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Am J Physiol Lung Cell Mol Physiol 281: L1219-L1231, 2001;
1040-0605/01 $5.00
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Vol. 281, Issue 5, L1219-L1231, November 2001

Translational activation and repression by distinct elements within the 5'-UTR of ENaC alpha -subunit mRNA

Gail Otulakowski1,2, Tanya Freywald2, Yanxia Wen2, and Hugh O'Brodovich1,2,3

2 Canadian Institutes of Health Research Group in Lung Development, Research Institute, Hospital for Sick Children, and Departments of 1 Paediatrics and 3 Physiology, University of Toronto, Ontario M5G 1X8, Canada

The rat amiloride-sensitive epithelial Na+ channel (rENaC), the rate-limiting step in epithelial Na+ transport, consists of three subunits, alpha , beta , and gamma . We hypothesized that alpha -rENaC translation is regulated via its 5'-untranslated region (UTR). Transient transfections of alpha -rENaC promoter-reporter constructs in representative epithelial cell lines demonstrated up to fivefold differences in activity among constructs containing different amounts of the alpha -rENaC 5'-UTR sequence. Differences in reporter protein activity did not parallel differences in reporter mRNA, demonstrating that 5'-UTR regulation must be at the level of translation. Specifically, translation was enhanced by a region extending from +53 to +211 bp downstream from the transcription start site and repressed by the region between +367 and +499 bp. Examination of the 5'-UTR sequence revealed an out-of-frame initiation codon within the repressive region, 43 bp upstream from the start of the alpha -rENaC open reading frame. Mutational analysis of this upstream start codon indicated that it plays, at most, a minor role in impeding translation both in vitro and in vivo, suggesting that additional mechanisms of translational regulation are operative.

rat epithelial sodium channel; 5'-untranslated region; lung fluid; ion transport; translational regulation


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