Effect of fatty acids on the synthesis and secretion of apolipoprotein B by rat hepatocytes

The modulation of apolipoprotein B synthesis and secretion by fatty acids in rat hepatocytes was studied. Maximum apolipoprotein B production was obtained in the case of oleic acid followed by linoleic, stearic and palmitic/linolenic acid when compared to control which was not supplemented with any fatty acids. Oleic acid was found to exert a concentration dependent increase in the secretion of [³H] apolipoprotein B into the medium while that associated with the cell layer was not affected. Pulse chase experiments in the presence of oleic acid showed that it caused an increase in the secretion of apolipoprotein B into the medium.¹⁴C-acetate incorporation into cholesterol and cholesteryl ester associated with the cell layer and secreted very low density lipoproteins also showed an increase in the presence of oleic acid indicating an increase in cholesterogenesis. The effect of oleic acid on [³H] apolipoprotein B and very low density lipoproteins secretion appeared to be mediated through cholesterol as (i) ketoconazole, an inhibitor of cholesterol synthesis caused significant reduction in the stimulatory effect of oleic acid on apolipoprotein secretion and (ii) mevinolin, another inhibitor of cholesterol synthesis also reversed the stimulatory effect of oleic acid on apolipoprotein B secretion. These results indicated that oleic acid may influence apolipoprotein B synthesis and secretion in hepatocytes probably by affecting cholesterol/cholesteryl ester formation which may be a critical component in the secretion of apolipoprotein B as lipoproteins     

Secretion of apolipoprotein B in serum-free cultures of human hepatoma cell line, HepG2

We have developed a defined medium which can maintain efficient growth of HepG2 cells sustaining the synthesis of a variety of plasma proteins including apolipoprotein B. This defined system was used to investigate long-term effects of insulin, estrogen, triiodothyronine, cholesterol, and oleate on the growth pattern of HepG2 cells and secretion rate of apolipoprotein B. Oleate and triiodothyronine caused significant increases in secretion of apolipoprotein B. The stimulatory effect of triiodothyronine was only observed after long (6 days) exposure of cells to the hormone. In contrast, insulin caused up to a 4-fold decrease in the secretion rate of apolipoprotein B during the early growth periods. This inhibitory effect appeared to be partially abolished after 6 days. Our data suggest that some important questions on regulation of apolipoprotein B expression can be addressed by the long-term culture of HepG2 cells in defined medium.     

Ferritins can regulate the secretion of apolipoprotein B

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

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.     

Recent studies of lipoprotein kinetics in the metabolic syndrome and related disorders

PURPOSE OF REVIEW: Dyslipoproteinemia is a cardinal feature of the metabolic syndrome that accelerates atherosclerosis. Recent in-vivo kinetic studies of dyslipidemia in the metabolic syndrome are reviewed here. RECENT FINDINGS: The dysregulation of lipoprotein metabolism may be caused by a combination of overproduction of VLDL apolipoprotein B-100, decreased catabolism of apolipoprotein B-containing particles, and increased catabolism of HDL apolipoprotein A-I particles. Nutritional modifications and increased physical exercise may favourably alter lipoprotein transport by collectively decreasing the hepatic secretion of VLDL apolipoprotein B and the catabolism of HDL apolipoprotein A-I, as well as by increasing the clearance of LDL apolipoprotein B. Conventional and new pharmacological treatments, such as statins, fibrates and cholesteryl ester transfer protein inhibitors, can also correct dyslipidemia by several mechanisms, including decreased secretion and increased catabolism of apolipoprotein B, as well as increased secretion and decreased catabolism of apolipoprotein A-I. SUMMARY: Kinetic studies provide a mechanistic insight into the dysregulation and therapy of lipid and lipoprotein disorders. Future research mandates the development of new tracer methodologies with practicable in-vivo protocols for investigating fatty acid turnover, macrophage reverse cholesterol transport, cholesterol transport in plasma, corporeal cholesterol balance, and the turnover of several subpopulations of HDL particles.     

