Tacrolimus regulates endoplasmic reticulum stress–mediated osteoclastogenesis and inflammation: In vitro and collagen‐induced arthritis mouse model

Abstract Tacrolimus is an immunosuppressive drug that inhibits the release of inflammatory cytokines involved in rheumatoid arthritis development by blocking T cell activation. “Endoplasmic reticulum stress,” an imbalance between protein folding load and capacity leading to the accumulation of unfolded proteins in the endoplasmic reticulum lumen, has been implicated in rheumatoid arthritis and other inflammatory and metabolic diseases. We aimed to investigate the effect of tacrolimus on endoplasmic reticulum stress–mediated osteoclastogenesis and inflammation and elucidate the underlying mechanisms. In vitro studies were performed using mouse bone marrow cells that were cultured with or without interleukin‐1β, thapsigargin, or tacrolimus to induce osteoclast differentiation. A mouse model of arthritis was established by immunizing mice with bovine type II collagen. Tacrolimus was orally administered to mice from day 20 to 45 following the initial immunization, and histopathological changes and expression of specific biomarkers of endoplasmic reticulum stress–mediated inflammatory signaling pathways were examined. In vitro, tacrolimus inhibited receptor activator of nuclear factor‐κB ligand‐mediated osteoclast formation augmented by interleukin‐1β, thapsigargin, or both. Furthermore, tacrolimus inhibited glucose‐regulated protein (GRP78), protein kinase R‐like endoplasmic reticulum kinase, inositol‐requiring enzyme 1 (IRE 1), and activating transcription factor 6 (ATF6) augmented by interleukin‐1β, thapsigargin, or both. Tacrolimus significantly ameliorated osteolysis and endoplasmic reticulum stress intensity in mice. Simultaneously, it reduced inflammatory cell infiltration, osteoclastogenesis, and inflammatory responses by inhibiting GRP78, IRE 1, and ATF6. These findings suggest that tacrolimus exhibits an anti‐inflammation effect in rheumatoid arthritis and might inhibit joint damage progression by inhibiting endoplasmic reticulum stress.