Identification of a heparin-releasable lipoprotein lipase binding protein from endothelial cells.
Triglycerides in circulating plasma lipoproteins are hydrolyzed by lipoprotein lipase ( LPL) which is thought to bind to proteoglycans on the luminal endothelial cell surface. Previous studies from this laboratory using LPL-Sepharose affinity chromatography identified a 220-kDa LPL binding proteoglycan. Using ligand blotting with 125I- LPL, we now report a 116-kDa LPL binding protein in plasma membrane preparations of endothelial cells. 125I- LPL binding to this protein was abolished by addition of unlabeled LPL. When the cell surface of endothelial cells was labeled with biotin, a 116-kDa protein was biotinylated. Furthermore, the biotinylated 116-kDa protein bound to LPL-Sepharose and eluted with 0.4 M NaCl suggesting that the 116-kDa LPL binding protein is present on the cell surface. When detergent extracts of endothelial cells were applied to LPL-Sepharose in the presence of 0.15 M NaCl, the 116-kDa, but not the 220-kDa, protein still bound to LPL-Sepharose. The 116-kDa protein was not labeled with 35SO4 and eluted from DEAE-cellulose prior to proteoglycans, suggesting that it is not a proteoglycan. However, a 116-kDa endothelial cell surface protein was metabolically labeled with [35S]methionine. This protein was dissociated from the cell surface by incubating cells with heparin (50 units/ml)-containing buffer. After heparin treatment of endothelial cells, LPL binding to and internalization by the cells decreased greater than 70% compared to control cells. These results suggest that endothelial cells synthesize a heparin-releasable, high affinity 116-kDa LPL binding protein. We postulate that this protein is associated with proteoglycans on luminal endothelial surfaces and mediates LPL binding, internalization, and recycling. We name this protein hrp (heparin-releasable protein)-116.[1]References
- Identification of a heparin-releasable lipoprotein lipase binding protein from endothelial cells. Sivaram, P., Klein, M.G., Goldberg, I.J. J. Biol. Chem. (1992) [Pubmed]
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