Tetrahydroaminoacridine-induced apoptosis in rat hepatocytes

Tacrine (tetrahydroaminoacridine, THA) is currently administered to thousands of patients for the treatment of Alzheimer's disease. Unfortunately, THA therapy is often limited by this drugs' propensity to induce reversible hepatotoxicity. In the present study we investigated the mechanism of THA cytotoxicity by measuring the effect of THA on cell viability, protein synthesis activity and the induction of apoptosis in suspensions of freshly isolated rat hepatocytes. Our experimental findings indicate that THA-mediated apoptosis is responsible for the acutein vitro hepatotoxicity observed with this aminoacridine derivative. We found that THA-treated hepatocytes (0.1, 0.25 and 0.5 mm) demonstrated a significant and dose-dependent reduction in cellular protein synthesis activity (84, 55 and 5% of control activity, respectively) after 1 hr of incubation. However, in hepatocytes exposed to 0.1 and 0.25 mm THA, the inhibition of protein synthesis was short-lived. In these treated cells, protein synthesis activity returned to control levels (100%) by the fifth hr of incubation without a significant increase in cellular lactate dehydrogenase (LDH) leakage or the induction of apoptosis. In hepatocytes exposed to 0.5 mm THA, the near complete inhibition of protein synthesis was not reversible and a dramatic increase in LDH leakage (necrosis) was observed after 6 hr of treatment. In 0.5 mm THA-treated hepatocytes the appearance of apoptotic nuclei and cells were observed with electron microscopy following 2 hr of treatment (12% of total hepatocytes analysed) and steadily increased to 42% by the fifth hr (compared with 4% for control cells). We speculate that THA's ability to inhibit hepatocyte protein synthesis (>50%) and induce apoptosis may have an important role in the hepatotoxic episodes experienced by Alzheimer's patients taking this drug. However, the role of apoptosis in clinical THA-induced hepatotoxicity and the relevance of using rat hepatocyte suspensions as anin vitro model for THA hepatotoxictyin vivo require additional investigation.