Polarized secretion of newly synthesized lipoproteins by the Caco-2 human intestinal cell line

Influence of mental stress and circadian cycle on postprandial lipemia

BACKGROUND

Mental stress produces alterations in serum lipids and lipoproteins.

OBJECTIVE

The objective was to assess the effect of mental stress during the day and night on postprandial lipoproteins.

DESIGN

Fourteen healthy subjects aged 26.6 +/- 5.0 y were given randomly the same meal either at night (0100) or during the day (1300), with or without (control session) a mental stress challenge. The meal contained 40% of estimated daily energy needs. The mental task was performed on a computer and consisted of a task of choice reaction. Blood samples were drawn at baseline and hourly for 7 h after the meal.

RESULTS

Urinary epinephrine concentrations were higher (P < 0.012) during the mental task than during the control sessions. Repeated-measures analysis of variance showed that mean postprandial triacylglycerol concentrations were significantly higher (P < 0.02) and total cholesterol (P < 0.0001) and HDL-cholesterol concentrations were significantly lower (P < 0.0001) at night than during the day. The mean postprandial VLDL-triacylglycerol concentration was significantly higher (P < 0.04) during the mental task than during the control sessions. Similarly, the VLDL-cholesterol response, calculated as the area under the postprandial curve, was significantly greater (P < 0.02) during the mental task than during the control sessions. There was no interaction between mental stress and nyctohemeral cycle on postprandial lipoprotein responses, suggesting that both indexes act independently on postprandial lipid metabolism.

CONCLUSIONS

Mental stress is associated with increased concentrations of postprandial triacylglycerol-rich lipoprotein fractions. Therefore, postprandial hyperlipidemia is one possible mechanism contributing to the higher risk of ischemic heart disease in stressed people.

Assembly and secretion of chylomicrons by differentiated Caco-2 cells. Nascent triglycerides and preformed phospholipids are preferentially used for lipoprotein assembly

To develop a cell culture model for chyclomicron (CM) assembly, the apical media of differentiated Caco-2 cells were supplemented with oleic acid (OA) together with either albumin or taurocholate (TC). The basolateral media were subjected to sequential density gradient ultracentrifugations to obtain large CM, small CM, and very low density lipoproteins (VLDL), and the distribution of apoB in these fractions was quantified. In the absence of OA, apoB was secreted as VLDL/LDL size particles. Addition of OA (>/=0.8 mM) with TC, but not with albumin, resulted in the secretion of one-third of apoB as CM. Lipid analysis revealed that half of the secreted phospholipids (PL) and triglycerides (TG) were associated with CM. In CM, TG were 7-11-fold higher than PL indicating that CM were TG-rich particles. Secreted CM contained apoB100, apoB48, and other apolipoproteins. Secretion of large CM was specifically inhibited by Pluronic L81, a detergent known to inhibit CM secretion in animals. These studies demonstrate that differentiated Caco-2 cells assemble and secrete CM in a manner similar to enterocytes in vivo. Next, experiments were performed to identify the sources of lipids used for lipoprotein assembly. Cells were labeled with [3H]glycerol for 12 h, washed, and supplemented with OA, TC, and [14C] glycerol for various times to induce CM assembly and to radiolabel nascent lipids. TG and PL were extracted from cells and media and the association of preformed and nascent lipids with lipoproteins was determined. All the lipoproteins contained higher amounts of preformed PL compared with nascent PL. VLDL contained equal amounts of nascent and preformed TG, whereas CM contained higher amounts of nascent TG even when nascent TG constituted a small fraction of the total cellular pool. These studies indicate that nascent TG and preformed PL are preferentially used for CM assembly and provide a molecular explanation for the in vivo observations that the fatty acid composition of TG, but not PL, of secreted CM reflects the composition of dietary fat. It is proposed that in the intestinal cells the preformed PL from the endoplasmic reticulum bud off with apoB as primordial particles and the assembly of larger lipoproteins is dependent on the synthesis and delivery of nascent TG to these particles.

Tetradecylthioacetic acid (a 3-thia fatty acid) impairs secretion of oleic acid-induced triacylglycerol-rich lipoproteins in CaCo-2 cells

The fatty acid analogue tetradecylthioacetic acid (TTA) has previously been shown to decrease triacylglycerol secretion in CaCo-2 cells (Gedde-Dahl et al., J. Lipid Res. 36 (1995) 535-543). The present study was designed to further elucidate the effect of TTA on lipoprotein production in CaCo-2 cells. TTA did not affect oleic acid-induced triacylglycerol synthesis, but it significantly decreased secretion of newly synthesized triacylglycerol when compared to cells incubated with oleic acid alone or oleic acid in combination with palmitic acid. In contrast, pulse-chase experiments showed no difference in the amount of labeled triacylglycerol secreted from cells exposed to either fatty acid combination during the chase period, indicating that TTA did not affect the secretory process in general. Cells incubated with TTA alone secreted triacylglycerol present at 1.025<rho<1.073 g/ml, corresponding to the low density lipoprotein/intermediate density lipoprotein density range. In contrast, cells supplemented with oleic acid or oleic acid in combination with TTA secreted triacylglycerol mainly in the very low density lipoprotein/chylomicron density range (rho<1.006 g/ml). Despite a marked decrease in triacylglycerol secretion, TTA treatment did not change secretion of apolipoprotein B nor the activity of microsomal triacylglycerol transfer protein (MTP) in the cells. Furthermore, the presence of TTA in cellular triacylglycerol had no effect on the ability of purified MTP to transfer triacylglycerol from donor to acceptor vesicles. Together, the above observations suggest that TTA interferes with other MTP-independent factors that regulate the intestinal lipoprotein secretion.

