AMP-activated protein kinase phosphorylation of endothelial NO synthase

Potential role of nitric oxide in contraction-stimulated glucose uptake and mitochondrial biogenesis in skeletal muscle

1. The present review discusses the potential role of nitric oxide (NO) in the: (i) regulation of skeletal muscle glucose uptake during exercise; and (ii) activation of mitochondrial biogenesis after exercise. 2. We have shown in humans that local infusion of an NO synthase inhibitor during exercise attenuates increases in skeletal muscle glucose uptake without affecting blood flow. Recent studies from our laboratory in rodents support these findings in humans, although rodent studies from other laboratories have yielded conflicting results. 3. There is clear evidence that NO increases mitochondrial biogenesis in non-contracting cells and that NO influences basal skeletal muscle mitochondrial biogenesis. However, there have been few studies examining the potential role of NO in the activation of mitochondrial biogenesis following an acute bout of exercise or in response to exercise training. Early indications are that NO is not involved in regulating the increase in mitochondrial biogenesis that occurs in response to exercise. 4. Exercise is considered the best prevention and treatment option for diabetes, but unfortunately many people with diabetes do not or cannot exercise regularly. Alternative therapies are therefore critical to effectively manage diabetes. If skeletal muscle NO is found to play an important role in regulating glucose uptake and/or mitochondrial biogenesis, pharmaceutical agents designed to mimic these effects of exercise may improve glycaemic control.

Metformin enhances the differentiation and mineralization of osteoblastic MC3T3-E1 cells via AMP kinase activation as well as eNOS and BMP-2 expression

It is unclear whether metformin, one of the anti-hyperglycemic agents commonly used for type 2 diabetes, could affect bone formation through activation of AMP-activated protein kinase (AMPK). In order to clarify this issue, we investigated the effects of metformin on the differentiation and mineralization of osteoblastic MC3T3-E1 cells as well as intracellular signal transduction. Metformin (50 microM) significantly increased collagen-I and osteocalcin mRNA expression, stimulated alkaline phosphatase activity, and enhanced cell mineralization. Moreover, metformin significantly activated AMPK in dose- and time-dependent manners, and induced endothelial nitric oxide synthase (eNOS) and bone morphogenetic protein-2 (BMP-2) expressions. Supplementation of Ara-A (0.1mM), a specific AMPK inhibitor, significantly reversed the metformin-induced eNOS and BMP-2 expressions. Our findings suggest that metformin can induce the differentiation and mineralization of osteoblasts via activation of the AMPK signaling pathway, and that this drug might be beneficial for not only diabetes but also osteoporosis by promoting bone formation.

Identification of nitric oxide as an endogenous activator of the AMP-activated protein kinase in vascular endothelial cells

In endothelial cells, the AMP-activated protein kinase (AMPK) is stimulated by sheer stress or growth factors that stimulate release of nitric oxide (NO). We hypothesized that NO might act as an endogenous activator of AMPK in endothelial cells. Exposure of human umbilical vein endothelial cells (HUVECs) to NO donors caused an increase in phosphorylation of both Thr-172 of AMPK and Ser-1177 of endothelial nitric oxide synthase, a downstream enzyme of AMPK. NO-induced activation of AMPK was not affected by inhibition of LKB1, an AMPK kinase. In contrast, inhibition of calcium calmodulin-dependent protein kinase kinase abolished the effect of NO in HUVECs. NO-induced AMPK activation in HeLa S3 cells was abolished by either 1H-(1,2,4)-oxadiazole[4,3-a]quinoxalon-1-one, a potent inhibitor for guanylyl cyclase, or 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethyl ester) (BAPTA-AM), an intracellular Ca(2+) chelator, indicating that NO-induced AMPK activation is guanylyl cyclase-mediated and calcium-dependent. Exposure of HUVECs or isolated mice aortas to either calcium ionophore A23187 or bradykinin significantly increased AMPK Thr-172 phosphorylation, which was abolished by N-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase. Finally, A23187- or bradykinin-enhanced AMPK activation was significantly greater in aortas from wild type mice than those in the aortas of endothelial nitric oxide synthase knock-out mice. Taken together, we conclude that NO might act as an endogenous AMPK activator.

Molecular mechanisms of ghrelin-mediated endothelial nitric oxide synthase activation

Metabolic syndrome accelerates the atherosclerotic process, and the earliest event of which is endothelial dysfunction. Ghrelin, a newly discovered gastric peptide, improves endothelial function and inhibits proatherogenic changes. In particular, low ghrelin concentration has been associated with several features of metabolic syndrome, including obesity, insulin resistance, and high blood pressure. However, the molecular mechanisms underlying ghrelin vascular actions remain largely unclear. Here, we showed that ghrelin activated endothelial nitric oxide (NO) synthase (eNOS) in cultured endothelial cells (ECs) and in intact vessels. Specifically, ghrelin rapidly induced phosphorylation of eNOS on an activation site and production of NO in human umbilical vein ECs and bovine aortic ECs. The eNOS phosphorylation was also observed in mouse aortas ex vivo perfused with ghrelin and in aortic tissues isolated from mice injected with ghrelin. Mechanistically, ghrelin stimulated AMP-activated protein kinase (AMPK) and Akt activation in cultured ECs and intact vessels. Inhibiting AMPK and Akt with their pharmacological inhibitors, small interference RNA and adenoviruses carried dominant-negative mutants, markedly attenuated ghrelin-induced eNOS activation, and NO production. Furthermore, ghrelin receptor/Gq protein/calcium-dependent pathway mediates activation of AMPK, Akt, and eNOS, and calmodulin-dependent kinase kinase is a potential convergent point to regulate Akt and AMPK activation in ghrelin signaling. Importantly, eNOS activation is critical for ghrelin inhibition of vascular inflammation. Together, both in vitro and in vivo data demonstrate a new role of ghrelin signaling for eNOS activation, and highlight the therapeutic potential for ghrelin to correct endothelial dysfunction associated with atherosclerotic vascular diseases and metabolic syndrome.

High molecular weight adiponectin activates AMPK and suppresses cytokine-induced NF-kappaB activation in vascular endothelial cells

Various isoforms of adiponectin circulate in the plasma. We purified high molecular weight (HMW) adiponectin from human plasma. HMW adiponectin was observed to activate AMP-activated protein kinase (AMPK), thereby increasing the phosphorylation of eNOS and NO production in endothelial cells. On the other hand, cells preincubated with HMW adiponectin had reduced TNFalpha-induced NF-kappaB activation. HMW adiponectin by itself was found to modestly activate NF-kappaB, which was significantly enhanced by inhibition of AMPK/eNOS activation. Thus, HMW adiponectin might have dual action, both pro and anti-inflammatory. An initial period of NF-kappaB activation by HMW adiponectin might be proinflammatory, but it could be counteracted by activation of AMPK/eNOS, which lead to a potential reduction in a second activation of NF-kappaB against inflammatory stimuli.

