Department of Nutrition, University of North Carolina, Chapel Hill, NC 27599, USA.
Proc Natl Acad Sci U S A. 2012 Jan 31;109(5):1667-72. doi: 10.1073/pnas.1110730109. Epub 2012 Jan 17.
Increased flux through the glycerolipid synthesis pathway impairs the ability of insulin to inhibit hepatic gluconeogenesis, but the exact mechanism remains unknown. To determine the mechanism by which glycerolipids impair insulin signaling, we overexpressed glycerol-3-phosphate acyltransferase-1 (GPAT1) in primary mouse hepatocytes. GPAT1 overexpression impaired insulin-stimulated phosphorylation of Akt-S473 and -T308, diminished insulin-suppression of glucose production, significantly inhibited mTOR complex 2 (mTORC2) activity and decreased the association of mTOR and rictor. Conversely, in hepatocytes from Gpat1(-/-) mice, mTOR-rictor association and mTORC2 activity were enhanced. However, this increase in mTORC2 activity in Gpat1(-/-) hepatocytes was ablated when rictor was knocked down. To determine which lipid intermediate was responsible for inactivating mTORC2, we overexpressed GPAT1, AGPAT, or lipin to increase the cellular content of lysophosphatidic acid (LPA), phosphatidic acid (PA), or diacylglycerol (DAG), respectively. The inhibition of mTOR/rictor binding and mTORC2 activity coincided with the levels of PA and DAG species that contained 16:0, the preferred substrate of GPAT1. Furthermore, di-16:0-PA strongly inhibited mTORC2 activity and disassociated mTOR/rictor in vitro. Taken together, these data reveal a signaling pathway by which phosphatidic acid synthesized via the glycerol-3-phosphate pathway inhibits mTORC2 activity by decreasing the association of rictor and mTOR, thereby down-regulating insulin action. These data demonstrate a critical link between nutrient excess, TAG synthesis, and hepatic insulin resistance.
甘油磷脂合成途径通量的增加会损害胰岛素抑制肝糖异生的能力，但确切的机制尚不清楚。为了确定甘油磷脂损害胰岛素信号的机制，我们在原代小鼠肝细胞中过表达了甘油-3-磷酸酰基转移酶-1（GPAT1）。GPAT1 的过表达会损害胰岛素刺激的 Akt-S473 和 -T308 的磷酸化，降低胰岛素对葡萄糖生成的抑制作用，显著抑制 mTOR 复合物 2（mTORC2）的活性，并减少 mTOR 和rictor 的结合。相反，在 Gpat1(-/-) 小鼠的肝细胞中，mTOR-rictor 结合和 mTORC2 活性增强。然而，当敲低 rictor 时，Gpat1(-/-) 肝细胞中 mTORC2 活性的增加被消除。为了确定哪种脂质中间体能使 mTORC2 失活，我们过表达了 GPAT1、AGPAT 或 lipin，分别增加了溶血磷脂酸（LPA）、磷脂酸（PA）或二酰基甘油（DAG）的细胞含量。mTOR/rictor 结合和 mTORC2 活性的抑制与含有 16:0 的 PA 和 DAG 物质的水平一致，16:0 是 GPAT1 的首选底物。此外，二-16:0-PA 强烈抑制 mTORC2 活性并在体外使 mTOR/rictor 解偶联。综上所述，这些数据揭示了一条信号通路，即通过甘油-3-磷酸途径合成的磷脂酸通过降低 rictor 和 mTOR 的结合来抑制 mTORC2 活性，从而下调胰岛素作用。这些数据表明，营养过剩、TAG 合成和肝胰岛素抵抗之间存在着关键联系。
