Activation of 5'-AMP-activated kinase is mediated through c-Src and phosphoinositide 3-kinase activity during hypoxia-reoxygenation of bovine aortic endothelial cells. Role of peroxynitrite

AMP-activated protein kinase activation as a strategy for protecting vascular endothelial function

1. AMP-activated protein kinase (AMPK) is a serine/threonine protein kinase involved in the regulation of cellular and organismal metabolism. AMPK has a heterotrimeric structure, consisting of a catalytic alpha-subunit and regulatory beta- and gamma-subunits, each of which has two or more isoforms that are differentially expressed in various tissues and that arise from distinct genes. The AMPK system acts as a sensor of cellular energy status that is conserved in all eukaryotic cells. In addition, AMPK is activated by physiological stimuli and oxidants. 2. The importance of AMPK in cardiovascular functions is best demonstrated by recent studies showing that widely used drugs, including statins, metformin and rosiglitazone, execute cardiovascular protective effects at least partly through the activation of AMPK. As a consequence, AMPK has been proposed as a candidate target for therapeutic intervention in the treatment of both Type 2 diabetes and metabolic syndrome owing to its central role in the regulation of energy balance; it may also have a role in weight control. 3. In the present brief review, we summarize the recent progress of AMPK signalling and regulation focusing on vascular endothelial cells. We further hypothesize that AMPK is a dual sensor for energy and redox status within a cell and AMPK may be a therapeutic target for protecting vascular endothelial function.

Thromboxane receptor activates the AMP-activated protein kinase in vascular smooth muscle cells via hydrogen peroxide

Thromboxane A2 receptor (TPr) stimulation induces cellular hypertrophy in vascular smooth muscle cells (VSMCs); however, regulation of VSMC hypertrophy remains poorly understood. Here we show that TPr stimulation activates AMP-activated kinase (AMPK), which in turn limits TPr-induced protein synthesis in VSMCs. Exposure of cultured VSMCs to either TPr agonists, IBOP and U46619, or exogenous hydrogen peroxide (H2O2) caused time- and dose-dependent AMPK activation, as evidenced by increased phosphorylation of both AMPK-Thr172 and acetyl-coenzyme A carboxylase-Ser79, a downstream enzyme of AMPK, whereas SQ29548, a selective TPr antagonist, significantly attenuated TPr-enhanced AMPK activation. In parallel, both IBOP and U46619 significantly increased the production of reactive oxygen species such as H2O2. Furthermore, adenoviral overexpression of catalase (an H2O2 scavenger) abolished, whereas superoxide dismutase (which catalyzes H2O2 formation) enhanced, IBOP-induced AMPK activation, suggesting that TPr-activated AMPK was mediated by H2O2. Consistently, exposure of VSMCs to either TPr agonists or exogenous H2O2 dose-dependently increased the phosphorylation of LKB1 (at serines 428 and 307), an AMPK kinase, as well as coimmunoprecipitation of AMPK with LKB1. In addition, direct mutagenesis of either Ser428 or Ser307 of LKB1 into alanine, like the kinase-dead LKB1 mutant, abolished both TPr-stimulated AMPK activation and coimmunoprecipitation. Finally, genetic inhibition of AMPK significantly accentuated IBOP-enhanced protein synthesis, whereas adenoviral overexpression of constitutively active AMPK abolished IBOP-enhance protein synthesis in VSMCs. We conclude that TPr stimulation triggers reactive oxygen species-mediated LKB1-dependent AMPK activation, which in return inhibits cellular protein synthesis in VSMCs.

Local nitric oxide synthase inhibition reduces skeletal muscle glucose uptake but not capillary blood flow during in situ muscle contraction in rats

OBJECTIVE

We have previously shown in humans that local infusion of a nitric oxide synthase (NOS) inhibitor into the femoral artery attenuates the increase in leg glucose uptake during exercise without influencing total leg blood flow. However, rodent studies examining the effect of NOS inhibition on contraction-stimulated skeletal muscle glucose uptake have yielded contradictory results. This study examined the effect of local infusion of an NOS inhibitor on skeletal muscle glucose uptake (2-deoxyglucose) and capillary blood flow (contrast-enhanced ultrasound) during in situ contractions in rats.

RESEARCH DESIGN AND METHODS

Male hooded Wistar rats were anesthetized and one hindleg electrically stimulated to contract (2 Hz, 0.1 ms) for 30 min while the other leg rested. After 10 min, the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) (arterial concentration of 5 micromol/l) or saline was infused into the epigastric artery of the contracting leg.

RESULTS

Local NOS inhibition had no effect on blood pressure, heart rate, or muscle contraction force. Contractions increased (P < 0.05) skeletal muscle NOS activity, and this was prevented by L-NAME infusion. NOS inhibition caused a modest significant (P < 0.05) attenuation of the increase in femoral blood flow during contractions, but importantly there was no effect on capillary recruitment. NOS inhibition attenuated (P < 0.05) the increase in contraction-stimulated skeletal muscle glucose uptake by approximately 35%, without affecting AMP-activated protein kinase (AMPK) activation.

CONCLUSIONS

NOS inhibition attenuated increases in skeletal muscle glucose uptake during contraction without influencing capillary recruitment, suggesting that NO is critical for part of the normal increase in skeletal muscle fiber glucose uptake during contraction.

Peroxynitrite inhibits the expression of G(i)alpha protein and adenylyl cyclase signaling in vascular smooth muscle cells

We previously showed that S-nitroso-N-acetylpenicillamine, a nitric oxide donor, decreased the levels and functions of G(i)alpha proteins by formation of peroxynitrite (ONOO(-)) in vascular smooth muscle cells (VSMC). The present studies were undertaken to investigate whether ONOO(-) can modulate the expression of G(i)alpha protein and associated adenylyl cyclase signaling in VSMC. Treatment of A-10 and aortic VSMC with ONOO(-) for 24 h decreased the expression of G(i)alpha-2 and G(i)alpha-3, but not G(s)alpha, protein in a concentration-dependent manner; expression was restored toward control levels by (111)Mn-tetralis(benzoic acid porphyrin) and uric acid, but not by 1H[1,2,4]oxadiazole[4,3-a]quinoxaline-1-one (ODQ) and KT-5823. cGMP levels were increased by approximately 50% and 150% by 0.1 and 0.5 mM ONOO(-), respectively, and attenuated toward control levels by ODQ. In addition, 0.5 mM ONOO(-) attenuated the inhibition of adenylyl cyclase by ANG II and C-type atrial natriuretic peptide (C-ANP(4-23)), as well as the inhibition of forskolin-stimulated adenylyl cyclase activity by GTPgammaS, whereas, the G(s)-mediated stimulations were augmented. In addition, 0.5 mM ONOO(-) decreased phosphorylation of ERK1/2 and p38 MAP kinase and enhanced JNK phosphorylation but did not affect AKT1/3 phosphorylation. These results suggest that ONOO(-) decreased the expression of G(i) proteins and associated functions in VSMC through a cGMP-independent mechanism and may involve the MAP kinase signaling pathway.

Reactive Nitrogen Species Induced by Hyperglycemia Suppresses Akt Signaling and Triggers Apoptosis by Upregulating Phosphatase PTEN (Phosphatase and Tensin Homologue Deleted on Chromosome 10) in an LKB1-Dependent Manner

Background— Oxidative stress plays a causal role in vascular injury in diabetes mellitus, but the mechanisms and targets remain poorly understood.

Methods and Results— Exposure of cultured human umbilical vein endothelial cells to either peroxynitrite (ONOO − ) or high glucose significantly inhibited both basal and insulin-stimulated Akt phosphorylation at Ser473 and Akt activity in parallel with increased apoptosis, phosphorylation, and activity of phosphatase and tensin homologue deleted on chromosome 10 (PTEN). Furthermore, protein kinase B/Akt inhibition induced by ONOO − or high glucose and apoptosis triggered by high glucose could be abolished by transfection of PTEN-specific small interfering RNA, suggesting that PTEN mediated the Akt inhibition by ONOO − . In addition, exposure of human umbilical vein endothelial cells to ONOO − or high glucose remarkably increased Ser428 phosphorylation of LKB1, a tumor suppressor. Interestingly, the ONOO − -enhanced PTEN phosphorylation and Akt inhibition can be blocked by LKB1-specific small interfering RNA. Consistently, LKB1 phosphorylated PTEN at Ser380/Thr382/383 in vitro, suggesting that LKB1 might act as an upstream kinase for PTEN. Compared with nondiabetic mice, the levels of PTEN, LKB1-Ser428 phosphorylation, and 3-nitrotyrosine (a biomarker of ONOO − ) were significantly increased in the aortas of streptozotocin-induced diabetic mice, which was in parallel with a reduction in Akt-Ser473 phosphorylation and an increase in apoptosis. Furthermore, administration of PTEN-specific small interfering RNA suppressed diabetes-enhanced apoptosis and Akt inhibition. Finally, treatment with Tempol, a superoxide dismutase mimetic, and insulin, both of which reduced the ONOO − formation, markedly reduced diabetes-enhanced LKB1-Ser428 phosphorylation, PTEN, and apoptosis in the endothelium of mouse aortas.

