here. Genotypes at each locus are examined for associations with quantitative lipid (total cholesterol (total-c), LDL-cholesterol (LDL-c), HDL-cholesterol (HDL-c), and triglycerides) and apolipoprotein (APO AI, APO AII, APO E, and APO B) levels. Genotype frequencies at all four loci are in Hardy-Weinberg equilibrium. The most common APO A-IV genotype (1-1) was observed in 252 American Samoans (97%). The three most common APO E genotypes were 3-3 (47%), 3-4 (30%), and 2-3 (12%). The most frequent APO H genotype was 2-2 (86%). The most common ACE genotype (I/I) was observed in 75% of sampled individuals, and 23% were I/D heterozygotes. APO E genotypic variation was associated with total-c, HDL-c, LDL-c, and all four quantitative apolipoproteins (AI, AII, E, and B). APO A-IV genotypes were associated significantly with total cholesterol, LDL-c, and APO-B levels. APO H showed little association with any quantitative lipid or apolipoprotein. ACE D/D homozygotes had higher AII levels. ACE showed a consistent association with APO AII levels, with either APO A-IV or APO E as a covariate. The interaction term between ACE and APO E was also significantly associated with total-c and APO E levels, and the ACE genotype showed a significant main effect on APO AI levels in multivariate analyses.     

Apolipoprotein E and A-IV polymorphisms in ethnic Russians living in Estonia

137 Russians living in Estonia was screened by isoelectric focusing and immunoblotting procedures to determine the distribution of genetic variations in apolipoprotein E (apoE) and apolipoprotein A-IV (apoA-IV) genes. The apoA-IV-2 allele and epsilon4 allele frequency of the Russians tended to be lower than in most other European populations.     

Apolipoprotein A-IV: a protein occurring in human mesenteric lymph chylomicrons and free in plasma. Isolation and quantification.

1. Human mesenteric lymph chylomicrons were isolated from chylous ascites fluid by ultra-centrifugation and agarose/gel chromatography and their apoprotein composition was analysed by dodecylsulfate/polyacrylamide gel electrophoresis, analytical isoelectric focusing and immuno-chemically. Major components of mesenteric lymph chylomicrons were apoprotein A-I, proteins of Mr less than 15 000 including the C-group apoproteins and a protein of Mr 46 000. Minor components were apoprotein E and a protein of Mr approximately equal to 200 000 (B-like protein). This apoprotein composition was qualitatively identical with that of chylomicrons from intestinal lymph of the rat, but was distinctly different from plasma chylomicrons of humans with fasting chylomicronaemia. 2. The protein of Mr approximately equal to 46 000 has been isolated by preparative dodecylsulfate/polyacrylamide gel electrophoresis from human and rat lymph chylomicrons and was compared to a protein of identical Mr present in rat high-density lipoproteins (apoplipoprotein A-IV) and in the rho less than 1.006 g/ml serum lipoprotein fraction of individual humans with alimentary hypertriglyceridaemia. In both species the 46 000-Mr proteins isolated from lymph and serum were identical according to amino acid composition and isoelectric point in 6 M urea. The human proteins from both sources were also immunologically identical. The similarities in the molecular properties of the human apolipoprotein and rat apolipoprotein A-IV indicate that these proteins are homologous. 3. Plasma levels of human apolipoprotein A-IV determined by electroimmunodiffusion were 14.15 +/- 3.66 mg/100 ml (n = 59), but greater than 90% of the protein was unassociated with the major lipoprotein fractions. It is concluded, that apolipoprotein A-IV is a main protein component of human lymph chylomicrons, that is removed from the particles in the plasma compartment.     