AMP-activated protein kinase phosphorylates and desensitizes smooth muscle myosin light chain kinase

Smooth muscle contraction is initiated by a rise in intracellular calcium, leading to activation of smooth muscle myosin light chain kinase (MLCK) via calcium/calmodulin (CaM). Activated MLCK then phosphorylates the regulatory myosin light chains, triggering cross-bridge cycling and contraction. Here, we show that MLCK is a substrate of AMP-activated protein kinase (AMPK). The phosphorylation site in chicken MLCK was identified by mass spectrometry to be located in the CaM-binding domain at Ser(815). Phosphorylation by AMPK desensitized MLCK by increasing the concentration of CaM required for half-maximal activation. In primary cultures of rat aortic smooth muscle cells, vasoconstrictors activated AMPK in a calcium-dependent manner via CaM-dependent protein kinase kinase-beta, a known upstream kinase of AMPK. Indeed, vasoconstrictor-induced AMPK activation was abrogated by the STO-609 CaM-dependent protein kinase kinase-beta inhibitor. Myosin light chain phosphorylation was increased under these conditions, suggesting that contraction would be potentiated by ablation of AMPK. Indeed, in aortic rings from mice in which alpha1, the major catalytic subunit isoform in arterial smooth muscle, had been deleted, KCl- or phenylephrine-induced contraction was increased. The findings suggest that AMPK attenuates contraction by phosphorylating and inactivating MLCK. This might contribute to reduced ATP turnover in the tonic phase of smooth muscle contraction.

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

OBJECTIVE

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

RESEARCH DESIGN AND METHODS AND RESULTS

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

CONCLUSIONS

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

Nitric oxide generation by endothelial cells exposed to shear stress in glass tubes perfused with red blood cell suspensions: role of aggregation

Endothelial function is modulated by wall shear stress acting on the vessel wall, which is determined by fluid velocity and the local viscosity near the vessel wall. Red blood cell (RBC) aggregation may affect the local viscosity by favoring axial migration. The aim of this study was to investigate the role of RBC aggregation, with or without altered plasma viscosity, in the mechanically induced nitric oxide (NO)-related mechanisms of endothelial cells. Human umbilical vein endothelial cells (HUVEC) were cultured on the inner surface of cylindrical glass capillaries that were perfused with RBC suspensions having normal and increased aggregation at a nominal shear stress of 15 dyn/cm(2). RBC aggregation was enhanced by two different approaches: 1) poloxamer-coated RBC suspended in normal, autologous plasma, resulting in enhanced aggregation but unchanged plasma viscosity and 2) normal RBC suspended in autologous plasma containing 0.5% dextran (mol mass 500 kDa), with a similar level of RBC aggregation but higher plasma viscosity. Compared with normal cells in unmodified plasma, perfusion with suspensions of poloxamer-coated RBC in normal plasma resulted in decreased levels of NO metabolites and serine 1177 phosphorylation of endothelial nitric oxide synthase (eNOS). Perfusion with normal RBC in plasma containing dextran resulted in a NO level that remained elevated, whereas only a modest decrease of phosphorylated eNOS level was observed. The results of this study suggest that increases of RBC aggregation tendency affect endothelial cell functions by altering local blood composition, especially if the alterations of RBC aggregation are due to modified cellular properties and not to plasma composition changes.

Acute metformin therapy confers cardioprotection against myocardial infarction via AMPK-eNOS-mediated signaling

OBJECTIVE

Clinical studies have reported that metformin reduces cardiovascular end points of type 2 diabetic subjects by actions that cannot solely be attributed to glucose-lowering effects. The therapeutic effects of metformin have been reported to be mediated by its activation of AMP-activated protein kinase (AMPK), a metabolite sensing protein kinase whose activation following myocardial ischemia has been suggested to be an endogenous protective signaling mechanism. We investigated the potential cardioprotective effects of a single, low-dose metformin treatment (i.e., 286-fold less than the maximum antihyperglycemic dose) in a murine model of myocardial ischemia-reperfusion (I/R) injury.

RESEARCH DESIGN AND METHODS

Nondiabetic and diabetic (db/db) mice were subjected to transient myocardial ischemia for a period of 30 min followed by reperfusion. Metformin (125 microg/kg) or vehicle (saline) was administered either before ischemia or at the time of reperfusion.

RESULTS

Administration of metformin before ischemia or at reperfusion decreased myocardial injury in both nondiabetic and diabetic mice. Importantly, metformin did not alter blood glucose levels. During early reperfusion, treatment with metformin augmented I/R-induced AMPK activation and significantly increased endothelial nitric oxide (eNOS) phosphorylation at residue serine 1177.

CONCLUSIONS

These findings provide important information that myocardial AMPK activation by metformin following I/R sets into motion events, including eNOS activation, which ultimately lead to cardioprotection.

Rosiglitazone stimulates nitric oxide synthesis in human aortic endothelial cells via AMP-activated protein kinase

The thiazolidinedione anti-diabetic drugs increase activation of endothelial nitric-oxide (NO) synthase by phosphorylation at Ser-1177 and increase NO bioavailability, yet the molecular mechanisms that underlie this remain poorly characterized. Several protein kinases, including AMP-activated protein kinase, have been demonstrated to phosphorylate endothelial NO synthase at Ser-1177. In the current study we determined the role of AMP-activated protein kinase in rosiglitazone-stimulated NO synthesis. Stimulation of human aortic endothelial cells with rosiglitazone resulted in the time- and dose-dependent stimulation of AMP-activated protein kinase activity and NO production with concomitant phosphorylation of endothelial NO synthase at Ser-1177. Rosiglitazone stimulated an increase in the ADP/ATP ratio in endothelial cells, and LKB1 was essential for rosiglitazone-stimulated AMPK activity in HeLa cells. Infection of endothelial cells with a virus encoding a dominant negative AMP-activated protein kinase mutant abrogated rosiglitazone-stimulated Ser-1177 phosphorylation and NO production. Furthermore, the stimulation of AMP-activated protein kinase and NO synthesis by rosiglitazone was unaffected by the peroxisome proliferator-activated receptor-gamma inhibitor GW9662. These studies demonstrate that rosiglitazone is able to acutely stimulate NO synthesis in cultured endothelial cells by an AMP-activated protein kinase-dependent mechanism, likely to be mediated by LKB1.

Resveratrol stimulates nitric oxide production by increasing estrogen receptor alpha-Src-caveolin-1 interaction and phosphorylation in human umbilical vein endothelial cells

Epidemiological studies correlate moderate red wine consumption to reduced incidence of cardiovascular disease. Resveratrol is a polyphenolic compound in red wine that has cardioprotective effects in rodents. Although endothelial cell (EC) studies indicate that micromolar resveratrol has diverse biological activities, these concentrations are not physiologically relevant because human oral ingestion provides only brief exposure to nanomolar plasma levels. Previously, we reported that nanomolar resveratrol activated ERK1/2 signaling in bovine aortic ECs (BAECs). The goal of this study was to determine the mechanisms by which nanomolar resveratrol rapidly activates endothelial nitric oxide synthase (eNOS) in human umbilical vein ECs (HUVECs). We report for the first time that resveratrol increased interaction between estrogen receptor alpha (ER alpha), caveolin-1 (Cav-1) and c-Src, and increased phosphorylation of Cav-1, c-Src, and eNOS. Pretreatment with the lipid raft disruptor beta-methyl cyclodextrin or G alpha inhibitor pertussis toxin blocked resveratrol- and E(2)-induced eNOS activation and NO production. Depletion of endogenous ER alpha, not ERbeta, by siRNA attenuated resveratrol- and E(2)-induced ERK1/2, Src, and eNOS phosphorylation. Our data demonstrate that nanomolar resveratrol induces ER alpha-Cav-1-c-SRC interaction, resulting in NO production through a G alpha-protein-coupled mechanism. This study provides important new insights into mechanisms for the beneficial effects of resveratrol in ECs.

