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

Mechanisms for the interaction of the milk fat globule membrane with the plasma membrane of gut epithelial cells

The milk fat globule membrane (MFGM) provides infants and adults with several health benefits. These are not derived solely from its unique composition, but also from arrangement of lipids in the MFGM that, in the case of newborns, could reach the intestine partially intact. Fluorochromes associated with lipid derivatives were used to prove a fusion process between the MFGM and the cellular membrane of differentiated Caco-2 cells. To explore the mechanism of this interaction, incubations of MFGM with Caco-2 cells were carried out in the presence of fusogenic agents or compounds that block other MFGM interaction pathways with cells. Confocal fluorescence microscopy provided visual evidence of the fusion process. Lastly, determination on the lipid profile of cells after their interaction with MFGM indicated a metabolic rearrangement of lipids leading to accumulation of triacylglycerols.

Comparative Pharmacokinetics of Commercially Available Cannabidiol Isolate, Broad-Spectrum, and Full-Spectrum Products

BACKGROUND AND OBJECTIVES

A wide variety of products containing cannabidiol (CBD) are available on the commercial market. One of the most common products, CBD oil, is administered to self-treat a variety of conditions. These oils are available as CBD isolate, broad-spectrum [all terpenes and minor cannabinoids except Δ-9-tetrahydrocannabinol (THC)], or full-spectrum (all terpenes and minor cannabinoids with THC < 0.3% dried weight) products. A systematic pharmacokinetic study was performed to determine whether there are differences in the pharmacokinetic parameters and systemic exposure of CBD after oral dosing as an isolate, broad-spectrum, or full-spectrum product.

METHODS

Male and female Sprague Dawley rats were treated with a single, equivalent oral dose of CBD delivered as isolate, broad-spectrum, or full-spectrum product. An additional study using an in-house preparation of CBD isolate plus 0.2% THC was performed. A permeability assay was also conducted to investigate whether the presence of THC alters the intestinal permeability of CBD.

RESULTS

There was an increase in the oral bioavailability of CBD (12% and 21% in male and female rats, respectively) when administered as a full-spectrum product compared with the isolate and broad-spectrum products. There was no difference in the bioavailability of CBD between the commercially available full-spectrum formulation (3.1% CBD; containing 0.2% THC plus terpenes and other minor cannabinoids) versus the in-house preparation of CBD full-spectrum (CBD isolate 3.2% plus 0.2% THC isolate). In vitro permeability assays demonstrated that the presence of THC increases permeability of CBD while also decreasing efflux through the gut wall.

CONCLUSIONS

The presence of 0.2% THC increased the oral bioavailability of CBD in male and female rats, indicating that full-spectrum products may produce increased effectiveness of CBD due to a greater exposure available systemically.

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.

Chylomicron production is repressed by RPTOR knockdown, R-α-lipoic acid and 4-phenylbutyric acid in human enterocyte-like Caco-2 cells

Although the role of mechanistic target of rapamycin complex 1 (mTORC1) in lipid metabolism has been the subject of previous research, its function in chylomicron production is not known. In this study, we created three stable human colorectal adenocarcinoma Caco-2 cell lines exhibiting normal, low or high mTORC1 kinase activity, and used these cells to investigate the consequences of manipulating mTORC1 activity on enterocyte differentiation and chylomicron-like particle production. Constitutively active mTORC1 induced Caco-2 cell proliferation and differentiation (as judged by alkaline phosphatase activity) but weakened transepithelial electrical resistance (TEER). Repressed mTORC1 activity due to the knockdown of RPTOR significantly decreased the expression of lipogenic genes FASN, DGAT1 and DGAT2, lipoprotein assembly genes APOB and MTTP, reduced protein expression of APOB, MTTP and FASN, downregulated the gene expression of very long-chain fatty acyl-CoA ligase (FATP2), acyl-CoA binding protein (DBI), and prechylomicron transport vesicle-associated proteins VAMP7 (vesicle-associated membrane protein 7) and SAR1B (secretion associated Ras related GTPase 1B) resulting in the repression of apoB-containing triacylglycerol-rich lipoprotein secretion. Exposure of Caco-2 cells harboring a constitutively active mTORC1 to short-chain fatty acid derivatives, R-α-lipoic acid and 4-phenylbutyric acid, downregulated chylomicron-like particle secretion by interfering with the lipidation and assembly of the particles, and concomitantly repressed mTORC1 activity with no change to Raptor abundance or PRAS40 (Thr246) phosphorylation. R-α-lipoic acid and 4-phenylbutyric acid may be useful to mitigate intestinal lipoprotein overproduction and associated postprandial inflammation.

Mechanisms Underlying the Impacts of Lipids on the Diverse Bioavailability of Per- and Polyfluoroalkyl Substances in Foods

Food is a major source of human exposure to per- and polyfluoroalkyl substances (PFASs), yet little is known about their bioavailability in food matrices. Here, the relative bioavailability (RBA) of PFASs in foods was determined using an in vivo mouse model. Pork, which had the highest lipid content, exhibited the greatest effect on bioavailability by increasing the RBAs of perfluoroalkyl acids (PFAAs) while reducing those of fluorotelomer phosphate diesters (diPAPs). During intestinal digestion of lipids, the bioaccessibility of PFAAs increased due to their greater partition into the stable mixed micelles. However, diPAPs were more likely to partition into the undigested oil phase due to their strong hydrophobicity. Both in vitro incubation and molecular docking results indicated that the PFAAs exhibited stronger binding affinities with mouse blood chylomicrons (CMs) than with diPAPs. Collectively, both lipid digestion in the intestine and the carrier effect of CMs played important roles in modulating the bioavailability of PFASs in food. More attention should be given to further evaluating the health risks of PFASs associated with the intake of high-lipid foods.

Dose Dependent Effects of Fructose and Glucose on de novo Palmitate and Glycerol Synthesis in an Enterocyte Cell Model

SCOPE

Fructose exacerbates post-prandial hypertriacylglycerolaemia; perhaps partly due to increased enterocyte de novo lipogenesis (DNL). It is unknown whether this is concentration-dependent or if fructose has a greater effect on lipid synthesis than glucose. Dose-dependent effects of fructose and glucose on DNL and de novo triacylglycerol (TAG)-glycerol synthesis are investigated in a Caco-2 cell model.

METHODS AND RESULTS

Caco-2 cells are treated for 96 h with 5, 25, or 50 mM fructose or glucose, or 12.5 mM fructose/12.5 mM glucose mix. DNL is measured following addition of [¹³ C₂ ]-acetate to apical media. Separately, [¹³ C₆ ]-fructose and [¹³ C₆ ]-glucose are used to measure DNL and de novo TAG-glycerol synthesis. DNL from [¹³ C₂ ]-acetate is detected following all treatments, with greater amounts in intracellular than secreted (media) samples (all p < 0.05). DNL from [¹³ C₆ ]-fructose and [¹³ C₆ ]-glucose is also measurable. Intracellular synthesis is concentration-dependent for both glucose (p = 0.003) and fructose (p = 0.034) tracers and is higher with 25 mM glucose than 25 mM fructose (p = 0.025). DNL from fructose and glucose is <1%, but up to 70% of de novo TAG-glycerol is synthesized from glucose or fructose.

CONCLUSION

Fructose is not a major source of DNL in Caco-2 cells but contributes substantially to de novo TAG-glycerol synthesis.

