A molecular approach to the concerted action of kinases involved in energy homoeostasis

AMP-activated protein kinase can be allosterically activated by ADP but AMP remains the key activating ligand

The AMP-activated protein kinase (AMPK) is a sensor of cellular energy status. When activated by increases in ADP:ATP and/or AMP:ATP ratios (signalling energy deficit), AMPK acts to restore energy balance. Binding of AMP to one or more of three CBS repeats (CBS1, CBS3, CBS4) on the AMPK-γ subunit activates the kinase complex by three complementary mechanisms: (i) promoting α-subunit Thr172 phosphorylation by the upstream kinase LKB1; (ii) protecting against Thr172 dephosphorylation; (iii) allosteric activation. Surprisingly, binding of ADP has been reported to mimic the first two effects, but not the third. We now show that at physiologically relevant concentrations of Mg.ATP2- (above those used in the standard assay) ADP binding does cause allosteric activation. However, ADP causes only a modest activation because (unlike AMP), at concentrations just above those where activation becomes evident, ADP starts to cause competitive inhibition at the catalytic site. Our results cast doubt on the physiological relevance of the effects of ADP and suggest that AMP is the primary activator in vivo. We have also made mutations to hydrophobic residues involved in binding adenine nucleotides at each of the three γ subunit CBS repeats of the human α2β2γ1 complex and examined their effects on regulation by AMP and ADP. Mutation of the CBS3 site has the largest effects on all three mechanisms of AMP activation, especially at lower ATP concentrations, while mutation of CBS4 reduces the sensitivity to AMP. All three sites appear to be required for allosteric activation by ADP.

New insights into the correlations between circulating tumor cells and target organ metastasis

Organ-specific metastasis is the primary cause of cancer patient death. The distant metastasis of tumor cells to specific organs depends on both the intrinsic characteristics of the tumor cells and extrinsic factors in their microenvironment. During an intermediate stage of metastasis, circulating tumor cells (CTCs) are released into the bloodstream from primary and metastatic tumors. CTCs harboring aggressive or metastatic features can extravasate to remote sites for continuous colonizing growth, leading to further lesions. In the past decade, numerous studies demonstrated that CTCs exhibited huge clinical value including predicting distant metastasis, assessing prognosis and monitoring treatment response et al. Furthermore, increasingly numerous experiments are dedicated to identifying the key molecules on or inside CTCs and exploring how they mediate CTC-related organ-specific metastasis. Based on the above molecules, more and more inhibitors are being developed to target CTCs and being utilized to completely clean CTCs, which should provide promising prospects to administer advanced tumor. Recently, the application of various nanomaterials and microfluidic technologies in CTCs enrichment technology has assisted to improve our deep insights into the phenotypic characteristics and biological functions of CTCs as a potential therapy target, which may pave the way for us to make practical clinical strategies. In the present review, we mainly focus on the role of CTCs being involved in targeted organ metastasis, especially the latest molecular mechanism research and clinical intervention strategies related to CTCs.

Involvement of the phosphoryl transfer network in gill bioenergetic imbalance of pacamã (Lophiosilurus alexandri) subjected to hypoxia: notable participation of creatine kinase

Hypoxia is among the most critical environmental stressors for fish in aquatic environments, and several energetic alterations have been associated with it. The aim of the present study was to evaluate the involvement of the phosphoryl transfer network and its effects on adenosine triphosphate (ATP)-dependent enzymes during hypoxia, as well as the role of oxidative stress in the activity of the phosphoryl transfer network in pacamã (Lophiosilurus alexandri) subjected to severe hypoxia. Branchial creatine kinase (CK; cytosolic and mitochondrial fractions), adenylate kinase (AK), and pyruvate kinase (PK) activities were inhibited after 72 h of exposure to hypoxia compared to their respective normoxia groups, and remained low (except for AK) after 24 and 72 h of re-oxygenation. Activities of the branchial sodium-potassium pump (Na⁺, K⁺-ATPase) and proton pump (H⁺-ATPase) were inhibited in fish exposed to 72 h of hypoxia compared to the normoxia group, remained inhibited after 24 h of re-oxygenation, and were restored to physiological levels after 72 h of re-oxygenation. Levels of branchial reactive oxygen species (ROS) were higher in fish exposed to hypoxia for 72 h compared to the normoxia group, and increased during re-oxygenation. Lipid peroxidation (LOOH) levels were higher in fish subjected to 72 h of hypoxia compared to the normoxia group, and remained higher during re-oxygenation. On the other hand, protein sulfhydryl (PSH) levels were lower in fish exposed to hypoxia for 72 h compared to the normoxia group, and remained low during re-oxygenation. Based on this evidence, inhibition of the activities of enzymes belonging to phosphoryl transfer network contributed to impairing energetic homeostasis linked to ATP production and ATP utilization in gills of pacamã subjected to hypoxia, and remained inhibited during re-oxygenation (except AK activity). Moreover, inhibition of the phosphoryl transfer network impaired activity of ATP-dependent enzymes, which can be mediated by ROS overproduction, lipid peroxidation, and oxidation of SH groups.

Nanospheres as a technological alternative to suppress hepatic cellular damage and impaired bioenergetics caused by nerolidol in Nile tilapia (Oreochromis niloticus)

Nerolidol is a sesquiterpene found in essential oils of several plant species. It is found commonly in human and animal diets and is approved by the US Food and Drug Administration as a flavoring agent. Nevertheless, recent studies have suggested that nerolidol has potent hepatotoxic effects. Because use of plant-based products in human and animal food has expanded considerably, it is essential to develop approaches such as nanotechnology to avoid or reduce hepatic toxic effects. Therefore, the aim of the study was to determine whether nerolidol dietary supplementation elicited hepatic damage associated with impairment of energy homeostasis, as well as whether supplementation with nerolidol-loaded in nanospheres prevented hepatotoxic effects in Nile tilapia (Oreochromis niloticus). Nile tilapia were divided into five groups (A-E, n = 10 per group) with four replicates each, as follows: group A received basal feed (without supplementation); group B received feed containing 0.5 mL free nerolidol/kg; group C received feed containing 1.0 mL free nerolidol/kg; group D received feed containing 0.5 mL nanospheres nerolidol/kg; and group E received feed containing 1.0 mL nanospheres nerolidol/kg. All groups received experimental feed once a day (10% total biomass) at 2 p.m. for 60 consecutive days. Hepatic liver weight and relative liver weight were significantly lower in fish fed 1.0 mL free nerolidol/kg feed than in fish given basal diet (control group). Hepatic pyruvate kinase (1.0 mL free nerolidol/kg) and adenylate kinase (0.5 and 1.0 mL free nerolidol/kg) activities were significantly lower than in the control group, while hepatic reactive oxygen species and lipid damage levels were significantly higher. Finally, the comet assay revealed significant increases in the frequency of damage and the damage index in fish given 0.5 and 1.0 mL free nerolidol/kg in a dose-dependent manner. Nerolidol-loaded in nanospheres prevented all alterations elicited by free nerolidol. Based on these data, we concluded that dietary supplementation with free nerolidol elicited severe impairment of hepatic bioenergetics homeostasis that appeared to be mediated by excessive ROS production and lipid damage, contributing to a genotoxic effect. Dietary supplementation with nerolidol-loaded in nanospheres did not elicit hepatic damage, and therefore, should be considered as a replacement so as to limit toxicity, permitting its continued use as a dietary supplement.

