SR-BI Undergoes Cholesterol-stimulated Transcytosis to the Bile Canaliculus in Polarized WIF-B Cells

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 cholesterolreplete 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. High density lipoproteins (HDL)2 have a functional role in the protection against cardiovascular disease. HDL mediates both cholesterol removal from lipid-laden macrophages and delivery to the liver for subsequent biliary secretion in a process termed "macrophage reverse cholesterol transport." The cholesterol removal is mediated, in part, by the well established HDL receptor, scavenger receptor class BI (SR-BI) (1) (reviewed in Ref. 2).SR-BI is a two-transmembrane domain cell surface glycoprotein with short intracellular N-and C-terminal domains (3). The receptor is ubiquitously expressed at low levels and is highly expressed in steroidogenic, intestinal, and hepatic tissues (1). SR-BI binds a large array of ligands, including HDL and native or modified low density lipoproteins (LDL).Hepatic SR-BI protects against atherosclerosis by promoting the final stages of macrophage reverse cholesterol transport (5). In contrast to the holo-particle uptake of the LDL receptor pathway, SR-BI mediates cholesterol, cholesteryl ester, and phospholipid uptake via a selective pathway whereby lipids are transferred down their concentration gradient through a hydrophobic channel into the membrane (6). Importantly, if the concentration gradient is reversed, SR-BI can also perform cholesterol efflux (7).Although lipid delivery to and from lipoprotein particles occurs mainly on the plasma membrane, both SR-BI and HDL have been shown to internalize into and recycle from endocytic compartments (8 -10). SR-BI also localizes on the apical domain in gall bladder epithelial cells (11), testicular Sertoli cells (12), isolated primary mouse hepatocytes (13), primary mouse hepatocyte couplets (8), and hepatic tissues sections (14) and has been shown to undergo regulated transcytotic movement in polarized Madin-Darby canine kidney cells (15).Our laboratory has recently demonstra...

Scavenger Receptor Class B Type I Mediates Biliary Cholesterol Secretion Independent of ATP-Binding Cassette Transporter g5/g8 in Mice†

“…34 In addition, a transcytotic route for SR-BI has been characterized, which mediates the movement of the receptor from basolateral to apical membranes in a microtubule-dependent fashion. 35,36 Although transcytosis of SR-BI was not formally investigated in our study, our data indicate that transcytotic pathways do not contribute to the SR-BI-mediated increase in biliary cholesterol secretion, because cholesterol output into bile was not reduced in response to colchicine, which blocks microtubule function.…”

Scavenger receptor class B type I localizes to a late endosomal compartment

“…It is also possible that in hepatocytes, lysosomal SR-BI is involved in the transfer of cholesterol from those organelles to the bile canaliculi (BC). Indeed, experiments in a hepatocyte cell line, WIF-B, showed that SR-BI and cholesterol follow the trafficking pathway (28). In addition, immunolabelling of lysosomal SNAREs, syntaxin 7 in the same cell type, revealed lysosomal structures in close vicinity to the BC (29), suggesting that cholesterol may traffic from lysosomes to BC and that this trafficking could be dependent on SR-BI.…”

Intestinal SR-BI does not impact cholesterol absorption or transintestinal cholesterol efflux in mice

“…However, the majority of cell-based work examining the subcellular traffi cking with this directional traffi cking pattern in polarized cells, hepatic overexpression or knockout of SR-BI either promotes or diminishes the movement of HDL-derived cholesterol into bile, respectively ( 37,(42)(43)(44)49 ). In fact all pattern of SR-BI has agreed that in polarized cells SR-BI sorts HDL-derived cholesterol from basolateral membranes to apical membranes (32)(33)(34)(35), and the receptor itself has shown a similar traffi cking pattern (32)(33)(34)(35). In agreement ( Figs.…”

“…The rationale for studying SR-BI stems from the well-known ability of SR-BI to traffi c cholesterol across hepatocytes in a unidirectional basolateral to apical manner (32)(33)(34)(35). We hypothesized that intestinal SR-BI may likewise traffi c plasma cholesterol across the enterocyte in a basolateral to apical manner thereby contributing to TICE.…”

“…SR-BI is expressed in a wide variety of tissues, with the highest levels of expression in those regulating cholesterol metabolism and steroid hormone production such as the liver and adrenal gland (29)(30)(31). It has been a matter of debate, but SR-BI seems to localize to both apical and basolateral membranes in polarized cells (32)(33)(34)(35)(36)(37), and likely undergoes sterol-dependent basolateral to apical transcytosis (32)(33)(34)(35)(36)(37). Importantly, SR-BI has been classifi ed as a major HDL receptor, facilitating a unique sterol transport process called selective uptake (38)(39)(40).…”