Comparison of the effects of bile acids on cell viability and DNA synthesis by rat hepatocytes in primary culture

Bile acid-induced inhibition of DNA synthesis by the regenerating rat liver in the absence of other manifestation of impairment in liver cell viabilit y has been reported. Because in experiments carried out on in vivo models b ile acids are rapidly taken up and secreted into bile, it is difficult to e stablish steady concentrations to which the hepatocytes are exposed. Thus, in this work, a dose-response study was carried out to investigate the in v itro cytotoxic effect of major unconjugated and tauro- (T) or glyco- (G) co njugated bile acids and to compare this as regards their ability to inhibit DNA synthesis. Viability of hepatocytes in primary culture was measured by Neutral red uptake and formazan formation after 6 h exposure of cells to b ile acids. The rate of DNA synthesis was determined by radiolabeled thymidi ne incorporation into DNA. Incubation of hepatocytes with different bile ac id species - cholic acid (CA), deoxycholic acid (DCA), chenodeoxycholic aci d (CDCA) and ursodeoxycholic acid (UDCA), in the range of 10-1000 mu M - re vealed that toxicity was stronger for the unconjugated forms of CDCA and DC A than for CA and UDCA. Conjugation markedly reduced the effects of bile ac ids on cell viability. By contrast, the ability to inhibit radiolabeled thy midine incorporation into DNA was only slightly lower for taurodeoxycholic acid (TDCA) and glycodeoxycholic acid (GDCA) than for DCA. When the effect of these bile acids on DNA synthesis and cell viability was compared, a cle ar dissociation was observed. Radiolabeled thymidine incorporation into DNA was significantly decreased (-50%) at TDCA concentrations at which cell vi ability was not affected. Lack of a cause-effect relationship between both processes was further supported by the fact that well-known hepatoprotectiv e compounds, such as tauroursodeoxycholic acid (TUDCA) and S-adenosylmethio nine (SAMe) failed to prevent the effect of bile acids on DNA synthesis. In summary, our results indicate that bile acid-induced reduction of DNA synt hesis does not require previous decreases in hepatocyte viability. This sug gests the existence of a high sensitivity to bile acids of cellular mechani sms that may affect the rate of DNA repair and/or proliferation, which is o f particular interest regarding the role of bile acids in the etiology of c ertain types of cancer. (C) 2000 Elsevier Science B.V. All rights reserved.