Deficiency of LKB1 in heart prevents ischemia-mediated activation of AMPKα2 but not AMPKα1

Abstract

. Deficiency of LKB1 in heart prevents ischemia-mediated activation of AMPK␣2 but not AMPK␣1. Am J Physiol Endocrinol Metab 290: E780 -E788, 2006. First published December 6, 2005 doi:10.1152/ajpendo.00443.2005.-Recent studies indicate that the LKB1 is a key regulator of the AMP-activated protein kinase (AMPK), which plays a crucial role in protecting cardiac muscle from damage during ischemia. We have employed mice that lack LKB1 in cardiac and skeletal muscle and studied how this affected the activity of cardiac AMPK␣1/␣2 under normoxic, ischemic, and anoxic conditions. In the heart lacking cardiac muscle LKB1, the basal activity of AMPK␣2 was vastly reduced and not increased by ischemia or anoxia. Phosphorylation of AMPK␣2 at the site of LKB1 phosphorylation (Thr 172 ) or phosphorylation of acetylCoA carboxylase-2, a downstream substrate of AMPK, was ablated in ischemic heart lacking cardiac LKB1. Ischemia was found to increase the ADP-to-ATP (ADP/ATP) and AMP-to-ATP ratios (AMP/ATP) to a greater extent in LKB1-deficient cardiac muscle than in LKB1-expressing muscle. In contrast to AMPK␣2, significant basal activity of AMPK␣1 was observed in the lysates from the hearts lacking cardiac muscle LKB1, as well as in cardiomyocytes that had been isolated from these hearts. In the heart lacking cardiac LKB1, ischemia or anoxia induced a marked activation and phosphorylation of AMPK␣1, to a level that was only moderately lower than observed in LKB1-expressing heart. Echocardiographic and morphological analysis of the cardiac LKB1-deficient hearts indicated that these hearts were not overtly dysfunctional, despite possessing a reduced weight and enlarged atria. These findings indicate that LKB1 plays a crucial role in regulating AMPK␣2 activation and acetyl-CoA carboxylase-2 phosphorylation and also regulating cellular energy levels in response to ischemia. They also provide genetic evidence that an alternative upstream kinase can activate AMPK␣1 in cardiac muscle. cellular energy metabolism; hypoxia; cardiovascular physiology; AMP-activated protein kinase THE AMP-ACTIVATED PROTEIN KINASE (AMPK) is switched on by increases in levels of AMP, resulting from reduced availability of ATP. AMPK functions to restore ATP concentrations by stimulating energy-producing processes, such as nutrient uptake and oxidation of fatty acids, and inhibiting unnecessary energy-consuming processes, such as protein synthesis and cell proliferation (reviewed in Refs. 8, 11). AMPK is a heterotrimeric complex comprising a catalytic ␣-subunit and regulatory ␤-and ␥-subunits. AMP activates the AMPK complex by binding to the Bateman domains made up of pairs of CBS sequences located on the ␥-subunit and by stimulating the phosphorylation of Thr172 in the T-loop of both mammalian AMPK␣ catalytic subunits, termed AMPK␣1 and AMPK␣2.

AMP-activated protein kinase control of energy metabolism in the ischemic heart

“…It is of interest to note that mice hearts deficient in LKB1 show a decrease in the phosphorylation and activation of AMPKa2 during ischemia, whereas the ischemia-induced activation of AMPKa1 remains unchanged. 10 This not only suggests the presence of additional AMPKKs, but also that LKB1 does play a role in ischemia-induced activation of AMPK. As in other tissues, it is possible that metabolic stress can make AMPK a better substrate for AMPKKs LKB1.…”

“…We first demonstrated that AMPK activation occurs during myocardial ischemia, 2,3 a finding now reproduced by numerous laboratories. 4,[7][8][9][10][11]23,24 This activation of AMPK presumably occurs as an approach to restore myocyte ATP levels, primarily turning on ATP generating pathways. It is now clear that AMPK activation can increase both glucose and fatty acid metabolism in the ischemic heart.…”

“…It is of interest to note that the ischemia-induced activation of AMPKa1 remains unchanged. 10 This suggests that LKB1 does contribute to the ischemia-induced activation of AMPK, but that additional AMPKKs are also involved in the ischemia-induced activation of AMPK. Alternatively, the a1 isoform of AMPK may be expressed at higher levels than the a2 isoform in endothelial cells within the heart and it is the endothelial cell-specific a1 isoform that is not affected by cardiomyocyte-specific loss of LKB1.…”

“…However, the majority of studies examining AMPK activation in the ischemic heart suggest that AMPK activation is an adaptive response that allows the heart to generate ATP and thereby protect cardiac tissues in the presence of oxygen deprivation. 4,7,10 Unfortunately, very few of these studies have actually examined what effect AMPK activation has directly on cardiac function.…”

“…Cardiac AMPK is rapidly activated during ischemia. [2][3][4][5][6][7][8][9][10] This activation of AMPK results in a stimulation of glucose uptake, glycolysis and fatty acid oxidation. 3,5,6,11,12 These metabolic effects can have both beneficial and harmful effects during ischemia and during reperfusion following ischemia.…”