5'-AMP-activated protein kinase increases glucose uptake independent of GLUT4 translocation in cardiac myocytes

Cardiac Metabolism, Reprogramming, and Diseases

Cardiovascular diseases (CVD) account for the largest bulk of deaths worldwide, posing a massive burden on societies and the global healthcare system. Besides, the incidence and prevalence of these diseases are on the rise, demanding imminent action to revert this trend. Cardiovascular pathogenesis harbors a variety of molecular and cellular mechanisms among which dysregulated metabolism is of significant importance and may even proceed other mechanisms. The healthy heart metabolism primarily relies on fatty acids for the ultimate production of energy through oxidative phosphorylation in mitochondria. Other metabolites such as glucose, amino acids, and ketone bodies come next. Under pathological conditions, there is a shift in metabolic pathways and the preference of metabolites, termed metabolic remodeling or reprogramming. In this review, we aim to summarize cardiovascular metabolism and remodeling in different subsets of CVD to come up with a new paradigm for understanding and treatment of these diseases.

Overexpression of Rhodopsin or Its Mutants Leads to Energy Metabolism Dysfunction in 661w Cells

Purpose

Retinitis pigmentosa (RP) is a heterogeneous group of inherited disorders characterized by photoreceptor degeneration. The rhodopsin gene (RHO) is the most frequent cause of autosomal dominant RP (ADRP), yet it remains unclear how RHO mutations cause heterogeneous phenotypes. Energy failure is a main cause of the secondary cone death during RP progression; however, its role in primary rod death induced by ADRP RHO mutants is unknown.

Methods

Three RHO missense mutations were chosen from different clinical classes. Wild-type (WT) RHO and its mutants, P23H (class B1), R135L (class A), and G188R (class B2), were overexpressed in 661w cells, a mouse photoreceptor cell line, and their effects on oxidative phosphorylation (OXPHOS) and aerobic glycolysis were compared separately.

Results

Here, we report that energy failure is an early event in the cell death caused by overexpression of WT RHO and its mutants. RHO overexpression leads to OXPHOS deficiency, which might be a result of mitochondrial loss. Nonetheless, only in WT RHO and P23H groups, energy stress triggers AMP-activated protein kinase activation and metabolic reprogramming to increase glycolysis. Metabolic reprogramming impairment in R135L and G188R groups might be the reason why energy failure and cell injury are much more severe in those groups.

Conclusions

Our results imply that overexpression of RHO missense mutants have distinct impacts on the two energy metabolic pathways, which might be related to their heterogeneous phenotypes.

Adipose Lipolysis Regulates Cardiac Glucose Uptake and Function in Mice under Cold Stress

The heart primarily uses fatty acids as energy substrates. Adipose lipolysis is a major source of fatty acids, particularly under stress conditions. In this study, we showed that mice with selective inactivation of the lipolytic coactivator comparative gene identification-58 (CGI-58) in adipose tissue (FAT-KO mice), relative to their littermate controls, had lower circulating FA levels in the fed and fasted states due to impaired adipose lipolysis. They preferentially utilized carbohydrates as energy fuels and were more insulin sensitive and glucose tolerant. Under cold stress, FAT-KO versus control mice had >10-fold increases in glucose uptake in the hearts but no increases in other tissues examined. Plasma concentrations of atrial natriuretic peptide and cardiac mRNAs for atrial and brain-type natriuretic peptides, two sensitive markers of cardiac remodeling, were also elevated. After one week of cold exposure, FAT-KO mice showed reduced cardiac expression of several mitochondrial oxidative phosphorylation proteins. After one month of cold exposure, hearts of these animals showed depressed functions, reduced SERCA2 protein, and increased proteins for MHC-β, collagen I proteins, Glut1, Glut4 and phospho-AMPK. Thus, CGI-58-dependent adipose lipolysis critically regulates cardiac metabolism and function, especially during cold adaptation. The adipose-heart axis may be targeted for the management of cardiac dysfunction.

