Rabbit apolipoprotein A-I mRNA and gene. Evidence that rabbit apolipoprotein A-I is synthesized in the intestine but not in the liver

Isolation and characterization of some novel genes of the apolipoprotein A-I family in Japanese eel, Anguilla japonica

Apolipoproteins such as apolipoprotein (apo) A-I, apoA-IV, and apoE are lipid binding proteins synthesized mainly in the liver and the intestine and play an important role in the transfer of exogenous or endogenous lipids through the circulatory system. To investigate the mechanism of lipid transport in fish, we have isolated some novel genes of the apoA-I family, apoIA-I (apoA-I isoform) 1–11, from Japanese eel by PCR amplification. Some of the isolated genes of apoIA-I corresponded to 28kDa-1 cDNAs which had already been deposited into the database and encoded an apolipoprotein with molecular weight of 28 kDa in the LDL, whereas others seemed to be novel genes. The structural organization of all apoIA-Is consisted of four exons separated by three introns. ApoIA-I10 had a total length of 3232 bp, whereas other genes except for apoIA-I9 ranged from 1280 to 1441 bp. The sequences of apoIA-Is at the exon-intron junctions were mostly consistent with the consensus sequence (GT/AG) at exon-intron boundaries, whereas the sequences of 3′ splice acceptor in intron 1 of apoIA-I1-7 were (AC) but not (AG). The deduced amino acid sequences of all apoIA-Is contained a putative signal peptide and a propeptide of 17 and 5 amino acid residues, respectively. The mature proteins of apoIA-I1-3, 7, and 8 consisted of 237 amino acids, whereas those of apoIA-I4-6 consisted of 239 amino acids. The mature apoIA-I10 sequence showed 65% identity to amino acid sequence of apoIA-I11 which was associated with an apolipoprotein with molecular weight of 23 kDa in the VLDL. All these mature apoIA-I sequences satisfied the common structural features depicted for the exchangeable apolipoproteins such as apoA-I, apoA-IV, and apoE but apoIA-I11 lacked internal repeats 7, 8, and 9 when compared with other members of apoA-I family. Phylogenetic analysis showed that these novel apoIA-Is isolated from Japanese eel were much closer to apoA-I than apoA-IV and apoE, suggesting new members of the apoA-I family.

Apolipoprotein A-I and lecithin:cholesterol acyltransferase transfer induce cholesterol unloading in complex atherosclerotic lesions

Plasma levels of high-density lipoprotein (HDL) cholesterol and its major apolipoprotein (apo), apo A-I, are inversely correlated with the incidence of ischemic cardiovascular diseases. Reverse cholesterol transport is likely the main mechanism underlying the atheroprotective effects of HDL. Here, we investigated whether increased HDL cholesterol following hepatocyte-directed adenoviral rabbit apo A-I (AdrA-I) or rabbit lecithin-cholesterol acyltransferase (LCAT) (AdrLCAT) transfer may induce cholesterol unloading in complex atherosclerotic lesions in heterozygous low-density lipoprotein receptor-deficient rabbits fed a 0.15% cholesterol diet for 420 days before and for 120 days after transfer. HDL cholesterol levels increased 2.0-fold (P<0.001) and 1.9-fold (P<0.001) in the 120 days after transfer with AdrA-I and AdrLCAT, respectively, compared to levels just before transfer whereas non-HDL cholesterol remained unchanged. Increased HDL cholesterol following AdrA-I and AdrLCAT transfer resulted in a 31% (P<0.05) reduction of the intima/media ratio in comparison with the control progression group. Compared to the baseline group killed after 420 days of cholesterol diet, AdrA-I and AdrLCAT transfer reduced the percentage of Oil Red O area 1.6-fold (P<0.001) and 1.4-fold (P<0.001), respectively. In conclusion, increased HDL cholesterol after AdrA-I and AdrLCAT transfer inhibits progression of atherosclerosis and induces cholesterol unloading in complex lesions in rabbits.

