Hypercapnia (elevated CO2 levels) occurs as a consequence of poor alveolar ventilation and impairs alveolar fluid reabsorption (AFR) by promoting Na,K-ATPase endocytosis. We studied the mechanisms regulating CO2-induced Na,K-ATPase endocytosis in alveolar epithelial cells (AECs) and alveolar epithelial dysfunction in rats. Elevated CO2 levels caused a rapid activation of AMP-activated protein kinase (AMPK) in AECs, a key regulator of metabolic homeostasis. Activation of AMPK was mediated by a CO2-triggered increase in intracellular Ca2+ concentration and Ca2+/calmodulin-dependent kinase kinase-β (CaMKK-β). Chelating intracellular Ca2+ or abrogating CaMKK-β function by gene silencing or chemical inhibition prevented the CO2-induced AMPK activation in AECs. Activation of AMPK or overexpression of constitutively active AMPK was sufficient to activate PKC-ζ and promote Na,K-ATPase endocytosis. Inhibition or downregulation of AMPK via adenoviral delivery of dominant-negative AMPK-α1 prevented CO2-induced Na,K-ATPase endocytosis. The hypercapnia effects were independent of intracellular ROS. Exposure of rats to hypercapnia for up to 7 days caused a sustained decrease in AFR. Pretreatment with a β-adrenergic agonist, isoproterenol, or a cAMP analog ameliorated the hypercapnia-induced impairment of AFR. Accordingly, we provide evidence that elevated CO2 levels are sensed by AECs and that AMPK mediates CO2-induced Na,K-ATPase endocytosis and alveolar epithelial dysfunction, which can be prevented with β-adrenergic agonists and cAMP.
István Vadász, Laura A. Dada, Arturo Briva, Humberto E. Trejo, Lynn C. Welch, Jiwang Chen, Péter T. Tóth, Emilia Lecuona, Lee A. Witters, Paul T. Schumacker, Navdeep S. Chandel, Werner Seeger, Jacob I. Sznajder