mTORC1 stimulates phosphatidylcholine synthesis to promote triglyceride secretion
William J. Quinn III, … , Morris J. Birnbaum, Paul M. Titchenell
William J. Quinn III, … , Morris J. Birnbaum, Paul M. Titchenell
Published October 16, 2017
Citation Information: J Clin Invest. 2017;127(11):4207-4215. https://doi.org/10.1172/JCI96036.
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Research Article Metabolism

mTORC1 stimulates phosphatidylcholine synthesis to promote triglyceride secretion

Abstract

Liver triacylglycerol (TAG) synthesis and secretion are closely linked to nutrient availability. After a meal, hepatic TAG formation from fatty acids is decreased, largely due to a reduction in circulating free fatty acids (FFA). Despite the postprandial decrease in FFA-driven esterification and oxidation, VLDL-TAG secretion is maintained to support peripheral lipid delivery and metabolism. The regulatory mechanisms underlying the postprandial control of VLDL-TAG secretion remain unclear. Here, we demonstrated that the mTOR complex 1 (mTORC1) is essential for this sustained VLDL-TAG secretion and lipid homeostasis. In murine models, the absence of hepatic mTORC1 reduced circulating TAG, despite hepatosteatosis, while activation of mTORC1 depleted liver TAG stores. Additionally, mTORC1 promoted TAG secretion by regulating phosphocholine cytidylyltransferase α (CCTα), the rate-limiting enzyme involved in the synthesis of phosphatidylcholine (PC). Increasing PC synthesis in mice lacking mTORC1 rescued hepatosteatosis and restored TAG secretion. These data identify mTORC1 as a major regulator of phospholipid biosynthesis and subsequent VLDL-TAG secretion, leading to increased postprandial TAG secretion.

Authors

William J. Quinn III, Min Wan, Swapnil V. Shewale, Rebecca Gelfer, Daniel J. Rader, Morris J. Birnbaum, Paul M. Titchenell

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Figure 6

Restoration of PC synthesis in the absence of mTORC1 is sufficient to regulate TAG secretion in vivo.

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Restoration of PC synthesis in the absence of mTORC1 is sufficient to re...
Six- to ten-week-old Raptorfl/fl animals were injected with either AAV-GFP (control, black) or AAV-CRE (L-Raptor–KO, white) and injected daily with saline (solid) or 150 mg/kg CDP-choline for 2 weeks. (A) Triglyceride secretion rates were determined in fasted animals by blocking triglyceride uptake via i.p. injection of poloxamer 407 and measuring the accumulation of triglyceride in the serum over time. n = 9–10. (B) Fasting serum triglyceride levels were measured. n = 9–10. (C) Fasting hepatic triglyceride levels were measured. n = 3. Isolated hepatocytes from treatment groups were cultured with 3H-glycerol ± CDP-choline (150 mg/kg) (D–G). (D) Secreted TAG was measured in the medium. n = 3. (E) Intracellular TAG was measured. n = 3. (F) Newly made secreted PC was measured in the medium. n = 3. (G) Intracellular newly made PC was measured. n = 3. For hepatocyte studies, hepatocytes from 3 mice were isolated and technical replicates pooled. Data represent 3 individual mice per condition. *P < 0.05; **P < 0.01 vs. control using 1-way ANOVA. †P < 0.01 using 1-way ANOVA.