Caffeine supplementation in diet mitigates Aeromonas hydrophila-induced impairment of the gill phosphotransfer network in grass carp Ctenopharyngodon idella

Some evidence suggests the involvement of phosphotransfer network in the pathogenesis of fish bacterial diseases, catalyzed by creatine kinase (CK), pyruvate kinase (PK) and adenylate kinase (AK); nevertheless, the effects on fish affected by Aeromonas hydrophila remain unknown. Recent evidence suggested a potent protective effect of caffeine on the branchial phosphotransfer network of fish subjected to challenge conditions. Therefore, the aim of this study was to evaluate whether A. hydrophila infection impaired branchial bioenergetics. We also determined whether dietary supplementation with caffeine protected against A. hydrophila-induced gill bioenergetic imbalance. We found that branchial cytosolic CK and AK activities were significant lower in fish experimentally infected with A. hydrophila than in uninfected fish, while mitochondrial CK activity was significant higher. Branchial lactate dehydrogenase (LDH) activity and lactate levels were significant higher in fish experimentally infected by A. hydrophila than in uninfected fish, while sodium-potassium ion pump (Na⁺, K⁺-ATPase) activity and adenosine triphosphate (ATP) levels were significant lower. No significant difference was observed between groups with respect to branchial PK activity. The dietary supplementation with 8% caffeine improved the branchial CK (cytosolic and mitochondrial), AK, and LDH activities, as well as ATP levels, but did not prevent increases in branchial lactate levels or the inhibition of Na⁺, K⁺-ATPase activity elicited by aeromonosis. Based on this evidence, we believe that reduction of CK (cytosolic) and AK activities contributes to impairment of bioenergetic homeostasis, while augmentation of mitochondrial CK activity can be considered an attempt to prevent or reduce the energetic imbalance during aeromonosis caused by A. hydrophila. The use of 8% caffeine dietary supplementation improved the energetic metabolism via protective effects on CK and AK activities, avoiding the necessity of using anaerobic metabolism. In summary, 8% dietary caffeine can be used to improve branchial energetic homeostasis during aeromonosis caused by A. hydrophila.

Synthetic energy sensor AMPfret deciphers adenylate-dependent AMPK activation mechanism

AMP-activated protein kinase AMPK senses and regulates cellular energy state. AMPK activation by increasing AMP and ADP concentrations involves a conformational switch within the heterotrimeric complex. This is exploited here for the construction of a synthetic sensor of cellular energetics and allosteric AMPK activation, AMPfret. Based on engineered AMPK fused to fluorescent proteins, the sensor allows direct, real-time readout of the AMPK conformational state by fluorescence resonance energy transfer (FRET). AMPfret faithfully and dynamically reports the binding of AMP and ADP to AMPK γ-CBS sites, competed by Mg²⁺-free ATP. FRET signals correlate with activation of AMPK by allosteric mechanisms and protection from dephosphorylation, attributed here to specific CBS sites, but does not require activation loop phosphorylation. Moreover, AMPfret detects binding of pharmacological compounds to the AMPK α/β-ADaM site enabling activator screening. Cellular assays demonstrate that AMPfret is applicable in vivo for spatiotemporal analysis of energy state and allosteric AMPK activation.

AMP-activated protein kinase AMPK senses and regulates cellular energy state. Here the authors engineer a synthetic sensor, AMPfret, that allows direct, real-time readout of the AMPK conformational state by fluorescence resonance energy transfer (FRET).

AMP-Activated Protein Kinase (AMPK)-Dependent Regulation of Renal Transport

AMP-activated kinase (AMPK) is a serine/threonine kinase that is expressed in most cells and activated by a high cellular AMP/ATP ratio (indicating energy deficiency) or by Ca²⁺. In general, AMPK turns on energy-generating pathways (e.g., glucose uptake, glycolysis, fatty acid oxidation) and stops energy-consuming processes (e.g., lipogenesis, glycogenesis), thereby helping cells survive low energy states. The functional element of the kidney, the nephron, consists of the glomerulus, where the primary urine is filtered, and the proximal tubule, Henle's loop, the distal tubule, and the collecting duct. In the tubular system of the kidney, the composition of primary urine is modified by the reabsorption and secretion of ions and molecules to yield final excreted urine. The underlying membrane transport processes are mainly energy-consuming (active transport) and in some cases passive. Since active transport accounts for a large part of the cell's ATP demands, it is an important target for AMPK. Here, we review the AMPK-dependent regulation of membrane transport along nephron segments and discuss physiological and pathophysiological implications.

Listeria monocytogenes impairs enzymes of the phosphotransfer network and alters antioxidant/oxidant status in cattle brain structures

Several evidences have suggested the involvement of enzymes belonging to the phosphotransfer network, formed by creatine kinase (CK), pyruvate kinase (PK) and adenylate kinase (AK), as well the oxidative stress on the pathogenesis of infectious diseases associated with the central nervous system (CNS). Thus, the aim of this study was to evaluate whether listeriosis alters the brain energy metabolism and/or causes oxidative stress in different brain structures of cattle experimentally infected by Listeria monocytogenes. The cytosolic CK activity was inhibited in the cerebral cortex, cerebellum, brainstem and hippocampus of infected animals compared to uninfected animals, while the mitochondrial CK activity was increased. The PK activity was inhibited in all brain structures of infected animals, while the AK activity was unchanged. Na⁺, K⁺-ATPase activity decreased in the cerebral cortex, cerebellum and hippocampus of animals infected by L. monocytogenes. Regarding the oxidative strees variables, the cerebellum and brainstem of infected animals showed increased thiobarbituric acid reactive substances, while the catalase activity was inhibited. Glutathione S-transferarase was inhibited in the cerebral cortex and brainstem of infected animals, and it was increased in the cerebellum. L. monocytogenes was quantified in the liver (n = 5/5) and cerebral cortex (n = 4/5) of the infected cattle. Based on these evidences, the nucleocytoplasmic communication between CK isoenzymes was insufficient to avoid an impairment of cerebral bioenergetics. Moreover, the inhibition on brain PK activity caused an impairment in the communication between sites of ATP generation and ATP utilization. The lipid peroxidation and alteration on antioxidant status observed in some brain structures were also involved during the disease. In summary, these alterations contribute to disease pathogenesis linked to CNS during cattle listeriosis.

Changes in the cerebral phosphotransfer network impair energetic homeostasis in an aflatoxin B1-contaminated diet

The phosphotransfer network system, through the enzymes creatine kinase (CK), adenylate kinase (AK), and pyruvate kinase (PK), contributes to efficient intracellular energetic communication between cellular adenosine triphosphate (ATP) consumption and production in tissues with high energetic demand, such as cerebral tissue. Thus, the aim of this study was to evaluate whether aflatoxin B₁ (AFB₁) intoxication in diet negatively affects the cerebral phosphotransfer network related to impairment of cerebral ATP levels in silver catfish (Rhamdia quelen). Brain cytosolic CK activity decreased in animals fed with a diet contaminated with AFB₁ on days 14 and 21 post-feeding, while mitochondrial CK activity increased, when compared to the control group (basal diet). Also, cerebral AK and PK activity decreased in animals fed with a diet contaminated with AFB₁ on days 14 and 21 post-feeding, similarly to the results observed for cerebral ATP levels. Based on this evidence, inhibition of cerebral cytosolic CK activity is compensated by stimulation of mitochondrial CK activity in an attempt to prevent impairment of communication between sites of ATP generation and ATP utilization. The inhibition of cerebral AK and PK activity leads to impairment of cerebral energy homeostasis, decreasing the brain's ATP availability. Moreover, the absence of a reciprocal compensatory mechanism between these enzymes contributes to cerebral energetic imbalance, which may contribute to disease pathophysiology.

Thiamethoxam induced hepatic energy changes in silver catfish via impairment of the phosphoryl transfer network pathway: Toxicological effects on energetics homeostasis

Precise coupling of spatially separated intracellular adenosine triphosphate (ATP)-producing and ATP-consuming processes exerts a pivotal role in bioenergetic homeostasis of living organisms, and the phosphotransfer network pathway, catalyzed by adenylate kinase (AK) and pyruvate kinase (PK), is fundamental in cellular and tissue energetic homeostasis. Measurement of the phosphotransfer network can provide new information for understanding the alterations in hepatic energetic metabolism during exposition to insecticides, such as thiamethoxam. Therefore, the aim of this study was to evaluate whether exposition to thiamethoxam negatively affects the hepatic enzymes of the phosphotransfer network in silver catfish (Rhamdia quelen). Hepatic AK and PK activities were inhibited at 3.75 μg L^-1 after 24 h of exposure and at 1.125 and 3.75 μg L^-1 after 96 h of exposure compared with the control group. The hepatic ATP levels were decreased following 3.75 μg L^-1 thiamethoxam treatment after 24 h of exposure and at 1.125 and 3.75 μg L^-1 after 96 h of exposure compared with the control group. The enzymatic activity of the phosphotransfer network and ATP levels did not recover after 48 h of recovery in clean water. Thus, the inhibition of hepatic AK and PK activities by thiamethoxam caused impairment of energy homeostasis in liver tissue, decreasing hepatic ATP availability. Moreover, the absence of a mutual compensatory mechanism between these enzymes directly contributes to ATP depletion and to a severe energetic dysregulation, which may contribute to toxic effects caused by thiamethoxam.

