Identification of potential therapeutic targets in prostate cancer through a cross-species approach.

Genetically engineered mouse models of cancer can be used to filter genome-wide expression datasets generated from human tumours and to identify gene expression alterations that are functionally important to cancer development and progression. In this study, we have generated RNAseq data from tumours arising in two established mouse models of prostate cancer, PB-Cre/PtenloxP/loxP and p53loxP/loxPRbloxP/loxP, and integrated this with published human prostate cancer expression data to pinpoint cancer-associated gene expression changes that are conserved between the two species. To identify potential therapeutic targets, we then filtered this information for genes that are either known or predicted to be druggable. Using this approach, we revealed a functional role for the kinase MELK as a driver and potential therapeutic target in prostate cancer. We found that MELK expression was required for cell survival, affected the expression of genes associated with prostate cancer progression and was associated with biochemical recurrence.

Epigenetic and oncogenic regulation of SLC16A7 (MCT2) results in protein over-expression, impacting on signalling and cellular phenotypes in prostate cancer.

Monocarboxylate Transporter 2 (MCT2) is a major pyruvate transporter encoded by the SLC16A7 gene. Recent studies pointed to a consistent overexpression of MCT2 in prostate cancer (PCa) suggesting MCT2 as a putative biomarker and molecular target. Despite the importance of this observation the mechanisms involved in MCT2 regulation are unknown. Through an integrative analysis we have discovered that selective demethylation of an internal SLC16A7/MCT2 promoter is a recurrent event in independent PCa cohorts. This demethylation is associated with expression of isoforms differing only in 5'-UTR translational control motifs, providing one contributing mechanism for MCT2 protein overexpression in PCa. Genes co-expressed with SLC16A7/MCT2 also clustered in oncogenic-related pathways and effectors of these signalling pathways were found to bind at the SLC16A7/MCT2 gene locus. Finally, MCT2 knock-down attenuated the growth of PCa cells. The present study unveils an unexpected epigenetic regulation of SLC16A7/MCT2 isoforms and identifies a link between SLC16A7/MCT2, Androgen Receptor (AR), ETS-related gene (ERG) and other oncogenic pathways in PCa. These results underscore the importance of combining data from epigenetic, transcriptomic and protein level changes to allow more comprehensive insights into the mechanisms underlying protein expression, that in our case provide additional weight to MCT2 as a candidate biomarker and molecular target in PCa.

A glycolytic phenotype is associated with prostate cancer progression and aggressiveness: a role for monocarboxylate transporters as metabolic targets for therapy.

Metabolic adaptation is considered an emerging hallmark of cancer, whereby cancer cells exhibit high rates of glucose consumption with consequent lactate production. To ensure rapid efflux of lactate, most cancer cells express high levels of monocarboxylate transporters (MCTs), which therefore may constitute suitable therapeutic targets. The impact of MCT inhibition, along with the clinical impact of altered cellular metabolism during prostate cancer (PCa) initiation and progression, has not been described. Using a large cohort of human prostate tissues of different grades, in silico data, in vitro and ex vivo studies, we demonstrate the metabolic heterogeneity of PCa and its clinical relevance. We show an increased glycolytic phenotype in advanced stages of PCa and its correlation with poor prognosis. Finally, we present evidence supporting MCTs as suitable targets in PCa, affecting not only cancer cell proliferation and survival but also the expression of a number of hypoxia-inducible factor target genes associated with poor prognosis. Herein, we suggest that patients with highly glycolytic tumours have poorer outcome, supporting the notion of targeting glycolytic tumour cells in prostate cancer through the use of MCT inhibitors.

Transcriptomic analysis reveals inhibition of androgen receptor activity by AMPK in prostate cancer cells.

