Angiogenesis in arthritis: role in disease pathogenesis and as a potential therapeutic target.
Rheumatoid arthritis (RA) is a chronic destructive musculo-skeletal disorder, associated with thickening of the synovial membrane lining the joints, inflammation and hyperproliferation of synovial cells, as well as a pro-inflammatory cytokine cascade, leukocyte infiltration, and tissue damage and bone resorption. An early event in RA is an alteration in blood vessel density and prominent neovascularisation. The hyperplasia of the synovium necessitates a compensatory increase in the number of blood vessels to nourish and oxygenate the tissue. However, angiogenesis may not keep pace with synovial proliferation, leading to regions of hypoperfusion and hypoxia. VEGF, a potent endothelial cell mitogen, is expressed in RA synovium and elevated in the serum of RA patients. We have reported that dissociated RA synovial membrane cells spontaneously secrete VEGF, and that release of VEGF by these cells is upregulated by cytokines and hypoxia. In a murine model of RA, VEGF is released from synovial cells isolated from the knees of arthritic but not healthy mice, and the extent of VEGF production correlates with the severity of arthritis. VEGF thus appears to play a key role in mediating alterations in synovial vessel density in arthritis. As a consequence, RA may be a potential target for anti-angiogenic therapy, and targeting VEGF may prove to be especially beneficial.