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 3

mTORC1 cell autonomously regulates TAG secretion.

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mTORC1 cell autonomously regulates TAG secretion. Six- to ten-week-old R...
Six- to ten-week-old Raptorfl/fl and Tsc1fl/fl animals were injected with either AAV-GFP (control, black) or AAV-CRE (L-Raptor–KO, white; L-TSC–KO, gray) for 2 weeks prior to sacrifice. (A) Serum TAG levels. n = 5–9. (B) Serum was subjected to FPLC analysis, and triglyceride content was measured in each of the eluted fractions. (C) 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 = 4–6 per group. (D–F) Six- to ten-week-old Raptorfl/fl animals were injected with AAV-GFP (black) or AAV-Cre (white) and rested 2 weeks to allow for gene excision. Hepatocytes were isolated and metabolically labeled for 4 hours in culture with 3H-glycerol. Medium and cellular fractions were separated. Samples were then fractionated by TLC and compared with lipid standards. (D) Intracellular TAG. (E) Secreted TAG. n = 6. (F) Intracellular DAG. n = 6. C57BL/6 hepatocytes were isolated and metabolically labeled with 3H-glycerol in the presence (white) or absence (black) of rapamycin (10 ng/ml) for 4 hours. Medium and cellular fractions were separated. Samples were then fractionated by TLC and compared with lipid standards. (G) Intracellular DAG. n = 3. (H) Intracellular TAG. n = 3. (I) Secreted TAG. n = 3. For hepatocyte studies in L-Raptor–KO or control, hepatocytes from 4 to 9 mice per group were isolated and technical replicates pooled. Data represent 4 to 9 individual mice per group. For hepatocyte studies using rapamycin, hepatocytes from 3 mice per group were isolated and technical replicates pooled. Data represent 3 individual mice per condition. *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001 vs. control condition using 2-way ANOVA (A and C) or Student’s t test (D–I).