SR-BI Undergoes Cholesterol-stimulated Transcytosis to the Bile Canaliculus in Polarized WIF-B Cells*
- Lipoprotein and Atherosclerosis Group, University of Ottawa Heart Institute, Ottawa, Ontario K1Y 4W7, Canada
- 1 To whom correspondence should be addressed: Lipoprotein and Atherosclerosis Group, University of Ottawa Heart Institute, Ottawa, Ontario K1Y 4W7, Canada. Tel.: 613-761-5256; Fax: 613-761-5281; E-mail: rmcpherson{at}ottawaheart.ca.
Abstract
The scavenger receptor BI (SR-BI) is highly expressed in hepatocytes, where it mediates the uptake of lipoprotein cholesterol, promotes the secretion of cholesterol into bile, and protects against atherosclerosis. Despite a strong correlation between the hepatic expression of SR-BI and biliary cholesterol secretion, little is known about SR-BI trafficking in response to changes in sterol availability. Using a well characterized polarized hepatocyte cell model, WIF-B, we determine that in cholesterol-depleted cells, SR-BI is extensively located on the basolateral surface, where it can access circulating lipoproteins. However, in response to cholesterol loading, SR-BI undergoes a slow transcytosis to the apical bile canaliculus independently of lipoprotein binding and new protein synthesis. In cholesterol-replete WIF-B cells, SR-BI that resides on the canalicular membrane is dynamically associated with defined microdomains and does not rapidly recycle to and from the subapical or basolateral regions. Taken together, these data demonstrate that hepatic SR-BI transcytosis is regulated by cholesterol and suggest that SR-BI has a stationary function on the bile canaliculus.
Footnotes
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↵2 The abbreviations used are: HDL, high density lipoprotein(s); Ad, adenovirus; ABC, ATP binding cassette; BC, bile canaliculus or canaliculi; BL, basolateral membrane; EM, immunoelectron microscopy; FRAP, fluorescence recovery after photobleaching; LDL, low density lipoprotein(s); MβCD, methyl β-cyclodextrin; SR-BI, scavenger receptor BI; YFP, yellow fluorescent protein; PBS, phosphate-buffered saline; BSA, bovine serum albumin; FBS, fetal bovine serum.
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↵* This work was supported by Canadian Institutes of Health Research Grants 44360 (to R. M.) and 43935 (to H. M. M.) and by an Ontario Graduate Scholarship in Science and Technology (to C. H.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. 1-3 and Movies 1-4.
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- Received May 15, 2006.
- Revision received October 20, 2006.
- The American Society for Biochemistry and Molecular Biology, Inc.