Citrobacter freundii impairs the phosphoryl transfer network in the gills of Rhamdia quelen: Impairment of bioenergetics homeostasis

The precise coupling of spatially separated intracellular adenosine triphosphate (ATP)-producing and ATP-consuming, catalyzed by creatine kinase (CK), adenylate kinase (AK), and pyruvate kinase (PK), is a critical process in the bioenergetics of tissues with high energy demand, such as the branchial tissue. The effects of Citrobacter freundii infection on gills remain poorly understood, limited only to histopathological studies. Thus, the aim of this study was to evaluate whether experimental infection by C. freundii impairs the enzymes of the phosphoryl transfer network in gills of silver catfish (Rhamdia quelen). The CK (cytosolic and mitochondrial) and AK activities decreased in infected compared to uninfected animals, while the PK activity did not differ between groups. The gill histopathology of infected animals revealed extensive degeneration with fusion and necrosis of secondary lamellae, detachment of superficial epithelium, aneurysm, vessel congestion and inflammatory process. Based on these evidences, the inhibition and absence of an efficient communication between CK compartments caused the impairment of the branchial bioenergetics homeostasis, which was not compensated by the augmentation on branchial AK activity in an attempt to restore energy homeostasis. In summary, these alterations contribute to disease pathogenesis linked to branchial tissue in animals infected with C. freundii.

Ichthyophthirius multifiliis impairs splenic enzymes of the phosphoryl transfer network in naturally infected Rhamdia quelen: effects on energetic homeostasis

Its integrated energetic and metabolic signaling roles place the phosphoryl transfer network, through the enzymes creatine kinase (CK), adenylate kinase (AK), and pyruvate kinase (PK), as a regulatory system coordinating components of the cellular bioenergetics network. Analysis of these enzymes provides new information and perspectives with which to understand disturbances in energetic metabolism between sites of adenosine triphosphate (ATP) generation and utilization. Thus, the aim of this study was to evaluate the involvement of the phosphoryl transfer network in splenic tissue linked with the pathogenesis of silver catfish naturally infected with Ichthyophthirius multifiliis. Splenic cytosolic and mitochondrial CK activities decreased in infected animals compared to uninfected animals, as was also observed for splenic PK activity and splenic ATP levels. In contrast, splenic AK activity increased in infected animals compared to uninfected animals. Based on this evidence, the inhibition and absence of efficient communication between CK isoenzymes cause the impairment of splenic bioenergetics, which is in turn compensated by the augmentation of splenic AK activity in an attempt to restore energy homeostasis. The inhibition of splenic PK activity impairs communication between sites of ATP generation and ATP utilization, as corroborated by splenic ATP depletion. In summary, these alterations contribute to disease pathogenesis linked to spleen tissue in animals infected with white spot disease.

Effect modification by region in the associations of LEP G2548A and LEPR Q223R polymorphisms with statin-induced CK elevation

We investigated the associations of LEP G2548A and LEPR Q223R polymorphisms with statin-induced creatine kinase (CK) elevation among Chinese patients with hyperlipidemia. A total of587 enrolled individuals were treated with 20 mg/d oral simvastatin for 8 consecutive weeks. Genotyping of LEP G2548A and LEPR Q223R were conducted using PCR-RFLP. Multiple regression analyses showed that, in the Dongzhi region only, patients carrying the LEP AA genotype had a significantly greater increase in CK levels compared to those carrying the AG+GG genotypes after four weeks (P = 0.004) and eight weeks (P < 0.001) consecutive simvastatin treatment. Patients were further divided into three groups based on the tertiles of the CK distribution. Compared to subjects in the lowest tertile of CK elevation, the adjusted relative odds of having the AG+GG genotypes among subjects in the highest tertile was 0.5 (95% CI, 0.3 to 0.7) and 0.4 (95% CI, 0.2 to 0.6) after the fourth and eighth weeks, respectively. The interaction terms between the Beijing or Dongzhi region and the LEP GA+AA genotypes were marginally significant for CK elevation at the fourth week (P = 0.057) and significant for CK elevation at the eighth week (P = 0.002). The adverse effect of the LEP G2548A polymorphism on increasing CK levels may be dependent on the environmental milieu. It suggests that lifestyle interventions might offset the side effects of simvastatin therapy among those with genetic susceptibility. Further research is needed to identify specific individual-level factors for clinical practice that modify the effect of genotype.

Genetically Encoded Fluorescent Biosensors to Explore AMPK Signaling and Energy Metabolism

Maintenance of energy homeostasis is a basic requirement for cell survival. Different mechanisms have evolved to cope with spatial and temporal mismatch between energy-providing and -consuming processes. Among these, signaling by AMP-activated protein kinase (AMPK) is one of the key players, regulated by and itself regulating cellular adenylate levels. Further understanding its complex cellular function requires deeper insight into its activation patterns in space and time at a single cell level. This may become possible with an increasing number of genetically encoded fluorescent biosensors, mostly based on fluorescence resonance energy transfer, which have been engineered to monitor metabolic parameters and kinase activities. Here, we review basic principles of biosensor design and function and the advantages and limitations of their use and provide an overview on existing FRET biosensors to monitor AMPK activation, ATP concentration, and ATP/ADP ratios, together with other key metabolites and parameters of energy metabolism.

Creatine kinase in cell cycle regulation and cancer

The phosphocreatine-creatine kinase (CK) shuttle system is increasingly recognized as a fundamental mechanism for ATP homeostasis in both excitable and non-excitable cells. Many intracellular processes are ATP dependent. Cell division is a process requiring a rapid rate of energy turnover. Cell cycle regulation is also a key point to understanding the mechanisms underlying cancer progression. It has been known for about 40 years that aberrant CK levels are associated with various cancers and for over 30 years that CK is involved in mitosis regulation. However, the underlying molecular mechanisms have not been investigated sufficiently until recently. By maintaining ATP at sites of high-energy demand, CK can regulate cell cycle progression by affecting the intracellular energy status as well as by influencing signaling pathways that are essential to activate cell division and cytoskeleton reorganization. Aberrant CK levels may impair cell viability under normal or stressed conditions and induce cell death. The involvement of CK in cell cycle regulation and cellular energy metabolism makes it a potential diagnostic biomarker and therapeutic target in cancer. To understand the multiple physiological/pathological functions of CK, it is necessary to identify CK-binding partners and regulators including proteins, non-coding RNAs and participating endogenous small molecular weight chemical compounds. This review will focus on molecular mechanisms of CK in cell cycle regulation and cancer progression. It will also discuss the implications of recent mechanistic studies, the emerging problems and future challenges of the multifunctional enzyme CK.

Regulation of brain-type creatine kinase by AMP-activated protein kinase: interaction, phosphorylation and ER localization

AMP-activated protein kinase (AMPK) and cytosolic brain-type creatine kinase (BCK) cooperate under energy stress to compensate for loss of adenosine triphosphate (ATP) by either stimulating ATP-generating and inhibiting ATP-consuming pathways, or by direct ATP regeneration from phosphocreatine, respectively. Here we report on AMPK-dependent phosphorylation of BCK from different species identified by in vitro screening for AMPK substrates in mouse brain. Mass spectrometry, protein sequencing, and site-directed mutagenesis identified Ser6 as a relevant residue with one site phosphorylated per BCK dimer. Yeast two-hybrid analysis revealed interaction of active AMPK specifically with non-phosphorylated BCK. Pharmacological activation of AMPK mimicking energy stress led to BCK phosphorylation in astrocytes and fibroblasts, as evidenced with a highly specific phospho-Ser6 antibody. BCK phosphorylation at Ser6 did not affect its enzymatic activity, but led to the appearance of the phosphorylated enzyme at the endoplasmic reticulum (ER), close to the ER calcium pump, a location known for muscle-type cytosolic creatine kinase (CK) to support Ca²⁺-pumping.

Sensing hypoxia by mitochondria: a unifying hypothesis involving S-nitrosation

SIGNIFICANCE

Sudden hypoxia requires a rapid response in tissues with high energy demand. Mitochondria are rapid sensors for a lack of oxygen, but no consistent mechanism for the sensing process and the subsequent counter-regulation has been described.

