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

AMP-activated kinase (AMPK) is activated by stress and in conditions associated with ATP depletion and a consequent increase in the AMP/ATP ratio. Once activated, AMPK phosphorylates several downstream substrates, the overall effect of which is to switch off ATP-consuming pathways (e.g., fatty acid synthesis and cholesterol synthesis) and to switch on ATP-generating pathways (Carling et al., 1989; Hardie, 2003; Carling, 2004). So far, it has been established that AMPK, directly or indirectly, regulate the activity/expression of many intracellular proteins, including acetyl-coenzyme A carboxylase, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, glycerol phosphate acyl transferase, glycogen synthase, elongation factor-2, mammalian target of rapamycin, p70 ribosomal protein S6 kinase, hormone-sensitive lipase, cystic fibrosis transmembrane regulator (CFTR), 6-phosphofructo-2-kinase, endothelial nitric oxide synthase, insulin receptor substrate-1, phosphatidylinositol 3-kinase, glucose transporter 1 and 4, phosphoenolpyruvate carboxykinase, glucose-6-phosphatase, fatty acid synthase, and probably many others that will be identified in the future (for detailed description of AMPK targets, see Hardie, 2003; Hardie et al., 2003). In the heart, AMPK is activated by different types of stress, both of those occurring under physiological and pathophysiological conditions (Kudo et al., 1995; Coven et al., 2003; Sakamoto et al., 2004; Young et al., 2005). AMPK is a heterotrimeric complex comprising of a catalytic α subunit and regulatory β and γ subunits. There are two isoforms of the catalytic subunit termed α1 and α2. α2 subunit is primarily expressed in liver, heart, and skeletal muscle (Hardie, 2003; Carling, 2004). It has been previously shown that mice overexpressing dominant negative α2 subunit in the heart respond to myocardial ischemia by accelerated ATP depletion, early development of myocardial contracture, and decreased glucose uptake (Xing et al., 2003). Murray et al. (1986) discovered that brief periods of blood vessel occlusion and reperfusion administered prior to a sustained ischemic episode lead to a reduction in infarct size. This cardioprotective phenomenon (now called early preconditioning) enhances the survival of cardiac cells under conditions that induce myocardial infarction. This and subsequent studies have indicated that brief ischemia or hypoxia are likely to switch on intracellular signaling pathways that ultimately result in an increased cellular tolerance to metabolic stress (Yellon and Downey, 2003). As AMPK is activated in the heart by hypoxia/ischemia (for review see Russell, 2003), we have hypothesized that this enzyme, and in particular α2 isoform that is predominant in the heart, might play a role in mediating preconditioning in the heart. Therefore, we have taken advantage of the cardiac phenotype overexpressing dominant negative form of α2 AMPK subunit to analyze the role, if any, that AMPK play in preconditioning in the heart.