Sensing of Dietary Lipids by Enterocytes: A New Role for SR-BI/CLA-1

Abstract

BackgroundThe 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 FindingsThe 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/SignificanceThese 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.

Micellar lipid composition affects micelle interaction with class B scavenger receptor extracellular loops

“…The interaction of micellar cholesterol with brush border membrane vesicles has been previously studied ( 20 ). The authors showed that although Niemann-Pick C1-like 1 has been described as the key mediator of intestinal cholesterol uptake, the active micelle binding was mediated by SR-BI, according to results obtained in Caco-2 cells ( 13 ). Here, the fact that both lSR-BI and lCD36 could effi ciently bind mixed micelles is consistent with these data.…”

“…recently been hypothesized that the main role of SR-BI was either to facilitate the uptake of other lipid molecules ( 1 ) or to mediate lipid sensing ( 13,14 ), even if a signifi cant involvement in lipid transport cannot be ruled out.…”

“…Numerous experiments on transfected cells have previously shown specifi c binding of lipoproteins, apolipoproteins, or phospholipids to SR-BI (15)(16)(17) or CD36 ( 18,19 ), and there have been few attempts to investigate micellar cholesterol binding with one or both SRs at the intestinal level ( 13,20,21 ). However, the molecular functions of these two multiligand proteins leading to selective dietary lipid transport remain largely unclear.…”

Serum-reduced and serum-free media for differentiation of Caco-2 cells

“…BSA-conjugated linoleic acid was employed as lipid supplement in a serum-free medium for Caco-2 differentiation, developed from a hybridoma serum-free medium (Halleux and Schneider, 1991). The BSA-cholesterol, palmitic and oleic acid mixture used as lipid supplement in this work has been used previously in studies aimed at clarifying the mechanisms of intestinal lipid absorption and metabolism in differentiated Caco-2 on filters (Beaslas et al, 2009) and was utilized by our group together with the ItS supplement to differentiate Caco-2 cell lines of different origins (Ranaldi et al, 2003). In the present study, when we directly compared the effects of lipids on ITS and MITO+™ supplemented media, lipid addition did not produce any significant improvement in the performance of the parameters under investigation.…”

“…Expression levels of PEPT1 than the standard FBS symmetric protocol. The asymmetric culture condition was originally used in previous studies (Morel et al, 2004;Beaslas et al, 2009) to mimic in vitro the physiological asymmetric intestinal environment existing in vivo, and a detailed protocol for its use has recently been published (Ferruzza et al, 2012b).…”