Abstract 271: Ca2+/Calmodulin-Dependent Kinase Kinase β (CaMKKβ) Contributes to AMPK Activation in the Heart during Hypoxia
AMP-activated protein kinase (AMPK) is a stress signaling enzyme that regulates important cellular responses to ischemic stress in the heart. AMPK stimulates glucose uptake and utilization during myocardial ischemia and limits cardiac apoptosis and contractile dysfunction associated with ischemia/reperfusion injury. Activation of AMPK involves allosteric regulation by AMP/ATP ratio as well as phosphorylation by upstream AMPK kinases (AMPKKs) including LKB1 and CaMKKβ. The identity of the AMPKKs involved in the activation of AMPK during myocardial ischemia is not fully defined. The aims of this study were to determine whether
CaMKKβ is expressed in the heart and in cardiomyocytes
CaMKKβ is activated during ischemia and
CaMKKβ contributes to AMPK activation during ischemia. CaMKKβ mRNA and protein were found to be expressed in mouse and rat hearts and isolated rat cardiomyocytes. To determine whether myocardial ischemia stimulates CaMKKβ activity in vivo, mouse hearts were subjected to regional ischemia by LAD occlusion, and assessed for phosphorylation of CaMKI (Thr177), a direct downstream substrate of CaMKKβ. There was a 4-fold increase in CaMKI phosphorylation on its activating Thr177 site within 2–5 min of ischemia (p<0.05 vs. control). CaMKKβ signaling was transiently activated and correlated with the initial rise in AMPK phosphorylation during the early phase of myocardial ischemia. To further determine whether CaMKKβ participates in ischemia-induced heart AMPK activation and glucose uptake, isolated rat left ventricular papillary muscles were subjected to hypoxia in the presence of a specific CaMKKβ inhibitor, STO-609 (2 μM). CaMKKβ inhibition resulted in a 60% reduction in hypoxia-induced AMPK activation (p<0.01 vs. hypoxia). In addition, inhibition of CaMKKβ yielded an 85% and 30% reduction in hypoxia-induced ACC phosphorylation and glucose uptake, respectively (p<0.05 vs. hypoxia), two well-known downstream consequences of AMPK activation.
CONCLUSION: Myocardial ischemia leads to an early, transient stimulation of the CaMKKβ pathway which correlates with the initial activation of AMPK during ischemic stress. CaMKKβ partially mediates hypoxia-induced activation of AMPK pathway in the heart.