H3K18la-driven methyltransferase 1-mediated upregulation of NeuroD1 m7G modification in promoting ferroptosis resistance in rheumatoid arthritis synovial fibroblasts

Activated rheumatoid arthritis synovial fibroblasts (RASFs) exhibit hyperplasia that significantly contributes to rheumatoid arthritis (RA)-related joint destruction. Lactate, previously considered merely a byproduct of glycolysis, is now recognized as a critical regulator in the progression of RA from inflammation to bone degradation. Histone lactylation, an epigenetic modification dependent on lactate, serves as a link between glycolytic metabolism and chromatin remodeling. However, the role of lactate-dependent histone lactylation in the hyperplasia of RASFs remains insufficiently understood. Our findings revealed that elevated lactate levels in the synovial fluid of RA patients enhanced the proliferation of RASFs. Moreover, stimulation with extracellular lactate significantly increased the expression of histone H3 lysine 18 lactylation (H3K18la), while the inhibition of H3K18la through P300 silencing reduced the lactate-induced proliferation in RASFs. Furthermore, the Chromatin Immunoprecipitation-qPCR assay revealed that methyltransferase 1 (METTL1) was a downstream target gene of H3K18la, and silencing of METTL1 counteracted the lactate-induced suppression of ferroptosis and further diminished the proliferation in RASFs. Mechanistically, METTL1 facilitated m7G modification on NeuroD1 mRNA, thereby enhancing its stability. Subsequently, NeuroD1 bound to the glutathione peroxidase 4 (GPX4) promoter to augment its transcriptional activity, thereby inhibiting lipid peroxidation in a manner that conferred ferroptosis resistance. Additionally, intra-articular delivery of lentivirus encoding shMETTL1 or shNeuroD1 markedly alleviated arthritis severity and inhibited the hyperplasia of synovial fibroblasts in a collagen-induced arthritis (CIA) mouse model. Our findings demonstrate that lactate-induced H3K18la promotes ferroptosis resistance to facilitate the hyperplasia of RASFs through the activation of the METTL1/NeuroD1/GPX4 signaling pathway in RA.