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Journal of Lipid Research, Vol. 45, 253-262, February 2004
Copyright © 2004 by American Society for Biochemistry and Molecular Biology

Scavenger receptor class B type I reduces cholesterol absorption in cultured enterocyte CaCo-2 cells¹

Lei Cai^2,*, Erik R. M. Eckhardt^2,*, Wei Shi^*, Zhenze Zhao^*, Munira Nasser, Willem J. S. de Villiers and Deneys R. van der Westhuyzen^3,*

^* Department of Internal Medicine, University of Kentucky Medical Center, Lexington, KY 40536
Divisions of Endocrinology and Molecular Medicine, and Gastroenterology, University of Kentucky Medical Center, Lexington, KY 40536
^a Department of Veterans Affairs Medical Center, Lexington, KY 40511

³ To whom correspondence should be addressed. e-mail: dvwest1{at}uky.edu

Scavenger receptor class B type I (SR-BI) mediates selective uptake of cholesteryl esters from HDL as well as efflux of cellular free cholesterol to HDL. It is unclear whether the receptor is involved in intestinal cholesterol absorption. We addressed this issue by studying [³H]cholesterol flux in differentiated CaCo-2 cells incubated at their apical side with mixed taurocholate/phosphatidylcholine/cholesterol micelles. Biotinylation and HDL binding experiments showed predominant apical expression of endogenous and overexpressed SR-BI. Mixed micellar cholesterol saturation affected the magnitude and direction of cholesterol flux with significant net uptake only from supersaturated micelles and net efflux from unsaturated micelles. Incubation with micelles that depleted cellular cholesterol resulted in a decrease of SR-BI protein, whereas incubation with cholesterol-loading micelles resulted in a significant increase of SR-BI protein. Apical cholesterol uptake by CaCo-2 cells was increased in the presence of a SR-BI-blocking antibody and by partial inhibition of SR-BI expression with small inhibitory RNA. Adenovirus-mediated overexpression of apical SR-BI did not affect cholesterol uptake but stimulated apical cholesterol efflux, even to supersaturated mixed micelles. Partial inhibition of SR-BI with small inhibitory RNA reduced apical cholesterol efflux.

Our data argue against a direct role for SR-BI in micellar cholesterol uptake. However, SR-BI might be involved in cholesterol absorption by facilitating cholesterol efflux to micelles.

Abbreviations: apoA-I, apolipoprotein A-I; eypc, egg yolk phosphatidylcholine; MBSTOG, MES-buffered saline with 1% Triton X-100 and 1.75% n-octyl-ß-D-glucopyranoside; SR-BI, scavenger receptor class B type I; tc, taurocholate

Supplementary key words micelles • cholesterol efflux • phospholipids • bile salts

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