AMPK in Health and Disease

The function and survival of all organisms is dependent on the dynamic control of energy metabolism, when energy demand is matched to energy supply. The AMP-activated protein kinase (AMPK) alphabetagamma heterotrimer has emerged as an important integrator of signals that control energy balance through the regulation of multiple biochemical pathways in all eukaryotes. In this review, we begin with the discovery of the AMPK family and discuss the recent structural studies that have revealed the molecular basis for AMP binding to the enzyme's gamma subunit. AMPK's regulation involves autoinhibitory features and phosphorylation of both the catalytic alpha subunit and the beta-targeting subunit. We review the role of AMPK at the cellular level through examination of its many substrates and discuss how it controls cellular energy balance. We look at how AMPK integrates stress responses such as exercise as well as nutrient and hormonal signals to control food intake, energy expenditure, and substrate utilization at the whole body level. Lastly, we review the possible role of AMPK in multiple common diseases and the role of the new age of drugs targeting AMPK signaling.

A parathyroid hormone-related protein implicated in malignant hypercalcemia: cloning and expression

Humoral hypercalcemia of malignancy is a common complication of lung and certain other cancers. The hypercalcemia results from the actions of tumor factors on bone and kidney. We report here the isolation of full-length complementary DNA clones of a putative hypercalcemia factor, and the expression from the cloned DNA of the active protein in mammalian cells. The clones encode a prepro peptide of 36 amino acids and a mature protein of 141 amino acids that has significant homology with parathyroid hormone in the amino-terminal region. This previously unrecognized hormone may be important in normal as well as abnormal calcium metabolism.

The Ca2+/calmodulin-dependent protein kinase kinases are AMP-activated protein kinase kinases

The AMP-activated protein kinase (AMPK) is an important regulator of cellular metabolism in response to metabolic stress and to other regulatory signals. AMPK activity is absolutely dependent upon phosphorylation of AMPKalphaThr-172 in its activation loop by one or more AMPK kinases (AMPKKs). The tumor suppressor kinase, LKB1, is a major AMPKK present in a variety of tissues and cells, but several lines of evidence point to the existence of other AMPKKs. We have employed three cell lines deficient in LKB1 to study AMPK regulation and phosphorylation, HeLa, A549, and murine embryo fibroblasts derived from LKB(-/-) mice. In HeLa and A549 cells, mannitol, 2-deoxyglucose, and ionomycin, but not 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR), treatment activates AMPK by alphaThr-172 phosphorylation. These responses, as well as the downstream effects of AMPK on the phosphorylation of acetyl-CoA carboxylase, are largely inhibited by the Ca(2+)/ calmodulin-dependent protein kinase kinase (CaMKK) inhibitor, STO-609. AMPKK activity in HeLa cell lysates measured in vitro is totally inhibited by STO-609 with an IC50 comparable with that of the known CaMKK isoforms, CaMKKalpha and CaMKKbeta. Furthermore, 2-deoxyglucose- and ionomycin-stimulated AMPK activity, alphaThr-172 phosphorylation, and acetyl-CoA carboxylase phosphorylation are substantially reduced in HeLa cells transfected with small interfering RNAs specific for CaMKKalpha and CaMKKbeta. Lastly, the activation of AMPK in response to ionomycin and 2-deoxyglucose is not impaired in LKB1(-/-) murine embryo fibroblasts. These data indicate that the CaMKKs function in intact cells as AMPKKs, predicting wider roles for these kinases in regulating AMPK activity in vivo.

Protein kinase C contains a pseudosubstrate prototope in its regulatory domain

The regulatory domain of protein kinase C contains an amino acid sequence between residues 19 and 36 that resembles a substrate phosphorylation site in its distribution of basic residue recognition determinants. The corresponding synthetic peptide (Arg19-Phe-Ala-Arg-Lys-Gly-Ala25-Leu-Arg-Gln-Lys-Asn-Val-His -Glu-Val-Lys-Asn36) acts as a potent substrate antagonist with an inhibitory constant of 147 +/- 9 nM. It is a specific inhibitor of protein kinase C and inhibits both autophosphorylation and protein substrate phosphorylation. Substitution of Ala25 with serine transforms the pseudosubstrate into a potent substrate. These results demonstrate that the conserved region of the regulatory domain (residues 19 to 36) of protein kinase C has the secondary structural features of a pseudosubstrate and may be responsible for maintaining the enzyme in the inactive form in the absence of allosteric activators such as phospholipids.