Effects of Pinus pinaster and Pinus koraiensis seed oil supplementation on lipoprotein metabolism in the rat

The aim of the present study was to assess the effect of vegetal oils obtained from Pinus pinaster and P. koraiensis seeds on plasma lipoprotein levels and apolipoprotein (apo) gene expression in rats. These oils contain two particular fatty acids of the delta5-unsaturated polymethylene-interrupted fatty acid (delta5-UPIFA) family: all-cis-5,9,12-1 8:3 (pinolenic) and/or all-cis-5,11,14-20:3 (sciadonic) acids. Rats were fed for 28 d a diet containing 5% (w/w) oil supplement. Two control diets were prepared to match the fatty acid composition of P. pinaster or P. koraiensis oils with the exception of delta5-UPIFA, which were replaced by oleic acid. Pinus pinaster seed oil decreased serum triglycerides by 30% (P < 0.02), very low density lipoprotein (VLDL)-triglycerides by 40% (P < 0.01), and VLDL-cholesterol by 33% (P < 0.03). Pinus koraiensis seed oil decreased serum triglycerides by 16% [not statistically significant (ns)] and VLDL-triglycerides by 21% (ns). Gel permeation chromatography and nondenaturating polyacrylamide gel electrophoresis showed a tendency of high density lipoprotein to shift toward larger particles in pine seed oil-supplemented rats. Finally, P. pinaster seed oil treatment was associated with a small decrease of liver apoC-III (P < 0.02) but not in apoE, apoA-I, or apoA-II mRNA levels. The levels of circulating apo were not affected by pine seed oil supplementation. In conclusion, P. pinaster seed oil has a triglyceride-lowering effect in rats, an effect that is due to a reduction in circulating VLDL.

Lipid and lipoprotein analysis of cats with lipoprotein lipase deficiency

BACKGROUND

We have previously described a colony of domestic cats with a naturally occurring mutation in the lipoprotein lipase (LPL) gene. We have now further characterized cats homozygous for LPL deficiency (LPL -/-, homozygotes), and have contrasted these with heterozygotes (LPL +/-) and normal cats (LPL +/+).

MATERIALS AND METHODS

Density gradient ultracentrifugation with subsequent lipid analysis, agarose and polyacrylamide gel electrophoresis was used to examine detailed liproprotein differences between the genotypes. Oral fat loading studies and breast milk fatty acid analysis were also performed to further characterize the phenotypic expression of LPL deficiency in this model system.

RESULTS

Several lipid abnormalities associated with homozygosity for LPL deficiency were evident. Triglyceride-rich lipoprotein-triglycerides (TRL-TG) and cholesterol (TRL-C) were higher (TRL-TG 2.09 +/- 1.14 vs. 0.15 +/- 0.04 mmol L-1, P < 0.001; TRL-C 0.42 +/- 0.30 vs. 0.11 +/- 0.16 mmol L-1, P < 0.05) in male -/- than in male +/+ cats, as was HDL-cholesterol (HDL-C, 1.75 +/- 0.24 vs. 1.41 +/- 0.14 mmol L-1, P < 0.05). LDL-C levels were lower in homozygous cats than in control cats, similar to what is seen in human LPL deficiency. Oral fat loading studies revealed that homozygous cats have a marked reduced ability to clear plasma TGs in terms of peak time (7 h vs. 3 h), peak height (9.36 vs. 1.1 mmol L-1), area under the TG clearance curve (AUC, 280.3 vs. 2.2 h mmol L-1) and time to return to baseline. Fasting lipid and lipoprotein levels were not significantly different between heterozygous and normal cats. However, oral fat loading in heterozygotes revealed an intermediate phenotype (peak of 2.35 mmol L-1 at 5 h, AUC 13.1 h mmol L-1), highlighting the impaired TG clearance in these animals.

CONCLUSION

Thus, LPL deficiency in the cat results in a lipid and lipoprotein phenotype that predominantly parallels human LPL deficiency, further validating the use of these animals in studies on the pathobiology of LPL.

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.

Method for simultaneous measurements of traces of heptadeuterated cholesterol and cholesterol by gas chromatography-mass spectrometry: application in humans

An assay was developed to quantify deuterated cholesterol (used as a tracer) and cholesterol using gas chromatography-mass spectrometry. Ergosterol and epicoprostanol were used as internal standards. Deuterated cholesterol was quantified by comparing its peak area to that of epicoprostanol and cholesterol to ergosterol. The mean absolute recovery in spiked serum was 99.96%; the precision was in the range 0.16-10.9% and accuracy 90.4-100%; the limit of detection in plasma was 3x10(-5) mmol l(-1). Using two internal standards, the method described herein seems particularly suitable for application in humans i.e., measuring traces of deuterated cholesterol (range: 0-6.26 x 10(-4) mmol l(-1)) along with natural cholesterol (range: 0.065-4.42 mmol l(-1)) in human plasma and lipid fractions postprandially.

