ABSTRACT
Overexpression of several aquaporins has been reported in different types of human cancer but the role of AQPs in human carcinogenesis has not yet been clearly defined. Here, we demonstrate that ectopic expression of human AQP5 (hAQP5), a water channel expressed in lung, salivary glands, and kidney, induces many phenotypic changes characteristic of transformation both in vitro and in vivo. Furthermore, the cell proliferative ability of AQP5 appears to be dependent upon the phosphorylation of a cAMP-protein kinase (PKA) consensus site located in a cytoplasmic loop of AQP5. In addition, phosphorylation of the PKA consensus site was found to be phosphorylated preferentially in tumors. These findings altogether indicate that hAQP5 plays an important role in human carcinogenesis and, furthermore, provide an attractive therapeutic target.
Subject(s)
Aquaporin 5/metabolism , Cell Transformation, Neoplastic , Neoplasms/drug therapy , Neoplasms/metabolism , Proto-Oncogene Proteins/metabolism , Animals , Aquaporin 5/drug effects , Aquaporin 5/genetics , Cell Line, Tumor , Cell Proliferation/drug effects , Cell Transformation, Neoplastic/genetics , Humans , Mice , NIH 3T3 Cells , Neoplasms/genetics , Phosphorylation , Proto-Oncogene Proteins/drug effects , RNA, Small Interfering/pharmacology , Up-Regulation/geneticsABSTRACT
Phosphorylation pathway has been identified as an important step in membrane trafficking for AQP5. We generated stably transfected BEAS-2B human bronchial epithelial cells with various over-expression constructs on permeable support. In stable cells with wild-type AQP5 and S156A (AQP5 mutant targeting PKA consensus sequence), AQP5 expression was predominantly polarized to the apical membrane, whereas stable cells with N185D (AQP5 mutant targeting second NPA motif), mainly localized to the cytoplasm. Treatment with H89 and/or chlorophenylthio-cAMP (cpt-cAMP) did not affect membrane expression of AQP5 in any of three stable cells. In cells with wild-type AQP5 and N185D, AQP5s were phosphorylated by PKA, while phosphorylation of AQP5 was not detected in cells with S156A. These results indicate that, in AQP5, serine156 may be phosphorylated by PKA, but membrane expression of AQP5 may not be regulated by PKA phosphorylation. We conclude that AQP5 membrane targeting can include more than one mechanism besides cAMP dependent phosphorylation.
