Rapid stimulation of apolipoprotein B secretion by oleate is not associated with cholesteryl ester biosynthesis in HepG2 cells

Pathophysiology of Diabetic Dyslipidemia

Accumulating clinical evidence has suggested serum triglyceride (TG) is a leading predictor of atherosclerotic cardiovascular disease, comparable to low-density lipoprotein (LDL)-cholesterol (C) in populations with type 2 diabetes, which exceeds the predictive power of hemoglobinA1c. Atherogenic dyslipidemia in diabetes consists of elevated serum concentrations of TG-rich lipoproteins (TRLs), a high prevalence of small dense low-density lipoprotein (LDL), and low concentrations of cholesterol-rich high-density lipoprotein (HDL)2-C. A central lipoprotein abnormality is an increase in large TG-rich very-low-density lipoprotein (VLDL)1, and other lipoprotein abnormalities are metabolically linked to increased TRLs. Insulin critically regulates serum VLDL concentrations by suppressing hepatic VLDL production and stimulating VLDL removal by activation of lipoprotein lipase. It is still debated whether hyperinsulinemia compensatory for insulin resistance is causally associated with the overproduction of VLDL. This review introduces experimental and clinical observations revealing that insulin resistance, but not hyperinsulinemia stimulates hepatic VLDL production. LDL and HDL consist of heterogeneous particles with different size and density. Cholesterol-depleted small dense LDL and cholesterol-rich HDL2 subspecies are particularly affected by insulin resistance and can be named “Metabolic LDL and HDL,” respectively. We established the direct assays for quantifying small dense LDL-C and small dense HDL(HDL3)-C, respectively. Subtracting HDL3-C from HDL-C gives HDL2-C. I will explain clinical relevance of measurements of LDL and HDL subspecies determined by our assays. Diabetic kidney disease (DKD) substantially worsens plasma lipid profile thereby potentiated atherogenic risk. Finally, I briefly overview pathophysiology of dyslipidemia associated with DKD, which has not been so much taken up by other review articles.

Evolutionary conservation of drug action on lipoprotein metabolism-related targets

Genetic analysis has shown that the slower than normal rhythmic defecation behavior of the clk-1 mutants of Caenorhabditis elegans is the result of altered lipoprotein metabolism. We show here that this phenotype can be suppressed by drugs that affect lipoprotein metabolism, including drugs that affect HMG-CoA reductase activity, reverse cholesterol transport, or HDL levels. These pharmacological effects are highly specific, as these drugs affect defecation only in clk-1 mutants and not in the wild-type and do not affect other behaviors of the mutants. Furthermore, drugs that affect processes not directly related to lipid metabolism show no or minimal activity. Based on these findings, we carried out a compound screen that identified 190 novel molecules that are active on clk-1 mutants, 15 of which also specifically decrease the secretion of apolipoprotein B (apoB) from HepG2 hepatoma cells. The other 175 compounds are potentially active on lipid-related processes that cannot be targeted in cell culture. One compound, CHGN005, was tested and found to be active at reducing apoB secretion in intestinal Caco-2 cells as well as in HepG2 cells. This compound was also tested in a mouse model of dyslipidemia and found to decrease plasma cholesterol and triglyceride levels. Thus, target processes for pharmacological intervention on lipoprotein synthesis, transport, and metabolism are conserved between nematodes and vertebrates, which allows the use of C. elegans for drug discovery.

