Role of the neutral lipid accessible pool in the regulation of secretion of apoB-100 lipoprotein particles by HepG2 cells

Lactoferrin attenuates fatty acid-induced lipotoxicity via Akt signaling in hepatocarcinoma cells

Nonalcoholic fatty liver disease (NAFLD) describes a spectrum of lesions ranging from simple steatosis to non-alcoholic steatohepatitis (NASH). The excess influx of fatty acids (FAs) into the liver is recognized as a main cause of simple steatosis formation and progression to NASH. Recently, administration of lactoferrin (LF), a glycoprotein present in milk, was suggested to prevent NAFLD development. However, the effect of LF on the contribution of FA to NAFLD development remains unclear. In this study, the effects of LF on FA mixture (FAm)-induced lipotoxicity using human hepatocarcinoma G2 cells were assessed. FAm significantly decreased cell viability and increased intracellular lipid accumulation, whereas LF significantly recovered cell viability without affecting lipid accumulation. FAm-induced lactic dehydrogenase (LDH) and caspase-3/7 activities were significantly decreased by LF and SP600125, a c-Jun N-terminal kinase (JNK) specific inhibitor. We also found that LF added to FAm-treated cells induced Akt phosphorylation, which contributed to inhibition of JNK signaling pathway-dependent apoptosis. Akt inhibitor VIII, an allosteric Akt inhibitor, significantly attenuated the effect of LF on LDH activity and abrogated the ones on cell viability and caspase-3/7 activity. In summary, the present study has revealed that LF has a protective effect on FAm-induced lipotoxicity in a HepG2 model of NAFLD and identified the activation of the Akt signaling pathway as a possibly major mechanism.

Distinct regulation of plasma LDL cholesterol by eicosapentaenoic acid and docosahexaenoic acid in high fat diet-fed hamsters: participation of cholesterol ester transfer protein and LDL receptor

Despite established anti-atherogenic action, previous reports have shown that fish oils or n-3 poly-unsaturated fatty acid (PUFA) increase plasma LDL-C in animals and humans. However, which component of n-3 PUFAs and what mechanisms contribute to this increase are unclear. We investigated the effects of the major components of n-3 PUFA, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), on plasma LDL-C in high fat diet-fed hamsters. While LDL-C increased significantly with n-3 PUFA oil and DHA, EPA had no effect on LDL-C. Interestingly, a positive correlation was found between plasma cholesterol ester transfer protein (CETP) activity and LDL-C. Only DHA increased plasma CETP activity and significantly decreased LDL receptor expression in the liver. Our data suggest that DHA, not EPA, is a major factor in the LDL-C increasing effect of n-3 PUFA oil. These differential effects on LDL-C may arise from differences in plasma CETP activity and LDL receptor expression.

Knockdown of FABP5 mRNA decreases cellular cholesterol levels and results in decreased apoB100 secretion and triglyceride accumulation in ARPE-19 cells

To maintain normal retinal function, retinal pigment epithelial (RPE) cells engulf photoreceptor outer segments (ROS) enriched in free fatty acids (FFAs). We have previously demonstrated fatty acid-binding protein 5 (FABP5) downregulation in the RPE/choroidal complex in a mouse model of aging and early age-related macular degeneration. FABPs are involved in intracellular transport of FFAs and their targeting to specific metabolic pathways. To elucidate the role of FABP5 in lipid metabolism, the production of the FABP5 protein in a human RPE cell line was inhibited using RNA interference technology. As a result, the levels of cholesterol and cholesterol ester were decreased by about 40%, whereas FFAs and triglycerides were increased by 18 and 67% after siRNA treatment, respectively. Some species of phospholipids were decreased in siRNA-treated cells. Cellular lipid droplets were evident and apoB secretion was decreased by 76% in these cells. Additionally, we discovered that ARPE-19 cells could synthesize and secrete Apolipoprotein B100 (apoB100), which may serve as a backbone structure for the formation of lipoprotein particles in these cells. Our results indicate that FABP5 mRNA knockdown results in the accumulation of cellular triglycerides, decreased cholesterol levels, and reduced secretion of apoB100 protein and lipoprotein-like particles. These observations indicated that FABP5 plays a critical role in lipid metabolism in RPE cells, suggesting that FABP5 downregulation in the RPE/choroid complex in vivo might contribute to aging and early age-related macular degeneration.

Altered composition of triglyceride-rich lipoproteins and coronary artery disease in a large case-control study

BACKGROUND

Traditional beta-quantification of plasma lipoproteins by ultracentrifugation separates triglyceride-rich lipoproteins (TGRL) from higher density lipoproteins. The cholesterol in the TGRL fraction is referred to as measured very low-density lipoprotein cholesterol (VLDL-C) recognizing that other TGRL may be present. The measured VLDL-C to total plasma triglyceride (VLDL-C/TG) has long been considered an index of average TGRL composition with abnormally high VLDL-C/TG ratios (>or=0.30 with TG>150mg/dL) indicative of atherogenic remnant accumulation (type III hyperlipidemia). However, virtually no reports are available which examine potential associations between CAD and VLDL-C/TG at the lower end of the spectrum.

METHODS AND RESULTS

We performed ultracentrifugation in 1170 cases with premature-onset, familial CAD and 1759 population-based controls and examined the VLDL-C/TG ratio as an index of TGRL composition. As expected, we found very high CAD risk associated with severe type III hyperlipidemia (OR 10.5, p=0.02). Unexpectedly, however, we found a robust, graded, and independent association between CAD risk and lower than average VLDL-C/TG ratios (p<0.0001 as ordered categories or as a continuous variable). Among those in the lowest VLDL-C/TG category (a ratio <0.12), CAD risk was clearly increased (OR 4.5, 95% CI 2.9-6.9) and remained significantly elevated in various subgroups including those with triglycerides below 200mg/dl, in males and females separately, as well as among those with no traditional CAD risk factors (OR 5.8, 95% CI 1.5-22). Significant compositional differences by case status were confirmed in a subset whose samples were re-spun with measurement of lipids and apolipoprotein B (apo B) in each subfraction.

