Objectives: The eventual role of a disintegrin and a metalloproteinase 8 (ADAM8) in osteoclastogenesis was studied in erosive rheumatoid arthritis (RA) and in vitro.
Methods: ADAM8 protein and mRNA expression was measured in RA pannus and synovitis and compared to osteoarthritic (OA) synovial membrane. Human monocytes were isolated and stimulated with proinflammatory cytokines and their ADAM8 expression and surface ADAM8 were measured. Human peripheral blood monocytes and RAW 264.7 mouse monocyte/macrophage cells were stimulated to osteclast like-cells, and their expression of ADAM8 and osteoclastic markers (calcitonin receptor, integrin β 3, cathepsin K, TRAP) were analysed. Transfection and small interfering RNA (siRNA) were used to assess the role of ADAM8 in formation of polykaryons.
Results: Increased numbers of ADAM8 positive cells were shown particularly in the pannus-cartilage/bone junction close or adjoining to TRAP positive multinucleate cells under formation (60 (2)% in pannus, 47 (2)% in synovitis vs 10 (1)% in OA, p<0.001). Human pannus contained high ADAM8 mRNA copy numbers (23 (7) in pannus, 14 (4) in synovitis vs 1.7 (0.3) in OA, p<0.001). Functional studies in vitro disclosed ADAM8 mRNA and protein, which was first converted to a proteolytically active and then to fusion-active form. Gene transfection and siRNA experiments enhanced and inhibited, respectively, expression of osteoclast markers and maturation of multinuclear cells.
Conclusions: ADAM8 may be involved in bone destruction in RA because it is upregulated in RA pannus adjacent to developing erosions and enhances maturation of osteoclast-like cells.
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Competing interests: None declared.
Funding: This study was supported by clinical EVO research grants, the Academy of Finland, Centre for Technological Advancement (TEKES), the Invalid Foundation, the Sigrid Juselius Foundation, Wilhelm och Else Stockmanns Stiftelse, Finska Läkaresällskapet, the PhD Graduate School on Biomaterials and Tissue Engineering of the Ministry of Education, Research and Science Foundation of Farmos and Biomedicum Helsinki Foundation.
Ethics approval: The research plan was approved by the ethical committee of the Helsinki University Central Hospital (Statement number 37/E6/04). Guidelines of the Declaration of Helsinki were followed.
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