Letter to the Editor Letters to the Editor will be published, if suitable, as space permits. They should not exceed 1000 words (typed double-spaced) in length and may be subject to editing or abridgment.
Response to Letter Regarding Article, “Myostatin Regulates Energy Homeostasis in the Heart and Prevents Heart Failure”
cardiac hypertrophy in our experimental setting. We are aware of the fact that AMPK is generally seen as inhibitor of protein synthesis, which seems to contradict a role of AMPK activation in promoting cardiac hypertrophy. However, point mutations in the nucleotide-binding region of the AMPK γ2-subunit, encoded by the Prkag2 gene (eg, the N488I allele) resulting in aberrant increase of kinase activity, cause cardiac hypertrophy probably via mechanistic target of rapamycin (mTOR) and FoxO (forkhead box O) signaling pathways independent of glycogen storage.6 Moreover, the expression of a kinase-dead version of AMPK-α2 mice in compound-heterozygous mice carrying the Prkag2 allele N488I, which increases AMPK activity, blocks cardiac hypertrophy and decreases enhanced glycogen storage.7 In further support of the prohypertrophic role of aberrant AMPK activity, we identified several new interaction partners of AMPK involved in hypertrophic signaling by coimmunoprecipitation. Therefore, a link seems to exist between myostatin, AMPK, and cardiac hypertrophy, which would fit to the well-known role of myostatin as inhibitor of Akt and mTOR signaling in heart and skeletal muscle.8 Neumann et al correctly state that most of the PRKAG2 mutations render AMPK insensitive to AMP, but cause increased AMPK activity, glycogen storage, and cardiac hypertrophy9,10 (see also paragraph above). We cannot see that these facts collide with our notion that PRKAG2 “mimics aspects of the myostatin phenotype,” which is characterized by increased AMPK activity, glycogen storage, and cardiac hypertrophy. Instead, the rescue of the PRKAG2 N4881 phenotype associated with increased AMPK activity by kinase-dead AMPK-α27 seems to phenocopy the effects of pharmacological inhibitors of AMPK in hearts of myostatin mutants with increased AMPK activity. Neumann et al reason that activation of AMPK after inactivation of myostatin in the heart might represent the reaction to metabolic alterations rather than the cause, which is a reasonable concern. However, we demonstrated several direct effects of myostatin signaling on AMPK: (1) we showed that cardiacspecific overexpression of myostatin reduces AMPK activity in vivo and (2) we demonstrated that treatment of adult primary cardiomyocytes with recombinant myostatin blocks AMPK phosphorylation after 10 minutes in vitro.1 Moreover, other groups also observed activation of AMPK in skeletal muscle and fat after inactivation of myostatin.11,12 Regulation of AMPK activity is complex and does not solely depend on the cellular energy status as indicated by the activation of AMPK via CamKKβ (calcium/ calmodulin-dependent protein kinase kinase beta).13 Similarly, we are just at the beginning of understanding the pleiotropic effects of AMPK in the heart in physiological and pathological conditions. Neumann et al point out that the stimulation of AMPK in cardiomyocytes upregulates glucose and fatty acid uptake as well as their utilization but did not mention that AMPK activation in left-ventricular hypertrophy activates glucose uptake and in parallel decreases β-oxidation.14 In our article, we described decreased interaction of AMPK with all major enzymes involved
In the article1 referred to by the letter of Neumann et al,2 we found that inactivation of myostatin in the adult heart caused a metabolic switch toward glycolysis and glycogen accumulation, cardiac hypertrophy, and heart failure, which was associated with massive activation of AMP-activated kinase (AMPK). Because pharmacological inhibition of AMPK in vitro and in vivo reversed several aspects of this phenotype, we concluded that myostatin-mediated repression of AMPK is required to maintain a mature aerobic energy metabolism in the heart and to prevent cardiac hypertrophy. Neumann et al challenge this conclusion arguing that activation of AMPK might represent the reaction to metabolic alterations rather than the cause. Furthermore, they postulate that increased transphosphorylation of protein kinase D observed in our study after inactivation of myostatin in the adult heart might explain increases in glucose utilization and uptake and cardiac hypertrophy by phosphorylation of HDAC5 (histone deacetylase 5) and subsequent deinhibition of myocyte enhancer factor 2 (MEF2). We share the skepticism expressed by Neumann et al on the use of pharmacological inhibitors such as the AMPK-inhibitor compound C, because many inhibitors exert off-target effects. To cope with this problem, we took advantage of a second inhibitor Ara-A, which is