Platelet-derived growth factor and transforming growth factor beta synergistically potentiate inflammatory mediator synthesis by fibroblast-like synoviocytes

Introduction: The objective of this study was to model the effects of transforming growth factor beta (TGF-beta) and platelet-derived growth factor (PDGF), both present in rheumatoid arthritis (RA) synovia, on the behavior of fibroblast-like synoviocytes (FLS) in response to pro-inflammatory cytokine (interleukin (IL) 1 beta, tumor necrosis factor-alpha (TNF alpha)) challenge. Methods: Gene and protein expression by fibroblast-like synoviocytes in vitro was studied by quantitative Polymerase Chain Reaction (qPCR), ELISA and multiplex bead cytokine assays. Intracellular signaling pathway activation was determined by Western blot for phospho-kinases and the use of specific inhibitors. Results: In combination, TGF-beta and PDGF (2GF) synergistically augmented TNF alpha- or IL1 beta-induced matrix metalloproteinase 3 (MMP3), IL6, IL8, and macrophage inflammatory protein 1 alpha (MIP1 alpha) secretion by FLS. Other FLS-derived mediators remained unaffected. Individually, neither growth factor significantly potentiated TNF alpha or IL1 beta-induced MMP3 secretion, and only slightly enhanced IL6. The effect of 2GF on TNF alpha-induced gene expression was transcriptionally mediated; blocked by imatinib mesylate; and occurred even if 2GF was added as much as four hours prior to TNF alpha. In addition, a 15-minute pulse of 2GF four hours prior to TNF alpha stimulation yielded a synergistic response. The extracellular-signal-regulated kinase (ERK) and phosphoinositide 3-kinase (PI3K) signaling pathways were induced for at least four hours by 2GF, as demonstrated by persistently upregulated levels of phospho-Akt and phospho-ERK. However, pharmacologic inhibitor studies demonstrated that the potentiating action of 2GF was dependent on PI3 kinase only, and not on ERK. Conclusions: The combination of PDGF and TGF-beta dramatically potentiates FLS response to cytokines in a receptor-mediated and PI3 kinase-dependent fashion. These data suggest that 2GF contribute to synovitis by directing synovial fibroblasts toward a more aggressive phenotype in response to TNF alpha. Therefore, inhibition of growth factor signaling may constitute a complementary therapeutic approach to cytokine-targeted treatments for RA.