Conclusion— We conclude that hyperglycemia triggers apoptosis by inhibiting Akt signaling via ONOO − -mediated LKB1-dependent PTEN activation.

AICAR induces phosphorylation of AMPK in an ATM-dependent, LKB1-independent manner

AMPK is an AMP-activated protein kinase that plays an important role in regulating cellular energy homeostasis. Metabolic stress, such as heat shock and glucose starvation, causes an energy deficiency in the cell and leads to elevated levels of intracellular AMP. This results in the phosphorylation and activation of AMPK. LKB1, a tumor suppressor, has been identified as an upstream kinase of AMPK. We found that in response to treatment with 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR), the LKB1 deficient cancer cell line, HeLa, exhibited AMPK-alpha phosphorylation. This indicates the existence of an LKB1-independent AMPK-alpha phosphorylation pathway. ATM is a protein that is deficient in the disease ataxia telangiectasia (A-T). We measured the activation of AMPK by AICAR in the normal mouse embryo fibroblast cell line, A29, and the mouse cell line lacking the ATM protein, A38. In A38 cells, the level of AICAR-induced AMPK-alpha phosphorylation was significantly lower than that found in A29 cells. Furthermore, phosphorylation of AMPK in HeLa and A29 cells was inhibited by an ATM specific inhibitor, KU-55933. Our results demonstrate that AICAR treatment could lead to phosphorylation of AMPK in an ATM-dependent and LKB1-independent manner. Thus, ATM may function as a potential AMPK kinase in response to AICAR treatment.

Agonist-modulated regulation of AMP-activated protein kinase (AMPK) in endothelial cells. Evidence for an AMPK -> Rac1 -> Akt -> endothelial nitric-oxide synthase pathway

The endothelial isoform of nitric-oxide synthase (eNOS), a key determinant of vascular homeostasis, is a calcium/calmodulin-dependent phosphoprotein regulated by diverse cell surface receptors. Vascular endothelial growth factor (VEGF) and sphingosine 1-phosphate (S1P) stimulate eNOS activity through Akt/phosphoinositide 3-kinase and calcium-dependent pathways. AMP-activated protein kinase (AMPK) also activates eNOS in endothelial cells; however, the molecular mechanisms linking agonist-mediated AMPK regulation with eNOS activation remain incompletely understood. We studied the role of AMPK in VEGF- and S1P-mediated eNOS activation and found that both agonists led to a striking increase in AMPK phosphorylation in pathways involving the calcium/calmodulin-dependent protein kinase kinase beta. Treatment with tyrosine kinase inhibitors or the phosphoinositide 3-kinase inhibitor wortmannin demonstrated differential effects of VEGF versus S1P. Small interfering RNA (siRNA)-mediated knockdown of AMPKalpha1or Akt1 impaired the stimulatory effects of both VEGF and S1P on eNOS activation. AMPKalpha1 knockdown impaired agonist-mediated Akt phosphorylation, whereas Akt1 knockdown did not affect AMPK activation, thus suggesting that AMPK lies upstream of Akt in the pathway leading from receptor activation to eNOS stimulation. Importantly, we found that siRNA-mediated knockdown of AMPKalpha1 abrogates agonist-mediated activation of the small GTPase Rac1. Conversely, siRNA-mediated knockdown of Rac1 decreased the agonist-mediated phosphorylation of AMPK substrates without affecting that of AMPK, implicating Rac1 as a molecular link between AMPK and Akt in agonist-mediated eNOS activation. Finally, siRNA-mediated knockdown of caveolin-1 significantly enhanced AMPK phosphorylation, suggesting that AMPK is negatively regulated by caveolin-1. Taken together, these results suggest that VEGF and S1P differentially regulate AMPK and establish a central role for an agonist-modulated AMPK --> Rac1 --> Akt axis in the control of eNOS in endothelial cells.

Long-term activation of adenosine monophosphate-activated protein kinase attenuates pressure-overload-induced cardiac hypertrophy

Recent in vitro studies suggest that adenosine monophosphate (AMP)-activated protein kinase (AMPK) exerts inhibitory effects on cardiac hypertrophy. However, it is unclear whether long-term activation of AMPK will affect cardiac hypertrophy in vivo. In these reports, we investigate the in vivo effects of long-term AMPK activation on cardiac hypertrophy and the related molecular mechanisms. To examine the effects of AMPK activation in the development of pressure overload-induced cardiac hypertrophy, we administered 5-aminoimidazole 1 carboxamide ribonucleoside (AICAR, 0.5 mg/g body wt), a specific activator of AMPK, to rats with transaortic constriction (TAC) for 7 weeks. We found that long-term AMPK activation attenuated cardiac hypertrophy, and improved cardiac function in rats subjected to TAC. Furthermore, long-term AMPK activation attenuated protein synthesis, diminished calcineurin-nuclear factor of activated T cells (NFAT) and nuclear factor kappaB (NF-kappaB) signaling in pressure overload-induced hypertrophic hearts. Our in vitro experiments further proved that activation of AMPK by infection of AdAMPK blocked cardiac hypertrophy and NFAT, NF-kappaB, and MAPK signal pathways. The present study demonstrates for the first time that pharmacological activation of AMPK inhibits cardiac hypertrophy in through blocking signaling transduction pathways that are involved in cardiac growth. It presents a potential therapy strategy to inhibit pathological cardiac hypertrophy by increasing the activity of AMPK.

Nitric oxide production is a proximal signaling event controlling exercise-induced mRNA expression in human skeletal muscle

Previous studies have described the magnitude and time course by which several genes are regulated within exercising skeletal muscle. These include interleukin-6 (IL-6), interleukin-8 (IL-8), heme oxygenase-1 (HO-1), and heat shock protein-72 (HSP72), which are involved in secondary signaling and preservation of intracellular environment. However, the primary signaling mechanisms coupling contraction to transcription are unknown. We hypothesized that exercise-induced nitric oxide (NO) production is an important signaling event for IL-6, IL-8, HO-1, and HSP72 expression in muscle. Twenty healthy males participated in the study. By real-time PCR, mRNA levels for 11 genes were determined in thigh muscle biopsies obtained 1) before and after 2 h knee extensor exercise without (control) and with concomitant NO synthase inhibition (nitro-L-arginine methyl ester, L-NAME, 5 mg x kg(-1)); or 2) before and after 2 h femoral artery infusion of the NO donor nitroglycerin (NTG, 1.5 microg x kg(-1) x min(-1)). L-NAME caused marked reductions in exercise-induced expression of 4 of 11 mRNAs including IL-6, IL-8, and HO-1. IL-6 protein release from the study leg to the circulation increased in the control but not in the L-NAME trial. NTG infusion significantly augmented expression of the mRNAs attenuated by L-NAME. These findings advance the novel concept that NO production contributes to regulation of gene expression in muscle during exercise. Subsequently, we sought evidence for involvement of AMP-activated kinase or nuclear factor kappa B, but found none.

Antidiabetes and antiobesity effect of cryptotanshinone via activation of AMP-activated protein kinase

Metabolic disorders, including type 2 diabetes and obesity, represent major health risks in industrialized countries. AMP-activated protein kinase (AMPK) has become the focus of a great deal of attention as a novel therapeutic target for the treatment of metabolic syndromes, because AMPK has been demonstrated to mediate, at least in part, the effects of a number of physiological and pharmacological factors that exert beneficial effects on these disorders. Thus, the identification of a compound that activates the AMPK pathway would contribute significantly to the treatment and management of such syndromes. In service of this goal, we have screened a variety of naturally occurring compounds and have identified one compound, cryptotanshinone, as a novel AMPK pathway activator. Cryptotanshinone was originally isolated from the dried roots of Salvia militorrhiza, an herb that is used extensively in Asian medicine and that is known to exert beneficial effects on the circulatory system. For the first time, in the present study, we have described the potent antidiabetic and antiobesity effects of cryptotanshinone, both in vitro and in vivo. Our findings suggest that the activation of the AMPK pathway might contribute to the development of novel therapeutic approaches for the treatment of metabolic disorders such as type 2 diabetes and obesity.

Peroxynitrite mediates VEGF's angiogenic signal and functionviaa nitration‐independent mechanism in endothelial cells

The modulation of angiogenic signaling by reactive oxygen species (ROS) is an emerging area of interest in cellular and vascular biology research. We provide evidence here that peroxynitrite, the powerful oxidizing and nitrating free radical, is critically involved in transduction of the VEGF signal. We tested the hypothesis that VEGF induces peroxynitrite formation, which causes tyrosine phosphorylation and mediates endothelial cell migration and tube formation, by studies of vascular endothelial cells in vitro and in a model of hypoxia-induced neovascularization in vivo. The specific peroxynitrite decomposition catalyst FeTPPs blocked VEGF-induced phosphorylation of VEGFR2 and c-Src and inhibited endothelial cell migration and tube formation. Furthermore, exogenous peroxynitrite mimicked VEGF activity in causing phosphorylation of VEGFR2 and stimulating endothelial cell growth and tube formation in vitro and new blood vessel growth in vivo. The selective nitration inhibitor epicatechin enhanced VEGF's angiogenic function in activating VEGFR2, c-Src, and promoting endothelial cell growth, migration, and tube formation in vitro and retinal neovascularization in vivo. Decomposing peroxynitrite with FeTPPs or blocking oxidation using the thiol donor NAC blocked VEGF's angiogenic functions in vitro and in vivo. In conclusion, peroxynitrite is critically involved in transducing VEGF's angiogenic signal via nitration-independent and oxidation-mediated tyrosine phosphorylation.

