Effect of tryptophan on isolated hepatocytes of rats

The addition of tryptophan to adult rat hepatocyte cultures stimulated DNA synthesis. The increase in DNA synthesis as measured by 3H-thymidine incorporation into DNA was observed on treatment of the cultures with tryptophan for 48 h but also as short as for 6 h in comparison with control cultures. An increase was also apparent at 30 h which was maintained for up to 48 h post treatment with tryptophan. The increase in DNA synthesis by tryptophan cannot be attributed to cell injury or to increased DNA degradation. Of the degradative enzymes added after harvesting the hepatocytes, only DNase decreased incorporation of 3H-thymidine. The observed effect was specific for tryptophan since treatment with kynurenine, isoleucine, methionine or serine failed to show a significant effect. Pretreatment of cultured hepatocytes with hydroxyurea prevented the tryptophan stimulated increase in DNA synthesis suggesting that the latter was due to replicative and not to reparative DNA synthesis. Experiments performed with the addition of diethylnitrosamine also alluded to tryptophan's role in replicative DNA synthesis. The mechanism of tryptophan-induced DNA synthesis is discussed.