RECENT ADVANCES

In the present hypothesis review, we suggest an oxygen-sensing mechanism by mitochondria that is initiated at low oxygen tension by electrons from the respiratory chain, leading to the reduction of intracellular nitrite to nitric oxide ((•)NO) that would subsequently compete with oxygen for binding to cytochrome c oxidase. This allows superoxide ((•)O2(-)) formation in hypoxic areas, leading to S-nitrosation and the inhibition of mitochondrial Krebs cycle enzymes. With more formation of (•)O2(-), peroxynitrite is generated and known to damage the connection between the mitochondrial matrix and the outer membrane.

CRITICAL ISSUES

A fundamental question on a regulatory mechanism is its reversibility. Readmission of oxygen and opening of the mitochondrial KATP-channel would allow electrons from glycerol-3-phosphate to selectively reduce the ubiquinone pool to generate (•)O2(-) at both sides of the inner mitochondrial membrane. On the cytosolic side, superoxide is dismutated and will support H2O2/Fe(2+)-dependent transcription processes and on the mitochondrial matrix side, it could lead to the one-electron reduction and reactivation of S-nitrosated proteins.

FUTURE DIRECTIONS

It remains to be elucidated up to which stage the herein proposed silencing of mitochondria remains reversible and when irreversible changes that ultimately lead to classical reperfusion injury are initiated.

AMPK modulates glucose-sensing in insulin-secreting cells by altered phosphotransfer to KATP channels

Glucose-sensing (GS) behaviour in pancreatic β-cells is dependent on ATP-sensitive K(+) channel (KATP) activity, which is controlled by the relative levels of the KATP ligands ATP and ADP, responsible for closing and opening KATP, respectively. However, the mechanism by which β-cells transfer energy status from mitochondria to KATP, and hence to altered electrical excitability and insulin secretion, is presently unclear. Recent work has demonstrated a critical role for AMP-activated protein kinase (AMPK) in GS behaviour of cells. Electrophysiological recordings, coupled with measurements of gene and protein expression were made from rat insulinoma cells to investigate whether AMPK activity regulates this energy transfer process. Using the whole-cell recording configuration with sufficient intracellular ATP to keep KATP closed, raised AMPK activity induced GS electrical behaviour. This effect was prevented by the AMPK inhibitor, compound C and required a phosphotransfer process. Indeed, high levels of intracellular phosphocreatine or the presence of the adenylate kinase (AK) inhibitor AP5A blocked this action of AMPK. Using conditions that maximised AMPK-induced KATP opening, there was a significant increase in AK1, AK2 and UCP2 mRNA expression. Thus we propose that KATP opening in response to lowered glucose concentration requires AMPK activity, perhaps in concert with increased AK and UCP2 to enable mitochondrial-derived ADP signals to be transferred to plasma membrane KATP by phosphotransfer cascades.

Macro CK2 accumulation in tenofovir-treated HIV patients is facilitated by CK oligomer stabilization but is not predictive for pathology

BACKGROUND

Ubiquitous mitochondrial creatine kinase (uMtCK) accumulates as macroenzyme creatine kinase type 2 (macro CK2) in the serum of HIV-infected patients under a tenofovir disoproxil fumarate (TDF)-containing antiretroviral regimen. The genesis and clinical significance of this finding is unclear.

METHODS

A prospective observational 5-year follow-up study was performed on those patients in which macro CK2 appearance was initially described ('TDF switch study' cohort). In addition, tenofovir (TFV), its prodrug TDF and its active, intracellular derivative TFV diphosphate (TDP) were tested in vitro for their effects on different key properties of uMtCK to clarify possible interactions of uMtCK with TFV compounds.

RESULTS

In just under 5 years of continuous TDF treatment, only 4/12 (33%) patients remained macro CK2-positive, whereas 8/12 (66%) originally positive patients were macro CK2-negative at the end of follow-up. Prospective clinical follow-up data indicate that macro CK2 appearance under TDF is not associated with significant cell damage or occurrence of malignancies. A trend towards grade 1 hypophosphataemia suggests subclinical proximal tubular dysfunction in macro-CK2-positive patients, although it was not associated with a significant decrease in estimated glomerular filtration rate. In vitro, TFV, TDF and TDP did not interfere with uMtCK enzyme activity as competitive inhibitors or pseudo-substrates, but TFV and TDF stabilized the native uMtCK octameric structure in dilute solutions.

CONCLUSIONS

Appearance of octameric uMtCK as macro CK2 in the serum of TDF-treated patients is suggested to result from a combination of low-level mitochondrial damage caused by subclinical renal tubular dysfunction together with possible compensatory uMtCK overexpression and a putative concomitant stabilization of uMtCK octamers by higher levels of TFV in proximal tubules.

Novel tyrosine phosphorylation sites in rat skeletal muscle revealed by phosphopeptide enrichment and HPLC-ESI-MS/MS

Tyrosine phosphorylation plays a fundamental role in many cellular processes including differentiation, growth and insulin signaling. In insulin resistant muscle, aberrant tyrosine phosphorylation of several proteins has been detected. However, due to the low abundance of tyrosine phosphorylation (<1% of total protein phosphorylation), only a few tyrosine phosphorylation sites have been identified in mammalian skeletal muscle to date. Here, we used immunoprecipitation of phosphotyrosine peptides prior to HPLC-ESI-MS/MS analysis to improve the discovery of tyrosine phosphorylation in relatively small skeletal muscle biopsies from rats. This resulted in the identification of 87 distinctly localized tyrosine phosphorylation sites in 46 muscle proteins. Among them, 31 appear to be novel. The tyrosine phosphorylated proteins included major enzymes in the glycolytic pathway and glycogen metabolism, sarcomeric proteins, and proteins involved in Ca(2+) homeostasis and phosphocreatine resynthesis. Among proteins regulated by insulin, we found tyrosine phosphorylation sites in glycogen synthase, and two of its inhibitors, GSK-3α and DYRK1A. Moreover, tyrosine phosphorylation sites were identified in several MAP kinases and a protein tyrosine phosphatase, SHPTP2. These results provide the largest catalogue of mammalian skeletal muscle tyrosine phosphorylation sites to date and provide novel targets for the investigation of human skeletal muscle phosphoproteins in various disease states.

A two-dimensional screen for AMPK substrates identifies tumor suppressor fumarate hydratase as a preferential AMPKα2 substrate

AMP-activated protein kinase (AMPK) is emerging as a central cellular signaling hub involved in energy homeostasis and proliferation. The kinase is considered as a suitable target for pharmacological intervention in several energy-related pathologies like diabetes type II and cancer, although its signaling network is still incompletely understood. Here we apply an original two-dimensional in vitro screening approach for AMPK substrates that combines biophysical interaction based on surface plasmon resonance with in vitro phosphorylation. By enriching for proteins that interact with a specific AMPK isoform, we aimed to identify substrates that are also preferentially phosphorylated by this specific AMPK isoform. Application of this screen to full-length AMPK α2β2γ1 and soluble rat liver proteins identified the tumor suppressor fumarate hydratase (FH). FH was confirmed to interact with and to be preferentially phosphorylated by the AMPKα2 isoform by using yeast-two-hybrid and in vitro phosphorylation assays. AMPK-mediated phosphorylation of FH significantly increased enzyme activity in vitro and in vivo, suggesting that it is a bona fide AMPK substrate. In vivo, AMPKα2 is supposed to target the cytosolic/nuclear pools of FH, whose tumor suppressor function relies on DNA damage repair and inhibition of HIF-1α-signaling.

