Degradation of newly synthesized apolipoprotein B-100 in a pre-Golgi compartment

Generation of new hepatocyte-like in vitro models better resembling human lipid metabolism

In contrast to human hepatocytes in vivo, which solely express acyl-coenzyme A:cholesterol acyltransferase (ACAT) 2, both ACAT1 and ACAT2 (encoded by SOAT1 and SOAT2) are expressed in primary human hepatocytes and in human hepatoma cell lines. Here, we aimed to create hepatocyte-like cells expressing the ACAT2, but not the ACAT1, protein to generate a model that - at least in this regard - resembles the human condition in vivo and to assess the effects on lipid metabolism. Using the Clustered Regularly Interspaced Short Palindromic Repeats technology, we knocked out SOAT1 in HepG2 and Huh7.5 cells. The wild type and SOAT2-only-cells were cultured with fetal bovine or human serum and the effects on lipoprotein and lipid metabolism were studied. In SOAT2-only-HepG2 cells, increased levels of cholesterol, triglycerides, apolipoprotein B and lipoprotein(a) in the cell media were detected; this was likely dependent of the increased expression of key genes involved in lipid metabolism (e.g. MTP, APOB, HMGCR, LDLR, ACACA, and DGAT2). Opposite effects were observed in SOAT2-only-Huh7.5 cells. Our study shows that the expression of SOAT1 in hepatocyte-like cells contributes to the distorted phenotype observed in HepG2 and Huh7.5 cells. As not only parameters of lipoprotein and lipid metabolism but also some markers of differentiation/maturation increase in the SOAT2-only-HepG2 cells cultured with HS, this cellular model represent an improved model for studies of lipid metabolism.

Relationship between Amphipathic β Structures in the β1 Domain of Apolipoprotein B and the Properties of the Secreted Lipoprotein Particles in McA-RH7777 Cells

Our previous studies demonstrated that the first 1000 amino acid residues (the βα₁ domain) of human apolipoprotein (apo) B-100, termed apoB:1000, are required for the initiation of lipoprotein assembly and the formation of a monodisperse stable phospholipid (PL)-rich particle. The objectives of this study were (a) to assess the effects on the properties of apoB truncates undergoing sequential inclusion of the amphipathic β strands in the 700 N-terminal residues of the β₁ domain of apoB-100 and (b) to identify the subdomain in the β₁ domain that is required for the formation of a microsomal triglyceride transfer protein (MTP)-dependent triacylglycerol (TAG)-rich apoB-containing particle. Characterization of particles secreted by stable transformants of McA-RH7777 cells demonstrated the following. (1) The presence of amphipathic β strands in the 200 N-terminal residues of the β₁ domain resulted in the secretion of apoB truncates (apoB:1050 to apoB:1200) as both lipidated and lipid-poor particles. (2) Inclusion of residues 300-700 of the β₁ domain led to the secretion of apoB:1300, apoB:1400, apoB:1500, and apoB:1700 predominantly as lipidated particles. (3) Particles containing residues 1050-1500 were all rich in PL. (4) There was a marked increase in the lipid loading capacity and TAG content of apoB:1700-containing particles. (5) Only the level of secretion of apoB:1700 was markedly diminished by MTP inhibitor BMS-197636. These results suggest that apoB:1700 marks the threshold for the formation of a TAG-rich particle and support the concept that MTP participates in apoB assembly and secretion at the stage where particles undergo a transition from PL-rich to TAG-rich.

Membrane Protein Quantity Control at the Endoplasmic Reticulum

The canonical function of the endoplasmic reticulum-associated degradation (ERAD) system is to enforce quality control among membrane-associated proteins by targeting misfolded secreted, intra-organellar, and intramembrane proteins for degradation. However, increasing evidence suggests that ERAD additionally functions in maintaining appropriate levels of a subset of membrane-associated proteins. In this 'quantity control' capacity, ERAD responds to environmental cues to regulate the proteasomal degradation of specific ERAD substrates according to cellular need. In this review, we discuss in detail seven proteins that are targeted by the ERAD quantity control system. Not surprisingly, ERAD-mediated protein degradation is a key regulatory feature of a variety of ER-resident proteins, including HMG-CoA reductase, cytochrome P450 3A4, IP3 receptor, and type II iodothyronine deiodinase. In addition, the ERAD quantity control system plays roles in maintaining the proper stoichiometry of multi-protein complexes by mediating the degradation of components that are produced in excess of the limiting subunit. Perhaps somewhat unexpectedly, recent evidence suggests that the ERAD quantity control system also contributes to the regulation of plasma membrane-localized signaling receptors, including the ErbB3 receptor tyrosine kinase and the GABA neurotransmitter receptors. For these substrates, a proportion of the newly synthesized yet properly folded receptors are diverted for degradation at the ER, and are unable to traffic to the plasma membrane. Given that receptor abundance or concentration within the plasma membrane plays key roles in determining signaling efficiency, these observations may point to a novel mechanism for modulating receptor-mediated cellular signaling.

Apolipoprotein B Secretory Regulation by Degradation

In this short review, we discuss apolipoprotein B100 and the assembly of very low-density lipoproteins. In particular, we address the nature and importance of co- and posttranslational degradation of apolipoprotein B100 during the assembly process. We also provide a short historical background to the development of the current model for the degradation of apolipoprotein B100.

The Gp78 ubiquitin ligase: probing endoplasmic reticulum complexity

The endoplasmic reticulum (ER) has been classically divided, based on electron microscopy analysis, into parallel ribosome-studded rough ER sheets and a tubular smooth ER network. Recent studies have identified molecular constituents of the ER, the reticulons and DP1, that drive ER tubule formation and whose expression determines expression of ER sheets and tubules and thereby rough and smooth ER. However, segregation of the ER into only two domains remains simplistic and multiple functionally distinct ER domains necessarily exist. In this review, we will discuss the sub-organization of the ER in different domains focusing on the localization and role of the gp78 ubiquitin ligase in the mitochondria-associated smooth ER and on the evidence for a quality control ERAD domain.