Turmeric, red pepper, and black pepper affect carotenoids solubilized micelles properties and bioaccessibility: Capsaicin/piperine improves and curcumin inhibits carotenoids uptake and transport in Caco-2 cells

This study aimed to evaluate the role of spices/spice active principles on physical, biochemical, and molecular targets of bioaccessibility/bioavailability. Carotenoids-rich micellar fraction obtained through simulated digestion of green leafy vegetables (GLV) with individual or two/three combinations were correlated to their influence on bioaccessibility, cellular uptake, and basolateral secretion of carotenoids in Caco-2 cells. Results suggest that carotenoids' bioaccessibility depends on micelles physicochemical properties, which is affected due to the presence of co-treated dietary spices and their composition. Increased bioaccessibility of β-carotene (BC) and lutein (LUT) is found in GLV (spinach) digested with turmeric (TM) than red pepper (RP) and black pepper (BP). In contrast, enhanced cellular uptake and secretion of BC and LUT-rich triglyceride-rich lipoprotein is observed in the presence of RP and BP compared to the control group. In contrast, TM inhibited absorption, while retinol levels significantly reduced in the presence of TM and RP than BP. Control cells have indicated higher cleavage of β-carotene to retinol than the spice-treated group. Besides, spice active principles modulate facilitated transport of carotenoids by scavenger receptor class B type 1 (SR-B1) protein. The effect of spices on carotenoids' bioavailability is validated with active spice principles. Overall, carotenoids' bioavailability (cellular uptake and basolateral secretion) was found in the following order of treatments; piperine > capsaicin > piperine + capsaicin > curcumin + capsaicin + piperine > control > turmeric. These findings suggested that the interaction of specific dietary factors, including spice ingredients at the enterocyte level, could provide greater insight into carotenoid absorption. PRACTICAL APPLICATION: Spices/spice active principles play a role in the digestion process by stimulating digestive enzymes and bile acids secretion. Since carotenoids are lipid soluble and have low bioavailability, spice ingredients' influence on intestinal absorption of carotenoids is considered crucial. Hence, understanding the interaction of co-consumed spices on the absorption process of carotenoids may help to develop functional foods/formulation of nutraceuticals to improve their health benefits.

Ezetimibe impairs transcellular lipid trafficking and induces large lipid droplet formation in intestinal absorptive epithelial cells

Ezetimibe inhibits Niemann-Pick C1-like 1 (NPC1L1) protein, which mediates intracellular cholesterol trafficking from the brush border membrane to the endoplasmic reticulum, where chylomicron assembly takes place in enterocytes or in the intestinal absorptive epithelial cells. Cholesterol is a minor lipid constituent of chylomicrons; however, whether or not a shortage of cholesterol attenuates chylomicron assembly is unknown. The aim of this study was to examine the effect of ezetimibe, a potent NPC1L1 inhibitor, on trans-epithelial lipid transport, and chylomicron assembly and secretion in enterocytes. Caco-2 cells, an absorptive epithelial model, grown onto culture inserts were given lipid micelles from the apical side, and chylomicron-like triacylglycerol-rich lipoprotein secreted basolaterally were analyzed after a 24-h incubation period in the presence of ezetimibe up to 50 μM. The secretion of lipoprotein and apolipoprotein B48 were reduced by adding ezetimibe (30% and 34%, respectively). Although ezetimibe allowed the cells to take up cholesterol normally, the esterification was abolished. Meanwhile, oleic acid esterification was unaffected. Moreover, ezetimibe activated sterol regulatory element-binding protein 2 by approximately 1.5-fold. These results suggest that ezetimibe limited cellular cholesterol mobilization required for lipoprotein assembly. In such conditions, large lipid droplet formation in Caco-2 cells and the enterocytes of mice were induced, implying that unprocessed triacylglycerol was sheltered in these compartments. Although ezetimibe did not reduce the post-prandial lipid surge appreciably in triolein-infused mice, the results of the present study indicated that pharmacological actions of ezetimibe may participate in a novel regulatory mechanism for the efficient chylomicron assembly and secretion.

Effects of undigested protein-rich ingredients on polarised small intestinal organoid monolayers

Abstract

Here, we describe the use of monolayers of intestinal epithelial cells derived from intestinal organoids and transcriptomics to investigate the direct effects of dietary protein sources on epithelial function. Mechanically dissociated 3D organoids of mouse duodenum were used to generate a polarized epithelium containing all cell types found in the tissue of origin. The organoid-derived cell monolayers were exposed to 4% (w/v) of 'undigested (non-hydrolysed)-soluble' fraction of protein sources used as feed ingredients [soybean meal (SBM) and casein], or alternative protein sources (spray dried plasma protein, and yellow meal worm), or controls for 6 h prior to RNA isolation and transcriptomics. All protein sources altered expression of unique biological processes in the epithelial cells. Exposure of intestinal organoids to SBM downregulated expression of retinol and retinoid metabolic processes as well as cholesterol and lipid biosynthetic pathways, consistent with the reported hypotriglyceridaemic effect of soy protein in vivo. These findings support the use of intestinal organoids as models to evaluate complex interactions between dietary ingredients and the intestinal epithelium and highlights some unique host effects of alternative protein sources in animal feed and potentially human food.

Graphical abstract

Schematic representation of the study. 3-dimensional organoids were generated from mouse duodenum (1). The organoids were subsequently dissociated into single cells (2) and grown as 2-dimensional polarised monolayers (3). Polarized monolayers of organoid cells were exposed to different protein sources [CAS, SBM, SDPP, YMW, or medium control (MC)] for 6 h (4) and further processed for imaging (5) gene expression (6), and biochemical assays (7), to investigate the effects of undigested protein sources on the duodenal epithelium.

Leptin-mediated differential regulation of microsomal triglyceride transfer protein in the intestine and liver affects plasma lipids

The hormone leptin regulates fat storage and metabolism by signaling through the brain and peripheral tissues. Lipids delivered to peripheral tissues originate mostly from the intestine and liver via synthesis and secretion of apolipoprotein B (apoB)-containing lipoproteins. An intracellular chaperone, microsomal triglyceride transfer protein (MTP), is required for the biosynthesis of these lipoproteins, and its regulation determines fat mobilization to different tissues. Using cell culture and animal models, here we sought to identify the effects of leptin on MTP expression in the intestine and liver. Leptin decreased MTP expression in differentiated intestinal Caco-2 cells, but increased expression in hepatic Huh7 cells. Similarly, acute and chronic leptin treatment of chow diet-fed WT mice decreased MTP expression in the intestine, increased it in the liver, and lowered plasma triglyceride levels. These leptin effects required the presence of leptin receptors (LEPRs). Further experiments also suggested that leptin interacted with long-form LEPR (ObRb), highly expressed in the intestine, to down-regulate MTP. In contrast, in the liver, leptin interacted with short-form LEPR (ObRa) to increase MTP expression. Mechanistic experiments disclosed that leptin activates signal transducer and activator of transcription 3 (STAT3) and mitogen-activated protein kinase (MAPK) signaling pathways in intestinal and hepatic cells, respectively, and thereby regulates divergent MTP expression. Our results also indicated that leptin-mediated MTP regulation in the intestine affects plasma lipid levels. In summary, our findings suggest that leptin regulates MTP expression differentially by engaging with different LEPR types and activating distinct signaling pathways in intestinal and hepatic cells.

ATP7B knockout disturbs copper and lipid metabolism in Caco-2 cells

Intestinal cells control delivery of lipids to the body by adsorption, storage and secretion. Copper (Cu) is an important trace element and has been shown to modulate lipid metabolism. Mutation of the liver Cu exporter ATP7B is the cause of Wilson disease and is associated with Cu accumulation in different tissues. To determine the relationship of Cu and lipid homeostasis in intestinal cells, a CRISPR/Cas9 knockout of ATP7B (KO) was introduced in Caco-2 cells. KO cells showed increased sensitivity to Cu, elevated intracellular Cu storage, and induction of genes regulating oxidative stress. Chylomicron structural protein ApoB48 was significantly downregulated in KO cells by Cu. Apolipoproteins ApoA1, ApoC3 and ApoE were constitutively induced by loss of ATP7B. Formation of small sized lipid droplets (LDs) was enhanced by Cu, whereas large sized LDs were reduced. Cu reduced triglyceride (TG) storage and secretion. Exposure of KO cells to oleic acid (OA) resulted in enhanced TG storage. The findings suggest that Cu represses intestinal TG lipogenesis, while loss of ATP7B results in OA-induced TG storage.