Tyrosine impairs enzymes of energy metabolism in cerebral cortex of rats

Tyrosine levels are abnormally elevated in tissues and physiological fluids of patients with inborn errors of tyrosine catabolism, especially in tyrosinemia type II, which is caused by deficiency of tyrosine aminotransferase and provokes eyes, skin, and central nervous system disturbances. Considering that the mechanisms of brain damage in these disorders are poorly known, in this study, we investigated the in vivo and in vitro effects of tyrosine on some parameters of energy metabolism in cerebral cortex of 14-day-old Wistar rats. We observed that 2 mM tyrosine inhibited in vitro the pyruvate kinase (PK) activity and that this inhibition was prevented by 1 mM reduced glutathione with 30, 60, and 90 min of preincubation. Moreover, administration of tyrosine methyl ester (TME) (0.5 mg/g of body weight) decreased the activity of PK and this reduction was prevented by pre-treatment with creatine (Cr). On the other hand, tyrosine did not alter adenylate kinase (AK) activity in vitro, but administration of TME enhanced AK activity not prevented by Cr pre-treatment. Finally, TME administration decreased the activity of CK from cytosolic and mitochondrial fractions and this diminution was prevented by Cr pre-treatment. The results suggest that tyrosine alters essential sulfhydryl groups necessary for CK and PK functions, possibly through oxidative stress. In case this also occurs in the patients, it is possible that energy metabolism alterations may contribute, along with other mechanisms, to the neurological dysfunction of hypertyrosinemias.

Creatine-kinase- and exercise-related muscle damage implications for muscle performance and recovery

The appearance of creatine kinase (CK) in blood has been generally considered to be an indirect marker of muscle damage, particularly for diagnosis of medical conditions such as myocardial infarction, muscular dystrophy, and cerebral diseases. However, there is controversy in the literature concerning its validity in reflecting muscle damage as a consequence of level and intensity of physical exercise. Nonmodifiable factors, for example, ethnicity, age, and gender, can also affect enzyme tissue activity and subsequent CK serum levels. The extent of effect suggests that acceptable upper limits of normal CK levels may need to be reset to recognise the impact of these factors. There is a need for standardisation of protocols and stronger guidelines which would facilitate greater scientific integrity. The purpose of this paper is to examine current evidence and opinion relating to the release of CK from skeletal muscle in response to physical activity and examine if elevated concentrations are a health concern.

A short review on creatine-creatine kinase system in relation to cancer and some experimental results on creatine as adjuvant in cancer therapy

The creatine/creatine kinase (CK) system plays a key role in cellular energy buffering and transport. In vertebrates, CK has four isoforms expressed in a tissue-specific manner. In the process of creatine biosynthesis several other important metabolites are formed. The anticancer effect of creatine had been reported in the past, and recent literature has reported low creatine content in several types of malignant cells. Furthermore, creatine can protect cardiac mitochondria from the deleterious effects of some anticancer compounds. Previous work from our laboratory showed progressive decrease of phosphocreatine, creatine and CK upon transformation of skeletal muscle into sarcoma. It was convincingly demonstrated that prominent expression of creatine-synthesizing enzymes L-arginine: glycine amidinotransferase and N-guanidinoacetate methyltransferase occurs in sarcoma, Ehrlich ascites carcinoma and sarcoma 180 cells; whereas, both these enzymes are virtually undetectable in skeletal muscle. Creatine transporter also remained unaltered in malignant cells. The anticancer effect of methylglyoxal had been known for a long time. The present work shows that this anticancer effect of methylglyoxal is significantly augmented in presence of creatine. On creatine supplementation the effect of methylglyoxal plus ascorbic acid was further augmented and there was no visible sign of tumor. Moreover, creatine and CK, which were very low in sarcoma tissue, were significantly elevated with the concomitant regression of tumor.

Autoactivation of transforming growth factor beta-activated kinase 1 is a sequential bimolecular process

Transforming growth factor-beta-activated kinase 1 (TAK1), an MAP3K, is a key player in processing a multitude of inflammatory stimuli. TAK1 autoactivation involves the interplay with TAK1-binding proteins (TAB), e.g. TAB1 and TAB2, and phosphorylation of several activation segment residues. However, the TAK1 autoactivation is not yet fully understood on the molecular level due to the static nature of available x-ray structural data and the complexity of cellular systems applied for investigation. Here, we established a bacterial expression system to generate recombinant mammalian TAK1 complexes. Co-expression of TAK1 and TAB1, but not TAB2, resulted in a functional and active TAK1-TAB1 complex capable of directly activating full-length heterotrimeric mammalian AMP-activated protein kinase (AMPK) in vitro. TAK1-dependent AMPK activation was mediated via hydrophobic residues of the AMPK kinase domain alphaG-helix as observed in vitro and in transfected cell culture. Co-immunoprecipitation of differently epitope-tagged TAK1 from transfected cells and mutation of hydrophobic alphaG-helix residues in TAK1 point to an intermolecular mechanism of TAB1-induced TAK1 autoactivation, as TAK1 autophosphorylation of the activation segment was impaired in these mutants. TAB1 phosphorylation was enhanced in a subset of these mutants, indicating a critical role of alphaG-helix residues in this process. Analyses of phosphorylation site mutants of the activation segment indicate that autophosphorylation of Ser-192 precedes TAB1 phosphorylation and is followed by sequential phosphorylation of Thr-178, Thr-187, and finally Thr-184. Finally, we present a model for the chronological order of events governing TAB1-induced TAK1 autoactivation.

Regulation of the creatine transporter by AMP-activated protein kinase in kidney epithelial cells

The metabolic sensor AMP-activated protein kinase (AMPK) regulates several transport proteins, potentially coupling transport activity to cellular stress and energy levels. The creatine transporter (CRT; SLC6A8) mediates creatine uptake into several cell types, including kidney epithelial cells, where it has been proposed that CRT is important for reclamation of filtered creatine, a process critical for total body creatine homeostasis. Creatine and phosphocreatine provide an intracellular, high-energy phosphate-buffering system essential for maintaining ATP supply in tissues with high energy demands. To test our hypothesis that CRT is regulated by AMPK in the kidney, we examined CRT and AMPK distribution in the kidney and the regulation of CRT by AMPK in cells. By immunofluorescence staining, we detected CRT at the apical pole in a polarized mouse S3 proximal tubule cell line and in native rat kidney proximal tubules, a distribution overlapping with AMPK. Two-electrode voltage-clamp (TEV) measurements of Na(+)-dependent creatine uptake into CRT-expressing Xenopus laevis oocytes demonstrated that AMPK inhibited CRT via a reduction in its Michaelis-Menten V(max) parameter. [(14)C]creatine uptake and apical surface biotinylation measurements in polarized S3 cells demonstrated parallel reductions in creatine influx and CRT apical membrane expression after AMPK activation with the AMP-mimetic compound 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside. In oocyte TEV experiments, rapamycin and the AMPK activator 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranosyl 5'-monophosphate (ZMP) inhibited CRT currents, but there was no additive inhibition of CRT by ZMP, suggesting that AMPK may inhibit CRT indirectly via the mammalian target of rapamycin pathway. We conclude that AMPK inhibits apical membrane CRT expression in kidney proximal tubule cells, which could be important in reducing cellular energy expenditure and unnecessary creatine reabsorption under conditions of local and whole body metabolic stress.

Homo-oligomerization and activation of AMP-activated protein kinase are mediated by the kinase domain alphaG-helix

AMP-activated protein kinase (AMPK) is a heterotrimeric complex playing a crucial role in maintaining cellular energy homeostasis. Recently, homodimerization of mammalian AMPK and yeast ortholog SNF1 was shown by us and others. In SNF1, it involved specific hydrophobic residues in the kinase domain alphaG-helix. Mutation of the corresponding AMPK alpha-subunit residues (Val-219 and Phe-223) to glutamate reduced the tendency of the kinase to form higher order homo-oligomers, as was determined by the following three independent techniques in vitro: (i) small angle x-ray scattering, (ii) surface plasmon resonance spectroscopy, and (iii) two-dimensional blue native/SDS-PAGE. Recombinant protein as well as AMPK in cell lysates of primary cells revealed distinct complexes of various sizes. In particular, the assembly of very high molecular mass complexes was dependent on both the alphaG-helix-mediated hydrophobic interactions and kinase activation. In vitro and when overexpressed in double knock-out (alpha1(-/-), alpha2(-/-)) mouse embryonic fibroblast cells, activation of mutant AMPK was impaired, indicating a critical role of the alphaG-helix residues for AMPK activation via its upstream kinases. Also inactivation by protein phosphatase 2Calpha was affected in mutant AMPK. Importantly, activation of mutant AMPK by LKB1 was restored by exchanging the corresponding and conserved hydrophobic alphaG-helix residues of LKB1 (Ile-260 and Phe-264) to positively charged amino acids. These mutations functionally rescued LKB1-dependent activation of mutant AMPK in vitro and in cell culture. Our data suggest a physiological role for the hydrophobic alphaG-helix residues in homo-oligomerization of heterotrimers and cellular interactions, in particular with upstream kinases, indicating an additional level of AMPK regulation.

