5-aminosalicylic acid suppresses osteoarthritis through the OSCAR-PPARγ axis.

Osteoarthritis (OA) is a progressive and irreversible degenerative joint disease that is characterized by cartilage destruction, osteophyte formation, subchondral bone remodeling, and synovitis. Despite affecting millions of patients, effective and safe disease-modifying osteoarthritis drugs are lacking. Here we reveal an unexpected role for the small molecule 5-aminosalicylic acid (5-ASA), which is used as an anti-inflammatory drug in ulcerative colitis. We show that 5-ASA competes with extracellular-matrix collagen-II to bind to osteoclast-associated receptor (OSCAR) on chondrocytes. Intra-articular 5-ASA injections ameliorate OA generated by surgery-induced medial-meniscus destabilization in male mice. Significantly, this effect is also observed when 5-ASA was administered well after OA onset. Moreover, mice with DMM-induced OA that are treated with 5-ASA at weeks 8-11 and sacrificed at week 12 have thicker cartilage than untreated mice that were sacrificed at week 8. Mechanistically, 5-ASA reverses OSCAR-mediated transcriptional repression of PPARγ in articular chondrocytes, thereby suppressing COX-2-related inflammation. It also improves chondrogenesis, strongly downregulates ECM catabolism, and promotes ECM anabolism. Our results suggest that 5-ASA could serve as a DMOAD.

BCAT1 promotes osteoclast maturation by regulating branched-chain amino acid metabolism.

Branched-chain aminotransferase 1 (BCAT1) transfers the amine group on branched-chain amino acids (BCAAs) to alpha-ketoglutarate. This generates glutamate along with alpha-keto acids that are eventually oxidized to provide the cell with energy. BCAT1 thus plays a critical role in sustaining BCAA concentrations and availability as an energy source. Osteoclasts have high metabolic needs during differentiation. When we assessed the levels of amino acids in bone marrow macrophages (BMMs) that were undergoing receptor activator of nuclear factor κB ligand (RANKL)-induced osteoclast differentiation, we found that the BCAA levels steadily increase during this process. In vitro analyses then showed that all three BCAAs but especially valine were needed for osteoclast maturation. Moreover, selective inhibition of BCAT1 with gabapentin significantly reduced osteoclast maturation. Expression of enzymatically dead BCAT1 also abrogated osteoclast maturation. Importantly, gabapentin inhibited lipopolysaccharide (LPS)-induced bone loss of calvaria in mice. These findings suggest that BCAT1 could serve as a therapeutic target that dampens osteoclast formation.

Tetraspanin 7 regulates osteoclast function through association with the RANK/αvß3 integrin complex.

Actin rings are unique structures that facilitate the attachment of osteoclasts to the bone matrix during bone resorption. Previous studies have shown that tetraspanin7 (TSPAN7) plays an important role in the reorganization of the cytoskeleton necessary for the bone-resorbing activity of osteoclasts. However, questions remain as to the mechanisms by which TSPAN7 regulates this cytoskeletal rearrangement. In this study, we investigated the roles of TSPAN7 in osteoclasts by deleting the Tm4sf2 gene in mice, which encodes TSPAN7. The Tm4sf2 global knockout model showed protective effects on pathological bone loss, but no discernible changes in bone phenotypes under physiological conditions. In vitro study revealed that ablation of Tm4sf2 caused significant defects in integrin-mediated actin ring formation, thereby leading to significantly decreased bone resorption. Additionally, we demonstrated an association between TSPAN7 and the receptor activator of nuclear factor-кB/αvß3 integrin. Overall, our findings suggest that TSPAN7 acts as a novel modulator regulating the bone-resorbing function of osteoclasts.

Early estrogen-induced gene 1 facilitates osteoclast formation through the inhibition of interferon regulatory factor 8 expression.

Osteoclast-mediated inflammatory bone resorption is a major cause of many inflammatory bone disorders, including rheumatoid arthritis and periodontitis. However, the mechanisms regulating osteoclast differentiation in inflammatory settings are not well understood. We demonstrate here that early estrogen-induced gene 1 (EEIG1)-deficient mice are protected from inflammatory bone loss as determined with the use of models of lipopolysaccharide (LPS)-induced bone destruction. EEIG1-deficient macrophages markedly decreased RANKL- and TNFα-mediated osteoclastogenesis due to the downregulation of the nuclear factor of activated T cells, cytoplasmic 1 (NFATc1), which is an essential transcription factor for osteoclast formation. In contrast, expression of interferon regulatory factor 8 (IRF8), a transcriptional repressor that blocks osteoclast differentiation, is elevated in EEIG1-deficient macrophages relative to wild-type cells. We found that reduced expression of B lymphocyte-induced maturation protein-1 (Blimp1) by siRNA downregulated RANKL-induced EEIG1 levels, whereas overexpression of Blimp1 potentiated EEIG1 levels. Mechanistic studies revealed that EEIG1 forms a complex with Blimp1 to negatively regulate the expression of the anti-osteoclastogenic gene, Irf8. We elucidated a novel mechanism by which EEIG1 restricts IRF8 expression and function, thereby enhancing the osteoclast formation by contributing to Blimp1-mediated IRF8 regulation. Together, these findings identify EEIG1 as a key regulator of osteoclastogenesis and a possible therapeutic target for pathological bone destruction.