The assembly and secretion of apoB 100 containing lipoproteins in Hep G2 cells. Evidence for different sites for protein synthesis and lipoprotein assembly

Metabolism of triglyceride-rich lipoproteins in health and dyslipidaemia

Accumulating evidence points to the causal role of triglyceride-rich lipoproteins and their cholesterol-enriched remnants in atherogenesis. Genetic studies in particular have not only revealed a relationship between plasma triglyceride levels and the risk of atherosclerotic cardiovascular disease, but have also identified key proteins responsible for the regulation of triglyceride transport. Kinetic studies in humans using stable isotope tracers have been especially useful in delineating the function of these proteins and revealing the hitherto unappreciated complexity of triglyceride-rich lipoprotein metabolism. Given that triglyceride is an essential energy source for mammals, triglyceride transport is regulated by numerous mechanisms that balance availability with the energy demands of the body. Ongoing investigations are focused on determining the consequences of dysregulation as a result of either dietary imprudence or genetic variation that increases the risk of atherosclerosis and pancreatitis. The identification of molecular control mechanisms involved in triglyceride metabolism has laid the groundwork for a 'precision-medicine' approach to therapy. Novel pharmacological agents under development have specific molecular targets within a regulatory framework, and their deployment heralds a new era in lipid-lowering-mediated prevention of disease. In this Review, we outline what is known about the dysregulation of triglyceride transport in human hypertriglyceridaemia.

Bioactivity of Dietary Polyphenols: The Role in LDL-C Lowering

Cardiovascular diseases are the leading causes of the death around the world. An elevation of the low-density lipoprotein cholesterol (LDL-C) level is one of the most important risk factors for cardiovascular diseases. To achieve optimal plasma LDL-C levels, clinal therapies were investigated which targeted different metabolism pathways. However, some therapies also caused various adverse effects. Thus, there is a need for new treatment options and/or combination therapies to inhibit the LDL-C level. Dietary polyphenols have received much attention in the prevention of cardiovascular diseases due to their potential LDL-C lowering effects. However, the effectiveness and potential mechanisms of polyphenols in lowering LDL-C is not comprehensively summarized. This review focused on dietary polyphenols that could reduce LDL-C and their mechanisms of action. This review also discussed the limitations and suggestions regarding previous studies.

Mitochondria-rough-ER contacts in the liver regulate systemic lipid homeostasis

Contacts between organelles create microdomains that play major roles in regulating key intracellular activities and signaling pathways, but whether they also regulate systemic functions remains unknown. Here, we report the ultrastructural organization and dynamics of the inter-organellar contact established by sheets of curved rough endoplasmic reticulum closely wrapped around the mitochondria (wrappER). To elucidate the in vivo function of this contact, mouse liver fractions enriched in wrappER-associated mitochondria are analyzed by transcriptomics, proteomics, and lipidomics. The biochemical signature of the wrappER points to a role in the biogenesis of very-low-density lipoproteins (VLDL). Altering wrappER-mitochondria contacts curtails VLDL secretion and increases hepatic fatty acids, lipid droplets, and neutral lipid content. Conversely, acute liver-specific ablation of Mttp, the most upstream regulator of VLDL biogenesis, recapitulates this hepatic dyslipidemia phenotype and promotes remodeling of the wrappER-mitochondria contact. The discovery that liver wrappER-mitochondria contacts participate in VLDL biology suggests an involvement of inter-organelle contacts in systemic lipid homeostasis.

Normal serum ApoB48 and red cells vitamin E concentrations after supplementation in a novel compound heterozygous case of abetalipoproteinemia

BACKGROUND AND AIMS

Abetalipoproteinemia (ABL) is a rare recessive monogenic disease due to MTTP (microsomal triglyceride transfer protein) mutations leading to the absence of plasma apoB-containing lipoproteins. Here we characterize a new ABL case with usual clinical phenotype, hypocholesterolemia, hypotriglyceridemia but normal serum apolipoprotein B48 (apoB48) and red blood cell vitamin E concentrations.

METHODS

Histology and MTP activity measurements were performed on intestinal biopsies. Mutations in MTTP were identified by Sanger sequencing, quantitative digital droplet and long-range PCR. Functional consequences of the variants were studied in vitro using a minigene splicing assay, measurement of MTP activity and apoB48 secretion.

RESULTS

Intestinal steatosis and the absence of measurable lipid transfer activity in intestinal protein extract supported the diagnosis of ABL. A novel MTTP c.1868G>T variant inherited from the patient's father was identified. This variant gives rise to three mRNA transcripts: one normally spliced, found at a low frequency in intestinal biopsy, carrying the p.(Arg623Leu) missense variant, producing in vitro 65% of normal MTP activity and apoB48 secretion, and two abnormally spliced transcripts resulting in a non-functional MTP protein. Digital droplet PCR and long-range sequencing revealed a previously described c.1067+1217_1141del allele inherited from the mother, removing exon 10. Thus, the patient is compound heterozygous for two dysfunctional MTTP alleles. The p.(Arg623Leu) variant may maintain residual secretion of apoB48.

CONCLUSIONS

Complex cases of primary dyslipidemia require the use of a cascade of different methodologies to establish the diagnosis in patients with non-classical biological phenotypes and provide better knowledge on the regulation of lipid metabolism.

Sort1, encoded by the cardiovascular risk locus 1p13.3, is a regulator of hepatic lipoprotein export

Recent genome-wide association studies (GWAS) have revealed strong association of hypercholesterolemia and myocardial infarction with SNPs on human chromosome 1p13.3. This locus covers three genes: SORT1, CELSR2, and PSRC1. We demonstrate that sortilin, encoded by SORT1, is an intracellular sorting receptor for apolipoprotein (apo) B100. It interacts with apoB100 in the Golgi and facilitates the formation and hepatic export of apoB100-containing lipoproteins, thereby regulating plasma low-density lipoprotein (LDL) cholesterol. Absence of sortilin in gene-targeted mice reduces secretion of lipoproteins from the liver and ameliorates hypercholesterolemia and atherosclerotic lesion formation in LDL receptor-deficient animals. In contrast, sortilin overexpression stimulates hepatic release of lipoproteins and increases plasma LDL levels. Our data have uncovered a regulatory pathway in hepatic lipoprotein export and suggest a molecular explanation for the cardiovascular risk being associated with 1p13.3.

Interactions between the endoplasmic reticulum, mitochondria, plasma membrane and other subcellular organelles

Several recent works show structurally and functionally dynamic contacts between mitochondria, the plasma membrane, the endoplasmic reticulum, and other subcellular organelles. Many cellular processes require proper cooperation between the plasma membrane, the nucleus and subcellular vesicular/tubular networks such as mitochondria and the endoplasmic reticulum. It has been suggested that such contacts are crucial for the synthesis and intracellular transport of phospholipids as well as for intracellular Ca(2+) homeostasis, controlling fundamental processes like motility and contraction, secretion, cell growth, proliferation and apoptosis. Close contacts between smooth sub-domains of the endoplasmic reticulum and mitochondria have been shown to be required also for maintaining mitochondrial structure. The overall distance between the associating organelle membranes as quantified by electron microscopy is small enough to allow contact formation by proteins present on their surfaces, allowing and regulating their interactions. In this review we give a historical overview of studies on organelle interactions, and summarize the present knowledge and hypotheses concerning their regulation and (patho)physiological consequences.