Metabolic alterations contribute to prostate cancer development and progression; however, the role of the central metabolic regulator AMP-activated protein kinase (AMPK) remains controversial. The androgen receptor (AR), a key driver of prostate cancer, regulates prostate cancer cell metabolism by driving the expression of a network of metabolic genes and activates AMPK through increasing the expression of one of its upstream kinases. To more clearly define the role of AMPK in prostate cancer, we performed expression profiling following pharmacologic activation of this kinase. We found that genes down-regulated upon AMPK activation were over-expressed in prostate cancer, consistent with a tumour suppressive function of AMPK. Strikingly, we identified the AR as one of the most significantly enriched transcription factors mediating gene expression changes downstream of AMPK signalling in prostate cancer cells. Activation of AMPK inhibited AR transcriptional activity and reduced androgen-dependent expression of known AR target genes. Conversely, knock-down of AMPK increased AR activity. Modulation of AR expression could not explain these effects. Instead, we observed that activation of AMPK reduced nuclear localisation of the AR. We thus propose the presence of a negative feedback loop in prostate cancer cells whereby AR activates AMPK and AMPK feeds back to limit AR-driven transcription.

The endoplasmic reticulum coat protein II transport machinery coordinates cellular lipid secretion and cholesterol biosynthesis.

Triglycerides and cholesterol are essential for life in most organisms. Triglycerides serve as the principal energy storage depot and, where vascular systems exist, as a means of energy transport. Cholesterol is essential for the functional integrity of all cellular membrane systems. The endoplasmic reticulum is the site of secretory lipoprotein production and de novo cholesterol synthesis, yet little is known about how these activities are coordinated with each other or with the activity of the COPII machinery, which transports endoplasmic reticulum cargo to the Golgi. The Sar1B component of this machinery is mutated in chylomicron retention disorder, indicating that this Sar1 isoform secures delivery of dietary lipids into the circulation. However, it is not known why some patients with chylomicron retention disorder develop hepatic steatosis, despite impaired intestinal fat malabsorption, and why very severe hypocholesterolemia develops in this condition. Here, we show that Sar1B also promotes hepatic apolipoprotein (apo) B lipoprotein secretion and that this promoting activity is coordinated with the processes regulating apoB expression and the transfer of triglycerides/cholesterol moieties onto this large lipid transport protein. We also show that although Sar1A antagonizes the lipoprotein secretion-promoting activity of Sar1B, both isoforms modulate the expression of genes encoding cholesterol biosynthetic enzymes and the synthesis of cholesterol de novo. These results not only establish that Sar1B promotes the secretion of hepatic lipids but also adds regulation of cholesterol synthesis to Sar1B's repertoire of transport functions.

CDKN2B expression in adipose tissue of familial combined hyperlipidemia patients.

The purpose of this study was to determine the core biological processes perturbed in the subcutaneous adipose tissue of familial combined hyperlipidemia (FCHL) patients. Annotation of FCHL and control microarray datasets revealed a distinctive FCHL transcriptome, characterized by gene expression changes regulating five overlapping systems: the cytoskeleton, cell adhesion and extracellular matrix; vesicular trafficking; lipid homeostasis; and cell cycle and apoptosis. Expression values for the cell-cycle inhibitor CDKN2B were increased, replicating data from an independent FCHL cohort. In 3T3-L1 cells, CDKN2B knockdown induced C/EBPα expression and lipid accumulation. The minor allele at SNP site rs1063192 (C) was predicted to create a perfect seed for the human miRNA-323b-5p. A miR-323b-5p mimic significantly reduced endogenous CDKN2B protein levels and the activity of a CDKN2B 3'UTR luciferase reporter carrying the rs1063192 C allele. Although the allele displayed suggestive evidence of association with reduced CDKN2B mRNA in the MuTHER adipose tissue dataset, family studies suggest the association between increased CDKN2B expression and FCHL-lipid abnormalities is driven by factors external to this gene locus. In conclusion, from a comparative annotation analysis of two separate FCHL adipose tissue transcriptomes and a subsequent focus on CDKN2B, we propose that dysfunctional adipogenesis forms an integral part of FCHL pathogenesis.

Downregulation of androgen receptor transcription by promoter g-quadruplex stabilization as a potential alternative treatment for castrate-resistant prostate cancer.