Insulin directly stimulates mitochondrial glucose oxidation in the heart

Background

Glucose oxidation is a major contributor to myocardial energy production and its contribution is orchestrated by insulin. While insulin can increase glucose oxidation indirectly by enhancing glucose uptake and glycolysis, it also directly stimulates mitochondrial glucose oxidation, independent of increasing glucose uptake or glycolysis, through activating mitochondrial pyruvate dehydrogenase (PDH), the rate-limiting enzyme of glucose oxidation. However, how insulin directly stimulates PDH is not known. To determine this, we characterized the impacts of modifying mitochondrial insulin signaling kinases, namely protein kinase B (Akt), protein kinase C-delta (PKC-δ) and glycogen synthase kinase-3 beta (GSK-3β), on the direct insulin stimulation of glucose oxidation.

Methods

We employed an isolated working mouse heart model to measure the effect of insulin on cardiac glycolysis, glucose oxidation and fatty acid oxidation and how that could be affected when mitochondrial Akt, PKC-δ or GSK-3β is disturbed using pharmacological modulators. We also used differential centrifugation to isolate mitochondrial and cytosol fraction to examine the activity of Akt, PKC-δ and GSK-3β between these fractions. Data were analyzed using unpaired t-test and two-way ANOVA.

Results

Here we show that insulin-stimulated phosphorylation of mitochondrial Akt is a prerequisite for transducing insulin’s direct stimulation of glucose oxidation. Inhibition of mitochondrial Akt completely abolishes insulin-stimulated glucose oxidation, independent of glucose uptake or glycolysis. We also show a novel role of mitochondrial PKC-δ in modulating mitochondrial glucose oxidation. Inhibition of mitochondrial PKC-δ mimics insulin stimulation of glucose oxidation and mitochondrial Akt. We also demonstrate that inhibition of mitochondrial GSK3β phosphorylation does not influence insulin-stimulated glucose oxidation.

Conclusion

We identify, for the first time, insulin-stimulated mitochondrial Akt as a prerequisite transmitter of the insulin signal that directly stimulates cardiac glucose oxidation. These novel findings suggest that targeting mitochondrial Akt is a potential therapeutic approach to enhance cardiac insulin sensitivity in condition such as heart failure, diabetes and obesity.

Structural and functional regulation of lactate dehydrogenase-A in cancer

Dysregulated metabolism is one of the hallmarks of cancer. Under normal physiological conditions, ATP is primarily generated by oxidative phosphorylation. Cancers commonly undergo a dramatic shift toward glycolysis, despite the presence of oxygen. This phenomenon is known as the Warburg effect, and requires the activity of LDHA. LDHA converts pyruvate to lactate in the final step of glycolysis and is often upregulated in cancer. LDHA inhibitors present a promising therapeutic option, as LDHA blockade leads to apoptosis in cancer cells. Despite this, existing LDHA inhibitors have shown limited clinical efficacy. Here, we review recent progress in LDHA structure, function and regulation as well as strategies to target this critical enzyme.

Effect of Rubus anatolicus Leaf Extract on Glucose Metabolism in HepG2, CRI-D2 and C2C12 Cell Lines

INTRODUCTION

The aim of this study was to assess the effects of Rubus anatolicus on glucose metabolism in HepG2, CRI-D2 and C2C12 cell lines.

MATERIALS AND METHODS

R. anatolicus was collected in Golestan province, Iran. Three different cell lines HepG2 (human liver cell), CRI-D2 (mice pancreatic cell) and C2C12 (rat myoblast) were used for cell culture experiments. Cell viability was measured using MTT assay. Cells were treated with various concentrations of the extract (6.25-400 μg/mL) and then the extracellular glucose level and intracellular glycogen content were measured using colorimetric methods. The insulin level of the culture medium was measured using the ELISA method.

RESULTS

Our findings showed that R. anatolicus extract enhances glucose uptake and consumption by all three cell lines. The R. anatolicus extract exposure also elevated cellular glycogen content in HepG2 and C2C12 cells (for 200 and 100 μg/mL) significantly. We found a significant increase in glucose uptake and consequently higher stimulation of insulin secretion in CRI-D2 cell pancreatic cells treated with R. anatolicus extract.

