Regulation of glucocorticoid receptor (GR) mRNA and protein levels by phorbol ester in MCF-7 cells. Mechanism of GR mRNA induction and decay.

Treatment of MCF-7 cells with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) (10(-7) M) was associated with a time-dependent increase in specific binding of [3H]dexamethasone (34.8 +/- 4.6 fmol/mg protein after 9 h of TPA treatment compared with 16.0 +/- 2.3 fmol/mg protein in control cells) as well as a transient induction in the level of glucocorticoid receptor (GR) mRNA (4- to 8-fold stimulation after 2-3 h, followed by a decline towards the control value after 6 h). In the presence of the transcription inhibitor actinomycin D (AMD) (5.0 micrograms/ml) the TPA-dependent induction of GR mRNA was completely abolished, and GR mRNA showed a gradual decline with a half-life of 2-3 h. In contrast, treatment with TPA and the protein synthesis inhibitor cycloheximide (50 microM) resulted in a superinduction of GR mRNA (> 50-fold after 6 h). Inhibition of a half-life of 2-3 h, which is identical to that observed in non-treated cells. We conclude that the increase in GR mRNA in the presence of TPA is dependent on ongoing transcription, whereas the rate by which GR transcripts are degraded, is not altered by TPA.