Protein kinase recognition sequence motifs

Protein kinases play a crucial role in the regulation of many cellular processes. They alter the functions of their target proteins by phosphorylating specific serine, threonine and tyrosine residues. Identification of phosphorylation site sequences and studies with corresponding model peptides have provided clues to how these important enzymes recognize their substrate proteins. This knowledge has made it possible to identify potential sites of phosphorylation in newly sequenced proteins as well as to construct specific model substrates and inhibitors.

Metformin, independent of AMPK, inhibits mTORC1 in a rag GTPase-dependent manner

Dysfunctional mTORC1 signaling is associated with a number of human pathologies owing to its central role in controlling cell growth, proliferation, and metabolism. Regulation of mTORC1 is achieved by the integration of multiple inputs, including those of mitogens, nutrients, and energy. It is thought that agents that increase the cellular AMP/ATP ratio, such as the antidiabetic biguanides metformin and phenformin, inhibit mTORC1 through AMPK activation of TSC1/2-dependent or -independent mechanisms. Unexpectedly, we found that biguanides inhibit mTORC1 signaling, not only in the absence of TSC1/2 but also in the absence of AMPK. Consistent with these observations, in two distinct preclinical models of cancer and diabetes, metformin acts to suppress mTORC1 signaling in an AMPK-independent manner. We found that the ability of biguanides to inhibit mTORC1 activation and signaling is, instead, dependent on the Rag GTPases.

AMP-activated protein kinase phosphorylation of endothelial NO synthase

The AMP-activated protein kinase (AMPK) in rat skeletal and cardiac muscle is activated by vigorous exercise and ischaemic stress. Under these conditions AMPK phosphorylates and inhibits acetyl-coenzyme A carboxylase causing increased oxidation of fatty acids. Here we show that AMPK co-immunoprecipitates with cardiac endothelial NO synthase (eNOS) and phosphorylates Ser-1177 in the presence of Ca2+-calmodulin (CaM) to activate eNOS both in vitro and during ischaemia in rat hearts. In the absence of Ca2+-calmodulin, AMPK also phosphorylates eNOS at Thr-495 in the CaM-binding sequence, resulting in inhibition of eNOS activity but Thr-495 phosphorylation is unchanged during ischaemia. Phosphorylation of eNOS by the AMPK in endothelial cells and myocytes provides a further regulatory link between metabolic stress and cardiovascular function.

Single phosphorylation sites in Acc1 and Acc2 regulate lipid homeostasis and the insulin-sensitizing effects of metformin

The obesity epidemic has led to an increased incidence of nonalcoholic fatty liver disease (NAFLD) and type 2 diabetes. AMP-activated protein kinase (Ampk) regulates energy homeostasis and is activated by cellular stress, hormones and the widely prescribed type 2 diabetes drug metformin. Ampk phosphorylates mouse acetyl-CoA carboxylase 1 (Acc1; refs. 3,4) at Ser79 and Acc2 at Ser212, inhibiting the conversion of acetyl-CoA to malonyl-CoA. The latter metabolite is a precursor in fatty acid synthesis and an allosteric inhibitor of fatty acid transport into mitochondria for oxidation. To test the physiological impact of these phosphorylation events, we generated mice with alanine knock-in mutations in both Acc1 (at Ser79) and Acc2 (at Ser212) (Acc double knock-in, AccDKI). Compared to wild-type mice, these mice have elevated lipogenesis and lower fatty acid oxidation, which contribute to the progression of insulin resistance, glucose intolerance and NAFLD, but not obesity. Notably, AccDKI mice made obese by high-fat feeding are refractory to the lipid-lowering and insulin-sensitizing effects of metformin. These findings establish that inhibitory phosphorylation of Acc by Ampk is essential for the control of lipid metabolism and, in the setting of obesity, for metformin-induced improvements in insulin action.