Ileal adenocarcinoma in a mild phenotype of abetalipoproteinemia

Abetalipoproteinemia (ABL) is a rare autosomal recessive disorder that is characterized by defective assembly and secretion of plasma apolipoprotein (apo) B-containing lipoproteins. This disorder results from mutations in the MTP gene encoding the microsomal triglyceride transfer protein. We report a 58-year-old male homozygote for a missense mutation, S590I, in MTP. The patient had a lifelong history of fat malabsorption, but was only diagnosed with ABL at age 52, based upon such classic features as absence of apo B-containing lipoproteins, acanthocytosis, atypical retinitis pigmentosa and markedly depressed serum beta-carotene concentration. However, his presentation was notable not only by survival to the sixth decade of life without specific treatment, but also by the absence of neurological involvement and by normal serum vitamin E concentration. He subsequently developed adenocarcinoma of the ileum, which required ileal resection. Therefore, this missense mutation appears to be associated with a late-presenting and relatively mild ABL phenotype that lacks some classical features, particularly neuropathy, but appears to be associated with other atypical features, specifically small intestinal cancer.

Apolipoprotein B metabolism: tracer kinetics, models, and metabolic studies

The study of apolipoprotein (apo) B metabolism is central to our understanding of lipoprotein metabolism. However, the assembly and secretion of apoB-containing lipoproteins is a complex process. Specialized techniques, developed and applied to in vitro and in vivo studies of apoB metabolism, have provided insights into the mechanisms involved in the regulation of this process. Moreover, these studies have important implications for understanding both the pathophysiology as well as the therapeutic options for the dyslipidemias. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the applications of kinetic analysis and multicompartmental modeling to the study of apoB metabolism. New developments and significant advances over the last decade are discussed.

Intracellular mechanisms mediating the inhibition of apoB-containing lipoprotein synthesis and secretion in HepG2 cells by avasimibe (CI-1011), a novel acyl-coenzyme A: cholesterol acyltransferase (ACAT) inhibitor

We have studied the cellular and molecular mechanisms involved in the suppression of apoB secretion from HepG2 cells following incubation with avasimibe (CI-1011), a novel inhibitor of acyl-coenzyme A: cholesterol acyltransferase (ACAT). Cellular lipid analysis revealed that avasimibe significantly decreased the synthesis of cholesterol and cholesteryl ester, and, at higher doses, of triglyceride. Time-course trypsin protection assays revealed that avasimibe induced the accumulation of translocationally arrested apoB intracellularly. Pulse-chase studies showed that the treatment with avasimibe induced a >75% decrease in apoB secretion relative to control, but initially enhanced the protein stability and cellular accumulation of apoB. Subcellular fractionation of microsomes further confirmed the accumulation of secretion-incompetent apoB-lipoproteins in the endoplasmic reticulum (ER) and Golgi compartments of avasimibe-treated HepG2 cells. Although incubation of drug-treated cells with carbobenzoxyl-leucinyl-leucinyl-leucinal (MG132), a potent proteasome inhibitor, increased cellular apoB (70%), it failed to increase apoB secretion. Drug treatment induced an accumulation of secretion-incompetent apoB-containing lipoprotein particles, the majority of which demonstrated a density in a range similar to that of high-density lipoprotein. However, studies in permeabilized cells demonstrated that, at longer chase times, intracellularly accumulated apoB was eventually degraded, indicating that the inhibition of degradation may be transient. Oleate treatment of avasimibe-treated cells partially restored apoB secretion but not to the levels seen in control cells. In summary, we hypothesize that avasimibe acutely blocks the secretion of apoB and its associated lipoproteins from HepG2 cells, transiently enhancing its membrane association and cellular accumulation with eventual intracellular degradation of accumulated apoB.

Translocational status of ApoB in the presence of an inhibitor of microsomal triglyceride transfer protein

Despite numerous studies demonstrating that microsomal triglyceride transfer protein (MTP) activity is critical to apoB secretion, there is still controversy as to whether MTP directly facilitates the translocation of apoB across the membrane of the endoplasmic reticulum (ER) through either the recruitment of lipids and/or chaperone activity. In the present study, a specific inhibitor of MTP (BMS 197636) was utilized in HepG2 cells to investigate whether a direct relationship exists between the translocation of apoB across the ER membrane and the lipid-transferring activity of MTP. Inhibition of MTP (with 10 and 50 nmol/L of the inhibitor) did not significantly affect the translocation of newly synthesized apoB (P = 0.77) or the translocational efficiency of the steady-state apoB mass (P = 0.45), despite a 49% decrease in apoB secretion and increased proteosomal degradation. These results compared well with subcellular fractionation experiments which showed no significant change in the fraction of apoB accumulated in the lumen of isolated microsomes in MTP-treated cells (P = 0.35). In summary, MTP lipid transfer activity does not appear to influence translocational status of apoB, but its inhibition is associated with an increased susceptibility to proteasome-mediated degradation and reduced assembly and secretion of apoB lipoprotein particles.

ADP-ribosylation factor 1 and its activation of phospholipase D are important for the assembly of very low density lipoproteins

The role of ADP-ribosylation factor 1 (ARF-1) in the assembly of very low density lipoproteins (VLDL) was investigated by expressing dominant-negative mutants in McA-RH7777 cells. Transient expression of ARF-1(T31N), a GDP-restrictive mutant, significantly inhibited apolipoprotein B-100 (apoB-100) VLDL production without influencing the biosynthesis of apoB-100 low density lipoproteins or total apoB production (indicating that it inhibited the second step of VLDL assembly) and without altering total protein production or biosynthesis of transferrin, phosphatidylcholine, or triglycerides. These effects were confirmed in stable inducible transfectants. In contrast, expression of an ARF-1 mutant lacking the N-terminal 17 amino acids, which has no myristoylation site and cannot interact with the microsomal membrane, did not affect VLDL assembly. Thus, active ARF-1 is needed for the second step of the process. To further explore these observations, we developed a cell-free system based on the postnuclear supernatant isolated from McA-RH7777 cells. In this system, 10-15% of the apoB-100 pool was converted to VLDL in a time- and temperature-dependent way. The assembly process was highly dependent on a heat-stable factor in the d > 1.21 g/ml infranatant of fetal calf serum; this factor was not present in low density lipoproteins or VLDL. Brefeldin A inhibited VLDL assembly in this system, as did a synthetic peptide (corresponding to N-terminal amino acids 2-17 of ARF-1) that displaces ARF-1 from the membrane. Thus, active ARF-1 is also needed for cell-free assembly of VLDL. Guanosine 5'-3-O-(thio)triphosphate also inhibited VLDL assembly in this system, indicating that the process requires ongoing hydrolysis of GTP. 1-Butanol, which inhibits the formation of phosphatidic acid (PA) and instead gives rise to phosphatidylbutanol, inhibited VLDL assembly, whereas 2-butanol, which does not inhibit PA formation, failed to do so. Thus, phospholipase D (PLD)-catalyzed formation of PA from phosphatidylcholine is essential for VLDL assembly. In support of this conclusion, exogenous PLD prevented brefeldin A from inhibiting the assembly process. Our results indicate that ARF-1 participates in the second step of VLDL assembly through a process that involves activation of PLD and production of PA.

