Intrahepatic assembly of very low density lipoproteins: immunologic characterization of apolipoprotein B in lipoproteins and hepatic membrane fractions and its intracellular distribution

Development and partial characterisation of an antiserum against apolipoprotein B of the short-finned eel, Anguilla australis

Despite its key role in transportation of triacylglycerides in blood, the distribution, localisation and molecular weight variants of apolipoprotein B (Apob) in teleost fish have essentially escaped study. To address this, a specific short-finned eel (Anguilla australis) Apob antiserum was produced by an immunised rabbit, purified and partially characterised. Localisation of Apob at both the mRNA (in situ hybridisation) and protein (immunohistochemistry) levels mirrored that of mammals; thus immunostaining was confined to the interstitial spaces of the liver and the vascular core of the intestinal villi. Immunostaining of proteins by Western blotting, followed by high-resolution LC-MS, indicated that peptide sequence coverage of Apob in low-density lipoproteins spanned the full-length protein. We conclude that only full-length Apob is produced by eels and that both liver and intestine are key sites for its synthesis.

Overexpression of the endoplasmic reticulum 60 protein ER-60 downregulates apoB100 secretion by inducing its intracellular degradation via a nonproteasomal pathway: evidence for an ER-60-mediated and pCMB-sensitive intracellular degradative pathway

Co- and posttranslational regulation of apolipoprotein B (apoB) has been postulated to involve degradation by both proteasomal and nonproteasomal pathways; however, nonproteasomal mechanisms of apoB degradation are currently unknown. We have previously demonstrated an intracellular association of newly synthesized apoB with endoplasmic reticulum (ER)-60, an ER-localized protein, possessing both proteolytic and chaperone activities. In the present paper, adenoviral expression vectors containing rat ER-60 cDNA were used to achieve dose- and time-dependent overexpression of ER-60 to investigate its role in apoB100 turnover. Overexpressed ER-60 accumulated in the microsomal lumen of HepG2 cells and was associated with apoB100 in dense lipoprotein particles. Overexpression of ER-60 in HepG2 cells significantly reduced both intracellular and secreted apoB100, with no effect on the secretion of a control protein, albumin. Similar results were obtained in McA-RH7777 rat hepatoma cells. ER-60-stimulated apoB100 degradation and inhibition of apoB100 secretion were sensitive to the protease inhibitor, p-chloromercuribenzoate (pCMB), in a dose-dependent manner but were unaffected by the proteasomal or lysosomal protease inhibitors, N-acetyl-leucinyl-leucinyl-nor-leucinal, E64, and leupeptin. Interestingly, enhanced expression of ER-60 induced apoB100 fragmentation in permeabilized HepG2 cells and resulted in detection of a unique 50 kDa degradation intermediate, a process that could be inhibited by pCMB. Intracellular stability and secretion of apoB100 in primary hamster hepatocytes were also found to be sensitive to pCMB. When taken together, the data suggest an important role for ER-60 in promoting apoB100 degradation via a pCMB-sensitive process in the ER. ER-60 may act directly as a protease or may be involved indirectly as a chaperone/protein factor targeting apoB100 to this nonproteasomal and pCMB-sensitive degradative pathway.