Induction of liver steatosis and lipid droplet formation in ATF6alpha-knockout mice burdened with pharmacological endoplasmic reticulum stress

We burdened mice with intraperitoneal injection of the endoplasmic reticulum stress-inducing reagent tunicamycin, and found that wild-type mice were able to recover from the insult, whereas ATF6α-knockout mice exhibited liver dysfunction and steatosis. Our results establish links between endoplasmic reticulum stress, lipid metabolism and steatosis

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) activates homeostatic responses collectively termed the unfolded protein response. Among the three principal signaling pathways operating in mammals, activating transcription factor (ATF)6α plays a pivotal role in transcriptional induction of ER-localized molecular chaperones and folding enzymes as well as components of ER-associated degradation, and thereby mouse embryonic fibroblasts deficient in ATF6α are sensitive to ER stress. However, ATF6α-knockout mice show no apparent phenotype under normal growing conditions. In this report, we burdened mice with intraperitoneal injection of the ER stress-inducing reagent tunicamycin and found that wild-type mice were able to recover from the insult, whereas ATF6α-knockout mice exhibited liver dysfunction and steatosis. Thus, ATF6α-knockout mice accumulated neutral lipids in the liver such as triacylglycerol and cholesterol, which was ascribable to blockage of β-oxidation of fatty acids caused by decreased mRNA levels of the enzymes involved in the process, suppression of very-low-density lipoprotein formation due to destabilized apolipoprotein B-100, and stimulation of lipid droplet formation resulting from transcriptional induction of adipose differentiation-related protein. Accordingly, the hepatocytes of tunicamycin-injected knockout mice were filled with many lipid droplets. These results establish links among ER stress, lipid metabolism, and steatosis.

Glucosamine-induced endoplasmic reticulum stress attenuates apolipoprotein B100 synthesis via PERK signaling

Glucosamine impairs hepatic apolipoprotein B100 (apoB100) production by inducing endoplasmic reticulum (ER) stress and enhancing cotranslational and posttranslational apoB100 degradation (Qiu, W., R. K. Avramoglu, A. C. Rutledge, J. Tsai, and K. Adeli. Mechanisms of glucosamine-induced suppression of the hepatic assembly and secretion of apolipoprotein B-100-containing lipoproteins. J. Lipid Res. 2006. 47: 1749-1761). Here, we report that glucosamine also regulates apoB100 protein synthesis via ER-stress-induced PERK activation. Short-term (4 h) glucosamine treatment of HepG2 cells reduced both cellular (by 62%) and secreted apoB100 (by 43%) without altering apoB100 mRNA. Treatment with proteasomal inhibitors only partially prevented the suppressive effects of glucosamine, suggesting that mechanisms other than proteasomal degradation may also be involved. Glucosamine-induced ER stress was associated with a significantly reduced apoB100 synthesis with no significant change in posttranslational decay rates, suggesting that glucosamine exerted its effect early during apoB biosynthesis. The role of PERK and its substrate, alpha-subunit of eukaryotic initiation factor 2 (eIF2alpha), in the suppressive effects of glucosamine on apoB synthesis was then investigated. Coexpression of apoB15 (normally resistant to intracellular degradation) with wild-type double stranded (ds) RNA activated protein kinase (PKR)-like endoplasmic reticulum kinase (PERK) in COS-7 cells resulted in a dramatic reduction in the levels of newly synthesized apoB15. Interestingly, cotransfection with apoB15 and a kinase inactive PERK mutant (K618A) increased apoB15 expression. In addition, short-term glucosamine treatment stimulated an increase in phosphorylation of PERK and eIF2alpha. Taken together, these data suggest that in addition to the induction of ER-associated degradation and other degradative pathways, ER stress is associated with suppression of apoB synthesis via a PERK-dependent mechanism.

Interfacial properties of a complex multi-domain 490 amino acid peptide derived from apolipoprotein B (residues 292-782)

ApolipoproteinB (ApoB) is a lipid binding protein that is a nonexchangeable component of chylomicrons, VLDL, and LDL. In the liver and intestinal cells ApoB recruits lipid to form nascent triacylglycerol rich particles cotranslationally in the endoplasmic reticulum membrane which are then processed and secreted to form plasma lipoproteins. The N-terminal domain, which comprises the first 22% of apoB, recruits lipid in a controlled manner. The first 6% (residues 1-291) of the N-terminus does not bind lipid. The first lipid binding domain, including residues 292-782 (B6-17), forms a lipid binding pocket which is predicted to consist of 17 alpha-helices and 6 beta-strands. A structural model based on the X-ray structure of the homologues protein lipovitellin suggests that the N-terminal 6-8 helices and the beta-sheet interact with lipid while the C-terminal helices form a structural unit stabilizing the beta-sheet. Using isothermal drop tensiometry we showed that ApoB6.4-17 is surface active and binds to a triolein/water interface and exerts 16-19 mN/m of pressure (Pi) on that surface. The protein initially adsorbs slowly from aqueous solution to the surface but following compression and re-expansion it reaches equilibrium much faster. When Pi exceeds 16.9 mN/m part of the protein is ejected from the surface, but when compressed to high Pi the protein is never completely ejected indicating that part of the peptide is irreversibly anchored to the interface. The surface dilation modulus (epsilon) varies between 25-38 mN/m, and is predominantly elastic with a small viscous component. When compressed at an air/water interface ApoB6.4-17 has a limiting area of approximately 11 A2 per amino acid at lift off and only approximately 7 A2 per amino acid at the collapse Pi (28 mN/m). These values are about half the anticipated values if all the residues are at the surface. This suggests that ApoB6.4-17 retains some globular structure at an interface and does not completely denature at the surface, as many other globular proteins do. We suggest that while bound to the surface ApoB6.4-17 exhibits properties of both alpha and beta structure giving it unique and versatile characteristics at a hydrophobic interface.