Double-Blind Comparison of Apolipoprotein and Lipoprotein Particle Lowering Effects of Atorvastatin and Pravastatin Monotherapy in Patients With Primary Hypercholesterolemia

METHODS AND RESULTS: A total of 305 subjects with primary hypercholesterolemia were randomized in a 3:1 ratio to receive either atorvastatin 10 mg daily or pravastatin 20 mg daily according to a 16-week double-blind comparative study of the effect on apolipoprotein and lipoprotein particle levels. All patients had low-density lipoprotein (LDL)-cholesterol levels between 4.2 and 6.6 mM and triglyceride concentrations below 4.5 mM at baseline. After 16 weeks of treatment, apoB (-27% and -16%; P <.001), apoE (-13.3% and -5.6%; P <.05) and the triglyceride-rich LpC-III:B particle (-33% and -26%; P <.05) levels were reduced to a significantly greater extent in the atorvastatin than in the pravastatin treatment group. Both atorvastatin and pravastatin increased apoA-I levels, an effect that was more pronounced in the pravastatin group (+7% and +11%; P <.002). The increased apoA-I levels predominated on LpA-I in the atorvastatin group (+11%) and on LpA-I:A-II in the pravastatin group (+13%). ApoA-II levels were decreased with atorvastatin to a greater extent than with pravastatin (-1% and +2.8%; P <.05). CONCLUSIONS: Although atorvastatin and pravastatin belong to the same therapeutic family, they produce different effects in apoliprotein concentrations in hypercholesterolemic patients. Atorvastatin, an agent of the new generation, appears to efficiently reduce apoB-containing lipoprotein particles containing apoC-III.

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.

A complete epithelial organization of Caco-2 cells induces I-FABP and potentializes apolipoprotein gene expression

The culture of Caco-2 cells on plastic support impairs the expression of several genes involved in lipid metabolism. We describe culture conditions that permit the expression of the I-FABP gene and better expression of the apolipoprotein A-I, C-III, and A-IV genes. Basal lamina deposited on filters as well as the nature of nutrients on the apical side differentially modulated mRNA expression of I-FABP, APOBEC-1, and apolipoprotein genes. Growing cells on a filter led to functional polarization, illustrated by a secretion of apo B at the basal side, which induced the expression of the I-FABP, APOBEC-1, and apo A-IV genes and highly increased the expression of the apo C-III gene. Moreover, basal lamina deposited on the filter enhances the mRNA expression of apo A-I. Apo C-III and A-IV mRNA levels were decreased when cells were grown on a filter covered with basal lamina in the presence of a medium deprived of protein and lipid on the apical side, whereas these conditions had no effect on I-FABP, apo A-I, and APOBEC-1 mRNA levels. The addition of lipid micelles on the apical side had various effects, according to the genes. Caco-2 cells cultured under the conditions described here closely resembled enterocytes and represent a useful tool for studying the regulation of genes involved in lipid metabolism.

Expression of enzymatically active CYP3A4 by Caco-2 cells grown on extracellular matrix-coated permeable supports in the presence of 1alpha,25-dihydroxyvitamin D3

The human colon carcinoma cell line, Caco-2, is widely used as a model for oral absorption of xenobiotics. The usefulness of Caco-2 cells has been limited, however, because they do not express appreciable quantities of CYP3A4, the principle cytochrome P450 present in human small bowel epithelial cells. We report that treatment of Caco-2 cells with 1 alpha,25-dihydroxyvitamin D3, beginning at confluence, results in a dose- and duration-dependent increase in CYP3A4 mRNA and protein, with little apparent effect on the expression of CYP3A5 or CYP3A7. This treatment also results in increases in NADPH cytochrome P450 reductase and P-glycoprotein (the MDR1 gene product) but has no detectable effect on expression of CYP1A1, CYP2D6, cytochrome b5, liver or intestinal fatty acid binding proteins, or villin. Maximal expression of CYP3A4 requires an extracellular matrix on a permeable support and the presence of serum. In the treated cells, the intrinsic formation clearance of 1'-hydroxymidazolam (a reaction characteristically catalyzed by CYP3A enzymes) was estimated to be somewhat lower than that of human jejunal mucosa (1.14 and 3.67 ml/min/g of cells, respectively). The 1'-OH-midazolam/4-OH-midazolam product ratio produced by the cells (approximately 5.3) is comparable to, but somewhat lower than, that observed in human jejunal microsomes (7.4-15.4), which may reflect the presence of CYP3A7 in the Caco-2 cells. 25-Hydroxyvitamin D3 is less efficacious but reproduces the effects of the dihydroxy compound, whereas unhydroxylated vitamin D is without appreciable effect. These observations, together with the time course of response, suggest that the vitamin D receptor may be involved in CYP3A4 regulation. The culture model we describe should prove useful in defining the role of CYP3A4 in limiting the oral bioavailability of many xenobiotics.

Clathrin-mediated endocytosis of high density lipoprotein3 in human intestinal Caco-2 cells. A post-embedding immunocytochemical study

The mechanism by which high density lipoprotein (HDL) removes excess cholesterol from intracellular sites has been the subject of much controversy. There is some evidence that HDL binds to specific cell surface receptors without internalization. Other evidence suggests that HDL is taken up by endocytosis, enters a pathway of endosomal trafficking and is resecreted from the cells (retroendocytsosis). In the present study, we investigated the distribution of apolipoprotein AI, the major protein constituent of HDL, in cultured intestinal Caco-2 cells employing post-embedding immunocytochemistry on LR White-embedded material. Cells grown under control conditions showed label for apolipoprotein AI in the endoplasmic reticulum. After incubation with native apolipoprotein E-free high density lipoprotein3 (HDL3) additional label for apolipoprotein AI was found in endosomes. These endosomes were observed near lipid droplets and in the basolateral cytoplasm. Further, it was demonstrated that label for apolipoprotein AI was colocalized with label for clathrin on the basolateral membrane. Our results support the concept that HDL3 is internalized and subsequently processed in an endosomal pathway in Caco-2 cells besides de novo synthesis of apolipoprotein AI.

