Biliary excretory function is regulated by canalicular membrane fluidity associated with phospholipid fatty acyl chains in the bilayer: implications for the pathophysiology of cholestasis

Current insights in the complexities underlying drug-induced cholestasis

Drug-induced cholestasis (DIC) poses a major challenge to the pharmaceutical industry and regulatory agencies. It causes both drug attrition and post-approval withdrawal of drugs. DIC represents itself as an impaired secretion and flow of bile, leading to the pathological hepatic and/or systemic accumulation of bile acids (BAs) and their conjugate bile salts. Due to the high number of mechanisms underlying DIC, predicting a compound's cholestatic potential during early stages of drug development remains elusive. A profound understanding of the different molecular mechanisms of DIC is, therefore, of utmost importance. Although many knowledge gaps and caveats still exist, it is generally accepted that alterations of certain hepatobiliary membrane transporters and changes in hepatocellular morphology may cause DIC. Consequently, liver models, which represent most of these mechanisms, are valuable tools to predict human DIC. Some of these models, such as membrane-based in vitro models, are exceptionally well-suited to investigate specific mechanisms (i.e. transporter inhibition) of DIC, while others, such as liver slices, encompass all relevant biological processes and, therefore, offer a better representation of the in vivo situation. In the current review, we highlight the principal molecular mechanisms associated with DIC and offer an overview and critical appraisal of the different liver models that are currently being used to predict the cholestatic potential of drugs.

Changes in GM1 ganglioside content and localization in cholestatic rat liver

(Glyco)sphingolipids (GSL) are believed to protect the cell against harmful environmental factors by increasing the rigidity of plasma membrane. Marked decrease of membrane fluidity in cholestatic hepatocytes was described but the role of GSL therein has not been investigated so far. In this study, localization in hepatocytes of a representative of GSL, the GM1 ganglioside, was compared between of rats with cholestasis induced by 17alpha-ethinylestradiol (EE) and vehicle propanediol treated or untreated animals. GM1 was monitored by histochemical reaction employing cholera toxin B-subunit. Our findings in normal rat liver tissue showed that GM1 was localized in sinusoidal and canalicular hepatocyte membranes in both peripheral and intermediate zones of the hepatic lobules, and was nearly absent in central zones. On the contrary, in EE-treated animals GM1 was also expressed in central lobular zones. Moreover, detailed densitometry analysis at high magnification showed greater difference of GM1 expression between sinusoidal surface areas and areas of adjacent cytoplasm, caused as well by increased sinusoidal staining in central lobular zone as by decreased staining in cytoplasm in peripheral zone. These differences correlated with serum bile acids as documented by linear regression analyses. Both GM1 content and mRNA corresponding to GM1-synthase remained unchanged in livers; the enhanced expression of GM1 at sinusoidal membrane thus seems to be due to re-distribution of cellular GM1 at limited biosynthesis and could be responsible for protection of hepatocytes against harmful effects of bile acids accumulated during cholestasis.

Hepatic canalicular membrane transport of bile salt in C57L/J and AKR/J mice: implications for cholesterol gallstone formation

C57L/J (gallstone-susceptible) and AKR/J (gallstone-resistant) mice have been utilized for quantitative trait loci (QTL) analysis to identify the Lith 1 locus for cholelithiasis. Abcb11 encodes for the liver canalicular membrane bile salt export pump (BSEP), which maps to this QTL and is a candidate gene for Lith 1. We investigated the transmembrane transport of taurocholate in canalicular liver membrane vesicles isolated from these murine strains. Canalicular liver plasma membranes (cLPM) and RNA were isolated from C57L/J and AKR/J mice livers, and were utilized for Northern and Western blot analysis and functional (3)H-taurocholate uptake studies. ATP-dependent (3)H-taurocholate uptake was significantly higher in AKR/J, compared to C57L/J mice. V(max) was 127 vs. 42 pmol TC/mg/s in the murine strains, respectively, while K(m) was unchanged. In contrast, gene and protein expression of hepatic Abcb11 was increased three-fold in C57L/J, compared to AKR/J mice. Thus, Abcb11 bile salt transport activity per unit protein was reduced nine-fold in the C57L/J, compared to AKR/J mice. In contrast, canalicular membrane cholesterol:phospholipid content was also significantly higher in the C57L/J mice. We conclude that gallstone-susceptible C57L/J mice demonstrate increased gene and canalicular membrane expression of Abcb11, however, taurocholate transport is functionally diminished. The latter may be due to the increased cholesterol membrane content of the cLPM in C57L/J mice. These findings may be important for the pathogenesis of gallstone formation.