Regulation of intestinal apolipoprotein B synthesis and secretion by Caco-2 cells. Lack of fatty acid effects and control by intracellular calcium ion

Dietary fat induced chylomicron-mediated LPS translocation in a bicameral Caco-2cell model

BACKGROUND

There is increasing evidence that dietary fat, especially saturated fat, promotes the translocation of lipopolysaccharide (LPS) via chylomicron production in the gut. Chylomicrons can subsequently transport LPS to other parts of the body, where they can induce low-grade chronic inflammation that is linked to various metabolic and gut-related diseases. To identify promising (food) compounds that can prevent or ameliorate LPS-related low-grade inflammation, we developed and optimized a bicameral in vitro model for dietary fat-induced LPS translocation that closely mimics the in vivo situation and facilitates high-throughput screening.

METHODS

Caco-2 cells were cultured in monolayers and differentiated to a small intestinal phenotype in 21 days. Thereafter, optimal conditions for fat-induced chylomicron production were determined by apical exposure of Caco-2 cells to a dilution range of in vitro digested palm oil and sunflower oil, optionally preceded by a 1-week apical FBS deprivation (cultured without apical fetal bovine serum). Chylomicron production was assessed by measuring basolateral levels of the chylomicron-related marker apolipoprotein B. Next, LPS was coincubated at various concentrations with the digested oils, and fat-induced LPS translocation to the basolateral side was assessed.

RESULTS

We found that dietary fat-induced LPS translocation in Caco-2 cells was optimal after apical exposure to digested oils at a 1:50 dilution in combination with 750 ng/mL LPS, preceded by 1 week of apical FBS deprivation. Coincubation with the chylomicron blocker Pluronic L81 confirmed that fat-induced LPS translocation is mediated via chylomicron production in this Caco-2 cell model.

CONCLUSION

We developed a robust Caco-2 cell model for dietary fat-induced LPS translocation that can be used for high-throughput screening of (food) compounds that can reduce LPS-related low-grade inflammation.

Effects of eicosapentaenoic acid and docosahexaenoic acid on chylomicron and VLDL synthesis and secretion in Caco-2 cells

The present research was undertaken to determine the effects of EPA (20 : 5 n-3) and DHA (22 : 6 n-3) on chylomicron and VLDL synthesis and secretion by Caco-2 cells. Cells were incubated for 12 to 36 h with 400 μM OA, EPA, and DHA; then 36 h was chosen for further study because EPA and DHA decreased de novo triglycerides synthesis in a longer incubation compared with OA  (P < 0.01). Neither the uptake nor oxidation was different in response to the respective fatty acids (P > 0.05). Compared with OA, intercellular and secreted nascent apolipoprotein B48 and B100 were decreased by EPA and DHA (P < 0.01). Both DHA and EPA resulted in a lower secretion of chylomicron and VLDL (P < 0.01). In contrast to OA, EPA and DHA were preferentially incorporated into phospholipids instead of triacylglycerols (P < 0.01). These discoveries demonstrated that exposure of DHA and EPA reduced the secretion of chylomicron and VLDL partly by regulating the synthesis of TG and apoB.

Common mechanisms of monoacylglycerol and fatty acid uptake by human intestinal Caco-2 cells

Free fatty acids (FFA) and sn-2-monoacylglycerol (sn-2-MG), the two hydrolysis products of dietary triacylglycerol, are absorbed from the lumen into polarized enterocytes that line the small intestine. Intensive studies regarding FFA transport across the brush-border membrane of the enterocyte are available; however, little is known about sn-2-MG transport. We therefore studied the kinetics of sn-2-MG transport, compared with those of long-chain FFA (LCFA), by human intestinal Caco-2 cells. To mimic postprandial luminal and plasma environments, we examined the uptake of taurocholate-mixed lipids and albumin-bound lipids at the apical (AP) and basolateral (BL) surfaces of Caco-2 cells, respectively. The results demonstrate that the uptake of sn-2-monoolein at both the AP and BL membranes appears to be a saturable function of the monomer concentration of sn-2-monoolein. Furthermore, trypsin preincubation inhibits sn-2-monoolein uptake at both AP and BL poles of cells. These results suggest that sn-2-monoolein uptake may be a protein-mediated process. Competition studies also support a protein-mediated mechanism and indicate that LCFA and LCMG may compete through the same membrane protein(s) at the AP surface of Caco-2 cells. The plasma membrane fatty acid-binding protein (FABP(pm)) is known to be expressed in Caco-2, and here we demonstrate that fatty acid transport protein (FATP) is also expressed. These putative plasma membrane LCFA transporters may be involved in the uptake of sn-2-monoolein into Caco-2 cells.