Postprandial lipaemia is associated with increased levels of apolipoprotein A-IV in the triacylglycerol-rich fraction and decreased levels in the denser plasma fractions

Apolipoprotein (apo) A-IV is primarily associated with HDL or with the lipoprotein-free fraction of plasma, and in small amounts with chylomicrons and VLDL. The aim of the present study was to assess the effect of a fatty meal on the postprandial variation in plasma apo A-IV and on its distribution among lipoprotein fractions following absorption of fat. Twenty healthy male subjects participated in the study. After an overnight fast, subjects were given a fatty breakfast containing 1 g fat/kg body weight (% energy: fat 65, carbohydrate 20; protein 15). Blood samples were taken every hour during the next 10 h. Apo A-IV was measured by ELISA. Postprandial lipaemia was associated with a moderate, although significant, increase in the plasma levels of apo A-IV. Apo A-IV increased from the median baseline value of 0.15 g/l to 0.165 g/l (median +17%; P < 0.01) 5 h after fat ingestion. The postprandial peak of apo A-IV occurred 1 h after the triacylglycerol peak. There were no statistically significant correlations between baseline lipids, baseline apo A-IV and postprandial changes in apo A-IV levels, or between postprandial changes in lipids and apo A-IV at any time. To assess apo A-IV distribution among lipoproteins, plasma was fractionated by fast performance liquid chromatography at baseline and 3, 6 and 10 h postprandially. There was a substantial heterogeneity in the apo A-IV distribution among lipoproteins following the fatty meal. At 3 h after fat ingestion, apo A-IV levels increased in the triacyglycerol-rich lipoprotein (TRL) fraction and decreased in the denser plasma fraction. At 6 h after the fatty meal, apo A-IV was still present in the TRL but was decreased in the HDL fractions. The findings of the present study support the concept that apo A-IV particles transfer from the denser plasma fraction to TRL during postprandial lipaemia.

Lipid, apolipoprotein, and lipoprotein synthesis and secretion during cellular differentiation in Caco-2 cells

Although Caco-2 cells are frequently employed for the study of enterocyte lipid metabolism, variable results have been reported regarding their ability to synthesize and secrete lipids and apolipoproteins. The major goal of this investigation is to examine the capacity of Caco-2 cells to elaborate and secrete lipids, lipoproteins, and apolipoproteins at different degrees of morphological and functional differentiation. Cells were cultured in medium with 5% fetal bovine serum (FBS), on permeable polycarbonate filters from 2 to 30 d in the presence of 14C-oleate or 35S-methionine. Cellular differentiation, as assessed by morphology (light and electron microscopy), transepithelial resistance, free fatty acid flux, and sucrase activity, progressed steadily up to 20 d of culture. Caco-2 cells esterified oleic acid mainly into phospholipids, triglycerides (TG), and smaller amounts of cholesterol esters. Lipid synthesis began as early as 2 d, and TG secretion was enhanced with increased duration of culture. However, very low efficiency of lipid export was observed at all levels of differentiation, reaching a maximum of only 6% of intracellular lipids. VLDL and LDL were the dominant lipoproteins secreted, with HDL comprising < 20% of the total. VLDL secretion increased, while LDL decreased, whereas the lipid composition of lipoproteins varied little with increasing duration of culture. Apoprotein B and A-I synthesis and secretion increased markedly from 11 to 20 d of culture. The ratio of apo B-100/B-48 decreased between 11 and 30 d, consistent with enhanced apo B editing of more mature enterocytes. Taken together, our data suggest that from 20 d of culture, Caco-2 cells are morphologically and functionally mature, capable of lipid esterification, and lipoprotein and apolipoprotein synthesis. However, despite their functional and morphological similarities to mature enterocytes, Caco-2 cells have a very limited lipid export capacity.

Insulin modulation of newly synthesized apolipoproteins B-100 and B-48 in human fetal intestine: gene expression and mRNA editing are not involved

We investigated insulin's effect on intestinal lipid, transport and, particularly, the biogenesis of apolipoproteins crucial to lipoprotein secretion. Adding insulin (3 mU) to the serum-free medium of cultured jejunal explants from human fetuses (17-20 weeks) reduced triglyceride and chylomicron production and inhibited apo B-48 and apo B-100 secretion. When apo B mRNA was assayed by RT-PCR and its editing by primer extension, no change was detectable following the addition of insulin. HDL lipid content, apo A-1 synthesis and RNA level were unaffected by insulin. Collectively, these results suggest that the insulin-stimulated decline in intestinal chylomicron output may involve apo B co- or post-translational modifications.

Reduced cholesterol esterification in CaCo-2 cells by indirect action of pravastatin

In microsomal preparations of CaCo-2 cells pravastatin decreased cholesterol esterifying activity at 25 micrograms/ml to 82.5% and at 800 micrograms/ml to 56.2% of controls. Pravastatin reduced cholesteryl ester formation dose-dependently also in viable CaCo-2 cells. However, the maximal inhibition was by 90.4% at pravastatin concentration of 25 micrograms/ml, half maximal inhibition occurred between concentrations of 5 and 10 micrograms/ml. Addition of mevalonolactone, which serves as endogenous source of cholesterol, antagonized this effect. At 10 mM mevalonolactone (MVL) even doses up to 200 micrograms/ml of pravastatin were ineffective. On the other hand, pravastatin suppressed cholesteryl ester formation when acyl-CoA cholesterol acyltransferase (ACAT) (E.C. 2.3.1.26) activity was stimulated by addition of exogenous liposomal or Low Density Lipoprotein (LDL)-derived cholesterol. This inhibition was refractory to increasing amounts of exogenous cholesterol up to 400 micrograms/ml. Therefore we conclude that only excessive doses of pravastatin suppress ACAT activity directly. In viable cells the observed inhibition of cholesteryl ester formation is due to the block in de novo synthesis of cholesterol, causing a lack of substrate for ACAT and of non-sterol products of mevalonic acid. Furthermore pravastatin interferes with the esterification and/or intracellular transport only of exogenous cholesterol, confirming former results of a compartmentalized cholesterol metabolism in the enterocyte.

