Fyn-dependent regulation of energy expenditure and body weight is mediated by tyrosine phosphorylation of LKB1

A critical role of Src family kinase in SDF-1/CXCR4-mediated bone-marrow progenitor cell recruitment to the ischemic heart

The G protein-coupled receptor CXCR4 and its ligand stromal-cell derived factor 1 (SDF-1) play a crucial role in directing progenitor cell (PC) homing to ischemic tissue. The Src family protein kinases (SFK) can be activated by, and serve as effectors of, G proteins. In this study we sought to determine whether SFK play a role in SDF-1/CXCR4-mediated PC homing. First, we investigated whether SDF-1/CXCR4 signaling activates SFK. Bone-marrow mononuclear cells (BM MNCs) were isolated from WT and BM-specific CXCR4-KO mice and treated with SDF-1 and/or CXCR4 antagonist AMD3100. SDF-1 treatment rapidly induced phosphorylation (activation) of hematopoietic Src (i.e., Lyn, Fgr, and Hck) in WT cells but not in AMD3100-treated cells or CXCR4-KO cells. Then, we investigated whether SFK are involved in SDF-1/CXCR4-mediated PC chemotaxis. In a combined chemotaxis and endothelial-progenitor-cell (EPC) colony assay, Src inhibitor SU6656 dose-dependently inhibited the SDF-1-induced migration of colony-forming EPCs. Next, we investigated whether SFK play a role in SDF-1/CXCR4-mediated BM PC homing to the ischemic heart. BM MNCs from CXCR4BAC:eGFP reporter mice were i.v. injected into WT and SDF-1BAC:SDF1-RFP transgenic mice following surgically-induced myocardial infarction (MI). eGFP(+) MNCs and eGFP(+)c-kit(+) PCs that were recruited in the infarct border zone in SDF-1BAC:SDF1-RFP recipients were significantly more than that in WT recipients. Treatments of mice with SU6656 significantly reduced eGFP(+) and eGFP(+)c-kit(+) cell recruitment in both WT and SDF-1BAC:RFP recipients and abrogated the difference between the two groups. Remarkably, PCs isolated from BM-specific C-terminal Src kinase (CSK)-KO (Src activated) mice were recruited more efficiently than PCs from WT PCs in the WT recipients. In conclusion, SFK are activated by SDF-1/CXCR4 signaling and play an essential role in SDF-1/CXCR4-mediated BM PC chemotactic response and ischemic cardiac recruitment.

Regulation of AMPK activation by CD36 links fatty acid uptake to β-oxidation

Increases in muscle energy needs activate AMPK and induce sarcolemmal recruitment of the fatty acid (FA) translocase CD36. The resulting rises in FA uptake and FA oxidation are tightly correlated, suggesting coordinated regulation. We explored the possibility that membrane CD36 signaling might influence AMPK activation. We show, using several cell types, including myocytes, that CD36 expression suppresses AMPK, keeping it quiescent, while it mediates AMPK activation by FA. These dual effects reflect the presence of CD36 in a protein complex with the AMPK kinase LKB1 (liver kinase B1) and the src kinase Fyn. This complex promotes Fyn phosphorylation of LKB1 and its nuclear sequestration, hindering LKB1 activation of AMPK. FA interaction with CD36 dissociates Fyn from the protein complex, allowing LKB1 to remain cytosolic and activate AMPK. Consistent with this, CD36(-/-) mice have constitutively active muscle and heart AMPK and enhanced FA oxidation of endogenous triglyceride stores. The molecular mechanism described, whereby CD36 suppresses AMPK, with FA binding to CD36 releasing this suppression, couples AMPK activation to FA availability and would be important for the maintenance of cellular FA homeostasis. Its dysfunction might contribute to the reported association of CD36 variants with metabolic complications of obesity in humans.