Monogenic Hypocholesterolaemic Lipid Disorders and Apolipoprotein B Metabolism

The study of apolipoprotein (apo) B metabolism is central to our understanding of human lipoprotein metabolism. Moreover, the assembly and secretion of apoB-containing lipoproteins is a complex process. Increased plasma concentrations of apoB-containing lipoproteins are an important risk factor for the development of atherosclerotic coronary heart disease. In contrast, decreased levels of, but not the absence of, these apoB-containing lipoproteins is associated with resistance to atherosclerosis and potential long life. The study of inherited monogenic dyslipidaemias has been an effective means to elucidate key metabolic steps and biologically relevant mechanisms. Naturally occurring gene mutations in affected families have been useful in identifying important domains of apoB and microsomal triglyceride transfer protein (MTP) governing the metabolism of apoB-containing lipoproteins. Truncation-causing mutations in the APOB gene cause familial hypobetalipoproteinaemia, whereas mutations in MTP result in abetalipoproteinaemia; both rare conditions are characterised by marked hypocholesterolaemia. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the key biochemical, clinical, metabolic and genetic features of the monogenic hypocholesterolaemic lipid disorders affecting apoB metabolism.

Regulation of ApoB secretion by the low density lipoprotein receptor requires exit from the endoplasmic reticulum and interaction with ApoE or ApoB

Apolipoprotein B (apoB) is required for the hepatic assembly and secretion of very low density lipoprotein (VLDL). The LDL receptor (LDLR) promotes post-translational degradation of apoB and thereby reduces VLDL particle secretion. We investigated the trafficking pathways and ligand requirements for the LDLR to promote degradation of apoB. We first tested whether the LDLR drives apoB degradation in an endoplasmic reticulum (ER)-associated pathway. Primary mouse hepatocytes harboring an ethyl-nitrosourea-induced, ER-retained mutant LDLR secreted comparable levels of apoB with LDLR-null hepatocytes, despite reduced secretion from cells expressing the wild-type LDLR. Additionally, treatment of cells with brefeldin A inhibited LDLR-dependent degradation. However, this rescue was reversible, and degradation of apoB occurred upon removal of brefeldin A. To characterize the lipoprotein reuptake pathway of degradation, we employed an LDLR mutant defective in constitutive endocytosis and internalization of apoB. This mutant was as effective in reducing apoB secretion as the wild-type LDLR. However, the effect was dependent on apolipoprotein E (apoE) as only the wild-type LDLR, and not the endocytic mutant, reduced apoB secretion in apoE-null cells. Treatment with heparin rescued a pool of apoB in cells expressing the endocytic mutant, indicating that reuptake of VLDL via apoE still occurs with this mutant. Finally, an LDLR mutant defective in binding apoB but not apoE reduced apoB secretion in an apoE-dependent manner. Together, these data suggest that the LDLR directs apoB to degradation in a post-ER compartment. Furthermore, the reuptake mechanism of degradation occurs via internalization of apoB through a constitutive endocytic pathway and apoE through a ligand-dependent pathway.

Glucosamine-induced endoplasmic reticulum stress promotes ApoB100 degradation: evidence for Grp78-mediated targeting to proteasomal degradation

OBJECTIVE

To investigate the role of glucosamine-mediated endoplasmic reticulum (ER) stress and Grp78 (BiP) in the intracellular degradation of apolipoprotein B100 (apoB100) in cultured hepatocytes.

METHODS AND RESULTS

Glucosamine treatment (2.5 to 10 mmol/L) of HepG2 cells increased levels of the ER chaperones, 78-kDa glucose-regulated protein (Grp78) and Grp94, in a dose-dependent manner and led to significant decreases in both cellular and secreted apoB100 by up to 97% (P<0.01). In contrast, no changes were observed in ER resident (ER60, PTP-1B) or secretory (albumin, apoE) control proteins. Glucosamine-induced apoB degradation was similarly observed in primary hamster hepatocytes and McA-RH7777 cells. Glucosamine treatment led to reduced tranlocational efficiency of apoB100 in the ER and enhanced its ubiquitination and proteasomal degradation. Adenoviral overexpression of Grp78 also led to significantly decreased levels of newly synthesized apoB100 in a dose-dependent manner (P<0.01). Grp78-induced downregulation of apoB100 was sensitive to inhibition by the proteasome inhibitor, lactacystin, but not lysosomal protease inhibitors, E64 and leupeptin, suggesting that overexpression of Grp78 selectively induced proteasomal degradation of apoB100.

CONCLUSIONS

These findings suggest that binding and retention by Grp78 may play a critical role in proteasomal targeting and the ER quality-control of misfolded apoB. Interaction with core lipoprotein lipids may facilitate apoB transport out of the ER by reducing Grp78-mediated ER retention.

Bile acid reduces the secretion of very low density lipoprotein by repressing microsomal triglyceride transfer protein gene expression mediated by hepatocyte nuclear factor-4

Microsomal triglyceride transfer protein (MTP) is involved in the transfer of triglycerides, cholesterol esters, and phospholipids to newly synthesized apolipoprotein (apo) B. It is therefore essential for lipoprotein synthesis and secretion in the liver and the small intestine. Although several recent experiments have revealed the transcriptional regulation of the MTP gene, little has been revealed to date about hepatocyte nuclear factor-4 (HNF-4)-dependent regulation. We here report that the human MTP gene promoter contains a pair of functional responsive elements for HNF-4 and HNF-1, the latter of which is another target gene of HNF-4. Chromatin immunoprecipitation assays provide evidence that endogenous HNF-4 and HNF-1 can bind these elements in chromatin. In Hep G2 cells overexpression of either a dominant negative form of HNF-4 or small interfering RNAs (siRNAs) against HNF-4 dramatically reduces the activities of both the wild type and the HNF-4 site mutant MTP promoter. This suggests that HNF-4 regulates MTP gene expression either directly or indirectly through elevated HNF-1 levels. When Hep G2 cells were cultured with chenodeoxycholic acid (CDCA), a ligand for the farnesoid X receptor (FXR), mRNA levels for MTP and apo B were reduced because of increased expression of the factor small heterodimer partner (SHP), which factor suppresses HNF-4 activities. Chenodeoxycholic acid, but not a synthetic FXR ligand, attenuated expression of HNF-4, bringing about a further suppression of MTP gene expression. Over time the intracellular MTP protein levels and apo B secretion in the culture medium significantly declined. These results indicate that two nuclear receptors, HNF-4 and FXR, are closely involved in MTP gene expression, and the results provide evidence for a novel interaction between bile acids and lipoprotein metabolism.

