The LDL Receptor-Related Protein 1 (LRP1) Regulates the PDGF Signaling Pathway by Binding the Protein Phosphatase SHP-2 and Modulating SHP-2- Mediated PDGF Signaling Events

BACKGROUND The PDGF signaling pathway plays a major role in several biological systems, including vascular remodeling that occurs following percutaneous transluminal coronary angioplasty. Recent studies have shown that the LDL receptor-related protein 1 (LRP1) is a physiological regulator of the PDGF signaling pathway. The underlying mechanistic details of how this regulation occurs have yet to be resolved. Activation of the PDGF receptor β (PDGFRβ) leads to tyrosine phosphorylation of the LRP1 cytoplasmic domain within endosomes and generates an LRP1 molecule with increased affinity for adaptor proteins such as SHP-2 that are involved in signaling pathways. SHP-2 is a protein tyrosine phosphatase that positively regulates the PDGFRβ pathway, and is required for PDGF-mediated chemotaxis. We investigated the possibility that LRP1 may regulate the PDGFRβ signaling pathway by binding SHP-2 and competing with the PDGFRβ for this molecule. METHODOLOGY/PRINCIPAL FINDINGS To quantify the interaction between SHP-2 and phosphorylated forms of the LRP1 intracellular domain, we utilized an ELISA with purified recombinant proteins. These studies revealed high affinity binding of SHP-2 to phosphorylated forms of both LRP1 intracellular domain and the PDGFRβ kinase domain. By employing the well characterized dynamin inhibitor, dynasore, we established that PDGF-induced SHP-2 phosphorylation primarily occurs within endosomal compartments, the same compartments in which LRP1 is tyrosine phosphorylated by activated PDGFRβ. Immunofluorescence studies revealed colocalization of LRP1 and phospho-SHP-2 following PDGF stimulation of fibroblasts. To define the contribution of LRP1 to SHP-2-mediated PDGF chemotaxis, we employed fibroblasts expressing LRP1 and deficient in LRP1 and a specific SHP-2 inhibitor, NSC-87877. Our results reveal that LRP1 modulates SHP-2-mediated PDGF-mediated chemotaxis. CONCLUSIONS/SIGNIFICANCE Our data demonstrate that phosphorylated forms of LRP1 and PDGFRβ compete for SHP-2 binding, and that expression of LRP1 attenuates SHP-2-mediated PDGF signaling events.