Racial differences in nitric oxide-dependent vasorelaxation

Along with the growing heterogeneity of the American population, ethnic/racial disparity is becoming a clear health issue in the United States. The awareness of ethnic/racial disparities has been growing because of considerable data gathered from recent clinical and epidemiological studies. These studies have highlighted the importance of addressing these differences in the diagnosis and treatment of various diseases potentially according to race. It is becoming particularly clear that there is a 2- to 3-fold racial difference in certain cardiovascular diseases (eg, preeclampsia) associated with dysfunctional nitric oxide-mediated vasodilation. In this review, the authors summarize the current literature on racial disparities in nitric oxide-mediated vasodilation in relation to cardiovascular health with an emphasis on vascular nitric oxide bioavailability as a balance between production via endothelial nitric oxide synthase and degradation through reactive oxygen species. The major hypotheses postulated on the biological basis of these differences are also highlighted.

Relative reduction of endothelial nitric-oxide synthase expression and transcription in atherosclerosis-prone regions of the mouse aorta and in an in vitro model of disturbed flow

Atherosclerosis develops in distinct regions of the arterial tree. Defining patterns and mechanisms of endothelial cell gene expression in different regions of normal arteries is key to understanding the initial molecular events in atherogenesis. In this study, we demonstrated that the expression of endothelial nitric-oxide synthase (eNOS), an atheroprotective gene, and its phosphorylation on Ser(1177), a marker of activity, were lower in regions of the normal mouse aorta that are predisposed to atherosclerosis. The same expression pattern was observed in mouse strains that are both susceptible and resistant to atherosclerosis, and the topography of eNOS expression was inverse to p65, the main nuclear factor-kappaB subunit. Modeling of disturbed and uniform laminar flow in vitro reproduced the expression patterns of eNOS and p65 that were found in vivo. Heterogeneous nuclear RNA expression and RNA polymerase II chromosome immunoprecipitation studies demonstrated that regulation of transcription contributed to increased eNOS expression in response to shear stress. In vivo, the transcription of eNOS was reduced in regions of the mouse aorta predisposed to atherosclerosis, as defined by reporter gene expression in eNOS promoter-beta-galactosidase reporter transgenic mice. These data suggest that disturbed hemodynamic patterns found at arterial branches and curvatures uniquely modulate endothelial cell gene expression by regulating transcription, potentially explaining why these regions preferentially develop atherosclerosis when risk factors such as hypercholesterolemia are introduced.

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.

Adiponectin prevents cerebral ischemic injury through endothelial nitric oxide synthase dependent mechanisms

BACKGROUND

Adiponectin is a fat-derived plasma protein that has beneficial actions on cardiovascular disorders. A low level of plasma adiponectin is associated with increased mortality after ischemic stroke; however, the causal role of adiponectin in ischemic stroke is unknown.

METHODS AND RESULTS

To explore the role of adiponectin in the development of acute cerebral injury, we subjected adiponectin-deficient (APN-KO) and wild-type (WT) mice to 1 hour of middle cerebral artery occlusion followed by 23 hours of reperfusion. APN-KO mice exhibited enlarged brain infarction and increased neurological deficits after ischemia-reperfusion compared with WT mice. Conversely, adenovirus-mediated supplementation of adiponectin significantly reduced cerebral infarct size in WT and APN-KO mice. APN-KO mice showed decreased cerebral blood flow during ischemia by laser speckle flowmetry methods. Adiponectin colocalized within the cerebral vascular endothelium under transient ischemic conditions by immunohistochemical analysis. Phosphorylation of endothelial nitric oxide synthase in ischemic brain tissues and the production of nitric oxide metabolites in plasma were attenuated in APN-KO mice compared with WT mice. Adenovirus-mediated administration of adiponectin stimulated endothelial nitric oxide synthase phosphorylation and nitric oxide metabolites during cerebral ischemia in both WT and APN-KO mice. Neuronal nitric oxide synthase expression during ischemia did not differ between WT and APN-KO mice. Adenovirus-mediated delivery of adiponectin did not affect brain infarction in mice deficient in endothelial nitric oxide synthase.

CONCLUSIONS

These data provide causal evidence that adiponectin exerts a cerebroprotective action through an endothelial nitric oxide synthase-dependent mechanism. Adiponectin could represent a molecular target for the prevention of ischemic stroke.

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.

Diminished NO release in chronic hypoxic human endothelial cells

The present study addressed whether chronic hypoxia is associated with reduced nitric oxide (NO) release due to decreased activation of endothelial NO synthase (eNOS). Primary cultures of endothelial cells from human umbilical veins (HUVECs) were used and exposed to different oxygen levels for 24 h, after which NO release, intracellular calcium, and eNOS activity and phosphorylation were measured after 24 h. Direct measurements using a NO microsensor showed that in contrast to 1-h exposure to 5% and 1% oxygen (acute hypoxia), histamine-evoked (10 μM) NO release from endothelial cells exposed to 5% and 1% oxygen for 24 h (chronic hypoxia) was reduced by, respectively, 58% and 40%. Furthermore, chronic hypoxia also lowered the amount and activity of eNOS enzyme. The decrease in activity could be accounted for by reduced intracellular calcium and altered eNOS phosphorylation. eNOS Ser1177 and eNOS Thr495 phosphorylations were reduced and increased, respectively, consistent with lowered enzyme activity. Akt kinase, which can phosphorylate eNOS Ser1177, was also decreased by hypoxia, regarding both total protein content and the phosphorylated (active) form. Moreover, the protein content of β- actin, which is known to influence the activity of eNOS, was almost halved by hypoxia, further supporting the fall in eNOS activity. In conclusion, chronic hypoxia in HUVECs reduces histamine-induced NO release as well as eNOS expression and activity. The decreased activity is most likely due to changed eNOS phosphorylation, which is supported by decreases in Akt expression and phosphorylation. By reducing NO, chronic hypoxia may accentuate endothelial dysfunction in cardiovascular disease.