Inhibition of apolipoprotein B secretion by taurocholate is controlled by the N-terminal end of the protein in rat hepatoma McArdle-RH7777 cells

Bile salts (BS) inhibit the secretion of apolipoprotein B (apoB) and triacylglycerol (TG) in primary rat, mouse and human hepatocytes and in mice in vivo. We investigated whether lipidation of apoB into a lipoprotein particle is required for this inhibitory action of BS. The sodium/taurocholate co-transporting polypeptide (Ntcp) was co-expressed in McArdle-RH7777 (McA-RH7777) cells stably expressing the full-length human apoB100 (h-apoB100, secreted as TG-rich lipoprotein particles) or carboxyl-truncated human apoB18 (h-apoB18, secreted in lipid-free form). The doubly transfected cell lines (h-apoB/r-Ntcp) effectively accumulated taurocholic acid (TC). TC incubation decreased the secretion of endogenous rat apoB100 (-50%) and h-apoB18 (-35%), but did not affect secretion of rat apoA-I. Pulse-chase experiments (35S-methionine) indicated that the impaired secretion of radiolabeled h-apoB18 and h-apoB100 was associated with accelerated intracellular degradation. The calpain protease inhibitor N-acetyl-leucyl-leucyl-norleucinal (ALLN) partially inhibited intracellular apoB degradation but did not affect the amount of either h-apoB18 or h-apoB100 secreted into the medium, indicating that inhibition of apoB secretion by TC is not due to calpain-dependent proteasomal degradation. We conclude that TC does not inhibit apoB secretion by interference with its lipidation, but rather involves a mechanism dependent on the N-terminal end of apoB.

Ferritins can regulate the secretion of apolipoprotein B

Apolipoprotein B is secreted with atherogenic lipids as lipoprotein particles from hepatocytes. Regulation of the secretion of apolipoprotein B is largely post-translational and reflects the balance between processes that leads to particle assembly or to intracellular degradation. Previously, we conducted a proteomic screen to find proteins that bind apolipoprotein B in rat liver microsomes. We identified ferritin heavy and light chains in this screen among other proteins and showed that the two ferritins bind apolipoprotein B directly in vitro. In hepatocytes and other cells, ferritin heavy and light chains form cytosolic cages that store iron. We now show that ferritin heavy or light chains post-translationally inhibit the secretion of apolipoprotein B without altering the export of other hepatic proteins including albumin, factor XIII, and apolipoprotein A-I. This inhibition of apolipoprotein B secretion is not due to diminished lipid synthesis and can be partially overcome by stimulating triglyceride synthesis. The block in apolipoprotein B secretion by ferritins leads to an increase in endoplasmic reticulum-associated degradation of the apolipoprotein. Thus, despite being cytosolic proteins without known chaperone activity, ferritins can specifically regulate the secretion of apolipoprotein B post-translationally. The metabolic pathways for iron storage and intercellular cholesterol and triglyceride transport could intersect.

Ubiquitin-dependent and -independent proteasomal degradation of apoB associated with endoplasmic reticulum and Golgi apparatus, respectively, in HepG2 cells

Studies in hepatocyte cultures indicate that apolipoprotein (apo) B-100 production is regulated largely by intracellular degradation and the proteasome pathway is a major mechanism for the degradation. In the present study, we have examined the detailed itinerary of apoB degradation through its secretory pathway in HepG2 cells. We found that ubiquitin-dependent proteasomal degradation of apoB largely occurred on the cytosolic surface of rough and smooth endoplasmic reticulum (ER) and that a small proportion of apoB was dislodged from the secretory organelles into the cytosolic compartment where it underwent ubiquitination for proteasomal degradation. The transmembrane conformation of apoB persisted as the protein was transported through the Golgi apparatus. We further demonstrated that proteasomal degradation of apoB was associated the Golgi apparatus but Golgi-associated apoB was not ubiquitinated, indicating an ubiquitin-independent proteasomal degradation of apoB is associated with this organelle. We conclude that apoB undergoes proteasomal degradation while going through different compartments of the secretory pathway; further, ER-associated proteasomal degradation of apoB in the ER is ubiquitin-dependent whereas that occurring in the Golgi is ubiquitin-independent.

Transmembrane lipid transfer is crucial for providing neutral lipids during very low density lipoprotein assembly in endoplasmic reticulum

Very low density lipoprotein (VLDL), a large particle containing apolipoprotein B (apoB) and large amounts of neutral lipids, is formed in the luminal space within the endoplasmic reticulum (ER) of hepatic cells. The assembly mechanism of VLDL particles is a tightly regulated process where apoB, associated with an insufficient amount of lipids, is selectively degraded intracellularly. In this study we found that treatment of HuH-7 human hepatoma cells with verapamil inhibited secretion of apoB-containing lipoprotein particles through increasing degradation of apoB. Addition of N-acetylleucyl-leucyl-norleucinal, an inhibitor of proteasome and other cysteinyl proteases that are responsible for apoB degradation, restored apoB recovery from verapamil-treated cells. De novo synthesis of lipids from [14C]acetate was increased in the presence of verapamil, suggesting that verapamil decreases lipid availability for apoB thus leading to the secretion of apoB-containing lipoprotein. We prepared cytosolic fractions from cells preincubated with [14C]acetate and used as a donor of radioactive lipids. When this cytosolic fraction was incubated with microsomes isolated separately, radioactive triglyceride (TG) accumulated in the luminal space of the microsomes. The transfer of radioactive TG from the cytosolic fraction to the microsomal lumen was inhibited in the presence of verapamil, suggesting that there is a verapamil-sensitive mechanism for TG transfer across ER membranes that is involved in formation of apoB-containing lipoprotein particles in ER. Verapamil showed no inhibitory effect on microsomal TG transfer protein, a well known lipid transfer protein in ER. We propose from these results that there is novel machinery for transmembrane movement of neutral lipids, which is involved in providing TG for apoB during VLDL assembly in ER.

