Carboxyl-terminal truncation impairs lipid recruitment by apolipoprotein B100 but does not affect secretion of the truncated apolipoprotein B-containing lipoproteins

A two-site model for ApoB degradation in HepG2 cells

Newly synthesized apolipoprotein B (apoB) undergoes rapid degradation in a pre-Golgi compartment in HepG2 cells. A major site of this early degradation seems to be on the cytosolic side of the endoplasmic reticulum (ER) membrane and is sensitive to N-acetyl-leucinyl-leucinyl-norleucinal (ALLN), which can inhibit neutral cysteine proteases and/or proteasome activity. Oleate (OA) treatment, which facilitates translocation of nascent apoB across the ER membrane, also reduces early degradation. In the present studies, we have used brefeldin A (BFA), which inhibits vesicular transport from the ER to the Golgi, to demonstrate that apoB can also be degraded by an ER luminal proteolytic activity that is distinct from the ALLN-sensitive proteases. Thus, when BFA-treated HepG2 cells were co-treated with ALLN, which protects apoB but does not facilitate its translocation into the ER lumen, degradation of newly synthesized apoB was significantly reduced compared with cells incubated with BFA alone. However, apoB degradation was rapid and complete when OA was added to media containing either BFA or ALLN/BFA. These results suggested that OA, by increasing translocation of nascent apoB into the ER lumen, exposed apoB to an ALLN-resistant proteolytic pathway. When we incubated HepG2 cells with dithiothreitol (DTT)/OA/BFA or DTT/OA/ALLN/BFA, degradation of apoB was inhibited. Furthermore, addition of DTT resulted in the accumulation of a 70-kDa amino-terminal fragment of apoB. Both full-length and amino-terminal apoB were degraded if DTT was removed from the incubation media; both were secreted if only BFA was removed. Thus, even after apoB is translocated into the ER lumen (thereby avoiding the initial proteolytic pathway), it can potentially be degraded by a lumenal proteolytic process that is ALLN-resistant but DTT-sensitive. The present results, together with previous studies, suggest that at least two distinct steps may be involved in the posttranslational degradation of apoB: 1) the first occurs while apoB is partially translocated and is ALLN-sensitive; and 2) the second occurs in the ER lumen and is DTT-sensitive. Finally, our results support the hypothesis that degradation of partially translocated apoB generates a 70-kDa amino-terminal fragment that is mainly degraded in the ER lumen by a DTT-sensitive pathway.

In vitro reconstitution of assembly of apolipoprotein B48-containing lipoproteins

Human apolipoprotein B48 (apoB48) and apoB15 (the NH2-terminal 48 and 15% of apoB100, respectively) were translated in vitro from their respective mRNAs using a rabbit reticulocyte lysate and microsomes derived from rat liver or dog pancreas. Synthesis of phosphatidylcholine and triacylglycerols was reconstituted in freshly isolated microsomes by the addition of precursors of these glycerolipids (acylcoenzyme A, glycerol 3-phosphate, and CDP-choline) before, during, or after translation. Assembly of apoB15 and apoB48 with newly synthesized phospholipids and triacylglycerols was favored by active, co-translational lipid synthesis. Moreover, translocation of apoB48 but not B15 into the microsomal lumen was increased in the presence of co-translational lipid synthesis. When apoB48 was translated in vitro, approximately 50% of apoB48 was buoyant at a density of <1.10 g/ml in the lumen of liver microsomes only when lipid synthesis was reconstituted during translation. Microsomal triacylglycerol transfer protein has been proposed to be essential for lipidation and/or translocation of apoB48. However, apoB48 was translocated into the lumen of dog pancreas microsomes in which the activity of the microsomal triacylglycerol transfer protein was not detectable. These data indicate that (i) apoB15 and apoB48 bind newly synthesized phosphatidylcholine during translocation; (ii) apoB48 but not apoB15 associates co-translationally with triacylglycerols; (iii) translocation of apoB48 but not apoB15 is stimulated by lipid synthesis; (iv) assembly of buoyant apoB48-containing lipoproteins can be reconstituted in vitro in the presence of active lipid synthesis; and (v) even in microsomes lacking microsomal triacylglycerol transfer protein activity, apoB48 is translocated into the lumen.

