Aims: To explore the synovial expression of mucin 1 (MUC1) and its role in rheumatoid arthritis (RA), as well as the possible downstream mechanisms. Methods: Patients with qualified synovium samples were recruited from a RA cohort. Synovium from patients diagnosed as non-inflammatory orthopaedic arthropathies was obtained as control. The expression and localization of MUC1 in synovium and fibroblast-like synoviocytes were assessed by immunohistochemistry and immunofluorescence. Small interfering RNA and MUC1 inhibitor GO-203 were adopted for inhibition of MUC1. Lysophosphatidic acid (LPA) was used as an activator of Rho-associated pathway. Expression of inflammatory cytokines, cell migration, and invasion were evaluated using quantitative real-time polymerase chain reaction (PCR) and Transwell chamber assay. Results: A total of 63 RA patients and ten controls were included. Expression of MUC1 was observed in both the synovial lining and sublining layer. The percentage of MUC1+ cells in the lining layer of synovium was significantly higher in RA than that in control, and positively correlated to joint destruction scores of RA. Meanwhile, MUC1+ cells in the sublining layer were positively correlated to the Krenn subscore of inflammatory infiltration. Knockdown of MUC1, rather than GO-203 treatment, ameliorated the expression of proinflammatory cytokines, cell migration, and invasion of rheumatoid synoviocytes. Knockdown of MUC1 decreased expression of RhoA, Cdc42, and Rac1. Treatment with LPA compromised the inhibition of migration and invasion, but not inflammation, of synoviocytes by MUC1 knockdown. Conclusion: Upregulated MUC1 promotes the aggression of rheumatoid synoviocytes via Rho guanosine triphosphatases (GTPases), thereby facilitating synovitis and joint destruction during the pathological process of RA. Cite this article: Bone Joint Res 2022;11(9):639–651.

Objectives: The role of Phosphoglycerate Dehydrogenase (PHGDH), the first key enzyme in the serine biosynthesis pathway, is important in controlling cancer survival; however, its role in rheumatoid arthritis (RA) remains unknown. Here, we investigated the functional involvement of PHGDH in RA pathogenesis, as well as its underlying molecular mechanisms. Methods: mRNA and protein expression in RA fibroblast-like synoviocytes (FLS) was measured by RT-qPCR and Western blot, respectively. Immunohistochemistry (IHC) was used to detect the protein expression in RA synovium. Cellular and tissue localization of the protein was assessed using IHC and immunofluorescence. The functional role of PHGDH in RA FLS was evaluated using multiple approaches: Transwell assays to assess cell migration and invasion, Annexin V/PI staining to detect apoptosis, and EdU assays to measure cell proliferation. Key downstream targets of PHGDH were identified via RNA sequencing (RNA-seq). The therapeutic potential of PHGDH targeting was further assessed in a rat collagen-induced arthritis (CIA) model following intra-articular administration of PHGDH-shRNA. Results: PHGDH expression was markedly elevated in RA synovial tissues and FLS. Functionally, knockdown of PHGDH suppressed proliferation, migration, invasion, and inflammatory cytokine production of RA FLS. Mechanistic studies revealed that PHGDH exerts its effects, at least in part, by inhibiting the expression of adrenoceptor alpha 2A (ADRA2A). The therapeutic relevance of these findings was further supported by in vivo experiments, where intra-articular delivery of PHGDH-shRNA significantly ameliorated the severity of arthritis in rats with CIA. Conclusions: Our results indicate that the PHGDH-ADRA2A axis critically drives the inflammatory and aggressive phenotype of RA FLS, suggesting that PHGDH might be as a promising therapeutic target for RA.