GLUCOCORTICOID REGULATION OF CORTICOTROPIN-RELEASING FACTOR(1) RECEPTOR EXPRESSION IN PITUITARY-DERIVED ATT-20 CELLS

Corticotropin-releasing factor (CRF) receptors represent one of the pr imary sites for negative feedback of the pituitary by adrenocortical g lucocorticoid hormones; however, the molecular mechanisms involved hav e yet to be elucidated. The present study examines the mechanisms by w hich glucocorticoids regulate CRF-RI expression in the pituitary cell line, AtT-20. Treatment of these cells with dexamethasone resulted in a concentration- and time-dependent inhibition of CRF-RI mRNA that was significant by 1 hr and maximal after 4 hr. Levels of CRF-R1 mRNA the n returned to control levels after 24 hr. Similar changes were observe d when the cells were treated with corticosterone. Pro-opiomelanocorti n mRNA was also decreased after dexamethasone pretreatment; however, t he time course was much slower with a significant effect only detected after 6 hr. Further analysis of the mechanisms that mediate glucocort icoid regulation of CRF-R1 mRNA was conducted. These studies demonstra ted that glucocorticoid incubation significantly decreases the rate of CRF-R1 gene transcription, as determined by nuclear run-on analysis. In addition, the results demonstrate that glucocorticoid incubation si gnificantly decreases CRF-RI mRNA stability by approximately 50%. The down-regulation of CRF-R1 mRNA was dependent on de novo protein synthe sis, as it was blocked by pretreatment with cycloheximide. This repres ents a novel mechanism for glucocorticoid negative feedback regulation of CRF-R1 expression.