structurally and mechanistically different from compound C.3 Ara-A had exactly the same effect as compound C in rescuing the metabolic phenotype after myostatin deletion. In addition, we found that the treatment of primary cardiomyocytes with compound C followed by real-time respirometry leads to decreased glycolytic capacity, whereas AICAR (5-aminoimidazole4-carboxamide ribonucleotide), an AMPK activator, increased the glycolytic capacity. We also demonstrated that either inhibitor blocked increased phosphorylation of AMPK at the activating phosphorylation site T172 after inactivation of myostatin. In our view these observations clearly indicate that the inactivation of myostatin directly causes AMPK activation. To further validate activation of AMPK, we monitored phosphorylation of 2 AMPK targets, acetyl-CoA carboxylase, and glycogen synthase (GS). Neumann et al speculate that phosphorylation of GS at S641 might have been caused by GS kinase 3 and not by AMPK. Of course, GS kinase 3 does also phosphorylate GS at Ser641. However, increase of GS phosphorylation at Ser641 closely correlated with increased AMPK activity in our study, whereas inactivation of AMPK results in decreased Ser641 phosphorylation4 indicating that AMPK targets GS. Moreover, activation of GS kinase 3 suppresses and not induces cardiac hypertrophy,5 which argues against functionally relevant GS kinase 3 activation in myostatin mutant mice. Based on the general assumption that AMPK is a safeguard and not a culprit when it comes to cardiac hypertrophy, Neumann et al challenge our hypothesis that excessive activation of AMPK contributes to cardiac hypertrophy. Yet, treatment of adult myostatin mutant mice with either compound C or Ara-A prevents (Circ Res. 2015;116:e97-e98. DOI: 10.1161/CIRCRESAHA.115.306486.) © 2015 American Heart Association, Inc. Circulation Research is available at http://circres.ahajournals.org
DOI: 10.1161/CIRCRESAHA.115.306486
e97
e98 Circulation Research May 8, 2015 in β-oxidation after inactivation of myostatin, whereas interaction with glycolytic proteins was increased.1 Taken together, our genetic, proteomic, and respiratory data argue against a simultaneous activation of glycolysis and β-oxidation after myostatin inactivation and subsequent AMPK activation but because we did not measure β-oxidation directly, we cannot tell whether increased AMPK activity after inactivation of myostatin significantly represses β-oxidation. In our article, we extensively discussed the controversial findings concerning the regulation of AMPK by TGF-beta activated kinase 1 (TAK1). Previous publications described decreased phosphorylation and activation of AMPK after expression of a dominant-negative version of TAK1 in neonatal cardiomyocytes,15 which seem to be at odds with our finding that TAK1 directly or indirectly inhibits AMPK. However, the same group failed to show that wild-type TAK1 overexpression activates AMPK in neonatal cardiomyocytes.15 In this context, it should be remembered that cardiac metabolism dramatically changes during early postnatal stages from a preference to glycolysis toward fatty acids in adult hearts,16 which might be a reason for the conflicting findings. We agree that additional studies are necessary to clarify the impact of TAK1 on the regulation of AMPK in the heart at different developmental stages characterized by distinct metabolic states. Finally, Neumann et al speculate that the increased transphosphorylation of protein kinase D observed in our study might provide a suitable explanation for enhanced glucose utilization and cardiac hypertrophy after inactivation of myostatin in the adult heart. We cannot exclude a contribution of protein kinase D but many of the observed metabolic effects such as glycogen accumulation, increased glycolytic capacity, inhibition of acetyl-CoA carboxylase, and increased lactate content are difficult to reconcile with protein kinase D activation. Moreover, cardiac-specific overexpression of PKD1 leads to systolic dysfunction and dilated cardiomyopathy17 and MEF2a transgenic mice do not develop cardiac hypertrophy but show signs of dilated cardiomyopathy and mechanical dysfunction,18 supporting our hypothesis that the Rgs2-PKD-MEF2 axis is involved in the systolic dysfunction after acute deletion of myostatin but not in development of the metabolic phenotype. We agree that our study raised several exciting questions about the regulation of AMPK by myostatin and its impact on cardiac metabolism, which should be addressed in future experiments. However, we stand by our hypothesis that excessive activation of AMPK as a result of myostatin inactivation in the adult heart is currently the most plausible explanation for the increased glycogen accumulation, increased glycolysis, and cardiac hypertrophy in myostatin mutants. Even a safeguard might sometimes turn into a culprit.
