Phosphorylation and ubiquitination are necessary for Na,K-ATPase endocytosis during hypoxia

As a cellular adaptative response, hypoxia decreases Na,K-ATPase activity by triggering the endocytosis of its alpha(1) subunit in alveolar epithelial cells. Here, we present evidence that the ubiquitin conjugating system is important in the Na,K-ATPase endocytosis during hypoxia and that ubiquitination of Na,K-ATPase alpha(1) subunit occurs at the basolateral membrane. Endocytosis and ubiquitination were prevented when the Ser 18 in the PKC phosphorylation motif of the Na,K-ATPase alpha(1) subunit was mutated to an alanine, suggesting that phosphorylation at Ser-18 is required for ubiquitination. Mutation of the four lysines surrounding Ser 18 to arginine prevented Na,K-ATPase ubiquitination and endocytosis during hypoxia; however, only one of them was sufficient to restore hypoxia-induced endocytosis. We provide evidence that ubiquitination plays an important role in cellular adaptation to hypoxia by regulating Na,K-ATPase alpha(1)-subunit endocytosis.