Apolipoprotein B sequence requirements for hepatic very low density lipoprotein assembly. Evidence that hydrophobic sequences within apolipoprotein B48 mediate lipid recruitment

We studied the structural requirements of apolipoprotein (apo) B for assembly of very low density lipoproteins (VLDL) using rat hepatoma McA-RH7777 cells expressing human apoB (h-apoB). Recombinant h-apoB48, like endogenous rat apoB48 (r-apoB48), was secreted as VLDL in addition to high density lipoproteins (HDL) by transfected cells, indicating that the N-terminal 48% of apoB contains sequences sufficient for VLDL assembly. Truncation of the C terminus of h-apo-B48 to -B42 or -B37 had little effect on the ability of apoB to assemble VLDL, whereas truncation to -B34 or -B29 markedly diminished or abolished VLDL formation. None of the truncations affected the integration of apoB into HDL. To determine whether the ability to assemble VLDL is governed by apoB length or by sequences beyond apoB29, we created chimeric proteins that contained human apoA-I and a segment derived from between the C-terminal 29 and 34%, 34 and 37%, or 37 and 42% of apoB100. The resulting chimeras, namely AI/B29-34, AI/B34-37, and AI/B37-42, were secreted by the transfected cells as lipoproteins with buoyant density (d < 1.006 g/ml), electrophoretic mobility (pre-beta), and size characteristics of human plasma VLDL. The chimeras could assemble discrete VLDL particles devoid of endogenous r-apoB100, and could actively recruit triglycerides and phospholipids into the lipoproteins. However, these chimeras were secreted inefficiently. Pulse-chase analysis showed that less than 5% of the newly synthesized AI/B proteins were secreted, and more than 70% was degraded intracellularly. Degradation of the chimeras could be blocked by the cysteine protease inhibitor N-acetyl-leucyl-leucyl-norleucinal, but the treatment did not enhance their secretion. Protease protection analysis of microsomes isolated from transfected cells indicated that >65% of AI/B chimeras (compared with <25% of r-apoB100) were inaccessible to exogenous trypsin. These data suggest that the recruitment of large quantities of triglycerides during VLDL formation is not governed simply by apoB length, but is mediated by short hydrophobic sequences ranging from 152 to 237 amino acids (3-5%) of apoB. The existence of multiple such hydrophobic sequences within apoB48 may facilitate efficient assembly of hepatic VLDL particles.

Interactions between microsomal triglyceride transfer protein and apolipoprotein B within the endoplasmic reticulum in a heterologous expression system

When apolipoprotein B (apoB) is expressed in heterologous cells, it is not secreted but retained and degraded within the endoplasmic reticulum (ER). We have previously characterized carboxyl-terminal truncated forms of apoB expressed in COS cells and have shown that these proteins were readily synthesized but retained within the ER and degraded, if the size of the truncated protein was larger than apoB 29. Below this size, the smaller the size of the apoB truncates, the greater the extent of secretion, although >50% of these smaller proteins were also degraded within the ER. In the present study, we demonstrate that this secretory defect can be overcome by coexpression with microsomal triglyceride transfer protein (MTP); moreover, this complementation is inversely related to the size of apoB. Secretion of apoBs larger than B29 required the coexpression of MTP and, in the presence of MTP, was oleate-responsive. MTP, in the presence or absence of oleate supplementation, had little or no effect on the secretion of the shorter truncates. We discovered, however, that MTP was physically associated with all forms of apoB intracellularly (B13-B41). The association of MTP with apoB 41 was stable to high salt washing, as well as to low pH, suggesting that these interactions may be hydrophobic in nature. In addition to the interaction with MTP, apoB was also found to be associated with calnexin, confirming previous studies, and with proteins bearing the KDEL retention signal. However, studies on overexpression of human calnexin and tunicamycin inhibition of glycosylation showed that interaction with calnexin was not necessary for the formation or secretion of apoB 41-containing lipoproteins; moreover, in the presence of MTP, the association of calnexin with apoB 41 was transient or absent. These data suggest that for apoB to attain a folded state sufficient to escape the quality control of the ER, it needs to obtain neutral lipid (supplied by MTP), as well as its ability to keep it packaged as a rudimentary lipoprotein, dependent on its size being larger than B29.

