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.

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.

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.

Introducing an acute pain service

An acute pain service in a new district general hospital is described. We have reported incidence of severe pain, common postoperative anaesthetic problems and patient satisfaction in relation to the analgesic technique. Over half the patients were treated by intermittent intramuscular injection of opioid, but increase in the use of continuous intravenous therapy and in particular patient-controlled analgesia, was welcomed by both medical and nursing staff.

Studies of introital colonization in women with recurrent urinary infections. IV. The role of local vaginal antibodies

Immunoglobulins IgG and IgA were determined in 94 collections of vaginal fluid from 10 patients before, during and after 17 episodes of bacteriuria. The results were compared to 49 collections from 6 volunteer controls who had never had bacteriuria. The mean IgG concentration in patients (48 plus or minus 54 mg. per cent) was no different from controls (50 plus or minus 33 mg. per cent). IgA concentrations were 2.6 plus or minus 2.1 and 2.6 plus or minus 2.4 mg. per cent, respectively. Bacterial agglutination titers in vaginal fluid to Escherichia coli colonizing the vaginal vestibule of the patients were no different (7.7 plus or minus 9.8, reciprocal tube titer) than the titers to be predominant Escherichia coli cultured from the rectum of the controls (9.7 plus or minus 5.0). Longitudinal studies in patients failed to show any relationship between vaginal colonization with changing serotypes of Escherichia coli and agglutination titers in vaginal fluid.