Common polymorphism in promoter of microsomal triglyceride transfer protein gene influences cholesterol, ApoB, and triglyceride levels in young african american men: results from the coronary artery risk development in young adults (CARDIA) study

The microsomal triglyceride transfer protein (MTP) plays a key role in the assembly of apolipoprotein B (apoB)-containing lipoproteins. We investigated the relation between lipid profiles and a common functional polymorphism (-493G/T) of the MTP gene in a large sample of young black men in the Coronary Artery Risk Development in Young Adults (CARDIA) Study. We performed serial cross-sectional analyses on lipids of 586 black men in 5 exams over 10 years of follow-up. Total cholesterol, LDL cholesterol, and apoB levels were very similar between the GT and GG genotypes; therefore, the GT and GG genotypes were combined as 1 group when the 3 phenotypes were analyzed. The results from ANCOVA showed that the TT group (prevalence 7%) had higher levels of apoB-related lipids than did the GT+GG group: the difference in total cholesterol ranged from 2 (P=0.79) to 19 (P=0.002) mg/dL in exams 1 to 5; the difference in LDL cholesterol ranged from 10 (P=0.14) to 17 (P=0.003) mg/dL in exams 1 to 4, but in examination 5, the difference became negligible. The TT group had higher levels of apoB, measured in only 2 exams, by 6 (P=0.12) and 9 (P=0.03) mg/dL. The TT group had higher levels of triglycerides than did the TG or GG group by 3 to 34 (P=0.02 to approximately 0.003) mg/dL in all 5 exams. HDL cholesterol and apolipoprotein A-I levels were similar among the 3 genotypes. Our serial cross-sectional analyses indicated that the TT genotype was associated with higher levels of total cholesterol, LDL cholesterol, triglycerides, and apoB in young black men. The broad effect of this polymorphism on several atherogenic traits suggests that the MTP gene could be influential in atherosclerosis.

The assembly and secretion of apolipoprotein B-48-containing very low density lipoproteins in McA-RH7777 cells

We have used an extraction procedure, which released membrane-bound apoB-100, to study the assembly of apoB-48 VLDL (very low density lipoproteins). This procedure released apoB-48, but not integral membrane proteins, from microsomes of McA-RH7777 cells. Upon gradient ultracentrifugation, the extracted apoB-48 migrated in the same position as the dense apoB-48-containing lipoprotein (apoB-48 HDL (high density lipoprotein)) secreted into the medium. Labeling studies with [(3)H]glycerol demonstrated that the HDL-like particle extracted from the microsomes contains both triglycerides and phosphatidylcholine. The estimated molar ratio between triglyceride and phosphatidylcholine was 0.70 +/- 0.09, supporting the possibility that the particle has a neutral lipid core. Pulse-chase experiments indicated that microsomal apoB-48 HDL can either be secreted as apoB-48 HDL or converted to apoB-48 VLDL. These results support the two-step model of VLDL assembly. To determine the size of apoB required to assemble HDL and VLDL, we produced apoB polypeptides of various lengths and followed their ability to assemble VLDL. Small amounts of apoB-40 were associated with VLDL, but most of the nascent chains associated with VLDL ranged from apoB-48 to apoB-100. Thus, efficient VLDL assembly requires apoB chains of at least apoB-48 size. Nascent polypeptides as small as apoB-20 were associated with particles in the HDL density range. Thus, the structural requirements of apoB to form HDL-like first-step particles differ from those to form second-step VLDL. Analysis of proteins in the d < 1.006 g/ml fraction after ultracentrifugation of the luminal content of the cells identified five chaperone proteins: binding protein, protein disulfide isomerase, calcium-binding protein 2, calreticulin, and glucose regulatory protein 94. Thus, intracellular VLDL is associated with a network of chaperones involved in protein folding. Pulse-chase and subcellular fractionation studies showed that apoB-48 VLDL did not accumulate in the rough endoplasmic reticulum. This finding indicates either that the two steps of apoB lipoprotein assembly occur in different compartment or that the assembled VLDL is transferred rapidly out of the rough endoplasmic reticulum.

Mechanisms of hepatic very low density lipoprotein overproduction in insulin resistance. Evidence for enhanced lipoprotein assembly, reduced intracellular ApoB degradation, and increased microsomal triglyceride transfer protein in a fructose-fed hamster model

A novel animal model of insulin resistance, the fructose-fed Syrian golden hamster, was employed to investigate the mechanisms mediating the overproduction of very low density lipoprotein (VLDL) in the insulin resistant state. Fructose feeding for a 2-week period induced significant hypertriglyceridemia and hyperinsulinemia, and the development of whole body insulin resistance was documented using the euglycemic-hyperinsulinemic clamp technique. In vivo Triton WR-1339 studies showed evidence of VLDL-apoB overproduction in the fructose-fed hamster. Fructose feeding induced a significant increase in cellular synthesis and secretion of total triglyceride (TG) as well as VLDL-TG by primary hamster hepatocytes. Increased TG secretion was accompanied by a 4.6-fold increase in VLDL-apoB secretion. Enhanced stability of nascent apoB in fructose-fed hepatocytes was evident in intact cells as well as in a permeabilized cell system. Analysis of newly formed lipoprotein particles in hepatic microsomes revealed significant differences in the pattern and density of lipoproteins, with hepatocytes derived from fructose-fed hamsters having higher levels of luminal lipoproteins at a density of VLDL versus controls. Immunoblot analysis of the intracellular mass of microsomal triglyceride transfer protein, a key enzyme involved in VLDL assembly, showed a striking 2.1-fold elevation in hepatocytes derived from fructose-fed versus control hamsters. Direct incubation of hamster hepatocytes with various concentrations of fructose failed to show any direct stimulation of its intracellular stability or extracellular secretion, further supporting the notion that the apoB overproduction in the fructose-fed hamster may be related to the fructose-induced insulin resistance in this animal model. In summary, hepatic VLDL-apoB overproduction in fructose-fed hamsters appears to result from increased intracellular stability of nascent apoB and an enhanced expression of MTP, which act to facilitate the assembly and secretion of apoB-containing lipoprotein particles.