Genome-wide methylation analysis identifies differentially methylated CpG loci associated with severe obesity in childhood

Childhood obesity is a major public health issue. Here we investigated whether differential DNA methylation was associated with childhood obesity. We studied DNA methylation profiles in whole blood from 78 obese children (mean BMI Z-score: 2.6) and 71 age- and sex-matched controls (mean BMI Z-score: 0.1). DNA samples from obese and control groups were pooled and analyzed using the Infinium HumanMethylation450 BeadChip array. Comparison of the methylation profiles between obese and control subjects revealed 129 differentially methylated CpG (DMCpG) loci associated with 80 unique genes that had a greater than 10% difference in methylation (P-value < 0.05). The top pathways enriched among the DMCpGs included developmental processes, immune system regulation, regulation of cell signaling, and small GTPase-mediated signal transduction. The associations between the methylation of selected DMCpGs with childhood obesity were validated using sodium bisulfite pyrosequencing across loci within the FYN, PIWIL4, and TAOK3 genes in individual subjects. Three CpG loci within FYN were hypermethylated in obese individuals (all P < 0.01), while obesity was associated with lower methylation of CpG loci within PIWIL4 (P = 0.003) and TAOK3 (P = 0.001). After building logistic regression models, we determined that a 1% increase in methylation in TAOK3, multiplicatively decreased the odds of being obese by 0.91 (95% CI: 0.86 - 0.97), and an increase of 1% methylation in FYN CpG3, multiplicatively increased the odds of being obese by 1.03 (95% CI: 0.99 - 1.07). In conclusion, these findings provide evidence that childhood obesity is associated with specific DNA methylation changes in whole blood, which may have utility as biomarkers of obesity risk.

Thy1 (CD90) controls adipogenesis by regulating activity of the Src family kinase, Fyn

Worldwide obesity rates are at epidemic levels, and new insight into the regulation of obesity and adipogenesis are required. Thy1 (CD90), a cell surface protein with an enigmatic function, is expressed on subsets of fibroblasts and stem cells. We used a diet-induced obesity model to show that Thy1-null mice gain weight at a faster rate and gain 30% more weight than control C57BL/6 mice. During adipogenesis, Thy1 expression is lost in mouse 3T3-L1 cells. Overexpression of Thy1 blocked adipocyte formation and reduced mRNA and protein expression of an adipocyte marker, fatty acid-binding protein 4, by 5-fold. Although preadipocyte fibroblasts expressed Thy1 mRNA and protein, adipocytes from mouse and human fat tissue had almost undetectable Thy1 levels. Thy1 decreases the activity of the adipogenic transcription factor PPARγ by more than 60% as shown by PPARγ-dependent reporter assays. Using both genetic and pharmacologic approaches, we show Thy1 expression dampens PPARγ by inhibiting the activity of the Src-family kinase, Fyn. Thus, these studies reveal Thy1 blocks adipogenesis and PPARγ by inhibiting Fyn and support the idea that Thy1 is a novel therapeutic target in obesity.

Mangiferin stimulates carbohydrate oxidation and protects against metabolic disorders induced by high-fat diets

Excessive dietary fat intake causes systemic metabolic toxicity, manifested in weight gain, hyperglycemia, and insulin resistance. In addition, carbohydrate utilization as a fuel is substantially inhibited. Correction or reversal of these effects during high-fat diet (HFD) intake is of exceptional interest in light of widespread occurrence of diet-associated metabolic disorders in global human populations. Here we report that mangiferin (MGF), a natural compound (the predominant constituent of Mangifera indica extract from the plant that produces mango), protected against HFD-induced weight gain, increased aerobic mitochondrial capacity and thermogenesis, and improved glucose and insulin profiles. To obtain mechanistic insight into the basis for these effects, we determined that mice exposed to an HFD combined with MGF exhibited a substantial shift in respiratory quotient from fatty acid toward carbohydrate utilization. MGF treatment significantly increased glucose oxidation in muscle of HFD-fed mice without changing fatty acid oxidation. These results indicate that MGF redirects fuel utilization toward carbohydrates. In cultured C2C12 myotubes, MGF increased glucose and pyruvate oxidation and ATP production without affecting fatty acid oxidation, confirming in vivo and ex vivo effects. Furthermore, MGF inhibited anaerobic metabolism of pyruvate to lactate but enhanced pyruvate oxidation. A key target of MGF appears to be pyruvate dehydrogenase, determined to be activated by MGF in a variety of assays. These findings underscore the therapeutic potential of activation of carbohydrate utilization in correction of metabolic syndrome and highlight the potential of MGF to serve as a model compound that can elicit fuel-switching effects.