Hepatic expression of the catalytic subunit of the apolipoprotein B mRNA editing enzyme (apobec-1) ameliorates hypercholesterolemia in LDL receptor-deficient rabbits

Apolipoprotein (apo) B48, a protein contained in intestinally derived lipoprotein particles, is synthesized by post-transcriptional editing of apoB100 mRNA. This reaction is mediated by an enzyme complex that includes the catalytic subunit, apobec-1. The liver of most mammals, by contrast, contains only unedited apoB mRNA and secretes apoB100, the major protein component of plasma low-density lipoprotein (LDL). Because rabbits, like humans, fail to edit hepatic apoB100 mRNA, we introduced a recombinant adenovirus encoding apobec-1 into the livers of LDL receptor-defective rabbits to determine the impact on lipoprotein metabolism of hepatic apoB48 secretion. Transgene expression was mainly confined to the liver and was sustained for up to 3 weeks following virus administration, as evidenced by the presence of apobec-1 mRNA and the ability of hepatic S100 extracts to edit a synthetic apoB RNA template in vitro. The transient induction of hepatic apoB mRNA editing accompanied alterations in very-low-density lipoprotein (VLDL) size, the presence of apoB48 in fractions spanning the VLDL and LDL range, and modest reductions in total plasma cholesterol levels.

Chylomicron assembly and catabolism: role of apolipoproteins and receptors

Chylomicrons are lipoproteins synthesized exclusively by the intestine to transport dietary fat and fat-soluble vitamins. Synthesis of apoB48, a translational product of the apob gene, is required for the assembly of chylomicrons. The apob gene transcription in the intestine results in 14 and 7 kb mRNAs. These mRNAs are post-transcriptionally edited creating a stop codon. The edited mRNAs chylomicrons from the shorter apoB48 peptide remains to be elucidated. In addition, the roles of proteins involved in the assembly pathway, e.g. apobec-1, MTP and apoA-IV, needs to be studied. Cloning of enzymes involved in the intestinal biosynthesis of triglycerides will be crucial to fully appreciate the assembly of chylomicrons. There is a need for cell culture and transgenic animal models that can be used for intestinal lipoprotein assembly. The catabolism of chylomicrons is far more complex and efficient than the catabolism of VLDL. Even though the major steps involved in the catabolism of chylomicrons are now known, the determinants for apolipoprotein exchange, processing of remnants in the space of Disse, as well as the mechanism of uptake of these particles by extra-hepatic tissue needs further exploration.

Elution of lipoprotein fractions containing apolipoproteins E and A-I in size exclusion on Superose 6 columns is sensitive to mobile phase pH and ionic strength

Separation of lipoproteins secreted from McA-RH7777 (rat hepatoma) cells by Superose 6 column size-exclusion chromatography, using PBS buffer (NaCl 150 mM, sodium phosphate 10 mM, pH 7.5, EDTA 1 mM), produced apolipoprotein (apo) E or A-I profiles that did not correlate with lipoproteins separated by density ultracentrifugation. By density ultracentrifugation, apoE and apoA-I were mostly (> 90%) confined to high-density lipoproteins (HDL, d = 1.063-1.023 g/ml), but by chromatography apoE and apoA-I were recovered in all lipoprotein classes, including low-density lipoproteins (LDL), HDL, and post-HDL. Moreover, the elution volume of phenol red on Superose 6 greatly exceeded the total column volume. These discrepancies were attributable to pH and ionic strength effects. In low ionic strength, high pH buffer (Tris 25 mM, pH 8.3), elution volumes of lipoproteins, albumin, and phenol red were minimized. Elution volumes increased 25-70% when buffer pH was lowered at constant ionic strength (Tris 25 mM, pH 7.4) or when ionic strength was increased at constant pH (Tris 25 mM, pH 8.3, NaCl 500 mM). Altered phase partition appeared to cause the altered elution volumes, since recovery (measured as analyte peak area), resolution (measured as peak width at half height), and column void volume varied little from buffer to buffer. In Superose 6 size-exclusion chromatography with PBS buffer, then, elution volumes vary with pH and ionic strength. We propose that TBE buffer (Tris-borate 89 mM, pH 8.3, EDTA 2 mM) may produce fewer artefacts than PBS. With TBE there were (i) better correlation between size-exclusion and ultracentrifugal fractions, (ii) lower elution volumes, and (iii) less ¿smearing¿ of McA-RH7777 apoE and apoA-I containing lipoprotein bands.

