ENaC activity and expression is decreased in the lungs of protein kinase C-α knockout mice

We used a PKC-alpha knockout model to investigate the regulation of alveolar epithelial Na+ channels (ENaC) by PKC. Primary alveolar type II (ATII) cells were subjected to cell-attached patch clamp. In the absence of PKC-alpha, the open probability (P-o) of ENaC was decreased by half compared with wild-type mice. The channel density (N) was also reduced in the knockout mice. Using in vivo biotinylation, membrane localization of all three ENaC subunits (alpha, beta, and gamma) was decreased in the PKC-alpha knockout lung, compared with the wild-type. Confocal microscopy of lung slices showed elevated levels of reactive oxygen species (ROS) in the lungs of the PKC-alpha knockout mice vs. the wild-type. High levels of ROS in the knockout lung can be explained by a decrease in both cytosolic and mitochondrial superoxide dismutase activity. Elevated levels of ROS in the knockout lung activates PKC-delta and leads to reduced dephosphorylation of ERK1/2 by MAP kinase phosphatase, which in turn causes increased internalization of ENaC via ubiquitination by the ubiquitin-ligase Nedd4-2. In addition, in the knockout lung, PKC-delta activates ERK, causing a decrease in ENaC density at the apical alveolar membrane. PKC-delta also phosphorylates MARCKS, leading to a decrease in ENaC P-o. The effects of ROS and PKC-delta were confirmed with patch-clamp experiments on isolated ATII cells in which the ROS scavenger, Tempol, or a PKC-delta-specific inhibitor added to patches reversed the observed decrease in ENaC apical channel density and P-o. These results explain the decrease in ENaC activity in PKC-alpha knockout lung.