Interleukin-6 increases insulin-stimulated glucose disposal in humans and glucose uptake and fatty acid oxidation in vitro via AMP-activated protein kinase

Although interleukin-6 (IL-6) has been associated with insulin resistance, little is known regarding the effects of IL-6 on insulin sensitivity in humans in vivo. Here, we show that IL-6 infusion increases glucose disposal without affecting the complete suppression of endogenous glucose production during a hyperinsulinemic-euglycemic clamp in healthy humans. Because skeletal muscle accounts for most of the insulin-stimulated glucose disposal in vivo, we examined the mechanism(s) by which IL-6 may affect muscle metabolism using L6 myotubes. IL-6 treatment increased fatty acid oxidation, basal and insulin-stimulated glucose uptake, and translocation of GLUT4 to the plasma membrane. Furthermore, IL-6 rapidly and markedly increased AMP-activated protein kinase (AMPK). To determine whether the activation of AMPK mediated cellular metabolic events, we conducted experiments using L6 myotubes infected with dominant-negative AMPK alpha-subunit. The effects described above were abrogated in AMPK dominant-negative-infected cells. Our results demonstrate that acute IL-6 treatment enhances insulin-stimulated glucose disposal in humans in vivo, while the effects of IL-6 on glucose and fatty acid metabolism in vitro appear to be mediated by AMPK.

The ancient drug salicylate directly activates AMP-activated protein kinase

Salicylate, a plant product, has been in medicinal use since ancient times. More recently, it has been replaced by synthetic derivatives such as aspirin and salsalate, both of which are rapidly broken down to salicylate in vivo. At concentrations reached in plasma after administration of salsalate or of aspirin at high doses, salicylate activates adenosine monophosphate-activated protein kinase (AMPK), a central regulator of cell growth and metabolism. Salicylate binds at the same site as the synthetic activator A-769662 to cause allosteric activation and inhibition of dephosphorylation of the activating phosphorylation site, threonine-172. In AMPK knockout mice, effects of salicylate to increase fat utilization and to lower plasma fatty acids in vivo were lost. Our results suggest that AMPK activation could explain some beneficial effects of salsalate and aspirin in humans.

Mammalian AMP-activated protein kinase subfamily

The mammalian 5'-AMP-activated protein kinase (AMPK) is related to a growing family of protein kinases in yeast and plants that are regulated by nutritional stress. We find the most prominent expressed form of the hepatic AMPK catalytic subunit (alpha 1) is distinct from the previously cloned kinase subunit (alpha 2). The alpha 1 (548 residues) and alpha 2 (552 residues) isoforms have 90% amino acid sequence identity within the catalytic core but only 61% identity elsewhere. The tissue distribution of the AMPK activity most closely parallels the low abundance 6-kilobase alpha 1 mRNA distribution and the alpha 1 immunoreactivity rather than alpha 2, with substantial amounts in kidney, liver, lung, heart, and brain. Both alpha 1 and alpha 2 isoforms are stimulated by AMP and contain noncatalytic beta and gamma subunits. The liver alpha 1 isoform accounts for approximately 94% of the enzyme activity measured using the SAMS peptide substrate. The tissue distribution of the alpha 2 immunoreactivity parallels the alpha 2 8.5-kilobase mRNA and is most prominent in skeletal muscle, heart, and liver. Isoforms of the beta and gamma subunits present in the human genome sequence reveal that the AMPK consists of a family of isoenzymes.

Phosphorylation of Thr(495) regulates Ca(2+)/calmodulin-dependent endothelial nitric oxide synthase activity

The activity of the endothelial nitric oxide synthase (eNOS) can be regulated independently of an increase in Ca(2+) by the phosphorylation of Ser(1177) but results only in a low nitric oxide (NO) output. In the present study, we assessed whether the agonist-induced (Ca(2+)-dependent, high-output) activation of eNOS is associated with changes in the phosphorylation of Thr(495) in the calmodulin (CaM)-binding domain. eNOS Thr(495) was constitutively phosphorylated in porcine aortic endothelial cells and was rapidly dephosphorylated after bradykinin stimulation. In the same cells, bradykinin enhanced the phosphorylation of Ser(1177), which was maximal after 5 minutes, and abolished by the CaM-dependent kinase II (CaMKII) inhibitor KN-93. Bradykinin also enhanced the association of CaMKII with eNOS. Phosphorylation of Thr(495) was attenuated by the protein kinase C (PKC) inhibitor Ro 31-8220 and after PKC downregulation using phorbol 12-myristate 13-acetate. The agonist-induced dephosphorylation of Thr(495) was completely Ca(2+)-dependent and inhibited by the PP1 inhibitor calyculin A. Little CaM was bound to eNOS immunoprecipitated from unstimulated cells, but the agonist-induced dephosphorylation of Thr(495) enhanced the association of CaM. Mutation of Thr(495) to alanine increased CaM binding to eNOS in the absence of cell stimulation, whereas the corresponding Asp(495) mutant bound almost no CaM. Accordingly, NO production by the Ala(495) mutant was more sensitive to Ca(2+)/CaM than the aspartate mutant. These results suggest that the dual phosphorylation of Ser(1177) and Thr(495) determines the activity of eNOS in agonist-stimulated endothelial cells. Moreover, the dephosphorylation of Thr(495) by PP1 precedes the phosphorylation of Ser(1177) by CaMKII. The full text of this article is available at http://www.circresaha.org.

