Transforming growth factor-beta1 decreases epithelial sodium channel functionality in renal collecting duct cells via a Smad4-dependent pathway.

BACKGROUND: Transformation growth factor-beta1 (TGF-beta1) inhibits transepithelial sodium transport and suppresses the epithelial sodium channel (ENaC) in many different types of epithelial cells; however, the molecular mechanism of this effect in the kidney is still not clear. The aim of this study was to examine the regulation of transepithelial sodium transport by TGF-beta1 in renal cells.
METHODS: We derived stable mouse cortical collecting duct cell lines that overexpressed Smad4 or N-termianl truncated Smad4, and studied the effects of TGF-beta1 on them. The equivalent electrical current (I(eq)) was taken as representing transepithelial current and the amiloride sensitive short circuit current (AmsIsc) as representing the ENaC activity. We used real-time PCR to quantify the expression of ENaC and measurement of the luciferase activity of cells transiently transfected with a mouse alpha-ENaC promoter to assess the alpha-ENaC promoter activity. Result. The administration of TGF-beta1 decreased I(eq), mainly as a result of the decrease of AmsIsc, and it correlated with inhibition of the alpha-ENaC mRNA expression. The overexpression of Smad4 led to a decrease in AmsIsc, alpha-ENaC mRNA and alpha-ENaC promoter activity, but the overexpression of the N-terminal truncated Smad4 did not induce these changes. The TGF-beta1-induced reduction of AmsIsc was alleviated in the N-terminal truncated Smad4-overexpressed cells.
CONCLUSION: It appears that the N-terminus region of Smad4 is indispensable in Smad4-mediated inhibition of the transepithelial sodium transport. TGF-beta1 may decrease the ENaC functionality via a Smad4-dependent pathway.