THE GLUCOCORTICOID RECEPTOR INHIBITS THE HUMAN PROLACTIN GENE-EXPRESSION BY INTERFERENCE WITH PIT-1 ACTIVITY

Glucocorticoids have been shown to inhibit the activity of the human p rolactin (hPRL) promoter. Using transient expression experiments in ra t pituitary cells, we located the sequence conferring glucocorticoid i nhibition to a region which contains Pit-1 binding sites, responsible for pituitary-specific expression, but does not seem to contain a gluc ocorticoid receptor (GR) binding site. Co-transfection experiments in non-pituitary cell lines, using expression vectors for Pit-1 and diffe rent mutants of the human GR show that inhibition of the hPRL gene is seen only in the presence of Pit-1 and GR, and that the DNA binding fu nction of the receptor is not required. Immunoprecipitation studies sh ow that either anti-GR or anti-Pit-1 antibodies are able to co-precipi tate GR and Pit-1, suggesting an interaction between these factors. We conclude that the activated GR functionally interferes with the pitui tary specific factor Pit-1, thereby leading to the observed transcript ional repression. (C) 1997 Elsevier Science Ireland Ltd.