Apolipoprotein A-IV polymorphism and its effect on plasma lipid and apolipoprotein concentrations

Recently, we determined the apolipoprotein E (apoE) phenotype distribution in 2,000 randomly selected 35-year-old male individuals by slab gel isoelectric focusing of delipidated plasma samples, followed by immunoblotting using anti-apoE antiserum. These blots have been successfully re-used for immunovisualization of apoA-IV isoelectric focusing patterns. In a population sample of 1,393 individuals, four distinct apoA-IV isoforms were detected, encoded by the alleles A-IV*0, A-IV*1, A-IV*2, and A-IV*3 with gene frequencies of 0.002, 0.901, 0.079, and 0.018, respectively. The mean of plasma cholesterol, triglyceride, apoB and E levels did not differ significantly among the different apoA-IV phenotype groups. For these lipoprotein parameters, less than 0.1% of the total phenotypic variance could be accounted for by the APOA-IV gene locus. Our results did not show any effect of apoA-IV polymorphism on plasma apoA-I levels nor could we find any correlation between plasma levels of apoA-I and apoA-IV within the different apoA-IV phenotype groups. The plasma level of apoA-IV in subjects bearing the A-IV*3 allele is significantly lower than in subjects without the A-IV*3 allele (5 mg/dl versus 14 mg/dl). We therefore conclude that, in contrast to the apoE polymorphism, the polymorphism at the APOA-IV locus does not influence any of the levels of the lipoprotein parameters considered except apoA-IV.     

Isolation and characterization of human apolipoprotein A-I Fukuoka (110 Glu----Lys). A novel apolipoprotein variant

A novel genetic variant of apolipoprotein(apo) A-I Fukuoka, has been identified in a Japanese family. This variant has a relative charge of +2 compared to normal apolipoprotein A-I (A-I4), on the isoelectric focusing gels and the same molecular mass and immunologic characteristics as normal apolipoprotein A-I. This variant, transmitted as an autosomal co-dominant inheritance was purified by preparative Immobiline isoelectric focusing. Sequence analysis after cleavage with lysyl endopeptidase and CNBr, followed by high-performance liquid chromatography revealed a single substitution of lysine at position 110, instead of the usual glutamic acid. This mutant apolipoprotein A-I has much the same potential as to activate lecithin-cholesterol acyltransferase.     

Polymorphisms of apolipoproteins A-IV and E in a Turkish population living in Germany

Human apolipoproteins (apo) E and apo A-IV are polymorphic with significantly different allele frequencies among different ethnic groups. Whereas the variation at the apo E gene locus affects plasma cholesterol levels in all populations studied so far and is associated with longevity in Caucasians, the influence of the common apo A-IV polymorphism on plasma lipoproteins has not been unanimously accepted. We have therefore determined the common apo E and apo A-IV polymorphisms by isoelectric focusing, calculated the respective allele frequencies and studied their effects on plasma lipoproteins in a random sample of 240 nonrelated Turkish subjects (141 males, 99 females) living in Germany and originating from central and eastern Anatolia. When compared with the German population and other Caucasians in Europe a prominence of the apo ɛ3 allele frequency (0.885) was accompanied by a decrease in the frequencies of both the apo ɛ2 allele (0.048) and the apo ɛ4 allele (0.067). Thus, the Turkish population studied here clustered with populations mainly from southern Europe and Japan, which have low ɛ2 and ɛ4 allele frequencies. Also, the frequency of the A-IV-1 allele was higher (0.967) and that of the A-IV-2 allele lower (0.033) in the Turkish subjects studied than in other populations. At an average level of total cholesterol of 194.5 ± 45 mg/dl, no significant influence of the A-IV alleles on plasma lipoproteins was seen. However, apo E and apo B differed significantly between apo E phenotypes, with high levels of apo E and low levels of cholesterol and apo B in carriers of the ɛ2 allele, and vice versa for the ɛ4 allele. The average cholesterol excess for the ɛ2 allele was –7.95 mg/dl, for the ɛ3 allele, –1.34, and for the ɛ4 allele, +14.15 mg/dl. Thus, despite the unusual frequency distribution of the apo E alleles, their effects on plasma lipoproteins are within the range reported for other populations in Europe. Received: 10 April 1995 / Revised: 25 March 1996     

Apolipoprotein A-IGiessen (Pro143----Arg). A mutant that is defective in activating lecithin:cholesterol acyltransferase

