Autocrine/paracrine role of the angiopoietin-1 and -2/Tie2 system in cell proliferation and chemotaxis of cultured fibroblastic synoviocytes in rheumatoid arthritis

Abstract

Hypervascularity is a characteristic synovial feature of rheumatoid arthritis (RA). We previously reported that Tie1 and Tie2, endothelium-specific tyrosine kinase receptors essential for angiogenesis, are expressed not only by vascular cells, but also by a subpopulation of synovial lining and stromal cells in the inflamed RA synovium. The present study used immunohistochemistry and in situ hybridization to examine whether angiopoietin-1 and -2 (Ang1 and Ang2), ligands for Tie2, are also expressed in the RA synovium. Ang1 and Ang2 were expressed in all of 15 RA synovial samples, and their distribution pattern was similar to that of Tie2. Intense staining was noted in synovial lining and stromal cells, as well as in small vessels in areas of papillary projection and high cell density. Double immunohistochemistry revealed coexpression of Ang1, Ang2, and Tie2 in synovial components exhibiting proliferating cell nuclear antigen immunoreactivity. In addition, Ang1 and Ang2 were preferentially expressed in vimentin-positive fibroblastic cells. To address the functional role of Ang/Tie signaling in RA pathophysiology, we carried out [(3)H]thymidine incorporation and transwell chemotaxis assays using cultured fibroblastic synoviocytes obtained from 2 RA patients. Incubation with various concentrations of recombinant Ang1 or Ang2 did not alter DNA synthesis, but the ligands enhanced chemotactic migration of RA fibroblastic synoviocytes. Our results suggest that the autocrine/paracrine signaling of the Ang/Tie2 system is important for the up-regulated angiogenesis in the RA synovium, as well as for synoviocyte behavior, by regulating chemotactic cell movement.