l-Tryptophan-mediated Enhancement of Susceptibility to Nonalcoholic Fatty Liver Disease Is Dependent on the Mammalian Target of Rapamycin

Nonalcoholic fatty liver disease is one of the most common liver diseases. l-Tryptophan and its metabolite serotonin are involved in hepatic lipid metabolism and inflammation. However, it is unclear whether l-tryptophan promotes hepatic steatosis. To explore this issue, we examined the role of l-tryptophan in mouse hepatic steatosis by using a high fat and high fructose diet (HFHFD) model. l-Tryptophan treatment in combination with an HFHFD exacerbated hepatic steatosis, expression of HNE-modified proteins, hydroxyproline content, and serum alanine aminotransaminase levels, whereas l-tryptophan alone did not result in these effects. We also found that l-tryptophan treatment increases serum serotonin levels. The introduction of adenoviral aromatic amino acid decarboxylase, which stimulates the serotonin synthesis from l-tryptophan, aggravated hepatic steatosis induced by the HFHFD. The fatty acid-induced accumulation of lipid was further increased by serotonin treatment in cultured hepatocytes. These results suggest that l-tryptophan increases the sensitivity to hepatic steatosis through serotonin production. Furthermore, l-tryptophan treatment, adenoviral AADC introduction, and serotonin treatment induced phosphorylation of the mammalian target of rapamycin (mTOR), and a potent mTOR inhibitor rapamycin attenuated hepatocyte lipid accumulation induced by fatty acid with serotonin. These results suggest the importance of mTOR activation for the exacerbation of hepatic steatosis. In conclusion, l-tryptophan exacerbates hepatic steatosis induced by HFHFD through serotonin-mediated activation of mTOR.