Effects of Sapropterin against Non-Alcoholic Fatty Liver Disease in mice

Tetrahydrobiopterin (BH₄) maintains nitric oxide synthase (NOS) coupling. However, its therapeutic potential in non-alcoholic fatty liver disease (NAFLD) remains unexplored. Male C57BL/6J mice were subjected to a high-fat diet (HFD) for 16 weeks to establish NAFLD, with oral administration of sapropterin (10 mg/kg/day) initiated at week 5. Sapropterin treatment significantly elevated both systemic and hepatic BH₄ concentrations and ameliorated HFD-induced dyslipidemia, as evidenced by reductions in circulating total cholesterol, triglycerides, and low-density lipoprotein cholesterol. Hepatocellular injury markers (ALT, AST) were also markedly decreased. Histopathological examination revealed substantial improvements in hepatic steatosis, ballooning degeneration, and macrovesicular lipid accumulation, with the NAFLD activity score reduced by 63.1% and hepatic lipid content by over 60%. Molecular analyses demonstrated that sapropterin suppressed lipogenic gene expression (Fasn, Cd36) and enhanced transcription of Ppara, promoting lipid catabolism. Concurrently, hepatic fibrotic burden was significantly reduced, accompanied by downregulation of Tgfb1 and diminished TGF-β protein expression. Anti-inflammatory effects were evidenced by decreased hepatic IL-6 and IL-1β levels and a 131.5% increase in IL-10. Additionally, sapropterin facilitated macrophage polarization toward an anti-inflammatory M2 phenotype (Cd206, Arg1), while suppressing pro-inflammatory M1 markers (Cd86). These findings indicated that sapropterin improvedNAFLD by regulatinglipid metabolism, inflammation, and fibrosis.