Characterization, cell-free synthesis, and processing of apolipoprotein A-I of rat high-density lipoproteins

High density lipoprotein turnover in patients with hypertension

Although hyperinsulinemia and decreased high density lipoprotein cholesterol concentration can occur in patients with hypertension, there is no information available concerning the dynamic state of high density lipoprotein metabolism. To address this issue, we quantified high density lipoprotein turnover in 12 patients with mild hypertension and 11 matched subjects with normal blood pressure. Patients with high blood pressure had lower high density lipoprotein cholesterol concentrations. Fractional catabolic rates of 125I-apolipoprotein AI (apoAI)/high density lipoprotein were faster in patients with hypertension (0.36 +/- 0.02 versus 0.26 +/- 0.02 l/day, p less than 0.001). Total synthetic rates of apoAI were also significantly greater in patients with high blood pressure (17.4 +/- 1.1 versus 13.2 +/- 0.6 mg/kg/day, p less than 0.001). Although significant correlation was observed between blood pressure and fractional catabolic rate of 125I-apoAI/high density lipoprotein in the experimental population (r = 0.52, p less than 0.01), no relation was found when patients with normal blood pressure or hypertension were considered separately. However, a highly significant positive correlation was found between 125I-apoAI/high density lipoprotein fractional catabolic rate and insulin concentration in the entire population (r = 0.72, p less than 0.001). In conclusion, the patients with mild hypertension studied were hyperinsulinemic, had a faster fractional catabolic rate of 125I-apoAI/high density lipoprotein, and a lower high density lipoprotein-cholesterol concentration. It is suggested that the changes seen in high density lipoprotein-cholesterol concentration and 125I-apoAI/high density lipoprotein fractional catabolic rates were secondary to the hyperinsulinemia and not due to the high blood pressure per se.

Optic nerve regeneration in adult fish and apolipoprotein A-I

Fish optic nerves, unlike mammalian optic nerves, are endowed with a high capacity to regenerate. Injury to fish optic nerves causes pronounced changes in the composition of pulse-labeled substances derived from the surrounding non-neuronal cells. The most prominent of these injury-induced changes is in a 28-kilodalton (kDa) polypeptide whose level increases after injury, as revealed by one-dimensional gel electrophoresis and autoradiography. The present study identified as apolipoprotein A-I (apo-A-I) a polypeptide of 28 kDa in media conditioned by regenerating fish optic nerves. The level of this polypeptide increased after injury by approximately 35%. Apo-A-I was isolated by gel-permeation chromatography from delipidated high-density lipoproteins (HDL) that had been obtained from carp plasma by sequential ultracentrifugation. Further identification of the purified protein as apo-A-I was based on its molecular mass (28 kDa) as determined by gel electrophoresis, amino acid composition, and microheterogeneity studies. The isolated protein was further analyzed by immunoblots of two-dimensional gels and was found to contain six isoforms. Western blot analysis using antibodies directed against the isolated plasma protein showed that the 28-kDa polypeptide in the preparation of soluble substances derived from the fish optic nerves (conditioned media, CM) cross-reacted immunologically with the isolated fish plasma apo-A-I. Immunoblots of two-dimensional gels revealed the presence of three apo-A-I isoforms in the CM of regenerating fish optic nerves (pIs: 6.49, 6.64, and 6.73). At least some of the apo-A-I found in the CM is derived from the nerve, as was shown by pulse labeling with [35S]methionine, followed by immunoprecipitation. The apo-A-I immunoactive polypeptides in the CM of the fish optic nerve were found in high molecular-weight, putative HDL-like particles. Immunocytochemical staining revealed that apo-A-I immunoreactive sites were present in the fish optic nerves. Higher labeling was found in injured nerves (between the site of injury and the brain) than in non-injured nerves. The accumulation of apo-A-I in nerves that are capable of regenerating may be similar to that of apo-E in sciatic nerves of mammals (a regenerative system); in contrast, although its synthesis is increased, apo-A-I does not accumulate in avian optic nerves nor does apo-E in rat optic nerves (two nonregenerative systems).

