Biosynthesis of human preproapolipoprotein A-II

Structure of HDL: particle subclasses and molecular components

A molecular understanding of high-density lipoprotein (HDL) will allow a more complete grasp of its interactions with key plasma remodelling factors and with cell-surface proteins that mediate HDL assembly and clearance. However, these particles are notoriously heterogeneous in terms of almost every physical, chemical and biological property. Furthermore, HDL particles have not lent themselves to high-resolution structural study through mainstream techniques like nuclear magnetic resonance and X-ray crystallography; investigators have therefore had to use a series of lower resolution methods to derive a general structural understanding of these enigmatic particles. This chapter reviews current knowledge of the composition, structure and heterogeneity of human plasma HDL. The multifaceted composition of the HDL proteome, the multiple major protein isoforms involving translational and posttranslational modifications, the rapidly expanding knowledge of the HDL lipidome, the highly complex world of HDL subclasses and putative models of HDL particle structure are extensively discussed. A brief history of structural studies of both plasma-derived and recombinant forms of HDL is presented with a focus on detailed structural models that have been derived from a range of techniques spanning mass spectrometry to molecular dynamics.

Influence of apolipoprotein A-I and apolipoprotein A-II availability on nascent HDL heterogeneity

It is important to understand HDL heterogeneity because various subspecies possess different functionalities. To understand the origins of HDL heterogeneity arising from the existence of particles containing only apoA-I (LpA-I) and particles containing both apoA-I and apoA-II (LpA-I+A-II), we compared the abilities of both proteins to promote ABCA1-mediated efflux of cholesterol from HepG2 cells and form nascent HDL particles. When added separately, exogenous apoA-I and apoA-II were equally effective in promoting cholesterol efflux, although the resultant LpA-I and LpA-II particles had different sizes. When apoA-I and apoA-II were mixed together at initial molar ratios ranging from 1:1 to 16:1 to generate nascent LpA-I+A-II HDL particles, the particle size distribution altered, and the two proteins were incorporated into the nascent HDL in proportion to their initial ratio. Both proteins formed nascent HDL particles with equal efficiency, and the relative amounts of apoA-I and apoA-II incorporation were driven by mass action. The ratio of lipid-free apoA-I and apoA-II available at the surface of ABCA1-expressing cells is a major factor in determining the contents of these proteins in nascent HDL. Manipulation of this ratio provides a means of altering the relative distribution of LpA-I and LpA-I+A-II HDL particles.

Serum Levels of an Isoform of Apolipoprotein A-II as a Potential Marker for Prostate Cancer

Purpose: We recently showed that protein expression profiling of serum using surface-enhanced laser desorption/ionization time-of-flight mass spectrometry (SELDI-TOF MS) has potential as a diagnostic approach for detection of prostate cancer. As a parallel effort, we have been pursuing the identification of the protein(s) comprising the individual discriminatory “peaks” and evaluating their utility as potential biomarkers for prostate disease.

Experimental Design: We employed liquid chromatography, gel electrophoresis and tandem mass spectroscopy to isolate and identify a protein that correlates with observed SELDI-TOF MS mass/charge (m/z) values. Immunodepletion, immunoassay, and Western analysis were used to verify that the identified protein generated the observed SELDI peak. Subsequent immunohistochemistry was used to examine the expression of the proteins in prostate tumors.

Results: An 8,946 m/z SELDI-TOF MS peak was found to retain discriminatory value in each of two separate data sets with an increased expression in the diseased state. Sequence identification by liquid chromatography-MS/MS and subsequent immunoassays verified that an isoform of apolipoprotein A-II (ApoA-II) is the observed 8,946 m/z SELDI peak. Immunohistochemistry revealed that ApoA-II is overexpressed in prostate tumors. SELDI-based immunoassay revealed that an 8.9-kDa isoform of ApoA-II is specifically overexpressed in serum from individuals with prostate cancer. ApoA-II was also overexpressed in the serum of individuals with prostate cancer who have normal prostate-specific antigen (0-4.0 ng/mL).

Conclusions: We have identified an isoform of ApoA-II giving rise to an 8.9K m/z SELDI “peak” that is specifically overexpressed in prostate disease. The ability of ApoA-II to detect disease in patients with normal prostate-specific antigen suggests potential utility of the marker in identifying indolent disease.