CONCLUSIONS

We found a strong, graded, independent, and robust association between CAD and lower VLDL-C/TG ratios. We consider this a novel, hypothesis-generating observation which will hopefully generate additional future studies to provide confirmation and further insight into potential mechanisms.

ACAT2 stimulates cholesteryl ester secretion in apoB-containing lipoproteins

Previous studies in nonhuman primates revealed a striking positive correlation between liver cholesteryl ester (CE) secretion rate and the development of coronary artery atherosclerosis. CE incorporated into hepatic VLDL is necessarily synthesized by ACAT2, the cholesterol-esterifying enzyme in hepatocytes. We tested the hypothesis that the level of ACAT2 expression, in concert with cellular cholesterol availability, affects the CE content of apolipoprotein B (apoB)-containing lipoproteins. In a model system of lipoprotein secretion using COS cells cotransfected with microsomal triglyceride transfer protein and truncated forms of apoB, ACAT2 expression resulted in a 3-fold increase in microsomal ACAT activity and a 4-fold increase in the radiolabeled CE content of apoB-lipoproteins. After cholesterol-cyclodextrin (Chol-CD) treatment, CE secretion was increased by 27-fold in ACAT2-transfected cells but by only 7-fold in control cells. Chol-CD treatment also caused the percentage of CE in the apoB-lipoproteins to increase from 3% to 33% in control cells and from 16% to 54% in ACAT2-transfected cells. In addition, ACAT2-transfected cells secreted 3-fold more apoB than control cells. These results indicate that under all conditions of cellular cholesterol availability tested, the relative level of ACAT2 expression affects the CE content and, hence, the potential atherogenicity, of nascent apoB-containing lipoproteins.

Effects of trans-10,cis-12 conjugated linoleic acid on cholesterol metabolism in hypercholesterolaemic hamsters

BACKGROUND

Conjugated linoleic acid (CLA) has received great attention in recent years because of its pleiotropic biological activities, but considerably fewer studies have been published addressing its role in serum lipids and atherosclerosis compared to other topics covered.

AIMS OF THE STUDY

The aim of the present study was to assess the effects of the trans-10,cis-12 isomer of CLA on cholesterolaemia and on several metabolic pathways involved in cholesterol metabolism in hamsters.

METHODS

Animals were fed atherogenic diets supplemented with 0.5% linoleic acid, 0.5% trans-10,cis-12 CLA or 1.0% trans-10,cis-12 CLA, for 6 weeks. Serum lipoproteins were separated by FPLC. Cholesterol in serum and liver, as well as triacylglycerols and phospholipids in liver were assessed by spectrophotometry. 3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMGCoAR), acyl-coenzyme A:cholesterol acyltransferase (ACAT) and cholesteryl ester hydrolase (CEH) activities were measured by radiometry, and LDL receptors were determined by Western blot.

RESULTS

trans-10,cis-12 CLA feeding did not modify food intake nor final body weight. Although serum total cholesterol remained unchanged, when cholesterol fractions were analyzed a significant decrease in VLDL-cholesterol was observed in CLA-fed animals, without changes in HDL-cholesterol or LDL-cholesterol. trans-10,cis-12 CLA decreased cholesterol ester content and increased free cholesterol in liver. The activity of HMGCoAR was not modified. In contrast, ACAT activity was reduced by both CLA doses and CEH was increased by the high CLA dose. LDL receptors were significantly reduced by trans-10,cis-12 feeding when expressed as arbitrary units per mg of protein, however, the total receptor mass remained unchanged.

CONCLUSIONS

These results suggest that, under the present experimental conditions, trans-10,cis-12 CLA feeding reduces cholesterol esterification in liver and decreases the minority serum VLDL-cholesterol fraction, but it does not show a hypocholesterolaemic effect. A dose-response effect was not observed.

Hepatic ABCG5/G8 overexpression reduces apoB-lipoproteins and atherosclerosis when cholesterol absorption is inhibited

We previously reported that liver-specific overexpression of ABCG5/G8 in mice is not atheroprotective, suggesting that increased biliary cholesterol secretion must be coupled with decreased intestinal cholesterol absorption to increase net sterol loss from the body and reduce atherosclerosis. To evaluate this hypothesis, we fed low density lipoprotein receptor-knockout (LDLr-KO) control and ABCG5/G8-transgenic (ABCG5/G8-Tg)xLDLr-KO mice, which overexpress ABCG5/G8 only in liver, a Western diet containing ezetimibe to reduce intestinal cholesterol absorption. On this dietary regimen, liver-specific ABCG5/G8 overexpression increased hepatobiliary cholesterol concentration and secretion rates (1.5-fold and 1.9-fold, respectively), resulting in 1.6-fold increased fecal cholesterol excretion, decreased hepatic cholesterol, and increased (4.4-fold) de novo hepatic cholesterol synthesis versus LDLr-KO mice. Plasma lipids decreased (total cholesterol, 32%; cholesteryl ester, 32%; free cholesterol, 30%), mostly as a result of reduced non-high density lipoprotein-cholesterol and apolipoprotein B (apoB; 36% and 25%, respectively). ApoB-containing lipoproteins were smaller and lipid-depleted in ABCG5/G8-TgxLDLr-KO mice. Kinetic studies revealed similar 125I-apoB intermediate density lipoprotein/LDL fractional catabolic rates, but apoB production rates were decreased 37% in ABCG5/G8-TgxLDLr-KO mice. Proximal aortic atherosclerosis decreased by 52% (male) and 59% (female) in ABCG5/G8-TgxLDLr-KO versus LDLr-KO mice fed the Western/ezetimibe diet. Thus, increased biliary secretion, resulting from hepatic ABCG5/G8 overexpression, reduces atherogenic risk in LDLr-KO mice fed a Western diet containing ezetimibe. These findings identify distinct roles for liver and intestinal ABCG5/G8 in modulating sterol metabolism and atherosclerosis.

