Interaction of mesenchymal stem cells with fibroblast-like synoviocytes via cadherin-11 promotes angiogenesis by enhanced secretion of placental growth factor.

Bone marrow-derived mesenchymal stem cells (MSC) exist in the synovium of patients with rheumatoid arthritis (RA), yet the role of MSC in RA is elusive. Placental growth factor (PlGF) expression is increased in RA synovial fluids, and blocking of PlGF attenuates progression of arthritis in mice. In this study, we observed that PlGF induced chemotaxis of MSC in a dose-dependent manner, which was blocked by anti-vascular endothelial growth factor receptor-1 peptide. MSC exposed to PlGF elicited increased phosphorylation of Akt and p38 MAPK. PlGF-mediated chemotaxis was inhibited by PI3K inhibitor (LY294002) and p38 MAPK inhibitor (SB203580), but not by ERK1/2 inhibitor (PD98059). Fibroblast-like synoviocytes (FLS) constitutively produced PlGF, but MSC released negligible amounts of PlGF. Of note, when FLS of RA patients and MSC were cocultured, PlGF production by FLS was significantly increased; such an increase was dependent on the number of added MSC. Moreover, coculture conditioned medium promoted chemotaxis of MSC and increased angiogenesis in Matrigel plugs assay, and these were suppressed by preincubation of the medium with anti-PlGF Ab. Transwell experiments revealed that MSC to FLS contact was required for the increase in PlGF production by coculture. Cadherin-11 was expressed both in FLS and MSC, and small interfering RNA knockdown of cadherin-11 in FLS significantly abrogated the enhanced PlGF production under coculture conditions. These data indicate that increased levels of PlGF in RA joints could induce the migration of MSC to the synovium, and interaction of migrated MSC with FLS via cadherin-11 may contribute to angiogenesis and chronic synovitis by enhancing the secretion of PlGF.