Introduction Activins and inhibins belong to the transforming growth factor β family. Activins are disulphide-linked homo- or heterodimers consisting of two inhibin β chains (βA, βB) that are expressed in many cell types. However, activin A (βA βA) is the only activin that is expressed in bone and cartilage. Moreover, activin A has been demonstrated not only to stimulate receptor activator of NF-κB ligand (RANKL)-induced osteoclast (OC) differentiation but also to inhibit osteoblast differentiation.

Objectives Here we investigate the impact of activin a on joint destruction in rheumatoid arthritis.

Methods Synovial tissue samples from rheumatoid arthritis (RA) and osteoarthritis (OA) patients were analysed by immunohistochemical staining. For in vitro experiments, bone marrow-derived macrophages (BMM) were isolated from femurs and tibias of WT mice and differentiated into osteoclasts in the presence of macrophage colony-stimulating factor (M-CSF) and RANKL with or without activin A. OC differentiation was characterised by TRAP staining. Resorption activity was determined by quantification of osteoclast-mediated pit formation on a calcium phosphate-coated plate. Proliferation of BMMs was evaluated using the CyQUANT Cell Proliferation Assay Kit. Furthermore, osteoclast-specific gene expression as well as the activation of SMAD2, MAPK and NF-kB signalling were analysed by immunoblotting. The interaction of phospho-SMAD2 with NFATc1 was evaluated by co-immunoprecipitation using Dynabeads.

Results We demonstrate that activin A is highly abundant in the synovium of RA but not of OA patients. In vitro, activin A strongly enhanced the RANKL-mediated differentiation of BMMs into mature OCs, reflected by a significantly increased OC number, OC size and number of nuclei per OC compared to the conventional treatment with RANKL alone. Moreover, concomitant administration of activin A led to a significant increase of the total resorption area as well as resorption area per pit, indicating an increased activity of individual OCs. Effects of activin A on BMMs were not caused by increased proliferation since no effect on M-CSF-driven proliferation was observed. Furthermore, activin A alone was not able to induce the expression of OC differentiation markers, but the RANKL-induced expression was enhanced by activin A. After stimulation with activin A, BMMs showed an activation of SMAD2, but not of MAPK p38, ERK, JNK or NF-kB. Finally, co-stimulation of RANKL and activin A resulted in an increased interaction of activated SMAD2 with NFATc1.

Conclusions The data strongly suggest that increased expression of activin A in the arthritic joint is most likely associated with enhanced osteoclast formation, promoting joint destruction in rheumatoid arthritis.

Disclosure of interest None declared