Hypoxia-induced reactive oxygen species formation in skeletal muscle

The existence of hypoxia-induced reactive oxygen species (ROS) production remains controversial. However, numerous observations with a variety of methods and in many cells and tissue types are supportive of this idea. Skeletal muscle appears to behave much like heart in that in the early stages of hypoxia there is a transient elevation in ROS, whereas in chronic exposure to very severe hypoxia there is evidence of ongoing oxidative stress. Important remaining questions that are addressed in this review include the following. Are there levels of PO2 in skeletal muscle, typical of physiological or mildly pathophysiological conditions, that are low enough to induce significant ROS production? Does the ROS associated with muscle contractile activity reflect imbalances in oxygen uptake and demand that drive the cell to a more reduced state? What are the possible molecular mechanisms by which ROS may be elevated in hypoxic skeletal muscle? Is the production of ROS in hypoxia of physiological significance, both with respect to cell signaling pathways promoting cell function and with respect to damaging effects of long-term exposure? Discussion of these and other topics leads to general conclusions that hypoxia-induced ROS may be a normal physiological response to imbalance in oxygen supply and demand or environmental stress and may play a yet undefined role in normal response mechanisms to these stimuli. However, in chronic and extreme hypoxic exposure, muscles may fail to maintain a normal redox homeostasis, resulting in cell injury or dysfunction.

AMP-activated protein kinase mediates VEGF-stimulated endothelial NO production

Vascular endothelial growth factor (VEGF) is an important regulator of endothelial cell function. VEGF stimulates NO production, proposed to be a result of phosphorylation and activation of endothelial NO synthase (eNOS) at Ser1177. Phosphorylation of eNOS at this site also occurs after activation of AMP-activated protein kinase (AMPK) in cultured endothelial cells. We therefore determined whether AMPK mediates VEGF-stimulated NO synthesis in endothelial cells. VEGF caused a rapid, dose-dependent stimulation of AMPK activity, with a concomitant increase in phosphorylation of eNOS at Ser1177. Infection of endothelial cells with an adenovirus expressing a dominant negative mutant AMPK partially inhibited both VEGF-stimulated eNOS Ser1177 phosphorylation and NO production. VEGF-stimulated AMPK activity was completely inhibited by the Ca²⁺/calmodulin-dependent protein kinase kinase inhibitor, STO-609. Stimulation of AMPK via Ca²⁺/calmodulin-dependent protein kinase kinase represents a novel signalling mechanism utilised by VEGF in endothelial cells that contributes to eNOS phosphorylation and NO production.

Nitric oxide and peroxynitrite in health and disease

The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review.

Selenium regulates cyclooxygenase-2 and extracellular signal-regulated kinase signaling pathways by activating AMP-activated protein kinase in colon cancer cells

Epidemiologic and experimental evidences indicate that selenium, an essential trace element, can reduce the risk of a variety of cancers. Protection against certain types of cancers, particularly colorectal cancers, is closely associated with pathways involving cyclooxygenase-2 (COX-2). We found that AMP-activated protein kinase (AMPK), which functions as a cellular energy sensor, mediates critical anticancer effects of selenium via a COX-2/prostaglandin E(2) signaling pathway. Selenium activated AMPK in tumor xenografts as well as in colon cancer cell lines, and this activation seemed to be essential to the decrease in COX-2 expressions. Transduction with dominant-negative AMPK into colon cancer cells or application of cox-2(-/-)-negative cells supported the evidence that AMPK is an upstream signal of COX-2 and inhibits cell proliferation. In HT-29 colon cancer cells, carcinogenic agent 12-O-tetradecanoylphorbol-13-acetate (TPA) activated extracellular signal-regulated kinase (ERK) that led to COX-2 expression and selenium blocked the TPA-induced ERK and COX-2 activation via AMPK. We also showed the role of a reactive oxygen species as an AMPK activation signal in selenium-treated cells. We propose that AMPK is a novel and critical regulatory component in selenium-induced cancer cell death, further implying AMPK as a prime target of tumorigenesis.

LKB1, an upstream AMPK kinase, regulates glucose and lipid metabolism in cultured liver and muscle cells

LKB1 is a 50 kDa serine/threonine kinase that phosphorylates and activates the catalytic subunit of AMPK at its T-loop residue Thr 172. We prepared adenoviruses expressing the constitutive active (wild-type) form (CA) or dominant negative (kinase inactive, D194A mutant) form (DN) of LKB1 and overexpressed these proteins in cultured myotubes (C2C12 cells) and rat hepatoma cells (FAO cells). When analyzed by immunoblotting with the antibody against Thr172-phosphorylated AMPK, the phosphorylation of AMPK was increased (2.5-fold) and decreased (0.4-fold) in cells expressing CA and DN LKB1, respectively, as compared with Lac-Z expressing control cells. Immunoprecipitation experiments, using isoform-specific antibody, revealed these alterations of AMPK phosphorylation to be attributable to altered phosphorylation of AMPK alpha2, but not alpha1 catalytic subunits, strongly suggesting the alpha2 catalytic subunit to be the major substrate for LKB1 in mammalian cells. In addition, adiponectin or AICAR-stimulated AMPK phosphorylation was inhibited by overexpression of DN LKB1, while phenformin-stimulated phosphorylation was unaffected. These results may explain the difference in AMPK activation mechanisms between AMP and phenformin, and also indicate that AMPK phosphorylation by LKB1 is involved in AMP-stimulated AMPK activation. As a downstream target for AMPK, AICAR-induced glucose uptake and ACCbeta phosphorylation were found to be significantly reduced in DN LKB1 expressing C2C12 cells. The expression of key enzymes for gluconeogenesis, glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, was also dependent on LKB1 activities in FAO cells. These results demonstrate that LKB1 is a crucial regulator of AMPK activation in muscle and liver cells and, therefore, that LKB1 activity is potentially of importance to our understanding of glucose and lipid metabolism.

Uncoupling of Endothelial Nitric Oxidase Synthase by Hypochlorous Acid

OBJECTIVE

The aim of the present study is to determine whether hypochlorous acid (HOCl), the major oxidant of leukocyte-derived myeloperoxidase (MPO), oxidizes the zinc-thiolate center of endothelial nitric oxide synthase (eNOS) and uncouples the enzyme.

METHODS AND RESULTS

Exposure of purified recombinant eNOS to HOCl (> or = 100 micromol/L) released zinc and disrupted the enzyme-active eNOS dimers. In parallel with increased detections of both O2*- and ONOO-, clinically relevant concentrations of HOCl disrupted eNOS dimers in cultured human umbilical vein endothelial cells (HUVEC) at concentration 10- to 100-fold lower than those required for recombinant eNOS. In HUVEC, HOCl increased the translocation of both p67(phox) and p47(phox) of NAD(P)H oxidase and the phosphorylation of atypical protein kinase C-zeta. Further, genetic or pharmacological inhibition of either NAD(P)H oxidase-derived O2*- or PKC-zeta or NOS abolished the effects of HOCl on eNOS dimers. Consistently, HOCl increased both O2*- and ONOO- and eNOS dimer oxidation in isolated mouse aortas from C57BL/6 but less in those of gp91(phox) knock-out mice. Finally, in human carotid atherosclerotic arteries, eNOS predominantly existed as monomers in parallel with increased staining of both MPO and 3-nitrotyrosine.

CONCLUSIONS

We conclude that HOCl uncouples eNOS by ONOO- generated from PKC-zeta-dependent NAD(P)H oxidase.