Adipocyte triglyceride lipase expression in human obesity

We have investigated the gene and protein expression of adipose triglyceride lipase (ATGL) and triglyceride (TG) lipase activity from subcutaneous and visceral adipose tissue of lean and obese subjects. Visceral and subcutaneous adipose tissue was obtained from 16 age-matched lean and obese subjects during abdominal surgery. Tissues were analyzed for mRNA expression of lipolytic enzymes by real-time quantitative PCR. ATGL protein content was assessed by Western blot and TG lipase activity by radiometric assessment. Subcutaneous and visceral adipose tissue of obese subjects had elevated mRNA expression of PNPLA2 (ATGL) and other lipases including PNPLA3, PNPLA4, CES1, and LYPLAL1 (P < 0.05). Surprisingly, ATGL protein expression and TG lipase activity were reduced in subcutaneous adipose tissue of obese subjects. Immunoprecipitation of ATGL reduced total TG lipase activity in adipose lysates by 70% in obese and 83% in lean subjects. No significant differences in the ATGL activator CGI-58 mRNA levels (ABHD5) were associated with obesity. These data demonstrate that ATGL is important for efficient TG lipase activity in humans. They also demonstrate reduced ATGL protein expression and TG lipase activity despite increased mRNA expression of ATGL and other novel lipolytic enzymes in obesity. The lack of correlation between ATGL protein content and in vitro TG lipase activity indicates that small decrements in ATGL protein expression are not responsible for the reduction in TG lipase activity observed here in obesity, and that posttranslational modifications may be important.

Nitric oxide increases GLUT4 expression and regulates AMPK signaling in skeletal muscle

Nitric oxide (NO) and 5'-AMP-activated protein kinase (AMPK) are involved in glucose transport and mitochondrial biogenesis in skeletal muscle. Here, we examined whether NO regulates the expression of the major glucose transporter in muscle (GLUT4) and whether it influences AMPK-induced upregulation of GLUT4. At low levels, the NO donor S-nitroso-N-penicillamine (SNAP, 1 and 10 microM) significantly increased GLUT4 mRNA ( approximately 3-fold; P < 0.05) in L6 myotubes, and cotreatment with the AMPK inhibitor compound C ablated this effect. The cGMP analog 8-bromo-cGMP (8-Br-cGMP, 2 mM) increased GLUT4 mRNA by approximately 50% (P < 0.05). GLUT4 protein expression was elevated 40% by 2 days treatment with 8-Br-cGMP, whereas 6 days treatment with 10 microM SNAP increased GLUT4 expression by 65%. Cotreatment of cultures with the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one prevented the SNAP-induced increase in GLUT4 protein. SNAP (10 microM) also induced significant phosphorylation of alpha-AMPK and acetyl-CoA carboxylase and translocation of phosphorylated alpha-AMPK to the nucleus. Furthermore, L6 myotubes exposed to 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) for 16 h presented an approximately ninefold increase in GLUT4 mRNA, whereas cotreatment with the non-isoform-specific NOS inhibitor N(G)-nitro-l-arginine methyl ester, prevented approximately 70% of this effect. In vivo, GLUT4 mRNA was increased 1.8-fold in the rat plantaris muscle 12 h after AICAR injection, and this induction was reduced by approximately 50% in animals cotreated with the neuronal and inducible nitric oxide synthases selective inhibitor 1-(2-trifluoromethyl-phenyl)-imidazole. We conclude that, in skeletal muscle, NO increases GLUT4 expression via a cGMP- and AMPK-dependent mechanism. The data are consistent with a role for NO in the regulation of AMPK, possibly via control of cellular activity of AMPK kinases and/or AMPK phosphatases.

Platelet-Derived Nitric Oxide Signaling and Regulation

Nitric oxide (NO) exerts important vasodilatory, antiplatelet, antioxidant, antiadhesive, and antiproliferative effects. Although endothelium derived NO has been shown to be of prime importance in cardio- and vasculoprotection, until recently little was known about the role of platelet-derived NO. New evidence suggests that NO synthesized by platelets regulates platelet functions, in particular suppressing platelet activation and intravascular thrombosis. Moreover, platelet NO biosynthesis may be decreased in patients with cardiovascular risk factors or with coronary heart disease, and this may contribute to arterial thrombotic disease in these patients. Here, we review the current state of knowledge as regards the role of platelet-derived NO, both in normal physiology and in cardiovascular disease states, and compare platelet NO signaling and regulation with that in endothelial cells.

Regulation of endothelial nitric oxide synthase: involvement of protein kinase G 1 beta, serine 116 phosphorylation and lipid structures

1. Endothelial nitric oxide synthase (NOS3) is important for vascular homeostasis. The role of protein kinase G (PKG) in regulation of NOS3 activity was studied in primary cultures of newborn lamb lung microvascular endothelial cells (LMVEC). 2. We determined the presence of PKG in fetal and neonatal LMVEC as well as subcellular localization of PKG isoforms in the neonatal cells by fluorescence immunohistochemistry. We used diaminofluorescein (DAF) fluorophore to measure nitric oxide (NO) production from neonatal LMVEC. We confirmed that NO measured was from constitutive NOS3 by inhibiting it with NOS inhibitors. 3. To identify a role for PKG in basal NO production, we measured NO release from LMVEC cells using 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM; 0.5-0.8 micromol/L) with and without prior stimulation with the PKG activator 8-bromo-cGMP (8-Br-cGMP; 0.3 and 3 micromol/L) or prior PKG inhibition with beta-phenyl-1,N2-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothionate (BPC; 0.3 and 3 micromol/L). With the same drugs, we determined the role of PKG on cellular expression of NOS3 and serine 116 phosphorylated NOS (pSer116-NOS) by qualitative and quantitative immunofluorescence assays, as well as western blotting. 4. Because PKG 1 beta was distributed throughout the cytosol in a punctate expression, we used 2 mmol/L cyclodextrin, a cholesterol extractor, to determine a role for lipid vesicles in PKG regulation of NO production. 5. Protein kinase G 1 beta gave a punctate appearance, indicating its presence in intracellular vesicles. Nitric oxide production decreased by approximately 20% with 300 nmol/L and 3 micromol/L 8-Br cGMP (P < 0.05) and increased by 20.8 +/- 3.7% with 3 micromol/L BPC (P < 0.001), indicating that both stimulated and basal PKG activity has inhibitory effects on basal NOS3 function. Nitric oxide synthase immunofluorescence and immunoblot expression were decreased and pSer116-NOS immunofluorescence was increased by 800 nmol/L 8-Br-cGMP and 170 micromol/L (Z)-1-[2-(2-aminoethyl)-N-(2-ammonio-ethyl)amino]diazen-1-ium-1, 2-diolate (DETANONOate). The effect of cyclodextrin indicated that cholesterol extraction interfered with PKG inhibition of NOS. Further examination of pSer116-NOS by immunohistochemistry showed it abundant in the endoplasmic reticulum and colocalized with PKG 1 beta, especially in nuclear vesicles. 6. We conclude that endothelial PKG is involved in endogenous regulation of basal NOS3 activity with the involvement of lipid structures, the endoplasmic reticulum and the nucleus. Protein kinase G 1 beta is colocalized with pSer116-NOS, indicating that PKG action may involve serine 116 phosphorylation on NOS.