Calcium increases apolipoprotein B mRNA editing

ApoB-100 and apoB-48 are major components of chylomicrons, very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL). The two proteins are generated from a single apoB mRNA by apoB mRNA editing which induces an in-frame stop codon in apoB mRNA. Apolipoprotein B (apoB) mRNA editing is an important determinant of the proportion of full-length (apoB-100) and truncated (apoB-48) proteins in total apoB metabolism. Calcium is involved in the regulation of secretion and synthesis of VLDL and apoB. In this paper, we demonstrate for the first time that the amount of edited apoB mRNA in the cultured cells Caco-2 and McA7777 is markedly increased by calcium. Increasing extracellular calcium concentration, calcium ionophore (A23187 and ionomycin) treatment, and depleting calcium stores and raising cytoplasmic calcium concentration by thapsigargin increase apoB mRNA editing up to threefold in a dose dependent manner. Calcium has no direct stimulative effect on apoB mRNA editing in an in vitro editing system. The editing increase by extracellular calcium is not related to alterations of APOBEC-1 mRNA expression. These data suggest that calcium is not only involved in the regulation of apolipoprotein metabolism but also apoB mRNA editing.

A proposed model for the assembly of chylomicrons

The intestine synthesizes very low density lipoproteins (VLDL) and chylomicrons (CM) to transport fat and fat-soluble vitamins into the blood. VLDL assembly occurs constitutively whereas CM assembly is a characteristic property of the enterocytes during the postprandial state. The secretion of CM is specifically inhibited by Pluronic L81. CM are very heterogeneously-sized particles that consist of a core of triglycerides (TG) and cholesterol esters and a monolayer of phospholipids (PL), cholesterol and proteins. The fatty acid composition of TG, but not PL, in CM mirrors the fatty acid composition of fat in the diet. CM assembly is deficient in abetalipoproteinemia and CM retention disease. Abetalipoproteinemia results due to mutation in the mttp gene and is characterized by the virtual absence of apoB-containing lipoproteins in the plasma. Patients suffer from neurologic disorders, visual impairment, and exhibit acanthocytosis. CM retention disease, an inherited recessive disorder, is characterized by chronic diarrhea with steatorrhea in infancy, abdominal distention and failure to thrive. It is caused by a specific defect in the secretion of intestinal lipoproteins; secretion of lipoproteins by the liver is not affected. Besides human disorders, mice that do not assemble intestinal lipoproteins have been developed. These mice are normal at birth, but defective in fat and fat-soluble vitamin absorption, and fail to thrive. Thus, fat and fat-soluble vitamin transport by the intestinal lipoproteins is essential for proper growth and development of neonates. Recently, differentiated Caco-2 cells and rabbit primary enterocytes have been described that synthesize and secrete CM. These cells can be valuable in distinguishing between the two different models proposed for the assembly of CM. In the first model, the assembly of VLDL and CM is proposed to occur by two 'independent' pathways. Second, CM assembly is proposed to be a product of 'core expansion' that results in the synthesis of lipoproteins of different sizes. According to this model, intestinal lipoprotein assembly begins with the synthesis of 'primordial' lipoprotein particles and involves release of the nascent apoB with PL derived from the endoplasmic reticulum (ER) membrane. In addition, TG-rich 'lipid droplets' of different sizes are formed independent of apoB synthesis. The fusion of lipid droplets and primordial lipoproteins results in the formation of different size lipoproteins due to the 'core expansion' of the primordial lipoproteins.

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.

Chylomicron assembly and catabolism: role of apolipoproteins and receptors

Chylomicrons are lipoproteins synthesized exclusively by the intestine to transport dietary fat and fat-soluble vitamins. Synthesis of apoB48, a translational product of the apob gene, is required for the assembly of chylomicrons. The apob gene transcription in the intestine results in 14 and 7 kb mRNAs. These mRNAs are post-transcriptionally edited creating a stop codon. The edited mRNAs chylomicrons from the shorter apoB48 peptide remains to be elucidated. In addition, the roles of proteins involved in the assembly pathway, e.g. apobec-1, MTP and apoA-IV, needs to be studied. Cloning of enzymes involved in the intestinal biosynthesis of triglycerides will be crucial to fully appreciate the assembly of chylomicrons. There is a need for cell culture and transgenic animal models that can be used for intestinal lipoprotein assembly. The catabolism of chylomicrons is far more complex and efficient than the catabolism of VLDL. Even though the major steps involved in the catabolism of chylomicrons are now known, the determinants for apolipoprotein exchange, processing of remnants in the space of Disse, as well as the mechanism of uptake of these particles by extra-hepatic tissue needs further exploration.

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.