ERK1/2 antagonize AMPK-dependent regulation of FcεRI-mediated mast cell activation and anaphylaxis

BACKGROUND

Extracellular signal-regulated kinases 1/2 (ERK1/2) make important contributions to allergic responses via their regulation of degranulation, eicosanoid production, and cytokine expression by mast cells, yet the mechanisms underlying their positive effects on FcεRI-dependent signaling are not fully understood. Recently, we reported that mast cell activation and anaphylaxis are negatively regulated by AMP-activated protein kinase (AMPK). However, little is known about the relationship between ERK1/2-mediated positive and the AMPK-mediated negative regulation of FcεRI signaling in mast cells.

OBJECTIVE

We investigated possible interactions between ERK1/2 and AMPK in the modulation of mast cell signaling and anaphylaxis.

METHODS

Wild-type or AMPKα2(-/-) mice, or bone marrow-derived mast cells obtained from these mice, were treated with either chemical agents or small interfering RNAs that modulated the activity or expression of ERK1/2 or AMPK to evaluate the functional interplay between ERK1/2 and AMPK in FcεRI-dependent signaling.

RESULTS

The ERK1/2 pathway inhibitor U0126 and the AMPK activator 5-aminoimidazole-4-carboxamide-1-β-4-ribofuranoside similarly inhibited FcεRI-mediated mast cell signals in vitro and anaphylaxis in vivo. ERK1/2-specific small interfering RNA also mimicked this effect on FcεRI signals. Moreover, AMPKα2 knockdown or deficiency led to increased FcεRI-mediated mast cell activation and anaphylaxis that were insensitive to U0126 or activator 5-aminoimidazole-4-carboxamide-1-β-4-ribofuranoside, suggesting that the suppression of FcεRI signals by the inhibition of the ERK1/2 pathway relies largely on AMPK activation. ERK1/2 controlled AMPK activity by regulating its subcellular translocation.

CONCLUSIONS

ERK1/2 ablated the AMPK-dependent negative regulatory axis, thereby activating FcεRI signals in mast cells.

Disruption of Fyn SH3 domain interaction with a proline-rich motif in liver kinase B1 results in activation of AMP-activated protein kinase

Fyn-deficient mice display increased AMP-activated Protein Kinase (AMPK) activity as a result of Fyn-dependent regulation of Liver Kinase B1 (LKB1) in skeletal muscle. Mutation of Fyn-specific tyrosine sites in LKB1 results in LKB1 export into the cytoplasm and increased AMPK activation site phosphorylation. This study characterizes the structural elements responsible for the physical interaction between Fyn and LKB1. Effects of point mutations in the Fyn SH2/SH3 domains and in the LKB1 proline-rich motif on 1) Fyn and LKB1 binding, 2) LKB1 subcellular localization and 3) AMPK phosphorylation were investigated in C2C12 muscle cells. Additionally, novel LKB1 proline-rich motif mimicking cell permeable peptides were generated to disrupt Fyn/LKB1 binding and investigate the consequences on AMPK activity in both C2C12 cells and mouse skeletal muscle. Mutation of either Fyn SH3 domain or the proline-rich motif of LKB1 resulted in the disruption of Fyn/LKB1 binding, re-localization of 70% of LKB1 signal in the cytoplasm and a 2-fold increase in AMPK phosphorylation. In vivo disruption of the Fyn/LKB1 interaction using LKB1 proline-rich motif mimicking cell permeable peptides recapitulated Fyn pharmacological inhibition. We have pinpointed the structural elements within Fyn and LKB1 that are responsible for their binding, demonstrating the functionality of this interaction in regulating AMPK activity.

Skeletal muscle autophagy: a new metabolic regulator

Autophagy classically functions as a physiological process to degrade cytoplasmic components, protein aggregates, and/or organelles, as a mechanism for nutrient breakdown, and as a regulator of cellular architecture. Proper autophagic flux is vital for both functional skeletal muscle, which controls the support and movement of the skeleton, and muscle metabolism. The role of autophagy as a metabolic regulator in muscle has been previously studied; however, the underlying molecular mechanisms that control autophagy in skeletal muscle have only recently begun to emerge. We review recent literature on the molecular pathways controlling skeletal muscle autophagy and discuss how they connect autophagy to metabolic regulation. We also focus on the implications these studies hold for understanding metabolic and muscle-wasting diseases.

