ABSTRACT: Inflammation-mediated oncogenesis has been implicated in a variety of cancer types. Rheumatoid synovial tissues can be viewed as a tumor-like mass, consisting of hyperplastic fibroblast-like synoviocytes (FLSs). FLSs of rheumatoid arthritis (RA) patients have pro-migratory and invasive characteristics, which may be caused by chronic exposure to genotoxic stimuli, including hypoxia and growth factors. We tested whether a transformed phenotype of RA-FLSs is associated with placental growth factor (PlGF), a representative angiogenic growth factor induced by hypoxia. Here, we identified PlGF-1 and PlGF-2 as the major PlGF isoforms in RA-FLSs. Global gene expression profiling revealed that cell proliferation, apoptosis, angiogenesis, and cell migration were mainly represented by differentially expressed genes in RA-FLSs transfected with siRNA for PlGF. Indeed, PlGF-deficient RA-FLSs showed a decrease in cell proliferation, migration, and invasion, but an increase in apoptotic death in vitro. PlGF gene overexpression resulted in the opposite effects. Moreover, exogeneous PlGF-1 and PlGF-2 increased survival, migration, and invasiveness of RA-FLSs by binding their receptors, Flt-1 and NP-1, up-regulating the expression of anti-apoptotic molecules, pErk and Bcl2. Knock-down of PlGF transcripts reduced RA-FLS proliferation in a xeno-transplantation model. Collectively, in addition to their role for neovascularization, PlGF-1 and -2 promote proliferation, survival, migration, and invasion of RA-FLSs in an autocrine and paracrine manner. These results demonstrated how primary cells of mesenchymal origin acquired an aggressive and transformed phenotype. PlGF and its receptors thus offer new targets for anti-FLS therapy. The FLSs were prepared from the synovial tissues of RA patients and incubated in DMEM supplemented with 10% FBS. Cells were cultured in RPMI supplemented with 10% FBS. There are 4 FLS samples treated with PlGF siRNA, and 4 FLS samples treated with control siRNA.
