Sensing of dietary lipids by enterocytes: a new role for SR-BI/CLA-1.

BACKGROUND: The intestine is responsible for absorbing dietary lipids and delivering them to the organism as triglyceride-rich lipoproteins (TRL). It is important to determine how this process is regulated in enterocytes, the absorptive cells of the intestine, as prolonged postprandial hypertriglyceridemia is a known risk factor for atherosclerosis. During the postprandial period, dietary lipids, mostly triglycerides (TG) hydrolyzed by pancreatic enzymes, are combined with bile products and reach the apical membrane of enterocytes as postprandial micelles (PPM). Our aim was to determine whether these micelles induce, in enterocytes, specific early cell signaling events that could control the processes leading to TRL secretion. METHODOLOGY/PRINCIPAL FINDINGS: The effects of supplying PPM to the apex of Caco-2/TC7 enterocytes were analyzed. Micelles devoid of TG hydrolysis products, like those present in the intestinal lumen in the interprandial period, were used as controls. The apical delivery of PPM specifically induced a number of cellular events that are not induced by interprandial micelles. These early events included the trafficking of apolipoprotein B, a structural component of TRL, from apical towards secretory domains, and the rapid, dose-dependent activation of ERK and p38MAPK. PPM supply induced the scavenger receptor SR-BI/CLA-1 to cluster at the apical brush border membrane and to move from non-raft to raft domains. Competition, inhibition or knockdown of SR-BI/CLA-1 impaired the PPM-dependent apoB trafficking and ERK activation. CONCLUSIONS/SIGNIFICANCE: These results are the first evidence that enterocytes specifically sense postprandial dietary lipid-containing micelles. SR-BI/CLA-1 is involved in this process and could be a target for further study with a view to modifying intestinal TRL secretion early in the control pathway.

Transcriptome response of enterocytes to dietary lipids: impact on cell architecture, signaling, and metabolism genes.

Intestine contributes to lipid homeostasis through the absorption of dietary lipids, which reach the apical pole of enterocytes as micelles. The present study aimed to identify the specific impact of these dietary lipid-containing micelles on gene expression in enterocytes. We analyzed, by microarray, the modulation of gene expression in Caco-2/TC7 cells in response to different lipid supply conditions that reproduced either the permanent presence of albumin-bound lipids at the basal pole of enterocytes or the physiological delivery, at the apical pole, of lipid micelles, which differ in their composition during the interprandial (IPM) or the postprandial (PPM) state. These different conditions led to distinct gene expression profiles. We observed that, contrary to lipids supplied at the basal pole, apical lipid micelles modulated a large number of genes. Moreover, compared with the apical supply of IPM, PPM specifically impacted 46 genes from three major cell function categories: signal transduction, lipid metabolism, and cell adhesion/architecture. Results from this first large-scale analysis underline the importance of the mode and polarity of lipid delivery on enterocyte gene expression. They demonstrate specific and coordinated transcriptional effects of dietary lipid-containing micelles that could impact the structure and polarization of enterocytes and their functions in nutrient transfer.

Preferential association of Hepatitis C virus with apolipoprotein B48-containing lipoproteins.

Hepatitis C virus (HCV) in cell culture has a density comparable to that of other members of the family Flaviviridae, whereas in vivo infectious particles are found partially in low-density fractions, associated with triacylglycerol (TG)-rich lipoproteins (TRLs). In the blood of infected patients, HCV circulates as heterogeneous particles, among which are lipo-viroparticles (LVPs), globular particles rich in TG and containing viral capsid and RNA. The dual viral and lipoprotein nature of LVPs was addressed further with respect to apolipoprotein composition and post-prandial dynamic lipid changes. The TRLs exchangeable apoE, -CII and -CIII, but not the high-density lipoprotein apoA-II, were present on LVPs, as well as the viral envelope proteins. apoB100 and -B48, the two isoforms of the non-exchangeable apoB, were represented equally on LVPs, despite the fact that apoB48 was barely detectable in the plasma of these fasting patients. This indicates that a significant fraction of plasma HCV was associated with apoB48-containing LVPs. Furthermore, LVPs were enriched dramatically and rapidly in triglycerides after a fat meal. As apoB48 is synthesized exclusively by the intestine, these data highlight the preferential association of HCV with chylomicrons, the intestine-derived TRLs. These data raise the question of the contribution of the intestine to the viral load and suggest that the virus could take advantage of TRL assembly and secretion for its own production and of TRL fate to be delivered to the liver.