Elevated hepatic apolipoprotein A-I transcription is associated with diet-induced hyperalphalipoproteinemia in rabbits

Past studies have shown that a high saturated fatty acid diet containing coconut oil elevates plasma HDL cholesterol and apolipoprotein A-I (apoA-1) in rabbits through a mechanism involving increased synthesis. We have extended those studies by investigating expression of the hepatic apolipoprotein A-I gene and other lipid related genes in that model. Rabbits fed a diet containing 14% coconut oil for 4 weeks showed HDL-C elevations of 170% to 250% over chow-fed controls with peak differences occurring at 1 week. Plasma apoA-I levels were also increased over this time frame (160% to 180%) reflecting the HDL-C changes. After 4 weeks, there were no differences in plasma VLDL-C or LDL-C levels in chow versus coconut oil-fed rabbits. Hepatic levels of apoA-I mRNA in coconut oil-fed animals were elevated 150% after 4 weeks compared to chow-fed controls; hepatic mRNA levels for ten other genes either decreased slightly (apoB, LCAT, hepatic lipase, albumin, ACAT, and HMG CoA reductase) or were unchanged (CETP, apoE, LDL-receptor, and acyl CoA oxidase). Nuclear run-on transcription assays revealed that coconut oil feeding for 4 weeks caused a 220% increase in hepatic apoA-I transcription rate compared to controls; no change was observed for CETP and apoE. Treatment of cultured rabbit liver cells with various saturated fatty acids and sera from chow-fed and coconut oil-fed rabbits did not alter apoA-I mRNA levels as observed in vivo. These data demonstrate that coconut oil elevates plasma HDL-C and apoA-I by increasing hepatic apoA-I transcription while expression of other genes involved in lipid metabolism are reduced or unchanged in response to coconut oil feeding.

The Antarctic toothfish (Dissostichus mawsoni) lacks plasma albumin and utilises high density lipoprotein as its major palmitate binding protein

Plasma from the Antarctic toothfish, Dissostichus mawsoni, a member of the advanced teleost Nototheniidae family, was analysed. Agarose gel electrophoresis showed a major diffuse anionic protein that bound [14C]palmitic acid but not 63Ni2+, and two more cationic proteins that bound 63Ni2+ but not palmitate. Oil Red O staining following cellulose acetate electrophoresis indicated that the palmitate binding protein was a lipoprotein. Two-dimensional electrophoresis showed that this palmitate binding band was composed of three proteins with M(r) of 11, 30, and 42 kDa, without any trace of material at approximately 65 kDa, the mass of albumin. N-terminal sequencing of the palmitate binding band gave a major sequence of DAAQPSQELR-, indicating a high degree of homology to apolipoprotein A-I (apo-AI), the major apolipoprotein of high density lipoprotein (HDL). N-terminal sequencing of the major nickel binding band produced a sequence with no homology to albumin. When ultracentrifugation was used to isolate the lipoproteins from Antarctic toothfish plasma, the palmitate binding protein was found solely in the lipoprotein fraction. In competitive binding experiments, added human albumin did not prevent palmitate binding to toothfish HDL. In conclusion, there is no evidence for albumin in Antarctic toothfish plasma and HDL assumes the role of fatty acid transport.

Beneficial effects of fibrates on apolipoprotein A-I metabolism occur independently of any peroxisome proliferative response

BACKGROUND

In humans, fibrates are frequently used normolipidemic drugs. Fibrates act by regulating genes involved in lipoprotein metabolism via activation of the peroxisome proliferator-activated receptor-alpha (PPARalpha) in liver. In rodents, however, fibrates induce a peroxisome proliferation, leading to hepatomegaly and possibly hepatocarcinogenesis. Although this peroxisome proliferative response appears not to occur in humans, it remains controversial whether the beneficial effects of fibrates on lipoprotein metabolism can occur dissociated from such undesirable peroxisomal response. Here, we assessed the influence of fenofibrate on lipoprotein metabolism and peroxisome proliferation in the rabbit, an animal that, contrary to rodents and similar to humans, is less sensitive to peroxisome proliferators.

