Protein kinase A and mitogen-activated protein kinase pathways mediate cAMP induction of α-epithelial Na⁺ channels (α-ENaC)

A major mechanism for Na⁺ transport across epithelia occurs through epithelial Na⁺ channels (ENaC). ENaC is a multimeric channel consisting of three subunits (α, β, and γ). The α-subunit is critical for ENaC function. In specific culture conditions, the rat submandibular gland epithelial cell line (SMG-C6) demonstrates minimal Na ⁺ transport properties and exposure to dibutyryl cAMP (DbcAMP) for up to 48 h caused an elevation of α-ENaC mRNA and protein expression and amiloride-sensitive short-circuit current (I_SC). Here we examined the early signaling pathways evoked by DbcAMP which contribute to the eventual increase in Na⁺ transport. Treatment with either of the protein kinase A (PKA) inhibitors KT5720 or H-89 followed by exposure to 1 mM DbcAMP for 24 h markedly attenuated DbcAMP-induced α-ENaC protein formation and I_SC. Exposure of SMG-C6 cells to 1 mM DbcAMP induced a rapid, transient phosphorylation of the cAMP response element binding protein (CREB). This response was attenuated in the presence of either KT5720 or H-89. Dominant-negative CREB decreased DbcAMP-induced α-ENaC expression. Suppression of the extracellular signal-regulated protein kinase (ERK 1,2) with PD98059 or the p38 mitogen-activated protein kinase (MAPK) pathway with SB203580 reduced DbcAMP-induced α-ENaC protein levels in SMG-C6 cells. DbcAMP-induced phosphorylation of CREB was markedly attenuated by PD98059 or SB203580. DbcAMP-induced activation of the either the p38 or the ERK 1,2 MAPK pathways was abolished by either of the PKA inhibitors, H-89 or KT5720. Cross talk between these signaling pathways induced by DbcAMP via the activation of CREB appears to contribute to increased levels of α-ENaC observed after 24 h of treatment in SMG-C6 epithelial cells.