Sources of Funding Dr Braun is supported by the Max-Planck-Society, the Deutsche Forschungsgemeinschaft (Excellence Cluster Cardio-Pulmonary System, and Sonderforschungsbereich TR81), the Landes-Offensive zur Entwicklung Wissenschaftlich-ökonomischer Exzellenz Center for Cell and Gene Therapy, and the German Center for Cardiovascular Research.
Disclosures None.
Nadine Biesemann Thomas Braun Department of Cardiac Development and Remodelling Max-Planck-Institute for Heart and Lung Research Bad Nauheim, Germany
References 1. Biesemann N, Mendler L, Wietelmann A, Hermann S, Schäfers M, Krüger M, Boettger T, Borchardt T, Braun T. Myostatin regulates energy homeostasis in the heart and prevents heart failure. Circ Res. 2014;115:296–310. doi: 10.1161/CIRCRESAHA.115.304185. 2. Neumann D, Luiken JJFP, Nabben M, Glatz JFC. Letter by Neumann et al regarding article, “Myostatin regulates energy homeostasis in the heart and prevents heart failure.” Circ Res. 2015;116:e95–e96. doi: 10.1161/ CIRCRESAHA.115.306463. 3. Henin N, Vincent MF, Van den Berghe G. Stimulation of rat liver AMPactivated protein kinase by AMP analogues. Biochim Biophys Acta. 1996;1290:197–203. 4. Hunter RW, Treebak JT, Wojtaszewski JF, Sakamoto K. Molecular mechanism by which AMP-activated protein kinase activation promotes glycogen accumulation in muscle. Diabetes. 2011;60:766–774. doi: 10.2337/ db10-1148. 5. Antos CL, McKinsey TA, Frey N, Kutschke W, McAnally J, Shelton JM, Richardson JA, Hill JA, Olson EN. Activated glycogen synthase-3 beta suppresses cardiac hypertrophy in vivo. Proc Natl Acad Sci U S A. 2002;99:907–912. doi: 10.1073/pnas.231619298. 6. Kim M, Hunter RW, Garcia-Menendez L, Gong G, Yang YY, Kolwicz SC Jr, Xu J, Sakamoto K, Wang W, Tian R. Mutation in the γ2-subunit of AMP-activated protein kinase stimulates cardiomyocyte proliferation and hypertrophy independent of glycogen storage. Circ Res. 2014;114:966– 975. doi: 10.1161/CIRCRESAHA.114.302364. 7. Ahmad F, Arad M, Musi N, et al. Increased alpha2 subunit-associated AMPK activity and PRKAG2 cardiomyopathy. Circulation. 2005;112:3140–3148. doi: 10.1161/CIRCULATIONAHA.105.550806. 8. Braun T, Gautel M. Transcriptional mechanisms regulating skeletal muscle differentiation, growth and homeostasis. Nat Rev Mol Cell Biol. 2011;12:349–361. doi: 10.1038/nrm3118. 9. Arad M, Moskowitz IP, Patel VV, et al. Transgenic mice overexpressing mutant PRKAG2 define the cause of Wolff-Parkinson-White syndrome in glycogen storage cardiomyopathy. Circulation. 2003;107:2850–2856. doi: 10.1161/01.CIR.0000075270.13497.2B. 10. Luptak I, Shen M, He H, Hirshman MF, Musi N, Goodyear LJ, Yan J, Wakimoto H, Morita H, Arad M, Seidman CE, Seidman JG, Ingwall JS, Balschi JA, Tian R. Aberrant activation of AMP-activated protein kinase remodels metabolic network in favor of cardiac glycogen storage. J Clin Invest. 2007;117:1432–1439. doi: 10.1172/JCI30658. 11. Zhang C, McFarlane C, Lokireddy S, Bonala S, Ge X, Masuda S, Gluckman PD, Sharma M, Kambadur R. Myostatin-deficient mice exhibit reduced insulin resistance through activating the AMP-activated protein kinase signalling pathway. Diabetologia. 2011;54:1491–1501. doi: 10.1007/s00125-011-2079-7. 12. Shan T, Liang X, Bi P, Kuang S. Myostatin knockout drives browning of white adipose tissue through activating the AMPK-PGC1α-Fndc5 pathway in muscle. FASEB J. 2013;27:1981–1989. doi: 10.1096/ fj.12-225755. 13. Horman S, Beauloye C, Vanoverschelde JL, Bertrand L. AMP-activated protein kinase in the control of cardiac metabolism and remodeling. Curr Heart Fail Rep. 2012;9:164–173. doi: 10.1007/s11897-012-0102-z. 14. Tian R, Musi N, D’Agostino J, Hirshman MF, Goodyear LJ. Increased adenosine monophosphate-activated protein kinase activity in rat hearts with pressure-overload hypertrophy. Circulation. 2001;104:1664–1669. 