METHODS AND RESULTS

First, we demonstrate that in normal rabbits, fenofibrate given at a high dose for 2 weeks does not influence serum concentrations or intestinal mRNA levels of the HDL apolipoprotein apoA-I. Therefore, the study was continued with human apoA-I transgenic rabbits that overexpress the human apoA-I gene under control of its homologous promoter, including its PPAR-response elements. In these animals, fenofibrate increases serum human apoA-I concentrations via an increased expression of the human apoA-I gene in liver. Interestingly, liver weight or mRNA levels and activity of fatty acyl-CoA oxidase, a rate-limiting and marker enzyme of peroxisomal beta-oxidation, remain unchanged after fenofibrate.

CONCLUSIONS

Expression of the human apoA-I transgene in rabbit liver suffices to confer fibrate-mediated induction of serum apoA-I. Furthermore, these data provide in vivo evidence that the beneficial effects of fibrates on lipoprotein metabolism occur mechanistically dissociated from any deleterious activity on peroxisome proliferation and possibly hepatocarcinogenesis.

High density lipoprotein (HDL), and not albumin, is the major palmitate binding protein in New Zealand long-finned (Anguilla dieffenbachii) and short-finned eel (Anguilla australis schmidtii) plasma

Plasma from two members of the teleost Anguillidae family, the New Zealand long-finned (Anguilla dieffenbachii) and short-finned eels (Anguilla australis schmidtii), were examined. Agarose gel electrophoresis showed both species had a major anionic diffuse protein band migrating at approximately the same position as human albumin, and autoradiography showed this protein bound [14C]palmitic acid, but not 63Ni2+. Cellulose acetate electrophoresis followed by Oil Red O staining suggested that this band was a lipoprotein. Two-dimensional electrophoresis of plasma showed the absence of a significant albumin band at approx. 65 kDa, and that the palmitate binding band appeared to be composed of at least three proteins, with the major protein running at 30 kDa. N-Terminal sequencing of the palmitate binding band indicated major sequences of DAPAPP(S)QLED- for long-finned eel and DAPAPPSQLEHV- for short-finned eel, confirming their identities as apo-AI, the major apolipoprotein of high density lipoprotein (HDL). When ultracentrifugation was used to separate the lipoproteins of each species, the anionic palmitate binding protein was found solely in the lipoprotein fractions. There was no evidence of albumin in plasma from either eel, and it appears that in its absence HDL takes on the role of fatty acid transport.

Cloning, characterisation and expression of the apolipoprotein A-I gene in the sea bream (Sparus aurata)

A full length cDNA clone representing apolipoprotein A-I was isolated from a sea bream (Sparus aurata) liver library. The clone encodes a 261 amino acid protein which shows highest amino acid identity (38%) with salmon apolipoprotein A-I. Northern blot analysis showed strong expression of a 1.4 kb transcript in liver with lower expression in intestine. Expression of apolipoprotein A-I in intestine was markedly reduced by treatment with triiodothyronine (T3).

Transgenic rabbits expressing human apolipoprotein A-I in the liver

Human apolipoprotein A-I (apo A-I) transgenic rabbits were created by use of an 11-kb genomic human apo A-I construct containing a liver-specific promoter. Five independent transgenic lines were obtained in which human apo A-I gene had integrated and was expressed. Plasma levels of human apo A-I ranged from 8 to 100 mg/dL for the founder and up to 175 mg/dL for the progeny. Rabbit apo A-I levels were substantially decreased in the transgenic rabbits. HDL cholesterol (HDL-C) levels were higher in two of the five transgenic rabbit lines than in controls (line 20 versus nontransgenic littermate, HDL-C = 80 +/- 7 versus 37 +/- 6 mg/dL; line 8 versus nontransgenic littermate, HDL-C = 54 +/- 16 versus 35 +/- 6 mg/dL). This resulted in less atherogenic lipoprotein profiles, with very low (VLDL + LDL-C)/HDL-C ratios. HDL size and protein and lipid compositions were similar between transgenic and littermate nontransgenic rabbits. However, a large amount of pre-beta apo A-I-containing lipoproteins was observed in the plasma of the highest human apo A-I expressor. Cell cholesterol efflux was evaluated with the incubation of whole serum from transgenic and control rabbits. Cell cholesterol efflux was highly correlated with HDL cholesterol, with apo A-I, and with the presence of pre-beta apo A-I-containing lipoproteins. These rabbits will be an extremely useful model for the evaluation of the effect of increased hepatic apo A-I expression on atherosclerosis.