AMPK is a direct adenylate charge-regulated protein kinase

The adenosine monophosphate (AMP)-activated protein kinase (AMPK) regulates whole-body and cellular energy balance in response to energy demand and supply. AMPK is an αβγ heterotrimer activated by decreasing concentrations of adenosine triphosphate (ATP) and increasing AMP concentrations. AMPK activation depends on phosphorylation of the α catalytic subunit on threonine-172 (Thr(172)) by kinases LKB1 or CaMKKβ, and this is promoted by AMP binding to the γ subunit. AMP sustains activity by inhibiting dephosphorylation of α-Thr(172), whereas ATP promotes dephosphorylation. Adenosine diphosphate (ADP), like AMP, bound to γ sites 1 and 3 and stimulated α-Thr(172) phosphorylation. However, in contrast to AMP, ADP did not directly activate phosphorylated AMPK. In this way, both ADP/ATP and AMP/ATP ratios contribute to AMPK regulation.

Coordinated control of endothelial nitric-oxide synthase phosphorylation by protein kinase C and the cAMP-dependent protein kinase

Endothelial nitric-oxide synthase (eNOS) is an important regulatory enzyme in the cardiovascular system catalyzing the production of NO from arginine. Multiple protein kinases including Akt/PKB, cAMP-dependent protein kinase (PKA), and the AMP-activated protein kinase (AMPK) activate eNOS by phosphorylating Ser-1177 in response to various stimuli. During VEGF signaling in endothelial cells, there is a transient increase in Ser-1177 phosphorylation coupled with a decrease in Thr-495 phosphorylation that reverses over 10 min. PKC signaling in endothelial cells inhibits eNOS activity by phosphorylating Thr-495 and dephosphorylating Ser-1177 whereas PKA signaling acts in reverse by increasing phosphorylation of Ser-1177 and dephosphorylation of Thr-495 to activate eNOS. Both phosphatases PP1 and PP2A are associated with eNOS. PP1 is responsible for dephosphorylation of Thr-495 based on its specificity for this site in both eNOS and the corresponding synthetic phosphopeptide whereas PP2A is responsible for dephosphorylation of Ser-1177. Treatment of endothelial cells with calyculin selectively blocks PKA-mediated dephosphorylation of Thr-495 whereas okadaic acid selectively blocks PKC-mediated dephosphorylation of Ser-1177. These results show that regulation of eNOS activity involves coordinated signaling through Ser-1177 and Thr-495 by multiple protein kinases and phosphatases.

Parathyroid hormone-related protein purified from a human lung cancer cell line

A protein with biological activities similar to parathyroid hormone (PTH) has been purified from serum-free culture medium obtained from a human lung cancer cell line (BEN). A major protein band of 18 kDa was obtained on NaDodSO4/polyacrylamide gels, with faint bands at 35 kDa and 67 kDa. Biological activity was associated only with the 18-kDa band. Amino acid sequence analysis of the material purified by HPLC revealed that 8 of the 16 residues were identical with those of human PTH. Antibody raised to a corresponding synthetic peptide recognized the PTH-related material but showed less than 1% cross-reactivity with human PTH amino-terminal peptides. BEN cells contained PTH DNA, but not PTH messenger RNA, indicating involvement of another gene. The purified PTH-related protein had a specific biological activity approximately equal to 6 times greater than that of bovine PTH(1-34). The PTH-related protein may have a role in the syndrome of humoral hypercalcemia of malignancy.

Dealing with energy demand: the AMP-activated protein kinase

The AMP-activated protein kinase (AMPK) is a member of a metabolite-sensing protein kinase family that is found in all eukaryotes. AMPK activity is regulated by vigorous exercise, nutrient starvation and ischemia/hypoxia, and modulates many aspects of mammalian cell metabolism. The AMPK yeast homolog, Snf1p, plays a major role in adaption to glucose deprivation. In mammals, AMPK also has diverse roles that extend from energy metabolism through to transcriptional control.

