Galina A. Gusarova

α1-AMP-Activated Protein Kinase Regulates Hypoxia-Induced Na,K-ATPase Endocytosis via Direct Phosphorylation of Protein Kinase Cζ

ABSTRACT Hypoxia promotes Na,K-ATPase endocytosis via protein kinase Cζ (PKCζ)-mediated phosphorylation of the Na,K-ATPase α subunit. Here, we report that hypoxia leads to the phosphorylation of 5′-AMP-activated protein kinase (AMPK) at Thr172 in rat alveolar epithelial cells. The overexpression of a dominant-negative AMPK α subunit (AMPK-DN) construct prevented the hypoxia-induced endocytosis of Na,K-ATPase. The overexpression of the reactive oxygen species (ROS) scavenger catalase prevented hypoxia-induced AMPK activation. Moreover, hypoxia failed to activate AMPK in mitochondrion-deficient ρ0-A549 cells, suggesting that mitochondrial ROS play an essential role in hypoxia-induced AMPK activation. Hypoxia-induced PKCζ translocation to the plasma membrane and phosphorylation at Thr410 were prevented by the pharmacological inhibition of AMPK or by the overexpression of the AMPK-DN construct. We found that AMPK α phosphorylates PKCζ on residue Thr410 within the PKCζ activation loop. Importantly, the activation of AMPK α was necessary for hypoxia-induced AMPK-PKCζ binding in alveolar epithelial cells. The overexpression of T410A mutant PKCζ prevented hypoxia-induced Na,K-ATPase endocytosis, confirming that PKCζ Thr410 phosphorylation is essential for this process. PKCζ activation by AMPK is isoform specific, as small interfering RNA targeting the α1 but not the α2 catalytic subunit prevented PKCζ activation. Accordingly, we provide the first evidence that hypoxia-generated mitochondrial ROS lead to the activation of the AMPK α1 isoform, which binds and directly phosphorylates PKCζ at Thr410, thereby promoting Na,K-ATPase endocytosis.

Hypoxia Leads to Na,K-ATPase Downregulation via Ca2+ Release-Activated Ca2+ Channels and AMPK Activation

ABSTRACT To maintain cellular ATP levels, hypoxia leads to Na,K-ATPase inhibition in a process dependent on reactive oxygen species (ROS) and the activation of AMP-activated kinase α1 (AMPK-α1). We report here that during hypoxia AMPK activation does not require the liver kinase B1 (LKB1) but requires the release of Ca2+ from the endoplasmic reticulum (ER) and redistribution of STIM1 to ER-plasma membrane junctions, leading to calcium entry via Ca2+ release-activated Ca2+ (CRAC) channels. This increase in intracellular Ca2+ induces Ca2+/calmodulin-dependent kinase kinase β (CaMKKβ)-mediated AMPK activation and Na,K-ATPase downregulation. Also, in cells unable to generate mitochondrial ROS, hypoxia failed to increase intracellular Ca2+ concentration while a STIM1 mutant rescued the AMPK activation, suggesting that ROS act upstream of Ca2+ signaling. Furthermore, inhibition of CRAC channel function in rat lungs prevented the impairment of alveolar fluid reabsorption caused by hypoxia. These data suggest that during hypoxia, calcium entry via CRAC channels leads to AMPK activation, Na,K-ATPase downregulation, and alveolar epithelial dysfunction.

α1-AMP-activated protein kinase (AMPK) regulates hypoxia-induced Na,K-ATPase endocytosis via direct phosphorylation of PKCζ

ABSTRACT Hypoxia promotes Na,K-ATPase endocytosis via protein kinase Cζ (PKCζ)-mediated phosphorylation of the Na,K-ATPase α subunit. Here, we report that hypoxia leads to the phosphorylation of 5′-AMP-activated protein kinase (AMPK) at Thr172 in rat alveolar epithelial cells. The overexpression of a dominant-negative AMPK α subunit (AMPK-DN) construct prevented the hypoxia-induced endocytosis of Na,K-ATPase. The overexpression of the reactive oxygen species (ROS) scavenger catalase prevented hypoxia-induced AMPK activation. Moreover, hypoxia failed to activate AMPK in mitochondrion-deficient ρ0-A549 cells, suggesting that mitochondrial ROS play an essential role in hypoxia-induced AMPK activation. Hypoxia-induced PKCζ translocation to the plasma membrane and phosphorylation at Thr410 were prevented by the pharmacological inhibition of AMPK or by the overexpression of the AMPK-DN construct. We found that AMPK α phosphorylates PKCζ on residue Thr410 within the PKCζ activation loop. Importantly, the activation of AMPK α was necessary for hypoxia-induced AMPK-PKCζ binding in alveolar epithelial cells. The overexpression of T410A mutant PKCζ prevented hypoxia-induced Na,K-ATPase endocytosis, confirming that PKCζ Thr410 phosphorylation is essential for this process. PKCζ activation by AMPK is isoform specific, as small interfering RNA targeting the α1 but not the α2 catalytic subunit prevented PKCζ activation. Accordingly, we provide the first evidence that hypoxia-generated mitochondrial ROS lead to the activation of the AMPK α1 isoform, which binds and directly phosphorylates PKCζ at Thr410, thereby promoting Na,K-ATPase endocytosis.

High CO2 Levels Cause Skeletal Muscle Atrophy via AMP-activated Kinase (AMPK), FoxO3a Protein, and Muscle-specific Ring Finger Protein 1 (MuRF1)

Background: CO2 retention and skeletal muscle atrophy occur in patients with lung diseases and are associated with poor clinical outcomes. Results: Hypercapnia leads to AMPK/FoxO3a/MuRF1-dependent muscle fiber size reduction. Conclusion: Hypercapnia activates a signaling pathway leading to skeletal muscle atrophy. Significance: High CO2 levels directly activate a proteolytic program of skeletal muscle atrophy which is of relevance to patients with lung diseases. Patients with chronic obstructive pulmonary disease, acute lung injury, and critical care illness may develop hypercapnia. Many of these patients often have muscle dysfunction which increases morbidity and impairs their quality of life. Here, we investigated whether hypercapnia leads to skeletal muscle atrophy. Mice exposed to high CO2 had decreased skeletal muscle wet weight, fiber diameter, and strength. Cultured myotubes exposed to high CO2 had reduced fiber diameter, protein/DNA ratios, and anabolic capacity. High CO2 induced the expression of MuRF1 in vivo and in vitro, whereas MuRF1−/− mice exposed to high CO2 did not develop muscle atrophy. AMP-activated kinase (AMPK), a metabolic sensor, was activated in myotubes exposed to high CO2, and loss-of-function studies showed that the AMPKα2 isoform is necessary for muscle-specific ring finger protein 1 (MuRF1) up-regulation and myofiber size reduction. High CO2 induced AMPKα2 activation, triggering the phosphorylation and nuclear translocation of FoxO3a, and leading to an increase in MuRF1 expression and myotube atrophy. Accordingly, we provide evidence that high CO2 activates skeletal muscle atrophy via AMPKα2-FoxO3a-MuRF1, which is of biological and potentially clinical significance in patients with lung diseases and hypercapnia.