Nicotinamide N-methyltransferase drives fibroblast activation and skin fibrosis in systemic sclerosis

Abstract Background In systemic sclerosis (SSc), an autoimmune response leads to progressive fibrosis of the skin and internal organs, driven by aberrant activation of fibroblasts. The mechanisms dictating persistent dermal fibroblast (DF) activation and production of extracellular matrix (ECM) remain poorly understood. Nicotinamide N-methyltransferase (NNMT), a SAM-consuming enzyme that modulates cellular methylation potential, has been implicated in fibrotic tissue remodelling in metabolic and malignant diseases. Here, we identify NNMT as a key determinant in DF activation and fibrosis in SSc. Methods We analyzed bulk, single-cell RNA-Seq and spatial transcriptomics datasets from SSc skin. Functional studies were performed in TGFβ-activated primary human DFs using siRNA-mediated NNMT knockdown (KD) combined with RNA-Seq, metabolite profiling, ELISA, and western blotting. The role of NNMT-regulated transcription factors was assessed by QuantSeq 3′ RNA-Seq following ATF4, SOX9, or SRF KD. Findings NNMT was markedly upregulated in SSc skin and enriched in disease-expanded SFRP2⁺/COL8A1⁺ myofibroblast states. NNMT KD restored methylation balance by increasing the SAM/SAH ratio and H3K27me3 levels, and abrogated TGFβ-induced profibrotic programs regulating ECM production and collagen synthesis. Mechanistically, NNMT was required for TGFβ-induced upregulation of the transcription factors ATF4, SOX9, and SRF, which together orchestrate ECM gene expression and COL1A1 secretion. Interpretation These findings define a previously unrecognized TGFβ–NNMT-ATF4/SOX9/SRF axis that coordinates profibrotic transcriptional programs in DFs. Accordingly, NNMT functions as a central effector linking TGFβ signaling to DF activation and ECM remodelling. Targeting NNMT may thus represent a promising therapeutic strategy to attenuate skin fibrosis in SSc.