Differential Regulation of Rat Aquaporin-5 Promoter/Enhancer Activities in Lung and Salivary Epithelial Cells

Aquaporin-5 (AQP5) is a water channel protein that is selectively expressed in respiratory, salivary, and lacrimal tissues. In order to establish the tissue-specific transcriptional programs that underlie its lung- and salivary-specific expression, a 4.5-kilobase pair DNA fragment encompassing the 5′-flanking region of the rat AQP5 gene has been characterized in detail. A major transcription start site utilized in lung and salivary glands has been localized downstream of a TATAA-like motif. Transient transfection assays of –4.3- and –1.7- AQP5 -luciferase constructs in AQP5-expressing lung (MLE-15) and salivary (Pa-4) cells and nonexpressing fibroblast (NIH3T3) and epithelial (HeLa) cells demonstrate preferential transcriptional enhancement of reporter activities in MLE-15 and Pa-4 cells. Transient transfection assays of a series of 5′ → 3′ deletion constructs of –4.3- AQP5 -luciferase suggest that a common salivary and lung enhancer is located between nucleotides –274 and –139, and a lung-specific enhancer is located between nucleotides –894 and –710. There is one putative lung-specific repressor located in the region of nucleotides –1003/–894 and a common lung and salivary repressor located at nucleotides –503/–385. Moreover, 3′ → 5′ deletions up to –171 and –127 base pairs almost abolish transcriptional activation in salivary and lung cells, respectively. Together, our findings indicate that the combination of enhancer/repressor elements within the proximal 5′-flanking region of rat AQP5 gene dictates its restricted expression in both lung and salivary cells.