Introduction Adiponectin is a strong effector molecule in the pathophysiology of RA, for example by contributing to the production of proinflammatory chemokines and matrix degrading enzymes synthesised by RA synovial fibroblasts. In addition, there is growing evidence that adiponectin influences bone metabolism. Our immunohistochemical analysis of RA bone tissue showed a co-localization of adiponectin with key cells of bone remodelling, i.e. osteoblasts and osteoclasts. Moreover, immunocytochemistry and RT-PCR confirmed the expression of adiponectin receptors by RA osteoblasts and osteoclasts. In this study, we therefore focussed on the influence of adiponectin on RA osteoblast and osteoclast activity, their differentiation as well as the immunomodulatory properties of adiponectin.

Methods Human osteoblasts and osteoclasts were isolated from bone tissue and blood samples of RA patients. OA patients and healthy donors served as control. Osteoblasts and osteoclasts were treated with adiponectin (10 µg/ ml). Adiponectin-mediated effects on osteoblasts and osteoclasts were analysed using realtime PCR and immunoassays. Effects of adiponectin on matrix-production by osteoblasts as well as differentiation and resorptive activity of osteoclasts were examined using Alizarin-Red S-, TRAP- and von Kossa-staining.

Results Stimulation with adiponectin induced the secretion of pro-inflammatory cytokines in RA osteoblasts (e.g. IL-6: 5.2 fold, p = 0.001; MCP-1: 7.6 fold, p = 0.2). Especially pre-osteoclasts responded with an increase in IL-8 production (12.9-fold, p = 0.041) in comparison to mature osteoclasts (2.6-fold, p = 0.06, n = 8). Gene expression analysis revealed an induction of OPG and an inhibition of osterix in RA osteoblasts by adiponectin (OPG: 2.7-fold; p = 0.04; osterix: -1.55-fold; p = 0.004). In osteoclasts, the expression of MMP-9 and TRAP was increased (MMP-9: 1.9-fold, p = 0.03; TRAP: 1.5-fold, p = 0.052). Moreover, differentiation of PBMCs in the presence of adiponectin resulted in an increased number of TRAP positive cells (131.2% ± 60.7, p = 0.3, n = 5) and a significantly increased resorptive activity (134% ± 16.3, p = 0.016; n = 5). In contrast, functional examinations of osteoblasts did not reveal differences with regard to mineralisation capacity in the presence of adiponectin.

Conclusions On one hand, the present study shows an increased release of cytokines after adiponectin treatment by osteoblasts and osteoclasts and thereby confirms the pro-inflammatory potential of adiponectin in RA. On the other hand, our findings revealed adiponectin-mediated alterations in gene expression, activity and differentiation of human osteoblasts and osteoclasts in RA. This supports the idea of adiponectin being involved in the pathophysiologic modulation of RA effector cells with regard to the process of bone remodelling in RA.

Acknowledgement Funded by the German Research Society (SPP1468, IMMUNOBONE, NE1174/6-1).