Characterization of the Rat NHE3 Promoter

NHE3, a transmembrane protein involved in transcellular ion transport, is expressed in the apical membrane of renal and gastrointestinal epithelia. Chronic regulation by multiple stimuli, including glucocorticoid-induced transcriptional regulation, has been demonstrated. To study the tissue-specific expression and transcriptional regulation of NHE3, the 5' flanking region of the rat NHE3 gene was cloned. Two genomic libraries were screened with the 5' end of the NHE3 cDNA. The 5' flanking region and first exon were isolated. Primer extension mapped a single transcription start site in stomach, colon and kidney. The NHE3 promoter near the transcription initiation site is characterized by the absence of TATA and CAAT sequences. Two Sp1 sites, one Egr-1 site, and an initiator with the sequence GGGATTAAA mark the area of transcription initiation. Upstream sequences include multiple DNA sequence elements recognized by the glucocorticoid and thyroid receptors, Sp1, atriopeptin-2, and several other transcription factors. Transcriptional regulation by glucocorticoids and chronic acidosis was demonstrated. Promoter activity was present in OKP cells, a renal proximal tubule cell line, but not in fibroblasts. This suggests that the NHE3 promoter contains elements conferring epithelial cell-specific expression.