Hormonal regulation of translatable mRNA of tryptophan 2,3-dioxygenase in primary cultures of adult rat hepatocytes

Tryptophan 2,3-dioxygenase [EC 1.13.11.11] in primary cultures of adult rat hepatocytes was induced 3-4 fold by 1 microM dexamethasone and 6-7 fold by dexamethasone plus glucagon (0.1 microM). Changes of the enzyme activity, amount of enzyme, measured by immunotitration, and rate of enzyme synthesis, assayed by measurement of [3H]leucine incorporation into the enzyme protein, were closely correlated. Furthermore, in a reticulocyte lysate system for cell-free protein synthesis, mRNA of the enzyme was translated to the protein corresponding to the subunit of tryptophan 2,3-dioxygenase, which was identified by SDS-polyacrylamide gel electrophoresis. The activity of translatable mRNA of the enzyme was increased more than 10-fold by dexamethasone and its final content in total mRNA was 0.34%. Glucagon alone did not increase mRNA activity, but dexamethasone plus glucagon increased mRNA activity to twice that with dexamethasone alone, the maximal content of the mRNA being 0.77% of the total mRNA content 12 h after addition of hormones. Insulin (0.1 microM) caused 75% inhibition of the maximum increase of mRNA activity of the enzyme induced by dexamethasone and glucagon. Epinephrine (10 microM) also caused 58% inhibition of the maximum increase. Insulin and epinephrine also suppressed increase of mRNA of tryptophan 2,3-dioxygenase induced by dexamethasone alone. Therefore, dexamethasone alone or together with glucagon stimulated transcription of tryptophan 2,3-dioxygenase increasing its mRNA and enzyme synthesis in hepatocytes. Conversely, insulin and epinephrine suppressed these increases of mRNA synthesis and thus decreased enzyme synthesis.