Androgen receptor (AR) signaling remains an important regulatory pathway in castrate-resistant prostate cancer, and its transcriptional downregulation could provide a new line of therapy. A number of small-molecule ligands have previously demonstrated the ability to stabilize G-quadruplex structures and affect gene transcription for those genes whose promoters contain a quadruplex-forming sequence. Herein, we report the probable formation of new G-quadruplex structure present in the AR promoter in a transcriptionally important location. NMR spectroscopy, circular dichroism, UV spectroscopy, and UV thermal melting experiments for this sequence are consistent with G-quadruplex formation. Fluorescence resonance energy transfer (FRET) melting studies have identified a novel compound, MM45, which appears to stabilize this G-quadruplex at submicromolar concentrations. The effects of MM45 have been investigated in prostate cancer cell lines where it has been shown to inhibit cell growth. A reporter assay intended to isolate the effect of MM45 on the G-quadruplex sequence showed dose-dependent transcriptional repression only when the AR promoter G-quadruplex sequence is present. Dose-dependent transcriptional repression of the AR by MM45 has been demonstrated at both a protein and mRNA level. This proof of concept study paves the route toward a potential alternative treatment pathway in castrate-resistant prostate cancer.

AMPK is essential for energy homeostasis regulation and glucose sensing by POMC and AgRP neurons.

Hypothalamic AMP-activated protein kinase (AMPK) has been suggested to act as a key sensing mechanism, responding to hormones and nutrients in the regulation of energy homeostasis. However, the precise neuronal populations and cellular mechanisms involved are unclear. The effects of long-term manipulation of hypothalamic AMPK on energy balance are also unknown. To directly address such issues, we generated POMC alpha 2KO and AgRP alpha 2KO mice lacking AMPK alpha2 in proopiomelanocortin- (POMC-) and agouti-related protein-expressing (AgRP-expressing) neurons, key regulators of energy homeostasis. POMC alpha 2KO mice developed obesity due to reduced energy expenditure and dysregulated food intake but remained sensitive to leptin. In contrast, AgRP alpha 2KO mice developed an age-dependent lean phenotype with increased sensitivity to a melanocortin agonist. Electrophysiological studies in AMPK alpha2-deficient POMC or AgRP neurons revealed normal leptin or insulin action but absent responses to alterations in extracellular glucose levels, showing that glucose-sensing signaling mechanisms in these neurons are distinct from those pathways utilized by leptin or insulin. Taken together with the divergent phenotypes of POMC alpha 2KO and AgRP alpha 2KO mice, our findings suggest that while AMPK plays a key role in hypothalamic function, it does not act as a general sensor and integrator of energy homeostasis in the mediobasal hypothalamus.

Thr2446 is a novel mammalian target of rapamycin (mTOR) phosphorylation site regulated by nutrient status.

The mammalian target of rapamycin (mTOR) is a key regulator of protein translation. Signaling via mTOR is increased by growth factors but decreased during nutrient deprivation. Previous studies have identified Ser2448 as a nutrient-regulated phosphorylation site located in the mTOR catalytic domain, insulin stimulates Ser2448 phosphorylation via protein kinase B (PKB), while Ser2448 phosphorylation is attenuated with amino acid starvation. Here we have identified Thr2446 as a novel nutrient-regulated phosphorylation site on mTOR. Thr2446 becomes phosphorylated when CHO-IR cells are nutrient-deprived, but phosphorylation is reduced by insulin stimulation. Nutrient deprivation activates AMP-activated protein kinase (AMPK). To test whether this could be involved in regulating phoshorylation of mTOR, we treated cultured murine myotubes with 5'-aminoimidazole-4-carboxamide ribonucleoside (AICAR) or dinitrophenol (DNP). Both treatments activated AMPK and also caused a concomitant increase in phosphorylation of Thr2446 and a parallel decrease in insulin's ability to phosphorylate p70 S6 kinase. In vitro kinase assays using peptides based on the sequence in amino acids 2440-2551 of mTOR found that PKB and AMPK are capable of phosphorylating sites in this region. However, phosphorylation by PKB is restricted when Thr2446 is mutated to an acidic residue mimicking phosphorylation. Conversely, AMP-kinase-induced phosphorylation is reduced when Ser2448 is phosphorylated. These data suggest differential phosphorylation Thr2446 and Ser2448 could act as a switch mechanism to integrate signals from nutrient status and growth factors to control the regulation of protein translation.

