A growing role for mTOR in promoting anabolic metabolism.

mTOR [mammalian (or mechanistic) target of rapamycin] is a protein kinase that, as part of mTORC1 (mTOR complex 1), acts as a critical molecular link between growth signals and the processes underlying cell growth. Although there has been intense interest in the upstream mechanisms regulating mTORC1, the full repertoire of downstream molecular events through which mTORC1 signalling promotes cell growth is only recently coming to light. It is now recognized that mTORC1 promotes cell growth and proliferation in large part through the activation of key anabolic processes. Through a variety of downstream targets, mTORC1 alters cellular metabolism to drive the biosynthesis of building blocks and macromolecules fundamentally essential for cell growth, including proteins, lipids and nucleic acids. In the present review, we focus on the metabolic functions of mTORC1 as they relate to the control of cell growth and proliferation. As mTORC1 is aberrantly activated in a number of tumour syndromes and up to 80% of human cancers, we also discuss the importance of this mTORC1-driven biosynthetic programme in tumour growth and progression.

Nuclear export-independent inhibition of Foxa2 by insulin.

The Forkhead box A2 transcription factor (Foxa2/HNF-3beta) has been shown to be a key regulator of genes involved in the maintenance of glucose and lipid homeostasis in the liver. It is constitutively inactivated in several hyperinsulinemic/obese mouse models, thereby enhancing their metabolic phenotypes. Foxa2 is activated under fasting conditions but is inhibited by insulin signaling via phosphatidylinositol 3-kinase/AKT in a phosphorylation-dependent manner, which results in its nuclear exclusion. However, the mechanism and relative importance of its nuclear export has not yet been elucidated. Here we show that Foxa2 contains a functional nuclear export signal and is excluded from the nucleus via a CRM1-dependent pathway in response to insulin signaling. Furthermore, direct evidence is provided that nuclear export-defective Foxa2 is phosphorylated and inactivated by insulin in vitro and in vivo. These data demonstrate for the first time that phosphorylation itself is the main event regulating the activity of Foxa2, suggesting that export-independent mechanisms have evolved to ensure inhibition of Foxa2 under conditions in which insulin signaling is present.

Discovery of Small-Molecule Selective mTORC1 Inhibitors via Direct Inhibition of Glucose Transporters.

The mechanistic target of rapamycin (mTOR) is a central regulator of cellular metabolic processes. Dysregulation of this kinase complex can result in a variety of human diseases. Rapamycin and its analogs target mTORC1 directly; however, chronic treatment in certain cell types and in vivo results in the inhibition of both mTORC1 and mTORC2. We have developed a high-throughput cell-based screen for the detection of phosphorylated forms of the mTORC1 (4E-BP1, S6K1) and mTORC2 (Akt) substrates and have identified and characterized a chemical scaffold that demonstrates a profile consistent with the selective inhibition of mTORC1. Stable isotope labeling of amino acids in cell culture-based proteomic target identification revealed that class I glucose transporters were the primary target for these compounds yielding potent inhibition of glucose uptake and, as a result, selective inhibition of mTORC1. The link between the glucose uptake and selective mTORC1 inhibition are discussed in the context of a yet-to-be discovered glucose sensor.

Metformin Inhibits Hepatic mTORC1 Signaling via Dose-Dependent Mechanisms Involving AMPK and the TSC Complex.

Metformin is the most widely prescribed drug for the treatment of type 2 diabetes. However, knowledge of the full effects of metformin on biochemical pathways and processes in its primary target tissue, the liver, is limited. One established effect of metformin is to decrease cellular energy levels. The AMP-activated protein kinase (AMPK) and mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) are key regulators of metabolism that are respectively activated and inhibited in acute response to cellular energy depletion. Here we show that metformin robustly inhibits mTORC1 in mouse liver tissue and primary hepatocytes. Using mouse genetics, we find that at the lowest concentrations of metformin that inhibit hepatic mTORC1 signaling, this inhibition is dependent on AMPK and the tuberous sclerosis complex (TSC) protein complex (TSC complex). Finally, we show that metformin profoundly inhibits hepatocyte protein synthesis in a manner that is largely dependent on its ability to suppress mTORC1 signaling.

Stimulation of de novo pyrimidine synthesis by growth signaling through mTOR and S6K1.

Cellular growth signals stimulate anabolic processes. The mechanistic target of rapamycin complex 1 (mTORC1) is a protein kinase that senses growth signals to regulate anabolic growth and proliferation. Activation of mTORC1 led to the acute stimulation of metabolic flux through the de novo pyrimidine synthesis pathway. mTORC1 signaling posttranslationally regulated this metabolic pathway via its downstream target ribosomal protein S6 kinase 1 (S6K1), which directly phosphorylates S1859 on CAD (carbamoyl-phosphate synthetase 2, aspartate transcarbamoylase, dihydroorotase), the enzyme that catalyzes the first three steps of de novo pyrimidine synthesis. Growth signaling through mTORC1 thus stimulates the production of new nucleotides to accommodate an increase in RNA and DNA synthesis needed for ribosome biogenesis and anabolic growth.