Overexpression of human diacylglycerol acyltransferase 1, acyl-coa:cholesterol acyltransferase 1, or acyl-CoA:cholesterol acyltransferase 2 stimulates secretion of apolipoprotein B-containing lipoproteins in McA-RH7777 cells

The relative importance of each core lipid in the assembly and secretion of very low density lipoproteins (VLDL) has been of interest over the past decade. The isolation of genes encoding diacylglycerol acyltransferase (DGAT) and acyl-CoA:cholesterol acyltransferases (ACAT1 and ACAT2) provided the opportunity to investigate the effects of isolated increases in triglycerides (TG) or cholesteryl esters (CE) on apolipoprotein B (apoB) lipoprotein biogenesis. Overexpression of human DGAT1 in rat hepatoma McA-RH7777 cells resulted in increased synthesis, cellular accumulation, and secretion of TG. These effects were associated with decreased intracellular degradation and increased secretion of newly synthesized apoB as VLDL. Similarly, overexpression of human ACAT1 or ACAT2 in McA-RH7777 cells resulted in increased synthesis, cellular accumulation, and secretion of CE. This led to decreased intracellular degradation and increased secretion of VLDL apoB. Overexpression of ACAT2 had a significantly greater impact upon assembly and secretion of VLDL from liver cells than did overexpression of ACAT1. The addition of oleic acid (OA) to media resulted in a further increase in VLDL secretion from cells expressing DGAT1, ACAT1, or ACAT2. VLDL secreted from DGAT1-expressing cells incubated in OA had a higher TG:CE ratio than VLDL secreted from ACAT1- and ACAT2-expressing cells treated with OA. These studies indicate that increasing DGAT1, ACAT1, or ACAT2 expression in McA-RH7777 cells stimulates the assembly and secretion of VLDL from liver cells and that the core composition of the secreted VLDL reflects the enzymatic activity that is elevated.

The hepatitis B virus X protein inhibits secretion of apolipoprotein B by enhancing the expression of N-acetylglucosaminyltransferase III

The X protein of hepatitis B virus (HBx) plays a major role on hepatocellular carcinoma (HCC). Apolipoprotein B (apoB) in the liver is an important glycoprotein for transportation of very low density lipoproteins and low density lipoproteins. Although lipid accumulation in the liver is known as one of the factors for the HCC, the relationship between HBx and apoB during the HCC development is poorly understood. To better understand the biological significance of HBx in HCC, liver Chang cells that specifically express HBx were established and characterized. In this study we demonstrate that overexpression of HBx significantly up-regulates the expression of UDP-N-acetylglucosamine:beta-d-mannoside-1,4-N-acetylglucosaminyltransferase-III (GnT-III), an enzyme that functions as a bisecting-N-acetylglucosamine (GlcNAc) transferase in apoB, and increases GnT-III promoter activity in a chloramphenicol acetyltransferase assay. GnT-III expression levels of HBx-transfected cells appeared to be higher than that of hepatocarcinoma cells as well as GnT-III-transfected cells, indicating that HBx may has a strong GnT-III promotor-enhancing activity. Intracellular levels of apoBs, which contained the increased bisecting GlcNAc, were accumulated in HBx-transfected liver cells. These cells as well as GnT-III-transfected liver cells revealed the inhibition of apoB secretion and the increased accumulation of intracellular triglyceride and cholesterol compared with vector-transfected cells. Moreover, overexpression of GnT-III and HBx in liver cells was shown to down-regulate the transcriptional level of microsomal triglyceride transfer protein, which regulates the assembly and secretion of apoB. Therefore, our study strongly suggested that the HBx increase in intracellular accumulation of aberrantly glycosylated apoB resulted in inhibition of secretion of apoB as well as intracellular lipid accumulation by elevating the expression of GnT-III.

Interaction between dietary protein and fat in triglyceride metabolism in the rat: effects of soy protein and menhaden oil

The objective of the present study was to determine the mechanisms by which dietary proteins interact with dietary lipids in the regulation of triglyceridemia in rats. Male Sprague-Dawley rats (n = 56) were subjected to 28-d experimental diets containing different combinations of proteins (20% w/w) and lipid sources (14% w/w): (i) casein-menhaden oil, (ii) casein-beef tallow, (iii) soy protein-menhaden oil, and (iv) soy protein-beef tallow. Significant protein-lipid interactions were observed on triglyceridemia and hepatic cholesterol in fasted rats. The combination of casein and beef tallow was associated with high plasma TG and hepatic cholesterol concentrations, which were reduced by substitution either of soy for casein or of menhaden oil for beef tallow. Therefore, triglyceridemia and liver cholesterol remained low with soy protein feeding, independently of the lipid source, as well as with menhaden oil feeding, regardless of the protein source. The menhaden oil diets reduced plasma cholesterol, hepatic TG, and TG secretion compared with beef tallow diets independently of the dietary protein source. Modifying the source of dietary proteins and lipids had no effect on post-heparin plasma lipoprotein lipase activity. These results demonstrate that soy protein can lower rat triglyceridemia relative to casein when associated with beef tallow consumption, whereas menhaden oil can attenuate hypertriglyceridemia when rats are fed casein. The data further suggest that part of the hypotriglyceridemic effect of soy protein in the rat may be mediated by reduced hepatic lipid synthesis, as is the case for menhaden oil.