Neuregulin signaling on glucose transport in muscle cells.

Neuregulin-1, a growth factor that potentiates myogenesis induces glucose transport through translocation of glucose transporters, in an additive manner to insulin, in muscle cells. In this study, we examined the signaling pathway required for a recombinant active neuregulin-1 isoform (rhHeregulin-beta(1), 177-244, HRG) to stimulate glucose uptake in L6E9 myotubes. The stimulatory effect of HRG required binding to ErbB3 in L6E9 myotubes. PI3K activity is required for HRG action in both muscle cells and tissue. In L6E9 myotubes, HRG stimulated PKBalpha, PKBgamma, and PKCzeta activities. TPCK, an inhibitor of PDK1, abolished both HRG- and insulin-induced glucose transport. To assess whether PKB was necessary for the effects of HRG on glucose uptake, cells were infected with adenoviruses encoding dominant negative mutants of PKBalpha. Dominant negative PKB reduced PKB activity and insulin-stimulated glucose transport but not HRG-induced glucose transport. In contrast, transduction of L6E9 myotubes with adenoviruses encoding a dominant negative kinase-inactive PKCzeta abolished both HRG- and insulin-stimulated glucose uptake. In soleus muscle, HRG induced PKCzeta, but not PKB phosphorylation. HRG also stimulated the activity of p70S6K, p38MAPK, and p42/p44MAPK and inhibition of p42/p44MAPK partially repressed HRG action on glucose uptake. HRG did not affect AMPKalpha(1) or AMPKalpha(2) activities. In all, HRG stimulated glucose transport in muscle cells by activation of a pathway that requires PI3K, PDK1, and PKCzeta, but not PKB, and that shows cross-talk with the MAPK pathway. The PI3K, PDK1, and PKCzeta pathway can be considered as an alternative mechanism, independent of insulin, to induce glucose uptake.

LKB1 is the upstream kinase in the AMP-activated protein kinase cascade.

Inactivating mutations in the protein kinase LKB1 lead to a dominantly inherited cancer in humans termed Peutz-Jeghers syndrome. The role of LKB1 is unclear, and only one target for LKB1 has been identified in vivo [3]. AMP-activated protein kinase (AMPK) is the downstream component of a protein kinase cascade that plays a pivotal role in energy homeostasis. AMPK may have a role in protecting the body from metabolic diseases including type 2 diabetes, obesity, and cardiac hypertrophy. We previously reported the identification of three protein kinases (Elm1, Pak1, and Tos3 [9]) that lie upstream of Snf1, the yeast homologue of AMPK. LKB1 shares sequence similarity with Elm1, Pak1, and Tos3, and we demonstrated that LKB1 phosphorylates AMPK on the activation loop threonine (Thr172) within the catalytic subunit and activates AMPK in vitro [9]. Here, we have investigated whether LKB1 corresponds to the major AMPKK activity present in cell extracts. AMPKK purified from rat liver corresponds to LKB1, and blocking LKB1 activity in cells abolishes AMPK activation in response to different stimuli. These results identify a link between two protein kinases, previously thought to lie in unrelated, distinct pathways, that are associated with human diseases.

Metabolic and mitogenic signal transduction in human skeletal muscle after intense cycling exercise.