Carboxyl-terminal domain truncation alters apolipoprotein A-I in vivo catabolism

Apolipoprotein A-I (apoA-I), the major protein of high density lipoproteins, facilitates reverse cholesterol transport from peripheral tissue to liver. To determine the structural motifs important for modulating the in vivo catabolism of human apoA-I (h-apoA-I), we generated carboxyl-terminal truncation mutants at residues 201 (apoA-I201), 217 (apoA-I217), and 226 (apoA-I226) by site-directed mutagenesis. ApoA-I was expressed in Escherichia coli as a fusion protein with the maltose binding protein, which was removed by factor Xa cleavage. The in vivo kinetic analysis of the radioiodinated apoA-I in normolipemic rabbits revealed a markedly increased rate of catabolism for the truncated forms of apoA-I. The fractional catabolic rates (FCR) of 9.10 +/- 1.28/day (+/- S.D.) for apoA-I201, 6.34 +/- 0.81/day for apoA-I217, and 4.42 +/- 0.51/day for apoA-I226 were much faster than the FCR of recombinant intact apoA-I (r-apoA-I, 0.93 +/- 0.07/day) and h-apoA-I (0.91 +/- 0.34/day). All the truncated forms of apoA-I were associated with very high density lipoproteins, whereas the intact recombinant apoA-I (r-apoA-I) and h-apoA-I associated with HDL2 and HDL3. Gel filtration chromatography revealed that in contrast to r-apoA-I, the mutant apoA-I201 associated with a phospholipid-rich rabbit apoA-I containing particle. Analysis by agarose gel electrophoresis demonstrated that the same mutant migrated in the pre-beta position, but not within the alpha position as did r-apoA-I. These results indicate that the carboxyl-terminal region (residue 227-243) of apoA-I is critical in modulating the association of apoA-I with lipoproteins and in vivo metabolism of apoA-I.

Transcriptional regulation of the apoAI gene by hepatic nuclear factor 4 in yeast

Hepatocyte Nuclear Factor 4 (HNF-4), a liver-enriched orphan receptor of the nuclear receptor superfamily, is required for the expression of a wide variety of liver-specific genes including apoAI. To explore the possibility that site A of the apoAI gene enhancer might also be the target for HNF-4 without the interference of endogenous mammalian cell proteins that also bind to site A, we tested the ability of HNF-4 to activate transcription from site A in yeast cells. Electrophoretic mobility shift assays (EMSA) and Scatchard plot analysis demonstrated that yeast produced HNF-4 binds to site A with an affinity two times higher than that of yeast produced RXR alpha. Mapping analysis indicated that the 5' portion of site A containing two imperfect direct repeats (TGAACCCTTGACC) and the sequence of the trinucleotide spacer (CCT) between these imperfect repeats are critical determinants for selective binding and transactivation by HNF-4. Similar observations were obtained when these mutated versions of site A were evaluated by transient cotransfection assays in CV1 cells. We conclude that the unique structural determinants of site A in conjunction with the differential binding affinity of HNF-4 for site A may play a fundamental role in apoAI gene regulation.

Characterization of the chicken apolipoprotein A-I gene 5'-flanking region

Apolipoprotein A-I (apoA-I) is a major protein component of plasma high-density lipoprotein in all species studied, and plays an important role in cholesterol homeostasis. In an earlier study, we cloned and structurally characterized the chicken apoA-I gene. In this study, the 5'-flanking region of the chicken apoA-I gene was sequenced and functionally characterized. Sequence analysis of the 510-nucleotide 5' upstream region revealed the presence of TATA and CCAAT boxes. In addition, we identified binding sites for several transcription factors such as Sp1, AP1, and NFI.2. When the 5' fragment was ligated into a promoterless CAT vector and transfected into a chicken hepatocarcinoma cell line (LMH), the bacterial chloramphenicol acetyl transferase (CAT) gene was expressed, suggesting transcriptional regulation is associated with this region. Transfection studies with other 5' deletion constructs revealed that the sequence spanning the region -82 to +87 contained the major transcriptional activity. DNase I footprinting, gel retardation, and Southwestern blot analyses showed that the fragment interacts with nuclear proteins.