A polymorphism in the human intestinal fatty acid binding protein alters fatty acid transport across Caco-2 cells

The human intestinal fatty acid binding protein (IFABP) binds long-chain fatty acids in vitro, but its intracellular function has remained speculative. A polymorphism in the gene that encodes IFABP results in an alanine (Ala54) to threonine (Thr54) substitution at codon 54 that alters the in vitro binding affinity of the protein for long-chain fatty acids. To identify potential functional variability between Ala54 and Thr54 IFABP, we established permanently transfected Caco-2 cell lines that express either Ala54 or Thr54 IFABP. We found that Caco-2 cells expressing Thr54 IFABP transport long-chain fatty acids and secrete triglycerides to a greater degree than Caco-2 cells expressing Ala54 IFABP. These results provide the first demonstration that IFABP participates in the intracellular transport of long-chain fatty acids. In addition, the observed increase in transport of fatty acids across cells expressing Thr54 IFABP suggests a plausible physiologic mechanism for our prior observation that Pima Indians with a Thr54 IFABP genotype have increased post-absorptive lipid oxidation rates and are more insulin-resistant than Pimas with a Ala54 IFABP genotype.

Effects of saturated, mono-, and polyunsaturated fatty acids on the secretion of apo B containing lipoproteins by Caco-2 cells

We studied the effects of addition of physiological concentrations (0.5 mM) of fatty acids i.e., palmitic (16:0), stearic (18:0), oleic (18:1) and linoleic acid (18:2) on lipoprotein secretion by polarized Caco-2 cells. With saturated fatty acids, secreted lipoproteins were at IDL/LDL density, 1.009 < d < 1.068 g/ml. The numbers of secreted lipoproteins, expressed as secreted apolipoprotein (apo) B, were comparable to control without fatty acid (palmitic acid, 551 +/- 185; stearic acid, 629 +/- 304 and control, 504 +/- 176 ng apo B/4.7 cm2 filter). With unsaturated fatty acids, apo B containing lipoproteins were secreted at chylomicron/VLDL density (d < 1.006 g/ml). Oleic acid caused a two-fold higher secretion of apo B than control (1058 +/- 87 vs. 504 +/- 176 ng/4.7 cm2 filter, P < 0.001). The increase in apo B secretion was attributed to a specific increase in apo B-48. Unsaturated fatty acid caused a two-fold higher secretion of triglyceride than saturated fatty acids but incorporation of newly synthesized lipid into the secreted lipoproteins, measured by incorporation of a fatty acid marker, was 10- to 20-fold higher, indicating preferential translocation of unsaturated triglycerides into lipoproteins. Mixtures rich in either polyunsaturated, monounsaturated, or saturated fatty acids, resembling nutritional fat and oils, were capable of a two-fold stimulation of secretion of apo B containing triglyceride-rich lipoproteins. The triglyceride/apo B ratio in the basolateral medium was higher with the monounsaturated 'olive oil' mixture (12 250 +/- 2000 mol/mol) than with the polyunsaturated 'corn oil' mixture (7830 +/- 2480 mol/mol) and incorporation of newly synthesized lipid into the secreted lipoproteins was 1.5-fold higher as well. In conclusion, unsaturated fatty acids were most potent in stimulating the secretion of apo B by specifically increasing apo B-48 secretion. Unsaturated triglycerides, that contain mainly oleic acid, were more efficiently incorporated into lipoproteins than saturated triglycerides, suggesting preferential translocation by microsomal triglyceride transfer protein.

N-3 and n-6 fatty acid metabolism in undifferentiated and differentiated human intestine cell line (Caco-2)

Metabolism of n-6 and n-3 fatty acids in the undifferentiated and differentiated human adenocarcinoma colon cell line (Caco-2) was studied. In cells incubated with either 18:2n-6 or 18:3n-3, no significant amounts of long chain n-6 and n-3 metabolites were found. Incubation with either 18:3n-6 or 18:4n-3 raised significantly the levels of 20:3n-6 and 20:4n-3, respectively. In the undifferentiated cells, significant proportions of 20:3n-6 and 20:4n-3 were further delta 5-desaturated to form 20:4n-6 and 20:5n-3, respectively. Incubation with either 20:4n-6 or 20:5n-3 raised the levels of their direct elongation products, 22:4n-6 and 22:5n-3, respectively. Incubation with 22:4n-6 or 22:5n-3 increased the levels of 20:4n-6 and 20:5n-6. These results suggest that delta 6-desaturation in the Caco-2 cells is less active in comparison with elongation, delta 5-desaturation and retro-conversion. These enzymes were modulated by the state of differentiation, and appeared to be non-specific to n-3 and n-6 fatty acids. When cells were incubated with 18:3n-6 and 18:4n-3 concomitantly, the levels of incorporation of total n-6 fatty acids into cellular lipids were greater than those of the n-3 fatty acids, whereas the ratios of 20+22 carbon metabolites to 18-carbon precursor favored n-3 over n-6 fatty acids. These results suggest that n-3 and n-6 fatty acids were not metabolized identically in Caco-2 cells.

