Hepatic de novo lipogenesis (DNL) is increased by both carbohydrate intake and protein consumption. In hepatic fat synthesis, a key role is played by the induction of the hepatic expression of lipogenic genes, including Fasn, Scd1, and Srebf1. Regarding carbohydrate intake, increased blood glucose and insulin levels promote the expression of hepatic lipogenic genes. However, although amino acids serve as a carbon source for hepatic DNL during protein consumption, their effects on hepatic lipogenic gene expression remain unclear. We investigated the effects of amino acids on hepatic lipogenic gene induction using primary cultured mouse hepatocytes and hepatic Fasn reporter (l-FasnGLuc) mice. In primary cultured hepatocytes, lipogenic gene expression (Fasn, Scd1, Srebf1) was induced under postprandial-mimicking conditions (treatment with insulin and LXR agonist). When hepatocytes were stimulated with an amino acid mixture containing 20 amino acids, the induction of lipogenic gene expression was enhanced, but this effect disappeared when proline was removed from the mixture. Furthermore, when each amino acid was tested individually, only proline potentiated the induction of lipogenic gene expression in hepatocytes under postprandial-mimicking conditions. In mouse liver, continuous proline infusion via osmotic pump increased Fasn gene expression and showed a trend toward increased Srebf1 expression. In l-FasnGLuc mice, continuous proline infusion resulted in sustained enhancement of hepatic Fasn transcription, measured by secreted luciferase activity. These results demonstrate that proline enhances the induction of hepatic lipogenic gene expression both in vitro and in vivo.
Keywords: Amino acid; Fatty acid synthase; Insulin; Liver X receptor; Proline; SREBP1; de novo lipogenesis.
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