Rabbit and rat liver nuclei both contain proteins which bind to the regions controlling apolipoprotein A-I gene expression

We have tested the 5' flanking region of the apolipoprotein A-I (apo A-I) gene, which controls its expression in hepatic cells, for the ability to bind protein factors from rat and rabbit liver nuclei. We found that nuclear extracts from each species contain proteins which bind to three sites in the region which have been shown to be important for control of apo A-I gene transcription. These results contrast with a previous report [Dai, P. H., Lan, S. S. F., Ding, X. H. & Chao, Y. S. (1990) Eur. J. Biochem. 190, 305-310] that no rabbit liver nuclear protein could be detected which binds to the rat apo A-I upstream region and that this lack of binding could explain the failure of the rabbit liver to express the apo A-I gene. We have also shown that the low levels of apo A-I mRNA in the rabbit liver correlate with decreased transcription. Our data suggest that the lack of apo A-I gene expression in liver is a result of transcriptional control but cannot be due to simple lack of protein binding to this region of DNA.

In vivo conversion of recombinant human proapolipoprotein AI (rh-Met-proapo AI) to apolipoprotein AI in rabbits

In vivo conversion of recombinant human proapolipoprotein AI (rh-Met-proapo AI) from E. coli to apolipoprotein (apo) AI was investigated. rh-Met-proapo AI was labeled with 125I, and then administered intravenously to rabbits. Blood was sampled periodically for 6 days. The plasma decay curves of radioiodinated rt-Met-proapo AI were similar to those of human mature apo AI (fractional catabolic rate (FCR); 1.018 +/- 0.090/day vs. 0.976 1 0.031/day, respectively). In vivo conversion of rh-Met-proapo AI to mature apo AI was examined by autoradiography of the isoelectric focusing (IEF) slab gel, i.e., the HDL fraction from each sampling point was semiquantitatively applied to IEF. It was found that the radioactivity of rh-Met-proapo AI migrated to more acidic isoproteins, the conversion was complete within 24 h, and the FCR of rh-Met-proapo AI was 9.20 +/- 1.34/day. Although the plasma decay curves of both human pro (rh-Met-proapo AI) and mature apo AI were significantly steeper than those of rabbit mature apo AI4 and apo AI5 (FCR; 0.703 +/- 0.027/day and 0.795 +/- 0.031/day, respectively), the conversion rate of human rt-Met-proapo AI to mature apo AI in rabbit was assumed to be 1:1. In vitro incubation of rh-Met-proapo AI with rabbit serum produced mature apo AI isoproteins, as determined by the apo AI immunoblotting method. Prediction of the amino acid sequence at the NH2 terminus of rabbit proapo AI showed that the prosegment consisted of an alpha helix with a high probability of a beta turn at Pro9, which is close to that in humans. Thus, (1) the proteolytic cleavage of proapo AI is an extracellular event, (2) the converting enzyme in rabbits can also process human proapo AI, (3) this converting enzyme does not specifically and directly attack the Gln6-Asp7 bond which links the carboxyl-terminal residue of the hexapeptide to the amino-terminal residue of human mature apo AI. The conformation of proapo AI at the NH2 terminus (alpha helix of the prosegment and a beta turn at Pro9) may have a key role in this cleavage, and (4) the examination of rh-Met-proapo AI in rabbits helps to explain the early events of HDL biogenesis.

Characterization of the mouse apolipoprotein Apoa-1/Apoc-3 gene locus: genomic, mRNA, and protein sequences with comparisons to other species