We determined whether mitogen-activated protein kinase (MAPK) and 5'-AMP-activated protein kinase (AMPK) signalling cascades are activated in response to intense exercise in skeletal muscle from six highly trained cyclists (peak O(2) uptake (.V(O2,peak)) 5.14 +/- 0.1 l min(-1)) and four control subjects (Vdot;(O(2))(,peak) 3.8 +/- 0.1 l min(-1)) matched for age and body mass. Trained subjects completed eight 5 min bouts of cycling at approximately 85% of .V(O2,peak) with 60 s recovery between work bouts. Control subjects performed four 5 min work bouts commencing at the same relative, but a lower absolute intensity, with a comparable rest interval. Vastus lateralis muscle biopsies were taken at rest and immediately after exercise. Extracellular regulated kinase (ERK1/2), p38 MAPK, histone H3, AMPK and acetyl CoA-carboxylase (ACC) phosphorylation was determined by immunoblot analysis using phosphospecific antibodies. Activity of mitogen and stress-activated kinase 1 (MSK1; a substrate of ERK1/2 and p38 MAPK) and alpha(1) and alpha(2) subunits of AMPK were determined by immune complex assay. ERK1/2 and p38 MAPK phosphorylation and MSK1 activity increased (P < 0.05) after exercise 2.6-, 2.1- and 2.0-fold, respectively, in control subjects and 1.5-, 1.6- and 1.4-fold, respectively, in trained subjects. Phosphorylation of histone H3, a substrate of MSK1, increased (P < 0.05) approximately 1.8-fold in both control and trained subject. AMPKalpha(2) activity increased (P < 0.05) after exercise 4.2- and 2.3-fold in control and trained subjects, respectively, whereas AMPKalpha(1) activity was not altered. Exercise increased ACC phosphorylation (P < 0.05) 1.9- and 2.8-fold in control and trained subjects. In conclusion, intense cycling exercise in subjects with a prolonged history of endurance training increases MAPK signalling to the downstream targets MSK1 and histone H3 and isoform-specific AMPK signalling to ACC. Importantly, exercise-induced signalling responses were greater in untrained men, even at the same relative exercise intensity, suggesting muscle from previously well-trained individuals requires a greater stimulus to activate signal transduction via these pathways.

Regulation of glycogen synthase by glucose and glycogen: a possible role for AMP-activated protein kinase.

We report here use of human myoblasts in culture to study the relationships between cellular glycogen concentrations and the activities of glycogen synthase (GS) and AMP-activated protein kinase (AMPK). Incubation of cells for 2 h in the absence of glucose led to a 25% decrease in glycogen content and a significant decrease in the fractional activity of GS. This was accompanied by stimulation of both the alpha1 and alpha2 isoforms of AMPK, without significant alterations in the ratios of adenine nucleotides. When glucose was added to glycogen-depleted cells, a rapid and substantial increase in GS activity was accompanied by inactivation of AMPK back to basal values. Inclusion of the glycogen phosphorylase inhibitor, CP-91149, prevented the loss of glycogen during glucose deprivation but not the activation of AMPK. However, in the absence of prior glycogen breakdown, glucose treatment failed to activate GS above control values, indicating the crucial role of glycogen content. Activation of AMPK by either 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside (AICAR) or hydrogen peroxide was also associated with a decrease in the activity ratio of GS. AICAR treatment had no effect on total cellular glycogen content but led to a modest increase in glucose uptake. These data support a role for AMPK in both stimulating glucose uptake and inhibiting GS in intact cells, thus promoting glucose flux through glycolysis.

Protein kinase inhibitors block the stimulation of the AMP-activated protein kinase by 5-amino-4-imidazolecarboxamide riboside.

The AMP-activated protein kinase (AMPK) is the central component of a protein kinase cascade that plays a major role in energy sensing. AMPK is activated pharmacologically by 5-amino-4-imidazolecarboxamide (AICA) riboside monophosphate (ZMP), which mimics the effects of AMP on the AMPK cascade. Here we show that uptake of AICA riboside into cells, mediated by the adenosine transport system, is blocked by a number of protein kinase inhibitors. Under these conditions, ZMP does not accumulate to sufficient levels to stimulate AMPK. Our results demonstrate that careful interpretation is required when using AICA riboside in conjunction with protein kinase inhibitors to investigate the physiological role of AMPK.