Alteration of de novo glucose production contributes to fasting hypoglycaemia in Fyn deficient mice

Previous studies have demonstrated that glucose disposal is increased in the Fyn knockout (FynKO) mice due to increased insulin sensitivity. FynKO mice also display fasting hypoglycaemia despite decreased insulin levels, which suggested that hepatic glucose production was unable to compensate for the increased basal glucose utilization. The present study investigates the basis for the reduction in plasma glucose levels and the reduced ability for the liver to produce glucose in response to gluconeogenic substrates. FynKO mice had a 5-fold reduction in phosphoenolpyruvate carboxykinase (PEPCK) gene and protein expression and a marked reduction in pyruvate, pyruvate/lactate-stimulated glucose output. Remarkably, de novo glucose production was also blunted using gluconeogenic substrates that bypass the PEPCK step. Impaired conversion of glycerol to glucose was observed in both glycerol tolerance test and determination of the conversion of (13)C-glycerol to glucose in the fasted state. α-glycerol phosphate levels were reduced but glycerol kinase protein expression levels were not changed. Fructose-driven glucose production was also diminished without alteration of fructokinase expression levels. The normal levels of dihydroxyacetone phosphate and glyceraldehyde-3-phosphate observed in the FynKO liver extracts suggested normal triose kinase function. Fructose-bisphosphate aldolase (aldolase) mRNA or protein levels were normal in the Fyn-deficient livers, however, there was a large reduction in liver fructose-6-phosphate (30-fold) and fructose-1,6-bisphosphate (7-fold) levels as well as a reduction in glucose-6-phosphate (2-fold) levels. These data suggest a mechanistic defect in the allosteric regulation of aldolase activity.

Fyn deficiency promotes a preferential increase in subcutaneous adipose tissue mass and decreased visceral adipose tissue inflammation

Previous studies have demonstrated that Fyn knockout (FynKO) mice on a standard chow diet display increased glucose clearance and whole-body insulin sensitivity associated with decreased adiposity resulting from increased fatty acid use and energy expenditure. Surprisingly, however, despite a similar extent of adipose tissue (AT) mass accumulation on a high-fat diet, the FynKO mice remained fully glucose tolerant and insulin sensitive. Physiologic analyses demonstrated that the FynKO mice had a combination of skewed AT expansion into the subcutaneous compartment rather than to the visceral depot, reduced AT inflammation associated with reduced T-cell and macrophage infiltration, and increased proportion of anti-inflammatory M2 macrophages. These data demonstrate that Fyn is an important regulator of whole-body integrative metabolism that coordinates AT expansion, inflammation, and insulin sensitivity in states of nutrient excess. These data further suggest that inhibition of Fyn function may provide a novel target to prevent AT inflammation, insulin resistance, and the dyslipidemia components of the metabolic syndrome.

Fyn regulates adipogenesis by promoting PIKE-A/STAT5a interaction

Fyn is a tyrosine kinase with multiple roles in a variety of cellular processes. Here we report that Fyn is a new kinase involved in adipocyte differentiation. Elevated Fyn protein is detected specifically in the adipocytes of obese mice. Moreover, Fyn expression increases progressively in 3T3-L1 cells during in vitro adipogenesis, which correlates with its kinase activity. Inhibition of Fyn by either genetic or pharmacological manipulation restrains the 3T3-L1 preadipocytes from fully differentiating into mature adipocytes. Mechanistically, Fyn regulates the activity of the adipogenic transcription factor signal transducer and activator of transcription 5a (STAT5a) through enhancing its interaction with the GTPase phosphoinositide 3-kinase enhancer A (PIKE-A). The STAT5a activity is therefore reduced in Fyn- or PIKE-ablated adipose tissues, leading to diminished expression of adipogenic markers and adipocyte differentiation. Our data thus demonstrate a novel functional interaction between Fyn, PIKE-A, and STAT5a in mediating adipogenesis.