Apolipoprotein A-IGiessen is a variant form of apo A-I that is displaced from the corresponding normal A-I isoforms on isoelectric focusing gels by a single charge unit towards the cathode [Utermann et al. (1982) J. Biol. Chem. 257, 501-507]. Three subjects heterozygous for the variant were detected in one family. The percentage of the total A-I in plasma represented by the A-IGiessen in these subjects ranged over 25-30%. The variant and normal major A-I isoforms from the proband (Y.J.) were purified by preparative isoelectric focusing and cleaved with CNBr. Analytical focusing of CNBr fragments demonstrated a charge difference between CB3Giessen and normal CB3. Sequence analysis of CB3Giessen revealed that a proline existing in normal A-I was replaced by an arginine in the variant A-I at residue 143. The ability of the mutant A-I to activate purified lecithin:cholesterol acyltransferase was determined in vitro. The cofactor activity of [Arg143]apolipoprotein A-I was about 60-70% of that demonstrated by control A-I. Residue 143 is in a putative beta-turn between two of the repeating amphiphilic helices in apolipoprotein A-I and may be a critical determinant of the protein's structure and function.     

Human apolipoprotein A-I polymorphism. Identification of amino acid substitutions in three electrophoretic variants of the Münster-3 type

Variant forms of apolipoprotein A-I (apo-A-I) have been shown to exist in the human population. One mutant form, referred to as apo-A-I-Münster-3, is one charge unit more basic than normal apo-A-I on isoelectric focusing gels. This variant has the same immunologic characteristics and molecular weight as normal apo-A-I. The apo-A-I-Münster-3 from subjects in three unrelated families (in two of which the trait has been shown to be transmitted as an autosomal co-dominant) has been analyzed by partial amino acid sequencing to define the cause of the electrophoretic abnormality. In the apo-A-I of family A, the abnormality was shown to occur in the smallest cyanogen bromide fragment, CB-2 (residues 87-112), and amino acid sequencing revealed asparagine instead of the usual aspartic acid at residue 103. Subjects with this mutant form have shown no signs of dyslipoproteinemia. The NH2-terminal cyanogen bromide fragment (CB-1, residues 1-86) from the apo-A-I of family B was shown to differ electrophoretically from normal CB-1, and amino acid sequencing revealed that a substitution of arginine for proline at residue 4 was responsible for this variant form. Analysis of the plasma lipids of one affected family B member demonstrated that the percentage of the total cholesterol that was esterified was somewhat lower than that normally observed. In a third family, family C, a variant having the same electrophoretic abnormality as the other two was determined to have an amino acid substitution at yet a different position. In this variant, histidine was found at residue 3 in the apo-A-I sequence, rather than the usual proline. In all three cases, the substitution could account for the electrophoretic abnormality. It is proposed that these three apo-A-I-Münster-3 variants be designated apo-A-I(Asp103----Asn), apo-A-I(Pro4----Arg), and apo-A-I(Pro3----His), respectively, to indicate the substitution that accounts for the abnormality in isoelectric focusing gels.     

Identification and partial characterization of discrete apolipoprotein A-containing lipoprotein particles secreted by human hepatoma cell line HepG2

The purpose of this study was to identify the apolipoprotein A-containing lipoprotein particles produced by HepG2 cells. The apolipoprotein A-containing lipoproteins separated from apolipoprotein B-containing lipoproteins by affinity chromatography of culture medium on concanavalin A were fractionated on an immunosorber with monoclonal antibodies to apolipoprotein A-II. The retained fraction contained apolipoproteins A-I, A-II and E, while the unretained fraction contained apolipoproteins A-I and E. Both fractions were characterized by free cholesterol as the major and triglycerides and cholesterol esters as the minor neutral lipids. Further chromatography of both fractions on an immunosorber with monoclonal antibodies to apolipoprotein A-I showed that 1) apolipoprotein A-II only occurs in association with apolipoprotein A-I, 2) apolipoprotein A-IV is only present as part of a separate lipoprotein family (lipoprotein A-IV), and 3) apolipoprotein E-enriched lipoprotein A-I:A-II and lipoprotein A-I are the main apolipoprotein A-containing lipoproteins secreted by HepG2 cells.     

Apolipoprotein E and A-IV Polymorphisms in Ethnic Russians Living in Estonia

To determine the distribution of genetic variations in apolipoprotein E (apoE) and apolipoprotein A-IV (apoA-IV) genes, 137 Russians living in Estonia was screened by isoelectric focusing and immunoblotting procedures. The apoA-IV-2 allele and apoE4 allele frequency of the Russians tended to be lower than in most other European populations.     