A proteomic approach identifies proteins in hepatocytes that bind nascent apolipoprotein B

The biogenesis of apolipoprotein B is quite complex in view of its huge size, hydrophobicity, obligate association with lipids such as cholesterol and triglycerides prior to secretion, and intracellular degradation of a substantial proportion of newly synthesized molecules. Multiple proteins likely serve roles as molecular chaperones to assist in folding, assembly with lipids, and regulation of the secretion of apolipoprotein B. In these studies, we developed a strategy to isolate proteins associated with apolipoprotein B in rat livers. The purification consisted of two stages: first, microsomes were prepared from rat liver and treated with chemical cross-linkers, and second, the solubilized proteins were co-immunoprecipitated with antibody against apolipoprotein B. We found that several proteins were cross-linked to apolipoprotein B. The proteins were digested with trypsin, and the released peptides were sequenced by tandem mass spectrometry. The sequences precisely matched 377 peptides in 99 unique proteins. We show that at least two of the identified proteins, ferritin heavy and light chains, can directly bind apolipoprotein B. These and possibly other proteins identified by this proteomic approach are novel candidates for proteins that affect apolipoprotein B during its biogenesis.     

Inhibition of apolipoprotein B net synthesis and secretion from cultured rat hepatocytes by the calcium-channel blocker diltiazem.

1. The effect of the Ca2+-channel blocker diltiazem on hepatic apolipoprotein B (apo B) synthesis and secretion was studied in 12-18 h cultures of collagenase-dispersed rat hepatocytes. 2. The presence of diltiazem in the medium decreased apo B secretion by hepatocytes in a concentration-dependent manner. At 25 microM, diltiazem inhibited apo B secretion by approx. 36%, but there was no evidence of intracellular accumulation of apo B. 3. The inhibition of apo B secretion by hepatocytes was significantly correlated with cell-associated diltiazem (r = 0.72, P less than 0.01). 4. The rate of apo B secretion remained linear over 16 h even in the presence of 50 microM-diltiazem. 5. At diltiazem concentrations in the medium which were inhibitory for apo B secretion, [14C]acetate incorporation into cellular lipids and [35S]methionine incorporation into protein were enhanced. 6. Diltiazem inhibited the secretion of the apo B variants with a preferential inhibition of the higher-molecular-mass form of apo B (apo BH) over the lower-molecular-mass form (apo BL) at diltiazem concentrations in the medium greater than 25 microM. 7. Together, these results suggest that Ca2+ may play an important role in the synthesis and secretion of apo B-containing lipoproteins.     

Synthesis,processing, and secretion of hepatic very low density lipoprotein

Very low density lipoprotein (VLDL) is the major vehicle in the plasma which carries triacylglycerol synthesized in the liver to peripheral tissues for utilization. Estrogen-induced chick parenchymal liver cells (hepatocytes) synthesize and secrete large amounts of VLDL. These cells, in a primary monolayer culture system developed in this laboratory, have been employed to study the operative and regulatory aspects of VLDL synthesis, assembly, and secretion. Some 10 min are required for the translation of the principle VLDL protein constituent, apolipoprotein B, and 30–35 min are required for the two newly translated chick VLDL apolipoproteins, apolipoprotein B and apolipoprotein II, to be secreted. Apolipoprotein B is synthesized on membrane-bound polysomes as a contiguous polypeptide chain of 350K molecular weight (MW) and is not assembled posttranslationally from smaller-peptide precursors. Translocation of puromycin-discharged apolipoprotein B nascent chains into the endoplasmic reticulum lumen and their subsequent secretion are independent of both ongoing protein synthesis and the attachment of the nascent peptides to ribosomes. Apolipoprotein B nascent chains discharged by puromycin assemble with glycerolipid (mainly triacylglycerol) and are secreted as immunoprecipitable VLDL. Core oligosaccharides are added to the apolipoprotein B nascent chain co-translationally in at least two stages, at molecular weights of ～ 120K and ～ 280K. Inhibition of N-linked glycosylation of apolipoprotein B with tunicamycin affects neither the assembly of glycerolipids into VLDL nor the secretion of the VLDL particle, indicating that aglyco-apolipoprotein B can serve as a functional component for VLDL assembly and secretion. Active synthesis of the VLDL apolipoproteins is required, however, for glycerolipid assembly into VLDL and secretion from the hepatocyte. The differential kinetics with which newly synthesized apolipoproteins and glycerolipids are secreted as VLDL and the timing of the effects of protein-synthesis inhibitors on their secretion indicate that VLDL constituents are assembled sequentially in the intact liver cell. The bulk of the VLDL triacylglycerol and some VLDL phosphoglyceride is introduced early in the secretory pathway proximal, yet subsequent to apopeptide synthesis, while a significant fraction of VLDL phosphoglyceride associates with the resulting triacylglycerol-rich lipid-protein complexes just prior to their secretion as mature VLDL. Within the context of current models for VLDL structure, the late assembly of phosphoglyceride into VLDL is taken to represent a surface maturation of the nascent VLDL particle.     

