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.

ACKNOWLEDGMENTS

We thank G. R. Scott Budinger and Angel Gonzalez for their help with the measurement of adenine nucleotide concentrations.

This work was supported in part by NIH grants PO1-HL071643 (Project 1) and NIH grants RO1CA123067 and PO1-HL071643 (Project 3).