Inhibition of cholesterol esterification by DuP 128 decreases hepatic apolipoprotein B secretion in vivo: effect of dietary fat and cholesterol

To further test the hypothesis that newly synthesized cholesteryl esters regulate hepatic apolipoprotein B (apoB) secretion into plasma, apoB kinetic studies were carried out in seven control miniature pigs and in seven animals after 21 days intravenous administration of the acyl coenzyme A:cholesterol acyltransferase (ACAT) inhibitor DuP 128 (2.2 mg/kg/day). Pigs were fed a fat (34% of calories; polyunsaturated/monounsaturated/saturated ratio, 1:1:1) and cholesterol (400 mg/day; 0.1%; 0.2 mg/kcal) containing pig chow based diet. DuP 128 significantly reduced total plasma triglyceride and very low density lipoprotein (VLDL) triglyceride concentrations by 36 and 31%, respectively (P<0.05). Autologous 131I-VLDL and 125I-LDL were injected simultaneously into each pig and apoB kinetic data was analyzed using multicompartmental analysis (SAAM II). The VLDL apoB pool size decreased by 26% (0.443 vs. 0.599 mg/kg; P<0. 001) which was due entirely to a 28% reduction in VLDL apoB production or secretion rate (1.831 vs. 2.548 mg/kg/h; P=0.006). The fractional catabolic rate (FCR) for VLDL apoB was unchanged. The LDL apoB pool size and production rate were unaffected by DuP 128 treatment. Hepatic microsomal ACAT activity decreased by 51% (0.44 vs. 0.90 nmol/min/mg; P<0.001). Although an increase in hepatic free cholesterol and subsequent decrease in both LDL receptor expression and LDL apoB FCR might be expected, this did not occur. The concentration of hepatic free cholesterol decreased 12% (P=0.008) and the LDL apoB FCR were unaffected by DuP 128 treatment. In addition, DuP 128 treatment did not alter the concentration of hepatic triglyceride or the activity of diacylglycerol acyltransferase, indicating a lack of effect of DuP 128 on hepatic triglyceride metabolism. In our previous studies, DuP 128 treatment of miniature pigs fed a low fat, cholesterol free diet, decreased VLDL apoB secretion by 65% resulting in a reduction in plasma apoB of 60%. We conclude that in miniature pigs fed a high fat, cholesterol containing diet, the inhibition of hepatic cholesteryl ester synthesis by DuP 128 decreases apoB secretion into plasma, but the effect is attenuated relative to a low fat, cholesterol free diet.

Dimerization of the human MUC2 mucin in the endoplasmic reticulum is followed by a N-glycosylation-dependent transfer of the mono- and dimers to the Golgi apparatus

Pulse-chase experiments in the colon cell line LS 174T combined with subcellular fractionation by sucrose density gradient centrifugation showed that the initial dimerization of the MUC2 apomucin started directly after translocation of the apomucin into the rough endoplasmic reticulum as detected by calnexin reactivity. As the mono- and dimers were chased, O-glycosylated MUC2 mono- and dimers were precipitated using an O-glycosylation-insensitive antiserum against the N-terminal domain of the MUC2 mucin. These O-glycosylated species were precipitated from the fractions that comigrated with the galactosyltransferase activity during the subcellular fractionation, indicating that not only MUC2 dimers but also a significant amount of monomers are transferred into the Golgi apparatus. Inhibition of N-glycosylation with tunicamycin treatment slowed down the rate of dimerization and introduced further oligomerization of the MUC2 apomucin in the endoplasmic reticulum. Results of two-dimensional gel electrophoresis demonstrated that these oligomers (putative tri- and tetramers) were stabilized by disulfide bonds. The non-N-glycosylated species of the MUC2 mucin were retained in the endoplasmic reticulum because no O-glycosylated species were precipitated after inhibition by tunicamycin. This suggests that N-glycans of MUC2 are necessary for the correct folding and dimerization of the MUC2 mucin.

Effects of atorvastatin on the intracellular stability and secretion of apolipoprotein B in HepG2 cells

We investigated the effects of atorvastatin, a new 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, on the biogenesis of apolipoprotein B (apoB) in intact and permeabilized HepG2 cells. Intact cells were pretreated either with single or multiple doses of atorvastatin (0.1 to 20 micromol/L) for periods of 6 to 20 hours and pulsed with [35S]methionine. In some cases the cells were permeabilized with digitonin. Experiments were performed to investigate the effects of atorvastatin on (1) the rates of lipid synthesis and secretion, (2) the synthesis and accumulation of apoB, (3) the intracellular stability of apoB, (4) the amount of apoB-containing lipoprotein particles assembled in HepG2 microsomes, and (5) the secretion and accumulation of apoB into the culture medium. ApoB synthesis, degradation, and secretion were measured by pulse-chase experiments with [35S]methionine in both intact and permeabilized HepG2 cells. Lipid synthesis was assessed by pulse-labeling experiments with [3H]acetate or [3H]oleate bound to bovine serum albumin. Comparisons were made under basal conditions and in the presence of oleate (0.36 micromol/L). Atorvastatin acutely inhibited the synthesis of cholesterol and cholesterol ester but did not have a significant effect on triglyceride or phospholipid synthesis. Atorvastatin did not affect the uptake of [35S]methionine by the cells nor did it influence the synthesis of apoB or a control protein, albumin. However, atorvastatin reduced the secretion of apoB into the culture medium, apparently by enhancing the degradation of apoB in the cell under basal and induced conditions with oleate. The stability of apoB associated with the lipoprotein particles was also significantly lowered by atorvastatin. The stimulated degradation of apoB in atorvastatin-treated cells was sensitive to MG132, a proteasome inhibitor. The net effect of atorvastatin was a reduction in the number of apoB-containing lipoprotein particles of different sizes isolated from microsomes and a reduction in apoB secretion into the culture medium. The data suggest that atorvastatin may impair the translocation of apoB into the lumen of the endoplasmic reticulum, thus increasing the amount of apoB degraded intracellularly. It is hypothesized that atorvastatin alters these parameters primarily as a result of inhibiting cholesterol synthesis and limiting the availability of cholesterol and/or cholesterol ester for the normal assembly of apoB-containing lipoprotein particles.

The microsomal triglyceride transfer protein catalyzes the post-translational assembly of apolipoprotein B-100 very low density lipoprotein in McA-RH7777 cells

In cells in which the lipoprotein assembly process had been inactivated by brefeldin A (BFA), membrane-associated apoB-100 disappeared without forming lipoproteins or being secreted, indicating that it was degraded. Reactivation of the assembly process by chasing the cells in the absence of BFA, gave rise to a quantitative recovery of the membrane-associated apoB-100 in the very low density lipoprotein (VLDL) fraction in the medium. These results indicate that the membrane-associated apoB-100 can be converted to VLDL. A new method was developed by which the major amount (88%) of microsomal apoB-100 but not integral membrane proteins could be extracted. The major effect of this method was to increase the recovery of apoB-100 that banded in the LDL and HDL density regions, suggesting that the membrane-associated form of apoB-100 is partially lipidated. We also investigated the role of the microsomal triglyceride transfer protein (MTP) in the assembly of apoB-100 VLDL using a photoactivatable MTP inhibitor (BMS-192951). This compound strongly inhibited the assembly and secretion of apoB-100 VLDL when present during the translation of the protein. To investigate the importance of MTP during the later stages in the assembly process, the cells were preincubated with BFA (to reversibly inhibit the assembly of apoB-100 VLDL) and pulse-labeled (+BFA) and chased (+BFA) for 30 min to obtain full-length apoB-100 associated with the microsomal membrane. Inhibition of MTP after the 30-min chase blocked assembly of VLDL. This indicates that MTP is important for the conversion of full-length apoB-100 into VLDL. Results from experiments in which a second chase (-BFA) was introduced before the inactivation of MTP indicated that only early events in this conversion of full-length apoB-100 into VLDL were blocked by the MTP inhibitor. Together these results indicate that there is a MTP-dependent "window" in the VLDL assembly process that occurs after the completion of apoB-100 but before the major amount of lipids is added to the VLDL particle. Thus the assembly of apoB-100 VLDL from membrane-associated apoB-100 involves an early MTP-dependent phase and a late MTP-independent phase, during which the major amount of lipid is added.

