Cell Adhesion Regulates Ubiquitin-mediated Degradation of the Platelet-derived Growth Factor Receptor β

Abstract

Cross-talk between integrin-mediated adhesion and growth factors has been described in many recent studies; however, the underlying mechanisms remain incompletely understood. We report here that detachment of cells from the extracellular matrix induced a decrease in both the autophosphorylation and protein levels of the platelet-derived growth factor receptor ␤ (PDGF-R ␤), which was completely reversed upon replating cells on fibronectin. The effect occurred in all cells examined but to a greater extent in primary fibroblasts compared with established cell lines. Decreased PDGF-R levels in suspended cells correlated with ubiquitination of the PDGF-R and was blocked by treatment with inhibitors of the proteasome pathway. Unlike PDGF-induced down-regulation, detachment-induced degradation did not require receptor autophosphorylation, internalization, or tyrosine kinase activity. We conclude that cell detachment results in cellular desensitization to PDGF that is mediated by degradation of the PDGF-R via a novel ubiquitin-dependent pathway.Cell adhesion to the extracellular matrix induces a wide array of intracellular signals mediated primarily by members of the integrin family of transmembrane receptors. Integrins act coordinately with growth factors to regulate migration, growth or differentiation, and survival (reviewed in Ref. 1). For instance, several recent studies reported a synergistic effect of PDGF 1 and cell adhesion on autophosphorylation of the PDGF-R and activation of the mitogen-activated protein kinase cascade (2, 3). Moreover, PDGF induces the association of its phosphorylated receptor with integrin ␣ v ␤ 3 in fibroblasts plated on vitronectin, which correlates with enhanced PDGFinduced cell proliferation (4). These effects appear to contribute to the anchorage dependence of growth (1).PDGF-Rs are transmembrane tyrosine kinases that mediate the effects of PDGF on growth and motility in cell types including fibroblasts and smooth muscle cells (5-7). Two structurally related receptors, ␣ and ␤, possess different affinities for the A and B PDGF isoforms and generate similar though not identical cellular responses. Thus, both the ␣ and ␤ receptors stimulate cell proliferation, membrane ruffling, and loss of actin stress fibers, but only the ␤ receptor promotes chemotaxis (5, 6). Binding of PDGF to its receptor induces receptor dimerization and autophosphorylation on several tyrosine residues, which provide docking sites for SH2-containing molecules, thereby initiating a variety of signaling pathways.c-Cbl, one of the numerous proteins that bind to the PDGF-R (8), has recently been identified as a proteasomal E3-like ubiquitin ligase that is involved in down-regulation of tyrosine kinase receptors (9 -11). PDGF-induced down-regulation is a complex process that involves receptor internalization through clathrin-coated pits and requires the receptor tyrosine kinase activity (12, 13). The 100-kDa GTPase dynamin is responsible for pinching off of the vesicles, which results in the delivery of receptors to l...

Merlin, a multi‐suppressor from cell membrane to the nucleus

“…A similar mechanism is also discussed for other growth factor receptors, such as ErbB2 and 3, and insulin-like growth factor 1 receptor [41]. In addition, integrins, which are overexpressed in human schwannomas, could also stabilize PDGFR and delay its degradation by enhancing its autophosphorylation [44].…”

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

“…Ligand-induced Ubiquitination of PDGFR␤-Ubiquitination of PDGFR␤ is a necessary step for receptor internalization and subsequent degradation in response to PDGF stimulation (19,20), which raises the possibility that the receptor might be hyper-ubiquitinated in MEF2 cells. Therefore, we compared the levels of ubiquitinated PDGFR␤ in MEF1 and MEF2 cells after PDGF-BB stimulation (Fig.…”

“…On the other hand, proteasomal activity has also been implicated (19,20). To de- termine which mechanism was responsible for the accelerated PDGFR degradation in MEF2 cells, we investigated the effects of MG132, a commonly used proteasome inhibitor, and chloroquine, a weak base that inhibits lysosomal proteolysis, on the down-regulation of PDGFR␤ in LRP1-deficient fibroblasts.…”

“…Ubiquitination of PDGFR␤ in response to ligand binding (18) has been implicated in the efficient degradation of the receptor through proteasomal activity (19,20). Other recent studies have also suggested that monoubiquitination of receptor tyrosine kinases, including PDGFR, mediates internalization and intracellular sorting of the receptor (21)(22)(23)(24).…”

The shaping of invasive glioma phenotype by the ubiquitin–proteasome system

“…All these pathways are regulated by the UPS in multiple points. EGFR, PDGFR, GDNFR and c-met proteins are proteasome substrates (Gur et al, 2004;Pierchala et al, 2006;Kim et al, 2008;Baron & Schwartz, 2000) and the same is true for intracellular proteins that take part in signal transduction such as Akt and ERK (Adachi et al, 2003;Mikalsen et al, 2005) as well as transcription target proteins of the pathways. Particularly for EGFR signaling pathway, mutant EGFRvIII induces a specifi c pattern of genes with pronounced roles in enhancement of invasion, such as ECM proteins, metalloproteases and a serine protease (Lal et al, 2002).…”