Accelerated Lipid Absorption in Mice Overexpressing Intestinal SR-BI

Florence Bietrix,Daoguang Yan,Michel Nauze,Corinne Rolland,Justine Bertrand-Michel,Christine Coméra,Stephane Schaak,Ronald Barbaras,Albert K Groen,Bertrand Perret,François Tercé,Xavier Collet

doi:10.1074/jbc.m508868200

Abstract

Dietary cholesterol absorption contributes to a large part of the circulating cholesterol. However, the mechanism of sterol intestinal uptake is not clearly elucidated. Scavenger receptor class B type I (SR-BI), major component in the control of cholesterol homeostasis, is expressed in the intestine, but its role in this organ remains unclear. We have generated transgenic mice overexpressing SR-BI primarily in the intestine by using the mouse SR-BI gene under the control of intestinal specific "apoC-III enhancer coupled with apoA-IV promoter." We found SR-BI overexpression with respect to the natural protein along the intestine and at the top of the villosities. After a meal containing [(14)C]cholesterol and [(3)H]triolein, SR-BI transgenic mice presented a rise in intestinal absorption of both lipids that was not due to a defect in chylomicron clearance nor to a change in the bile flow or the bile acid content. Nevertheless, SR-BI transgenic mice showed a decrease of total cholesterol but an increase of triglyceride content in plasma without any change in the high density lipoprotein apoA-I level. Thus, we described for the first time a functional role in vivo for SR-BI in cholesterol but also in triglyceride intestinal absorption.

Highlights

The mechanisms by which enterocyte lipid absorption occurs are still controversial
Characterization of the Transgenic Mice Overexpressing Scavenger receptor class B type I (SR-BI) in the Intestine—We have generated transgenic mice overexpressing SR-BI under the control of the apolipoprotein C-III enhancer coupled to the apolipoprotein A-IV promoter, and we analyzed the presence of SR-BI in tissue extracts (Fig. 1)
SR-BI protein was classically expressed mostly in the liver among other tissues, whereas transgenic mice displayed a dramatic overexpression of a mature (82 kDa) SR-BI almost totally restricted to the intestine (Fig. 1, A and B)

Summary

EXPERIMENTAL PROCEDURES

Generation of Transgenic Mice—The murine SR-BI cDNA was obtained by digestion of pRC/CMV vector by restriction enzymes HindIII and XbaI. Plasma Lipid Analysis—Blood samples were collected from mice, and plasma HDL were isolated by LDL/VLDL precipitation with phosphotungstate/MgCl2 reagent (HDL-cholesterol; Sigma). Membrane Preparation and Protein Analysis—Animals were sacrificed, and different tissues were collected and immediately frozen. Proteins from the different tissue extracts were separated on 10% SDS-polyacrylamide gel (Bio-Rad) and transferred to nitrocellulose membrane. Intestinal Absorption of Lipids—To determine dietary lipid absorption, mice were fasted overnight and fed by gavage with a mixture of 2 ␮Ci of [4-14C]cholesterol/1 ␮Ci of [9,10-3H]triolein (PerkinElmer Life Sciences) and 2 g/liter cholesterol (Sigma) suspended in 100 ␮l of corn oil. The different samples were homogenized in methanol/water (2:1, v/v), and tissue distribution of radioactivity was determined by scintillation counting. MM), methanol, acetonitrile, 35:60:5 v/v/v for 25 min move to 24:53:23 in 5 min, plateau for 20 min

RESULTS

DISCUSSION

Cholesteryl ester

Liver iWAT eWAT