In this report we present the genomic, cDNA, and predicted protein sequences for mouse apolipoproteins A-I and CIII, as well as sequence comparisons with other species. The genes for these apolipoproteins are within 2.5 kb of each other and convergently transcribed. The almost 9 kb of genomic sequence presented extends from 1298 bp 5' to the apolipoprotein A-I (Apoa-1) gene to 1249 bp 5' to the apolipoprotein CIII (Apoc-3) gene. The mouse Apoa-1 gene is 1.76 kb in length with four exons and three introns. The 5' flanking region contains TATA and CCAAT box sequences, an interferon responsive element homology, and potential binding sites for transcription factors CTF/NF1 and HNF4. Translation of the cDNA predicts that the mouse Apoa-1 primary transcript is 264 amino acids. The mouse Apoc-3 gene is 2.2 kb in length and also consists of four exons and three introns. The 5' flanking region contains TATA and CCAAT box sequences, RXR-1 and ARP-1 binding sites, and potential binding sites for transcription factors HNF4, NFkB, AP-1, and CTF/NF1. Translation of the cDNA predicts that the mouse Apoc-3 primary transcript is 99 amino acids. The clustering and genomic organization of the mouse Apoa-1 and Apoc-3 genes are similar to those of the rat and human genes. Significant sequence homologies between species exist for the proximal promoter and exonic regions of each gene, but not for the intronic or intergenic regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Concentration and distribution of apolipoproteins A-I and E in normolipidemic, WHHL and diet-induced hyperlipidemic rabbit sera

Two sandwich-type enzyme immunoassays have been developed to measure apolipoproteins A-I and E in rabbit serum. Specific goat antibodies were purified by affinity chromatography and used both for coating and for preparing antibody-peroxydase conjugates. The sensitivity of these assays is sufficient to allow studies of apo A-I and E distribution in lipoproteins fractionated by gel filtration from 50 microliters of serum. In WHHL rabbits, apo A-I is 5-fold lower (5.2 +/- 2.5 mg/dl) and apo E is 8-fold higher (9.9 +/- 3.5 mg/dl) than in normolipidemic rabbits (29 +/- 4.3 mg/dl and 1.3 +/- 0.5 mg/dl, respectively). In hyperlipidemic rabbits, fed 2 months on a 0.5% cholesterol diet, the apo A-I level was similar (32 +/- 12 mg/dl) to that of normolipidemic rabbits, but the apo E level is 12-fold higher (15.1 +/- 5.5 mg/dl). In addition, HDL particles were enriched with cholesterol and apo E. The bulk of apo E and cholesterol is located in large beta-VLDL in diet-induced hyperlipidemia, whereas they are mainly located in smaller size beta-VLDL in WHHL rabbits. In normolipidemic rabbits apo E occurs mainly in HDL, and cholesterol is distributed in the main three lipoprotein fractions VLDL, LDL and HDL. Interestingly, HDL of WHHL rabbit are deficient in apo A-I. These results are compatible with profound perturbations of lipoprotein composition and metabolism in atherogenic hyperlipidemia.

Hypercholesterolemia induces differential expression of rabbit apolipoprotein A and C genes

We have compared steady-state mRNA levels of apolipoproteins AI, AII, AIV, CI, CII and CIII in liver and small intestine of rabbits fed on a cholesterol-rich diet for up to 16 weeks. Apolipoprotein (apo) AIV mRNA was detected in both liver and small intestine, while apo AII was not detected in either organ. Apo CI, apo CII and apo CIII were expressed only in liver and apo AI mRNA was detected only in small intestine. In small intestine, apo AIV and apo AI mRNA levels increased to a maximum at the 4th and 12th week of treatment, respectively. In liver there was a parallel increase in the mRNA levels of apo AIV, apo CI, apo CII and apo CIII, with maximum levels after 4 weeks of treatment. A 3-fold increase was found in apo CII and apo CIII hepatic transcription rates between hypercholesterolemic and control rabbits after 4 weeks of treatment, no longer detectable after 8 weeks. However, no changes were found in apo AIV and apo CI transcription rates. Changes in apolipoprotein mRNA levels were accompanied by changes in plasma lipoprotein levels. Overall, these changes correlate well with the variations detected in the expression of the different apolipoprotein genes. Our results indicate that dietary cholesterol plays an important role in the regulation of these genes and that this regulation is tissue dependent.