Inhibition of phosphatidylcholine synthesis via the phosphatidylethanolamine methylation pathway impairs incorporation of bulk lipids into VLDL in cultured rat hepatocytes

Inhibition of phosphatidylcholine (PC) synthesis via the phosphatidylethanolamine (PE) methylation pathway was shown to decrease the secretion of VLDL from primary rat hepatocytes (Nishimaki-Mogami et al. 1996. BIOCHIM: Biophys. Acta. 1304: 21-31). To understand further the role of PE methylation, we determined the effect of bezafibrate, an inhibitor of PE methylation, on VLDL assembly within the microsomal lumen. Bezafibrate was shown to decrease VLDL (triacylglycerol) secretion only when cellular PE methylation was active in the presence of methionine. Pulse-chase experiments showed that bezafibrate treatment did not impair the movement of [(35)S]apolipoprotein (apo)B-48 from microsomal membranes into the lumen. However, bezafibrate treatment resulted in reduced VLDL-[(35)S]apoB-48 and increased [(35)S]apoB-48-containing particles in the HDL density range (HDL-[(35)S]apoB-48) within the lumen. Inhibition of PE methylation by bezafibrate or 3-deazaadenosine after the completion of HDL-[(35)S]apoB-48 assembly effectively decreased VLDL-[(35)S]apoB-48 secretion with a concomitant increase in HDL-[(35)S]apoB-48 secretion. These findings suggest that inhibition of PC synthesis via the PE methylation pathway impairs the stage of bulk triacylglycerol incorporation during the assembly of VLDL.

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

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

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

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

Prolonged inhibition of cholesterol synthesis by atorvastatin inhibits apo B-100 and triglyceride secretion from HepG2 cells

Atorvastatin is a new HMG-CoA reductase inhibitor that strongly lowers plasma cholesterol and triglyceride (TG) levels in humans and animals. Since previous data indicated that atorvastatin has prolonged inhibition of hepatic cholesterol synthesis, we tested whether this longer duration of inhibitory effect on cholesterol synthesis decreased hepatic lipoprotein secretion in vitro. We used the HepG2 hepatoma cell line to: (1) determine the time required until levels of secreted apo B-100 and TG declined significantly, (2) examine the relation to the mass of cellular cholesteryl ester (CE) and (3) test microsomal triglyceride transfer protein (MTP) activity which leads to decreased apo B-100 production. Although atorvastatin significantly inhibited cholesterol synthesis in HepG2 cells regardless of treatment duration (1, 14 or 24 h), it did not inhibit TG synthesis. Apo B-100 and TG secretion were unchanged after 1-h atorvastatin treatment, but declined significantly after 24-h treatment. Atorvastatin treatment also reduced cellular CE mass, exhibiting both time- and dose-dependency. Mevalonolactone, a product of HMG-CoA reductase, attenuated the inhibitory effects of atorvastatin. Atorvastatin strongly reduced mRNA levels of MTP, whereas it did not inhibit MTP activity as measured by TG transfer assay between liposomes. Simvastatin also induced treatment- and time-dependent reductions in apo B-100, whereas the MTP inhibitor BMS-201038 exhibited no time dependency, instead inhibiting this variable even on 1-h treatment. These results indicate that reduced apo B-100 secretion caused by atorvastatin is a secondary result owing to decreased lipid availability, and that atorvastatin's efficacy depends on the duration of cholesterol synthesis inhibition in the liver.

Pharmacological profile of F 12511, (S)-2',3', 5'-trimethyl-4'-hydroxy-alpha-dodecylthioacetanilide a powerful and systemic acylcoenzyme A: cholesterol acyltransferase inhibitor

The pharmacological profile of F 12511 (S)-2',3', 5'-trimethyl-4'-hydroxy-alpha-dodecylthio-phenylacetanilide, a new inhibitor of acyl-CoA: cholesterol acyltransferase (EC 2.3.1.26; ACAT), was evaluated by using different in vitro and in vivo models. In vitro, F 12511 was shown to be a highly potent inhibitor of ACAT activity in microsomal preparations from various animal species as well as of cholesterol esterification in relevant human cell lines in culture. The concentrations of F 12511 required to produce 50% inhibition of ACAT activity (IC(50) values) in microsomal preparations ranged from 41nM for hypercholesterolemic rabbit intestine to 223 nM for normocholesterolemic hamster liver. In whole cell assays using hepatic (Hep G2), intestinal (CaCo-2) and macrophagic (THP-1) cell lines, F 12511 inhibited ACAT activity with IC(50) values of 3, 7, and 71 nM, respectively. In vivo, orally administered F 12511 displayed high potency and efficacy as an antihypercholesterolemic compound in different cholesterol-fed animals (rat, guinea-pig, rabbit). For instance, in guinea-pigs the dose required to reduce plasma cholesterol levels by 30% (ED(30) value) was 0.008 mg.kg(-1.) In rabbits, an animal species prone to atherosclerosis, the hypocholesterolemic effect was accompanied by a dose-related reduction in the incidence of aortic fatty streaks that reached asymptote at 2.5 mg.kg(-1) and by an improvement of the impaired endothelial function. When given orally to chow-fed hamsters, F 12511 elicited a dose-related decrease in plasma cholesterol from 9% at 0.63 mg.kg(-1) up to 31% at 40 mg.kg(-1) associated with a preferential reduction in atherogenic lipoproteins, very low density lipoproteins (VLDL) and low density lipoproteins (LDL). Moreover, in the same dose range, F 12511 decreased hepatic cholesteryl ester concentrations and reduced liver ex vivo ACAT activity. By using a bioassay, ACAT inhibitory activity was present in plasma of treated hamsters 1 hr after oral administration of F 12511. Hence, the results in chow-fed hamsters are consistent with systemic and direct hepatic effects of F 12511. In guinea-pigs, an adreno-sensitive species, F 12511 did not impair the adrenal function (adrenocorticotrophic hormone challenge) at doses up to 2.5 mg.kg(-1,) far higher than those eliciting hypocholesterolemic effects in the same species. In conclusion, the results suggest that F 12511, a powerful and systemic ACAT inhibitor, constitutes an appropriate tool to determine whether the inhibition of ACAT constitutes an effective therapy for the treatment of hypercholesterolemia and of atherosclerosis in man.

