The Effect of L-Ornithine on the Phosphorylation of mTORC1 Downstream Targets in Rat Liver

Abstract

A non-protein amino acid, L-ornithine (Orn), has been shown to stimulate the urea cycle and tissue protein synthesis in the liver. The purpose of the current study was to assess whether Orn affects the mammalian target of rapamycin (mTOR) complex 1 (mTORC1) pathway, which is involved in protein synthesis. Primary cultured cells isolated from Wistar rat liver were incubated in an amino acid-free medium, followed by addition of Orn for 3 h. The cell lysate was subjected to immunoblotting to evaluate the phosphorylation of downstream targets of mTORC1, including p70S6K, S6, and 4EBP1. To assess the involvement of mTORC1 for the effect of Orn, the cells were pretreated with the mTOR inhibitor rapamycin before the addition of Orn and the cell lysate was subjected to immunoblotting. We next examined whether the effects of Orn were exerted in vivo. Orn was orally administered to 18 h food-deprived rats, the blood and the livers were collected at 1 and 3 h after administration for immunoblotting. Orn treatment for primary cultured cells for 3 h enhanced the phosphorylation of p70S6K, S6, and 4EBP1. In addition, rapamycin blocked the effects of Orn completely (p70S6K and S6) or partially (4EBP1). The oral administration of Orn to the rat also augmented the phosphorylation of mTORC1 downstream targets notably in S6 at 1 h. Our findings demonstrate that Orn has the potential to induce the phosphorylation of downstream targets of mTORC1 in the rat liver. This may be mediated by the augmentation of mTORC1 activity.

A novel amino acid signaling process governs glucose-6-phosphatase transcription

“…mTORC1 is the primary representative of a signal-mediating molecule related to nutrient-sensing. Leucine, arginine, and methionine (or S-adenosylmethionine) have been demonstrated to stimulate (Wolfson et al, 2015;Chantranupong et al, 2016;Gu et al, 2017), and another report has shown that ornithine also activates mTORC1 (Kokubo et al, 2015). Thus, we initially suspected that mTORC1 is also involved in the amino-acid-dependent regulation of G6Pase.…”

“…Similar to leucine, ornithine also activates mTORC1 (Kokubo et al, 2015). We, therefore, treated cells with torin 1 again and evaluated the involvement of mTORC1 in the ornithine-mediated G6Pase transcription regulation.…”