Adenylate kinase and AMP signaling networks: metabolic monitoring, signal communication and body energy sensing

Adenylate kinase and downstream AMP signaling is an integrated metabolic monitoring system which reads the cellular energy state in order to tune and report signals to metabolic sensors. A network of adenylate kinase isoforms (AK1-AK7) are distributed throughout intracellular compartments, interstitial space and body fluids to regulate energetic and metabolic signaling circuits, securing efficient cell energy economy, signal communication and stress response. The dynamics of adenylate kinase-catalyzed phosphotransfer regulates multiple intracellular and extracellular energy-dependent and nucleotide signaling processes, including excitation-contraction coupling, hormone secretion, cell and ciliary motility, nuclear transport, energetics of cell cycle, DNA synthesis and repair, and developmental programming. Metabolomic analyses indicate that cellular, interstitial and blood AMP levels are potential metabolic signals associated with vital functions including body energy sensing, sleep, hibernation and food intake. Either low or excess AMP signaling has been linked to human disease such as diabetes, obesity and hypertrophic cardiomyopathy. Recent studies indicate that derangements in adenylate kinase-mediated energetic signaling due to mutations in AK1, AK2 or AK7 isoforms are associated with hemolytic anemia, reticular dysgenesis and ciliary dyskinesia. Moreover, hormonal, food and antidiabetic drug actions are frequently coupled to alterations of cellular AMP levels and associated signaling. Thus, by monitoring energy state and generating and distributing AMP metabolic signals adenylate kinase represents a unique hub within the cellular homeostatic network.

Structural properties of AMP-activated protein kinase: dimerization, molecular shape, and changes upon ligand binding

Heterotrimeric AMP-activated protein kinase (AMPK) is crucial for energy homeostasis of eukaryotic cells and organisms. Here we report on (i) bacterial expression of untagged mammalian AMPK isoform combinations, all containing gamma(1), (ii) an automated four-dimensional purification protocol, and (iii) biophysical characterization of AMPK heterotrimers by small angle x-ray scattering in solution (SAXS), transmission and scanning transmission electron microscopy (TEM, STEM), and mass spectrometry (MS). AMPK in solution at low concentrations (~1 mg/ml) largely consisted of individual heterotrimers in TEM analysis, revealed a precise 1:1:1 stoichiometry of the three subunits in MS, and behaved as an ideal solution in SAXS. At higher AMPK concentrations, SAXS revealed concentration-dependent, reversible dimerization of AMPK heterotrimers and formation of higher oligomers, also confirmed by STEM mass measurements. Single particle reconstruction and averaging by SAXS and TEM, respectively, revealed similar elongated, flat AMPK particles with protrusions and an indentation. In the lower AMPK concentration range, addition of AMP resulted in a significant decrease of the radius of gyration by approximately 5% in SAXS, which indicates a conformational switch in AMPK induced by ligand binding. We propose a structural model involving a ligand-induced relative movement of the kinase domain resulting in a more compact heterotrimer and a conformational change in the kinase domain that protects AMPK from dephosphorylation of Thr(172), thus positively affecting AMPK activity.

Co-expression of LKB1, MO25alpha and STRADalpha in bacteria yield the functional and active heterotrimeric complex

The tumour suppressor LKB1 plays a critical role in cell proliferation, polarity and energy metabolism. LKB1 is a Ser/Thr protein kinase that is associated with STRAD and MO25 in vivo. Here, we describe the individual expression of the three components of the LKB1 complex using monocistronic vectors and their co-expression using tricistronic vectors that were constructed from monocistronic vectors using a fully modular cloning approach. The data show that among the three individually expressed components of the LKB1 complex, only MO25alpha can be expressed in soluble form, whereas the other two, LKB1 and STRADalpha are found almost exclusively in inclusion bodies. However, using the tricistronic vector system, functional LKB1-MO25alpha-STRADalpha complex was expressed and purified from soluble extracts by sequential immobilized-metal affinity and heparin chromatography, as shown by Western blotting using specific antibodies. In size exclusion chromatography, MO25alpha and STRADalpha exactly co-elute with LKB1 with an apparent molecular weight of the heterotrimeric complex of 160 kDa. The specific activity in the peak fraction of the size exclusion chromatography was 250 U/mg at approximately 25% purity. As shown by autoradiography, LKB1 and STRADalpha, both strongly autophosphorylate in vitro. Moreover, recombinant LKB1 complex activates AMPK by phosphorylation of the alpha-subunit at the Thr-172 site as shown (i) by Western blotting using phospho-specific antibodies after LKB1-dependent phosphorylation, (ii) by LKB1-dependent incorporation of radioactive phosphate into the alpha-subunit of kinase dead AMPK heterotrimer, and (iii) by activity determination of AMPK. Functional mammalian LKB1 complex is constitutively active, and when enriched from bacteria should prove to be a valuable tool for studying its molecular function and regulation.

CK levels in unmedicated bipolar patients

Various reports have described increased serum creatine kinase (CK) activity in the majority of hospitalized acutely disturbed schizophrenics and patients with affective psychoses. We investigated CK serum levels of 52 unmedicated bipolar inpatients, in manic versus depressive states. Additional 17 patients were evaluated in both states. Hamilton Rating Scale for Depression and Young Mania Rating Scale were used and blood samples were obtained from new admitted patients. Higher CK level was found in the manic patients compared with the depressed ones. Likewise, the CK level was higher in the manic phase than in the depressive one, when tested within the same patient. Our results suggest that the clinical differences between mania and depression states are supported by contrasting levels of CK. The lack of correlations between CK level and motor items suggest that CK level in mania versus depression could emphasize the "thinking speed" and not the motor one.

Characterization of the human skeletal muscle proteome by one-dimensional gel electrophoresis and HPLC-ESI-MS/MS

Changes in protein abundance in skeletal muscle are central to a large number of metabolic and other disorders, including, and perhaps most commonly, insulin resistance. Proteomics analysis of human muscle is an important approach for gaining insight into the biochemical basis for normal and pathophysiological conditions. However, to date, the number of proteins identified by this approach has been limited, with 107 different proteins being the maximum reported so far. Using a combination of one-dimensional gel electrophoresis and high performance liquid chromatography electrospray ionization tandem mass spectrometry, we identified 954 different proteins in human vastus lateralis muscle obtained from three healthy, nonobese subjects. In addition to a large number of isoforms of contractile proteins, we detected all proteins involved in the major pathways of glucose and lipid metabolism in skeletal muscle. Mitochondrial proteins accounted for 22% of all proteins identified, including 55 subunits of the respiratory complexes I-V. Moreover, a number of enzymes involved in endocrine and metabolic signaling pathways as well as calcium homeostasis were identified. These results provide the most comprehensive characterization of the human skeletal muscle proteome to date. These data hold promise for future global assessment of quantitative changes in the muscle proteome of patients affected by disorders involving skeletal muscle.

Adaptation of energy metabolism in breast cancer brain metastases

Brain metastases are among the most feared complications in breast cancer, as no therapy exists that prevents or eliminates breast cancer spreading to the brain. New therapeutic strategies depend on specific knowledge of tumor cell properties that allow breast cancer cell growth within the brain tissue. To provide information in this direction, we established a human breast cancer cell model for brain metastasis based on circulating tumor cells from a breast cancer patient and variants of these cells derived from bone or brain lesions in immunodeficient mice. The brain-derived cells showed an increased potential for brain metastasis in vivo and exhibited a unique protein expression profile identified by large-scale proteomic analysis. This protein profile is consistent with either a selection of predisposed cells or bioenergetic adaptation of the tumor cells to the unique energy metabolism of the brain. Increased expression of enzymes involved in glycolysis, tricarboxylic acid cycle, and oxidative phosphorylation pathways suggests that the brain metastatic cells derive energy from glucose oxidation. The cells further showed enhanced activation of the pentose phosphate pathway and the glutathione system, which can minimize production of reactive oxygen species resulting from an enhanced oxidative metabolism. These changes promoted resistance of brain metastatic cells to drugs that affect the cellular redox balance. Importantly, the metabolic alterations are associated with strongly enhanced tumor cell survival and proliferation in the brain microenvironment. Thus, our data support the hypothesis that predisposition or adaptation of the tumor cell energy metabolism is a key element in breast cancer brain metastasis, and raise the possibility of targeting the functional differentiation in breast cancer brain lesions as a novel therapeutic strategy.