Regulation of apolipoprotein secretion by long-chain polyunsaturated fatty acids in newborn swine enterocytes

Long-chain polyunsaturated fatty acids (LC-PUFA) are important in the development of the immature nervous system, and adding these fatty acids to infant formula has been proposed. To determine the effect of n-3 LC-PUFA on apolipoprotein secretion and lipid synthesis in newborn swine enterocytes, differentiated IPEC-1 cells were incubated for 24 h with docosahexaenoic acid (DHA; 22:6) or eicosapentaenoic acid (EPA; 20:5) complexed with albumin at a fatty acid concentration of 0.8 mM or albumin alone (control) added to the apical medium. Oleic acid (OA; 18:1) was used a control for lipid-labeling studies. Both DHA and EPA reduced apolipoprotein (apo) B secretion by one-half, whereas EPA increased apo A-I secretion. The increased apo A-I secretion occurred primarily in the high-density lipoprotein fraction. These changes in apoprotein secretion were not accompanied by significant changes in synthesis. Modest decreases in apo B mRNA levels were observed for DHA and EPA, whereas there were no changes in apo A-I mRNA abundance. EPA reduced cellular triacylglycerol labeling by one-half, and DHA and EPA decreased cellular phospholipid labeling compared with OA. Labeled triacylglycerol secretion was decreased 75% by EPA, and DHA doubled labeled phospholipid secretion. If present in vivo, these effects should be considered before supplementing infant formula with these fatty acids.     

Evolution and mechanism of apolipoprotein B-containing lipoprotein assembly

PURPOSE OF REVIEW: Apolipoprotein B-containing lipoprotein assembly and secretion is critical for lipid absorption and triglyceride homeostasis, and plays a role in atherogenesis and the pathobiology of type 2 diabetes and obesity. This review highlights recent insights into the evolutionary, structural, and cell biology of hepatic and intestinal pathways for lipid mobilization, and the mechanisms and regulation of lipoprotein assembly and secretion. RECENT FINDINGS: Until recently it was assumed that microsomal triglyceride transfer protein-dependent apolipoprotein B-containing lipoprotein assembly was a unique adaptation associated with vertebrate lipid homeostasis. However, it is now clear that microsomal triglyceride transfer protein (MTP) exists in species whose last common ancestor diverged over 550 million years ago. In its long evolutionary history, the MTP gene has given rise to a series of paralogous lipid transport proteins, all of which require MTP for their biogenesis. During its evolution, MTP has acquired new functions, enabling it to participate in a disparate array of lipid mobilization and transport pathways, ranging from primitive lipoprotein assembly to antigenic lipid presentation. In addition to the complex and multifunctional role of MTP in apolipoprotein B assembly, other factors responsible for the generation of secretion-coupled lipids and the modulation of apolipoprotein B production are emerging. SUMMARY: The phylogenic dissection of MTP and apolipoprotein B function, coupled with ongoing structural and biochemical analyses, provide significant insights into the mechanisms of lipid mobilization and secretion. Some of these factors and processes may be targeted therapeutically to modulate the quantitative and qualitative aspects of apolipoprotein B production.     

HIV protease inhibitors protect apolipoprotein B from degradation by the proteasome: a potential mechanism for protease inhibitor-induced hyperlipidemia.

