Glycerolipid signals alter mTOR complex 2 (mTORC2) to diminish insulin signaling

Feb 7, 2012Proceedings of the National Academy of Sciences of the United States of America

Fat molecule signals change mTOR complex 2 to reduce insulin response

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Abstract

Overexpression of glycerol-3-phosphate acyltransferase-1 (GPAT1) in primary mouse hepatocytes significantly impaired insulin-stimulated phosphorylation of Akt.

Increased GPAT1 levels diminished insulin's ability to suppress glucose production.
GPAT1 overexpression significantly inhibited mTOR complex 2 (mTORC2) activity.
The association between mTOR and rictor decreased with elevated GPAT1.
Hepatocytes lacking GPAT1 (Gpat1(-/-) mice) exhibited enhanced mTOR-rictor association and mTORC2 activity.
Inhibition of mTORC2 activity was associated with increased levels of phosphatidic acid (PA) and diacylglycerol (DAG) containing 16:0.
Di-16:0-PA specifically inhibited mTORC2 activity and disrupted mTOR/rictor binding in vitro.

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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.

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Glycerolipid signals alter mTOR complex 2 (mTORC2) to diminish insulin signaling

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