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.

The effects of fatty acids on apolipoprotein B secretion by human hepatoma cells (HEP G2)

We have investigated the effect of fatty acids on the rate of apolipoprotein B (apo B) secretion by human hepatoma cells (Hep G2). When Hep G2 cells were maintained in tissue culture flasks oleic acid up to 0.4 mM increased apo B secretion in a dose-dependent manner, whereas increases in triacylglycerol (TG) were smaller and dose dependency was less evident. In the absence of oleic acid, apo B accumulating in the tissue culture medium was predominantly in lipoproteins of higher density than very low density lipoproteins (VLDL). However, when the rate of secretion was stimulated with oleic acid the apo B-containing lipoproteins became lower in density. We postulated that there was a high rate of lipolysis of newly secreted VLDL by Hep G2 cells, which would account both for the relatively smaller effect of oleic acid on TG as opposed to apo B accumulating in the culture medium and the predominance of apo B in lipoproteins of a higher density than VLDL, which became less evident when VLDL secretory rates were stimulated by oleic acid. To test this hypothesis, cultured Hep G2 cells were transferred to columns containing Cytodex beads, permitting their continuous perfusion with culture medium so that newly secreted VLDL did not remain in contact with the cells. Apo B recovered from the perfusate was largely in VLDL range lipoproteins and the TG measured in the perfusate indicated that the true secretory rate of TG-rich lipoproteins was substantially higher than had been reflected by TG accumulating in culture medium left in contact with cells. Apo B measured in the culture medium of Hep G2 cells may thus be a better reflection of VLDL secretion, even though it is contained in higher density lipoproteins due to removal of TG by lipolysis. The effects of saturated fatty acids (SFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) on apo B (apo B) secretion by Hep G2 cells maintained in tissue culture flasks were next investigated. SFA (0.4 mM), with the exception of stearic acid (C18:0), increased apo B secretion. Lauric acid (C12:0) increased apo B secretion by 32%, myristic acid (C14:0) by 41% (P<0.005), palmitic acid (C16:0) by 154% (P<0.025), and arachidic acid (C20:0) by 186% (P<0.005). The effect of MUFA (0.4 mM) was to increase apo B secretion, oleic acid (C18:1) by 239% ((P<0.0005) and palmitoleic acid (C16: 1) by 125% (P<0.005). Of the PUFA investigated, linolenic acid (C18:3) (0.4 mM) did not have any significant effect on apo B secretion, whereas linoleic acid (C18:2) (0.4mM) arachidonic acid (C20:4) (0.1 mM) and eicosapentaenoic acid (C20:5) (0.1 mM) caused significant increases of 164, 171 and 171%, respectively (P<0.005). The fatty acids studied increased intracellular TG and cholesteryl ester concentrations to varying extents. The increase in intracellular TG produced by the different fatty acids correlated with the rate of apo B secretion (r=0.6; P<0.05). In this human hepatoma cell line, with the exception of the saturated fatty acids, the rate of secretion of apo B-containing lipoproteins does not follow the same pattern as changes in circulating low density lipoprotein (LDL) concentrations reported with dietary manipulation in man. If our findings reflect the in vivo situation, we suggest that whilst the dietary effects of SFA on serum LDL may in part be determined by the hepatic apo B secretory rate, the effects of MUFA and PUFA must be largely mediated through a catabolic effect rather than an effect on hepatic secretion. The marked increase in apo B secretion with the more highly polyunsaturated fatty acids, such as eicosapentaenoic acid, may also explain why they do not lower circulating LDL, despite reports of their apparently favourable effect on LDL-receptor mediated clearance.

Manipulation of cholesterol and cholesteryl ester synthesis has multiple effects on the metabolism of apolipoprotein B and the secretion of very-low-density lipoprotein by primary hepatocyte cultures

Inhibition of esterified and non-esterified cholesterol synthesis by lovastatin in primary rat hepatocytes suppressed the net synthesis and very-low-density lipoprotein (VLDL) secretion of apolipoprotein B (apoB)-48 and apoB-100. Lovastatin did not alter the rates of apoB-48 and apoB-100 post-translational degradation. 25-Hydroxycholesterol, which inhibited non-esterified cholesterol synthesis but increased the synthesis of cholesteryl ester, showed differential effects on the metabolism of apoB-48 and apoB-100. Whereas the secretion of apoB-48 VLDL was suppressed there was no effect on the secretion of apoB-100 VLDL. The post-translational degradation of apoB-48, but not of apoB-100, was enhanced by 25-hydroxycholesterol. The net synthesis rates of apoB-48 and apoB-100 were unaffected by 25-hydroxycholesterol. The inhibitory effect of lovastatin alone on the net synthesis of apoB-48 and apoB-100 was reversed by the simultaneous presence of 25-hydroxycholesterol, suggesting a role for newly synthesised cholesteryl ester. Prevention of the reversal effect by the acyl-CoA: cholesterol acyltransferase (ACAT) inhibitor YM 17E supported this interpretation. In the presence of lovastatin, restoration of the net synthesis of apoB by 25-hydroxycholesterol was not accompanied by an increased VLDL output of apoB-48 and apoB-100. However, under these conditions there was an increased post-translational degradation of apoB-48 and apoB-100. These results suggest that interference with intracellular cholesterol and cholesteryl ester metabolism interrupts VLDL assembly at sites of both apoB net synthesis and post-translational degradation.