Enhanced antioxidant effect of prenylated polyphenols as Fyn inhibitor

Polyphenols have antioxidant effects. In view of the diverse biological activities of prenylated natural products, this study investigated whether polyphenols with prenyl residues have improved antioxidant and cytoprotective activity against oxidative stress, and explored the underlying basis for this effect. A set of structurally related polyphenols exhibited varying degrees of antioxidant effect in HepG2 cells, as evidenced by increases in cell viability against oxidative injury; kazinol E and C with three prenyls had greater potency than other kazinols having fewer prenyl chains. Polyphenols without prenyl (tupichinol C and resveratrol) showed weaker potency. Treatment with kazinol E diminished H(2)O(2) production and enabled cells to protect the mitochondria, as indicated by the inhibition of mitochondrial fragmentation, mitochondrial permeability transition, and cytochrome c release. Moreover, kazinol E activated LKB1 by its phosphorylation and cytoplasmic translocation, contributing to the protection of mitochondria via AMPK. In vitro or in a cell-based assay, tyrosine phosphorylation of Fyn was prohibited by kazinol E, which led to LKB1 activation, as shown by the experiments using Fyn over-expression construct or siRNA. SU6656, a known Fyn inhibitor, had a similar effect. Moreover, oxidative stress facilitated Fyn phosphorylation with repression of AMPKα and GSK3β phosphorylation, which was abolished by kazinol E treatment. The role of Fyn inhibition by kazinol E in AMPK-mediated protection of the cell viability and mitochondrial function was strengthened by ectopically expressed Fyn's reversal of these effects. In conclusion, kazinols as multi-prenylated polyphenols possess increased antioxidant and cytoprotective activity, which depends on the activation of LKB1-AMPK pathway downstream of Fyn inhibition.

Mouse skeletal muscle fiber-type-specific macroautophagy and muscle wasting are regulated by a Fyn/STAT3/Vps34 signaling pathway

Skeletal muscle atrophy induced by aging (sarcopenia), inactivity, and prolonged fasting states (starvation) is predominantly restricted to glycolytic type II muscle fibers and typical spares oxidative type I fibers. However, the mechanisms accounting for muscle fiber-type specificity of atrophy have remained enigmatic. In the current study, although the Fyn tyrosine kinase activated the mTORC1 signaling complex, it also induced marked atrophy of glycolytic fibers with relatively less effect on oxidative muscle fibers. This was due to inhibition of macroautophagy via an mTORC1-independent but STAT3-dependent reduction in Vps34 protein levels and decreased Vps34/p150/Beclin1/Atg14 complex 1. Physiologically, in the fed state endogenous Fyn kinase activity was increased in glycolytic but not oxidative skeletal muscle. In parallel, Y705-STAT3 phosphorylation increased with decreased Vps34 protein levels. Moreover, fed/starved regulation of Y705-STAT3 phosphorylation and Vps34 protein levels was prevented in skeletal muscle of Fyn null mice. These data demonstrate a Fyn/STAT3/Vps34 pathway that is responsible for fiber-type-specific regulation of macroautophagy and skeletal muscle atrophy.

MLR-1023 is a potent and selective allosteric activator of Lyn kinase in vitro that improves glucose tolerance in vivo

2(1H)-pyrimidinone,5-(3-methylphenoxy) (MLR-1023) is a candidate for the treatment of type 2 diabetes. The current studies were aimed at determining the mechanism by which MLR-1023 mediates glycemic control. In these studies, we showed that MLR-1023 reduced blood glucose levels without increasing insulin secretion in vivo. We have further determined that MLR-1023 did not activate peroxisome proliferator-activated α, δ, and γ receptors or glucagon-like peptide-1 receptors or inhibit dipeptidyl peptidase-4 or α-glucosidase enzyme activity. However, in an in vitro broad kinase screen MLR-1023 activated the nonreceptor-linked Src-related tyrosine kinase Lyn. MLR-1023 increased the V(max) of Lyn with an EC(50) of 63 nM. This Lyn kinase activation was ATP binding site independent, indicating that MLR-1023 regulated the kinase through an allosteric mechanism. We have established a link between Lyn activation and blood glucose lowering with studies showing that the glucose-lowering effects of MLR-1023 were abolished in Lyn knockout mice, consistent with existing literature linking Lyn kinase and the insulin-signaling pathway. In summary, these studies describe MLR-1023 as a unique blood glucose-lowering agent and show that MLR-1023-mediated blood glucose lowering depends on Lyn kinase activity. These results, coupled with other results (J Pharmacol Exp Ther 342:23-32, 2012), suggest that MLR-1023 and Lyn kinase activation may be a new treatment modality for type 2 diabetes.