15. Xie M, Zhang D, Dyck JR, Li Y, Zhang H, Morishima M, Mann DL, Taffet GE, Baldini A, Khoury DS, Schneider MD. A pivotal role for endogenous TGF-beta-activated kinase-1 in the LKB1/AMP-activated protein kinase energy-sensor pathway. Proc Natl Acad Sci U S A. 2006;103:17378– 17383. doi: 10.1073/pnas.0604708103. 16. Pohjoismäki JL, Krüger M, Al-Furoukh N, Lagerstedt A, Karhunen PJ, Braun T. Postnatal cardiomyocyte growth and mitochondrial reorganization cause multiple changes in the proteome of human cardiomyocytes. Mol Biosyst. 2013;9:1210–1219. doi: 10.1039/c3mb25556e. 17. Harrison BC, Kim MS, van Rooij E, Plato CF, Papst PJ, Vega RB, McAnally JA, Richardson JA, Bassel-Duby R, Olson EN, McKinsey TA. Regulation of cardiac stress signaling by protein kinase d1. Mol Cell Biol. 2006;26:3875–3888. doi: 10.1128/MCB.26.10.3875-3888.2006. 18. van Oort RJ, van Rooij E, Bourajjaj M, Schimmel J, Jansen MA, van der Nagel R, Doevendans PA, Schneider MD, van Echteld CJ, De Windt LJ. MEF2 activates a genetic program promoting chamber dilation and contractile dysfunction in calcineurin-induced heart failure. Circulation. 2006;114:298–308. doi: 10.1161/CIRCULATIONAHA.105.608968.
Response to Letter Regarding Article, “Myostatin Regulates Energy Homeostasis in the Heart and Prevents Heart Failure”
cardiac hypertrophy in our experimental setting. We are aware of the fact that AMPK is generally seen as inhibitor of protein synthesis, which seems to contradict a role of AMPK activation in promoting cardiac hypertrophy. However, point mutations in the nucleotide-binding region of the AMPK γ2-subunit, encoded by the Prkag2 gene (eg, the N488I allele) resulting in aberrant increase of kinase activity, cause cardiac hypertrophy probably via mechanistic target of rapamycin (mTOR) and FoxO (forkhead box O) signaling pathways independent of glycogen storage.6 Moreover, the expression of a kinase-dead version of AMPK-α2 mice in compound-heterozygous mice carrying the Prkag2 allele N488I, which increases AMPK activity, blocks cardiac hypertrophy and decreases enhanced glycogen storage.7 In further support of the prohypertrophic role of aberrant AMPK activity, we identified several new interaction partners of AMPK involved in hypertrophic signaling by coimmunoprecipitation. Therefore, a link seems to exist between myostatin, AMPK, and cardiac hypertrophy, which would fit to the well-known role of myostatin as inhibitor of Akt and mTOR signaling in heart and skeletal muscle.8 Neumann et al correctly state that most of the PRKAG2 mutations render AMPK insensitive to AMP, but cause increased AMPK activity, glycogen storage, and cardiac hypertrophy9,10 (see also paragraph above). We cannot see that these facts collide with our notion that PRKAG2 “mimics aspects of the myostatin phenotype,” which is characterized by increased AMPK activity, glycogen storage, and cardiac hypertrophy. Instead, the rescue of the PRKAG2 N4881 phenotype associated with increased AMPK activity by kinase-dead AMPK-α27 seems to phenocopy the effects of pharmacological inhibitors of AMPK in hearts of myostatin mutants with increased AMPK activity. Neumann et al reason that activation of AMPK after inactivation of myostatin in the heart might represent the reaction to metabolic alterations rather than the cause, which is a reasonable concern. However, we demonstrated several direct effects of myostatin signaling on AMPK: (1) we showed that cardiacspecific overexpression of myostatin reduces AMPK activity in vivo and (2) we demonstrated that treatment of adult primary cardiomyocytes with recombinant myostatin blocks AMPK phosphorylation after 10 minutes in vitro.1 Moreover, other groups also observed activation of AMPK in skeletal muscle and fat after inactivation of myostatin.11,12 Regulation of AMPK activity is complex and does not solely depend on the cellular energy status as indicated by the activation of AMPK via CamKKβ (calcium/ calmodulin-dependent protein kinase kinase beta).13 Similarly, we are just at the beginning of understanding the pleiotropic effects of AMPK in the heart in physiological and pathological conditions. Neumann et al point out that the stimulation of AMPK in cardiomyocytes upregulates glucose and fatty acid uptake as well as their utilization but did not mention that AMPK activation in left-ventricular hypertrophy activates glucose uptake and in parallel decreases β-oxidation.14 In our article, we described decreased interaction of AMPK with all major enzymes involved
