The role of AMP-activated protein kinase in fuel selection by the stressed heart

Glucose-free conditions induce the expression of AMPK in dental pulp cells

OBJECTIVES

This study is aimed to test whether glucose-free conditions induce the activation of adenosine monophosphate-activated protein kinase (AMPK) and, to investigate association with AMPK expression and cell viability in human dental pulp cells.

DESIGN

Human dental pulp cells were initially maintained in culture medium containing glucose and the medium was subsequently changed to glucose-free medium. To evaluate the expression of AMPK, quantitative real-time RT-PCR, Western blot analysis and immunofluorescence were carried out. Cell viability was evaluated by MTT assay.

RESULTS

The expression of AMPK mRNA in glucose free conditions was 2.0-2.5 fold higher than the control at 1, 2 and 3 h (P<0.01). The expression of phosphorylated-AMPK was characterized by Western blot analysis and by immunofluorescence. Compound C-pre-treated group showed a decline of both AMPK expression and cell viability, while AICAR-pre-treated group showed an increase of AMPK and maintain of cell viability at regular level.

CONCLUSIONS

AMPK plays an important role on fluctuating in accordance with glucose availability and protects cell viability from glucose free condition in human dental pulp cells.

Berberine attenuates ischemia-reperfusion injury via regulation of adenosine-5'-monophosphate kinase activity in both non-ischemic and ischemic areas of the rat heart

BACKGROUND

Berberine exhibits numerous pharmacological effects, but the mechanism for its protective effects against ischemia-reperfusion cardiac injury is unknown.

METHODS

Male Wistar rats were treated with berberine (100 mg/Kg/day, ig) for 14 days and controls treated with water. Hearts were isolated in vitro and perfused in the Langendorff mode and subjected to 30 min of global ischemia followed by 30 min of reperfusion and hemodynamic data examined. In a separate set of experiments, hearts were subjected in vivo to left anterior descending coronary artery ligation for 30 min followed by 120 min reperfusion and hemodynamic data, type and duration of arrhythmias, and myocardial infarct size determined. AMP-activated protein kinase (AMPK) level, ADP/ATP and AMP/ATP ratios were examined in non-ischemic areas and risk areas of the heart.

RESULTS

Subsequent to ischemia-reperfusion injury, left ventricular developed pressure, left ventricular end diastolic pressure and maximum rate of intraventricular pressure contractility and relaxation were significantly improved in the berberine treatment groups compared to controls. Berberine treatment decreased infarct size and diminished the duration and incidence of arrhythmias compared to controls. Berberine treatment significantly decreased AMPK protein concentration, and the ratio of ADP/ATP and AMP/ATP in the myocardial risk areas. In contrast, berberine treatment significantly increased AMPK protein concentration, and the ratio of ADP/ATP and AMP/ATP in the non-ischemia areas compared to controls.

CONCLUSION

These findings suggest that berberine may exert its cardioprotective effect on ischemia-reperfusion injury via regulation of AMPK activity in both non-ischemic areas and risk areas of the heart.

Targeting energetic metabolism: a new frontier in the pathogenesis and treatment of pulmonary hypertension

This perspective highlights advances in the understanding of the role of cellular metabolism in the pathogenesis of pulmonary hypertension. Insights gained in the past 20 years have revealed several similarities between the cellular processes underlying the pulmonary vascular remodeling in pulmonary hypertension and those seen in cancer processes. In line with these insights, there is increasing recognition that abnormal cellular metabolism, notably of aerobic glycolysis (the "Warburg effect"), the potential involvement of hypoxia-inducible factor in this process, and alterations in mitochondrial function, are key elements in the pathogenesis of this disease. The glycolytic shift may underlie the resistance to apoptosis and increased vascular cell proliferation, which are hallmarks of pulmonary hypertension. These investigations have led to novel approaches in the diagnosis and therapy of pulmonary hypertension.

Absence of equilibrative nucleoside transporter 1 in ENT1 knockout mice leads to altered nucleoside levels following hypoxic challenge

AIMS

Equilibrative nucleoside transporters (ENT) modulate the flux of adenosine. The ENT1-null (KO) mouse heart is endogenously cardioprotected but the cellular basis of this phenotype is unknown. Therefore, we investigated the cellular mechanisms underlying ENT1-mediated cardioprotection.

MAIN METHODS

Circulating adenosine levels were measured in WT and KO mice. Cellular levels of nucleosides and nucleotides were investigated in isolated adult cardiomyocytes from WT and KO mice using HPLC following hypoxic challenge (30 min, 2% O(2)). Changes in hypoxic gene expression were analyzed by PCR arrays and cAMP levels were measured to investigate contributions from adenosine receptors.

KEY FINDINGS

Circulating adenosine levels were significantly higher in KO (416±42nmol/l, n=12) compared to WT animals (208±21, n=13, p<0.001). Absence of ENT1 led to an elevated expression of genes involved in cardioprotective pathways compared to WT cardiomyocytes. Following hypoxic challenge, extracellular adenosine levels were significantly elevated in KO (4360±1840 pmol/mg protein) versus WT cardiomyocytes (3035±730 pmol/mg protein, n≥12, p<0.05). This effect was enhanced in the presence of dipyridamole (30 μM), which inhibits ENT1 and ENT2. Enhanced extracellular adenosine levels in ENT1-null cardiomyocytes appeared to come from a pool of extracellular nucleotides including IMP, AMP and ADP. Adenosine receptor (AR) activation mimicked increases in cAMP levels due to hypoxic challenge suggesting that ENT1 modulates AR-dependent signaling.

SIGNIFICANCE

ENT1 contributes to modulation of extracellular adenosine levels and subsequent purinergic signaling via ARs. ENT1-null mice possess elevated circulating adenosine levels and reduced cellular uptake resulting in a perpetually cardioprotected phenotype.

