The role of tryptophan hydroxylase-1 in metabolic dysfunction-associated steatotic liver disease.

IMPORTANCE: Metabolic dysfunction-associated steatotic liver disease (MASLD) is caused by dysregulated lipid metabolism, inflammation, and mitochondrial dysfunction. Given the rising burden of MASLD in children and adolescents, identifying experimentally tractable mechanisms relevant to pediatric diseases is of considerable interest. Tryptophan hydroxylase-1 (TPH1), the rate-limiting enzyme for peripheral serotonin synthesis, has been implicated in metabolic disorders, but its hepatic role in MASLD remains unclear. OBJECTIVE: To investigate whether Tph1 suppression alleviates lipid accumulation, mitochondrial dysfunction, and hepatocyte injury through changes associated with Parkin-mediated mitophagy. METHODS: MASLD was induced in young adult male C57BL/6J mice by a 12-week high-fat diet (HFD). At HFD onset, mice received a single tail-vein injection of liver-tropic AAV8-shTph1 or the control AAV8. The HFD feeding was continued for 12 weeks until the end of the experiment. Hepatic histology, serum biochemistry, oxidative stress, inflammatory responses, apoptosis, and mitochondrial function were also evaluated. Mechanistic studies were conducted in palmitic acid/oleic acid-treated AML12 hepatocytes using siRNA targeting Tph1 and Parkin. RESULTS: Hepatic TPH1 expression was elevated in MASLD mice. Tph1 knockdown alleviated diet-induced steatosis, reduced serum triglycerides, total cholesterol, low-density lipoprotein cholesterol, and transaminase levels, suppressed oxidative stress, and attenuated inflammatory and fibrotic markers. Tph1 silencing decreased hepatocyte apoptosis and restored mitochondrial function. Transmission electron microscopy and fractionated Western blotting analyses revealed increased Parkin-associated mitophagy. In AML12 cells, siTph1 reduced lipid accumulation and apoptosis, whereas co-silencing Parkin partially reversed these effects. INTERPRETATION: Hepatic TPH1 is associated with MASLD progression and impaired Parkin-mediated mitophagy. Tph1 suppression may represent a potential therapeutic strategy for experimental MASLD, with possible translational relevance to pediatric MASLD, though direct clinical validation is still needed.