Molecular Regulation of Bone Turnover in Juvenile Idiopathic Arthritis: Animal Models, Cellular Features and TNFα

We review the abnormal bone turnover that is the basis of idiopathic inflammatory or rheumatoid arthritis and bone loss, with emphasis on Tumor Necrosis Factor-alpha (TNFα)-related mechanisms. We review selected data on idiopathic arthritis in juvenile human disease, and discuss mouse models focusing on induction of bone resorbing cells by TNFα and Receptor Activator of Nuclear Factor kappa B Ligand (RANKL). In both humans and animal models, macrophage-derived cells in the joint, particularly in the synovium and periosteum, degrade bone and cartilage. Mouse models of rheumatoid arthritis share with human disease bone resorbing cells and strong relation to TNFα expression. In humans, differences in therapy and prognosis of arthritis vary with age, and results from early intervention for inflammatory cytokines in juvenile patients are particularly interesting. Mechanisms that contribute to inflammatory arthritis reflect, in large part, inflammatory cytokines that play minor roles in normal bone turnover. Changes in inflammatory cytokines, particularly TNFα, are many times larger, and presented in different locations, than cytokines that regulate normal bone turnover. Recent data from in vitro and mouse models include novel mechanisms described in differentiation of bone resorbing cells in inflammatory arthritis dependent on the Transient Receptor Potential Channel (TRPC) family of calcium channels. Low-molecular weight (MW) inhibitors of TRPC channels add to their potential importance. Associations with inflammatory arthritis unrelated to TNFα are briefly summarized as pointing to alternative mechanisms. We suggest that early detection and monoclonal antibodies targeting cytokines mediating disease progression deserves emphasis.