Molecular cloning and sequence of the cynomolgus monkey apolipoprotein A-II gene

A clone containing the coding region for cynomolgus monkey (Macaca fascicularis) apolipoprotein A-II has been isolated from a cynomolgus genomic DNA library. The gene spans 1.4 kilobases (kb). The complete nucleic acid sequence of the apolipoprotein A-II gene has been determined, establishing that the gene is interrupted by three intervening sequences of 170, 273 and 394 bp, respectively. The open reading frame encodes a protein of 100 amino acids, and shows 94% sequence similarity with its human equivalent. Both apolipoproteins have identical signal peptide. A noticeable feature is the substitution of mature human Cys-6 for Ser. This change explains the existence of cynomolgus apolipoprotein A-II as a monomer and may have important consequences in the kinetics of this apolipoprotein.

Isolation and partial characterization of lipoprotein A-II (LP-A-II) particles of human plasma

High density lipoproteins (HDL) consist of a mixture of chemically and functionally distinct families of particles defined by their characteristic apolipoprotein (Apo) composition. The two major lipoprotein families are lipoprotein A-I (LP-A-I) and lipoprotein A-I:A-II (LP-A-I:A-II). This study describes the isolation of a third minor HDL family of particles referred to as lipoprotein A-II (LP-A-II) because it lacks ApoA-I and contains ApoA-II as its main or sole apolipoprotein constituent. Because ApoA-II is an integral protein constituent of three distinct lipoprotein families (LP-A-I:A-II, LP-A-II: B:C:D:E and LP-A-II), LP-A-II particles were isolated from whole plasma by sequential immunoaffinity chromatography on immunosorbers with antisera to ApoA-II, ApoB and ApoA-I, respectively. In normolipidemic subjects, the concentration of LP-A-II particles, based on ApoA-II content, is 4-18 mg/dl accounting for 5-20% of the total ApoA-II not associated with ApoB-containing lipoproteins. The lipid composition of LP-A-II particles is characterized by low percentage of triglycerides and cholesterol esters and a high percentage of phospholipids in comparison with lipid composition of LP-A-I and LP-A-II: A-II. The major part of LP-A-II particles contain ApoA-II as the sole apolipoprotein constituent; however, small subsets of LP-A-II particles may also contain ApoD and other minor apolipoproteins. The lipid/protein ratio of LP-A-II is higher than those of LP-A-I and LP-A-I:A-II. In homozygous ApoA-I and ApoA-I/ApoC-III deficiencies, LP-A-II particles are the only ApoA-containing high density lipoprotein with levels found to be within the same range (7-13 mg/dl) as those of normolipidemic subjects. However, in contrast to normal LP-A-II, their lipid composition is characterized by higher percentages of triglycerides and cholesterol esters and a lower percentage of phospholipids and their apolipoprotein composition by the presence of ApoC-peptides and ApoE in addition to ApoA-II and ApoD. These results show that LP-A-II particles are a minor HDL family and suggest that, in the absence of ApoA-I-containing lipoproteins, they become an efficient acceptor/donor of ApoC-peptides and ApoE required for a normal metabolism of triglyceride-rich lipoproteins. Their other possible functional roles in lipid transport remain to be established in future experiments.

Lignin peroxidase from the basidiomycete Phanerochaete chrysosporium is synthesized as a preproenzyme

The cDNA clone L18 encoding lignin peroxidase LiP2, the most highly expressed LiP isozyme from Phanerochaete chrysosporium strain OGC101, was isolated and sequenced. Comparison of the cDNA sequence with the N-terminal sequence of the mature LiP2 protein isolated from culture medium suggests that the mature protein contains 343 amino acids (aa) and is preceded by a 28-aa leader sequence. In vitro transcription followed by in vitro translation and processing by signal peptidase resulted in cleavage at a site following the Ala21 (counted from the N-terminal Met1 of the initial translation product). The resultant protein contains a 7-aa propeptide, indicating that LiP is synthesized as a preproenzyme.

The absorption and transport of lipids by the small intestine

Dietary lipid provides as much as 40% of the caloric intake in the Western diet. Triacylglycerol is the main dietary fat. The human small intestine is also presented daily with 11-12 g of phospholipid, predominantly phosphatidylcholine. The predominant sterol in the Western diet is cholesterol, which is derived from animal fat. Plant sterols account for up to 20-25% of total dietary sterol. This paper reviews our current understanding of the process and the factors that regulate the absorption and transport of different dietary lipids by the human small intestine.