In the article1 referred to by the letter of Neumann et al,2 we found that inactivation of myostatin in the adult heart caused a metabolic switch toward glycolysis and glycogen accumulation, cardiac hypertrophy, and heart failure, which was associated with massive activation of AMP-activated kinase (AMPK). Because pharmacological inhibition of AMPK in vitro and in vivo reversed several aspects of this phenotype, we concluded that myostatin-mediated repression of AMPK is required to maintain a mature aerobic energy metabolism in the heart and to prevent cardiac hypertrophy. Neumann et al challenge this conclusion arguing that activation of AMPK might represent the reaction to metabolic alterations rather than the cause. Furthermore, they postulate that increased transphosphorylation of protein kinase D observed in our study after inactivation of myostatin in the adult heart might explain increases in glucose utilization and uptake and cardiac hypertrophy by phosphorylation of HDAC5 (histone deacetylase 5) and subsequent deinhibition of myocyte enhancer factor 2 (MEF2). We share the skepticism expressed by Neumann et al on the use of pharmacological inhibitors such as the AMPK-inhibitor compound C, because many inhibitors exert off-target effects. To cope with this problem, we took advantage of a second inhibitor Ara-A, which is structurally and mechanistically different from compound C.3 Ara-A had exactly the same effect as compound C in rescuing the metabolic phenotype after myostatin deletion. In addition, we found that the treatment of primary cardiomyocytes with compound C followed by real-time respirometry leads to decreased glycolytic capacity, whereas AICAR (5-aminoimidazole4-carboxamide ribonucleotide), an AMPK activator, increased the glycolytic capacity. We also demonstrated that either inhibitor blocked increased phosphorylation of AMPK at the activating phosphorylation site T172 after inactivation of myostatin. In our view these observations clearly indicate that the inactivation of myostatin directly causes AMPK activation. To further validate activation of AMPK, we monitored phosphorylation of 2 AMPK targets, acetyl-CoA carboxylase, and glycogen synthase (GS). Neumann et al speculate that phosphorylation of GS at S641 might have been caused by GS kinase 3 and not by AMPK. Of course, GS kinase 3 does also phosphorylate GS at Ser641. However, increase of GS phosphorylation at Ser641 closely correlated with increased AMPK activity in our study, whereas inactivation of AMPK results in decreased Ser641 phosphorylation4 indicating that AMPK targets GS. Moreover, activation of GS kinase 3 suppresses and not induces cardiac hypertrophy,5 which argues against functionally relevant GS kinase 3 activation in myostatin mutant mice. Based on the general assumption that AMPK is a safeguard and not a culprit when it comes to cardiac hypertrophy, Neumann et al challenge our hypothesis that excessive activation of AMPK contributes to cardiac hypertrophy. Yet, treatment of adult myostatin mutant mice with either compound C or Ara-A prevents (Circ Res. 2015;116:e97-e98. DOI: 10.1161/CIRCRESAHA.115.306486.) © 2015 American Heart Association, Inc. Circulation Research is available at http://circres.ahajournals.org
DOI: 10.1161/CIRCRESAHA.115.306486
e97