2000 George Lyman Duff Memorial Lecture: atherosclerosis is a liver disease of the heart

The production of apolipoprotein B (apoB)-containing lipoproteins by the liver is regulated by a complex series of processes involving apoB being cotranslationally translocated across the endoplasmic reticulum and assembled into a lipoprotein particle. The translocation of apoB across the endoplasmic reticulum is facilitated by the intraluminal chaperone, microsomal triglyceride transfer protein (MTP). MTP facilitates the translocation and folding of apoB, as well as the addition of lipid to lipid-binding domains (which consist of amphipathic beta sheets and alpha helices). In the absence of MTP or sufficient lipid, apoB exhibits translocation arrest. Thus, apoB translation, translocation, and assembly with lipids to form a core-containing lipoprotein particle occur as concerted processes. Abrogation of >/=1 of these processes diverts apoB into a degradation pathway that is dependent on conjugation with ubiquitin and proteolysis by the proteasome. The nascent core-containing lipoprotein particle that forms within the lumen of the endoplasmic reticulum can be "enlarged" to form a mature very low density lipoprotein particle. Additional studies show that the assembly and secretion of apoB-containing lipoproteins are linked to the cholesterol/bile acid synthetic pathway controlled by cholesterol 7alpha-hydroxylase. Studies in cultured cells and transgenic mice indicate that the expression of cholesterol 7alpha-hydroxylase indirectly regulates the expression of lipogenic enzymes through changes in the cellular content of mature sterol response element binding proteins. Oxysterols and bile acids may also act via the ligand-activated nuclear receptors LXR and FXR to link the metabolic pathways controlling energy balance and lipid metabolism to nutritional state.

The amino-terminal domain of apolipoprotein B does not undergo retrograde translocation from the endoplasmic reticulum to the cytosol. Proteasomal degradation of nascent apolipoprotein B begins at the carboxyl terminus of the protein, while apolipoprotein B is still in its original translocon

We studied the sequential topology of the NH(2) and COOH termini of apoB during translocation by expressing, in Chinese hamster ovary (CHO) and HepG2 cells, an apoB42 construct with c-Myc and hemagglutinin (HA) tags at 2 and 41% (relative to apoB100) of its amino acid sequence. We conducted similar studies using monoclonal antibodies against the NH(2) and COOH termini of apoB100 in HepG2 cells. After radiolabeling, microsomes were immunoisolated from transfected CHO cells using anti-c-Myc or anti-HA antibodies. Throughout a 60-min chase in the presence of N-acetyl-leucyl-norleucinal, more than 90% of microsomes were isolated by anti-HA antibodies, whereas less than 10% were isolated by anti-c-Myc antibodies. Proteinase K digestion of total microsomes consistently generated two fragments ( approximately 70 and approximately 120 kDa) of apoB42 containing the NH(2) terminus throughout the chase; no fragments containing the COOH terminus were detected. Immunofluorescent studies of transfected CHO cells were consistent with results from the labeling studies. Essentially identical results were obtained from pulse-chase studies in both native and apoB42-transfected HepG2 cells. The present studies support a model in which, in the absence of adequate core lipid synthesis, there is partial translocation of apoB leading to cytosolic exposure, ubiquitination, and proteasomal degradation directly from the original translocation channel.

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.

The role of the LDL receptor in apolipoprotein B secretion

Familial hypercholesterolemia is caused by mutations in the LDL receptor gene (Ldlr). Elevated plasma LDL levels result from slower LDL catabolism and a paradoxical lipoprotein overproduction. We explored the relationship between the presence of the LDL receptor and lipoprotein secretion in hepatocytes from both wild-type and LDL receptor-deficient mice. Ldlr(-/-) hepatocytes secreted apoB100 at a 3.5-fold higher rate than did wild-type hepatocytes. ApoB mRNA abundance, initial apoB synthetic rate, and abundance of the microsomal triglyceride transfer protein 97-kDa subunit did not differ between wild-type and Ldlr(-/-) cells. Pulse-chase analysis and multicompartmental modeling revealed that in wild-type hepatocytes, approximately 55% of newly synthesized apoB100 was degraded. However, in Ldlr(-/-) cells, less than 20% of apoB was degraded. In wild-type hepatocytes, approximately equal amounts of LDL receptor-dependent apoB100 degradation occured via reuptake and presecretory mechanisms. Adenovirus-mediated overexpression of the LDL receptor in Ldlr(-/-) cells resulted in degradation of approximately 90% of newly synthesized apoB100. These studies show that the LDL receptor alters the proportion of apoB that escapes co- or post-translational presecretory degradation and mediates the reuptake of newly secreted apoB-containing lipoprotein particles.

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.

