👤 Fumihiko Hakuno

The liver is a central metabolic organ, and nutritional status, such as protein/amino acid malnutrition, significantly affects metabolic homeostasis. When animals are fed an amino acid-restricted diet, triglyceride-rich very low-density lipoprotein (VLDL) secretion is lowered, leading to fatty liver development. Therefore, we have explored the effects of amino acids on the expression of Apolipoprotein b (Apob), the main VLDL component, using a hepatoma cell culture model. When H4IIE rat hepatoma cells were cultured in an amino acid-depleted medium, Apob mRNA levels were significantly lower than those in control cells. In addition, when cells were cultured in media deprived of a single amino acid, aspartic or glutamic acid deprivation decreased Apob mRNA levels, whereas depletion of lysine, histidine, threonine, leucine, or isoleucine increased it. To understand the interrelationship between these extracellular amino acids and Apob transcription, metabolome analysis of these cells was performed. The intracellular methionine, adenine, and ornithine levels were positively correlated with Apob mRNA levels. Among them, only ornithine significantly enhanced Apob transcription, when added to the amino acid-depleted medium. In summary, these results suggest that ornithine plays a key role in Apob transcriptional regulation, corresponding to changes in extracellular amino acid concentrations. Show less

Dietary protein deficiency and amino acid imbalance cause hepatic fat accumulation. We previously demonstrated that only arginine deficiency or total amino acid deficiency in a diet caused significant hepatic triglyceride (TG) accumulation in young Wistar rats. In this study, we explored the mechanisms of fatty liver formation in these models. We fed 6-week-old male Wistar rats a control diet (containing an amino acid mixture equivalent to 15% protein), a low-total-amino acid diet (equivalent to 5% protein; 5PAA), and a low-arginine diet (only the arginine content is as low as that of the 5PAA diet) for 2 weeks. Much greater hepatic TG accumulation was observed in the low-arginine group than in the low-total-amino acid group. The lipid consumption rate and fatty acid uptake in the liver did not significantly differ between the groups. In contrast, the low-total-amino acid diet potentiated insulin sensitivity and related signaling in the liver and enhanced de novo lipogenesis. The low-arginine diet also inhibited hepatic very-low-density lipoprotein secretion without affecting hepatic insulin signaling and lipogenesis. Although the arginine content of the low-arginine diet was as low as that of the low-total-amino acid diet, the two diets caused fatty liver via completely different mechanisms. Enhanced lipogenesis was the primary cause of a low-protein diet-induced fatty liver, whereas lower very-low-density lipoprotein secretion caused low-arginine diet-induced fatty liver. Show less