Effects of 3-phosphoglycerate and other metabolites on the activation of AMP-activated protein kinase by LKB1-STRAD-MO25

Skeletal muscle contraction results in the phosphorylation and activation of the AMP-activated protein kinase (AMPK) by an upstream kinase (AMPKK). The LKB1-STE-related adaptor (STRAD)-mouse protein 25 (MO25) complex is the major AMPKK in skeletal muscle; however, LKB1-STRAD-MO25 activity is not increased by muscle contraction. This relationship suggests that phosphorylation of AMPK by LKB1-STRAD-MO25 during skeletal muscle contraction may be regulated by allosteric mechanisms. In this study, we tested an array of metabolites including, glucose 6-phosphate, fructose 6-phosphate, fructose 1,6-bisphosphate, 3-phosphoglycerate (3-PG), glucose 1-phosphate, glucose 1,6-bisphosphate, ADP, carnitine, acetylcarnitine, IMP, inosine, and ammonia for allosteric regulation. ADP inhibited both AMPK and LKB1-STRAD-MO25 actions, but probably is not important physiologically because of the low free ADP inside the muscle fiber. We found that 3-PG stimulated LKB1-STRAD-MO25 activity and allowed for increased AMPK phosphorylation. 3-PG did not stimulate LKB1-STRAD-MO25 activity toward the peptide substrate LKB1tide. These results have identified 3-PG as an AMPK-specific regulator of AMPK phosphorylation and activation by LKB1-STRAD-MO25.

Dissecting the role of 5'-AMP for allosteric stimulation, activation, and deactivation of AMP-activated protein kinase

AMP-activated protein kinase (AMPK) is a heterotrimeric protein kinase that is crucial for cellular energy homeostasis of eukaryotic cells and organisms. Here we report on the activation of AMPK alpha1beta1gamma1 and alpha2beta2gamma1 by their upstream kinases (Ca(2+)/calmodulin-dependent protein kinase kinase-beta and LKB1-MO25alpha-STRADalpha), the deactivation by protein phosphatase 2Calpha, and on the extent of stimulation of AMPK by its allosteric activator AMP, using purified recombinant enzyme preparations. An accurate high pressure liquid chromatography-based method for AMPK activity measurements was established, which allowed for direct quantitation of the unphosphorylated and phosphorylated artificial peptide substrate, as well as the adenine nucleotides. Our results show a 1000-fold activation of AMPK by the combined effects of upstream kinase and saturating concentrations of AMP. The two AMPK isoforms exhibit similar specific activities (6 mumol/min/mg) and do not differ significantly by their responsiveness to AMP. Due to the inherent instability of ATP and ADP, it proved impossible to assay AMPK activity in the absolute absence of AMP. However, the half-maximal stimulatory effect of AMP is reached below 2 microm. AMP does not appear to augment phosphorylation by upstream kinases in the purified in vitro system, but deactivation by dephosphorylation of AMPK alpha-subunits at Thr-172 by protein phosphatase 2Calpha is attenuated by AMP. Furthermore, it is shown that neither purified NAD(+) nor NADH alters the activity of AMPK in a concentration range of 0-300 microm, respectively. Finally, evidence is provided that ZMP, a compound formed in 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside-treated cells to activate AMPK in vivo, allosterically activates purified AMPK in vitro, but compared with AMP, maximal activity is not reached. These data shed new light on physiologically important aspects of AMPK regulation.

New insights into doxorubicin-induced cardiotoxicity: the critical role of cellular energetics

Cardiotoxic side-effects represent a serious complication of anticancer therapy with anthracyclines, in particular with doxorubicin (DXR) being the leading drug of the group. Different hypotheses, accentuating various mechanisms and/or targets, have been proposed to explain DXR-induced cardiotoxicity. This review focuses on the myocardial energetic network as a target of DXR toxic action in heart and highlights the recent advances in understanding its role in development of the DXR related cardiac dysfunction. We present a survey of DXR-induced defects in different steps of cardiac energy metabolism, including reduction of oxidative capacity of mitochondria, changes in the profile of energy substrate utilization, disturbance of energy transfer between sites of energy production and consumption, as well as defects in energy signaling. Considering the wide spectrum and diversity of the changes reported, we attempt to integrate these facts into a common framework and to discuss important functional and temporal relationships between DXR-induced events and the possible underlying molecular mechanisms.

Mitoenergetic failure in Alzheimer disease

Brain cells are highly energy dependent for maintaining ion homeostasis during high metabolic activity. During active periods, full mitochondrial function is essential to generate ATP from electrons that originate with the oxidation of NADH. Decreasing brain metabolism is a significant cause of cognitive abnormalities of Alzheimer disease (AD), but it remains uncertain whether this is the cause of further pathology or whether synaptic loss results in a lower energy demand. Synapses are the first to show pathological symptoms in AD before the onset of clinical symptoms. Because synaptic function has high energy demands, interruption in mitochondrial energy supply could be the major factor in synaptic failure in AD. A newly discovered age-related decline in neuronal NADH and redox ratio may jeopardize this function. Mitochondrial dehydrogenases and several mutations affecting energy transfer are frequently altered in aging and AD. Thus, with the accumulation of genetic defects in mitochondria at the level of energy transfer, the issue of neuronal susceptibility to damage as a function of age and age-related disease becomes important. In an aging rat neuron model, mitochondria are both chronically depolarized and produce more reactive oxygen species with age. These concepts suggest that multiple treatment targets may be needed to reverse this multifactorial disease. This review summarizes new insights based on the interaction of mitoenergetic failure, glutamate excitotoxicity, and amyloid toxicity in the exacerbation of AD.

Alterations in myocardial energy metabolism induced by the anti-cancer drug doxorubicin

Doxorubicin and other anthracyclines are among the most potent chemotherapeutic drugs for the treatment of acute leukaemia, lymphomas and different types of solid tumours such as breast, liver and lung cancers. Their clinical use is, however, limited by the risk of severe cardiotoxicity, which can lead to irreversible congestive heart failure. There is increasing evidence that essential components of myocardial energy metabolism are among the highly sensitive and early targets of doxorubicin-induced damage. Here we review doxorubicin-induced detrimental changes in cardiac energetics, with an emphasis on the emerging importance of defects in energy-transferring and -signalling systems, like creatine kinase and AMP-activated protein kinase.

Decrease in creatine kinase messenger RNA expression in the hippocampus and dorsolateral prefrontal cortex in bipolar disorder

OBJECTIVES

Bipolar disorder (BPD) affects more than 2 million adults in the USA and ranks among the top 10 causes of worldwide disabilities. Despite its prevalence, very little is known about the etiology of BPD. Recent evidence suggests that cellular energy metabolism is disturbed in BPD. Mitochondrial function is altered, and levels of high-energy phosphates, such as phosphocreatine (PCr), are reduced in the brain. This evidence has led to the hypothesis that deficiencies in energy metabolism could account for some of the pathophysiology observed in BPD. To further explore this hypothesis, we examined levels of creatine kinase (CK) mRNA, the enzyme involved in synthesis and metabolism of PCr, in the hippocampus (HIP) and dorsolateral prefrontal cortex (DLPFC) of control, BPD and schizophrenia subjects.

METHODS

Tissue was obtained from the Harvard Brain Tissue Resource Center. Real-time quantitative polymerase chain reaction (HIP, DLPFC) and gene expression microarrays (HIP) were employed to compare the brain and mitochondrial 1 isoforms of CK.

RESULTS

Both CK isoforms were downregulated in BPD. Furthermore, mRNA transcripts for oligodendrocyte-specific proteins were downregulated in the DLPFC, whereas the mRNA for the neuron-specific protein microtubule-associated protein 2 was downregulated in the HIP.

CONCLUSION

Although some of the downregulation of CK might be explained by cell loss, a more general mechanism seems to be responsible. The downregulation of CK transcripts, if translated into protein levels, could explain the reduction of high-energy phosphates previously observed in BPD.

