The role of free radicals in the pathogenesis of rheumatoid arthritis

Free radicals are reactive chemical species that differ from other compounds in that they have unpaired electrons in their outer orbitals. They are capable of damaging cellular components, and accumulating evidence suggests that they may contribute to various disease entities including inflammatory joint disease. Reactive oxygen species (ROS) as well as reactive nitrogen species (RNS) can directly or indirectly damage basic articular constituents and lead to the clinical expression of the inflammatory arthritis. Hydroxyl radicals degrade isolated proteoglycans, and HOCl fragments collagen. Hydrogen peroxide, which is very diffusible, readily inhibits cartilage proteoglycan synthesis, e.g. by interfering with ATP synthesis, in part by inhibiting the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase in chondrocytes, aggravating the effects of proteolytic and free-radical-mediated cartilage degradation. Peroxynitrite and HOCl may facilitate cartilage damages by inactivating TIMPs. TIMP-1 inhibits stromelysins, collagenases and gelatinases and this ability is lost after ONOO- or HOCl treatment. HOCl can also activate latent forms of neutrophil collagenases and gelatinase with obvious consequences. Hypochlorous acid, ONOO- and O-2(.-) react with ascorbate, which is essential for cartilage function, leading to low levels of ascorbate in synovial fluid. Low concentrations of H2O2,O-2(.-) or both, accelerate bone resorption by osteoclasts, whereas NO. inhibits it. NO. promotes chondrocyte apoptosis, inhibits proteoglycan synthesis and activates latent metalloproteinases and cyclooxygenase. ROS, produced by activated phagocytes, could alter the antigenic behaviour of immunoglobulin G, producing fluorescent protein aggregates that can further activate phagocytic cells. Radical-exposed IgG. is able to bind rheumatoid factor and results in the generation of C3alpha. This reaction may be self-perpetuating within the rheumatoid joint, suggesting that free radicals play a role in the chronicity of the inflammatory reaction which is a key question regarding to which extent free radicals contribute to the consequences of inflammation, such as the cartilage and bone destruction. Reactive oxygen intermediates can also function as signaling messengers to activate transcription factors, like NFkappaB and AP-1, and induce gene expression. All this knowledge might serve to apply a rational selection of antioxidants for possible therapeutic purposes, enforcing combination therapy of the inflammatory joint disease.