Inhibition of intestinal glucose transport by polyphenols: a mechanism for indirect attenuation of cholesterol absorption?

Cholesterol uptake and chylomicron synthesis are promoted by increasing glucose concentrations in both healthy and diabetic individuals during the postprandial phase. The goal of this study was to test whether acute inhibition of glucose uptake could impact cholesterol absorption in differentiated human intestinal Caco-2 cells. As expected, high glucose upregulated intestinal cholesterol metabolism promoting its uptake and incorporation in lipoproteins. This was accompanied by an increase in the gene expression of Niemann-Pick C1 Like 1 and proprotein convertase subtillisin/kexin type 9. Cholesterol uptake was attenuated by acute inhibition of glucose absorption by cytochalasin B, by a chamomile extract and by one of its main constituent polyphenols, apigenin 7-O-glucoside; however, chylomicron secretion was only reduced by the chamomile extract. These data support a potential indirect role for bioactives in modulating intestinal lipid pathways through effects on intestinal glucose uptake. This working hypothesis warrants further testing in an in vivo setting such as in hypercholesterolaemic or prediabetic individuals.

Only α-Gal bound to lipids, but not to proteins, is transported across enterocytes as an IgE-reactive molecule that can induce effector cell activation

BACKGROUND

The oligosaccharide galactose-α-1,3-galactose (α-Gal), present in mammalian proteins and lipids, causes an unusual delayed allergic reaction 3 to 6 hours after ingestion of mammalian meat in individuals with IgE antibodies against α-Gal. To better understand the delayed onset of allergic symptoms and investigate whether protein-bound or lipid-bound α-Gal causes these symptoms, we analyzed the capacity of α-Gal conjugated proteins and lipids to cross a monolayer of intestinal cells.

METHODS

Extracts of proteins and lipids from beef were prepared, subjected to in vitro digestions, and added to Caco-2 cells grown on permeable supports. The presence of α-Gal in the basolateral medium was investigated by immunoblotting, thin-layer chromatography with immunostaining and ELISA, and its allergenic activity was analyzed in a basophil activation test.

RESULTS

After addition of beef proteins to the apical side of Caco-2 cells, α-Gal containing peptides were not detected in the basolateral medium. Those peptides that crossed the Caco-2 monolayer did not activate basophils from an α-Gal allergic patient. Instead, when Caco-2 cells were incubated with lipids extracted from beef, α-Gal was detected in the basolateral medium. Furthermore, these α-Gal lipids were able to activate the basophils of an α-Gal allergic patient in a dose-dependent manner.

CONCLUSION

Only α-Gal bound to lipids, but not to proteins, is able to cross the intestinal monolayer and trigger an allergic reaction. This suggests that the slower digestion and absorption of lipids might be responsible for the unusual delayed allergic reactions in α-Gal allergic patients and identifies glycolipids as potential allergenic molecules.

Lipin 2/3 phosphatidic acid phosphatases maintain phospholipid homeostasis to regulate chylomicron synthesis

The lipin phosphatidic acid phosphatase (PAP) enzymes are required for triacylglycerol (TAG) synthesis from glycerol 3-phosphate in most mammalian tissues. The 3 lipin proteins (lipin 1, lipin 2, and lipin 3) each have PAP activity, but have distinct tissue distributions, with lipin 1 being the predominant PAP enzyme in many metabolic tissues. One exception is the small intestine, which is unique in expressing exclusively lipin 2 and lipin 3. TAG synthesis in small intestinal enterocytes utilizes 2-monoacylglycerol and does not require the PAP reaction, making the role of lipin proteins in enterocytes unclear. Enterocyte TAGs are stored transiently as cytosolic lipid droplets or incorporated into lipoproteins (chylomicrons) for secretion. We determined that lipin enzymes are critical for chylomicron biogenesis, through regulation of membrane phospholipid composition and association of apolipoprotein B48 with nascent chylomicron particles. Lipin 2/3 deficiency caused phosphatidic acid accumulation and mammalian target of rapamycin complex 1 (mTORC1) activation, which were associated with enhanced protein levels of a key phospholipid biosynthetic enzyme (CTP:phosphocholine cytidylyltransferase α) and altered membrane phospholipid composition. Impaired chylomicron synthesis in lipin 2/3 deficiency could be rescued by normalizing phospholipid synthesis levels. These data implicate lipin 2/3 as a control point for enterocyte phospholipid homeostasis and chylomicron biogenesis.

Oleoylethanolamide differentially regulates glycerolipid synthesis and lipoprotein secretion in intestine and liver

Dietary fat absorption takes place in the intestine, and the liver mobilizes endogenous fat to other tissues by synthesizing lipoproteins that require apoB and microsomal triglyceride transfer protein (MTP). Dietary fat triggers the synthesis of oleoylethanolamide (OEA), a regulatory fatty acid that signals satiety to reduce food intake mainly by enhancing neural PPARα activity, in enterocytes. We explored OEA's roles in the assembly of lipoproteins in WT and Ppara ^-/- mouse enterocytes and hepatocytes, Caco-2 cells, and human liver-derived cells. In differentiated Caco-2 cells, OEA increased synthesis and secretion of triacylglycerols, apoB secretion in chylomicrons, and MTP expression in a dose-dependent manner. OEA also increased MTP activity and triacylglycerol secretion in WT and knockout primary enterocytes. In contrast to its intestinal cell effects, OEA reduced synthesis and secretion of triacylglycerols, apoB secretion, and MTP expression and activity in human hepatoma Huh-7 and HepG2 cells. Also, OEA reduced MTP expression and triacylglycerol secretion in WT, but not knockout, primary hepatocytes. These studies indicate differential effects of OEA on lipid synthesis and lipoprotein assembly: in enterocytes, OEA augments glycerolipid synthesis and lipoprotein assembly independent of PPARα. Conversely, in hepatocytes, OEA reduces MTP expression, glycerolipid synthesis, and lipoprotein secretion through PPARα-dependent mechanisms.

Influence of PEGylation of Vitamin-K-Loaded Mixed Micelles on the Uptake by and Transport through Caco-2 Cells

The aim of the study is to investigate the uptake by and transport through Caco-2 cells of two mixed micelle formulations (based on egg phosphatidylcholine and glycocholic acid) of vitamin K, i.e., with and without DSPE-PEG2000. The uptake of vitamin K and fluorescently labeled mixed micelles with and without PEG coating showed similar kinetics and their uptake ratio remained constant over time. Together with the fact that an inhibitor of scavenger receptor B1 (BLT-1) decreased cellular uptake of vitamin K by ∼80% compared to the uptake in the absence of this inhibitor, we conclude that both types of micelles loaded with vitamin K can be taken up intactly by Caco-2 cells via this scavenger receptor. The amount of vitamin K in chylomicrons fraction from Caco-2 cell monolayers further indicates that mixed micelles (with or without PEGylation) are likely packed into chylomicrons after internalization by Caco-2 cells. Uptake of vitamin K from PEGylated mixed micelles increased four- to five-fold at simulated gastrointestinal conditions. In conclusion, PEGylated mixed micelles are stable upon exposure to simulated gastric conditions, and as a result, they do show overall a higher cellular uptake efficiency of vitamin K as compared to mixed micelles without PEG coating.

