Abstract 16551: The Scavenger Receptor Class B Type I (SR-BI) Chaperones the Syndecan-1 Heparan Sulfate Proteoglycan to the Surface of Hepatocytes: Novel Role in the Clearance of Atherogenic Postprandial Remnant Lipoproteins
BACKGROUND: Syndecan-1 is a highly conserved receptor for postprandial remnant lipoproteins, directly mediating their endocytosis via a novel raft-dependent pathway (JCI 1997;100:1611; JBC 2013;288:13988). Syndecan-1 displays roughly a million binding sites per hepatocyte (JCI 2009;119:3236), yet SR-BI, which displays far fewer binding sites, has been reported to play a quantitatively significant role in remnant lipoprotein clearance.
HYPOTHESIS: SR-BI may not act primarily as a receptor for remnant lipoproteins, but instead plays a supportive role in syndecan-1-mediated binding and endocytosis.
METHODS: We used McArdle 7777 hepatoma cells, which express syndecan-1 and SR-BI. We also used McArdle cells expressing a chimera, FcR-Synd1, that consists of an IgG Fc receptor ectodomain linked to the transmembrane and cytoplasmic regions of syndecan-1. The chimera undergoes the same endocytic pathway as syndecan-1, but its ligand, non-immune IgG, does not bind SR-BI.
RESULTS: We found that SR-BI robustly co-immunoprecipitates with syndecan-1 and with FcR-Synd1. Addition of lipoproteins did not affect these associations. Neither SR-BI, syndecan-1, nor FcR-Synd1 co-IP’d with the LDL receptor, indicating specificity. Use of siRNA to knock-down SR-BI decreased the catabolism of model remnant lipoproteins, consistent with prior findings in SR-BI knock-out mice. Importantly, SR-BI knock-down in McArdle hepatocytes expressing FcR-Synd1 did not affect total cellular content of the chimera, but decreased its display on the cell surface (assessed by binding of 125I-labeled IgG at 4°C). We reported that the RMKKK-->5A mutant of FcR-Synd1 is expressed but fails to reach the cell surface, while the MKKK-->4A mutant traffics to the cell surface but fails to undergo efficient endocytosis. Here, we found that RMKKK-->5A does not co-IP with SR-BI, while MKKK-->4A does.
CONCLUSIONS: SR-BI associates with the transmembrane and cytoplasmic domains of syndecan-1, specifically requiring the Arg (R) that is the first cytoplasmic residue. SR-BI serves a novel role as a chaperone to bring syndecan-1 and FcR-Synd1 to the cell surface. These findings may clarify how SR-BI participates in the catabolism of remnant lipoproteins and other ligands for syndecan-1.
- © 2013 by American Heart Association, Inc.