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

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

Short- and long-term effects of atorvastatin, lovastatin and simvastatin on the cellular metabolism of cholesteryl esters and VLDL secretion in rat hepatocytes

The short- and long-term in vitro effects of the hydroxymethylglutaryl-CoA reductase inhibitor atorvastatin, compared with lovastatin and simvastatin on VLDL secretion, and on the formation and the neutral and acid lysosomal hydrolysis of cholesteryl esters was investigated in rat liver hepatocytes maintained in suspension (2 or 4 h) or cultured in monolayers (24 h). All statins time-dependently reduced [14C]oleate incorporation into cholesteryl esters, but when exogenous cholesterol was added only atorvastatin caused an immediate transient decrease in hepatocyte ACAT activity. Activity of the lysosomal, microsomal and cytosolic CEH isoforms was unaffected by the hepatocyte treatments. Statins reduced free and esterified cholesterol mass in hepatocyte microsomes after 2 h, and this was followed by a modest decline in VLDL cholesteryl esters, whilst secretion of VLDL apoB and triglycerides was unaltered. However, after 24 h of treatment, statins caused generalized 20-40% decreases in the secretion of VLDL apoB, cholesterol and triglycerides, with the reduction in apoB48 secretion being significantly superior to that caused in apoB100. The mean diameter of secreted VLDL was not modified by either duration or drug treatment. Additional studies with subcellular fractions demonstrated that statins have a direct selective effect on the enzymes governing the cholesterol-cholesteryl ester cycle, with the exception of the microsomal CEH. Atorvastatin, lovastatin and simvastatin inhibited ACAT activity in microsomes by 50% at doses of 250, 100 and 50 microM, respectively. The cytosolic CEH elicited a biphasic profile of activity with activations up to 100 microM statin and inhibitions above 250 microM, and the lysosomal CEH was only inhibited by atorvastatin at a dose of 100 microM or more. We conclude that a prolonged, but not a short, limited availability of hepatocyte cholesterol derived from the endogenous synthesis reduces VLDL secretion, and that reactivity of statins at the cellular level are more similar than reactivity at the subcellular level as regards the cholesterol-cholesteryl ester cycle.

Enhanced secretion of ApoB by transfected HepG2 cells overexpressing fibrinogen

HepG2 cells stably transfected with cDNA-encoding single fibrinogen chains overexpress fibrinogen and have increased (4-fold) secretion of apolipoprotein B. Overexpression of fibrinogen does not affect the secretion of three representative acute-phase proteins but causes a small increase in albumin secretion. Enhanced apolipoprotein B secretion is due to less intracellular degradation and not to increased expression. The increased secretion of apolipoprotein B is independent of the acute-phase response, since stimulation of fibrinogen gene expression by interleukin 6 did not affect secretion. HepG2 cells overexpressing fibrinogen chains had increased 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA levels, enhanced cholesterol production but normal levels of triglyceride and phospholipid synthesis and of sterol response binding proteins. These results, that associate overexpression of fibrinogen with enhance apolipoprotein B secretion, may be significant since epidemiological studies indicate that elevated levels of fibrinogen and lipids are independent risk factors in coronary artery disease.

NK-104, a potent 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, decreases apolipoprotein B-100 secretion from Hep G2 cells

Intracellular cholesterol biosynthesis may play a key role in supplying cholesterol (as cholesteryl ester) for the neutral core of very low density lipoprotein (VLDL), thus modulating the secretion of apolipoprotein B-100 (apo B-100) from hepatocytes. The effect of compound NK-104 was studied, a new competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA-reductase), on apo B-100 synthesis and secretion from the human hepatoma cell line Hep G2. Cells were preincubated with NK-104 (0.01-5 microM) in the presence or absence of oleate (0.8 mM). Apo B-100 in the medium was determined by an enzyme-linked immunosorbent assay (ELISA). Incubation of Hep G2 with NK-104 resulted in a marked inhibition of cholesterogenesis (up to 95%), determined as incorporation of [14C]acetate into sterols, and decreased in a dose-dependent manner apo B-100 secretion, both in basal conditions (from 110 to 82 ng/mg cell protein, P < 0.01) and after incubation with oleate (from 227 to 165 ng/mg cell protein, P < 0.01). Density gradient for distribution of apo B-100 secreted, showed that this decrease was essentially due to a reduction of apo B-100 associated with lipoproteins in the density range of low density lipoproteins (LDL). Pulse chase experiment demonstrated that NK-104 did not affect the synthetic rate of apo B-100 but increased intracellular degradation of newly synthesized protein. The compound had only marginal effect on the mass of intracellular triglyceride but significantly decreased intracellular mass of free cholesterol and cholesteryl ester (P < 0.01). It is speculated that the ability of compound NK-104 to decrease apo B-100 secretion from Hep G2 cells is due to a decreased intracellular cholesterol availability.