[Differentiation of human colon cancer cells: a new approach to colon cancer]

Our purpose is to demonstrate that some human colon cancer cells are able to express the same differentiation features as normal intestinal epithelial cells and to report experimental data which suggest that these particular cells are spontaneously resistant and adaptable to anticancer drugs. The concept of normal differentiation of colon cancer cells is supported by observations made with two cultured cell lines, Caco-2 and HT-29. The cell line Caco-2 expresses spontaneously and homogeneously an enterocytic differentiation. The cell line HT-29 is heterogeneous as it contains a small proportion (< 5%) of differentiated cells of either enterocytic or mucus-secreting type. Homogeneous populations of differentiated HT-29 cells of either type have been isolated by pressure selection. In order to investigate whether the pressure associated with anti-cancer drugs would result in the selection of differentiated populations, HT-29 cells were cultured in the presence of increasing concentrations of methotrexate and 5-fluorouracil. The resulting resistant populations were found to be totally differentiated. This supports the view that the small proportion of parental cells which are able to differentiate are also able to spontaneously resist to drug pressure. These results imply that the concept of cellular differentiation should be considered by pathologists. They also imply that basic research should be developed in order to unravel which mechanisms of drug resistance are specifically associated with the ability of the cells to differentiate.

Apolipoprotein expression and cellular differentiation in Caco-2 intestinal cells

Caco-2 cells, cultured for 18 days on porous filter supports and conventional plastic culture dishes, were used to study the effects of cellular differentiation on the expression of apolipoprotein (apo) genes. Media of filter-grown cells accumulated more apo B as apo B-48 and contained three times the amount of edited apo B mRNA compared with plastic-grown cells. The accumulation of apo A-I by media of plastic-grown cells was higher than accumulation by filter-grown cells, despite similar concentrations of apo A-I mRNA. The apo A-IV was detectable in the culture media earlier with filter-grown cells compared with plastic-grown cells, despite similar apo A-IV mRNA concentrations. Plastic-grown cells contained more apo E mRNA, and their media accumulated more apo E than filter-grown cells. With the exception of apo A-I, apo gene expression changed with Caco-2 cell differentiation to resemble more closely the patterns seen in adult enterocytes. There were no effects or minimal effects of added retinoic acid, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], or thyroid hormone on apo accumulation in media of filter-grown cultures of Caco-2 cells. However, 1,25(OH)2D3 and thyroid hormone increased apo B, apo A-IV, and apo A-I mRNA concentrations, retinoic acid increased apo B mRNA concentrations alone, and all three reduced apo E mRNA concentrations. Ratios of edited to unedited apo B mRNA were unaffected. In conclusion, culture substratum importantly influences Caco-2 cell differentiation. Soluble factors that influence cellular differentiation may affect apo gene expression over and above effects mediated by the culture substratum.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypercholesterolemia induces differential expression of rabbit apolipoprotein A and C genes

We have compared steady-state mRNA levels of apolipoproteins AI, AII, AIV, CI, CII and CIII in liver and small intestine of rabbits fed on a cholesterol-rich diet for up to 16 weeks. Apolipoprotein (apo) AIV mRNA was detected in both liver and small intestine, while apo AII was not detected in either organ. Apo CI, apo CII and apo CIII were expressed only in liver and apo AI mRNA was detected only in small intestine. In small intestine, apo AIV and apo AI mRNA levels increased to a maximum at the 4th and 12th week of treatment, respectively. In liver there was a parallel increase in the mRNA levels of apo AIV, apo CI, apo CII and apo CIII, with maximum levels after 4 weeks of treatment. A 3-fold increase was found in apo CII and apo CIII hepatic transcription rates between hypercholesterolemic and control rabbits after 4 weeks of treatment, no longer detectable after 8 weeks. However, no changes were found in apo AIV and apo CI transcription rates. Changes in apolipoprotein mRNA levels were accompanied by changes in plasma lipoprotein levels. Overall, these changes correlate well with the variations detected in the expression of the different apolipoprotein genes. Our results indicate that dietary cholesterol plays an important role in the regulation of these genes and that this regulation is tissue dependent.

High level expression of human apolipoprotein A-I in transgenic rats raises total serum high density lipoprotein cholesterol and lowers rat apolipoprotein A-I

To examine the consequences of increased apolipoprotein A-I production on cholesterol and lipoprotein metabolism, we have produced two lines of transgenic rats; one expressing moderate and one very high levels of human apolipoprotein A-I. The rats were produced by microinjection of a 13 kbp DNA fragment containing the human apolipoprotein A-I gene plus 10 kbp of its 5' flanking sequence and 1 kbp of its 3' flanking sequence. Both lines of transgenic rats express human apolipoprotein A-I mRNA in liver and human apolipoprotein A-I in plasma. Sera from these rats contain significantly higher levels of total apolipoprotein A-I, high density lipoprotein cholesterol and phospholipid than sera from non-transgenic littermates. Transgenic rats expressing high levels of human apolipoprotein A-I have reduced levels of serum rat apolipoprotein A-I suggesting a mechanism exists to down-regulate apolipoprotein A-I production. These transgenic rats provide a unique animal model to examine the effects of increased apolipoprotein A-I production on lipid and lipoprotein metabolism.