Studies on degradative mechanisms mediating post-translational fragmentation of apolipoprotein B and the generation of the 70-kDa fragment

It has been well established that the biogenesis of apoB is mediated co-translationally by the cytosolic proteasome. Here, however, we investigated the role of both the cytosolic proteasome as well as non-proteasome-mediated degradation systems in the post-translational degradation of apoB. In pulse-chase labeling experiments, co-translational (0-h chase) apoB degradation in both intact and permeabilized cells was sensitive to proteasome inhibitors. Interestingly, turnover of apoB in intact cells over a 2-h chase was partially inhibitable by lactacystin, thus suggesting a role for the cytosolic proteasome in the post-translational degradation of apoB. In permeabilized cells, however, there was no post-translational protection of apoB by lactacystin. Further investigations of proteasomal activity in HepG2 cells revealed that, following permeabilization, there was a dramatic loss of the 20 S proteasomal subunits, and consequently the cells exhibited no detectable lactacystin-inhibitable activity. Thus, apoB fragmentation and the generation of the 70-kDa apoB degradation fragment, characteristic of permeabilized cells, continued to occur in these cells despite the absence of functional cytosolic proteasome. Similar results were observed when we used a derivative of lactacystin, clastolactacystin beta-lactone, which represents the active species of the inhibitor. Interestingly, however, the abundance of the 70-kDa fragment could be modulated by the microsomal triglyceride transfer protein inhibitor, BMS-197636, as well as by pretreatment of the permeabilized cells with dithiothreitol. These data thus suggest that although the cytosolic proteasome appears to be involved in the post-translational turnover of apoB in intact cells, the specific post-translational fragmentation of apoB generating the 70-kDa fragment observed in permeabilized cells occurs independent of the cytosolic proteasome.

Translocation efficiency, susceptibility to proteasomal degradation, and lipid responsiveness of apolipoprotein B are determined by the presence of beta sheet domains

Apolipoprotein (apo) B100 is an atypical secretory protein in that its translocation across the endoplasmic reticulum membrane is inefficient, resulting in the partial translocation and exposure of apoB100 on the cytoplasmic surface of the endoplasmic reticulum. Cytosolic exposure leads to the association of nascent apoB with heat shock protein 70 and to its predisposition to ubiquitination and proteasomal degradation. The basis for the inefficient translocation of apoB100 remains unclear and controversial. To test the hypothesis that beta sheet domains present in apoB100 contribute to its inefficient translocation, we created human apoB chimeric constructs apoB13,16 and apoB13,13,16, which contain amino-terminal alpha globular domains but no beta sheet domains, and apoB13,16,beta, which has an amphipathic beta sheet domain of apoB100 inserted into apoB13,16. These constructs, along with carboxyl-terminal truncations of apoB100, apoB34 and apoB42, were used to transfect HepG2 and Chinese hamster ovary cells. In contrast to the lack of effect of proteinase K on apoB13,16 and apoB13,13,16, the levels of apoB34, apoB42, and apoB13,16,beta were decreased by 70-85% after proteinase K-induced proteolysis in both HepG2 and Chinese hamster ovary cells. Either oleic acid or proteasomal inhibitors (N-acetyl-leucinyl-leucinyl-norleucinal and lactacystin) significantly increased the cell levels of apoB13,16,beta, apoB34, apoB42, and full-length apoB100 but had no effect on the cell levels of apoB13,16 and apoB13,13,16. When HepG2 cells were incubated with a microsomal triglyceride transfer protein inhibitor, the cellular levels of apoB13,16,beta, apoB34, and apoB42 were decreased by 70-80%, whereas the levels of apoB13,16 and apoB13,13,16 were unaffected. The effects of microsomal triglyceride transfer protein inhibition were reversed by lactacystin. Our results clearly demonstrate that the translocation efficiency, susceptibility to proteasomal degradation, and lipid responsiveness of apoB were determined by the presence of a lipid binding beta sheet domain. It is possible that beta sheet domains may at least transiently facilitate the interaction of apoB with the lipid bilayer surrounding the translocation channel.

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.

