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.


CONCLUSIONS
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.
Department of Nephrology, Chang Gung Memorial Hospital, 5 Fu-Shin Street, Kwei-shan 333, Taiwan.
Chiz-Tzung
CT
Chang
Cheng-Chieh
CC
Hung
Yung-Chang
YC
Chen
Tzung-Hai
TH
Yen
Mai-Szu
MS
Wu
Chih-Wei
CW
Yang
Aled
A
Phillips
Ya-Chung
YC
Tian
D000818:Animals; D015153:Blotting, Western; D002292:Carcinoma, Renal Cell / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D045744:Cell Line, Tumor; D004791:Enzyme Inhibitors / Q000494:pharmacology; D062686:Epithelial Sodium Channel Blockers; D053503:Epithelial Sodium Channels / Q000235:genetics* / Q000378:metabolism; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects*; D017136:Ion Transport / Q000187:drug effects; D007680:Kidney Neoplasms / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D007685:Kidney Tubules, Collecting / Q000378:metabolism* / Q000473:pathology; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D008856:Microscopy, Fluorescence; D009374:Neoplasms, Experimental / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D018408:Patch-Clamp Techniques; D016133:Polymerase Chain Reaction; D011401:Promoter Regions, Genetic; D012334:RNA, Neoplasm / Q000235:genetics*; D051901:Smad4 Protein / Q000187:drug effects / Q000235:genetics*; D012964:Sodium / Q000378:metabolism*; D053773:Transforming Growth Factor beta1 / Q000494:pharmacology*; D019459:Urothelium / Q000378:metabolism / Q000473:pathology
D004791:Enzyme Inhibitors; D062686:Epithelial Sodium Channel Blockers; D053503:Epithelial Sodium Channels; D012334:RNA, Neoplasm; D051901:Smad4 Protein; C490810:Smad4 protein, mouse; D053773:Transforming Growth Factor beta1; D012964:Sodium