Phosphatidylcholine synthesis regulates triglyceride storage and chylomicron secretion by Caco2 cells

Intracellular lipid droplets (LDs) supply fatty acids for energy, membrane biogenesis, and lipoprotein secretion. The surface monolayer of LDs is composed of phospholipids, primarily phosphatidylcholine (PC), that stabilize the neutral lipid core of triglyceride (TG). To determine the relationship between PC synthesis and TG storage and secretion in chylomicrons, we used a model of intestinal-derived human epithelial colorectal adenocarcinoma (Caco2) cells with knockout of PCYT1A, which encodes the rate-limiting enzyme CTP:phosphocholine cytidylyltransferase (CCT)α in the CDP-choline pathway, that were treated with the fatty acid oleate. CRISPR/Cas9 knockout of CCTα in Caco2 cells (Caco2-KO cells) reduced PC synthesis by 50%. Compared with Caco2 cells, Caco2-KO cells exposed to oleate had fewer and larger LDs and greater TG accumulation as a result. The addition of exogenous lysophosphatidylcholine to Caco2-KO cells reversed the LD morphology defect. Caco2-KO cells, differentiated into epithelial monolayers, accumulated intracellular TG and had deficient TG and chylomicron-associated apoB48 secretion; apoB100 secretion was unaffected by CCTα knockout or oleate. Metabolic-labeling and LD imaging of Caco2-KO cells indicated preferential shuttling of de novo synthesized TG into larger LDs rather than into chylomicrons. Thus, reduced de novo PC synthesis in Caco2 cells enhances TG storage in large LDs and inhibits apoB48 chylomicron secretion.

Transcriptional development of phospholipid and lipoprotein metabolism in different intestinal regions of Atlantic salmon (Salmo salar) fry

Background

It has been suggested that the high phospholipid (PL) requirement in Atlantic salmon (Salmo salar) fry is due to insufficient intestinal de-novo synthesis causing low lipoprotein (LP) production and reduced transport capacity of dietary lipids. However, in-depth ontogenetic analysis of intestinal PL and LP synthesis with the development of salmon has yet to be performed. Therefore, in this paper we used RNA-Seq technology to investigate the expression of genes involved in PL synthesis and LP formation throughout early developmental stages and associate insufficient expression of synthesis pathways in salmon fry with its higher dietary PL requirement. There was a special focus on the understanding homologous genes, especially those from salmonid-specific fourth vertebrate whole-genome duplication (Ss4R), and their contribution to salmonid specific features of regulation of PL metabolic pathways. Salmon fry were sampled at 0.16 g (1 day before first-feeding), 2.5 and 10 g stages of development and transcriptomic analysis was applied separately on stomach, pyloric caeca and hindgut of the fish.

Results

In general, we found up-regulated pathways involved in synthesis of phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn), and LP in pyloric caeca of salmon between 0.16 and 10 g. Thirteen differentially expressed genes (q < 0.05) in these pathways were highly up-regulated in 2.5 g salmon compared to 0.16 g, while only five more differentially expressed (q < 0.05) genes were found when the fish grew up to 10 g. Different homologous genes were found dominating in stomach, pyloric caeca and hindgut. However, the expression of dominating genes in pathways of PL and LP synthesis were much higher in pyloric caeca than stomach and hindgut. Salmon-specific homologous genes (Ss4R) had similar expression during development, while other homologs had more diverged expression.

Conclusions

The up-regulation of the de-novo PtdCho and PtdEtn pathways confirm that salmon have decreasing requirement for dietary PL as the fish develops. The similar expressions between Ss4R homologous genes suggest that the functional divergence of these genes was incomplete compared to homologs derived from other genome duplication. The results of the present study have provided new information on the molecular mechanisms of phospholipid synthesis and lipoprotein formation in fish.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4651-8) contains supplementary material, which is available to authorized users.

Gastrointestinal factors regulating lipid droplet formation in the intestine

Cytoplasmic lipid droplets (CLD) are considered as neutral lipid reservoirs, which protect cells from lipotoxicity. It became clear that these fascinating dynamic organelles play a role not only in energy storage and metabolism, but also in cellular lipid and protein handling, inter-organelle communication, and signaling among diverse functions. Their dysregulation is associated with multiple disorders, including obesity, liver steatosis and cardiovascular diseases. The central aim of this review is to highlight the link between intra-enterocyte CLD dynamics and the formation of chylomicrons, the main intestinal dietary lipid vehicle, after overviewing the morphology, molecular composition, biogenesis and functions of CLD.

Effect of sonication on bioaccessibility and cellular uptake of carotenoids from preparations of photoautotrophic Phaeodactylum tricornutum

With regard to its cost-effective cultivation and the composition of high-value nutrients, the diatom Phaeodactylum tricornutum (P. tricornutum) attracts interest for the use in human nutrition. Besides a number of important nutrients, it is rich in carotenoids. Therefore, this study aimed to investigate the potential of P. tricornutum as a carotenoid source for human nutrition. In photoautotrophically produced P. tricornutum biomass the carotenoid constitution, bioaccessibility (in vitro digestion model) and cellular uptake in differentiated Caco-2 cells (Transwell model system) was determined. Furthermore, the influence of sonication on these parameters was investigated. The results indicate that β-carotene, zeaxanthin and fucoxanthin were the main carotenoids found in P. tricornutum. Moreover, these carotenoids showed a good bioaccessibility (β-carotene: 25%, zeaxanthin: 27%, fucoxanthin: 57%), which is further improved by sonication for β-carotene and fucoxanthin. In line with the good bioaccessibility, fucoxanthin was the most abundant carotenoid in Caco-2 cells followed by zeaxanthin. In contrast, β-carotene could not be detected in the cells. The present study demonstrated that P. tricornutum represents a good source of carotenoids, particularly fucoxanthin. Thus, this diatom can contribute to the intake of bioaccessible carotenoids, even without processing. In addition, sonication might be a useful tool to improve the carotenoid bioaccessibility.

Cloning and characterization of an insect apolipoprotein (apolipophorin-II/I) involved in the host immune response of Antheraea pernyi

Apolipoproteins are protein components of lipoprotein particles, and are increasingly recognized to be functioning in the innate immune systems of both insects and mammals. Mammalian apolipoprotein B (apoB) is associated with a diverse range of innate immune defenses including suppression of bacterial pathogenesis, virus toxicity neutralization, and inhibition of cytokine releases from immune cells. However, little is known about apoB homologous insect apolipophorin-II/I (apoLp-II/I) in controlling of specific pathogen-host encounters such as microbial infections. In the present study, we describe cDNA cloning and characterization of an apoLp-II/I from Chinese oak silk worm, Antheraea pernyi. The apoLp-II/I cDNA is 10237bp in length, which possesses an open reading frame encoding 3305 amino acids. A consensus cleavage site RGRR presenting from Arg⁷¹⁰ to Arg⁷¹³ implies posttranslational cleavage of this protein. Ap-apoLp-II/I shares high sequence identities with apoLp-II/I in lepidoptera and other insects. In addition, considerable similarities also exist between Ap-apoLp-II/I and human apoB, which basically positioned in first 1000 residues of the amino termini. Tissue distribution and time-course expression results demonstrate that Ap-apoLp-II/I transcripts were detected predominantly in the fat body, less in epidermis and rarely in midgut, while the synthetic apoLp-II/I protein was abundant in hemocytes and plasma instead of the fatbody. Expression of Ap-apoLp-II/I was stimulated in response to bacterial challenge. In addition, our preliminary studies established a novel role for Ap-apoLp-II/I in regulating prophenoloxidase activation system. Taken together, apoLp-II/I may play an essential role in innate responses of Antheraea pernyi.

