Hypoxia Triggers AMPK Activation through Reactive Oxygen Species-Mediated Activation of Calcium Release-Activated Calcium Channels

Abstract

AMP-activated protein kinase (AMPK) is an energy sensor activated by increases in [AMP] or by oxidant stress (reactive oxygen species [ROS]). Hypoxia increases cellular ROSEnergy-dependent cellular processes require ATP, which is derived from mitochondrial oxidative phosphorylation and/or from glycolysis. Conditions that limit the cellular oxygen supply to the point that oxidative metabolism suffers can threaten cell survival because they undermine the ability of the cell to sustain essential ATP-dependent processes. Given that ATP supply is critical for survival, multiple systems have evolved to protect cells from the consequences of oxygen supply limitation and metabolic substrate deprivation. In this regard, defense of cellular energy substrates is mediated by the AMPactivated protein kinase (AMPK) system, which has been described as a "fuel gauge of the cell" (17). Activation of AMPK leads to increased glucose uptake via translocation of the transporter GLUT4 to the plasma membrane (42) and activation of glycolytic enzymes such as 6-phosphofructo-2-kinase, leading to enhanced glycolytic capacity via increases in fructose-2,6-bisphosphate (36). These and other responses triggered by AMPK activation confer protection against hypoxic injury in tissues such as the heart by preserving energy supply, mitochondrial metabolism, and glycolytic flux (43).AMPK activation requires phosphorylation of Thr-172 in the activation loop of its ␣ subunit (21) and is regulated by increases in cellular AMP and decreases in ATP via allosteric mechanisms and by the activity of upstream kinases and phosphatases that control AMPK phosphorylation and dephosphorylation at the critical threonine residue (8,23). Perhaps the best-described AMPK kinase is the tumor suppressor LKB1, which phosphorylates the catalytic ␣ subunit of AMPK in an AMP-dependent manner (20,53). A second activation pathway occurs independently of cellular AMP levels and involves Ca 2ϩ /calmodulin-dependent protein kinase kinase (CaMKK␤) (25,52). In that pathway, activation is initiated by a rise in [Ca 2ϩ ] i which increases the activity of CaMKK␤, which then phosphorylates AMPK. In vitro studies using both endogenous and recombinant proteins show that LKB1 and CaMKK␤ phosphorylate AMPK at 20,52). Activation of AMPK can also occur in response to oxidant stress (26,47), as demonstrated by the observation that H 2 O 2 administration triggers phosphorylation of AMPK and its downstream target, acyl coenzyme A carboxylase (ACC) (7, 28). However, the mechanism underlying this reactive oxygen species (ROS)-dependent activation of AMPK is not known.During severe oxygen deprivation, limitations in mitochondrial respiration lead to increases in the AMP/ATP ratio, which then trigger AMPK activation. However, AMPK-mediated protection might be greater if the kinase was activated before the cell reached the point where the AMP/ATP ratio has increased. Some beneficial effects of AMPK signaling, including the activation of gene expression or mitochondrial biogenesis (39), are not...

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