GLUCOCORTICOID RECEPTOR-BINDING SITES IN THE PROMOTER REGION OF MILK PROTEIN GENES

The action of glucocorticoids on the induction of the two milk protein genes beta-casein and whey acidic protein (WAP) is delayed and appear s to be indirect. The response requires a co-stimulation of cells with prolactin, is restricted to mammary epithelial cells and is dependent on cis-acting sequences localized in the promoter region of the two g enes. We have searched for glucocorticoid receptor (GR) binding sites in these hormone response regions. In vitro DNaseI footprinting experi ments were performed with a purified preparation of the GR from rat li ver. The sequences between -231 and -7 and between -250 and -79 of mou se WAP and rat beta-casein gene promoter, respectively, were found to contain multiple sites which were protected from the attack of DNasel by the GR preparation. The sites comprise sequence motifs related to t he delayed secondary glucocorticoid response elements described (Chan et al., J. Biol. Chem. 266, 22,634-22,644, 1991), which represent half sites of classical GR binding sites. In the WAP gene promoter, the mo tifs are arranged head to tail with irregular spacing. The GR binding sites are in close proximity or even overlap with the binding sites fo r candidate factors involved in mammary cell specific gene expression. The results suggest a direct co-operation between GR and mammary cell specific transcription factors in mediating the expression of milk pr otein genes.