TLR-3 enhances osteoclastogenesis through upregulation of RANKL expression from fibroblast-like synoviocytes in patients with rheumatoid arthritis.

This study was undertaken to determine the effect of toll-like receptor-3 (TLR3) on the regulation of osteoclastogenic activity in rheumatoid arthritis (RA) fibroblast-like synoviocytes (FLS). The expression of receptor activator of nuclear factor kappa B ligand (RANKL) mRNA and protein in RA-FLS after TLR3 activation was determined using RT-PCR, real-time PCR, western blot analysis, and immunohistochemistry. Human monocytes were cocultured with RA-FLS that had been prestimulated by the TLR3 ligand polyriboinosinic-polyribocytidylic acid and then stained for tartrate-resistant acid phosphatase (TRAP) activity. Other markers of osteoclasts were measured using RT-PCR and real-time PCR. The expression of TLR3 and RANKL was much higher in the RA synovium than in the osteoarthritis (OA) synovium. TLR3 activation induced RANKL expression in RA-FLS, but not in OA-FLS or in normal skin fibroblasts. TLR3 activation also induced the production of IL-1beta but had no effect on IL-17 or TNF-alpha production in RA-FLS. Inhibition of IL-1beta reversed the TLR3-induced upregulation of RANKL expression. Coculture of human monocytes with TLR3-activated RA-FLS or TLR3 ligand-stimulated human monocytes increased the expression of TRAP, RANK, cathepsin K, calcitonin receptor, and MMP-9, reflecting the differentiation of monocytes into osteoclasts. Our results suggest that TLR3 promotes osteoclastogenesis in the RA synovium both directly and indirectly. TLR3 stimulates human monocytes directly to promote osteoclast differentiation. TLR3 induces RANKL expression indirectly in RA-FLS, and the expression of RANKL promotes the differentiation of osteoclasts in the RA synovium. Targeting the TLR3 pathway may be a promising approach to preventing inflammatory bone destruction in RA.