Putative mechanisms of action of probucol on high-density lipoprotein apolipoprotein A-I and its isoproteins kinetics in rabbits

Probucol is a widely prescribed lipid-lowering agent, the major effects of which are to lower cholesterol in both low- and high-density lipoproteins (LDL and HDL, respectively). The mechanism of action of probucol on HDL apolipoprotein (apo) A-I kinetics was investigated in rabbits, with or without cholesterol feeding. 125I-labeled HDL was injected intravenously, and blood samples were taken periodically for 6 days. Kinetic parameters were calculated from the apo A-I-specific radioactivity decay curves. Fractional catabolic rate (FCR) and synthetic rate (SR) of apo A-I in rabbits fed a normal chow and normal chow with 1% probucol were similar. Apo A-I FCR of the rabbits fed 0.5% cholesterol was significantly increased but there were no changes in SR, compared to findings in the normal chow-fed group. Apo A-I FCR of the rabbits fed 1% probucol with 0.5% cholesterol (both 1 month and 2 months) was significantly increased compared to findings in rabbits fed the normal chow as well as 0.5% cholesterol diet group, while SR of apo A-I was significantly reduced in the former groups. Kinetics at 1 month after discontinuation of 1% probucol (under cholesterol feeding) showed a similar FCR of HDL-apo A-I to that of the rabbits fed 0.5% cholesterol, but the SR of apo A-I remained lower. Apo A-I isoproteins kinetics assessed by autoradiography of isoelectric focusing slab gels showed that the synthesis of proapo A-I was significantly reduced in the 1% probucol with 0.5% cholesterol administered, compared to the 0.5% cholesterol group. Thus, the action of probucol on HDL apo A-I kinetics was only prominent in case of higher serum cholesterol levels. The decreased HDL or apo A-I seen with probucol was apparently the result of an increase in FCR and a decrease in SR of HDL-apo A-I. A decreased synthesis of apo A-I remained evident even 1 month after discontinuing probucol. The action of probucol on the intracellular synthetic processes of apo A-I was revealed by the reduced synthesis of proapo A-I.

Binding of nuclear proteins to the enhancer elements of the rat apolipoprotein A-I gene

To determine the cis- and trans-regulatory elements which control the expression of the apolipoprotein (apo) A-I gene, several DNA-protein binding assays, namely, gel mobility shift, exonuclease III protection, and exonuclease III footprinting assays, were employed to identify these elements. It is demonstrated that nuclear proteins of Hep G2 cells bind to five regions of DNA sequences between 252 and 149 base pairs upstream from the transcription initiation site of the rat apo A-I gene. Using South-Western blot analysis, it is determined that DNA-binding protein has a molecular mass of approximately 90 kDa. It is also shown that the DNA-binding protein was present in Hep G2 cells and rat livers but absent in rabbit livers. The results suggest that the lack of expression of the apo A-I gene in rabbit livers is due to the absence of this DNA-binding protein.

Expression, location and cross-reactivity of two antigenic sites on the amino terminal region of rabbit and human apolipoprotein A-I

Rabbit apolipoprotein A-I (apo A-I) of molecular weight 27,612 contained 241 amino acids. In contrast to its human counterpart which has 3 methionine residues, the rabbit protein possesses only one and therefore produces 2 fragments after CNBr cleavage (CNBr I and II, NH2- and COOH-terminal, respectively). From a series of monoclonal antibodies raised against human apo A-I, 2 (A05 and A16) cross-reacted with rabbit apo A-I. In the present study, we show that A05 recognizes the rabbit CNBr I fragment while the integrity of the intermediate region between the 2 CNBr fragments (including the methionine residue) is required for the expression of the A16 antigenic determinant. Competition experiments were performed between human 125I-labelled high density lipoprotein (HDL) and a variety of preparations of human and rabbit apo A-I (including the purified and delipidated protein, complexes of dimyristoylphosphatidylcholine (DMPC) containing apo A-I, HDL subfractions and whole serum). The A05 antigenic determinant was expressed identically in all these fractions of both species. In contrast the A16 showed poor reactivity with delipidated apo A-I, the apparent affinity constant being about 100 times less than for HDL. These data suggest that phospholipids improve the recognition of apo A-I by the A16 antibody. The similar immunoreactivity of the human and rabbit proteins in the present study is consistent with the view that the NH2-terminal region contains the major portion activating lecithin:cholesterol acyltransferase.