Fyn inhibition by cycloalkane-fused 1,2-dithiole-3-thiones enhances antioxidant capacity and protects mitochondria from oxidative injury

Fyn kinase has emerged as a regulator of diverse pathological processes. However, therapeutic Fyn inhibitors are not available. This study investigated the potential of a series of cycloalkane-fused dithiolethiones (CDTs) or other congeners to increase antioxidant capacity in association with Fyn inhibition, as well as the molecular basis for this effect. Treatment of HepG2 cells with each agent protected the mitochondria from oxidative injury elicited by arachidonic acid and iron, which increased cell viability; 4,5,6,7-tetrahydrobenzo-1,2-dithiole-3-thione (SNU1A) and 5,6-dihydro-4H-cyclopenta-1,2-dithiole-3-thione (SNU2A) were the most effective, whereas 5-methyl-1,2-dithiole-3-thione (SNU3A) was less active. 5-(Quinolin-2-yl)-1,2-dithiole-3-thione (SNU3E) had a minimal effect. SNU1A treatment decreased mitochondrial superoxide production and enabled cells to restore mitochondrial membrane permeability. Oxidative injury caused by arachidonic acid and iron enhanced Fyn phosphorylation at a tyrosine residue, which was decreased by SNU1A treatment. 2,3-Dihydro-N,N-dimethyl-2-oxo-3-[(4,5,6,7-tetrahydro-1H-indol-2-yl)methylene]-1H-indole-5-sulfonamide (SU6656), a known Fyn inhibitor, had a similar effect. Fyn inhibition contributed to protecting mitochondria from injury through AMP-activated protein kinase (AMPK), as supported by reversal of this effect with Fyn overexpression. Consistently, Fyn overexpression attenuated AMPK activation by SNU1A, which strengthens the inhibitory role of Fyn in AMPK activity. CDTs had antioxidant effects, as shown by increases in GSH contents and inhibition of H(2)O(2) production. They also had the ability to activate nuclear factor E2-related factor 2 (Nrf2), a key antioxidant transcription factor. Fyn overexpression decreased the Nrf2 activation induced by SNU1A. Our results demonstrate that CDTs exert cytoprotective effects by protecting mitochondria and increasing the cellular antioxidant capacity, which may result not only from Fyn inhibition leading to AMPK activation but also from Nrf2 activation.

Targeted therapies of the LKB1/AMPK pathway for the treatment of insulin resistance

Type II diabetes is characterized by elevated serum glucose levels and altered lipid metabolism due to peripheral insulin resistance and defects of insulin secretion in the pancreatic β-cells. While some cases of obesity and Type II diabetes result from genetic dysfunction, the increased worldwide incidence of these two disorders strongly suggest that the contribution of environmental factors such as sedentary lifestyles and high-calorie intake may disrupt energy balance. AMP-activated protein kinase and its upstream kinase liver kinase B1 are conserved serine/threonine kinases regulating anabolic and catabolic metabolic processes, therefore representing attractive therapeutic targets for the treatment of obesity and Type II diabetes. In this review, we will discuss the advantages of targeting the liver kinase B1/AMP-activated protein kinase pathway for the treatment of metabolic diseases.

What we have learnt about PIKE from the knockout mice

Phosphoinositide 3-kinase enhancer (PIKE) is a group of GTPase that belongs to the centaurin superfamily. These proteins have been discovered for more than a decade but our understandings on their functions are still limited. Studies from our research group and others have revealed some of their functions in a cellular context but their roles in organ development or systemic homeostasis just begin to unveil. The generation of PIKE knockout mice thus provides the valuable model to delineate the physiological roles of PIKE. In addition to being a PI3K/Akt enhancer, phenotypic characterization of the PIKE knockout mice demonstrates that the proteins are involved in multiple signaling cascades including Janus kinase (JAK)/ Signal Transducer and Activator of Transcription (STAT), AMP-activated protein kinase (AMPK)/Acetyl-CoA carboxylase (ACC) and insulin receptor (IR)/Akt. In this article, we will review the current findings from the PIKE knockout mice studies and will discuss how these in vivo observations lead to the identifications of novel signaling cascades regulated by PIKE.