Regulation of HepG2 cell apolipoprotein B metabolism by the citrus flavanones hesperetin and naringenin

Our previous studies showed that replacing the drinking water of rabbits fed a casein-containing diet with either orange juice or grapefruit juice reduced serum low density lipoprotein cholesterol and hepatic cholesteryl ester concentrations. To determine whether the changes observed in rabbits were due to flavonoids present in the juices acting directly on the liver, the effects of hesperetin and naringenin on net apolipoprotein B (apoB) secretion by HepG2 cells were investigated. These flavanones dose-dependently reduced net apoB secretion by up to 81% after a 24 h incubation, while doses of 60 micrograms/mL reduced net apoB secretion by 50% after 4 h. Coincubation with the proteasome inhibitor, MG-132, did not alter the ability of the flavonoids to reduce net apoB secretion over 4 h, suggesting that the flavonoid-induced changes in apoB metabolism were not due to a direct increase in proteasomal activity. However, the flavonoids were unable to reduce net apoB secretion after 4 h in the presence of oleate, suggesting that these compounds may interfere with the availability of neutral lipids for lipoprotein assembly. Furthermore, our 14C-acetate-labeling studies showed a 50% reduction in cholesteryl ester synthesis in the presence of either flavonoid, which could account for the reduction in net apoB secretion caused by incubation with these compounds. These in vitro studies suggest that hesperetin and naringenin may, in part, reduce net apoB secretion by HepG2 cells by inhibiting cholesteryl ester synthesis and that these compounds are good candidates for further in vivo studies to determine whether they are responsible for the cholesterol-lowering properties of dietary citrus juices.

ApoB100 secretion from HepG2 cells is decreased by the ACAT inhibitor CI-1011: an effect associated with enhanced intracellular degradation of ApoB

The concept that hepatic cholesteryl ester (CE) mass and the rate of cholesterol esterification regulate hepatocyte assembly and secretion of apoB-containing lipoproteins remains controversial. The present study was carried out in HepG2 cells to correlate the rate of cholesterol esterification and CE mass with apoB secretion by CI-1011, an acyl CoA:cholesterol acyltransferase (ACAT) inhibitor that is known to decrease apoB secretion, in vivo, in miniature pigs. HepG2 cells were incubated with CI-1011 (10 nmol/L, 1 micromol/L, and 10 micromol/L) for 24 hours. ApoB secretion into media was decreased by 25%, 27%, and 43%, respectively (P<0.0012). CI-1011 (10 micromol/L) inhibited HepG2 cell ACAT activity by 79% (P<0.002) and cellular CE mass by 32% (P<0.05). In contrast, another ACAT inhibitor, DuP 128 (10 micromol/L), decreased cellular ACAT activity and CE mass by 85% (P<0.002) and 42% (P=0.01), respectively, but had no effect on apoB secretion into media. To characterize the reduction in apoB secretion by CI-1011, pulse-chase experiments were performed and analyzed by multicompartmental modelling using SAAM II. CI-1011 did not affect the synthesis of apoB or albumin. However, apoB secretion into the media was decreased by 42% (P=0.019). Intracellular apoB degradation increased proportionately (P=0.019). The secretion of albumin and cellular reuptake of labeled lipoproteins were unchanged. CI-1011 and DuP 128 did not affect apoB mRNA concentrations. These results show that CI-1011 decreases apoB secretion by a mechanism that involves an enhanced intracellular degradation of apoB. This study demonstrates that ACAT inhibitors can exert differential effects on apoB secretion from HepG2 cells that do not reflect their efficacy in inhibiting cholesterol esterification.