Intracellular modification of human apolipoprotein AII (apoAII) and sites of apoAII mRNA synthesis: comparison of apoAII with apoCII and apoCIII isoproteins

We have studied the intracellular modifications of human apoAII by pulse-chase labeling of HepG2 cell cultures with [35S]methionine or [3H]arginine followed by two-dimensional analysis and autoradiography of the radiolabeled apoAII isoproteins. A short (5.0-min) pulse showed the presence of a precursor form of apoAII (pI = 5.75) designated proapoAII or apoAII3. A 5-10-min chase resulted in a decrease in the relative concentration of the proapoAII coupled with an increase in the relative concentration of a new form (pI = 5.3) designated modified proapoAII or apoAII1. Longer chase resulted in the appearance of the plasma apoAII form and at least five other acidic apoAII isoproteins in the cell lysate and the culture medium. Labeling with [3H]arginine showed that apoAII isoproteins designated 3, 1, -1, and -3 contained the prosegment whereas isoproteins designated 1a, 0, -1a, -2a, -3a, and -4a did not. Comparison of nascent apoAII, apoCII, and apoCIII isoproteins revealed that they were distinctly different on the two-dimensional gels. Neuraminidase treatment converted the acidic apoAII isoproteins to isoproteins 1a and 0 (modified and plasma apoAII forms). The combined data are consistent with the following intra- and/or extracellular modifications of apoAII: (a) modification of the apoAII which results in the net loss of two positive charges; (b) glycosylation of the modified proapoAII with carbohydrate chains containing sialic acid; (c) proteolytic removal of the prosegment and cyclization of the N-terminal glutamine. Analysis of apoAII mRNA distribution in 13 fetal human tissues as well as in cell lines of human origin showed abundance of apoAII mRNA in liver and HepG2 cells and only traces in the intestine.

Plasma lipoprotein lipids in relation to the MspI polymorphism of the apolipoprotein AII gene in Caucasian men. Lack of association with plasma triglyceride concentration

Digestion of the human apolipoprotein (apo) A-II gene with the endonuclease MspI produces fragments of 3.0 or 3.7 kb, reflecting the presence or absence of a polymorphic site within an Alu sequence 3' to the gene. Patients with hypertriglyceridemia have been shown to have an increased prevalence of the 3.0 kb allele. To explore this observation further, plasma lipoprotein lipids were studied in a random sample of fasted middle-aged Caucasian men, of which 59 were 3.0 kb homozygotes, 24 were heterozygotes, and 2 were 3.7 kb homozygotes. After adjusting for the effects of age, height, weight, alcohol intake and cigarette consumption by covariance analysis, no statistically significant associations were present between genotype and the concentrations of triglyceride in whole plasma or the d less than 1.019 g/ml fraction of plasma (i.e., VLDL + IDL). Nor were the cholesterol concentrations in VLDL + IDL, low density lipoprotein (LDL, d = 1.019-1.063 g/ml), high density lipoprotein (HDL), HDL2 or HDL3 related to genotype. In an independent comparison of eight 3.0 kb homozygotes and eight 3.7 kb homozygotes (all Caucasians) drawn from a different community, genotype was unrelated to the triglyceride or cholesterol concentrations in VLDL (d less than 1.006 g/ml), IDL + LDL (d = 1.006-1.063 g/ml) or HDL, after adjustment for the effects of covariates. These results suggest that the MspI polymorphism of the apo A-II gene is not associated with genetic variation that significantly affects triglyceride transport in the majority of men.(ABSTRACT TRUNCATED AT 250 WORDS)

Secretion of cholesteryl ester transfer protein-lipoprotein complexes by human HepG2 hepatocytes

We have employed immunoaffinity chromatography to characterize the distribution of cholesteryl ester transfer activity in particles secreted by HepG2 hepatocytes. HepG2-secreted cholesteryl ester transfer activity is associated with apoprotein (apo) A-I (58%) as well as apo A-II (55%), and is not associated with apo B or E. In contrast, our previous studies have shown that most (88%) cholesteryl ester transfer activity in human plasma is associated with apo A-I whereas very little (7%) is associated with apo A-II. Thus, the distribution of cholesteryl ester transfer activity in plasma particles likely reflects active remodeling of nascent particles in the plasma compartment. Further data suggested that HepG2 cells secrete a lipid transfer inhibitor activity which is associated with apo E-containing lipoprotein particles. This inhibitory activity is heat labile.