Meiotic induction by heat stress in mouse oocytes: involvement of AMP-activated protein kinase and MAPK family members

In this study, we examined the effect of heat pulsing on oocyte maturation and assessed the possible role of stress-activated enzymes during heat stress-induced meiotic maturation. Denuded oocytes from immature eCG-primed mice were pulsed for 30 min at increasing temperatures from 40 degrees C to 43 degrees C in dibutyryl cAMP-containing medium and were subsequently cultured at 37 degrees C for a total incubation time of 17-18 h. Oocytes exposed to 42 degrees C showed the greatest stimulation of maturation, with no effect at 43 degrees C. A heat pulse did not compromise progression to metaphase II as observed by polar body (PB) formation. The AMP-activated protein kinase (PRKA) inhibitors compound C and Ara-A each blocked the meiosis-stimulating effects of heat. Western blots showed that acetyl-CoA carboxylase, an important substrate of PRKA, was phosphorylated in heat-treated germinal vesicle-stage oocytes, indicating activation of PRKA before maturation. The mitogen-activated protein 2 kinase (MAP2K1) inhibitor PD98059 also prevented heat-induced maturation, but this effect was unrelated to MAPK1/3 activation, which was not observed until after germinal vesicle breakdown (GVB). Phosphorylated MAPK14 was not detected in the oocyte under any experimental condition, and only high concentrations of the MAPK14 inhibitor SB203580 blocked heat-stimulated maturation, suggesting that MAPK14 is not involved in meiotic induction. MAPK8/9 was activated by heat, and the MAPK8/9 inhibitor SP600125, but not JUN N-terminal kinase I, blocked heat-induced maturation. Heat treatment transiently suppressed GVB and PB formation in spontaneously maturing oocytes by a mechanism that is apparently different from its meiosis-inducing action. Collectively, these data show that an acute heat pulse stimulates GVB in meiotically arrested oocytes and suggest that this effect is mediated through the activation of PRKA.

Free Radicals, Mitochondria, and Oxidized Lipids

Mitochondria have long been known to play a critical role in maintaining the bioenergetic status of cells under physiological conditions. It was also recognized early in mitochondrial research that the reduction of oxygen to generate the free radical superoxide occurs at various sites in the respiratory chain and was postulated that this could lead to mitochondrial dysfunction in a variety of disease states. Over recent years, this view has broadened substantially with the discovery that reactive oxygen, nitrogen, and lipid species can also modulate physiological cell function through a process known as redox cell signaling. These redox active second messengers are formed through regulated enzymatic pathways, including those in the mitochondrion, and result in the posttranslational modification of mitochondrial proteins and DNA. In some cases, the signaling pathways lead to cytotoxicity. Under physiological conditions, the same mediators at low concentrations activate the cytoprotective signaling pathways that increase cellular antioxidants. Thus, it is critical to understand the mechanisms by which these pathways are distinguished to develop strategies that will lead to the prevention of cardiovascular disease. In this review, we describe recent evidence that supports the hypothesis that mitochondria have an important role in cell signaling, and so contribute to both the adaptation to oxidative stress and the development of vascular diseases.

Thrombin activates AMP-activated protein kinase in endothelial cells via a pathway involving Ca2+/calmodulin-dependent protein kinase kinase beta

AMP-activated protein kinase (AMPK) is a sensor of cellular energy state in response to metabolic stress and other regulatory signals. AMPK is controlled by upstream kinases which have recently been identified as LKB1 or Ca2+/calmodulin-dependent protein kinase kinase beta (CaMKKbeta). Our study of human endothelial cells shows that AMPK is activated by thrombin through a Ca2+-dependent mechanism involving the thrombin receptor protease-activated receptor 1 and Gq-protein-mediated phospholipase C activation. Inhibition of CaMKK with STO-609 or downregulation of CaMKKbeta using RNA interference decreased thrombin-induced AMPK activation significantly, indicating that CaMKKbeta was the responsible AMPK kinase. In contrast, downregulation of LKB1 did not affect thrombin-induced AMPK activation but abolished phosphorylation of AMPK with 5-aminoimidazole-4-carboxamide ribonucleoside. Thrombin stimulation led to phosphorylation of acetyl coenzyme A carboxylase (ACC) and endothelial nitric oxide synthase (eNOS), two downstream targets of AMPK. Inhibition or downregulation of CaMKKbeta or AMPK abolished phosphorylation of ACC in response to thrombin but had no effect on eNOS phosphorylation, indicating that thrombin-stimulated phosphorylation of eNOS is not mediated by AMPK. Our results underline the role of Ca2+ as a regulator of AMPK activation in response to a physiologic stimulation. We also demonstrate that endothelial cells possess two pathways to activate AMPK, one Ca2+/CaMKKbeta dependent and one AMP/LKB1 dependent.

Regulatory role for the arginine–nitric oxide pathway in metabolism of energy substrates

Nitric oxide (NO) is synthesized from L-arginine by NO synthase in virtually all cell types. Emerging evidence shows that NO regulates the metabolism of glucose, fatty acids and amino acids in mammals. As an oxidant, pathological levels of NO inhibit nearly all enzyme-catalyzed reactions through protein oxidation. However, as a signaling molecule, physiological levels of NO stimulate glucose uptake as well as glucose and fatty acid oxidation in skeletal muscle, heart, liver and adipose tissue; inhibit the synthesis of glucose, glycogen, and fat in target tissues (e.g., liver and adipose); and enhance lipolysis in adipocytes. Thus, an inhibition of NO synthesis causes hyperlipidemia and fat accretion in rats, whereas dietary arginine supplementation reduces fat mass in diabetic fatty rats. The putative underlying mechanisms may involve multiple cyclic guanosine-3',5'-monophosphate-dependent pathways. First, NO stimulates the phosphorylation of adenosine-3',5'-monophosphate-activated protein kinase, resulting in (1) a decreased level of malonyl-CoA via inhibition of acetyl-CoA carboxylase and activation of malonyl-CoA decarboxylase and (2) a decreased expression of genes related to lipogenesis and gluconeogenesis (glycerol-3-phosphate acyltransferase, sterol regulatory element binding protein-1c and phosphoenolpyruvate carboxykinase). Second, NO increases the phosphorylation of hormone-sensitive lipase and perilipins, leading to the translocation of the lipase to the neutral lipid droplets and, hence, the stimulation of lipolysis. Third, NO activates expression of peroxisome proliferator-activated receptor-gamma coactivator-1alpha, thereby enhancing mitochondrial biogenesis and oxidative phosphorylation. Fourth, NO increases blood flow to insulin-sensitive tissues, promoting substrate uptake and product removal via the circulation. Modulation of the arginine-NO pathway through dietary supplementation with L-arginine or L-citrulline may aid in the prevention and treatment of the metabolic syndrome in obese humans and companion animals, and in reducing unfavorable fat mass in animals of agricultural importance.

Mitochondria as signaling organelles in the vascular endothelium

Vascular endothelial cells are highly glycolytic and consume relatively low amounts of oxygen (O(2)) compared with other cells. We have confirmed that oxidative phosphorylation is not the main source of ATP generation in these cells. We also show that at a low O(2) concentration (<1%) endogenous NO plays a key role in preventing the accumulation of the alpha-subunit of hypoxia-inducible factor 1. At higher O(2) concentrations (1-3%) NO facilitates the production of mitochondrial reactive oxygen species. This production activates the AMP-activated protein kinase by a mechanism independent of nucleotide concentrations. Thus, the primary role of mitochondria in vascular endothelial cells may not be to generate ATP but, under the control of NO, to act as signaling organelles using either O(2) or O(2)-derived species as signaling molecules. Diversion of O(2) away from endothelial cell mitochondria by NO might also facilitate oxygenation of vascular smooth muscle cells.

Low-intensity contraction activates the alpha1-isoform of 5'-AMP-activated protein kinase in rat skeletal muscle

Skeletal muscle expresses two catalytic subunits, alpha1 and alpha2, of the 5'-AMP-activated protein kinase (AMPK), which has been implicated in contraction-stimulated glucose transport and fatty acid oxidation. Muscle contraction activates the alpha2-containing AMPK complex (AMPKalpha2), but this activation may occur with or without activation of the alpha1-containing AMPK complex (AMPKalpha1), suggesting that AMPKalpha2 is the major isoform responsible for contraction-induced metabolic events in skeletal muscle. We report for the first time that AMPKalpha1, but not AMPKalpha2, can be activated in contracting skeletal muscle. Rat epitrochlearis muscles were isolated and incubated in Krebs-Ringer bicarbonate buffer containing pyruvate. In muscles stimulated to contract at a frequency of 1 and 2 Hz during the last 2 min of incubation, AMPKalpha1 activity increased twofold and AMPKalpha2 activity remained unchanged. Muscle stimulation did not change the muscle AMP concentration or the AMP-to-ATP ratio. AMPK activation was associated with increased phosphorylation of Thr(172) of the alpha-subunit, the primary activation site. Muscle stimulation increased the phosphorylation of acetyl-CoA carboxylase (ACC), a downstream target of AMPK, and the rate of 3-O-methyl-d-glucose transport. In contrast, increasing the frequency (>or=5 Hz) or duration (>or=5 min) of contraction activated AMPKalpha1 and AMPKalpha2 and increased AMP concentration and the AMP/ATP ratio. These results suggest that 1) AMPKalpha1 is the predominant isoform activated by AMP-independent phosphorylation in low-intensity contracting muscle, 2) AMPKalpha2 is activated by an AMP-dependent mechanism in high-intensity contracting muscle, and 3) activation of each isoform enhances glucose transport and ACC phosphorylation in skeletal muscle.

