NADPH oxidases in bone and cartilage homeostasis and disease: A promising therapeutic target.

Reactive oxygen species (ROS) generated by the NADPH oxidase (Nox) enzymes are important short-range signaling molecules. They have been extensively studied in the physiology and pathophysiology of the cardiovascular system, where they have important roles in vascular inflammation, angiogenesis, hypertension, cardiac injury, stroke, and aging. Increasing evidence demonstrates that ROS and Nox enzymes also affect bone homeostasis and osteoporosis, and more recent studies implicate ROS and Nox enzymes in both inflammatory arthritis and osteoarthritis. Mechanistically, this connection may be through the effects of ROS on signal transduction. ROS affect both transforming growth factor-β/Smad signaling, interleukin-1β/nuclear factor-kappa B signaling, and the resulting changes in matrix metalloproteinase expression. The purpose of this review is to describe the role of Nox enzymes in the physiology and pathobiology of bone and joints and to highlight the potential of therapeutically targeting the Nox enzymes.