Oxysterols and Their Cellular Effectors

Oxysterols are oxidized 27-carbon cholesterol derivatives or by-products of cholesterol biosynthesis, with a spectrum of biologic activities. Several oxysterols have cytotoxic and pro-apoptotic activities, the ability to interfere with the lateral domain organization, and packing of membrane lipids. These properties may account for their suggested roles in the pathology of diseases such as atherosclerosis, age-onset macular degeneration and Alzheimer’s disease. Oxysterols also have the capacity to induce inflammatory responses and play roles in cell differentiation processes. The functions of oxysterols as intermediates in the synthesis of bile acids and steroid hormones, and as readily transportable forms of sterol, are well established. Furthermore, their actions as endogenous regulators of gene expression in lipid metabolism via liver X receptors and the Insig (insulin-induced gene) proteins have been investigated in detail. The cytoplasmic oxysterol-binding protein (OSBP) homologues form a group of oxysterol/cholesterol sensors that has recently attracted a lot of attention. However, their mode of action is, as yet, poorly understood. Retinoic acid receptor-related orphan receptors (ROR) α and γ, and Epstein-Barr virus induced gene 2 (EBI2) have been identified as novel oxysterol receptors, revealing new physiologic oxysterol effector mechanisms in development, metabolism, and immunity, and evoking enhanced interest in these compounds in the field of biomedicine.Peer reviewe

OSBP-Related Protein 8 (ORP8) Regulates Plasma and Liver Tissue Lipid Levels and Interacts with the Nucleoporin Nup62

Peer reviewe

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

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 triglyceriderich 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 lipidcontainin

Signaling pathways specifically induced by postprandial micelles.

<p>(A) Kinetworks™ phosphoprotein immunoblots from lysates of Caco-2/TC7 cells cultured on filters without micelles (control) or with PPM or IPM for 5 min. Duplicate immunoblots using specific antibodies directed against phosphoproteins were analyzed by Kinexus. Bands corresponding to phosphoproteins specifically up-regulated by PPM are indicated by arrows. The antibodies recognized the phosphosites: S144/S141/S154 for PAK1/2/3, T180/Y182 for p38αMAPK, S129/S133 for CREB1, T202/Y204 for ERK1, T185/Y187 for ERK2 and S338 for PKA Cβ. The phosphosites analyzed for MEK1 are indicated in the figure. Histograms show the amounts of phosphoproteins expressed as the ratio of phosphorylation in the PPM versus the IPM sample (from three independent experiments). (B) Caco-2/TC7 cells cultured on semi-permeable filters were incubated in the absence or presence of PPM or IPM in the apical compartment for the indicated times. Cell lysates were analyzed by immunoblot with antibodies against phospho-ERK1/2 (P-ERK), and phospho-p38MAPK (P-p38 MAPK). Total ERK, p38MAPK and E-cadherin (E-cadh) were used as loading controls. (C) Quantification of normalized P-ERK and P-p38MAPK levels (from three independent experiments) in the absence (c) or presence of PPM or IPM for 10 min (for P-ERK and ERK) or for 5 min (for P-p38MAPK and p38MAPK), **p<0.01. (D) Caco-2/TC7 cells were cultured in the presence of various amounts of PPM supplied in the apical compartment for 10 min. Cell lysates were analyzed by immunoblot with the same antibodies as in (B) and a blot representative of three independent analyses is shown.</p

PPM supply induces movement of SR-BI/CLA-1 towards raft microdomains.

<p>(A) Caco-2/TC7 cells were harvested in the presence of Triton X-100 and the lysate fractionated on a 5–40% sucrose gradient. Eleven fractions were collected for immunoblots of SR-BI/CLA-1, EEA1 (early endosome antigen 1) and flottilin-1 (raft marker). (B) Caco-2/TC7 cells were cultured in the absence (control) or presence of PPM or IPM for 10 min and then harvested in the presence of Triton X-100. Cell lysates were applied to a 5–40% sucrose gradient and eleven fractions collected. Fractions 3 to 8 were analyzed by immunoblotting with antibodies against SR-BI/CLA-1 (left panel) and flottilin-1 (right panel). (C) Immunolocalization of SR-BI/CLA-1 and alkaline phosphatase (PLAP, used as raft marker) in the brush border of Caco-2/TC7 cells supplied with PPM. SR-BI/CLA-1 is labelled with anti-rabbit immunoglobulin-gold complexes (18-nm particles) and PLAP with anti-sheep immunoglobulin-gold complexes (12-nm particles). MV, microvilli; bar, 100 nm.</p

Subcellular localization of SR-BI/CLA-1 after the supply of postprandial micelles.

<p>(A) Immunoelectron micrograph of SR-BI/CLA-1 in untreated differentiated Caco-2/TC7 cells. MV, microvilli; TW, terminal web (bar, 0.5 µm). Note the significant amount of intracellular trafficking SR-BI/CLA-1 in addition to its main apical localization (arrowheads). (B) Immunolocalization of SR-BI/CLA-1 (green channel) and sucrase isomaltase (SI, red channel) in differentiated Caco-2/TC7 cells before (T0) and after 5, 10 and 15 min of apical PPM supply. Panels represent XY acquisitions at the apical level (bar, 10 µm). Arrowheads show clusters of SR-BI/CLA-1. (C) Immunolocalization of SR-BI/CLA-1 in differentiated Caco-2/TC7 cells in the absence (control) or presence of PPM or IPM for 20 min (bar, 20 µm). Arrowheads show clusters of SR-BI/CLA-1 (D) Immunoelectron micrograph of SR-BI/CLA-1 in Caco-2/TC7 cells supplied with PPM (MV, microvilli). Arrowheads indicate SR-BI/CLA-1 clusters (bar, 100 nm). (E) Cell surface biotinylation assay for apical SR-BI/CLA-1. Caco-2/TC7 cells were cultured in the absence (0) or presence of PPM for the indicated times. Cells were then selectively labeled with non-permeant biotin at the apical (left panel) or basal surface (right panel). Biotinylated fractions were obtained as described in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0004278#s2" target="_blank">Material and Methods</a>. Total cell lysates (total), apical and basal biotinylated fractions (left and right panel respectively) and non-apical fractions (non-apical) were analyzed in immunoblots of SR-BI/CLA-1, E-cadherin being used as a basolateral membrane marker.</p