Soft fibrin matrix instructs macrophage M2 polarization via FABP4-mediated enhancement of fatty acid β-oxidation to alleviate MASH

BACKGROUND: Macrophage-driven inflammation is a key pathogenic factor in metabolic dysfunction-associated steatohepatitis (MASH). While fibrin deposition is observed in MASH livers, its role in regulating macrophage polarization and disease progression remains unclear. METHODS: We investigated fibrin deposition in liver tissues from MASH patients and high-fat diet (HFD)-induced MASH mice. Macrophages were cultured in soft or stiff 3D fibrin matrices to assess polarization phenotypes. In vivo, macrophages pre-cultured in soft or stiff fibrin were adoptively transferred into HFD-fed mice. Mechanistic studies involved siRNA-mediated knockdown, overexpression, chromatin immunoprecipitation, co-immunoprecipitation, and metabolomic analyses. Macrophage-specific FABP4 knockdown was achieved using an AAV8 vector expressing F4/80-driven FABP4 shRNA and eGFP. RESULTS: Fibrin deposition was significantly elevated in MASH livers and correlated with pro-inflammatory macrophage infiltration. Macrophages cultured in soft fibrin matrices exhibited enhanced anti-inflammatory (M2) polarization, characterized by increased Arg1 and CD206 and decreased iNOS and TNFα expression. Adoptive transfer of soft fibrin-cultured macrophages alleviated hepatic steatosis, insulin resistance, and oxidative stress in MASH mice. Mechanistically, soft fibrin downregulated FABP4 expression by inhibiting ITGB2/STAT1 signaling. FABP4 competitively bound to PPARα, thereby sterically hindering the access of USP53, which functions as a deubiquitinase, to PPARα. This displacement prevented USP53-mediated deubiquitination, resulting in increased PPARα ubiquitination and proteasomal degradation, ultimately leading to suppression of fatty acid β-oxidation. Macrophage-specific FABP4 knockdown mirrored the therapeutic effects of soft fibrin, ameliorating MASH pathology. CONCLUSIONS: Our findings reveal that soft fibrin matrices promote anti-inflammatory macrophage polarization and mitigate MASH by enhancing fatty acid β-oxidation via downregulation of FABP4 and stabilization of PPARα. Targeting the Fibrin-FABP4-PPARα axis may offer a novel therapeutic strategy for MASH.