Infant digestion physiology and the relevance of in vitro biochemical models to test infant formula lipid digestion

Lipids play an important role in the diet of preterm and term infants providing a key energy source and essential lipid components for development. While a lot is known about adult lipid digestion, our understanding of infant digestion physiology is still incomplete, the greatest gap being on the biochemistry of the small intestine, particularly the activity and relative importance of the various lipases active in the intestine. The literature has been reviewed to identify the characteristics of lipid digestion of preterm and term infants, but also to better understand the physiology of the infant gastrointestinal tract compared to adults that impacts the absorption of lipids. The main differences are a higher gastric pH, submicellar bile salt concentration, a far more important role of gastric lipases as well as differences at the level of the intestinal barrier. Importantly, the consequences of improper in vitro replication of gastric digestions conditions (pH and lipase specificity) are demonstrated using examples from the most recent of studies. It is true that some animal models could be adapted to study infant lipid digestion physiology, however the ethical relevance of such models is questionable, hence the development of accurate in vitro models is a must. In vitro models that combine up to date knowledge of digestion biochemistry with intestinal cells in culture are the best choice to replicate digestion and absorption in infant population, this would allow the adaptation of infant formula for a better digestion and absorption of dietary lipids by preterm and term infants.

FABP1 knockdown in human enterocytes impairs proliferation and alters lipid metabolism

Fatty Acid-Binding Proteins (FABPs) are abundant intracellular proteins that bind long chain fatty acids (FA) and have been related with inmunometabolic diseases. Intestinal epithelial cells express two isoforms of FABPs: liver FABP (LFABP or FABP1) and intestinal FABP (IFABP or FABP2). They are thought to be associated with intracellular dietary lipid transport and trafficking towards diverse cell fates. But still their specific functions are not well understood. To study FABP1's functions, we generated an FABP1 knockdown model in Caco-2 cell line by stable antisense cDNA transfection (FABP1as). In these cells FABP1 expression was reduced up to 87%. No compensatory increase in FABP2 was observed, strengthening the idea of differential functions of both isoforms. In differentiated FABP1as cells, apical administration of oleate showed a decrease in its initial uptake rate and in long term incorporation compared with control cells. FABP1 depletion also reduced basolateral oleate secretion. The secreted oleate distribution showed an increase in FA/triacylglyceride ratio compared to control cells, probably due to FABP1's role in chylomicron assembly. Interestingly, FABP1as cells exhibited a dramatic decrease in proliferation rate. A reduction in oleate uptake as well as a decrease in its incorporation into the phospholipid fraction was observed in proliferating cells. Overall, our studies indicate that FABP1 is essential for proper lipid metabolism in differentiated enterocytes, particularly concerning fatty acids uptake and its basolateral secretion. Moreover, we show that FABP1 is required for enterocyte proliferation, suggesting that it may contribute to intestinal homeostasis.

Intestinal bile acid receptors are key regulators of glucose homeostasis

In addition to their well-known function as dietary lipid detergents, bile acids have emerged as important signalling molecules that regulate energy homeostasis. Recent studies have highlighted that disrupted bile acid metabolism is associated with metabolism disorders such as dyslipidaemia, intestinal chronic inflammatory diseases and obesity. In particular, type 2 diabetes (T2D) is associated with quantitative and qualitative modifications in bile acid metabolism. Bile acids bind and modulate the activity of transmembrane and nuclear receptors (NR). Among these receptors, the G-protein-coupled bile acid receptor 1 (TGR5) and the NR farnesoid X receptor (FXR) are implicated in the regulation of bile acid, lipid, glucose and energy homeostasis. The role of these receptors in the intestine in energy metabolism regulation has been recently highlighted. More precisely, recent studies have shown that FXR is important for glucose homeostasis in particular in metabolic disorders such as T2D and obesity. This review highlights the growing importance of the bile acid receptors TGR5 and FXR in the intestine as key regulators of glucose metabolism and their potential as therapeutic targets.

Bioaccessibility and intestinal cell uptake of astaxanthin from salmon and commercial supplements

Although the keto-carotenoid astaxanthin (Ast) is not typically present in human plasma due to its relative scarcity in the typical diet, global consumption of salmon, the primary source of Ast in food, and Ast supplements continues to increase. The first objective of the present study was to investigate the bioaccessibility of Ast from uncooked and cooked fillets of wild and aquacultured salmon, Ast-supplements and krill oil, during simulated gastric and small intestinal digestion. Uptake of E-Ast from micelles generated during digestion of wild salmon by monolayers of Caco-2 was also monitored. Both wild and aquacultured salmon flesh contained E-Ast and Z-isomers of unesterified Ast, whereas Ast esters were the predominant form of the carotenoid in commercial supplements and krill oil. Flesh from wild salmon contained approximately 10 times more Ast than aquacultured salmon. Common styles of cooking flesh from wild and aquacultured salmon decreased Ast content by 48-57% and 35-47%, respectively. Ast in salmon flesh, supplements and krill oil was relatively stable (>80% recovery) during in vitro digestion. The efficiency of transfer of Ast into mixed micelles during digestion of uncooked wild salmon was 43%, but only 12% for uncooked acquacultured salmon. Cooking wild salmon significantly decreased Ast bioaccessibility. The relative bioaccessibility of Ast (41-67%) after digestion of oil vehicle in commercial supplements was inversely proportional to carotenoid content (3-10mg/capsule), whereas bioaccessibility of endogenous Ast in phospholipid-rich krill oil supplement was 68%. >95% of Ast in mixed micelles generated during digestion of supplements and krill oil was unesterified. Caco-2 intestinal cells accumulated 11-14% of E-Ast delivered in mixed micelles generated from digested wild salmon. Apical uptake and basolateral secretion of E-Ast by Caco-2 cells grown on inserts were greater after digestion of Ast-enriched krill oil compared to uncooked wild salmon. These data suggest that the bioacessibility of Ast in wild salmon and soft-gel capsules is greater than that in aquacultured salmon, and that uptake and basolateral secretion of the carotenoid by enterocyte-like cells is enhanced by the digestion products of phospholipid-rich krill oil.

New Insights In Intestinal Sar1B GTPase Regulation and Role in Cholesterol Homeostasis

Sar1B GTPase is a key component of Coat protein complex II (COPII)-coated vesicles that bud from the endoplasmic reticulum to export newly synthesized proteins. The aims of this study were to determine whether Sar1B responds to lipid regulation and to evaluate its role in cholesterol (CHOL) homeostasis. The influence of lipids on Sar1B protein expression was analyzed in Caco-2/15 cells by Western blot. Our results showed that the presence of CHOL (200 μM) and oleic acid (0.5 mM), bound to albumin, increases Sar1B protein expression. Similarly, supplementation of the medium with micelles composed of taurocholate with monooleylglycerol or oleic acid also stimulated Sar1B expression, but the addition of CHOL (200 μM) to micelle content did not modify its regulation. On the other hand, overexpression of Sar1B impacted on CHOL transport and metabolism in view of the reduced cellular CHOL content along with elevated secretion when incubated with oleic acid-containing micelles for 24 h, thereby disclosing induced CHOL transport. This was accompanied with higher secretion of free- and esterified-CHOL within chylomicrons, which was not the case when oleic acid was replaced with monooleylglycerol or when albumin-bound CHOL was given alone. The aforementioned cellular CHOL depletion was accompanied with a low phosphorylated/non phosphorylated HMG-CoA reductase ratio, indicating elevated enzymatic activity. Combination of Sar1B overexpression with micelle incubation led to reduction in intestinal CHOL transporters (NPC1L1, SR-BI) and metabolic regulators (PCSK9 and LDLR). The present work showed that Sar1B is regulated in a time- and concentration-dependent manner by dietary lipids, suggesting an adaptation to alimentary lipid flux. Our data also suggest that Sar1B overexpression contributes to regulation of CHOL transport and metabolism by facilitating rapid uptake and transport of CHOL.