The phosphorylation state of eNOS modulates vascular reactivity and outcome of cerebral ischemia in vivo

NO plays critical roles in vascular function. We show that modulation of the eNOS serine 1179 (S1179) phosphorylation site affects vascular reactivity and determines stroke size in vivo. Transgenic mice expressing only a phosphomimetic (S1179D) form of eNOS show greater vascular reactivity, develop less severe strokes, and have improved cerebral blood flow in a middle cerebral artery occlusion model than mice expressing an unphosphorylatable (S1179A) form. These results provide a molecular mechanism by which multiple diverse cardiovascular risks, such as diabetes and obesity, may be centrally integrated by eNOS phosphorylation in vivo to influence blood flow and cardiovascular disease. They also demonstrate the in vivo relevance of posttranslational modification of eNOS in vascular function.

Fatty acid incorporation in endothelial cells and effects on endothelial nitric oxide synthase

BACKGROUND

The nature of fatty acids provided by the diet as well as plasma lipid metabolism can modify the composition and properties of plasma membrane and thus the activity of membrane proteins. In humans, as well as in experimental models, diabetes is associated with both an alteration in serum lipid profile and a documented endothelial dysfunction. This in vitro study investigated on an immortalized human endothelial cell line (EA.hy 926) the specific effects of several free fatty acids (FFAs) on the composition of cellular membranes and the regulation of endothelial nitric oxide synthase (eNOS).

MATERIALS AND METHODS

0.1% of lipid deprived serum was added to the incubation medium with 25 mM glucose in order to study the effects of individual fatty acids: myristic acid, palmitic acid, stearic acid, oleic acid or linoleic acid at 100 microM bound with albumin. The effects of the FFAs on the endothelial nitric oxide synthase were investigated on mRNA level by quantitative PCR, on protein level and Ser1177 phosphorylation by Western blot and on enzymatic activity on living cells using radiolabelled arginine.

RESULTS

Free linoleic acid increased the membrane content in n-6 fatty acids (mainly C18: n-6 and its metabolites) with a decrease in saturated and monounsaturated fatty acids. These conditions decreased the basal eNOS activity and reduced the phosphorylation of eNOS-Ser1177 due to activation by histamine. Free palmitic acid enriched the membranes with 16 : 0 with a slight decrease in monounsaturated fatty acids. These conditions increased eNOS activation without increasing Ser1177 phosphorylation upon histamine activation. The addition of the other FFAs also resulted in modifications of membrane composition, which did not to affect eNOS-Ser1177 phosphorylation.

CONCLUSION

Among the fatty acids used, only modification of the membrane composition due to linoleic acid supply disturbed the basal enzymatic activity and Ser1177 phosphorylation of eNOS in a way that limited the role of histamine activation. Linoleic acid might involve the dysfunction of both eNOS basal activity and its phosphorylation status and may then contribute to an impaired vasodilatation in vivo.

The AMP-activated protein kinase: more than an energy sensor

The AMPK (AMP-activated protein kinase) is a highly conserved eukaryotic protein serine/threonine kinase. It mediates a nutrient signalling pathway that senses cellular energy status and was appropriately called the fuel gauge of the cell. At the cellular level, AMPK controls energy homoeostasis by switching on catabolic ATP-generating pathways, while switching off anabolic ATP-consuming processes. Its effect on energy balance extends to whole-body energy homoeostasis, because, in the hypothalamus, it integrates nutritional and hormonal signals that control food intake and body weight. The interest in AMPK also stems from the demonstration of its insulin-independent stimulation of glucose transport in skeletal muscle during exercise. Moreover, the potential importance of AMPK in metabolic diseases is supported by the notion that AMPK mediates the anti-diabetic action of biguanides and thiazolidinediones and that it might be involved in the metabolic syndrome. Finally, the more recent demonstration that AMPK activation could occur independently of changes in cellular energy status, suggests that AMPK action extends to the control of non-metabolic functions.

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.

The developmental stage and cell type dependent phosphorylation of eNOS in murine enteric mucosa and myenteric plexus

In order to clarify the developmental regulation of the eNOS activity in intestine by phosphorylation, we examined the immunohistochemical localizations of the eNOS phosphorylation sites at Ser(1177), Ser(116) and at Thr(495) in cells of the mouse enteric mucosa and myenteric plexus at E13.5, E14.5, E16.5, E18.5, E20.5 and P3. In addition, in cells of the E16.5 stage the protein levels of eNOS and the phosphorylation sites of eNOS at Ser(1177), Ser(116) and at Thr(495) were investigated by immunoblot. From E14.5 to P3, phosphorylation residues of eNOS at Ser(1177) and at Ser(116) were detected with different staining intensities in the enteric mucosa epithelium. In ganglion cells of the myenteric plexus Ser(116) was identified at E18.5 to P3. The absence of phosphorylated Thr(495) in cells of intestine during all developmental stages, was confirmed by immunoblot at E16.5. The immunoblot levels of eNOS and eNOS phosphorylated at Ser(1177) and at Ser(116) were comparable with the immunohistochemical results of E16.5 mouse intestine. It was concluded that development of epithelial cells of the enteric mucosa may be modulated by phosphorylation of eNOS at Ser(1177) and at Ser(116). The phosphorylation of eNOS in cells of the myenteric plexus is modulated at Ser(116). These data suggest that there is a developmental stage and cell type dependent phosphorylation of eNOS.

Nongenomic estrogen effects on nitric oxide synthase activity in rat adipocytes

Estrogens exert multiple genomic effects on adipose tissue through binding to nuclear estrogen receptors. However, there is evidence for additional nongenomic mechanisms whereby estrogens may exert their control on adipose tissue metabolism through rapid activation of various membrane-initiated kinase cascades. Here, we tested rapid effects of estrogens on nitric oxide production in white adipose tissue using 17-beta estradiol (E2) and its membrane impermeant albumin conjugated form (17-beta estradiol hemisuccinate BSA, E2-BSA). We found that both E2 and E2-BSA stimulate nitric oxide synthase (NOS) activity in adipocytes. These effects were abolished by 1) ICI 182-780, a selective estrogen receptor antagonist; 2) wortmannin, an inhibitor of phosphatidylinositol 3-kinase; and 3) N-[2-(p-bromocinnamylamino) ethyl]-5-isoquinolinesulfonamide (H-89) an inhibitor of protein kinase A. In contrast to NOS activation by E2, E2-BSA-induced NOS activity was abolished by UO126, an inhibitor of MAPK kinase/ERK (p42/p44 MAPKs). Immunoblotting studies have shown that both estrogens phosphorylate endothelial NOS (NOS III) on Ser(1179), an effect that is prevented by wortmannin and H89, suggesting that NOS III is the target for estrogen-induced NOS activity. Furthermore, only the E2-BSA-induced NOS III phosphorylation on Ser(1179) was totally abolished by UO126. These results indicate that the signaling cascades involved in adipocyte NOS stimulation by estrogens are different depending on whether estrogens are free or conjugated to albumin and therefore underline the importance of estrogen receptor locations in the nongenomic actions of estrogens in these cells.