The Akt kinase signals directly to endothelial nitric oxide synthase

Endothelial nitric oxide synthase (eNOS) is an important modulator of angiogenesis and vascular tone [1]. It is stimulated by treatment of endothelial cells in a phosphatidylinositol 3-kinase (PI 3-kinase)-dependent fashion by insulin-like growth factor-1 (IGF-1) and vascular endothelial growth factor (VEGF) [2] [3] and is activated by phosphorylation at Ser1177 in the sequence RIRTQS(1177)F (in the single-letter amino acid code) [4]. The protein kinase Akt is an important downstream target of PI 3-kinase [5] [6], regulating VEGF-stimulated endothelial cell survival [7]. Akt phosphorylates substrates within a defined motif [8], which is present in the sequence surrounding Ser1177 in eNOS. Both Akt [5] [6] and eNOS [9] are localized to, and activated at, the plasma membrane. We found that purified Akt phosphorylated cardiac eNOS at Ser1177, resulting in activation of eNOS. Phosphorylation at this site was stimulated by treatment of bovine aortic endothelial cells (BAECs) with VEGF or IGF-1, and Akt was activated in parallel. Preincubation with wortmannin, an inhibitor of Akt signalling, reduced VEGF- or IGF-1-induced Akt activity and eNOS phosphorylation. Akt was detected in immunoprecipitates of eNOS from BAECs, and eNOS in immunoprecipitates of Akt, indicating that the two enzymes associate in vivo. It is thus apparent that Akt directly activates eNOS in endothelial cells. These results strongly suggest that Akt has an important role in the regulation of normal angiogenesis and raise the possibility that the enhanced activity of this kinase that occurs in carcinomas may contribute to tumor vascularization and survival.

AMP-activated protein kinase, super metabolic regulator

The AMP-activated protein kinase (AMPK) is a metabolic-stress-sensing protein kinase that regulates metabolism in response to energy demand and supply by directly phosphorylating rate-limiting enzymes in metabolic pathways as well as controlling gene expression.

Regulation of endothelial and myocardial NO synthesis by multi-site eNOS phosphorylation

The controlled regulation of nitric oxide (NO) synthesis in endothelial cells and cardiomyocytes by the endothelial form of nitric oxide synthase (eNOS or NOS3) is essential for cardiovascular health. In recent years, a picture of complex and precise regulation of eNOS activity involving multi-site phosphorylation of specific serine and threonine residues has emerged. Regulation of endothelial NO synthesis by multi-site eNOS phosphorylation occurs in response to a wide variety of humoral, mechanical and pharmacological stimuli. This regulation involves numerous kinases and phosphatases, as well as interactions with other aspects of eNOS regulation such as Ca(2+) flux, protein-protein interactions and regulation of subcellular localization. Phosphorylation of eNOS-Ser(1177) close to the carboxy-terminal is a critical requirement for eNOS activation. In addition, phosphorylation of eNOS-Ser(633) in the flavin mononucleotide (FMN) binding domain also increases eNOS activity and appears particularly important for the maintenance of NO synthesis after initial activation by Ca(2+) flux and Ser(1177) phosphorylation. In contrast, NO synthesis is inhibited by phosphorylation of eNOS-Thr(495), which interferes with the binding of calmodulin to the eNOS calmodulin-binding domain. Regulated phosphorylation of eNOS also occurs at eNOS-Ser(114) and eNOS-Ser(615); however, the functions of these phosphorylation sites remain controversial. This review summarizes the present knowledge of the regulation of NO synthesis by multi-site eNOS phosphorylation and its relationship to other mechanisms of eNOS regulation. This progress in understanding important mechanisms controlling endothelial NO synthesis creates new opportunities to understand and potentially treat cardiovascular diseases characterized by deficient NO synthesis.

AMP-activated protein kinase regulates GLUT4 transcription by phosphorylating histone deacetylase 5

OBJECTIVE

Insulin resistance associated with obesity and diabetes is ameliorated by specific overexpression of GLUT4 in skeletal muscle. The molecular mechanisms regulating skeletal muscle GLUT4 expression remain to be elucidated. The purpose of this study was to examine these mechanisms.

