The Degradation of Phosphoenolpyruvate Carboxykinase (GTP) mRNA Is Regulated by Cyclic AMP But Not by Dexamethasone

The relationship between the control by cAMP of phosphoenolpyruvate carboxykinase (GTP) mRNA synthesis and breakdown was examined. The half-life of phosphoenolpyruvate carboxykinase mRNA was estimated from its decay rate in cells exposed to insulin (a specific inhibitor of phosphoenolpyruvate carboxykinase gene transcription) in the presence or absence of 8-chlorophenylthio-cyclic AMP after high levels of the mRNA had been induced by the cyclic nucleotide. The mRNA decayed with a half-life of 300-350 mm in cells treated with 8-chlorophenylthio-cyclic AMP and insulin whereas the T1/2 was 30-40 min in cells exposed to insulin alone. Similar results were obtained when 5,6-dichloro-1-β-ribofuranosyl-benzimidazole, a general inhibitor of mRNA synthesis, was used. The mRNA decayed at comparable rates in cells treated with insulin or 5,6-dichloro-1-β-ribofuranosyl-benzimidazole or in cells exposed to insulin-free medium after withdrawal of the inducer of gene transcription. These results suggest that insulin does not modulate phosphoenolpyruvate carboxykinase mRNA breakdown. 8-Chlorophenylthio-cyclic AMP reversed the rapid decay of the mRNA in cells preexposed to insulin for 1 h even when over 60% of the mRNA had been degraded. In contrast to cAMP, no alterations in the rate of phosphoenolpyruvate carboxykinase mRNA decay were noted in cells exposed to the synthetic glucocorticoid dexamethasone. The half-life of phosphoenolpyruvate carboxykinase mRNA, expressed from a hybrid gene driven by the non-cAMP regulated promoter of the mouse metallothionein-I gene, was also increased in cells treated with 8-chlorophenylthio-cyclic AMP. These results suggest that the control by cAMP of phosphoenolpyruvate carboxykinase mRNA breakdown is accomplished by a rapid mechanism which is independent of the induction of gene transcription.