Highly active anti-retroviral therapies, which incorporate HIV protease inhibitors, resolve many AIDS-defining illnesses. However, patients receiving protease inhibitors develop a marked lipodystrophy and hyperlipidemia. Using cultured human and rat hepatoma cells and primary hepatocytes from transgenic mice, we demonstrate that protease inhibitor treatment inhibits proteasomal degradation of nascent apolipoprotein B, the principal protein component of triglyceride and cholesterol-rich plasma lipoproteins. Unexpectedly, protease inhibitors also inhibited the secretion of apolipoprotein B. This was associated with inhibition of cholesteryl-ester synthesis and microsomal triglyceride transfer-protein activity. However, in the presence of oleic acid, which stimulates neutral-lipid biosynthesis, protease-inhibitor treatment increased secretion of apolipoprotein B-lipoproteins above controls. These findings suggest a molecular basis for protease-inhibitor-associated hyperlipidemia, a serious adverse effect of an otherwise efficacious treatment for HIV infection.     

地塞米松对HepG2细胞分泌载脂蛋白的影响

以培养的人肝癌细胞系HepG2为研究对象,采用“冻干浓缩培养液载脂蛋白测定法”,考察了地塞米松对HepG2细胞载脂蛋白(apolipoprotein,apo)AⅠ、AⅡ、CⅢ、B100及E分泌的影响.结果表明：地塞米松对HepG2细胞apoAⅠ和apoE的分泌有促进作用,对apoAⅡ、apoB100和apoCⅢ的分泌有抑制作用,且这种作用随地塞米松浓度的增加而增强.当培养液中地塞米松的浓度为5.5×10^－5mol/L时,apoAⅠ和apoE的分泌分别增加36.6%和49.4%（P＜0.01）,apoAⅡ、apoB100和apoCⅢ的分泌分别减少38.9%、31.9%和29.8%（P＜0.01）.     

Astrocytes synthesize apolipoprotein E and metabolize apolipoprotein E-containing lipoproteins

We have previously demonstrated that astrocytes synthesize and secrete apolipoprotein E in situ. In the present work, primary cultures of rat brain astrocytes were used to study apolipoprotein E synthesis, secretion, and metabolism in vitro. The astrocytes in culture contained immunoreactive apolipoprotein E in the area of the Golgi apparatus. Incubation of the astrocytes with [35S]methionine resulted in the secretion of labeled immunoprecipitable apolipoprotein E, which constituted 1-3% of the total secreted proteins. The apolipoprotein E secreted in culture and the apolipoprotein E in rat brain extracts differed from serum apolipoprotein E in two respects: both had a slightly higher apparent molecular weight (approx. 36,000) and more acidic isoforms than serum apolipoprotein E. Sialylation of the newly secreted apolipoprotein accounted for the difference in both the apparent molecular weight and isoelectric focusing pattern of newly secreted apolipoprotein E and plasma apolipoprotein E. The astrocytes possessed apolipoprotein B,E(LDL) receptors capable of binding and internalizing apolipoprotein E-containing lipoproteins. The uptake of lipoproteins by the cells led to a reduction in the number of cell surface receptors and to the intracellular accumulation of cholesteryl esters. Since apolipoprotein E is present within the brain, and since brain cells can express apolipoprotein B,E(LDL) receptors, apolipoprotein E-containing lipoproteins may function to redistribute lipid and regulate cholesterol homeostasis within the brain.     

Dietary cholesterol does not affect the synthesis of apolipoproteins B and E by rat hepatocytes.

To examine the unproved hypothesis that dietary cholesterol affects the synthesis of apolipoprotein B and E, we fed rats a cholesterol-rich diet that has been shown to alter dramatically the serum concentrations of these apolipoproteins. Rats fed for 4 weeks on a cholesterol-rich diet accumulate increased concentrations of low Mr apolipoprotein B (+2.7-fold) and decreased concentrations of apolipoprotein E (-40%) in their serum. Hepatocytes obtained from similarly treated rats were placed in monolayer culture and the rate of synthesis de novo of apolipoproteins was determined. Although cells from cholesterol-fed rats remained filled with lipid droplets throughout the experimental period, there was no difference in plating efficiency or viability, compared with cells obtained from chow-fed control rats. Both groups of cells synthesized and secreted immunoprecipitable apolipoproteins B and E at similar rates throughout the 18 h experiment. Thus there was a discordance between the effects of dietary cholesterol on serum apolipoprotein concentrations and hepatocyte synthesis and secretion. The data indicate that altered hepatic apolipoprotein synthesis cannot account for the changes in serum apolipoprotein concentrations caused by dietary cholesterol.     