Lipid micelles stimulate the secretion of triglyceride-enriched apolipoprotein B48-containing lipoproteins by Caco-2 cells.

Intestinal triglyceride-rich lipoproteins (TRL) are synthesized from dietary lipids. This study was designed to evaluate the effects of lipid micelles, mimicking post-digestive duodenal micelles, on the fate of apolipoprotein B (apoB)48-containing lipoproteins by Caco-2 cells. Such micelles, consisting of oleic acid (OA), taurocholate, 2-monooleoylglycerol (2-MO), cholesterol (Chol), and L-alpha-lysophospatidylcholine, were the most efficient inducers of OA uptake and esterification. The efficiency of TG and apoB48 secretion increased specifically as a function of cell differentiation. PAGE analysis of secreted lipoproteins separated by sequential ultracentrifugation after [35S] labeling revealed differences in the secretion of apoB100- and apoB48-containing lipoproteins. In absence of micelles, apoB48 was secreted mostly in "HDL-like" particles, as observed in enterocytes in vivo. Micelle application increased 2.7-fold the secretion of apoB, resulting in 53 times more apoB48 being recovered as TG-enriched lipoproteins at d < 1.006 g/ml. Electron microscopy revealed the presence of lipid droplets in the secretory pathway and the accumulation of newly synthesized TG in cytoplasmic lipid droplets, as in enterocytes in vivo. We showed that these droplets could be used for secretion. However, apoB48 preferentially bound to newly synthesized TG in the presence of micelles, accounting in part for the functional advantage of apoB editing in the intestine. While Caco-2 cells express both apoB isoforms, our results show that the apical supply of complex lipid micelles favors the physiological route of apoB48-containing TG-enriched lipoproteins.

HNF-4-dependent induction of apolipoprotein A-IV gene transcription by an apical supply of lipid micelles in intestinal cells.

Apolipoprotein (apo) A-IV, a component of triglyceride-rich lipoproteins secreted by the small intestine, has been shown to play an important role in the control of lipid homeostasis. Numerous studies have described the induction of apoA-IV gene expression by lipids, but the molecular mechanisms involved in this process remain unknown. In this study, we have demonstrated that a lipid bolus induced transcription of the apoA-IV gene in transgenic mice and that the regulatory region of the apoA-IV gene, composed of the apoC-III enhancer and the apoA-IV promoter (eC3-A4), was responsible for this induction. In enterocyte Caco-2/TC7 cells, a permanent supply of lipids at the basal pole induced expression of the apoA-IV gene both at the transcriptional level and through mRNA stabilization. ApoA-IV gene transcription and protein secretion were further induced by an apical supply of complex lipid micelles mimicking the composition of duodenal micelles, and this effect was not reproduced by apical delivery of different combinations of micelle components. Only induction of the apoA-IV gene by lipid micelles involved the participation of hepatic nuclear factor (HNF)-4, as demonstrated using a dominant negative form of this transcription factor. Accordingly, lipid micelles increased the DNA binding activity of HNF-4 on the eC3-A4 region. These results emphasize the importance of physiological delivery of dietary lipids on apoA-IV gene expression and the implication of HNF-4 in this regulation.

Lipid-dependent bidirectional traffic of apolipoprotein B in polarized enterocytes.

Enterocytes are highly polarized cells that transfer nutrients across the intestinal epithelium from the apical to the basolateral pole. Apolipoprotein B (apoB) is a secretory protein that plays a key role in the transepithelial transport of dietary fatty acids as triacylglycerol. The evaluation of the control of apoB traffic by lipids is therefore of particular interest. To get a dynamic insight into this process, we used the enterocytic Caco-2 cells cultured on microporous filters, a system in which the apical and basal compartments can be delimited. Combining biochemical and morphological approaches, our results showed that, besides their role in protection from degradation, lipids control the intracellular traffic of apoB in enterocytes. A supply of fatty acids and cholesterol is sufficient for the export of apoB from the endoplasmic reticulum and its post-Golgi traffic up to the apical brush-border domain, where it remains until an apical supply of complex lipid micelles signals its chase down to the basolateral secretory domain. This downward traffic of apoB involves a microtubule-dependent process. Our results demonstrate an enterocyte-specific bidirectional process for the lipid-dependent traffic of a secretory protein.