To study a cAMP-mediated signaling pathway in the regulation of amiloride-sensitive Na+ transport in rat fetal distal lung epithelial cells, we measured an amiloride-sensitive short-circuit current (Na+ transport). Forskolin, which increases the cytosolic cAMP concentration, stimulated the Na+ transport. Forskolin also activated cAMP-dependent protein kinase (PKA). A β-adrenergic agonist and cAMP mimicked the forskolin action. PKA inhibitors KT-5720, H-8, and myristoylated PKA-inhibitory peptide amide-(14—22) did not influence the forskolin action. These results suggest that forskolin stimulates Na+ transport through a PKA-independent pathway. Furthermore, forskolin increased tyrosine phosphorylation of ∼70- to 80-, ∼97-, and ∼110- to 120-kDa proteins. Protein tyrosine kinase (PTK) inhibitors (tyrphostin A23 and genistein) abolished the forskolin action. Moreover, 5-nitro-2-(3-phenylpropylamino)benzoate (a Cl−-channel blocker) prevented the stimulatory action of forskolin on Na+ transport via abolishment of the forskolin-induced cell shrinkage and tyrosine phosphorylation. Based on these results, we conclude that forskolin (and cAMP) stimulates Na+ transport in a PTK-dependent but not a PKA-dependent pathway by causing cell shrinkage, which activates PTK in rat fetal distal lung epithelial cells.
33 Sodium 4-phenylbutyrate induces IL-8 expression in CF lung epithelial cells through an ERK1/2-dependent pathway