Plectin scaffolds recruit energy-controlling AMP-activated protein kinase (AMPK) in differentiated myofibres

Plectin, a cytolinker protein greater than 500 kDa in size, has an important role as a mechanical stabiliser of cells. It interlinks the various cytoskeletal filament systems and anchors intermediate filaments to peripheral junctional complexes. In addition, there is increasing evidence that plectin acts as a scaffolding platform that controls the spatial and temporal localisation and interaction of signaling proteins. In this study we show that, in differentiated mouse myotubes, plectin binds to the regulatory gamma1 subunit of AMP-activated protein kinase (AMPK), the key regulatory enzyme of energy homeostasis. No interaction was observed in undifferentiated myoblasts, and plectin-deficient myotubes showed altered positioning of gamma1-AMPK. In addition we found that plectin affects the subunit composition of AMPK, because isoform alpha1 of the catalytic subunit decreased in proportion to isoform alpha2 during in vitro differentiation of plectin(-/-) myotubes. In plectin-deficient myocytes we could also detect a higher level of activated (Thr172-phosphorylated) AMPK, compared with wild-type cells. Our data suggest a differentiation-dependent association of plectin with AMPK, where plectin selectively stabilises alpha1-gamma1 AMPK complexes by binding to the gamma1 regulatory subunit. The distinct plectin expression patterns in different fibre types combined with its involvement in the regulation of isoform compositions of AMPK complexes could provide a mechanism whereby cytoarchitecture influences energy homeostasis.

Evidence against regulation of AMP-activated protein kinase and LKB1/STRAD/MO25 activity by creatine phosphate

Muscle contraction results in phosphorylation and activation of the AMP-activated protein kinase (AMPK) by an AMPK kinase (AMPKK). LKB1/STRAD/MO25 (LKB1) is the major AMPKK in skeletal muscle; however, the activity of LKB1 is not increased by muscle contraction. This finding suggests that phosphorylation of AMPK by LKB1 is regulated by allosteric mechanisms. Creatine phosphate is depleted during skeletal muscle contraction to replenish ATP. Thus the concentration of creatine phosphate is an indicator of cellular energy status. A previous report found that creatine phosphate inhibits AMPK activity. The purpose of this study was to determine whether creatine phosphate would inhibit 1) phosphorylation of AMPK by LKB1 and 2) AMPK activity after phosphorylation by LKB1. We found that creatine phosphate did not inhibit phosphorylation of either recombinant or purified rat liver AMPK by LKB1. We also found that creatine phosphate did not inhibit 1) active recombinant alpha1beta1gamma1 or alpha2beta2gamma2 AMPK, 2) AMPK immunoprecipitated from rat liver extracts by either the alpha1 or alpha2 subunit, or 3) AMPK chromatographically purified from rat liver. Inhibition of skeletal muscle AMPK by creatine phosphate was greatly reduced or eliminated with increased AMPK purity. In conclusion, these results suggest that creatine phosphate is not a direct regulator of LKB1 or AMPK activity. Creatine phosphate may indirectly modulate AMPK activity by replenishing ATP at the onset of muscle contraction.

Cardiac system bioenergetics: metabolic basis of the Frank-Starling law

The fundamental principle of cardiac behaviour is described by the Frank-Starling law relating force of contraction during systole with end-diastolic volume. While both work and respiration rates increase linearly with imposed load, the basis of mechano-energetic coupling in heart muscle has remained a long-standing enigma. Here, we highlight advances made in understanding of complex cellular and molecular mechanisms that orchestrate coupling of mitochondrial oxidative phosphorylation with ATP utilization for muscle contraction. Cardiac system bioenergetics critically depends on an interrelated metabolic infrastructure regulating mitochondrial respiration and energy fluxes throughout cellular compartments. The data reviewed indicate the significance of two interrelated systems regulating mitochondrial respiration and energy fluxes in cells: (1) the creatine kinase, adenylate kinase and glycolytic pathways that communicate flux changes generated by cellular ATPases within structurally organized enzymatic modules and networks; and (2) a secondary system based on mitochondrial participation in cellular calcium cycle, which adjusts substrate oxidation and energy-transducing processes to meet increasing cellular energy demands. By conveying energetic signals to metabolic sensors, coupled phosphotransfer reactions provide a high-fidelity regulation of the excitation-contraction cycle. Such integration of energetics with calcium signalling systems provides the basis for 'metabolic pacing', synchronizing the cellular electrical and mechanical activities with energy supply processes.

Long-chain acyl-CoA esters inhibit phosphorylation of AMP-activated protein kinase at threonine-172 by LKB1/STRAD/MO25

Activation of the AMP-activated protein kinase (AMPK) results in acute changes in cellular metabolism and transcriptional events that make the cell more robust when encountering an energy challenge. AMPK is thought to be inhibited by glycogen, the major storage form of intracellular carbohydrate. We hypothesized that long-chain acyl-CoA esters (LCACEs) might also inhibit AMPK signaling. Cytosolic LCACEs are available for immediate transport and oxidation within the mitochondria and accordingly may be representative of the lipid energy charge of the cell. We found that LCACEs inhibited phosphorylation of AMPK by the recombinant AMPK kinase (AMPKK) LKB1/STRAD/MO25 in a concentration-dependent manner. Palmitoyl-CoA (PCoA) did not affect the activity of phosphothreonine-172 AMPK. PCoA potently inhibited AMPKK purified from liver. Conversely, PCoA stimulated the kinase activity of LKB1/STRAD/MO25 toward the peptide substrate LKB1tide. Octanoyl-CoA, palmitate, and palmitoylcarnitine did not inhibit AMPKK activity. Removal of AMP from the reaction mixture resulted in reduced AMPKK activity in the presence of PCoA. In conclusion, these results demonstrate that the AMPKK activity of LKB1/STRAD/MO25 is substrate specific and distinct from the kinase activity of LKB1/STRAD/MO25 toward the peptide substrate LKB1tide. They also demonstrate that LCACEs inhibit the AMPKK activity of LKB1/STRAD/MO25 in a specific manner with a dependence on both a long fatty chain and a CoA moiety. These results suggest that the AMPK signaling cascade may directly sense and respond to the lipid energy charge of the cell.

Epithelial Sodium Channel Inhibition by AMP-activated Protein Kinase in Oocytes and Polarized Renal Epithelial Cells

The epithelial Na(+) channel (ENaC) regulates epithelial salt and water reabsorption, processes that require significant expenditure of cellular energy. To test whether the ubiquitous metabolic sensor AMP-activated kinase (AMPK) regulates ENaC, we examined the effects of AMPK activation on amiloride-sensitive currents in Xenopus oocytes and polarized mouse collecting duct mpkCCD(c14) cells. Microinjection of oocytes expressing mouse ENaC (mENaC) with either active AMPK protein or an AMPK activator inhibited mENaC currents relative to controls as measured by two-electrode voltage-clamp studies. Similarly, pharmacological AMPK activation or overexpression of an activating AMPK mutant in mpkCCD(c14) cells inhibited amiloride-sensitive short circuit currents. Expression of a degenerin mutant beta-mENaC subunit (S518K) along with wild type alpha and gamma increased the channel open probability (P(o)) to approximately 1. However, AMPK activation inhibited currents similarly with expression of either degenerin mutant or wild type mENaC. Single channel recordings under these conditions demonstrated that neither P(o) nor channel conductance was affected by AMPK activation. Moreover, expression of a Liddle's syndrome-type beta-mENaC mutant (Y618A) greatly enhanced ENaC whole cell currents relative to wild type ENaC controls and prevented AMPK-dependent inhibition. These findings indicate that AMPK-dependent ENaC inhibition is mediated through a decrease in the number of active channels at the plasma membrane (N), presumably through enhanced Nedd4-2-dependent ENaC endocytosis. The AMPK-ENaC interaction appears to be indirect; AMPK did not bind ENaC in cells, as assessed by in vivo pull-down assays, nor did it phosphorylate ENaC in vitro. In summary, these results suggest a novel mechanism for coupling ENaC activity and renal Na(+) handling to cellular metabolic status through AMPK, which may help prevent cellular Na(+) loading under hypoxic or ischemic conditions.