Hepatotoxicity of tacrine: Occurrence of membrane fluidity alterations without involvement of lipid peroxidation

Tacrine (THA), used in the treatment of Alzheimer's disease, is known to in duce hepatotoxicity, the mechanisms of which remain to be fully established . We have previously shown that THA reduced intracellular glutathione conce ntration in rat hepatocytes in primary culture, thus pointing to a possible role for oxidative stress in THA toxicity. To test this, the effects of an tioxidant molecules, namely, the flavonoids silibinin, silibinin dihydrogen succinate, and silymarin, were evaluated on the toxicity of THA in cultured rat hepatocytes. This toxicity was investigated after a 24-h treatment ove r a concentration range from 0 to 1 mM, in the presence or absence of antio xidant (1 and 10 mu M). We found that simultaneous treatment of hepatocytes with any of the antioxidants and THA remained ineffective on the lactate d ehydrogenase release induced by THA. Then, the production of lipid-derived radicals (to estimate lipid peroxidation) was measured in THA (0.05-0.50 mM )-treated cells using a spin-trapping technique coupled to electron paramag netic resonance (EPR) spectroscopy. No increase of the EPR signal was obser ved over the period of 30 min to 24 h. In contrast, treatment of cells with the spin label 12-doxyl stearic acid followed by EPR spectroscopy showed t hat THA (0.05 and 0.25 mM) rapidly increased hepatocyte membrane fluidity. Extracellular application of GM1 ganglioside (60 mu M) both reversed this i ncrease in fluidity and partially reduced lactate dehydrogenase release on THA exposure. In conclusion, this work indicates that early alterations of membrane fluidity, not resulting from lipid peroxidation, are likely to pla y an important role in the development of THA toxicity.