CMTM6 promotes synovial proliferation and macrophage polarization by preventing ubiquitination of TAK1 in rheumatoid arthritis.

OBJECTIVE: Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by synovial inflammation and bone destruction. Abnormal TNF/TNFR signaling leads to various dysregulations in Fibroblast-like synoviocytes (FLSs), including proliferation, migration, and cytokine production. CKLF like MARVEL transmembrane domain containing 6 (CMTM6) plays a crucial role in tumor progression and immune escape. This study aimed to research the function of CMTM6 in fibroblast-like synoviocytes (FLSs) of rheumatoid arthritis (RA). MATERIALS AND METHODS: The level of CMTM6 in serum and synovial fluid was detected by enzyme-linked immunosorbent assays (ELISA). CMTM6 was silenced by transfecting FLSs with small interfering RNAs (siRNAs). Proliferation, migration, and apoptosis were assessed using a Cell counting kit-8 (CCK-8), Matrigel invasion assays and flow cytometry respectively. The levels of interleukin-1β (IL-1β) and interleukin-6 (IL-6) of cell culture supernatant were detectd with ELISA. The protein expression level of TNF/TNFR pathway and its downstream NF-κB/MAPK signaling were detected through RNA-sequencing and determined by western blotting (WB). Co-Immunoprecipitation, immunofluorescence and pymol analysis were used to confirm the interaction between CMTM6 and TGF-β-activated kinase 1 (TAK1). We used ubiquitination assay and Cycloheximide (CHX) assay to detect degradation of TAK1. The collagen II-induced arthritis (CIA) mouse model were established in DBA/1 mice and treated with Cmtm6 shRNA (AAV-shCmtm6) or rAAV vector (AAV-Ctrl). Micro-CT and histological analyses evaluated the severity of arthritis. RESULTS: Increased levels of CMTM6 in the serum, synovial fluid and synovial tissues were observed in RA patients. Silencing CMTM6 suppressed proliferation, migration and inflammatory cytokines secretion and promote the apoptosis of FLSs. Mechanistically, CMTM6 maintains the stability of TAK1 by inhibiting its ubiquitin-proteasome degradation, leading to the activation of TNF/TNFR pathway and its downstream NF-κB/MAPK signaling. In addition, CMTM6 promoted the M1-macrophage polarization by producing more pro-inflammation cytokines in the micro-environment of joints. AAV-shCmtm6 treatment attenuated the severity of joint damage of CIA mice and reduced the Cmtm6/Tak1 expression similarly to TNF-α antibody. CONCLUSIONS: CMTM6 participates in the proliferation, migration, and apoptosis of FLSs and exacerbates joint inflammation. Mechanistically, CMTM6 activates the NF-κB/MAPK signaling pathway by protecting TAK1 from ubiquitination degradation. THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE: This study shows that CMTM6 mediates inflammatory signatures such as proliferation and migration of FLSs through TAK1. Targeting CMTM6 may become a potential therapeutic target for RA.