Platelet-derived growth factor receptor-β (PDGFR-β) activation promotes its association with the low density lipoprotein receptor-related protein (LRP): Evidence for co-receptor function

Abstract

Activation of the platelet-derived growth factor receptor-β (PDGFR-β) leads to tyrosine phosphorylation of the cytoplasmic domain of LRP and alters its association with adaptor and signaling proteins, such as Shc. The mechanism of the PDGF-induced LRP tyrosine phosphorylation is not well understood, especially since PDGF not only activates PDGF receptor but also binds directly to LRP. To gain insight into this mechanism, we used a chimeric receptor in which the ligand binding domain of the PDGFR-β was replaced with that from the macrophage colony-stimulating factor (M-CSF) receptor, a highly related receptor tyrosine kinase of the same subfamily, but with different ligand specificity. Activation of the chimeric receptor upon the addition of M-CSF readily mediated the tyrosine phosphorylation of LRP. Since M-CSF is not recognized by LRP, these results indicated that growth factor binding to LRP is not necessary for this phosphorylation event. Using a panel of cytoplasmic domain mutants of the chimeric M-CSF/PDGFR-β, we confirmed that the kinase domain of PDGFR-β is absolutely required for LRP tyrosine phosphorylation but that PDGFR-β-mediated activation of phosphatidylinositol 3-kinase, RasGAP, SHP-2, phospholipase C-γ, and Src are not necessary for LRP tyrosine phosphorylation. To identify the cellular compartment where LRP and the PDGFR-β may interact, we employed immunofluorescence and immunogold electron microscopy. In WI-38 fibroblasts, these two receptors co-localized in coated pits and endosomal compartments following PDGF stimulation. Further, phosphorylated forms of the PDGFR-β coimmunoprecipitated with LRP following PDGF treatment. Together, these studies revealed close association between activated PDGFR-β and LRP, suggesting that LRP functions as a co-receptor capable of modulating the signal transduction pathways initiated by the PDGF receptor from endosomes. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.