OBJECTIVES: Inflammation in diseases such as rheumatoid arthritis (RA) stimulates osteoclast-mediated articular bone erosion and inhibits osteoblast-mediated bone formation, leading to a net loss of bone. Pro-inflammatory cytokines and antagonists of the Wnt signalling pathway have been implicated in the inhibition of osteoblast differentiation and activity in RA, contributing to the erosive process and impairing erosion healing. Importantly, osteoblast differentiation and function are also regulated by the osteogenic bone morphogenetic protein (BMP) signalling pathway, which is antagonized by BMP3. We therefore examined the potential role of BMP3 in inflammatory arthritis. METHOD: Two murine models of RA, K/BxN serum transfer arthritis (STA) and antigen-induced arthritis (AIA), were used to establish the temporal expression of BMP3 and the cellular sources of BMP3 mRNA and protein in inflammatory arthritis. To determine the effects of inflammation on the expression of BMP3 in osteoblasts, murine calvarial osteoblasts were treated with pro-inflammatory cytokines and BMP3 expression was assessed. RESULTS: In both murine models of RA, BMP3 mRNA and protein are highly expressed by osteoblasts lining inflammation-bone interfaces late in the course of arthritis. Synovial tissues are not a significant source of BMP3. BMP3 expression is induced in osteocalcin-expressing osteoblasts in vitro following stimulation by tumour necrosis factor (TNF). CONCLUSIONS: These data implicate BMP3 as a novel factor that may act locally to contribute to the erosive process and inhibit the repair of articular bone in RA through inhibition of osteoblast differentiation and function.

Articular bone erosion in rheumatoid arthritis (RA) is mediated by the interaction between inflammation and pathways regulating bone metabolism. Inflammation promotes osteoclastogenesis and also inhibits osteoblast function, further contributing to the persistence of erosions. MicroRNAs (miRNAs) are important regulators of skeletal remodeling and play a role in RA pathogenesis. We therefore determined the expression of miRNAs in inflamed synovial tissue and the role they play in pathways regulating osteoblast and osteoclast function. Using the serum transfer mouse model of RA in C57BL/6 mice, we performed Fluidigm high-throughput qPCR-based screening of miRNAs from nonarthritic and arthritic mice. Global gene expression profiling was also performed on Affymetrix microarrays from these same synovial samples. miRNA and mRNA expression profiles were subjected to comparative bioinformatics. A total of 536 upregulated genes and 417 downregulated genes were identified that are predicted targets of miRNAs with reciprocal expression changes. Gene ontology analysis of these genes revealed significant enrichment in skeletal pathways. Of the 22 miRNAs whose expression was most significantly changed (p < 0.01) between nonarthritic and arthritic mice, we identified their targets that both inhibit and promote bone formation. These miRNAs are predicted to target Wnt and BMP signaling pathway components. We validated miRNA array findings and demonstrated that secretion of miR-221-3p in exosomes was upregulated by synovial fibroblasts treated with the proinflammatory cytokine TNF. Overexpression of miR-221-3p suppressed calvarial osteoblast differentiation and mineralization in vitro. These results suggest that miRNAs derived from inflamed synovial tissues may regulate signaling pathways at erosion sites that affect bone loss and potentially also compensatory bone formation.