OBJECTIVE The accumulation of endogenous bile acids contributes to hepatocellular damage during cholestatic liver disease. To evaluate the potential role of apoptotic cell death due to increased concentrations of bile acids, primary human hepatocytes were treated with hydrophobic and hydrophilic bile acids. Because the Fas receptor-ligand system may mediate apoptosis in human liver cells, the effect of toxic bile acids on hepatocellular Fas receptor expression was evaluated. METHODS Primary human hepatocytes were incubated with 50 and 100 microM glycochenodeoxycholic acid (GCDCA) and co-incubated with equimolar concentrations of tauroursodeoxycholic acid (TUDCA). To evaluate cytolytic and apoptotic effects, morphological alterations, hepatocellular enzyme release, nuclear DNA fragmentation and hepatocellular Fas receptor expression were evaluated. RESULTS Apoptotic cell death was significantly increased after exposure to 50 microM GCDCA. Bile acid-induced apoptosis was not accompanied by hepatocellular Fas receptor overexpression. Tauroursodeoxycholic acid reduced apoptosis, as indicated by a significant reduction of oligonucleosomal DNA cleavage. Fas receptor expression was not significantly affected by tauroursodeoxycholic acid. At higher concentrations, direct cytolytic cell destruction was observed. CONCLUSIONS Primary human hepatocytes represent a suitable model to study bile acid-induced apoptotic cell death. In these hepatocytes, already low bile acid concentrations might induce apoptotic cell death, which is not triggered by hepatocellular Fas receptor overexpression. Apoptotic DNA fragmentation was significantly reduced by co-incubation with tauroursodeoxycholic acid. The reduction of bile acid-induced apoptosis by ursodeoxycholic acid and its conjugates may contribute to the beneficial effects of these hydrophilic bile acids used for medical treatment of several cholestatic liver diseases.