Insulin effects on apolipoprotein B production by normal, diabetic and treated-diabetic rat liver and cultured rat hepatocytes.

1. The effect of insulin on apolipoprotein (apo B) secretion was studied in 24 h recirculating liver perfusions of isolated normal, diabetic and insulin-treated diabetic rats. In single perfusions from each group apo B accumulated in the media in a linear fashion. 2 In perfusions of normal rat livers, when the medium contained insulin plus cortisol, apo B production was significantly inhibited (by 35.8%), demonstrating a hormone effect on apo B secretion. 3. In perfusions of diabetic-rat livers, apo B production was decreased to 11.8% of normal when the medium contained no hormones, and was not significantly changed by the addition of insulin plus cortisol to the medium, suggesting that the hormone effect on apo B secretion is missing in long-term hypoinsulinaemic states. 4. Treatment of diabetic rats with daily insulin injection restored apo B production and restored the effect of insulin plus cortisol in the medium to inhibit apo B secretion during perfusion. 5. Parallel studies of apo B secretion with insulin alone, cortisol alone and insulin plus cortisol in the medium were performed in primary cultures of hepatocytes to compare results from liver perfusions. 6. Apo B secretion by hepatocytes from normal, diabetic and treated-diabetic rats was inhibited (by 36.8%, 57.1% and 57.9% respectively) when insulin alone was added to the medium. 7. Insulin plus cortisol inhibited apo B secretion by hepatocytes from normal and treated diabetic rats (by 30.2% and 47.2% respectively), but failed to inhibit apo B secretion by hepatocytes from diabetic rats.     

Description of two different patients with abetalipoproteinemia: synthesis of a normal-sized apolipoprotein B-48 in intestinal organ culture

We describe here two patients, M. P. and S. L., with recessive abetalipoproteinemia. Analysis of restriction fragments of DNA from both patients using cDNA probes spanning the entire apolipoprotein B gene revealed no major insertions or deletions. Further, as defined by restriction fragment length polymorphism, abetalipoproteinemia, in these patients, did not appear associated with particular alleles of apolipoprotein B. Northern and dot blot analysis of intestinal mRNA of one patient (M. P.) revealed a normal-sized apolipoprotein B mRNA which was present in slightly reduced amounts. At the cellular level apolipoprotein B was detected in both intestinal and hepatic biopsies, of one patient (S. L.), by immunoenzymatic techniques using polyclonal and monoclonal antibodies to apolipoprotein B-48 and/or B-100. The level of apolipoprotein B-48 appeared to increase in the intestine after a fatty meal. In the other patient (M. P.), although no apolipoprotein B was detected in the enterocytes using similar immunoenzymatic techniques, organ culture experiments using [35S]methionine demonstrated the synthesis of a normal-sized apolipoprotein B-48 which appeared to be normally glycosylated. The glycosylation and processing of two intestinal membrane enzymes, sucrase-isomaltase and aminopeptidase N, were also normal. Although lipids and apolipoprotein B-48 were present intracellularly, no lipoprotein-like particles were observed by electron microscopy in the endoplasmic reticulum, the Golgi apparatus, or in the intercellular spaces of intestinal biopsies obtained in the fasted (M. P. and S. L.) or fed state (S. L.). The defect in these cases of abetalipoproteinemia, therefore, does not appear to involve the apolipoprotein B gene nor the synthesis or the glycosylation of the apolipoprotein but instead appears to involve some aspect of lipoprotein assembly or secretion.     

Lipoprotein secretion and triglyceride stores in the heart

The genes for apolipoprotein B and microsomal triglyceride transfer protein are expressed in mouse and human heart tissue. Why the heart would express these "lipoprotein assembly" genes has been unclear. Here we demonstrate that the beating mouse heart actually secretes spherical lipoproteins. Moreover, increased cardiac production of lipoproteins (e.g., in mice that express a human apolipoprotein B transgene) was associated with increased triglyceride secretion from the heart and decreased stores of triglycerides within the heart. Increased cardiac production of lipoproteins also reduced the pathological accumulation of triglycerides that occurs in the hearts of mice lacking long-chain acyl coenzyme A dehydrogenase. In contrast, blocking heart lipoprotein secretion (e.g., in heart-specific microsomal triglyceride transfer protein knockout mice) increased cardiac triglyceride stores. Thus, heart lipoprotein secretion helps regulate cardiac triglyceride stores and may protect the heart from the detrimental effects of surplus lipids.     