Intracellular accumulation of antithrombin Morioka (C95R), a novel mutation causing type I antithrombin deficiency

Antithrombin (AT) is a major plasma protease inhibitor with three intramolecular disulfide bonds, and its deficiency is associated with increased venous thrombosis. Recently, we found a novel missense mutation named AT Morioka (C95R), which causes the loss of one of the three disulfide bonds. In this study, we prepared Chinese hamster ovary cells stably overexpressing wild type or mutant AT and examined the intracellular fate of the ATs. In pulse-chase experiments, newly synthesized wild type AT was secreted into the medium with a half-life of approximately 1.5 h. In contrast, most of the mutant type AT was not secreted during the chase period of 9 h and, surprisingly, was not degraded in the cells. The kinetics of the secretion suggests that the mutant was secreted about 50 times more slowly into the medium. Most of the mutant AT in the cells had high mannose type oligosaccharides, suggesting that it was retained in the endoplasmic reticulum (ER). In addition, half of the mutant AT existed in a dimeric form with an intermolecular disulfide bond. On immunoelectron microscopy, the mutant AT was found to have accumulated in variously sized structures surrounded by a single membrane in the cytoplasm. Immunogold particles exhibiting calnexin immunoreactivity were detected on the membranes. Ribosomes were attached to some of the small structures that had accumulated the mutant AT. Further, we prepared Chinese hamster ovary cells stably overexpressing another mutant AT in which two cysteine residues at 21 and 95, responsible for disulfide bond formation, were substituted for arginines. In pulse-chase experiments, the mutant AT (C21C,C95R) was secreted faster than that of AT Morioka (C95R) into the medium. These results suggest that AT Morioka remained for a long time in ER without being degraded and accumulated in newly formed membrane structures derived from the ER. The dimerization of AT Morioka (C95R) through Cys-21 seems to be critical for its intracellular accumulation.

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.

The triple threat to nascent apolipoprotein B. Evidence for multiple, distinct degradative pathways

We previously showed that Omega-3 fatty acids reduce secretion of apolipoprotein B (apoB) from cultured hepatocytes by stimulating post-translational degradation. In this report, we now characterize this process, particularly in regard to the two known processes that degrade newly synthesized apoB, endoplasmic reticulum (ER)-associated degradation and re-uptake from the cell surface. First, we found that Omega-3-induced degradation preferentially reduces the secretion of large, assembled apoB-lipoprotein particles, and apoB polypeptide length is not a determinant. Second, based on several experimental approaches, ER-associated degradation is not involved. Third, re-uptake, the only process known to destroy fully assembled nascent lipoproteins, was clearly active in primary hepatocytes, but Omega-3-induced degradation of apoB continued even when re-uptake was blocked. Cell fractionation showed that Omega-3 fatty acids induced a striking loss of apoB100 from the Golgi, while sparing apoB100 in the ER, indicating a post-ER process. To determine the signaling involved, we used wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, which blocked most, if not all, of the Omega-3 fatty acid effect. Therefore, nascent apoB is subject to ER-associated degradation, re-uptake, and a third distinct degradative pathway that appears to target lipoproteins after considerable assembly and involves a post-ER compartment and PI3K signaling. Physiologic, pathophysiologic, and pharmacologic regulation of net apoB secretion may involve alterations in any of these three degradative steps.

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.

Apolipoprotein B, a paradigm for proteins regulated by intracellular degradation, does not undergo intracellular degradation in CaCo2 cells

Studies in different liver-derived cells in culture indicate that apolipoprotein (apo) B-100 production is regulated largely by intracellular degradation and the ubiquitin-proteasome pathway is a major mechanism for the degradation. The proteasomal degradation of apoB-100 was postulated to be an intrinsic property of the protein that occurs even in the presence of optimal amounts of lipids supplied to the cell. We examined apoB-100 and apoB-48 biogenesis in CaCo2, a human colon carcinoma cell line. To our surprise, apoB-100 and apoB-48 were quantitatively secreted by CaCo2 cells; essentially none of the newly synthesized apoB was degraded before secretion in a 2-h period whether the cells were cultured on filter or on plastic. Furthermore, although ubiquitin immunoreactivity was readily detected in the intracellular apoB isolated from HepG2 cells, little or no ubiquitin was detectable in the intracellular apoB from CaCo2 cells. The amounts of free ubiquitin and total and non-apoB ubiquitinated proteins were comparable in HepG2 and CaCo2 cells, indicating that CaCo2 cells have the necessary machinery for tagging ubiquitin chains onto cellular proteins for proteasomal degradation. Incubation in lipoprotein-deficient serum did not induce apoB degradation, but the addition of a microsomal triglyceride transfer protein inhibitor led to apoB degradation in CaCo2 cells. Finally, similar proportions of apoB polypeptide in isolated microsomes from CaCo2 and HepG2 cells were accessible to exogenously added trypsin, indicating that the mere exposure of apoB nascent chains to the cytosolic compartment is insufficient to cause the proteasomal degradation. Therefore, the intracellular degradation of apoB is not an intrinsic property of the protein, and the phenomenon is neither universal nor inevitable. The unconditional use of apoB as a paradigm for intracellular protein degradation is not warranted.

Sterol regulatory element-binding protein negatively regulates microsomal triglyceride transfer protein gene transcription

We herein report that mRNA expression of microsomal triglyceride transfer protein (MTP) and its protein synthesis decline in response to sterol depletion in HepG2 cells, and we functionally characterized the MTP gene promoter in an effort to investigate the molecular mechanisms by which MTP gene transcription is regulated. Luciferase assays using truncated versions of the reporter gene revealed that the region at -124 to +33 base pairs of the human promoter contains the elements required for the suppression of transcription by sterol depletion. Enforced expression of an active form of sterol regulatory element-binding protein (SREBP)-1 (amino acids 1-487) or -2 (amino acids 1-481), both of which are activated under sterol-depleted conditions, is able to mimic sterol-mediated down-regulation. Either further truncation of the promoter region or mutation of the putative SREBP-binding sequence (5'-GCAGCCCAC-3', -124 to -116 base pairs) abolishes the sterol- and SREBP-dependent transcriptional regulation. Gel mobility shift assay showed that recombinant SREBP-2-(1-481) is able to bind the sequence. Enforced expression of a truncated form of SREBP-2 (amino acids 31-481), which acts as an inhibitor of transcription of the low density lipoprotein receptor gene because it lacks the transcriptional activation domain, also diminishes the luciferase activity, suggesting that direct binding to the promoter region might be sufficient and that the mechanism by which SREBPs inhibit MTP gene expression is distinct from that for the transcriptional stimulation of sterol-regulated genes. Although the SREBP-binding site overlaps a negative insulin-responsive element, insulin negatively regulates MTP gene expression even when the amount of the active form of SREBPs is quite low under the sterol-loaded conditions, indicating that SREBPs only slightly mediate, if at all, the insulin effects. Overall, we conclude that SREBPs are responsible for regulation of lipoprotein secretion via their control of MTP gene expression. Moreover, our results describe for the first time a novel mechanism by which SREBPs negatively regulate expression of the gene encoding the protein involved in lipid metabolism.

