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Hypercapnia-induces IRE1α-driven Endoplasmic Reticulum-associated Degradation of the Na,K-ATPase β-subunit.

Acute respiratory distress syndrome (ARDS) is often associated with elevated levels of CO2 (hypercapnia) and impaired alveolar fluid clearance. Misfolding of the Na,K-ATPase (NKA), a key molecule involved in both alveolar epithelial barrier tightness and in resolution of alveolar edema, in the endoplasmic reticulum (ER) may decrease plasma membrane (PM) abundance of the transporter. Here, we investigated how hypercapnia affects the NKA β-subunit (NKA-β) in the ER. Exposing murine precision-cut lung slices (PCLS) and human alveolar epithelial A549 cells to elevated CO2 levels led to a rapid decrease of NKA-β abundance in the ER and at the cell surface. Knockdown of ER alpha-mannosidase I (MAN1B1) and ER degradation enhancing alpha-mannosidase like protein 1 by siRNA or treatment with the MAN1B1 inhibitor, kifunensine rescued loss of NKA-β in the ER, suggesting ER-associated degradation (ERAD) of the enzyme. Furthermore, hypercapnia activated the unfolded protein response (UPR) by promoting phosphorylation of inositol-requiring enzyme 1α (IRE1α) and treatment with a siRNA against IRE1α prevented the decrease of NKA-β in the ER. Of note, the hypercapnia-induced phosphorylation of IRE1α was triggered by a Ca2+-dependent mechanism. Additionally, inhibition of the inositol trisphosphate receptor decreased phosphorylation levels of IRE1α in PCLS and A549 cells, suggesting that Ca2+ efflux from the ER might be responsible for IRE1α activation and ERAD of NKA-β. In conclusion, here we provide evidence that hypercapnia attenuates maturation of the regulatory subunit of NKA by activating IRE1α and promoting ERAD, which may contribute to impaired alveolar epithelial integrity in patients with ARDS and hypercapnia.