Level of apolipoprotein B mRNA has an important effect of the synthesis and secretion of apolipoprotein B-containing lipoproteins. Studies on transfected hepatoma cell lines expressing recombinant human apolipoprotein B

The effect of apoB mRNA level on hepatic apoB production has not been studied extensively, primarily because the steady state level of apoB mRNA cannot be altered on a short-term basis. We studied the effect of vastly different apoB mRNA levels on the synthesis and secretion of apoB-containing lipoproteins using rat hepatoma (McA-RH7777) cell lines transfected with cDNA constructs encoding human apoB53 (the amino-terminal 53% of the protein; hapoB53) or apoB100 (hapoB100). Among the three hapoB53-transfected cell lines, the relative steady state levels of the hapoB53 mRNA were 10:2.5: < 0.1. Correspondingly, the relative concentration of the intracellular hapoB53 protein was 8:3:1 and of the medium hapoB53 (accumulated over a period of 18 hours) was 12:4:1, which positively correlates with the hapoB53 (d = 1.06 to 1.21 g/mL) or endogenous rat apoB100 (d < 1.06 g/mL). When cell lines containing high or intermediate hapoB53 mRNA levels were compared, there was an eightfold increase in the synthesis and a twofold increase in the secretion efficiency of hapoB53. Analysis of the synthesis and secretion of lipids revealed that in cells producing high levels of hapoB53, triglyceride synthesis (twofold) and secretion (twofold to threefold) were also increased. Furthermore, with the three hapoB100-transfected cells we also observed an increase in apoB100 synthesis (three-fold), apoB100 secretion efficiency (twofold), triglyceride synthesis (fourfold to fivefold), and triglyceride secretion (fourfold to fivefold) in the cells expressing high levels of hapoB100. In all the cell lines examined, secretion efficiency of endogenous rat apoA-I was not affected by transfection. Together these data suggest that secretion of apoB-containing triglyceride-rich lipoproteins can be influenced by the level of apoB mRNA or the rate of apoB translation.

Effects of dexamethasone on the synthesis, degradation, and secretion of apolipoprotein B in cultured rat hepatocytes

Oversecretion of apoB and decreased removal of apoB-containing lipoproteins by the liver results in hyperapobetalipoproteinemia, which is a risk factor for atherosclerosis. We investigated how dexamethasone, a synthetic glucocorticoid, affects the synthesis, degradation, and secretion of apoB-100 and apoB-48. Primary rat hepatocytes were incubated with dexamethasone for 16 hours. Incorporation of [35S]methionine into apoB-48 and apoB-100 was increased by 36% and 50%, respectively, with 10 nmol/L dexamethasone, despite a 28% decrease of incorporation into total cell proteins. However, Northern blot analysis revealed that dexamethasone (1 to 1000 nmol/L) did not significantly alter the steady-state concentrations of apoB mRNA, suggesting that the net increase in apoB synthesis may involve increased translational efficiency. The intracellular retention and the rate and efficiency of apoB secretion were determined by pulse-chase experiments in which the hepatocytes were labeled with [35S]methionine for 10 minutes or 1 hour, and the disappearance of labeled apoB from the cells and its accumulation in the medium were monitored. Degradation of labeled apoB-100 after a 3-hour chase in both protocols was decreased from about 50% to 30%, whereas degradation of apoB-48 was decreased from 30% to 10% to 20% by treatment with 10 or 100 nmol/L dexamethasone. Additionally, the half-life of decay (time required for 50% of labeled cell apoB-100 to disappear from the peak of radioactivity following a 10-minute pulse) was increased by treatment with 10 nmol/L dexamethasone from 77 to 112 minutes, and the value for apoB-48 increased from 145 to 250 minutes. Treatment with 100 nmol/L dexamethasone also stimulated secretion of 35S-labeled apoB-100 and apoB-48 by twofold and 1.5-fold, respectively. The increased secretion of apoB-100 and apoB-48 after dexamethasone treatment was confirmed by immunoblot analysis for apoB mass, and the effect was relatively specific since albumin secretion was not significantly changed. We conclude that glucocorticoids promote the secretion of hepatic apoB-containing lipoproteins by increasing the net synthesis of apoB-100 and apoB-48 and by decreasing the intracellular degradation of newly synthesized apoB. An increased action of glucocorticoids coupled with a decreased ability of insulin to suppress these effects in insulin resistance can lead to hyperapobetalipoproteinemia and an increased risk of atherosclerosis.