Cell and molecular biology of the assembly and secretion of apolipoprotein B-containing lipoproteins by the liver

Triglycerides are one of the most efficient storage forms of free energy. Because of their insolubility in biological fluids, their transport between cells and tissues requires that they be assembled into lipoprotein particles. Genetic disruption of the lipoprotein assembly/secretion pathway leads to several human disorders associated with malnutrition and developmental abnormalities. In contrast, patients displaying inappropriately high rates of lipoprotein production display increased risk for the development of atherosclerotic cardiovascular disease. Insights provided by diverse experimental approaches describe an elegant biological adaptation of basic chemical interactions required to overcome the thermodynamic dilemma of producing a stable emulsion vehicle for the transport and tissue targeting of triglycerides. The mammalian lipoprotein assembly/secretion pathway shows an absolute requirement for: (1) the unique amphipathic protein: apolipoprotein B, in a form that is sufficiently large to assemble a lipoprotein particle containing a neutral lipid core; and, (2) a lipid transfer protein (microsomal triglyceride transfer protein-MTP). In the endoplasmic reticulum apolipoprotein B has two distinct metabolic fates: (1) entrance into the lipoprotein assembly pathway within the lumen of the endoplasmic reticulum; or, (2) degradation in the cytoplasm by the ubiquitin-dependent proteasome. The destiny of apolipoprotein B is determined by the relative availability of individual lipids and level of expression of MTP. The dynamically varied expression of cholesterol-7alpha-hydroxylase indirectly influences the rate of lipid biosynthesis and the assembly and secretion lipoprotein particles by the liver.

Cytochrome P450 3A degradation in isolated rat hepatocytes: 26S proteasome inhibitors as probes

Mechanism-based inactivation of liver microsomal cytochromes P450 3A (CYP 3A, P450s 3A) in vivo and/or in vitro, via heme modification of the protein, results in accelerated proteolytic degradation of the enzyme that is preceded by the ubiquitination of the protein, thereby implicating the ubiquitin-ATP-dependent 26S proteasomal system. In this study, this involvement is confirmed with the use of the proteasomal inhibitors aclarubicin and MG-132 as probes, in isolated rat hepatocytes treated with the P450 3A mechanism-based inactivator, 3,5-dicarbethoxy-2,6-dimethyl-4-ethyl-1, 4-dihydropyridine (DDEP). In addition, the findings reveal that during the course of this proteolysis, the endoplasmic reticulum (ER)-anchored DDEP-inactivated P450 3A is translocated from the ER to the cytosol in a brefeldin A-insensitive manner.

Niacin accelerates intracellular ApoB degradation by inhibiting triacylglycerol synthesis in human hepatoblastoma (HepG2) cells

The mechanism by which the potent drug niacin decreases apoB-containing atherogenic lipoproteins and prevents coronary disease is unclear. Utilizing human hepatoblastoma (HepG2) cells as an in vitro model, we have examined the effect of niacin on intracellular degradation of apoB and the regulatory mechanisms involved in apoB processing. Niacin significantly increased apoB degradation in a dose- and time-dependent manner. Treatment of HepG2 cells with calpain inhibitor I [N-acetyl-leucyl-leucyl-norleucinal (ALLN), an inhibitor of certain protease-mediated apoB degradation], did not alter niacin-induced apoB degradation. Niacin decreased inhibition of oleate-mediated apoB degradation. Niacin dose-dependently inhibited the synthesis of both fatty acids and triacylglycerol (TG) by 20% to 40% as determined by the incorporation of 14C-acetate and 3H-glycerol into fatty acids and TG, respectively. Incubation of HepG2 cells with niacin significantly inhibited (by 12% to 15%) fatty acid esterification to produce TG as assessed by the incorporation of 3H-oleic acid into TG. 14C-acetate incorporation into cholesterol and phospholipids was unchanged. The activity of microsomal triglyceride transfer protein (MTP), a carrier protein for lipids, was not altered by pretreatment of cells with niacin. ApoB mRNA expression and 125I-LDL protein uptake were also unchanged. These data indicate that niacin accelerates hepatic intracellular post-translational degradation of apoB by selectively reducing triglyceride synthesis (through inhibiting both fatty acid synthesis and fatty acid esterification to produce TG) without affecting ALLN-inhibitable protease- or MTP-mediated intracellular apoB processing, resulting in decreased apoB secretion and hence lower circulating levels of the atherogenic lipoproteins.

Role of the enterohepatic circulation of bile salts in lipoprotein metabolism

The enterohepatic circulation of bile salts and cholesterol plays a central role in maintaining whole body cholesterol homeostasis. Hepatic lipoprotein metabolism is reviewed and the role of disturbances in bile salt metabolism in the pathogenesis of dyslipidemias is discussed. Further, the manipulation of bile salt metabolism to treat dyslipidemia is reviewed.

Effects of gamma-tocotrienol on ApoB synthesis, degradation, and secretion in HepG2 cells