RESEARCH DESIGN AND METHODS AND RESULTS

Here, we report that AMP-activated protein kinase (AMPK) regulates GLUT4 transcription through the histone deacetylase (HDAC)5 transcriptional repressor. Overexpression of HDAC5 represses GLUT4 reporter gene expression, and HDAC inhibition in human primary myotubes increases endogenous GLUT4 gene expression. In vitro kinase assays, site-directed mutagenesis, and site-specific phospho-antibodies establish AMPK as an HDAC5 kinase that targets S259 and S498. Constitutively active but not dominant-negative AMPK and 5-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside (AICAR) treatment in human primary myotubes results in HDAC5 phosphorylation at S259 and S498, association with 14-3-3 isoforms, and H3 acetylation. This reduces HDAC5 association with the GLUT4 promoter, as assessed through chromatin immunoprecipitation assays and HDAC5 nuclear export, concomitant with increases in GLUT4 gene expression. Gene reporter assays also confirm that the HDAC5 S259 and S498 sites are required for AICAR induction of GLUT4 transcription.

CONCLUSIONS

These data reveal a signal transduction pathway linking cellular energy charge to gene transcription directed at restoring cellular and whole-body energy balance and provide new therapeutic targets for the treatment and management of insulin resistance and type 2 diabetes.

Contraction-induced changes in acetyl-CoA carboxylase and 5'-AMP-activated kinase in skeletal muscle

The concentration of malonyl-CoA, a negative regulator of fatty acid oxidation, diminishes acutely in contracting skeletal muscle. To determine how this occurs, the activity and properties of acetyl-CoA carboxylase beta (ACC-beta), the skeletal muscle isozyme that catalyzes malonyl-CoA formation, were examined in rat gastrocnemius-soleus muscles at rest and during contractions induced by electrical stimulation of the sciatic nerve. To avoid the problem of contamination of the muscle extract by mitochondrial carboxylases, an assay was developed in which ACC-beta was first purified by immunoprecipitation with a monoclonal antibody. ACC-beta was quantitatively recovered in the immunopellet and exhibited a high sensitivity to citrate (12-fold activation) and a Km for acetyl-CoA (120 microM) similar to that reported for ACC-beta purified by other means. After 5 min of contraction, ACC-beta activity was decreased by 90% despite an apparent increase in the cytosolic concentration of citrate, a positive regulator of ACC. SDS-polyacrylamide gel electrophoresis of both homogenates and immunopellets from these muscles showed a decrease in the electrophoretic mobility of ACC, suggesting that phosphorylation could account for the decrease in ACC activity. In keeping with this notion, citrate activation of ACC purified from contracting muscle was markedly depressed. In addition, homogenization of the muscles in a buffer free of phosphatase inhibitors and containing the phosphatase activators glutamate and MgCl2 or treatment of immunoprecipitated ACC-beta with purified protein phosphatase 2A abolished the decreases in both ACC-beta activity and electrophoretic mobility caused by contraction. The rapid decrease in ACC-beta activity after the onset of contractions (50% by 20 s) and its slow restoration to initial values during recovery (60-90 min) were paralleled temporally by reciprocal changes in the activity of the alpha2 but not the alpha1 isoform of 5'-AMP-activated protein kinase (AMPK). In conclusion, the results suggest that the decrease in ACC activity during muscle contraction is caused by an increase in its phosphorylation, most probably due, at least in part, to activation of the alpha2 isoform of AMPK. They also suggest a dual mechanism for ACC regulation in muscle in which inhibition by phosphorylation takes precedence over activation by citrate. These alterations in ACC and AMPK activity, by diminishing the concentration of malonyl-CoA, could be responsible for the increase in fatty acid oxidation observed in skeletal muscle during exercise.

Tumor necrosis factor alpha-induced skeletal muscle insulin resistance involves suppression of AMP-kinase signaling

Elevated levels of tumor necrosis factor (TNFalpha) are implicated in the development of insulin resistance, but the mechanisms mediating these chronic effects are not completely understood. We demonstrate that TNFalpha signaling through TNF receptor (TNFR) 1 suppresses AMPK activity via transcriptional upregulation of protein phosphatase 2C (PP2C). This in turn reduces ACC phosphorylation, suppressing fatty-acid oxidation, increasing intramuscular diacylglycerol accumulation, and causing insulin resistance in skeletal muscle, effects observed both in vitro and in vivo. Importantly even at pathologically elevated levels of TNFalpha observed in obesity, the suppressive effects of TNFalpha on AMPK signaling are reversed in mice null for both TNFR1 and 2 or following treatment with a TNFalpha neutralizing antibody. Our data demonstrate that AMPK is an important TNFalpha signaling target and is a contributing factor to the suppression of fatty-acid oxidation and the development of lipid-induced insulin resistance in obesity.