Adiponectin-induced endothelial nitric oxide synthase activation and nitric oxide production are mediated by APPL1 in endothelial cells

Adiponectin protects the vascular system partly through stimulation of endothelial nitric oxide (NO) production and endothelium-dependent vasodilation. The current study investigated the role of two recently identified adiponectin receptors, AdipoR1 and -R2, and their downstream effectors in mediating the endothelium actions of adiponectin. In human umbilical vein endothelial cells, adiponectin-induced phosphorylation of endothelial NO synthase (eNOS) at Ser(1177) and NO production were abrogated when expression of AdipoR1 and -R2 were simultaneously suppressed. Proteomic analysis demonstrated that the cytoplasmic tails of both AdipoR1 and -R2 interacted with APPL1, an adaptor protein that contains a PH (pleckstrin homology) domain, a PTB (phosphotyrosine-binding) domain, and a Leucine zipper motif. Suppression of APPL1 expression by RNA interference significantly attenuated adiponectin-induced phosphorylation of AMP-activated protein kinase (AMPK) at Thr(172) and eNOS at Ser(1177), and the complex formation between eNOS and heat shock protein 90, resulting in a marked reduction of NO production. Adenovirus-mediated overexpression of a constitutively active version of AMPK reversed these changes. In db/db diabetic mice, both APPL1 expression and adiponectin-induced vasodilation were significantly decreased compared with their lean littermates. Taken together, these results suggest that APPL1 acts as a common downstream effector of AdipoR1 and -R2, mediating adiponectin-evoked endothelial NO production and endothelium-dependent vasodilation.

Exercise-induced activation of cardiac sympathetic nerve triggers cardioprotection via redox-sensitive activation of eNOS and upregulation of iNOS

We investigated the mechanism of exercise-induced late cardioprotection against ischemia-reperfusion (I/R) injury. C57BL/6 mice received treadmill exercise (60 min/day) for 7 days at a work rate of 60–70% maximal oxygen uptake. Exercise transiently increased oxidative stress and activated endothelial isoform of nitric oxide synthase (eNOS) during exercise and increased expression of inducible isoform of NOS (iNOS) in the heart after 7 days of exercise. The mice were subjected to regional ischemia by 30 min of occlusion of the left coronary artery, followed by 2 h of reperfusion. Infarct size was significantly smaller in the exercised mice. Ablation of cardiac sympathetic nerve by topical application of phenol abolished oxidative stress, activation of eNOS, upregulation of iNOS, and cardioprotection mediated by exercise. Treatment with the antioxidant N-(2-mercaptopropionyl)-glycine during exercise also inhibited activation of eNOS, upregulation of iNOS, and cardioprotection. In eNOS−/− mice, exercise-induced oxidative stress was conserved, but upregulation of iNOS and cardioprotection was lost. Exercise did not confer cardioprotection when the iNOS selective inhibitor 1400W was administered just before coronary artery occlusion or when iNOS−/− mice were employed. These results suggest that exercise stimulates cardiac sympathetic nerves that provoke redox-sensitive activation of eNOS, leading to upregulation of iNOS, which acts as a mediator of late cardioprotection against I/R injury.

Biochemical consequences of the NOS3 Glu298Asp variation in human endothelium: altered caveolar localization and impaired response to shear

Human endothelial nitric oxide synthase (NOS3) gene polymorphism at Exon 7 (Glu298Asp) has been linked to vascular endothelial dysfunction, but the mechanisms are not defined. Shear is a key modulator of NOS3 function in vivo and association with caveolae is important for the control of NOS3 protein activity. Here we tested the hypothesis that altered enrichment of NOS3 in the caveolar membrane defines Glu298Asp genotype-specific responses and NOS3 activity. Basal caveolar membrane enrichment was carried out to quantitate the NOS3 enrichment in caveolae. Cells were subjected to shear and NOS3 protein levels, phosphorylation, enzyme function were investigated. Variant genotypes had lower NOx production pre- and post-shear, but no genotype-dependent alterations in pNOS3 were observed. Asp variants had significantly lower NOS3 enrichment in the caveolar membrane fraction. Further, immunoprecipitation studies demonstrated that Asp variants had substantially less NOS3/Cav-1 association (approximately 40%) during static conditions. Furthermore, acute shear causes impaired NOS3/Cav-1 dissociation in Asp variants. The results from immunoprecipitation studies were in complete agreement with caveolar membrane preparation findings. Collectively, these data demonstrate functional consequences of the Glu298Asp NOS3 variation and further define disruption of NOS3 caveolar localization and shear-induced mobilization as the primary mechanism responsible for these differences.

Activation of AMP kinase alpha1 subunit induces aortic vasorelaxation in mice

Vasodilatation is a vital mechanism of systemic blood flow regulation that occurs during periods of increased energy demand. The AMP-dependent protein kinase (AMPK) is a serine/threonine kinase that is activated by conditions that increase the AMP-to-ATP ratio, such as exercise and metabolic stress. We hypothesized that AMPK could trigger vasodilatation and participate in blood flow regulation. Rings of thoracic aorta were isolated from C57Bl6 mice and mice deficient in the AMPK catalytic alpha1 (AMPKalpha1-/-) or alpha2 (AMPKalpha2-/-) subunit and their littermate controls, and mounted in an organ bath. Aortas were preconstricted with phenylephrine (1 microM) and activation of AMPK was induced by addition of increasing concentrations of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR). AICAR (0.1-3 mM) dose-dependently induced relaxation of precontracted C57BL6, AMPKalpha1+/+ and alpha2+/+ aorta (P<0.001, n=5-7 per group). This AICAR induced vasorelaxation was not inhibited by the addition of adenosine receptor antagonists. Moreover, when aortic rings were freed of endothelium by gentle rubbing, AICAR still induced aortic ring relaxation, suggesting a direct effect of AICAR on smooth muscle cells. When aortic rings were pretreated with L-NMMA (30 microM) to inhibit nitric oxide synthase activity, AICAR still induced relaxation. Western blot analysis of C57Bl6 mice denuded aorta showed that AMPK was phosphorylated after incubation with AICAR and that AMPKalpha1 was the main catalytic subunit expressed. Finally, AICAR-induced relaxation of aortic rings was completely abolished in AMPKalpha1-/- but not AMPKalpha2-/- mice. Taken together, the results show that activation of AMPKalpha1 but not AMPKalpha2 is able to induce aortic relaxation in mice, in an endothelium- and eNOS-independent manner.

Therapeutic Potential of H11 Kinase for the Ischemic Heart

H11 kinase (H11K) is a small heat shock protein expressed predominantly in the heart and skeletal muscle, which plays a critical role in the maintenance of cardiac cell survival and in promoting cell growth through the activation of complementary signaling pathways. An overexpression of H11K was detected in various forms of heart disease, both in animal models and in patients, including acute and chronic ventricular dysfunction, and myocardial hypertrophy. Overexpression of H11K was reproduced in a cardiac-specific transgenic model, which led to significant progress in understanding the role and mechanism of action of the protein. Increased expression of H11K confers a cardioprotection that is equivalent to ischemic preconditioning; it promotes cardiac hypertrophy while maintaining contractile function. The overexpression of H11K is sufficient to activate most of the signaling pathways involved in cardiac cell growth and survival, including the phosphatidylinositol-3-kinase/Akt pathway, the AMP-dependent protein kinase, the PKCepsilon pathway of ischemic preconditioning, the nitric oxide pathway of delayed cardioprotection, and the mTOR pathway of cell growth. As a result, the survival response triggered by H11K in the heart includes antiapoptosis, cytoprotection, preconditioning, growth, and metabolic stimulation. In addition to activating signaling pathways, H11K promotes the subcellular translocation and crosstalk of intracellular messengers. This review discusses the biological function of H11K, its molecular mechanisms of action, and its potential therapeutic relevance. In particular, we discuss how preemptive conditioning of the heart by H11K might be beneficial for patients with ischemic heart disease who would be at risk of further irreversible cardiac damage.