Extracellular nucleotides and adenosine independently activate AMP-activated protein kinase in endothelial cells: involvement of P2 receptors and adenosine transporters

AMP-activated protein kinase (AMPK) plays a key role in the regulation of energy homeostasis and is activated in response to cellular stress, including hypoxia/ischemia and hyperglycemia. The stress events are accompanied by rapid release of extracellular nucleotides from damaged tissues or activated endothelial cells (EC) and platelets. We demonstrate that extracellular nucleotides (ATP, ADP, and UTP, but not UDP) and adenosine independently induce phosphorylation and activation of AMPK in human umbilical vein EC (HUVEC) by the mechanism that is not linked to changes in AMP:ATP ratio. HUVEC express NTPDases, as well as 5'-nucleotidase; hence, nucleotides can be metabolized to adenosine. However, inhibition of 5'-nucleotidase had no effect on ATP/ADP/UTP-induced phospho- rylation of AMPK, indicating that AMPK activation occurred as a direct response to nucleotides. Nucleotide-evoked phosphorylation of AMPK in HUVEC was mediated by P2Y1, P2Y2, and/or P2Y4 receptors, whereas P2Y6, P2Y11, and P2X receptors were not involved. The nucleotide-induced phosphorylation of AMPK was affected by changes in the concentration of intracellular Ca2+ and by Ca2+/calmodulin-dependent kinase kinase (CaMKK), although most likely it was not dependent on LKB1 kinase. Adenosine-induced phosphorylation of AMPK was not mediated by P1 receptors but required adenosine uptake by equilibrative nucleoside transporters followed by its (intracellular) metabolism to AMP. Moreover, adenosine effect was Ca2+ and CaMKK independent, although probably associated with upstream LKB1. We hypothesize that P2 receptors and adenosine transporters could be novel targets for the pharmacological regulation of AMPK activity and its downstream effects on EC function.

Activation of the AMP-activated kinase by antidiabetes drug metformin stimulates nitric oxide synthesis in vivo by promoting the association of heat shock protein 90 and endothelial nitric oxide synthase

Metformin, one of most commonly used drugs for the treatment of type 2 diabetes, improves vascular endothelial functions and reduces cardiovascular events in patients with type 2 diabetes, although its mechanisms remain unknown. The current study aimed to elucidate how metformin improves endothelial functions. Exposure of cultured bovine aortic endothelial cells (BAECs) to clinically relevant concentrations of metformin (50-500 micromol/l) dose-dependently increased serine-1179 (Ser1179) phosphorylation (equal to human Ser1179) of endothelial nitric oxide (NO) synthase (eNOS) as well as its association with heat shock protein (hsp)-90, resulting in increased activation of eNOS and NO bioactivity (cyclic GMP). These effects of metformin were mimicked or completely abrogated by adenoviral overexpression of a constitutively active 5'-AMP-activated kinase (AMPK) mutant or a kinase-inactive AMPK-alpha, respectively. Furthermore, administration of metformin as well as 5-aminoimidazole-4-carboxamide ribonucleoside, an AMPK agonist, significantly increased eNOS Ser1179 phosphorylation, NO bioactivity, and coimmunoprecipitation of eNOS with hsp90 in wild-type C57BL6 mice but not in AMPK-alpha1 knockout mice, suggesting that AMPK is required for metformin-enhanced eNOS activation in vivo. Finally, incubation of BAECs with clinically relevant concentrations of metformin dramatically attenuated high-glucose (30 mmol/l)-induced reduction in the association of hsp90 with eNOS, which resulted in increased NO bioactivity with a reduction in overexpression of adhesion molecules and endothelial apoptosis caused by high-glucose exposure. Taken together, our results indicate that metformin might improve vascular endothelial functions in diabetes by increasing AMPK-dependent, hsp90-mediated eNOS activation.

Activation of protein kinase C zeta by peroxynitrite regulates LKB1-dependent AMP-activated protein kinase in cultured endothelial cells

We previously reported the phosphoinositide 3-kinase-dependent activation of the 5'-AMP-activated kinase (AMPK) by peroxynitrite (ONOO-) and hypoxia-reoxygenation in cultured endothelial cells. Here we show the molecular mechanism of activation of this pathway. Exposure of bovine aortic endothelial cells to ONOO- significantly increased the phosphorylation of both Thr172 of AMPK and Ser1179 of endothelial nitric-oxide synthase, a known downstream enzyme of AMPK. In addition, activation of AMPK by ONOO- was accompanied by increased phosphorylation of protein kinase Czeta (PKCzeta) (Thr410/403) and translocation of cytosolic PKCzeta into the membrane. Further, inhibition of PKCzeta abrogated ONOO- -induced AMPK-Thr172 phosphorylation as that of endothelial nitric-oxide synthase. Furthermore, overexpression of a constitutively active PKCzeta mutant enhanced the phosphorylation of AMPK-Thr172, suggesting that PKCzeta is upstream of AMPK activation. In contrast, ONOO- activated PKCzeta in LKB1-deficient HeLa-S3 but affected neither AMPK-Thr172 nor AMPK activity. These data suggest that LKB1 is required for PKCzeta-enhanced AMPK activation. In vitro, recombinant PKCzeta phosphorylated LKB1 at Ser428, resulting in phosphorylation of AMPK at Thr172. Further, direct mutation of Ser428 of LKB1 into alanine, like the kinase-inactive LKB1 mutant, abolished ONOO- -induced AMPK activation. In several cell types originating from human, rat, and mouse, inhibition of PKCzeta significantly attenuated the phosphorylation of both LKB1-Ser428 and AMPK-Thr172 that were enhanced by ONOO-. Taken together, we conclude that PKCzeta can regulate AMPK activity by increasing the Ser428 phosphorylation of LKB1, resulting in association of LKB1 with AMPK and consequent AMPK Thr172 phosphorylation by LKB1.

Up-regulation of AMP-activated kinase by dysfunctional cystic fibrosis transmembrane conductance regulator in cystic fibrosis airway epithelial cells mitigates excessive inflammation

AMP-activated kinase (AMPK) is a ubiquitous metabolic sensor that inhibits the cystic fibrosis (CF) transmembrane conductance regulator (CFTR). To determine whether CFTR reciprocally regulates AMPK function in airway epithelia and whether such regulation is involved in lung inflammation, AMPK localization, expression, and activity and cellular metabolic profiles were compared as a function of CFTR status in CF and non-CF primary human bronchial epithelial (HBE) cells. As compared with non-CF HBE cells, CF cells had greater and more diffuse AMPK staining and had greater AMPK activity than their morphologically matched non-CF counterparts. The cellular [AMP]/[ATP] ratio was higher in undifferentiated than in differentiated non-CF cells, which correlated with AMPK activity under these conditions. However, this nucleotide ratio did not predict AMPK activity in differentiating CF cells. Inhibiting channel activity in non-CF cells did not affect AMPK activity or metabolic status, but expressing functional CFTR in CF cells reduced AMPK activity without affecting cellular [AMP]/[ATP]. Therefore, lack of functional CFTR expression and not loss of channel activity in CF cells appears to up-regulate AMPK activity in CF HBE cells, presumably through non-metabolic effects on upstream regulatory pathways. Compared with wild-type CFTR-expressing immortalized CF bronchial epithelial (CFBE) cells, DeltaF508-CFTR-expressing CFBE cells had greater AMPK activity and greater secretion of tumor necrosis factor-alpha and the interleukins IL-6 and IL-8. Further pharmacologic AMPK activation inhibited inflammatory mediator secretion in both wild type- and DeltaF508-expressing cells, suggesting that AMPK activation in CF airway cells is an adaptive response that reduces inflammation. We propose that therapies to activate AMPK in the CF airway may be beneficial in reducing excessive airway inflammation, a major cause of CF morbidity.

Stress stimulates AMP-activated protein kinase and meiotic resumption in mouse oocytes

This study examined the effects of three different cellular stresses on oocyte maturation in meiotically arrested mouse oocytes. Cumulus-cell enclosed oocytes (CEO) or denuded oocytes (DO) from immature, eCG-primed mice were cultured for 17-18 h in dbcAMP-containing medium plus increasing concentrations of the metabolic poison, sodium arsenite, or the free radical-generating agent, menadione. Alternatively, oocytes were exposed to osmotic stress by pulsing with sorbitol and returned to control inhibitory conditions for the duration of culture. Arsenite and menadione each dose-dependently induced germinal vesicle breakdown (GVB) in both DO and CEO. DO, but not CEO, pulsed for 60 min with 500 mM sorbitol were stimulated to resume maturation. The lack of effect in CEO suggests that the cumulus cells may be playing a protective role in osmotic stress-induced GVB. The AMP-activated protein kinase (PRKA; formerly known as AMPK) inhibitors, compound C and araA, completely blocked the meiosis-stimulating effects of all the tested stresses. Western blots showed that acetyl-CoA carboxylase, an important substrate of PRKA, was phosphorylated before GVB, supporting a role for PRKA in stress-induced maturation. Together, these data show that a variety of stresses stimulate GVB in meiotically arrested mouse oocytes in vitro and suggest that this effect is mediated through activation of PRKA.

