Cyclic adenonosine monophosphate does not affect the stability of the messenger ribonucleic acid for tyrosine aminotransferase in cultured hepatoma cells.

Cyclic AMP has been shown to stimulate synthesis of tyrosine aminotransferase (L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) by increasing the amount of its mRNA through an increase in initiation of transcription. However, cAMP also has posttranscriptional effects on the enzyme's synthesis, as evidenced by the 4- to 5-fold enhanced decline seen when cultured hepatoma cells are exposed to cAMP and transcription is inhibited. As a direct test of the possibility that cAMP exerts this effect by destabilizing the mRNA for tyrosine aminotransferase, we analyzed the rate of decay of the mRNA using the transcriptional inhibitor 5,6-dichlororibofuranosylbenzimidazole, Northern blot analysis, and an internal standard consisting of prelabeled rRNA. It was found that the half-life of the mRNA (2.0 +/- 0.2 h) was not changed by treatment of cultured hepatoma cells under conditions which increase intracellular cAMP levels. These mRNA half-life values were not significantly different from the decline in the rate of synthesis of the enzyme after induction in dexamethasone-treated cells. We conclude that cAMP does not affect the stability of the mRNA for tyrosine aminotransferase and discuss other possible explanations for the paradoxical effect of cAMP on deinduction of this enzyme.