The role of factors that regulate the synthesis and secretion of very-low-density lipoprotein by hepatocytes

Lipoproteins are particles that contribute to overall metabolic homeostasis by transporting hydrophobic lipids in the blood plasma to and from different tissues in the body. Very-low-density lipoprotein (VLDL) is the principal vehicle for the transport of endogenous triglyceride (TG), and, ultimately, through its metabolic product, low-density lipoprotein (LDL), of cholesterol as well. It is synthesized mainly in hepatocytes, with small amounts also being produced by enterocytes in the fasting state. The mechanism of VLDL assembly is complex and is regulated at different levels by a variety of factors. The main structural protein of VLDL is called apolipoprotein B-100 (Apo B). Apo B formation and degradation therefore represent two major points of regulation of VLDL secretion. Hepatic levels of lipids such as phosphatidylcholine (PC), cholesteryl ester (CE), fatty acids (FA), and TG also affect VLDL synthesis. There are different views as to the specific mechanism by which each lipid class affects VLDL particle formation. In general, PC appears to promote the translocation of apo B from the cytosol to the lumen of the endoplasmic reticulum, a step that is crucial in the early stages of VLDL assembly. Apo B degradation is suppressed, and therefore VLDL secretion is enhanced, in the presence of elevated CE levels. For TG to be incorporated into the lipoprotein, it requires the action of a protein called microsomal triglyceride transfer protein (MTP). MTP might have a preference for TG comprised of FA with a certain degree of saturation. It becomes apparent that changes in diet that are accompanied by variations in the type of fats that are ingested affect VLDL formation and secretion. Regulation also occurs post-prandially in response to elevations in plasma insulin levels. Acute elevations in insulin inhibit VLDL secretion by promoting the degradation of apo B. This action is consistent with insulin's anabolic properties as it allows for the hepatic storage of lipid rather than for its distribution in VLDL to other tissues for fuel. Many studies have attempted to unravel the mechanisms of VLDL formation and secretion. The fact that so many factors are involved complicates the issue. The purpose of this article is to describe the relationship between different factors involved in VLDL assembly and secretion so that a better understanding of its metabolic regulation may be achieved.

S-phase DNA damage checkpoint in budding yeast

Eukaryotic cells must be able to coordinate DNA repair, replication and cell cycle progression in response to DNA damage. A failure to activate the checkpoints which delay the cell cycle in response to internal and external cues and to repair the DNA lesions results in an increase in genetic instability and cancer predisposition. The use of the yeast Saccharomyces cerevisiae has been invaluable in isolating many of the genes required for the DNA damage response, although the molecular mechanisms which couple this regulatory pathway to different DNA transactions are still largely unknown. In analogy with prokaryotes, we propose that DNA strand breaks, caused by genotoxic agents or by replication-related lesions, trigger a replication coupled repair mechanism, dependent upon recombination, which is induced by the checkpoint acting during S-phase.

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.

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.

Coordinate regulation of lipogenesis, the assembly and secretion of apolipoprotein B-containing lipoproteins by sterol response element binding protein 1

Stable plasmid-driven expression of the liver-specific gene product cholesterol 7alpha-hydroxylase (7alpha-hydroxylase) was used to alter the cellular content of transcriptionally active sterol response element binding protein 1 (SREBP1). As a result of stable expression of 7alpha-hydroxylase, individual single cell clones expressed varying amounts of mature SREBP1 protein. These single cell clones provided an opportunity to identify SREBP1-regulated genes that may influence the assembly and secretion of apoB-containing lipoproteins. Our results show that in McArdle rat hepatoma cells, which normally do not express 7alpha-hydroxylase, plasmid-driven expression of 7alpha-hydroxylase results in the following: 1) a linear relationship between (i) the cellular content of mature SREBP1 and 7alpha-hydroxylase protein, (ii) the relative expression of 7alpha-hydroxylase mRNA and the mRNA's encoding the enzymes regulating fatty acid, i.e. acetyl-CoA carboxylase and sterol synthesis, i.e. HMG-CoA reductase, (iii) the relative expression of 7alpha-hydroxylase mRNA and microsomal triglyceride transfer protein mRNA, a gene product that is essential for the assembly and secretion of apoB-containing lipoproteins; 2) increased synthesis of all lipoprotein lipids (cholesterol, cholesterol esters, triglycerides, and phospholipids); and 3) increased secretion of apoB100 without any change in apoB mRNA. Cells expressing 7alpha-hydroxylase contained significantly less cholesterol (both free and esterified). The increased cellular content of mature SREBP1 and increased secretion of apoB100 were concomitantly reversed by 25-hydroxycholesterol, suggesting that the content of mature SREBP1, known to be decreased by 25-hydroxycholesterol, mediates the changes in the lipoprotein assembly and secretion pathway that are caused by 7alpha-hydroxylase. These data suggest that several steps in the assembly and secretion of apoB-containing lipoproteins by McArdle hepatoma cells may be coordinately linked through the cellular content of mature SREBP1.