Proteolytic processing and compartmentalization of the primary translation products of mammalian apolipoprotein mRNAs

The steps involved in the initial assembly of apolipoproteins and lipids into supramolecular arrays (nascent lipoprotein particles) are largely unknown. Examination of the proteolytic processing and compartmentalization of the primary translation products of apolipoprotein mRNAs represents one approach to deciphering the molecular details of lipoprotein assembly. The structures of the primary translation products of seven mammalian apolipoprotein mRNAs has been determined in the past several years. The organization of apolipoprotein signal peptides is typical of eukaryotic prepeptides, although an unusual degree of sequence conservation is present among the signal segments of apo AI, AIV, and E. For those apolipoprotein sequences studied in detail, SRP-dependent cotranslational translocation and proteolytic processing appears to be highly efficient and results in sequestration of the processed protein within the lumen of the endoplasmic reticulum (ER). However the mechanism by which these lipid-binding proteins avoid arrest during their translocation through the lipid bilayer of the ER membrane remains obscure. The two principal human HDL apolipoproteins undergo novel extracellular post-translational proteolytic processing, which results in removal of nonhomologous propeptides. The proteases responsible for proapo AI and AII processing appear to be different. The processing of these proapolipoproteins provides a potential series of steps for regulating the ordered assembly of HDL constituents.

Biosynthesis of high density lipoprotein by chicken liver: intracellular transport and proteolytic processing of nascent apolipoprotein A-1

To study the in vivo processing and secretion of Apolipoprotein A-I (Apo A-I), young chickens were administered individual L-[3H]amino acids intravenously and the time of intracellular transport of nascent Apo A-I from rough endoplasmic reticulum (RER) to the Golgi apparatus was measured. Within 3 to 9 min there was maximal incorporation of radioactivity into Apo A-I in both the RER and the Golgi cell fractions. By contrast, the majority of radioactive albumin was also present in the RER by 3 to 9 min, but did not reach peak amounts in the Golgi fraction until 9 to 25 min. Both radioactive Apo A-I and albumin appeared in the blood at about the same time (between 20 and 30 min). NH2-terminal amino acid sequence analysis of nascent intracellular Apo A-I showed that it contains a pro-hexapeptide extension identical to that of human Apo A-I. After 30 min of administration of radioactive amino acids radioactive Apo A-I was isolated by immunoprecipitation from the liver and serum. NH2-terminal sequence analysis of 20 amino acids indicated that chicken liver contained an equal mixture of nascent pro-Apo A-I and fully processed Apo A-I, whereas the serum only contained processed Apo A-I. Further studies showed that the RER only contained pro-Apo A-I, whereas a mixture of pro-Apo A-I and processed Apo A-I was found in the Golgi complex. These results indicate that, in chicken hepatocytes, there is a more rapid transport of Apo A-I than of albumin from the RER to the Golgi cell fractions, and that Apo A-I remains in the Golgi apparatus for a longer period of time before it is secreted into the blood. In addition these studies show that the in vivo proteolytic processing of chicken pro-Apo A-I to Apo A-I occurs in the Golgi cell fractions.

Isoforms of rat apolipoprotein A-I isolated from the lipoproteins of hepatic Golgi apparatus and plasma

We compared apo A-I isolated from the lipoproteins of the Golgi apparatus of rat liver with apo A-I found in plasma lipoproteins. Golgi apo A-I consists of 3 main isoforms with a molecular weight of approximately 28000 and isoelectric points (pI) of 5.97, 5.88 and 5.76, respectively. Plasma apo A-I consists of 4 major and 3 minor isoforms with a molecular weight of 27000. The pI of the major isoforms (numbered 4-7) is 5.88, 5.80, 5.70 and 5.60, respectively. In order to investigate which of the plasma isoforms derived directly from Golgi apo A-I, [35S]methionine was injected into the portal vein and Golgi and plasma apo A-I were isolated shortly thereafter. While all Golgi isoforms were labelled only 3 isoforms of plasma apo A-I (namely isoforms 5, 6 and 7) were found to be labelled. The major plasma isoform (isoform 4 which accounts for more than 60% of apo A-I mass of plasma HDL) was found to be unlabelled. However, when 35S plasma lipoproteins newly secreted by the liver were incubated in vitro in the presence of heparinized plasma, labelled isoform 4 appeared suggesting that heparinized plasma contained some factor capable of converting isoforms 5-7 into isoform 4. This plasma factor appears to be a protease as the in vitro formation of isoform 4 is prevented by protease inhibitors.

