The role of nitric oxide in arthritic joints: a therapeutic target?

Abstract Nitric oxide (NO) is produced by many cell types in the joint, and its expression is delicately regulated. Depending on its concentration and cellular origin, NO appears to have both pro- and anti-inflammatory potential in the joint. Constitutively expressed nitric oxide synthase (NOS) produces small amounts of NO, which is essential for normal physiological homeostasis. However, inflammatory stimuli such as endotoxins, cytokines, and growth factors promote inducible NOS (iNOS) expression, initially as an anti-inflammatory response, and catalyse a high output of NO. Excessive NO can amplify inflammatory pathways and contribute to the development and maintenance of arthritis. Consequently, proper regulation of NO synthesis can lead to a novel therapeutic approach for inflammatory joint diseases. Further careful study will be necessary to develop new drugs to regulate the NO pathway and to determine the dosage, timing of administration, and duration of treatment in order to avoid both undesirable immunostimulatory effects and immunosuppressive effects.