Conformational changes in apolipoprotein B modulate intracellular assembly and degradation of ApoB-containing lipoprotein particles in HepG2 cells

The linkage between the conformation of apolipoprotein B100 (apoB) and the intracellular assembly and degradation of apoB-containing lipoproteins was investigated in the present study. Disruption of disulfide bond formation in newly synthesized apoB molecules through the use of the reducing agent DTT resulted in a decrease in the secretion of apoB-containing lipoproteins from HepG2 cells compared with control cells. The synthesis of total apoB (apoB100 plus nascent chains), as well as a number of control proteins, such as albumin and alpha 1-antitrypsin, was decreased significantly in DTT-treated cells. However, the intracellular accumulation of full-length apoB100 molecules was not inhibited in the presence of DTT. Subcellular fractionation indicated that apoB molecules isolated from the microsomes of DTT-treated cells had an increased association with the microsomal membrane compared with apoB isolated from untreated cells. Analysis of the distribution of apoB-containing lipoproteins from the lumen of isolated microsomes demonstrated that in the presence of DTT, there was a shift in the distribution, such that there was a decrease in the formation of HDL-sized (lipid-poor) apoB-containing lipoproteins and a decrease in the formation of LDL/VLDL apoB particles. Alterations in apoB conformation and their impact on degradation were also investigated by using DTT and by inhibiting N-linked glycosylation with tunicamycin. DTT appeared to change the rate and pattern of apoB degradation. Degradation was accelerated in both intact and permeabilized HepG2 cells. ApoB degradation occurred in DTT-treated permeabilized cells without the usual generation of the 70-kD and 335-kD fragments and was largely N-acetyl-leucyl-leucyl-norleucinal (ALLN) insensitive. In tunicamycin-treated cells, DTT further accelerated the degradation of unglycosylated apoB. Overall, the data suggest that the misfolding of apoB may prevent the proper association of apoB with lipids, resulting in impairment of the assembly of mature apoB-containing lipoproteins. Alteration in the conformation of apoB also appears to alter the degradation pathway of apoB, such that the protein is degraded through a pathway that is at least in part ALLN insensitive.

Measurement of apolipoprotein B in various cell lines: correlation between intracellular levels and rates of secretion

We have standardized simple but sensitive enzyme-linked immunoassays to understand a relationship between intracellular levels and secretion rates of apoB. The assays were based on commercially available antibodies and were specific to human apoB. A monoclonal antibody, 1D1, was immobilized on microtiter wells and incubated with different amounts of low density lipoproteins to obtain a standard curve. Conditioned media were added to other wells in parallel, and the amount of apoB was quantitated from a linear regression curve. To standardize conditions for the measurement of intracellular apoB, cells were homogenized and solubilized with different concentrations of taurocholate. We found that 0.5% taurocholate was sufficient to solubilize all the apoB in HepG2, Caco-2, and McA-RH7777 cells. Next, a standard curve was prepared in the presence of taurocholate and used to determine intracellular levels of apoB in different cell lines. The intracellular levels (pmol/mg cell protein) and the rates of secretion (pmol/mg/h) of apoB100 were positively correlated (r2 = 0.81, P = 0.0009) in HepG2 cells. Furthermore, a positive correlation (r2 = 0.88, P < 0.0001) was found between intracellular and secreted apoB42 in stably transfected McA-RH7777 cells. In contrast, no correlation was observed for human apoB28 and apoB18 in stably transfected cells that were secreted either partially associated or completely unassociated with lipoproteins. These studies indicated that the rate of secretion of lipid-associated apoB, but not the lipid-free apoB, was tightly controlled.

Apolipoprotein B is intracellularly associated with an ER-60 protease homologue in HepG2 cells

Two ALLN (N-acetyl-leucyl-leucyl-norleucinal)-sensitive endoplasmic reticulum (ER)-localized proteases (ER-60 and ER-72) were recently purified from rat liver. We used an antibody to rat ER-60 to investigate the possible role of this protease in apolipoprotein B (apoB) degradation. First, immunoprecipitation and immunoblotting experiments with the anti-rat ER-60 antibody suggested that HepG2 cells contain a homologue of ER-60 with an approximate molecular mass of 58-60 kDa. The ER-60 homologue was mostly associated with the luminal contents of HepG2 microsomes. Evidence from co-immunoprecipitation and cross-linking experiments appear to suggest that the ER-60 homologue in HepG2 cells is associated with apoB intracellularly. A small pool of apoB was recovered when HepG2 lysates were subjected to immunoprecipitation with anti-rat ER-60 antibody followed by a second immunoprecipitation with anti-apoB antibody. Furthermore, cross-linking of permeabilized cells with dithiobis(succinimidylpropionate) further demonstrated association of apoB with the ER-60 homologue in HepG2 cells. Three polypeptides with molecular masses of 78, 66, and 50 kDa were consistently found to be associated with apoB as well as the 58-kDa ER-60 homologue. The 78-kDa protein associated with both apoB and ER-60 appeared to represent immunoglobulin heavy chain-binding protein (BiP) based on immunoprecipitation with a monoclonal antibody. Cross-linking and immunoblotting experiments suggested the association of the 78-kDa BiP with both the 58-kDa ER-60 homologue as well as the 550-kDa apoB. In summary, the data suggests that HepG2 cells contain a 58-kDa protein which is homologous to the rat liver ER-60 in size, antigenecity, and intracellular localization. The ER-60 homologue in HepG2 cells appears to be closely associated with apoB, as well as other proteins possibly representing ER chaperones such as BiP. We hypothesize that the ER-60 homologue may be involved in the degradation of apoB in the ER lumen of HepG2 cells.

Co-translational degradation of apolipoprotein B100 by the proteasome is prevented by microsomal triglyceride transfer protein. Synchronized translation studies on HepG2 cells treated with an inhibitor of microsomal triglyceride transfer protein

We studied the effect of inhibition of microsomal triglyceride transfer protein (MTP) on apolipoprotein (apo) B100 translation and secretion using HepG2 cells. The MTP-mediated lipid transfer activity was reduced using a specific MTP inhibitor. ApoB100 translation was synchronized by treatment with puromycin prior to L-[35S]methionine pulse-chase labeling. During the first 4 min of chase, synthesis of apoB polypeptides the size of 100-200 kDa was insensitive to the inhibitor, suggesting that inhibition of MTP did not affect the initiation of apoB100 translation. After 15 min of chase, the 100-200-kDa species were chased into polypeptides larger than 320 kDa (i.e. apoB65 or 65% of full-length apoB100) in both control and inhibitor-treated cells. However, the amount of these polypeptides decreased (by 36% for apoB65-75, by 64% for apoB75-85, by 76% for apoB85-95, and by 77% for apoB100) upon MTP inhibition. No accumulation of smaller polypeptides was observed, but total immunoprecipitable apoB radioactivity was decreased suggesting that apoB could undergo co-translational degradation when MTP activity was reduced. Inhibitors of the multicatalytic proteinase complex (proteasome) such as lactacystin or MG-115 could prevent apoB co-translational degradation. Nevertheless, MG-115 could not avoid the MTP inhibitor decreasing apoB100 secretion but rather induced the accumulation of secretion-incompetent apoB100 in the cell. These results indicate that MTP activity is required during the elongation of apoB100 polypeptides, particularly at the sequences downstream of carboxyl terminus of apoB65. Co-translational degradation might constitute a more general mechanism of early quality control for large or complex proteins.