gamma-Tocotrienol (gamma-T3), a naturally occurring analog of tocopherol (vitamin E), has been shown to have a hypocholesterolemic effect in animals and humans. Unlike tocopherol, it has also been shown to reduce plasma apoB levels in hypercholesterolemic subjects. The aim of this study was to define the mechanism of action of gamma-T3 on hepatic modulation of apoB production using cultured HepG2 cells as the model system. HepG2 cells preincubated with gamma-T3 were initially shown to inhibit the rate of incorporation of [14C]acetate into cholesterol in a concentration- and time-dependent manner, with a maximum 86+/-3% inhibition at 50 micromol/L observed within 6 hours. gamma-T3, on the other hand, had no significant effect on the uptake of [14C]glycerol into pools of cellular triacylglycerol and phospholipid relative to untreated control. The rate of apoB synthesis and secretion was then studied by an [35S]methionine pulse-labeling experiment and quantified by immunoprecipitating apoB on chasing up to 3 hours. An average reduction of 24+/-3% in labeled apoB in the media was apparent with gamma-T3 despite a 60+/-2% increase in apoB synthesis. Fractionation of secreted apoB revealed a relatively denser lipoprotein particle, suggesting a less stable particle. Using a digitonin-permeabilized HepG2 cell system, the effects of gamma-T3 on apoB translocation and degradation in the endoplasmic reticulum were further investigated. The generation of a specific N-terminal 70-kDa proteolytic fragment proved to be a sensitive measure of the rate of apoB translocation and degradation. The abundance of this fragment increased significantly in gamma-T3-treated cells relative to untreated control cells (50+/-21%) after 2 hours of chase. In addition, the presence of gamma-T3 resulted in an average decrease of 64+/-8% in intact apoB. Taken together, the data suggest that gamma-T3 stimulates apoB degradation possibly as the result of decreased apoB translocation into the endoplasmic reticulum lumen. It is speculated that the lack of cholesterol availability reduces the number of secreted apoB-containing lipoprotein particles by limiting translocation of apoB into the endoplasmic reticulum lumen.

Ubiquitin-proteasome-dependent degradation of apolipoprotein B100 in vitro

Apolipoprotein B100 (apoB) is a large secretory protein that forms very low density lipoprotein in liver. An in vitro degradation assay was developed using rabbit reticulocyte (RR) lysate in order to investigate the mechanism of intracellular degradation of newly synthesized apoB by the ubiquitin-proteasome pathway. [3H]apoB, isolated from [3H]leucine pulsed/chased Hep G2 cells, was degraded 51% when incubated for 2 h at 37 degreesC in an assay mixture that included RR lysate (source of the ubiquitin conjugation system and proteasome) and an exogenous ATP regenerating system. ApoB degradation was ATP-dependent and degradation fragments were not observed suggesting that the very large apoB molecule was extensively degraded. ApoB degradation was decreased to 50% when potent proteasome inhibitors, clasto-lactacystin beta-lactone (10 microM) or MG-132 (50 microM), were added to the reaction mixture, but was not affected by the cysteine protease inhibitor, E-64, or the serine protease inhibitor, phenylmethylsulfonyl fluoride. ApoB degradation was inhibited by the mutant ubiquitin protein K48R and by ubiquitin aldehyde, an inhibitor of ubiquitin-protein isopeptidases. During incubation ubiquitination of apoB increased even as apoB was being degraded. These results suggest that in vitro degradation of apoB, a large secretory protein that is normally found in the endoplasmic reticulum (ER) lumen or associated with the ER membrane, was proteasome-dependent and involved both ubiquitination and deubiquitination steps.

Administration of n-3 fatty acids in the diets of rats or directly to hepatocyte cultures results in different effects on hepatocellular ApoB metabolism and secretion

Hepatocytes derived either from rats fed a diet enriched in n-3 fatty acids or from rats fed a low-fat diet and cultured with an n-3 fatty acid (eicosapentaenoic acid, EPA) in vitro were used to distinguish between the dietary effects and the direct effects of n-3 fatty acids on hepatocellular apolipoprotein (apo) B metabolism and secretion. ApoB-48 and apoB-100 synthesis, degradation, and secretion as large (d<1.006) and small (d>1.006) particles were determined after a pulse label with [35S]methionine. These effects were compared with changes in triacylglycerol (TAG) synthesis and secretion and with changes in de novo fatty acid synthesis (using 3H2O incorporation) under identical conditions. When n-3 fatty acid was given via the dietary route, apoB-48 very low density lipoprotein (VLDL) secretion was inhibited, but there was no effect on the secretion of apoB-100 VLDL. There was no effect on the secretion of either apoB-48 or apoB-100 as small, dense particles (d>1.006). Cellular TAG synthesis was significantly inhibited under these conditions, and fatty acid synthesis de novo was inhibited by 80%. By contrast, after direct addition of EPA to hepatocytes from normal rats, the secretion of both apoB-48 and apoB-100 VLDL was suppressed. The secretion of apoB-48, but not of apoB-100, as dense particles was also inhibited. However, there was little or no effect on TAG synthesis nor on fatty acid synthesis de novo. In addition, whereas dietary administration of n-3 fatty acid gave rise to decreased net synthesis and degradation of apoB-48, direct administration in vitro resulted in increased degradation with no effect on net synthesis. We conclude that the effects of n-3 fatty acids on hepatic lipid and apoB metabolism differ according to whether they are administered in vivo, via the dietary route, or in vitro, via direct addition to hepatocyte cultures.

Intracellular translocation and stability of apolipoprotein B are inversely proportional to the length of the nascent polypeptide

We have studied the relationship between the length of apolipoprotein B (apoB) and its intracellular translocation and stability using McArdle RH7777 (McA-RH7777) cells expressing recombinant human apoB variants, ranging in size from B15 to B100. The translocational status of apoB was assessed based on trypsin sensitivity of apoB using isolated microsomes as well as permeabilized cells. In isolated microsomes, shorter apoB variants (</=B48) were 75-100% resistant to exogenous trypsin digestion, whereas apoB variants larger than B48 were less than 40% trypsin-resistant. Experiments with hepatic microsomes isolated from rat or transgenic mice expressing human B48 and B100 also confirmed the high trypsin accessibility of B100 compared with B48. In permeabilized cells, apoB variants shorter than B48 were relatively resistant to exogenous trypsin (percentage of trypsin-resistant apoB greater than 70%) in contrast to recombinant human B72 and B100, which were only 55 and 42% trypsin-resistant, respectively. The trypsin sensitivity of human B100 was comparable with that of endogenous rat B100 in McA-RH7777 cells as well as endogenous B100 in HepG2 cells (percentages of trypsin-resistant cells were as follows: for human B100 construct, 42 +/- 7.5%; for endogenous McA-RH7777 B100, 52 +/- 2.9%; and for endogenous HepG2 B100, 46 +/- 6.3%). Overall, an inverse correlation between the length of apoB and its resistance to exogenous trypsin was evident irrespective of the model system examined. An inverse relationship was also observed between the size of apoB and its co-translational resistance to proteasomal degradation. Truncated apoB constructs were relatively insensitive to proteasome inhibition by MG132 co-translationally (during the pulse) compared with the full-length B100, which was highly sensitive (apoB recovered in the presence of MG132 as a percentage of control was as follows: B15, 127%; B29, 94%; B48, 110%; B72, 140%; B100, 282%). Post-translationally (over a 2-h chase), a similar inverse relationship was found, with B100 being the least stable in comparison with truncated apoB variants. In summary, as the size of the nascent apoB chain increases, there appears to be a greater cytosolic exposure of the polypeptide, leading to a higher sensitivity to proteasomal degradation.