Role of cholesterol ester mass in regulation of secretion of ApoB100 lipoprotein particles by hamster hepatocytes and effects of statins on that relationship

Our understanding of the factors that regulate the secretion of apoB100 lipoproteins remains incomplete with considerable debate as to the role, if any, for cholesterol ester in this process. This study examines this issue in primary cultures of hamster hepatocytes, a species in which both cholesterol and apoB100 metabolism are very similar to man. Addition of oleate to medium increased the mass of triglyceride and cholesterol ester within the hepatocyte and also increased the secretion of triglycerides, cholesterol ester, and apoB100 into the medium. Next, the responses of hamster hepatocytes to addition of either an HMG-CoA reductase inhibitor (lovastatin) or an acyl-CoA cholesterol acyltransferase inhibitor (58-035) to the medium, with or without added oleate, were determined. Effects of either agent were only evident in the oleate-supplemented medium in which cholesterol ester mass had been increased above basal. If oleate was not added to the medium, neither agent reduced apoB100 secretion; equally important, over the 24-hour incubation, neither agent, at the concentration used, produced any detectable change in intracellular cholesterol ester mass. However, in contrast to the estimates of mass, which were unchanged, under the same conditions radioisotopic estimates of cholesterol ester synthesis were markedly reduced. Any conclusion as to the relation of cholesterol ester mass to apoB100 secretion would therefore depend on which of the 2 methods was used. Overall, the data indicate a close correlation between the mass of cholesterol ester within the hepatocyte and apoB100 secretion from it and they go far to explain previous apparently contradictory data as to this relation. More importantly, though, taken with other available data, they indicate that the primary response of the liver to increased delivery of lipid is increased secretion rather than decreased uptake. These results point, therefore, to a hierarchy of hepatic responses to increased flux of fatty acids and increased synthesis of cholesterol that in turn suggests a more dynamic model of cholesterol homeostasis in the liver than has been appreciated in the past.

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.

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.

Hepatic responses to inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase: a comparison of atorvastatin and simvastatin

We have compared the cellular responses to simvastatin (Simva) and atorvastatin (Atorva), two potent HMG-CoA reductase inhibitors. The two drugs exhibited similar IC50's for inhibition of either rat or human reductase, and single oral dosing in rats showed the compounds to be nearly equipotent at inhibiting hepatic cholesterol synthesis. Treatment of rats with Simva or Atorva in the feed for four days yielded comparable inductions of hepatic reductase activity and reductase protein. For example, 0.05% Simva induced reductase activity 27.3 +/- 9.1 fold and 0.05% Atorva induced activity 26.9 +/- 4.7 fold. This adaptive response was also studied in HepG2 cells, a human hepatoblastoma line, cultured for 24 h in delipidated serum and then for an additional 24 h with Simva or Atorva. Over a broad range (10 nM-10 microM), both drugs caused similar inductions of reductase activity, reductase protein, and reductase mRNA. Under all conditions, the drugs induced similar changes in the ratio of mRNA/protein suggesting that Simva and Atorva have similar effects on both transcriptional and post-transcriptional regulatory machinery. Moreover, reductase in cells treated with Simva or Atorva for 22 h responded similarly to subsequent challenge with 25-hydroxycholesterol. Finally, we measured the ability of the two reductase inhibitors to reduce ApoB secretion by HepG2 cells. Simva and Atorva at 0.5 microM inhibited ApoB secretion nearly identically, 38% and 42% respectively. We conclude that these two drugs induce similar adaptive responses in cells and that their actions are qualitatively and mechanistically identical. Human studies have shown that plasma is cleared of Atorva much more slowly than it is of Simva. The large pharmacokinetic difference in man, rather than some difference in mechanism, is the most likely explanation for the finding that the equipotent dose ratio for cholesterol lowering in humans of Simva to Atorva is about 2/1.

