Low Density Lipoprotein Receptor-related Protein 1 (LRP1) Controls Endocytosis and c-CBL-mediated Ubiquitination of the Platelet-derived Growth Factor Receptor β (PDGFRβ)

Abstract

The low density lipoprotein receptor-related protein 1 (LRP1) has been implicated in intracellular signaling functions as well as in lipid metabolism. Recent in vivo and in vitro studies suggest that LRP1 is a physiological modulator of the platelet-derived growth factor (PDGF) signaling pathway. Here we show that in mouse fibroblasts LRP1 modulates PDGF-BB signaling by controlling endocytosis and ligand-induced down-regulation of the PDGF receptor ␤ (PDGFR␤). In LRP1-deficient fibroblasts, basal PDGFR␤ tyrosine kinase activity was derepressed, and PDGF-BB-induced endocytosis and degradation of PDGFR␤ were accelerated as compared with control cells. This was accompanied by rapid uptake of receptor-bound PDGF-BB into the cells and by attenuated ERK activation in response to PDGF-BB stimulation. Pulse-chase analysis indicated that the steady-state turnover rate of PDGFR␤ was also accelerated in LRP-deficient fibroblasts. The rapid degradation of PDGFR␤ in the LRP1-deficient fibroblasts was prevented by MG132 and chloroquine. Furthermore, the association of PDGFR␤ with c-Cbl, a ubiquitin E3-ligase, as well as the ligand-induced ubiquitination of PDGFR␤ were increased in LRP1-deficient fibroblasts. We show that LRP1 can directly interact with c-Cbl, suggesting a Sprouty-like role for LRP1 in regulating the access of the PDGFR␤ to the ubiquitination machinery. Thus, LRP1 modulates PDGF signaling by controlling ubiquitination and endocytosis of the PDGFR␤.The low density lipoprotein receptor-related protein 1 (LRP1) 1 is a member of the low density lipoprotein receptor gene family. It consists of a 515-kDa heavy chain containing four clusters of ligand binding domains and a non-covalently associated 85-kDa light chain containing a trans-membrane and cytoplasmic domain. LRP1 cooperates with the low density lipoprotein receptor in the endocytosis and clearance of cholesterol-rich chylomicron remnants from the circulation (1, 2). However, the large number and functional diversity of LRP1 ligands (3) and the lethality in LRP1 conventional knock-out mice during early to mid-gestation (4) suggest that LRP1 is involved in essential physiological processes other than lipid metabolism.In contrast to the low density lipoprotein receptor, there is now substantial evidence indicating a role for LRP1 in cellular signaling pathways. For instance, LRP1 has been shown to form complexes with other cell surface signaling proteins, such as the urokinase-type plasminogen activator (uPA) and the uPA receptor (uPAR) complex (5, 6). Furthermore, the cytoplasmic domain of LRP1 also interacts with cytoplasmic adaptor proteins that are involved in the regulation of MAP kinase activity, cytoskeletal reorganization, and cell adhesion. These proteins include Shc, FE65, PSD-95, and c-Jun amino-terminal kinase-interacting proteins (JIPs) (7-9). Recent studies indicate that LRP1 is required for the activation of MAP kinase and phosphatidylinositol 3-kinase and for focal adhesion disassembly induced by thrombospondin binding to calreticulin (10) as w...

Heterozygous germline mutations in A2ML1 are associated with a disorder clinically related to Noonan syndrome

“…21 Additionally, LRP1 directly interacts with CBL. 22 Taken together, these reports suggest that A2ML1 may act upstream of signaling pathways known to be involved in NS.…”

Role of platelet-derived growth factors in physiology and medicine

“…Thus, LRP1 normally limits PDGFR-␤ expression and function in vascular SMC. This function appears to involve at least two independent and synergizing mechanisms: (1) LRP1 inhibition of thrombospondin-mediated TGF␤ activation and signaling through Smad2/3, leading to decreased PDGFR-␤ expression (Boucher et al 2007); and (2) LRP1-dependent endocytosis and c-Cbl-mediated ubiquitinylation of PDGFR-␤ (Takayama et al 2005). Inhibition of PDGFR signaling by imatinib had a major protective effect on atherosclerosis development in SMC-specific LRP1 knockouts.…”

The role of ubiquitylation in receptor endocytosis and endosomal sorting

“…The activity of CBL ubiquitin ligases is negatively regulated by several mechanisms, such as self-ubiquitylation of CBL, which promotes its proteasomal degradation, and specific modulators that can inhibit CBL-mediated ubiquitylation, including Sprouty 2 (SPRY2), suppressor of T-cell receptor signaling 2 (STS2, also known as UBS3A and UBASH3A) and low-density-lipoprotein-related protein 1 (LRP1) (Kowanetz et al, 2004;Takayama et al, 2005;Wong et al, 2002b). Thus, robust mechanisms are in place to regulate RTK ubiquitylation, endocytosis and degradation, and ultimately control the duration of RTK activation, thereby modulating receptor signaling output.…”