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.

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.

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

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

In HepG2 cells, translocation, not degradation, determines the fate of the de novo synthesized apolipoprotein B

Previous studies show that translocation and degradation of apolipoprotein B (apoB), two processes occurring on or within the endoplasmic reticulum, determine how much de novo synthesized apoB is secreted. We determined which of these processes regulates the intracellular fate of apoB by examining whether degradation determines how much apoB is translocated or if translocation determines how much apoB is degraded. HepG2 cells, treated with the cysteine active site protease inhibitor ALLN, previously shown to block the degradation of translocation-arrested apoB in Chinese hamster ovary cells (Du, E., Kurth, J., Wang, S.-L., Humiston, P., and Davis, R.A. (1994) J. Biol. Chem. 269, 24169-24176), showed a 10-fold increase in the accumulation of de novo synthesized [35S]methionine-labeled apoB. The majority (80%) of the apoB accumulated in response to ALLN was in the microsomal fraction. In contrast, ALLN did not effect apoB secretion. Since ALLN did not effect the intracellular accumulation of [35S]methionine-labeled albumin and other proteins (trichloroacetic acid-precipitable [35S]methionine-labeled proteins), its effect on apoB was specific. Pulse-chase studies showed that ALLN dramatically reduced the first-order rate of removal of [35S]methionine-labeled apoB from the cell but did not effect its rate of secretion. The finding that ALLN caused the intracellular accumulation of incompletely translated chains of apoB suggests that at least some of the degradation occurs at the ribosomal level. Moreover, 85% of the apoB that accumulated in isolated microsomes in response to ALLN was accessible to exogenous trypsin, indicating this pool of apoB was incompletely translocated. The combined data suggest that translocation, not degradation, determines the intracellular fate of de novo synthesized apoB.

Apoprotein B100, an inefficiently translocated secretory protein, is bound to the cytosolic chaperone, heat shock protein 70

Apoprotein B100 (apoB) is a secretory protein that appears to be constitutively translated but inefficiently translocated into the lumen of the endoplasmic reticulum. Using several experimental approaches, we found that apoB is bound to the cytosolic chaperone protein, heat shock protein 72/73 (commonly referred to as Hsp70). Similar to other chaperone-protein interactions, this binding was transient and ATP-sensitive. The binding of apoB to Hsp70 in HepG2 cells was decreased by treatment with oleic acid, which increases both translocation and secretion of apoB, and was increased by N-acetyl-leucyl-leucyl-norleucinal, a protease inhibitor which efficiently protects apoB from cellular degradation without affecting translocation. The N-terminal 16% of apoB, which is efficiently translocated into the endoplasmic reticulum lumen in stably transfected Chinese hamster ovary (CHO) cells, showed minimal, if any, binding to Hsp70. The N-terminal 50% of apoB, which is very poorly translocated in CHO cells, was found to bind significantly to Hsp70. These results suggest that domains of nascent apoB localized on the C-terminal regions of the molecule are transiently exposed to the cytosol during translation and/or translocation, and that Hsp70 functions as a molecular chaperone to maintain apoB in a translocational competent conformation until translocation is completed.