LKB1 regulates TCR-mediated PLCγ1 activation and thymocyte positive selection

The serine/threonine kinase LKB1 is a tumour suppressor that regulates cell growth, polarity, and proliferation in many different cell types. We previously demonstrated that LKB1 controls thymocyte survival via regulation of AMPK activation. In this study, we show that LKB1 was also involved in thymocyte positive selection through regulation of T cell receptor (TCR) signalling. Both Lck-Cre- and CD4-Cre-mediated deletion of LKB1 impaired the generation of mature CD4 and CD8 single positive (SP) thymocytes that might have resulted from the attenuated tyrosine phosphorylation of phospholipase C-γ 1 (PLCγ1) in the absence of LKB1. We found that LKB1 was directly phosphorylated by Lck at tyrosine residues 36, 261, and 365 and predominately interacted with LAT and PLCγ1 following TCR stimulation. Loss of LKB1 led to impaired recruitment of PLCγ1 to the LAT signalosome. Correlatively, LKB1-deficient thymocytes failed to upregulate lineage-specifying factors, and to differentiate into SP thymocytes even if their impaired survival was rescued. These observations indicated that LKB1 is a critical component involved in TCR signalling, and our studies provide novel insights into the mechanisms of LKB1-mediated thymocyte development.

Molecular mechanisms of tumor suppression by LKB1

The LKB1 tumor suppressor gene is frequently mutated in sporadic lung adenocarcinomas and cervical cancers and germline mutations are causative for Peutz-Jeghers syndrome characterized by gastrointestinal polyposis. The intracellular LKB1 kinase is implicated in regulating polarity, metabolism, cell differentiation, and proliferation - all functions potentially contributing to tumor suppression. LKB1 acts as an activating kinase of at least 14 kinases mediating LKB1 functions in a complex signaling network with partial overlaps. Regulation of the LKB1 signaling network is highly context dependent, and spatially organized in various cellular compartments. Also the mechanisms by which LKB1 activity suppresses tumorigenesis is context dependent, where recent observations are providing hints on the molecular mechanisms involved.

AMPK α2 subunit is involved in platelet signaling, clot retraction, and thrombus stability

The adenosine monophosphate (AMP)–activated protein kinase (AMPK) is a regulator of energy balance at the cellular and whole-body levels, but little is known about the role of AMPK in platelet activation. We report that both the α1 and α2 AMPK isoforms are expressed by human and murine platelets and that thrombin elicits the phosphorylation of AMPKα as well as the upstream kinase, liver kinase B1 (LKB1). In human platelets, the kinase inhibitors iodotubercidin and compound C significantly inhibited thrombin-induced platelet aggregation and clot retraction without affecting the initial increase in [Ca2+]i. Clot retraction was also impaired in platelets from AMPKα2−/− mice but not from wild-type littermates or AMPKα1−/− mice. Moreover, rebleeding was more frequent in AMPKα2−/− mice, and the FeCl3-induced thrombi formed in AMPKα2−/− mice were unstable. Mechanistically, AMPKα2 was found to phosphorylate in vitro the Src-family kinase, Fyn, and isoform deletion resulted in the attenuated threonine phosphorylation of Fyn as well as the subsequent tyrosine phosphorylation of its substrate, β3 integrin. These data indicate that AMPKα2—by affecting Fyn phosphorylation and activity—plays a key role in platelet αIIbβ3 integrin signaling, leading to clot retraction and thrombus stability.

AMPK inhibition in health and disease

All living organisms depend on dynamic mechanisms that repeatedly reassess the status of amassed energy, in order to adapt energy supply to demand. The AMP-activated protein kinase (AMPK) alphabetagamma heterotrimer has emerged as an important integrator of signals managing energy balance. Control of AMPK activity involves allosteric AMP and ATP regulation, auto-inhibitory features and phosphorylation of its catalytic (alpha) and regulatory (beta and gamma) subunits. AMPK has a prominent role not only as a peripheral sensor but also in the central nervous system as a multifunctional metabolic regulator. AMPK represents an ideal second messenger for reporting cellular energy state. For this reason, activated AMPK acts as a protective response to energy stress in numerous systems. However, AMPK inhibition also actively participates in the control of whole body energy homeostasis. In this review, we discuss recent findings that support the role and function of AMPK inhibition under physiological and pathological states.