Plasma high density lipoprotein (HDL) is the principal carrier of plasma cholesterol for biliary cholesterol secretion. The HDL receptor Scavenger Receptor B-l (SR-BI) mediates the uptake of HDL cholesterol and cholesteryl ester for excretion into bile, and its expression correlates with biliary cholesterol secretion. SR-BI knockout mice have a significant decrease in biliary cholesterol secretion, indicating an important role of SR-BI in that process. However, little is known regarding the molecular mechanisms by which SR-BI mediates hepatobiliary secretion of cholesterol. SR-BI is expressed on both sinusoidal and canalicular membranes in liver, and undergoes transcytosisto the canalicular membrane. The sorting signals necessary for SR-BI basolateral targeting and transcytosis to the canalicular membrane are unknown. We have defined three important factors that may regulate SR-BI subcellular sorting in liver and thus SR-BI- dependent biliary cholesterol secretion. One factor is PDZK1, a PDZ domain protein that has been shown to interact with SR-BI and to be essential to maintain hepatic SR-BI levels. A second factor is a basolateral targeting signal in the C-terminus of SR-BI, and a third factor is a cholesterol binding domain in the C- terminal transmembrane domain of SR-BI. The C-terminal transmembrane domain of SR-BI can directly bind cholesterol but is not important for SR-BI-mediated selective uptake or cholesterol efflux. Cholesterol binding by SR-BI in liver may play a role in SR-BI transcytosis.
Specific Aim 1 will determine the role of PDZK1 in SR-BI sorting. We have generated a PDZK1 knockout mouse model and will use both PDZK1 deficient mice and in vitro polarized cells models to examine the role of PDZK1 in SR-BI sorting.
Specific Aim 2 will delimit the basolateral targeting signal of SR-BI and test the role of this sequence in SR-BI sorting in vivo and in vitro as well as in biliary cholesterol secretion in vivo.
Specific Aim 3 will test the hypothesis that the SR-BI cholesterol binding domain plays a role in SR-BI transcytosis. We will test the role of this domain in biliary cholesterol secretion in vivo and SR-BI transcytosis in vitro. These studies will provide fundamental insights into the mechanisms regulating SR-BI sorting and biliary cholesterol secretion.
|Berger, Justin H; Charron, Maureen J; Silver, David L (2012) Major facilitator superfamily domain-containing protein 2a (MFSD2A) has roles in body growth, motor function, and lipid metabolism. PLoS One 7:e50629|
|Gross, David A; Zhan, Chenyang; Silver, David L (2011) Direct binding of triglyceride to fat storage-inducing transmembrane proteins 1 and 2 is important for lipid droplet formation. Proc Natl Acad Sci U S A 108:19581-6|
|Miranda, Diego A; Koves, Timothy R; Gross, David A et al. (2011) Re-patterning of skeletal muscle energy metabolism by fat storage-inducing transmembrane protein 2. J Biol Chem 286:42188-99|
|Zhu, Weifei; Saddar, Sonika; Seetharam, Divya et al. (2008) The scavenger receptor class B type I adaptor protein PDZK1 maintains endothelial monolayer integrity. Circ Res 102:480-7|