e98 Circulation Research May 8, 2015 in β-oxidation after inactivation of myostatin, whereas interaction with glycolytic proteins was increased.1 Taken together, our genetic, proteomic, and respiratory data argue against a simultaneous activation of glycolysis and β-oxidation after myostatin inactivation and subsequent AMPK activation but because we did not measure β-oxidation directly, we cannot tell whether increased AMPK activity after inactivation of myostatin significantly represses β-oxidation. In our article, we extensively discussed the controversial findings concerning the regulation of AMPK by TGF-beta activated kinase 1 (TAK1). Previous publications described decreased phosphorylation and activation of AMPK after expression of a dominant-negative version of TAK1 in neonatal cardiomyocytes,15 which seem to be at odds with our finding that TAK1 directly or indirectly inhibits AMPK. However, the same group failed to show that wild-type TAK1 overexpression activates AMPK in neonatal cardiomyocytes.15 In this context, it should be remembered that cardiac metabolism dramatically changes during early postnatal stages from a preference to glycolysis toward fatty acids in adult hearts,16 which might be a reason for the conflicting findings. We agree that additional studies are necessary to clarify the impact of TAK1 on the regulation of AMPK in the heart at different developmental stages characterized by distinct metabolic states. Finally, Neumann et al speculate that the increased transphosphorylation of protein kinase D observed in our study might provide a suitable explanation for enhanced glucose utilization and cardiac hypertrophy after inactivation of myostatin in the adult heart. We cannot exclude a contribution of protein kinase D but many of the observed metabolic effects such as glycogen accumulation, increased glycolytic capacity, inhibition of acetyl-CoA carboxylase, and increased lactate content are difficult to reconcile with protein kinase D activation. Moreover, cardiac-specific overexpression of PKD1 leads to systolic dysfunction and dilated cardiomyopathy17 and MEF2a transgenic mice do not develop cardiac hypertrophy but show signs of dilated cardiomyopathy and mechanical dysfunction,18 supporting our hypothesis that the Rgs2-PKD-MEF2 axis is involved in the systolic dysfunction after acute deletion of myostatin but not in development of the metabolic phenotype. We agree that our study raised several exciting questions about the regulation of AMPK by myostatin and its impact on cardiac metabolism, which should be addressed in future experiments. However, we stand by our hypothesis that excessive activation of AMPK as a result of myostatin inactivation in the adult heart is currently the most plausible explanation for the increased glycogen accumulation, increased glycolysis, and cardiac hypertrophy in myostatin mutants. Even a safeguard might sometimes turn into a culprit.
Sources of Funding Dr Braun is supported by the Max-Planck-Society, the Deutsche Forschungsgemeinschaft (Excellence Cluster Cardio-Pulmonary System, and Sonderforschungsbereich TR81), the Landes-Offensive zur Entwicklung Wissenschaftlich-ökonomischer Exzellenz Center for Cell and Gene Therapy, and the German Center for Cardiovascular Research.
Disclosures None.
Nadine Biesemann Thomas Braun Department of Cardiac Development and Remodelling Max-Planck-Institute for Heart and Lung Research Bad Nauheim, Germany