Isoform-specific regulation of 5' AMP-activated protein kinase in skeletal muscle from obese Zucker (fa/fa) rats in response to contraction.

Glucose transport can be activated in skeletal muscle in response to insulin via activation of phosphoinositide (PI) 3-kinase and in response to contractions or hypoxia, presumably via activation of 5' AMP-activated protein kinase (AMPK). We determined the effects of insulin and muscle contraction/hypoxia on PI 3-kinase, AMPK, and glucose transport activity in epitrochlearis skeletal muscle from insulin-resistant Zucker (fa/ fa) rats. Insulin-stimulated glucose transport in isolated skeletal muscle was reduced 47% in obese versus lean rats, with a parallel 42% reduction in tyrosine-associated PI 3-kinase activity. Contraction and hypoxia elicited normal responses for glucose transport in skeletal muscle from insulin-resistant obese rats. Isoform-specific AMPK activity was measured in skeletal muscle in response to insulin, contraction, or hypoxia. Contraction increased AMPKalpha1 activity 2.3-fold in lean rats, whereas no effect was noted in obese rats. Hypoxia increased AMPKalpha1 activity to a similar extent (more than sixfold) in lean and obese rats. Regardless of genotype, contraction, and hypoxia, each increased AMPKalpha2 activity more than fivefold, whereas insulin did not alter either AMPKalpha1 or -alpha2 activity in skeletal muscle. In conclusion, obesity-related insulin resistance is associated with an isoform-specific impairment in AMPKalpha1 in response to contraction. However, this impairment does not appear to affect contraction-stimulated glucose transport. Activation of AMPKalpha2 in response to muscle contraction/ exercise is associated with a parallel and normal increase in glucose transport in insulin-resistant skeletal muscle.

Activation of GLUT1 by metabolic and osmotic stress: potential involvement of AMP-activated protein kinase (AMPK).

In the rat liver epithelial cell line Clone 9, the V(max) for glucose uptake is acutely increased by inhibition of oxidative phosphorylation and by osmotic stress. By using a membrane-impermeant photoaffinity labelling reagent together with an isoform-specific antibody, we have, for the first time, provided direct evidence for the involvement of the GLUT1 glucose transporter isoform in this response. Transport stimulation was found to be associated with enhanced accessibility of GLUT1 to its substrate and with photolabelling of formerly 'cryptic' exofacial substrate binding sites in GLUT1 molecules. The total amount of cell surface GLUT1 remained constant. The precise mechanism for this binding site 'unmasking' is unclear but appears to involve AMP-activated protein kinase: in the current study, osmotic and metabolic stresses were found to result in activation of the alpha 1 isoform of AMP-activated protein kinase, and transport stimulation could be mimicked both by 5-aminoimidazole-4-carboxamide ribonucleoside and by infection of cells with a recombinant adenovirus encoding constitutively active AMP-activated protein kinase. The effect of 5-aminoimidazole-4-carboxamide ribonucleoside, as for metabolic stress, was on the V(max) rather than on the K(m) for transport and did not affect the cell-surface concentration of GLUT1. The relevant downstream target(s) of AMP-activated protein kinase have not yet been identified, but stimulation of transport by inhibition of oxidative phosphorylation or by 5-aminoimidazole-4-carboxamide ribonucleoside was not prevented by either inhibitors of conventional and novel protein kinase C isoforms or inhibitors of nitric oxide synthase. These enzymes, which have been implicated in stress-regulated pathways in other cell types, are therefore unlikely to play a role in transport regulation by stress in Clone 9 cells.

The Anti-diabetic drugs rosiglitazone and metformin stimulate AMP-activated protein kinase through distinct signaling pathways.