Demonstration that the enzyme that converts precursor of apolipoprotein A-I to apolipoprotein A-I is secreted by the hepatocarcinoma cell line Hep G2

The conversion of the precursor of apolipoprotein A-I (proapoA-I) to apolipoprotein A-I (apoA-I) is known to occur extracellularly by an enzyme that has been shown to be present in plasma. The hepatocarcinoma-derived cell line Hep G2, when grown in culture, secretes proapoA-I. We now show that this cell line also secretes the converting enzyme that correctly processes proapoA-I to mature apoA-I as determined by radio-sequence analyses. The secreted enzyme is inhibited by EDTA and 1,10-phenanthroline, is activated by Ca2+ and is unaffected by both phenylmethylsulfonyl fluoride and diisoprophylfluorophosphate in the same way as the converting enzyme previously described in the plasma. The conversion of proapoA-I to apoA-I effected by this enzyme obeys first-order kinetics and is linear over the first 4 h with a calculated initial velocity of 3.3% conversion per hour. The converting activity is secreted in a time-dependent fashion and parallels the mass of total secreted protein.

Structural requirements of human preproapolipoprotein AI for translocation and processing studied by site-directed mutagenesis in vitro

A full length human serum apolipoprotein AI (apo AI) cDNA clone was isolated from a human liver cDNA library. The EcoRI insertion fragment was cloned into expression vectors pDS5 and pDS12 for in vitro transcription and translation. The primary translation product is correctly translocated and the N-terminal signal sequence of the primary translation product of the wild type apo AI cleaved in the presence of dog pancreatic endoplasmic reticulum (ER) membranes releasing proapo AI. Ala-7 at the C-terminus of the signal sequence and Gln-1 of the prosequence were transposed by site-directed mutagenesis thus mutually exchanging the C-termini Gln-8-Ala-7 of the presequence and Gln-2-Gln-1 of the prosequence. The primary translation product of this mutated preproapo AI cDNA is correctly cotranslationally translocated into the lumen of the ER membranes and remains uncleaved by the signal peptidase. Deletion of the hexapeptide prosequence by site-directed mutagenesis in the preproapo AI cDNA led to a primary translation product which is cotranslationally translocated with processing to the mature apo AI polypeptide. We conclude that neither the proteolytic cleavage of the presequence nor the presence of the prosequence are structurally essential for the cotranslational translocation of apo AI. The amino-acid sequence bordering the cleavage site at the C-terminus of the presequence is without influence for the specificity of the signal peptidase.

Proapolipoprotein-converting enzymes and high-density lipoprotein early events in biogenesis

The early events in high-density lipoprotein biogenesis involve the extracellular action of two converting enzymes affecting the cleavage of the prosegment of either proapolipoprotein A-I or proapolipoprotein A-II and the generation of mature apolipoprotein (apo) A-I and apo A-II, the main apolipoprotein of high-density lipoproteins. These two converting enzymes differ from each other in mechanism of action and specificity. The observation that they can be secreted by human hepatocarcinoma G2 cells in culture provides an experimental basis for examining the possible coordination between the synthesis and secretion of these two converting enzymes and the events attending the production and cellular export of apo A-I and apo A-II.

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.

Isolation and characterization of apolipoproteins A-I, A-II, and A-IV

A number of different analytical techniques are now available for the isolation of apoA-I, apoA-II, and apoA-IV. The choice of a particular technique is dependent on the instrumentation available, and the quantity of isolated apolipoprotein required. The isolation and characterization of the separate isoforms and the precursor isoproteins of the individual apolipoproteins are detailed, and methods for the evaluation of the purity of the separate apolipoproteins presented. A method for the evaluation of apolipoproteins in plasma is now available which permits the identification of structural variants of plasma apolipoproteins in patients with dyslipoproteinemias.

The molecular biology of human apoA-I, apoA-II, apoC-II and apoB

The application of molecular biology techniques has enabled us to determine the gene sequence, organization, transcription and processing of apolipoprotein genes. Consequently, new insights have been gained in the biosynthesis and processing of these proteins. In addition to apoA-I, apoA-II and apoC-III reported here, other apolipoprotein genes such as apoC-II and apoE genes were found to share common intron-exon organizations. The results suggest that these genes most probably arise from a common ancestral gene. Utilizing cDNA as hybridization probes, we have localized apoA-I, apoA-II, apoC-II, apoC-III, apoE and apoB to specific locations of individual chromosomes (for review, see ref. 6). There is no clear relationship between currently known physiological function and the organization of the apolipoproteins in the chromosomes with the exception of the LDL receptor and its ligand, apoE which are localized to chromosome 19. However, apoB-100, the major ligand for the LDL receptor is on chromosome 2 and not in synteny with the apoE and the LDL receptor genes. The cloning of the major human apolipoprotein genes have also allowed us to initiate studies on the molecular defects leading to various dyslipoproteinemias including Tangier disease and abetalipoproteinemia. Undoubtedly, information derived from these studies will provide the basis for future in vitro and in vivo studies on patients with dyslipoproteinemia and premature atherosclerosis.

