PHGDH Promotes Synovial Aggression and Inflammation via Upregulating ADRA2A Expression in Rheumatoid Arthritis

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.