Although bacterial lipopolysaccharides (LPS) are known to cause cholestasis in sepsis, the molecular mechanisms accounting for this effect are only partially known. Because aquaporin-8 (AQP8) seems to facilitate the canalicular osmotic water movement during hepatocyte bile formation, we studied its gene and functional expression in LPS-induced cholestasis. 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, expression and subcellular localization of hepatocyte sinusoidal AQP9 were unaffected. Immunohistochemistry for liver AQPs confirmed these observations. Osmotic water permeability (Pf) of canalicular membranes, measured by stopped-flow spectrophotometry, was significantly reduced (65 ± 1 vs. 49 ± 1 μm/s) by LPS, consistent with defective canalicular AQP8 functional expression. By Northern blot analysis, we found that 1.5-kb AQP8 mRNA expression was increased by 80%, suggesting a posttranscriptional mechanism of protein reduction. The tumor necrosis factor-α (TNF-α) receptor fusion protein TNFp75:Fc prevented the LPS-induced impairment of AQP8 expression and bile flow, suggesting the cytokine TNF-α as a major mediator of LPS effect. Accordingly, studies in hepatocyte primary cultures indicated that recombinant TNF-α downregulated AQP8. The effect of TNF-α was prevented by the lysosomal protease inhibitors leupeptin or chloroquine or by the proteasome inhibitors MG132 or lactacystin, suggesting a cytokine-induced AQP8 proteolysis. In conclusion, our data suggest that LPS induces the TNF-α-mediated posttranscriptional downregulation of AQP8 functional expression in hepatocytes, a mechanism potentially relevant to the molecular pathogenesis of sepsis-associated cholestasis.
|Original language||English (US)|
|Journal||American Journal of Physiology - Gastrointestinal and Liver Physiology|
|State||Published - Feb 1 2008|
- Bile secretion
- Water channels
- Water transport