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.

Regulation of apolipoprotein B biogenesis in human hepatocytes: posttranscriptional control mechanisms that determine the hepatic production of apolipoprotein B-containing lipoproteins

OBJECTIVES

Hepatic overproduction of apolipoprotein B (apoB)-containing lipoproteins appears to be a common cause of hyperlipoproteinemia in humans. Patients with overproduction states secrete denser cholesterol ester-rich lipoprotein particles which are highly atherogenic. The formation of apoB particles involves a very complex process that requires the coordinated synthesis and assembly of apoB, triglycerides, cholesterol esters, phospholipids, and other components. ApoB expression is an important prerequisite for the assembly and secretion of apoB particles. Evidence to date appears to suggest that apoB expression is regulated posttranscriptionally. ApoB secretion rate is determined at the levels of apoB translocation into the endoplasmic reticulum (ER) as well as degradation within the ER.

RESULTS AND HYPOTHESIS

Based on available data, we postulate that the rate of apoB particle secretion is determined at the critical point where newly-synthesized apoB interacts with core lipids, particularly triglycerides. The supply of these lipids determines the rate of translocation of the apoB molecule across the ER membrane and into the ER lumen. Lipidation of apoB facilitates its proper folding, its assembly into a lipoprotein particle, and its extracellular secretion. In the absence of lipids, apoB is misfolded resulting in the abortion of ER translocation and subsequent degradation by an apoB specific protease.

CONCLUSIONS

The balance between intracellular degradation and extracellular secretion determines the rate at which the human liver secretes apoB particles.

Intracellular degradation of apolipoprotein B generates an N-terminal 70 kDa fragment in the endoplasmic reticulum

Regulated apoB degradation in HepG2 cells occurs in the endoplasmic reticulum (ER), is catalyzed by an N-acetylleucylleucylnorleucinal (ALLN)-sensitive proteinase, and generates a specific 70 kDa fragment (Adeli, K., 1994, J. Biol. Chem. 269, 9166-9175) [corrected]. In the present report, we have characterized the 70 kDa fragment by immunoprecipitation of permeabilized HepG2 cells with a battery of monoclonal antibodies against various sites on the apoB molecule. N-Terminal monoclonal antibodies (1D1 and 2D8) were capable of binding to the 70 kDa fragment suggesting that this polypeptide is an N-terminal fragment of the intact apoB. Subcellular fractionation of permeabilized cells and carbonate extraction resulted in the detection of the 70 kDa fragment in the ER lumen. Endoglycosidase H treatment confirmed that the fragment is N-linked glycosylated. We hypothesize that the ALLN-sensitive proteinase which may be located on the luminal side of the ER membrane, catalyzes an initial cleavage of apoB near the N-terminus generating a 70 kDa fragment, which is then released into the ER lumen.

Inhibition of hydroxymethylglutaryl coenzyme A reductase activity does not affect the secretion rate of apolipoproteins B and AI by CaCo-2 cells