Iron absorption by intestinal epithelial cells: 1. CaCo2 cells cultivated in serum-free medium, on polyethyleneterephthalate microporous membranes, as an in vitro model

Iron absorption by intestinal epithelial cells, passage onto plasmatic apotransferrin, and regulation of the process remain largely misunderstood. To investigate this problem, we have set up an in vitro model, consisting in CaCo2 cells (a human colon adenocarcinoma line, which upon cultivation displays numerous differentiation criteria of small intestine epithelial cells). Cells are cultivated in a serum-free medium, containing 1 microgram/ml insulin, 1 ng/ml epidermal growth factor, 10 micrograms/ml albumin-linoleic acid, 100 nM hydrocortisone, and 2 nM T3 on new, transparent, Cyclopore polyethyleneterephthalate microporous membranes coated with type I collagen. Cells rapidly adhere, grow, and form confluent monolayers; after 15 days, scanning electron microscopy reveals numerous uniform microvilli. Domes, which develop on nonporous substrata, are absent on high porosity membranes. Culture medium from upper and lower compartments of microplate inserts and cell lysates were immunoprecipitated after labeling with [3H]glucosamine and leucine; analysis was done by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), followed by autoradiography. [3H]transferrin is found mainly in the lower compartment and in cells; [3H]apolipoprotein B is released in both compartments, and fibronectin almost entirely recovered in the lower compartment; [3H]transferrin receptors and ferritin are only present in cell lysates. Binding experiments also show that transferrin receptors are accessible from the lower compartment. These results suggest that CaCo2 cells, cultivated in synthetic medium on membranes of appropriate porosity, could provide an in vitro model of the intestinal barrier, with the upper compartment of the culture insert corresponding to the apical pole facing the intestinal lumen and the lower one to the basal pole in contact with blood.

The assembly of lipids into lipoproteins during secretion

The process of assembly and secretion of lipoproteins is discussed with particular reference to the role of lipids. The majority of circulating lipoproteins is produced by the liver (80%) with the remainder being supplied by the intestine. The liver secretes both very low density lipoproteins and high density lipoproteins, but the assembly and secretion of these two types of particles may follow different routes. The major lipid components of lipoproteins are triacylglycerols, cholesterol, cholesterol esters and phospholipids. The biosynthesis of these lipids occurs on membranes of the endoplasmic reticulum, with many of the enzymes also being present in the Golgi; the roles of these two subcellular organelles in the assembly of lipoproteins are discussed. There appears to be a compartmentalization of lipids in cells, such that defined pools, often those newly-synthesized, are preferred, or even required, for lipoprotein assembly. The process of hepatic very low density lipoprotein secretion appears to be regulated by the supply of lipids. Indeed, the synthesis of new lipid may be a major driving force in lipoprotein assembly and secretion.

Oleic acid stimulation of apolipoprotein B secretion from HepG2 and Caco-2 cells occurs post-transcriptionally

HepG2 and Caco-2 cells were studied to compare the regulation of liver and intestinal apolipoprotein (apo) biosynthesis and secretion by fatty acids. Incubation with fatty acid consistently stimulated apoB production by both HepG2 and Caco-2 cells. Media concentrations of apoB, determined by radioimmunoassay, were approx. 3-fold greater for cells incubated for 24 h in serum-free medium containing oleic acid bound to albumin than for cells incubated with albumin alone. Oleic acid also resulted in a 2-3-fold accumulation of cellular triacylglycerol for HepG2 cells and Caco-2 cells. Cellular apoB and media and cellular apoA-I concentrations were not affected by oleic acid. Immunoblotting with a monoclonal antibody against human apoB confirmed a greater mass of apoB in media from HepG2 and Caco-2 cells incubated with oleic acid. Radiolabeled apoB-100 was also increased in media from HepG2 and Caco-2 cells incubated with [35S]methionine for 24 h in the presence of oleic acid, suggesting enhanced apoB synthesis. However, apoB mRNA concentrations were unchanged in response to oleic acid. Gel filtration of media by fast protein liquid chromatography (FPLC) revealed a redistribution of apoB from LDL-sized particles to VLDL or chylomicrons in media from Caco-2 cells incubated with oleic acid, whereas apoB remained in LDL for HepG2 cells. ApoA-I in media from HepG2 and Caco-2 cells eluted as free or lipid-poor apoA-I, and the apoA-I distribution was unaltered by incubation with oleic acid. These data demonstrate that HepG2 and Caco-2 cells maintained in supplemented serum-free medium respond to oleic acid by a similar post-transcriptional increase in apoB synthesis, but different packaging of apoB into triacylglycerol-rich lipoproteins.