Author:
Carreras Flavia I.,Lehmann Guillermo L.,Ferri Domenico,Tioni Mariana F.,Calamita Giuseppe,Marinelli Raúl A.
Abstract
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 17α-ethinylestradiol administration (5 mg·kg body wt−1·day−1for 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, 17α-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 ( Pf) 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.
Publisher
American Physiological Society
Subject
Physiology (medical),Gastroenterology,Hepatology,Physiology
Cited by
44 articles.
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