Acyl coenzyme A: cholesterol acyltransferase inhibition and hepatic apolipoprotein B secretion

Acyl coenzyme A: cholesterol acyltransferase (ACAT) is postulated to play a role in hepatic and intestinal lipoprotein secretion. There is accumulating evidence, both in vitro and in vivo, that cholesterol and/or cholesteryl ester availability can regulate hepatic VLDL secretion. How ACAT inhibition regulates the assembly and secretion of apolipoprotein (apo) B containing lipoproteins within the hepatocyte has not been clearly established. ApoB kinetic studies performed in animals indicate that reduction in VLDL apoB secretion is an important mechanism whereby ACAT inhibitors decrease the plasma concentrations of these lipoproteins. However, in cultured hepatocytes, the effect of ACAT inhibition on apoB secretion has been inconsistent. Recent evidence has suggested the existence of more than one ACAT enzyme in mammals, which has culminated in the recent cloning of ACAT2. ACAT1 and ACAT2 respond differently to ACAT inhibitors of differing structures and classes. ACAT2 is present in the liver and intestine, the sites of apoB containing lipoprotein secretion and may represent the enzyme responsible for generating cholesteryl esters destined for lipoprotein assembly and secretion.

Plasma acylation stimulating protein, adipsin and lipids in non-obese and obese populations

BACKGROUND

Acylation stimulating protein (ASP) is a potent stimulator of TG synthesis in human adipocytes.

DESIGN

In the present study, we have analysed plasma ASP and adipsin levels and their relationships to plasma lipids in non-obese and obese groups.

RESULTS

The results show that the frequency distribution of ASP is skewed but that of adipsin is normal in both groups. In the non-obese population, the mean levels of plasma ASP and adipsin were 20.2 nmol L-1 (median) and 66.6 +/- 19 nmol L-1 (mean) respectively. No difference was observed between men and women for each of the parameters. In the obese population, the median plasma ASP was increased by 246% (69.9 nmol L-1) and adipsin by 31% (87.0 +/- 22.7 nmol L-1) above that of the control group. Although the levels for men and women were not statistically different for adipsin, the median ASP plasma concentration was 1.9-fold higher in obese women than in obese men (71.8 nmol L-1 vs. 37.6 nmol L-1, P < 0.05). Best subset regression analysis provided a model with variables that best predict plasma ASP [r2 = 0.160, P < 0.008 for body mass index (BMI), P < 0.05 for triacylglycerol (TG), P < 0.03 for free fatty acid (FFA)] and plasma adipsin (r2 = 0.057, P < 0.017 for BMI) in a non-obese population. In obese subjects, the model was different for plasma ASP (P = NS for any of the variables) and plasma adipsin (r2 = 0.356, P < 0.008 for FFA, P < 0.0002 for BMI, P < 0.02 for age). There was no correlation between ASP and adipsin in either the non-obese or the obese group.

CONCLUSION

The present data suggest involvement of the ASP/adipsin pathway in the pathogenesis of obesity.

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.

Chylomicron synthesis by intestinal cells in vitro and in vivo

Synthesis and secretion of chylomicrons by the intestine is essential to transport dietary fats in the circulation and to deliver these fats to the appropriate peripheral tissues. The assembly of chylomicrons within the enterocyte and the subsequent secretion of these lipoprotein particles into the lymph is a complex, multi-step process that includes absorption of lipids by the enterocytes, cellular lipid (re)synthesis and translocation of cellular lipid pools, synthesis and post-translational modification of various apolipoproteins and, finally, the assembly of lipid and lipoprotein components into a chylomicron. The key process in chylomicron synthesis is the intracellular association of apolipoprotein (apo)B48, the structural protein of chylomicrons, with lipids.

Dietary soluble fiber lowers plasma LDL cholesterol concentrations by altering lipoprotein metabolism in female guinea pigs

This experiment was designed to evaluate the effects of pectin (PE), guar gum (GG) and psyllium (PSY) intake on VLDL and LDL metabolism in female guinea pigs fed high dietary cholesterol. Guinea pigs were fed a 15 g/100 g fat diet containing 0.25 g/100 g cholesterol with 12.5 g/100 g PE, 12.5 g/100 g GG, 7.5 g/100 g PSY or 12.5 g/100 g cellulose (control diet) for 4 wk. Plasma cholesterol concentrations were 29, 43 and 39% lower in guinea pigs fed PE, GG or PSY, respectively, compared with the control group (P < 0.0001). Plasma apolipoprotein (apo) B concentrations were 16-22% lower in the groups fed soluble fiber compared with the control group (P < 0.01). In contrast, hepatic cholesterol and triglyceride concentrations were not different among the PE, GG, PSY and control groups. No differences in triacylglycerol (TAG) or apo B secretion rates, measured by blocking VLDL catabolism by triton (WR 1339) injection, were observed, whereas plasma LDL apo B fractional catabolic rates (FCR), determined by injection of radiolabeled LDL, were higher in guinea pigs fed GG or PSY than in those from the control group. All sources of dietary soluble fiber reduced LDL apo B flux (P < 0.05). These results suggest that the mechanisms of plasma LDL cholesterol lowering by dietary soluble fiber are distinctive for each fiber source and result in specific alterations in lipoprotein metabolism in female guinea pigs. Differences between male and female guinea pigs in response to these diets are discussed.

Dietary proteins modulate the effects of fish oil on triglyceridemia in the rat

Sprague-Dawley rats were fed purified diets varying in both protein (20%) and lipid (11%) content for 28 d to verify the independent and interactive effects of dietary proteins and lipids on serum and hepatic lipids, and on tissue lipoprotein lipase (LPL) activity in both fasted and postprandial states. These diets consisted of either casein-menhaden oil, casein-coconut oil, soy protein-menhaden oil (SPMO), soy protein-coconut oil, cod protein-menhaden oil, or cod protein-coconut oil. A randomized 3 x 2 factorial design was used. A significant protein-lipid interaction was seen on serum triglyceride levels: menhaden oil, compared with coconut oil, induced a decrease in serum triglyceride levels when combined with soy protein but not when combined with cod protein and casein. The lower serum triglyceride concentrations observed in the SPMO-fed rats could be the result of decreased hepatic triglycerides when soy protein was compared with casein and when menhaden oil was compared with coconut oil. Total LPL activity in the heart was higher in menhaden oil-fed rats than in coconut oil-fed rats in the postprandial state. The higher LPL activity in the heart could, however, explain only 10% of the reduction of serum triglycerides, contributing slightly to the lowering effects of SPMO diet on serum triglycerides. Therefore, the present results indicate that dietary proteins can modulate the effects of fish oil on triglyceridemia in the rat, and that could be mainly related to specific alterations in hepatic lipid concentrations.