Use of cyclodextrin to deliver lipids and to modulate apolipoprotein B-100 production in HepG2 cells

2-Hydroxypropyl-beta-cyclodextrin (cyclodextrin), cyclodextrin-solubilized oleate, and cyclodextrin-solubilized cholesterol were used to modulate proteolysis and secretion of newly-synthesized apolipoprotein B-100 (apoB) in HepG2 cells. Following cyclodextrin and lipid treatments, cells were pulse-labeled with [3H] leucine, and quantitative immunoprecipitation was used to measure apoB synthesis, apoB secreted into the medium, and the cellular content of undegraded apoB that was not secreted. Three-hour treatment with cyclodextrin-solubilized oleate (0.2 mM) increased secreted apoB from 4% (control cells) to 32% and cellular undegraded apoB from 15% (control cells to 64% of apoB synthesized, which is consistent with earlier studies using bovine serum albumin to complex exogenous oleate. Prolonged daily (4 d or more) administration of 0.5% (3.5 mM) cyclodextrin with medium containing 10% fetal bovine serum increased the secretion of nascent apoB from 5-10% (control) to 17-28% and cellular undegraded apoB from 15-20% (control) to 25-31% of apoB synthesized, respectively. Subsequent administration of cyclodextrin solubilized cholesterol (10-40 micrograms) for only 3 h reversed the cyclodextrin-mediated increase in apoB secretion. The application of 0.5% cyclodextrin to HepG2 cells can rapidly (within minutes) stimulate cholesterol efflux, and transiently (over a 1-2 d period) increase cholesterol synthesis. In the current studies, the cyclodextrin-mediated increase in cholesterol synthesis was not concurrent with the increase in apoB secretion. However, prolonged (15 d) administration of cyclodextrin was shown to increase the cellular free cholesterol concentration by 25-41%, reduce the cellular triglyceride concentration by 59%, and increase apoB secretion 3- to 4-fold, without affecting the cellular cholesteryl ester concentration. In comparison, 14-d treatment with cyclodextrin-solubilized cholesterol (20 micrograms/mL) followed by 1-d equilibration without cholesterol was shown to increase the cellular free cholesterol and cholesteryl ester concentrations by 76% and 10-fold, respectively, although apoB secretion was not affected. It is hypothesized that chronic daily administration of 0.5% cyclodextrin increased the cellular cholesterol concentration and flux in discrete putative regulatory compartments, which "shielded" nascent apoB from rapid proteolysis and facilitated apoB secretion. In conclusion, cyclodextrin was used independently and in combination with cholesterol or oleate to modulate apoB proteolysis and secretion. We speculate that subcellular changes in cholesterol concentration and flux may modulate apoB production in HepG2 cells.

Decreased production and increased catabolism of apolipoprotein B-100 in apolipoprotein B-67/B-100 heterozygotes

Apolipoprotein (apo) B-67 is a truncated form of apoB-100 due to deletion of an adenine at cDNA 9327. Heterozygotes have one allele making apoB-100; therefore, plasma apoB levels would be predicted to be at least 50% of normal. However, apoB-67 heterozygotes have total plasma apoB levels that are 24% of normal. To determine the mechanisms responsible for the lower-than-expected levels of apoB, in vivo kinetics of apoB-100 were performed in three apoB-67/apoB-100 heterozygotes and compared with those of six control subjects by using a primed-constant infusion of [5,5,5-2H3]leucine in the fed state. Kinetic parameters were calculated by multicompartmental modeling of the data. The mean total apoB plasma concentration of the apoB-67 subjects was 21.8+/-6.1 mg/dL, or 24% of that of control subjects (89.6+/-24.1 mg/dL, P=.002). ApoB-67 subjects had lower mean VLDL apoB-100 production rates (3.6+/-1.2 versus 13.9+/-3.5 mg x kg(-1) x d(-1), P=.002) and lower mean transport rates of apoB-100 into LDL (3.5+/-1.4 versus 12.6+/-4.1 mg x kg(-1) x d(-1), P=.008) compared with control subjects. The transport rate into IDL was not significantly different (1.2+/-0.5 versus 6.2+/-4.0 mg x kg(-1) x d(-1), P=.07). The fractional catabolic rate of VLDL apoB-100 was significantly higher in apoB-67 subjects than in control subjects (18.1+/-8.6 versus 7.6+/-1.6 mg x kg(-1) x d(-1), P=.017). ApoB-100 IDL and LDL fractional catabolic rates were not significantly different. VLDL apoB-100 pool size in apoB-67 subjects was 11% of that of control subjects (15.8+/-7.7 versus 141.6+/-33.7 mg, P=.0004) due to a 74% lower production rate (26% of control values) and a 2.4-fold higher fractional catabolic rate. LDL apoB-100 pool size in apoB-67 subjects was 22% of that of control subjects (665.3+/-192.4 versus 2968.3+/-765.2 mg, P=.002) due primarily to a lower production rate (27% of control values). Thus, both decreased production of VLDL and LDL apoB-100 and increased catabolism of VLDL apoB-100 are responsible for the low levels of apoB-100 in apoB-67 subjects.

Role of lipid synthesis, chaperone proteins and proteasomes in the assembly and secretion of apoprotein B-containing lipoproteins from cultured liver cells

1. Apolipoprotein B (apoB) is necessary for the assembly and secretion of both chylomicrons from the small intestine and very low-density lipoproteins (VLDL) from the liver. ApoB is also the major protein in low-density lipoproteins (LDL) and is the ligand for the LDL receptor. Studies in humans suggest that increased production of apoB-containing lipoproteins, particularly VLDL, is a common abnormality in dyslipidaemias. 2. Studies in primary and long-term cultures of hepatocytes and hepatoma cells indicate that a significant proportion of newly synthesized apoB is rapidly degraded and that this is the major mechanism for regulation of apoB secretion. The availability of newly synthesized lipids, particularly triglyceride and cholesteryl ester, appears to be a critical factor in targeting apoB for secretion rather than degradation. 3. ApoB is an atypical secretory protein in that cotranslational translocation across the endoplasmic reticulum membrane, a feature of all secretory proteins, seems to slow or stop in the absence of adequate lipid availability (or in the absence of microsomal triglyceride transfer protein), allowing for rapid degradation of apoB. 4. The degradation of apoB seems to be facilitated by the association of nascent apoB with the major cytosolic chaperone protein, heat shock protein 70. Additionally, degradation of nascent apoB appears to occur, to a large degree, via the proteasomal pathway for degradation of cytosolic proteins.

