Differential regulation of rat aquaporin-5 promoter/enhancer activities inlung 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 salivar y-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 m ajor 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-1 5) and salivary (Pa-4) cells and nonexpressing fibroblast (NIH3T3) and epit helial (HeLa) cells demonstrate preferential transcriptional enhancement of reporter activities in MLE-15 and Pa-4 cells. Transient transfection assay s of a series of 5' -> 3' deletion constructs of -4.3-AQP5-luciferase sugge st 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 a nd -127 base pairs almost abolish transcriptional activation in salivary an d lung cells, respectively. Together, our findings indicate that the combin ation of enhancer/repressor elements within the proximal 5'-flankinng regio n of rat AQP5 gene dictates its restricted expression in both lung and sali vary cells.