Toll-like receptor-mediated IRE1α activation as a therapeutic target for inflammatory arthritis

Abstract

In rheumatoid arthritis (RA), macrophage is one of the major sources of inflammatory mediators. Macrophages produce inflammatory cytokines through toll‐like receptor (TLR)‐mediated signalling during RA. Herein, we studied macrophages from the synovial fluid of RA patients and observed a significant increase in activation of inositol‐requiring enzyme 1α (IRE1α), a primary unfolded protein response (UPR) transducer. Myeloid‐specific deletion of the IRE1α gene protected mice from inflammatory arthritis, and treatment with the IRE1α‐specific inhibitor 4U8C attenuated joint inflammation in mice. IRE1α was required for optimal production of pro‐inflammatory cytokines as evidenced by impaired TLR‐induced cytokine production in IRE1α‐null macrophages and neutrophils. Further analyses demonstrated that tumour necrosis factor (TNF) receptor‐associated factor 6 (TRAF6) plays a key role in TLR‐mediated IRE1α activation by catalysing IRE1α ubiquitination and blocking the recruitment of protein phosphatase 2A (PP2A), a phosphatase that inhibits IRE1α phosphorylation. In summary, we discovered a novel regulatory axis through TRAF6‐mediated IRE1α ubiquitination in regulating TLR‐induced IRE1α activation in pro‐inflammatory cytokine production, and demonstrated that IRE1α is a potential therapeutic target for inflammatory arthritis.

Targeting IRE1 with small molecules counteracts progression of atherosclerosis

“…4C). Analogous experiments using 4μ8c in the same animal model [using previously published doses that showed no toxicity (33,37,40,48)] produced similar results (Fig. 4 D-F): 4μ8c treatment led to a significant reduction (45.2%; P < 0.001) in atherosclerotic lesion area in en face aorta preparations (Fig.…”

“…Consistent with these observations, treatment with IRE1 inhibitors led to a marked suppression of hyperlipidemia-induced Th-1 immune responses in these mice. We observed no differences in T-cell numbers in adventia/lesions between the IRE1 inhibitor treatment and control groups, and previous studies have shown the IRE1 inhibitor used in this study does not affect Treg cells (33,37). Collectively, these findings show that prevention of inflammasome-associated cytokine production by IRE1 inhibitors in vivo has dramatic effects on counteracting atherosclerotic disease progression.…”

“…The action of both compounds is wellunderstood mechanistically. Both form a Schiff base with a specific lysine positioned in the active site of the IRE1 RNase domain, blocking its function, and both show no overt toxicity when administered systemically (21,32,33). Thus, we reasoned that these drugs might have therapeutic applicability in atherosclerosis.…”

“…Third, experimentally sustained Xbp1 mRNA splicing in the vessel wall promotes atherosclerosis, whereas its ablation ameliorates hypercholesterolemia in obese or apolipoprotein E-deficient (ApoE −/− ) mice (10,30). Two small molecules, STF-083010 and 4μ8C, which selectively inhibit IRE1's RNase function, have been used in cells and animals to produce favorable therapeutic outcomes in other disease settings: STF-083010 reduced growth of multiple myeloma (21,24,31,32), and 4μ8c suppressed inflammation in a murine arthritis model (33). The action of both compounds is wellunderstood mechanistically.…”

Protein misfolding and dysregulated protein homeostasis in autoinflammatory diseases and beyond

“…There have been a number of interesting connections identified in RA between aberrant UPR and increased inflammatory responses. As in TRAPS, synovial macrophages, synovial fibroblasts and PBMCs from RA patients were found to have increased Xbp-1 splicing but not increased expression of classical UPR response genes, leading to increased production of pro-inflammatory cytokines, IL-6 and TNF [74,75]. Endogenous TLR ligands which have been found in the joint, such as SNAPIN, may act to induce this aberrant Xbp-1 splicing and sustain inflammatory responses [76].…”