Cloning and structure analysis of the rat apolipoprotein A-I cDNA

Apolipoprotein A-I, the major protein in mammalian high-density lipoprotein, acts as a cofactor for lecithin-cholesterol acyltransferase during the formation of cholesterol ester and as such, is thought to promote cholesterol efflux from peripheral cells to the liver. In this paper, we report the partial purification of rat liver apolipoprotein A-I mRNA by a polysome immunoadsorption technique, and its cDNA cloning. Isolation of two overlapping cDNA clones enabled us to derive the whole rat apolipoprotein A-I cDNA coding sequence. Comparison of the deduced protein sequence with its human counterpart reveals a striking homology between the prepropeptide precursors. Both mature protein amino-terminal regions are very homologous, suggesting that this particular domain could be involved in lipid/protein binding or lecithin-cholesterol acyltransferase activation.

Separation of the isoprotein forms of apoprotein A-I of rat, rabbit and human HDL by combined isoelectrofocusing and SDS-polyacrylamide gel electrophoresis

The distribution and the relative content of the isoprotein forms (isoforms) of apoprotein A-I (apo A-I) of HDL isolated from rat, rabbit and human plasma were studied by combined isoelectrofocusing and SDS-polyacrylamide gel electrophoresis. Rat apo A-I consists of seven isoforms having the same molecular weight (27,000) and moving in the 6.44-5.58 pH range. Isoforms 4, 5 and 6 are the major ones. Both rat HDL2 (1.090-1.210 g/ml) and purified rat apo A-I contain additional minor bands (designated 4a, 5a and 6a) which have the same isoelectric point as isoforms 4-6 but higher molecular weight (27,900). It is suggested that they might represent precursors of the main apo A-I isoforms. Rabbit apo A-I contains five isoforms focusing in the 5.69-5.34 pH range. Isoform 4 accounts for about 90% of apo A-I mass. Human apo A-I consists of five isoforms focusing in the pH range 5.91-5.0. Isoforms 3 and 4 are the main ones: their respective contents show high degrees of individual variation.

Nucleotide sequence and the encoded amino acids of human apolipoprotein A-I mRNA

The cDNA clones encoding the precursor form of human liver apolipoprotein A-I (apoA-I), preproapoA-I, have been isolated from a cDNA library. A 17-base synthetic oligonucleotide based on residues 108-113 of apoA-I and a 26-base primer-extended, dideoxynucleotide-terminated cDNA were used as hybridization probes to select for recombinant plasmids bearing the apoA-I sequence. The complete nucleic acid sequence of human liver preproapoA-I has been determined by analysis of the cloned cDNA. The sequence is composed of 801 nucleotides encoding 267 amino acid residues. PreproapoA-I contains an 18-amino-acid prepeptide and a 6-amino-acid propeptide connected to the amino terminus of the 243-amino acid mature apoA-I. Southern blotting analysis of chromosomal DNA obtained from peripheral blood indicated the apoA-I gene is contained in a 2.1-kilobase-pair Pst I fragment and there is no gross difference in structural organization between the normal apoA-I gene and the Tangier disease apoA-I gene.

Localization of the structural gene for human apolipoprotein A-I on the long arm of human chromosome 11

Apolipoprotein A-I (apo A-I), the major apolipoprotein in human high density lipoproteins, is involved in the disease atherosclerosis. Cloned apo A-I cDNA (pA1-3) was used as a probe in chromosome mapping studies to detect the human apo A-I structural gene sequence in human-Chinese hamster cell hybrids. Southern blot analysis of 13 hybrids localized the gene to human chromosome 11. Confirmation of the chromosomal assignment was obtained by analysis of a hybrid (J1) containing a single human chromosome, no. 11. Regional mapping was achieved by using deletion subclones of J1 that localized the human apo A-I structural gene to the region 11q13 leads to qter. Since the human apolipoprotein C-III (apo C-III) structural gene is closely linked to apo A-I, it can be assigned to the same region on the long arm of chromosome 11. By extension of methods previously described, it now appears possible to carry out fine-structure analysis of this and related gene regions on chromosome 11 and to study the biochemical concomitants of these genes and of genes on other chromosomes for analysis of their role in atherosclerosis.

