Activated synovial fibroblasts are key players in the pathogenesis of RA by driving inflammation and joint destruction. Numerous molecules including cytokines and Toll-like receptor (TLR) ligands induce pro-inflammatory signalling and gene expression through a hierarchical network of kinases. Upstream mitogen-activated protein kinase kinase kinases (MAP3Ks) represent an attractive target for RA treatment. In this study, we sought to determine the role of the MAP3K TGF-β-activated kinase 1 (TAK1) in cytokine and TLR-mediated signalling.
TAK1 activity was inhibited using either a small molecule inhibitor or lentivirally overexpressed kinase-inactive TAK1-K63W mutant in murine embryonic and human dermal and synovial fibroblasts. Fibroblasts were stimulated with IL-1, TNF, TLR2 or TLR4 agonists and responses were evaluated using transcriptional reporters, western blotting and analysis of gene expression of collagenases (MMP3 and MMP13), cytokines (IL-1β and IL-6) and chemokines (IL-8 and MCP-1).
TAK1 inhibition abrogated cytokine- and TLR-induced nuclear factor-κB (NF-κB) and Saa3-promoter reporter activation in murine and human dermal fibroblasts. In synovial fibroblasts, TAK1 regulated IL-1 and TNF-mediated NF-κB, but not Saa3-promoter reporter activation. Inducible mRNA expression of cytokines, collagenases and chemokines, except MCP-1, was TAK1 dependent for IL-1, TNF and TLR2 signalling. Unexpectedly, TLR4-mediated NF-κB reporter activation and inducible mRNA expression was fully TAK1 independent. Accordingly, NF-κB p65 and p38 MAPK phosphorylation was unaffected by TAK1 inhibition.
In general, TAK1 crucially regulates IL-1 and TNF signalling in fibroblasts. Interestingly, TLR4 signalling is specifically TAK1 independent in synovial fibroblasts. Consequently, therapeutic TAK1 inhibition in arthropathies may not dampen the damage-associated molecular pattern-mediated TLR4 activation of synovial fibroblasts.