Studies on intracellular translocation of apolipoprotein B in a permeabilized HepG2 system

Recent evidence suggests that the rate of apolipoprotein B-100 (apoB) translocation may be a key regulatory point in the production of apoB-containing lipoproteins. We have developed an in vitro system to measure the translocation rate of apoB in HepG2 cells. Intact cells were initially pretreated with oleate and N-acetyl-Leu-Leu-norleucinal to maximize the translocation rate while minimizing degradation. Cells were pulsed with [35S]methionine, chased (5-30 min), and then permeabilized with digitonin (75 microg/ml). Permeabilized cells were incubated with or without trypsin (200 microg/ml) for 10 min, and digestion was halted with soybean trypsin inhibitor (2 mg/ml). The rate of translocation was determined by comparing the amount of immunoprecipitable intact apoB in trypsin-treated cells with that in control cells at each time point. Under these conditions, two control proteins, alpha1-antitrypsin and transferrin, were fully protected from trypsin digestion, confirming the integrity of the secretory pathway in permeabilized cells. The percentage of apoB translocated steadily increased from 36% after 5 min to 71% after a 30-min chase (mean percentage, n = 3). A characteristic apoB fragmentation pattern resulted from trypsin digestion, and protected fragments of various size including N-terminal 60-70-kDa fragments were identified. Subcellular fractionation of the cells confirmed that the apoB pool protected from trypsin digestion was luminal in nature, confirming its translocation. ApoB translocation was significantly increased in oleate-treated cells compared with untreated cells. Inhibition of peptidylprolyl isomerase through the use of cyclosporin A and disruption of disulfide bond formation using dithiothreitol reduced the percentage of translocated apoB by 37 and 63%, respectively. Dithiothreitol induced specific changes in the pattern of protected apoB fragments, suggesting a conformational change in apoB that may hinder its translocation. Inhibition of N-linked glycosylation with tunicamycin did not significantly alter the rate of apoB translocation but appeared to stimulate its degradation. Together, the data suggest that the rate of apoB translocation across the membrane of the ER is determined by both lipid availability as well as the correct conformation of nascent apoB molecules.

Intracellular assembly and degradation of apolipoprotein B-100-containing lipoproteins in digitonin-permeabilized HEP G2 cells

Permeabilized Hep G2 cells have been used to investigate the turnover of apolipoprotein B-100 (apoB-100). When such cells were chased in the presence of buffer, there was no biosynthesis of apoB-100, nor was the protein secreted from the cells. Thus the turnover of apoB-100 in these cells reflected the posttranslational degradation of the protein. Pulse-chase studies indicated that apoB-100 was degraded both when associated with the membrane and when present as lipoproteins in the secretory pathway. Neither albumin nor alpha1-antitrypsin showed any significant posttranslational intracellular degradation under the same condition. The kinetics for the turnover of apoB-100 in the luminal content differed from that of apoB-100 that was associated with the microsomal membrane. Moreover, while the degradation of the luminal apoB-100 was inhibited by N-acetyl-leucyl-leucyl-norleucinal (ALLN), this was not the case for the membrane-associated protein. Together these results suggest the existence of different pathways for the degradation of luminal apoB-100 and membrane-associated apoB-100. This was further supported by results from pulse-chase studies in intact cells, showing that ALLN increased the amount of radioactive apoB-100 that associated with the microsomal membrane during the pulse-labeling of the cells. However, ALLN did not influence the rate of turnover of the membrane-associated apoB-100. The presence of an ATP-generating system during the chase of the permeabilized cells prevented the disappearance of pulse-labeled apoB-100 from the luminal lipoprotein-associated pool. The ATP-generating system combined with cytosol protected the total apoB-100 in the system from being degraded. The cells cultured in the presence of oleic acid and chased after permeabilization in the presence of cytosol and the ATP-generating system showed an increase in the amount of apoB-100 present on dense ("high density lipoprotein-like") particles. This increase was linear during the time investigated (i. e. from 0 to 2 h chase) and independent of protein biosynthesis. Our results indicate that the dense particle was generated by a redistribution of apoB-100 within the secretory pathway and that it most likely was assembled from the membrane- associated form of apoB-100. These results indicate that the release of apoB-100 from this membrane-associated form to the microsomal lumen is dependent on cytosolic factors and a source of metabolic energy.

The role of phosphatidylethanolamine methylation in the secretion of very low density lipoproteins by cultured rat hepatocytes: rapid inhibition of phosphatidylethanolamine methylation by bezafibrate increases the density of apolipoprotein B48-containing lipoproteins

The role of phosphatidylcholine (PC) synthesis via the phosphatidylethanolamine (PE) methylation pathway in the secretion of very low density lipoproteins (VLDL) by cultured rat hepatocytes has been investigated by determining the effects of inhibitors. We have shown that bezafibrate and clofibric acid, known hypolipidemic agents, are potent inhibitors of PE methylation (see accompanying paper by Nishimaki-Mogami et al. (1996) Biochim. Biophys. Acta 1304). In hepatocytes incubated with ethanolamine, which maintained cellular PE levels and PE methylation activity, bezafibrate (200 microM) decreased the secretion of triacylglycerol (TG), PC, apolipoproteins B48, and E in VLDL by 50-75%. In contrast, bezafibrate at this concentration had marginal effect on VLDL secretion (83-115% of control) by hepatocytes that had been cultured in the absence of ethanolamine. In these cells PE levels and PE methylation activity had decreased by approx. 40%. VLDL secretion was decreased at concentrations similar to those required to inhibit PE methylation, and was accompanied by an increase in cellular TG levels. The same ethanolamine-dependent effects were produced by clofibric acid and also by 3-deazaadenosine (DZA), an inhibitor of cellular methylation reactions. These results indicate that PC synthesis via the PE methylation pathway plays a significant role in VLDL secretion in rat hepatocytes if the pathway is maintained at levels comparable to those in vivo. The reductions of PE methylation by bezafibrate and DZA did not affect the total amount of apolipoprotein B48 secreted into the medium. The decrease in apolipoprotein B48 in VLDL caused by bezafibrate was accompanied by an increase in apolipoprotein B48 in the HDL density range. In contrast, the amount of apolipoprotein B100 in VLDL and density determined by sequential flotation were unaffected. These findings indicate that rapid reduction of PC synthesis via PE methylation does not affect the secretion of apolipoprotein B48- or B100-containing lipoprotein particles, but does impair the lipidation of particles containing apolipoprotein B48, but not apolipoprotein B100.

Ubiquitin-proteasome pathway mediates intracellular degradation of apolipoprotein B

Newly synthesized apolipoprotein B (apoB) is degraded by a proteolytic process in the pre-Golgi compartment that can be inhibited by N-acetyl-L-leucinyl-L-leucinyl-L-norleucinal (ALLN) but not by several other protease inhibitors. We have tested the hypothesis that the ubiquitin-proteasome pathway is involved in the intracellular degradation of apoB in liver cells. We found that inhibitors of proteasomes blocked the degradation of apoB in cultured human hepatoma (HepG2) cells. Protein degradation by proteasomes is ATP-dependent, and ATP depletion by dinitrophenol and 2-deoxyglucose also inhibited apoB degradation in these cells. Furthermore, the intracellular human apoB isolated by immunoprecipitation was shown to react specifically with anti-ubiquitin antibody by immunoblotting. This result was corroborated by sequential immunoprecipitation of [35S]methionine-labeled proteins by anti-human apoB and anti-ubiquitin antisera. In contrast, secreted apoB was not ubiquitinated. The amount of intracellular ubiquitinated apoB was increased by the proteasome inhibitors, ALLN and carbobenzoxyl-leucinyl-leucinyl-norvalinal-H (MG115). Our findings suggest that the ubiquitin-proteasome pathway is one mechanism for the intracellular degradation of apoB.