Hypocholesterolemic activity of a novel inhibitor of cholesterol absorption, SCH 48461

The amount of cholesterol that circulates in the plasma as lipoproteins can be affected by the balance of cholesterol metabolism within and between the intestines and liver. In the present report, we describe a novel hypocholesterolemic agent and document its pharmacological effects in animal models of hypercholesterolemia. The oral administration of (3R,4S)-1,4-bis-(4-methoxyphenyl)-3-(3-phenylpropyl)-2-azetidinone (SCH 48461) reduced plasma cholesterol concentrations in cholesterol-fed hamsters, rats and rhesus monkeys with ED50s of 1, 2 and 0.2 mg/kg per day, respectively, SCH 48461 was also highly effective in reducing hepatic cholesteryl ester accumulation in cholesterol-fed hamsters and rats after 7 days of treatment. In one 3 week study, rhesus monkeys were fed a 0.25% cholesterol/22% saturated fat diet with or without SCH 48461. At the end of the 3 week period the control group's VLDL + LDL-cholesterol increased to 180 Mg/dl from a baseline of approximately 65 mg/dl while plasma apolipoprotein B levels had doubled. Animals treated daily with 1 mg/kg SCH 48461 maintained their baseline levels of VLDL + LDL-cholesterol, HDL-cholesterol, and plasma apolipoproteins B and A-I. After 3 weeks the diets of the two groups were switched. Within 1 week SCH 48461 (1 mg/kg per day) rapidly reversed the elevated VLDL + LDL-cholesterol levels of the previous control group to near baseline values. SCH 48461 exerted its hypocholesterolemic effect through the inhibition of cholesterol absorption. A dose of 10 mg/kg per day inhibited cholesterol absorption in cholesterol-fed hamsters by 68% while a similar reduction was achieved in chow-fed monkeys with 3 mg/kg per day. This latter dose inhibited cholesterol absorption in cholesterol-fed monkeys by 95%. Treatment of cholesterol-fed monkeys with 10 mg/kg per day SCH 48461 significantly increased fecal neutral sterol excretion (52 vs. 32 mg/kg) but had no effect on acidic sterol excretion. Using a 2-h absorption model in cholesterol-fed hamsters, SCH 48461 caused a 46% inhibition of unesterified [14C]cholesterol accumulation in the intestinal wall and a 90% inhibition of cholesteryl ester formation at a dose of 10 mg/kg. Similar data were observed when the plasma radioactivity was assessed, indicating inhibition of both free (61%) and esterified (85%) cholesterol appearance. In contrast, CI-976, a potent acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, did not affect the uptake of free cholesterol into the intestines while inhibiting cholesterol esterification (98% inhibition).(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

HDL3-mediated cholesterol efflux from cultured enterocytes: the role of apoproteins A-I and A-II

High density lipoproteins (HDL) were recently demonstrated in an enterocyte model (CaCo-2 cells) to mediate reverse cholesterol transport by retroendocytosis. The present study was carried out to define the role of the major HDL apoproteins (apo) A-I and apo A-II in this pathway. HDL3 was fractionated by heparin affinity chromatography into the two main fractions containing either apo A-I only (fraction A) or both apo A-I and apo A-II (fraction B). In addition, liposomes were reconstituted from purified apo A-I or apo A-II and dimyristoyl phosphatidylcholine. The cell binding properties and cholesterol efflux potential were studied in the lipoprotein fractions and the liposomes. Both fractions exhibited similar maximal binding capacities of 4427 (A) and 5041 (B) ng/mg cell protein, but their dissociation constants differed (40.5 and 167.7 micrograms/mL, respectively). Fraction A induced cholesterol efflux and stimulated cholesterol synthesis more than did fraction B. Fraction A mobilized both cellular free and esterified cholesterol, whereas fraction B preferentially mobilized cholesteryl esters. Liposomes, containing either apo A-I or apo A-II, showed specific binding, endocytosis and endosomal transport, and were released as intact particles. Apo A-I liposomes also mediated cholesterol efflux. In conclusion, there is evidence that the HDL3 subfractions A and B, as well as reconstituted liposomes containing either apo A-I or apo A-II, were specifically bound and entered a retroendocytosis pathway which was directly linked to cholesterol efflux. Quantitatively, the apo A-I subfraction appeared to play the dominant role in normal enterocytes. The apo A-II content of fraction B was related to the mobilization of cholesteryl esters.

Fatty acid uptake and metabolism in CaCo-2 cells: eicosapentaenoic acid (20:5(n-3)) and oleic acid (18:1(n-9)) presented in association with micelles or albumin

In vivo, long-chain fatty acids are incorporated into bile salt micelles, which solubilize the hydrophobic fatty acids before they are transported across the unstirred water layer to the intestinal brush border membrane. In the present study we have used CaCo-2 cells, cultured on filter membranes as a model of human enterocytes, and compared presentation of fatty acids bound to albumin with a micellar form. Absorption of eicosapentaenoic acid and oleic acid from micellar solutions was 4-times faster than from fatty acid-albumin complexes after 5 h incubation, and resulted in a corresponding increase in triacylglycerol synthesis and secretion. Mass determination of newly synthesized, cell-associated triacylglycerol after 5 h incubation, indicated a 5-fold increase in cells exposed to a micellar solution versus albumin-complexed fatty acids. A 2-fold larger fraction of the absorbed fatty acids was incorporated into triacylglycerol than into phospholipids when the fatty acids were presented as micelles. Analysis by resistive pulse technique showed that secreted lipoproteins of density less than 1.006 g/ml were in the same size-range as chylomicrons derived from human plasma. In spite of an increased amount of secreted triacylglycerol from cells supplemented with micellar fatty acids, there was no increase in the mean size of these particles. Synthesis and secretion of triacylglycerol in cells exposed to eicosapentaenoic acid and oleic acid were similar regardless of whether the fatty acids were presented to the cells associated with albumin or micelles, although the total amount of triacylglycerol synthesized and secreted was highest with micelles. When incubating CaCo-2 monolayers with eicosapentaenoic acid or oleic acid bound to albumin, a similar amount of radioactivity was released as CO2 and acid soluble products into the medium with less than 3% of the lipids being oxidized after 5 h of incubation. The oxidation rate of fatty acids in cells incubated with micelles was increased 40 to 100%. In conclusion, micellar fatty acids are absorbed, metabolized and influence secretion of lipoprotein particles to a higher extent than albumin-bound fatty acids in CaCo-2 cells, and there is no major difference between eicosapentaenoic acid and oleic acid.

