Angiopoietin- I (Ang-1) and Ang-2 are ligands for the receptor tyrosine kinase, Tie-2. Ang-1, a Tie-2 agonist, may have a vascular stabilizing role in angiogenesis, while Ang-2, an endogenous antagonist of Tie-2, may have an early role in angiogenesis, destabilizing existing vasculature. We show that these ligands are expressed by rheumatoid synovial fibroblasts (RSF) and investigate whether their expression was modulated by proinflammatory cytokines present in the joint in rheumatoid arthritis (RA).
Using quantitative PCR we determined the level of expression of these 2 ligands in RSF and chronic inflamed synovial tissue. The level of expression of these ligands after treatment with proinflammatory cytokines and hypoxia was also determined.
We observed constitutive expression of Ang-1 and Ang-2 in RSF and chronic inflamed synovial tissue. Ang-1 was the most highly expressed ligand in late stage RA synovial fibroblasts; however, in chronic inflamed synovial tissue, Ang-2 was predominant and was expressed at strikingly high levels (70 to 120-fold increase). We observed that tumor necrosis factor-alpha (TNF-alpha) and transforming growth factor-beta (TGF-beta), but not interleukin 1beta or hypoxia, stimulated Ang-1 gene expression in RSE This was confirmed at the protein level as media from TNF-alpha treated RSF resulted in increased autophosphorylation of Tie-2. In contrast, TNF-alpha and TGF-beta had no effect on Ang-2 expression in RSF, but augmented expression of Ang-2 in normal synovial fibroblasts.
The angiopoietins are important angiogenic factors constitutively present in RA, and their expression is modulated by certain cytokines. Ang-2 may have an important role in rheumatoid tissue where vigorous angiogenesis is occurring.