Background and Objectives Rheumatoid arthritis (RA) is a polygenic disorder usually arising from combined genetic predisposition and environmental influences with associated dysfunctional immune responses. The X-box binding protein 1 (XBP1) transcription factor is a central regulator of the endoplasmic reticulum (ER) stress response. It is induced via activation of the IRE1 stress sensor as part of the unfolded protein response (UPR) and has been implicated in several diseases processes including RA. XBP1 can also be activated in direct response to Toll-like Receptor (TLR) ligation independently of the UPR but the pathogenic significance of this mode of XBP1 activation is not well understood. Our aim was to investigate the relevance of interactions between UPR and TLR signalling pathways in the serum and synovial fibroblasts (SFs) of patients with RA, using samples from healthy individuals and patients with osteoarthritis (OA) as controls
Materials and Methods Peripheral blood mononuclear cells (PBMC) were obtained from control groups: healthy individuals (n = 24) and RA patients, comprising active disease (n = 47) and remission (n = 12). SFs from RA and OA patients were isolated by digestion of synovial biopsies. Gene expression profiling was performed using qPCR for the detection of sXBP1 and enzyme-linked immunosorbent assays (ELISA) to quantify levels of pro-inflammatory cytokines, IL-6 and TNF. siRNA targeting of XBP1 was used for knockdown experiments in SFs.
Results We investigated the expression of ER stress response genes in patients with active RA and patients in remission. We show that TLR-dependent XBP1 activation is operative in the SFs of patients with active RA. The active (spliced) form of (s)XBP1 was significantly overexpressed in the active RA group compared to healthy controls and patients in remission (p = 0.01). Paradoxically, expression of nine other ER stress response genes was reduced in active RA compared to patients in remission, suggestive of a UPR-independent process. However, sXBP1 was induced in SFs by TLR4 and TLR2 stimulation, resulting in sXBP1-dependent IL-6 and TNF production. We also show that TNF itself induces sXBP1 in SFs, thus generating a potential feedback loop for sustained activation of these cells.
Conclusions sXBP1 plays a central role in two different cellular processes that may first appear unconnected. However, linking inflammatory pathways with ER stress provides SFs with a timely, coordinated and protective response. XBP1 activation may therefore be a suitable target in the treatment of RA, since it forms a cornerstone of two different molecular processes implicated in the pathogenesis of RA.