Apolipoprotein E is secreted by cultured lipocytes of the rat liver

Hepatic lipocytes, the retinoid-storing cells of the liver, share several characteristics with vascular smooth muscle cells. To determine whether they also share the characteristic of apolipoprotein E secretion, we have compared the relative mRNA expression and protein secretion of apolipoprotein E, apolipoprotein A-I, and apolipoprotein A-IV in early primary cultures of lipocytes, hepatocytes, and Kupffer cells. Expression of apolipoprotein mRNAs was detected using the polymerase chain reaction and oligonucleotide primers specific for apolipoprotein E, apolipoprotein A-I, and apolipoprotein A-IV. Cellular mRNA concentrations were compared by dot blot analysis, and apolipoprotein secretion was assessed by immunoblot analysis of culture media. Apolipoprotein E mRNA was found in all three cell types, whereas apolipoprotein A-I and A-IV mRNAs were detected only in hepatocytes. Hepatocyte, lipocyte, and Kupffer cell media all contained a Mr approximately 36,000 protein identified by an antibody specific for rat apolipoprotein E. The relative concentration of apolipoprotein E mRNA per microgram of total cellular RNA in lipocytes, hepatocytes, and Kupffer cells was 1.0, 3.0, and 1.6, respectively. The relative secretion of apolipoprotein E per cell was also lowest in lipocytes, being twofold greater in hepatocytes and 1.4-fold greater in Kupffer cells. The secretion of apolipoprotein E by lipocytes is not only an additional smooth muscle cell-like characteristic of the hepatic lipocyte, but also raises the possibility of retinol mobilization upon apolipoprotein secretion.     

Influence of eicosapentaenoic acid (20:5, n-3) on secretion of lipoproteins in CaCo-2 cells.

CaCo-2 cells, grown on filter membranes, were used to study the effects of fatty acids on cellular metabolism of triacylglycerol and phospholipids. The rate of triacylglycerol secretion was enhanced more than 2-fold, from 1 to 2 weeks after reaching confluency, in the presence of 0.6 mM fatty acids. Triacylglycerol secretion and oxidation of oleic acid increased 2- and 9-fold, respectively, with this culture system, as compared to cells grown on conventional plastic dishes. Eicosapentaenoic acid (20:5 n-3), when compared to oleic acid, did not reduce formation of triacylglycerol or enhance phospholipid synthesis in CaCo-2 cells during short term (less than 24 h) experiments, when the cells resided on membranes, regardless of what type of radioisotopes were used as precursors in the incubation media. However, the n-3 fatty acid was preferentially incorporated into phosphatidylinositol, lysophosphatidylcholine, and sphingomyelin, as compared to oleic acid. The disappearance from the apical medium and cellular uptake of labeled eicosapentaenoic and oleic acid were similar during incubations up to 24 h, and the metabolism of these fatty acids to acid-soluble materials and CO2 was equal. Light scattering analysis indicated that secreted lipoproteins of density less than 1.006 g/ml were in the same size-range as chylomicrons derived from human plasma. Assessment of secreted apolipoprotein B showed that by incubating CaCo-2 cells with oleic acid, apolipoprotein B levels increased approximately 1.4-fold when compared to cells incubated with eicosapentaenoic acid, whereas the amount of triacylglycerol and size-range of particles were similar for the two fatty acids. Our data indicate that CaCo-2 cells grown on filter membranes exhibit enterocyte-like characteristics with the ability to synthesize and secrete chylomicrons. Eicosapentaenoic acid and oleic acid are absorbed, metabolized, and influence secretion of lipoprotein particles in a similar way, except for some differences in incorporation of the fatty acids into certain phospholipid classes and a reduced secretion of apolipoprotein B. The culture conditions, including time after confluency and cellular support, are critical for the rate of secretion in the presence of eicosapentaenoic acid and oleic acid.