Abstract
BACKGROUND: L-Phenylalanine (L-Phe) levels are elevated in patients with metabolic dysfunction-associated steatotic liver disease (MASLD). However, whether L-Phe induces liver steatosis and the underlying mechanism remain unknown. This study aimed to investigate the mechanism through which L-Phe promotes liver steatosis. METHODS: We utilized human data from the UK Biobank and SPECT-China studies. Plasma/serum samples were collected for metabolomic testing to measure L-Phe levels. A rat model with L-Phe in the drinking water was established to investigate changes in hepatic lipid metabolism. In addition, BNIP3 was overexpressed both in vitro and in vivo to validate the role of L-Phe in BNIP3-mediated mitophagy associated with liver steatosis. RESULTS: In both populations, elevated L-Phe quartiles were associated with increased body mass index, triglyceride, and transaminase levels and increased odds of MASLD (all p < 0.05). Rats exposed to L-Phe had increased hepatic lipid deposition and decreased mitophagy in the liver. Differentially expressed proteins were enriched in the PPARα and fatty acid β-oxidation signalling pathways, with downregulation of the mitophagy marker BNIP3. Mitophagy was activated by rapamycin and then inhibited by L-Phe, indicating that elevated L-Phe promoted lipid accumulation by suppressing mitophagy. BNIP3 overexpression effectively mitigated L-Phe-induced hepatic steatosis by restoring mitophagy. Moreover, L-Phe regulates the BNIP3-mediated PPARα and AMPK/mTOR signalling pathways to promote hepatic steatosis. CONCLUSIONS: Our study revealed the role of L-Phe in regulating lipid metabolism and promoting liver steatosis via BNIP3-mediated mitophagy. These findings provide novel insights into the link between L-Phe and liver steatosis, suggesting potential nutritional intervention strategies for preventing MASLD.