It is believed that the major mechanisms by which hydroxymethylglutaryl coenzyme A reductase inhibitors lower plasma cholesterol levels are by inducing hepatic low-density lipoprotein (LDL) receptor activity and by decreasing apolipoprotein B (apoB) secretion by the liver. However, the intestine is also an important cholesterogenic organ and the possibility that this class of drugs may alter lipoprotein secretion by the intestine has not been fully studied. The purpose of the present study was to examine the possible role of cholesterol in regulating apoB secretion by the intestine by testing if the suppression of cholesterol synthesis by the reductase inhibitor lovastatin affected the secretion of apoB by CaCo-2 human intestinal cells. Differentiated post-confluent CaCo-2 cells were incubated for 24-72 h in serum-free medium in the presence or absence of 5 microM lovastatin, and the secretion rate of lipids, as well as apoB and apolipoprotein AI (apoAI) into the medium, was measured. Lovastatin markedly inhibited the incorporation of [1-14C]acetate into cholesterol for at least 48 h, lowered the content of esterified cholesterol in cells, and reduced their rate of cholesterol secretion. However, under basal conditions lovastatin had no effect on the secretion rate of apoB. After stimulation of apoB secretion by addition of 0.8 mM oleic acid to the medium, lovastatin did not alter apoB secretion in the first 2 days of incubation, but reduced the content of apoB in media from the 3rd day by 30%. This could not be explained by an increase in the rate of LDL degradation. Furthermore, supplementation with mevalonic acid only reversed about one-half of the effect of lovastatin, suggesting that this effect was at least parly nonspecific or unrelated to inhibition of cholesterol biosynthesis. There was also no specific effect of lovastatin on apoAI secretion. When cells were cultured with [1-14C]acetate for 24 or 72 h, the specific activity of cholesterol in medium at the end of the incubation was the same as in cells, suggesting that cholesterol used for lipoprotein secretion was in equilibrium with bulk cellular cholesterol and was not from a segregated compartment derived from newly synthesized cholesterol. This may explain why apoB secretion by CaCo-2 cells was unaffected by inhibition of cholesterol synthesis with lovastatin.

Factors affecting the regulation of apo B secretion by liver cells

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

Insulin regulation of triacylglycerol-rich lipoprotein synthesis and secretion

This review has considered a number of observations obtained from studies of insulin in perfused liver, hepatocytes, transformed liver cells and in vivo and each of the experimental systems offers advantages. The evaluation of insulin effects on component lipid synthesis suggests that overall, lipid synthesis is positively influenced by insulin. Short-term high levels of insulin through stimulation of intracellular degradation of freshly translated apo B and effects on synthesis limit the ability of hepatocytes to form and secrete TRL. The intracellular site of apo B degradation may involve membrane-bound apo B, cytoplasmic apo B and apo B which has entered the ER lumen. How insulin favors intracellular apo B degradation is not known. An area of recent investigation is in insulin-stimulated phosphorylation of intracellular substrates such as IRS-1 which activates insulin specific cellular signaling molecules [245]. Candidate molecules to study insulin action on apo B include IRS-1 and SH2-containing signaling molecules. Insulin dysregulation in carbohydrate metabolism occurs in non-insulin-dependent diabetes mellitus due to an imbalance between insulin sensitivity of tissue and pancreatic insulin secretion (reviewed in Refs. [307,308]). Insulin resistance in the liver results in the inability to suppress hepatic glucose production; in muscle, in impaired glucose uptake and oxidation and in adipose tissue, in the inability to suppress release of free FA. This lack of appropriate sensitivity towards insulin action leads to hyperglycemia which in turn stimulates compensatory insulin secretion by the pancreas leading to hyperinsulinemia. Ultimately, there may be failure of the pancreas to fully compensate, hyperglycemia worsens and diabetes develops. The etiology of insulin resistance is being intensively studied for the primary defect may be over secretion of insulin by the pancreas or tissue insulin resistance and both of these defects may be genetically predetermined. We suggest that, in addition to effects in carbohydrate metabolism, insulin resistance in liver results in the inability of first phase insulin to suppress hepatic TRL production which results in hypertriglyceridemia leading to high levels of plasma FA which accentuate insulin resistance in other target organs. As recently reviewed [17,254] the role of insulin as a stimulator of hepatic lipogenesis and TRL production has been long established. Several lines of evidence support that insulin is stimulatory to the production of hepatic TRL in vivo. First, population based studies support a positive relationship between plasma insulin and total TG and VLDL [253]. Second, there is a strong association between chronic hyperinsulinemia and VLDL overproduction [309].(ABSTRACT TRUNCATED AT 400 WORDS)

Synthesis and secretion of hepatic apolipoprotein B-containing lipoproteins

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

Influence of triacylglycerol biosynthesis rate on the assembly of apoB-100-containing lipoproteins in Hep G2 cells

