COORDINATE REGULATION OF GLUCOCORTICOID RECEPTOR AND C-JUN MESSENGER-RNA LEVELS - EVIDENCE FOR CROSS-TALK BETWEEN 2 SIGNALING PATHWAYS AT THE TRANSCRIPTIONAL LEVEL

Autologous regulations of steroid receptors by their cognate ligands h as been demonstrated for a number of nuclear receptor family members. To determine the molecular mechanism for glucocorticoid receptor (GR) autoregulation, the expression of glucocorticoid receptor mRNA and pro tein levels were examined in the mouse AtT-20 pituitary tumor cell lin e. The expression of c-jun and c-fos mRNA and protein was also examine d in the same cell extracts. A rapid down-regulation of the GR protein was observed after treatment with the glucocorticoid analog, triamcin olone acetonide (TA). An oscillatory, parallel regulation of both GR a nd c-jun mRNA levels occurred. In contrast, POMC mRNA levels remained at a stable, low level during chronic TA treatment. Dose-response anal yses also revealed a coordinate down-regulation of GR and c-jun (but n ot POMC or c-fos) mRNA levels. FOS protein levels were unaffected by T A treatment. Surprisingly, JUN protein levels were increased by TA, ev en when the c-jun mRNA translation occurs after TA treatment, and this may be due to GR/JUN heteromer formation interfering JUN repression o f c-jun mRNA translation. The effect of TA on GR and c-jun gene expres sion was a primary effect, as it occurred rapidly and was not inhibite d by cycloheximide (CHX). Nuclear run-on transcription assays revealed a rapid (15 min) down-regulation in both GR and c-jun gene transcript ion rates, while POMC gene transcription was unaffected at this early time. Treatment of AtT-20 cells with all-trans retinoic acid gave diff erent kinetics for GR and c-jun mRNA regulation than obtained with TA; however, the GR and c-jun mRNA levels were still coordinately regulat ed after retinoic acid treatment. Based upon these data, the promoter structures of the GR and c-jun genes, and previously published results , a novel mechanism for the coupled regulation of GR and c-jun transcr iption, via a direct transcriptional interference with AP-1 (FOS/JUN) activity, is proposed.