Homocysteine stimulates the production and secretion of cholesterol in hepatic cells

Homocysteinemia and hypercholesterolemia are important risk factors associated with the occurrence of arteriosclerotic vascular diseases. A positive correlation between plasma levels of homocysteine and cholesterol was found in homocysteinemic patients as well as in experimental animals. In the present study, the effect of homocysteine on the production and secretion of cholesterol in human hepatoma cell line HepG2 cells was investigated. When cells were incubated with 4 mM homocysteine, the amounts of total cholesterol produced as well as the cholesterol secreted by these cells were significantly increased (from 32 +/- 5 to 74 +/- 5 nmol/mg cellular protein). Further biochemical analyses revealed that the increase in cholesterol was resulted from an enhancement in the production and secretion of the unesterified cholesterol with no concomitant change in the level of cholesteryl esters. The activity of intracellular 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase was markedly elevated by 131% and 190% after cells were incubated with homocysteine for 24 and 48 h. Homocysteine also stimulated the secretion of apo B100 by HepG2 cells (from 0.84 +/- 0.11 to 1.37 +/- 0.12 micrograms apolipoprotein B/mg cellular protein). Our results demonstrate that homocysteine stimulates the production and secretion of cholesterol and apolipoprotein B100 in HepG2 cells. The increase in the production of cholesterol induced by homocysteine may contribute to the pathogenesis of arteriosclerosis.

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.

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.

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.

Divergent responses of the liver to increased delivery of glucose or fatty acids: implications for the pathogenesis of type IV hyperlipoproteinemia

This study examines the patterns of response of primary cultures of hamster hepatocytes to increased delivery of glucose or oleate. Increased glucose in the medium produced: (1) increased triglyceride in the cells and the medium; (2) no change in cholesterol ester in the cells or the medium; (3) no change in apo B100 secreted into the medium; (4) more apo B100 particles within the VLDL range with an increase in the VLDL triglyceride to apo B100 ratio. By contrast, increased oleate in the medium resulted in: (1) increased triglyceride in the cells and the medium; (2) increased cholesterol ester in the cells and the medium; and (3) increased apo B100 secreted into the medium. Important differences in the intracellular metabolism of triglyceride and cholesterol ester were also documented. Under all circumstances, there was substantially more radiolabelled triglyceride (overall eight times more) in the cell than in the medium, indicating that up to 90% of the newly synthesized triglyceride enters the cellular pool rather than being secreted with apo B100. By contrast, almost half of the newly synthesized cholesterol ester molecules were secreted with apo B100, pointing to an equal probability of entering the cell storage pool as opposed to being secreted. The data establish therefore two patterns of response of the liver to increased triglyceride synthesis depending on whether the substrate drive is glucose or oleate.

Characterization and modulation of LP(a) in human hepatoma HEPG2 cells

HepG2 cells have been widely used to study factors which affect the secretion of apoB100 lipoprotein particles. The objectives of this study were to determine if Lp(a) particles were present in conditioned medium from HepG2 cells and if so, was this accumulation affected by factors which alter apoB100 lipoprotein metabolism. The data demonstrate that Lp(a) accumulated in the medium in a time dependent manner over a 48 h incubation period. Ultracentrifugation fractionation and Western blot analysis demonstrated that lipoprotein particles containing apo(a) in complex with apoB100 were present at a density consistent with human plasma Lp(a). Incubation of the HepG2 cells with LDL or VLDL caused increases in Lp(a) accumulation in the medium (+33% +/- 14%, P NS and 56% +/- 21%, P < 0.05, respectively). In contrast, apo(a) mRNA decreased (-17% +/- 3%, P < 0.01 for both LDL and VLDL incubation). Increasing concentrations of amino acids in the medium resulted in progressively less Lp(a) and apoB100 in the medium, the effect being greater on apoB100. ApoB100 mRNA levels decreased with incubation of HepG2 cells with amino acids (-22% +/- 10%, P < 0.06) whereas apo(a) mRNA levels increased significantly (+47% +/- 14%, P < 0.005). Taken together, our data show that HepG2 cells express mRNA for apo(a), and accumulate Lp(a) in the medium. The close correlation of medium Lp(a) levels with medium apoB100 levels, and not with apo(a) mRNA levels, suggests that medium Lp(a) accumulation may be a function of lipoprotein synthesis and secretion and is consistent with extracellular assembly of Lp(a) lipoprotein particles.

Inhibition of HMG-CoA reductase by atorvastatin decreases both VLDL and LDL apolipoprotein B production in miniature pigs

