Stimulation of the epithelial sodium channel (ENaC) by cAMP involves putative ERK phosphorylation sites in the C termini of the channel's β- and γ-subunit

The mechanisms involved in the regulation of the epithelial sodium channel ( ENaC) via the cAMP pathway are not yet completely understood. The aim of the present study was to investigate cAMP-mediated ENaC regulation in Xenopus laevis oocytes heterologously expressing the three subunits (alpha beta gamma) of rat ENaC and to determine the ENaC regions important for mediating the stimulatory effect of cAMP. In oocytes treated for about 24 h with 1 mM 3-isobutyl-1-methylxanthine ( IBMX) and 1 mu M forskolin (FSK) so as to increase intracellular cAMP, the amiloride-sensitive whole cell current (Delta I-Ami) was on average 10-fold larger than Delta I-Ami in matched control oocytes. This effect on Delta I-Ami was paralleled by an increase in ENaC surface expression caused by a reduced rate of ENaC retrieval. In addition, IBMX/FSK also enhanced ENaC open probability from about 0.2 to 0.5. The stimulatory effect of IBMX/FSK was dependent on the presence of intact PY motifs in the C termini of the channel. Mutagenesis of putative protein kinase A and CK-2 consensus motifs in the cytosolic domains of the channel did not reveal critical sites involved in mediating the stimulatory effect of IBMX/FSK. In contrast, site-directed mutagenesis of two putative ERK-consensus motifs (T613A in beta ENaC and T623A in gamma ENaC) largely reduced the stimulatory effect of IBMX/FSK. Phosphorylation of these ERK sites has previously been reported to enhance the interaction of ENaC and Nedd4 (Shi, H., Asher, C., Chigaev, A., Yung, Y., Reuveny, E., Seger, R., and Garty, H. ( 2002) J. Biol. Chem. 277, 13539 - 13547). Using co-expression experiments we demonstrated that mutating the two ERK sites attenuates the inhibitory effect of Nedd4-2 on ENaC currents. We conclude that an increase in intracellular cAMP favors the dephosphorylation of the two ERK sites, which reduces channel retrieval and increases PO by modulating ENaC/Nedd4 interaction. This defines a novel regulatory pathway likely to be relevant for cAMP-induced stimulation of ENaC in vivo.