mTORC1 stimulates phosphatidylcholine synthesis to promote triglyceride secretion
William J. Quinn III, Min Wan, Swapnil V. Shewale, Rebecca Gelfer, Daniel J. Rader, Morris J. Birnbaum, Paul M. Titchenell
William J. Quinn III, Min Wan, Swapnil V. Shewale, Rebecca Gelfer, Daniel J. Rader, Morris J. Birnbaum, Paul M. Titchenell
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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 1

mTORC1 activity is both required and sufficient to induce hepatic steatosis.

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mTORC1 activity is both required and sufficient to induce hepatic steato...
Fasted 8- to 12-week-old C57BL/6 mice were injected with 20 mg/kg of rapamycin (A) liver TAG; n = 4. Veh., vehicle; Rapa, rapamycin. (B–G) 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. Cohorts were either fasted for 18 hours or fasted and refed for 4 hours. (B) Livers from mice of the indicated genotypes. (C) Immunoblot for indicated proteins. (D) Hepatic TAG was measured. (E) Total hepatic TAG normalized to liver. (F) Serum FFAs. (G) Serum ketones. n = 5–12 per group. *P < 0.05; **P < 0.01 vs. control. #P < 0.05 vs. fasted (when comparing refed vs. fasting). †P < 0.01 vs. fasted (when comparing refed vs. fasting). Two-way ANOVA.