Recent discoveries on absorption of dietary fat: Presence, synthesis, and metabolism of cytoplasmic lipid droplets within enterocytes

Dietary fat provides essential nutrients, contributes to energy balance, and regulates blood lipid concentrations. These functions are important to health, but can also become dysregulated and contribute to diseases such as obesity, diabetes, cardiovascular disease, and cancer. Within enterocytes, the digestive products of dietary fat are re-synthesized into triacylglycerol, which is either secreted on chylomicrons or stored within cytoplasmic lipid droplets (CLDs). CLDs were originally thought to be inert stores of neutral lipids, but are now recognized as dynamic organelles that function in multiple cellular processes in addition to lipid metabolism. This review will highlight recent discoveries related to dietary fat absorption with an emphasis on the presence, synthesis, and metabolism of CLDs within this process.

Characteristics and functions of lipid droplets and associated proteins in enterocytes

Cytosolic lipid droplets (LDs) are observed in enterocytes of jejunum during lipid absorption. One important function of the intestine is to secrete chylomicrons, which provide dietary lipids throughout the body, from digested lipids in meals. The current hypothesis is that cytosolic LDs in enterocytes constitute a transient pool of stored lipids that provides lipids during interprandial period while lowering chylomicron production during the post-prandial phase. This smoothens the magnitude of peaks of hypertriglyceridemia. Here, we review the composition and functions of lipids and associated proteins of enterocyte LDs, the known physiological functions of LDs as well as the role of LDs in pathological processes in the context of the intestine.

Using Caco-2 Cells to Study Lipid Transport by the Intestine

Studies of dietary fat absorption are generally conducted by using an animal model equipped with a lymph cannula. Although this animal model is widely accepted as the in vivo model of dietary fat absorption, the surgical techniques involved are challenging and expensive. Genetic manipulation of the animal model is also costly and time consuming. The alternative in vitro model is arguably more affordable, timesaving, and less challenging. Importantly, the in vitro model allows investigators to examine the enterocytes as an isolated system, reducing the complexity inherent in the whole organism model. This paper describes how human colon carcinoma cells (Caco-2) can serve as an in vitro model to study the enterocyte transport of lipids, and lipid-soluble drugs and vitamins. It explains the proper maintenance of Caco-2 cells and the preparation of their lipid mixture; and it further discusses the valuable option of using the permeable membrane system. Since differentiated Caco-2 cells are polarized, the main advantage of using the permeable membrane system is that it separates the apical from the basolateral compartment. Consequently, the lipid mixture can be added to the apical compartment while the lipoproteins can be collected from the basolateral compartment. In addition, the effectiveness of the lentivirus expression system in upregulating gene expression in Caco-2 cells is discussed. Lastly, this paper describes how to confirm the successful isolation of intestinal lipoproteins by transmission electron microscopy (TEM).

Lycopene bioavailability is associated with a combination of genetic variants

The intake of tomatoes and tomato products, which constitute the main dietary source of the red pigment lycopene (LYC), has been associated with a reduced risk of prostate cancer and cardiovascular disease, suggesting a protective role of this carotenoid. However, LYC bioavailability displays high interindividual variability. This variability may lead to varying biological effects following LYC consumption. Based on recent results obtained with two other carotenoids, we assumed that this variability was due, at least in part, to several single nucleotide polymorphisms (SNPs) in genes involved in LYC and lipid metabolism. Thus, we aimed at identifying a combination of SNPs significantly associated with the variability in LYC bioavailability. In a postprandial study, 33 healthy male volunteers consumed a test meal containing 100g tomato puree, which provided 9.7 mg all-trans LYC. LYC concentrations were measured in plasma chylomicrons (CM) isolated at regular time intervals over 8 h postprandially. For the study 1885 SNPs in 49 candidate genes, i.e., genes assumed to play a role in LYC bioavailability, were selected. Multivariate statistical analysis (partial least squares regression) was used to identify and validate the combination of SNPs most closely associated with postprandial CM LYC response. The postprandial CM LYC response to the meal was notably variable with a CV of 70%. A significant (P=0.037) and validated partial least squares regression model, which included 28 SNPs in 16 genes, explained 72% of the variance in the postprandial CM LYC response. The postprandial CM LYC response was also positively correlated to fasting plasma LYC concentrations (r=0.37, P<0.05). The ability to respond to LYC is explained, at least partly, by a combination of 28 SNPs in 16 genes. Interindividual variability in bioavailability apparently affects the long-term blood LYC status, which could ultimately modulate the biological response following LYC supplementation.

Characterization of the proteome of cytoplasmic lipid droplets in mouse enterocytes after a dietary fat challenge

Dietary fat absorption by the small intestine is a multistep process that regulates the uptake and delivery of essential nutrients and energy. One step of this process is the temporary storage of dietary fat in cytoplasmic lipid droplets (CLDs). The storage and mobilization of dietary fat is thought to be regulated by proteins that associate with the CLD; however, mechanistic details of this process are currently unknown. In this study we analyzed the proteome of CLDs isolated from enterocytes harvested from the small intestine of mice following a dietary fat challenge. In this analysis we identified 181 proteins associated with the CLD fraction, of which 37 are associated with known lipid related metabolic pathways. We confirmed the localization of several of these proteins on or around the CLD through confocal and electron microscopy, including perilipin 3, apolipoprotein A-IV, and acyl-CoA synthetase long-chain family member 5. The identification of the enterocyte CLD proteome provides new insight into potential regulators of CLD metabolism and the process of dietary fat absorption.

Regulation of intestinal lipid absorption by clock genes

Plasma levels of triacylglycerols and diacylglycerols, the lipoproteins that transport them, and proteins involved in their absorption from the intestinal lumen fluctuate in a circadian manner. These changes are likely controlled by clock genes expressed in the intestine that are probably synchronized by neuronal and humoral signals from the suprachiasmatic nuclei, which constitute a master clock entrained by light signals from the eyes and from the environment, e.g., food availability. Acute changes in circadian rhythms--e.g., due to nonsynchronous work schedules or a transcontinental flight--may trigger intestinal discomfort. Chronic disruptions in circadian control mechanisms may predispose the individual to irritable bowel syndrome, gastroesophageal reflux disease, and peptic ulcer disease. A more detailed understanding of the molecular mechanisms underlying temporal changes in intestinal activity might allow us to identify novel targets for developing therapeutic approaches to these disorders.

TNF-alpha and IL-6 inhibit apolipoprotein A-IV production induced by linoleic acid in human intestinal Caco2 cells

Background

Apolipoprotein A-IV (apoA-IV) is a protein mainly synthesized by enterocytes in the intestine. Its gene expression is suppressed during fasting and stimulated during active fat absorption. Chronic feeding of a high-fat (HF) diet abolishes the differential expression between fasting and fat-feeding and therefore may contribute to diet-induced obesity since apoA-IV is a potent satiety factor. It is well established that the circulating pro-inflammatory cytokines TNF-α and IL-6 are increased by HF feeding.

Methods

To determine whether pro-inflammatory cytokines are involved in the diminished response of apoA-IV gene expression to fat-feeding, different concentrations of linoleic acid (LA), an important dietary fatty acid, was used to stimulate apoA-IV expression in human intestinal Caco2 cells. Cells were pre-treated with or without human recombinant TNF-α, IL-6 or their combination before the addition of LA. Real-time PCR and ELISA were used to detect and quantify RNA transcripts and proteins of apoA-IV and the cytokines.