Characterization of lipoproteins containing a truncated form of apolipoprotein B, apolipoprotein B32

We have characterized the lipoproteins isolated from the plasma of a human subject who was heterozygous for a mutation yielding a truncated apolipoprotein B (apo-B32). The apo-B32 lipoproteins were isolated from the plasma high density lipoprotein (HDL) fraction by heparin-Sepharose chromatography. Although the chemical composition of the apo-B32-containing lipoproteins was similar to that of normal HDL, the mean diameter of the apo-B32 lipoproteins was larger than typical apo-AI-containing HDL particles. On agarose gels, the apo-B32 lipoproteins had pre-beta mobility similar to that of normal very low density lipoproteins. Analysis of the purified apo-B32 lipoproteins by SDS-polyacrylamide gel electrophoresis revealed the presence of both apo-AI and apo-E. In order to further analyze the properties of apo-B32, we developed an apo-B32 expression vector and generated stable rat hepatoma cell lines expressing apo-B32. In these cell lines, the apo-B32 protein was secreted in a d > 1.21 g/ml lipoprotein. Oleic acid supplementation of the cell-culture media had no measurable affect on the density distribution of the apo-B32 lipoproteins that were secreted by the cells.

Inhibition of secretion of truncated apolipoproteins B by monomethylethanolamine is independent of the length of the apolipoprotein

Translocation of apolipoprotein (apo) B across the endoplasmic reticulum membrane is a likely site for regulation of secretion of very low density lipoproteins from the liver. When primary rat hepatocytes are enriched with the phospholipid phosphatidylmonomethylethanolamine, the secretion of apoB, but not other proteins such as apoprotein A1 and albumin, is disrupted (Vance, J. E. (1991) J. Lipid Res. 32, 1971-1982). Moreover, less apoB enters the microsomal lumen and the intracellular degradation of apoB is increased (Rusiñol, A. E., Chan, E. Y. W., and Vance, J. E. (1993a) J. Biol. Chem. 268, 25168-25175). In the present study we have used McArdle 7777 rat hepatoma cells stably transfected with carboxyl-terminal-truncated variants of human apoB100 and have demonstrated that the reduction in apoB secretion induced by phosphatidylmonomethylethanolamine is not a function of assembly of the apoB into a buoyant lipoprotein particle. In addition, inhibition of the intracellular degradation of the apoproteins B does not restore apoB secretion, suggesting that the effect of phosphatidylmonomethylethanolamine enrichment on apoB degradation is secondary to the effect on translocation of the protein into the endoplasmic reticulum lumen. Furthermore, supplementation of the culture medium with oleic acid does not increase apoB secretion, reduce the intracellular degradation of apoB or reverse the effects of phosphatidylmonomethylethanolamine enrichment on these processes. Our data support the hypothesis that translocation of apoB protein across the endoplasmic reticulum membrane, regardless of the association of the apoB with neutral lipids, may be a key regulatory step in very low density lipoprotein secretion.

The enhanced association of apolipoprotein E with apolipoprotein B-containing lipoproteins in serum-stimulated hepatocytes occurs intracellularly

Synthesis and secretion of VLDL in HepG2 cells are stimulated by several lipogenic factors, including serum. We previously found that the amount of apolipoprotein (apo) E associated with large lipoproteins such as VLDL increased under conditions of stimulated lipogenesis. The present study was designed to determine if the increased apoE association with VLDL occurs intracellularly or after secretion. In addition to HepG2, we studied rat hepatoma McA-RH7777 cells for production of endogenous rat apoE and transfected human apoE3. In both hepatoma cell lines stimulation of lipogenesis and production of large apoB-containing lipoproteins by serum resulted in increased apoE association with these particles and in decreased apoE association with HDL without affecting the total apoE output. Although evidence of apoE redistribution was seen among lipoproteins in the media, the apoE newly secreted under conditions of stimulated lipogenesis mainly associated with apoB-containing lipoproteins at the expense of its association with HDL. However, this effect was not attributable to reduced HDL lipid and apoA-I mass. Finally, redistribution of apoE from HDL to apoB-containing lipoproteins was also observed intracellularly in both HepG2 and transfected McA-RH7777 cells expressing human apoE3. These data suggest that the redistribution of apoE from HDL to apoB-containing lipoproteins upon activated lipogenesis in hepatoma cells occurs intracellularly and is not attributable to a decrease in HDL production.