Beta-adrenoceptors, but not alpha-adrenoceptors, stimulate AMP-activated protein kinase in brown adipocytes independently of uncoupling protein-1

AIMS/HYPOTHESIS

Brown adipocytes provide a potentially important model system for understanding AMP-activated protein kinase (AMPK) regulation, where adrenergic stimulation leads to mitochondrial uncoupling through uncoupling protein-1 (UCP1) activity. AMPK is a sensor of energy homeostasis and has been implicated in glucose and lipid metabolism in several insulin-sensitive tissues. The aim of this study was to characterise the potential role of AMPK in adrenergically mediated glucose uptake and to find out whether UCP1 is involved in the adrenergic activation of AMPK.

METHODS

We used primary brown adipocytes differentiated in culture and measured AMPK phosphorylation and glucose uptake following adrenergic activation.

RESULTS

Treatment of adipocytes with noradrenaline (norepinephrine) caused phosphorylation of AMPK via beta-adrenoceptors and not alpha(1)- or alpha(2)-adrenoceptors. This effect was not beta(3)-adrenoceptor specific, since responses remained intact in adipocytes from beta(3)-adrenoceptor knock-out mice. These effects were also mimicked by forskolin and cAMP analogues. Treatment of cells with adenine 8-beta-D-arabinofuranoside, an AMPK inhibitor, partially blocked beta-adrenoceptor-mediated increases in glucose uptake. Brown adipocytes are characterised by the production of UCP1, which can uncouple the mitochondria. Using adipocytes from Ucp1(+/+) and Ucp1(-/-) mice, we showed that noradrenaline-mediated phosphorylation of AMPK does not require the presence or activity of UCP1.

CONCLUSIONS/INTERPRETATION

These results suggest a pathway where increases in cAMP mediated by beta-adrenoceptors leads to activation of AMPK in brown adipocytes, which contributes in part to beta-adrenoceptor-mediated increases in glucose uptake, an effect independent of the presence or function of UCP1.

AICAR stimulates IL-6 production via p38 MAPK in cardiac fibroblasts in adult mice: a possible role for AMPK

Though known as a sensor of energy balance, AMP-activated protein kinase (AMPK) was recently shown to limit damage and apoptotic activity and contribute to the late preconditioning in heart. Interleukin-6 was also reported to involve in anti-apoptosis and cardio-protection in myocardium. Interestingly, both AMPK activity and IL-6 level were increased in response to ischemia, hypertrophy and oxidative stress. To determine whether AMPK activation will promote IL-6 production, cardiac fibroblasts (CFs) from mice were incubated with AMPK activator, 5-aminoimidazole-4-carboxamide-1-4-ribofuranoside (AICAR). The results demonstrated that AICAR time and dose-dependently stimulated IL-6 production by ELISA and immunofluorescence. Pretreatment with p38 mitogen-activated protein kinase (MAPK) inhibitor blocked AICAR-induced IL-6 production; furthermore, AICAR-activated p38 MAPK phosphorylation by Western blot. To confirm that the increase in IL-6 production is ascribed to AMPK activation, we used another known AMPK activator, metformin. It also dose-dependently potentiated IL-6 production in CFs, and this potentiation could be reversed by p38 MAPK inhibitor. In conclusion, AMPK activation promoted IL-6 production in CFs via p38 MAPK-dependent pathway.

Role of PECAM-1 in the shear-stress-induced activation of Akt and the endothelial nitric oxide synthase (eNOS) in endothelial cells

The application of fluid shear stress to endothelial cells elicits the formation of nitric oxide (NO) and phosphorylation of the endothelial NO synthase (eNOS). Shear stress also elicits the enhanced tyrosine phosphorylation of endothelial proteins, especially of those situated in the vicinity of cell-cell contacts. Since a major constituent of these endothelial cell-cell contacts is the platelet endothelial cell adhesion molecule-1 (PECAM-1) we assessed the role of PECAM-1 in the activation of eNOS.

In human endothelial cells, shear stress induced the tyrosine phosphorylation of PECAM-1 and enhanced the association of PECAM-1 with eNOS. Endothelial cell stimulation with shear stress elicited the phosphorylation of Akt and eNOS as well as of the AMP-activated protein kinase (AMPK). While the shear-stress-induced tyrosine phosphorylation of PECAM-1 as well as the serine phosphorylation of Akt and eNOS were abolished by the pre-treatment of cells with the tyrosine kinase inhibitor PP1 the phosphorylation of AMPK was unaffected. Down-regulation of PECAM-1 using a siRNA approach attenuated the shear-stress-induced phosphorylation of Akt and eNOS, as well as the shear-stress-induced accumulation of cyclic GMP levels while the shear-stress-induced phosphorylation of AMPK remained intact. A comparable attenuation of Akt and eNOS (but not AMPK) phosphorylation and NO production was also observed in endothelial cells generated from PECAM-1-deficient mice.

These data indicate that the shear-stress-induced activation of Akt and eNOS in endothelial cells is modulated by the tyrosine phosphorylation of PECAM-1 whereas the shear-stress-induced phosphorylation of AMPK is controlled by an alternative signaling pathway.

Peroxynitrite activates ERK via Raf-1 and MEK, independently from EGF receptor and p21Ras in H9C2 cardiomyocytes

Peroxynitrite is a potent oxidant and nitrating species proposed as a direct effector of myocardial damage in a wide range of cardiac diseases. Whether peroxynitrite also acts indirectly, by modulating cell signal transduction pathways in the myocardium, has not been investigated. Here, we examined the ability of peroxynitrite to activate extracellular signal-related kinase (ERK), a MAP kinase which has been linked with hypertrophic and anti-apoptotic responses in the heart, in cultured H9C2 cardiomyocytes. Peroxynitrite elicited a concentration- and time-dependent activation of ERK, secondary to the upstream activation of MEK 1 (ERK kinase). Activation of MEK-ERK by peroxynitrite was related to the upstream activation of Raf-1 kinase, as ERK and MEK phosphorylation were prevented by the Raf-1 inhibitor BAY43-9006. These effects of peroxynitrite were not associated with the activation of p21(Ras), known as a common signaling target of cellular oxidative stress. In contrast to ERK activation mediated by the epidermal growth factor (EGF), ERK activation by peroxynitrite was not prevented by AG1478 (EGF receptor inhibitor). Peroxynitrite acted through oxidative, but not nitrative chemistry, as ERK remained activated while nitration was prevented by the flavanol epicatechin. In addition to ERK, peroxynitrite also potently activated two additional members of the MAP kinase family of signaling proteins, JNK and p38. Thus, peroxynitrite activates ERK in cardiomyocytes through an unusual signaling cascade involving Raf-1 and MEK 1, independently from EGFR and P21(Ras), and also acts as a potent activator of JNK and p38. These results provide the novel concept that peroxynitrite may represent a previously unrecognized signaling molecule in various cardiac pathologies.

Myristic acid stimulates endothelial nitric-oxide synthase in a CD36- and an AMP kinase-dependent manner

Dietary free fatty acids have been reported to have various effects on the endothelium including the generation of nitric oxide. The goal of the current study was to determine the mechanism whereby free fatty acid causes an increase in nitric oxide synthesis. The specific hypothesis tested was that free fatty acid association with CD36, a class B scavenger receptor, induces the activation of endothelial nitric-oxide synthase (eNOS). A human microvascular endothelial cell line and a transfected Chinese hamster ovary cell system were used to determine which free fatty acids stimulate eNOS. Surprisingly, only myristic acid, and to a lesser extent palmitic acid, stimulated eNOS. The stimulation of eNOS was dose- and time-dependent. Competition experiments with other free fatty acids and with a CD36-blocking antibody demonstrated that the effects of myristic acid on eNOS required association with CD36. Further mechanistic studies demonstrated that the effects of myristic acid on eNOS function were not dependent on PI 3-kinase, Akt kinase, or calcium. Pharmacological studies and dominant negative constructs were used to demonstrate that myristic acid/CD36 stimulation of eNOS activity was dependent on the activation of AMP kinase. These data demonstrate an unexpected link among myristic acid, CD36, AMP kinase, and eNOS activity.

Estradiol-mediated endothelial nitric oxide synthase association with heat shock protein 90 requires adenosine monophosphate-dependent protein kinase

BACKGROUND

Estradiol activates endothelial nitric oxide synthase (eNOS) by mechanisms that involve estrogen receptor-alpha (ERalpha), protein kinase B/Akt, mitogen-activated protein kinases, and heat shock protein 90 (HSP90). Recently, AMP-activated protein kinase (AMPK), an enzyme that plays a crucial role in cellular adaptation to metabolic stress, has been implicated in physiological eNOS activation by the hormones adiponectin and insulin. We therefore investigated whether AMPK is activated by estradiol in endothelial cells and plays a role in estradiol-induced eNOS activation.

METHODS AND RESULTS

Porcine aortic endothelial cells exhibited time- and concentration-dependent AMPK activation as determined by phosphorylation of AMPK and its downstream target acetyl coenzyme A carboxylase in response to estradiol (1 nmol/L to 10 micromol/L, 1 to 30 minutes). AMPK activation by estradiol was independent of both AMP levels and ERalpha but required estradiol conversion to its catechol metabolites. Estradiol treatment increased eNOS catalytic activity, an effect that was largely reversed when endothelial cells were infected with an AMPK dominant-negative adenovirus. However, inhibition of AMPK did not alter estradiol-induced eNOS phosphorylation at serine 1177 or threonine 495 but decreased eNOS interaction with HSP90. Consistent with this observation, blood vessels from alpha1-AMPK-null mice exhibited defective eNOS-mediated NO production in response to estradiol.