AMP-activated protein kinase (AMPK) is activated within the cell in response to multiple stresses that increase the intracellular AMP:ATP ratio. Here we show that incubation of muscle cells with the thiazolidinedione, rosiglitazone, leads to a dramatic increase in this ratio with the concomitant activation of AMPK. This finding raises the possibility that a number of the beneficial effects of the thiazolidinediones could be mediated via activation of AMPK. Furthermore, we show that in addition to the classical activation pathway, AMPK can also be stimulated without changing the levels of adenine nucleotides. In muscle cells, both hyperosmotic stress and the anti-diabetic agent, metformin, activate AMPK in the absence of any increase in the AMP:ATP ratio. However, although activation is no longer dependent on this ratio, it still involves increased phosphorylation of threonine 172 within the catalytic (alpha) subunit. AMPK stimulation in response to hyperosmotic stress does not appear to involve phosphatidylinositol 3-phosphate kinase, protein kinase C, mitogen-activated protein (MAP) kinase kinase, or p38 MAP kinase alpha or beta. Our results demonstrate that AMPK can be activated by at least two distinct signaling mechanisms and suggest that it may play a wider role in the cellular stress response than was previously understood.

Characterization of the role of the AMP-activated protein kinase in the stimulation of glucose transport in skeletal muscle cells.

Stimulation of AMP-activated protein kinase (AMPK) in skeletal muscle has been correlated with an increase in glucose transport. Here, we demonstrate that adenoviral-mediated expression of a constitutively active mutant of AMPK alpha leads to activation of glucose transport in a skeletal-muscle cell line, similar to that seen following treatment with 5-amino-imidazolecarboxamide (AICA) riboside, hyperosmotic stress or insulin. In contrast, expression of a dominant-negative form of AMPK blocked the stimulation of glucose transport by both AICA riboside and hyperosmotic stress, but was without effect on either insulin or phorbol-ester-stimulated transport. These results demonstrate that activation of AMPK is sufficient for stimulation of glucose uptake into muscle cells, and is a necessary component of the AICA riboside- and hyperosmotic-stress-induced pathway leading to increased glucose uptake. On the other hand, AMPK is not required in the insulin- or phorbol-ester-mediated pathways. Long-term (5 days) expression of the constitutively active AMPK mutant increased protein expression of GLUT1, GLUT4 and hexokinase II, consistent with previous reports on the chronic treatment of rats with AICA riboside. Expression of constitutively active AMPK had no detectable effect on p38 mitogen-activated protein kinase levels, although interestingly the level of protein kinase B was decreased. These results demonstrate that long-term activation of AMPK is sufficient to cause increased expression of specific proteins in muscle. Our results add further support to the hypothesis that long-term activation of AMPK is involved in the adaptive response of muscle to exercise training.

Leptin stimulates fatty-acid oxidation by activating AMP-activated protein kinase.

Leptin is a hormone secreted by adipocytes that plays a pivotal role in regulating food intake, energy expenditure and neuroendocrine function. Leptin stimulates the oxidation of fatty acids and the uptake of glucose, and prevents the accumulation of lipids in nonadipose tissues, which can lead to functional impairments known as "lipotoxicity". The signalling pathways that mediate the metabolic effects of leptin remain undefined. The 5'-AMP-activated protein kinase (AMPK) potently stimulates fatty-acid oxidation in muscle by inhibiting the activity of acetyl coenzyme A carboxylase (ACC). AMPK is a heterotrimeric enzyme that is conserved from yeast to humans and functions as a 'fuel gauge' to monitor the status of cellular energy. Here we show that leptin selectively stimulates phosphorylation and activation of the alpha2 catalytic subunit of AMPK (alpha2 AMPK) in skeletal muscle, thus establishing a previously unknown signalling pathway for leptin. Early activation of AMPK occurs by leptin acting directly on muscle, whereas later activation depends on leptin functioning through the hypothalamic-sympathetic nervous system axis. In parallel with its activation of AMPK, leptin suppresses the activity of ACC, thereby stimulating the oxidation of fatty acids in muscle. Blocking AMPK activation inhibits the phosphorylation of ACC stimulated by leptin. Our data identify AMPK as a principal mediator of the effects of leptin on fatty-acid metabolism in muscle.