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

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

Decreased intracellular degradation and increased secretion of apo B-100 in Hep G2 cells after inhibition of cholesteryl ester synthesis

Control of apolipoprotein B (apo B) secretion in hepatocytes occurs partly at the post-translational level. The key step in this process appears to be intracellular degradation of newly synthesized apo B. The aim of this paper was to investigate the mechanisms that regulate apo B secretion by Hep G2 cells, in response to the inhibition of Acyl-CoA Acyltransferase (ACAT) by the compound Sandoz 58035 (S-58035). S-58035 (20 microM) reduced cholesteryl ester synthesis from [14C]oleate by 95%, and increased significantly, in a dose-dependent manner, (2-100 microM) apo B secretion, either in control conditions (from 78 +/- 4.3 to 126 +/- 6.1 ng apo B-100/mg cell protein/4 h) or upon stimulation of apo B secretion by oleate (from 134 +/- 4.23 to 177 +/- 4.3 ng apo B/mg cell protein/4 h). This increased secretion of newly synthesized apo B-100 was confirmed by pulse experiments and by gradient ultracentrifugation of the media. Moreover pulse-chase experiments showed that the addition of S-58035 reduced intracellular degradation of apo B-100, both in control conditions and in the presence of oleate. S-58035 (20 microM) did not affect total cellular cholesterol content, but free cholesterol increased with a concomitant decrease of cholesteryl ester (-20%). S-58035 increased cellular triglyceride mass, which was observed in basal conditions (from 12.8 +/- 1.09 to 22.7 +/- 2.7 micrograms TG/mg cellular protein) and also in presence of oleate (from 48 +/- 0.53 to 59 +/- 6.3 micrograms TG/mg cellular protein). This effect is due to a stimulation of triglyceride synthesis, as determined by incorporation of [3H]glycerol into cellular triglycerides. From these data we conclude that, under our experimental conditions, triglyceride synthesis and/or availability is likely to control intracellular degradation of apo B.

Inhibitory effect of fluvastatin, an HMG-CoA reductase inhibitor, on the expression of adhesion molecules on human monocyte cell line

The effect of fluvastatin, an HMG-CoA reductase inhibitor, was investigated on the adhesive interaction between U937 cells, the human monocyte cell line, and human umbilical vein endothelial cells (HUVEC), focusing on the expression of adhesion molecules. U937 treated with fluvastatin lowered the capacity for binding to HUVEC. Fluvastatin at 0.1 microM or more inhibited the expression of lymphocyte function associated antigen-1 (LFA-1) on U937 and intercellular adhesion molecule-1 (ICAM-1) on U937. The expression of ICAM-1 on HUVEC was not inhibited by fluvastatin. The inhibitory effects of fluvastatin on the expression of adhesion molecules on U937 were completely reversed by the addition of mevalonate. Because fluvastatin did not affect the expression of other cell surface markers, CD4 and CD71, the inhibitory effects of fluvastatin on adhesion molecule expression could not be attributed to the non-specific suppression of the cell. It is conceivable that cellular interaction between monocytes and endothelial cells is inhibited by fluvastatin, mediated via reducing the expression of adhesion molecules, particularly in the side of monocyte.

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

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

Cholesterol esterification is not essential for secretion of lipoprotein components by HepG2 cells

Hepatic acyl CoA:cholesterol acyltransferase (ACAT) activity may determine storage of cholesterol and supply of cholesteryl esters for the neutral lipid core of very low density lipoprotein. Inhibition of cholesterol esterification in HepG2 cells, by the ACAT inhibitor 447C88, partially reduced the secretion of labelled total cholesterol, but the secretion of apoprotein B mass, and of radiolabelled triacylglycerol and phosphatidylcholine were unaffected. Furthermore, this compound was shown to substantially deplete the intracellular cholesteryl ester mass without affecting secretion of lipoprotein components. In contrast, the less potent ACAT inhibitor, CL277,082, significantly decreased secretion of labelled triacylglycerol, phosphatidylcholine and total cholesterol, in a manner which mirrored the decreases in secretion of apoB. This study clearly illustrates that ACAT inhibitors can exert differential effects on secretion of apoB-containing lipoproteins, which do not correlate with their efficacy in inhibiting ACAT, arguing that cholesterol esterification is not essential for lipoprotein secretion from these cells.