CONCLUSIONS

Taken together, these data indicate that AMPK activity is essential for estradiol-induced eNOS activation via the promotion of eNOS interaction with HSP90. These data point to a novel role for AMPK in modulating endothelial cell NO bioactivity and HSP90 function.

Oxidative stress inactivates VEGF survival signaling in retinal endothelial cells via PI 3-kinase tyrosine nitration

In diabetic retinopathy, endothelial cell apoptosis is paradoxically increased despite upregulation of the potent pro-survival factor VEGF. We tested the hypothesis that elevated glucose levels disrupt VEGF's pro-survival function via peroxynitrite-mediated alteration of the Akt-1/p38 MAP kinase signaling pathway by studies of retinal endothelial cells in vitro. High glucose or exogenous peroxynitrite caused significant increases in apoptosis in the presence or absence of VEGF. Treatment with a peroxynitrite decomposition catalyst blocked these effects, implying a causal role of peroxynitrite. Peroxynitrite or high glucose treatment also increased phosphorylation of p38 MAP kinase, whereas phosphorylation of Akt-1 was significantly decreased in basal or VEGF-stimulated conditions. High glucose- or peroxynitrite-treated cells also showed significant increases in tyrosine nitration on the p85 subunit of PI 3-kinase that blocked PI 3-kinase and Akt-1 kinase activity. Inhibiting peroxynitrite formation or blocking tyrosine nitration of p85 restored the activity of PI 3-kinase and Akt-1 kinase, blocked phosphorylation of p38 MAP kinase and normalized pro-survival function. Transfecting the cells with constitutively active Akt-1 or inhibiting activity of p38 MAP kinase completely masked the pro-apoptotic effects of high glucose and exogenous peroxynitrite, suggesting an interaction between the Akt-1 and p38 MAP kinase pathways. In conclusion, high glucose treatment blocks the pro-survival effect of VEGF and causes accelerated endothelial cell apoptosis via the action of peroxynitrite in causing tyrosine nitration of PI 3-kinase, inhibiting activity of Akt-1 kinase and increasing the activity of p38 MAP kinase.

Inhibition of superoxide generation and associated nitrosative damage is involved in metallothionein prevention of diabetic cardiomyopathy

The mechanisms of metallothionein prevention of diabetic cardiomyopathy are largely unknown. The present study was performed to test whether inhibition of nitrosative damage is involved in metallothionein prevention of diabetic cardiomyopathy. Cardiac-specific metallothionein-overexpressing transgenic (MT-TG) mice and wild-type littermate controls were treated with streptozotocin (STZ) by a single intraperitoneal injection, and both developed diabetes. However, the development of diabetic cardiomyopathy, revealed by histopathological and ultrastructural examination, serum creatine phosphokinase, and cardiac hemodynamic analysis, was significantly observed only in the wild-type, but not in MT-TG, diabetic mice 2 weeks and 6 months after STZ treatment. Formations of superoxide and 3-nitrotyrosine (3-NT), a marker for peroxynitrite-induced protein damage, were detected only in the heart of wild-type diabetic mice. Furthermore, primary cultures of cardiomyocytes from wild-type and MT-TG mice were exposed to lipopolysaccharide/tumor necrosis factor-alpha for generating intracellular peroxynitrite. Increases in 3-NT formation and cytotoxicity were observed in wild-type, but not in MT-TG, cardiomyocytes. Either urate, a peroxynitrite-specific scavenger, or Mn(111) tetrakis 1-methyl 4-pyridyl porphyrin pentachloride (MnTMPyP), a superoxide dismutase mimic, significantly inhibited the formation of 3-NT along with a significant prevention of cytotoxicity. These results thus suggest that metallothionein prevention of diabetic cardiomyopathy is mediated, at least in part, by suppression of superoxide generation and associated nitrosative damage.

Pharmacological inhibition of AMP-activated protein kinase provides neuroprotection in stroke

The restoration of energy balance during ischemia is critical to cellular survival; however, relatively little is known concerning the regulation of neuronal metabolic pathways in response to central nervous system ischemia. AMP-activated protein kinase (AMPK), a master sensor of energy balance in peripheral tissues, is phosphorylated and activated when energy balance is low. We investigated whether AMPK might also modulate neuronal energy homeostasis during ischemia. We utilized two model systems of ischemia, middle cerebral artery occlusion in vivo and oxygen-glucose deprivation in vitro, to delineate changes in AMPK activity incurred from a metabolic stress. AMPK is highly expressed in cortical and hippocampal neurons under both normal and ischemic conditions. AMPK activity, as assessed by phosphorylation status, is increased following both middle cerebral artery occlusion and oxygen-glucose deprivation. Pharmacological inhibition of AMPK by either C75, a known modulator of neuronal ATP levels, or compound C reduced stroke damage. In contrast, activation of AMPK by 5-aminoimidazole-4-carboxamide ribonucleoside exacerbated damage. Mice deficient in neuronal nitric-oxide synthase demonstrated a decrease in both stroke damage and AMPK activation compared with wild type, suggesting a possible interaction between NO and AMPK activation in stroke. These data demonstrate a role for AMPK in the response of neurons during metabolic stress and suggest that in ischemia the activation of AMPK is deleterious. The ability to manipulate pharmacologically neuronal energy balance during ischemia represents an innovative approach to neuroprotection.

Acute toxicity of doxorubicin on isolated perfused heart: response of kinases regulating energy supply

Doxorubicin (DXR) is a widely used and efficient anticancer drug. However, its application is limited by the risk of severe cardiotoxicity. Impairment of cardiac high-energy phosphate homeostasis is an important manifestation of both acute and chronic DXR cardiotoxic action. Using the Langendorff model of the perfused rat heart, we characterized the acute effects of 1-h perfusion with 2 or 20 microM DXR on two key kinases in cardiac energy metabolism, creatine kinase (CK) and AMP-activated protein kinase (AMPK), and related them to functional responses of the perfused heart and structural integrity of the contractile apparatus as well as drug accumulation in cardiomyocytes. DXR-induced changes in CK were dependent on the isoenzyme, with a shift in protein levels of cytosolic isoenzymes from muscle-type CK to brain-type CK, and a destabilization of octamers of the mitochondrial isoenzyme (sarcometric mitochondrial CK) accompanied by drug accumulation in mitochondria. Interestingly, DXR rapidly reduced the protein level and phosphorylation of AMPK as well as phosphorylation of its target, acetyl-CoA-carboxylase. AMPK was strongly affected already at 2 microM DXR, even before substantial cardiac dysfunction occurred. Impairment of CK isoenzymes was mostly moderate but became significant at 20 microM DXR. Only at 2 microM DXR did upregulation of brain-type CK compensate for inactivation of other isoenzymes. These results suggest that an impairment of kinase systems regulating cellular energy homeostasis is involved in the development of DXR cardiotoxicity.

Hypoxia and AMP independently regulate AMP-activated protein kinase activity in heart

The hypothesis was tested that hypoxia increases AMP-activated protein kinase (AMPK) activity independently of AMP concentration ([AMP]) in heart. In isolated perfused rat hearts, cytosolic [AMP] was changed from 0.2 to 16 microM using metabolic inhibitors during both normal oxygenation (95% O2-5% CO2, normoxia) and limited oxygenation (95% N2-5% CO2, hypoxia). Total AMPK activity measured in vitro ranged from 2 to 40 pmol.min(-1).mg protein(-1) in normoxic hearts and from 5 to 55 pmol.min(-1).mg protein(-1) in hypoxic hearts. The dependence of the in vitro total AMPK activity on the in vivo cytosolic [AMP] was determined by fitting the measurements from individual hearts to a hyperbolic equation. The [AMP] resulting in half-maximal total AMPK activity (A0.5) was 3 +/- 1 microM for hypoxic hearts and 28 +/- 13 microM for normoxic hearts. The A0.5 for alpha2-isoform AMPK activity was 2 +/- 1 microM for hypoxic hearts and 13 +/- 8 microM for normoxic hearts. Total AMPK activity correlated with the phosphorylation of the Thr172 residue of the AMPK alpha-subunit. In potassium-arrested hearts perfused with variable O2 content, alpha-subunit Thr172 phosphorylation increased at O2 < or = 21% even though [AMP] was <0.3 microM. Thus hypoxia or O2 < or = 21% increased AMPK phosphorylation and activity independently of cytosolic [AMP]. The hypoxic increase in AMPK activity may result from either direct phosphorylation of Thr172 by an upstream kinase or reduction in the A0.5 for [AMP].

