Background The local bone loss around the inflamed joints is found from the early stage of rheumatoid arthritis. We previously reported that cell-surface expression of alpha-enolase (ENO1), multifunctional glycolytic protein, is increased in monocytes and macrophages isolated from RA patients and related to production of proinflammatory cytokines (1).
Objectives This study was performed to examine the role of surface ENO1 on osteoclastogenesis in rheumatoid arthritis.
Methods Monocytes isolated from RA patients and healthy control were used to examine the surface level of ENO1 by flow cytometry and confocal microscopy. U937, human monocytic cell line, was treated with ENO1-targeting siRNA (siENO1) to down-regulate ENO1 expression, then osteoclast differentiation were induced by RANKL and the level of ENO1 and NFATc1 was measured. Also, monocytes from RA patients were differentiated into macrophages and transfected with siENO1 to assess the effect on osteoclast differentiation.
Results We found the surface expression level of ENO1 was higher in monocytes isolated from RA patients than healthy control and gradually decreased during osteoclastogenesis in both cells, while the total level of ENO1 remained unchanged. ENO1-specific siRNA significantly decreased the surface ENO1 level in U937, but did not affect the viability and proliferation of the cells. When osteoclast formation was induced by RANKL, osteoclast-specific transcription factor, nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1) expression was markedly inhibited in ENO1 down-regulated cells compared to control cells. We also examined the effect of down regulation of ENO1 in osteoclast precursor cells isolated from RA patients (N=10). The surface ENO1 level was significantly decreased and RANKL-induced osteoclast differentiation was markedly inhibited in ENO1 down-regulated cells compared to control cells.
Conclusions These results suggest that increased expression level of ENO1 on the surface of monocytes/macrophages have positive effect on osteoclast formation in RA.
S. Bae, H. Kim, N. Lee, C. Won, H-R. Kim, Y. Hwang, Y W. Song, J S. Kang, and W J. Lee. A-Enolase expressed on the surfaces of monocytes and macrophages induces robust synovial inflammation in rheumatoid arthritis. The Journal of Immunology. (2012) 189: 365–372.
Disclosure of Interest None declared