The pattern of apolipoprotein B100 containing lipoprotein subclasses produced by the isolated visceral rat yolk sac depends on developmental stage and fatty acid availability

Explants of visceral rat yolk sacs from gestational days 16, 18 and 22 were used for studying developmental changes of secretion and density distribution of lipoproteins, particularly of those containing apoB. Moreover, the influence of fatty acid supply on the amount and density distribution of secreted apolipoproteins was studied on day 18 of gestation. Active lipoprotein production was observed in yolk sacs taken on days 16 and 18 of gestation. It declined considerably on day 22 of gestation in parallel with the production of total protein, triacylglycerols and cholesterol. On all gestational days, apoB floated mainly in the LDL range ( > or = 70%) with differences in the distribution pattern of LDL subclasses. The lowest density of secreted LDL was found on day 18 of gestation (peak at d = 1.025 g/ml) followed by day 16 (peak at d = 1.035 g/ml) and day 22 of gestation (peak at d = 1.045 g/ml). ApoAIV, apoE and apoAI floated exclusively in the HDL range with a peak at d = 1.089 g/ml independently of the gestational day. After incubation of yolk sacs from the 18th day of gestation with 0.4 mM or 0.8 mM oleate, the density of secreted apoB containing particles was decreased (peaks in the VLDL and IDL density range), whereas palmitate in the same concentrations caused a redistribution of secreted apoB toward higher densities (peaks at d > or = 1.032 g/ml). Taken together, the data provide evidence that the density of LDL subclasses produced by isolated yolk sacs between days 16 and 22 of gestation depended on the gestational stage. Moreover, addition of unsaturated or saturated fatty acids to the organ culture differently affected the secretory rate and the density of lipoproteins delivered by yolk sacs on day 18 of gestation.

Intracellular apoprotein B degradation is suppressed by decreased albumin concentration in Hep G2 cells

It is generally accepted that hepatic secretion of apoprotein (apo) B-containing lipoproteins is substantially increased in nephrosis. To elucidate the mechanisms for the oversecretion of apo B, we investigated the effect of a various concentration of albumin on apo B kinetics in the absence or presence of oleate in Hep G2 cells. Hep G2 cells were labeled with [3H]-leucine in leucine-free medium containing 0, 1.5, 3.0 or 4.5% BSA for 180 minutes, and the secreted radiolabeled apo B, apo A1 and albumin were isolated by immunoprecipitation and counted. The secretions of apo B and albumin were suppressed by BSA (bovine serum albumin) in a dose-dependent manner, but the secretion of apo A1 was not suppressed significantly. Oleate (0.4 mM) increased the rate of apo B secretion by 2.5-fold when oleate was bound to 1.5% BSA, but at higher concentrations of BSA (3.0 or 4.5%), apo B secretion was less responsive to oleate. A pulse-chase study indicated that early apo B degradation was significantly suppressed in cells incubated with lower concentrations of BSA (0 or 1.5% BSA), thereby rapidly stimulating apo B secretion. Oleate (0.4 mM) potently inhibited apo B degradation when oleate was bound to 1.5% BSA, whereas the inhibition was not observed when oleate was bound to 4.5% BSA. Intracellular albumin synthesis was stimulated in BSA-free medium, but intracellular decay of albumin was essentially unaffected by concentration of BSA. Similar to BSA, a higher concentration of dextran (3.0 or 4.5%) reduced apo B secretion, and this was the result of increased early apo B degradation in the cells. These results indicate that reduced albumin suppresses intracellular apo B degradation, and the inhibition of apo B degradation by oleate is manifested only at a low concentration of albumin. Therefore, the present study suggests that free fatty acids bound to low concentration of albumin in the circulating plasma play an important role on hepatic oversecretion of apo B-containing lipoprotein in hypoalbuminemic state, such as nephrotic syndrome.

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)