Results

LA stimulated gene and protein expression of apoA-IV in a dose and time dependent manner. Pre-treatment with the cytokines for 72 h significantly inhibited the increased expression of apoA-IV gene and protein induced by LA. Furthermore, the cytokines, especially TNF-α, also positively up-regulate the cytokine themselves in Caco2 cells.

Conclusions

Our data indicate that the pro-inflammatory cytokines may be responsible for the reduced apoA-IV production in response to fat feeding. Because of apoA-IV’s role in satiety, we propose the inhibitory effect of circulating pro-inflammatory cytokines on apoA-IV production contributes to diet-induced obesity.

Lipids around the Clock: Focus on Circadian Rhythms and Lipid Metabolism

Disorders of lipid and lipoprotein metabolism and transport are responsible for the development of a large spectrum of pathologies, ranging from cardiovascular diseases, to metabolic syndrome, even to tumour development. Recently, a deeper knowledge of the molecular mechanisms that control our biological clock and circadian rhythms has been achieved. From these studies it has clearly emerged how the molecular clock tightly regulates every aspect of our lives, including our metabolism. This review analyses the organisation and functioning of the circadian clock and its relevance in the regulation of physiological processes. We also describe metabolism and transport of lipids and lipoproteins as an essential aspect for our health, and we will focus on how the circadian clock and lipid metabolism are greatly interconnected. Finally, we discuss how a deeper knowledge of this relationship might be useful to improve the recent spread of metabolic diseases.

Increased bioavailability of ubiquinol compared to that of ubiquinone is due to more efficient micellarization during digestion and greater GSH-dependent uptake and basolateral secretion by Caco-2 cells

The oral bioavailability of ubiquinol recently has been reported to be greater than that of ubiquinone in healthy adults. The basis for this influence of redox state of coenzyme Q (CoQ) on bioavailability has been investigated using the coupled in vitro digestion/Caco-2 cell model. Solubilized ubiquinol and ubiquinone were added to yogurt and subjected to simulated gastric and small intestinal digestion. Partitioning of CoQ in mixed micelles during small intestinal digestion was significantly greater during digestion of yogurt enriched with ubiquinol. Similarly, apical uptake from mixed micelles and transepithelial transport of CoQ by Caco-2 cells were significantly greater after digestion of the ubiquinol-rich yogurt compared to digested ubiquinone-rich yogurt. Reduction of cellular GSH significantly decreased cell uptake and basolateral secretion of both ubiquinol and ubiquinone, although the adverse impact was much greater for ubiquinol. These data suggest that the enhanced bioaccessibility and bioavailability of ubiquinol compared to ubiquinone results from reduced coenzyme being more efficiently incorporated into mixed micelles during digestion and its greater uptake and basolateral secretion in a glutathione-dependent mechanism.

Lxr-driven enterocyte lipid droplet formation delays transport of ingested lipids

Liver X receptors (Lxrs) are master regulators of cholesterol catabolism, driving the elimination of cholesterol from the periphery to the lumen of the intestine. Development of pharmacological agents to activate Lxrs has been hindered by synthetic Lxr agonists' induction of hepatic lipogenesis and hypertriglyceridemia. Elucidating the function of Lxrs in regulating enterocyte lipid handling might identify novel aspects of lipid metabolism that are pharmacologically amenable. We took a genetic approach centered on the single Lxr gene nr1h3 in zebrafish to study the role of Lxr in enterocyte lipid metabolism. Loss of nr1h3 function causes anticipated gene regulatory changes and cholesterol intolerance, collectively reflecting high evolutionary conservation of zebrafish Lxra function. Intestinal nr1h3 activation delays transport of absorbed neutral lipids, with accumulation of neutral lipids in enterocyte cytoplasmic droplets. This delay in transport of ingested neutral lipids protects animals from hypercholesterolemia and hepatic steatosis induced by a high-fat diet. On a gene regulatory level, Lxra induces expression of acsl3a, which encodes acyl-CoA synthetase long-chain family member 3a, a lipid droplet-anchored protein that directs fatty acyl chains into lipids. Forced overexpression of acls3a in enterocytes delays, in part, the appearance of neutral lipids in the vasculature of zebrafish larvae. Activation of Lxr in the intestine cell-autonomously regulates the rate of delivery of absorbed lipids by inducting a temporary lipid intestinal droplet storage depot.

Interindividual variability of lutein bioavailability in healthy men: characterization, genetic variants involved, and relation with fasting plasma lutein concentration

BACKGROUND

Lutein accumulates in the macula and brain, where it is assumed to play physiologic roles. The bioavailability of lutein is assumed to display a high interindividual variability that has been hypothesized to be attributable, at least partly, to genetic polymorphisms.

OBJECTIVES

We characterized the interindividual variability in lutein bioavailability in humans, assessed the relation between this variability and the fasting blood lutein concentration, and identified single nucleotide polymorphisms (SNPs) involved in this phenomenon.

DESIGN

In a randomized, 2-way crossover study, 39 healthy men consumed a meal that contained a lutein supplement or the same meal for which lutein was provided through a tomato puree. The lutein concentration was measured in plasma chylomicrons isolated at regular time intervals over 8 h postprandially. Multivariate statistical analyses were used to identify a combination of SNPs associated with the postprandial chylomicron lutein response (0-8-h area under the curve). A total of 1785 SNPs in 51 candidate genes were selected.

RESULTS

Postprandial chylomicron lutein responses to meals were very variable (CV of 75% and 137% for the lutein-supplement meal and the meal with tomato-sourced lutein, respectively). Postprandial chylomicron lutein responses measured after the 2 meals were positively correlated (r = 0.68, P < 0.0001) and positively correlated to the fasting plasma lutein concentration (r = 0.51, P < 0.005 for the lutein-supplement-containing meal). A significant (P = 1.9 × 10(-4)) and validated partial least-squares regression model, which included 29 SNPs in 15 genes, explained most of the variance in the postprandial chylomicron lutein response.

CONCLUSIONS

The ability to respond to lutein appears to be, at least in part, genetically determined. The ability is explained, in large part, by a combination of SNPs in 15 genes related to both lutein and chylomicron metabolism. Finally, our results suggest that the ability to respond to lutein and blood lutein status are related. This trial was registered at clinicaltrials.gov as NCT02100774.

Chylomicrons produced by Caco-2 cells contained ApoB-48 with diameter of 80-200 nm

The small intestine generally transports dietary fats to circulation in triglyceride (TG)‐rich lipoproteins. The two main intestinal lipoproteins are chylomicron (CM) and very low‐density lipoprotein (VLDL). Unfortunately, studies on the CM biogenesis and intestinal transport of dietary fats have been hampered by the lack of an adequate in vitro model. In this study, we investigated the possible factors that might increase the efficiency of CM production by Caco‐2 cells. We utilized sequential NaCl gradient ultracentrifugation to isolate the CMs that were secreted by the Caco‐2 cells. To confirm the successful isolation of the CMs, we performed Fat Red 7B staining, TG reading, apolipoprotein B (ApoB) measurement, and transmission electron microcopy (TEM) analysis. We then tested the effects of cell differentiation, oleic acid, mono‐olein, egg lecithin, incubation time, and collagen matrix on CM secretion. We found that cell differentiation, oleic acid, and lecithin were critical for CM secretion. Using the Transwell system, we further confirmed that the CMs produced by our Caco‐2 cells contained significant amount of TGs and ApoB‐48 such that they could be detected without the use of isotope labeling. In conclusion, when fully differentiated Caco‐2 were challenged with oleic acid, lecithin, and sodium taurocholate, 21% of their total number of lipoproteins were CMs with the diameter of 80–200 nm.

