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This Article Full Text Full Text (PDF) All Versions of this Article: 292/3/G905    most recent 00386.2006v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (6) Citing Articles via Google Scholar Google Scholar Articles by Carreras, F. I. Articles by Marinelli, R. A. Search for Related Content PubMed PubMed Citation Articles by Carreras, F. I. Articles by Marinelli, R. A.

LIVER AND BILIARY TRACT

Defective hepatocyte aquaporin-8 expression and reduced canalicular membrane water permeability in estrogen-induced cholestasis

¹Instituto de Fisiología Experimental and ³Instituto de Biología Molecular y Celular de Rosario, Consejo Nacional de Investigaciones Científicas y Técnicas, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Rosario, Argentina; ²Dipartimento di Zoologia and ⁴Dipartimento di Fisiologia Generale ed Ambientale, Università degli Studi di Bari, Bari, Italy

Submitted 18 August 2006 ; accepted in final form 13 November 2006

Our previous work supports a role for aquaporin-8 (AQP8) water channels in rat hepatocyte bile formation mainly by facilitating the osmotically driven canalicular secretion of water. In this study, we tested whether a condition with compromised canalicular bile secretion, i.e., the estrogen-induced intrahepatic cholestasis, displays defective hepatocyte AQP8 functional expression. After 17-ethinylestradiol administration (5 mg·kg body wt^–1·day^–1 for 5 days) to rats, the bile flow was reduced by 58% (P < 0.05). By subcellular fractionation and immunoblotting analysis, we found that 34 kDa AQP8 was significantly decreased by 70% in plasma (canalicular) and intracellular (vesicular) liver membranes. However, 17-ethinylestradiol-induced cholestasis did not significantly affect the protein level or the subcellular localization of sinusoidal AQP9. Immunohistochemistry for liver AQPs confirmed these observations. Osmotic water permeability (P_f) of canalicular membranes, measured by stopped-flow spectrophotometry, was significantly reduced (73 ± 1 vs. 57 ± 2 µm/s) in cholestasis, consistent with defective canalicular AQP8 functional expression. By Northern blotting, we found that AQP8 mRNA expression was increased by 115% in cholestasis, suggesting a posttranscriptional mechanism of protein level reduction. Accordingly, studies in primary cultured rat hepatocytes indicated that the lysosomal protease inhibitor leupeptin prevented the estrogen-induced AQP8 downregulation. In conclusion, hepatocyte AQP8 protein expression is downregulated in estrogen-induced intrahepatic cholestasis, presumably by lysosomal-mediated degradation. Reduced canalicular membrane AQP8 expression is associated with impaired osmotic membrane water permeability. Our data support the novel notion that a defective expression of canalicular AQP8 contributes as a mechanism for bile secretory dysfunction of cholestatic hepatocytes.

aquaporins; intrahepatic cholestasis; water transport; liver