Regulation of Aquaporin-4 Water Channels by Phorbol Ester-dependent Protein Phosphorylation*
- From the Department of Ophthalmology and Visual Sciences, Washington University School of Medicine, St. Louis, Missouri 63110
Abstract
The molecular mechanisms for regulating water balance in many tissues are unknown. Like the kidney, the eye contains multiple water channel proteins (aquaporins) that transport water through membranes, including two (AQP1 and AQP4) in the ciliary body, the site of aqueous humor production. However, because humans with defective AQP1 are phenotypically normal and because the ocular application of phorbol esters reduce intraocular pressure, we postulated that the water channel activity of AQP4 may be regulated by these agents. We now report that protein kinase C activators, phorbol 12,13-dibutyrate, and phorbol 12-myristate 13-acetate strongly stimulate the phosphorylation of AQP4 and inhibit its activity in a dose-dependent manner. Phorbol 12,13-dibutyrate (10 μm) and phorbol 12-myristate 13-acetate (10 nm) reduced the rate of AQP4-expressing oocyte swelling by 87 and 92%, respectively. Further, phorbol 12,13-dibutyrate significantly increased the amount of phosphorylated AQP4. These results demonstrate that protein kinase C can regulate the activity of AQP4 through a mechanism involving protein phosphorylation. Moreover, they suggest important potential roles for AQP4 in several clinical disorders involving rapid water transport such as glaucoma, brain edema, and swelling of premature infant lungs.
Footnotes
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↵* This work was supported by Grants EY10423 (to R. V. P.) and EY06810 (to M. B. W) from the National Eye Institute, a Core Grant for Vision Research, Grant EY02687 from the National Eye Institute, and an unrestricted grant from Research to Prevent Blindness, Inc.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‡ To whom correspondence and reprint requests should be addressed: Dept. of Ophthalmology and Visual Sciences, Washington University School of Medicine, 660 South Euclid, St. Louis, MO 63110. Tel.: 314-362-3770; Fax: 314-362-3638; E-mail:patil{at}am.seer.wustl.edu.
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↵1 The abbreviations used are: PKC, protein kinase C; PDBu, phorbol 12,13-dibutyrate; PMA, phorbol 12-myristate 13-acetate; 4α-PDD, 4α-phorbol 12,13-didecanoate; PAGE, polyacrylamide gel electrophoresis.
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- Received December 19, 1997.
- Revision received January 17, 1998.
- The American Society for Biochemistry and Molecular Biology, Inc.