Apolipoprotein B-100 (apoB-100) appears in three forms in the endoplasmic reticulum of Hep G2 cells: (1) tightly bound to the membrane, ie, not extractable by sodium carbonate. This form is glycosylated but protease sensitive when present in intact microsomes, suggesting that it is only partially translocated to the microsomal lumen; (2) extractable by sodium carbonate and present on low-density lipoprotein-very-low-density lipoprotein (LDL-VLDL)-like particles. This form is glycosylated and secreted into the medium; and (3) extractable by sodium carbonate but having a higher density than the LDL-VLDL-like particles. This form, referred to as Fraction I, is glycosylated and protected against proteases when present in intact microsomal vesicles, indicating that it is completely translocated to the luminal side of the microsomal membrane. Fraction I is not secreted into the medium, but it disappears with time from the cell, suggesting that it is degraded. Oleic acid induced a 2.7-fold increase in the rate of the biosynthesis of triacylglycerol but not of phosphatidylcholine in Hep G2 cells. Incubation of the cells with oleic acid had no significant effect on the rate of initiation of the apoB-100-containing lipoproteins, nor did it influence the amount of apoB-100 that was associated with the membrane or the turnover of apoB-100 in the membrane. Instead, it increased the proportion of the nascent apoB polypeptides on initiated lipoproteins that was converted into full-length apoB-100 on LDL-VLDL-like particles, giving rise to an increased amount of these particles in the lumen of the secretory pathway. Pulse-chase experiments showed that incubation with oleic acid gave rise to an increased formation of LDL-VLDL-like particles on behalf of the formation of Fraction I. This effect of oleic acid could partially explain the protective effect of the fatty acid on apoB-100, preventing it from undergoing posttranslational degradation.

Lipid absorption and transport in ruminants

The objective of this paper is to review new insights on the biological mechanisms of absorption and transport of lipid in ruminants, especially the modern concepts and analytical methods used in studies on structural properties and intravascular and tissue metabolism of lipoproteins and their factors of variation. The intestinal absorption of lipids (including long-chain fatty acids) is detailed, and variations in the qualitative and the quantitative aspects of absorption with diet composition, especially for high fat diets, are presented. Also, structural properties and distribution characteristics of lipoprotein classes in different lymphatic and blood vessels are compared across several animal species. Physicochemical and hydrodynamic properties of the lipoprotein particles and their apolipoprotein moieties are given for the main classes of lipoproteins. Finally, lipoprotein metabolism is discussed in relation to development and physiological, nutritional, and hormonal status. Intravascular metabolism of lipoproteins, including the role of lipolytic enzymes and lipid transfer proteins, is presented. Characteristics of the intestinal and hepatic synthesis of lipoproteins and apolipoprotein fractions are compared, especially through experiments stimulating the hepatic secretion of triglyceride-rich lipoproteins. Different methods of measurement of lipoprotein tissue uptake or secretion in ruminants are discussed.

Cloning and gene defects in microsomal triglyceride transfer protein associated with abetalipoproteinaemia

The microsomal triglyceride transfer protein (MTP), which catalyses the transport of triglyceride, cholesteryl ester and phospholipid between phospholipid surfaces, is a heterodimer composed of the multifunctional protein, protein disulphide isomerase, and a unique large subunit with an apparent M(r) of 88K (refs 1-3). It is isolated as a soluble protein from the lumen of the microsomal fraction of liver and intestine. The large subunit of MTP was not detectable in four unrelated subjects with abetalipoproteinaemia, a rare autosomal recessive disease characterized by a defect in the assembly or secretion of plasma lipoproteins that contain apolipoprotein B (ref. 6). We report here the isolation and sequencing of complementary DNA encoding the large subunit of MTP. A comparison of this sequence to corresponding genomic sequences from two abetalipoproteinaemic subjects revealed a homozygous frameshift mutation in one subject and a homozygous nonsense mutation in the other. The results indicate that a defect in the gene for the large subunit of MTP is the proximal cause of abetalipoproteinaemia in these two subjects, and that MTP is required for the secretion of plasma lipoproteins that contain apolipoprotein B.