Objective: To determine whether angiopoietin 1 (Ang-1) potentiates overgrowth of the synovium and joint degradation in rheumatoid arthritis (RA), and to clarify the cell-signaling mechanisms of Ang-1 in the rheumatoid joint.
Methods: Expression of Ang-1, TIE-2 (a receptor for Ang-1), and matrix metalloproteinase 3 (MMP-3) was studied by immunohistochemistry. Activation of the ERK/MAPK and phosphatidylinositol (PI) 3-kinase/Akt pathways and of NF-kappaB was determined by Western blotting and an NF-kappaB p65 DNA binding activity assay, respectively. Induction of apoptosis was evaluated by nuclear staining, cell viability assay, and Western blotting of caspases. Synovial cell migration was evaluated by actin polymerization, Western blotting of Rho family proteins, and affinity purification with Rhotekin-Rho and p21-activated kinase 1. Matrix degradation was examined by induction of proMMP-3 secretion from synovial cells followed by in vitro cartilaginous matrix degradation assay.
Results: Ang-1 stimulated the ERK/MAPK and PI 3-kinase/Akt pathways in a cooperative but independent manner, which enhanced rheumatoid synovium overgrowth and joint destruction. In addition, Ang-1 activated NF-kappaB via Akt to promote cell growth, but also inhibited cell apoptosis via ERK and Akt. Ang-1 directly potentiated the extension of synovial cells in an ERK- and Akt-dependent manner by up-regulating Rho family proteins, which attenuated Rac signaling and led to membrane ruffling. Ang-1 induced proMMP-3 secretion from synovial cells, which resulted in direct degradation of the cartilaginous matrix.
Conclusion: Ang-1 stimulates the ERK/MAPK and PI 3-kinase/Akt pathways cooperatively, but in a manner independent of each other, to directly potentiate synovium overgrowth and joint destruction in RA. In addition to inflammatory cytokines, Ang-1/TIE-2 signaling appears to be an independent factor that contributes to the destruction of the rheumatoid joint.