When fully differentiated Caco‐2 cells were challenged with oleic acid, lecithin, and sodium taurocholate, they produced 21% chylomicrons that were 80–200 nm in diameter. These chylomicrons contained significant amount of ApoB‐48 and triglycerides such that they could be detected without the use of isotope labeling. Our studies identified factors that were important for chylomicron secretion.

Intestinal lipid absorption and lipoprotein formation

PURPOSE OF REVIEW

To summarize the evidence for the presence of two lipid absorption pathways and their regulation.

RECENT FINDINGS

Lipid absorption involves hydrolysis of dietary fat in the lumen of the intestine, followed by the uptake of hydrolyzed products by enterocytes. Lipids are resynthesized in the endoplasmic reticulum and are either secreted with chylomicrons and HDLs or stored as cytoplasmic lipid droplets. Lipids in the droplets are hydrolyzed and are secreted at a later time. Secretion of lipids by the chylomicron and HDL pathways are dependent on microsomal triglyceride transfer protein (MTP) and ATP-binding cassette family A protein 1, respectively, and are regulated independently. Gene-ablation studies showed that MTP function and chylomicron assembly is essential for the absorption of triglycerides. Ablation of MTP abolishes triglyceride absorption and results in massive triglyceride accumulation in enterocytes. Although the majority of phospholipid, cholesterol, and vitamin E are absorbed through the chylomicron pathway, a significant amount of these lipids are also absorbed via the HDL pathway. Chylomicron assembly and secretion is increased by the enhanced availability of fatty acids, whereas the HDL pathway is upregulated by liver X receptor agonists.

SUMMARY

Triglycerides are exclusively transported with chylomicrons and this process is critically dependent on MTP. In addition to chylomicrons, absorption of phospholipids, free cholesterol, retinol, and vitamin E also involves HDLs. These two pathways are complementary and are regulated independently. They may be targeted to lower lipid absorption in order to control hyperlipidemia, obesity, metabolic syndrome, steatosis, insulin resistance, atherosclerosis, and other disorders.

Unsaturated fatty acids promote bioaccessibility and basolateral secretion of carotenoids and α-tocopherol by Caco-2 cells

Bioavailability of carotenoids and tocopherols from foods is determined by the efficiency of transfer from food/meal to mixed micelles during digestion, incorporation into chylomicrons for trans-epithelial transport to lymphatic/blood system, and distribution to target tissues.

Demonstration of ATP-dependent, transcellular transport of lipid across the lymphatic endothelium using an in vitro model of the lacteal

PURPOSE

The lymphatic system plays crucial roles in tissue fluid balance, trafficking of immune cells, and the uptake of dietary lipid from the intestine. Given these roles there has been an interest in targeting lymphatics through oral lipid-based formulations or intradermal delivery of drug carrier systems. However the mechanisms regulating lipid uptake by lymphatics remain unknown. Thus we sought to modify a previously developed in vitro model to investigate the role of ATP in lipid uptake into the lymphatics.

METHODS

Lymphatic endothelial cells were cultured on a transwell membrane and the effective permeability to free fatty acid and Caco-2 cell-secreted lipid was calculated in the presence or absence of the ATP inhibitor sodium azide.

RESULTS

ATP inhibition reduced Caco-2 cell-secreted lipid transport, but not dextran transport. FFA transport was ATP-dependent primarily during early periods of ATP inhibition, while Caco-2 cell-secreted lipid transport was lowered at all time points studied. Furthermore, the transcellular component of transport was highly ATP-dependent, a mechanism not observed in fibroblasts, suggesting these mechanisms are unique to lymphatics. Total transport of Caco-2 cell-secreted lipid was dose-dependently reduced by ATP inhibition, and transcellular lipoprotein transport was completely attenuated.

CONCLUSION

The transport of lipid across the lymphatic endothelium as demonstrated with this in vitro model occurs in part by an ATP-dependent, transcellular route independent of passive permeability. It remains to be determined the extent that this mechanism exists in vivo and future work should be directed in this area.

Vitamin E secretion by Caco-2 monolayers to APOA1, but not to HDL, is vitamer selective

The aim of this study was to characterize the pathways of basolateral secretion of common dietary tocopherols from polarized Caco-2 monolayers, a model of intestinal absorption. Given differences in structure and physical properties, we hypothesized that secretion may differ between different forms of vitamin E, thus potentially contribute to the selectivity seen in vivo. Monolayers were incubated apically and simultaneously with 10 μmol/L α-, γ-, and δ-tocopherol (1:1:1) in lipid micelles. Treatment with the microsomal triglyceride transfer protein inhibitor BMS201038 revealed that the triglyceride-rich particle secretory pathway (apolipoprotein B-dependent pathway) accounted for ~ 80% of total tocopherol secretion, without selectivity among the three forms of vitamin E. Apolipoprotein B-independent secretion of tocopherols (and cholesterol) was greatly enhanced by the liver X receptor agonist T0901317. T0901317 induced ATP-binding cassette transporter A1 (ABCA1) protein expression and basolateral secretion of tocopherols to apolipoprotein A1. ABCA1-dependent secretion demonstrated vitamer selectivity such that efficiency of secretion of α- and γ-tocopherols exceeded that of δ-tocopherol. Basal addition of HDL stimulated vitamin E secretion but without selectivity among the three forms, whereas LDL had no effect. Basal addition of scavenger receptor class B member I (SR-BI) blocking antibody, which inhibits the interaction between SR-BI and HDL, increased basal accumulation of all tocopherols, demonstrating a role for SR-BI in cellular re-uptake of secreted vitamin E. These findings demonstrated that vitamin E and cholesterol utilize common pathways of secretion and that secretion via the ABCA1 pathway favors certain forms of vitamin E.

Impaired cholesterol metabolism and enhanced atherosclerosis in clock mutant mice

BACKGROUND

Clock is a key transcription factor that positively controls circadian regulation. However, its role in plasma cholesterol homeostasis and atherosclerosis has not been studied.

METHODS AND RESULTS

We show for the first time that dominant-negative Clock mutant protein (Clock(Δ19/Δ19)) enhances plasma cholesterol and atherosclerosis in 3 different mouse models. Detailed analyses revealed that Clk(Δ19/Δ19)Apoe(-/-) mice display hypercholesterolemia resulting from the accumulation of apolipoprotein B48-containing cholesteryl ester-rich lipoproteins. Physiological studies showed that enhanced cholesterol absorption by the intestine contributes to hypercholesterolemia. Molecular studies indicated that the expression of Niemann Pick C1 Like 1, Acyl-CoA:Cholesterol acyltransferase 1, and microsomal triglyceride transfer protein in the intestines of Clk(Δ19/Δ19)Apoe(-/-) mice was high and that enterocytes assembled and secreted more chylomicrons. Furthermore, we identified macrophage dysfunction as another potential cause of increased atherosclerosis in Clk(Δ19/Δ19)Apoe(-/-) mice. Macrophages from Clk(Δ19/Δ19)Apoe(-/-) mice expressed higher levels of scavenger receptors and took up more modified lipoproteins compared with Apoe(-/-) mice, but they expressed low levels of ATP binding casette protein family A member 1 and were defective in cholesterol efflux. Molecular studies revealed that Clock regulates ATP binding casette protein family A member 1 expression in macrophages by modulating upstream transcription factor 2 expression.

CONCLUSIONS

Clock(Δ19/Δ19) protein enhances atherosclerosis by increasing intestinal cholesterol absorption, augmenting uptake of modified lipoproteins by macrophages, and reducing cholesterol efflux from macrophages. These studies establish that circadian Clock activity is crucial in maintaining low plasma cholesterol levels and in reducing atherogenesis in mice.