AICAR activates AMPK and alters PIP2 association with the epithelial sodium channel ENaC to inhibit Na+ transport in H441 lung epithelial cells

Changes in amiloride-sensitive epithelial Na+ channel (ENaC) activity (NPo) in the lung lead to pathologies associated with dysregulation of lung fluid balance. UTP activation of purinergic receptors and hydrolysis of PIP2 via activation of phospholipase C (PLC) or AICAR activation of AMP-activated protein kinase (AMPK) inhibited amiloride-sensitive Na+ transport across human H441 epithelial cell monolayers. Neither treatment altered alpha, beta or gamma ENaC subunit abundance (N) in the apical membrane indicating that the mechanism of inhibition was via a change in channel open state probability (P-o). We found that UTP depleted PIP2 abundance in the apical membrane whilst activation of AMPK prevented the binding of beta and gamma ENaC subunits to PIP2. The association of PIP2 with the ENaC subunits is required to maintain channel activity via P-o. Thus, these data show for the first time that AICAR activation of AMPK inhibits Na+ transport via a mechanism that perturbs the PIP2-ENaC channel interaction to alter P-o. In addition, we show that dissociation of PIP2 from ENaC together with activation of AMPK further reduced Na+ transport by a secondary effect that correlated with ENaC subunit internalization. Thus, when PIP2-ENaC subunit interactions were compromised, ENaC protein retrieval was initiated, indicating that AMPK can modulate ENaC P-o and N.