CONCLUSION

The R. anatolicus appears to activate glucose uptake and cellular glycogen synthesis probably by activating the glycogenesis or inhibition of glycogenolysis pathways. The extract enhances insulin secretion in the pancreatic cells by increased glucose uptake.

Dandelion Chloroform Extract Promotes Glucose Uptake via the AMPK/GLUT4 Pathway in L6 Cells

The number of patients with type 2 diabetes mellitus (T2DM) is increasing rapidly worldwide. Glucose transporter 4 (GLUT4) is one of the main proteins that transport blood glucose into the cells and is a target in the treatment of T2DM. In this study, we investigated the mechanism of action of dandelion chloroform extract (DCE) on glucose uptake in L6 cells. The glucose consumption of L6 cell culture supernatant was measured by a glucose uptake assay kit, and the dynamic changes of intracellular GLUT4 and calcium (Ca²⁺) levels were monitored by laser scanning confocal microscopy in L6 cell lines stably expressing IRAP-mOrange. The GLUT4 fusion with the plasma membrane (PM) was traced via myc-GLUT4-mOrange. GLUT4 expression and AMP-activated protein kinase (AMPK), protein kinase B (PKB/Akt), protein kinase C (PKC), and phosphorylation levels were determined by performing western blotting. GLUT4 mRNA expression was detected by real-time PCR. DCE up-regulated GLUT4 expression, promoted GLUT4 translocation and fusion to the membrane eventually leading to glucose uptake, and induced AMPK phosphorylation in L6 cells. The AMPK inhibitory compound C significantly inhibited DCE-induced GLUT4 expression and translocation while no inhibitory effect was observed by the phosphatidylinositol 3-kinase (PI3K) inhibitor Wortmannin and PKC inhibitor Gö6983. These data suggested that DCE promoted GLUT4 expression and transport to the membrane through the AMPK signaling pathway, thereby stimulating GLUT4 fusion with PM to enhance glucose uptake in L6 cells. DCE-induced GLUT4 translocation was also found to be Ca²⁺-independent. Together, these findings indicate that DCE could be a new hypoglycemic agent for the treatment of T2DM.

Relaxin activates AMPK-AKT signaling and increases glucose uptake by cultured cardiomyocytes

PURPOSE

Many evidences show that the hormone relaxin plays a pivotal role in the physiology and pathology of the cardiovascular system. This pleiotropic hormone exerts regulatory functions through specific receptors in cardiovascular tissues: in experimental animal models it was shown to induce coronary vasodilation, prevent cardiac damage induced by ischemia/reperfusion and revert cardiac hypertrophy and fibrosis. A tight relationship between this hormone and important metabolic pathways has been suggested, but it is at present unknown if relaxin could regulate cardiac metabolism. Our aim was to study the possible effects of relaxin on cardiomyocyte metabolism.

METHODS

Neonatal rat cardiomyocytes were treated with relaxin and (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assays (MTT) were performed to assess metabolic activity; while 2-deoxy-D-[³H] glucose and BODIPY-labelled fatty acid incorporations were analyzed to measure glucose and fatty acid uptakes, and western blot was utilized to study the intracellular signaling pathways activated by the hormone.

RESULTS

We observed that relaxin at 10 ng/ml was able to increase the level of metabolic activity of cultured neonatal rat cardiomyocytes; the rate of 2-deoxy-D-[³H]glucose incorporation demonstrated that relaxin also induced an increase in glucose uptake. First evidence is also offered that relaxin can activate the master energy sensor and regulator AMPK in cardiomyocytes. Moreover, the treatment of cardiomyocytes with relaxin also induced dose-dependent increases in ERK1/2, AKT, and AS160 phosphorylation. That raise in AS160 phosphorylation induced by relaxin was prevented by the pretreatment with AMPK and AKT pathways inhibitors, indicating that both molecules play important roles in the relaxin effects reported.

CONCLUSION

Relaxin can regulate cardiomyocyte metabolism and activate AMPK, the central sensor of energy status that maintains cellular energy homeostasis, and also ERK and AKT, two molecular sensing nodes that coordinate dynamic responses of the cell's metabolic responses.

