Hypertonicity Regulates the Aquaporin-2 Promoter Independently of Arginine Vasopressin
| Title: | Hypertonicity Regulates the Aquaporin-2 Promoter Independently of Arginine Vasopressin |
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| Authors: | Kasono, Keizo; Saito, Takako; Saito, Tomoyuki; Tamemoto, Hiroyuki; Yanagidate, Chieko; Uchida, Shinichi; Kawakami, Masanobu; Sasaki, Sei; Ishikawa, San-e |
| Publisher: | Nephrology Dialysis Transplantation |
| Date Published: | January 01, 2005 |
| Reference Number: | 684 |
BACKGROUND: Aquaporin-2 (AQP-2) is an arginine vasopressin (AVP)-regulated water channel in kidney collecting duct cells. The present study was undertaken to determine whether a change in tonicity could directly regulate the AQP-2 gene in an in vitro experiment. METHODS: Various fragments of the 5'-flanking region of the murine AQP-2 gene up to -9.5 kb were cloned into a luciferase (Luc) reporter plasmid, and they were transiently transfected into Madin-Darby canine kidney cells. RESULTS: Hypertonicity significantly increased the Luc activity of the constructs containing >6.1 kb of the 5'-flanking region of the AQP-2 gene (-6.1AQP2). However, promoter regions <4.3 kb in length containing the tonicity-responsive enhancer (TonE) at bp -570 to -560 were not stimulated by hypertonicity. The TonE-deleted construct which contains -9.5 to -1.1 kb of the 5' side of the AQP-2 gene, 8.4AQP2, was also stimulated by hypertonicity. Mitogen-activated protein (MAP) kinase inhibitors SB203580 and U0126 did not affect the Luc activity of -6.1AQP2 induced by hypertonicity. In addition, the vector expressing dominant-negative TonE-binding protein (TonEBP) did not affect the hypertonicity-induced Luc activity of -6.1AQP2. The Luc activity of -6.1AQP2 was stimulated by the overexpression of TonEBP. Hypertonicity further increased the Luc activity of -6.1AQP2 under the overexpression of TonEBP. CONCLUSION: These findings indicate that hypertonicity regulates AQP-2 promoter activity via an AVP-independent mechanism, and that the tonicity-responsive element resides between the -6.1 and -4.3 kb 5'-flanking region of the AQP-2 gene, in which the structure and mechanism of response to hypertonicity could be distinct from those of TonE.
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This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)
In this study, Kasono, et al., investigated as to whether AQP2 was regulated by other bodily mechanisms in addition to AVP. In specific, they investigated as to whether the body's osmotic condition had a controlling function in the AQP2 gene's synthesis of AQP2. Osmosis is the passage of a pure solvent across a membrane from a solution of lesser to one of greater solute concentration. That is, in osmosis, the flow of the solvent is from the less concentrated to the more concentrated solution so that the solutions on either side of the membrane become more balanced in relation to each other. By bathing laboratory cell cultures containing AQP2 genes in a hypertonic solution, i.e., one that caused a net flow of water across the cell membrane out of the cell, the researchers could detect whether the condition of hypertonicity had a regulating effect on the AQP2 gene's synthesis of AQP2.
After exposing the cell culture to a hypertonic solution for six hours, the researchers saw an increase in AQP2s. Further refining their search, they discovered that the part of the AQP2 gene that was stimulated by the hypertonic conditions was its promoter region, that segment of DNA located upstream from the gene's coding region. The promoter region acts as a controlling element in the synthesis of the gene.
Thus, the researchers provided evidence that hypertonicty directly regulates the expression of AQP2. Though this dynamic is independent from AVP-induced regulation of AQP2 expression, it has a synergistic effect with it in upping the number of AQP2s found in the body.