Cell type-dependent effect of phospholipid and cholesterol on bile salt cytotoxicity

The effect of phosphatidylcholine and cholesterol on bile salt-induced cytotoxicity was investigated. Experiments were performed in both human erythrocytes and cultured CaCo-2 cells, a model system for gastrointestinal epithelium. Hemolysis induced by 50 mmol/L sodium-taurocholate was reduced by both lecithin and cholesterol in a concentration-dependent manner. Cholesterol was 10 times more effective than phosphatidylcholine. Addition of only small amounts of the sterol to phosphatidylcholine/taurocholate solutions eliminated all cytotoxicity. The protective influence of cholesterol is probably mediated through a direct effect on the membrane. Incubation of erythrocytes with a cholesterol/taurocholate mixture greatly increased the cholesterol content of the membrane. With respect to sensitivity to bile salts and the protective effect of lecithin, CaCo-2 cells behaved very similar to erythrocytes. However, cholesterol failed to have any cytoprotective effect when used in combination with taurocholate or phosphatidylcholine/taurocholate solutions. Interestingly, at relatively high concentrations of cholesterol (cholesterol saturation index greater than 1.0), the sterol even increased cytotoxicity. This correlated with a cholesterol-induced shift of phosphatidylcholine from micelles to vesicles, which normally occurs in supersaturated model and human bile. The different sensitivity of the two cell types to the effect of cholesterol on bile salt damage might be caused by the difference in lipid membrane composition. In conclusion, CaCo-2 cells represent a physiologically more relevant model system to study bile cytotoxicity than erythrocytes. When extrapolated to gallbladder epithelial cells, these results could be relevant for the pathogenesis of gallstone disease. The increased cytotoxicity might be the signal by means of which supersaturated bile induces mucin hypersecretion by gallbladder epithelial cells.