IGF-1 phosphorylates AMPK-alpha subunit in ATM-dependent and LKB1-independent manner

Serine/threonine protein kinase AMP-activated protein kinase (AMPK) is a key metabolic stress-responsive factor that promotes the adaptation of cells to their microenvironment. Elevated concentrations of intracellular AMP, caused by metabolic stress, are known to activate AMPK by phosphorylation of the catalytic subunit. Recently, the tumor suppressor serine/threonine protein kinase LKB1 was identified as an upstream kinases, AMPKKs. In the current study, we found that stimulation with growth factors also caused AMPK-alpha subunit phosphorylation. Interestingly, even an LKB1-nonexpressing cancer cell line, HeLa, exhibited growth factor-stimulated AMPK-alpha subunit phosphorylation, suggesting the presence of an LKB1-independent pathway for AMPK-alpha subunit phosphorylation. In the human pancreatic cancer cell line PANC-1, AMPK-alpha subunit phosphorylation promoted by IGF-1 was suppressed by antisense ataxia telangiectasia mutated (ATM) expression. We found that IGF-1 also induced AMPK-alpha subunit phosphorylation in the human normal fibroblast TIG103 cell line, but failed to do so in a human fibroblast AT2-KY cell line lacking ATM. Immunoprecipitates of ATM collected from IGF-1-stimulated cells also caused the phosphorylation of the AMPK-alpha subunit in vitro. IGF-1-stimulated ATM phosphorylation at both threonine and tyrosine residues, and our results demonstrated that the phosphorylation of tyrosine in the ATM molecule is important for AMPK-alpha subunit phosphorylation during IGF-1 signaling. These results suggest that IGF-1 induces AMPK-alpha subunit phosphorylation via an ATM-dependent and LKB1-independent pathway.

Metabolic activation of AMP kinase in vascular smooth muscle

AMP-activated kinase (AMPK) is a highly conserved heterotrimeric kinase that functions as a metabolic master switch to coordinate cellular enzymes involved in carbohydrate and fat metabolism that regulate ATP conservation and synthesis. AMPK is activated by conditions that increase AMP-to-ATP ratio, such as exercise and metabolic stress. In the present study, we probed whether AMPK was expressed in vascular smooth muscle and would be activated by metabolic stress. Endothelium-denuded porcine carotid artery segments were metabolically challenged with 2-deoxyglucose (10 mM) plus N(2) (N(2)-2DG). These vessels exhibited a rapid increase in AMPK activity by 1 min that was near maximal by 20 min. AMPK inactivation on return to normal physiological saline was approximately 50% in 1 min and fully recovered by 5 min. Immunoprecipitation of the alpha(1)- and alpha(2)-catalytic subunit followed by immunoblot analysis for [P]Thr(172)-AMPK indicates that alpha(1)-AMPK accounts for all activity. Little if any alpha(2)-AMPK was detected in carotid smooth muscle. AMPK activity was not increased by contractile agonist (endothelin-1) or by the reported AMPK activators 5-aminoimidazole-4-carboxamide ribofuranoside (2 mM), metformin (2 mM), or phenformin (0.2 mM). AMPK activation by N(2)-2DG was associated with a rapid and pronounced reduction in endothelin-induced force and reduced phosphorylation of Akt and Erk 1/2. These data demonstrate that AMPK expression differs in vascular smooth muscle compared with striated muscles and that activation and inactivation after metabolic stress occur rapidly and are associated with signaling pathways that may regulate smooth-muscle contraction.

Activation of the AMP-activated protein kinase by the anti-diabetic drug metformin in vivo. Role of mitochondrial reactive nitrogen species

Metformin, one of the most commonly used drugs for the treatment of type II diabetes, was recently found to exert its therapeutic effects, at least in part, by activating the AMP-activated protein kinase (AMPK). However, the site of its action, as well as the mechanism to activate AMPK, remains elusive. Here we report how metformin activates AMPK. In cultured bovine aortic endothelial cells, metformin dose-dependently activated AMPK in parallel with increased detection of reactive nitrogen species (RNS). Further, either depletion of mitochondria or adenoviral overexpression of superoxide dismutases, as well as inhibition of nitric-oxide synthase, abolished the metformin-enhanced phosphorylations and activities of AMPK, implicating that activation of AMPK by metformin might be mediated by the mitochondria-derived RNS. Furthermore, administration of metformin, which increased 3-nitrotyrosine staining in hearts of C57BL6, resulted in parallel activation of AMPK in the aorta and hearts of C57BL6 mice but not in those of endothelial nitric-oxide synthase (eNOS) knockout mice in which metformin had no effect on 3-nitrotyrosine staining. Because the eNOS knockout mice expressed normal levels of AMPK-alpha that was activated by 5-aminoimidazole-4-carboxamide riboside, an AMPK agonist, these data indicate that RNS generated by metformin is required for AMPK activation in vivo. In addition, metformin significantly increased the co-immunoprecipitation of AMPK and its upstream kinase, LKB1, in C57BL6 mice administered to metformin in vivo. Using pharmacological and genetic inhibitors, we found that inhibition of either c-Src or PI3K abolished AMPK that was enhanced by metformin. We conclude that activation of AMPK by metformin might be mediated by mitochondria-derived RNS, and activation of the c-Src/PI3K pathway might generate a metabolite or other molecule inside the cell to promote AMPK activation by the LKB1 complex.

Possible involvement of the alpha1 isoform of 5'AMP-activated protein kinase in oxidative stress-stimulated glucose transport in skeletal muscle

Recent studies have suggested that 5'AMP-activated protein kinase (AMPK) is activated in response to metabolic stresses, such as contraction, hypoxia, and the inhibition of oxidative phosphorylation, which leads to insulin-independent glucose transport in skeletal muscle. In the present study, we hypothesized that acute oxidative stress increases the rate of glucose transport via an AMPK-mediated mechanism. When rat epitrochlearis muscles were isolated and incubated in vitro in Krebs buffer containing the oxidative agent H(2)O(2), AMPKalpha1 activity increased in a time- and dose-dependent manner, whereas AMPKalpha2 activity remained unchanged. The activation of AMPKalpha1 was associated with phosphorylation of AMPK Thr(172), suggesting that an upstream kinase is involved in the activation process. H(2)O(2)-induced AMPKalpha1 activation was blocked in the presence of the antioxidant N-acetyl-l-cysteine (NAC), and H(2)O(2) significantly increased the ratio of oxidized glutathione to glutathione (GSSG/GSH) concentrations, a sensitive marker of oxidative stress. H(2)O(2) did not cause an increase in the conventional parameters of AMPK activation, such as AMP and AMP/ATP. H(2)O(2) increased 3-O-methyl-d-glucose transport, and this increase was partially, but significantly, blocked in the presence of NAC. Results were similar when the muscles were incubated in a superoxide-generating system using hypoxanthine and xanthine oxidase. Taken together, our data suggest that acute oxidative stress activates AMPKalpha1 in skeletal muscle via an AMP-independent mechanism and leads to an increase in the rate of glucose transport, at least in part, via an AMPKalpha1-mediated mechanism.

Adiponectin stimulates production of nitric oxide in vascular endothelial cells

Adiponectin is secreted by adipose cells and mimics many metabolic actions of insulin. However, mechanisms by which adiponectin acts are poorly understood. The vascular action of insulin to stimulate endothelial production of nitric oxide (NO), leading to vasodilation and increased blood flow is an important component of insulin-stimulated whole body glucose utilization. Therefore, we hypothesized that adiponectin may also stimulate production of NO in endothelium. Bovine aortic endothelial cells in primary culture loaded with the NO-specific fluorescent dye 4,5-diaminofluorescein diacetate (DAF-2 DA) were treated with lysophosphatidic acid (LPA) (a calcium-releasing agonist) or adiponectin (10 microg/ml bacterially produced full-length adiponectin). LPA treatment increased production of NO by approximately 4-fold. Interestingly, adiponectin treatment significantly increased production of NO by approximately 3-fold. Preincubation of cells with wortmannin (phosphatidylinositol 3-kinase inhibitor) blocked only adiponectin- but not LPA-mediated production of NO. Using phospho-specific antibodies, we observed that either adiponectin or insulin treatment (but not LPA treatment) caused phosphorylation of both Akt at Ser473 and endothelial nitric-oxide synthase (eNOS) at Ser1179 that was inhibitable by wortmannin. We next transfected bovine aortic endothelial cells with dominant-inhibitory mutants of Akt (Akt-AAA) or AMP-activated protein kinase (AMPK) (AMPKK45R). Neither mutant affected production of NO in response to LPA treatment. Importantly, only AMPKK45R, but not Akt-AAA, caused a significant partial inhibition of NO production in response to adiponectin. Moreover, AMPK-K45R inhibited phosphorylation of eNOS at Ser1179 in response to adiponectin but not in response to insulin. We conclude that adiponectin has novel vascular actions to directly stimulate production of NO in endothelial cells using phosphatidylinositol 3-kinase-dependent pathways involving phosphorylation of eNOS at Ser1179 by AMPK. Thus, the effects of adiponectin to augment metabolic actions of insulin in vivo may be due, in part, to vasodilator actions of adiponectin.