Antidiabetic Activity of Ergosterol from Pleurotus Ostreatus in KK-Ay Mice with Spontaneous Type 2 Diabetes Mellitus

SCOPE

The number of people with diabetes is increasing rapidly in the world. In the present study, the hypoglycemic activity and potential mechanism of ergosterol (ERG), a phytosterol derived from the edible mushroom Pleurotus ostreatus are investigated in vitro and in vivo.

METHODS AND RESULTS

ERG is isolated from Pleurotus ostreatus and identified by NMR spectra. The effects of ERG on the glucose uptake, glucose transporter 4 (GLUT4) translocation, GLUT4 expression, and the phosphorylation of AMPK, Akt and PKC in L6 cells are evaluated. ERG enhances glucose uptake and displays a GLUT4 translocation activity with up-regulating GLUT4 expression and phosphorylation of Akt and PKC in L6 cells. In vivo, antidiabetic activity of ERG is examined. The phosphorylation of Akt and PKC in different tissues from KK-A^y mice is assessed. ERG significantly improves insulin resistance and blood lipid indices while reducing fasting blood glucose levels and protecting pancreas and liver in the mice. Moreover, the phosphorylation of Akt and PKC is increased in different tissues.

CONCLUSION

The results suggest that ERG may be a potential hypoglycemic agent for the treatment of T2DM with the probable mechanism of stimulating GLUT4 translocation and expression modulated by the PI3K/Akt pathway and PKC pathway.

Molecular mechanisms of cardiac pathology in diabetes - Experimental insights

Diabetic cardiomyopathy is a distinct pathology independent of co-morbidities such as coronary artery disease and hypertension. Diminished glucose uptake due to impaired insulin signaling and decreased expression of glucose transporters is associated with a shift towards increased reliance on fatty acid oxidation and reduced cardiac efficiency in diabetic hearts. The cardiac metabolic profile in diabetes is influenced by disturbances in circulating glucose, insulin and fatty acids, and alterations in cardiomyocyte signaling. In this review, we focus on recent preclinical advances in understanding the molecular mechanisms of diabetic cardiomyopathy. Genetic manipulation of cardiomyocyte insulin signaling intermediates has demonstrated that partial cardiac functional rescue can be achieved by upregulation of the insulin signaling pathway in diabetic hearts. Inconsistent findings have been reported relating to the role of cardiac AMPK and β-adrenergic signaling in diabetes, and systemic administration of agents targeting these pathways appear to elicit some cardiac benefit, but whether these effects are related to direct cardiac actions is uncertain. Overload of cardiomyocyte fuel storage is evident in the diabetic heart, with accumulation of glycogen and lipid droplets. Cardiac metabolic dysregulation in diabetes has been linked with oxidative stress and autophagy disturbance, which may lead to cell death induction, fibrotic 'backfill' and cardiac dysfunction. This review examines the weight of evidence relating to the molecular mechanisms of diabetic cardiomyopathy, with a particular focus on metabolic and signaling pathways. Areas of uncertainty in the field are highlighted and important knowledge gaps for further investigation are identified. This article is part of a Special issue entitled Cardiac adaptations to obesity, diabetes and insulin resistance, edited by Professors Jan F.C. Glatz, Jason R.B. Dyck and Christine Des Rosiers.

Hypoglycemic and hypolipidemic effects of a polysaccharide from Lachnum YM240 and its derivatives in mice, induced by a high fat diet and low dose STZ

