Introduction: Bone morphogenetic proteins (BMPs) are multifunctional secreted growth factors regulating a broad spectrum of functions in numerous systems. An increased expression and production of specific BMPs have been described in the rheumatoid arthritis (RA) synovium. The aim of this study was to analyze the involvement of the BMP signaling pathway in RA synoviocytes in response to interleukin-17 (IL-17) and tumor necrosis factor-alpha (TNF-α).
Methods: The expression of components of the BMP signaling pathway (BMP receptors, BMP ligands, BMP signal transducers, and BMP antagonists) was analyzed by quantitative polymerase chain reaction before and after treatment of RA synoviocytes with TNF-α or IL-17 or both. Regulation was studied in the presence of the specific BMP inhibitor DMH1 (dorsomorphin homologue 1) or an exogenous BMP ligand, BMP6. Expression and production of pro-inflammatory cytokines (IL-6 and granulocyte-macrophage colony-stimulating factor), chemokines (IL-8, CCL2, CCL5, and CXCL10), and matrix metalloproteinases (MMP-1, -2, -3, -9, and -13) were analyzed.
Results: RA synoviocytes express BMP receptors (mainly BMPRIA, ACTRIA, and BMPRII), signal transducers of the Smad family (Smad1 and 5 and co-Smad4), and different BMP antagonists. The modulation of the expression of the BMP target genes-Id (inhibitor of DNA-binding/differentiation) proteins and Runx (Runt-related transcription factor) transcription factors-after the addition of exogenous BMP shows that the BMP signaling pathway is active. RA synoviocytes also express BMP ligands (BMP2, BMP6, and BMP7) which are highly upregulated after activation with TNF-α and IL-17. Autocrine BMP signaling pathway can be blocked by treatment with the inhibitor DMH1, leading to an increase in the upregulated expression of pro-inflammatory cytokines, chemokines, and MMPs induced by the activation of RA synoviocytes with TNF-α and IL-17. Conversely, the additional stimulation of the BMP pathway with the exogenous addition of the BMP6 ligand decreases the expression of those pro-inflammatory and pro-destructive factors.
Conclusion: The results indicate that the canonical BMP pathway is functionally active in human RA synoviocytes and that the inhibition of autocrine BMP signaling exacerbates the pro-inflammatory phenotype induced in RA synoviocytes by the stimulation with IL-17 and TNF-α.