REGULATION OF GLUCOCORTICOID RECEPTOR (GR) MESSENGER-RNA AND PROTEIN-LEVELS BY PHORBOL ESTER IN MCF-7 CELLS - MECHANISM OF GR MESSENGER-RNAINDUCTION AND DECAY

Treatment of MCF-7 cells with the phorbol ester 12-O-tetradecanoylphor bol-13-acetate (TPA) (10(-7) M) was associated with a time-dependent i ncrease in specific binding of [H-3]dexamethasone (34.8 +/- 4.6 fmol/m g protein after 9 h of TPA treatment compared with 16.0 +/- 2.3 fmol/m g protein in control cells) as well as a transient induction in the le vel of glucocorticoid receptor (GR) mRNA (4- to 8-fold stimulation aft er 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 mu g/ml) the TPA-dependent induction of GR mRNA was completely aboli shed, 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 cy cloheximide (50 mu M) resulted in a superinduction of GR mRNA (>50-fol d after 6 h). Inhibition of transcription by AMD after 3 h of TPA trea tment was associated with a decay of GR mRNA with a half-life of 2-3 h , which is identical to that observed in non-treated cells. We conclud e that the increase in GR mRNA in the presence of TPA is dependent on ongoing transcription, whereas the rate by which GR transcripts are de graded, is not altered by TPA.