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Abstract
The low density lipoprotein receptor-related protein (LRP) functions in the catabolism of numerous ligands including proteinases, proteinase inhibitor complexes, and lipoproteins. In the current study we provide evidence indicating an expanded role for LRP in modulating cellular signaling events. Our results show that platelet-derived growth factor (PDGF) BB induces a transient tyrosine phosphorylation of the LRP cytoplasmic domain in a process dependent on PDGF receptor activation and c-Src family kinase activity. Other growth factors, including basic fibroblast growth factor, epidermal growth factor, insulin-like growth factor-1, were unable to mediate tyrosine phosphorylation of LRP. The basis for this selectivity may result from the ability of LRP to bind PDGFBB, because surface plasmon resonance experiments demonstrated that only PDGF, and not basic fibroblast growth factor, epidermal growth factor, or insulin-like growth factor-1, bound to purified LRP immobilized on a sensor chip. The use of LRP mini-receptor mutants as well as in vitro phosphorylation studies demonstrated that the tyrosine located within the second NPXY motif found in the LRP cytoplasmic domain is the primary site of tyrosine phosphorylation by Src and Src family kinases. Co-immunoprecipitation experiments revealed that PDGF-mediated tyrosine phosphorylation of LRPs cytoplasmic domain results in increased association of the adaptor protein Shc with LRP and that Shc recognizes the second NPXY motif within LRPs cytoplasmic domain. In the accompanying paper, Boucher et al. (Boucher, P., Liu, P. V., Gotthardt, M., Hiesberger, T., Anderson, R. G. W., and Herz, J. (2002) J. Biol. Chem. 275, 15507-15513) reveal that LRP is found in caveolae along with the PDGF receptor. Together, these studies suggest that LRP functions as a co-receptor that modulates signal transduction pathways initiated by the PDGF receptor.
Highlights
The low density lipoprotein receptor-related protein (LRP) functions in the catabolism of numerous ligands including proteinases, proteinase inhibitor complexes, and lipoproteins
Our results show that platelet-derived growth factor (PDGF) BB induces a transient tyrosine phosphorylation of the LRP cytoplasmic domain in a process dependent on PDGF receptor activation and c-Src family kinase activity
Other growth factors, including basic fibroblast growth factor, epidermal growth factor, insulin-like growth factor-1, were unable to mediate tyrosine phosphorylation of LRP. The basis for this selectivity may result from the ability of LRP to bind PDGFBB, because surface plasmon resonance experiments demonstrated that only PDGF, and not basic fibroblast growth factor, epidermal growth factor, or insulin-like growth factor-1, bound to purified LRP immobilized on a sensor chip
Summary
Antibodies, and Expression Constructs—A rabbit polyclonal IgG prepared against purified human LRP (R2629) was affinity-purified over LRP-Sepharose as described [21]. Surface Plasmon Resonance—Binding of PDGFBB, bFGF, EGF, IGF-1 to purified LRP was measured using a BIA 3000 optical biosensor (Biacore AB, Uppsala, Sweden). For these studies, the BIAcore sensor chip (type CM5; Biacore AB) was activated with a 1:1 mixture of 0.2 M N-ethyl-NЈ-(3-dimethylaminopropyl)carbodiimide and 0.05 M N-hydroxysuccinimide in water as described by the manufacturer. The media was replaced with DMEM containing either 0.1% fetal bovine serum or 1% Nutridoma® NS, and the cells were incubated with this media for an additional 18 h. Cell layers were washed 3 times with serum-free medium and incubated in 1% Nutridoma containing DMEM overnight. Whole cell extracts (2%) were analyzed by immunoblotting for LRP- expression using anti-Myc IgG and for c-Src expression using anti-Src IgG
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