Dynamic interfacial properties of human apolipoproteins A-IV and B-17 at the air/water and oil/water interface

Viscoelastic behavior of proteins at interfaces is a critical determinant of their ability to stabilize emulsions. We therefore used air bubble surfactometry and drop volume tensiometry to examine the dynamic interfacial properties of two plasma apolipoproteins involved in chylomicron assembly: apolipoprotein A-IV and apolipoprotein B-17, a recombinant, truncated apolipoprotein B. At the air/water interface apolipoproteins A-IV and B-17 displayed wide area - tension loops with positive phase angles indicative of viscoelastic behavior, and suggesting that they undergo rate-dependent changes in surface conformation in response to changes in interfacial area. At the triolein/water interface apolipoprotein A-IV displayed maximal surface activity only at long interface ages, with an adsorption rate constant of 1.0 3 10(-)(3) sec(-)(1), whereas apolipoprotein B-17 lowered interfacial tension even at the shortest interface ages, with an adsorption rate constant of 9.3 3 10(-)(3) sec(-)(1). Apolipoprotein A-IV displayed an expanded conformation at the air/water interface and a biphasic compression isotherm, suggesting that its hydrophilic amphipathic helices move in and out of the interface in response to changes in surface pressure.We conclude that apolipoproteins A-IV and B-17 display a combination of interfacial activity and elasticity particularly suited to stabilizing the surface of expanding triglyceride-rich particles.     

Peptide YY stimulates the expression of apolipoprotein A-IV gene in Caco-2 intestinal cells

The effect of peptide YY, a gastrointestinal hormone, on the expression of the apolipoprotein A-IV gene in the intestinal epithelial cell line Caco-2 was examined by semiquantitative RT-PCR followed by Southern hybridization with an inner oligonucleotide probe. Apolipoprotein A-IV mRNA levels were increased in response to peptide YY in a dose- and time-dependent fashion. Western blotting revealed that the exogenous peptide YY increased the intracellular concentration of apolipoprotein A-IV. In contrast, apolipoprotein A-I, B, and C-III mRNA did not respond to peptide YY. Differentiated Caco-2 cells expressed Y1- but not Y2- and Y5-receptor subtype mRNA. The present results suggest that peptide YY modulates apolipoprotein A-IV gene expression, likely via the Y1-receptor subtype in intestinal epithelial cells.     

Correlation between apolipoprotein A-IV and triglyceride concentrations in human sera

Apolipoprotein A-IV concentration was measured by a newly developed competitive enzyme immunoassay in sera from fasted human subjects (n = 105) whose triglyceride concentrations ranged from 20 to 474 mg/dl (total cholesterol below 260 mg/dl) and in which chylomicrons could not be detected. Mean (+/- SD) apolipoprotein A-IV concentration was 13.0 +/- 2.6 mg/dl in sera with triglyceride levels ranging from 20 to 100 mg/dl, 16.9 +/- 3.7 mg/dl in sera with triglyceride levels ranging from 101 to 250 mg/dl, and 22.7 +/- 6.7 mg/dl in sera with triglyceride levels ranging from 251 to 474 mg/dl. The differences among the three groups were highly significant (P less than 0.001). Moreover, variations of apolipoprotein A-IV concentrations according to the triglyceride levels were noted within the normo-triglyceridemic population. Apolipoprotein A-IV concentration was 12.8 +/- 2.1 mg/dl for triglyceride levels ranging from 20 to 75 mg/dl and 16.4 +/- 3.8 mg/dl for triglyceride levels ranging from 76 to 150 mg/dl (P less than 0.01). In the entire population that was studied there was a significant linear correlation (r = 0.61, P less than 0.001) between the concentrations of serum apolipoprotein A-IV and triglyceride. Although the hypothesis of an unknown factor independently influencing both very low density lipoproteins and apolipoprotein A-IV cannot be ruled out, and although no apolipoprotein A-IV was found in the triglyceride-rich lipoprotein fraction after separation by gel filtration, these data suggest that, in fasting subjects, the secretion of very low density lipoproteins could contribute to the plasma apolipoprotein A-IV level.