Carboxymethylated and sulfated polysaccharides (CLEP and SLEP) were prepared from an exopolysaccharide previously obtained from Lachnum YM240 (LEP) by chemical modifications. Two doses (50 mg kg-1 and 200 mg kg-1 b. w.) of LEP, CLEP and SLEP were orally administered to normal mice and type 2 diabetic mice (T2DM) that were induced by streptozotocin (STZ) and a high fat diet, respectively. The hypoglycemic effect was evaluated by testing the oral glucose tolerance, fasting blood glucose (FBG) levels, fasting serum insulin (FINS), glycosylated hemoglobin A1c (HbA1c), and the hypolipidemic effect was evaluated by the body, spleen, pancreas, liver and kidney weights, as well as serum triglycerides (TG), cholesterol (TC) and free fatty acids (FFA). After four weeks of administration, LEP, CLEP and SLEP showed a marked FBG fall rate of 11.2%, 44.0% and 42.5% for the high-dose and 7.43%, 38.5% and 33.1% for the low-dose, respectively, as compared to the DC group. Moreover, compared with DC mice, TC concentrations in the high-dose groups of LEP, CLEP and SLEP were significantly decreased by 29.6%, 38.7% (P < 0.05), 33.0% (P < 0.05), and TG concentrations decreased by 18.9%, 43.9% (P < 0.01), 29.0% (P < 0.05), respectively. In addition, LEP and the derivatives significantly upregulated the expression of glucokinase (GK) and adenosine monophosphate-activated protein kinase (AMPK) in the liver, AMPK and glucose transporter 4 (Glut4) in skeletal muscle and peroxysome proliferator-activated receptor (PPAR-γ) in adipose tissue, whereas downregulated the expression of glucose-6-phosphatase (G6P) in the liver; these were examined using ELISA detection kits. These results for FBG and serum lipids indicate that LEP and its derivatives possess significant hypoglycemic and hypolipidemic effects and carboxymethylation improved the hypoglycemic and hypolipidemic effects more effectively than sulfation. Therefore, the carboxymethylated and sulfated modifications were effective ways to enhance the hypoglycemic and hypolipidemic activities of polysaccharides.

Lysophosphatidic Acid Pretreatment Attenuates Myocardial Ischemia/Reperfusion Injury in the Immature Hearts of Rats

The cardioprotection of the immature heart during cardiac surgery remains controversial due to the differences between the adult heart and the newborn heart. Lysophosphatidic acid (LPA) is a small bioactive molecule with diverse functions including cell proliferation and survival via its receptor: LPA₁–LPA₆. We previously reported that the expressions of LPA₁ and LPA₃ in rat hearts were much higher in immature hearts and then declined rapidly with age. In this study, we aimed to investigate whether LPA signaling plays a potential protective role in immature hearts which had experienced ischemia/reperfusion (I/R) injury. The results showed that in Langendorff-perfused immature rat hearts (2 weeks), compared to I/R group, LPA pretreatment significantly enhanced the cardiac function, attenuated myocardial infarct size and CK-MB release, decreased myocardial apoptosis and increased the expression of pro-survival signaling molecules. All these effects could be abolished by Ki16425, an antagonist to LPA₁ and LPA₃. Similarly, LPA pretreatment protected H9C2 from hypoxia-reoxygenation (H/R) induced apoptosis and necrosis in vitro. The mechanisms underlying the anti-apoptosis effects were related to activation of the phosphatidylinositol 3-kinase (PI3K)/protein kinas B (AKT) signaling pathways as well as phosphorylation of the downstream effector of AKT, glycogen synthase kinase 3 beta (GSK3β), through LPA₁ and/or LPA₃. What's more, we found that LPA preconditioning increased glucose uptake of H9C2 subjected to H/R by the activation of AMP-Activated Protein Kinase (AMPK) but not the translocation of GLUT4. In conclusion, our study indicates that LPA is a potent survival factor for immature hearts against I/R injuries and has the potential therapeutic function as a cardioplegia additive for infantile cardiac surgery.

Antidiabetic activity of perylenequinonoid-rich extract from Shiraia bambusicola in KK-Ay mice with spontaneous type 2 diabetes mellitus

ETHNOPHARMACOLOGICAL RELEVANCE

Bitter and cold traditional Chinese medicines (TCMs) have been long used to treat diabetes mellitus (DM) based on unique medical theory system since ancient China. As one of bitter and cold TCMs, the stromatas of Shiraia bambusicola have been used for the treatment of DM and exerted clinical effects to a certain extent. However, the corresponding active principles and antidiabetic mechanism of the TCM still remain unknown. Therefore, the aim of the present investigation was to evaluate the potential antidiabetic effect of the active Shiraia bambusicola EtOAc extract (SB-EtOAc) in vitro and in vivo, and elucidate its probable antidiabetic mechanism.

