Platelet-Derived Growth Factor Receptor Activation Promotes the Prodestructive Invadosome-Forming Phenotype of Synoviocytes from Patients with Rheumatoid Arthritis

Fibroblast-like synoviocytes (FLS) play a major role in invasive joint destruction in rheumatoid arthritis (RA). This prodestructive phenotype has been shown to involve autocrine TGF-β that triggers formation of matrix-degrading invadosomes through molecular mechanisms that are not fully elucidated. The platelet-derived growth factor (PDGF) receptor (PDGFR) family of receptor tyrosine kinases (RTK) has been shown to cooperate with TGF-β in various pathological conditions. We therefore sought to determine whether RTK activity played a role in invadosome biogenesis. We demonstrated that, among the common RTKs, PDGFR-αβ was specifically phosphorylated in FLS from RA patients. Phosphorylation of PDGFR-αβ was also elevated in RA synovial tissues. Interference with PDGFR activation or PDGF neutralization inhibited invadosome formation in RA synoviocytes, indicating the presence of an autocrine PDGFR activation loop that involved endogenous PDGF. Among the PDGF-A-D isoforms, only PDGF-B was found both significantly elevated in FLS lines from RA patients, and related to high-invadosome forming cells. Addition of TGF-β upregulated invadosome formation, PDGF-B mRNA expression, and phosphorylation of PDGFR. All of these functions were efficiently suppressed by TGF-β neutralization or interference with the Smad/TβR1or PI3K/Akt pathway. Among the class 1 PI3K family proteins known to be expressed in RA synoviocytes, PI3Kα was selectively involved in PDGF-B expression, whereas both PI3Kα and PI3Kδ participated in invadosome formation. Our findings demonstrate that PDGFR is a critical RTK required for the prodestructive phenotype of RA synovial cells. They also provide evidence for an association between autocrine TGF-β and PDGFR-mediated invadosome formation in RA synoviocytes that involves the production of PDGF-B induced by TGF-β.