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

Abstract

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, mTORrictor 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.triacylglycerol | palmitate | hepatic steatosis

Hepatic Gluconeogenesis Is Enhanced by Phosphatidic Acid Which Remains Uninhibited by Insulin in Lipodystrophic Agpat2−/− Mice

“…Two previous reports have attempted to demonstrate a role of PA in insulin signaling (34,35). Zhang et al (35) observed an increase in total LPA, PA, and DAG levels and also a modest increase in glucose output in primary mouse hepatocytes from WT mice overexpressing GPAT1.…”

“…Zhang et al (35) observed an increase in total LPA, PA, and DAG levels and also a modest increase in glucose output in primary mouse hepatocytes from WT mice overexpressing GPAT1. However, it is unclear if this is specifically due to PA, LPA, or DAG or a combination of these lipids (Fig.…”

Phospholipase D and the Maintenance of Phosphatidic Acid Levels for Regulation of Mammalian Target of Rapamycin (mTOR)

“…As indicated above, there is an apparent requirement for an unsaturated fatty acid in order to achieve mTOR complex stability (30,54). Thus, it will be of interest to determine whether there are significant differences in the acyl composition of PA obtained from the different sources.…”

“…Although this study did not connect the need for desaturated lipids and the dependence of mTOR on PA, it did provide further evidence for a lipid dependence of mTOR and potentially a dependence on desaturated lipids. Coleman and colleagues (54) recently reported that the mTORC2 complex falls apart in the presence of dipalmitoyl-PA, which consists of two saturated fatty acids. This is in stark contrast to the effect of PA containing palmitate (saturated) and oleate (mono-unsaturated), which stabilized both mTORC1 and mTORC2 complexes in cells where PA production by PLD was suppressed (30).…”

Ectopic fat, insulin resistance and non-alcoholic fatty liver disease

“…In contrast, mTORC2 has a positive effect on glucose uptake and glucose tolerance (30) . Lipid synthesis in the liver may also cause insulin resistance via disruption of mTORC2 signalling (31) . These authors showed that overexpression of acyl-CoA:glycerol 3 phosphate acyl transferase that catalyses the formation of LPA and which is the rate limiting step in TAG synthesis (Fig.…”