MATERIALS AND METHODS

A LC-PDA-ESIMS protocol was developed to determine the chemical principles of the active EtOAc extract rapidly and unambiguously. The effect of SB-EtOAc on the glucose transporter type 4 (GLUT4) translocation and glucose uptake in L6 cells was examined. SB-EtOAc was orally administration at the dose of 30, 60 and 120mg/kg/d in KK-Ay mice, for 21 days. Body weight, plasma glucose, oral glucose tolerance test, fasted blood glucose levels, oral glucose tolerance test and insulin tolerance test, serum insulin and blood-lipid indexes were measured. GLUT4 on L6 cells membrane and phosphorylation of the AMP-activated protein kinase (p-AMPK) expression in L6 cells were measured. The GLUT4 and p-AMPK expression in KK-Ay mice skeletal muscle were measured. Phosphorylation of the acetyl-CoA carboxylase (p-ACC) and p-AMPK were measured.

RESULTS

In vitro, SB-EtOAc exhibited a strong effect of stimulation on GLUT4 translocation by 3.2 fold in L6 cells compared with basal group, however, the selective AMPK inhibitor compound C can completely inhibit the AMPK pathway and prevent the GLUT4 translocation caused by SB-EtOAc. The further western blotting experiments showed that SB-EtOAc can stimulate AMPK phosphorylation in L6 cells and improve the expression of GLUT4. In vivo, SB-EtOAc can improve the KK-Ay mice insulin resistant and oral glucose tolerance to a certain extent. And the body weight, blood glucose levels and the serum TC, TG, FFA, AST, ALT and LDL-C were significantly reduced and HDL-C were increased after 3 weeks treatment. Mechanistically, phosphorylation of the AMPK and ACC had been improved obviously and the levels of AMPK phosphorylation and GLUT4 had been also enhanced.

CONCLUSION

In vitro, SB-EtOAc exhibited a strong effect of stimulation on GLUT4 translocation and improved significantly the glucose uptake. In vivo, SB-EtOAc significantly improved oral glucose tolerance and the insulin resistant as well as glucolipid metabolism. In this study, SB-EtOAc displayed promising positive antidiabetic activity in vitro and in vivo, partly by modulating AMPK-GLUT4 and AMPK-ACC signaling pathways.

The Effect of a High-Protein Diet and Exercise on Cardiac AQP7 and GLUT4 Gene Expression

High-protein (HP) diets are commonly consumed by athletes despite their potential health hazard, which is postulated to enforce a negative effect on bone and renal health. However, its effects on heart have not been known yet. Aquaporin-7 (AQP7) is an aquaglyceroporin that facilitates glycerol and water transport. Glycerol is an important cardiac energy production substrate, especially during exercise, in conjunction with fatty acids and glucose. Glucose transporter 4 (GLUT4) is an insulin-sensitive glucose transporter in heart. We aimed to investigate the effect of HPD on AQP7 and GLUT4 levels in the rat heart subjected to exercise. Male Sprague-Dawley rats were divided into control (n = 12), exercise (E) training (n = 10), HPD (n = 12), and HPD-E training (n = 9) groups. The HPD groups were fed a 45 % protein-containing diet 5 weeks. The HPD-E and E groups were performed the treadmill exercise during the 5-week study period. Real-time polymerase chain reaction and immunohistochemistry techniques were used to determine the gene expression and localization of AQP7 and GLUT4 in heart tissue. Results of relative gene expression were calculated by the 'Pfaffl' mathematical method using the REST program. Differences in AQP7 and GLUT4 gene expression were expressed as fold change compared to the control group. Heart weight/tibia ratio and ventricular wall thickness were evaluated as markers of cardiac hypertrophy. Further, serum glucose, glycerol, and insulin levels were also measured. AQP7 gene expression was found to be increased in the E (3.47-fold, p < 0.001), HPD (5.59-fold, p < 0.001), and HPD-E (3.87-fold, p < 0.001) groups compared to the control group. AQP7 protein expression was also increased in the HPD and HPD-E groups (p < 0.001). Additionally, cardiac mRNA expression levels of GLUT4 showed a significant increase in the E (2.16-fold, p < 0.003), HPD (7.14-fold, p < 0.001), and HPD-E (3.43-fold, p < 0.001) groups compared to the control group. GLUT4 protein expression was significantly increased in the E, HPD, and HPD-E groups compared to the control group (p = 0.024, p < 0.001, and p < 0.001, respectively). Furthermore, Serum glucose levels were significantly different between groups (p < 0.005). This difference was observed between the HPD groups and normal-protein diet groups (C and E). Serum insulin levels were higher for HPD groups compared with the normal-protein diet groups (p < 0.001), whereas no differences were observed between the exercise and sedentary groups (p = 0.111). Serum glycerol levels were significantly increased in the HPD groups compared with control and E groups (p < 0.05 and p < 0.05, respectively). Consumption of HPD supplementation caused the increased effects on AQP7 and GLUT4 expression in rat heart.