ApoB-100 secretion by HepG2 cells is regulated by the rate of triglyceride biosynthesis but not by intracellular lipid pools

Triglycerides (TGs), cholesteryl esters (CEs), cholesterol, and phosphatidylcholine have been independently proposed as playing regulatory roles in apoB-100 secretion; the results depended on the cellular model used. In this study, we reinvestigate the role of lipids in apoB-100 production in HepG2 cells and in particular, we clarify the respective roles of intracellular mass and the biosynthesis of lipids in the regulation of apoB-100 production. In a first set of experiments, the pool size of cholesterol, CEs, and TGs was modulated by a 3-day treatment with either lipid precursors or inhibitors of enzymes involved in lipid synthesis. We used simvastatin (a hydroxymethylglutaryl coenzyme A reductase inhibitor), 58-035 (an acyl coenzyme A cholesterol acyltransferase inhibitor), 5-tetradecyloxy-2-furancarboxylic acid (TOFA, an inhibitor of fatty acid synthesis), and oleic acid. The secretion rate of apoB-100 was not affected by the large modulation of lipid mass induced by these various pre-treatments. In a second set of experiments, the same lipid modulators were added during a 4-hour labeling period. Simvastatin and 58-035 inhibited cholesterol and CE synthesis without affecting apoB-100 secretion. By contrast, treatment of HepG2 cells with TOFA resulted in the inhibition of TG synthesis and apoB-100 secretion. This effect was highly specific for apoB-100 and was reversed by adding oleic acid, which stimulated both TG synthesis and apoB-100 secretion. Moreover, a combination of oleic acid and 58-035 inhibited CE biosynthesis and increased both TG synthesis and apoB-100 secretion. These results show that in HepG2 cells TG biosynthesis regulates apoB-100 secretion, whereas the rate of cholesterol or CE biosynthesis has no effect.

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.

Selective recruitment of ApoB-48 for the assembly of VLDL in rat triacylglycerol-enriched hepatocytes

Primary rat hepatocyte cultures were enriched in cellular triacylglycerol (TAG) by exposure to extracellular oleate for 3 days. Control cells were cultured for the same time without oleate. The large increase in TAG secretion into the medium of TAG-enriched cells during the final 24 hours (225 +/- 30 versus 40 +/- 10 micrograms/mg cell protein [control cells], P < .01) was not accompanied by a similar change in apolipoprotein B (apoB) secretion (4.22 +/- 0.94 versus 3.72 +/- 0.75 micrograms/mg per 24 hours, respectively). Instead, TAG-enriched cells recruited a larger proportion of apoB for the synthesis of very low density lipoprotein (VLDL), the secretion of which was substantially higher under these circumstances (1.46 +/- 0.39 versus 0.34 +/- 0.06 microgram apoB per milligram cell protein per 24 hours, P < .05). The increase in VLDL assembly was accompanied by a selective 2.5-fold increase (P < .05) in the specific recruitment of apoB-48. There was no significant increase in the amount of apoB-100, which appeared in the VLDL fraction when cells were enriched with TAG. Under these circumstances there was an increase in net cellular synthesis of apoB-48 (5524 +/- 667 versus 2505 +/- 598 disintegrations per minute per milligram protein per hour, P < .05). The net cellular synthesis of apoB-100 was unchanged compared with that observed in control cell cultures (1548 +/- 237 versus 2000 +/- 897 dpm/ mg per hour, respectively). A large proportion of the total secreted apoB was associated with small particles of density higher than VLDL, even when VLDL output was maximally stimulated, suggesting that apoB was oversecreted and in excess of the cells' requirement to transport TAG.

Brefeldin A reversibly inhibits the assembly of apoB containing lipoproteins in McA-RH7777 cells

BFA inhibited in a dose dependent way the assembly of apoB-48 very low density lipoprotein (VLDL) but allowed a normal rate of biosynthesis of the apolipoprotein and of the assembly of the dense ("high density lipoprotein (HDL)-like") apoB-48 particle (apoB-48 HDL). The inhibition of the assembly of apoB-48 VLDL occurred at BFA levels that allowed a major secretion of both transferrin and apoB-48 HDL. The assembly of apoB-100 containing lipoproteins was also inhibited by BFA but could be reactivated by a 30-60 min chase in the absence of BFA, which agreed with the time that was estimated to be needed to restore the secretory pathway (approximately 60 min). Also the assembly of apoB-48 VLDL was reversible. Both apoB-48 and apoB-100 that was labeled in the presence of BFA assembled VLDL after removal of the BFA. Both apoB-100 and apoB-48 were associated with the membrane pellet of the microsomes. Virtually all (122 +/- 30%) of the membrane associated pulse-labeled apoB-48 remained in the membrane after a 180-min chase in the presence of BFA, compared to only 21 +/- 2% in normal cells (mean +/- S.D., n = 4). The corresponding figures for apoB-100 was 40 +/- 7% in BFA-treated cells and 9 +/- 7% in normal cells (mean +/- S.D., n = 4). Pulse-chase experiments with BFA offered conditions to selectively follow the turnover of membrane-associated apoB-100. Such experiments indicated that this apoB-100 pool is a precursor to VLDL.

Degradation of apolipoprotein B in cultured rat hepatocytes occurs in a post-endoplasmic reticulum compartment

The site of apolipoprotein B (apoB) degradation was investigated in cultured rat hepatocytes. Brefeldin A plus nocodazole completely blocked apoB degradation suggesting the involvement of a post-endoplasmic reticulum (ER) compartment. Monensin inhibited apoB degradation by 40% implying that a post-Golgi compartment could be involved in degradation of apoB. Ammonium chloride or chloroquine inhibited partially the degradation of apoB100 and apoB48, indicating some degradation in lysosomes, or in an acidic compartment such as trans-Golgi or endosomes. The degradations of apoB100 and apoB48 were blocked completely by (2S,3S)-trans-epoxysuccinyl-L-leucylamido-3-methylbutane ethyl ester (EST) during a chase of 90 min demonstrating that a cysteine protease was responsible for apoB degradation. Chymostatin, leupeptin, pepstatin, phenylmethylsulfonyl fluoride, and aprotinin had no significant effect on the degradation of apoB48. However, leupeptin and pepstatin decreased the degradation of apoB100 by 20-30%. Degradation of apoB100 and apoB48 occurred in isolated Golgi fractions with little degradation in heavy or light ER. Degradation of apoB in Golgi fractions was inhibited by EST and by preincubating hepatocytes with 10 nM dexamethasone. Immunofluorescent microscopy revealed that apoB accumulated in the Golgi region after EST treatment. It is concluded that a major part of apoB degradation in rat hepatocytes occurs in a post-ER compartment via the action of a cysteine protease that is regulated by glucocorticoids.