Submitted on May 30, 2008
Revised on February 19, 2009
Accepted on March 12, 2009
BIPHASIC REGULATION OF ENaC BY TGF
AND EGF IN RENAL EPITHELIAL CELLS
Lian Liu1*, Billie Jeanne Duke2, Bela Malik3, Qiang Yue1, and Douglas C Eaton1
1 Emory University
2 Emory University Medical School
3 Emory
* To whom correspondence should be addressed. E-mail: lliu6{at}emory.edu.
The epithelial sodium channel (ENaC) is regulated by epidermal growth factor (EGF). We investigate whether ENaC is regulated by another EGF receptor (EGFR) ligand, transforming growth factor-alpha (TGF
). We show that chronic (24 h) treatment with TGF inhibits ENaC in Xenopus kidney cells 20 times more strongly than EGF. By using single channel measurements, we show that TGF significantly reduces the number of ENaC per patch. The open probability (Po) is unchanged by 24 hour treatment with TGF. , 
and 
ENaC mRNA levels are significantly reduced by TGF or EGF. TGF or EGF reduces and ENaC proteins in the membrane; however, ENaC is unchanged. TGF or EGF inhibits ENaC by activating EGFR since the EGFR inhibitor, AG1478, blocks the effects of both. The mitogen-activated or extracellular signal-regulated protein kinase 1/2 inhibitor, U0126, also blocks the effect of TGF or EGF on ENaC, indicating that MAPK1/2 pathway is involved in the TGF or EGF-induced inhibition of ENaC. Interestingly, acute treatment (<1h) with TGF or EGF does not inhibit ENaC current; it enhances ENaC activity by increasing Po. Pre-treatment of the cells with U0126 potentiates the acute TGF or EGF-induced stimulation of ENaC. This TGF or EGF-induced increase in sodium current is abolished by a phosphatidylinositol 3-kinase (PI-3 kinase) inhibitor, LY294002, suggesting that PI-3 kinase is involved in the activation of sodium transport. In conclusion, chronic treatment with TGF or EGF inhibits ENaC by decreasing the number of channels in the membrane transcriptionally through MAPK1/2 pathways; but acute treatment with TGF or EGF activates ENaC by increasing Po via PI-3 kinase.