Sorting of a secretory protein (gp80) to the apical surface of Caco-2 cells

We have investigated the synthesis and polarized secretion of the exogenous gp80 glycoprotein complex in the human epithelial adenocarcinoma cell line, Caco-2. gp80 is secreted at the apical surface of Madin-Darby canine kidney (MDCK) cells and should, therefore, display the signal(s) required for sorting into the apical exocytic pathway. In Caco-2 cells, no bona fide secretory protein released preferentially at the apical surface has been described so far. To address the question of whether Caco-2 cells possess a machinery capable of delivery of secretory proteins at the apical surface, we stably transfected the cells with a recombinant gene coding for the gp80 glycoprotein complex. Pulse-chase analysis showed that stably transfected Caco-2 cells secrete gp80 quantitatively into the medium. In polarized layers of filter-grown Caco-2 cells, the protein was secreted predominantly at the apical surface, demonstrating the ability of the cells to efficiently sort secretory proteins directly into the apical exocytic pathway. Our results further demonstrate that the apical targeting information of gp80 recognized by MDCK cells is also recognized by Caco-2 cells.

Effects of NH4Cl and nocodazole on polarized fibronectin secretion vary amongst different epithelial cell types

The extracellular matrix protein fibronectin was found to be secreted by three polarized epithelial cell lines Madin-Darby canine kidney (MDCK), Caco-2 and LLC-PK1. About 54 and 46% of fibronectin was secreted from the apical and basolateral cell surfaces, respectively, in MDCK cells. In Caco-2 and LLC-PK1 cells, the majority (about 92-93%) of fibronectin secretion occurs from the basolateral cell surface, with the remaining 7-8% from the apical surface. In all three cell types, NH4Cl was found to inhibit basolateral secretion (resulting in enhanced apical secretion), while total fibronectin secretion was not significantly affected (although a delay in secretion was observed). Nocodazole reduced total fibronectin secretion to about 70% of control levels in MDCK and Caco-2 cells, with significant inhibition on secretion from both surfaces. In contrast, total fibronectin secretion was enhanced by nocodazole in LLC-PK1 cells. Furthermore, the majority of fibronectin secretion was redirected to the apical cell surface in LLC-PK1 cells. These observations demonstrate that the nature as well as the extent of the effects of NH4-Cl and nocodazole on polarized fibronectin secretion varies amongst different epithelial cell types.

Retinoic-acid-induced augmentation of molecular species carrying sialosyl Lewis(a) antigen in colorectal-carcinoma cell cultures

In order to study the effects of vitamin-A metabolites on long-term carcinoma-antigen secretion, colorectal-carcinoma cells SW1116 were cultured on membrane filters in totally synthetic media with 0 to 2.6 microM retinoic acid (RA). RA altered cell division, cell size and soluble-sialosyl Le(a) (S-Le(a) secretion and S-Le(a) accumulation within cells and apical-membrane domains. Cultures treated with RA for 10-12 days grew to lower cell densities (60% of controls) and contained more protein per cell (140% of controls). RA treated cells also had 5-fold higher levels of S-Le(a) in cells and secreted 9-fold more S-Le(a) into culture media assayed per 24 hr by (ELISA) 19-9 monoclonal antibody binding. As total media S-Le(a) increased, polarity of non-lipid S-Le(a) antigen secretion increased toward the interior (apical) media. High-performance thin-layer immunobinding showed that ganglioside S-Le(a) was higher in RA-fed cells, but could not be detected in apical media of RA-fed or control cells after 24 hr. Western blots indicated that non-lipid sialosyl Lewis(a) was bound to 150- to 180-kDA molecular species principally in cells, but 210- to 300-kDa molecular species appeared in the non-lipid extract of media. Thus, the above RA alterations, monitored by 3 immunochemical techniques, include up to 9-fold stimulation of "constitutive" 150- to 300-kDa sialosyl-Lewis(a) secretion, but ganglioside Lewis(a) is sorted differently and retained by apical membranes.

Effects of NB-598, a potent squalene epoxidase inhibitor, on the apical membrane uptake of cholesterol and basolateral membrane secretion of lipids in Caco-2 cells

Caco-2 cells grown on membrane filters were used as a model to study the effects of NB-598, an inhibitor of squalene epoxidase, on cholesterol absorption from the intestinal epithelia. NB-598 (10 microM) inhibited the synthesis of sterol and sterol ester from [14C]acetate without affecting the synthesis of other lipids such as phospholipids (PL), free fatty acids (FFA) and triacylglycerol (TG). When labeled lipid was apically loaded as a micellar lipid solution into Caco-2 cell cultures, NB-598 reduced basolaterally secreted radioactivity in cholesterol, cholesterol ester, PL and TG. Furthermore, NB-598 suppressed the basolateral secretion of apolipoprotein (apo) B. When microsomes prepared from control Caco-2 cells were incubated with 10 microM NB-598, acyl CoA:cholesterol acyltransferase (ACAT) activity was inhibited slightly. After incubating Caco-2 cells with 10 microM NB-598, a slight reduction in cellular ACAT activity was also observed. These results suggest that suppression of the secretion of particles containing apo B and reduction of cellular ACAT activity in the intestinal epithelia are part of the mechanism of the cholesterol-lowering effect of NB-598.