Intra- and extracellular modifications of apolipoproteins

This chapter outlined the methods used to study intra- and extracellular modifications of apolipoproteins. These and other related studies have shown that several of the apolipoproteins undergo a series of intra- and extracellular modifications as follows: All apolipoproteins studied contain an 18-26 long signal peptide which is cleaved cotranslationally by the signal peptidase of the rough endoplasmic reticulum. ApoE is further modified intracellularly with carbohydrate chains containing sialic acid and is secreted in the modified form designated apoEs. The modified apoE is subsequently desialated in plasma. ApoA-I is secreted in a proapoA-I form, which consists of 249 amino acids. The N-terminal hexapeptide of proapoA-I is cleaved extracellularly by a proapoA-I to plasma apoA-I converting protease. This cleavage generates the plasma apoA-I form which consists of 243 amino acids. Other known apolipoprotein modifications include the modification of apoB, apoC-III, and apoD with carbohydrate chains that contain sialic acid and the proteolytic cleavage of the proapoA-II segment. At the present time we are able to distinguish several isoprotein forms for a particular apolipoprotein. In addition, we began to understand the biochemical changes which lead to a few of these isoproteins. Future research should be directed toward a better understanding not only of the structure but most importantly of the physiological significance of the different apolipoprotein forms.

Processing of proapolipoprotein AI requires specific conformation

Apolipoprotein AI of human high-density lipoproteins is secreted by hepatocytes as a proapolipoprotein with a N-terminal hexapeptide sequence (Arg-His-Phe-Trp-Gln-Gln-) which differs from the prosequence of rat apolipoprotein AI (Trp-Asp-Phe-Trp-Gln-Gln). The two proteins have in common the unusual cleavage site -Gln-Gln-Asp-Glu-. It is hydrolysed by a specific serum proteinase with the release of mature apo AI. We synthesized a model substrate for the study of the final processing of pro-apo AI by the serum proteinase. It is an undecapeptide embracing the human pro-hexapeptide sequence and the first five N-terminal residues of apo AI, covalently linked to a hydrophilic resin. The N-terminal arginine residue was 3H-labelled. [formula; see text] This sequence was not cleaved by human serum under the conditions under which rat serum processes the pro-form of apo AI secreted by rat hepatocytes. Pepsin and chymotrypsin fragmented the undecapeptide at sites characteristic for these proteinases. We conclude that the proteolytic cleavage at the specific site (-Gln-Gln-Asp-Glu-) requires the correct conformation in addition to the specific amino-acid sequence.

Synthesis and processing of human serum apolipoprotein AII in vitro and in Hep G2 cells

The synthesis and structure of the primary translation product of apo AII in a human liver poly(A+) mRNA primed cell-free system and its cotranslational modification was studied parallel to studies in vivo with Hep G2 cells, a human hepatoma cell line. The primary translation product is a preproprotein containing 100 amino acid residues, which is cleaved by the signal peptidase of endoplasmic reticulum to pro-apo AII with the loss of the N-terminal pre-sequence consisting of 18 amino acid residues. Hep G2 cells contain about equal amounts of the proform of apolipoprotein AII and of mature apo AII. Approximately in the same ratio pro- and mature apo AII are secreted into the medium. Determination of the partial amino-acid sequence by automated Edman degradation of the labelled prepro- and proforms of apo AII led to the segmentation of the N-terminus of the primary translation product, consisting of 23 amino acid residues, into the pre-sequence (18 residues) and the pro-sequence (5 residues) with terminal Arg-Arg-residues at the cleavage site to apo AII. We must therefore correct our previously postulated 17 and 6 residues containing segmentation. So far no information has been obtained in which compartment and at what stage of posttranslational events the dimerization occurs by formation of the single disulfide bond at position Cys6 in the mature apo AII structure, leading to the symmetrical molecule.