Chronic activation of mTOR complex 1 is sufficient to cause hepatocellular carcinoma in mice.

The mammalian target of rapamycin (mTOR) complex 1 (mTORC1) is a nutrient-sensitive protein kinase that is aberrantly activated in many human cancers. Whether dysregulation of mTORC1 signaling in normal tissues increases the risk for cancer, however, is unknown. We focused on hepatocellular carcinoma, which has been linked to environmental factors that affect mTORC1 activity, including diet. Ablation of the gene encoding TSC1 (tuberous sclerosis complex 1), which as part of the TSC1-TSC2 complex is an upstream inhibitor of mTORC1, results in constitutively increased mTORC1 signaling, an effect on this pathway similar to that of obesity. We found that mice with liver-specific knockout of Tsc1 developed sporadic hepatocellular carcinoma with heterogeneous histological and biochemical features. The spontaneous development of hepatocellular carcinoma in this mouse model was preceded by a series of pathological changes that accompany the primary etiologies of this cancer in humans, including liver damage, inflammation, necrosis, and regeneration. Chronic mTORC1 signaling led to unresolved endoplasmic reticulum stress and defects in autophagy, factors that contributed to hepatocyte damage and hepatocellular carcinoma development. Therefore, we conclude that increased activation of mTORC1 can promote carcinogenesis and may thus represent a key molecular link between cancer risk and environmental factors, such as diet.

mTOR couples cellular nutrient sensing to organismal metabolic homeostasis.

The mammalian target of rapamycin complex 1 (mTORC1) has the ability to sense a variety of essential nutrients and respond by altering cellular metabolic processes. Hence, this protein kinase complex is poised to influence adaptive changes to nutrient fluctuations toward the maintenance of whole-body metabolic homeostasis. Defects in mTORC1 regulation, arising from either physiological or genetic conditions, are believed to contribute to the metabolic dysfunction underlying a variety of human diseases, including type 2 diabetes. We are just now beginning to gain insights into the complex tissue-specific functions of mTORC1. In this review, we detail the current knowledge of the physiological functions of mTORC1 in controlling systemic metabolism, with a focus on advances obtained through genetic mouse models.

Foxa2 activity increases plasma high density lipoprotein levels by regulating apolipoprotein M.

Obesity, diabetes, insulin resistance, and hyperinsulinemia are frequently associated with a cluster of closely related lipid abnormalities such as low plasma levels of high density lipoprotein (HDL) and elevated levels of triglyceride, both known to increase the risk of developing atherosclerotic disease. The molecular mechanisms linking obesity, insulin resistance, and hyperinsulinemia to low HDL levels are incompletely understood. Here we demonstrate that insulin, through a Foxa2-mediated mechanism, inhibited the expression of apolipoprotein M (apoM), an important determinant of plasma pre-beta-HDL and alpha-HDL concentrations. Obese mice had decreased apoM expression and plasma pre-beta-HDL levels due to inactivation of Foxa2 in hyperinsulinemic states. Nuclear reexpression of Foxa2 with a phosphorylation-deficient mutant Foxa2T156A (Ad-T156A) activated apoM expression and increased plasma pre-beta-HDL and alpha-HDL levels. In contrast, haploinsufficient Foxa2(+/-) mice exhibited decreased hepatic apoM expression and plasma pre-beta-HDL and HDL levels. The increase in plasma HDL levels and pre-beta-HDL formation by Foxa2 was mediated exclusively by apoM, as constitutive active expression of Foxa2 in apoM(-/-) mice had no effect on plasma HDL levels. Our results identify a fundamental mechanism by which insulin regulates plasma HDL levels in physiological and insulin-resistant states and thus have important implications for novel therapeutic approaches to prevent atherosclerosis.

A novel hypoxia-inducible factor-independent hypoxic response regulating mammalian target of rapamycin and its targets.

Hypoxia triggers a reversible inhibition of protein synthesis thought to be important for energy conservation in O2-deficient environments. The mammalian target of rapamycin (mTOR) pathway integrates multiple environmental cues to regulate translation in response to nutrient availability and stress, suggesting it as a candidate for O2 regulation. We show here that hypoxia rapidly and reversibly triggers hypophosphorylation of mTOR and its effectors 4E-BP1, p70S6K, rpS6, and eukaryotic initiation factor 4G. Hypoxic regulation of these translational control proteins is dominant to activation via multiple distinct signaling pathways such as insulin, amino acids, phorbol esters, and serum and is independent of Akt/protein kinase B and AMP-activated protein kinase phosphorylation, ATP levels, ATP:ADP ratios, and hypoxia-inducible factor-1 (HIF-1). Finally, hypoxia appears to repress phosphorylation of translational control proteins in a manner analogous to rapamycin and independent of phosphatase 2A (PP2A) activity. These data demonstrate a new mode of regulation of the mTOR pathway and position this pathway as a powerful point of control by O2 of cellular metabolism and energetics.