Acute Downregulation of ENaC by EGF Involves the PY Motif and Putative ERK Phosphorylation Site

The epithelial sodium channel (ENaC) is expressed in a variety of tissues, including the renal collecting duct, where it constitutes the rate-limiting step for sodium reabsorption. Liddle's syndrome is caused by gain-of-function mutations in the β and γ subunits of ENaC, resulting in enhanced Na reabsorption and hypertension. Epidermal growth factor (EGF) causes acute inhibition of Na absorption in collecting duct principal cells via an extracellular signal–regulated kinase (ERK)–dependent mechanism. In experiments with primary cultures of collecting duct cells derived from a mouse model of Liddle's disease (β-ENaC truncation), it was found that EGF inhibited short-circuit current (Isc) by 24 ± 5% in wild-type cells but only by 6 ± 3% in homozygous mutant cells. In order to elucidate the role of specific regions of the β-ENaC C terminus, Madin-Darby canine kidney (MDCK) cell lines that express β-ENaC with mutation of the PY motif (P616L), the ERK phosphorylation site (T613A), and C terminus truncation (R564stop) were created using the Phoenix retroviral system. All three mutants exhibited significant attenuation of the EGF-induced inhibition of sodium current. In MDCK cells with wild-type β-ENaC, EGF-induced inhibition of Isc (<30 min) was fully reversed by exposure to an ERK kinase inhibitor and occurred with no change in ENaC surface expression, indicative of an effect on channel open probability (P_o). At later times (>30 min), EGF-induced inhibition of Isc was not reversed by an ERK kinase inhibitor and was accompanied by a decrease in ENaC surface expression. Our results are consistent with an ERK-mediated decrease in ENaC open probability and enhanced retrieval of sodium channels from the apical membrane.