Mevinolin, a competitive inhibitor of hydroxymethylglutaryl coenzyme A reductase, suppresses enterocyte esterification of exogenous but not endogenous cholesterol

Mevinolin (lovastatin), a competitive inhibitor of hydroxymethylglutaryl-coenzyme A reductase, directly inhibited acyl-CoA cholesteryl acyltransferase in rabbit intestinal microsomes at a dose of 20 micrograms/ml or more. Lineweaver-Burk analysis showed a competitive type of inhibition with respect to oleoyl-CoA. In cultured intestinal Caco-2 cells, mevinolin reduced [14C]oleate incorporation into cholesteryl-esters by 86% of controls at doses as low as 0.1 micrograms/ml. However, in cells whose activity of acyl-CoA cholesteryl acyltransferase was stimulated 7-fold by 10 mM mevalonolactone, a significant inhibitory effect on cholesteryl-ester formation could not be detected, even at 40 micrograms/ml of mevinolin. In contrast, cells supplied with liposomal cholesterol or cholesterol derived from low-density lipoproteins showed a marked reduction of cholesteryl-ester formation in the presence of 10 or 0.1 micrograms/ml of mevinolin, respectively. It is concluded that the observed suppressive effects of mevinolin on cholesterol esterification in cultured Caco-2 cells are indirect and possibly caused by changes in the acyl-CoA cholesteryl acyltransferase substrate pool or intracellular cholesterol transport.

High-density lipoprotein 3 retroendocytosis: a new lipoprotein pathway in the enterocyte (Caco-2)

The present study in Caco-2 cells, derived from a human colon carcinoma and capable of enterocyte differentiation in culture, describes a retroendocytotic pathway for high-density lipoprotein 3 (HDL3). These cells exhibit specific binding of apolipoprotein E-free HDL3 which was competed for by HDL3 but not by low-density lipoproteins. At 37 degrees C, degradation was negligible and intact particles were internalized and resecreted into the medium within 2 hours. Electron microscopy showed binding and internalization of gold-labeled HDL3 in coated pit regions and transport in endosomes distinct from lysosomes to lipid droplets. The fusion of these endosomes with lipid droplets was followed by their dissolution and the subsequent extrusion of HDL particles from the cells. Fluorescence labeling studies of HDL3 supported cytosolic transport in vesicles. Specific binding showed negative feedback regulation by HDL3, was modulated by alterations in cellular cholesterol content, and increased with the cellular state of differentiation. HDL3 mediated efflux of endogenously labeled cholesterol. It is concluded that intact HDL3 is bound specifically by Caco-2 cells, leading to a subsequent intracellular passage and resecretion through a process of retroendocytosis effecting the efflux of cellular cholesterol.

Fate of exogenous and newly synthesized cholesterol in intestinal cell lines

1. The current study was undertaken to test the existence of functionally distinct intracellular pools of cholesterol depending on the origin: neosynthesis or exogenous. 2. This was performed on two subpopulations, either differentiated or undifferentiated, of the HT29 cell line. 3. A parallel study was also carried out on Caco-2 cells. 4. First we checked the ability of differentiated HT29 cells to secrete lipids into the medium and found that lipid production was efficient but less so than in Caco-2 cells. 5. In contrast, undifferentiated HT29 cells were unable to secrete lipids into the medium. 6. Then we studied the fate of [14C]cholesterol incorporated into micellar preparations and of [14C]mevalonate in the different models. 7. The data obtained with labelled exogenous cholesterol show that it enters the membrane cholesterol pool as well as, for the differentiated models, the cholesteryl ester pool. 8. Similarly, labelled newly synthesized cholesterol could be used for membrane formation as well as for incorporation into cholesteryl esters. 9. Thus, in HT29 subpopulations as well as in Caco-2 cells, the results suggest the existence of a common pool of cholesterol whatever its origin.

Synthesis and secretion of apolipoprotein E by human placenta and choriocarcinoma cell lines

The synthesis and secretion of apolipoproteins (apos) by cells from a human choriocarcinoma cell line, JAR, were examined by [35S]-methionine labeling followed by immunoprecipitation and SDS/PAGE. Apo E, but not apos A-I, A-IV, or B, was synthesized and secreted. Apo E was also synthesized by fragments of chorionic villi from human placenta and by another choriocarcinoma line, BeWo. Pulse-chase experiments with JAR cells revealed that apoE was initially synthesized as a 33 kDa protein followed by a 34 KDa protein, probably the result of glycosylation. The latter was secreted into the medium where it was detected coincident with a 21/22 kDa doublet, possibly proteolytic fragments of apo E. Approximately 50 per cent of the apo E in the medium was complexed with lipid as indicated by ultracentrifugation at a density of 1.21 g/ml. The amount of apo E produced by JAR was not affected by preincubation with dibutyryl cAMP and theophylline, or by the cholesterol content of the cells. Following perfusion of an isolated lobule of human placenta with [14C]-amino acids, [14C]-apo E was detected by immunoprecipitation of the maternal and fetal perfusates with 88 per cent in the maternal perfusate. These studies suggest that apo E, which promotes receptor-mediated lipoprotein uptake, is secreted by the trophoblast to facilitate uptake of maternal lipoproteins.