AMPK Attenuates Bupivacaine-induced Neurotoxicity

Bupivacaine has been widely used as a long-acting local anesthetic. However, evidence strongly suggests that bupivacaine causes apoptosis. AMP-activated protein kinase (AMPK) regulates metabolic homeostasis and mediates cellular protection from stress. We hypothesized that AMPK may be cytoprotective in bupivacaine-treated Schwann cells. To explore this, we applied bupivacaine to the RT4-D6P2T Schwann cell line. The expression of phosphorylated AMPK was compared after bupivacaine treatment. Bupivacaine induced cell death in a time- and dose- [50% lethal dose (LD50) = 316 µM] dependent manner, and increased expression of phosphorylated AMPK after bupivacaine treatment. Bupivacaine-induced cytotoxicity was attenuated by AICAR (an AMPK activator), whereas compound C (an AMPK inhibitor) enhanced it. The cytoprotective effect of AICAR was reversed in the presence of iodotubercidin, an AICAR inhibitor. Our results suggest that the AMPK pathway may protect Schwann cells from bupivacaine-induced cytotoxicity.

Cellular energy sensing and signaling by AMP-activated protein kinase

AMP-activated protein kinase (AMPK) is an energy sensing/signaling protein that, when activated, increases ATP production by stimulating glucose uptake and fatty acid oxidation while at the same time inhibiting ATP = consuming processes such as protein synthesis. Chronic activation of AMPK inhibits expression of lipogenic enzymes in the liver and enhances expression of mitochondrial oxidative enzymes in skeletal muscle. Deficiency of muscle LKB1, the upstream kinase of AMPK, results in greater fluctuation in energy charge during muscle contraction and decreased capacity for exercise at higher work rates. Because AMPK enhances both glucose uptake and fatty acid oxidation in skeletal muscle, it has become a target for prevention and treatment of type 2 diabetes and obesity.

AMP-activated protein kinase mediates preconditioning in cardiomyocytes by regulating activity and trafficking of sarcolemmal ATP-sensitive K(+) channels

Brief periods of ischemia and reperfusion that precede sustained ischemia lead to a reduction in myocardial infarct size. This phenomenon, known as ischemic preconditioning, is mediated by signaling pathway(s) that is complex and yet to be fully defined. AMP-activated kinase (AMPK) is activated in cells under conditions associated with ATP depletion and increased AMP/ATP ratio. In the present study, we have taken advantage of a cardiac phenotype overexpressing a dominant negative form of the alpha2 subunit of AMPK to analyze the role, if any, that AMPK plays in preconditioning the heart. We have found that myocardial preconditioning activates AMPK in wild type, but not transgenic mice. Cardiac cells from transgenic mice could not be preconditioned, as opposed to cells from the wild type. The cytoprotective effect of AMPK was not related to the effect that preconditioning has on mitochondrial membrane potential as revealed by JC-1, a mitochondrial membrane potential-sensitive dye, and laser confocal microscopy. In contrast, experiments with di-8-ANEPPS, a sarcolemmal-potential sensitive dye, has demonstrated that intact AMPK activity is required for preconditioning-induced shortening of the action membrane potential. The preconditioning-induced activation of sarcolemmal K(ATP) channels was observed in wild type, but not in transgenic mice. HMR 1098, a selective inhibitor of sarcolemmal K(ATP) channels opening, inhibited preconditioning-induced shortening of action membrane potential as well as cardioprotection afforded by AMPK. Immunoprecipitation followed by Western blotting has shown that AMPK is essential for preconditioning-induced recruitment of sarcolemmal K(ATP) channels. Based on the obtained results, we conclude that AMPK mediates preconditioning in cardiac cells by regulating the activity and recruitment of sarcolemmal K(ATP) channels without being a part of signaling pathway that regulates mitochondrial membrane potential.

Insulin signalling downstream of protein kinase B is potentiated by 5'AMP-activated protein kinase in rat hearts in vivo

AIMS/HYPOTHESIS

5'AMP-activated protein kinase (AMPK) and insulin stimulate glucose transport in heart and muscle. AMPK acts in an additive manner with insulin to increase glucose uptake, thereby suggesting that AMPK activation may be a useful strategy for ameliorating glucose uptake, especially in cases of insulin resistance. In order to characterise interactions between the insulin- and AMPK-signalling pathways, we investigated the effects of AMPK activation on insulin signalling in the rat heart in vivo.

METHODS

Male rats (350-400 g) were injected with 1 g/kg 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) or 250 mg/kg metformin in order to activate AMPK. Rats were administered insulin 30 min later and after another 30 min their hearts were removed. The activities and phosphorylation levels of components of the insulin-signalling pathway were subsequently analysed in individual rat hearts.

RESULTS

AICAR and metformin administration activated AMPK and enhanced insulin signalling downstream of protein kinase B in rat hearts in vivo. Insulin-induced phosphorylation of glycogen synthase kinase 3 (GSK3) beta, p70 S6 kinase (p70S6K)(Thr389) and IRS1(Ser636/639) were significantly increased following AMPK activation. To the best of our knowledge, this is the first report of heightened insulin responses of GSK3beta and p70S6K following AMPK activation. In addition, we found that AMPK inhibits insulin stimulation of IRS1-associated phosphatidylinositol 3-kinase activity, and that AMPK activates atypical protein kinase C and extracellular signal-regulated kinase in the heart.

CONCLUSIONS/INTERPRETATIONS

Our data are indicative of differential effects of AMPK on the activation of components in the cardiac insulin-signalling pathway. These intriguing observations are critical for characterisation of the crosstalk between AMPK and insulin signalling.