In the present studies, the 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor atorvastatin was used to test the hypothesis that inhibition of cholesterol biosynthesis in vivo with a consequent reduction in the availability of hepatic cholesterol for lipoprotein synthesis, would (1) reduce very low density lipoprotein (VLDL) apolipoprotein B (apoB) secretion into the plasma, (2) reduce the conversion of VLDL apoB to LDL apoB, and (3) reduce LDL apoB direct synthesis. ApoB kinetic studies were carried out in six control miniature pigs and in six animals after 21 days of administration of atorvastatin (3 mg/kg per day). Pigs were fed a fat- (34% of calories; polyunsaturated to monounsaturated to saturated ratio, 1:1:1) and cholesterol- (400 mg/d cholesterol; 0.1%; 0.2 mg/kcal) containing pig chow-based diet. Atorvastatin treatment significantly reduced plasma total cholesterol, LDL cholesterol, total triglyceride, and VLDL triglyceride concentrations by 16%, 31%, 19%, and 28%, respectively (P < .01). Autologous 131I-VLDL, 125I-LDL, and [3H]leucine were injected simultaneously into each pig, and apoB kinetic data were analyzed using multicompartmental analysis (SAAM II). The VLDL apoB pool size decreased by 29% (0.46 versus 0.65 mg/kg; P = .002), which was entirely due to a 34% reduction in the VLDL apoB production rate (PR) (1.43 versus 2.19 mg/kg per hour; P = .027). The fractional catabolic rate (FCR) was unchanged. The LDL apoB pool size decreased by 30% (4.74 versus 6.75 mg/kg; P = .0004), which was due to a 22% reduction in the LDL apoB PR (0.236 versus 0.301 mg/kg per hour; P = .004), since the FCR was unchanged. The reduction in LDL apoB PR was primarily due to a 34% decrease in conversion of VLDL apoB to LDL apoB; however, this reduction was not statistically significant (P = .114). Hepatic apoB mRNA abundance quantitated by RNase protection assay was decreased by 13% in the atorvastatin-treated animals (P = .003). Hepatic and intestinal LDL receptor mRNA abundances were not affected. We conclude that inhibition of hepatic HMG-CoA reductase by atorvastatin reduces both VLDL and LDL apoB concentrations, primarily by decreasing apoB secretion into the plasma and not by an increase in hepatic LDL receptor expression. This decrease in apoB secretion may, in part, be due to a reduction in apoB mRNA abundance.

Assembly and secretion of VLDL in nondifferentiated Caco-2 cells stably transfected with human recombinant ApoB48 cDNA

Intestinal cells secrete apoB48-containing very low density lipoproteins (VLDLs) and chylomicrons for the transport of biliary and dietary lipids. The molecular mechanisms regulating the assembly of intestinal lipoproteins are not known due to a lack of reliable and specific cell culture models. Caco-2 (a human colon carcinoma) cells have been used to study intestinal lipid metabolism. These cells have been shown to secrete both apoB100- and apoB48-containing triglyceride (TG)-rich lipoproteins only after differentiation into enterocyte-like cells. To study lipoprotein assembly in nondifferentiated Caco-2 cells, we stably expressed human recombinant apoB48 cDNA under the control of a constitutive cytomegalovirus promoter. Pulse-chase analysis revealed that the majority (> 50%) of apoB48 synthesized was degraded intracellularly in the presence or absence of oleic acid. Transfected nondifferentiated cells secreted lipoproteins with flotation densities similar to those of plasma HDL or LDL when cultured in serum-free or serum-containing media, respectively. Incubation of cells with media containing serum and oleic acid resulted in the secretion of VLDL-like particles. Secretion of VLDL was inhibited (> 80%) by triacsin C due to > 60% inhibition of oleate-induced TG synthesis. However, inhibition of cholesteryl ester synthesis by 70% with an acyl coenzyme A:cholesterol acyltransferase inhibitor did not affect VLDL secretion. Efficient assembly of lipoproteins usually requires the microsomal TG transfer protein (MTP). The presence of MTP in transfected Caco-2 cells was investigated by measuring TG transfer activity in microsomal fractions. Microsomal fractions had 0.2% TG transfer activity per hour per microgram of protein, which corresponds to 30% to 60% of the MTP activity present in liver-derived cells. To determine whether MTP activity was required for lipoprotein assembly, transfected cells were incubated in the presence of the MTP inhibitor CP-10,447. This compound completely abolished the secretion of apoB. These data show that the transfected cell lines secrete lipoproteins of different densities under different culture conditions and that the assembly of larger VLDL particles requires active TG synthesis and MTP activity. Thus, in nondifferentiated Caco-2 cells, the amount of apoB secreted and not the MTP activity is the limiting factor for lipoprotein assembly.

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.

Plasma Acylation-Stimulating Protein in Coronary Artery Disease

To date, plasma levels of acylation-stimulating protein (ASP) have been determined only in normal and obese individuals. Accordingly, ASP was measured in fasting samples obtained from 59 age-matched controls and 208 patients with documented coronary artery disease (CAD). Overall, plasma ASP was significantly higher in the CAD subjects compared to the control subjects (55.3±1.8 nmol/L CAD versus 32.0±2.6 nmol/L control, P <.QQQ5). In the control group, the distribution of plasma ASP values was unimodal whereas in the coronary group it was significantly skewed to the right. The coronary group was subdivided into those with pronounced elevation of apoB (a marked type II phenotype, n=13), those with hypertriglyceridemia with a normal apoB (n=17), and the remaining CAD subjects (n=178). In the first two groups, ASP did not differ significantly from control subjects (43±2.8 nmol/L and 49±4.4 nmol/L respectively). By contrast, in the remaining CAD subjects, both the mean ASP level (56.8±2.0 nmol/L, P <.001 by ANOVA) and the proportion of patients with a markedly elevated ASP (25.3% were >95th percentile, P <.005 versus control by χ 2 ) w e r e significantly increased. When this third group was divided into tertiles by plasma apoB and triglyceride, there was a direct relationship between plasma ASP and these two parameters. Linear regression analysis demonstrated an association between plasma ASP and plasma triglyceride ( P <.05), VLDL cholesterol ( P <.025), and VLDL apoB ( P <.05). Finally, when all of the CAD subjects were divided by apoE phenotype, there appeared to be a relationship between plasma ASP and apoE phenotype such that ASP was higher in E2 subjects, intermediate in E3 subjects, and lower in E4 subjects. The present data document plasma ASP levels in a number of dyslipoproteinemic states and suggest a relation between the adipsin-ASP pathway and other metabolic determinants of lipoprotein metabolism. ( Arterioscler Thromb Vase Biol. 1997;17:1239-1244.)

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.