Life history of eNOS: partners and pathways

The complex regulation of eNOS (endothelial nitric oxide synthase) in cardiovascular physiology occurs at multiple stages. eNOS mRNA levels are controlled both at the transcriptional and post-transcriptional phases, and epigenetic mechanisms appear to modulate tissue-specific eNOS expression. The eNOS enzyme reversibly associates with a diverse family of protein partners that regulate eNOS sub-cellular localization, catalytic function, and biological activity. eNOS enzyme activity and sub-cellular localization are intimately controlled by post-translational modifications including phosphorylation, nitrosylation, and acylation. The multiple extra-cellular stimuli affecting eNOS function coordinate their efforts through these key modifications to dynamically control eNOS and NO bioactivity in the vessel wall. This review will focus on the biochemical partners and perturbations of the eNOS protein as this vital enzyme undergoes modulation by diverse signal transduction pathways in the vascular endothelium.

AMP-activated protein kinase in metabolic control and insulin signaling

The AMP-activated protein kinase (AMPK) system acts as a sensor of cellular energy status that is conserved in all eukaryotic cells. It is activated by increases in the cellular AMP:ATP ratio caused by metabolic stresses that either interfere with ATP production (eg, deprivation for glucose or oxygen) or that accelerate ATP consumption (eg, muscle contraction). Activation in response to increases in AMP involves phosphorylation by an upstream kinase, the tumor suppressor LKB1. In certain cells (eg, neurones, endothelial cells, and lymphocytes), AMPK can also be activated by a Ca(2+)-dependent and AMP-independent process involving phosphorylation by an alternate upstream kinase, CaMKKbeta. Once activated, AMPK switches on catabolic pathways that generate ATP, while switching off ATP-consuming processes such as biosynthesis and cell growth and proliferation. The AMPK complex contains 3 subunits, with the alpha subunit being catalytic, the beta subunit containing a glycogen-sensing domain, and the gamma subunits containing 2 regulatory sites that bind the activating and inhibitory nucleotides AMP and ATP. Although it may have evolved to respond to metabolic stress at the cellular level, hormones and cytokines such as insulin, leptin, and adiponectin can interact with the system, and it now appears to play a key role in maintaining energy balance at the whole body level. The AMPK system may be partly responsible for the health benefits of exercise and is the target for the antidiabetic drug metformin. It is a key player in the development of new treatments for obesity, type 2 diabetes, and the metabolic syndrome.

Addition of adenosine monophosphate-activated protein kinase activators to University of Wisconsin solution: a way of protecting rat steatotic livers

This study investigates how the addition of trimetazidine (TMZ) and aminoimidazole-4-carboxamide ribonucleoside (AICAR) to University of Wisconsin (UW) solution protects steatotic livers. Steatotic and nonsteatotic livers were preserved for 24 hours at 4 degrees C in UW and UW with TMZ and AICAR (separately or in combination) and then perfused ex vivo for 2 hours at 37 degrees C. Adenosine monophosphate-activated protein kinase (AMPK) or nitric oxide (NO) synthesis inhibition in livers preserved in UW with TMZ was also investigated. Hepatic injury and function (transaminases, bile production, and sulfobromophthalein clearance) and factors potentially involved in the susceptibility of steatotic livers to ischemia-reperfusion (I/R), including vascular resistance, mitochondrial damage, adenosine triphosphate depletion, and oxidative stress were evaluated. AMPK, NO synthase (NOS), nitrate, and nitrite levels were also determined. The addition of TMZ and AICAR (separately or in combination) to UW reduced hepatic injury, improved functionality, and protected against the mechanisms responsible for the vulnerability of steatotic livers to I/R. Like AICAR, TMZ increased AMPK, constitutive NOS, and nitrates and nitrites, and conversely, AMPK or NO synthesis inhibition abolished the benefits of TMZ. In conclusion, TMZ, by means of AMPK, increased NO, thus protecting steatotic livers against their vulnerability to I/R injury. TMZ and AICAR may constitute new additives to UW solution in steatotic liver preservation, whereas a combination of both seems unnecessary.

The effect of n-3 PUFA on eNOS activity and expression in Ea hy 926 cells

The aim of this study was to evaluate the effects of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on NO synthase (eNOS) activation in Ea hy 926 endothelial cells. EPA or DHA (0-80 microM), added to the culture medium during 24h, were dose-dependently incorporated into the cells. In control medium, eNOS activity (evaluated by the citrulline assay) and eNOS phosphorylation on Ser 1177 were correlated. They were increased by 10 microM histamine and prevented by 20 microM lysophosphatidylcholine (LPC). By contrast, EPA or DHA increased basal phosphorylation without affecting eNOS activity in non-stimulated cells, but dose-dependently decreased this activity in histamine-stimulated cells without modifying the phosphorylation level. Furthermore, EPA and DHA did not prevent the deleterious effects of LPC on histamine stimulation. In conclusion, incorporation of EPA and DHA could be deleterious for endothelial cells by deregulating the activation of eNOS and preventing NO liberation.

Fluid Shear Stress and NO Decrease the Activity of the Hydroxy-Methylglutaryl Coenzyme A Reductase in Endothelial Cells via the AMP-Activated Protein Kinase and FoxO1

The rate-limiting enzyme for cholesterol synthesis, the hydroxy-methylglutaryl coenzyme A reductase (HCR), is phosphorylated by the AMP-activated protein kinase (AMPK). As shear stress activates the AMPK in endothelial cells, we determined whether it affects HCR activity and subsequent HCR-dependent signaling. Shear stress (12 dynes cm(-2)) rapidly increased the phosphorylation and activity (6.5- and 4-fold, respectively) of the AMPK in cultured endothelial cells and the activated AMPK phosphorylated the HCR in vitro. Moreover, shear stress and the AMPK activator 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) attenuated endothelial HCR activity by 37% and 33%, respectively. Inhibition of NO production attenuated the acute shear stress-induced phosphorylation of the AMPK and the decrease in HCR activity. Prolonged shear stress (18 hours) led to a significant (50%) decrease in HCR mRNA expression that was dependent on NO, AMPK, and the subsequent phosphorylation and degradation of FoxO1a. Correspondingly, the downregulation of FoxO (small interfering RNA) decreased HCR expression. Prolonged shear stress also attenuated the bradykinin-induced activation of Ras and extracellular signal-regulated kinase 1/2, a phenomenon that was comparable to the effects of cerivastatin and that was reversed by mevalonate and thus attributed to HCR inhibition. A decrease (35%) in HCR expression was also detected in femoral arteries from mice following voluntary exercise, and the bradykinin-induced vasodilatation of the mouse hindlimb was attenuated by both exercise and the HCR inhibitor cerivastatin. These data indicate that fluid shear stress regulates the activity and expression of the HCR in endothelial cells and determines responsiveness to stimuli, such as bradykinin via a mechanism involving NO, AMPK, FoxO1a, and p21Ras.

