Resistance of rheumatoid arthritis synovial fibroblasts to p38 MAP-kinase inhibition of pro-destructive functions mediated by tumor necrosis factor alpha/tumor necrosis factor receptor-1

In rheumatoid arthritis (RA), tumor necrosis factor (TNF) alpha is a major inductor of the proinflammatory/pro-destructive functions of synovial fibroblasts (SFB). These effects are predominantly mediated via the TNF receptor-1 (TNFR1). In addition to the NF-κB pathway, the p38 MAP kinase seems to play a central role for the underlying signal transduction. In the present study, RA-SFB were compared with osteoarthritis (OA)-SFB concerning the TNF-α/TNFR1/2-induced secretion of IL-6, IL-8, prostaglandin E₂ (PGE₂), and matrix metalloproteinase-1/tissue inhibitor of matrix metalloproteinase-1 (MMP-1/TIMP-1), as well as the sensitivity to p38 MAP-kinase inhibition.

Early-passage (second) RA-SFB and OA-SFB were analyzed for TNFR expression by FACS. The cells were then stimulated with TNF-α (10 ng/ml) or agonistic anti-TNFR1 (HTR-9) or anti-TNFR2 monoclonal antibodies (UTR-1; 10 μg/ml each) with/without inhibition of the p38 kinase by SB203580 (1 μM). Secretion of IL-6, IL-8, PGE₂, MMP-1, and TIMP-1 was analyzed by ELISA.

RA-SFB and OA-SFB both expressed TNFR1 and TNFR2 on their surface, without significant differences between the two groups. Secretion of IL-6, IL-8, PGE₂, and MMP-1, but not TIMP-1, was significantly augmented by stimulation of RA-SFB and OA-SFB with TNF-α. Except for PGE₂ (induced via both TNFRs), these effects were exclusively mediated via the TNFR1. Inhibition of p38 kinase reduced the secretion of IL-6 and PGE₂ significantly and equally well in RA-SFB and OA-SFB. However, the secretion of MMP-1 was significantly suppressed only in OA-SFB, whereas RA-SFB were insensitive to the inhibition of MMP-1 secretion by p38 inhibition.

In early-passage RA-SFB and OA-SFB, TNF-α-induced proinflammatory/pro-destructive functions are predominantly mediated by TNFR1. Strikingly, RA-SFB are partially resistant to the suppression of pro-destructive MMP-1 by p38 MAP-kinase inhibition. The underlying structural or functional alterations of the p38 MAP kinase in RA-SFB may contribute to the pathogenesis and/or therapeutic sensitivity of RA.