Fibroblast-like synoviocytes (FLSs) are critical to synovial aggression and joint destruction in rheumatoid arthritis (RA). The role of long noncoding RNAs (lncRNAs) in RA is largely unknown. Here, we identified a lncRNA, LERFS (lowly expressed in rheumatoid fibroblast-like synoviocytes), that negatively regulates the migration, invasion, and proliferation of FLSs through interaction with heterogeneous nuclear ribonucleoprotein Q (hnRNP Q). Under healthy conditions, by binding to the mRNA of RhoA, Rac1, and CDC42 — the small GTPase proteins that control the motility and proliferation of FLSs — the LERFS–hnRNP Q complex decreased the stability or translation of target mRNAs and downregulated their protein levels. But in RA FLSs, decreased LERFS levels induced a reduction of the LERFS–hnRNP Q complex, which reduced the binding of hnRNP Q to target mRNA and therefore increased the stability or translation of target mRNA. These findings suggest that a decrease in synovial LERFS may contribute to synovial aggression and joint destruction in RA and that targeting the lncRNA LERFS may have therapeutic potential in patients with RA.

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.