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The induction of tyrosine aminotransferase (TAT) in the HTC hepatoma cell line is observed after a single administration of an active steroid. A few minutes of contact between the cells and dexamethasone or corticosterone is sufficient to induce TAT synthesis to its maximal level. When radiolabeled hormones are used, no radioactivity is found in the cell one hour after removal of the hormone from the culture medium, whereas TAT activity remains optimal. Thus, the hormone behaves like a start signal for the optimal synthesis of the enzyme and its continuous presence in the medium is not necessary during the whole induction cycle.

Sodium butyrate in a 5 mM concentration prevents the induction of tyrosine aminotransferase in hepatoma culture cells, without affecting the basal level of the enzyme. This effect is reversible immediately after the removal of butyrate, or after a lag, if butyrate was present for more than 2 h. Neither the amount of cellular RNA nor the rate of total RNA synthesis were affected by sodium butyrate. Furthermore, butyrate does not inhibit protein synthesis: [35S]methionine incorporation into proteins, measured in a reticulocyte lysate system, shows no significant difference between the translation capacity of the RNAs from butyrate-treated cells and from dexamethasone-induced or uninduced cells. Nevertheless, when tyrosine aminotransferase was isolated from the translation products by its specific antiserum and analyzed by gel electrophoresis, we observed that the amount of the enzyme synthetized in the presence of RNAs from dexamethasone/butyrate-treated cells was strongly diminished relative to that synthesized in the presence of RNA from dexamethasone-induced cells. These experiments indicate that the treatment of the cells with butyrate decreases the activity of the specific messenger RNA for tyrosine aminotransferase to a level close to the basal level.