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.

Calnexin and other factors that alter translocation affect the rapid binding of ubiquitin to apoB in the Sec61 complex

Several secretory proteins, including apolipoprotein B, have been shown to undergo degradation by proteasomes. We found that the rapid degradation of nascent apolipoprotein B in HepG2 cells was diminished but not abolished by the addition of any of three different inhibitors of proteasomes. Ubiquitin is conjugated to apolipoprotein B that is not assembled with sufficient lipids either during or soon after synthesis. In addition, we found that apolipoprotein B that has entered the endoplasmic reticulum sufficiently to become glycosylated can be degraded by proteasomes. Furthermore, we detected ubiquitin-apolipoprotein B that is associated with the Sec61 complex, the major constituent of the translocational channel. Treatment of cells with monomethylethanolamine or dithiothreitol decreased the translocation of apolipoprotein B and increased the proportion of ubiquitin-conjugated molecules associated with Sec61. Conversely, treatment of cells with oleic acid, which increased the proportion of translocated apolipoprotein B, decreased the amount of ubiquitin-apolipoprotein B in the Sec61 complex. Finally, we found that inhibition of the interaction between calnexin and apolipoprotein B decreases the translocation of apolipoprotein B, increases the ubiquitin-apolipoprotein B in the Sec61 complex, and increases the proteasomal degradation of glycosylated apolipoprotein B. Thus, ubiquitin can be attached to unassembled apolipoprotein B in the Sec61 complex, and this process is affected by factors including calnexin that alter the translocation of apolipoprotein B.

Identification of two regions in apolipoprotein B100 that are exposed on the cytosolic side of the endoplasmic reticulum membrane

Protease protection assays of apolipoprotein B100 (apoB) in digitonin-permeabilized HepG2 cells indicated that multiple domains of apoB are exposed to the cytosol through an extensive portion of the secretory pathway. The intracellular orientation of apoB in the secretory pathway was confirmed by immunocytochemistry using antibodies recognizing specific domains of apoB in streptolysin-O (STP-O)- and saponin-permeabilized HepG2 cells. Lumenal epitopes on marker proteins in secretory pathway compartments (p63, p53, and galactosyltransferase) were not stained by antibodies in STP-O-treated cells, but were brightly stained in saponin-treated cells, confirming that internal membranes were not perforated in STP-O-treated cells. An anti-apoB peptide antibody (B4) recognizing amino acids 3221-3240 caused intense staining in close proximity to the nuclear membrane, and less intensely throughout the secretory pathway in STP-O-permeabilized cells. Staining with this antibody was similar in STP-O- and saponin-treated cells, indicating that this epitope in apoB is exposed to the cytosol at the site of apoB synthesis and throughout most of the remaining secretory pathway. Similar results indicating a cytosolic orientation were obtained with monoclonal antibody CC3.4, which recognizes amino acids 690-797 (79-91 kD) in apoB. Two polyclonal antibodies made to human LDL and two monoclonal antibodies recognizing amino acids 1878-2148 (D7.2) and 3214-3506 (B1B6) in apoB did not produce a strong reticular signal for apoB in STP-O-treated cells. The anti-LDL and B1B6 antibodies produced almost identical punctate patterns in STP-O-treated cells that overlapped with LAMP-1, a membrane marker for lysosomes. These observations suggest that the B1B6 epitope of apoB is exposed on the surface of the lysosome. The results identify two specific regions in apoB that are exposed to the cytosol in the secretory pathway.

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.

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.

Demonstration of biphasic effects of docosahexaenoic acid on apolipoprotein B secretion in HepG2 cells

Oleic acid (OA) stimulates apolipoprotein B (apoB) secretion from HepG2 cells by protecting the nascent protein from rapid intracellular degradation. In contrast, the n-3 fatty acids, docosahexaenoic acid (DHA) and eicosapentaenoic acid, have been shown to reduce apoB secretion by increasing its intracellular degradation in rat hepatocytes. We attempted to determine if OA and DHA have these opposite effects at the same point in the secretory pathway for apoB or if they act at different points in HepG2 cells. Unexpectedly, we found that when DHA (0.2 mmol/L) was incubated with HepG2 cells for 2 hours, it stimulated both triglyceride (TG) synthesis and apoB secretion significantly (the "stimulatory effect"). The stimulatory effect of DHA on apoB secretion was associated with decreased intracellular degradation of newly synthesized apoB. These acute effects of DHA on TG synthesis and apoB secretion paralleled those previously demonstrated with OA. After DHA was removed from the medium, however, both TG synthesis and apoB secretion rapidly decreased to a level that was significantly less than the control level (the "inhibitory effect"). At the same time, intracellular apoB degradation was significantly increased, and this degradation was efficiently prevented by proteasome inhibitors. Removal of DHA from the incubation resulted in inhibition of the incorporation of endogenous fatty acids into TG. In contrast, removal of OA from the media was not associated with any such inhibitory effect. The inhibitory effect of DHA on basal apoB secretion persisted at least 8 hours. These studies suggest that incubation of HepG2 cells with DHA has biphasic effects on TG synthesis and apoB secretion: an initial stimulation of TG synthesis is followed by inhibition of TG synthesis and increased apoB degradation. Although the stimulatory effect of DHA is apparent during short incubations of HepG2 cells, both effects would be expected to occur during long incubations, since fatty acid uptake by cells is rapid and efficient. Thus, long incubations of HepG2 cells with DHA could result in overall reduced apoB secretion compared with cells incubated in bovine serum albumin. If these findings are extrapolated to the in vivo situation, they can explain the ability of dietary n-3 fatty acids, which would be delivered to the liver intermittently, to reduce very low density lipoprotein secretion.

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.