Characterization of recombinant human apoB-48-containing lipoproteins in rat hepatoma McA-RH7777 cells transfected with apoB-48 cDNA. Overexpression of apoB-48 decreases synthesis of endogenous apoB-100

We studied the effect of overexpression of apolipoprotein (apo) B-48 on the synthesis and secretion of endogenous apoB-100 in rat hepatoma McA-RH7777 cell lines stably transfected with human apoB-48 cDNA under the control of the cytomegalovirus promoter. Three cell lines that secrete 40 to 60 ng human apoB.mg cell protein-1.h-1 were used. The recombinant human apoB-48 exhibited physicochemical characteristics (buoyant density, 1.06 to 1.21 g/mL; beta-electrophoretic mobility and diameters, 16 to 20 nm) indistinguishable from those of endogenous rat apoB-48. Overexpression of the recombinant human apoB-48 resulted in a 50% decrease in the secretion of endogenous apoB-100 but did not affect the secretion of apoE or apoA-I. Several possible mechanisms for the decreased secretion of apoB-100 were evaluated. First, recruitment of lipids into lipoproteins was shown to be unaffected since no major changes in the physicochemical properties of apoB-100-containing lipoproteins were observed. Second, the intracellular degradation of apoB-100 was not altered as the intracellular retention half-time and secretion efficiency remained unaffected by apoB-48 overexpression. Third, the posttranslational regulatory mechanisms for apoB-100 remained normal, as demonstrated by a twofold increase in apoB-100 secretion after supplementation with oleic acid. Unexpectedly, a 35% to 50% decrease in the steady-state synthesis of endogenous apoB-100 was observed in apoB-48-transfected cells compared with control cells. These data suggested that decreased secretion of apoB-100 was secondary to decreased synthesis. The decreased apoB-100 synthesis was not due to decreased steady-state levels of rat apoB-100 mRNA. These results suggest that overexpression of recombinant human apoB-48 may interfere with posttranscriptional events, possibly at the translation-translocation level, and decrease translational yield of apoB-100. These posttranscriptional events prior to the complete synthesis of the apoB-100 polypeptide can be important in the control of apoB-100 secretion.

Factors affecting the regulation of apo B secretion by liver cells

The concentration of apo B is an important risk factor for atherosclerosis, and thus its reduction is associated with a reduction in CHD mortality. In order to reduce apo B concentrations effectively, we must understand how plasma apo B concentration is regulated. Apo B is synthesized, assembled, and secreted by the liver, controlling this process will reduce the number of particles that eventually enter the plasma compartment. The assembly of apo B into a VLDL particle is a complex process which occurs through several stages: peptide synthesis, translocation, accumulation of lipid, and transport through the secretory pathway. Multiple control points regulate the synthesis and secretion of apolipoproteins. Modulation of transcription, translation and intracellular degradation represent independent regulatory mechanisms. The ability of the lipoprotein to bind cotranslationally to lipid appears to be crucial to the formation of a secreted particle. This process may be regulated solely by MTP, or may be modified by the activity of the lipid-synthesizing enzymes. A great deal of evidence supports the role of TG and CE synthesis, although the relative importance of these two lipids is a source of major controversy. In summary, all the lipoprotein components can be limiting for apo B and VLDL synthesis when their availability is substantially decreased. The rate-limiting component in vivo has still not been identified. By understanding how lipoprotein synthesis and assembly are regulated, it should become possible to design new ways of altering these processes in a beneficial manner.

Carboxyl-terminal truncation of apolipoproteinB-100 inhibits lipoprotein(a) particle formation

Recombinant expression systems for both apo(a) and apoB were used to identify sequences in apoB which are required for Lp(a) formation. Incubation of a [35S]Cys-labelled 17-kringle form of apo(a) with supernatants from rat hepatoma (McA-RH7777) cells expressing apoB-88, apoB-94 and apoB-100 resulted in covalent r-Lp(a) formation only with apoB-100. Additionally, apoB-86 present in the LDL of a hypobetalipoproteinemic subject did not associate with a 12-kringle form of recombinant apo(a) to form r-Lp(a) complexes. Our data suggest that sequences within the C-terminal 6% of apoB-100 are essential for Lp(a) assembly.

Synthesis and secretion of hepatic apolipoprotein B-containing lipoproteins

Human apolipoprotein (apo) B-100 is required for the synthesis and secretion of hepatic triacyglycerol-rich lipoproteins. This review summarizes recent developments in understanding the interaction of cis-acting DNA sequences and trans-acting protein factors in regulation of apo B gene expression and apo B mRNA editing, and the role of structural determinants of apo B-100 in the assembly and secretion of hepatic lipoproteins. In particular, experimental results obtained from cell culture studies using techniques of molecular and cellular biology are described and discussed. The relationship between apo B length and its ability to recruit lipids is presented, and the involvement of factors other than apo B in hepatic triacylglycerol-rich lipoprotein production is discussed.