References 1. Biesemann N, Mendler L, Wietelmann A, Hermann S, Schäfers M, Krüger M, Boettger T, Borchardt T, Braun T. Myostatin regulates energy homeostasis in the heart and prevents heart failure. Circ Res. 2014;115:296–310. doi: 10.1161/CIRCRESAHA.115.304185. 2. Neumann D, Luiken JJFP, Nabben M, Glatz JFC. Letter by Neumann et al regarding article, “Myostatin regulates energy homeostasis in the heart and prevents heart failure.” Circ Res. 2015;116:e95–e96. doi: 10.1161/ CIRCRESAHA.115.306463. 3. Henin N, Vincent MF, Van den Berghe G. Stimulation of rat liver AMPactivated protein kinase by AMP analogues. Biochim Biophys Acta. 1996;1290:197–203. 4. Hunter RW, Treebak JT, Wojtaszewski JF, Sakamoto K. Molecular mechanism by which AMP-activated protein kinase activation promotes glycogen accumulation in muscle. Diabetes. 2011;60:766–774. doi: 10.2337/ db10-1148. 5. Antos CL, McKinsey TA, Frey N, Kutschke W, McAnally J, Shelton JM, Richardson JA, Hill JA, Olson EN. Activated glycogen synthase-3 beta suppresses cardiac hypertrophy in vivo. Proc Natl Acad Sci U S A. 2002;99:907–912. doi: 10.1073/pnas.231619298. 6. Kim M, Hunter RW, Garcia-Menendez L, Gong G, Yang YY, Kolwicz SC Jr, Xu J, Sakamoto K, Wang W, Tian R. Mutation in the γ2-subunit of AMP-activated protein kinase stimulates cardiomyocyte proliferation and hypertrophy independent of glycogen storage. Circ Res. 2014;114:966– 975. doi: 10.1161/CIRCRESAHA.114.302364. 7. Ahmad F, Arad M, Musi N, et al. Increased alpha2 subunit-associated AMPK activity and PRKAG2 cardiomyopathy. Circulation. 2005;112:3140–3148. doi: 10.1161/CIRCULATIONAHA.105.550806. 8. Braun T, Gautel M. Transcriptional mechanisms regulating skeletal muscle differentiation, growth and homeostasis. Nat Rev Mol Cell Biol. 2011;12:349–361. doi: 10.1038/nrm3118. 9. Arad M, Moskowitz IP, Patel VV, et al. Transgenic mice overexpressing mutant PRKAG2 define the cause of Wolff-Parkinson-White syndrome in glycogen storage cardiomyopathy. Circulation. 2003;107:2850–2856. doi: 10.1161/01.CIR.0000075270.13497.2B. 10. Luptak I, Shen M, He H, Hirshman MF, Musi N, Goodyear LJ, Yan J, Wakimoto H, Morita H, Arad M, Seidman CE, Seidman JG, Ingwall JS, Balschi JA, Tian R. Aberrant activation of AMP-activated protein kinase remodels metabolic network in favor of cardiac glycogen storage. J Clin Invest. 2007;117:1432–1439. doi: 10.1172/JCI30658. 11. Zhang C, McFarlane C, Lokireddy S, Bonala S, Ge X, Masuda S, Gluckman PD, Sharma M, Kambadur R. Myostatin-deficient mice exhibit reduced insulin resistance through activating the AMP-activated protein kinase signalling pathway. Diabetologia. 2011;54:1491–1501. doi: 10.1007/s00125-011-2079-7. 12. Shan T, Liang X, Bi P, Kuang S. Myostatin knockout drives browning of white adipose tissue through activating the AMPK-PGC1α-Fndc5 pathway in muscle. FASEB J. 2013;27:1981–1989. doi: 10.1096/ fj.12-225755. 13. Horman S, Beauloye C, Vanoverschelde JL, Bertrand L. AMP-activated protein kinase in the control of cardiac metabolism and remodeling. Curr Heart Fail Rep. 2012;9:164–173. doi: 10.1007/s11897-012-0102-z. 14. Tian R, Musi N, D’Agostino J, Hirshman MF, Goodyear LJ. Increased adenosine monophosphate-activated protein kinase activity in rat hearts with pressure-overload hypertrophy. Circulation. 2001;104:1664–1669. 15. Xie M, Zhang D, Dyck JR, Li Y, Zhang H, Morishima M, Mann DL, Taffet GE, Baldini A, Khoury DS, Schneider MD. A pivotal role for endogenous TGF-beta-activated kinase-1 in the LKB1/AMP-activated protein kinase energy-sensor pathway. Proc Natl Acad Sci U S A. 2006;103:17378– 17383. doi: 10.1073/pnas.0604708103. 16. Pohjoismäki JL, Krüger M, Al-Furoukh N, Lagerstedt A, Karhunen PJ, Braun T. Postnatal cardiomyocyte growth and mitochondrial reorganization cause multiple changes in the proteome of human cardiomyocytes. Mol Biosyst. 2013;9:1210–1219. doi: 10.1039/c3mb25556e. 17. Harrison BC, Kim MS, van Rooij E, Plato CF, Papst PJ, Vega RB, McAnally JA, Richardson JA, Bassel-Duby R, Olson EN, McKinsey TA. Regulation of cardiac stress signaling by protein kinase d1. Mol Cell Biol. 2006;26:3875–3888. doi: 10.1128/MCB.26.10.3875-3888.2006. 18. van Oort RJ, van Rooij E, Bourajjaj M, Schimmel J, Jansen MA, van der Nagel R, Doevendans PA, Schneider MD, van Echteld CJ, De Windt LJ. MEF2 activates a genetic program promoting chamber dilation and contractile dysfunction in calcineurin-induced heart failure. Circulation. 2006;114:298–308. doi: 10.1161/CIRCULATIONAHA.105.608968.