Deletion of MLIP (muscle-enriched A-type lamin-interacting protein) leads to cardiac hyperactivation of Akt/mammalian target of rapamycin (mTOR) and impaired cardiac adaptation

Aging and diseases generally result from tissue inability to maintain homeostasis through adaptation. The adult heart is particularly vulnerable to disequilibrium in homeostasis because its regenerative abilities are limited. Here, we report that MLIP (muscle enriched A-type lamin-interacting protein), a unique protein of unknown function, is required for proper cardiac adaptation. Mlip(-/-) mice exhibited normal cardiac function despite myocardial metabolic abnormalities and cardiac-specific overactivation of Akt/mTOR pathways. Cardiac-specific MLIP overexpression led to an inhibition of Akt/mTOR, providing evidence of a direct impact of MLIP on these key signaling pathways. Mlip(-/-) hearts showed an impaired capacity to adapt to stress (isoproterenol-induced hypertrophy), likely because of deregulated Akt/mTOR activity. Genome-wide association studies showed a genetic association between Mlip and early response to cardiac stress, supporting the role of MLIP in cardiac adaptation. Together, these results revealed that MLIP is required for normal myocardial adaptation to stress through integrated regulation of the Akt/mTOR pathways.

AMPK Activation Affects Glutamate Metabolism in Astrocytes

Mammalian AMP-activated protein kinase (AMPK) functions as a metabolic switch. It is composed of 3 different subunits and its activation depends on phosphorylation of a threonine residue (Thr172) in the α-subunit. This phosphorylation can be brought about by 5-aminoimidazole-4-carboxamide 1-β-D-ribofuranoside (AICAR) which in the cells is converted to a monophosphorylated nucleotide mimicking the effect of AMP. We show that the preparation of cultured astrocytes used for metabolic studies expresses AMPK, which could be phosphorylated by exposure of the cells to AICAR. The effect of AMPK activation on glutamate metabolism in astrocytes was studied using primary cultures of these cells from mouse cerebral cortex during incubation in media containing 2.5 mM glucose and 100 µM [U-(13)C]glutamate. The metabolism of glutamate including a detailed analysis of its metabolic pathways involving the tricarboxylic acid (TCA) cycle was studied using high-performance liquid chromatography analysis supplemented with gas chromatography-mass spectrometry technology. It was found that AMPK activation had profound effects on the pathways involved in glutamate metabolism since the entrance of the glutamate carbon skeleton into the TCA cycle was reduced. On the other hand, glutamate uptake into the astrocytes as well as its conversion to glutamine catalyzed by glutamine synthetase was not affected by AMPK activation. Interestingly, synthesis and release of citrate, which are hallmarks of astrocytic function, were affected by a reduction of the flux of glutamate derived carbon through the malic enzyme and pyruvate carboxylase catalyzed reactions. Finally, it was found that in the presence of glutamate as an additional substrate, glucose metabolism monitored by the use of tritiated deoxyglucose was unaffected by AMPK activation. Accordingly, the effects of AMPK activation appeared to be specific for certain key processes involved in glutamate metabolism.