Isolation and characterization of the human apolipoprotein A-I gene

We have recently shown that an inherited polymorphism occurring in the human apolipoprotein A-I (apo A-I) gene is related to decreased high density lipoprotein and apo A-I levels in the plasma of two patients with severe premature atherosclerosis. Analysis of the molecular basis of this polymorphism and its possible effects on apo A-I gene expression requires direct comparison of both normal and polymorphic apo A-I alleles. Here we report the isolation and characterization of the normal human apo A-I gene and we show that the gene is interrupted by three intervening sequences, IVS-1, IVS-2, and IVS-3, occurring in the 5' noncoding region of apo A-I mRNA, the mRNA sequence coding for the signal peptide of apo A-I, and the sequence coding for the mature protein, respectively. In addition, the nucleotide sequence analysis of the apo A-I gene allowed determination of the complete amino acid sequence of the primary translation product of apo A-I mRNA. This amino acid sequence consists of 267 residues including a 24-residue-long amino-terminal extension (preprosegment). Finally, we show that the apo A-I gene contains six 66-base-pair-long tandemly repeated DNA segments, which suggests that the gene may have evolved by intragenic duplication events.

Effects of dietary proteins on the intestinal synthesis and transport of cholesterol and apolipoprotein A-I in rats

Feeding a low-fat diet free of cholesterol to rats resulted in a low level of serum cholesterol and apolipoprotein A-1 (apoA-I) when dietary protein was soybean protein as compared to casein. Since the small intestine is an important synthetic site for lipoproteins, the effects of dietary protein sources on the intestinal synthesis in vitro of cholesterol and apoA-I and the concentration of these components in the mesenteric lymph were studied. In rats fed a 1% fat diet, the apoA-I concentration in the lymph and the de novo synthesis of apoA-I in the small intestine segment were both low for animals fed a soybean protein diet as compared to a casein diet. The intestinal hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase activity was high in animals fed soybean protein, whereas the lymph cholesterol level was comparable. Even in rats fed a 5% fat diet, concentrations of serum cholesterol and apoA-I and lymphatic apoA-I were again significantly low for rats fed vegetable protein. These results indicate that the intestine plays an important role in the dietary protein-dependent regulation of serum cholesterol and apoA-I levels.

Nucleotide sequence of cloned cDNA of human apolipoprotein A-I

ApoA-I is the major human HDL apoprotein. By oligonucleotide hybridization, we have isolated 5 dscDNA clones to human hepatic apo A-I mRNA. One of these clones (pA1-3) was completely sequenced. It has 878 bp plus a poly A tail of 48 and includes all the coding and 3'-untranslated regions of the mRNA and part of the 5'-untranslated region. It predicts a peptide sequence of 267 amino acids (including the 24 amino acid prepropeptides) which is very similar to the sequence reported by Brewer et al., (1978) Biochem. Biophys. Res. Commun. 80:623-630. The predicted signal peptide sequence is highly homologous to the rat apoA-I signal peptide. There is no evidence for any internally repeated segments in apoA-I either at the amino acid or at the DNA level. Using pA1-3 as a probe, we have detected on Northern gels apo A-I mRNA sequences of approximately 1100 nucleotides in human hepatic and baboon hepatic and intestinal RNAs, but not in RNAs from baboon skeletal muscle, kidney or spleen. The demonstration of apo A-I mRNA sequences in specific organs is important to our concept of "reverse cholesterol transport".

Intestinal Biosynthesis of apolipoproteins in the rat: apoE and apoA-I mRNA translation and regulation

Rat intestinal poly(A) RNA was translated in wheat germ and reticulocyte lysate systems in vitro. ApoA-I and apoE were demonstrated to be specific products by immunoprecipitation and fractionation on sodium dodecyl sulfate acrylamide gels. They were identical in size to the respective products from rat liver. In pulse-labeling studies, apoE was shown to be synthesized by slices of rat intestine in situ. Furthermore, a high cholesterol diet stimulated the synthesis of apoE and apoA-I at the pretranslational level.

Synthesis in vitro and translocation of apolipoprotein AI across microsomal vesicles

Apolipoprotein AI of rats has been synthesized in a cell-free wheat germ system and cotranslationally translocated into dog pancreas microsomal vesicles. Translocation is accompanied by cleavage of a signal sequence of 18 amino acid residues and is dependent on the recently purified signal recognition protein.