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

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

AMP-activated protein kinase 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.

Fenofibrate regulates retinal endothelial cell survival through the AMPK signal transduction pathway

Fenofibrate, a widely used hypolipidemic drug, has anti-inflammatory and anti-atherosclerotic effects in the vessel wall. In the present study, we report an anti-apoptotic property of fenofibrate in human retinal endothelial cells (HRECs) and describe an underlying molecular mechanism. Treatment with fenofibrate protected HRECs from apoptosis in response to serum deprivation in a dose-dependent manner. This inhibition of apoptosis by fenofibrate was not altered by peroxisome proliferator-activated receptor alpha (PPARalpha) antagonist MK 886, and selective agonist for PPARalpha, WY-14643 had no beneficial effects on serum deprivation-induced cell death. Fenofibrate potently induced a sustained activation of AMP-activated protein kinase (AMPK) and vascular endothelial growth factor (VEGF) mRNA expression. Furthermore, compound C, a specific AMPK inhibitor, almost completely blocked the fenofibrate-induced survival effect as well as VEGF mRNA expression. Taken together, these results suggest that fenofibrate prevents apoptotic cell death induced by serum deprivation through PPARalpha-independent, but AMPK-dependent pathway. Thus fenofibrate may have a novel therapeutic property that can control unwanted cell death found in diabetic retinopathy.

Regulation of endothelial nitric-oxide synthase activity through phosphorylation in response to epoxyeicosatrienoic acids

Endothelial nitric oxide synthase (eNOS) is a key enzyme in NO-mediated cardiovascular homeostasis and its activity is modulated by a variety of hormonal and mechanical stimuli via phosphorylation modification. Our previous study has demonstrated that epoxyeicosatrienoic acids (EETs), the cytochrome P450 (CYP)-dependent metabolites of arachidonic acid, could robustly up-regulate eNOS expression. However, the molecular mechanism underlying the effects of EETs on eNOS remains elusive. Particularly, whether and how EETs affect eNOS phosphorylation is unknown. In the present study, we investigated the effects of EETs on eNOS phosphorylation with cultured bovine aortic endothelial cells (BAECs). BAECs were either treated with exogenous EETs or infected with recombinant adeno-associated virus (rAAV) carrying CYP2C11-CYPOR, CYP102 F87V mutant and CYP2J2, respectively, to increase endogenous EETs. Both addition of EETs and CYP epoxygenase transfection markedly increased eNOS phosphorylation at its Ser1179 and Thr497 residues. Inhibition of phosphatidylinositol 3-kinase (PI3K) with LY294002 prevented EETs-induced increases of eNOS-Ser(P)1179 but had no effect on the phosphorylation status of Thr497. However, inhibitors of protein kinase B (Akt), mitogen-activated protein kinase (MAPK) and MAPK kinase could block phosphorylation of eNOS at both sites. Inhibition of these kinases also attenuated the up-regulation of eNOS expression by EETs. Finally, administration of viral CYP epoxygenases expression vectors into rats enhanced eNOS phosphorylation and function in vivo. Thus, in addition to up-regulating eNOS expression, EETs also augment eNOS function by enhancing eNOS phosphorylation. EETs-induced up-regulation of eNOS phosphorylation and expression appears to involve in both PI3K/Akt and MAPK pathways.

Glucose, insulin, and leptin signaling pathways modulate nitric oxide synthesis in glucose-inhibited neurons in the ventromedial hypothalamus

Glucose-sensing neurons in the ventromedial hypothalamus (VMH) are involved in the regulation of glucose homeostasis. Glucose-sensing neurons alter their action potential frequency in response to physiological changes in extracellular glucose, insulin, and leptin. Glucose-excited neurons decrease, whereas glucose-inhibited (GI) neurons increase, their action potential frequency when extracellular glucose is reduced. Central nitric oxide (NO) synthesis is regulated by changes in local fuel availability, as well as insulin and leptin. NO is involved in the regulation of food intake and is altered in obesity and diabetes. Thus this study tests the hypothesis that NO synthesis is a site of convergence for glucose, leptin, and insulin signaling in VMH glucose-sensing neurons. With the use of the NO-sensitive dye 4-amino-5-methylamino-2',7'-difluorofluorescein in conjunction with the membrane potential-sensitive dye fluorometric imaging plate reader, we found that glucose and leptin suppress, whereas insulin stimulates neuronal nitric oxide synthase (nNOS)-dependent NO production in cultured VMH GI neurons. The effects of glucose and leptin were mediated by suppression of AMP-activated protein kinase (AMPK). The AMPK activator 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR) increased both NO production and neuronal activity in GI neurons. In contrast, the effects of insulin on NO production were blocked by the phosphoinositide 3-kinase inhibitors wortmannin and LY-294002. Furthermore, decreased glucose, insulin, and AICAR increase the phosphorylation of VMH nNOS, whereas leptin decreases it. Finally, VMH neurons express soluble guanylyl cyclase, a downstream mediator of NO signaling. Thus NO may mediate, in part, glucose, leptin, and insulin signaling in VMH glucose-sensing neurons.

Stress signaling in the heart by AMP-activated protein kinase

The stress-signaling protein, adenosine monophosphate-activated protein kinase (AMPK), regulates a variety of pathways in cells that 1) increase the provision and utilization of energy-providing substrates such as glucose and fatty acids, 2) inhibit energy-requiring pathways such as cholesterol biosynthesis and protein synthesis, and 3) increase the transcription of genes involved in energy metabolism and mitochondrial biogenesis. In the heart, AMPK therefore becomes very important in protecting against ischemia-reperfusion injury and regulating substrate metabolism in the face of changes in workload. This review summarizes the regulation of AMPK activity in the heart and discusses the effects of AMPK activation.

Adiponectin actions in the cardiovascular system

Obesity is strongly associated with the pathogenesis of type 2 diabetes, hypertension, and cardiovascular disease. Levels of the hormone adiponectin are downregulated in obese individuals, and several experimental studies show that adiponectin protects against the development of various obesity-related metabolic and cardiovascular diseases. Adiponectin exhibits favorable effects on atherogenesis, endothelial function, and vascular remodeling by modulation of signaling cascades in cells of the vasculature. More recent findings have shown that adiponectin directly affects signaling in cardiac cells and is beneficial in the setting of pathological cardiac remodeling and acute cardiac injury. Several of these effects of adiponectin have been attributed to the activation of the 5' AMP-activated protein kinase signaling cascade and other signaling proteins. This review will discuss the epidemiological and experimental studies that have elucidated the role of adiponectin in a variety of cardiovascular 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.